ppb-Level Selective Hydrogen Gas Detection of Pd-Functionalized In2O3-Loaded ZnO Nanofiber Gas Sensors

Pd nanoparticle-functionalized, xIn2O3 (x = 0.05, 0.1, and 0.15)-loaded ZnO nanofibers were synthesized by an electrospinning and ultraviolet (UV) irradiation method and assessed for their hydrogen gas sensing properties. Morphological and chemical analyses revealed the desired morphology and chemical composition of the synthesized nanofibers. The optimal gas sensor namely Pd-functionalized, 0.1In2O3-loaded ZnO nanofibers showed a very strong response to 172–50 ppb hydrogen gas at 350 °C, which is regarded as the optimal sensing temperature. Furthermore, the gas sensors showed excellent selectivity to hydrogen gas due to the much lower response to CO and NO2 gases. The enhanced gas response was attributed to the excellent catalytic activity of Pd to hydrogen gas, and the formation of Pd/ZnO and In2O3/ZnO heterojunctions, ZnO–ZnO homojunction, as well as the formation of PdHx. Overall, highly sensitive and selective hydrogen gas sensors can be produced based on a simple methodology using a synergistic effect from Pd functionalization and In2O3 loading in ZnO nanofibers.


Introduction
Hydrogen (H 2 ) gas is a green, renewable, and sustainable energy source [1]. The main advantage of H 2 is its abundance and its final combustion product being H 2 O, which is again a source of H 2 [2]. On the other hand, despite the high practicality of H 2 , its flammability over a wide range of concentrations (4%-75%) and an auto-ignition temperature of 250-400 • C makes it extremely dangerous [3]. Furthermore, it is a colorless, odorless, and tasteless gas that cannot be detected by human senses [4]. Accordingly, during storage, transport and use, an unexpected leakage of hydrogen can be catastrophic. As the use of hydrogen is becoming increasingly commonplace in different industries, the early detection of hydrogen gas using gas sensors is of prime importance [5].
Chemiresistor-based gas sensors are one of the most popular types of sensors for hydrogen gas detection [6]. Among them, those based on ZnO are often reported for the sensing of hydrogen gas [7][8][9]. Indeed, ZnO, with n-type conductivity, has some advantages for use as a hydrogen sensing layer. ZnO has a wide band gap of 3.37 eV, high mobility of charge carriers, high thermochemical stability, and low production cost [10]. On the other hand, to enhance the hydrogen sensing properties of ZnO-based gas sensors, including their response to hydrogen gas and selectivity, different strategies,

Preparation of the Solution for Electrospinning
The synthesis of a viscous solution for the preparation of In 2 O 3 -loaded ZnO NFs was based on previous papers [35,36]. Analytical grades of polyvinyl alcohol (PVA, MW = 80,000), zinc chloride dihydrate (ZnCl 2 ·2H 2 O), and indium nitrate hydrate (In(NO 3 ) 3 ·4.5H 2 O) were provided by Sigma-Aldrich (St. Louis, MO, USA). Deionized (DI) water was used as a solvent for the preparation of all solutions. First, PVA was added to DI water and stirred (400 rpm) at 70 • C for 4 h to produce a 10 wt.% PVA aqueous solution. Subsequently, 1 g of ZnCl 2 ·2H 2 O and calculated amounts (0.05-0.15 molar ratio) of In(NO 3 ) 3 ·4.5H 2 O were added to the polymer solution and stirred (400 rpm) for 12 h at 70 • C. The final viscous solutions (280 mPa.s) for electrospinning were then prepared. The amount of In precursor was changed to prepare solutions with different nominal compositions of In 2 O 3 (0.05, 0.1, and 0.15 molar ratio) for the final products.

Electrospinning Process
The electrospinning solution was loaded into a plastic syringe with a metallic needle with an inner diameter of 0.021 mm. To initiate and accelerate electrospinning, a large positive voltage (+15 kV) and large negative voltage (−10 kV) were applied to the needle and Al collector, respectively. During electrospinning, the distance between the tip of the needle and collector was fixed to 0.2 m, and the feed rate was set to 0.01 mL/h. The as-synthesized NFs were annealed at 600 • C for 2 h to remove the organic matter and water to obtain crystalline phases.

