Chapter - 8. Summary and Conclusion

Transcription

1 Chapter - 8 Summary and Conclusion The present research explains the synthesis process of two transition metal oxide semiconductors SnO 2 and V 2 O 5 thin films with different morphologies and studies their electrochemical performances. The nanostructured SnO 2 thin films were successfully prepared by constant potentiostaic electrodeposition method and V 2 O 5 thin films by simple sol-gel dip coating method. The prepared thin films were methodically characterized by, different analytical techniques such as X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FTIR), Raman spectroscopy, UV-Visible spectroscopy (UV-Vis), Energy dispersive X-ray analysis, Field emission scanning electron microscope (FESEM), cyclic voltammetry and chronoamperometry analysis. The copper substrates were anodized by using sulphuric acid solution before the film deposition and it has been used as working electrode during electrodeposition. Surface modification after anodization helps nanostructure growth and increases corrosion resistance and the electrode potential, time, molar concentration of the electrolytes for the electrodeposition is optimized. Nanostructured SnO 2 thin films were prepared by altering the deposition voltage, applying bath temperature and introducing supporting electrolytes. Crystallintiy, chemical bonding and vibrational modes were analysed by XRD, FTIR and Raman analysis. Morphological evolution was examined by FESEM analysis. X-ray diffraction pattern confirms the obtained SnO 2 nanostructures were of orthorhombic and mixed phase of orthorhombic-tetragonal crystal structures. Nanopyramids, nanoplates, hexagonal nanoplates, macroporous structure and nanoneedles were obtained by varying the deposition voltage. Cyclic voltammetry analysis was used to investigate alloying and de-alloying reactions of the obtained SnO 2 nanostructures with lithium ions in aqueous solutions containing dissolved lithium ions between the potential window of 0.6 V and -0.8 V at different scan rates from 5 to 100 mv/s. The operating potential extended up to 0.6 V in LiOH.H 2 O and 219

2 Li 2 CO 3 electrolyte solutions and the H 2 /O 2 evolution appears at -1V and 0.7 V. In LiNO 3 and Li 2 SO 4 electrolyte solution, the operating potentials were restricted between -0.8 and 0.1V. Hexagonal nanoplates exhibit highest current density in LiOH.H 2 O aqueous electrolytes resembling battery characteristic of a material. Bath temperature has been applied to electrolyte solution and its influence on morphology of the SnO 2 thin films are investigated. Flower like architectures and plates, porous chain like structure and porous nanocubes were obtained by applying the bath temperature. The CV curve of flower-like SnO 2 thin films did not show any wellresolved redox peaks in LiOH.H 2 O and Li 2 CO 3 electrolytes indicating that no alloying and de-alloying reactions were occurring in the flower like architectures. The CV curves exhibits capacitance behaviour instead of battery behaviour in both the electrolytes. It exhibits maximum specific capacitance of 64 F/g at a scan rate of 5 mv/s in Na 2 SO 4 electrolyte. Porous chain like structure show reversible lithium ion alloying and de-alloying in all four electrolyte solution, it offers a larger area for interaction with the electrolyte by the porous layer, and the compact layer avoids the interaction of electrolyte directly and thus eliminates the disadvantage of the shortening of electrolyte to the substrate. Porous SnO 2 nanocubes were good in supercapacitor application than other morphologies. Porous SnO 2 nanocubes exhibit maximum specific capacitance of F/g at a scan rate of 5 mv/s. The occurrence of redox peaks in Na 2 SO 4 implies the charge storage mechanism is mainly based on redox reaction. Effects of supporting electrolytes in electrodeposition baths on morphologies of SnO 2 thin films were also investigated. The electrodeposition bath contains SnCl 2.2H 2 O (25mM) and HNO 3 (75mM) as host composition. The CH 4 N 2 O, NaNO 3, KNO 3 and CH 3 COONH 4 respectively were added into baths for studying the effects of NO - 3, CH3COO -, anions on the morphologies of various SnO 2. Microspheres, leaf-like structure, dense nanosheets and microcubes were obtained by the influence of supporting electrolytes. The anions interact and adsorb on SnO 2 and suppress the growth along different direction and adapt to grow in a particular morphology. The electrodes exhibit pseudo capacitance behaviour that the charge storage mechanism is mainly based on faradic reaction. Microspheres made up of nanocubes exhibit 220

