Low-Temperature H2S Detection with Hierarchical Cr-Doped WO3

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Low Temperature HS Detection with Hierarchical Cr-doped WO Microspheres Yanrong Wang, Bin Liu, Songhua Xiao, Xinghui Wang, Leimeng Sun, Han Li, Wuyuan Xie, Qiuhong Li, Qing Zhang, and Taihong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12857 • Publication Date (Web): 23 Mar 2016 Downloaded from http://pubs.acs.org on March 31, 2016

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ACS Applied Materials & Interfaces

Low Temperature H2S Detection with Hierarchical Cr-doped WO3 Microspheres Yanrong Wang, a, b Bin Liu, a Songhua Xiao, a Xinghui Wang, b Leimeng Sun, b Han Li, a Wuyuan Xie, a Qiuhong Li, a Qing Zhang,*b Taihong Wang *a a. Department of Physics/Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China. b. NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore KEYWORDS WO3, Cr dopant, hierarchical, H2S gas sensors, low temperature

ABSTRACT Hierarchical Cr-doped WO3 microspheres have been successfully synthesized for efficient sensing of H2S gas at low temperatures. The hierarchical structures provide an effective gas diffusion path via well-aligned micro-, meso- and macroporous architectures, resulting in significant enhancement in sensing response to H2S. The temperature and gas concentration dependence on the sensing properties elucidate that Cr dopants remarkably improve the response and lower the sensor’ operating temperature down to 80 ̊C. Under 0.1 vol% H2S, the response of Cr-doped WO3 sensor is 6 times larger than pristine WO3 sensor at 80 ̊C. We suggest the increasing number of oxygen vacancies created by Cr dopants to be the underlying reason for enhancement of charge carrier density and accelerated reactions with H2S.

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1 INTRODUCTION Nowadays, atmospheric environmental monitoring requires advanced sensing techniques to detect major air polluters. Hydrogen sulfide is a malodorous toxic, inflammable and corrosive gas, which is widely utilized in industry, such as manufacturing of paper products, coal mining and natural gas exploitation, etc. Therefore, highly sensitive, reliable and rapid H2S sensors are in great demand. Gas sensors based on metal oxide semiconductors (MOSs) present interesting advantages in their simple implementation, low cost and good reliability for real-time monitoring. Currently, H2S gas can be monitored using MOSs sensors, such as tin oxide,1,2 tungsten oxide,3 zinc oxide,4,5 stannic oxide,6,7 cupric oxide,8 α-Fe2O3,9 β-AgVO3,10 etc. However, most of the sensors make use of adsorption and desorption of the gas or chemical reactions at the surfaces of MOSs, so that they can only function at elevated operating temperatures. This leads to high power consumption and poor durability. Many efforts have been put to reduce the operating temperature, including doping MOSs with suitable catalytic nanomaterials,11,12 making use of MEMS technology3, surface modification13 etc. It has been noted that dopants could modify the electronic structure of MOSs by forming impurity levels in the bandgaps. Thus, doping is the most commonly employed technologies to enhance the responses of MOSs sensors to specific gases. Chromium, as a member of transition metals, exhibits a wide range of possible oxidation states. And it is a promising candidate for efficient doping materials in sensor applications. There have been some reports showing the performance of Cr dopant in ammonia sensors14-16, oxygen sensors17, NO2 sensors18,19 , acetone sensors20,21 humidity sensor 22 and so on.


