Holes, and

Aug 25, 2014 - Materials, Xuchang University, Henan 461000, P. R. China. ‡. Center for Food Safety and Applied Nutrition, U.S. Food and Drug Adminis...
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Generation of Reactive Oxygen Species, Electrons/Holes, and Photocatalytic Degradation of Rhodamine B by Photoexcited CdS and Ag2S Micro-Nano Structures Huimin Jia,† Weiwei He,*,†,‡ Wayne G. Wamer,‡ Xiangna Han,† Beibei Zhang,† Shu Zhang,§ Zhi Zheng,*,† Yong Xiang,§ and Jun-Jie Yin*,‡ †

Key Laboratory for Micro-Nano Energy Storage and Conversion Materials of Henan Province, Institute of Surface Micro and Nano Materials, Xuchang University, Henan 461000, P. R. China ‡ Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, College Park, Maryland 20740, United States § State Key Laboratory of Electronic Thin Film & Integrated Devices, School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, P. R. China S Supporting Information *

ABSTRACT: Identifying the dominantly active species and their reactive behaviors in semiconductor photocatalysis is important for developing a full understanding of their photochemical and photophysical properties. Here we report an effective method for studying the reactive oxygen species (ROS) and charge carriers generated by irradiating single crystalline CdS and Ag2S micro-nano structures (MNS). Our method, based on electron spin resonance spectroscopy (ESR) combined with spin trapping and spin labeling techniques, confirmed the generation of superoxide and charge carriers and their contribution to photocatalytic degradation of rhodamine B elicited by CdS and Ag2S MNS. The electronic band structures determined the reactivity of photogenerated holes/electrons and the generation of reactive oxygen species. Our comparison of CdS and Ag2S MNS showed that, because of the large difference between their band edge positions, these two sulfides differ greatly in ROS production and in the reactivity of photoinduced electrons and holes. Our ESR method not only provides specific mechanistic information, but also can predict the photocatalytic activity for metal sulfide and possible metal oxide micro-nano structures.

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INTRODUCTION

Metal sulfide semiconductor micro/nano structures have received a great deal of interest because of their unique optical and electrical properties and broad applications in photocatalysis,1,2 light-emitting diodes, and solar cells.3,4 For example, as a classic II−VI semiconductor, cadmium sulfide (CdS) micro/nano structures can act as visible light photocatalysts to degrade organic dyes or produce hydrogen.5,6 In recent years, researchers have made many attempts to improve the photocatalytic activity of CdS nanoparticles by controlling their size and shape7 and crystal structures,8 as well as by constructing hybrid composite nanostructures with metals or carbon.9,10 To optimize the photocatalytic activity of metal sulfide semiconductors, researchers must be able to identify the ROS and charge carriers produced and understand their relative roles in photocatalytic reactions. Studies to date indicate that the following mechanism can explain the photocatalytic activity of metal sulfides, for example, of CdS:11−13 +

−

CdS + light → CdS holes (h ) + CdS electrons (e ) © 2014 American Chemical Society

e− + O2 → O2−•

(2)

h+ + H 2O → •OH + H+

(3)

2e− + O2 + 2H+ → H 2O2

(4)

H 2O2 + light → •OH + •OH

(5)

H 2O2 + O2−• → •OH + OH− + O2

(6)

As indicated above, the electron/hole pairs and reactive oxygen species (ROS), including H2O2, O2−•, and •OH, generated during irradiation should be the main active species responsible for photocatalytic reactions including both photoreduction and photo-oxidation. In most previous reports, generation of ROS has been recognized as the critical step for explaining the photocatalytic activity of semiconductor nanoparticles.14,15 For instance, studies indicate that hydroxyl radicals are the primary oxidants in photocatalysis by semiconductor nanoparticles.16 Received: June 10, 2014 Revised: August 23, 2014 Published: August 25, 2014

