Zinc Germanium Oxynitride as a Photocatalyst for Overall Water

Dec 14, 2006 - Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo. 113-8656, Japan, Chemical Resources ...
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J. Phys. Chem. C 2007, 111, 1042-1048

Zinc Germanium Oxynitride as a Photocatalyst for Overall Water Splitting under Visible Light Yungi Lee,† Hiroaki Terashima,‡ Yoshiki Shimodaira,§ Kentaro Teramura,† Michikazu Hara,‡ Hisayoshi Kobayashi,| Kazunari Domen,*,†,⊥ and Masatomo Yashima∞ Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, Department of Applied Chemistry, Faculty of Science, Science UniVersity of Tokyo, 1-3 Kagurazaka, Shinjyuku-ku, Tokyo 162-8601, Japan, Department of Chemistry and Materials Technology, Faculty of Engineering and Design, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan, Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, Solution-Oriented Research for Science and Technology (SORST), Japan Science and Technology Co. (JST), 2-1-13 Higashiueno, Taito-ku, Tokyo 110-0015, Japan ReceiVed: August 31, 2006; In Final Form: October 18, 2006

A solid solution of zinc oxide and germanium nitride (Zn1+xGe)(N2Ox) (x ) 0.44) is demonstrated to be an effective photocatalyst for overall water splitting under ultraviolet and visible light. The catalyst is prepared by reaction of GeO2 and ZnO under ammonia flow (20 mL‚min-1) at 1123 K for 15 h. The crystal structure of the material is investigated by a combination of Rietveld analysis and the maximum-entropy method using neutron powder diffraction data. The (Zn1.44Ge)(N2.08O0.38) catalyst is confirmed to have a wurtzite-type structure (space group P63mc) and to be the solid solution where the oxygen atoms are substituted for nitrogen atoms. The (Zn1.44Ge)(N2.08O0.38) catalysts thus prepared exhibit a band gap of ca. 2.7 eV and corresponding activity at visible wavelengths. The decrease in band gap compared to the starting materials is attributed to larger valance band dispersion resulting from the energy difference between O2p and N2p orbitals and from the p-d repulsion between Zn3d and N2p+O2p electrons in the upper valance band, which raises the top of the valance band. (Zn1.44Ge)(N2.08O0.38) powder modified by surface loading with RuO2 nanoparticles at 5 wt % achieves overall water splitting under both ultraviolet and visible irradiation.

Introduction Photocatalysts for overall water splitting have been studied extensively for the purpose of solar energy conversion of hydrogen as an energy carrier. To improve the utilization of sunlight, substantial effort has been devoted to the sensitization of photocatalysts for the abundant visible region. Although nonoxide materials such as CdS1 and CdSe2 have been examined as visible-light-active photocatalysts, the inherent instability of these catalysts has obstructed development. Some of the present authors recently reported that certain oxynitrides exhibit potential for overall water splitting under visible light. Oxynitrides of early transition metals with d0 electronic configuration, such as Ta3N5,3 TaON,4 and LaTiO2N,5 or typical metals with d10 electronic configuration, such as (Ga1-xZnx)(N1-xOx)6 and Ge3N47 have been found to be promising candidates. Among these materials, typical metal oxynitrides * To whom correspondence should be addressed. Phone: +81-3-58411148. Fax: +81-3-5841-8838. E-mail: [email protected]. † Department of Chemical System Engineering, The University of Tokyo. ‡ Chemical Resources Laboratory, Tokyo Institute of Technology. § Department of Applied Chemistry, Faculty of Science, Science University of Tokyo. | Department of Chemistry and Materials Technology, Faculty of Engineering and Design, Kyoto Institute of Technology. ⊥ Solution-Oriented Research for Science and Technology (SORST), Japan Science and Technology Co. (JST). ∞ Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology.

