Preparation, Conversion, and Comparison of the Photocatalytic and

Nov 24, 2006 - ABSTRACT: In the present paper, we report the successful synthesis of a ZnS(en)0.5 complex (en ) ethylenediamine) with a cuboid morphol...
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Preparation, Conversion, and Comparison of the Photocatalytic and Electrochemical Properties of ZnS(en)0.5, ZnS, and ZnO Ni,*,†

Cao,†

Yonghong Xiaofeng Yonghong Chen,‡ and Jun Xu‡

Guangzhi

Hu,†

Zhousheng

Yang,†

Xianwen

Wei,†

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 2 280-285

College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal UniVersity, Wuhu 241000, P. R. China, and Department of Chemistry, Huainan Normal College, Huainan 232000, P. R. China ReceiVed May 27, 2006; ReVised Manuscript ReceiVed NoVember 24, 2006

ABSTRACT: In the present paper, we report the successful synthesis of a ZnS(en)0.5 complex (en ) ethylenediamine) with a cuboid morphology by a solvothermal method employing zinc powder and sulfur powder as the reactants in ethylenediamine and further conversion to ZnS and ZnO. Research showed that the morphology hardly changed after the ZnS(en)0.5 complex had been converted into ZnS in a vacuum at 450 °C for 40 min and that nearly spherical ZnO grains with a mean diameter of 300 nm were obtained after the ZnS(en)0.5 complex was oxidized in air at 650 °C for 40 min. Also, the experiments indicated that ZnS(en)0.5, ZnS, and ZnO could degrade safranine T, an organic dye, under irradiation of 254 nm UV light and that ZnS had better photocatalytic degradation properties than ZnS(en)0.5 and ZnO after irradiation for 30 min. The electrochemical studies showed that ZnS and ZnO had a stronger ability to promote electron transfers between hemoglobin (Hb) and the Au electrode than ZnS(en)0.5 but had a weaker ability to accelerate electron transfers between catechol and the Au electrode than ZnS(en)0.5. The possible mechanisms are discussed. 1. Introduction Transition metal complexes are a class of important compounds owing to their good catalytic properties in organic synthesis and some potential uses as sensitizers for photolysis of water.1 At the same time, they can be used as precursors for preparation of some semiconductors including ZnS,2,3 ZnSe,3 ZnO,2,4 CuS,5 and Cu2S6 via thermal decomposition, solvothermal or hydrothermal process, solid-state reaction, and so on. For example, X. Chen et al.5 synthesized copper sulfide hollow spheres via a hydrothermal route utilizing a copper(II)-thiourea complex as the precursor. C. Chen et al.6 reported the preparation of Cu2S nanoribbons from Cu(thiocarbamide)Cl‚1/2H2O complex nanowires by solid-state reaction at room temperature without any surfactant. S. Yu and co-workers2 obtained wurtzite ZnS single-crystal nanosheets and ZnO flake-like dendrites from a lamellar molecular precursor, ZnS(en)0.5. It is well-known that some compounds such as amines are apt to coordinate with metal ions, especially transition metal ions. An ethylenediamine (en) molecule contains two N atoms, so it is a good bidentate chelating ligand and can be extensively used as a structure-directing reagent in synthesizing some semiconductors. Employing en as the structure-directing reagent, for instance, B. Liu et al.7 and U. Pal et al.8 prepared ZnO nanorods with a diameter of 50 nm and ZnO nanostructures with different morphologies, respectively, via a hydrothermal route. Also en is a good solvent and can dissolve chalcogen powders, so it has been widely used to synthesize metal chalcogenides, especially transition metal chalcogenides.4,9-12 However, the preparation of ZnE (E ) S, Se) nanocrystals fails when en is employed as solvent and structure-directing agent. Research from Li’s group showed that ZnE (E ) S, Se) could not be obtained in the en system using zinc powders or Zn2+ salts as zinc sources since a stable complex, ZnE(en)0.5, is produced.3 Yu et al. considered that ZnS(en)0.5 was a lamellar compound and could convert into ZnS nanosheets and ZnO * Corresponding author. E-mail: [email protected]. † Anhui Normal University. ‡ Huainan Normal College.

