Noble Metal-Decorated In₂O₃ for NO₂ Gas Sensor: An Experimental and DFT Study

Abstract

Indium oxide-based gas sensors have been proven to be a promising material for detecting nitrogen dioxide (NO₂) gas because of its wide bandgap and stability. In this paper, the enhancement mechanism for the sensitivity of indium oxide NO₂ gas sensors was systematically investigated using density functional theory (DFT) calculations and experimental validation with noble metals like Au, Ag, Pt, Pd, and Cu. We have fabricated a GaN nanowire-based NO₂ gas sensor functionalized with In₂O₃ and decorated with noble metals using a standard fabrication technique. Experimental tests showed that Au/In₂O₃ sensors exhibited the highest response of 38.9% followed by bare In₂O₃ with 10% for 10 ppm NO₂ at room temperature. The sensing properties were mainly attributed to a spillover effect or catalytic performance of Au with In₂O₃. The adsorption energies, charge transfers, and band gap confirm the enhanced sensing capability of Au-decorated Indium oxide for a NO₂ gas sensor.

1. Introduction

The early detection and continuous monitoring of toxic gases is of paramount importance, due to their potential harmful effects on both human health and the environment. Many industrial processes, vehicular emissions, and natural events contribute to the release of hazardous gases such as nitrogen dioxide (NO₂), carbon monoxide (CO), chlorine (Cl₂), and sulfur dioxide (SO₂). Prolonged exposure to these pollutants can lead to respiratory diseases, cardiovascular issues, and environmental degradation including acid rain and photochemical smog. Consequently, there is a growing need for reliable, sensitive, and cost-effective gas sensors that can operate in diverse environmental conditions and detect low concentrations of harmful gases with high accuracy [1,2,3,4,5].

Among various types of gas sensors, metal oxide semiconductor (MOS)-based sensors have garnered significant attention in recent years. Their popularity stems from several advantages, such as high surface-to-volume ratio, low cost, fast response and recovery times, ease of fabrication, and tunable sensitivity through structural and compositional modifications. These sensors operate on a simple yet effective principle: the change in the electrical resistance of the sensing material upon exposure to target gases. In ambient air, oxygen molecules are adsorbed onto the surface of the metal oxide and capture free electrons from the conduction band, forming ionized oxygen species. This process leads to the formation of an electron depletion layer at the surface, which increases the sensor’s resistance. Upon exposure to reducing gases like CO or SO₂, the gases react with the adsorbed oxygen, releasing the trapped electrons back into the conduction band, thereby reducing the resistance. In contrast, oxidizing gases like NO₂ and O₃ can further extract electrons, increasing the resistance [5,6,7,8,9,10].

Enhancing the performance of MOS gas sensors is a major area of research. Structural modifications such as transforming the sensing layer into nanowires, nanoribbons, ultrathin nanotubes, or two-dimensional flakes significantly increase the surface area available for gas interaction. Additionally, doping or decorating the sensor surface with noble or transition metals such as Ag, Pd, Cu, and Au improves the sensor’s response, due to phenomena such as the spillover effect, electronic sensitization, and catalytic enhancement. These modifications not only increase the number of active sites for gas adsorption but also alter the electronic structure of the sensing material, thereby improving sensitivity and selectivity. Light activation is another method used to enhance gas-sensing performance by generating additional charge carriers or activating the catalyst at lower operating temperatures [11,12,13,14,15].

Commonly used metal oxides in gas sensing include SnO₂, In₂O₃, WO₃, ZnO, and TiO₂. Among them, indium oxide (In₂O₃), an n-type semiconductor with a wide bandgap (Eg = 3.55–3.75 eV), is particularly attractive due to its excellent photocatalytic activity, chemical stability, and high electron mobility [16,17,18,19,20]. Several studies have demonstrated that modifying In₂O₃ with noble or transition metals significantly enhances its sensing performance. For example, Ag-decorated In₂O₃ exhibited improved adsorption energy for formaldehyde (HCHO), changing from −1.6 eV to −0.84 eV, owing to an increase in surface area and enhanced catalytic activity [21]. Similarly, In₂O₃ decorated with CuO nanowires via hydrothermal synthesis showed improved gas response due to the formation of p-n heterojunctions and increased active sites [22,23].

