Stephen M. Francis

Site selectivity in the growth of copper islands on Au (111)

The room temperature deposition of copper onto a Au(111)-(22◊ p 3) reconstructed surface has been investigated using scanning tunnelling microscopy (STM), up to a copper coverage of approximately 0.7monolayer (ML). At extremely low coverage ( 0.02ML), preferential adsorption is observed to occur by displacement of gold atoms and incorporation of copper into the top gold layer at alternate herringbone elbows along the h112i directions. Both fcc regions and hcp regions are occupied. With increasing coverage, incorporation of copper continues but copper is also deposited on top of the incorporated copper islands. When full coverage of these islands to monolayer thickness is reached, further deposition leads to preferential growth of those islands located in hcp regions through both the deposition process and migration of copper from other elbows, predominantly those in fcc regions. Eventually, a critical island size is reached above which atomically thick copper islands exhibit a reconstructed surface similar, in essence, to that of the clean gold surface. Models for the initial adsorption mechanism, island formation and the eventual reconstruction of the copper islands are discussed qualitatively in terms of surface strain within the gold and copper surfaces.

Crystal Structure, Oxygen Nonstoichiometry, and Conductivity of Mixed Ionic–Electronic Conducting Perovskite Composites with CeO2

Conductivity, structure, and oxygen nonstoichiometry of compositions in the following series (100 - x) La 0.6 Sr 0.4 CoO 3±δ ·xCeO 2 (LSCCe) and (100 ― x) La 0.8 Sr 0.2 MnO 3±δ ·xCeO 2 (LSMCe), where x = 0, 2, 5, and 10 mol %, are studied. CeO 2 has a low solubility limit in the rhombohedral structure. Compositions containing more than 2 mol % ceria are two-phase and consist of a perovskite constituent with rhombohedral structure and ceria with cubic structure. Small additions of CeO 2 influence the conductivity of the modified perovskites in a different way. The conductivity in the LSCCe series dramatically decreases with an addition of up to 10 mol % CeO 2 . However, the total conductivity in the LSMCe series only changes slightly. The conductivity of the perovskites as a function of oxygen partial pressure (0.21 to 1 × 10- 4 atm) is discussed. Oxygen nonstoichiometry (δ) depending on temperature and oxygen partial pressure varies in the LSCCe series in a wider range (0 ≤ δ ≤ 0.23) compared to that in the LSMCe series (0.06 ≤ δ < 0.14). In contrast to the LSMCe series, the compositions in the LSCCe series show a strong relationship between lattice parameters, oxygen nonstoichiometry, and conductivity. X-ray photoelectron spectroscopy indicates that distortion of Co-O bonds takes place, which could affect electronic conductivity of perovskites.

Assembly of a chiral amino acid on an unreactive surface: (S)-proline on Au(111).

The adsorption of (S)-proline on Au(111) at 300 K was studied by low-temperature scanning tunnelling microscopy, X-ray photoelectron spectroscopy, and high resolution electron energy loss spectroscopy. (S)-proline adsorbs to produce a 2-D gas phase at 300 K, which can be condensed to form ordered molecular assemblies on cooling to 77 K. The chemical nature of the self-assembled structures is discussed in light of the information provided by photoelectron and vibrational spectroscopies.

Red-ox behaviour in the La0.6Sr0.4CoO3±δ-CeO2 system

The compositions in the (100 − x)La0.6Sr0.4CoO3±δ-xCeO2 (LSCC) system with 5 ≤ x ≤ 76 are two-phase at room temperature. They consist of the modified perovskite with rhombohedral symmetry (Rc) and modified ceria with fluorite structure (Fmm). The cross-dissolution of La, Sr, Co and Ce cations between the initial La0.6Sr0.4CoO3±δ (LSC) and CeO2 takes place and results in the modification of the initial phases. This is particularly important for the modified ceria. The lattice parameter of the modified ceria increases due to the dissolution of La and Sr cations with larger ionic radii, thereby changing noticeably the oxygen sublattice in the fluorite structure. Above 300 °C LSCCx composites are three-phase due to the reversible change in the symmetry from rhombohedral (Rc) to cubic (Pmm) within the perovskite phase. Red-ox behaviour of the LSCC composites has been explored under air and argon atmospheres in terms of evolution of the chemical composition at the grains surface and phase interfaces, formation of oxygen vacancies and thermochemistry of this process. Reversible red-ox behaviour was observed in LSCCx with x = 8–37 most probably due to an observed high surface concentration of Co cations, that can be easily involved in the reduction/re-oxidation cycle. The increase in the surface concentration of Ce4+ cations together with the decrease in surface concentration of Co cations seems to result in the differences in the reduction and oxidation behaviour under air in LSCCx with x = 57–76. Formation of oxygen vacancies in LSC, LSCC02 and LSCCx with x = 5–76 in air was not accompanied by any distinct thermal events. This process becomes more endothermic with further increase in oxygen nonstoichiometry (δ) above certain values: δ > 0.08 in LSC, δ > 0.13 in LSCC02, and LSCC with x = 5–76. The LSCCx with x = 5–37 and with x = 57–76 show slightly different reduction behaviour under a(o2) = 7.4 × 10−5. In the composites with a relatively low CeO2 content, the extent of the reduction is proportional to the Co content in a composition, whereas the reduction of the LSCCx with x = 57–76 was more significant than expected. The changes in the enthalpy of oxygen vacancy formation and the kinetics of reduction have been discussed.

