J. Phys. Chem. C 2009, 113, 15877–15881
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Evidence of Localized Water Molecules and Their Role in the Gasochromic Effect of WO3 Nanowire Films Jian Yi Luo, Shao Zhi Deng, Yu Ting Tao, Fu Li Zhao, Lian Feng Zhu, Li Gong, Jian Chen, and Ning Sheng Xu* State Key Laboratory of Optoelectronic Materials and Technologies and Guangdong ProVince Key Laboratory for Display Material and Technology, School of Physics and Engineering, Sun Yat-sen UniVersity, Guangzhou 510275, China ReceiVed: April 19, 2009; ReVised Manuscript ReceiVed: July 2, 2009
The gasochromic effect has important applications in energy savings, for example, in a smart window. This study reveals evidence of the existence of localized water molecules in colored WO3 nanowires and their important role in the gasochromic effect. Such water molecules can be moved out from the nanowires. The coexistence of the water molecules and oxygen vacancies in nanowires leads to a defect band in the band gap. Coloration is attributed to absorptions of photons involving this defect band. These findings deepen the understanding of the physical mechanism underlying the gasochromic effect of WO3. Experimental Section
Introduction There have been continuous efforts in exploring the chromogenic phenomena observed with tungsten oxides in potential device applications, such as gas sensors, displays, solar energy cells, optical switches,1-5 and, recently, new memory devices.6 However, the fundamental understanding of the gasochromic effect of WO3 remains controversial,5,7 mainly about what changes may happen to its material structure and electronic property during coloration and bleaching. A number of physical models have been proposed to explain the gasochromic effect of WO3. Geory et al.8,9 suggested that the absorbed H ions will react with the surface oxygen of porous WO3 film so that H2O and oxygen vacancies are created. The oxygen vacancy then diffuses into the body of the material and leads to the coloration. The oxygen vacancy will be filled with a foreign oxygen atom from air during self-bleaching. However, Lee et al.10 did not detect such foreign oxygen atoms in their dedicated experiment, in which isotopic heavy oxygen (18O) was used during bleaching. Thus, Lee supported another widely accepted model,1,11 based on the double injection of hydrogen ions and electrons during the coloration. More recently, Deb5 et al. suggested that the origin of the color center in amorphous WO3 films is due to the injected electrons trapped by oxygen vacancies, but the origin of oxygen vacancies was not clearly identified. Our previous work12 demonstrated that the double injection can change the valence of tungsten ions from W6+ to W5+ and the lattice structure from the monoclinic phase of WO3 to the tetragonal phase of HxWO3. In this paper, we report evidence revealing that the H2O molecules may be created in the body of the colored WO3 nanowires, and the formation of a H2O molecule will locally shift the O ion from its original lattice position, thus leading to the creation of an oxygen vacancy. The resultant electronic structure can be used to explain the photon absorption process. * To whom correspondence should be addressed. E-mail: stsdsz@ mail.sysu.edu.cn (S.Z.D),
[email protected] (N.S.X.).
We used films of crystalline nanowires of WO3 prepared on both a silicon substrate and a quartz substrate as our samples, and the detailed description of their synthesis process has been reported elsewhere.12 A thin layer of Pt nanoparticles as catalyst was deposited onto the surfaces of WO3 nanowires by sputtering, which was characterized by high-resolution transmission electron microscopy (HRTEM). The study of the gasochromic effect of nanowire films on a quartz substrate was carried out in a quartz box, and the states of coloring and bleaching were observed by introducing pure H2 gas and dry air, respectively. Their transmission and absorption spectra were recorded by using a UV-3101PC (SHIMADZU) double-beam spectrophotometer. The changes in the material structure of nanowires during coloration were studied by using X-ray diffraction (XRD) spectroscopy, micro-Raman spectroscopy, and a thermogravimetric (TG) apparatus (model STA 449C/6/G Jupiter) connected with a commercial mass spectrometer. To study the change in electronic structure after coloration, X-ray photoelectron spectroscopy (XPS) was used to detect the occupied electronic states around the Fermi level of both colored and bleached samples, where the catalyst Pt layers were deposited beneath the nanowire films. Results and Discussion The typical SEM images (as shown in Figure 1a and the inset) show that the film is composed of quasi-aligned nanowires with an average diameter of 50 nm and a length of about 2 µm. Figure 1b shows the Raman spectrum of the prepared film on the quartz substrate that contains the characteristic Raman peaks of the monoclinic phase of WO3. The typical TEM and HRTEM images (Figure 2) of a single nanowire show that Pt nanoparticles with the average size of 3 nm are uniformly coated around the surface of the nanowire. When such nanostructures are exposed to H2 gas, the absorbed H2 molecules on the Pt particles will be dissociated into H atoms or ions. These H species may then transfer from the surface of the Pt particle to the surface of the WO3 nanowire and subsequently diffuse into the nanowire body, resulting in the coloration of the film.13,14 The dissociation
10.1021/jp903581s CCC: $40.75 2009 American Chemical Society Published on Web 08/18/2009
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Figure 1. (a) Typical top-view SEM image. Inset: cross-sectional view SEM image of WO3 nanowire film. (b) Raman spectrum of WO3 nanowire film on the quartz substrate.
