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Visible-Light-Responsive CuLi1/3Ti2/3O2 Powders Prepared by a Molten CuCl Treatment of Li2TiO3 for Photocatalytic H2 Evolution and Z-Schematic Water Splitting Katsuya Iwashina, Akihide Iwase, Shunsuke Nozawa, Shin-ichi Adachi, and Akihiko Kudo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01557 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 20, 2016

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Chemistry of Materials

Visible-Light-Responsive CuLi1/3Ti2/3O2 Powders Prepared by a Molten CuCl Treatment of Li2TiO3 for Photocatalytic H2 Evolution and Z-Schematic Water Splitting Katsuya Iwashina,a Akihide Iwase,a,b Shunsuke Nozawa,c Shin-ichi Adachi,c,d Akihiko Kudoa,b,* a

Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan b Photocatalysis International Research Center, Research Institute for Science and Technology, Tokyo University of Science, 2641 Noda-shi, Yamazaki, Chiba-ken, Japan c Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 3050801, Japan d Department of Materials Structure Science, School of High Energy Accelerator Science, The Graduate University for Advanced Studies, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan ABSTRACT: CuLi1/3Ti2/3O2 powders with hexagonal- and trigonal-delafossite structures were prepared by treating cubic-Li2TiO3 and monoclinic-Li2TiO3 with a molten CuCl, respectively. It was confirmed that Li+ at the interlayer of Li2TiO3 was replaced with Cu(I). Both hexagonal- and trigonal-CuLi1/3Ti2/3O2 showed photocatalytic activities for sacrificial H2 evolution using visible light up to 600 nm which corresponds to their absorption edges (BG 2.1 eV), when Ru- and Pt-cocatalysts were loaded. The activity of the hexagonal-CuLi1/3Ti2/3O2 was higher than that of the trigonal-CuLi1/3Ti2/3O2. The apparent quantum yield of the sacrificial H2 evolution from an aqueous K2SO3+Na2S solution over the hexagonal-CuLi1/3Ti2/3O2 loaded with Ru-cocatalyst was 3.3% at 420 nm. Moreover, Z-schematic water splitting was achieved, when hexagonal- and trigonal-CuLi1/3Ti2/3O2 with a Pt-cocatalyst as H2evolving photocatalysts were combined with a reduced graphene oxide (RGO)-TiO2 composite as an O2-evolving photocatalyst.

INTRODUCTION Solar hydrogen production using photocatalysts is a potential candidate to address energy and environmental issues. Many photocatalysts for overall water splitting have been reported, while most of them respond to only UV light.1-5 Development of visible-light-driven photocatalysts is a key issue to achieve highly efficient solar hydrogen production. Formation of shallow valence bands is a promising strategy to reduce band gap energy.2 Cu(I),6-15 Ag(I),16,17 Sn(II),18 Pb(II),19 and Bi(III)20 are useful components to form new and shallow valence bands in oxides. Among them, Cu(I) forms the shallowest valence bands. Cu(I)-containing oxides should be synthesized in the absence of oxygen, because Cu(I) is easily oxidized to Cu(II). Cu(I)-containing niobates and tantalates were synthesized by a solid-state reaction and a flux method with a CuCl flux in the absence of air.6-14 In the flux method, the objective material is formed from Cu2O and metal oxide precursors through dissolution-precipitation processes in a CuCl flux. We have also reported the synthesis of Cu(I)substituted NaTaO3 and K2La2Ti3O10 by treating NaTaO3 and K2La2Ti3O10 powders with a molten CuCl.15 In the molten CuCl treatment, the objective material is formed through an ion-exchange process for Na+ and K+ of monovalent cations in the molten CuCl flux. Thus, the preparation of Cu(I)containing materials by Cu(I)-substitution in the molten CuCl flux is a different technique from that by the flux method with CuCl. Those Cu(I)-containing photocatalysts showed activities

for H2 evolution from an aqueous solution containing sacrificial reagents under visible light irradiation.9,13-15 Thus, the Cu(I) is a useful component to develop visible-light-driven metal oxide photocatalysts. We have previously reported that AgLi1/3Ti2/3O2 with delafossite structure was able to be synthesized by treating Li2TiO3 powders with a molten AgNO3.17 The AgLi1/3Ti2/3O2 responds to visible light due to the valence band formed by Ag(I) and shows photocatalytic activity for sacrificial O2 evolution under visible light irradiation. However, the band gap (2.7 eV) of the AgLi1/3Ti2/3O2 is still too wide to utilize the sunlight efficiently. Band gap narrowing can be expected by replacing Ag(I) with Cu(I). As mentioned above, the molten CuCl treatment is a useful technique to synthesize Cu(I)containing oxides. Cu(I) can stably located in the hexagonaland trigonal-delafossite structures, as observed for CuFeO2 shown in Figure 1(c,d).21,22 Although the crystal structure has not been fully clarified yet, CuLi1/3Ti2/3O2 with a hexagonaldelafossite structure was recently prepared from Cu2O and Li4Ti5O12 by a flux method using CuCl and showed photocatalytic activity for sacrificial H2 evolution under visible light irradiation.14 However, a single phase of CuLi1/3Ti2/3O2 with a trigonal-delafossite structure is not obtained. These facts motivate us to synthesize selectively the CuLi1/3Ti2/3O2 with a hexagonal-delafossite structure and with a trigonal-delafossite structure by treating Li2TiO3 powders with a molten CuCl. The Li2TiO3 precursors with cubic and monoclinic phases

