Strategies to boost chemiresistive sensing performance of In₂O₃-based gas sensors: an overview

Abstract

The development of effective and efficient materials for the selective and sensitive detection of toxic gases and volatile organic compounds is crucial for protecting human health and the environment. In this respect, a well-known n-type semiconducting material, namely indium oxide (In₂O₃), has attracted significant attention because of its gas-sensing applications. The rapid advances in various synthesis techniques have enabled researchers to explore numerous novel nanostructures and their integration into smart gas-sensing devices. Despite sustainable development, the application of In₂O₃ in gas sensing is limited by its poor selectivity, high working temperature, and response deterioration under humid conditions. This review outlines various strategies, such as morphology and interface engineering, catalytic functionalization, shell structure and thickness, and doping, for improving the gas detection performance of In₂O₃-based chemiresistive gas sensors. The significant influence of the nanostructures with different morphologies on the gas-sensing performance of In₂O₃-based sensors is also demonstrated. Pristine In₂O₃ nanomaterials with zero-dimensional (0D) to three-dimensional (3D) morphologies are reviewed. Different composites of In₂O₃, including In₂O₃/metal oxides (p-type and n-type), In₂O₃/noble metal loading and encapsulation, In₂O₃/elemental doping, In₂O₃/conducting polymers, and In₂O₃/carbonaceous materials, were evaluated to improve their sensing performances. Finally, a future outlook on the further progress of the In₂O₃ gas sensors is suggested.

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Sanjit Manohar Majhi

Sanjit Manohar Majhi received his M.Sc. degree from Utkal University, India in 2008, and his Ph.D. degree from Jeonbuk National University (JBNU), South Korea in Aug 2017. In March 2018, he joined as a Postdoctoral Fellow at King Abdullah University of Science and Technology (KAUST), Kingdom of Saudi Arabia. In Sept 2019, he joined as a Postdoctoral Researcher at Hanyang University, Seoul, South Korea. Since Jan 2021, he has been working as a Research Associate at United Arab Emirates University (UAEU), UAE. He is interested in the synthesis of various nanostructures including core@shell NPs, MOFs, 1D metal–oxide NPs, and 2D MXene-based materials for gas sensing applications.

Sachin T. Navale

Sachin T. Navale received his M.Sc. and Ph.D. degrees in Physics from the School of Physical Sciences at Solapur University, India in 2012 and 2015, respectively. After that, he worked as a Postdoctoral Research Associate at Shenzhen University in China (2015–2019). Then, he worked as a Postdoctoral Researcher at Hanyang University in Seoul and Inha University in Incheon, South Korea (Feb. 2021 to Dec. 2021). Currently, he is working as a Marie Sklodowska-Curie Individual Researcher at the University of Barcelona in Spain. His research is focused on synthesizing various nanostructures of metal oxides, conducting polymers, organic/inorganic nanocomposites, and metal–organic frameworks for chemiresistive gas sensor applications.

Ali Mirzaei

Ali Mirzaei received his B.Sc. from Isfahan University of Technology in 2006 and his Master and Ph.D. from Shiraz University in 2009 and 2016, respectively. He was a visiting student at Messina University, Italy in 2015. From 2016 to 2018 he was a postdoc researcher at Hanyang University in Seoul, Korea under the supervision of Prof. H. W. Kim. In 2022, he was selected for the Brain Pool program at Hanyang University. Currently, he is an assistant professor at the Shiraz University of Technology, Shiraz, Iran. He has authored and co-authored 157 peer-reviewed papers. He is interested in the synthesis and characterization of nanomaterials and gas sensors.

Hyoun Woo Kim

Hyoun Woo Kim joined the Division of Materials Science and Engineering at Hanyang University as a full professor in 2011. He received his B.S. and M.S. degrees from Seoul National University and his Ph.D. degree from Massachusetts Institute of Technology (MIT) in electronic materials in 1986, 1988, and 1994, respectively. He was a senior researcher at Samsung Electronics Co., Ltd from 1994 to 2000. He was a professor of materials science and engineering at Inha University from 2000 to 2010. He was a visiting professor at the Department of Chemistry of Michigan State University, in 2009. His research interests include the dimensional nanostructures, nanosheets, and gas sensors.

Sang Sub Kim

Sang Sub Kim joined the Department of Materials Science and Engineering, at Inha University, in 2007 as a full professor. He received his B.S. degree from Seoul National University and his M.S. and Ph.D. from Pohang University of Science and Technology (POSTECH) in Material Science and Engineering in 1987, 1990, and 1994, respectively. He was a visiting researcher at the National Research in Inorganic Materials (currently NIMS), Japan for 2 years each in 1995 and in 2000. In 2006, he was a visiting professor at Department of Chemistry, University of Alberta, Canada. In 2010, he also served as a cooperative professor at Nagaoka University of Technology, Japan. His research interests include the synthesis and applications of nanomaterials such as nanowires and nanofibers, functional thin films, and surface and interfacial characterizations.

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1. Introduction to chemiresistive gas sensors

Owing to the development of industries, a range of toxic gases, such as sulfur dioxide (SO₂), carbon monoxide (CO), hydrogen sulfide (H₂S), ammonia (NH₃), nitrogen dioxide (NO₂), and volatile organic compounds (VOCs) are emitted into the environment, causing alarming damage and threatening the sustainability of human society.^1–3 Exposure to toxic gases and vapors, even at low concentrations, causes severe health risks to human beings. Therefore, many health- and safety-related agencies have set guidelines for exposure limits regarding these toxic gases for human beings.^4–6 The physical properties of these toxic gases and VOCs, their uses, exposure limits, and human health effects have been presented in various reports.^7–24 The increasing worldwide concerns regarding environmental pollution have driven society to seek out gas detection systems for monitoring and quantifying these dangerous and life-threatening gases. Gas detectors have been exploited in several areas, including industries, automobiles, and mining, for environmental monitoring, both indoors and outdoors.^25,26 Compared to conventional detection methods, chemiresistive gas sensors based on metal–oxide semiconductors (MOS) are extensively utilized because of their excellent attributes, such as low acquisition price, miniature form, robustness, simple fabrication, and versatility in detecting wide ranges of gases with high sensitivity.^27,28 In the case of chemiresistive gas sensors, sensing materials change their electrical resistance as the analyte or the gas interacts. Therefore, the resistance of chemiresistive sensors depends mainly on the direct chemical interactions between the detection material and the target gas. According to a literature survey,²⁹ more than 90% of studies on MOS gas sensors used n-type MOS. This is because n-type MOS sensors generally exhibit higher sensitivity than its counterpart (i.e., p-MOS sensor) owing to the larger number and higher mobility of charge carriers.³⁰ Among the many n-type MOS gas sensors, indium oxide (In₂O₃) is a capable chemiresistive sensor material because of its exceptional optical, physicochemical, and electrical properties and high electron mobility (160 cm² V s⁻¹).³¹ In addition to its gas-sensing applications, In₂O₃ (band-gap of 3.5 eV with good transparency) is a sought-after material for many applications, such as energy storage, catalysis, microelectronics, and solar cells.^32–35

1.1 Key challenges of current gas sensors

The key parameters of a chemiresistive MOS gas sensor are sensitivity, selectivity, and stability, which are collectively known as “3S”.^36,37 An ideal gas sensor should meet the above ‘3S’ requirement criteria, such as high response and selectivity, towards a target gas together with long-term response stability. Furthermore, low working temperature is an advantage. On the other hand, despite the extensive research on MOS-based sensors, existing gas sensors have shown comparatively low selectivity towards various test gases. Moreover, their high operating temperature results in high energy consumption. In addition, one of the challenges with the chemiresistive sensors is the inadequate response of MOS-based gas sensors under high humidity conditions. As a result, recent challenges in the MOS-based sensing field drive extremely active research towards enhancing the sensitivity or the response, achieving higher selectivity towards target gas, and miniaturizing the sensors, making them portable and easy to integrate into smartphones.³⁸ Thus, several strategies are being developed to advance the sensing properties of MOS-based chemiresistive sensors and meet the required standards of modern life. Various influencing factors play a major role in achieving distinctive sensing characteristics of the MOS-based sensors, including surface morphology, microstructure, surface area, nanomaterials porosity, the structure of nanocomposites, defect structures, and working temperature.^39–42

Several synthetic strategies have been used to synthesize In₂O₃ with various surface morphologies from 0D to 3D morphologies with many attractive nanostructures.^43–46 Furthermore, other approaches, e.g., the utilization of p–n and n–n heterojunction nanostructures, elemental doping, noble metal loading, and utilization of composites with conducting polymers or carbon materials, have been developed. Scheme 1 shows various In₂O₃-based gas-sensing materials from the morphologically controlled synthesis of different nanostructures (0D to 3D) to different composite nanostructures.

Scheme 1 Different morphologies of In₂O₃-based gas sensors.

There are no detailed reports on the In₂O₃-based nanostructures for gas detection applications to the best of the authors’ knowledge. The current review comprehensively presents a state-of-the-art design of In₂O₃ nanomaterials and their detailed gas-sensing properties. First, this paper outlines the basic principle of the sensing mechanism of a chemiresistive sensor. Subsequently, the gas-sensing properties of various interesting morphologies (0D to 3D) emanating from the morphological engineering of In₂O₃ nanomaterials using different synthetic techniques are summarized methodically. Finally, the current state of progress of the different strategies for preparing composite nanostructures based on In₂O₃ to improve the gas detection properties is described.

1.2 Gas sensing mechanisms

A typical MOS gas detection device consists of three components: (i) the sensing layer (to recognize the target gas), (ii) electrodes (to detect changes in resistance), and (iii) a heating device (to reach the operating temperature of the sensor).⁴⁷ The most widely accepted sensing mechanism of MOS-based gas sensors depends on the variation of the electrical resistances of sensing materials (n- or p-type) upon the interaction of various oxidizing and reducing gases. In the case of n-type MOS gas sensors, the electrical resistance decreases when exposed to reducing gases, such as NH₃, H₂, H₂S, and CO. On the other hand, the electrical resistance increases upon the interaction of oxidizing gases, such as CO₂, NO₂, SO₂, and (ozone) O₃. On the other hand, the inverse effect applies to p-type MOS gas sensors.^47–49 This phenomenon can be explained schematically in Fig. 1. The sensitivity or the response of a MOS gas sensor is strongly affected by the interactions among the detection layer, the adsorbed oxygen, and the target gas.⁵⁰ In general, there are two types of adsorption: physical (physisorption) and chemical (chemisorption). During the physisorption process, the target gas molecules are adsorbed on the sensing layer at low working temperatures without charge transfer between the gas and sensing layer. On the other hand, during the chemisorption process at higher temperatures, charge transfer by the bond configuration relating to the adsorbed gas analytes and the sensor surface atoms.⁵⁰ In general, when an n-type In₂O₃ MOS sensor is exposed to air, the oxygen molecules from the air are adsorbed onto the material surface. This causes the removal of electrons from the conduction band (CB) of In₂O₃. This process increases the amount of negatively charged oxygen ions, such as O²⁻, O₂⁻, and O⁻, on the In₂O₃ surface. The formation of these chemisorbed oxygen species depends largely on the working temperature of the sensor.⁵¹

Fig. 1 Sensing mechanism of n-type and p-type gas sensors based on MOS. Reproduced with permission from the ref. 29, copyright 2020, Elsevier.

For example, at lower working temperatures (room temperature to 150 °C), the adsorption of oxygen molecules takes place in the following manner:

O_2(g) + e⁻ ⇌ O₂⁻_(ads) (1)

At higher working temperatures, however, the O₂⁻ anions are dissociated with single/double oxygen ions by taking a further electron from the CB of the sensor, according to the following reactions:

O₂⁻_(ads) + e⁻ ⇌ 2O⁻_{(ads) (150–300 °C)} (2)

O⁻_(ads) + e⁻ ⇌ O²⁻_{(ads) (>300 °C)} (3)

As a result, an electron depletion layer or a space charge layer (Δ_air) is produced because of the lower amounts of electrons. This results in higher sensor resistance in the presence of air. The high sensor resistance decreases upon the interaction with reducing gases, such as H₂ (shown in Fig. 2a). Generally, the Debye length (L_d) is demonstrated as the distance between the surface of the MOS sensor at which the electrons are removed.⁵² The L_d of the MOS sensor depends largely on the operating temperature and concentration of charge carriers,⁵³ as follows:

where ε, K_b, T, q (1.6 × 10⁻¹⁹ C), and N_d are the dielectric constant, Boltzmann constant, operating temperature, electron charge, and the concentration of charge carriers, respectively.⁵⁴ A depletion layer on the surface results in band bending, as shown in Fig. 2b. The flow of electrons from one grain to another is hindered by the polycrystalline sensing materials being interconnected with abundant grain boundaries. This process results in the formation of a potential barrier (V) on the surface that increases the electrical resistance of the sensor (Fig. 2c).

Fig. 2 (a) Schematic diagram of the change in the resistance of n-MOS in the presence of a reducing gas (H₂); (b) model of the surface charge layer; (c) model of grain boundary barrier. Reproduced from the ref. 51 under the Creative Commons Attribution 4.0 License (CC BY 4.0/). Copyright 2020, IOP Science.

The potential barrier height depends mainly on the quantity of oxygen molecules adsorbed.⁵⁵ The dynamic interactions between a reducing gas (e.g., H₂) and chemisorbed oxygen result in a change in carrier concentrations by releasing the electrons back to the sensor surface. This process leads to the appearance of a sensing signal. Overall, the reaction can be shown as follows:^51,56

X + Oⁿ⁻_ads → X′ + ne⁻ (5)

where X, X′, and n are the target gas, product gas, and the number of released electrons, respectively.

The sensing properties of the gas sensors are controlled primarily by three factors: the receptor function, the transducer function, and the utility factor.⁵⁷

The primary role of the receptor function of the gas sensor is to recognize the oxygen molecules and the target gas in the surrounding environment through a sensing layer. The adsorption of gas molecules depends significantly on the surface area of the detection materials. On the other hand, the surface can be modified according to the change in shape, morphology, and size of the synthesized materials. The transducer function is accountable for transforming the electronic changes that occur due to the interactions among the detection materials and the target gases with respect to a signal output as a change in resistance. Finally, the utility factor facilitates the access of gas molecules by the pores of the detection materials, which influences the response. Control over these factors (Fig. 3) can guide the development of new materials with an appropriate structure for realizing reliable gas sensors.⁵⁸

Fig. 3 Schematic illustration of the receptor, transducer function, and utility factors of MOS gas sensors. Reproduced from the ref. 58 under the Creative Commons Attribution 4.0 License (CC BY 4.0/). Copyright 2020, MDPI.

