Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2021 Jul 1;6(27):17442–17454. doi: 10.1021/acsomega.1c01831

Nanostructured Indium Oxide Thin Films as a Room Temperature Toluene Sensor

Sunil Gavaskar Dasari †, Pothukanuri Nagaraju ‡,*, Vijayakumar Yelsani §, Sreekanth Tirumala ∥, Ramana Reddy M V †
PMCID: PMC8280699  PMID: 34278130

Abstract

graphic file with name ao1c01831_0021.jpg

Toluene gas is the most toxic and affects the respiratory system of humans, and thereby, its detection at lower levels is an important task. Herein, we report a room temperature-operatable indium oxide-based chemiresistive gas sensor, which detects 50 ppm toluene vapors. Nanocrystalline indium oxide (In2O3) films were sprayed on a pre-cleaned glass substrate using a cost-effective spray pyrolysis method at different substrate temperatures in the range of 350–500 °C. The X-ray diffraction studies confirmed that the sprayed thin films, which were deposited at different substrate temperatures, exhibit a cubic structure. The preferred orientation was aligned along the (222) orientation. Average crystallite size calculation based on the Scherrer formula indicates that the crystallite size increases with the enhancement of substrate temperature. FESEM analysis showed that the indium oxide thin films possess uniform grain distribution, which persists over the entire substrate. As the substrate temperature is increased, a partial agglomeration in the film morphology was observed. The deposited film’s nanostructured nature was confirmed by transmission electron microscopy, and the polycrystalline nature was confirmed from the selected area electron diffraction pattern. Root mean square roughness of the samples was determined from the atomic force microscopy studies. From the Raman spectra, characteristic vibrational modes appeared at 558.61, 802.85, and 1097.18 cm–1 in all the samples, which confirms the cubic structure of indium oxide thin films. Photoluminescence emission spectra have been recorded with an excitation wavelength of 280 nm. The optical band gap was measured using the Tauc plot. The band gap was found to decrease with an increase in the substrate temperature. The gas-sensing performance of indium oxide films sprayed at various substrate temperatures has demonstrated a better response toward 50 ppm toluene gas at room temperature with good stability, and the response and recovery times were determined using a transient response curve.

Introduction

Metal oxide-based semiconductors have been widely investigated as sensing materials for a long time due to their abundant features, such as high sensitivity and selectivity toward volatile organic compounds.1 Among them, indium oxide (In2O3) has been found to have pronounced sensitivity toward ammonia, methane, acetone, and other species due to its outstanding performance in electrical conductivity and due to abundant oxygen vacancies.2,3 It is an n-type semiconductor with high stability and low specific resistance. Due to the direct band gap of ∼3.6 eV, it is a significant transparent conducting oxide material. It has received much attention in the fields of solar cells, optoelectronic devices, organic light-emitting diodes, photocatalysts, architectural glasses, field-emission catalysis, and sensors.4−8 In2O3 has been prepared using thermal hydrolysis, the sol–gel technique, thermal decomposition, microemulsion, mechanical and chemical processing, pulsed laser deposition, and spray pyrolysis.9−16 Among all the techniques described above, spray pyrolysis is a cost-effective and straightforward method. It has been investigated for the large-area deposition applications.

Gas sensors are designed to trace the low concentration of hazardous gases in the environment. These will play a vital role in many fields such as industrial control systems, household safety and security, fuel emission, and environmental pollution monitoring. Recently, various kinds of gas sensors are fabricated based on different sensing materials and their transduction principles. Among all the gas sensors, chemiresistive-type semiconducting metal oxide-based gas sensors are the essential materials to trace low concentrations of toxic gases such as benzene, xylene, acetone, ethylbenzene, toluene, and so forth. Among all these gases, toluene (C7 H8) is the most harmful air pollutant with its wide range of applications as diluents and adhesive material in the decoration of interiors, which is firmly associated with our day-to-day lives and finally causes severe harm to human health with direct long-term exposure.17 Consumption of toluene in low to moderate levels can cause weakness, tiredness, drunken-type actions, confusion, nausea, loss of appetite, memory and hearing loss, and even color vision loss. Few of these syndromes generally vanish when inhalation is impeded. Inhaling massive levels of toluene in a short time may cause unconsciousness, nausea, sleepiness, light headedness, and even death.18 According to the suggestions of the OSHA, the short-term exposure limit of toluene is 100 ppm. It is associated with nasopharyngeal cancer, bronchial and chronic asthma, and other various subjective health problems. Also, toluene can be treated as an essential lung cancer biomarker.19,20

In this present work, an attempt was made to fabricate a cost-effective indium oxide sensor to trace low concentrations of toluene at room temperature.

Results and Discussion

Thickness Measurement

The thickness of the thin films deposited at different substrate temperatures is depicted in Figure 1. As the substrate temperature is increased, the thickness of the films is found to decrease. It may be due to the probability of the decomposition of the aerosol solution droplets near the preheated glass substrate, leading to a reduction in the particles’ transportation in the direction of the substrate, and also, the sticking coefficient of an atom may be significantly less at higher substrate temperatures.21

Figure 1.

Figure 1

Open in a new tab

Variation of indium oxide thin-film thickness with substrate temperature.

