Hierarchical Assembly, Modification And Photoactivity Of Novel Photocatalytic Nanomaterials

Energy and environmental issues are two main topics of scientific research in the 21 century. Semiconductor photocatalytic materials showed great potentials in solar energy conversion and environmental protection,and were widely applied in air and water purification,H₂ production by water splitting as well as dye-sensitized solar cells, etc..However, the performance of most photocatalysts required to be further enhanced in part due to its narrow light-response range as well as its low quantum efficiency.Consequently, numerous works focused on enhancing the photocatalytic performance of TiO₂ by guest incorporation, for example, by doping, heterojunction interfacial coupling, surface sensitization,etc.,and on exploiting novel photocatalytic material alternatives.Our ideas are enhancing the photocatalytic performances by multi-level structural design of low dimensional photocatalytic nanostructures without destroying the basic chemical structures.Firstly, flower-like Bi2WO₆ hierarchical assemblies are synthesized under hydrothermal conditions with the cooperative effects of polymeric PSS,based on the organic-inorganic interface cooperative self-assembly and in-situ self-transformation (OIICSS) approach.The Bi2WO₆ flowers are constructed by the multi-level assembly of tiny Bi2WO₆ nanoplates, and thus have multilevel porous structures. As compared to the separated Bi₂WO₆ nanoplates, the Bi2WO₆ flowers show better light-harvesting and enhanced optical absorption.However, the photocatalytic activities Bi2WO₆ flowers was not satisfying, which may be related the low degree of crystallinity and high surface defective density as a result of the inhibiting effect of organic additive on nanocrystal coarsening. Besides, the controlled synthesis of BiFeO₃ and Au@Fe₂O3 hierarchical superstructures were also investigated based on a similar OIICSS strategy.Secondly, porous TiO₂ hollow microspheres are fabricated in high yield under hydrothermal conditions with the promoting effect of urea and fluoride, based on newly developed chemically-induced self-assembly and in situ transformation (CIST) strategy.The TiO₂ hollow microspheres are constructed by the multi-level assembly of tiny anatase nanocrystals with size of 10-20 nm, with consequence of multilevel porous structures within them.The porous TiO₂ hollow microspheres show better photocatalytic activity as compared with the solid TiO₂ microspheres and dispersed TiO2 nanocrystals (P25),which may be related to the better surface adsorption of pollutant molecules and enhanced light-harvesting within porous TiO₂ hollow microspheres. Based on the time-dependent evolution and a series of control experiments, two important processes are disclosed.One is the multi-level assembly of primary nanobuilding blocks (TiO₂ nanoclusters)into metastable spherical aggregates;the other is such metastable spherical aggregates evolving into their crystalline hollow counterparts after a series of confined chemical and structural transformation.The first process relied on the control over the nucleation dynamics and surface states (surface functional groups and charges) of TiO2 nanoclusters,which could be tuned by adding urea as base catalyst.The second process based on the thermodynamically instability of metastable spherical aggregates and the positive effect of active additive(free fluoride ion),promoting the confined mater transfer with consequence of simultaneous interior hollowing and exterior crystallizing. Furthermore, the CIST strategy was also extended to the controlled synthesis of other functional oxide hollow micro/nano-structures.such as SnO₂, Sn₃O₄/SnO₂, FeOOH. CoOOH, etc..Finally, the cooperative effects of textural design and other modification methods (for example, doping) in promoting TiO₂ photocatalytic activity were investigated. Using Sn⁴⁺ doping as an example, we showed that moderate doping did not change the phase structures, porous structures as well as hollow mincrostructures, but would change the surface and electronic microstructures.Below a certain doping level,the textural and doping modification could be compatible and cooperative in promoting the photoactivity. Besides, the synergetic codoping within porous TiO₂ hollow microspheres was also investigated.The Zr/F synergetic codoping by electron transfer from Zr 3d to F Is contribute to decreasing surface defect density and increasing surface ordering, promoting lattice substitutive doping, flexible textural tuning, and thus help to increase the photocatalytic activity.As a conclusion, we showed that the multi-level microstructural design is an effective approach in promoting the photoactivity of semiconductor photocatalytic matcrials.But we need to known that the textural control is usually accompanying with the possible changing phase, surface states and electronic microstructures.Thus, we have to consider the comprehensive effect of various aspects, to control the desired nanobuilding blocks with specific phase and surface states, and to control their multilevel collective assembly, and to gain the highly photoactive hierarchical superstructures.