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purchased from Shanghai Chemical Reagent Co. Ltd. The spintrap 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO) was purchased from Applied Bioanalytical Laboratories (Sarasota, FL). 5,5-Dimethy-1-pyrroline N-oxide (DMPO) was purchased from Dojindo laboratories (Japan). Rhodamine B (RhB) and superoxide dismutase (SOD) was purchased from Sigma-Aldrich Co. (St. Louis, MO). 1Hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine (CPH) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) were purchased from Alexis, Enzo Life Sciences, Inc. (NY, USA). Synthesis of CdS and Ag2S Micro-Nano Structures. For our synthesis, 0.5 mmol of CdCl2·2.5H2O and 0.5 mmol of sulfur powder were added into a 28 mL Teflon-lined stainless steel autoclave. Then, 21 mL of absolute ethanol was added and the mixture was vigorously stirred. The autoclave was sealed, heated at 180 °C for 12 h, and air-cooled to room temperature. Finally, the bright yellow precipitate was collected by centrifugation, washed several times with double-distilled water and ethanol, and dried under vacuum at 50 °C. For synthesis of Ag2S, 1.0 mmol of AgNO3 and 0.5 mmol of L-cystine were mixed with 21 mL of deionized water. Then the mixture was transferred into a 28 mL Teflon-lined stainless steel autoclave, which was sealed, heated at 180 °C for 12 h, and then air-cooled to room temperature. Finally, the black precipitate was collected by centrifugation, washed several times with double-distilled water and ethanol, and dried under vacuum at 50 °C. Characterization. The crystal structure of as-synthesized products was characterized by X-ray diffraction (XRD, Bruker D8 Advance diffractometer), using monochromatized Cu Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM, Zeiss EVO LS-15) was used to characterize the morphology of CdS and Ag2S MNS. Energy dispersed X-ray analysis and element mapping were performed by a SEM/EDX (FEI Quanta FEG 250). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected-area electron diffraction were performed with a Tecnai G2 F20 U-TWIN. The samples for TEM analysis were prepared by adding a drop of colloidal solution onto standard holey carbon-coated copper grids. The grids were dried in air at room temperature. The UV−vis absorption spectrum of as-prepared CdS was recorded on a Varian Cary 5000 UV−vis−NIR spectrophotometer. A Renishaw inVia-Reflex was used to record the Raman spectrum of the as-prepared CdS samples by depositing a film of CdS MNS on a glass slide at room temperature. The BET surface areas of CdS and Ag2S micro-nano structures were determined with use of a Micromeritics Gemini 2380 specific area analyzer by measuring nitrogen adsorption. The photocatalytic activities of CdS and Ag2S micro-nano structures were evaluated by measuring the degradation of rhodamine B (RhB) in aqueous solutions. A 0.1 mg/mL sample of as-prepared CdS or Ag2S photocatalyst was dispersed in 20 mL of aqueous solution containing 10 mg/L RhB. The solution was continuously stirred in the dark for 30 min to ensure that an adsorption−desorption equilibrium between the photocatalyst and RhB was established. Then, the suspension was irradiated by using a 850 W xenon lamp with sunlight simulating filter. During irradiation, the solution was stirred to maintain a suspension. Aliquots of suspension were taken from the reactor and centrifuged at 15 min intervals. The residual concentration of the organic dye in the supernatant was monitored with a Varian Cary 300 spectrophotometer. Total organic carbon (TOC) measurement was performed on a