rate of H2 molecules will greatly increase as the catalyst particle size decreases to several nanometers.15,16 If the density of the particle is high, for example, as shown in Figure 2, H species may cover the entire surface of a nanowire.17 Thus, our Ptcoated WO3 nanowire films are expected to have better performances, for example, higher contrast and faster responding time of coloration in the gasochromic effect, as compared with those of conventional WO3 thin films. The typical photographs of the bleached and colored WO3 nanowire films on quartz substrates are shown in Figure 3a,b, respectively, and their corresponding transmission spectra are shown in Figure 3c. The coloration contrast may be determined by calculating an optical density change from the bleached state to the colored state, as defined below18-21
∆O.D.(λ) ) log(T0(λ)/Tmin(λ))
(1)
where T0 and Tmin represent the optical transmission of the film at the bleached state and the minimum transmission reached during coloration, respectively. The optical density change, 4O.D., is a function of the wavelength of transmitting light, and its minimum value of 0.2 just appears at 1.95 eV (633 nm) for our sample. However, the ∆O.D. of our sample is higher
Figure 3. Photographs of WO3 nanowire film on the quartz substrate at the bleached state (a) and the colored state (b). The corresponding transmission spectra (c) and absorption spectra (d) of WO3 nanowire film at the colored state and the bleached state. (e) The relative absorption spectrum by subtracting the absorption spectrum of the bleached state from that of the colored state and its two-peak Gauss fitting curves.
than 0.75 at 1.03 eV (1200 nm), which has been better than most of the amorphous and crystalline WO3 films.18-21 To highlight the change brought about by coloration, we obtained the relative absorption spectrum22 (shown in Figure 3e) by subtracting the absorption spectrum of the bleached state from that of the colored state; both of them are shown in Figure 3d, which were determined by the approximate expression of R(λ) ) ln{[1 - R(λ)]/T(λ)}. Here, R(λ) and R(λ) are the absorption coefficient and reflectance intensity, respectively. It should be noted that the intensities of the reflectance spectra (see Figure s1 in the Supporting Information) of the nanowire film at the bleached and colored states are very weak (less than 5%) and the difference is small before and after coloration. Thus, R(λ), in the above expression, is ignored. There are two wide absorption bands in the relative absorption spectrum, including the near-infrared absorption band centered at 1.0 eV (1240 nm) and the visible-light absorption band centered at 2.57 eV (480 nm). To study the mechanism responsible for the gasochromic effect, we first studied the structural change by comparing the
Figure 2. Typical TEM image (a) of a single nanowire coated by Pt nanoparticles and its HRTEM image (b) of the area (denoted by a white dashed ring) in (a).
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Figure 4. (a) The XRD spectra of the WO3 nanowire film at the bleached state and the colored state. (b) The evolvement of the Raman spectra recorded during self-bleaching; curves a-e are obtained in the sequence of time during the bleaching. (c) The fitting results of the Raman spectra of curves b-d of (b) from up to down, respectively. (d) An illustration showing how a localized water molecule and an oxygen vacancy are created during coloration.
