Xingyi Deng

Electron-Induced Dissociation of CO2 on TiO2(110)

The electron-induced dissociation of CO(2) adsorbed at the oxygen vacancy defect on the TiO(2)(110) surface has been investigated at the single-molecular level using scanning tunneling microscopy (STM). Electron injection from the STM tip into the adsorbed CO(2) induces the dissociation of CO(2). The oxygen vacancy defect is found to be healed by the oxygen atom released during the dissociation process. Statistical analysis shows that the dissociation of CO(2) is one-electron process. The bias-dependent dissociation yield reveals that the threshold energy for electron-induced dissociation of CO(2) is 1.4 eV above the conduction-band minimum of TiO(2). The formation of a transient negative ion by the injected electron is considered to be the key process in CO(2) dissociation.

Water Chain Formation on TiO2(110)

The adsorption of water on a reduced rutile TiO2(110)-(1×1) surface has been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM measurements show that at a temperature of 50 K, an isolated water monomer adsorbs on top of a Ti(5f) atom on the Ti row in agreement with earlier studies. As the coverage increases, water molecules start to form one-dimensional chain structures along the Ti row direction. Supporting DFT calculations show that the formation of an H-bonded one-dimensional water chain is energetically favorable compared to monomer adsorption. In the chain, there are H-bonds between adjacent water molecules, and the water molecules also form H-bonds to neighboring bridging oxygens of TiO2(110). Thermal annealing at T = 190 K leads to the formation of longer chains facilitated by the diffusion of water on the surface. The results provide insight into the nature of the hydrogen bonding in the initial stage of wetting of TiO2.

Tunable Lattice Constant and Band Gap of Single- and Few-Layer ZnO

Single and few-layer ZnO(0001) (ZnO(nL), n = 1-4) grown on Au(111) have been characterized via scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. We find that the in-plane lattice constants of the ZnO(nL, n ≤ 3) are expanded compared to that of the bulk wurtzite ZnO(0001). The lattice constant reaches a maximum expansion of 3% in the ZnO(2L) and decreases to the bulk wurtzite ZnO value in the ZnO(4L). The band gap decreases monotonically with increasing number of ZnO layers from 4.48 eV (ZnO(1L)) to 3.42 eV (ZnO(4L)). These results suggest that a transition from a planar to the bulk-like ZnO structure occurs around the thickness of ZnO(4L). The work also demonstrates that the lattice constant and the band gap in ultrathin ZnO can be tuned by controlling the number of layers, providing a basis for further investigation of this material.

In Situ Observation of Water Dissociation with Lattice Incorporation at FeO Particle Edges Using Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy

The dissociation of H2O and formation of adsorbed hydroxyl groups on FeO particles grown on Au(111) were identified with in situ X-ray photoelectron spectroscopy (XPS) at water pressures ranging from 3 × 10(-8) to 0.1 Torr. The facile dissociation of H2O takes place at FeO particle edges, and it was successfully observed in situ with atomically resolved scanning tunneling microscopy (STM). The in situ STM studies show that adsorbed hydroxyl groups were formed exclusively along the edges of the FeO particles with the O atom becoming directly incorporated into the oxide crystalline lattice. The STM results are consistent with coordinatively unsaturated ferrous (CUF) sites along the FeO particle edge causing the observed reactivity with H2O. Our results also directly illustrate how structural defects and under-coordinated sites participate in chemical reactions.

Probing active site chemistry with differently charged Au25q nanoclusters (q = −1, 0, +1)

Charged active sites are hypothesized to participate in heterogeneously-catalyzed reactions. For example, Auδ+ species at the catalyst surface or catalyst–support interface are thought to promote the thermally-driven CO oxidation reaction. However, the concept of charged active sites is rarely extended to electrochemical systems. We used atomically precise Au25q nanoclusters with different ground state charges (q = −1, 0, +1) to study the role of charged active sites in Au-catalyzed electrochemical reactions. Au25q clusters showed charge state-dependent electrocatalytic activity for CO2 reduction, CO oxidation and O2 reduction reactions in aqueous media. Experimental studies and density functional theory identified a relationship between the Au25q charge state, the stability of adsorbed reactants or products, and the catalytic reaction rate. Anionic Au25− promoted CO2 reduction by stabilizing coadsorbed CO2 and H+ reactants. Cationic Au25+ promoted CO oxidation by stabilizing coadsorbed CO and OH− reactants. Finally, stronger product adsorption at Au25+ inhibited O2 reduction rates. The participation of H+ and OH− in numerous aqueous electrocatalytic reactions likely extends the concept of charge state-mediated reactivity to a wide range of applications, including fuel cells, water splitting, batteries, and sensors. Au25q clusters have also shown photocatalytic and more traditional thermocatalytic activity, and the concept of charge state-mediated reactivity may create new opportunities for tuning reactant, intermediate and product interactions in catalytic systems extending beyond the field of electrochemistry.

Diffusion of CO2on the Rutile TiO2(110) Surface

The diffusion of CO{sub 2} molecules on a reduced rutile TiO{sub 2}(110)-(1×1) surface has been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM feature associated with a CO{sub 2} molecule at an oxygen vacancy (V{sub O}) becomes increasingly streaky with increasing temperature, indicating thermally activated CO{sub 2} diffusion from the V{sub O} site. From temperature-dependent tunneling current measurements, the barrier for diffusion of CO{sub 2} from the V{sub O} site is estimated to be 3.31 ± 0.23 kcal/mol. The corresponding value from the DFT calculations is 3.80 kcal/mol. In addition, the DFT calculations give a barrier for diffusion of CO{sub 2} along Ti rows of only 1.33 kcal/mol.

Diffusion of CO2 on the Rutile TiO2(110) Surface

The diffusion of CO{sub 2} molecules on a reduced rutile TiO{sub 2}(110)-(1×1) surface has been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM feature associated with a CO{sub 2} molecule at an oxygen vacancy (V{sub O}) becomes increasingly streaky with increasing temperature, indicating thermally activated CO{sub 2} diffusion from the V{sub O} site. From temperature-dependent tunneling current measurements, the barrier for diffusion of CO{sub 2} from the V{sub O} site is estimated to be 3.31 ± 0.23 kcal/mol. The corresponding value from the DFT calculations is 3.80 kcal/mol. In addition, the DFT calculations give a barrier for diffusion of CO{sub 2} along Ti rows of only 1.33 kcal/mol.

Diffusion of CO{sub 2} on Rutile TiO{sub 2}(110) Surface

The diffusion of CO{sub 2} molecules on a reduced rutile TiO{sub 2}(110)-(1×1) surface has been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM feature associated with a CO{sub 2} molecule at an oxygen vacancy (V{sub O}) becomes increasingly streaky with increasing temperature, indicating thermally activated CO{sub 2} diffusion from the V{sub O} site. From temperature-dependent tunneling current measurements, the barrier for diffusion of CO{sub 2} from the V{sub O} site is estimated to be 3.31 ± 0.23 kcal/mol. The corresponding value from the DFT calculations is 3.80 kcal/mol. In addition, the DFT calculations give a barrier for diffusion of CO{sub 2} along Ti rows of only 1.33 kcal/mol.