Particle Size Dependence of Carrier Dynamics and Reactivity of

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Particle Size Dependence of Carrier Dynamics and Reactivity of Photocatalyst BiVO Probed with Single-Particle Transient Absorption Microscopy 4

Mitsunori Yabuta, Atsuhiro Takeda, Toshiki Sugimoto, Kazuya Watanabe, Akihiko Kudo, and Yoshiyasu Matsumoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06230 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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The Journal of Physical Chemistry

Particle Size Dependence of Carrier Dynamics and Reactivity of Photocatalyst BiVO4 Probed with Single-Particle Transient Absorption Microscopy Mitsunori Yabuta,† Atsuhiro Takeda,‡ Toshiki Sugimoto,‡ Kazuya Watanabe,‡ Akihiko Kudo,¶ and Yoshiyasu Matsumoto∗,‡ Kyoto University, Graduate Shool of Science, Department of Chemistry, Kyoto 606-8502, Japan, Kyoto University, Graduate School of Science, Department of Chemistry, Kyoto 606-8502, Japan, and Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. E-mail: [email protected]

Abstract Heterogeneous photocatalytic water splitting under the irradiation of sunlight is an attractive method for generating hydrogen from water. While the photocatalytic mechanism has been extensively studied, most of experimental studies have been performed with an ensemble of photocatalyst particles with various sizes, morphologies, and secondary structures. To gain a deeper understanding of the mechanism of photocatalysis, it is indispensable to clarify how the geometric structure of photocatalyst ∗

To whom correspondence should be addressed Kyoto University ‡ Kyoto University ¶ Tokyo University of Science †

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affects the kinetics of photogenerated carriers and redox reactions. In this study, the hole decay characteristics and photocatalytic activity of BiVO4 , a promising photocatalyst for oxygen evolution with visible light, have been investigated with single-particle transient absorption microscopy. Upon the irradiation with 527 nm light, well-faceted non-aggregated crystallites show fast hole decay and little reactivity for Fe3+ reduction. In contrast, aggregated particles with grain boundaries between small primary crystallites show slower hole decay and higher reactivity for Fe3+ reduction than the non-aggregated crystallites. This behavior is increasingly pronounced as the secondary particle size of aggregated crystallite increases. This indicates that grain boundaries in aggregated particles do not work as recombination centers, but play an important role in elongation of carrier lifetime and thus in enhancing the reactivity of photocatalyst through trap-detrap processes.

Introduction Water splitting with heterogeneous photocatalysts driven under sunlight is one of the promising artifical photosynthetic methods for generating clean environment-friendly fuel: hydrogen. 1–8 Many factors have to be concerned to fabricate photocatalysts with better photon energy conversion efficiency, including the band structure, the size and morphology, the facet orientation, and the chemical stability of photocatalyst particles. Moreover, one has to design photocatalytic systems from the view point of kinetics. Although carriers generated by photoexcitation across the band gap of a photocatalyst can induce redox reactions, a large number of photogenerated carriers are lost as a result of effective electron-hole recombination. This is a major energy loss pathway in photocatalysis. In particular, the slow kinetics of oxygen evolution reaction in water splitting requires photogenerated holes to be substantially long lived. Thus, reducing the electron-hole recombination rate is of paramount importance to improve photon energy conversion efficiency in water splitting. Heterogeneous photocatalysts are usually composed of an ensemble of particles with


