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Synthesis of N, C Codoped Hierarchical Porous Microsphere ZnS As a Visible Light-Responsive Photocatalyst Manickavachagam Muruganandham and Yoshihumi Kusumoto* Department of Chemistry and Bioscience, Graduate School of Science and Engineering, Kagoshima UniVersity, 1-21-35 Korimoto, Kagoshima 890-0065, Japan ReceiVed: May 7, 2009

A new N, C codoped visible light responsive hierarchical porous ZnS photocatalysts have been successfully synthesized by using a simple procedure in large scale without using catalyst or template. This is a first example of a nonmetal doped sulfide semiconductor photocatalyst working under visible light irradiation. N, C codoped ZnS catalysts were formed by thermal decomposition of zinc isothiocyanate at 300, 400, and 500 °C under open atmospheric conditions. The influences of various experimental conditions on the morphology of the catalysts have been investigated. The synthesized catalysts were characterized by the required analytical techniques. The N, C codoped catalysts possess well-crystallized wurtzite structure and have excellent visible light absorption when compared to undoped ZnS. Elemental and XPS analyses confirmed the presence of both nitrogen and carbon elements in ZnS. The chemical natures of nitrogen and carbon have been discussed on the basis of FTIR and XPS results. The visible light induced photocatalytic activity of these catalysts were investigated by using AO7 as a model pollutant and results showed that the synthesized catalysts have excellent photocatalytic activity. A possible growth mechanism of the hierarchical porous photocatalysts was proposed. Introduction The development of new visible light-active photocatalysts is one of the most important topics in photocatalysis research. Doping of metal ions or nonmetals such as N, C, S, F, and P into wide band gap metal oxide semiconductors has been utilized to prepare visible light-active photocatalysts.1 However, not much attention has been paid to such nonmetals doping into sulfide semiconductors by using conventional chemical methods because it is difficult to prepare such kinds of material. Being a large band gap semiconductor (3.6 eV), ZnS is active only in the UV region. However, visible light-active ZnS was obtained by doping with metal ions like Ni, Cu, and Pb and also by preparing ZnS-based solid solutions, sensitized by thiocyanuric acid.2,3 Hollow materials with hierarchical pore structures and complex morphologies have attracted much attention in terms of fundamental interest in biomineralization and their potential applications in catalysis, separation, and controlled release.4 Hierarchical porous photocatalysts have been found to be highly efficient in photocatalytic decomposition of pollutants.5 Fabrication of such porous materials without using template or catalyst in a simple methodology in large-scale production is a highly challenging job for chemists and material scientists. To the best of our knowledge, nonmetal doped ZnS semiconductor photocatalysts working under visible light irradiation have not been reported so far. For the first time we report visible light-active nitrogen and carbon codoped hierarchical porous wurtzite ZnS by using a simple procedure. Thiocyanate ion (SCN-) is an interesting and widely studied ligand, which may coordinate with a variety of metal ions and is used to prepare metal sulfides.6 Coordination between thiocyanate ions and different metal ions has been investigated from the interesting ambient nature of SCN- ion, which can coordinate with metal ions through either the hard nitrogen or soft sulfur end depending on the hardness * To whom correspondence should be addressed. E-mail: kusumoto@ sci.kagoshima-u.ac.jp. Phone/fax: +81-99-285-8914.

