Gallium Oxynitride Photocatalysts Synthesized from Ga(OH)3

Nov 9, 2010 - Department of Chemical Engineering and Center for Micro/Nano Science ..... argument that the structure of Ga(OH)3 facilitates ionic tran...
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J. Phys. Chem. C 2010, 114, 20100–20106

Gallium Oxynitride Photocatalysts Synthesized from Ga(OH)3 for Water Splitting under Visible Light Irradiation Che-Chia Hu and Hsisheng Teng* Department of Chemical Engineering and Center for Micro/Nano Science and Technology, National Cheng Kung UniVersity, Tainan 70101, Taiwan ReceiVed: July 27, 2010; ReVised Manuscript ReceiVed: October 18, 2010

We report the synthesis of wurtzite-like gallium oxynitride (GaON) photocatalysts by nitridation of Ga(OH)3 with NH3 at temperatures between 550 and 900 °C. Ga(OH)3 is a more suitable precursor for GaON synthesis than Ga2O3, because its crystal lattice contains unoccupied 12-coordinate sites that facilitate ionic transportation during nitridation. The prepared GaON catalysts had band gap energies from 2.2 to 2.8 eV and showed significant activities in the visible-light promoted evolution of H2 and O2 gases from methanol and AgNO3 solutions, respectively. The maximum H2 and O2 evolution rates occurred for catalysts synthesized at 625 and 700 °C, respectively. These active catalysts had an N/O atomic ratio close to unity, suggesting that extensive hybridization of N2p and O2p orbitals promotes charge mobility, and thus enhances photocatalytic activity. This study highlights the interesting possibility of synthesizing a large diversity of visible-light active, IIIoxynitride catalysts using this Ga(OH)3 route. Introduction Water cleavage into H2 and O2 using photocatalysts with solar irradiation is a promising technique for producing clean energy without pollution or by products.1-5 A large proportion of metal oxide photocatalysts can be activated only by UV light due to large band gap energy.6-18 Many reports focus on developing visible-light active photocatalysts, with d10 electronic configurations involving Ga3+, In3+, Ge4+, Se4+, and Sb5+ ions.19-30 The d10 conduction bands of semiconducting materials are formed by hybridized sp orbitals with large band dispersion, indicative of high electron mobility and hence high photocatalytic performances.22,23 Group III nitrides with wurtzite structures have been extensively studied, due to their superior optical and semiconducting properties.31-40 Among these nitrides, GaN is a blue-light emitting semiconductor, and its conduction and valence band levels are suitable for generating H2 and O2 gases from photocatalytic water splitting. Additionally, GaN has excellent chemical stability in both acidic and basic solutions, which may enable continuous H2 and O2 productions without catalytic degradation.19-23 However, GaN has a large band gap of ca. 3.4 eV and is thereby insensitive to visible light irradiation. Modification of GaN to narrow the band gap is a possible route for synthesizing water-splitting photocatalysts. Phase-pure GaN showed a poor ability at water cleavage under UV irradiation.21-23 Poor photocatalytic activity may arise from the N2p-characterized valence band. Doping divalent metal ions (such as Mg2+, Zn2+, and Be2+) to produce acceptor levels may increase hole mobility. Indeed, GaN photocatalysts with dispersed RuO2 as a cocatalyst showed promising activity for overall water splitting under UV light irradiation.22,23 This indicates that hole mobility in the valence band of GaN governs the photocatalytic activity. A solid solution of GaN and ZnO loaded with RuO2 or Rh-Cr mixed oxide cocatalyst showed outstanding overall water cleavage performance under visible light irriadiation.41-50 The performance of this solid solution * To whom correspondence should be addressed. Tel.: 886-6-2385371. Fax: 886-6-2344496. E-mail: [email protected].

