Amorphous carbon material of daily carbon ink: emerging applications in pressure, strain, and humidity sensors

Abstract

Carbon-based materials have been widely used in various sensors. In addition to the widely reported graphene, carbon nanotubes and their composites, low-cost amorphous carbon materials are also sought after. In the different amorphous carbon materials, daily carbon ink (DCI) containing carbon black nanoparticles has a long history in writing. Interestingly, researchers found that DCI has promising applications in various sensors (pressure, strain, and humidity sensors) due to its many unique characteristics and advantages, such as conductive, good dispersion, strong adhesion, black, and low cost. In this review, the state-of-the-art advances in the amorphous material of DCI for pressure, strain, and humidity sensors are presented and discussed. Initially, combined with the characterization results, the physical and chemical properties of DCI are introduced in detail. Then, the research progresses of DCI in the field of pressure, strain, and humidity sensors are systematically reviewed and analyzed. In addition, the applications of DCI in temperature sensors and actuators closely related to sensors are discussed. Finally, the perspectives are illustrated in this field. We hope that this comprehensive review will provide new insights for the applications of DCI in various sensors and other electronic devices.

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Zaihua Duan

Zaihua Duan received his PhD degree in optical engineering from University of Electronic Science and Technology of China (UESTC) in 2022. Currently, he is a lecturer of School of Optoelectronic Science and Engineering at UESTC. His research interests include sensing functional materials and their applications in gas, humidity, pressure, and strain sensors.

Zhen Yuan

Zhen Yuan received his PhD degree in optical engineering from UESTC in 2020. He is currently an associate professor at UESTC. His research interest includes microstructure sensors, functional materials, and their applications such as in gas, strain and sweat sensing.

Yadong Jiang

Yadong Jiang received his PhD degree in Materials Physics and Chemistry from UESTC in 2001. He is a Changjiang Professor and Dean of School of Optoelectronic Science and Engineering at UESTC. His major research interests include optoelectronic materials and devices.

Liu Yuan

Liu Yuan received his PhD degree in Physical Chemistry from University of Chinese Academy of Sciences in 2016. Then, he worked at National University of Singapore as a postdoc during 2016–2019. Currently, he is an associate professor at the School of Optoelectronic Science and Engineering, UESTC. His research focuses on organic semiconductors and their applications in optoelectronics and sensors.

Huiling Tai

Huiling Tai received her PhD degree in optical engineering from UESTC in 2009. Currently, she is a professor at the School of Optoelectronic Science and Engineering, UESTC. Her major interests are sensing functional materials and sensors, and she has published over 120 peer-reviewed articles.

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1. Introduction

Well-known carbon nanomaterials such as graphene, carbon nanotubes, and their derivatives have been widely developed in various sensors because of their unique physical and chemical properties.^1–4 However, these carbon nanomaterials usually involve complex preparation techniques, high temperature treatment, and high costs. Amorphous carbon, as an important member of the carbon material family, was once in parallel with graphite and diamond in history, and was considered to be one of the three existing states of carbon element.⁵ It can be synthesized at low temperature with low cost and is widely used in electronics.^6–10 For the widely used resistive sensors, their basic characteristic is that the conductivity is adjustable; thus, the sensing materials used to fabricate resistive sensors are expected to have appropriate conductivity (i.e., semiconductor characteristics). Amorphous carbon materials contain different types of orbital configurations (sp, sp², and sp³), which endow them with different physical and chemical properties (such as adjustable conductivity).^6,11 Carbon black, as a typical low-cost amorphous carbon material, is widely used in various sensors (e.g., pressure, strain, humidity, temperature, and biosensors).^12–25 However, the dispersion of pristine carbon black is poor, and it is difficult to be directly used to fabricate sensors; thus, it needs to be compounded with other materials and solutions.¹² In recent years, another commercial and low-cost amorphous carbon nanomaterial, daily carbon ink containing carbon black nanoparticles (DCI, it should be noted that it is different from the “carbon ink” or “ink” containing other electronic materials specially developed for electronic devices), commonly used for writing (Fig. 1),^26,27 has attracted attention in the field of sensors.

Fig. 1 Conventional application of DCI in writing with pen (a) and (b) brush. Reproduced with permission from ref. 26 and 27. Copyright 2019 American Chemical Society and 2021 Elsevier.

The properties of materials determine their applications. Unlike pristine carbon black, DCI is required to have good dispersion and adhesion to ensure smooth writing and long-term preservation of words. Therefore, carbon black nanoparticles are introduced in the production of DCI, which endows it with DCI with another important characteristic of conductivity.^26–30 Interestingly, the conductivity, good dispersion, and strong adhesion of the DCI are also important properties for fabricating sensors.^{26,27,31–35} In particular, compared with other carbon materials (e.g., graphene, carbon nanotubes, and their derivatives), the commercial and industrial DCI has the advantage of extremely low cost. As shown in Fig. 2, DCI can be used to fabricate pressure, strain, and humidity sensors.^{26,27,32–34} For example, our research group previously used the conductive property of DCI to develop various resistive-type pressure, strain, and humidity sensors.^{26,27,31,33,34}

Fig. 2 Emerging applications of DCI in various sensors. Reproduced with permission from ref. 26, 27 and 32–34. Copyright 2019 American Chemical Society, 2021 Elsevier, 2022 American Chemical Society, 2021 Springer and 2021 The Royal Society of Chemistry.

As mentioned above, DCI has attracted the extensive attention of researchers in the field of sensors. Feng et al. previously reviewed the progress of DCI in the preparation of electrochemical electrodes and mainly introduced the synthesis methods of DCI.³⁷ However, there is a lack of a comprehensive overview to systematically review the progress of DCI in various sensors. In this review, we firstly summarize and analyze the basic physical and chemical properties of DCI based on its characterization results. Then, the research progresses of DCI in sensors are comprehensively reviewed and analyzed. Finally, the prospects of DCI for sensors and other electronic devices are proposed.

2. Characteristics of DCI

The composition of DCI is complex, and the preparation methods of different kinds of DCI are also different. The physical and chemical properties of materials are closely related to their applications; thus, it is necessary to systematically review and analyze the characteristics of DCI. It should be noted that in addition to the zeta potential, the following other characterization results are for solid carbon nanoparticles in DCI.

