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A Systematical Study on Photocatalytic Properties of AgMO2 (M ) Al, Ga, In): Effects of Chemical Compositions, Crystal Structures, and Electronic Structures Shuxin Ouyang,†,‡,§ Naoki Kikugawa,† Di Chen,† Zhigang Zou,*,‡ and Jinhua Ye*,† Photocatalytic Material Center (PCMC), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, Ecomaterials and Renewable Energy Research Center (ERERC), Department of Physics, Nanjing UniVersity, 22 Hankou Road, Nanjing 210093, People’s Republic of China, and Department of Materials Science and Engineering, Nanjing UniVersity, 22 Hankou Road, Nanjing 210093, People’s Republic of China ReceiVed: July 23, 2008; ReVised Manuscript ReceiVed: NoVember 14, 2008
Four Ag-based semiconductor oxides with visible-light absorption, R-AgGaO2, R-AgInO2, β-AgAlO2, and β-AgGaO2, were prepared to investigate the influences of chemical composition and the crystal structure on the electronic structures and photocatalytic properties of AgMO2 (M ) Al, Ga, In). The catalytic efficiencies of these oxides were characterized by testing the photooxidation of gaseous 2-propanol to acetone under visible-light irradiation. The ranking of the activity was R-AgGaO2 > β-AgAlO2 > β-AgGaO2 > R-AgInO2. The electronic structures of these compounds were investigated in terms of density functional theory. These experimental and computational studies of these materials reveal the following: (1) regarding chemical compositions, the conduction bands constructed through hybridization of the Ag 5s5p states with Ga 4s4p states and Al 3s3p states are necessary for promotion of photocatalytic activities under visible light for R-phase and β-phase, respectively, and (2) regarding crystal structures, the Ag 4d states in the absence of crystal-field splitting in the R-phase are favorable for a well-dispersed valence band that is responsible for higher photocatalytic activity. 1. Introduction During recent decades, semiconductor photocatalysis has attracted much attention because it is a promising technology for production of clean hydrogen energy and remediation of environmental pollution using solar energy. However, because of a band gap wider than 3.2 eV, the traditional photocatalyst, TiO2, absorbs only ultraviolet (UV) light, which accounts for approximately 4% of all sunlight. That characteristic restricts the practical applications of UV-active TiO2. Therefore, much effort has been undertaken to develop visible-light-sensitive photocatalysts. For environmental purification, two kinds of material designs have received particular attention in the relevant literature: anion-doped TiO2 7-10 and multimetal oxides.11-19 Recently, Ag-containing multimetal oxides15-23 with the capability of eliminating organic compounds have attracted much interest. We developed a new visible-light-sensitive photocatalyst, AgAlO2 (β-phase), and reported details of the crystal structure and electronic structure of the material.15 The potential of the valence band (VB) constructed through hybridization of Ag 4d and O 2p states is competent for oxidizing the organic compounds. Furthermore, Maruyama and co-workers described crystal structures, electronic structures, and photocatalytic properties of the new photocatalysts, AgGaO2 (R-phase and β-phase).16 They reported that, in contrast to the VB of the β-phase, the dispersive VB of the R-phase was advantageous for higher mobility of the photogenerated holes, which imparted stronger oxidization ability. To gain insight into Ag-based
photocatalysts, we systematically investigated AgAlO2, AgCrO2, and Ag2CrO4.18 The influence of 3d states on their electronic structures and photocatalytic properties has been clarified among these Ag-based oxides: comparison of the electronic structure between Ag2CrO4 and AgCrO2 supported the argument that a shifting down of the unoccupied 3d states to the bottom of the conduction band (CB), as Ag2CrO4 has, is advantageous for promoting photocatalytic activity. Another strategy to tune the potentials of CB of the Ag-based oxides is to introduce different unoccupied s,p-states. However, this has not been studied to date. For this study, we specifically examined AgIMIIIO2 (M ) Al, Ga, In), for which it was expected that the trivalent cations possess various unoccupied s,p-states. Using results of this study, we expect to investigate Ag-based photocatalysts systematically. To study their photophysical and photocatalytic properties, R-AgGaO2, R-AgInO2, β-AgAlO2, and β-AgGaO2 samples were prepared. Their photocatalytic activities were evaluated using photooxidation of gaseous 2-propanol (IPA) under visible-light irradiation. These results, combined with calculations of the electronic structures for these materials, indicate two important issues: (1) regarding chemical compositions, the substitution of trivalent cations affects the electronic structures and photophysical and photocatalytic properties of AgIMIIIO2 (M ) Al, Ga, In) and (2) with respect to crystal structures, the crystal fields around Ag ions induce differences of band structures and photocatalytic activities between the R-phase and β-phase.
