High-Temperature Anodized WO3 Nanoplatelet Films for

Jul 24, 2009 - The 514.5 nm line from an argon ion laser was used as the excitation source. ..... All samples were tested using a broadband UV-vis spo...
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High-Temperature Anodized WO3 Nanoplatelet Films for Photosensitive Devices )

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Abu Z. Sadek,† Haidong Zheng,† Michael Breedon,† Vipul Bansal,‡ Suresh K. Bhargava,‡ Kay Latham,‡ Jianmin Zhu,§ Leshu Yu, Zheng Hu, Paul G. Spizzirri,^ Wojtek Wlodarski,† and Kourosh Kalantar-zadeh*,† School of Electrical and Computer Engineering, RMIT University, Melbourne, Australia, ‡School of Applied Sciences, Applied Chemistry, RMIT University, Melbourne, Australia, §National Laboratory of Solid State Microstructures, Physics Department, Nanjing University, Nanjing 210093, China, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China, and ^School of Physics, University of Melbourne, Melbourne, Australia )

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Received March 4, 2009. Revised Manuscript Received July 2, 2009 Anodization at elevated temperatures in nitric acid has been used for the production of highly porous and thick tungsten trioxide nanostructured films for photosensitive device applications. The anodization process resulted in platelet crystals with thicknesses of 20-60 nm and lengths of 100-1000 nm. Maximum thicknesses of ∼2.4 μm were obtained after 4 h of anodization at 20 V. X-ray diffraction analysis revealed that the as-prepared anodized samples contain predominantly hydrated tungstite phases depending on voltage, while films annealed at 400 C for 4 h are predominantly orthorhombic WO3 phase. Photocurrent measurements revealed that the current density of the 2.4 μm nanostructured anodized film was 6 times larger than the nonanodized films. Dye-sensitized solar cells developed using these films produced 0.33 V and 0.65 mA/cm2 in open- and short-circuit conditions.

Introduction Tungsten trioxide (WO3) is a popular metal oxide with many interesting electrical, optical, structural, and defect properties.1 It has been extensively used in electrochromic (EC) devices,2,3 gas sensors,4-6 water splitting,7 and batteries.8 Numerous applications of WO3 in chromic devices include electrochromic smart windows,9,10 displays,1 and tunable EC photonic crystals.11,12 For the fabrication of many such devices, and to increase their efficiencies, nanostructured WO3 is required to provide an enhanced surface-to-volume ratio. In photosensitive devices such as dye-sensitized solar cells (DSCs), photosensors, displays, and smart windows, thick and porous nanostructured metal oxide layers are also required to provide sufficient interaction volume for impinging photons.13,14 Nanostructures increase the surface activity, and higher film thicknesses provide a large interaction *Corresponding author: e-mail [email protected] Tel +61 3 9925 3254, Fax +61 3 9925 2007.

(1) Deb, S. K. Sol. Energy Mater. Sol. Cells 2008, 92 (2), 245-258. (2) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2000, 60, (3), 201-262. (3) Rauh, R. D. Electrochim. Acta 1999, 44 (18), 3165-3176. (4) Sberveglieri, G.; Depero, L.; Groppelli, S.; Nelli, P. Sens. Actuators, B 1995, 26 (1-3), 89-92. (5) Galatsis, K.; Li, Y. X.; Wlodarski, W.; Kalantar-zadeh, K. Sens. Actuators, B 2001, 77 (1-2), 478-483. (6) Ippolito, S. J.; Kandasamy, S.; Kalantar-Zadeh, K.; Wlodarski, W. Sens. Actuators, B 2005, 108 (1-2), 553-557. (7) Hamilton, J. W. J.; Byrne, J. A.; Dunlop, P. S. M.; Brown, N. M. D. Int. J. Photoenergy 2008. (8) Chen, L. C.; Tseng, K. S.; Huang, Y. H.; Ho, K. C. J. New Mater. Electrochem. Syst. 2002, 5 (3), 213-221. (9) Bechinger, C.; Ferrer, S.; Zaban, A.; Sprague, J.; Gregg, B. A. Nature 1996, 383 (6601), 608-610. (10) Lee, S. H.; Gao, W.; Tracy, C. E.; Branz, H. M.; Benson, D. K.; Deb, S. J. Electrochem. Soc. 1998, 145 (10), 3545-3550. (11) Kuai, S. L.; Bader, G.; Ashrit, P. V. Appl. Phys. Lett. 2005, 86 (22). (12) Sumida, T.; Wada, Y.; Kitamura, T.; Yanagida, S. Chem. Lett. 2002 (2), 180-181. (13) Gratzel, M. Prog. Photovoltaics 2000, 8 (1), 171-185. (14) Gratzel, M. J. Photochem. Photobiol., C 2003, 4 (2), 145-153.

