Xavier Torrelles

Surface characterization of sulfur and alkanethiol self-assembled monolayers on Au(111)

In the last two decades surface science techniques have decisively contributed to our present knowledge of alkanethiol self-assembled monolayers (SAMs) on solid surfaces. These organic layers have been a challenge for surface scientists, in particular because of the soft nature of the organic material (which can be easily damaged by irradiation), the large number of atoms present in the molecules, and the complex physical chemistry involved in the self-assembly process. This challenge has been motivated by the appealing technological applications of SAMs that cover many fields of the emerging area of nanotechnology. Sulfur (S) is closely related to alkanethiols and can be used to understand basic aspects of the surface structure of SAMs. In this review we focus on the atomic/molecular structures of S-containing SAMs on Au(111). Particular emphasis is given to the substrate, adsorption sites, chemical state of the S–metal bond and also to the experimental and theoretical tools used to study these structures at the atomic or molecular levels.

Structure of a model TiO2 photocatalytic interface

The interaction of water with TiO2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis.

Structure of the clean NiAl(1 1 0) surface and the Al2O3/NiAl(1 1 0) interface by measurements of crystal truncation rods

The clean NiAl(1 1 0) surface and the Al2O3/NiAl(1 1 0) interface have been investigated by synchrotron X-ray diffraction experiments. In the case of the oxide surface the analysis of the NiAl(1 1 0) crystal truncation rods (CTR) provide information about the interface between the Al2O3 film and the NiAl substrate. The analysis of the CTR-data shows clearly a rippled Ni-Al topmost surface with an amplitude value of RNi/Al = 0.16 ± 0.01 A for the clean surface and RNi/Al = 0.18 ± 0.02 A for the oxide covered surface. On the clean surface the Al sites are expanded by +3.8\% (outwards) and the Ni sites are contracted by -3.2\% (inwards) respect to the unrelaxed interlayer separation. For the oxide covered surface an increase of the expansion of the outermost Al atoms (+7.3\%) relative to their bulk positions has been found, while the Ni atoms remain (-0.9\%) at the bulk position. On both cases, an ideal surface stoichiometry (1:1) was obtained. However, some intermixing (chemical disorder) of one specimen in the sites of the other and vice versa was present (less than 4\%). This chemical disorder was not enhanced by the presence of the Al2O3 overlayer. Neither rippling nor oscillatory relaxation in deeper layers was detectable.

Structure determination of the blue mineral pigment aerinite from synchrotron powder diffraction data The solution of an old riddle

The structure of aerinite, a blue fibrous silicate mineral associated with the alteration of ophitic rocks in the southern Pyrenees, has been determined by applying the direct methods modulus sum function to synchrotron powder diffraction data. This mineral was the blue pigment commonly used in most Catalan romanic paintings between the XI-XV centuries. The studied specimen comes from the Camporrells-Estopanya area (Huesca, Spain). The unit cell dimensions are a = b = 16.8820(9), c = 5.2251(3) A, the space group is P 3 c 1 and the structural formula is (Ca 5.1 Na 0.5 )(Fe 3+ AlFe 2+ 1.7 Mg 0.3 )(Al 5.1 Mg 0.7 )[Si 12 O 36 (OH) 12 H]·[(CO 3 ) 1.2 (H 2 O) 12 ] with Z = 1 and D c = 2.52 g/cm 3 (Fe 2+ /Fe 3+ ratio from Mossbauer spectroscopy; carbonate content confirmed by infrared spectroscopy). The model of the structure obtained by direct methods was refined with the Rietveld method to the residual value R wp = 0.0937 (χ 2 = 1.05). A bond valence analysis shows the plausibility of the refined model. The crystal structure of aerinite can be best understood by introducing cylindrical basic building units consisting on three pyroxene chains pointing inwards to accommodate tri- and divalent metal cations at the centres of the resulting face-sharing octahedra. The average composition of these cationic sites is Fe 3+ 0.25 ,Al 3+ 0.25 ,Fe 2+ 0.43 ,Mg 2+ 0.07 , the mean cation-oxygen bond length is 2.054(9) A and the intercationic distance is 2.61 A. Out of the three symmetry-independent three-fold rotation axes in the unit cell, two are occupied by such cylindrical units and the third by CO 3 groups. Consequently, each unit is surrounded by three similar ones which are, however, shifted by 0.93 A along c . Between two such units, i.e. , tangential to both cylindrical envelopes, a four-row wide slab of a brucite-like layer is found. The two inner octahedra are predominantly filled with Al and Mg atoms, the two outer with Ca, Na and some vacancies [average values: d (Al-O) = 1.936(53) A (6x), ∠(O-Al-O) = 90.1(6.5)° (12x) and 172.7(4.1)° (3x); d (Ca-O) = 2.42(6) A (6x), ∠(O-Ca-O) = 90(23)° (12x) and 158(14)° (3x)]. The internal O atoms of the brucite-like layer are hydroxyl groups, the intermediate are unshared basal O atoms of the neighbouring pyroxene chains, while the external ones are water molecules forming relatively strong H-bridges with the partially disordered CO 3 groups. Presumably, the hydroxyl groups in the brucite-like layer also form H-bridges with the apical O atoms of the neighbouring pyroxene chains to compensate for the defect of charge caused by the presence of divalent cations in the face-sharing octahedra.