* Authors to whom correspondence should be addressed, jinhua.ye@ nims.go.jp and
[email protected]. † Photocatalytic Material Center, National Institute for Materials Science. ‡ Ecomaterials and Renewable Energy Research Center, Department of Physics, Nanjing University. § Department of Materials Science and Engineering, Nanjing University.
2. Experimental Methods 2.1. Material Synthesis. Polycrystalline samples of β-AgAlO2,24 β-AgGaO2,25 and R-AgInO226 were synthesized via the cation-exchange method of treating precursors with molten
1-6
10.1021/jp806513t CCC: $40.75 2009 American Chemical Society Published on Web 01/08/2009
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Figure 1. AgMO2 (M ) Al, Ga, In) system.
Figure 2. Crystal structures of AgMO2 (M ) Al, Ga, In) oxides with a delafossite structure and cristobalite-related structure.
Figure 3. XRD patterns of the R-AgGaO2-HT, R-AgInO2, β-AgAlO2, and β-AgGaO2 samples.
AgNO3. The R-AgGaO2 specimen was prepared using the cation exchange reaction in aqueous solution. (1) β-AgAlO2 and β-AgGaO2. First, the precursors, NaAlO2 and NaGaO2, were prepared using a solid-state reaction. The reagents, Al2O3 (99.0%) (Ga2O3 (99.99%)) and Na2CO3 (99.5%), were weighed out for stoichiometric NaAlO2 (NaGaO2), then they were calcined at 900 °C for 12 h. The NaAlO2 sample was reground and sintered at 1200 °C for 24 h. Then a cationexchange reaction was carried out by heating the mixture of NaAlO2 (NaGaO2), AgNO3 (99.8%), and KNO3 (99.0%), with
a molar ratio of 1.00:1.03:1.00, at 180-200 °C for 20 h. The product was washed repeatedly with distilled water to remove NaNO3, KNO3, and excess AgNO3. Finally, the β-AgAlO2 (βAgGaO2) sample was dried in air at room temperature. (2) r-AgInO2. For preparation of the NaInO2 precursor, In2O3 (99.99%) and Na2CO3 were mixed at a molar ratio of 1.00: 1.05 and then heated at 900 °C for 12 h. The obtained NaInO2 was ground together with AgNO3 and KNO3 at a 1:10:6 ratio and then heated at 300-320 °C for 20 h to execute the cation exchange reaction. The resulting sample was washed carefully and dried in air at room temperature. (3) r-AgGaO2. The AgNO3 and NaGaO2 were each dissolved in distilled water (the NaGaO2 solution was a suspended-solid solution). Then the solution of NaGaO2 was dripped slowly into the AgNO3 solution. The R-AgGaO2 powder crystallized directly during this procedure, but it was treated further in a conical flask at 50 °C for 20 h or in an autoclave at 100 °C for 5 h to improve its crystallinity. For clarification, the sample treated using the hydrothermal technique is designated hereinafter as R-AgGaO2-HT. Finally, this sample was also dried in air at room temperature. It is noteworthy that this method differs entirely from the phase transition reaction method reported by Vanaja25 and Maruyama,16 and the other former methods for AgMO2 preparation.24-30 2.2. Sample Characterization. Structural features of the specimens were determined using powder X-ray diffraction (RINT; Rigaku Corp., Japan) with Cu KR1 radiation. The diffuse reflectance spectra of the samples were recorded on a UV-visible spectrophotometer (UV-2500PC; Shimadzu Corp., Japan) with barium sulfate as the reference. Then the absorption spectra were obtained from the reflectance spectra by means of Kubelka-Munk transformations. The Brunauer-Emmett-Teller (BET) surface areas were measured via nitrogen physisorption (Gemini2360; Shimadzu Corp., Japan). 