and softness of the central cation involved. For example, Zn2+ and Hg2+ ions behave as hard and soft acids, respectively, and a Cd2+ ion has an intermediate character.7 Earlier studies confirmed that thiocyanate ions could coordinate with a Zn2+ ion through the nitrogen atom, but with a Hg2+ ion through the sulfur atom. Thus, it is expected that the zinc ion coordinates with nitrogen, which results in a [Zn (NCS)2] complex. Experimental Section Unhydrous zinc chloride, potassium thiocyanate, zinc nitrate hexahydrate, zinc acetate dihydrate, zinc sulfate, and ethanol were obtained from Wako Pure Chemical Ind., Ltd. (Osaka, Japan). Acid orange 7 (AO7) dye was received from Tokyo kasei kogyo Ltd. (Tokyo, Japan). These chemicals were used without further purification. For all experimental work Milli Q-Plus water (resistance ) 18.2 MΩ) was used. Zinc chloride and potassium thiocyanate were used as the source of zinc and thiocyanate, respectively. In a typical experiment, 12.51 g of anhydrous ZnCl2 was dissolved in 25 mL of water and then 50 mL of ethanol (99.99%) was added with constant stirring (solution 1); 20.07 g of potassium thiocyanate was separately dissolved in another 25 mL of water (solution 2). Under magnetic stirring, solution 2 was then added drop by drop (30-45 min) into solution 1. The clear solution was stirred for another 30 min and then solvents were evaporated on a hot plate until they become a semisolid, which is transferred into a porcelain dish for decomposition. Thermal decomposition of the final residue was carried out under open atmospheric conditions at 300, 400, and 500 °C for 2 h and denoted as S-1, S-2, and S-3 catalyst, respectively. After thermal decomposition at the desired temperature, samples were washed with plenty of water and methanol and finally dried in an oven at 120 °C for 20 min. The synthesized catalyst was characterized by using appropriate analytical techniques. A 50 mL borosilicate glass beaker containing dye solution was used for irradiation. A xenon lamp (500 W) was used as a

10.1021/jp904253u CCC: $40.75  2009 American Chemical Society Published on Web 08/13/2009

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Figure 1. FESEM pictures of N, C codoped ZnS photocatalyst (a and a1) prepared after 2 h of decomposition at 300, (b) 400, and (c) 500 °C, (d) prepared without ethanol at 500 °C, (e) prepared at acidic medium at 300 °C, and (f) prepared under N2 atmospheric conditions at 500 °C.

visible light source with an L42 cutoff filter that allows only visible light (λ g 400 nm). The experiments have been carried out at room temperature without purging oxygen or air. In all cases 40 mL of a 2.5 mg/L of AO7 dye solution containing 1.25 g/L of the catalyst was used. The resulting solution was then stirred continuously in the dark for 45 min to achieve an

adsorption equilibrium of AO7 on the catalysts. Then, the photocatalytic run was started under visible light illumination. Samples were withdrawn from the reactor at the desired time intervals in the course of the experiments. The catalyst was centrifuged and filtrated through microfiber filter paper in order to remove the catalyst. A 1 mL sample of the centrifugate was

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TABLE 1: Band Gap, Elemental Analysis, and Surface Properties of N, C Codoped ZnS Photocatalysts Prepared at Various Decomposition Temperatures dec temp calcd band % of % of % of BET surface pore pore vol (°C) gap (eV) N C S area (m2/g) size (Å) (cm3/g) 300 400 500

2.18 1.95 2.60

8.28 4.67 23.36 14.20 8.21 21.12 8.24 4.58 25.05

27.3 17.7 48.1

34.6 86.4 64.8

0.023 0.038 0.078

diluted to 5 mL and its absorbance at 486 nm for AO7 was measured. The absorbance at 486 nm is due to the color of the dye solution and it is used to monitor the decolorization of the dye. During the illumination time, no volatility of the solvent was observed. The decolorization of AO7 was determined by measuring the change in absorbance of AO7 during irradiation, using a Shimadzu MPS-2000 spectrophotometer. The X-ray diffraction (XRD) patterns were recorded with a X’ Pert PRO PAN analytical diffractometer, with a scanned angle from 10° to 100°. High-resolution transmission electron microscope (HR-TEM) images were recorded with a JEOL JEML-2010. Samples for HR-TEM were prepared by ultrasonically dispersing the catalyst into ethanol, then placing a drop of this suspension onto a copper grid and then drying in air. The working voltage of TEM was 200 kV. The morphology of the catalyst was examined with a Hitachi S-4100 scanning electron microscope (SEM). Prior to SEM measurements, the samples were mounted on a carbon platform that was then coated by platinum by using a magnetron sputter. The plate containing the sample was then placed in SEM for the analysis with the desired magnifications. Elemental analysis was carried out with a Vario EL III CHNOS elemental analyzer (Germany). The X-ray photoelectron spectra were collected on an ESCA1000 X-ray photoelectron spectrometer (XPS), using a Mg KR X-ray as the excitation source. UV-visible diffuse reflectance spectra were recorded with a Shimadzu MPS-2000 spectrophotometer and barium sulfate was used as a standard. Results and Discussion Initially, zinc isothiocyanate complex thermal decompositions have been carried out at 300, 400, and 500 °C under open air or nitrogen atmospheric conditions and hereafter denoted as S-1, S-2, and S-3 catalysts, respectively. The decomposition at both conditions results in N, C codoped ZnS and its XRD patterns and morphology (Figure 1f) are quite similar, confirming the suitability of air atmospheric conditions for the N, C codoped ZnS preparation, and therefore all decompositions have been done under open atmospheric conditions. The synthesized catalysts were characterized by using XRD, FESEM, TEM, and other required analytical techniques. Initially, catalyst synthesis was investigated under various experimental conditions by monitoring the morphology of the catalysts. Thermal decomposition of zinc isothiocyante complex at 300, 400, and 500 °C for 2 h in atmospheric conditions results in hierarchical porous microspheres and their sizes rang from 0.1 to 3 µm as shown in Figure 1, panels a, b, and c, respectively. Hierarchical porous microsphere formation could not facilitate if we use zinc nitrate and zinc sulfate as a zinc source, which results in porous aggregates. Though zinc acetate facilitates hierarchical porous microspheres, the catalysts appear to be black powder. Surface analysis results showed that N, C codoped ZnS photocatalysts prepared at three different temperatures have different surface properties as shown in Table 1. While keeping the total volume of the solvent mixture (100 mL), changing the water-ethanol volume ratios did not influence the morphology of the catalysts.

