Sanjit Manohar
Majhi†
ab,
Sachin T.
Navale†
abc,
Ali
Mirzaei
d,
Hyoun Woo
Kim
*a and
Sang Sub
Kim
*b
aDivision of Materials Science and Engineering and The Research Institute of Industrial Science, Hanyang University, Seoul 04763, South Korea. E-mail: hyounwoo@hanyang.ac.kr
bDepartment of Materials Science and Engineering, Inha University, Incheon 22212, South Korea. E-mail: sangsub@inha.ac.kr
cDepartment of Electronic and Biomedical Engineering and Institute of Nanoscience and Nanotechnology, University of Barcelona, 08028 Barcelona, Spain
dDepartment of Materials Science and Engineering, Shiraz University of Technology, Shiraz 715557-13876, Iran
First published on 3rd May 2023
The development of effective and efficient materials for the selective and sensitive detection of toxic gases and volatile organic compounds is crucial for protecting human health and the environment. In this respect, a well-known n-type semiconducting material, namely indium oxide (In2O3), has attracted significant attention because of its gas-sensing applications. The rapid advances in various synthesis techniques have enabled researchers to explore numerous novel nanostructures and their integration into smart gas-sensing devices. Despite sustainable development, the application of In2O3 in gas sensing is limited by its poor selectivity, high working temperature, and response deterioration under humid conditions. This review outlines various strategies, such as morphology and interface engineering, catalytic functionalization, shell structure and thickness, and doping, for improving the gas detection performance of In2O3-based chemiresistive gas sensors. The significant influence of the nanostructures with different morphologies on the gas-sensing performance of In2O3-based sensors is also demonstrated. Pristine In2O3 nanomaterials with zero-dimensional (0D) to three-dimensional (3D) morphologies are reviewed. Different composites of In2O3, including In2O3/metal oxides (p-type and n-type), In2O3/noble metal loading and encapsulation, In2O3/elemental doping, In2O3/conducting polymers, and In2O3/carbonaceous materials, were evaluated to improve their sensing performances. Finally, a future outlook on the further progress of the In2O3 gas sensors is suggested.
Several synthetic strategies have been used to synthesize In2O3 with various surface morphologies from 0D to 3D morphologies with many attractive nanostructures.43–46 Furthermore, other approaches, e.g., the utilization of p–n and n–n heterojunction nanostructures, elemental doping, noble metal loading, and utilization of composites with conducting polymers or carbon materials, have been developed. Scheme 1 shows various In2O3-based gas-sensing materials from the morphologically controlled synthesis of different nanostructures (0D to 3D) to different composite nanostructures.
There are no detailed reports on the In2O3-based nanostructures for gas detection applications to the best of the authors’ knowledge. The current review comprehensively presents a state-of-the-art design of In2O3 nanomaterials and their detailed gas-sensing properties. First, this paper outlines the basic principle of the sensing mechanism of a chemiresistive sensor. Subsequently, the gas-sensing properties of various interesting morphologies (0D to 3D) emanating from the morphological engineering of In2O3 nanomaterials using different synthetic techniques are summarized methodically. Finally, the current state of progress of the different strategies for preparing composite nanostructures based on In2O3 to improve the gas detection properties is described.
Fig. 1 Sensing mechanism of n-type and p-type gas sensors based on MOS. Reproduced with permission from the ref. 29, copyright 2020, Elsevier. |
For example, at lower working temperatures (room temperature to 150 °C), the adsorption of oxygen molecules takes place in the following manner:
O2(g) + e− ⇌ O2−(ads) | (1) |
At higher working temperatures, however, the O2− anions are dissociated with single/double oxygen ions by taking a further electron from the CB of the sensor, according to the following reactions:
O2−(ads) + e− ⇌ 2O−(ads) (150–300 °C) | (2) |
O−(ads) + e− ⇌ O2−(ads) (>300 °C) | (3) |
As a result, an electron depletion layer or a space charge layer (Δair) is produced because of the lower amounts of electrons. This results in higher sensor resistance in the presence of air. The high sensor resistance decreases upon the interaction with reducing gases, such as H2 (shown in Fig. 2a). Generally, the Debye length (Ld) is demonstrated as the distance between the surface of the MOS sensor at which the electrons are removed.52 The Ld of the MOS sensor depends largely on the operating temperature and concentration of charge carriers,53 as follows:
(4) |
Fig. 2 (a) Schematic diagram of the change in the resistance of n-MOS in the presence of a reducing gas (H2); (b) model of the surface charge layer; (c) model of grain boundary barrier. Reproduced from the ref. 51 under the Creative Commons Attribution 4.0 License (CC BY 4.0/). Copyright 2020, IOP Science. |
The potential barrier height depends mainly on the quantity of oxygen molecules adsorbed.55 The dynamic interactions between a reducing gas (e.g., H2) and chemisorbed oxygen result in a change in carrier concentrations by releasing the electrons back to the sensor surface. This process leads to the appearance of a sensing signal. Overall, the reaction can be shown as follows:51,56
X + On−ads → X′ + ne− | (5) |
The sensing properties of the gas sensors are controlled primarily by three factors: the receptor function, the transducer function, and the utility factor.57
The primary role of the receptor function of the gas sensor is to recognize the oxygen molecules and the target gas in the surrounding environment through a sensing layer. The adsorption of gas molecules depends significantly on the surface area of the detection materials. On the other hand, the surface can be modified according to the change in shape, morphology, and size of the synthesized materials. The transducer function is accountable for transforming the electronic changes that occur due to the interactions among the detection materials and the target gases with respect to a signal output as a change in resistance. Finally, the utility factor facilitates the access of gas molecules by the pores of the detection materials, which influences the response. Control over these factors (Fig. 3) can guide the development of new materials with an appropriate structure for realizing reliable gas sensors.58
Fig. 3 Schematic illustration of the receptor, transducer function, and utility factors of MOS gas sensors. Reproduced from the ref. 58 under the Creative Commons Attribution 4.0 License (CC BY 4.0/). Copyright 2020, MDPI. |
(i) Nitrogen oxide (NO 2 ): NO2, with a threshold limit value (TLV) of 3 ppm,69 is a highly toxic and oxidizing gas that is emitted mainly from car exhausts and different combustion processes.70 Exposure to NO2 causes lung irritations and a decrease in the fixation of oxygen molecules on red blood corpuscles. Furthermore, it causes acid rain.71 At concentrations higher than 10 ppm, it can be highly harmful to the respiratory tract, resulting in emphysema, asthma, and chronic bronchitis.72
(ii) Hydrogen sulfide (H 2 S): H2S is a highly poisonous and flammable gas that commonly collects in basements, manholes, and sewer lines. At concentrations >100 ppm, the gas quickly paralyzes the olfactory nerves, and the sense of odor disappears. Exposure to 250 ppm causes the alveolar membranes to exude fluids that interfere with the normal exchange of gases. The principal symptom is asphyxia, which may lead to suffocation. Inhalation of high concentrations (1000 ppm) of H2S paralyzes the respiratory nerve center, which can also lead to suffocation. Physical collapse may occur without warning.73 Thus, from a safety standpoint, monitoring of H2S is essential in the petrochemical and coal industries.74 Furthermore, H2S is a biomarker. For example, a H2S concentration above 250 ppb in a person's exhaled breath indicates the presence of periodontal disease.75
(iii) Carbon monoxide (CO): CO is an odorless, colorless, and tasteless gas that can kill people silently. It is produced by the incomplete combustion of fuels.76 The inhalation of >15 ppm CO is dangerous, and the TLV of CO is 25 ppm set by the American Conference of Governmental Industrial Hygienists (ACGIH).77 Exposure to 0.5% CO for 10 min can cause immediate death. Currently, more than 15000 intentional CO poisoning occurs each year.78 Furthermore, CO is a biomarker, where the CO concentration in the exhaled breath of sick people is higher than those of normal people.79
(iv) Ammonia (NH 3 ): NH3 is one of the most widely used chemicals. It is emitted mainly from chemical plants and motor vehicles.80 The TLV of NH3 gas is set to 25 ppm.19 Inhaling more than the safe level of NH3 can lead to life-threatening illnesses because of its highly toxic and corrosive properties to the skin, eyes, and lungs.81
(v) Sulfur dioxide (SO 2 ): SO2 is a toxic and corrosive gas that is emitted into the atmosphere from many industrial processes and through natural phenomena.82,83 The TLV of SO2 was set to 5 ppm.84 It is one of the most hazardous air pollutants because it can transform into sulfuric acids, resulting in acid rain. Acid rain can decrease agricultural productivity, affecting flora and fauna and damaging the overall plant ecosystem.85 Human exposure to high levels of SO2 gas can result in cardiovascular disease, respiratory illnesses, and breathing problems.86
(vi) Methane (CH 4 ): CH4 is a colorless, odorless, and flammable gas that is explosive at high concentrations, from 5 to 15% in air. It is mainly used as fuel to make heat and light. Therefore, it is the main component of natural gas.87 In addition, it is a potent greenhouse gas because it is nearly 25 times more powerful than CO2.88 CH4 explosions and CO poisoning are two common gas disasters in coal mining.89
(vii) Carbon dioxide (CO 2 ): CO2 is a greenhouse gas that is emitted mainly from fossil fuels in automobiles, industries, and electric power generators.90 CO2 is used widely in the food industry, fire extinguishers, lasers, and refrigerants owing to its unique physicochemical characteristics (inert and water-soluble, and high density).91 It absorbs the short wavelengths reflected from the earth to space, which increases the earth's temperature, producing droughts, floods, and ice melting at the poles.92 In greenhouse planting, there is a need to detect CO2 in low concentrations (<300 ppm), while in modified atmosphere packaging for fruit and vegetables, high CO2 concentrations (up to 25%) need to be monitored.93
(viii) VOCs and other hazardous gases: In general, VOCs are organic compounds with high vapor pressures and low boiling points, which causes them to be volatile.15,16 The most important VOC pollutants are n-butanol, xylene, acetone, toluene, ethanol, benzene, methanol, and formaldehyde. Commonly used examples of toxic gases include SO2, NH3, CO, H2S, NO2, and trimethylamine. In addition, hydrogen (H2) and methane are considered explosive gases when combined with air. All of these gases are hazardous to the environment and human health. Hence, the design of portable and high-performance sensors is essential for the early detection and monitoring of these potentially lethal gases.
The present review paper discusses the gas-detection properties of In2O3 to different gases, mainly VOCs (such as n-butanol, xylene, acetone, toluene, ethanol, benzene, methanol, and formaldehyde) and toxic gases (such as SO2, NH3, CO, H2S, NO2, and trimethylamine). It is essential to know the details of these gases and their possible effects before discussing their gas detection properties. Thus, the physical properties of these gases, their uses, guidelines on the IDLH (immediately dangerous to life or health),17 and TLV (threshold limit value)18 are presented in Table 1, which summarizes the properties of VOCs and toxic gases and their effects on human health.
