A Study of the Optical Properties in ZnWO<sub>4</sub> Nanorods Synthesized by Hydrothermal Method

doi:10.4236/msa.2011.28133

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Materials Sciences and Applicatio n, 2011, 2, 988-992

doi:10.4236/msa.2011.28133 Published Online August 2011 (http://www.SciRP.org/journal/msa)

A Study of the Optical Properties in ZnWO4

Nanorods Synthesized by Hydrothermal Method#

Nguyen Van Minh1*, Nguyen Manh Hung1,2

1Center for Nano Science and Technology, and Department of Physics, Hanoi National University of Education, Hanoi, Vietnam; 1,2

Hanoi University of Mining and Geology, Dong Ngac, Tu Liem, Hanoi, Vietnam.

Email: *[email protected]; [email protected]

Received May 17th, 2011; revised April 20th, 2011; accepted June 7th, 2011.

ABSTRACT

We investigate the effect of synthesized time on the structure, as well as optical properties in ZnWO4 nano rod prepared

by hydrothermal method. The prepared rods were characterized by X-ray diffraction (XRD), scanning electron micros-

copy (SEM), Raman scattering, absorption and photoluminescent (PL) spectra techniques. The size and morphology of

ZnWO4 nano-rod can be controlled by adjusting the reaction time. The resultant sample is a pure phase of ZnWO4

without any impurities. The results showed that the optical property of ZnWO4 nanoparticles obviously relied on their

rod sizes.

Keywords: ZnWO4 Nano Rod, Raman Spectroscopy, Absorption, Photoluminescence

1. Introduction

Zinc tungstate (ZnWO4) with a wolframite structure has

been of practical interest for a long time because of its

attractive luminescence [1]. ZnWO4 has been applied as

a possible new material for microwave amplification by

stimulated emission of radiation [2], scintillator [3] and

optical hole burning lattice material [4], etc. Recently,

new applications for this material have emerged, includ-

ing large-volume scintillators for high-energy physics [5]

In particular, ZnWO4, also known by its mineral name

sanmartinite, is a wide-gap semiconductor, with band gap

energy close to 4 eV [6], and is a promising material for

the new generation of radiation detectors.

ZnWO4 has been prepared by different routes such as

the Czochralski method [7], sintering of WO3 and ZnO

or ZnCO4 powders [8], reaction in aqueous solution fol-

lowed by heating of the precipitate [9], heating of ZnO

thin films with WO3 vapor [10], sol-gel reaction [11],

and hydrothermal reaction over an extensive period [12].

However, ZnWO4 particles prepared by these routes are

relatively large in particle size and irregular in morphol-

ogy. Furthermore, higher calcining temperature is still

needed. It is very significant whether in fundamental or

applied field to explore new routes to ZnWO4, especially

for ZnWO4 crystallites with nanometer size, which would

have unique properties compared to traditional products

[13,14]. To obtain nanosized powders, the solid state

methods have several problems, because the WO3 has a

tendency to vaporize at high temperatures [15], nonho-

mogeneous compounds might be easily formed during

the solid-state and melting processing and the tempera-

ture for the solid state reaction is relatively high [16].

These problems could be solved by applying the hydro-

thermal method. However, a few studies on the chemical

synthesis of zinc tungstate by the hydrothermal method

have been reported. Furthermore, very few papers were

concerned with effects of the size and morphology on the

optical properties of ZnWO4 nanoparticles.

In this work, we report the synthesis of ZnWO4 nano

rod by hydrothermal method at a low temperature of

180˚C and investigate their structure, Raman scattering,

absorption and photoluminescence.

2. Experiment

Zinc tungstate (ZnWO4) nanoparticles were prepared by

the hydrothermal reaction of Zn(NO3)2·6H2O and

Na2WO4·2H2O at temperature of 180˚C, and various re-

action times (2, 4, 6 and 8 h). In a typical procedure for

the preparation of sample, Zn(NO3)2·6H2O (1 mmol) in

water (10 ml) was added Na2WO4·2H2O (1 mmol) in

water (20 ml) with vigorous stirring. H2O was added to

make 40 ml of the solution, and pH of the solution was

#This work has been supported by The Vietnam’s National Foundation

for Science and Technology Development (NAFOSTED).

A Study of the Optical Properties in ZnWONanorods Synthesized by Hydrothermal Method989

adjusted to 6.68, respectively, with dilute of 30%

NH3·H2O solution. The solution was then added into a

Teﬂon-lined stainless steel autoclave of 100 ml capacity.

The autoclave was heated to 180˚C for 2, 4, 6 and 8 h,

respectively, without shaking or stirring. Afterwards, the

autoclave was allowed to cool to room temperature gradu-

ally. The white precipitate collected was washed with

distilled water four times. The solid was then heated at

80˚C and dried under vacuum for 2.5 h.

