Synthesis and Oxidation Behavior of Pd-Ir@CeO₂ Core-shell Nanoparticles for Hydrogen Gas Sensor

1. INTRODUCTION

The global trend of transitioning from fossil fuels to renewable and clean energy sources stems from concerns about environmental pollution and resource depletion. Among the promising alternative energy sources, hydrogen stands out because of its environmental friendliness, high energy density, and inexhaustibility [1-3]. Nonetheless, hydrogen is a colorless, odorless, and leak-prone gas with a wide explosive concentration range, making it important to develop reliable and inexpensive gas detection systems [4-6].

Metal-oxide semiconductor (MOS) gas sensors have attracted significant attention for gas detection [7-9]. These materials are favored because of their high stability, simple operating principles, and responses to most gases. In particular, CeO₂ has emerged as a leading gas sensing material owing to its 4f electron shell, low redox potential between Ce³⁺ and Ce⁴⁺ states, and high density of oxygen vacancies [10-12]. For example, Hussain et al. synthesized polyhedral CeO₂ nanoparticles (NPs) by hydrothermal method, which exhibited high sensitivity and selectivity to 150 ppm HCHO at 220^oC [13]. Dao et al. fabricated CeO₂ NPs by the ionic liquid-supported method, which showed a high response to 100 ppm ethanol at 400^oC [14]. Despite their potential, MOS gas sensors face challenges such as high operating temperatures, low sensitivity, and poor selectivity for hydrogen gas [15-17]. To address these issues, researchers have integrated noble metal catalysts into MOS sensors to enhance their gas-sensing properties through catalytic effects containing chemical and electronic sensitization [18-20].

Palladium (Pd) has been widely studied as a catalyst for hydrogen gas sensing because of its unique ability to absorb hydrogen gas and form metal hydrides, thereby changing the electric charge distribution via chemical reactions on the surface [21-24]. For instance, Cai et al. reported Pd/Fe₂O₃-NiO nanofibers prepared by the electrospinning method, which exhibited high hydrogen selectivity against NO₂, ethanol, CO, toluene, H₂S, and acetone gases at 250^oC [25]. Meng et al. reported that Pd/SnO₂ nanoparticles, synthesized via hydrothermal and chemical reduction methods, exhibited a high response toward 500 ppm H₂ at a low temperature of 125^oC [26]. Thathsara et al. decorated Pd nanoparticles on hollow TiO₂ nanospheres via a chemical reduction method using SiO₂ nanospheres as a template, which exhibited high sensitivity and selectivity in a wide concentration range from 50 to 10,000 ppm H₂ with irradiation of UV light at 80^oC [27].

However, the Pd-based MOS gas sensor has the drawback that the Pd catalyst NPs are easily oxidized to PdO at an operating temperature above 200^oC. Hence, they are not yet suitable for industrial applications [28-31]. To address these issues, researchers have explored the use of Pd-based alloys as catalysts. For instance, Pandey et al. reported that the Pd-Ag@SnO/SnO₂ nanosheet synthesized by hydrothermal and chemical reduction methods showed a high response toward 100 ppm H₂ at 225^oC and high selectivity toward hydrogen over ethanol, NH₃, NO₂, CO, and CO₂ gases [32]. Choi et al. discovered that Au-Pd/SnO₂ nanosheets showed a high sensor signal response to hydrogen and high selectivity of hydrogen toward CH₄ gas [33]. Li et al. reported that PdPt@In₂O₃ nanocomposites demonstrated a significant detection performance for 100 ppm H₂ at room temperature with high selectivity for ammonia, benzene, acetone, ethanol, formaldehyde, and ether gases [34]. Among Pd-based alloys, Pd–iridium (Ir) alloys have been reported to exhibit superior hydrogen absorption performance [35-37]. Nevertheless, because Ir is resistant to growth beyond 2 nm, controlling its size and shape at the nanoscale poses significant challenges [38,39]. However, the hydrogen absorption capacity diminishes with decreasing particle size at the nanoscale [40,41], limiting the use of Pd-Ir alloy NPs as catalysts for hydrogen gas detection. Furthermore, Ir is broadly immiscible with most other metals in the periodic table [42]. Consequently, there have been no reports on the synthesis of Pd-Ir alloy NPs with diameters larger than 10 nm.

