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Here, we demonstrate a two-step electrosynthesis approach for the preparation of silver pyrovanadate, Ag 4 V 2 O 7 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 CO 2 -infused sodium orthovanadate (Na 3 VO 4 ) 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 Ag 4 V 2 O 7 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 Ag 4 V 2 O 7 semiconductor. Finally, the electrochemical and photoelectrochemical properties of the electrosynthesized Ag 4 V 2 O 7 thin films were studied in both aqueous and non-aqueous electrolytes. 

This Perspective addresses the current state-of-the-art with the development of multinary oxides—a family of compounds that has long interested Prof. John B. Goodenough. Specifically, here we focus on their use as photoelectrodes for solar fuels generation. Using optical data and assuming an idealized 100% incident photon-to-electron conversion efficiency, it is possible to project the maximum short circuit photocurrent efficiency to be expected for a given oxide semiconductor. The performance gap between this theoretical value and that realized experimentally, is shown to be sizable for all but a couple of candidates. The technical issues underlying this gap and strategies for closing it are presented below.