Recent advancements in heterostructure engineering have further boosted the performance of In₂O₃-based sensors. Rb-loaded ZnO/In₂O₃ heterojunctions demonstrated a sensor response of 24.2 for 1 ppm NO₂ with a rapid response time of just 55 s [8]. Another notable example includes In₂O₃–graphene–Cu nanocomposites, which achieved a sensitivity of 46 ppm to NO₂, attributed to the synergistic effect of highly defective graphene and the catalytic nature of Cu nanoparticles [6]. Similarly, a modified WS₂ microsheet with In₂O₃ and Au yielded an impressive response of 46.91 for 50 ppm NO₂, mainly due to the increased ionized oxygen adsorption sites [9]. Furthermore, the performance of a Pd-AlGaN/GaN NO₂ sensor was enhanced by optimizing the gate bias voltage, achieving a sensitivity of 91.6% with a reduced response time of just 9 s at −1V bias [24]. ITO thin films deposited on AlN ceramic substrates exhibited maximum responses of 2.21 to 2000 ppm H₂ at 400 °C, 2.39 to 1000 ppm NH₃ at 350 °C, and 2.14 to 100 ppm NO₂ at 350 °C, demonstrating their effective gas-sensing performance at elevated temperatures [25]. NO₂ gas sensors based on laser-induced graphene (LIG) and its heterostructure with SnO₂ were developed for room-temperature operation. The sensors feature a highly porous architecture synthesized through a one-step laser-scribing process. The LIG/SnO₂ hybrid sensor demonstrated effective NO₂ detection at room temperature. With increasing temperature, the sensor’s response improved; however, its sensitivity to different NO₂ concentrations decreased. Additionally, in humid environments, the response rate increased. Selectivity tests using CO₂ showed no notable response, confirming the sensor’s specificity toward NO₂ [26,27]. This study investigates the gas-sensing performance of tin oxide (SnO₂) nanoparticles for the detection of NO₂, NH₃, CO, and H₂S across a range of operating temperatures. SnO₂ powders were synthesized via the sol–gel method, and thin films were deposited using spin coating. At room temperature, the sensor exhibited sensitivity to NO₂ at a low concentration of 2 ppm, with response and recovery times of 184 s and 432 s [28]. Light-enhanced ultra-sensitive NO₂ gas detection was achieved using flexible In₂O₃/MXene-based sensors, demonstrating excellent linearity in the 5–100 ppb range and a remarkably low detection limit of 0.79 ppb [29].

To gain a deeper understanding of the sensing mechanisms and to guide the design of novel sensor materials, first-principles calculations such as with the application of density functional theory (DFT) have been extensively employed. DFT provides valuable insights into the electronic and structural properties of materials, including adsorption energy, charge transfer, adsorption distance, energy gap, and density of states (DOS). These computational studies allow researchers to evaluate various combinations of sensing materials and predict their sensing behavior before experimental realization. By simulating gas adsorption on different material surfaces, DFT helps in identifying the optimal doping levels, suitable metal decorations, and appropriate heterostructure configurations for enhanced sensitivity and selectivity.

In this study, we focus on improving the NO₂ gas-sensing performance of indium oxide through decoration with noble metals. Our aim is to systematically investigate the interaction of NO₂ with modified In₂O₃ surfaces using both experimental characterization and DFT-based simulations. The outcomes of this research are expected to contribute to the development of next-generation gas sensors that are not only highly sensitive and selective but also energy-efficient and capable of real-time monitoring in practical environments.

2. Device Fabrication and Characterization

The GaN on silicon wafers were purchased from EpiGaN. All the metal-oxide sputtering targets were obtained from Kurt J. Lesker company.

The sensors were fabricated following steps similar to those of our prior paper [30]. Sensor fabrication was started with a standard RCA cleaning process, followed by a stepper lithography-assisted dry-etching process to form GaN nanowires of width 200–400 nm. To protect GaN nanowires during inductively coupled plasma etching, process patterned metals were used. With a standard electron beam evaporator, ohmic contacts were formed. Then, plasma-enhanced chemical vapor deposition was used to deposit the SiO₂ layer, and also to protect nanowires, which are metal contacts that can be damaged by high-temperature processing or etching. Active area on GaN nanowire was created by a reactive ion-etching process to functionalize it with metal oxide. A thin layer of In₂O₃ was deposited by RF magnetron sputtering, and then noble metals (Au, Ag, Pt, Pd and Cu) were deposited to form a sensor, shown in Figure 1a. The In₂O₃ layer thickness was adjusted by controlling the sputtering deposition by varying the doping concentration from 1 × 10¹⁷ cm⁻³ to 1 × 10²⁰ cm⁻³, depending on the sputtering and annealing conditions.

