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Fabrication, Characterization and Properties of Flowerlike ZnS-ZnO Heterogeneous Microstructures Built Up by ZnS-Particle-Strewn ZnO Microrods Yonghong Ni,*,† Sen Yang,† Jianming Hong,‡ Lei Zhang,† Weili Wu,† and Zhousheng Yang† College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal UniVersity, Wuhu, 241000, P. R. China, and Center of Materials Analysis, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed: December 7, 2007; ReVised Manuscript ReceiVed: March 11, 2008
In the present paper, flowerlike ZnS-ZnO heterogeneous microstructures built up by ZnS-particle-strewn ZnO microrods were successfully obtained at 120 °C for 10 h via a simple one-pot hydrothermal route without the assistance of any surfactant or template, employing S, Zn(CH3COO)2 · 2H2O, NaOH, and NH3 · H2O as reactants. The optical properties of the as-prepared product are different from those of pure ZnO nanocrystals, which were prepared under the same conditions. Meanwhile, the electrochemical research of the products showed that flowerlike ZnS-ZnO heterogeneous microstructures had a strong ability to promote electron transfers between Hb and the Au electrode. The phase and morphologies of the products were characterized by X-ray powder diffraction, transmission electron microscopy, field emission scanning electron microscopy and X-ray energy dispersive spectra. 1. Introduction Over the past decade, II-VI semiconductor nanomaterials have been attracting extensive research interest due to their unique physical and chemical properties and potential wide applications in many fields. Zinc oxide is an important, direct band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV),1 and has been considered as a promising material for UV light emission diodes, laser diodes, transparent electronics, integrated sensors, optical waveguides and solar cell windows,2–6 due to its exciting electrical, optoelectronic and photochemical properties.7 With the development of materials science, it is believed that ZnO has further application in many fields.8 Up to now, well-defined ZnO nanostructures with an abundant variety of morphologies have been achieved through vapor-based techniques or the chemical solution route. ZnO nanonails,9 nanowires,10 nanotubes,11 nanorings12 and nanoneedles13 have been prepared via various methods including the sol-gel route, template-assisted growth, chemical vapor deposition and hydrothermal synthesis, etc.14 Among the above methods to prepare ZnO, the hydrothermal synthesis route, as an important method for wet chemistry, has been attracting materials chemists’ attention. Employing this method, ZnO hollow nanospheres were synthesized at 140 °C for 24 h by Xie and co-workers15 using the decomposition of the precursor obtained with ZnSO4, KSCN and C10H8N2. Tong et al.16 successfully synthesized ZnO nanotowers and nanocubes by a hydrothermal route without any templates and catalysts. Our group has also synthesized columnlike ZnO microcrystals by a one-pot hydrothermal route employing the CTAB as the structure-directing reagent.17 Recently, flowerlike ZnO18 nanostructures have been synthesized via solution routes under the assistance of various surfactants. Generally, the templates or organic compounds (as surfactants) are considered to be * Corresponding author. E-mail:
[email protected]. Fax: 86-5533869302. † Anhui Normal University. ‡ Nanjing University.
necessary for fabrication of flowerlike ZnO nanostructures in chemical reaction.19 However to date, in a simple aqueous system without the assistance of surfactants, no report of flowerlike ZnS-ZnO heterogeneous microstructures built up by ZnS-particle-strewn ZnO microrods is found in literatures. In the present paper, we successfully prepared flowerlike ZnS-ZnO heterogeneous microstructures built up by ZnSparticle-strewn ZnO microrods via a simple one-pot hydrothermal route without the assistance of any surfactant or template. Sulfur powder (S), Zn(CH3COO)2 · 2H2O, NaOH, and NH3 · H2O were used as reactants. In a basic system containing NH3 · H2O, the sulfur element can be easily converted to S2- ion, which led to the formation of flowerlike ZnS-ZnO heterogeneous microstructures built up by ZnS-particle-strewn ZnO microrods. Compared with the pure ZnO microrods synthesized in the system without sulfur powders, flowerlike ZnS-ZnO heterogeneous microstructures have obviously different optical and electrochemical properties. 2. Experimental Section In a typical experiment, all reagents were analytically pure and used without further purification. 2.5 mmol of Zn(CH3COO)2 · 2H2O and 5 mmol of NaOH were dissolved into a small amount of distilled water, respectively, and a white floccule appeared as soon as they were mixed. After 5 mL of NH3 · H2O(25%) was introduced into the mixture under stirring, the white floccule was completely dissolved. Then, small amounts of sulfur powders were dissolved into the above solution under magnetic stirring. The formed blue solution was transferred into a Teflon-lined stainless steel autoclave of 30 mL up to 80% volume. Hydrothermal treatments were carried out at 120 °C for 10 h. After that, the autoclave was allowed to cool down to room temperature naturally. White precipitates were collected, and washed with CS2, distilled water, and ethanol several times to remove impurities. Finally, the precipitates were dried in air at 60 °C for 5 h. As controls, the above process was repeated in the absence of S and NH3 · H2O, or only S in the same conditions,
10.1021/jp711539u CCC: $40.75 2008 American Chemical Society Published on Web 05/03/2008
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Figure 2. The XRD patterns of the as-prepared product: (a) flowerlike ZnS-ZnO heterogeneous microstructures and (b) pure ZnO.
