Cobalt Oxide Nanoclusters on Rutile Titania as Bifunctional Units for

Jan 24, 2017 - Modification of the R-TiO2 with 2.0 wt % Co followed by heating at 423 K for 1 h resulted in the highest photocatalytic activity with g...
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Cobalt Oxide Nanoclusters on Rutile Titania as Bifunctional Units for Water Oxidation Catalysis and Visible Light Absorption: Understanding the Structure−Activity Relationship Kazuhiko Maeda,*,† Koki Ishimaki,† Megumi Okazaki,† Tomoki Kanazawa,† Daling Lu,‡ Shunsuke Nozawa,§ Hideki Kato,∥ and Masato Kakihana∥ †

Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan ‡ Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan § Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan ∥ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *

ABSTRACT: The structure of cobalt oxide (CoOx) nanoparticles dispersed on rutile TiO2 (R-TiO2) was characterized by X-ray diffraction, UV−vis−NIR diffuse reflectance spectroscopy, highresolution transmission electron microscopy, X-ray absorption finestructure spectroscopy, and X-ray photoelectron spectroscopy. The CoOx nanoparticles were loaded onto R-TiO2 by an impregnation method from an aqueous solution containing Co(NO3)2·6H2O followed by heating in air. Modification of the R-TiO2 with 2.0 wt % Co followed by heating at 423 K for 1 h resulted in the highest photocatalytic activity with good reproducibility. Structural analyses revealed that the activity of this photocatalyst depended strongly on the generation of Co3O4 nanoclusters with an optimal distribution. These nanoclusters are thought to interact with the R-TiO2 surface, resulting in visible light absorption and active sites for water oxidation. KEYWORDS: artificial photosynthesis, cobalt, photocatalyst, solar energy conversion, water splitting

1. INTRODUCTION

Very recently, our group developed a new photocatalytic system based on rutile TiO2 and Co(OH)2 nanoclusters that is able to oxidize water to generate O2 under visible light illumination up to 850 nm.8 In contrast to the typical semiconductor photocatalysis process, it appears that, in our new system, electrons migrate from the Co(OH)2 nanoclusters to the metal oxide support, and water oxidation occurs on the electron-deficient cobalt species, as illustrated in Scheme 1B. That is, the loaded cobalt species play two roles, simultaneously serving as light-absorption centers and water oxidation sites. For many years now, the preparation of TiO2-based photocatalysts capable of functioning in response to visible light has been extensively studied. Among the various strategies that have been proposed, surface modification techniques using metal oxide clusters represent simple but effective approaches to sensitizing wide-gap TiO2 materials to visible light.9−16 Almost all of the work to date has been aimed at decomposing harmful organic compounds, such that our new system is the

Photocatalytic water splitting utilizing solar energy has attracted considerable attention in recent years as a means of producing hydrogen.1−4 Development of water oxidation systems that operate under a wide range of visible light is a critically important subject with regard to both water splitting and CO2 fixation5−7 because water oxidation to form O2 involves a complicated four-electron process with slow kinetics. Heterogeneous photocatalysis is one of the most promising approaches to realizing efficient water oxidation systems.1−4 It is well-known that semiconductor photocatalysis consists of the three steps summarized in Scheme 1A:1−4 (1) a semiconductor absorbs light with energy greater than its band gap, generating electrons and holes in the conduction and valence bands, respectively, (2) the photogenerated carriers move to the surface, and (3) surface-adsorbed species are reduced and oxidized to release products. In some cases, nanoparticulate cocatalysts are loaded to promote water oxidation/reduction reactions. The majority of semiconductor photocatalysts reported to be active for water oxidation operates via this mechanism. © 2017 American Chemical Society

Received: December 9, 2016 Accepted: January 24, 2017 Published: January 24, 2017 6114

DOI: 10.1021/acsami.6b15804 ACS Appl. Mater. Interfaces 2017, 9, 6114−6122

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ACS Applied Materials & Interfaces Scheme 1. Semiconductor Photocatalysis of Water Oxidationa

a

(A) Band-gap excitation of a semiconductor and (B) charge transfer from a cobalt compound to the conduction band of a semiconductor combined with water oxidation catalysis by the cobalt species.

