Applied Science and Convergence Technology 2023; 32(4): 100-103
Published online July 30, 2023
Copyright © The Korean Vacuum Society.
aInstitute of Advanced Composite Materials, Korea Institute of Science and Technology, Wanju 55324, Republic of Korea
bDepartment of Flexible and Printable Electronics, LANL‐JBNU Engineering Institute, Jeonbuk National University, Jeonju 54896, Republic of Korea
cDepartment of JBNU-KIST Industry-Academia Convergence Research, Jeonbuk National University, Jeonju 54896, Republic of Korea
dSchool of Materials Science and Engineering, Pusan National University, Busan 46241, Republic of Korea
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In this study, H2 sensors were developed with Pd-doped porous WO3 (Pd/WO3) as the sensing material using different amounts (wt%) of Pd. Pd/WO3 was synthesized using an in-situ high-temperature solvothermal method. The surface morphology and nanostructure of Pd/WO3 were characterized through X-ray diffraction and high-resolution transmission electron microscopy. 1.0 wt% Pd/WO3 exhibited the highest hydrogen-sensing ability compared to pure WO3 and 0.5 wt% and 1.5 wt% Pd/WO3 at 100 °C. The Pd/WO3-based gas sensor demonstrated promising H2-sensing characteristics, such as high selectivity and good repeatability. Therefore, this sensor is an excellent candidate for use in applications that require high-performance low-temperature H2 sensing.
Keywords: Pd, WO3, In-situ synthesis, Low operating temperature, Hydrogen gas sensor
Hydrogen is the most common element in the universe, and it has gathered considerable attention as one of the most promising green and renewable energy sources for various industrial applications [1,2]. In particular, it can be used as a clean alternative fuel, making it a promising renewable energy option. However, hydrogen is highly explosive and has a low flammability point (4 vol%) in air . Furthermore, H2 is odorless, tasteless, and colorless, making it difficult to detect in the atmosphere in everyday life . Therefore, a rapid and precise leakage detection and monitoring system is necessary for safe H2 production, storage, and use. H2 is detected using various sensors, including catalytic, electrochemical, mechanical, optical, acoustic, thermal-conductivity, and chemiresistive sensors. Among these, chemiresistive sensors with metal oxide semiconductors, such as SnO2, ZnO, TiO2, and WO3 doped with noble metals, are considerably effective in sensing H2 . WO3 is widely used in H2 sensors because of its high oxygen vacancy diffusion coefficient and good chemical stability . Pristine WO3 with Pd-functionalized or Pt-functionalized nanostructures can effectively enhance the sensitivity and response of sensors for hydrogen detection [7,8]. However, the operating temperature of such sensors is quite high (150−300 °C), which limits their practical application . This issue is addressed by employing two methods to reduce the operating temperature, i.e., the in-situ functionalization of noble metals on the surface of WO3 and the development of a porous WO3 architecture. In-situ functionalization results in strong orbital interaction with WO3, which enhances electron-transport pathways. The porous architecture of WO3 nanoparticles enhances the surfaceto- volume ratio, thereby increasing the surface area. This results in efficient physisorption and chemisorption of H2 and reduces the operating temperature. In this study, pure WO3 and Pd/WO3 nanocomposites [with various amounts (wt%) of Pd] were synthesized in situ using a high temperature solvothermal process. The morphologies, structures, and H2-sensing properties of the as-synthesized nanocomposites were systematically investigated. This study developed a simple method for synthesizing sensing devices that can be expanded to create other metal-oxide-sensing devices.
