Size-dependent visible photoluminescence from ZnO nanoparticles prepared via SiO2 network

https://doi.org/10.1016/j.spmi.2012.04.024Get rights and content

Abstract

This study reports a simple method for the synthesis of different size of wurtzite ZnO nanoparticles in assistance of tetraethyl orthosilicate (TEOS). With the increase of the amount of TEOS added, the average size of ZnO nanoparticles was found decreased from ∼14.6 to ∼1.9 nm by characterization of X-ray diffraction (XRD) and high-resolution electron microscopy (HRTEM). The growth of ZnO nanoparticles is proposed to be controlled by the density of the SiO2 chain mesh which is determined by TEOS amount in precursor. Ultraviolet–visible (UV–VIS) absorption and photoluminescence (PL) spectra show both shift to higher energy in cut-off edge and in visible emission bands respectively. The electron transition process in the mechanism of the visible emission shift was described and related to quantum size effect in ZnO nanoparticles.

Highlights

► The simple synthesis of ZnO nanoparticles was developed in the SiO2 network. ► The average size decreases with the amount increase of TEOS in precursor. ► The visible emission peaks blue shifts with the increase of TEOS amount. ► Quantum size effect has been explained the visible emission band shift.

Introduction

With excellent optical and electronic properties, nanostructured ZnO have attracted considerable attention in numerous applications, including light emitting diodes [1], solar energy conversion [2], bio/gas sensors [3], piezoelectric [4] and photocatalytic activity [5]. Actually, the size and shape control are critical to understand the properties and behavior of ZnO nanoparticles for their applications in the devices. To this end, many different synthetic routes have been developed for preparing ZnO nanoparticles with different size, including sol–gel [6], template [7], vapor deposition [8] and precipitation [9]. With sol–gel [10] or a modified precipitation method [11], near-UV photoluminescence (PL) spectra were found to blue shift by varying the size of ZnO nanoparticles. Luyuan Zhang et al. [12] reported that ZnO nanoparticles with the average size from 2.2 to 7.8 nm exhibited size-dependent behavior in the visible range as the reaction time increased from 5 min to 4 days. Also, similar results were observed when the nanoparticles size was tailored by controlling the synthesis temperature due to quantum confinement effect [13].

Tetraethyl orthosilicate (TEOS) was considered to produce a dense SiO2 network via hydrolysis–condensation reaction [14] and permits in situ growth control of nanostructured materials [15], [16]. In this paper, the simple and direct synthesis of ZnO nanoparticles was carried out in the SiO2 network formed by hydrolysis of TEOS. PL and photoluminescence excitation (PLE) spectra were used to investigate the electron transition process for the mechanism of the visible emission shift in ZnO nanoparticles.

Section snippets

Materials and methods

ZnO nanoparticles were synthesized by a simple precipitation method in the presence of TEOS. Various volumes of TEOS in 0, 0.05, 0.1, 0.5, 1 and 5 ml were added to the complete mixture of 0.13 M zinc chloride (ZnCl2) and 100 ml ethanol. The obtained products were labeled A–F and stirred respectively for 2 h at room temperature. Then 0.26 M sodium hydroxide (NaOH) dissolved in 100 ml ethanol was dropped into each solution and was stirred for 3 h at 80 °C. Finally, each product was isolated by

Results and discussion

Fig. 1 shows XRD patterns of the ZnO nanoparticles prepared with different TEOS amount. All diffraction peaks of the samples match well with the wurtzite ZnO crystal structure [17]. The weak and broad envelope between 20–30° corresponds to the character of amorphous SiO2. For the samples with a large amount of TEOS additions, the reduced intensity of XRD peak suggests a decrease in the degree of crystallinity of ZnO phase. Also the broadened XRD peak predicts a reduction in grain size. By using

Conclusion

In summary, the wurtzite ZnO nanoparticles were synthesized by chemical precipitation method in assistance of TEOS. The average size of ZnO nanoparticles was estimated. UV–VIS absorption and PL spectra revealed both shift to higher energy with the increase of TEOS amount. The possible influencing factors for the blue shift of visible emission band, including amorphous SiO2, defect density ratio and size of ZnO nanoparticles, were carefully discussed. Quantum size effect in ZnO nanoparticles

Acknowledgements

This research has been supported by the National Science Foundation of China (NSFC Grant No 60877029, 10904109, 60977035 and 60907021), the Natural Science Foundation of Tianjin (Grant Nos 06TXTJJC14600 and 07JCYBJC06400) and Tianjin Key Subject for Materials Physics and Chemistry.

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    • Preparation and characterization of ZnO-SiO<inf>2</inf> thin films as highly efficient photocatalyst

      2014, Journal of Photochemistry and Photobiology A: Chemistry
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      The first one could be related to ZnO, and this value (Eg = 3.3 eV) is in agreement with the same value obtained for ZnO [2,62,63]. On the other hand, Eg = 3.3 eV is slightly smaller than that of obtained ZnO nanoparticles prepared via SiO2 network (Eg = 3.41–3.75 eV at various volumes of TEOS) [64]. In a previous work reported by Sharma et al. [65] a blue shifted as compared to the bulk band gap of 3.32 eV was observed.

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