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Study of Physical and Chemical Processes of H2 Sensing of Pt-Coated WO3 Nanowire Films Lian Feng Zhu, Jun Cong She, Jian Yi Luo, Shao Zhi Deng,* Jun Chen, and Ning Sheng Xu* State Key Lab of Optoelectronic Materials and Technologies, Guangdong ProVince Key Lab of Display Material and Technology, School of Physics and Engineering, Sun Yat-sen UniVersity, Guangzhou 510275, People’s Republic of China ReceiVed: July 13, 2010; ReVised Manuscript ReceiVed: August 9, 2010
H2 sensors able to operate at room temperature are very important for safe detection of H2 leakage. We report the large electrical response to H2 of Pt-coated WO3 (Pt-WO3) nanowire films without the need of using an external heater. More important, hydrogen sensing processes have been investigated under various conditions, including in air, vacuum filled with pure gas, and a mixture of H2 and other gases. This is carried out with both electrical and optical methods. The evidence shows that hydrogen will inject into the nanowire and create oxygen vacancy, in addition to reducing adsorbed oxygen at the surface, as is often recognized. It is experimentally demonstrated that the increase of electrical conductivity resulting from the reaction with hydrogen is hampered by coadsorption of O2, while N2 has no such effect. A model has been developed to combine these new findings to give a clearer understanding of the mechanism responsible for H2 sensing behavior. Introduction
Experimental Section
Chemical sensors based on tungsten oxides have received more and more attention as they can monitor low concentrations of various kinds of gases. They are reported to be favorable choices for detecting NO2, H2S, O3, NH3, and liquefied petroleum gas when operated at elevated temperatures of 200-300 °C.1-5 It is unique that a layer of WO3 coated with catalyst (Pt or Pd) can change its optical6-10 and electrical11-13 properties upon exposure to H2. Through re-exposure to air or oxygen, their original properties can be restored. This process can take place at room temperature. Such a phenomenon is very useful in the application of hydrogen-safe detection. Nakagawa et al. carried out a pioneering study of the chemiresister-type Pt-doped WO3 sensor operating at room temperature.11 They found that the conductivity of the sensor increased greatly when exposed to H2 at the temperature of 13 °C. A similar conductivity increase of Pt-modified WO3 induced by H2 at room temperature was also reported by Shanak et al.12 and Chen et al.13 All of these authors suggested that the increase of conductivity might be involved in double injection of H+ and electron into the Pt-doped WO3 film,11-13 which is widely used to explain the gasochromic effect of WO3 induced by H2.14-22
The synthesis of WO3 nanowires was carried out following the technique that we have developed, and the detailed description may be found elsewhere.23-26 A thin layer of Pt nanoparticles as the catalyst was deposited onto the surfaces of WO3 nanowires by sputtering. The nanowires were analyzed by scanning electron microscopy (SEM, Raith e-line), X-ray diffraction spectroscopy (XRD, D/max 2200 vpc apparatus with Cu KR radiation), transmission electron microscopy (TEM, Phillips EM200 operated at 160 KV), and energy dispersive X-ray spectroscopy (EDX, installed in Phillips EM200). In the present study, a device structure was designed, as illustrated in Figure 1a. To carry out electrical measurements, the Pt-WO3 nanowire film was fabricated on a 1.5 × 1.5 mm2 ceramic substrate with interdigitated electrodes, and the spacing between two adjacent electrodes was 50 µm. The change in electrical current of the nanowire film was monitored using a picoammeter (Keithley 6487), with an applied voltage of 1 V. The hydrogen sensing behaviors were investigated both in atmosphere and in vacuum. When the study was carried out in atmosphere, the sample was located in a test cell (50 mL volume) with gas inlet and outlet ports. The gas concentration was controlled by the gas flow-through method. Pure hydrogen and reference gas (dry air or pure N2) could flow through the test cell simultaneously. Both flow rates of hydrogen and reference gas could be regulated. Thus, the concentration of H2 was determined by the flow rate ratio of pure hydrogen and the total gas mixture. The total flow rate of the gas mixture was kept constant at 500 mL/min during the whole test. When the study was carried out in a vacuum chamber, the pressure of the pure gases (H2 and O2) could be controlled strictly; thus, effects caused by H2 or O2 molecule adsorption could be finely investigated. The sample had been pretreated with a surface clean process each time before electrical measurement was carried out. This was done by UV irradiation and annealing at 150 °C for 30 min in vacuum (about 10-3 Pa), followed by cooling of the sample to room temperature (23 °C).