Gas name | IDLH | TLV | Human health issues | Properties | Ref. |
---|---|---|---|---|---|
Acetone | 25.00 ppm | 750 ppm | Muscle weakness, dryness in the mouth, fatigue, dizziness, nausea, and narcosis harmful to the nerve system | A colorless liquid, solvent, pharmaceuticals, pungent odor/used to dissolve plastics, as laboratory reagents | 94 |
Formaldehyde | 20 ppm (OSHA) | 0.1–0.3 ppm | Human carcinogen (nasopharyngeal cancer) pulmonary damage and leukemia at 6 ppm | Colorless, flammable | 95 and 96 |
Ethanol | 3300 ppm (NiOSH) | 1000 ppm – STEL (ACGIH) | Difficulty in breathing, eye irritation, drowsiness, and headache | Volatile, colorless, inflammable | 97 |
NO2 | 13 ppm (NIOSH) | 0.3 ppm (ACGIH) | Lung damage, irritating to eyes and produce ozone, | Pungent odor, not flammable | 98 and 99 |
H2S | 100 ppm (NIOSH) | 1–5 ppm (ACGIH) | Highly reactive with hemoglobin causing fatal damage to the olfactory system | Rotten egg smell, colorless, toxic | 100–102 |
NH3 | 300 ppm (NIOSH) | 25 ppm (ACGIH) | Hazardous to skin/eyes/respiratory tract of humans, corrosive and irritant | Colorless, corrosive, and hazardous | 103 |
Cl2 | 10 ppm (NIOSH) | 0.1–0.4 ppm (ACGIH) | Eye irritation (>5 ppm), | Irritating odor, harmful, quickly reacts with water to form corrosive acid, present in the ozone layer, a fatal to a human being | 104 |
Acetic acid | 50 ppm (NIOSH) | 10–15 ppm (ACGIH) | Eyes/nose/throat irritation, cough, chest tightness, fever, confusion, headache | Colorless, heavily vinegar-like smell, flammable | 105 |
CO | 1200 ppm (NIOSH) | 50 ppm (OSHA) and 35 ppm (NIOSH) | Headache, loss of consciousness, fatal death due to the binding of hemoglobin and reduction oxygen transportation, dizziness collapse, nausea | Colorless, odorless, tasteless, non-irritating/— | 106 and 107 |
Trimethylamine (TMA) | — | 15 ppm – STEL (OSHA) | Irritation to the throat, nose, skin, and eyes | Blurred vision, irritation of the eyes/skin/nose/throat, respiratory system, and cough | 108 and 109 |
H2 | NA | NA | Stinging of the nose and throat, vomiting, pure form is a chemical asphyxiant, dizziness, headaches, drowsiness, and nausea | Highly flammable and colorless, metal smelting, tasteless, low minimum ignition energy (0.017 mJ), nontoxic, explosive/used as fuel in vehicles, petroleum extraction, glass making | 110 |
2H2S + 3O2− ⇌ 2H2O(g) + 2SO2(g) + 3e− | (6) |
When an In2O3 NP sensor is exposed to H2S, it interacts with O2− ions to discharge the initially trapped electrons to increase the conductivity and decrease the resistance. Sulfurization occurs on the In2O3 NP sensor surface. When the sensor is exposed to H2S gas, an In2S3 layer can be formed on the In2O3 sensor surface:
In2O3(s) + 3H2S(g) → In2S3(s) + 3H2O(g) | (7) |
The change in Gibbs free energies for the formation of In2S3 from In2O3 is −161.7 kJ mol−1 at room temperature, which is spontaneous and thermodynamically stable.113 The good conductivity of In2S3 increases the electrical current remarkably and yields a strong response to H2S. Therefore, the enhanced response of porous In2O3 NPs to H2S is due to the oxidation of surface adsorption of oxygen and the development of In2S3 through the sulfuration mechanism (eqn (7)).
Oxygen vacancies (VO) are one of the defect structures of MOS that plays an important role as far as the sensing performance is concerned. On the other hand, designing nanomaterials with improved detection performance by manipulating VO is a significant challenge because the role of surface/bulk VO on the detection performance of MOS is unclear. Gu et al.43 studied the roles of different types of VO (bulk and surface) on the sensing performances of the In2O3 NP sensor. Four In2O3 NP sensors were prepared by heating at 550 °C in the presence of H2 and He at different time intervals (0/10/15/30 min). Among the different sensors, the In2O3-based sensor, heat treated for 10 min (called In2O3-H10), exhibited a strong response to formaldehyde (HCHO) at 230 °C with a rapid τres of 100 s and τrec of 70 s. Fig. 4 presents a schematic diagram of the HCHO sensing mechanism of H2-treated In2O3.
Fig. 4 Schematic diagram of HCHO sensing on the surface of the H2-treated In2O3. Reproduced from ref. 43 with permission from the American Chemical Society, Copyright 2018. |
The HCHO gas-sensing reaction can be expressed as follows:
HCHO(g) + 2O−(ads) → CO2(g) + H2O(g) + 3e− | (8) |
The enhanced HCHO sensing performance of In2O3-H10 is due mainly to the presence of bulk VO. In contrast, surface VO does not help improve the HCHO performance. This is because the bulk VO contains a narrow bandgap and a small energy barrier, which promote electron mobility and the development of chemisorbed oxygen ions that contribute to an increased HCHO response. On the other hand, high surface VO contents are not needed to enhance the detection performance.
Li et al.114 examined the advantages of mesoporous In2O3 NPs prepared through the hydrothermal process for gas-sensing studies. The pore size and surface area of the prepared In2O3 were 31 nm and 23.5 m2 g−1, respectively. The response and τres/τrec of the In2O3 sensor at 260 °C were 18 and 1.7 s/1.5 s for 500 ppm H2, respectively, with a detection limit of 0.01 ppm. The good H2 gas-sensing characteristics were credited to the unique mesoporous structure, large surface area, and a large amount of chemisorbed oxygen on the surface of porous In2O3.
Xiao et al.115 synthesized 40–60 nm sized In2O3 nanospheres packed with numerous NPs (8 nm in size) via a solvothermal process for NO2 sensing applications. The developed sensor exhibited an excellent response (Ra/Rg) of 217.5 to 500 ppb NO2 gas with good selectivity, excellent repeatability, superior stability, and a lower detection limit of 10 ppb at a working temperature of 120 °C. The improved gas response was attributed mainly to the higher surface area and porous structure of the synthesized powders. The results indicated that it could be exploited as a capable sensing material for low-ppm NO2 gas sensors.
Acetaldehyde, a VOC gas, can cause severe health-related problems.116 In this respect, acetaldehyde gas-sensing was studied using In2O3 NPs fabricated using a microwave hydrothermal method.117a The prepared In2O3 NP sensor exhibited a strong response (Ra/Rg = 11.06) to acetaldehyde (to 100 ppm), with a detection limit as low as 1 ppm, at a working temperature of 300 °C.
Despite its importance, realizing dual gas selectivity on a chemiresistive gas sensor based on metal oxides remains challenging. In this regard, Jin et al.117b reported the potential of In2O3 hierarchical porous nanospheres (HPNSs) for achieving dual gas selectivity to C2H5OH and TEA. In2O3 HPNSs were fabricated using a sacrificial template method by annealing In (OH)3 NPs at different temperatures (400 to 600 °C). The highest oxygen vacancy content was observed at 500 °C, resulting in excellent gas sensitivity to C2H5OH and TEA compared to 400 and 600 °C. The observed dual selectivity at different operating temperatures was attributed to different diffusion behaviors and specific adsorption properties of C2H5OH and TEA molecules. The results highlighted the significance of hierarchical porous structures in increasing the sensitivity of metal oxides and achieving dual gas selectivity for sensing different gases.
Chao et al.117c used a nitric acid-assisted solvothermal method to transform In2O3 nanosheets into spheres, improving the C2H5OH gas-sensing properties. Specifically, the spheres with partially broken structures and a high surface area of 60.6 m2 g−1 demonstrated the most favorable response of 250 (Ra/Rg) to 50 ppm C2H5OH at 250 °C, along with short response–recovery times of 16 and 14 s, respectively. These In2O3 spheres exhibited good selectivity, reproducibility, and high stability to 50 ppm C2H5OH, which was attributed mainly to the unique structure of the spheres.
In recent years, colloidal quantum dots (CQDs) have attracted considerable attention for their potential in optoelectronic devices. CQDs possess excellent characteristics, such as solution processability, flexibility to deposit onto various substrates, excellent crystallinity, large surface area, and high surface chemistry.118,119 In addition, the outstanding crystallinity and higher surface-to-volume ratios of CQDs guarantee an enormous active site for the solid–gas interaction. In this respect, Yang et al.119 reported a room temperature (RT) operable H2S gas sensor based on In2O3 CQDs. The CQD sensor revealed a maximum response of 90 to H2S (5 ppm), demonstrating the potential of CQDs in RT gas-sensing studies. The observed RT sensing performance was attributed to the size and surface effects of CQDs that are proficient for advanced surface activities and offer reasonable energy for the adsorption and conversion of oxygen. Furthermore, the dangling bonds on the surface of CQDs are promising for the adsorption of oxygen, which may provide appropriate activation energy for low-temperature transformation into O2− to O− species.119
Fig. 5 Schematic presentation of sensing of In2O3 NFs and In2O3 FNFs to acetone and HCHO. Reproduced from ref. 126, Copyright 2020, Elsevier. |
Wang et al. described acetic acid sensing behaviors of In2O3 NFs.128 Acetic acid irritates the eyes/skin and respiratory system. Hence, the development of acetic acid sensors is important. In2O3 NFs prepared through electrospinning were assessed for their ability to detect acetic acid, which yielded a response of 66.7 for 2000 ppm, and fast τres/τrec (25 s/37 s), tested at 250 °C.128 A typical MOS sensing mechanism regarding the change in sensor resistance upon the interaction of gas was used to clarify the possible interactions between In2O3 NFs and acetic acid gas molecules.
Fig. 6 Schematic diagram of the sensing mechanism of In2O3 nanosheets to reducing and oxidizing gases. Reproduced from ref. 132, Copyright 2020, Elsevier. |
In this case, the sensing mechanism was explained according to the variation of sensor resistance upon the interaction of reducing or oxidizing gases. For example, the gas-sensing reaction for Cl2 (oxidizing gas) can be explained as follows:
Cl2 + 4O2−(ads) ⇌ 2Cl−(ads) + 4O2 + 2e− | (9) |
Cl2 + 2O2−(latt) ⇌ 2Cl−(latt) + O2 + 2e− | (10) |
Herein, the lattice oxygen could be involved at an elevated temperature that causes a reaction in eqn (10). Based on the oxidation capability, Cl2 can also be adsorbed onto the oxygen vacancies of the material surface in the following way:
Cl2 + 2e− ⇌ 2Cl−(surface) | (11) |
Cl2 + 2O(vac) + 2e− ⇌ 2Cl−o | (12) |
Accordingly, the sensor resistance increases as the electrons are captured from the conducting-region. Thus, the detection mechanism of oxidizing gases is dominated by eqn (11) and (12). Herein, oxidizing Cl2 can react with the surface and oxygen vacancies. While reducing gases can only react with pre-adsorbed oxygen species. As a result, oxidizing gases have a stronger response than reducing gases.
Sun et al.45 synthesized In2O3-based nanosheets with porous structures using a calcination process after liquid reflux. The porous In2O3 nanosheet sensor showed a high response and fast τres of 89.48 and 16.6 s, respectively, for 97.0 ppm NO2 at RT with good selectivity. The enhanced performance was attributed to a unique 2D nanosheet-like porous structure with a single-crystal phase, which was helpful for the effective gas diffusion of gas molecules, enabling high sensitivity.
