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

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 themore » 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.« less
Authors:
 [1] ;  [2] ;  [1] ;  [3] ; ORCiD logo [1] ;  [1] ;  [4] ;  [1] ;  [5] ;  [6] ; ORCiD logo [7] ; ORCiD logo [8] ;  [9] ;  [10] ;  [11] ;  [11] ;  [12]
  1. Karlsruhe Inst. of Technology (KIT) (Germany)
  2. Univ. of Melbourne (Australia)
  3. Max Bergmann Center for Biomaterials, Dresden (Germany)
  4. Karlsruhe Inst. of Technology (KIT) (Germany); Indian Inst. of Science Education and Research Kolkata, West Bengal (India)
  5. Indian Institute of Science Education and Research Kolkata,; Brenk Systemplanung GmbH, Aachen (Germany)
  6. Univ. of Chicago, IL (United States)
  7. Washington Univ., St. Louis, MO (United States)
  8. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Univ. of Gothenburg (Sweden)
  9. Univ. of Zagreb (Croatia)
  10. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  11. Harbin Inst. of Technology (China)
  12. Westfälische Wilhelms-Universität Münster (Germany)
Publication Date:
Grant/Contract Number:
AC05-00OR22725
Type:
Accepted Manuscript
Journal Name:
Advances in Colloid and Interface Science
Additional Journal Information:
Journal Volume: 251; Journal Issue: C; Journal ID: ISSN 0001-8686
Research Org:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
OSTI Identifier:
1476427

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., and Braunschweig, B.. The surface chemistry of sapphire-c: A literature review and a study on various factors influencing its IEP. United States: N. p., Web. doi:10.1016/j.cis.2017.12.004.
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.. The surface chemistry of sapphire-c: A literature review and a study on various factors influencing its IEP. United States. doi:10.1016/j.cis.2017.12.004.
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., and Braunschweig, B.. 2017. "The surface chemistry of sapphire-c: A literature review and a study on various factors influencing its IEP". United States. doi:10.1016/j.cis.2017.12.004. https://www.osti.gov/servlets/purl/1476427.
@article{osti_1476427,
title = {The surface chemistry of sapphire-c: A literature review and a study on various factors influencing its IEP},
author = {Lützenkirchen, J. and Franks, G. V. and Plaschke, M. and Zimmermann, R. and Heberling, F. and Abdelmonem, A. and Darbha, G. K. and Schild, D. and Filby, A. and Eng, P. and Catalano, J. G. and Rosenqvist, J. and Preocanin, T. and Aytug, T. and Zhang, D. and Gan, Y. and Braunschweig, B.},
abstractNote = {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.},
doi = {10.1016/j.cis.2017.12.004},
journal = {Advances in Colloid and Interface Science},
number = C,
volume = 251,
place = {United States},
year = {2017},
month = {12}
}