However, questions remain about the mechanisms underlying this activity. For example, researchers do not know if the generation of ROS is necessary for semiconductor photocatalysis or what types of ROS are generated. It is also unclear to what extent ROS contribute to photocatalysis compared to photoinduced charge carriers. Answers to these questions are critical for building a full understanding of the mechanism of semiconductor photocatalysis. Although many reports have been published on determining ROS by indirect spectroscopy or ESR,17,18 as well as on measuring charge carrier dynamics by using transient spectroscopic methods,19,20 the underlying mechanisms are still not fully understood. Answering these questions will require developing an effective method to accurately identify ROS and describe the roles of ROS and charge carriers in the overall photoreaction. Electron spin resonance spectroscopy (ESR) is a powerful technique for studying molecules and materials with unpaired electrons, and has broad applications for examining biological systems, polymer chemistry, and naturally occurring substances.21,22 ESR together with spin trapping is the most reliable and direct method for identifying and quantifying shortlived free radicals. Spin trapping is based on the reaction between an unstable free radical and the spin trap molecule, which produces a relatively stable spin adduct. This technique has been used to study a variety of nanomaterials (e.g., Au, Ag, TiO2, and ZnO) to detect free radical intermediates involved in biologically relevant systems or photoinduced reactions.23−27 Spin labeling is another ESR technique that uses spin label molecules (molecules with unpaired electrons) to study the local dynamics of proteins or biological membranes.28 The interaction between spin labels and neighboring molecules can be monitored by ESR and reveal important mechanistic information. Recently, we have introduced the spin label 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) to study electron transfer during photoexcitation of ZnO/Au hybrid nanostructures.29 By combining spin trapping and spin labeling techniques, we can get specific information about both ROS and electron/hole behavior of nanomaterials. In this paper, we report on an effective method to study the ROS intermediates and charge carriers produced during photocatalysis by CdS and Ag2S micro-nano structures. Our method is based on electron spin resonance spectroscopy with spin trapping and spin labeling techniques. There have been many reports on preparing CdS and Ag2S nanostructures with different shapes and sizes. To make this study representative on a micro/nano scale, we used a simple solvothermal method to prepare CdS and Ag2S with combined micro and nano structures. Rhodamine B dye (RhB), a typical textile industry pollutant, was selected to evaluate the photocatalytic activity of CdS and Ag2S measured by photodegradation and mineralization of RhB. Using the ESR method we have developed, we demonstrate the generation of electron/hole pairs and superoxide during photoexcitation of CdS MNS, and show that holes and superoxide dominate the photo-oxidative degradation of RhB. We also examined Ag2S with micro-nano structures to demonstrate the wide applicability of the ESR-spin trapping and spin labeling method in examining specific ROS and holes/electrons generated during irradiation of semiconductor structures.

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EXPERIMENTAL SECTION Chemical and Materials. Cadmium chloride, AgNO3, sulfur, L-cystine, and absolute ethanol were analytically pure and 21448

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Figure 1. XRD pattern (a) and SEM image at a low (b) and high (c) magnification, TEM image at a low magnification (d), high-resolution TEM image (e) from the red square in panel d, and selected-area electron diffraction pattern (f) of CdS MNS obtained at 180 °C for 12 h. The inset in panel c is the image of an actual snowflake.

TOC analyzer (TOC-VCPH, Shimadzu, Japan) by filtration of the sample through a 0.22 μm microporous membrane after irradiation at selected times. The chemical oxygen demand (COD) of RhB at different levels of degradation was determined according to the Standard Test Methods for Chemical Oxygen Demand of Water by Sealed Digestion (ASTM D1252−06). Electron Spin Resonance Spectroscopy. The ESR measurements were carried out with a Bruker EMX ESR spectrometer (Billerica, MA) at ambient temperature. A light system consisting of a 450 W xenon lamp coupled with optical filters (20CGA-400) was used to generate light above 400 nm. Fifty microliter aliquots of control or sample solutions were put in quartz capillary tubes with internal diameters of 0.9 mm which were then sealed. The capillary tubes were put into the ESR cavity, and the spectra were recorded after irradiation at selected times. All ESR measurements were carried out by using three settings to detect the spin adducts: 20 mW microwave power, 100 G scan range, and 1 G field modulation. The spin traps BMPO and DMPO were used to verify the formation of superoxide (•OOH). CPH and TEMPO were used as spin labels for probing the holes and electrons generated during photoexcitation of CdS or Ag2S. In this study, the intensity of the ESR signal was measured as the peak-topeak height of the second line of the ESR spectrum. The final concentration of each component is described in each figure caption.

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possible reasons for this improvement with these structures. The higher photocatalytic activity of micro-nano structures has been attributed to their larger active surface areas, increased efficiency of light collection and charge carrier separation, and a higher efficiency for production of hydroxyl radicals than corresponding particles. In this work, therefore, CdS with micro-nano structures were prepared and used as a model to study the ROS intermediates and charge carriers activity in enhanced photocatalytic activity. Typically, CdS micro-nano structures (MNS) were synthesized by using a simple solvothermal method. Figure 1 summarizes the results obtained from X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The XRD pattern (Figure 1a) 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 crystalinity of the as-prepared CdS MNS. Figure 1b displays a typical SEM image of the CdS MNS, exhibiting a tree-like superstructure, composed of dendrite branches, with an average size of 3.0−5.0 μm. Under high magnification (Figure 1c), it is clear to see that CdS has hierarchical structures. Two typical structures were clearly observed. One was a quasi-two-dimensional plate-like structure with sixbranched arms and 6-fold symmetry. A dendrite structure is observed for each branch. This hexagonal structure closely resembles an actual hexgonal snowflake (inset in Figure 1c). The second hierarchical structure was a complicated structure composed of dendrite flakes, displaying 3D snowflake-like structures. By using SEM, the thickness of each flake was determined to be 10−20 nm. The obtained CdS structures had overall dimensions on a micro scale and fine structure at the nano scale, so we labeled them micro-nano structures. The

