Effect of hydrothermal reaction temperature on growth, photoluminescence and photoelectrochemical properties of ZnO nanorod arrays

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Abstract

Well-aligned ZnO nanorod arrays have been successfully prepared by using low-temperature hydrothermal methods. The influence of the reaction temperature on the hydrothermal growth, photoluminescence and photoelectrochemical properties of the ZnO nanorod arrays was studied in detail. The ultraviolet emission peak increases and shows a slightly red shift, while the photoelectrochemical performance was improved with the growth temperature increasing. The overall conversion efficiency of 0.31% is obtained for the dye-sensitized solar cells based on the ZnO nanorod arrays as the photoelectrode under the AM1.5 simulated solar light.

Introduction

Zinc oxide (ZnO) is a wide band-gap semiconductor, which has a direct wide band gap (3.37 eV) and large exciton binding energy (60 meV). Due to the excellent properties, ZnO has also been investigated and used for many applied devices such as transparent conductor [1], light emitting diodes [2], ultraviolet devices [3], [4], solar cells [5], gas sensors [6], nanoresonators [7] and piezoelectric devices [8]. With the promising utilization, the one-dimensional ZnO attracts greatest interest because of the better properties than other morphological ZnO materials. Especially, the ZnO nanorod arrays are more suitable for the future nanoscale devices and expected to enhance the performance.

Hitherto, various ZnO nanostructures are synthesized by numerous fabrication techniques and reported in some literature, such as chemical vapor deposition [9], metal–organic chemical vapor deposition [10], pulsed laser deposition [11], electrochemical deposition [12] and hydrothermal method [13], [14], [15]. Hydrothermal methods are not only more suitable and economical than others in getting uniform and high-quality nanomaterials, but also recognized as the excellent procedure for the controllable preparation of the ZnO nanostructure materials in the further applications. The influence of reaction time [16] and pre-coated seed layer [17] has been investigated extensively. However, the influence of the reaction temperature on the morphology, photoluminescence (PL) and photoelectrochemical properties of the well-aligned ZnO nanorod arrays is not explored much in detail.

In this paper, we discuss the growth, photoluminescence and photoelectrochemical properties of the high quality, large area and well-oriented ZnO nanorod arrays, which were successfully fabricated on conductive transparent oxide substrates by low-temperature hydrothermal method. A radio frequency magnetron sputtering technique was applied to prepare ZnO film as the seed layer coated on substrates for subsequent growth of highly oriented ZnO nanorods. It is found that the reaction temperature has a strong influence on the morphology, diameter and orientation of the ZnO nanorod arrays. The PL properties of the ZnO nanorod arrays are also investigated in this paper. To be worthy of attention, this investigation may play an important role in the further application of ultraviolet laser devices and solar cells.

Section snippets

Experimental

Radio frequency magnetron sputtering was used for the pre-treatment of the substrates. The sputter target material and the working power were the ZnO ceramic (99.99% purity) and 100 W respectively. Indium tin oxide (ITO, 10 Ω cm−2) glass plates were cut to 1.5 cm × 2.0 cm and used as substrates. All the substrates were ultrasonically cleaned in acetone, and then in alcohol several times, finally rinsed in deionized water and dried in flowing nitrogen gas before deposition. During the deposition, the

Results and discussions

The AFM image shows that the ZnO film deposited on ITO glass is dense and homogeneous (Fig. 1). The average scale of the small ZnO islands is about 50 nm, which serve as nuclei for the growth of ZnO nanorod arrays. According to our knowledge, the pre-coated ZnO layers play an important role in fabricating the well-oriented ZnO nanorod arrays. The well alignment grown on ZnO film is due to the matching lattice structure and the polar nature of the ZnO surface [18].

Fig. 2 shows the XRD patterns of

Conclusions

In summary, well-aligned ZnO nanorod arrays have been successfully synthesized by the low-temperature hydrothermal method. The reaction temperature is crucial to the morphology, orientation, PL property and photoelectrochemical performance of the ZnO nanorods. With the reaction temperature increasing, ZnO nanorods have a faster c-axis growth rate and better vertical orientation. The UV emission peaks of the ZnO nanorods show a slightly red shift, and the peak intensity ratios of UV to visible

Acknowledgements

This work was supported by the National Basic Research Program of China (No. 2007CB936201), the Major Project of International Cooperation and Exchanges (Nos. 50620120439, 2006DFB51000), the National Natural Science Foundation of China (Nos. 10876001, 50772011) and other science foundation (Nos. 2082015, NCET-07-0066).

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