3.1. Structural and morphological characteristics
The crystal structure and phase purity of the WMSs with different Pt concentrations were investigated by XRD technology, and the results are illustrated in Fig. 1. As seen in this figure, the pure and Pt-doped WMSs have obvious characteristic peaks at the diffraction angle between 10o and 70o, which are consistent with those of the standard data file (JCPDS No. 35-1001), indicating that all the as-synthesized products are WO3·0.33H2O with hexagonal crystal structure. There are no other diffraction peaks related to impurity components in the XRD patterns, reflecting that the as-synthesized products show high purity. It can also be observed that the main diffraction peaks of all samples are strong and narrow, demonstrating that the as-synthesized samples are in good crystallinity, and grow along (200) plane preferentially. By comparison, it is found that the diffraction peak intensities of (200) planes of all Pt-doped samples are greater than that of the pure one. The diffraction peak intensity gradually increases with the increase of Pt concentration in the range of 1−1.0 mol%, and then decreases with further increasing the concentration, indicating that Pt doping can promote its growth along the main crystal plane. In addition, the relevant diffraction peaks of Pt are not found in all XRD patterns, which maybe mainly due to the low amount of Pt in the WMSs samples.
Fig. 2 shows the SEM images of the WMSs with different Pt concentrations. It can be seen that the pure and Pt-doped WMSs with the diameter of 0.8−1.9 µm are composed of well-dispersed microshuttles, which are self-assembled from numerous nanorods. These nanorods with the diameter of 17−62 nm are closely packed in the same direction, and the length of the nanorods is gradually shortened from the middle position to the outer position, eventually resulting in the formation of the microshuttles. Moreover, with increasing Pt concentration, the surface of the obtained microshuttles becomes smoother, indicating that Pt doping may promote the growth of nanorods in a more dense direction to a certain extent.
The microstructure, crystallinity and elemental distribution of 1.0 mol% Pt-doped WMSs were further measured by TEM and EDS technologies, and the corresponding results are illustrated in Fig.3. Figs. 3(a-c) are the low-resolution TEM images of this sample, which further prove that the as-synthesized sample with good dispersion is composed of WMSs self-assembled by nanorods. The nanorods are closely packed in the same direction, and the nanorods existed at the middle of the microshutters are obviously longer, which is consistent with the observation results of SEM images. Such kind of coarse structure can effectively increase the specific surface area of the sensing material, which will be beneficial to the chemisorption of gaseous molecules as well as the subsequent reactions with the material surfaces. Fig. 3(d) is the high-resolution TEM image of the WMSs. Some very clear lattice fringes can be found, indicating a high crystallinity of the sample. Among them, the lattice spacing of 0.365 nm can match well with the (110) crystal plane of hexagonal WO3·0.33H2O, and the lattice spacing of 0.227 nm can match well with the (020) crystal plane of PtO2. Fig. 3(e) illustrates the selected area electron diffraction (SAED) image of the WMSs. As observed in this figure, there are many neat and regular diffraction spots in the entire field of view, indicating that the microshutters with excellent crystallization are composed of single-crystal nanoparticles. Figs. 3(f-i) are the distribution images of main elements of Pt-doped WMSs. As observed in these figures, the three elements of W, O and Pt are the main elements of the WMSs, and each element is uniformly distributed in the microshutters, indicating that Pt element is successfully doped into the microshutters by the in-situ hydrothermal method in the form of its oxidate.
The chemical valence state and elemental compositions of the 1.0 mol% Pt-doped WMSs characterized by XPS technology are presented in Fig. 4. The C 1s peak (284.8 eV) was regarded as a reference to calibrate the binding energies of the Pt-doped sample. The full scanning XPS spectrum of this sample shown in Fig. 4(a) indicates that the surface elements are comprised of primarily C and some W and O. As observed in Fig. 4(b), the peak located at 531.05 eV corresponds to O 1s, confirming the presence of lattice oxygen (O2─), which can be bonded with W . As seen in Fig. 4(c), the W 4f spectra of the WMSs can be deconvoluted into two strong peaks of W 4f5/2 (37.43 eV) and W 4f7/2 (35.33 eV), demonstrating that the tungsten in WO3·0.33H2O crystal mainly exists in the form of W6+ state . In addition, a single peak at 40.98 eV can be observed, which originates from a weak emission of W 5P3/2 . Fig. 4(d) shows the XPS spectrum of Pt 4f. There is an obvious absorption peak located at 79.46 eV, and its binding energy is corresponding to Pt 4f, indicating the presence of PtO2 in the Pt-doped WMSs .
The main functional groups and chemical bonds of pure and Pt-doped WMSs were investigated by FTIR technology, and the results are shown in Fig. 5. It is found that the four samples have similar FTIR spectrum, namely that the positions and intensities of the characteristic peaks are basically same. For each sample, two characteristic peaks appear at 713 and 881 cm-1, both of which are ascribed to the stretching vibration of the bridging oxygen (W-O-W) . A weak peak can be found at 1389 cm-1, which can be contributed to the stretching vibration of W-OH . The bending vibration band of O-H can be found at 1619 cm-1 for the sample . The stretching vibration band of –OH appears at 3452 cm-1 . In addition, the characteristic peaks of other impurities are not found, which further indicates that the four samples have higher purity. Simultaneously, there are no characteristic peaks related to Pt in the FTIR spectra, which may be attributed to the low amount of Pt in the samples.
