Controllable Growth Of In₂O₃ Crystal Facet And Facet-Dependent Photoelectrochemical Performance

Semiconductor photoelectrocatalytic material has been known to be the first choice to solve the world energy crisis, which is lost-cost and environmentally friendly and directly transfers the inexhaustible solar energy into chemical energy. Central to the device is semiconductor photoelectrode, whose material plays critical roles in the improving conversion efficiency. The exposed crystal facets of photoelectrode cast profound influence on the photoelectrocatalytic activity because different crystal facets possess different surface atomic arrangements and electronic band structures. However, the poor performance of current semiconductor photoelectrodes is dicated by several drawbacks such as limited light-absorption wavelength range, the high surface recombination rate and poor photostability. The goal of this dissertation is to explor delicately controllable growth of In2O3 nano/microstuctures terminated in specific crystal facet by understanding in depeth the underlying growth mechanism. Based on the unique morphology and hydrogen treatment, application of these In2O3 micro/nanostructures has been further developed as photoanode materials for photoelectrochemical (PEC) water splitting. The calculated and experimental results show that In2O3 photoelectrodes with exposed {001} facets exhibit superior PEC water splitting activity and excellent chemical stability, and the prospect for which is promising. The obtained main results are described as follows:1. A series of polyhedral In2O3 microcrystals with single morphology and size are produced by the modified CVD route based on the VS growth mechanism. The morphological evolution during chemical vapor deposition is investigated and the new knowledge enables precise facet cutting. The synthesized cubic In2O3 microparticles possess superior photoelectrocatalytic activity and excellent chemical and structural stability in oxygen evolution reaction because that{001} facets have the capability to dissociate adsorbed H2O molecules into H+ and OH- and accumulate photogenerated holes. Our results reveal that it is feasible to promote the photolectrochemical water splitting efficiency of photoanode materials by controlling the growth on specific crystal facets. The technique and concept can be extended to other facet-specific materials in applications such as sensors, solar cells, and lithium batteries.2. One-dimensional nanowires (NWs) have been widely used in PEC water splitting because of the enhanced light absorption and charge transport. It has been theoretically predicted that the{001} facets of body center cubic (bcc) In2O3 microcrystals can effectively accumulate photogenerated holes under illumination, but it is unclear whether facet cutting of NWs can enhance the efficiency of PEC water splitting. In chapter four, a large quantity of square and cylindrical In2O3 NWs with diameter of 100-200 nm and lengths up to hundreds of nanometers are fabricated by modified CVD routes based on VLS and VS growth mechanism. From FE-SEM and FE-TEM images we can conclude that square NWs grow along the [100] direction and terminate in the {001} crystal facets. The photocurrent of square In2O3 NWs with four{001} facets is observed to be an order of magnitude larger than that of cylindrical In2O3 NWs under the same conditions. The results provide experimental evidence of the important role of facet cutting, which is promising in the design and fabrication of NW-based photoelectric devices.3. Although the performance of In2O3 NWs pertaining to water splitting can be enhanced by facet cutting, the drawback is that only a small portion of the solar spectrum is utilized in the process. In order to narrow the band gap energy, we introduce disorder to surface of square In2O3 NWs by hydrogen treatment. The hydrogen treatment further promotes the PEC water splitting performance of the NWs. The optimized hydrogenated In2O3 NWs yield a photocurrent density of 1.2 mA/cm2 at 0.22 V versus Ag/AgCl, which is 5 times higher than that of the pristine In2O3 under the same conditions. The enhanced PEC properties can be attributed to the reduced band gap due to merging of the disordered layer-induced band tail states with the valence band as well as improved separation of the photogenerated electrons/holes between the In2O3 crystal core and disordered layer interface. The results demonstrate the excellent PEC water splitting performance of the hydrogenated square In2O3 NWs and the methodology can be extended to the design and fabrication of other NW-based core-shell structures in photoelectric devices.