Electrochemical Solar Cell Based on the d-Band Semiconductor Tungsten-Diselenide

Long-term stability and tree-ring oxidation of WSe2 using phase-contrast AFM

In this work, we use atomic force microscopy (AFM) to investigate the long-term evolution of oxidative defects of tungsten diselenide (WSe₂) in ambient conditions over a period of 75 months, which is the longest such study performed on any layered material. In particular, we find that phase-imaging AFM of mechanically exfoliated WSe₂ crystals provides convenient, direct identification of exposed and covered step-edges, and together with topographic thickness measurements allows complete determination of the layer arrangement in a multilayer flake. Step-edges with low or no phase-contrast consistently exhibit long-term stability in ambient conditions, indicating that they are covered and effectively protected by above-lying WSe₂ layers. On the contrary, step-edges with initial high phase-contrast are clearly degraded after medium- to long-term exposure to ambient conditions (up to six months), indicating that these are not covered by other layers. Similar behaviour was observed for MoTe₂ and MoS₂. The correlation between phase-contrast and step order was confirmed by cross-sectional transmission electron microscopy. By comparing the phase-contrast line-traces in different locations and at different times, we find that long-term storage in ambient conditions led to evolution of a distinct ring-like pattern resembling the tree-lines arising from seasonal changes. Indeed the phase-contrast showed correlation with the average amount of sun-hours registered at the storage location. Storage in darkness slowed down the evolution of the tree-ring lines, in accordance with this explanation. Our work provides a unique dataset on long-term degradation of one of the most stable transition metal dichalcogenides, as well as insights into the conditions causing acceleration or inhibition of the degradation process.

Single Nanoflake Photoelectrochemistry Reveals Intrananoflake Doping Heterogeneity That Explains Ensemble-Level Photoelectrochemical Behavior

Transition metal dichalcogenide (TMD) nanoflake thin films are attractive electrode materials for photoelectrochemical (PEC) solar energy conversion and sensing applications, but their photocurrent quantum yields are generally lower than those of bulk TMD electrodes. The poor PEC performance has been primarily attributed to enhanced charge carrier recombination at exposed defect and edge sites introduced by the exfoliation process. Here, a single nanoflake PEC approach reveals how an alternative effect, doping heterogeneity, limits ensemble-level PEC performance. Photocurrent mapping and local photocurrent-potential (i-E) measurements of MoS2 nanoflakes exfoliated from naturally occurring bulk crystals revealed the presence of n- and p-type domains within the same nanoflake. Interestingly, the n- and p-type domains in the natural MoS2 nanoflakes were equally efficient for iodide oxidation and tri-iodide reduction (IQE values exceed 80%). At the single domain-level, the natural MoS2 nanoflakes were nearly as efficient as nanoflakes exfoliated from synthetic n-type MoS2 crystals. Single domain-level i-E measurements explain why natural MoS2 nanoflakes exhibit an n-type to p-type photocurrent switching effect in ensemble-level measurements: the n- and p-type diode currents from individual domains oppose each other upon illuminating the entire nanoflake, resulting in zero photocurrent at the switching potential. The doping heterogeneity effect is likely due to nonideal stoichiometry, where p-type domains are S-rich according to XPS measurements. Although this doping heterogeneity effect limits photoanode or photocathode performance, these findings open the possibility to synthesize efficient TMD nanoflake photocatalysts with well-defined lateral p- and n-type domains for enhanced charge separation.

