Plasmon-induced photonic and energy-transfer enhancement of solar water splitting by a hematite nanorod array

Nanoengineered Metasurface Immunosensor with over 1000-Fold Electrochemiluminescence Enhancement for Ultra-sensitive Bioassay

Enhancing electrochemiluminescence (ECL) with plasmonic materials is promising but still a long-standing barrier to improve its sensitivity for ultrasensitive bioassays, due to the lack of comprehensive understanding and effective strategies to fully utilize plasmonic effects for ECL enhancement. Herein, by insulating gold nanoparticles with silica shells (Au@SiO₂ NPs), and finely tuning their core/shell sizes and controlling interparticle spacing via assembling them into a dense nanomembrane, we develop a novel 2D metasurface. Due to well-controlled high density “hot spots” and 2D ordered arrangement of the unit NPs in the nanomembrane, the metasurfaced ECL electrode shows over 1,000-fold plasmonic ECL enhancement for the classical Ru(bpy)₃²⁺-tripropylamine system, which is two orders of magnitude higher than ever reported (<30-fold). Such fabricated ECL biosensor demonstrates superior detection performance for prostate-specific antigen with a detection limit of 3 fg mL⁻¹. Our results provide understanding of plasmonic effects for ECL enhancement and will benefit for biosensor construction for ultrasensitive bioassays.

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A unique Au@SiO₂ NP-based 2D metamaterial was constructed

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The plasmon effects were fully utilized to enhance ECL excitation

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The as-fabricated metasurfaced ECL electrode shows over 1,000-fold enhancement

Fabrication of Nickel Oxide Nanopillar Arrays on Flexible Electrodes for Highly Efficient Perovskite Solar Cells

Semiconductor nanomaterials with controlled morphologies and architectures are of critical importance for high-performance optoelectronic devices. However, the fabrication of such nanomaterials on polymer-based flexible electrodes is particularly challenging due to degradation of the flexible electrodes at a high temperature. Here we report the fabrication of nickel oxide nanopillar arrays (NiO _x NaPAs) on a flexible electrode by vapor deposition, which enables highly efficient perovskite solar cells (PSCs). The NiO _x NaPAs exhibit an enhanced light transmittance for light harvesting, prohibit exciton recombination, promote irradiation-generated hole transport and collection, and facilitate the formation of large perovskite grains. These advantageous features result in a high efficiency of 20% and 17% for the rigid and flexible PSCs, respectively. Additionally, the NaPAs show no cracking after 500 times of bending, consistent with the mechanic simulation results. This robust fabrication opens a new opportunity for the fabrication of a large area of high-performance flexible optoelectronic devices.

Epitaxial Bi2FeCrO6 Multiferroic Thin-Film Photoanodes with Ultrathin p-Type NiO Layers for Improved Solar Water Oxidation

The photoelectric properties of multiferroic double-perovskite Bi₂FeCrO₆ (BFCO), such as above-band gap photovoltages, switchable photocurrents, and bulk photovoltaic effects, have recently been explored for potential applications in solar technology. Here, we report the fabrication of photoelectrodes based on n-type ferroelectric (FE) semiconductor BFCO heterojunctions coated with p-type transparent conducting oxides (TCOs) by pulsed laser deposition and their application for photoelectrochemical (PEC) water oxidation. The photocatalytic properties of the bare BFCO photoanodes can be improved by controlling the FE polarization state. However, the charge recombination as well as the limited charge transfer kinetics in the photoanode/electrolyte cause major energy loss and thus hinder the PEC performance. We show that this problem may be addressed by the deposition of an ultrathin p-type NiO layer on the photoanode to enhance the charge transport kinetics and reduce charge recombination at surface-trapped states for increased surface band bending. A fourfold enhancement of photocurrent density, up to 0.4 mA cm^-2 (at +1.23 V vs RHE), a best performance of stability over 4 h, and a high incident photon-to-current efficiency (∼3.7%) were achieved under 1 sun illumination in such p-NiO/n-BFCO heterojunction photoanodes. These studies reveal the optimization of PEC performance by polarization switching of BFCO and the successful achievement of p-TCOs/n-FE heterojunction photoanodes that are able to sustain water oxidation that is stable for many hours.

Wide spectrum photocatalytic activity in lanthanide-doped upconversion nanophosphors coated with porous TiO2 and Ag-Cu bimetallic nanoparticles

Approaches towards maximum utilization of solar light spectrum for photocatalysis have currently attracted great interest. The combination of profoundly different properties, such as, upconversion, semiconducting and plasmonic properties can produce a favorable path in efficient utilization of the different regions of solar light reaching to earth. In this regard, design and fabrication of microstructures consisting of upconverting lanthanide doped nanophosphors coated with porous semiconducting material, TiO₂ and decorated with plasmonic Ag-Cu bimetallic nanoparticles is presented in this work. These microstructures display great stability and exceptional photocatalytic activity by absorbing wide spectrum from ultraviolet to near infrared. The photocatalytic activity could be attributed to the synergistic effects between the different components and the efficient energy transfer between them. The development of such sort of hybrid microstructures could pave way for the development of new materials for the efficient utilization of the wide spectrum of sunlight.