However, without ethanol the microsphere formed in aqueous solvent at 300 and 400 °C were less in hierarchical porous structure and seems to be facilitating microsphere aggregation. Nevertheless, microspheres formed at 500 °C were similar to one formed in the presence of ethanol cosolvent as shown in Figure 1d. Obviously, changing the initial pH of the solvent mixture and the final solvent evaporation mode did not influence the morphology of the catalysts (Figure 1e). The zinc isothiocyanate complexes have been prepared in three different methods and the decomposition results show similar morphology, suggesting that the preparation method of the above complexes did not influence the morphology of the catalysts. Similarly, while keeping constant the volume (100 mL) of water-ethanol solvents and Zn2+:SCN- ratios at 1:2, changing the amount of initial reactants could not influence the morphology of the catalysts. The aforementioned discussions clearly show that the catalysts can be prepared under wide experimental conditions which open new vistas for the large-scale and industrially applicable preparation. The XRD results showed that the final residue was completely decomposed to give ZnS in 30 min as shown in Figure 2. All XRD peaks belong to the hexagonal wurtzite type, indicating the formation of wurtzite ZnS nanocrystallites. The XRD peaks of S-1 and S-2 catalysts can be indexed to hexagonal wurtzite2H ZnS with the lattice parameters of a ) 3.82 Å and c ) 6.25 Å [S. G. P63mc (186)] (JCPDS 36-1450). The (002) peak intensity increased over that of a (100) peak when the sample was prepared at 500 °C, which belongs to wurtzite-6H ZnS. This is due to change in the stacking sequence of the closepacked planes of the ZnS crystal. The XRD patterns of the S-1 and S-2 catalysts show that the (100) peak has the highest intensity and the wurtzite (100), (002), and (101) peaks strongly overlap, which is in contrast to the three well-resolved peaks shown in the S-3 catalyst, implying the formation of wellcrystallized ZnS. Decomposition under an N2 atmosphere also yields wurtzite ZnS. This implies that the atmospheric conditions have not influenced the crystal structure. All three catalysts show a yellow-to-orange color, suggesting their ability to absorb light in the visible region. The energydispersive X-ray microanalysis (EDX) demonstrates that the crystal consists of Zn, S, N, and C elements as shown in Figure 3a. The quantitative analysis of the Zn:S molar ratio is nearly equal to 1:1, which is consistent with stoichiometric ZnS, and therefore it would be suitable to write N, C codoped ZnS photocatalyst instead of writing a ZnS2-x,yNxCy photocatalyst. However, we were not able to get consistent results on the nitrogen and carbon quantitative estimation in the EDX analysis and therefore we used the elemental analysis to estimate the nitrogen and carbon contents. The elemental analysis results indicated that nitrogen and carbon codoped ZnS is formed, which is shown in Table 1. UV-visible diffuse reflectance spectra showed that nitrogen and carbon codoping shifts the absorption wavelength from the UV region to the visible region as shown in Figure 4. The shape of the diffuse reflectance spectrum of N, C codoped ZnS indicates that the codoped nitrogen and carbon form a new energy level in the band structure of ZnS. The onset absorption wavelength and corresponding band gap of S-1, S-2, and S-3 catalysts were found to be 568 (2.18 eV), 634 (1.95 eV), and 476 nm (2.60 eV), respectively. The above result suggests that the incorporation of both N and C impurities is responsible for the visible light photoresponse of the synthesized ZnS photocatalyst. FTIR spectra of the three synthesized catalysts have shown similar absorption at wide range and the intensity of the

