Ritu
Malik
a,
Nirav
Joshi
b and
Vijay K.
Tomer
*c
aDepartment of Physics, D.C.R. University of Science & Technology, Murthal-131039, Haryana, India
bSão Carlos Institute of Physics, University of São Paulo, CP 369, São Carlos 13560-970, São Paulo, Brazil
cDepartment of Materials Science & Nanotechnology, D.C.R. University of Science & Technology, Murthal-131039, Haryana, India. E-mail: vjtomer@gmail.com
First published on 1st June 2021
The rapid expansion and development of industrial sectors and corridors pose a significant threat to the world today owing to the deteriorating air quality resulting from the release of harmful and toxic gases into the atmosphere. To combat and tackle air pollution, reliable and precise sub-ppm detection of these gases is highly desirable for human safety and the environment. For a gas sensor to perform its level best, the choice of nanomaterials is a critical factor that can significantly impact the robustness, stability, cost-effectiveness, sensitivity, and selectivity of the sensing device. Molybdenum trioxide (MoO3), as an n-type semiconducting metal oxide, has been rated as a research hotpot material in recent years due to its utility in a wide range of important technological applications. Owing to the advancement of synthetic techniques, it has been made possible to explore numerous novel nanostructures and integrate them into smart gas sensing devices. In this quest, this review is an effort to highlight the various nanostructures of MoO3 and the influence of these morphologies on the gas sensing performance. A detailed morphological overview of pristine MoO3 nanomaterials ranging from one-dimensional (1-D) to three-dimensional (3-D) nanostructure formation, followed by the preparation of different heterostructures including MoO3/metal oxides (p-type and n-type), MoO3/noble metal decoration, and MoO3/2D materials in the thematic domain of gas sensing, has been presented. Finally, a future outlook on the further progress of MoO3 gas sensors based on the current scenario is also suggested.
Molybdenum trioxide (MoO3) is an intriguing wide band gap n-type semiconductor with three unique polymorphous crystalline forms—orthorhombic α-MoO3 (thermodynamically stable phase), monoclinic β-MoO3 (low temperature metastable phase), and hexagonal h-MoO3 phases.36–40 MoO3 has several merits such as a unique layered structure, tunable band gap (2.8–3.6 eV), high electron mobility, low cost-phase-controlled synthesis, non-toxicity, and excellent electrochemical property, which prompts increasing interest in this fascinating material. In particular, α-MoO3 has excited the research community with its layered anisotropic structure wherein the layers are parallel to the (010) crystal plane.41–43 Every layer is composed of two further sublayers, which are formed by corner-sharing MoO6 octahedra along the [001] and [100] directions. The two sublayers then bind together with van der Waals forces by sharing the edges of the octahedra along the [001] direction to form MoO6 octahedra layers.44,45 These layers then alternately stack along the [010] direction to form an α-MoO3 structure.46–48 This unique layered structure of α-MoO3 increases the content of pentavalent Mo5+ ions, which possesses strong affinity to oxygen.49,50 Since gas sensors function by the reaction between oxygen and the adsorbed analyte gas molecules, the presence of Mo5+ increases the adsorption effect, thus resulting in enhanced gas sensor response.49,51–54 In recent times, MoO3-based gas sensors have been intensively investigated for determining the trace concentration of toxic gases such as NO2,55 H2,56 ethanol,57 CO,58 NH3,59 and triethylamine.60 The main governing factors that affect the sensitivity of a gas sensor are its size, morphology, and structure.61,62 Thus, great efforts are being invested so as to improve the gas-sensing performance of MoO3 by incorporating tailored nanostructures with controlled shapes, sizes, and morphologies. Owing to this, nanostructured MoO3 with zero-dimensional (0-D), one dimensional (1-D), two dimensional (2-D), and three dimensional (3-D) morphologies have been synthesized (Scheme 1) employing various synthesis methods such as hydrothermal,63 solvothermal,64 sol–gel,65,66 co-precipitation,67 physical vapor deposition,68 thermal evaporation,69 RF magnetic sputtering,54 chemical vapor deposition,70 and spray-pyrolysis.71 Also, because of its intrinsic structural anisotropy, α-MoO3 has demonstrated rich nanostructured morphologies; therefore, in the past few years, efforts have been made to enhance the sensitivity and enable the sensor to perform at low concentration by designing nanostructures that not only possess high crystallinity but also high surface to volume (S/V) ratio such as nanobelts,72,73 nanorods,74,75 nanofibers,76,77 nanoplates,78,79 nanosheets,80,81 nanowires,82 and nanoflowers.83,84 In particular, a high S/V ratio enables the swift diffusion of analyte gas molecules into the sensor layer, thus resulting in quicker response, higher selectivity, better sensitivity, and lower power consumption; however, the selectivity and operation temperature remain major constraints for MoO3-based gas sensors. To overcome these challenges, great progress has been achieved in the recent past, which is reflected in the increased number of scientific articles primarily focusing on the formation of heterostructures, surface functionalization with a noble metal, and use of light illumination.85–89
Considering these aspects, a review article is the need of hour, which can summarize the latest happenings in gas sensing technology while putting forth a detailed morphological investigation of nanostructured MoO3 in the current scenario of the sensing domain. This review has been drafted in line with the holistic coverage of MoO3-based gas sensors since it is one of the metal oxide materials that have experienced excessively increased interest in the recent years (Scheme 2). Although there are many reports on the detection of harmful and toxic gases using resistive gas sensors,25,29,30,90–95 none such review on MoO3 could be located in the research database, which can provide a holistic overview of the sensing abilities and performances of this rising star of the SMOx family. The review is divided into three main sections, which provide an in-depth overview of the detection of hazardous gases using different MoO3 nanostructures (1-D, 2-D, and 3-D). Different morphologies of MoO3 in relation to their gas sensing attributes are addressed, while key challenges and future research perspectives have been discussed, which could serve as a roadmap for exploring this fascinating material not only for gas sensing but also for other technologically important applications.
Scheme 2 Yearly publications demonstrating the increasing interest in MoO3-based gas sensors in the last three decades (Source: ISI Web of Science database and search criteria ‘MoO3 + gas sensor’). |
Class | Material | Synthesis method | Gas | Conc. (ppm) | Operating temp. (°C) | Response | Resp./Reco. time (s/s) | Ref. |
---|---|---|---|---|---|---|---|---|
a Not given. | ||||||||
Nanobelt | MoO3/Fe2(MoO4)3 | Hydrothermal | Toluene | 50 | 250 | 5.3 | <30/<30 | 191 |
MoO3/ZnO | Hydrothermal | Ethanol | 100 | 250 | 19 | 2.5/3.5 | 192 | |
MoO3 | Hydrothermal | Ethanol | 800 | 300 | 174 | 42a/4a | 193 | |
Zr/MoO3 | Hydrothermal | Xylene | 100 | 206 | 7.99 | 32/264 | 194 | |
MoO3/Fe2O3 | Hydrothermal | Xylene | 100 | 206 | 6.9 | 87/190 | 195 | |
Ce/MoO3 | Hydrothermal | TMA | 50 | 240 | 17.4 | <10/<20 | 196 | |
MoO3 | Chemical spray pyrolysis | NO2 | 100 | 200 | 68(%) | 15/150 | 55 | |
Fe/MoO3 | Hydrothermal | Xylene | 100 | 206 | 6.1 | 20/75 | 117 | |
Au/MoO3 | Hydrothermal | 1-butyl amine | 100 | 240 | 300 | 23/388 | 197 | |
In2O3/MoO3 | Hydrothermal + chemical synthesis | TMA | 10 | 260 | 31.69 | 6/9 | 198 | |
Zn–MoO3 | Hydrothermal | Ethanol | 1000 | 240 | 321 | 15/121 (100 ppm) | 103 | |
RuO2/MoO3 | Chemical synthesis | TEA | 1 | 260 | 12.8 | 2/10 | 60 | |
Fe2O3/MoO3 | 2-Step hydrothermal | Xylene | 100 | 233.5 | 22.48 | 4/102 | 104 | |