RESULTS AND DISCUSSION

Formation and Characterization of CdS Micro-Nano Structures. Micro-nano structures have been reported to exhibit better photocatalytic activity than corresponding microor nanoscale particles.30−33 Researchers have proposed several 21449

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composition of the CdS MNS was verified by EDX and elemental mapping analysis (Figure S1, Supporting Information), and the Cd/S molar ratio was determined to be 1.1 (averaged from three measurements), consistent with the stoichiometric ratio expected for CdS. The formation process of CdS MNS was studied by stopping the solvothermal reaction at selected times and characterizing reaction products by SEM and XRD (Figures S2 and S3, Supporting Information). The snowflake-like MNS probably evolved from walnut-like nuclei through sidebranching and branch growth.34 The reaction time affects not only the structural evolution but also crystallization of CdS. The typical diffraction peaks of CdS MNS become clearer and stronger at prolonged reaction times. This observation matched quite well with the SEM images taken at each stage of product formation. The TEM, HRTEM, and selected area electron diffraction (SAED) pattern from a CdS MNS branch tip are displayed in Figure 1, panels d, e, and f, respectively. The HRTEM was obtained from area outlined by the red square in Figure 1d. The well-resolved lattice fringes with a hexagonal arrangement confirmed the single crystalline structure of CdS dendrite branches (Figure 1e). The lattice spacing of 0.36 nm corresponds to the distance between [100] planes. The SAED pattern shows a similar hexgonal arrangement and indicates that the electron beam is aligned along the [001] direction (Figure 1f). Diffraction spots demonstrate that the dendrite branch is associated with CdS single crystals exposed by [001] facets with the three directions along [010], [100], and [110]. These results support the contention that each fine dendrite in every branched arm of CdS MNS has a well-defined single-crystalline structure. The single-crystalline structure and exposure of [001] facet with high surface energy was reported to enhance the transfer of photogenerated electrons/holes and lead to greater production of hydroxyl radical.8,35 The UV−vis diffuse reflectance spectra of CdS micro-nano structures was measured (Figure 2a). The as-prepared CdS MNS exhibited strong photoabsorption from 250 to 525 nm, followed by relatively low absorption of up to 700 nm. These results demonstrate that the snowflake-like CdS MNS can absorb both UV and visible light, making CdS MNS a potential visible light photocatalyst. The CdS MNS have an absorption edge of 543.8 nm and band gap estimated to be 2.28 eV by extrapolating the linear region of the plot of the absorbance squared versus energy (inset of Figure 2a). Figure 2b shows the low-frequency part of the Raman spectra for CdS structures. Two typical peaks positioned near 300 (1LO) and 600 cm−1 (2LO), which correspond to the first order longitudinal optical (1LO) and second order longitudinal optical (2LO) phonon peak, respectively. From the 1LO peak, a noticeable asymmetric broadening at the low frequency side was observed, indicating the effect of phonon confinement probably due to the unique structures consisting of micro/nano hybrid structures.36 Identification of Superoxide Induced by CdS MNS with Use of ESR-Spin Trapping. Superoxide and hydroxyl radicals have been recognized as the predominant ROS generated during photocatalysis by metal oxides or sulfides.37 These radicals cannot be detected directly by ESR because of their short lifetimes. However, they readily react with diamagnetic nitrone spin traps, forming stable free radicals (spin adducts) that can be identified from the magnetic parameters of spin adduct’s ESR spectrum.38 To verify the generation of superoxide or hydroxyl radicals induced by CdS

Figure 2. UV−vis diffuse reflectance spectrum (a) and Raman spectrum (b) with excitation at 532 nm of the as-prepared CdS MNS. The inset in panel a shows the plot of (αhν)2 vs hν corresponding to the absorbance spectrum.