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volume with liquid electrolytes or gaseous target analytes, which are both necessary for increasing the efficiency of the photosensitive devices. Additionally, fast response and recovery of such devices can be achieved when nanostructured films are used. Here we introduce thick nanostructured arrays of WO3 platelets with a large active surface area capable of interacting efficiently with incident photons. It is well-known that the thick nanostructured films facilitate large surface area for the adsorption of the dye sensitizer, assisting in the fabrication of efficient DSCs.13,14 A number of methods have been developed for the synthesis of nanostructured WO3 thin films, such as chemical vapor deposition,15,16 physical vapor deposition (thermal17,18 and electron beam19,20 evaporation, sputtering4,18), electrodeposition,21 and sol-gel techniques.22 Recently, fast and inexpensive liquid synthesis methods such as anodization23-26 and high-temperature acid etching27 have been developed to obtain nanostructured WO3. (15) Palgrave, R. G.; Parkin, I. P. J. Mater. Chem. 2004, 14 (19), 2864-2867. (16) Tagtstrom, P.; Jansson, U. Thin Solid Films 1999, 352 (1-2), 107-113. (17) Cantalini, C.; Sun, H. T.; Faccio, M.; Pelino, M.; Santucci, S.; Lozzi, L.; Passacantando, M. Sens. Actuators, B 1996, 31 (1-2), 81-87. (18) Cantalini, C.; Wlodarski, W.; Li, Y.; Passacantando, M.; Santucci, S.; Comini, E.; Faglia, G.; Sberveglieri, G. Sens. Actuators, B 2000, 64, 182-188. (19) LeGore, L. J.; Greenwood, O. D.; Paulus, J. W.; Frankel, D. J.; Lad, R. J. J. Vac. Sci. Technol., A 1997, 15 (3), 1223-1227. (20) vonRottkay, K.; Rubin, M.; Wen, S. J. Thin Solid Films 1997, 306 (1), 1016. (21) Santato, C.; Odziemkowski, M.; Ulmann, M.; Augustynski, J. J. Am. Chem. Soc. 2001, 123 (43), 10639-10649. (22) Choi, Y. G.; Sakai, G.; Shimanoe, K.; Teraoka, Y.; Miura, N.; Yamazoe, N. Sens. Actuators, B 2003, 93 (1-3), 486-494. (23) Berger, S.; Tsuchiya, H.; Ghicov, A.; Schmuki, P. Appl. Phys. Lett. 2006, 88 (20), 203119. (24) Hahn, R.; Macak, J. M.; Schmuki, P. Electrochem. Commun. 2007, 9 (5), 947-952. (25) Tsuchiya, H.; Macak, J. M.; Sieber, I.; Taveira, L.; Ghicov, A.; Sirotna, K.; Schmuki, P. Electrochem. Commun. 2005, 7 (3), 295-298. (26) Zheng, H.; Sadek, A. Z.; Latham, K.; Kalantar-Zadeh, K. Electrochem. Commun. 2009, 11, 768-771. (27) Widenkvist, E.; Quinlan, R. A.; Holloway, B. C.; Grennberg, H.; Jansson, U. Cryst. Growth Des. 2008, 8 (10), 3750-3753.

