Predicting and identifying reactive oxygen species and electrons for photocatalytic metal sulfide micro–nano structures

Journal of Catalysis 320 (2014) 97–105

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Predicting and identifying reactive oxygen species and electrons for photocatalytic metal sulfide micro–nano structures Weiwei He a,b, Huimin Jia a, Wayne G. Wamer c, Zhi Zheng a,⇑, Pinjiang Li a, John H. Callahan b, Jun-Jie Yin b,⇑ a Key Laboratory of Micro-Nano Materials for Energy Storage and Conversion of Henan Province, Institute of Surface Micro and Nano Materials, Xuchang University, Henan 461000, PR China b Division of Analytical Chemistry, Office of Regulatory Science, Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, College Park, MD 20740, United States c Division of Bioanalytical Chemistry, Office of Regulatory Science, Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, College Park, MD 20740, United States

a r t i c l e

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Article history: Received 20 July 2014 Revised 3 October 2014 Accepted 7 October 2014 Available online 28 October 2014 Keywords: Reactive oxygen species Electrons transfer Photocatalytic Electron spin resonance Metal sulfides

a b s t r a c t A broadly applicable theoretical and experimental framework was developed for understanding the photocatalytic mechanism of semiconductors. Using this framework, we found that it is possible to predict the type and reactivity of reactive oxygen species and electrons produced during photoexcitation of semiconductors by comparing the band edge energies of semiconductors with the redox potentials of relevant species. In addition, we could experimentally verify these predictions using electron spin resonance spectroscopy (ESR) with spin trapping and spin labeling techniques. We selected four types of metal sulfides (CdS, ZnS, In2S3, and Bi2S3) to elucidate the applicability of this model system. Using ESR technique, we found that these four sulfides are significantly different in the types of produced reactive oxygen species.  1 When irradiated, ZnS can generate superoxide (O 2 ), hydroxyl radicals ( OH), and singlet oxygen ( O2); , while irradiation of Bi S generates none of these reactive oxygen species. CdS and In2S3 can produce O 2 2 3 These results are correlated with the photocatalytic oxidation and reduction activities of metal sulfide structures. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction The increasing importance of semiconductor photoactivity in environmental remediation, energy conversion, and health care has stimulated a great deal of interest in understanding the fundamental photophysical and photochemical mechanisms underlying these applications [1]. The increasing of use of semiconductor nanomaterials has also led to new environment, health, and safety concerns [2,3]. Addressing these safety concerns also requires a better understanding of the mechanisms underlying the photoactivity of semiconductors. Light-driven generation of charge carriers (holes/electrons) and electron transfer are the initial crucial steps that determine the interaction between semiconductors and surrounding substances. The products of electron transfer, reactive oxygen species, including hydroxyl radical (OH), superoxide 1 (O 2 ), and singlet oxygen ( O2), can lead to oxidative damage and ⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Zheng), [email protected] (J.-J. Yin). http://dx.doi.org/10.1016/j.jcat.2014.10.004 0021-9517/Ó 2014 Elsevier Inc. All rights reserved.

a variety of biological effects. These reactive intermediates are frequently produced photocatalytically following excitation of the semiconductor [4,5]. Reactive oxygen species and charge carriers, therefore, have been recognized as the main intermediates responsible for the photocatalysis as well as the antibacterial activity and cellular toxicity of semiconductors [5–10]. Because of the fundamental importance of reactive oxygen species in determining the photoactivity of semiconductors, the generation of reactive oxygen species can be considered, along with particle size, shape, band gap, and crystal structure, as an important intrinsic parameter determining the photochemical properties of semiconductors. If a framework can be built to identify and predict the generation of reactive oxygen species and reactivity of photoinduced electrons, it will be of great use in understanding the mechanism of semiconductor photoactivity and in evaluating or designing new materials. This current study aimed to develop such a theoretical and experimental framework. Reactive oxygen species are not a single entity but represent a broad range of chemically distinct, reactive species with diverse oxidizing reactivity. Chemiluminescence and fluorescence are the

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most frequently used indirect methods for measuring reactive oxygen species. Chemiluminescent assays can measure OH, O 2 and H2O2 levels but cannot distinguish these species [11]. Fluorescent  1 probes can be easily used to detect O 2 , OH, and O2, but interference from other oxidants frequently requires additional HPLC analyses to definitively identify the reactive oxygen species [12]. Electron spin resonance spectroscopy (ESR) together with spin trapping is the most reliable and direct method for identification and quantification of short-lived free radicals and reactive oxygen species [13,14]. However, the lack of a standardized and systematic approach can lead to inaccurate identification of reactive oxygen species. For example, one study reported that photoexcited TiO2 generated superoxide but another did not [15,16]. Moreover, it is difficult to identify photoinduced electrons using spin trapping. 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO), a ESR-spin label, has been used to elucidate electron transfer and the reactivity of electrons generated during photoexcitation of ZnO and ZnO/Au hybrid nanostructures [17,18]. The model system we have developed utilizes ESR combined with spin trapping and spin labeling techniques and can provide precise information about reactive oxygen species and electron behavior. The theoretical approach used in the proposed model system relies on the link between band energy structures of semiconductors and chemical reduction potential of each kind of reactive oxygen species. Several investigators, using indirect methods, have reported that electronic band edge energies of a metal oxide strongly predict generation of reactive oxygen species [19–21]. The pioneering work of E. Burello and A. Worth demonstrated theoretically that the relative oxidative stress potential of metal oxide nanoparticles could be predicted from their band edge energies and the redox potentials of reactive oxygen species [19]. It has also been reported that the band edge energy of several types of metal oxide nanostructures is correlated with both generation of reactive oxygen species (measured with fluorescent indicators), and their photoinduced antibacterial activity [20]. While metal oxide semiconductors have received much attention, little research has focused on the generation of reactive oxygen species in metal sulfides [22,23]. Therefore, much is unknown about photogeneration of reactive oxygen species for metal sulfides. Metal sulfide micro–nano structures have received a great deal of interest due to their excellent solar spectrum responses, proper positions of the conduction and valence bands and high photocatalytic activities [24,25]. For example, more than 30 kinds of sulfide structures have been employed as photocatalysts [25]. We have therefore developed a framework in which photogeneration of reactive oxygen species and charge carriers can be experimentally determined by ESR and the formation of these species can be predicted theoretically using the band edge energy for study of metal sulfides. For our studies, we synthesized four types of metal sulfide micro–nano structures (MNs) (e.g. CdS, ZnS, In2S3, and Bi2S3). The chemical compositions of these materials were selected because they are metal sulfides widely used in photocatalytic applications. Spherical or quasi-spherical metal sulfides with combined microand nanostructures were prepared by a solvothermal method. Our primary objective was to establish a model system which combined theoretical and experimental analyses. Theoretical predictions were based on comparison of the band edge energy of a metal sulfide with the redox potential of individual reactive oxygen species. Experimental demonstration of the formation of reactive oxygen species was accomplished using ESR coupled with spin trapping and spin labeling techniques. This model was tested by examining the four selected metal sulfides semiconductors. Furthermore, we investigated correlations between the photocatalytic reduction and oxidation abilities and photogenerated electrons or reactive oxygen species in metal sulfides.