1.3 Gas-sensing performance parameters

The main parameters of the chemiresistive gas sensor are selectivity, sensitivity/response, response time (τ_res), recovery time (τ_rec), long-term response stability, repeatability, and limit of detection (LOD).⁵⁹ The term sensitivity (S) is expressed as the change in resistance (ΔR) with the change in concentration of a target gas (ΔC). The response time pertains to a 90% change in the sensor resistance in the presence of analyte gas. The recovery time is the time needed to achieve 90% of the initial resistance after stopping the target gas.⁶⁰ Moreover, selectivity is an important parameter of chemiresistive sensors. If the sensor is sensitive to some gases, it often leads to a false alarm in measuring gas. The selectivity of a gas-sensing device is the ability to determine a specific gas in a mixture of interfering gases.⁶¹ In addition, the repeatability of a chemiresistive sensor is the ability of a gas sensor to provide reproducible results when repeated measurements are performed under similar measuring conditions. The LOD is defined as the ability to detect the lowest possible concentration of an analyte using a sensor at a particular operating temperature.⁶² Finally, the stability of a chemiresistive sensor is defined as the capability of a gas sensor to provide the same results over a specified time period.⁴⁸

2. General information about hazardous gases

Air pollution results from CO, NO₂, O₃, and SO₂ gases along with particulate matter (PM) in the air.⁶³ Climate change and air pollution problems are two significant challenges for all countries.⁶⁴ More than 90% of the people live in areas with poor air quality, leading to exposure to a high risk of disease or death caused by air pollution.⁶⁵ Accordingly, air pollution is a major factor in the global disease burden, with ∼12% of all deaths in 2019 attributed to outdoor and household air pollution.⁶⁶ In India, for example, ∼1.67 million deaths were related to air pollution in 2019, which was ∼17.8% of the total deaths in the country.⁶⁷ In addition, air pollution is the 4^th highest-ranking risk factor for mortality, even higher than high body mass index, high LDL cholesterol, or alcohol use.⁶⁸ An overview of the properties of significant hazardous gases and VOCs detected by In₂O₃-based nanostructures, along with their impact on human health and the environment, are discussed as follows:

(i) Nitrogen oxide (NO ₂ ): NO₂, with a threshold limit value (TLV) of 3 ppm,⁶⁹ is a highly toxic and oxidizing gas that is emitted mainly from car exhausts and different combustion processes.⁷⁰ Exposure to NO₂ causes lung irritations and a decrease in the fixation of oxygen molecules on red blood corpuscles. Furthermore, it causes acid rain.⁷¹ At concentrations higher than 10 ppm, it can be highly harmful to the respiratory tract, resulting in emphysema, asthma, and chronic bronchitis.⁷²

(ii) Hydrogen sulfide (H ₂ S): H₂S is a highly poisonous and flammable gas that commonly collects in basements, manholes, and sewer lines. At concentrations >100 ppm, the gas quickly paralyzes the olfactory nerves, and the sense of odor disappears. Exposure to 250 ppm causes the alveolar membranes to exude fluids that interfere with the normal exchange of gases. The principal symptom is asphyxia, which may lead to suffocation. Inhalation of high concentrations (1000 ppm) of H₂S paralyzes the respiratory nerve center, which can also lead to suffocation. Physical collapse may occur without warning.⁷³ Thus, from a safety standpoint, monitoring of H₂S is essential in the petrochemical and coal industries.⁷⁴ Furthermore, H₂S is a biomarker. For example, a H₂S concentration above 250 ppb in a person's exhaled breath indicates the presence of periodontal disease.⁷⁵

(iii) Carbon monoxide (CO): CO is an odorless, colorless, and tasteless gas that can kill people silently. It is produced by the incomplete combustion of fuels.⁷⁶ The inhalation of >15 ppm CO is dangerous, and the TLV of CO is 25 ppm set by the American Conference of Governmental Industrial Hygienists (ACGIH).⁷⁷ Exposure to 0.5% CO for 10 min can cause immediate death. Currently, more than 15 000 intentional CO poisoning occurs each year.⁷⁸ Furthermore, CO is a biomarker, where the CO concentration in the exhaled breath of sick people is higher than those of normal people.⁷⁹

(iv) Ammonia (NH ₃ ): NH₃ is one of the most widely used chemicals. It is emitted mainly from chemical plants and motor vehicles.⁸⁰ The TLV of NH₃ gas is set to 25 ppm.¹⁹ Inhaling more than the safe level of NH₃ can lead to life-threatening illnesses because of its highly toxic and corrosive properties to the skin, eyes, and lungs.⁸¹

(v) Sulfur dioxide (SO ₂ ): SO₂ is a toxic and corrosive gas that is emitted into the atmosphere from many industrial processes and through natural phenomena.^82,83 The TLV of SO₂ was set to 5 ppm.⁸⁴ It is one of the most hazardous air pollutants because it can transform into sulfuric acids, resulting in acid rain. Acid rain can decrease agricultural productivity, affecting flora and fauna and damaging the overall plant ecosystem.⁸⁵ Human exposure to high levels of SO₂ gas can result in cardiovascular disease, respiratory illnesses, and breathing problems.⁸⁶

(vi) Methane (CH ₄ ): CH₄ is a colorless, odorless, and flammable gas that is explosive at high concentrations, from 5 to 15% in air. It is mainly used as fuel to make heat and light. Therefore, it is the main component of natural gas.⁸⁷ In addition, it is a potent greenhouse gas because it is nearly 25 times more powerful than CO₂.⁸⁸ CH₄ explosions and CO poisoning are two common gas disasters in coal mining.⁸⁹

(vii) Carbon dioxide (CO ₂ ): CO₂ is a greenhouse gas that is emitted mainly from fossil fuels in automobiles, industries, and electric power generators.⁹⁰ CO₂ is used widely in the food industry, fire extinguishers, lasers, and refrigerants owing to its unique physicochemical characteristics (inert and water-soluble, and high density).⁹¹ It absorbs the short wavelengths reflected from the earth to space, which increases the earth's temperature, producing droughts, floods, and ice melting at the poles.⁹² In greenhouse planting, there is a need to detect CO₂ in low concentrations (<300 ppm), while in modified atmosphere packaging for fruit and vegetables, high CO₂ concentrations (up to 25%) need to be monitored.⁹³

(viii) VOCs and other hazardous gases: In general, VOCs are organic compounds with high vapor pressures and low boiling points, which causes them to be volatile.^15,16 The most important VOC pollutants are n-butanol, xylene, acetone, toluene, ethanol, benzene, methanol, and formaldehyde. Commonly used examples of toxic gases include SO₂, NH₃, CO, H₂S, NO₂, and trimethylamine. In addition, hydrogen (H₂) and methane are considered explosive gases when combined with air. All of these gases are hazardous to the environment and human health. Hence, the design of portable and high-performance sensors is essential for the early detection and monitoring of these potentially lethal gases.

The present review paper discusses the gas-detection properties of In₂O₃ to different gases, mainly VOCs (such as n-butanol, xylene, acetone, toluene, ethanol, benzene, methanol, and formaldehyde) and toxic gases (such as SO₂, NH₃, CO, H₂S, NO₂, and trimethylamine). It is essential to know the details of these gases and their possible effects before discussing their gas detection properties. Thus, the physical properties of these gases, their uses, guidelines on the IDLH (immediately dangerous to life or health),¹⁷ and TLV (threshold limit value)¹⁸ are presented in Table 1, which summarizes the properties of VOCs and toxic gases and their effects on human health.

Table 1 Properties of different toxic VOCs and gases

Gas name	IDLH	TLV	Human health issues	Properties	Ref.
Acetone	25.00 ppm	750 ppm	Muscle weakness, dryness in the mouth, fatigue, dizziness, nausea, and narcosis harmful to the nerve system	A colorless liquid, solvent, pharmaceuticals, pungent odor/used to dissolve plastics, as laboratory reagents	94
Formaldehyde	20 ppm (OSHA)	0.1–0.3 ppm	Human carcinogen (nasopharyngeal cancer) pulmonary damage and leukemia at 6 ppm	Colorless, flammable	95 and 96
Ethanol	3300 ppm (NiOSH)	1000 ppm – STEL (ACGIH)	Difficulty in breathing, eye irritation, drowsiness, and headache	Volatile, colorless, inflammable	97
NO2	13 ppm (NIOSH)	0.3 ppm (ACGIH)	Lung damage, irritating to eyes and produce ozone,	Pungent odor, not flammable	98 and 99
H2S	100 ppm (NIOSH)	1–5 ppm (ACGIH)	Highly reactive with hemoglobin causing fatal damage to the olfactory system	Rotten egg smell, colorless, toxic	100–102
NH3	300 ppm (NIOSH)	25 ppm (ACGIH)	Hazardous to skin/eyes/respiratory tract of humans, corrosive and irritant	Colorless, corrosive, and hazardous	103
Cl2	10 ppm (NIOSH)	0.1–0.4 ppm (ACGIH)	Eye irritation (>5 ppm),	Irritating odor, harmful, quickly reacts with water to form corrosive acid, present in the ozone layer, a fatal to a human being	104
Acetic acid	50 ppm (NIOSH)	10–15 ppm (ACGIH)	Eyes/nose/throat irritation, cough, chest tightness, fever, confusion, headache	Colorless, heavily vinegar-like smell, flammable	105
CO	1200 ppm (NIOSH)	50 ppm (OSHA) and 35 ppm (NIOSH)	Headache, loss of consciousness, fatal death due to the binding of hemoglobin and reduction oxygen transportation, dizziness collapse, nausea	Colorless, odorless, tasteless, non-irritating/—	106 and 107
Trimethylamine (TMA)	—	15 ppm – STEL (OSHA)	Irritation to the throat, nose, skin, and eyes	Blurred vision, irritation of the eyes/skin/nose/throat, respiratory system, and cough	108 and 109
H2	NA	NA	Stinging of the nose and throat, vomiting, pure form is a chemical asphyxiant, dizziness, headaches, drowsiness, and nausea	Highly flammable and colorless, metal smelting, tasteless, low minimum ignition energy (0.017 mJ), nontoxic, explosive/used as fuel in vehicles, petroleum extraction, glass making	110

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3. Strategies to boost gas sensing performance of In₂O₃

The surface morphology and microstructure of nanomaterials play a significant role in achieving superior gas detection performance. Several efforts have been made to design various In₂O₃ morphologies to improve the gas detection performance. The morphologies of In₂O₃-based gas sensors can be classified based on different dimensionalities, i.e., zero-dimensional to three-dimensional (0D to 3D). The type of nanostructure greatly affects the properties of nanomaterials. Nanomaterials, such as nanosheets, nanowires, and nanoparticles, show enhanced gas-sensing properties owing to their surface-active sites and gas adsorption affinity.³⁰ In this regard, the gas detection properties of different morphologies of In₂O₃ nanostructures are discussed.

3.1 Morphology engineering

The unique properties of nanostructured materials are derived from the fact that their properties are different from those of bulk materials when reduced to the nanometric scale.²¹ Considerable attention has been paid to adapting nanomaterials properties and fabricating innovative multifunctional nanostructures. Gas detection properties, such as selectivity, sensitivity, and response/recovery, generally depend on the surface-to-volume ratios, crystallinity, surface morphology, microstructure, and electronic properties of nanomaterials. With a basic understanding of the critical factors contributing to the sensitivity of a sensor, efforts have been made to engineer different morphologies to reach a highly sensitive gas sensor.^21,22 This section addresses the different morphologies, from 0D to 3D, and examines their gas-detecting properties corresponding to various individual gases such as H₂S, CO, NO₂, acetone, H₂, and ethanol.

3.1.1 In₂O₃ sensors based on 0D nanomaterials. 0D nanostructures comprise nanoparticles (NPs), nanospheres, quantum dots, and mesoporous NPs, and are used widely to improve sensor performance.¹¹¹ These mesoporous materials possess unique electrical properties and large surface areas that are highly beneficial in gas detection applications.³⁶ Moreover, porous nanostructures have larger areas and better adsorption sites relative to dense structures, which are highly advantageous for producing surface-adsorbed oxygen species that are often the significant factor in improving gas-sensing performances.^36,111 In this regard, Zhijie et al. described the synthesis of porous In₂O₃ NPs with a large surface area of 60.6 m² g⁻¹ using the hydrothermal procedure followed by a calcination process for H₂S gas-sensing application.¹¹² The H₂S response (R_a/R_g) of the sensor towards 1 ppm H₂S was 26 268.5 at 25 °C, which is considerably high for H₂S sensing. Moreover, the sensor showed excellent stability, reversibility, and selectivity. The unique porous nanostructure of In₂O₃ has provided an effective way to facilitate the diffusion of H₂S molecules to the entire porous surface, leading to an ultra-high response. The reaction of In₂O₃ with H₂S gas can be explained using the following equation:

2H₂S + 3O₂⁻ ⇌ 2H₂O(g) + 2SO₂(g) + 3e⁻ (6)

When an In₂O₃ NP sensor is exposed to H₂S, it interacts with O₂⁻ ions to discharge the initially trapped electrons to increase the conductivity and decrease the resistance. Sulfurization occurs on the In₂O₃ NP sensor surface. When the sensor is exposed to H₂S gas, an In₂S₃ layer can be formed on the In₂O₃ sensor surface:

In₂O_3(s) + 3H₂S_(g) → In₂S_3(s) + 3H₂O_(g) (7)

The change in Gibbs free energies for the formation of In₂S₃ from In₂O₃ is −161.7 kJ mol⁻¹ at room temperature, which is spontaneous and thermodynamically stable.¹¹³ The good conductivity of In₂S₃ increases the electrical current remarkably and yields a strong response to H₂S. Therefore, the enhanced response of porous In₂O₃ NPs to H₂S is due to the oxidation of surface adsorption of oxygen and the development of In₂S₃ through the sulfuration mechanism (eqn (7)).

Oxygen vacancies (V_O) are one of the defect structures of MOS that plays an important role as far as the sensing performance is concerned. On the other hand, designing nanomaterials with improved detection performance by manipulating V_O is a significant challenge because the role of surface/bulk V_O on the detection performance of MOS is unclear. Gu et al.⁴³ studied the roles of different types of V_O (bulk and surface) on the sensing performances of the In₂O₃ NP sensor. Four In₂O₃ NP sensors were prepared by heating at 550 °C in the presence of H₂ and He at different time intervals (0/10/15/30 min). Among the different sensors, the In₂O₃-based sensor, heat treated for 10 min (called In₂O₃-H10), exhibited a strong response to formaldehyde (HCHO) at 230 °C with a rapid τ_res of 100 s and τ_rec of 70 s. Fig. 4 presents a schematic diagram of the HCHO sensing mechanism of H₂-treated In₂O₃.

Fig. 4 Schematic diagram of HCHO sensing on the surface of the H₂-treated In₂O₃. Reproduced from ref. 43 with permission from the American Chemical Society, Copyright 2018.

The HCHO gas-sensing reaction can be expressed as follows:

HCHO_(g) + 2O⁻_(ads) → CO_2(g) + H₂O_(g) + 3e⁻ (8)

The enhanced HCHO sensing performance of In₂O₃-H10 is due mainly to the presence of bulk V_O. In contrast, surface V_O does not help improve the HCHO performance. This is because the bulk V_O contains a narrow bandgap and a small energy barrier, which promote electron mobility and the development of chemisorbed oxygen ions that contribute to an increased HCHO response. On the other hand, high surface V_O contents are not needed to enhance the detection performance.

Li et al.¹¹⁴ examined the advantages of mesoporous In₂O₃ NPs prepared through the hydrothermal process for gas-sensing studies. The pore size and surface area of the prepared In₂O₃ were 31 nm and 23.5 m² g⁻¹, respectively. The response and τ_res/τ_rec of the In₂O₃ sensor at 260 °C were 18 and 1.7 s/1.5 s for 500 ppm H₂, respectively, with a detection limit of 0.01 ppm. The good H₂ gas-sensing characteristics were credited to the unique mesoporous structure, large surface area, and a large amount of chemisorbed oxygen on the surface of porous In₂O₃.

