- Email: [email protected]
Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing Zhongjun Han a, Jun Ren a, Junjing Zhou a, Shiyun Zhang a, Zili Zhang b, Liu Yang c, Chenbo Yin a,* a
College of Mechanical and Power Engineering, Nanjing Tech University, Nanjing, 211816, PR China College of Mechanical and Electrical Engineering, Jinling Institute of Technology, Nanjing, 211169, PR China c College of Mechanical Engineering, Nanjing Institute of Technology, Nanjing, 211167, PR China b
highlights
graphical abstract
Porous Pd-WO3 composite films were prepared for hydrogen leak detection. The optimal molar ratio of Pd: W was 1 mol% for porous Pd-WO3 composite films. The PeN heterojunction enhances H2 response characteristics. Porous structure with additional active sites enhances H2 sensing performance.
article info
abstract
Article history:
Based on sol-gel, multilayer porous Pd-WO3 composite thin films have been successfully
Received 21 September 2019
prepared by utilizing a layer-by-layer deposition strategy. The crystal structure and
Received in revised form
microstructure were analyzed by various characterization methods. The sensing perfor-
4 December 2019
mance of the prepared films at different hydrogen concentrations and the temperature was
Accepted 20 December 2019
studied. Results show that the performance of the hydrogen sensor can be improved
Available online xxx
greatly by the use of composite structure and soft template (Pluronic F127). The response performance of 1 mol% porous Pd-WO3 composite films was better than that of 2e10 mol%
Keywords:
films. The porous Pd-WO3 composite films showed a high sensitivity (~346.5 times better
Nanocomposites
than the sensitivity of pure WO3 films for 1000 ppm H2) and a fast response time (7 s) at the
Porous structure
temperature of 250 C. Porous Pd-WO3 composite films had good selectivity for hydrogen
Tungsten trioxide
and stable sensing performance. The different response and recovery behavior of samples
Palladium oxide
were contrasted and discussed to explain the effect of the special composite structure and
Sol-gel
porous structure on hydrogen sensing performance.
H2 response characteristics
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (C. Yin). https://doi.org/10.1016/j.ijhydene.2019.12.149 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
2
international journal of hydrogen energy xxx (xxxx) xxx
Introduction Hydrogen has been widely concerned as a kind of clean energy with high efficiency, high energy, reproducibility, and nonpollution [1]. However, the development and application of hydrogen are hindered by its characteristics of easy diffusion, wide combustion concentration of 4e75% and easy escape from most storage materials [2,3]. Therefore, once hydrogen is leaked during storage and transportation, accidents are very likely to happen. As a result, hydrogen sensors with excellent performances such as sensitivity, selectivity, stability, and durability are necessary to detect and monitor hydrogen leakage in various fields of hydrogen application [4]. In the field of hydrogen sensor, metal oxide semiconductors (MOS) such as ZnO [5], SnO2 [6] and Al2O3 [7] are suitable for the preparation of hydrogen sensitive materials and these gas sensors have the advantages of simple structure, low production cost and good sensing performance [8]. WO3 as an n-type MOS material has been used in the detection of various gases because of its unique chemical and physical property and excellent gas-sensing character [9]. Up to now, many effective methods have been applied to fabricate WO3 thin films successfully, such as sol-gel mothed [10,11], magnetron sputtering [12], chemical vapor deposition [13] and electron beam evaporation [14]. Among various techniques, sol-gel deposition has the advantages of low cost, ordinary equipment, low processing temperature and easy access to molecular level uniformity (or doping uniformity). So, sol-gel is an economical and effective method for preparing porous WO3 films. WO3 is generally used at temperatures between 200 and 450 C, but it can be doped and modified to improve gas sensitivity and reduce operating temperature [15]. It has been shown that the dopants can be used as catalysts or surface sites to absorb gases and optimize the sensing mechanism. Therefore, the reaction between the film and the target gas was catalyzed and the active sites of the film were improved by adding the right amount of metal additives (Pd, Ag, Pt) [16e18] or different metal oxides (CuO, SnO2, In2O3) [19e21]. It is an important way to improve the gas sensitivity of MOS. At the same time, the grain structure characteristic of WO3 also has an important influence on the H2 sensing performance. Nowadays, various types of structural WO3 such as nanowires [22], nanosheets [23], nanotubes [24], nanorod [25] and hollow sphere [26] have been studied to improve gas sensitivity. The porous crystal with high specific surface areas is more sensitive than the dense grain structure film. And the high surface volume ratio of the porous crystal makes the gas diffusion easy. Therefore, gas sensing performance of MOS was improved by adding surfactants such as F127 [11] and P123 [27] or using hard templates such as SBA-15 [28] and KIT-6 [29] to form the porous structure. In our previous work, the reaction characteristics of SnO2 to H2 doped with different metal oxides [30e33] and WO3eSnO2 composite thin film [20] to H2 were studied. Many studies have reported methods to improve gas sensitivity by doping, surface modification, and increasing porous structure. However, most of the studies focus on doping of single-layer film or surface modification of the top layer of films to improve gas sensing properties [34e38]
and there are relatively few reports of hydrogen sensors utilizing multilayer porous WO3 composite thin films. In the paper, we successfully prepared multilayer porous Pd-WO3 composite thin films by a sol-gel method utilizing F127 as a soft template. The hydrogen sensing performance of pure WO3 films and Pd-WO3 composite films were compared and the optimal compound molar ratio was studied among the composite thin films. The prepared films were characterized to obtain microstructure and morphology characteristics by various characterization methods. The hydrogen sensitivity of composite films was greater than pure WO3 films and the Pd-WO3 composite films with porous structure had better sensing performance. The gas selectivity test shows that porous Pd-WO3 composite films had good selectivity to H2, which was 20 times higher than CO and CH4 in the air of 200 ppm H2, and porous Pd-WO3 composite films exhibited reproducibility and short-term stability.
Experimental Synthesis of porous Pd-WO3 composite thin film 1 g F127 (Sigma Aldrich) was added to 30 ml absolute ethanol (Sino Pharm Chemical Reagent) and stirred for 10 min. Then,1 g WCl6 (Sino Pharm Chemical Reagent) was dissolved. This solution was stirred for 1 h at 45 C and the stirred solution was aged for 24 h at room temperature (RT). For the preparation of porous Pd-WO3 composite films, different quantities of PdCl2 (Sino Pharm Chemical Reagent) was added to the solution with 10 ml absolute ethanol, 0.165 g F127 and 1 ml deionized water to obtain different molar ratios of Pd: W which were 1%, 2%, 5% and 10%. The solution was stirred for 1 h at 35 C and the stirred solution was aged for 24 h at RT. This process is shown in Fig. 1 (Solution preparation). At the same time, the solution of WCl6 and PdCl2 without F127 were used to prepare pure WO3 films and Pd-WO3 composite films. The films were spin-coated on the silicon substrates with Pt electrodes. The silicon substrates were cleaned by acetone, anhydrous ethanol and deionized water in an ultrasonic cleaner for 4e5 min respectively to remove organic stains. The prepared solution was rotationally coated and deposited on the cleaned silicon substrates. Spin-coating speed was kept at 600 rpm for 6 s and 3000 rpm for 30 s. For the preparation of pure WO3 films, when a layer was deposited, the film was placed at 75 C for 5 min, and then a new layer was spincoated. This deposited process was repeated five times. After the last layer was deposited, the films were aged at 30e40% relative humidity (RH) for 72 h at RT. For the preparation of porous Pd-WO3 composite films, the schematic illustration is shown in Fig. 1. The solutions of WCl6 (add F127) and PdCl2 (add F127) were deposited on the substrate alternately. The spin-coating process was carried out 9 times. The films were dried under 75 C for 5 min after 1, 3, 5 and 7-layer deposition. The last two layers were dried at 30e40% relative humidity (RH) for 72 h at RT. All thin films were sintered at 500 C for 2 h after drying and aging. The samples with F127 were fired at 400 C for one more 1 h. At the hour of 400 C sintering, oxygen was added to remove the template residue.
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
international journal of hydrogen energy xxx (xxxx) xxx
3
Fig. 1 e Schematic illustration for the preparation of multilayer porous Pd-WO3 composite films. Six samples were prepared for hydrogen sensitive testing. Sample S1 represented pure WO3 film without porous structures. Sample S2 stood for 1 mol% Pd-WO3 composite film without porous structures. Sample S3, S4, S5, and S6 represented porous Pd-WO3 composite film with a Pd: W molar ratio of 1%, 2%, 5%, and 10%, respectively.
represents the resistance of the sensor in air and Rg stands for the resistance of the sensor in the air where the target gas exists. The response time was defined as the time to reach the minimum stable value of 90% and recovery time was defined as the time to reach the maximum stable value of 90%.
Material characterizations
Results and discussion
The powders (pure WO3 and porous WO3) for material characterization were prepared by aging, drying, and annealing in the same way as the films. The prepared powders and films were characterized to obtain microstructure and morphology characteristics by various characterization methods. The diffraction peak and phase composition of powders and films can be obtained by X-ray diffraction (XRD). The surface and cross-section morphologies of the films can be studied by field emission scanning electron microscope (FE-SEM, FEI Sirion 200). Energy dispersive spectroscopy (EDS) which made use of different X-ray photon characteristic energies of different elements is adopted to analyze the composition of thin films. The microstructures of the powders are analyzed and characterized by transmission electron microscopy (TEM). The analysis of N2 adsorption isotherm and pore diameter distribution of the materials are obtained by Brunauer-EmmettTeller (BET) and Barrett-Joyner-Halenda (BJH) methods, and BET and BJH equations are used to calculate the specific surface area and pore diameter of the material.