2.1 Name, brand, price, color, morphology, and specific surface area

To distinguish from the “carbon ink” or “ink” specially developed for electronic devices,^38–45 this review refers to the conventional carbon ink used for daily writing as DCI. In terms of appellation, DCI has many names, including mo (in China), sumi (in Japan), ink, carbon ink, pen ink, fountain pen ink, and Chinese ink.^29,37,46,47 Although there are many brands of DCI (such as Guizhou Boss,^{26,27,31–34} Shanghai Hero,⁴⁸ and Tianjin Ostrich⁴⁹), their main components are basically the same, including carbon black nanoparticles, deionized water, surfactants, and anti-sedimentation agents. There are certain differences in the prices of DCI with different brands and types. Most DCI on the market is less than $1/50 mL, which is far cheaper than other carbon materials.^{26,27,31–34} In terms of color, DCI is mainly black for easy identification due to the introduction of carbon black nanoparticles (Fig. 1). Fig. 3 shows the transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of DCI with different brands.^27,48,49 Although the size of solid particles in DCI with different brands is different, they are all nanoscale (<50 nm). Cai et al. characterized the specific surface area of the carbon nanoparticles in DCI, indicating that it has a large specific surface area of 177.5 m² g⁻¹.⁵⁰

Fig. 3 TEM and SEM images of DCI with different brands. (a) Guizhou Boss (size: 20–40 nm). Reproduced with permission from ref. 27. Copyright 2021 Elsevier. (b) Shanghai Hero (size: about 20 nm). Reproduced with permission from ref. 48. Copyright 2012 Wiley-VCH. (c) Tianjin Ostrich (size: around 50 nm). Reproduced with permission from ref. 49. Copyright 2015 Wiley-VCH.

2.2 Crystal structure, composition, and conductivity

It should be noted that there are slight differences in the characteristics of DCI with different brands. In this review, we take the DCI produced by Guizhou Boss, which is widely used in sensors, as an example to introduce its crystal structure, composition, and conductivity.^{26,27,31–34} To clarify the crystal structure of the carbon nanoparticles in DCI, it is characterized by X-ray diffraction (XRD), as shown in Fig. 4(a). There is a broad diffraction peak (10–35°) in the XRD patterns, indicating that the carbon nanoparticles in DCI are mainly amorphous carbon.^27,51 The weak diffraction peak (35–50°) is attributed to the a-axis of the graphite structure.^27,51 The mixed structure (amorphous carbon and graphite structure) of carbon nanoparticles in DCI endows it with a certain conductivity.

Fig. 4 (a) XRD patterns, (b) FTIR spectrum, and (c) C 1s spectrum of DCI. Reproduced with permission from ref. 27. Copyright 2021 Elsevier. (d) Conductivity test of DCI (applied voltage: 20 V). Reproduced with permission from ref. 26. Copyright 2019 American Chemical Society.

As shown in Fig. 4(b) and (c),²⁷ Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) show that there are many oxygen-containing functional groups in DCI. To illustrate the conductivity of DCI, the resistance of DCI painted on paper is tested, as shown in Fig. 4(d), and its resistance is less than 0.17 MΩ.²⁶ In addition, its sheet resistance on paper is about 10⁵ Ω sq⁻¹.³¹ To visually demonstrate the conductivity of DCI, it can be coated on the paper surface as a paper-based circuit to light a light-emitting diode (LED) (Fig. 4(d)).²⁶ The conductivity of DCI is very important for fabricating resistive sensors.

2.3 Dispersibility, stability, and hydrophilicity

The dispersity and stability of nanomaterials are very important for the formation of good and uniform electronic films.^12,27,33,37 As shown in Fig. 5(a), the particle size of DCI is mainly about 36 nm, which is basically consistent with the TEM result (Fig. 3(a)). In particular, the inset in Fig. 5(a) shows that the zeta potential of DCI is about −52 mV, proving that the DCI is of good dispersibility and stability.²⁷ According to the above FTIR and XPS characterizations (Fig. 4(b) and (c)), DCI contains rich hydrophilic oxygen-containing functional groups. The contact angle of the DCI film can quickly reach a very small angle of 9°, indicating that it has good hydrophilicity (Fig. 5(b)).²⁷ Similarly, Wang et al. confirmed that the film formed by DCI has an excellent hydrophilicity (Fig. 5(c)).⁵² Most hydrophobic nanomaterials are difficult to combine with both hydrophilic and hydrophobic matrixes directly. For example, the pristine carbon black nanoparticles are hydrophobic and difficult to combine with a hydrophobic matrix.¹² Importantly, the hydrophilicity of the DCI is conducive to its combination with other hydrophilic materials (such as cellulose paper, fabric, wood, and foam).^26,31–36 For hydrophobic matrixes, the hydrophilic treatment on their surfaces can be considered to facilitate combination with DCI (for example, introducing hydrophilic functional groups on the surface of hydrophobic matrix).⁵³ In addition, the humidity sensor can be fabricated using the hydrophilic characteristic of the DCI.²⁷

Fig. 5 (a) Size distribution of the DCI, and the inset shows its zeta potential. (b) and (c) Contact angles of the DCI with different reports. Reproduced with permission from ref. 27 and 52. Copyright 2020 and 2021 Elsevier.

3. Applications of the DCI in sensors

The physical and chemical properties of the DCI are reviewed and analyzed above. According to these characteristics of DCI, the following will focus on summarizing and discussing its research progress in pressure, strain, and humidity sensors. In addition, the applications of the DCI in the temperature sensor and actuator closely related to sensors are discussed.

3.1 Pressure sensor

With the development of wearable electronic systems, the flexible pressure sensor has received great attention and made remarkable achievements.^54–58 Among all kinds of flexible pressure sensors (e.g., resistance, capacitance, piezoelectricity, and triboelectricity), resistance-type flexible pressure sensor is widely developed because of its simple structure, high linearity, and simple read-out signal.⁵⁸ The resistance-type flexible pressure sensor is based on the piezoresistive sensing mechanism; thus, the sensing active layer is required to have a certain conductivity, and the conductive material is indispensable for fabricating the resistance-type flexible pressure sensor. Carbon materials have a wide variety and adjustable conductivity; thus, they are widely used in resistance-type flexible pressure sensors.⁵⁸ The appropriate conductivity of the DCI, combined with its good dispersion and low cost, has attracted attention in the field of resistance-type flexible pressure sensors.

In 2021, we reported a simple and low-cost flexible pressure sensor based on DCI and paper.³¹ Combined with the rough surface microstructure of the paper and the conductivity of DCI, a flexible paper-based pressure sensor is prepared through simple dipping, drying, and pasting processes. Under pressure, the area of the rough microstructure interface formed between the upper and lower conductive DCI paper increases, resulting in pressure-sensing response in a wide pressure range (0.1–50 kPa). In the pressure range of 0.1–6 kPa, the sensitivity of the sensor is 0.614 kPa⁻¹. To further improve the pressure-sensing performance of the above DCI paper-based pressure sensor, soon afterward, we constructed a paper-based pressure sensor with double-layer microstructure interface (Fig. 6(a)).³³ Compared with the above single-layer microstructure paper-based pressure sensor, the results show that the double-layer microstructure interface significantly improved the response range and sensitivity of the sensor (Fig. 6(b) and (c)). In the pressure range of 0.5–5 kPa, the sensitivity of the sensor reaches 5.54 kPa⁻¹, which is superior to many flexible pressure sensors based on other conductive materials. Benefitting from the good pressure-sensing response, flexibility, and environmental protection, the DCI paper-based pressure sensor can be used for multiple human body-related pressure detections (e.g., wrist pulse, speech recognition, finger bending, abdominal respiration rates, and insole pressing). Fig. 6(d) and (e) show the typical application demonstration for wrist pulse monitoring.

