Slow photons for solar fuels

doi:10.1016/S1872-2067(17)62930-9

Chinese Journal of Catalysis

Volume 39, Issue 3, March 2018, Pages 379-389

https://doi.org/10.1016/S1872-2067(17)62930-9 Get rights and content

Abstract

Converting solar energy into hydrogen and hydrocarbon fuels through photocatalytic H₂ production and CO₂ photoreduction is a highly promising approach to address growing demand for clean and renewable energy resources. However, solar-to-fuel conversion efficiencies of current photocatalysts are not sufficient to meet commercial requirements. The narrow window of solar energy that can be used has been identified as a key reason behind such low photocatalytic reaction efficiencies. The use of photonic crystals, formed from multiple material components, has been demonstrated to be an effective way of improving light harvesting. Within these nanostructures, the slow-photon effect, a manifestation of light-propagation control, considerably enhances the interaction between light and the semiconductor components. This article reviews recent developments in the applications of photonic crystals to photocatalytic H₂ production and CO₂ reduction based on slow photons. These advances show great promise for improving light harvesting in solar-energy conversion technologies.

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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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].
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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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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).

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Review (Special Issue of Photocatalysis for Solar Fuels)Slow photons for solar fuels

Abstract

Graphical Abstract

Introduction

Section snippets

Photonic crystals and slow photons

Solar-to-H2 energy conversion

CO2 photoreduction

Conclusions and perspectives

Energy Buildings

Progr. Energy Combust.

Sol. Energy Mater. Sol. C

Appl. Catal. B

Opt. Commun.

Appl. Catal. B

J. Alloy. Compd.

Appl. Catal. B

Appl. Catal. B

Appl. Catal. B

Catal. Today

Energy Environ. Sci.

Science

Nature

Nature

Energy Environ. Sci.

Energy Environ. Sci.

Nanoscale

J. Phys. Chem. B

Chem. Commun.

Chem. Soc. Rev.

Adv. Mater.

Energy Environ. Sci.

Molecular Photochemistry-Various Aspects

Energy Environ. Sci.

J. Photon. Energy

Nature

J. Am. Chem. Soc.

Phys. Chem. Chem. Phys.

Part. Part. Syst. Char.

J. Am. Chem. Soc.

Sci. Technol. Adv. Mat.

J. Mater. Chem. A

Environ. Sci. Technol.

J. Phys. Chem. B

Phys. Chem. Chem. Phys.

Appl. Phys. Lett.

Phys. Rev. Lett.

Opt. Express

Opt. Express

Chin. Phys. Lett.

Opt. Express

Appl. Phys. B

Review (Special Issue of Photocatalysis for Solar Fuels)
Slow photons for solar fuels

Solar-to-H₂ energy conversion

CO₂ photoreduction