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Figure 2. XRD spectra of N, C codoped ZnS photocatalyst (a) prepared at various temperatures under atmospheric conditions at (A) 500, (B) 400, and (C) 300 °C for 2 h, (b) prepared under nitrogen atmospheric conditions at various temperatures at (A) 500, (B) 400, and (C) 300 °C for 2 h, and (c) prepared at various decomposition times at 300 °C under atmospheric conditions.

absorption was different among the three catalysts as shown in Figure 3b. However, slight absorption shifts were noted in some regions. The strong absorption at 1624 and 795 cm-1 can be attributed to CdN and CdS stretching vibrations, respectively.8,9 Generally, the CN stretching vibrations appeared in the range of 2000-2200 (CtN) and 1100-1300 cm-1 (CsN), and the weak absorption in the above region of the three catalysts confirmed the presence of CN bonds.10 The absorption bands

J. Phys. Chem. C, Vol. 113, No. 36, 2009 16147 around 1400-1500 cm-1 can be assigned to NsCsN and CdS stretching vibrations, and strong absorption peaks appearing in this region indicated the presence of NsCsN and or CdS bonds.11 Moreover, the NsCsN stretching vibration can also be noted in the 1100-1300 cm-1 region and the strong absorption suggests the presence of NsCsN type bonding in the catalysts. The absence of bending vibration absorption peaks at 445.61 and 469.65 cm-1 confirms the absence of SCN type bonding in the catalysts.11 The aforementioned FTIR data could provide useful information for understanding the bonding nature of ZnS with doped N and C elements. XPS studies also confirmed the presence of both nitrogen and carbon in ZnS catalysts prepared at three different temperatures. The N 1s and C 1s spectra are deconvoluted into various bands, with each associated with a different binding energy as shown in Figure S1 in the Supporting Information. Due to the large number of figures, deconvoluted XPS pictures are presented in the Supporting Information as well. An N 1s XPS peak found at around 400.6 eV in all three samples (Figure 5a) can be attributed to CdN bonds, which is consistent with FTIR results and literature reports assigned for the CdN bond.12,13 However, the deconvolution analysis of the N1s peak indicated additional two peaks which appeared at around 399.3 and 402.2 eV in the S-1 and S-2 catalysts. Moreover, in addition to the above-mentioned three peaks one more peak appeared at 397.0 eV in the S-3 catalyst. The peak at 399.3 eV could be attributed to ZnsN and/or CtN bonds which may usually appear at this position.13 The N 1s peak of the ZnsN bond of Zn3N2 and ZnO1-xNx materials was found to be at 396.2 and 399.1 eV, respectively. This difference in the N 1s binding energy was due to more positive charge of N in the later than the former.14,15 From the aforementioned discussion it is concluded that the N 1s binding energy spectra can change in the local chemical and structural environments. Therefore we believe that some chemical shifts of the peak position appear when compared to other materials used for comparison. This is because of surface strain and lattice distortion induced by the incorporation of nitrogen and carbon.16 To the best of our knowledge, the N 1s peak of the ZnsN bond in ZnS1-xNx materials has not been reported so far. Therefore it is not possible to compare the results with those of our N, C codoped catalysts. Therefore it is reasonable to assign the peak at 399.3 eV to the ZnsN bonding in our catalysts. The peak appeared at 402.2 eV, which is suspected to arise from an oxidized form of nitrogen or molecularly chemisorbed nitrogen (γ-N2), and the one at 297 eV arises from molecular nitrogen (R-N2) or atomic nitrogen (β-N).14,17 The C 1s XPS spectra showed two peaks at around 286 and 290.5 eV. However, the deconvolution analysis of the C 1s spectrum showed additional peaks at 283.7 and 285.0 eV in the S-1 catalyst. Similarly, in the S-2 catalyst peaks at 287.6, 289.0, and 291.8 eV and in the S-3 catalyst peaks at 287.5, 289.4, 292.5, and 288.8 eV were noted. Generally, the contribution of CsN, CdN, and CtN peaks will appear from 285.6 to 287.6 eV.13 All three catalysts showed characteristic peaks in this particular area. Therefore the peaks at 286.0 and 287.5 eV can be attributed to CsN and CtN bonds, respectively. However, the peaks appearing from 289.4 to 292.5 eV can be suspected to be due to CsO and OdCsO bonds which could exist as impurities. However, the CsO and OdCsO bonds are not chromophores and are not responsible for the visible-light photocatalytic activity.18 There is no clear evidence for the presence of ZnsC and the elemental carbon (CsC) bond which usually appeared from 280 to 284 eV except in the S-3 catalyst where it appeared at 283.7 eV.19 XPS peaks