Figure 1. (A) Variations in the O2 evolution rates over R-TiO2 prepared with various cobalt loadings at a common processing temperature of 423 K and (B) variations in the O2 evolution rates over 2.0 wt % cobalt-modified R-TiO2 with processing temperature. Reaction conditions: catalyst, 100 mg; reactant solution, aqueous AgNO3 (10 mM, 140 mL); light source, 300 W xenon lamp with a cutoff filter.

first example of the application of surface-modified TiO2 to water oxidation with possible applications to light energy conversion schemes. It is very important to understand the structure−activity relationship of a given photocatalytic material to optimize its efficiency. For this reason, there has been a significant amount of research attempting to clarify the structure−activity relationships in typical photocatalyst systems composed of oxides,17−20 oxynitrides,21,22 and nitrides.23−25 However, there is very little information regarding this new photocatalytic system in terms of the structure of the loaded cobalt species even though it is clear that these species play important roles in the water oxidation reaction. Because this type of system involves a new photocatalytic water oxidation mechanism, investigating the structure−activity relationship is of great interest and is an important subject in functional materials research. It is known that cobalt oxide-based materials are good catalysts for electrochemical water oxidation depending on their composition and textural properties.26,27 It is thus likely that the photocatalytic activity for water oxidation in our system results at least partly from the physicochemical properties of the loaded cobalt species. In the present study, R-TiO2 modified with cobalt oxide species was prepared under different conditions and subsequently characterized by X-ray diffraction (XRD), UV−vis−NIR diffuse reflectance spectroscopy (DRS),

high-resolution transmission electron microscopy (HR-TEM), X-ray absorption fine-structure spectroscopy (XAFS), and Xray photoelectron spectroscopy (XPS). The relationship between the structure of the photocatalyst and its activity is discussed based on the results of this structural characterization.

2. RESULTS AND DISCUSSION 2.1. Photocatalytic Activity. Figure 1A shows the rates of O2 evolution over cobalt-modified R-TiO2 specimens under visible light (λ > 500 nm) as a function of the amount of cobalt loaded, applying a temperature of 423 K during preparation of the samples. The original R-TiO2 did not evolve O2 under the present reaction conditions because its band gap was too large to harvest photons above 500 nm. With increases in the amount of cobalt loaded, the O2 evolution rate was improved to a maximum at 2.0 wt % cobalt, after which the rate gradually decreased. In a similar manner, the effect of the processing temperature was investigated using 2.0 wt % cobaltimpregnated R-TiO2. As shown in Figure 1B, increasing the temperature enhanced the O2 evolution activity of the material. The maximum activity was obtained at 423 K, beyond which the reaction rate began to drop. Thus, it is evident that both the extent of cobalt loading and the processing temperature are important parameters when attempting to maximize the photocatalytic activity of cobalt-modified R-TiO2. 6115

DOI: 10.1021/acsami.6b15804 ACS Appl. Mater. Interfaces 2017, 9, 6114−6122

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Figure 2. (A) Typical time course of O2 evolution under visible light irradiation (λ > 500 nm) over 50 mg of 2.0 wt % cobalt-modified R-TiO2 processed at 423 K together with data for CoTiO3 (100 mg) for comparison purposes and (B) dependence of the O2 evolution rate obtained from the same material (100 mg) on the cutoff wavelength of the incident light. The red curve in this panel presents the DRS spectrum of the same material. Reaction conditions: catalyst, 50−100 mg; reactant solution, aqueous AgNO3 (10 mM, 140 mL); light source, 300 W xenon lamp with a cutoff filter.

Figure 3. XRD patterns obtained from (A) R-TiO2 prepared with various cobalt loadings and a common temperature of 423 K and (B) 2.0 wt % cobalt-modified R-TiO2 after calcination at various temperatures.

Figure 4. TEM images of cobalt-modified R-TiO2 specimens processed at 423 K with two different cobalt loadings.