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Figure 1. Crystal structures of Li2TiO3 with (a) cubic (PDF: 1-751602) and (b,b’) monoclinic (PDF: 1-71-2348) phases and CuFeO2 with (c) hexagonal (PDF: 1-79-1546) and (d) trigonal (PDF: 1-75-2146) phases. (c) and (d) are proposed models of hexagonal- and trigonal-CuLi1/3Ti2/3O2, respectively.

possess bulky and layered crystal structures, respectively, as shown in Figure 1(a,b). The layered monoclinic Li2TiO3 (Figure 1b) can be described as (LiLi1/3Ti2/3O2)1.5 in which TiO6 and LiO6 octahedrons-ordered layers (Figure 1b’) and Li ions in interlayers exist. On the basis of the similarly ordered layers in the monoclinic Li2TiO3 (Figure 1b) and in the CuFeO2 with a trigonal-delafossite structure (Figure 1d), the CuLi1/3Ti2/3O2 with a trigonal-delafossite structure is expected to be prepared by substitution of Li+ in the monoclinic Li2TiO3 with Cu(I) in a molten CuCl. In the present study, we selectively prepared single phases of CuLi1/3Ti2/3O2 with hexagonal- and trigonal-delafossite structures by treating cubic and monoclinic Li2TiO3 powders with a molten CuCl. Their photocatalytic activities for the sacrificial H2 evolution were investigated under visible light irradiation. We have also demonstrated Z-schematic water splitting into H2 and O2 using the CuLi1/3Ti2/3O2 as an H2evolving photocatalyst.

EXPERIMENTAL SECTION Preparation of metal oxides. Cubic Li2TiO3 (Li2TiO3(cub)) and monoclinic Li2TiO3 (Li2TiO3(mon)) powders were prepared by a polymerized complex method at different temperature.23 Li2CO3 (Kojundo Chemical; 99.99%), Ti(OC4H9)4 (Kanto Chemical; 97%), propylene glycol (Kanto Chemical; 99.5%), and citric acid (Wako Pure Chemical; 98.0%) were used as staring materials for preparation of Li2TiO3(cub) and Li2TiO3(mon). Those starting metal compounds and citric acid were dissolved in a mixed solvent of methanol and propylene glycol. The mixed solution was heated at 393 K to obtain a

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precursor via sol and gel states. The precursor was calcined using an alumina crucible in air at 773 K for 1 h and 973 K for 10 h to obtain Li2TiO3(cub) of a low temperature phase and Li2TiO3(mon) of a high temperature phase, respectively. CuCl was prepared from CuCl2 (Wako Pure Chemical; 99.0%) and metallic Cu. The CuCl2 and metallic Cu were added into boiled hydrochloric acid (3–4 mol L–1), resulting in formation of CuCl powders. The Li2TiO3(cub) and Li2TiO3(mon) powders were immersed in a molten CuCl at 773–873 K for 10 h in a quartz ampoule tube under vacuum to obtain CuLi1/3Ti2/3O2 powders with hexagonal- and trigonal-delafossite structures. After the molten CuCl treatment, an excess CuCl was removed using an aqueous NH3 solution. For comparison, CuLi1/3Ti2/3O2 was also prepared by a solid-state reaction. Starting materials of Cu2O (Wako Pure Chemical; 99.5%), Li2CO3, and TiO2 (Kojundo Chemical; 99.99%) were mixed in a ratio of Cu:Li:Ti=3:1.1:2. The mixture was calcined in N2 at 873–1273 K for 10 h in an aluminum boat. An RGO-TiO2 composite of an O2-evolving photocatalyst in Z-scheme was prepared by photocatalytic reduction of graphene oxide over TiO2.24 0.025 g of graphene oxide prepared by Hummers’ method25 and 0.5 g of TiO2 (Kojundo Chemical, 99.99%) were dispersed in 40 mL of an aqueous methanol (Kanto Chemical, 99.8%) solution (50 vol%). The suspension was irradiated with UV light for 4 h in N2 to form RGO-TiO2 composite. The obtained RGO-TiO2 composite was collected by a filtration. Characterization. The crystal structures of obtained metal oxides were analyzed on an X-ray diffractometer (Rigaku; Mini Flex). The bulk composition of CuLi1/3Ti2/3O2 was analyzed with Inductively Coupled Plasma Atomic Emission Spectrometry (HITACHI; P-4010). The samples were dissolved in aqua regia for the ICP measurement. The morphologies and shapes of Li2TiO3 and CuLi1/3Ti2/3O2 particles were observed with a scanning electron microscope (JEOL; JSM7400F). The surface chemical states of CuLi1/3Ti2/3O2 before and after photocatalytic reaction were investigated using XPS and Auger spectra collected by an X-ray photoelectron spectrometer (Shimazu; ESCA-3400). Diffuse reflectance spectra were obtained by a UV-vis-NIR spectrometer equipped with an integrating sphere (Jasco; UbestV-570) and converted from reflection to K-M function by Kubelka-Munk method. Ti and Cu K-edge XAFS spectra were collected in the transmission mode at the BL12C of the Photon Factory (PF) in Tsukuba, Japan. The incident X-ray was monochromatized by a Si(111) double-crystal monochromator, and higher-order harmonics were removed with a harmonic-rejection mirror. After removing the background fitted by a Victreen function, all spectra were normalized for the average intensities of the spectral regions from 5015 eV to 5035 eV for the Ti K-edge and from 9030 eV to 9050 eV for the Cu K-edge. Photocatalytic reaction. Photocatalytic H2 evolution from aqueous K2SO3(0.5 mol L–1)+Na2S(0.1 mol L–1) and methanol (10 vol%) solutions and O2 evolution from an aqueous AgNO3(0.02 mol L–1) solution were carried out using a gasclosed-circulation system. Photocatalyst powder (0.1 g) was dispersed in the reactant solutions (120 mL) in a topirradiation cell with a Pyrex window. Ru and Pt cocatalysts (0.3 wt%) were loaded in situ by a photodeposition method, using aqueous RuCl3 and H2PtCl6 solutions, respectively. A 300 W Xe arc lamp (Perkin-Elmer: Cermax-PE300BF) with a long-pass filter (λ>440 nm, HOYA) was employed as a light