Spherical and cubic particles have been reported in sensing studies. Zhou et al.137 prepared In2O3 microcubes with a cubic structure using a hydrothermal process and a surface area of 30.3634 m2 g−1. The fabricated sensor exhibited high sensing performance with a high response (Ra/Rg = 85) to 1000 ppm CH3COCH3 at 210 °C, demonstrating high potential for sensing applications. H. Ma et al.138 synthesized walnut-like In2O3 nanostructures using a simple solvothermal process followed by thermal treatment and evaluated their photoelectric-based gas-sensing properties. The as-prepared walnut-like In2O3 nanostructures exhibited a strong response of 219 to 50 ppm NO2 in a UV light (λ = 365 nm) irradiation (1.2 mW cm−2). The in situ growth of such thin films improved the contact between the sensing material and Ag interdigitated electrode substrate. A high surface area with walnut-like In2O3 structures containing numerous active edge sites improved the performance of NO2. Hollow particles generally have a huge specific surface area caused by the hollowness of their structure. A solvothermal method was used to synthesize urchin-like In2O3 hollow structures with an In (OH)3 sacrificial template.139 After annealing at 350 °C, the as-prepared initial structure of In (OH)3 was transformed into an urchin-like In2O3 structure exhibiting a surface area of 122.02 m2 g−1. The sensor showed excellent sensitivity (20.9) for 1 ppm HCHO, selectivity, fast τres, and LOD (50 ppb) at 140 °C. The sensing result was attributed to the unique structure with a small crystallite size (∼17 nm) and large surface area. In addition, the presence of a loose shell in the urchin-like hollow structure was conducive to gas diffusion.
While many synthesis strategies are based on various templates and surfactants, there are still adverse effects, such as the cost of the template materials and the subsequent removal of templates, which is a tedious process. As a result, simple and cost-effective synthesis of nanostructures that negates the need for templates is a tremendous scientific advantage. Recently, biomolecules, such as amino acids, as reducing and shape-directing agents were used to prepare In2O3 micro/nano structures.31,140 Brick-shaped In2O3 was fabricated using a simplistic hydrothermal technique via a green approach with amino acid (L-aniline).141 The sensor responded strongly (600) to 100 ppm NO2 gas at a low working temperature of 100 °C. The morphology, microstructure, structural defects, and porosity of the In2O3 bricks were responsible for the high sensing toward NO2 gas. On the other hand, the high selectivity to NO2 gas was due mainly to the reactive nature of NO2 because of the presence of a lone pair electron.
Another study used a hydrothermal method to synthesize hollow In2O3 microcubes (HIMC) using L-alanine as a shape-controlling agent.31 The Kirkendall effect was responsible for the evolution of these hollow structures. The hollow In2O3 microcube sensor exhibited an unprecedented high sensitivity (S = 1401) and high selectivity to 100 ppm NO2 at 100 °C. Various factors, such as variation in barrier height in NO2 ambiance, thick electron depletion layer, and high adsorption and the gas-sensing reaction of hollow In2O3 microcube structures, were responsible for the strong response to NO2. Fig. 7(a) shows FESEM images of HIMCs, and Fig. 7(b) presents a schematic diagram of the sensing mechanism of a hollow In2O3 microcube gas sensor.
Fig. 7 (a–c) FESEM images of HIMC film and (d) schematic diagram of NO2 sensing mechanism of the HIMC sensor. Reprinted with permission from ref. 31, Copyright 2020, Elsevier. |
In a different study, In2O3 microcubes were produced using a low-temperature solution chemical technique to investigate NO2 gas sensing. The In2O3 microcubes sensor displayed a strong response (Rg/Ra = 1884) to NO2 (30 ppm) at 60 °C and detected NO2 gas at concentrations as low as 500 ppb,142 which is very interesting for practical applications. Moreover, Zhang et al.143a reported the synthesis of hollow In2O3 microspheres prepared using a hydrothermal process. The In2O3 microcubes sensor showed a good response value of 20 to 10 ppm HCHO at a working temperature of 200 °C with τres/τrec of 4 s/8 s. The In2O3 hollow microspheres possessed abundant pores, a surface area of 40.94 m2 g−1, and a hollow structure that facilitated the diffusion and adsorption of the target gas to the active sites of the detection materials.
Han et al.143b recently fabricated a flower-like In2O3 material with a high surface area (77.38 m2 g−1) to detect trace isoprene, a biomarker of liver disease. The In2O3 sensor showed a strong response to isoprene at 190 °C with good repeatability and selectivity over other biomarkers. The degree of the reaction of active oxygen with isoprene on the surface of the In2O3 crystal determines the amount of adsorbed oxygen ions, which strongly influences the sensor performance. In particular, the In2O3 sensor could detect 5 ppb isoprene and has potential use in portable breath isoprene detectors for the noninvasive screening and grading of chronic liver disease.
Chen et al.144 used another template-free, solvothermal method to synthesize cubic–rhombohedral In2O3(bcc-rh-In2O3) NPs (Fig. 8(a)–(f)) for exhaled breath analysis. Both the orthorhombic phase of InOOH and the cubic phase of In(OH)3were present in the products. The presence of the InOOH and In(OH)3 phases promoted the configuration of the rh-In2O3 phase and bcc-In2O3 phases, respectively. The presence of glycerol in the reactant solution formed both phases in the final product. The bcc-rh-In2O3 sensor exhibited a response of Ra/Rg = 12, 2 s of response time, and LOD of 10 ppb towards 50 ppm acetone. The main reason for the superior performance of the abovementioned sensor was the establishment of the n–n heterojunction between bcc-In2O3 and rh-In2O3 (Fig. 8g and h).
Fig. 8 SEM images of the sample (a, d) before calcination, (b, e) after calcination at 400 °C, and (c, f) commercial In2O3. Sensing mechanism with energy band model of (g) pure rh-In2O3 phase and (h) bcc-rh-In2O3 phase. Reproduced from ref. 144 with permission from Elsevier (Copyright 2020). |
The work functions of cubic In2O3 and rhombohedral In2O3 are 5 eV and 4.3 eV, respectively. An n-homojunction structure formed after they came into contact with each other. The transfer of electrons occurred from rh-In2O3 to bcc-In2O3 until the Fermi levels realized equilibrium owing to the higher position of Fermi levels of rh-In2O3 compared to those of bcc-In2O3. Accordingly, a high depletion layer of electrons (WD3 > WD1) was produced on the rh-In2O3 surface, resulting in a potential energy barrier (V3 > V1). Modulation of the potential barrier and depletion layer resulted in a high response value for the bcc-rh-In2O3 sensor.
Various p-type oxides have been coupled with In2O3 to design p–n heterojunction-based gas sensors.149–151 For example, Park et al.152 demonstrated the synergistic effects of p-type Co3O4 and n-Fe2O3 NPs co-decorated on In2O3 NRs for ethanol (C2H5OH) gas-sensing properties. First, they synthesized In2O3 NRs by thermal evaporation followed by spin coating of Co3O4 and Fe2O3 NPs (Fig. 9a), which were synthesized separately using a hydrothermal method. Among the different VOCs, the Fe2O3–Co3O4-codecorated In2O3 NRs sensor showed the maximum selectivity to C2H5OH, followed by the Co3O4 NP-decorated In2O3 NRs sensor (Fig. 9b). The effects of the p–n heterojunctions (Fig. 9c) were evidenced by the change in the baseline resistances in the Co3O4-decorated In2O3 NRs sensor and the Fe2O3–Co3O4-codecorated In2O3 NP sensors, as compared to those of the pristine one. Modulating the width of the depleting layer and the height of the potential barrier at the p–n junction occurs within the sensor associated with the adsorption/desorption process of C2H5OH. The electrocatalytic attributes of the Fe2O3–Co3O4-codecorated In2O3 NRs sensing materials facilitate the oxidation of C2H5OH, which may explain the good selectivity to C2H5OH over other gases at an operating temperature of 200 °C. Overall, the primary cause of the synergistic effects of co-decoration on the C2H5OH detection performance of In2O3 NRs is not the development of compounds/nanoalloys between the oxides but the development of the p–n junctions among two different kinds of decorated oxides, such as Fe2O3–Co3O4 and Co3O4–In2O3 p–n junctions.
Fig. 9 (a) TEM image of Co3O4 and Fe2O3 co-decoration on In2O3 NRs, (b) selectivity pattern of pristine Fe2O3, Co3O4, In2O3, and Fe2O3–Co3O4 NPs decorated In2O3 NRs sensors, and (c) the gas sensing mechanism of Fe2O3–Co3O4 NPs decorated In2O3 NRs sensor. Reproduced with permission from the ref. 152, copyright 2020, Elsevier. |
Wang et al.153a produced In2O3 microspheres with a 3D-inverse opal structure via an ultrasonic spray-pyrolysis technique using a sulfonated polystyrene sphere. Subsequently, PdO NPs were loaded on it using a simple impregnation method. The PdO NPs-loaded 3D-inverse opal In2O3 microsphere sensor showed excellent selectivity to 100 ppm acetone with a four-fold stronger response (Ra/Rg = 50.9) than the corresponding pristine gas sensor at a working temperature of 250 °C. The improved gas-sensing properties of the PdO–In2O3 sensor were attributed to the novel 3D microporous structure, the highly 3D interconnections among the grains, large specific surface area, and size-controlled bimodal pores. These interesting morphological features are usually beneficial for increasing the adsorption sites for gas molecules with high permeability and diffusion.
Most importantly, the PdO catalyst NPs act as strong acceptors of electrons; hence, electron transfer from In2O3 to PdO produces a strong depletion layer at the PdO–In2O3 p–n heterojunction interface. Furthermore, the addition of PdO catalyst NPs helps increase the number of chemisorbed oxygen ions on the PdO–In2O3 sensor surface. Consequently, significant variation in resistance occurs for the PdO–In2O3 sensor, further enhancing the sensitivity. In addition, the partial reduction of PdO (Pd2+) to Pd0 during the acetone gas-sensing test can be attributed to the catalytic dissociation of acetone gas molecules and the lowering of the activation energy of the reaction between chemisorbed oxygen ions and acetone gas molecules, which eventually increases sensing performance.
MXenes are a promising class of 2D inorganic compounds composed of atomically thin layers of transition metal carbides, carbonitrides, and nitrides.153b These materials possess functional groups that can serve as active sites for the adsorption of gas molecules, even at room temperature. This property highlights the potential of MXenes in the design of low-power gas sensors.153c,d In this respect, Liu et al. synthesized a hybrid p-MXene/n-In2O3 material to produce a chemiresistive-type sensor for NH3 detection at room temperature.153b The developed hybrid sensor exhibited a 28-fold larger response, improved selectivity, better detection limit, and excellent stability compared to the pure MXene (Ti3C2Tx) sensor. The enhanced sensing performance was attributed to the heterostructure formed between MXene and In2O3 and the richer functional groups of the 2D Ti3C2Tx MXene with a high specific surface area. A sensor based on this hybrid material also exhibited faster response/recovery times, highly repeatable signals, and long-term stability, highlighting the potential for development as a wireless passive NH3 sensor. Fig. 10 shows the energy band diagram of a hybrid sensor before and after the contact. The Fermi level of n-type In2O3 is higher than that of p-type MXene (Ti3C2Tx), causing electrons to transfer from In2O3 to MXene, leading to the formation of a heterostructure at the interface of MXene and In2O3. This transfer of electrons causes an equilibrium in the Fermi levels of both materials. As a result, there is a higher concentration of free electrons in MXene, which enhances the pre-adsorption of oxygen in the air onto the surface of the sensing films. The pre-adsorbed oxygen captures electrons to form ionized oxygen molecules (O2−). The redox reaction is favored because of the increased adsorbed O2−, resulting in an enhanced response (Fig. 10b).