Gold-Sensitized Silicon/ZnO Core/Shell Nanowire Array for Solar Water Splitting

Solar water splitting represents one of the most promising strategies in the quest for clean and renewable energy. However, low conversion efficiency, use of sacrificial agents, and external bias for current water splitting system limit its practical application. Here, a gold-sensitized Si/ZnOcore/shell nanowire photoelectrochemical (PEC) cell is reported for efficient solar water oxidation. We demonstrated gold-sensitized n-Si/n-ZnO nanowire arrays exhibited higher energy conversion efficiency than gold-sensitized p-Si/n-ZnO nanowire arrays due to the favorable energy-band alignment characteristics. Without any assistance from an external electrical source and sacrificial reagents, gold-sensitized n-Si/n-ZnO core/shell nanowire array photoanode achieved unbiased water splitting under simulated solar light illumination. This method opens a promising venue to cost-efficient production of solar fuels.

Photodetection by Hot Electrons or Hot Holes: A Comparable Study on Physics and Performances

Hot-carrier photodetectors are drawing significant attention; nevertheless, current researches focus mostly on the hot-electron devices, which normally show low quantum efficiencies. In contrast, hot-hole photodetectors usually have lower barriers and can provide a wide spectral range of photodetection and an improved photoconversion efficiency. Here, we report a comparable study of the hot-electron and hot-hole photodetectors from both underlying physics and optoelectronic performance perspectives. Taking the typical Au/Si Schottky contact as an example, we find obvious differences in the energy band diagram and the sequent hot-carrier generation/transport/emission processes, leading to very distinguished photodetection performances. Compared with hot electrons, hot holes show higher density below the Fermi level, the longer mean free path arising under the lower electron-electron and electron-phonon scatterings, a lower barrier height, and a lighter effective mass in Si, all of which lead to larger number of high-energy hot holes, larger transport probability, higher emission efficiency, and higher photoresponsivity. However, the low barrier height can cause poor performances of hot-hole device in dark current density and detectivity. The study elucidates the intrinsic physical differences and compares the key performance parameters of the hot-hole and hot-electron photodetections, with the objective of providing complete information for designing hot-carrier devices.

Superlocalized Three-Dimensional Live Imaging of Mitochondrial Dynamics in Neurons Using Plasmonic Nanohole Arrays

We investigated the transport of neuronal mitochondria using superlocalized near-fields with plasmonic nanohole arrays (PNAs). Compared to traditional imaging techniques, PNAs create a massive array of superlocalized light beams and allow 3D mitochondrial dynamics to be sampled and extracted almost in real time. In this work, mitochondrial fluorescence excited by the PNAs was captured by an optical microscope using dual objective lenses, which produced superlocalized dynamics while minimizing light scattering by the plasmonic substrate. It was found that mitochondria move with an average velocity 0.33 ± 0.26 μm/s, a significant part of which, by almost 50%, was contributed by the movement along the depth axis ( z-axis). Mitochondrial positions were acquired with superlocalized precision (σ _x = 5.7 nm and σ _y = 11.8 nm) in the lateral plane and σ _z = 78.7 nm in the z-axis, which presents an enhancement by 12.7-fold in resolution compared to confocal fluorescence microscopy. The approach is expected to serve as a way to provide 3D information on molecular dynamics in real time.

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

Hydrogen is a promising alternative renewable fuel for meeting the growing energy demands of the world. Over the past few decades, photoelectrochemical water splitting has been widely studied as a viable technology for the production of hydrogen utilizing solar energy. A solar-to-hydrogen (STH) efficiency of 10% is considered to be sufficient for practical applications. Amongst the wide class of semiconductors that have been studied for their application in solar water splitting, iron oxide (α-Fe₂O₃), or hematite, is one of the more promising candidate materials, with a theoretical STH efficiency of 15%. In this work, we show experimentally that by utilizing gold nanostructures that support gap-plasmon resonances together with a hematite layer, we can increase the water oxidation photocurrent by two times over that demonstrated by a bare hematite film at wavelengths above the hematite bandgap. Moreover, we achieve a six-fold increase in the oxidation photocurrent at near-infrared wavelengths, which is attributed to hot electron generation and decay in the gap-plasmon nanostructures. Theoretical simulations confirmed that the metamaterial geometry with gap plasmons that was used allows us to confine electromagnetic fields inside the hematite semiconductor and to enhance the surface photochemistry.