respectively. Furthermore, different reaction temperatures (100 °C, 180 °C) were investigated. Powder X-ray diffraction (XRD) of the product was carried out on a Shimadzu XRD-6000 X-ray diffractometer equipped with Cu KR radiation (λ ) 0.154060 nm), employing a scanning rate of 0.02°s-1 and 2θ ranges from 25° to 70°. The TEM images were obtained on a JEOL JEM 200CX transmission electron microscope, employing an accelerating voltage of 200 kV. SEM images and EDS were taken on a JEOL-6340F and S-4800 field-emission scanning electron microscope operated at 5.0 kV. The fluorescence spectra were measured with an F-4500 spectrofluorometer (Hitachi) with a quartz cell of 1 cm, through dispersing certain amounts of the samples into the deionic water under the assistance of ultrasonic. A Xe lamp with a filter was used as the excitation light source and the exciting wave is 325 nm. The UV-vis absorption spectra were carried out on a Shimadzu UV-2450 spectrophotometer. The samples were homogeneously covered on BaSO4 powders which were used as the background. The electrochemical measurement was performed on a CHI 660A electrochemical workstation (CH Instruments, Chenhua Corp., Shanghai, China) with threeelectrode system consisting of a AgCl/Ag electrode as a reference electrode, a platinum wire as a counter electrode and a bare or modified Au electrode as working electrodes employing a scanning rate of 0.1 V/s and the rest time of 2 s. 3. Results and Discussion
Figure 1. (a) A typical TEM image of ZnO crystals prepared from the system containing ZnAc2, NaOH, NH3 · H2O and small amount of S powders at 120 °C for 10 h; (b) and (c) EDS analyses took from the rod and the sphere, respectively; (d) the SAED pattern of small particles shown in Figure 1a.
Figure 1a is a representative TEM image of the as-prepared product. Several microrods strewn by some small particles with a mean size of ∼150 nm can be easily found. The EDS analyses taken from microrods and small particles, respectively, showed that microrods were ZnO and small particles were ZnS (see Figures 1b and 1c), indicating that the as-obtained products were ZnO microrods strewn by ZnS particles. The generation of ZnS was further confirmed by the selected area electron diffraction (SAED) pattern of small particles shown in Figure 1a. Three concentric rings can be indexed as (111), (220) and (311) plane of cubic ZnS form, respectively (see Figure 1d). Figure 2 shows the XRD analysis of the product. All diffraction peaks at 2θ degrees ranging from 30° to 70° can be indexed as hexagonal ZnO phase (Wurtzite structure) by comparison with the JCPDS cards file No. 36-1451. The strong and narrow diffraction peaks indicate that the products have good crystallinity and relatively
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Figure 4. TEM and SEM images of the products prepared at 120 °C for 10 h under in the absence/presence of NH3 · H2O: (a) (CH3COO)2Zn · 2H2O + NaOH and (b) (CH3COO)2Zn · 2H2O + NaOH + NH3 · H2O.
