Pd/Pt-Bimetallic-Nanoparticle-Doped In2O3 Hollow Microspheres for Rapid and Sensitive H2S Sensing at Low Temperature

H2S is a poisonous gas that is widespread in nature and human activities. Its rapid and sensitive detection is essential to prevent it from damaging health. Herein, we report Pd- and Pt-bimetallic-nanoparticle-doped In2O3 hollow microspheres that are synthesized using solvothermal and in situ reduction methods for H2S detection. The structure of as-synthesized 1 at% Pd/Pt-In2O3 comprises porous hollow microspheres assembled from In2O3 nanosheets with Pd and Pt bimetallic nanoparticles loaded on its surface. The response of 1 at% Pd/Pt-In2O3 to 5 ppm H2S is 140 (70 times that of pure In2O3), and the response time is 3 s at a low temperature of 50 °C. In addition, it can detect trace H2S (as low as 50 ppb) and has superior selectivity and an excellent anti-interference ability. These outstanding gas-sensing performances of 1 at% Pd/Pt-In2O3 are attributed to the chemical sensitization of Pt, the electronic sensitization of Pd, and the synergistic effect between them. This work supplements the research of In2O3-based H2S sensors and proves that Pd- and Pt-bimetallic-doped In2O3 can be applied in the detection of H2S.


Introduction
Hydrogen sulfide (H 2 S) is a common gas. In addition to its natural source, a large amount of H 2 S is also produced in petroleum refining, sewage treatment, textile treatment, and other human activities [1]. However, H 2 S is highly poisonous and corrosive. Exposure to H 2 S at concentrations above 250 ppm can be fatal. Exposure to low concentrations of H 2 S (10 ppm) also stimulates the visual and olfactory systems, resulting in tears, dizziness, etc. The safety critical concentration of H 2 S in relevant industries is specified as 10 ppm [2]. Therefore, the sensitive and rapid detection of H 2 S is critical to ensure the safety and preservation of relevant workers' health. According to different mechanisms, gas sensors can be divided into surface acoustic wave sensors [3], chemosensitive-based sensors [4], and metal oxide semiconductor (MOS) sensors, etc. Among them, metal oxide semiconductors, such as SnO 2 , In 2 O 3 , TiO 2 , and IGZO [5][6][7][8][9], are some of the most extensively studied gas-sensing materials, and they have become widely used in gas sensors across the world due to their advantages of a low cost, small size, and high response [10].
Indium oxide (In 2 O 3 ) is a typical wide-band-gap (3.5-3.7 eV) n-type semiconductor [11], and it has been utilized in various applications, such as lithium-ion batteries [12] and photocatalysis [13]. For gas sensing, the advantages of a wide band gap and a high conductivity make In 2 O 3 have potential in the detection of H 2 , HCHO, and CO [14][15][16]. However, the research on In 2 O 3 -based H 2 S sensors is still relatively scarce. Pristine In 2 O 3 -based gas sensors have the disadvantages of a low response, poor selectivity, and a high operating temperature [17]. Numerous studies have proven that synthesizing two-dimensional (2D) or three-dimensional (3D) nanocomposites and doping metals are effective methods to improve the gas-sensing performance of In 2 O 3 .
