First, samarium oxide, ferric nitrate, palladium chloride, DMF (99.5%), PVP (Mw = 1,300,000), C₂H₅OH (99.7%), and HNO3 in a stoichiometric ratio were weighed and mixed in deionized water (Figure 1A). The mixed solution was heated in the water bath at 60°C with magnetic stirring until it became transparent, in order to obtain the electrospinning precursor solution (Figure 1B). Then, the as-prepared precursor solution was transferred into a 10 mL syringe. The voltage was maintained at 12 kV during the spinning process, the distance between needle tip and the collector was about 20 cm, and the injection rate of the syringe was 0.4 mL/h (Figure 1C). The obtained nanofibers were sintered at 800°C for 6 h in a muffle furnace (Figure 1D). Finally, the X (0, 1, 3, 5) wt% Au–PrFeO₃ powder was obtained.

The Au–PrFeO₃ powder was mixed with deionized water to make a paste, which was then placed on gas-sensing film (Figure 1E). The as-prepared plane electrode plate was aged on an aging platform for 48 h, in order to dry it out. At this time, a qualified sensor was ready. The front side of the electrode plate has two electrodes, which are used to detect the resistance value of the gas-sensing material. On the back of electrode plate is a heating plate, which enables the gas-sensing material to reach a higher operating temperature.

The gas sensor structure diagram is shown in Figure 1E. The Au–PrFeO₃ is coated onto the sensing film. The V C is the supply voltage, which was kept constant at 5 V. The R g is calculated by the following formula: R g =   U I = V l / R l   (1)

The gas-sensing response, S, is defined as R g / R a , where R a is the resistance of the sensor in air and R g is that when in the tested gas. The response time is defined as the time taken to attain 90% of the maximum value in ascending phase, while the recovery time is the time taken to regain 10% of the base value in the descent phase. For the experimental environment, the RH was 20% and the temperature was 20°C.

The whole experiment was carried out in a closed glass chamber, into which H₂S gas was injected with a microinjector. The injection amount of H₂S liquid was calculated as follows (Deng et al., 2013): V l i q u i d = V s C g a s M 22.4 ρ d (2) where V l i q u i d is the volume of the injected H₂S liquid; V s is the volume of the test chamber; C g a s (ppm) is the concentration of the target gas; and M , ρ , and d are the molecular weight, density, and purity of the injected liquid, respectively. When a gas with PPM concentration is obtained, the gas with PPB concentration can be obtained by diluting it ten times using a microinjector.

Figure 2A shows the X-ray diffraction analysis (XRD; Bruker D8 ADVANCE with the CuKα amount of 1.5405 Å at 40 kV and 40 mA) results of X (0, 1, 3, 5) wt% Au–PrFeO₃. Compared with the standard card (PDF card: 37-1493), it shows a single-phase. The average particle size can be calculated using the Scherrer method. The Scherrer equation is as follows: D =   k λ β c o s θ (3) where λ is the X-ray wavelength, β is the integral width of diffraction peaks, and θ is the Bragg diffraction angle. The average particle size of 3 wt% Au–PrFeO₃ is about 73.8 nm. Due to the low Au doping amount, its characteristic peak could not be reflected in the XRD pattern; therefore, X-ray Photoelectron Spectroscopy (XPS; Thermo Scientific™ K-Alpha™⁺ spectrometer equipped with a monochromatic Al Kα X-ray source at 1486.6 eV operating at 100 W) was performed on 3 wt% Au–PrFeO₃ to confirm the presence of Au. As can be seen from Figure 2B, the Au element was doped in the material. Figures 2C–F show the fine spectra obtained by XPS analysis for each element. In Figure 2C, the peaks located at about 84.0 and 88.3 eV can be assigned to Au 4f_7/2 and Au 4f_5/2; in Figure 2D, the peaks located at about 932.2 and 953.1 eV can be assigned to Pr 3d5 and 3d3; in Figure 2E, the peaks located at about 708.6 and 724.1 eV can be assigned to 2p_3/2 and 2p_1/2 of Fe³⁺; and, in Figure 2F, the peaks located at about 528.6 and 530.8 eV can be assigned to lattice O1s and adsorbed O1s.

