Chelating Agent Functionalized Substrates for the Formation of Thick Films via Electrophoretic Deposition

Abstract

Incorporating nanoparticles into devices for a wide range of applications often requires the formation of thick films, which is particularly necessary for improving magnetic power storage, microwave properties, and sensor performance. One approach to assembling nanoparticles into films is the use of electrophoretic deposition (EPD). This work seeks to develop methods to increase film thickness and stability in EPD by increasing film-substrate interactions via functionalizing conductive substrates with various chelating agents. Here, we deposited iron oxide nanoparticles onto conductive substrates functionalized with three chelating agents with different functional moieties and differing chelating strengths. We show that increasing chelating strength can increase film-substrate interactions, resulting in thicker films when compared to traditional EPD. Results will also be presented on how the chelating strength relates to film formation as a function of deposition conditions. Yield for EPD is influenced by deposition conditions including applied electric field, particle concentration, and deposition time. This work shows that the functionalization of substrates with chelating agents that coordinate strongly with nanoparticles (phosphonic acid and dopamine) overcome parameters that traditionally hinder the deposition of thicker and more stable films, such as applied electric field and high particle concentration. We show that functionalizing substrates with chelating agents is a promising method to fabricate thick, stable films of nanoparticles deposited via EPD over a larger processing space by increasing film-substrate interactions.

Keywords: assembly; chelating ligands; electrophorectic deposition; magnetic nanopartcles; nanomaterials.

FIGURE 1

Schematic illustrating coordination bonding between chelating agents of various dents and the hydrogens of the surface −OH (hydroxyl) groups of a metal oxide nanoparticle.

FIGURE 2

(A) Illustration of the substrate functionalized with chelating agent molecules for use during the electrophoretic deposition of iron oxide nanoparticles. (B) Expected structure of the chelating agent molecules as they are functionalized on an Au/Ti/Si substrate, including a thiol head group, a PEG3400 spacer, and functional group (R).

FIGURE 3

Reaction scheme for synthesizing (A) SH-PEG-PA and (B) SH-PEG-DPA (n = 3,400).

FIGURE 4

FTIR spectra of the two starting materials and the resulting chelating agent molecule for (A) SH-PEG-PA and (B) SH-PEG-DPA. Both (A) and (B) show the presence of an amide bond at 1700 cm⁻¹ (noted by the star) and the disappearance of the peaks associated with the NHS group, seen around 1750–1850 cm⁻¹ in the SH-PEG-NHS spectra but not in the final product in (A) or (B).

FIGURE 5

Panel of cross-sectional SEM micrographs showing the films for substrates that are non-functionalized and those functionalized with NTA, DPA, or PA for the (A) low particle concentration and moderate field and (B) high particle concentration and high field conditions. The “X” in panel (B) indicates that cross-sectional SEM images were not able to be obtained for the non-functionalized samples. The average thickness of each film is indicated by “T” in the top-right corner of each SEM image.

FIGURE 6

Plot showing the normalized thicknesses (defined as the average functionalized thickness divided by the average non-functionalized thickness) comparing the films that were both non-functionalized and functionalized with NTA, PA, or DPA for the low particle concentration and moderate field condition.

FIGURE 7

Effect of SH-PEG-DPA chelating agent in the suspension on the mass deposited for EPD on non-functionalized and SH-PEG-DPA functionalized substrates. The concentration of SH-PEG-DPA in the suspension was 0 or 0.5 g/L SH-PEG-DPA.

FIGURE 8

Comparison of mass loss between non-functionalized substrates and substrates functionalized with SH-PEG-DPA after the Tape Test (ASTM-D3359-17) was performed on films deposited with the (A) low particle concentration and moderate field and b) high particle concentration and high field conditions.

FIGURE 9

Field (E) vs. particle concentration (C) plot showing the E*C processing area, shaded in solid, for the parameter space used in this work. The solid-dashed line area represents the additional parameter space that is available by functionalizing substrates with chelating agents.