Photoinduced Charge Separation in a Colloidal System of Exfoliated

J. Phys. Chem. B 2009, 113, 1323–1331

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Photoinduced Charge Separation in a Colloidal System of Exfoliated Layered Semiconductor Controlled by Coexisting Aluminosilicate Clay Teruyuki Nakato,*,† Yoshimi Yamada,† and Nobuyoshi Miyamoto‡ DiVision of Bio-Applications and Systems Engineering (BASE), Institute of Symbiotic Science and Technology, Tokyo UniVersity of Agriculture and Technology, 2-24-16 Naka-cho, Koganei-shi, Tokyo 184-8588, Japan, and Department of Life, EnVironment, and Materials Science, Faculty of Engineering, Fukuoka Institute of Technology, 3-30-1 Wajiro-higashi, Higashi-ku, Fukuoka-shi, Fukuoka 811-0295, Japan ReceiVed: August 13, 2008; ReVised Manuscript ReceiVed: October 16, 2008

We investigated photoinduced charge separation occurring in a multicomponent colloidal system composed of oxide nanosheets of photocatalytically active niobate and photochemically inert clay and electron accepting methylviologen dications (MV2+). The inorganic nanosheets were obtained by exfoliation of layered hexaniobate and hectorite clay. The niobate and clay nanosheets were spatially separated in the colloidally dispersed state, and the MV2+ molecules were selectively adsorbed on the clay platelets. UV irradiation of the colloids led to electron transfer from the niobate nanosheets to the MV2+ molecules adsorbed on clay. The photoinduced electron transfer produced methylviologen radical cations (MV•+), which was characterized by high yield and long lifetime. The yield and stability of the MV•+ species were found to depend strongly on the clay content of the colloid: from a few mol % to ∼70 mol % of the yield and several tens of minutes to more than 40 h of the lifetime. The contents of the niobate nanosheets and MV2+ molecules and the aging of the colloid also affected the photoinduced charge separation. In the absence of MV2+ molecules in the colloid, UV irradiation induced electron accumulation in the niobate nanosheets. The stability of the electron-accumulated state also depended on the clay content. The variation in the photochemical behavior is discussed in relation to the viscosity of the colloid. Introduction Photoinduced charge separation has been actively investigated for a long time as a key reaction of photoenergy conversion in various systems.1 Stable and efficient charge separation is realized if charge recombination (backward reaction) is suppressed after the forward electron transfer from the electron donor to the acceptor. Natural photosynthetic systems realize this situation with spatial separation of the donor and acceptor molecules by utilizing biological membranes as heterogeneous matrices.2 Inspired by the natural systems, various photoinduced charge separation systems have been organized with the concept of spatial separation, and numerous heterogeneous media have been examined as matrices for these systems. Examples are organic polymers,3 self-assembled mono- and multilayers,4 sol-gel glasses,5 and porous inorganic solids such as zeolites,6 layered compounds,7,8 and mesoporous materials.9 Photoinduced charge separation is controlled not only with the location of the donor and acceptor molecules but also by their diffusion. Fast diffusion of the molecules generally facilitates efficient charge transfer from the donor to the acceptor. However, the backward reaction is also accelerated in highly diffusible media such as homogeneous solution to destabilize the charge-separated state. Therefore, heterogeneous media that can regulate both the location and diffusion of the components are required for efficient photoinduced charge separation. This requirement is satisfied by soft matter (or soft materials) because its softness allows both molecular immobilization and diffusion. In fact, natural photosynthetic * Corresponding author: Fax +81-42-388-7344; e-mail [email protected]. † Tokyo University of Agriculture and Technology. ‡ Fukuoka Institute of Technology.

systems are regarded as typical soft matter. In natural systems, diffusion of molecular electron carriers such as quinone derivatives facilitate efficient charge separation.2 Artificial systems constructed on some organic matrices such as micelles10 and polymer solutions11 also have similar properties. On the contrary, inorganic-based systems seem disadvantageous to mimic soft organic molecular assemblies because most of inorganic media are characterized by rigid structures that tend to tightly immobilize the donor and acceptor molecules. Reported systems of photoinduced charge separation, constructed on rigid inorganic matrices,5-9 have been designed to precisely and statically immobilize the donor and acceptor molecules on appropriate locations. However, one may realize novel inorganic-based soft matter for efficient and stable photoinduced charge separation if one can organize inorganic building units into flexible superstructures that ensure both the spatial regulation and mobility of the constituents. Hence, we pay attention to inorganic structured colloids. In particular, colloids of anisotropic particles such as plates and rods are promising because they form various ordered structures exemplified by mesophases given by orientational ordering of the particles.12 If the colloids are composed of two morphologically different particles, they show phase separation with demixing of dispersed particles.13 Thus, colloidal systems of anisotropic particles can be utilized to regulate the location of functional molecules by the use of their characteristic superstructures maintaining the mobility of the components. Inorganic nanosheets prepared by exfoliation of layered crystals14,15 are usable for building units of structured colloids because of their highly anisotropic nature represented by thickness of a few nanometers and lateral dimension of up to several micrometers.16,17 Nanosheets are obtained from inorganic

10.1021/jp807214w CCC: $40.75  2009 American Chemical Society Published on Web 01/13/2009

1324 J. Phys. Chem. B, Vol. 113, No. 5, 2009

Figure 1. Schematic structures of (a) methylviologen dication (MV2+), (b) layered hexaniobate K4Nb6O17, and (c) hectorite clay.

layered materials that possess intercalating capability. When such materials are intercalated with extra molecules, their interlayer spaces are expanded. If the interlayer expansion occurs infinitely with intercalation of a huge amount of solvents, the inorganic layers are delaminated to form colloidally dispersed nanosheets. The colloidal nanosheets show lyotropic liquid crystallinity, and their high aspect (lateral length to thickness) ratio stabilizes the liquid crystalline phases.18-23 However, the “nanosheet colloids” have attracted little attention except for clay colloids that have been studied for a long time,18,24-26 although exfoliated nanosheets themselves are actively investigated as building blocks of novel solid nanomaterials such as layer-by-layer assemblies.8,15,27,28 Nanosheet colloids other than those of clay minerals had rarely been studied in modern chemistry, until H3Sb3P2O14,21 layered double hydroxides,23,29 and layered niobates (and titanates)22,30,31 were found in this century to form mesophases in colloidal systems of exfoliated layers. Among the exfoliative inorganic materials, layered niobates and titanates are characterized as wide band gap semiconductors.32 They are photoexcited with UV light to form electrons and holes that can be utilized for photocatalytic water splitting.33 When electron acceptor molecules like methylviologen dications (MV2+, Figure 1a) are intercalated into these materials, photoinduced electron transfer occurs from the semiconducting oxide layers to the intercalated MV2+ molecules, yielding methylviologen radical cations (MV•+).28,34 This charge-separated state is stable, as demonstrated by long-lived MV•+ species, the behavior which is characteristic of the donor-acceptor systems of hard matter organized on rigid matrices. If the photoactive layered niobates and titanates are utilized as soft matter like the colloids of exfoliated nanosheets, we may obtain novel photoinduced charge separation systems, whose behavior is different from that of conventional solid systems such as intercalation compounds because of the fluidity of semiconductor particles. We propose here a multicomponent colloidal system composed of exfoliated nanosheets of hexaniobate (Figure 1b) and synthetic hectorite clay (Laponite, Figure 1c) and MV2+ molecules, as an example of inorganic-based soft matter that can undergo efficient photoinduced charge separation. In this system, the colloidally dispersed niobate and clay nanosheets construct a hierarchical superstructure based on morphological dissimilarity of the two nanosheets. The niobate and clay nanosheets are phase-separated and assembled into microdomains, where organic cations such as the MV2+ species are selectively adsorbed on the clay nanosheets.35 We have recently