suggests that the presence of Zn3d and N2p orbitals in the upper valence band provides p-d repulsion to shift valence band maximum upward, and thus reducing the band gap.41,44,49 In addition, the large dispersion of hybridized Zn3d, N2p, and O2p orbitals may lead to increased photogenerated hole mobility in the valence band and thus promoted photocatalytic activity. In view of the advantage of this orbital-hybridizing effect, we focus on the dispersing effect resulting from hybridization of N2p and O2p with Ga3d orbitals in the valence band of gallium oxynitride (GaON) photocatalysts, without considering hybridization of transition-metal d-orbitals. This expected hybridization may raise the top of the valence band due to p-d repulsion49-54 and enhance hole mobility. Ammonolysis of Ga2O3 is preferred to the oxidation of GaN for preparation of GaON photocatalysts with a relatively homogeneous framework, because the former route destroys the original structure via homogeneous formation of GaN nuclei, while the latter grows a Ga2O3 layer to cover the complete GaN crystal.55 However, factors affecting the activity of GaON photocatalysts synthesized using ammonolysis remain unclear. Within the above scope, we researched the development of a method to synthesize visible-light responding GaON photocatalysts using crystalline Ga(OH)3 powders. These photocatalysts exhibit noteworthy activity for production of H2 and O2 gases in the presence of sacrificial reagents under visible light irradiation. We also interpret the origin of visible-light response of GaON catalysts. The physicochemical properties of the synthesized GaON photocatalysts and optimal preparation conditions are investigated in an attempt to improve the activity of these oxynitride catalysts. Experimental Section GaON photocatalysts were prepared by nitridation of Ga(OH)3 with NH3, that is, via ammonolysis. In the Ga(OH)3 preparation, 6 g of gallium nitrate (Ga(NO3)3, Alfa Aesar, U.S.) was dissolved in 80 mL of deionized water and precipitated from solution by slow addition of 80 mL of ammonium hydroxide

10.1021/jp1070083  2010 American Chemical Society Published on Web 11/09/2010

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solution (NH4OH, 25 vol %, Sigma Aldrich, Germany). Crystalline Ga(OH)3 powder was obtained by autoclaving the asprepared solution at 120 °C for 6 h followed by filtration and drying at 40 °C for 24 h. Nitridation of the crystalline Ga(OH)3 was performed in a flow of NH3 gas (60 mL min-1) at varying temperatures of 550-900 °C for 15 h, using a tubular furnace. The obtained GaON materials were ground and used without further purification. The crystalline structure of the GaON specimens was characterized by powder X-ray diffraction (XRD) using a diffractometer (Rigaku RINT 2100, Japan) with Cu KR radiation (λ ) 1.5418 Å) at 40 kV and 40 mA. The XRD patterns were collected with a step interval of 0.01° and a scan rate of 4 deg min-1 in the 2-theta range of 20-70°. The microstructure was explored with a scanning electron microscope (SEM; JEOL JSM-6700F, Japan). Elemental composition was determined with energy dispersive X-ray spectroscopy (EDS) attached to the SEM. The electron binding energy of the photocatalyst was studied with X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, ULVAC-PHI, Japan) using Al KR radiation. The binding energy was calibrated using the carbon 1s peak as the reference energy at 284.8 eV. Diffuse reflection spectra were measured for the specimens using an ultraviolet-visible-nearinfrared spectrometer (Hitachi U-4100, Japan) equipped with an integration sphere. These spectra were converted from reflection to absorbance using the Kubelka-Munk method.56 The N2 surface area of the specimens was determined with the Brunauer-Emmett-Teller (BET) equation at -196 °C using an adsorption apparatus (Micromeritics ASAP 2010, U.S.). For photocatalytic gas evolution, the GaON catalysts were photodeposited with nanoparticulate Pt metal as cocatalyst.21 The cocatalyst is deposited on GaON surface via reduction with photogenerated electrons in the GaON conduction band. This photodeposition was achieved using a 400 W high-pressure mercury lamp (SEN HL400EH-5, Japan) as the light source. GaON catalyst (500 mg) was magnetically stirred in 1000 mL of 20 vol % aqueous methanol solution, containing 6.64 mg of H2PtCl6 · 6H2O (Alfa Aesar, U.S.) for 180 min. The resulting Pt-deposited photocatalyst was filtered and subjected to photocatalytic reaction measurements under visible light irradiation. Photodeposition generally gives a uniform dispersion of Pt on the catalyst surface because no thermal treatment is required. Photocatalytic reactions were conducted at approximately 25 °C in a gas-closed inner irradiation system. The light source was a 400 W high-pressure mercury lamp. A jacket between the mercury lamp and the reaction chamber was filled with flowing, thermostatted aqueous NaNO2 solution (1 M) as a filter to block UV light (λ < 400 nm).41 The photocatalyst powders (500 mg) were suspended in the reaction chamber containing 1000 mL of methanol solution (20 vol %) for H2 evolution or AgNO3 solution (0.02 M) for O2 evolution tests. The amounts of H2 and O2 evolved were determined using gas chromatography (Hewlett-Packard 7890, U.S.; molecular sieve 5A column, thermal conductivity detector, argon carrier gas). The light spectrum irradiated on the photocatalytic reaction system was obtained using a photodetector (Oriel, model 71964, U.S.) coupled with a Cornerstone 130 monochromator (Oriel) having a bandwidth of 10 nm. This study calculated the photon flux of each wavelength interval and thus the total photon flux by incorporating the irradiation spectrum and the light power irradiated on the reacting system.