Xiao et al.¹¹⁵ synthesized 40–60 nm sized In₂O₃ nanospheres packed with numerous NPs (8 nm in size) via a solvothermal process for NO₂ sensing applications. The developed sensor exhibited an excellent response (R_a/R_g) of 217.5 to 500 ppb NO₂ gas with good selectivity, excellent repeatability, superior stability, and a lower detection limit of 10 ppb at a working temperature of 120 °C. The improved gas response was attributed mainly to the higher surface area and porous structure of the synthesized powders. The results indicated that it could be exploited as a capable sensing material for low-ppm NO₂ gas sensors.

Acetaldehyde, a VOC gas, can cause severe health-related problems.¹¹⁶ In this respect, acetaldehyde gas-sensing was studied using In₂O₃ NPs fabricated using a microwave hydrothermal method.^117a The prepared In₂O₃ NP sensor exhibited a strong response (R_a/R_g = 11.06) to acetaldehyde (to 100 ppm), with a detection limit as low as 1 ppm, at a working temperature of 300 °C.

Despite its importance, realizing dual gas selectivity on a chemiresistive gas sensor based on metal oxides remains challenging. In this regard, Jin et al.^117b reported the potential of In₂O₃ hierarchical porous nanospheres (HPNSs) for achieving dual gas selectivity to C₂H₅OH and TEA. In₂O₃ HPNSs were fabricated using a sacrificial template method by annealing In (OH)₃ NPs at different temperatures (400 to 600 °C). The highest oxygen vacancy content was observed at 500 °C, resulting in excellent gas sensitivity to C₂H₅OH and TEA compared to 400 and 600 °C. The observed dual selectivity at different operating temperatures was attributed to different diffusion behaviors and specific adsorption properties of C₂H₅OH and TEA molecules. The results highlighted the significance of hierarchical porous structures in increasing the sensitivity of metal oxides and achieving dual gas selectivity for sensing different gases.

Chao et al.^117c used a nitric acid-assisted solvothermal method to transform In₂O₃ nanosheets into spheres, improving the C₂H₅OH gas-sensing properties. Specifically, the spheres with partially broken structures and a high surface area of 60.6 m² g⁻¹ demonstrated the most favorable response of 250 (R_a/R_g) to 50 ppm C₂H₅OH at 250 °C, along with short response–recovery times of 16 and 14 s, respectively. These In₂O₃ spheres exhibited good selectivity, reproducibility, and high stability to 50 ppm C₂H₅OH, which was attributed mainly to the unique structure of the spheres.

In recent years, colloidal quantum dots (CQDs) have attracted considerable attention for their potential in optoelectronic devices. CQDs possess excellent characteristics, such as solution processability, flexibility to deposit onto various substrates, excellent crystallinity, large surface area, and high surface chemistry.^118,119 In addition, the outstanding crystallinity and higher surface-to-volume ratios of CQDs guarantee an enormous active site for the solid–gas interaction. In this respect, Yang et al.¹¹⁹ reported a room temperature (RT) operable H₂S gas sensor based on In₂O₃ CQDs. The CQD sensor revealed a maximum response of 90 to H₂S (5 ppm), demonstrating the potential of CQDs in RT gas-sensing studies. The observed RT sensing performance was attributed to the size and surface effects of CQDs that are proficient for advanced surface activities and offer reasonable energy for the adsorption and conversion of oxygen. Furthermore, the dangling bonds on the surface of CQDs are promising for the adsorption of oxygen, which may provide appropriate activation energy for low-temperature transformation into O₂⁻ to O⁻ species.¹¹⁹

3.1.2 In₂O₃ sensors based on one-dimensional (1D) nanomaterials. 1D nanomaterials are prominent gas-sensing materials owing to their greater surface-to-volume ratios and active transformation of the chemical species on the surface.^120–125 Over the past few years, 1D In₂O₃-based nanostructures, such as nanowires (NWs),^120,121 nanofibers (NFs),^122,123 nanorods (NRs),¹²⁴ and nanotubes (NTs),¹²⁵ have been exploited as high-performance gas sensors because of their several benefits, e.g., large surface area to volume, tremendous accessibility for gas molecules, and more thermal-activated carriers for conduction. Cao et al.¹²⁶ reported the synthesis of In₂O₃ NFs and flower-like NFs (FNFs) and assessed their effect on the sensing performances. They reported that the In₂O₃ NFs sensor displayed the maximum gas-sensing performance (R_a/R_g = 72, to 100 ppm) and good selectivity towards acetone (CH₃COCH₃) at 180 °C, with very fast τ_res (1 s). In contrast, the In₂O₃ FNFs showed enhanced sensitivity to HCHO. Fig. 5 shows a schematic presentation of the sensing interactions between In₂O₃ NFs and In₂O₃ FNFs sensors towards CH₃COCH₃ and HCHO. The observed sensing performance was attributed mainly to the difference in the oxidation capacities of the absorbed oxygen species stimulated by the different phase structures. Their oxidation ability was weak because there were more chemisorbed oxygen molecules in In₂O₃ FNFs.¹²⁷ Hence, compared to In₂O₃ NFs, In₂O₃ FNFs oxidized strongly to reducing gases, resulting in high sensitivity to HCHO. The sensor response was unaffected by the specific surface area. On the other hand, the oxidation capability of adsorbed oxygen species at the sensor surface led to the selectivity and response of the sensing material. Overall, engineering the crystalline phase structure of oxide-semiconductors is a suitable technique for improving the gas-sensing performance.

Fig. 5 Schematic presentation of sensing of In₂O₃ NFs and In₂O₃ FNFs to acetone and HCHO. Reproduced from ref. 126, Copyright 2020, Elsevier.

Wang et al. described acetic acid sensing behaviors of In₂O₃ NFs.¹²⁸ Acetic acid irritates the eyes/skin and respiratory system. Hence, the development of acetic acid sensors is important. In₂O₃ NFs prepared through electrospinning were assessed for their ability to detect acetic acid, which yielded a response of 66.7 for 2000 ppm, and fast τ_res/τ_rec (25 s/37 s), tested at 250 °C.¹²⁸ A typical MOS sensing mechanism regarding the change in sensor resistance upon the interaction of gas was used to clarify the possible interactions between In₂O₃ NFs and acetic acid gas molecules.

3.1.3 In₂O₃ sensors based on two-dimensional (2D) nanomaterials. In the past few years, two-dimensional (2D) nanomaterials, such as nanosheets, have been studied extensively for their distinctive attributes, e.g., nanoscale thickness, surface area, superior sensing performances, tunable electrical properties, excellent thermal and chemical stability, in addition to the potential of doping or surface functionalization with foreign elements.^129,130 Nanosheet-type structures have additional electron transfer channels because of the abundance of active sites and strong intersects that may enhance the reaction ratio among the sensing materials and the target gas.⁴⁵ In this respect, 2D nanostructures based on In₂O₃ have been developed for high-performance gas sensing, even though they are less reported than other structures. Chlorine (Cl₂) gas leads to severe health-related issues that are fatal to the human body, highlighting the need for detection.¹³¹ Ma et al. designed a Cl₂ gas sensor from In₂O₃ nanosheets, synthesized from a melamine–formaldehyde (MF)-based template.¹³² As-prepared In₂O₃ nanosheets sensor showed an ultra-high response (R_a/R_g = 2353.4) to 3 ppm Cl₂ at 200 °C compared to the pristine In₂O₃ sensor (14.9), with a LOD value of 0.037 ppb and τ_res/τ_rec of 53 s/17 s, this is an indication of a promising sensor. The improved gas-detecting properties of the In₂O₃-based nanosheet sensor were primarily credited to the particular nanosheet type of morphology, high BET surface area, and abundant oxygen vacancies. Fig. 6 presents a schematic diagram of the interactions between the In₂O₃ nanosheets and reducing/oxidizing gases.

Fig. 6 Schematic diagram of the sensing mechanism of In₂O₃ nanosheets to reducing and oxidizing gases. Reproduced from ref. 132, Copyright 2020, Elsevier.

In this case, the sensing mechanism was explained according to the variation of sensor resistance upon the interaction of reducing or oxidizing gases. For example, the gas-sensing reaction for Cl₂ (oxidizing gas) can be explained as follows:

Cl₂ + 4O₂⁻_(ads) ⇌ 2Cl⁻_(ads) + 4O₂ + 2e⁻ (9)

Cl₂ + 2O²⁻_(latt) ⇌ 2Cl⁻_(latt) + O₂ + 2e⁻ (10)

Herein, the lattice oxygen could be involved at an elevated temperature that causes a reaction in eqn (10). Based on the oxidation capability, Cl₂ can also be adsorbed onto the oxygen vacancies of the material surface in the following way:

Cl₂ + 2e⁻ ⇌ 2Cl⁻_(surface) (11)

Cl₂ + 2O_(vac) + 2e⁻ ⇌ 2Cl⁻_o (12)

Accordingly, the sensor resistance increases as the electrons are captured from the conducting-region. Thus, the detection mechanism of oxidizing gases is dominated by eqn (11) and (12). Herein, oxidizing Cl₂ can react with the surface and oxygen vacancies. While reducing gases can only react with pre-adsorbed oxygen species. As a result, oxidizing gases have a stronger response than reducing gases.

Sun et al.⁴⁵ synthesized In₂O₃-based nanosheets with porous structures using a calcination process after liquid reflux. The porous In₂O₃ nanosheet sensor showed a high response and fast τ_res of 89.48 and 16.6 s, respectively, for 97.0 ppm NO₂ at RT with good selectivity. The enhanced performance was attributed to a unique 2D nanosheet-like porous structure with a single-crystal phase, which was helpful for the effective gas diffusion of gas molecules, enabling high sensitivity.

3.1.4 In₂O₃ sensors based on 3D nanomaterials. 3D nanostructures have attracted considerable research interest owing to their large surface area, unique morphology, and rich porosity. These structures are mostly prepared by assembling low-dimensional nanostructures, such as NP, nanosheets, and NRs, to form hierarchical nano architectures in the form of micro/macrospheres, mesoporous structures, urchin-like structures, and micro/nano-flowers.^133,134 The sensing performance can be significantly enhanced with respect to increasing the utility factor of the gas-sensing material.¹³⁵ Numerous structured directing agents, such as sodium hydroxide, urea, hexamethyl tetraamine, glucose, sucrose, and methylammonium bromide, have been employed to synthesize various In₂O₃-based 3D nano/microstructures. hydrothermal,^136,137 solvothermal,^138,139 and microwave-based¹⁴⁰ methods have mainly been used to synthesize these structures. The following gives an overview of some of the reported In₂O₃ sensors with 3D structures.

Spherical and cubic particles have been reported in sensing studies. Zhou et al.¹³⁷ prepared In₂O₃ microcubes with a cubic structure using a hydrothermal process and a surface area of 30.3634 m² g⁻¹. The fabricated sensor exhibited high sensing performance with a high response (R_a/R_g = 85) to 1000 ppm CH₃COCH₃ at 210 °C, demonstrating high potential for sensing applications. H. Ma et al.¹³⁸ synthesized walnut-like In₂O₃ nanostructures using a simple solvothermal process followed by thermal treatment and evaluated their photoelectric-based gas-sensing properties. The as-prepared walnut-like In₂O₃ nanostructures exhibited a strong response of 219 to 50 ppm NO₂ in a UV light (λ = 365 nm) irradiation (1.2 mW cm⁻²). The in situ growth of such thin films improved the contact between the sensing material and Ag interdigitated electrode substrate. A high surface area with walnut-like In₂O₃ structures containing numerous active edge sites improved the performance of NO₂. Hollow particles generally have a huge specific surface area caused by the hollowness of their structure. A solvothermal method was used to synthesize urchin-like In₂O₃ hollow structures with an In (OH)₃ sacrificial template.¹³⁹ After annealing at 350 °C, the as-prepared initial structure of In (OH)₃ was transformed into an urchin-like In₂O₃ structure exhibiting a surface area of 122.02 m² g⁻¹. The sensor showed excellent sensitivity (20.9) for 1 ppm HCHO, selectivity, fast τ_res, and LOD (50 ppb) at 140 °C. The sensing result was attributed to the unique structure with a small crystallite size (∼17 nm) and large surface area. In addition, the presence of a loose shell in the urchin-like hollow structure was conducive to gas diffusion.

While many synthesis strategies are based on various templates and surfactants, there are still adverse effects, such as the cost of the template materials and the subsequent removal of templates, which is a tedious process. As a result, simple and cost-effective synthesis of nanostructures that negates the need for templates is a tremendous scientific advantage. Recently, biomolecules, such as amino acids, as reducing and shape-directing agents were used to prepare In₂O₃ micro/nano structures.^31,140 Brick-shaped In₂O₃ was fabricated using a simplistic hydrothermal technique via a green approach with amino acid (L-aniline).¹⁴¹ The sensor responded strongly (600) to 100 ppm NO₂ gas at a low working temperature of 100 °C. The morphology, microstructure, structural defects, and porosity of the In₂O₃ bricks were responsible for the high sensing toward NO₂ gas. On the other hand, the high selectivity to NO₂ gas was due mainly to the reactive nature of NO₂ because of the presence of a lone pair electron.

Another study used a hydrothermal method to synthesize hollow In₂O₃ microcubes (HIMC) using L-alanine as a shape-controlling agent.³¹ The Kirkendall effect was responsible for the evolution of these hollow structures. The hollow In₂O₃ microcube sensor exhibited an unprecedented high sensitivity (S = 1401) and high selectivity to 100 ppm NO₂ at 100 °C. Various factors, such as variation in barrier height in NO₂ ambiance, thick electron depletion layer, and high adsorption and the gas-sensing reaction of hollow In₂O₃ microcube structures, were responsible for the strong response to NO₂. Fig. 7(a) shows FESEM images of HIMCs, and Fig. 7(b) presents a schematic diagram of the sensing mechanism of a hollow In₂O₃ microcube gas sensor.

Fig. 7 (a–c) FESEM images of HIMC film and (d) schematic diagram of NO₂ sensing mechanism of the HIMC sensor. Reprinted with permission from ref. 31, Copyright 2020, Elsevier.

In a different study, In₂O₃ microcubes were produced using a low-temperature solution chemical technique to investigate NO₂ gas sensing. The In₂O₃ microcubes sensor displayed a strong response (R_g/R_a = 1884) to NO₂ (30 ppm) at 60 °C and detected NO₂ gas at concentrations as low as 500 ppb,¹⁴² which is very interesting for practical applications. Moreover, Zhang et al.^143a reported the synthesis of hollow In₂O₃ microspheres prepared using a hydrothermal process. The In₂O₃ microcubes sensor showed a good response value of 20 to 10 ppm HCHO at a working temperature of 200 °C with τ_res/τ_rec of 4 s/8 s. The In₂O₃ hollow microspheres possessed abundant pores, a surface area of 40.94 m² g⁻¹, and a hollow structure that facilitated the diffusion and adsorption of the target gas to the active sites of the detection materials.