Nakato et al. discovered that the colloidal system causes electron transfer from the niobate nanosheets to the MV2+ molecules upon irradiation of UV light accompanied by a long-lived charge-separated state.36 This stable charge separation has been ascribed to the spatial separation between the electron donor (niobate nanosheets) and the acceptor (MV2+ molecules); this behavior has shown that inorganic soft matter consisting of anisotropic colloidal particles can be developed as a unique system for photoenergy conversion. In this paper, we report on a systematic examination of the experimental parameters that affect the photochemical behavior of this multicomponent colloidal system. The results demonstrate that both the stability and efficiency of the charge separation are controllable over a wide range of the contents of clay nanosheets, which play vital roles for materializing both donor-acceptor spatial separation and appropriate fluidity of the system. Experimental Method Materials. Niobate nanosheets were prepared by exfoliation of tetrapotassium hexaniobate (K4Nb6O17) by the method reported previously.16,31 Briefly, single crystalline K4Nb6O17 prepared by a flux method37 was allowed to react with an aqueous solution of propylamine hydrochloride (Wako Pure Chemical Co.). This treatment displaced the K+ ions located between the Nb6O174- layers for propylammonium ions to delaminate the layered crystals. The exfoliated material was recovered by centrifugation, and the deposit obtained was redispersed in water. The suspension was dialyzed and diluted with water to yield a colloid sample of niobate nanosheets. Laponite RD, synthetic hectorite clay (ideal formula Na0.7Si8Mg5.4Li0.4H4O24), was supplied by Wilbur-Ellis Co. and used as received. The cation exchange capacity (CEC) of the clay is 0.075 equiv per 100 mg clay.38 It was dispersed in water to yield a colloidal sample of clay nanosheets. The lateral dimension of the niobate nanosheets is larger than 1 µm,31 whereas that of Laponite is around 25 nm.39 MV2+ dichloride was purchased from Tokyo Kasei Co. and used as received. Multicomponent colloids composed of hexaniobate, hectorite clay, and MV2+ were prepared by the following procedure. An aqueous solution of MV2+ was slowly added to a clay colloid. Then, the MV2+-containing clay colloid was instantly added to the niobate colloid. The compositions of the standard sample of MV/clay-niobate colloid were [niobate] ) 1 g L-1, [clay] ) 10 g L-1, and [MV2+] ) 0.1 mmol L-1; the niobate concentration [niobate] is given with the mass of K4Nb6O17. This composition corresponds to the molar ratio of [Nb6O17]4-: clay:MV2+ ) 9.6:75:1.0, where the amount of clay is represented by that of the exchangeable cations determined by CEC; this ratio indicates that the amount of electron donor (niobate) is much larger than that of acceptor (MV2+). The amount of each component was varied in order to examine the influence of the composition on the photochemical behavior of the colloid samples. We designate the obtained multicomponent samples as MV/clay-niobate colloids. Colloidal mixtures of hectorite and niobate nanosheets were also prepared in the absence of MV2+. This sample is hereafter called clay-niobate colloids. The colloid samples prepared were subjected to photochemical experiments 1 day after the preparation, unless noted otherwise, because the photochemical behavior was found to be greatly affected by the aging of the colloids as described below. Observation of the Photochemical Behavior. The colloids were placed in a water-cooled (25 °C) quartz cell (5 mm in thickness) capped with a rubber septum. After the colloid was bubbled with water-saturated nitrogen gas for more than 30 min,

Colloidal System of Exfoliated Layered Semiconductor

Figure 2. Visible absorption spectra of the MV/clay-niobate colloid ([clay] ) 10 g L-1, [niobate] ) 1 g L-1, [MV2+] ) 0.1 mmol L-1) (a) before UV irradiation and (b) 30, (c) 60, and (d) 120 min after termination of the irradiation for 8 min.

the sample was irradiated by an Ushio SX-UI500XQ 500 W Xe lamp for 8 min. After the irradiation was stopped, the cell was stood with flowing N2 in the headspace, and visible spectra of the samples were measured repeatedly. During the experiment, the cell was kept as static as possible because agitation of the colloid dramatically destabilized the charge-separated state. Analyses. Visible absorption spectra were recorded on a Shimadzu UV-2450 spectrophotometer before and after the irradiation. The amount of the photogenerated MV•+ species was determined from the characteristic absorption of the radical cations at 600 nm. Background scattering by the colloidally dispersed nanosheets was subtracted to estimate the absorbance due to the MV•+ species based on the spectrum before the UV irradiation and absorbance at 800 nm, where the absorption of the MV•+ species is negligible. The viscosity of the colloid samples was analyzed by a Brookfield DV-II+ viscometer with Couette geometry. The values of viscosity and yield stress were estimated from the flow curves measured with increasing shear rate. Small-angle neutron scattering (SANS) measurements were carried out with SANS-J located at the JRR-3 atomic reactor of Japan Atomic Energy Agency (JAEA), Tokai-mura, Japan;40 the samples in a 1 mm thick quartz cell were measured with a two-dimensional 3He detector positioned at 10 m from the sample. The wavelength of the incident neutron beam was 0.65 nm. Results Photochemical Behavior of the MV/Clay-Niobate Colloids. The MV2+ dications added to the colloidal mixture of clay and niobate nanosheets were reduced by UV irradiation to form MV•+ radical cations, which were detected by visible spectroscopy and the naked eye with blue color characteristic of the MV•+ species. Figure 2 indicates typical spectral response of an MV/clay-niobate colloid. The irradiated colloid exhibited characteristic absorption bands around 400 and 600 nm assigned to the MV•+ species.41 This photoresponse demonstrates photoinduced electron transfer from the photocatalytically active semiconducting niobate nanosheets to the MV2+ cations adsorbed on the photochemically inert clay nanosheets, as reported in our preceding paper.36 In this system, the niobate and clay nanosheets are spatially separated by phase separation to form their microdomains, which

J. Phys. Chem. B, Vol. 113, No. 5, 2009 1325

Figure 3. Small-angle neutron scattering (SANS) profiles of (a) niobate colloid ([niobate] ) 62.0 g L-1) and (b, c) clay-niobate colloids with [niobate] ) 62.0 g L-1 and [clay] ) 10 g L-1 (b) and 17.5 g L-1 (c).