Functionalization of Pd Nanoparticles
Pd NPs were deposited on the surface of In 2 O 3 -loaded ZnO NFs using a UV reduction method. A Pd 2+ -containing solution was prepared by dissolving 0.4 g PdCl 2 (Kojima chemicals Co. Ltd., Saitama, Japan) in 20 g of isopropanol. The In 2 O 3 -loaded ZnO NFs were immersed completely in the Pd 2+ -containing solution. Subsequently, the solution was irradiated with UV light at 360 nm using a halogen lamp at room temperature. The intensity of UV light was fixed to 0.17 mW/cm 2 for 60 s. After the UV exposure, the NFs were removed from the solution and annealed at 500 • C for 1 h to remove any residual solvent.

Materials Characterization
The morphology of the products was observed by field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4200, Tokyo, Japan) and transmission electron microscopy (JEM-3010, Tokyo, Japan). The elemental composition was determined by energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) using Cu Kα radiation (λ = 0.15094 nm) was used to examine the crystallinity and phase formation. Ultraviolet photoelectron spectroscopy (UPS, Thermo Fisher Scientific Co., K-Alpha+, Waltham, USA) was carried out in an ultrahigh vacuum (UHV) chamber through the irradiation of HeI 21.2 eV light to construct the electronic band structures of the components by obtaining the work functions.

Gas Sensing Measurement
The sensing measurements performed in this study are explained in detail elsewhere [35,36]. Here, only the basic points are described. First, Ti (50 nm thick) and Pt (100 nm thick) bilayer electrodes were sputter-deposited onto a SiO 2 -grown Si substrate and NFs were then drop-casted onto the substrate with a solvent of 2-propanol. For pure Pd sensors, Pd NPs were synthesized on Si wafers by adopting the same experimental conditions used for the functionalization of Pd NPs. The synthesized Pd NPs were drop-casted on the same Si substrate equipped with electrodes and the gas sensing properties were studied.
Second, for the sensing measurements, the fabricated sensor devices were placed into a tube furnace, equipped with a gas chamber that can control the temperature. The gas concentrations were controlled by varying the ratios of the desired gases to dry air using mass flow controllers. After recording the resistance in air (R a ) and the resistance in the presence of the target gas (R g ) using a Keithley source meter, the sensor response (R) was calculated as R = R a /R g (for a reducing gas, such as H 2 and CO) and R = R g /R a (for an oxidizing gas, such as NO 2 ). The response and recovery times were defined as the time required for the resistance to reach 90 % of its final value upon exposure to target gas and air, respectively [37]. The insets in these figures show higher-magnification FE-SEM images. Overall, the diameter of the synthesized NFs was less than 100 nm and the smooth morphology observed on the as-spun NFs was changed to an irregular and rough morphology, even though the overall web-like structure of NFs had been maintained. The rough morphology was attributed to the evaporation and decomposition of water and inorganic materials during the calcination process. Figure 1e presents FE-SEM images of Pd NP-functionalized 0.1 In 2 O 3 -loaded ZnO NFs. Similar to the pristine In 2 O 3 -loaded ZnO NFs, the surface morphology became rough. In addition, the overall diameter was less than 100 nm, demonstrating a nanoscale range of synthesized NFs. Figure 1f Sensors 2019, 19 [38]) and crystalline ZnO (JCPDS card no. 89-0511 [39]). For Pd-functionalized 0.1 In 2 O 3 -loaded ZnO nanofibers an additional peak related to Pd (JCPDS card no. 87-0641 [40]) appears. This demonstrates the formation and co-existence of crystalline Pd, In 2 O 3 , and ZnO phases. In addition, no unwanted phases were observed, demonstrating the high purity of the starting materials and the correctness of the procedure employed for the preparation of NFs.  Figure 1e presents FE-SEM images of Pd NP-functionalized 0.1 In2O3-loaded ZnO NFs. Similar to the pristine In2O3-loaded ZnO NFs, the surface morphology became rough. In addition, the overall diameter was less than 100 nm, demonstrating a nanoscale range of synthesized NFs. Figure  1f shows XRD patterns of 0.1 In2O3-loaded ZnO nanofibers and Pd-functionalized 0.1 In2O3-loaded ZnO nanofibers. The XRD pattern of the 0.1 In2O3-loaded ZnO nanofibers revealed crystalline In2O3 (JCPDS(joint committee on powder diffraction standards) card no. 89-4595 [38]) and crystalline ZnO (JCPDS card no. 89-0511 [39]). For Pd-functionalized 0.1 In2O3-loaded ZnO nanofibers an additional peak related to Pd (JCPDS card no. 87-0641 [40]) appears. This demonstrates the formation and co-existence of crystalline Pd, In2O3, and ZnO phases. In addition, no unwanted phases were observed, demonstrating the high purity of the starting materials and the correctness of the procedure employed for the preparation of NFs.

