Article pubs.acs.org/JPCC
Effects of V‑Ion Doping on the Photoelectrochemical Properties of Epitaxial TiO2(110) Thin Films on Nb-Doped TiO2 (110) Single Crystals Atsushi Hachiya, Shintaro Takata, Yutaro Komuro, and Yuji Matsumoto* Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta Midori-ku, Yokohama, 226-8503 Japan ABSTRACT: V-doped TiO2(110) epitaxial films were grown on atomically flat Nb-doped TiO2 (110) single-crystal substrates by a pulsed laser deposition method, and their photoelectrochemical properties were investigated. In the photocurrent−potential curves measured in a 0.1 M HClO4 electrolyte for V-doped TiO2/Nb-doped TiO2 photoelectrodes, the photocurrent was almost proportional to the electrode potential, and there was a good linear relationship between the inverse of the photocurrent density obtained at +0.6 V versus Ag/AgCl KCl (sat.) and the V doping level. The potential at which the photocurrent appeared, that is, the flat-band potential, was about +0.33 V irrespective of the doping level, whereas it was about −0.25 V for the nondoped one. When a 0.01 M AgNO3 electrolyte was used, a sizable amount of Ag metal particles was deposited at +0.25 V on the electrode surfaces at V doping levels above 6 at %. The V-doping effects on these photoelectrochemical properties are discussed in terms of the in-gap states induced by the V impurities in TiO2 as well as their acting not only as a recombination center but also as a mediator of interfacial electron transfer.
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INTRODUCTION Since the discovery of the photoelectrolysis of water over a single-crystal n-type TiO2 (rutile) in 1969,1 there have been a number of studies on TiO2 during the past decades for possible applications such as solar to chemical and electricity conversion2−4 as well as water and air remediation.5,6 The chemical doping of TiO2 with one or more metal elements and their oxides is one of the essential strategies to enhance the photocatalytic activity.6,7 For the doped metal elements, on the one hand, they may form an active site for the oxidizing and/or reducing reactions at the surface5 and, on the other hand, they would form in-gap states or hybridize with either the conduction band or the valence band through which the visible-light excitation is possible.7−9 However, the results on the effects of doping on the photocatalytic activity have ended up in controversies between different research groups, one of the reasons for which may be the unfavorable surface speciation and defects due to the specific preparation methods, chemical reactions used to verify photoactivity, and different experimental conditions, as discussed by Hoffmann et al.7 One of the most effective ways to exclude such complicated factors, particularly for the structural control of metal-iondoped TiO2 photocatalysts, is to prepare them in the form of epitaxial thin films. Pulsed laser deposition (PLD), which has now become commonly available in various research areas, is a suitable method for the growth of such metal-ion-doped TiO2 thin films. The key to a good reproducible growth of TiO2 (rutile) thin films in a layer-by-layer fashion was the use of atomically flat TiO2 (rutile) single-crystal substrates;10,11 for the present purpose, the film quality can cater to the requirements of a TiO2-based model photocatalyst, whose composition and structure are atomically well-defined. The surface morphology © 2012 American Chemical Society
effect on the photocatalytic activity is thus negligible owing to the uniformity of atomically flat thin film surfaces, almost identical to that of the substrate. In our previous works, we prepared epitaxial TiO2 films doped with different transition-metal ions on the atomically flat Nb-doped TiO2(110) substrates by PLD and compared the effects of different dopant elements on the resulting physicochemical properties (e.g., a crystal structure and a valence state of each dopant) and their corresponding activity for photodeposition of Ag metal particles from a AgNO3 solution. It has been found that most transition-metal ions can trap excess electrons from the Ti3d band like an acceptor, and thus the Fermi level is lowered, the magnitude of which depends greatly on the kinds of transition metals as well as their valence states.12 Furthermore, among such transition-metaldoped