Characteristics of powders and films
Gas sensing measurement In our previous research, the test system for gas sensing measurement of thin films has been mentioned [30]. The flow of dry air and hydrogen were controlled by two mass flow controllers severally. The Agilent 34972a Data collector was used to test the resistance of the film at a frequency of once per second. A heating system with pulse current was used to keep the operating temperature. The response magnitude (S) of the sensor to gas was defined as S ¼ Ra/Rg, where Ra
Fig. 2 shows the representative XRD pattern of pure WO3 (a), Pd-WO3 composite film (b) and porous Pd-WO3 composite film (c). All well-resolved diffraction peaks are basically the same as WO3 with monoclinic structure (JCPDS No.83-0950). No obvious diffraction peaks of Pd, PdO or PdO2 can be observed in Fig. 1(b), which may be due to the low Pd concentration of S2. The diffraction peak of porous Pd-WO3 composite films was sharper than that of pure WO3 and Pd-WO3 composite films, which indicated that porous Pd-WO3 composite films had a higher crystallinity. Meanwhile, the additional peak around 61.7 in the XRD diffraction spectrum of porous PdWO3 composite film (S6) was due to the (103) reflection of Palladium oxide (JCPDS No.75-0200). Fig. 3 shows surface morphology of pure WO3 film, Pd-WO3 composite film and porous Pd-WO3 composite film and the EDS spectra of Pd-WO3 composite film. The surface grain structure of pure WO3 film and Pd-WO3 composite film is dense, while there are pores between grains on the surface of porous Pd-WO3 composite film, so grain structure of the porous composite film is conducive to the diffusion of gas molecules in the films. The illustration (inset) of Fig. 3(b) shows that the thickness of porous composite films is about 500 nm. The thickness of the composite film is larger than that of the film prepared in the previous study. This may result in low gas sensitivity at the beginning of the gas sensing test. Fig. 3(c) shows the relative content of W and Pd in the composite film and further confirmed the existence of Pd diffraction peaks in XRD images.
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 2 e XRD patterns of pure WO3 (a), Pd-WO3 composite film (S2) (b) and porous Pd-WO3 composite film (S6) (c).
Fig. 3 e Surface morphology of pure WO3 film (S1) (a), Pd-WO3 composite film (S2) (a) and porous Pd-WO3 composite film (S3) (b) and the EDS spectra of Pd-WO3 composite film (S3) (c). Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 4 e N2 adsorption/desorption isotherms and pore size distribution (illustration) of pure WO3 (a) and porous WO3 (b). Fig. 4 shows both N2 adsorption-desorption isotherms had hysteresis loops, which indicates the existence of porous structure. From the BET summary, the specific surface areas of porous WO3 (25.25 m2/g) was larger than that of pure WO3 (12.73 m2/g). According to the BJH diagrams, the pore diameter distribution of porous WO3 was relatively concentrated, about 3e4 nm. The average pore diameter of pure WO3 was smaller than that of porous WO3, which was 3.71 nm and 3.82 nm respectively. The pore volume of porous WO3 and pure WO3 were 0.121 cc/g and 0.0248 cc/g, respectively. These results show that the specific surface area of metal oxide is improved effectively and the better porous structure is formed by adding a templating agent. Fig. 5 shows the TEM and HRTEM images of pure WO3 and porous WO3 and the SAED image of porous WO3. In a certain range, porous WO3 can be observed as a periodic porous structure with a linear array, as shown in Fig. 5(d). However, pure WO3 has only a single arrangement of grain structure (Fig. 5(b)). Most of the nanoparticles had a uniform diameter of 5e6 nm. BET results show that the pore volume of porous WO3 is much larger than that of pure WO3. In general, the light shade areas can be regarded as porous regions on a bright background. Therefore, porous WO3 in the porous Pd-WO3 composite films has porous structures. As can be seen from Fig. 5(d), the marked crystal plane spacing in the HRTEM image is about 0.384 nm, 0.375 nm and 0.366 nm, corresponding to the plane (002), (020) and (200), respectively. The crystalline structure of porous WO3 is also validated by SAED (Fig. 5(e)). These results are consistent with the XRD pattern (Fig. 2(c)).
H2 sensing characteristics Fig. 6 shows the initial resistance of six samples mentioned above at different temperatures in air. It shows that the initial resistance of S1 and S2 decreases as the temperature increases and the resistance of S2 decrease faster with temperature than that of S1. This result may be due to the addition of Pd which enhances the transportation of electrons between two WO3 thin films. However, the resistances of S3~S6 increased first and then decreased with the increase of temperature. This is due to the fact that the carrier and mobility of
doped semiconductor change with the increase of temperature. When the temperature rises in a certain range, the carrier in the composite films does not change with the temperature, and the mobility decreases with the increase of temperature, so the resistance increases with the increase of temperature. When the temperature continues to rise, the rate of carrier production in the composite films is greater than the mobility, so the resistance decreases with the increase of temperature. The resistance of S6 was much higher than those of other samples (S3~S5) and the resistance of porous composite film increased with the increase of Pd concentration. This may be due to the fact that the effective carrier concentration decreases with the high concentration of Pd thin films and then resistance increases. The response magnitudes of six samples at 200e250 C to 1000 ppm H2 are shown in Fig. 7. While the operating temperature was below 200 C, the response and recovery of the porous Pd-WO3 composite films were so slow that there was no actual meaning to test below 200 C. In the range of 200e250 C, the six curves in the figure indicated that the response magnitude of the samples was the largest at 250 C except S4, which was at 225 C. The maximum response magnitudes of sample S1~S6 were 2.76, 9.72, 956.52, 195.02, 82.55 and 109.57, respectively. To some extent, the addition of composite structure improved the hydrogen sensitivity of WO3 films. Furthermore, the samples with porous structures (S3~S6) had better sensing performance than the samples without porous structure (S1~S2). Sample S3 showed the highest response magnitude in the porous composite films and the response magnitude was 98.4 times that of sample S2. At the same time, the response magnitude of sample S2 was 3.5 times that of sample S1. This proved that composite structure and porous structure effectively improved the gas sensing performance of WO3 thin films. Fig. 8 shows the response magnitudes of the six samples to H2 ranging from 50 ppm to 2000 ppm at 250 C. With the increase of the hydrogen concentration, the response magnitudes of the six samples also increased. When H2 concentration was lower than 100 ppm, the composite film did not have good hydrogen sensitivity. It is possible that at low concentrations of H2, the reaction only occurred on the surface of the composite film, but not in the film. When H2
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 5 e TEM and HRTEM images of pure WO3 (S1) (a, b), porous WO3 (S3) (c, d) and SAED image (e) of porous WO3.
Fig. 6 e Initial resistances of six samples at different operating temperatures (S1: pure WO3; S2: 1 mol% Pd-WO3; S3: porous 1 mol% Pd-WO3; S4: porous 2 mol% Pd-WO3; S5: porous 5 mol% Pd-WO3; S6: porous 10 mol% Pd-WO3).
Fig. 7 e The response magnitudes of six samples to 1000 ppm H2 at different operating temperature (S1: pure WO3; S2: 1 mol% Pd-WO3; S3: porous 1 mol% Pd-WO3; S4: porous 2 mol% Pd-WO3; S5: porous 5 mol% Pd-WO3; S6: porous 10 mol% Pd-WO3).
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 8 e The response magnitudes of six samples towards different H2 concentration at 250 C (S1: pure WO3; S2: 1 mol% Pd-WO3; S3: porous 1 mol% Pd-WO3; S4: porous 2 mol% Pd-WO3; S5: porous 5 mol% Pd-WO3; S6: porous 10 mol% Pd-WO3).
concentration was higher than 200 ppm, the hydrogen sensitivity of the composite film was greatly improved. The response magnitudes of S3~S6 to 2000 ppm H2 were 2144.10, 369.71, 253.33 and 223.82, respectively. The response magnitude of sample S3 was higher than that of other samples. On the one hand, composite and porous structure enhanced the hydrogen sensing performance of WO3 films. On the other hand, the hydrogen sensing performance of films increased with the decrease of film thickness in a certain range for porous Pd-WO3 composite films [39] (the thickness of Pd-WO3 composite films were affected by the PdCl6 solution of different concentrations). Therefore, the optimal molar ratio of Pd: W was 1 mol% for porous Pd-WO3 composite film. The response time and the recovery time of six samples to 50e2000 ppm H2 at 250 C are shown in Fig. 9. The response time of six samples at 2000 ppm was the shortest, which was 75 s, 21 s, 7 s, 7 s, 13 s, and 9 s, respectively. However, the recovery time of six samples after reaction with 2000 ppm H2 was relatively long, and they were 209 s, 239 s, 299 s, 252 s,
7
244 s, and 235 s, respectively. The response time of sample S1 was longer than the sample S2. It may be attributed to the fact that the reaction between the film and the target gas is catalyzed by adding thin films of Pd. The response time of samples (S3~S6) was shorter than that of the samples without porous structure (S1~S2). This may be due to the numerous pores that allow H2 molecules to penetrate into the membrane and accelerate the reduction reaction. Fig. 9(a) also showed that the response time increased first and then decreased with the increase of H2 concentration. This phenomenon was mentioned and explained in our previous report [30]. In the case of the sample without porous structure (S1, S2), the peak appeared at around 500 ppm. For the samples (S3, S4, S5, S6) with better porous structures, the peak appeared at enhances 200 ppm. This may be due to the large specific area and a large number of pores that allow the H2 molecules to penetrate into the film at lower concentrations. This led to a decrease in response times above 200 ppm. However, the recovery time of samples with porous composite films (S3~S6) was slower than that of sample S1 and S2, especially sample S3. This may be due to the fact that H2 diffuses faster than O2 in porous materials. When H2 was broken off, a certain amount of H2 was still diffusing or adsorbed in the film, the oxygen molecules were not able to diffuse into the film. And the larger stoichiometry changes in the porous films required a longer time to return to the initial state [23,40]. Fig. 10 shows the real-time resistance variation of the optimum sample S3 with various H2 concentrations. It further illustrated the response and recovery characteristics of porous Pd-WO3 composite films. To evaluate the selectivity of porous Pd-WO3 composite films for hydrogen, sensitivity tests for other gases were carried out. Fig. 11 shows the comparison of sensitivities of pure WO3 film and porous Pd-WO3 composite film to different kinds of analyte gases at the same operating temperature of 250 C. The result indicated that porous Pd-WO3 composite film had remarkable selectivity for H2 (the sensitivity was about 20 times than that of CO and CH4 at 200 ppm) and pure WO3 film was not very sensitive to all the gases. So, porous and composite structures improved the selectivity of WO3 films to H2. Finally, the stability of Pd-WO3 composite films in the air was tested. Fig. 12 shows the changes in response magnitudes (a) and response-recovery characteristics (b) of sample S3 to 1000 ppm H2 at 250 C within 6 months. The results showed
Fig. 9 e The response (a) and recovery times (b) of six samples vs. H2 concentration at 250 C (S1: pure WO3; S2: 1 mol% PdWO3; S3: porous 1 mol% Pd-WO3; S4: porous 2 mol% Pd-WO3; S5: porous 5 mol% Pd-WO3; S6: porous 10 mol% Pd-WO3). Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
8
international journal of hydrogen energy xxx (xxxx) xxx
that the sensitivity of porous Pd-WO3 composite films was basically the same and the maximum error was kept within 4%. The response-recovery characteristics were similar, so the porous composite film had stable sensing characteristics. The comparison of hydrogen sensitivity parameters of PdWO3 composite films with WO3 materials in other kinds of literature is shown in Table 1. It can be seen from the table that compared with other studies, Pd-WO3 composite films had a faster response and higher sensitivity, indicating that Pd-WO3 composite films prepared in this paper were an effective H2 detection sensing material.