Despite that a number of studies have been done on the roomtemperature electric response of hydrogen gas sensors using WO3, the hydrogen sensing mechanism in fact has not been systematically investigated. A number of key questions remain unanswered. Double injection of H+ and electron gives no net charge carriers; then, how does the conductivity of WO3 increase? Also, a number of gases often coexist during hydrogen sensing in air; therefore, how do they affect the sensing process? In this paper, we report on our study of these issues and propose a physical model based on our new findings. * To whom correspondence should be addressed. E-mail: stsdsz@ mail.sysu.edu.cn (S.Z.D.);
[email protected] (N.S.X.). Tel: 86-208411-0916. Fax: 86-20- 8403-7855.
10.1021/jp106460w 2010 American Chemical Society Published on Web 08/23/2010
H2 Sensing of Pt-Coated WO3 Nanowire Films
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Figure 2. (a) Typical TEM image of a single Pt-WO3 nanowire coated with Pt nanoparticles, and the area denoted by a white dash ring is enlarged in (c) as the HRTEM image. (b) EDS spectrum of the Pt-WO3 nanowire.
Figure 1. (a) Sketch of the device structure for electrical measurement of Pt-WO3 nanowire films. (b) Typical SEM image of the as-grown WO3 nanowire film. (c) Typical XRD spectrum of the as-grown WO3 nanowire film.
Optical changes due to exposure to H2 were also investigated. In this case, the Pt-WO3 nanowire film fabricated on a quartz substrate was exposed to H2 of different concentrations (10, 30, and 50%, air diluted) for 15 min; then, their transmission spectra were recorded immediately in atmosphere by using a UV3101PC (SHIMADZU) double-beam spectrophotometer. Additionally, electrical changes of such a sample caused by 10, 30, and 50% H2 diluted by air were recorded. During measurement, the applied voltage was fixed at 1 V. Results and Discussion The typical SEM image shows that the film is composed of quasi-aligned nanowires with an average diameter of 50 nm and length of about 2 µm (Figure 1b). The nanowires overlap each other and form a network structure. Such nanowire networks provide large surface area in three-dimensional (3D) space, so that each nanowire in the network can, in principle, be exposed to the gas analyte thoroughly, unlike thin film of pressed grain material, in which only the surface of the upper layer can be exposed to gas species. In addition, the nanowires have a large surface to volume ratio, which is favorable for gas adsorption. The XRD spectrum reveals that the nanowires are highly crystalline (Figure 1c). The diffraction peaks are identified to belong to the monoclinic WO3 with lattice parameters of a ) 7.297 Å, b ) 7.539 Å, and c ) 7.688 Å and the ac angle of 90.91° (JCPDS: 43-1035). The typical TEM image of a single Pt-WO3 nanowire shows that Pt nanoparticles are
Figure 3. Responses of the Pt-WO3 nanowire film to different concentrations of H2 diluted by air.