A direct phasing method based on the origin-free modulus sum function and the FFT algorithm. XII.

An alternative way of refining phases with the origin-free modulus sum function S is shown that, instead of applying the tangent formula in sequential mode [Rius (1993). Acta Cryst. A49, 406-409], applies it in parallel mode with the help of the fast Fourier transform (FFT) algorithm. The test calculations performed on intensity data of small crystal structures at atomic resolution prove the convergence and hence the viability of the procedure. This new procedure called S-FFT is valid for all space groups and especially competitive for low-symmetry ones. It works well when the charge-density peaks in the crystal structure have the same sign, i.e. either positive or negative.

Investigation of the {104} surface of calcite under dry and humid atmospheric conditions with grazing incidence X-ray diffraction (GIXRD)

Using grazing incidence X-ray diffraction (GIXRD) the three-dimensional surface and interface structure of the {104} calcite surface was investigated under dry and humid atmospheric conditions. The measurement of specular and non-specular crystal truncation rods (CTR) provided information about the periodic order of the vertical and lateral surface structure on atomic scale. The calcite surface is nearly ideally terminated with relaxation of the atoms in the topmost layers down to about 12 A depth. In dry environment the calcite surface structure is dominantly determined by the large shift, rotation and tilt of the surface carbonate groups. In humid atmosphere a laterally ordered monolayer of water is formed on the surface. The truncated coordination site of the surface Ca 2+ ions are filled by a water molecule in a distance of (2.1 ± 0.2) A. The relaxation of the structure is significantly changed, which results in a strong distortion of the Ca-O coordination octahedron. The refinement of water as complete rigid H 2 O species positioned flat on the surface with one rotation parameter resulted in the formation of hydrogen bonds to the adjacent surface carbonate groups. No evidence for a surface reconstruction was found.

A Quantitative Structural Investigation of the 0.1 wt % Nb–SrTiO3(001)/H2O Interface

Surface X-ray diffraction has been employed to elucidate the structure of the interface between a well-characterized (001) surface of 0.1 wt % Nb–SrTiO3 and liquid H2O. Results are reported for the clean surface, the surface in contact with a drop of liquid water, and the surface after the water droplet has been removed with a flow of nitrogen. The investigation revealed that the clean surface, prepared via annealing in 1 × 10–2 mbar O2 partial pressure, is unreconstructed and rough on a short length scale. The surface is covered with large terraces, the topmost layer of which is either TiO2 or SrO with an area ratio of about 7/3. For the surface in contact with water, our results reveal that associative H2O adsorption is favored for the TiO2-terminated terrace whereas adsorption is dissociative for the SrO-terminated terrace, which validates recent first-principles calculations. After removal of the water droplet, the surface largely resembles the water-covered surface but now with a disordered overlayer of water present on the surface.

A tangent formula derived from Patterson-function arguments. VII. Solution of inorganic structures from powder data with accidental overlap

Accidental overlap constitutes one of the principal limitations for the solution of crystal structures from powder diffraction data, since it reduces the number of available intensities for direct-methods application. In this work, the field of application of the direct-methods sum function is extended to cope with powder patterns with relatively large amounts of accidental overlap. This is achieved by refining not only the phases of the structure factors but also the estimated intensities of the severely overlapped peaks during the structure solution process. This procedure has been specifically devised for inorganic compounds with uncertain cell contents and with probable severe atomic disorder, a situation often found when studying complex minerals with limited crystallinity. It has been successfully applied to the solution of the previously unknown crystal structure of the mineral tinticite. Finally, an estimation of the smallest ratio (number of observations to number of variables) for the procedure to be successful is given.

Geometry of α-Cr2O3(0001) as a Function of H2O Partial Pressure.

Surface X-ray diffraction has been employed to elucidate the surface structure of α-Cr2O3(0001) as a function of water partial pressure at room temperature. In ultra high vacuum, following exposure to ∼2000 Langmuir of H2O, the surface is found to be terminated by a partially occupied double layer of chromium atoms. No evidence of adsorbed OH/H2O is found, which is likely due to either adsorption at minority sites, or X-ray induced desorption. At a water partial pressure of ∼30 mbar, a single OH/H2O species is found to be bound atop each surface Cr atom. This adsorption geometry does not agree with that predicted by ab initio calculations, which may be a result of some differences between the experimental conditions and those modeled.

A new interpretation and practical aspects of the direct-methods modulus sum function. VIII

Since the first publication of the direct-methods modulus sum function [Rius (1993). Acta Cryst. A49, 406-409], the application of this function to a variety of situations has been shown in a series of seven subsequent papers. In this way, much experience about this function and its practical use has been gained. It is thought by the authors that it is now the right moment to publish a more complete study of this function which also considers most of this practical knowledge. The first part of the study relates, thanks to a new interpretation, this function to other existing phase-refinement functions, while the second shows, with the help of test calculations on a selection of crystal structures, the behaviour of the function for two different control parameters. In this study, the principal interest is focused on the function itself and not on the optimization procedure which is based on a conventional sequential tangent formula refinement. The results obtained are quite satisfactory and seem to indicate that, when combined with more sophisticated optimization algorithms, the application field of this function could be extended to larger structures than those used for the test calculations.