Materials Letters 64 (2010) 962–965 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / m a t l e t The microwave-assisted synthesis and characterization of Zn1 − xCoxO nanopowders Luc Huy Hoang a,⁎, Pham Van Hai a, Nguyen Hoang Hai b, Pham Van Vinh a, Xiang-Bai Chen c,⁎, In-Sang Yang c a b c Faculty of Physics, Hanoi National University of Education, 136 Xuanthuy, Caugiay, Hanoi, Viet Nam Center for Materials Science, Hanoi University of Science, 334 Nguyen Trai, Hanoi, Viet Nam Department of Physics, Ewha Womans University, Seoul, 120-750, South Korea a r t i c l e i n f o Article history: Received September 2009 Accepted 27 January 2010 Available online February 2010 Keywords: Microwave-assisted synthesis Zn1 − xCoxO nanopowders Optical properties a b s t r a c t In this paper, we present a simple microwave-assisted synthesis of Zn1 − xCoxO nanopowders With the advantages of the microwave-assisted method, we have successfully synthesized good crystalline quality and good surface morphology Zn1 − xCoxO nanopowders The nanopowders are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–VIS absorption, and micro-Raman spectroscopy We found, in the synthesis process, the surfactant Triethanolamine (TEA) plays an important role on the morphology of Zn1 − xCoxO nanoparticles The XRD study shows that for Co doping up to 5%, Co2+ ions are successfully incorporated into the ZnO host matrix The absorption spectra of Zn1 − xCoxO (x=1–5%) nanopowders show several peaks at 660, 611 and 565 nm, indicating the presence of Co2+ ions in the tetrahedral sites The Raman study shows that the linewidth of Elow mode increases with Co concentration, which further indicates the incorporation of Co2+ ions into the ZnO host matrix © 2010 Elsevier B.V All rights reserved Introduction ZnO nanopowders have attracted considerable interest due to the potential applications including photonic devices, chemical and biological sensors, light emitting diodes, laser diodes, ultravioletprotection, etc [1–5] Moreover, among II–VI semiconductors, ZnO has been considered as one of the promising candidates for fabricating diluted magnetic semiconductor (DMS), due to its high solubility for transition metals (TM) and superior semiconductor properties A number of methods have been used for synthesizing ZnO nanopowders [6–12] In recent years, a new method has been reported: microwave-assisted synthesis Due to its unique features such as short reaction time, enhanced reaction selectivity, energy saving, and high reaction rate [5,13], the application of microwave-assisted synthesis of ZnO nanoparticles has been rapidly growing [13–17] Co doping into the Zn-site of the wurtzite ZnO structure homogeneously without changing the crystal structure is crucial not only for clarifying the contradictory claims among different groups about the high-temperature ferromagnetism in this material, but also for potential applications of the noble properties of this DMS material Co-doped ZnO nanopowders have been synthesized in various methods, including a simple chemical method [18], and a co-precipitation technique [19] However, synthesis of Codoped ZnO nanopowders taking the advantage of the microwave assistance has not been reported yet In this paper, we present a simple microwave-assisted chemical method to produce Zn1 − xCoxO nano- ⁎ Correspondence authors Hoang is to be contacted at Faculty of Physics, Hanoi National University of Education, 136 Xuanthuy, Caugiay, Hanoi, Viet Nam Chen, Department of Physics, Ewha Womans University, Seoul, 120-750, South Korea E-mail addresses: hoanglhsp@hnue.edu.vn (L.H Hoang), xchen@ewha.ac.kr (X.-B Chen) 0167-577X/$ – see front matter © 2010 Elsevier B.V All rights reserved doi:10.1016/j.matlet.2010.01.074 powders, using zinc acetate dihydrate and cobalt acetate tetrahydrate as precursors We have produced single phase Zn1 − xCoxO nanopowders of uniform surface morphology and good crystalline quality with average particle size ∼50 nm The nanopowders are characterized by XRD, SEM, UV–VIS absorption, and Raman scattering Experiments Sodium hydroxide (NaOH), zinc acetate dehydrate [Zn(CH3COO)2· 2H2O], cobalt acetate tetrahydrate [Co(CH3COO)2·4H2O], and Triethanolamine (TEA) were purchased from Aldrich All the reagents were analytically pure and were used without further purification In a typical experiment, Zn(CH3COO)2·2H2O and Co(CH3COO)2·4H2O were separately dissolved in distilled water The two solutions were mixed in a proportion to obtain a mixture solution with 0, 1, 2, 5, or 7% Co2+, and NaOH was slowly added into the mixture solution Then TEA was added drop by drop