X-ray Diffraction

The structural characterization of the indium oxide thin films sprayed at different substrate temperatures was studied using an X-ray diffractometer in a 2θ range of 15–80°. The X-ray diffraction (XRD) patterns of indium oxide films are shown in Figure 2. It was noticed that indium oxide thin films are polycrystalline with a cubic structure without any additional impurities. All the diffraction peaks are listed using the Joint Committee on Powder Diffraction Standards, card no: 88-2160. The diffraction peaks appeared at 21.59, 30.66, 35.48, 45.78, 51.21, and 60.86, corresponding to (211), (222), (400), (431), (440), and (622) miller index planes, respectively. The intensity of the (222) peak is decreased with an increase in the substrate temperature. The average crystallite size has been calculated with Scherer’s equation. The crystallite size increased with increasing substrate temperature, as shown in Figure 3. It might be due to the recrystallization process during the deposition22

graphic file with name ao1c01831_m001.jpg 1

where “θ” is the diffraction angle, “β” is full width at half maxima, and “λ” is the monochromatic X-ray wavelength.

Figure 2.

Figure 2

Open in a new tab

XRD spectra of In2O3 thin films.

Figure 3.

Figure 3

Open in a new tab

Average crystallite size of In2O3 thin films at different substrate temperatures.

The dislocation density (δ) is defined as the length of dislocation lines per unit volume of the crystal and was determined using the following relation23,24

graphic file with name ao1c01831_m002.jpg 2

Dislocation density is decreased with increasing substrate temperature, as the dislocation density indicates the dislocation network in the indium oxide thin films. The reduction in dislocation density reflects the emergence of good-quality thin films at higher deposition temperatures.25 The variation of dislocation density is shown in Figure 4. The dislocation density is significantly low at high substrate temperature, leading to more carrier density and surface oxygen adsorption, which leads to enhancement of the gas-sensing properties of indium oxide thin films.

Figure 4.

Figure 4

Open in a new tab

Dislocation density of the In2O3 thin films.

FESEM with EDX

The surface morphology of the thin films depends on the preparation technique and its deposition parameters. The FESEM images of indium oxide films which are sprayed at different deposition temperatures are depicted in Figure 5. All the thin films are found to be well adhered to the substrate without any pinholes. All the films possess uniform grain distribution throughout the surface of the samples. The size of the grains is enhanced with increasing substrate temperature due to the agglomeration.26,27 The elemental analysis of thin films has been carried out using the energy-dispersive X-ray (EDX) spectrum, which validates the presence of indium and oxygen atoms only. The EDX spectra are shown in Figure 5a3–d3.

Figure 5.

Figure 5

Open in a new tab

FESEM, grain size, and EDX images of In2O3 thin films sprayed at (a) 350, (b) 400, (c) 450, and (d) 500 °C.

Atomic Force Microscopy

Atomic force microscopy (AFM) is extensively used to investigate the topographical features of the deposited indium oxide thin films at different substrate temperatures. Mainly, AFM studies assist in computing the effect of substrate temperature on the surface nature and investigating the crystal growth mechanism of the thin film. The topological properties will play an essential role in the present gas-sensing characterization. Two-dimensional images of indium oxide thin films deposited at different substrate temperatures are shown in Figure 6. The root mean square (rms) roughness of the deposited thin films is analyzed using Nanoscope E software, and it is found to decrease with increasing substrate temperature. Calculated rms roughness values are tabulated in Table 1. As substrate temperature is increasing, adatom mobility will be increased, which leads to a decrease in the surface roughness of the indium oxide thin film. Also, the film which is deposited at higher substrate temperature has perfect crystals with larger crystallite size, which will provide stronger interactions between target gas molecules and the indium thin film, in turn improving the sensitivity of the sensor element.

Figure 6.

Figure 6

Open in a new tab

AFM images of indium oxide thin films deposited at (a) 350, (b) 400, (c) 450, and (d) 500 °C.

Table 1. rms Roughness of Indium Oxide Films Sprayed at Various Deposition Temperatures.

s. no. deposition temperature (°C) rms roughness (nm)
1 350 16.8
2 400 14.3
3 450 6.04
4 500 1.33
Open in a new tab

Brunauer–Emmett–Teller Surface Area Analysis

To explain internal architectures, nitrogen adsorption–desorption investigations of the indium oxide thin film along with the corresponding pore diameter versus pore volume plot which is deposited at a substrate temperature of 500 °C are presented in Figure 7. In a mesoporous material, during the adsorption process, the molecules fill the higher energy sites which are near to the pore wall first and then lower energy sites which are away from the pore wall. When the adsorbed molecules in the opposing walls get closer, they collapse into thermodynamically lower energy states (capillary condensation), and during the desorption process, these collapsed molecules at lower energy need higher pressure drop to get desorbed, which results in hysteresis in the adsorption and desorption isotherm.28 Hence, it can be seen that the isotherm exhibits the IV-type mesoporous nature of the deposited indium oxide thin film. The indium oxide thin film sample’s BET surface is about 37.6 m2/g, which could provide large reaction sites to facilitate target gas molecules in the gas-sensing mechanism.29,30 Pore size distribution of the sample was determined by the BJH method, and it was found to be 14.6 nm.

Figure 7.

Figure 7

Open in a new tab

(a) BJH, (b) BET, and (c) N2 adsorption–desorption isotherm of the indium oxide thin film deposited at 500 °C.

Raman Spectroscopy

Raman spectroscopy is a non-destructive advanced technique for investigating metal oxides’ structural information and bringing out helpful information. Figure 8 shows the room temperature Raman spectra of indium oxide films sprayed at different temperatures. The active Raman modes are observed at a Raman shift of 558.61, 802.85, and 1097.18 cm–1. The peak that appeared at 558.61 cm–1 is most likely attributed to In2O3. Raman modes are observed at a Raman shift of 802.85 and 1097.18 cm–1 and are assigned to phonons associated with the cubic-structured indium oxide thin films.31,32 As the substrate temperature is enhanced, the full width half maxima are decreased. It is due to the reduction in the dislocation density due to a decrease in the intergranular volume fraction, which leads to increment in the size of the crystallite, which is in accordance with the XRD investigations.32

Figure 8.