3.2. Gas sensing characteristics
Fig. 6 illustrates the responses of the WMSs with various Pt concentrations to 1000 ppm NH3 gas in the temperature range of 25−225 oC. As seen in Fig. 6, except for the pure one, the responses of all Pt-doped WMSs show an obvious trend of “first rising and then descending” with increasing operating temperature. This is mainly ascribed to the chemical activities of the gas sensing materials and gas molecules, and the adsorption-desorption process of the gaseous molecules on the nanomaterial surfaces is greatly affected by the operating temperature [38-43]. The responses of all Pt-doped WMSs increase as the temperature increases in the range of 25−175 oC, and then decrease as the temperature value exceeds 175 oC. Therefore, the optimal operating temperature of Pt-doped WMSs towards NH3 gas is 175 oC. Compared with Pt-doped WMSs, the pure one exhibits higher response at a lower temperature ranging from 25 to 100 oC, and the response reaches the maximum value at 50 oC. However, the response stability of the pure sample is relatively poor, and there is basically no response to NH3 gas in the high temperature range. The responses of pure, 0.7, 1.0 and 1.3 mol% Pt-doped WMSs to 1000 ppm NH3 gas at optimal operating temperatures are 7.8, 21.2, 28.2 and 3.8, respectively. Obviously, although the pure WMSs can obtain the maximum response at a relatively lower temperature, the response is far less than those of 0.7 and 1.0 mol% Pt-doped ones. The enhanced response of the WMSs by Pt doping may be ascribed to the excellent catalytic activity of PtO2 nanoparticles [44, 45]. When Pt concentration is low, the catalyst dispersed on the surface of sensing material can only catalyze a part of NH3 molecular. As Pt concentration further increases, the finely dispersed PtO2 nanoparticles will trap more electrons, leading to the improvement in the N-H bond dissociation.
Fig. 7 shows the response-recovery curves of the WMSs to 1000 ppm NH3 gas at their optimal operating temperatures. As seen in Fig. 7, when the four gas sensors are placed in the NH3 gas atmosphere, the resistance values of the gas sensors drop sharply, and then trend to be stable. When the NH3 gas is released, the resistance values of all the gas sensors can be completely recovered to their initial values, demonstrating that the WMSs are n-type MOS materials. According to the analysis, the response and recovery times of pure WMSs to 1000 ppm NH3 gas at 50 oC are 45 and 74 s, respectively. The response times of 0.7, 1.0 and 1.3 mol% Pt-doped WMSs to the same concentration of NH3 gas at 175 oC are 54, 39 and 42 s, respectively, while the recovery times are 401, 300 and 182 s, respectively. The results show that a certain amount of Pt doping can not only shorten the response time of the gas sensing material, but also greatly improve the response value, which will be very beneficial to the real-time detection of NH3 gas.
Fig. 8 presents the response-recovery curves of the WMSs to various concentrations (10, 30, 50, 100, 300, 500, and 1000 ppm) of NH3 gas at optimal operating temperatures. As observed in Fig. 8, the amplitude changes of the resistance values of the four gas sensors obviously show a similar stepwise increasing trend with increasing NH3 concentration, indicating that the responses also show the same increase trends. Among them, the pure WMSs have almost no response to the low concentration of NH3 gas. In the case of in-situ Pt doping, the gas sensing materials show a greater response to low-concentration NH3 gas, indicating that Pt doping can detect lower concentration of NH3 gas. In addition, the amplitude change of the resistance value of 1.0 mol% Pt-doped WMSs for various concentrations of NH3 gas are significantly higher than those of pure and other Pt-doped ones.
Fig. 9 illustrates the responses of the WMSs with different Pt concentrations towards 10−1000 ppm NH3 gas at the optimal operating temperatures. As observed in this figure, the responses of the four sensing materials are continuously enhanced with increasing NH3 concentration. Especially, 1.0 mol% Pt-doped WMSs show higher response values to different concentrations of NH3 gas, while 1.3 mol% Pt-doped WMSs show poor response to NH3 gas, indicating that the appropriate concentration of Pt doping is helpful to improve the response of gas sensing material. The high concentration of Pt doping may inhibit the gas sensing reactions on the material surface to a certain extent, thus reducing the sensor response. As observed in this figure, 1.0 mol% Pt-doped WMSs does not reach the saturation stage for the chemisorption of NH3 molecules when the NH3 concentration is 1000 ppm, indicating that the as-prepared gas sensing material can supply more active sites for the chemisorption of gaseous molecules, and has a wider detection range for NH3 gas. The responses of 1.0 mol% Pt-doped WMSs to the above different concentrations of NH3 gas are 1.9, 3.7, 5.0, 8.9, 13.2, 16.8 and 28.2, respectively.