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Figure 3. (a) EDX analysis and (b) FTIR spectra of N, C codoped ZnS photocatalyst.

of Zn 2p and S 2p in the three catalysts are similar to that of pure ZnS. After argon sputtering for 10 s, no obvious shift was observed in N 1s, S 2p, and Zn 2p peaks, implying good stability of the catalysts and dopants. However, all the catalysts have highly structured surfaces and therefore the roughness will make it difficult to clean the surface by ion etching. Hence we do not present XPS results after etching the catalysts. From the XPS studies it is concluded that the doping of carbon and nitrogen is substitutional doping and also there is a possibility of interstitial doping since the concentrations of dopants are relatively higher in the catalysts. From the XPS results, EDX analysis of the Zn:S ratio and the N:C ratio in the elemental analysis suggests the possibility of Zn-N-C-N type linkage. Both the Zn and S atoms in the wurtzite are four-coordinated and the structure is composed of alternating planes of four coordinated S2- and Zn2+ ions.20 Therefore we speculate that the N-C-N linkage could bond with zinc and/or sulfur atom in the same plane or two adjacent planes and could also exist in the interface of the planes, which may open a door for the possibility that N and C induce a higher energy band contributed by the localized N 2p and C 2p states. The amount of nitrogen and carbon is relatively higher in the synthesized catalysts, which may strongly reduce the band gap of ZnS from 3.6 to 1.95 eV.

Figure 4. Diffuse reflectance spectra of (1) pure commercial ZnS, (2) ZnS prepared at 300 °C, (3) ZnS prepared at 400 °C, and (4) ZnS prepared at 500 °C; the inset picture shows the corresponding catalysts.

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Figure 5. (a and a1) HR-TEM lattice fringes of as-synthesized ZnS and (b) SAED pattern of ZnS prepared at 300 °C.

Theoretical studies also confirmed the formation of Nimpurity bands in ZnS at the top of the valence band due to the overlap of the N-acceptor orbital.21 Tang et al. reported surface defects (zinc and sulfur vacancy)-induced donor and acceptor levels which reduce the band gap of ZnS.22 The abovementioned example also confirms the possibility of mid band gap donor-acceptor band formation in ZnS. We could not find any nonmetal-doped sulfide semiconductor photocatalyst to support our band gap reduction induced by the N, C codoping. However, many studies have been done on TiO2 photocatalsyts. Both substitutional and interstitial nitrogen impurities in the matrix of anatase TiO2 were shown to induce the formation of localized states which lie above the valence band.23 The formation of N, C codoped hierarchical porous microspheres could be explained by the self-assembled mechanism. The thermal decomposition of the zinc isothiocyanate complex at the desired temperature results in weakening of the coordination of the Zn-NCS complex and Zn2+ will be released gradually. On the other hand, due to thermal effect the CdS and CdN double bonds of thiocyanate will be broken to release S2- anions while at the same time various nitrogen and carbon species could also be generated during decomposition. Then the active S2reacts with Zn2+ to generate the ZnS nuclei. However, the zinc ion could also bond with nitrogen species, which is due to higher affinity with nitrogen. The aforementioned speculation could also be supported by an earlier observation reported by Martianez et al. They found the preferential bonding of zinc to N- or S-containing functional groups, which provide higher affinity sites than O-containing functional groups.24 Therefore it is expected that nitrogen and carbon could also be incorporated into ZnS during the crystal growth. Due to the high surface energy of the ZnS particles it can be aggregated or selfassembled in order to minimize its surface energy, which results in flexible thin nanosheets. These nanosheets can easily form a folded structure, yielding the hierarchical morphology25 as