To complete the fabrication process, a rapid thermal annealing at 600–700 °C was performed to crystallize the noble metal-decorated In₂O₃. This also helps to improve the ohmic contacts. The microstructure and surface morphology of the fabricated metal-oxide/GaN sensors were analyzed using field-emission scanning electron microscopy (FESEM). Imaging was conducted with a Zeiss Ultra 60 FESEM to investigate the structural features of the synthesized samples. Elemental composition of the GaN nanowires was confirmed via energy-dispersive X-ray spectroscopy (EDS). The crystallographic structure and phase purity of the metal oxide films were assessed using X-ray diffraction, (XRD) performed on a Rigaku SmartLab system, Virgina, USA equipped with Cu-Kα radiation (

Table 1. Elemental mixing ratio.

	In2O3		Au/In2O3
Elements	Weight %	Atomic %	Weight %	Atomic %
N	31.84	69.93	33.73	71.73
Ga	68.16	30.07	66.15	28.26
Au	0	0	0.11	0.02

). Additionally, the surface topography and roughness of the metal-oxide/GaN sensors were characterized using an Asylum Cypher high-resolution atomic force microscope (AFM). Device EDS analysis (Figure 1c) and a sensor SEM micrograph are shown in Figure 1b.

The fabricated device was placed in the designated gas stainless steel chamber for gas-sensing testing. Then, NO₂ and compressed air were let in to the sensing apparatus while maintaining the net flow at 100 SCCM. The flow rate was controlled by a mass flow controller, and the device was biased with a 5 V constant voltage supply. The sensor was illuminated with a 365 nm, 470 mW/cm² UV light power source. The photon energy of a 365 nm UV source is 3.4 eV which is equal to the GaN band gap and necessary for electron hole pair generation; UV power helps to operate the sensor at room temperature, as under dark conditions the sensor response was lower, due to insufficient active sites. The UV source helps to save power by operating the sensor at room temperature. All our experimental measurements were carried out at room temperature.

With NO₂ present, change in the resistance leads to the change in the sensitivity S

$$\mathrm{S}={\displaystyle \frac{{\mathrm{R}}_{{\mathrm{N}\mathrm{O}}_2}-{\mathrm{R}}_{\mathrm{a}\mathrm{i}\mathrm{r}}}{{\mathrm{R}}_{\mathrm{a}\mathrm{i}\mathrm{r}}}}$$

(1)

where R_air and R_NO2 represent baseline resistance without NO₂ and with NO₂. A transient response of 10 ppm NO₂ with various noble metals is shown in Figure 2a,b. Throughout the gas-sensing measurement, the device was allowed to regain the baseline (i.e., recover) after it was exposed to NO₂ within 10 min of “buffer” time. NO_2, upon exposure to the Au catalyst layer, dissociated as NO and Oxygen ions and then transferred to the In₂O₃ layer. This charge transfer changes the resistance of the sensor [24,25,26,27,28,29,30,31,32]. The relative change in resistance for Au/In₂O₃ is from 1.37 M to 1.9 M and bare In₂O₃ is from 800K to 883K in the presence of NO₂ gas. In room temperature, the change in resistance or sensitivity is higher for Au when compared to other noble metals. Au/In₂O₃ sensors exhibited the highest response, at 38.9%, followed by bare In₂O₃ at 10%. Nanoparticle decoration increases the adsorption capabilities of NO₂ on an In₂O₃ surface, due to a spill-over effect [24,25,26,27,28,29,30,31,32]. Increased sensitivity of Au/In₂O₃ is attributed to an increased area of active site, and increased electron transfer is due to gold decoration on In₂O₃. In air, a few free electrons in the conduction band of the sensor are captured by the oxygen, forming a depletion layer on the In₂O₃ surface. As Au/In₂O₃ has more free electrons, the depletion layer widens. Under NO₂ exposure, negatively charged NO₂ species are formed on the In₂O₃, and the depletion layer thickness increases. Adsorption of negatively charged NO₂ by Au/In₂O₃ is stronger than by bare In₂O_3, since there are more free electrons in Au/In₂O₃ than in bare In₂O₃. The observed superiority of Au noble metal is further studied with the help of density functional theory.

