LETTER pubs.acs.org/JPCL
Charge Carrier Dynamics on Mesoporous WO3 during Water Splitting Federico M. Pesci,† Alexander J. Cowan,† Bruce D. Alexander,‡ James R. Durrant,† and David R. Klug*,† † ‡
Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom School of Science, University of Greenwich, Central Avenue, Chatham Maritime, Kent, ME4 4TB, United Kingdom
bS Supporting Information ABSTRACT: Transient absorption spectroscopy (TAS) has been employed to identify the spectra of photogenerated electrons and holes on WO3. WO3 is a widely studied photoanode for the oxidation of water, and by measuring the decay of photoholes on the milliseconds to seconds time scale in the presence of an electron scavenger, we provide a lower limit for the required lifetime of holes for water oxidation on WO3. The rate of electron/hole recombination on WO3 has also been examined at a range of excitation intensities and is found to be nonlinear with excitation intensity. SECTION: Kinetics, Spectroscopy
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oncerns over atmospheric CO2 levels and decreases in proven reserves of fossil fuels have led to a focus on the production of carbon-free forms of energy. Photocatalytic water splitting using semiconductors is potentially an inexpensive route to the storage of solar energy. A range of semiconducting materials have been studied since the first report of photoelectrochemical water splitting using titanium dioxide electrodes.1 TiO2, Fe2O3, and WO3 have been widely studied due to their high stability and low cost, although the large band gap of TiO2 (anatase, 3.2 eV) leads to a low theoretical maximum solar-tohydrogen yield.2 4 There is considerable interest in the use of WO3 as a photoanode in photoelectrochemical cells due to the reported high incident-photon-to-current efficiencies5 7 and the ability of WO3 to absorb a greater portion of the solar spectrum than TiO2 (WO3 Eg ≈ 2.7 eV). Despite significant interest in developing new photocatalytic materials, there is limited experimental data on the factors determining the efficiency of materials such as WO3. TAS is a useful tool for the study of charge carriers dynamics in semiconductor films, and we have recently employed TAS to identify the dynamics of key processes in R-Fe2O3 and TiO2.8 11 Spectroelectrochemical measurements on WO3 have identified an absorption, increasing with wavelength, at negative applied potentials, which has been assigned to conduction band electrons.12,13 A similar UV vis spectrum of excess electrons on WO3 colloids has been reported in pulse radiolysis and photolysis experiments of WO3 in the presence of formic acid, an efficient hole scavenger.14 The broad photoelectron absorption spectrum of WO3 is consistent with the presence of free charge carriers, similar to that observed on several other semiconductors including TiO2 and ZnO;15 17 however, previous TAS experiments have measured that electron trapping on WO3 occurs very rapidly (k ≈ 1010 s 1).12 r 2011 American Chemical Society
To the best of our knowledge, the absorption spectrum of the photoholes on WO3 has not been assigned. The formation of long-lived holes is a prerequisite for water oxidation on TiO2 and R-Fe2O3 (∼0.3 and ∼3 s, respectively).8,9,11,18 To understand the behavior of WO3 photoelectrodes for water oxidation, it is desirable to monitor the photohole dynamics. In this Letter, we present a TAS study of photoelectrons and photoholes in mesoporous WO3 films. Identification of the kinetics of the key processes determining the efficiency of WO3 photoelectrodes for water photooxidation is an important goal in aiding the development of this material. Mesoporous WO3 films were prepared using the method described by Santato et al.7 and were characterized using Raman microscopy and XRD and were found to consist of monoclinic WO3. Details of experimental conditions and instrumentation employed for the transient measurements can be found in the Supporting Information. TAS experiments are recorded following the UV excitation (355 nm) of WO3; rapid scavenging of the electrons or holes can then be employed to aid charge carrier characterization. In previous TAS studies on TiO2, the fast scavenging of photoholes by methanol has been used to identify the absorption spectrum of the photoelectrons,19 and the hole scavenging effect of CH3OH on WO3 has been investigated electrochemically.20 The spectrum recorded at 10 μs (Figure 1a) in the presence of methanol shows a strong enhancement at longer wavelengths compared with the spectrum recorded in an argon atmosphere, indicating that scavenging of photoholes by methanol occurs significantly faster than we can observe (∼1 μs). Received: June 21, 2011 Accepted: July 14, 2011 Published: July 14, 2011 1900
dx.doi.org/10.1021/jz200839n | J. Phys. Chem. Lett. 2011, 2, 1900–1903
The Journal of Physical Chemistry Letters
LETTER
Figure 1. TAS spectra recorded in the absence/presence of (a) CH3OH as a hole scavenger and (b) AgNO3 as an electron scavenger following UV excitation (355 nm, ∼250 μJ/cm2) of WO3 from the substrate side.