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Herein we report a facile one-step hydrothermal method to synthesize Cr-doped WO3 microspheres, whose hollow nanostructures provide a large pore size for chemical reactions, effective diffusion of chemical species into the interface/surface. All these advantages of hierarchical and hollow structure would greatly enhance the sensing performance of the gas sensors.23-27 At 80 °C, we have achieved a significant improvement in sensing performance towards hydrogen sulfide gases with a low cross-selectivity and short response time. The enhanced sensing response can be attributed to the increases in surface area, pore size, and oxygen vacancies in WO3 microspheres by Cr doping. 2 EXPERIMENTAL PROCEDURES 2.1 Materials All chemicals used in this work were purchased from Sinoplarm Chemical Reagent CO.LTD and used without further purification. Deionized water was used throughout the experiments. 2.2 Synthesis of materials WO3 microspheres were synthesized using a facial in-situ hydrothermal method followed by calcination.28 In a typical synthesis procedure, 1g of WCl6 was added into 50mL of absolute ethanol and the mixture was then stirred to form a homogeneous solution. Afterwards, the prepared clear yellow solution was transferred to a Teflon-lion stainless steel autoclave which was sealed and kept in an oven at 180 ̊C for 24 h. After naturally cooling down to room temperature, the resulting solid products were centrifuged, rinsed with distilled water and ethanol several times and finally dried in air at 60 ̊C.



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Cr-doped WO3 microspheres were also synthesized through an in situ redox reaction between weakly reductive WO2.72 and oxidative metal salts in aqueous solution. Typically, 1g WCl6 and 20 mg of Cr(acac)2 were added in 50 mL of ethanol and after 10 min stirring, the homogenous mixture solution was then transferred into a Teflon-lion autoclave which was sealed and kept at 180 ̊C for 24 h. After cooling down to room temperature, the solid products were collected using the same process described above and dried at 60 ̊C. Finally, the as-prepared dry product was annealed in a Muffle furnace at 500 ̊C for 4h for the following measurements. 2.3 Fabrication and measurement of gas sensors In a typical experiment, a small amount of the sensing material powder was mixed with terpineol to form a pastes and then coated uniformly onto the outside surface of a ceramic tube. The schematic structure of the sensors is shown in Figure 1a. A small Ni-Cr alloy coil was placed through the tube as a heater.29 Detection temperature was tuned from room temperature up to 500 ̊C through the heater. The ambient humidity was about 50% RH and the response of sensor was measured by monitoring the relative variation of DC conductance, Rair/Rgas, with a ZhongKe NS-4003 smart sensor analyzer. The response time is defined as the period in which the sensor output change reached 90% of the stabilized value, similarly, the recovery time is defined as the time required for the sensor to reach 10% of the initial steady-state value after the gas was removed. The interdigital electrodes used for I-V measurement, as shown in Figure 1b, were cleaned and dried at 60 ̊C. The sensing materials were suspended in 5 mL ethanol by intense sonication, and then drop-casted 3µL suspension on the pre-cleaned electrodes. The two-probe measurements were conducted for the samples under controlled environments in a metal shielded chamber using a semiconductor parameter analyzer.


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2.4 Characterization of materials Scanning electron microscopy (SEM) images were recorded using a LEO 1550 Gemini SEM. The transmission electron microscopy images (TEM) and energy dispersive spectra (EDS) were obtained using JEM-2010. X-ray powder diffraction data of the as-synthesized materials were collected using Cu-Kα radiation at 40kV and 30mA with the Philips X’Pert pro diffractometer. The Raman spectra were performed using the WITEC CRM200 Raman system. A semiconductor parameter analyser, (Agilent 4156C) was employed to measure the current with a measurement resolution of 1 fA. 3 RESULTS The overall morphology of pristine WO3 microspheres is shown in Figure 2a, indicating that the sample was composed of a large number of microspheres with good mono-dispersity, and all microspheres were roughly uniform with an average diameter of 1.5µm. From Figure 2b, the enlarged view of hierarchical microsphere surface shows clearly a porous surface with an average pore size of 23.8nm (see in Figure S1a in Supporting Information). The broken sites and exposed hollow interiors of the microspheres provide a direct evidence of hollow structure of pristine WO3 microsphere (Figure S2a). Figure 2c presents the low magnification SEM images of the annealed Cr-doped WO3 samples, which displayed a similar spherical morphology to pristine WO3 samples. After Cr-doping, a 15% surface area increases and the average adsorption pore size increases from 23.8nm to 46.9nm. (Figure S1b). In addition, SEM observations also reveal that hollow Cr-doped WO3 microspheres were assembled in a more complex manner (Figure 2d and S2b). These findings corresponding to the related publications30-35, which confirm