uniformly coated around the surface (Figure 2a). EDS analysis found that, with some weak signals from the copper TEM grid (Cu and C) and the Cr sample holder (Cr), the main signals are from W, O, and Pt (Figure 2b). The high-resolution TEM image shows that Pt nanoparticles are about 3-5 nm in diameter (Figure 2c). As the catalyst particle size decreases to several nanometers, the dissociation rate of H2 molecules will greatly increase.27,28 The hydrogen sensing process of the Pt-WO3 nanowire film was first studied in atmosphere. As may be seen in Figure 3, currents of the nanowire film sample increased when exposed to various concentrations of H2 diluted by air. The sample was able to detect H2 down to 0.1%. The response time of the electrical current signal was typically 15 min, which is defined as the time for reaching 90% of the full response after introduction of H2 gas. According to the previous reports,11-13 the electrical current change of the Pt-doped WO3 thin film involved H+ and electron double injection. However, this cannot explain clearly the physical mechanism of the electrical current response since it seems that the injection gives no net charge carriers. In order to study the physical origin of the electrical response, we investigated optical changes of the nanowire sample by transmission spectra. The sample was exposed to H2 diluted by air of different concentrations (10, 30, and 50%) for
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Figure 4. (a) Light transmittance spectra of the Pt-WO3 nanowire film before and after being exposed to 10, 30, and 50% H2 diluted by air. (b) Current changes of the sample upon exposure to 10, 30, and 50% H2 diluted by air.
Figure 5. (a) Response of the Pt-WO3 nanowire film to sequential exposure of pure H2 at 0.05 Pa, H2 diluted by 5 Pa of oxygen, and pure O2 at 5 Pa. (b) Response of the Pt-WO3 nanowire film exposed to 0.5% H2 diluted by air and by N2.
15 min, and the changes of their transmission spectra were recorded. As shown in Figure 4a, a very high percentage of light of 800-2000 nm in wavelength can transmit through the Pt-WO3 nanowire film before being exposed to H2. The photon energy of the light is in the range of 1.6-0.6 eV. Their energies are not enough to excite the electrons from the valence band to the conduction band of WO3 (the electronic band gap of WO3 is about 2.7 eV), and therefore, they will not be adsorbed. However, as the concentration of H2 is increased, light absorption is enhanced. This is due to the fact that the energy band structure of the WO3 nanowire films is changed, as reported by us earlier;29 thus, photons of relatively low energies begin to be adsorbed. We also measured the electrical current changes of such a sample upon exposure to 10, 30, and 50% H2 diluted by air. As can be seen in Figure 4b, great increase in the current flowing through the Pt-WO3 nanowire film is observed. Note that the current increases about 104 times when exposed to 50% H2 diluted by air. It is clear that the above optical and electrical changes of the WO3 nanowire film were related to the exposure of H2. In fact, the above experiments have omitted the coadsorption of O2 during a real H2 sensing process. The effect of O2 coadsorption was studied by comparing the H2 sensing behaviors with or without O2. First, electrical measurement of the Pt-WO3 nanowire film was carried out in a vacuum chamber, where quantities of pure gases could be strictly controlled. As shown in Figure 5a, the electrical current of the sample rises when pure H2 of 0.05 Pa is introduced, but the process is slowed down right after H2 is diluted by 5 Pa of O2. The current stops increasing when the mixture of H2 and O2 is evacuated. Also, the current decreases when the sample is exposed to pure O2 of 5 Pa. Second, the study was carried out in atmosphere, comparing the sensing behaviors of the sample exposed to H2