Published on Web 07/24/2009

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Figure 1. SEM images of W foils anodized at (A) 20, (B) 30, and (C) 40 V for 1 h. Insets are the corresponding cross-sectional image (45 tilted view) and anodization current variation with time and (D) at 20 V for 4 h. Inset is the cross-sectional image (45 tilted view).

Anodization in fluoride-containing electrolyte results in meso/ nanotubular or porous films26 while high-temperature etching results in nanoplatelet surfaces.27 The surface morphology of the nanostructured WO3 prepared via anodization methods can be altered by changing the anodization potential, duration, and electrolyte concentration23-26 or by varying the etching temperature and acid concentration for high-temperature etching.27 The main limitation of such methods is that the films produced are very thin as the rate of nanostructure formation in anodization, and the rate of etching in liquid acid media reaches steady state quickly.27 As previously reported,27 it has been observed that the difference in film thickness of samples immersed in acidic media between 12 and 24 h is insignificant. In both anodization23-26,28 and acid etching,27 nanostructured layers of on average less than 1 μm were obtained, which is less than ideal for photovoltaic and photochromic applications, for which a much thicker layer is required for the fabrication of efficient devices. In acid etching, the length of the platelets were typically in the range of 0.15-2 μm; however, the average thickness of the films was less than 1 μm. Having nanostructured films of several micrometers thickness can provide enhanced capabilities for the fabricated photochromic and photovoltaic devices. To date, most research on (28) Mukherjee, N.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Mater. Res. 2003, 18 (10), 2296-2299.

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DSCs has been focused on TiO2,14,29 SiO2,30 and ZnO,31 though Cheng et al.32 have recently reported that when TiO2 films were modified with 0.5 wt % WO3, an improvement of 18% conversion efficiency as compared to pure TiO2 can be achieved. However, despite having a real potential for the fabrication of tungsten trioxide (WO3)-based DSCs, such a device has not been reported. In this work, we have investigated the anodization of tungsten foil at high temperatures in a nitric acid environment. By combining anodization techniques with high-temperature acid etching, the authors were able to obtain relatively thick (∼2.4 μm) nanostructured films comprised of nanoplatelets. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy were utilized to characterize these films, and the photocurrent responses obtained from the nanoplatelet-containing WO3 surfaces are presented in this paper. The films were also employed for the fabrication of WO3-based DSCs, and the results are presented herein. (29) Oregan, B.; Gratzel, M. Nature 1991, 353 (6346), 737-740. (30) Ito, S.; Makari, Y.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Mater. Chem. 2004, 14 (3), 385-390. (31) Zhang, Y. Z.; Wu, L. H.; Liu, Y. P.; Xie, E. Q. J. Phys. D: Appl. Phys. 2009, 42 (8). (32) Cheng, P.; Deng, C. S.; Dai, X. M.; Li, B.; Liu, D. N.; Xu, J. M. J. Photochem. Photobiol., A 2008, 195 (1), 144-150.

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Figure 2. XRD patterns of anodized samples with different applied potentials. The labeled peaks correspond to R-W (3, ICDD no. 04-0806), hydrotungstite, WO3 3 2H2O ([, ICDD no. 18-1420), tungstite, WO3 3 H2O (/, ICDD no. 43-0679), and minor phase, W18O49 (b, ICDD no. 36-0101).

Experimental Section Tungsten (W) foil of 99.5% purity was purchased from SigmaAldrich and cut into 11.5 cm2 samples. The samples were cleaned with acetone, isopropanol (IPA), and DI water before anodization. Then, the samples were placed in a reaction flask containing 200 mL of 1.5 M HNO3 solution, and the flask was kept at 50 C during the process. Anodization was carried out by a conventional anode (tungsten foil)-cathode (platinum plate) system, where a dc voltage was applied. Anodization current was recorded using a computer with data logger software. After anodization, samples were washed with DI water and dried in a stream of N2. Several of these samples were annealed at 400 C for 4 h in 90% O2, 10% Ar environment. SEM characterization was carried out using a FEI Nova NanoSEM. Samples for TEM were prepared by sonicating the films for 1 h in DI water and subsequently dropcasting the sonicated samples onto a Cu TEM grid. X-ray diffraction analysis was carried out on a Bruker D8 Discover microdiffractometer fitted with a GADDS (general area detector diffraction system). Data were collected at room temperature using Cu KR radiation (λ=1.541 78 A˚) with a potential of 40 kV and a current of 40 mA, and filtered with a graphite monochromator in the parallel mode (175 mm collimator with 0.5 mm pinholes). Micro-Raman measurements were performed on a Renishaw RM1000 spectrometer in a backscattering geometry. The 514.5 nm line from an argon ion laser was used as the excitation source. The notch filter spectral response profile of the instrument prevented measurement below ∼100 cm-1.