Silicon-Based Embedded Trenches of Active Antennas for High-Responsivity Omnidirectional Photodetection at Telecommunication Wavelengths

Although the use of plasmonic nanostructures for photodetection below the band gap energy of the semiconductor has been intensively investigated recently, efficiencies of such hot electron-based devices have, unfortunately, remained low because of the inevitable energy loss of the hot electrons as they move and transfer in active antennas based on metallic nanostructures. In this work, we demonstrate the concept of high-refractive-index material-embedded trench-like (ETL) active antennas that could be used to achieve almost 100% absorbance within the ultrashallow region (approximately 10 nm) beneath the metal-semiconductor interface, which is a much smaller distance compared with the hot electrons' mean free path in the noble metal layer. Taking advantage of these ETL-based active antennas, we obtained photoresponsivities under zero bias at wavelengths of 1310 and 1550 nm of 5854 and 693 nA mW^-1, respectively-values higher than most those previously reported for active antenna-based silicon (Si) photodetectors that operate at optical telecommunication wavelengths. Furthermore, the ETL antenna strategy allowed us to preserve an omnidirectional and broadband photoresponse, with a superior degree of detection linearity of R² = 0.98889 under the light of low power density (down to 11.1 μW cm^-2). The photoresponses of the ETL antenna-based device varied by less than 10% upon changing the incident angle from normal incidence to 60°. Because these ETL-based devices provide high responsivity and omnidirectional detection over a broad bandwidth, they show promising potentials for use in hot electron-based optoelectronics for many applications (e.g., Si photonics, energy harvesting, photocatalysis, and sensing devices).

Schottky junction effect enhanced plasmonic photocatalysis by TaON@Ni NP heterostructures

The localized surface plasmon resonance and Schottky junction in the TaON@Ni hybrid photocatalyst improve the light harvesting and promote the electron–hole separation and transport of TaON.

A methodological review on material growth and synthesis of solar-driven water splitting photoelectrochemical cells

As a renewable and sustainable energy source and an alternative to fossil fuels, solar-driven water splitting with photoelectrochemical (PEC) cell is a promising approach to obtain hydrogen fuel with its near-zero carbon emission pathway by transforming incident sunlight, the most abundant energy source. Because of its importance and future prospects, a number of architectures with their own features have been formed by various synthesis and growth methods. Because the materials themselves are one of the most dominant components, they determine the solar-to-hydrogen efficiency of the PEC cells. Thus, several representative PEC cells were reviewed by categorizing them as per synthesis and/or growth methods such as physical vapor deposition, chemical vapor deposition, electrochemical deposition, etc. This review provides researchers with an overview and acts as a guide for research on solar-driven water splitting PEC cells.

Solar-driven PEC cell is a promising approach to obtain hydrogen with near-zero carbon emission pathway. In this article, PEC cell was reviewed as per growth/synthesis methods. This review provides an overview and a guide for research on PEC cell.

Boosting the Efficiency of Photoelectrolysis by the Addition of Non-Noble Plasmonic Metals: Al & Cu

Solar water splitting by semiconductor based photoanodes and photocathodes is one of the most promising strategies to convert solar energy to chemical energy to meet the high demand for energy consumption in modern society. However, the state-of-the-art efficiency is too low to fulfill the demand. To overcome this challenge and thus enable the industrial realization of a solar water splitting device, different approaches have been taken to enhance the overall device efficiency, one of which is the incorporation of plasmonic nanostructures. Photoanodes and photocathodes coupled to the optimized plasmonic nanostructures, matching the absorption wavelength of the semiconductors, can exhibit a significantly increased efficiency. So far, gold and silver have been extensively explored to plasmonically enhance water splitting efficiency, with disadvantages of high cost and low enhancement. Instead, non-noble plasmonic metals such as aluminum and copper, are earth-abundant and low cost. In this article, we review their potentials in photoelectrolysis, towards scalable applications.

Unified theory of plasmon-induced resonance energy transfer and hot electron injection processes for enhanced photocurrent efficiency

Plasmons in metal nanoparticles (MNPs) promise to enhance solar energy conversion in semiconductors. Two essential mechanisms of enhancement in the near-field regime are hot electron injection (HEI) and plasmon-induced resonance energy transfer (PIRET). Individual studies of both mechanisms indicate that the PIRET efficiency is limited by the short lifetime of the plasmon, whereas the hot electrons result from the plasmon decay. The development of a unified theory of the coupled HEI and PIRET processes is fundamentally interesting and necessary for making reliable predictions but is complicated by the multiple interactions between various components that participate in the enhancement process. In this paper, we use the model-Hamiltonian approach to develop a combined theoretical framework including both PIRET and HEI. The coupled dynamics as well as the time evolution of hot electron energy distribution are studied. The theory further predicts an interference-induced asymmetry in the spectral dependence of PIRET, which can be used to distinguish it from HEI. As the relative contributions of PIRET and HEI strongly depend on the size of the MNPs, this presents itself as a simple route to control the strength of their contributions. The results presented here can further guide future applications of plasmonic solar energy harvesting.