was confirmed by small-angle neutron scattering (SANS) measurements of clay-niobate colloids.36 The SANS profile of niobate nanosheet colloid that does not contain clay nanosheets with [niobate] ) 62 g L-1 42 (Figure 3a) shows peaks at q ) 0.12 and 0.23 nm-1. These peaks are ascribed to the lamellar structure with the basal spacing of d ) 2π/q ) 55 nm given by theliquid-crystallineorderingofexfoliatedniobatenanosheets.22,31,43 However, the basal spacing shrinks by introduction of the clay nanosheets into the colloid. The SANS profiles of clay-niobate colloids with [clay] ) 10.0 and 17.5 g L-1 (Figure 3b,c) show that the basal spacings were 40 and 35 nm, respectively, as evidenced by the peaks at higher q values. The reduction of the basal spacing indicates the formation of clay microdomains with microphase separation between the clay and niobate nanosheets and compression of the liquid crystalline niobate phase by intrusion of the clay phase. Since the MV2+ cations are selectively adsorbed on the clay nanosheets in the MV/ clay-niobate colloids, microsegregation of the niobate and clay nanosheets leads to spatial separation of the electron donor (niobate nanosheets) and the acceptor (MV2+ molecules on the clay nanosheets) of the photoinduced electron transfer. The charge-separated state caused by the photoinduced electron transfer was characterized by high stability of the photogenerated MV•+ species. Figure 4 shows the time dependence of the MV•+ concentration ([MV•+]) in the MV/ clay-niobate colloids with varied clay concentrations (1-40 g L-1) at constant niobate and MV2+ contents ([niobate] ) 1 g L-1, [MV2+] ) 0.1 mmol L-1). The time courses show an increase in [MV•+] after the irradiation was stopped at t ) 0, indicating that the amount of radical cations continues to increase for a while after the irradiation. They also show a very slow decrease in [MV•+] after the irradiation is stopped; thus, the photoproduct is kept stably. The maximum concentration of the MV•+ species ([MV•+]max) in the time courses reaches ∼0.07 mmol L-1 for some samples, indicating rather high conversion (∼70%) of MV2+ to MV•+. Effects of the Clay Concentration. The time courses shown in Figure 4 indicate that the photochemical behavior of the MV/ clay-niobate colloids greatly depends on the composition of the colloids. We evaluate the photochemical behavior of the photogenerated MV•+ species with three characteristic values: the maximum amount ([MV•+]max) and the time constants of the generation (τg) and decay (τd). Table 1 lists these values estimated from the time courses shown in Figure 4. The value of [MV•+]max, corresponding to the yield of photoproduct, is


Nakato et al.

Figure 4. Time courses of the concentration of MV•+ species observed after termination of UV irradiation in the MV/clay-niobate colloids with composition of [clay] ) 1 (a, open diamonds), 5 (b, filled diamonds), 10 (c, open circles), 20 (d, filled circles), 25 (e, open triangles), and 30 g L-1 (f, filled triangles), [niobate] ) 1 g L-1, and [MV2+] ) 0.1 mmol L-1. The lines overlapping with plot (c) indicate first-order kinetics obtained by fitting the plot.

TABLE 1: Maximum Concentration ([MV•+]max) and Time Constants of the Generation (τg) and Decay (τd) of the MV•+ Species Photogenerated in MV/Clay-Niobate Colloids sample

results

[MV2+]/ mmol L-1

[niobate]/ g L-1

[clay]/ g L-1

[MV+•]max/ mmol L-1

τg/h

τd/h

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.01 1 0.1 0.1

1 1 1 1 1 1 1 1 1 0.5 2

1 5 10 20 25 30 40 10 10 10 10

0.0076 0.069 0.066 0.020 0.017 0.0088 0b 0.0070 0.069 0.0057 0.042

sa 0.5 0.3 0.2 0.1 0.1 sb sc sa sa sc

0.2 10 11 8 42 0.7 sb 1 5 1 19

a [MV•+] did not increase after termination of UV irradiation. MV•+ species was not detected. c No reliable results were obtained because of insufficiency of data points or the shapes of time-dependence curves. b

the maximum [MV•+] in each time course. The time constants τg and τd were determined by fitting the time courses of MV•+ generating and decaying regions, respectively, with first-order kinetics. Table 1 demonstrates that both of the yield ([MV•+]max) and the stability (τd) of the MV•+ species greatly varied with the clay content ([clay]) of the colloids. For the colloid with [clay] ) 1 g L-1, the [MV•+]max and τd values were small, while the τg value was not measurable, indicating that the reduction of MV2+ to MV•+ was completed during the irradiation. When [clay] increased to 5-10 g L-1, both the [MV•+]max and τd values exceeded 10 times those for [clay] ) 1 g L-1, and the formation of MV•+ continued after the irradiation was stopped to give the τg values. The sample with [clay] ) 25 g L-1 provided the MV•+ species of higher stability and lower yield than those with [clay] ) 5-10 g L-1. Much larger [clay] (30 g L-1) destabilized the MV•+ species with a lower yield, and the sample with [clay] ) 40 g L-1 did not yield a detectable amount of the MV•+ species with the experimental setup of the present study. These results

Figure 5. Time courses of the concentration of MV•+ species observed after termination of UV irradiation in the MV/clay-niobate colloids with the composition of [MV2+] ) 0.01 (a, open diamonds), 0.1 (b, open circles), and 1 mmol L-1 (c, filled diamonds), [clay] ) 10 g L-1, and [niobate] ) 1 g L-1.

indicate that the stability and efficiency of the photoinduced charge separation in the MV/clay-niobate colloids are controllable over a wide range by varying the [clay] around 1 mass % (10 g L-1); too small or too large [clay] is not favorable for the charge separation. Effects of the Concentration of Other Constituents. The concentration of the MV2+ species in the colloid also affected the photochemical process, although the influence was less than that of [clay]. Figure 5 compares the time courses of [MV•+] in the MV/niobate-clay colloids with different [MV2+] (0.01-1 mmol L-1) at constant concentrations of the nanosheets ([niobate] ) 1 g L-1 and [clay] ) 10 g L-1). When [MV2+] decreased from 0.1 to 0.01 mmol L-1, [MV•+]max was reduced to about 1/10 (namely, the yield of MV•+ was nearly equal), and the radical cations were somewhat destabilized. However, the increase in [MV2+] from 0.1 to 1 mmol L-1 did not affect the maximum concentration and stability of MV•+ (namely, the MV•+ yield was reduced); the decay curve of the sample with [MV2+] ) 1 mmol L-1 almost overlaps that of the colloid with [MV2+] ) 0.1 mmol L-1. On the other hand, no increase in [MV•+] was observed for the sample with [MV2+] ) 1 mmol L-1 after the irradiation was stopped; namely, the reduction of MV2+ to MV•+ was completed within the irradiation period (8 min). The concentration of niobate nanosheets also affected the time courses of [MV•+] alike that of [MV2+]. Figure 6 shows the time courses of [MV•+] with different [niobate] (0.5-2 g L-1) at constant concentrations of clay (10 g L-1) and MV2+ (0.1 mmol L-1). At a small [niobate] (0.5 g L-1), [MV•+]max and τd were smaller than those at [niobate] ) 1 g L-1. However, the increase in [niobate] from 1 to 2 g L-1 did not improve these parameters. Effects of Electrolyte Addition and Aging. It is generally known that a coexisting electrolyte influences the dispersed state of electrically charged colloidal particles including the niobate and clay nanosheets. Thus, the photochemical behavior of the MV/clay-niobate colloids is altered with the coexistence of electrolytes. Figure 7 indicates the time courses of [MV•+] for the colloids added by various amounts of KCl as the coexisting electrolyte, and Table 2 summarizes the [MV•+]max, τg, and τd values obtained from the time courses. The value of [MV•+]max decreased as the concentration of KCl increased. However, the lifetime τd was longer at higher KCl concentrations.