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that Cr-doped WO3 hierarchical surface and mesoporous morphology well promote the diffusion and transport of gases into sensing layer, thus obtaining better sensor performance. The hierarchical hollow structure of Cr-doped WO3 microspheres was further investigated using TEM, as shown in Figure 3a and 3b. The rough surface of microspheres can be clearly observed, and the hollow interior can be confirmed by the contrast between dark edges and the pale center, as shown in Figure 3b. From Figure 3c, the hierarchical structure surface were assembled by nanoplates with irregular sizes around 25-80nm. A representative high-resolution TEM image of a portion of Cr-doped WO3 samples is shown in Figure 3d. The image inset of Figure 3d shows that the lattice fringes of samples had a spacing of 0.232nm and 0.261nm, respectively, reasonably matching with the interplanar distance of (110) plane of monoclinic WO3 and (220) plane of Cr2O3. The STEM–energy-dispersive X-ray (EDX) spectroscopy, as shown in Figure 3e, indicated that only three elements, O, W and Cr, were found, in addition to Cu element which was from the Cu foil substrate. 2.8 at. % Cr doped WO3 is used for our whole measurement. From the related publication14,16,36, it is appreciable that different doping amount of Cr will greatly influence growth kinetics and surface energy of crystalline planes, thereby altering the morphology. We know that morphology has a great effect on the gas sensing performance, due to co-effects of exposed crystal facets, activation energy, catalyst effect and particle size on the surface reaction and electron transport units. So it has complex factors to make a comparison with pristine WO3 sensor and different doping amount samples to H2S detection. We will take this vertical comparison as our next research direction to make a full and deep analysis. Figure 4a shows the X-ray diffraction pattern of Cr-doped WO3 microspheres. It can be concluded that the crystal structure and morphology were barely changed after doping chromium element. All peaks of monoclinic WO3 and the highest peaks of Cr2O3 are preserved and no


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diffraction peaks from any other impurities were detected. Figure 4b shows the measured resonant Raman scattering spectra of pristine WO3 and Cr-doped WO3 samples. The significant peaks centered at around 85, 135, 273, 324, 715, 805 cm-1 can be attributed to the monoclinic γ phase, which is the stable form of WO3 at room temperature.37 Cr-doped WO3 samples presented a Raman vibration centered at 992 cm−1, which can be indexed as the stretching mode of Cr═O terminal bond of dehydrated monochromates, revealing the existence of chromium.38 The rest peaks were caused by usual WO3 Raman vibration modes. The surface/near-surface chemical composition of pristine WO3 and Cr-doped WO3 were examined by X-ray photoelectron spectroscopy (XPS). The full-range XPS spectrum of Crdoped WO3 reveals that W, Cr and O are main elements in the sample with a lower level of carbon contamination (Figure 5a). More detailed information on chemical state of these elements can be obtained from the high-resolution XPS spectra of Cr 2p, O 1s and W 4f, as shown in Figure 5b-5d. The peaks of W4f7/2 at 35.8 eV and W4f5/2 at 37.9eV can be assigned to the existence of WO3 chemical state at the surface in pristine WO3 sample.39 The shift of these peaks towards lower binding energy in Cr-doped WO3 sample can be attributed to photoelectrons emitted from lower oxidation states of tungsten (sub-stoichiometric WO3-x).40 Various chromium oxide can be distinguished from each other by XPS. The binding energy of Cr 2p3/2 is 577.8 eV and Cr 2p3/2-Cr 2p1/2 binding energy separation is 9.6 eV (Figure 5b). Since both of them are consistent with Cr2O3, the oxidation state of dopant can be assigned to Cr3+.

