Zn-Doped CdS Nanoarchitectures Prepared by Hydrothermal

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Zn-Doped CdS Nanoarchitectures Prepared by Hydrothermal Synthesis: Mechanism for Enhanced Photocatalytic Activity and Stability under Visible Light Fan Yang,†,‡,⊥ Nan-Nan Yan,†,⊥ Sheng Huang,† Qiang Sun,§ Li-Zhi Zhang,∥ and Ying Yu*,† †

Institute of Nanoscience and Nanotechnology, Central China Normal University, Wuhan 430079, China School of Electronic and Electrical Engineering, Wuhan Textile University, Wuhan 430073, China § School of Physics and Engineering, Zhengzhou University, Zhengzhou 450001, China ∥ College of Chemistry, Central China Normal University, Wuhan 430079, China ‡

ABSTRACT: Zn-doped CdS nanoarchitectures with different Zn content are synthesized by a simple hydrothermal method with water as the only solvent. The prepared samples are characterized by X-ray powder diffraction, scanning electron microscopy, UV−vis diffuse reflectance spectra, Brunauer−Emmett−Teller measurement, and X-ray photoelectron spectroscopy, while the photocatalytic activities are tested by photocatalytic degradation of rhodamine-B under visible-light irradiation. The results show that CdS with small amount of Zn doping can lead to an enhanced photocatalytic activity. Zn-doped CdS sample derived at 160 °C for 12 h with the molar ratio of Zn/Cd = 1:10 exhibits the best photocatalytic activity, which is much higher than that of pure CdS. Moreover, there is almost no loss of photocatalytic activity after four cycles of repeated experiments. So, Zn2+ doping indeed improves the photocatalytic activity and stability of CdS. Theoretical calculation indicates that Zn doping into a CdS crystal lattice can result in the shift of the valence band of CdS to a positive direction. It may lead to its higher oxidative ability than pure CdS, which is important for organic pollutant degradation under visible-light irradiation. Furthermore, the low formation energy for Zn-doped CdS systems demonstrates that the stability of CdS with Zn2+ doping can be improved. Experimentally and theoretically, this study will be useful for the improvement of photocatalytic activity and stability of CdS through the method of metal ion doping.

1. INTRODUCTION Since photoelectrochemical water splitting into H2 and O2 on a TiO2 electrode was reported in 1972,1 TiO2 photocatalysis has received a great deal of attention in all aspects for the past several decades, especially in water purification technology.2−5 However, because of the wide band gap of TiO2, its practical application is limited due to the need of an ultraviolet excitation source, which accounts for only small part (3−5%) of solar light. Therefore, developing visible-light-driven photocatalysts is necessary for photocatalysis application under solar energy. CdS is a traditional II−VI semiconductor with a direct band gap of 2.4 eV, which can perform as an excellent photocatalytic material sensitive to visible light. Yet, it is well-known that pure CdS is unstable upon light illumination, and photocorrosion or photodissolution may occur on a photocatalyst surface during photocatalytic reaction.6,7 Additionally, the photocatalytic activity of CdS has not reached high enough for practical © 2012 American Chemical Society

application. There are several ways to modify CdS to overcome its drawbacks such as the hybridization of CdS with other semiconductors8,9 or polymer,10 metal ion doping,11,12 CdS deposition onto the materials with high surface area,13 and so on. In recent years, a lot of attention has been focused on metal ion doping, which is believed to improve the photocatalytic activity and stability of semiconductor effectively.3,14 It is because metal ion doping not only can affect the photophysical behavior of semiconductor but also facilitate the photochemical reaction.15,16 Metal ion may influence surface property by the generation of a Schottky barrier through the metal in contact with the semiconductor surface, which acts as an electron trap. Received: January 30, 2012 Revised: April 3, 2012 Published: April 3, 2012 9078

