Review (Special Issue of Photocatalysis for Solar Fuels)Slow photons for solar fuels
Graphical Abstract
This article reviews recent developments in the applications of photonic crystals to photocatalytic H2 production and CO2 reduction based on slow photons, highlighting promising approaches towards improving light harvesting in solar-energy-conversion technologies.
Introduction
Human civilization is powered mainly by fossil fuels such as oil, coal, and natural gas. However, the combustion of these fuels causes environmental pollution and considerable CO2 emissions. Solar power is widely recognized as a promising alternative to fossil-fuel-based energy sources. Considerable efforts have been made in the study, design, and synthesis of novel materials that can convert solar energy into heat, electricity, and chemical energy 1, 2, 3, 4. One such strategy is photocatalytic splitting of water molecules to generate hydrogen or to drive the reduction of CO2 into valuable hydrocarbon fuels 5, 6, 7, 8. Currently, a myriad of materials has been shown to be potential candidates for such photocatalytic reactions. However, owing to limited light absorption and high rates of charge carrier recombination, the conversion efficiencies of current photocatalysts remain too low to meet commercial requirements. To enhance light harvesting, one important approach is to improve the interaction of light with the semiconductor, by manipulating light propagation in the various material structures. For instance, multiple scattering can be used to induce more photons to be absorbed under given incident light conditions 9, 10. Random scattering by large particles [11] or spherical voids [12] has also been applied in dye-sensitized solar cells. Furthermore, hierarchically structured porous materials provide interconnected porosity at different length scales, which is favorable for light harvesting 13, 14, 15. Photonic crystals are formed by specific periodic arrangements of dielectric materials, which have a unique role in regulating light, through light reflection, scattering, and the slow photon effect. The phenomena allow control over light propagation in the medium structure.
Photonic crystals are the best materials devised for light manipulation. Several papers have reviewed photochemical applications of photonic crystals, focused mainly on photocatalysis 14, 16, 17, 18 and photovoltaics 14, 17. The slow photon effect has also been reviewed from the viewpoint of photocatalytic degradation 14, 16. In this review, we discuss both theoretically and experimentally the importance of the slow-photon effect in light-harvesting enhancement and its applications in solar-to-fuel energy conversion, i.e., photocatalytic H2 production and CO2 photoreduction. The photoreactivity enhancement of slow photons is highlighted and we discuss the potential for making considerable improvements to light harvesting through several strategies, which are likely to attract attention in the near future.
Section snippets
Photonic crystals and slow photons
Photonic crystals are periodic ordered structures in space composed of two or more materials with different dielectric constants. When light propagates inside a photonic crystal, the periodic Bragg scattering modulates light to form a photonic band gap (PBG), which is analogous to the electronic bandgap in semiconductor materials. In photonic crystals, light with certain frequencies is forbidden from propagating owing to the photonic stop band arising from the periodic modulation of the
Solar-to-H2 energy conversion
Energy and environmental problems are well-known currently issues. Hydrogen fuel is a clean energy source that could provide the ultimate solution to many pollution problems. In particular, hydrogen evolution from water splitting powered by renewable solar energy represents a promising but challenging method to a clean, sustainable and affordable energy system. In this review, photocatalysts are structured into photonic crystal architectures to maximize the usage and conversion of light energy.
CO2 photoreduction
Large amounts of anthropogenic CO2 emissions associated with increased fossil fuel consumption have led to global warming and an energy crisis. Photocatalytic reduction of CO2 into solar fuels such as methane or methanol is believed to be one of the best methods to address these two problems. In addition to light harvesting and charge separation, the adsorption/activation and reduction of CO2 at the surface of photocatalysts remains a critical challenge, which greatly limits overall
Conclusions and perspectives
To date, considerable achievements have been made in the design and fabrication of photonic crystal photocatalysts for efficient photocatalytic H2 production and CO2 reduction. Photonic crystals show great promise for improving solar-to-fuel conversion efficiencies through the slow photon effect, which has tremendous benefits for light harvesting. To enhance the slow photon effect, the photonic crystal stop band is tuned to the semiconductor band edge or the excitation maxima of the sensitizer
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2020, Applied Surface ScienceCitation Excerpt :Light utilization rate is an important parameter in photocatalysis. Photonic crystal (PC) is an orderly periodic structure that can confine the light propagation in a certain range of wavelength called photonic band gap (PBG) [32,33]. On the score of Bragg diffraction and scattering, incident light within the wavelength region of PBG will be reflected repeatedly in the PC structure and finally go outside [34,35].
Design of a ZnO/Poly(vinylidene fluoride) inverse opal film for photon localization-assisted full solar spectrum photocatalysis
2020, Chinese Journal of CatalysisCitation Excerpt :Therefore, incorporating materials with good heat-absorbing abilities into photocatalysts and drawing upon visible light and infrared radiation which photocatalysts cannot absorb, could be a feasible solution to the problem [28–30]. Photonic crystal (PC) is a periodic structure that can confine light propagation to the spectral region called the photonic band gap (PBG) [31,32]. Among the different types of PC, opal and inverse opal (IO) structures have been widely researched due to their excellent optical performance.
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2019, Chinese Journal of CatalysisCitation Excerpt :The result showed that the addition of surfactants played an important role in the formation of mesopores on the 3DOM skeleton. As a kind of photonic crystals, the three-dimensionally ordered spatial lattice structures of 3DOM materials denote obvious photonic band gap effect [39] and the slow photon effect [40,41]. The incident light with certain wavelengths will be hindered from propagating through the 3DOM materials along a specific crystal direction results in the stop-band reflection, also called photonic band gap [42].
Ni-P cluster modified carbon nitride toward efficient photocatalytic hydrogen production
2019, Chinese Journal of CatalysisCitation Excerpt :With increasing concerns of global energy shortage, it is realized urgent for the development of sustainable technology for producing renewable energy. One of the attractive approaches is the H2 generation from proton reduction driven by semiconductor photocatalysis [1–5]. The choice of a high-performance and stable photocatalyst is critical in this approach.
Published 5 March 2018
This work was supported by the National Natural Science Foundation of China (21607027, 21507011 and 21677037), and Ministry of Science and Technology of the People's Republic of China (2016YFE0112200).