41

The O 1s peak

showed at 530.2 eV in pristine WO3 and a slight shift to lower binding energy with Cr-doped WO3 sample (Figure 5c), suggesting the increased defective or incomplete W-O binding via Cr addition.42



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In order to measure its electrical performance to background atmosphere, we performed a twoprobe measurement with a pair of IDT electrodes (Figure 1b). Figure S3a shows I-V curve between -2.5V and 2.5V at different pressure of air and nitrogen. The distinction of different air pressure is ambient humidity and atmospheric oxygen content, then we exclude oxygen influence, under different nitrogen pressure, ambient humidity could be the sole impact. The results indicate that electrical properties of pristine WO3 was sensitive to background gases. While for Cr-doped WO3 samples, the influence of ambient humidity can be ignored but atmospheric oxygen has a slight influence in conductance. Figure 6a and 6b summarize the responses of pristine WO3 and Cr-doped WO3 sensors to 0.1vol% H2S at different temperatures, respectively. It is clearly seen that the responses exhibited an ‘increase-maximum-decrease’ characteristics. The higher reaction activity and the conversion of surface absorbed oxygen species, the higher response to H2S was obtained. The highest response to 0.1vol% H2S was 21.58 and 153.55 for pristine WO3 sensor and Cr-doped WO3 sensor, respectively. The optimal detection temperature was 360 ̊C for the former and 80 ̊C for the latter. Above the optimal temperature, both sensor showed a low utilization rate due to low adsorption ability to H2S molecules. Typical response curve towards 0.05 vol% H2S at 80 ̊C was shown in Figure 7a, the conductance of Cr-doped WO3 sensor increased rapidly in H2S atmosphere. In comparison, the recovery process was a bit slow. Various H2S concentrations at 250 ̊C were also measured, as shown in Figure 7b. As expected, the response has a positive relationship with gas concentration, and fast response at high temperature was observed, due to faster surface chemical reactions and higher charge carrier mobility. Furthermore, we demonstrated the response curves to different H2S concentration of Cr-doped WO3 sensor at 80 ̊C and 250 ̊C, as shown in Figure 8b, the response to all concentrations of H2S at 80 ̊C are


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higher than those at 250 ̊C The measured response to 0.1vol% H2S was around 153.2 at 80 ̊C and 48.7 at 250 ̊C, suggesting that Cr-doped WO3 was of a significant improvement in sensing performance at low operating temperature. As shown in Figure 8a, the response and recovery time of Cr doped WO3 sensor decreased sharply with increasing temperature. At 80 ̊C with 0.1vol % H2S, the response time and recovery time of the sensor were 336s and 300s, respectively, while they reduced to 14s and 28s at 250 ̊C. The fast response and recovery processes at higher temperatures could attribute to fast surfacechemical reactions, high charge carrier mobility or enhanced gas-phase diffusion. Furthermore, the ionic form of adsorbed oxygen that may change from O2− to O2– at such a high temperature [O2-+ 3e-→ 2O2− (ads)] 29. The binding energy of O2− is expected to be greater than that of O2–; thus, desorption of the former is more difficult, leading to longer response time. Additionally, at higher temperatures, the direct decomposition of H2S on WO3 surface can be accelerated. Considering all factors mentioned here, 80 ̊C was selected as the operation temperature because it has relativity high response and acceptable response and recovery time, moreover, low operating temperature is essential for flammable and explosive gas detection and also corresponds to low power consumption.. To further investigate the influences of background gases on sensing performance, we conducted the measurement at different humidity and found there was no obvious change in resistance under different humidities (Figure S4). The reason could be attributed that under high temperatures, few layers of water molecules cover the material due to desorption of water from the surface.43 Therefore, we focused on the influence of atmospheric oxygen, the experiment was performed under different O2 concentration with certain concentration of H2S gas (0.1vol %), the detailed procedures and results are explained in Supporting Information. From Figure S5, low