MNS, we chose two spin traps, BMPO and DMPO, that are often used to capture hydroxyl radical and superoxide. Figure 3 shows the ESR spectra obtained from solutions containing BMPO and CdS micro-nano structures before and during irradiation above 400 nm. For samples without irradiation, a characteristic ESR signal was absent (Figure 3a). After irradiation for 3 min, we clearly observed a four-line spectrum with relative intensities of 1:1:1:1 and hyperfine splitting parameters of aN = 13.4, aβH = 12.1 G, which is the characteristic spectrum for the adduct, BMPO/•OOH.39 An ESR spectrum characteristic for the DMPO/•OOH spin adduct (four lines with relative intensities of 1:1:1:1 and hyperfine splitting parameters of aN = 14.25, aH = 12.45 G) was also observed when DMPO was the spin trap (Figure S4, Supporting Information).39 These results indicate that superoxide is generated by CdS MNS during irradiation. When more CdS MNS was added (Figure 3b−d), the BMPO/•OOH signal intensity increased, indicating that generation of superoxide was dependent on the concentration of CdS MNS. DMPO and BMPO are also spin traps frequently used for capturing hydroxyl radicals. However, no characteristic ESR signal for the adduct DMPO/•OH or BMPO/•OH was observed. To further determine whether the ESR signal in part comes from hydroxyl radicals, we investigated the scavenging effect of superoxide dismutase (SOD) which can efficiently catalyze the dismutation of superoxide. When 4 U/mL SOD was added (Figure 3e), the ESR signal was totally inhibited compared with 21450

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samples without SOD (Figure 3d). This result confirms that the observed ESR signal is from superoxide without the involvement of hydroxyl radicals. It is noteworthy that the spin adduct DMPO/•OOH was unstable and decayed to the spin adduct DMPO/•OH after irradiation ended (Figure S4, Supporting Information). This observation may explain previous reports of the generation of hydroxyl radicals when using DMPO as a spin trap. Identifying and Measuring the Reactivity of Photoinduced Electrons and Holes by ESR-Spin Labeling. During irradiation of a semiconductor, electrons in the valence band are excited to the conduction band by the adsorbed energy producing charge carriers. The formation of electrons and holes is fundamental to photocatalysis by semiconductor nanoparticles, as they have strong reductive and oxidative abilities and also result in the generation of reactive oxygen species with highly oxidative properties. Here, taking CdS micro-nano structures as an example for the proof of concept, we use ESR with spin labeling to study the redox properties of charge carriers induced by irradiation. In this study, two spin probes 1-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine (CPH) and 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), which are often used in ESR studies of electron transfer in biological system,40,41 were selected to investigate the behavior of electrons and holes. CPH itself is an ESR silent probe, but exhibits characteristic three-line ESR signals after being oxidized (e.g, by holes associated with photoexcited semiconductors) to CP* (CP-nitroxide). In contrast, TEMPO is a typical spin label molecule that can be reduced (e.g., by electrons associated with photoexcited semiconductors) to give a hydroxyl amine, TEMPOH, accompanied by flattening of the

Figure 3. Generation of superoxide during irradiation (λ > 400 nm) of CdS MNS. ESR spectra were obtained at room temperature from samples containing 25 mM BMPO and CdS MNS before (a) and during (b−e) irradiation. Samples contained 0.1 (b), 0.2 (C), or 0.5 mg/mL CdS (e); panel e is the same as panel d but with the addition of 4 U/mL SOD; panel a is the same as panel d but without irradiation. All the spectra were recorded after 3 min of irradiation.

Figure 4. Demonstration of electron and hole generation during irradiation of CdS micro-nano structures with light above 400 nm. (A) The interaction between spin labels CPH or TEMPO with photoinduced holes or electrons results in a change of redox state of spin labels detectable by ESR; (B) ESR spectra of CPH were recorded from samples containing 0.1 mM CPH, 0.1 mg/mL CdS, and 10 U/mL SOD before and after irradiation for 3 min. (C) The ESR signal intensity of TEMPO, in the presence of 0.1 mg/mL CdS MNS, was dependent on the irradiation time. Inset shows the ESR spectra of 0.1 mM TEMPO in the absence and presence of 0.1 mg/mL CdS MNS both after irradiation for 8 min. 21451