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various sizes, where primary particles aggregate in complex secondary structures. Aggregated photocatalyst particles have carrier trap sites in the bulk of a primary particle, at the interface with water, and at grain boundaries between primary particles. Transport and quenching of photogenerated carriers, and redox reactions at the surface of photocatalyst take place while carriers diffuse through a sequence of trap-detrap processes. Thus, the energetic and spatial distributions of trap states greatly affect the performance of a photocatalyst. In fact, it has been known that the activity of a photocatalyst depends on its particle size and the degree of aggregation. 9–15 Thus, the performance of photocatalyst is governed not only by the inherent properties of a photocatalyst primary particle but also by the nature of particle aggregation. However, those studies concerning the size dependence of photocatalytic performances have been performed with ensembles of photocatalyst particles with a wide range of size and hence the degree of aggregation. Those ensemble averaged measurements provide some qualitative size dependence of photocatalytic reaction yields, but it is not clear how the aggregation of primary particles influences the kinetics of charge recombination and photocatalytic reactions. Therefore, more systematic and quantitative measurements are needed. Bismuth vanadate (BiVO4 ) is a choice of photocatalyst in this study. It has a bandgap of 2.4 eV and has been considered as a promising photocatalyst for oxygen evolution with visible light. 16–21 The photocatalytic activities 17–28 and the charge dynamics 29–34 of BiVO4 have been measured in the form of thin films or the ensemble of BiVO4 particles dispersed in water. In those measurements, the observables are averaged over the ensemble of particles with various sizes, morphologies, and secondary structures. Thus, information regarding the size and the degree of aggregation on the reactivity is indirect and thus unclear. A couple of studies have focused on well faceted single non-aggregated BiVO4 particles, stimulated by the facet dependence of water splitting: the crystallites with {010} facets dominating over {110} and {011} facets show greater photooxidation reactivity. 33,35,36 Tachikawa et al. have elucidated photoluminescence microscopy to clarify the facet dependence of charge dynamics



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in single crystallites of BiVO4 . 37 They confirmed that the facet dependence of redox reactions is due to preferential hole accumulation at {110} facets, which has been also attributed in the studies of facet-selective photo-deposition of metal ions 38 and direct imaging of surface photovoltage. 39 However, those studies focusing on well-faceted non-aggregated crystallites do not provide information on how electron-hole recombination depends on the size and the secondary structure of aggregated particles. This paper reports the systematic measurements of charge dynamics in single BiVO4 crystallites with various sizes and secondary structures. We have conducted transient absorption (TA) microscopy by focusing individual non-aggregated as well as aggregated crystallites to clarify how photohole lifetime and the reactivity with respect to the reduction of Fe3+ depend on the size of primary and aggregated crystallites. Surprisingly non-aggregated crystallites with well-developed {010} facets, supposed to be preferable for reduction, do not show any reactivity, while aggregated crystallites with large secondary particle sizes show much superior reactivity.

Materials and Methods BiVO4 crystallites were synthesized by a microwave (MW)-assisted homogeneous-precipitation method (MW-Urea method). Urea (KANTO CHEMICAL, 99.0%) was added into the 1.3 mol L−1 of an aqueous nitric acid solution dissolving Bi(NO3 )3 · 5H2 O (KANTO CHEMICAL, 99.9%) and NH4 VO3 (KANTO CHEMICAL, 99.0%) with a molar ratio of Bi:V:urea = 1:1:28. This mixed aqueous solution was irradiated with MW (EYELA, MWO-1000s). The crystal structure of obtained BiVO4 was identified by XRD (Rigaku, MiniFlex, Fig. S1). Photocatalytic O2 evolution from an aqueous FeCl3 solution was carried out using a gas-closed circulation system. BiVO4 powder (0.2 g) was dispersed in an aqueous FeCl3 solution (2 mmol L−1 , 120 mL) at pH=2.4 adjusted with H2 SO4 . The solution contained in a top-irradiation cell with a Pyrex window was irradiated with visible light of a 300 W Xe