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The trap can capture e− or h+ and thereafter inhibit e−/h+ recombination.2 In addition, there are also many noble metals, such as Pt, Au, and Ag that have been shown to promote the separation of photogenerated electron and hole for the improvement of photocatalytic activity.17−20 Most recently, ZnxCd1−xS solid solutions with the morphology of nanoparticles and nanorods were reported by Fu and Li’s group.7,21 They had much better photocatalytic activity for dye degradation than pure CdS and N-doped TiO2, and the degradation pathway for organic pollutants were also explored by liquid chromatography-mass spectrometry (LC-MS). Zn and Cd are in the same element group. Zn hybridization with CdS has resulted in positive effect. However, from the viewpoint of materials, the question of why Zn influences CdS in such a way still remain unanswered. The unstability problem of CdS has not been solved yet. There are many methods to synthesize CdS materials such as chemical bath deposition,22,23 microwave method,11,21 solvothermal method,7,39 and so forth. Among all the preparation methods, the hydrothermal method is a popular one for lowtemperature synthesis, and it is easy to operate. Through the change of the hydrothermal conditions, such as temperature, reaction time, and reactant concentration, the prepared samples with different structure, morphology, and compositions can be tuned. Moreover, band gap energy and band positions that drive both oxidation and reduction under visible-light irradiation can also be adjusted by changing hydrothermal conditions.24,25 For nanomaterials, environmentally benign preparation method and nonorganic solvent are important for their future application, since the crisis of environment around the world is becoming more and more serious. Therefore, the hydrothermal method will be promising in future. Here, in order to address the issue about CdS used for organic pollutant degradation, a metal ion doping method has been adopted with Zn2+ as a model. That is, Zn-doped CdS nanoarchitecture samples with different Zn content have been synthesized via a simple hydrothermal route with water as single solvent. The preparation condition has been optimized by the change of reaction temperature and time. The photocatalytic activity of the prepared samples has been evaluated by the degradation of rhodamine-B (RhB) as a model organic pollutant under visible-light irradiation. Furthermore, through characterization of the prepared samples and theoretical calculation based on density functional theory (DFT), the reason why the photocatalytic activity and stability of CdS is enhanced with Zn2+ doping has been explored and described. To the best of our knowledge, this is the first time to systematically investigate the mechanism for the enhanced photocatalytic activity and stability of CdS with the presence of suitable amount of metal ion.

through centrifugation, washing for at least 3 times with distilled water and absolute alcohol, and being dried in air at 80 °C for 10 h. To prepare different samples, the experimental parameters, such as molar ratio of Zn/Cd, reaction temperature, and time had been tuned during the synthesis. All hydrothermal processes were conducted in the absence of any surfactant or organic solvent. 2.2. Characterization of Photocatalysts. X-ray powder diffraction (XRD) patterns of the prepared samples were analyzed by a Y-2000 diffractometer (D/max 30 kv) using Cu Kα radiation (λ = 0.154178 nm). Data were recorded at a scanning rate of 0.04°/s for 2θ in the range of 10° ≤ 2θ ≤70°. The morphology of the sample was investigated by field emission scanning electron microscopy (SEM) (JEOL JSM6700F). The UV−vis diffuse reflectance spectra (UV−vis DRS) of the samples were recorded by PerkinElmer Lambda35 Spectrophotometer in the range of 200−800 nm. The chemical composition analysis of the sample surfaces was made by X-ray photoelectron spectra (XPS) (VG Multiab-2000) using a PHI Quantum 2000 XPS system with a monochromatic Al Kα source and charge neutralizer. All of the spectra were calibrated to C 1s peak at 284.6 eV. The Bruauer−Emmett−Teller (BET) surface area (SBET) of the samples was determined by a high speed automated area and pore size analyzer (Bel Minisorp). 2.3. Photocatalytic Activity Test. Photocatalytic activities of the prepared samples were evaluated by the degradation of RhB under visible-light irradiation. A 350 W xenon lamp (Lap Pu, XQ) was used as a light source, which can generate visible light target-background environment. The photocatalytic reaction was conducted in an homemade beakerlike glassware with double wall for cooling the reactor by tap water. The photocatalytic process was as follows. Photocatalyst (0.1 g) was added into 100 mL of RhB solution (1.0 × 10−5 mol·L−1). Before the irradiation, the photocatalyst was dispersed in the solution by magnetically stirring in the dark for about 30 min to reach an adsorption/desorption equilibrium between the photocatalysts and organic pollutant. Then, the suspensions were exposed to visible-light irradiation. About 0.5 mL of suspension was sampled every 30 min and centrifuged to separate the photocatalyst. The filtrates were analyzed by using a UV−vis spectrophotometer (Shimadzu UV-1700), and the concentration of RhB was monitored at 553 nm. The calculation of the photocatalytic efficiency was referred to ref 7. 2.4. Theoretical Calculation Method. The electronic structures of the materials with Zn doping with different concentrations were calculated based on DFT26 by using the Vienna ab initio simulation package (VASP).27,28 The local density approximation (LDA) was used with the Ceperly− Alder form of the exchange and correlation parametrized by J. Perdew and Zunger.29,30 To simulate the doping system, a 3 × 2 × 2 CdS supercell with 48 atoms was employed, which is the same as that for the (CuIn)xCd2(1−x)S2 system in ref 14 that we calculated before. The plane wave cutoff energy 350 eV was used here for all calculations to ensure the accurate results. For Brillouin zone integration, a 5 × 5 × 5 Monkhorst−Pack kpoint sampling scheme31 was used for the supercell.

