ARTICLE pubs.acs.org/JPCC
Influence of Indium Doping on the Activity of Gallium Oxynitride for Water Splitting under Visible Light Irradiation Che-Chia Hu, Yuh-Lang Lee, and Hsisheng Teng* Department of Chemical Engineering and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan ABSTRACT: Visible-light-active indium-doped gallium oxynitrides with a wurtzite-like structure were synthesized from nitridation of In(OH)3containing Ga(OH)3 under NH3 flow at 625 °C and used for photocatalytic water splitting. This synthesis method yielded a homogeneous In distribution in gallium oxynitride solid solutions for Ga replacement levels of up to 1%. An appropriate amount of In substitution for Ga, approximately 0.5%, significantly enhanced the activity of gallium oxynitride in the visible-lightinduced evolutions of H2 and O2 gases from methanol and AgNO3 solutions, respectively. X-ray photoelectron spectroscopic analysis indicated that In doping increased the dispersion of hybridized orbitals in the valence band of gallium oxynitride. This can enhance charge mobility in gallium oxynitride and thus photocatalytic activity. A higher degree of In doping resulted in nucleation of InN-like oxynitride on the gallium oxynitride surface and degraded the photocatalytic activity. This study demonstrates that band structure engineering of gallium oxynitride powders with In doping is a facile procedure for obtaining visible-light sensitive photocatalysts with high activities.
’ INTRODUCTION Photocatalytic water cleavage with solar irradiation is becoming an attractive renewable energy technology.1,2 Many semiconducting powders such as TiO2,3-5 NaTaO3,6-8 Ta-related compounds,9 SrTiO3,10 metal sulfides,11 and GaN12,13 are capable of effectively splitting water under irradiation when they are suspended in pure water or aqueous solutions. However, most of materials respond only to UV light, the energy of which makes up only a small fraction of the solar spectrum.14 In addition to the absorption spectrum, the mobility of photogenerated charges is critical to photocatalytic activity. Metal compounds associated with d10 electronic configurations (such as Ga3þ, In3þ, Ge4þ, Se4þ, and Sb5þ ions) are attractive because their conduction and valence bands are formed by hybridized sp orbitals with large band dispersion that would lead to high charge mobility and, hence, high photocatalytic performance.12c,12d Visible-light active InGaN alloy compounds with wurtzite structures are one of the candidate materials for this application.13d However, these compounds are synthesized mostly as films via deposition on substrates, and the powders of these nitride alloy compounds are rarely synthesized due to the low volatilization temperature of In. Instead, III-oxynitride compounds that can be synthesized from wet methods at relatively low temperatures are an alternative to InxGa1-xN compounds.12d,13c GaN, a semiconductor with a wide band gap of 3.4 eV, is a wellknown blue-light-emitting semiconductor and catalytic material.15-18 InN has a band gap of 0.7 eV, and its hexagonal wurtzite structure is identical to GaN.19 The band gap energy of InN was measured r 2011 American Chemical Society
to be 1.9 eV in previous reports due to the Moss-Burstein effect.19b,19c Many researchers have focused on InxGa1-xN solid solution due to its tunable and promising optical properties.20,21 InN also possesses superior transport properties to those of GaN, and the charge mobility of InxGa1-xN, therefore, increases when the In composition is increased.22 However, the properties of indium-doped gallium oxynitride, which can be synthesized with wet methods, have been rarely discussed. An oxynitride compound of a solid solution of GaN with ZnO (i.e., (Ga1-xZnx)(N1-xOx)) exhibited outstanding overall water cleavage performance under visible light irradiation.23,24 The presence of the Zn3d orbital in the oxynitride reduced the band gap and dispersed the hybridized orbitals to enhance charge mobility. Our previous study has found that the dispersion resulting from hybridization of N2p and O2p with Ga3d orbitals in the valence band of gallium oxynitride (GaON) was effective without the presence of transition-metal d orbitals.25 Doping In into GaON may tune the properties, further improve the visible-light absorption, and charge mobility. This study synthesizes visible-light-responding In-doped GaON photocatalysts via nitridation of In(OH)3-containing Ga(OH)3 with NH3. An appropriate amount of In doping in the GaON framework significantly improves the photocatalytic activity for H2 and O2 productions from sacrificial-reagent aqueous solutions under visible-light irradiation. The activity enhancement Received: November 5, 2010 Revised: December 29, 2010 Published: January 20, 2011 2805
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may be attributed to the In presence to enhance the band dispersion of GaON. The physicochemical properties of the synthesized In-doped GaON photocatalysts and the optimal synthesis conditions are investigated in an attempt to improve the activity of these III-oxynitride catalysts.