Figure 8

Open in a new tab

Raman spectra of In2O3 thin films.

Photoluminescence Characterization

Photoluminescence (PL) spectra obtained at room temperature of the indium oxide thin film deposited at 500 °C are investigated using a xenon source with an excitation wavelength of 280 nm, as depicted in Figure 9. The standard indium oxide has a strong and broad emission peak near 330 nm. The peak near 387 nm is evoked owing to the exciton recombination process, which plays a vital role in influencing the optical emission.34,35 The emission at this wavelength might be due to the oxygen vacancies existing in the sample. The formation of oxygen vacancies in the deposited nanostructured In2O3 thin films can be described as follows. At the time of deposition, a few oxygen sites evolve into oxygen-deficient sites, or maybe, several intrinsic imperfections appear, which leads to the creation of oxygen vacancies. The crackdown of defect-related emission of indium oxide is related to the reconstruction of imperfect nanostructures. These generated oxygen vacancies will create a new energy level close to or within the indium oxide thin films’ energy band gap, which might generally be acting as deep defect donor levels. The ultraviolet emission of the indium oxide thin films could be the radiative recombination process of the electrons and photoexcited holes absorbed by oxygen vacancies.36

Figure 9.

Figure 9

Open in a new tab

PL spectra of the indium oxide thin film deposited at 500 °C.

Transmission Electron Microscopy

To understand the microstructure of the films, transmission electron microscopy (TEM) is carried out in detail. Figure 10a shows the TEM images of the indium oxide thin film deposited at a substrate temperature of 500 °C. Figure 10b depicts the selected area electron diffraction (SAED) pattern. It shows a set of diffraction rings, which indicates the polycrystalline nature of the prepared indium oxide thin films. The characteristic planes (211), (222), (431), and (400) correspond to the simple cubic structure of indium oxide, which is in agreement with XRD studies.

Figure 10.

Figure 10

Open in a new tab

TEM images of the In2O3 thin film deposited at 500 °C (a). (b) SAED of the In2O3 thin film deposited at 500 °C.

The lattice spacing (d) might be determined using the following formula37

graphic file with name ao1c01831_m003.jpg 3

where “λ” is the electron wavelength (0.0027 nm), L is the length of the camera (100 mm), and R is the radius of the concentric ring, which is estimated from the central bright spot. The calculated lattice spacing (d) values using the abovementioned eq 3 are good in agreement with the (222) orientation of the XRD studies.

Optical Properties

The optical properties of indium oxide thin films strongly depend on the microstructure, film thickness, and deposition parameters. Figure 11 shows the absorption spectra of indium oxide films sprayed at different deposition temperatures. Figure 12 depicts the transmittance spectra of indium oxide thin films prepared at various substrate temperatures. The average transmittance of the indium oxide films is more than 72% in the visible region. The transmittance is found to rise by enhancing the deposition temperature. It may be owing to the increased carrier concentration because of oxygen deficiency.11

Figure 11.

Figure 11

Open in a new tab

Absorption spectra of indium oxide thin films.

Figure 12.

Figure 12

Open in a new tab

Transmittance spectra of indium oxide films.

The optical band gap (Eg) of indium oxide thin films sprayed at various substrate temperatures is determined by adopting the Tauc expression using the absorption spectra.38

graphic file with name ao1c01831_m004.jpg 4

Tauc plots of indium oxide films are depicted in Figure 13. The optical band gap (Eg) will be determined by estimating the linear segment of the absorption line to the horizontal axis. Estimated optical energy band gaps of indium oxide thin films sprayed at various deposition temperatures are tabulated in Table 2. The optical band gap is decreased with increasing substrate temperature, and it is due to the quantum confinement effect when the particle size is in the nanoscale.

Figure 13.

Figure 13

Open in a new tab

Tauc plot of the indium oxide thin films deposited at (a) 350, (b) 400, (c) 450, and (d) 500 °C.

Table 2. Optical Band Gap of Indium Oxide Films.

s. no. deposition temperature (°C) optical band gap (eV)
1 350 3.72
2 400 3.69
3 450 3.67
4 500 3.62
Open in a new tab

Gas-Sensing Measurements

Sensitivity and Selectivity

The room temperature sensitivity of the indium oxide films, which are sprayed at different substrate temperatures toward 50 ppm toluene, is determined using eq 1. In comparison with toluene, several other gases such as methanol, ethanol, acetone, n-butanol, and benzene are tested at a concentration of 50 ppm toward different types of sensor elements sprayed at various substrate temperatures. The selectivity characteristics of indium oxide thin film sensors toward other gases are depicted in Figure 14. Thus, it is concluded that the sensor element, which is sprayed at a substrate temperature of 500 °C, has shown outstanding selectivity and sensitivity toward toluene at room temperature. It is well known that the gas-sensing mechanism in oxide-based materials is surface-controlled, and each chemical reaction depends on the activation energy of the material. An increase in film sensitivity with an increase in substrate temperature may be due to the decrease in activation energy at increased substrate temperature. Thus, a relatively higher sensitivity has been observed for the sample which is deposited at 500 °C.39 Also, toluene has shown the best selectivity due to its lower dissociation energy in comparison with that of other vapors. Due to the lower dissociation energy, bonds in toluene can be easily broken to behave with the sensing element; subsequently, a considerable number of free electrons are liberated at the time of the reaction, which causes enough change in the resistance of the sensor, affecting the high sensitivity toward toluene.40 Dong et al.41 reported hierarchical rosette-like In2O3 microsphere- and hollow microsphere-based sensors to detect toluene vapors at an operating temperature of 350 °C. Xiao et al.42 synthesized indium oxide nanotubes using an electrospinning method for toluene-sensing properties at an optimum operating temperature of 340 °C toward 100 ppm. Xu et al.43 investigated gas-sensing properties of hexagonal indium oxide nanorods prepared by a solvothermal method. They reported that indium oxide nanorods are sensitive toward different vapors at an operating temperature of 330 °C. Their sensor element has shown a response of 1.8 toward 500 ppm toluene. To the best of our knowledge and belief, majority of the indium oxide-based sensors are operated relatively at high operating temperatures to trace high concentrations of toluene vapors. Hence, our indium oxide thin film sensor, which is deposited with a cost-effective spray pyrolysis technique, is further utilized to study other gas-sensing characterizations such as stability, repeatability, and dynamic response.

