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Study of optical and magnetic properties of graphene wrapped ZnO nanoparticle hybrids Arpita Jana, and Elke Scheer Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02953 • Publication Date (Web): 31 Dec 2017 Downloaded from http://pubs.acs.org on January 1, 2018
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Study of optical and magnetic properties of graphene wrapped ZnO nanoparticle hybrids
Arpita Jana*, Elke Scheer
AUTHOR INFORMATION: Department of Physics, University of Konstanz, 78457 Konstanz, Germany Email: Arpita Jana* -
[email protected],
[email protected] * Corresponding author
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ABSTRACT: In this work, we report a one-step method for the preparation of graphene-wrapped zinc oxide (ZnO) nanoparticle (NP) (ZnO@G) hybrids. These hybrids are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, optical absorption measurements, photoluminescence emission spectroscopy (PL) and M-H hysteresis measurements. All the results reveal that the ZnO NPs are entirely covered with graphene sheets. In the PL spectra the quenching of the band gap emission and the enhanced green emission serves as evidence of the electron transfer from the ZnO NPs to the graphene layer. The increase of the room temperature magnetization of the hybrid, compared to pure ZnO NPs is due to the increasing defect concentration. We suggest a band diagram model that accounts for these observations. We present the simple wet-chemical synthesis procedure to open a new way for the synthesis of NP-graphene hybrid systems having magnetic properties giving the large manifold potential application.
KEYWORDS: Graphene, ZnO nanoparticles, Magnetic Properties.
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INTRODUCTION
ZnO-graphene hybrids have attracted great attention for their combined interesting properties, where the drawbacks of ZnO have been compensated by graphene and vice versa. The most common drawbacks of ZnO, like low conductivity and high recombination rate of photogenerated electron-hole pair leads to the interest of formation of ZnO-graphene hybrid materials. ZnO is an II-VI, wide band gap (3.37 eV) semiconductor having large exciton binding energy (60 meV) at room temperature. ZnO has attracted considerable interest from a few past years due to its unique optical properties and electrical properties.1,2 ZnO has a great potential for wide range of applications in UV laser, field-effect transistors, photodetectors, gas sensors, solar cells photocatalyst.3 Graphene is a highly promising material for different application due to its outstanding optical, electrical and mechanical properties. Graphene attracts great interest for the scientific community because of its unique high crystalline structure, high transparency, large surface area, excellent flexibility, extraordinary electronic quality and good thermal and mechanical properties.4,5 Though graphene has large surface area but the use of entire surface area of graphene is not possible as graphene has a strong tendency to stack due to van der Waals interactions between the graphene layers. The other drawbacks of graphene is its zero band gap which limits its optical application. Those drawbacks can be removed by preparing ZnO-graphene hybrid. Recently graphene-based hybrids have been investigated intensively as this synergistic combination of graphene with semiconductor NPs can be used as a building block for future nanodevices owing to their superior electronic, mechanical, thermal properties and also for their good chemical stability. For example when graphene has 3 ACS Paragon Plus Environment
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been hybridized with ZnO, the strong interaction between ZnO and graphene accelerates the transfer of photo-generated electrons from ZnO to graphene, suppresses the recombination of photo-generated electron-hole pairs, thereby increasing the photocatalytic activity.6-8 Thus graphene-ZnO hybrids give improved photocatalytic properties. Apart from enhancing the properties, the ZnO NPs act as stabilizers against the aggregation of the graphene sheets, caused by the strong van der Waals interaction between the graphene layers. Inhibiting the aggregation of the graphene layers enhances the lithium storage capacity.9,10 The combination of graphene and ZnO NP significantly improves electrode materials in supercapacitors due to the enhanced specific surface area, improved electrical and ionic conductivity, good cyclic stability, excellent specific capacity, increased energy and power density.11 To take advantage of graphene in this work, our aim is to prepare graphene wrapped ZnO NPs. In this context some work has been done to prepare graphene wrapped ZnO NPs for improved performance in optoelectronics applications. ZnOgraphene quasi-core shell quantum dots have been prepared by solution method by dissolving zinc acetate solution in DMF solution along with graphene oxide (GO), followed by heating and stirring the solution at 95⁰ C for 5 hrs.12-14 Whereas ZnOgraphene core shell structures have been prepared by surface modification of ZnO NPs with amine groups, coating with GO and then conversion of GO to graphene.15 For the amine functionalization of ZnO NPs poly(allyamine hydrochloride)6,15 or (3aminopropyl) triethoxysilane16 has been used widely. For the conversion of GO to graphene hydrothermal methods were used mostly.6,17 Another method of preparing graphene wrapped ZnO nanospheres is lyophilization leading to enhanced photocatalytic activity of the resulting hybrids.18 The superior performance of graphene encapsulated porous carbon and ZnO particles is that, it not only enhanced the 4 ACS Paragon Plus Environment
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electrical conductivity of the overall electrode but also helps to avoid aggregation of the materials during cycling.19 In the present work, we introduce a one step method for the preparation of ZnO@G NPs core shell structure. In this process we have prepared ZnO@G NPs by in-situ reaction. In brief, First the Zn2+ is chemisorbed on the GO, then by using a reduction process, GO is converted to graphene resulting in the formation of ZnO@G NPs. The crystal structure was studied by XRD, structure information of the hybrid was examined by TEM, the luminescence properties of the hybrid have been investigated by PL spectroscopy and the influence of charge transfer was explained in detail by its energy band diagram. The magnetic properties of the hybrid were studied at room temperature and was compared with the pure ZnO NPs. The divergence between zero filed cooled and field cooled response was also studied for these two samples. As close contact between ZnO and graphene is favorable for charge transfer, this hybrid has large potential for the application in photocatalysis and photoelectronics.
EXPERIMENTAL SECTION
Materials: All the chemicals, graphite powder with a particle size