In this study, we report, for the first time, a hydrothermal method for preparing Pd–Ir alloy NPs with an average size of 19 nm. The Pd–Ir NPs were coated with CeO₂, which protects them from oxidation and agglomeration and serves as the main site for gas reaction, to fabricate Pd–Ir@CeO₂ core-shell nanoparticles (CSNPs), and the oxidation behaviors of the Pd–Ir alloy cores during the heat treatment at 500°C were investigated. The degree of oxidation of the Pd-Ir alloy cores is an important factor for hydrogen gas sensing. The hydrogen gas response and selectivity of the Pd–Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs after heat treatment were investigated in detail, and a reaction mechanism was proposed based on these results.

2. EXPERIMENTAL

2.3 Synthesis of Pd-Ir@CeO₂ core-shell nanoparticles

A schematic of the synthesis procedure for the Pd-Ir@CeO₂ CSNPs is shown in Fig. 1(a). The prepared Pd-Ir NPs were dispersed in 90 mL of deionized (DI) water. A mixture containing 60 mL of the Pd-Ir alloy NPs colloid, 6 mL of Na₂CO₃ (50 mM), and 4.8 mL of Ce(NO₃)₃ (50 mM) was stirred for 10 min. Subsequently, the mixture was refluxed at 90^oC for 12 hours to synthesize Pd-Ir@CeO₂ CSNPs. The samples were washed several times with water and ethanol, and then dried. Afterward, Pd-Ir@CeO₂ CSNPs were calcinated at 500^oC in Ar and air atmosphere to investigate the alloy formation and oxidation behavior of Pd-Ir NPs. The samples calcinated in air and Ar are denoted PdIrOx@CeO₂ CSNPs and Pd-Ir(alloy)@CeO₂ CSNPs, respectively. Additionally, the sample calcinated in Ar was further calcinated in air, and the core was partially oxidized during this process; thus, it was named Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs.

Fig. 1.

(a) Schematic illustration for the formation procedure of Pd-Ir(alloy)@CeO2 CSNPs and PdO-IrO2@CeO2 CSNPs; TEM images of (b) Pd-Ir NPs, (c) Pd-Ir@CeO2 CSNPs, (d) PdOIrO2@ CeO2 CSNPs, and (e) Pd-Ir(alloy)@CeO2 CSNPs; (f) HAADF image and the corresponding EDS mapping of Pd-Ir(alloy)@CeO2 CSNPs.

3. RESULTS AND DISCUSSIONS

3.1 Material characterizations

The morphologies and sizes of the synthesized nanoparticles were examined using TEM. As shown in Fig. 1(b), the TEM image reveals that the as-synthesized Pd-Ir NPs are aggregates of primary particles with an average diameter of 4.5 nm, and the average diameter of these aggregates is 19 nm. Fig. 1(c) presents a TEM image of the as-synthesized Pd-Ir@CeO₂ CSNPs, which exhibit a core-shell structure with sizes ranging from 36 nm to 42 nm. Fig. 1(d) shows the TEM image of PdO-IrO₂@CeO₂ CSNPs formed after air calcination at 500°C, confirming that the size of these nanoparticles decreased to 28-37 nm (from 36-42 nm of Pd-Ir@CeO₂) as a result of thermal shrinkage and the removal of organic groups. Fig. 1(e) illustrates the TEM image of Pd-Ir(alloy)@CeO₂ CSNPs formed after Ar calcination at 500°C, demonstrating that core morphology changed into a hollow structure. The shape transformation can be attributed to the Kirkendall effect, which occurs because of the difference in diffusion rates between the Pd and Ir atoms [43]. In addition, the size of these nanoparticles decreased to 27-32 nm. To analyze the nanostructure of the core-shell nanoparticles in more detail, highangle annular dark-field (HAADF) and mapping analyses using energy-dispersive X-ray spectroscopy (EDS) were performed as shown in Fig. 1(f), where the presence of O, Ce, Ir, and Pd elements is highlighted in green, yellow, blue, and red, respectively. The results indicate that Pd and Ir elements were concentrated in the core, whereas Ce and O were distributed around the core.