2. EXPERIMENTAL SECTION 2.1. Preparation of Photocatalysts. All chemicals used in the study were purchased from Shanghai Guoyao Chemical Co. They were analytical grade and used without further treatment. A typical synthesis process for Zn-doped CdS was as follows. Stoichiometric amount of Cd(CH3COO)2, Zn(CH3COO)2, and thiourea was dissolved in 80 mL of deionized water. Followed by further stirring at room temperature for about 10 min, the mixture was transferred into a Teflon-lined stainless steel autoclave of 100 mL capacity, which was maintained at 120−180 °C for different hours. After being cooled down to room temperature naturally, the products were obtained

3. RESULTS AND DISCUSSION 3.1. Characteristic of Zn-Doped CdS. Figure 1 displays XRD patterns of the samples obtained at different molar ratio of Zn/Cd for reaction time of 5 h and temperature of 160 °C. The reflections of all samples were well-indexed to the hexagonal phase of CdS (JCPDS card no. 41-1049). No 9079

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Figure 2 shows SEM images of the samples prepared at 160 °C with the molar ratio of Zn/Cd = 1:10 for different reaction times. It can be clearly seen that the samples basically looked like cauliflowers with the size ranging from 1.5 to 3 μm, which was mainly assembled by nanoparticles. With the increase of hydrothermal reaction time, the microstructures changed. When the reaction time was 0.5 h, the obtained nanoarchitecture had the smallest size. The prepared nanoarchitectures gradually grew up as the reaction time increased. When the hydrothermal reaction lasted for 5−8 h, the morphology was stable but the size of the nanoarchitecture was much larger than that prepared with 2 h. Additionally, the assembled particle size also became larger. When the reaction lasted for 12 h, a real flowerlike structure appeared (Figure 2g), which means that small nanoparticles were assembled into petals of a flower. The growth mechanism is unknown now, and it is under investigation. After 24 h reaction, the beautiful flower disappeared and changed back to cauliflower. So, the hydrothermal reaction time had effect on the morphology of the prepared samples, and the sample derived at 160 °C for 12 h mostly consisted of obviously hierarchical flowerlike microstructures, which may be helpful for its photocatalytic activity. UV−vis diffuse reflectance spectrum is used to probe the optical absorption property, and the band structure of materials, since UV−vis light excitation creates photogenerated electrons and holes. Figure 3 shows the diffuse reflectance absorption spectra of Zn-doped CdS prepared at various molar ratio of Zn/Cd. All of the samples demonstrated excellent visible-light absorption from 554 to 569 nm and slight shift of the absorption edge to blue light with an elevation of the molar ratio of Zn/Cd. For a crystalline semiconductor, it is known that the optical absorption near the band edge follows eq 1:

Figure 1. XRD patterns of Zn-doped CdS samples with different molar ratio of Zn/Cd: (a) pure CdS, (b) 1:4, (c) 1:3, (d) 1:2, and (e) 1:1. The figure below is the enlarged peaks corresponding to (112) and (110) crystal plane for Zn-doped CdS samples with different molar ratio of Zn/Cd.

peaks of any other phases or impurities were detected. So, there was no ZnS in the prepared samples. From the enlarged peaks attributed to (112) and (110) crystal plane for Zn-doped CdS samples with different molar ratio of Zn/Cd, it can be seen that the diffraction peaks were shifted to a low degree side as the molar ratio of Zn/Cd decreased. The successive shift of the peaks in the XRD patterns indicates that the prepared samples were not mixtures of CdS and ZnS phases but Zn ions doped CdS.32 The shift is reasonable because the ion radii of Zn2+ (0.74 Å) is just slightly smaller than that of Cd2+ (0.97 Å), and the doped metal ions may likely be incorporated into CdS matrix by substituting for Cd2+ lattice sites.