Fig. 6 (a) Fabrication process of the flexible paper-based pressure sensor. (b) Response and recovery curves of the flexible paper-based pressure sensor under different pressures. (c) Linear fitting lines. (d) and (e) Application demonstration for wrist pulse monitoring. Reproduced with permission from ref. 33. Copyright 2021 IOP Publishing.

In addition to good compatibility with paper, DCI can also form a composite conductive elastomer with foam.^36,59,60 Recently, Zhang et al. reported a flexible pressure sensor based on DCI-decorated melamine foam.³⁶ When pressure is applied to the sensor, the three-dimensional porous conductive network will be compressed, resulting in an increase in the contact points of the conductive network and pressure-sensing response. The sensitivity of the sensor is only 0.056 kPa⁻¹ (0–7.84 kPa). Compared with the above paper-based flexible pressure sensors,^31,33 the pressure-sensing response range of the DCI decorated foam pressure sensor is narrower and the sensitivity is lower. From this point of view, the pressure-sensing performance of the DCI-decorated three-dimensional structure pressure sensor still needs to be improved.

In addition, the self-powered triboelectric nanogenerator (TENG) has potential applications in pressure sensors. Xia et al. demonstrated that the conductivity of DCI can be used to fabricate self-powered TENG sensor for pressure detection.⁶¹Fig. 7(a) shows the fabrication process of the stacked sliding paper TENG (SP-TENG) sensor, including simple cutting, brushing, bending, and pasting steps. As shown in Fig. 7(b), the stacked SP-TENG sensor can output voltages up to hundreds of volts under varying vertical pressures. Through appropriate structural changes, the SP-TENG sensor can be used to detect the moving speed in the horizontal sliding mode (Fig. 7(c)). For the TENG force sensor, the DCI is mainly used as a conductive material to fabricate electrodes.

Fig. 7 (a) Fabrication process of the stacked SP-TENG sensor. (b) Output voltage of the stacked SP-TENG sensor under different forces. (c) Output voltage of the SP-TENG sensor under different speeds. Reproduced with permission from ref. 61. Copyright 2018 Elsevier.

3.2 Strain sensor

Strain sensor also has important applications in wearable electronic systems and has received extensive attention in recent years.^62–65 According to the strain conversion signal, the strain sensor is mainly resistance type. Therefore, the conductive active material is indispensable in the strain sensor. In 2019, our research group firstly reported a simple and low-cost strain sensor based on DCI and elastic core-spun yarn.²⁶ As shown in Fig. 8(a), the fabrication processes of the strain sensor are very simple, mainly including sonication, drying, rinsing, and pasting electrodes. During the stretching process, the fabric covered with DCI on the outer layer of the elastic core-spun yarn will separate, which will reduce the electron penetration and produce strain sensing response. Fig. 8(b) shows the response-strain linear fitting line, indicating that the strain sensor has a good linear response within the strain range of 0.5–20%. Benefiting from the good strain sensing performance of the sensor, it has been proved to be used for a variety of human-related strain detections such as vertebral curvature, abdominal respiration rates, finger bending (gesture recognition), and knee bending (Fig. 8(c)). Similarly, Yang et al. prepared a strain sensor by combining DCI with an elastic fabric.⁶⁶ The results show that the strain sensor has a high gauge factor (∼62.9, 0–30% stain) and can be used to monitor human motions (e.g., joint movement, chest respiration rates, and wrist pulse).

Fig. 8 (a) Fabrication process and the optical photograph of the strain sensor. (b) Linear fitting line. (c) Various application demonstrations of the strain sensor. Reproduced with permission from ref. 26. Copyright 2019 American Chemical Society.

The strain sensor mentioned above belongs to the tensile strain working mode and is also the mainstream strain sensor type. In addition, there is a strain sensor that works in the bending strain mode, which has important applications in bending angle detection.^67–71 Our group proposed a high-response, waterproof, and low-cost paper-based bending strain sensor using DCI as the conductive active material.³⁴Fig. 9(a) shows the fabrication process of the bending strain sensor. Firstly, the paper is soaked in DCI, and the conductive DCI paper is obtained by drying. Secondly, two pieces of DCI paper are pasted together by polyimide tape. Finally, the preparation of the bending strain sensor is completed by pasting the electrodes. As shown in Fig. 9(b), the surface SEM image of the bending strain sensor indicates that a contact gap is formed between the two DCI papers. When the sensor is subjected to bending, the contact gap between the DCI papers will become smaller, thereby promoting conduction and generating bending strain sensing response. Fig. 9(c) shows the sensitivities of the strain sensor are 1.59/° (14.4–41.41°) and 0.3/° (41.4–68°), which are much larger than that reported in previous studies. Fig. 9(d) and (e) show the typical applications of the bending strain sensor for cervical spines and finger bending monitoring.

Fig. 9 (a) Fabrication process of the bending strain sensor. (b) Surface SEM image of the bending strain sensor. (c) Linear fitting lines. Application demonstrations of (d) cervical spines and (e) finger bending monitoring. Reproduced with permission from ref. 34. Copyright 2021 The Royal Society of Chemistry.

3.3 Humidity sensor

Humidity sensor has been widely used in industry, agriculture, meteorology, home, and other environmental fields,^72–75 and they also have great application prospects in human body-related humidity detections.^76–79 There are many kinds of humidity sensors (such as resistance, capacitance, impedance, optics, voltage, quartz crystal microbalance, and surface acoustic wave),⁷⁶ among which conductive materials have important applications in resistance-type humidity sensor. In the above characterization part, it has been shown that the DCI has conductivity, hydrophilic oxygen-containing functional groups, and good hydrophilicity (small contact angle). These characteristics show that DCI is expected to be used as a humidity sensing material to fabricate humidity sensors.

