DSpace at VNU: 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 2

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 mi-croscopy (TEM), high-resolution transmission electron mimi-croscopy (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 diffrac-tion, 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

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

op-tical 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

sta-ble 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

inves-tigated 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

The ZnO nanoparticles were prepared by precipitation

from solution using Zn(CH3CO2)2.H2O and NaOH The

overall reaction for the synthesis of ZnO nanoparticles

from Zn(II) acetate can be written as follows:

Zn(CH3COO)2+ NaOH→ ZnO + CH3COONa + H2O

(1) The used solvent was isopropanol (Merk 99%) The

sol-vent was used as received without further purification In

∗This paper was presented at the International Workshop on

Ad-vanced Materials and Nanotechnology 2009 (IWAMN2009), Hanoi

University of Science, VNU, Hanoi, Vietnam, 24-25 November, 2009.

†Corresponding author: canhtd@vnu.edu.vn

a typical procedure, 2.194 g Zn(CH3CO2)2.H2O (Merk,

99 %) was first dissolved in 50 ml isopropanol with con-tinuous stirring until a homogeneous solution was ob-tained Various amounts of polyvinylpyrrolidone (PVP,

MW 40,000) were then added into previous zinc precur-sor solutions in order to investigate the role of the PVP

in controlling the shape and size of the ZnO nanopar-ticles Finally, 1.6 g NaOH (Merk, 99% purity) was dissolved in 50ml isopropanol and then this NaOH so-lution was slowly added to the PVP-modified zinc pre-cursor solutions For doping, appropriate amounts of Ni(CH3CO2)2.H2O (99%) were added to zinc acetate so-lution until the concentration of the dopant was 3% The resulting solution was then placed in a conventional mi-crowave oven The mimi-crowave 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 ab-solute ethanol and distilled water Finally, the products were dried at 70◦C in air for 4 hours The

morpholo-gies and structures of the products were investigated by SEM (JEOL- J8M5410 LV), TEM (JEOL JEM 1010, Japan), X-ray diffractometer (Bruker-AXSD5005) Ra-man scattering spectra at room temperature in the en-ergy 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 RESULTS AND DISCUSSION

Various amounts of PVP are used to investigate the ef-fect of surfactant on the ZnO:Ni particle size and shape The results are shown in Fig 1, where the zinc ion con-centration was 0.09 M and the reaction was performed

FIG 1: TEM images of ZnO:Ni nanoparticles a) ZnO:Ni

nanoparticles without PVP and ZnO: Ni nanorods with

differ-ent R (Zn2+/PVP weight ratio) values: b) R = 0.6; c) R =

0.9; d) R = 1.2

under the same conditions When PVP isn’t used as

sur-factant, 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

control-ling 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 2

The spectra exhibit a strong absorption with an

on-set 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

de-scribed the absorption coefficient for the allowed direct

c b

Wavelength (nm) a

FIG 2: UV-vis spectra of the prepared ZnO:Ni with different

R (Zn2+/PVP weight ratio) values: a) R = 0.6; b) R = 0.9; c) R = 1.2

c b

E g

=3.45 eV

E g

=3.40 eV

Energy (eV)

E g

=3.38 eV

a

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)2= 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

con-stant 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 op-tical 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

ob-tained powder-type products A typical XRD pattern of the products is shown in Fig 4 It can be seen that there are seven diffraction peaks corresponding to the (100),

30 40 50 60 70

a

2 (deg.)

b

FIG 4: XRD patterns of (a) non capped ZnO spherical

nanoparticles and (b) ZnO:Ni nanorods capped with PVP (R

= 0.6)

2nd order

332

(2-E 2 )

Raman shift (cm

-1 )

104

(E

2

, low)

383 (A 1 , TO)

437

(E 2 , high)

574

LO

FIG 5: Typical room-temperature micro-Raman spectrum of

the synthesized sample

(002), (101), (102), (110), (102) and (112) crystalline

lat-tice planes The calculated latlat-tice 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 A1+ E1+ 2E2 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 A1 The

other two weaker and broader peaks at 203 and 333 cm−1

FIG 6: Energy dispersion spectrum of ZnO:Ni nanorods

can be assigned to the secondary Raman scattering aris-ing 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 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 microstruc-ture 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 par-ticles 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 ultra-sonication 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 trans-mission 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 cor-responds 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 corre-sponds to the singly ionized oxygen vacancy in ZnO, and this emission results from the recombination of a photo-generated hole with the singly ionized charge state of the specific defect [10, 11] Strong intensity of the green emis-sion may be due to the high density of oxygen vacancies during the preparation of the ZnO:Ni powders

Figure 9 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.

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

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

-4

-3

-2

-1

0

1

2

3

4

Magnetic field (kOe)

FIG 9: Magnetization vs magnetic field of the ZnO:Ni

nanorods measured at room temperature

Microwave-assisted synthesis is generally character-ized 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 prepa-ration of the Ni-doped ZnO nanoparticles XRD results showed that the obtained ZnO:Ni nanoparticles were com-posed of hexagonal wurtzite phase with very good crys-tallinity By varying the Zn2+/PVP weight ratio, we can control the ZnO:Ni particle size (the nanorods with di-ameters of 8-10 nm and lengths of 35-45 nm, when R = 0.6) When PVP isn’t used as surfactant, the nanoprod-ucts ZnO:Ni have spherical shape with mean diameter of about 10 nm The nanoscale ZnO:Ni powders are ferro-magnetic at room temperature The ZnO:Ni nanopow-ders 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 precur-sors can be extended to prepare nanocrystalline powders

of other interesting metal oxide powders

Acknowledgments

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

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