Controllable Preparation And Photoelectric Applications Of Semiconductive Metal-oxide Nanoparticles And Thermosensitive Polymer Microgels

The quality of human life depends to a large degree on the availability of energy. However, with the rapid development of the global economy, energy crisis, environmental pollution and climate change have become the most serious problems, which will restrict the sustainable development of human society. As a kind of energy saving and environment friendly energy, solar energy is really inexhaustible and natural renewable, which makes it one of the most attractive research fields. The utilization of solar energy is mainly concentrated on transforming into other forms of energy, such as electrical energy and chemical energy. In the last few years, research fields in solar energy have been developed, such as solar cells, photocatalysis, optical storage, photoelectric switch, and building energy conservation.The controllable synthesis of semiconductive metal-oxides and thermosensitive polymer microgels with special photoelectric properties, and their applications in energy conservation and environmental protection, were studied in this thesis. Because of their special band structures, semiconductive metal-oxides can be applied as photocatalysts for the degradation of organic dyes and transparent electrodes for solar cells. Their photoelectric properties can be effectively regulated by element doping and process improvement. Poly(N-isopropylamide) microgels, which possess the lower critical solution temperature (LCST), undergo a reversible swelling-deswelling process upon heating and cooling. The great change of their optical properties during the phase transition makes them potential materials for thermochromic smart windows. Due to the synergistic effect, inorganic-organic composite materials usually exhibit better performance than that of a single material. Thus, at the last charpter of this thesis, inorganic-organic composite microgels were synthesized from thermochromic materials. Their phase transition behavior, composite mechanism and the application in smart windows were also investigated. Details of this thesis are outlined as follows:1. Nanosized SnO2with different morphologies were synthesized via a simple hydrothermal process at180℃, using polyvinylpyrrolidone (PVP), sodium dodecyl sulfonate (SDS), cetyl trimethyl ammonium bromide (CTAB) or tetrapropyl ammonium bromide (TPAB) as surfactant. All the prepared SnO2are of a tetragonal crystal structure, and nanocubes, nanorods, nanosheets, nanobelts and nanoparticles were prepared when changing the type and dosage of organic surfactants. The effect of surfactants on the morphology of SnO2is probably attributed to the electrostatic interactions and Van der Waals’forces, which can change the growth oriention of SnO2nanocrystals during the hydrothermal process. So the effect of surfactant on the morphology of SnO2is significantly dependent on the solvent type:water or alcohol. With the same solvent, the addition of anionic surfactant (SDS) and cationic surfactant (CTAB or TPAB) can both largely affect the morphology of SnO2nanocrystals, but showing really different results. While, with the same kind of surfactant, resulting morphologies of SnO2were also different under different solvents. The non-ionic surfactant (PVP) can also change the morphology of SnO2like SDS, but showing less obvious impact.2. Pure rutile-phase SnO2nanoparticles with different3D microstructures (e.g., flower-like architectures and microspheres) were prepared via a feasible surfactant-free hydrothermal process. These microstructures were composed of nanosheets, nanocubes or nanopyramids. By changing the amount of ethanol additives, the growth orientation and size of the SnO2nanocrystals were changed, which resulted in the formation of different growth patterns. The effect of ethanol content on the morphological evolution of the SnO2nanoparticles, and the formation mechanism of the flower-like architectures and microspheres were explored. Furthermore, the microstructure morphology of SnO2showed great effect on its surface area. Both the adsorption and photocatalytic properties are mainly dependent on the surface area of SnO2and slightly influenced by the band gap energy. Compared to the flower-like architectures, the SnO2microspheres exhibited higher adsorption capacity and photocatalytic activity toward the degradation of acid fuchsine. When the ratio of ethanol/water in the solvent was3:1, the products were broken SnO2microspheres with a surface area of58.2m/g and a photocatalytic activity of0.073min-1. They showed a removal efficiency of acid fuchsine over80%in35min. With excellent recycling performance, these microspheres can be used as efficient water treatment materials.3. Transparent and conductive tungsten-doped tin oxide (SnO2:W) thin films with different thickness (from60to600±10nm) were fabricated on quartz glass substrates by a spin-coating method. A stable precursor solution was prepared from tin chloride and ammonium tungstate, together with polyvinyl alcohol as a film-forming promoter. It was found that all the synthesized films showed homogeneous composition, smooth surface with no cracks and high transparency with an optical band gap ranging from3.93to4.31eV. The effect of tungsten concentration, spin rate and annealing temperature on the morphological, electrical and optical properties of the films were investigated. W doping had a strong impact on the microstructure and the conductivity of SnO2thin films. The lowest resistivity of2.8×10-3Ω·cm was obtained for a SnO2:3at%W film, which was prepared at3000rpm and annealed at800℃in air. An eight-layer film with a sheet resistance of60Ω and a thickness of606nm was fabricated by multiple coating operation, which exhibited an optical transmittance over80%in the visible region from400to760nm, suitable for transparent electrodes of solar cells.4. Thermosensitive poly(N-isopropylacrylamide)(PNIPAm) microgel colloids were prepared by a simple emulsion polymerization method, and a model house was made to test their energy-saving performance for smart windows. The solar modulation ability of PNIPAm microgels can be largely influenced by the monomer concentration, the crosslinker type and dose, and emulsifiers, which showed no obvious effect on the LCST. While, the addition of cosolvent could effectively change the LCST of PNIPAm microgels. By using glycerol as cosolvent, PNIPAm microgel colloids with a LCST between20.4and32.2℃, a freezing point between-18.1and-32℃, were obtained. And they displayed an excellent solar modulation ability (more than60%), resulting in a temperature reduction of20℃compared to float glass, under irradiation. More importantly, the prepared PNIPAm microgel colloids also exhibited high response rate (less than150s), suitable viscosity, restrained evaporation rate, which make them ideal candidates for smart windows.5. Thermochromic VO2(M) nanopowders were prepared via a hydrothermal route, and nanoparticles filled or core-shell VO2-NIPAm composite microgels were synthesized for the first time by in-situ emulsion polymerization. The influences of inorganic nanoparticles on the polymer network structure and the optical properties of PNIPAm-based microgels were studied, and the composite mechanism was investigated. The solar modulation ability of VO2-PNIPAm composite microgels was dependent on the control of PNIPAm network structure by VO2nanoparticles. With appropriate amount of VO2, VO2-PNIPAm composite microgels with a solar modulation ability of88%can be obtained by the combination of VO2nanoparticles with a solar modulation abilty of13%and PNIPAm microgels with a solar modulation of35%.