Elsevier

Carbon

Volume 47, Issue 8, July 2009, Pages 2054-2059
Carbon

Graphene–nanocrystalline metal sulphide composites produced by a one-pot reaction starting from graphite oxide

https://doi.org/10.1016/j.carbon.2009.03.055Get rights and content

Abstract

Graphene–nanocrystalline metal sulphide composites were prepared by a one-pot reaction. A dispersion of graphite oxide layers in an aqueous solution of metal ions (Cd2+/Zn2+) was reacted with H2S gas, which acts as a sulphide source as well as a reducing agent, resulting in the formation of metal sulphide nanoparticles and simultaneous reduction of graphite oxide sheets to graphene sheets. The surface defect related emissions shown by free metal sulphide particles are quenched in the composites due to the interaction of the surface of the nanoparticles with graphene sheets.

Introduction

Graphite oxide (GO) is a lamellar solid obtained on oxidation of neutral graphite sheets. GO consists of graphite sheets covalently bonded with oxygen containing functional groups like hydroxy and epoxide groups on basal planes and carboxyl groups at the edges [1]. Thus the GO platelets are strongly hydrophilic and dispersible in water resulting in colloidal dispersions of negatively charged graphene oxide sheets [2]. Negative charges originate on GO layers due to the dissociation of the acidic groups located at the edges of GO sheets [3]. The protons of the acidic groups are also exchangeable for other cations [2], [4], [5]. As in the case of clays and other layered solids, the interlamellar space and the ion exchange property of GO has been utilized to synthesize graphite based materials such as graphite-polymer [5], nanoporous graphite [6], graphite–metal [7], graphite–metal oxide nanoparticle composites [8], and battery electrode materials [9].

Semiconductor nanoparticles have gained a lot of interest due to their unique optical and electronic properties arising from the quantum confinement of electrons and large surface area [10], [11]. With the decrease in the particle size the band gap increases resulting in a blue shift of the absorption onset [12]. The surface of the semiconductor nanoparticles is highly defective which directly influences the band gap of the nanoparticles [11]. Therefore for various applications, the band gap of nanoparticles can be tuned either by controlling the particle size [13], [14], [15] or the interactions of the particles with the surrounding matrix [16], [17], [18]. Semiconductor nanoparticles of CdS and ZnS have been extensively studied due to their potential applications in field effect transistors (FET), light emitting diodes (LED), photocatalysis, solar cells and biological sensors [19], [20], [21], [22].

Metal sulphide nanoparticles have also been grown within the layers of layered hosts such as layered silicates [23], [24] smectites [25], [26], hydrotalcite [27] and K2Ti4O9[28]. In most of the reported work, a precursor is first intercalated within the layers and then it is converted into metal sulphide nanoparticles by a suitable post treatment. In a typical method for the preparation of smectite-nanoparticle composite, the smectite clay is first ion-exchanged with the relevant metal ion and the exchanged solid is reacted with H2S to form the metal sulfide nanoparticles in the interlayer region. In all the cases the observed optical behavior has been attributed to particle size effects rather than the nanoparticle–matrix interactions.

Though attempts have been made to grow semiconductor nanoparticles on graphite [29] and synthesize carbon nanotubes encapsulated with CdS/ZnS, [30], [31] graphene–nanocrystalline CdS/ZnS composites have not been reported so far. In this article we report the synthesis and optical behavior of such composites. GO was dispersed in metal (Cd2+/Zn2+) nitrate solution. This dispersion was reacted with H2S gas, which acts as sulphide source as well as reducing agent, resulting in the precipitation of CdS/ZnS and simultaneous reduction of GO sheets to graphene sheets.

Section snippets

Synthesis of GO

The method due to Hummers and Offeman [32] was adopted to prepare GO from graphite powder. About 1 g of graphite powder was added to 23 ml of cooled (0 °C) concentrated H2SO4. About 3 g of KMnO4 was added gradually with stirring and cooling, so that the temperature of the mixture was maintained below 20 °C. The mixture was then stirred at 35 °C for 30 min. After this, 46 ml of distilled water was slowly added to cause an increase in temperature to 98 °C and the mixture was maintained at that temperature

Results and discussion

Fig. 1a shows the pXRD pattern of the pristine GO. The basal spacing of GO calculated from the 001 reflection is 9.0 Å and it matches well with the values reported in the literature [33]. The pXRD pattern of free CdS nanoparticles obtained in the control experiment (Fig. 1b) matches well with that of hexagonal greenockite phase (JCPDS PDF 6-0314). The pXRD pattern of the G–CdS composite (Fig. 1c) shows peaks only due to greenockite CdS and no peaks due to GO or graphite indicating complete

Conclusions

The reaction of a mixture of GO layers and metal ions with H2S yields, in one step, graphene–metal sulphide composite in which the metal sulphide nanoparticles are surrounded by chemically modified graphene layers. Here H2S acts as a sulphide source for the formation of the metal sulphide nanoparticles and as a reducing agent to reduce GO sheets to graphene sheets. The capping of the particles by graphene sheets quenches the surface defect related emissions of the nanoparticles.

Acknowledgements

This work was funded by DST, New Delhi. C. N. thanks CSIR, New Delhi for the award of Senior Research Fellowship. C. N. and M. R. thank UGC, New Delhi for having provided IR spectrometer through CPE scheme.

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