Highly sensitive detection of exosomes by 3D plasmonic photonic crystal biosensor

In this study, two-dimensional (2D), quasi-three-dimensional (3D), and 3D plasmonic photonic crystal (PPC) nanostructures with point-defect cavities were developed and fabricated using direct and reversal nanoimprint lithography. As a result of the hybrid coupling of localized surface plasmon resonance and Fabry-Perot cavity modes, the quasi-3D plasmonic nanoholes showed higher electromagnetic field intensity and sensitivity than the 2D plasmonic nanoholes. Specifically, the sensitivity of the quasi-3D plasmonic nanoholes was 483 nm per refractive index unit (RIU), whereas that of the 2D plasmonic nanoholes was 276 nm RIU-1. In addition, by enhancing electromagnetic field intensity around corners and generating an additional subradiant dark mode, the symmetrical breakage of the quasi-3D plasmonic nanoholes further increased the sensitivity to 946 nm RIU-1. Among all the nanostructures developed in the study, the 3D PPC nanostructures with point-defect cavities showed the highest sensitivity up to 1376 nm RIU-1 and highest figure of merit of 11.6 as the result of the hybrid coupling of plasmonics and photonic crystal modes with multilayered plasmonic nanostructures. The spacing between the 3D PPC nanostructures was comparable with the average size of exosomes derived from fibroblast L cells, which allowed the exosomes to spread around the 3D PPC nanostructures with increased sensing area. This effect further enhanced the detection sensitivity with a large peak shift of 9 nm when using the 3D PPC biosensor to detect exosomes at the concentration of 1 × 104 particles per ml, and the peak shift increased to 102 nm as exosome concentration increased to 1 × 1011 particles per ml.

Limitation of Fermi level shifts by polaron defect states in hematite photoelectrodes

The optical band gap is a major selection criterion for an absorber in photocatalytic water splitting. Due to its ideal value hematite has been intensively investigated without reaching the expectation, yet. In this work, the Fermi level positions in hematite due to doping and contact formation are investigated. An upper boundary for the Fermi level position at 1.8 eV above the valence band maximum due to the formation of polarons is identified. This results in a different concept of the effective band gap for hematite which we believe is transferable to any material with competing polaron formation after optical excitation: the optical band gap of 2.2 eV deviates from an effective electronic band gap of 1.75 eV. The polaron state acts as a limit in (quasi-)Fermi level shift, restricting the potential of charge transfer reactions. Additionally, it has led to an incorrect determination of the band edge positions of hematite in electrochemical contacts.

Hematite has been proposed as a suitable photocatalyst for water splitting based on its stability and appealing optical band gap, but its performance has not reached theoretical expectations. Here the authors show that this is due to intra-gap polaronic states that reduce the effective electronic band gap.

Plasmonic Au-Loaded Hierarchical Hollow Porous TiO2 Spheres: Synergistic Catalysts for Nitroaromatic Reduction

Plasmonic Au nanoparticle (NP)-loaded hierarchical hollow porous TiO₂ spheres are designed and synthesized with the purpose of enhancing the overall catalytic activity by introducing the Au plasmonic effect into the system, where Au NPs themselves are catalytically active. The constructed nanohybrid exhibits both high activity in 4-nitrophenol reduction, compared to all of the previously reported Au-based catalysts, and high selectivity. The synergy of the inherent catalytic property of Au NPs and the plasmonic effect (mainly via hot electron transfer) under irradiation is confirmed by a series of control experiments. The specifically designed, porous hollow structure also greatly contributes to the good catalytic activity because it provides a large surface area, facilitates reactant adsorption, and hinders charge recombination. In addition, theoretical calculations reveal that such a structure also leads to an increase in light absorption of about 21% in the range of 400-800 nm with respect to a uniform water-TiO₂ background featuring the same filling factor. This work provides insight into the rational design of plasmon-enhanced catalysts that will show their versatility in various electro-/photocatalysis.

Additive-Free Rice Starch-Assisted Synthesis of Spherical Nanostructured Hematite for Degradation of Dye Contaminant

Nanostructured hematite materials for advanced applications are conventionally prepared with the presence of additives, tainting its purity with remnants of copolymer surfactants, active chelating molecules, stabilizing agents, or co-precipitating salts. Thus, preparing nanostructured hematite via additive-free and green synthesis methods remains a huge hurdle. This study presents an environmentally friendly and facile synthesis of spherical nanostructured hematite (Sp-HNP) using rice starch-assisted synthesis. The physicochemical properties of the Sp-HNP were investigated by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DR UV-Vis), and nitrogen adsorption–desorption analysis. The Sp-HNP showed a well-crystallized structure of pure rhombohedral phase, having a spherical-shaped morphology from 24 to 48 nm, and a surface area of 20.04 m²/g. Moreover, the Sp-HNP exhibited enhanced photocatalytic degradation of methylene blue dye, owing to the large surface-to-volume ratio. The current work has provided a sustainable synthesis route to produce spherical nanostructured hematite without the use of any hazardous agents or toxic additives, in agreement with the principles of green chemistry for the degradation of dye contaminant.

Efficient Light Absorption by GaN Truncated Nanocones for High Performance Water Splitting Applications

Despite the importance of gallium nitride (GaN) nanostructures for photocatalytic activity, relatively little attention has been paid to their geometrical optimization on the basis of wave optics. In this study, we present GaN truncated nanocones to provide a strategy for improving solar water splitting efficiencies, compared to the efficiency provided by the conventional geometries (i.e., flat surface, cylindrical, and cone shapes). Computational results with a finite difference time domain (FDTD) method and a rigorous coupled-wave analysis (RCWA) reveal important aspects of truncated nanocones, which effectively concentrate light in the center of the nanostructures. The introduction of nanostructures is highly recommended to address the strong light reflection of photocatalytic materials and carrier lifetime issues. To fabricate the truncated nanocones at low cost and with large-area, a dry etching method was employed with thermally dewetted metal nanoparticles, which enables controllability of desired features on a wafer scale. Experimental results exhibit that the photocurrent density of truncated nanocones is improved about three times higher compared to that of planar GaN.

