Dopant Effects on the Photocatalytic Activity of Colloidal Zinc Sulfide

Jan 4, 2010 - Department of Chemistry, Lebanon Valley College, 101 N. College AVenue, AnnVille, PennsylVania 17003. ReceiVed: July 24, 2009; ReVised ...
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Dopant Effects on the Photocatalytic Activity of Colloidal Zinc Sulfide Semiconductor Nanocrystals for the Oxidation of 2-Chlorophenol Michael W. Porambo, Heather R. Howard, and Anderson L. Marsh* Department of Chemistry, Lebanon Valley College, 101 N. College AVenue, AnnVille, PennsylVania 17003 ReceiVed: July 24, 2009; ReVised Manuscript ReceiVed: December 7, 2009

In this work, colloidal zinc sulfide nanocrystals, prepared with selected concentrations of manganese(II) ions in the range of 0-2% and stabilized with a polymeric capping agent, were used in the photocatalytic degradation of 2-chlorophenol in aqueous solutions. Particle sizes for the synthesized nanocrystals were determined using UV-vis absorbance spectroscopy and X-ray diffraction (XRD). UV-vis spectra for all of the synthesized nanocrystals display an excitonic peak at around 290 nm, which corresponds to a particle size of approximately 3 nm, in agreement with the XRD analysis. Photoluminescence emission spectra recorded for undoped ZnS nanocrystals exhibit an emission peak at 460 nm in the spectrum, whereas peaks corresponding to the dopant ion are red-shifted to 590 nm. With increasing addition of the dopant ion, the area of the peak at 460 nm first increases and then decreases for higher dopant concentrations, whereas the area of the peak at 590 nm increases. This finding clearly indicates that the presence of the dopant ion alters the electronic states of the nanocrystals, which, in turn, affects the photocatalytic properties of the colloidal nanocrystals. Apparent rate constants for the photodegradation of 2-chlorophenol in aqueous solution utilizing undoped and doped colloidal ZnS nanocrystals were calculated using pseudo-first-order kinetic analysis. As the dopant concentration used during synthesis was increased, the apparent rate constants increased initially and then decreased. Results from kinetic experiments performed after nitrogen purging showed a decrease in the apparent rate constant, suggesting that the reaction mechanism involves a reactive oxygen species such as superoxide. Taken together, these results provide further insight into the effects of dopants on the activity of colloidal semiconductor nanocrystals employed as photocatalysts in environmental remediation applications. 1. Introduction Chlorinated phenols released into aqueous systems in the environment continue to be a pollution concern because of their high toxicity and stability, and for this reason, these compounds are listed as priority pollutants by the U.S. Environmental Protection Agency.1 They find their way into the environment primarily through their use as preservative agents and disinfectants, as well as in the manufacturing of various chemical commodities such as insecticides and pharmaceuticals. Because of health concerns related to exposure to chlorinated phenols, it is essential that these compounds be removed during waste processing or from the environment if they are already present. There is a growing concern about the proper methods to be used when eliminating these toxic organic substances from aqueous systems. Hence, the removal of these contaminants by various processes remains an active area of current research. Photocatalytic methods have been widely used to decompose toxic organic compounds in aqueous environments, with ZnS being employed as a photocatalytic material in some instances.2 Nanocrystalline zinc sulfide has been shown to be active for a number of different photocatalytic reactions.3-14 In particular, ZnS nanocrystals capped with amino acids have been tested as photocatalysts for the removal of 4-nitrophenol from aqueous environments.4,5 Reaction conditions that were varied included substrate-to-nanocrystal ratio, solution pH, capping agent, and sulfide-to-zinc ratio during nanocrystal synthesis. Nanocrystalline ZnS prepared in N,N-dimethylformamide has also been utilized for the dehalogenation of halogenated benzene deriva* Corresponding author. Phone: 1-717-867-6149. E-mail: [email protected].