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concentration of additional O2 led to a higher response (red line in Figure S5 refers to the response of 0.1 vol% H2S with no additional oxygen introduced into chamber), and eventually reached to a saturated value. As O2 concentration increased beyond 1vol% O2, the sensing response dropped apparently. Therefore, adsorbed oxygen species are not solely responsible for the surface resistance of oxide, oxygen partial pressure should have a profound effect upon the sensor resistance.44,45 A power-law dependence of resistance on oxygen concentration in ambient atmosphere has been well used: R=R0PO2β , where R0 is the resistance in pure oxygen, and β is the exponent.46-48 The ionosorption of atmospheric oxygen typically proceeds along with consumption of the conduction band electrons, while reaction with reducing H2S leads to liberation of these electrons via adsorbing oxygen.49 When O2 concentration, PO2, is low, PH2S is substantial. Cr-doped WO3 microspheres’ surface is likely directly reduced by adsorbed H2S, leading to an increase in conduction band electrons or the conductance. Upon increasing oxygen concentration to a certain value, a large change in the surface concentration of oxygen species would occur, depending upon the competition between the reaction rate of H2S with oxygen species and oxygen adsorption rate. When O2 concentration is further increased, oxidation at the surface of Cr-doped WO3 is not balanced as oxidation is a dominate process over reduction process.50,51 Therefore, an addition of Po2 could lead to a weak response. From Figure 9a, at same gas concentration of 0.1vol %, the response of Cr-doped WO3 sensor was much larger to H2S (nearly 50 times) than to other gases such as H2, C2H5OH, and CO2 at 80 ̊C. Additionally, it is obvious that Cr-doped WO3 sensor performed a better performance to all of gases than pristine WO3 sensor at 80 ̊C. This means that Cr-doped WO3 sensor processed a good selectivity to H2S and Cr dopant played an important role in enhancing the sensing performance. Besides, Cr-doped WO3 sensors have been cycled up to five times and good


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reversibility can be observed from Figure 9b. The experiment was carried out in 0.05vol% H2S at 80 ̊C and 250 ̊C, respectively. The maximum deviation was less than 15%, an acceptable range ensuring a high stability and repeatability. To understand more about H2S sensing performance, we compared our results with some reported ones based on doped metal oxides. As summarized in Table 1, it is clearly seen that the response of this sensor at 80 ̊C is apparently advantageous over other sensors. 4 DISCUSSION It is known that the sensing mechanism of oxide materials is a surface controlled process in which grain size, surface states and oxygen adsorption play important roles.56,57 At first stage, as shown in Figure 10a, it takes place in air atmosphere, O2- is dominated oxygen species at low temperature, the formation of atmospheric oxygen adsorbates on the surface of WO3 nanostructures results in a thick electron-depletion surface layer, owing to strong electron withdrawal power of oxygen adsorbates. Compared with pristine WO3, the increase in resistance of Cr-doped WO3 sample in air can be ascribed to incorporation of Cr in WO3 matrix. It was found that the effective ionic radius of Cr+3 and W+6 are 61.5 pm and 60 pm, respectively, and they are comparable. Therefore, Cr ions can penetrate into WO3 lattice as substitutional dopants without affecting the crystallography of WO3. The trivalent Cr could act as an acceptor type impurity which can be expressed as: 17 WO3 Cr2O3  → 2CrW ''' + 3OO × + 2VO •••

(1)

Where we have adopted the Kroger-Vink’s notation for the defects: Vo represents oxygen vacancies with three positive charge, CrW is Cr substitution in W sites, Oo× is an oxygen ion in its



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regular site with neutral charge. Doping with suitable cations provides the shortest route to alter the electronic and catalytic properties of Cr-doped WO3 for gas interaction at the interface. The concentration of oxygen vacancies is critical in this stage as oxygen vacancies can trap ionosorbed atmospheric oxygen. The reaction kinetics is as follows: 58, 59 1 O2 (g) + e′(CB) + VO • ↔ O×o 2