into the above solution, which was stirred with a magnetic stirrer, until pH reached and the solution became colorless or green The obtained solution was heated by a Sanyo microwave oven at a power of 300 W for 20 After microwave processing, the solution was cooled to room temperature The resulted precipitate was separated by centrifugation, then washed with deionized water and acetone for several times, and finally dried in an oven at 60 °C for 24 h and annealed at 600 °C for h in air XRD of Zn1 − xCoxO nanopowders was carried out on a Siemens D5500 X-ray diffractometer SEM images were taken on a Hitachi S-4800 field-emission scanning electron microscope The JASCO V670 UV–VIS spectrophotometer, equipped with diffuse reflectance accessory (DRA), was employed to record the electronic spectra of the powder samples in the region 200–900 nm The diffuse reflectance measurements were converted into absorption using Kubelka–Munk function (f(R∞) = L.H Hoang et al / Materials Letters 64 (2010) 962–965 (1 − R∞)2 / 2R∞) The Raman scattering was performed using a Jobin–Yvon T64000 micro-Raman system in the back scattering geometry with a 532 nm laser excitation Results and discussions Fig shows the XRD patterns of all the Zn1 − xCoxO nanopowders, in which, Fig 1a is for ZnO powders prepared without TEA, Fig 1b is for ZnO powders prepared with TEA, and Fig 1c–f is for Co-doped (1–7%) ZnO powders The diffraction peaks are indexed as those from the known wurtzite ZnO with lattice constants a) 0.325 nm and c) 0.521 nm, within experimental error (JCPDS, file no 36-1451) It can be seen from Fig 1a and b, TEA has no significant effect on the structure of ZnO powders While, we will show in our later discussion, TEA has an important effect on the morphology of ZnO powders No characteristic peaks of other phases or impurities were observed with Co doping up to 5% (Fig 1c, d, e) comparing with those of ZnO powders, indicating a single hexagonal phase However, it can be seen in the inset of Fig 1, the peak position increases with Co concentration, which indicates the decrease of lattice parameters This phenomenon presumably results from the substitution of Co ions with a small ionic radius of 0.58 Å for Zn (0.60 Å) sites For Co doping of 7%, a secondary impurity phase corresponding to Co3O4 was clearly observed, as marked by filled circles in Fig 1f On the basis of the linewidths of (100), (002) and (101) diffraction peaks, the mean particle size of Zn1 − xCoxO nanopowders were calculated according to Scherrer equation, the results are shown in Table As can be seen in Table 1, without TEA, ZnO nanoparticles of ∼70 nm were produced, while with TEA, the particle size decreased to ∼54 nm In previous studies, it has been shown that TEA plays as an organic capping agent in the reaction media [20], hindered the crystal growth [20,21], and also controls the pH of preliminary solution [22] The crystalline size decreases further when Co is doped, which suggests Co incorporation into the ZnO lattice, as observed in other systems [23,24] The size and morphology of Zn1 − xCoxO nanoparticles were further analyzed by SEM studies, which are represented in Fig As can be 963 Table Nanoparticle size calculated from the (100), (002) and (101) peaks using Scherrer equation Sample Nanoparticle size (nm)a Note ZnO ZnO ZnO ZnO ZnO 70 54 42 36 40 Without TEA surfactant With TEA surfactant With TEA surfactant With TEA surfactant With TEA surfactant a powder powder doped 1% Co doped 2% Co doped 5% Co After adjusting for instrumental broadening seen in Fig 2a and b, TEA produces a significant effect on the morphology of the ZnO nanopowders Without TEA, various nonuniform particles are produced While, with TEA, uniform spherical particles are produced The particle size is about 50–70 nm, which is in good agreement with the XRD data The reduction of particle size with Co doping was also observed in the SEM images (Fig 2c and d), again agrees with the XRD data The absorption spectra of the Zn1 − xCoxO nanopowders, obtained from diffuse reflectance measurement at room temperature, are presented in Fig We found, the band edge energy redshifts with Co doping The band edge energies of pure ZnO and 1%, 2%, 5% Co doping, determined from the optical absorption spectra, are 3.27, 3.26, 3.23, and 3.22 eV, respectively The redshift is due to sp–d exchange interactions between the band electrons and the localized d electrons of Co2+ cations [25,26] As can be seen in Fig 3, the absorption spectra of Co-doped ZnO nanopowders show three absorption peaks at 660, 611, and 565 nm These absorption peaks have been identified