tives, with benzene being the final product.6 More recently, the effects of manganese, nickel, and copper dopants have been investigated for the photodegradation of organic dyes using undoped and doped zinc sulfide nanocrystals capped with mercaptoethanol.11 In this study, the dopant-to-zinc ratio was varied to find the stoichiometry of each doped photocatalyst that exhibited the maximum degradation efficiency, and the pH of the solution was changed to find the optimum operating conditions. Certain stoichiometries of each doped ZnS photocatalyst displayed higher degradation rate constants than nanocrystalline ZnS, suggesting that synthetic conditions can be used to tune photocatalytic behavior. Extensive work has been carried out on the synthesis and characterization of doped colloidal zinc sulfide semiconductor nanocrystals with selected optoelectronic properties.15 A range of ZnS nanocrystals doped with different transition-metal and rare-earth-metal ions have been prepared using a variety of synthetic methods, resulting in well-defined optoelectronic properties. For example, with the Mn2+ ion as a dopant, the photoluminescence of the nanocrystal can be red-shifted from the blue region of the visible spectrum to the orange region.16-21 In addition to dopants, capping agents with organic functional groups have also been shown to modify photoluminescent properties. For example, enhanced photoluminescence has been reported for doped ZnS nanocrystals capped with poly(vinylpyrrolidone) (PVP), a water-soluble polymer.22 The authors suggested that efficient energy transfer occurs between the polymer functional group adsorbed at the surface and the dopant centers in the nanocrystal. This result indicates that PVP might prove to be a suitable capping agent for semiconductor nanocrystals,

10.1021/jp907061d  2010 American Chemical Society Published on Web 01/04/2010

Dopant Effects on the Photocatalytic Activity of ZnS

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particularly those targeted for applications such as photocatalysis in aqueous systems. Synthetic methods for producing PVPcapped, Mn-doped ZnS nanocrystals have recently been reported.23,24 To better understand the synthetic conditions needed to prepare these PVP-capped nanocrystals with selected photocatalytic properties for 2-chlorophenol oxidation, experiments systematically varying the dopant concentration present during synthesis must be performed. In this work, we report on the synthesis of colloidal zinc sulfide semiconductor nanocrystals capped with PVP and prepared using 0-2% concentrations of Mn2+ ion as a dopant, as well as their subsequent utilization in the photocatalytic removal of 2-chlorophenol from aqueous solution. Particle size determinations using excitonic peak positions in UV-vis absorption spectra, as well as peak positions and peak widths from X-ray diffraction (XRD) patterns, indicate that all synthesized nanocrystals were in the 2-3-nm size range. Regarding photoluminescent properties, undoped colloidal ZnS nanocrystals exhibit an emission peak centered at around 460 nm, whereas doped colloidal ZnS nanocrystals display a strong peak that is red-shifted to a higher wavelength, with this peak centered at around 590 nm increasing in intensity with increasing concentration of the Mn2+ dopant ion present during synthesis. These results signify that PVP-capped zinc sulfide semiconductor nanocrystals can be synthesized with various Mn dopant compositions that modify their optoelectronic properties, without changing the particle sizes of the synthesized nanocrystals. The photocatalytic oxidation of 2-chlorophenol (2-CP) was used to gain further insight into the mechanism by which these colloidal semiconductor nanocrystals act as photocatalysts. Apparent pseudo-first-order rate constants measured for the reaction were found to increase initially with increasing Mn dopant concentration but then decrease for higher dopant concentrations. Based on this decrease in the apparent rate constant, coupled with a decrease upon nitrogen purging of the reaction system, it is suggested that oxidation occurs through a pathway involving superoxide. These findings were compared with those in the literature to develop a molecular-level mechanism by which colloidal ZnS semiconductor nanocrystals act as photocatalysts in aqueous systems.