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Figure 5. (A) XRD pattern and SEM image (inset) of as prepared Ag2S MNS; (B) ESR spectra obtained from sample containing 25 mM BMPO and 0.1 mg/mL Ag2S MNS before (control) and after irradiation with light above 400 nm in the absence and presence of SOD; (C) ESR spectra obtained from a sample containing 0.1 mM CPH and 0.1 or 0.2 mg/mL Ag2S MNS before (control) and after irradiation with light above 400 nm; (D) the ESR signal intensity of TEMPO, in the presence of 0.1 mg/mL Ag2S structures, was dependent on the irradiation time (the inset shows the ESR spectra of TEMPO in the presence of 0.1 mg/mL Ag2S structures before and after irradiation for 25 min). The ESR spectra in parts B and C were recorded after 8 min of irradiation.

1:1:1. This spectrum is characteristic for nitrogen-centered free radical of the spin label TEMPO. TEMPO can be reduced by photogenerated electrons to form TEMPOH acormpanied by a reduction of the ESR signal intensity.42 The signal intensity was unchanged after mixing with CdS before irradiation. However, the signal intensity rapidly decreased within 1 min of irradiation. With extended irradiation times, a greater reduction was observed (Figure 4C). The free radical TEMPO does not react with oxidizing species (superoxide and holes) or unirradiated CdS particles. Therefore, the formation of TEMPOH from TEMPO must involve the transfer of electrons from the surface of CdS to the spin label. The level of electrons generated during irradiation of CdS can be estimated from the results shown in Figure 4C. Near complete consumption of TEMPO was observed during 11 min of irradiation of a 50 μL solution containing 0.1 mM TEMPO. The one-electron reduction of TEMPO leading to the results in Figure 4C would require that approximately 5 × 10−9 mol of free electrons were generated during irradiation of 0.005 mg of CdS MNS for 11 min.

ESR signal (Figure 4A). Therefore, the interaction between these spin labels and electrons/holes can be readily assessed by using ESR. First, we used the ESR silent probe CPH to study oxidants generated during photoexcitation of CdS. Figure 4B demonstrates the interaction between CPH and photonduced holes. We observed no ESR signals for CPH alone during irradiation or for a mixture of CPH and CdS structures held in the dark. After irradiating for 3 min, a strong three-line ESR signal of CP* with intensity ratio of 1:1:1 was detected, implying the oxidation of CPH. However, the oxidizing intermediate was not clear because in this experimental system, both superoxide and holes are potentially oxidants.31 To find the dominant oxidant, we added 10 U/mL SOD to the above mixture to eliminate any superoxide. Only a slight decrease (about 8%) of ESR signal resulted, strongly supporting the involvement of photoinduced holes as the dominant reactive species leading to the oxidation of CPH. Second, we used TEMPO to probe the reactivity of electrons generated concomitantly with holes in photoexcited CdS MNS (Figure 4C). The ESR spectrum of an aqueous solution of TEMPO shows a signal having three peaks with intensity of 21452

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Figure 6. Photocatalytic activity of CdS and Ag2S for oxidative decomposition of RhB during irradiation. The temporal concentration change of RhB (A) and −ln(C/C0) (B) as a function of irradiation time during the degradation of RhB. Both photocatalysts were present at 0.1 mg/mL.

Identification of the Intermediates for Ag2S MNS. The described spin trapping and labeling techniques were used to identify and distinguish the reactive oxygen species and electrons/holes in photoreactions for other metal sulfides. Ag2S micro-nano structures were selected due to their ability to absorb longwave light and their facile synthesis by using the described solvothermal methods that we have developed. Figure 5A shows a typical XRD pattern and an imbedded SEM image of Ag2S micro-nano structures. The XRD pattern shows that all the diffraction peaks can be indexed to monoclinic acanthite silver sulfide (JCPDS no. 14−72), indicating the high crystalinity of as-prepared Ag2S MNS. The SEM image shows that the prepared Ag2S MNS have a irregular shape with an average diameter of about 200−300 nm. The presence of both Ag and S was demonstrated by EDX analysis (Figure S5, Supporting Information). The atomic ratio of Ag/S was determined to be 1.96 ± 0.1 (n = 3). The EDX maps of the elements, Ag and S, also verified that more Ag signal was detected than S signal (Figure S6, Supporting Information). The specific surface area of Ag2S was measured to be 10.1 m2/ g, slightly lower than that of CdS (13.5 m2/g). Figure 5B gives the ESR spectra of Ag2S obtained during irradiation in the presence of BMPO. We observed a characteristic, albeit not strong, ESR spectrum that was similar to the BMPO/•OOH signal generated in the presence of CdS structures. After the addition of 4 U/mL SOD, the ESR signals were totally quenched. These results indicate that photoexcited Ag2S can generate superoxide, although only weakly. When mixed with the spin probe, CPH, photoexcited Ag2S readily oxidizes CPH to form CP* identified by its characteristic ESR signal (Figure 5C). The ESR signal intensity of CP* increased proportionally with the concentration of Ag2S, and was also dependent on the irradiation time (Figure S7, Supporting Information). Since Ag2S only weakly generates superoxide when irradiated and unirradiated Ag2S has no effect on CPH, the CP* produced by Ag2S should be attributable to the holes generated during

