TiO2 thin films were prepared on fused quartz by the liquid-phase deposition (LPD) method from a (NH4)2TiF6 aqueous solution upon addition of boric acid (H3BO3) and calcined at various temperatures. The as-prepared films were characterized with thermogravimetry (TG), Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), UV−Visible spectrophotometry (UV−Vis), scanning electron microscopy (SEM), photoluminescence spectra (PL), and X-ray photoelectron spectroscopy (XPS), respectively. The photocatalytic activity of the samples was evaluated by photocatalytic decolorization of methyl orange aqueous solution. It was found that the as-prepared TiO2 thin films contained not only Ti and O elements, but also a small amount of F, N, and Si elements. The F and N came from the precursor solution, and the amount of F decreased with increasing calcination temperature. Two sources of Si were identified. One was from the SiF6 2- ions, which were formed by a reaction between the treatment solution and quartz substrate. The other was attributed to the diffusion of Si from the surface of quartz substrate into TiO2 thin film at 700 °C or higher calcination temperatures. With increasing calcination temperature, the photocatalytic activity of the TiO2 thin films gradually increased due to the improvement of crystallization of the anatase TiO2 thin films. At 700 °C, the TiO2 thin film showed the highest photocatalytic activity due to the increasing amount of SiO2 as an adsorbent center and better crystallization of TiO2 in the composite thin film. Moreover, the SiO2/TiO2 composite thin film showed the lowest PL intensity due to a decrease in the recombination rate of photogenerated electrons and holes under UV light irradiation, which further confirms the film with the highest photocatalytic activity at 700 °C. When the calcination temperature is higher than 700 °C, the decrease in photocatalytic activity is due to the formation of rutile and the sintering and growth of TiO2 crystallites resulting in the decrease of surface area.

strategies in addressing the low-efficiency issue will be discussed.Photocatalysis mainly deals with electron and energy transfer processes. Prior to the discussion, it is necessary to learn the basic behaviors of the excited state of a molecule, which can enhance the understanding on electron transfer and energy dissipation in semiconductors. Generally, the first process is the absorption of a photon by a molecule (in the time scale of femtoseconds (fs)), where the ground state is lifted energetically to the first excited singlet state. Subsequently, a simplified Jablonski diagram is reconsidered, [2] as shown in Figure 1. The charge carriers generated in one molecule experience several possible processes with varying possibilities: [3] a) Vibrational relaxation (VR): it is relaxation of excited state electrons to the lowest energy level which generally occurs in picoseconds (ps). It can happen from each excited state to each non-excited state including the ground state. This VR results in loss of energy because excessive vibrational energy is converted into heat. b) Fluorescence: After the relaxation to the lowest vibrational level, the excited molecule can finally get back to the ground state by emitting a photon. This is named as fluorescence, which occurs in a relatively long time, ranging from ps to nanoseconds (ns). c) Internal conversion (IC): it is a crossover process in which an electronically excited molecule moves from one electronic state to a lower one of the same multiplicity (singlet-to-singlet or triplet-to-triplet states) and can be measured from ps to fs. [4] d) Intersystem crossing (ISC): A transition from one electronic state to another one with a different spin multiplicity is called ISC. e) Phosphorescence: After the molecule transitions through ISC to the triplet state, further deactivation occurs through phosphorescence. And its lifetime ranges from one millisecond (ms) to hundreds of seconds. Apart from these, other processes such as vibrational cooling are also possible, which are not illustrated here.The above-mentioned processes in a single molecule can be used as a reference when discussing a semiconductor photocatalyst. Analogously, electrons are excited to the conduction band (CB) after absorption. Afterward, they will undergo several decay processes or they will finally migrate to the surface and participate in a specific redox reaction. These decay processes are briefly introduced here by comparing with those in a molecule (Figure 1): a) Relaxation of electrons to the lowest CB energy states. b) Radiative recombination of electrons and holes via emitted as fluorescence; c) Non-radiative decay, also referred to as Photocatalysis is a green technology to use ubiquitous and intermittent sunlight. The emerging S-scheme heterojunction has demonstrated its superiority in photocatalysis. This article covers the state-of-the-art progress and provides new insights into its general designing criteria. It starts with the challenges confronted by single photocatalyst from the perspective of...

We have prepared a TiO 2 -based novel visible-light-sensitive photocatalyst, in which Fe(III) species were grafted on a rutile TiO 2 surface (denoted as Fe(III)/TiO 2 ). With use of X-ray absorption fine structure analysis, the grafted iron species were determined to be in the 3+ state and adopt an amorphous FeO(OH)-like structure. Fe(III)/TiO 2 displayed optical absorption in the visible light range over 400 nm, which was assigned to the interfacial charge transfer from the valence band of TiO 2 to the surface Fe(III) species. Its photocatalytic activity was evaluated by the decomposition of gaseous 2-propanol under visible light (400-530 nm), which revealed a high quantum efficiency (QE) of 22%. Monochromatic light experiments indicated that the effective wavelength region was extended as far as 580 nm while maintaining a QE of greater than 10%. On the basis of the analogy to Cu(II)-grafted TiO 2 photocatalyst et al. Chem. Phys. Lett. 2008, 457, 202), we speculate that the high performance of the present photocatalyst is derived from the photoproduced holes that are generated in the valence band of TiO 2 and contribute to the oxidative decomposition of 2-propanol, and the catalytic reduction of oxygen (presumably multielectron reduction) by photoproduced Fe(II) species on TiO 2 .