Figure 6. Photocatalytic decomposition of AO7 under visible-light irradiation. [AO7] ) 2.5 mg/L, [catalyst] ) 1 g/L. (1) Commercial pure ZnS and N, C codoped ZnS photocatalysts prepared at (2) 300, (3) 400, and (4) 500 °C.

shown in HR-TEM images in Figure 5a,a1. The microsphere sizes ranging from 0.1 to 3 µm and the lattice fringes are clear even with the irregular folding of the nanosheets. The SAED analysis (Figure 5b) shows a ring pattern, confirming the polycrystalline nature of the N, C codoped ZnS photocatalyst. The nitrogen adsorption analysis showed that the pore size and the pore volume of the S-2 and S-3 catalysts are higher than the S-1 catalyst, which may influence the adsorption and photocatalytic activity of the catalysts. The surface area of the S-2 catalyst is lower than those of the other two catalysts, which may be due to the presence of a higher concentration of the dopant which could reduce the surface area of the catalyst. The photocatalytic activity of N, C codoped ZnS catalysts under visible light irradiation (λ g 400 nm) was investigated with Acid Orange 7 (AO7) as a model pollutant. Figure 6 depicts AO7 absorbance ratios (A/A0) against irradiation time. Here, A and A0 denote absorbance after irradiation time and at initial time, respectively. Initial experiments showed that AO7 dye degradation was not facilitated (i) without visible light irradiation

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for all synthesized catalysts, (ii) for commercially available undoped pure ZnS under visible light irradiation, which confirms the absence of sensitized degradation of AO7 dye, and (iii) without catalysts, which also confirms the stability of dye under visible light irradiation. Under similar experimental conditions N, C codoped ZnS photocatalyst showed a significant AO7 degradation under visible light irradiation, demonstrating that the synthesized materials are effective visible light-responsive photocatalysts. The adsorption and degradation efficiency of these photocatalysts were found to be in the following order S-3 > S-2 > S-1. The higher adsorption and photocatalytic activity was noted in the S-3 catalyst, which may be due to its higher surface area, pore volume, and higher degree of crystalline nature when compared to the other two catalysts. Though the surface area of the S-2 catalyst is less than that of the S-1 catalyst, the photocatalytic degradation efficiency of the S-2 catalyst is higher than that of the S-1 catalyst, which may be due to its higher pore volume. However, it is very hard to compare the photocatalytic efficiency of all three synthesized catalysts and make a general conclusion for higher photocatalytic activity. The above results were reproduced by using repeated preparations starting with fresh batches of precursors. Conclusions In summary, effective visible light-active hierarchical porous N, C codoped ZnS photocatalysts have been successfully synthesized for the first time by using a reproducible method. The synthesized catalyst belongs to the wurtzite structure and has strong visible light absorption. The FTIR and XPS analysis revealed nitrogen and carbon bonding in the catalysts. The surface properties of the catalyst and the calcination temperature control the photocatalytic activity. All three catalysts were found to be effective in photocatalytic degradation of AO7 under visible light irradiation. Moreover, this synthesis method may be economical for a large-scale process, and offers a new material platform for an environmental remediation process. Acknowledgment. We gratefully acknowledge the Japan Society for the Promotion of Science (JSPS) for the postdoctoral fellowship award to one of the authors (M.M.). This work was partly supported by Grant-in-Aid for scientific research (B) (No. 19360367) and Grant-in-Aid for JSPS Fellows (No.19.07413). Supporting Information Available: XPS spectra of N, C codoped ZnS photocatalysts prepared at various decomposition