The crystallographic properties and oxidation behavior of the synthesized materials were investigated by X-ray diffraction (XRD). Fig. 2(a) shows the XRD profiles of three samples of Pd-Ir NPs heat-treated at 500°C in different atmospheres: 1) in Ar atmosphere for 2 h, 2) in air atmosphere for 2 h, and 3) in Ar atmosphere for 2 h followed by an additional heat treatment in air. The peaks at 40.9°, 45.8°, and 69° correspond to the (111), (200), and (220) planes of the standard X-ray diffraction pattern of fcc-Ir (JCPDS-06-0598), and the peaks at 40.1°, 46.6°, 68.1°, and 82.1° correspond to the (111), (200), (220), and (311) planes of the standard X-ray diffraction pattern of fcc-Pd (JCPDS-46-1043). The XRD profiles of the as-synthesized Pd-Ir NPs were compared with the individual patterns of Pd and Ir. The peak position of the (111) plane in the as-synthesized Pd-Ir NPs was observed at 40.7°, which was situated between the peak positions of the (111) planes of Pd and Ir. This suggests the formation of a Pd-Ir alloy between the metallic phases. However, for the PdO-IrO₂ NPs calcinated in air, the diffraction peaks of Pd and Ir disappeared and new peaks appeared at different diffraction positions. These new peaks are consistent with the X-ray diffraction peaks of tetragonal-IrO₂ (JCPDS-88-0288) and tetragonal PdO (JCPDS-85-0713), suggesting that the Pd-Ir NPs were oxidized. This indicates that the alloying of the as-synthesized Pd-Ir NPs was incomplete. Subsequently, we calcinated the synthesized Pd-Ir NPs at 500°C in an Ar atmosphere to promote alloying (resulting in Pd-Ir alloy NPs). The results showed a decrease in the full width at half maximum (FWHM) and an increase in the peak intensity compared to those of the as-synthesized Pd-Ir NPs. To further confirm the alloying, the sample was further heat-treated at 500°C in an air atmosphere (resulting in Pd-Ir(alloy)/PdO-IrO₂ NPs). Compared to the PdO-IrO₂ NPs calcinated in air, the Pd-Ir(alloy)/PdO-IrO₂ NPs exhibited various X-ray diffraction peaks of Pd, Ir, PdO, and IrO₂. Furthermore, the peak of the (111) plane of the Pd-Ir alloy appeared at 40.7°, which matched that of the as-synthesized Pd-Ir NPs. Therefore, the results indicate that the calcination process in an Ar atmosphere facilitated the alloying of Pd-Ir NPs and consequently reduced oxidation. Fig. 2(b) shows the XRD profiles of the calcinated pure CeO₂ NPs and two other samples that heat-treated Pd-Ir@CeO₂ CSNPs at 500°C in different atmospheres: 1) in air atmosphere for 2 h, and 2) in Ar atmosphere for 2 h followed by an additional heat treatment in air. In these profiles, the X-ray diffraction peaks at 28.55°, 33.07°, 47.48°, 56.34°, 59.09°, 69.41°, 76.70°, and 79.07° correspond to the (111), (200), (220), (311), (222), (400), (331), and (420) planes of standard fcc-CeO₂ (JCPDS-43-1002) from. The CeO₂ NPs exhibited X-ray diffraction peaks consistent with fcc-CeO₂ crystals, while PdO-IrO₂@CeO₂ CSNPs calcinated in air atmosphere displayed X-ray diffraction peaks consistent with IrO₂, PdO, and CeO₂ crystals. Moreover, the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs calcinated in both Ar and air atmospheres exhibited X-ray diffraction peaks of Pd and Ir metals as well as metal oxides including PdO, IrO₂, and CeO₂. It indicates that alloying of the Pd-Ir core progressed through Ar calcination, resulting in relatively low oxidation by the alloying effect.

