Large amounts of anthropogenic CO2 emissions associated with increased fossil fuel consumption have led to global warming and an energy crisis. The 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 on the surface of heterogeneous catalysts remain a scientifically critical challenge, which greatly limits the overall photoconversion efficiency and selectivity of CO2 reduction. This review describes recent advances in the fundamental understanding of CO2 photoreduction on the surface of heterogeneous catalysts and particularly provides an overview of enhancing the adsorption/activation of CO2 molecules. The reaction mechanism and pathways of CO2 reduction as well as their dependent factors are also analyzed and discussed, which is expected to enable an increase in the overall efficiency of CO2 reduction through minimizing the reaction barriers and controlling the selectivity towards the desired products. The challenges and perspectives of CO2 photoreduction over heterogeneous catalysts are presented as well.

CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts

The generation of fuels and chemicals from sunlight offers a promising approach for the storage of solar energy. Natural photosynthesis is the best-known example for converting solar energy into chemical energy. In the light-dependent reactions of photosynthesis, the photoinduced electron transport chain from water (H2O) to reduced nicotinamide adenine dinucleotide phosphate (NADPH) is a kind of Z-scheme energetics diagram, which involves two photoexcitation steps and a downhill electron flow from photosystem II (PSII) to photosystem I (PSI) (Figure 1). The Z-scheme pathway is advantageous in realizing efficient charge separation and producing active species with strong reducibility for subsequent reduction of CO2 to hydrocarbons. In practice, artificial photosynthesis is an alternative approach to generate hydrogen energy using sunlight to split Photoelectrocatalytic water splitting offers a promising approach to convert sunlight into sustainable hydrogen energy. A thorough understanding of the relationships between the properties and functions of photoelectrocatalytic materials plays a crucial role in the design and fabrication of efficient photoelectrochemical systems for water splitting. This review presents the advances in the development of efficient photoelectrocatalytic materials. First, the fundamentals involved in the photoelectrocatalytic water splitting are elaborated. Then, the critical properties of photoelectrocatalytic materials are classified and discussed according to the associated photoelectrochemical processes, including light absorption, charge separation, charge transportation, and photoelectrocatalytic reactions. The importance of heterointerfaces in photoelectrodes is also mentioned in conjunction with the illustration of some functional interlayer materials. Finally, some strategies that can be employed in material screening and optimization for the construction of highly efficient photoelectrochemical devices for water splitting are also discussed. Water Splitting

Photoelectrocatalytic Materials for Solar Water Splitting

Water photolysis is a sustainable technology to convert natural solar energy and water into chemical fuels and is thus considered a thorough solution to the forthcoming energy crises. Unassisted water splitting that could directly harvest solar light and subsequently split water in a single device has become an important research theme. Three types of tandem devices including photoelectrochemical (PEC), photovoltaic (PV) cell/PEC and PV/electrolyser tandem cells are proposed to realize water photolysis at different levels of integration and component. Recent progress in tandem water splitting devices is summarized, and crucial issues on device optimization from the perspective of each photo-absorber functionalities in band edge potential, light absorptivity and transmittance are discussed. By increasing the performances of stand-alone PEC or PV devices, a 20% solar to hydrogen efficiency is predicted that is a significant value towards further application in practice. Accordingly, the challenges for materials development and configuration optimization are further outlined.

Water Splitting Progress in Tandem Devices: Moving Photolysis beyond Electrolysis

Photovoltaic cells based on arrays of semiconductor nanowires promise efficiencies comparable or even better than their planar counterparts with much less material. One reason for the high efficiencies is their large absorption cross section, but until recently the photocurrent has been limited to less than 70% of the theoretical maximum. Here we enhance the absorption in indium phosphide (InP) nanowire solar cells by employing broadband forward scattering of self-aligned nanoparticles on top of the transparent top contact layer. This results in a nanowire solar cell with a photovoltaic conversion efficiency of 17.8% and a short-circuit current of 29.3 mA/cm2 under 1 sun illumination, which is the highest reported so far for nanowire solar cells and among the highest reported for III–V solar cells. We also measure the angle-dependent photocurrent, using time-reversed Fourier microscopy, and demonstrate a broadband and omnidirectional absorption enhancement for unpolarized light up to 60° with a wavelength...