XRD spectra obtained in the bleached and colored states. As may be seen from Figure 4a, the lattice structure of the nanowires is only the monoclinic phase of WO3 before coloration and has, additionally, the tetragonal phase of hydrogen tungsten oxide (H0.23WO3) after. This agrees with our previous work.12 We further studied the changes of the W-O vibration modes by monitoring the evolution of Raman spectra (Figure 4b) during bleaching. We slowed down the bleaching process by controlling the amount of oxygen to react with the sample. The changes of Raman modes (enlarged in Figure 4c) may be seen as below: (i) Two new modes at 664 and 180 cm-1 are observed, and the mode at 714 cm-1 is very weak at its colored state, compared with those at its bleached state. (ii) The mode at 810 cm-1 at the colored state shifts back to the mode at 802 cm-1 during bleaching. According to the previous reports,23,24 the Raman modes at 714 and 802 cm-1 can be assigned to the W-O stretching vibrations, which strongly depend on the lengths of the W-O bonds in pure monoclinic WO3. The Raman mode at 664 cm-1 can be assigned to the W-O stretching vibration of the bridging oxygen in the residual hydrated tungsten oxide (WO3 · H2O), as already reported by the other authors, who studied the WO3 nanostructure fabricated by a low-temperature sol-gel method.25-27 Another new mode at 180 cm-1 might be assigned to the W5+-W5+ stretching vibration with an oxygen vacancy because 180 cm-1 is close to the theoretical value of 200 cm-1 of the W5+-W5+ stretching vibration with the oxygen vacancy predicted by the first-principles approach calculation.28 Accordingly, both water molecules and oxygen vacancies are seen to coexist in the colored WO3 nanowires. We propose the following to explain the above findings from the Raman spectra, using the illustration of Figure 4d. Two injected hydrogen ions will react with an oxygen ion at the c axis of WO3 (denoted as O (1) in (I) of Figure 3d), and consequently, a water molecule forms ((II) of Figure 4d). This causes a shift of the oxygen ion from its original position, resulting in the creation of an oxygen vacancy, and weakens the W-O (1) bonds ((III) of Figure 4d). Thus, the tungsten ion relaxes toward the opposite axial direction and this results in
shorter W-O (3) bonds and longer W-O (1) and W-O (2) bonds. Finally, the long W-O (1) bond might be easily broken by infrared photons or by induced phonons, and thus, the H2O molecule around might be released, leaving alone an oxygen vacancy. This process is similar to the laser coloration of WO3 · H2O.25 Therefore, we suggest that the new structure responsible for coloration in the gasochromic effect of WO3 nanowires may be denoted as WO3-x · xH2O to indicate the coexistence of localized water molecules and oxygen vacancies. We further designed the following experiment to see if one can remove the water molecules from the nanowires. This was carried out in the vacuum chamber of the thermogravimetric apparatus (model STA 449C/6/G Jupiter) connected with a commercial mass spectrometer flowing with protecting argon gas. As indicated in Figure 5a, we first preheated the sample at 200 °C for 6 h to eliminate absorbed water. We then let the sample temperature come down to 50 °C, at which the sample reacted with H2 gas for 20 min. This allowed the occurrence of coloration of the sample. H2 gas was then pumped out and argon gas was inlet. The sample was then gradually heated to 400 °C and kept at 400 °C for 30 min. Simultaneously, the signal of released water molecules during the whole procedure was recorded by the mass spectrometer. From Figure 5a, during the preheating, the ion current of H2O decreased gradually to a low point before the inlet of H2 gas. Further, Figure 5b shows that the ion current of H2O increases obviously during the input of H2 gas and decreases after turning off the input of H2 gas until the beginning of the second heating. During the early period of ∼40 min of the second heating, the ion current of H2O undergoes a small increase (∼10 min) and then a much slower decrease than that observed before the second heating begins. After they cooled back to room temperature, the nanowire films were found to turn to a deep blue color. This colored state remained stable even after being exposed to air, as evident from the photograph (right part) shown in the inset of Figure 5a, but the coloration did not occur to the part without the Pt coating of the same WO3 nanowire sample (left part). It should be noted here that, without the second heating of up to 400 °C, the colored state will change in several hours back to the bleached state.
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Figure 5. (a) The sample temperature (red) and the ion current of H2O (black) recorded during the procedure of the MSA experiment described in the text. Inset: The photograph showing the colors of the sample. The left part corresponds to the area without Pt coating (yellow) and the right part with Pt coating (deep blue) of the same sample after the MSA experiment. (b) Enlarged section of the dashed rectangle of (a) highlighting the detailed changes in the intensity of the ion current of released water. (c) An illustration showing how water molecules are released from a nanowire coated with Pt particles after the input of H2 gas.
Figure 6. (a) The valence-band XPS spectra of the WO3 nanowire films at the colored state and bleached state. (b) The top of the valence bands and Fermi levels at the colored state and bleached state. (c) Illustration of the energy band structure of WO3-x · xH2O. EC, EV, and EF denote the bottom of the conduction band, the top of the valence band, and the Fermi level, respectively.