’ EXPERIMENTAL SECTION In-doped GaON photocatalysts were prepared by nitridation of In(OH)3-containing Ga(OH)3 with NH3 flow, that is, via ammonolysis. In the In(OH)3-containing Ga(OH)3 preparation, reagent-grade gallium nitrate (Ga(NO3)3, 99.9% Alfa Aesar, Ward Hill, MA) and indium nitrate (In(NO3)3, 99.9%, Alfa Aesar) were used as the starting materials. Appropriate amounts of gallium nitrate and indium nitrate were dissolved in 80 mL of deionized water and precipitated from the solution by slow addition of 80 mL of ammonium hydroxide solution (NH4OH, 25 vol %, Sigma Aldrich, Germany). The In atomic percentage of the group III elements (cIn) ranged from 0-25% for the nitrate solutions. A crystalline In(OH)3-containing Ga(OH)3 powder was obtained by autoclaving the as-prepared solution at 120 °C for 6 h followed by filtration and drying at 40 °C for 24 h. Nitridation of the crystalline In(OH)3-containing Ga(OH)3 was performed in a flow of NH3 gas (60 mL min-1) at 625 °C for 15 h, using a tubular furnace. The obtained In-doped GaON materials were ground and used without further purification. The crystalline structure of the obtained catalysts was characterized with 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 at 2θ angles of 20-70°. The 2θ step size was 0.01°, and the scan rate was 4 deg min-1. The elemental composition was measured with energy dispersive X-ray spectroscopy (EDS; JEOL JSM6700F, Japan) attached to a scanning electron microscope (SEM; JEOL JSM-6700F, Japan). The chemical composition and electron binding energy of the catalysts were studied with X-ray photoelectron spectroscopy (XPS; PHI 5000 VersaProbe, Electron Spectroscopy for Chemical Analysis, Japan) with Al KR radiation. The binding energy was calibrated using the carbon 1s peak as the reference energy at 284.8 eV. The diffuse reflection spectra were measured using an ultraviolet-visible-near-infrared (UV-vis-NIR) spectrometer (Hitachi U-4100, Japan) equipped with an integration sphere and converted these spectra from reflection to absorbance using the Kubelka-Munk method.26 The microstructures of the catalysts were explored with a high-resolution transmission electron microscope (HRTEM; FEI Tecnai, G2 F20, Hillsboro, OR). For photocatalytic gas evolution, the In-doped GaON catalysts were photodeposited with nanoparticulate Pt metal as cocatalyst.13a Photodeposition was achieved using a 400 W highpressure mercury lamp (SEN HL400EH-5, Japan) as the light source. An In-doped GaON catalyst (500 mg) was magnetically stirred in 1000 mL of 20 vol % aqueous methanol solution, containing 6.64 mg of H2PtCl6 3 6H2O (Alfa Aesar) for 180 min. The resulting 0.5 wt % Pt-deposited photocatalyst was filtered and subjected to photocatalytic reaction measurements under visible light irradiation. 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).23a The photocatalyst powders (500 mg)
Figure 1. Powder X-ray diffraction (XRD) patterns of the In-doped GaON catalysts obtained at nitridation temperature of 625 °C with varying In atomic percentages (cIn) in the catalyst precursors. The standard diffraction patterns of wurtzite GaN (JCPDS 76-0703) and InN (JCPDS 50-1239) data are shown at the top and bottom, respectively.
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 evolved H2 and O2 were determined using gas chromatography (HewlettPackard 7890, Palo Alto, CA; molecular sieve 5A column, thermal conductivity detector, argon carrier gas).