High-Efficiency Nanowire Solar Cells with Omnidirectionally Enhanced Absorption Due to Self-Aligned Indium–Tin–Oxide Mie Scatterers

The clean and sustainable CO2 reutilization toward products of higher value is of great interest in a background of established environmental concerns and reducing the use of fossil fuels. As promising alternative fuels, hydrocarbons are more valuable than CO, alcohols or formate and can be directly used in existing infrastructures with high energy densities. The prominent development of catalysts capable of selectively converting CO2 into hydrocarbons, from methane to short olefins and long carbon-chains, has been reflected in an expanding volume of exploratory works, which suitably demand interpretive and continuous revision. In the past decades, conventional studies on the thermochemical conversion of CO2 have consistently unlocked meaningful pathways toward the synthesis of hydrocarbons covering a fairly wide range of molecular weights. Conversely, both electrochemically and photochemically driven reactions have only now started to unveil encouraging results, with an extensive number of critical citations outlining the continuous emergence of very recently published reports. In a field in need of urgent development, the authors provide, in a clear form, a detailed retrospective on benchmark catalysts, pioneering approaches and competitive developments in this subject, mechanistic difficulties, emerging stability issues, and reactor design, while highlighting the latest noteworthy reports. Most importantly, this review highlights the advances toward an increase in the hydrocarbon chain-length in the synthesis of highly competitive alternative fuels. Comparisons of valuable thermochemical, electrochemical and photochemically driven strategies in the conversion of CO2 to hydrocarbons are expected to serve as guidelines to disclose promising pathways in a field where mechanistic uncertainties remain a bottleneck for determining the product selectivity. The authors summarize leading and inquisitive perspectives with a focus on the viability and practicability of each approach at a larger scale, while tentatively paving the way to stimulate progress in this field.

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From CO2 methanation to ambitious long-chain hydrocarbons: alternative fuels paving the path to sustainability

The solar energy conversion efficiency of photoelectrochemical (PEC) devices is usually limited by poor interface energetics, limiting the onset potential, and light reflection losses. Here, a three-pronged approach to obtain excellent performance of an InP-based photoelectrode for water reduction is presented. First, a buried p-n+ junction is fabricated, which shifts the valence band edge favorably with respect to the hydrogen redox potential. Photoelectron spectroscopy substantiates that the shift of the surface photovoltage is mainly determined by the buried junction. Second, a periodic array of InP nanopillars is created at the surface of the photoelectrode to substantially reduce the optical reflection losses. This device displays an unprecedented photocathodic power-saved efficiency of 15.8% for single junction water reduction. Third, a thin TiO2 protection layer significantly increases the stability of the InP-based photoelectrode. Careful design of the interface energetics based on surface photovoltage spectroscopy allows obtaining a PEC cell with stable record performance in water reduction. The efficiency of photoelectrochemical (PEC) water splitting devices is usually limited by interface energetics mismatch between the semiconductor and the electrolyte and insufficient light absorption. The highest photocathodic power-saved efficiency of 15.8% reported so far for single junction PEC water reduction is obtained through interface energetics design and photon management of an InP-based photoelectrode.

High‐Efficiency InP‐Based Photocathode for Hydrogen Production by Interface Energetics Design and Photon Management

Superconducting nanowire single-photon detectors with peak efficiencies above 90% and unrivalled timing jitter (<30 ps) have emerged as a potent technology for quantum information and sensing applications. However, their high cost and cryogenic operation limit their widespread applicability. Here, we present an approach using tapered InP nanowire p–n junction arrays for high-efficiency, broadband and high-speed photodetection without the need for cryogenic cooling. The truncated conical nanowire shape enables a broadband, linear photoresponse in the ultraviolet to near-infrared range (~500 nm bandwidth) with external quantum efficiencies exceeding 85%. The devices exhibit a high gain beyond 105, such that a single photon per pulse can be distinguished from the dark noise, while simultaneously showing a fast pulse rise time (<1 ns) and excellent timing jitter (<20 ps). Such detectors open up new possibilities for applications in remote sensing, dose monitoring for cancer treatment, three-dimensional imaging and quantum communication. Tapered InP nanowire p–n junction arrays enable efficient broadband and high-speed room-temperature photodetection.

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Tapered InP nanowire arrays for efficient broadband high-speed single-photon detection

Understanding light absorption in individual nanostructures is crucial for optimizing the light-matter interaction at the nanoscale. Here, we introduce a technique named time-reversed Fourier microscopy that enables the measurement of the angle-dependent light absorption in dilute arrays of uncoupled semiconductor nanowires. Because of their large separation, the nanowires have a response that can be described in terms of individual nanostructures. The geometry of individual nanowires makes them behave as nanoantennas that show a strong interaction with the incident light. The angle-dependent absorption measurements, which are compared to numerical simulations and Mie scattering calculations, show the transition from guided-mode to Mie-resonance absorption in individual nanowires and the relative efficiency of these two absorption mechanisms in the same nanostructures. Mie theory fails to describe the absorption in finite-length vertical nanowires illuminated at small angles with respect to their axis. At...

Dick van Dam

Papers

High-Efficiency Nanowire Solar Cells with Omnidirectionally Enhanced Absorption Due to Self-Aligned Indium–Tin–Oxide Mie Scatterers

High‐Efficiency InP‐Based Photocathode for Hydrogen Production by Interface Energetics Design and Photon Management

Tapered InP nanowire arrays for efficient broadband high-speed single-photon detection

Nanowire Antenna Absorption Probed with Time-Reversed Fourier Microscopy