Effects of Cocatalyst on Carrier Dynamics of a Titanate Photocatalyst

Apr 30, 2014 - We have measured the time profiles of transient absorption (TA) of BLT with and without loading NiOx cocatalyst in air and in water in ...
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Effects of Cocatalyst on Carrier Dynamics of a Titanate Photocatalyst with Layered Perovskite Structure Mitsunori Yabuta,† Tomoaki Takayama,‡ Kenji Shirai,† Kazuya Watanabe,† Akihiko Kudo,‡ Toshiki Sugimoto,† and Yoshiyasu Matsumoto*,† †

Graduate School of Science, Department of Chemistry, Kyoto University, Kyoto 606-8502, Japan Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan

‡

S Supporting Information *

ABSTRACT: Cocatalysts are usually needed to improve photoconversion efficiency in water splitting with heterogeneous photocatalysts. Here, we show that NiOx (0 < x < 1) nanoparticles loaded on a layered perovskite, BaLa4Ti4O15 (BLT), serve as an effective electron sink. We have measured the time profiles of transient absorption (TA) of BLT with and without loading NiOx cocatalyst in air and in water in a wide wavelength range from 400 nm to 4 μm upon excitation with a femtosecond pulse at 266 nm. TA at 4 μm indicates that photogenerated electrons are rapidly transferred to cocatalyst within 1 ps. The time profile of TA at 402 nm from sub-μs to 10 ms contributed by surface photoholes is affected by loading of cocatalyst and redox reactions with water. Fittings of the decay profiles of TA at 402 nm with a trap−detrap kinetic model indicate that the oxidation of water appreciably starts at around 1 ms after the pump pulse, while the reduction of water takes place prior to oxidation in much early time domains. This implies that the redox reactions take place under substantial imbalance between electron and hole densities.

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INTRODUCTION Heterogeneous photocatalysts for water splitting have attracted much attention because of their potential for artificial photosynthesis without any burden to the environment.1−8 Photocatalytic water splitting with semiconductor-based powders is composed of a couple of major processes: generation of electrons and holes by optical transitions across the band gap of semiconductor, charge separation and transportation in semiconductor particles, and redox reactions of water at the surface of photocatalyst. Although extensive efforts have been made for synthesizing and testing various kinds of photocatalysts, the present efficiency is not sufficient for practical use. Thus, there are many challenges of improving the efficiency of water splitting.3 One of the most important functions of photocatalysts for water splitting is to accelerate redox reactions of water. Water splitting is a thermodynamically uphill reaction and proceeds via multiple processes. Thus, photocatalysts should be capable to effectively lower the activation barrier of a rate-determining process in the redox reactions. Another important function is to retard charge recombination rates; charges with enough redox power have to survive until they are consumed in the redox reactions. Otherwise, charge recombination results in a direct loss of photons absorbed by photocatalysts. It is ideal to have a photocatalyst that harvests light effectively and catalyzes the redox reactions at its own surface. However, this is very demanding, and such a photocatalyst has not been synthesized so far. A realistic approach for practical water splitting is to separate functions with the aid of cocatalysts.9 In © XXXX American Chemical Society

view of the functions needed for effective photocatalysis, cocatalysts have to have two major roles in photocatalysis: first, cocatalysts have to provide active reaction sites to lower the activation barrier for redox reactions; second, cocatalysts have to be a sink of charges, leading to spatial separation between electrons and holes and reduction of the charge recombination rate. Many cocatalysts in the semiconductor-based photocatalysts have been tested so far, including noble metals, metal oxides, metal sulfides, and biomimetic hydrogenase.9 In fact, substantial improvements in the overall energy conversion efficiency have been reported with loading of cocatalysts. Nickel oxide has been known as a promising cocatalyst for water splitting since the early stage of water splitting studies. Domen and co-workers have found that NiO-loaded SrTiO3 (STO) powder steadily decomposes water photocatalytically.10−12 NiO cocatalysts are often “activated” for improving photoconversion efficiency. In the activation process, NiO loaded on STO powder is first reduced with hydrogen and then partially oxidized. The cocatalysts treated in this way is denoted as NiOx (0 < x < 1). They have investigated the structure of NiOx-loaded STO powder and reported that nickel metal exists at the interface between NiO and STO.13 On the basis of the structural information, they have proposed that the Ni metal layers at the interface play an important role in transferring photogenerated electrons from STO to NiOx, resulting in the Received: March 20, 2014 Revised: April 30, 2014