Han et al.^143b recently fabricated a flower-like In₂O₃ material with a high surface area (77.38 m² g⁻¹) to detect trace isoprene, a biomarker of liver disease. The In₂O₃ sensor showed a strong response to isoprene at 190 °C with good repeatability and selectivity over other biomarkers. The degree of the reaction of active oxygen with isoprene on the surface of the In₂O₃ crystal determines the amount of adsorbed oxygen ions, which strongly influences the sensor performance. In particular, the In₂O₃ sensor could detect 5 ppb isoprene and has potential use in portable breath isoprene detectors for the noninvasive screening and grading of chronic liver disease.

Chen et al.¹⁴⁴ used another template-free, solvothermal method to synthesize cubic–rhombohedral In₂O₃(bcc-rh-In₂O₃) NPs (Fig. 8(a)–(f)) for exhaled breath analysis. Both the orthorhombic phase of InOOH and the cubic phase of In(OH)₃were present in the products. The presence of the InOOH and In(OH)₃ phases promoted the configuration of the rh-In₂O₃ phase and bcc-In₂O₃ phases, respectively. The presence of glycerol in the reactant solution formed both phases in the final product. The bcc-rh-In₂O₃ sensor exhibited a response of R_a/R_g = 12, 2 s of response time, and LOD of 10 ppb towards 50 ppm acetone. The main reason for the superior performance of the abovementioned sensor was the establishment of the n–n heterojunction between bcc-In₂O₃ and rh-In₂O₃ (Fig. 8g and h).

Fig. 8 SEM images of the sample (a, d) before calcination, (b, e) after calcination at 400 °C, and (c, f) commercial In₂O₃. Sensing mechanism with energy band model of (g) pure rh-In₂O₃ phase and (h) bcc-rh-In₂O₃ phase. Reproduced from ref. 144 with permission from Elsevier (Copyright 2020).

The work functions of cubic In₂O₃ and rhombohedral In₂O₃ are 5 eV and 4.3 eV, respectively. An n-homojunction structure formed after they came into contact with each other. The transfer of electrons occurred from rh-In₂O₃ to bcc-In₂O₃ until the Fermi levels realized equilibrium owing to the higher position of Fermi levels of rh-In₂O₃ compared to those of bcc-In₂O₃. Accordingly, a high depletion layer of electrons (W_D3 > W_D1) was produced on the rh-In₂O₃ surface, resulting in a potential energy barrier (V₃ > V₁). Modulation of the potential barrier and depletion layer resulted in a high response value for the bcc-rh-In₂O₃ sensor.

3.2 Interface engineering

Efforts have been made to design different nanocomposite materials and improve the sensing properties.^145,146 Such composite gas-sensing materials can be constructed by incorporating two dissimilar materials, which are referred to as heterostructures. On the other hand, the physical interface that results between these two individual components is known as a heterojunction.⁴⁷ When designing a heterostructure, the presence of the second component, with the host In₂O₃, significantly boosts the sensing properties. Compared to the individual component, the newly designed nanocomposite materials usually exhibit improved gas-detecting properties because of the synergistic effects between them.⁴⁷ The In₂O₃-based composites can be prepared by coupling In₂O₃ with other metal oxides (p-type and n-type), noble metal (Ag, Au, Pd, and Pt) functionalization, and encapsulation onto In₂O₃, elemental doping (e.g., Sn, Pb, Ca, La, and Zn), coupling with several conducting polymers, and utilization of carbon nanomaterials (e.g., CNT, RGO, and graphene).³ Therefore, the following section reviews the current development of In₂O₃-based composite materials to determine their enhanced sensing properties.

3.2.1 Heterojunction-based In₂O₃ nanomaterials. A heterojunction means the contact transition zone developed at the interface between two semiconductors with distinct band gaps. Among the different In₂O₃-based composite materials, the heterostructures formed by coupling two kinds of metal oxides (n-type and p-type) have been promising strategies for improving the sensing properties. Generally, two kinds of heterojunction sensing materials are formed. A p–n heterojunction-type material is formed when In₂O₃ is coupled with a p-type oxide. On the other hand, an n–n heterojunction-type material is formed when n-type MOS are coupled with In₂O₃. Because both oxides have differences in Fermi energy levels, the electrical contact between them resulted in the flow of electrons from the higher to the lower Fermi energy levels, which lasts until equilibrium is reached. The abovementioned phenomenon induces either a charge depletion layer (for n-type) or an accumulation layer (p-type) at the interface of the heterostructures, resulting in an enormous change in electrical resistance in the heterostructures.^47,147 These p–n or n–n heterojunctions improve the sensor performances. Based on the literature, different p-type and n-type metal oxides have been coupled with In₂O₃ when designing various heterojunction nanostructures. The following section discusses the current progress of In₂O₃-based heterojunction nanostructures.
3.2.1.1 Combination with p-type oxide (p–n heterojunction). MOS, without modification, generally demonstrate low sensitivity and selectivity because of the insufficient amount of active oxygen ions at the surface. Typically, n-type MOS demonstrate high sensitivity compared to p-type MOS. On the other hand, p-type MOS materials have been exploited continuously as catalysts for potential sensors and selective detection variety of VOCs.^29,148 On the other hand, combining low-sensitivity p-type MOS with n-type In₂O₃ for synthesizing p–n heterojunctions has enhanced sensing abilities. Therefore, heterostructures from In₂O₃ coupled with p-type MOS will provide a promising strategy for developing gas sensors with remarkable performance. The reason for the enhanced sensing properties from such p–n heterojunctions is presented in the subsequent section.

Various p-type oxides have been coupled with In₂O₃ to design p–n heterojunction-based gas sensors.^149–151 For example, Park et al.¹⁵² demonstrated the synergistic effects of p-type Co₃O₄ and n-Fe₂O₃ NPs co-decorated on In₂O₃ NRs for ethanol (C₂H₅OH) gas-sensing properties. First, they synthesized In₂O₃ NRs by thermal evaporation followed by spin coating of Co₃O₄ and Fe₂O₃ NPs (Fig. 9a), which were synthesized separately using a hydrothermal method. Among the different VOCs, the Fe₂O₃–Co₃O₄-codecorated In₂O₃ NRs sensor showed the maximum selectivity to C₂H₅OH, followed by the Co₃O₄ NP-decorated In₂O₃ NRs sensor (Fig. 9b). The effects of the p–n heterojunctions (Fig. 9c) were evidenced by the change in the baseline resistances in the Co₃O₄-decorated In₂O₃ NRs sensor and the Fe₂O₃–Co₃O₄-codecorated In₂O₃ NP sensors, as compared to those of the pristine one. Modulating the width of the depleting layer and the height of the potential barrier at the p–n junction occurs within the sensor associated with the adsorption/desorption process of C₂H₅OH. The electrocatalytic attributes of the Fe₂O₃–Co₃O₄-codecorated In₂O₃ NRs sensing materials facilitate the oxidation of C₂H₅OH, which may explain the good selectivity to C₂H₅OH over other gases at an operating temperature of 200 °C. Overall, the primary cause of the synergistic effects of co-decoration on the C₂H₅OH detection performance of In₂O₃ NRs is not the development of compounds/nanoalloys between the oxides but the development of the p–n junctions among two different kinds of decorated oxides, such as Fe₂O₃–Co₃O₄ and Co₃O₄–In₂O₃ p–n junctions.

Fig. 9 (a) TEM image of Co₃O₄ and Fe₂O₃ co-decoration on In₂O₃ NRs, (b) selectivity pattern of pristine Fe₂O₃, Co₃O₄, In₂O₃, and Fe₂O₃–Co₃O₄ NPs decorated In₂O₃ NRs sensors, and (c) the gas sensing mechanism of Fe₂O₃–Co₃O₄ NPs decorated In₂O₃ NRs sensor. Reproduced with permission from the ref. 152, copyright 2020, Elsevier.

Wang et al.^153a produced In₂O₃ microspheres with a 3D-inverse opal structure via an ultrasonic spray-pyrolysis technique using a sulfonated polystyrene sphere. Subsequently, PdO NPs were loaded on it using a simple impregnation method. The PdO NPs-loaded 3D-inverse opal In₂O₃ microsphere sensor showed excellent selectivity to 100 ppm acetone with a four-fold stronger response (R_a/R_g = 50.9) than the corresponding pristine gas sensor at a working temperature of 250 °C. The improved gas-sensing properties of the PdO–In₂O₃ sensor were attributed to the novel 3D microporous structure, the highly 3D interconnections among the grains, large specific surface area, and size-controlled bimodal pores. These interesting morphological features are usually beneficial for increasing the adsorption sites for gas molecules with high permeability and diffusion.

Most importantly, the PdO catalyst NPs act as strong acceptors of electrons; hence, electron transfer from In₂O₃ to PdO produces a strong depletion layer at the PdO–In₂O₃ p–n heterojunction interface. Furthermore, the addition of PdO catalyst NPs helps increase the number of chemisorbed oxygen ions on the PdO–In₂O₃ sensor surface. Consequently, significant variation in resistance occurs for the PdO–In₂O₃ sensor, further enhancing the sensitivity. In addition, the partial reduction of PdO (Pd²⁺) to Pd⁰ during the acetone gas-sensing test can be attributed to the catalytic dissociation of acetone gas molecules and the lowering of the activation energy of the reaction between chemisorbed oxygen ions and acetone gas molecules, which eventually increases sensing performance.

MXenes are a promising class of 2D inorganic compounds composed of atomically thin layers of transition metal carbides, carbonitrides, and nitrides.^153b These materials possess functional groups that can serve as active sites for the adsorption of gas molecules, even at room temperature. This property highlights the potential of MXenes in the design of low-power gas sensors.^153c,d In this respect, Liu et al. synthesized a hybrid p-MXene/n-In₂O₃ material to produce a chemiresistive-type sensor for NH₃ detection at room temperature.^153b The developed hybrid sensor exhibited a 28-fold larger response, improved selectivity, better detection limit, and excellent stability compared to the pure MXene (Ti₃C₂T_x) sensor. The enhanced sensing performance was attributed to the heterostructure formed between MXene and In₂O₃ and the richer functional groups of the 2D Ti₃C₂T_x MXene with a high specific surface area. A sensor based on this hybrid material also exhibited faster response/recovery times, highly repeatable signals, and long-term stability, highlighting the potential for development as a wireless passive NH₃ sensor. Fig. 10 shows the energy band diagram of a hybrid sensor before and after the contact. The Fermi level of n-type In₂O₃ is higher than that of p-type MXene (Ti₃C₂T_x), causing electrons to transfer from In₂O₃ to MXene, leading to the formation of a heterostructure at the interface of MXene and In₂O₃. This transfer of electrons causes an equilibrium in the Fermi levels of both materials. As a result, there is a higher concentration of free electrons in MXene, which enhances the pre-adsorption of oxygen in the air onto the surface of the sensing films. The pre-adsorbed oxygen captures electrons to form ionized oxygen molecules (O₂⁻). The redox reaction is favored because of the increased adsorbed O₂⁻, resulting in an enhanced response (Fig. 10b).

Fig. 10 (a) Energy band diagram of the MXene/In₂O₃ heterostructure before and after contact, (b) adsorption of NH₃ and oxygen molecules on the heterostructure surface. Reproduced with permission from the ref. 153b.

Liu et al.^153e synthesized MXene/In₂O₃ (p–n) nanocomposites using a hydrothermal method for formaldehyde gas detection. The formaldehyde sensing test showed that the nanocomposite sensor had an optimal working temperature of 100 °C, which was lower than that of the pure In₂O₃ sensor (300 °C). In addition, the response of the nanocomposite to 100 ppm formaldehyde increased from 66.06 to 106.8%. The gas sensor prepared by MXene/In₂O₃ nanocomposites demonstrated good stability, rapid response (1 s), and recovery (1 s) times, as observed in repeatable tests. In MXene/In₂O₃ nanocomposites, the high sensing performance is credited mainly to their large specific surface area, which offered many sites for oxygen and the adsorption of formaldehyde gas. In particular, the functional groups (–OH and –F) on the MXene surface contained active sites for the adsorption of formaldehyde gas molecules, leading to an extended range of electron changes on the surface and a significantly increased response. The Schottky junctions formed at the interface between MXene and In₂O₃ and their respective work functions of 3.9 eV and 4.83 eV caused electrons to transfer from MXene to In₂O₃ to achieve equilibrium between their Fermi energy levels.

3.2.1.2 Combination with n-type oxide (n–n homojunction). Compared to In₂O₃/p-type oxide-based composite materials, n-type oxides have been widely studied. According to the literature, this oxide type leads to the fabrication of n–n homojunction nanostructures with In₂O₃. Thus far, various n-type oxides have been combined with n-type In₂O₃ to construct an n–n homojunction structure as promising gas sensors. For example, Wan et al.¹⁵⁴ accounted the hierarchical structure of In₂O₃@SnO₂ core–shell (C–S) NFs for HCHO sensing. First, they produced In₂O₃ NFs using an electrospinning method (Fig. 11a), and highly ordered SnO₂ nanosheet arrays then grew vertically on the In₂O₃ NFs surface to design the C–S morphology. From Fig. 11b, it was observed that the In₂O₃@SnO₂ C–S NFs sensor displayed an ultra-high response value (180.1 to 100 ppm HCHO) compared to the pristine SnO₂ NFs (R_a/R_g = 33.2) and pristine In₂O₃ NFs (19.7) with fast τ_res/τ_rec (3 s/3.6 s) and the lowest detection limit (1.9) to HCHO (10 ppb) at 120 °C.

Fig. 11 (a) Schematic diagram of the preparation of the hierarchical structure of 3D In₂O₃@SnO₂ C–S NFs. (b) Selectivity patterns of all prepared sensors to various gases (100 ppm) at 120 °C. (c) Morphology and active sites on the surface of gas sensor and (d–f) gas sensing mechanism. Reproduced from ref. 154 with permission from the American Chemical Society, Copyright 2019.

In addition to its large surface area (Fig. 11c), the outstanding sensing performances of the In₂O₃@SnO₂ C–S NFs were assigned to the following reasons (Fig. 11d–f): (i) the increased baseline resistances due to the configuration of the depletion layers and higher potential barriers caused by heterogeneous and homogeneous interfaces compared to the single homogeneous interfaces of In₂O₃ and SnO₂; (ii) the increase in electron concentration caused by unique electron transfer from In₂O₃ (φ = 4.83 eV) to SnO₂ (φ = 5.13 eV), which improves the gas-sensing reaction; (iii) configuration of potential barriers on the junctions between the NWs; (iv) the availability of abundant adsorbed oxygen species and large surface area (31.4 m² g⁻¹) of the as-prepared In₂O₃@SnO₂ C–S NFs as well as the unique hierarchical morphology with connected SnO₂ nanosheet arrays.

Zinc oxide (ZnO), a promising sensing material, can be combined with In₂O₃ to form In₂O₃–ZnO C–S NWs gas sensors. Park et al.¹⁵⁵ reported on the role of interfaces of multiple-networked In₂O₃ NWs coated with a ZnO shell for C₂H₅OH gas-sensing application. A thermal evaporation method synthesized the vertically grown In₂O₃@ZnO C–S NWs. The ZnO shell thickness varied from 10 to 53 nm to examine the impact of the ZnO shell layer on the sensing properties. The ZnO shall layer with a size of 44 nm yielded the strongest response. The enhanced response to C₂H₅OH is elucidated based on the wider depletion layer formed by the In₂O₃–ZnO interface and the outer shell of the ZnO in the In₂O₃/ZnO C–S NWs. Moreover, the superior response can be credited to the modulation of the potential barriers at the In₂O₃–ZnO interface, at the ZnO–ZnO homojunction, and the ZnO grain boundaries.