(β-Ge3N4)andasolidsolutionofGaNandZnO[(Ga1-xZnx)(N1-xOx)] with coloaded RuO2 nanoparticles have achieved potentially useful functionality as photocatalysts for overall water splitting.6,7 The activity of β-Ge3N4 is limited to the ultraviolet (UV) region due to the large band gap energy of the catalyst (ca. 3.8 eV). In contrast, (Ga1-xZnx)(N1-xOx) has been shown to decompose water under visible light.7 GaN and ZnO are wellknown materials with wurtzite structures and band gaps of ca. 3.4 and 3.2 eV, respectively, providing response only in the UV region.8,9 However, the solid solution of the oxide and nitride, (Ga1-xZnx)(N1-xOx), which retains the wurtzite structure, exhibits a band gap of 2.6-2.8 eV, suitable for response in the visible region. The small band gap is attributed to repulsion of N2p-Zn3d electrons in the upper valance band, thus defining the valance band maximum of (Ga1-xZnx)(N1-xOx).10 The photocatalytic activity of (Ga1-xZnx)(N1-xOx) for water splitting under visible light (at wavelengths longer than 400 nm) is greatly enhanced by loading the catalyst surface with nanoparticles of a mixed oxide of rhodium and chromium.11 ZnGeN2 is a wide band gap II-VI-N2 semiconductor with monoclinic phase and β angle of 118°53′, which is very close to the value of β ) 120° for the wurtzite structure.12 The band gap of ZnGeN2 ranges from ca. 2.66 to 4.31 eV, depending on the crystal structure and composition.13 The atomic arrangement of the (001) surface in an orthorhombic crystal structure with space group Pna21 is hexagonal, which leads to variation of lattice parameters over the range 3.052-3.213 Å on the a-axis

10.1021/jp0656532 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006

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Figure 2. TEM images and electron diffraction pattern of (Zn1.44Ge)(N2.08O0.38). Figure 1. XRD patterns of (A) ZnO, (B) ZnGeN2, and (C) (Zn1.44Ge)(N2.08O0.38).

and 4.976-5.147 Å on the c-axis according to the alloy composition.14 These parameters are potentially well-matched with the lattice constants of GaN (a ) 3.189 Å and c ) 5.185 Å) and ZnO (a ) 3.253 Å and c ) 5.213 Å) with the wurtzite structure.15,16 In this study, the solid solution between ZnO and ZnGeN2, (Zn1+xGe)(N2Ox), is prepared and investigated as a photocatalyst for overall water splitting. Experimental Section Preparation of (Zn1+xGe)(N2Ox) Solid Solution. The (Zn1+xGe)(N2Ox) solid solution and ZnGeN2 powder were prepared by heating a mixture of GeO2 (Kanto Chemicals, 99.99%) and ZnO (Kanto Chemicals, 99%) powders (ca. 2 g) under NH3 flow at high temperature. The (Zn1+xGe)(N2Ox) solid solution was prepared with a molar ratio of Zn to Ge (Zn/Ge) of 5 and subsequently nitrided at 1123 K under NH3 flow (20 mL‚min-1) for 15 h. ZnGeN2 powder with Zn/Ge ) 2 (molar) was prepared at 1153 K under NH3 flow (100 mL‚min-1) for 10 h. After nitridation, the sample was cooled to room temperature under NH3 flow. Characterization. The structures were characterized by X-ray diffractometry (XRD; RINT 2100, Rigaku) using Cu KR radiation at 40 kV and 40 mA. The XRD patterns were collected at 2θ angles of 10-70° at a scan rate of 1° min-1. The surface morphologies and average particle sizes of products were observed by field-emission scanning electron microscopy (SEM; S-4700, Hitachi) and field-emission transmission electron microscopy (TEM; JEM-2010F, JEOL). The chemical composition was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES; Iris Advantage DuO, Thermo Elemental Co.) and using a TC436 oxygen and nitrogen determinator (LECO Co.). The surface of the (Zn1.44Ge)(N2.08O0.38) solid solution was examined by X-ray photoelectron spectroscopy (XPS; ESCA 3200, Shimadzu). All binding energies were corrected using the binding energy of Au 4f7/2 (83.8 eV). UV-visible diffuse reflectance spectrometry (DRS; V-560, Jasco) was performed in reference to BaSO4 powder. For neutron diffraction analysis, the (Zn1.44Ge)(N2.08O0.38) powder was placed in a 10 mm × 50 mm (φ) vanadium cylinder. Neutron diffraction data were collected at 299 K using a multidetector fixed-wavelength powder diffractometer (HERMES) of the Institute of Materials Research, Tohoku University, installed at the JRR-3M research reactor of Tokai Research Laboratories, Japan Atomic Energy Association (JAEA).17 A neutron beam with wavelength of 1.813 86 Å was obtained using the (331) plane of a germanium monochromator. Profile data were measured by scanning at intervals of 0.10° in the 2θ range of 5.00-130.00° using 150 detectors of the HERMES diffrac-