2. Materials and methods 2.1. Chemical and materials Cadmium chloride, indium (III) chloride, zinc nitrate, bismuth chloride, sulfur, and absolute ethanol were analytically pure and purchased from Shanghai Chemical Reagent Co. Ltd. The spin trap, 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO), was purchased from Applied Bioanalytical Labs (Sarasota, FL). 4oxo-2,2,6,6-Tetramethyl-1-piperidinyloxy (4-oxo-TEMP), 3,30 ,5, 50 -tetramethylbenzidine (TMB), NaN3, and superoxide dismutase (SOD) were purchased from Sigma Chemical Co. (St. Louis, MO). 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) was purchased from Alexis, Enzo Life Sciences, Inc. (Farmingdale, NY). Milli-Q water (18 MX cm) was used for preparation of all solutions. 2.2. Synthesis of metal sulfide micro–nano structures (MNs) In a typical synthesis, 0.5 mmol metal ions (CdCl2 or Zn(NO3)2, or InCl3) and 0.5 mmol sulfur powder (1 mmol thioacetamide for InCl3) were added into a 28-ml Teflon-lined stainless steel autoclave. Then 21 ml absolute ethanol was added and the mixture was vigorously stirred. The autoclave was sealed, heated at 180 °C for 2–12 h, and air-cooled to room temperature. Finally, the precipitate was collected by centrifugation, washed several times with double distilled water and ethanol, and dried under vacuum at 50 °C. Bi2S3 micro–nano structures were prepared similarly by solvothermal methods [26]. 2.3. Characterization The crystal structure of as-synthesized products was characterized by X-ray diffraction (XRD, Bruker D8 Advance diffractometer) using monochromatized Cu Ka radiation (k = 1.5418 Å). Scanning electron microscopy (SEM, Zeiss EVO LS-15) was used to characterize the morphology of metal sulfide micro–nano structures. Transmission electron microscopy and electron diffraction were performed with FEI Tecnai G2 20 at 200 kV accelerated voltage. The UV–vis absorption spectra were recorded on a Varian Cary 5000 UV–vis–NIR spectrophotometer. The BET surface areas of different metal sulfide micro–nano structures were determined by a Micromeritics Gemini 2380 specific area analyzer by measuring nitrogen adsorption. For the comparison of reduction photocatalyzed by CdS, ZnS, In2S3 and Bi2S3 MNS, we examined the photoreduction of the TMB oxidization product. 5 ml 0.1 mg/ml TMB was firstly oxidized to TMB oxidation product (TMB⁄) by UV light (220 nm < k < 320 nm). The TMB⁄ solution was mixed well with 0.1 mg/ml photocatalyst and then irradiated with a WG320-filtered 450 W Xenon lamp to initiate the photoreduction reaction. The residual concentration of TMB⁄ was determined after 5 min of irradiation by UV–vis spectroscopy. The photocatalytic degradation of the methyl orange (MO) or rhodamine B (RhB) by ZnS, CdS, In2S3, and Bi2S3 was evaluated in aqueous solutions. 20 mg of as-prepared photocatalyst was dispersed in a 50 ml aqueous solution containing 20 mg/l MO (or RhB). The solution was continuously stirred in the dark for 30 min to ensure that an adsorption–desorption equilibrium between the photocatalyst and dye molecules was established. Then, the suspension was irradiated using a 500 W Xenon lamp. During irradiation, the solution was stirred to maintain a suspension and the temperature was controlled by circulation of room temperature water around the sample. The pH values were about 6.0 and remained nearly unchanged before and after photocatalytic reaction for the four different metal sulfides. A 2 ml aliquot of the