H2 sensing mechanism
Fig. 10 e Real-time resistance variation of the optimum sample S3 with various H2 concentrations.
Fig. 11 e Comparison of sensitivities of pure WO3 film (S1) and porous Pd-WO3 composite film (S3) to different kinds of analyte gases (200 ppm for H2, CO, and CH4) at the same operating temperature of 250 C.
The gas sensing mechanism of WO3 is based on the changes of conductivity attributed to the reactions of hydrogen molecules, adsorbed oxygen and lattice oxygen. In Hua et al.’s research about the sensing mechanism of WO3 [41], the main mechanism which explains the decrease in the sensor resistance is the reaction of hydrogen molecules with adsorbed oxygen ions that results in the increase of free charge carriers and the decrease of space charge layer. For the Pd-WO3 com2through a posite films, oxygen is converted from O 2 to O series of reactions [4,29,42]. At the same time, the PeN heterojunction formed by the reaction of PdO and WO3 enhances the electronic interaction and the reaction between O2- and Pd accelerates the formation of PdO, and then the accumulation of charges forms the space charge layer. When H2 is added, surface lattice oxygen (O2 L ) is eliminated by H2 and oxygen vacancies (OV ) and W5þ ions are formed near Pd sites. So, the resistance of the sensor decreases. After H2 is off, the vacancies are re-oxidized by oxygen molecules, and then sensor resistance increases gradually. According to the surface redox reaction, the reaction process to H2 can be described by the equations below: 5þ þ OV þ e H2 þ W6þ þ O2 L /H2 O þ W
(1)
1 O2 þ W5þ þ OV þ e/W6þ þ O2 L 2
(2)
Therefore, the amount of reacted lattice oxygen in the reactions determines the change of the sensing resistance in H2. Fig. 13 shows the schematic model of the H2 sensing process
Fig. 12 e The changes in response magnitudes (a) and response-recovery characteristics (b) of sample S3 to 1000 ppm H2 at 250 C within 6 months. Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
9
international journal of hydrogen energy xxx (xxxx) xxx
Table 1 e The comparison of hydrogen sensitivity parameters of Pd-WO3 composite films with WO3 materials in other literatures. Material Multilayer porous Pd-WO3 composite thin films Pd doped WO3 films Pt-loaded WO3 sensing layers WO3eSnO2 dual-layer thin films 2D WO3 nanosheets WO3 nanotubes functionalized with bioinspired Pd catalysts Palladium-doped mesoporous WO3 Pt/WO3 thin films
Operating temp. ( C)
Concentration Response Response time Recovery time (ppm) (S) (T90, s) (Tr90, s)
Ref.
250
2000
2000~
7
~299
This work
RT 200 225 250 450
1300 500 2000 1%vol 500
2.5 104 ~3 ~53 ~72 ~17.6
~100 e 6.6~ 120~ 25
3600~ e e 235~ e
[16] [18] [20] [23] [24]
RT 80
5000 0.1%vol
~11.78 ~16
80 40~
10 45~
[29] [36]
d no measure.
and energy band of Pd-WO3 composite films. On the one hand, the films of Pd promote the electron transfer by PeN heterojunction between PdO and WO3 [43]. Moreover, the catalytic property and oxygen adsorption capacity of the p-type oxide semiconductors is higher than that of n-type semiconductors, resulting in more active sites inside composite film for the reaction between hydrogen molecules and oxygen ions. This behavior not only optimizes the reaction mechanism of surface lattice oxygen of WO3 but also enhances H2 sensing response. On the other hand, new adsorption points are formed and the sensing properties are improved because the
arrangement of ions in the composite materials has been changed [44]. In addition, the microstructure and specific surface area also have certain effects on the gas sensitivity of WO3 films. According to the experimental characterization results, the hydrogen sensitivity of the porous composite film in this paper is better than that of pure WO3 film, which further indicates that enough active sites for gas adsorption were provided and the gas sensing performance was improved by the porous structures.
Conclusion Multilayer porous Pd-WO3 composite thin films had been successfully prepared by a layer-by-layer deposition method utilizing Pluronic F127 as a soft template. The material characterization results manifest that the addition of soft template F127 improves the porous structure and increases specific surface areas. 1 mol% porous Pd-WO3 composite film had better hydrogen sensitivity than other films in the hydrogen sensing test. The response magnitude of the porous Pd-WO3 composite film (S3) was 956.52e1000 ppm H2 at 250 C. The sensitivity was about 346.5 times that of pure WO3 film (S1). The special composite structure and the porous structure were discussed to explain the H2 sensing mechanism. The great H2 sensing performance can be due to the PeN heterojunction between PdO and WO3, the redistribution of ions in the composite and the improvement of porosity.
Acknowledgment This research is sponsored by the National Natural Science Foundation of China (No. 51575255). Suggestions were made for this research and the use of several types of analytical equipment was supplied to our research team by Professor Chunhai Cao at the Department of Electronic Science and Engineering of Nanjing University and Professor Chongqing Wang at College of Chemistry and Chemical of Nanjing Tech University. Fig. 13 e Schematic model of H2 sensing process (a) and energy band (b) of Pd-WO3 composite films. Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
10
international journal of hydrogen energy xxx (xxxx) xxx
references [16] [1] Mazloomi K, Gomes C. Hydrogen as an energy carrier: prospects and challenges. Renew Sustain Energy Rev 2012. https://doi.org/10.1016/j.rser.2012.02.028. [2] Najjar YSH. Hydrogen safety: the road toward green technology. Int J Hydrogen Energy 2013. https://doi.org/ 10.1016/j.ijhydene.2013.05.126. [3] Boon-Brett L, Bousek J, Black G, Moretto P, Castello P, Hu¨bert T, Banach U. Identifying performance gaps in hydrogen safety sensor technology for automotive and stationary applications. Int J Hydrogen Energy 2010. https:// doi.org/10.1016/j.ijhydene.2009.10.064. [4] Luo Y, Zhang C, Zheng B, Geng X, Debliquy M. Hydrogen sensors based on noble metal doped metal-oxide semiconductor: a review. Int J Hydrogen Energy 2017. https:// doi.org/10.1016/j.ijhydene.2017.06.066. [5] Lavanya N, Sekar C, Fazio E, Neri F, Leonardi SG, Neri G. Development of a selective hydrogen leak sensor based on chemically doped SnO2 for automotive applications. Int J Hydrogen Energy 2017. https://doi.org/10.1016/ j.ijhydene.2017.03.027. [6] Kolhe PS, Shinde AB, Kulkarni SG, Maiti N, Koinkar PM, Sonawane KM. Gas sensing performance of Al doped ZnO thin film for H2S detection. J Alloy Comp 2018. https://doi.org/ 10.1016/j.jallcom.2018.03.123. [7] Nagai D, Nishibori M, Itoh T, Kawabe T, Sato K, Shin W. Ppm level methane detection using micro-thermoelectric gas sensors with Pd/Al2O3 combustion catalyst films. Sens Actuators B Chem 2015;206:488e94. https://doi.org/10.1016/ j.snb.2014.09.059. [8] Wetchakun K, Samerjai T, Tamaekong N, Liewhiran C, Siriwong C, Kruefu V, Wisitsoraat A, Tuantranont A, Phanichphant S. Semiconducting metal oxides as sensors for environmentally hazardous gases. Sens Actuators B Chem 2011. https://doi.org/10.1016/j.snb.2011.08.032. [9] Galatsis K, Li YX, Wlodarski W, Comini E, Sberveglieri G, Cantalini C, Santucci S, Passacantando M. Comparison of single and binary oxide MoO3, TiO2 and WO3 sol-gel gas sensors. Sens Actuators B Chem 2002. https://doi.org/ 10.1016/S0925-4005(01)01072-3. [10] Naseri N, Yousefzadeh S, Daryaei E, Moshfegh AZ. Photoresponse and H2 production of topographically controlled PEG assisted Sol-gel WO3 nanocrystalline thin films. Int J Hydrogen Energy 2011. https://doi.org/10.1016/ j.ijhydene.2011.07.129. cs M, Pap AE. Gas sensitivity of sol-gel prepared [11] Taka mesoporous WO3 thin film. Procedia Eng 2016. https:// doi.org/10.1016/j.proeng.2016.11.197. [12] Rydosz A, Szkudlarek A, Ziabka M, Domanski K, Maziarz W, Pisarkiewicz T. Performance of Si-doped WO3 thin films for acetone sensing prepared by glancing angle DC magnetron sputtering. IEEE Sens J 2016. https://doi.org/10.1109/ JSEN.2015.2496212. M, Dı´az F, Borull JF, [13] Qadri MU, Annanouch FE, Aguilo Pujol MC, Llobet E. Metal decorated WO3 nanoneedles fabricated by aerosol assisted chemical vapor deposition for optical gas sensing. J Nanosci Nanotechnol 2016. https:// doi.org/10.1166/jnn.2016.12840. [14] Arfaoui A, Touihri S, Mhamdi A, Labidi A, Manoubi T. Structural, morphological, gas sensing and photocatalytic characterization of MoO3 and WO3 thin films prepared by the thermal vacuum evaporation technique. Appl Surf Sci 2015. https://doi.org/10.1016/j.apsusc.2015.09.124. [15] Teoh LG, Hon YM, Shieh J, Lai WH, Hon MH. Sensitivity properties of a novel NO2 gas sensor based on mesoporous