The O2 evolution evidently decreased as the cutoff wavelength was increased, falling to almost nil above 850 nm. This change in activity is consistent with the DRS data for this same sample. These results clearly indicate that the observed O2 evolution is a photocatalytic process. Qu et al. reported that CoTiO3 is an active photocatalyst for O2 evolution from an aqueous AgNO3 solution under visible light irradiation (λ > 420 nm).28 However, our data indicate that the activity of CoTiO3 under the present reaction

Figure 2A presents a typical time course of O2 evolution using the optimized material (2.0 wt % cobalt/R-TiO2 heated at 423 K) under visible light (λ > 500 nm). Although the O2 evolution can be seen to have gradually declined over time, due primarily to the deposition of Ag resulting from the photoreduction of Ag+ with electrons, the total amount of O2 generated by the cobalt-loaded material was 3.4 times greater than that of the loaded cobalt. Figure 2B plots the dependence of the O2 evolution rate on the incident light cutoff wavelength. 6116

DOI: 10.1021/acsami.6b15804 ACS Appl. Mater. Interfaces 2017, 9, 6114−6122

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Figure 5. (A) Co−K edge XANES spectra of R-TiO2 samples prepared with various cobalt loadings and a common temperature of 423 K and (B) the Fourier transforms of the k3-weighted Co−K edge EXAFS spectra of the same samples. Some reference data are also shown.

important observation is that the position of the TiO2 diffraction peak was shifted slightly to higher 2θ angles with increasing temperature. The ionic radii of Co2+ and Co3+ in a 6coordination environment are 0.79 and 0.69 Å, respectively. Therefore, this peak shift is reasonable based on the substitution of smaller Co3+ ions for Ti4+ (ionic radius of 0.75 Å) in the TiO2 lattice. The presence of Co3+ in the prepared samples is also supported by the XAFS data discussed below. The valence states of the cobalt species in the prepared materials were studied by XAFS, and Figure 5A shows the Co− K edge X-ray absorption near-edge structure (XANES) spectra for 2.0 and 10.0 wt % cobalt-modified R-TiO2 after calcination at 423 K. Some reference data are also provided for comparison purposes. The Co−K edge spectra of the cobalt-loaded samples are seen to be similar to one another. The shapes of these spectra somewhat resemble that of Co3O4 but are dissimilar to those of Co(NO3)2, CoO, and CoTiO3. The Co3O4 XANES has the characteristic cobalt 4p structures at 7724 and 7729 eV (see dotted lines in Figure 5A), which are attributed to two cobalt sites in Co3O4:octahedral (Co(oct)) and tetrahedral (Co(tetra)).31,32 The characteristic structures of the 2.0 wt % sample are much weaker and broader than that of the 10.0 wt % sample. The behavior with decreasing the particle size can be explained by the disorder of the local structure around cobalt sites, as exemplified by Chen et al. in TiO2 nanocrystals.33 This assignment is supported by the Fourier transforms of the k3-weighted Co−K edge EXAFS spectra of the same samples. As shown in Figure 5B, the spectrum of the 10.0 wt % sample is a very close match to that of Co3O4 with peaks at approximately 1.5, 2.5, 3.0, and 4.7 Å. These results demonstrate that the cobalt species in the 10.0 wt % sample were very close to Co3O4 in their chemical composition. According to a previous report,32 the peaks at approximately 1.5 and 2.5 Å can be assigned to Co−O and Co(oct)−Co(oct), respectively. The third and fourth peaks in the vicinities of 3.0 and 4.7 Å, respectively, are assigned to Co(tetra)−Co(oct) and Co(tetra)−Co(tetra), and Co(oct)−Co(oct) behind the

conditions (>500 nm irradiation) was very low (Figure 2A). It is also noted that the present photocatalyst prepared by an impregnation method exhibited activity higher than that of the previous Co(OH)2/TiO2 system (Figure S1) and that the present impregnation method is highly reproducible not only when using R-TiO2 but also when applied to other oxide semiconductors such as SrTiO3 (Figure S2). Another noticeable fact is that the optimized material produced O2 even in the presence of a significant amount of methanol (Figure S3), which is much more susceptible to oxidation than water. This result indicates high water oxidation activity of cobalt-modified R-TiO2, clearly different from other visible-light-responsive photocatalysts such as GaN:ZnO and TaON, where O2 evolution was completely suppressed in the presence of methanol.29,30 2.2. Structures of Photocatalysts. To understand the structure−activity relationship during photocatalytic water oxidation by these cobalt-impregnated R-TiO2 photocatalysts, the synthesized materials were investigated by XRD, UV−vis− NIR DRS, HR-TEM, XAFS, and XPS. Figure 3A shows the XRD patterns generated by cobalt-impregnated R-TiO 2 specimens with varying cobalt loadings and processed at a common temperature of 423 K. No peaks other than those attributable to rutile TiO2 are seen, regardless of the loading amount. This result suggests that the loaded cobalt species were amorphous and/or in the form of nanoparticles smaller than the diffraction limit. Support for this finding is provided by TEM observations (Figure 4), which indicate that the loaded cobalt species was in the form of nanoparticles with primary sizes in the range of 3−4 nm but without lattice fringes. At higher loadings, the nanoparticles evidently assembled to form larger agglomerates. Figure 3B presents the XRD patterns of 2.0 wt % cobaltimpregnated TiO2 samples processed at different temperatures. No significant changes in the patterns are observed up to 773 K. However, at 973 K, additional peaks assigned to CoTiO3 appear, indicating that the impregnated cobalt species reacted with the TiO2 surface to form a mixed oxide. Another 6117