The synthesis of PVP-capped colloidal ZnS nanocrystals was carried out based on a previously established wet chemical procedure.24 First, aqueous solutions of 1.0 M Zn(C2H3O2)2, 0.85 M Na2S, and 0.010 M Mn(C2H3O2)2 were prepared in 18.2 MΩ deionized water. Next, 0.56 g of PVP was dissolved in 5 mL of the zinc acetate solution, resulting in a PVP concentration of 1.0 M based on the monomer unit. To this solution was added a selected volume of the manganese acetate solution, corresponding to the desired dopant concentration. Then, 5 mL of the sodium sulfide solution was added slowly with stirring, leading to precipitation of the nanocrystals. An aliquot of the nanocrystal solution was then centrifuged at low speed for 10 min, after which it was washed with deionized water. The centrifugation was repeated two more times, with a wash in between. After the final centrifugation, the nanocrystals were redispered in water to make a 0.01 M (based on ZnS units) solution. For reaction studies, 0.2 mL of the ZnS solution was placed in a 1-cm-path-length UV-vis cuvette. Next, 0.2 mL of a 1 mM aqueous solution of 2-CP was added to this solution, giving a 10:1 ZnS/2-CP mole ratio. The ZnS/2-CP reaction solution was placed in a dark box and irradiated with 365-nm light from a low-intensity lamp. Absorbance spectra of the reaction mixture were recorded every 3 min for 30 min, with agitation also being performed to ensure a uniform dispersion. The concentration of 2-CP was tracked by monitoring the absorbance at a wavelength of 273 nm. Because the ZnS nanocrystals were still suspended in the reaction solution during analysis, the absorbance measured at 273 nm was the sum of the absorbance due to 2-CP and that due to the nanocrystals. The absorbance at 273 nm at time t, A273 nm,t, subtracted from the absorbance at time 0, A273 nm,0, gives

A273 nm,0 - A273 nm,t ) (Anc,0 + A2-CP,0) - (Anc,t + A2-CP,t) (2) Assuming that the concentration of nanocrystals does not change over the course of the reaction, Anc remains constant, yielding the expression

A273 nm,0 - A273 nm,t ) A2-CP,0 - A2-CP,t

2. Experimental Section Zinc acetate dihydrate, sodium sulfide nonahydrate, manganese(II) acetate tetrahydrate, poly(vinylpyrrolidone) (MW ) 29000), and 2-chlorophenol were purchased from Sigma-Aldrich and used without further purification. UV-vis absorbance spectra were recorded on a Thermo-Electron Evolution 300 rapid scanning UV-vis spectrophotometer. Photoluminescence spectra were acquired on a Shimadzu RF-5301PC spectrofluorophotometer. X-ray diffraction patterns were collected using a Phillips 3520 X-ray diffraction system operating with a monochromator and Cu KR radiation. Samples for XRD analysis were prepared by casting thin films onto quartz microscope slides. After carefully accounting for contributions from the substrate, peak positions and peak widths were used to calculate particle sizes through the Debye-Scherrer equation

d)

Kλ β cos θ

(1)

where d is the particle size, K is the shape factor (0.9), λ is the wavelength of the Cu X-rays (0.154 nm), β is the full width at half-maximum of the peak in radians, and θ is the position of the peak.

(3)

The Beer-Lambert law, A ) εbc, where b is the path length, c is the concentration, and ε is the molar absorptivity, can then be used to convert A2-CP into the concentration of 2-chlorophenol, [2-CP]. Rearranging eq 3 to solve for A2-CP,t and subsequently employing the Beer-Lambert law gives

[2-CP]t ) [2-CP]0 -

A273 nm,0 - A273 nm,t εb

(4)

It is this expression for [2-CP]t that was then used to construct pseudo-first-order plots to determine apparent rate constants. For the calculations, the initial concentration of 2-CP was 5 × 10-4 M, the path length was taken to be 1 cm, and the molar absorptivity of 2-chlorophenol at 273 nm was determined experimentally to be 1780 M-1 cm-1. 3. Results Colloidal zinc sulfide nanocrystals capped with PVP were synthesized using selected concentrations of Mn2+ ions as dopants and were then characterized using UV-vis absorption

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Figure 1. UV-vis absorbance spectrum of undoped ZnS nanocrystals.

Figure 2. XRD pattern of undoped ZnS nanocrystals.