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suspension was taken from the reactor and centrifuged at 15 min intervals (30 min for TOC measurement). The residual concentration of MO or RhB in the supernatant was monitored using a Varian Cary 5000 spectrophotometer. Total organic carbon (TOC) changes were measured with a TOC analyser (TOC-VCPH, Shimadzu, Japan) at different levels of degradation of methyl orange. 2.4. Electron spin resonance spectroscopy The ESR measurements were taken using a Bruker EMX ESR spectrometer (Billerica, MA) at ambient temperature. A light system consisting of a 450 W Xenon lamp coupled with optical filters (WG320) was used to generate light having wavelengths longer than 350 nm. For ESR measurements, fifty microliter aliquots of control or sample solutions were put in quartz capillary tubes with internal diameters of 0.9 mm and sealed. The capillary tubes were inserted in the ESR cavity, and the spectra were recorded after irradiation at selected times. All ESR measurements were taken using the following settings for detection of the spin adducts: 20 mW microwave power, 100 G scan range and 1 G field modulation. For the detection of individual reactive oxygen species, each spin trap or spin label reagent at fixed concentration was mixed well with 0.1 mg/ml metal sulfide MNs in a 50-ll quartz tube. ESR spectra were recorded during irradiation at 0, 1, 3, 5, and 8 min. To identify each type of reactive oxygen species, 25 mM BMPO was employed to verify the formation of superoxide (OOH) and hydroxyl radicals (OH), 10 mM 4-oxo-TEMP was used to detect singlet oxygen, and 0.02 mM TEMPO was used to characterize photogenerated electrons. To confirm and distinguish reactive oxygen species, the effects of specific scavengers, such as SOD for confirming superoxide and NaN3 for confirming singlet oxygen, on the ESR signal for spin adducts were examined. 3. Results and discussion 3.1. The framework based on combining theoretical and ESR analyses Semiconductors have band energy structures in which the relevant energy levels are the top of the valence band (Ev) and the bottom of the conduction band (Ec). The band gap (Eg) refers to the energy difference between Ev and Ec. The energy level of adsorbed substances can be approximated by their standard redox potential (E0). The feasibility of electron transfer between semiconductors and adsorbed substances will be determined by the relative energy position of Ev or Ec versus E0. In aqueous media, the redox potential for dissolved oxygen/superoxide couple (O2(aq.)/O 2 ) is 0.16 eV [27], singlet oxygen/dioxygen (1O2/O2) is 1.88 eV and for the H2O/OH couple is 2.2 eV [20]. In aqueous environment, the conduction band edge and valence band edge of metal sulfides are dependent on pH because of band bending induced by the Helmholtz layer. The ESR measurements and photocatalytic reductions were conducted in aqueous solution at approximately pH 6.0 in this work. Therefore, the relevant Ev and Ec of metal sulfide MNs at pH 6.0 were calculated by the following equations [28,29]:

Ev ;6 ¼ X sulfide þ 0:5Eg þ 0:059 ðpHZPC  pHÞ þ E0 Ec;6 ¼ Ev ;6  Eg ¼ X sulfide  0:5Eg þ 0:059 ðpHZPC  pHÞ þ E0 where Ev,6 refers to valence band edge energy at pH 6.0; Xsulfides is the absolute electronegativity for each metal sulfide, Xsulfide (MxSy) = (XMxXSy)1/(x+y), where XM and XS are the absolute electronegativity of the atoms M and S; pHZPC is the point of zero zetapotential for each metal sulfide, which is available in the literature; E0 is scale factor to the absolute vacuum scale (E0 = 0.45 eV for normal hydrogen electrode). The calculated Ev and Ec (with respect

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to NHE) of some metal sulfides are listed in Table S1 and illustrated in Fig. 1A. The four metal sulfides in the light blue area of Fig. 1A were examined in this study. By comparing the Ev or Ec with E0 of  1 (O2(aq.)/O 2 ), (H2O/ OH) and ( O2/O2), it can be easily determined whether the formation of a particular reactive oxygen species is thermodynamically favorable. Photogenerated electrons, needed to form superoxide, must therefore have a potential less than 0.16 eV. Photogenerated holes, needed to form hydroxyl radicals, must have a potential greater than 2.2 eV. To form singlet oxygen from oxygen, photogenerated holes must have a potential greater than 1.88 eV. Fig. 1B outlines the experimental method using ESR to distinguish the reactive oxygen species and electrons generated during photoexcitation of metal sulfides. Spin trap adducts, formed by the reaction between spin traps and reactive oxygen species, have characteristic ESR spectra which can be used to identify and quantify particular reactive oxygen species. BMPO and DMPO are typical spin traps use to study the formation of hydroxyl radicals and superoxide. TEMP and 4-oxo-TEMP are specific traps for singlet oxygen to yield a nitroxide radical TEMPONE with stable ESR signal. Spin labels refer to the molecules having an ESR signal with unpaired electrons, e.g. TEMPO. In addition to the directly trapping reactive oxygen species and identifying them by their characteristic ESR spectra, we also used indirect tools into distinguish precisely each species by examined scavenging effect of DMSO, SOD and NaN3, which is specific to hydroxyl radical, superoxide and singlet oxygen, respectively. We will discuss these techniques in detail when presenting the results of the ESR experiments. 3.2. Synthesis of selected metal sulfides micro–nano structures In this work, four types of metal sulfides with micro–nano structures were prepared and used as model compounds to study the photogeneration of reactive oxygen species intermediates and electrons. CdS, ZnS, In2S3 and Bi2S3 MNs were synthesized using a simple solvothermal method. Fig. 2 summarizes the results obtained from X-ray powder diffraction (XRD). The XRD pattern from CdS samples shows that all the diffraction peaks can be indexed to hexagonal wurtzite CdS (JCPDS no. 41-1069). No other characteristic peaks of impurities were detected indicating the excellent crystallinity of as-prepared CdS MNs. The XRD patterns for ZnS, In2S3 and Bi2S3 all matched very well with hexagonal wurtzite ZnS (JCPDS no. 3-1093), tetragonal indium sulfide (JCPDS no. 73-1366) and orthorhombic bismuthinite (JCPDS no. 2-391), respectively. These data support the formation of pure metal sulfides with highly crystalline structures. Representative SEM images of these four metal sulfide products are displayed in Fig. S1. They all showed microscale sphere/quasi-sphere like morphologies but with different details. The average size for CdS, ZnS, In2S3 and Bi2S3 was calculated to be approximately 1.5, 2.0, 4.0 and 2.0 lm, respectively. Thus we named them micro–nano structures (MNs). This size allows us to make valid approximation for calculation of band energies in this work without considering the size effect and surface states on band edge positions, because they behave like bulk materials as particle size exceeds 20–30 nm [30]. To investigate the surface morphology and crystal structure, the TEM and electron diffraction (ED) patterns were recorded (Fig. S2). The TEM images of a single particle indicate the spherical shape and diameters in microscale, which are consistent with the SEM observation. The solid center resulted in a high contrast in TEM image of ZnS, CdS and In2S3. The TEM image of Bi2S3 shows clearly the porous structure assembled by nanorods, the diameter of the marked nanorod was calculated to be 64 nm. All the corresponding ED patterns show that diffraction spots are superimposed on the rings for ZnS, CdS, In2S3 and Bi2S3, indicating the polycrystalline structure of the single particle. The scraggy surface