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Figure 3. A flowchart of the selective synthesis of trigonal- and hexagonal-CuLi1/3Ti2/3O2 from Li2TiO3 by a molten CuCl treatment. LT and HT represent low and high temperature phases, respectively.

Figure 2. X-ray diffraction patterns of (a) Li2TiO3(cub), (b,c) samples prepared by treating Li2TiO3(cub) with a molten CuCl, (d) Li2TiO3(mon), (e,f) samples prepared by treating Li2TiO3(mon) with a molten CuCl, (g) sample prepared by calcining sample (e) at 1273 K in N2. Treated at (b,e) 773 K and (c,f) 873 K.

source. Z-schematic water splitting was carried out using an Ar flow system. 0.05-0.1 g of Pt-loaded CuLi1/3Ti2/3O2 and 0.05 g of an RGO-TiO2 composite were dispersed in water (120 mL) in a top-irradiation cell with a Pyrex window. A 300 W Xe arc lamp and solar simulator (Yamashita Denso; YSS80QA, 100 mW cm-2) were employed as light sources. Evolved H2 and O2 were determined using an online gas chromatograph (Shimazu; GC-8A, MS-5A column, TCD, Ar carrier). An apparent quantum yield was measured using a 300 W Xe arc lamp (Asahi Spectra; LAX-102 and MAX-303) with band-pass filters (Asahi spectra), a photodiode head (OPHIRA; PD300-UV) and a NOVA power monitor, according to the following equation (1). [Apparent quantum yield %]=100 x [The number of reacted electrons or holes]/[The number of incident photons] (1) Solar energy conversion efficiency was calculated by the following equation (2). [Solar energy conversion efficiency %]=100x[⊿G0(H2O)/J mol–1]x[Rate of H2 evolution/mol h–1]/(3600x[Solar energy (AM1.5)/W cm–2]x[Irradiation area/cm2] (2) Here, ⊿G0(H2O)= 237 J mol–1, solar energy (AM1.5)=0.1 W cm–2, irradiation area=33 cm2.

RESULTS AND DISCUSSION Preparation and characterization of CuLi1/3Ti2/3O2 powders by a molten CuCl treatment. XRD measurement revealed that Li2TiO3(cub) of a low temperature phase and Li2TiO3(mon) of a high temperature phase were obtained at

