Conference-IWAMN 2009-Synthesis and Characteristics of Single-Crystal Ni-Doped ZnO Nanorods Prepared by a Microwave Irradiation Method

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]


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][2][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(CH 3 CO 2 ) 2 .H 2 O and NaOH.The overall reaction for the synthesis of ZnO nanoparticles from Zn(II) acetate can be written as follows: (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(CH 3 CO 2 ) 2 .H 2 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(CH 3 CO 2 ) 2 .H 2 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 5 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 4 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 = 632.817nm) 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. 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 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 Zn 2+ /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 Zn 2+ /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 Zn 2+ 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 Zn 2+ /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 (Zn 2+ /PVP weight ratio) values are shown in Fig. 2.
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  transitions: where α is the optical absorption coefficient, hν is the photon energy, E g is the direct band gap and A is a constant.Figure 3 shows the plots of (αhν) 2 versus hν for ZnO:Ni powders.The linear portion of the curves when extrapolating to α = 0 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  (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 5 shows a micro-Raman scattering spectrum of the synthesized sample.ZnO:Ni nanorods have a wurtzite crystal structure, which belongs to C 6v group.According to the group theory analysis, the A 1 + E 1 + 2E 2 modes are Raman active.The two higher peaks at 103 and 438 cm −1 can be assigned to E 2 modes, characteristic of the wurtzite lattice.The much weaker peak at 379 cm −1 is attributed to the transverse optical modes of A 1 .The other two weaker and broader peaks at 203 and 333 cm −1 can be assigned to the secondary Raman scattering arising from zero-boundary phonons 2-TA (M), and 2-E 2 (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 6 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.

IV. 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 Zn 2+ /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.