The apparent surface free energy of rare earth oxides is governed by hydrocarbon adsorption

Junho Oh; Daniel Orejon; Wooyoung Park; Hyeongyun Cha; Soumyadip Sett; Yukihiro Yokoyama; Vincent Thoreton; Yasuyuki Takata; Nenad Miljkovic

doi:10.1016/j.isci.2021.103691

The apparent surface free energy of rare earth oxides is governed by hydrocarbon adsorption

iScience. 2021 Dec 25;25(1):103691. doi: 10.1016/j.isci.2021.103691. eCollection 2022 Jan 21.

Authors

Junho Oh^{1

2}, Daniel Orejon^{3

4}, Wooyoung Park¹, Hyeongyun Cha¹, Soumyadip Sett¹, Yukihiro Yokoyama¹, Vincent Thoreton^{3

5}, Yasuyuki Takata³, Nenad Miljkovic^{1

3

6

7}

Affiliations

¹ Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
² Department of Mechanical Engineering, BK21 FOUR ERICA-ACE Center, Hanyang University, Ansan, Gyeonggi 15588, Republic of Korea.
³ International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan.
⁴ Institute for Multiscale Thermofluids, School of Engineering, University of Edinburgh, Edinburgh, EH9 3FD, UK.
⁵ Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway.
⁶ Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
⁷ Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.

Abstract

The surface free energy of rare earth oxides (REOs) has been debated during the last decade, with some reporting REOs to be intrinsically hydrophilic and others reporting hydrophobic. Here, we investigate the wettability and surface chemistry of pristine and smooth REO surfaces, conclusively showing that hydrophobicity stems from wettability transition due to volatile organic compound adsorption. We show that, for indoor ambient atmospheres and well-controlled saturated hydrocarbon atmospheres, the apparent advancing and receding contact angles of water increase with exposure time. We examined the surfaces comprehensively with multiple surface analysis techniques to confirm hydrocarbon adsorption and correlate it to wettability transition mechanisms. We demonstrate that both physisorption and chemisorption occur on the surface, with chemisorbed hydrocarbons promoting further physisorption due to their high affinity with similar hydrocarbon molecules. This study offers a better understanding of the intrinsic wettability of REOs and provides design guidelines for REO-based durable hydrophobic coatings.

Keywords: Inorganic materials; Materials application; Materials characterization; Materials chemistry; Materials science.