Figure 3. SEM image of (Zn1.44Ge)(N2.08O0.38).

tometer. The crystal structure of the (Zn1.44Ge)(N2.08O0.38) powder was refined by Ritveld analysis of the neutron diffraction data using the computer program RIETAN-200018 with the neutron scattering lengths of Ge (8.185 fm), Zn (5.60 fm), N (9.36 fm), and O (5.803 fm). The peak shape was assumed to be a modified split-type pseudo-Voigt function with asymmetry. The background was approximated by a 12-parameter polynomial in 2θn (n ) 0, 1, ..., 11). The n parameters were refined simultaneously with the unit cell, scale, peak width/shape/ asymmetry, and crystal structural parameters. Nuclear density distributions were obtained by a maximum-entropy method (MEM) for the structure factors obtained in the Reitveld analysis. The calculations were performed using the MEM analysis computer program PRIMA19 with 64 × 64 × 104 pixel cells. Plane-wave-based density functional theory (DFT) calculations were carried out for ZnGeN2 and (Zn1+xGe)(N2Ox) (x ) 0.33) using the CASTEP program.20 The core electrons were replaced with ultrasoft core potentials,21 and the valence electronic configurations for Zn, Ge, N, and O atoms were set at 3d104s2, 4s2sp2, 2s22p3, and 2s22p4, respectively. The calculations were carried out using the conventional unit cells of [ZnGeN2]4 for Zn4Ge4N8 and [Zn1.33GeN2O0.33]24 for (Zn1.33Ge)(N2.00O0.33). The total numbers of electrons for Zn4Ge4N8 and (Zn1.33Ge)(N2.00O0.33) were 104 and 776, and the numbers of occupied orbitals were 52 and 388, respectively. The kinetic energy cutoffs were taken to be 300 eV. The DFT exchangecorrelation potentials used in the calculations were the gradientcorrected (GGA) functional22 and the Perdew-Bruke-Enzerhof (PBE) functional.23 Modification of (Zn1+xGe)(N2Ox) Solid Solution. RuO2 was loaded onto the as-prepared (Zn1.44Ge)(N2.08O0.38) powder as a cocatalyst by immersing the (Zn1.44Ge)(N2.08O0.38) powder in a tetrahydrofuran (THF) solution containing dissolved Ru3(CO)12 (Aldrich Chemical Co., 99%) and stirring at 333 K for 5 h. The solution was then dried under reduced pressure by heating

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Figure 5. Rietveld pattern of neutron diffraction data for (Zn1.44Ge)(N2.08O0.38): crosses denote observed data and solid line denotes calculated pattern. The lower trace is a difference plot (observed minus calculated). Vertical lines denote possible Bragg peaks of the hexagonal phase.

TABLE 2: Refined Crystallographic Parameters and Reliability Factors for (Zn1.44Ge)(N2.08O0.38)a Occupancy

Figure 4. XPS spectra for (A) ZnO, (B) ZnGeN2, and (C) (Zn1.44Ge)(N2.08O0.38).

TABLE 1: Atomic Ratio of Zn, N, and O to Ge in Bulk and Surface of (Zn1.44Ge)(N2.08O0.38) element concentration to Ge bulka surfaceb

Zn/Ge

N/Ge

O/Ge

1.44 1.51

2.08 2.00

0.38 0.41

a Measured by element analysis. b Calculated from the areas under peaks in the XPS spectra.