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lamp (ParkinElmer, CERMAX-PE300F) through a cutoff filter (Kenko, L42). Evolved O2 quantities (Fig. S2) were determined with a gas chromatography (Shimadzu, GC-8A; TCD, MS-5A, Ar carrier gas). The quantum yield of O2 generation at 440 nm was 18.3%. 28 Transient absorption microscope measurements were conducted with an inverted optical microscope (Nikon, Ti-U). The excitation light source was a pulsed laser at 527 nm with a duration of ca. 400 ns. The fluence of pump pulse was in the range from 0.05 to 1.25 mJ cm−2 . While the whole area of an individual particle was irradiated homogeneously with pump pulses through a condense lens, the probe beam at 633 nm of a HeNe cw-laser was focused with a 10× objective lens (Nikon, CFI Plan Fluor, NA = 0.3). Scattered light of the probe beam from the particle was accumulated by the objective lens and detected with a photomultiplier tube (Hamamatsu, R585). Time profiles of transient absorption of individual particles with various sizes and morphologies were recorded. BiVO4 particles were spin coated on the glass plate of a bottom dish and calcinated at 150◦ ; this immobilized particles on the glass. Focusing on the same particle, we measured TA time profiles first in pure water and in a 2 mM Fe3+ aqueous solution for comparison.

Results and discussion Sample structure and hole decay kinetics It has been known that single crystal particles of BiVO4 in the monoclinic scheelite phase are formed by the hydrothermal method through dissolution–recrystallization processes. 18,40,41 The crystal structure of BiVO4 sample was confirmed by XRD whose pattern (Fig. S1) indicates that the sample is in the monoclinic scheelite phase. Morphologies of BiVO4 particles were observed with a scanning electron microscope (SEM). Figure 1a and 1b show the SEM images of typical non-aggregated and aggregated particles of BiVO4 . As shown in Fig. 1a, a non-aggregated BiVO4 particle is a crystallite in a square or rectangular flat shape with a top facet of {010} and side facets of {110} and {011}. 36 The lengths of non5 ACS Paragon Plus Environment


c ΔI (Normalized)

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Delay (μs) Figure 1: Typical SEM images of (a) a non-aggregated crystallite and (b) an aggregated BiVO4 particle. (c) Time profiles of transient absorption (black circles) at 633 nm of a nonaggregated single crystallite and an aggregated particle. The particles were excited by pump pulses at 527 nm. Red curves are fitting results with eq. (2) at a fixed value of τs = 0.8 µs with β= 0.31 for the non-aggregated particle and β= 0.26 for the aggregated particle. aggregated BiVO4 crystallites along (110) and (011) directions range from 3 to 10 µm. Aggregated particles (Figure 1b) are composed of crystallites with relatively wide {010} facets similar to the non-aggregated crystallites. Note that most of primary particles in the aggregated particle are much smaller than the non-aggregated one (Fig. S3). Moreover, we confirmed with SEM that the aggregated particles observed in this study do not show any porous structures. A fraction of non-aggregated crystallites in the sample was less than 5% of all particles observed. Thus, most of particles are aggregated. BiVO4 is an n-type semiconductor without any intentional doping. 27,30,39,42 The absorption band of non-aggregated crystallites shows an onset at 529–536 nm (2.34 – 2.36 eV) with a tail extending to ∼560 nm. According to the DFT calculations on the electronic structure and the optical properties of monoclinic BiVO4 , 43 this absorption edge is attributed to the electronic transition from O 2p + Bi 6s occupied states to Bi 6p + V 3d unoccupied states.


