Visible Light-Sensitive ZnGe Oxynitride Catalysts for the

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Environ. Sci. Technol. 2010, 44, 3500–3504

Visible Light-Sensitive ZnGe Oxynitride Catalysts for the Decomposition of Organic Pollutants in Water J I A N H U I H U A N G , †,‡ Y A N J U A N C U I , † A N D X I N C H E N W A N G * ,† Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350002, People’s Republic of China, and Environmental and Life Sciences Department, Putian University, Putian 351100, People’s Republic of China

Received January 13, 2010. Revised manuscript received March 31, 2010. Accepted April 5, 2010.

A ZnGe oxynitride semiconductor was prepared by a solidstate reaction using GeO2 and ZnO under NH3 flow. The catalyst was used as a visible-light photocatalyst for the decomposition of organic compounds in polluted water. The physicochemical properties of the synthesized ZnGe oxynitride photocatalysts were characterized by several techniques, and aqueous photocatalytic activity was evaluated via the decomposition of model organic compounds including Rhodamine B, Methyl orange, Methylene blue, 4-chlorophenol, and salicylic acid. The results demonstrate that ZnGe oxynitride can photocatalytically oxidize organic pollutants in aqueous solution under visible light irradiation, suffering no obvious catalyst deactivation during reaction testing. The possible active species in the photocatalytic process are also discussed.

Introduction Heterogeneous photocatalysis allows for the direct use of abundant sunlight or artificial light to drive a series of important chemical reactions (e.g., water splitting, carbon dioxide fixation, mineralization of organic compounds, and selective transformation of organic functional groups). Over the past decades, the treatment of polluted air and water by photocatalysis has been actively investigated and considered as a promising candidate to complement advanced oxidation technologies in environmental applications (1-4). Despite significant progress in environmental photocatalysis, the development of stable photocatalysts active in the visible light region is still a major goal and remains a significant challenge. The exploration of visible light-sensitive catalysts has evolved from modified-TiO2 (5-8) to non-TiO2 based semiconductor materials, mainly including metal oxides, nitrides, sulfides, and their mixed solid solutions (9-14). Very recently, we found that some miscellaneous metal germanate oxides, such as Zn2GeO4 and Cd2Ge2O6, are photocatalytically active for organic pollutant decomposition in both the liquid and gas phases (15-17). These materials were introduced as photocatalysts by Inoue et al. for the overall water photosplitting reaction (18). Thus, numerous well-explored water * Corresponding author e-mail: [email protected]. † Fuzhou University. ‡ Putian University. 3500

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splitting photocatalysts could be examined as photocatalysts for environmental remediation applications, especially after further textural and surface modifications. Unfortunately most of these metal oxide-based photocatalysts, including metal germanate semiconductors, work only under specific UV light wavelengths because of their intrinsically wide band gap of typically larger than 3 eV, hence the requirement for higher photon energy of the incident light for excitation. Fabrication of a solid-state solution between two semiconductors possessing similar crystal structures is a useful approach to adjust semiconductor material band structure and therefore to obtain catalytic semiconductors with desired photoelectrochemical properties. The electronic structure of the solid solution can be facilely controlled by altering the chemical composition of the precursor system. Based on this strategy, many solid solution materials have been developed and used in the synthesis of visible light harvesting semiconductors for photocatalysis applications, including GaN-ZnO (19, 20), ZnS-CuInS2-AgInS2, AgInZn7S9, and BaBi2Mo4O16 (21-23). Recently, Domen and co-workers have reported that the monoclinic structure (β angle ) 118°53′) of ZnGeN2, a wide band gap semiconductor, is very close to another wide band gap semiconductor, ZnO, with a wurtzite structure. The small lattice mismatch between them ensures the potential formation of a solid-state solution of ZnGeN2 and ZnO under a thermal driving force. The introduction of additional Zn to the ZnGeN2 crystal creates Zn 3d levels in the valence band of ZnGeN2. The d-p repulsion between the introduced Zn 3d and the N 2p is reported to narrow the band gap of ZnGeN2 from 3.3 to 2.7 eV. It was demonstrated that this solid solution was a stable and active catalyst for overall water splitting with visible light (λ > 420 nm), even in the absence of any external sacrificial agents (24, 25). This photocatalytic system enables stable production of H2 and O2 from pure water under visible light irradiation, with a nonoptimized quantum yield of ca. 3% at 420-460 nm. However, the use of such a ZnGe oxynitride solid-state solution photocatalyst, denoted herein as (Zn1+xGe)(N2Ox), for pollutant elimination with visible light has not yet been reported. Here, detailed descriptions are given for the preparation of (Zn1+xGe)(N2Ox) and its surface modification for water purification. The prepared samples were characterized by various analytical techniques, while the active species in the photocatalytic process are also discussed.