Rapid electron trapping is known to occur on WO3;12 therefore, the TAS signal at wavelengths greater than 750 nm is assigned to trapped photoelectrons, in agreement with previous studies.12,21 The electron absorption decays very slowly (ms s) in the presence of methanol because electron/hole recombination is prevented and reduction of dissolved oxygen by photoelectrons is also minimized by purging the solution with argon. To identify the photoholes spectrum, we have investigated WO3 in the presence of an efficient electron scavenger. Literature examples of electron scavengers include Fe3+ and Ag+.22,23 The oxidation of Fe2+ by photoholes is a significant loss pathway. Therefore, we have employed Ag+ as an electron scavenger, despite the deposition of silver nanoparticles potentially leading to distinct UV absorption features.24 The TAS spectrum recorded following the immersion for 12 h of a WO3 film in 0.01 M AgNO3(aq) in the dark shows a strong enhancement at short wavelengths compared with the spectrum recorded under an argon atmosphere, Figure 1b. The absorption with a maximum at ∼475 nm is assigned to photoholes on WO3 as rapid electron scavenging occurs in this experiment. A previous spectroelectrochemical study observed a similar absorption feature on WO3 films held at strongly negative potentials in a perchlorate electrolyte;13 however, this was an irreversible process, and the reversibility of our transient absorption signal at ∼475 nm indicates that we are observing a different spectral feature. We have also ruled out the possibility of this transient absorption being due to the presence of silver nanoparticles by carrying out experiments in the presence of both a hole and an electron scavenger (see Supporting Information, Figure S1). In the absence of chemical scavengers, rapid electron/hole recombination is expected to occur. The TAS spectrum of WO3 under an argon atmosphere is shown in Figure 1a,b. The very broad TAS spectra recorded are due to the overlap of a combination of the photoholes and photoelectrons spectra. Under argon, the transient absorption signal at 475 nm is significantly weaker at 10 μs than that in the presence of silver ions, and we estimate that more than 95% of the charge carriers have recombined by 10 μs, in agreement with a previous transient microwave study that measured ∼80% recombination within 500 ns of UV excitation (∼2.3 mJ/cm2).25 The dependence of the electron/hole recombination rate on the intensity of the incident laser excitation has been monitored by recording TAS signals of the photoelectrons at 900 nm in electrolyte (NaClO4 0.5 M) at pH 1.89 (HClO4) in the absence of chemical scavengers between 18 and 1500 μJ/cm2, Figure 2. The electron/hole recombination displays a nonlinear dependence
Figure 2. Dependence of the WO3 photoelectron transient absorption signal (900 nm) on the UV (355 nm) excitation intensity. The traces were recorded between 18 and 1500 μJ/cm2 in NaClO4, 0.5 M, pH 1.89 (HClO4). ΔAbsorbance values were recorded at 10 μs after laser excitation. Fit lines to a linear (blue) and nonlinear (red) function of the form y = Axb (b ≈ 0.5) are shown.
on the laser intensity, and the signal amplitude (900 nm) at 10 μs is well fitted by a power function of the form y = Axb. The nonlinear dependence of the recombination rate on the laser intensity highlights the need for transient experimental studies to be carried out at excitation intensities that are suitably low enough to mimic solar fluxes. It has been reported that irradiation of WO3 in the presence of AgNO3 leads to water oxidation;23 therefore, in order to confirm that oxygen production is occurring in the solutions employed during our TAS experiments, we have measured the dissolved oxygen concentration using an oxygen membrane polarographic electrode (details in Supporting Information). In the absence of AgNO3, the oxygen reading decreases very slowly under Xe lamp illumination, suggesting that a very low level of oxygen photoreduction occurs. Significantly, under these conditions, no oxygen production occurs, and this supports our TAS experiments that show that rapid (