Figure 14.

Figure 14

Open in a new tab

Response of In2O3 thin films deposited at different substrate temperatures (350, 400, 450, and 500 °C) toward 50 ppm of various gases at room temperature.

Stability and Repeatability

The long-term stability and repeatability of a sensor element will play an essential role in real-time gas sensor applications. The long-term stability of the indium oxide sensor deposited at 500 °C has been reported over a period of 30 days, as depicted in Figure 15. The sensor element has shown almost a stable response value during the period, as mentioned earlier, which indicates that the sensor has good stability. To investigate the repeatability, the gas-sensing test has been carried out continuously for four cycles toward 50 ppm toluene at room temperature, as shown in Figure 16; the response values have shown a negligible variation during the repeated cycles. Hence, we can conclude that the fabricated sensor has excellent repeatability property.

Figure 15.

Figure 15

Open in a new tab

Long-term stability of the pure In2O3 gas sensor deposited at 500 °C toward 50 ppm toluene at room temperature.

Figure 16.

Figure 16

Open in a new tab

Repeatability of indium oxide thin films sprayed at 500 °C toward 50 ppm toluene.

Transient Response Characteristics of the Sensor

The response–recovery studies investigate essential parameters in real-time application to detect harmful gases. The transient response toward 50 ppm toluene is studied at room temperature, as depicted in Figure 17. As noticed from the results, the indium oxide sensor deposited at 500 °C shows a classical n-type sensing behavior with large resistance in the presence of air, and the resistance of the sensor drops down when exposed to a reducing test gas (toluene). From the figure, it is clear that recovery and response times are 26 and 28 s, respectively. Comparison of toluene-sensing properties of different metal oxides available in the literature along with the present work is tabulated in Table 3.

Figure 17.

Figure 17

Open in a new tab

Transient response curves of indium oxide thin films which are deposited at 500 °C.

Table 3. Comparison of Toluene-Sensing Properties of Different Metal Oxides Available in the Literature along with the Present Work.
gas-sensing materials toluene concentration (ppm) operating temperature (°C) response/recovery times reference
Au/ZnO NPs 100 ppm 377 20 min/6 min (44)
clustering SnO2 NPs 50 ppm 250   (45)
CdS–TiO2 5000 ppm 27 70 s/125 s (46)
Ce–SnO2 coral-like 50 ppm 190   (47)
Pt NP-decorated In2O3 NFs 10 ppm 250   (48)
Au–ZnO NWs 500 ppm 340 32 s/57 s (49)
SnO2 NFs 1–300 ppm 350 1 s/5 s (50)
TiO2–ZnO Nanoflowers 100 ppm 290   (51)
MWCNT/SnO2 NCs 1000 ppm 250 24 s/14 s (52)
C-WO3 1000 ppb 320 40 s/10 s (53)
Au/ZnO nanoparticles 100 ppm 377 NA/300 s (54)
Co3O4 nanorods   200 90/55 (55)
pure In2O3 50 ppm 27 28 s/26 s present work
Open in a new tab

Conclusions

Indium oxide thin films are prepared using a cost-effective chemical spray pyrolysis technique. The effects of substrate temperature on microstructural, morphological, optical, and gas-sensing characteristics are systematically investigated. Crystallite size is increased and dislocation density is reduced with increasing deposition temperature. The nanostructured nature of the deposited thin film is confirmed using TEM. Atomic force microscopy studies revealed that the thin film sprayed at a deposition temperature of 500 °C exhibited the least rms roughness. The band gap of films is determined using the Tauc plot, and it is decreased with increasing substrate temperature. Gas-sensing characterization of indium oxide films is studied toward various volatile organic compounds such as acetone, methanol, ethanol, benzene, n-butanol, and toluene at room temperature. The film, which is sprayed at 500 °C, has exhibited the best sensitivity toward toluene at room temperature. It is also observed that the In2O3 thin film has shown excellent stability and repeatability with response and recovery times of 28 and 26 s, respectively.

Experimental Details

Materials and Thin-Film Preparation

Analytical-grade indium acetate (99.9% pure) was purchased from Sigma-Aldrich (India), and it is used as a starting precursor without further purification. The required amount of indium acetate was dissolved in deionized water and stirred on a magnetic stirrer for 30 min at room temperature. After 10 min, few drops of acetic acid were added to the abovementioned solution to obtain a clear solution. The glass substrates (Blue Star-India with a thickness of 1 mm) are cleaned with particle-free solution, then followed by ultrasonication in acetone and ethanol for 20 min, and cleaned with double-distilled water and heated at 100 °C using a programmable furnace for 30 min and dried in a hot air oven. The obtained solution is filled into a 50 mL quartz spray container. The solution is deposited with a computer-interfaced spray pyrolysis system at different substrate (deposition) temperatures ranging from 350 to 500 °C with a flow rate of 1 mL/min and deposited for 10 min. The nozzle-to-substrate distance was maintained at 25 cm, and filtered compressed air has been used as a carrier gas at a pressure of 1 bar. A schematic diagram of the spray pyrolysis system is depicted in Figure 18.