Compared to traditionally synthesized ZnO NPs, newly developed ZnO nanostructures from metal–organic frameworks (MOFs), a highly crystalline and porous material, have recently attracted attention for gas-sensing applications. Liu et al.¹⁵⁶ fabricated In₂O₃ NFs coated with zeolitic imidazole framework-8 (ZIF-8)-based gas sensors for NO₂ detection. Initially, In₂O₃@ZnO NFs were synthesized using an electrospinning method, where ZIF-8 was used as a template and a Zn²⁺ source (Fig. 12a–e).

Fig. 12 (a) Schematic diagram of the synthesis procedure of In₂O₃/ZIF-8 composites (structure of ZIF-8 is shown in (b)) with varying ratios of In/Zn²⁺ to control the morphologies (c–e), (f) TEM image of the In₂O₃/ZIF-8 interface and a humidity-sensing mechanism showing the repellence of H₂O molecules by ZIF-8, (g) response values of various sensors, including pure In₂O₃ NFs and In₂O₃/ZIF-8 composites, with respect to the operating temperatures, and (h) response values of sensors with respect to the different humidity conditions. Reproduced from ref. 156 with permission from the American Chemical Society (Copyright 2020).

Various morphologies of In₂O₃/ZIF-8 C–S nanostructures were obtained by changing the molar amount of Zn²⁺, such as 8 : 1, 4 : 1, and 2 : 1 (Fig. 12c–e presents TEM images). Fig. 12f shows a high-resolution TEM image and a schematic diagram of the sensing surface. The as-prepared In₂O₃/ZIF-8 NFs sensor displayed a strong response of R_g/R_a = 16.4 to 1 ppm NO₂against other interfering gases, rapid response, and recovery time of 80 s/133 s, and LOD value of 4.9, which is in contrast to the pristine In₂O₃NFs sensor, at a lower optimal temperature of 140 °C (Fig. 12g). The In₂O₃@ZIF-8 C–S NFs exhibited excellent humidity resistance because of the hydrophobicity of ZIF-8 (Fig. 12h) because it acted as an effective gas enhancement medium that encouraged the adsorption capability of NO₂ gas molecules and prevented the diffusion of water molecules. The high response value was attributed to the enhanced surface area (700 m² g⁻¹) and porous morphology of the composite. Furthermore, ZIF-8 possesses the excellent adsorption capability of NO₂.¹⁵⁷

Tungsten oxide (WO₃), as an n-type MOS, has been utilized broadly as a gas-detecting material. Cao et al.¹⁵⁸ reported n–n heterostructures of ultra-thin In₂O₃ nanosheets decorated with WO₃ clusters for HCHO sensing. Different WO₃ clusters (2–16 wt%) were incorporated on In₂O₃ nanosheets using the impregnation method to study their impacts on gas-sensing performance. The sensing results showed that the 4 wt%-WO₃-loaded In₂O₃ sensor exhibited an outstanding sensing response (R_a/R_g = 25), excellent selectivity, rapid τ_res/τ_rec (1 s/67 s), and a LOD of 0.1 ppm HCHO at 170 °C. The high performance was attributed to the heterojunction-type structures, large surface area, porous ultra-thin nanosheets, and electronic transduction at the heterojunction interface.

Feng et al.¹⁵⁹ fabricated a series of WO₃-NP-loaded In₂O₃ NFs for CH₃COCH₃ gas sensing. The 1.5% In₂O₃–WO₃ NFs (S3) sensor displayed the strongest response (29) to 200 ppm CH₃COCH₃ at 275 °C with a LOD of 0.4 ppm and a response of 1.28. The mechanism of the sensing performance was attributed to the establishment of an n–n heterojunction between In₂O₃ and WO₃, which increased the resistance of the In₂O₃–WO₃ NFs sensor. On the other hand, the surface of the 1.5% In₂O₃–WO₃ NFs (S3) sensor was rough, which can facilitate the adsorption of the test gas and improve the sensing reaction of the test gas with adsorbed oxygen species.¹³⁸ The 1.5% In₂O₃–WO₃ NFs (S3) sensor possessed a higher utility factor because of the presence of well-connected grain boundaries or grain junctions, which highly contributed to the gas response.⁵⁸ Therefore, the response of the 1.5% In₂O₃–WO₃ NFs sensor was higher than the other sensors (S1, S2 and S4). Hence, tuning the morphology helps enhance the sensing response.

3.3 Catalytic functionalization

Several noble metals (such as Pt, Au, Ag, and Pd) can be integrated into the host material to increase the gas-sensing properties and reduce the working temperature. Recently, the following three types of strategies have been utilized the most for noble metals on In₂O₃ gas sensors: (i) noble metal functionalization or decoration directly on the surface of In₂O₃-based NPs (noble metal-functionalized In₂O₃); (ii) noble metal encapsulation inside a thin or thick solid shell of In₂O₃, which is called a C–S structure (metal@In₂O₃ core–shell); (iii) noble metal encapsulation inside a thin or thick hollow shell, which is a yolk–shell or rattle-type structure (metal@In₂O₃ yolk–shell). This section evaluates three types of In₂O₃-based nanostructured sensing materials listed above.

Noble metal functionalization or decoration on the surface of MOS is a popular and promising strategy for boosting the gas-detecting properties. Noble metals, which act as catalysts, have been used to modify the surface of MOS to enhance the gas-sensing properties.¹⁶⁰ The effects can be divided into two types, electronic and chemical sensitization.¹⁶¹ In electronic sensitization, when the noble metals and metal oxides come into contact, the flow of electrons occurs from the MOS to the noble metal, ensuing in a space charge layer on the MOS surface as well as a Schottky potential barrier and band-bending in the MOS sensor. Therefore, the overall sensor resistance increases. After introducing the target gas, it interacts with the chemisorbed oxygen species on the sensor surface, and electrons are released back to the conduction band of the sensor. Subsequently, both the sensor resistance and the potential barrier height decrease. In chemical sensitization, noble metals can dissociate O₂ into atomic species because of their high catalytic nature, which then are spilled over on the sensor surface to be chemisorbed. Therefore, this effect is also called the “spillover effect”. Fundamentally, a greater number of oxygen ions are found on the surface of the sensor. In addition, the target gases can be dissociated and adsorbed onto the surface-active sites of the noble metal catalysts for a possible reaction with the oxygen ions that have already been chemisorbed. Hence, chemical sensitization or the spillover effect helps accelerate the sensing process.

Among various noble metal NPs, Au NPs have been used extensively to boost the gas-sensing properties of MOS materials. Xu et al.¹⁶⁰ reported an in situ synthesized Au-loaded In₂O₃NFs gas sensor with different amounts of Au (0.1 to 0.4 wt%) for C₂H₅OH gas-sensing applications. The 0.2 wt% Au-loaded In₂O₃sensor displayed a high response (13.8) and fast τ_res/τ_rec (12 s/24 s) at 140 °C. The improved C₂H₅OH response of In₂O₃NFs was related to the electronic sensitization of Au NPs. The Au NPs acted as an efficient catalyst that generated more active sites on the sensor surface and contributed to enhancing surface reactions, which are essential for improving the sensor response. They proposed that this device can be exploited as a portable high-performance C₂H₅OH gas sensor because of its relatively lower power consumption and fast τ_res/τ_rec. In a different study, Au NPs were chosen to decorate mesoporous In₂O₃ to study the relationship between the different decoration methods of Au and their corresponding gas-sensing performance.^162a A 2D hexagonal mesoporous silica template of SBA-15 was synthesized initially for constructing In₂O₃ mesoporous architectures. Au NPs were loaded using two techniques: adding 0.5 mol% of Au to the precursor solution during synthesis followed by calcinating obtained products to obtain Au-decorated mesoporous In₂O₃ (IO-Au-D); the other was produced by loading 0.5 mol% Au on the pure calcined mesoporous In₂O₃ (IO-Au-L). The Au NPs were dispersed uniformly on the IO-Au-D composite, while the Au NPs were aggregated in the IO-Au-L composite. The Au-decorated mesoporous In₂O₃ sensor and 0.5 mol% Au-loaded In₂O₃ sensor displayed responses of 19.01 and 12.25 towards CH₃COCH₃ (100 ppm) at 250 °C, respectively, compared to pure In₂O₃ (8.38) at 275 °C. The good sensing performance exhibited by the Au-decorated mesoporous In₂O₃ sensor was attributed to the expanded depletion layer compared to the Au-loaded In₂O₃ sensor. In this case, both the electronic and chemical sensitization effects of Au NPs were responsible for increasing the CH₃COCH₃ sensing response of the In₂O₃ sensor. As an active catalyst, Au formed additional active sites important for boosting the CH₃COCH₃ response. On the other hand, Au NPs can catalyze the dissociation of oxygen molecules. Thus, the activated oxygen species formed spill onto the In₂O₃ surface, leading to more reactive oxygen species for further reactions with CH₃COCH₃ gas molecules.

Tuning sensing materials has been the focus of significant efforts to enhance the gas selectivity in chemiresistive gas sensors. On the other hand, selective detection of less reactive gases still poses a challenge. In this respect, Lee et al.^162b presented a novel method for the selective detection of low-reactive gases using patterned In₂O₃ NFs and tuning the electrode catalytic properties. The approach involved direct-write near-field electrospinning onto interdigitated electrodes (IDE) with low sensing material coverage, allowing exposure to analyte gases. Sensors made of entangled In₂O₃ NFs sensors were prepared on Pt_IDE and Au/Pt_IDE electrode substrates to study the effect of gas access to the electrode on gas sensing characteristics.

The sensing properties of the fabricated gas sensors rely on the catalytic activity of the electrode, with Pt_IDE indicating an excellent selective response to xylene. Fig. 13 presents the gas sensing mechanisms of In₂O₃@Au/Pt_IDE and In₂O₃@Pt_IDE sensors with the respective response plots of the sensors [In₂O₃@Pt_IDE (Fig. 13a), In₂O₃@AuPt_IDE (Fig. 13c), Au@In₂O₃@Pt_IDE (Fig. 13e), and Au–In₂O₃@AuPt_IDE (Fig. 13f)]. When subjected to the highly catalytic electrode, namely 8Au/Pt_IDE, C₂H₅OH gas molecules underwent instantaneous reactions with the electrode (Fig. 13d1), resulting in complete oxidation and an inadequate C₂H₅OH response. On the other hand, xylene gas was reformed into highly reactive and smaller intermediates through its reaction with 8Au/Pt_IDE (Fig. 13d2), which were later transported to In₂O₃ NFs, leading to an improved sensor response. On the other hand, when only Pt_IDE substrates were used, the analyte gases, such as C₂H₅OH and xylene, were not fully oxidized or reformed because of the lower catalytic activity of Pt_IDE compared to 8Au/Pt_IDE (Fig. 13b1 and b2).

Fig. 13 Gas sensing characteristics of (a) In₂O₃ @Pt_IDE, (c) Au–In₂O₃@8Au/Pt_IDE, and (e) Au–In₂O₃@Pt_IDE, (f) Au–In₂O₃@8Au/Pt_IDE to 5 ppm of analyte gases at 325 °C. Schematic diagram of the gas sensing mechanism of (b) In₂O₃@Pt_IDE, and (d) In₂O₃@8Au/Pt_IDE. Reproduced from ref. 162b with permission from Elsevier.

Modification by noble metal NPs also dramatically improves the selectivity and τ_res/τ_rec speed. For example, Xing et al. reported the loading of Au NPs using different loading methods on a unique 3D inverse opal In₂O₃ microporous film with a honeycomb architecture¹⁶³ for a noninvasive diagnosis of CH₃COCH₃ gas in diabetic patients. The sensor made from such structures could sense up to 5 ppm CH₃COCH₃ with a strong response of 42.4 and a rapid response time (11 s) at 340 °C. The excellent sensing performance can be attributed to the spillover of Au NPs on the 3D inverse opal In₂O₃ microporous film and its unique structure. A microporous honeycomb structure from In₂O₃ provided immense gas accessibility that enables CH₃COCH₃ gas molecules to access easily and quickly, making sensors more active for gas-sensing. The preceding study can lead to a promising sensing material for monitoring CH₃COCH₃via exhaled breath, benefitting diabetic patients. In another study, a room-temperature sensor was fabricated using Ag–In₂O₃ NRs composites, prepared via a facile hydrothermal process, for H₂S gas-sensing applications. The sensor made from 13.6 wt% Ag–In₂O₃ NRs displayed an outstanding and super sensitive response of 93 719 to 20 ppm H₂S with a LOD of 0.005 ppm. The chemical sensitization effect of Ag NPs increased the number of chemisorbed oxygen ions by withdrawing electrons from the conduction band of In₂O₃, which in turn increased the depletion layers of In₂O₃ sensing materials and the response performance.¹⁶⁴ Similarly, Au NPs of various amounts have been loaded on sensing materials of In₂O₃ microspheres,¹⁶⁵ hollow nanospheres,¹⁶⁶ and porous nanocubes,¹⁶⁷ for detecting trimethylamine (TMA), butylamine, and HCHO at 340 °C, 280 °C, and 240 °C, respectively.

Thus far, various noble metals (Pd, Au, and Pt) have been functionalized on In₂O₃ to design prospective sensing materials. On the other hand, these noble metal NPs are expensive, increasing the ultimate device cost. Ag, another noble metal, has been used in low-cost material sensors.^168–170 Wang et al.¹⁶⁸ reported a 3D hierarchical structure made from In₂O₃ and subsequent Ag functionalization using a NaBH₄ reducing agent to construct 6 wt%-Ag-loaded In₂O₃ nanoflower composite materials for HCHO sensing. The sensor exhibited enhanced sensitivity (R_a/R_g = 11.3) to 20 ppm gas, rapid response/recovery time (0.9 s/14 s), and high selectivity. The enhanced sensitivity was attributed to the nonporous and branched nanorods connected in a sunflower-like morphology that exhibited a relatively larger surface area and induced abundant dynamic sites for the gas adsorption, diffusion, and surface reactions through the catalytic dissociation of Ag NPs via the spillover effect, which triggered the sensing reactions and electronic sensitization effect of Ag and In₂O₃.

Generally, the MOS-based gas sensors are operated at high temperatures. On the other hand, sensing measurement at a high temperature has several practical limitations, such as an increase in energy consumption, size, and cost of devices, as well as the degradation of sensing performance due to microstructural changes at an elevated temperature. Importantly, detecting flammable and explosive gases at such high temperatures has a high risk of exploding. Therefore, the research on developing room temperature (RT)-based gas sensors has a beneficial impact on.¹⁷¹ At RT, some gases, such as H₂ and CO, are often difficult to detect using pristine MOS gas sensors. This problem can be avoided by modifying the noble metal catalysts on the sensor surface. Singh et al.¹⁷² employed an APhS-self-assembly monolayer (SAM) approach in decorating In₂O₃ NWs with Au NPs for CO sensing at RT. They prepared In₂O₃ nanowires using a thermal evaporation technique and immobilized the citrate-capped Au NPs on the amine-terminated surface of In₂O₃ NWs to form Au-functionalized In₂O₃ NWs sensing materials (Fig. 14a). The effect of the loading time (10–60 min) of the Au NPs on the In₂O₃ NWs was studied (Fig. 14b). With a loading time of Au NPs of 60 min, the senor response increased and became the highest. It displayed a response (R_a/R_g) of 110 to 5 ppm CO gas (Fig. 14c). Fig. 14d compares the response of various sensors to 5 ppm CO gas. The high exposure of the Au NPs on the sensor surface increased CO oxidation to CO₂ because of the increased spillover zone via the catalytic activation of Au, increasing the conductance in the sensor. The small size of the Au NPs also resulted in a high density of reactive defect sites for gas molecule adsorption and dissociation. The functionalization of the AphS-SAM layer helps densify the Au NPs on the surface of the In₂O₃ NWs and increases the depletion layer region beneath the nanowire and NPs. Furthermore, modulation of the Schottky barrier height caused by the configuration of the depletion layer at the Au NPs and In₂O₃ NWs interface contributes to sensor enhancement (Fig. 14e).