Experimental Section Preparation of Catalysts. (Zn1+xGe)(N2Ox) solid solutions were prepared by heating a mixture of ZnO (Sinopharm Chemical Reagent, >99.0%) and GeO2 (Sinopharm Chemical Reagent, >99.999%) powders at a molar ratio of 1:5 under NH3 flow (20 mL min-1) as described previously (24, 25) at different temperatures for 15 h. (Zn1+xGe)(N2Ox) loaded with Pt was prepared by impregnating (Zn1+xGe)(N2Ox) with an aqueous solution of H2PtCl6. The initial ratio of Pt to (Zn1+xGe)(N2Ox) was fixed at 1 wt %. The impregnated sample was dried at 110 °C and then calcined at 350 °C for 3 h. The resulting solid was subsequently reduced with a NaBH4 solution (0.1 M) to produce Pt/(Zn1+xGe)(N2Ox). The N-TiO2 sample was prepared according to the reported method (26). Characterization. X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance X-ray diffractometor (Cu KR1 irradiation, λ ) 1.5406 Å). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL model JEM 2010 EX instrument at an accelerating voltage of 10.1021/es1001264

 2010 American Chemical Society

Published on Web 04/13/2010

200 kV. A Varian Cary 500 Scan UV/vis system equipped with a Labsphere diffuse reflectance accessory was used to obtain the reflectance spectra of the catalysts. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scientific) at 3.0 × 10-10 mbar using an Al Ka X-ray beam (1486.6ev). The amount of OH · produced in the photocatalytic reaction was detected by a terephthalic acid (TA) fluorescence (FL) probe method (27, 28). An aqueous solution (80.0 mL) containing 0.01 M NaOH and 5 mM TA was prepared, followed by addition of 40 mg (Zn1+xGe)(N2Ox) powder. The excitation light source for (Zn1+xGe)(N2Ox) was a 500 W halogen lamp (Philips Electronics) with two cutoff filters (420 and 800 nm). The OH · radicals formed in the system react with (TA) and generate 2-hydroxyterephthalic acid (HTA), the fluorescence of which is directly proportional to the generated OH · . The fluorescence spectra were measured using an Edinburgh Analytical Instruments FL/FSTCSPC920 fluorescence spectrophotometer. The analysis of intermediates in the decompsion of organic pollutants was conducted by a high performance liquid chromatography-mass spectrometry system (Agilent 1100). The analysis of total organic carbon (TOC) values of the solutions have been achieved with a Shimadzu TOC-VCPH total carbon analyzer. All experiments were carried out under ambient conditions. Activity Measurements. The photocatalytic activities of the catalysts investigated were evaluated via Rhodamine B (RhB), Methyl orange (MO), Methylene blue (MB), salicylic acid (SA), and 4-chlorophenol (4CP) decomposition in aqueous solution. The initial concentrations of RhB, MO, MB, SA, and 4CP in aqueous solution are 4.8, 10, 10, 10, and 20 ppm, respectively. An 80 mg amount of catalyst was suspended in a 100 mL Pyrex glass vessel containing 80 mL of contaminated aqueous solution. The visible light source was a 500 W halogen lamp (Philips Electronics) positioned beside a cylindrical reaction vessel with a flat side. The system was water and air cooled to maintain the temperature. A 420 nm cutoff filter was placed in front of the vessel to ensure irradiation by visible light wavelengths. The suspension was stirred in darkness for 12 h to achieve adsorption equilibrium, and then the reactor was irradiated to induce photocatalyzed decomposition reactions. The temperatures of reaction suspension under the irradiation of visible light were kept at 39 ( 2 °C. At given irradiation time intervals, 3 mL of the reaction suspension was collected and centrifuged to remove the catalyst. The degraded solution was analyzed using a Varian Cary 50 Scan UV/vis spectrophotometer.