Figure 18.

Figure 18

Open in a new tab

Schematic diagram of spray pyrolysis equipment.

Material Characterization

Thin-film thickness was determined using a stylus profiler (SJ-301 Mitutoyo Surface Profilometer). The structural characterization and crystallinity of the indium oxide thin films sprayed at various substrate temperatures were investigated using a Bruker X-ray diffractometer (Bruker D8) with Cu Kα as a radiation source (0.154 nm) in the grazing incident mode with a rate of scanning 2°/min. The morphology of the indium oxide thin films has been investigated by FESEM (Carl ZEISS EVO-18, Germany), and elemental analysis of thin films was carried out using EDX. The surface roughness of the indium oxide films has been determined by AFM (Innova, Bruker). The indium oxide thin films’ optical properties and energy band gap were investigated using a UV–Vis spectrophotometer (Analytical jena, specord 210 Plus) in the spectral range of 400–800 nm. The variation in crystal defects of indium oxide thin films was investigated using Raman spectroscopy (Labram-HR800). PL spectra have been obtained using the HORIBA Fluorolog 3. TEM was carried out to explain the nanostructure of the thin films using the TECNAI 20G2 operating at 200 kV. The TEM investigations were performed in both the image and diffraction modes.

Fabrication of the Thin-Film Gas Sensor

To study the gas-sensing characterization of deposited indium oxide thin films, Ohmic contacts were made on both ends of the film with silver paste and copper wires. These sensor elements were sintered for 2 h at 150 °C to ensure good contact of electrodes. They were utilized to study the gas-sensing characteristics of different gases such as ethanol, acetone formaldehyde, ammonia, methanol, and toluene with a concentration of 50 ppm at room temperature. A schematic figure of the gas-sensing system is depicted in Figure 19. Humidity will decrease the stability of the sensor; hence, we have maintained the relative humidity in the chamber at 60% with the help of a digital humidity controller (Humitherm, India) during the gas-sensing measurements.56,57

graphic file with name ao1c01831_m005.jpg 5

where Rg = resistance of the sensor in the presence of target gas and Ra = resistance of the sensor in the presence of air.

Figure 19.

Figure 19

Open in a new tab

Schematic diagram of the gas-sensing system.

Acknowledgments

The authors thank the Head, Department of Physics, Osmania University, Hyderabad for providing necessary experimental facilities to carryout this work. DSG thankful to UGC-New Delhi, India for providing fincancial assistance in the form of NET-JRF during the tenure of this work. R.R.M.V. sincerely thanks science and engineering research board, New Delhi, for providing the necessary financial support under a research project with the file number (file no.: EMR/2017/002651) to carry out the present work. P.N. wishes to acknowledge the JNTU-Hyderabad for supporting the current work with reference number JNTUH/TEQIP-III/CRS/2019/Physics 01, and V.Y. wishes to acknowledge the JNTU-Hyderabad for funding the present work with reference number JNTUH/TEQIP-III/CRS/2019/Physics 03. The authors also thank the UGC-NRC, School of Physics, University of Hyderabad for providing FESEM facility. The authors would like express their sincere grattitude to Dr. N. P. Lalla, Scientist-H, Dr. Vasant Sathe, Scientist-H, UGC-DAE-CSR, Indore, for providing experimental facilities in their laboratories. The authors also express their sincere grattitude to Dr. P. S. Reddy, NIT-Goa, for providing AFM measurments.

The authors declare no competing financial interest.