In 2021, our research group reported a DCI humidity sensor.²⁷ As shown in Fig. 10(a), the fabrication of the DCI humidity sensor is completed by directly coating the DCI on the surface of the polyethylene terephthalate interdigital electrodes with a painting brush. Fig. 10(b) shows the dynamic response curve of the DCI humidity sensor under different relative humidity (RH), indicating that it has wide detection range (0–95% RH), low detection limit (2% RH), and high detection resolution (1% RH). The excellent humidity sensing performances of the DCI humidity sensor are mainly attributed to the good hydrophilicity and dispersibility of DCI, which are conducive to the adsorption of water molecules and the expansion between carbon nanoparticles. For most resistance-type humidity sensors, the resistance of the humidity sensor will decrease due to the enhancement of ion conduction (OH⁻, H₃O⁺) after the adsorption of water molecules.^72–75 Interestingly, the resistance of the DCI humidity sensor increases with the increase in RH (Fig. 10(b)). It should be noted that the humidity sensing film formed by DCI has a very small resistance of about 530 Ω (dry air). Fig. 10(c) shows the schematic diagram of the humidity sensing mechanism of the DCI humidity sensor. Under dry conditions, the conduction between carbon nanoparticles is very easy. When water molecules are adsorbed on the surface of carbon nanoparticles, the molecular water membrane will be formed on the surface of carbon nanoparticles to hinder electron tunneling. Therefore, the DCI humidity sensor presents a positive humidity sensing response. In addition, the conductive characteristic of DCI can be used to fabricate the electrodes of the humidity sensor. We fabricated a high-performance flexible paper-based humidity sensor by depositing DCI on the paper surface as the electrodes.^31,80

Fig. 10 (a) Fabrication process of the DCI humidity sensor. (b) Dynamic response curve of the DCI humidity sensor under different RH. (c) Schematic diagram of the humidity sensing mechanism of the DCI humidity sensor. Reproduced with permission from ref. 27. Copyright 2021 Elsevier.

In addition to resistance-type humidity sensors, voltage-type humidity sensors with potential self-powered capability have also attracted much attention in recent years.^81–86 Recently, Li et al. reported a self-powered DCI/paper flexible humidity sensor.³²Fig. 11(a) shows the facile fabrication process of the self-powered DCI/paper humidity sensor, including dipping, drying, and pasting steps. As shown in Fig. 11(b) and (c), the humidity sensor has good linear and dynamic responses in the range of 11–98% RH. The humidity sensing response of the self-powered DCI/paper humidity sensor mainly depends on ion diffusion. As shown in Fig. 11(d), the water molecules adsorbed on the surface of DCI/paper dissociate to form H⁺, and then H⁺ diffuses from top to bottom to form an ion gradient, thus forming a voltage between the upper and lower electrodes and generating humidity sensing response. Benefiting from the good humidity sensing performance and human body compatibility, the DCI/paper humidity sensor can be used to monitor respiratory rates (Fig. 11(e)). In addition, Zhang et al. developed a DCI/wood humidity power generation sensor by combining DCI with wood and polyethylene terephthalate (PET) mesh, which can generate 250 mV at 50–60% RH.³⁵ DCI mainly acts as a conductive channel in the self-powered humidity sensor, which is used to reduce the resistance of the sensor and then increase the output current and power.

Fig. 11 (a) Fabrication process of the self-powered DCI/paper humidity sensor. (b) Linear response fitting line. (c) Dynamic output current curve of the sensor under different RH. (d) Sensing mechanism of the self-powered DCI/paper humidity sensor. (e) Real-time response of the self-powered DCI/paper humidity sensor to different respiratory rates. Reproduced with permission from ref. 32. Copyright 2022 American Chemical Society.

3.4 Others

The above content mainly reviewed and discussed the applications of the DCI in common pressure, strain, and humidity sensors. In addition, DCI can also be used to fabricate other sensing devices, such as temperature sensors and various actuators.^87–90 For example, Liu et al. reported a temperature sensor based on DCI/glue composites.⁸⁷ The DCI/glue conductive composites are obtained by a simple mixing method. Fig. 12(a) shows the schematic diagram of the DCI/glue sensor for temperature detection. Accordingly, Fig. 12(b) shows the response of the DCI/glue sensor under different temperatures, indicating that the DCI/glue sensor with a 1 : 1 ratio has a good linear response in 15–80 °C. With the increase in temperature, the resistance of the DCI/glue sensor gradually decreases because the DCI/glue conductive network shrinks due to heat, and electron tunneling between carbon nanoparticles becomes easier, resulting in the decrease in the resistance of the sensor with the increase in temperature. Further, Fig. 12(c) shows a simple application demonstration circuit diagram of the DCI/glue sensor. By observing the brightness of the LED, we can know the temperature of the environment (Fig. 12(d)).

Fig. 12 (a) Schematic diagram of the DCI/glue sensor for temperature detection. (b) Response of the DCI/glue sensor under different temperatures. (c) Application demonstration circuit diagram and (d) optical photographs of the DCI/glue sensor. Reproduced with permission from ref. 87. Copyright 2022 American Chemical Society.

Actuators that can sense external stimuli (e.g., light, heat, humidity, magnetism, and electricity) have important application prospects in software robots, human–machine interaction, and other fields.^91–94 DCI can be used in light-driven actuators because of its good light absorption and photothermal conversion characteristics.^88–90 On the basis of previous research on graphene and carbon nanotube-based actuators,^95,96 Chen's research group took the lead in studying DCI-based actuators.^88,89 In 2021, they proposed a dual driven (light and humidity) DCI/graphene oxide (GO) actuator.⁸⁸ The DCI/GO double-layer actuator is fabricated by vacuum filtration. As shown in Fig. 13(a), under near-infrared (NIR) light irradiation, the DCI/GO actuator will bend toward the GO side, which is caused by the mismatch of the volume change between the DCI and GO film. Conversely, under humidity irradiation, the DCI/GO actuator will bend toward the DCI side, which is caused by the mismatch of the hygroscopic expansion between the DCI and GO film (Fig. 13(b)). Compared with DCI, the GO film has stronger water molecule absorption and expansion capacity. Inspired by natural creatures, humidity-driven artificial mimosa can be prepared using the DCI/GO double-layer film. As shown in Fig. 13(c), when the finger (the surface of the finger contains a certain amount of water) is close to the bionic mimosa, the bionic leaves will bend but the gloved fingers will not. The DCI/GO actuator is expected to be used in non-contact switch and human–machine interaction. Soon afterward, Chen's research group proposed a DCI/waterborne polyurethane composite for reprogrammable, light-driven, and sensing actuators.⁸⁹ Combined with the NIR light-driven function of the DCI/waterborne polyurethane, a somatosensory light-driven butterfly robot is developed.

Fig. 13 Schematic diagram and optical photographs of (a) light-driven and (b) humidity-driven DCI/GO actuator. (c) Application of humidity-driven DCI/GO actuator. Reproduced with permission from ref. 88. Copyright 2021 The Royal Society of Chemistry.