Fig. 14 (a) Schematic diagram of the synthesis process of citrate stabilized Au functionalization on In₂O₃ NWs, (b) Au functionalized In₂O₃ NWs with different loading times (10–60 min), (c) CO gas response for Au functionalized In₂O₃ NWs (60 min loading time), (d) response comparison for 5 ppm CO with different sensing devices, and (e) energy band model of a Au-functionalized In₂O₃ NWs sensor. Reprinted from ref. 172 with permission from the American Chemical Society, (Copyright 2011).

Zhou et al.¹⁷³ fabricated an RT-based ultrasensitive HCHO sensor made from 5 wt% Ag nanoparticle sensitized-In₂O₃ rod-type nanograin structures. The sensor displayed an outstanding response of 135 to 1 ppm HCHO gas with τ_res/τ_rec of 102 s/157 s and a LOD of 0.05 ppm at RT. The excellent sensing activities were attributed to the chemical and electronic sensitization effects of the Ag additive.

Pd is an excellent catalyst for gas-sensing applications. In this respect, Wang et al.¹⁷⁴ synthesized a 3DOM-In₂O₃ sensor loaded with 0.5 wt% Pd (Pd_0.5In) for an RT study of NO₂ sensing. The Pd_0.5In sensor exhibited high sensitivity of 980 compared to 3DOM-In₂O₃, for 500 ppb of NO₂ gas at RT, with quick τ_res/τ_rec and good stability. The excellent sensing results of Pd_0.5In were attributed to the catalytic effect of Pd NPs and their synergistic effect with In₂O₃, which increased the concentration of electrons and surface defects. Fig. 15 shows the sensing mechanism of the Pd₀In and Pd_0.5In sensors in the presence of air and NO₂ gas. In this case, the NO₂ sensing mechanism of the Pd₀In and Pd_0.5In sensors was elucidated using the variation of the depletion layer, which results primarily from the surface modification on the 3DOM-In₂O₃ when loading Pd. In addition, the higher carrier concentration of the Pd-loaded 3DOM In₂O₃ sensor, induced by highly conductive Pd loading, also facilitated the increased response to NO₂ gas. The enhanced carrier concentration of the Pd-loaded 3DOM In₂O₃ sensor stimulated additional chemisorbed oxygen species by capturing more electrons to widen the depletion layer.

Fig. 15 Schematic diagram of the gas detecting mechanism of pristine 3DOM In₂O₃ (Pd₀In) and 0.5 wt% Pd-loaded 3DOM In₂O₃ (Pd_0.5In) with their corresponding energy band models. Reproduced from ref. 174 with permission from Elsevier (Copyright 2020).

3.4 Shell structure

Numerous nanomaterials based on noble metal-functionalized MOS gas sensors were reviewed in the previous section. Despite their good sensing properties because of the effective catalytic activity of metal NPs, there are structural drawbacks, such as sintering at higher operating temperatures during the sensing tests, which often lead to reduced sensing properties.^175,176 To overcome such problems, researchers have recently started exploring a unique type of morphology known as the C–S nanostructure, which offers many advantageous features in gas-sensing applications.¹⁷⁷ Several applications of the C–S nanostructure have been explored, including its gas-sensing application.^178–180 Among them, a noble metal–oxide C–S system has become a sought-after material for gas-sensing applications. In the case of metal–oxide C–S systems, metal NPs are encapsulated in a thick oxide shell material. In this new morphology, metal NPs can exhibit unique functionality compared to noble metal-deposited or functionalized MOS nanostructure-based gas sensors. The physicochemical and sensing properties can be tuned by varying core and shell materials. Their effective size and optical properties can be tuned by judiciously controlling their synthetic conditions. The specialty of this structure is that the metal NPs can be prevented from sintering, aggregation, and coarsening; hence, this structure has received more research attention regarding gas-sensing applications.^181,182 Most importantly, the noble metal–oxide interface must be maximized to facilitate electronic interactions in terms of charge transfer, leading to enhanced sensing properties.¹⁸⁰ This section summarizes the current progress on In₂O₃-based C–S nanostructures in gas-sensing applications was summarized.

Li et al.¹⁸³ conducted pioneering work on noble metal@In₂O₃-based C–S nanostructure-based gas sensors. They prepared Au@In₂O₃ C–S nanostructures using a two-step method. First, an Au@carbon (Au@C) sphere template was synthesized via a facile hydrothermal technique at 180 °C using HAuCl₄ and a glucose mixture solution. Second, the complete Au@In₂O₃ C–S nanostructures were formed after the Au@C sphere was decorated with an In₂O₃ shell via a solution method followed by a simultaneous RT aging and high-temperature calcination process. The as-prepared Au@In₂O₃C–S structures were tested for their potential gas-sensing properties, which exhibited high sensitivity (R_a/R_g = 17.0) and selectivity to HCHO at an optimal temperature of 200 °C with a fast τ_res/τ_rec (7 s/135 s). The improved sensing result of the Au@ In₂O₃C-SNPs was attributed to their high electron depletion layer, which resulted from the catalytic activity of the Au core. Wang et al.¹⁸⁴ carried out further research on this material. An Au@In₂O₃ C–S NPs was fabricated using a sol–gel strategy. The sensing performance was assessed for various gases at varied temperatures. The Au@In₂O₃ NP gas sensor exhibited excellent selectivity and the maximum sensing response (R_a/R_g) of 36.14 toward C₂H₅OH (100 ppm) at an optimal temperature of 160 °C with rapid τ_res/τ_rec (4 s/2 s). The good sensing performance was attributed to the configuration of the Schottky junction because of the Au/In₂O₃ interface and catalytic activity of the Au metal core.

Liu et al.¹⁸⁵ reported Ag@In₂O₃ C–S nanostructures for C₂H₅OH gas-sensing. The synthesis of Ag@In₂O₃ C–S nanostructures was similar to Li et al.¹⁸³ This time, however, the noble metal core Ag was introduced through encapsulation inside the In₂O₃ shell. The authors chose Ag as the core material owing to the high cost of Au noble metal, as well as the high electrical and thermal conductivity, antibacterial properties, and nontoxicity of Ag noble metal. The typical Ag@In₂O₃ C–S nanostructure was synthesized from the sacrificial template of Ag@C nanospheres (Fig. 16a). The Ag@In₂O₃ C–S nanostructures (Fig. 16b) exhibited an increased response (72.56), and high selectivity to 50 ppm C₂H₅OH at an operating temperature of 220 °C (Fig. 16c) with a rapid response/recovery speed (13 s/8 s) and excellent transient values, even at high-humidity (25 to 90%). As shown in Fig. 16d, the high selectivity to C₂H₅OH, alongside other volatile gases, was attributed to the low bond energy and high absorbing ability of O–H in C₂H₅OH molecules.¹⁸⁶Fig. 16e presents the sensing mechanism, where a Schottky junction was established near Ag and In₂O₃ interface after their contact. Electron transfer occurred from In₂O₃ (φ = 4.3 eV) to Ag (φ = 4.6 eV), resulting in the bending of the In₂O₃ band and the subsequent extension of the electron depletion layer. Hence, the resistance of the Ag@In₂O₃ C–S sensor increased. When the C₂H₅OH gas was exposed to the Ag@In₂O₃ C–S gas sensor, C₂H₅OH gas molecules were oxidized by a reaction with the adsorbed oxygen ions, releasing the trapped electrons back to the Ag@In₂O₃ system (Fig. 16f). This resulted in a significant increase in charge carriers, lowering the sensor resistance. In addition, the Ag catalyst provided sufficient dynamic sites for the binding and dissociation of O₂ molecules and accelerated the gas-sensing reaction via the spillover effect.

Fig. 16 (a) Schematic diagram of the synthesis procedure of Ag@In₂O₃ C–S NPs, (b) TEM image of Ag@In₂O₃ C–S NPs, (c) sensor responses with respect to different operating temperatures toward 50 ppm C₂H₅OH, (d) selectivity comparison of Ag@In₂O₃ C–S, as well as solid and hollow In₂O₃ NP sensors at the optimal temperature of 220 °C for 5 ppm C₂H₅OH, and (e and f) sensing mechanism of Ag@In₂O₃ C–S NPs in air and C₂H₅OH atmospheres with the corresponding energy band model. Reproduced from ref. 185 with permission from Elsevier (Copyright 2020).

Chava et al.¹⁸⁷ prepared Au@In₂O₃ C–S NPs using a hydrothermal technique followed by calcination. Flower-shaped Au@In₂O₃ C–S NPs with porous structures were formed after the subsequent calcination of Au@InOOH C–S NPs at 350 °C. The Au@In₂O₃ C–S nanospheres were studied for the potential applicability of H₂ gas detection at different temperatures. They exhibited a high sensing response value of 34.38 compared to the pure In₂O₃ nanospheres at 300 °C. The improved sensing properties of Au@In₂O₃ C–S nanostructures were attributed to the electronic sensitization and chemical sensitization of Au. The work functions of the Au NPs and In₂O₃ are 5.1 eV and 4.8 eV, respectively. As a result, the charge carriers from the CB of In₂O₃ were transferred to the Au metal until the Fermi levels reached equilibrium. This phenomenon led to a large depletion layer near the Au NPs and In₂O₃ interface, leading to band bending and the formation of a Schottky barrier, thereby further increasing the resistance of the system. Regarding the sensitization effect, the Au noble metal provided active sites for the adsorption of oxygen and H₂ molecules and their dissociation into different oxygen species and H atoms on the In₂O₃ surface, as well as the subsequent spillover on the same. This process resulted in the extraction of more electrons from the CB of In₂O₃. The increased number of adsorbed oxygen ions resulted in the sensitivity of Au@In₂O₃ C–S to H₂ gas, ensuring a strong response. Moreover, the spillover effect of Au NPs facilitated the reaction speed-up concerning the oxygen species and H₂ atoms. Thus, the Au@In₂O₃C–S NP sensor exhibited enhanced sensing performance compared to the bare In₂O₃. The spillover reactions between H₂ and O₂ on Au surfaces can be expressed as

2Au + H₂ → 2Au/H (13)

2H + O⁻ → H₂O + e⁻ (14)

2Au + O₂ → 2Au/O (15)

O + e⁻ → O⁻ (16)

Another category of C–S nanostructures, known as yolk–shell or rattle-type structures, is in great demand. The typical yolk–shell nanostructures represent a unique type of C–S structure with a characteristic core–hollow–shell configuration. In this structure, the noble metal core is movable inside a hollow metal oxide shell, enabling the complete exposure of all active sites to come in contact with target analytes during the sensing reaction. This section summarizes the recent developments in metal–In₂O₃-based yolk–shell nanostructures for their potential gas-sensing applications.

Thus far, there are only a few reports on metal@In₂O₃ yolk–shell nanostructures for gas-sensing applications. Rai et al.^188a examined Pd@In₂O₃ yolk–shell (Y–S) NPs for sensing reducing and oxidizing gases. They used a typical glucose-assisted synthesis technique to prepare Pd@In₂O₃Y–S nanostructures. Initially, Pd@C, as a sacrificial template, was used to synthesize Pd@In₂O₃Y–S NPs. The Pd@In₂O₃Y-SNP sensor exhibited a C₂H₅OH response of 159.02 (5 ppm) at 350 °C compared to pure In₂O₃hollow NSs at 350 °C. The strong response of the Pd@In₂O₃Y–S NPs could be attributed to the hollow space of Pd@In₂O₃Y–S NPs, which maximized the accessibility of Pd NPs with gas molecules, with an increase in the number of oxygen ions adsorbed owing to the catalytic activity in Pd NPs. Enhancing the performance of semiconductor oxide-based gas sensors using bimetallic nanoparticles is a promising approach. Sun et al.^188b enhanced the trimethylamine (TMA) sensing performance of pure In₂O₃ nanosphere sensor by decorating the In₂O₃ nanospheres with bimetallic AuPd NPs. Compared to pure In₂O₃, the AuPd–In₂O₃ sensor showed a stronger response (R_a/R_g = 367), low operating temperature (175 °C), and short response time (2 s) towards TMA. In addition, it showed good repeatability, long-term stability, and cross-selectivity. The improved TMA sensing performance of the AuPd–In₂O₃ sensor was attributed to the synergistic effect of AuPd and that AuPd–In₂O₃ inherits the chemical sensitization of Au and the electronic sensitization of Pd. Therefore, modifying the surface of In₂O₃ with AuPd NPs is an efficient way to enhance the gas-sensitive performance.

3.5 Doping

Doping a metal oxide crystal with specific impurities is a standard method to alter the gas-sensing properties. Doping in metal oxides influences the electronic and structural properties, such as charge carrier concentration, grain size, shape, and morphology, thereby significantly enhancing the gas-sensing performances. In recent years, elemental doping has been considered an efficient way of improving the sensing properties. The doping process judicially increases the defects and number of active sites in the metal oxide, influencing the gas-sensing properties.¹⁷¹ In addition to the transition metal element, rare earth metal elements (e.g., La and Ce) with available 4f shells are considered effective dopants for enhancing the sensing performance. Thus far, various transition metals and rare earth metal elements have been reported. This section presents an overview of doped In₂O₃ gas sensors. Zhao et al. reported the synthesis of mesoporous Fe-doped In₂O₃ nanostructures prepared via a nano-casting route using a template 3D cubic KIT-6 mesoporous silica, as well as indium and ferric nitrate precursors for low-temperature NO₂ gas sensing.¹⁸⁹ The Fe-doped In₂O₃ mesoporous sensor was tested at 100 °C, and showed a response value of R_s = 116 for 1 ppm NO₂, compared to pure In₂O₃ (R_s = 27). In addition, the mesoporous Fe-doped In₂O₃ sensor exhibited excellent selectivity to NO₂ gas when tested against other gases. The excellent NO₂ sensing response observed in the Fe-doped In₂O₃ was attributed to its superior surface area and pore volume. This causes extremely active surface interactions between surface-active sites and NO₂ gas molecules. In addition, the doped Fe-species in the In₂O₃ sensor promoted the efficient adsorption of NO₂ gas molecules at the sensor surface, leading to an enhanced response. Yang et al. further improved the NO₂ sensing properties by preparing Zr-doped mesoporous In₂O₃ nanostructures.¹⁹⁰ The NO₂ sensor was tested at a low optimal temperature of 75 °C, which yielded a response value of 169 for 1 ppm NO₂. The typical n-type MOS detection mechanism, based on the depletion layer, was used to examine the sensing interactions between the target NO₂ gas and the Zr–In₂O₃ sensor; Fig. 17 shows a schematic diagram of the equivalent sensing mechanism.

