Thesis conclusions and future work
6.1 Summary
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Chapter Six
E.T. Mombeshora Page 142 policies are discussed within this chapter pointing out how both developing and developed countries aims to face a potential energy crisis through various roadmaps.
Chapter Two further explains how nanotechnology has the potential to solve the real world’s problem, “energy crisis” via generation of nanomaterials with higher efficiencies and band gap engineering of current nanomaterials on the market. Nanoscience and nanotechnology which are terms often confused were defined and differentiated. Carbon was recognized as the key player and thus carbon allotropes, including CNTs and their chemistry are described within this chapter. Properties and applications of MWCNTs are also presented in greater detail within this chapter. MWCNTs require modifications in order to manipulate them as functional moieties and therefore possible modifications of MWCNTs were identified. Chapter Two further narrowed down to MWCNTs in MWCNT-titania nanocomposites.
Background information on titania in terms of properties, forms, band gap modifications and exciton generation mechanism was also explained in this chapter. The survey showed that MWCNTs had a potential to tune physicochemical properties of tinania nanoparticles towards higher light-harvesting capabilities. MWCNT-titania nanocomposites synthesis methods were identified. In addition, analytical techniques used for characterization of nanomaterials are discussed in terms of how they are crucial to the study presented in the subsequent chapters. The theory behind DSSC as a photo-electrochemistry system was explained within the chapter. The background on some important parameters involved in PV systems are also stated and explained. Chapter Two also gives some reported work done in the field of DSSCs which is relevant to our approach in order to have a rough idea of what was on the ground when the work was started.
Chapter Three is an experimental section. Chapter Three presents the details of; reagents, solvents, materials and experimental procedures involved in the acid treatment of MWCNTs, synthesis of nanocomposites as well as in fabrication of DSSCs. The details of;
reagents, solvents and procedures involved in electrolyte synthesis were also given within this chapter. This chapter also outlines details of the two synthetic methods used in this study, i.e. sol-gel and CVD. Details of the experimental procedures such as use of an ultrasonic water bath, stirrer, solvents and how the precursors were mixed by the sol-gel
E.T. Mombeshora Page 143 method are also given in greater detail. The information of the parts used in constructing the CVD reactor, temperature program involved and how precursors were mixed together were also presented within this chapter. Range of MWCNTs ratio to titania in the MWCNT- titania nanocomposites was from 2 to 98 wt.%. Instrumental and software details for the physicochemical characterisation, i.e. SEM, TEM, HR-TEM, TGA, Raman spectroscopy, powder XRD, FTIR spectroscopy and textural characteristics, and light-harvesting characterization techniques, i.e. diffuse reflectance, PL, UV-Vis and solar simulator, engaged in this work are given within the chapter. The diagram showing components and the design of DSSC devices used in this work, and a brief view of how measurements were carried out in this work is given. In short, this chapter gives all experimental steps involved, i.e. from acid treatment of MWCNTs via MWCNT-titania nanocomposite synthesis, physicochemical characterization to application of nanocomposites in DSSCs.
Chapter Four is the core of the study. It discusses each method and the results obtained in this work. Morphology and dimensional studies were done by SEM, TEM and HRTEM. The three dimensional images from SEM shows that uniformity of titania coating on MWCNTs varied with differences in wt.% of MWCNTs. The TEM gave good two dimensional images showing that titania coated MWCNTs. EDS was used as a qualitative technique in identifying the elements present as titanium and carbon. This technique also gave a rough view of how the two elements identified were distributed within the nanocomposites. ICP-OES quantified the amount of titanium in each nanocomposites culminating in the calculation of the wt.% ratios of MWCNTs to titania. The FTIR spectroscopy was the method used to investigate the relationship of titania and MWCNTs in the nanocomposites and other chemical bonds present. This technique also identified the phase present in the nanocomposites as anatase. Thermal stability of MWCNTs in the synthesized nanocomposites was investigated by TGA and according to the results from this technique, high titania wt.% reduced MWCNTs resistance to oxidation and it also showed that experimental wt.% ratios of MWCNTs:titania correlated with those obtained by means of ICP-OES. The graphitic quality trend of MWCNTs in the nanocomposites was investigated using Raman spectroscopy which also agreed with FTIR spectroscopy on anatase being the only phase present. This was further confirmed by powder XRD which also gave the surface crystal arrangements in addition to its ability to reveal the high degree of crystalinity in the
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.