The 3D structures assembled from 2D nanosheets can maintain the ultra-thin structures of nanosheets while avoiding the performance loss caused by the stacking of the nanosheets [18]. Nanosheet-assembled structures are porous and have large specific surface areas. A larger specific surface area means that there are more atoms on the surface of the material, which can provide more gas adsorption sites and make more gas molecules interact with the material to enhance the gas-sensing property. Liu et al. [19] synthesized an In 2 O 3 porous pompon assembled from 2D nanosheets using the hydrothermal method, and its response to 50 ppm ethanol was 3.4. Qin et al. [20] synthesized porous In 2 O 3 nanosheet-assembled microflowers, which had a response of 66 to 100 ppm ethanol and could respond (12.4 s) and recover (10.4 s) quickly. This was because the porous structures provided more gas adsorption sites and allowed gas molecules to diffuse to the depth of the material. Doping metal can change the carrier concentration in In 2 O 3 by improving the properties of the electron donors or acceptors, changing the structure of In 2 O 3 or sensitization to enhance the gas-sensing performance [21,22]. Platinum (Pt) and palladium (Pd) are important dopants because Pt possess strong catalytic performance, and Pd can change the carrier concentration of a material through electron sensitization [21]. Chen et al. [23] synthesized Pd-In 2 O 3 nanofibers via electrospinning, and their response to 10,000 ppm H 2 at room temperature was 146.8 times that of pure In 2 O 3 ; this was because Pd provided more O 2 absorption sites and could form a Schottky barrier with In 2 O 3 . Song et al. [24] improved the response of a sensor to 10 ppm acetone from 12 to 113 by modifying Pt on the surface of In 2 O 3 nanotubes, which benefited from more adsorbed oxygen molecules because of the catalytic effect of Pt. On the basis of monometallic catalysts, some studies have shown that, when several metals are doped into a material in a specific proportion, the gas-sensing property of the material is further improved compared to that of a material doped with a single metal. In bimetallic catalysts, the two metals may exhibit synergistic effects due to geometric effects, the interaction of electrons, or chemical reactions, and they exhibit a better gas-sensing property than monometallic catalysts [25,26]. The work of Huang et al. [15] showed that the response of In 2 O 3 modified by Ag nanoparticles (NPs) and Au nanocages (NCs) to 100 ppm HCHO could reach 3400. The response of Ag NP-In 2 O 3 was 519, and that of Au NC-In 2 O 3 was 240 under the same condition. This was mainly because the electrons in Au transferred to Ag, enhancing the spillover effect of Ag.
In this work, we synthesized In 2 O 3 hollow microspheres doped with Pd and Pt bimetallic nanoparticles (Pd/Pt NPs) using the solvothermal and in situ reduction methods. The structure, morphology, and elemental composition of the microspheres were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption and desorption, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Afterward, a sensor based on this material was prepared, and a gas test was carried out. The working temperature, response/recovery time, dynamic characteristics, limit of detection (LOD), repeatability, and selectivity of the sensor were investigated via the gas tests. The response of the as-synthesized 1 at% Pd/Pt-NP-doped In 2 O 3 hollow microspheres (1 at% Pd/Pt-In 2 O 3 ) to 5 ppm H 2 S at 50 • C was 140, which is 70 times that of the pure In 2 O 3 . The response time of the sensor was 3 s, and the LOD was 50 ppb; moreover, the sensor had a strong linearity, superb repeatability, and great selectivity. However, the recovery time of the sensor was relatively long. It was necessary to increase the working temperature of the sensor to 400 • C through a voltage pulse to speed up the desorption process. The resistance of the sensor could quickly recover to the original value after removing the voltage pulse. In addition, the sensing mechanism of the sensor was also analyzed. The chemical sensitization of Pt, the electronic sensitization of Pd, and the synergistic effect of Pd and Pt were the important factors underlying the excellent gas-sensing performance of the sensor.

Synthesis of Pd/Pt Bimetallic Nanoparticles Doped In 2 O 3 Hollow Microspheres
In 2 O 3 hollow microspheres were synthesized using the solvothermal method. First, 0.6 g indium nitrate hydrate (In(NO 3 ) 3 ·4.5H 2 O) was added into a beaker containing 60 mL isopropanol and 20 g glycerin. Subsequently, the solution was vigorously stirred at room temperature for 3 h. The obtained solution was transferred into a Teflon-lined stainless-steel autoclave for reaction at 180 • C for 6 h. After naturally cooling to room temperature, the products were collected via centrifugation at a speed of 3500 r/min, and they were washed with absolute ethanol three times. The white In 2 O 3 precursors were obtained after drying at 60 • C for 24 h. The precursors were then transferred to a ceramic crucible and calcined at 400 • C for 6 h in an air atmosphere in order to obtain the In 2 O 3 hollow microspheres.