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Fig. 1. (A) Band edge positions of selected metal sulfides compared with standard redox potential of superoxide, singlet oxygen and hydroxyl radical. (The sulfides in light blue area were selected for examination in this work.) (B) schematic of the ESR approach for identifying and distinguishing reactive oxygen species and electrons generated during photoexcitation of metal sulfide MNs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

a theoretical examination of their valence band edge energy and conduction band edge energy. For the selected four metal sulfides with varying band edge structures, we expect a large difference in the photogenerated reactive oxygen species. We predict that ZnS can generate 3 kinds of reactive oxygen species including hydroxyl radicals, superoxide and singlet oxygen; CdS and In2S3 will only favor the formation of superoxide; Bi2S3 may not produce any reactive oxygen species. We then used ESR techniques to confirm these predictions experimentally. These radicals cannot be detected directly by ESR because of their short lifetimes. However, they readily react with diamagnetic nitrone spin traps, forming relatively stable free radicals (spin adducts) that can be identified from the magnetic parameters of the spin adduct’s ESR spectrum [31]. To verify the generation of superoxide or hydroxyl radicals induced by metal sulfides MNs, we chose the spin trap, BMPO, that is often

Fig. 2. XRD patterns of CdS, ZnS, In2S3 and Bi2S3 micro–nano structures prepared under solvothermal conditions.

sub-branches have different spatial orientations, thus indicating the polycrystalline structure of the entire particle. The ED patterns of CdS and Bi2S3 show relatively ordered diffraction spots suggesting the preferential spatial orientation in the whole CdS and Bi2S3 particles. The optical band gaps of the four metal sulfide micro–nano structures have been measured by UV–vis–NIR diffuse reflection spectra (Fig. S3). The band gap energy was calculated by the formula, ahv = A (hv  Eg)1/2, where a is the absorption coefficient, A is a constant, hv is the photon energy. The band gaps were determined to be 3.54, 2.31, 2.03 and 1.37 eV for ZnS, CdS, In2S3 and Bi2S3, respectively. The measured optical band gaps matched well with the theoretical band gap in Table S1. 3.3. Identification of superoxide, hydroxyl radical and singlet oxygen induced by metal sulfide MNs using ESR spectroscopy Our expectations for the types of reactive oxygen species formed during photoexcitation of metal sulfide MNs resulted from

Fig. 3. Generation of superoxide and/or hydroxyl radical from three types of metal sulfides during irradiation (k > 350 nm). ESR spectra obtained at room temperature from samples containing 25 mM BMPO only with irradiation (a), 25 mM BMPO and 0.1 mg/ml metal sulfide MNs before (b) and during (c) irradiation (d) represents the same condition as (c) but with the addition of 4 U/ml SOD. All the spectra were recorded at after 3 min of irradiation.

