J. Phys. Chem. C 2010, 114, 1809–1814
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In Situ XRD Studies of ZnO/GaN Mixtures at High Pressure and High Temperature: Synthesis of Zn-Rich (Ga1-xZnx)(N1-xOx) Photocatalysts Haiyan Chen,† Liping Wang,‡ Jianming Bai,§ Jonathan C. Hanson,† John B. Warren,⊥ James T. Muckerman,†,# Etsuko Fujita,† and Jose A. Rodriguez*,† Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973, Mineral Physics Institute & Department of Geosciences, Stony Brook UniVersity, Stony Brook, New York 11794, UniVersity of Tennessee, KnoxVille, Tennessee 37996, Instrumentation DiVision, BrookhaVen National Laboratory, Upton, New York 11973, and Center for Functional Nanomaterials, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: October 8, 2009; ReVised Manuscript ReceiVed: December 9, 2009
The high-pressure, high-temperature conditions for the synthesis of Zn-rich (Ga1-xZnx)(N1-xOx) solid solutions from mixtures of ZnO/GaN were explored using synchrotron-based in situ time-resolved X-ray diffraction (XRD). Following a new synthetic path, (Ga1-xZnx)(N1-xOx) solid solutions with a Zn content up to ∼75% were prepared for the first time. The structures of the (Ga1-xZnx)(N1-xOx) solid solutions were characterized by XRD and X-ray absorption fine structure (XAFS) analyses and were in excellent agreement with the predictions of density functional calculations. These materials adopt a wurtzite crystal structure with metal-N or metal-O bond distances in the range of 1.95-1.98 Å. Although the (Ga1-xZnx)(N1-xOx) solid solutions seem to be stable over the full range of compositions, no ideal solid solution formation was observed. In all cases, the lattice parameters were larger than those of ideal solid solutions. The variation of the lattice parameter c showed an upward double bowing curve, as was predicted by theoretical calculations. Also, no ideal behavior was observed in the electronic properties of the (Ga1-xZnx)(N1-xOx) solid solutions. X-ray absorption spectra at the Zn and Ga K-edges of the (Ga1-xZnx)(N1-xOx) systems showed significant electronic perturbations with respect to ZnO and GaN. The synthesized (Ga1-xZnx)(N1-xOx) solid solution with a Zn content of 50% displayed the ability to absorb visible light well above 500 nm. This material has a great potential for splitting water under visible light irradiation. The availability of (Ga1-xZnx)(N1-xOx) solid solutions with a high Zn content opens the door to fully explore the application of these materials in photocatalysis. I. Introduction As we face severe global energy and environmental challenges, the efficient utilization of renewable solar energy becomes imperative. The production of hydrogen from the direct splitting of water (H2O f H2 + 0.5O2) is one of the most promising ways of using solar irradiation to generate a clean fuel.1 To accomplish photocatalytic water splitting, inorganic catalysts, especially metal oxides and doped metal oxides, have been extensively explored.2 Recently, wurtzite gallium zinc oxynitride (Ga1-xZnx)(N1-xOx), a solid solution of ZnO and GaN, was found to be one of a few photocatalysts that can accomplish overall water splitting under visible light irradiation.3,4 The most recent report shows an apparent quantum efficiency for water splitting as high as 5.9% in the 420-440 nm wavelength range. This was achieved without the presence of sacrificial regents.5 The efficiency of water splitting was correlated with the content of Zn in the solid solution.6 High Zn content is believed to result in a lower defect density within the (Ga1-xZnx)(N1-xOx) and therefore enhanced activity.5 According to first-principles density functional (DF) calculations for (Ga1-xZnx)(N1-xOx) solid solutions,7 the photocatalytic activity of the material is expected to * To whom correspondence should be addressed. Phone: (+1) 631-3442246. Fax: (+1) 631-344-5815. E-mail:
[email protected]. † Chemistry Department, Brookhaven National Laboratory. ‡ Stony Brook University. § University of Tennessee. ⊥ Instrumentation Division, Brookhaven National Laboratory. # Center for Functional Nanomaterials, Brookhaven National Laboratory.