Figure 1. (a) Powder XRD pattern and (b) SEM image of the Ga(OH)3 powder obtained from hydrothermal synthesis. The standard diffraction pattern of Ga(OH)3 (JCPDS 18-0532) is inset at the bottom of panel (a).

Results and Discussion Figure 1a shows the XRD pattern of the Ga(OH)3 product obtained from 120 °C autoclaving in ammonium hydroxide solution. The diffraction peaks are indexed according to the cubic phase Ga(OH)3 (Im3, a0 ) 0.7462 nm; JCPDS 18-0532) shown at the bottom of the figure. The XRD pattern also shows that this Ga(OH)3 powder contained Ga2O3 as an impurity phase. Figure 1b shows a SEM image of the Ga(OH)3 powder. This powder had particle sizes of ca. 20-40 nm, indicating a high surface area. Figure 2a shows the unit cell and the refined crystal structure of Ga(OH)3. This crystal structure is regarded as a three-dimensional framework with corner-shared octahedral Ga(OH)6. The 12-coordinate interstices in this framework are unoccupied, which may provide facile ionic transport within the structure. A previous study characterized the ammonolysis of Ga2O3 with a mechanism involving nucleation and subsequent growth of GaN nuclei.55 Transport of nitrogen and oxygen ions takes place during ammonolysis. The transport efficiency of these ions during ammonolysis would govern the quality of the resulting GaON catalysts. For comparison, Figure 2b shows the crystal structure of Ga2O3 (monoclinic, C2/m, a0 ) 1.224 nm, b0 ) 0.304 nm, c0 ) 0.580 nm, β ) 103.83°; JCPDS 87-1901), which is compact relative to that of Ga(OH)3. Figure 3 shows the X-ray diffraction patterns of the GaON specimens. The codes GaON5, GaON6, GaON7, GaON8, and GaON9 denote the specimens obtained from nitridation of Ga(OH)3 at 550, 625, 700, 800, and 900 °C. Temperature increase significantly enhanced the efficiency of Ga(OH)3 nitridation. Ga(OH)3 converted into the wurtzite phase GaN (hexagonal, P63mc; a0 ) 0.319 nm, c0 ) 0.5189 nm; JCPDS 76-0703); the standard pattern of wurtzite GaN is inset at the bottom of Figure 3. Figure 2c provides the crystal structure of GaN, in which GaN4 tetrahedra are linked at their corners to form a three-dimensional array. This compact wurtzite structure explains why oxidation of GaN grows a Ga2O3 layer that covers the GaN crystal, rather than forming a gallium oxynitride compound.55 This also supports the argument that the compactness and interstitial spaces of the precursor structure influence

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Figure 4. SEM images of the GaON catalysts obtained from various nitridation temperatures: (a) GaON5 (550 °C); (b) GaON6 (625 °C); (c) GaON7 (700 °C); and (d) GaON9 (900 °C).

