Solution-based synthesis of graphene nanosheets and functional TiO2 nanoparticles in high-performance dye-sensitized solar cells

Abstract

Graphene-based materials such as graphene nanosheets, graphene/metal nanoparticles (NPs) composite, graphene foam/aerogel, have been investigated extensively because of their low cost, high surface area, wide availability, nontoxicity, high chemical stability and excellent electrical conductivity, leading to the realization of multifunctional surfaces with a wide range of applications such as electrochemical sensors, oil absorbers, supercapacitors and dye-sensitized solar cells (DSSCs).

In Chapter 1, I systematically introduce the synthesis methods and various applications of graphene and its derivatives, for example, N-doped graphene nanosheets and graphene foam/aerogel. I also present the fundamental mechanism, the state-of-art of DSSCs and propose the feasible solution to address the technical issues. In addition, DSSCs under low-intensity illumination was investigated to figure out the different electron transfer process compared to that one illuminated in 1 full sun condition.

In Chapter 2, I introduce a graphene/Pt composite film employed as flexible counter electrode in DSSCs. A facile and scalable aqueous process is developed to synthesize the water-soluble microwave-exfoliated graphene (MEG)/platinum NPs (PtNPs) composite, which has a relative low defect level and can be readily dispersed in deionized water without adding surfactants. This low cost synthesis method is applicable in flexible DSSCs.

To further meet the needs of low cost and wide applications, high electrocatalytic and wettable nitrogen-doped graphene (NG) nanosheets are used as Pt-free CE for DSSCs and PSSCs, which are coupled with iodide/triiodide and Co(polypyridyl) redox electrolyte, respectively. The pyrrolic and pyridinic N atoms are doped into the carbon conjugated lattice to enhance electrocatalytic activity. Moreover, the NG CE exhibits a lower value of charge transfer resistance (Rct) for the Co(polypyridyl) redox couple and significantly better electrochemical stability during prolonged potential cycles than those for the Pt CE. The details of the NG fabrication process and applications are discussed in Chapter 3 of the thesis.

Besides application in DSSCs, N-doped graphene, its derivative N-doped graphene foam (NGF), which is oleophilic and has a mesoporous structure with a high surface area, has been demonstrated to be an effective absorber of a wide variety of oils and chemical waste. In Chapter 4, I present an environment-friendly water vapor treatment for preparing a highly hydrophobic (CA ≈ 147.5°) and oleophilic NGF for efficiently removing oil from oil/water emulsions.

In order to address the three technical issues including porphyrin sensitizer aggregation, interfacial recombination and ionic diffusion in cobalt-mediated, porphyrin-sensitized system, I proposed a simple and cost-effective method to solve the above technical issues at once by blending home-made TiO2 NPs (~ 50 nm) with exposed {001} facets into commercial 20 nm TiO2 paste to fabricate a new TiO2 photoanode. The technical details of this method and the performance of devices are discussed in the Chapter 5. In addition, I systematically investigated the photovoltaic performance of the DSSCs incorporating TiO2 compact layer (CL) under both low-intensity light and indoor fluorescent light illumination and clarified the controversy over the blocking effect of TiO2 CL for the efficiency increment.