The chemical state of a catalyst in operando is particularly important for catalysts that target minority species, such as atmospheric CO2 which has a concentration of only 400 ppm. A reaction can be promoted by the selective binding of reactants or hindered by molecules that block active sites. We show that adsorbed CO2, a very weakly bonded species on TiO2, is unlikely to play the key role in CO2 photoreduction under ambient conditions, at least on rutile (110), as the vast majority of unsaturated Ti sites are terminated by a different, much more strongly bound carbonaceous species: adsorbed bicarbonate (HCO3). Using a combination of scanning tunneling microscopy (STM) and surface spectroscopies, we show that atmospheric CO2 readily and stably displaces adsorbed H2O on rutile (110), creating a self-assembled monolayer of HCO3 and H that is stable at room temperature even in vacuum. This reaction occurs on near-ideal, stoichiometric rutile (110) and does not require surface defects, such as O vacancies, Ti interstitials, or steps. This reaction is promoted both by the strong bidentate bonding of HCO3 as well as the nanoscale H2O film that spontaneously forms on TiO2 under ambient conditions. Density functional theory calculations show that the nanoscale water layer adsorbed to rutile (110) solvates the products and changes the reaction energetics significantly. The chemical state of the catalyst in operando will also be affected by the half-monolayer of adsorbed H produced by the reactive dissociation of H2O.

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High-quality, self-assembled benzoate monolayers were synthesized on rutile (110) using simple aqueous reactions. Sputtering and annealing cycles, which create surface and subsurface defects, were not needed. The monolayers were hydrophobic and remained largely contaminant free during exposures to laboratory air for tens of minutes. During this period, infrared spectroscopy showed that the monolayers did not spontaneously adsorb airborne hydrocarbons or other adventitious aliphatic species. Scanning tunneling microscopy (STM) images, infrared and X-ray photoemission spectra, Monte Carlo simulations, and ab initio calculations were all consistent with benzoate molecules adopting an edge-to-face ring geometry with their four nearest neighborsa tetrameric bonding geometry. This bonding is further stabilized by a pairing interaction between adjacent benzoate molecules, a pairing that has previously been interpreted as dimerization. The coexistence of paired and unpaired regions of the monolayer is consistent with the relatively small additional energy gained by pairing and the cooperative nature of the pairing interaction. Monolayer stability is driven both by the strong bidentate bonding to unsaturated Ti atoms on the surface as well as by π–π interactions between adsorbates.

Hybrid organic–inorganic perovskites (HOIPs), such as CH3NH3PbI3 (MAPbI3), are attractive for inexpensive, high-performance solar cells. Controlling HOIP thin-film quality and morphology, which is essential to achieve consistent solar-cell efficiencies, requires a fundamental understanding of the link between solution chemistry and crystallization pathways. To elucidate the effect of solvent choice and solution speciation on the crystallization pathway, we combined computational modeling of molecular-level solvent–solute interactions in HOIP growth solutions with experimental monitoring of film phase evolution. Using density functional theory calculations and a Bayesian optimization-based approach (PAL), we exhaustively searched the HOIP/solvent combinatorial space to obtain a ranked list of increasing HOIP/solvent intermolecular binding energy. Then, using in situ X-ray diffraction, we tested solvents of varying coordinating abilities with MAPbI3 to correlate the PAL-generated ranking with the crystallization pathway. Weakly coordinating solvents (e.g., N-methyl-2-pyrrolidone) formed the perovskite via a two-step crystallization pathway, where a crystalline intermediate formed and was subsequently transformed to perovskite with heating, whereas strongly coordinating solvents (e.g., tetrahydrothiophene 1-oxide) formed the perovskite directly from solution. We propose that the solution coordination chemistry determines the crystallization pathway. Our integrated experimental–computational approach could be applied to study the interplay between solution chemistry and crystallization pathways for other solution-grown materials.