with d–d transition of the high spin Co2+ 3d7-4F ion in tetrahedral oxygen coordination [26,27] The absorption peaks at 660, 611, and 565 nm, are corresponding to the transitions from 4A2 to 4T1(4P), 2E(2G), and T1(4F), respectively The appearance of these transitions confirms that Co2+ ions have substituted the Zn2+ ions in the tetrahedral sites The XRD and UV–VIS absorption studies discussed above have revealed that Co was successfully incorporated into the ZnO lattice without changing the host wurtzite structure To gain further information Fig XRD patterns of ZnO nanopowders obtained without TEA (a), with TEA (b), and Zn1 − xCoxO nanopowders of x = 1% (c), 2% (d), 5% (e) and 7% (f) All the Co-doped nanopowders are obtained with TEA 964 L.H Hoang et al / Materials Letters 64 (2010) 962–965 Fig SEM images of ZnO nanopowders obtained without TEA (a), with TEA (b), and Zn1 − xCoxO nanopowders with x = 1% (c) and 7% (d) The SEM images of 2% and 5% Co doping are very similar to that of 1% and 7%, which are not presented here on the Zn1 − xCoxO nanopowders, we then performed Raman scattering study Fig shows first-order Raman spectra obtained at room temperature The obtained phonon frequencies of ZnO powders (Fig 4a) are consistent with previous studies [28–31] The peaks at 99, low high 200, 332, 437, and 580 cm− can be assigned to Elow − Elow , 2E2 , (E2 ), Ehigh , and A (LO), respectively [30,31] The observed intense and sharp E(high) and E(low) peaks confirm good crystallinity of the ZnO nanopow2 ders The Elow mode decreases in intensity and shifts to lower frequency with increasing Co concentration Since the Elow mode involves mainly Zn motion, the shifting and broadening of this peak can be associated with the substitution of Co to Zn in the host lattice [28] The systematic broadening of Elow peak confirms that the substitution of Co at the Zn-site Fig Absorption spectra of ZnO nanopowders (a), and Zn1 − xCoxO nanopowders of x = 1% (b), 2% (c), and 5% (d) L.H Hoang et al / Materials Letters 64 (2010) 962–965 965 Fig Raman spectra of ZnO nanopowders (a), and Zn1 − xCoxO nanopowders of x = 1% (b), 2% (c), 5% (d), and 7% (e) is proportional to the Co concentration up to 5% Moreover, the observation of the broad peak at ∼552 cm− in the Raman spectra of Zn1 − xCoxO nanopowders (Fig 4b, c, d), not observed in ZnO nanopowders (Fig 4a), gives a clear evidence for the Co substitution in ZnO host lattice [32] As the Co content increases to 7%, the Ehigh peak intensity decreases quickly, which can be attributed to the disordering of cations around oxygen In addition, the Raman spectrum of 7% Co doping shows several additional peaks at ∼486, 523, and 625 cm− 1, indicating the formation of a secondary phase of Co3O4 [33,34], which is consistent with the XRD results presented in Fig As can be seen in Fig 4, no detectable peaks corresponding to secondary phases were presented in Zn1 − xCoxO nanopowders up to 5% However, the possibility of the existence of hidden secondary phases still cannot be simply ruled out Our recent study shows that due to the inhomogeneity of the Zn1 − xCoxO nanopowders, hidden secondary phases are also presented in Co doping below 5%, the details of this study will be presented elsewhere Hidden secondary phases of Zn1 − yCo3 − yO4 were also detected in 4.5% Co-doped ZnO nanorods in a recent report [35] Conclusion Zn1 − xCoxO nanopowders are successfully prepared using a simple microwave-assisted method We find that the surfactant TEA has negligible influence on the phase of the final product, while it affects the morphology significantly The average particle size of Zn1 − xCoxO nanopowders with TEA is ∼50 nm, determined by SEM and XRD analysis The successful incorporation of Co into ZnO is evidenced by XRD, UV–VIS absorption, and micro-Raman scattering, which show that Co is homogeneously incorporated into the Zn-site without changing the host wurtzite structure for Co doping up to 5% Acknowledgments This work was supported by NAFOSTED Grant 103.03.93.09, 2010 Key Project of Vietnam National University, Hanoi, Viet Nam and Quantum Metamaterials Research Center financed by Korea Science and Engineering Foundation Grant (R11-2008-503-03001) References [1] Peiro AM, Ravirajan P, Govender K, Boyle DS, O'Brien P, Bradley DDC, Nelson J, Durrant JR J Mater Chem 2006;16:2088 [2] Peiro AM, Domingo C, Peral J, Domenech X, Vigil E, Hernandez- Fenollosa MA, Mollar M, Mari B, Ayllon JA Thin Solid Films 2005;483:79 [3] Baek S, Song J, Lim S Physica B 2007;399:101 [4] Wu L, Wu Y, Pan X, Kong F Opt Mater 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