Near-Complete Suppression of Oxygen Evolution for Photoelectrochemical H2O Oxidative H2O2 Synthesis.

Solar energy-assisted water oxidative hydrogen peroxide (H2O2) production on an anode combined with H2 production on a cathode increases the value of solar water splitting, but the challenge of the dominant oxidative product, O2, needs to be overcome. Here, we report a SnO2-x overlayer coated BiVO4 photoanode, which demonstrates the great ability to near-completely suppress O2 evolution for photoelectrochemical (PEC) H2O oxidative H2O2 evolution. Based on the surface hole accumulation measured by surface photovoltage, downward quasi-hole Fermi energy at the photoanode/electrolyte interface and thermodynamic Gibbs free energy between 2-electron and 4-electron competitive reactions, we are able to consider the photoinduced holes of BiVO4 that migrate to the SnO2-x overlayer kinetically favor H2O2 evolution with great selectivity by reduced band bending. The formation of H2O2 may be mediated by the formation of hydroxyl radicals (OH·), from 1-electron water oxidation reactions, as evidenced by spin-trapping electron paramagnetic resonance (EPR) studies conducted herein. In addition to the H2O oxidative H2O2 evolution from PEC water splitting, the SnO2-x/BiVO4 photoanode can also inhibit H2O2 decomposition into O2 under either electrocatalysis or photocatalysis conditions for continuous H2O2 accumulation. Overall, the SnO2-x/BiVO4 photoanode achieves a Faraday efficiency (FE) of over 86% for H2O2 generation in a wide potential region (0.6-2.1 V vs reversible hydrogen electrode (RHE)) and an H2O2 evolution rate averaging 0.825 µmol/min/cm2 at 1.23 V vs RHE under AM 1.5 illumination, corresponding to a solar to H2O2 efficiency of ∼5.6%; this performance surpasses almost all previous solar energy-assisted H2O2 evolution performances. Because of the simultaneous production of H2O2 and H2 by solar water splitting in the PEC cells, our results highlight a potentially greener and more cost-effective approach for "solar-to-fuel" conversion.

Aligned Heterointerface-Induced 1T-MoS2 Monolayer with Near-Ideal Gibbs Free for Stable Hydrogen Evolution Reaction.

1T-phase molybdenum disulfide (1T-MoS2 ) exhibits superior hydrogen evolution reaction (HER) over 2H-phase MoS2 (2H-MoS2 ). However, its thermodynamic instability is the main drawback impeding its practical application. In this work, a stable 1T-MoS2 monolayer formed at edge-aligned 2H-MoS2 and a reduced graphene oxide heterointerface (EA-2H/1T/RGO) using a precursor-in-solvent synthesis strategy are reported. Theoretical prediction indicates that the edge-aligned layer stacking can induce heterointerfacial charge transfer, which results in a phase transition of the interfacial monolayer from 2H to 1T that realizes thermodynamic stability based on the adhesion energy between MoS2 and graphene. As an electrocatalyst for HER, EA-2H/1T/RGO displays an onset potential of -103 mV versus RHE, a Tafel slope of 46 mV dec-1 and 10 h stability in acidic electrolyte. The unexpected activity of EA-2H/1T/RGO beyond 1T-MoS2 is due to an inherent defect caused by the gliding of S atoms during the phase transition from 2H to 1T, leading the Gibbs free energy of hydrogen adsorption (ΔGH* ) to decrease from 0.13 to 0.07 eV, which is closest to the ideal value (0.06 eV) of 2H-MoS2 . The presented work provides fundamental insights into the impressive electrochemical properties of HER and opens new avenues for phase transitions at 2D/2D hybrid interfaces.

Defect-Induced Epitaxial Growth for Efficient Solar Hydrogen Production.

Epitaxial growth suffers from the mismatches in lattice and dangling bonds arising from different crystal structures or unit cell parameters. Here, we demonstrate the epitaxial growth of 2D MoS2 ribbon on 1D CdS nanowires (NWs) via surface and subsurface defects. The interstitial Cd0 in the (12̅10) crystal plane of the [0001]-oriented CdS NWs are found to serve as nucleation sites for interatomically bonded [001]-oriented MoS2, where the perfect lattice match (∼99.7%) between the (101̅1) plane of CdS and the (002)-faceted in-plane MoS2 result in coaxial MoS2 ribbon/CdS NWs heterojunction. The coaxial but heterotropic epitaxial MoS2 ribbon on the surface of CdS NWs induces delocalized interface states that facilitate charge transport and the reduced surface state. A less than 5-fold ribbon width of MoS2 as hydrogen evolution cocatalyst exhibits a ∼10-fold H2 evolution enhancement than state of the art Pt in an acidic electrolyte, and apparent quantum yields of 79.7% at 420 nm, 53.1% at 450 nm, and 9.67% at 520 nm, respectively.

Rationally embedded zinc oxide nanospheres serving as electron transport channels in bismuth vanadate/zinc oxide heterostructures for improved photoelectrochemical efficiency.

