Laser Annealing Improves the Photoelectrochemical Activity of Ultrathin MoSe2 Photoelectrodes

Direct Laser Irradiation and Modification of 2D Te for Development of Volatile Memristor

Laser irradiation, as a kind of post-fabrication method for two-dimensional (2D) materials, is a promising way to tune the properties of materials and the performance of corresponding nano-devices. As the memristor has been regarded as an excellent candidate for in-memory devices in next-generation computing system, the application of laser irradiation in developing excellent memristor based on 2D materials should be explored deeply. Here, tellurene (Te) flakes are exposed to a 532 nm laser in the air atmosphere to investigate the evolutions of the surface morphology and atom structures under different irradiation parameters. Laser is capable of thinning the flakes, inducing amorphous structures, oxides and defects, and forming nanostructures by controlling the irradiation power and time. Furthermore, the laser-induced oxides and defects promote the migration of metal ions in Te, resulting in the formation of the conductive filaments, which provides the switching behavers of volatile memristor, opening a route to the development of next-generation nano-devices.

Understanding Wavelength-Dependent Synergies between Morphology and Photonic Design in TiO2-Based Solar Powered Redox Cells

Solar powered redox cells (SPRCs) are promising for large-scale and long-term storage of solar-energy, particularly when coupled with redox flow batteries (RFBs). While efforts have primarily focused on heterostructure engineering, the potential of synergistic morphology and photonic design has not been carefully studied. Here, we investigate the wavelength-dependent effects of light-absorption and charge transfer characteristics on the performance of gold decorated TiO₂-based SPRC photoanodes operating with RFB-compatible redox couples. Through an in-depth optical and photoelectrochemical characterization of three complementary TiO₂ microstructures, namely nanotubes, honeycombs, and nanoparticles, we elucidate the combined effects of nanometer-scale semiconductor morphology and plasmonic design across the visible spectrum. In particular, thin-walled TiO₂ nanotubes exhibit a ∼ 50% increase in solar-to-chemical efficiency (STC) compared to thick-walled TiO₂ honeycombs thanks to improved charge transfer. Au nanoparticles both increase generation and interfacial charge transfer (above bandgap) and promote hot carrier injection (below bandgap) leading to a further 25% increase in STC. Overall, Au/TiO₂ nanotubes achieve a high photocurrent at 0.098 mA/cm² and an excellent STC of 0.06%, among the highest with respect to the theoretical limit. The incident photon to current efficiency and internal quantum efficiency are also superior to those of bare TiO₂ showing maximum values of 54.7% and 67%, respectively. Overall, nanophotonic engineering that synergistically combines morphology optimization and plasmonic sensitization schemes offer new avenues for improving rechargeable solar-energy technologies such as solar redox flow batteries.

Vacancy Defects in 2D Transition Metal Dichalcogenide Electrocatalysts: From Aggregated to Atomic Configuration

Vacancy defect engineering has been well leveraged to flexibly shape comprehensive physicochemical properties of diverse catalysts. In particular, growing research effort has been devoted to engineering chalcogen anionic vacancies (S/Se/Te) of 2D transition metal dichalcogenides (2D TMDs) toward the ultimate performance limit of electrocatalytic hydrogen evolution reaction (HER). In spite of remarkable progress achieved in the past decade, systematic and in-depth insights into the state-of-the-art vacancy engineering for 2D-TMDs-based electrocatalysis are still lacking. Herein, this review delivers a full picture of vacancy engineering evolving from aggregated to atomic configurations covering their development background, controllable manufacturing, thorough characterization, and representative HER application. Of particular interest, the deep-seated correlations between specific vacancy regulation routes and resulting catalytic performance improvement are logically clarified in terms of atomic rearrangement, charge redistribution, energy band variation, intermediate adsorption-desorption optimization, and charge/mass transfer facilitation. Beyond that, a broader vision is cast into the cutting-edge research fields of vacancy-engineering-based single-atom catalysis and dynamic structure-performance correlations across catalyst service lifetime. Together with critical discussion on residual challenges and future prospects, this review sheds new light on the rational design of advanced defect catalysts and navigates their broader application in high-efficiency energy conversion and storage fields.