Gas Sensing Studies
In the first step, an attempt was made to find the optimal sensing temperature using the 0.1 In2O3-loaded ZnO NF gas sensor while exposing it to low concentrations of hydrogen gas (50 ppb-5 ppm) at a range of sensing temperatures. Figure 2a shows the transient resistance curves at 250-400 °C. At all temperatures tested, the sensor exhibited very high reproducibility of the resistance signal, where after stopping hydrogen gas, the resistance returned to its initial value. In addition, the sensor exhibited n-type sensing behavior, originating from the n-type conductive nature of both ZnO and

Gas Sensing Studies
In the first step, an attempt was made to find the optimal sensing temperature using the 0.1 In 2 O 3 -loaded ZnO NF gas sensor while exposing it to low concentrations of hydrogen gas (50 ppb-5 ppm) at a range of sensing temperatures. Figure 2a shows the transient resistance curves at 250-400 • C. At all temperatures tested, the sensor exhibited very high reproducibility of the resistance signal, where after stopping hydrogen gas, the resistance returned to its initial value. In addition, the sensor exhibited n-type sensing behavior, originating from the n-type conductive nature of both ZnO and In 2 O 3 . To examine the temperature dependence of the gas response, the response of the gas sensor at various hydrogen concentrations versus the sensing temperature was plotted, as shown in Figure 2b. From this plot, the optimal sensing temperature was determined to be 350 • C. At lower temperatures, the thermal energy is insufficient to provide the complete energy of adsorption, and at 400 • C hydrogen gas may escape before adsorption. At the optimal sensing temperature, the adsorption and desorption rates are equal. The responses to 50 ppb, 100 ppb, 1 ppm, and 5 ppm H 2 gas were 72, 88, 96, and 114, respectively. This demonstrates the strong response of the gas sensor to hydrogen gas. In2O3. To examine the temperature dependence of the gas response, the response of the gas sensor at various hydrogen concentrations versus the sensing temperature was plotted, as shown in Figure 2b. From this plot, the optimal sensing temperature was determined to be 350 °C. At lower temperatures, the thermal energy is insufficient to provide the complete energy of adsorption, and at 400 °C hydrogen gas may escape before adsorption. At the optimal sensing temperature, the adsorption and desorption rates are equal. The responses to 50 ppb, 100 ppb, 1 ppm, and 5 ppm H2 gas were 72, 88, 96, and 114, respectively. This demonstrates the strong response of the gas sensor to hydrogen gas.  In the next step, the gas sensors with various amounts of In2O3 were exposed to hydrogen gas at 350 °C to determine the optimal composition of the gas sensor. Figure 2c,d shows the transient resistance curves of different sensors and corresponding calibration curves, respectively. Note that In the next step, the gas sensors with various amounts of In 2 O 3 were exposed to hydrogen gas at 350 • C to determine the optimal composition of the gas sensor. Figure 2c,d shows the transient resistance curves of different sensors and corresponding calibration curves, respectively. Note that the data for the pristine ZnO NF gas sensor were taken from reference [42]. Based on the presented results, the 0.1 In 2 O 3 -loaded ZnO NF gas sensor showed the strongest response to all concentrations of H 2 gas. The responses of the ZnO nanofibers loaded with 0.05, 0.1, and 0.15 In 2 O 3 toward 50 ppb H 2 gas were 62, 72, and 67 respectively, again clarifying the trend.
It should be noted that we also studied the sensing results for the pure Pd sensor. As shown in Figure 2d, the sensing response of the pure Pd sensor is inferior to the other sensors. Figure S1a,b in Supplementary Materials shows the dynamic resistance curves and calibration curves of the pure Pd gas sensor to H 2 gas at different operating temperatures, respectively. At 200 • C, the best sensing performance was obtained and compared to the other type gases in Figure 2d.