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were detected with a photomultiplier after passing through a monochromator with a bandwidth of about 50 nm. Probe pulses at 4 μm were detected with a mercury cadmium telluride detector after passing through a Ge filter and a monochromator. The pump beam was chopped at a frequency of 500 Hz, and TA signals were detected by taking the differences between signal intensities with and without pump. For TA measurements in the μs−ms time range, the repetition frequency of the regenerative amplifier was set to 1 kHz or 19 Hz, depending on the time range of interest. A photodiode or a photomultiplier was used for detection of the scattered light of the continuous wave lasers or the halogen lamp. Samples. BLT and NiOx−BLT showing activity for water splitting under UV light irradiation were employed as samples for the measurements of carrier dynamics. These samples were prepared by the same method as previously reported.15 CaCO3 (Kanto Chemical; 99%), SrCO3 (Kanto Chemical;99.9%), BaCO3 (Kanto Chemical; 99%), La(NO3)3·6H2O (Wako Pure Chemical; 99.9%), Ti(OC4H9)4 (Kanto Chemical; 97%), citric acid (Sigma-Aldrich Japan; 99.5%), and propylene glycol (Kanto Chemical; 99.0%) were employed as starting materials for the preparation by a polymerized complex method. These metal compounds and a citric acid were dissolved in a mixed solvent of ethanol and propylene glycol. The mixed solution was aged at 353 K for 2 h. The resultant solution was dehydrated to form gel by polymerization at 403 K for 6 h. A precursor was obtained by calcination of the gel using a mantle heater. BLT was prepared by calcination of the precursor at 1373 K for 10 h. NiO-loaded BLT was prepared by an impregnation method from an aqueous Ni(NO3)2 solution. The amount of NiO loading was 0.5 wt %. The powder impregnated with Ni(NO3)2 was calcined at 573 K for 1 h in air. NiOx−BLT was prepared from NiO-loaded BLT by reduction with 200 Torr of H2 at 773 K for 2 h and subsequent oxidation with 100 Torr of O2 at 573 K for 1 h. NiOx obtained by the pretreatment represents a core−shell structure of metallic Ni inside and NiO outside.13 The activity for water splitting of BLT is substantially enhanced by loading of NiOx cocatalyst. NiOx−BLT (0.5 g) produced H2 and O2 at the rates of 2.3 and 1.14 mmol h−1 form pure water using an inner irradiation reaction cell made of quartz equipped with a 400 W high-pressure mercury lamp, while nonloaded BLT gives those at the rates of 0.005 and 0.002 mmol h−1.