CoMoO4/MoO3 | Hydrothermal + dipping-annealing process | TEA | 100 | 220 | 104.8 | <10/<10 | 105 | |
Au/MoO3 | Hydrothermal + chemical synthesis | TMA | 50 | 280 | 70 | 6/9 | 72 | |
Pt/MoO3 | Hydrothermal + chemical reduction | Formaldehyde | 200 | RT | 39.3 | 17.8/10.5 (100 ppm) | 73 | |
W/MoO3 | Hydrothermal | TMA | 50 | 200 | 13.8 | 6/11 | 101 | |
Pd/MoO3 | Spray pyrolysis + chemical dip | NO2 | 100 | 200 | 95.2(%) | 74/297 | 102 | |
Cd/MoO3 | Hydrothermal | H2S | 100 | 140 | 378.5 | 23/45 (50 ppm) | 96 | |
MoO3 | Hydrothermal | TMA | 50 | 240 | 582 | 15/50 (1 ppm) | 99 | |
Nanoribbon | MoO3 | Hydrothermal | H2 | 1000 | 200 | 14.1 | 21/75 | 100 |
MoO3/Graphene | Hydrothermal | H2 | 1000 | RT | 20.5 | 10/30 | 106 | |
MoO3 | Hydrothermal | H2 | 1000 | RT | 17.3 | 10.9/30.4 | 56 | |
MoO3 | Hydrothermal | H2 | 100 | RT | 3.2a | 3/16 | 107 | |
MoO3 | Hydrothermal | NH3 | 25 | 450 | 60 | 21/216.9 (5 ppm) | 59 | |
Nanorod | NiCo2O4/MoO3 | Hydrothermal + chemical deposition | Ethanol | 1 | 350 | 20 | N.G. | 74 |
Ag–MoO3 | Hydrothermal + chemical reduction | TEA | 100 | 200 | 408.6 | 3/107 | 75 | |
MoO3 | Hydrothermal | TEA | 100 | 300 | 101.74 | 4/88 | 97 | |
MoO3/BiVO4 | Hydrothermal + metal organic deposition | TEA | 20 | 125 | 1.86 | 15/110 | 108 | |
MoO3/GO | Solvothermal + annealing | NH3 | 100 | 200 | 15.3 | 5/84 | 109 | |
MoO3 | Hydrothermal | NO2 | 20 | 110 | 84 | 20/45 | 110 | |
h-MoO3 | Chemical bath technique | NH3 | 50 | 200 | 67 | 183/202 | 111 | |
p-Si/MoO3 | Hydrothermal + physical vapor deposition | CO2 | 100 | 250 | 12.08 | 8/15 | 112 | |
rGO/MoO3 | Hydrothermal + in situ microwave | H2S | 40 | 110 | 44.7 | 109/36 | 113 | |
Nanofiber | SnO2/MoO3 | Hydrothermal + wet chemical | CO | 300 | 300 | 2.4 | 1430/1524 | 76 |
MoO3 | Hydrothermal | Ethanol | 100 | 275 | 25 | 45/138 | 114 | |
Nanowire | MoO3 | Hydrothermal | H2 | 1.5(%) | 260 | 0.85 | 28/42 (500 ppm) | 82 |
Microrod | MoO3 | Hydrothermal | Ethanol | 500 | 332 | 8.24 | N.G. | 98 |
MoO3 | Probe sonication | TMA | 1000 | 200 | 2533 | 8/9 (1 ppm) | 115 | |
h-MoO3 | Microwave assisted hydrothermal | Acetone | 10 | 200 | 1.48 | 60/500 | 116 |
Fig. 1 (a–f) FESEM images of pure MoO3 NBs and ZM-1–ZM-5; (g) the average thicknesses of pure MoO3 NBs and Zn-doped MoO3 NBs; (h) gas sensing responses of the sensors based on pure MoO3 NBs and Zn-doped MoO3 NBs to 1000 ppm ethanol at different OTs; (i) response and recovery curves of the sensors based on pure MoO3 NBs and ZM-3 to different concentrations of ethanol at the OT of 240 °C; (j) cross-sensitivity to various gases at different temperatures; reproduced with permission from ref. 103, copyright 2017 Elsevier. (k and l) FESEM images of pure MoO3 NBs; (m and n) FESEM images of RuO2/MoO3 NBs; (o) schematic diagram of the possible gas sensing mechanisms of RuO2/MoO3 NBs; (p) response to 10 ppm TEA gas versus OT; (q) response transient to 10 ppm TEA at 300 °C; (r) responses of pristine and RuO2/MoO3 NBs gas sensors to different gases (10 ppm) at 300 °C. Reproduced with permission from ref. 60, copyright 2019 Elsevier. |
Xylene is a toxic and colorless VOC, whose over exposure results in cardiovascular and kidney problems. To detect xylene, the formation of n–n heterostructures is a novel approach, following which the group of Qu et al.104 detected xylene gas by preparing an n–n type heterostructure comprising of Fe2O3 NPs and MoO3 NBs by a two-step HT method. The morphological analysis results in Fig. 2a and b shows that the MoO3 NBs are ∼200–300 nm in width while ∼2–3 mm in length; also, the nanobelt structure of MoO3 was retained even after uniformly doping Fe2O3 NPs (Fig. 2c and d). The sensor response to xylene gas at different OTs in Fig. 2e displayed that although pure MoO3 detected xylene gas at a lower temperature than the Fe2O3/MoO3 NBs, yet the former exhibited a lower maximum response than the nanocomposite. The selectivity results in Fig. 2f reveal that the response to xylene gas was the highest for Fe2O3/MoO3 NBs, and ∼250% improvement in the sensitivity was observed than that using pure MoO3 NBs. The research group attributed the superior sensing performance to the unique n-type heterojunction between MoO3 NBs and Fe2O3 nanospheres. Another example was reported by Wang and co-workers105 on very known p–n heterostructures because of their effective charge separation, long-life of the charge carrier to make them more favorable to achieve high sensor response, and selective sensing toward target analytes. They utilized a simple dipping-annealing process and developed p–n heterojunctions of CoMoO4 and MoO3. The MoO3 NBs were smooth, having 100 nm thickness, 100–300 nm width, and a few micrometers length (Fig. 2g), while the rough surface of CoMoO4/MoO3 nanocomposites in Fig. 2h illustrated the successful growth of the CoMoO4 NPs on the MoO3 NBs. The TEM results in Fig. 2i and j further confirm the uniform dispersion of CoMoO4 nanoparticles (20–50 nm diameter) on the surface of the MoO3 NBs. The sensing results toward triethylamine (TMA) gas in Fig. 2k reveal that the CoMoO4/MoO3 nanocomposites-based sensors show better sensing performance while causing a reduction of 60 °C in the optimum temperature as compared to the pristine MoO3 NBs. The sensing response as a function of TMA concentration in Fig. 2l reveals the stronger response (4-fold) of the nanocomposite to 200 ppm TMA than pure MoO3, while the dynamic responses in Fig. 2m seem to be perfectly repeatable and reproducible during 3 cycles of switch ‘on’ and ‘off’ measurement. It was concluded that the formation of a potential barrier between CoMoO4 (p-type) and MoO3 (n-type), the stronger oxygen adsorption of CoMoO4, and the formation of crystallographic defects all together resulted in superior sensing performance.
Fig. 2 (a and b) SEM and TEM image of pure MoO3 NBs; (c and d) SEM and TEM image of Fe2O3/MoO3 NBs; (e) OT dependent response of the sensors to 100 ppm xylene; (f) response of the sensors to 100 ppm various gases at their optimum OT. Reproduced with permission from ref. 104, copyright 2019 Elsevier. (g) FESEM image of pure MoO3 NBs; (h) FESEM image of the CoMoO4/MoO3 nanocomposites; (i and j) low and high-magnification TEM images of the CoMoO4/MoO3 nanocomposites; (k) response of the sensors to 10 ppm of TMA at different OTs; (l) response and recovery curves toward different concentrations of TMA; (m) response and recovery curves of the sensor based on the CoMoO4/MoO3 nanocomposites to 5 ppm TMA after 3 cycles of gas on and off at 220 °C. Reproduced with permission from ref. 105, copyright 2018 Elsevier. |
Noble metal NPs such as Ag, Au, Pt, and Pd, with their outstanding catalytic effect, have been known to improve the sensing attributes of the MoO3 NBs. For instance, Zhang et al.72 prepared catalytic Au NPs doped MoO3 NBs via the hydrothermal method and displayed superior sensing performance to TMA gas. The MoO3 NBs were 100–300 nm wide and 10–20 μm in size (Fig. 3(a and b)), while the FESEM image in Fig. 3c revealed that several Au NPs were stuck on the surface of the NBs. In addition, the HRTEM image in Fig. 3d displayed the non-continuous distribution of Au NPs (diameter ∼10–20 nm) on the MoO3 NBs surface, which confirms the high crystallinity of the MoO3 NBs and Au NPs. The sensing response of the prepared materials toward TMA gas demonstrated an increase-maximum-decay (IMD) type of pattern with the increase in the OT (Fig. 3e). The OT has a considerable impact on the sensing performance of the material owing to the thermal energy of the analyte gas molecule for clearing the energy barrier of the surface reaction and later converting the adsorbed oxygen for further attracting the electrons from the semiconductor. The dynamic response-recovery curves in Fig. 3f displayed a fast response (Tres ∼ 7 s) and recovery (Trec ∼ 10 s) time for Au@MoO3 NBs toward 10 ppm TMA gas. Besides, the as-prepared materials also demonstrated the highest response to the TMA gas (Fig. 3g). Overall, the improved sensing performance was accredited to the catalytic Au NPs, which, with the help of reactive oxygen species, improves the electron exchange process between Au NPs and MoO3.
