Influence of Defects in Graphene-Like Network of Diamond-Like Carbon on Silica Scale Adhesion

At a Glance

Metadata	Details
Publication Date	2022-12-26
Journal	Tribology Letters
Authors	Yuya Nakashima, Noritsugu Umehara, Hiroyuki Kousaka, Takayuki TOKOROYAMA, Motoyuki Murashima
Institutions	Fuji Electric (Japan), Tohoku University
Citations	5
Analysis	Full AI Review Included

Executive Summary

This study investigates the fundamental mechanism of silica scale adhesion onto Diamond-Like Carbon (DLC) coatings, establishing a new model crucial for developing high-efficiency anti-scaling surfaces for geothermal power plant equipment.

Core Mechanism Identified: Silica adhesion occurs selectively at defects (specifically, dangling bonds) within the sp²-hybridized graphene-like network of the DLC coating, rather than adhering to the defect-free carbon atoms themselves.
Adsorption Mode: Dangling bonds facilitate chemical adsorption of silicic acid ions, resulting in a high adsorption energy (-1.04 eV) and strong adhesion force.
Suppression Strategy: Terminating these dangling bonds with hydrogen shifts the adhesion mode from chemical to physical adsorption, significantly reducing the adsorption energy to -0.69 eV.
Experimental Validation: In-lens Scanning Electron Microscopy (SE-SEM) confirmed that silica particles adhered preferentially to black-lined (defective) patterns on CVD graphene, while adhesion was minimal on defect-free Highly Oriented Pyrolytic Graphite (HOPG).
Design Implication: To suppress silica scaling, DLC coatings must be engineered to either minimize the number of defects in the graphene-like network or ensure that existing defects are fully saturated (terminated) with hydrogen.
Context: This mechanism explains why previous studies found DLC coatings with low sp² fractions and high hydrogen content were most effective at suppressing silica scale.

Technical Specifications

Parameter	Value	Unit	Context
Adsorption Energy (Defect-free)	-0.25	eV	Silicic acid ion on defect-free graphene (Physical Adsorption)
Adsorption Energy (Dangling Bonds)	-1.04	eV	Silicic acid ion on defective graphene (Chemical Adsorption)
Adsorption Energy (H-Terminated)	-0.69	eV	Silicic acid ion on hydrogen-terminated defects (Physical Adsorption)
Interatomic Distance (Dangling Bonds)	1.59	Angstrom	O^- of silicic acid ion to C atom with dangling bond (Chemically adsorbed state)
Interatomic Distance (H-Terminated)	2.08	Angstrom	O^- of silicic acid ion to H atom terminating dangling bond
Silicic Acid Ion to C Atom Distance (Defect-free)	2.66	Angstrom	Calculated distance after relaxation
DLC sp² Fraction (ta-CNx)	0.13	N/A	Low sp² fraction, hydrogen-free DLC
DLC Hydrogen Content (a-C:H)	45-47	%	High hydrogen content DLC
Silica Adhesion Test Temperature	50	°C	Imitated geothermal brine conditions
Silica Adhesion Test pH	8.5	N/A	Adjusted with HCl
Calculation Model Size	17.19 x 17.01 x 31.70	Angstrom³	Cell size for 112 carbon atoms graphene sheet model
Raman D Band	1350	cm^-1	Indicates presence of defects in CVD graphene
Raman G Band	1580	cm^-1	Indicates sp² carbon structure (present in HOPG and CVD graphene)

Key Methodologies

The study combined experimental silica adhesion tests on simplified carbon models with first-principles calculations to elucidate the adhesion mechanism.

Graphene Specimen Preparation:
- Defect-Free Model: Highly Oriented Pyrolytic Graphite (HOPG) was used, confirmed by the absence of the D band in Raman spectra.
- Defective Model: CVD-synthesized monolayer graphene (CVD graphene) was used, confirmed by the presence of the D band and black-lined patterns in in-lens SE images.
Structural and Defect Analysis:
- Raman Spectroscopy: Used to confirm the presence (CVD graphene) or absence (HOPG) of defects (D band at 1350 cm^-1).
- Field-Emission Scanning Electron Microscopy (FE-SEM): Used with secondary electron (SE) and in-lens SE detectors. In-lens detection visualized work function differences, correlating black-lined patterns on CVD graphene to high-work-function, defective sites.
Silica Adhesion Tests (Imitated Geothermal Brine):
- Samples were dipped in an imitated geothermal brine solution (NaSiO₃-9H₂O, NaCl) at 50 °C for 1 hour.
- The solution pH was maintained at 8.5, a condition where silicic acid ions (precursors to silica scale) are present.
- Morphological observation confirmed that silica adhered selectively to the defective patterns on CVD graphene, but adhered uniformly and minimally to HOPG.
First-Principles Calculations (Density Functional Theory):
- Calculations used plane-wave basis sets and the generalized gradient approximation (GGA) with van der Waals density functional (vdW-DF) corrections.
- Three Graphene Models were established to calculate the adsorption energy (ΔE) of a silicic acid ion:
  - Defect-free graphene.
  - Graphene with dangling bonds (simulating defects).
  - Graphene with hydrogen-terminated dangling bonds (simulating hydrogenated DLC).
Adsorption Mode Confirmation:
- Bader Charge Analysis was performed on the calculated models.
- Charge sharing between the silicic acid ion and the carbon atom confirmed chemical adsorption for the dangling bond model.
- Lack of charge sharing confirmed physical adsorption for the defect-free and hydrogen-terminated models.

Commercial Applications

This research directly supports the development and optimization of anti-scaling coatings, primarily for energy infrastructure operating in high-silica environments.

Geothermal Power Generation:
- Developing highly resistant DLC coatings for heat exchangers, pipes, and turbine components to prevent silica scale accumulation, thereby maintaining power efficiency and eliminating costly plant shutdowns required for mechanical or chemical cleaning.
High-Performance Tribological Coatings:
- The findings provide a fundamental understanding of how surface defects in carbon-based coatings (like ta-CNx and a-C:H) interact chemically with environmental species. This knowledge is transferable to optimizing DLC for low-friction and wear applications where chemical stability is critical.
Water Treatment and Desalination:
- Designing anti-fouling surfaces for membranes, filters, and equipment used in industrial water recycling or desalination processes where mineral scaling (including silica) reduces throughput and lifespan.
Advanced Material Design:
- Guiding the synthesis of DLC coatings by controlling the sp²/sp³ ratio and hydrogen content to minimize or passivate reactive dangling bonds, ensuring the resulting material possesses superior anti-adhesion properties.

View Original Abstract

Abstract Silica scale adhesion onto geothermal power plant equipment reduces the power efficiency. In our previous study, diamond-like carbon (DLC) coatings with low sp 2 fractions and high hydrogen contents were found to suppress silica adhesion. Therefore, the present study was aimed at clarifying the mechanism of silica adhesion onto the graphene-like network of DLC. In-lens scanning electron microscopic imaging of silica adhered onto defective graphene indicated that the adhesion occurred on defects in the graphene-like network. First-principles calculations revealed that the graphene with hydrogen-terminated defects exhibited reduced adsorption energy between silica and the graphene-like network. Overall, the simulations and experiments helped establish a silica adhesion model in which defects in the graphene-like network of DLC behave as silica adhesion sites. Graphical Abstract

Tech Support

Original Source

DOI: https://doi.org/10.1007/s11249-022-01690-4