Surface Cleaning And Microstructure Control And Aqueous Charging Properties Of Alumina Single Crystals

Alpha alumina (α-Al2O3)-one of most important inorganic oxide materials, has been widely used for structural/fine ceramic applications, LED substrates and for aqueous geochemistry and catalysts study. The point of zero charge (PZC), which measures the surface’s affinity towards interacting electrons or protons in aqueous solutions, is the most important parameter in the charging property characterization. Recent PZC results, however, revealed serious inconsistency such as significantly lower PZC values of single-crystals than powders’, unexpected large variation of PZC values among single-crystals-even for the stable and well-known (0001) model system, and failure of traditional surface models for predicting experimental PZC values. We proposed to critically re-examine and cautiously address these issues, trying to gain new insights into surface structure/charging property relationships.We critically evaluated methods reported in the literatures using comprehensive surface analysis techniques including atomic force microscopy, XPS and contact angle measurements. We found that reported methods did not perform well in terms of removing organic and particulate contaminants from the (0001) surface, though some methods are only good at removing particular types of contaminants. After thoroughly examining the cleaning effect of various solvents/solutions and UV light and plasma irradiation, and based on modified RCA cleaning protocols, we developed a new wet-cleaning method showing outstanding cleaning performance. The new method is capable of removing major organic and particulate contaminants, revealing distinct step-terrace structures. With this method, the first two steps of soaking in ethanol and sonicated in detergent solution were used to remove bulky dissolvable contaminants and particulate matters. The SPM (H2SO4+H2O2) treatment was used to remove organic contaminants. The next two steps of modified SCI (NH3+H2O2+H2O) and SC2(HCl+H2O2+H2O) treatments-both with higher volume ratio of acid/base and peroxide in the solution against the standard RCA method, were used to remove metal contamination and particulate materials. This new reliable method will benefit the next-step study of surface charging properties of α-Al2O3single-crystals.Evolution of step-terrace structure of series of cleaned c-plane sapphire substrates with varying off-cut angles, treated with low-pressure air plasma irradiation as well as vacuum annealing was studied with AFM. By varying off-cut angles (0.1°～10°), the terrace width can be tuned between1.3nm-80nm to realize statistically different population percentage of different hydroxyl groups-from99.93%to96.3%for doubly coordinated Al2OH groups. Depending on plasma irradiation time, original structure with uniform terrace width and sharp steps undergoes distinct morphology changes with slight change in apparent percentage of Al2OH groups (as low as ca.75%but with low accuracy due to limited AFM resolution). With longer plasma irradiation up to30min, pairing of neighboring terraces into alternating wider and narrower terraces, step roughening and terrace etching, and "step-terrace free" morphology with etched pits were observed. Annealing at1273K for1h and1673K for10h resulted in slight difference in Al2OH percentage-99.84%vs.99.77%-due to combination of vicinal steps.Aqueous interfacial charging properties of (0001) single-crystals with controlled surface step-terrace structures and evaluation of protonation models were studied. Using AFM and colloidal probe technique, we obtained pH dependent force curves in10-3mol/L KNO3solutions on samples with well-defined distribution of hydroxyl groups. Derivation of surface potential from force curves was accomplished by fitting force curves with a double layer interaction model, and PZC values were thus determined from the surface charge-pH curves. The key findings were that, first, the intrinsic protonation constant of Al2OH is5.4as derived from samples with large terrace width, second, samples with lower percentage of Al2OH showed higher PZC with a highest value of6.9for plasma treated samples. Our results strongly support the recent Bickmore model and the non-orthodox-yet more compelling in recent years-viewpoint of Al2OH groups’ protonation activity over mild pH range. The implications of a modified model and the siginificance of consistently explaining controversial experimental results were discussed in details. Suggestions were given for future investigations in this field.