Abstract
Electrochemical surface science of oxides is an emerging field with expected high impact in developing, for instance, rationally designed catalysts. The aim in such catalysts is to replace noble metals by earth-abundant elements, yet without sacrificing activity. Gaining an atomic-level understanding of such systems hinges on the use of experimental surface characterization techniques such as scanning tunneling microscopy (STM), in which tungsten tips have been the most widely used probes, both in vacuum and under electrochemical conditions. Here, we present an in situ STM study with atomic resolution that shows how tungsten(VI) oxide, spontaneously generated at a W STM tip, forms 1D adsorbates on oxide substrates. By comparing the behavior of rutile TiO2(110) and magnetite Fe3O4(001) in aqueous solution, we hypothesize that, below the point of zero charge of the oxide substrate, electrostatics causes water-soluble WO3 to efficiently adsorb and form linear chains in a self-limiting manner up to submonolayer coverage. The 1D oligomers can be manipulated and nanopatterned in situ with a scanning probe tip. As WO3 spontaneously forms under all conditions of potential and pH at the tungsten–aqueous solution interface, this phenomenon also identifies an important caveat regarding the usability of tungsten tips in electrochemical surface science of oxides and other highly adsorptive materials.
Highlights
Metal oxides abundant and robust are the prime material candidates for energy-related applications in electro, photo, and heterogeneous catalysis.[1]
As WO3 spontaneously forms under all conditions of potential and pH at the tungsten−aqueous solution interface, this phenomenon identifies an important caveat regarding the usability of tungsten tips in electrochemical surface science of oxides and other highly adsorptive materials
Electrochemical surface science pursues an atomic-level understanding of structure and changes thereof under electrochemical conditions,[4,5] with electrochemical scanning tunneling microscopy (EC-STM) as a main experimental tool
Summary
Metal oxides abundant and robust are the prime material candidates for energy-related applications in electro-, photo-, and heterogeneous catalysis.[1] Establishing structure−reactivity relationships, to allow rational design of improved catalysts, requires a fundamental understanding of the structural basis of the processes involved, and ideally atomic-level control over defects and dopants. As different tip metals have different electrochemical stability windows, tip material and coating are decided on the basis of the system under study.[12−14] For tungsten, during the etching process, anodic oxidation yields a tungstate (WO42−) that dissolves efficiently in the etching solution at high pH.[9] The close proximity of the STM tip to the surface under study as a prerequisite for the tunneling process implies very short diffusion paths for material originating at the tip to reach the substrate. Other modes of neardirect contact between tip and substrate have been explored for ultralocal surface modification, including alloy formation,[17] substrate micromachining,[18] and controlled scission of bonds in covalently grafted species.[19]
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