In this work, we demonstrate the fabrication of Mo, W co-doped BiVO4/ZnO nanosphere (Mo, W: BVO/ZnO NS) heterostructures using a simple solution dry-out method that uniformly embeds ZnO NSs in Mo, W: BVO nanocrystals. Photoelectrochemical (PEC) examination confirmed that Mo, W: BVO/ZnO NSs exhibit higher PEC performance than pure Mo, W: BVO, and this improved performance of Mo, W: BVO/ZnO NSs also depends on decreased ZnO NS size. Moreover, a layered ZnO/Mo, W: BVO (ZnO film coated with Mo, W: BVO layer) heterostructure was prepared using a simple physical piled method that exhibited far lower PEC performances than those of Mo, W: BVO/ZnO NS heterostructures. These results clearly revealed that the formation of the Mo, W: BVO/ZnO NS heterostructure could increase the charge carrier density and it possessed a much larger contact area. These improvements were favorable for the transfer and transport of photoexcited charge carriers. This work offers an effective strategy to fabricate heterostructures with effective charge separation by a simple solution dry-out method that is suitable for other heterostructure-engineered photoanodes in solar water-splitting applications.

Rationally Designed Copper-Modified Polymeric Carbon Nitride as a Photocathode for Solar Water Splitting.

Polymeric carbon nitride has been considered to be an active photocathode for catalyzing the generation of H2 through water splitting. However, the application of this material in photoelectrochemical cells remains a challenge owing to the intrinsically sluggish kinetics of charge separation. Herein, a facile salt-melt method is developed for fabricating Cu-modified polymeric carbon nitride as an effective photocathode material for solar water splitting. Various characterization data confirm that Cu-modified polymeric carbon nitride contains both free CuCl, derived from precursors, and coordinated Cu species incorporated into the polymeric carbon nitride, which can generate type-II heterojunctions. This special heterojunction energy structure contributes to a significantly enhanced photocurrent density for hydrogen evolution. The proposed strategy for synthesizing the Cu-modified polymeric carbon nitride can stimulate research for the development of highly efficient visible-light-active photocathodes.

Black phosphorene as a hole extraction layer boosting solar water splitting of oxygen evolution catalysts.

As the development of oxygen evolution co-catalysts (OECs) is being actively undertaken, the tailored integration of those OECs with photoanodes is expected to be a plausible avenue for achieving highly efficient solar-assisted water splitting. Here, we demonstrate that a black phosphorene (BP) layer, inserted between the OEC and BiVO4 can improve the photoelectrochemical performance of pre-optimized OEC/BiVO4 (OEC: NiOOH, MnOx, and CoOOH) systems by 1.2∼1.6-fold, while the OEC overlayer, in turn, can suppress BP self-oxidation to achieve a high durability. A photocurrent density of 4.48 mA·cm-2 at 1.23 V vs reversible hydrogen electrode (RHE) is achieved by the NiOOH/BP/BiVO4 photoanode. It is found that the intrinsic p-type BP can boost hole extraction from BiVO4 and prolong holes trapping lifetime on BiVO4 surface. This work sheds light on the design of BP-based devices for application in solar to fuel conversion, and also suggests a promising nexus between semiconductor and electrocatalyst.

Amorphous Phosphorus-Incorporated Cobalt Molybdenum Sulfide on Carbon Cloth: An Efficient and Stable Electrocatalyst for Enhanced Overall Water Splitting over Entire pH Values.

The development of economical, proficient, and highly stable catalysts to substitute the expensive noble metal electrodes for electrocatalytic water-splitting applications is exceedingly desirable. In this context, the most fascinating and challenging approach is the rational design of a nanocomposite encompassing multiple components with unique functionalities. Herein, we describe the fabrication of a strongly catalytic and superb durable phosphorus-incorporated cobalt molybdenum sulfide electrocatalyst grown on carbon cloth (P-CoMoS/CC). The hybrid material exhibited excellent activity for hydrogen and oxygen evolution reactions over a wide range of pH (1-14) with extremely high stability (∼90% retention of the initial current density) after 24 h of electrolysis. Importantly, when P-CoMoS/CC was used as both cathode and anode for overall water splitting, a very low cell voltage of 1.54 V is required to attain the 10 mA cm-2 current density, and the hybrid material exhibited a long-term stability (89.8% activity retention after 100 h). The outstanding overall water-splitting performance compared to an electrolyzer consisting of the noble-metal-based catalysts Pt/C and RuO2 makes P-CoMoS one of the most efficient earth-abundant water-splitting catalysts. Phosphorus incorporation was proved to be a vital aspect for the improved charge-transfer properties and catalytic durability of the P-CoMoS/CC catalyst.

Electric field-directed growth and photoelectrochemical properties of cross-linked Au-ZnO hetero-nanowire arrays.

We demonstrate a simple and effective strategy for vertically growing Au nanowires over ZnO nanowire arrays to construct a cross-linked hierarchical architecture under the synergistic effects of the electric field direction and photoreduction, which exhibit excellent photoelectrochemical performances under visible light irradiation.