spectroscopy to determine nanocrystal particle sizes. The UV-vis absorbance spectrum of freshly prepared, undoped colloidal ZnS nanocrystals capped by PVP is shown in Figure 1. As can be seen in the figure, the spectrum exhibits an excitonic peak centered at around 290 nm. Using the Brus equation,25 this wavelength was used to calculate an approximate particle size of 3 nm for these nanocrystals. UV-vis absorbance spectra were also recorded for all of the doped ZnS nanocrystals, and all doped samples exhibited an excitonic peak at approximately the same wavelength as observed for the undoped sample, indicating that, at these concentrations, doping does not affect the size of the nanocrystals. Particle sizes were also determined by XRD analysis. Displayed in Figure 2 is an XRD pattern of the synthesized undoped nanocrystals. As can be observed in the figure, a strong peak centered at around 2θ ) 29.6° appears for the (111) reflection. A smaller peak corresponding to the (220) reflection is located at around 2θ ) 47.5°, likely indicating that the ZnS nanocrystals have a cubic structure.18,21,23 For all synthesized nanocrystals, undoped and doped, calculations using the (111) reflection revealed that the particle sizes were in the 2-3-nm size range. By considering these results in conjunction with those from UV-vis spectroscopy, one can discern the effect of doping on the size of the nanocrystals. Because of the relative similarity in size of the Mn2+ dopant ions to Zn2+, as well as the small concentrations used during the nanocrystal synthesis, there is little to no change in the lattice size within the nanocrystal. As a result, the size of

Porambo et al.

Figure 3. Photoluminescence of ZnS nanocrystals prepared with selected percentages of Mn dopant.

the nanocrystal stays relatively constant over the range of dopant concentrations used in this work. In addition to particle size characterization, the photoluminescent properties of these colloidal ZnS semiconductor nanocrystals capped with PVP were also investigated using photoluminescence emission spectroscopy. The photoluminescence emission spectrum of undoped nanocrystals is displayed in Figure 3. For undoped colloidal ZnS nanocrystals excited by light at a wavelength of 310 nm, a broad photoluminescence emission peak centered at around 460 nm was observed in the spectrum. The addition of Mn2+ dopant ions in the ZnS crystal lattice altered the photoluminescent emission properties of the nanocrystals, however. Photoluminescence emission spectra for Mn-doped colloidal ZnS nanocrystals capped by PVP are also included in Figure 3. As can be observed in the figure, the presence of the Mn2+ dopant results in a very intense emission peak centered at around 590 nm, while still retaining the photoluminescence emission peak of the undoped colloidal ZnS nanocrystals at 460 nm. This finding is consistent with those reported previously for Mn-doped ZnS nanocrystals, with the emission centered at around 590 nm proposed to be due to the 4 T1-6A1 transition in Mn2+.17-19,24 The spectra show that, as the concentration of Mn2+ ions present during synthesis is increased, the intensity of the 590-nm peak is increased. For PVP-capped ZnS nanocrystals doped with Mn in much larger concentrations (10-40%), no blue emission was observed in the spectrum.23 In the current work, however, the blue emission (peak centered at around 460 nm) was still present because of the low concentration of dopant used. To study the effects of the dopants on photocatalytic activity, these undoped and doped colloidal semiconductor nanocrystals were then utilized in the degradation of 2-chlorophenol in aqueous solution. For kinetics experiments involving these colloidal ZnS nanocrystals, the decrease of 2-chlorophenol concentration over time was monitored using the procedure outlined in the Experimental Section. Using a pseudo-first-order kinetic analysis, apparent rate constants for the degradation of 2-chlorophenol in aqueous solution were determined for reactions employing as photocatalysts colloidal ZnS nanocrystals prepared with selected Mn2+ ion concentrations present during the synthesis. Shown in Figure 4 are plots of ln([2-CP]t/[2-CP]0) versus time for selected concentrations of Mn dopant used in the synthesis of the colloidal ZnS nanocrystals. Solid lines are best fits from a regression analysis, and the opposite of the slope of each line equals the apparent rate constant, kapp. The exact kapp values,

Dopant Effects on the Photocatalytic Activity of ZnS

Figure 4. Pseudo-first-order plot showing ln([2-CP]t/[2-CP]0) versus time for 2-chlorophenol oxidation carried out in the presence of ZnS nanocrystals prepared using selected concentrations of Mn: (b) 0%, (9) 0.1%, (2) 0.4%, (1) 1%, and (() 2%.