Results and Discussion XRD patterns of samples obtained by heating a mixture of GeO2 and ZnO (mole ratio ) 1: 5) in NH3 gas at different temperatures indicate that the nitridation temperature plays an important role in controlling the crystal structure and composition of final products (Figure 1). At 900 °C, a single monoclinic phase of ZnGeN2 is obtained, whereas the sample treated at 850 °C shows a single solid solution phase of (Zn1+xGe)(N2Ox) with the same wurtzite phase as ZnO. Upon lowering the temperature to less than 800 °C, a large proportion of unreacted ZnO was found. The diffraction peaks of (Zn1+xGe)(N2Ox) located between those of ZnGeN2 and ZnO indicate that (Zn1+xGe)(N2Ox) represents the solid solution between ZnGeN2 and ZnO. The XPS spectrum of the (Zn1+xGe)(N2Ox) exhibits the characteristic spin-orbit splitings for the orbitals Ge 3d, Zn 2p, N 1s, and O 1s signals, further reflecting the formation of a metal oxyonitride-based semiconductor (Figure S1). The XPS results also show the surface atomic ratios (Zn/Ge ) 1.7, N/Ge ) 1.6, and O/Ge ) 0.5) of the (Zn1+xGe)(N2Ox). TEM images of the (Zn1+xGe)(N2Ox) sample reveals that the particle size of (Zn1+xGe)(N2Ox) material is in the range

FIGURE 1. XRD patterns of catalysts prepared at 800, 850, and 900 °C.

FIGURE 2. TEM image (a), HRTEM image (b), and electron diffraction pattern (c) of (Zn1+xGe)(N2Ox).

150-300 nm (Figure 2a). HRTEM images show well-resolved lattice fringes according to a (002) d wurtzite spacing for (Zn1+xGe)(N2Ox) (Figure 2b). The electron diffraction pattern coupled with the HRTEM image indicates that the sample consists of a well-crystallized structure (Figure 2c). The light absorption properties of the (Zn1+xGe)(N2Ox) and ZnGeN2 are presented alongside ZnO data as a comparison, while the wavelength at the absorption edge, λ, is determined as the intercept on the wavelength axis for a tangent line drawn on the absorption spectra (Figure 3). The gray ZnGeN2 powder is characterized by a sharp absorption edge at ∼375 nm due to the band gap transition. The long tail in the visible region is attributed to small amounts of Zn impurity formed during the high temperature ZnO reduction in the NH3. However, (Zn1+xGe)(N2Ox) prepared at 850 °C was found to absorb visible light with wavelengths g460 nm. The band gap energy of (Zn1+xGe)(N2Ox) is estimated to be 2.7 eV. The absorption edge of (Zn1+xGe)(N2Ox) shifts to longer wavelengths compared to both ZnO and ZnGeN2, believed to be caused by N2p and Zn3d orbital hybridization leading to a rise in the top of the material valence band (25). In addition, (Zn1+xGe)(N2Ox) film electrode coating on a ITO glass could generate a photocurrent over a wide potential range (-0.3 to 0.8 V vs Ag/AgCl) with visible light (λ > 420 VOL. 44, NO. 9, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. UV-vis diffuse reflectance spectra of ZnO, (Zn1+xGe)(N2Ox), and ZnGeN2. Inset: Photograph of (Zn1+xGe)(N2Ox).

FIGURE 4. RhB reduction without catalyst as compared to the RhB photocatalytic decomposition under visible light irradiation using catalysts prepared at different temperatures, where C is the concentration of RhB at different irradiation time and C0 is the concentration in the adsorption equilibrium of the photocatalyst before irradiation. nm), reflecting the potential of (Zn1+xGe)(N2Ox) for visible light photocatalysis (Figure S3). The degradation of aqueous RhB was initially examined as a model reaction to evaluate the photocatalytic activity of the synthesized samples under visible light (λ > 420 nm) illumination. The reference experiment demonstrated that RhB dye is stable in aqueous solution under visible light irradiation in the absence of a photocatalyst, while changes in the UV-vis spectra during the photodegradation of RhB in the presence of (Zn1+xGe)(N2Ox) and visible light indicated a decrease in absorbance as RhB undergoes pronounced photocatalytic degradation (Figure S4). After 6 h of photoreaction, ca. 100% RhB was degraded by (Zn1+xGe)(N2Ox). Note that the absorbance maximum exhibited a marked blue shift (from λ ) 554 to λ ) 499 nm) during the reaction, which is reported to be due to N-deethylation preferentially occurring in the photocatalytic process (29-31). A first-order linear relationship was revealed by plots of ln(C/C0) versus irradiation time for catalysts prepared at different temperatures (Figure 4). Via the first-order linear fit, the determined reaction-rate constants, k, were calculated to be 0.1383, 0.5618, 0.0338 h-1, respectively, for catalysts prepared at 800, 850, and 900 °C. A blank experiment (i.e., without catalyst) was also performed, in which RhB decomposition was found to be negligible. These results indicate 3502