Figure 2. Unit-cell and refined crystal structures of (a) Ga(OH)3, (b) Ga2O3, and (c) wurtzite GaN. Dashed line indicates the primitive unit cell.

Figure 3. Powder XRD patterns of the GaON catalysts obtained at various nitridation temperatures. The standard diffraction pattern of wurtzite GaN (JCPDS 76-0703) is inset at the bottom.

ionic diffusion, and thus the homogeneity of resulting gallium oxynitride compounds. We also carried out nitridation of Ga2O3 powders to obtain gallium oxynitride. The color of the compound and the XRD pattern of the Ga2O3-derived products differed from those of the GaON specimens derived by nitridation of Ga(OH)3 (see the Supporting Information). The differences are attributed to the fact that the Ga(OH)3 framework (Figure 2a) contains vacant 12-coordinate interstices, which allow nitrogen ion penetration and so leads to formation of homogeneous oxynitrides. On the other hand, Ga2O3 has a defect-spinel structure (Figure 2b), which is more compact than the Ga(OH)3 structure. This spatial restriction may explain why nitridation of Ga2O3 was less efficient. Thermogravimetric analysis on the nitridation of Ga(OH)3 and Ga2O3 was conducted (see the Supporting Information). The results show that a stabilized GaON mass was obtained at ca. 800 °C in Ga(OH)3 nitridation, whereas in

Ga2O3 nitridation we could not obtain a stabilized GaON at temperatures as high as 850 °C. Figure 4 shows SEM images of some GaON specimens. The irregularly shaped particles of each specimen coagulated as nitridation temperature was increased. The XRD patterns in Figure 3 also indicate increase in crystal size with nitridation temperature. Table 1 shows that the GaON specimens obtained at temperatures below 800 °C had specific surface areas of around 30 m2 g-1. When the nitridation was conducted at 900 °C, coagulation caused a significant decrease in surface area, which may affect the photocatalytic activity.8,10 The ease of Ga(OH)3 nitridation is advantageous for the synthesis of GaON at lower temperatures, because this avoids problems with structure agglomeration and surface area reduction. Figure 5a shows the absorbance spectra of the GaON specimens converted from reflection using the Kubelka-Munk method.56 The absorption band edges of ca. 500-650 nm indicate that visible light is capable of exciting electrons in the GaON catalysts from the valence band to the conduction band. A plot of the square of the absorption energy (aE, where a is the absorbance) against photon energy (E) provides the energy for direct gap transition. From a linear extrapolation, Figure 5b shows that catalysts GaON5 to GaON9 had direct gap transition energies of ca. 2.5, 2.3, 2.2, 2.3, and 2.8 eV. On the other hand, the oxide and nitride of gallium compounds, Ga2O3 and GaN, have band gap energies of 4.7 and 3.4 eV, respectively,19-23,57,58 and do not absorb visible light. The variation in GaON band gap with changing nitridation temperature is associated with their oxygen and nitrogen compositions. Figure 6a shows the chemical composition of the GaON catalysts determined with EDS. As expected, oxygen content decreased, and nitrogen content increased, with increase in nitridation temperature. The gallium content decreased with increasing temperature to a minimum of 700 °C, and then increased with further temperature increases. The observed variation in composition may result in electronic structure changes in the active catalysts. Figure 6b summarizes band gap variation for the GaON catalysts with nitridation temperature, based on the data provided by Figure 5b, and includes the band gap energies of Ga2O3 (4.7 eV) and GaN (3.4 eV). The GaON7 catalysts had the smallest band gap, probably due to the presence of high nitrogen and oxygen contents, which would cause a high degree of O2p with N2p orbital hybridization and shift upward the valence band maximum via p-d repulsion.59-61 Figure 7a shows a schematic energy diagram of the orbitals constituting the valence band of GaN.62 The valence states of GaN consist mainly of the nitrogen 2s and 2p electrons, and

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TABLE 1: Surface Area, Photocatalytic Activity, and Stoichiometric Ratio of GaON Catalysts Obtained from Nitridation of Ga(OH)3 at Various Temperatures photocatalytic activity (µmol h-1)/(µmol h-1 m-2)

stoichiometric ratio

catalyst code

nitridation temperature (°C)

surface area (m2 g-1)