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The absorption tail is likely due to the transitions in which in-gap states are involved. Thus, the excitation with pump pulses at 527 nm utilized in the current work generates electrons and holes at the conduction and valence band edges and also in the surface in-gap states. Measurements of TA spctra induced by ultraviolet light with and without electron- or hole-scavengers 29,30 indicate that the transient absorption band appearing from 500 to 900 nm with a peak around 550 nm is attributed to photogenerated holes. Thus, the TA decay characteristics observed at 633 nm represent the time profile of holes in a BiVO4 particle generated by a sub-µs pump pulse at 527 nm. Because the absorbance of BiVO4 at 633 nm is negligibly small, the probe beam detects holes in the entire particle volume. Figure 1c shows the TA time profiles of a non-aggregated and an aggregated BiVO4 particle, where ∆I(t) is the amplitude of TA signal at a delay time t normalized at t= 0 µs. The TA amplitude of the non-aggregated crystallite decays much faster than that of the aggregated crystallite. Although the TA amplitude increased with the fluence of pump pulse F , the decay characteristics did not change with F used in this study: F = 0.05– 1.25 mJ cm−2 (Fig. S4). The characteristics of TA time profiles significantly depend on particle size. We have measured TA time profiles of individual crystallites with various sizes. Because all the decay curves normalized at t=0 µs can be described with a multi-exponential decay function: ∑ ∑ ∆I(t) = i Ai exp(−t/τi ), where i Ai =1, integrating a normalized decay curve from t= 0 to tf , where ∆I(tf ) ≃ 0, gives the average lifetime of a crystallite ⟨τ ⟩, ∫ ⟨τ ⟩ =

tf

∆I(t)dt = 0

∑

Ai τi .

(1)

i

In Figure 2 we plot the average hole lifetimes of individual crystallite particles. Here the particle size of primary particle in the case of non-aggregated crystallite or of secondary one in the case of aggregated crystallite is represented by the cross-section area of a particle imaged by the microscope. Because the height of each particle could not be evaluated precisely by its


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Average lifetime (μs)


Particle size (μm 2 ) Figure 2: Size dependence of hole lifetime. The average lifetimes of individual BiVO4 crystallites (non-aggregated crystallites: red, aggregated crystallites: black) immersed in pure water are estimated from TA profiles measured at 633 nm. microscope image, we simply assumed that all the crystallites have the same height. While the largest particle size of non-aggregated crystallites was limited to ∼50 µm2 , we could measure the decay of hole in aggregates crystallites whose secondary particle size is as large as 300 µm2 . The obtained results clearly indicate that the hole lifetime increases with the particle size of both non-aggregated and aggregated crystallites. The lifetimes with a similar particle size somewhat scatter around an average value. This may be due to the fact that they have different heights and hence volumes although they have a similar cross-section area in microscope images. Although the range of the particle size of non-aggregated crystallites is rather narrow, Figure 2 clearly shows that the hole lifetime increases with increasing particle size. If photogenerated holes are trapped and quenched in the bulk of a crystallite, the lifetime of holes generated under uniform excitation with pump light should be independent of particle size. In contrast, if holes are trapped and quenched at the surface of a particle interfacing with water, the lifetime should increase with increasing particle size because the surface/bulk ratio of a non-aggregated crystallite decreases with increasing particle size. Thus, the observed size dependence indicates that holes are trapped mostly at the surface of a crystallite. This is reasonable because in-gap states for carrier trapping are likely localized at the surface of a particle. The similar trend of carrier lifetime has been also observed in the ensemble


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averaged measurements of WO3 crystallites. 44 The hole lifetimes of aggregated crystallites also increase with secondary particle size. The aggregated particles observed in this study are composed of primary crystallites whose size is much smaller than the non-aggregated crystallites (Fig. S3). If the hole decay characteristics were solely determined by the size of primary particles as in the non-aggregated particles, holes in the aggregated crystallites would decay much faster than the non-aggregated crystallites; this is not consistent with the observed results. Grain boundaries between primary crystallites in aggregated particles play an important role in the hole decay kinetics. It has been well known that effective carrier trapping and recombination at grain boundaries cast profound problems in various fields of technology: 45 field effect transistors, 46 varistors, 47–52 and solar cells. 53 Grain boundaries with in-gap states do not allow carriers migrate freely among the primary crystallites of an aggregated particle. BiVO4 is n-type semiconductor, 27,39,42 and a density of majority carrier of ∼ 1018 cm−3 and a space-charge layer length of ∼60 nm without intentional doping have been reported. 30,54 When this kind of BiVO4 primary particles aggregate, a double Schottky barrier where the bands are bent upward as a result of electron occupation in in-gap states at the grain boundaries. Thus, photogenerated minority carriers (holes) can be trapped at grain boundaries. If the hole trap sites at gain boundaries worked as electron–hole recombination centers, the hole density in aggregated particles would decay faster than those in non-aggregated particles; however, we observed the hole lifetime becomes longer with increasing the particle size and thus increasing the number of grain boundaries. Thus, the hole trap sites at the grain boundaries do not work as effective carrier recombination centers so that holes are mostly lost by carrier recombination at the outer surface of aggregated particles as in the case of non-aggregated crystallites. This suggests that the energy distribution of in-gap states and the barrier height for electron at the outer surface interfacing with water is different from those at grain boundaries in aggregated crystallites as a result of specific interactions between primary particles. Therefore, the size dependence of hole lifetime of aggregated



