Adsorption of Al13–Keggin clusters to sapphire c-plane single crystals: Kinetic observations by streaming current measurements (original) (raw)
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Advances in Colloid and Interface Science, 2018
A wide range of isoelectric points (IEPs) has been reported in the literature for sapphire-c (alumina), also referred to as basal plane, (001) or (0001), single crystals. Interestingly, the available data suggest that the variation of IEPs is comparable to the range of IEPs encountered for particles, although single crystals should be much better defined in terms of surface structure. One explanation for the range of IEPs might be the obvious danger of contaminating the small surface areas of single crystal samples while exposing them to comparatively large solution reservoirs. Literature suggests that factors like origin of the sample, sample treatment or the method of investigation all have an influence on the surfaces and it is difficult to clearly separate the respective, individual effects. In the present study, we investigate cause-effect relationships to better understand the indivicual effects. The reference IEP of our samples is between 4 and 4.5. High temperature treatment tends to decrease the IEP of sapphire-c as does UV treatment. Increasing the initial miscut (i.e. the divergence from the expected orientation of the crystal) tends to increase the IEP as does plasma cleaning, which can be understood assuming that the surfaces have become less hydrophobic due to the presence of more and/or larger steps with increasing miscut or due to amorphisation of the surface caused by plasma cleaning. Pre-treatment at very high pH caused an increase in the IEP. Surface treatments that led to IEPs different from the stable value of reference samples typically resulted in surfaces that were strongly affected by subsequent exposure to water. The streaming potential data appear to relax to the reference sample behavior after a period of time of water exposure. Combination of the zeta-potential measurements with AFM investigations support the idea that atomically smooth surfaces exhibit lower IEPs, while rougher surfaces (roughness on the order of nanometers) result in higher IEPs compared to reference samples. Two supplementary investigations resulted in either surprising or ambiguous results. On very rough surfaces (roughness on the order of micrometers) the IEP lowered compared to the reference sample with nanometer-scale roughness and transient behavior of the rough surfaces was observed. Furthermore, differences in the IEP as obtained from streaming potential and static colloid adhesion measurements may suggest that hydrodynamics play a role in streaming potential experiments. We finally relate surface diffraction data from previous studies to possible interpretations of our electrokinetic data to corroborate the presence of a water film that can explain the low IEP. Calculations show that the surface diffraction data are in line with the presence of a water film, however, they do not allow to unambiguously resolve critical features of this film which might explain the observed surface chemical characteristics like the dangling OH-bond reported in sum frequency generation studies. A broad literature review on properties of related surfaces shows that the presence of such water films could in many cases affect the interfacial properties. Persistence or not of the water film can be crucial. The presence of the water film can in principle affect important processes like ice-nucleation, wetting behavior, electric charging, etc.
Langmuir, 2010
We report an extensive first-principles study of the structure and electronic properties of Ag n (n = 1-8) clusters isolated in gas phase and deposited on the R-Al 2 O 3 surface. We have used the plane wave based pseudopotential method within the framework of density functional theory. The electron ion interaction has been described using projector augmented wave (PAW), and the spin-polarized GGA scheme was used for the exchange correlation energy. The results reveal that, albeit interacting with support alumina, the Ag atoms prefers to remain bonded together suggesting an island growth motif is preferred over wetting the surface. When compared the equilibrium structures of Ag clusters between free and on alumina substrate, a significant difference was observed starting from n = 7 onward. While Ag 7 forms a three-dimensional (3D) pentagonal bipyramid in the isolated gas phase, on alumina support it forms a planar hexagonal structure parallel to the surface plane. Moreover, the spin moment of the Ag 7 cluster was found to be fully quenched. This has been attributed to higher delocalization of electron density as the size of the cluster increases. Furthermore, a comparison of chemical bonding analysis through electronic density of state (EDOS) shows that the EDOS of the deposited Ag n cluster is significantly broader, which has been ascribed to the enhanced spd hybridization. On the basis of the energetics, it is found that the adsorption energy of Ag clusters on the R-Al 2 O 3 surface decreases with cluster size.
Molecular cluster models of aluminum oxide and aluminum hydroxide surfaces
American Mineralogist
Ab initio, molecular orbital calculations for two different Hartree-Fock basis levels were performed on clusters in the system Al-O-H, and tested by comparing derived vibrational frequencies to the measured values for aluminum oxides and aluminum oxyhydroxide minerals. Models were chosen to reflect surface groups that may be present on aluminous minerals such as ␣-Al 2 O 3 (corundum) and Al(OH) 3 (gibbsite). Protonation and deprotonation reactions on bridging and terminal oxygen or hydroxyl sites were modeled to investigate the effects of solution pH on the structure of surface groups. Relative deprotonation energies, including solvation effects, were calculated for each site using the self-consistent isodensity polarized continuum model of , and then used to predict the order in which each particular surface group protonates.