2.3. Photocatalytic Activity Evaluation. A 300 W Xe arc lamp (7 A imported current, focused through a 45 mm × 45 mm shutter window) was used as the light source of photocatalytic reaction. The light beam was passed through a set of glass filters (HA30 + U390 + L42, 400 nm < λ < 530 nm) and a water filter (removing the infrared radiation) before reaching the reactor. The reactor volume was 500 mL; it was equipped with a Pyrex lid as a window. Under such conditions, the incident light intensity was about 0.9 mW/cm2. The light intensities in the photocatalytic reaction were measured using a spectroradiometer (USR-40; Ushio Inc., Japan). The light intensity data were collected from 200 to 800 nm. In photocatalytic degradation of IPA, the 400 mg of sample was spread uniformly in an 8.5 cm2 plate that was located in the bottom of the reactor. The IPA was injected into the reactor to produce a concentration of 300-400 ppm. Before irradiation, the reactor was kept in the dark for 120-135 min to ensure an adsorptiondesorption equilibrium of gaseous reactants on the sample. The
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TABLE 1: Crystal Structures and Photophysical and Photocatalytic Properties of r-AgGaO2, r-AgInO2, β-AgAlO2, and β-AgGaO2 materials
crystal systems
space groups R3jm
R-AgGaO2
rhombohedral
R-AgGaO2-HT
rhombohedral
R3jm
R-AgInO2
rhombohedral
R3jm
β-AgAlO2
orthorhombic
Pna21
β-AgGaO2
orthorhombic
Pna21
lattice parameters a ) b ) 2.991(1) Å c ) 18.641(2) Å a ) b ) 2.992(1) Å c ) 18.640(5) Å a ) b ) 3.278(1) Å c ) 18.903(5) Å a ) 5.427(5) Å b ) 6.995(5) Å c ) 5.381(3) Å a ) 5.571(1) Å b ) 7.163(1) Å c ) 5.478(1) Å
concentrations of IPA and acetone were detected on a gas chromatograph (GC-14B; Shimadzu Corp., Japan) with an FID detector (details: Porapak Q and PEG1000 column; injection port, 120 °C; column, 60 °C; detection, 200 °C; maximum error ∼7%). The measurements of apparent photonic efficiency were performed under similar conditions except for the wavelength regions of irradiation light. Various glass filters combined with cutoff and band-pass filters were used to control the wavelength regions of irradiation light. For every wavelength region, the irradiation lasted for 60 min. 2.4. Theoretical Calculations. Electronic structures of the five models, R-AgAlO2, R-AgGaO2, R-AgInO2, β-AgAlO2, and β-AgGaO2, were investigated via the plane-wave-pseudopotential approach based on the density functional theory (DFT). All structures were optimized. Then their electronic structures were calculated using a standard Cambridge serial total energy package (CASTEP) code.31 The electron-core interaction was represented via ultrasoft pseudopotentials. The electronic exchange-correlation energy was treated within the framework of the local density approximation (LDA). Detailed computational parameters for these five models are listed in the Supporting Information (SI-1). 