enhancement of hydrogen evolution at the surface of NiOx cocatalyst. However, experimental evidence for supporting the role of NiO and NiOx as an electron sink is scarce. Yamakata et al. have measured mid-infrared transient absorption measurements of NiO cocatalysts loaded on NaTaO3;14 the limited time resolution in their measurements only gave them an upper limit of time constant of electron transfer from NaTaO3 particles to NiO cocatalysts, i.e., 1 μs. NiO also enhances the photocatalytic water splitting activities of various photocatalysts with a (111) plane-type layered perovskite structure,15 including A5Nb4O15 (A = Sr, Ba), Ba3LaNb3O12, ALa4Ti4O15 (A = Ca, Sr, Ba), and La4Ti3O12. The anisotropic structures of perovskites with interlayers are believed to be advantageous to separate reaction sites for oxidation of water from those for reduction; these structures allow to prevent reverse reactions. Among them, BaLa4Ti4O15 with NiOx nanoparticles is one of the best photocatalysts showing high conversion efficiency; NiOx loading improves the efficiency of water splitting by almost 3 orders of magnitude to reach the quantum yield of 15% at 270 nm.15 This paper reports a systematic study of transient absorption of BaLa4Ti4O15 and NiOx-loaded BaLa4Ti4O15 photocatalysts (denoted as BLT and NiOx−BLT, respectively) in a wide wavelength range from 400 nm to 4 μm and a wide time range from subpicoseconds to a few tens of milliseconds. Comparison of transient absorption of BLT with and without loading NiOx provides detailed information on electron transfer from light harvesting photocatalyst BLT to NiOx cocatalyst. Furthermore, analyzing temporal decays of transient absorption of NiOx− BLT in air and water with a kinetic model, we obtain the time scales of electron and hole transfers for the redox reactions.

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MATERIALS AND METHODS Light Sources. The light source for transient absorption (TA) spectroscopy in the fs−ps time range was a femtosecond Ti:sapphire laser with a regenerative amplifier (Spitfire, SpectraPhysics, 800 nm, 170 fs). The fundamental output of the regenerative amplifier was divided into two beams. One of the beams was used for pump pulses at 266 nm, and the other was used for probe pulses for ultrafast measurements at 400 nm, 645 nm, and 4 μm. Pump pulses at 266 nm were generated with barium borate (BBO) crystals by frequency-tripling of fundamental pulses of the regenerative amplifier. The probe pulses at 400 nm were generated with a BBO crystal by frequency-doubling of the fundamental pulses. The probe pulses at 645 nm and 4 μm were prepared by generation of second harmonic and difference frequency of output pulses of a home-built optical parametric amplifier16 pumped by the regenerative amplifier, respectively. For TA measurements in the μs−ms time range, pump pulses were the same as used in the ultrafast measurements, i.e., fs pulses at 266 nm, while continuous light sources were used for probe: a halogen lamp, a diode laser at 402 nm, and a He−Ne laser at 633 nm. Diffuse Reflectance Transient Absorption. We measured TA time profiles at several wavelengths using a diffuse reflectance configuration. A powder of BLT was placed in air or precipitated in distilled water. Pump and probe beams were overlapped at the surface of the sample powder, and the probe light scattered from the sample was collected with a lens and detected as a function of delay time with respect to the pump pulse. For TA measurements in the fs−ps time range, the repetition frequency of the regenerative amplifier was set to 1 kHz. Probe pulses at 400 or 645 nm scattered from the sample

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RESULTS AND DISCUSSION Effects of NiOx Loading on Transient Absorption. Pump pulses at 266 nm (4.66 eV) excite electrons in the valence band to the conduction band of BLT across the band gap, 3.85 eV.15 Transient absorption (TA) originating in photogenerated carriers was observed in a wide wavelength range from 400 nm to 4 μm. First, we concentrate on carrier dynamics within a delay time of 50 ps. Figure 1 shows the temporal profiles of transient absorption of BLT in air at a couple of representative probe wavelengths: 400 nm, 645 nm, and 4 μm. In this time range, the TA time profiles at 402 and 645 nm decayed almost linearly with delay time, whereas the profile at 4 μm clearly showed a fast and a slow decay components. Moreover, loading of NiOx on BLT made a remarkable difference in the TA time profile at 4 μm as shown in Figure 1a. While the TA time profiles of both bare BLT and NiOx−BLT have two decay components, the relative contributions are very different. Clearly the TA of NiOx−BLT decays faster than that of bare BLT: τ = 0.49 vs 1.2 ps, B

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Figure 1. Time profiles of transient absorption of bare BLT (black, filled circle) and NiOx−BLT (red, triangle) measured in air at (a) 4 μm, (b) 400 nm, and (c) 645 nm. In (a), the decay curves obtained with a pump fluence of 3 (solid line) and 12 mJ cm−2 (dashed line) are depicted.

