Effect of the Agglomerated State on the Photocatalytic Hydrogen Production with in Situ Agglomeration of Colloidal TiO2 Nanoparticles

Abstract

The photocatalytic production of H-2 in aqueous TiO2 colloid (with methanol as an electron donor) was greatly accelerated by the in situ agglomeration of the colloid although such an agglomeration should reduce the photocatalytic activity in most other cases because of the reduction of the surface area. The in situ agglomeration occurred after an induction period of 3 h and was ascribed to the pH increase which was resulted from the photocatalytic reduction of nitrate (incorporated from the synthetic step of TiO2 sol) to ammonia. The agglomeration occurred at pH close to the isoelectric point of colloidal TiO2 which was 6.9 as measured by the zeta-potential. It is proposed that the charge separation is facilitated by electron hopping from particle to particle when TiO2 nanoparticles are connected with each other within the agglomerates. This behavior was further supported by the photocurrent collection measurement (mediated by the methyl viologen MV2+/MV+ redox couple in the colloidal solution), which also showed a rapid increase in the photocurrent after the agglomeration of TiO2 nanoparticles. When the colloid of TiO2 was initially coagulated at around pH 6, the production of hydrogen increased linearly with time without showing an induction period and the collected photocurrent showed an immediate increase upon irradiation. To understand the role of the agglomerated state, the colloidal TiO2 (well-dispersed) and the suspension of commercial TiO2 (agglomerated) systems were compared and discussed for their photocatalytic behaviors. The present study demonstrates that the degree of agglomeration of TiO2 nanoparticles is a critical parameter in determining the efficiency of the charge separation and the photocatalytic hydrogen production.