High Performance Semiconducting Nanosheets via a Scalable Powder-Based Electrochemical Exfoliation Technique

The liquid-phase exfoliation of semiconducting transition metal dichalcogenide (TMD) powders into 2D nanosheets represents a promising route toward the scalable production of ultrathin high-performance optoelectronic devices. However, the harsh conditions required negatively affect the semiconducting properties, leading to poor device performance. Herein we demonstrate a gentle exfoliation method employing standard bulk MoS₂ powder (pressed into pellets) together with the electrochemical intercalation of a quaternary alkyl ammonium. The resulting nanosheets are produced in high yield (32%) and consist primarily of mono-, bi-, triatomic layers with large lateral dimensions (>1 μm), while retaining the semiconducting polymorph. Exceptional optoelectronic performance of nanosheet thin-films is observed, such as enhanced photoluminescence, charge carrier mobility (up to 0.2 cm² V^-1 s^-1 in a multisheet device), and photon-to-current efficiency while maintaining high transparency (>80%). Specifically, as a photoanode for iodide oxidation, an internal quantum efficiency up to 90% (at +0.3 V vs Pt) is achieved (compared to only 12% for MoS₂ nanosheets produced via ultrasonication). Further using a combination of fluorescence microscopy and high-resolution scanning transmission electron microscopy (STEM), we show that our gently exfoliated nanosheets possess a defect density (2.33 × 10¹³ cm^-2) comparable to monolayer MoS₂ prepared by vacuum-based techniques and at least three times less than ultrasonicated MoS₂ nanoflakes. Finally, we expand this method toward other TMDs (WS₂, WSe₂) to demonstrate its versatility toward high-performance and fully scalable van der Waals heterojunction devices.

Structural Features Dictate the Photoelectrochemical Activities of Two-Dimensional MoSe2 and WSe2 Nanostructures

The exfoliation of layered materials into two-dimensional (2D) semiconductors creates new structural domains, for example, basal planes, defect-rich in-planes, and edge sites. These surface species affect the photoelectrochemical (PEC) performance, which in turn determines their applicability in solar energy conversion technologies. In this study, a custom-designed microdroplet cell-based spatially resolved PEC approach was employed to identify the structural parts and to measure the PEC activity of the mechanically exfoliated MoSe₂ and WSe₂ nanosheets for bulk, few-layer, and monolayer specimens. The PEC performance decreased with the decreasing thickness of nanoflakes, and the relative PEC activity (photo/total current) reduced by introducing more defects to the 2D flakes: 1-3% loss was found for in-plane defects and 30-40% for edge defects. While edge sites act as charge carrier recombination centers, their electrocatalytic activity is higher than that of the basal planes. The comparison of PEC activity of micromechanically and liquid phase exfoliated bulk and few-layer MoSe₂ and WSe₂ flakes further confirmed that the PEC performance of 2D flakes decreases with an increasing number of edge sites.

Directly visualizing carrier transport and recombination at individual defects within 2D semiconductors

Two-dimensional semiconductors (2DSCs) are promising materials for a wide range of optoelectronic applications. While the fabrication of 2DSCs with thicknesses down to the monolayer limit has been demonstrated through a variety of routes, a robust understanding of carrier transport within these materials is needed to guide the rational design of improved practical devices. In particular, the influence of different types of structural defects on transport is critical, but difficult to interrogate experimentally. Here, a new approach to visualizing carrier transport within 2DSCs, Carrier Generation-Tip Collection Scanning Electrochemical Cell Microscopy (CG-TC SECCM), is described which is capable of providing information at the single-defect level. In this approach, carriers are locally generated within a material using a focused light source and detected as they drive photoelectrochemical reactions at a spatially-offset electrolyte interface created through contact with a pipet-based probe, allowing carrier transport across well-defined, µm-scale paths within a material to be directly interrogated. The efficacy of this approach is demonstrated through studies of minority carrier transport within mechanically-exfoliated n-type WSe2 nanosheets. CG-TC SECCM imaging experiments carried out within pristine basal planes revealed highly anisotropic hole transport, with in-plane and out-of-plane hole diffusion lengths of 2.8 µm and 5.8 nm, respectively. Experiments were also carried out to probe recombination across individual step edge defects within n-WSe2 which suggest a significant surface charge (∼5 mC m-2) exists at these defects, significantly influencing carrier transport. Together, these studies demonstrate a powerful new approach to visualizing carrier transport and recombination within 2DSCs, down to the single-defect level.

