Tunable Electrocatalytic Activity of Atomically Thin Film Platinum Catalyst Electrodeposited on Sub-Stoichiometric Titania

Abstract

Platinum (Pt) is an essential catalyst for many reactions including, but not limited to, oxidation/reduction, hydrogenation, and water gas shift. Despite its common use, the major disadvantages of platinum lie within its high cost due to its low abundance, and propensity for deactivation through poisoning. One way to address this setback is to reduce its loading content in catalyst design. Recently, our group has successfully synthesized a wetted atomic layer- by- layer Pt on carbon-based and noble metal supports via the surface limited redox replacement mechanism. This methodological approach produces atomically thin platinum films with the unprecedented electronic tuning of the catalyst activity induced by the supporting material. Implementing transition metal oxides as the support with tuned electronic properties (such as work function) can dictate the activity of the surface catalyst atoms through vicinity effects. This research project focuses on the electrochemical deposition and structural characterization of atomically thin-filmed platinum supported on oxygen deficient titanium oxide (TixOy) using imaging and spectroscopic techniques. TixOy supports are used to alleviate corrosion issues caused by the Vulcan XC-72, a known carbon support that is commonly used in fuel cells. TixOy films of varying oxygen vacancies are synthesized via pulsed laser deposition at elevated substrate temperatures in a forming gas environment. Four-point probe measurements, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and ultraviolet photoelectron spectroscopy (UPS) are used to examine the chemical and electrical properties of the TixOy supports. It has been revealed that the films deposited at 400-600oC display a Ti2O3-like phase while the films deposited at 700oC revealed a Ti3O5-like phase. The resistivity of stoichiometric titania films was immeasurable; however, by increasing the substrate deposition temperatures the films became more conductive. UPS demonstrated deviations within the work function of the TiO2, Ti2O3, and Ti3O5-like films. Dissimilarities of the work function values demonstrate transition metal oxide tunability in the energy required for moving an electron from the surface of the solid to an overlying material. To implement the growth of the atomically thin-filmed Pt monolayers, a single layer of graphene is transferred onto TixOy supports prior to the electrochemical deposition of Pt. Pt monolayers were synthesized using surface limited redox replacement mechanism, a spontaneous technique which initially relies on the electrochemical deposition of a sacrificial metal. XPS confirmed the growth of Pt overlayers and the cyclic voltammogram confirmed the low loading of platinum. Future experimental studies are geared towards probing the catalytic activity and durability of the Pt catalyst as a function of varying oxygen concentrations within the TixOy support. The novel electrocatalysts alleviate some of the major problems within fuel cell technology by simultaneously reducing the amount of platinum catalyst needed and modifying its performance through support induced catalytic activity tunability.