In addition, Gao et al. proposed a humidity-driven actuator based on polyvinyl butyral (PVB)/DCI/cellulose paper (PDC) sandwiched structure.⁹⁰ The humidity-driven response of the PDC actuator originates from the hygroscopicity difference between the hydrophobic PVB and hydrophilic cellulose paper. Under the humidity condition, cellulose paper will swell after absorbing water molecules; thus, the PDC actuator will bend to the side of PVB. The DCI in the middle layer of the PDC actuator is used for photothermal conversion. Under NIR irradiation, the temperature of the PDC actuator increases, which is conducive to the desorption of water molecules so that the PDC actuator can recover quickly. Finally, the potential application of the PDC actuator in bionic flower was verified by cutting it into a flower shape and simulating the opening process of a flower.

4. Conclusion and outlooks

In this review, we systematically summarized and discussed the applications of DCI in various sensors. Initially, the basic characteristics of DCI are introduced and discussed: (1) name, brand, price, and color of DCI; (2) the morphology, specific surface area, crystal structure, composition, conductivity, dispersibility, stability, and hydrophilicity of DCI are analyzed based on various characterization results. Secondly, the advances of DCI in sensors (including pressure, strain, humidity, and temperature sensors) and actuators are particularly reviewed and discussed based on its the physical and chemical characteristics. In all, DCI, whose main solid component is carbon black nanoparticles, has the basic properties of amorphous carbon material; thus, it has potential application prospects in sensors and related electronic devices. Compared with other carbon materials (graphene, carbon nanotubes, and their derivatives), DCI has obvious advantages, such as mature industrial preparation technology, extremely low cost, good dispersion, and stability. It can be predicted that DCI can be applied to low-cost electronic devices in the future in addition to being used for writing. However, opportunities and challenges coexist. For example, DCI has good hydrophilicity, which may affect the electrical properties and long-term stability of DCI-based pressure and strain sensors. It is useful to construct a hydrophobic film on the DCI sensing active layer to prevent the influence of humidity and water environment.^97–99 Combined with the above analysis, we finally put forward some prospects in this field (Fig. 14).

Fig. 14 Opportunities and prospects of DCI beyond writing.

According to various characteristics (conductivity, dispersion, adhesion, hydrophilicity, and black) of the DCI, the DCI-based pressure, strain, and humidity sensors and actuators for sensing have been developed successively. In addition to continuing to explore the application of these known characteristics of DCI in other electronic devices, we can also refer to other characteristics of carbon materials (especially carbon black) to investigate its unknown characteristics and new applications.

In view of the differences among different DCI brands and types, it is necessary to develop an e-ink purposefully and on demand according to the industrial production process of DCI. For example, referring to the composition of DCI, Lu's research group synthesized carbon ink specially for fabricating sensors.^38–40 In addition, it is worth noting that the reported DCI-based sensors usually use DCI without further processing. Compared with other excellent carbon composites, the research on DCI composite sensing functional materials is rarely reported. In future research, we can consider combining the good dispersion of DCI with other functional materials, such as emerging two-dimensional materials,^100–104 to improve the sensing performance of DCI-based sensors and expand their new sensing applications.

Carbon materials have been proved to have a wide range of applications.^105–107 Similarly, the applications of DCI in other fields also deserves attention. For example, the applications of DCI in supercapacitors,^108–110 batteries,^111,112 solar thermal collection,^113–115 solar steam generation,¹¹⁶ hydrogen evolution reaction,¹¹⁷ photothermal therapy,¹¹⁸ and water desalination¹¹⁹ have been reported. Tracking the application of carbon materials in other fields, we believe that DCI will show emerging applications in more fields in the future.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work is supported by the National Science Fund for Distinguished Young Scholars (grant no. 62225106), Natural Science Foundation of China (grant no. U19A2070 and 62101105), and Chinese Postdoctoral Science Foundation (grant no. 2020M683289 and 2021T140089).