H2a

O2b

N/Ga

O/Ga

GaON5 GaON6 GaON7 GaON8 GaON9

550 625 700 800 900

30 33 30 27 12

11/0.73 18/1.1 14/0.93 1.6/0.12 1.1/0.18

13/0.87 23/1.4 30/2.0 23/1.7 9.2/1.5

0.19 0.43 1.0 1.1 0.98

0.56 0.43 0.51 0.21 0.14

a Determined from reactions in 20 vol % methanol solution under visible light (λ > 400 nm) illumination. b Determined from reactions in 0.02 M AgNO3 solution under visible light (λ > 400 nm) illumination.

Figure 5. (a) Diffuse reflectance spectra of the GaON catalysts obtained from various nitridation temperatures; (b) plots of (aE)2 versus photon energy (E) for direct gap transition of the GaON catalysts. a represents absorbance.

the gallium 3d, 4s, and 4p electrons.61-64 Hybridization of these states gives rise to the band structure, the conduction band being made up from the antibonding orbitals and the valence band from the bonding orbitals of the respective covalent bonds.62 Figure 7a illustrates that the upper part of the valence band is composed of the hybrid Ga4p-N2p and Ga4s-N2p states, and the lower part originates from hybridized Ga3d-N2s states. The p-d repulsion between N2p and Ga3d is negligible because of the large energy separation between these two levels, and therefore the GaN band gap is relatively wide. With the introduction of oxygen into GaN to form GaON, Figure 7b shows that the O2p orbital is represented in the upper part of the valence band level via hybridization with Ga4s. The presence of the O2p orbital reduces the energy difference between the upper and lower valence levels, and p-d repulsion thus becomes more significant and shifts upward the top of the valence band level, thus reducing the GaON band gap. To justify the above argument, we used XPS measurements to probe the local structure of the GaON catalysts.57,58 Figure 8 shows the valence state spectra (indicated by solid lines) of GaON9, GaON7, and GaON5 as examples. We decomposed these spectra into three peaks (indicated by the dashed lines) and fitted them using a combination of Gaussian (80%) and Lorentzian (20%) functions (that is, the Voigt function). The three peaks were due to hybrid Ga4p-N2p, Ga4s-N2p, and Ga4s-O2p states.57,60,62 With lower nitridation temperatures, oxygen content is increased and the Ga4s-O2p peak shows a concomitant intensity enhancement. This explicitly evidences

Figure 6. Variation of (a) chemical composition and (b) band gap energy of the GaON catalysts with nitridation temperature. The band gap energies of Ga2O3 and wurtzite GaN are also provided in panel (b).

Figure 7. Schematic energy levels of (a) GaN and (b) GaON, displaying the regions for the valence state.62

that the valence state of the GaON catalysts involves hybridization of the Ga and O orbitals. Figure 9 shows the Ga3d spectra of the GaON catalysts. By reducing nitridation temperature from 900 to 700 °C, the Ga3d state showed a small increase in the electron binding energy, as a result of oxygen introduction, to enhance p-d repulsion. The repulsion reached a maximum at 700 °C, resulting in the highest binding energy for the Ga3d electrons (Figure 9), and the smallest band gap (Figure 6b) because of the extensive hybridization of the Ga, N, and O orbitals. With further lowering of nitridation temperature to reduce the nitrogen content, p-d repulsion between the upper

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Figure 10. Time courses of (a) photocatalytic H2 evolution from 1000 mL of 20 vol % methanol solution and (b) photocatalytic O2 evolution from 1000 mL of 0.02 M AgNO3 solution, with 500 mg of suspended Pt/GaON catalyst under visible light (λ > 400 nm) irradiation. 0.5 wt % of Pt was loaded as cocatalyst by photodeposition method. Figure 8. XPS spectra of the valence state for GaON9, GaON7, and GaON5 (indicated by the solid lines). These spectra were decomposed into three peaks (indicated by the dashed lines) that were fitted using a combination of Gaussian (80%) and Lorentzian (20%) functions.