’ RESULTS AND DISCUSSION Crystal Structure of In-Doped GaON Catalysts. The present study prepared the In-doped GaON catalysts using the In(OH)3-containing Ga(OH)3 powders as the precursors. The catalysts were obtained by nitridation of the precursors under NH3 flow at 625 °C, above which significant vaporization of In metal occurred. Figure 1 shows the X-ray diffraction (XRD) spectra of In-doped GaON catalysts with various indium contents. The In concentration (cIn) shown in Figure 1 denotes the In atomic percentage of the group III elements (In and Ga) contained in the hydroxide precursors. The standard patterns of wurtzite GaN (JCPDS 76-0703) and InN (JCPDS 50-1239) are inset at the top and bottom of Figure 1. The In-doped GaON powders with cIn < 3% exhibited a wurtzite-like structure similar to that of the standard wurtzite GaN. This indicates the formation of homogeneous wurtzite-like InGaON solid solutions at low degrees of In doping. Elemental analysis with energy dispersive X-ray spectroscopy (EDS) shows that the catalysts with low In contents (cIn < 3%) had a similar chemical composition of 40 at. % gallium, 24 at. % oxygen, and 36 at. % nitrogen. The actual indium contents in the catalysts are discussed later along with the catalyst properties. The compounds with high indium contents (cIn g 3%) exhibited dual-phase diffraction patterns that were similar to InN and GaN (Figure 1). This implies the segregation of InN-like and GaN-like oxynitrides in the In-doped GaON compounds, reflecting that In incorporation into GaON is difficult.20b Photocatalytic Activity of In-Doped GaON Catalysts. The In-doped GaON photocatalysts were loaded with Pt as a 2806
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Figure 3. Variation of gas evolution rates and band gap energies of the In-doped GaON catalysts with the In atomic percentage (cIn) in the catalyst precursors.
Figure 2. 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 In-doped GaON catalysts under visible light (λ > 400 nm) irradiation. The catalysts were synthesized with varying In percentages (cIn) in their precursors. Pt (0.5 wt %) was loaded as cocatalyst by the photodeposition method.
cocatalyst for the photocatalytic reaction. Half-reactions of photoreduction and oxidation under visible light irradiation were conducted in the presence of an electron donor (methanol) and an acceptor (Agþ), respectively, as the sacrificial reagents. Prior to conducting the photocatalytic reactions with visible-light irradiation, 0.5 wt % of Pt was loaded on the GaON catalysts by photodeposition using mercury-lamp irradiation. Figure 2a shows the time course of H2 gas evolution from a methanol solution (20 vol %) over 500 mg of the In-doped GaON catalysts under visible light (λ > 400 nm) irradiation. The activity of the catalysts increased with In doping to reach a maximum at cIn = 0.5%, followed by a decrease of further In doping. The gas evolution results of the catalysts with cIn > 3% are not shown in the figure because of the low activities of these catalysts. Figure 2b shows the O2 evolution from an AgNO3 solution (0.02 M) over 500 mg of the catalysts. The O2 evolution degraded after 4 h of irradiation due to deposition of Ag metal covering the catalyst surface.23d The influence of In doping on O2 evolution was similar to that of H2 evolution; that is, an In-doping degree of cIn = 0.5% gave a maximum O2 evolution rate. Figure 3 summarizes the stable H2 gas evolution rates and the mean O2 evolution rates within the initial 4 h under irradiation.
Figure 4. (a) Diffuse reflectance spectra of the In-doped GaON catalysts with varying cIn values. (b) Plots of (RE)2 vs photon energy (E) of the In-doped GaON catalysts with varying cIn values. R represents the absorbance.
Results indicate that the bare GaON is active in reducing or oxidizing water in the presence of sacrificial reagents under visible light irradiation, and an appropriate In doping leads to a significant activity promotion. A clearer perspective on the physicochemical property change of GaON caused by In doping would be valuable for developing visible-light sensitive photocatalysts of high activities. Influence of In Doping on GaON Structure. To understand how In doping affects the optical properties of GaON, diffuse reflectance spectroscopic analysis was used to analyze the absorption performance of the catalysts. Figure 4a shows the absorbance spectra that were converted from reflection via the Kubelka-Munk method.25 The absorption bands have onset wavelengths larger than 550 nm, indicating that visible light is capable of exciting electrons in the bare and In-doped GaON catalysts from the valence band to the conduction band. The visible-light response character of the bare GaON catalyst can be 2807
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Figure 6. Dependence of the In:(Ga þ In) atomic ratios (XIn) on the In-doped GaON catalysts with the cIn value in the precursor. The XIn values were obtained with EDS and XPS analyses.