In the next step, Pd NP-functionalized 0.1 In 2 O 3 -loaded ZnO NFs were exposed to various concentrations of H 2 gas. Figure 2e shows the transient response curves of the 0.1 In 2 O 3 -loaded ZnO NFs and Pd-functionalized In 2 O 3 -loaded ZnO NFs. The corresponding calibration curves presented in Figure 2f shows that the gas sensor functionalized with Pd NPs showed a much improved response to H 2 gas compared to the In 2 O 3 -loaded ZnO NFs without Pt functionalization. Accordingly, the Pd-functionalized 0.1 In 2 O 3 -loaded ZnO NF gas sensor was selected for the selectivity study. Figure 2g shows the dynamic resistance curves of the Pd-functionalized 0.1 In 2 O 3 -loaded ZnO MF gas sensor to 50 ppb-5 ppm NO 2 , CO, and H 2 gases. Figure 2h shows the corresponding selectivity pattern, demonstrating the excellent selectivity of gas sensor to hydrogen gas. Considering 50 ppb gases, the responses of the gas sensor to H 2 , NO 2 , and CO gases were 152, 14.6, and 13.48, respectively. Long-term stability of sensors was also tested. Figure 3a,b shows the dynamic resistance curves and corresponding calibration curves of fresh and 12-months-aged Pd NPs-functionalized 0.1 In 2 O 3 -loaded ZnO NFs gas sensors toward 50 ppb-5 ppm of H 2 gas, respectively. As is evident, after 12 months the sensors showed almost the same sensing response, demonstrating the high stability of the gas sensor which is critical for practical applications. Long-term stabilities of the other gas sensors are shown in Figure S2 in Supplementary Materials. the data for the pristine ZnO NF gas sensor were taken from reference [42]. Based on the presented results, the 0.1 In2O3-loaded ZnO NF gas sensor showed the strongest response to all concentrations of H2 gas. The responses of the ZnO nanofibers loaded with 0.05, 0.1, and 0.15 In2O3 toward 50 ppb H2 gas were 62, 72, and 67 respectively, again clarifying the trend. It should be noted that we also studied the sensing results for the pure Pd sensor. As shown in Figure 2d, the sensing response of the pure Pd sensor is inferior to the other sensors. Figure S1a,b in Supplementary Materials shows the dynamic resistance curves and calibration curves of the pure Pd gas sensor to H2 gas at different operating temperatures, respectively. At 200 °C , the best sensing performance was obtained and compared to the other type gases in Figure 2d.
In the next step, Pd NP-functionalized 0.1 In2O3-loaded ZnO NFs were exposed to various concentrations of H2 gas. Figure 2e shows the transient response curves of the 0.1 In2O3-loaded ZnO NFs and Pd-functionalized In2O3-loaded ZnO NFs. The corresponding calibration curves presented in Figure 2f shows that the gas sensor functionalized with Pd NPs showed a much improved response to H2 gas compared to the In2O3-loaded ZnO NFs without Pt functionalization. Accordingly, the Pd-functionalized 0.1 In2O3-loaded ZnO NF gas sensor was selected for the selectivity study. Figure 2g shows the dynamic resistance curves of the Pd-functionalized 0.1 In2O3-loaded ZnO MF gas sensor to 50 ppb-5 ppm NO2, CO, and H2 gases. Figure 2h shows the corresponding selectivity pattern, demonstrating the excellent selectivity of gas sensor to hydrogen gas. Considering 50 ppb gases, the responses of the gas sensor to H2, NO2, and CO gases were 152, 14.6, and 13.48, respectively. Long-term stability of sensors was also tested. Figure 3a,b shows the dynamic resistance curves and corresponding calibration curves of fresh and 12-months-aged Pd NPs-functionalized 0.1 In2O3-loaded ZnO NFs gas sensors toward 50 ppb-5 ppm of H2 gas, respectively. As is evident, after 12 months the sensors showed almost the same sensing response, demonstrating the high stability of the gas sensor which is critical for practical applications. Long-term stabilities of the other gas sensors are shown in Figure S2 in Supplementary Materials.

