Photoluminescence Spectroscopic and Computational Investigation of

Aug 23, 2010 - ... of the GaN component. Photoluminescence (PL) bands of (Ga1−xZnx)(N1−xOx) photocatalysts were observed at 480 and 650 nm, sugges...
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J. Phys. Chem. C 2010, 114, 15510–15515

Photoluminescence Spectroscopic and Computational Investigation of the Origin of the Visible Light Response of (Ga1-xZnx)(N1-xOx) Photocatalyst for Overall Water Splitting Masaaki Yoshida,† Takeshi Hirai,‡ Kazuhiko Maeda,† Nobuo Saito,§ Jun Kubota,† Hisayoshi Kobayashi,| Yasunobu Inoue,§ and Kazunari Domen*,† Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, College of Science and Engineering, Ritsumeikan UniVersity, 1-1-1 Noji Higashi, Kusatsu, Shiga 525-8577, Japan, Department of Chemistry, Nagaoka UniVersity of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan, and Department of Chemistry and Materials Technology, Faculty of Engineering and Design, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan ReceiVed: January 5, 2010; ReVised Manuscript ReceiVed: August 10, 2010

The electronic structure of a solid solution between GaN and ZnO, GaN-rich (Ga1-xZnx)(N1-xOx), was investigated by photoluminescence spectroscopy and a plane wave based density functional method. Photoluminescence excitation spectra (PLE) of (Ga1-xZnx)(N1-xOx) photocatalysts (x ) 0.05-0.11) at 20 K had PLE edges in the UV region in addition to those of undoped GaN and Zn-doped GaN. However, the absorption edges appeared in the visible region, indicating that the intrinsic band gap of GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions is derived from that of the GaN component. Photoluminescence (PL) bands of (Ga1-xZnx)(N1-xOx) photocatalysts were observed at 480 and 650 nm, suggesting that the luminescence originated from electron transitions from the conduction band to Ga vacancies as native defects, or to Zn acceptor levels as impurity levels. GaN-rich (Ga1-xZnx)(N1-xOx) material containing a large amount of oxygen is likely to produce an O donor level slightly below the conduction band minimum (CBM). The Zn acceptor level is likely to be filled with electrons derived from O donor levels or thermal excitation, suggesting that the absorption in the visible light region of this material occurs via electron transitions from the Zn acceptor level to the conduction band. The electronic structure and band gap narrowing are discussed in terms of density functional theory calculations using local nonstoichiometric defect structures modeled by Zn atom replacement, O atom replacement, or Ga atom vacancies. 1. Introduction Photocatalytic overall water splitting into hydrogen and oxygen using solar energy is an attractive candidate for the sustainable production of hydrogen gas as an energy carrier.1,2 Many photocatalysts for overall water splitting under UV light have already been established.2 However, for efficient utilization of visible solar radiation for the production of storable energy, visible-light-driven photocatalysts have long been desired.2–4 Our group has recently demonstrated that a GaN-rich solid solution of GaN and ZnO, (Ga1-xZnx)(N1-xOx), provides overall water splitting through photocatalytic reactions under visible irradiation (>400 nm) when loaded with a suitable cocatalyst.3 The most important finding was that (Ga1-xZnx)(N1-xOx) can absorb visible light, although pure GaN and ZnO cannot.3 Clarifying the origin of the visible light absorption in (Ga1-xZnx)(N1-xOx) solid solutions is a key to further development of visible-light-driven photocatalysts. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra at low temperatures are very useful for the identification of the origins of the light emission.5 Our group has tentatively reported that the visible light absorption of (Ga1-xZnx)(N1-xOx) photocatalyst originates from the impurity levels formed by the substitution of Zn for Ga and O for N in * To whom correspondence should be addressed. E-mail: domen@ chemsys.t.u-tokyo.ac.jp. Phone: +81-3-5841-1148. Fax: +81-3-5841-8838. † The University of Tokyo. ‡ Ritsumeikan University. § Nagaoka University of Technology. | Kyoto Institute of Technology.