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particles cannot be accounted for by the simplistic surface/bulk ratio discussed for isolated non-aggregated particles; more careful considerations are necessary.

Trap-detrap kinetics To gain more insight into carrier recombination kinetics in aggregated particles, we analyzed the hole decay profiles in terms of a trap-detrap model. The decay time profiles particularly of large aggregated crystallites cannot be represented by a first order kinetics with a single exponential decay nor a second order kinetics, indicating that the recombination kinetics of carriers are governed by the diffusion-limited processes where both electrons and holes diffuse by repeating trapping and detrapping at trap sites of in-gap states. The trap and detrap processes of carriers in organic and inorganic materials have been extensively modeled. 55–59 In these processes, the kinetics of carrier density, i.e., ∆I(t) in the current case, are frequently represented by either a power law ∆I(t) ∝ t−β or a stretched exponential decay, ∆I(t) ∝ exp[−(t/ts )β ],

(2)

where ts is a scaling factor for time delay and β (0 ≤ β ≤ 1) is a parameter representing the energy or spatial distribution of carrier trap sites of the system of interest. As a matter of fact, Figure 1c shows the typical fitting results of decay curves in the longer delay times (t ≥ 10 µs) with a stretched exponential function at a fixed value of ts = 0.8 µs. All decay curves of non-aggregated and aggregated crystallites were fitted to this function and the obtained values of β are plotted in Figure 3 as a function of particle size. It is clear that β correlates with particle size: β decreases with increasing the particle size of primary or secondary particle. There are two ways for interpreting the parameter β. First, β represents the energy


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β

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Particle size (μm 2 ) Figure 3: Size dependence of β obtained by fitting the decay curves of non-aggregated (red) and aggregated (black) crystallites of BiVO4 with a stretched exponential function exp[−(t/ts )β ] at a fixed value of ts = 0.8 µs. distribution of trap levels g(E) in the band gap of a semiconductor particle as ( ) βE β exp − g(E) = , kB T kB T

(3)

where the valence-band edge energy is set to be zero for hole traps. In the case of β = 1, holes are delocalized in the valence bands. 56 If ne (0) = nh (0), where ni (0) (i = e, h) is the initial density of electron or hole, respectively, the carrier densities decay with a normal second-order electron–hole recombination kinetics: ni (t) = ni (0)/(1 + kt), where k is a rate constant independent of time. In the case of β < 1, the energy distribution of carrier trap states has a longer exponential tail in the band gap and k is no longer constant but depends on time. If ne (0) > nh (0), the hole density decreases exponentially: nh (t) = nh (0) exp[−(t/ts )β ]. 60 Thus, the monotonous decrease in β with increasing particle size in Figure 3 would be interpreted with changes in the energy distribution of hole trap levels; they are distributed deeper in the band gap as the particle size increases. Because a double Schottky barrier is formed at a grain boundary of semiconductor crystallites, 61,62 the energy distribution of trap levels at grain boundaries could be different from that at outer surface interfacing with water. Thus, an aggregated particle composed of a few primary crystallites may have a trap energy distribution much different from that of a non-aggregated