Structural characterization was performed by means of

X-ray diffraction using a D5005 diffractometer with Cu

Kα radiation. The FE-SEM observation was carried out

by using a S4800 (Hitachi) microscope. Raman meas-

urements were performed in a back scattering geometry

using Jobin Yvon T 64000 triple spectrometer equipped

with a cryogenic charge-coupled device (CCD) array

detector, and the 514.5 nm line of Ar ion laser. The ab-

sorption spectra were recorded by using Jasco 670 UV-vis

spectrometer and the room temperature luminescent

spectra were recorded on a spectrofluorometer (PL,

Fluorolog-3, Jobin Yvon Inc, USA).

3. Result and Discussion

3.1. Structure

Figure 1 shows the XRD patterns of ZnWO4 powders

heated for 2, 4, 6 and 8 h as a function of the reaction

time. It is noted that the ZnWO4 single phase could be

observed in all XRD patterns. All diffraction peaks of

ZnWO4 crystal appeared when the sample was prepared

at 180˚C, which could be easily indexed as a pure,

monoclinic wolframite tungstate structure according to

the standard card (JCPDS Card number: 73-0554). It was

found that, the optimum temperature for the production

2345678

Particle size (nm)

Time (h)

y( )

20 30 40 50 60 70

2-theta (degree)

Figure 1. XRD patterns of nanosamples with various reac-

tion times. The inset shows the particle size vs. reaction

time.

of the high-quality crystal was as high as 180˚C [17].

The morphologies and microstructures of the samples

were then investigated with SEM. Figure 2 shows that

the morphologies and dimensions of the samples were

strongly dependent on the reaction time. SEM micro-

graph for the sample synthesized for 2 h was basically

irregular (Figure 2a). With the increase of the reaction

time to 4 h, the rod-shaped crystals can be seen (Figure

2b), and the rod size was in the range from several na-

nometers to several tens of nanometers. For 6 h, the

crystal was basically rod-like (Figure 2c). When the re-

action time was raised to 8 h, the rod-shaped crystals

grew larger and longer and a majority of the crystals

have exceeded 50 nm in length, the sizes of the rods be-

came homogeneous (Figure 2d). However, it was shown

that, for longer reactive times, such as 48 h, the crystal

phase instead became inhomogeneous, which could be

due to the breakage of the large crystal under such condi-

tions, suggesting the worse crystallinity [12]. The inset of

Figure 1 shows the average crystallite sizes for the

heat-treated powders calculated by XRD line broadening

method [18]. The calculated average crystallite sizes

were 22.02, 25.00, 26.05 and 26.00 nm for the heat-treat-

ed powders at 2, 4, 6 and 8 h, respectively. These are

corresponding to the SEM observation in Figure 2

showing an ordinary tendency to increase with the reac-

tion time from 2 h to 8 h. However, Zhao et al. [19] has

investigated the calcinations time and concluded that the

calcination time plays little effect on the crystal phase of

ZnWO4. This comment was contrary to our result.

3.2. Absorption Spectroscopy

Figure 3 shows a diffuse reﬂection spectrum of ZnWO4

nanopowder. Steep shape of the spectra indicated that the

UV light absorption was due to the band-gap transition

instead of the transition from the impurity level. For a

Intensity (arb.units)

Figure 2. SEM images of the nanosamples synthesized in

different time: for 2 h (a), 4 h (b), 6 h.

A Study of the Optical Properties in ZnWONanorods Synthesized by Hydrothermal Method

990 4

8 h

2 h

Ab so rp tio n (a rb . u n its)

2468

3.74

3.76

3.78

Band gap (eV)

Time (h)

250 300 350 400 450

Wavelength (nm)

Figure 3. Diffuse reﬂection spectra of ZnWO4 nanopowders.

The inset shows the optic al band gap vs. reaction time.

crystalline semiconductor, the optical absorption near the

band edge follows the equation: ah



= A(h



– Eg)1/n,

where a,



, Eg and A are absorption coefficient, light

frequency, band gap, and a constant, respectively [20].

For the ZnWO4, n is determined to be 2. Thus, the band

gaps of the ZnWO4 nanopowders were roughly estimated

to be 3.78, 3.76, 3.73 and 3.72 eV, as shown in the inset

of Figure 3. Bonanni et al. [21] reported that the band

gap of ZnWO4 was 3.75 eV, which is in agreement to our

experimental values. However, the band gap becomes

narrower in the sample with longer reaction time. There-

fore, the crystallization degree may contribute to the ab-

sorption edge shift.

3.3. Raman Spectroscopy

Figure 4 illustrates the Raman spectra for ZnWO4

nanopowdes. It is clear that, the peak shifts to higher

frequency as increasing the reaction time. Studies of the

optical properties and the Raman spectra of ZnWO4 at

room temperature have been reported in the literature

[22]. ZnWO4 has the monoclinic wolframite structure

880 900 920

200 400 600 8001000

8 h

6 h

4 h

2 h

Intensity (Arb. units)

Raman shift (cm

-1

)

Figure 4. Raman spectra of ZnWO4 nanopowders.

with C2h point group symmetry and P2/c space group. It

has two formula units per unit cell. The W-O interatomic

distance is substantially smaller than that of Zn-O, there-

fore, to a first order approximation, the lattices can be

separated into internal vibrations of the octahedra and the

external vibrations in which an octahedron vibrates as a

unit. A group theoretical calculation of the ZnWO4

structure yields 36 lattices modes, of which 18 are Ra-

man active (8Ag + 10Bg).