Figure 9. XPS spectra of the Ga3d state for the GaON catalysts. The peak binding energy positions are indicated in the figure.

and lower valence state levels becomes lessened, because of the lowered degree of O2p and N2p hybridization. In addition to p-d repulsion in the valence state for reducing the GaON catalyst band gap, interband level creation may also be responsible for the catalyst visible-light sensitivity characteristics. The catalysts with high N content had crystal lattices similar to wurtzite GaN (Figure 3), oxygen substitution for nitrogen in these catalysts was likely to function as a donor dopant, which created a donor level near the conduction band, so enhancing the visible-light absorbance.44 We used sacrificial reagents for water splitting half-reactions, to evaluate photocatalytic activity of the GaON specimens under visible light irradiation. In this case, a cocatalyst such as Pt, which serves as an electron trap, is required to assist reduction of water or other acceptor species. Prior to the visible-light experiments, 0.5 wt % of Pt was loaded as a cocatalyst by photodeposition using mercury-lamp irradiation. The presence of the Pt cocatalyst on GaON would facilitate separation of photogenerated charges and thus improved the generation of H2 and O2. Figure 10 shows the evolution of H2 and O2 gases over time, from the methanol and silver nitrate solutions over the GaON catalysts under visible light irradiation. H2 gas generation was stable, and with no activity decay, for all

catalysts during the experimental period (Figure 10a). This indicated that the GaON catalysts are active under visible light irradiation, and their conduction band levels are high enough for water reduction to produce H2. The GaON6 catalyst (625 °C nitridation) produced the maximum H2 gas evolution rate. Other nitridation temperatures resulted in deactivation of this GaON-type catalyst. We also conducted a photocatalytic reaction over the catalyst obtained from nitridation of Ga2O3 at 700 °C (see the Supporting Information). The photocatalytic H2 evolution over the Ga2O3-derived catalyst was much lower than that over the GaON7 catalyst derived from nitridation of Ga(OH)3 under the same temperature conditions. This result supports the argument that the structure of Ga(OH)3 facilitates ionic transport during nitridation and leads to formation of active GaON catalysts. Table 1 summarizes the stable H2 gas evolution rates for the GaON catalysts. The results show that the ranking of the catalytic activity based on the catalyst surface area (in units of µmol h-1 m-2) is almost identical to those based on the same catalyst mass (in units of µmol h-1). The activity variation must be associated with the electronic and/or optical properties of the catalysts. For O2 evolution, Figure 10b shows that the GaON7 catalyst (700 °C nitridation) produced the maximum evolution rate and altering the nitridation temperature deactivated the catalysts. The evolution of O2 degraded after approximately 4 h irradiation due to deposition of Ag metal covering the catalyst surface.45 Table 1 summarizes the mean O2 evolution rates within the initial 4 h irradiation. The ability to producing O2 reflects that the valence band level of the GaON catalysts lies below the water oxidation potential. Although stable photocatalytic performance was observed in both of the half-reactions, it did not lead to a similarly high photocatalytic activity in overall water splitting in the absence of sacrificial reagents. Nevertheless, the study of the GaON catalyst half-reactions and their physicochemical properties is still valuable for developing visible-light sensitive photocatalysts. Correlation of the gas evolution rates (Figure 10) and the chemical compositions of the catalysts (Figure 6a) indicates that the photocatalytic activity reached a maximum when dispersion, due to cation/anion orbital hybridization, was maximized at