Fig. 10 (a) Energy band diagram of the MXene/In2O3 heterostructure before and after contact, (b) adsorption of NH3 and oxygen molecules on the heterostructure surface. Reproduced with permission from the ref. 153b. |
Liu et al.153e synthesized MXene/In2O3 (p–n) nanocomposites using a hydrothermal method for formaldehyde gas detection. The formaldehyde sensing test showed that the nanocomposite sensor had an optimal working temperature of 100 °C, which was lower than that of the pure In2O3 sensor (300 °C). In addition, the response of the nanocomposite to 100 ppm formaldehyde increased from 66.06 to 106.8%. The gas sensor prepared by MXene/In2O3 nanocomposites demonstrated good stability, rapid response (1 s), and recovery (1 s) times, as observed in repeatable tests. In MXene/In2O3 nanocomposites, the high sensing performance is credited mainly to their large specific surface area, which offered many sites for oxygen and the adsorption of formaldehyde gas. In particular, the functional groups (–OH and –F) on the MXene surface contained active sites for the adsorption of formaldehyde gas molecules, leading to an extended range of electron changes on the surface and a significantly increased response. The Schottky junctions formed at the interface between MXene and In2O3 and their respective work functions of 3.9 eV and 4.83 eV caused electrons to transfer from MXene to In2O3 to achieve equilibrium between their Fermi energy levels.
Fig. 11 (a) Schematic diagram of the preparation of the hierarchical structure of 3D In2O3@SnO2 C–S NFs. (b) Selectivity patterns of all prepared sensors to various gases (100 ppm) at 120 °C. (c) Morphology and active sites on the surface of gas sensor and (d–f) gas sensing mechanism. Reproduced from ref. 154 with permission from the American Chemical Society, Copyright 2019. |
In addition to its large surface area (Fig. 11c), the outstanding sensing performances of the In2O3@SnO2 C–S NFs were assigned to the following reasons (Fig. 11d–f): (i) the increased baseline resistances due to the configuration of the depletion layers and higher potential barriers caused by heterogeneous and homogeneous interfaces compared to the single homogeneous interfaces of In2O3 and SnO2; (ii) the increase in electron concentration caused by unique electron transfer from In2O3 (φ = 4.83 eV) to SnO2 (φ = 5.13 eV), which improves the gas-sensing reaction; (iii) configuration of potential barriers on the junctions between the NWs; (iv) the availability of abundant adsorbed oxygen species and large surface area (31.4 m2 g−1) of the as-prepared In2O3@SnO2 C–S NFs as well as the unique hierarchical morphology with connected SnO2 nanosheet arrays.
Zinc oxide (ZnO), a promising sensing material, can be combined with In2O3 to form In2O3–ZnO C–S NWs gas sensors. Park et al.155 reported on the role of interfaces of multiple-networked In2O3 NWs coated with a ZnO shell for C2H5OH gas-sensing application. A thermal evaporation method synthesized the vertically grown In2O3@ZnO C–S NWs. The ZnO shell thickness varied from 10 to 53 nm to examine the impact of the ZnO shell layer on the sensing properties. The ZnO shall layer with a size of 44 nm yielded the strongest response. The enhanced response to C2H5OH is elucidated based on the wider depletion layer formed by the In2O3–ZnO interface and the outer shell of the ZnO in the In2O3/ZnO C–S NWs. Moreover, the superior response can be credited to the modulation of the potential barriers at the In2O3–ZnO interface, at the ZnO–ZnO homojunction, and the ZnO grain boundaries.
Compared to traditionally synthesized ZnO NPs, newly developed ZnO nanostructures from metal–organic frameworks (MOFs), a highly crystalline and porous material, have recently attracted attention for gas-sensing applications. Liu et al.156 fabricated In2O3 NFs coated with zeolitic imidazole framework-8 (ZIF-8)-based gas sensors for NO2 detection. Initially, In2O3@ZnO NFs were synthesized using an electrospinning method, where ZIF-8 was used as a template and a Zn2+ source (Fig. 12a–e).
Fig. 12 (a) Schematic diagram of the synthesis procedure of In2O3/ZIF-8 composites (structure of ZIF-8 is shown in (b)) with varying ratios of In/Zn2+ to control the morphologies (c–e), (f) TEM image of the In2O3/ZIF-8 interface and a humidity-sensing mechanism showing the repellence of H2O molecules by ZIF-8, (g) response values of various sensors, including pure In2O3 NFs and In2O3/ZIF-8 composites, with respect to the operating temperatures, and (h) response values of sensors with respect to the different humidity conditions. Reproduced from ref. 156 with permission from the American Chemical Society (Copyright 2020). |
Various morphologies of In2O3/ZIF-8 C–S nanostructures were obtained by changing the molar amount of Zn2+, such as 8:1, 4:1, and 2:1 (Fig. 12c–e presents TEM images). Fig. 12f shows a high-resolution TEM image and a schematic diagram of the sensing surface. The as-prepared In2O3/ZIF-8 NFs sensor displayed a strong response of Rg/Ra = 16.4 to 1 ppm NO2against other interfering gases, rapid response, and recovery time of 80 s/133 s, and LOD value of 4.9, which is in contrast to the pristine In2O3NFs sensor, at a lower optimal temperature of 140 °C (Fig. 12g). The In2O3@ZIF-8 C–S NFs exhibited excellent humidity resistance because of the hydrophobicity of ZIF-8 (Fig. 12h) because it acted as an effective gas enhancement medium that encouraged the adsorption capability of NO2 gas molecules and prevented the diffusion of water molecules. The high response value was attributed to the enhanced surface area (700 m2 g−1) and porous morphology of the composite. Furthermore, ZIF-8 possesses the excellent adsorption capability of NO2.157
Tungsten oxide (WO3), as an n-type MOS, has been utilized broadly as a gas-detecting material. Cao et al.158 reported n–n heterostructures of ultra-thin In2O3 nanosheets decorated with WO3 clusters for HCHO sensing. Different WO3 clusters (2–16 wt%) were incorporated on In2O3 nanosheets using the impregnation method to study their impacts on gas-sensing performance. The sensing results showed that the 4 wt%-WO3-loaded In2O3 sensor exhibited an outstanding sensing response (Ra/Rg = 25), excellent selectivity, rapid τres/τrec (1 s/67 s), and a LOD of 0.1 ppm HCHO at 170 °C. The high performance was attributed to the heterojunction-type structures, large surface area, porous ultra-thin nanosheets, and electronic transduction at the heterojunction interface.
Feng et al.159 fabricated a series of WO3-NP-loaded In2O3 NFs for CH3COCH3 gas sensing. The 1.5% In2O3–WO3 NFs (S3) sensor displayed the strongest response (29) to 200 ppm CH3COCH3 at 275 °C with a LOD of 0.4 ppm and a response of 1.28. The mechanism of the sensing performance was attributed to the establishment of an n–n heterojunction between In2O3 and WO3, which increased the resistance of the In2O3–WO3 NFs sensor. On the other hand, the surface of the 1.5% In2O3–WO3 NFs (S3) sensor was rough, which can facilitate the adsorption of the test gas and improve the sensing reaction of the test gas with adsorbed oxygen species.138 The 1.5% In2O3–WO3 NFs (S3) sensor possessed a higher utility factor because of the presence of well-connected grain boundaries or grain junctions, which highly contributed to the gas response.58 Therefore, the response of the 1.5% In2O3–WO3 NFs sensor was higher than the other sensors (S1, S2 and S4). Hence, tuning the morphology helps enhance the sensing response.
Noble metal functionalization or decoration on the surface of MOS is a popular and promising strategy for boosting the gas-detecting properties. Noble metals, which act as catalysts, have been used to modify the surface of MOS to enhance the gas-sensing properties.160 The effects can be divided into two types, electronic and chemical sensitization.161 In electronic sensitization, when the noble metals and metal oxides come into contact, the flow of electrons occurs from the MOS to the noble metal, ensuing in a space charge layer on the MOS surface as well as a Schottky potential barrier and band-bending in the MOS sensor. Therefore, the overall sensor resistance increases. After introducing the target gas, it interacts with the chemisorbed oxygen species on the sensor surface, and electrons are released back to the conduction band of the sensor. Subsequently, both the sensor resistance and the potential barrier height decrease. In chemical sensitization, noble metals can dissociate O2 into atomic species because of their high catalytic nature, which then are spilled over on the sensor surface to be chemisorbed. Therefore, this effect is also called the “spillover effect”. Fundamentally, a greater number of oxygen ions are found on the surface of the sensor. In addition, the target gases can be dissociated and adsorbed onto the surface-active sites of the noble metal catalysts for a possible reaction with the oxygen ions that have already been chemisorbed. Hence, chemical sensitization or the spillover effect helps accelerate the sensing process.
Among various noble metal NPs, Au NPs have been used extensively to boost the gas-sensing properties of MOS materials. Xu et al.160 reported an in situ synthesized Au-loaded In2O3NFs gas sensor with different amounts of Au (0.1 to 0.4 wt%) for C2H5OH gas-sensing applications. The 0.2 wt% Au-loaded In2O3sensor displayed a high response (13.8) and fast τres/τrec (12 s/24 s) at 140 °C. The improved C2H5OH response of In2O3NFs was related to the electronic sensitization of Au NPs. The Au NPs acted as an efficient catalyst that generated more active sites on the sensor surface and contributed to enhancing surface reactions, which are essential for improving the sensor response. They proposed that this device can be exploited as a portable high-performance C2H5OH gas sensor because of its relatively lower power consumption and fast τres/τrec. In a different study, Au NPs were chosen to decorate mesoporous In2O3 to study the relationship between the different decoration methods of Au and their corresponding gas-sensing performance.162a A 2D hexagonal mesoporous silica template of SBA-15 was synthesized initially for constructing In2O3 mesoporous architectures. Au NPs were loaded using two techniques: adding 0.5 mol% of Au to the precursor solution during synthesis followed by calcinating obtained products to obtain Au-decorated mesoporous In2O3 (IO-Au-D); the other was produced by loading 0.5 mol% Au on the pure calcined mesoporous In2O3 (IO-Au-L). The Au NPs were dispersed uniformly on the IO-Au-D composite, while the Au NPs were aggregated in the IO-Au-L composite. The Au-decorated mesoporous In2O3 sensor and 0.5 mol% Au-loaded In2O3 sensor displayed responses of 19.01 and 12.25 towards CH3COCH3 (100 ppm) at 250 °C, respectively, compared to pure In2O3 (8.38) at 275 °C. The good sensing performance exhibited by the Au-decorated mesoporous In2O3 sensor was attributed to the expanded depletion layer compared to the Au-loaded In2O3 sensor. In this case, both the electronic and chemical sensitization effects of Au NPs were responsible for increasing the CH3COCH3 sensing response of the In2O3 sensor. As an active catalyst, Au formed additional active sites important for boosting the CH3COCH3 response. On the other hand, Au NPs can catalyze the dissociation of oxygen molecules. Thus, the activated oxygen species formed spill onto the In2O3 surface, leading to more reactive oxygen species for further reactions with CH3COCH3 gas molecules.