3. Results and Discussion 3.1. Crystal Structures. In the AgMO2 (M ) Al, Ga, In) system, although nine materials with three different crystal structures are present, reportedly only five materials were synthesized24-27 (with underlined remarks in Figure 1). Here, the R-3R-AgGaO2, R-3R-AgInO2, β-AgAlO2, and β-AgGaO2 specimens (with frame remarks in Figure 1) were investigated, except for the white R-3R-AgAlO2 compound32 because we specifically examined the visible-light-sensitive materials. The schematic crystal structures of the AgMO2 (M ) Al, Ga, In) oxides are presented in Figure 2. The delafossite structure and cristobaliterelated structure are defined respectively as R-phase27 and β-phase.33 The delafossite structure contains two subtypes: 2Hsubtype and 3R-subtype. In this work, both theoretical and experimental studies of R-phase are based on the 3R-subtype of delafossite structure; thereby R-AgMO2 (M ) Al, Ga, In) represents R-3R-AgMO2 (M ) Al, Ga, In) in the subsequent text. The crystal structures of the AgMO2 (M ) Al, Ga, In) samples were investigated using the powder X-ray diffraction method. Figure 3 depicts X-ray diffraction (XRD) patterns of the AgMO2 (M ) Al, Ga, In) oxides. The XRD pattern of R-AgGaO2-HT sample resembles that described in an earlier report related to AgGaO2;16 it mainly shows characteristic of 3R-subtype (XRD patterns of R-AgGaO2 and R-AgGaO2-HT are similar). The other patterns show that the R-AgInO2 sample
band gaps (eV)
surface areas (m2/g)
acetone evolution (ppm/h)
2.38
2.2
16.8
2.38
1.4
88.8
1.90
1.8
1.3
2.95
0.7
5.3
2.18
1.1
2.2
crystallizes in a rhombohedral system with the space group R3jm and that both β-AgAlO2 and β-AgGaO2 belong to the orthorhombic system with the space group Pna21 because the indexed results are consistent with the JCPDS database card numbers of 21-1077, 21-1070, and 21-1076, respectively, for R-AgInO2, β-AgAlO2, and β-AgGaO2. Herein, these XRD patterns are also used for deducing the lattice parameters of these compounds, as listed in Table 1. 3.2. Photophysical and Photocatalytic Properties. Figure 4 displays UV-visible absorption spectra of the R-AgGaO2-HT, R-AgInO2, β-AgAlO2, and β-AgGaO2 samples. All of these oxides possess absorption edges that extend into the visible region. Their band gaps are calculated as RhV ) A(hV - Eg)n/2, in which R, V, A, and Eg signify the absorption coefficient, light frequency, proportionality constant, and band gap, respectively.34 In that equation, n depends on whether the transition is direct (n ) 1) or indirect (n ) 4);34 here n ) 4 because the theoretical calculations indicate these materials are all indirect-gap semiconductors. The band gaps of R-AgGaO2, R-AgInO2, β-AgAlO2, and β-AgGaO2 are, respectively, 2.38, 1.90, 2.95, and 2.18 eV. The photocatalytic activity was evaluated by the degradation of gaseous IPA into acetone under visible-light irradiation. As portrayed in Figure 5a, the acetone was detected to produce over all the four materials when the lamp was turned on, whereas little acetone evolution was observed under dark condition. The R-AgGaO2 sample exhibited the best activity in photooxidation for IPA. The ranking of the activity among the samples was R-AgGaO2 > β-AgAlO2 > β-AgGaO2 > R-AgInO2; their rates
Figure 4. UV-visible absorption spectra of the R-AgGaO2-HT, R-AgInO2, β-AgAlO2, and β-AgGaO2 samples.
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Figure 6. Apparent photonic efficiencies of acetone evolution over the R-AgGaO2-HT sample at various light wavelength regions consistent with the UV-visible absorption spectrum.
Figure 5. IPA degradation activities of R-AgGaO2, R-AgInO2, β-AgAlO2, and β-AgGaO2: (a) activities of different materials; (b) activities of different synthesis methods for R-AgGaO2.