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
WO3 thin film. Sens Actuators B Chem 2003. https://doi.org/ 10.1016/S0925-4005(03)00528-8. Fardindoost S, Iraji zad A, Rahimi F, Ghasempour R. Pd doped WO3 films prepared by sol-gel process for hydrogen sensing. Int J Hydrogen Energy 2010. https://doi.org/10.1016/ j.ijhydene.2009.11.033. Naseri N, Kim H, Choi W, Moshfegh AZ. Implementation of Ag nanoparticle incorporated WO3 thin film photoanode for hydrogen production. Int J Hydrogen Energy 2013. https:// doi.org/10.1016/j.ijhydene.2012.11.132. Jolly Bose R, Illyaskutty N, Tan KS, Rawat RS, Matham MV, Kohler H, Mahadevan Pillai VP. Hydrogen sensors based on Pt-loaded WO3 sensing layers. EPL 2016. https://doi.org/ 10.1209/0295-5075/114/66002. Kumar N, Dvora S, k F, Duchon T, Haviar S, Capek J, Batkova M, Zeman P. Hydrogen gas sensing properties of WO3 Fialova sputter-deposited thin films enhanced by on-top deposited CuO nanoclusters. Int J Hydrogen Energy 2018. https:// doi.org/10.1016/j.ijhydene.2018.10.127. Zhang Z, Yin C, Yang L, Jia W, Zhou J, Xu H, Cao D. H2 response characteristics for solegel-derived WO3-SnO2 duallayer thin films. Ceram Int 2017. https://doi.org/10.1016/ j.ceramint.2017.02.065. Feng C, Li X, Ma J, Sun Y, Wang C, Sun P, Zheng J, Lu G. Facile synthesis and gas sensing properties of In2O3-WO3 heterojunction nanofibers. Sens Actuators B Chem 2015. https://doi.org/10.1016/j.snb.2014.12.019. Cai Z-X, Li H-Y, Yang X-N, Guo X. NO sensing by single crystalline WO3 nanowires. Sens Actuators B Chem 2015. https://doi.org/10.1016/j.snb.2015.05.036. Rahmani MB, Yaacob MH, Sabri YM. Hydrogen sensors based on 2D WO3 nanosheets prepared by anodization. Sens Actuators B Chem 2017. https://doi.org/10.1016/ j.snb.2017.05.029. Choi SJ, Chattopadhyay S, Kim JJ, Kim SJ, Tuller HL, Rutledge GC, Kim ID. Coaxial electrospinning of WO3 nanotubes functionalized with bio-inspired Pd catalysts and their superior hydrogen sensing performance. Nanoscale 2016. https://doi.org/10.1039/c5nr06611e. Kabcum S, Channei D, Tuantranont A, Wisitsoraat A, Liewhiran C, Phanichphant S. Ultra-responsive hydrogen gas sensors based on PdO nanoparticle-decorated WO3 nanorods synthesized by precipitation and impregnation methods. Sens Actuators B Chem 2016. https://doi.org/10.1016/ j.snb.2015.11.120. Li J, Liu X, Cui J, Sun J. Hydrothermal synthesis of selfassembled hierarchical tungsten oxides hollow spheres and their gas sensing properties. ACS Appl Mater Interfaces 2015. https://doi.org/10.1021/am508121p. Liu KI, Hsueh YC, Su CY, et al. Photoelectrochemical application of mesoporous TiO2/WO3 nanohoneycomb prepared by sol-gel method. Int J Hydrogen Energy 2013. https://doi.org/10.1016/j.ijhydene.2013.04.046. Tomer VK, Devi S, Malik R, Nehra SP, Duhan S. Highly sensitive and selective volatile organic amine (VOA) sensors using mesoporous WO3-SnO2 nanohybrids. Sens Actuators B Chem 2016. https://doi.org/10.1016/j.snb.2016.01.124. Wu CH, Zhu Z, Huang SY, Wu RJ. Preparation of palladiumdoped mesoporous WO3 for hydrogen gas sensors. J Alloy Comp 2019. https://doi.org/10.1016/j.jallcom.2018.10.372. Bin Z, Chenbo Y, Zili Z, Chunmin T, Liu Y. Investigation of the hydrogen response characteristics for sol-gel-derived Pddoped, Fe-doped and PEG-added SnO2 nano-thin films. Sens Actuators B Chem 2013. https://doi.org/10.1016/ j.snb.2012.12.101. Zhang Z, Yin C, Yang L, Jiang J, Guo Y. Optimizing the gas sensing characteristics of Co-doped SnO2 thin film based
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
international journal of hydrogen energy xxx (xxxx) xxx
[32]
[33]
[34]
[35]
[36]
[37]
[38]
hydrogen sensor. J Alloy Comp 2019. https://doi.org/10.1016/ j.jallcom.2019.01.244. Yang L, Yin C, Zhang Z, Zhu B. A study of hydrogen sensing properties and microstructure for highly dispersed Pd SnO2 thin films with high response magnitude. Appl Surf Sci 2014. https://doi.org/10.1016/j.apsusc.2014.05.003. Zhang S, Yin C, Yang L, Zhang Z, Han Z. Investigation of the H2 sensing properties of multilayer mesoporous pure and Pddoped SnO2 thin film. Sens Actuators B Chem 2019. https:// doi.org/10.1016/j.snb.2018.12.051. Sadek AZ, Wlodarski W, Shin K, Kaner RB, Kalantar-zadeh K. A polyaniline/WO3 nanofiber composite-based ZnO/64 YX LiNbO3 SAW hydrogen gas sensor. Synth Met 2008. https:// doi.org/10.1016/j.synthmet.2007.11.008. Yang M, Sun Y, Zhang D, Jiang D. Using Pd/WO3 composite thin films as sensing materials for optical fiber hydrogen sensors. Sens Actuators B Chem 2010;143(2):750e3. https:// doi.org/10.1016/j.snb.2009.10.017. Amani E, Khojier K, Zoriasatain S. Improving the hydrogen gas sensitivity of WO3 thin films by modifying the deposition angle and thickness of different promoter layers. Int J Hydrogen Energy 2017. https://doi.org/10.1016/ j.ijhydene.2017.10.027. Shi J, Cheng Z, Gao L, Zhang Y, Xu J, Zhao H. Facile synthesis of reduced graphene oxide/hexagonal WO3 nanosheets composites with enhanced H2S sensing properties. Sens Actuators B Chem 2016;230:736e45. https://doi.org/10.1016/ j.snb.2016.02.134. Zhang H, Wang Y, Zhu X, Li Y, Cai W. Bilayer Au nanoparticle-decorated WO3 porous thin films: on-chip
[39]
[40]
[41]
[42]
[43]
[44]
11
fabrication and enhanced NO2 gas sensing performances with high selectivity. Sens Actuators B Chem 2019. https:// doi.org/10.1016/j.snb.2018.10.065. Teimoori F, Khojier K, Dehnavi NZ. On the dependence of H2 gas sensitivity of ZnO thin films on film thickness. Procedia Mater Sci 2015. https://doi.org/10.1016/j.mspro.2015.11.061. Kadir RA, Zhang W, Wang Y, Ou JZ, Wlodarski W, O'Mullane AP, Bryant G, Taylor M, Kalantar-Zadeh K. Anodized nanoporous WO3 Schottky contact structures for hydrogen and ethanol sensing. J Mater Chem 2015. https:// doi.org/10.1039/c4ta06286h. Hua Z, Yuasa M, Kida T, Yamazoe N, Shimanoe K. H2 sensing mechanism of Pd-loaded WO3 nanoparticle gas sensors. Chem Lett 2014. https://doi.org/10.1246/cl.140396. Poongodi S, Kumar PS, Mangalaraj D, Ponpandian N, Meena P, Masuda Y, Lee C. Electrodeposition of WO3 nanostructured thin films for electrochromic and H2S gas sensor applications. J Alloy Comp 2017. https://doi.org/ 10.1016/j.jallcom.2017.05.122. Mirzaei A, Yousefi HR, Falsafi F, Bonyani M, Lee JH, Kim JH, Kim HW, Kim SS. An overview on how Pd on resistive-based nanomaterial gas sensors can enhance response toward hydrogen gas. Int J Hydrogen Energy 2019. https://doi.org/ 10.1016/j.ijhydene.2019.05.180. Gerasimov GN, Gromov VF, Ilegbusi OJ, Trakhtenberg LI. The mechanisms of sensory phenomena in binary metal-oxide nanocomposites. Sens Actuators B Chem 2017. https:// doi.org/10.1016/j.snb.2016.09.007.