Journal of Colloid and Interface Science, 2005
Surface charges of gibbsite particles were probed by potentiometric titration and subsequently analyzed to estimate intrinsic proton affinity constants of OH surface groups. A detailed spectroscopic characterization of the molecular structure of surface OH groups yielded estimates of bond lengths and bond valences of O-H surface sites. Based on these results, the effects of the setting parameters of a MUSIC calculation have been shown in comparison with previous predictions yielding higher pK a values (2 < −pK a < 4) for the protonation of basal doubly coordinated OH surface groups and lower pK a values (7.9 < −pK a < 9.9) for the protonation of lateral singly coordinated OH surface groups. Comparison with experimental data is complicated by reproducible hysteresis between acid and base addition in optimal raw potentiometric titration curves at different ionic strengths. Such effects prevented the determination of a univocal intersection point to provide the global point of zero charge of gibbsite particles, even though the ionic strength dependence of the point of zero net proton charge and the different crossovers between curves indicated that the point of zero charge could be estimated between 8.1 and 9.6, in relative agreement with the lateral affinity constant calculated with the MUSIC model. Still, two main drawbacks remained to differentiate the reactivity of lateral singly and basal doubly coordinated surface groups. First, significant kinetic effects observed in acidic media indicated a dissolution process and/or protonation of basal surface groups. Second, the choice of specific surface areas, especially for a heterogeneous sample, led to several cases for the calculation of the absolute surface charge of particles. Therefore, our results demonstrated the heterogeneous reactivity of gibbsite particles and that the prediction and the experimental determination of respective surface groups are still complex even if some trends emerge.
The Journal of Chemical Physics, 2010
We have used reactive force field ͑ReaxFF͒ to investigate the mechanism of interaction of alanes on Al͑111͒ surface. Our simulations show that, on the Al͑111͒ surface, alanes oligomerize into larger alanes. In addition, from our simulations, adsorption of atomic hydrogen on Al͑111͒ surface leads to the formation of alanes via H-induced etching of aluminum atoms from the surface. The alanes then agglomerate at the step edges forming stringlike conformations. The identification of these stringlike intermediates as a precursor to the bulk hydride phase allows us to explain the loss of resolution in surface IR experiments with increasing hydrogen coverage on single crystal Al͑111͒ surface. This is in excellent agreement with the experimental works of Go et al. ͓E. Go, K. Thuermer, and J. E. Reutt-Robey, Surf. Sci. 437, 377 ͑1999͔͒. The mobility of alanes molecules has been studied using molecular dynamics and it is found that the migration energy barrier of Al 2 H 6 is 2.99 kcal/mol while the prefactor is D 0 = 2.82ϫ 10 −3 cm 2 / s. We further investigated the interaction between an alane and an aluminum vacancy using classical molecular dynamics simulations. We found that a vacancy acts as a trap for alane, and eventually fractionates/annihilates it. These results show that ReaxFF can be used, in conjunction with ab initio methods, to study complex reactions on surfaces at both ambient and elevated temperature conditions.
Analysis of solution-deposited alkali ions by cluster surface collisions
Analytical Chemistry, 2003
Sulfur dioxide clusters (SO2)x of mean size 〈x〉 = 1.7 × 103 collide at a velocity of 1.6 km s-1 with a metal surface that has been pretreated with dilute salt solutions containing Li+, Na+, K+, Cl-, and Br- ions in different concentrations. In the scattered gas plume, positive and negative cluster fragments of the formal composition (SO2)x(Li+, Na+, K+, SO2-) are detected. While the amount of charge observed in the various cationic channels correlates with the concentration of the respective cations in the solutions, no cluster fragments carrying chloride or bromide anions have been observed. This indicates, that the observed free charge carriers are not formed by a direct pickup of ions from the surface. Based on the assumption that the physical state for alkali adsorbates on metal surfaces is independent of the charge state of the adsorbing precursor, the findings are explained in terms of a known charge separation effect in cluster surface collisions involving neutrally deposited...