Fig. 17 Schematic diagram of the NO₂ sensing mechanism of pure and Zr doped In₂O₃ sensor. Reproduced from ref. 190 with permission from Elsevier (Copyright 2017).

Outstanding NO₂ sensing properties of Fe-doped In₂O₃ nanostructures can be explained. First, the charge carrier concentration increased due to Zr-doping in In₂O₃, which increased the chemical adsorption of oxygen molecules on its surface. The ionic radius of Zr⁴⁺ and In³⁺ is 0.074 nm and 0.081 nm, respectively. Therefore, Zr⁴⁺ ions readily substituted in the In³⁺ sites,¹⁹¹ generating one electron per Zr. Hence, the electron concentration of Zr-doped In₂O₃ increases with increasing chemisorbed oxygen species.¹⁹⁰ Moreover, the crystal size of In₂O₃ decreased due to Zr⁴⁺ doping, possibly enhancing the response. As an oxidizing gas, NO₂ can capture electrons from In₂O₃ and react with oxygen species to generate O₂ molecules. The reduction in grain size increased the depletion layer and the barriers of grain boundaries, which further increased the resistance of the Zr-doped In₂O₃ sensor, thereby improving the sensitivity. Furthermore, the ordered mesopores and huge specific surface area make the adsorption and diffusion of target gas molecules easy.

Rare earth metals have also been used to dope In₂O₃. Liu et al.¹⁹² reported the synthesis of Ce-doped In₂O₃ nanostructures for glycol-sensing properties. The Ce-doped In₂O₃ porous NS sensor revealed a high response and superior selectivity to glycol at a testing temperature of 240 °C. Kim et al.¹⁹³ reported the fabrication of In₂O₃ macroporous spheres with 3–12 at% Pr-doping for the study of CH₃COCH₃ gas-sensing. The sensor demonstrated excellent sensitivity (R_a/R_g = 20) with negligible fluctuations in response even at high relative humidity (RH = 80%). Among the other earth metals, La has been exploited as a dopant in gas-sensing applications. Wei et al.⁴⁶ synthesized 1 to 5 mol% La-doped In₂O₃ microspheres for H₂S gas-sensing. Their gas-sensing outcomes showed that the 3 mol% La-doped In₂O₃ showed a response of 17.8 to 10 ppm H₂S at 200 °C, with outstanding selectivity, excellent stability, and good repeatability. Such excellent sensitivity to H₂S can be elucidated through the large surface area, the changes in crystallite size, and the different oxygen components caused by La doping. Zhao et al.¹²⁵ reported improved H₂S sensing properties using Mg-doped In₂O₃ NTs. Mg-doped In₂O₃ NTs exhibited high sensitivity (R_a/R_g = 173.14) and selectivity towards H₂S tested at 150 °C compared to bare In₂O₃ NTs (R_a/R_g = 12.31). The oxygen vacancies and sulfuration–desulfuration were two reasons for such an improved sensing performance. Zhang et al.^194a designed an In₂O₃ nanostructure modified by Ni doping (2.5 to 7.5 mol%) and surface functionalization (3 to 9 wt%) with Ag NPs. First, the Ni-doped In₂O₃ NRs were synthesized using an oil bath and annealing method, followed by Ag-functionalization through a chemical reduction method. The gas detecting results showed that the 6 wt% Ag functionalized with 5 mol% Ni-doped In₂O₃ exhibited an excellent response to 100 ppm HCHO at 160 °C. Furthermore, the 6%-Ag@Ni_5.0In₂O₃ sensor showed excellent selectivity to HCHO, with a quick response speed (1.45 s) and high stability. The superior sensing performance was attributed to the large surface area, the catalytic and electronic sensitization effects of Ag NP, and relative O_V and O_C contents caused by the elemental doping of Ni.

Jin et al.^194b synthesized Ni-doped In₂O₃ NPs using a metal–organic framework-derived solvothermal method, forming ultrafine particles with a mean size of 13 nm. Gas sensing tests showed that the Ni-doped In₂O₃ (2 mol%) NPs had a fast response/recovery time of 2 s to 10 ppm NO₂, with a response value of ∼70 at 200 °C. Furthermore, the Ni-doped In₂O₃ sensor had a low detection limit of 5 ppb, making it an excellent candidate for rapid NO₂ detection. The enhanced sensing properties were attributed to the abundant adsorption sites, high oxygen vacancies content, and catalytic action of Ni²⁺. Recently, Zhang et al.^194c synthesized hierarchical S-doped In₂O₃ networks with GO as a template and annealed them at different temperatures to examine their effect on the structure and the surface morphology. The resulting S-doped In₂O₃ networks exhibited excellent gas sensing properties, with the sample annealed at 500 °C showing the best response (74.4 to 50 ppm C₂H₅OH at 180 °C). The observed response was approximately two times higher than S-doped In₂O₃ prepared without a GO template. The excellent properties were attributed to the unique hierarchical structure with a surface area of 295.23 m² g⁻¹, S-doping, and abundant oxygen vacancies because it provided abundant active sites for C₂H₅OH molecules to enhance diffusion and adsorption, facilitating gas response. Specifically, S-doping narrows the band gap, introduces surface oxygen defects, and improves the gas-sensing properties. SO₄²⁻ adsorption on S-doped In₂O₃ produces acidic sites that enhance the chemical adsorption of C₂H₅OH and oxygen.

3.6 Loading of carbonaceous materials

A new class of sensing materials based on carbonaceous nanoparticles, including graphene, graphene oxide (GO), reduced GO (rGO), carbon nanotubes (CNTs/MWCNTs), and carbon quantum dots, have been immerged.^195–197 These carbon nanomaterials possessed excellent advantages, such as good electrical properties, high mechanical strength, and high carrier mobility at RT, making them promising materials for other applications, including gas sensing.^198–200 In their pristine form, however, carbon nanomaterials usually show poor response characteristics with a sluggish response/recovery time at RT. These materials are often combined with other secondary nanomaterials, such as noble metals and metal–oxides, to enhance their sensing performances further. The following section review the In₂O₃/carbon material composites and discuss their potential application for gas sensing.

Since its discovery, GO has been used as a remarkable sensing material owing to its unique 2D architecture, rapid electron transport properties, and large surface area (2630 m² g⁻¹), which facilitates the adsorption of gas molecules and surface reaction because of its sufficient number of surface-active sites.^201–203 The hydrothermal process is an effective, facile, and low-cost method for obtaining various micro/nanomaterials with controlled morphologies. Mishra et al. reported the synthesis of In₂O₃ nanocubes and In₂O₃/RGO heterostructures using a hydrothermal technique with hexamethyldisilazane (HMDS) surfactant.²⁰⁴ The sensor comprised of In₂O₃/RGO heterostructures showed a high response to 25 ppm each of HCHO (88%) and CH₃COCH₃ (85%) at operating temperatures of 225 °C and 175 °C, respectively. The strong response to HCHO was attributed to the electrophilicity of HCHO molecules. Its Lewis acid characteristics allowed the acceptance of more electrons. Moreover, the improved sensing characteristics of In₂O₃–RGO heterostructures can be attributed to the synergistic reactions among the In₂O₃ nanocubes and rGO. In another study, In₂O₃ cubes/rGO sheet composites were prepared from InN NWs and GO as the primary precursors using a microwave-assisted hydrothermal technique.²⁰⁵ Here, graphene displayed a key role in determining the transformation of InN-NWs to In₂O₃ nanocubes. Different InN to GO mass ratios (i.e., 0.1 : 1, 0.5 : 1, 1 : 1, 3 : 1, and 5 : 1) were used to produce various composite structures and distribute In₂O₃ on the graphene matrix. The In₂O₃ cubes/rGO (1 : 1) sensor showed the strongest response (R_a/R_g − 1 = 60.8%) in the presence of 50% RH at RT, with good selectivity to NO₂ (1 ppm). The high sensing performance of the In₂O₃ cubes/rGO sensors to NO₂ was attributed to the numerous wrinkles in the crumpled graphene sheets, where In₂O₃ cubes were distributed uniformly over the matrix. In addition, the electron retreating nature of NO₂, the oxygen atom-rich functional groups, and the defect sites of In₂O₃ cubes/rGO also resulted in the high selectivity of NO₂ sensing.

Gu et al.²⁰⁶ developed In₂O₃–GO nanostructures using hydrazine hydrate-reduced GO as the primary precursor. The response of the In₂O₃–GO nanostructures sensor to 30 ppm NO₂ was 8.25 at RT with a long response time/recovery time (4 min/24 min). Fang et al.²⁰⁷ reported NO₂ gas sensing based on In₂O₃ NRs decorated on rGO (In₂O₃ NRs/rGO), synthesized from the reflux process using In³⁺, GO, and urea precursors solution, followed by calcination (550 °C). The sensor showed a strong response to NO₂ at RT. The good performance was attributed to the gas diffusion of NO₂ in the mesopores of In₂O₃NRs and the highly efficient electron mobility of rGO. Yan et al.^208a developed 1D porous In₂O₃ NFs and rGO heterojunction composite (rGO–In₂O₃ NFs) to examine NO₂ gas sensing. Different amounts of rGO (1.1 to 3.6 wt%)–In₂O₃ NF composites were synthesized by varying the amount of rGO using an electrospinning technique. The gas detection properties showed that 2.2 wt% rGO–In₂O₃heterojunction NFs exhibited improved results compared to pure In₂O₃NFs. On the other hand, an excess of rGO caused a reduction in the sensing response.

Shah et al.^208b synthesized In₂O₃@GO nanocomposites using a simple precipitation method for NO₂ gas sensing applications. The gas sensing performance was optimized by tuning the percentage of GO in the nanocomposite. At 4 wt% GO, the sensor exhibited a high response of 78 to 40 ppm NO₂ gas at 225 °C, which was 13 times higher than pure In₂O₃ NPs. This was attributed to the high surface area (1336 m² g⁻¹) and formation of p–n heterojunctions in the In₂O₃@GO nanocomposites. In particular, the uniform distribution of In₂O₃ NPs onto the GO-surface formed junctions for ion–electron interaction reactions. The work function of GO (4.39 eV) was higher than that of In₂O₃ NPs (4.15 eV), forming a heterojunction. The crumpling and overlapping regions of GO sheets offer a high surface area that facilitates the operative access of gases to the In₂O₃ surface. Hence, In₂O₃@GO nanocomposites exhibited improved sensing performance towards NO₂ gas.

Nowadays, portable devices capable of detecting and analyzing environmental signals have shown numerous applications in modern electronics. In this regard, flexible gas sensors have been increasingly attractive because of their several advantages, such as portability, miniaturization, and wearable features. You et al.²⁰⁹ reported a unique laser writing technology to synthesize In₂O₃@rGO composites for RT-operated NO₂ sensing. This process assists in the photo reduction of GO and enables a programmable patterning on the flexible substrates, e.g., polyimide (PI), within a short period. The resulting In₂O₃@rGO sensor showed good sensitivity (31.6%), high selectivity towards 1 ppm NO₂, and excellent stability at RT. These results were caused by the porous structure of graphene, which resulted from the laser writing.

Na et al.²¹⁰ prepared rGO–In₂O₃ nanocomposites (Fig. 18a) using a solvothermal reaction of In³⁺ and rGO precursors, followed by drop-casting on a flexible PI substrate made from ITO/Ag–Pd–Cu electrodes for the analysis of its NO₂ gas-sensing properties (Fig. 18b). The rGO–In₂O₃ nanocomposites sensor demonstrated a strong response value (35.7) to 500 ppb NO₂, which is in contrast to that of the pristine In₂O₃ sensor (2.6) at 120 °C (Fig. 18c). Fig. 18d and e shows the sensing response of pristine and rGO–In₂O₃ nanocomposite gas sensor. rGO has outstanding electron-acceptor properties. Thus, reducing the conduction path along In₂O₃ NPs by intensifying the depletion layer close to the p–n interface among In₂O₃ and rGO was assumed to have a probable cause for increasing the sensor resistance (Fig. 18d and e).

Fig. 18 (a) Morphology of the rGO–In₂O₃ nanocomposites, (b) schematic diagram of the flexible gas sensor, (c) response of the gas sensor to different gases at diverse temperatures, (d) gas-sensing mechanism of the pristine In₂O₃ sensor, and (e) gas-sensing mechanism of rGO–In₂O₃ nanocomposite sensor. Reproduced from ref. 210 with permission from Elsevier (Copyright 2020).

The authors attributed the enhancement in response performance to the higher electron depletion layer in In₂O₃ NPs caused by the development of a p–n junction interface between the p-type rGO and n-type In₂O₃. In the air environment, O₂ ions were formed by capturing electrons from p-type In₂O₃–rGO hybrids. Because NO₂ gas has high electron affinity, it can capture the electrons from In₂O₃–rGO hybrids after exposure. As a result, larger hole accumulation layers with higher potential barriers formed. Hence, the final resistance decreased immediately, leading to a high response value and rapid response time. The equations for the sensing reaction are as follows:

NO_2(g) + e⁻ → NO₂⁻_(surface) (17)

2NO_2(g) + O₂⁻_(ads) + e⁻ → 2NO₃⁻_(ads) (18)

MOS-based chemiresistive sensors work at relatively high temperatures, which increases material instability and reduces the response. In addition, they enhance the risk associated with detecting combustible gases at high temperatures. Hence, polymer-based gas-detecting materials can be used as an alternative sensing material, given that they reduce the risk of these problems, allowing sensing measurement at RT. In this respect, the conducting polymers (CPs), e.g., polyaniline (PANI), poly(3-hexylthiophene) (P3HT), polypyrrole (PPy), polythiophene (PTh), and poly(3,4-ethylenedioxythiophene) (PEDOT), have attracted significant attention because of their excellent electrical and mechanical characteristics. These CPs contain π-bonds in their backbone chain, facilitating electron mobility through the polymeric chain. The presence of delocalized p-electrons on the backbone of these CPs is responsible for their unique electrical properties. Since 1980, some polymers have been used as sensing materials because of their electrical conductivity. Generally, CPs have been coupled with inorganic oxides in designing novel organic–inorganic heterostructures.^211–213 Doping and the incorporation of metal–oxides into these polymers can improve gas-sensing characteristics.²¹⁴ On the other hand, few studies have reported In₂O₃-based composites with CPs for gas-sensing applications. Moreover, PANI was explored in most of these reports. For example, Nie et al. reported²¹⁵ an In₂O₃/PANI composite NF-based sensor synthesized through in situ polymerization by varying the aniline to In₂O₃ mass ratios (i.e., 1 : 1, 2 : 1, and 4 : 1). The In₂O₃/PANI-2 NFs sensor showed the strongest response (53.20) to 1000 ppm NH₃ at RT. The improvement in the sensing characteristics of the In₂O₃/PANI nanofiber sensor was attributed to p–n junction formation among PANI and In₂O₃.

Li et al.²¹⁶ prepared a Au-loaded In₂O₃(2 to 50 mol%) @PANI C–S hybrid-structured sensing nanocomposite material (PANI/In₂O₃) (FESEM and TEM images are shown in Fig. 19a–d) using a combination of an in situ polymerization and a facile hydrothermal process for NH₃ gas sensing. The Au-loaded PANI/In₂O₃ sensing device was fabricated on a flexible PET substrate and tested at RT. The 1 at% Au-loaded 20 mol% In₂O₃nanosphere@PANI (1Au–2In₂O₃/PANI) C–S hybrid sensor displayed a response of approximately 46 to NH₃(100 ppm) at RT. The enhanced NH₃ sensing performance was attributed to the developed p–n junction between PANI/In₂O₃ and the catalytic activities of Au.

