α-Al2O3 Surface Research Articles

Formation of Interfacial Reaction Layers in Al2O3/SS 430 Brazed Joints Using Cu-7Al-3.5Zr Alloys

The formation of interfacial reaction layers was investigated in an α-Al2O3/430 stainless steel (SS430) joint brazed using a Cu-7Al-3.5Zr active brazing alloy. Brazing was conducted at above its eutectic temperature of 945 °C and below liquidus 1045 °C, where liquid and solid phases of the brazing alloys coexists. At 1000 °C, the liquid phase of the brazing alloy was wet onto the α-Al2O3 surface. Zr in the liquid phase reduced α-Al2O3 to form a continuous ZrO2 layer. As the dwell time increased, Zr in the liquid phases near α-Al2O3 interface was used up to thicken the reaction layers. The growth kinetics of the layer obeys the parabolic rate law with a rate constant of 9.25 × 10−6 cm·s−1/2. It was observed that a number of low yield strength Cu-rich particles were dispersed over the reaction layer, which can release the residual stress of the joint resulting in reduction of crack occurrence.

Humic acids adsorption and decomposition on Mn2O3 and α-Al2O3 nanoparticles in aqueous suspensions in the presence of ozone

The removal and decomposition of humic acids (HAs) in the presence of ozone and aqueous suspensions of Mn2O3 and α-alumina (Al2O3) nanoparticles was investigated. Mn2O3 presented lower BET specific surface area (15.6 m2 g−1 vs 45.8 m2 g−1) but a higher point of zero charge (PZC) (5.9 vs 4.2) than α-Al2O3. Solution pH played a key role in the adsorption of HAs and catalytic oxidation on the surface of α-Al2O3 and Mn2O3 nanoparticles. The adsorption capacity of α-Al2O3 at the natural pH of HAs in water (pH 5.5) was up to 2.903 gHAs g−1, but no adsorption occurred onto the Mn2O3 nanoparticles, due to the unfavorable surface charge at pH 5.5. In consequence, although Mn2O3 was a more efficient catalyst (khet = 0.7 L−1 min−1 g−1) than α-Al2O3 (khet = 0.2 L−1 min−1 g−1) for the decomposition of O3, Mn2O3 did not exhibited catalytic action during the ozonation of HAs at pH 5.5. Instead, the Mn2O3 catalytic action was significant at pH equal to PZC (catalytic rate constant ratio k1-HAcat/ k1-HA = 1.562). Overall, α-Al2O3 exhibited the highest catalytic removal rate of HAs during ozonation (k1-HAcat/ k1-HA = 2.298) due to favorable surface charge and larger specific surface area. The main mechanism for HAs removal in the presence of α-Al2O3 involves simultaneous adsorption of both HAs and O3, the reaction of ozone from the bulk solution and the catalytic decomposition of HAs on the solid surface by ROS, through complex series-parallel reactions. The α-Al2O3 dosage up to 0.5 g L−1 required to remove HAs by catalytic ozonation was significantly lower than in other studies employing granular activated carbon, iron coated zeolite or γ-alumina catalysts.

Theoretical investigation of the effect of phosphate doping on the aggregation of Au atoms on an Al2O3 (0001) surface

Using density functional theory, the adsorption of Au atoms and Au dimers onto α-Al2O3 (0001) surfaces with various phosphate coverages was investigated. When the phosphate coverage is low (0% or 25%), Au dimers are more stable than isolated Au atoms on the surfaces, and therefore, aggregation of Au atoms into Au dimers is exothermic. In contrast, when the phosphate coverage is high (50% or 100%), isolated Au atoms are more stable than Au dimers on the surface, and Au atom aggregation is endothermic. This difference is caused by differences in the distance between adjacent phosphate groups; at certain distances, the phosphate groups are capable of bridging Au atoms. The Au-Au bond of a Au dimer at a phosphate-phosphate bridge site is destabilized by hole transfer from the Al2O3 surfaces to the bonding σ orbital of the Au dimer. Isolated phosphate groups cannot cause the destabilization. These results provide a good explanation for the mechanism of the suppression of aggregation of Au particles caused by phosphate doping on an Al2O3 surface, and show the importance of charge transfer from a surface to Au atoms for suppressing Au sintering.

