Rama Gaur and P. Jeevanandam* Thermal decomposition approach for the synthesis of ZnO-CdS nanocomposites and studies on their optical properties Rama Gaur and P. Jeevanandam* Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India *Corresponding author; Email: jeevafcy@itr.ac.in Abstract ZnO-CdS nanocomposites with tunable band gap are useful in a variety of applications. The present study aims at producing ZnO-CdS nanocomposites with better optical properties compared to the constituents. Various physical and chemical methods have been reported for the synthesis of ZnO-CdS nanocomposites. In the present study, ZnO-CdS nanocomposites have been synthesised by a novel thermal decomposition approach. ZnO was first synthesized by homogenous precipitation and the nanocomposites were prepared by refluxing cadmium acetate and thiourea at 200°C in diphenyl ether in the presence of ZnO. The nanocomposites were characterized using a variety of analytical techniques. Presence of wurtzite ZnO and cubic CdS in the nanocomposites has been confirmed by XRD results. The nanocomposites consist of particles with flower-like morphology for ZnO with CdS nanoparticles deposited on it. The EDXA results indicate the uniform distribution of elements in the nanocomposites. The optical properties of nanocomposites lie in between that of ZnO and CdS. The band gap of CdS in the nanocomposites varies between 2.4-2.6 eV. The nanocomposites show a blue shift of emission band due to sulphur vacancies with respect to pure CdS. The ZnO-CdS nanocomposites are expected to be useful in photocatalysis and optoelectronic applications. Introduction 2 Synthesis Characterization 3 1 Nanocomposites: mixing two or more dissimilar materials at the nanoscale, to control and develop new and improved structures and properties.  The properties of nanocomposites depend upon the individual components used , the morphology and the interfacial characteristics.  Zinc oxide (ZnO), a direct wide band gap (3.37 eV) semiconductor, possess high photosensitivity and stability . CdS is a narrow band gap semiconductor(2.4 eV), acts as photosensitizer for wide band-gap semiconductor. Leads to the sensitization of wide bandgap semiconductor with narrow bandgap material, maximize the utilization of the solar spectrum. ZnO/CdS nanocomposites are promising candidate for variety of application Centrifuged washed and dried Add NaOH dropwise Ammonia Solution Trisodium citrate 850C for 12 h Zinc acetate dihydrate in distilled water Cadmium acetate dihydrate Scanning electron microscopy Refluxed at 2000C for 60 mins Zinc oxide nanoparticles Thiourea Precipitated using Methanol Centrifuged washed and dried Transmission electron microscopy X-Ray diffraction Characterization tools [A] Synthesis of Zinc oxide hierarchical nanostructures [B] Synthesis of ZnO/CdS nanocomposites Schematic: Cd(Ac)2 + CS(NH2)2 200oC, 60 mins 80oC, 12h Self assembly NaOH Trisodium citrate Zn(Ac)2 Photoluminescence spectroscopy Diffuse Reflectance spectroscopy Photocatalysis Gas sensing Solar cell applications Environmental remediation ZnO Hierarchical nanostructure ZnO-CdS nanocomposite Results and discussion 4 XRD Results SEM & TEM Results 5 EDXA Results ZC-4 ZC-1 ZnO CdS Sample Crystallite Size(nm) CdS ZnO 2.04 -- 24.32 ZC-1R 2.77 22.96 ZC-2 R 2.68 20.3 ZC-3 R 2.39 19.2 ZC-4 24.3 Sample Id Zn O Cd S Wt% At% ZC-1 17.46 10.05 14.15 37.10 49.75 20.04 12.18 ZC-2 68.13 38.33 25.97 59.11 5.12 1.68 0.77 ZC-3 67.50 36.40 28.44 62.08 3.79 1.20 0.25 ZC-4 68.87 37.23 28.01 61.78 3.10 0.98 Hierarchical assembly of ZnO nanoparticles show flower like morphology and agglomerated spheres made up of small particles for CdS SEM images for nanocomposites show deposition of CdS nanoparticles on ZnO hierarchical nanostructures. EDX analysis confirmed the presence of CdS on ZnO nanostructures. TEM images of nanocomposites also confirms the presence of CdS nanoparticles on the ZnO hierarchical nanostructures. Peaks of due to ZnO and CdS confirms the presence of both in the nanocomposites. 6 DRS Results Photoluminiscence Results 7 Sample Band Gap (eV) CdS ZnO CdS-R 2.49 -- ZnO-R 3.2 ZC-1R 2.50 3.20 ZC-2R 2.46 3.21 ZC-3R 2.58 ZC-4R 2.55 ZnO- 388 nm (near band edge emission), 468 nm (oxygen vacancies) and CdS- 526 nm (Sulfur vacancies) Enhanced peak emission at 388 nm with decreasing cadmium content in nanocomposites PL peaks blue shifted with respect to pure CdS nanoparticles. DRS spectra shows two band gap absorption corresponding to CdS and ZnO. Nanocomposites show blue shift of CdS band with respect to pure CdS nanoparticles. Conclusions: 8 XRD confirmed the formation of ZnO, CdS, and presence of ZnO and CdS in the nanocomposites, average crystallite size calculated from XRD is ~2nm for CdS and ~24 nm for ZnO in nanocomposites DRS spectra shows the transitional behavior of nanocomposites, with their band lying in between ZnO and CdS, and their band gaps are between 2.4 to 3.2 eV PL studies were conducted at λexc=350nm. PL spectra show shift of peak towards lower wavelength with decrease in cadmium concentration SEM images show flower like morphology for pure ZnO, and small spherical particles for CdS. Nanocomposites show deposition of CdS nanoparticles on ZnO hierarchical nanostructures 9 Acknowledgements Sincere acknowledgement to the Council of Scientific and Industrial Research, Government of India for the award of Junior Research Fellowship (JRF)