Tuning Optical\nand Electrical Properties of Vanadium\nOxide with Topochemical Reduction and Substitutional Tin

Vanadium oxides are widely tunable materials, with many\nthermodynamically\nstable phases suitable for applications spanning catalysis to neuromorphic\ncomputing. The stability of vanadium in a range of oxidation states\nenables mixed-valence polymorphs of kinetically accessible metastable\nmaterials. Low-temperature synthetic routes to, and the properties\nof, these metastable materials are poorly understood and may unlock\nnew optoelectronic and magnetic functionalities for expanded applications.\nIn this work, we demonstrate topochemical reduction of α-V<sub>2</sub>O<sub>5</sub> to produce metastable vanadium oxide phases\nwith tunable oxygen vacancies (>6%) and simultaneous substitutional\ntin incorporation (>3.5%). The chemistry is carried out at low\ntemperature\n(65 °C) with solution-phase SnCl<sub>2</sub>, where Sn<sup>2+</sup> is oxidized to Sn<sup>4+</sup> as V<sup>5+</sup> sites are reduced\nto V<sup>4+</sup> during oxygen vacancy formation. Despite high oxygen\nvacancy and tin concentrations, the transformations are topochemical\nin that the symmetry of the parent crystal remains intact, although\nthe unit cell expands. Band structure calculations show that these\nvacancies contribute electrons to the lattice, whereas substitutional\ntin contributes holes, yielding a compensation doping effect and control\nover the electronic properties. The SnCl<sub>2</sub> redox chemistry\nis effective on both solution-processed V<sub>2</sub>O<sub>5</sub> nanoparticle inks and mesoporous films cast from untreated inks,\nenabling versatile routes toward functional films with tunable optical\nand electronic properties. The electrical conductance rises concomitantly\nwith the SnCl<sub>2</sub> concentration and treatment time, indicating\na net increase in density of free electrons in the host lattice. This\nwork provides a valuable demonstration of kinetic tailoring of electronic\nproperties of vanadium–oxygen systems through top-down chemical\nmanipulation from known thermodynamic phases.