An in-situ high-temperature solvothermal growth reaction was used to synthesize Pd/WO3 nanocomposites. First, 0.5 g of terephthalic acid (98 %, Sigma-Aldrich) and 0.3 g of D-glucose (≥ 99.9 %, Sigma- Aldrich) were dissolved in 40 mL ethanol and 10 mL dimethylformamide followed by the addition of an appropriate amount of WCl6 (≥ 99.9 %, Sigma-Aldrich) such that the final concentration of WCl6 was 0.016 mM. Then, a PdCl2 solution (with a Pd/W ratio of 0.5, 1.0, and 1.5) was added, and the mixture was transferred to a 50 mL autoclave for 24 h at 180 °C. The final precipitates were filtered and washed with acetone and dried in air at 60 °C for 24 h. Thereafter, all the samples were calcined in air at 500 °C for 2 h to obtain the final sensing product. Pure WO3 was prepared using the same procedure but without the addition of the PdCl2 solution.
All the dried powder samples were mixed with ethanol to create a homogeneous thick solution, which was dropped onto a ceramic substrate with Pt interdigital electrodes using a micropipette to form a uniform gas-sensing layer. The obtained gas sensor was aged at 50 °C for 24 h to improve thermal stability. The devices were then placed in a Nextron 100 cc microprobe station connected to a Keithley 2400 multimeter that controlled the voltage with a temperature controller. The mass flow controller maintained a constant total flow of the gas mixture (100 sccm). The gas response (Rs) was calculated using Eq. (1).
where Ra and Rg are the sensor resistances in air and the target gas, respectively.
The crystal structures of WO3 and Pd/WO3 were examined using an X-ray diffraction (XRD) (Rigaku SmartLab) analyzer with Cu Kα radiation (40 kV, 100 mA) at a scanning speed of 0.5°/min. The morphologies and microstructures of the samples were investigated using high-resolution transmission electron microscopy (HRTEM) (Tecnai G2 F20, FEI, USA).
Figure 1 shows the representative XRD patterns of pure WO3 and the doped Pd/WO3 nanocomposites. All the diffraction peaks have the same fundamental characteristics as WO3 with a monoclinic structure (JCPDS No.43-1035) . The diffraction peaks at 2θ = 23.1, 23.7, and 24.2° correspond to the (002), (020), and (200) monoclinic phases of WO3 crystal planes, respectively. These peaks are quite sharp, revealing the crystalline character of the nanocomposites . No discernible diffraction peaks of Pd or PdO are observed, which may be due to the low Pd concentration . The diffraction peak intensity of the doped Pd/WO3 composite films is slightly lower than that of pure WO3, indicating successful functionalization of Pd atoms on the surface of the WO3 composite with high crystallinity. No other impurity peaks are observed under these experimental conditions, suggesting that the as-synthesized product is highly pure.
Figure 2 depicts the HRTEM images of the 1.0 wt% Pd/WO3 nanocomposite. As shown in Fig. 2(a), WO3 particles with sizes of 50−100 nm are arbitrarily arranged, and they cluster to form a WO3 structure. Figure 2(b) depicts the selected area electron diffraction (SAED) pattern, which clearly shows that the multiple rings correspond to crystalline WO3 [10,11]. A high-magnification image of the 1.0 wt% Pd/WO3 nanocomposite shows the presence of Pd functionalized on the surface of WO3. The HRTEM images [Figs. 2(c) and 2(d)] show lattice fringes, indicating the high crystallinity of Pd/WO3. The lattice fringe spacing values are 0.364 and 0.238 nm, which match the d-spacing values of the monoclinic WO3 (020) and Pd (111) facets, respectively [5,11].