The doping of Pd/Pt NPs was completed via an in situ reduction. A total of 0.2 g of the In 2 O 3 hollow microspheres synthesized in the first step was added to 40 mL deionized water. The canary color solution was obtained after carrying out ultrasonic irradiation for 15 min and vigorous stirring at room temperature for 3 h. Pt with a content of 1 at% and Pd with a content of 1.8 at% were added to the solution in turn. After 15 min of ultrasonic irradiation, the solution was stirred until there was no obvious sediment in the beaker. Slightly excess ascorbic acid solution (0.01 M) was then slowly dropped into the beaker. The solution was vigorously stirred at room temperature for 3 h after it turned black. The products were collected via centrifugation at a speed of 3500 r/min, and they were washed with deionized water and absolute ethanol three times. Finally, 1 at% Pd/Pt-In 2 O 3 was obtained after drying at 60 • C for 24 h.

Material Characterization
The changing trend of the mass of the samples with temperature was characterized using a thermal gravimetric analyzer (TG, NETZSCH STA 449 F5/F3 Jupiter, NETZSCH, Selb, Germany). The surface morphology and microstructure of the samples were observed using scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy (SEM, TEM, and HRTEM, respectively; SU8020, Hitachi, Tokyo, Japan, and FEI Tecnai G2 F30, FEI, Hillsboro, OR, USA). The specific surface area and pore size of the as-synthesized samples were determined via nitrogen adsorptiondesorption isotherms (Micromeritics TriStar III 3020, Micromeritics Instrument Corporation, Norcross, GA, USA). The composition of the material and the phase composition of the samples were determined using X-ray diffraction (XRD, Bruker D8 Advance, Bruker, Billerica, MA, USA) with CuKα Irradiation (λ = 0.154 nm). The scanning range (2θ) was 10-80 • , and the scanning speed was 6 • /min. The elemental and combined state composition of the surface of the samples were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi, Thermo Fisher Scientific, Waltham, MA, USA). All binding energies were calibrated according to the C 1s peak of surface indeterminate carbon at 284.8 eV.

Gas Sensor Fabrication and Measurement
A gas sensor was obtained by coating the gas-sensing material on a micro-hotplate. The micro-hotplate is a kind of microheater prepared using MEMS technology. It consists of a substrate layer, a heating layer, an insulating layer, and a testing layer. The heating layer comprises a heating circuit wire and a pair of heating electrodes composed of Pt. The insulating layer comprises an insulating medium that separates the heating layer from the test layer. The test layer comprises an interdigital electrode and a pair of test electrodes composed of Au wires. When the sensor is working, the heating layer provides the corresponding temperature for the gas-sensing material coated on the test layer according to the voltage applied to the heating electrodes. The interdigital electrode on the test layer will be conductive after being coated with the gas-sensing material, and the resistance change in the gas-sensing material can be detected through the test electrodes.
The gas sensor was prepared in the following steps: The as-synthesized 1 at% Pd/Pt-In 2 O 3 was mixed with an appropriate amount of anhydrous ethanol. The uniformly mixed paste was uniformly coated on the interdigital electrode in the center of the test layer. After the paste dried, the micro-hotplate and the base were combined to obtain a gas sensor. The static gas distribution method was used to measure the gas response of the sensor. The sensor was aged at 100 • C for 24 h to improve its stability before gas tests. A static gas distribution method was applied to obtain the desired concentration of the target gas in the test chamber, and the air was used as the background gas. The response of the sensor was calculated by detecting the resistance change in the sensor. The response of the sensor (S) is defined according to Equation (1): where R a and R g are the resistances in the air and the target gas, respectively. The response time (T Response ) and recovery time (T Recovery ) of the sensor are defined as the time spent when the sensor resistance changes from the initial value to 90% of the total resistance change after the target gas is applied and released.