Figure 2. Time profiles of transient absorption of bare BLT (black) and NiOx/BLT(red) measured in air at (a) 402 and (c) 633 nm. The intensity is normalized at 10 ns.

respectively, and the amplitude of the fast component of NiOx−BLT is about twice as large as that of bare BLT. The time profiles of TA of NiOx−BLT at 4 μm did not depend on the fluence of pump pulses in the range from 3 to 12 mJ cm−2. According to Yamakata et al.,14 we assume that TA signals at 4 μm mostly originate from photogenerated charges in BLT rather than in NiOx. Since transient absorption signals in the mid-infrared region observed in pump−probe measurements of TiO217−21 and tantalates14,22,23 have been attributed to photogenerated electrons, we assigned the TA at 4 μm to photogenerated electrons in BLT. The larger contribution of the fast decay component in TA decay characteristics of NiOx− BLT than that of bare BLT indicates that photogenerated electrons in BLT are effectively transferred to NiOx cocatalyst within the time range of 1 ps. This supports the proposal made by Domen and co-workers13,24 in the study of photocatalytic decomposition of water with NiOx−STO: the metallic Ni layer between NiO and BLT created in the activation pretreatment makes an Ohmic contact. The time profiles of TA both at 400 and 645 nm are very different from that at 4 μm, as shown in Figure 1b,c. They did not have a fast decay component. Moreover, they were little affected by loading of NiOx. These results indicate that photogenerated electrons do not contribute to TA at 400 and 645 nm. Hence, TA in these visible wavelengths is mainly contributed by photogenerated holes. Time profiles of TA in a long delay time scale provide a clue to the nature of holes responsible for the transient absorption at the visible wavelengths. Figure 2 shows TA profiles probed at 402 and 633 nm in the time range from 10 ns to 10 ms. The TA profile at 402 nm was significantly decelerated by loading of NiOx, whereas the TA profile at 633 nm was little affected. This can be interpreted as follows. As shown in Figure 1a, photoexcited electrons near the surface of BLT are effectively transferred to cocatalyst particles and electron densities in BLT and NiOx reach a pseudoequilibrium rapidly because of the

Ohmic contact between NiOx and BLT. Although the same densities of electrons and holes are initially created both in the bulk and at the surface of BLT by the electronic transition across the band gap, the rapid electron transfer between BLT and NiOx makes the electron density at the surface of BLT much less than that of bare BLT; thus, the imbalance between electron and hole densities at the surface of BLT makes electron−hole recombination slower. Consequently, the TA profile at 402 nm is attributable to holes near the surface. In contrast, the fact that the decay characteristics at 633 nm are not altered significantly by NiOx loading indicates that the TA at this wavelength is mainly contributed by holes in bulk BLT. This clear wavelength dependence and sensitivity to cocatalyst loading suggest that the mobility of holes in BLT is poor; holes in the bulk BLT are not transported to the surface even in the time range of 10 ms. Figures 3a and 3b show the probe wavelength dependence of TA time profiles of bare BLT and NiOx−BLT, respectively, in the time range from 1 μs to 40 ms. The wavelength dependence is little affected by loading of NiOx. In both the samples, the decay rate of TA increases with probe wavelength. Assuming that the TA profile at 400 nm is mainly contributed by surface holes whereas that at 600 nm by bulk holes, we decomposed the time profiles of TA in the wavelength range from 400 to 600 nm into the two decay curves at 400 nm, I400, and 600 nm, I600: I(t;λ) = α(λ)I400(t) + β(λ)I600(t). Figures 3c and 3d show the plot of the coefficients, α(λ) and β(λ), as a function of probe wavelength. These plots indicate that the contributions of surface and bulk holes change almost linearly with probed wavelength. In this longer delay time range, photogenerated holes survived without suffering from recombination with electrons likely form small polarons at lattice oxygen to form O− whose energy depends on its local environment: whether they are in the bulk or the surface layers. Recent calculations of interpolaron optical transitions of anatase TiO2 have shown C