Tuning sensing materials has been the focus of significant efforts to enhance the gas selectivity in chemiresistive gas sensors. On the other hand, selective detection of less reactive gases still poses a challenge. In this respect, Lee et al.162b presented a novel method for the selective detection of low-reactive gases using patterned In2O3 NFs and tuning the electrode catalytic properties. The approach involved direct-write near-field electrospinning onto interdigitated electrodes (IDE) with low sensing material coverage, allowing exposure to analyte gases. Sensors made of entangled In2O3 NFs sensors were prepared on PtIDE and Au/PtIDE electrode substrates to study the effect of gas access to the electrode on gas sensing characteristics.
The sensing properties of the fabricated gas sensors rely on the catalytic activity of the electrode, with PtIDE indicating an excellent selective response to xylene. Fig. 13 presents the gas sensing mechanisms of In2O3@Au/PtIDE and In2O3@PtIDE sensors with the respective response plots of the sensors [In2O3@PtIDE (Fig. 13a), In2O3@AuPtIDE (Fig. 13c), Au@In2O3@PtIDE (Fig. 13e), and Au–In2O3@AuPtIDE (Fig. 13f)]. When subjected to the highly catalytic electrode, namely 8Au/PtIDE, C2H5OH gas molecules underwent instantaneous reactions with the electrode (Fig. 13d1), resulting in complete oxidation and an inadequate C2H5OH response. On the other hand, xylene gas was reformed into highly reactive and smaller intermediates through its reaction with 8Au/PtIDE (Fig. 13d2), which were later transported to In2O3 NFs, leading to an improved sensor response. On the other hand, when only PtIDE substrates were used, the analyte gases, such as C2H5OH and xylene, were not fully oxidized or reformed because of the lower catalytic activity of PtIDE compared to 8Au/PtIDE (Fig. 13b1 and b2).
Fig. 13 Gas sensing characteristics of (a) In2O3 @PtIDE, (c) Au–In2O3@8Au/PtIDE, and (e) Au–In2O3@PtIDE, (f) Au–In2O3@8Au/PtIDE to 5 ppm of analyte gases at 325 °C. Schematic diagram of the gas sensing mechanism of (b) In2O3@PtIDE, and (d) In2O3@8Au/PtIDE. Reproduced from ref. 162b with permission from Elsevier. |
Modification by noble metal NPs also dramatically improves the selectivity and τres/τrec speed. For example, Xing et al. reported the loading of Au NPs using different loading methods on a unique 3D inverse opal In2O3 microporous film with a honeycomb architecture163 for a noninvasive diagnosis of CH3COCH3 gas in diabetic patients. The sensor made from such structures could sense up to 5 ppm CH3COCH3 with a strong response of 42.4 and a rapid response time (11 s) at 340 °C. The excellent sensing performance can be attributed to the spillover of Au NPs on the 3D inverse opal In2O3 microporous film and its unique structure. A microporous honeycomb structure from In2O3 provided immense gas accessibility that enables CH3COCH3 gas molecules to access easily and quickly, making sensors more active for gas-sensing. The preceding study can lead to a promising sensing material for monitoring CH3COCH3via exhaled breath, benefitting diabetic patients. In another study, a room-temperature sensor was fabricated using Ag–In2O3 NRs composites, prepared via a facile hydrothermal process, for H2S gas-sensing applications. The sensor made from 13.6 wt% Ag–In2O3 NRs displayed an outstanding and super sensitive response of 93719 to 20 ppm H2S with a LOD of 0.005 ppm. The chemical sensitization effect of Ag NPs increased the number of chemisorbed oxygen ions by withdrawing electrons from the conduction band of In2O3, which in turn increased the depletion layers of In2O3 sensing materials and the response performance.164 Similarly, Au NPs of various amounts have been loaded on sensing materials of In2O3 microspheres,165 hollow nanospheres,166 and porous nanocubes,167 for detecting trimethylamine (TMA), butylamine, and HCHO at 340 °C, 280 °C, and 240 °C, respectively.
Thus far, various noble metals (Pd, Au, and Pt) have been functionalized on In2O3 to design prospective sensing materials. On the other hand, these noble metal NPs are expensive, increasing the ultimate device cost. Ag, another noble metal, has been used in low-cost material sensors.168–170 Wang et al.168 reported a 3D hierarchical structure made from In2O3 and subsequent Ag functionalization using a NaBH4 reducing agent to construct 6 wt%-Ag-loaded In2O3 nanoflower composite materials for HCHO sensing. The sensor exhibited enhanced sensitivity (Ra/Rg = 11.3) to 20 ppm gas, rapid response/recovery time (0.9 s/14 s), and high selectivity. The enhanced sensitivity was attributed to the nonporous and branched nanorods connected in a sunflower-like morphology that exhibited a relatively larger surface area and induced abundant dynamic sites for the gas adsorption, diffusion, and surface reactions through the catalytic dissociation of Ag NPs via the spillover effect, which triggered the sensing reactions and electronic sensitization effect of Ag and In2O3.
Generally, the MOS-based gas sensors are operated at high temperatures. On the other hand, sensing measurement at a high temperature has several practical limitations, such as an increase in energy consumption, size, and cost of devices, as well as the degradation of sensing performance due to microstructural changes at an elevated temperature. Importantly, detecting flammable and explosive gases at such high temperatures has a high risk of exploding. Therefore, the research on developing room temperature (RT)-based gas sensors has a beneficial impact on.171 At RT, some gases, such as H2 and CO, are often difficult to detect using pristine MOS gas sensors. This problem can be avoided by modifying the noble metal catalysts on the sensor surface. Singh et al.172 employed an APhS-self-assembly monolayer (SAM) approach in decorating In2O3 NWs with Au NPs for CO sensing at RT. They prepared In2O3 nanowires using a thermal evaporation technique and immobilized the citrate-capped Au NPs on the amine-terminated surface of In2O3 NWs to form Au-functionalized In2O3 NWs sensing materials (Fig. 14a). The effect of the loading time (10–60 min) of the Au NPs on the In2O3 NWs was studied (Fig. 14b). With a loading time of Au NPs of 60 min, the senor response increased and became the highest. It displayed a response (Ra/Rg) of 110 to 5 ppm CO gas (Fig. 14c). Fig. 14d compares the response of various sensors to 5 ppm CO gas. The high exposure of the Au NPs on the sensor surface increased CO oxidation to CO2 because of the increased spillover zone via the catalytic activation of Au, increasing the conductance in the sensor. The small size of the Au NPs also resulted in a high density of reactive defect sites for gas molecule adsorption and dissociation. The functionalization of the AphS-SAM layer helps densify the Au NPs on the surface of the In2O3 NWs and increases the depletion layer region beneath the nanowire and NPs. Furthermore, modulation of the Schottky barrier height caused by the configuration of the depletion layer at the Au NPs and In2O3 NWs interface contributes to sensor enhancement (Fig. 14e).
Fig. 14 (a) Schematic diagram of the synthesis process of citrate stabilized Au functionalization on In2O3 NWs, (b) Au functionalized In2O3 NWs with different loading times (10–60 min), (c) CO gas response for Au functionalized In2O3 NWs (60 min loading time), (d) response comparison for 5 ppm CO with different sensing devices, and (e) energy band model of a Au-functionalized In2O3 NWs sensor. Reprinted from ref. 172 with permission from the American Chemical Society, (Copyright 2011). |
Zhou et al.173 fabricated an RT-based ultrasensitive HCHO sensor made from 5 wt% Ag nanoparticle sensitized-In2O3 rod-type nanograin structures. The sensor displayed an outstanding response of 135 to 1 ppm HCHO gas with τres/τrec of 102 s/157 s and a LOD of 0.05 ppm at RT. The excellent sensing activities were attributed to the chemical and electronic sensitization effects of the Ag additive.
Pd is an excellent catalyst for gas-sensing applications. In this respect, Wang et al.174 synthesized a 3DOM-In2O3 sensor loaded with 0.5 wt% Pd (Pd0.5In) for an RT study of NO2 sensing. The Pd0.5In sensor exhibited high sensitivity of 980 compared to 3DOM-In2O3, for 500 ppb of NO2 gas at RT, with quick τres/τrec and good stability. The excellent sensing results of Pd0.5In were attributed to the catalytic effect of Pd NPs and their synergistic effect with In2O3, which increased the concentration of electrons and surface defects. Fig. 15 shows the sensing mechanism of the Pd0In and Pd0.5In sensors in the presence of air and NO2 gas. In this case, the NO2 sensing mechanism of the Pd0In and Pd0.5In sensors was elucidated using the variation of the depletion layer, which results primarily from the surface modification on the 3DOM-In2O3 when loading Pd. In addition, the higher carrier concentration of the Pd-loaded 3DOM In2O3 sensor, induced by highly conductive Pd loading, also facilitated the increased response to NO2 gas. The enhanced carrier concentration of the Pd-loaded 3DOM In2O3 sensor stimulated additional chemisorbed oxygen species by capturing more electrons to widen the depletion layer.
Fig. 15 Schematic diagram of the gas detecting mechanism of pristine 3DOM In2O3 (Pd0In) and 0.5 wt% Pd-loaded 3DOM In2O3 (Pd0.5In) with their corresponding energy band models. Reproduced from ref. 174 with permission from Elsevier (Copyright 2020). |
Li et al.183 conducted pioneering work on noble metal@In2O3-based C–S nanostructure-based gas sensors. They prepared Au@In2O3 C–S nanostructures using a two-step method. First, an Au@carbon (Au@C) sphere template was synthesized via a facile hydrothermal technique at 180 °C using HAuCl4 and a glucose mixture solution. Second, the complete Au@In2O3 C–S nanostructures were formed after the Au@C sphere was decorated with an In2O3 shell via a solution method followed by a simultaneous RT aging and high-temperature calcination process. The as-prepared Au@In2O3C–S structures were tested for their potential gas-sensing properties, which exhibited high sensitivity (Ra/Rg = 17.0) and selectivity to HCHO at an optimal temperature of 200 °C with a fast τres/τrec (7 s/135 s). The improved sensing result of the Au@ In2O3C-SNPs was attributed to their high electron depletion layer, which resulted from the catalytic activity of the Au core. Wang et al.184 carried out further research on this material. An Au@In2O3 C–S NPs was fabricated using a sol–gel strategy. The sensing performance was assessed for various gases at varied temperatures. The Au@In2O3 NP gas sensor exhibited excellent selectivity and the maximum sensing response (Ra/Rg) of 36.14 toward C2H5OH (100 ppm) at an optimal temperature of 160 °C with rapid τres/τrec (4 s/2 s). The good sensing performance was attributed to the configuration of the Schottky junction because of the Au/In2O3 interface and catalytic activity of the Au metal core.