Figures 3A,B shows the Scanning Electron Microscope (SEM; Japan HITACHI SU8010, 8.0 kV) images of 3 wt% Au–PrFeO₃ under different magnifications (PrFeO₃ was synthesized by a sol-gel method and sintered at 800°C; Supplementary Figure S1). The pure PrFeO₃ presented a common perovskite structure, while 3 wt% Au–PrFeO₃ presented a nanotube-like microstructure. In the material preparation stage, after sintering, the surface of the material becomes rough and the nanotubes become hollow as the PVP decomposes at high temperature.

In order to understand which microstructure provides more favorable properties to the gas-sensing material, it is necessary to figure out which structure has higher specific surface area and porosity. The specific surface area and porosity of the 3 wt% Au–PrFeO₃ hollow nanofibers were further analyzed by nitrogen adsorption–desorption analysis. Figure 3C shows the BET curves for 3 wt% Au–PrFeO₃ and the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution (inset). The specific surface area of 3 wt% Au–PrFeO₃ was 23.67 m²/g and the average pore size was 10.2 nm. The specific surface areas of PrFeO₃ with different amounts of Au element doping are shown in Figure 3D. It can be seen that, when the doping amount of Au element was 3 wt%, the composite powder presented the largest specific surface area. This occurred as Au doping can inhibit the growth of MOS grains (the smaller the grain size, the larger the specific surface area); however, when the Au doping amount is too high, the particles will appear in a small range of agglomeration, and the specific surface area of the material will decreased. Considering the sensing properties of materials, the specific surface area is an important factor. A high specific surface area can provide more adsorption sites, which can enhance the reactions between the sensing material and gas molecules, leading to a high response to the test gas.

Figure 4A and Supplementary Figure S2 show the response curves of PrFeO₃ with different amount of Au doping to 1 ppm H₂S at various operating temperatures. For all samples, the highest responses were obtained at 120°C. The highest responses to 1 ppm H₂S were 6.93 (0 wt% Au), 38.16 (1 wt% Au), 72.86 (3 wt% Au), and 56.29 (5 wt% Au). It can be seen that the response was more than 10 times higher when using the best Au-doped sample, compared with the pure sample. Moreover, Table 1 shows the H₂S sensing properties of some typical gas-sensing materials for reference. By comparison, 3 wt% Au–PrFeO₃ exhibited an extremely high response value while ensuring a short response–recovery time.

The relationship between the material’s sensitivity and the gas concentration is very important, and a good fitting relationship can be used to predict the response value at a given gas concentration. Figure 4B and Supplementary Figure S3 show the relationship between the response of Au–PrFeO₃ and multiple H₂S concentrations. It can be seen that, for both undoped and Au–doped PrFeO₃, the response had a good linear relationship with the gas concentration, with all R 2 values greater than 96%. Additionally, the response values of Au–doped PrFeO₃ to H₂S are given in Table 2. It can be seen that the detection limit of pure PrFeO₃ was 50 ppb; meanwhile, after Au doping, the Au–PrFeO₃ could detect a much lower concentration (10 ppb) of H₂S.

Repeatability is another important property that determines whether a gas-sensing material is excellent or not. For Au–PrFeO₃, the repeatability of responses to different concentrations of H₂S gas are shown in Figures 5A–C and Supplementary Figure S4. The repeated processes were carried out as follows: when the resistance value of the gas-sensing material had stabilized, the H₂S gas was injected into the reaction chamber, and the resistance of the material increased immediately. After a period of time, the resistance stabilized, following which the H₂S gas is removed and the resistance of the material decreased immediately, restoring it to the initial state. It can be seen that, for H₂S gas at different concentrations, the resistance of the gas-sensing material could be restored to the initial value every time after the H₂S gas was removed, indicating that the material has excellent repeatability. Additionally, the response of all samples changed upon exposure to 1 ppm H₂S gas, as shown in Figures 5D–F. It can be seen that the gas response of samples had no obvious change after a 3-cycle response–recovery test, indicating the high operating stability of the designed Au–PrFeO₃ sensor. Additionally, the gas-sensing reproducibility of Au-PrFeO₃ is about 38.16 ± 4% (1 wt%), 72.86 ± 2% (3 wt%), 56.29 ± 3.6% (5 wt%).