Gas Sensing Mechanism
When chemiresistor-based gas sensors are exposed to different gases, their resistance changes, which is the basis of gas detection in these types of gas sensors. Any resistance modulation source can contribute to the sensor signal. Let us start with the electron depletion layer (EDL) concept [43]. When chemiresistor-based gas sensors are exposed to air, oxygen gases can be adsorbed easily on the surface and take the electrons from the sensor materials. As a result, the oxygen ion species (O2 − , O − , and O 2− ) become chemisorbed on the sensor surface. On the other hand, due to the depletion of the outer layer of the sensing material, the concentration of electrons decrease, which leads to the appearance of a so-called EDL. When the resistance of gas sensor in the air was compared with that in the vacuum, the resistance of sensor in the air was higher due to the presence of the EDL. Upon

Gas Sensing Mechanism
When chemiresistor-based gas sensors are exposed to different gases, their resistance changes, which is the basis of gas detection in these types of gas sensors. Any resistance modulation source can contribute to the sensor signal. Let us start with the electron depletion layer (EDL) concept [43]. When chemiresistor-based gas sensors are exposed to air, oxygen gases can be adsorbed easily on the surface and take the electrons from the sensor materials. As a result, the oxygen ion species (O 2 − , O − , and O 2− ) become chemisorbed on the sensor surface. On the other hand, due to the depletion of the outer layer of the sensing material, the concentration of electrons decrease, which leads to the appearance of a so-called EDL. When the resistance of gas sensor in the air was compared with that in the vacuum, the resistance of sensor in the air was higher due to the presence of the EDL. Upon exposure of the gas sensor to a toxic gas, the toxic gas molecules will react with already adsorbed oxygen ion species and in the case of reducing gases, they will release electrons to the surface of the gas sensor. As a result, the width of the EDL decreases, leading to resistance modulation. This eventually  [44]. exposure of the gas sensor to a toxic gas, the toxic gas molecules will react with already adsorbed oxygen ion species and in the case of reducing gases, they will release electrons to the surface of the gas sensor. As a result, the width of the EDL decreases, leading to resistance modulation. This eventually leads to the appearance of a sensor signal. In the In2O3-loaded ZnO nanofibers in the surface of each individual grain of ZnO and In2O3, which are exposed to air, EDLs form and upon exposure to H2 gas, the following reaction, H2 + O − → H2O + e − , will cause the electrons to return to the surface of the gas sensor. Accordingly, the resistance of the sensor changes. In addition, in In2O3-loaded ZnO nanofibers, heterojunctions between In2O3 and ZnO form because of the co-existence of In2O3 and ZnO and the difference in work function. The work function values were obtained according to the procedure described elsewhere [44].  According to the calculated work functions, the energy levels can be established. As shown in Figure 5a, upon intimate contact between In2O3 and ZnO, and to equate the Fermi levels, electrons flow from In2O3 to ZnO and band bending occurs. This will result in a potential barrier for the flow of electrons. When the sensor is exposed to H2 gas, the electrons released return to the surface of the According to the calculated work functions, the energy levels can be established. As shown in Figure 5a, upon intimate contact between In 2 O 3 and ZnO, and to equate the Fermi levels, electrons flow from In 2 O 3 to ZnO and band bending occurs. This will result in a potential barrier for the flow of electrons. When the sensor is exposed to H 2 gas, the electrons released return to the surface of the sensing layer, eventually decreasing the height of the potential barrier. This leads to a decrease in the sensing layer, eventually decreasing the height of the potential barrier. This leads to a decrease in the sensor resistance, which contributes to the sensor signal. In addition, ZnO-In2O3 heterojunctions, ZnO-ZnO homojunctions, and In2O3-In2O3 homojunctions also can be formed. On the other hand, because ZnO is the major component, the contribution of ZnO-ZnO homojunctions to the sensing signal is significantly higher. At ZnO-ZnO homojunctions, the height of the potential barriers in air changes greatly in a hydrogen atmosphere, contributing predominantly to the sensor signal. Because of the high sensing temperature and high reducing power of hydrogen gas, it is possible that zinc oxide can be converted to metallic zinc on the surface of ZnO (Figure 5b). This conversion is one of the main reasons for the enhanced sensing response because metallic Zn has a significantly higher conductivity than ZnO [33]. When ZnO is in a H2 atmosphere, the continuous ultra-thin layer on the outer surfaces of ZnO will be reduced to metallic Zn, where a semiconductor-to-metallic conversion will occur [42]. Because the resistance of ZnO is significantly higher than that of metallic Zn, remarkable resistance modulation occurs upon this conversion, which contributes to the sensing signal [3]. Based on the literature, [45] metallization of the ZnO surface can occur by the adsorption of H atoms onto the O sites on the nonpolar surfaces of ZnO, resulting in the metallization of ZnO. Hybridization occurs between the s-orbitals of H atoms and the p-orbitals of O atoms (in ZnO), resulting in a shift in the energy states of these O p-orbitals shift to lower energy states. Accordingly, delocalization of the charges between Zn and the O-H bond occurs, which can lead to the metallization of Zn atoms.