Template-Guided Programmable Janus Heteronanostructure Arrays for Efficient Plasmonic Photocatalysis

Janus heteronanostructures (HNs), as an important class of anisotropic nanomaterials, could facilitate synergistic coupling of diverse functions inherited by their comprised nanocomponents. Nowadays, synthesizing deterministically targeted Janus HNs remains a challenge. Here, a general yet scalable technique is utilized to fabricate an array of programmable Janus HNs based on anodic aluminum oxide binary-pore templates. By designing and employing an overetching process to partially expose four-edges of one set of nanocomponents in a binary-pore template, selective deposition and interfacing of the other set of nanocomponents is successfully achieved along the exposed four-edges to form a densely packed array of Janus HNs on a large scale. In combination with an upgraded two-step anodization, the synthesis provides high degrees of freedom for both nanocomponents of the Janus HNs, including morphologies, compositions, dimensions, and interfacial junctions. Arrays of TiO₂-Au and TiO₂/Pt NPs-Au Janus HNs are designed, fabricated, and demonstrated about 2.2 times photocurrent density and 4.6 times H₂ evolution rate of that obtained from their TiO₂ counterparts. The enhancement was mainly determined as a result of localized surface plasmon resonance induced direct hot electron injection and strong plasmon resonance energy transfer near the interfaces of TiO₂ nanotubes and Au nanorods. This study may represent a promising step forward to pursue customized Janus HNs, leading to novel physicochemical effects and device applications.

Plasmon-driven water splitting enhancement on plasmonic metal-insulator-semiconductor hetero-nanostructures: unraveling the crucial role of interfacial engineering

Understanding and controlling the charge transfer behavior across the interface/junction in hybrid nanostructures is essential for various plasmon-enhanced catalytic reactions. The rational design of plasmonic nanostructures offers a unique capability for eliminating the daunting complexity of the electronic effect induced by interfacial interactions and maximizing the conversion efficiency of solar energy into chemical energy by surface coupling. Herein, we tactfully construct a new type of plasmon-driven photoanode based on plasmonic metal-insulator-semiconductor (PMIS) hetero-nanostructures (Au@SiO₂NP-decorated α-Fe₂O₃ nanorod array), by using Fe₂O₃ nanoarrays as model semiconductor structures and Au@SiO₂ NPs as photosensitizers, for optimizing the photoelectrochemical (PEC) water splitting performance. The thin insulating layer (SiO₂) of the hetero-nanostructure has been found to play a crucial role in significantly enhancing the plasmon-driven water splitting performance via eliminating the negative effect of surface states (resulting in Fermi level pinning and recombination) at the metal-semiconductor interface, suppressing the recombination of current carriers, as well as maximizing the metal-semiconductor barrier height. This study provides new insight into a novel plasmonic nanocatalyst design by rational interface engineering and will be of benefit for a better understanding of manipulating the interfacial electronic properties between plasmonic nanocrystals and semiconductors for catalytic applications.

Recent Advances on Black Phosphorus for Energy Storage, Catalysis, and Sensor Applications

As a new type of 2D semiconductor, black phosphorus (BP) possesses high charge-carrier mobility and theoretical capacity, thickness-dependent bandgap, and anisotropic structure, which has attracted tremendous attention since early 2014. To explore its full application in all aspects, studies based on BP nanostructures are swiftly expanding from the electronic field to energy storage and even biochemistry. The mechanism and application of BP in Li-/Na-ion battery anodes, oxygen evolution reaction/hydrogen evolution reaction catalysis, photocatalytic hydrogen production, and selective sensors are summarized. Based on the solid research on this topic, feasible improvements and constructive suggestions regarding these four fields are put forward.

NiO Nanoparticles Anchored on Phosphorus-Doped α-Fe2 O3 Nanoarrays: An Efficient Hole Extraction p-n Heterojunction Photoanode for Water Oxidation

The photoelectrochemical (PEC) water-splitting efficiency of a hematite-based photoanode is still far from the theoretical value due to its poor surface reaction kinetics and high density of surface trapping states. To solve these drawbacks, a photoanode consisting of NiO nanoparticles anchored on a gradient phosphorus-doped α-Fe₂ O₃ nanorod (NR) array (NiO/P-α-Fe₂ O₃ ) was fabricated to achieve optimal light absorption and charge separation, as well as rapid surface reaction kinetics. Specifically, a photoanode with the NR array structure allowed a high mass-transport rate to be achieved, while phosphorus doping effectively decreased the number of surface trapping sites and improved the electrical conductivity of α-Fe₂ O₃ . Furthermore, the p-n junction that forms between NiO and P-α-Fe₂ O₃ can further improve the PEC performance due to efficient hole extraction and the water oxidization catalytic activity of NiO. Consequently, the NiO/P-α-Fe₂ O₃ NR photoanode produced a high photocurrent density of 2.08 mA cm^-2 at 1.23 V versus a reversible hydrogen electrode and a 110 mV cathodic shift of the onset potential. This rational design of structure offers a new perspective in exploring high-performance PEC photoanodes.