An illustration of surface chemical reactions is given in Figure 5c to help one understand the origins of water molecules. As illustrated in (I) of Figure 5c, absorbed oxygen species exist on the surfaces of Pt particles and nanowires.29-33 When the H2 gas is inlet, it will be dissociated into hydrogen species14,34 (II) (Figure 5c), which may subsequently react with the absorbed oxygen species to form H2O molecules. They may be released from the surfaces because the sample was at 50 °C. The hydrogen species may also undergo a reaction with the oxygen atom of WO3 to form localized water molecules, as illustrated in Figure 4d. These may be removed by high-temperature heating, as we did in the second heating ((III) in Figure 5c). If
the removal of these molecules was not occurring, the coloration should not be retained after the sample was exposed to air again. More experimental studies are required to obtain further direct evidence. Now, let us consider what effect the structural change has on the electronic property of the material. We studied the occupied electronic states around the Fermi level of both colored and bleached samples using X-ray photoelectron spectroscopy (XPS). To eliminate the effect of the Pt coating on the XPS spectra, we deposited Pt beneath the nanowire film. As shown in Figure 6a, an additional sub-band near the Fermi level (binding energy ) 0) appears in the valence-band XPS spectrum
Gasochromic Effect of WO3 Nanowire Films of the colored WO3 nanowires. Additionally, as shown in Figure 6b, the separation between the top of the valence band and the Fermi level at the colored state decreases to 2.0 eV, compared with 2.50 eV at the bleached state. We now can explain how the optical absorption occurs during coloration, with reference to Figure 6c. First of all, the electronic band gap of WO3-x · xH2O is assumed to be 2.7 eV in the rigidband approximation; but, in principle, the corresponding optical gaps are smaller due to the extended states resulting from defects in nanowires. Defect states like surface states will be significant in the nanostructures. Thus, two small tails above Ev and below EC extending into the band gap appear due to the Anderson localization effect. In addition, the separation between the experimentally determined Fermi level (EF) and the assumed bottom of the conduction band (EC) at the colored state is around 0.7 eV. Because the filled states in the defect band must be below the Fermi level, the electrons in these states will be excited to the states above the bottom of the conduction band when they absorb a photon with energies larger than 0.7 eV (e.g., hν1 in Figure 6c). This explains why the peak energy of near-infrared photons of absorption is centered at about 1.0 eV in the relative absorption spectrum (as shown in Figure 3e). Finally, two small tails above Ev and below EC will expand after coloration due to the higher degree of structural disorder in the colored nanowires. This allows an electronic transition to occur when absorbing a photon with energies much below 2.7 eV (e.g., hν2 in Figure 6c). This explains why the peak energy of the visible-light absorption is centered at 2.57 eV (480 nm). Thus, coloration is attributed to both absorptions of photons involving both the defect band in the band gap and the two tails of the valence and conduction bands. Conclusion In conclusion, we have carried out various experiments to show the existence of localized water molecules in colored WO3 nanowires and their important role in the gasochromic effect. The injection of hydrogen ions into nanowires reacts with an oxygen ion to form water molecules, and this thus creates oxygen vacancies. Such water molecules can be moved out from the nanowires by heating in vacuum with protecting gases. XPS analysis shows that the coexistence of the water molecules and oxygen vacancies in nanowires leads to a defect band in the band gap. The coloration