J. Phys. Chem. B, Vol. 113, No. 5, 2009 1327

Figure 6. Time courses of the concentration of MV•+ species observed after termination of UV irradiation in the MV/clay-niobate colloids with the composition of [niobate] ) 0.5 (a, open diamonds), 1 (b, open circles), and 2 mmol L-1 (c, filled diamonds), [clay] ) 10 g L-1, and [MV2+] ) 0.1 mmol L-1.

Figure 8. Time courses of the concentration of MV•+ species observed after termination of UV irradiation in the MV/clay-niobate colloids with the composition of [clay] ) 10 g L-1, [niobate] ) 1 g L-1, and [MV2+] ) 0.1 mmol L-1 observed 1 day (a, open circles), 6 months (b, open diamonds), and 1 year (c, filled diamonds) after preparation of the colloids.

TABLE 3: Maximum Concentration ([MV•+]max) and Time Constants of the Generation (τg) and Decay (τd) of the MV•+ Species Photogenerated in the MV/Clay-Niobate Colloids after Aging sample

results +•

2+

[MV ]/ [niobate]/ [clay]/ aged period/ [MV ]max/ τ /h τd/h mmol L-1 g L-1 g L-1 month mmol L-1 g 0.1 0.1 0.1 a

Figure 7. Time courses of the concentration of MV•+ species observed after termination of UV irradiation in the MV/clay-niobate colloids with the composition of [clay] ) 10 g L-1, [niobate] ) 1 g L-1, and [MV2+] ) 0.1 mmol L-1 added by 0 (a, open circles) 0.1 (b, open diamonds), 1 (c, filled diamonds), and 10 mmol L-1 (d, filled circles) of KCl.

TABLE 2: Maximum Concentration ([MV•+]max) and Time Constants of the Generation (τg) and Decay (τd) of the MV•+ Species Photogenerated in the MV/Clay-Niobate Colloids Added by KCl sample

results

[KCl]/ [MV•+]max/ [MV2+]/ [niobate]/ [clay]/ τg/h τd/h mmol L-1 g L-1 g L-1 mmol L-1 mmol L-1 0.1 0.1 0.1

1 1 1

10 10 10

0.1 1 10

0.050 0.038 0.0069

0.4 0.3 0.2

16 30 380

Aging of the colloids also influenced the photochemical behavior. Aqueous colloids of exfoliated smectite-type clays are known to undergo very slow gelation over the time range of years.24,39,44,45 Thus, the dispersed state of clay nanosheets in the MV/clay-niobate colloids should be gradually altered by aging to modify the photoprocess. We examined the photochemical behavior of the MV/clay-niobate colloids of [clay] ) 10 and 25 g L-1 before and after storage of the samples for half a year under ambient conditions. Figure 8

1 1 1

10 10 25

6 12 6

0.047 0.047 0

0.1 0.2 sa

9 6 sa

MV•+ species was not detected.

compares the time courses of the sample of [clay] ) 10 g L-1 with aging. The colloid exhibited decreases in [MV•+]max and τd after aging, as given in Table 3. Moreover, we discerned that the sample of [clay] ) 25 g L-1 did not generate MV•+ after aging. Photochemical Behavior of the Clay-Niobate Colloids in the Absence of MV2+. UV irradiation of the clay-niobate colloids, which are colloidal mixtures of the niobate and clay nanosheets lacking the MV2+ cations, caused reduction of the niobate nanosheets. Spectral response of the colloids indicated that the electrons photogenerated in the semiconducting niobate nanosheets were accumulated in the nanosheets. Figure 9 shows typical visible spectra of a clay-niobate colloid before and after the irradiation. The sample turned green with the irradiation, and the spectrum of the colored sample exhibited a broad absorption band. This band was assigned to lower-valent niobium species produced by accumulation of the conductionband electrons in the niobate nanosheets upon band gap excitation with UV light;46 the reduced niobate species is hereafter denoted as niobate(e-). Figure 10 indicates typical time courses of the amount of niobate(e-) species photogenerated in the clay-niobate colloids monitored by the increase in absorbance at 600 nm with the irradiation. The niobate(e-) species was stably kept in the clay-niobate colloids after the irradiation was stopped, as indicated by the slow decay occurring over hours. Its generation was completed during the irradiation period (8 min), as evidenced by the amount of niobate(e-) that started to decrease immediately after the irradiation was stopped at t ) 0.


Nakato et al.

Figure 9. Visible absorption spectra of the clay-niobate colloid ([clay] ) 10 g L-1, [niobate] ) 1 g L-1, MV2+ is absent) (a) before UV irradiation and (b) 1, (c) 30, and (d) 60 min after termination of UV irradiation for 8 min.

Figure 11. Flow curves of the MV/clay-niobate colloids with the composition of [clay] ) 5 (A), 10 (B), 20 (C), 25 (D), and 30 g L-1 (E), [niobate] ) 1 g L-1, and [MV2+] ) 0.1 mmol L-1. The open and filled symbols correspond to the flow curves obtained with increasing and decreasing shear rates, respectively.

TABLE 4: Viscosity and Yield Stress of the MV/ Clay-Niobate Colloids Figure 10. Time courses of the absorption due to niobate(e-) species observed after termination of UV irradiation in the clay-niobate colloids with the composition of [clay] ) 0 (a, open diamonds), 5 (b, filled diamonds), 10 (c, open circles), and 20 g L-1 (d, filled circles) and [niobate] ) 1 g L-1. The “∆abs” value was obtained by subtracting the absorbance of the sample before irradiation from that after irradiation.