dendrites under heating conditions in vacuum and in air. The optical properties of ZnS(en)0.5 and as-converted ZnS and ZnO were also studied.2 In the published works, however, the formation process of the lamellar compound was not studied, and there is no report of ZnS(en)0.5 microcrystals with a regular cuboid structure and the photocatalytic and electrochemical property studies of ZnS(en)0.5 and as-converted ZnS and ZnO, yet. In this paper, we investigate synthesis of ZnS(en)0.5 microcrystals with a cuboid morphology in en solution employing zinc powders and sulfur powders as the initiating reactants and further conversion to ZnS and ZnO. Our research interest mainly focused on the formation process of ZnS(en)0.5 microcrystals with a cuboid morphology and comparison of their photocatalytic activity and electrochemical response to the same compounds. 2. Experimental Section All reagents were purchased from Shanghai Chemical Company and used without further purification. ZnS(en)0.5 was fabricated similar to refs 2 and 3. In a typical preparation procedure, zinc powder (0.001 mol, AR) and sulfur powder (0.002 mol, CP) were dispersed in 20 mL of en under stirring. The color of the solvent was primrose yellow and then gradually became light green when the sulfur powders were completely dissolved in en. The mixed solutions were transferred into a Teflon inner autoclave with total capacity of 90% and maintained at 200 °C for 15 h and, then, allowed to cool to room temperature naturally. A white precipitate was collected, washed with distilled water and absolute ethanol several times, and then dried in a vacuum at 60 °C for 5 h. The thermal decomposition of the precursor was performed in a vacuum furnace with a temperature controller. Pure hexagonal ZnS and ZnO phases were obtained after the precursor had been heated in a vacuum at 450 °C and in air at 650 °C, respectively, for 40 min. For measuring the catalytic degradation of the precursor, ZnS, and ZnO of organic dyes, 20 mg of ZnS(en)0.5, ZnS, and ZnO were dispersed into safranine T aqueous solutions with a concentration of 10 mg/L and irradiated by the 254 nm UV light for 30 min at room temperature. To study the electrochemical properties of the above three compounds, two 0.1 mol/L phosphate buffer solutions (PBS) with 1.0 × 10-6 mol/L hemoglobin (Hb) and 1.0 × 10-4 mol/L catechol were prepared by mixing the stock standard solutions of K2HPO4 and KH2PO4. The pH values were adjusted to 7.0 with 0.1 mol/L H3PO4 or NaOH. All

10.1021/cg060312z CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007

Properties of ZnS(en)0.5, ZnS, and ZnO

Crystal Growth & Design, Vol. 7, No. 2, 2007 281 lyzer Sirion 200 or a KYKY-EM-3200 scanning electron microscope employing an operating voltage of 25 kV. Energy dispersion spectra (EDS) were carried out on a high-resolution transmission microscope, JEOL-2010, employing an accelerating voltage of 200 kV. The optical property changes of dyes are recorded on a Hitachi U-3010 UV-vis absorption spectrophotometer (Tokyo, Japan) and F-4500 spectrofluorometer with a quartz cell of 1 cm, using an exciting wavelength of 530 nm. Electrochemical responses were performed on a CHI660-A electrochemical workstation (CH Instruments, Chenhua Corp., Shanghai, China) with a three-electrode system consisting of a saturated calomel electrode (SCE) as a reference electrode, a platinum wire as a counter electrode, and a bare or modified Au electrode (φ ) 2 mm) as a working electrode, employing a scanning rate of 0.1 V/s and a rest time of 2 s. To prepare Au electrodes modified by the products, 1 mg of the precursor, ZnS, and ZnO was dispersed into the twice-distilled water under ultrasound. Then, 5 mL solutions were dropped onto the Au electrodes and dried in air at room temperature.

3. Results and Discussion

Figure 1. The XRD patterns of the products: (a) ZnS(en)0.5; (b) ZnS obtained from thermal decomposition of ZnS(en)0.5 in vacuum at 450 °C for 40 min; (c) ZnO prepared via heating ZnS(en)0.5 in air at 650 °C for 40 min. solutions were prepared with twice-distilled water, deoxygenated by bubbling highly pure nitrogen for 15 min, and maintained under nitrogen atmosphere during measurements at room temperature. X-ray powder diffraction (XRD) patterns of the products were carried out on a Shimadzu XRD-6000 X-ray diffractometer equipped with Cu KR radiation (λ ) 0.154 060 nm), employing a scanning rate of 0.02° s-1 and 2θ ranges from 5° to 70°. Scanning electron microscopy (SEM) images were measured on a field emission scanning electron microana-

Figure 1a shows the XRD pattern of the precursor prepared from the en solutions. It is in good agreement with that of ZnS(en)0.5 reported in refs 2, 4, and 11, indicating that ZnS(en)0.5 has been successfully fabricated under the current experimental conditions. The strong and sharp diffraction peaks imply good crystallinity and big particle size of the product. SEM observations clearly showed the regular cuboid shape of the product (see Figure 2a). However, the sizes of these cuboids were not homogeneous. In the published works only lamellar ZnS(en)0.5 crystals were reported.2 Comparing our work with the literature, we find there are two differences: reaction temperature and time. The lamellar product was prepared at 120-180 °C for 6-12 h in ref 2, while our experiment was carried out at 200 °C for 15

Figure 2. SEM images of ZnS(en)0.5 microcrystals obtained at the different temperatures for 15 h: (a) 200; (b) 180; (c) 150; (d) 120 °C.