References

1. Y. Guo, X. Wei, S. Gao, W. Yue, Y. Li and G. Shen, Adv. Funct. Mater., 2021, 31, 2104288.
2. T. Yan, Z. Wang and Z. J. Pan, Curr. Opin. Solid State Mater. Sci., 2018, 22, 213–228.
3. F. R. Baptista, S. A. Belhout, S. Giordani and S. J. Quinn, Chem. Soc. Rev., 2015, 44, 4433–4453.
4. C. Wang, K. Xia, H. Wang, X. Liang, Z. Yin and Y. Zhang, Adv. Mater., 2019, 31, 1801072.
5. V. Georgakilas, J. A. Perman, J. Tucek and R. Zboril, Chem. Rev., 2015, 115, 4744–4822.
6. I. S. Kim, C. E. Shim, S. W. Kim, C. S. Lee, J. Kwon, K. E. Byun and U. Jeong, Adv. Mater., 2022, 34, 2204912.
7. D. Zhu, H. Li, L. Ji, H. Zhou and J. Chen, Chem. Commun., 2021, 57, 11776–11786.
8. S. Sarkar, S. Roy, Y. Hou, S. Sun, J. Zhang and Y. Zhao, ChemSusChem, 2021, 14, 3693–3723.
9. J. Nai, X. Zhao, H. Yuan, X. Tao and L. Guo, Nano Res., 2021, 14, 2053–2066.
10. X. Ma, P. Guo, X. Tong, Y. Zhao, Q. Zhang, P. Ke and A. Wang, Appl. Phys. Lett., 2019, 114, 253502.
11. M. S. Choi, H. G. Na, J. H. Bang, A. Mirzaei, S. Han, H. Y. Lee, S. S. Kim, H. W. Kim and C. Jin, Sens. Actuators, B, 2021, 326, 128801.
12. Z. Han, H. Li, J. Xiao, H. Song, B. Li, S. Cai, Y. Chen, Y. Ma and X. Feng, ACS Appl. Mater. Interfaces, 2019, 11, 33370–33379.
13. X. Chen, F. Wang, L. Shu, X. Tao, L. Wei, X. Xu, Q. Zeng and G. Huang, Mater. Des., 2022, 221, 110926.
14. Y. Liu, H. Zheng and M. Liu, Mater. Des., 2018, 141, 276–285.
15. X. Wang, X. Liu and D. W. Schubert, Nano-Micro Lett., 2021, 13, 64.
16. Y. Zhai, Y. Yu, K. Zhou, Z. Yun, W. Huang, H. Liu, Q. Xia, K. Dai, G. Zheng, C. Liu and C. Shen, Chem. Eng. J., 2020, 382, 122985.
17. W. Dong, W. Li, N. Lu, F. Qu, K. Vessalas and D. Sheng, Composites, Part B, 2019, 178, 107488.
18. M. Hu, Y. Gao, Y. Jiang, H. Zeng, S. Zeng, M. Zhu, G. Xu and L. Sun, Adv. Compos. Hybrid Mater., 2021, 4, 514–520.
19. J. Hu, J. Yu, Y. Li, X. Liao, X. Yan and L. Li, Nanomaterials, 2020, 10, 664.
20. X. Lu, Y. Qin, X. Chen, C. Peng, Y. Yang and Y. Zeng, J. Mater. Chem. C, 2022, 10, 6296–6305.
21. S. Sun, Y. Liu, X. Chang, Y. Jiang, D. Wang, C. Tang, S. He, M. Wang, L. Guo and Y. Gao, J. Mater. Chem. C, 2020, 8, 2074–2085.
22. J. Kim, J. H. Cho, H. M. Lee and S. M. Hong, Sensors, 2021, 21, 1974.
23. Y. Xiao, S. Jiang, Y. Li and W. Zhang, Smart Mater. Struct., 2021, 30, 025035.
24. F. Arduini, S. Cinti, V. Mazzaracchio, V. Scognamiglio, A. Amine and D. Moscone, Biosens. Bioelectron., 2020, 156, 112033.
25. T. A. Silva, F. C. Moraes, B. C. Janegitz and O. Fatibello-Filho, J. Nanomater., 2017, 2017, 1–14.
26. Z. Duan, Y. Jiang, S. Wang, Z. Yuan, Q. Zhao, G. Xie, X. Du and H. Tai, ACS Sustainable Chem. Eng., 2019, 7, 17474–17481.
27. Z. Duan, Y. Jiang, Q. Zhao, Q. Huang, S. Wang, Y. Zhang, Y. Wu, B. Liu, Z. Yuan and H. Tai, Sens. Actuators, B, 2021, 339, 129884.
28. J. Martín-Gil, M. C. Ramos-Sánchez and F. J. Martín-Gil, J. Chem. Educ., 2006, 83, 1476.
29. J. R. Swider, V. A. Hackley and J. Winter, J. Cult. Herit., 2003, 4, 175–186.
30. X. Li, C. Guan, H. Yan, M. Shi, W. Yang, Q. Ding and X. Zuo, IOP Conf. Ser.: Mater. Sci. Eng., 2020, 555, 012121,  DOI:10.1088/1755-1315/555/1/012121.
31. Z. Duan, Y. Jiang, Q. Huang, S. Wang, Y. Wang, H. Pan, Q. Zhao, G. Xie, X. Du and H. Tai, Smart Mater. Struct., 2021, 30, 055012.
32. X. Li, Y. Guo, J. Meng, X. Li, M. Li and D. Gao, Langmuir, 2022, 38, 8232–8240.
33. Z. Duan, Y. Jiang, Q. Huang, S. Wang, Q. Zhao, Y. Zhang, B. Liu, Z. Yuan, Y. Wang and H. Tai, Cellulose, 2021, 28, 6389–6402.
34. Z. Duan, Y. Jiang, Q. Huang, Q. Zhao, Z. Yuan, Y. Zhang, S. Wang, B. Liu and H. Tai, J. Mater. Chem. C, 2021, 9, 14003–14011.
35. J. Zhang, Y. Hou, Y. Li and S. Hu, J. Mater. Res. Technol., 2022, 17, 1822–1830.
36. X. Zhang, J. Li, J. Lin, W. Li, W. Chu and X. Wang, J. Mater. Sci.: Mater. Electron., 2022, 33, 13731–13742.
37. Y. Feng, J. Li, R. Tian and J. Yao, J. Energy Chem., 2021, 53, 433–440.
38. S. Bi, L. Hou, H. Zhao, L. Zhu and Y. Lu, J. Mater. Chem. A, 2018, 6, 16556–16565.
39. S. Bi, L. Hou, W. Dong and Y. Lu, ACS Appl. Mater. Interfaces, 2021, 13, 2100–2109.
40. S. Bi, W. Dong, B. Lan, H. Zhao, L. Hou, L. Zhu, Y. Xu and Y. Lu, Composites, Part A, 2019, 124, 105452.
41. S. J. Kahng, C. Cerwyn, B. M. Dincau, J. H. Kim, I. V. Novosselov, M. P. Anantram and J. H. Chung, Nanotechnology, 2018, 29, 335304.
42. Y. Yi, A. Samara and B. Wang, J. Mater. Sci., 2021, 56, 607–614.
43. L. Y. Xu, G. Y. Yang, H. Y. Jing, J. Wei and Y. D. Han, Nanotechnology, 2014, 25, 055201.