Figure 5. High-resolution TEM images of the In-doped GaON catalysts with varying cIn values: (a) cIn = 0%; (b) cIn = 0.5%; (c) cIn = 3%. The upper and lower insets of each panel show the corresponding dark field image and selected area electron nanobeam diffraction pattern.
attributed to the hybridization of Ga, N, and O orbitals, and p-d repulsion between N2p/O2p and Ga3d hybridized orbitals in the valence state.19b,23a Figure 4a also shows that the onset wavelength for the absorption bands increases gradually with the amount of In doping as a result of the narrow gap of InN. The continuum pattern of the absorption bands indicates the dissolution of In3þ ions into the GaON framework. Figure 4b shows the square of the absorption energy (RE, where R is the absorbance) against photon energy (E) for determining the energy for direct gap transition. The band gap variation was obvious only for catalysts with cIn > 3%, which had poor activities in water splitting. From a linear extrapolation, Figure 3 summarizes the band gap energies of the catalysts with cIn values of 0-3%. In comparison with the activity in water splitting, the negligible change in the gap energy cannot be the reason for activity variation with In doping. To verify further the dissolution of In3þ ions into the GaON framework, Figure 5 shows the high-resolution transmission
electron microscopy (HR-TEM) images, the corresponding selected area electron nanobeam diffraction (SAENBD) patterns of the bare GaON, and the In-doped patterns at cIn = 0.5% and 3%. All the catalysts show clear lattice fringes that can be assigned to the (0001) planes of GaN-like wurtzite. In the [0001] direction, the effective mass of InN is 0.042 m0, which is smaller than that of GaN (0.2 m0).27a This indicates In doping on GaON is capable of reducing the effective mass and improves the carrier mobility along the [0001] direction. Therefore, the larger gases evolution rate exhibited by In-doped GaON can be, at least partially, attributed to the more efficient transport of charge carriers. The dark field image shown in the upper inset of each panel in Figure 5 confirms the high content of crystalline domains (indicated by the bright spots) in these catalysts. The TEM images also show that the d-spacing of the lattice planes increases with the degree of In doping. This corresponds to expansion of the GaON lattice because of replacements of some Ga3þ cations (0.62 Å) with larger In3þ (0.8 Å). The SAENBD pattern shown in the lower inset of each panel in Figure 5 verifies the dissolution of In to expand the lattice spacing. However, the pattern of the catalyst with cIn = 3% shows the appearance of InN-like wurtzite diffraction spots. This indicates the nucleation of In compounds from GaON at a higher level of In doping. The growth of InN-like wurtzite must have been driven by the large lattice mismatch between InN and GaN; and the ionic size difference.20a The present study analyzed the distribution of In cations in the In-doped GaON catalysts using EDS and X-ray photoelectron spectroscopy (XPS). Figure 6 shows the In:(Ga þ In) atomic ratios (XIn) for different catalysts. The values of the catalysts with XIn < 1% were too small to be accurately determined. For the results with cIn = 1%, the XIn data determined from EDS and XPS were similar, indicating a homogeneous distribution of In3þ cations in GaON. For cIn > 1%, the In:(Ga þ In) atomic ratios determined from XPS were significantly larger than those from EDS. Due to the fact that XPS only analyzes the external portion within several nanometers below the solid surface, the difference between the XPS and EDS data reflects that the interior of GaON cannot accommodate a high degree of In doping and nucleation 2808
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Figure 7. Schematic energy level diagrams of (a) GaON and (b) Indoped GaON catalysts in the valence state region.
of InN-like wurtzite most likely InON, occurring preferentially at the outer part of the catalyst particles. This also explains why the activity of the catalysts significantly degraded with high In doping (cIn > 1%). Band Structure Interpretation of In-Doped GaON Catalysts. The above analyses demonstrate that a small amount of In3þ cations ( 1%) resulted in segregation of crystalline phases in the catalysts and a corresponding degradation of the photocatalytic activity. To verify the argument that In doping improves the dispersion of hybridized orbitals in the valence band, we used XPS measurements to probe the local structure of the oxynitride catalysts.21b-21d Figure 8 shows the valence state spectra of the bare GaON and the In-doped GaON with cIn = 0.5% and 3%. The spectrum of the bare
Figure 8. XPS spectra of the valence state for the In-doped GaON catalysts with different cIn values (indicated by the solid lines). These spectra were decomposed into three peaks (indicated by the dashed lines) that were fitted to a symmetric Gaussian function.