X-ray photoelectron spectroscopy (XPS) was used to determine the surface compositions and chemical states of the as-synthesized pure WO3 and doped Pd/WO3 nanocomposites, as shown in Fig. 3. The presence of W, O, and Pd was confirmed by the full-range XPS spectra of all nanocomposites [Fig. 3(a)], which show highly pure nanocomposites. This is supported by the XRD data (Fig. 1). The deconvoluted W 4f spectrum [Fig. 3(b)] consists of peaks at 34.96 and 37.10 eV for pure WO3, which correspond to W 4f7/2 and W 4f5/2, respectively (pink solid line), and indicate the existence of W6+ in the WO3 crystal lattice . The addition of Pd to the WO3 nanocomposite shifts the W 4f peaks to higher binding energy sites (cyan dotted line). The W 4f7/2 and W 4f5/2 peaks in all the Pd/WO3 nanocomposites are located at approximately 35.14 and 37.27 eV, respectively . The shift of the W 4f peak to a higher binding energy confirms that Pd is doped into the WO3 lattice. Figure 3(c) shows the deconvoluted Pd 3d core spectrum of the 1.0 wt% Pd/WO3 nanocomposite. The peaks at 336.67 and 342.30 eV correspond to Pd 3d5/2 and Pd 3d3/2, respectively [8,9]. These peaks correspond to metallic Pd (Pd0). The small peak at 337.69 eV, corresponds to the Pd2+ state, which is due to the partial oxidation of Pd during calcination. Figure 3(d) shows the deconvolution of the O 1s peak of the 1.0 wt% Pd/WO3 nanocomposite into two peaks: one corresponding to the lattice oxygen (OL) in WO3 (530.06 eV) and the other at approximately 531.22 eV corresponding to the surface chemisorbed oxygen (OA) [10–12]. OA interacts with target gas molecules on the surfaces of metal oxides; therefore, it is essential for improving the efficiency of gas sensing. The area percentages of OL and OA for all samples are shown in the inset of Fig. 3(d). As Pd increases, OA increases and reaches the maximum at 1.0 wt% Pd/WO3. Then, OA decreases as Pd increases further owing to the catalytic poisoning caused by excess Pd.
Figure 4 shows the gas-sensing analysis of the as-synthesized nanocomposites. In gas-sensing measurements, the operating temperature is a critical parameter that must be tuned for all sensing materials. All of the sensors in this study were evaluated by increasing the operating temperature from 50 to 150 °C at an interval of 25 °C. Figure 4(a) depicts the temperature-dependent response of all sensors to 100 ppm H2 for different amounts of Pd. All the nanocomposites show a volcano-shaped response. The optimal operating temperature is 100 °C for all the Pd-functionalized samples  and 125 °C for pure WO3. This proves that Pd functionalization has positive effects on the operating temperature. At low temperatures, the sensing processes are slow owing to a lack of energy. The number of free electrons increases with the temperature; this entices surface oxygen molecules and enhances sensing reactions . At temperatures higher than the optimum range, the rate of oxygen molecule desorption becomes more than that of adsorption, which reduces the sensing response of the surface . The highest response value is 12.25 for the 1.0 wt% Pd/WO3 nanocomposite at 100 °C. Figure 4(b) shows the dynamic transient curve at 100 °C for different concentrations of H2 (5−100 ppm). As WO3 is an n-type semiconductor gas sensor, the 1.0 wt% Pd/WO3 sensor resistance decreases and increases when it is exposed to H2 (reducing gas) and air, respectively. Figure 4(c) shows the corresponding calibration curves, which depict a linear relationship for all concentrations. The linear relationship curve is extremely useful for calibrating and determining hydrogen concentration in real-world applications .
The times required for the total resistance to change by 90 % after exposure to H2 and air are defined as the response time (T
Pd/WO3 nanocomposites with different amounts of Pd (wt%) were successfully synthesized using a high-temperature solvothermal method, and their H2-sensing properties were measured. Compared to pure WO3, the 1.0 wt% Pd/WO3 sensor exhibited a fairly high response of 12.25 to 100 ppm hydrogen at an optimal operating temperature of 100 °C. Furthermore, it exhibited satisfactory selectivity for H2 compared to NH3 and NO2. The sensor showed good stability, which demonstrated its potential for practical applications. The improvement in the H2-sensing property was attributed to the successful functionalization of Pd on the surface of WO3, which effectively catalyzed the spillover phenomenon of Pd nanoparticles, and to the porous structure of Pd/WO3, which provided active sites. The results of this study show that Pd/WO3 nanocomposites exhibit improved hydrogen-sensing properties, and they can potentially be used for detecting hydrogen.
This work was financially supported by the Korea Institute of Science and Technology (KIST) institutional program.
The authors declare no conflicts of interest.