(2)

1 O2 (g) + 2e′ (CB) + VO •• ↔ O×o 2

(3)

Where Vo•• is an oxygen vacancy with double positive charge and e’ is electron in the conduction band. From Figure 5c and 5d, the shift of O 1s peaks and W 4f peaks to low binding energy in Cr-doped WO3 sample demonstrated the defective of WO3 and sub-stoichiometric WO3-x. It is appreciably to be the increasing number of oxygen vacancies, which could result in higher absorption of oxygen and enhance sensing response. In addition, the increase in oxygen vacancies could also reduce activation energy of Cr-doped WO3, facilitating the sensor to respond at low temperatures. 14 When reducing H2S is introduced, as second stage of Figure 10b, the surface chemical reaction can be molded as:

H 2 S(g) + 3OO× ⇔ SO2(g) + H 2O(g) + 3VO×

(4)

VO× ↔ VO• + e ' ( CB )

(5)

Once H2S gas molecules react with the pre-absorbed oxygen ions and release the trapped electrons back to conduction band, the conductance of Cr doped WO3 increases. In addition, at a


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high concentration of H2S, the chemical conversion through the direct reaction is known to be an important mechanism that changes the conductivity and yields a high response. For example, the chemical conversion of CuO nanoparticles to metallic CuS,60 Au nanoparticle reaction with H2S to form AuS,61 and also several of complex reaction with ZnO to fabricated ZnS nanostructures.62,63 In our experiment, the consequent reduction of W+6 to W+4 to release electrons into WO3 also contributed to the response.42,64-66 WO3 + H 2 S(g) ↔WS 2 + SO2(g) + H 2O(g)

(6)

It is this process that causes the formation of a shallow donor level and promotes a drastic increase in conductance. Since the test conditions are identical for pristine WO3 and Cr-doped WO3 microspheres, the enhanced sensing performance of Cr-doped WO3 sensor could be directly related to the following aspects. Firstly, the concentration of oxygen vacancies. Doping of Cr to WO3 could dramatically influence the defect chemistry and sintering behavior of WO3, and create more oxygen vacancies in material, leading to a decrease in free electron concentration and an increase in response. Cr can create abundant sites on surface of WO3 nanostructures to adsorb O2 and H2S, accelerating electron exchange between the sensor and H2S, leading to a high response at low temperature.52 Secondly, average adsorption pore size is increase from 23nm to 46 nm for pristine WO3 and Cr-doped WO3 microspheres, respectively. Also, an increase in the surface area by 15% due to Cr dopant, providing more efficient and rapid inlets of target gas to sensing material surface and improving sensor response. Furthermore, a strong bonding interaction exists between H2S and chromium oxide at 350K, which depends on electrostatic interactions between the dipole of molecule and the ionic field generated by the charges of oxide plus orbital mixing.67



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Sulfur atoms could be decomposed from H2S into HS or S at the certain temperature and deposited on the metal cations of Cr2O3.68,69 This may play a secondary role in the adsorption process and may be the reason for high selectivity and response to H2S in Cr-doped WO3 sensor. However, considering that the concentration of dopant Cr and analyte H2S are low, a further detailed study is necessary to substantiate this. 5 CONCLUSIONS We have developed a simple approach to synthesize Cr-doped WO3 hierarchical hollow microspheres. Compared with pristine WO3, Cr-doped WO3 sensor shows a remarkable response to H2S at low temperatures. The response of Cr-doped WO3 to 0.1 vol% H2S is 6 fold increase than that of pristine WO3 sensor at 80 ̊C. The improved sensing response at low temperatures can be attributed to the increase in surface area, pore size and oxygen vacancies in WO3 nanostructures by Cr doping. Besides, the interaction between chromium oxide and H2S may confirm the high selectivity and response to H2S. All the synergistic effects of Cr dopant and WO3 species, not only on chemical compositions but also on microstructures, could be essential to improve the low-temperature gas-sensing property of the sensing materials. Furthermore, this simple cost effective method can be used for synthesizing of other doped nanostructured semiconducting oxides for catalytic as well as gas sensing application. ASSOCIATED CONTENT Supporting Information.