GaN using PL and PLE spectra.6 In this work, the interpretation of the PL and PLE spectra is discussed in further detail, with comparisons drawn between the GaN, Zn-doped GaN, and GaNrich (Ga1-xZnx)(N1-xOx) samples. As to the theoretical work, Jensen et al.,7 Wei et al.,8 Huda et al.,9 and Valentin10 investigated the electronic structures of (Ga1-xZnx)(N1-xOx) by treating as a solid solution of GaN and ZnO stoichiometrically (stoichiometric model), suggesting that (Ga1-xZnx)(N1-xOx) has a narrower band gap than GaN because of strong p-d coupling between the N 2p and Zn 3d states. On the other hand, Valentin examined the electronic structures arising from a restricted set of modifications to the stoichiometric model by introducing a substitutional Zn atom in the place of Ga or a substitutional O atom in the place of N (so-called nonstoichiometric model), implying that the calculation result can explain our previous work obtained by photoluminescence experiments to some extent.6,10 We believe that both the stoichiometric and nonstoichiometric models are effective to interpret the new experimental results described in this work. The (Ga1-xZnx)(N1-xOx) materials with higher ZnO concentration (until 75%) can absorb visible light with longer wavelength, as shown in the previous reports.3d,e On the other hand, the overall water splitting by these alloys has been reported at only lower ZnO concentration (below 22%).3b Therefore, we discuss the band structures in the GaN-rich region (the ZnO molar ratio is 11% at maximum). In this study, the origin of the visible light response of GaN-rich (Ga1-xZnx)(N1-xOx) solid

10.1021/jp100106y  2010 American Chemical Society Published on Web 08/23/2010

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solutions is revealed by a combination of photoluminescence spectroscopy and density functional theory (DFT) calculations. 2. Experimental Section The (Ga1-xZnx)(N1-xOx) solid solutions were prepared from mixtures of β-Ga2O3 (High Purity Chemicals, 99.9%) and ZnO (Kanto Chemicals, 99%) powders by nitridation at 1123 K under NH3 gas flow (250 mL/min) for 10, 15, and 30 h according to the method described in previous reports.3 The molar ratios of Zn to Ga in the mixtures were 1.5 in the mixture as starting materials. The chemical compositions (x) of the (Ga1-xZnx)(N1-xOx) samples prepared by nitridation for 10, 15, and 30 h were simply estimated to be 0.11, 0.09, and 0.05, respectively, from the ratios between Ga and Zn compositions by energy dispersive X-ray spectroscopy (EDX; Emax-7000, Horiba), although the ratios of Ga to N and Zn to O compositions are a little different from 1 in fact.3a Zn-doped GaN was prepared from a mixture of Ga2S3 (High Purity Chemicals, 99.99%) and ZnS (High Purity Chemicals, 99.999%) with a molar ratio of Ga:Zn ) 1:2, followed by nitridation in a NH3 flow at 1273 K for 15 h using a rotary kiln-type electric furnace fabricated inhouse.11 The five samples of GaN (prepared from elemental gallium by Mitsubishi Chemicals),12 Zn-doped GaN, and GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions (x ) 0.05, 0.09, and 0.11) were examined by powder X-ray diffraction (XRD; RINTUltimaIII, Rigaku; Cu KR) analysis and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS; V-560, Jasco). The PL and PLE spectra at 20 K were measured by a photoluminescence spectrometer (FP-6600, Jasco) using a closed-cycle helium cryostat system (Cryo Mini, Iwatani Industrial Gases Co). 3. Method of Calculation The electronic band structure of GaN-rich (Ga1-xZnx)(N1-xOx) was calculated using a plane wave based DFT program, CASTEP.13 The electronic structures of the GaN-rich solid solutions were calculated employing 27 times (3 × 3 × 3) super cells, i.e., Ga54N54 as a reference nondefect cell. In preliminary calculations, primitive cells containing Ga2N2 and Zn2O2 and GaZnNO were used. The present study deals with substitutional defects but not interstitial atoms. Four types of lattice defects were considered: (1) Ga atom replacement by Zn (Ga53ZnN54), (2) N atom replacement by O (Ga54N53O), (3) Ga atom vacancy (Ga53N54), and (4) combinations of 1-3. Elementary defect structures 1, 2, and 3 were introduced at the central primitive cell of the 3 × 3 × 3 super cell, and those structures were surrounded by 26 normal primitive cells. Solid solutions with a high oxygen concentration made the electronic structure an n-type semiconductor, because of electron donation by oxygen atoms. It has been reported that the formation energy for a N atom vacancy is higher than that for a Ga atom vacancy in this situation,14 indicating that the N atom vacancy can be ignored. The Perdew-Burke-Ernzerhof functional15 was employed in this work, along with the ultrasoft core potential.16 The cutoff value of the plane wave basis set was set to 295-340 eV. The valence electron configurations considered were 3d104s24p1 for Ga, 2s22p3 for N, 3d104s2 for Zn, and 2s22p6 for O. Geometry was optimized with respect to the fractional coordinates of atoms, and the lattice constants were fixed to those of the nondefect super cell derived from the crystalline data. Although both neutral and charged cells were investigated, we present only the results for neutral cells, assuming that the defects were

