Jenny Schneider

Understanding TiO2 Photocatalysis: Mechanisms and Materials

Jenny Schneider,*,† Masaya Matsuoka,‡ Masato Takeuchi,‡ Jinlong Zhang, Yu Horiuchi,‡ Masakazu Anpo,‡ and Detlef W. Bahnemann*,† †Institut fur Technische Chemie, Leibniz Universitaẗ Hannover, Callinstrasse 3, D-30167 Hannover, Germany ‡Faculty of Engineering, Osaka Prefecture University, 1 Gakuen-cho, Sakai Osaka 599-8531, Japan Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China

Charge carrier dynamics and photocatalytic behavior of TiO2 nanopowders submitted to hydrothermal or conventional heat treatment

The sol–gel technique followed by conventional (TiO2-1) and hydrothermal (TiO2-2) thermal treatment was employed to prepare TiO2-based photocatalysts with distinct particle sizes and crystalline structures. The as prepared metal oxides were evaluated as photocatalysts for gaseous HCHO degradation, methanol, and dye oxidation reactions. Additionally, metallic platinum was deposited on the TiO2 surfaces and H2 evolution measurements were performed. The photocatalytic activities were rationalized in terms of morphologic parameters along with the electron/hole dynamics obtained from transient absorption spectroscopy (TAS). TiO2-2 exhibits smaller particle size, poorer crystallinity, and higher surface area than TiO2-1. Moreover the hydrothermal treatment leads to formation of the metastable brookite phase, while TiO2-1 exhibits only the anatase phase. TAS measurements show that the electron/hole recombination of TiO2-2 is faster than that of the latter. Despite that, TiO2-2 exhibits higher photonic efficiencies for photocatalytic oxidation reactions, which is attributed to its larger surface area that compensates for the decrease of the surface charge carrier concentration. For H2 evolution, it was found that the surface area has only a minor effect and the photocatalyst performance is controlled by the efficiency of the electron transfer to the platinum islands. This process is facilitated by the higher crystallinity of TiO2-1, which exhibits higher photonic efficiency for H2 evolution than that observed for TiO2-2. The results found here provide new insights into the correlations between thermal treatment conditions and photocatalytic activity and will be useful for the design of high performance photocatalysts.

Characterization of a highly efficient N-doped TiO2 photocatalyst prepared via factorial design

The preparation of titanium dioxide nanoparticles doped with nitrogen for application as a photocatalyst in the decomposition of azo dyes was optimized by factorial planning. Five variables were evaluated and the results showed that the stirring method of the reaction medium, the nitrogen source and the calcination temperature are the determining parameters that affect the photocatalytic activity. With this methodology, it was possible to obtain an optimized photocatalyst (K1) with high surface area and high mineralization efficiency (100%) of the dye Ponceau 4R under solar irradiation. K1, its non-doped version and the worst photocatalyst obtained by the factorial planning (K2) were characterized by several techniques to rationalize the different behaviors. The observed mineralization rate constants under artificial UV-A radiation were in the order of 10−2, 10−4 and 10−3 min−1, respectively, for K1, K2 and the non-doped oxide. As shown by N2 sorption isotherms, the powders exhibited large variations in porosity as well as in the specific surface area, with values ranging from 63.03 m2 g−1 for K1 to 12.82 m2 g−1 for K2. Infrared spectra showed that the calcination of the doped oxides between 300 and 500 °C leads to considerable loss of the nitrogen content, which is corroborated by XPS measurements that also indicate the presence of oxygen vacancies on their surfaces. Nanosecond transient absorption measurements show that the electron–hole half-lifetime in K1 is 870 ns, ca. two times longer than that observed for the other photocatalysts. Additionally, dye degradation studies under solar radiation reveal that K1 is ca. 28% faster than the non-doped TiO2 under similar conditions. This higher photoactivity for K1 is attributed to its extended visible light absorption and the optimized morphological and electronic properties.

CHAPTER 2:Understanding the Chemistry of Photocatalytic Processes

Much effort has been devoted to the understanding of the fundamental and the engineering aspects of semiconductor photocatalysis with the goal of improving its efficiency. Remarkably, the quantum yields of the photocatalytic reactions are still very low. On the one hand, all charge carriers generated upon light absorption have to escape recombination and reach the semiconductor surface. Here they are either trapped or directly transferred to preadsorbed substrate molecules. Finally, a diffusion controlled charge transfer process to suitable reductants or oxidants needs to compete with the surface recombination processes of the trapped charge carriers. Since, on the other hand, many one-electron transfer steps are required to fully oxidize organic substrates into CO2, various chemical intermediates will be formed that subsequently need to be oxidized before leaving the photocatalyst surface. Otherwise, undesired products can be generated in such complex photocatalytic reactions, making control of the overall processes very difficult.

Solar Photocatalytic Hydrogen Production: Current Status and Future Challenges

Due to the increase of the worldwide demand for energy along with the global warming and the increasing level of atmospheric CO2, solar hydrogen has been proposed as an optimal fuel as it can be produced from water using solar energy which emerges as the most promising energy source in terms of abundance and sustainability. So far the main commercial process for producing hydrogen is steam reforming of hydrocarbons which is connected with a CO2 emission disadvantage. Carbon free hydrogen production can be achieved by water splitting through an electrolyser powered by photovoltaics, but a potentially more cost effective route is to perform direct photocatalytic water splitting using semiconductor photocatalysts. Herein, the authors will present the principles of this process, the current progress in the field, and future challenges.

New insights into the surface plasmon resonance (SPR) driven photocatalytic H2 production of Au–TiO2

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.

Understanding charge transfer processes on metal oxides: a laser-flash-photolysis study

In the focus of this study, mixtures of commercially available TiO2 powders were created and their photocatalytic activity concerning the acetaldehyde degradation in the gas phase was tested. Further, the lifetime of the photogenerated charge carriers was analyzed by Laser-Flash-Photolysis-Spectroscopy. The acetaldehyde degradation experiments of the mixed powders lead to positive and negative deviations from the expected weighted mean. Nevertheless, their photocatalytic activity could be correlated with the lifetime of the charge carriers. A longer charge carrier lifetime at ambient conditions correlated with a lower fractional conversion of acetaldehyde. The advantageous activities of the samples were associated with a charge transfer reaction between larger and smaller particles comparable to the antenna mechanism.1