Synthesis and luminescent properties of ZnNb2O6 nanocrystals for solar cell

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Synthesis and luminescent properties of ZnNb

2

O

6

nanocrystals for solar cell

Yu-Jen Hsiao

a

_{, Te-Hua Fang}

b,

⁎

_{, Liang-Wen Ji}

c

a

National Nano Device Laboratories, Tainan 741, Taiwan

b

Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan

c_{Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan}

a b s t r a c t

a r t i c l e i n f o

Article history: Received 29 June 2010 Accepted 17 August 2010 Available online 23 August 2010 Keywords:

Luminescence Sol–gel preparation

Optical materials and properties

The phase formation, morphology and luminescent properties of ZnNb2O6nanocrystals by the sol–gel

method were investigated at a lower temperature than that of the traditional solid-state reaction method. The products were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), photoluminescence spectroscopy (PL) and absorption spectra. The activation energy of ZnNb2O6grain growth is obtained about 18.4 kJ/mol. The diameters of the nanocrystals are in the range of

20–40 nm. The PL spectra excited at 276 nm have a broad and strong blue emission band maximum at 450 nm, corresponding to the self-activated luminescence of the niobate octahedra group [NbO6]7−. The

optical absorption spectrum of the sample at a calcination temperature of 800 °C has a band gap energy of 3.68 eV.

1. Introduction

The electro-optical properties of the metal niobates of ANb2O6

(A = Mg and Ca) have been studied extensively in recent years[1,2]. It is known that various compositions are possible in the Zn–Nb–O system. To date, three possible zinc niobium oxides have been identiﬁed: ZnNb2O6, Zn2Nb34O87and Zn3Nb2O8[3–5]. Among these

compounds, zinc niobate (ZnNb2O6) is one of the most well-known

materials. ZnNb2O6 ceramics have excellent dielectric properties:

Q × f = 87 300 GHz, orεr= 25 andτf= 56 ppm/°C[6]. It is investigated

for the applications in microwave dielectric resonators and low-temperature co-ﬁred ceramics (LTCCs) [7]. ZnNb2O6 exhibits very

strong blue luminescence with excitation by ultraviolet radiation of 375 nm at room temperature[8]. The absorption wavelength of the general solar cell is about 400–1000 nm, however, photolumines-cence would be beneﬁcial if the absorption wavelengths were shorter than 400 nm[9]. The photoluminescence of the ZnNb2O6

nanostruc-ture is very promising for application to solar cells because of the absorption wavelengths of less than 400 nm. Currently, the nanos-tructures of zinc niobate (ZnNb2O6) have been synthesized by the

rapid vibro-milling technique[4], the combustion synthesis method [8], and the molten salt route[10]. This study is to explore a sol–gel synthetic route for the preparation of single phase ZnNb2O6oxides.

Chemically synthesized ceramic powders often have better chemical homogeneity and a ﬁner particle together with better control of particle morphology than those produced by the mixed oxide route

[11]. To our knowledge, the works of the luminescence behavior of the niobate-based complex formed by the citric gel method are few. This fact motivates this work which discusses the phase formation, morphology and luminescent properties of ZnNb2O6nanocrystals.

2. Experiments

Pure ZnNb2O6 powders were prepared by the sol–gel method

using zinc nitrate [Zn(NO3)2 6H2O], niobium chloride (NbCl5),

ethylene glycol (EG) and citric acid anhydrous (CA). Their purities are over 99.9%. First, a stoichiometric amount of zinc nitrate, and niobium ethoxide were dissolved in distilled water. Niobium ethoxide, Nb(OC2H5)5, was synthesized from niobium chloride and

ethanol, C2H5OH, according to the general reaction:

NbCl5＋ 5C2H5OH→Nb OCð 2H5Þ5＋5HCl: ð1Þ

A sufﬁcient amount of citric acid was added to the former solution as a chelating agent to form a solution. Citric acid to the total metal ions in the molar ratio of 3:2 was used for this purpose. EG was also added to the above solution as a stabilizing agent. The precursor was dried in an oven at 120 °C for 10 h and then the powders were obtained after calcinations at 450–800 °C for 3 h in air. All of the above measurements were taken at room temperature.

3. Results and discussion

XRD patterns of the precursor powders at heat-treatment temperatures of 450–800 °C for 3 h are shown in Fig. 1. All the peaks can be well-indexed to a pure orthorhombic phase of ZnNb2O6

Materials Letters 64 (2010) 2563–2565

⁎ Corresponding author: Tel.: +886 7 381 4526x5336; fax: +886 7 3831373. E-mail address:[email protected](T.-H. Fang).

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photon energy. We have measured and estimated the band gap from the absorption. For a direct band gap semiconductor, the absorbance in the vicinity of the onset due to the electronic transition is given by the following equation[17]:

α = C hν−Eg

1= 2

hν ð4Þ

whereα is the absorption coefﬁcient, C is the constant, hν is the photon energy and Egis the band gap. The inset ofFig. 4shows the

relationship of (αhν)2

and hν. Extrapolation of the linear region gives a band gap of 3.68 eV.

4. Conclusion

The well-crystallized orthorhombic ZnNb2O6can be obtained by

heat-treatment at 500 °C from XRD. The activation energy of the ZnNb2O6nanocrystal grain growth can be obtained about 18.4 kJ/mol.

The diameters of the nanocrystals are in the range of 20–40 nm. The excitation wavelengths at about 276 nm, were associated with the charge transfer bands of [NbO6]7−. The PL spectra under 276 nm

excitation that showed a broad and strong blue emission peaks at about 450 nm, were originated from the niobate octahedra group. The

visible light absorption edge of the sample calcined at 800 °C was corresponded to a band gap energy of 3.68 eV.

References

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spectra of pure ZnNb2O6.

Fig. 4. Absorption spectra of ZnNb2O6at 800 °C. The inset is the function of photon

energy.

2565 Y.-J. Hsiao et al. / Materials Letters 64 (2010) 2563–2565