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Figure 3. SEM images of ZnS(en)0.5 microcrystals obtained at 200 °C for the different times: (a) 12; (b) 8 h.

h. In order to investigate the effect of temperature on the morphology of the product, ZnS(en)0.5 crystals were prepared at 180, 150, and 120 °C, keeping the rest of the conditions constant. SEM observations showed that the products still were cuboids with various sizes (Figure 2b-d). This fact indicated that the temperature is not the main factor to affect the morphology. Furthermore, we also prepared ZnS(en)0.5 crystals at 200 °C for shorter times, ca. 12 and 8 h. Many large lamellar ZnS(en)0.5 crystals were found in SEM images (see Figure 3), and some of them were breaking into small cuboids (see the arrows in Figure 3b). So, we can conclude that the cuboidshaped ZnS(en)0.5 crystals were obtained due to the breaking of the lamellar ZnS(en)0.5 crystals. As a precursor, ZnS(en)0.5 can be decomposed to produce ZnS in vacuum or ZnO in air at desired temperatures. Figure 1b,c depict the XRD patterns of ZnS and ZnO obtained by heating the precursor in vacuum at 450 °C and in air at 650 °C, respectively, for 40 min. By comparison with the JCPDS cards no. 36-1450 and no. 80-0075, they can be indexed as hexagonal ZnS and ZnO phases. Further evidence of the successful conversion of ZnS(en)0.5 to ZnS and ZnO comes from EDS analyses of the products (see Figure 4). The three spectra are obviously dissimilar. In the spectrum of the precursor (Figure 4a), the peaks of S, Zn, and C can be easily found. Unfortunately, the N peak cannot be seen. Possibly, the strong C peak

Figure 4. EDS analyses of the products: (a) ZnS(en)0.5; (b) ZnS obtained from thermal decomposition of ZnS(en)0.5 in vacuum at 450 °C for 40 min; (c) ZnO prepared via heating ZnS(en)0.5 in air at 650 °C for 40 min.

covers it. After the precursor has been heated in vacuum at 450 °C for 40 min, the C peak disappears (Figure 4b), and after it has been heated in air at 650 °C for 40 min, the S peak disappears and the O peak appears (Figure 4c). The Cu peak is ascribed to the grid of copper in all spectra. SEM observations showed that the morphology of ZnS was also cuboid, similar to that of the precursor, and the sizes of ZnS particles were smaller than that of the precursor (see Figure 5a), which should be attributed to the loss of en in the precursor molecule. Figure 5b is a SEM image of as-obtained ZnO particles. Comparing

Properties of ZnS(en)0.5, ZnS, and ZnO

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Figure 5. SEM images of (a) ZnS obtained from thermal decomposition of ZnS(en)0.5 in vacuum at 450 °C for 40 min and (b) ZnO prepared via heating ZnS(en)0.5 in air at 650 °C for 40 min.

them with the SEM image of the precursor, one can find that the morphology of ZnO has changed into quasi-spheres and the sizes of the particles reduce and become more homogeneous. The above change of the morphology and size should be a direct result of the reaction between ZnS(en)0.5 and oxygen in air. During the annealing of the precursor in air, en molecules were first eliminated, and then, oxygen atoms substituted sulfur atoms in ZnS to produce ZnO. Since an oxygen atom is smaller than a sulfur atom, ZnO could not maintain the morphology and size of ZnS. Also, generally, a spherical particle has a smaller surface energy. Thus, the quasi-spherical ZnO particles were finally obtained. Li et al. studied the conversion mechanism of ZnS(en)0.5 to ZnS.4 Yu et al. investigated the optical properties of ZnS(en)0.5 and its conversion products, ZnS and ZnO.2 However, few studies are found in the literature on the photocatalytic degradation and electrochemical properties of the above three compounds. Usually, transition metal compounds are known to have good catalytic properties. Some metal oxides such as TiO213 and ZnO13,14 are recognized as good photocatalysts and have extensive potential uses for environmental protection. In our work, we investigated the photocatalytic degradation of an organic dye, safranine T, under irradiation of 254 nm light employing the above three compounds as catalysts. Figure 6a,b shows the UV-vis absorption and photoluminescence (PL) spectra of safranine T after being irradiated for 30 min in the presence of different catalysts. A same order was found, indicating that all three compounds could photocatalytically degrade safranine T, but ZnS possessed the strongest ability,