αhν = A(hν − Eg )1/2

(1)

where α, ν, Eg, and A are the absorption coefficient, light frequency, band gap, and constant, respectively.33 According to this equation, the band gap of pure CdS is estimated to be 2.20 eV from the onset of the absorption edge, which is slightly smaller than 2.4 eV reported in literature.34 The reason might be that CdS prepared here was assembled by larger particles than normal bulk CdS.35 Thus, the band gap of CdS became smaller, and the absorption peaks of CdS microstructures notably red shifted. On the other hand, it might be resulted

Figure 2. SEM images of Zn-doped CdS prepared at 160 °C with the hydrothermal reaction time of (a) 0.5 h, (b) 1 h, (c) 1.5 h, (d) 2 h, (e) 5 h, (f) 8 h, (g) 12 h, and (h) 24 h. All of the images have the same scale bar of 200 nm. 9080

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Figure 3. UV−vis diffuse reflectance spectra of Zn-doped CdS samples with various molar ratio of Zn/Cd. The insert is plots of [F(R)]2 versus photo energy, which is the demonstration of how band gap can be obtained with pure CdS as an example. Figure 4. High resolution XPS spectra of (a) Cd 3d in pure CdS, (b) Cd 3d, and (c) Zn 2p in Zn-doped CdS (Zn/Cd = 1:1).

from systematic errors. The band gap of other samples for corresponding doping proportion is listed in Table 1. It can be

binding energy shifts imply that the local chemical state was a little influenced by Zn2+ incorporation into the CdS lattice.38,39 Furthermore, from the XPS spectrum results, it is found that the superficial constant ratio of Zn 2p and Cd 3d was around 1:12 for the sample with molar ratio of Zn/Cd = 1:10, which is basically consistent with the actual proportion in preparation. 3.2. Photocatalytic Activity and Stability. The photocatalytic activities of various samples prepared at different preparation conditions were evaluated by the degradation of RhB in aqueous solution under visible-light irradiation. To explore the optimum conditions of photocatalytic activity, the degradation process of RhB were studied using different photocatalysts. The remaining concentration of RhB at various time intervals was investigated by examining the absorption in UV−vis spectra at 553 nm. Figure 5 shows the degradation efficiency of RhB over different Zn-doped CdS photocatalysts under visible-light irradiation after the adsorption/desorption

Table 1. BET Surface Area (SBET) and Band Gap of the ZnDoped CdS Samples with Various Molar Ratios of Zn/Cd Zn-doped CdS with different molar ratios of Zn/Cd sample

0:1

1:25

1:10

1:5

1:1

BET (m2·g−1) band gap (eV)

0.27 2.20

0.43 2.21

0.59 2.23

0.65 2.24

1.80 2.26

seen that, with the increase of the molar ratio of Zn/Cd, the band gap for Zn-doped CdS increased slightly and gradually. It may be due to the effect of different amount of Zn ion doping. In addition, BET surface area of the prepared samples is displayed in Table 1 as well. It can be found that the surface area of Zn-doped CdS samples was larger than that of pure CdS. For Zn-doped CdS samples, BET surface area increased slightly with the increase of the molar ratio of Zn/Cd. It is wellknown that a high specific surface area is usually beneficial for reactant adsorption, which can increase the contact chance between photogenerated carriers and the reactants and then lead to a better photocatalytic activity.36 As shown in Table 1, although all of the prepared samples had small BET surface area, they exhibited an excellent photocatalytic activity (shown later in Section 3.3), especially the sample derived at 160 °C for 12 h with the molar ratio of Zn/Cd = 1:10 possessed the best photocatalytic performance. In this case, it seems that the BET surface area is not the decisive factor in the photocatalytic reactions. So, the photocatalytic activity is not the result of adsorption relevant to the surface area but originated from their inner structure. XPS experiments were performed to analyze the composition of the samples and the chemical state of the atoms. Figure 4 demonstrates the high-resolution XPS spectra for the Cd 3d and Zn 2p regions in pure CdS and Zn-doped CdS sample (Zn/Cd = 1:1). The two peaks observed at 406.6 and 413.2 eV were attributed to Cd 3d5/2 and Cd 3d3/2 in Zn-doped sample, respectively. Substantially, the two peaks had a binding energy of 1.5 eV higher than that of pure CdS. Additionally, the binding energy positions located at 1023.8 and 1046.8 eV were ascribed to Zn 2p3/2 and Zn 2p1/2 in Zn-doped CdS sample, respectively, which also slightly shifted toward higher energies compared to the values in database.37 Both of the above