Adsorption of lithium polysulfides on an anatase (1 0 1) and an [formula omitted]-Al2O3 (0 0 0 1) surface under external electric field with first principles calculations

The “shuttle effect” induced by the dissolution and diffusion of the Li2Sx (4⩽x⩽8) into the electrolyte is a crucial problem of a LiS battery. Anatase or Al2O3 can be used as an additive to suppress the “shuttle effect”. First-principles approach coupled with van der Waals (vdW) interaction is used to calculate the adsorption energies of Li2Sx (x=4,6,8) molecules on an anatase (1 0 1) or an α-Al2O3 (0 0 0 1) surface. The results show that the adsorption of the molecules on an α-Al2O3 (0 0 0 1) is stronger than that on an anatase (1 0 1) surface. Furthermore, owing to the existence of an electric field inside the battery during the charge-discharge process, the external electric field effects on the adsorption are investigated. As a result, the adsorption energy (absolute value) almost linearly increases with the increase of electric field intensity in the range from -0.16 to 0.16 V/Å. In addition, the responses of different Li2Sx molecules on an anatase (1 0 1) surface or an α-Al2O3 (0 0 0 1) surface to the external electric fields are different.

Vibrational spectroscopy of hydroxylated α-Al2O3(0001) surfaces with and without water: An ab initio molecular dynamics study.

Using gradient- and dispersion-corrected density functional theory in connection with ab initio molecular dynamics and efficient, parametrized Velocity-Velocity Autocorrelation Function (VVAF) methodology, we study the vibrational spectra (Vibrational Sum Frequency, VSF, and infrared, IR) of hydroxylated α-Al2O3(0001) surfaces with and without additional water. Specifically, by considering a naked hydroxylated surface and the same surface with a particularly stable, "ice-like" hexagonal water later allows us to identify and disentangle main spectroscopic bands of OH bonds, their orientation and dynamics, and the role of water adsorption. In particular, we assign spectroscopic signals around 3700 cm-1 as being dominated by perpendicularly oriented non-hydrogen bonded aluminol groups, with and without additional water. Furthermore, the thin water layer gives spectroscopic signals which are already comparable to previous theoretical and experimental findings for the solid/(bulk) liquid interface, showing that water molecules closest to the surface play a decisive role in the vibrational response of these systems. From a methodological point of view, the effects of temperature, anharmonicity, hydrogen-bonding, and structural dynamics are taken into account and analyzed, allowing us to compare the calculated IR and VSF spectra with the ones based on normal mode analysis and vibrational density of states. The VVAF approach employed in this work appears to be a computationally accurate yet feasible method to address the vibrational fingerprints and dynamical properties of water/metal oxide interfaces.

Water Molecular Beam Scattering at α-Al2O3(0001): An Ab Initio Molecular Dynamics Study

Recent molecular beam experiments have shown that water may adsorb molecularly or dissociatively on an α-Al2O3(0001) surface, with enhanced dissociation probability compared to “pinhole dosing”, i.e., adsorption under thermal equilibrium conditions. However, precise information on the ongoing reactions and their relative probabilities is missing. In order to shed light on molecular beam scattering for this system, we perform ab initio molecular dynamics calculations to simulate water colliding with α-Al2O3(0001). We find that single water molecules hitting a cold, clean surface from the gas phase are either reflected, molecularly adsorbed, or dissociated (so-called 1–2 dissociation only). A certain minimum translational energy (above 0.1 eV) seems to be required to enforce dissociation, which may explain the higher dissociation probability in molecular beam experiments. When the surface is heated and/or when refined surface and beam models are applied (preadsorption with water or water fragments, clusteri...