Homogeneous localized surface plasmon resonance inflection points for enhanced sensitivity and tracking plasmon damping in single gold bipyramids

The most polarizable localized surface plasmon resonance (LSPR) longitudinal mode of anisotropic metallic nanoparticles, such as gold bipyramids (AuBPs), is of high prominence. This optical response has tremendous applications from spectroscopy to photonics and energy devices to sensing. In conventional LSPR-based sensing, broadening and asymmetry in peaks due to chemical and instrument noise hinder obtaining a precise insight on shift positions, accordingly limiting the effectiveness and impact of LSPR sensors. Further, when investigating LSPR properties, utilizing more simplistic frequency dependent dielectric-type models can aberrantly impact the reliability of fundamental properties used for designing and fabricating efficient optical devices. For instance, more approximations can effectively limit screening intra-band and inter-band (IB) electronic transition contributions and other related optical properties. With an aim to find alternative methods to further improve their efficiency, as a first report, we devoted a particular focus on LSPR scattering inflection points (IFs) of single AuBPs. The findings reveal that tracking LSPR IFs exhibit high sensitivity over their counterpart LSPR peak shift locations. In addition, we newly detected IB transition contributions near the resonance energy in the range (1.50 eV-2.00 eV) dominated by intra-band transitions. A small increase in the local RI effectively enhances the LSPR quality factor due to IB transitions. Therefore, while neglecting IB transitions in the range below 2.4 eV can work for local air refractive index (RI), in high local RI media it can be aberrantly underestimated. Demonstrated by the use of the dielectric function based on Kramers-Kronig consistent Lorentz oscillators, our findings are in good agreement with the enhancing RI sensitivity effect. The results of this investigation support the idea that tracking curvature changes of an optical signal can be effectively used for LSPR longitudinal peak RI sensing as well as damping in the local RI environment of a single AuBP.

Influence of Surface Roughness on Strong Light-Matter Interaction of a Quantum Emitter-Metallic Nanoparticle System

We investigate the quantum optical properties of strong light-matter interaction between a quantum emitter and a metallic nanoparticle beyond idealized structures with a smooth surface. Based on the local coupling strength and macroscopic Green’s function, we derived an exact quantum optics approach to obtain the field enhancement and light-emission spectrum of a quantum emitter. Numerical simulations show that the surface roughness has a greater effect on the near-field than on the far-field, and slightly increases the vacuum Rabi splitting on average. Further, we verified that the near-field enhancement is mainly determined by the surface features of hot-spot area.

Supported black phosphorus nanosheets as hydrogen-evolving photocatalyst achieving 5.4% energy conversion efficiency at 353 K

Solar-driven water splitting using powdered catalysts is considered as the most economical means for hydrogen generation. However, four-electron-driven oxidation half-reaction showing slow kinetics, accompanying with insufficient light absorption and rapid carrier combination in photocatalysts leads to low solar-to-hydrogen energy conversion efficiency. Here, we report amorphous cobalt phosphide (Co-P)-supported black phosphorus nanosheets employed as photocatalysts can simultaneously address these issues. The nanosheets exhibit robust hydrogen evolution from pure water (pH = 6.8) without bias and hole scavengers, achieving an apparent quantum efficiency of 42.55% at 430 nm and energy conversion efficiency of over 5.4% at 353 K. This photocatalytic activity is attributed to extremely efficient utilization of solar energy (~75% of solar energy) by black phosphorus nanosheets and high-carrier separation efficiency by amorphous Co-P. The hybrid material design realizes efficient solar-to-chemical energy conversion in suspension, demonstrating the potential of black phosphorus-based materials as catalysts for solar hydrogen production.

In order to displace fossil fuel technologies, it is crucial to develop efficient solar-to-fuel conversion materials using abundant, cheap elements. Here, the authors prepare few-layer black phosphorous with amorphous cobalt phosphide and produce hydrogen gas with light at high efficiencies.

Hydrogenated TiO2 nanotube photonic crystals for enhanced photoelectrochemical water splitting

We report the design, fabrication and characterization of novel TiO₂ nanotube photonic crystals with a crystalline core/disordered shell structure as well as substantial oxygen vacancies for photoelectrochemical (PEC) water splitting. The novel TiO₂ nanotube photonic crystals are fabricated by annealing of anodized TiO₂ nanotube photonic crystals in hydrogen atmosphere at various temperatures. The optimized novel TiO₂ nanotube photonic crystals produce a maximal photocurrent density of 2.2 mA cm^-2 at 0.22 V versus Ag/AgCl, which is two times higher that of the TiO₂ nanotube photonic crystals annealed in air. Such significant PEC performance improvement can be ascribed to synergistic effects of the disordered surface layer and oxygen vacancies. The reduced band gap owing to the disordered surface layer and localized states induced by oxygen vacancies can enhance the efficient utilization of visible light. In addition, the disordered surface layer and substantial oxygen vacancies can promote the efficiency for separation and transport of the photogenerated carriers. This work may open up new opportunities for the design and construction of the high efficient and low-cost PEC water splitting system.

