5686
J. Phys. Chem. B 2000, 104, 5686-5696
Photolelectrochemistry of Nanostructured WO3 Thin Film Electrodes for Water Oxidation: Mechanism of Electron Transport Heli Wang, Torbjo1 rn Lindgren, Jianjun He, Anders Hagfeldt, and Sten-Eric Lindquist* Department of Physical Chemistry, UniVersity of Uppsala, Box 532, S-75121 Uppsala, Sweden ReceiVed: January 21, 2000; In Final Form: March 10, 2000
Nanostructured WO3 thin films were prepared, and photooxidation of water at such films was studied in a pH 4.68 solution. The cathodic current at potentials below -100 mV versus a saturated Ag/AgCl electrode was related to the reversible intercalation of H+ and/or Na+. The photocurrent onset was at ∼100 mV, and the saturation photocurrent was at potentials >800 mV. In the range 300-1000 mV, photocurrent increased linearly with the increasing light intensity, indicating that charge carrier generation dominates the photoelectrochemical cell. Under illumination, linear log|i| versus potential (Tafel) behavior was registered in the range 300-650 mV. Tafel slopes and exchange current densities are reported. The incident photonto-current efficiency (IPCE) and the quantum yield (Φ) were high, regardless of the incidence of the light (front side, EE, or backside, SE, illumination). Both IPCEEE and IPCESE increased with film thickness. The low wavelength edge of the action spectra was red-shifted and moved toward the absorption band edge. Both ΦEE and ΦSE reached a plateau region at shorter wavelength. In the plateau, ΦSE was close to 1 and independent of the film thickness, whereas ΦEE was ∼20% lower and decreasing with increasing film thickness. Adopting a simple diffusion model for the electron transport, the diffusion length of electrons (L) was estimated to be 6.7 µm for a 5.0-µm thick film. Higher activation energies, EA, were obtained at lower potentials (e.g., 0.60 eV at 200 mV and 0.32 eV at 300 mV). The EA was 2.0 V versus * Corresponding author.
a normal hydrogen electrode (NHE) at pH 0 which means that WO3 has an overpotential of ∼0.8 V.2 In AM1 sunlight, oxygen evolution at the same electrode starts at 0.6 V versus NHE, a shift of 1.4 V.2 A similar result was reported by Augustynski3 for WO3 electrodes under illumination of monochromatic light. Photooxidation of water with visible light at WO3 powders has been reported.4,5 However, in both reports, either an Fe3+/ Fe2+ or an Ag+/Ag redox couple was utilized for assisting the reaction. For this reason, hydrogen evolution was not the cathodic reaction. With visible light, photooxidation of water only occurred when both WO3 and the electron acceptor (Fe3+ or Ag+) were present. At a 10-20-µm-thick WO3 electrode made from the same powders, the quantum yield at 405 nm was 0.058, which is much higher than that at WO3 particles where the quantum yield was 0.020 at 405 nm.4 Lower efficiency at particles in dispersions was assigned to the charges recombination.4 The results indicate the advantage of using electrodes over particles. Photoelectrolysis of water at polycrystalline WO3 film electrodes was investigated by Butler, and a model for charge carriers separation at an polycrystalline photoelectrode was established for the semiconductor/electrolyte interface (SEI).6 According to Butler, the photoelectrochemical cell was dominated by the properties of the semiconductor. Photoexcited charge carriers were separated by the electric field in the depletion region of the SC and transferred to participate in the anodic and cathodic reactions. In that report,6 photocurrent density (iphoto) versus light intensity (Jlight) showed a linear relationship up to 10 suns, suggesting that charge carrier generation dominates the photoelectrochemical cell. Thus,
10.1021/jp0002751 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/24/2000
Photoelectrochemistry of WO3 Thin Film Electrodes reaction kinetics at the SEI had very little effect because the overpotential for Reaction 1-1 is determined by the position of the VB, which is ideally independent of the applied potential. However, it should be mentioned that in these experiments a bias as high as 5 V was applied in 1 M sodium acetate as well as in 0.5 M phosphoric acid solutions. If we consider that the VB level of WO3 is at 3.15-3.2 V versus NHE in a pH 0 solution,6,7 a 5 V bias corresponds to a Fermi level well below the bottom of the VB edge of the SC. Nanostructured semiconductor film electrodes are characterized by a large internal surface area. The electrodes are built up by interconnected nanoparticles in a porous structure. Recently, photocurrents up to ∼2 mA/cm2 were reported8 for a 4-µm thick nanostructured WO3 film in 3 M H2SO4 solution at 1.0 V versus NHE (or 0.8 V versus saturated Ag/AgCl electrode). The latter investigation highlights the advantage of nanostructured semiconductors for photooxidation of water. Although it is generally accepted that there is very little band bending in nanostructured SC films and the majority of the charge carrier (the electron) is transferred mainly by means of diffusion,9,10 the charge separation and transport in nanostructured semiconductors is far from fully understood. With a change in the external