References

  1. Williams D. E. Semiconducting oxides as gas-sensitive resistors. Sens. Actuators, B 1999, 57, 1–16. 10.1016/s0925-4005(99)00133-1. [DOI] [Google Scholar]
  2. Li C.; Zhang D.; Liu X.; Han S.; Tang T.; Han J.; Zhou C. In2O3 nanowires as chemical sensors. Appl. Phys. Lett. 2003, 82, 1613–1615. 10.1063/1.1559438. [DOI] [Google Scholar]
  3. Soulantica K.; Erades L.; Sauvan M.; et al. Synthesis of indium and indium oxidenanoparticles from indium cyclopentadienyl precursor and their application for gas sensing. Adv. Funct. Mater. 2003, 13, 553–557. 10.1002/adfm.200304291. [DOI] [Google Scholar]
  4. Li X.; Yao S.; Liu J.; Sun P.; Sun Y.; Gao Y.; Lu G. Vitamin C-assisted synthesisand gas sensing properties of coaxial In 2O3 nanorod bundles. Sens. Actuators, B 2015, 220, 68–74. 10.1016/j.snb.2015.05.038. [DOI] [Google Scholar]
  5. Koundourakis G.; Rockstuhl C.; Papazoglou D.; Klini A.; Zergioti I.; Vainos N. A.; Fotakis C. Laser printing of active optical microstructures. Appl. Phys. Lett. 2001, 78, 868–870. 10.1063/1.1348321. [DOI] [Google Scholar]
  6. Babu P. M.; Radhakrishnaa B.; Venkata Rao G.; Sreedhara Reddy P.; Uthanna S. J. Optoelectron. Adv. Mater. 2004, 6, 205–210. [Google Scholar]
  7. Gîrtan M.; Rusu G. On the Size Effect in In2O3 Thin Films. Mater. Sci. Eng., B 1999, XLV–XLVI, 166–172. [Google Scholar]
  8. Mailis S.; Boutsikaris L.; Vainos N. A.; Xirouhaki C.; Vasiliou G.; Garawal N.; Kiriakidis G.; Fritzsche H. Holographic recording in indium–oxide (In2O3) and indium–tin–oxide (In2O3:Sn) thin films. Appl. Phys. Lett. 1996, 69, 2459–2461. 10.1063/1.117497. [DOI] [Google Scholar]
  9. Veeraswamy Y.; Sunil Gavaskar D.; Choudhary R. J.; Phase D. M.; Ramana Reddy M. V. Structural and photoluminescence properties of Ta: In2O3 thin films grown by pulsed laser deposition technique. AIP Conf. Proc. 2018, 2005, 060003. 10.1063/1.5050757. [DOI] [Google Scholar]
  10. Senthilkumar V.; Vickraman P. Annealing temperature dependent on structural, optical and electrical properties of indium oxide thin films deposited by electron beam evaporation method. Curr. Appl. Phys. 2010, 10, 880–885. 10.1016/j.cap.2009.10.014. [DOI] [Google Scholar]
  11. Xu J.; Yang Z.; Zhang X.; Yang L.; Xu H.; Wang H. Study on Needles and Cracks of Tin-doped Indium Oxide Tablets for Electron Beam Evaporation Process. Mater. Res. 2015, 18, 519–524. 10.1590/1516-1439.310714. [DOI] [Google Scholar]
  12. Chung W.-Y. Gas-sensing properties of spin coted indium oxide film on various substrates. J. Mater. Sci.: Mater. Elec. 2001, 12, 591–596. 10.1023/A:1012497704963. [DOI] [Google Scholar]
  13. Sunde T. O. L.; Garskaite E.; Otter B.; Fossheim H. E.; Sæterli R.; Holmestad R.; Einarsrud M.-A.; Grande T. Transparent and conducting ITO thin films by spin coating of an aqueous precursor solution. J. Mater. Chem. 2012, 22, 15740–15749. 10.1039/c2jm32000b. [DOI] [Google Scholar]
  14. Ramanathan G.; Murali K. R. Dip Coated Indium Oxide Films and Their Optical Constants. Trans. Electr. Electron. Mater. 2020, 21, 513–518. 10.1007/s42341-020-00202-2. [DOI] [Google Scholar]
  15. Seki Y.; Sawada Y.; Wang M. H.; Lei H.; Hoshi Y.; Uchida T. Electrical properties of tin-doped indium oxide thin films prepared by a dip coating. Ceram. Int. 2012, 38, S613–S616. 10.1016/j.ceramint.2011.05.109. [DOI] [Google Scholar]
  16. Pramod N. G.; Pandey S. N.; Sahay P. P. Sn-Doped In2O3 Nanocrystalline Thin Films Deposited by Spray Pyrolysis: Microstructural, Optical, Electrical, and Formaldehyde-Sensing Characteristics. J. Therm. Spray Technol. 2013, 22, 1035–1043. 10.1007/s11666-013-9933-8. [DOI] [Google Scholar]
  17. Qiao L.; Bing Y.; Wang Y.; Yu S.; Liang Z.; Zeng Y. Enhanced toluene sensing performances of Pd- loaded SnO2 cubic nanocages with porous nanoparticle-assembled shells. Sens. Actuators, B 2017, 241, 1121–1129. 10.1016/j.snb.2016.10.024. [DOI] [Google Scholar]
  18. Xu F.; Guo S.; Luo Y.-L. Novel THTBN/MWNTs-OH polyurethane conducting composite thin films for applications in detection of volatile organic compounds. Mater. Chem. Phys. 2014, 145, 222–231. 10.1016/j.matchemphys.2014.02.006. [DOI] [Google Scholar]
  19. Nagaraju P.; Vijayakumar Y.; Reddy M. V. R.; Deshpande U. P. Effect of vanadium pentoxide concentration in ZnO/V2O5 nanostructured composite thin films for toluene detection. RSC Adv. 2019, 9, 16515–16524. 10.1039/c9ra02356a. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Kou L.; Zhang D.; Liu D. A Novel Medical E-Nose Signal Analysis System. Sensors 2017, 17, 402. 10.3390/s17040402. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Nagaraju P.; Vijayakumar Y.; Reddy G. L. N.; Ramana Reddy M. V. ZnO wrinkled nanostructures: enhanced BTX sensing. J. Mater. Sci.: Mater. Electron. 2018, 29, 11457–11465. 10.1007/s10854-018-9238-2. [DOI] [Google Scholar]
