Article pubs.acs.org/JPCC
Strain-Induced Tunable Band Gap and Morphology-Dependent Photocurrent in RGO−CdS Nanostructures Supriya Mondal,† Suparna Sudhu,‡ Shatabda Bhattacharya, and Shyamal K. Saha* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata-700 032, India S Supporting Information *
ABSTRACT: In situ CdS nanostructures (nanorods, nanoparticles, nanoclusters) are grown on a reduced graphene oxide (RGO) surface to tune the photocurrent generated due to transfer of excited charge from CdS to RGO. The highest change in photocurrent is achieved in the case of nanoclusters, while nanorods show the lowest. Rietveld analysis has been done to find the microstrain present in three nanocomposites. UV−vis spectroscopy reveals the modulation in band gap due to different growth morphology. From the band structure, it is seen that in nanorod structure strain-induced localized states lower the conduction band which essentially decreases the charge transfer from CdS to RGO, resulting in a smaller change in the photocurrent, while in the case of a nanocluster the photocurrent is maximum due to the lowest strain. This is also consistent with the photoluminescence (PL) quenching as obtained from PL spectra.
1. INTRODUCTION Graphene, a monolayer of carbon atoms with hexagonal lattice structure having two-dimensional large surface area, ballistic transport behavior, and excellent tensile strength with superior conductivity and transmittivity,1−6 has been considered as one of the most useful materials in field effect transistors, organic photovoltaics, and transparent conducting7−13 coating for optoelectronic devices. On the other hand, CdS is a wellknown optical material for its size-dependent band gap modulation.14−17 Several strategies have been developed to enhance the optical property of graphene oxide by different functionalized techniques.17−21 Intensive research has been carried out on major applications of quantum dot solar cells and photocatalytic reduction using, on CdS, CdSe (II−VI semiconductors)/reduced graphene oxide composites.22−28 In spite of such elaborate experiments, photoconductivity and microstrain study of in situ grown CdS nanorods, nanoparticles, and nanoclusters on the reduced graphene oxide (RGO) surface had not yet been observed. Here we have introduced a strain-modulated band gap, improved charge separation, and enhanced photoconductivity efficiency for the RGO−CdS nanorod, nanoparticle, and nanocluster. Therefore, by varying the morphology of CdS nanostructures grown on the RGO surface we have achieved enhanced photoconductivity behavior and variation in band gap. These three composite semiconductors absorb UV energy and are able to transfer electrons from CdS to RGO. A corresponding change in photoconductivity of the CdS/RGO nanorod, nanoparticle, and nanocluster under UV light illumination is also observed. The significant change in photocurrent in the three composites is also consistent with the PL quenching, UV−vis spectra, and the strain generated in the nanostructures as estimated from the Rietveld analysis. The nonlinear shape of the I−V curves is © XXXX American Chemical Society
observed both in the dark and in light for all the systems, and the RGO−CdS nanocluster shows maximum nonlinearity due to better formation of the heterostructure at the interface. All photoluminescence studies of pure CdS nanostructures and RGO−CdS composites are performed at a fixed particular absorption (0.21) from UV−vis spectroscopy. The maximum change in photoconductivity in the case of the RGO−CdS nanocluster is due to the greater difference between the conduction band of CdS and Fermi level of RGO. With increasing microstrain this reduces the conduction band for the nanoparticle and nanorod. Hence, the transition rate decreases, and the corresponding change in photoconductivity is the least for the nanorod.
2. EXPERIMENTAL SECTION 2.1. Materials. Ultrafine graphite powder (Loba), sodium chloride (Merck), potassium permanganate (Merck), concentrated sulfuric acid (Merck, 98% pure), hydrogen peroxide (30%, Merck), hydrochloric acid (35%, Merck), oleylamine (70%, Aldrich), trioctylphosphine oxide (TOPO) (70%, Aldrich), cadmium chloride (Loba Chemie, extra pure), sulfur powder (Merck), ethanol (99.9%, Merck), toluene (Merck), chloroform (Merck), and double distilled water were used. 2.2. Synthesis Procedure. Graphene oxide (GO) was synthesized from natural graphite by the modified Hummers method.27 Step 1. Synthesis of GO−Cd. The GO−Cd was synthesized by using the adsorptive properties of GO toward divalent metal ions.28−31 The experiment was carried out with aqueous Received: August 20, 2015 Revised: November 12, 2015
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DOI: 10.1021/acs.jpcc.5b08116 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Schematic diagram of the synthesis process of the three nanocomposites with varied conditions.
at 100 °C under Ar atmosphere and used for the growth of the CdS NP on the RGO surface. The reaction mixture was aged for 3 h at 150 °C under an Ar atmosphere. The RGO−CdS NP composite was collected using the same procedure as before and redispersed in chloroform. Synthesis of the RGO−CdS Nanocluster Composite (RGO− CdS NC). For the preparation of the RGO−CdS nanocluster composite, the nucleation step is also similar to that described above. In this case also, Cd−precursor solution was prepared by dissolving 0.2013 g (0.1 mM) of CdCl2 in 10 mL of oleylamine at 100 °C under Ar atmosphere and used for the growth of CdS NC on the RGO surface. The reaction mixture was aged for 2 days at 60 °C under an Ar atmosphere. The RGO−CdS NC composite was collected using the same procedure as before and redispersed in chloroform. 2.3. Characterization. The crystal structure of the samples was determined by an X-ray diffractometer (Seifert 3000P) using Cu Kα radiation (λ = 1.54178 Å). The morphology and the detailed structural features were investigated by a highresolution transmission electron microscope (HRTEM; JEOL 2100). Raman spectra at room temperature were recorded in solution via 90° scattering by exciting the sample with an Ar+ ion laser source of 48 mW power (T64000 model made of Horiba Jobin Yvon). Raman study with different irradiation time was carried out using a white light source compiled with a
