The remarkable performance of lead halide perovskites in solar cells can be attributed to the long carrier lifetimes and low non-radiative recombination rates, the same physical properties that are ideal for semiconductor lasers. Here, we show room-temperature and wavelength-tunable lasing from single-crystal lead halide perovskite nanowires with very low lasing thresholds (220 nJ cm(-2)) and high quality factors (Q ∼ 3,600). The lasing threshold corresponds to a charge carrier density as low as 1.5 × 10(16) cm(-3). Kinetic analysis based on time-resolved fluorescence reveals little charge carrier trapping in these single-crystal nanowires and gives estimated lasing quantum yields approaching 100%. Such lasing performance, coupled with the facile solution growth of single-crystal nanowires and the broad stoichiometry-dependent tunability of emission colour, makes lead halide perovskites ideal materials for the development of nanophotonics, in parallel with the rapid development in photovoltaics from the same materials.

The development of efficient and robust earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) is an ongoing challenge. We report metallic cobalt pyrite (cobalt disulfide, CoS2) as one such high-activity candidate material and demonstrate that its specific morphology--film, microwire, or nanowire, made available through controlled synthesis--plays a crucial role in determining its overall catalytic efficacy. The increase in effective electrode surface area that accompanies CoS2 micro- and nanostructuring substantially boosts its HER catalytic performance, with CoS2 nanowire electrodes achieving geometric current densities of -10 mA cm(-2) at overpotentials as low as -145 mV vs the reversible hydrogen electrode. Moreover, micro- and nanostructuring of the CoS2 material has the synergistic effect of increasing its operational stability, cyclability, and maximum achievable rate of hydrogen generation by promoting the release of evolved gas bubbles from the electrode surface. The benefits of catalyst micro- and nanostructuring are further demonstrated by the increased electrocatalytic activity of CoS2 nanowire electrodes over planar film electrodes toward polysulfide and triiodide reduction, which suggests a straightforward way to improve the performance of quantum dot- and dye-sensitized solar cells, respectively. Extension of this micro- and nanostructuring strategy to other earth-abundant materials could similarly enable inexpensive electrocatalysts that lack the high intrinsic activity of the noble metals.

The scalable and sustainable production of hydrogen fuel through water splitting demands efficient and robust Earth-abundant catalysts for the hydrogen evolution reaction (HER). Building on promising metal compounds with high HER catalytic activity, such as pyrite structure cobalt disulphide (CoS2), and substituting non-metal elements to tune the hydrogen adsorption free energy could lead to further improvements in catalytic activity. Here we present a combined theoretical and experimental study to establish ternary pyrite-type cobalt phosphosulphide (CoPS) as a high-performance Earth-abundant catalyst for electrochemical and photoelectrochemical hydrogen production. Nanostructured CoPS electrodes achieved a geometrical catalytic current density of 10 mA cm(-2) at overpotentials as low as 48 mV, with outstanding long-term operational stability. Integrated photocathodes of CoPS on n(+)-p-p(+) silicon micropyramids achieved photocurrents up to 35 mA cm(-2) at 0 V versus the reversible hydrogen electrode (RHE), onset photovoltages as high as 450 mV versus RHE, and the most efficient solar-driven hydrogen generation from Earth-abundant systems.

Lead halide perovskite nanowires (NWs) are emerging as a class of inexpensive semiconductors with broad bandgap tunability for optoelectronics, such as tunable NW lasers. Despite exciting progress, the current organic-inorganic hybrid perovskite NW lasers suffer from limited tunable wavelength range and poor material stability. Herein, we report facile solution growth of single-crystal NWs of inorganic perovskite CsPbX3 (X = Br, Cl) and their alloys [CsPb(Br,Cl)3] and a low-temperature vapor-phase halide exchange method to convert CsPbBr3 NWs into perovskite phase CsPb(Br,I)3 alloys and metastable CsPbI3 with well-preserved perovskite crystal lattice and NW morphology. These single crystalline NWs with smooth end facets and subwavelength dimensions are ideal Fabry-Perot cavities for NW lasers. Optically pumped tunable lasing across the entire visible spectrum (420-710 nm) is demonstrated at room temperature from these NWs with low lasing thresholds and high-quality factors. Such highly efficient lasing similar to what can be achieved with organic-inorganic hybrid perovskites indicates that organic cation is not essential for light emission application from these lead halide perovskite materials. Furthermore, the CsPbBr3 NW lasers show stable lasing emission with no measurable degradation after at least 8 h or 7.2 × 10(9) laser shots under continuous illumination, which are substantially more robust than their organic-inorganic counterparts. The Cs-based perovskites offer a stable material platform for tunable NW lasers and other nanoscale optoelectronic devices.

