Capturing the Light Fantastic

Abstract

Artificial photosynthesis began as a serious research topic in the late 1970s as the oil crisis reached a critical stage. Early work focussed on the identification of photochemical systems for the evolution of either hydrogen gas or oxygen gas and usually involved precious metal catalysts and respective sacrificial electron donors or acceptors. The photochemical reduction of CO2 was soon recognised as being a highly prized goal but the experimental conditions needed for an effective system were unsuited for practical exploitation. Parallel to this rather applied research ran a theme to design and synthesize highly imaginative bio-inspired mimics that resembled the structures and functions of natural photosynthesis. This work led to massive advances in our understanding of electronic energy transfer and light-induced electron transfer between weakly coupled donor–acceptor pairs. As the price of oil stabilised and then fell, research in artificial photosynthesis took on a more fundamental nature and the synthesis of evermore complicated molecular architectures replaced the optimisation of solar fuel production. Photovoltaic cells, not normally associated with artificial photosynthesis, took centre stage in terms of realistic approaches for the harvesting of sunlight. Over the intervening years, there has been a progressive growth and diversification of photovoltaic devices. We have also witnessed the rapid rise of public concerns about climate change and global warming due to greenhouse gases. There has been a growing recognition that solar electricity production is insufficient for our needs and that there must be an avenue for the sustainable production of chemical feedstocks and fuels from readily available materials driven entirely from renewable energy sources. It is hard to imagine any sustainable photochemical cycle that does not include water as the electron and proton source, such as hydrogen evolution, or reduction of CO2 as well as N2 fixation. Options exist, however, for the complementary reductive process as long as the product can be collected, stored or utilised on-site. To be competitive with existing protocols, solar fuels production has to be transformed from the test tube to an industrial plant and from the microliter to the cubic meter scale. These are critical problems that demand urgent solutions. This special issue of ChemPhotoChem represents an opportunity to establish the current state-of-the-art in the field of Artificial Photosynthesis and to identify both barriers and promising routes for future development. Contributions address each of the key subjects, including both heterogeneous (i.e., semiconductor-based) and homogeneous (i.e., photosensitizer-based) systems. The photochemical reduction of CO2 receives well-deserved attention as an alternative to hydrogen production but we still lack ideas on how to reduce nitrogen under ambient conditions. Hybrid photosystems and the application of dedicated light-harvesting machinery are advancing quickly and could offer important benefits in terms of stability and versatility. Energy storage in the form of chemical transformations, often overlooked in the quest to design innovative strategies for solar energy conversion, makes a timely entry. Water oxidation, possibly the most important single step in the overall energy transduction cycle, gets a fresh look. These contributions are a valuable resource and offer a potential stepping stone to a future fuelled by energy from the sun. Anthony Harriman started research into bioinspired artificial photosynthesis in the late 1970s, working with Lord George Porter at the Royal Institution in London. Research centred on using metalloporphyrins to sensitise water reduction or oxidation and on the development of effective catalysts for oxygen evolution. Subsequent studies were aimed at seeking correlations between rates of light-induced electron or energy transfer and molecular topology. On-going work looks at ways to construct artificial light-harvesting arrays as universal solar concentrators. Dr. Haruo Inoue is a specially appointed full professor of Applied Chemistry at Tokyo Metropolitan University and serves also as a Director of the Center for Artificial Photosynthesis. He has been serving as the Research Supervisor of Precursory Research for Embryonic Science and Technology Project (PRSETO/JST) on “Chemical Conversion of Light Energy” and has been leading the All-Nippon Artificial Photosynthesis Project for Living Earth (AnApple: MEXT/Japan). Prof. Licheng Sun received his Ph.D. in 1990 from Dalian University of Technology (DUT), and went to Germany as a postdoc at Max-Planck-Institut für Strahlenchemie with Dr. Helmut Görner (1992–1993), and then as an Alexander von Humboldt postdoc at Freie Universität Berlin (1993–1995) with Prof. Dr. Harry Kurreck. He moved to KTH Royal Institute of Technology, Stockholm in 1995 as a postdoc with Prof. Björn Åkermark, became assistant professor in 1997, associate professor in 1999 at Stockholm University and full professor in 2004 at KTH. He is presently also a distinguished professor at DUT. His research interests cover artificial photosynthesis, including dye sensitized solar cells, perovskite solar cells, bio-inspired catalysts for water oxidation and hydrogen generation, nanomaterials and photoelectrochemical cells for water splitting and CO2 reduction.