X-ray photoelectron and infrared spectroscopies of quartz samples of contrasting toxicity

An exploratory XPS and FTIR investigation of the surfaces of bulk quartz powders widely used in toxicological studies (DQ12 and Min-U-Sil 5) was carried with the aim of correlating surface features with toxicity as reflected by indicators of biological response. Some patches of amorphous silica were identified as well as varying amounts of calcium but none of these features correlated with biological response. No evidence of widely-quoted surface silanol (SiOH) structures was found in this investigation and the possibility that FTIR artefacts have been previously misidentified as silanol structures is discussed.

Formation of bioinorganic complexes by the corrosive adsorption of (S)-proline on Ni/Au(111)

Nickel nanoparticles modified by the adsorption of chiral amino acids are known to be effective enantioselective heterogeneous catalysts. The leaching of nickel by amino acids has a number of potential effects including the induction of chirality in the nickel atoms left behind in the nanoparticle and the creation of catalytically active nickel complexes. The adsorption of (S)-proline onto Au(111) precovered by two-dimensional nickel nanoclusters was investigated by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and high-resolution electron energy loss spectroscopy. Adsorption of (S)-proline at 300 K resulted in the corrosion of the nickel clusters, the oxidation of the leached nickel, and the on-surface formation of bioinorganic complexes, which are concluded to contain three prolinate species in an octahedral arrangement around the central Ni ion. Two distinguishable forms of nickel prolinate complexes were identified. One form self-assembles into 1-D chains, and the other form gives rise to porous 2-D islands. Octahedral complexes of the type M(AB)3 are intrinsically chiral, resulting in two pairs of enantiomers. The mirror symmetry of each pair of enantiomers is broken when, as in this study, the bidentate ligand itself possesses a chiral center. DFT calculations are used to examine the relative energies of each Ni(prolinate)3 complex as isolated gas phase species and isolated adsorbed species.

The Interaction of Sublimed Iminodiacetic Acid with a Cu{110} Surface

Iminodiacetic acid (IMDA), NH(CH2CO2H)2, whose molecular structure is shown in figure 1, is a small molecule but has a very large scope for interacting with copper due to its two carboxylic acid and imine functionalities, which have been shown to bind strongly to copper [1,2,3]. The electronegative oxygen and nitrogen atoms present also make this a good candidate for hydrogen bonding, which often leads to very stable ordered two dimensional (2D) structures being formed across surfaces.

Self-Assembly of Di(pyrazol-1-yl)Pyridine-Benzoic Acid on Underpotentially Deposited Ag from Solution

Monolayers of 4-(2,6-di(1 H-pyrazol-1-yl)pyridine-4-yl)benzoic acid (DPP-BA) on Au substrates modified by an underpotential-deposited bilayer of Ag were studied by X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy, and scanning tunneling microscopy. Highly crystalline layers are formed with molecules coordinatively bonding to the surface through the carboxylate moiety in a bidentate configuration. The molecules assemble to rows characterized by densely packed upright-orientated DPP units occupying an area of 41 Å2. The DPP units adopt a trans-trans conformation as inferred from the N 1s XPS spectra taken at different photon energies. Their alignment alternates between adjacent rows, giving rise to a herringbone pattern. The pronouncedly different structure of the weakly commensurate self-assembled monolayer (SAM) of DPP-BA on Ag compared to the commensurate SAM of a DPP thiol on Au is a manifestation of the shift from substrate-directed assembly in the latter case to assembly dominated by intermolecular interactions in the former case.

Synthesis of ZIF-93/11 Hybrid Nanoparticles via Post-Synthetic Modification of ZIF-93 and Their Use for H2/CO2 Separation

The present work shows the synthesis of nano-sized hybrid zeolitic imidazolate frameworks (ZIFs) with the rho topology based on a mixture of the linkers benzimidazole (bIm) and 4-methyl-5-imidazolecarboxaldehyde (4-m-5-ica). The hybrid ZIF was obtained by post-synthetic modification of ZIF-93 in a bIm solution. The use of different solvents, MeOH and N,N-dimethylacetamide (DMAc), and reaction times led to differences in the quantity of bIm incorporated to the framework, from 7.4 to 23 % according to solution-state NMR spectroscopy. XPS analysis showed that the mixture of linkers was also present at the surface of the particles. The inclusion of bIm to the ZIF-93 nanoparticles improved the thermal stability of the framework and also increased the hydrophobicity according to water adsorption results. N2 and CO2 adsorption experiments revealed that the hybrid material has an intermediate adsorption capacity, between those of ZIF-93 and ZIF-11. Finally, ZIF-93/11 hybrid materials were applied as fillers in polybenzimidazole (PBI) mixed matrix membranes (MMMs). These MMMs were used for H2 /CO2 separation (at 180 °C) reaching values of 207 Barrer of H2 and a H2 /CO2 selectivity of 7.7 that clearly surpassed the Robeson upper bound (corrected for this temperature).

Adsorption of a dihydro-TTF derivative on Au(111) via a thiolate bonding complex to gold adatoms

A dihydro-TTF derivative with four acetyl-protected thiol ligands was synthesised and adsorbed on Au(111) under UHV conditions. Scanning Tunnelling Microscopy (STM) and Infrared (IR) spectroscopy show that self-organised structures are formed following annealing to 333 K, with each pair of bidentate thiolate ligands bonding to a single gold adatom in a S-Auad-S complex. Due to the lack of a direct orbital overlap between the dihydro-TTF moieties and the surface, relatively little charge transfer between TAT-TTF and the gold surface occurs.