773K and 973K by a polymerized complex method, respectively, as shown in Figure 2(a,d). The Li2TiO3(cub) and Li2TiO3(mon) were treated with a molten CuCl at 773 K and 873 K to substitute Li+ with Cu+. All of the treated samples showed different XRD patterns from non-treated Li2TiO3(cub) and Li2TiO3(mon). The Li2TiO3(cub) treated at 873 K (Figure 2c) and Li2TiO3(mon) treated at 773 K (Figure 2e) gave similar XRD patterns to CuFeO2 with a hexagonal-delafossite structure (PDF 1-79-1546) and with a trigonal-delafossite structure (PDF 1-75-2146), respectively. Additionally, ICPAES measurement revealed that molar ratios of Cu/Li/Ti were 1.0:0.30:0.68 for Li2TiO3(cub) treated at 873 K and 1.0:0.47:0.79 for Li2TiO3(mon) treated at 773 K, while the theoretical ratio of Cu/Li/Ti=1.00:0.33:0.67. On the basis of the XRD and ICP-AES results, the Li2TiO3(cub) treated at 873 K and Li2TiO3(mon) treated at 773 K were identified as CuLi1/3Ti2/3O2 with a hexagonal-delafossite structure (CuLi1/3Ti2/3O2(hex)) and with a trigonal-delafossite structure (CuLi1/3Ti2/3O2(tri)), respectively. Slightly higher ratio of the Li and Ti in the CuLi1/3Ti2/3O2(tri) will be due to the remaining of the unreacted Li2TiO3 and/or partially remained Li ions in interlayers. The change in crystal structure from Li2TiO3 to the delafossite structure was observed for the AgLi1/3Ti2/3O2 prepared by treating Li2TiO3 powders with a molten AgNO3.17 The XRD pattern of the CuLi1/3Ti2/3O2(hex) shown in Figure 2c was the same as that of CuLi1/3Ti2/3O2 with hexagonaldelafossite structure prepared by a flux method at 973K in the reference.14 In contrast, The Li2TiO3(cub) treated at 773 K and Li2TiO3(mon) treated at 873 K were the mixture of hexagonaland trigonal-CuLi1/3Ti2/3O2 (Figure 2b, f). The CuLi1/3Ti2/3O2(tri) became hexagonal-CuLi1/3Ti2/3O2 upon calcining at 1273 K, as shown in Figure 2g. This phase change indicates that trigonal and hexagonal phases are low and high temperature phases of CuLi1/3Ti2/3O2, respectively. Above selective preparation processes of the single phases of the trigonal- and hexagonal-CuLi1/3Ti2/3O2 were summarized in Figure 3. A solid-state reaction was also tried to prepare single phases of trigonal- and hexagonal-CuLi1/3Ti2/3O2. When starting materials of Cu2O, Li2CO3, and TiO2 were calcined at 673–1073 K in N2, the mixtures of Cu2O, TiO2, trigonal-CuLi1/3Ti2/3O2, and hexagonal-CuLi1/3Ti2/3O2 were obtained (Figure S1). When calcined at 1273 K, almost the single phase of hexagonalCuLi1/3Ti2/3O2 was obtained. Thus, the single phase of trigonal-CuLi1/3Ti2/3O2 was not obtained by a solid-state reaction and a flux method.14 These facts clearly indicate the advantage



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Figure 4. (A) Ti K-edge XANES spectra of (a) CuLi1/3Ti2/3O2(hex) and (b) CuLi1/3Ti2/3O2(tri) compared with Ti(0)foil, Ti(III)2O3, and rutile-typed Ti(IV)O2 standards. (B) Cu Kedge XANES spectra of (a) CuLi1/3Ti2/3O2(hex) and (b) CuLi1/3Ti2/3O2(tri) compared with of Cu(0)-foil, Cu(I)2O and Cu(II)O standards. CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) were prepared by treating Li2TiO3(cub) at 873 K and Li2TiO3(mon) at 773 K with a molten CuCl, respectively.

of the molten CuCl treatment in the selective synthesis of the single phases of trigonal- and hexagonal CuLi1/3Ti2/3O2. The key factor for the selective synthesis of the single phaseof hexagonal- and trigonal-CuLi1/3Ti2/3O2 lies in the crystal structures of Li2TiO3(cub) and Li2TiO3(mon) of the precursors. The Li2TiO3(cub) possesses a bulky structure, while Li2TiO3(mon) possesses a layered structure, as shown in Figure 1(a,b). Hexagonal- and trigonal-CuLi1/3Ti2/3O2 should be similar structures to hexagonal- and trigonal CuFeO2 shown in Figure 1(c,d), judging from the similar XRD patterns (Figure 2). The layers in hexagonal- and trigonal-CuLi1/3Ti2/3O2 consist of TiO6 and LiO6 octahedrons. The layer and its reversed layer are interlaminated in the hexagonal-CuLi1/3Ti2/3O2, while only one type of the layer consisting of TiO6 and LiO6 octahedrons was laminated in the trigonal-CuLi1/3Ti2/3O2. The order of the layers consisting of TiO6 and LiO6 octahedrons in the trigonal-CuLi1/3Ti2/3O2 is similar to that in the Li2TiO3(mon). Accordingly, the trigonal-CuLi1/3Ti2/3O2 was easily obtained from Li2TiO3(mon) by substituting Cu(I) for Li+ in interlayer at low temperature of 773 K, keeping the main frameworks of the Li2TiO3(mon). Although the increase in the particle size during the phase change from Li2TiO3(mon) to CuLi1/3Ti2/3O2(tri) was observed as seen in later (Figure 6), it will be due to the crystallization and/or the increase in the crystallinity. In contrast, a molten CuCl treatment of Li2TiO3(cub) with non-orientational bulky structure at 773 K gave mixed phases of hexagonal- and trigonal-CuLi1/3Ti2/3O2 (Figure 2b). Not only the substitution of ions but also the reconstruction of the frameworks are necessary to form the layered CuLi1/3Ti2/3O2 from bulky Li2TiO3(cub). During this process, the high temperature phase would form even at 773K. To obtain direct evidence about the presence of Cu(I) in the CuLi1/3Ti2/3O2(tri) and CuLi1/3Ti2/3O2(hex) structures, XAFS measurements were conducted. The Ti and Cu K-edge XANES spectra for the CuLi1/3Ti2/3O2(tri) and CuLi1/3Ti2/3O2(hex) are shown in Figure 4. The 1s-4p absorption edge indicated by dashed lines were used for the determination of the oxidation states of the metal valence. The energy of absorption edges in the XANES analysis clearly indicate that the Cu and Ti in the CuLi1/3Ti2/3O2(tri) and CuLi1/3Ti2/3O2(hex)