3. DFT Calculation: Results and Discussion

All density functional theory calculations were performed using quantum espresso with generalized gradient approximation (GGA) and ultra-soft pseudo potential [33,34]. We designed our substrate using the In₂O₃ (111) surface, comprising 140 O atoms and 96 In atoms. We kept a vacuum layer of 15 Å to avoid adjacent layer interactions. All the atoms were relaxed until the force between them is less than 0.003 Ry/Bohr using the Broyden–Fletcher–Goldfarb– Shanno (BFGS) algorithm [35]. The Monkhorst–Pack k-point grid of 2 × 2 × 1 was used for geometrical optimization and a denser k-point of 6 × 6 × 1 was used to evaluate the electronic properties. Cutoff for wave function and charge was set to 55 Ry and 550 Ry.

To understand the adsorption behavior and site preference of noble metals on an In₂O₃ substrate, we first calculated the binding energy (Eb). This provides insight into the thermodynamic favorability of metal adsorption and helps identify the most stable configuration. The expression used to evaluate Eb is as follows [36]:

$${\mathrm{E}}_{\mathrm{b}}={\mathrm{E}}_{\mathrm{I}\mathrm{n}2\mathrm{O}3}+{\mathrm{E}}_{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}-{\mathrm{E}}_{\mathrm{I}\mathrm{n}2\mathrm{O}3+\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}$$

(2)

where ${\mathrm{E}}_{\mathrm{I}\mathrm{n}2\mathrm{O}3+\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}$, ${\mathrm{E}}_{\mathrm{I}\mathrm{n}2\mathrm{O}3}$, ${\mathrm{E}}_{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}$ denote the total energies of the noble metal adsorbed In₂O₃, bare In₂O₃ and energy of non-interacting noble metals (Au, Ag, Pt, Pd and Cu). When greater, the absolute value of the binding energy is the most optimal site for noble metal. The resulting binding energies range from −3.5 eV to −4.0 eV across the different metals. These negative values indicate a spontaneous and favorable adsorption process and underscore a strong metal–support interaction. A higher absolute binding energy suggests a stronger bond and, thus, a more stable adsorption site.

Following the binding energy analysis, we explored the interaction between NO₂ gas and the metal-decorated In₂O₃ surfaces by calculating the adsorption energy (Eads) [36]:

$${\mathrm{E}}_{\mathrm{a}\mathrm{d}\mathrm{s}}={\mathrm{E}}_{{\displaystyle \frac{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}{\mathrm{I}\mathrm{n}2\mathrm{O}3}}+\mathrm{N}\mathrm{O}2}-\left({\mathrm{E}}_{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}/\mathrm{I}\mathrm{n}2\mathrm{O}3}+{\mathrm{E}}_{\mathrm{N}\mathrm{O}2}\right)$$

(3)

where ${\mathrm{E}}_{{\displaystyle \frac{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}{\mathrm{I}\mathrm{n}2\mathrm{O}3}}+\mathrm{N}\mathrm{O}2}$, ${\mathrm{E}}_{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}/\mathrm{I}\mathrm{n}2\mathrm{O}3}$ and ${\mathrm{E}}_{\mathrm{N}\mathrm{O}2}$ are the total energies of the gas molecules adsorbed on different structures with noble metal, metal-attached structures and isolated gas molecules, respectively.

These energies are essential to assess the thermodynamics of gas adsorption and the interaction strength between the gas molecule and the surface.

The optimized configurations of noble metal-decorated In₂O₃ surfaces with NO₂ adsorption are depicted in Figure 3, while the corresponding geometric parameters, such as bond lengths and bond angles, are listed in

Table 2. Adsorption parameter of bare and noble metal decorated In₂O_3, including adsorption energy and charge transfer along with gas molecules bond length, bond angle, and distance.