Results and Discussion The high-temperature anodized films were characterized using SEM, XRD, TEM, Raman, and photocurrent measurements. The SEM images in Figure 1 show the top layer of the WO3 surface obtained at different anodization voltages. The SEM images revealed that the surface morphology of the nanostructured WO3 does not change significantly with the anodization Langmuir 2009, 25(16), 9545–9551

potential. Additionally, no distinct variation was observed between the surface morphologies of the high-temperature acidetched27 samples and the anodized WO3. In all cases, randomly oriented square and rectangular shape platelets with lengths of 100-1000 nm were observed. The thicknesses of the platelets were in the order of 20-60 nm. It was also observed that many of the platelets were aligned normal to the substrates’ surfaces. The cross-sectional SEM images of the samples anodized at 20, 30, and 40 V for 1 h are shown in the inset of parts A, B, and C of Figure 1, respectively, to highlight the variation of film thickness with the different anodization potentials. The cross sectional image revealed that thickness of the nanoplatelet layer was ∼0.7 μm after anodization at 20 V for 1 h. When anodization potentials were greater than 30 V, a bilayered structure of ∼1.2 μm thick for the 30 V 1 h sample and 4 μm thick for the 40 V 1 h sample was formed after 1 h, in which only the very top layer (700 nm) contained nanoplatelets aligned normal to the surface (inset of Figure 1B,C). The rest of the compact anodized layer (referred to as “intermediate layer”) was made of compressed platelets with reduced porosity. The intermediate layers consist of both hexagonal and rectangular nanoplatelets with dimensions smaller than the top layer platelets (inset of Figure 1C). The top nanoplatelet layer was similar irrespective of anodization voltage; however, the intermediate layer was only formed at high anodization voltage (>30 V). Additionally, many cracks were also observed on the film obtained at 40 V anodization due to the large volume expansion and stress mismatch on the foil surface and the formed WO3 at higher potentials. The SEM images depicting cracks in the oxide layer at high anodization potentials (30 and 40 V) are shown in the Supporting Information. Analysis of the cross-sectional images suggests that 20 V is the optimum potential for the growth of porous nanoplatelet layer without having any intermediate compact layer. Additionally, in 20 V anodization, it was observed that with the help of a longer DOI: 10.1021/la901944x

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Figure 3. Comparison of XRD patterns for W foil and different 20 V 1 h anodized samples with and without annealing. The labeled peaks correspond to R-W (3, ICDD no. 04-0806); WO3 with orthorhombic structure (b, ICDD no. 20-1324), hydrotungstite, WO3 3 2H2O ([, ICDD no. 18-1420), and WO3 3 1/3H2O (/, ICDD no. 35-0270).

