Construction And Photocatalytic Performance Of The TiO₂ Surface Heterojunction

With the development of photocatalytic technology, photocatalytic technology has been recognized by more and more researcher and used to solve the energy crisis and environmental pollution. The key point is how to improve the activity of catalysts. Compared with other semiconductor materials, Ti O₂ has many excellent advantages such as non-toxic, stability, high corrosion resistance, biocompatibility and low-cost and thus was extensively used in the field of photocatalysis. However, it also suffered from a hurdle of low quantum efficiency when Ti O₂ or its composite materials were used as photocatalyst due to the fast electron-hole recombination rate and limited harvesting of visible light（caused by its wide band gap, 3.2 e V）. The common method of resolving the problems above is to construct surface heterojunction. In this paper, three new surface heterostructures from different angles of dimensions, planes and levels were synthesized and studied. The features of as-synthesied materials such as morphology and lattice structure, crystal phase and textural properties, band gap and chemical state analysis were characterized by SEM, TEM, XRD, Raman, Uv-vis, XPS, respectively. In order to evaluate the photocatalytic performance of materials, the potocatalytic degradation, photocatalytic hydrogen and photocurrent were chosen as model reactions and all the results were summarized as follows:（1） The dimensions of materials: A simple hydrothermal method was used to construct a new kind of 0D/1D heterostructure through loading 0D Ti O₂ nanoparticles（NP） on the surface of 1D Ti O₂ nanowires（NW）, where the NP and NW were both anatase crystal and enclosed with the {101} facets, It excludes the differences of the Fermi levels caused by different crystal phases and planes of NP and NW. The close contact and the different dimensions between NP and NW were the most important reason for forming the surface heterojunction of 0D/1D. Compared with NP and NW alone, the NP/NW heterostructure showed better photocatalytic performance, the ratio of NP to NW is an important factor to affect the performance of 0D/1D heterostructure, when the ratio of NP to NW was 50:50, the NP/NW heterostructure showed an excellent cycling capability for Rhdamine B degradation（93% retention over 6 cycles） and superior activity for both photodegradation of Rhdamine B（degradation rate was two times as fast as NW） and water splitting to H2（Photocatalytic hydrogen production rate was 5 times as fast as NW）.（2） The planes of materials: Based on the dimensions of materials, the influence of different crystals on the performance of the heterojunction materials was further considered. A simple hydrothermal method was used to construct a new kind of {101}/{100} heterostructure through patching Ti O₂ nanoparticles（NP） on nanorods（NR）, where the NP and NR were predominately enclosed with the {101} and {100} facets, although NP and NR were both anatase crystal, the different dimensions and planes lead to the difference on Fermi levels between NP and NR. Compared with NP and NR alone, the NP/NR heterostructure showed better photocatalytic performance, the ratio of NP to NR is an important factor to affect the performance of {101}/{100} heterostructure, when the ratio of NP to NR was 50:50, the NP/NR heterostructure showed an excellent cycling capability for methyl orange degradation（99% retention over 6 cycles） and superior activity for both the photodegradation of multiple organics, like Methyl Orange, Methyl Blue, Rhodamine B, Malachite Green and Bisphenol A（the degradation rate of methyl orange was 4 times as fast as NR） and water splitting to H2（Photocatalytic hydrogen production rate was 13 times as fast as NR）. It is worth noting that the heterojunction combined with dimensions and planes has a greater exaltation on photocatalytic performance than that combined with dimensions alone.（3） The levels of materials: Based on the dimensions of materials, the influence of different levels on the performance of the heterojunction materials was further considered. A simple deposition-precipitation method was used to construct a new kind of Au-Cu/Ti Ns heterostructure through gradually depositing Au and Cu on Ti O₂ nanosheets（Ti Ns）, on the one hand, constructing heterostructure can improve visible light absorption, on the other hand the schottky barrier between Au-Cu alloy（lower Fermi level） and Ti Ns（higher Fermi level） can suppress the recombination of carrier to improve the photocatalytic activity. The ratio of Au to Cu, sequence and method of deposition were also the important factors to affect the performance of Au-Cu/Ti Ns heterostructure, when the ratio of Au to Cu was 1:1, the Au-Cu/Ti Ns heterostructure showed the best photocatalytic performance on water splitting to H2（over 9 times than Ti Ns, over 1.47 times than Au/Ti Ns, over 1.75 times than Cu/Ti Ns）.