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing Zhongjun Han a, Jun Ren a, Junjing Zhou a, Shiyun Zhang a, Zili Zhang b, Liu Yang c, Chenbo Yin a,* a
College of Mechanical and Power Engineering, Nanjing Tech University, Nanjing, 211816, PR China College of Mechanical and Electrical Engineering, Jinling Institute of Technology, Nanjing, 211169, PR China c College of Mechanical Engineering, Nanjing Institute of Technology, Nanjing, 211167, PR China b
highlights
graphical abstract
Porous Pd-WO3 composite films were prepared for hydrogen leak detection. The optimal molar ratio of Pd: W was 1 mol% for porous Pd-WO3 composite films. The PeN heterojunction enhances H2 response characteristics. Porous structure with additional active sites enhances H2 sensing performance.
article info
abstract
Article history:
Based on sol-gel, multilayer porous Pd-WO3 composite thin films have been successfully
Received 21 September 2019
prepared by utilizing a layer-by-layer deposition strategy. The crystal structure and
Received in revised form
microstructure were analyzed by various characterization methods. The sensing perfor-
4 December 2019
mance of the prepared films at different hydrogen concentrations and the temperature was
Accepted 20 December 2019
studied. Results show that the performance of the hydrogen sensor can be improved
Available online xxx
greatly by the use of composite structure and soft template (Pluronic F127). The response performance of 1 mol% porous Pd-WO3 composite films was better than that of 2e10 mol%
Keywords:
films. The porous Pd-WO3 composite films showed a high sensitivity (~346.5 times better
Nanocomposites
than the sensitivity of pure WO3 films for 1000 ppm H2) and a fast response time (7 s) at the
Porous structure
temperature of 250 C. Porous Pd-WO3 composite films had good selectivity for hydrogen
Tungsten trioxide
and stable sensing performance. The different response and recovery behavior of samples
Palladium oxide
were contrasted and discussed to explain the effect of the special composite structure and
Sol-gel
porous structure on hydrogen sensing performance.
H2 response characteristics
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (C. Yin). https://doi.org/10.1016/j.ijhydene.2019.12.149 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
2
international journal of hydrogen energy xxx (xxxx) xxx
Introduction Hydrogen has been widely concerned as a kind of clean energy with high efficiency, high energy, reproducibility, and nonpollution [1]. However, the development and application of hydrogen are hindered by its characteristics of easy diffusion, wide combustion concentration of 4e75% and easy escape from most storage materials [2,3]. Therefore, once hydrogen is leaked during storage and transportation, accidents are very likely to happen. As a result, hydrogen sensors with excellent performances such as sensitivity, selectivity, stability, and durability are necessary to detect and monitor hydrogen leakage in various fields of hydrogen application [4]. In the field of hydrogen sensor, metal oxide semiconductors (MOS) such as ZnO [5], SnO2 [6] and Al2O3 [7] are suitable for the preparation of hydrogen sensitive materials and these gas sensors have the advantages of simple structure, low production cost and good sensing performance [8]. WO3 as an n-type MOS material has been used in the detection of various gases because of its unique chemical and physical property and excellent gas-sensing character [9]. Up to now, many effective methods have been applied to fabricate WO3 thin films successfully, such as sol-gel mothed [10,11], magnetron sputtering [12], chemical vapor deposition [13] and electron beam evaporation [14]. Among various techniques, sol-gel deposition has the advantages of low cost, ordinary equipment, low processing temperature and easy access to molecular level uniformity (or doping uniformity). So, sol-gel is an economical and effective method for preparing porous WO3 films. WO3 is generally used at temperatures between 200 and 450 C, but it can be doped and modified to improve gas sensitivity and reduce operating temperature [15]. It has been shown that the dopants can be used as catalysts or surface sites to absorb gases and optimize the sensing mechanism. Therefore, the reaction between the film and the target gas was catalyzed and the active sites of the film were improved by adding the right amount of metal additives (Pd, Ag, Pt) [16e18] or different metal oxides (CuO, SnO2, In2O3) [19e21]. It is an important way to improve the gas sensitivity of MOS. At the same time, the grain structure characteristic of WO3 also has an important influence on the H2 sensing performance. Nowadays, various types of structural WO3 such as nanowires [22], nanosheets [23], nanotubes [24], nanorod [25] and hollow sphere [26] have been studied to improve gas sensitivity. The porous crystal with high specific surface areas is more sensitive than the dense grain structure film. And the high surface volume ratio of the porous crystal makes the gas diffusion easy. Therefore, gas sensing performance of MOS was improved by adding surfactants such as F127 [11] and P123 [27] or using hard templates such as SBA-15 [28] and KIT-6 [29] to form the porous structure. In our previous work, the reaction characteristics of SnO2 to H2 doped with different metal oxides [30e33] and WO3eSnO2 composite thin film [20] to H2 were studied. Many studies have reported methods to improve gas sensitivity by doping, surface modification, and increasing porous structure. However, most of the studies focus on doping of single-layer film or surface modification of the top layer of films to improve gas sensing properties [34e38]
and there are relatively few reports of hydrogen sensors utilizing multilayer porous WO3 composite thin films. In the paper, we successfully prepared multilayer porous Pd-WO3 composite thin films by a sol-gel method utilizing F127 as a soft template. The hydrogen sensing performance of pure WO3 films and Pd-WO3 composite films were compared and the optimal compound molar ratio was studied among the composite thin films. The prepared films were characterized to obtain microstructure and morphology characteristics by various characterization methods. The hydrogen sensitivity of composite films was greater than pure WO3 films and the Pd-WO3 composite films with porous structure had better sensing performance. The gas selectivity test shows that porous Pd-WO3 composite films had good selectivity to H2, which was 20 times higher than CO and CH4 in the air of 200 ppm H2, and porous Pd-WO3 composite films exhibited reproducibility and short-term stability.
Experimental Synthesis of porous Pd-WO3 composite thin film 1 g F127 (Sigma Aldrich) was added to 30 ml absolute ethanol (Sino Pharm Chemical Reagent) and stirred for 10 min. Then,1 g WCl6 (Sino Pharm Chemical Reagent) was dissolved. This solution was stirred for 1 h at 45 C and the stirred solution was aged for 24 h at room temperature (RT). For the preparation of porous Pd-WO3 composite films, different quantities of PdCl2 (Sino Pharm Chemical Reagent) was added to the solution with 10 ml absolute ethanol, 0.165 g F127 and 1 ml deionized water to obtain different molar ratios of Pd: W which were 1%, 2%, 5% and 10%. The solution was stirred for 1 h at 35 C and the stirred solution was aged for 24 h at RT. This process is shown in Fig. 1 (Solution preparation). At the same time, the solution of WCl6 and PdCl2 without F127 were used to prepare pure WO3 films and Pd-WO3 composite films. The films were spin-coated on the silicon substrates with Pt electrodes. The silicon substrates were cleaned by acetone, anhydrous ethanol and deionized water in an ultrasonic cleaner for 4e5 min respectively to remove organic stains. The prepared solution was rotationally coated and deposited on the cleaned silicon substrates. Spin-coating speed was kept at 600 rpm for 6 s and 3000 rpm for 30 s. For the preparation of pure WO3 films, when a layer was deposited, the film was placed at 75 C for 5 min, and then a new layer was spincoated. This deposited process was repeated five times. After the last layer was deposited, the films were aged at 30e40% relative humidity (RH) for 72 h at RT. For the preparation of porous Pd-WO3 composite films, the schematic illustration is shown in Fig. 1. The solutions of WCl6 (add F127) and PdCl2 (add F127) were deposited on the substrate alternately. The spin-coating process was carried out 9 times. The films were dried under 75 C for 5 min after 1, 3, 5 and 7-layer deposition. The last two layers were dried at 30e40% relative humidity (RH) for 72 h at RT. All thin films were sintered at 500 C for 2 h after drying and aging. The samples with F127 were fired at 400 C for one more 1 h. At the hour of 400 C sintering, oxygen was added to remove the template residue.
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
international journal of hydrogen energy xxx (xxxx) xxx
3
Fig. 1 e Schematic illustration for the preparation of multilayer porous Pd-WO3 composite films. Six samples were prepared for hydrogen sensitive testing. Sample S1 represented pure WO3 film without porous structures. Sample S2 stood for 1 mol% Pd-WO3 composite film without porous structures. Sample S3, S4, S5, and S6 represented porous Pd-WO3 composite film with a Pd: W molar ratio of 1%, 2%, 5%, and 10%, respectively.
represents the resistance of the sensor in air and Rg stands for the resistance of the sensor in the air where the target gas exists. The response time was defined as the time to reach the minimum stable value of 90% and recovery time was defined as the time to reach the maximum stable value of 90%.
Material characterizations
Results and discussion
The powders (pure WO3 and porous WO3) for material characterization were prepared by aging, drying, and annealing in the same way as the films. The prepared powders and films were characterized to obtain microstructure and morphology characteristics by various characterization methods. The diffraction peak and phase composition of powders and films can be obtained by X-ray diffraction (XRD). The surface and cross-section morphologies of the films can be studied by field emission scanning electron microscope (FE-SEM, FEI Sirion 200). Energy dispersive spectroscopy (EDS) which made use of different X-ray photon characteristic energies of different elements is adopted to analyze the composition of thin films. The microstructures of the powders are analyzed and characterized by transmission electron microscopy (TEM). The analysis of N2 adsorption isotherm and pore diameter distribution of the materials are obtained by Brunauer-EmmettTeller (BET) and Barrett-Joyner-Halenda (BJH) methods, and BET and BJH equations are used to calculate the specific surface area and pore diameter of the material.
