e-Journal of Surface Science and Nanotechnology 27 December 2011 Conference - IWAMN2009 - e-J Surf Sci Nanotech Vol (2011) 472-476 Synthesis and Characteristics of Single-Crystal Ni-Doped ZnO Nanorods Prepared by a Microwave Irradiation Method∗ Nguyen Viet Tuyen, Nguyen Ngoc Long, and Ta Dinh Canh† Faculty of Physics, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam (Received 24 November 2009; Accepted 10 May 2010; Published 27 December 2011) Straight single-crystal Ni-doped zinc oxide (ZnO:Ni) nanorods are prepared in large quantities via microwave irradiation by using zinc acetate and polyvinyl pyrrolidone (PVP) as precursors The nanocrystals of the ZnO:Ni with hexagonal wurtzite structure are characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and UV-Vis absorption techniques Highly straight ZnO:Ni nanorods with 8-10 nm diameter and 35-45 nm length are produced The X-ray diffraction, transmission electron micrograph and magnetization hysteresis loops of nickel-doped ZnO nanocrystals were presented to confirm that the nickel impurities are embedded inside the nanocrystal Comparison of the amount of ZnO:Ni nanorods prepared in the presence or absence of PVP reveals that the PVP plays an important role in preparing large quantities of ZnO:Ni nanorods [DOI: 10.1380/ejssnt.2011.472] Keywords: Microwave irradiation; Ni-doped ZnO; Nanorods; PVP; Magnetic property I INTRODUCTION Recently, nanocrystalline powders with uniform size and shape, in particular nanocrystalline metal oxides, have shown interesting properties due to their numerous important properties such as catalytic, electrical and optical properties as well as distinguishable differences in these properties from macroscopic, microscopic and bulk materials [1–3] Zinc oxide (ZnO) is technologically an important material due to its wide band gap (3.37 eV) and a large exciton binding energy (60 meV) The stable structure of ZnO is wurtzite, in which four atoms of oxygen locate in tetrahedral coordination surround each atom of zinc Recently, Ni-doped ZnO have been investigated for possible application as a spintronic material [5, 6] Synthesis of these materials is often accomplished by sputtering [7], chemical vapor deposition [8, 9], sol-gel technique [12] and vapor-phase transports process [13] At the present, there are a few reports focusing on the role of PVP (molecular weight of 40,000) in controlling the morphological ZnO powder In this study, we used PVP as a capping agent because PVP dissolves very well in many organic solvents and it is expected it can control the growth of inorganic crystal II EXPERIMENTAL The ZnO nanoparticles were prepared by precipitation from solution using Zn(CH3 CO2 )2 H2 O and NaOH The overall reaction for the synthesis of ZnO nanoparticles from Zn(II) acetate can be written as follows: Zn(CH3 COO)2 + NaOH → ZnO + CH3 COONa + H2 O (1) The used solvent was isopropanol (Merk 99%) The solvent was used as received without further purification In a typical procedure, 2.194 g Zn(CH3 CO2 )2 H2 O (Merk, 99 %) was first dissolved in 50 ml isopropanol with continuous stirring until a homogeneous solution was obtained Various amounts of polyvinylpyrrolidone (PVP, MW 40,000) were then added into previous zinc precursor solutions in order to investigate the role of the PVP in controlling the shape and size of the ZnO nanoparticles Finally, 1.6 g NaOH (Merk, 99% purity) was dissolved in 50ml isopropanol and then this NaOH solution was slowly added to the PVP-modified zinc precursor solutions For doping, appropriate amounts of Ni(CH3 CO2 )2 H2 O (99%) were added to zinc acetate solution until the concentration of the dopant was 3% The resulting solution was then placed in a conventional microwave oven The microwave power was set to 150 W The reaction time was minutes During microwave irradiation the temperature of the solution reached up 60◦ C After reaction time, the transparent solution yields white products, which was washed several times with absolute ethanol and distilled water Finally, the products were dried at 70◦ C in air for hours The morphologies and structures of the products were investigated by SEM (JEOL- J8M5410 LV), TEM (JEOL JEM 1010, Japan), X-ray diffractometer (Bruker-AXSD5005) Raman scattering spectra at room temperature in the energy region between 100 and 1000 cm−1 were recorded by a micro-Raman spectrograph LABRAM-1B equipped with a He-Ne laser (λ = 632.817 nm) with a power of 11 mW High-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL- 2010 TEM Room temperature photoluminescence (PL) spectrum of Ni-doped ZnO powders was acquired using 325 nm line of a He-Cd laser as excitation source A Shimadzu UV 2450 PC spectrometer was used to record the UV-visible absorption spectra III ∗ This paper was presented at the International Workshop on Advanced Materials and Nanotechnology 2009 (IWAMN2009), Hanoi University of Science, VNU, Hanoi, Vietnam, 24-25 November, 2009 † Corresponding author: canhtd@vnu.edu.vn RESULTS AND DISCUSSION Various amounts of PVP are used to investigate the effect of surfactant on the ZnO:Ni particle size and shape The results are shown in Fig 1, where the zinc ion concentration was 0.09 M and the reaction was performed c 2011 The Surface Science Society of Japan (http://www.sssj.org/ejssnt) ISSN 1348-0391 ⃝ 472 e-Journal of Surface Science and Nanotechnology Volume (2011) a Absorption b c 300 400 500 600 700 800 Wavelength (nm) FIG 2: UV-vis spectra of the prepared ZnO:Ni with different R (Zn 2+ /PVP weight ratio) values: a) R = 0.6; b) R = 0.9; c) R = 1.2 FIG 1: TEM images of ZnO:Ni nanoparticles a) ZnO:Ni nanoparticles without PVP and ZnO: Ni nanorods with different R (Zn2+ /PVP weight ratio) values: b) R = 0.6; c) R = 0.9; d) R = 1.2 E =3.45 eV g a E =3.40 eV g ( h ) under the same conditions When PVP isn’t used as surfactant, the nanoproducts ZnO:Ni have spherical shape with mean diameter of about 10 nm as shown in Fig 1(a) In Fig 1(b), the Zn2+ /PVP weight ratio (denoted as R) was equal 0.6; it can be seen clearly that the diameter and length of particles are 8-10 nm and 35-45 nm, respectively Compared with Fig 1(b), with increasing the Zn2+ /PVP ratio (R = 1.2), the ZnO:Ni nanorod sizes increase (see Fig 1(d)) Their diameters are up to 30-40 nm and the lengths are up to 60-70 nm The results demonstrate that the surfactant PVP plays two important roles in controlling the ZnO:Ni size and shape First, PVP promotes the reaction of Zn2+ ions with NaOH by generating the OH− groups in solution, favoring more reaction and grain growth Secondly, PVP acts as stabilizer or capping agent when the Zn2+ /PVP weight ratio was smaller than R = 1.2 Therefore, PVP can encapsulate the ZnO particles at higher concentration to suppress the grain growth In this study, the morphology of ZnO powder was changed from spherical to a rod shape, when adding PVP into solution because of the adsorption protonated PVP species on the (100) negative plane, so the grains can grow in the ⟨001⟩ direction [4] The room temperature UV-visible spectra were also measured The UV-visible spectra of the prepared ZnO:Ni colloidal suspensions with different R (Zn2+ /PVP weight ratio) values are shown in Fig The spectra exhibit a strong absorption with an onset around 355 nm It is known that the bulk ZnO has absorption edge at 375 nm in the UV-visible spectrum, which is obviously larger than that of the prepared ZnO nanostructures This is interpreted as blue shift of the absorption band edge with decreasing the particle size Based on the absorption spectra, we could estimate the band gap of ZnO:Ni powders from the relationship described the absorption coefficient for the allowed direct b E =3.38 eV g c 2.0 2.5 3.0 3.5 4.0 4.5 Energy (eV) FIG 3: Plots of (αhν)2 vs h ν for ZnO:Ni nanorods modified PVP with different R (Zn2+ /PVP weight ratio) values: a) R = 0.6; b) R = 0.9; c) R = 1.2 transitions: (αhν) = A (hν − Eg ) , (2) where α is the optical absorption coefficient, hν is the photon energy, Eg is the direct band gap and A is a constant Figure shows the plots of (αhν)2 versus hν for ZnO:Ni powders The linear portion of the curves when extrapolating to α = was the optical band gap value of ZnO:Ni powders In this study, we obtained the optical band gap of about 3.45; 3.40 and 3.38 eV at R = 0.6, R = 0.9 and R = 1.2, respectively The band gap values in this study are larger than the band gap value of ZnO (3.37 eV) in [4] It is clear that