Figure 5. Fourier transformed EXAFS spectra of (A) Ti-K edge and (B) Cu-K edge for (a) CuLi1/3Ti2/3O2(hex), (b) CuLi1/3Ti2/3O2(tri), (a’) Li2TiO3(cub), and (b’) Li2TiO3(mon). CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) were prepared by treating Li2TiO3(cub) at 873 K and Li2TiO3(mon) at 773 K with a molten CuCl, respectively. In the both Ti- and Cu-K edges, it is difficult to measure Li components due to the weak photoelectron scattering from the light Li atom.

were monovalent and tetravalent, respectively. The result consists with the XPS measurement (Figure S2) which indicates the presence of Cu(I) at the surface of CuLi1/3Ti2/3O2(tri) and CuLi1/3Ti2/3O2(hex). Additionally, the Cu K-edge XANES spectra also give an information about coordination environments of Cu species.26 Generally, in Cu complexes which has linear and plane local structures, 1s-4pπ and 1s-4pσ transitions are shown to be the anisotropy of chemical bonding. Moreover, the ~6 eV splitting of the 4pπ and 4pσ XANES components are observed in divalent Cu compounds due to the screening effect in 3d hole attributed to the ligand-to-metal charge transfer. In the Cu oxide reference samples, although the divalent CuO which has the plane 4-coodinated local structure shows the ~6eV splitting, the monovalent CuO2 which has the linear 2-coordinated local structure does not show the ~6 eV splitting due to its the 3d closed shell electronic configuration. Therefore, from the point of view of the Cu XANES structures, it is suggested that the Cu(I) is formed with the linear 2-coordinated structure in CuLi1/3Ti2/3O2(tri) and CuLi1/3Ti2/3O2(hex). Additionally, in order to discuss the local structure around


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Figure 6. SEM images of (a) Li2TiO3(cub), (b) Li2TiO3(mon), (c) CuLi1/3Ti2/3O2(hex), and (d) CuLi1/3Ti2/3O2(tri) prepared by a molten CuCl treatment. CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) were prepared by treating Li2TiO3(cub) at 873 K and Li2TiO3(mon) at 773 K with a molten CuCl, respectively.

Ti and Cu sites in CuLi1/3Ti2/3O2(tri) and CuLi1/3Ti2/3O2(hex), EXAFS spectra were measured. The Fourier transformed EXAFS of Ti K- and Cu K-edges for Li2TiO3(cub), Li2TiO3(mon), CuLi1/3Ti2/3O2(tri), and CuLi1/3Ti2/3O2(hex) by the k range from 2 to 12 Å−1 are shown in Figure 5. The assignment of peaks indicated by dashed lines, which correspond to the atomic distances of Ti-O, Ti-Ti, and Ti-Cu for the Ti K-edge EXAFS and Cu-O, Cu-Cu, and Cu-Ti for the Cu Kedge EXAFS, are well explained by the crystal structure where Li+ in the Li2TiO3 was replaced with Cu(I). Figure 6 shows the SEM images of Li2TiO3(cub), Li2TiO3(mon), CuLi1/3Ti2/3O2(hex), and CuLi1/3Ti2/3O2(tri). Small particles with the size of 100 nm were aggregated for Li2TiO3(cub). Sintered angular particles were observed for Li2TiO3(mon). The particle size of CuLi1/3Ti2/3O2(hex) was larger than that of CuLi1/3Ti2/3O2(tri), because the CuLi1/3Ti2/3O2(hex) was prepared at higher temperature than the CuLi1/3Ti2/3O2(tri). A part of CuLi1/3Ti2/3O2(hex) and

Figure 7. Diffuse reflectance spectra of (a) Li2TiO3(cub), (b) Li2TiO3(mon), (c) CuLi1/3Ti2/3O2(hex), and (d) CuLi1/3Ti2/3O2(tri). CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) were prepared by treating Li2TiO3(cub) at 873 K and Li2TiO3(mon) at 773 K with a molten CuCl, respectively.