Characteristics of powders and films
Gas sensing measurement In our previous research, the test system for gas sensing measurement of thin films has been mentioned [30]. The flow of dry air and hydrogen were controlled by two mass flow controllers severally. The Agilent 34972a Data collector was used to test the resistance of the film at a frequency of once per second. A heating system with pulse current was used to keep the operating temperature. The response magnitude (S) of the sensor to gas was defined as S ¼ Ra/Rg, where Ra
Fig. 2 shows the representative XRD pattern of pure WO3 (a), Pd-WO3 composite film (b) and porous Pd-WO3 composite film (c). All well-resolved diffraction peaks are basically the same as WO3 with monoclinic structure (JCPDS No.83-0950). No obvious diffraction peaks of Pd, PdO or PdO2 can be observed in Fig. 1(b), which may be due to the low Pd concentration of S2. The diffraction peak of porous Pd-WO3 composite films was sharper than that of pure WO3 and Pd-WO3 composite films, which indicated that porous Pd-WO3 composite films had a higher crystallinity. Meanwhile, the additional peak around 61.7 in the XRD diffraction spectrum of porous PdWO3 composite film (S6) was due to the (103) reflection of Palladium oxide (JCPDS No.75-0200). Fig. 3 shows surface morphology of pure WO3 film, Pd-WO3 composite film and porous Pd-WO3 composite film and the EDS spectra of Pd-WO3 composite film. The surface grain structure of pure WO3 film and Pd-WO3 composite film is dense, while there are pores between grains on the surface of porous Pd-WO3 composite film, so grain structure of the porous composite film is conducive to the diffusion of gas molecules in the films. The illustration (inset) of Fig. 3(b) shows that the thickness of porous composite films is about 500 nm. The thickness of the composite film is larger than that of the film prepared in the previous study. This may result in low gas sensitivity at the beginning of the gas sensing test. Fig. 3(c) shows the relative content of W and Pd in the composite film and further confirmed the existence of Pd diffraction peaks in XRD images.
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 2 e XRD patterns of pure WO3 (a), Pd-WO3 composite film (S2) (b) and porous Pd-WO3 composite film (S6) (c).
Fig. 3 e Surface morphology of pure WO3 film (S1) (a), Pd-WO3 composite film (S2) (a) and porous Pd-WO3 composite film (S3) (b) and the EDS spectra of Pd-WO3 composite film (S3) (c). Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 4 e N2 adsorption/desorption isotherms and pore size distribution (illustration) of pure WO3 (a) and porous WO3 (b). Fig. 4 shows both N2 adsorption-desorption isotherms had hysteresis loops, which indicates the existence of porous structure. From the BET summary, the specific surface areas of porous WO3 (25.25 m2/g) was larger than that of pure WO3 (12.73 m2/g). According to the BJH diagrams, the pore diameter distribution of porous WO3 was relatively concentrated, about 3e4 nm. The average pore diameter of pure WO3 was smaller than that of porous WO3, which was 3.71 nm and 3.82 nm respectively. The pore volume of porous WO3 and pure WO3 were 0.121 cc/g and 0.0248 cc/g, respectively. These results show that the specific surface area of metal oxide is improved effectively and the better porous structure is formed by adding a templating agent. Fig. 5 shows the TEM and HRTEM images of pure WO3 and porous WO3 and the SAED image of porous WO3. In a certain range, porous WO3 can be observed as a periodic porous structure with a linear array, as shown in Fig. 5(d). However, pure WO3 has only a single arrangement of grain structure (Fig. 5(b)). Most of the nanoparticles had a uniform diameter of 5e6 nm. BET results show that the pore volume of porous WO3 is much larger than that of pure WO3. In general, the light shade areas can be regarded as porous regions on a bright background. Therefore, porous WO3 in the porous Pd-WO3 composite films has porous structures. As can be seen from Fig. 5(d), the marked crystal plane spacing in the HRTEM image is about 0.384 nm, 0.375 nm and 0.366 nm, corresponding to the plane (002), (020) and (200), respectively. The crystalline structure of porous WO3 is also validated by SAED (Fig. 5(e)). These results are consistent with the XRD pattern (Fig. 2(c)).
H2 sensing characteristics Fig. 6 shows the initial resistance of six samples mentioned above at different temperatures in air. It shows that the initial resistance of S1 and S2 decreases as the temperature increases and the resistance of S2 decrease faster with temperature than that of S1. This result may be due to the addition of Pd which enhances the transportation of electrons between two WO3 thin films. However, the resistances of S3~S6 increased first and then decreased with the increase of temperature. This is due to the fact that the carrier and mobility of
doped semiconductor change with the increase of temperature. When the temperature rises in a certain range, the carrier in the composite films does not change with the temperature, and the mobility decreases with the increase of temperature, so the resistance increases with the increase of temperature. When the temperature continues to rise, the rate of carrier production in the composite films is greater than the mobility, so the resistance decreases with the increase of temperature. The resistance of S6 was much higher than those of other samples (S3~S5) and the resistance of porous composite film increased with the increase of Pd concentration. This may be due to the fact that the effective carrier concentration decreases with the high concentration of Pd thin films and then resistance increases. The response magnitudes of six samples at 200e250 C to 1000 ppm H2 are shown in Fig. 7. While the operating temperature was below 200 C, the response and recovery of the porous Pd-WO3 composite films were so slow that there was no actual meaning to test below 200 C. In the range of 200e250 C, the six curves in the figure indicated that the response magnitude of the samples was the largest at 250 C except S4, which was at 225 C. The maximum response magnitudes of sample S1~S6 were 2.76, 9.72, 956.52, 195.02, 82.55 and 109.57, respectively. To some extent, the addition of composite structure improved the hydrogen sensitivity of WO3 films. Furthermore, the samples with porous structures (S3~S6) had better sensing performance than the samples without porous structure (S1~S2). Sample S3 showed the highest response magnitude in the porous composite films and the response magnitude was 98.4 times that of sample S2. At the same time, the response magnitude of sample S2 was 3.5 times that of sample S1. This proved that composite structure and porous structure effectively improved the gas sensing performance of WO3 thin films. Fig. 8 shows the response magnitudes of the six samples to H2 ranging from 50 ppm to 2000 ppm at 250 C. With the increase of the hydrogen concentration, the response magnitudes of the six samples also increased. When H2 concentration was lower than 100 ppm, the composite film did not have good hydrogen sensitivity. It is possible that at low concentrations of H2, the reaction only occurred on the surface of the composite film, but not in the film. When H2
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 5 e TEM and HRTEM images of pure WO3 (S1) (a, b), porous WO3 (S3) (c, d) and SAED image (e) of porous WO3.
Fig. 6 e Initial resistances of six samples at different operating temperatures (S1: pure WO3; S2: 1 mol% Pd-WO3; S3: porous 1 mol% Pd-WO3; S4: porous 2 mol% Pd-WO3; S5: porous 5 mol% Pd-WO3; S6: porous 10 mol% Pd-WO3).
Fig. 7 e The response magnitudes of six samples to 1000 ppm H2 at different operating temperature (S1: pure WO3; S2: 1 mol% Pd-WO3; S3: porous 1 mol% Pd-WO3; S4: porous 2 mol% Pd-WO3; S5: porous 5 mol% Pd-WO3; S6: porous 10 mol% Pd-WO3).
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 8 e The response magnitudes of six samples towards different H2 concentration at 250 C (S1: pure WO3; S2: 1 mol% Pd-WO3; S3: porous 1 mol% Pd-WO3; S4: porous 2 mol% Pd-WO3; S5: porous 5 mol% Pd-WO3; S6: porous 10 mol% Pd-WO3).
concentration was higher than 200 ppm, the hydrogen sensitivity of the composite film was greatly improved. The response magnitudes of S3~S6 to 2000 ppm H2 were 2144.10, 369.71, 253.33 and 223.82, respectively. The response magnitude of sample S3 was higher than that of other samples. On the one hand, composite and porous structure enhanced the hydrogen sensing performance of WO3 films. On the other hand, the hydrogen sensing performance of films increased with the decrease of film thickness in a certain range for porous Pd-WO3 composite films [39] (the thickness of Pd-WO3 composite films were affected by the PdCl6 solution of different concentrations). Therefore, the optimal molar ratio of Pd: W was 1 mol% for porous Pd-WO3 composite film. The response time and the recovery time of six samples to 50e2000 ppm H2 at 250 C are shown in Fig. 9. The response time of six samples at 2000 ppm was the shortest, which was 75 s, 21 s, 7 s, 7 s, 13 s, and 9 s, respectively. However, the recovery time of six samples after reaction with 2000 ppm H2 was relatively long, and they were 209 s, 239 s, 299 s, 252 s,
7
244 s, and 235 s, respectively. The response time of sample S1 was longer than the sample S2. It may be attributed to the fact that the reaction between the film and the target gas is catalyzed by adding thin films of Pd. The response time of samples (S3~S6) was shorter than that of the samples without porous structure (S1~S2). This may be due to the numerous pores that allow H2 molecules to penetrate into the membrane and accelerate the reduction reaction. Fig. 9(a) also showed that the response time increased first and then decreased with the increase of H2 concentration. This phenomenon was mentioned and explained in our previous report [30]. In the case of the sample without porous structure (S1, S2), the peak appeared at around 500 ppm. For the samples (S3, S4, S5, S6) with better porous structures, the peak appeared at enhances 200 ppm. This may be due to the large specific area and a large number of pores that allow the H2 molecules to penetrate into the film at lower concentrations. This led to a decrease in response times above 200 ppm. However, the recovery time of samples with porous composite films (S3~S6) was slower than that of sample S1 and S2, especially sample S3. This may be due to the fact that H2 diffuses faster than O2 in porous materials. When H2 was broken off, a certain amount of H2 was still diffusing or adsorbed in the film, the oxygen molecules were not able to diffuse into the film. And the larger stoichiometry changes in the porous films required a longer time to return to the initial state [23,40]. Fig. 10 shows the real-time resistance variation of the optimum sample S3 with various H2 concentrations. It further illustrated the response and recovery characteristics of porous Pd-WO3 composite films. To evaluate the selectivity of porous Pd-WO3 composite films for hydrogen, sensitivity tests for other gases were carried out. Fig. 11 shows the comparison of sensitivities of pure WO3 film and porous Pd-WO3 composite film to different kinds of analyte gases at the same operating temperature of 250 C. The result indicated that porous Pd-WO3 composite film had remarkable selectivity for H2 (the sensitivity was about 20 times than that of CO and CH4 at 200 ppm) and pure WO3 film was not very sensitive to all the gases. So, porous and composite structures improved the selectivity of WO3 films to H2. Finally, the stability of Pd-WO3 composite films in the air was tested. Fig. 12 shows the changes in response magnitudes (a) and response-recovery characteristics (b) of sample S3 to 1000 ppm H2 at 250 C within 6 months. The results showed
Fig. 9 e The response (a) and recovery times (b) of six samples vs. H2 concentration at 250 C (S1: pure WO3; S2: 1 mol% PdWO3; S3: porous 1 mol% Pd-WO3; S4: porous 2 mol% Pd-WO3; S5: porous 5 mol% Pd-WO3; S6: porous 10 mol% Pd-WO3). Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
8
international journal of hydrogen energy xxx (xxxx) xxx
that the sensitivity of porous Pd-WO3 composite films was basically the same and the maximum error was kept within 4%. The response-recovery characteristics were similar, so the porous composite film had stable sensing characteristics. The comparison of hydrogen sensitivity parameters of PdWO3 composite films with WO3 materials in other kinds of literature is shown in Table 1. It can be seen from the table that compared with other studies, Pd-WO3 composite films had a faster response and higher sensitivity, indicating that Pd-WO3 composite films prepared in this paper were an effective H2 detection sensing material.
