Abstract— ZnOs with flower- and rod- like shapes were

prepared by the simple hydrolysis of Zn metal powder. The

starting material of Zn nano powder was put into distilled

water. Small about of acetic acid was added, and the solution

was then refluxed at 60 ℃ for 8 h. Cu and Fe-doped ZnO

nanorods were synthesized by a very simple process employing

a hydrolysis of metal powders. Zn, Cu, and Fe nano-powders

were used as starting materials and incorporated into distilled

water. The solution was refluxed at 60 ℃ for 24 h to obtain the

precipitates from the hydrolysis of Zn, and the dopants (Cu

and Fe). The TEM results for ZnO with and without metal

doping showed that the produced powders had a rod-like

shape. The rod shape was attributable to the zinc oxide from

the hydrolysis of Zn. With an increase in the doping content,

the UV–vis spectra were shifted to a long wave length and this

result indicates that the band gap was changed by an metal

-doping.

I. INTRODUCTION

Nanocrystalline zinc oxide (ZnO) were applied as catalysts, surface acoustic wave devices, cosmetic pigments, varistor, ultraviolet (UV) absorbers, optical materials, gas sensors and window material for displays and solar cells, etc.[1] The microstructure and chemical properties of ZnO powders depend upon the synthesis method of this material. Different synthesis methods were used to fabricate ZnO particles with various sizes and morphologies. Accordingly, the synthesis of ZnO fine particles is of great importance for basic research. The size-dependent effects are correlated to the physical properties and structure of the system, e.g. the size dependence of electron– phonone coupling, the size dependence of surface luminiscenece of ZnO nanowires, the compressibility and the transition pressure [2]. Metal-doped ZnO is generally investigated in the form of diluted magnetic semiconductor (DMS) materials and a photo-catalyst, because it shows much higher Curie temperature than room temperature, along with strong stability in UV light [1-2]. A visible - rays - active photocatalyst is very important with respect to solar energy and interior lighting applications. For practical application, it has been reported that the enhancement of photocatalytic activity can be achieved by introducing foreign metal ions into ZnO or creating oxygen

* Young Rang Uhm (e-mail:uyrang @kaeri.re.kr)

vacancies with hydrogen plasma or X-ray irradiation. Therefore, many scientist have been studying the methods to introduce foreign metal ions, such as W

6+, V

5+, and Fe

3+, etc.

into ZnO. In particular, the Fe-doped case has been widely examined [3-4]. To obtain metal-doped ZnO powders suitable for their intended applications, the control of particular properties including chemical composition, purity, morphology, and particle size is very important. ZnO powder has various shapes such as prismatic, ellipsoidal, bi-pyramidal and dumbbell-like, nanowire, nano-rod and so on by different synthesis methods [3]. There are several methods for the synthesis of ZnO nanopowder, such as a sol-gel method, hydrothermal process, gas condensation method and spray pyrolysis, etc [5-8]. Among them, the hydrolysis synthetic route has the advantage to simply obtain high-crystallized powders. The present paper describes the processing details to synthesize a ZnO nanoflower and rods as well as their particulate morphologies such as the phase, size, and shape.

In this work, we synthesized Fe and Cu-doped ZnO nano-rods using a simple process employing the hydrolysis of Zn, Fe, and Cu nanopowders, which were produced by pulsed wire evaporatioin (PWE) of a metal wire [9]. The present paper describes the processing details for the synthesis of Fe and Cu -doped ZnO nano-rods as well as the particulate properties of the produced powders such as the phase, size, and phtocatalytic effect.

II. EXPERIMENTAL TECHNIQUE

High purity Zn, Fe, and Cu nanopowders were synthesized by a pulsed wire evaporation (PWE) method. The Zn, Fe, and Cu nanopowders have spherical shapes and average sizes of about 80-120nm. For a precondition of the hydrolysis reaction, the nanopowders were immersed into distilled water at a regular rate (Fe or Cu : 0, 2, 5, 8, 10 wt.%) and ultrasonically treated for 10 min.[10]. A small amount of acetic acid was added into the solution, where the acid played a role of promoting the hydrolysis reaction between the nanopowders and H2O. Hydrolysis has been carried out at 60 oC for 24 h to produce the precipitation of the iron doped zinc

hydroxide gel. The produced gel has been drawn through filtering using a 0.2 μm filter and subsequently dried in an oven at 60

oC for 12 h, which yielded solid precipitates with a

whitish to dark yellow color. After that, the gel powders were

Synthesis of Cu and Fe-doped ZnO nano-rod by hydrolysis of metal

powders

Young Rang Uhm1, Chang Kyu Rhee

2, and Sun Ju Choi

1,

1Radioisotope Research Division, Korea Atomic Energy Research Institute (KAERI) 989 Daedukdaero,

