(Invited) Atomic-Scale Investigations of TiO2
surfaces in Ambient and Solution Environments
Authors: Hines Melissa A. 
Published: February 2020

Earth-abundant metal oxide nanocrystals have attracted a great deal of attention for environmental remediation and photocatalysis. These materials are attractive because they are inexpensive, they are typically very stable under reactive conditions, and their semiconducting nature enables efficient generation of long-lived photocarriers that can initiate chemical reactions. Nevertheless, an atomic-scale understanding of their reactivity in ambient and solution environments has proven elusive for two reasons. First, the complex milieu of reactants in these environments makes the discrimination of surface chemistry from adventitious contamination difficult. Second, the small size of nanocatalysts prohibits face-specific studies with most surface science techniques. In this talk, I will review our work in developing “new eyes” for nanocatalysis that address both issues.
Over the past few years, researchers around the world have observed the formation of mysterious molecularly ordered structures on the surface of TiO2 photocatalysts in air and solution. These structures have been attributed to many causes, including a new ordered state of adsorbed H2O — persistent in vacuum at room temperature — in which one H2O molecule adsorbs to every other surface Ti atom. Using a combination of atomic-scale microscopy and spectroscopy, we show that these structures are due to the highly selective adsorption of atmospheric formic and acetic acids that are typically present in parts-per-billion concentrations. These surfaces effectively repel other adsorbates, such as alcohols, present in much higher concentrations. The high affinity of the surface towards carboxylic acids is attributed to their bidentate binding. The self-assembled organic acid monolayers have the unusual property of being both hydrophobic and highly water soluble, which may contribute to the self-cleaning properties of TiO2. Interestingly, these monolayers block the undercoordinated surface cation sites typically implicated in photocatalysis.
In related work, we have shown that hydrothermal crystal growth techniques can produce anatase nanocrystals suitable for study at the atomic-scale with STM and other surface science techniques, as shown in the accompanying figure. Despite being synthesized in solution, the nanocrystal surfaces are very clean and passivated by a protecting monolayer. Using these crystals, we have shown that the most commonly used functionalization chemistry for oxide nanocatalysts, a carboxylic acid solution, causes the spontaneous reorganization of the nanocatalyst, leading to a five-fold increase in the number of reactive sites and revealing the atomic-scale origins of catalyst activation.
Finally, we have investigated the structure of solution-deposited monolayers on TiO2 and demonstrated a rational approach to tuning intermolecular interactions and enabling long-range ordering. We show that simple electrostatic insights can be used to engineer away unfavorable intermolecular interactions, producing monolayers with exceptional long-range ordering. Quantitative measurements of the intermolecular interaction energies from molecularly resolved STM images are a factor of ~7 larger than those predicted by dispersion-corrected density functional theory (DFT). The discrepancy between experiment and simulation is not currently understood. This finding suggests a new path to the production of highly ordered monolayers and superstructures of large molecules.

Figure 1