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Figure 3. Probe wavelength dependence of time profiles of transient absorption of (a) bare BLT and (b) NiOx−BLT measured in air. Coefficients α(λ) and β(λ) obtained by decomposing the time profiles of transient absorption into two decay curves at 400 and 600 nm are plotted in (c) for bare BLT and (d) for NiOx−BLT as a function of probe wavelength.

Figure 4. Time profiles of transient absorption decay profiles of NiOx−BLT in water (red) in comparison with those in air (black) measured at (a) 402 and (b) 633 nm. The intensity is normalized at 10 ns.

that the transition energy of O− in the surface layer is larger than that in the bulk25 because the energy difference between the O− states involved in the interpolaron transition is larger at near the surface than in the bulk; this reproduces a blue-shift of the band of holes after excitation in ps transient absorption measurements of anatase TiO2.26−29 Assuming the similar trend of energy of holes in the current system, we can understand why the TA at shorter wavelengths (∼400 nm) is mainly contributed by surface holes, while that at longer wavelengths (∼600 nm) is contributed by bulk holes. Effects of Surrounding Water on Transient Absorption. Measurements of BLT in water are useful for obtaining the time scale of charge transfer at the interface between BLT and water. We measured the transient absorption of both bare BLT and NiOx−BLT in water in comparison with those in air. The decay profiles at 402 and 633 nm of bare BLT in water were not different from those in air (Figure S1 in the Supporting Information), indicating that charge transfer at the interface leading to redox reactions is very slow in the case of bare BLT. Because the evolution rates of H2 and O2 are larger by almost 3 orders of magnitude than those of bare BLT, the decay profiles of NiOx−BLT in water were expected to deviate from those in air. Indeed, deviations were found in the case of NiOx−BLT. Figure 4 shows the decay profiles of TA of NiOx− BLT in water in comparison with those in air. Although there is little difference between the results in water and in air at 633 nm, the decay at 402 nm in water deviates from that in air particularly in the range from 10 μs to 10 ms. The fact that the TA time profile at 633 nm is insensitive to the environment of BLT, whether BLT is in air or in water, is also indicative that bulk holes are responsible for the TA profile at this wavelength. If the hole mobility in BLT is small, consumption of carriers at water/photocatalyst interfaces affect little the density of holes in the bulk. In contrast, the deviation of TA time profile at 402 nm in water from that in air reinforces the assignment that surface holes are responsible for TA at 402 nm. Reduction reactions taking place at the interface between

water and NiOx cocatalyst consume electrons at the surface; thus, this makes electron density near the surface of BLT lower than that in air because electrons at the BLT surface can be effectively transferred to cocatalyst through the Ohmic contact at the NiOx−BLT interface. Consequently, the imbalance between the densities of electrons and holes near the surface becomes more significant than NiOx−BLT in air, resulting in deceleration of a charge recombination rate. In contrast to reduction reactions, oxidation at the BLT surface accelerates the decay of surface holes. Therefore, the redox reactions influence the decay of surface holes in the opposite ways: reduction at the NiOx surface decelerates the decay of surface hole density, whereas oxidation at the BLT surface accelerates the decay. In the next section, we describe the analysis of TA time profiles of NiOx−BLT at 402 nm with a kinetic model to extract information on time scales where redox reactions take place effectively. Analysis with a Trap−Detrap Model. Elementary steps in the kinetic model used in this study are depicted in Scheme 1, and the rate equations corresponding to this scheme are given in the Supporting Information. In this kinetic model, we only deal with charges at the surface of catalyst because the wavelength dependence of TA profiles indicates that the mobilities of holes and electrons are small; photogenerated Scheme 1. A Schematic Representation of the Kinetic Model Employed