Fig. 10(a) shows the reproducibility of 1.0 mol% Pt-doped WMSs to 1000 ppm NH3 at 175 oC. It can be seen that after the introduction of 1000 ppm NH3 gas in five cycles, the Pt-doped WMSs exhibit approximately the same amplitude change of the resistance values, reflecting good reproducibility. In addition, when the NH3 gas is released, the Pt-doped WMSs can completely recover the initial resistance value, showing excellent detection reversibility. The long-term stability illustrated in Fig. 10(b) demonstrates that the response values of the as-synthesized sensing material to 1000 ppm NH3 gas has been fluctuating around 28 in the whole test period of 30 days, indicating that the present gas sensor has a good long-term stability.
Fig. 11 illustrates the selectivity of pure and 1.0 mol% Pt-doped WMSs towards NH3 gas in different kinds of gas atmospheres, such as methanol, NO2, acetone, methylbenzene, SO2 and methanal. As seen in Fig. 11, the response of Pt-doped WMSs towards 1000 ppm NH3 at 175 oC is as high as 28.2, while the response values are relatively low and not higher than 3 to 1000 ppm methanol, 5 ppm NO2, 100 ppm methylbenzene, 100 ppm methanal, 100 ppm acetone and 100 ppm SO2, demonstrating that Pt doping can significantly improve the response of the sensing material to NH3 gas, but it has no obviously enhanced effect on the responses of other interfering gases.
In order to further clarify the superiority of the present sensor, the gas sensing properties of the as-synthesized WMSs was compared with those of other WO3-based gas sensors, mainly including the peak response, the response/recovery time and the optimal operating temperature, and the corresponding results are shown in Table 4. It can be seen that most of NH3 gas sensors based on WO3 sensing materials have the optimal operating temperature in a higher temperature range of 250−350 oC, while the WMSs-based gas sensors prepared in this study show lower operating temperature, especially the Pt-doped WMSs gas sensors. In addition, the as-synthesized Pt-doped WMSs sensing material has lower operating temperature and higher response to the same concentration of NH3 gas compared with the Pt-decorated WO3 thin film in the literature. Although the as-prepared Pt-doped WMSs exhibit a lower response to a higher concentration of NH3 gas compared with the WO3 flower-like nanostructures, the operating temperature of the as-prepared sensing material is far lower than that of the reported sensing materials. Therefore, the as-synthesized Pt-doped WMSs show good comprehensive performance.
3.3. Gas sensing mechanism
The WMSs belong to a kind of surface-controlled gas sensing material. The gas sensing properties of the WMSs is greatly affected by the type and quantity of the chemisorbed oxygen on the material surface. The gas sensing reaction process of the WMSs mainly includes the following two stages. In the first stage, oxygen molecules in air adsorb on the surface of the WMSs at a certain operating temperature, capturing electrons from the conduction band of the sensing material, and forming a variety of chemical adsorbed oxygen ions on the material surface, such as O2─ (less than 100 oC), O─ (100−300 oC), and O2─ (more than 300 oC) [50, 51]. At this time, the depletion layer width of the WMSs increases, so the resistance value of the material increases. In the second stage, when the WMSs are placed in the NH3 atmosphere, the highly reactive oxygen anions will react with the NH3 molecules to produce smaller group molecules such as N2, NO and NO2, and release free electrons . These free electrons are transferred to the conduction band of the WMSs, and the depletion layer width of the sensing material becomes narrow, thus the resistance value of the material decreases significantly [47, 53].
On the basis of the gas sensing performance results of the as-synthesized WMSs, the appropriate concentration of Pt doping can significantly enhance the gas sensing performance to NH3 gas. The enhancement in gas sensing properties of the semiconductor materials induced by the noble metal nanoparticles is mainly ascribed to the electronic sensitization and chemical sensitization mechanism of the noble metal nanoparticles, which can play a catalytic role in the electron transport and transfer on the material surface, thereby improving the gas sensing properties to the reducing NH3 gas . In the process of electron sensitization, noble metal oxide nanoparticles can act as effective electron acceptors to capture free electrons from the surface of the WMSs and form electron depletion layer at the contact interface of the two materials. When the reducing NH3 molecules contact with the noble metal nanoparticles, the noble metal oxides are reduced and the extra electrons are released and returned to the material surface, which is shown as the enhancement in the response. The chemical sensitization process is mainly due to the catalytic oxidation of noble metal nanoparticles on the material surface. The noble metal nanoparticles can supply active adsorption and reaction sites for NH3 molecules on the WMSs surface. On the one hand, the introduction of noble metal element will accelerate the formation of chemisorbed oxygen on the material surface. On the other hand, it will make it easier for electrons to transfer from the active adsorption sites to the surfaces of the sensing materials, and react with the oxygen anion on the surfaces of the sensing materials, so as to enhance the gas sensing performance to NH3 gas.