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particle. However, the contributions of grain boundaries cannot account for the monotonous size dependence of β particularly for larger aggregated crystallites in the following reason. The trap energy distribution at a grain boundary would depend on the combination of facets of primary crystallites facing to each other. Because a primary particle has only limited kinds of facets: {010}, {110}, {011}, etc., the grain boundaries in an aggregated particle are formed with a few different combinations of facets. This implies that the trap energy distribution of aggregated particles would not persistently change as secondary particle size increases although the number of grain boundaries increases. Thus, the energy distribution of trap levels in the band gap due to grain boundaries partly accounts for the size dependence of β, but is not totally responsible for this dependence. Another possible contribution to the size dependence of β can be the inhomogeneous spatial distribution of hole trap sites in a particle. To simplify the problem, let us assume that the energy distribution of trap levels does not depend on particle size. Let Lmin be the minimum diffusion length of the minority carrier, i.e., hole. At the early stage of carrier density decay, photogenerated holes inside a primary particle are lost by recombination with intrinsic majority carriers located within Lmin and by non-geminate recombination with photogenerated electrons. Thus, the recombination processes within the particle dominate at the early stage. Holes without suffering from the recombination processes migrate through the space charge layer and are trapped at the surface or at a grain boundary of the particle. These trapped holes are eventually recombined with electrons in the same primary particle or those migrating from other primary particles. In this time domain, the diffusion of electrons is involved in recombination; thus, the recombination rate coefficient becomes time dependent and the decay of hole density is stretched in time by a factor of β < 1. 55 In the case of electron-hole recombination, β depends on the dimension of a system d: β= 3/4, 1/2, 1/4 for d= 3, 2, 1, respectively. 63 As the particle size increases, the recombination of trapped holes at the surface and the grain boundaries of an aggregated particle become less frequent. The variation of β from 0.4 to 0.25 may imply that the dimension for carrier


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diffusion is reduced as the particle size increases, which retards the hole decay. Consequently, the spatially inhomogeneous distribution of trapped holes can also be responsible for the size dependence of β.

Size dependence of reactivity Focusing on the reduction of Fe3+ by photogenerated electrons in BiVO4 , we have investigated how the particle morphology and size affect the reactivity of a particle for reduction of Fe3+ . Photogenerated electrons are annihilated by recombination with photogenerated holes. When BiVO4 crystallites are immersed in Fe3+ aqueous solution, another path of electron loss opens: the reduction of Fe3+ . Because the recombination rate is proportional to the product of electron and hole densities, this additional electron loss path makes the rate of electron-hole recombination slower, resulting in the elongation of hole lifetime. Thus, the deviation of the average hole lifetime in a Fe3+ aqueous solution from that in pure water is a good measure of the reduction reactivity. In Figure 4, the average lifetimes of hole in a 2 mM Fe3+ aqueous solution ⟨τh ⟩ are plotted against those in pure water ⟨τh◦ ⟩, showing that ⟨τh ⟩ deviates from ⟨τh◦ ⟩ in the range of ⟨τh◦ ⟩ > 10 µs and the deviation increases with ⟨τh ⟩. This implies that (1) the reduction of Fe3+ does not take place effectively at crystallites showing ⟨τh◦ ⟩ ≤ 10 µs; (2) the reduction takes place effectively at the crystallites showing ⟨τh◦ ⟩ ≥ 10 µs and its yield increases with ⟨τh◦ ⟩. Note that hole consumption due to oxidation of water does not effectively takes place in this time domain; 30 if this took place, the hole lifetime should decrease rather than increase in comparison with one observed in pure water. This slow oxidation rate does not contradict with a high photoconversion efficiency of 18.3% of the sample because the duty ratio of the pump pulse laser less than 10−3 is too small for accumulation of holes at the surface of photocatalysts to proceed oxygen evolution effectively. As shown in Figure 2, ⟨τh◦ ⟩= 10 µs corresponds to particles with a particle size of 50 µm2 , which are mostly non-aggregated crystallites. Thus, the current results indicate that the size of aggregated crystallites has to be larger than 50 µm2 for the reduction of Fe3+ 13 ACS Paragon Plus Environment