Daejeon 305-353, Republic of Korea 2Nuclear Materials Research Division, Korea Atomic Energy Research Institute (KAERI), 989

Daedukdaero, Daejeon 305-353, Republic of Korea

Proceedings of the 13thIEEE International Conference on NanotechnologyBeijing, China, August 5-8, 2013

978-1-4799-0676-5/13/$31.00 ©2013 IEEE 764

heat-treated at 300 oC for 1 h[11]. To investigate the structural

properties of the samples produced after the hydrolysis process, a X-ray diffractometer (RIGAKU D/MAX-3C) with CuKα radiation was used. The particle size and morphology of the particulate samples were examined using a MTE10 transmission electron micro-scope (TEM) at accelerating voltages of up to 300kV. The particles were also analyzed by selected area diffraction. Absorption spectra of the samples were recorded using a UV-visible spectrometer.

III. RESULTA AND DISCUSSION

A. Synthesis and properties of Fe-doped ZnO nano-rod

The ZnO nano particles were synthesized by hydrolysis of the nano metal powders as represented on the section of experimental technique. Typical TEM images of the as-prepared ZnO nanostructures are shown in Fig 1. The ZnO nano powder was prepared by hydrolysis of Zn nano powders as shown in Fig 1(a). Initially, a hydrolysis reaction in the pure distilled water did not occur. However, the cohesion of Zn metal was started immediately after the acetic acid was added into the reaction. The fibrous forms shown in Figs. 1(b), and 1(c), which were prepared by a hydrolysis reaction goes on for 1.5 h. The shape of a chestnut bur was formed as shown in Fig. 1(d), which was an intermediate form from the fibrous to flower-like shape. The mixed Zn and ZnO phase was observed by a phase analysis of this intermediate state. The flower-like ZnO nanocrystals was obtained by a hydrolysis

reaction in distilled water at 60 ℃, as shown in Fig. 1(e).The

nanopowders, which are in a flower-like shape were destroyed, since the hydrolysis reaction goes on for 8 h [12]. The nanorods shown in Figs. 1(f) were prepared by a

hydrolysis reaction in distilled water at 60 ℃ for 24 h. These

images of ZnO showed that the produced powders possess a tetrahedron and rod-like shape with a diameter of 30 nm and length of 100 nm. The aspect ratio of the ZnO with rod-shape was from 1:5 to 1:15. The TEM results for ZnO showed that the produced powders had a flower-like shape. It is clearly observed that the morphology changes the ZnO with the reaction. And the ZnO crystal belongs to the hexagonal space group P63mc. The lattice constants are a = 0.3249 nm and c = 0.5205 nm. The width of each diffraction peak for this sample is narrow, indicating the high degree of crystalline [10]. At previous study, Infrared spectrum is an important record, which provides information about the structure of a compound. The shape of the IR spectrum of ZnO particles is generally influenced by particle size and morphology, the degree of particles aggregation, or the crystal structure of the ZnO particles. The maximum band broadens and often splits into two maxima if the particle morphology changes from a spherical to a needle-like shape. [12].

B. Synthesis and properties of Fe-doped ZnO nano-rod

Fe-doped ZnO nano-rods (Fe= 0, 2, 5, 8, and 10 wt. %) were synthesized by the hydrolysis of Fe nanopowders. By increasing the Fe contents, the color of the samples gradually changed from white to dark yellow. Characterizations of the crystal structure for the samples synthesized by the hydrolysis process were carried out by XRD. The results are presented

in Fig. 2. The sharp diffraction peaks imply the good crystallization of the samples. The positions and relative intensities of all the main diffraction peaks were in good agreement with those of the standard JCPDS card (JCPDS No.89-1397, 89-0511, 89-0510 ) of ZnO having the prominent diffration peaks from crystal planes such as (100), (002) and (101). When Zn metal particles are hydrolyzed with distilled water, the ZnO phase is formed by the following reaction [10]:

Zn + H2O → ZnO + H2 (1)

Fig. 1 (a) SEM image for Zn nanoparticles, TEM images for ZnO phases during hydrolysis ; (b) inert acetic acid and after 20 min, (c) after 45 min. (d) after 2h, (e) after 6h, and (f) after hydrolysis completion, 24 h.