CuLi1/3Ti2/3O2(tri) particles were plate which reflects their delafossite layered structures. Figure 7 shows the diffuse reflectance spectra of Li2TiO3(cub), Li2TiO3(mon), CuLi1/3Ti2/3O2(hex), and CuLi1/3Ti2/3O2(tri). The absorption edges of Li2TiO3(cub) and Li2TiO3(mon) at 310 and 300 nm, respectively, were drastically red-shifted into the visible light region at around 590 nm by a molten CuCl treatment. The steep absorption edges of CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) indicated that the Cu(I) in the CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) formed not doping level but band structure. The band gaps of CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) were estimated to be 2.1 eV, which was about 2.0 eV narrower than those of Li2TiO3(cub) and Li2TiO3(mon). Sacrificial H2 evolution over CuLi1/3Ti2/3O2 photocatalysts under visible light irradiation. Photocatalytic activity for sacrificial H2 evolution over CuLi1/3Ti2/3O2 prepared by a molten CuCl treatment and a solid-state reaction was evaluated under visible light irradiation, as shown in Table 1.

Table 1. Photocatalytic H2 evolution from aqueous solutions containing sacrificial reagents under visible light irradiation over CuLi1/3Ti2/3O2 prepared by a molten CuCl treatment (MT) and a solid-state reaction (SSR) Entry

Preparation method

Starting materials

Preparation temperature/K

Productsa)

Sacrificial reagents

H2 evolution /µmol h–1

1 2 3 4 5 6

MT MT MT MT MT MT

CuCl, Li2TiO3(cub) CuCl, Li2TiO3(cub) CuCl, Li2TiO3(cub) CuCl, Li2TiO3(mon) CuCl, Li2TiO3(mon) CuCl, Li2TiO3(mon)

773 873 873 773 773 873

T, H H H T, (Li2TiO3) T, (Li2TiO3) T, H

K2SO3 + Na2S b) K2SO3 + Na2S b) Methanol c) K2SO3 + Na2S b) Methanol c) K2SO3 + Na2S b)

143 130 11 105 6.6 136

7

MT + Anneal

CuLi1/3Ti2/3O2(tri)

1273

H, unknown

K2SO3 + Na2S b)

trace

H, unknown

b)

8

SSR

Cu2O, Li2CO3, TiO2

1273

K2SO3 + Na2S

a) T: trigonal-CuLi1/3Ti2/3O2, H: hexagonal-CuLi1/3Ti2/3O2 b) Cocatalyst: Ru(0.3 wt%), solution: 0.5 mol L-1 K2SO3aq. + 0.1 mol L-1 Na2Saq. (120 mL) c) Cocatalyst: Pt(0.3 wt%), solution: 10 vol% MeOHaq. at pH 10 adjusted by NaOH (120 mL) Catalysts: 0.1 g; light source: 300 W Xe lamp (λ > 440 nm); cell: top-irradiation cell with a Pyrex window.


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Figure 8. Photocatalytic H2 evolution from an aqueous K2SO3 + Na2S solution under visible light irradiation over Ru(0.3 wt%)loaded (a) CuLi1/3Ti2/3O2(hex) and (b) CuLi1/3Ti2/3O2(tri). Catalyst: 0.1 g, solution: 0.5 mol L-1 K2SO3aq. + 0.1 mol L-1 Na2Saq. (120 mL), light source: 300 W Xe lamp (λ > 440 nm), cell: topirradiation cell with a Pyrex window. CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) were prepared by treating Li2TiO3(cub) at 873 K and Li2TiO3(mon) at 773 K with a molten CuCl, respectively.

Entries 2-3 and 4-5 correspond to the activities of the CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri), respectively. All of the CuLi1/3Ti2/3O2 photocatalysts produced H2 from an aqueous solution containing K2SO3+Na2S of a sacrificial reagent under visible light irradiation, when Ru-cocatalyst was loaded. The CuLi1/3Ti2/3O2 photocatalysts prepared by a molten CuCl treatment (entries 1,2,4,6) showed higher H2 evolution activity than that prepared by a solid-state reaction (entry 8). Moreover, hexagonal-CuLi1/3Ti2/3O2 prepared by calcining CuLi1/3Ti2/3O2(tri) showed negligible activity (entry 7). The low activities of hexagonal-CuLi1/3Ti2/3O2 are probably due to the presence of Cu(II) in the CuLi1/3Ti2/3O2 judging from the tailedabsorption in diffused reflectance spectra (Figure S3). Both CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) steadily produced H2 from an aqueous K2SO3+Na2S solution, as shown in Figure 8. The activity of CuLi1/3Ti2/3O2(hex) was slightly higher than that of CuLi1/3Ti2/3O2(tri). Turnover numbers of the number of reacted electrons estimated from the amount of evolved H2 over CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) per the number of the contained Cu+ ions were 2.5 and 1.8 at 8 h, respectively. The turnover numbers greater than unity indicate that the H2 evolution photocatalytically proceeded. Methanol is usually used as a hole scavenger to evaluate the H2 evolution ability of metal oxide photocatalysts. The CuLi1/3Ti2/3O2 photocatalysts prepared by a molten CuCl treatment also produced H2 from an aqueous methanol solution with pH 10 (entries 3,5), when Pt-cocatalyst was loaded. The reason why an aqueous methanol solution with pH 10 was used is that the oxidation of the methanol was promoted under alkaline conditions.13,27 The activities (entries 3,5) were lower than those from an aqueous K2SO3+Na2S solution (entries 2,4). This is because the oxidation of SO32– and S2– is thermodynamically more favorable than that of methanol. In fact, the baselines in the diffuse reflectance spectra of CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) were elevated after H2 evolution from an aqueous methanol solution probably due to the oxidation of the photocatalysts, while they did not almost change after the reaction from an aqueous K2SO3+Na2S solution (Figure S4). Moreover, the presence of Cu(II) was

