Transition metal oxide (TMO) nanomaterials are an important class of materials that exhibit a variety of elemental stoichiometries and thus crystal structures and chemical properties. Often these nanomaterials are applied to energy harvesting, sensing, and catalytic applications due to their band gap manipulability and diverse electronic and magnetic properties. One such sought after optoelectronic property is a light absorption phenomenon known as local surface plasmon resonance (LSPR) which originates from excess conduction band electrons, free carriers, deriving from defects in TMO nanomaterials. Specifically, oxygen deficient tungsten oxide, WO3-x, nanomaterials have demonstrated controllable LSPR absorption peaks based upon morphology and total amount of oxygen vacancies. Currently, there is little known about how the surface of these nanomaterials impact free carrier density (Ne/cm-3), thus their LSPR response. Recently, our group reported that colloidally synthesized 2D WO3-x ultrathin nanoplatelets provide remarkably high free carrier density, 3.95 x 1022 cm-3, due to the large number of surface oxygen vacancies. However, the removal of surface oxygen creates dangling bonds, and a possible electron reorganization can occur to create W+5 where previously a W+6 atom existed. This redox event is expected to decrease the free carrier density in the WO3-x system. Here we present that an in-situ aliphatic phosphonic acid, a multi-coordinated ligand, treatment minimizes the formation of W+5 on our WO3-x nanoplatelets by passivating the dangling bonds. Utilizing electron paramagnetic, X-ray photoelectron, and ultraviolet photoelectron spectroscopies, we demonstrate our ability to curtail the formation of W+5 at the nanoplatelet surface and thus maximize our free carrier density to unprecedented values, as comparable to noble metal nanomaterials. Therefore, we believe that these uniquely prepared hybrid ligand-passivated WO3-x nanoplatelets can be a viable alternative to expensive noble metal nanomaterials for future LSPR-based solid-state devices and solar energy conversion applications.

Surface ligand chemistry-controlled maximization of free carrier density in 2D WO3-x nanoplatelets

ACS Fall 2021

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