Water Dissociative Adsorption on α-Al2O3(112̅0) Is Controlled by Surface Site Undercoordination, Density, and Topology

α-Al2O3 surfaces are common in a wide variety of applications and useful models of more complicated, environmentally abundant, alumino-silicate surfaces. While decades of work have clarified that all properties of these surfaces depend sensitively on the crystal face and the presence of even small amounts of water, quantitative insight into this dependence has proven challenging. Overcoming this challenge requires systematic study of the mechanism by which water interacts with various α-Al2O3 surfaces. Such insight is most easily gained for the interaction of small amounts of water with surfaces in ultra high vacuum. In this study, we continue our combined theoretical and experimental approach to this problem, previously applied to water interaction with the α-Al2O3 (0001) and (11̅02) surfaces, now to water interaction with the third most stable surface, that is, the (112̅0). Because we characterize all three surfaces using similar tools, it is straightforward to conclude that the (112̅0) is most reactive with water. The most important factor explaining its increased reactivity is that the high density of undercoordinated surface Al atoms on the (112̅0) surface allows the bidentate adsorption of OH fragments originating from dissociatively adsorbed water, while only monodentate adsorption is possible on the (0001) and (11̅02) surfaces: the reactivity of α-Al2O3 surfaces with water depends strongly, and nonlinearly, on the density of undercoordinated surface Al atoms.

Molecular structure of octadecylphosphonic acids during their self-assembly on α-Al2O3(0001).

The formation of octadecylphosphonic acid (ODPA) self-assembled monolayers (SAMs) from 2-propanol solutions on hydroxylated α-Al2O3(0001) surfaces was studied in situ and in real time at the solid/liquid interface. Time-resolved vibrational spectra from sum-frequency generation (SFG) of C-H stretching modes revealed contributions from ODPA's alkyl backbone and the terminal methyl group as well as vibrational bands that originated from the presence of 2-propanol molecules at the α-Al2O3 surface. 2-Propanol signatures in SFG spectra decreased during SAM formation. This is due to adsorption of ODPA molecules which trigger desorption of 2-propanol from the α-Al2O3(0001) surface, so that these sites can be occupied by ODPA molecules. SAM formation was studied for different bulk concentrations of ODPA which changed substantially both the quality and the coverage of the final SAM. At initial stages of SAM growth, SFG spectra are dominated by methylene contributions and are indicative for a low molecular order and coverage of ODPA molecules. For concentrations of ODPA ≤2 mM this situation did not change within reasonable adsorption times (∼16 h) while for 5 and 30 mM concentrations a dramatic increase in molecular order and coverage within the first 2 h of adsorption is observed. Thermodynamic analysis using Langmuir adsorption kinetics provided equilibrium constants and the Gibbs free energy of adsorption between -24 and -28 kJ mol-1.

DFT investigation on the adsorption of munition compounds on α-Fe2O3: similarity and differences with α-Al2O3.

Arid environments have long been a testing and training ground for novel munitions. However, these activities leave behind unknown quantities of munition residues with unknown impact on local flora and fauna. In particular, arid soil contains Lewis acidic metal oxides which bind and catalyze the electron rich substituent groups commonly found in munition compounds, although the exact mechanisms are poorly understood. The current study remedies this lack of knowledge by utilizing density functional theory (DFT) to explore various orientations of four important munition compounds on the α-Fe2O3(0001) and α-Al2O3(0001) surfaces. Our findings reveal that while α-Fe2O3 binds the munition compounds more strongly than α-Al2O3, all four compounds experienced elongation of their nitro (-NO2) groups, indicating their susceptibility towards degradation on these surfaces.

Electronic transport properties of graphene/Al2O3 (0001) interface

The electronic structure and transport properties of a single layer of graphene (Gr) on α-Al2O3 surface are studied using the density functional theory (DFT). We present three models that take into account the atom at the termination of the alumina surface: a) Al atoms, with the center of the Gr hexagon directly over an Al atom; b) Al atoms, with a carbon directly positioned above an Al atom; c) oxygen atoms. Two processes of geometric optimization were used: (i) All the atoms of the supercell were allowed to move in accordance with the BFGS quasi-Newton algorithm; (ii) The atoms of the three topmost layers of the α-Al2O3 (0001) slab, including the C atoms, were allowed to move, whereas the atoms of the remaining layers were frozen in their respective atomic bulk positions. The first two models preserve qualitatively the electronic structure of the pristine Gr using the geometric optimization process (i) whereas, in the third model this structure was lost due to a significant charge transfer between the carbon and oxygen atoms irrespective of the optimization procedure. However, models (a) and (b) with the optimization (ii) reveal a p-type semiconducting behavior.