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used to capture hydroxyl radical and superoxide. Fig. 3 shows the ESR spectra obtained from solutions containing BMPO and metal sulfide MNs before and during irradiation with light above 350 nm in wavelength. For an irradiated BMPO solution or mixed samples without irradiation, a characteristic ESR signal was absent (Fig. 3 a and b). Upon irradiation for 3 min in the presence of CdS or In2S3 MNs, we clearly observed a four-line spectrum with relative intensities of 1:1:1:1 and hyperfine splitting parameters of aN = 13.4, abH = 12.1 G, which is the characteristic spectrum for the adduct formed between BMPO and superoxide, BMPO/OOH [31]. No evident ESR signal was detected from irradiated Bi2S3 MNs. In the case of ZnS MNs, a four-line ESR spectrum appeared with similar hyperfine splitting parameters but different relative peak intensities from CdS and In2S3. BMPO is also a spin trap frequently used for capturing hydroxyl radicals, the spectra for adducts of BMPO/OOH and BMPO/OH overlap. For ZnS, the ESR spectrum using BMPO may be the mixture of these two signals. It is necessary to further determine whether the ESR signal from these 3 metal sulfides in part comes from hydroxyl radicals, either directly or through intermediate radical reactions involving hydroxyl radicals. Superoxide dismutase (SOD) can efficiently and specifically reduce superoxide by catalyzing its dismutation, but has no interaction with hydroxyl radicals. We investigated the scavenging effect of SOD on the ESR signal. When 4 U/ml SOD was added (Fig. 3d), the ESR signal was totally inhibited compared with samples without SOD for CdS and In2S3. This result confirms that the ESR signals produced in CdS and In2S3 are from superoxide without the involvement of hydroxyl radicals. A different result was observed for ZnS MNs. After addition of SOD a new four-line spectrum clearly appeared with relative intensities of 1:2:2:1 and hyperfine splitting parameters of aN = 13.56, abH = 12.30, acH = 0.66, which is the characteristic spectrum for the adduct formed between BMPO and the hydroxyl radical, BMPO/OH [31]. These results demonstrate directly that for these four metal sulfides MNs, both hydroxyl radicals and superoxide are generated by photoexcited ZnS MNs, only superoxide was produced during photoexcitation of CdS and In2S3, and neither hydroxyl radical nor superoxide was formed from during photoexcitation of Bi2S3. Fig. 1 shows the Ec energy of CdS, ZnS and In2S3 (0.76, 1.27 and 1.04 V with respect to NHE) are less than 0.16 eV (E0 of O2(aq.)/O 2 ). These relative energies predict that photoexcited CdS, ZnS and In2S3 MNs can transfer electrons from the conduction band to dioxygen molecules resulting in the formation of superoxide. This prediction is in agreement with our ESR results. The calculated Ec of Bi2S3 (+0.17 eV) is much higher than 0.16 eV, which is not sufficiently negative to reduce O2. Thus no production of superoxide is expected. The Ev energy of CdS, ZnS, In2S3 and Bi2S3 is calculated to be 1.64, 2.33, 0.96 and 1.47 eV with respect to NHE, respectively. Compared with E0 of H2O/OH (+2.2 eV), only ZnS has a more positive valence band edge (+2.33 eV) than H2O/OH. Therefore, reaction of holes in the valence band with water to form hydroxyl radical is thermodynamically favorable only for photoexcited ZnS. These theoretical predictions are in good agreement with the ESR observations for photogeneration of superoxide and hydroxyl radicals. Singlet oxygen formation in the presence of photoexcited metal sulfides was detected by ESR using 4-oxo-TEMP. It has been previously reported that 4-oxo-TEMP reacts with singlet oxygen to produce 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPONE), a nitroxide radical detectable by its distinctive ESR spectrum [32]. Fig. 4 displays the ESR spectra using 4-oxo-TEMP as spin probe. These spectra can be used to clearly demonstrate which of the four metal sulfides photocatalyze the formation of singlet oxygen. No ESR signal was observed for samples containing only the spin probe or unirradiated metal sulfide. In contrast, a three line spectrum with relative intensity ratio of 1:1:1 and calculated hyperfine

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Fig. 4. Identification of singlet oxygen during photoexcitation (k > 350 nm) of the four metal sulfide MNs. ESR spectra were obtained at room temperature from samples containing 10 mM 4-oxo-TEMP only with irradiation (a), 10 mM 4-oxoTEMP mixed with 0.1 mg/ml metal sulfide MNs before (b) and during irradiation of CdS (c), ZnS (d), In2S3 (e) and Bi2S3 (f). All the spectra were recorded after irradiation for 5 min.

splitting 15.85 G characteristic for the nitroxide radical, TEMPONE, was observed for samples containing CdS or ZnS. No ESR signal was evident in the presence of In2S3 or Bi2S3. To confirm the role of singlet oxygen in the formation of TEMPONE, we further investigated the effect of NaN3, a typical singlet oxygen quencher, on the TEMPONE ESR signal. We observed that addition of 10 mM NaN3 resulted in the disappearance of the TEMPONE ESR signal during photoexcitation of CdS or ZnS (Fig. S4). Taken together, these results demonstrate that CdS and ZnS photocatalyze the formation of singlet oxygen, but In2S3 and Bi2S3 do not. As indicated by Fig. 1A, only ZnS has the ability of to directly photo-oxidize oxygen to singlet oxygen, for other three sulfides, the hole appearing in their valence bands during photoexcitation have insufficient electrochemical potentials to drive the formation of singlet oxygen. However, CdS MNs readily generated strong singlet oxygen as demonstrated by ESR. To understand this unexpected result, we more closely examined the mechanism of singlet oxygen formation during photoexcitation of metal sulfides. Singlet oxygen also can be produced via the electron transfer between superoxide and cation species with appropriate oxidizing power [33]. Photogenerated holes in semiconductor nanoparticles could be such cation species to oxidize superoxide. This mechanism for the production of 1O2 during photoexcitation of TiO2 has been reported [32]. We observed the interesting and noteworthy phenomenon that the ESR signal attributed to the photogeneration of singlet oxygen in samples containing ZnS or CdS MNs diminished significantly when SOD was added (Fig. S4). Superoxide and hydroxyl radicals do not appreciably react with 4-oxo-TEMP to produce a TEMPONE ESR signal. This may be clearly seen by the lack of a TEMPONE ESR signal in samples containing In2S3 NMs, which, as we have shown, photocatalyzes the formation of superoxide. In addition, SOD is a specific scavenger for superoxide and does not react with singlet oxygen. Therefore, the effects of SOD on the photogeneration of 1 O2 indicate superoxide must be involved in 1O2 generation. Namely, that the formation of 1O2 during photoexcitation of ZnS and CdS is due to electron transfer between superoxide and holes. Two conditions are needed, therefore, to generate 1O2 in sulfides: appropriately high Ev and superoxide formation. This dual requirement explains why In2S3 and Bi2S3 cannot photocatalyze the formation of singlet oxygen, because these sulfides do not meet the two requirements. Although ZnS had a Ev greater than 1O2/O2, 1 1 O2/O 2 has a lower redox potential than O2/O2 [33], therefore,

