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

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 Zn 2+ 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.

A Universal Strategy of Perovskite Ink ‐ Substrate Interaction to Overcome the Poor Wettability of a Self‐Assembled Monolayer for Reproducible Perovskite Solar Cells

“…This might result in a non‐uniform, random and dense orientation of Me‐4PACz on the glass part of the ITO substrate. Previously, Nie et al [ 65 ] and Nakamura et al [ 66 ] showed that the use of polar solvents to dissolve SAM results in no SAM layer formation on some oxide surfaces including glass. From this it indicates that the use of polar solvent might be the reason for poor/no assembly of Me‐4PACz SAM on the glass part of the patterned ITO substrate, thus leading to the formation of a dense Me‐4PACz layer and thus, no perovskite layer formation.…”

Understanding the Heterointerfaces in Perovskite Solar Cells via Hole Selective Layer Surface Functionalization

“…Phosphonic acids are known to induce superior surface modifications compared to chemisorbed carboxylic acids, particularly when subjected to annealing to maximize the incorporation of P-O bonds into the metal oxide lattice. [6] Better formation of octadecylphosphonic acid (ODPA) SAM on the oxide layer using solvents with lower polarity is reported by Nakamura et al [7] Enhancing device performance can be achieved by molecularlevel interface modification using a self-assembly monolayer. [8] NiO x surface treatment by different types of phenyl-phosphonic acids by dipping the substrates in SAM solution is well explored by Singh et al and they observe an enhancement of the power conversion efficiency (PCE) to 18.45%.…”

“…[ 6 ] Better formation of octadecylphosphonic acid (ODPA) SAM on the oxide layer using solvents with lower polarity is reported by Nakamura et al. [ 7 ] Enhancing device performance can be achieved by molecular‐level interface modification using a self‐assembly monolayer. [ 8 ] NiO x surface treatment by different types of phenyl‐phosphonic acids by dipping the substrates in SAM solution is well explored by Singh et al.…”