In a previous study, during the hydrolysis reaction of an iron metal powder, several different phases of iron hydroxide gels such as Fe(OH)2, Fe(OH)3, and FeOOH were formed[11]. However, when the iron powder was co-hydrolyzed with zinc in distilled water, no goethite or maghemite forms were observed as shown in Fig. 2, indicating that the doped-irons were well substituted into Zn sites without changing the crystal structure. As for the Fe-doped case, a rod shape with a diameter of 40 nm and a

765

length of 270 nm is observed, in which the large aspect ratio of the shape is attributable to the hydrolysis of iron [10].

Figure 3 shows the UV-vis spectra for the pure and Fe-doped ZnO nano-rods. In the spectrum of the Fe 2wt.%-doped ZnO, it was observed that the absorbance between 400 and 500 nm begins to increase, when compared with the undoped one. When ZnO is doped above 5 wt% Fe, the spectra shows that the absorption edge shifts to a long wavelength.

20 30 40 50 60 70 80

(202)

(004)

(201)

(200)

(110)

(103)

(112)

(102)

(101)

(002)

10 wt.%Fe

5 wt.%Fe

ZnO

Inte

ns

ity

(Arb

.Un

it)

2 (degrees)

(100)

Fig. 2 X-ray diffraction patterns for the Fe-doped ZnO nano-rods synthesized by the hydrolysis process.

400 500 600 700 800

ZnO

Ab

so

rba

nc

e (

a.u

)

wavelength(nm)

8 wt.%

5 wt.%

2 wt.%

Fig. 3 UV–vis absorbance of the Fe-doped ZnO nano-rods synthesized by hydrolysis process.

The introduction of Fe into ZnO by substituting the Zn sites with Fe ions leads to the appearance of additional absorption bands [2]. These bands are due to the transitions involving crystal field levels in the Fe ions. These transitions are observed in the Fe-doped ZnO with a doping rate of above 5 wt% [13-14]. At previous study, the absorption spectrum of Fe 5 wt.%-doped ZnO sample consists of the weak sextet and the central doublet, suggesting that the magnetically ordered and the paramagnetic phases coexist [15]. Magnetic hysteresis curves (not shown) of Fe-doped samples measured by vibrating sample magnetometer (VSM) indicated that, whereas the sample with 5 wt.% Fe-doping showed the ferromagnetic behavior with the coercive field of 210 Oe at room temperature, the ferromagnetic properties disappeared at the samples with above 5 wt.% Fe-doping [15].

C. Synthesis and properties of Cu-doped ZnO nano-rod

Cu-doped ZnO nano-rods (Fe= 0, 2, 5, 8, and 10 wt. %) have been synthesized by the hydrolysis of Cu nanopowders.. Characterizations of the crystal structure for the samples synthesized by the hydrolysis process were carried out by XRD, and the results are presented in Fig. 4. However, when the copper powder was co-hydrolyzed with zinc in distilled water, no cuprite, tenorite and Cu-hydroxides forms were observed as shown in Fig. 4, indicating that the doped-irons were well substituted into Zn sites without changing the crystal structure. Figure 5 shows the UV-vis spectra for the pure and Cu-doped ZnO nano-rods. In the spectrum of the Cu 8wt.%-doped ZnO, it was observed that the absorbance between 400 and 500 nm begins to increase. When ZnO is doped with above 10 wt% Cu, the spectra shows that the absorption edge shifts to a long wavelength.

20 30 40 50 60 70 80

10 wt.%

5 wt.%

ZnO

In

ten

sit

y(A

rb.U

nit

)

2 (degree)

(202)

(201)

(112)

(200)

(103)

(110)

(102)

(101)

(002)

(100)

Fig. 4 X-ray diffraction patterns for the Cu-doped ZnO nano-rods synthesized by the hydrolysis process.

The introduction of Cu into ZnO by substituting the Zn sites with Cu ions leads to the appearance of additional absorption

766

bands involving crystal field levels in the Cu ions [2]. The transitions are observed in a doping rate of above 8 wt% [12-13]. Figure 6 shows the transmission electron microscopy (TEM) image for the 5 wt% Cu-doped ZnO nano-rod by the hydrolysis process.As for the Cu-doped case, a rod shape with a diameter of 60 nm and a length of 400 nm is observed, in which the large aspect ratio of the shape is attributable to the hydrolysis of copper.