3+

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Average lifetime in Fe aq(μs)


Average lifetime in water (μs)

Figure 4: Plot of average hole lifetimes in a 2mM Fe3+ aqueous solution ⟨τh ⟩ against those in pure water ⟨τh◦ ⟩. The solid line indicates the case of ⟨τh ⟩ = ⟨τh◦ ⟩. to occur. This trend in reactivity can be understood on the basis of the trap-detrap kinetics described in the previous section. The reduction of Fe3+ at the surface of a photocatalyst particle competes with electron-hole recombination. In small non-aggregated particles, electron is effectively annihilated through fast recombination with hole. In contrast, as the size of aggregated particle increases, carrier diffusion is more retarded because of trap-detrap processes at grain boundaries; this makes annihilation of electron at the surface less effective, so that the electron lifetime can be long enough to promote the reduction of Fe3+ . Superior reactivity of BiVO4 crystallites with well-developed {010} facets has been reported previously. 23,33,36,64 This can be understood because electrons and holes are preferentially accumulated at {010} and {110} facets, respectively, so that they can be well separated in the crystallites. 38,39 The fact that the well-faceted non-aggregated particles show the reactivity inferior to the aggregated particles in this study does not necessarily contradict the results in those previous reports. All the works testing correlation between the photocatalytic reactivity and the ratio of exposed {010} and {110} facet areas have been done for ensembles of BiVO4 crystallites, where most of the particles are likely aggregated; thus, the lack of direct comparison of reactivity between non-aggregated and aggregated crystallites in those studies makes impossible to clarify the effect of aggregation on charge kinetics and reactivity. Consequently, in addition to a proper ratio of {010} and {110} facet areas of 14 ACS Paragon Plus Environment

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primary BiVO4 crystallites, this study uncovers a new factor important for realizing high photocatalytic reactivity of BiVO4 crystallites, i.e., the effect of aggregation on retardation of the electron–hole recombination rate and enhancement of the reactivity.

Summary We have measured the time profiles of photogenerated holes in individual BiVO4 crystallites by means of single-particle transient absorption microscopy. Photogenerated holes in a BiVO4 crystallite, in particular aggregated ones, decay more slowly as particle size increases. The decay kinetics in the time range longer than 10 µs are well described with stretched exponential decay functions, indicating that electron-hole recombination is coupled with carrier diffusion in which carrier trap-detrap processes at the surface and grain boundaries of a particle are involved. The trap-detrap kinetics of photogenerated carriers strongly affect the reactivity of BiVO4 crystallites. While the non-aggregated single crystallites with {010} and {110} facets measured in the current study do not show any reactivity for the reduction of Fe3+ , the reactivity of aggregated crystallites increases with increasing secondary particle size. Thus the trapping of carriers at the grain boundaries play an important role in promoting the reduction of Fe3+ at the surface of photocatalyst crystallites. Clarifying the correlation between geometric and spectroscopic properties will lead to a more deeper understanding of the mechanism of heterogeneous photocatalysis, which is underway in our laboratory.

Acknowledgement We thank S. Norioka for his assistance of measurements. This work was supported by the Grant-in-Aid for Scientific Research (A) from the Japanese Society for the Promotion of Sciences (Grant No. 25248006 and 16H02249).



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Supporting Information Available XRD pattern, photocatalytic activity, facet-area distributions of primary particles in aggregated crystallites, pump fluence dependence of transient absorption time profile This material is available free of charge via the Internet at http://pubs.acs.org/.

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Graphical TOC Entry Hole Lifetime

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Particle Size


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Particle Size Dependence of Carrier Dynamics and Reactivity of

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