The amount and lifetime of the niobate(e-) species both depended on the clay content of the colloids. The largest yield and lifetime of niobate(e-) were obtained at [clay] ) 10 g L-1, whereas formation of the niobate(e-) species was not observed in the colloid of [clay] ) 30 g L-1. Too low or too high [clay] was inappropriate to obtain the niobate(e-) species with high yield and stability. These results demonstrate that photoreduction of the niobate nanosheets in the clay-niobate colloids is controlled by the concentration of coexisting clay nanosheets similarly to the photogeneration of MV•+ species in the MV/ clay-colloids. Viscosity Measurements. The MV/clay-niobate colloids exhibited Bingham-like (pseudoplastic) flow behavior, while colloids consisting only of niobate nanosheets were almost Newtonian.47 Figure 11 shows flow curves of some MV/ clay-niobate colloids. They show that the flow behavior of the colloids greatly depends on [clay]. Table 4 summarizes the viscosity and the yield stress of the samples estimated from the shear-increasing branch of the flow curves. The viscosity was estimated by linear fitting of the flow curves in the fluid region.

sample

results

[MV2+]/ mmol L-1

[niobate]/ g L-1

[clay]/ g L-1

viscosity/ mPa s

yield stress/Pa

0.1 0.1 0.1 0.1 0.1

1 1 1 1 1

5 10 20 25 30

16 18 1020 482 858

0.16 0.20 28 76 188

As shown in Table 4, colloids with small [clay] (e10 g L-1) are fluid, as evidenced by their small viscosity and yield stress values. In contrast, the samples containing large [clay] (g20 g L-1) were rather viscous with large yield stress. The differences between the colloids with small and large [clay] values (e10 and g20 g L-1) correspond to the phase transition of Laponite colloid, which has been reported to cause sol-gel transition at [clay] ) 10-20 g L-1.19,45,48 The flow behavior of the MV/clay-niobate colloids was affected by aging, reflecting the slow gelation of clay colloids, as indicated by the flow curves of Figure 12. The MV/ clay-niobate colloid ([clay] ) 10 g L-1) aged for half a year gave increased viscosity but nearly equal yield stress in comparison with the sample before aging. On the other hand, aging of the colloids with a large [clay] value (25 g L-1) greatly stiffened the sample, as evidenced by the increased yield stress. The sample of the colloid aged for half a year, which did not undergo the photoinduced electron transfer, gave the yield stress


Figure 12. Flow curves of the MV/clay-niobate colloids with the composition of [clay] ) 10 (A) and 25 (B) g L-1, [niobate] ) 1 g L-1, and [MV2+] ) 0.1 mmol L-1. The circles and diamonds correspond to the flow curves obtained 1 day and 6 months, respectively, after preparation of the colloids. The open and filled symbols correspond to the flow curves obtained with increasing and decreasing shear rates, respectively.

Figure 13. Schematic representation of the photoinduced electron transfer occurring in the MV/clay-niobate colloids.

of 480 Pa. This value is more than 7 times larger than that before the aging and also larger than that of the sample with [clay] ) 30 g L-1 measured 1 day after the preparation. Discussion Photoinduced Charge Separation between the Niobate Nanosheets and MV2+ Cations. The photochemistry of the multicomponent colloidal system, composed of the niobate and clay nanosheets and the MV2+ molecules, is characterized by the efficient reduction of the MV2+ cations and the stable photogenerated MV•+ species; both the yield and stability of the MV•+ species are easily controllable with the colloidal parameters of the system. The lifetime τd of the MV•+ ions formed by the electron transfer from the niobate nanosheets varies from minutes to several tens of hours, while the maximum concentration of MV•+ ([MV•+]max) ranges from a few mol % to ∼70 mol %. Although long-lived MV•+ species with lifetimes comparable to that in the present system have been obtained in various solid systems, such as inclusion compounds with inorganic oxides34,49 and macromolecular hybrids with organic matrices,50 these solid systems have difficulties in obtaining photoproducts with widely controllable yields and stabilities because they do not allow compositional and structural variations in a broad range. Figure 13 illustrates the photoprocess in the organized structure of the MV/clay-colloids. As shown by the SANS profiles (Figure 3), the multicomponent colloids build up microdomain structures, where the niobate and clay nanosheets, being different from each other in their size and crystal structure, form their domains with microphase separation.35,36 In this situation, the electron donor (niobate nanosheets) and the acceptor (MV2+ molecules adsorbed on the clay nanosheets) are spatially separated. This structural condition suppresses the backward charge transfer between the donor and the acceptor.

J. Phys. Chem. B, Vol. 113, No. 5, 2009 1329 The variability of the charge-separation behavior originates from the structural flexibility of the samples. This is given by fluidity of the colloidal system that allows the niobate and clay nanosheet domains to diffuse and penetrate into each other. This property assists the donor and acceptor nanosheets to meet and thus facilitate electron transfer in both forward and backward directions. However, we suppose from the rheological data (Figure 11 and Table 4) that the niobate and clay domains have different molecular transport properties. The clay domain should be more viscous than the niobate domain because the viscosity of the samples increases with [clay]. Consequently, an appropriate balance of these immobilizing and diffusive properties is likely to be responsible for the generation and decay of the MV•+ species occurring in different time scales. Since the dispersed state and diffusion of the niobate and clay nanosheets are greatly altered under different conditions of the colloids, the photochemical behavior should vary widely with the colloidal conditions. Role of the Coexisting Clay Nanosheets. The clay content is the most important experimental parameter examined in the present study. The clay nanosheets form acceptor microdomains by adsorbing MV2+ cations and are separated from electrondonating niobate nanosheets. Moreover, the present results indicate that the clay component influences the fluidity of the colloid and greatly alter the reaction rate of the photoinduced charge separation. These features are probably related to the viscosity-enhancing and gelling effects of colloidally dispersed smectite-type clay minerals.24,25,39,51 This idea is supported by the results of viscosity measurements. According to the flow curves (Figure 11), colloids with [clay] e 10 g L-1 are fluid. Among the MV/clay-niobate colloids examined in this study, moderately viscous samples with [clay] ) 5-10 g L-1 give both the high yields and long lifetimes of the MV•+ species. Thus, the appropriate fluidity for the efficient and stable photoinduced charge separation is expected when the diffusion of the nanosheets is retarded but not greatly suppressed. In such colloids, the fluidity ensures the diffusion of nanosheets to cause efficient photoinduced electron transfer whereas the increased viscosity given by the clay component helps to suppress the backward electron transfer to elongate the lifetime of the charge-separated state. On the other hand, the samples with larger clay contents ([clay] g 20 g L-1) can be regarded as gels, as indicated by their large yield stress and viscosity. The colloid with [clay] ) 25 g L-1 gives reduced conversion and increased lifetime of the MV•+ species. In this sample, diffusion of the nanosheets should be more largely suppressed because of its viscosity. This situation suggests that the niobate and clay nanosheets contact less frequently than loose colloids with lower [clay] so that backward as well as forward electron transfer is somewhat suppressed. When [clay] exceeds 30 g L-1, the yield and lifetime of MV•+ species both decrease, and photoinduced electron transfer is not observable at [clay] ) 40 g L-1; there the electron transfer is suppressed by the stiffness of the colloids, and the charge-separated state can be destabilized because the clay nanosheets on which the MV•+ radical cations formed do not escape fast from the niobate nanosheets. The flow behavior of the MV/clay-niobate colloids is altered by the clay content and also by the aging of the samples. This is ascribed to the slow gelation of the colloids induced by the clay nanosheets.24,39,44,45 The colloids are stiffened by aging and greatly suppress the diffusion of the nanosheets; thus, the efficiency and stability of the charge-separated state are reduced in the aged colloids. In particular, the yield stress of the colloid