Model	Bond Length (O-N) Å	Bond Angle (O-N-O) Deg	Adsorption Energy (ev)	Charge Transfer (e)	Distance (Å)
In2O3–NO2	1.21, 1.28	121.46	−0.77	0.03	2.86
Au/In2O3–NO2	1.22, 1.3	119.72	−1.97	0.23	2.06
Ag/In2O3–NO2	1.22, 1.26	124.34	−0.94	0.1	2.17
Pt/In2O3–NO2	1.19, 1.42	115.02	−0.99	0.09	1.95
Pd/In2O3–NO2	1.22, 1.28	124.07	−0.98	0.07	1.99
Cu/In2O3–NO2	1.21, 1.29	122.17	−0.85	0.05	1.89

. The bonding patterns are distinctive for each noble metal:

• Au and Pd each bond with one surface oxygen atom.
• Pt forms a bond with one oxygen atom and one indium atom.
• Ag binds to one indium atom.
• Cu establishes bonds with two oxygen atoms and one indium atom.

The bond lengths and angles in the Au–NO₂ and Pt–NO₂ systems showed more significant deviation from the pristine In₂O₃ structure than in other cases. This observation suggests that NO₂ undergoes molecular activation in the presence of these metals, thereby demonstrating their catalytic potential.

The computed NO₂ adsorption energies for different systems are as follows:

• Bare In₂O₃: −0.77 eV
• Au/In₂O₃: −1.97 eV
• Pt/In₂O₃: −0.99 eV
• Pd/In₂O₃: −0.98 eV
• Ag/In₂O₃: −0.94 eV
• Cu/In₂O₃: −0.85 eV

Clearly, all noble metals enhance the adsorption energy compared to the bare substrate, with Au showing the most pronounced effect. The adsorption distance also decreases with metal decoration, indicating stronger interactions. These adsorption energies reflect that NO₂ adsorption on Au is most favorable and likely involves chemisorption, while weaker values for Ag, Pt, Pd, and Cu indicate physisorption. The highly negative values confirm the spontaneous nature of the process.

To further elucidate the adsorption mechanism, we examined charge transfer between NO₂ and the In₂O₃ surface using Löwdin population analysis. This analysis helps quantify the electronic redistribution during adsorption. As an electron acceptor, NO₂ draws electrons from the surface, forming an electron depletion region that influences the material’s electrical properties.

The charge transferred to NO₂ from the surface for each noble metal system is as follows:

• Au/In₂O₃: 0.23 e
• Pt/In₂O₃: 0.09 e
• Pd/In₂O₃: 0.07 e
• Ag/In₂O₃: 0.10 e
• Cu/In₂O₃: 0.05 e

This transfer reduces the conduction band electron density, leading to an increase in surface resistance—especially relevant for resistive-type gas sensors. The largest transfer observed for Au implies that Au enhances the electron exchange and facilitates stronger NO₂ adsorption. This effect widens the depletion layer and reduces carrier mobility, thus boosting sensor sensitivity. These trends align well with the calculated adsorption energies. The summary of adsorption energy, adsorption distance and charge transfer are all listed in Table 1.

To analyze the electronic structure modifications upon NO₂ adsorption, we computed the density of states (DOS) before and after gas exposure. As illustrated in Figure 4, the DOS of Ag, Pt, Pd, and Cu systems showed minimal change, suggesting weak electronic interactions. In contrast, the Au-decorated system exhibited a significant DOS alteration near the Fermi level, affirming robust charge transfer and a stronger interaction with NO₂ [26,27,28,29,30,31].

This finding is also consistent with adsorption energy and charge transfer calculation. We have also calculated charge density difference using [26]

$$∆\mathrm{\rho }={\mathrm{\rho }}_{{\displaystyle \frac{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}{\mathrm{I}\mathrm{n}2\mathrm{O}3}}+\mathrm{N}\mathrm{O}2}-{\mathrm{\rho }}_{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}/\mathrm{I}\mathrm{n}2\mathrm{O}3}-{\mathrm{\rho }}_{\mathrm{N}\mathrm{O}2}$$

(4)

where ${\mathrm{\rho }}_{{\displaystyle \frac{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}}{\mathrm{I}\mathrm{n}2\mathrm{O}3}}+\mathrm{N}\mathrm{O}2}$, ${\mathrm{\rho }}_{\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}/\mathrm{I}\mathrm{n}2\mathrm{O}3}$ and ${\mathrm{\rho }}_{\mathrm{N}\mathrm{O}2}$ are the charge density of the gas adsorbed In₂O₃ with noble metal, and In₂O₃ with noble metal and gas molecule. The charge density difference in all the structures is displayed in Figure 5. Figure 5 reveals blue regions (electron depletion) and yellow regions (electron accumulation). The most intense charge redistribution is observed for Au/In₂O₃, again confirming that Au induces a higher degree of electronic reorganization, consistent with its catalytic role [22,23,24,25,26,27,28,29,30,31,32]. The findings support the hypothesis that Au serves as an excellent mediator for NO₂. This result is also consistent with adsorption energy, charge transfer, and DOS plot.