are presented in Table 1. The surface area and crystallinity of samples are two key factors affecting the photocatalytic performance. Considering the factor of surface area, the activities of the R-AgGaO2, R-AgInO2, β-AgAlO2, and β-AgGaO2 are, 19.1, 1.8, 18.9, and 5.0 ppm/(h · m2), respectively. Therefore, the ranking of photocatalytic activities does not change. On the other hand, the crystallinity of R-AgGaO2 and β-AgAlO2 was poor. Their performance would be somewhat enhanced if we were to improve the crystallinity of these two samples. Consequently, considering both the surface areas and the crystallinity of the samples, the ranking of photocatalytic activities probably remained as R-AgGaO2 > β-AgAlO2 > β-AgGaO2 > R-AgInO2. Figure 5b portrays activities of IPA photooxidation over R-AgGaO2 and R-AgGaO2-HT; the specimens were reprocessed using different methods. The observed rates of acetone evolution were 88.8 and 16.8 ppm/h, for R-AgGaO2-HT and R-AgGaO2, respectively. The hydrothermal treatment probably affected the surfacial state of the R-AgGaO2 sample and enhanced its activity. We consider that the different surfacial state originated from the surfacial microstructure and/or radical. Additional study of that inference is now in progress. As shown in Figure 6, for the sample with highest activity, R-AgGaO2-HT, we measured the apparent photonic efficiency (APE) of IPA converting to acetone (details related to measurements of apparent photonic efficiency are described in the Supporting Information, SI-2). The achieved APE was 0.03 under the condition of λ ) 425 ( 12 nm indicating the R-AgGaO2-HT sample prepared by the new method was active under visible-
light irradiation. Taking a commercial TiO2-xNx (TPS201; Sumitomo Corp., Japan) as reference, the R-AgGaO2-HT is half the activity of the TiO2-xNx (Supporting Information, SI-3). However, the BET surface area of the former is only 2% as small as that of latter. Therefore, the activity of R-AgGaO2 will be promoted if its surface area is improved. This work is in progress. The XRD, UV-vis absorption spectra, and ICPAES techniques were used for the stability test of the R-AgGaO2-HT specimen (results are listed in Supporting Information SI-4). The results show that the crystal structures, band gaps, and chemical compositions of the as-prepared and postreaction samples are not obviously changed. The somewhat uplifted background in the UV-vis absorption spectrum (λ > 500 nm) is suggested as attributable to a slight increase of the metallic Ag or deficiency on the surface of photocatalyst after irradiation. 3.3. Discussion. 3.3.1. Effect of M Elements on Electronic Structures and Photocatalytic Properties of AgMO2 (M ) Al, Ga, In): CB Potential. 3.3.1(a) R-AgMO2 (M ) Al, Ga, In). This section describes the photophysical and photocatalytic properties of the materials based on their electronic structures. The calculated band structures and density of states (DOS) of the R-AgMO2 (M ) Al, Ga, In) models are presented in Figure 7. The results show the following: (1) the energy bands of these compounds present indirect gaps; (2) their CBs are mainly constructed from the Ag 5s5p and M s,p states, whereas all VBs consist of the Ag 4d and O 2p states (see TDOS images in Figure 7); (3) when M changes from Al to In, the s,p states from M elements contribute to shifting of the energy level of the conduction band minimum (CBM) to a more negative potential, as shown for the PDOS-Al, PDOS-Ga, and PDOS-In in Figure 7. The findings of (2) and (3) suggest that the potentials of the valence band maximum (VBM) in these materials are similar, although those of CBM are affected systematically through hybridization between the Ag 5s5p and M s,p states, and consequently Eg(R-AgAlO2) > Eg(R-AgGaO2) > Eg(RAgInO2), as shown in Figure 7. This calculation is consistent with the optically measured band gaps of the R-AgAlO2,32 R-AgGaO2, and R-AgInO2 compounds. The band gap of R-AgAlO2 is as wide as 3.6 eV.32 Therefore, its light absorption is limited to within the UV region. Although the light absorption of R-AgInO2 in the visible region is much stronger than that of R-AgGaO2, the activity of the former is lower than that of the latter. This result can be reasonably attributed to the different
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Figure 8. Electronic structures of β-AgMO2 (M ) Al, Ga).