DOI: 10.1021/acsami.6b15804 ACS Appl. Mater. Interfaces 2017, 9, 6114−6122

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Figure 6. (A) Co−K edge XANES spectra of 2.0 wt % cobalt-impregnated R-TiO2 samples processed at various temperatures and (B) Fourier transforms of the k3-weighted Co−K edge EXAFS spectra of the same samples. Some reference data are also shown.

Figure 7. UV−vis−NIR diffuse reflectance spectra of (A) R-TiO2 prepared with various CoOx loadings and a common processing temperature of 423 K and (B) 2.0 wt % CoOx-loaded R-TiO2 after calcination at various temperatures.

Co(NO3)2·6H2O spectrum. The sample spectra also exhibit a change above a processing temperature of 423 K, after which they become increasingly similar to the Co3O4 spectra as the temperature increases to 773 K (as noted above). These results suggest that the impregnated cobalt species on the R-TiO2 surfaces decomposed and converted to Co3O4 at elevated temperatures. At 973 K, the sample spectrum is a good match to that of CoTiO3. Figure 6B provides the Fourier transforms of the k3-weighted Co−K edge EXAFS spectra of the same samples. Some spectral features of Co(NO3)2·6H2O (for example, a peak at 1.7 Å) are still present to some extent in those samples heated below 373 K, indicating that some Co(NO3)2·6H2O was preserved up to this temperature. However, these characteristic peaks are undetectable at 423 K and above. Peaks that are indicative of Co3O4, at 1.5, 2.5, 3.0, and 4.7 Å, remain almost unchanged up to 773 K, although a slight shift in the shell peaks is ongoing from 423 to 773 K, which suggests the growth of the loaded Co3O4 with increasing temperature. At 973 K, however, the EXAFS spectrum becomes similar to that of CoTiO3. These

nearest neighboring Co(oct). It is worth noting that the peak positions of the 10.0 wt % sample appear at slightly shorter distances relative to those of the bulk Co3O4. The shell peak in the radial structure function is known to shift to shorter distances with decreasing particle size, as demonstrated by Chen et al.33 Therefore, the present results can be reasonably interpreted to show that the 10.0 wt % sample contained aggregated Co 3O 4 nanoparticles but not at the same concentration as in a bulk type specimen. Figure 5B also shows that the spectral features of the 2.0 wt % cobalt sample resemble those of the 10.0 wt % sample but with much weaker peaks (especially at longer distances) and more pronounced peak shifts. These data indicate that the loaded cobalt species in the 2.0 wt % sample had particle sizes smaller than those in the 10.0 wt % specimen, which is supported by the results of TEM observations (Figure 4). Figure 6A presents the Co−K edge XANES spectra for 2.0 wt % cobalt specimens processed at various temperatures along with some reference data. The Co−K edge spectra of the impregnated samples clearly differ from those generated by CoO, Co3O4, and CoTiO3 but are very similar to the 6118

