Thesis conclusions and future work
6.2 Overall conclusion
E.T. Mombeshora Page 144 nanocomposites. According to the textural characteristics obtained, the MWCNTs improved surface area by reducing titania agglomeration, pore volume of titania and showed that the nanocomposites were mesoporous. In short, Chapter Four presents and compares a thorough characterization approach to MWCNT-titania nanocomposites synthesized from the sol-gel and CVD methods.
Chapter Five gave a presentation of some electrical characterizations of the MWCNT-titania nanocomposites, i.e. band gap and e-/h+ recombination dynamics using diffuse reflectance and photoluminescence respectively. A brief explanation of the DSSCs and the properties of an ideal dye were also presented. UV-Vis was used to investigate absorption maxima of the eosin B dye. From this technique it was observed that eosin B was potentially suitable for DSSCs applications because it absorbed in the visible region. The light-harvesting experiments results and discussion are presented in greater detail within this chapter. The main aim of this chapter is to apply the MWCNT-titania nanocomposites in light-harvesting.
The nanocomposites synthetic method influences physicochemical properties such as defects, surface area and interfacial contact on the MWCNT/titania interface and hence affects e- transport. The nanocomposites prepared by the CVD method performed better in DSSCs than those made by sol-gel techniques and band gap engineering is not the only factor than can enhance light harvesting capabilities.
A summary of the work or conclusions are given in Chapter Six. A possible description of possible future is also given.
E.T. Mombeshora Page 145 synthetic method influenced physicochemical properties such as nanostructure, ultimate morphology and surface area. According to EDX mapping, CVD approach gave a better distribution of titania dispersion on MWCNTs than sol-gel method. From Raman spectroscopy results, CVD nanocomposites were less defective i.e. had smaller IID
G ratio.
Titania sat on the defects thereby reducing defect intensity on the MWCNT walls.
Thermal stability of the MWCNTs improved in MWCNT-titania nanocomposites from both sol-gel and CVD method relative to that of acid-treated MWCNTs. In addition to this, coating MWCNTs caused tube bending which created weak points for thermal oxidation at elevated temperatures. TGA and ICP-OES suggested that CVD approach was a better titania loading method since the actual loaded titania wt.% had better precision with the theoretically expected values than sol-gel. According to the FTIR spectrum obtained, titania was covalently linked to MWCNTs via the oxygen-containing groups on the tube walls. The FTIR and other techniques such as Raman spectroscopy, photoluminescence and powder XRD suggested that the only phase present in the MWCNT-titania nanocomposites from both CVD and sol-gel methods was anatase. According to XRD diffractogram, CVD nanocomposites were more crystalline. Also, from the textural characterization done, the sol-gel method gave large surface area of nanocomposites than those from CVD method.
CVD was found to be a better synthetic method than sol-gel. The vacuum effect involved in the CVD method is critical for better quality of MWCNT-titania nanocomposites than sol-gel.
The MWCNT-titania nanocomposites produced by the CVD method have more uniform coating, higher thermal stability, and larger pore volume and size, less defects and were more crystalline compared to those made by sol-gel process.
Some interesting conclusions were made from observed electronic properties and light- harvesting experiments. To begin with, MWCNTs reduced e-/h+ recombination and band gap when loaded to titania. The e-/h+ recombination was more in sol-gel MWCNT-titania nanocomposites than CVD. Band gap shifted towards that of MWCNTs above 50 wt.% of MWCNTs ratio whereas below 50 wt.%, MWCNTs reduced band gap of titania. The conclusion from the redox system synthesized was that the gel state electrolyte was more stable and less volatile than iodine electrolyte alone. Hence, the gel state electrolyte was more ideal for the DSSC design used than liquid iodine. The gel state electrolyte unlike
E.T. Mombeshora Page 146 iodine was able to regenerate dye molecules in the DSSC design used in this work. Eosin B absorbed in the UV-Vis region and hence, is a suitable metal-free sensitizer for DSSC light- harvesting.
From the physicochemical characteristics and light-harvesting experiments the conclusions deduced are elucidated below. The nanostructures synthesized were working photo-anode materials with a positive rectification and therefore MWCNT-titania nanocomposites can be applied in DSSCs. The devices have high potential to do work. Also, low wt.% ratios of MWCNTs to titania in MWCNT-titania nanocomposites from both sol-gel and CVD methods performed better in DSSCs than high ratios. From the experiments carried out, the optimum MWCNTs wt.% in DSSCs applications from CVD method was 15 and 10 wt.% for sol-gel method. CVD method nanocomposites performed better in DSSCs than sol-gel nanocomposites. This is because of number reasons such that CVD nanocomposites had larger pore size and pore volume which is available for dye to fill in. Also, CVD nanocomposites were more crystalline and therefore had less number of nanoparticle boundaries. Crystalinity improve electrical conductivity. Therefore, CVD nanocomposites had minimal chances of e-/h+ recombination at nanoparticle boundaries. An intimate contact between TiO2 and MWCNTs caused relative positions of MWCNTs CB edge to allow e- transfer from TiO2 to MWCNTs at the titania/MWCNTs interface. In addition to that, the CO-Ti link improved intimacy of titania and MWCNTs more in CVD. The deductions are made from the more intense CO-Ti peak from FTIR spectroscopy observed in CVD approach than sol-gel method. This also improved light-harvesting capabilities in DSSC devices fabricated using nanocomposites by the CVD method.
In short, titanium optical properties were tailored to enhance its light harvesting capabilities and the ideal properties for better light-harvesting performance deduced from this work include crystalinity, large pore size and volume, uniform morphology, existence of chemical bonds between components of nanocomposites and a defect-free nature. VOC is influenced by CB of MWCNT-titania nanocomposite. Band gap engineering is not the only factor vital for enhancement of light harvesting capabilities. Absorption properties of DSSC components rather than nanocomposites also affect device performances. The high e- conductivity nature of MWCNTs interferes with e- transfer from the MWCNT/titania
E.T. Mombeshora Page 147 interface to the counter electrode in nanocomposites with high MWCNTs wt.% thereby causing DSSCs deterioration.