Liu et al.185 reported Ag@In2O3 C–S nanostructures for C2H5OH gas-sensing. The synthesis of Ag@In2O3 C–S nanostructures was similar to Li et al.183 This time, however, the noble metal core Ag was introduced through encapsulation inside the In2O3 shell. The authors chose Ag as the core material owing to the high cost of Au noble metal, as well as the high electrical and thermal conductivity, antibacterial properties, and nontoxicity of Ag noble metal. The typical Ag@In2O3 C–S nanostructure was synthesized from the sacrificial template of Ag@C nanospheres (Fig. 16a). The Ag@In2O3 C–S nanostructures (Fig. 16b) exhibited an increased response (72.56), and high selectivity to 50 ppm C2H5OH at an operating temperature of 220 °C (Fig. 16c) with a rapid response/recovery speed (13 s/8 s) and excellent transient values, even at high-humidity (25 to 90%). As shown in Fig. 16d, the high selectivity to C2H5OH, alongside other volatile gases, was attributed to the low bond energy and high absorbing ability of O–H in C2H5OH molecules.186Fig. 16e presents the sensing mechanism, where a Schottky junction was established near Ag and In2O3 interface after their contact. Electron transfer occurred from In2O3 (φ = 4.3 eV) to Ag (φ = 4.6 eV), resulting in the bending of the In2O3 band and the subsequent extension of the electron depletion layer. Hence, the resistance of the Ag@In2O3 C–S sensor increased. When the C2H5OH gas was exposed to the Ag@In2O3 C–S gas sensor, C2H5OH gas molecules were oxidized by a reaction with the adsorbed oxygen ions, releasing the trapped electrons back to the Ag@In2O3 system (Fig. 16f). This resulted in a significant increase in charge carriers, lowering the sensor resistance. In addition, the Ag catalyst provided sufficient dynamic sites for the binding and dissociation of O2 molecules and accelerated the gas-sensing reaction via the spillover effect.
Fig. 16 (a) Schematic diagram of the synthesis procedure of Ag@In2O3 C–S NPs, (b) TEM image of Ag@In2O3 C–S NPs, (c) sensor responses with respect to different operating temperatures toward 50 ppm C2H5OH, (d) selectivity comparison of Ag@In2O3 C–S, as well as solid and hollow In2O3 NP sensors at the optimal temperature of 220 °C for 5 ppm C2H5OH, and (e and f) sensing mechanism of Ag@In2O3 C–S NPs in air and C2H5OH atmospheres with the corresponding energy band model. Reproduced from ref. 185 with permission from Elsevier (Copyright 2020). |
Chava et al.187 prepared Au@In2O3 C–S NPs using a hydrothermal technique followed by calcination. Flower-shaped Au@In2O3 C–S NPs with porous structures were formed after the subsequent calcination of Au@InOOH C–S NPs at 350 °C. The Au@In2O3 C–S nanospheres were studied for the potential applicability of H2 gas detection at different temperatures. They exhibited a high sensing response value of 34.38 compared to the pure In2O3 nanospheres at 300 °C. The improved sensing properties of Au@In2O3 C–S nanostructures were attributed to the electronic sensitization and chemical sensitization of Au. The work functions of the Au NPs and In2O3 are 5.1 eV and 4.8 eV, respectively. As a result, the charge carriers from the CB of In2O3 were transferred to the Au metal until the Fermi levels reached equilibrium. This phenomenon led to a large depletion layer near the Au NPs and In2O3 interface, leading to band bending and the formation of a Schottky barrier, thereby further increasing the resistance of the system. Regarding the sensitization effect, the Au noble metal provided active sites for the adsorption of oxygen and H2 molecules and their dissociation into different oxygen species and H atoms on the In2O3 surface, as well as the subsequent spillover on the same. This process resulted in the extraction of more electrons from the CB of In2O3. The increased number of adsorbed oxygen ions resulted in the sensitivity of Au@In2O3 C–S to H2 gas, ensuring a strong response. Moreover, the spillover effect of Au NPs facilitated the reaction speed-up concerning the oxygen species and H2 atoms. Thus, the Au@In2O3C–S NP sensor exhibited enhanced sensing performance compared to the bare In2O3. The spillover reactions between H2 and O2 on Au surfaces can be expressed as
2Au + H2 → 2Au/H | (13) |
2H + O− → H2O + e− | (14) |
2Au + O2 → 2Au/O | (15) |
O + e− → O− | (16) |
Another category of C–S nanostructures, known as yolk–shell or rattle-type structures, is in great demand. The typical yolk–shell nanostructures represent a unique type of C–S structure with a characteristic core–hollow–shell configuration. In this structure, the noble metal core is movable inside a hollow metal oxide shell, enabling the complete exposure of all active sites to come in contact with target analytes during the sensing reaction. This section summarizes the recent developments in metal–In2O3-based yolk–shell nanostructures for their potential gas-sensing applications.
Thus far, there are only a few reports on metal@In2O3 yolk–shell nanostructures for gas-sensing applications. Rai et al.188a examined Pd@In2O3 yolk–shell (Y–S) NPs for sensing reducing and oxidizing gases. They used a typical glucose-assisted synthesis technique to prepare Pd@In2O3Y–S nanostructures. Initially, Pd@C, as a sacrificial template, was used to synthesize Pd@In2O3Y–S NPs. The Pd@In2O3Y-SNP sensor exhibited a C2H5OH response of 159.02 (5 ppm) at 350 °C compared to pure In2O3hollow NSs at 350 °C. The strong response of the Pd@In2O3Y–S NPs could be attributed to the hollow space of Pd@In2O3Y–S NPs, which maximized the accessibility of Pd NPs with gas molecules, with an increase in the number of oxygen ions adsorbed owing to the catalytic activity in Pd NPs. Enhancing the performance of semiconductor oxide-based gas sensors using bimetallic nanoparticles is a promising approach. Sun et al.188b enhanced the trimethylamine (TMA) sensing performance of pure In2O3 nanosphere sensor by decorating the In2O3 nanospheres with bimetallic AuPd NPs. Compared to pure In2O3, the AuPd–In2O3 sensor showed a stronger response (Ra/Rg = 367), low operating temperature (175 °C), and short response time (2 s) towards TMA. In addition, it showed good repeatability, long-term stability, and cross-selectivity. The improved TMA sensing performance of the AuPd–In2O3 sensor was attributed to the synergistic effect of AuPd and that AuPd–In2O3 inherits the chemical sensitization of Au and the electronic sensitization of Pd. Therefore, modifying the surface of In2O3 with AuPd NPs is an efficient way to enhance the gas-sensitive performance.
Fig. 17 Schematic diagram of the NO2 sensing mechanism of pure and Zr doped In2O3 sensor. Reproduced from ref. 190 with permission from Elsevier (Copyright 2017). |
Outstanding NO2 sensing properties of Fe-doped In2O3 nanostructures can be explained. First, the charge carrier concentration increased due to Zr-doping in In2O3, which increased the chemical adsorption of oxygen molecules on its surface. The ionic radius of Zr4+ and In3+ is 0.074 nm and 0.081 nm, respectively. Therefore, Zr4+ ions readily substituted in the In3+ sites,191 generating one electron per Zr. Hence, the electron concentration of Zr-doped In2O3 increases with increasing chemisorbed oxygen species.190 Moreover, the crystal size of In2O3 decreased due to Zr4+ doping, possibly enhancing the response. As an oxidizing gas, NO2 can capture electrons from In2O3 and react with oxygen species to generate O2 molecules. The reduction in grain size increased the depletion layer and the barriers of grain boundaries, which further increased the resistance of the Zr-doped In2O3 sensor, thereby improving the sensitivity. Furthermore, the ordered mesopores and huge specific surface area make the adsorption and diffusion of target gas molecules easy.
Rare earth metals have also been used to dope In2O3. Liu et al.192 reported the synthesis of Ce-doped In2O3 nanostructures for glycol-sensing properties. The Ce-doped In2O3 porous NS sensor revealed a high response and superior selectivity to glycol at a testing temperature of 240 °C. Kim et al.193 reported the fabrication of In2O3 macroporous spheres with 3–12 at% Pr-doping for the study of CH3COCH3 gas-sensing. The sensor demonstrated excellent sensitivity (Ra/Rg = 20) with negligible fluctuations in response even at high relative humidity (RH = 80%). Among the other earth metals, La has been exploited as a dopant in gas-sensing applications. Wei et al.46 synthesized 1 to 5 mol% La-doped In2O3 microspheres for H2S gas-sensing. Their gas-sensing outcomes showed that the 3 mol% La-doped In2O3 showed a response of 17.8 to 10 ppm H2S at 200 °C, with outstanding selectivity, excellent stability, and good repeatability. Such excellent sensitivity to H2S can be elucidated through the large surface area, the changes in crystallite size, and the different oxygen components caused by La doping. Zhao et al.125 reported improved H2S sensing properties using Mg-doped In2O3 NTs. Mg-doped In2O3 NTs exhibited high sensitivity (Ra/Rg = 173.14) and selectivity towards H2S tested at 150 °C compared to bare In2O3 NTs (Ra/Rg = 12.31). The oxygen vacancies and sulfuration–desulfuration were two reasons for such an improved sensing performance. Zhang et al.194a designed an In2O3 nanostructure modified by Ni doping (2.5 to 7.5 mol%) and surface functionalization (3 to 9 wt%) with Ag NPs. First, the Ni-doped In2O3 NRs were synthesized using an oil bath and annealing method, followed by Ag-functionalization through a chemical reduction method. The gas detecting results showed that the 6 wt% Ag functionalized with 5 mol% Ni-doped In2O3 exhibited an excellent response to 100 ppm HCHO at 160 °C. Furthermore, the 6%-Ag@Ni5.0In2O3 sensor showed excellent selectivity to HCHO, with a quick response speed (1.45 s) and high stability. The superior sensing performance was attributed to the large surface area, the catalytic and electronic sensitization effects of Ag NP, and relative OV and OC contents caused by the elemental doping of Ni.
Jin et al.194b synthesized Ni-doped In2O3 NPs using a metal–organic framework-derived solvothermal method, forming ultrafine particles with a mean size of 13 nm. Gas sensing tests showed that the Ni-doped In2O3 (2 mol%) NPs had a fast response/recovery time of 2 s to 10 ppm NO2, with a response value of ∼70 at 200 °C. Furthermore, the Ni-doped In2O3 sensor had a low detection limit of 5 ppb, making it an excellent candidate for rapid NO2 detection. The enhanced sensing properties were attributed to the abundant adsorption sites, high oxygen vacancies content, and catalytic action of Ni2+. Recently, Zhang et al.194c synthesized hierarchical S-doped In2O3 networks with GO as a template and annealed them at different temperatures to examine their effect on the structure and the surface morphology. The resulting S-doped In2O3 networks exhibited excellent gas sensing properties, with the sample annealed at 500 °C showing the best response (74.4 to 50 ppm C2H5OH at 180 °C). The observed response was approximately two times higher than S-doped In2O3 prepared without a GO template. The excellent properties were attributed to the unique hierarchical structure with a surface area of 295.23 m2 g−1, S-doping, and abundant oxygen vacancies because it provided abundant active sites for C2H5OH molecules to enhance diffusion and adsorption, facilitating gas response. Specifically, S-doping narrows the band gap, introduces surface oxygen defects, and improves the gas-sensing properties. SO42− adsorption on S-doped In2O3 produces acidic sites that enhance the chemical adsorption of C2H5OH and oxygen.
