pH-dependent surface properties of the gallium nitride - Solution interface mapped by surfactant adsorption

Jianan Wang; Xing Zhang; Constance Wang; Hua Li; Haoran Li; Stacia Keller; Umesh K Mishra; Brett D Nener; Giacinta Parish; Rob Atkin

doi:10.1016/j.jcis.2019.08.079

pH-dependent surface properties of the gallium nitride - Solution interface mapped by surfactant adsorption

J Colloid Interface Sci. 2019 Nov 15:556:680-688. doi: 10.1016/j.jcis.2019.08.079. Epub 2019 Aug 23.

Authors

Jianan Wang¹, Xing Zhang², Constance Wang³, Hua Li⁴, Haoran Li⁵, Stacia Keller⁶, Umesh K Mishra⁷, Brett D Nener⁸, Giacinta Parish⁹, Rob Atkin¹⁰

Affiliations

¹ School of Engineering, The University of Western Australia, Perth, WA 6009, Australia; School of Molecular Sciences, The University of Western Australia, Perth, WA 6009, Australia. Electronic address: jianan.wang@research.uwa.edu.au.
² School of Engineering, The University of Western Australia, Perth, WA 6009, Australia. Electronic address: 21844631@student.uwa.edu.au.
³ School of Engineering, The University of Western Australia, Perth, WA 6009, Australia; Material Science and Engineering Department, University of Washington, Seattle, WA 98195-2120, United States. Electronic address: wangcon0@uw.edu.
⁴ School of Molecular Sciences, The University of Western Australia, Perth, WA 6009, Australia. Electronic address: hua.li@uwa.edu.au.
⁵ Department of Electrical and Computer Engineering, University of California Santa Barbara, Santa Barbara, CA 93106-9560, United States. Electronic address: haoran_li@umail.ucsb.edu.
⁶ Department of Electrical and Computer Engineering, University of California Santa Barbara, Santa Barbara, CA 93106-9560, United States. Electronic address: stacia@ece.ucsb.edu.
⁷ Department of Electrical and Computer Engineering, University of California Santa Barbara, Santa Barbara, CA 93106-9560, United States. Electronic address: mishra@ece.ucsb.edu.
⁸ School of Engineering, The University of Western Australia, Perth, WA 6009, Australia. Electronic address: brett.nener@uwa.edu.au.
⁹ School of Engineering, The University of Western Australia, Perth, WA 6009, Australia. Electronic address: giacinta.parish@uwa.edu.au.
¹⁰ School of Molecular Sciences, The University of Western Australia, Perth, WA 6009, Australia. Electronic address: rob.atkin@uwa.edu.au.

PMID: 31499439
DOI: 10.1016/j.jcis.2019.08.079

Abstract

Hypothesis: The surface charge of gallium nitride (GaN) in contact with solution is controlled by pH via surface protonation and deprotonation, similar to silica. Ionic surfactants adsorb on surfaces via electrostatic and hydrophobic interactions and can be utilized to reflect the surface charge of GaN.

Experiments: The surface charge properties of Ga-polar GaN in solution were probed as a function of pH using atomic force microscopy (AFM). AFM soft-contact images and force curves were used to study the pH-dependent adsorption of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and anionic surfactant sodium dodecylsulfate (SDS) on GaN surfaces. To further confirm the AFM results, GaN/AlGaN/GaN heterostructure-based ion sensing devices were used to measure the surfactant adsorption over the same pH range.

Findings: SDS aggregates adsorb on GaN below pH 2.75 while CTAB aggregates adsorb above pH 10. This shows that the GaN surface carries substantial net positive charge at low pH, and negative charge at high pH. There is no clear SDS or CTAB adsorption on the GaN surface between pH 3 and 9.75, which indicates the surface is weakly charged. GaN/AlGaN/GaN heterostructure-based devices confirm these results, and demonstrate the utility of these devices for measuring surfactant adsorption.

Keywords: AFM; Adsorption; Sensor; Solid/liquid; Surfactant.