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the electron transfer between superoxide and holes may be the preferential reaction to produce singlet oxygen in current study. These results demonstrate the utility of a model incorporating theoretical and experimental analysis for understanding the photoactivity of metal semiconductors. The use of this model can dramatically reduce testing requirements during the development of photocatalysts for a variety of uses. We also calculated Ev and Ec of additional metal sulfides, such as FeS, CuS, MnS, SnS, HgS, and PbS. MnS and HgS have a lower Ec than the E0 of O2(aq.)/O 2 , which indicates MnS and HgS, when photoexcited, may be expected to generate superoxide. In addition, MnS and HgS have relatively high Ev of 1.57 and 1.78 eV and, therefore, the possibility of generating superoxide, which indicates that formation of singlet oxygen, photocatalyzed by MnS and HgS, are thermodynamically favorable. The Ev for all of these metal sulfides is less than 2.2 eV, indicating that they are not expected to photocatalyze the formation of hydroxyl radicals. These predictions need further experimental confirmation. In addition, the band edge energies of semiconductors are dependent on the micro-environmental pH, therefore, the generation of reactive oxygen species also may vary under different pH conditions.

rapidly decreased within 3 min of irradiation, indicating electrons were generated and reduced the spin label TEMPO to an ESR-silent product. The four metal sulfides exhibited different abilities in generating electrons and reducing TEMPO. The greatest decline of ESR signal intensity was caused by CdS MNs, where TEMPO was totally reduced. ZnS had the next highest level of photoreducing activity. The order of ability in reducing TEMPO is CdS > ZnS > Bi2S3 > In2S3, which indicates the relative ability of these metal sulfides to photocatalyze reductions. With extended irradiation times, more reduction was observed (data not shown). The level of electrons generated during irradiation of metal sulfides can be estimated. Complete consumption of TEMPO was observed, for example, during 3 min of irradiation of a 50 ll solution containing 0.02 mM TEMPO and 0.1 mg/ml CdS. The one electron reduction of TEMPO leading to the results in Fig. 5 would require that approximately 1  109 mol of electrons were available for photoreduction during irradiation of 5 lg of CdS MNs for 3 min. The use of ESR together with spin trapping and spin labeling techniques provides an effective and facile way to give a clear picture of the photoreaction mechanism for metal sulfide MNs. 3.5. The photocatalytic reduction activity of metal sulfide MNs

3.4. Identifying and measuring the reactivity of photo-induced electrons by ESR-spin labeling Electrons, excited to the conduction band of semiconductors during irradiation, play a fundamental role in photocatalyzed reductions. We used ESR with the spin labeling technique to examine the selected four metal sulfides for reducing behavior attributable to photogenerated electrons. TEMPO is often used in ESR studies of dynamic and structural features in biological system [34]. TEMPO can be reduced (e.g. by reducing biomolecules and electrons) to give a hydroxyl amine, TEMPOH, accompanied by flattening of the ESR signal (Fig. 5). The reaction between TEMPO and electrons produced from excitation of semiconductor nanostructures has been demonstrated in previous works [17,18,35]. In this study, the free radical TEMPO does not react with oxidizing species (reactive oxygen species and holes) and unirradiated metal sulfide particles. The transfer of electrons from photoexcited metal sulfides to TEMPO is an obligatory and determining step in the formation of TEMPOH. Therefore, we used TEMPO to probe the reactivity of electrons generated in photoexcited metal sulfide MNs (Fig. 5). The ESR spectrum of an aqueous solution of TEMPO has three peaks with an intensity of 1:1:1. The signal is stable and the intensity remains unchanged after mixing with metal sulfide MNs before irradiation. However, the signal intensity from the mixtures

Fig. 5. Demonstration of electron generation during irradiation of metal sulfides with light (k > 350 nm). ESR spectra obtained at room temperature from samples containing 0.02 mM TEMPO in the absence (control) and presence of 0.1 mg/ml metal sulfide MNs. All the spectra were recorded at 3 min during irradiation.

The four selected metal sulfide MNs differ in photocatalytic activity. This difference in reactivity is attributable to differences in the ability to photoinduce reactive oxygen species and electrons. Can these results obtained from ESR-spin trapping and spin labeling be correlated with their photochemical activities, such as photoreduction and photo-oxidation? To answer this question we firstly investigated the correlation between photocatalytic reduction activity and the results from ESR spectroscopy. 3,30 ,5,50 -Tetramethylbenzidine (TMB), a chromogenic substrate often used in enzyme-linked immunosorbent assays [36], was chosen to compare the photoreductive ability of the metal sulfide MNs (Fig. 6). TMB can act as an electrons donor for the reduction of O2 or hydrogen peroxide and produce a diimine, TMB⁄ (the oxidized product), with a blue color. TMB⁄ shows the characteristic new absorbance peaks at 370 nm and 650 nm (Fig. S5). TMB⁄ acting as

Fig. 6. Ability of CdS, ZnS, In2S3 and Bi2S3 MNs to photocatalyze the reduction of TMB oxidized product (TMB⁄). UV–vis spectra of TMB⁄ after irradiation (k > 350) for 5 min in the absence (control) and presence of 0.1 mg/ml metal sulfide MNs. Inset shows the color evolution from TMB to TMB⁄ when the photocatalysts, CdS, ZnS, In2S3 and Bi2S3, are added. The chemical equation indicates the molecular transformation of TMB to form the oxidation product, TMB⁄.