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FIGURE 5. 4-Chlorophenol concentration changes during photocatalytic reaction over 1% wt. Pt-deposited (Zn1+xGe)(N2Ox) under visible light irradiation. that the formation of well crystallized (Zn1+xGe)(N2Ox) solid solution possesses the highest activity for this model photodegradation study. The photocatalytic activity of (Zn1+xGe)(N2Ox) photocatalyst was found not to decrease significantly in the RhB decomposition even after six successive operations under visible light irradiation (Figure S5). The XRD patterns and XPS spectra of (Zn1+xGe)(N2Ox) (Figures S6 and S7) also indicate that there is no remarkable alteration in the crystal structure and the surface of the catalyst before and after the photoreaction. The activity of (Zn1+xGe)(N2Ox) and Pt/ (Zn1+xGe)(N2Ox) was also compared with a typical nitrogendoped TiO2 visible light photocatalyst (Figure S8), with results indicating that (Zn1+xGe)(N2Ox) especially after Pt deposition has higher activity than that of N-TiO2. We also extended the application of this new photocatalyst to treat 4-CP, a model compound known to be transparent in the visible light region (Figure 5). Results revealed that Pt deposited on (Zn1+xGe)(N2Ox) could also degrade 4-CP efficiently under visible light irradiation. After a reaction time of 7 h, ∼90% 4-CP was found to be degraded. It is important to note that unmodified (Zn1+xGe)(N2Ox) has limited photocatalytic activity for the decomposition of 4-CP. We have also examined other metals as cocatalysts for promoting the degradation of 4-CP with visible light. It was found that Pt is the best promoter among the metal examined (Figure S9). In addition, various organic pollutants including MO, MB, and SA were also found to be effectively degraded by Pt/ (Zn1+xGe)(N2Ox) (Figures S10, S11, and S12). The determination of TOC indicates that more than 90% TOC could be eliminated from the RhB solution after photocatalytic reaction in the presence of Pt/(Zn1+xGe)(N2Ox). LC-MS analysis detected the intermediate in the MO decomposition process (Figure S13), with results indicating that MO was photocatalytically degraded by Pt/ (Zn1+xGe)(N2Ox) through a series process of hydroxylation or demethylation. To examine the active species during the photocatalytic reaction, an additional experiment was performed to examine the capability of light excited (Zn1+xGe)(N2Ox) to induce the formation of hydroxyl radicals, using the fluorescence spectrum of the (Zn1+xGe)(N2Ox)/terephthalic acid solution under visible light irradiation to monitor reaction progress (Figure 6). For comparison, a blank experiment (without catalyst) was also conducted. The fluorescence intensity was found to increase linearly with the irradiation time for the system containing catalyst, while the intensity is almost unchanged for the system without the catalyst. Thus, it is clear that formation of hydroxyl radicals were induced by bulk (Zn1+xGe)(N2Ox) under visible light irradiation. The

FIGURE 6. (a) Fluorescence spectra obtained for the supernatant liquid of the irradiated (Zn1+xGe)(N2Ox) suspension containing terephthalic acid (5.0 mmol L-1) at various irradiation periods. (b) Plots of the induced fluorescence intensity (426 nm) against light irradiation time. results further demonstrate that solid solution photocatalysts, here exemplified by (Zn1+xGe)(N2Ox), are capable of realizing a combination of charge separation and heterogeneous redox chemistry. It is expected that the activity of ZnGe oxynitrides could be further improved and the cost could be reduced by applying onto inert heterogeneous supporters such as mesoporous silica.

Acknowledgments This work was financially supported by the National Natural ScienceFoundationofChina,the973Program(2007CB613306), the Program for New Century Excellent Talents in University of China (NCET-07-0192), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT0818), the Natural Science Foundation of Fujian Province (2008H0089), and the SRPEB of Fujian (JK2009031). We are indebted to Dr. R. J. White for editing the text.

Supporting Information Available Detailed synthesis, characterization, and performance analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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