The response–recovery time of all samples differed at different operating temperatures, indicating that the operating temperature affects the chemical reaction on the material’s surface. The response–recovery times of all samples are shown in Supplementary Table S1, Figures 5G–I, and Supplementary Figure S5. It can be seen that the response–recovery time increased with the operating temperature up to 120°C; then, after 120°C, the response–recovery time decreased with any further increase in the operating temperature. This may be due to the fact that, before the optimum operating temperature, the adsorption rate of gas molecules is higher than the desorption rate, and the number of oxygen ions and H₂S gas molecules adsorbed on the material’s surface are increased, leading to an increased reaction time. With an increase in the operating temperature, the adsorption and desorption rates are balanced at the optimum operating temperature, and the number of H₂S gas molecules and adsorbed oxygen ions on the material’s surface reach a maximum. At this operating temperature, the reaction time also reaches its maximum. With a further increase in operating temperature, the desorption rate of gas molecules is higher than the adsorption rate, the reaction reactants become less, and the reaction time is shortened. In addition, Au doping can increase the surface activity of the material and improve the reaction rate; therefore, the response–recovery time of Au–PrFeO₃ was shorter than that of pure PrFeO₃.

In practical application, it is very common to detect a certain gas in a mixture, such as H₂S gas in an individual’s exhaled breath. Therefore, the selectivity of a gas-sensing material to a certain gas determines its practical application value. The selectivity comparison of Au–PrFeO₃ to 1 ppm H₂S and several other common gases in a person’s exhaled breath is shown in Figures 6A–C and Supplementary Figure S6. It can be seen that, compared with other gases, Au–PrFeO₃ presented high selectivity for H₂S gas. In particular, for N₂, O₂, NO, CO₂, CO, and other common gases present in exhaled breath, the response was negligible, such that the H₂S in the exhaled breath can be detected more accurately.

The relative humidity (RH) in the environment is also a factor that cannot be ignored in the application of gas sensors. Figures 7A and Supplementary Figure S7 show the responses of Au–PrFeO₃ to 1 ppm H₂S with varying RH. It can be seen that the response decreased with increasing RH: before 50% RH, the response was little affected by it; however, above 50% RH, the responses decreased sharply. This means that the gas sensor in this study can be used in a low-RH environment without considering the influence of RH. This will greatly expand its practical application field. Figures 7B and Supplementary Figure S8 show the resistance change of Au–PrFeO3 with RH. For Au–PrFeO₃, the resistance decreased with RH, but the proportion of decrease differed. In the 20–90% RH range, the proportion of decreases were 53.21% (0 wt% Au), 48.6% (1 wt% Au), 41.8% (3 wt% Au), and 47.09% (5 wt% Au). Thus, the resistance of 3 wt% Au–PrFeO₃ presented the highest RH adaptability.

Long-term stability is another important property for gas-sensing materials. The higher the long-term stability, the longer the replacement cycle of the gas-sensing material and, so, the more economic and energy advantages it has. Figures 7C–F show the long-term stability of Au–PrFeO₃ under different RH over 30 days. The experimental data were obtained every 2 days. It can be seen that all of the responses decreased slightly with time, but the proportion of decrease was lowest when the sensor was kept at under 20% RH. The proportions of decrease when the sensor was kept at under 20% RH were 34.9% (0 wt% Au), 13.3% (1 wt% Au), 3.7% (3 wt% Au), and 5.7% (5 wt% Au). It can be seen that the long-term stability of 3 wt% Au–PrFeO₃ was more than 9 times that of pure PrFeO₃. Therefore,Au-doped PrFeO₃ demonstrated advantages, in terms of long-term stability. Other types of sensors, such as MOX (Pashami et al., 2012; Li et al., 2021) and MWCNT (Pistone et al., 2013; Barthwal and Singh, 2020) have been shown to have good stability under high RH environments. However, MOS, MOX, and MWCNT gas sensors are affected by RH in practical applications; therefore, improving their RH adaptability is a keyway to broaden their application field.