Figure 5. Photodegradation of RhB solution (1.0 × 10−5 M, 100 mL) in the presence of pure CdS and Zn-doped CdS photocatalysts (0.1 g) under visible-light irradiation. (a) Zn/Cd mole ratio effect on the photocatalytic activity of prepared samples, (b) hydrothermal temperature effect, (c) reaction time effect, and (d) photocatalytic activity comparison in different conditions. 9081

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Cd = 1:10. The photodegradation behavior of RhB by the photocatalyst for four cycles under the same conditions is displayed in Figure 6. Before each experiment, the used

equilibrium for 30 min in dark. Since metal ion dopant can act as a mediator of interfacial charge transfer or as a recombination center, the activity or efficiency of doped CdS is changeable. Therefore, the doping concentration has an optimal value.4 Figure 5a illustrates the photocatalytic activity of photocatalysts derived at 180 °C for 5 h by the change of Zn/Cd molar ratio. It can be clearly seen that the samples doped with appropriate amount of Zn ion had much higher photocatalytic activity compared with pure CdS, especially the sample with the molar ratio of Zn/Cd = 1:10, which exhibited the best photocatalytic activity. Zn2+ dopant can serve as charge traps retarding electron−hole combination rate and thereafter enhancing the interfacial charge transfer for RhB degradation within a suitable molar ratio of Zn/Cd.5 When the molar ratio of Zn/Cd was too low, the photocatalytic activity decreased. The possible reason is that the Zn2+ ion dopant cannot act as a mediator of interfacial charge transfer if the molar ratio of Zn/ Cd is lower than the optimal value. Meanwhile, Zn2+ doping also became detrimental when the molar ratio of Zn/Cd was higher than the optimal value. It is because the recombination rate may increase when the distance between trapping sites in a particle decreases.11 It is well-known that the hydrothermal temperature plays an important role in the formation of crystal structure, shape, and size,40 which may influence photocatalytic efficiency. Figure 5b displays the photocatalytic activity of the samples prepared at various hydrothermal temperature for 5 h at the molar ratio of Zn/Cd = 1:10. It can be seen that the sample obtained at 160 °C had the highest photocatalytic activity, which is most likely due to outstanding crystal quality and suitable surface area under this temperature. Although the sample prepared at 120 °C also exhibited good activity, sample yield was quite low. So, the reaction temperature of 160 °C was used for the preparation of samples for further experiment. In Figure 5c, the photocatalytic activity of RhB over Zndoped CdS samples derived at 160 °C with different hydrothermal reaction time is demonstrated. As can be seen from the graph, the photocatalytic activity of the sample derived at 12 h is significantly higher than those derived at any other reaction time. In a word, based on all of the above photodegradation experiments, it can be concluded that the sample prepared at 160 °C for 12 h with the molar ratio of Zn/ Cd = 1:10 possessed the best photocatalytic activity for the degradation of RhB under visible-light irradiation and the photocatalytic degradation efficiency reached 99% after 2 h. In order to properly illustrate the higher photocatalytic activity of the best sample we prepared, some comparative experiments were conducted under different conditions, and the results are shown in Figure 5d. From the chart, it can be clearly seen that there was almost no degradation without any catalysts under visible-light irradiation. Substantially, under identical conditions, the sample of Zn-doped CdS demonstrated significantly higher photocatalytic activity than that of pure CdS. The low activity of pure CdS might be probably due to the poor stability of pure CdS and photocorrosion or photodissolution present on the surface of catalysts in the photocatalytic reaction.7 Therefore, the above results indicate that Zn2+ dopant indeed can play an important role for the improvement of photocatalytic activity of CdS. The capability for reuse is one of the most critical factors for an ideal photocatalyst. So, we examined the stability of the sample prepared at 160 °C for 5 h with the molar ratio of Zn/

Figure 6. Repeated experiments of photocatalytic degradation of RhB over the Zn-doped CdS sample prepared at 160 °C for 5 h with the molar ratio of Zn/Cd = 1:10 under visible-light irradiation (concentration of RhB, 1 × 10−5 mol/L).