Stability of two orientations of MoS2 on α-Al2O3(0001)

We have studied the morphology of MoS2 on α-Al2O3(0001) using first-principles calculations. We found that the binding energy between MoS2 and the OH-terminated α-Al2O3(0001) surface is weaker than that for the Al-terminated surface. The band gap reduction of MoS2 is also smaller on the OH-terminated surface than on the Al-terminated surface. The strong chemical interaction between MoS2 and α-Al2O3(0001) seems to result in the larger binding energy and the larger band gap reduction on the Al-terminated surface. Despite the different binding characteristics between the OH- and Al-terminated surfaces, the total energy difference between the two orientations of the MoS2 monolayer related by a 60° rotation is quite small for both surfaces, indicating that the two orientations of MoS2 exist with almost the same amount on the α-Al2O3(0001) regardless of the termination. This result suggests that it is essentially difficult to obtain large-scale MoS2 with a single domain on the α-Al2O3(0001) by chemical vapor deposition.

Surface-charge-induced orientation of interfacial water suppresses heterogeneous ice nucleation on <i>α</i>-alumina (0001)

Abstract. Surface charge is one of the surface properties of atmospheric aerosols, which has been linked to heterogeneous ice nucleation and hence cloud formation, microphysics, and optical properties. Despite the importance of surface charge for ice nucleation, many questions remain on the molecular-level mechanisms at work. Here, we combine droplet-freezing assay studies with vibrational sum frequency generation (SFG) spectroscopy to correlate interfacial water structure to surface nucleation strength. We study immersion freezing of aqueous solutions of various pHs on the atmospherically relevant aluminum oxide α-Al2O3 (0001) surface using an isolated droplet on the surface. The high-pH solutions freeze at temperatures higher than that of the low-pH solution, while the neutral pH has the highest freezing temperature. On the molecular level, the SFG spectrum of the interfacial water changes substantially upon freezing. At all pHs, crystallization leads to a reduction of intensity of the 3400 cm−1 water resonance, while the 3200 cm−1 intensity drops for low pH but increases for neutral and high pHs. We find that charge-induced surface templating suppresses nucleation, irrespective of the sign of the surface charge. Heterogeneous nucleation is most efficient for the nominally neutral surface.

Born-Oppenheimer molecular dynamics simulation of pentanoic acid adsorption on α-Al2O3

Adsorption of a single pentanoic acid (C5H10O2) molecule on (0001) α-Al2O3 in a vacuum was explored with the aid of Born-Oppenheimer molecular dynamics simulations. Computer simulations were carried out considering two different situations, namely a clean Al/O-terminated surface and, also, a (0001) α-Al2O3 surface saturated with doubly-coordinated, isolated hydroxyls. In the first case, pentanoic acid adsorbs dissociatively, with the creation of an isolated surface hydroxyl, while the oxygen from the molecule's former carbonyl makes a bond to a nearby surface Al. On the other hand, pentanoic acid adsorbs on hydroxylated alumina by making a strong hydrogen bond to a surface oxygen, with the molecule aligning itself nearly parallel to the surface after full relaxation. For each case (i.e., pentanoic acid adsorption on Al/O-terminated or hydroxylated corundum surface), the different adsorption mechanism has a marked impact on the respective calculated infrared absorption spectrum, which can be of further use as an analytical tool to determine the underlying adsorption mechanism in actual experiments.

Substrate induced reconstruction and activation of platinum clusters: A systematic DFT study

The fundamental understanding of the electronic and geometric structures of small platinum clusters on metal oxide support is important to design the futuristic Pt-based novel materials for heterogeneous catalysis. Here we report a systematic theoretical study on the trend in the structural and electronic properties of alumina supported Ptn (n=1–7 and 10) clusters with a focus to highlight the effect on the substrate. All calculations were carried out using the plane wave based pseudo-potential approach. The model for the support has been designed by using Al-terminated α-Al2O3 (0001) surface which is the most stable surface termination under ultrahigh vacuum conditions. The results show that the binding of Pt atom with the Al2O3 surface releases 1.84eV energy which is significantly higher than atomic adsorption energy of other noble metal atoms (Ag, Au, and Pd). As a consequence, the equilibrium geometries of free Ptn clusters (n=3–7) are significantly altered on the alumina surface. Whilst Pt10 cluster favors tetracapped prism structure in the gas phase, on alumina support it prefers a layered structure. The geometrical changes of Pt clusters on the alumina surface have been attributed to the energy balance between the Pt-Pt and Pt-substrate interactions. The nature of interaction between the Ptn clusters and surface has been verified using the electronic density of states analysis. Surface induced electronic charge on the deposited cluster results in red shift in its energy levels, indicating electron rich activation of platinum clusters. The inclusion of spin-orbit coupling(SOC) significantly changes the electronic structure of gas phase platinum cluster; however, the extent of SOC influence reduces due to interfacial bonding with alumina support.