Fig. 3 (a and b) FESEM images of pure MoO3 NBs, (c) FESEM image Au@MoO3 nanocomposites; (d) TEM images of Au@MoO3 nanocomposites; (e) response curves of the sensors to 10 ppm trimethylamine gas at different OTs; (f) response and recovery time curves of the sensors based on Au@MoO3 NBs to 10 ppm TMA at different OTs; (g) response values of the Au@MoO3 nanocomposites sensor toward 10 ppm different gases at the working temperature of 280 °C. Reproduced with permission from ref. 72, copyright 2016 Elsevier. (h) The SEM images of the pristine nanowires, the inset picture is a low-magnified SEM image; (i–m) SEM images of Pt/MoO3 NBs; (n and o) TEM images of Pt-decorated MoO3 NBs; (p) the dynamic sensor response of pristine and Pt/MoO3 NBs toward HCHO gas of 200 ppm at 27 °C; (q) sensor response to different gases with concentrations of 200 ppm; (r) the schematic diagram for the HCHO sensing behavior of Pt/MoO3 NBs in air and in HCHO-containing atmosphere. Reproduced with permission from ref. 73, copyright 2019 Elsevier. |
In the last couple of decades, the detection of VOCs in indoor environments has received much attention. VOCs are produced as a result of gaseous emission from commonly uses household products such as nail paints, wall paints, furniture, and cleansers, and cause both short- and long-term effects on human health. Formaldehyde (HCHO) is one of the VOCs found in many daily usage products, such as carpets, wood, and other plastic products widely used in every household. There have been several ways to enhance the selectivity of a sensor for a specific VOC and have remained a great topic of interest among researchers. For example, Gu and coworkers73 prepared MoO3 NBs using a facile HT method and surface-decorated them with Pt NPs to illustrate superior sensing response toward HCHO gas. The SEM image in Fig. 3h reveals the width of pristine MoO3 NBs to be ∼200–400 nm with very minute thickness (inset). The other SEM images (Fig. 3(i–m)) further displayed the increasing presence of Pt NPs with its loading amount on the MoO3 NBs. The TEM images in Fig. 3n and o for M-Pt3 (Pt-loading amount = 0.61%) not only confirm the presence of the highly crystalline MoO3 NBs but also reveal the occurrence of small Pt NPs on the surface of the MoO3 NBs. The response results in Fig. 3p expose the poor performance of pure MoO3 NPs, while with the appropriate Pt%-decorated MoO3 (M-Pt3) NBs cause an improved sensing response to formaldehyde gas. An outstanding selectivity to formaldehyde gas among other interferent gases is illustrated in Fig. 3q. The M-Pt3 sensor shows no response to ethanol, while a negligible response of 0.1% acetone was observed. The sensing mechanism in Fig. 3r reveals that the astounding sensing performance was due to the presence of highly catalytic Pt NPs, which reduces the adsorption activation energy of HCHO on the surface of MoO3 and also assists in forming the spillover region around the Pt NPs on the surface of the MoO3 NBs. Overall, according to the research reported by Xu et al.,117 the oxygen species tends to preferably adsorb on Mo5+, causing an increase in the intensity of chemisorbed oxygen on the surface of the NBs, thus enhancing the gas sensing response. In addition, the higher specific surface area of the NBs provides more absorption sites and contributes positively to gas sensor response.
Fig. 4 (a) SEM images of the as-prepared samples synthesized at different OT; (b) schematic diagram of the H2 sensing mechanism of MoO3 NRbs; (c) dynamic response of different MoO3 sensors toward 500 ppm of H2; (inset) schematic illustration of the fabrication process of the sensor; (d) sensitivity, response time, and recovery time of different MoO3 sensors; (e) relation among the sensitivity (S), Mo content, and Ochem content of the MoO3 NRbs. Reproduced with permission from ref. 100, copyright 2015 American Chemical Society. (f-1) TEM image and schematic diagram of graphene oxide used as the hydrothermal precursor; (f-2) SEM image of the MoO3 NRbs/graphene nanocomposite; (f-3) HRTEM images of individual MoO3 NRbs; (g) the RT response curves of the MoO3 NRbs/graphene to 500 ppm H2 in air; (h) selectivity to 1000 ppm H2 of M/G-1.5 against other gases with the same concentration; (i) schematic diagram of pure MoO3 NRbs and MoO3 NRbs/graphene under air and H2-containing atmospheres. Reproduced with permission from ref. 106, copyright 2017 Elsevier. (j) SEM images of the original MoO3; (k) SEM image of MoO3 treated at 400 °C; (l) the dynamic response curves toward 750 ppm H2 gas at 25 °C; (m) the selectivity of S-H3 to different gases with concentrations of 1000 ppm; (n) the schematic diagram of the H2 sensing mechanism of the sensor based on MoO3 NRbs both in air and in H2 atmosphere. Reproduced with permission from ref. 56, copyright 2019 Elsevier. |
Recently, 2-D materials have mostly been in focus for the development of gas sensors due to their outstanding electronic properties.118–121 A suitable combination of these 2-D material with oxide nanostructures offers spontaneous electron transfer and ensures that the diffusion of gas molecules results in an improvement in the sensor response along with the optimization of the response/recovery times of the gas sensor.122 Yang et al.106 demonstrated a one-step HT method for uniformly loading orthorhombic MoO3 NRbs on the exfoliated graphene oxide (GO) supporting layers (Fig. 4f–1). The SEM image in Fig. 4f–2 revealed that the as-synthesized products were composed of large amounts of NRbs (length ∼10 μm) loaded on the graphene nanosheets, while MoO3 NRbs were also clearly located on either side of the graphene nanosheet, as shown in Fig. 4f–3. The hydrogen sensing responses in Fig. 4g displayed the negligible response of pure graphene nanosheets despite the fact that a little addition of graphene in MoO3 NRbs enhances the response and reduces the Tres/Trec time considerably. The selectivity results for the GO/MoO3 NRbs in Fig. 4h further attest to the fact that the sensor responded perfectly to H2 gas. Such a high response was credited to the formation of innumerable MoO3/graphene heterojunctions (Fig. 4i) and also the high surface area of the nanocomposite as a result of adding the graphene networks, which not only loosened the structure but also enhanced the conductivity of the sensor.
Yang et al.123 also prepared Fe-doped orthorhombic MoO3 (α-MoO3) nanoribbons and demonstrated superior H2 sensing performance. The first-principles density functional theory (DFT) calculations were used to calculate the adsorption of O2 and H2 molecules on the surface of Fe/MoO3 (Scheme 3). It was observed that oxygen was adsorbed parallel to the surface of Fe/MoO3 in three modes, i.e., along the x-axis (mode O-1) with adsorption energy of −0.539 eV, y-axes (mode O-2) with adsorption energy of −0.461, or perpendicular to the plane of the Fe-doped MoO3 (mode O-3) with adsorption energy of −0.673 eV. The results in Scheme 3a reveal that oxygen preferred to adsorb on Fe-doped MoO3 in the O-3 mode. In addition, the H2 molecules adsorbed parallelly to the oxygen ions on the surface of the Fe/MoO3 while interacting with the pre-adsorbed oxygen ions. The difference in the electronic density in Scheme 3b indicated that the charges to the oxygen atoms numbered 1 was −0.09e, and those numbered 2 was −0.06e in the adsorbed oxygen molecule. This causes the transfer of −0.15e from the Fe–MoO3 to the adsorbed oxygen molecule. These theoretical results further confirmed the capturing of the electrons from the adsorbed oxygen on the surface of Fe/MoO3.
Scheme 3 The optimized structure (a) and the electronic density difference (b) of the adsorption of two H2 on Fe-doped MoO3 with one pre-adsorbed oxygen ion. The red, blue, and white balls represent the O, Mo, and H atoms. Reproduced with permission from ref. 123, copyright 2021 Elsevier. |
In most cases, metal oxide-based gas sensors operate at high temperatures (100–200 °C), which hinders the monitoring of the gas composition in explosive species environment since high temperatures could trigger an explosion. In this way, RT sensors are more favorable due to low power consumption, simplified manufacturing processes, and reduced operating costs.124–128 Yang et al.56 utilized a simple HT method to prepare MoO3 NRbs and demonstrated superior hydrogen sensing performance. A ribbon-like morphology was observed in the SEM image (Fig. 4j) for pure MoO3 with an average thickness, width, and length of ∼90 nm, 270 nm, and 20 μm, respectively; however, the sample calcined in hydrogen gas atmosphere at 300 °C (Fig. 4k) demonstrated a depreciated size in all the dimensions as compared to the pristine MoO3 NRbs. The room temperature response transients to H2 gas in Fig. 4l revealed a typically n-type sensing performance and a fast response/recovery speed. Besides, the histogram results in Fig. 4m further indicated the excellent selectivity of the MoO3 NRbs sensors toward H2 gas. It was also revealed that the sample annealed in a hydrogen atmosphere was ∼2.5 more responsive to H2 gas compared to pristine MoO3. The reason was the higher concentrations of Mo5+ and chemisorbed oxygen ions in MoO3 treated at 300 °C (Fig. 4n) under hydrogen atmosphere, which triggered the redox reactions due to increased collision between H2 and O2−.
Fig. 5 (a) Schematic illustration of the synthesis of the NiCo2O4/α-MoO3 nanocomposite; (b-1 and b-2) SEM image of pristine α-MoO3; (b-3) magnified SEM image of the NiCo2O4/α-MoO3 composites; (c) dynamic response transients to different concentrations of ethanol; (d) long-term stability of the NiCo2O4/α-MoO3 nanocomposites toward 1 ppm ethanol at 350 °C; (e) selectivity toward 1 ppm interfering gases at 350 °C. Reproduced with permission from ref. 74, copyright 2019 Elsevier. FESEM images of the pure α-MoO3 NRs (f-1) and AgNPs-decorated α-MoO3 nanorods (f-2); (g) TEM images of 2%Ag–MoO3; (h) sensing response toward 100 ppm of TEA at varied OT; (i) sensing transients of the 2%Ag–MoO3 sensor toward 100 ppm of TEA; (j) cross-response of 2%Ag–MoO3 at 200 °C. Reproduced with permission from ref. 75, copyright 2019 Elsevier. |
As stated earlier, the surface doping of SMOx with noble metals is also considered a brilliant approach owing to their higher catalytic activity.129–134 In particular, Ag NPs, being comparatively cheaper and having higher catalytic performance, have been extensively explored in promoting the sensing performance of oxide-based sensors. Considering this, Tian et al.75 successfully demonstrated the decoration of Ag NPs on the surfaces of α-MoO3 NRs. The morphological results in Fig. 5f-1 reveal the presence of α-MoO3 NRs with smoother surfaces having lengths and diameters of about 10 μm and 200–300 nm, respectively. Ag NPs of ∼20 nm size were clearly observed in the Ag–MoO3 sample (Fig. 5f-2), which was further confirmed in the TEM results in Fig. 5g-1 and g-2. The sensing response results in Fig. 5h unveiled the utility of Ag decoration on pure α-MoO3 for enhancing the response toward TEA gas. The effect of temperature on the dynamic transient response curves (Fig. 5i) pointed out the incomplete recovery of response to its baseline due to the slower desorption of the gases. The cross-response results in Fig. 5j confirm the excellent selectivity of the sensor toward TEA gas among a variety of other tested gases due to the interaction between the basic nature of TEA gas and the acidic MoO3 surface. Besides, the electronic and chemical sensitization of the Ag NPs was also considered as a major factor for realizing the high response of Ag/α-MoO3 NRs.