Figure 7. Electronic structures of R-AgMO2 (M ) Al, Ga, In).
construction in CBs. On the basis of the band-structure characteristics of the present materials, the lower level of CBM enhances the visible light absorption, but it generally suppresses the reduction potential. As an overall effect of the light absorption and the appropriate reduction potential, the R-AgGaO2 sample exhibits higher activity for IPA photodegradation. Conversely, the R-AgInO2 oxide possesses strong absorption in the visible region but very weak reduction ability. The photocatalytic decomposition of gaseous organic compounds in the presence of oxygen is known to include mainly the following processes: (1) excitation of photoexcited electrons from VB to CB by absorption of the incoming photons coincides with the excitation of the hole in VB; (2) the electrons contribute to reduction of O2 to form •O2- species; (3) the organic molecules are oxidized by the holes or the surface •OH radicals.4,11,23 The durative redox reaction is required such that the VB holes are used through the oxidization reaction of organic compounds, and the CB electrons are consumed by O2. The poor photocatalytic activity of R-AgInO2 is attributed to the fact that the reduction potential of CB is too low to generate active electrons;
consequently, the electrons cannot be consumed efficiently and instead recombine with the VB holes. Therefore, the CB potential of R-AgGaO2 is more favorable for the activity under visible-light irradiation. 3.3.1(b) β-AgMO2 (M ) Al, Ga). Figure 8 displays calculations of β-AgMO2 (M ) Al, Ga) models. The crystal structure of β-AgMO2 differs from that of R-AgMO2, but a similar feature of the electronic structures was observed in the β-AgMO2 systems: the potential of Ga 4s4p states is lower than that of Al 3s3p states (see the PDOS-Al, PDOS-Ga in Figure 8). This results in Eg(β-AgAlO2) > Eg(β-AgGaO2), which also concurs with results obtained from the optical absorbance spectra. Similarly, the higher photocatalytic performance on β-AgAlO2 is explained in terms of the suitable level of the CBM for the promotion of the activity. 3.3.2. Effect of Crystal Structures on Electronic Structures and Photocatalytic Properties of AgGaO2 Oxides: Crystal Field. The chemical compositions of R-AgGaO2 and β-AgGaO2 are identical. Nevertheless, although the difference of their band gaps is only 0.2 eV, the activity of R-AgGaO2 is dramatically higher than that of β-AgGaO2. This oddity compels us to investigate what factor induces the different activities of the R-phase and β-phase. Figure 9 presents the electronic structures of the two AgGaO2 compounds. A notable discrepancy is readily apparent in the tops of their VBs. The top of VB in the R-phase is well dispersive, although that in the β-phase is flat, which indicates that the photogenerated holes of the R-phase possess smaller effective mass and therefore higher migration ability. This fact is consistent with our experiment that R-AgGaO2 shows more active photooxidation for IPA than β-AgGaO2. Maruyama and coauthors proposed that the differences of VBs between the R-phase and β-phase were attributed to the different
Photocatalytic Properties of AgMO2
J. Phys. Chem. C, Vol. 113, No. 4, 2009 1565 the Ag 4d bands induced by the crystal field is disadvantageous for photocatalytic activity. 4. Conclusions Polycrystalline specimens of R-AgGaO2, R-AgInO2, β-AgAlO2, and β-AgGaO2 with visible-light absorption were synthesized to study their photophysical and photocatalytic properties. All four of these materials were active for photooxidation of IPA under visible light. A new process, cation exchange reaction in aqueous solution, was developed to prepare the R-AgGaO2 compound for this study. Particularly, the sample further treated using a hydrothermal technique exhibited the best performance; its apparent photonic efficiency achieved about 0.03 at 425 ( 12 nm. Electronic structure calculations give a reasonable explanation for the narrowing of the band gaps of AgMO2 (M ) Al, Ga, In) system with varying M from Al to In. On the basis of systematical investigations of the photocatalytic properties and the electronic structures for these compounds, two important issues were clarified in detail for the Ag-base oxides: (1) the reduction potential of conduction band plays a crucial role in the photodegradation of organic compounds; (2) the crystal field around the Ag ions is a key factor for the valenceband structure and the subsequent photocatalytic performance. This study presented evidence that photoinduced chemical behavior for IPA degradation is closely related to the crystal structures and electronic structures of semiconductor oxides. The discussions presented herein will provide useful information for further study of materials with delafossite and cristobalite-related structures.