Figure 3. SEM images of the as-obtained product under the present experiment conditions.
big particle size. Some weak peaks below 30° in Figure 2a were attributed to the contribution of ZnS particles. SEM observations of the product confirmed the result of TEM. Figure 3 depicts typical SEM images of the product. Some flowerlike structures built up by many rods can be clearly seen (see Figures 3a, 3b). The sizes of these rods are not uniform. Their lengths are several microns, and the mean diameter is ∼600 nm. However, these rods are not smooth, and a large amount of small particles is strewn on their surfaces (see Figure 3c). The average diameter of small particles is ∼150 nm, which is in good agreement with the result of TEM observation. It was found that S powders, NH3 · H2O and the reaction temperature were main factors influencing the formation of flowerlike ZnS-ZnO heterogeneous microstructures. When S powders and NH3 · H2O did not exist in the system, only
aggregated ZnO nanoparticles were produced (see Figure 4a); When only S powders did not exist, ZnO microrods with smooth surfaces were obtained (Figure 4b). These microrods connected with each other at one end to form flowerlike structures (see the circle in Figure 4b). And when the reaction temperature was 100 °C, only aggregated ZnO nanoparticles were prepared (Figure 5a); while the temperature was above 120 °C, ca. 180 °C, flowerlike ZnS-ZnO heterogeneous microstructures could also be formed (Figure 5b). Figure 6 depicts UV-vis absorption and PL spectra of the obtained product. Compared with those of the pure ZnO microrods prepared from the system without S powders, distinct differences could be found. In the UV-vis absorption spectrum of the pure ZnO microrods, an absorption peak centered at 369 nm could be easily seen, which had a slight blue-shift compared with that of the bulk; while in that of flowerlike ZnS-ZnO heterogeneous microstructures, the intensity of the peak at 369 nm decreased and a new strong peak at 322 nm appeared, which should be ascribed to the influence of ZnS particles (Figure 6a). Similar phenomena were also found in PL spectra (see Figure 6b). Under the excitation of 325 nm light, two peaks centered at 408 nm and 440 nm, respectively, were found in the PL spectrum of pure ZnO mircorods. The former should be ascribed to the recombination of free excitons, and the latter should come from the oxygen vacancy of ZnO structures.20 However, in the PL spectrum of flowerlike ZnS-ZnO heterogeneous micro-
Flowerlike ZnS-ZnO Heterogeneous Microstructures
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Figure 5. SEM images of products prepared at different temperatures for 10 h: (a) 100 °C and (b) 180 °C.
structures, the intensities of two peaks obviously enhanced and the situations red-shifted to 412 nm and 452 nm, respectively. The above facts indicated that more defects might exist in ZnO crystals due to the presence of ZnS particles: The larger sulfur atoms substituted oxygen atoms, which led to the change of the optical properties of ZnO. Also, the presence of ZnS could affect the electrochemical properties of ZnO. Figure 7 presents the electrochemical properties of ZnO microrods and flowerlike ZnS-ZnO heterogeneous microstructures in 0.1 mol/L phosphate buffer solution (pH 7.0) with hemoglobin (Hb). A pair of weak redox peaks was observed when a bare Au electrode was used as the working electrode (curve a). When the Au electrode modified with ZnO microrods was used, the peak current obviously increased (curve b), indicating that ZnO microrods could promote the electron transfers between Hb and the Au electrode. However, a pair of stronger redox peaks appeared after the bare Au electrode was substituted by flowerlike ZnS-ZnO heterogeneous microstructure-modified Au electrode and the separation between the anodic peak potential (Epa) and the cathodic peak potential (Epc) increased about 270 mV (curve c). This fact implied that the flowerlike ZnS-ZnO heterogeneous microstructures prepared by the simple hydrothermal route owned stronger ability to accelerate the electron transfers between Hb and the Au electrode. We considered that the presence of ZnS on the surface of ZnO microrods increased their surface area, which could adsorb more Hb molecules. As a result, the electron transfers between Hb and the Au electrode were greatly promoted. The
Figure 6. (a) UV-vis spectra and (b) PL spectra of pure ZnO crystals and flowerlike ZnS-ZnO heterogeneous microstructures prepared under the same experimental conditions.
Figure 7. Cyclic voltammograms of different electrodes in 0.1mol/L phosphate buffer solution (pH 7.0) containing hemoglobin (Hb): (a) bare Au electrode, (b) pure ZnO crystals/Au electrode, (c) Au electrode modified by flowerlike ZnS-ZnO heterogeneous microstructures.
above experiment implies that the novel microstructures have potential application in electrochemical detection of some proteins.