Fig. 19 FESEM images of (a) In₂O₃ nanosphere, (b) pristine PANI, (3) Au-loaded In₂O₃/PANI, and (d) TEM image of Au-loaded In₂O₃/PANI. (e) Schematic diagram of the sensing mechanism of In₂O₃@PANI and Au-loaded In₂O₃@PANI. Reproduced from ref. 216 with permission from Elsevier (Copyright 2020).

Fig. 19e presents the sensing mechanism, which indicates the development of PANI–In₂O₃ heterojunctions and the chemical sensitization effect caused by Au. In this case, the NH₃ detection mechanism of the PANI sensor was explained using a well-known protonation/deprotonation process.^216–219 Briefly, when the PANI sensor is exposed to NH₃ gas, NH₃ captures H-ions from the acidified PANi. In this way, the PANI is reduced to the intrinsically non-conducting emeraldine base state from the conducting emerald salt state, which then causes a change in the resistance of PANI. On the other hand, the mechanism for enhancing the response of the PANI/In₂O₃ sensor was described using a spill-over mechanism resulting from catalytic Au NPs (Fig. 19e). In this case, NH₃ molecules could spill at the Au surface and adsorb with the interaction among the p-orbital of NH₃ and d-orbital of Au atoms (NH₃ → NH₂ + H, NH₂ → NH + H). Subsequently, the formed H-atoms have greater reducibility, favoring an efficient PANI reaction and enhancing the sensor response.

Amu-Darko et al.²²⁰ developed a PANI-doped In₂O₃ nanosheet heterojunction using a hydrothermal method for NO₂ sensing applications. The porous sheets provided numerous surface adsorption sites and gas diffusion paths, enhancing the gas-sensing properties. The In₂O₃/PANI-1 composite exhibited significant sensitivity (R = 341.5 to 30 ppm), good selectivity, and a low detection limit (0.3 ppm) for NO₂ gas at 250 °C. Fig. 20 shows a schematic diagram of the NO₂ sensing mechanism of the In₂O₃/PANI nanosheets. In air, oxygen is adsorbed onto the surface of the composite material and generates ionized oxygen ions (O₂⁻_(ads)) by capturing electrons from the conduction band. This forms active sites on the material surface interacting efficiently with NO₂ gas. When the sensor is exposed to NO₂ gas, the NO₂ gas molecules compete with O₂ molecules for adsorption onto the surface. Owing to the high electron affinity of NO₂, it can draw electrons from the surface, causing an increase in the sensor resistance and the depletion layer thickness. The heterojunction interaction between the In₂O₃ and PANI materials is critical in this process. Heterojunctions develop at the contact between In₂O₃ and PANI, with electrons and holes diffusing in opposite directions because of the variation of the same charge density. The system has comparable Fermi levels when all the charge dispersions are balanced, resulting in the blending of the bands. This blending of the bands enables efficient interaction between the surface-active sites and the NO₂ gas molecules. Upon re-exposure to air, the captured electrons are released from the NO₂ gas molecules and revert to the conduction band of the material. This causes the resistance of the sensor to return to its initial value.

Fig. 20 Schematic diagram of the NO₂ gas sensing mechanism of In₂O₃/PANI nanosheets. Reproduced from ref. 220 with permission from Elsevier.

3.7 Comparison of the sensing performance

Finally, Table 2 summarizes the sensing performances of the above-mentioned papers. This demonstrates the high potential of In₂O₃-based gas sensors in terms of detecting a range of toxic gases and VOCs.

Table 2 Sensing performance of pure, composite, and metal functionalized In₂O₃ gas sensors

Morphology	Gas and conc. (ppm or ppb)	T (°C) or room tempt. (RT)	Response (Ra/Rg) or (Rg/Ra)* or [(Ra − Rg/Ra) × 100]**	Response time (s)/recovery time (s)	Ref.
Pristine In 2 O 3 gas sensors
In2O3 microcubes	NO2/100 ppm	100	1401	16 s/165 s	31
In2O3 NPs	HCHO/100 ppm	230	80	100 s/70 s	43
In2O3 nanosheets	NO2/97 ppm	RT	89.48*	16.6 s/NA	45
In2O3 NPs	H2S/1 ppm	25	26 268.5	NA	112
In2O3 mesoporous NPs	H2/500 ppm	260	18	1.7 s/1.5 s	114
In2O3 NPs	NO2/1 ppm	120	371.9	148 s/72 s	115
In2O3 nanospheres	TEA/50 ppm	320	400	1 s/71 s	117b
In2O3 nanospheres	C2H5OH/200	240	180	1 s/93 s	117b
In2O3 spheres	C2H5OH/50	250	250	16 s/14 s	117c
In2O3 quantum dots	H2S/5 ppm	RT	90	72 s/200 s	119
In2O3 NFs	CH3COCH3/100 ppm	180	72	1 s/NA	126
In2O3 NFs	Acetic acid/2000 ppm	250	66.7	25 s/37 s	128
In2O3 nanosheets	Cl2/3 ppm	200	2353.4*	53 s/17 s	132
In2O3 microcubes	C2H5OH/100	210	23	NA	137
In2O3 walnut-like	NO2/50	RT	219*	89 s/80 s	138
In2O3 urchin-like hollow spheres	HCHO/1 ppm	140	20.9	50 s/NA	139
In2O3 bricks	NO2/100	100	600*	12 s/148 s	141
In2O3 microcubes	NO2/10 ppm	60	348.3	NA	142
In2O3 hollow microspheres	HCHO/10 ppm	200	20	4 s/8 s	143a
Flower-like In2O3	Isoprene/0.5 ppm	190	3.1	53 s/299 s	143b
In2O3 hierarchical porous flower-like	CH3COCH3/50 ppm	250	12	2 s/46 s	144
Composite or metal-doped/functionalized In 2 O 3 gas sensors
La-doped In2O3 hollow microspheres	H2S/10 ppm	200	17.8	NA	46
Mg-doped In2O3 NTs	H2S/10 ppm	150	173.14	NA	125
Co3O4/In2O3 NRs	C2H5OH/200 ppm	200	18	25 s/145 s	152
PdO/In2O3 microspheres	CH3COCH3/100 ppm	250	50.9	1 s/26 s	153a
MXene/In2O3 NPs	NH3/20 ppm	RT	100.7%	60 s/300 s	153b
MXene/In2O3 nanocomposite	HCHO/100 ppm	100	106.82%	1 s/1 s	153c
In2O3@SnO2 C–S NFs	HCHO/100 ppm	120	180.1	3 s/3.6 s	154
ZnO/In2O3 C–S NWs	C2H5OH/1000 ppm	300	196%	NA	155
ZIF-8@In2O3 NFs	NO2/1 ppm	140	16.4	80 s/133 s	156
WO3/In2O3 nanosheets	HCHO/100 ppm	170	25	1 s/67 s	158
WO3/In2O3 NFs	CH3COCH3/200 ppm	275	29	NA	159
Au/In2O3 NFs	C2H5OH/NA	140	13.8	12 s/24 s	160
Au/mesoporous In2O3	CH3COCH3/100 ppm	250	19.01	25 s/31 s	162a
In2O3@AuIDE	Xylene/5 ppm	325	∼400	NA	162b
In2O3@PtIDE	C2H5OH/5 ppm	350	∼400	NA	162b
Au/3DOM In2O3	CH3COCH3/5 ppm	340	42.4	11 s/NA	163
Ag/In2O3 NRs	H2S/20 ppm	RT	93 719	84 s/186 s	164
Ag/In2O3 sunflowers	HCHO/20 ppm	240	11.3	0.9 s/14 s	168
Au/In2O3 NWs	CO/NA	RT	104	130 s/50 s	172
Ag/In2O3 nanograin	HCHO/1 ppm	RT	135	102 s/157 s	173
Pd/3DOM In2O3	NO2/500 ppb	RT	980	270 s/286 s	174
Au@In2O3 C–S NPs	HCHO/	200	17	7 s/135 s	183
Au@In2O3 C–S NPs	C2H5OH/100 ppm	160	36.14	4 s/2 s	184
Ag@In2O3 C–S NPs	C2H5OH/50 ppm	220	72.56	13 s/8 s	185
Au@In2O3 C–S NPs	H2/100 ppm	300	34.38	31 s/10 min	187
Pd@In2O3 yolk–shell NPs	C2H5OH/5 ppm	350	159.02	NA	188a
AuPd/In2O3	TMA/100 ppm	175	367	2 s/300 s	188b
Mesoporous Fe-doped In2O3	NO2/1 ppm	100	116	4.6 min/2.5 min	189
Mesoporous Zr-doped In2O3	NO2/1 ppm	75	169	NA	190
Ce-doped In2O3nanospheres	Glycol/100 ppm	240	63.4	NA	192
Ag/Ni-doped In2O3 NRs	HCHO/10 ppm	160	123.97	1.45 s/58.2 s	194a
Ni–In2O3 NPs	NO2/10 ppm	200	70	2 s/2 s	194b
S-doped In2O3 networks	C2H5OH/50 ppm	180	74.4	NA	194c
RGO/In2O3 nanocubes	CH3COCH3 and HCHO/25 ppm	175 and 225	85% and 88%**	NA	204
rGO/In2O3 cubes	NO2/5 ppm	RT	37.81%	NA	205
rGO/In2O3 NRs	NO2/97 ppm	RT	1.45	25 s/NA	207
In2O3–rGO NFs	NO2/5 ppm	50	42	261 s/698 s	208a
In2O3–GO	NO2/40 ppm	225	78	106 s/42 s	208b
In2O3@GO films	NO2/1 ppm	RT	31.6%	4.2 min/13.3 min	209
rGO/In2O3 nanospheres	NO2/500 ppb	120	35.7	137 s/540 s	210
In2O3/PANI NFs	NH3/1000 ppm	RT	53.20	NA	215
PANI/In2O3 C–S NPs (PAIn20A1)	NH3/100 ppm	RT	46	118 s/144 s	216
In2O3/PANI nanosheets	NO2/30 ppm	250	341.5	24 s/53 s	220

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Overall, In₂O₃-based nanomaterials as sensing materials exhibited improved detection performance towards various target gases using different strategies, such as surface and interface engineering, noble metal doping, and loading with carbonaceous materials. Almost all the strategies outlined above to boost the gas-sensing performance of In₂O₃-based nanomaterials are highly effective. In terms of the synthesis of nanomaterials (from 0D to 3D), the synthesis of 2D In₂O₃-based nanomaterials is more challenging. Thus, the researchers privileged other types of structures because of their relatively moderate synthesis conditions. In particular, chemical methods, such as hydrothermal and solvothermal, are used widely for synthesizing different In₂O₃ nanostructures to detect various toxic gases and VOCs. The most radially detected gases by In₂O₃-based nanomaterials are NO₂ and H₂S, followed by VOCs. Among the various sensors listed in Table 2, the sensors based on In₂O₃ NPs¹¹² and Ag–In₂O₃ NRs¹⁶⁴ displayed extraordinary responses of 26 268.5 (1 ppm) and 93 719 (20 ppm) towards H₂S at room temperature, which is considered outstanding in the area of chemiresistive gas-sensing.

4. Conclusions and future perspectives

Research on high-performance gas sensors with excellent novel nanomaterials is expanding exponentially. This paper reviewed the recent advancements in In₂O₃ nanostructures, encompassing their synthesis and gas-sensing applications. Different promising strategies for improving In₂O₃ gas-detecting properties have been reviewed and summarized. The sensing performance of pure In₂O₃ can be improved by structural and morphological engineering, e.g., using In₂O₃ NPs, nanospheres, NRs, NFs, nanosheets, microspheres, and nano/micro flowers. Many versatile synthetic techniques, e.g., hydrothermal and solvothermal techniques and electrospinning, have been used to prepare mesoporous In₂O₃ materials with different morphologies, as discussed in this review. The porous and hollow nanoarchitecture with large surfaces showed improved sensing performance mainly by facilitating the diffusion of gas molecules. Oxygen vacancies were determined to be another efficient way of boosting the gas-sensing performances. Among the various strategies for enhancing the sensing properties of In₂O₃-based gas sensors, the potential incorporation of noble metals and heterojunctions stimulated several pioneering studies. The electronic sensitization (Fermi energy control) and catalytic oxidation (spillover effect) by noble metals were the primary motives for upgrading the sensitivity and selectivity and reducing the operating temperature through the functionalization using noble metals. In₂O₃-based composite nanostructures are promising sensing materials, exhibiting better-sensing properties than their single counterparts. The synergistic effect between the n-type and p-type gas sensors, larger depletion layer, and potential barrier modulation were the main reasons for the superior sensing properties in p–n heterojunction gas sensors. Compared to p-type MOS, more n-type MOS on In₂O₃ were used effectively to construct p–n heterojunction gas sensors.

Despite the significant advances in designing In₂O₃-based gas sensors, many challenges and problems remain to be overcome before high sensing properties can be achieved. Moreover, the high operating temperature is the main obstacle to its practical application. As a result, attention should be given to developing ambient temperature sensors to reduce energy consumption. In this respect, In₂O₃ with carbon and polymer materials was identified as an excellent choice for operation at low-temperature sensing. Nevertheless, there was a lack of available technologies to manufacture and integrate flexible sensors using nanomaterials, considering that flexible and wearable sensors could be used as direct points of care for real-time recognition of analytes. The advantages of RT operation could open the door for the design of wearable/flexible devices based on In₂O₃ sensors, but more emphasis on research is needed.

Furthermore, the detection capabilities and response/recovery speeds of In₂O₃-based sensors need to be enhanced for dynamic sensor design. Importantly, the relative humidity is a major concern because it strongly affects the response of In₂O₃ sensors. Therefore, continuous efforts are needed to design robust sensors based on In₂O₃ to maintain high performance across multiple cycles under different environmental conditions. In addition, it is vital to explore the improved mechanism, particularly in terms of selectivity in heterojunction nanostructures. Most of the studies reviewed explained that synergistic effects enhanced the sensing performance, but the reason for selectivity toward a particular gas was rarely described. Pattern recognition analysis via different sensing layers for designing sensor arrays was another important strategy in enabling accuracy for specific gases. This means that an appropriate technology or realistic models need to be developed to understand better the gas detection mechanism of nanomaterials towards a target gas. Finally, research in chemiresistive gas sensors is growing to boost the gas-sensing performances of MOS-based materials, and the number of related publications is increasing constantly. In this regard, establishing a database, accepting standardized sensing protocols, and integrating laboratory-made sensing devices into practical applications are necessary. Major collective efforts are necessary to address the abovementioned challenges. Nevertheless, In₂O₃-based nanomaterials have great potential as gas-sensing materials to detect various toxic gases and VOCs.

Conflicts of interest

The authors declare no conflict of interest with this work.

Acknowledgements

This research was supported by the Korea Polar Research Institute (KOPRI) grant funded by the Ministry of Oceans and Fisheries (KOPRI project No. PE22900) and a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2021R1A2C1009790).

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Footnote

† These authors contributed equally to this work.

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