the optical band gap shifted to higher energy (blue shift) with increasing PVP concentrations or decreasing the grain size In order to prepare ZnO:Ni nanorods with R = 0.6, the solution of ZnO:Ni/PVP was dried at 100◦ C in air After that we obtained powder-type products A typical XRD pattern of the products is shown in Fig It can be seen that there are seven diffraction peaks corresponding to the (100), http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) 473 Tuyen, et al (112) b (103) (110) (102) (002) (101) Intensity (a.u.) (100) Volume (2011) a 30 FIG 6: Energy dispersion spectrum of ZnO:Ni nanorods 40 50 60 70 (deg.) FIG 4: XRD patterns of (a) non capped ZnO spherical nanoparticles and (b) ZnO:Ni nanorods capped with PVP (R = 0.6) 437 (E , high) 383 (A , TO) Intensity (a.u.) 2nd order 332 (2-E ) 104 (E , low) 574 LO 200 400 600 800 -1 Raman shift (cm ) FIG 5: Typical room-temperature micro-Raman spectrum of the synthesized sample (002), (101), (102), (110), (102) and (112) crystalline lattice planes The calculated lattice constants a = 0.325 nm and c = 0.521 nm are consistent with the standard values No characteristic peaks from other impurities are detected All the diffraction peaks can be indexed to the hexagonal structured of ZnO:Ni That the morphology of the particles changes from spherical to rod form can be observed clearly in the XRD patterns In PVP capped sample, the FWHM of (002) peak is much smaller than those of other peaks, this suggests that the PVP capped nanoparticles may not have the spherical symmetry but have a preferred growth direction along (002) direction Figure shows a micro-Raman scattering spectrum of the synthesized sample ZnO:Ni nanorods have a wurtzite crystal structure, which belongs to C6v group According to the group theory analysis, the A1 + E1 + 2E2 modes are Raman active The two higher peaks at 103 and 438 cm−1 can be assigned to E2 modes, characteristic of the wurtzite lattice The much weaker peak at 379 cm−1 is attributed to the transverse optical modes of A1 The other two weaker and broader peaks at 203 and 333 cm−1 474 can be assigned to the secondary Raman scattering arising from zero-boundary phonons 2-TA (M), and 2-E2 (M), respectively [10] The presence of the E1 (LO, 580 cm−1 ) mode of oxygen deficiency indicates that there are oxygen vacancies in our ZnO:Ni nanorods The XRD and Raman spectra reveal good crystal quality Figure shows the EDS spectrum from Ni-doped ZnO nanorods The sample has an oxygen peak at 0.53 keV and Zn signal at 1.03, 8.64 and 9.58 keV The Ni signal at 7.49 keV was observed in the Ni-doped ZnO nanorods TEM image gives us more details about the microstructure of the ZnO:Ni nanorods with R = 0.6 It can be seen from Fig 7(a) that the ZnO:Ni nanopowders are of good transparency to the electron beam The particles appeared to be well separated from each other Figure 6(a) shows the magnified TEM image of ZnO:Ni nanorods, synthesized in isopropanol The nanorods are very straight and have a high regularity Note that the short ZnO:Ni nanorods could be observed when the ultrasonication was used in the sample preparation for TEM analysis The selected area electron diffraction pattern (SAED) shown in Fig 7(b) and the high-resolution transmission microscopy (HRTEM) image shown in Fig 7(c) indicate a single-crystal structure of ZnO:Ni product The fringe spacing is about 0.28 nm, which corresponds to that of (100) crystal planes in ZnO crystal (Fig 7c) The room PL spectrum of the ZnO:Ni nanorods mainly consist of three emission bands : a weak and narrow UV emission band at ∼ 382 nm (3.25 eV), a weak blue-green band at 470 nm (2.64 eV), and strong green band at ∼ 542 nm (2.29 eV) The weak and narrow UV emission corresponds to the excition recombination related near-band edge emission of ZnO The weak blue-green emission is possibly due to surface defect in the ZnO nanopowders as in the [10] A strong and broad green band emission corresponds to the singly ionized oxygen vacancy in ZnO, and this emission results from the recombination of a photogenerated hole with the singly ionized charge state of the specific defect [10, 11] Strong intensity of the green emission may be due to the high density of oxygen