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charges in the bulk do not affect the kinetics of charges at the surface. Excitation across the band gap of BLT by ultraviolet light produces electrons in the BLT conduction band and holes in the BLT valence band. They relax rapidly to each of the band edges. A pseudoequilibrium of electron densities is rapidly established between BLT and cocatalyst because of the Ohmic contact at the interface between a nanoparticle of cocatalyst and a BLT particle.13,24 Electrons in the conduction band and holes in the valence band are trapped and detrapped at electron and hole trapping sites, respectively, at the surface of BLT. Carrier densities created by the electronic excitation at the surface of catalyst are decreased through recombination between detrapped electrons and holes. Thus, the charged species involved in this model are in the following: electrons in shallow trap sites (e−s ), electrons in cocatalyst NiOx (e−c ), electrons in deep trap sites (e−d ), holes in shallow trap sites (h+s ), and holes in deep trap sites (h+d ). A couple of assumptions were made. First, we assume that electrons transferred to cocatalyst are responsible for reduction.13,24 Second, we assume that holes in shallow trap sites rather than holes in deep trap sites are responsible for oxidation. Finally, we assume that only holes in shallow trap sites contribute to transient absorption at 402 nm. The kinetic model under these assumptions is denoted as model 1. First, we fitted the decay curve of NiOx−BLT in air at 402 nm. Because redox reactions do not take place in air, we kept the rate constants k5 and k6 to be zero. Using the experimental results in the ps time range (Figure 1a), we obtained the rate constants of k2 and k−2 to be 1.0 × 1012 and 0.5 × 1012 s−1, respectively, and they were fixed in the following fitting procedures. The fitting quality is satisfactory as shown in Figure 5a, and the rate constants obtained are tabulated in Table S1 (Supporting Information). The fitting result indicates that the densities of both electrons in shallow trap sites and in cocatalyst substantially decreases in a few μs because of effective trapping at deep trap sites of BLT. Electrons trapped in deep trap sites maintain high densities until ∼1 ms. Because TA measurements were performed in air, some fraction of electrons may be trapped by O230−32 as O2− at the surface of BLT in addition to electron trap sites at the surface. The density of holes in deep trap sites rises around 10 μs, reaches the maximum at ∼300 μs, and decays afterward. Second, we fitted the decay curve of NiOx−BLT in water at 402 nm. All the rate constants except for those of reduction and oxidation: k5 and k6, respectively, were fixed at the values obtained in the fitting of the decay in air. The quality of fitting is acceptable but poorer than that in air, particularly in the time range around 5−100 μs, as shown in Figure 5b. The rate constants obtained are k5 = (3.8 ± 0.1) × 105 s−1 and k6 = (1.02 ± 0.02) × 102 s−1. In comparison with the decay in air, the density of electron in deep trap sites is smaller because electrons in cocatalyst are consumed by reduction of water. In water, oxygen coverage could be smaller because of smaller O2 density in water than in air. Moreover, the energy levels of charge trapping states of photocatalysts may change from those in air because of the interaction with water. Thus, fitting was made by relaxing the constraint; the rate constants other than k1, k2, and k−2 were varied in addition to k5 and k6. As shown in Figure 5c, the fitting quality is improved. Although the rate constants changed from those obtained in the fitting of the sample in air (Table S1 in the Supporting Information), the

Figure 5. Fitting results of the time profiles of transient absorption decay profiles at 402 nm; (a) NiOx−BLT in air, (b) in water fitted with free parameters of k5 and k6, while k1, k±2, k±3, and k±4 were fixed at the values obtained by the fitting of data in air, and (c) in water fitted with all parameters except for k1 and k±2. The time evolutions of density of each charged species are also plotted, which are calculated with the rate constants obtained by the fittings.