1330 J. Phys. Chem. B, Vol. 113, No. 5, 2009 of [clay] ) 25 g L-1 is enhanced by aging for half a year and exceeds that of the unaged colloid of [clay] ) 30 g L-1. The absence of photoinduced electron transfer in the aged colloid with [clay] ) 25 g L-1 is rationalized by the high yield stress that strongly restricts the diffusion of nanosheets in the colloid. Note that we lack evidence for the dispersed structure of clay nanosheets and the distribution of the MV2+ species on the nanosheets. Although the clay nanosheets do not present as submillimeter level aggregates according to our previous optical microscope observations,35 their actual diepersed state is not clarified. Also, location and diffusibility of the MV2+ molecules on the clay nanosheets are still unsettled problems. The detailed photoprocess controlled by the clay nanosheets in the MV/ clay-niobate colloids awaits clarification of the structure of clay microdomains in relation to colloidal compositions. Photoresponse of the Clay-Niobate Colloids Compared with That of the MV/Clay-Niobate Colloids. The photochemical behavior of the clay-niobate colloids lacking MV2+ species largely depends on the clay content similarly to the MV/ clay-niobate colloids. In the clay-niobate colloids, the band gap photoexcitation of the niobate nanosheets accumulates the conduction-band electrons in the niobate nanosheets to give the niobate(e-) species. Larger amounts of the niobate(e-) species with long lifetimes are obtained in the clay-niobate colloids with large clay contents ([clay] g 5 g L-1). However, too large clay contents suppress the generation of niobate(e-) species. The resemblance of the clay-dependent photoresponse of the niobate(e-) and MV•+ species in the clay-niobate and MV/ clay-niobate colloids, respectively, suggests that the photochemical reactions of these colloids are basically governed by the photogeneration and accumulation behavior of the conduction-band electrons in the niobate nanosheets, i.e., the behavior which is greatly affected by coexisting photochemically inert clay nanosheets. Nevertheless, the absence of MV2+ cations causes some differences in the photoresponse of the colloids. The niobate(e-) species photogenerated in the clay-niobate colloids decays monotonically, whereas the formation of MV•+ cations in the MV/clay-niobate colloids continues after the irradiation is stopped. This difference is probably related to interparticle electron transfer. The niobate(e-) species is generated with reduction of the photoexcited niobate nanosheets themselves. On the other hand, MV•+ radical cations form as a result of electron transfer from the niobate nanosheets to the clay nanosheets carrying the MV2+ cations. The latter reaction requires an approach of the niobate nanosheets which accumulate photogenerated electrons to the clay nanosheets before electron transfer. Since the MV/clay-colloids are viscous, the travel of the nanosheets should be slower than the accumulation of electrons. Thus, the electron transfer is expected to continue after the irradiation is stopped, during which the niobate nanosheets are photoreduced. The slow diffusion of the nanosheets is also reflected in the photoresponse of the MV/clay-niobate colloids with varied amounts of niobate nanosheets and MV2+ (Figures 4 and 5). When [niobate] or [MV2+] increases relative to a constant value of [clay], the generation period of the MV•+ species disappears, indicating that the electron transfer to MV2+ is completed within the period of irradiation. These results can be explained by multiplication of the opportunities for the niobate nanosheets and MV2+ molecules to meet under the increased [niobate] and [MV2+]; thus, the electron transfer from the donor to the acceptor is likely to be completed within the period of irradiation.

Nakato et al. Conclusions The present study demonstrates that the multicomponent colloidal system of the semiconducting niobate nanosheets and the clay nanosheets carrying the electron-accepting MV2+ molecules causes stable photoinduced charge separation and that the stability and efficiency are greatly controlled by the clay content. The photochemical behavior is explained by structural and dynamic properties characteristic to soft matter: donoracceptor spatial separation given by the hierarchical microdomain structure of the colloid and appropriate diffusion of the nanosheets given by the fluidity of the colloids. When the acceptor molecules are absent in the colloids, electrons photogenerated in the niobate nanosheets are accumulated there, and the coexisting clay nanosheets influence the stability of electronaccumulated state. The viscosity measurements indicate the contribution of the viscosity-enhancing effect and sol-gel transition of clay colloids, showing the characteristic features of the colloidally dispersed clay nanosheets as reaction media. These results represent versatility of inorganic nanosheets as advanced inorganic soft materials, which are different from conventional hard materials of inorganic layered crystals such as intercalation compounds and layer-by-layer assemblies. Acknowledgment. We thank Professor Kozo Kuchitsu (Tokyo University of Agriculture and Technology) for his valuable comments on English style and Dr. Satoshi Koizumi (Japan Atomic Energy Agency) for his assistance with the SANS experiments. This work was supported by a Grant-in-Aid for Scientific Research (No. 20550171) from Japan Society for the Promotion of Science. One of the authors (N. M.) thanks financial support by Reimei Research Program of Japan Atomic Energy Agency. References and Notes (1) (a) Solid State and Surface Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 2000. (b) AlstrumAcevedo, J. H.; Brennaman, M. K.; Meyer, T. J. Inorg. Chem. 2005, 44, 6802–6827. (c) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834–2860. (2) (a) Axelrod, H. L.; Okamura, M. Y. Photosynth. Res. 2005, 85, 101–114. (b) Melkozernov, A. N.; Barber, J.; Blankenship, R. E. Biochemistry 2006, 45, 331–344. (3) (a) Huynha, M. H. V.; Dattelbauma, D. M.; Meyer, T. J. Coord. Chem. ReV. 2005, 249, 457–483. (b) Thompson, B. C.; Frećhet, J. M. J. Angew. Chem., Int. Ed. 2008, 58–77. (4) (a) Nagamura, T. Colloids Surf., A 1997, 123-124, 457–471. (b) Kondo, T.; Uosaki, K. J. Photochem. Photobiol. C 2007, 8, 1–17. (c) Lu, Y.; Xu, J.; Liu, B.; Kong, J. Biosens. Bioelectron. 2007, 22, 1173–1185. (5) Thomas, J. K.; Ellison, E. H. AdV. Colloid Interface Sci. 2001, 8990, 195–238. (6) (a) Yoon, K. B. Chem. ReV. 1993, 93, 321–339. (b) Corma, A.; Garcia, H. Chem. Commun. 2004, 2004, 1443–1459. (7) (a) Ogawa, M.; Kuroda, K. Chem. ReV. 1995, 95, 399–438. (b) Shichi, T.; Takagi, K. J. Photochem. Photobiol. C 2000, 1, 113–130. (c) Takagi, S.; Eguchi, M.; Tryk, D. A.; Inoue, H. J. Photochem. Photobiol. C 2006, 7, 104–126. (8) Hoertz, P. G.; Mallouk, T. E. Inorg. Chem. 2005, 44, 6828–6840. (9) (a) Ogawa, M. J. Photochem. Photobiol. C 2002, 3, 129–146. (b) Angelos, S.; Johansson, E.; Stoddart, J. F.; Zink, J. I. AdV. Funct. Mater. 2007, 17, 2261–2271. (10) Behera, G. B.; Mishra, B. K.; Behera, P. K.; Panda, M. AdV. Colloid Interface Sci. 1999, 82, 1–42. (11) (a) Baxter, S. M., Jr.; Danielson, E.; Worl, L.; Strouse, G.; Younathan, J.; Meyer, T. J. Coord. Chem. ReV. 1991, 111, 47–71. (b) Miyasaka, H.; Khan, S. R.; Itaya, A. J. Photochem. Photobiol. C 2003, 4, 195–214. (12) (a) Onsager, L. Ann. N.Y. Acad. Sci. 1949, 51, 627–659. (b) Vroege, G. J.; Lekkerkerker, H. N. W. Rep. Prog. Phys. 1992, 55, 1241–1309. (c) Sonin, A. S. J. Mater. Chem. 1998, 8, 2557–2574. (d) Gabriel, J.-C. P.; Davidson, P. AdV. Mater. 2000, 12, 9–20. (e) Davidson, P.; Gabriel, J.C. P. Curr. Opin. Colloid Interface Sci. 2005, 9, 377–383. (f) Kajiwara, K.; Donkai, N.; Hiragi, Y.; Inagaki, H. Makromol. Chem. 1986, 187, 2883– 2893. (g) Buining, P. A.; Lekkerkerker, H. N. W. J. Phys. Chem. 1993, 97,