photoexcitation of Ag2S. When TEMPO was used as a spin label, a reduction of the ESR signal of TEMPO was observed during photoexcitation of Ag2S (Figure 5D). This reduction in ESR signal is attributable to the electrons elevated to the conduction band during irradiation of Ag2S. The reduction of the TEMPO signal intensity was dependent on irradiation time, with a reduction of about 30% in 25 min of irradiation. These ESR results indicate that CdS structures exhibited a much stronger ability than Ag2S in producing superoxide and hole/ electron pairs. These results predict that CdS has a higher a photocatalytic activity than Ag2S. Photocatalytic Activity of CdS and Ag2S MNS. Can these results obtained by using spin trapping and spin labeling be used to predict the photocatalytic activities of CdS and Ag2S? To determine this, we further investigated the correlation between photocatalyic activity and the results from ESR spectroscopy. We examined the photodegradation of Rhodamine B (RhB), a representative organic dye, to evaluate the photocatalytic activity of CdS and Ag2S (Figure 6). In the absence of a photocatalyst, RhB is resistant to photodegradation (Figure 6A, control). In the presence of CdS, 62% of RhB was degraded after irradiation for 75 min, while only 8.5% of RhB was photodegraded in the presence of Ag2S (Figure 6A). The degradation of RhB over CdS and Ag2S was found to be a pseudo-first-order kinetic process (Figure 6B). The degradation rate constants of CdS and Ag2S were calculated as 0.012 and 0.0011 min−1, respectively. Thus, CdS photocatalyzes the degradation of RhB at a rate 10 times higher than Ag2S. The TOC and COD values for RhB before and after irradiation with CdS and Ag2S were determined after irradiation at selected times. The relative TOC and COD changes of RhB were compared (Figure S8, Supporting Information). The reduction of TOC and COD for the RhB dye solution indicates the mineralization of RhB along with the color removal under phtocatalysis. The decrease in TOC and COD in the presence of CdS is much higher than Ag2S, consistent with the results 21453

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Scheme 1. Schematic Illustration for the Generation of Charge Carriers and Reactive Oxygen Species by CdS and Ag2S Structures during Irradiation, and Their Interactions with Surrounding Moleculesa

a

The red cross-mark means the generation of hydroxyl radical is unfavorable in the case of CdS and Ag2S MNS.

and have a band gap of 2.4 and 0.92 eV, respectively. The ECB,pH6 and EVB,pH6 were calculated to be −0.76 and 1.64 eV for CdS, and −0.23 and 0.69 eV for Ag2S (vs NHE), respectively, as illustrated in Scheme 1A. Comparing the band edge energy level with redox potentials of ROS, it is obvious that the excited eletrons in the conduction band of CdS and Ag2S were sufficiently negative to reduce oxygen and to generate superoxide anions. However, holes in the valence band cannot oxidize water to produce hydroyxl radicals since the valence band edge potential is less than E0 of (H2O/•OH). We observed CdS generated more superoxide than Ag2S during photoexcitation, and exhibited higher photocatalytic activity than Ag2S. These characteristics may be attibuted to a more negative conduction band edge and more positive valence band edge energy in CdS than in Ag2S.