3 highest specific capacitance of 110 F/g and SnO 2 microcubes exhibits lowest specific capacitance of 42.8 F/g. This significant difference in specific capacitance of SnO 2 can be attributed to its difference in morphology. The electrochemical performance of nanostructured SnO 2 thin films in different aqueous electrolytes suggests that the electrode can be applied as an anode material in aqueous lithium-ion batteries. Electrochemical detection of ascorbic acid (AA) at the surface of the nanostructured SnO 2 thin films was investigated in phosphate buffer solution by cyclic voltammetry and differential pulse voltammetry. The electrode shows selective detection towards AA. Nanopyramids, nanoplates and flower-like plates do not exhibit any redox behaviour indicating charge transfer processes. Aggregation of particles affects the diffusion of electroactive species at the surface of the electrode. SnO 2 electrodes with porous nature exhibits improved detection due to the promotion of electron transfer processes. The porous microstructure is expected to be beneficial for the electrolyte diffusion. The oxidation behaviour suggests that the SnO 2 electrode can be a useful sensor for the control measurement of ascorbic acid. Other mediators used for the preparation of ascorbic acid sensors also provide good performance, but they are often more costly and more poisonous. β-v 2 O 5 nanorods with an average diameter of 500 nm have been synthesized by a simple sol gel process. V 2 O 5 films annealed at 500 C exhibit improved crystallinity and nanorod like morphology. The annealing temperature significantly impact optical and microstructural properties of nanostructured V 2 O 5 thin films. Surface diffusion phenomenon contributes an important role in the growth process of V 2 O 5 nanorods. It is evident from the AFM images that the pore structures of films annealed at 500 C can potentially improve charge/discharge rate capability. Electrochemical data confirm that nanorods appear to provide good intercalation path ways for lithium ions. The V 2 O 5 film annealed at 500 C can be used as cathode material in Lithium-ion batteries and exhibit better electrochemical performance in terms of better cycling compared to the as prepared V 2 O 5 film. 221

4 Future work The work reported in this thesis describes various techniques that are used to help for characterizing and understanding the electrochemical performance of SnO 2 thin films in aqueous electrolyte solutions. Charge-discharge analysis needs to be carried out to find out the discharge capacity and rate capability of the electrode. 222

5 PUBLICATIONS 1. Effect of annealing and electrochemical properties of sol gel dip coated nanocrystalline V 2 O 5 thin films. D.Vasanth Raj, N.Ponpandian, D.Mangalaraj, C.Viswanathan, Material Science in Semiconductor Processing, 2013, 16, Electrochemical Performance of SnO 2 hexagonal nanoplates. D.Vasanth Raj, N.Ponpandian, D.Mangalaraj, A.Balamurugan, C.Viswanathan, Ionics, Electrodeposition of SnO 2 nanoneedles on anodized copper substrates and its electrochemical performance. D.Vasanth Raj, N.Ponpandian, D.Mangalaraj and C.Viswanathan, AIP Conf. Proc. 1536, (2013). 4. Electrodeposition of V 2 O 5 nanorods on current collector substrate. D.Vasanth Raj, V.Nandhinidevi, D.Mangalaraj and C.Viswanathan, AIP Conf. Proc.1447, 435 (2012). 5. Electrochemical behavior of nanostructured SnO 2 thin films in aqueous electrolyte solutions. D.Vasanth Raj, N.Ponpandian, D.Mangalaraj, C.Viswanathan, Material Science in Semiconductor Processing (Revision submitted) 6. Electrodeposition of hierarchical SnO 2 microspheres with the assistance of urea and its electrochemical performance. D.Vasanth Raj, N.Ponpandian, D.Mangalaraj, A.Balamurugan, C.Viswanathan, Solid State Ionics (Under review)