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Figure 9. Action spectra for H2 evolution from an aqueous K2SO3 + Na2S solution and diffuse reflectance spectra. (a,c) CuLi1/3Ti2/3O2(hex) and (b,d) CuLi1/3Ti2/3O2(tri). CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) were prepared by treating Li2TiO3(cub) at 873 K and Li2TiO3(mon) at 773 K with a molten CuCl, respectively.

Figure 10. Proposed band structures of monoclinic-Li2TiO3, hexagonal-CuLi1/3Ti2/3O2, trigonal-CuLi1/3Ti2/3O2, and AgLi1/3Ti2/3O2.

confirmed by XPS for the CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) after H2 evolution from an aqueous methanol solution, but not after the reaction from an aqueous K2SO3+Na2S solution (Figure S2). Good affinity or adsorption property of such sulfur species to Cu species would also be the reason. The CuLi1/3Ti2/3O2(hex) showed higher H2 evolution activity than the CuLi1/3Ti2/3O2(tri) even from an aqueous methanol solution. Figure 9 shows the action spectra for H2 evolution over Ruloaded CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) from an aqueous K2SO3+Na2S solution. The apparent quantum yields of the sacrificial H2 evolution from an aqueous K2SO3+Na2S solution over the Ru-loaded CuLi1/3Ti2/3O2(hex) and Ru-loaded CuLi1/3Ti2/3O2(tri) were 3.3% and 1.0% at 420 nm, respectively. Both photocatalysts responded to visible light up to 600 nm. The onsets of the action spectra for H2 evolution agreed with those of the diffuse reflectance spectra of the CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri). This agreement indicates that the H2 evolution over these photocatalysts proceeds accompanied by photoexcitation from the valence bands consisting of Cu 3d orbitals in Cu(I) to the conduction bands consisting of Ti 3d orbitals. All of the CuLi1/3Ti2/3O2 did not show activity for O2 evolution from an aqueous silver nitrate solution, being different from AgLi1/3Ti2/3O2 photocatalyst.17 This is probably due to


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Figure 11 Z-schematic solar water splitting using Pt(0.3 wt%)loaded CuLi1/3Ti2/3O2(hex) and RGO-TiO2 composite. Catalyst: 0.1 g of Pt(0.3 wt%)-loaded CuLi1/3Ti2/3O2(hex) and 0.05 g of RGO(5 wt%)-TiO2 composite, solution: water without pH adjustment (120 mL); light source: solar simulator with an AM1.5 filter (100 mW cm-1), cell: top-irradiation cell with a Pyrex window; irradiated area: 33 cm2. CuLi1/3Ti2/3O2(hex) was prepared by treating Li2TiO3(cub) at 873 K with a molten CuCl.

Table 2. Overall water splitting using Z-scheme systems consisting of CuLi1/3Ti2/3O2 photocatalysts of an H2evolving photocatalyst and RGO-TiO2 composite of an O2evolving photocatalyst H2-evolving photocatalyst Pt(0.3 wt%)-loaded CuLi1/3Ti2/3O2(hex) Pt(0.3 wt%)-loaded CuLi1/3Ti2/3O2(tri)

Activity/µmol h–1 H2

O2

3.0 1.0

1.5 0.5

Catalyst: 0.05 g each, solution: water without pH adjustment (120 mL), light source: 300 W Xe lamp; cell: top-irradiation cell with a Pyrex window. CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) were prepared by treating Li2TiO3(cub) at 873 K and Li2TiO3(mon) at 773 K with a molten CuCl, respectively.

the levels of the valence band maxima formed by Cu(I) which are more negative than that for water oxidation to O2, as shown in Figure 10. Considering that the band gap of monoclinic-Li2TiO3 is 4.1 eV and the top of the valence band consisting of O2p orbitals usually lies at around +3.0 V vs NHE (pH 0),28 the bottom of the conduction band of monoclinicLi2TiO3 lies at around –1.1 V vs NHE (pH 0). The bottom of the conduction bands of CuLi1/3Ti2/3O2(hex), CuLi1/3Ti2/3O2(tri), and AgLi1/3Ti2/3O2 can be assumed to be located at around –1.1 V vs NHE (pH 0), because their conduction bands consist of Ti3d and their crystal structure is similar to that of the monoclinic-Li2TiO3. Therefore, the top of the valence band will lie at around +1.6 V vs NHE (pH 0) for AgLi1/3Ti2/3O2 and at around +1.0 V vs NHE (pH 0) for CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri). Thus, CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) will be thermodynamically impossible to produce O2 from water, being different form AgLi1/3Ti2/3O2. Construction of Z-scheme system using the CuLi1/3Ti2/3O2 as an H2-evolving photocatalyst. We tried to apply the CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) as H2evolving photocatalysts to the Z-scheme system with reduced graphene oxide (RGO) of an electron mediator and TiO2 of an

