ABSTRACT
The effects of the particle size distribution on the charge carrier dynamics and the photocatalytic activity of mixed titanium dioxide (TiO2) powder samples were investigated in this work. Instead of the synthesis of the small semiconductor particles, the binary particle size distributions of the powders were obtained by mixing commercially available TiO2 powders with different particle sizes. The pure anatase samples (average diameters: 7, 20, and 125 nm, respectively) were created via ultrasound treatment and discreet drying. The photocatalytic activity of the powder samples was assessed by the degradation of nitric oxide (NO) and acetaldehyde in the gas phase. Furthermore, the charge carrier kinetics was determined using transient absorption spectroscopy following pulsed laser excitation. Importantly, a recently published model based on fractal dimensions was used to fit the transient signals of the photo generated charge carriers in the TiO2 powder samples. The effects of the particle size on the acetaldehyde degradation could be explained by the formation of agglomerates, which reduce the available surface area of smaller particles. The fast oxidation of acetaldehyde on the surface of TiO2 by direct hole transfer was further independent of the observed charge carrier lifetimes on the microsecond time scale. The photocatalytic NO degradation, on the other hand, increased for samples containing larger amounts of small particles. The corresponding photonic efficiencies correlated well with the charge carrier lifetimes determined by the time-resolved studies. Hence, it was concluded that a long charge carrier lifetime generally leads to higher fractional conversions of NO. The employed fractal fit function was proved to be beneficial for the kinetic analysis of charge carrier recombination in TiO2, in direct comparison with a second order fit function.
ABSTRACT
The Surface Plasmon Resonance (SPR) driven photocatalytic H2 production upon visible light illumination (≥500 nm) was investigated on gold-loaded TiO2 (Au-TiO2). It has been clearly shown that the Au-SPR can directly lead to photocatalytic H2 evolution under illumination (≥500 nm). However, there are still some open issues about the underlying mechanism for the SPR-driven photocatalytic H2 production, especially the explanation of the resonance energy transfer (RET) theory and the direct electron transfer (DET) theory. In this contribution, by means of the EPR and laser flash photolysis spectroscopy, we clearly showed the signals for different species formed by trapped electrons and holes in TiO2 upon visible light illumination (≥500 nm). However, the energy of the Au-SPR is insufficient to overcome the bandgap of TiO2. The signals of the trapped electrons and holes originate from two distinct processes, rather than the simple electron-hole pair excitation. Results obtained by Laser Flash Photolysis spectroscopy evidenced that, due to the Au-SPR effect, Au NPs can inject electrons to the conduction band of TiO2 and the Au-SPR can also initiate e-/h+ pair generation (interfacial charge transfer process) upon visible light illumination (≥500 nm). Moreover, the Density Functional Theory (DFT) calculation provided direct evidence that, due to the Au-SPR, new impurity energy levels occurred, thus further theoretically elaborating the proposed mechanisms.