  22. Nagaraju P.; Vijayakumar Y.; Phase D. M.; Choudary R. J.; Ramana Reddy M. V. Microstructural, optical and gas sensing characterisation of laser ablated nanostructured ceria thin films. J. Mater. Sci.: Mater. Electron. 2016, 27, 651–658. 10.1007/s10854-015-3801-x. [DOI] [Google Scholar]
  23. Nagaraju P.; VijayaKumar Y.; Radhika P.; Choudhary R. J.; RamanaReddy M. V. Structural, morphological, optical and gas sensing properties of nanocrystalline ceria thin films. Mater. Today: Proc. 2016, 3, 4009–4018. 10.1016/j.matpr.2016.11.065. [DOI] [Google Scholar]
  24. Nagaraju P.; VijayaKumar Y.; Phase D.; Reddy V.; Ramana Reddy M. Preparation and microstructural characterisation of Si(100) Ce1–xGdxO2−δ thin films prepared by pulsed laser deposition technique. Mater. Sci. 2014, 32, 541–546. 10.2478/s13536-014-0246-5. [DOI] [Google Scholar]
  25. Nagaraju P.; Vijayakumar Y.; Choudhary R. J.; Ramana Reddy M. V. Preparation and characterisation of nanostructured Gd doped cerium oxide thin films by pulsed laser deposition for acetone sensor application. Mater. Sci. Eng., B 2017, 226, 99–106. 10.1016/j.mseb.2017.09.002. [DOI] [Google Scholar]
  26. Zhang L.; Lan J.; Yang J.; Guo S.; Peng J.; Zhang L.; Tian S.; Ju S.; Xie W. Study on the physical properties of indium tin oxide thin films deposited by microwave-assisted spray pyrolysis. J. Alloys Compd. 2017, 728, 1338–1345. 10.1016/j.jallcom.2017.09.024. [DOI] [Google Scholar]
  27. Heo K. C.; Son P. K.; Sohn Y.; Yi J.; Kwon J. H.; Gwag J. S. Characteristics of Ion Beam Assisted ITO Thin Films Deposited by RF Magnetron Sputtering. Mol. Cryst. Liq. Cryst. 2014, 601, 57–63. 10.1080/15421406.2014.940493. [DOI] [Google Scholar]
  28. Gondal M. A.; Dastageer M. A.; Oloore L. E.; Baig U.; Rashid S. G. Enhanced photo-catalytic activity of ordered mesoporous indium oxide nanocrystals in the conversion of CO2 into methanol. J. Environ. Sci. Health 2017, 52, 785–793. 10.1080/10934529.2017.1305173. [DOI] [PubMed] [Google Scholar]
  29. Wang Z.; Hou C.; De Q.; Gu F.; Han D. One-step Synthesis of Co-doped In2O3 Nanorods for High Response of Formaldehyde Sensor at Low Temperature. ACS Sens. 2018, 3, 468–475. 10.1021/acssensors.7b00896. [DOI] [PubMed] [Google Scholar]
  30. Zhang X.; Song D.; Liu Q.; Chen R.; Hou J.; Liu J.; Zhang H.; Yu J.; Liu P.; Wang J. Designed synthesis of Ag-functionalised Ni-doped In2O3 nanorods with enhanced formaldehyde gas sensing properties. J. Mater. Chem. C 2019, 7, 7219–7229. 10.1039/c9tc00978g. [DOI] [Google Scholar]
  31. Gwamuri J.; Marikkannan M.; Mayandi J.; Bowen P.; Pearce J. Influence of Oxygen Concentration on the Performance of Ultra-Thin RF Magnetron Sputter Deposited Indium Tin Oxide Films as a Top Electrode for Photovoltaic Devices. Materials 2016, 9, 63. 10.3390/ma9010063. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Chandrasekhar R.; Choy K. L. Innovative and cost-effective synthesis of indium tin oxide films. Thin Solid Films 2001, 398–399, 59. 10.1016/s0040-6090(01)01434-1. [DOI] [Google Scholar]
  33. Gu F.; Wang S. F.; Lü M. K.; Zhou G. J.; Xu D.; Yuan D. R. Photoluminescence Properties of SnO 2 Nanoparticles Synthesised by Sol–Gel Method. J. Phys. Chem. B 2004, 108, 8119–8123. 10.1021/jp036741e. [DOI] [Google Scholar]
  34. Narmada A.; Kathirvel P.; Mohan L.; Saravanakumar S.; Marnadu R.; Chandrasekaran J. Jet Nebuliser sray pyrolysed Indium Oxide and Nickel doped Indium Oxide thin films for photodiode application. Optik 2020, 202, 163701. 10.1016/j.ijleo.2019.163701. [DOI] [Google Scholar]
  35. Ullah H.; Yamani Z. H.; Qurashi A.; Iqbal J.; Safeen K. Study of the optical and gas sensing properties of In2O3 nanoparticles synthesised by rapid sonochemical method. J. Mater. Sci.: Mater. Electron. 2020, 31, 17474–17481. 10.1007/s10854-020-04303-9. [DOI] [Google Scholar]
  36. Vijayakumar Y.; Nagaraju P.; Yaragani V.; Parne S. R.; Awwad N. S.; Ramana Reddy M. V. Nanostructured Al and Fe co-doped ZnO thin films for enhanced ammonia detection. Phys. B 2020, 581, 411976. 10.1016/j.physb.2019.411976. [DOI] [Google Scholar]
  37. Bhat P.; Naveen Kumar S. K.; Nagaraju P. Fabrication of ultrasensitive hexagonal disc structured ZnO thin film sensor to trace nitric oxide. J. Asian Ceram. Soc. 2021, 9, 96. 10.1080/21870764.2020.1848036. [DOI] [Google Scholar]