photocatalyst was cleaned through ultrasonication. The repeated experiments show that the photocatalyst was very stable, since the photocatalytic efficiency almost remained unchanged with cycles going on. The slight decrease of the efficiency after 2 h degradation in the third and fourth cycles may come from the residual contamination on the surface of the catalyst. Moreover, the crystal structure of the catalyst before and after degradation of RhB was demonstrated in Figure 7. It indicates that the position and intensity of

Figure 7. XRD pattern comparison for Zn-doped CdS before RhB degradation and after RhB degradation for the fourth cycle.

diffraction peaks was almost the same although there was a small peak located at 68.0° after degradation for the fourth cycle. It probably resulted from the contamination on the surface of the catalysts. Overall, Zn2+-doped CdS samples exhibited excellent stability and did not decompose or photocorrode during the photocatalytic reaction. 3.3. Mechanism of Enchanced Photocatalytic Activity. As for the mechanism of the enhanced photocatalytic activity of Zn-doped CdS, it has been only described by Fu and Li’s group with the viewpoint of reactive species produced by photocatalysts under visible-light irradiation for dye degradation.7,21 Terephthalic acid photoluminescence probing technique and an electron spin resonance (ESR) instrument were used to explore 9082

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the reactive species evolved during the photocatalytic reaction process, and it is found that active oxygen species (O2‑•, OH•, •HO2) or the hVB+ were responsible for the degradation. However, the photocatalytic activity of a semiconductor for the degradation of organic pollutants is also closely relevant to its band structure from the viewpoint of materials, since the active oxygen species can be produced by photocatalysts with suitable band gap. For further investigation, we performed DFT calculation, which is well-known for its accurate calculations for crystal and electronic structure of materials. The model used for wurtzite crystalline structure CdS is the same as (CuIn)xCd2(1−x)S2 we calculated before in ref 14. Here, the band structure and formation energy of CdS doped with Zn atom under different concentrations were calculated. Besides, the evolvement of crystalline structure of CdS under different doping concentrations was also explored. We considered two doping levels as Zn/Cd = 1:23 and Zn/ Cd = 1:11. The calculated stable structures are shown in Figure 8. For both of the doping systems, the calculated average bond

Figure 9. Calculated band structure for Zn-doped CdS with different molar ratio of Zn/Cd.

for Zn-doped CdS is very important for organic pollutant oxidation, since the photogenerated holes can have high oxidative potential for direct organic pollutant degradation or the easy formation of oxygen reactive species,17,21 which may lead to higher photocatalytic activity of Zn-doped CdS than that of pure CdS. That is when photocatalyst was irradiated by visible light, an electron (e−) in the VB can be excited to the CB, and a hole (h+) is generated in the VB simultaneously. Herein, h+ is considered as a reactive species during photocatalytic reactions,20 due to the lower VB energy level, the stronger oxidation ability of h+ for direct organic pollutant degradation, or the formation of oxygen reactive species. Besides, the formation energies were also explored. Herein, the formation energy of the defect can be given as follows: Eform = ECdS_nZn + nECd − EpureCdS − nEZn, where E means total energy of the supercell systems calculated. The smaller Eform indicates the easier formation of this doping system. The calculated results were 1.89 eV for the Zn/Cd = 1:23 system and 0.89 eV for the Zn/Cd = 1:11 system, respectively. Thus, the Zn-doped CdS sample with mole ratio of Zn/Cd around 1:11 is easier to be formed compared to that of Zn/Cd around 1:23. Generally speaking, the easier for the sample to be formed, the more stable the sample. It can be used to explain why the Zn-doped CdS with Zn/Cd = 1:10 had good stability for RhB degradation. Additionally, the lower formation energy together with the lower band level of VB indicate that the Zn/ Cd = 1:11 system is suitable for organic pollutant degradation. The theoretical calculation result is also in good agreement with the experimental result, in which the sample with Zn/Cd = 1: 10 had the best photocatalytic activity in all doping samples.