Colloidal System of Exfoliated Layered Semiconductor 11510–11516. (h) Maeda, H.; Maeda, Y. Langmuir 1996, 12, 1446–1452. (i) Somoza, A. M.; Sagui, C.; Roland, C. Phys. ReV. B 2001, 63, 081403. (j) Li, L.-S.; Alivisatos, A. P. AdV. Mater. 2003, 15, 408–411. (k) Dessombz, A.; Chiche, D.; Davidson, P.; Panine, P.; Chaneác, C.; Jolivet, J.-P. J. Am. Chem. Soc. 2007, 129, 5904–5909. (l) van der Kooij, F. M.; Lekkerkerker, H. N. W. J. Phys. Chem. B 1998, 102, 7829–7832. (m) van der Kooij, F. M.; Kassapidou, K.; Lekkerkerker, H. N. W. Nature (London) 2000, 406, 868–871. (n) Petukhov, A. V.; van der Beek, D.; Dullens, R. P. A.; Dolbnya, I. P.; Vroege, G. J.; Lekkerkerker, H. N. W. Phys. ReV. Lett. 2005, 95, 077801. (o) van der Beek, D.; Radstake, P. B.; Petukhov, A. V.; Lekkerkerker, H. N. W. Langmuir 2007, 23, 11343–11346. (13) (a) Adams, M.; Dogic, Z.; Keller, S. L.; Fraden, S. Nature (London) 1998, 393, 349–352. (b) Vliegenthart, G. A.; Lekkerkerker, H. N. W. J. Chem. Phys. 1999, 111. (c) van der Kooij, F. M.; Lekkerkerker, H. N. W. Phys. ReV. Lett. 2000, 84, 781–784. (d) Wensink, H. H.; Vroege, G. J.; Lekkerkerker, H. N. W. J. Chem. Phys. 2001, 115, 7319–7329. (e) Varga, S.; Galindo, A.; Jackson, G. J. Chem. Phys. 2002, 117, 7207–7221. (f) Oversteegen, S. M.; Lekkerkerker, H. N. W. Phys. ReV. E 2003, 68, 021404. (14) Jacobson, A. J. In ComprehensiVe Supramolecular Chemistry; Alberti, G., Bein, T., Eds.; Pergamon: New York, 1996; Vol. 7, pp 315335. (15) Sasaki, T. J. Ceram. Soc. Jpn. 2007, 115, 9–16. (16) Miyamoto, N.; Yamamoto, H.; Kaito, R.; Kuroda, K. Chem. Commun. 2002, 2378–2379. (17) Tanaka, T.; Ebina, Y.; Takada, K.; Kurashima, K.; Sasaki, T. Chem. Mater. 2003, 15, 3564–3568. (18) Langmuir, I. J. Chem. Phys. 1938, 6, 873–896. (19) Gabriel, J.-C. P.; Sanchez, C.; Davidson, P. J. Phys. Chem. 1996, 100, 11139–11143. (20) (a) Michot, L. J.; Bihannic, I.; Maddi, S.; Funari, S. S.; Baravian, C.; Levitz, P.; Davidson, P. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16101– 16104. (b) Michot, L. J.; Bihannic, I.; Maddi, S.; Baravian, C.; Levitz, P.; Davidson, P. Langmuir 2008, 24, 3127–3139. (c) Camerel, F. Chem. Commun. 2002, 1926–1927. (d) Backov, R.; Morga, A. N.; Lane, S.; PerezCordero, E. E.; Williams, K.; Meisel, M. W.; Sanchez, C.; Talham, D. R. Mol. Cryst. Liq. Cryst. 2002, 376, 127–134. (21) Gabriel, J.-C. P.; Camerel, F.; Lemaire, B. J.; Desvaux, H.; Davidson, P.; Batail, P. Nature (London) 2001, 413, 504–508. (22) Miyamoto, N.; Nakato, T. AdV. Mater. 2002, 14, 1267–1270. (23) Liu, S.; Zhang, J.; Wang, N.; Liu, W.; Zhang, C.; Sun, D. Chem. Mater. 2003, 15, 3240–3241. (24) van Olphen, H. Clay Colloid Chemistry, 2nd ed.; Krieger: Malabar, 1991. (25) Clay-Water Interface and Its Rhelogical Implications; Gu¨ven, N.; Pollastro, R. M., Eds.; The Clay Minerals Society: Aurora, 1992. (26) Smalley, M. Clay Swelling and Colloid Stability; Taylor & Francis: Boca Raton, FL, 2006. (27) (a) Ras, R. H. A.; Umemura, Y.; Johnston, C. T.; Yamagishi, A.; Schoonheydt, R. A. Phys. Chem. Chem. Phys. 2007, 9, 918–932. (b) Keller, S. W.; Kim, H.-N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817– 8818. (c) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370–373. (d) Kotov, N. A.; De´kańy, I.; Fendler, J. H. AdV. Mater. 1996, 8, 637–641. (e) Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M.; Matsumoto, M. Chem. Mater. 2000, 12, 854–856. (f) Sasaki, T.; Ebina, Y.; Tanaka, T.; Harada, M.; Watanabe, M.; Decher, G. Chem. Mater. 2001, 13, 4661–4667. (g) Omomo, Y.; Sasaki, T.; Wang, L.; Watanabe, M. J. Am. Chem. Soc. 2003, 125, 3568–3575. (h) Li, L.; Ma, R.; Ebina, Y.; Iyi, N.; Sasaki, T. Chem. Mater. 2005, 17, 4386–4391. (i) Umemura, Y.; Shinohara, E.; Koura, A.; Nishioka, T.; Sasaki, T. Langmuir 2006, 22, 3870–3877. (j) Saruwatari, K.; Sato, H.; Kogure, T.; Wakayama, T.; Iitake, M.; Akatsuka, K.; Haga, M.; Sasaki, T.; Yamagishi, A. Langmuir 2006, 22, 10066–10071. (28) Kaschak, D. M.; Lean, J. T.; Waraksa, C. C.; Saupe, G. B.; Usami, H.; Mallouk, T. E. J. Am. Chem. Soc. 1999, 121, 3435–3445. (29) (a) Wang, N.; Liu, S.; Zhang, J.; Wu, Z.; Chen, J.; Sun, D. Soft Matter 2005, 1, 428–430. (b) Zhang, J.; Luan, L.; Zhu, W.; Liu, S.; Sun, D. Langmuir 2007, 23, 5331–5337. (30) (a) Nakato, T.; Miyamoto, N.; Harada, A. Chem. Commun. 2004, 78–79. (b) Nakato, T.; Yamashita, Y.; Kuroda, K. Thin Solid Films 2006, 495, 24–28. (c) Nakato, T.; Hashimoto, S. Chem. Lett. 2007, 36, 1240– 1241.