In addition, the presence of Pd can increase the H2 response of the gas sensor significantly. Pd can dissociate oxygen molecules to atomic oxygen and increase the rate of oxygen adsorption on the surface of the gas sensor [4]. According to the UPS measurements, the work function of Pd was measured to be 5.60 eV. Therefore, Schottky barriers are formed at the interface between Pd and ZnO due to electron transfer from ZnO to Pd. This causes an overall increase in the resistance of the gas sensor. On the other hand, upon exposure to H2 gas, it can be dissociated on and dissolved into Because of the high sensing temperature and high reducing power of hydrogen gas, it is possible that zinc oxide can be converted to metallic zinc on the surface of ZnO (Figure 5b). This conversion is one of the main reasons for the enhanced sensing response because metallic Zn has a significantly higher conductivity than ZnO [33]. When ZnO is in a H 2 atmosphere, the continuous ultra-thin layer on the outer surfaces of ZnO will be reduced to metallic Zn, where a semiconductor-to-metallic conversion will occur [42]. Because the resistance of ZnO is significantly higher than that of metallic Zn, remarkable resistance modulation occurs upon this conversion, which contributes to the sensing signal [3]. Based on the literature, [45] metallization of the ZnO surface can occur by the adsorption of H atoms onto the O sites on the nonpolar surfaces of ZnO, resulting in the metallization of ZnO. Hybridization occurs between the s-orbitals of H atoms and the p-orbitals of O atoms (in ZnO), resulting in a shift in the energy states of these O p-orbitals shift to lower energy states. Accordingly, delocalization of the charges between Zn and the O-H bond occurs, which can lead to the metallization of Zn atoms.
In addition, the presence of Pd can increase the H 2 response of the gas sensor significantly. Pd can dissociate oxygen molecules to atomic oxygen and increase the rate of oxygen adsorption on the surface of the gas sensor [4]. According to the UPS measurements, the work function of Pd was measured to be 5.60 eV. Therefore, Schottky barriers are formed at the interface between Pd and ZnO due to electron transfer from ZnO to Pd. This causes an overall increase in the resistance of the gas sensor. On the other hand, upon exposure to H 2 gas, it can be dissociated on and dissolved into the Pd. Pd can uptake more than 600 times its own volume of H 2 gas [46]. Accordingly, Pd can adsorb H 2 and form PdH x . The formation of PdH x with a very different work function (3.7 eV) than Pd will destroy the Schottky barriers to form Ohmic contact (Figure 5c) [33], which can eventually modulate the sensor resistance. The formation of PdH x can be shown as follows [46]: At the same time, partial H atoms formed dissociatively in Pd migrate to the interface of Pd and the sensing layer via a so-called spill-over effect (Figure 5d). The formed PdH x and migrated H atoms potentially react with the adsorbed oxygen and inject electrons into the sensing layer, resulting in a decrease in resistance. The reduced concentration of H atoms at the interface of Pd and the sensing layer may assist in the dissolution of H to Pd, which certainly promotes the sensor performance. The following chemical formulae are a possible explanation of the complete reaction [47].
Another possible mechanism can be related to the kinetics of gas molecules. The strong response to H 2 gas can also be related to the smaller kinetic diameter of H 2 gas (2.89 Å) [48] relative to CO (3.76 Å) and NO 2 gas (4.01-5.02 Å) [49]. Accordingly, H 2 gas molecules can diffuse to the deeper parts of the sensing layer. Therefore, their adsorption volume can be significantly larger than that of other gases. Table 1 lists the H 2 sensing properties of the present sensor with some ZnO-based gas sensors reported elsewhere. Overall, the present sensor showed a superior response to H 2 gas, demonstrating its practical applications. Table 1. Comparison of the hydrogen sensing performances of some ZnO-based gas sensors with the present optimized gas sensor.

Sensing Material
Conc.

Conclusions
Pd-functionalized xIn 2 O 3 (x = 0.05, 0.1, and 0.15)-loaded ZnO NFs were prepared for H 2 gas sensing applications. FE-SEM, TEM, and XRD were performed to confirm the desired morphology and chemical composition of the products. The gas sensing results showed that the optimal gas sensor with Pd-functionalized 0.1 In 2 O 3 -loaded ZnO NFs showed a very strong response to 152-50 ppb H 2 gas at 350 • C. The enhanced gas response and excellent selectivity were attributed to the excellent catalytic activity of Pd to H 2 gas, formation of PdH x , formation of Pd/ZnO Schottky junctions, formation of In 2 O 3 /ZnO heterojunctions, as well as ZnO-ZnO homojunctions. The optimized gas sensor has almost all the features of a good sensor for practical applications. Moreover, based on the results, further enhancement in the gas response may be achieved by optimizing the content of Pd NPs.