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Ni et al. prismlike ZnO crystals.21 In our work, due to the stronger coordination of NH3 molecules to Zn2+ ions, it is possible that NH3 molecules were adsorbed at the circumference of Zn2+ ions in ZnO nuclei. Because of the spatial hindrance ZnO grew only along certain directions. With the increase of ammonia concentration, this led to the formation of ZnO crystals with certain morphology, such as nanoflakes (1 mL), mixtures of nanoflakes and microrods (3 mL), and flowers built up by microrods (above 5 mL). Maleki et al. considered that a coordination reaction would occur after sulfur powders dissolved into ethylenediamine:22
SN + 2NH2CH2CH2NH2 f [NH2CH2CH2NH - SN-1 NHCH2CH2NH2] + H2S Due to the property similarity between ammonia and ethylenediamine, a similar reaction would take place when S powders existed in the present system (see below eq 1).
SN + 2NH3 f [NH2 - SN-1 - NH2] + H2S
(1)
H2S + 2NH3 · H2O ) (NH4)2S + 2H2O
(2)
ZnO + H2O + S
2-
-
) ZnS + 2OH
(3)
Equation 1 was promoted at raised temperature. The produced S2- ions attacked ZnO on the surface of ZnO rods owing to smaller solubility of ZnS than ZnO in a basic solution (see eq 3). As a result, flowerlike microstructures built up by ZnSparticle-strewn ZnO microrods were obtained. 4. Conclusions
Figure 8. SEM images of the product prepared from the system with various volumes of NH3 · H2O: (a) 1 mL and (b) 3 mL.
In this work, Zn2+ ions first reacted with OH- ions to form Zn(OH)2, which was then dissolved by NH3 molecules due to the formation of [Zn(NH3)4]2+. After the system was heated, [Zn(NH3)4]2+ units were attacked by OH- ions to produce ZnO nuclei:
2OH- + Zn(NH3)42+ f ZnO + H2O + 4NH3 Figure 4a showed that only aggregated ZnO nanoparticles were obtained in the system without ammonia, while ZnO microrods were prepared in the system with ammonia, indicating that NH3 molecules played a key role in the shape-control of ZnO crystals. Further investigations found that the production of ZnO microrods and the formation of flowerlike ZnO structures were related to the amount of ammonia used in the system. Figure 8 shows SEM images of ZnO prepared in the system with various volumes of NH3 · H2O. When 1 mL of NH3 · H2O was used, the product was ZnO nanoflakes with the mean thickness of 30 nm (Figure 8a). After 3 mL of NH3 · H2O was introduced, the product was a mixture of ZnO nanoflakes and microrods (Figure 8b). When the system contained 5 mL of NH3 · H2O, pure ZnO microrods were obtained (Figure 4b). When the volume of NH3 · H2O was above 5 mL, ca. 7 mL, the morphology of the product was still microrods. Also, when the volume of NH3 · H2O was 5 mL or higher, the products presented flowerlike structures. L. D. Sun et al. considered that NH3 molecules could prevent from the amalgamation of ZnO nuclei in the supersaturated solvent, which led to the growth of
In summary, flowerlike ZnS-ZnO heterogeneous microstructures have been successfully prepared at 120 °C for 10 h without the assistance of any surfactant or template via the hydrothermal method, employing sulfur powder, Zn(CH3COO)2 · 2H2O, NaOH and NH3 · H2O as reactants. Experiments showed that NH3 · H2O played a crucial role in the formation of flowerlike ZnS-ZnO heterogeneous microstructures. The presence of ZnS particles on the surface of ZnO microrods greatly changed the optical and electrochemical properties of ZnO microrods. Furthermore, the as-prepared microstructures have potential application in the electrochemical detection of some proteins. And the present method can also be expanded for preparation of other zinc chalcogenide-strewn ZnO crystals. Acknowledgment. The authors thank the National Natural Science Foundation of China (20771005 and 20571002), the Natural Science Foundation of Anhui Province (05021024), the Education Department of Anhui Province (2005kj123 and No. 2006KJ006TD), Young Teachers Program of Anhui Education Department (No. 2005jq1047), and the special fund of Anhui Normal University (2005xzx17) for the fund support. References and Notes (1) Liang, S.; Sheng, H.; Liu, Y.; Hio, Z; Lu, Y.; Shen, H. J. Cryst. Growth 2001, 225, 110. (2) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (3) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (4) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. AdV. Funct. Mater. 2003, 13, 9. (5) Huang, M. H.; Mao, S.; Feick, H. N.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (6) Saito, N.; Haneda, H.; Sekiguchi, T.; Ohashi, N.; Sakaguchi, I.; Koumoto, K. AdV. Mater. 2002, 14, 418. (7) Ohshima, E.; Ogino, H.; Niikura, I.; Maeda, K.; Sato, M.; Ito, M.; Fukuda, T. J. Cryst. Growth 2004, 260, 166.