Dual Effects of Nanostructuring and Oxygen Vacancy on Photoelectrochemical Water Oxidation Activity of Superstructured and Defective Hematite Nanorods

An Ar atmospheric treatment is rationally used to etch and activate hematite nanoflakes (NFs) as photoanodes toward enhanced photoelectrochemical water oxidation. The formation of a highly ordered hematite nanorods (NRs) array containing a high density of oxygen vacancy is successfully prepared through in situ reduction of NFs in Ar atmosphere. Furthermore, a hematite (104) plane and an iron suboxide layer at the absorber/back-contact interface are formed. The material defects produced by a thermal oxidation method can be critical for the morphology transformation from 2D NFs to 1D NRs. The resulting hematite NR photoanodes show high efficiency toward solar water splitting with improved light harvesting capabilities, leading to an enhanced photoresponse due to the artificially formed oxygen vacancies.

High-Performance Silicon Photoanode Enhanced by Gold Nanoparticles for Efficient Water Oxidation

Ni catalyst is a low-cost catalyst for oxygen evolution reaction (OER) on silicon metal-insulator-semiconductor photoanode. We found that Au nanoparticles incorporated with Ni nanoparticles can enhance the OER activity and stability of Ni nanoparticles due to the local surface plasmon resonance (LSPR) effect of the Au nanoparticles. The efficiency of NiAu/TiO₂/n-Si photoanode can be boosted at least three times under the illumination (100 mW/cm²) by LSPR effect of the Au nanoparticles. A small onset potential of 1.03 V versus reversible hydrogen electrode (overpotential, η₀ = -0.20 V) and a current density of 18.80 mA/cm² at 1.23 V versus reversible hydrogen electrode can be obtained. The NiAu/TiO₂/n-Si photoanode exhibits a high saturation current density of 35 mA/cm², which is greater than that of most of the state-of-the-art silicon photoanodes.

Plasmonic metal-semiconductor photocatalysts and photoelectrochemical cells: a review

The incorporation of plasmonic metals into semiconductors is a promising route to improve the performance of photocatalysts and photoelectrochemical cells. This article summarizes the three major mechanisms of plasmonic energy transfer from a metal to a semiconductor, including light scattering/trapping, plasmon-induced resonance energy transfer (PIRET) and hot electron injection (also called direct electron transfer (DET)). It also discusses the rational design of plasmonic metal-semiconductor heterojunctions based on the underlying plasmonic energy transfer mechanisms. Moreover, this article highlights the applications of plasmonic photocatalysts and photoelectrochemical cells in solar water splitting, carbon dioxide reduction and environmental pollutant decomposition.

Multipolar Nanocube Plasmon Mode-Mixing in Finite Substrates

Facile control of the radiative and nonradiative properties of plasmonic nanostructures is of practical importance to a wide range of applications in the biological, chemical, optical, information, and energy sciences. For example, the ability to easily tune not only the plasmon spectrum but also the degree of coupling to light and/or heat, quality factor, and optical mode volume would aid the performance and function of nanophotonic devices and molecular sensors that rely upon plasmonic elements to confine and manipulate light at nanoscopic dimensions. While many routes exist to tune these properties, identifying new approaches-especially when they are simple to apply experimentally-is an important task. Here, we demonstrate the significant and underappreciated effects that substrate thickness and dielectric composition can have upon plasmon hybridization as well as downstream properties that depend upon this hybridization. We find that even substrates as thin as ∼10 nm can nontrivially mix free-space plasmon modes, imparting bright character to those that are dark (and vice versa) and, thereby, modifying the plasmonic density of states as well as the system's near- and far-field optical properties. A combination of electron energy-loss spectroscopy (EELS) experiment, numerical simulation, and analytical modeling is used to elucidate this behavior in the finite substrate-induced mixing of dipole, quadrupole, and octupole corner-localized plasmon resonances of individual silver nanocubes.

Balancing Catalytic Activity and Interface Energetics of Electrocatalyst-Coated Photoanodes for Photoelectrochemical Water Splitting

For photoelectrochemical (PEC) water splitting, the interface interactions among semiconductors, electrocatalysts, and electrolytes affect the charge separation and catalysis in turn. Here, through the changing of the bath temperature, Co-based oxygen evolution catalysts (OEC) with different crystallinities were electrochemically deposited on Ti-doped Fe₂O₃ (Ti-Fe₂O₃) photoanodes. We found: (1) the OEC with low crystallinity is highly ion-permeable, decreasing the interactions between OEC and photoanode due to the intimate interaction between semiconductor and electrolyte; (2) the OEC with high crystallinity is nearly ion-impermeable, is beneficial to form a constant buried junction with semiconductor, and exhibits the low OEC catalytic activity; and (3) the OEC with moderate crystallinity is partially electrolyte-screened, thus contributing to the formation of ideal band bending underneath surface of semiconductor for charge separation and the highly electrocatalytic activity of OEC for lowering over-potentials of water oxidation. Our results demonstrate that to balance the water oxidation activity of OEC and OEC-semiconductor interface energetics is crucial for highly efficient solar energy conversion; in particular, the energy transducer is a semiconductor with a shallow or moderate valence-band level.