TEA is, as we know, a toxic, volatile, and explosive gas used in the fish processing industry.4,21 It is, therefore, of utmost importance to design superiorly responsive gas sensors for the real-time detection of TEA at low OT. For example, Yang and coworkers97 utilized a facile hydrothermal method for preparing α-MoO3 NRs (Fig. 6a) for detecting TEA gas at low operating temperatures. α-MoO3 NRs with clean and smooth surfaces and having length of 10 mm and diameter in the range of 200–300 nm are observed in Fig. 6b1–b3. The sensing results for TEA gas in Fig. 6c reveals that MoO3 with NRs-type morphology possesses a higher response than the particle-based MoO3 at the same OT. The histograms revealing the response-recovery time in Fig. 6d concluded that a high concentration of TEA causes the Tres to be less than 10 s with longer recovery times and vice versa. The cross-response results of the sensor for determining its discrimination ability in Fig. 6e revealed that the sensor distinguishably detects TEA gas among other tested gases under identical testing conditions. It was concluded that the high TEA response was not only a result of the attractive forces between the acidic and basic nature of MoO3 surfaces and TEA molecules, respectively, but was also due to the highly active lattice oxygen and fast adsorption/desorption kinetics from the sensor surface.
Fig. 6 (a) Schematic illustration for the preparation of two MoO3 products and sensing measurement of the actual sensors; (b) FESEM images of the MoO3 NRs; (c) sensor responses to 100 ppm TEA vapor as a function of OT from 100 to 350 °C; (d) response and recovery times of the sensor at various concentrations of TEA gas; (e) sensor responses of the MoO3 NRs to various gases with an identical concentration (100 ppm) at 300 °C. Reproduced with permission from ref. 97, copyright 2019 Elsevier. (f) Schematic diagram of the α-MoO3/BiVO4 composite synthetic process; (g) SEM and TEM image of the 16Mo/Bi composite; (h) responses at different OT to 20 ppm TEA; (i) response-recovery time to 20 ppm TEA; (j) response to 20 ppm of different gases at 125 °C. Reproduced from ref. 108, copyright 2020 with permission from the Centre National de la Recherche Scientifique (CNRS) and The Royal Society of Chemistry. |
As discussed earlier, heterostructure formation plays a key role in the interface to enhance the sensing performance.21,135,136 Therefore, designing 1D-MoO3 heterostructures with appropriate counterparts is of great importance in order to achieve excellent TEA sensing performance. Thus, it was further revealed by Bai et al.108 that α-MoO3 can dissociate the C–N bond present in TEA at the desired temperature. They synthesized the n–n heterojunction of α-MoO3/BiVO4via the metal–organic decomposition method (Fig. 6f) and showed improved sensitivity toward TEA gas. The SEM and TEM results in Fig. 6g-1 and g-2 clearly show the development of the BiVO4/MoO3 composite as nanorods and also the growth of BiVO4 nanoparticles on MoO3 nanorods. The response curves in Fig. 6h showed an IMD trend for all the materials; however, the response of the BiVO4/MoO3 composite was much better than that of others. However, the longer Trec for the BiVO4/MoO3 composite was due to the strong binding of the TEA molecules on the surface of α-MoO3, which ultimately resulted in a poor desorption rate (Fig. 6i). In the end, the excellent selectivity results in Fig. 6j pointed to the fact that TEA, due to its lower C–N bond energy, gets oxidized very easily. It was finally concluded that the n–n heterostructured MoO3/BiVO4 composite was primarily responsible for the increased sensor response.
Fig. 7 (a) Schematic illustration of the growth mechanism of SnO2-doped MoO3 NFbs; (b) response–recovery curves of SnO2-doped MoO3 NFbs at 300 °C; (c) responses as a function of the CO concentration at 300 °C; (d) selectivity of pristine α-MoO3 NBs (red) and SnO2/MoO3 NFbs (blue) toward various gases of 300 ppm at 300 °C, (inset) high-magnification SEM micrographs of SnO2/MoO3 NFbs. Reproduced with permission from ref. 76, copyright 2017 Elsevier. (e) SEM image of M4 grown at a constant temperature; (f) response of M1–M8 devices in exposure to 100 ppm ethanol at different OTs; (g) selectivity study toward 200 ppm of different VOCs. Reproduced with permission from ref. 114, copyright 2019 Elsevier. |
In another work, Mondal et al.114 tailored highly crystalline and ultra-long MoO3 NFbs by applying the temperature pulsing method during HT growth. The SEM image in Fig. 7e demonstrates the presence of highly crystalline ultra-long nanofibers having several tens of micrometer of length and width in the range of 200–300 nm. The sensing results in Fig. 7f confirmed that MoO3 with NFbs morphology (M5–M8) possesses better response at lower operating temperature than MoO3 with the NBs structure (M1–M4) toward ethanol gas. Owing to the high surface area and more surface defects in MoO3 NFbs, a high selectivity (Fig. 7g) to ethanol gas was observed among the other tested VOCs. At last, it was concluded that the large surface area and presence of surface defects were the prime reasons for the improved sensing performance of the MoO3 NFs.
Fig. 8 (a) SEM and TEM images of the as-synthesized α-MoO3 NWs; (b) the sensitivities of different sensors to 1.5% H2; (c) response–recovery of the sensor to 1.5% H2. (d) Repeatability toward 1.5% H2; (e) dynamic response resistances of the flexible NWs paper for different H2 concentrations (100 ppm – 1.5%). Reproduced with permission from ref. 82, copyright 2017 Elsevier. (f) Typical SEM image of h-MoO3 MRds; (g) the selectivity of the h-MoO3 MRds gas sensor to 500 ppm gases at the OT of 332 °C and 380 °C; (h) sensitivity of the sensors based on h-MoO3 MRds to the exposure of different concentrations of ethanol at different OTs. Reproduced with permission from ref. 98, copyright 2015 Elsevier. |
Class | Material | Synthesis route/morphology | Gas | Conc. (ppm) | Operating temp. (°C) | Response | Resp./Reco. time (s/s) | Ref. |
---|---|---|---|---|---|---|---|---|
a Not given. | ||||||||
Nanosheet | MoO3 | Grinding + sonication | Alcohol | 100 | 300 | 33 | 21/10 | 80 |
MoO3 | Hydrothermal + calcination | Xylene | 10 | 400 | 9.1 | 7.1/6.8 | 81 | |
MoO3 | Solvothermal + annealing | TMA | 50 | 133 | 51.47 | 12/200 | 64 | |
Au/MoO3 | Hydrothermal + calcination + chemical reduction | Ethanol | 200 | 280 | 169 | 22/5 | 145 | |
Nanoflake | MoO3/rGO | Hydrothermal + calcination | Ethanol | 100 | 310 | 53 | 6/54 | 146 |
MoO3/SnO2 | Chemical synthesis | H2S | 10 | 115 | 43.5 | 22/10 | 147 | |
Au/MoO3 | Thermal evaporation + sputtering | H2S | 15 | 400 | 260 | 60/480 | 148 | |
Nanoplate | MoO3 | Chemical synthesis | Ethanol | 800 | 300 | 58 | 10a/40a | 149 |
MoO3 | Polymeric solution method | NO2 | 100 | 250 | 47.9 | N.G. | 150 | |
Nano lamella | Ni/MoO3 | Solvothermal | Formaldehyde | 100 | 255 | 41 | 4/12 | 151 |
Thin film | MoO3/V2O5 | Chemical spray pyrolysis | NO2 | 100 | 200 | 80(%) | 118/1182 | 154 |
MoO3 | Hydrothermal | Ethanol | 100 | 260 | 23(%) | 111/66 | 155 | |
Microsheet | MoO3 | Hydrothermal + thermal oxidation | TEA | 100 | 275 | 27.1 | 3/50 | 157 |
Micro plank | MoO3 | Chemical synthesis | CO | 100 | 150 | 74.87 | 80/110 | 158 |
Fig. 9 (a-1) Three-step liquid exfoliation process; (a-2) schematic illustration of the fabricated sensor and the image of the operation principle; (b) TEM image showing the layered nature of the MoO3 NSs; (c) the results of sensor response using bulk MoO3 and MoO3 NSs toward 100 ppm alcohol vapor at different OTs; (d) transient sensor response toward 100 ppm alcohol vapor at different temperatures, (inset) response and recovery curves at its optimum OT; (e) responses of sensors made of MoO3 NSs toward 100 ppm VOCs. Reproduced with permission from ref. 80, copyright 2016 Royal Society of Chemistry. (f) SEM images of the MoO3 NSs-400 samples; (g) sensor responses at varied OTs toward 10 ppm xylene; (h) response/recovery curve of the MoO3 NSs sensor toward 10 ppm xylene at 300 °C; (i) responses of the MoO3 NSs sensor to 10 ppm of different gases at 300 °C. Reproduced with permission from ref. 81, copyright 2020 Elsevier. |
Another poisonous and colorless VOC, i.e., TMA, has drawn significant attention because initially, the smell seems pungent but it can rapidly paralyze the olfactory system and cause unawareness and headache. Shen and coworkers64 detected TMA by developing porous α-MoO3 ultrathin NSs using a one-step solvothermal (ST) route, followed by calcination. The main process of the synthetic procedure and the NSs (size between 500 and 800 nm) obtained at different calcination temperatures are presented in Fig. 10a. The OT-based response of α-MoO3 NSs to 50 ppm TMA in Fig. 10b depicts an IMD trend for all the materials, yet the NSs obtained at 400 °C calcination temperature shows the highest response among others. The cross-sensitivity results (Fig. 10c) of α-MoO3-400 to different gases revealed that the response to TMA gas was way higher than to the other test gases. In addition, the sensor displayed minute fluctuation to TMA gas (Fig. 10d) over the course of 3 months, indicating its great stability and reproducibility. This high response was accredited to the porous and ultrathin configuration of α-MoO3-400, which provides numerous active sites for the adsorption of TMA molecules faster on the sensor surface. The adsorption energies for TMA and O2 on α-MoO3 NSs were estimated to be −2.16 and −0.5 eV, respectively, by DFT calculations. The results in Scheme 4a1 and b1 display that the energetically favorable adsorption positions of TMA on α-MoO3 and the α-MoO3 containing O-vacancy (Ov-α-MoO3) are Mo atom and O-vacancy, respectively. Due to the steric hindrance, TMA is primarily physically adsorbed on two oxygen atoms and forms a bridge-like structure (O-TMA-O) in TMA-Ov-α-MoO3. The adsorption energies of TMA on α-MoO3 and Ov-α-MoO3 are −2.16 and −0.25 eV, respectively. The density of states (DOS) results in Scheme 4b1 and b2 exhibit that on introducing the oxygen vacancy, the Fermi level enters the conduction band, thus causing a lowering of the band gap of TMA-Ov-α-MoO3 than that of TMA-α-MoO3. Since the smaller band gap is more favorable for electron transfer, the TMA states appear in the conduction band near the Fermi level, indicating that the interaction between TMA and Ov-α-MoO3 is enhanced.