Laser-Induced Crystalline-Phase Transformation for Hematite Nanorod Photoelectrochemical Cells

Generally, a high-temperature postannealing process is required to enhance the photoelectrochemical (PEC) performance of hematite nanorod (NR) photoanodes. However, the thermal annealing time is limited to a short duration as thermal annealing at high temperatures can result in some critical problems, such as conductivity degradation of the fluorine-doped tin oxide film and deformation of the glass substrate. In this study, selective laser processing is introduced for hematite-based PEC cells as an alternative annealing process. The developed laser-induced phase transformation (LIPT) process yields hematite NRs with enhanced optical, chemical, and electrical properties for application in hematite NR-based PEC cells. Owing to its improved properties, the LIPT-processed hematite NR PEC cell exhibits an enhanced water oxidation performance compared to that processed by the conventional annealing process. As the LIPT process is conducted under ambient conditions, it would be an excellent alternative annealing technique for heat-sensitive flexible substrates in the future.

Disentangling oxygen and water vapor effects on optoelectronic properties of monolayer tungsten disulfide

By understanding how the environmental composition impacts the optoelectronic properties of transition metal dichalcogenide monolayers, we demonstrate that simple photoluminescence (PL) measurements of tungsten disulfide (WS₂) monolayers can differentiate relative humidity environments. In this paper, we examine the PL and photoconductivity of chemical vapor deposition grown WS₂ monolayers under three carefully controlled environments: inert gas (N₂), dry air (O₂ in N₂), and humid nitrogen (H₂O vapor in N₂). The WS₂ PL is measured as a function of 532 nm laser power and exposure time and can be decomposed into the exciton, trion, and lower energy state(s) contributions. Under continuous illumination in either O₂ or H₂O vapor environment, we find dramatic (and reversible) increases in PL intensity relative to the PL in an inert environment. The PL bathochromically shifts in an O₂ environment and is dominated by increased trion emission and diminished exciton emission. In contrast, the WS₂ PL increase in a H₂O environment results from an overall increase in emission from all spectral components where the exciton contribution dominates. The drastic increases in PL are anticorrelated with corresponding decreases in photoconductivity, as measured by time-resolved microwave conductivity. The results suggest that both O₂ and H₂O react photochemically with the WS₂ monolayer surface, modifying the optoelectronic properties, but do so via distinct pathways. Thus, we use these optoelectronic differences to differentiate the amount of humidity in the air, which we show with 0%, 40%, and 80% relative humidity environments. This deeper understanding of how ambient conditions impact WS₂ monolayers enables novel humidity sensors as well as a better understanding of the correlation between TMDC surface chemistry, light emission, and photoconductivity. Moreover, these WS₂ measurements highlight the importance of considering the impact of the local environment on reported results.