qualitative features of time-dependent variations of charged species did not change. In these fittings, we assumed that no charges are trapped prior to photoexcitation. As in the case of TiO2, NiOx−BLT could be an n-type semiconductor due to donor states in the band gap provided by, for example, surface oxygen vacancies. To examine the effect of donor states on the rate constants in the kinetic model, we fitted the decay curve of NiOx−BLT in water at 402 nm, increasing a fraction of electrons preoccupied in the deep trap sites up to 20% of photoexcited electrons. The rate constants obtained by fittings are given in Table S1 (Supporting Information). In these conditions, both the rate constants k3 and k−3 decrease with increasing initially trapped electron density, while the ratio k3/k−3 is almost constant around 6.0, indicating that the time-dependent density ratio between shallow and deep trapped electrons is not altered by the preoccupied deep trapped electrons. Thus, the initially trapped electrons in the donor states do not change significantly the other rate constants. Accordingly, we can safely neglect the effects of preoccupied donor states in the following kinetic analysis. We can estimate how redox reactions evolve using the fitting results. Time evolutions of the amounts of electrons and holes lost by redox reactions per an excitation light pulse, Ne(t) and Nh(t), respectively, are represented as Ne(t ) = E

∫0

t

k5[e−c (t ′)] dt ′

(1)

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∫0

t

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trap sites as holes transferred to NiO that are responsible for water oxidation, whereas e−c as electrons in Ni nanoparticles that are responsible for water reduction in their scheme, we could also fit the time profile of TA at 402 nm obtained from NiOx−BLT in water. Fitting results are depicted in Figure S4 (Supporting Information). Although the rate constants (Table S3 in the Supporting Information) are different from those obtained in models 1 and 2, the time scales of charge consumption in redox reactions do not change, as shown in Figure 6: the reduction of water proceeds much faster than the oxidation of water. The kinetic analysis indicates that holes have to have a long lifetime at least a few ms to be involved in water oxidation. The similar long photohole lifetimes required for oxygen evolution have been reported in the studies of nanocrystalline TiO2 film: 0.27 s at pH ∼ 6.5;34 ∼30 ms at pH 12.7.35 Because oxygen evolution requires multiple holes at a single reaction site, photoholes have to sustain long enough and be transported to the site to proceed the reaction. Furthermore, a ratedetermining step among the complicated series of four-electron and four-proton processes requires a long hole lifetime. Recent chemical dynamics calculations36 have shown that the first proton-coupled electron transfer process

(2)

where [e−c ] and [h+s ] are the densities of e−c and e+s , respectively. Figure 6 shows the calculated results of electrons and holes

Figure 6. Time profiles of densities of electron (solid line) and hole (dotted line) consumed in redox reactions estimated from the fitting results with all rate constants except for k1 and k±2 that were fixed (red). Model 1 (blue): h+s oxidizes water and contribute to the transient absorption. Model 2 (green): h+s oxidizes water and both h+s and h+d contribute to the transient absorption. Model 3 (red): h+d oxidizes water and h+s contributes to the transient absorption.