44. A. A. Arbab, A. A. Memon, K. C. Sun, J. Y. Choi, N. Mengal, I. A. Sahito and S. H. Jeong, J. Colloid Interface Sci., 2019, 539, 95–106.
45. J. W. Han, B. Kim, J. Li and M. Meyyappan, Mater. Res. Bull., 2014, 50, 249–253.
46. H. Kimura, Y. Nakayama, A. Tsuchida and T. Okubo, Colloids Surf., B, 2007, 56, 236–240.
47. S. Wei, X. Fang, J. Yang, X. Cao, V. Pintus, M. Schreiner and G. Song, J. Cult. Herit., 2012, 13, 448–452.
48. Y. Fu, X. Cai, H. Wu, Z. Lv, S. Hou, M. Peng, X. Yu and D. Zou, Adv. Mater., 2012, 24, 5713–5718.
49. Q. C. Liu, L. Li, J. J. Xu, Z. W. Chang, D. Xu, Y. B. Yin, X. Y. Yang, T. Liu, Y. S. Jiang, J. M. Yan and X. B. Zhang, Adv. Mater., 2015, 27, 8095–8101.
50. X. Cai, Z. Lv, H. Wu, S. Hou and D. Zou, J. Mater. Chem., 2012, 22, 9639.
51. M. Okamura, A. Takagaki, M. Toda, J. N. Kondo, K. Domen, T. Tatsumi, M. Hara and S. Hayashi, Chem. Mater., 2006, 18, 3039–3045.
52. M. Wang, Z. Xu, Y. Hou, P. Li, H. Sun and Q. J. Niu, Sep. Purif. Technol., 2020, 251, 117408.
53. X. Duan, Z. Duan, Y. Zhang, B. Liu, X. Li, Q. Zhao, Z. Yuan, Y. Jiang and H. Tai, Sens. Actuators, B, 2022, 369, 132302.
54. H. Wang, Z. Li, Z. Liu, J. Fu, T. Shan, X. Yang, Q. Lei, Y. Yang and D. Li, J. Mater. Chem. C, 2022, 10, 1594–1605.
55. Y. He, M. Zhou, M. H. H. Mahmoud, X. Lu, G. He, L. Zhang, M. Huang, A. Y. Elnaggar, Q. Lei, H. Liu, C. Liu and I. H. E. Azab, Adv. Compos. Hybrid Mater., 2022, 5, 1939–1950.
56. Y. Zhao, L. Liu, Z. Li, F. Wang, X. Chen, J. Liu, C. Song and J. Yao, J. Mater. Chem. C, 2021, 9, 12605–12614.
57. Z. Duan, Y. Jiang, Q. Huang, Z. Yuan, Q. Zhao, S. Wang, Y. Zhang and H. Tai, J. Mater. Chem. C, 2021, 9, 13659–13667.
58. Q. Zheng, J. H. Lee, X. Shen, X. Chen and J. K. Kim, Mater. Today, 2020, 36, 158–179.
59. C. Liu, Q. Tan, L. Kong, Y. Deng, X. Ma, L. Xu, Y. Xu, W. Zhou, L. Zhong, Q. Qiang, T. Han, W. Chen and B. Liu, Mater. Lett., 2023, 330, 133331.
60. X. Cao, H. Liu, J. Cai, L. Chen, X. Yang and M. Liu, Composites, Part A, 2021, 149, 106535.
61. K. Xia, C. Du, Z. Zhu, R. Wang, H. Zhang and Z. Xu, Appl. Mater. Today, 2018, 13, 190–197.
62. M. Zhu, X. Du, S. Liu, J. Li, Z. Wang and T. Ono, J. Mater. Chem. C, 2021, 9, 9083–9101.
63. Q. Huang, Y. Jiang, Z. Duan, Z. Yuan, B. Liu, Q. Zhao, Y. Zhang, Y. Sun, P. Sun and H. Tai, J. Phys. D: Appl. Phys., 2021, 54, 284003.
64. H. Wu, H. Qi, X. Wang, Y. Qiu, K. Shi, H. Zhang, Z. Zhang, W. Zhang and Y. Tian, J. Mater. Chem. C, 2022, 10, 8206–8217.
65. D. Zhang, R. Yin, Y. Zheng, Q. Li, H. Liu, C. Liu and C. Shen, Chem. Eng. J., 2022, 438, 135587.
66. S. Yang, C. Li, X. Chen, Y. Zhao, H. Zhang, N. Wen, Z. Fan and L. Pan, ACS Appl. Mater. Interfaces, 2020, 12, 19874–19881.
67. E. Aslanidis, E. Skotadis and D. Tsoukalas, Nanoscale, 2021, 13, 3263–3274.
68. B. Park, J. Kim, D. Kang, C. Jeong, K. S. Kim, J. U. Kim, P. J. Yoo and T. I. Kim, Adv. Mater., 2016, 28, 8130–8137.
69. C. Zhang, J. Sun, Y. Lu and J. Liu, J. Mater. Chem. C, 2021, 9, 754–772.
70. Y. Bu, T. Shen, W. Yang, S. Yang, Y. Zhao, H. Liu, Y. Zheng, C. Liu and C. Shen, Sci. Bull., 2021, 66, 1849–1857.
71. Q. Li, H. Liu, S. Zhang, D. Zhang, X. Liu, Y. He, L. Mi, J. Zhang, C. Liu, C. Shen and Z. Guo, ACS Appl. Mater. Interfaces, 2019, 11, 21904–21914.
72. Z. Duan, Q. Zhao, S. Wang, Q. Huang, Z. Yuan, Y. Zhang, Y. Jiang and H. Tai, Sens. Actuators, B, 2020, 317, 128204.
73. C. Pi, W. Chen, W. Zhou, S. Yan, Z. Liu, C. Wang, Q. Guo, J. Qiu, X. Yu, B. Liu and X. Xu, J. Mater. Chem. C, 2021, 9, 11299–11305.
74. Z. Duan, Y. Jiang, Q. Zhao, S. Wang, Z. Yuan, Y. Zhang, B. Liu and H. Tai, Nanotechnology, 2020, 31, 355501.
75. Z. Duan, Q. Zhao, S. Wang, Z. Yuan, Y. Zhang, X. Li, Y. Wu, Y. Jiang and H. Tai, Sens. Actuators, B, 2020, 305, 127534.
76. Z. Duan, Y. Jiang and H. Tai, J. Mater. Chem. C, 2021, 9, 14963–14980.
77. Z. Duan, Y. Jiang, M. Yan, S. Wang, Z. Yuan, Q. Zhao, P. Sun, G. Xie, X. Du and H. Tai, ACS Appl. Mater. Interfaces, 2019, 11, 21840–21849.
78. Y. Zhang, Y. Wu, Z. Duan, B. Liu, Q. Zhao, Z. Yuan, S. Li, J. Liang, Y. Jiang and H. Tai, Appl. Surf. Sci., 2022, 585, 152698.
79. Y. Jiang, Z. Duan, Z. Fan, P. Yao, Z. Yuan, Y. Jiang, Y. Cao and H. Tai, Sens. Actuators, B, 2023, 380, 133396.
80. Z. Duan, Y. Jiang, S. Wang, Q. Zhao, Y. Zhang, Z. Yuan, H. Tai and J. Liu, Proc. SPIE, 2020, 1161715,  DOI:10.1117/12.2585014.
81. Z. Duan, Z. Yuan, Y. Jiang, Q. Zhao, Q. Huang, Y. Zhang, B. Liu and H. Tai, Chem. Eng. J., 2022, 446, 136910.
82. D. Zhang, Z. Xu, Z. Yang and X. Song, Nano Energy, 2020, 67, 104251.
83. D. Wang, D. Zhang, P. Li, Z. Yang, Q. Mi and L. Yu, Nano-Micro Lett., 2021, 13, 57.