Fig. 2.

XRD patterns of the as-synthesized (a) Pd-Ir NPs, (b) pure CeO2 and Pd-Ir@CeO2 CSNPs after the calcination process in Ar, air, and both Ar and air.

We conducted X-ray photoelectron spectroscopy (XPS) to investigate the electronic states of the surface elements. Fig. 3(a) illustrates the comprehensive XPS spectrum of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs including the Pd 3d, Ir 4f, Ce 3d, and O 1s spectra. In Fig. 3(b), the Pd 3d spectra of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs containing the 3d_5/2 and 3d_3/2 components are fitted to four peaks. The peaks at 335.63 and 340.93 eV represent metallic Pd (Pd⁰), while the peaks at 336.41 and 341.95 eV represent Pd²⁺, reflecting Pd oxidation [44]. The percentage of Pd⁰ was 63.65%, whereas that of Pd²⁺ was 36.35%. Because Pd oxidation proceeds from the surface [28], these ratios indicate that oxidation is been significantly suppressed by alloying. Fig. 3(c) shows that the Ir 4f spectra of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs containing the 4f7/2 and 4f5/2 components were segmented into six peaks. The peaks observed at 60.35 and 63.20 eV correspond to metallic Ir (Ir⁰), while the peaks at 60.83 and 63.84 eV are attributed to Ir⁴⁺, and the peaks at 61.64 and 64.60 eV correspond to Ir³⁺ indicating the presence of IrO₂ [45]. The percentage ratios of Ir⁰, Ir³⁺, and Ir⁴⁺ are 9.38, 26.63, and 60.31%, respectively. Considering that Ir is not extensively miscible with other metals, this indicates that Ir in the core is less alloyed than Pd [38]. Fig 3(d) shows the Ce 3d spectra of pure CeO₂ NPs and Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs, containing 3d_5/2 and 3d_3/2 components. There are eight prominent peaks at energy levels of 882.5, 885.6, 889, 898.4, 901.1, 903.7, 917.6, and 916.8 eV, which are associated with Ce 3d_3/2 and Ce 3d_5/2 orbitals [46]. In addition, these peaks indicated the presence of both Ce³⁺ and Ce⁴⁺ oxidation states. The fraction of Ce³⁺ in the CeO₂ NPs is 18.19%, while Ce³⁺ in Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs is 38.54%. The results showed that the core-shell structure had a higher Ce³⁺ content owing to the interactions between CeO₂ and the Pd-Ir(alloy)/PdO-IrO₂ core. Typically, the production of Ce³⁺ is accompanied by the formation of oxygen vacancies, indicating that the oxygen vacancy content of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs is higher than that of pure CeO₂ NPs [47]. Fig. 3(e) illustrates the O 1s spectra of pure CeO₂ NPs and Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs incorporating lattice oxygen (O_L), oxygen vacancies (O_v), and chemisorbed oxygen (O_c). The peaks observed at 529.8, 531.5, and 532.9 eV correspond to O_L, O_v, and O_c, respectively [48]. The sensing mechanism of the MOS gas sensor relies on the reaction between the adsorbed oxygen and the target gas, with the oxygen vacancies providing carriers. Therefore, it is crucial to confirm the contents of O_v and O_c [49-51]. The results indicated that the percentages of O_c and O_v were both significant in the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs compared to those in pure CeO₂ NPs as shown in Fig. 3(f). Furthermore, the increased amount of O_v aligns with the high Ce³⁺ content of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs in the Ce 3d spectra.

