Graphene oxide coated flower-shaped ZnO nanorods: Optoelectronic properties

https://doi.org/10.1016/j.jallcom.2020.154874Get rights and content

Highlights

  • ZnO nanorods have been fabricated by the hydrothermal method.

  • ZnO NRs:GO was composed of flower-like ZnO nanorods coated by GO layer.

  • ZnO NRs:GO nanocomposite has an important optical properties.

  • CIE diagram and luminescence properties.

Abstract

In this paper, the influence of graphene oxide coating on optical and photoluminescence properties of zinc oxide nanorods (ZnO NRs) has been investigated. ZnO NRs were prepared using a hydrothermal route (from zinc nitrate hexahydrate and hexamethylenetetramine), graphene oxide (GO) was fabricated by Hummer’s method, while for synthesis of ZnO nanorods:graphene oxide nanocomposite (ZnO NRs:GO) on Si (100) substrate, a facile technique (drop coating) was proposed. The structural, morphological, optical and luminescence properties of the films were investigated using X-ray diffraction (XRD) technique, scanning electron microscopy (SEM), together with Fourier transform infrared (FT-IR), Raman, ultraviolet–visible–near-infrared (UV/VIS/NIR) and photoluminescence (PL) spectroscopies. As revealed by XRD analysis, composites display a hexagonal wurtzite type structure with a (101) preferred grain orientation. The average crystallite sizes decrease from 45 to 40 nm after GO coating. The SEM study confirms successful coating of GO layers on flower-like ZnO nanostructures. The FTIR and Raman analyses validated the hybridization of nanocomposite and the strong interaction between ZnO NRs and GO. The band gap of the ZnO NRs:GO nanocomposite is lower (2.95eV) compared to that of ZnO NRs (3.11 eV), as determined from the analysis of UV absorbance spectra. The ZnO NRs:GO nanocomposite exhibits a broad PL band, from ∼450 nm to ∼750 nm, with a nearly white-light integrated emission and a chromaticity coordinate of (0.25, 0.34). Gaussian deconvoluted broad PL band exhibits three distinct sub-bands, associated with radiative recombinations in ZnO and GO.

Introduction

Recently, interest in graphene-based materials hasexploded due to many important applications in diverse fields of nanotechnology, because of their unique electric, thermal, mechanical properties and high theoretical specific surface area [1]. In particular, graphene oxide (GO), which can be prepared from graphite powder by a modified Hummers’ method [2], has attracted considerable interest as a promising building block for the fabrication of functional carbon-based nanomaterials, because of its significant advantages such as low cost, easy preparation, and good solubility [3]. In addition, GO is highly oxygenated by hydroxyl and epoxide functional groups at their basal planes, acting as nucleation centres to anchor active nanomaterials at the surface of GO [4]. Especially, incombination with ZnO nanostructures, the delocalized conjugated π systems of GO enable high mobility of charge carriers and relatively low recombination rate of graphene oxide [5,6].

Due to their excellent properties, wide band gap (3.37 eV), high exciton binding energy (60 meV), low cost and easy synthesis, one-dimensional ZnO nanostructures display a huge application potentialin solid-state device technology (optoelectronic devices, sensors, solar cells, and supercapacitors) [7]. A number of methods has been developed to synthesize ZnO nanostructures with different morphologies, such as hydrothermal technique, thermal evaporation, metal-organic chemical vapor deposition, pulsed laser deposition and atomic layer deposition [[8], [9], [10]]. The hydrothermal synthesis implies a simple process that occurs under easy conditions, while it is beneficial for ZnO crystal growth along the a and c axes [[11], [12], [13]]. Furthermore, the formation of ZnO nanostructures with a wide variety of morphologies involves control of the ratio of precursors, pH solution, reaction time and temperature [[14], [15], [16]].

In recent times, the conversion of one dimensional nanostructures on high-surface area substrates has attracted much attention in virtue of their exceptional and enhanced properties, due to synergistic interactions, size effects and abundant active sites on its surface [17], which can be efficiently used for linking with other materials such as graphene derivatives, carbon nanotubes, and polymers [18]. In particular, the combination of ZnO nanostructures and GO is expected to lead to improved performances in the case of photocatalysts [19], biosensors, batteries [20], supercapacitors [21]. To date, severalworks on the synthesis of GO coated ZnO nanostructures have been reported. Thuanet al. [22] synthesized ZnO quantum dots with different diameters, linked to the surface of GO, by using the sol-gel method. Kumar et al. [23] investigated the optical properties of ZnO decorated graphene oxide and of reduced graphene oxide synthesized by the hydrolysis approach. In our previous work [24] we successfully synthesized GO/ZnO NRs/GO sandwich structures via a simple hydrothermal method and investigated their photoluminescence (PL) mechanisms.

In this work, we report the preparation of ZnO NRs and ZnO NRs:GO nanocomposites by hydrothermal method and drop coating, respectively. The structural, morphological, vibrational, optical and luminescence properties of ZnO NRs and ZnO NRs:GO nanocomposites were investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), as well as micro-Raman, FTIR, UV–Vis–NIR (ultraviolet–visible–near infrared) and PL spectroscopies. Our results indicate that the addition of graphene oxide significantly contributes to the growth of high quality ZnO nanorods and improves their photocatalytic performance, together with their application potential in low-cost optoelectronic devices.

Section snippets

Synthesis of ZnO NRs and ZnO NRs:GO nanocomposite

The procedure to fabricate high quality ZnO nanorods is shown in Fig. 1(a). Initially, appropriate amounts of zinc nitrate hexahydrate (0.1 M, 73 ml), hexamethylenetetramine (0.1 M, 73 ml) and NaOH (73 mg, 10 ml) aqueous solutions were separately prepared in deionized water. Then NaOH solution was dripped into the mixture solution under constant stirring. After that, the mixed solution was added in a 200 ml teflon-sealed autoclave and hydrothermally grown at 90 °C for 24 h, then cooled to room

Structural analysis

Fig. 2 illustrates the XRD patterns of as-synthesized ZnO NRs and ZnO NRs:GO nanocomposite. The sharp diffraction peaks indicated high crystallinity of ZnO NRs based samples. The diffraction lines of ZnO NRs at 31.8°, 34.5°, 36.3°, 47.6°, 56.6°, 62.9°, 66.4°, 68.0°, 69.1°, 72.6° and 77.0°, can be assigned to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) planes, respectively, of the hexagonal (wurtzite) structure of ZnO (JCPDS Card no. 36-1451) with P63mc space

Conclusion

ZnO NRs and ZnO NRs:GO nanocomposite have been successfully prepared via the hydrothermal method and drop coating process, respectively. The effect of GO on the structural and optical properties of ZnO nanorods was investigated. The formation of the hexagonal crystal structure of ZnO without any other impurity phases was observed in the prepared samples. SEM images revealed that ZnO nanorods were covered by graphene oxide layers. Optical transmission and reflectance of GO coated ZnO NRs were

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Special thanks to the Africa Graphene Center, University of South Africa, Department of Physics, and Alexandru Ioan Cuza University of Iasi, Faculty of Physics (Romania).

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