H2 sensing mechanism
Fig. 10 e Real-time resistance variation of the optimum sample S3 with various H2 concentrations.
Fig. 11 e Comparison of sensitivities of pure WO3 film (S1) and porous Pd-WO3 composite film (S3) to different kinds of analyte gases (200 ppm for H2, CO, and CH4) at the same operating temperature of 250 C.
The gas sensing mechanism of WO3 is based on the changes of conductivity attributed to the reactions of hydrogen molecules, adsorbed oxygen and lattice oxygen. In Hua et al.’s research about the sensing mechanism of WO3 [41], the main mechanism which explains the decrease in the sensor resistance is the reaction of hydrogen molecules with adsorbed oxygen ions that results in the increase of free charge carriers and the decrease of space charge layer. For the Pd-WO3 com2through a posite films, oxygen is converted from O 2 to O series of reactions [4,29,42]. At the same time, the PeN heterojunction formed by the reaction of PdO and WO3 enhances the electronic interaction and the reaction between O2- and Pd accelerates the formation of PdO, and then the accumulation of charges forms the space charge layer. When H2 is added, surface lattice oxygen (O2 L ) is eliminated by H2 and oxygen vacancies (OV ) and W5þ ions are formed near Pd sites. So, the resistance of the sensor decreases. After H2 is off, the vacancies are re-oxidized by oxygen molecules, and then sensor resistance increases gradually. According to the surface redox reaction, the reaction process to H2 can be described by the equations below: 5þ þ OV þ e H2 þ W6þ þ O2 L /H2 O þ W
(1)
1 O2 þ W5þ þ OV þ e/W6þ þ O2 L 2
(2)
Therefore, the amount of reacted lattice oxygen in the reactions determines the change of the sensing resistance in H2. Fig. 13 shows the schematic model of the H2 sensing process
Fig. 12 e The changes in response magnitudes (a) and response-recovery characteristics (b) of sample S3 to 1000 ppm H2 at 250 C within 6 months. Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
9
international journal of hydrogen energy xxx (xxxx) xxx
Table 1 e The comparison of hydrogen sensitivity parameters of Pd-WO3 composite films with WO3 materials in other literatures. Material Multilayer porous Pd-WO3 composite thin films Pd doped WO3 films Pt-loaded WO3 sensing layers WO3eSnO2 dual-layer thin films 2D WO3 nanosheets WO3 nanotubes functionalized with bioinspired Pd catalysts Palladium-doped mesoporous WO3 Pt/WO3 thin films
Operating temp. ( C)
Concentration Response Response time Recovery time (ppm) (S) (T90, s) (Tr90, s)
Ref.
250
2000
2000~
7
~299
This work
RT 200 225 250 450
1300 500 2000 1%vol 500
2.5 104 ~3 ~53 ~72 ~17.6
~100 e 6.6~ 120~ 25
3600~ e e 235~ e
[16] [18] [20] [23] [24]
RT 80
5000 0.1%vol
~11.78 ~16
80 40~
10 45~
[29] [36]
d no measure.
and energy band of Pd-WO3 composite films. On the one hand, the films of Pd promote the electron transfer by PeN heterojunction between PdO and WO3 [43]. Moreover, the catalytic property and oxygen adsorption capacity of the p-type oxide semiconductors is higher than that of n-type semiconductors, resulting in more active sites inside composite film for the reaction between hydrogen molecules and oxygen ions. This behavior not only optimizes the reaction mechanism of surface lattice oxygen of WO3 but also enhances H2 sensing response. On the other hand, new adsorption points are formed and the sensing properties are improved because the
arrangement of ions in the composite materials has been changed [44]. In addition, the microstructure and specific surface area also have certain effects on the gas sensitivity of WO3 films. According to the experimental characterization results, the hydrogen sensitivity of the porous composite film in this paper is better than that of pure WO3 film, which further indicates that enough active sites for gas adsorption were provided and the gas sensing performance was improved by the porous structures.
Conclusion Multilayer porous Pd-WO3 composite thin films had been successfully prepared by a layer-by-layer deposition method utilizing Pluronic F127 as a soft template. The material characterization results manifest that the addition of soft template F127 improves the porous structure and increases specific surface areas. 1 mol% porous Pd-WO3 composite film had better hydrogen sensitivity than other films in the hydrogen sensing test. The response magnitude of the porous Pd-WO3 composite film (S3) was 956.52e1000 ppm H2 at 250 C. The sensitivity was about 346.5 times that of pure WO3 film (S1). The special composite structure and the porous structure were discussed to explain the H2 sensing mechanism. The great H2 sensing performance can be due to the PeN heterojunction between PdO and WO3, the redistribution of ions in the composite and the improvement of porosity.
Acknowledgment This research is sponsored by the National Natural Science Foundation of China (No. 51575255). Suggestions were made for this research and the use of several types of analytical equipment was supplied to our research team by Professor Chunhai Cao at the Department of Electronic Science and Engineering of Nanjing University and Professor Chongqing Wang at College of Chemistry and Chemical of Nanjing Tech University. Fig. 13 e Schematic model of H2 sensing process (a) and energy band (b) of Pd-WO3 composite films. Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
10
international journal of hydrogen energy xxx (xxxx) xxx
references [16] [1] Mazloomi K, Gomes C. Hydrogen as an energy carrier: prospects and challenges. Renew Sustain Energy Rev 2012. https://doi.org/10.1016/j.rser.2012.02.028. [2] Najjar YSH. Hydrogen safety: the road toward green technology. Int J Hydrogen Energy 2013. https://doi.org/ 10.1016/j.ijhydene.2013.05.126. [3] Boon-Brett L, Bousek J, Black G, Moretto P, Castello P, Hu¨bert T, Banach U. Identifying performance gaps in hydrogen safety sensor technology for automotive and stationary applications. Int J Hydrogen Energy 2010. https:// doi.org/10.1016/j.ijhydene.2009.10.064. [4] Luo Y, Zhang C, Zheng B, Geng X, Debliquy M. Hydrogen sensors based on noble metal doped metal-oxide semiconductor: a review. Int J Hydrogen Energy 2017. https:// doi.org/10.1016/j.ijhydene.2017.06.066. [5] Lavanya N, Sekar C, Fazio E, Neri F, Leonardi SG, Neri G. Development of a selective hydrogen leak sensor based on chemically doped SnO2 for automotive applications. Int J Hydrogen Energy 2017. https://doi.org/10.1016/ j.ijhydene.2017.03.027. [6] Kolhe PS, Shinde AB, Kulkarni SG, Maiti N, Koinkar PM, Sonawane KM. Gas sensing performance of Al doped ZnO thin film for H2S detection. J Alloy Comp 2018. https://doi.org/ 10.1016/j.jallcom.2018.03.123. [7] Nagai D, Nishibori M, Itoh T, Kawabe T, Sato K, Shin W. Ppm level methane detection using micro-thermoelectric gas sensors with Pd/Al2O3 combustion catalyst films. Sens Actuators B Chem 2015;206:488e94. https://doi.org/10.1016/ j.snb.2014.09.059. [8] Wetchakun K, Samerjai T, Tamaekong N, Liewhiran C, Siriwong C, Kruefu V, Wisitsoraat A, Tuantranont A, Phanichphant S. Semiconducting metal oxides as sensors for environmentally hazardous gases. Sens Actuators B Chem 2011. https://doi.org/10.1016/j.snb.2011.08.032. [9] Galatsis K, Li YX, Wlodarski W, Comini E, Sberveglieri G, Cantalini C, Santucci S, Passacantando M. Comparison of single and binary oxide MoO3, TiO2 and WO3 sol-gel gas sensors. Sens Actuators B Chem 2002. https://doi.org/ 10.1016/S0925-4005(01)01072-3. [10] Naseri N, Yousefzadeh S, Daryaei E, Moshfegh AZ. Photoresponse and H2 production of topographically controlled PEG assisted Sol-gel WO3 nanocrystalline thin films. Int J Hydrogen Energy 2011. https://doi.org/10.1016/ j.ijhydene.2011.07.129. cs M, Pap AE. Gas sensitivity of sol-gel prepared [11] Taka mesoporous WO3 thin film. Procedia Eng 2016. https:// doi.org/10.1016/j.proeng.2016.11.197. [12] Rydosz A, Szkudlarek A, Ziabka M, Domanski K, Maziarz W, Pisarkiewicz T. Performance of Si-doped WO3 thin films for acetone sensing prepared by glancing angle DC magnetron sputtering. IEEE Sens J 2016. https://doi.org/10.1109/ JSEN.2015.2496212. M, Dı´az F, Borull JF, [13] Qadri MU, Annanouch FE, Aguilo Pujol MC, Llobet E. Metal decorated WO3 nanoneedles fabricated by aerosol assisted chemical vapor deposition for optical gas sensing. J Nanosci Nanotechnol 2016. https:// doi.org/10.1166/jnn.2016.12840. [14] Arfaoui A, Touihri S, Mhamdi A, Labidi A, Manoubi T. Structural, morphological, gas sensing and photocatalytic characterization of MoO3 and WO3 thin films prepared by the thermal vacuum evaporation technique. Appl Surf Sci 2015. https://doi.org/10.1016/j.apsusc.2015.09.124. [15] Teoh LG, Hon YM, Shieh J, Lai WH, Hon MH. Sensitivity properties of a novel NO2 gas sensor based on mesoporous