consumed in redox reactions in model 1. The loss of electron, on the one hand, becomes appreciable in the sub-μs range, increases again at around 100 μs, and then saturated at around 3 ms; the first rise of Ne(t) corresponds to the consumption of e−s , while the second rise corresponds to the consumption of e−d . On the other hand, the loss of holes becomes appreciable at around a few ms and saturated at around 30 ms. Namely, the loss of electron due to reduction of water proceeds earlier than that of hole due to oxidation of water by 3−4 orders of magnitude in delay time. The calculated results using the rate constants obtained by fitting with the relaxed constraint did not change the delay time dependence of charges consumed (see Figure S2 in the Supporting Information). The kinetic model to fit the TA profiles is not unique. In model 1, we assumed that only holes in shallow trap sites of BLT near the surface contribute to absorption at 402 nm. We found that the time profile of TA can be fitted as well with the kinetic model assuming that both h+s and h+d contribute to the absorption (model 2, see Figure S3 in the Supporting Information). However, as shown in Figure 6, the evolutions of densities of charges consumed in redox reactions reproduce the similar trend: reduction proceeds much faster than oxidation. Another ambiguity in the kinetic model stems from the controversy in the reaction scheme of water splitting with NiOx-loaded STO. Townsend et al. have recently investigated the roles of NiOx cocatalyst on STO powders by surface photovoltage and electrochemical measurements.33 They claimed that NiOx-loaded STO is actually composed of three components: STO powders, nanoparticles of metallic Ni, and NiO. They concluded that electrons are transferred to metallic Ni nanoparticles, while holes are transferred to NiO nanoparticles; Ni and NiO nanoparticles are involved in reduction and oxidation reactions, respectively. This contradicts the traditional reaction scheme proposed by Domen and coworkers:10−12 water reduction takes place at partially reduced NiOx nanoparticles, while water oxidation does at the surface of STO. Our kinetic model can adapt the reaction scheme proposed by Townsend et al. as well (model 3). Regarding holes in deep

H 2O + h+ → OH + H+

(3)

which is a possible process responsible for the large overpotential, takes place in sequential processes: proton transfer followed by electron transfer. Particularly at low pH, the proton transfer is rate determining because of its energy barrier. Thus, a hole at a 3-fold coordinated oxygen atom needs to wait for a proton being transferred from an adsorbed water to a water molecule hydrogen bonding to it.36 A set of rate constants obtained from the kinetic analysis of TA temporal profiles provides how photocatalyst particles are charged under continuous irradiation. Figure 7 shows the

Figure 7. Densities of charged species as a function of continuous irradiation intensity calculated with the rate constants determined in model 1, where shallow trapped holes are assumed to be responsible for oxidation of water.

densities of charged species as a function of intensity of continuous light. Because the reduction of water at NiOx takes place faster than oxidation, the stationary densities of electrons in shallow trap sites and in NiOx are substantially smaller than those of the others. The densities of holes in shallow and deep trap sites are dominated over that of electron in deep trap sites. Therefore, the calculations based on the kinetic model suggest that BLT particles are in average positively charged during the irradiation. Charge distribution on photocatalyst surfaces could be spatially inhomogeneous; positive charges are mainly distributed at the surface of BLT, while negative charges are F

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in NiOx nanoparticles. This inhomogeneous charge, and hence surface voltage distribution drives the redox reactions.

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CONCLUSION We have measured time profiles of transient absorption of BLT with and without NiOx in a wide wavelength range from 400 nm to 4 μm. Photogenerated electrons in the conduction band of BLT are distributed in BLT and NiOx within 1 ps because of the Ohmic contact at the interface. This is a clear piece of evidence that NiOx nanoparticles serve as an effective electron sink, which has been proposed in the earlier studies on NiOxSTO.10−12 While holes in bulk contribute to the transient absorption at around 600 nm, holes at surfaces contribute that at around 400 nm. Fittings of the decay profiles of transient absorption in air and water with kinetic models indicate that the reduction of water proceeds faster than the oxidation of water by almost 3 orders of magnitude. Thus, the faster consumption of electron than that of hole results in photocatalyst powders of BLT positively charged, where the oxidation of water takes place with holes survived without suffering recombination. Hole migration from bulk to surface is very slow. Thus, reducing the size of BLT would be effective for improving the photoconversion efficiency in water splitting.

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ASSOCIATED CONTENT

* Supporting Information S

Time profiles of transient absorption of bare BLT, the details of kinetic model with the rate equations, and the rate constants determined by fittings. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Phone +81-75753-4047 (Y.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (A) from the Japan Society for the Promotion of Sciences (Grant No. 25248006) and by a Grant-in-Aid for Advanced Research, Tokyo University of Science.

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REFERENCES

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