W. He et al. / Journal of Catalysis 320 (2014) 97–105

electron acceptor can be reduced to TMB by photoinduced electrons companied with reduction of absorbance at 650 nm. TMB, being sensitive to light, especially UV light (220 nm < k < 320 nm), was first oxidized to TMB⁄ by UV irradiation (Fig. S5). It is noteworthy that neither TMB nor TMB⁄ are affected by irradiation for up to 15 min with light having wavelengths above 350 nm. In the presence of metal sulfide MNs photocatalysts, TMB⁄ was reduced gradually. After 5 min of irradiation, these four types of metal sulfides MNs showed differences in their abilities to photocatalytically reduce TMB⁄, where photoreduction was observed both by a color change and a change in the UV–vis spectrum (Fig. 6). We observed the following order in photoreductive ability: CdS > ZnS > Bi2S3 > In2S3 (Fig. S6). The results from measurements of photodegradation (Fig. 6) are consistent with results from ESR-spin labeling. This is expected because the photogenerated electrons from metal sulfides are the active species dominating the reduction of spin label TEMPO as well as the TMB⁄. 3.6. The photocatalytic activity of metal sulfide MNs toward degradation of methyl orange The photocatalytic degradation of the representative organic dye pollutant, methyl orange (MO), was determined to evaluate the photocatalytic oxidative activity of the four metal sulfides (Fig. 7). The control experiments show that MO is resistant to degradation during irradiation without photocatalyst. The four metal sulfides MNs show considerable catalytic ability for the degradation of MO during irradiation. For example, more than 90% MO is degraded by ZnS after 45 min irradiation (Fig. 7 left). The four metal sulfides show the following photocatalytic trend for photodegradation of MO: ZnS > CdS > In2S3 > Bi2S3. The degradation of MO in the presence of ZnS, CdS, In2S3 and Bi2S3 was found to be a pseudo-first order kinetic process (Fig. 7 right). The MO degradation rate constants for ZnS, CdS, In2S3 and Bi2S3 were determined to be 0.051, 0.022, 0.015 and 0.0077 min1, respectively. We have selected another representative organic dye, rhodamine B, as a substrate to compare the photocatalytic oxidative activity of the metal sulfide micro–nano structures (Fig. S7). The four metal

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sulfides showed the same photocatalytic trend for photodegradation of RhB as that for photodegradation of MO. In addition, the total organic carbon (TOC) analysis was performed by a TOC analyzer. The relative TOC changes for MO photocatalyzed by ZnS, CdS, In2S3 and Bi2S3 at different degradation degrees have been compared (Fig. S8). The reduction of TOC for the MO solution indicates the mineralization of MO along with the color removal during photocatalysis. The removal rate of TOC shared a same trend with photodegradation activity of MO for the four metal sulfides: ZnS > CdS > In2S3 > Bi2S3. Table 1 compares the photocatalytic performances of selected four metal sulfides for photoreduction of TMB⁄ and photo-oxidative degradation of MO. The photocatalytic efficiency (PE), mass normalized reactivity (TORm), surface area normalized reactivity (TORS) and reaction rate (RR) are listed in Table 1. The relative apparent photocatalytic efficiency per unit time is defined by g = (A0  A)/A0, where A0 and A are the initial and final absorbance of reactant after irradiation for 5 min in photoreduction or 30 min in photo-oxidation, respectively [37]. Rather than estimation of an absolute quantum yield for photocatalysis, our intent is to compare the relative photocatalytic activities for the four metal sulfides. The BET surface areas of metal sulfides were determined to calculate the TORS. In comparison of PE, it is found that the selected four metal sulfides show different catalytic trends for photodegradation of MO (ZnS > CdS > In2S3 > Bi2S3) and photoreduction of TMB⁄ (CdS > ZnS > Bi2S3 > In2S3). The TORm values shared the same order to PE both for photoreduction and photo-oxidation, since same amount of metal sulfides used as photocatalysts. When compared the activity normalized by surface area, the photo-oxidation showed same trend to TORm. For the photoreduction, Bi2S3, having highest surface area, possesses the lowest TORS indicating the lower activity of Bi2S3 for photoreduction reaction. In addition, the TOR values indicate that the selected metal sulfide photocatalysts are more active in photoreduction, about 2 orders of magnitude, than photo-oxidation. Metal sulfides generally possess higher energy conduction band positions than those of metal oxides, which make them more suitable for photocatalysis of reduction reactions.

Fig. 7. Photocatalytic activity of ZnS, CdS, In2S3 and Bi2S3 for the degradation of methyl orange. The figure shows the concentration change of methyl orange (left) and ln(C/ C0) as a function of irradiation time during the degradation (right).

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Table 1 Comparison of 4 metal sulfide photocatalysts for photoreduction and photo-oxidation. Photocatalyst

BET (m2/g)

Photoreduction (TMB⁄) PE (%)

ZnS CdS In2S3 Bi2S3

12.5 13.2 10.1 16.6

90 100 45 65

TORm (lmol/g/min) 2

5.7  10 6.4  102 2.9  102 4.2  102

Photo-oxidation (MO) 2

TORS (lmol/m /min)

PE (%)

TORm (lmol/g/min)

TORS (lmol/m2/min)

RR (min1)