400 500 600 700 800

ZnO

5 wt.%

8 wt.%

10 wt.%

Ab

sorb

ance

(a.u

)

wavelength(nm)

Fig. 5 UV–vis absorbance of the Cu-doped ZnO nano-rods synthesized by hydrolysis process.

Fig. 6 TEM images and EDS for 5 wt.% Cu-doped ZnO nano-rod.

IV. CONCLUSION

Fe and Cu-doped ZnO nano-rods have been synthesized

using a simple process employing the hydrolysis of Zn, Fe,

and Cu nanopowders. TEM results showed that the produced

samples had a rod shape. The acetic acid was a key-material

for the hydrolysis reaction of Zn metal powder with a thin

oxide surface layer. The sequence of the shape changes of

ZnO during a hydrolysis reaction in an aqueous solution are

chestnut bur → flower → tetrahedron → rod. By increasing

doping contents, the UV-vis spectra were shifted to a long

wave length, and the substitution of Cu2+

and Fe3+

into Zn2+

led to the appearance of additional absorption bands.

ACKNOWLEDGMENT

This work has been performed under the KAERI Major

R&D Program.

REFERENCES

[1] D. C. Look, “Recent advances in ZnO materials and device”, Mater. Sci. Eng. B, vol.80, pp. 383-387, March 2001.

[2] X. Wu, Z. Wu, L. Guo, C. Liu, J. Liu, X. Li, and H.Xu,

“Pressure-induced phase transformation in controlled shape ZnO nanorods”,Solid Stat. Comm, vol.135, pp. 780-784, September 2005.

[3] M. Quintana, J. Rodriquez, J. Solis, and W. Estrada, Photochem.

Photobiol., vol.81, pp.783-788, July 2005. [4] Q. A. Pankhurst, J. Connolly,S.K. Jones and J. Dobson, “Application

of magnetic nanoparticles in biomedicine”, J. Phys. D, vol. 36 (13), pp.

R167-R181, July 2003. [5] C. Lu and C. Yeh, “influence of hydrothermal conditions on the

morphology and particle size of zinc oxide powder” Ceram. Int., vol. 26(4), pp. 351-357, May 2000.

[6] B. G. Wang, E. W. Shi, and W. Z. Zhong, “Growth habits and

mechanism of several polar organic crystals in various solvents” Acta

Chimica Sinica vol.33, pp. 937, 1998.

[7] J. Zhang, L. Sun, H. Pan, C. Liao, and C. Yan, “Control of ZnO

morphology via a simple solution route” Chemistry of Mater. Vol. 14,

pp.4172-4177, October 2002. [8] C. Xu, G. Xu, Y. Liu, and G. Wang, “A simple and novel route for the

preparation of ZnO nanorods” Solid State Commun., vol.122,

pp.175-179, April 2002. [9] J. H. Park, M. K. Lee, C. K. Rhee, and W. W. Kim, Mater. Sci. Eng. A,

vol.375–377, pp.1263, 2004.

[10] B. S. Han, Y. R. Uhm, G. M. Kim, and C. K. Rhee, “Novel synthesis of nanorod ZnO and Fe-Doped ZnO by the hydrolysis of metal powders” J. Nano Sci. and Nano Tech., vol. 7, pp.1-3, July 2007.

[11] Y. R. Uhm, W. W. Kim, and C. K. Rhee, “A study of synthesis and phase transition of nanofibrous Fe2O3 derived from hydrolysis of Fe

nanopowders”, Scr. Mater., Vol.50, pp.561, 2004.

[12] B.S.Han, Y. R. Uhm, and C. K. Rhee, “Synthesis for nanoflower and rod of ZnO by a surfactant free and low temperature method”, Surface

Review and Letters ,Vol. 17, pp. 1-4 January (2010).

[13] Y. Lin, Ο. Jiang, F. Lin, Q. Shi, X. Ma, “Fe-doped ZnO magnetic semiconductor by mechanical alloying”, J. Alloys Compd., vol. 436,

pp.30-33, June 2007.

[14] M. Jakani, G. Campet, J. Claverie, D. Fichou, J, Pouliquen, and Kossanyi,”Photoelectrochemical properties of zinc oxide doped with

3d elements”, J. Sol. Sta. Chemi., vol.56, pp.269, March 1985.

[15] Y. R. Uum, B. S. Han, H. M. Lee, S. M. Hong, G. M. Kim, and C. K. Rhee, “Magnetic and photocatalytic effect of Fe-doped nano-rod ZnO

synthesized by the hydrolysis of metal powders”, phys. stat. sol.c, vol.

4(12), pp. 4408–4411, December 2007

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