Synthesis and Characterization of ZnO-CdS Core-Shell Nanohybrids by Thermal Decomposition Method and Studies on Their Charge Transfer Characteristics Rama Gaur and P. Jeevanandam*, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, India * Corresponding author’s E-mail: jeevafcy@iitr.ac.in Abstract Core-shell nanohybrids are multi-functional materials, which are formed by combining two or more dissimilar components at the nanoscale. Coupling the components usually results in improved and enhanced properties compared to both the parent constituents. The present study aims at the synthesis of ZnO-CdS core shell nanohybrids with enhanced charge transport characteristics. The ZnO-CdS core-shell nanohybrids were synthesized by a facile thermal decomposition approach. ZnO nanorods were first synthesized by solid state thermal decomposition of zinc acetate at 300 °C and then the nanorods were surface modified using ammonium oxalate. The core-shell nanohybrids were prepared by heating cadmium acetate and thiourea in different molar ratios at 150 °C in diphenyl ether in the presence of surface modified ZnO nanorods. The nanohybrids were characterized using an array of analytical techniques. The charge transfer characteristics on the nanohybrids were investigated by cyclic voltammetry measurements. 1 Introduction 2 Methods & Materials 3 Characterization Core-shell nanohybrids: Multi-functional materials, formed by combination of two or more dissimilar components at the nanoscale, and exhibit improved and enhanced properties compared to both the parent constituents. The properties of core-shell nanohybrids depend upon individual components morphology interfacial characteristics.  Zinc oxide (ZnO), a direct wide band gap (3.37 eV) semiconductor, possesses high photosensitivity and stability and CdS is a narrow band gap semiconductor(2.4 eV), and acts as a sensitizer for wide band-gap semiconductor. Sensitization of a wide band gap semiconductor with a narrow band gap material, enhances charge transport characteristics in ZnO-CdS core shell nanohybrids. ZnO-CdS core-shell nanohybrids are promising candidates for a variety of applications such as solar cells, photocatalysis, optoelectronics and sensors. I II Scanning electron microscopy SM-ZnO Cadmium acetate + Thiourea Diphenyl ether(10 mL) 150 °C, 1h Slurry Precipitate ZnO/CdS core-shell nanohybrids Addition of methanol Centrifuge and wash Zinc acetate dihydrate ground well in mortar pestle 300°C , 3h, 2°/min Surface modification by NH4(C2O4)2 Surface modified ZnO nanorods SM-ZnO Transmission electron microscopy X-Ray diffraction Characterization tools ZnO nanorods Schematic ZnO nanorods Surface modified ZnO nanorods Deposition of Cd+2 and S-2 ions on SM-ZnO ZnO-CdS core-shell nanohybrid ZnO CdS COO- ions Cadmium ions Sulfur ions Diffuse Reflectance spectroscopy Photoluminescence spectroscopy Peaks due to ZnO and CdS confirm the presence of both in the ZnO-CdS core-shell nanohybrids The crystallite size of ZnO lies in the range of 25.5 nm to 27 .5 nm. XRD Results 4 Results and Discussion FE-SEM & TEM Results ZnO nanorods CdS nanoparticles ZnO-CdS core-shell nanohybrids ZnO nanorods CdS nanoparticles ZnO-CdS core-shell nanohybrids Sample Crystallite Size (nm) ZnO 30.02 CdS 2.55 ZnO-CdS-(0.125mM) 25.56 ZnO-CdS-(0.25mM) 26.69 ZnO-CdS-(0.0625mM) 27.52 FE-SEM and TEM results confirm the formation of shell of CdS on ZnO nanorods (ZnO nanorods:- diameter ~ 40 nm and length ~ 600 nm). The selected area electron diffraction patterns (SAED) for pure ZnO nanorods and CdS nanoparticles indicate polycrystalline nature of the materials. The SAED pattern for ZnO-CdS core-shell nanohybrid exhibits two distinct sets of diffraction spots due to both the phases. Diffuse Reflectance Spectroscopy Results The optical properties of the core- shell nanohybrids are different compared to the pure systems. DRS spectra show two band gap absorptions corresponding to CdS and ZnO. Nanohybrids show blue shift of CdS band gap with respect to pure CdS nanoparticles. The band gap for the CdS in the core shell nanohybrids varies from 2.46 to 2.66 eV Photoluminescence Results ZnO: 388 nm (near band edge emission), 468 nm (oxygen vacancies) CdS: 526 nm (sulfur vacancies). The PL spectra for the ZnO-CdS core- shell nanohybrids show emission bands due to both CdS and ZnO. Reduction in the intensity of band edge emission of ZnO nanorods (at 388 nm) in the ZnO-CdS core shell nanohybrids. Charge Transfer Studies Peak separation value (ΔEp) and cathodic and anodic peak current for modified electrodes (coated with ZnO nanorod/CdS nanohybrids) Sample Id ΔEp IpC IpA ZnO 0.276 -29.7 12.01 CdS 0.235 -22.98 20.7 ZnO-CdS (0.125mM) 0.126 -30.17 27.48 Transition from capacitive behavior for pure ZnO nanorods (large peak separation (ΔEp = 276 mV)) to electrochemically reversible behavior (smaller ΔEp (126 mV) and higher peak current (27.5 μA)) in the core-shell nanohybrids. Smaller ΔEp for the ZnO-CdS core-shell nanohybrids indicates faster charge transfer kinetics which makes it suitable for various applications (e.g. solar cells, photocatalysis, optoelectronics and sensors). 5 Conclusions Acknowledgements 7 ZnO-CdS core-shell nanohybrids were successfully synthesized using a facile thermal decomposition approach. XRD results confirm the presence of ZnO and CdS in the ZnO-CdS core-shell nanohybrids. FE-SEM and TEM images show nanorods for pure ZnO, and small spherical particles for CdS and the formation of CdS shell on the surface of ZnO nanorods. The band gap for the CdS in the core shell nanohybrids varies from 2.46 to 2.66 eV. PL studies show reduction in the intensity of band edge emission of ZnO (at 388 nm) in the ZnO-CdS core shell nanohybrids. Cyclic voltammetry studies show enhanced charge transfer characteristics in the case of ZnO-CdS core-shell nanohybrids compared to pure ZnO and CdS. Thanks are due to the Council of Scientific and Industrial Research, Government of India for the award of Junior Research Fellowship to Ms. Rama Gaur.