45.6 48.5 28.7 25.3

72 43 31 20

3.6 2.3 1.55 1.0

0.288 0.174 0.153 0.06

0.051 0.022 0.015 0.0077

Generally, reactive oxygen species and photogenerated holes contributed to photo-oxidation because of their higher oxidizing potential [5,38]. The electrons in the conduction band of photoexcited semiconductors are responsible for the photoreduction activity of metal sulfide MNs. Hydroxyl radicals, singlet oxygen and superoxide, have been recognized as the active oxygen species involved in photocatalytic reactions. The 3 kinds of ROS differ greatly in chemical reactivity. By allying the standard chemical reduction potentials of OH/H2O (2.2 eV), 1O2/O2 (1.88 eV), and O 2 /O2 (0.16 eV) couples, it is easy to understand an order in oxidation reactivity: hydroxyl radical > singlet oxygen > superoxide. In the described ESR experiments, we observed that ZnS can gener 1  1 ate superoxide O 2 , OH and O2; CdS can produce O2 and O2, In2S3  can produce only O2 while Bi2S3 generates none of these reactive oxygen species during irradiation. These results are in agreement with the photocatalytic activity trend observed for degradation of MO and RhB during irradiation with a Xenon light source: ZnS > CdS > In2S3 > Bi2S3, because reactive oxygen species with high oxidative potential play a large role in photocatalytic degradation [39]. Comparing the Ec of the four metal sulfides MNs, the reducing power should be ordered by ZnS > In2S3 > CdS > Bi2S3. However, experimental results both from ESR-spin labeling and photoreduction of TMB⁄ support the activity order: CdS > ZnS > Bi2S3 > In2S3, or CdS > ZnS > In2S3 > Bi2S3. Apart from the conduction band edge position, other factors such as band gap, incident light and crystal structures may affect the photoreduction activity. A wider band gap generally produces holes and electrons with higher activity toward oxidation and reduction reactions. The light source used in photoreduction was a Xenon lamp coupled with WG320 filter emits light having wavelengths above 350 nm. This light source is fully suitable for CdS, In2S3 and Bi2S3, but is not the match best for ZnS because its equivalent wavelength is 344 nm according to band gap 3.6 V. Light having wavelengths predominately above 350 nm cannot fully excite the ZnS MNs. We have found that ZnS showed greater ability in reducing the TEMPO ESR signal than CdS when exposed to 340 nm monochromatic light (data not shown). Although the wavelength of light may not affect the type of reactive oxygen species produced, it is expected to decrease the reducing activity of electrons available for photoreduction under irradiation with longer wavelength. Crystal structures also play important roles in affecting photocatalytic reduction for some semiconductors. Fu and co-workers have reported that tetragonal In2S3 was inactive for photoreduction in hydrogen generation, whereas the cubic In2S3 exhibited superior photoactivity [40]. In2S3 MNs prepared in this work have tetragonal crystal phase. This may lead to the inaction of In2S3 in photoreduction. Therefore, several material properties in addition to band energy positions must be considered when predicting the photoreductive activity of metal sulfides and other semiconductors.

4. Conclusions In summary, we have developed a framework incorporating ESR experimental results and theoretical analysis, for predicting and identifying the reactive oxygen species and electrons generated

by selected four metal sulfide micro–nano structures. Four types of metal sulfides, CdS, ZnS, In2S3 and Bi2S3 were synthesized and used as case study. ZnS MNs generated hydroxyl radicals, superoxide and singlet oxygen, CdS photocatalyzed the formation of both superoxide and singlet oxygen. In contrast, photoexcitation of In2S3 resulted in only the formation of superoxide. Photoexcitation of Bi2S3 did not produce any of these reactive oxygen species. The theoretical prediction and ESR experimental confirmation were in remarkable agreement for the generation of reactive oxygen species from the selected metal sulfides. Furthermore, the photochemical reductive and oxidative activity of four metal sulfides was comparatively studied by ESR and photodegradation. These results indicate that electrons initiate the photoreduction while reactive oxygen species correlate to photo-oxidation toward degradation of methyl orange and rhodamine B. Because of the great importance of reactive oxygen species and electron transfer in photocatalysis, photocurrent conversion and biological systems, the empirical model system we have developed will be valuable for designing and screening semiconductors for these applications and effects. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant Nos. 21303153, 61204009, 21273192), Program for Science & Technology Innovation Talents in Universities of Henan Province (14HASTIT008), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 144200510014), and a regulatory science grant under the FDA Nanotechnology CORES Program and by the Office of Cosmetics and Colors, CFSAN/FDA. We appreciate Dr. Wang Li for her assistance with TOC measurement. This article is not an official US Food and Drug Administration (FDA) guidance or policy statement. No official support or endorsement by the US FDA is intended or should be inferred. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2014.10.004. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [2] J. Lee, S. Mahendra, P.J.J. Alvarez, ACS Nano 4 (2010) 3580. [3] D. He, C.J. Miller, T.D. Waite, J. Catal. 317 (2014) 198. [4] A. Nel, T. Xia, L. Madler, N. Li, Science 311 (2006) 622. [5] I.K. Konstantinou, T.A. Albanis, Appl. Catal. B: Environ. 49 (2004) 1. [6] Y. Kikuchia, K. Sunada, T. Iyoda, K. Hashimoto, A. Fujishima, J. Photochem. Photobiol. A: Chem. 106 (1997) 51. [7] R.A. Palominos, M.A. Mondaca, A. Giraldo, G. Peñuela, M. Pérez-Moya, H.D. Mansilla, Catal. Today 144 (2009) 100. [8] H. Zhang, X. He, Z. Zhang, P. Zhang, Y. Li, Y. Ma, Y. Kuang, Y. Zhao, Z. Chai, Environ. Sci. Technol. 45 (2011) 3725. [9] W. Wang, T.W. Ng, W.K. Ho, J. Huang, S. Liang, T. An, G. Li, J.C. Yu, P.K. Wong, Appl. Catal. B: Environ. 129 (2013) 482. [10] M.Y. Guo, A.M.C. Ng, F. Liu, A.B. Djurišic´, W.K. Chan, Appl. Catal. B: Environ. 107 (2011) 150.

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