Design And Synthesis Of Binary And Ternary Copper Chakogenides Nanostructures For Energy Conversion

As one of the transition metal chalcogenides (TMCs), copper chalcogenides, owing to their superior properties from nanostructre than bulk materials, have presented in broad applications of thermoelectric conversion, quantum-dot-sensitized solar cells (QDSSCs), lithium-ion or sodium-ion battery, photocatalysis and photoacoustic imaging. In order to enhance the properties, tuning the stoichiometric ratio, size or morphology of copper chalcogenides are strategies. A number of techniques and strategies, such as hydrothermal/solvothermal approach, sonochemistry, electrochemical-deposition, microwave-assistant route, ball milling technique, and chemical welding method have been developed to prepare nanocrystals, nanowires/tubes, nanocages, dendrimers, and nanosheets of copper chalcogenide. However, design and synthesis hierarchical architecture of copper chalcogenide for enhancing their performances in area-as above is still a challenge.In this thesis, design and synthesis of binary and ternary copper chalcogenides in the applications of thermoelectric conversion and QDSSCs have been researched. Copper selenide, p-type semiconductor, with high electrical conductivity, low thermal conductivity, and unique crystal structrue (i.e., Cu-deficiency) can explore as thermoelectric conversion materials. However, most methods (i.e., melting method, ball milling, hydrothermal/solvothernal method) are not able to produce morphology-controlled and large-scaled samples for thermoelectric research. Thus, a facile synthesis of copper selenide on a large scale with controlled morphology is desirable. In the application of QDSSCs, the conventional noble metal counter electrodes (Pt or Au) are unsuitable for QDSSC applications when polysulfide electrolytes are used, mainly because sulfur-containing (S2" or thiol) compounds are absorbed preferentially and strongly on the Pt or Au surface, leading to surface passivation and decrease in the conductivity of electrodes. In contrast, copper-chalcogenide-based CEs show lower resistance and higher electrocatalytic activity towards the redox reaction of polysulfide, thus improving the conversion efficiency of QDSSCs. Current investigations in this field are mainly focus on copper surfide CEs, however, there have been few reports on binary copper selenide CEs and no reports on ternary copper silver selenide CEs in QDSSCs, especially using their designed nanostructures for enhancing conversion efficiency of QDSSCs.In chapater 2, the research is about the systhesis of large yield surfactant-free copper selenide (Cu2-xSe) nanowires and the thermoelectric properties of these Cu2-xSe powders by the spark plasma sintering (SPS) technique. Uniform surfactant-free Cu2-xSe nanowires were prepared via an aqueous route. The effects of reaction parameters such as Cu/Se precursor ratio, Se/NaOH ratio, and reaction time on the formation of nanowires were comprehensively investigated. The results show that Cu2-xSe nanowires were formed through the assembling of CuSe nanoplates, accompanied by their self-redox reactions. The resultant Cu2-xSe nanowires were explored as a potential thermoelectric candidate in comparison with commercial copper selenide (Cu2Se) powder. Both synthetic and commercial samples have a similar performance and their figures of merit (ZT) are 0.29 and 0.38 at 480℃, respectively. Although the ZT of Cu2-xSe has not been observably increased compared to the ZT of commercial Cu2Se, the introduction of nanocrystals in Cu2-xSe decreased the thermal conductivity, which due to the enhancement of phonon scattering of nanocrystals can decrease the lattice thermal conductivity at high temperature. Besides, the Cu-deficiency in Cu2-xSe increased the electrical conductivity compared to that in commercial Cu2Se.In chapater 3, based on previous chapter, searching for facile and high yield fabrication method of copper selenide (Cu2-xSe) is an issue for thermoelectric application. Grams of Cu2-xSe nanostructures were prepared from commercial copper and selenium powders in the presence of thiol ligands by a one-pot reaction at room temperature. The morphology of the nanostructures is strongly dependent on the ratio of thiol ligand to selenium powder. The resultant Cu2-xSe nanostructures were treated with hydrazine solution to remove the surface ligands and then explored as a potential thermoelectric candidate in comparison with commercial copper selenide powders. The results demonstrate that the synthetic samples from our novel approach exhibit similar thermoelectric properties to commercial Cu2Se, which is similar as the result in previous chapater. However, Cu2-xSe retained cubic structure, while commercial Cu2Se converted to mixture of cubic and tetragonal structure after high temperature measurement, the resultant Cu2-xSe have more stable crystal structure than commercial Cu2Se.In chapater 4, the universal synthesis method mentioned in chapter 3 has been demonstrated by computational method. Using this ambient facile method, we prepared Cu2-xE (E=S,Se) micro-/nanotubes (NTs) with a hierarchical architecture and assembled them as counter electrodes (CEs) of quantum-dot-sensitized solar cells (QDSSCs). Cu2-xSe and Cu2-xS NTs were fabricated by using copper nanowires (Cu NWs), stable sulfur and selenium powder as precursors at room temperature. The influence of reaction parameters (e.g., precursor ratio, ligands, ligand ratio, and reaction time) on the formation of nanotubes was comprehensively investigated. The resultant Cu2-xE (E=S, Se) NTs were used as CE of QDSSCs to achieve a conversion efficiency (η) of 5.02 and 6.25%, respectively, much higher than that of QDSSCs made with Au CE (η=2.94%). The reasons are as follows, first, the hierarchical architecture of the nanotube structure provides larger surface area for adsorption of polysulfide electrolyte than bulk metal electrode. Second, Cu2-xSe and Cu2-xS NTs are more stable than metal electrodes in polysulfide electrolyte, as they have high resistance to corrosion and passivation. Third, Cu2-xSe and Cu2-xS are essentially heavily self-doped semiconductors and exhibit excellent conductivity.In chapater 5, one-dimensional/two-dimensional (1D/2D) ternary CuAgSe nanotubes (NTs) were successfully prepared from copper selenide (Cu2-xSe) NTs in chapater 4 at room temperature within a short reaction time by the facile cation exchange approach. The cation exchange leads to the transformation of the crystal structure from cubic into orthorhombic and/or tetragonal with well retention of morphology. The exchange reactions are spontaneous due to their large negative changes of the Gibbs free energy (△G). The effects of parameters such as reaction time, precursor source, and precursor ratio on the exchange reaction were investigated. The resultant CuAgSe NTs were explored as CEs of QDSSCs and achieved much higher conversion efficiency (η=5.61%) than those of QDSSCs containing the noble-metal Au CE (3.32%), which due to the higher mobility of both Cu+ and Ag+ ions in the CuAgSe NTs and the higher stability of metal selenides in polysulfide electrolyte by avoiding corrosion and passivation than that of the Au CE.Chapater 6 is the conclusion of the whole thesis.