diluted by air or by N2. As shown in Figure 5b, the increase of the electrical current of the sample exposed to H2 diluted by N2 is 5 orders of magnitude larger than that diluted by air. This further reinforces the strong effect of O2 on the H2 sensing process. The effect of O2 absorption decreases the electrical current, but N2 has no effect. The results in Figure 5a and b reveal that during hydrogen sensing, surface reduction is greatly hindered by coabsorption of O2. In order to eliminate the coabsorption of oxygen during the hydrogen sensing process, we further carried out the following experiments. The electrical measurement was done in vacuum, and the sample was sequentially exposed to pure H2 and O2. The nanowire film in vacuum (base pressure of 10-3 Pa) was first exposed to pure H2 of a specified pressure, that is, 0.005, 0.01, 0.05, and 0.1 Pa for 5 min, and then, H2 was evacuated. Subsequently, O2 of 5 Pa was introduced and lasted for 5 min. Figure 6a shows the current changes obtained when sequentially exposing the Pt-WO3 nanowire film to H2 and O2 of specified pressures at room temperature. On the basis of these data, the electrical response of the sample on each exposure can be calculated. The electrical response is expressed as ∆R/Ro ) (Ro - Rg)/Ro. Ro is the resistance of the sample before hydrogen exposure. Rg is defined as the resistance of the sample right after H2 exposure for 5 min. This normalized expression can evaluate the extent of resistance decrease for samples under different conditions. As shown in Figure 6b, the relation of electrical response against the pressure of hydrogen is plotted. In the environment where only H2 is presented, the electrical response of the Pt-WO3 nanowire film increases with the pressure of hydrogen at a ratio of 2.729 × 10-1. However, as shown in Figure 6c, when there is coadsorption of O2 during the H2 sensing process, the ratio of electrical response to the pressure of H2 (1.313 × 10-4) is 3 orders of magnitude smaller
H2 Sensing of Pt-Coated WO3 Nanowire Films
Figure 6. (a) Current changes when sequentially exposing the Pt-WO3 nanowire film to H2 and O2 of specified pressures at room temperature. (b) Plot of electrical response as a function of pressure of pure hydrogen. (c) Plot of electrical response as a function of the partial pressure of hydrogen in air.
than that determined for pure H2. The plots in Figure 6c are based on the data shown in Figure 3. The above results reveal that during H2 sensing, surface reduction is much more significant without coadsorption of O2. In the following, we propose a model to give some detailed description of the physical and chemical process, which is a development of the early one proposed by Nakagawa et al.11 As shown in Figure 7a (I), when H2 gas is let in, the hydrogen molecules will be dissociated into H atoms on the surface of the catalyst (Pt).30,31 Then, the hydrogen atoms will spill over onto and diffuse along the surface of the WO3 nanowire. The dissociation of the H2 molecule into H atoms is a chemisorption process. The chemisorption of hydrogen on the surface is exothermic, and it provides small activation energy for the subsequent surface diffusion.31 The H atom will hop on the surface by repeatedly breaking and forming equivalent bonds with lattice oxygen ions of WO3 at the surface, as illustrated in Figure 7a (II), where two half MO6 octahedron blocks at the surface of WO3 are also shown (an MO6 octahedron block is composed of a centered tungsten ion and six surrounded oxygen
J. Phys. Chem. C, Vol. 114, No. 36, 2010 15507 ions). During the surface diffusion of H atoms, they will reduce some charged oxygen (O2-), which may have previously adsorbed chemically with the WO3 nanowire in air (Figure 7a (III)). Then, the charged oxygen will be removed in the manner of H2O desorption. The electrons will be released and transfer back into the conduction band. This process will lead to increase of the electrical conductivity of WO3 nanowire films. On the other hand, as shown in Figure 7b (I), the H atom (composed of H+ and an electron) will also diffuse into the surface layer of WO3. When two H atoms approach an oxygen ion of WO3, the bond between the O ion and W ion will be weakened. A localized H2O molecule can form, and consequently, the oxygen ion will be shifted from its origin lattice site, and an oxygen vacancy will be formed. This result has been confirmed in our previous study of the gasochromic effect of the WO3 nanowire films.23 Upon formation of the localized H2O, two electrons are trapped in the oxygen vacancy. They may become mobile by receiving photons of small energies (Figure 4a) or by thermal activation (Figure 7b (II)). The energy band associated with oxygen vacancies is centered at 1.0 eV below the bottom of the conduction band, as shown in Figure 7b.23 The trapped electrons can be excited into the conduction band when absorbing photons to become free electrons or can hop from one vacancy to another. When the trapped electrons are released, the carrier density of the WO3 nanowire film increases, leading to an increase of the electrical