is explained by considering the absorptions of photons involving both this defect band and the tails of valence and conduction bands. These findings deepen the understanding of the physical mechanism underlying the gasochromic effect of WO3. Acknowledgment. The authors gratefully acknowledge the financial support of the project from the National Natural Science Foundation of China (Grant Nos. U0634002, 50725206, 60571035, and 50672135), the Science and Technology Ministry of China (National Basic Research Program of China, Grant Nos. 2003CB314701, 2007CB935501, and 2008AA03A314), the Science and Technology Department of Guangdong Prov-
J. Phys. Chem. C, Vol. 113, No. 36, 2009 15881 ince, the Department of Information Industry of Guangdong Province, and the Science and Technology Department of Guangzhou City. Supporting Information Available: The reflectance spectra of the WO3 nanowire film at the bleached and colored states in the gasochromic effect. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2000, 60, 201. (2) Grt¨zel, M. Nature 2001, 414, 338. (3) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2007, 91, 1529. (4) Bange, K. Sol. Energy Mater. Sol. Cells 1999, 58, 1. (5) Deb, S. K. Sol. Energy Mater. Sol. Cells 2008, 92, 245. (6) Zhang, Y.; Lee, S.-H.; Mascarenhas, A.; Deb, S. K. Appl. Phys. Lett. 2008, 93, 203508. (7) Niklasson, G. A.; Berggren, L.; Larsson, A.-L. Sol. Energy Mater. Sol. Cells 2004, 84, 315. (8) Krasˇovec, U. O.; Orel, B.; Georg, A.; Wittwer, V. Sol. Energy 2000, 68, 541. (9) Georg, A.; Graf, W.; Neumann, R.; Wittwer, V. Solid State Ionics 2000, 127, 319. (10) Lee, S.-H.; Cheong, H. M.; Liu, P.; Smith, D.; Tracy, C. E.; Mascarenhas, A.; Pitts, J. R.; Deb, S. K. Electrochim. Acta 2001, 46, 1995. Lee, S.-H.; Cheong, H. M.; Liu, P.; Smith, D.; Tracy, C. E.; Mascarenhas, A.; Pitts, J. R.; Deb, S. K. J. Appl. Phys. 2000, 88, 3076. (11) Faughnan, B. W.; Crandall, R. S.; Heyman, P. M. RCA ReV. 1975, 36, 177. (12) Chen, H. J.; Xu, N. S.; Deng, S. Z.; Lu, D. Y.; Li, Z. L.; Zhou, J.; Chen, J. Nanotechnology 2007, 18, 205701. (13) Khoobiar, S. J. Chem. Phys. 1964, 68, 411. (14) Roland, U.; Braunschweig, T.; Roessner, F. J. Mol. Catal. A: Chem. 1997, 127, 61. (15) Yetter, R. A.; Risha, G. A.; Son, S. F. Proc. Combust. Inst. 2009, 32, 1819. (16) Campbell, C. T. Surf. Sci. Rep. 1997, 27, 1. (17) Kolmakov, A.; Klenov, D. O.; Lilach, Y.; Stemmer, S.; Moskovits, M. Nano Lett. 2005, 5, 667. (18) Meda, L.; Breitkopf, R. C.; Haas, T. E.; Kirss, R. U. Thin Solid Films 2002, 402, 126. (19) Shanak, H.; Schmitt, H.; Nowoczin, J.; Ziebert, C. Solid State Ionics 2004, 171, 99. (20) Nowoczin, J.; Shanak, H.; Ziebert, C.; Schmitt, H.; Ehses, K. H. Phys. Status Solidi A 2005, 202, 1073. (21) Okumu, J.; Koerfer, F.; Salinga, C.; Pedersen, T. P.; Wutting, M. Thin Solid Films 2006, 515, 1327. (22) Berggren, L.; Niklasson, G. A. Appl. Phys. Lett. 2006, 88, 081906. (23) Lu, D. Y.; Chen, J.; Zhou, J.; Deng, S. Z.; Xu, N. S.; Xu, J. B. J. Raman Spectrosc. 2007, 38, 176. (24) Lu, D. Y.; Chen, J.; Chen, H. J.; Gong, L.; Deng, S. Z.; Xu, N. S.; Liu, Y. L. Appl. Phys. Lett. 2007, 90, 041919. (25) He, Y. P.; Zhao, Y. P. J. Phys. Chem. C 2008, 112, 61. (26) Djaoued, Y.; Priya, S.; Balaji, S. J. Non-Cryst. Solids 2008, 354, 673. (27) Xiao, Z. D.; Zhang, L. D.; Wang, Z. Y.; Lu, Q. F.; Tian, X. K.; Zeng, H. B. Mater. Lett. 2007, 61, 1718. (28) de Wijs, G. A.; de Groot, R. A. Electrochim. Acta 2001, 46, 1989. ¨ zen, I˙.; U ¨ ner, M. Appl. Catal., A 2003, (29) Uner, D.; Tapan, N. A.; O 251, 225. (30) Parker, D. H.; Koel, B. E. J. Vac. Sci. Technol., A 1990, 8, 2585. (31) Yeo, Y. Y.; Vattuone, L.; King, D. A. J. Chem. Phys. 1997, 106, 392. (32) Bennett, R. A.; Stone, P.; Bowker, M. Catal. Lett. 1999, 59, 99. (33) Falconer, J. L.; Magrini-Bair, K. A. J. Catal. 1998, 179, 171. (34) Conner, W. C.; Falconer, J. L. Chem. ReV. 1995, 95, 759 .
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