Fig. 10 (a) Schematic diagram for the preparation of α-MoO3 along with the SEM images of different materials; (b) responses versus the OT to 50 ppm TMA gas; (c) responses versus 50 ppm of various gases at 133 °C; (d) responses of the α-MoO3-400 sensor to 10 ppm TMA at 133 °C for 90 days. Reproduced with permission from ref. 64, copyright 2019 American Chemical Society. (e) TEM images of Au@MoO3 nanocomposite samples; (f) response to 200 ppm ethanol at different OTs; (g) gas responses to 200 ppm of the test gases at 280 °C. Reproduced with permission from ref. 145, copyright 2016 Elsevier. |
Scheme 4 Structures (top view) of TMA-α-MoO3 (a1) and TMA-Ov-α-MoO3 (b1). The density of states of TMA-MoO3 (a2) and TMA-Ov-MoO3 (b2). Reproduced with permission from ref. 64, copyright 2019 American Chemical Society. |
Surface modification with noble metals is a known practice in the sensing field. Essentially, noble metal NPs can facilitate the adsorption of oxygen molecules and accelerate the transfer of electrons to metal oxide surfaces.34 For instance, the Yan et al.145 used a chemical reduction method for depositing Au NPs onto the MoO3 surface to prepare Au-loaded MoO3 NSs. The TEM image in Fig. 10e displays the average distribution of Au NPs of size 10–15 nm on the surface of MoO3 NSs. The temperature-dependent response curves for ethanol in Fig. 10f exhibit an ‘increase-maximum-decrease’ tendency, whereas the Au@MoO3 NSs possess ∼12 times better response than that of pristine MoO3. The same behavior is illustrated in Fig. 10g, wherein the Au@MoO3 NSs show the highest response toward ethanol gas, among others. This high sensing response is accredited to the presence of highly catalytic Au NPs, which assist in enhancing gas diffusion on the sensor surface.
Fig. 11 (a) Schematic diagram of the preparation of MoO3-rGO; (b) SEM and TEM images of MoO3-rGO; (c) response to 100 ppm of ethanol under different OTs; (d) response of the sensors toward different VOCs. Reproduced with permission from ref. 146, copyright 2019 Elsevier. (e) Illustration for the preparation process of MoO3/SnO2 NFs; (f) SEM images of MoSn-S2; (g) sensor responses to 10 ppm H2S concentration; (h) response to 10 ppm of various test gases. Reproduced with permission from ref. 147, copyright 2019 American Chemical Society. (i) Response toward various gases at different OTs. (Left) Pure MoO3 NF and (Right) Au–MoO3 NFks, (inset) SEM image of Au–MoO3 NFks; (j) dynamic response of Au–MoO3 toward H2 (100, 250, 500 ppm), acetone (25, 50, 100 ppm), and ethanol (10, 25, 50 ppm) at 450 °C. RH = 40% at 20 °C, with an applied voltage equal to 1 V. Reproduced with permission from ref. 148, copyright 2018 Elsevier. |
Considering the potential of the heterostructured nanocomposites consisting of two or more types of metal oxides due to their individual synergetic effect in enhancing the sensing response,31,159,160 Gao and coworkers147 prepared porous MoO3/SnO2 NFks using graphene sheets (G) as sacrificial templates (Fig. 11e) and revealed the excellent sensing performance toward H2S, which is a poisonous, corrosive, and flammable gas. The SEM micrographs of MoO3/SnO2 NFks in Fig. 11f demonstrated the presence of an agglomerated structure with porous SnO2 NFks (diameter ∼8–12 nm). The results in Fig. 11g present that the MoO3/SnO2 NFks demonstrate better sensing properties (∼5 times response) than that of pure SnO2, that too at lower working temperature. The cross-sensitivity results in Fig. 11h further attests to the superior sensing response of the MoO3/SnO2 NFks to H2S gas as compared to the pure SnO2-based sensor. The superior sensing response of the MoO3/SnO2 NFs was credited to the presence of a large surface area and n–n heterojunctions, which favor faster gas diffusion, thus resulting in improved H2S sensing performance. In another work, the group of Comini et al.148 utilized an evaporation-condensation method to prepare Au-loaded MoO3 NFks for the detection of H2S gas. Owing to the smaller dimension and better separation, a large number of H2S molecules became absorbed on the surface of Au–MoO3 NFks, ultimately resulting in enhanced sensing performance. The TEM micrographs in the inset of Fig. 11i reveal the clear presence of homogenized Au NPs (diameter ∼10–12 nm) on the surface of the MoO3 NFks. The OT dependent response of pure MoO3 NFks (Fig. 11i-1) and Au–MoO3 NFks (Fig. 11i-2) indicated the increased response of the MoO3 NFks with Au functionalization. The results further revealed 10 times better response for the Au–MoO3 sensor than pure MoO3 NF, while the optimum OT was reduced from 450 °C to 400 °C. The dynamic response/recovery transients in Fig. 11j depict the excellent reversible response, which, when exposed to reducing gas, increased and restored the initial values on account of exposure to natural air. From the results, it was concluded that the Au–MoO3 NFks, owing to their enhanced surface area and formation of Schottky barriers at the interface between Au and MoO3, promote gaseous oxygen dissociation, which ultimately resulted in enhanced sensor response toward H2S gas.
Fig. 12 (a) TEM image of α-MoO3 NPts; (b) the sensitivities of the α-MoO3 NPts sensors as a function of the concentrations of different reagent vapors; (c) the sensitivity changes of the α-MoO3 NPts sensors at various OTs under ethanol vapor in the concentration range of 5–800 ppm. Reproduced with permission from ref. 149, copyright 2011 Royal Society of Chemistry. (d-1) SEM images of MoO3; (d-2) SEM image of the MoO3-Pd nanocomposite. The insets show the corresponding low-resolution images; (e) HRTEM image of the MoO3–Pd nanocomposite; (f) current–voltage characteristics of the MoO3–Pd nanocomposite in the presence of different concentrations of H2 gas at RT; (g) proposed mechanism of the interaction of H2 gas with the MoO3–Pd nanocomposite. Reproduced with permission from ref. 79, copyright 2017 Elsevier. (h) SEM image of the as-grown MoO3 nanostructures prepared on silicon substrates at 400 °C for 1 h with 3-layer depositions; (i) transient gas sensing response as a function of the NO2 concentration at 250 °C for the layered α-MoO3 nanoplates, (inset) transient H2 sensing response as a function of the concentration at 250 °C; (j) sensor signal as a function of the OT at different NO2 concentrations. Reproduced with permission from ref. 150, copyright 2020 Royal Society of Chemistry. |
In addition, UV light illumination is one of the alternative ways to improve the recovery speed. On illuminating the sensor with UV light, the adsorbed oxygen ions on the surface are removed, thus providing a clean surface with more fresh interaction sites that are readily available for interaction with the target gas. Using this method, Kalanur et al.79 prepared an H2 sensor by depositing Pd NPs on hydrothermally synthesized MoO3 NPts. The SEM images in Fig. 12d-1 for pure MoO3 NPts revealed their length, width, and thicknesses to be in the range of 1–4 μm, 100–150 nm, and 10–20 nm, respectively. Also, the NPts-type morphology of MoO3 was found to remain unchanged after Pd deposition and UV exposure (Fig. 12d-2). The TEM image (Fig. 12e) further supported the SEM results wherein the Pd NPs (diameter ∼5–20 nm) were clearly deposited on the smooth surface of MoO3. The RT I–V characteristics of the sensor in open air (Fig. 12f) displayed that the current level increased with H2 concentration. The studied mechanism in Fig. 12g indicated the chemochromic effect as a result of the structural changes from H2 gas. This indicated the co-occurrence of oxygen vacancies and water molecules in the MoO3 crystal. Due to the spillover effect, the Pd NPs dissociate the absorbed H2 molecules into H2 atoms, which are transferred onto MoO3 NPts and assisted in generating oxygen vacancies.