Figure 1. XRD patterns of undoped GaN (a), Zn-doped GaN (b), and GaN-rich (Ga1-xZnx)(N1-xOx) for x ) 0.05 (c), 0.09 (d), and 0.11 (e).

Figure 2. UV-visible diffuse reflectance spectra at 300 K of undoped GaN (a), Zn-doped GaN (b), and GaN-rich (Ga1-xZnx)(N1-xOx) for x ) 0.05 (c), 0.09 (d), and 0.11 (e).

produced by removing a neutral atom or by replacing a neutral atom with another neutral atom. 4. Results and Discussion 4.1. Spectroscopic Investigation of Band Structure. The XRD patterns of undoped GaN, Zn-doped GaN, and GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions (x ) 0.05, 0.09, and 0.11) were obtained, as shown in Figure 1. The patterns of all prepared samples exhibited a single phase of wurtzite structure, confirming the formation of a solid solution. The 2θ peak at ∼32 shifted to lower angles with increasing Zn concentration, in agreement with previous reports.3 Element analyses show that the ratios of Ga to N and Zn to O compositions in (Ga1-xZnx)(N1-xOx) materials are a little different from 1 in fact because samples include excess O element more than stoichiometric one, as shown in a previous report (representative example, Ga:38.3, Zn:13.3, N:32.7, O:15.7 atom %),3a although the prepared samples exhibited a single phase of wurtzite structure by XRD. These measurements surely suggest that (Ga1-xZnx)(N1-xOx) materials have local inhomogeneity derived from structure defects in the samples. Figure 2 shows the UV-visible diffuse reflectance spectra (350-550 nm) of the prepared five samples at 300 K. The spectra were normalized with respect to the maximum absorption. The spectrum of GaN had a long tail in the visible region, because the GaN sample contained elemental Ga impurity, since it was prepared from Ga metal.11 The absorption edges of undoped GaN and Zn-doped GaN were estimated to be 380 and 400 nm, respectively, close to the same value.10 On the other hand, the absorption edges of all GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions (x ) 0.05, 0.09, and 0.11) appeared in the visible region, and shifted to slightly longer wavelengths

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Figure 3. Photoluminescence excitation spectra at 20 K of undoped GaN (a), Zn-doped GaN (b), and GaN-rich (Ga1-xZnx)(N1-xOx) for x ) 0.05 (c), 0.09 (d), and 0.11 (e) detected at 550, 550, 480, 510, and 530 nm, respectively. The spectra were measured at a light intensity of ca. 1.8 W cm-2 by neutral density filters. Dashed lines show background intensities of each sample.