Role of Photogenerated Iodine on the Energy-Conversion Properties of MoSe2 Nanoflake Liquid Junction Photovoltaics

Transition metal dichalcogenides (TMDs) such as MoSe₂ and WSe₂ are efficient materials for converting solar energy to electrical energy in photoelectrochemical photovoltaic cells. One limiting factor of these liquid junction solar cells is that photogenerated oxidation products accumulate on the electrode surface and decrease the photocurrent efficiency. However, it is unclear where the reaction products accumulate on the electrode surface and how they impact the local photoelectrochemical response. This open question is especially important for the structurally heterogeneous TMD nanoflake thin-film electrodes that are promising for large-area solar energy conversion applications. Here, we use a single-nanoflake photoelectrochemical and Raman microscopy approach to probe how the photogenerated I₂/I₃^- products impact the photocurrent collection efficiency and the onset potential in MoSe₂-nanoflake|I^-/I₂|Pt photoelectrochemical solar cells. We observed localized I₂/I₃^- deposition on all types of MoSe₂ nanoflake surface motifs, including basal planes, perimeter edges, and interior step edges. Illuminated nanoflake spots with the highest photocurrent collection efficiency are the first to be limited by I₂/I₃^- formation under high-intensity illumination. Interestingly, I₂/I₃^- formation occurs on illuminated surface spots that have the lowest photocurrent onset potential for iodide oxidation, corresponding to the highest open circuit voltage ( V_OC). The V_OC shifts could be attributed to variations in the surface reaction kinetics or doping density across the nanoflake. Our results highlight important limiting factors of nanoflake thin-film TMD liquid junction photovoltaics under concentrated solar illumination intensities.

Highly (001)-textured p-type WSe2 Thin Films as Efficient Large-Area Photocathodes for Solar Hydrogen Evolution

Highly (001)-textured, photoactive WSe₂ thin films have been prepared by an amorphous solid-liquid-crystalline solid process promoted by palladium. By increasing the thickness of the Pd promoter film (≥10 nm) the structure and texture of the WSe₂ films can be improved significantly. However, these as-crystallized WSe₂ films are only weakly photoactive in a 0.5 М H₂SO₄ electrolyte under AM 1.5 solar irradiation which we attribute to an inefficient photogenerated charge transfer across the WSe₂/electrolyte interface via the prevailing van der Waals planes of the WSe₂ crystallites. In this work photochemically deposited platinum on the p-type WSe₂ photocathode is used for an efficient electron transfer thus inducing the hydrogen evolution reaction. Upon illuminating the WSe₂ photocathodes in a Pt-ion containing electrolyte, the photogenerated electrons reduce Pt⁺ to Pt leading to the precipitation of Pt islands, preferentially at edge steps of the WSe₂, i.e. at the grain boundaries of the WSe₂ crystallites. The increasing amount of Pt islands at the grain boundaries linearly enhances the photocurrent density up to 2.5 mA cm⁻² at 0 V_RHE in sulfuric acid, the highest reported value up to now for WSe₂ thin films.

Tunable donor and acceptor impurity states in a WSe2 monolayer by adsorption of common gas molecules

A gas sensor of common gas molecules, such as CO, H₂O, NH₃, O₂, NO and NO₂ on a WSe₂ monolayer is investigated systematically by using first-principle calculations.

Hydrogen-Evolving Solar Cells

Sunlight is directly converted to chemical energy in hydrogen-evolving photoelectrochemical cells with semiconductor electrodes. Their Gibbs free energy efficiency of solar-to-hydrogen conversion, 13.3 percent, exceeds the solar-to-fuel conversion efficiency of green plants and approaches the solar-to-electrical conversion efficiency of the best p-n junction cells. In hydrogen-evolving photoelectrodes, electron-hole pairs photogenerated in the semiconductor are separated at electrical microcontacts between the semiconductor and group VIII metal catalyst islands. Conversion is efficient when the island diameters are small relative to the wave-lengths of sunlight exciting the semiconductor; when the island spacings are smaller than the diffusion length of electrons at the semiconductor surface; when the height of the potential energy barriers that separate the photogenerated electrons from holes at the semiconductor surface is raised by hydrogen alloying of the islands; when radiationless recombination of electron-hole pairs at the semiconductor-solution interface between the islands is suppressed by controlling the semiconductor surface chemistry; and when the semiconductor has an appropriate band gap (1.0 to 1.8 electron volts) for efficient solar conversion.