O2-evolving photocatalyst29 to achieve water splitting of an energy conversion reaction (Figure S5). H2 and O2 evolved in a stoichiometric amount under visible light irradiation, when either Pt-loaded CuLi1/3Ti2/3O2(hex) or Pt-loaded CuLi1/3Ti2/3O2(tri) was used, as shown in Table 2. The water splitting activity with the Pt-loaded CuLi1/3Ti2/3O2(hex) was higher than that with the Pt-loaded CuLi1/3Ti2/3O2(tri), which the magnitude relation was the same order as the sacrificial H2 evolution shown in Figure 8. The Z-scheme system consisting of Pt-loaded CuLi1/3Ti2/3O2(hex) and RGO-TiO2 produced almost stoichiometric H2 and O2 continuously for 70 hours (Figure S6). The turnover numbers of reacted electrons to carbon atoms in RGO (assumed to contain pristine graphitic carbon) and Cu atoms at the surface of CuLi1/3Ti2/3O2(hex) were 1.1 and 70 at 70 h, respectively. These turnover numbers greater than unity indicate that the Z-schematic water splitting photocatalytically proceeded. Figure 11 shows steady evolution of H2 and O2 with a stoichiometric amount even under simulated sunlight irradiation after 3 hours with an obvious induction period. The higher O2 evolution rate in the induction period will be due to the further reduction of RGO. The solar energy conversion efficiency was calculated to be 0.002%. We have previously suggested that the combination of a ptype semiconductor as an H2-evolving photocatalyst with an ntype semiconductor as an O2-evolving photocatalyst is an important factor for the Z-schematic system using RGO.29 TiO2 is a well-known n-type semiconductor, giving an anodic photocurrent.30 In contrast, the CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) gave cathodic photocurrents under visible light irradiation (Figure S7), indicating a p-type semiconductor character. Moreover, the potential at which the CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) gave the cathodic photocurrent overlapped with the potential at which the TiO2 gave the anodic photocurrent. The presence of overlapped potential indicates that photogenerated electrons in the TiO2 particles can migrate to CuLi1/3Ti2/3O2(hex) and CuLi1/3Ti2/3O2(tri) particles to react with holes, being similar to a photoelectrochemical system. Consequently, we can conclude that the present Zschematic water splitting proceeded accompanied by the electron migration from TiO2 to Pt-loaded CuLi1/3Ti2/3O2(hex) and Pt-loaded CuLi1/3Ti2/3O2(tri) via RGO, as observed for previous systems.29,31

CONCLUSIONS Single phases of trigonal-CuLi1/3Ti2/3O2 and hexagonalCuLi1/3Ti2/3O2 with a delafossite structure were successfully prepared by treating monoclinic-Li2TiO3 and cubic-Li2TiO3 with a molten CuCl, respectively. The key factor for the selective synthesis of the single phase lies in the crystal structure of monoclinic-Li2TiO3 and cubic-Li2TiO3 of the precursors. The cubic-Li2TiO3 possesses a bulky structure, while the monoclinic-Li2TiO3 possesses a layered structure which layers are laminated with similar order to the trigonal-CuLi1/3Ti2/3O2 with a delafossite structure. The band gaps of trigonalCuLi1/3Ti2/3O2 and hexagonal-CuLi1/3Ti2/3O2 were estimated to be 2.1 eV. Ru- and Pt-loaded trigonal-CuLi1/3Ti2/3O2 and hexagonal-CuLi1/3Ti2/3O2 produced H2 from aqueous K2SO3+Na2S and methanol solutions under visible light irradiation. The activity of hexagonal-CuLi1/3Ti2/3O2 was higher than that of trigonal-CuLi1/3Ti2/3O2. The Pt-loaded trigonal-CuLi1/3Ti2/3O2 and Pt-loaded hexagonal-CuLi1/3Ti2/3O2 functioned as H2evolving photocatalysts in a Z-scheme system with RGO-TiO2 composite of an O2-evolving photocatalyst to split water into



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H2 and O2. Thus, we successfully developed newly active Cu(I)-containing metal oxide photocatalysts responding to visible light up to 600 nm for H2 evolution and constructed a new Z-scheme system for water splitting.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional experimental condition about photoelectrochemical measurements and additional data including X-ray diffraction patterns, X-ray photoelectron spectra, diffuse reflectance spectra, photocatalytic and photoelectrochemical properties (PDF)

AUTHOR INFORMATION Corresponding Author * (A.K.) E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by a Grant in Aid (no. 24107001, 24107004, and 15H00890) for Science Research on Innovative Areas (Area no. 2406) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan. The XAFS experiment were performed at beamline BL12C of KEK with the approval of the Photon Factory Program Advisory Committee (2014S2-006).

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3O2 Powders Prepared by a

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