Since its discovery, GO has been used as a remarkable sensing material owing to its unique 2D architecture, rapid electron transport properties, and large surface area (2630 m2 g−1), which facilitates the adsorption of gas molecules and surface reaction because of its sufficient number of surface-active sites.201–203 The hydrothermal process is an effective, facile, and low-cost method for obtaining various micro/nanomaterials with controlled morphologies. Mishra et al. reported the synthesis of In2O3 nanocubes and In2O3/RGO heterostructures using a hydrothermal technique with hexamethyldisilazane (HMDS) surfactant.204 The sensor comprised of In2O3/RGO heterostructures showed a high response to 25 ppm each of HCHO (88%) and CH3COCH3 (85%) at operating temperatures of 225 °C and 175 °C, respectively. The strong response to HCHO was attributed to the electrophilicity of HCHO molecules. Its Lewis acid characteristics allowed the acceptance of more electrons. Moreover, the improved sensing characteristics of In2O3–RGO heterostructures can be attributed to the synergistic reactions among the In2O3 nanocubes and rGO. In another study, In2O3 cubes/rGO sheet composites were prepared from InN NWs and GO as the primary precursors using a microwave-assisted hydrothermal technique.205 Here, graphene displayed a key role in determining the transformation of InN-NWs to In2O3 nanocubes. Different InN to GO mass ratios (i.e., 0.1:1, 0.5:1, 1:1, 3:1, and 5:1) were used to produce various composite structures and distribute In2O3 on the graphene matrix. The In2O3 cubes/rGO (1:1) sensor showed the strongest response (Ra/Rg − 1 = 60.8%) in the presence of 50% RH at RT, with good selectivity to NO2 (1 ppm). The high sensing performance of the In2O3 cubes/rGO sensors to NO2 was attributed to the numerous wrinkles in the crumpled graphene sheets, where In2O3 cubes were distributed uniformly over the matrix. In addition, the electron retreating nature of NO2, the oxygen atom-rich functional groups, and the defect sites of In2O3 cubes/rGO also resulted in the high selectivity of NO2 sensing.
Gu et al.206 developed In2O3–GO nanostructures using hydrazine hydrate-reduced GO as the primary precursor. The response of the In2O3–GO nanostructures sensor to 30 ppm NO2 was 8.25 at RT with a long response time/recovery time (4 min/24 min). Fang et al.207 reported NO2 gas sensing based on In2O3 NRs decorated on rGO (In2O3 NRs/rGO), synthesized from the reflux process using In3+, GO, and urea precursors solution, followed by calcination (550 °C). The sensor showed a strong response to NO2 at RT. The good performance was attributed to the gas diffusion of NO2 in the mesopores of In2O3NRs and the highly efficient electron mobility of rGO. Yan et al.208a developed 1D porous In2O3 NFs and rGO heterojunction composite (rGO–In2O3 NFs) to examine NO2 gas sensing. Different amounts of rGO (1.1 to 3.6 wt%)–In2O3 NF composites were synthesized by varying the amount of rGO using an electrospinning technique. The gas detection properties showed that 2.2 wt% rGO–In2O3heterojunction NFs exhibited improved results compared to pure In2O3NFs. On the other hand, an excess of rGO caused a reduction in the sensing response.
Shah et al.208b synthesized In2O3@GO nanocomposites using a simple precipitation method for NO2 gas sensing applications. The gas sensing performance was optimized by tuning the percentage of GO in the nanocomposite. At 4 wt% GO, the sensor exhibited a high response of 78 to 40 ppm NO2 gas at 225 °C, which was 13 times higher than pure In2O3 NPs. This was attributed to the high surface area (1336 m2 g−1) and formation of p–n heterojunctions in the In2O3@GO nanocomposites. In particular, the uniform distribution of In2O3 NPs onto the GO-surface formed junctions for ion–electron interaction reactions. The work function of GO (4.39 eV) was higher than that of In2O3 NPs (4.15 eV), forming a heterojunction. The crumpling and overlapping regions of GO sheets offer a high surface area that facilitates the operative access of gases to the In2O3 surface. Hence, In2O3@GO nanocomposites exhibited improved sensing performance towards NO2 gas.
Nowadays, portable devices capable of detecting and analyzing environmental signals have shown numerous applications in modern electronics. In this regard, flexible gas sensors have been increasingly attractive because of their several advantages, such as portability, miniaturization, and wearable features. You et al.209 reported a unique laser writing technology to synthesize In2O3@rGO composites for RT-operated NO2 sensing. This process assists in the photo reduction of GO and enables a programmable patterning on the flexible substrates, e.g., polyimide (PI), within a short period. The resulting In2O3@rGO sensor showed good sensitivity (31.6%), high selectivity towards 1 ppm NO2, and excellent stability at RT. These results were caused by the porous structure of graphene, which resulted from the laser writing.
Na et al.210 prepared rGO–In2O3 nanocomposites (Fig. 18a) using a solvothermal reaction of In3+ and rGO precursors, followed by drop-casting on a flexible PI substrate made from ITO/Ag–Pd–Cu electrodes for the analysis of its NO2 gas-sensing properties (Fig. 18b). The rGO–In2O3 nanocomposites sensor demonstrated a strong response value (35.7) to 500 ppb NO2, which is in contrast to that of the pristine In2O3 sensor (2.6) at 120 °C (Fig. 18c). Fig. 18d and e shows the sensing response of pristine and rGO–In2O3 nanocomposite gas sensor. rGO has outstanding electron-acceptor properties. Thus, reducing the conduction path along In2O3 NPs by intensifying the depletion layer close to the p–n interface among In2O3 and rGO was assumed to have a probable cause for increasing the sensor resistance (Fig. 18d and e).
Fig. 18 (a) Morphology of the rGO–In2O3 nanocomposites, (b) schematic diagram of the flexible gas sensor, (c) response of the gas sensor to different gases at diverse temperatures, (d) gas-sensing mechanism of the pristine In2O3 sensor, and (e) gas-sensing mechanism of rGO–In2O3 nanocomposite sensor. Reproduced from ref. 210 with permission from Elsevier (Copyright 2020). |
The authors attributed the enhancement in response performance to the higher electron depletion layer in In2O3 NPs caused by the development of a p–n junction interface between the p-type rGO and n-type In2O3. In the air environment, O2 ions were formed by capturing electrons from p-type In2O3–rGO hybrids. Because NO2 gas has high electron affinity, it can capture the electrons from In2O3–rGO hybrids after exposure. As a result, larger hole accumulation layers with higher potential barriers formed. Hence, the final resistance decreased immediately, leading to a high response value and rapid response time. The equations for the sensing reaction are as follows:
NO2(g) + e− → NO2−(surface) | (17) |
2NO2(g) + O2−(ads) + e− → 2NO3−(ads) | (18) |
MOS-based chemiresistive sensors work at relatively high temperatures, which increases material instability and reduces the response. In addition, they enhance the risk associated with detecting combustible gases at high temperatures. Hence, polymer-based gas-detecting materials can be used as an alternative sensing material, given that they reduce the risk of these problems, allowing sensing measurement at RT. In this respect, the conducting polymers (CPs), e.g., polyaniline (PANI), poly(3-hexylthiophene) (P3HT), polypyrrole (PPy), polythiophene (PTh), and poly(3,4-ethylenedioxythiophene) (PEDOT), have attracted significant attention because of their excellent electrical and mechanical characteristics. These CPs contain π-bonds in their backbone chain, facilitating electron mobility through the polymeric chain. The presence of delocalized p-electrons on the backbone of these CPs is responsible for their unique electrical properties. Since 1980, some polymers have been used as sensing materials because of their electrical conductivity. Generally, CPs have been coupled with inorganic oxides in designing novel organic–inorganic heterostructures.211–213 Doping and the incorporation of metal–oxides into these polymers can improve gas-sensing characteristics.214 On the other hand, few studies have reported In2O3-based composites with CPs for gas-sensing applications. Moreover, PANI was explored in most of these reports. For example, Nie et al. reported215 an In2O3/PANI composite NF-based sensor synthesized through in situ polymerization by varying the aniline to In2O3 mass ratios (i.e., 1:1, 2:1, and 4:1). The In2O3/PANI-2 NFs sensor showed the strongest response (53.20) to 1000 ppm NH3 at RT. The improvement in the sensing characteristics of the In2O3/PANI nanofiber sensor was attributed to p–n junction formation among PANI and In2O3.
Li et al.216 prepared a Au-loaded In2O3(2 to 50 mol%) @PANI C–S hybrid-structured sensing nanocomposite material (PANI/In2O3) (FESEM and TEM images are shown in Fig. 19a–d) using a combination of an in situ polymerization and a facile hydrothermal process for NH3 gas sensing. The Au-loaded PANI/In2O3 sensing device was fabricated on a flexible PET substrate and tested at RT. The 1 at% Au-loaded 20 mol% In2O3nanosphere@PANI (1Au–2In2O3/PANI) C–S hybrid sensor displayed a response of approximately 46 to NH3(100 ppm) at RT. The enhanced NH3 sensing performance was attributed to the developed p–n junction between PANI/In2O3 and the catalytic activities of Au.
Fig. 19 FESEM images of (a) In2O3 nanosphere, (b) pristine PANI, (3) Au-loaded In2O3/PANI, and (d) TEM image of Au-loaded In2O3/PANI. (e) Schematic diagram of the sensing mechanism of In2O3@PANI and Au-loaded In2O3@PANI. Reproduced from ref. 216 with permission from Elsevier (Copyright 2020). |
Fig. 19e presents the sensing mechanism, which indicates the development of PANI–In2O3 heterojunctions and the chemical sensitization effect caused by Au. In this case, the NH3 detection mechanism of the PANI sensor was explained using a well-known protonation/deprotonation process.216–219 Briefly, when the PANI sensor is exposed to NH3 gas, NH3 captures H-ions from the acidified PANi. In this way, the PANI is reduced to the intrinsically non-conducting emeraldine base state from the conducting emerald salt state, which then causes a change in the resistance of PANI. On the other hand, the mechanism for enhancing the response of the PANI/In2O3 sensor was described using a spill-over mechanism resulting from catalytic Au NPs (Fig. 19e). In this case, NH3 molecules could spill at the Au surface and adsorb with the interaction among the p-orbital of NH3 and d-orbital of Au atoms (NH3 → NH2 + H, NH2 → NH + H). Subsequently, the formed H-atoms have greater reducibility, favoring an efficient PANI reaction and enhancing the sensor response.
Amu-Darko et al.220 developed a PANI-doped In2O3 nanosheet heterojunction using a hydrothermal method for NO2 sensing applications. The porous sheets provided numerous surface adsorption sites and gas diffusion paths, enhancing the gas-sensing properties. The In2O3/PANI-1 composite exhibited significant sensitivity (R = 341.5 to 30 ppm), good selectivity, and a low detection limit (0.3 ppm) for NO2 gas at 250 °C. Fig. 20 shows a schematic diagram of the NO2 sensing mechanism of the In2O3/PANI nanosheets. In air, oxygen is adsorbed onto the surface of the composite material and generates ionized oxygen ions (O2−(ads)) by capturing electrons from the conduction band. This forms active sites on the material surface interacting efficiently with NO2 gas. When the sensor is exposed to NO2 gas, the NO2 gas molecules compete with O2 molecules for adsorption onto the surface. Owing to the high electron affinity of NO2, it can draw electrons from the surface, causing an increase in the sensor resistance and the depletion layer thickness. The heterojunction interaction between the In2O3 and PANI materials is critical in this process. Heterojunctions develop at the contact between In2O3 and PANI, with electrons and holes diffusing in opposite directions because of the variation of the same charge density. The system has comparable Fermi levels when all the charge dispersions are balanced, resulting in the blending of the bands. This blending of the bands enables efficient interaction between the surface-active sites and the NO2 gas molecules. Upon re-exposure to air, the captured electrons are released from the NO2 gas molecules and revert to the conduction band of the material. This causes the resistance of the sensor to return to its initial value.
