Surface Dissociation Effect on Phosphonic Acid Self-Assembled Monolayer Formation on ZnO Nanowires

Kentaro Nakamura; Tsunaki Takahashi; Takuro Hosomi; Yu Yamaguchi; Wataru Tanaka; Jiangyang Liu; Masaki Kanai; Kazuki Nagashima; Takeshi Yanagida

doi:10.1021/acsomega.1c06183

Surface Dissociation Effect on Phosphonic Acid Self-Assembled Monolayer Formation on ZnO Nanowires

ACS Omega. 2021 Dec 27;7(1):1462-1467. doi: 10.1021/acsomega.1c06183. eCollection 2022 Jan 11.

Authors

Kentaro Nakamura^{1

2}, Tsunaki Takahashi^{1

3}, Takuro Hosomi^{1

3}, Yu Yamaguchi¹, Wataru Tanaka¹, Jiangyang Liu¹, Masaki Kanai², Kazuki Nagashima^{1

3}, Takeshi Yanagida^{1

2}

Affiliations

¹ Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan.
² Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan.
³ JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.

Abstract

Understanding the formation process of self-assembled monolayers (SAMs) of organophosphonic acids on ZnO surfaces is essential to designing their various applications, including solar cells, heterogeneous catalysts, and molecular sensors. Here, we report the significant effect of surface dissociation on SAM formation of organophosphonic acids on single-crystalline ZnO nanowire surfaces using infrared spectroscopy. When employing the most conventional solvent-methanol (relative permittivity ε_r = 32.6), the production of undesired byproducts (layered zinc compounds) on the surface was identified by infrared spectral data and microscopy. On the other hand, a well-defined SAM structure with a tridentate coordination of phosphonic acids on the surface was confirmed when employing toluene (ε_r = 2.379) or tert-butyl alcohol (ε_r = 11.22-11.50). The observation of layered zinc compounds as byproducts highlights that the degree of Zn²⁺ dissociation from the ZnO solid surface into a solvent significantly affects the surface coordination of phosphonic acids during the SAM formation process. Although the ZnO nanowire surface (m-plane) is hydrophilic, the present results suggest that a weaker solvent polarity is preferred to form well-defined phosphonic acid SAMs on ZnO nanowire surfaces without detrimental surface byproducts.