Gallium Oxynitride Photocatalysts nitridation temperatures of 625 and 700 °C. The GaON6 and GaON7 catalysts had an O/N atomic ratio considerably closer to unity, which would ensure a high degree of orbital hybridization (see the valence-state XPS data in Figure 8). As to the individual H2 or O2 production, the vacancy type likely affected the activity in water reduction for H2 evolution or oxidation for O2 production. Table 1 lists the catalyst stoichiometric ratios, N/Ga and O/Ga, according to the data in Figure 6a. The GaON6 catalyst contained a certain quantity of defects at the anionic sites and might induce n-type conductivity. Thus, the high activity in H2 production was associated with increasing electron concentration in GaON6. On the other hand, the GaON7 catalyst had Ga3+ vacancies, which would result in p-type conductivity and high mobility for holes to enable O2 production. This is a situation similar to doping GaN with divalent ions for promoting hole mobility and thus O2 evolution.22,23 We calculated the photon flux of the irradiation with wavelength below 560 nm, which corresponds to the absorption edge of the GaON7 catalyst. The incident photon rate on the photocatalytic reaction system was 11 mmol h-1. The apparent quantum efficiency for gas evolution was calculated using the equation:

apparent quantum efficiency (%) ) [(number of gas molecules evolved × n)/ (number of incident photons)] × 100 where n ) 2 and 4 for H2 and O2, respectively. The apparent quantum efficiencies for H2 evolution over GaON6 from the methanol solution and O2 evolution over GaON7 from the AgNO3 solution (Table 1) were calculated to be 0.3% and 1%, respectively. This positive response to visible-light irradiation is encouraging and reflects the potential of the GaON catalysts in solar energy conversion. This study demonstrates that GaON materials offer the potential for engineering the band gap and conductivity characteristics of photocatalysts. The specific crystalline framework of the Ga(OH)3 precursor can facilitate nitridation to form highquality GaON catalysts and possibly provide an effective route for doping of other metals, such as zinc and indium, in similar oxynitride compounds. Conclusions The present study demonstrates that gallium oxynitride compounds derived from ammonolysis of Ga(OH)3 were capable of steadily catalyzing H2 and O2 evolution from methanol and AgNO3 solutions under visible light irradiation. The Ga(OH)3 precursor can be more readily converted to wurtzite-phase GaON than Ga2O3, probably due to its unoccupied 12-coordinate interstices allowing facile ionic transportation during nitridation. The visible-light response character of the GaON catalysts results from the hybridization of Ga, N, and O orbitals, and p-d repulsion between N2p/O2p and Ga3d hybridized orbitals in the valence state. The repulsion increased the maximum energy of the valence band. The band gap energy had a minimum for the GaON catalyst with nitridation at 700 °C, which gave an N/O atomic ratio close to unity and thus a high degree of orbital hybridization. The highest activity for H2 evolution occurred for the GaON with nitridation at 625 °C, which produced an anion deficient structure, so promoting electron mobility for water reduction. The highest activity of O2 evolution occurred for nitridation at 700 °C. This yielded a Ga-deficient structure, to promote hole mobility for water oxidation. Our experimental