anodization process, the nanostructured film thickness can be increased. It was found that optimum duration for obtaining maximum nanoplatelet layer thickness occurs after 4 h of anodization at 20 V. Beyond 4 h of anodization, thickness of the nanoplatelet layers remained similar, suggesting that the growth rate of the platelets decreased drastically after a certain film thickness was formed. The thickness of the nanoplateleted layer was ∼2.4 μm after 4 h of anodization at 20 V (inset of Figure 1D). In this film, the platelets were mostly normal to the surface with meso/microdimensional gaps in between them, providing high porosity essential for the fabrication of efficient surface sensitive devices. It was also observed that the average thicknesses of the nanoplatelets were ∼32% reduced after annealing of the anodized samples at more than 400 C. This shrinkage in thickness signifies the dehydration of water from the tungstite platelets after annealing which was identified during the XRD, TEM, and Raman analysis of the sample. The transient currents for 20, 30, and 40 V anodization have been shown in the inset of parts A, B, and C of Figure 1, respectively. Out of these three transient currents, only the 20 V case conforms to the typical anodization current pattern of materials such as titanium,33 where the anodization current drops rapidly from a large magnitude at the beginning and then increases until reaching a peak which is then followed by a gradual decrease with time. For higher anodization voltages (30 and 40 V), as can be seen in the inset of parts B and C of Figure 1, respectively, the beginning of the process follows conventional titanium anodization transient currents. However, the anodization current increases exponentially with time after the initial rapid drop. In the case of 30 V, the anodization current also follows the second step of the standard anodization process (33) Macak, J.; Taveira, L. V.; Tsuchiya, H.; Sirotna, K.; Macak, J.; Schmuki, P. J. Electroceram. 2006, 16 (1), 29-34.

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(within 20 min) where current shows a small peak before a gradual decrease. However, after the second stage, the anodization current increases rapidly. For the 40 V case, the rate of increase of the anodization current after initial rapid drop is much higher and it reaches 20 mA after 20 min. This large current is likely due to the breakdown of the rapidly formed oxide layer (Supporting Information). XRD patterns for samples anodized for 1 h at 20, 30, and 40 V are given in Figure 2. The XRD patterns of the 30 and 40 V samples were different from the XRD pattern of the 20 V sample, revealing a significant reduction in the intensity of the metallic W peaks, as the anodization potential was increased from 20 to 30 V and then to 40 V. This implies that as anodization voltage is increased the rate of oxidation of W is increased, and further reduction of W peaks results from the emergence of the intermediate WO3 layer. The non-W peaks in the XRD patterns of the anodized samples match almost exclusively to tungstite (both hydrotungstite WO3 3 2H2O and tungstite WO3 3 H2O). The type of hydratedWO3 phase present in the samples is a function of anodization voltage, with 20 and 30 V samples containing WO3 3 2H2O (ICDD no. 18-1420) and 40 V samples containing significant amounts of both WO3 3 2H2O and WO3 3 H2O (ICDD no. 43-0679) (Figure 2). A monoclinic, nonstoichiometric oxide (W18O49, no. 36-0101) is also present as a minor phase in all anodized samples. The XRD patterns of the W foil before and after anodization at 20 V for 1 h, and after annealing, are shown in Figure 3. The annealing temperature was 400 C for 4 h with a 2 C/min upward and downward ramping. The W peak at ∼58.5 2θ has been normalized in all the samples for the ease of comparison between the samples. It can be seen clearly that the dominant W signature peaks corresponding to stable R-phase with a few very weak signatures from metastable β-phase are present in all the samples including the W foil, after anodization, or even after annealing at Langmuir 2009, 25(16), 9545–9551

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Figure 4. (A) HRTEM image of WO3 nanoplatelets anodized at 20 V for 1 h, and insets are the Fourier transform (top) and corresponding SAED patterns of a nanoplatelet (bottom). (B) HRTEM image of a nanoplatelet after annealed at more than 400 C, and inset is the corresponding SAED patterns of a nanoplatelet.

400 C. The W signals in both the anodized and annealed samples can be attributed to the penetration of X-rays into the W substrate due to the highly porous nature of the nanoplatelets and the thin film thicknesses. Distinct WO3 peaks are apparent in annealed samples, which match predominantly to orthorhombic WO3, with a minor contribution from WO3 3 1/3H2O. Additionally, the high intensity of the WO3 crystallographic signatures of the (200) and (220) planes explain the presence of WO3 nanoplatelets in anodized and annealed samples. TEM images for the 20 V anodized samples before and after annealing at 400 C are shown in parts A and B of Figure 4, respectively. Without annealing, selected area electron diffraction (SAED) pattern of a platelet presents a set of bright diffraction spots indicating the single crystal feature (inset in Figure 4A, bottom), which could be indexed to the hydrated orthorhombic WO3.34 A Fourier analysis of the HRTEM image of an anodized nanoplatelet is given in the inset of Figure 4A (top). Single dislocation and larger zones of misalignments of several planes can be identified. Such crystal defects suggest that platelet formation presumably occurs by oriented attachment of large number of small crystallites, leading to a two-dimensional arrangement of nanoplatelets.34 Apparently, this short-range lattice order with a lattice discontinuities at terminal WdO groups signify a sharp Raman peak at about 960 cm-1 which will be observed in Figure 5. From the HRTEM image in Figure 4B, the lattice fringes can be clearly observed, indicating the much improved crystallinity of the sample after annealing. The corresponding SAED pattern demonstrates that the annealed sample consists of single crystal orthorhombic WO3 platelets (inset of Figure 4B). The lattice spacing of 3.90 A˚ between the adjacent fringes corresponds to the d-spacing of (002) planes of the orthorhombic WO3. The Raman technique is well-suited to the analysis of tungsten oxide phases (allotropes) as well as the detection of intercalated (structural) and surface adsorbed H2O for the hydrates (WO3 3 xH2O).35,36 The bending and stretching vibrations of WO3 occur around 600-960 and 200-500 cm-1, respectively, (34) Polleux, J.; Pinna, N.; Antonietti, M.; Niederberger, M. J. Am. Chem. Soc. 2005, 127 (44), 15595-15601. (35) Daniel, M. F.; Desbat, B.; Lassegues, J. C.; Gerand, B.; Figlarz, M. J. Solid State Chem. 1987, 67 (2), 235-247. (36) Salje, E. Acta Crystallogr., Sect. A 1975, A31 (May 1), 360-363. (37) Nonaka, K.; Takase, A.; Miyakawa, K. J. Mater. Sci. Lett. 1993, 12 (5), 274-277.

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with lattice modes evident below 200 cm-1.35-37 Because of the transmission properties of the notch filter used for these measurements, lattice modes could not be studied in these materials. Recently, Boulova et al.38 studied the influence of nanocrystallite size on the structural transitions of WO3 as a function of annealing temperature and pressure. At 300 K, the Raman bands broaden as the nanoparticle size decreases, which is consistent with phonon confinement effects. Results for the series of WO3 preparations are shown in Figure 5 where the anodization time has been varied. The lower three traces have been obtained from samples which have not received thermal processing (i.e., annealing) and are therefore, measurements of the materials as prepared. A reference sample, prepared by boiling metallic tungsten in acid, is also included. All spectra exhibit broad features with bands indicative of hydrous, amorphous, or nanocrystalline WO3 3 xH2O.35 Peaks arising from OH stretches were observed (not shown) around 3200 cm-1, which further support this assignment. The peak observed at 378 cm-1, which can be attributed to stretching modes arising from (W-OH2), is also consistent with presence of hydrous WO3. The band around 960 cm-1 is attributed to the symmetric stretching mode of a terminal W6+dO bond.35-37 This peak is also a spectral marker for amorphous material since it represents lattice discontinuities at terminal WdO groups which lead to short-range (lattice) order. Bridging (O-W-O) stretching vibrations, which occur around 700 cm-1 (with bending modes around 230 and 270 cm-1), are influenced significantly by hydration, and as result, the 660 cm-1 band can be used to identify the hydration level of the crystal.35 Appearing as a broad transition around 645 cm-1, the spectrum resembles that of the hydrate WO3 3 H2O, whereas when it appears as a doublet (i.e., 660 and 680 cm-1), the material is identified as WO3 3 2H2O.35 Care should be exercised using this approach since the crystalline hexagonal WO3 phase exhibits bands at similar frequencies. Characterizing these (nonannealed) materials using this approach, the 1 and 4 h anodization both appear to have produced WO3 3 2H2O, whereas the acid boiled reference sample produced WO3 3 H2O, which is consistent with ref 27. Following thermal annealing at 400 C for both 1 and 4 h, the spectral features of these materials changed dramatically with Raman bands becoming sharper, a sign of increasing crystallinity and order. The terminal WdO stretching mode at 950 cm-1 is (38) Boulova, M.; Lucazeau, G. J. Solid State Chem. 2002, 167 (2), 425-434.

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Figure 5. Room temperature Raman spectra of WO3 samples prepared using various anodization times at 20 V as well as an acid-treated reference sample. The lower three measurements were taken from materials as prepared whereas the top three had been thermally annealed at 400 C for 4 h.

mostly absent, and the broad feature around 700 cm-1 has been replaced by two strong, narrow bands at around 710 and 805 cm1 for each material. In addition to these changes at higher frequencies, two new peaks appear at 270 and 320 cm-1. From previously reported work on crystalline WO3,38,39 we may attribute these spectral features to the presence of the phase transition from monoclinic to orthorhombic WO3 in these preparations with the intense modes (710 and 805 cm-1) assigned to W-O stretching in the WO6 octahedral units39 while the lower frequency bands (270 and 320 cm-1) can be assigned to W-O-W bending vibrations.35 As the phase transition of WO3 from monoclinic to orthorhombic occurs above 320 C,38 it can be concluded that the dominant phase of the annealed films is orthorhombic. Photocurrent Measurements. Photocurrent measurements of the nanostructured WO3 films were conducted in a 0.1 M Na2SO4 solution with a Pt counter electrode. All samples were tested using a broadband UV-vis spotlight source with a spectrum similar to sunlight, which was focused on the samples through an optical fiber having ∼1.2 mW/cm2 optical power in the illuminated area of ∼0.18 cm2. Photocurrent measurements were carried out after annealing the anodized samples at 400 C for 4 h as nonannealed samples produced negligible photocurrent responses. The results were normalized against the current produced by the sample anodized at 20 V for 1 h at no applied voltage and shown in Figure 6. As can be seen, when no bias voltage is applied, samples anodized at 20 V for 4 h produced the largest photocurrents. For both 20 and 30 V samples, the 4 h anodized samples showed higher photocurrents than that of the 1 h anodized sample and the nonanodized acidetched samples. It was therefore concluded that the thicker nanostructured film has increased the photon interaction with the volume. Even though both high-temperature acid-etching and hightemperature anodization produced similar nanoplatelets on the surface, the lower photocurrent (6 times) for the high-temperature acid-etched films verifies that anodization has increased the nanostructured film thickness. At high biasing voltages (0.23(39) Souza, A. G.; Freire, V. N.; Sasaki, J. M.; Mendes, J.; Juliao, J. F.; Gomes, U. U. J. Raman Spectrosc. 2000, 31 (6), 451-454.

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0.5 V), 20 V anodized samples produced higher photocurrent than that of 30 V anodized samples. Samples anodized at 40 V for 1 h produced negligible photocurrent, and the 20 and 30 V anodized samples showed slightly different photocurrent characteristics. The negligible photocurrent at 40 V might be due to the formation of the thick and compact intermediate layer at higher anodization potentials. In this case, the generated photoelectrons on the surface of WO3 have to diffuse into a thick compact layer, which increases the chance of recombination and hence reducing the current. Additionally, the surface breakdown of the anodized WO3 at 40 V (and to some extent at 30 V) produces short-circuited paths which deteriorate the generation of voltage and consequently measurable current. The decrease in the photocurrent can also be caused by the reduction of surface to volume ratio compared to the optimum porous nanoplateleted layer (anodized at 20 V for 4 h), which also reduces the photocurrent generation rate. DSC Performance Assessment. For fabrication of the DSCs, the anodized WO3 platelet electrodes were soaked in an acetonitrile solution of a ruthenium dye (RuL2(NCS)2; L=2,20 bipyridyl-4,40 -dicarboxylic acid, 0.3 mM (B4(N3), Dyesol)) for 12 h at room temperature. The soaked electrodes were washed with ethanol to remove nonanchored dye molecules and then dried in air. Patterned Pt-sputtered glasses were used as a counter electrode, where Ti was first deposited to improve adhesion of Pt to the substrate. The authors employed Pt as the counter electrode; the application of Au-coated glass, ITO (indium-tin oxide), and FTO (fluorine-doped tin oxide) glass as a counter electrode did not produce any voltage. Also, the application of FTO glass with a thin film of Pt catalyst, which is generally used in standard DSCs, did not produce any voltage either. The cells were tested in simulated sunlight conditions, and the current density (mA/cm2) and voltage (V) were measured. The Pt counter electrode and the dye-anchored WO3 electrode were assembled into a sealed sandwich type cell with a 200 μm thick film gasket. Before placing the Pt counter electrode, an iodide redox electrolyte solution (high performance electrolyte: EL-HPE, Dyesol, Australia) was incorporated between the electrodes with a circular surface area of ∼0.25 cm2. The schematic of the developed solar cell is given in Figure 7. Langmuir 2009, 25(16), 9545–9551

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Figure 7. Schematic diagram of the developed DSC cell.

Conclusion Figure 6. Normalized (with respect to 20 V 1 h anodized sample photocurrent at no bias voltage) photocurrent measurements for different anodized WO3 samples using a 0.1 M Na2SO4 solution.

It was observed that the sample anodized at 20 V for 4 h had a higher efficiency than the sample anodized at 20 V for 1 h as this sample has a larger film thickness. With the same conditions, annealed 20 V 4 h anodized solar cells produced 0.33 V and 0.65 mA/cm2 in open- and short-circuit conditions, respectively. The efficiency of the developed cells was low (less than 0.2%) due to the small thickness of the nanoplateleted layer (2.4 μm), the Pt grids which block part of the light, and the small voltage produced. The 20 V 1 h anodized solar cell produced 0.35 V and 0.4 mA/cm2 in open- and short-circuit conditions, respectively. Hence, the developed WO3 nanoplatelet films were found to produce photocurrent almost proportionally to their thickness. In addition to the aforementioned cells, DSCs using WO3 films prepared from sputtering, thermal evaporation, electro-deposition, and liquid-deposition techniques were also fabricated. However, no significant current was produced from such films. Only high-temperature anodized-highly porous nanostructured WO3 surfaces produced discernible photovoltaic properties as demonstrated in the developed DSCs. This may be due to the fact that the nanostructured films are thick and highly porous which allow large interactions of the active surface with electrolyte and also that a thick and porous surface allows efficient coverage of dye molecules, which in turn increases the photon-dye interaction.

Langmuir 2009, 25(16), 9545–9551

Nanoplatelets with thicknesses of less than 60 nm and lengths of 100-1000 nm were obtained when W foil was anodized in 1.5 M HNO3 solution at 50 C temperature. SEM and XRD analysis and photocurrent measurements revealed that the optimum anodization potential for obtaining a porous thick nanoplatelet layer was 20 V. Analysis of XRD, TEM, and Raman results of the as-anodized samples suggests that the presence of hydrated tungstite phases (WO3 3 xH2O, where x = 1 and/or 2) depends on the voltage. WO3 films of ∼2.4 μm thick were obtained using 20 V anodization potential for 4 h with a photocurrent density 6 times larger than nonanodized samples (at no bias voltage). Additionally, dye-sensitized solar cells were developed based on these WO3 nanoplatelets. The 20 V 4 h anodized cells produced 0.33 V and 0.65 mA/cm2 for open- and shortcircuit conditions. The suggested high-temperature anodization process is a viable and low-cost method for the manipulation of metallic surfaces in order to obtain thick nanostructured metal oxide layers. Further study is warranted to investigate the effect of the anodization temperature and other acids with different concentrations to obtain larger nanostructured film thicknesses of up to several tens of micrometers. Supporting Information Available: SEM images depicting cracks in the oxide layer at high anodization potentials (30 and 40 V). This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la901944x

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