1. 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 , Uttarakhand, India
* Corresponding author’s
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 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 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.