Fig. 3.

High-resolution XPS spectra for (a) full range (b) Pd 3d, (c) Ir 4f in Pd-Ir(alloy)/PdO-IrO2@CeO2 CSNPs; (d) Ce 3d region and (e) O 1s region in the CeO2 NPs and Pd-Ir(alloy)/PdO-IrO2@CeO2 CSNPs; (f) the percentage of O 1s components in the CeO2 NPs and Pd-Ir(alloy)/PdO-IrO2@CeO2 CSNPs.

3.2 Gas sensing performances

As MOS gas sensors operate thermally, it is essential to conduct sensing tests at various temperatures and gas concentrations to determine the optimal operating temperature. Fig. 4(a) shows the resistance change of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs sensors to hydrogen gas at various operating temperatures (200-400°C) and concentrations (2-100 ppm). These results confirmed the R_a and R_g values, which were measured by evaluating the saturated sensor resistance in 10.5% oxygen after the injection of the test gas. Furthermore, the sensor response was calculated using the formula R_s = (R_a - R_g) / R_g × 100 (%), as shown in Fig. 4(b). The results exhibit that the optimum operating temperatures for Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs, PdO-IrO₂@CeO₂ CSNPs, and CeO₂ NPs gas sensors are 300°C, 200°C, and 350°C, respectively. In addition, the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs sensor showed the highest response value (R_s = 111%) toward 100 ppm H₂ gas at the test temperature of 300°C, while the response of PdO-IrO₂@CeO₂ CSNPs sensor at 200°C was even lower than that of pure CeO₂ NPs sensor at 350°C. This indicates that preventing the oxidation of Pd-Ir NPs is crucial for the design of hydrogen gas sensing materials. Fig. 4(c) shows the sensitivities of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs and pure CeO₂ NPs sensors under different hydrogen gas concentrations (2-100 ppm) at their optimum temperatures. The Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs sensor displayed a linear response to hydrogen concentrations ranging from 2 to 100 ppm and exhibited improved sensitivity compared to the pristine CeO₂ sensor. For practical applications of gas sensors, high hydrogen selectivity toward target gases, as opposed to interfering gases, is crucial. To evaluate selectivity, the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs sensor was tested at 300°C with 100 ppm of hydrogen, ethanol, acetone, acetaldehyde, and carbon monoxide gases as shown in Figs. 4(de). The responses of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs gas sensor to hydrogen, ethanol, acetone, methane, and carbon monoxide were 111, 63, 41, 39, and 10%, respectively. The responses of the pure CeO₂ NPs sensor to hydrogen, ethanol, acetone, methane, and carbon monoxide were 13.3, 14.9, 13.8, 15.9, and 12.7, respectively. These results indicate that the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs gas sensor exhibits higher hydrogen selectivity than the pure CeO₂ NPs sensor. To verify the stability of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs gas sensor, ten cycles of 100 ppm H₂ gas injection were performed at the optimal operating temperature as shown in Fig. 4(f). Each cycle exhibited almost the same resistance change. This demonstrates that the resistance returned to the initial level when the hydrogen supply was stopped, confirming that the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs gas sensor has excellent repeatability.

Fig. 4.

Gas sensing properties of Pd-Ir(alloy)/PdO-IrO2@CeO2 CSNPs gas sensor with pure CeO2 NPs or PdO-IrO2@CeO2 CSNPs sensors; (a) resistance changes for hydrogen gas; (b) response changes to 100 ppm H2 at various operating temperatures with other sensors; (c) responses at various hydrogen concentrations at optimal working temperatures; (d) resistance changes at optimal working temperature depending upon gas concentration (2-100 ppm) of ethanol, acetone, acetaldehyde, and carbon monoxide; (e) hydrogen selectivity to different kinds of gasses at 100 ppm under the optimal working temperature; (f) ten cycle stability.