in air at 373 K for 1 h to remove THF. The resulting powder was finally heated in air at 623 K for 1 h. Photocatalytic Activity. Reactions were carried out in a Pyrex inner irradiation-type reaction vessel connected to a glass closed gas circulation system. The reaction was performed in distilled water containing 0.20 g of the cocatalyst-loaded sample. The reactant solution was initially evacuated several times to remove air completely and then irradiated under a 450-W highpressure Hg lamp (λ > 300 nm). For visible-light irradiation (λ > 400 nm), a Pyrex tube filled with aqueous NaNO2 solution was inserted between the lamp and the sample to block UV light. The dependence of photocatalytic activity on the wavelength of the incident light was measured using a top irradiationtype reaction vessel made of Pyrex and a 300-W Xe lamp with cutoff filter. The evolved gases were analyzed by gas chromatography with a thermal conductivity detector (GC-8A, Shimadzu; Ar carrier gas). Results and Discussion XRD Patterns and TEM and SEM Images. Figure 1 showed the XRD pattern of the prepared sample, along with ZnO and ZnGeN2 data for comparison. Figure 1C shows the XRD pattern of the sample prepared at 1123 K for 15 h under NH3 flow of 20 mL‚min-1. The prepared sample exhibits a single-phase diffraction pattern, indicating that the sample was entirely of the wurtzite phase without other impurity phases or residues of ZnO or GeO2. The prepared ZnGeN2 exhibits an orthorhombic phase (Figure 1B). ZnGeN2 alone does not display a wurtzite structure, whereas in solid solution with ZnO the

atomic coordinates

atom

site

g

x

y

z

Ge0.409Zn0.586 N O

2b 2b 2b

1.0 0.87(3) 0.13(3)b

1/3 1/2 1/2

2/3 2/3 2/3

0.3777(4) 0 0

a Unit cell hexagonal, P63mc, a ) b ) 0.32022(2) nm, c ) 0.52026(1) nm, R ) β ) 90°, γ ) 120°, V ) 0.046120(3) nm3. Atomic displacement parameters: B(Ge0.409Zn0.586) ) B(N) ) B(O) ) 0.0013(5) nm2. Reliability factors in the Rietveld analysis: Rwp ) 3.22%, GOF ) 2.44, RI ) 1.99%, RF ) 0.87%. b g(N) + g(O) ) 1.00.

wurtzite structure is obtained. The diffraction peak for the (Zn1.44Ge)(N2.08O0.38) solid solution is shifted to higher angles compared to wurtzite ZnO (Figure 1A), which is reasonable given the smaller ionic radius of Ge4+ (0.39 Å) compared to Zn2+ (0.60 Å). TEM images of the prepared (Zn1.44Ge)(N2.08O0.38) powder are shown in Figure 2. The TEM images, in combination with the electron diffraction pattern, indicate that the sample consisted of primary well-crystallized submicrometer-order particles with a wurtzite structure. The SEM image shown in Figure 3 reveals that primary particles formed aggregates of ca. 200-300 nm in size. Bulk and Surface State of (Zn1+xGe)(N2Ox) (x ) 0.44) Solid Solution. The bulk chemical composition of the prepared (Zn1.44Ge)(N2.08O0.38) solid solution was found by chemical analysis to be Ge0.409(1)Zn0.586(1)N0.850(1)O0.155(1), where the values in parentheses denote standard error. The surface state of the prepared (Zn1.44Ge)(N2.08O0.38) solid solution was investigated by XPS. The narrow-scan XPS spectra of the Ge3d, Zn2p3/2, N1s, and O1s peaks for the prepared sample are shown in Figure 4, along with the XPS spectra for ZnO and ZnGeN2 for comparison. The Ge3d peak in the (Zn1.44Ge)(N2.08O0.38) solid solution was observed at 30.8 eV, which is located between that for metallic Ge (29.0 eV24) and GeO2 (32.1 eV25). This difference in Ge3d peak position among Ge, GeO2, and the (Zn1.44Ge)(N2.08O0.38) solid solution is probably due to the difference in bond polarity among Ge-Ge, Ge-O, and Ge-N bonds. The Ge3d peak for ZnGeN2 appeared at 30.9 eV, which is almost coincident with the position of the Ge3d peak for the (Zn1.44Ge)(N2.08O0.38) solid solution. The Zn2p3/2 peaks appeared at 1021.9, 1022.0, and 1022.3 eV for (Zn1.44Ge)(N2.08O0.38), ZnGeN2, and ZnO, respectively, indicating no significant chemical shift. This lack of shift can be attributed to the weaker sensitivity of the Zn2p3/2 peak toward the chemical environment compared to the Ge3d peak.26 The N1s peaks for

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Figure 6. (A) Refined crystal structure of (Zn1.44Ge)(N2.08O0.38) drawn with the (Ge,Zn)(N,O)4 tetrahedron. (B) Equicontour surface of scattering amplitude distribution at 10 fm‚Å-3.

