Phosphorene, a two-dimensional (2D) monolayer of black phosphorus, has attracted considerable theoretical interest, although the experimental realization of monolayer, bilayer, and few-layer flakes has been a significant challenge. Here we systematically survey conditions for liquid exfoliation to achieve the first large-scale production of monolayer, bilayer, and few-layer phosphorus, with exfoliation demonstrated at the 10-gram scale. We describe a rapid approach for quantifying the thickness of 2D phosphorus and show that monolayer and few-layer flakes produced by our approach are crystalline and unoxidized, while air exposure leads to rapid oxidation and the production of acid. With large quantities of 2D phosphorus now available, we perform the first quantitative measurements of the material's absorption edgewhich is nearly identical to the material's band gap under our experimental conditions-as a function of flake thickness. Our interpretation of the absorbance spectrum relies on an analytical method introduced in this work, allowing the accurate determination of the absorption edge in polydisperse samples of quantum-confined semiconductors. Using this method, we found that the band gap of black phosphorus increased from 0.33 ± 0.02 eV in bulk to 1.88 ± 0.24 eV in bilayers, a range that is larger than any other 2D material. In addition, we quantified a higherenergy optical transition (VB-1 to CB), which changes from 2.0 eV in bulk to 3.23 eV in bilayers. This work describes several methods for producing and analyzing 2D phosphorus while also yielding a class of 2D materials with unprecedented optoelectronic properties.

Phosphorene is emerging as an important two-dimensional semiconductor, but controlling the surface chemistry of phosphorene remains a significant challenge. Here, we show that controlled oxidation of phosphorene determines the composition and spatial distribution of the resulting oxide. We used X-ray photoemission spectroscopy to measure the binding energy shifts that accompany oxidation. We interpreted these spectra by calculating the binding energy shift for 24 likely bonding configurations, including phosphorus oxides and hydroxides located on the basal surface or edges of flakes. After brief exposure to high-purity oxygen or high-purity water vapor at room temperature, we observed phosphorus in the +1 and +2 oxidation states; longer exposures led to a large population of phosphorus in the +3 oxidation state. To provide insight into the spatial distribution of the oxide, transmission electron microscopy was performed at several stages during the oxidation. We found crucial differences between oxygen and water oxidants: while pure oxygen produced an oxide layer on the van der Waals surface, water oxidized the material at pre-existing defects such as edges or steps. We propose a mechanism based on the thermodynamics of electron transfer to interpret these observations. This work opens a route to functionalize the basal surface or edges of two-dimensional (2D) black phosphorus through site-selective chemical reactions and presents the opportunity to explore the synthesis of 2D phosphorene oxide by oxidation.

Zinc spinel ferrite, ZnFe O (ZFO), is an emerging photoanode material for photoelectrochemical (PEC) solar fuel production. However, a lack of fundamental insight into the factors limiting the photocurrent has prevented substantial advance in its performance. Herein, it is found that ZFO nanorod array photoelectrodes with varying crystallinity exhibit vastly different PEC properties. Using a sacrificial hole scavenger (H O ), spatially defined carrier generation, and electrochemical impedance spectroscopy, it is shown that ZFO with a relatively poor crystallinity but a higher spinel inversion degree (due to cation disorder) exhibits superior photogenerated charge separation efficiency and improved majority charge carrier transport compared to ZFO with higher crystallinity and a lower inversion degree. Conversely, the latter condition leads to better charge injection efficiency. Optimization of these factors, and the addition of a nickel-iron oxide cocatalyst overlayer, leads to a new benchmark solar photocurrent for ZFO of 1.0 mA cm at 1.23 V versus reversible hydrogen electrode (RHE) and 1.7 mA cm at 1.6 V versus RHE. Importantly, the observed correlation between the cation disorder and the PEC performance represents a new insight into the factors important to the PEC performance of the spinel ferrites and suggests a path to further improvement.

As organic semiconductors attract increasing attention to application in the fields of bioelectronics and artificial photosynthesis, understanding the factors that determine their robust operation in direct contact with aqueous electrolytes becomes a critical task. Herein we uncover critical factors that influence the operational stability of donor:acceptor bulk heterojunction photocathodes for solar hydrogen production and significantly advance their performance under operational conditions. First, using the direct photoelectrochemical reduction of aqueous Eu 3+ and impedance spectroscopy, we determine that replacing the commonly used fullerene-based electron acceptor with a perylene diimide-based polymer drastically increases operational stability and identify that limiting the photogenerated electron accumulation at the organic/water interface to values of ca. 100 nC cm −2 is required for stable operation (>12 h). These insights are extended to solar-driven hydrogen production using MoS 3 , MoP, or RuO 2 water reduction catalyst overlayers where it is found that the catalyst morphology strongly affects performance due to differences in charge extraction. Optimized performance of bulk heterojunction photocathodes coated with a MoS 3 :MoP composite gave 1 Sun photocurrent density up to 8.7 mA cm −2 at 0 V vs RHE (pH 1). However, increased stability was gained with RuO 2 where initial photocurrent density (>8 mA cm −2 ) deceased only 15% or 33% during continuous operation for 8 or 20 h, respectively, thus demonstrating unprecedented robustness without a protection layer. This performance represents a new benchmark for organic semiconductor photocathodes for solar fuel production and advances the understanding of stability criteria for organic semiconductor/water-junction-based devices.

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Exfoliated transition metal dichalcogenide (TMD) nanomaterials possess remarkable and tunable semiconducting properties which make them competitive for use in ultrathin, flexible devices such as photodetectors, sensors, and photoelectrodes for solar energy conversion. Until now, the large-scale production of such devices has presented a challenge that limits their commercialization and broad application. We demonstrate here, for the first time, a procedure for the roll-to-roll (R2R) deposition of two-dimensional (2D) TMD nanoflake thin films via a continuous liquid–liquid interfacial self-assembly method. Remarkably, no solvent removal is required during film deposition, nor is there any nanoflake accumulation in the solvent bath, making continuous operation feasible. In our prototype deposition system described herein, TMD nanoflakes (9 nm average flake thickness, 50–500 nm long) are self-assembled into large area films up to 100 mm in width that are reproducibly printed at 10 mm s–1 with nanoflake loadings of 35 mg m–2. Optically uniform coverage on transparent conductive oxide-coated flexible plastic substrates is shown, and key printing parameters to afford the required single-flake-layer deposition are identified. Photoelectrochemical testing verifies optoelectronic activity of the MoS2 nanoflake films achieving photocurrent densities for iodide oxidation of 40 μA cm–2 at +0.1 V (vs an Ag/Ag+ reference) under 1 sun illumination. In addition, we show that these R2R films are robust and that our method works for multiple (successive) flake layer depositions by preparing large area MoS2/WSe2 heterojunction nanoflake films. This demonstration represents an important milestone in advancing large-scale production of 2D TMD films toward low-cost, high-performance optoelectronic devices.

Organic semiconductors (OSs) are emerging candidates as light-harvesting materials in photoelectrochemical (PEC) cells for direct solar-to-chemical conversion. Despite significant recent progress with OS-based photocathodes, the development of efficient and stable...

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 MoS 2 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 cm 2 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 MoS 2 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 × 10 13 cm −2 ) comparable to monolayer MoS 2 prepared by vacuum-based techniques and at least three times less than ultrasonicated MoS 2 nanoflakes. Finally, we expand this method toward other TMDs (WS 2 , WSe 2 ) to demonstrate its versatility toward high-performance and fully scalable van der Waals heterojunction devices.