Adsorption of substituted benzoic acids onto α-Al2O3 surface in mixed-adsorbate mode: 2,4-Dihydroxybenzoic acid, 2,6-dihydroxybenzoic acid and 1,2,4-benzenetricarboxylic acid (trimellitic acid)

Influence of position and type of functional groups (COOH and phenolic OH groups) in a benzene ring on the adsorption of three natural organic matter (NOM) analogues e.g. 2,4-dihydroxybenzoic acid (2,4-DHBA), 2,6-dihydroxybenzoic acid (2,6-DHBA) and trimellitic acid (1,2,4-benzenetricarboxylic acid) onto α-alumina surface are explored in terms of adsorption kinetics and isotherms in single- and mixed-adsorbate mode. Focusing at the positional functional groups, adsorption profile of 2,4-DHBA and 2,6-DHBA is compared in single- and mixed-adsorbate mode. 2,4-DHBA produces higher adsorption density than 2,6-DHBA in both single- and mixed-adsorbate mode, governing factor is the ortho-para position of phenoilc OH group in 2,4-DHBA for higher adsorption density. Concerning type of functional groups, adsorption profile of 2,4-DHBA is also compared with trimellitic acid. Here again 2,4-DHBA produces higher adsorption density than trimellitic acid in single-adsorbate mode. In contrast, in mixed-adsorbate mode both adsorbates exhibit roughly equal adsorption densities indicating competition of co-operative nature, which is also supported by their activation energy values in mixed-adsorbate mode. Nevertheless, there exists a competition for the same surface sites of α-alumina that results in reduction of adsorption densities in mixed-adsorbate mode than that of the single-adsorbate mode.

Hydrogen diffusion mechanism on α-AlPO4 (0001)/α-Al2O3 (0001) interface: A first-principles study

α-AlPO4 (0001)/α-Al2O3 (0001) fabricated by selective laser sintering could improve the performance at protection against H permeation compared with that of single phase α-Al2O3 and α-AlPO4. In this study, the possible interfacial structures, thermodynamics and kinetics of hydrogen diffusion in heterojunction α-AlPO4 (0001)/α-Al2O3 (0001) coating were investigated based on the density functional theory. The adsorption energy and the adsorption sites are calculated in order to obtain the most stable configuration. The hydrogen isotopic diffusion in heterojunction α-AlPO4 (0001)/α-Al2O3 (0001) is also compared with that in α-Al2O3 (0001) or α-AlPO4 (0001) surface. The interfacial binding illustrates that hydrogen is more difficult to diffuse through the grain compare with α-Al2O3 (0001) or α-AlPO4 (0001). Due to the higher saddle point energy of hydrogen migration and potential well, the hydrogen resistivity property of α-AlPO4 (0001)/α-Al2O3 (0001) is better than the single phase materials. The interface of α-AlPO4 (0001)/α-Al2O3 (0001) is predicted to be effective at suppressing hydrogen permeation.

Molecular adsorption at electrolyte/α-Al2O3 interface of aluminum electrolytic capacitor revealed by sum frequency vibrational spectroscopy.

The operating voltage of an aluminum electrolytic capacitor is determined by the breakdown voltage (Ub) of the Al2O3 anode. Ub is related to the molecular adsorption at the Al2O3/electrolyte interface. Therefore, we have employed sum-frequency vibrational spectroscopy (SFVS) to study the adsorption states of a simple electrolyte, ethylene glycol (EG) solution with ammonium adipate, on an α-Al2O3 surface. In an acidic electrolyte (pH < 6), the Al2O3 surface is positively charged. The observed SFVS spectra show that long chain molecules poly ethylene glycol and ethylene glycol adipate adopt a "lying" orientation at the interface. In an alkaline electrolyte (pH > 8), the Al2O3 surface is negatively charged and the short chain EG molecules adopt a "tilting" orientation. The Ub results exhibit a much higher value at pH < 6 compared with that at pH > 8. Since the "lying" long chain molecules cover and protect the Al2O3 surface, Ub increases with a decrease of pH. These findings provide new insights to study the breakdown mechanisms and to develop new electrolytes for high operating voltage capacitors.