Interface Manipulation to Improve Plasmon-Coupled Photoelectrochemical Water Splitting on α-Fe2 O3 Photoanodes

The plasmon resonance effect of metal nanoparticles (NPs) offers a promising route to improve the solar energy conversion efficiency of semiconductors. In this study, it is revealed that hot electrons generated by the plasmon resonance effect of Au NPs tend to inject into the surface states instead of the conduction band of Fe₂ O₃ photoanodes, and then severe surface recombination occurs. Such an electron-transfer process seems to be independent of external applied potentials, but is sensitive to metal-semiconductor interface properties. Passivating the surface states of Fe₂ O₃ with a noncatalytic Al₂ O₃ layer can construct an effective resonant energy-transfer interface between Ti-doped Fe₂ O₃ (Ti-Fe₂ O₃ ) and Au NPs. In such a Ti-Fe₂ O₃ /Al₂ O₃ /Au electrode configuration, the enhanced photoelectrochemical (PEC) water-splitting performance can be attributed to the following two factors: 1) in the non-light-responsive wavelength range of Au NPs, both the relaxing Fermi pinning effect of the Al₂ O₃ passivation layer and the higher work function of Au enlarge band bending; thus promoting the charge separation; and 2) in the light-responsive wavelength range of Au NPs, the effective resonant energy transfer contributes to light harvesting and conversion. The interface manipulation proposed herein may provide a new route to design efficient plasmonic PEC devices for energy conversion.

Origin of Plasmon Lineshape and Enhanced Hot Electron Generation in Metal Nanoparticles

Plasmon-generated hot carriers are currently being studied intensively for their role in enhancing the efficiency of photovoltaic and photocatalytic processes. Theoretical studies of the hot electrons subsystem have generated insight, but we show that a unified quantum-mechanical treatment of the plasmon and hot electrons reveals new physical phenomena. Instead of a unidirectional energy transfer process in Landau damping, back energy transfer is predicted in small metal nanoparticles (MNPs) within a model-Hamiltonian approach. As a result, the single Lorentzian plasmonic line shape is modulated by a multipeak structure, whose individual line width provides a direct way to probe the electronic dephasing. More importantly, the hot electron generation can be enhanced greatly by matching the incident energy to the peaks of the modulated line shape.

Photoelectrochemical overall water splitting with textured CuBi2O4as a photocathode

Tailoring CuBi₂O₄photocathodes demonstrates their applicability in a photoelectrochemical tandem cell for entirely solar-driven overall water splitting.

Light-assisted surface reactions on metal nanoparticles

Light-assisted surface reaction can lower reaction temperature, potentially reducing the energy use by providing light together with heat.

Ferroelectric polarization-enhanced charge separation in a vanadium-doped ZnO photoelectrochemical system

A ferroelectric polarization protocol was demonstrated to be favorable for photoinduced charge separation in the V-doped ZnO based photoelectrochemical system.

Near-Infrared-to-Ultraviolet Light-Mediated Photoelectrochemical Aptasensing Platform for Cancer Biomarker Based on Core-Shell NaYF4:Yb,Tm@TiO2 Upconversion Microrods

Titanium dioxide (TiO₂; as a potential photosensitizer) has good photocurrent performance and chemical stability but often exhibits low utilization efficiency under ultraviolet (UV) region excitation. Herein, we devised a near-infrared light-to-UV light-mediated photoelectrochemical (PEC) aptasensing platform for the sensitive detection of carcinoembryonic antigen (CEA) based on core-shell NaYF₄:Yb,Tm@TiO₂ upconversion microrods by coupling with target-triggered rolling circle amplification (RCA). The upconversion microrods synthesized through the hydrothermal reaction could act as a photosensing platform to convert the near-infrared (near-IR) excitation into UV emission for generation of photoinduced electrons. The target analyte was determined on a functional magnetic bead by using the corresponding aptamers with a sandwich-type assay format. Upon target CEA introduction, a complex was first formed between capture aptamer-1-conjugated magnetic bead (Apt1-MB) and aptamer-2-primer DNA (Apt2-pDNA). Thereafter, the carried primer DNA by the aptamer-2 paired with linear padlock DNA to trigger the RCA reaction. The guanine (G)-rich product by RCA reaction was cleaved by exonuclease I and exonuclease III (Exos I/III), thereby resulting in the formation of numerous individual guanine bases to enhance the photocurrent of core-shell NaYF₄:Yb,Tm@TiO₂ upconversion microrods under near-IR illumination (980 nm). Under optimal conditions, the near-IR light-mediated PEC aptasensing system could exhibit good photoelectrochemical response toward target CEA and allowed for the detection of target CEA as low as 3.6 pg mL^-1. High reproducibility and good accuracy were achieved for analysis of human serum specimens. Importantly, the near-IR-activated PEC aptasensing scheme provides a promising platform for ultrasensitive detection of other biomolecules.