ZnGeN2 and (Zn1.44Ge)(N2.08O0.38) solid solution were observed at 397.0 eV. This N1s peak position differs from that for Zn3N2 (395.8 eV),27 indicating that only the Zn-N bond was absent on the surface of ZnGeN2 and the (Zn1.44Ge)(N2.08O0.38) solid solution. The O1s peaks were observed at 530.6 and 531.8 eV, representing lattice oxygen and surface OH groups in the (Zn1.44Ge)(N2.08O0.38) solid solution, respectively. The existence of lattice oxygen in the (Zn1.44Ge)(N2.08O0.38) solid solution, which was very minor on the surface of the ZnGeN2 powder, indicates that the oxygen in the (Zn1.44Ge)(N2.08O0.38) solid solution formed bonds with Zn and Ge, although the states of Zn, Ge, and N on the surface of (Zn1.44Ge)(N2.08O0.38) solid solution were similar to those on ZnGeN2. The atomic ratio of Ge calculated from the areas of the XPS peaks (Table 1) reveals that the surficial Zn concentration of the prepared (Zn1.44Ge)(N2.08O0.38) solid solution is higher than in the bulk (measured by element analysis), indicating no volatilization of Zn from the surface during the nitridation process. The N/Ge and O/Ge ratios of the prepared sample were 2.0 and 0.4, respectively, both in the bulk and on the surface, indicating that the surface of the prepared (Zn1.44Ge)(N2.08O0.38) solid solution did not readily oxidize in air. Rietveld Analysis of Neutron Diffraction Data. Neutron diffraction data for (Zn1.44Ge)(N2.08O0.38) were analyzed by assuming a single hexagonal wurtzite-type phase with space group P63mc (Figure 5). In a preliminary analysis, the occupancy factors of Ge [g(Ge)] and Zn [g(Zn)] atoms were refined using the linear constraint g(Ge) + g(Zn) ) 1, which is in good agreement with the results of chemical analysis. In another preliminary calculation, independent refinement of the z coordinates of Ge and Zn atoms [z(Ge) and z(Zn)] yielded essentially identical z coordinates. The occupancy factors were thus fixed at g(Ge) ) 0.409 and g(Zn) ) 0.586, and the constraint z(Ge) ) z(Zn) was applied in the final analysis. Table 2 shows the refined crystal parameters and reliability factors obtained by the final Rietveld analysis. The refined unit cell parameters were a ) 0.32022(2) nm and c ) 0.52026(1) nm, which are slightly smaller than for wurtzite ZnO (a ) 0.3253(1) nm and c ) 0.5213(1) nm).16 The small cell parameters of the (Zn1.44Ge)(N2.08O0.38) solid solution compared to ZnO are ascribed to the difference in ionic radius between Zn2+ (0.60 Å) and Ge4+ (0.39 Å) and is good agreement with the shift of the XRD peaks between the two compounds. The refined fractional coordinate z for the (Ge, Zn) cation in (Zn1.44Ge)(N2.08O0.38), 0.3777(4), is close to that for the (Ga0.93Zn0.07)(N0.90O0.10) solid solution [0.3782(2)]28,29 and GaN (0.377(1)).15 The (Ge,Zn) cation is

Figure 7. UV-visible DRS traces for (A) ZnO, (B) ZnGeN2, and (C) (Zn1.44Ge)(N2.08O0.38).

coordinated with four anions (N,O) to form a (Ge,Zn)(N,O)4 tetrahedron (Figure 6A), and the interatomic distances between the cation and anions ranges from 1.955 to 1.965 nm. The estimated quadratic elongation and angle variance are 1.000 and 0.27 (deg2), respectively, indicating that the (Ge,Zn)(N,O)4 tetrahedron is regular.30 The refined occupancies of N and O atoms determined by Rietveld analysis are g(N) ) 0.87(3) and g(O) ) 0.13(3) (Table 2) given the g(N) + g(O) ) 1.000 constraint. The corresponding chemical formula, Ge0.409Zn0.585N0.87(3)O0.13(3), agrees well with that determined by chemical analysis, Ge0.409(1)Zn0.586(1)N0.850(1)O0.155(1). Oxygen atoms therefore substitute for nitrogen atoms in this material. The validity of the refined crystal structure of (Zn1.44Ge)(N2.08O0.38) was investigated by calculating the scattering amplitude distribution using a maximum-entropy method (MEM). Figure 6 shows the crystal structure and equicontour surface of the nuclear density distribution at 10 fm‚Å-3. The MEM equicontour map successfully reproduced the atomic positions determined by Rietveld analysis, demonstrating the validity of the refined crystal structure. These results provide direct evidence of the substitution of oxygen atoms for nitrogen atoms and confirm the formation of the solid solution (Zn1.44Ge)(N2.08O0.38). Visible-Light Response of (Zn1+xGe)(N2Ox) (x ) 0.44) Solid Solution. Figure 7 shows the UV-visible diffuse reflectance spectrum of the (Zn1.44Ge)(N2.08O0.38) solid solution, along with ZnO and ZnGeN2 data for comparison. ZnO has a band gap of 3.2 eV (Figure 7A) and does not absorb light at visible wavelengths. ZnGeN2 exhibits two peaks (Figure 7B), a strong absorption in the UV region shorter than 370 nm, and