Fig. 20 Schematic diagram of the NO2 gas sensing mechanism of In2O3/PANI nanosheets. Reproduced from ref. 220 with permission from Elsevier. |
Morphology | Gas and conc. (ppm or ppb) | T (°C) or room tempt. (RT) | Response (Ra/Rg) or (Rg/Ra)* or [(Ra − Rg/Ra) × 100]** | Response time (s)/recovery time (s) | Ref. |
---|---|---|---|---|---|
Pristine In 2 O 3 gas sensors | |||||
In2O3 microcubes | NO2/100 ppm | 100 | 1401 | 16 s/165 s | 31 |
In2O3 NPs | HCHO/100 ppm | 230 | 80 | 100 s/70 s | 43 |
In2O3 nanosheets | NO2/97 ppm | RT | 89.48* | 16.6 s/NA | 45 |
In2O3 NPs | H2S/1 ppm | 25 | 26268.5 | NA | 112 |
In2O3 mesoporous NPs | H2/500 ppm | 260 | 18 | 1.7 s/1.5 s | 114 |
In2O3 NPs | NO2/1 ppm | 120 | 371.9 | 148 s/72 s | 115 |
In2O3 nanospheres | TEA/50 ppm | 320 | 400 | 1 s/71 s | 117b |
In2O3 nanospheres | C2H5OH/200 | 240 | 180 | 1 s/93 s | 117b |
In2O3 spheres | C2H5OH/50 | 250 | 250 | 16 s/14 s | 117c |
In2O3 quantum dots | H2S/5 ppm | RT | 90 | 72 s/200 s | 119 |
In2O3 NFs | CH3COCH3/100 ppm | 180 | 72 | 1 s/NA | 126 |
In2O3 NFs | Acetic acid/2000 ppm | 250 | 66.7 | 25 s/37 s | 128 |
In2O3 nanosheets | Cl2/3 ppm | 200 | 2353.4* | 53 s/17 s | 132 |
In2O3 microcubes | C2H5OH/100 | 210 | 23 | NA | 137 |
In2O3 walnut-like | NO2/50 | RT | 219* | 89 s/80 s | 138 |
In2O3 urchin-like hollow spheres | HCHO/1 ppm | 140 | 20.9 | 50 s/NA | 139 |
In2O3 bricks | NO2/100 | 100 | 600* | 12 s/148 s | 141 |
In2O3 microcubes | NO2/10 ppm | 60 | 348.3 | NA | 142 |
In2O3 hollow microspheres | HCHO/10 ppm | 200 | 20 | 4 s/8 s | 143a |
Flower-like In2O3 | Isoprene/0.5 ppm | 190 | 3.1 | 53 s/299 s | 143b |
In2O3 hierarchical porous flower-like | CH3COCH3/50 ppm | 250 | 12 | 2 s/46 s | 144 |
Composite or metal-doped/functionalized In 2 O 3 gas sensors | |||||
La-doped In2O3 hollow microspheres | H2S/10 ppm | 200 | 17.8 | NA | 46 |
Mg-doped In2O3 NTs | H2S/10 ppm | 150 | 173.14 | NA | 125 |
Co3O4/In2O3 NRs | C2H5OH/200 ppm | 200 | 18 | 25 s/145 s | 152 |
PdO/In2O3 microspheres | CH3COCH3/100 ppm | 250 | 50.9 | 1 s/26 s | 153a |
MXene/In2O3 NPs | NH3/20 ppm | RT | 100.7% | 60 s/300 s | 153b |
MXene/In2O3 nanocomposite | HCHO/100 ppm | 100 | 106.82% | 1 s/1 s | 153c |
In2O3@SnO2 C–S NFs | HCHO/100 ppm | 120 | 180.1 | 3 s/3.6 s | 154 |
ZnO/In2O3 C–S NWs | C2H5OH/1000 ppm | 300 | 196% | NA | 155 |
ZIF-8@In2O3 NFs | NO2/1 ppm | 140 | 16.4 | 80 s/133 s | 156 |
WO3/In2O3 nanosheets | HCHO/100 ppm | 170 | 25 | 1 s/67 s | 158 |
WO3/In2O3 NFs | CH3COCH3/200 ppm | 275 | 29 | NA | 159 |
Au/In2O3 NFs | C2H5OH/NA | 140 | 13.8 | 12 s/24 s | 160 |
Au/mesoporous In2O3 | CH3COCH3/100 ppm | 250 | 19.01 | 25 s/31 s | 162a |
In2O3@AuIDE | Xylene/5 ppm | 325 | ∼400 | NA | 162b |
In2O3@PtIDE | C2H5OH/5 ppm | 350 | ∼400 | NA | 162b |
Au/3DOM In2O3 | CH3COCH3/5 ppm | 340 | 42.4 | 11 s/NA | 163 |
Ag/In2O3 NRs | H2S/20 ppm | RT | 93719 | 84 s/186 s | 164 |
Ag/In2O3 sunflowers | HCHO/20 ppm | 240 | 11.3 | 0.9 s/14 s | 168 |
Au/In2O3 NWs | CO/NA | RT | 104 | 130 s/50 s | 172 |
Ag/In2O3 nanograin | HCHO/1 ppm | RT | 135 | 102 s/157 s | 173 |
Pd/3DOM In2O3 | NO2/500 ppb | RT | 980 | 270 s/286 s | 174 |
Au@In2O3 C–S NPs | HCHO/ | 200 | 17 | 7 s/135 s | 183 |
Au@In2O3 C–S NPs | C2H5OH/100 ppm | 160 | 36.14 | 4 s/2 s | 184 |
Ag@In2O3 C–S NPs | C2H5OH/50 ppm | 220 | 72.56 | 13 s/8 s | 185 |
Au@In2O3 C–S NPs | H2/100 ppm | 300 | 34.38 | 31 s/10 min | 187 |
Pd@In2O3 yolk–shell NPs | C2H5OH/5 ppm | 350 | 159.02 | NA | 188a |
AuPd/In2O3 | TMA/100 ppm | 175 | 367 | 2 s/300 s | 188b |
Mesoporous Fe-doped In2O3 | NO2/1 ppm | 100 | 116 | 4.6 min/2.5 min | 189 |
Mesoporous Zr-doped In2O3 | NO2/1 ppm | 75 | 169 | NA | 190 |
Ce-doped In2O3nanospheres | Glycol/100 ppm | 240 | 63.4 | NA | 192 |
Ag/Ni-doped In2O3 NRs | HCHO/10 ppm | 160 | 123.97 | 1.45 s/58.2 s | 194a |
Ni–In2O3 NPs | NO2/10 ppm | 200 | 70 | 2 s/2 s | 194b |
S-doped In2O3 networks | C2H5OH/50 ppm | 180 | 74.4 | NA | 194c |
RGO/In2O3 nanocubes | CH3COCH3 and HCHO/25 ppm | 175 and 225 | 85% and 88%** | NA | 204 |
rGO/In2O3 cubes | NO2/5 ppm | RT | 37.81% | NA | 205 |
rGO/In2O3 NRs | NO2/97 ppm | RT | 1.45 | 25 s/NA | 207 |
In2O3–rGO NFs | NO2/5 ppm | 50 | 42 | 261 s/698 s | 208a |
In2O3–GO | NO2/40 ppm | 225 | 78 | 106 s/42 s | 208b |
In2O3@GO films | NO2/1 ppm | RT | 31.6% | 4.2 min/13.3 min | 209 |
rGO/In2O3 nanospheres | NO2/500 ppb | 120 | 35.7 | 137 s/540 s | 210 |
In2O3/PANI NFs | NH3/1000 ppm | RT | 53.20 | NA | 215 |
PANI/In2O3 C–S NPs (PAIn20A1) | NH3/100 ppm | RT | 46 | 118 s/144 s | 216 |
In2O3/PANI nanosheets | NO2/30 ppm | 250 | 341.5 | 24 s/53 s | 220 |
Overall, In2O3-based nanomaterials as sensing materials exhibited improved detection performance towards various target gases using different strategies, such as surface and interface engineering, noble metal doping, and loading with carbonaceous materials. Almost all the strategies outlined above to boost the gas-sensing performance of In2O3-based nanomaterials are highly effective. In terms of the synthesis of nanomaterials (from 0D to 3D), the synthesis of 2D In2O3-based nanomaterials is more challenging. Thus, the researchers privileged other types of structures because of their relatively moderate synthesis conditions. In particular, chemical methods, such as hydrothermal and solvothermal, are used widely for synthesizing different In2O3 nanostructures to detect various toxic gases and VOCs. The most radially detected gases by In2O3-based nanomaterials are NO2 and H2S, followed by VOCs. Among the various sensors listed in Table 2, the sensors based on In2O3 NPs112 and Ag–In2O3 NRs164 displayed extraordinary responses of 26268.5 (1 ppm) and 93719 (20 ppm) towards H2S at room temperature, which is considered outstanding in the area of chemiresistive gas-sensing.
Despite the significant advances in designing In2O3-based gas sensors, many challenges and problems remain to be overcome before high sensing properties can be achieved. Moreover, the high operating temperature is the main obstacle to its practical application. As a result, attention should be given to developing ambient temperature sensors to reduce energy consumption. In this respect, In2O3 with carbon and polymer materials was identified as an excellent choice for operation at low-temperature sensing. Nevertheless, there was a lack of available technologies to manufacture and integrate flexible sensors using nanomaterials, considering that flexible and wearable sensors could be used as direct points of care for real-time recognition of analytes. The advantages of RT operation could open the door for the design of wearable/flexible devices based on In2O3 sensors, but more emphasis on research is needed.
Furthermore, the detection capabilities and response/recovery speeds of In2O3-based sensors need to be enhanced for dynamic sensor design. Importantly, the relative humidity is a major concern because it strongly affects the response of In2O3 sensors. Therefore, continuous efforts are needed to design robust sensors based on In2O3 to maintain high performance across multiple cycles under different environmental conditions. In addition, it is vital to explore the improved mechanism, particularly in terms of selectivity in heterojunction nanostructures. Most of the studies reviewed explained that synergistic effects enhanced the sensing performance, but the reason for selectivity toward a particular gas was rarely described. Pattern recognition analysis via different sensing layers for designing sensor arrays was another important strategy in enabling accuracy for specific gases. This means that an appropriate technology or realistic models need to be developed to understand better the gas detection mechanism of nanomaterials towards a target gas. Finally, research in chemiresistive gas sensors is growing to boost the gas-sensing performances of MOS-based materials, and the number of related publications is increasing constantly. In this regard, establishing a database, accepting standardized sensing protocols, and integrating laboratory-made sensing devices into practical applications are necessary. Major collective efforts are necessary to address the abovementioned challenges. Nevertheless, In2O3-based nanomaterials have great potential as gas-sensing materials to detect various toxic gases and VOCs.
Footnote |
† These authors contributed equally to this work. |
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