Muruganandham and Kusumoto temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (b) Murase, T.; Irie, H.; Hashimoto, K. J. Phys. Chem. B 2004, 108, 15803. (2) (a) Hamanoi, O.; Kudo, A. Chem. Lett. 2002, 31, 838. (b) Kudo, A.; Sekizawa, M. Chem. Commun. 2000, 15, 1371. (c) Arai, T.; Senda, S. I.; Sato, Y.; Takahashi, H.; Shinoda, K.; Jeyadevan, B.; Tohji, K. Chem. Mater. 2008, 20, 1997. (3) (a) Reber, J. F.; Rusek, M. J. Phys. Chem. 1986, 90, 824. (b) Youn, H. C.; Baral, S.; Fendler, J. J. Phys. Chem. 1988, 92, 6320. (c) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. J. Am. Chem. Soc. 2004, 126, 13406. (4) (a) Botterhuis, N. E.; Sun, Q. Y.; Magusin, P.; van Sante´, A.; Sommerdijk, N. Chem.sEur. J. 2006, 12, 1448. (b) Wang, H. N.; Wang, Y. H.; Zhou, X. F.; Zhou, L.; Tang, J. W.; Lei, J.; Yu, C. Z. AdV. Funct. Mater. 2007, 17, 613. (5) (a) Lu, F.; Cai, W.; Zhang, Y. AdV. Funct. Mater. 2008, 18, 1. (b) Yu, J. G.; Su, Y. R.; Cheng, B. AdV. Mater. 2007, 17, 1984. (6) (a) Bahta, A.; Parker, G.-A.; Tuck, D.-G. Pure Appl. Chem. 1997, 69, 1489. (b) Shi, Y.; Chen, J.; Shen, P. J. Alloys Compd. 2007, 441, 337. (7) Yamaguchi, T.; Yamamoto, K.; Ohtaki, H. Bull. Chem. Soc. Jpn. 1985, 58, 3235. (8) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: New York, 1991. (9) Agarwal, R. K.; Prakash, B.; Kumar, V.; Aslam Khan, A. J. Iran. Chem. Soc. 2007, 4, 114. (10) Singh, P. P.; Sharma, S. B. Can. J. Chem. 1976, 54, 1563. (11) Jayalakshmi, D. Kumar. J. Cryst. Res. Technol. 2006, 37, 41. (12) Chen, G. L.; Li, Y.; Lin, J.; Huan, C. H.; Guo, Y. P. J. Phys. D: Appl. Phys. 1999, 32, 195. (13) Cheng, Y. H.; Tay, B. K.; Lau, S. P.; Shi, X.; Chua, H. C.; Qiao, X. L.; Chen, J. G.; Wu, Y. P.; Xie, C. S. Diamond Relat. Mater. 2000, 9, 2010. (14) Tabet, N.; Faiz, M.; Al-Oteibi, A. J. Electron Spectroc. Relat. Phenom. 2008, 163, 15. (15) Mapa, M.; Gopinath, C. S. Chem. Mater. 2009, 21, 351. (16) Yang, J.; Bai, H.; Tan, X.; Lian, J. Appl. Surf. Sci. 2006, 253, 1988. (17) Chen, L.-C.; Tu, Y.-J.; Wang, Y.-S.; Kan, R.-S.; Huang, C.-M. J. Photochem. Photobiol. A 2008, 199, 170. (18) Tseng, Y.-H.; Kuo, C.-S.; Huang, C.-H.; Li, Y.-H.; Chou, P.-W.; Cheng, C.-L.; Wong, M.-S. Nanotechnology 2006, 17, 2490. (19) Pan, H.; Yi, J. B.; Shen, L.; Wu, R. Q.; Yang, J. H.; Lin, J. Y.; Feng, Y. P.; Ding, J.; Van, L. H.; Yin, J. H. Phys. ReV. Lett. 2007, 99, 127201. (20) Moore, D.; Wang, Z. L. J. Mater. Chem. 2006, 16, 3898. (21) Yamamoto, T. Jpn. J. Appl. Phys. 2003, 42, 514. (22) Tang, H.; Xu, G.; Weng, L.; Pan, L.; Wang, L. Acta Mater. 2004, 52, 1489. (23) Di Valentin, C.; Pacchioni, G.; Selloni, A. Chem. Mater. 2005, 17, 6656. (24) Martianez, C.; Bazilevsaya, K.; Lanzirotti, A. EnViron. Sci. Technol. 2006, 40, 5688. (25) Zhang, H.; Qi, L. Nanotechnology 2006, 17, 3984.

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