Flowerlike ZnS-ZnO Heterogeneous Microstructures (8) (a) Verardi, P.; Nastase, N.; Gherasim, C.; Ghica, C.; Dinescu, M.; Dinu, R.; Flueraru, C. J. Cryst. Growth 1999, 197, 523. (b) Rogach, A. L.; Talapin, D. V.; Shevchenko, E. V.; Weller, M. AdV. Funct. Mater. 2002, 12, 653. (9) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. Nano Lett. 2003, 3, 235. (10) Li, Y.; Meng, G. W.; Zhang, L. D.; Phillipp, F. Appl. Phys. Lett. 2000, 76, 2997. (11) Hu, J. Q.; Li, Q.; Meng, X. M.; Lee, C. S.; Lee, S. T. Chem. Mater. 2003, 15, 305. (12) Kong, X. Y.; Ding, Y.; Yang, R.; Wang, Z. L. Science 2004, 303, 1348. (13) Park, W. T.; Yi, G. C.; Kim, M.; Pennycook, S. J. AdV. Mater. 2002, 14, 1841. (14) (a) Lanf, R. J.; Bond, W. D. Am. Ceram. Soc. Bull. 1984, 63, 278. (b) Lee, N. Y.; Kim, M. S. J. Mater. Sci. 1991, 26, 1126. (c) Lu, C. H.; Yeh, C. H. Ceram. Int. 2000, 26, 351. (15) Li, Z. Q.; Xie, Y.; Xiong, Y. J.; Zhang, R. New J. Chem. 2003, 27, 1518. (16) Tong, Y. H.; Liu, Y. C.; Shao, C. L.; Liu, Y. X.; Xu, C. S.; Zhang, J. Z.; Lu, Y. M.; Shen, D. Z.; Fan, W. X. J. Phys. Chem. B 2006, 110, 14714.
J. Phys. Chem. C, Vol. 112, No. 22, 2008 8205 (17) Ni, Y. H.; Wei, X. W.; Ma, X.; Hong, J. M. J. Cryst. Growth 2005, 283, 48. (18) (a) Zhang, J.; Sun, L.; Yin, J.; Su, H.; Liao, C.; Yan, C. Chem. Mater. 2002, 14, 4172. (b) Zhang, H.; Yang, D.; Ji, Y.; Ma, X.; Xu, J.; Que, D. J. Phys. Chem. B 2004, 108, 3955. (c) Wang, Z.; Qian, X. F.; Yin, J.; Zhu, Z. K. Langmuir 2004, 20, 3441. (d) Gao, X.; Li, X.; Yu, W. J. Phys. Chem. B 2005, 109, 1155. (19) (a) Vayssieres, L. AdV. Mater. 2003, 5, 464. (b) Yang, H. F.; Lu, Q. Y.; Gao, F.; Shi, Q. H.; Yan, Y.; Zhang, F. Q.; Xie, S. H.; Tu, B.; Zhao, D. Y. AdV. Funct. Mater. 2005, 15, 1377. (c) Gao, P. X.; Mai, W. J.; Wang, Z. L. Nano Lett. 2006, 6, 2536. (20) (a) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. J. Appl. Phys. 1996, 79, 7983. (b) Sun, Y.; Fuge, G. M.; Ashfold, M. N. R. Chem. Phys. Lett. 2004, 396, 21. (c) Xu, C. L.; Qin, D. H.; Li, H.; Guo, Y.; Xu, T.; Li, H. L. Mater. Lett. 2004, 58, 3976. (21) Zhang, J.; Sun, L. D.; Yin, J. L.; Su, H. L.; Liao, C. S.; Yan, C. H. Chem. Mater. 2002, 14, 4172. (22) Maleki, M.; Mirdamadi, Sh.; Ghasemzadeh, R.; Ghamsari, M. S. Mater. Lett. 2008, 62, 1993.
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