Abstract
Energy shortages and global warming have become two major issues closely associated with the tremendous consumption of non-renewable fossil fuels. As a sustainable and economical route, photocatalytic reduction of CO2 conversion, the so-called artificial photosynthesis, provides an alluring strategy to realize the twofold benefits with respect to closing carbon cycle and producing renewable fuels/chemicals, thereby solving the above issues. TiO2 photocatalysts have attracted widespread attention in CO2 reduction reactions owing to their low cost, high stability, and environmental safety. Nevertheless, the limited absorption ability in the visible light range and fast recombination of photogenerated electrons and holes are the two main drawbacks impeding practical applications. This minireview summarizes the fabrication methodologies of nanostructured TiO2 (especially focused on the 1D, 2D, and 3D nanostructures), discusses the fundamentals of photocatalytic CO2 reduction to value-added chemicals, and draws a comparison of photocatalytic performances from modified TiO2 nanostructures. In further contexts, the opportunities and challenges for nanostructured TiO2 based materials on CO2 conversion are proposed.
Highlights
With the development of industrialization and rapid growth of population, the utilization of fossil fuels is ever increasing, accompanied with the accumulative emissions of greenhouse gas, which intensifies the global energy crisis and anthropogenic climate change.[1,2] Developing effective strategies to solve energy shortage and environmental pollution is, of urgent importance
Mol−1) to be higher than that of C–H (∼430 kJ mol−1) and C–C (∼336 kJ mol−1) bonds,[9] (2) CO2 is chemically inert with a low electron affinity and a large energy gap (13.7 eV) between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO),[10] and (3) CO2 exhibits a low solubility in water.[11]
The typical Photocatalytic CO2 conversion (PCC) process with H2O over TiO2 involves the following three main steps [Fig. 3(b)]:87 (1) H2O and CO2 are adsorbed on the surface of TiO2, (2) the electrons in the valence band (VB) are excited to the conduction band (CB) of TiO2 to produce the photogenerated e−–h+ pairs, and (3) the excited electrons transfer onto the surface of TiO2 and reduce the adsorbed CO2 into solar fuels (e.g., CO, CH4, CH3OH, and HCOOH), while the photogenerated holes oxidize H2O to O2
Summary
With the development of industrialization and rapid growth of population, the utilization of fossil fuels is ever increasing, accompanied with the accumulative emissions of greenhouse gas, which intensifies the global energy crisis and anthropogenic climate change.[1,2] Developing effective strategies to solve energy shortage and environmental pollution is, of urgent importance. As an extensively used semiconductor, TiO2 shows merits in good photostability, high resistance toward corrosion, low cost, environmental friendliness, and easy availability.[18,19,20,21,22] due to the wide bandgap of 3.2 eV and fast recombination of photogenerated electron–hole pairs, TiO2 still suffers from a low quantum efficiency, slow charge transfer, and poor light trapping (3%–5% of the entire solar) on applications of energy harvesting, photocatalysis, and storage.[23,24,25,26] To surmount these aforementioned limitations, numerous strategies [e.g., loading metals or doping with non-metal elements, constructing defects with O vacancies or Ti3+ (black TiO2), and designing heterojunctions with low bandgap materials] have been attempted to improve the performance for PCC in the broad visible light region. Perspectives and challenges for advancing the photocatalytic CO2 conversion over nanostructured TiO2 are covered
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