TABLE 1: Apparent Pseudo-First-Order Rate Constants Obtained for Reactions Performed Using ZnS Nanocrystals Prepared with Selected Mn Dopant Concentrations Mn concentration (%)

kapp (min-1)

0 0.1 0.4 1 2

0.016 ( 0.001 0.021 ( 0.004 0.031 ( 0.001 0.013 ( 0.001 0.0086 ( 0.0018

along with errors reported at the 95% confidence interval, can be found in Table 1. As can be seen in the figure and table, the apparent rate constant is dependent on the concentration of Mn2+ ions used during the synthesis of the nanocrystals. The highest kapp value occurred for a Mn concentration of 0.4% (0.031 ( 0.001 min-1), whereas the lowest kapp value was found for a concentration of 2% (0.0086 ( 0.0018 min-1). Although the errors reported for rate constants obtained using the 0.1% and 2% Mn-doped nanocrystals are on the higher side, the trend observed in this work and discussed below still falls within the range of the reported experimental errors. The differences in observed apparent rate constants might be related to the varying photoluminescent emission properties resulting from the introduction of the Mn dopant during synthesis. To elucidate the effects of the presence of the dopant on photocatalytic activity, in the following section, these findings are discussed and compared to known results in the literature for the photocatalytic degradation of 2-chlorophenol in aqueous solutions. 4. Discussion The oxidation of 2-chlorophenol using a variety of conditions and catalysts has been investigated previously. For photocatalytic processes, direct and indirect pathways have been proposed.1 In the direct pathway, 2-CP is adsorbed on the surface of the excited catalyst and oxidizes into product. Both Langmuir-Hinshelwood and Eley-Rideal processes have been proposed to model this pathway. In the indirect pathway, the photocatalyst is used as a sensitizer of molecular oxygen to produce singlet oxygen (1O2), superoxide (O2-), or hydroxyl radicals (•OH), although it still might provide the role of a substrate for the reaction to take place. Much research has been done on the reaction between singlet oxygen and 2-chlorophenol,26-31 with rose bengal (RB)

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Figure 5. Pseudo-first-order kinetic plot of the degradation of 2-chlorophenol with and without N2 purging of the system.

frequently used as a photosensitizer.26-29 During experiments, the singlet oxygen concentration is assumed to be in the steady state, which leads to a pseudo-first-order disappearance for 2-chlorophenol.26 The observed rate constant is then the product of the rate constant for the reaction of singlet oxygen with 2-chlorophenol and the steady-state concentration of singlet oxygen. Depending on the pH of the solution, the 2-chlorophenol might be dissociated to produce a chlorophenolate anion, which can also react with singlet oxygen. However, under the conditions used in this work, the fraction of chlorophenolate anion present in solution was negligible, so it was assumed that only undissociated chlorophenol reacted with an active oxygen species. In the observations reported here, if 2-chlorophenol were undergoing reaction with some form of active oxygen species through an indirect mechanism, then the partial pressure of oxygen dissolved in solution should affect the rate. Bubbling nitrogen through solution before initiating photocatalysis should decrease the reaction rate, as this step would decrease the concentration of dissolved oxygen in solution. Displayed in Figure 5 are pseudo-first-order plots comparing the degradation of 2-chlorophenol in the presence of undoped colloidal ZnS nanocrystals with and without nitrogen purging prior to the kinetic run. A least-squares analysis revealed a decrease in the apparent rate constant when N2 was bubbled through the reaction solution prior to irradiation. With an air-saturated solution, the apparent rate constant was calculated to be 0.016 ( 0.001 min-1, whereas for a nitrogen-purged solution, the apparent rate constant was determined to be 0.0087 ( 0.0007 min-1, resulting in a decrease of approximately 45%. Therefore, molecular oxygen, possibly in the form of superoxide or singlet oxygen, appears to be an important reactant involved in the degradation of 2-chlorophenol. Because the bubbling of N2 would not remove H2O, the amount of hydroxyl radical should not be affected. However, the removal of molecular oxygen would affect the amount of 1O2 or O2- species available for reaction. The data presented in Figure 5 clearly indicate the presence of an indirect mechanism using singlet oxygen or superoxide. Colloidal semiconductor nanocrystals have been shown to generate singlet oxygen species in solution, albeit in very low quantities, when coupled to a photosensitizer.32-34 Because PVP is not a photosensitizer, it is unlikely that singlet oxygen is a reactive species involved in the reaction probed in this work. Instead, superoxide is likely the active oxygen species produced that reacts with 2-chlorophenol through an indirect mechanism.