GaON can be decomposed into three hybrid states (indicated by the dashed lines): the Ga4p-N2p state peaking at 4.5 eV, Ga4s-N2p at 8.2 eV, and Ga4s-O2p at 11 eV.15c,21b The three peaks merge together as a result of In doping, explicitly justifying the preceding argument that In doping enhances the dispersion of hybrid orbitals in the valence band. In addition, the binding energy of the valence electrons, indicated by the intercept of the tangent line on the baseline, decreases with In doping, in agreement with the observed band gap energy reduction. 2809
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The Journal of Physical Chemistry C This study demonstrates that, even though a large difference in ionic size restrained group III Ga and In in forming a homogeneous wurtzite-like compound, a small amount of In doping to replace Ga tremendously enhanced the photocatalytic activity of GaON. In addition, our auxiliary experiments showed that the limiting of In-doping levels in InGaON solid solutions were low when group III oxides, that is, Ga2O3 and In2O3, were used as the precursors for subsequent nitridation, and the resulting catalysts showed poor activities. The specific crystalline framework of the Ga(OH)3 precursor not only facilitates nitridation to form highquality GaON catalysts25 but also provides an effective route for doping to give well-dispersed In3þ cations in GaON.
’ CONCLUSIONS The present study showed that introduction of In3þ cations into visible-light-sensitive GaON significantly enhanced the activities of H2 and O2 evolutions from water splitting in the presence of sacrificial reagents. Incorporating In3þ with GaON was achieved via nitridation of In(OH)3-containing Ga(OH)3 precursors at 625 °C. Both the bare and In-doped GaON catalysts have a hexagonal wurtzite-like structure and only 1% of the Ga3þ cations in GaON can be replaced by In3þ to form a homogeneous In-doped GaON solid solution. A higher degree of In doping led to nucleation of InN-like oxynitride at the outer part of GaON particles. With a Ga replacement of ca. 0.5%, the Indoped GaON exhibited the highest activity in water splitting under visible light irradiation. X-ray photoelectron spectroscopy (XPS) analysis on the valence band of the catalysts showed that the introduction of the In3þ valence orbitals into the GaON valence band improved the dispersion of the hybridized orbitals and, therefore, enhanced photocatalytic activity. Developing a synthesis method for homogeneous In3þ incorporation is the critical issue for improving the applicability of GaN-like oxynitride powders in visible light-driven photocatalysis.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This research is supported by the National Science Council of Taiwan (NSC98-2221-E-006-110-MY3, NSC98-2622-E-006-012CC2, NSC98-3114-E-007-011, NSC98-3114-E-007-005, and NSC98-2221-E-006-112-MY2) and the Bureau of Energy, Ministry of Economic Affairs, Taiwan (99-D0204-2). We also thank Mrs. Liang-Chu Wang of the Joint Center for High Valued Instruments in National Sun Yat-Sen University for her help in the HRTEM analysis. ’ REFERENCES (1) (a) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851. (b) Balzani, V.; Credi, A.; Venturi, M. ChemSusChem 2008, 1, 26. (c) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253. (d) Li, J.; Zhang, J. Z. Coord. Chem. Rev. 2009, 253, 3015. (e) Serrano, E.; Rus, G.; Garcia-Martinez, J. Renewable Sustainable Energy Rev. 2009, 13, 2373. (2) (a) Nian, J. N.; Hu, C. C.; Teng, H. Int. J. Hydrogen Energy 2008, 33, 2897. (b) Hu, C. C.; Nian, J. N.; Teng, H. Sol. Energy Mater. Sol. Cells 2008, 92, 1071. (c) Li, T. L.; Teng, H. J. Mater. Chem. 2010, 20, 3656. (d) Yeh, T. F.; Syu, J. M.; Cheng, C.; Chang, T. H.; Teng, H. Adv. Funct. Mater. 2010, 20, 2255. (e) Yerga, R. M. N.; Galvan, M. C. A.; del Valle, F.; de la Mano, J. A. V.; Fierro, J. L. G. ChemSusChem 2009, 2, 471.
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dx.doi.org/10.1021/jp1105983 |J. Phys. Chem. C 2011, 115, 2805–2811