Figure 6. The UV-vis absorption spectra (a) and the PL spectra (b) of safranine T dyes degraded under the irradiation of 254 nm light for 30 min in the presence of different catalysts.

then ZnO, and ZnS(en)0.5 the weakest. The above order should be related to the structures of these compounds. After ZnS(en)0.5 has converted into ZnS in vacuum at high temperature, the sites occupied by en molecules were liberated, which led to the increase of the special surface. While the precursor converted into ZnO in air under the higher temperature, the special surface would also increase, but below that of ZnS because of the changes of the morphology and the particle size. Namely, the order of the special surfaces of the particles was ZnS > ZnO > ZnS(en)0.5. As a result, ZnS was the strongest photocatalyst for degradation of safranine T, then ZnO, and ZnS(en)0.5 the weakest. Also, the electrochemical properties of the three compounds were studied. The cyclic voltammograms of different electrodes in 0.1 mol/L phosphate buffer solution (PBS) with Hb (pH ) 7.0) are shown in Figure 7. Since hemoglobin (Hb) hardly shows a pair of redox peaks at solid electrodes,15 no redox peak is observed when a bare Au electrode is used (curve a). Only a weak oxidation peak at -46.7 mV can be found using the Au electrode modified with ZnS(en)0.5 particles (curve b), indicating that ZnS(en)0.5 particles have weak promotion of the electron transfer between Hb and the Au electrode. However, a strong oxidation peak appears employing the Au electrode modified with ZnS particles (curve c), and the oxidation peak potential shifts to -94.5 mV. The above phenomenon implies that the

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Figure 7. Cyclic voltammograms of different electrodes in 0.1 mol/L phosphate buffer solution (PBS) with Hb (pH ) 7.0). Curves were obtained using different electrodes: (a) bare Au electrode; (b) ZnS(en)/Au electrode; (c) ZnS/Au electrode; (d) ZnO/Au electrode.

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were obviously enhanced, indicating that ZnS(en)0.5 particles could strongly promote the electron transfer between catechol and the Au electrode (curve b). Curve c was obtained using the ZnS/Au electrode instead of the bare Au electrode. By comparison with curve b, the redox peak sites had a small shift, but the oxidation peak current sharply reduced and the curve was more symmetrical. When the ZnO/Au electrode was used as the work electrode, a big shift of the redox peaks was shown in the current-voltage curve. Simultaneously, the symmetry of the curve became the best (curve d). We consider that the above experimental phenomena should be related to en molecules. It is probable that a H-bond existed between en and catechol molecules owing to the presence of -NH and -OH groups. So the redox peaks were strongly enhanced when the ZnS(en)0.5/ Au electrode was used. After ZnS(en)0.5 molecules decomposed into ZnS, the interaction (H-bond) between en and catechol molecules disappeared. The ability to accelerate the electron transfer between catechol and the Au electrode decreased, and the peak sites had a small change. At the same time, the redox reaction became reversible due to the native characteristic of ZnS particles. Furthermore, since the morphology and size of ZnO partices had changed, the redox peak sites had a big shift. Similarly, the redox reaction became more reversible due to the native characteristic of ZnO particles, which led to the increase of the symmetry of the I-E curve. 4. Conclusions

Figure 8. Cyclic voltammograms of different electrodes in 0.1 mol/L phosphate buffer solution (PBS) with catechol (pH ) 7.0). Curves were obtained using different electrodes: (a) bare Au electrode; (b) ZnS(en)/Au electrode; (c) ZnS/Au electrode; (d) ZnO/Au electrode.