with increasing x composition. The gradual increases in absorption from ca. 500 to ca. 370 nm were a likely indication that the (Ga1-xZnx)(N1-xOx) samples were not direct band gap semiconductors with intrinsic absorption edges. PLE spectra (300-450 nm) at 20 K of undoped GaN, Zndoped GaN, and GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions (x ) 0.05, 0.09, and 0.11) detected at 550, 550, 480, 510, and 530 nm, respectively, are shown in Figure 3. The PLE intensities of the five samples decreased with increasing wavelength from 350 to 400 nm. PLE measurements at extremely low temperature are very useful in identifying the origins of light absorption.5,6 The PLE edges of GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions (x ) 0.05, 0.09, and 0.11) appeared in the UV region, although the absorption edges appeared in the visible region. This difference suggests that the intrinsic band gap of GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions is derived from that of GaN component, and the visible light absorption of this material occurs via Zn-related acceptor levels. However, because the atomic density of Zn doping in (Ga1-xZnx)(N1-xOx) solid solutions was too high (10%) compared with many photocatlyst powders (under several percent),2,3 the doping level is likely to function as an impurity band with a high density of states. PL spectra (400-800 nm) under excitation at 325 or 410 nm at 20 K for undoped GaN, Zn-doped GaN, and GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions (x ) 0.05, 0.09, and 0.11) are shown in Figure 4. In the PL spectra under excitation at 325 nm for undoped GaN and Zn-doped GaN, a yellow band at ca. 570 nm and a blue band at ca. 480 nm were observed, which was consistent with previous reports.5,11 The yellow and blue bands originate from electron transitions from the conduction band to a Ga vacancy as a native defect or to a Zn acceptor level as an impurity level, respectively.5,11 It should be noted that a GaN sample prepared by different precursors can have different PL spectra by impurity level derived from initial material.5a,11,17 In the PL spectra under excitation at 410 nm for GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions (x ) 0.05, 0.09, and 0.11), PL bands at ca. 650 nm were observed, suggesting that the longer PL bands are likely to correspond to those observed with undoped GaN. It has been reported that oxygen substituted for N sites in GaN can function as a donor impurity at the bottom of the conduction band,6,18 indicating that the luminescence of GaN-rich (Ga1-xZnx)(N1-xOx) material containing a large amount of oxygen is likely to occur from an O donor level at the bottom of the conduction band. Thus, we tentatively

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Figure 4. Photoluminescence spectra at 20 K of undoped GaN (a), Zn-doped GaN (b), and GaN-rich (Ga1-xZnx)(N1-xOx) for x ) 0.05 (c), 0.09 (d), and 0.11 (e) under excitation at 325 (solid line) or 410 (dashed line) nm. The spectra were measured at a light intensity of ca. 1.8 W cm-2 by neutral density filters. Dashed lines show the background intensities of each sample.

Figure 5. Expected energy level diagram for impurity levels in undoped GaN, Zn-doped GaN, and GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions. Arrows denote photoabsorbtion, photoluminescence, and thermal relaxation.

assign the origin of the luminescence to the electron transition from an O donor level to a Ga vacancy as a native defect. In the PL spectra under excitation at 325 nm for GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions (x ) 0.05, 0.09, and 0.11), very broad PL bands with two local maximum points were observed, indicating that the PL bands originated from two types of electron transitions at 480 and 650 nm. The new PL band at 480 nm for GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions presumably corresponds with the PL band observed for Zndoped GaN, suggesting that the luminescence originates from an electron transition from the conduction band to a Zn acceptor level. The PL bands at 480 nm slightly shifted to higher wavelength with increasing Zn composition, indicating that the energy distribution of the Zn acceptor level becomes wider by higher Zn doping. The expected energy level diagrams for impurity levels in undoped GaN, Zn-doped GaN, and GaNrich (Ga1-xZnx)(N1-xOx) solid solutions are shown in Figure 5. The radiative recombination of electrons bound to donors with holes bound to acceptors is known as donor-acceptor pair (DAP) recombination, and the lifetime of each DAP recombination is different.5 Figure 6 shows the dependence of PL spectra on the intensity of excitation light under excitation at 325 nm at 20 K for GaN-rich (Ga1-xZnx)(N1-xOx) (x ) 0.09) with the light intensity changed by neutral density filters. The PL band

