Dr Yiyang Li

Abstract:

Solar light-driven water splitting provides a promising way to store and use abundant solar energy in the form of gaseous hydrogen which is the cleanest chemical fuel for mankind; therefore this field has been attracting increasing attention over the past decades. The fundamental steps for efficient photocatalyst for water splitting include uptake of photons of targeted energy range by appropriate electronic band structure, excited electrons and holes (excitons) migration, as well as recombination and selective conversion excited electrons for H+ reduction to H2 and holes and OH− to O2 on catalyst surface. Each step if not efficiently taken place could hamper the overall photocatalytic activity. Numerous semiconductors with appropriate bandgaps have mainly been developed as candidates for effective solar energy capture, whereas at present, their low quantum efficiency still remains as the major obstacle in further applications. In this minireview, we will disentangle the progress to develop photocatalysts with good photon uptake from photocatalytic water splitting performance. In accordance with the thermodynamic and kinetic considerations of the photocatalytic water splitting reaction, different strategies for improving the fundamental processes have been briefly reviewed. Some recent advances in facilitating charge carriers separation have also been presented. Photocatalytic water splitting at elevated temperatures is emphasized as a novel approach to suppress photo-excitons recombination on catalyst surface owing to adsorption of enhanced concentration of ionic species including H+ and OH− to create their local polarization to the excitons. Stronger polarization to hinder the excitons recombination can also be obtained by using polar-faceted support materials to the active phase of semiconductor. It is clearly demonstrated in this minireview that such high temperature–promoted photocatalytic water splitting systems could open up a new direction and provide a new innovation to this field.

Abstract:

Photocatalytic water splitting is attracting enormous interest for the storage of solar energy but no practical method has yet been identified. In the past decades, various systems have been developed but most of them suffer from low activities, a narrow range of absorption and poor quantum efficiencies (Q.E.) due to fast recombination of charge carriers. Here we report a dramatic suppression of electron-hole pair recombination on the surface of N-doped TiO2 based nanocatalysts under enhanced concentrations of H+ and OH−, and local electric field polarization of a MgO (111) support during photolysis of water at elevated temperatures. Thus, a broad optical absorption is seen, producing O2 and H2 in a 1:2 molar ratio with a H2 evolution rate of over 11,000 μmol g−1 h−1 without any sacrificial reagents at 270 °C. An exceptional range of Q.E. from 81.8% at 437 nm to 3.2% at 1000 nm is also reported.

Abstract:

The high activity of nanocrystallites is commonly attributed to the terminal high-energy facets. However, we demonstrate that the high activity of the anatase TiO2(001) facet in photocatalytic H2 evolution is not due to its high intrinsic surface energy, but local electronic effects created by surface features on the facet.

Abstract:

Environmental and energy issues have become one of the most important issues that human beings are facing, and carbon neutrality is a communal target worldwide nowadays. With the strong international incentives to decarbonise our fuels and chemicals, the solar-light-driven photocatalytic overall water splitting (POWS) reaction provides a promising and renewable way to store and utilise the abundant solar energy in the form of hydrogen which is the cleanest chemical fuel for mankind. Numerous materials and catalytic systems have been developed for more effective solar energy conversion, however, the performance achieved so far is still unsatisfactory.

In this context, this thesis has particularly focused on the design and development of novel POWS systems at elevated temperatures. Different semiconductor materials such as TiO2 and MoS2 were systematically studied in this thesis. In- depth catalytic activity studies illustrated the temperature effect on the POWS performance. Comprehensive surface characterisations combined with a series of spectroscopic, imaging and computational techniques were conducted to unravel the relationship between the activity and the structures of the catalysts.

Subsequently, various strategies have been proposed for improving the POWS activity at elevated temperatures, including the use of local electric field and local magnetic field. Remarkable charge polarisation effects have been identified in each case. Finally, it is demonstrated that the POWS system studied in this thesis could split seawater into H2 and O2 with extraordinarily high solar-to-hydrogen conversion efficiency of 20.3 % at 270 oC, exhibiting high potential for future practical applications.