The Dependence of Scanning Tunneling Microscope Topography of Carboxylates on Their Terminal Groups

Carboxylate Adsorption on Rutile TiO2(100): Role of Coulomb Repulsion, Relaxation, and Steric Hindrance

Understanding the adsorption and photoactivity of acetic acid and trimethyl acetic acid on TiO₂ surfaces is important for improving the performance of photocatalysts and dye-sensitized solar cells. Here we present a structural study of adsorption on rutile TiO₂(100)-1 × 1 and -1 × 3 using Scanning Tunnelling Microscopy and Density Functional Theory calculations. Exposure of both terminations to acetic acid gives rise to a ×2 periodicity in the [001] direction (i.e., along Ti rows), with a majority ordered c(2 × 2) phase in the case of the 1 × 1 termination. The DFT calculations suggest that the preference of c(2 × 2) over the 2 × 1 periodicity found for TiO₂(110)-1 × 1 can be attributed to an increase in interadsorbate Coulomb repulsion. Exposure of TiO₂(100)-1 × 1 and -1 × 3 to trimethyl acetic acid gives rise to largely disordered structures due to steric effects, with quasi-order occurring in small areas and near step edges where these effects are reduced.

Adsorbate induced restructuring of TiO2(011)-(2×1) leads to one-dimensional nanocluster formation

Metal oxide surfaces have been thought to be fairly rigid. On the example of rutile TiO2(011) we show that this is not necessarily the case. This surface restructures by interacting with molecules. The synergic effect of adsorbates causes a strictly directional reorganization of the substrate, which results in one-dimensional adsorbate cluster formation. The increase in the surface energy of the restructured surface is compensated for by the larger molecular adsorption energy. The reversible change of the surface structure suggests a dynamic surface that may change its properties in response to adsorbed molecules.

Scanning tunneling microscopy study of black dye and deoxycholic acid adsorbed on a rutile TiO2(110)

Trithiocyanato(4,4',4''-tricarboxy-2,2':6',2''-terpyridine)ruthenium(II), "black dye", was adsorbed on a rutile TiO(2)(110) surface and imaged by an ultrahigh vacuum scanning tunneling microscope. The TiO(2)(110)-(1 x 1) surface was prepared in a vacuum, covered with pivalate monolayer, and immersed in acetonitrile containing black dye. Black dyes exchanging preadsorbed pivalates were observed on the surface as protrusions with lateral dimensions from 2 to 10 nm. Protrusions with a minimum lateral dimension of 2 nm were assigned to single, isolated black dyes, and larger protrusions were attributed to aggregated dyes. When deoxycholic acid was added to the dye solution, the number ratio of the single dyes to the aggregated dyes increased, while adsorbed deoxycholic acid was not observed.

STM Observation of a Ruthenium Dye Adsorbed on a TiO2(110) Surface

Individual Ru(4,4'-dicarboxy-2,2'-bipyridine)2(NCS)2 molecules, commonly known as N3, adsorbed on a TiO2 surface were visualized with a scanning tunneling microscope (STM) operated in ultrahigh vacuum. A TiO2(110)-(1 x 1) crystal was taken out from the vacuum vessel and immersed into an acetonitrile solution of N3. A monolayer of pivalate ((CH3)3CCOO-) ions was used to protect the (1 x 1) surface from contamination during the wetting process of the N3 adsorption. The N3 molecules adsorbed on the flat terraces protruded by 0.65 nm from the pivalate monolayer. The image height difference of the admolecules could be understood with the assumption that the N3 molecules anchor to the TiO2 surface via two carboxyl groups. The number density of the N3 molecules on the steps was higher than that on the terraces. The poorly coordinated Ti atoms exposed at the step edges form preferential sites where the carboxyl groups can approach, due to a lower steric obstacle or because the structure of the adsorbed N3 molecules suffers less distortion.