J. Phys. Chem. C, Vol. 114, No. 47, 2010 20105 results suggest that the degree of orbital hybridization is the primary factor governing the photocatalytic activity of the GaON compounds, while the defect type, either anionic or cationic, superimposes on the primary factor to tune individual H2 or O2 gas evolution. Acknowledgment. This research is supported by the National Science Council of Taiwan (NSC98-2221-E-006-110-MY3, 982622-E-006-012-CC2, 98-3114-E-007-011, 98-3114-E-007-005, and 98-2221-E-006-112-MY2) and the Bureau of Energy, Ministry of Economic Affairs, Taiwan (98-D0204-2). Supporting Information Available: Comparison of the XRD patterns, diffuse reflectance spectra, external appearances, and photocatalytic activities of the gallium oxynitride catalysts synthesized from nitridation of Ga(OH)3 and Ga2O3 powders at 700 °C; thermogravimetric analysis on the nitridation of Ga(OH)3 and Ga2O3 powders. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851. (2) Kudo, A.; Miseki, Y. Chem. Soc. ReV. 2009, 38, 253. (3) Tributsch, H. Int. J. Hydrogen Energy 2008, 33, 5911. (4) Li, J.; Zhang, J. Z. Coord. Chem. ReV. 2009, 253, 3015. (5) Serrano, E.; Rus, G.; Garcia-Martinez, J. Renewable Sustainable Energy ReV. 2009, 13, 2373. (6) Li, Y.; Chen, G.; Zhou, C.; Li, Z. Catal. Lett. 2008, 123, 80. (7) Zhang, S.; Zhang, G.; Yu, S.; Chen, X.; Zhang, X. J. Phys. Chem. C 2009, 113, 20029. (8) Hu, C. C.; Teng, H. Appl. Catal., A 2007, 331, 44. (9) Liu, J. W.; Chen, G.; Li, Z. H.; Zhang, Z. G. Int. J. Hydrogen Energy 2007, 32, 2269. (10) Hu, C. C.; Tsai, C. C.; Teng, H. J. Am. Ceram. Soc. 2009, 92, 460. (11) Li, X.; Zang, J. J. Phys. Chem. C 2009, 113, 19411. (12) Yang, M.; Huang, X.; Yan, S.; Li, Z.; Yu, T.; Zou, Z. Mater. Chem. Phys. 2010, 121, 506. (13) Lv, J.; Kako, T.; Li, Z.; Zou, Z.; Ye, J. J. Phys. Chem. C 2010, 114, 6157. (14) Hu, C. C.; Teng, H. J. Catal. 2010, 272, 1. (15) Nian, J. N.; Hu, C. C.; Teng, H. Int. J. Hydrogen Energy 2008, 33, 2897. (16) Hu, C. C.; Nian, J. N.; Teng, H. Sol. Energy Mater. Sol. Cells 2008, 92, 1071. (17) Lee, C. K.; Lyu, M. D.; Liu, S. S.; Chen, H. C. J. Taiwan Inst. Chem. Eng. 2009, 40, 463. (18) Hsiao, P. T.; Tung, Y. L.; Teng, H. J. Phys. Chem. C 2010, 114, 6762. (19) Kocha, S. S.; Peterson, M. W.; Arent, D. J.; Redwing, J. M.; Tischler, M. A.; Turner, J. A. J. Electrochem. Soc. 1995, 142, L238. (20) Huygens, I. M.; Strubbe, K.; Gomes, W. P. J. Electrochem. Soc. 2000, 147, 1797. (21) Kida, T.; Minami, Y.; Guan, G.; Nagano, M.; Akiyama, M.; Yoshida, A. J. Mater. Sci. 2006, 41, 3527. (22) Arai, N.; Saito, N.; Nishiyama, H.; Inoue, Y.; Domen, K.; Sato, K. Chem. Lett. 2006, 35, 796. (23) Arai, N.; Saito, N.; Nishiyama, H.; Domen, K.; Kobayashi, H.; Sato, K.; Inoue, Y. Catal. Today 2007, 129, 407. (24) Fujii, K.; Kusakabe, K.; Ohkawa, K. Jpn. J. Appl. Phys. 2005, 44, 7433. (25) Kamata, K.; Maeda, K.; Lu, D.; Kako, Y.; Domen, K. Chem. Phys. Lett. 2009, 470, 90. (26) Kadowaki, H.; Sato, J.; Kobayashi, H.; Saito, N.; Nishiyama, H.; Simodaira, Y.; Inoue, Y. J. Phys. Chem. B 2005, 109, 22995. (27) Sato, J.; Saito, N.; Nishiyama, H.; Inoue, Y. J. Phys. Chem. B 2001, 105, 6061. (28) Wu, C. C.; Cheng, K. W.; Chang, W. S.; Lee, T. C. J. Taiwan Inst. Chem. Eng. 2009, 40, 180. (29) Ouyang, S.; Kikugawa, N.; Chen, D.; Zou, Z.; Ye, J. J. Phys. Chem. C 2009, 113, 1560. (30) Li, T. L.; Teng, H. J. Mater. Chem. 2010, 20, 3656. (31) Davis, R. F. Proc. IEEE 1991, 79, 702. (32) Chen, C. C.; Yeh, C. C. AdV. Mater. 2000, 12, 738. (33) Martinez, G. L.; Curiel, M. R.; Skromme, B. J.; Molnar, R. J. J. Electron. Mater. 2000, 29, 325. (34) Garcı´a, R.; Hirata, G. A.; Farı´as, M. H.; McKittrick, J. Mater. Sci. Eng., B 2002, 90, 7.

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