Air quality issues caused by exhaust gases from rapid industrialization have become a serious problem worldwide in recent years.161 In particular, nitrogen dioxide (NO2), which causes photochemical smog and acid rain, is one of the toxic gases emitted during combustion in industries. The high risk of respiratory and lung diseases will increase when exposed to this gas. Therefore, a reliable NO2 sensor for air quality monitoring is needed; in this regard, Felix and coworkers150 utilized a facile polymeric solution method to prepare rectangular α-MoO3 NPts and demonstrated good NO2 sensing performance. The SEM image in Fig. 12h illustrated well-faceted rectangular NPts morphology of the α-MoO3 samples prepared at 400 °C for 1 h as a function of the number of layers deposition. The transient response curves in Fig. 12i reveal that the resistance for NO2 increased, while an opposite behavior was observed for H2 gas. Also, the sensor was able to exhibit a highly reversible response down to sub-ppm values for both gases. The response results as a function of the OT and NO2 concentration in Fig. 12j revealed an increase-maximum-decrease kind of pattern for all the tested concentration ranges. The results indicate that the unique synthetic method assisted in the growth of 2-D MoO3 NPts on crystalline substrates, thus eliminating the requirement of any transfer process and leading to the development of a high-performance gas sensor.
Fig. 13 (a) SEM images of 5 mol% Ni-doped α-MoO3; (b) response vs. optimum OT of the sensors to formaldehyde gas at a concentration of 100 ppm; (c) gas-sensing transients to 5–100 ppm formaldehyde operated at optimal OT; (d) comparison of the response of the sensor to 100 ppm of different test gases at 255 °C. Reproduced with permission from ref. 151, copyright 2017 Elsevier. (e) Schematic figure of the MoO3-NPr gas sensor; (f) SEM images of the MoO3-NPr gas sensor; (g) responses of the MoO3-NPr sensor to 5 ppm TMA and H2S with respect to the OT; (h) selectivity of the MoO3-NPr sensor at 325 °C; (i) response–recovery curves of the MoO3-nanopaper sensor to different H2S concentrations at 250 °C and 325 °C, (insets) the response of the sensor to various gas concentrations. Reproduced with permission from ref. 152, copyright 2017 Royal Society of Chemistry. |
Fig. 14 (a) SEM micrograph of spray-deposited α-MoO3 TFm with a thickness of 520 nm; (b) response of α-MoO3 film for different TMA concentrations with error bars; (c) transient response of α-MoO3 film for different TMA concentrations; (d) transient TMA sensing response of α-MoO3 film in mixed amine environment. Reproduced with permission from ref. 153, copyright 2014 Elsevier. (e) The technological flow for the MoO3–V2O5 gas sensor device fabrication; (f) the variation in response of (MoO3)0.4(V2O5)0.6 TFm at different OTs; (g) the transient response curves of typical (MoO3)0.4(V2O5)0.6 TFm; (h) the gas response and selectivity coefficient study of typical (MoO3)0.4(V2O5)0.6 TFm operating at an OT of 200 °C for 100 ppm concentration of various gases. Reproduced with permission from ref. 154, copyright 2018 Elsevier. (i) Schematic device structure of the transparent humidity sensor; (j) SEM images of the α-MoO3 TFm on the surface of channel and FTO electrode; (k) photograph of the transparent device; (l) schematic diagram of the humidity sensing mechanism for the α-MoO3 TFm; (m) dynamic response/recovery curve for one cycle; (n) sensitivity of the device to different analytes. Reproduced with permission from ref. 156, copyright 2020 Royal Society of Chemistry. |
In order to further enhance the sensor response compared to the above-mentioned gas sensors, it is essential to adjust the morphology, modify the surface, or combine other oxide and 2-D materials. The group of Mane et al.154 prepared MoO3/V2O5 TFm using the chemical spray pyrolysis (CSP) deposition method (Fig. 14e) for the detection of NO2 gas. The variation in response of MoO3/V2O5 TFm toward NO2 at different OT in Fig. 14f presents an IMD pattern, while the maximum response was observed at 200 °C. The dynamic response curves of MoO3/V2O5 in Fig. 14g revealed that both Tres and Trec decrease with an increase in the OT, whereas the larger Trec at lower OT was ascribed to more prominently adsorbed O2− species on the sensor surface. The gas response results in Fig. 14h show that the response of gases for MoO3/V2O5 TFm varied in the pattern of CO < CO2 = H2S < NH3 < SO2 < NO2. The higher gas response of 80% for NO2 gas could be due to the unpaired e− in nitrogen, which forms the bond with the oxygen present on the surface and subsequently promotes chemisorption.
Flexible and wearable gas sensors using flexible substrates have been an active area of research to overcome the problem of the operating temperature. In addition, most of the sensors fabricated so far have been based on the deposition of sensing layers on mechanically rigid substrates such as alumina, glass, quartz, or silicon. In addition, the precise detection of humidity in the indoor climate has also emerged as a research hotpot in recent times;136,140,162–164 thus, Ma and coworkers156 prepared α-MoO3 TFm by a simple solution method and prepared a transparent humidity sensor, which consisted of laser-etched fluorine-doped tin oxide (FTO) electrode, onto which the annealed α-MoO3 thin film was coated (Fig. 14i). The SEM image in Fig. 14j revealed that the FTO substrate has an important influence on the film formation process as the morphologies of the film on the channels and FTO electrodes are not exactly the same. Fig. 14k represented the excellent transmittance of the prepared sensing device derived from the thin α-MoO3 film and transparent FTO substrate, which are particularly helpful in achieving superior response to humidity under ambient conditions (Fig. 14l). The dynamic current–time curve in Fig. 14m revealed the superfast Tres and Trec of the humidity sensor. In addition, the sensor possesses good anti-interference ability as its selectivity for moisture was much high than toward other test gases (Fig. 14n) under similar testing conditions.
Fig. 15 (a) Typical FESEM image of the MoO3 MSh; (b) responses of the sensors to 100 ppm TEA in the OT range of 200–325 °C; (c) responses of the sensors to different gases at 275 °C; (d) dynamic response of MoO3 MSh-based sensors at 275 °C during 5 months exposure to 100 ppm TEA. Reproduced with permission from ref. 157, copyright 2018 Royal Society of Chemistry. (e) SEM micrographs of Cu9; (f) variation of resistance versus temperature for different samples; (g) sensitivity versus temperature for different gases at varied OT; (h) sensitivity versus time for various CO gas concentrations in ppm. Reproduced with permission from ref. 158, copyright 2019 IOP Publishing. |
Class | Material | Synthesis route/morphology | Gas | Conc. (ppm) | Operating temp. (°C) | Response | Resp./Reco. time (s/s) | Ref. |
---|---|---|---|---|---|---|---|---|
a Not given. | ||||||||
Nano/Micro flower | MoO3 | Solvothermal + calcination | TEA | 100 | 250 | 416 | 3/1283 | 83 |
MoO3 | Hydrothermal | Ethanol | 400 | 300 | 40 | 7/12 | 84 | |
MoO3 | Hydrothermal | Ethanol | 100 | 350 | 22a | 20/25 (300 ppm) | 166 | |
MoO3 | Hydrothermal | Ethanol | 300 | 300 | 37.1 | N.G. | 167 | |
MoO3/WO3 | Hydrothermal | Ethanol | 100 | 320 | 28.5 | 13/10 | 168 | |
Zn/MoO3 | Hydrothermal | CO | 50 | 240 | 31.23 | 10/14 | 170 | |
Hierarchical nanostructures | MoO3 | Hydrothermal | Ethanol | 200 | 340 | 34a | N.G. | 171 |
MoO3 | Solvothermal | TEA | 10 | 170 | 931.2 | N.G. | 172 | |
MoO3 | Hydrothermal | Ethanol | 100 | 250 | 19.8 | 15/15 | 173 | |
MoO3 | Hydrothermal | Ethanol | 400 | 300 | 32 | 3.2/2.4 | 77 | |
SnO2/MoO3 | Chemical synthesis | Ethanol | 100 | 260 | 814 | 1/8 | 174 | |
Fe2O3/MoO3 | Sacrificial template | TMA | 100 | 240 | 18.6 | 12/106 | 175 | |
MoO3 | Hydrothermal + calcination | Ethanol | 200 | 260 | 80 | 16/10 | 57 | |
MoO3/In2O3 | Hydrothermal | Ethanol | 100 | 185 | 7 | 11/94 | 176 | |
Nanoarrays | Y/MoO3 | Solid state chemical reaction | Xylene | 100 | 370 | 28.3 | 1/15 | 177 |
MoO3 | Solid state chemical reaction | Xylene | 100 | 370 | 19.2 | 1/15 | 178 | |
Fe/MoO3 | Solid state chemical reaction | Xylene | 100 | 340 | 28.5 | 2/21 | 179 | |
Hollow spheres | Au/MoO3 | Solvothermal | Xylene | 100 | 250 | 22.1 | 118/289 | 180 |
MoO3/Bi2Mo3O12 | Solvothermal | TMA | 50 | 170 | 25.8 | 7.1/N.G. (100 ppm) | 181 | |
Core–shell | MoO3/ZnO | Hydrothermal + atomic layer deposition | Ethanol | 200 | 350 | 7.6 | 44.8/119/8 | 182 |
MoO3/NiO | Hydrothermal + sintering | Acetone | 100 | 350 | 20.3 | 17/131 | 183 | |
MoO3/Fe2(MoO4)3 | Hydrothermal + calcination | H2S | 1 | 70 | 1.7 | 20/70 | 184 | |
Microcage | MoO3 | Hydrothermal | Ethanol | 200 | 350 | 43 | N.G. | 185 |
Nano pompon | Ni/MoO3 | Solvothermal | Xylene | 100 | 250 | 62.6 | 1/50 | 186 |
Microbox | MoO3 | Hydrothermal | Ethanol | 100 | 260 | 78 | 15/5 | 187 |
Fig. 16 (a) Typical SEM and TEM images of α-MoO3 flower-like HNS; (b) the responses of the α-MoO3 thick-film sensor versus the OT to 100 ppm TEA gas; (c) the responses of the α-MoO3 thick-film sensor to 100 ppm gases at different OTs; (d) the transient response-recovery characteristics of the α-MoO3 sensor with different concentrations of TEA gas at the OT of 250 °C. Reproduced with permission from ref. 83, copyright 2015 Elsevier. (e) Schematic illustration of the possible formation process for the MoO3 NFL architectures and the SEM image of the aggregated state of NFLs; (f) gas response to ethanol (400 ppm) at a series of OT; (g) gas response to varied ethanol concentrations at OT of 300 °C; (h) gas response and recovery curves of the sensors; (i) repeat response–recovery characteristics of the sensors. Reproduced with permission from ref. 84, copyright 2016 Springer. |
In addition, the availability of different precursors also allows different nanostructures to be synthesized using a single-step or double-step HT procedure. For example, the flower-like morphology exhibits numerous edge sites, which interact strongly with gas analytes due to their high catalytic reactivity, thus providing a high sensing response.188 Liu et al.84 utilized sodium citrate and PEG-assisted HT method to prepare hierarchical MoO3 NFLs (Fig. 16e) for ethanol gas sensing. The SEM image in Fig. 16e displays the hierarchical and rose-like NFls architectures composed of densely packed thin porous NSs (thickness ∼15–18 nm) arranged in a multilayered stacked structure. The OT-dependent response of MoO3 NFLs to ethanol in Fig. 16f presents an IMD response pattern with the maximum at 300 °C. Besides, the response depicts no sign of saturation with the gas concentration increasing from 100 to 700 ppm, as shown in Fig. 16g. The voltage of the sensor under different OT revealed the highest voltage from 40 to 80 °C (Fig. 16h), whereas the voltage sharply increases when ethanol gas is purged in the chamber while it returns to its original state when the ethanol gas is out of the measurement chamber. The repetitive response results in Fig. 16i demonstrated no significant change in the sensor response while delivering a fast Tres and Trec for the sensor. The excellent response to ethanol was accredited to the high SSA and high S/V ratio of MoO3 NFLs, which assisted in improving the reaction sites for gas analytes.