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Porambo et al. result, the observed rate would decrease, as was also found for nitrogen purging of the system. Both of these effects lead to a decrease in the available concentration of superoxide species in solution, which suggests that a key step in the reaction is the formation of superoxide from adsorbed molecular oxygen. This superoxide species subsequently goes on to react with 2-chlorophenol. 5. Conclusions

Figure 6. Dependence of (b) apparent pseudo-first-order rate constants, (9) 460-nm photoluminescence emission peak area, and (2) 590-nm photoluminescence emission peak area on percent Mn dopant concentration.

The dependence of kapp on Mn concentration, which is shown in Figure 6, might indicate that superoxide is produced at the surface of the colloidal ZnS nanocrystal. As observed in the figure and discussed previously, the apparent rate constant initially increases with an increase in the Mn concentration used during nanocrystal synthesis, but it then decreases with the introduction of higher Mn concentrations. Also shown in Figure 6 is the dependence of the area of the photoluminescence emission peaks on Mn concentration. The area of the peak corresponding to the 590-nm photoluminescence emission increases with increasing Mn dopant concentration. The area of the peak corresponding to the 460-nm emission, however, increases initially with increasing Mn concentration and then decreases with a further increase in Mn dopant concentration. This trend is similar to that observed for the apparent rate constant and its dependence on the Mn dopant concentration. The 460-nm emission results from transitions involving sulfur vacancies, whereas the 590-nm emission is due to the Mn dopant.19 For both, the first step in the photoluminescence process is absorption of a photon to promote an electron from the valence band to the conduction band of the nanocrystal, resulting in the production of an electron-hole pair

ZnS + hν f ZnS (e-/h+)

(5)

In the case of the undoped nanocrystal, this electron relaxes to a sulfur vacancy before undergoing radiative emission to produce the photoluminenscence at 460 nm. If molecular oxygen is adsorbed at the surface of the nanocrystal, it can trap the electron to form the superoxide species16

e- + O2 f O2-

(6)

For the lower concentrations, the introduction of Mn dopant into the nanocrystal lattice might promote the formation of electron-hole pairs,21 as can be seen in the initial increase in the 460-nm photoluminescence peak area. This increase in the formation of electron-hole pairs would then account for the observed increase in the apparent rate constant. As the Mn dopant concentration is increased, however, the trapping of the electron by the excited Mn state might be favored over the trapping of the electron by adsorbed molecular oxygen. As a