vacancies during the preparation of the ZnO:Ni powders Figure shows magnetic hysteresis (M-H) curves of the ZnO:Ni nanorods measured at room temperature The ferromagnetic hysteresis loop was clearly observed from the M-H curves, the remanence (Mr ) and the coercive (Hc ) of the ZnO:Ni nanorods are ∼ 4.28 × 10−4 emu/g and ∼ 77 Oe at room temperature http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology Volume (2011) FIG 7: (a) Magnified TEM image of ZnO:Ni nanorods, (b) the corresponding electron diffraction pattern and (c) HR-TEM image of single ZnO:Ni nanorods showed (100) crystalline planes 382 nm Intensiy (a.u.) 470 nm 542 nm IV 400 450 500 550 600 650 700 Wavelenght (nm) FIG 8: Room temperature photoluminescence spectra of the synthesized ZnO:Ni nanorods under 325 nm light excitation Magnetization (10 -3 emu/g ) CONCLUSION Microwave-assisted synthesis is generally characterized by significant reduction of reaction time because of solvent-superheating effect, which cannot be generally achieved by traditional heating sources The easy and very fast microwave-assisted approach was used for preparation of the Ni-doped ZnO nanoparticles XRD results showed that the obtained ZnO:Ni nanoparticles were composed of hexagonal wurtzite phase with very good crystallinity By varying the Zn2+ /PVP weight ratio, we can control the ZnO:Ni particle size (the nanorods with diameters of 8-10 nm and lengths of 35-45 nm, when R = 0.6) When PVP isn’t used as surfactant, the nanoproducts ZnO:Ni have spherical shape with mean diameter of about 10 nm The nanoscale ZnO:Ni powders are ferromagnetic at room temperature The ZnO:Ni nanopowders also exhibited room temperature PL, having a weak and narrow UV emission at 3.25 eV, weak blue-green band at 2.64 eV, and a strong green band at 2.29 eV The current simple synthesis method using cheap precursors can be extended to prepare nanocrystalline powders of other interesting metal oxide powders -1 -2 Acknowledgments -3 -4 -15 -10 -5 10 15 Magnetic field (kOe) FIG 9: Magnetization vs magnetic field of the ZnO:Ni nanorods measured at room temperature [1] M R Vaezi and S K Sadrnezhaad, Nature 28, 515 (2007) [2] Y J Kwon, K H Kim, C S Lim, and K B Shim, J Ceramic Processing Res 3, 146 (2006) [3] J G Lu, Z Z Ye, J Y Huang, L P, Zhu, B, H, Zhao, Z L Wang, and Sz Fujita, Appl Phys Lett 88, (2006) This work is completed with financial support by the Vietnam National University, Hanoi (Key Project QG 09 05 and Project TN 09 09) Authors of this paper would like to thank the Center for Materials Science (CMS), Faculty of Physics, Hanoi University of Science,VNU for permission to use its equipments [4] K K Caswell, C M Bender, and C J Murphy, Nano Lett 3, 667 (2003) [5] Z Hu, G Oskam, and P C Searson, J Colloid and Interface Sci 263, 454 (2003) [6] G Glaspell, P Dutta, and A Manivannan, J Cluster Sci 16, 523 (2005) http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) 475 Tuyen, et al Volume (2011) [7] A D Yoffe, Adv Phys 51, 799 (2002) [8] T T Q Hoa, T D Canh, N N Long, N V Tuyen and N D Phuong, ASEAN J on Sci and Tech for Development 24, 131 (2007) [9] N F Hamedani and F Farzaneh, J Sci Islamic Republic of Iran 17, 231 (2006) [10] J Wang and L Gao, Solid State Commun 132, 269 (2004) [11] B D Yao, Y F Chan, and N Wang, Appl Phys Lett 476 81, 757 (2002) [12] N V Tuyen, N N Long, T T Q Hoa, N X Nghia, D H Chi, K Higashimine, T Mitani, and T D Canh, J Exp Nanoscience 4, 243 (2009) [13] N V Tuyen, T D Canh, N N Long, N X Nghia, B N Q Trinh, and Z Shen, J Phys Conf Series 187, 012020 (2009) http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) ... spectrum of the ZnO: Ni nanorods mainly consist of three emission bands : a weak and narrow UV emission band at ∼ 382 nm (3.25 eV), a weak blue-green band at 470 nm (2.64 eV), and strong green band at. .. generally achieved by traditional heating sources The easy and very fast microwave- assisted approach was used for preparation of the Ni-doped ZnO nanoparticles XRD results showed that the obtained... powders are ferromagnetic at room temperature The ZnO: Ni nanopowders also exhibited room temperature PL, having a weak and narrow UV emission at 3.25 eV, weak blue-green band at 2.64 eV, and a strong