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This material is available free of charge via the Internet at http://pubs.acs.org.” For instructions on what should be included in the Supporting Information, as well as how to prepare this material for publication, refer to the journal’s Instructions for Authors. AUTHOR INFORMATION Corresponding Author *Taihong Wang, *Qing Zhang E-mail: [email protected]; [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources Any funds used to support the research of the manuscript should be placed here (per journal style). Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This research is supported by the National Natural Science Foundation of China (Grant No. 61376073, 61574118), Key Project of Science and Technology Plan of Fujian Province (Grant No.2015H0038). REFERENCES



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Fig. 1 Schematic representation of (a) gas sensor element and (b) interdigital electrodes for I-V measurement.

Fig. 2 (a and c) Typical SEM images of pristine WO3 microspheres and Cr-doped WO3 microspheres at low magnification; (b and d) the enlarged view of hierarchical pristine WO3 surface and Cr-doped WO3 surface.


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Fig. 3 (a, b and c) TEM images, (d) HRTEM images and (e) EDS pattern of Cr-doped WO3 microspheres.



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Fig. 4 (a) XRD pattern of Cr-doped WO3 microspheres (reference from ICDD databases), (b) Raman scattering spectra of pristine WO3 and Cr-doped WO3 microspheres.

Fig.5 (a) Full spectrum and (b) XPS Cr 2p spectra of Cr-doped WO3 microspheres; (c) XPS W 4f and (d) O 1s spectra of pristine WO3 and Cr-doped WO3 samples.


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Fig. 6 The corresponding response of (a) pristine WO3 and (b) Cr-doped WO3 sensors under 0.1vol% H2S at different temperatures.

Fig. 7 (a) Response curve of Cr-doped WO3 sensor to 0.05 vol% H2S at 80 ̊C; (b) transient response curves of Cr-doped WO3 sensor at various H2S concentrations at 250 ̊C



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Fig. 8 (a) Response and recovery time of Cr-doped WO3 sensors to 0.1vol% H2S as a function of operating temperature, (b) response as the function of H2S concentration at 80 ̊C and 250 ̊C for Cr-doped WO3 gas sensor

Fig. 9 (a) Responses of pristine WO3 and Cr-doped WO3 sensor on exposure to 0.1 vol% of several gases at 80 ̊C; (b) Repeatability of Cr-doped WO3 sensor on successive exposure (5 cycles) to 0.05 vol% H2S at 80 ̊C and 250 ̊C.

Fig. 10 Band diagrams and schematic images of (a) oxygen inosorption surface before sensing and (b) H2S gas adsorption and surface reaction with surface oxygen


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Tables Table 1 Comparison of the gas sensing properties of several doped metal oxide sensors to H2S

Sensing material

Concentration

Rair / Rgas

Working Temperature

Ref.

Mo doped ZnO nanowires

5ppm

14

300̊ C

52

Pt doped WO3 film

1ppm

23

300̊ C

3

Au doped WO3 film

100 ppm

100

291̊ C

42

Al2O3 doped TiO2 film

600ppm

7.24

225̊ C

2

Pt doped α-Fe2O3 film

100 ppm

326

160 ̊C

53

CuO modified SnO2

50 ppm

180

160̊ C

54

La doped In2O3 nanocubes

50 ppm

7.14

125̊ C

55

100 ppm

153

80 ̊C

50 ppm

89.3

80 ̊C

this work

Cr doped WO3 microsphere



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Table of Contents/Abstract Graphic


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Low-Temperature H2S Detection with Hierarchical Cr-Doped WO3

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