The use of undoped and Mn-doped ZnS nanocrystals as photocatalysts in the photodegradation of the common aquatic pollutant 2-chlorophenol has been examined. Doping of the nanocrystals with Mn leads to an initial increase in the apparent rate constants and then to a decrease for higher dopant concentrations used during synthesis. This finding, combined with the decrease in the apparent rate constant observed upon purging oxygen from the reaction mixture, likely indicates that the reactive oxygen species involved is superoxide. Taken together, these results indicate that electronic properties of nanoscale photocatalysts must be considered when designing materials for optimized activity. Acknowledgment. This work was supported by the Neidig Endowed Research Fund of the Lebanon Valley College Department of Chemistry. The authors thank Karen Mertzman of the Franklin & Marshall College X-ray Laboratory for help in obtaining XRD data. References and Notes (1) Pera-Titus, M.; Garcı´a-Molina, V.; Ban˜os, M. A.; Gime´nez, J.; Esplugas, S. Appl. Catal. B 2004, 47, 219. (2) Kabra, K.; Chaudhary, R.; Sawhney, R. L. Ind. Eng. Chem. Res. 2004, 43, 7683. (3) Fujiwara, H.; Hosokawa, H.; Murakoshi, K.; Wada, Y.; Yanagida, S. Langmuir 1998, 14, 5154. (4) Kho, R.; Torres-Martinez, C. L.; Mehra, R. K. J. Colloid Interface Sci. 2000, 227, 561. (5) Torres-Martinez, C. L.; Kho, R.; Mian, O. I.; Mehra, R. K. J. Colloid Interface Sci. 2001, 240, 525. (6) Yin, H.; Wada, Y.; Kitamura, T.; Yanagida, S. EnViron. Sci. Technol. 2001, 35, 227. (7) Gou, Y.; Su, Z.; Xue, Z. Mater. Res. Bull. 2004, 39, 2203. (8) Chow, J.; Dingra, N. N.; Baker, E.; Helmly, B. C.; Nivens, D. A.; Lynch, W. E. J. Photochem. Photobiol. A 2005, 173, 156. (9) Hu, J.-S.; Ren, L.-L.; Guo, Y.-G.; Liang, H.-P.; Cao, A.-M.; Wan, L.-J.; Bai, C.-L. Angew. Chem., Int. Ed. 2005, 44, 1269. (10) Li, Y.; He, X.; Cao, M. Mater. Res. Bull. 2008, 43, 3100. (11) Xie, Y.; Zhang, C.; Miao, S.; Liu, Z.; Ding, K.; Miao, Z.; An, G.; Yang, Z. J. Colloid Interface Sci. 2008, 318, 110. (12) Wu, H.; Wang, Q.; Yao, Y.; Qian, C.; Zhang, Z.; Wei, X. J. Phys. Chem. C 2008, 112, 16779. (13) Yu, X.; Yu, J.; Cheng, B.; Huang, B. Chem.sEur. J. 2009, 15, 6731. (14) Pouretedal, H.; Norozi, A.; Keshavarz, M. H.; Semnani, A. J. Hazard. Mater. 2009, 162, 674. (15) Hu, H.; Zhang, W. Opt. Mater. 2006, 28, 536. (16) Becker, W. G.; Bard, A. J. J. Phys. Chem. 1983, 87, 4888. (17) Bhargava, R. N.; Gallagher, D.; Hong, X.; Nurmikko, A. Phys. ReV. Lett. 1994, 72, 416. (18) Khosravi, A. A.; Kundu, M.; Kuruvilla, B. A.; Shekhawat, G. S.; Gupta, R. P.; Sharma, A. K.; Vyas, P. D.; Kulkami, S. K. Appl. Phys. Lett. 1995, 67, 2506. (19) Sooklal, K.; Cullum, B. S.; Angel, S. M.; Murphy, C. J. J. Phys. Chem. 1996, 100, 4551. (20) Murase, N.; Jagannathan, R.; Kanematsu, Y.; Watanabe, M.; Kurita, A.; Hirata, K.; Yazawa, T.; Kushida, T. J. Phys. Chem. B 1999, 103, 754. (21) Suyver, J. F.; Wuister, S. F.; Kelly, J. J.; Meijerink, A. Nano Lett. 2001, 1, 429. (22) Manzoor, K.; Vadera, S. R.; Kumar, N.; Kutty, T. R. N. Solid State Commun. 2004, 129, 469. (23) Karar, N.; Singh, F.; Mehta, B. R. J. Appl. Phys. 2004, 95, 656. (24) Porambo, M. W.; Marsh, A. L. Opt. Mater. 2009, 31, 1631. (25) Brus, L. J. Phys. Chem. 1986, 90, 2555. (26) Tratnyek, P. G.; Holgne´, J. EnViron. Sci. Technol. 1991, 25, 1596.

Dopant Effects on the Photocatalytic Activity of ZnS (27) Gerdes, R.; Wo¨hrle, D.; Spiller, W.; Schneider, G.; Schnurpfeil, G.; Schulz-Ekloff, G. J. Photochem. Photobiol. A 1997, 111, 65. (28) Miller, J. S. Wat. Res. 2005, 39, 412. (29) Gryglik, D.; Miller, J. S.; Ledakowicz, S. J. Hazard. Mat. 2007, 146, 502. (30) Kluson, P.; Drobek, M.; Krejcikova, S.; Krysa, J.; Kalaji, A.; Cajthaml, T.; Rakusan, J. Appl. Catal. B 2008, 80, 321. (31) Zhang, J.; Song, Q.; Hu, X.; Zhang, E.; Gao, H. J. Lumin. 2008, 128, 1880.

J. Phys. Chem. C, Vol. 114, No. 3, 2010 1585 (32) Shi, L.; Hernandez, B.; Selke, M. J. Am. Chem. Soc. 2006, 128, 6278. (33) Tsay, J. M.; Trzoss, M.; Shi, L.; Kong, X.; Selke, M.; Jung, M. E.; Weiss, S. J. Am. Chem. Soc. 2007, 129, 6865. (34) Ma, J.; Chen, J.-Y.; Idowu, M.; Nyokong, T. J. Phys. Chem. B 2008, 112, 4465.

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