Improving the Water Oxidation Efficiency with a Light-Induced Electric Field in Nanograting Photoanodes

Enhanced Light Absorption and Efficient Carrier Collection in MoS2 Monolayers on Au Nanopillars

We fabricated hybrid nanostructures consisting of MoS₂ monolayers and Au nanopillar (Au-NP) arrays. The surface morphology and Raman spectra showed that the MoS₂ flakes transferred onto the Au-NPs were very flat and nonstrained. The Raman and photoluminescence intensities of MoS₂/Au-NP were 3- and 20-fold larger than those of MoS₂ flakes on a flat Au thin film, respectively. The finite-difference time-domain calculations showed that the Au-NPs significantly concentrated the incident light near their surfaces, leading to broadband absorption enhancement in the MoS₂ flakes. Compared with a flat Au thin film, the Au-NPs enabled a 6-fold increase in the absorption in the MoS₂ monolayer at a wavelength of 615 nm. The contact potential difference mapping showed that the electric potential at the MoS₂/Au contact region was higher than that of the suspended MoS₂ region by 85 mV. Such potential modulation enabled the Au-NPs to efficiently collect photogenerated electrons from the MoS₂ flakes, as revealed by the uniform positive surface photovoltage signals throughout the MoS₂ surface.

Black 3D-TiO2 Nanotube Arrays on Ti Meshes for Boosted Photoelectrochemical Water Splitting

Black 3D-TiO2 nanotube arrays are successfully fabricated on the Ti meshes through a facile electrochemical reduction method. The optimized black 3D-TiO2 nanotubes arrays yield a maximal photocurrent density of 1.6 mA/cm2 at 0.22 V vs. Ag/AgCl with Faradic efficiency of 100%, which is about four times larger than that of the pristine 3D-TiO2 NTAs (0.4 mA/cm2). Such boosted PEC water splitting activity primarily originates from the introduction of the oxygen vacancies, which results in the bandgap shrinkage of the 3D-TiO2 NTAs, boosting the utilization efficiency of visible light including the incident, reflected and/or refracted visible light captured by the 3D configuration. Moreover, the oxygen vacancies (Ti3+) can work as electron donors, which leads to the enhanced electronic conductivity and upward shift of the Fermi energy level, and thereby facilitating the transfer and separation of the photogenerated charge carrier at the semiconductor-electrolyte interface. This work offers a new opportunity to promote the PEC water splitting activity of TiO2-based photoelectrodes.

Nanoarray Structures for Artificial Photosynthesis

Conversion and storage of solar energy into fuels and chemicals by artificial photosynthesis has been considered as one of the promising methods to address the global energy crisis. However, it is still far from the practical applications on a large scale. Nanoarray structures that combine the advantages of nanosize and array alignment have demonstrated great potential to improve solar energy conversion efficiency, stability, and selectivity. This article provides a comprehensive review on the utilization of nanoarray structures in artificial photosynthesis of renewable fuels and high value-added chemicals. First, basic principles of solar energy conversion and superiorities of using nanoarray structures in this field are described. Recent research progress on nanoarray structures in both abiotic and abiotic-biotic hybrid systems is then outlined, highlighting contributions to light absorption, charge transport and transfer, and catalytic reactions (including kinetics and selectivity). Finally, conclusions and outlooks on future research directions of nanoarray structures for artificial photosynthesis are presented.

Plasmon-Enhanced Resonant Photoemission Using Atomically Thick Dielectric Coatings

By proposing an atomically thick dielectric coating on a metal nanoemitter, we theoretically show that the optical field tunneling of ultrafast-laser-induced photoemission can occur at an ultralow incident field strength of 0.03 V/nm. This coating strongly confines plasmonic fields and provides secondary field enhancement beyond the geometrical plasmon field enhancement effect, which can substantially reduce the barrier and enable more efficient photoemission. We numerically demonstrate that a 1 nm thick layer of SiO₂ around a Au-nanopyramid will enhance the resonant photoemission current density by 2 orders of magnitude, where the transition from multiphoton absorption to optical field tunneling is accessed at an incident laser intensity at least 10 times lower than that of the bare nanoemitter. The effects of the coating properties such as refractive index, thickness, and geometrical settings are studied, and tunable photoemission is numerically demonstrated by using different ultrafast lasers. Our approach can also directly be extended to nonmetal emitters, to-for example-2D material coatings, and to plasmon-induced hot carrier generation.