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Figure 6. Dependence of PL spectra on the intensity of excitation light under excitation at 325 nm at 20 K for the GaN-rich (Ga1-xZnx)(N1-xOx) for x ) 0.09 with changing the light intensity by neutral density filters. Dashed lines show the background intensities of each sample.

intensity at 480 nm than at 650 nm increased with increasing intensity of excitation light, indicating that more photogenerated holes remain at the Zn acceptor energy levels at higher light intensity, because the energy levels of the Ga vacancies are easily filled with photogenerated holes due to the low density of states. GaN-rich (Ga1-xZnx)(N1-xOx) is known to function as a photocatalyst for overall water splitting under visible light.1b,3 Thus, the origin of the visible light photocatalytic reaction is most likely the electron transition from the Zn acceptor level to the conduction band. On the other hand, the native Ga vacancy level is likely to act as a recombination center for photogenerated electrons and holes, and decrease the activity for photocatalytic overall water splitting. 4.2. DFT Calculation of the Band Structure. Preliminary Calculations. To form a qualitative picture of the electronic structure of GaN, ZnO, and GaNZnO, the density of states (DOS) were calculated using the respective primitive cells, as shown in Figure 7 (see Figure S1 of the Supporting Information for details). In these calculations, the energy of the valence band maximum (VBM) was set to zero energy. The valence bands consist of the N2p and O2p orbitals, respectively, for GaN and ZnO. The Ga3d band is located at a much lower energy than the N2p band, whereas the Zn3d band partially overlaps with the O2p band. The conduction bands consist of the Ga4s, 4p and Zn4s, 4p orbitals, respectively. Figure 7c shows the density of states of a virtual solid solution, with 50% ZnO. Qualitatively, the density of states of GaNZnO was reproduced by superposing those of GaN and ZnO. The O2s and N2s bands are clearly separated, whereas the O2p and N2p bands are rather hybridized. The Ga3d band is energetically separate, and has a stronger peak in the region of -13 eV. The Zn3d band overlaps with the lower part of the valence band consisting of the O2p and N2p orbitals, and becomes more broad. We have carried out the calculations of stoichiometric solid solution models with Ga16N16 super cells, and confirmed the following. (1) The configurations where substituted Zn and O atoms are located in the nearest neighbor are energetically more stable than the distant locations. (2) The band gap is sensitively dependent on the individual substitution sites but reduces at least until 20% ZnO when it is compared among the lowest energy configurations. (3) The band gap narrowing is explained in terms of the antibonding mixing between N2p and Zn3d orbitals at the upper part of valence band. These calculation results and their interpretation are almost the same as those reported with

Figure 7. DOS calculated for Ga2N2 (a), Zn2O2 (b), and GaNZnO (c) primitive cells.