Results of Characterization
A thermogravimetric (TG) analyzer was used to detect the change in the quality of the precursors with the increase in the calcination temperature. According to Figure 1, the quality of the precursors drops sharply in the range of 230-360 • C, which is mainly due to the thermal decomposition of the organic components in the precursors. Therefore, the calcination temperature was set to 400 • C. The surface morphology and microstructure of 1 at% Pd/Pt-In 2 O 3 were observed using FESEM, TEM, and HRTEM, as shown in Figure 2. It can be observed from the low-resolution SEM image in Figure 2a that the material is composed of many ellipsoidal microspheres with a rough surface, and the size of the microspheres is about 600-900 nm. It can be further observed from the high-resolution SEM image shown in Figure 2b   The specific surface area and pore size of the pure In 2 O 3 and 1 at% Pd/Pt-In 2 O 3 were characterized via nitrogen adsorption and desorption isotherms. In the nitrogen adsorption and desorption isotherms presented in Figure 3a, a hysteresis loop is observed in the isotherms of both materials, indicating mesoporous structures in the microspheres [25]. The specific surface area of the material was measured using the Brunauer-Emmett-Teller method (BET). The specific surface area of the pure In 2 O 3 is 61.5061 m 2 /g, while that of 1 at% Pd/Pt-In 2 O 3 is 58.3404 m 2 /g. Figure 3b indicates the pore size distribution of the pure In 2 O 3 and 1 at% Pd/Pt-In 2 O 3 . The pore size of the pure In 2 O 3 is concentrated in the range of 35-51 nm, while that of 1 at% Pd/Pt-In 2 O 3 is concentrated in the range of 25-35 nm. The specific surface area and pore size of the doped microspheres decrease slightly because the Pd/Pt NPs loaded on the surface of the microspheres block some of the pores. In general, doping does not significantly affect the gas adsorption capacity of the microspheres. Figure 3c presents the XRD patterns that were obtained for the pure In The elemental and combined state compositions of the surface of the sample were analyzed using XPS, and the results are shown in Figure 4. Figure 4a presents an XPS survey spectrum containing multiple spectral lines of O, In, and C elements. The diffraction peaks corresponding to Pd and Pt are not obvious due to the low content of Pd and Pt. Figure 4b shows the XPS spectra of In 3d. The In 3d peak can be divided into two peaks at 444.2 and 451.8 eV, corresponding to the binding energies of In 3d 3/2 and In 3d 5/2 valence states, respectively [28,29]. This result shows that the indium element in the sample is mainly in the form of In 3+ [30]. Figure 4c illustrates the peak related to the O element. The peak can be divided into 529.6 and 531.2 eV peaks, corresponding to lattice oxygen (O lattice ) and chemisorbed oxygen O x − (O − , O 2 − , and O 2− ), respectively [31]. After performing calculations according to the peak area, it is found that the proportion of chemisorbed oxygen is about 51.5%. The Pt 4f spectra shown in Figure 4d can be obtained from the highresolution XPS spectra. It can be divided into two peaks at 70.4 and 76.3 eV, corresponding to the binding energies of Pt 4f 7/2 and Pt 4f 5/2 valence states, respectively, assigned to Pt. Figure 4e presents the high-resolution spectra of Pd 3d, and they show two characteristic peaks with binding energies of about 334.8 and 340.4 eV, corresponding to the binding energies of Pd 3d 5/2 and Pd 3d 3/2 , respectively. This result proves the existence of Pd [32].   Figure 5 illustrates the responses of the pure In 2 O 3 and 1 at% Pd/Pt-In 2 O 3 to 5 ppm H 2 S at different operating temperatures. Since the minimum working temperature of the micro-hotplate in the sensor is 50 • C, the temperature gradient starts from 50 • C to ensure the stability of the working temperature of the sensor. As shown in Figure 5, when the operating temperature is 50 • C, 1 at% Pd/Pt-In 2 O 3 has the highest response to H 2 S, and the response decreases with the increase in the operating temperature. The response of the pure In 2 O 3 to H 2 S increases with the temperature, and it reaches the maximum at 250 • C. The optimal operating temperature of 1 at% Pd/Pt-In 2 O 3 is significantly lower than that of the pure In 2 O 3 because of the catalysis of Pt [33]. In addition, the response of 1 at% Pd/Pt-In 2 O 3 (140) is close to 70 times that of the pure In 2 O 3 (2.1) at 50 • C, which indicates that the introduction of Pd/Pt NPs significantly improves the gas sensitivity of the microspheres to H2S. The electronic sensitization of Pd, the chemical sensitization of Pt, and the synergistic effect between them are the main reasons for the enhanced response of 1 at% Pd/Pt-In 2 O 3 .  