Optical And Photocatalytic Properties Investigation Of N Doping And Facet-controlled SnO₂ Nanostructures

Tin dioxide (SnO2) is an important semiconductor with a large direct bandgap with extensive applications in transparent conducting electrodes, photocatalysis, photodetector, gas sensors, lithium-ion batteries, supercapacitors and so on. Semiconductor performances are influenced by size, shape, doping. Intentional doping is an effective method for altering physical properties, such as bandgap, conductivity, sensing properties, magnetic properties. Different crystal facets possess different surface actomic arrangements and electronic band structures. Facet dependent photocatalytic, gas sensing, and other surface-related properties are influenced by exposed facets. Controlling the morphology of single crystals with defined facets is another effective method for improving semiconductor performance. Optical and electronic properties of N doped nanocrystals and facets dependent photocatalytic properties of SnO2 micropolyhedron fabricated by chemical vapor depositon were investigated and presented in this dissertation. The obtained main results are as follows:1. A facile and economical chemical route was used to synthesize nitrogen-doped tin oxide (SnO2) nanocrystals (NCs). Raman spectral examinations reveal the existence of oxygen vacancies and local disorder. Temperature-dependent photoluminescence (PL) measurements display a broad band at 640 nm at room temperature and it shifts to a higher energy at lower measurement temperature. Excitation wavelength-dependent PL spectra show that the band blueshifts and its linewidth decreases as the excitation wavelength is reduced. The PL peak also blueshifts when the annealing temperature is increased. Spectral analysis and theoretical calculation suggest that the PL band stems from the mutual effects of oxygen vacancies and nitrogen dopants. This PL investigation on N-doped SnO2 NCs provides more insights about the optical properties and will promote further applications of SnO2 NCs.2. Different SnO2 polyhedrons with different facets were fabricated by chemical vapor deposition using tin and carbon powers as source and reducing agent, respectively. In this study, by adjusting the deposition temperature during chemical vapor deposition, octahedral SnO2 with the exposed (101) facet and two other kinds of SnO2 polyhedrons with (101) and (100) facets with different ratios are fabricated controllably based on the vapor-solid growth mechanism. As the deposition temperature is increased, the surface energy of the reduced (101) facet with Sn termination diminishes. A slight increase in the deposition temperature from 1030 to 1070℃ leads to the formation of polyhedrons with different area ratios of (101) to (100) facets. Carbon acting as a reducing agent mainly in the reaction is a guarantee of fabricating plenty of SnO2 polyhedrons successfully.3. By adopting the teraphalic acid fluorescent method, we estimate photocatalytic activity of the SnO2 polyhedrons. The SnO2 polyhedrons with the completely exposed {101} facets have the highest photocatalytic activity due to generation of active OH groups. With increasing{101} area ratios, the photocatalytic activity increases due to the formation of surface states induced by 5s electrons of bivalent Sn on the (101) surface. The photogenerated holes in the valance band can be easily transferred to the surface states of the (101) surface resulting in reduced spatial overlapping of the photoexcited electrons and holes and reducing the recombination probability. Transient photocurrent response spectra and electrochemical impedance spectra (EIS) all dominated that SnO2 octahedrons have the highest photogenerated charge separation efficiency due to the enhanced hole trapping ability by the surface states.The results reveal that the photocatalytic properties of SnO2 microcrystals can be enhanced by facet-controlled synthesis.