Enhanced photocatalytic activity of {1 0 1}-oriented bipyramidal TiO2 agglomerates through interparticle charge transfer

Abstract

Uniformly packed dense TiO2 bipyramid agglomerates (BPA) with the dominantly exposed {101} facets were designed and synthesized by using a simple solvothermal process. From scanning electron microscopy (SEM) image, BPA consists of uniformly and densely packed nanoparticles forming secondary mesopores. BPA structure was intentionally crushed by ball-milling to obtain c-BPA, the photoactivity of which was compared with BPA. The photocatalytic activities of as-prepared BPA and c-BPA were investigated for hydrogen production and 4-chlorophenol degradation and compared with P25 as a reference TiO2. The BPA showed much higher photocatalytic activities than those of c-BPA and P25 (randomly aggregated commercial TiO2 nanoparticles). The marked efficiency improvement in BPA can be ascribed to the noble nanostructural features: (i) the agglomerated bipyramid (BP) units suppressing the electron-hole recombination through efficient interparticle charge transfer, (ii) the well-developed mesopores allowing the efficient diffusion of reactants and products, (iii) the highly {101}-oriented BP units that help efficient charge separation, and (iv) a good crystallinity of the BP units. These superior properties of BPA were also confirmed in their photoelectrochemical behaviors: the electrode of BPA exhibited the highest photocurrent under UV light, the slowest decay of open-circuit potential after turning off the light, and the lowest charge transfer resistance from the electrochemical impedance spectroscopic (EIS) measurement, which implies the retarded recombination of charge pairs and efficient interparticle charge transfer on BPA. The controlled agglomerated structure of the nano-sized TiO2 can enhance the photocatalytic and photoelectrochemical properties through efficient interparticle charge transfer.