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ACS Applied Materials & Interfaces changes are consistent with those observed in the XANES spectra (Figure 6A). On the basis of the XAFS results, it can be concluded that processing at 423 K transforms the loaded cobalt species on RTiO2 to Co3O4 nanoparticles, but these undergo aggregation at higher cobalt loadings, as demonstrated by TEM observations (Figure 4). In addition, further increases in the heating temperature promote the reaction of the loaded cobalt species with the R-TiO2 surface to produce a mixed oxide phase. For the sake of simplicity, the prepared materials will be represented hereafter as CoOx/R-TiO2. Variations in the cobalt loading and processing temperature both caused a significant change in the light absorption properties of the material as well. Figure 7A shows the UV− vis−NIR DRS data obtained from R-TiO2 prepared with various cobalt loadings and a common temperature of 423 K. Increasing the loading amount resulted in the generation of new visible light absorption bands extending to 900 nm. The absorption band around 700 nm became especially pronounced at loading amounts above 5.0 wt %. The spectral features in this vicinity are somewhat similar to those seen in the spectra of CoO-R-TiO2 and Co3O4-R-TiO2 physical mixtures. This result suggests the generation of bulk-like CoO and/or Co3O4 species in the CoOx/R-TiO2 at higher loadings, which is consistent with the results of TEM observations and XAFS studies (Figures 4 and 5). It should be noted here that the XAFS results (Figures 5 and 6) appear to show that, at higher cobalt loadings, the loaded cobalt species were closer to Co3O4 than CoO. However, the spectra of CoO-R-TiO2 and Co3O4-R-TiO2 physical mixtures do not exhibit a shoulder at 400−600 nm, in sharp contrast to the spectra produced by the 10.0 wt % CoOx/ R-TiO2. Therefore, there is an electronic interaction between the loaded cobalt species and the TiO2 in CoOx/R-TiO2, which is almost completely absent in simple physical combinations of R-TiO2 and Co3O4 (or CoO). In this regard, the prepared CoOx/TiO2 was distinct from a basic physical mixture between the two components. In fact, we confirmed that neither of these physical mixtures, nor the bulk cobalt oxides, showed any activity during O2 evolution under the present reaction conditions. Thus, the electronic interactions between CoOx and R-TiO2 are essential to inducing visible light absorption and promoting the resulting water oxidation activity. The effects of the temperature applied after impregnation on light absorption properties were also examined. As shown in Figure 7B, Co(NO3)2-impregnated R-TiO2 showed little visible light absorption. However, heating the impregnated sample resulted in the generation of new absorption bands, which were more pronounced at elevated temperatures even though the same amount of cobalt was loaded on the R-TiO2 surface. This trend continued up to 773 K with a small rise at 700 nm, possibly due to the formation of aggregated cobalt species as discussed earlier. From these results, it is highly likely that electronic interactions between the loaded CoOx and R-TiO2 were strengthened upon heating. However, above 773 K, the intensity of the 400−600 nm band decreases, and at 973 K, a new peak assignable to CoTiO3 is observed, consistent with the XRD and XAFS results (Figures 3 and 6). XPS analyses were conducted to obtain more structural information. According to the XAFS data, the cobalt species in the 2.0 wt % sample heated at 423 K were in the form of Co3O4 nanoclusters (Figures 5 and 6). This material exhibited photoelectron signals similar to those of bulk Co3O4, as shown in Figure 8. However, the peak positions of the same

Figure 8. Co 2p XPS spectra of 2.0 wt % cobalt-loaded R-TiO2 after heating at various temperatures.

material were shifted slightly to higher binding energies, suggesting that the loaded CoOx species were more cationic (that is, in an electron-deficient state). This is not unreasonable, considering that there were electronic interactions between the loaded CoOx species and the R-TiO2, as indicated by the UV− vis−NIR DRS results (Figure 7). Another important observation is that the cobalt-derived photoelectron signal tended to become weaker as the processing temperature was increased. This is evident from the plot of the surface Co/Ti atomic ratio as a function of temperature (Figure S4). From this result, it seems that the loaded cobalt species underwent a reaction (or doping) with the surface of the R-TiO2, a conclusion that is supported by the XRD and XAFS results (Figures 3 and 6). 2.3. Relationship between Photocatalyst Structure and Activity. The results of photocatalytic reaction studies and structural characterization indicate that the photocatalytic activity for water oxidation is dependent on the physicochemical state of the final material. As shown in Figure 1A, the photocatalytic activity was found to vary greatly with the extent of cobalt loading. In the range between 0 and 2.0−3.0 wt % cobalt content, over which the activity increased significantly, the data demonstrate that the visible light absorption became more pronounced (Figure 7), and the loaded cobalt species transitioned from the precursor form to Co3O4 nanoclusters (Figures 4−6). Therefore, the increase in activity with increasing cobalt content is considered to be associated with the formation of Co3O4 nanoclusters, which are capable of inducing visible light absorption. However, further loading with more than 3.0 wt % cobalt resulted in a decrease in activity attributable to the aggregation of the primary particles, as observed in TEM images (Figure 4). It is generally accepted that the rate of a catalytic reaction over a solid material is decreased with increases in the particle size of that substance. In the present photocatalytic system, catalytic water oxidation occurring on the loaded Co3O4 would therefore become inefficient with increases in the Co3O4 particle size. The processing temperature after impregnation was also shown to be an important factor contributing to the enhancement of photocatalytic activity. The structural changes undergone by the catalyst in the various treatment steps are 6119