J. Phys. Chem. B, Vol. 113, No. 5, 2009 1331 (31) Miyamoto, N.; Nakato, T. J. Phys. Chem. B 2004, 108, 6152–6159. (32) Domen, K.; Kondo, J. N.; Hara, M.; Takata, T. Bull. Chem. Soc. Jpn. 2000, 73, 1307–1331. (33) (a) Domen, K.; Kudo, A.; Shinozaki, A.; Tanaka, A.; Maruya, K.; Onishi, T. J. Chem. Soc., Chem. Commun. 1986, 356–7. (b) Shibata, M.; Kudo, A.; Tanaka, A.; Domen, K.; Maruya, K.; Onishi, T. Chem. Lett. 1987, 1017–18. (c) Kudo, A.; Sayama, K.; Tanaka, A.; Asakura, K.; Domen, K.; Maruya, K.; Onishi, T. J. Catal. 1989, 120, 337–52. (d) Domen, K.; Yoshimura, J.; Sekine, T.; Tanaka, A.; Onishi, T. Catal. Lett. 1990, 4, 339– 43. (e) Inoue, Y.; Kubokawa, T.; Sato, K. J. Phys. Chem. 1991, 95, 4059– 4063. (34) (a) Miyata, H.; Sugahara, Y.; Kuroda, K.; Kato, C. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2677–82. (b) Nakato, T.; Kuroda, K.; Kato, C. Chem. Mater. 1992, 4, 128–32. (c) Tong, Z.; Shichi, T.; Takagi, K. J. Phys. Chem. B 2002, 106, 13306–13310. (d) Yui, T.; Mori, Y.; Tsuchino, T.; Itoh, T.; Hattori, T.; Fukushima, Y.; Takagi, K. Chem. Mater. 2005, 17, 206–211. (e) Yui, T.; Tsuchino, T.; Itoh, T.; Ogawa, M.; Fukushima, Y.; Takagi, K. Langmuir 2005, 21, 2644–2646. (35) Miyamoto, N.; Nakato, T. Langmuir 2003, 19, 8057–8064. (36) Miyamoto, N.; Yamada, Y.; Koizumi, S.; Nakato, T. Angew. Chem., Int. Ed. 2007, 46, 4123–4127. (37) Nassau, K.; Shiever, J. W.; Bernstein, J. L. J. Electrochem. Soc. 1969, 116, 348–353. (38) Levitz, P.; Lecolier, E.; Mourchid, A.; Delville, A.; Lyonnard, S. Europhys. Lett. 2000, 49, 672–677. (39) Ramsay, J. D. F. J. Colloid Interface Sci. 1986, 109, 441–447. (40) Koizumi, S.; Iwase, H.; Suzuki, J.-i.; Oku, T.; Motokawa, R.; Sasao, H.; Tanaka, H.; Yamaguchi, D.; Shimizu, H. M.; Hashimoto, T. Physica B 2006, 385-386, 1000–1006. (41) Kosower, E.; Cotter, J. L. J. Am. Chem. Soc. 1964, 86, 5524–5527. (42) The SANS measurements were carried out for the colloids with very high [niobate] compared with that for the photochemical investigations in order to obtain clear scattering contrast. (43) Yamaguchi, D.; Miyamoto, N.; Koizumi, S.; Nakato, T.; Hashimoto, T. J. Appl. Crystallogr. 2007, 40, s101-s105. (44) (a) Tanaka, H.; Jabbari-Farouji, S.; Meunier, J.; Bonn, D. Phys. ReV. E 2005, 71, 021402. (b) Cummins, H. Z. J. Non-Cryst. Solids 2007, 353, 3891–3905. (45) Mongondry, P.; Tassin, J. F.; Nicolai, T. J. Colloid Interface Sci. 2005, 283, 397–405. (46) (a) Wada, Y.; Morikawa, A. Bull. Chem. Soc. Jpn. 1987, 60, 3509– 3513. (b) Yao, J. N.; Loo, B. H.; Hashimoto, K. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 699–701. (c) Carroll, E. C.; Compton, O. C.; Madsen, D.; Osterloh, F. E.; Larsen, D. S. J. Phys. Chem. C 2008, 112, 2394–2403. (47) Nakato, T.; Miyamoto, N. J. Mater. Chem. 2002, 12, 1245–1246. (48) (a) Mourchid, A.; Lećolier, H.; Van Damme, H.; Levitz, P. Langmuir 1998, 14, 4718–4723. (b) DiMasi, E.; Fossum, J. O.; Gog, T.; Venkataraman, C. Phys. ReV. E 2001, 061704. (49) (a) Miyata, H.; Sugahara, Y.; Kuroda, K.; Kato, C. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1851–8. (b) Vermeulen, L. A.; Thompson, M. E. Nature (London) 1992, 358, 656–658. (c) Dutta, P. K.; Incavo, J. A. J. Phys. Chem. 1987, 91, 4443–6. (d) Borja, M.; Dutta, P. K. Nature (London) 1993, 362, 43–45. (e) Sykora, M.; Kincaid, J. R. Nature (London) 1997, 387, 162–164. (f) Park, Y. S.; Lee, E. J.; Chun, Y. S.; Yoon, Y. D.; Yoon, K. B. J. Am. Chem. Soc. 2002, 124, 7123–7135. (50) (a) Kaneko, M.; Motoyoshi, J.; Yamada, A. Nature (London) 1980, 285, 468–470. (b) Kamogawa, H.; Suzuki, T. J. Chem. Soc., Chem. Commun. 1985, 1985, 525–526. (c) Slama-Schwok, A.; Ottolenghi, M.; Avnir, D. Nature (London) 1992, 355, 240–242. (d) Sampanthar, J. T.; Neoh, K. G.; Ng, S. W.; Kang, E. T.; Tan, K. L. AdV. Mater. 2000, 12, 1536– 1539. (51) (a) Brandenburg, U.; Lagaly, G. Appl. Clay Sci. 1988, 3, 263–279. (b) Malfoy, C.; Pantet, A.; Monnet, P.; Righi, D. Clays Clay Miner. 2003, 51, 656–663. (c) ten Brinke, A. J. W.; Bailey, L.; Lekkerkerker, H. N. W.; Maitland, G. C. Soft Matter 2007, 3, 1145–1162.

JP807214W

Photoinduced Charge Separation in a Colloidal System of Exfoliated

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