Influence of the Substrate on the Optical and Photo-electrochemical Properties of Monolayer MoS2

Substrates influence the electrical and optical properties of monolayer (ML) MoS₂ in field-effect transistors and photodetectors. Photoluminescence (PL) and Raman spectroscopy measurements have shown that conducting substrates can vary the doping concentration and influence exciton decay channels in ML-MoS₂. Doping and exciton decay dynamics are expected to play a major role in the efficiency of light-driven chemical reactions, but it is unclear to what extent these factors contribute to the photo(electro)catalytic properties of ML-MoS₂. Here, we report spatially resolved PL, Raman, and photo-electrochemical current measurements of 5-10 μm-wide ML-MoS₂ triangles deposited on pairs of indium-doped tin oxide (ITO) electrodes that are separated by a narrow insulating quartz channel [i.e., an ITO interdigitated array (IDA) electrode]. Optical microscopy images and atomic force microscopy measurements revealed that the ML-MoS₂ triangles lie conformally on the quartz and ITO substrates. The PL spectrum of MoS₂ shifts and decreases in intensity in the ITO region, which can be attributed to differences in nonradiative and radiative exciton decay channels. Raman spectra showed no significant peak shifts on the two substrates that would have indicated a substrate-induced doping effect. We spatially resolved the photo-electrochemical current because of hole-induced iodide oxidation and observed that ML-MoS₂ produces lower photocurrents in the quartz region than in the ITO region. The correlated PL, Raman, and photocurrent mapping data show that the MoS₂/quartz interface diminishes fast nonradiative exciton decay pathways but the photocurrent response in the quartz region is likely limited by inefficient in-plane carrier transport to the ITO electrode because of carrier recombination with S vacancies in MoS₂ or charged impurities in the quartz substrate. Nonetheless, the experimental methodology presented herein provides a framework to evaluate substrate engineering strategies to tune the (photo)electrocatalytic properties of two-dimensional materials.

Interference Provides Clarity: Direct Observation of 2D Materials at Fluid-Fluid Interfaces

Monolayer particles of two-dimensional (2D) materials represent a scientifically and technologically interesting class of anisotropic particles with colloidal-scale lateral sizes but sub-nanometer thicknesses. This atomic-scale thickness leads to interesting phenomena that can be exploited in next-generation thin-film technologies, and fluid-fluid interfaces provide a potentially scalable platform to confine, assemble, and deposit functional thin films of 2D materials. However, directly observing how these materials interact and assemble into a given film morphology is experimentally challenging because of their sub-nanometer thicknesses. Here, we demonstrate the ability to directly observe graphene, molybdenum disulfide (MoS₂), and hexagonal boron nitride (h-BN) particles at fluid-fluid interfaces using interference reflection microscopy (IRM). Monolayer MoS₂ and graphene particles demonstrated >10% optical contrast at an air-water interface, which allowed us to quantitatively analyze in situ images of self-assembled MoS₂ particles and to map trajectories of interacting graphene particles. Additionally, the Brownian motion of a graphene particle was tracked and analyzed in the context of passive microrheology theory for 2D particle probes. Our results demonstrate how IRM can be used to obtain quantitative spatiotemporal information regarding the self-assembly and dynamics of 2D materials at fluid-fluid interfaces. It will have a significant impact on our ability to investigate systems of atomically thin particles at fluid-fluid interfaces, an area that has fundamental scientific importance and materials science applications but has suffered from a lack of direct, in situ observation techniques.

Probing Charge Carrier Transport and Recombination Pathways in Monolayer MoS2/WS2 Heterojunction Photoelectrodes

Monolayer heterojunctions such as MoS₂/WS₂ are attractive for solar energy conversion applications because the interfacial electric field spatially separates charge carriers in less than 100 fs. Photoelectrochemical cells represent an intriguing platform to collect the spatially separated carriers. However, the recombination, transport, and interfacial charge transfer processes that take place following the ultrafast charge separation step have not been investigated. Here we demonstrate novel charge recombination and transport pathways in monolayer MoS₂/WS₂ photoelectrochemical cells by spatially resolving the net collection of carriers (i.e., the photocurrent) at the single nanosheet level. We discovered an excitation-wavelength-dependent recombination pathway that depends on the heterojunction stacking configuration and the carrier generation profile in the heterostructure. Photocurrent mapping measurements revealed that charge transport occurs parallel to the layers over micrometer-scale distances even though the indium tin oxide electrode and liquid electrolyte provide efficient charge extraction pathways via intimate electron- and hole-selective contacts. Our results reveal how composition heterogeneity influences the performance of bulk heterojunction electrodes made from randomly oriented nanosheets and provide critical insight into the design of efficient heterojunction photoelectrodes for solar energy conversion applications.