Figure 8. Crystal structure for Zn-doped CdS with different molar ratio of Zn/Cd calculated by the density functional method. Zn: green (black) sphere; Cd: pink (gray) sphere; S: yellow (french gray) sphere.

length of Cd−S was 2.55 Å, while the bond length of Zn−S decreased to 2.377, 2.379, 2.390, and 2.390 Å and 2.375, 2.383, 2.393, and 2.395 Å for Zn/Cd = 1:23 and Zn/Cd = 1:11 systems, respectively. It can be concluded that the bond length of Zn−S is shorter than that of Cd−S, which results in the distortion of inner structure of crystalline around dopants. The obtained structures of the doping systems were in good agreement with the peak shifts in the XRD pattern shown in Figure 1. Therefore, it can be concluded that, in this case, a Zn ion can be incorporated into CdS and take the place of the Cd ion. Figure 9 shows the calculated band structure of Zn-doped CdS with different doping concentrations in comparison with pure CdS. It can be seen that the band gap of CdS had no significant changes with the increase of doping concentration. The calculated band gap of the three systems were all about 0.9 eV (0.975, 0.914, and 0.910 eV for pure CdS, Zn/Cd = 1:23 system, and Zn/Cd = 1:11 system, respectively). Although the calculated results of the band gap was smaller than the experiment data, which is because of well-known LDA error,41 the almost changeless band gaps were also in accordance with the UV−vis data we discussed in Figure 3 and Table 1. Interestingly, it is found that the band level of valence band (VB) and conduction band (CB) moved to low energy direction with the increase of doping concentration in comparison with pure CdS. The low energy of the top of VB

4. CONCLUSIONS A simple hydrothermal route can be successfully used for the preparation of Zn-doped CdS samples with the morphology of flowerlike nanoarchitecture. Among all the samples, the one derived at 160 °C for 12 h with the molar ratio of Zn/Cd = 1:10 has the best photocatalytic activity for the degradation of RhB under visible-light irradiation. Additionally, the Zn-doped CdS sample has excellent reuse ability over pure CdS, since its photocatalytic activity almost remained constant even after four cycle reuse. So, the photocatalytic activity and stability have been improved significantly by Zn2+ doping. Theoretical calculation based on DFT supports the experimental data. A suitable amount of Zn ion incorporated into CdS can lead to the shift of the valence band of CdS to the direction with high oxidative ability and the easy formation of Zn-doped CdS. 9083

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(25) Ren, L.; Jin, L.; Wang, J. B.; Yang, F.; Qiu, M. Q.; Yu, Y. Nanotechnology 2009, 20, 115603. (26) Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140, A1133. (27) Kohn, W.; Becke, A. D.; Parr, R. G. J. Phys. Chem. 1996, 100, 12974. (28) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 48, 13115. (29) Ceperley, D. M.; Alder, B. J. Phys. Rev. Lett. 1980, 45, 566. (30) Perdew, J. P.; Zunger, A. Phys. Rev. B 1981, 23, 5048. (31) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. (32) Yu, J. G.; Zhang, J.; Jaroniec, M. Green Chem. 2010, 12, 1611− 14. (33) Fu, H. B.; Pan, C. S.; Yao, W. Q. J. Phys. Chem. B 2005, 109, 22432. (34) Weller, H. Angew. Chem., Int. Ed. 1993, 32, 41. (35) Unni, C.; Philip, D.; Smitha, S. L.; Nissamudeen, K. M.; Gopchandran, K. G. Spectrochim. Acta A 2009, 72, 827. (36) Bao, N.; Shen, L.; Takata, T.; Domen, K. Chem. Mater. 2008, 20, 110. (37) Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation, Physical Electronics Division: Waltham, MA. (38) Nagaveni, K.; Hegde, M. S.; Ravishankar, N.; Subbanna, G. N.; Madras, G. Langmuir 2004, 20, 2900. (39) Erdem, B.; Hunsicker, R. A.; Simmons, G. W.; Sudol, E. D.; Dimonie, V. L.; El-Aasser, M. S. Langmuir 2001, 17, 2664. (40) Jang, J. S.; Joshi, U. A.; Lee, J. S. J. Phys. Chem. C 2007, 111, 13280. (41) Vogel, D.; Krüger, P.; Pollmann, J. Phys. Rev. B 1996, 54, 5495.

Furthermore, the high photocatalytic performance of the Zndoped CdS sample implies its potential photocatalytic application. The hydrothermal synthesis route using water as the only solvent can also supply an environmentally benign method for photocatalyst preparation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; fax: 86-27-67861185. Author Contributions ⊥

The first two authors have contributed to the work equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (no. 20973070), the National Basic Research Program of China (no. 2009CB939704), the Key Project of Natural Science Foundation of Hubei Province (no. 2011CDA092), and self-determined research funds of CCNU from the colleges’ basic research and operation of MOE.

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dx.doi.org/10.1021/jp300939q | J. Phys. Chem. C 2012, 116, 9078−9084