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
WO3 thin film. Sens Actuators B Chem 2003. https://doi.org/ 10.1016/S0925-4005(03)00528-8. Fardindoost S, Iraji zad A, Rahimi F, Ghasempour R. Pd doped WO3 films prepared by sol-gel process for hydrogen sensing. Int J Hydrogen Energy 2010. https://doi.org/10.1016/ j.ijhydene.2009.11.033. Naseri N, Kim H, Choi W, Moshfegh AZ. Implementation of Ag nanoparticle incorporated WO3 thin film photoanode for hydrogen production. Int J Hydrogen Energy 2013. https:// doi.org/10.1016/j.ijhydene.2012.11.132. Jolly Bose R, Illyaskutty N, Tan KS, Rawat RS, Matham MV, Kohler H, Mahadevan Pillai VP. Hydrogen sensors based on Pt-loaded WO3 sensing layers. EPL 2016. https://doi.org/ 10.1209/0295-5075/114/66002. Kumar N, Dvora S, k F, Duchon T, Haviar S, Capek J, Batkova M, Zeman P. Hydrogen gas sensing properties of WO3 Fialova sputter-deposited thin films enhanced by on-top deposited CuO nanoclusters. Int J Hydrogen Energy 2018. https:// doi.org/10.1016/j.ijhydene.2018.10.127. Zhang Z, Yin C, Yang L, Jia W, Zhou J, Xu H, Cao D. H2 response characteristics for solegel-derived WO3-SnO2 duallayer thin films. Ceram Int 2017. https://doi.org/10.1016/ j.ceramint.2017.02.065. Feng C, Li X, Ma J, Sun Y, Wang C, Sun P, Zheng J, Lu G. Facile synthesis and gas sensing properties of In2O3-WO3 heterojunction nanofibers. Sens Actuators B Chem 2015. https://doi.org/10.1016/j.snb.2014.12.019. Cai Z-X, Li H-Y, Yang X-N, Guo X. NO sensing by single crystalline WO3 nanowires. Sens Actuators B Chem 2015. https://doi.org/10.1016/j.snb.2015.05.036. Rahmani MB, Yaacob MH, Sabri YM. Hydrogen sensors based on 2D WO3 nanosheets prepared by anodization. Sens Actuators B Chem 2017. https://doi.org/10.1016/ j.snb.2017.05.029. Choi SJ, Chattopadhyay S, Kim JJ, Kim SJ, Tuller HL, Rutledge GC, Kim ID. Coaxial electrospinning of WO3 nanotubes functionalized with bio-inspired Pd catalysts and their superior hydrogen sensing performance. Nanoscale 2016. https://doi.org/10.1039/c5nr06611e. Kabcum S, Channei D, Tuantranont A, Wisitsoraat A, Liewhiran C, Phanichphant S. Ultra-responsive hydrogen gas sensors based on PdO nanoparticle-decorated WO3 nanorods synthesized by precipitation and impregnation methods. Sens Actuators B Chem 2016. https://doi.org/10.1016/ j.snb.2015.11.120. Li J, Liu X, Cui J, Sun J. Hydrothermal synthesis of selfassembled hierarchical tungsten oxides hollow spheres and their gas sensing properties. ACS Appl Mater Interfaces 2015. https://doi.org/10.1021/am508121p. Liu KI, Hsueh YC, Su CY, et al. Photoelectrochemical application of mesoporous TiO2/WO3 nanohoneycomb prepared by sol-gel method. Int J Hydrogen Energy 2013. https://doi.org/10.1016/j.ijhydene.2013.04.046. Tomer VK, Devi S, Malik R, Nehra SP, Duhan S. Highly sensitive and selective volatile organic amine (VOA) sensors using mesoporous WO3-SnO2 nanohybrids. Sens Actuators B Chem 2016. https://doi.org/10.1016/j.snb.2016.01.124. Wu CH, Zhu Z, Huang SY, Wu RJ. Preparation of palladiumdoped mesoporous WO3 for hydrogen gas sensors. J Alloy Comp 2019. https://doi.org/10.1016/j.jallcom.2018.10.372. Bin Z, Chenbo Y, Zili Z, Chunmin T, Liu Y. Investigation of the hydrogen response characteristics for sol-gel-derived Pddoped, Fe-doped and PEG-added SnO2 nano-thin films. Sens Actuators B Chem 2013. https://doi.org/10.1016/ j.snb.2012.12.101. Zhang Z, Yin C, Yang L, Jiang J, Guo Y. Optimizing the gas sensing characteristics of Co-doped SnO2 thin film based
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149
international journal of hydrogen energy xxx (xxxx) xxx
[32]
[33]
[34]
[35]
[36]
[37]
[38]
hydrogen sensor. J Alloy Comp 2019. https://doi.org/10.1016/ j.jallcom.2019.01.244. Yang L, Yin C, Zhang Z, Zhu B. A study of hydrogen sensing properties and microstructure for highly dispersed Pd SnO2 thin films with high response magnitude. Appl Surf Sci 2014. https://doi.org/10.1016/j.apsusc.2014.05.003. Zhang S, Yin C, Yang L, Zhang Z, Han Z. Investigation of the H2 sensing properties of multilayer mesoporous pure and Pddoped SnO2 thin film. Sens Actuators B Chem 2019. https:// doi.org/10.1016/j.snb.2018.12.051. Sadek AZ, Wlodarski W, Shin K, Kaner RB, Kalantar-zadeh K. A polyaniline/WO3 nanofiber composite-based ZnO/64 YX LiNbO3 SAW hydrogen gas sensor. Synth Met 2008. https:// doi.org/10.1016/j.synthmet.2007.11.008. Yang M, Sun Y, Zhang D, Jiang D. Using Pd/WO3 composite thin films as sensing materials for optical fiber hydrogen sensors. Sens Actuators B Chem 2010;143(2):750e3. https:// doi.org/10.1016/j.snb.2009.10.017. Amani E, Khojier K, Zoriasatain S. Improving the hydrogen gas sensitivity of WO3 thin films by modifying the deposition angle and thickness of different promoter layers. Int J Hydrogen Energy 2017. https://doi.org/10.1016/ j.ijhydene.2017.10.027. Shi J, Cheng Z, Gao L, Zhang Y, Xu J, Zhao H. Facile synthesis of reduced graphene oxide/hexagonal WO3 nanosheets composites with enhanced H2S sensing properties. Sens Actuators B Chem 2016;230:736e45. https://doi.org/10.1016/ j.snb.2016.02.134. Zhang H, Wang Y, Zhu X, Li Y, Cai W. Bilayer Au nanoparticle-decorated WO3 porous thin films: on-chip
[39]
[40]
[41]
[42]
[43]
[44]
11
fabrication and enhanced NO2 gas sensing performances with high selectivity. Sens Actuators B Chem 2019. https:// doi.org/10.1016/j.snb.2018.10.065. Teimoori F, Khojier K, Dehnavi NZ. On the dependence of H2 gas sensitivity of ZnO thin films on film thickness. Procedia Mater Sci 2015. https://doi.org/10.1016/j.mspro.2015.11.061. Kadir RA, Zhang W, Wang Y, Ou JZ, Wlodarski W, O'Mullane AP, Bryant G, Taylor M, Kalantar-Zadeh K. Anodized nanoporous WO3 Schottky contact structures for hydrogen and ethanol sensing. J Mater Chem 2015. https:// doi.org/10.1039/c4ta06286h. Hua Z, Yuasa M, Kida T, Yamazoe N, Shimanoe K. H2 sensing mechanism of Pd-loaded WO3 nanoparticle gas sensors. Chem Lett 2014. https://doi.org/10.1246/cl.140396. Poongodi S, Kumar PS, Mangalaraj D, Ponpandian N, Meena P, Masuda Y, Lee C. Electrodeposition of WO3 nanostructured thin films for electrochromic and H2S gas sensor applications. J Alloy Comp 2017. https://doi.org/ 10.1016/j.jallcom.2017.05.122. Mirzaei A, Yousefi HR, Falsafi F, Bonyani M, Lee JH, Kim JH, Kim HW, Kim SS. An overview on how Pd on resistive-based nanomaterial gas sensors can enhance response toward hydrogen gas. Int J Hydrogen Energy 2019. https://doi.org/ 10.1016/j.ijhydene.2019.05.180. Gerasimov GN, Gromov VF, Ilegbusi OJ, Trakhtenberg LI. The mechanisms of sensory phenomena in binary metal-oxide nanocomposites. Sens Actuators B Chem 2017. https:// doi.org/10.1016/j.snb.2016.09.007.
Please cite this article as: Han Z et al., Multilayer porous Pd-WO3 composite thin films prepared by sol-gel process for hydrogen sensing, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.149