The excellent intrinsic optoelectronic properties of methylammonium lead halide perovskites (MAPbX3, X = Br, I), such as high photoluminescence quantum efficiency, long carrier lifetime, and high gain coupled with the facile solution growth of nanowires make them promising new materials for ultralow-threshold nanowire lasers. However, their photo and thermal stabilities need to be improved for practical applications. Herein, we report a low-temperature solution growth of single crystal nanowires of formamidinium lead halide perovskites (FAPbX3) that feature red-shifted emission and better thermal stability compared to MAPbX3. We demonstrate optically pumped room-temperature near-infrared (∼820 nm) and green lasing (∼560 nm) from FAPbI3 (and MABr-stabilized FAPbI3) and FAPbBr3 nanowires with low lasing thresholds of several microjoules per square centimeter and high quality factors of about 1500-2300. More remarkably, the FAPbI3 and MABr-stabilized FAPbI3 nanowires display durable room-temperature lasing under ∼10(8) shots of sustained illumination of 402 nm pulsed laser excitation (150 fs, 250 kHz), substantially exceeding the stability of MAPbI3 (∼10(7) laser shots). We further demonstrate tunable nanowire lasers in wider wavelength region from FA-based lead halide perovskite alloys (FA,MA)PbI3 and (FA,MA)Pb(I,Br)3 through cation and anion substitutions. The results suggest that formamidinium lead halide perovskite nanostructures could be more promising and stable materials for the development of light-emitting diodes and continuous-wave lasers.

Many materials have been explored as potential hydrogen evolution reaction (HER) electrocatalysts to generate clean hydrogen fuel via water electrolysis, but none so far compete with the highly efficient and stable (but cost prohibitive) noble metals. Similarly, noble metals often excel as electrocatalytic counter electrode materials in regenerative liquid-junction photoelectrochemical solar cells, such as quantum dot-sensitized solar cells (QDSSCs) that employ the sulfide/polysulfide redox electrolyte as the hole mediator. Here, we systematically investigate thin films of the earth-abundant pyrite-phase transition metal disulfides (FeS2, CoS2, NiS2, and their alloys) as promising alternative electrocatalysts for both the HER and polysulfide reduction. Their electrocatalytic activity toward the HER is correlated to their composition and morphology. The emergent trends in their performance suggest that cobalt plays an important role in facilitating the HER, with CoS2 exhibiting highest overall performance. Additionally, we demonstrate the high activity of the transition metal pyrites toward polysulfide reduction and highlight the particularly high intrinsic activity of NiS2, which could enable improved QDSSC performance. Furthermore, structural disorder introduced by alloying different transition metal pyrites could increase their areal density of active sites for catalysis, leading to enhanced performance.

Producing hydrogen fuel through environmentally friendly electrochemical and solar-driven photoelectrochemical (PEC) water splitting is a very promising approach for providing affordable clean energy. The scalable and sustainable production of hydrogen demands efficient and robust earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) beyond platinum and other precious-metal catalysts. This review provides an overview of molybdenum disulfide (MoS 2) and related compounds as inexpensive alternative electrocatalysts for HER catalysis and PEC water splitting. After a background introduction, we discuss the important approaches to improving the intrinsic catalytic activity and overall catalytic performance of MoS 2. We further review the key developments in combining MoS 2 with semiconductors for integrated PEC systems for direct solar-to-fuel conversion and provide insights on how to design efficient solar-driven water-splitting systems. Our perspectives on the key challenges and future directions for development of earth-abundant HER electrocatalysts and PEC water splitting are also discussed.

We report the preparation and characterization of highly efficient and robust photocathodes based on heterostructures of chemically exfoliated metallic 1T-MoS2 and planar p-type Si for solar-driven hydrogen production. Photocurrents up to 17.6 mA/cm(2) at 0 V vs reversible hydrogen electrode were achieved under simulated 1 sun irradiation, and excellent stability was demonstrated over long-term operation. Electrochemical impedance spectroscopy revealed low charge-transfer resistances at the semiconductor/catalyst and catalyst/electrolyte interfaces, and surface photoresponse measurements also demonstrated slow carrier recombination dynamics and consequently efficient charge carrier separation, providing further evidence for the superior performance. Our results suggest that chemically exfoliated 1T-MoS2/Si heterostructures are promising earth-abundant alternatives to photocathodes based on noble metal catalysts for solar-driven hydrogen production.