Figure 9. Electronic structures of AgGaO2 (R-phase and β-phase).
geometrical configuration of the Ag+ networks and the Ag-O bonds.16 To explain this phenomenon further, we show evidence of the relation among the crystal structures, DOS, and energy bands here. Results show that the energy region of -2.0 to -2.5 eV in VB of β-AgGaO2 has a small gap. In Maruyama’s study,16 although they used another program (Vienna Ab initio Simulation Program, VASP) to calculate the electronic structure of β-AgGaO2, the same gap can also be found (the energy region was -1.8 to -2.0 eV). Furthermore, other β-phase materials such as β-AgAlO2 possess analogous features (see Figure 11a of ref 18). However, such a characteristic gap was not observed in VBs of R-AgGaO2 or the other R-phase oxides. The DOS of R-AgGaO2 and β-AgGaO2 reflect the corresponding features (see the PDOS-Ag curves in Figure 9), namely, the distribution of the Ag orbitals divides into two parts with energy of -2.5 eV as demarcation in β-phase, whereas the contribution of the Ag states mainly concentrates in the middle region of VB in the R-phase. These distinctions had a close relation with the different coordination environments around the Ag ions. The Ag ions are coordinated by two and four O ions for R-AgGaO2 and β-AgGaO2, respectively (see Figure 1 and Figure 2). Therefore, the tetrahedral crystal field splitting engenders separation of Ag 4d bands into the Ag 4d-t2 and Ag 4d-e groups in β-AgGaO2, although no such case in R-AgGaO2 exists. This fact provides a reasonable explanation for the separated DOS of β-AgGaO2 and continuous DOS of R-AgGaO2 in their VBs. Furthermore, regarding band-structure features, the top of VB in the β-phase is flat, which arises from the greater contribution of the localized Ag 4d orbitals to that part. However, the converse case occurs in the R-phase. The different activities and valence-band structures of the two phases originate from the various crystal fields around the Ag ions. The splitting of
Acknowledgment. This work was partially supported by the Global Environment Research Fund from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government, and the Strategic International Cooperative Program, Japan Science and Technology Agency (JST). Professor Zhigang Zou would like to thank the National Basic Research Program of China (973 Program, 2007CB613301, 2007CB613305). Mr. Shuxin Ouyang is grateful to Dr. Xiukai Li for his assistance in the writing of this manuscript. The authors would like to thank three anonymous reviewers for their helpful comments on an earlier draft of this paper. Supporting Information Available: Parameters for electronic structures calculation, details related to measurements of apparent photonic efficiency, photocatalytic activities of R-AgGaO2-HT and commercial TiO2-xNx, and tests of stability.This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Honda, K.; Fujishima, A. Nature 1972, 238, 37–38. (2) Formenti, M.; Courbon, H.; Juillet, F.; Lissatchenko, A.; Martin, J. R.; Meriaudeau, P.; Teichner, S. J. J. Vac. Sci. Technol. 1972, 9, 947– 952. (3) Agrios, A. G.; Pichat, P. J. Appl. Electrochem. 2005, 35, 655–663. (4) Tompkins, D. T.; Lawnicki, B. J.; Zeltner, W. A.; Anderson, M. A. ASHRAE Trans. 2005, 111, 60–84. (5) Robertson, P. K. J.; Bahnemann, D. W.; Robertson, J. M. C.; Wood, F. The Handbook of EnVironmental Chemistry; Springer: Berlin and Heidelberg, 2005; Volume 2M:Environmental Photochemistry Part II, pp 367-423. (6) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625– 627. (7) Sato, S. Chem. Phys. Lett. 1986, 123, 126–128. (8) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, T. Science 2001, 293, 269–271. (9) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908– 4911.
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