Wang et al.170 experimentally carried out and theoretically verified (with DFT) the detection of CO using Zn-doped MoO3 hierarchical microflowers using density of states (Scheme 5). As can be seen, a significant amount of deformation of the Zn–MoO3 surface was observed on account of the adsorption of CO, and a charge transfer value of 0.451e was obtained, thus indicating strong chemisorption. Electron transfer during the adsorption process was verified from the distinct continuous region in Scheme 5b, which is related to the formation of new chemical bonds. A shifting to the lower energy after adsorption was observed in the DOS curves, which indicate the chemical action in this system. Finally, the introduction of Zn atom promoted the adsorption ability of MoO3(010), which supports the experimental results showing the superior gas-sensing performance of Zn–MoO3 samples to CO.
Scheme 5 (a) The structure, (b) deformation charge density, (c) HOMO, (d) LUMO, (e) DOS, and (f) PDOS of CO-adsorbed MoO3 (010) adsorption system. Reproduced with permission from ref. 170, copyright 2020 Elsevier. |
Fig. 17 (a) SEM and TEM images of nest-like MoO3; (b) response vs. temperature toward 200 ppm ethanol; (c) ethanol concentration vs. response property at 340 °C, blue line symbol for nest-like MoO3 and red for monodispersed MoO3 NBs. Reproduced with permission from ref. 171, copyright 2015 Elsevier. (d) Schematic of the formation process of the α-MoO3 NFLs; (e-1 to e-4) TEM micrographs of the intermediate products collected at different reaction stages; (e-5) SEM images of the α-MoO3 product prepared for 8 h; (f) the response vs. OT to 10 ppm TEA gas; (g) the response vs. varied TEA concentration; (h) the response to 10 ppm of various gases, reproduced with permission from ref. 172, copyright 2015 Royal Society of Chemistry. |
In another work, Sui and coworkers172 prepared a 3-D flower-like α-MoO3 HNS from 1-D single-crystalline NBs via a one-step template-free ST method and achieved the fast detection of TEA at 170 °C. The growth process of α-MoO3 HNS, as depicted in Fig. 17d, revealed the formation of flower-like nanostructures from precursor nanoparticles and nanobelts. The mechanism of formation was further supported by the TEM images in Fig. 17e1 to e4. The SEM image in Fig. 17e5 displays the presence of numerous NBs (thickness ∼20–30 nm) with interconnected sharp tips and rough rims, which are radially assembled into flower-like shapes. The temperature-dependent response in Fig. 17f indicated a decrease in the responses of the sensors toward TEA gas with an increase in the OT 170 to 290 °C. The response of the sensor as a function of TEA concentrations in Fig. 17g presented that the MoO3 HNS possesses the highest response among other MoO3 nanostructures. The cross-sensitivity results in Fig. 17h reveal that an excellent response was observed toward TEA gas. The research group ascribed the superior sensing performance to the following reasons: (1) the fast oxidation of TEA gas, (2) superior electronic conduction of MoO3, (3) 3-D hierarchically assembled structures, and (4) large SSA and pore volume of the α-MoO3 HNS.
As one of the important VOCs, ethanol still requires a reliable and robust sensor for applications in breath analysis, food industries, and the biomedical field. Several approaches have been used to improve the sensing performance of metal oxide-based ethanol sensors. Xia et al.173 utilized the facile HT method to prepare 3-D porous α-MoO3 sponges with 1-D NRs as the building blocks. The schematic in Fig. 18a illustrates the formation of porous sponges, wherein some NRs randomly grown on the lateral surface of other NRs evolve into a branch, which is well criss-crossed to form porous sponges. The SEM image in Fig. 18b indicates the monodispersed NRs (length of 100–200 nm) assembled into porous, spongy-like hierarchical structure with abundant interconnected hollow spaces. The gas-sensing performance in Fig. 18c reveals that the 3-D porous α-MoO3 sponges possess ∼2 times greater response than monodispersed MoO3 nanorods in all the concentration ranges. Fig. 18d further indicates that both the sensors possess similar response–recovery dynamics. It was later concluded that the improved sensing performance was not only due to the interconnected porous structures but also due to the significant fraction of the atoms participating in ethanol gas-sensing reaction.
Fig. 18 (a) Schematic illustration of the evolution process of porous α-MoO3 sponges; (b) SEM images of porous α-MoO3 sponges; (c) responses to ethanol with different concentration at 250 °C, (inset) response versus the operating temperature of the two sensors exposed to 100 ppm methanol; (d) dynamic ethanol sensing transient toward 100 ppm ethanol at 250 °C. Reproduced with permission from ref. 173, copyright 2016 Elsevier. (e) Schematic diagram of the possible formation mechanism for the NFbs-assembled and the NSs-assembled hierarchical MoO3 structures; (f) SEM images of the NFbs-assembled hierarchical MoO3 structures; (g) SEM images of the NSs-assembled hierarchical MoO3 structures; (h) the gas response to 400 ppm ethanol at different OTs; (i) response–recovery characteristics of the sensors at 300 °C to 400 ppm ethanol. Reproduced with permission from ref. 77, copyright 2019 Elsevier. |
Similarly, Li and coworkers77 used CTAB and polyvinyl pyrrolidone (PVP) to respectively prepare NFbs and NSs assembled MoO3 HNS under HT conditions (Fig. 18e). The SEM images of the NFs-assembled MoO3 HNS in Fig. 18f-1 and f-2 clearly demonstrate the presence of a large number of NFs having size in the range of 20–25 μm and assembled around an invisible center. For the NSs-assembled MoO3 HNS in Fig. 18g-1 and g-2, smooth NSs (thickness ∼20–30 nm) were arbitrarily arranged on the surface of the sphere, while numerous NSs were found to cut across each other. The OT-dependent response in Fig. 18h reveals the better ethanol sensing performance of NS-MoO3 HNS than that of NF-MoO3 HNS. This case was the same as that in the case with the response/recovery results, given in Fig. 18i. It was later concluded at the high response of NS-MoO3 HNS was due to its higher SSA and the scattered intersection of various NSs on the spherical surface, which causes the formation of semi-closed spaces on the surface of NS-MoO3 HNS.
Fig. 19 (a) The SEM image of 1% Y-doped sample; (b) the response curve of the samples to 100 ppm of different gases at the OT of 370 °C; (c) the response/recovery curve of the samples for 100 ppm xylene at 370 °C; (d) the stability of the samples for 100 ppm xylene at 370 °C. Reproduced with permission from ref. 177, copyright 2017 Elsevier. (e) The response to 100 ppm gas at different OTs, (inset) the SEM images of samples S2; (f) the dynamic response curves to 100 ppm xylene at 370 °C; (g) the stability of the samples to 100 ppm xylene at 370 °C; (h) the response of sample S2 to 100 ppm gas at the different OTs. Reproduced with permission from ref. 178, copyright 2017 Elsevier. (i) A schematic representation for the synthesis of Fe-doped MoO3 NARs; (j) SEM images of MoOFe-03; (k) the response curves of the samples to 100 ppm xylene at different OTs; (l) the response and recovery curves for 100 ppm xylene at 340 °C; (m) the response of the samples to 100 ppm of different gases at 340 °C. Reproduced with permission from ref. 179, copyright 2020 Elsevier. |
Similarly, Qin and coworkers178 utilized a solid-state chemical reaction route to prepare α-MoO3 2-D nanoplates (thicknesses ∼50 nm) and developed self-assembled orderly NARs (inset of Fig. 19e). The OT-dependent response to xylene in Fig. 19e indicated that the sensor based on S2 (NARs) exhibited better sensing performance than the sensor based on S1 (NPts) but a higher OT due to the close array structure of S2. The transient response in Fig. 19f indicated a fast response/recovery time for both the sensors, wherein the response time was close to 1 s toward xylene gas. The stability results in Fig. 19g reveal that the response was the highest on the 5th day, which observed a sharp drop on the 10th day and stabilized on the 20th day. Besides, the cross-sensitivity results in Fig. 19h demonstrate the highest response toward xylene gas. This excellent sensing performance was attributed to the relatively larger SSA and preferential exposure of the active crystal face.