observed for photocatalytic decoloration of RhB. We have also done the experiments involving the effects of SOD on photocatalysis (data not shown). A slight reduction in catalytic activity of CdS during degradation of RhB is observed. Less effect was observed with Ag2S, probably because of the lower catalytic activity of Ag2S and lower level of superoxide generated by Ag2S than CdS. The results from these photodegradation measurements (Figure 6) are consistent with results from ESR measurements, confirming the correlation between the photocatalytic activity and reactive intermediates such as superoxides, holes, and electrons generated by irradiated CdS and Ag2S. Mechanims of Formation of Reactive Oxygen Species. Taken together, our results demonstrate that ESR with spin trapping and spin labeling techniques can be used to clarify tthe photocatalytic reaction mechanism for CdS and Ag2S micronano structures, as depicted in Scheme 1. Absorption of light by semiconductor MNS is accompanied by promotion of electrons from the valence band to the conduction band. This absorption of light results in the formation of an electron (conduction band) and hole (valence band) pair. The electrons and holes may recombine and yield no net photocatalysis. Alternatively, electrons and holes may react to form ROS or directly react with other substrates. By comparing the energy levels of valence band (EVB) and conduction band (ECB) with the standard redox potential (E0) of adsorbed substances, it is feasible to predict the photoreduction/oxidation ability and the generation of ROS. The redox potential for the dissolved oxygen/superoxide couple is −0.16 eV (E0(O2/O2−•), vs NHE), and for the H2O/ •OH couple it is 2.32 eV (E0(H2O/•OH), vs NHE).43 Photogenerated electrons from metal sulfide MNS, needed to form superoxide, therefore must have a potential less than −0.16 eV, and holes must have a potential greater than 2.32 eV to form hydroxyl radicals. In aqueous environment, the EVB and ECB of metal sulfides are affected by pH.44 Therefore, the relevant conduction band edge (ECB) and valence band edge (EVB) position at pH ∼6.0 were calculated by the following:44,45

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CONCLUSIONS We developed a method for identifying and distinguishing the ROS and charge carriers generated by CdS and Ag2S micronano structures during photoexcitation by using electron spin resonance spectroscopy with spin trapping and spin labeling techniques. This method provides specific information that increases our understanding of the photocatalytic mechanisms for semiconductor photocatalysts. The utility of this method was demonstrated in studies on CdS and Ag2S, both of which have micro-nano structures. Photoinduced charge carriers with high reactivity and superoxide were generated during irradiation of CdS, and were characterized by spin trapping and spin labeling techniques. Holes and superoxide were the dominantly active species for the photocatalytic degradation. The Ag2S micro-nano structures behave in ways similar to CdS in generating reactive oxygen species and charge carriers but with a lower ability in producing superoxide and hole/electron pairs, which consequently results in their lower photocatalytic activity. The described ESR spectroscopic method may be a powerful tool for obtaining detailed mechanistic information for photocatalysis and predicting the photocatalytic activity of metal sulfides or oxide micro-nano structures.

ECB,pH6 = Ec o + 0.059(pH ZPC − pH)

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E VB,pH6 = Ev o + 0.059(pH ZPC − pH)

ASSOCIATED CONTENT

S Supporting Information *

where the pHZPC refers to the point of zero ζ potential of the semiconductor. This value was found to be 2.0 for both CdS and Ag2S.45 CdS and Ag2S MNS behave like the bulk system

SEM image, SEM-EDX element mapping and energy dispersive X-ray spectrum of CdS (Figure S1); evolution of CdS micronano structure morphology at different solvothermal times 21454

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(Figure S2); XRD pattern of the CdS product obtained at different reaction times (Figure S3); demonstration of superoxide/DMPO adduct decay to hydroxyl radical/DMPO adduct (Figure S4); EDX spectrum of Ag2S nanostructures (Figure S5); SEM-EDX mapping of element Ag and S of Ag2S nanostructures (Figure S6); the irradiation time dependence of ESR signal intensity from CPH in the presence of Ag2S (Figure S7); and normalized TOC and COD changes as a function of irradiation time (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.H.). *E-mail: [email protected] (Z.Z.). *E-mail: [email protected] (J.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant Nos. 21303153, 61204009, and 21273192), Program for Science & Technology Innovation Talents in Universities of Henan Province (Grant No. 14HASTIT008), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 144200510014), and Program for Science and Technology of Henan Province (Grant Nos. 124300510055 and 12A150022). We would like to thank Dr. Philippa J. Benson for editorial assistance. 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.

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