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Figure 8. Band dispersion and DOS for (A) Zn4Ge4N8 ()[ZnGeN2]4) and (B) Zn32Ge24N48O8 ()[ZnGeN2]24 + [ZnO]8 ) [Zn1.33GeN2O0.33]24) from DFT calculations.

a broad absorption extending into the visible region. Absorption by ZnGeN2 in the visible region is considered to be due to the presence of impurities or defects, which has been confirmed by previous investigations of the optical properties of ZnGeN2.31,32 The band gap of ZnGeN2 is estimated from the absorption edge in the UV region to be 3.3 eV, which is in good agreement with the reported band gap of ZnGeN2.12,31,32 Thus, ZnO and ZnGeN2 are wide band gap materials that only respond to excitation in the UV region. In contrast, the solid solution of these two materials, (Zn1.44Ge)(N2.08O0.38), absorbs visible light with an absorption edge at longer wavelengths than either ZnO or ZnGeN2 (Figure 7C). The band gap of the solid solution is estimated to be approximately 2.7 eV based on the onset of the diffuse reflectance spectrum. The electronic structures of ZnGeN2 and (Zn1.33Ge)(N2.00O0.33) were investigated by plane-wave DFT calculations. Figure 8 shows the band structure and the density of states (DOS) for ZnGeN2 and (Zn1.33Ge)(N2O0.33). From the DOS, the bottom of the conduction band of both compounds is composed of hybridized Ge4s4p orbitals, whereas the top of the valence band consists of N2p orbitals followed by Zn3d orbitals in the case of ZnGeN2 and N2p+O2p orbitals followed by Zn3d orbitals in the case of (Zn1.33Ge)(N2O0.33). The band gaps are estimated to be 3.15 and 2.62 eV for ZnGeN2 and (Zn1.33Ge)(N2O0.33) and each band gap has a good agreement with that obtained experimentally, 3.3 and 2.7 eV, respectively. The presence of O2p and Zn3d orbitals in the top of the valance band is considered likely to play an important role in promoting the visible-light response of (Zn1+xGe)(N2Ox). The valence band of (Zn1.33Ge)(N2.00O0.33) is comprised of O2p orbitals on the lower-energy side and N2p orbitals on the higher-energy side followed by Zn3d orbitals. The energy difference between N2p and O2p orbitals in the valance band causes large band dispersion, resulting in a narrowing of the band gap. It has also been reported for LaTiO2N that the valance band consists of O2p orbitals on the lower-energy side and N2p on the higherenergy side, and that the band gap is narrower than for La2Ti2O7, which contains a single O anion.33 The experimentally determined band gaps decrease in the order Ge3N4 (3.8 eV) > ZnGeN2 (3.3 eV) > (Zn1.44Ge)(N2.08O0.38) (2.7 eV), corresponding to the increase in Zn content [Ge3N4 (Zn/Ge ) 0) < ZnGeN2

Figure 9. Dependence of photocatalytic activity of (Zn1.44Ge)(N2.08O0.38) under UV irradiation (λ > 300 nm) on coloaded RuO2 content. Circles denote H2 evolution, and squares denote O2 evolution.