the previous calculation.7–10 The stoichiometric models are useful and adequate to describe the change in electronics structures over the whole solid solution range. However, here we try to specifically investigate the influences by deviation from the stoichiometry. Nonstoichiometric Super Cells. To explain the origin of abnormally low-energy excitation in the observed PL and PLE spectra, four nonstoichiometric solid solution models were investigated using the 27 times (3 × 3 × 3) super cell. In the discussion of the electronic structure of defective super cells, the band dispersion for the Ga54N54 super cell was taken as a reference, and shown in Figure 8a. In contrast to the dispersion of Ga2N2, the dispersion for the valence band was very flat, but that of the conduction band undulated. In the current DFT calculations, the band gaps were underestimated, but their relative comparison is reliable. For pure GaN, the band gap was calculated to be 1.60 eV from Ga2N2 and Ga54N54 cells. Zn Atom Replacement. Figure 8b shows the band dispersion of the β-band for the Ga53ZnN54 super cell (see Figure S2 in the Supporting Information for details). This super cell is an odd electron system. The R-band is almost the same as that of Ga54N54, whereas the β-band is formed by removing one electron from the R-band. It is probable that a very shallow acceptor level exists just above the valence band. Since the two orbitals (β-#485 and β-#486) are almost degenerate, both of them are partially occupied due to the thermal excitation of electrons. Figure 9a shows the projected density of states for the Zn atom. A contribution from the Zn3d and 4p orbitals is recognizable at the VBM, which suggests existence of an acceptor level derived from the Zn atom. Figure 9b and c shows the projected density of states for the N atoms nearest neighbor to the Zn atom and the other N atoms distant to the Zn atom, respectively.

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Figure 8. Band dispersion for undoped and three defective super cells. Ga54N54 (a), β-band for Ga53ZnN54 produced by replacing a Ga atom with a Zn atom (b), R-band for Ga54N53O produced by replacing a N atom with an O atom (c), and β-band for Ga53N54 produced by removing one Ga atom (d). The orbital numbers are indicated at the VBM and CBM. “HO” and “LU” mean the highest occupied and lowest unoccupied orbitals.

The former density has a shoulder peak at the VBM, and this means that the Zn-centered acceptor level extends to the nearest neighbor N atoms. This is a complementary explanation for the p-d hybridization presented by the previous calculations with the stoichiometric models,7–10 where the hybridized orbital is filled by electrons. The estimated band gap was 1.50 eV, which is narrower than the 1.60 eV for GaN. Although this narrowing is small, it is expected that higher concentrations of Zn atoms would still decrease the band gap even in either the stoichiometric or non-stoichiometric models. O Atom Replacement. Figure 8c shows the band dispersion for the Ga54N53O super cell (see Figure S3 in the Supporting Information for details). This super cell is also an odd electron system. For the R-band, one electron is added to the conduction band minimum (CBM) of Ga54N54, whereas the β-band is essentially the same as that of Ga54N54. The highest occupied (HO) orbital in the R-band is a very shallow donor level just below the conduction band, its density localizes at the O atom and the neighboring four Ga atoms, and it represents the donor level introduced by the O atom. On the other hand, the density in the β-HO orbital delocalizes over many N atoms in the cell. Ga Atom Vacancy. This super cell was constructed by removing one Ga atom. It is an odd electron system, and the lowest spin state is a doublet. However, in the calculated doublet state, the HO β-orbital was higher than the lowest unoccupied (LU) R-orbital, and we present the band structure in the quartet state, as shown in Figure 8d (see Figure S4 in the Supporting Information for details), where the total energy difference

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Figure 9. Projected DOS calculated for Ga53ZnN54. DOS for the substituted Zn atom (a), the N atoms nearest neighbor to the Zn atom (b), and the other N atoms distant to the Zn atom (c).