its) Absorption (arb.un

It is assigned to the Ag mode observed near 907 cm–1

since, as is the case of the regular octahedron, the sym-

metric stretch is expected to have the highest frequency

of all the internal modes. The Eg mode (asymmetric

stretch) of the regular octahedron splits into Ag + Bg by

the crystal field. Again, these modes are expected to have

frequencies that are higher than those of the bending

mode (T2g) of the regular octahedron. The obvious

choices are the Bg and Ag modes observed near 786 and

709 cm–1, respectively. The remaining modes 2Ag + Bg

with frequencies of 407, 342 and 190 cm–1 are assigned

to the T2g mode of the regular octahedron.

In a first attempt to identify the six internal stretching

modes of the W-O atoms in the distorted WO6 octahedra

of ZnWO4, Liu et al. [23] assigned them to the modes at

906, 787 and 407 cm–1 on the basis of the bond lengths

and Raman frequencies in the WO6 group. Afterwards,

Wang et al. [24] assigned the internal stretching modes

to the phonons observed near 906, 787, 709, 407, 342,

and 190 cm–1 on the basis of the temperature dependence

of the Raman frequencies. However, this assignment is in

contradiction with the fact that the frequencies of the

internal modes are expected to be higher than those of

the external modes. These authors argue in favor of their

assignment that the oxygen sharing between WO6 and

ZnO6 octahedra may cause a considerable overlap in the

frequency range for the two types of vibrations.

3.4. Photoluminescence

Figure 5 shows the representative PL spectra of the

ZnWO4 crystallites synthesized by the hydrothermal

method for 2, 4, 6 and 8 h. With the exited wavelength at

320 nm, the corresponding emission peaks centered at

~500 nm can be observed. Obviously, the ZnWO4 nano-

rods, prepared at the same calcining temperature (180˚C)

with a shorter holding time, exhibit lower emission in-

tensity than that from longer time. It implies PL proper-

ties of the ZnWO4 nano-rods are strongly affected by

their long scale. This broad emission band had a shoulder

in the blue region, indicating it consisted of more than

one emission band. The PL spectra were ﬁt to three peaks

using a Gauss function as shown in Figure 6.

The PL spectrum for the ZnWO4 ﬁlm also consisted of

an emission band at 2.50 eV (495 nm) and two emission

A Study of the Optical Properties in ZnWONanorods Synthesized by Hydrothermal Method991

400 500 600

8 h

6 h

4 h

2 h

PL intensity (arb. units)

Wavelength (nm)

Figure 5. PL spectra ZnWO4 nanopowders.

400 500 600 700

PL Intensity (Arb. units)

W avelength (nm)

Figure 6. The PL spectra were ﬁt to three peaks using a

Gauss function. To clarify, we shows only fitting for PL of

the sample with reaction time of 8 h.

bands at 2.80 eV (448 nm) and 2.28 eV (545 nm). To

clarify, we show only the fitting figure of the sample

with reaction time for 8 h.

It is well established that the complex and a

slight deviation from perfect order in the crystal structure

are responsible for the emission bands [25]. But there

exist different opinions concerning the origin of these

bands. Lammers [26] and Grigorjeva [27] believed that

the blue and green emissions originated from the intrinsic

complex with a double emission from one and the

same center (3T1u1A1g), whereas the yellow emission is

due to recombination of e-h pairs localized at oxygen

atom deﬁcient tungstate ions. However, Ovechkin [28]

ascribed the blue band to the self-trapped exciton in

tungstenite crystals with strong electron–phonon cou-

pling, and the green and yellow bands to the transitions

of T2uT2g and T1gT2g in the complex. Almost

all investigations in luminescent properties of ZnWO4

have been carried out for large crystals and ﬁlms. The

nanosized ZnWO4 prepared via a hydrothermal route in

the current work showed luminescent properties similar

to those of bulk ZnWO4. The luminescence intensities

varied from sample synthesized for 2 h to sample synthe-

sized for 8 h. Nanorods with a longer synthesized time

exhibited a strong luminescence, and nanorods with a

shorter synthesized time gave weak luminescence. These

results suggest that morphologies and sizes of nanoparti-

cles may aﬀect their luminescence characteristics. Fur-

ther studies on PL property of the as-prepared ZnWO4

crystallites are in progress.

W

4. Conclusions

In conclusion, ZnWO4 nano-particles were successfully

synthesized at 180˚C by a hydrothermal route. The mor-

phology and dimension of the ZnWO4 crystallites were

affected by synthesized time. The optical band gap be-

comes narrower as increasing the reaction time. The im-

proved PL properties of the ZnWO4 crystallites can be

obtained with the increase of the rod scale. Because the

crystallite size of ZnWO4 for the nano rod is far large,

therefore, the absorption edge shift cannot result from

quantum size effect but prolonged rod.

5. Acknowledgements

This work has been supported by The Vietnam’s Na-

tional Foundation for Science and Technology Develop-

ment (NAFOSTED).

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