Figure 6 shows the response/recovery time of 1 at% Pd/Pt-In 2 O 3 for 5 ppm H 2 S at 50 • C. The response time (T Response ) and recovery time (T Recovery ) of the sensor are defined as the time spent when the sensor resistance changes from the initial value to 90% of the total resistance change after the target gas is applied and released. The sensor responds very quickly after contacting H 2 S, and the response time is only 3 s (T Response = 3 s). According to the characterization, the structure of 1 at% Pd/Pt-In 2 O 3 is porous and has a specific surface area of 58.3704 m 2 /g, which enables the gas to diffuse into the interior of the material; there are many active sites on the surface of the material where hydrogen sulfide can be bound, which accelerates the response speed [20]. In addition, a large amount of chemically adsorbed oxygen is distributed on the material surface due to the overflow effect of the Pt NPs, which further shortens the response time [25]. However, after H 2 S is released, the sensor recovers very slowly. After 1000 s, the resistance of the sensor only recovers from 10% to 28% of the original resistance. This slow recovery may be related to the existence of many cracks and pores in the NP-loaded materials, which hinder the desorption of the H 2 O and SO 2 generated in the reaction between the H 2 S and the chemically adsorbed oxygen [34,35]. To solve this problem, a voltage pulse is applied to the sensor to increase the working temperature of the sensor to 400 • C in order to promote the desorption of H 2 S. As shown in Figure 6, the sensor resistance value surges after the pulse is applied, and then it quickly returns to the original value. This proves that the application of a voltage pulse can effectively shorten the recovery time of the sensor because the high temperature accelerates the desorption of H 2 O and SO 2 .   Figure 7b shows the relationship between the response of 1 at% Pd/Pt-In 2 O 3 and the concentration of H 2 S. The response of the sensor has a strong linear relationship with the logarithm of the concentration of H 2 S, which indicates that the sensor can be applied to H 2 S sensing. As shown in Figure 7c, the response of the sensor to 50 ppb H 2 S is 1.6 at 50 • C. Therefore, the LOD of the sensor is 50 ppb.  Figure 8a shows the effect of humidity on the H 2 S sensing of 1 at% Pd/Pt-In 2 O 3 . The response value of the sensor decreases significantly with the increase in humidity. When the relative humidity increases from 25% to 75%, the response of the sensor decreases by 47%. The change trend of the response of 1 at% Pd/Pt-In 2 O 3 with humidity and that of the normalized resistance of a humidity sensor based on metal oxide are similar, so the performance loss may be caused by the influence of the humidity on the resistance of the sensor [36]. In addition, the occupation of some active sites by H 2 O molecules affects the adsorption of gas on the surface of the material and causes a further decline in the response [37]. Figure 8b indicates the response of the sensor to five groups of 5 ppm H 2 S at 50 • C. The responses of the sensor to the five groups of introduced H 2 S are the same, and their standard deviation is 1.4519, demonstrating the superb repeatability of the sensor. Figure 8c presents the response of 1 at% Pd/Pt-In 2 O 3 to various gases. When the working temperature is 50 • C, the response of the sensor to 5 ppm H 2 S is significantly higher than its response to various gases with concentrations of 50 ppm. Therefore, the sensor has a strong selectivity and can effectively avoid the influence of interference gas.

Gas-Sensing Characteristics
The long-term stability of 1 at% Pd/Pt-In 2 O 3 is measured by repeatedly testing its response to 5 ppm H 2 S at 50 • C within 20 days. According to Figure 8d, the response of the sensor is almost stable within 20 days and has potential for practical applications.