TiO2 films on Nb-doped TiO2(110) substrates, we discovered that the photodeposition of Ag metal particles from a AgNO3 solution was much enhanced on the film surface of Vdoped TiO2, whereas almost no photodeposition occurred on the nondoped film surface.13 However, the mechanism has not been yet clarified, although there have been some reports on the theoretical calculation of electronic structure of V-doped TiO2.14−16 With this background, in the present work, we investigated the effects of V-ion doping not only on the photocurrent behavior but also on the photodeposition of Ag metal particles using V-doped TiO2/Nb-doped TiO2(110) epitaxial photoelectrodes with a well-controlled electrode potential under UVillumination, and they are discussed in terms of the in-gap states Received: July 19, 2012 Published: July 26, 2012 16951
dx.doi.org/10.1021/jp307185d | J. Phys. Chem. C 2012, 116, 16951−16956
The Journal of Physical Chemistry C
Article
Fermi level in TiO2 was located at almost the same position as in the case when the 0.1 M HClO4 electrolyte was used, with respect to the conduction band level at the electrode/ electrolyte interface. The material photodeposited in the 0.01 M AgNO3 electrolyte was characterized by the conventional powder X-ray diffraction (XRD: Cu Kα, λ = 0.15406 nm) and scanning electron microscope (SEM: Hitachi S-4000 at acceleration voltage of 20 kV). A high-pressure Hg lamp (USH-150SC (Ushio)) was used as a UV light source, and the light intensity measured using an integral light counter for 254 nm (UIT-150-A (Ushio)) was 20 mW/cm2.19
induced by the V impurities in TiO2 as well as their acting as a recombination center or as a mediator of interfacial electron transfer.
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EXPERIMENTAL SECTION The sample preparations were carried out in an ultra-high vacuum (UHV) PLD chamber equipped with in situ surface characterization tools of Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED).17 The base pressure was 5 × 10−9 Torr in the PLD chamber and 2 × 10−10 Torr in the analysis chamber, respectively. An HF-treated rutile Nb-doped (0.5 wt %) TiO 2 (110) single-crystal (Shinkosha) was used because the HF-treated surface is ready to be atomically flat and free from any impurities that would be brought about during the conventional polishing process, except for carbon.10 10 nm thick TiO2 films with different V doping levels were deposited at 400 °C in 1 × 10−5 Torr O2 on clean Nb-doped TiO2(110) substrates by PLD with KrF excimer laser (λ = 248 nm, ∼1J/cm2, 5 Hz) using TiO2 single-crystal and V-doped TiO2-sintered ceramics targets. After the deposition, the growth of epitaxial films was confirmed by the LEED observation of a sharp (1 × 1) spot pattern, as shown in Figure 1a. The V-
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RESULTS AND DISCUSSION Effects of the V Doping and Electrode Potential on the Photocurrent Density. The photocurrent density Iph (mAcm−2) at an electrode potential U of +0.6 V versus Ag/ AgCl KCl (sat.) was plotted as a function of the V doping level NV (at %), as shown in Figure 2. Iph is found to decrease with
Figure 2. Photocurrent density Iph (mAcm−2) at an electrode potential U of +0.6 V versus Ag/AgCl KCl (sat.) plotted as a function of the V doping level NV (at %). The inset shows a good linear relationship between the inverse of Iph and NV. Figure 1. LEED patterns before and after deposition of V-doped TiO2 by PLD (a) and the corresponding AFM images (b), still exhibiting an atomically flat surface after the deposition.
increasing NV, as was reported by A. I. Kokorin et al.20 As shown in the inset of Figure 2, for NV up to 6 at % there is found a good linear relationship between the inverse of Iph and NV. The red filled circles denote the data from V-doped TiO2 films prepared by a one-step deposition process directly from V-doped sintered ceramics targets, whereas the blue filled circles are the data from those prepared by a two-step deposition process: 8 at % V-doped TiO2 films were first prepared, on which a few nanometers thick TiO2 films were then deposited. As a result, the AES peak intensity of V LMM was much reduced according to the amount of TiO2 films deposited but still could be detected. Taking the effective attenuation length of Auger electrons in this energy range (
