AVS2016 Session SS+HC-TuA: Photocatalysis and Photochemistry at Surfaces

The surface chemistry of N-containing heteroaromatics, molecular co-catalysts that enable the selective electrochemical reduction of CO₂ to fuels, is discussed. The presented experimental results focus on elucidating the role of the electrode surface in CO₂ reduction reactions that are co-catalyzed by pyridine. For this catalysis, exceptionally high selectivity for reduced fuels has been reported when the reaction occurs at the surface a GaP photocathode. For this reason, experimental emphasis is placed on assessing preferential adsorption sites and bonding interactions of adsorbates on surfaces of GaP. A surface science approach is used, whereby ultra-high vacuum conditions facilitate the fabrication of highly characterizable electrode-adsorbate systems. The use of single crystal surfaces permits analysis of surface chemistry independent of complicating factors such as grain boundaries and morphology. Surface-sensitive core-level and vibrational spectroscopy techniques, including high-resolution X-ray photoelectron spectroscopy, synchrotron-based photoemission, and high-resolution electron energy loss spectroscopy, are used to probe adsorbate-substrate and adsorbate-adsorbate interactions for pyridine, water, hydrogen, and carbon dioxide on GaP. Scanning tunneling microscopy was used to obtain molecular orbital-resolved images of adsorbed molecules. Conclusions from experimental results on these model systems are supported by calculations using density functional theory. This work assists in generating a molecular-level understanding of the heterogeneous processes important to the reaction mechanisms involved in the efficient photoelectrocatalytic generation of carbon-containing fuels with high energy densities.

The photoreactivity of organic self-assembled monolayers (SAMs) on TiO₂ surfaces is of considerable importance to applications such as dye-sensitized solar cells and photocatalytic environmental remediation. Despite extensive research, there remains little information about the reactivity of well characterized TiO₂ surfaces under ambient conditions. Here, we study the surface structure and photoreactivity of near-ideal benzoate monolayers prepared from dilute aqueous solutions and reacted at atmospheric pressure on anatase (001) and rutile (110) surfaces using scanning tunneling microscopy, infrared and x-ray photoelectron spectroscopies, and density functional theory.

We show that self-assembled monolayers of benzoate, an analogue of the organic linkage used in dye-sensitized solar cells, undergo rapid photodecomposition on both rutile (110) and anatase (001) under ultraviolet illumination in ambient and oxygen-rich conditions. Interestingly, while the two surfaces have similar, although not identical, reactivities, they differ in their reaction products, with the anatase polymorph producing a surface-bound ketene.

Quantum confined semiconductor nanocrystals have been widely investigated as light harvesting and charge separation components in photovoltaic and photocatalytic devices. The efficiency of these semiconductor nanocrystal-based devices depends on many processes, including light harvesting, carrier relaxation, charge separation and charge recombination. The competition between these processes determines the overall solar energy conversion (solar to electricity or fuel) efficiency. Compared with single component quantum dots (QDs), semiconductor nanoheterostructures, combining two or more materials, offer additional opportunities to control their charge separation properties by tailoring their compositions and dimensions through relative alignment of conduction and valence bands. Further integration of catalysts (heterogeneous or homogeneous) to these materials form multifunctional nanoheterostructures. Using CdSe/CdS/Pt, dot-in-rod nanorods(NRs) with Pt tips, as a model system, we are examining the mechanism of long-lived charge separation and H₂ generation in the presence of sacrificial electron donor. The rates of electron transfer, hole transfer and charge recombination are directly monitored by transient absorption and time-resolved fluorescence spectroscopy. In this talk, we will discuss the mechanism of exciton dissociation, the dependence of the rates of elementary charge transfer processes on the dimension (size and length) and band alignment in these materials, and how these rates affect the overall H2 generation efficiency.

Using temperature programmed desorption (TPD) and photon stimulated desorption (PSD), we show that coadsorbates of varying binding energies on the rutile TiO₂(110) surface exert a commensurate inhibiting influence on the hole-mediated photodesorption of adsorbed O₂. A variety of coadsorbates (Ar, Kr, Xe, N₂, CO, CO₂, CH₄, N₂O, acetone, methanol or water) were shown to quench O₂ photoactivity, with the extent correlating with the coadsorbate’s gas phase basicity, which in turn determines the strength of the coadsorbate-Ti⁴⁺ bond. Coadsorbed rare gases inhibited the photodesorption of O₂ by ~10-25%, whereas strongly bound species (water, methanol and acetone) nearly completely inhibited O₂ PSD. We suggest that coadsorption of these molecules inhibit the arrival probability of holes to the surface. Band bending effects, which vary with the extent of charge transfer between the coadsorbate and the TiO₂(110) surface, are not expected to be significant in the cases of the rare gases and physisorbed species. These results indicate that neutral coadsorbates can exert a significant influence on charge transfer events by altering the interfacial dipole in the vicinity of the target molecule.

By employing photoluminescence (PL) spectroscopy, it was found that oxygen adsorption on powdered TiO₂ changes the band bending of anatase in different ways. On the one hand, oxygen exposure leads to molecular chemisorption of oxygen molecules, which take up negative charge, but can be reversibly removed by ultraviolet photodesorption from the TiO₂ surface. Molecular chemisorption of oxygen molecules increases the upward band bending of TiO₂ and decreases the PL emission. On the one hand, oxygen can adsorb through irreversible reaction with defects, which occurs through an intermediate state of molecularly chemisorption, reducing the intrinsic upward band bending at the TiO₂ surface resulting in PL emission increase. Since band bending plays an important role in charge carrier migration to the surface, this finding that oxygen adsorption can have two different effects on the band bending of TiO₂ provides a new perspective on how oxygen and oxygen vacancies may modulate photocatalytic reaction efficiencies.

The hydroxylation-dependent permeability of bilayer SiO₂ supported on Ru(0001) was investigated by XPS and TDS studies in a temperature range of 100 K to 600 K. For this, the thermal behavior of Pd evaporated at 100 K, which results in surface and sub-surface (Ru-supported) binding arrangements, was examined relative to the extent of pre-hydroxylation. Samples containing only defect-mediated hydroxyls showed no effect on Pd diffusion through the film at low temperature. If, instead, the concentration of strongly bound hydroxyl groups and associated weakly bound water molecules was enriched by an electron-assisted hydroxylation procedure, the probability for Pd diffusion through the film is decreased via a pore-blocking mechanism. Above room temperature, all samples showed similar behavior, reflective of particle nucleation above the film and eventual agglomeration with any metal atoms initially binding beneath the film. When depositing Pd onto the same SiO₂/Ru model support via adsorption of [Pd(NH₃)₄]Cl₂ from alkaline (pH 12) precursor solution, we observe notably different adsorption and nucleation mechanisms. The resultant Pd adsorption complexes follow established decomposition pathways to produce model catalyst systems compatible with those created exclusively within UHV despite lacking the ability to penetrate the film due to the increased size of the initial Pd precursor groups.