increase with Zn content. At x ≈ 0.5, the solid solution is predicted to have a minimum band gap of ∼2.4 eV.7 Currently, (Ga1-xZnx)(N1-xOx) solid solutions are synthesized at ambient pressure from the reaction of NH3 with a mixture of ZnO and Ga2O3 at 850 °C.3,4,8 Under optimum conditions, the typical Zn content in the solid solution is below 1/3. To understand the mechanism of the solid solution formation, we performed detailed in situ X-ray diffraction (XRD) studies of the reaction of ZnO/Ga2O3 with NH3.8 Our results revealed the formation of a spinel ZnGa2O4 intermediate from which wurtzite (Ga1-xZnx)(N1-xOx) is produced.8 Since the zinc content is 1/3 in the spinel ZnGa2O4, it is impossible to produce wurtzite (Ga1-xZnx)(N1-xOx) with a zinc content higher than 1/3 using this synthetic route. Theoretical calculations indicate that the (Ga1-xZnx)(N1-xOx) solid solution is thermodynamically stable in a full range of compositions (0 e x e 1).7 To increase the Zn content in (Ga1-xZnx)(N1-xOx) to as high as 0.5, new synthesis approaches must be explored. In this study, we report the successful synthesis of a series of Zn-rich (Ga1-xZnx) (N1-xOx) solid solutions by means of a high-pressure, high-temperature solid-state reaction of ZnO and GaN powder
(1 - x)GaN + xZnO f (Ga1-xZnx)(N1-xOx) Direct reaction of ZnO and GaN to produce a solid solution under ambient pressure at high temperature is not possible
10.1021/jp909649n 2010 American Chemical Society Published on Web 01/07/2010
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J. Phys. Chem. C, Vol. 114, No. 4, 2010
Chen et al.
because the GaN decomposes to Ga and N2 at 770 °C before the reaction takes place.9 However, the stability of GaN increases with pressure; for example, at 7.5 GPa, the decomposition of GaN begins at ∼2300 °C.10 Thus, high pressures were applied in our experiments to stabilize GaN and to achieve the reaction between ZnO and GaN. The formation of the (Ga1-xZnx)(N1-xOx) solid solutions was monitored using synchrotron-based in situ time-resolved XRD. Our results of X-ray absorption near-edge spectroscopy (XANES) and diffuse reflectance spectroscopy (DRS) show that the Zn-rich (Ga1-xZnx)(N1-xOx) systems display distinctive electronic properties. II. Experimetal Procedures To prevent wurtzite-to-rock-salt phase transitions, which occur at 9.2 GPa for ZnO11 and 55 GPa for GaN12 (at room temperature), pressures below 9.2 GPa were used in our experiments. The high-pressure, high-temperature (HPHT) synthesis of the solid solutions of GaN and ZnO was achieved using a cubic-anvil high-pressure apparatus (SAM85),13 at beamline X17B2 of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). The sample was irradiated with 14-130 keV X-rays (White Beam), and in situ diffraction patterns were detected with an energy-dispersive solid-state detector. High-purity GaN powder (99.99%, Alfa Aesar) and ZnO powder (99.99%, Sigma Aldrich) were used without further purification. ZnO and GaN were mechanically mixed at three different ZnO/GaN molar ratios (1:3, 1:1, 3:1), and then, the as-prepared mixtures were loaded into a boron nitride sleeve with NaCl, which served as an internal pressure standard. The estimated standard deviation for pressure measurements at X17B2 is typically 0.1 GPa. The pressure medium was a mixture of amorphous boron and epoxy resin. The assembled cubic DIA cell13 was then compressed at room temperature, and XRD patterns of the sample and NaCl were taken at different pressures until the desired pressure was reached. Resistive heating (graphite heater) was applied to the DIA cell at high pressure13 until a single wurtzite phase was formed. The sample was then cooled to room temperature and decompressed to ambient pressure. Additional samples of the solid solution series were synthesized at the High Pressure Laboratory of the Mineral Physics Institute, Stony Brook University, using a 2000 ton split-sphere multianvil apparatus (USSA-2000).14 A standard 14/8 cell assembly was used, which consists of eight WC cubes (25 mm) with the 8 mm truncations as the second-stage anvils, a ceramic MgO octahedron with the edge length of 14 mm as the pressure medium, a graphite sleeve as the resistive furnace, and a gold capsule as the sample container. The pressure and temperature conditions were similar to those employed for the in situ XRD experiments. The synthesized pellet was crushed and ground to powder for further characterization by XRD, XAFS, energydispersive X-ray (EDX) spectra, and optical absorption measurements. The X-ray diffraction experiments on the synthesized samples were carried out on beamline X7B (λ ) 0.26470 Å) of the NSLS at BNL. The diffraction patterns were collected with a MAR345 image plate detector. XRD patterns were analyzed using the software GSAS (General Structure Analysis System code developed by A. C. Larson and R. B. Von Dreele at Los Alamos National Laboratory, Report LAUR 86-784, 2004) interfaced with EXPGUI.15 Gallium and zinc K-edge XANES and EXAFS data were collected at beamline X18B of the NSLS using the fluorescence detection mode, and the data were processed using the Athena and Artemis software packages.16
Figure 1. Upper panel: Room-temperature energy-dispersive XRD patterns of a 1:1 mixture of ZnO and GaN at different pressures. Lower panel: The energy dispersive in situ XRD patterns during a temperature increase at a constant press loading. The actual cell pressure was calculated using the NaCl internal standard. At the end, three strong peaks are seen for a (Ga1-xZnx)(N1-xOx) solid solution, and a weak peak (d ≈ 2.5 Å) is seen for a small amount (