potential, the Fermi level at the back contact/ semiconductor interface will change and consequently the charge carrier concentration in the SC film will also change.11 In the present work, the action spectra as well as the dark and the photocurrent characteristics as a function of electrode potential, light intensity, and temperature have been studied in a threeelectrode system to elucidate the mechanism of charge transport in nanostructured WO3 films, with the aim of understand the rate-limiting steps in the system. Experimental Section Electrolyte and Electrode Preparation. Reagent grade chemicals and Milli-Q water (Millipore Corp.) were used in the electrode and electrolyte preparation. The electrolyte selected in the present work was 0.5 N NaClO4 at pH 4.68. The pH of the electrolyte was buffered with K2HPO4 and KH2PO4. The preparation of colloidal solution of WO3 was adapted from a well-established method12 in which the colloidal WO3 solution was obtained by passing 0.25 M Na2WO4 through a column filled with proton-exchange resin in which Na+ was substituted by H+. To check the completion of the ion exchange during the processing, a few drops of colloidal solution was deposited onto a piece of glass and heated at 500 °C for 5 to 10 min to check the color of the solid product. Dark and blue colors indicate higher Na+ concentration in the colloidal solution. A slow dropping speed of the solution from the column is preferred to obtain a greenish-yellow solid product in the checking procedure just described, which indicates the completion of ion exchange. To obtain a good porous film and to assist the wetting of the substrate, 0.0520 g of carbowax per 10 mL of the solution and 1 drop of Triton X-100 per milliliter of the solution was added into the colloidal solution, which then was stirred for >6 h. Such colloidal solutions can be used for 2 to 3 weeks if stored in a refrigerator. Nanostructured WO3 film was obtained by depositing the carbowax-WO3 mixture onto a conducting glass substrate (Tec15, Hartford Glass Company Inc., 15 to 20 Ω/0), using a Scotch tape as a spacer to determine the film thickness, and raking off the excess of the suspension with a glass rod. Dried films were first heated at 140 °C for 15 min and then sintered in air at 500 °C for 60 min. Up to 5-µmthick films were obtained by repeating this procedure. Thicker films had a tendency to crack. Optical features and action spectra
J. Phys. Chem. B, Vol. 104, No. 24, 2000 5687 were recorded and analyzed for films of different thickness. Otherwise, the results are mainly from a ∼3.5-µm-thick film. Electrodes were cut from larger pieces of conducting glass substrate with WO3 films. The electrodes were connected to copper wires on a WO3-film-free area by a silver conductive paint. The exposed back contact and edges were sealed with epoxy resin. The sealing procedure was repeated a few times to have an exposed area of ∼1.0 cm2 for each sample. Characterization of WO3 Films. The morphology of the films was investigated by scanning electron microscopy (SEM), which revealed that the films were nanostructured, with particle sizes in the range 30-50 nm (Figure 1). The X-ray diffraction pattern of the film shown in Figure 2 indicates the main characteristic peaks of monoclinic structured WO3. The roughness factor of the film, obtained by adsorption and desorption of cis-di(thiocyanato)-N,N-bis(2,2′-bipyridyl-4,4′dicarboxylic acid)-ruthenium (II), was ∼70 for a 3.5-µm-thick film. This value only gives a lower limit value of the roughness factor because a full coverage of the inner surface area by the dye cannot be guaranteed. No tendency of agglomeration of the dye in the pores of the film was observed, so a monolayer of the dye at the WO3 surface was assumed. Transmittance and Reflectance Measurements. The transmittance and reflectance measurements, both for samples and for substrate Tec15, were made with a Beckman Spectrophotometer UV5240 equipped with an integrating sphere. These measurements were carried out for dry films, and no correction was made for the optical effects due to the presence of the electrolyte. Following the method in ref 13, the total transmittance (Tλ) and total reflectance (Rλ) were calculated. The absorption profile of the films (Aλ), shown in Figure 3, was calculated according to:
Aλ ) 1 - (Tλ + Rλ)
(2)
The absorption edge of the films is at ∼480 nm, corresponding to a band gap of 2.60 eV, which is in good agreement with previous report for WO3 colloids.14 Photoelectrochemistry. The experimental setup was the same as in ref 15. The WO3 electrodes served as working electrodes in the three-electrode system. Because illumination from the substrate/electrode interface (SE illumination) generally showed higher quantum yield than that from the electrolyte/electrode interface (EE illumination) commonly found for several nanostructured semiconductor systems,9 we employed SE illumination in most experiments. Except in action spectra, measurements in which a 450 W Xe lamp with a Schoeffel GM 252 monochromator was used as light source, we employed a 1000 W Xe lamp. According to the supplier’s specification for the latter lamp, light intensity at a wavelength