Tuning Adhesion at Metal/Oxide Interfaces by Surface Hydroxylation

–The control of adhesion at metal/oxide interfaces is of key importance in modern applications, whenever three-dimensional metal clusters or two-dimensional metal overlayers are to be synthesized on an oxide support. By focusing on the zinc/alumina system, we address here one of the long-standing issues in this context, which is the poor wetting of wide band gap oxides by noble and post-transition metals. It has recently been recognized to have detrimental industrial consequences for the adhesion of anticorrosive zinc coatings to new high strength steel grades. We have combined photoemission, thermal desorption, and plasmonics with atomistic simulation to describe the energetics of zinc deposits on dry and hydroxylated α-Al2O3(0001) surfaces. Both experimental and computational results show that an activated reaction of the metal with the OH-covered surface, followed by hydrogen desorption, produces dispersed interfacial moieties involving both oxidized Zn species and under-coordinated oxygen ions that le...

Insights on Interfacial Structure, Dynamics, and Proton Transfer from Ultrafast Vibrational Sum Frequency Generation Spectroscopy of the Alumina(0001)/Water Interface

Steady-state and time-resolved vibrational sum frequency generation (vSFG) were used to investigate the structure and dynamics of water at the α-Al2O3(0001) surface. The vSFG spectra of the O—H stretch of water next to the Al2O3(0001) surface are blue-shifted compared to the Al2O3(1120) surface, indicating its weaker hydrogen bonding network. Consequently, the vibrational dynamics of the O—H stretch of the neutral Al2O3(0001) surface is two times slower than the neutral Al2O3(1120) surface. Furthermore, the vibrational dynamics of the O—H stretch of water next to charged Al2O3 surfaces is observed to be faster than that in bulk water and at charged SiO2 surfaces, which could be due to (a) fast proton transfer dominating the vibrational relaxation and/or, (b) efficient coupling between the O—H stretch and the bend overtone via the presence of low frequency (∼3000 cm–1) O—H stretching modes. Lastly, the addition of excess ions (0.1 M NaCl) seems to have little to no effect on the time scale of vibrational...

Adsorptive removal of ammonium ion from aqueous solution using surfactant-modified alumina

Environmental contextAmmonium ion, an inorganic pollutant in agricultural land, can induce eutrophication, impacting on water quality. We investigate the adsorption of ammonium ion on surfactant-modified alumina and demonstrate highly efficient removal of ammonium ions by the alumina from two agricultural water samples. Adsorption mechanisms are also proposed based on adsorption isotherms, surface modification and the change in surface charge. AbstractThe adsorptive removal of ammonium ions (NH4+) from aqueous solution using surfactant-modified alumina (SMA) was investigated. The optimum NH4+ adsorption removal conditions on SMA were systematically studied and found to be pH 4, contact time 180min, adsorbent dosage 30mgmL–1 and ionic strength 1mM NaCl. The equilibrium concentration of NH4+ was measured by capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C4D) and spectrophotometry. Surface modification of α-Al2O3 with the anionic surfactant sodium dodecyl sulfate (SDS) at high salt concentration induced a significant increase of removal efficiency. The change in surface charge and surface modification of α-Al2O3 by pre-adsorption of SDS and subsequent adsorption of NH4+ were evaluated by zeta potential measurements and Fourier-transform infrared spectroscopy. Under optimum adsorption conditions, NH4+ removal from two agricultural water samples achieved very high removal efficiencies of 99.5 and 96.5%. The adsorption of NH4+ onto SMA increases with decreasing NaCl concentration because desorption of SDS from the α-Al2O3 surface is minimised. Experimental results of NH4+–SMA adsorption isotherms at different ionic strengths can be represented well by a two-step adsorption model. Based on adsorption isotherms, surface charge effect and surface modification, we suggest that the adsorption mechanism of NH4+ onto SMA was mainly electrostatic attraction between cationic NH4+ and the negatively charged SMA surface.