(Zn/Ge ) 1.0) < (Zn1.44Ge)(N2.08O0.38) (Zn/Ge ) 1.44)]. Sato et al. reported for Ge3N4 that the bottom of the conduction band consists of 4s and 4p orbitals of Ge, while the top of the valance band consists of N2p orbitals.7 It has been reported that p-d repulsion in II-VI semiconductors shifts the valence-band maximum upward without affecting the conduction-band minimum.34 These results suggest that the presence of Zn3d and N2p (or +O2p) electrons in the upper valence band of ZnGeN2 and (Zn1+xGe)(N2Ox) provides p-d repulsion, thereby lifting the top of the valance band and narrowing the band gap. The band gap decreases with increasing Zn content in each of the present compounds. Photocatalytic Activity of (Zn1+xGe)(N2Ox) (x ) 0.44) Solid Solution. Figure 9 shows the dependence of the photocatalytic activity of (Zn1.44Ge)(N2.08O0.38) solid solution for overall water splitting under UV light (λ > 300 nm) on the coloaded RuO2 content. The (Zn1.44Ge)(N2.08O0.38) solid solution alone exhibits little photocatalytic activity for water decomposition. However, when loaded with a suitable cocatalyst, the photocatalytic activity is remarkably enhanced. The photocatalytic activity increased with increasing RuO2 content to a maximum at ca. 5 wt %, above which the activity dropped gradually. The decrease in photocatalytic activity with higher RuO2 content indicates that loading with excess RuO2 causes agglomeration and reduces the number density of photocatalytic active sites. The same

Zinc Germanium Oxynitride as a Photocatalyst

Figure 10. Dependence of rates of H2 and O2 evolution by 5 wt % RuO2-loaded (Zn1.44Ge)(N2.08O0.38) on cutoff wavelength of incident light and the corresponding UV-visible DRS trace. Circles denote H2 evolution, and squares denote O2 evolution.

tendency has been reported for RuO2-loaded (Ga1-xZnx)(N1-xOx) solid solution.35 Neither ZnO nor ZnGeN2 alone display appreciable photocatalytic activity for water splitting under UV light, even when loaded with 5 wt % RuO2 as a cocatalyst. Figure 10 shows the dependence of the steady rate of H2 and O2 evolution by 5 wt % RuO2-loaded (Zn1.44Ge)(N2.08O0.38) on the cutoff wavelength of incident light. Filters were inserted between the light source and the sample in order to cut off light at wavelengths shorter than those indicated in the figure. The UV-visible diffuse reflectance spectrum of the (Zn1.44Ge)(N2.08O0.38) solid solution is also shown in the figure as a reference for the position of the absorption edge. Evolution of H2 and O2 was observed up to ca. 460-480 nm, with a gradual decrease in evolution rate from 420 to 480 nm. This longwavelength limit corresponds to the band gap transition of the (Zn1.44Ge)(N2.08O0.38) solid solution, confirming that these photoreactions proceed via the band gap transition. Figure 11 shows the time course of H2 and O2 evolution over 5 wt % RuO2-loaded (Zn1.44Ge)(N2.08O0.38) under UV and/or visible irradiation. The photocatalytic performance of (Zn1.44Ge)(N2.08O0.38) under UV irradiation (λ > 300 nm) is very stable (Figure 11A), proceeding with stoichiometric H2/O2 production (ratio of 2 within experimental error) and no N2 evolution. The rate of H2 and O2 evolution in the second run with intermittent evacuation was unchanged compared to the first run, and the

