Gap-plasmon enhanced water splitting with ultrathin hematite films: the role of plasmonic-based light trapping and hot electrons

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Molecular Plasmonics with Metamaterials

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

Latest Advances in Metasurfaces for SERS and SEIRA Sensors as Well as Photocatalysis

Metasurfaces can enable the confinement of electromagnetic fields on huge surfaces and zones, and they can thus be applied to biochemical sensing by using surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA). Indeed, these metasurfaces have been examined for SERS and SEIRA sensing thanks to the presence of a wide density of hotspots and confined optical modes within their structures. Moreover, some metasurfaces allow an accurate enhancement of the excitation and emission processes for the SERS effect by supporting resonances at frequencies of these processes. Finally, the metasurfaces allow the enhancement of the absorption capacity of the solar light and the generation of a great number of catalytic active sites in order to more quickly produce the surface reactions. Here, we outline the latest advances in metasurfaces for SERS and SEIRA sensors as well as photocatalysis.

Broadband Tamm Plasmons in Chirped Photonic Crystals for Light-Induced Water Splitting

An electrode of a light-induced cell for water splitting based on a broadband Tamm plasmon polariton localized at the interface between a thin TiN layer and a chirped photonic crystal has been developed. To facilitate the injection of hot electrons from the metal layer by decreasing the Schottky barrier, a thin n-Si film is embedded between the metal layer and multilayer mirror. The chipping of a multilayer mirror provides a large band gap and, as a result, leads to an increase in the integral absorption from 52 to 60 percent in the wavelength range from 700 to 1400 nm. It was shown that the photoresponsivity of the device is 32.1 mA/W, and solar to hydrogen efficiency is 3.95%.

Host/Guest Nanostructured Photoanodes Integrated with Targeted Enhancement Strategies for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) hydrogen production from water splitting is a green technology that can solve the environmental and energy problems through converting solar energy into renewable hydrogen fuel. The construction of host/guest architecture in semiconductor photoanodes has proven to be an effective strategy to improve solar-to-fuel conversion efficiency dramatically. In host/guest photoanodes, the absorber layer is deposited onto a high-surface-area electron collector, resulting in a significant enhancements in light-harvesting as well as charge collection and separation efficiency. The present review aims to summarize and highlight recent state-of-the-art progresses in the architecture designing of host/guest photoanodes with integrated enhancement strategies, including i) light trapping effect; ii) optimization of conductive host scaffolds; iii) hierarchical structure engineering. The challenges and prospects for the future development of host/guest nanostructured photoanodes are also presented.

Preparation of multilayer periodic nanopatterned WO3-based photoanode by reverse nanoimprinting for water splitting

Photoelectrochemical (PEC) water splitting has been studied extensively as an environmentally friendly technology for hydrogen production using solar energy. WO₃is considered a promising semiconducting material for photoanodes due to its high electron mobility, good hole diffusion length, and chemical stability. Periodic nanostructures of WO₃have been investigated for enhancing the PEC performance of WO₃-based photoanodes. In this study, facile fabrication of periodic nanostructures of WO₃was achieved using reverse nanoimprint lithography, and the multilayer stacking of nanopatterned WO₃film was also confirmed. The multilayer nanopatterned WO₃films were used as photoanodes for PEC water splitting. The performance of the fabricated photoanode in PEC was 2 times higher than that of planar WO₃film due to its higher light absorbance and lower charge transfer resistance.

Plasmon-Induced Water Splitting-through Flexible Hybrid 2D Architecture up to Hydrogen from Seawater under NIR Light

The efficient utilization of solar energy is the actual task of the present and near future. Thus, the preparation of appropriate materials that are able to harvest and utilize the broad wavelength range of solar light (especially commonly ignored near-infrared light region-NIR) is the high-priority challenging mission. Our study provides a rationally designed two-dimensional (2D) flexible heterostructures with photocatalytic activity for the production of "clean" hydrogen under NIR illumination, with the hydrogen production rate exceeding most 2D materials and the ability to use the seawater as a starting material. The proposed design utilizes the hybrid bimetallic (Au/Pt) periodic structure, which is further covalently grafted with a metal-organic framework MIL-101(Cr). The periodic gold structure is able to efficiently support the plasmon-polariton wave and to excite the hot electrons, which is further injected in the Pt and MIL-101(Cr) layers. The Pt and MIL-101(Cr) structures provide catalytic sites, which are saturated with hot electrons and efficiently initiate water splitting and hydrogen production. The MIL-101(Cr) layer also serves for repelling generated hydrogen bubbles. The mechanistic studies reveal the catalytic role of every element of the 2D flexible heterostructures. The maximum hydrogen output was achieved under plasmon resonance excitation in the NIR range, and it could be actively controlled by the applied LED wavelength.

Optimizing and Implementing Light Trapping in Thin-Film, Mesostructured Photoanodes

Stable semiconductor photoelectrodes for water splitting often exhibit long absorption lengths and poor properties for the efficient separation and transport of photogenerated charges. We propose a combination of resonant and geometric light trapping for thin-film, mesostructured α-Fe₂O₃ photoanodes to engineer enhanced light management and increase the photocurrent density. Simulations of the electromagnetic wave propagation on accurate mesostructures were used to optimize the semiconductor film thickness and the electrode morphology for maximum light absorption. Local photocurrent densities at the semiconductor-electrolyte interface were calculated via a probabilistic charge collection model. The findings of the numerical model were translated into photoanodes by a novel fabrication process based on template stripping. The developed experimental platform is versatile and enables to fabricate electrodes with various shapes and precise control on the mesostructure. We successfully demonstrated the fabrication of α-Fe₂O₃ photoanodes with arrays of wedge structures in the micrometer range on a flexible substrate that benefits from resonant and geometric light trapping.

Synthesis of Plasmonic Photocatalysts for Water Splitting

Production of H2, O2, and some useful chemicals by solar water splitting is widely expected to be one of the ultimate technologies in solving energy and environmental problems worldwide. Plasmonic enhancement of photocatalytic water splitting is attracting much attention. However, the enhancement factors reported so far are not as high as expected. Hence, further investigation of the plasmonic photocatalysts for water splitting is now needed. In this paper, recent work demonstrating plasmonic photocatalytic water splitting is reviewed. Particular emphasis is given to the fabrication process and the morphological features of the plasmonic photocatalysts.