Single nanoparticle photoelectrochemistry: What is next?

Semiconductor photoelectrochemistry is a fascinating field that deals with the chemistry and physics of photodriven reactions at solid/liquid interfaces. The interdisciplinary field attracts (electro)chemists, materials scientists, spectroscopists, and theorists to study fundamental and applied problems such as carrier dynamics at illuminated electrode/electrolyte interfaces and solar energy conversion to electricity or chemical fuels. In the pursuit of practical photoelectrochemical energy conversion systems, researchers are exploring inexpensive, solution-processed semiconductor nanomaterials as light absorbers. Harnessing the enormous potential of nanomaterials for energy conversion applications requires a fundamental understanding of charge carrier generation, separation, transport, and interfacial charge transfer at heterogeneous nanoscale interfaces. Our current understanding of these processes is derived mainly from ensemble-average measurements of nanoparticle electrodes that report on the average behavior of trillions of nanoparticles. Ensemble-average measurements conceal how nanoparticle heterogeneity (e.g., differences in particle size, shape, and surface structure) contributes to the overall photoelectrochemical response. This perspective article focuses on the emerging area of single particle photoelectrochemistry, which has opened up an exciting new frontier: direct investigations of photodriven reactions on individual nanomaterials, with the ability to elucidate the role of particle-dependent properties on the photoelectrochemical behavior. Here, we (1) review the basic principles of photoelectrochemical cells, (2) point out the potential advantages and differences between bulk and nanoelectrodes, (3) introduce approaches to single nanoparticle photoelectrochemistry and highlight key findings, and (4) provide our perspective on future research directions.

In Situ Integration of ReS2/Ni3S2 p-n Heterostructure for Enhanced Photoelectrocatalytic Performance

The excellent light absorption, low electron-hole recombination rate, and fast reaction kinetics of photogenerated charges are urgently needed for photoelectrochemical (PEC) water splitting. Herein, a novel p-n heterostructure photoelectrode (ReS₂/Ni₃S₂) is constructed via a one-step hydrothermal method, which shows remarkable HER activity under illumination such as a low overpotential (η₁₀) of 106 mV, high IPCE of 10-15%, and good stability. High-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) reveal that the intimate interface and strong electron interaction between ReS₂ and Ni₃S₂ can enhance the light adsorption and provide abundant active sites. The transient absorption (TA) spectroscopy and impedance spectroscopy analyses (EIS) demonstrate the prolonged carrier lifetime and fast charge transfer. All of these are responsible for the improvement of reaction kinetics. This work provides a brand new avenue to explore efficient photoelectrocatalysts for water splitting.

Directly Mapping Photoelectrochemical Behavior within Individual Transition Metal Dichalcogenide Nanosheets

Spatial variations in photoelectrochemical reaction rates within individual p-type WSe₂ nanosheets were mapped through the application of scanning electrochemical cell microscopy (SECCM). The simultaneous topographical and electrochemical information provided via SECCM directly revealed how both sheet thickness and the presence of defect structures affect the local rate of photoelectrochemical reactions for both outer sphere and inner sphere redox couples. Sheet thickness was found to play a dramatic role in reaction rates, with onset potentials shifting by as much as 0.5 V over thicknesses of 20-120 nm, attributable to the inability of thin sheets to support independent space charge layers. Step/edge features were found to play a detrimental role for the outer sphere redox couple investigated (Ru(NH₃)₆³⁺ reduction), with taller steps having larger effects on performance. Shorter step features were found to be beneficial for hydrogen evolution, showing a controlled density of defect features is desirable for inner sphere processes. The studies presented here not only provide valuable, quantitative insights into the behavior of transitional metal dichalcogenide materials but also demonstrate the power of applying SECCM to the study of photoelectrochemical systems, particularly those involving two-dimensional (2D) materials.