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preparation (68 μmol at a maximum for the 2.0 wt % case) for visible-light water oxidation reaction.

depicted schematically in Figure 9. The results of XAFS measurements indicate that the majority of the loaded cobalt

3. CONCLUSIONS This work studied the formation and structural characteristics of cobalt species loaded on R-TiO2 as catalytic water oxidation and visible light absorption sites. Cobalt oxide nanoclusters formed on the R-TiO2 surface using Co(NO3)2 as the precursor resulted in visible light absorption due to electronic interactions between the two components. The formation of a Co3O4/RTiO2 heterojunction following the suitably high dispersion of Co3O4 on the R-TiO2 surfaces as well as stronger visible light absorption and the suppression of the diffusion of Co3+ ions in the R-TiO2 are the most important factors affecting the enhancement of the photocatalytic activity during water oxidation. 4. EXPERIMENTAL SECTION 4.1. Materials and Regents. Rutile TiO2 (R-TiO2) powder (JRCTIO-6) was supplied by the Catalysis Society of Japan and was used without further purification. Co(NO3)2·6H2O (≥99.95%; Kanto Chemicals Co.) was employed as the precursor for the loading of nanoparticulate cobalt species. When acquiring XAFS and XPS spectra, CoO (99.7%; Kojundo Chemicals Co.) and Co 3 O 4 (≥99.95%; Kanto Chemicals Co.) were used as references. CoTiO3 and SrTiO3 were prepared by the polymerized complex method using ethylene glycol (99.5%; Kanto Chemicals) and anhydrous citric acid (98.0%; Wako Pure Chemicals) as polymerization agents.34 The Sr and Ti sources were SrCO3 (96%; Kanto Chemicals Co.) and titanium tetra-isopropoxide (97.0%; Kanto Chemicals), respectively. The final calcination temperature was 1073 K (2 h in air) in both cases. The production of single-phase CoTiO3 and SrTiO3 was confirmed by XRD analysis (Figure S7). 4.2. Modification of Rutile TiO2 with Nanoparticulate Cobalt Species. Nanoparticulate cobalt species were loaded onto R-TiO2 using an impregnation method. In this process, 0.25 g of R-TiO2 powder and 1 mL of distilled water containing an appropriate amount of Co(NO3)2·6H2O were transferred into an evaporation dish which was subsequently placed in a water bath. The suspension was stirred using a glass rod until the water was completely evaporated. The resulting powder was collected and heated in air at 373−973 K for 1 h, with the exception of one set of samples that was not heated. In this paper, the cobalt loading is given based on the amount of metallic cobalt. 4.3. Characterization. The prepared samples were characterized by XRD (Rigaku MiniFlex600; Cu Kα), HR-TEM (JEM-2010F, JEOL), UV−vis−NIR DRS (JASCO, V-565 spectrophotometer), FTIR (JASCO; FT-IR-610), XAFS, and XPS (ESCA-3400, Shimadzu). The binding energies determined by XPS were calibrated with respect to the C 1s peak (285.0 eV) for each sample. The Brunauer− Emmett−Teller (BET) surface areas were measured at 77 K using a BELSORP-mini instrument (BEL Japan). XAFS measurements of Co−K edge spectra were carried out at the BL9A beamline of the Photon Factory (High Energy Accelerator Research Organization, Tsukuba, Japan) under the approval of the Photon Factory Advisory Committee (Proposal no. 2014S2-006). The X-ray energy was varied using a Si(111) double-crystal monochromator and calibrated by the absorption edge energy of cobalt foil (7709.6 eV). At room temperature, XAFS spectra were collected in the fluorescence mode using a 19-element Ge solid-state array detector (CANBERRA, United States). The data for X-ray absorption near-edge structure (XANES) spectra were processed using the Athena. 4.4. Photocatalytic Water Oxidation. Water oxidation reactions were performed at room temperature using a method reported previously.8 Briefly, a top-irradiation type Pyrex cell connected to an airtight closed gas circulation system was immersed in a cold water bath. The reaction solution consisted of 140 mL of an aqueous solution containing 100 mg of cobalt-modified R-TiO2 and 10 mM of