3.3 Gas sensing mechanism

This section discusses the H₂ gas sensing mechanism of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs sensor by comparing it with that of a pure CeO₂ NPs sensor, as shown in Fig. 5. The H₂ gas sensing mechanism is based on well-established principles for n-type semiconductor gas sensors [52-54]. When the sensor is exposed to air, oxygen molecules are adsorbed on the surface of CeO₂ and capture electrons in the conduction band (CB), resulting in different oxygen species (O^2-, O^-, and O^2-) [55]. The type of ionized oxygen species depends on the operating temperature, as expressed by Eqs. (1-4). Since the optimal operating temperature of the sensing material was over 300°C, it is believed that O^2- is the major adsorbed oxygen species.

Fig. 5.

Illustration for hydrogen gas sensing mechanism of Pd-Ir(alloy)/PdO-IrO2@CeO2 CSNPs, when exposed to air and hydrogen.

$\[ {\mathrm{O}}_2\mathrm{ }\left(\mathrm{g}\mathrm{a}\mathrm{s}\right)\mathrm{ }\to 2\mathrm{O}\mathrm{ }\left(\mathrm{a}\mathrm{d}\mathrm{s}\right) \]$

(1)

(2)

(3)

$\[ \mathrm{O}\mathrm{ }\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)\mathrm{ }+2{\mathrm{e}}^{-}\to {\mathrm{O}}^{2-}\mathrm{ }\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)\mathrm{ }\left(\mathrm{T}>{300}^{\circ }\mathrm{C}\right) \]$

(4)

These processes led to the formation of a depletion layer on the CeO₂ surface, ultimately increasing the baseline resistance (R_a). When H₂ gas was injected, the formed oxygen ions reacted with the H₂ molecules, releasing the captured electrons back into CeO₂. Consequently, the depletion layer becomes thinner, leading to a decrease in electrical resistance. The relevant reactions between the H₂ gas and oxygen ions are as follows:

$\[ {\mathrm{H}}_2\mathrm{ }\left(\mathrm{g}\mathrm{a}\mathrm{s}\right)\mathrm{ }\to {\mathrm{H}}_2\mathrm{ }\left(\mathrm{a}\mathrm{d}\mathrm{s}\right) \]$

(5)

$\[ {\mathrm{H}}_2\mathrm{ }\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)\mathrm{ }+{\mathrm{O}}^{2-}\to {\mathrm{H}}_2\mathrm{O}\mathrm{ }\left(\mathrm{a}\mathrm{d}\mathrm{s}\right)\mathrm{ }+2{\mathrm{e}}^{-} \]$

(6)

In addition, the enhanced gas sensing performance of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs sensor toward H₂ gas as compared to the pure CeO₂ NPs can be explained by two effects of the Pd-Ir alloy catalyst. The first is the catalytic effect [56-58] and the second is related to the hydrogen absorption effect (hydride formation) [59-61].

The catalytic effect includes both electronic and chemical sensitization effects. The electronic sensitization effect arises from the fact that the work function of CeO₂ (3.4 eV) is lower than those of Pd (5.2 eV) and Ir (5.1 eV) [46,62]. While combining these two processes, the electrons in the conduction band of CeO₂ are transferred to the surface of the Pd-Ir alloy NPs, resulting in the formation of a Schottky barrier at the interface. Consequently, the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs sensor exhibited a higher electrical resistance (R_a) in air. However, the chemical sensitization effect facilitates the easy dissociation of adsorbed hydrogen molecules into atoms by the Pd-Ir alloy nanoparticles, as shown in Eq. (7) [63-65]. The dissociated hydrogen atoms then migrate to the surface of CeO₂, where they react with active oxygen ions to form water, as shown in Eq. (8).

