Hybrid Cathodic/Anodic Electrosynthesis of Phase Pure Ag4V2O7 Thin Films

Here, we demonstrate a two-step electrosynthesis approach for the preparation of silver pyrovanadate, Ag4V2O7 in thin-film form. In the first, cathodic step, polycrystalline Ag was deposited on fluorine doped tin oxide (FTO) substrate from a non-aqueous bath. Aqueous pyrovanadate species were then generated by aging of a CO2-infused sodium orthovanadate (Na3VO4) solution for three weeks. Silver ions were subsequently generated in situ in this medium using anodic stripping of the Ag/ITO films from the first step. Interfacial precipitation of the Ag+ ions with the pyrovanadate species afforded the targeted product in phase pure form. The various stages of the electrosynthesis were monitored in situ via the combined use of voltammetry, electrochemical quartz crystal nanogravimetry (EQCN), and coulometry. The Ag4V2O7 thin films were characterized by a variety of experimental techniques, including X-ray diffraction, laser Raman spectroscopy, diffuse reflectance spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements afforded information on the energy band structure of the p-type Ag4V2O7 semiconductor. Finally, the electrochemical and photoelectrochemical properties of the electrosynthesized Ag4V2O7 thin films were studied in both aqueous and non-aqueous electrolytes

Peeling off the surface: Pt-decoration of WSe2 nanoflakes results in exceptional photoelectrochemical HER activity

Photoelectrochemical (PEC) hydrogen evolution reaction (HER) was studied on exfoliated, pristine and Pt-decorated tungsten diselenide (p-WSe2) nanoflake samples, using a previously developed microdroplet PEC microscopy approach. The WSe2 nanoflakes had well-defined thicknesses as measured by atomic force microscopy, and the Pt nanoparticles (NPs) were deposited by a variable number of atomic layer deposition (ALD) cycles. An exceptionally high photocurrent density of 49.6 mA cm−2 (under 220 mW cm−2 irradiation) and internal-photon-to-electron-conversion efficiency (∼90% at 550 nm) were demonstrated on these Pt-decorated WSe2 (WSe2-Pt) photocathodes. The Pt NP loading and thickness of WSe2 nanoflakes (in the 24–235 nm range) were used to fine-tune their PEC activity for HER. We found similar charge transfer and surface recombination kinetics of pristine and WSe2-Pt specimens (as assessed by intensity-modulated photocurrent spectroscopy), which indicated significant differences in their bulk properties. X-ray and ultraviolet photoelectron spectroscopies were performed to identify defect states and quantify the density of states around the valence band of WSe2. The elevated temperature of the ALD process and the evolving Pt NP phase conspired to passivate the sub-surface (i.e., bulk) defects in the WSe2 nanoflakes, resulting in their vastly improved PEC performance