Figure 9. Changes in the structures of cobalt species on the rutile TiO2 surface upon heating at different temperatures.

species remained in the precursor form after impregnation and at processing temperatures below 373 K (Figure 6). The decomposition of the impregnated precursors began at 423 K, forming cobalt oxide species on the R-TiO2 that allowed visible light absorption (Figures 6 and 7). With increases in the processing temperature up to 773 K, the visible light absorption of the CoOx/R-TiO2 became more pronounced. XRD analysis demonstrated that elevated temperatures served to dope some portion of the Co3+ ions into the R-TiO2 lattice (Figure 3B) with varying degrees of aggregation of the loaded CoOx (Figure 6B). Accordingly, the increase in activity up to 423 K is considered to be primarily due to the formation of Co3O4/RTiO2 capable of visible light absorption as well as the generation of catalytic sites suitable for water oxidation. Conversely, the decrease in activity above 423 K can be attributed to the aggregation of the cobalt nanoparticles as well as surface doping that eventually produces an inactive CoTiO3 phase. We further examined the possible contribution of the doped cobalt species in R-TiO2 to visible-light water oxidation activity. Cobalt-doped R-TiO2 (Ti0.99Co0.01O2 that contained 0.74 wt % cobalt) was synthesized by heating Co(NO3)2impregnated R-TiO2 at 1373 K for 10 h in air. The production of single-phase rutile structure was confirmed by XRD (Figure S5A) as well as its visible-light-absorption capability in UV− visible diffuse reflectance spectrum (Figure S5B). However, the as-prepared Ti0.99Co0.01O2 exhibited no activity for water oxidation under visible light (λ > 500 nm), again confirming that the composite structure of Co3O4 and R-TiO2 was essential to induce the activity for visible-light water oxidation. It is also noted that a peak due to nitrate ions was observed at 1384 cm−1 in the FT-IR spectrum of the optimal sample (Figure S6). Although we could not quantify the amount of the nitrate residue on the R-TiO2 surface, it is likely that the residue had little impact on water oxidation activity because the asprepared material was dispersed in silver nitrate aqueous solution whose concentration (10 mM; 1.4 mmol in 140 mL solution) was much higher than that in the photocatalyst 6120

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Research Article

ACS Applied Materials & Interfaces

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AgNO3 buffered to a pH of 8.0−8.5 using 200 mg of La2O3. After the solution was evacuated several times to completely remove residual air, the reaction cell was exposed to light from a 300 W xenon lamp (Cermax, PE300BF) which first passed through a water filter, applying an output current of 10 A unless otherwise stated. The wavelength of the incident light was controlled with a cutoff filter (Y-50, R-66, IR-76, IR-80, and IR-85). The gases evolved in the reaction system were analyzed by online gas chromatography (Shimadzu GC-8A with a thermal conductivity detector and an MS-5A column with argon carrier gas).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15804. Additional characterization and reaction data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kazuhiko Maeda: 0000-0001-7245-8318 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aids for Challenging Exploratory Research (Project no. JP15K14220), Young Scientists (A) (Project no. JP16H06130), and Scientific Research on Innovative Areas (Project nos. JP16H06439 and JP16H06441; Mixed Anion). The authors would also like to acknowledge The Hosokawa Powder Technology Foundation and The Noguchi Institute. The work presented herein was also supported in part by the Photon and Quantum Basic Research Coordinated Development Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and the “Chemical Conversion of Light Energy” program of PRESTO/Japan Science and Technology Agency (JST).

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REFERENCES

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DOI: 10.1021/acsami.6b15804 ACS Appl. Mater. Interfaces 2017, 9, 6114−6122

Research Article

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DOI: 10.1021/acsami.6b15804 ACS Appl. Mater. Interfaces 2017, 9, 6114−6122