84. Q. Zhao, Y. Jiang, Z. Duan, Z. Yuan, J. Zha, Z. Wu, Q. Huang, Z. Zhou, H. Li, F. He, Y. Su, C. Tan and H. Tai, Chem. Eng. J., 2022, 438, 135588.
85. Q. Zhao, Z. Duan, Y. Wu, B. Liu, Z. Yuan, Y. Jiang and H. Tai, Sens. Actuators, B, 2022, 370, 132369.
86. F. Zhao, H. Cheng, Z. Zhang, L. Jiang and L. Qu, Adv. Mater., 2015, 27, 4351–4357.
87. B. Liu, Y. Zhou, L. Chen, Y. Feng and M. Liu, ACS Sustainable Chem. Eng., 2022, 10, 1847–1856.
88. J. Lin, P. Zhou, Z. Wen, W. Zhang, Z. Luo and L. Chen, Nanoscale, 2021, 13, 20134–20143.
89. J. Lin, P. Zhou, Q. Chen, W. Zhang, Z. Luo and L. Chen, Sens. Actuators, B, 2022, 362, 131776.
90. J. Gao, X. Zhao, J. Wen, D. Hu, R. Li and K. Wang, Adv. Mater. Technol., 2021, 6, 2100044.
91. N. El-Atab, R. B. Mishra, F. Al-Modaf, L. Joharji, A. A. Alsharif, H. Alamoudi, M. Diaz, N. Qaiser and M. M. Hussain, Adv. Intell. Syst., 2020, 2, 2000128.
92. Y. Chen, J. Yang, X. Zhang, Y. Feng, H. Zeng, L. Wang and W. Feng, Mater. Horiz., 2021, 8, 728–757.
93. J. Walker, T. Zidek, C. Harbel, S. Yoon, F. S. Strickland, S. Kumar and M. Shin, Actuators, 2020, 9, 3.
94. M. Zou, S. Li, X. Hu, X. Leng, R. Wang, X. Zhou and Z. Liu, Adv. Funct. Mater., 2021, 31, 2007437.
95. L. Chen, M. Weng, P. Zhou, F. Huang, C. Liu, S. Fan and W. Zhang, Adv. Funct. Mater., 2019, 29, 1806057.
96. Y. Xiao, J. Lin, J. Xiao, M. Weng, W. Zhang, P. Zhou, Z. Luo and L. Chen, Nanoscale, 2021, 13, 6259–6265.
97. H. Liu, Q. Li, Y. Bu, N. Zhang, C. Wang, C. Pan, L. Mi, Z. Guo, C. Liu and C. Shen, Nano Energy, 2019, 66, 104143.
98. S. Yang, W. Yang, R. Yin, H. Liu, H. Sun, C. Pan, C. Liu and C. Shen, Chem. Eng. J., 2023, 453, 139716.
99. H. Sun, Y. Bu, H. Liu, J. Wang, W. Yang, Q. Li, Z. Guo, C. Liu and C. Shen, Sci. Bull., 2022, 67, 1669–1678.
100. T. Tang, Z. Li, Y. F. Cheng, K. Xu, H. G. Xie, X. X. Wang, X. Y. Hu, H. Yu, B. Y. Zhang, X. W. Tao, C. M. Hung, N. D. Hoa, G. Y. Chen, Y. X. Li and J. Z. Ou, J. Mater. Chem. A, 2023, 11, 6361–6374.
101. H. Xie, Z. Li, L. Cheng, A. A. Haidry, J. Tao, Y. Xu, K. Xu and J. Z. Ou, iScience, 2022, 25, 103598.
102. Y. Cheng, Z. Li, T. Tang, K. Xu, H. Yu, X. Tao, C. M. Hung, N. D. Hoa, Y. Fang, B. Ren, H. Chen and J. Z. Ou, Appl. Mater. Today, 2022, 26, 101355.
103. Q. Zhao, Y. Zhang, Z. Duan, S. Wang, C. Liu, Y. Jiang and H. Tai, Rare Met., 2021, 40, 1459–1476.
104. T. Tang, Z. Li, Y. F. Cheng, H. G. Xie, X. X. Wang, Y. L. Chen, L. Cheng, Y. Liang, X. Y. Hu, C. M. Hung, N. D. Hoa, H. Yu, B. Y. Zhang, K. Xu and J. Z. Ou, J. Hazard. Mater., 2023, 451, 131184.
105. Q. Yi, B. Ren, P. Han, Y. Luan, K. Xu, Y. Yang, H. Yu, F. Yang, B. Y. Zhang, I. Jeerapan, C. Thammakhet-Buranachai and J. Z. Ou, J. Colloid Interface Sci., 2022, 629, 960–969.
106. B. Ren, Q. Yi, F. Yang, Y. Cheng, H. Yu, P. Han, Y. Yang, G. Chen, I. Jeerapan, Z. Li and J. Z. Ou, Energy Fuels, 2022, 36, 8381–8390.
107. E. Haque, A. Zavabeti, N. Uddin, Y. Wang, M. A. Rahim, N. Syed, K. Xu, A. Jannat, F. Haque, B. Y. Zhang, M. A. Shoaib, S. Shamsuddin, M. Nurunnabi, A. I. Minett, J. Z. Ou and A. T. Harris, Chem. Mater., 2020, 32, 1384–1392.
108. D. Harrison, F. Qiu, J. Fyson, Y. Xu, P. Evans and D. Southee, Phys. Chem. Chem. Phys., 2013, 15, 12215–12219.
109. X. Dong, J. Liang, H. Li, Z. Wu, L. Zhang, Y. Deng, H. Yu, Y. Tao and Q. H. Yang, Nano Energy, 2021, 80, 105523.
110. C. Shi, Q. Zhao, H. Li, Z. M. Liao and D. Yu, Nano Energy, 2014, 6, 82–91.
111. L. Zhou, L. Sun, P. Fu, C. Yang and Y. Yuan, J. Mater. Chem. A, 2017, 5, 14741–14747.
112. H. Wu, H. Tan, L. Chen, B. Yang, Y. Hou, L. Lei and Z. Li, Chin. Chem. Lett., 2021, 32, 2499–2502.
113. H. Wang, W. Yang, L. Cheng, C. Guan and H. Yan, Sol. Energy Mater. Sol. Cells, 2018, 176, 374–380.
114. H. Wang, Y. An, L. Cheng, H. Yan, C. Guan and W. Yang, Sol. Energy Mater. Sol. Cells, 2018, 183, 151–156.
115. Q. Ding, C. Guan, H. Li, M. Shi, W. Yang, H. Yan, X. Zuo, Y. An, S. Ramakrishna, P. Mohankumar and F. Zhang, Sol. Energy, 2020, 195, 636–643.
116. H. C. Yang, Z. Chen, Y. Xie, J. Wang, J. W. Elam, W. Li and S. B. Darling, Adv. Mater. Interfaces, 2018, 6, 1801252.
117. X. Fan, H. Zhang, B. Gao, H. Lu, L. Zheng, X. Yang, Y. Zhang, Q. Gao and Y. Tang, Carbon, 2020, 170, 558–566.
118. S. Wang, Y. Cao, Q. Zhang, H. Peng, L. Liang, Q. Li, S. Shen, A. Tuerdi, Y. Xu, S. Cai and W. Yang, ACS Omega, 2017, 2, 5170–5178.
119. X. F. Zhang, Z. Wang, L. Song, Y. Feng and J. Yao, Desalination, 2020, 496, 114727.

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