A classic relationship between band gap (Eg) and conductivity is as follows [26]

$$\mathrm{\sigma }\mathrm{\alpha }\mathrm{A}\mathrm{\ast }\mathrm{e}\mathrm{x}\mathrm{p}\left({\displaystyle \frac{-\mathrm{E}\mathrm{g}}{2\mathrm{k}\mathrm{T}}}\right)$$

(5)

where σ is the conductivity, k is the Boltzmann constant (1.38 × 10⁻²³ J/K), A is a fixed parameter and T is the temperature. As like in other papers, the band gap of In₂O₃ is less than the experimental value, since the DFT calculation underestimates the band gap [27].

The bare In₂O₃ exhibited a band gap of 0.8725 eV, which increased to 0.8899 eV after NO₂ adsorption. This shift implies a decrease in conductivity, as expected when electrons are withdrawn by an oxidizing gas like NO₂ [22,23,24,25,26,27,28,29,30,31,32].

Noble metal decoration reduced the band gap to 0.77 eV, enabling better electron mobility and enhancing NO₂ capture. Post adsorption, the band gap of Au/In₂O₃ increased to 0.8155 eV, signifying that NO₂ adsorption results in reduced conductivity—a typical signal in chemiresistive sensors (

Table 3. Band gap value of bare and noble metal decorated In₂O_3, before and after gas adsorption.

Structure	Band Gap (eV)
Bare/In2O3	0.8725
Bare/In2O3–NO2	0.8899
Au/In2O3	0.77
Au/In2O3–NO2	0.8155
Ag/In2O3	0.8361
Ag/In2O3–NO2	0.8248
Pt/In2O3	0.8112
Pt/In2O3–NO2	0.8196
Pd/In2O3	0.8294
Pd/In2O3–NO2	0.8194
Cu/In2O3	0.8534
Cu/In2O3–NO2	0.8503

). Band gap variations for other metals were less prominent, correlating with weaker charge transfer.

The changes in Eg before and after NO₂ adsorption are visualized in Figure 6. The largest ΔEg was observed for Au/In₂O₃, reinforcing Au’s pivotal role in promoting high NO₂ sensitivity.

Through comprehensive DFT-based analysis, this study demonstrates the superior performance of Au-decorated In₂O₃ in NO₂-sensing applications. The combination of high binding and adsorption energies, significant charge transfer, marked DOS variation, charge redistribution, and band gap modulation all converge to establish Au as the most effective catalyst among the noble metals studied.

This work provides theoretical validation for the enhanced sensing capability of Au/In₂O₃ composites, supporting their practical application in the development of next-generation, high-sensitivity NO₂ gas sensors.

4. Conclusions

In this work, we have successfully fabricated a GaN nanowire-based NO₂ gas sensor, functionalized with In₂O₃ and decorated by noble metals. The sensing properties of bare Indium oxide and that decorated with noble metals were investigated using density functional theory and experimental tests. Experimental tests showed that Au/In₂O₃ sensors exhibited the highest response of 38.9%, followed by bare In₂O₃ with 10% for 10 ppm NO₂ at room temperature. The adsorption energy (Eads) calculated for bare/In₂O₃, Au/In₂O₃, Pt/In₂O₃, Pd/In₂O₃, Ag/In₂O₃, and Cu/In₂O₃ are −0.77, −1.97, −0.99, −0.98, −0.94, and −0.85, respectively. The charge transfers from substrate to gas were about 0.23, 0.09, 0.07, 0.1, and 0.05 e for Au/In₂O₃, Pt/In₂O₃, Pd/In₂O₃, Ag/In₂O₃, and Cu/In₂O_3, respectively. For Au/In₂O₃, Eg increased from 0.77 to 0.8155 eV, indicating a higher amount of NO₂ adsorbed on it when compared to other noble metals. Both experimental and DFT results proved the enhancement of In₂O₃ with Au for NO₂ gas sensors.