We have studied the impact of dissolved aluminum on interfacial properties of two aluminum bearing minerals, corundum and kaolinite. The effect of intentionally adding dissolved aluminum on electrokinetic potential of basal plane surfaces of sapphire was studied by streaming potential measurements as a function of pH and was complemented by a second harmonic generation (SHG) study at pH 6. The electrokinetic data show a similar trend as the SHG data, suggesting that the SHG electric field correlates to zeta-potential. A comparable study was carried out on kaolinite particles. In this case electrophoretic mobility was measured as a function of pH. In both systems the addition of dissolved aluminum caused significant changes in the charging behavior. The isoelectric point consistently shifted to higher pH values, the extent of the shift depending on the amount of aluminum present or added. The experimental results imply that published isoelectric points of clay minerals may have been affected by this phenomenon. The presence of dissolved aluminum in experimental studies may be caused by particular pre-treatment methods (such as washing in acids and subsequent adsorption of dissolved aluminum) or even simply by starting a series of measurements from extreme pH (causing dissolution), and subsequently varying the pH in the very same batch. This results in interactions of dissolved aluminum with the target surface. A possible interpretation of the experimental results could be that at low aluminum concentrations adatoms of aluminum (we will refer to adsorbed mineral constituents as adatoms) can form at the sapphire basal plane, which can be rather easily removed. Simultaneously, once the surface has been exposed to sufficiently high aluminum concentration, a visible change of the surface is seen by AFM which is attributed to a surface precipitate that cannot be removed under the conditions employed in the current study.
Segregation of Aluminium at Nickel-Sapphire Interfaces
2001
Sessile drop experiments of pure liquid Ni on the basal surface of pure sapphire were conducted under controlled atmosphere and temperature. This system has been traditionally considered as non-reactive, based on thermodynamic assessments. However, the results of this study demonstrate that a capillary driven interaction exists between the pure liquid Ni and the sapphire, which causes the dissolution of the sapphire substrate mainly at the triple junction. Oxygen and Al resulting from the dissolution process diffuse into Ni and segregate at its interfaces with the atmosphere and the sapphire (probably as Al x O y clusters), which reduces the interface energy. It is considered that this reduction is beneficial for the adhesion of both liquid and solid Ni on sapphire. The amount of Al introduced into the drop, and hence the segregation of Al that affects the interface energy (and adhesion), are related to the size of the sessile drop.
Microscopy Research and Technique, 1992
Annealed (0001) surfaces of single-crystal sapphire (a-A1203) rod have been studied in the electron microscope using reflection electron microscopy (REM), scanning reflection electron microscopy (SREM), and reflection high energy electron diffraction (RHEED). Annealed surfaces of (0001) sapphire are vicinal and characterized by close-packed (0001)-oriented terraces separated by faceted multiple-height steps, with edges parallel to energetically preferred low-index directions (<10TO> and <1120>). These structural features are not seen on cleaved surfaces or polished surfaces treated at temperatures < 1,250"C. Oxygen-annealing produces clean surfaces which prove useful for investigating the interaction of deposited metals with the (0001) sapphire. Both REM and SREM (with microdiffraction spots) techniques have been used to observe fine structure of flat Ag islands on the scale of 1-100 nm on the (0001)-oriented terraces as well as aggregations at the steps. A preliminary result on interaction with Cu i.s also included.
Reactivity of aluminum cluster anions with ammonia: Selective etching of Al11− and Al12−
The Journal of Chemical Physics, 2009
Reactivity of aluminum cluster anions toward ammonia was studied via mass spectrometry. Highly selective etching of Al 11 − and Al 12 − was observed at low concentrations of ammonia. However, at sufficiently high concentrations of ammonia, all other sizes of aluminum cluster anions, except for Al 13 − , were also observed to deplete. The disappearance of Al 11 − and Al 12 − was accompanied by concurrent production of Al 11 NH 3 − and Al 12 NH 3 − species, respectively. Theoretical simulations of the photoelectron spectrum of Al 11 NH 3 − showed conclusively that its ammonia moiety is chemisorbed without dissociation, although in the case of Al 12 NH 3 − , dissociation of the ammonia moiety could not be excluded. Moreover, since differences in calculated Al n − +NH 3 ͑n=9-12͒ reaction energies were not able to explain the observed selective etching of Al 11 − and Al 12 − , we concluded that thermodynamics plays only a minor role in determining the observed reactivity pattern, and that kinetics is the more influential factor. In particular, the conversion from the physisorbed Al n − ͑NH 3 ͒ to chemisorbed Al n NH 3 − species is proposed as the likely rate-limiting step.