between the two spin states is a few kJ mol-1. The R-band is essentially the same as that of Ga54N54, but three acceptor levels appear in the band gap region for the β-band. The lowest acceptor level is above the VBM by 0.64 eV, and the highest acceptor level is below the CBM by 0.74 eV. In the acceptor levels, the electron density is largely localized on the four N atoms that are the nearest neighbors of the removed Ga atom (see Figure S4c,d in the Supporting Information). The origin of the acceptor level due to the Ga vacancy is the N2p orbitals surrounding the vacancy site. When a Ga atom is removed to form the Ga53N54 super cell, three electrons are removed from the valence band and three acceptor levels appear in the band gap, as seen in Figure 8d. Thus, only the Ga atom vacancy affords deep acceptor levels, which are the indispensable factor to explain the current experimental findings. Combination of Zn and O Atom Replacements and Ga Atom Vacancy. Three factors responsible for the nonstoichiometric solid solution have been examined separately. Here, the effects of their combination are discussed. The band structures for Ga52ZnN50O4 and Ga52ZnN48O6 super cells are shown in Figure 10a and b, respectively (see Figure S5 in the Supporting Information for details). The changes in valence electron numbers are 0 and +2 for Ga52ZnN50O4 and Ga52ZnN48O6, respectively, compared to that of Ga54N54. In Ga52ZnN50O4, the band structure is rather similar to that of Ga54N54, although the higher lying orbitals in the valence band are destabilized and the band gap is narrower. In Ga52ZnN48O6, the number of valence electrons increases by two, and the orbital located at the CBM in Ga52ZnN50O4 is occupied by electrons and becomes the donor level. In real solid solutions, the three factors may work in an interlaced fashion. The replaced Zn atom and Ga atom vacancy decrease the number of electrons in the valence band. On the

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J. Phys. Chem. C, Vol. 114, No. 36, 2010 15515 of Education, Culture, Sports, Science and Technology (MEXT) of Japan. Acknowledgement is also extended to the Global Center of Excellence (GCOE) Program for Chemistry Innovation. We wish to thank Tokyo Metropolitan Collaboration of Regional Entities for the Advancement of Technological Excellence, Japan Science and Technology Agency (JST), for their partial financial support. Supporting Information Available: Figures showing band dispersion, density of states, and orbital density contour maps. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

Figure 10. Band dispersion for Ga52ZnN50O4 (a) and Ga52ZnN48O6 (b) super cells produced by removing one Ga atom, replacing another Ga atom with a Zn atom, and replacing four or six N atoms with O atoms, respectively.

other hand, the replaced O atom increases it. Therefore, if a Ga vacancy and/or Zn atom exists, electrons in the O-derived donor levels transfer to the acceptor levels formed by the Ga vacancy and/or Zn atom, as in the Ga52ZnN50O4 model. These acceptor levels serve as the initial state for photoexcitation at longer wavelengths. More O atoms are required to produce occupied donor levels at the bottom of the conduction band, as in the Ga52ZnN48O6 model. Regardless of their electron occupation, the oxygen-derived levels serve as the initial state for the photoluminescence. 5. Conclusion Photoluminescence analysis and plane wave based density functional theory calculations revealed the electronic structures of solid solutions of GaN and ZnO. Photoluminescence and photoluminescence excitation spectra of undoped GaN, Zndoped GaN, and (Ga1-xZnx)(N1-xOx) photocatalysts (x ) 0.05, 0.09, 0.11) at 20 K showed that the intrinsic band gap of GaNrich (Ga1-xZnx)(N1-xOx) solid solutions originates from the GaN semiconductor. The PL spectra derive from electron transitions from the O donor level at the bottom of the conduction band to Ga vacancies as native defects or to Zn acceptor levels as impurity levels. This suggests that the visible light absorption of GaN-rich (Ga1-xZnx)(N1-xOx) solid solutions occurs via electron transition from the Zn acceptor level to the conduction band. (This acceptor level will be partially occupied due to thermal excitation.) In order to understand the electronic structure, density functional theory calculations with nonstoichiometric models were discussed. First, a substituted Zn atom produced an acceptor level just above the valence band, as demonstrated with the Ga53ZnN54 super cell (Zn atom replacement). The band gap narrowed, compared to pure GaN. Second, donor levels were introduced by O atoms just below the conduction band, as shown with the Ga54N53O super cell (O atom replacement). Finally, the Ga53N54 model (Ga atom vacancy) indicated that acceptor levels were formed by the Ga vacancy in the middle of the band gap. The observed experimental results can be explained by a combination of Zn acceptor levels, O donor levels, and Ga vacancies. Acknowledgment. This work was supported by the Research and Development in a New Interdisciplinary Field Based on Nanotechnology and Materials Science program of the Ministry

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