The surface chemistry of sapphire-c: A literature review and a study on various factors influencing its IEP

• Published in: Advances in colloid and interface science Vol. 251; no. C; pp. 1 - 25
• Main Authors: Lützenkirchen, J., Franks, G.V., Plaschke, M., Zimmermann, R., Heberling, F., Abdelmonem, A., Darbha, G.K., Schild, D., Filby, A., Eng, P., Catalano, J.G., Rosenqvist, J., Preocanin, T., Aytug, T., Zhang, D., Gan, Y., Braunschweig, B.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01-01-2018
• Subjects: 1964; 1993; 2004; acid-base; AFM; alpha-al2o3 0001 surface; alpha-fe2o3 crystal faces; Aluminum oxide basal plane; Annan medicinsk grundvetenskap; aquatic sciences; aqueous-electrolyte solutions; arlet l; atomic-force microscopy; australian journal of chemistry; c 03-05; cation-exchange capacity; Chemistry; france; heterogeneous; ice nucleation; Isoelectric point; molecular-dynamics simulations; optical 2nd-harmonic generation; Other Basic Medicine; p291; p53; p828; properties; reet n; Streaming potential; sum-frequency generation; Surface charge; Surface preparation; toulouse; v17; v55; Aluminum oxide basal plane; AFM; Surface charge; Streaming potential; Isoelectric point; Surface preparation
• DOI: 10.1016/j.cis.2017.12.004
• EISSN: 1873-3727
• ISSN: 0001-8686; 1873-3727

A wide range of isoelectric points (IEPs) has been reported in the literature for sapphire-c (α-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 individual 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.
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•The vast majority of sapphire-c plane samples exhibits isoelectric points between pH3.5 and 4.5.•It is best interpreted by physical adsorption of hydroxide ion in preference to hydronium at the interfacial water film.•Plasma cleaning increases the IEP. UV cleaning and annealing decrease it. All generate surfaces that evolve with time in water.•Different surface treatments cause contradictory spectroscopic sum frequency generation data.•Microscale roughness causes lower IEP and might cause slip boundary conditions.