current. It is reasonable to believe that a higher concentration of H2 will lead to a higher density of defect states. Thus, the electrical response of Pt-WO3 nanowire films can be enhanced as the concentration of H2 increases (Figure 6b). On the other hand, coadsorption of O2 molecules in air will simultaneously happen during H2 sensing, as shown in Figure 7a (IV). The oxygen molecules will adsorb on the surface of Pt particles. They will oxidize the H atoms and form H2O molecules on the surface. These surface water molecules will desorb from the surface. Due to the competing absorption of O2, parts of the disassociated H atoms are consumed. Therefore, the response of the Pt-WO3 nanowire film to H2 is weakened (Figure 5a). Briefly, the above processes can be described by the following four processes
2H + O-(S) f H2O(S) + e
(1)
2H + O2-(L) f H2O(L) + Vo**
(2)
Vo** f Vo + 2e
(3)
4H + O2(S) f 2H2O(S)
(4)
Here, L refers to lattice of WO3, S refers to surface, the asterisk (*) refers to the electron, and Vo is the oxygen vacancy. Process 1 describes the reduction of surface-charged oxygen, which contributes to the increase of the electrical conduction of the nanowire films. Process 2 refers to the formation of a localized H2O molecule and oxygen vacancy. This reaction can either happen at the surface or in the surface layer of the WO3 nanowire. Here, we suggest that the new product responsible for the reaction of H atoms and WO3 may be denoted as WO3-x · xH2O to indicate the coexistence of localized water molecules and oxygen vacancies. Process 3 leads to electrical conduction of the nanowire film increase. Process 4 refers to
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Figure 7. (a) Illustrations of the H2 sensing model of the Pt-WO3 nanowire in air. (b) The energy band structure of the H-injected WO3. EC and EV denote the bottom of the conduction band and the top of the valence band, respectively. Also, the formation of a localized H2O molecule and oxygen vacancy in WO3 is illustrated in the right of (b). (c) Illustration of the Pt-WO3 nanowire recovering in air.
the reaction of the H atom and absorbed O2 molecule on the surface. It will reduce the density of the surface H atoms. Processes of the sample recovering in air are illustrated in Figure 7c and described by processes 4-8. When the inlet of H2 is stopped, hydrogen atoms remaining at the surface will react with the oxygen spilling over (Figure 7c (I) and process 4). As the density of the surface oxygen increases and with the resultant decrease of density of surface H atoms, the localized H2O molecules in the body of WO3 might be decomposed (process 5). The H atoms will diffuse back to the surface (Figure 7c (II) and process 6), while the oxygen atoms will occupy the vacancy and capture two electrons, forming new bonds with W ions (Figure 7c (III) and process 7). Finally, oxygen molecules in the air will adsorb chemically with the WO3, forming charged oxygen at the surface (process 8). Processes 7 and 8 will reduce the carrier density of the WO3 nanowire; thus, the electrical current of the WO3 nanowire film decreases (Figure 6a).
H2O(L) f 2H(L) + O(L)
originates from at least two processes, (i) forming of water molecules with absorbed oxygen and (ii) forming of water molecules with the oxygen of WO3 of nanowires. During H2 sensing in air, coadsorption of oxygen weakens the electrical response of Pt-WO3. These findings deepen the understanding of the physical and chemical mechanism underlying the H2 sensing behaviors of Pt-WO3. The model is developed to describe the H2 sensing process. Acknowledgment. The authors gratefully acknowledge the financial support of the project from the National Natural Science Foundation of China (Grant No. U0634002, 50672135, 50725206), Science and Technology Ministry of China (National Basic Research Program of China: Grant No. 2007CB935501, 2010CB327703, and 2003CB314701), the Science and Technology Department of Guangdong Province, the Economic and Information Industry Commission of Guangdong Province, and the Science & Technology and Information Department of Guangzhou City.
(5) References and Notes
2H(L) f 2H(S)
(6)
O(L) + 2e + Vo f O2-(L)
(7)
O2(S) + 2e f 2O-(S)
(8)
Conclusion The details of physical and chemical processes of hydrogen sensing of the Pt-coated WO3 (Pt-WO3) nanowire film have been investigated. The significant increase of the electrical conduction current of the nanowire film upon exposure to H2
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