To further enhance the sensor performance compared to pristine α-MoO3, Qin and his group179 used a facile solid-state chemical reaction method to develop Fe-doped α-MoO3 (Fig. 19i). The orderly array structure self-assembled by the NPTs is observed in the SEM images in Fig. 19j. The OT-dependent response to xylene gas in Fig. 19k displayed an IMD pattern, whereas the Fe-loaded samples showed better response at a relatively lower temperature than that of pristine MoO3. The response–recovery transients in Fig. 19l demonstrated a fast response time for all the Fe doped α-MoO3 materials because the presence of Fe ions generates more oxygen vacancies on the surface of the sensor, which is not the case with pure MoO3. The cross-sensitivity results in Fig. 19m revealed that the Fe-doped α-MoO3 NARs displayed the highest response to xylene gas due to the presence of the benzene ring-like structure, which provides a greater number of electrons for reacting with the oxygen species absorbed on the surface of the sensor. It was concluded that the excellent sensing features of Fe-doped MoO3 NARs arise from their larger SSA, which generates more reactive sites, thus resulting in enhanced sensing response to xylene gas.
Fig. 20 (a) The SEM images of a single sphere obtained by calcination; (b) the responses vs. the OT to 100 ppm toluene gas; (c) the responses vs. 100 ppm various gases at their relative optimized OTs; (d) the responses of 2.04Au/α-MoO3 sensor versus 100 ppm gases. Reproduced with permission from ref. 180, copyright 2017 American Chemical Society. (e) SEM and TEM images of the calcined MoO3/Bi2Mo3O12 spheres; (f) responses of the sensors toward 50 ppm of TMA at different OTs; (g) the gas response–recovery characteristics of the MoO3/Bi2Mo3O12 sensor to varied concentrations of TMA measured at 170 °C; (h) the responses of the MoO3/Bi2Mo3O12 sensor toward 100 ppm of various gases. Reproduced with permission from ref. 181, copyright 2019 American Chemical Society. |
Fig. 21 (a) Schematic representation of the synthesis of MoO3-ZnO CSh; (b) SEM and TEM images of MoO3-ZnO CSh NRs; (c) response of the prepared materials to 200 ppm ethanol at different OTs; (d) selectivity of the pristine MoO3 and MoO3-ZnO CSh NR sensor at 350 °C; (e) long-term stability of the MoO3–ZnO CSh NR sensor at 350 °C. Reproduced with permission from ref. 182, copyright 2018 Elsevier. (f) Schematic illustration of the synthesis of α-MoO3@NiO composites; (g) SEM images of the as-prepared materials; (h) response vs. OT for 100 ppm acetone gas; (i) selectivity toward 100 ppm various gases at their optimal OTs. Reproduced with permission from ref. 183, copyright 2019 Royal Society of Chemistry. |
Similarly, in another work, Xu and coworkers183 anchored NiO porous NSs on α-MoO3 NBs using a simple method and developed the α-MoO3@NiO CSh P–N heterostructured nanocomposite (Fig. 21f). The SEM images in Fig. 21g revealed the structure of α-MoO3, which is composed of 1D NBs with a uniform size distribution. This structure of NBs was well-maintained in α-MoO3@NiO CSh; however, the surface becomes coarser than that of pristine α-MoO3 NBs, which could be due to the successful growth of NiO NSs on its surface in the process of forming the CSh structure. The sensing results in Fig. 21h exhibit an IMD kind of volcano-shaped pattern toward acetone gas, wherein the optimized α-MoO3@NiO CSh nanocomposite sensor demonstrated the highest response at its optimal OT due to the highest amount of heterogeneous interfacial bonds. Besides, the response to acetone gas was also the highest (Fig. 21i) among other interfering test gases. The high sensing performance of α-MoO3@NiO CSh was attributed to its larger SSA, which was readily available for surface reactions and also the formation of p–n heterojunction among NiO and α-MoO3.
Fig. 22 (a) Growth mechanism of hollow MoO3 cage; (b-1) SEM images of solid MoO3 polyhedrons; (b-2) SEM images of hollow MoO3 MCg; (c) the gas response vs. OT for 200 ppm ethanol gas; (d) the gas response of the two sensors toward different gas concentrations at 350 °C. Reproduced with permission from ref. 185, copyright 2019 Elsevier. (e) FESEM images of S-5% NPMn, (inset) TEM image of S-5%; (f) responses vs. OTs to 100 ppm xylene gas; (g) response and recovery curves of S-0 and S-5% sensors; (h) responses to 100 ppm different test gases (X: xylene, E: ethanol, M: methanol, A: acetone, F: formaldehyde, H: hydrogen sulfide, T: toluene, B: benzene) at 250 °C, reproduced with permission from ref. 186, copyright 2019 Elsevier. (i) Schematic illustration of the formation process of hierarchical MoO3 MBx; (j) FESEM image of hierarchical MoO3 MBx; (k) response of the sensor based on hierarchical MoO3 MBx to 100 ppm of ethanol at different OTs; (l) response/recovery curves of sensors to 100 ppm ethanol at 260 °C; (m) gas response of hierarchical MoO3 MBx to 100 ppm test gases at 260 °C; (n) selectivity to ethanol (Sethanol and Sgas, gas responses to ethanol and other gases, respectively). Reproduced with permission from ref. 187, copyright 2017 Elsevier. |
(a) The response could be enhanced using porous and hollow structures as they can provide fast adsorption–desorption, diffusion, and transmission of gas molecules to achieve high sensor response at low concentrations.
(b) Surface modification using noble metals or doping of metal ions could increase the number of reaction sites on the surface, which results in more oxygen vacancies to interact with the target gas and enhance the selectivity.
(c) The formation of heterostructures or composites with other oxides or 2-D materials can form abundant oxygen vacancies and create more active sites for the interaction. This can control the Fermi level and transfer the electrons from a higher energy level to a lower energy level, leading to increased response value and fast response.
(d) Light illumination/irradiation could help to achieve high sensor response and fast Tres/Trec at RT by generating photo-induced electron–hole pairs on the MoO3 surface via the chemisorption process.
Although there has been extraordinary progress in designing gas sensors using novel nanostructures, there are still many challenges and problems toward achieving high sensing performance, reproducible synthesis process, high selectivity, miniaturization of the sensor, and power consumption/operation temperature. This is very important because the mass-scale production of sensor devices requires a reliable and reproducible process. One of the key challenges is the durability of the sensor at RT since humidity is the main interference in a room temperature sensor. Thus, from a practical point of view, it is important to investigate the sensing performance under different humid conditions to establish the relationship between the sensing properties and the environmental conditions. Another key challenge is the selectivity or the interference of gases, which can often hinder the sensing performance. There have been few reports on gas sensors for detecting specific gases but not all gases, and sometimes, the detection of a specific gas out of a mixture of gases is a major concern. In the case of resistance-based sensors, it is difficult to discriminate the gases that can give similar resistive change/response. To enhance the selectivity and remove undesirable confounders, a diffusion filter layer can be made with microporous materials (e.g., zeolite and metal–organic frameworks, active carbon, and polymers (e.g., Nafion65)).189 In this case, only the filtered molecule can reach the surface of the sensing material. Güntner et al.190 demonstrated superior selective sensing toward formaldehyde using zeolite membranes. With a zeolite Mobile-Five (MFI)/Al2O3 membrane, the Pd-doped SnO2 sensor displayed astounding selectivity (>100) for formaldehyde (down to 30 ppb) at 90% relative humidity.
Lastly, the important aspect to consider is OT in the case of MoO3-based gas sensors. Undoubtedly, temperature plays a vital role in SMOx-based gas sensors to achieve fast response/recovery speed and high sensitivity. At low temperatures, the reaction rate on the metal oxide surface is sluggish, resulting in poor sensor response. By increasing the temperature, the thermal energy given is high enough to overcome the activation energy barrier for the surface reaction; thus, the reaction rate increases and the sensor displays increasing response to the target gas. However, in order to achieve high sensor response, we need to compromise the power consumption. To solve this issue, the idea of a low-powered or self-powered microheater has been proposed with advanced MEMS technology so as to achieve the best possible sensing performance with less power consumption. In addition, an in-depth study of the gas sensing mechanism of MoO3 nanostructures and metal oxides is still needed. Several models and hypotheses on the metal oxide-based gas sensing system are described by researchers but there is no such model that works for an wide range of gas molecules. Additional analysis may be needed by some advanced tools such as DFT or first principles study.
In conclusion, the key motive of this review was not only to summarize the state-of-the-art but also to inspire readers and excite curiosity, driving them to further investigate MoO3-based gas sensors. Future directions for understanding more about MoO3-based nanostructures, their sensing mechanism, and future applications have been discussed.
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