J. Phys. Chem. C, Vol. 111, No. 2, 2007 1047 reaction continued to proceed steadily with activities of 54.3 µmol‚h-1 for H2 and 27.5 µmol‚h-1 for O2. Stoichiometric H2 and O2 evolution was also confirmed under visible light only (λ > 400 nm) (Figure 11B), achieving production rates of 14.2 µmol‚h-1 for H2 and 7.4 µmol‚h-1 for O2. Up to 3.9 µmol of N2 was detected in the first run, but no N2 was detectable by gas chromatography in the second run. Compared with the activities of 5 wt % RuO2-loaded (Ga1-xZnx)(N1-xOx) powder shown in ref 10, the photocatalytic activities for overall water splitting over 5 wt % RuO2-loaded (Zn1.44Ge)(N2.08O0.38) are 20 times lower than that of (Ga1-xZnx)(N1-xOx) powder under UV and visible irradiation (λ > 300 nm) and still 4 times lower under visible irradiation (λ > 400 nm).10 However, the less difference in photocatalytic activity under visible irradiation (λ > 400 nm) indicates that (Zn1.44Ge)(N2.08O0.38) powder can utilize the photons from visible light region more efficiently than that of (Ga1-xZnx)(N1-xOx). So although the activity of (Zn1+xGe)(N2Ox) was low in the present study, (Zn1+xGe)(N2Ox) is a promising material as a highly efficient visible-light active photocatalyst. The total amount of H2 and O2 evolved over 30 h under UV irradiation was 2.3 mmol, greater than the amount of catalyst employed [0.20 g, 0.99 mmol of (Zn1.44Ge)(N2.08O0.38)]. No changes in the XRD patterns were detected following these photocatalytic reactions. These results clearly demonstrate that the surface-modified (Zn1.44Ge)(N2.08O0.38) solid solution is a photocatalyst suitable for overall water splitting under UV or visible light irradiation. Conclusions Zinc germanium oxynitride, (Zn1.44Ge)(N2.08O0.38), prepared as a solid solution by reaction of GeO2 and ZnO under NH3 flow (20 mL‚min-1) at 1123 K for 15 h, was shown to be an effective photocatalyst for overall water splitting. The crystal structure of (Zn1.44Ge)(N2.08O0.38) was determined by refinement of neutron powder diffraction data through Rietveld analyses. The validity of the refined crystal structure was confirmed by the scattering amplitude distribution obtained by the maximumentropy method. The (Zn1.44Ge)(N2.08O0.38) was thus confirmed to have a wurtzite-type structure (space group P63mc) and to be a solid solution in which oxygen substitutes for nitrogen in the crystal structure. Nuclear density mapping successfully reproduced the atomic positions determined by Rietveld analysis, further demonstrating the validity of the refined crystal structure. The (Zn1.44Ge)(N2.08O0.38) solid solution was shown to be responsive to visible light, with a band gap of ca. 2.7 eV,

Figure 11. Time courses of gas evolution for 5 wt % RuO2-loaded (Zn1.44Ge)(N2.08O0.38) under (A) UV-visible light (λ > 300 nm) and (B) visible light only (λ > 400 nm). Circles denote H2 evolution, squares denote O2 evolution, and triangles denote N2 production.

1048 J. Phys. Chem. C, Vol. 111, No. 2, 2007 narrower than for either ZnO (ca. 3.2 eV) or ZnGeN2 (ca. 3.3 eV). The small band gap of this material is attributable to the large valance band dispersion resulting from the energy difference between O2p and N2p orbitals and p-d repulsion between Zn3d and N2p+O2p electrons in the upper valance band, which raises the top of the valance band. Modification of (Zn1.44Ge)(N2.08O0.38) by loading with RuO2 as a cocatalyst resulted in an effective photocatalyst for overall water decomposition, achieving stoichiometric and stable H2 and O2 production under both ultraviolet and visible irradiation. Acknowledgment. The authors gratefully acknowledge the assistance of Professor K. Ohoyama and Mr. K. Nemoto for use of the HERMES diffractometer. Neutron diffraction work was carried out under the user programs 5.038 and 6.043 of the Institute for Solid-State Physics, University of Tokyo. Figure 6 was prepared using the VENUS program developed by Dr. R. A. Dilanian and Dr. F. Izumi. Thanks are extended to Dr. T. Ashino of the Analytic Research Core for advanced Materials, Institute for Materials Research, Tohoku University, for elemental analyses. This work was supported by the Solution Oriented Research for Science and Technology (SORST) program of the Japan Science and Technology Corporation (JST). References and Notes (1) Frank, S. N.; Bard, A. J. J. Phys. Chem. 1977, 81, 1484. (2) Mills, A. J. Chem. Soc., Chem. Commun. 1982, 367. (3) Hitoki, G.; Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Domen, K. Chem. Lett. 2002, 736-737. (4) Hitoki, G.; Takata, T.; Kondo, J. N.; Hara, M.; Domen, K. Chem. Commun. 2002, 1698-1699. (5) Kasahara, A.; Nukumizu, K.; Hitoki, G.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. J. Phys. Chem. A 2002, 106, 67506753. (6) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2005, 127, 8286-8287. (7) Sato, J.; Saito, N.; Yamada, Y.; Maeda, K.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K.; Inoue, Y. J. Am. Chem. Soc. 2005, 127, 4150-4151.

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