$\[ {\mathrm{H}}_2\mathrm{ }\left(\mathrm{g}\mathrm{a}\mathrm{s}\right)\mathrm{ }\to 2\mathrm{H} \]$

(7)

$\[ 2\mathrm{H}+{\mathrm{O}}^{2-}\to {\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-} \]$

(8)

As a result of this reaction, the fundamental gas-sensing mechanism is enhanced. Ultimately, this leads to a further decrease in electrical resistance in the presence of the target gases (R_g).

The second factor contributing to the enhanced hydrogen sensing performance of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs sensor is the hydrogen absorption capability of the Pd-Ir alloy nanoparticles [35,66,67]. Duś et al. noted that the work function decreases when Pd absorbs hydrogen to form Pd hydride [24]. This change in the work function is influenced by the redistribution of the electric charge resulting from chemical reactions on the surface. A decrease in the work function has been observed for several transition-metal hydrides, which is attributed to the presence of highly polarized hydrogen species on the surface. Therefore, when the Pd-Ir alloy nanoparticles absorb hydrogen and are partially converted to Pd-Ir hydride on their surfaces, as described in Eq. (9), the work function of the Pd-Ir alloy nanoparticles is reduced.

$\[ \mathrm{P}\mathrm{d}\mathrm{I}\mathrm{r}+\mathrm{x}\mathrm{H}\to {\mathrm{P}\mathrm{d}\mathrm{I}\mathrm{r}\mathrm{H}}_{\mathrm{x}} \]$

(9)

Consequently, the height of the Schottky barrier between the Pd and Ir alloy and CeO₂ was reduced, leading to a decrease in the electric depletion layer. This effect lowers the electrical resistance in the presence of hydrogen gas. The reaction described in Eq. (9) occurs selectively for hydrogen gas, which further explains the high selectivity of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs sensor toward H₂.

In addition, as described above, the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs with a core-shell structure showed significantly higher amounts of Ce³⁺ and oxygen vacancies than pure CeO₂ NPs. The presence of these oxygen vacancies plays a crucial role in gas-sensing performance because they are capable of generating electrons. Moreover, these vacancies served as active sites for the adsorption of both oxygen and hydrogen gases. Therefore, the increased number of oxygen vacancies helps create an environment that facilitates interactions between oxygen and hydrogen gas, such as adsorption onto the CeO₂ surface. Hence, this demonstrates the importance of structural modifications in enhancing gas-sensing performance.

4. CONCLUSIONS

Pd-Ir composite NPs with an average diameter of 19 nm were successfully synthesized by a hydrothermal method and used as seeds to obtain Pd-Ir@CeO₂ CSNPs with particle sizes in the range of 36-42 nm. For alloying the Pd-Ir core, heat treatments were performed sequentially at 500°C in an Ar atmosphere for 2 h and in an air atmosphere for 2 h, and as a result, Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs with a hollow alloy core were obtained.

The Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNP sensor showed a response of 111% to 100 ppm H₂ gas, which was significantly higher than those of fully oxidized PdO-IrO₂@CeO₂ CSNPs and pure CeO₂ NP sensors. The selectivity for hydrogen gas was significantly higher than that for ethanol, acetone, acetaldehyde, and CO. The enhanced hydrogen gas sensing properties of the Pd-Ir(alloy)/PdO-IrO₂@CeO₂ CSNPs sensor are attributed to the hydrogen absorption and catalytic effects of the Pd-Ir(alloy) core on the hydrogen oxidation reaction. In addition, PdIrH_x formed by hydrogen adsorption lowers the Schottky barrier between PdIr and CeO₂, resulting in a decrease in the resistance of the sensor and an increase in sensitivity.

Acknowledgments

This work was supported by 1) BK21-FOUR programs of the Ministry of Education and Human-Resource Development, South Korea; 2) National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2021R1A2C2008447); 3) "Regional Innovation Strategy (RIS)" through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (MOE) (2023RIS-008).

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