  38. Shewale P. S.; Kamble B. N.; Moholkar A. V.; Kim J. H.; Uplane M. D. Influence of Substrate Temperature on H2S Gas Sensing Properties of Nanocrystalline Zinc Oxide Thin Films Prepared by Advanced Spray Pyrolysis. IEEE Sens. J. 2013, 13, 1992–1998. 10.1109/jsen.2013.2246760. [DOI] [Google Scholar]
  39. Wang L.; Li Y.; Yue W.; Gao S.; Zhang C.; Chen Z. High-Performance Formaldehyde Gas Sensor Based on Cu-doped Sn3O4 Hierarchical Nanoflowers. IEEE Sens. J. 2020, 20, 6945–6953. 10.1109/jsen.2020.2977972. [DOI] [Google Scholar]
  40. Dong H.; Liu Y.; Li G.; Wang X.; Xu D.; Chen Z.; Zhang T.; Wang J.; Zhang L. Hierarchically rosette-like In2O3 microspheres for volatile organic compounds gas sensors. Sens. Actuators, B 2013, 178, 302–309. 10.1016/j.snb.2012.12.103. [DOI] [Google Scholar]
  41. Chi X.; Liu C.; Zhang J.; Liu L.; Li H.; He Y.; Bo X.; Liu L. Toluene-sensing properties of In2O3 nanotubes synthesized by electrospinning. J. Semicond. 2014, 35, 064005. 10.1088/1674-4926/35/6/064005. [DOI] [Google Scholar]
  42. Xu J.; Chen Y.; Shen J. Ethanol sensor based on hexagonal indium oxide nanorods prepared by solvothermal methods. Mater. Lett. 2008, 62, 1363–1365. 10.1016/j.matlet.2007.08.054. [DOI] [Google Scholar]
  43. Suematsu K.; Watanabe K.; Tou A.; Sun Y.; Shimanoe K. Ultraselective Toluene Gas Sensor: Nanosized Gold Loaded on Zinc Oxide Nanoparticles. Anal. Chem. 2018, 90, 1959–1966. 10.1021/acs.analchem.7b04048. [DOI] [PubMed] [Google Scholar]
  44. Suematsu K.; Shin Y.; Hua Z.; Yoshida K.; Yuasa M.; Kida T.; Shimanoe K. Nanoparticle Cluster Gas Sensor: Controlled Clustering of SnO2 Nanoparticles for Highly Sensitive Toluene Detection. ACS Appl. Mater. Interfaces 2014, 6, 5319–5326. 10.1021/am500944a. [DOI] [PubMed] [Google Scholar]
  45. Partridge M.; Wong R.; James S. W.; Davis F.; Higson S. P. J.; Tatam R. P. Long period grating based toluene sensor for use with water contamination. Sens. Actuators, B 2014, 203, 621–625. 10.1016/j.snb.2014.06.121. [DOI] [Google Scholar]
  46. Qu Y.; Wang H.; Chen H.; xiao J.; Lin Z.; Dai K. Highly sensitive and selective toluene sensor of Ce-doped coral-like SnO2. RSC Adv. 2015, 5, 16446–16449. 10.1039/C4RA12315H. [DOI] [Google Scholar]
  47. Im D.; Park K.-B.; Kang J.-G.; Park J.-S.; Kwak S.-M.; Kwon S.-H.; Chun M.; Jung H. Toluene Gas Sensing Properties of Pt-Nanoparticle-Decorated Indium Oxide Nanofibers on a Low-Power Consumable Bridge-Type Micro-Platform. J. Korean Phys. Soc. 2019, 74, 600–606. 10.3938/jkps.74.600. [DOI] [Google Scholar]
  48. Wang L.; Wang S.; Xu M.; Hu X.; Zhang H.; Wang Y.; Huang W. A Au-functionalized ZnO nanowire gas sensor for detection of benzene and toluene. Phys. Chem. Chem. Phys. 2013, 15, 17179–17186. 10.1039/c3cp52392f. [DOI] [PubMed] [Google Scholar]
  49. Qi Q.; Zhang T.; Liu L.; Zheng X. Synthesis and toluene sensing properties of SnO2 nanofibers. Sens. Actuators, B 2009, 137, 471–475. 10.1016/j.snb.2008.11.042. [DOI] [Google Scholar]
  50. Zeng Y.; Zhang T.; Wang L.; Kang M.; Fan H.; Wang R.; He Y. Enhanced toluene sensing characteristics of TiO2-doped flowerlike ZnO nanostructures. Sens. Actuators, B 2009, 140, 73–78. 10.1016/j.snb.2009.03.071. [DOI] [Google Scholar]
  51. Aroutiounian V. M.; Adamyan Z. N.; sayunts A. G.; Khachaturyan E. A.; Adamyan A. Z.. Study of MWCNT/SnO2 Nanocomposite Acetone and Toluene vapor Sensors. Proceedings Sensor, 2015; pp 836–841.
  52. Deng L.; Ding X.; Zeng D.; Zhang S.; Xie C. High Sensitivity and Selectivity of C-Doped WO3 Gas Sensors Toward Toluene and Xylene. IEEE Sens. J. 2012, 12, 2209–2214. 10.1109/jsen.2011.2179980. [DOI] [Google Scholar]
  53. Suematsu K.; Watanabe K.; Tou A.; Sun Y.; Shimanoe K. Ultraselective toluene-gas sensor: Nanosized gold loaded on zinc oxide nanoparticles. Anal. Chem. 2018, 90, 1959–1966. 10.1021/acs.analchem.7b04048. [DOI] [PubMed] [Google Scholar]
  54. Wang L.; Deng J.; Lou Z.; Zhang T. Nanoparticles-assembled Co3O4 nanorods p-type nanomaterials: One-pot synthesis and toluene-sensing properties. Sens. Actuators, B 2014, 201, 1–6. 10.1016/j.snb.2014.04.074. [DOI] [Google Scholar]
  55. Yuan T.; Li Z.; Zhang W.; Xue Z.; Wang X.; Ma Z.; Fan Y.; Xu J.; Wu Y. Highly sensitive ethanol gas sensor based on ultrathin nanosheets assembled Bi2WO6 with composite phase. Sci. Bull. 2019, 64, 595–602. 10.1016/j.scib.2019.04.014. [DOI] [PubMed] [Google Scholar]
  56. Xue Z.; Yan M.; Yu X.; Tong Y.; Zhou H.; Zhao Y.; Wang Z.; Zhang Y.; Xiong C.; Yang J.; Hong X.; Luo J.; Lin Y.; Huang W.; Li Y.; Wu Y. One-Dimensional Segregated Single Au Sites on Step-Rich ZnO Ladder for Ultrasensitive NO2 Sensors. Chem 2020, 6, 3364–3373. 10.1016/j.chempr.2020.09.026. [DOI] [Google Scholar]

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES