NANO LETTERS
Controlled Electrodeposition of Nanoparticulate Tungsten Oxide
2002 Vol. 2, No. 8 831-834
S. H. Baeck,† T. Jaramillo,† G. D. Stucky,‡ and E. W. McFarland*,† Department of Chemical Engineering and Department of Chemistry, UniVersity of California, Santa Barbara, California 93106-5080 Received April 19, 2002; Revised Manuscript Received June 2, 2002
ABSTRACT Nanoparticulate tungsten oxide films were synthesized by pulsed electrodeposition. Particle sizes between 45 and ∼330 nm were achieved by varying pulse duration from 5 to 500 ms. Shorter pulses increased the rate of new particle nucleation above the rate of existing particle growth, allowing for the observed variations in size. Cathodic deposition voltage (−1 ∼ −3 V) had little effect on particle size. Compared to films prepared by continuous electrodeposition, nanoparticulate tungsten oxide films showed a higher photoactivity and greater current density for the hydrogen intercalation reaction. Functional improvements are explained by the smaller particle size and larger surface area of nanocrystalline tungsten oxide.
Introduction. Semiconductor nanoparticles have been investigated for dye sensitized solar cells, electrochromic materials, and catalytic materials.1,2 The improved properties of nanoparticles usually result from their enormous surface area, which provides for large quantities of adsorbates and, in the case of photochemical reactions, internal light scattering.3 Charge transport between interconnected particles may be also facilitated.4 Tungsten oxide (WO3) is an indirect band gap semiconductor with interesting photoconductive behavior used in electrochromic and sensor devices.5-8 It has demonstrated promise as a low-cost material for solar energy applications, but it is limited by its relatively high electrical resistivity. Improvements have been observed with nanoparticulate tungsten oxide, which has been prepared by both physical and chemical methods. High-density nanoparticulate WO3 films have been synthesized by RF sputtering9 and electron beam evaporation.10 Synthesis of more desirable low-density films involves dispersing colloidal suspensions of particles in a solvent and uniformly coating a substrate.4 Unfortunately, however, the use of organic stabilizers often contaminates the films with carbonaceous deposits following thermal treatment. Electrochemical methods have been employed for the synthesis of WO3 films. Continuous electrochemical deposition methods have utilized solutions of Na2WO4 in sulfuric acid11 or tungsten powder in hydrogen peroxide.12,13 Continuous electrochemical methods have advantages in terms of cost effectiveness and the potential for synthesizing large * Corresponding author. † Department of Chemical Engineering. ‡ Department of Chemistry. 10.1021/nl025587p CCC: $22.00 Published on Web 06/18/2002
© 2002 American Chemical Society
area WO3 films; however, nanoparticulate materials are difficult to form in a controlled manner. Nanoparticulate materials have been created by pulsed electrodeposition, which utilizes short, high potential pulses to nucleate particle growth at a much higher number of sites than lower voltage continuous deposition. This has been applied to metal alloys that have been deposited as nanoparticulate films with improved mechanical and physical properties.14,15 Metal oxide nanoparticles have not been previously synthesized by pulse cathodic electrodeposition. Such materials would have the major advantage of high surface area and large film areas without the disadvantages of carbonaceous contaminants. We employed pulsed electrodeposition methods in our studies of WO3 to address the following questions: (1) Can high surface area, porous, nanocrystalline tungsten oxide films be fabricated by direct cathodic electrodeposition? (2) Can the morphology and particle size be controlled by pulse parameters? (3) Can these properties improve the photocatalytic behavior of tungsten oxide? Experimental Section. Deposition electrolytes were prepared by dissolving 1.8 g of tungsten powder in 60 mL of 30% hydrogen peroxide. The excess hydrogen peroxide was subsequently decomposed with platinum black. The solution was diluted to 50 mM with a 50:50 water and 2-propanol mixture. Both Ti foil and ITO-coated glass were used as cathodic substrates after cleaning with aqueous detergent, acetone, and 2-propanol. The cathodic-potentiostatic deposition was performed using a conventional three-electrode system (EG&G 273A) with Pt as a counter electrode and SCE as a reference electrode. Pulsed electrodeposition was performed under a variety of selected conditions. Square waveform pulses were applied with a duty cycle of 0.5, i.e.,
Figure 1. X-ray diffraction patterns of tungsten oxide on Ti foil calcined in air at 450 °C.
the same length of time was used for ton (cathodic pulse time) as toff (time between pulses). Pulse time was varied from 5 ms to 500 ms, and the total deposition time was 20 min in every case. After deposition, all samples were calcined at 450 °C for 6 h in air. Following synthesis, representative films underwent more detailed quantitative analysis. Scanning electron microscopy (FEI, XL-30) was performed to give surface morphology and particle size, and X-ray diffraction (Scintag, X2) was used to examine crystal structure. To calculate particle size and distribution, the SEM images were processed with commercial image analysis software (Adobe Photoshop and NIH Image). Photoelectrochemical screening of the samples for the measurement of zero bias photocurrent as well as potentiodynamic photocurrent was performed with a electrochemical cell described in detail elsewhere.16 In brief, the cell has Pt counter and reference electrodes. A 0.1 M sodium acetate electrolyte is pumped into the cell and current-voltage data obtained while the surface is illuminated with a chopped light source (Oriel, Xe 150 W, chopped at 100 Hz during 0.03 V/sec voltage ramp). Due to losses through the optical fiber, the light intensity incident on the sample was approximately 25 mW/cm2. For measurement of hydrogen intercalation, Pt was used as a counter electrode and SCE as a reference electrode in 0.5 M H2SO4. Results and Discussion. As-deposited tungsten oxide films were found to be amorphous by XRD and crystalline after calcination in air at 450 °C (Figure 1). Morphologies for several samples are shown in Figure 2. Films prepared by continuous electrodeposition had an average particle size of approximately 375 nm. As the pulse time decreased, particle size decreased as well, Figure 3. For a 5 ms pulsed deposition, the average particle size was approximately 45 nm (Figure 2c). We checked the particle size with respect to deposition time (30 s to 30 min, that is, 3000 to 180 000 pulses) and found that particle size was independent of total number of pulses; the total number of pulses seemed to affect only film thickness and not the final particle size (not shown). The growth of electrodeposited tungsten oxide films has been described as a five-step nucleation-coalescence mech832
anism:17 (1) formation of isolated nuclei, (2) growth to larger particles, (3) coalescence of larger particles, (4) formation of a linked network, and (5) formation of a continuous deposit. Smaller particle sizes can be explained by a higher nucleation rate with a decrease in the crystallite coalescence. Weil et al. reported that pulsed electrodeposition of a nickelmolybdenum alloy reduced the internal stresses that typically result from crystallite coalescence.15 Nucleation occurs instantly as the potential is switched on. Our results are consistent with a model whereby growth and coalescence of existing larger particles is less favorable than growth of the new small particle nuclei, thus during a single pulse cycle an individual particle is nucleated and grown to its maximum size. The effect of deposition voltage was also investigated. Above 1.3 V hydrogen is produced, which influences the macroscopic morphology of the deposited film. The particle size, however, was found to be nearly independent of deposition voltage between -1 and -3 V (not shown). Photoactivities for both continuous and pulse deposited films on ITO were measured under chopped illumination. The current-voltage behavior was typical of n-type semiconductor electrodes for all electrodeposited WO3. In the case of films prepared by continuous electrodeposition, zerobias photocurrent (an indirect means of screening for photocatalytic activity) increased with increased deposition time, and it reached a maximum value of approximately 8.5 ( 0.5 µA/ cm2 after 20 min of deposition. The photocurrent density of nanoparticulate tungsten oxide films was higher than that of the continuous WO3 films and found to increase with decreased particle size. The improvement in photoactivity may be due to combinations of increased light absorbance, more efficient charge transport, and increased surface redox activity. The values of zerobias photocurrent for films prepared on ITO coated glass by pulsed and continuous methods were 12.4 and 8.6 µA/cm2, respectively, Figure 4. This increase in photoactivity for nanoparticulate WO3 is particularly impressive considering that the film prepared by continuous electrodeposition was thicker (2.8 µm) than that prepared by pulsed deposition (1.9 µm), due to the longer integrated deposition time. The improvement in performance for nanoparticulate WO3 is consistent with the findings of Lindquist et al.,4 where nanoparticulate tungsten oxide was synthesized by dispersing colloidal suspensions with the aid of an organic stabilizer. They reported that nanocrystalline tungsten oxide showed a very low rate of internal electron and hole recombination, thus facilitating net electron transport across the film. The intercalation of hydrogen in WO3 films is shown in Figure 5. As the films were cathodically reduced in H2SO4, the color of the film changed from light green to blue, and the blue color was intensified with increased cathodic potential. When the blue films were anodized, they were bleached back to the original green color. This color change was reproducible without degradation, even after 20 cycles (data not shown). The electrochromic process of tungsten oxide has been explained on the basis of the double intercalation of a proton and an electron to form a colored tungsten bronze.6,11 Nano Lett., Vol. 2, No. 8, 2002
Figure 2. Electron micrographs of tungsten oxide films prepared cathodically at -1 V by (a) normal continuous electrodeposition, (b) pulsed deposition (100 ms pulses), and (c) pulsed deposition (5 ms pulses).
Figure 3. Dependence of particle size of tungsten oxide films on pulse duration. Error bars indicate one standard deviation above and below the mean.
Figure 4. Potentiodynamic scans under chopped illumination for (a) tungsten oxide prepared by pulsed electrodeposition (solid line), (b) tungsten oxide prepared by continuous electrodeposition (dashed line). The films were prepared on indium-tin oxide glass substrates.
xH+ + xe- + WO3 S HxWO3 Other cations, Li, Na, and K, can also be intercalated into the lattice of tungsten oxide.6 The cathodic current (coloring current) density indicates the amount of hydrogen intercalated within the tungsten bronze. Nano Lett., Vol. 2, No. 8, 2002
Figure 5. Cyclic voltammogram for hydrogen intercalation/ deintercalation in tungsten oxide films prepared by pulsed electrodeposition (solid line) and continuous electrodeposition (dashed line).
When comparing the coloring current of tungsten oxides prepared by both pulsed and continuous electrodeposition, the nanocrystalline tungsten oxide film on Ti foil shows significantly higher current density, Figure 5. This suggests that even though the films deposited continuously were thicker, for the same cathodic current, a lower potential can be used for the nanocrystalline tungsten oxide films. The improvement achieved in the hydrogen intercalation current and photoactivity can be explained by the significant increase of effective surface area due to the surface morphology and smaller particle size of nanocrystalline tungsten oxide. Conclusion. High surface area, porous, nanoparticulate tungsten oxide was successfully synthesized by pulsed electrodeposition. Particle sizes could be controlled by pulse duration; as pulse duration decreased, particle size decreased, and this effect was insensitive to deposition voltage. Compared to tungsten oxide films prepared by continuous electrodeposition, nanoparticulate tungsten oxide showed higher photoactivity and current density for hydrogen intercalation. Functional improvements are due to the smaller particle size and larger surface area of nanocrystalline tungsten oxide, leading to increased light absorbance, facilitated charge transport and increased surface redox activity. It is likely that other oxide materials may be similarly deposited as nanoparticles. 833
Acknowledgment. Major funding was supported by the Hydrogen Program of the U.S. Department of Energy (DOE, Grant# DER-FC36-01G011092) and the Cycad Group (Santa Barbara, CA). Partial funding and facilities were provided by the NSF-MRSEC funded Materials Research Laboratory (UCSB) and the California Energy Commission (CEC, Grant#51539A/99-36). The authors acknowledge the invaluable technical assistance of Anna Ivanovskaya. References (1) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (2) Vinodgopal, K.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993, 97, 9040. (3) Murali, A, Barve, A.; Leppert, V. J.; Risbud, S. H.; Kennedy, I. M.; Lee, H. W. H. Nano Lett. 2001, 1(6), 287. (4) Wang, H.; Lindgren, T.; He, J.; Hagfeldt, A.; Lindquist, S. E. J. Phys. Chem. B 2000, 104, 5686. (5) Yao, J. N.; Chen. D.; Fujishima, A. J. Electroanal. Chem. 1996, 406, 223.
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(6) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2000, 60, 201. (7) Ladouceur, M.; Dodelet, J. P.; Tourillon, G.; Parent, L.; Dallaire, S. J. Phys. Chem. 1990, 94, 4579. (8) Green, M.; Hussain, Z. J. Appl. Phys. 1993, 74(5), 3451. (9) Le Bellac, D.; Azens, A.; Granqvist, C. G. Appl. Phys. Lett. 1995, 66, 1715. (10) Solis, J. L.; Hoel, A.; Lantto, V.; Granqvist, C. G. J. Appl. Phys. 2001, 89(5), 2727. (11) Su, L.; Zhang, L.; Fang, J.; Xu M.; Lu, Z. Sol. Energy Mater. Sol. Cells 1999, 58, 133. (12) Shen, P. K.; Tseung, C. C. J. Mater. Chem. 1992, 2(11), 1141. (13) Meulenkamp, E. A. J. Electrochem. Soc. 1997, 144, 1664. (14) Pagotto, S. O., Jr.; Feire, C. M. A.; Ballester, M. Surf. Coat. Technol. 1999, 122, 10. (15) Nee, C. C.; Kim, W.; Weil, R. J. Electrochem. Soc. 1988, 135(5), 1100. (16) McFarland, E. W.; Baeck, S. H.; Brandli, C.; Ivanovskaya, A.; Jaramillo, T. F. Proc. of 11th Can. Hydro. Conf. 2001, in press. (17) Shen, P.; Chi, N.; Chan. K. Y. J. Mater. Chem. 2000, 1, 697.
NL025587P
Nano Lett., Vol. 2, No. 8, 2002