In conclusion, 1 at% Pd/Pt-In 2 O 3 is characterized by a low operating temperature, a high sensitivity (140 for 5 ppm H 2 S), a fast response, a low LOD, a linear response, superior selectivity, and repeatability. Table 1 demonstrates a comparison of the H 2 S sensing performance of In 2 O 3 -based sensors in the recently reported literature and in our work. The response time of 1 at% Pd/Pt-In 2 O 3 to 5 ppm H 2 S at 50 • C is 3 s, which is much faster than that of the other materials listed in the table. Additionally, the response of 1 at% Pd/Pt-In 2 O 3 under the same conditions is 140. The other materials have either a higher operating temperature or a lower response at 50 • C. Therefore, the gas-sensing material synthesized in this paper has an excellent low-temperature performance and can be used for the rapid and sensitive detection of H 2 S at a low temperature.

Gas-Sensing Mechanism
The gas-sensing mechanism of In 2 O 3 is a surface-controlled type, as shown in Figure 9 [43]. When In 2 O 3 belonging to an n-type semiconductor is exposed to air, the oxygen in the air will adsorb on the surface of In 2 O 3 to form adsorbed oxygen, as shown in Equation (2). The adsorbed oxygen will capture the conduction-band electrons on the surface of In 2 O 3 to generate chemisorbed oxygen, as shown in Equations (3)-(5) [44]. The types of chemisorbed oxygen that form are different according to the different ambient temperatures [45]. As electrons are trapped, the thickness of the electronic depletion layer increases, the carrier mobility of the surface of In 2 O 3 decreases, and the resistance increases.
Since the working temperature of 1 at% Pd/Pt-In 2 O 3 is 50 • C, the type of chemisorbed oxygen should be mainly O 2 − [46]. When the reductive H 2 S contacts the chemisorbed oxygen, H 2 S will react with the chemisorbed oxygen, as shown in Equation (6).
In this reaction, the electrons captured by the chemisorbed oxygen return to the conduction band of In 2 O 3 . The thickness of the electronic depletion layer decreases, the carrier mobility of In 2 O 3 recovers, and the resistance decreases. According to the measurements of the specific surface areas of the microspheres, the specific surface area of 1 at% Pd/Pt-In 2 O 3 is slightly smaller than that of the pure In 2 O 3 . In general, the gas-sensing performances of materials with larger specific surface areas are enhanced. However, the response of 1 at% Pd/Pt-In 2 O 3 to H 2 S is much higher than that of the pure In 2 O 3 , which is contrary to the regular pattern. We believe that this is because the introduction of Pd/Pt NPs can improve the gas-sensing performance of the material much more than the loss caused by a reduction in the specific surface area, which demonstrates that the introduction of Pd/Pt NPs can effectively improve the gas-sensing properties of materials. Pt NPs mainly improve gas sensitivity through chemical sensitization. Pt can promote the formation of chemisorbed oxygen through chemical sensitization. Due to the influence of Pt, a large amount of chemisorbed oxygen overflows to the surface of the material, which improves the sensitivity of the material [33]. Pd improves the gas-sensing property through electronic sensitization. As the work function of Pd NPs (5.5 eV) is greater than that of In 2 O 3 (5.0 eV) [47], a Schottky barrier forms between the Pd NPs and In 2 O 3 . Compared to monometallic catalysts, Pd/Pt bimetallic NPs can further improve the catalytic activity because of the synergistic effects between them. As shown in Equation (6), water is generated in the reaction of H 2 S and chemically adsorbed oxygen. It reacts with Pd to form Pd(OH) 2 with low catalytic activity, which reduces the activity of Pd [48]. Doping an appropriate amount of Pt into a material can inhibit this process and reduce the influence of water on Pd [49].

Conclusions
In summary, In 2 O 3 hollow microspheres doped with Pd/Pt NPs were successfully synthesized using the solvothermal method and the in situ reduction method. Due to the excellent physical properties of In 2 O 3 and the sensitization and synergistic effect of the dopants, the gas sensor based on 1 at% Pd/Pt-In 2 O 3 has the advantages of a low operating temperature, a high sensitivity, a low LOD, a fast response, superior selectivity, etc. Specifically, the response of the sensor to 5 ppm H 2 S at 50 • C can reach 140, and the response time is only 3 s. The disadvantage of the slow recovery is also solved by applying a voltage pulse during the recovery process. The material proposed in this paper has potential application prospects in H 2 S sensors.