ZnS particles prepared by decomposition of ZnS(en)0.5 in vacuum can greatly improve the electron transfer between Hb and the Au electrode, which should be a direct result of the large special surface of ZnS particles, too. ZnO particles also can promote the electron transfer between Hb and the Au electrode, and the oxidation peak potential locates at -565.7 mV. Compared with the oxidation peak potential of ZnS, a big shift occurs in the I-E curve recorded by using a ZnO-modified Au electrode as the work electrode. This phenomenon also can be explained by the morphology and special surface of ZnO particles. The above electrochemical experiments show that the order of the ability to accelerate the electron transfer between Hb and the Au electrode from great to small is ZnS > ZnO > ZnS(en)0.5. This is in good agreement with the order of photocatalytic degradation. However, the above order is not unchanged all the time. Figure 8 depicts cyclic voltammograms of different electrodes in 0.1 mol/L phosphate buffer solution (PBS) with catechol (pH ) 7.0). A pair of weak redox peaks could be found using the bare Au electrode (curve a). When a ZnS(en)0.5-modified Au electrode was used, the redox peaks

We have successfully synthesized the ZnS(en)0.5 complex with a cuboid morphology by a simple solvothermal method. SEM observations showed that the cuboids were formed due to the breaking of the lamellar products. By annealing in a vacuum and in air, ZnS(en)0.5 could be converted into the wutzite ZnS and ZnO. The research from the photocatalytic degradation of safranine T showed that ZnS was the strongest photocatalyst, then ZnO, and ZnS(en)0.5 the weakest. Moreover, ZnS(en)0.5, ZnS, and ZnO could promote the electron transfers between Hb or catechol and the Au electrode. However, the abilities to accelerate the electron transfer of various modified electrodes are different in different systems. The ability in the system containing Hb is ZnS > ZnO > ZnS(en)0.5, which should be attributed to the change of the special surface. In the system including catechol, ZnS(en)0.5 has a stronger ability to accelerate the electron transfer, which should be related to the presence of en molecules. Acknowledgment. The authors thank the National Natural Science Foundation of China (Grant 20571002), the Natural Science Foundation of Anhui Province (Grant 05021024), the Education Department of Anhui Province (Grants 2005kj123 and 2006KJ006TD), and the special fund of Anhui Normal University (Grant 2005xzx17) for the financial support. Prof. X. W. Wei thanks Anhui Provincial Excellent Young Scholars Foundation (Grant No. 04046065) for financial support. References (1) (a) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141. (b) Hara, M.; Waraksa, C. C.; Lean, J. T.; Lewis, B. A.; Mallouk, T. E. J. Phys. Chem. A 2000, 104, 5275. (2) (a) Yu, S. H.; Yoshimura, M. AdV. Mater. 2002, 14, 296. (b) Yu, S. H.; Yang, J.; Qian, Y. T.; Yoshimura, M. Chem. Phys. Lett. 2005, 361, 362. (3) Deng, Z. X.; Wang, C.; Sun, X. M.; Li, Y. D. Inorg. Chem. 2002, 41, 869. (4) Gao, X. D.; Li, X. M.; Yu, W. D. J. Phys. Chem. B 2005, 109, 1155.

Properties of ZnS(en)0.5, ZnS, and ZnO (5) Chen, X. Y.; Wang, Z. H.; Wang, X.; Zhang, R.; Liu, X. Y.; Lin, W. J.; Qian, Y. T. J. Cryst. Growth 2004, 263, 570. (6) Chen, C. N.; Zhu, C. L.; Hao, L. Y.; Hu, Y.; Z. Chen, Y. Chem. Lett. 2004, 33, 898. (7) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430. (8) Pal, U.; Santiago, P. J. Phys. Chem. B 2005, 109, 15317. (9) Deng, Z. X.; Li, L. B.; Li, Y. D. Inorg. Chem. 2003, 42, 2331. (10) Dai, J.; Jiang, Z. J.; Li, W. G.; Bian, G. Q.; Zhu, Q. Y. Mater. Lett. 2002, 55, 383. (11) Yu, S. H.; Wu, W. S.; Yang, J.; Han, Z. H.; Xie, Y.; Qian, Y. T.; Liu, X. M. Chem. Mater. 1998, 10, 2309.

Crystal Growth & Design, Vol. 7, No. 2, 2007 285 (12) Yu, S. H.; Yang, J.; Han, Z. H.; Zhou, Y.; Yang, R. Y.; Qian, Y. T.; Zhang, Y. H. J. Mater. Chem. 1999, 9, 1283. (13) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (14) Marci, G.; Augugliaro, V.; Lopez-Munoz, M. J.; Martin, C.; Palmisano, L.; Rives, V.; Schiavello, M.; Tilley, R. J. D.; Venezia, A. M. J. Phys. Chem. B 2001, 105, 1026. (15) Wang, L.; Hu, N. Bioelectrochemistry 2001, 53, 205.

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