High-Temperature Oxidation of Heavy Boron-Doped Diamond Electrodes - Microstructural and Electrochemical Performance Modification

At a Glance

Metadata	Details
Publication Date	2020-02-21
Journal	Materials
Authors	Jacek Ryl, Mateusz Cieślik, A. Zieliński, Mateusz Ficek, Bartłomiej Dec
Institutions	Gdańsk University of Technology
Citations	26
Analysis	Full AI Review Included

Executive Summary

Corrosion Mechanism Confirmed: High-temperature treatment (600 °C in air) of heavy boron-doped diamond (BDD) electrodes induces rapid and permanent structural modifications, leading to irreversible corrosion and degradation of electrochemical performance.
Kinetics Deterioration: Charge transfer kinetics were significantly hindered, demonstrated by a decrease in the anodic peak current (i_A) by a factor of 4 after 10 minutes of oxidation, and a nearly 50% reduction in the standard reaction rate constant (k⁰) after just 3 minutes.
Surface Heterogeneity: Scanning Electrochemical Microscopy (SECM) and Scanning Spreading Resistance Microscopy (SSRM) revealed that oxidation is highly heterogeneous, resulting in a bimodal conductivity distribution and locally variable electrochemical activity across the polycrystalline surface.
Structural Defects: Prolonged exposure (30-90 min) leads to irreversible structural defects, including the formation of small, shallow etch pits, indicating degradation of the BDD grain structure.
Surface Chemistry Shift: X-ray Photoelectron Spectroscopy (XPS) confirmed a transition from Hydrogen-Terminated (HT-BDD) to Oxygen-Terminated (OT-BDD), with hydroxyl species (C-OH) becoming dominant (up to 36.8% share).
Irreversible Damage: Subsequent rehydrogenation in plasma, while improving surface chemistry (reducing OT-BDD share), failed to fully restore the original electrochemical and electrical properties, confirming the corrosive nature of the high-temperature treatment.

Technical Specifications

Parameter	Value	Unit	Context
Oxidation Temperature	600	°C	Treatment performed in air.
BDD Thermal Conductivity	~700	W/mK	High intrinsic property of BDD.
Standard CV Scan Rate	50	mV/s	Used for kinetic analysis (Table 1).
Redox Couple	[Fe(CN)₆]^3-/4-	N/A	Inner-sphere electron transfer (ISET) probe.
Untreated k⁰	3.58 x 10^-3	cm/s	Standard reaction rate constant (HT-BDD).
90 min Oxidized k⁰	1.91 x 10^-3	cm/s	Significant kinetic reduction (OT-BDD).
Untreated Peak Separation (ΔE)	0.35	V	As-prepared BDD.
30 min Oxidized Peak Separation (ΔE)	1.04	V	Indicates highly irreversible process kinetics.
Untreated Mean Surface Resistance (R_s)	0.1505	MΩ	Measured by SSRM.
90 min Oxidized Mean Surface Resistance (R_s)	16.1148	MΩ	100-fold increase due to oxidation.
Untreated OT-BDD Surface Share	5.7	%	Based on XPS analysis.
90 min Oxidized OT-BDD Surface Share	52.4	%	Maximum oxidation plateau reached.
Dominant Oxidized Species (30 min)	35.6	%	Hydroxyl (C-OH) species share of C1s peak.
Rehydrogenated OT-BDD Share (90 min oxidized)	20.6	%	Post-plasma treatment; still significantly higher than untreated.

Key Methodologies

BDD Synthesis: Heavy boron-doped diamond thin films were synthesized on Si substrates using Microwave Plasma-Assisted Chemical Vapor Deposition (MPCVD).
Doping Level: Boron doping was high, with a B/C ratio of 2 x 10⁴ atoms cm^-3, resulting in electrically conductive nanocrystalline layers.
Initial Cleaning: As-prepared electrodes were cleaned using a Piranha solution (H₂O₂:H₂SO₄/1:3) at 90 °C, followed by a 10-minute hydrogen plasma treatment to achieve initial Hydrogen Termination (HT-BDD).
High-Temperature Oxidation: Samples were subjected to thermal treatment in air at 600 °C for durations of 3, 10, 30, and 90 minutes.
Reversibility Test: Oxidized samples were subsequently rehydrogenated using microwave hydrogen plasma for 10 minutes to assess the reversibility of the corrosion.
Electrochemical Characterization: Cyclic Voltammetry (CV) and Scanning Electrochemical Microscopy (SECM) were performed using the [Fe(CN)₆]^3-/4- redox couple in 0.5 M Na₂SO₄ electrolyte.
Surface Analysis: Microstructural changes were monitored using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Surface conductivity and termination were analyzed using Scanning Spreading Resistance Microscopy (SSRM) and high-resolution X-ray Photoelectron Spectroscopy (XPS).

Commercial Applications

The findings regarding the stability and degradation of BDD electrodes under high thermal stress are critical for applications where diamond materials are exposed to elevated temperatures or aggressive oxidative environments.

Advanced Oxidation Processes (AOP) / Water Treatment:
- BDD electrodes are used for wastewater treatment requiring high potentials and current densities, which can induce localized heating and oxidation. Understanding thermal corrosion limits operational stability.
High-Temperature Electronics and RF Devices:
- BDD layers are utilized as heat spreaders (~700 W/mK thermal conductivity) in high-power RF/microwave devices and as diamond-based Schottky barrier diodes, where thermal stability is paramount.
Electrochemical Sensing and Energy Storage:
- The irreversible loss of electrochemical activity and increased surface resistance directly impacts the long-term reliability and sensitivity of BDD-based sensors and energy storage components operating in thermally demanding conditions.
Protective Coatings:
- BDD layers are applied as corrosion-resistant protective coatings (e.g., for Si photoelectrodes in photoelectrochemical cells). The study defines the thermal limits before structural integrity is compromised by etch pit formation.

View Original Abstract

In this work, we reveal in detail the effects of high-temperature treatment in air at 600 °C on the microstructure as well as the physico-chemical and electrochemical properties of boron-doped diamond (BDD) electrodes. The thermal treatment of freshly grown BDD electrodes was applied, resulting in permanent structural modifications of surface depending on the exposure time. High temperature affects material corrosion, inducing crystal defects. The oxidized BDD surfaces were studied by means of cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM), revealing a significant decrease in the electrode activity and local heterogeneity of areas owing to various standard rate constants. This effect was correlated with a resultant increase of surface resistance heterogeneity by scanning spreading resistance microscopy (SSRM). The X-ray photoelectron spectroscopy (XPS) confirmed the rate and heterogeneity of the oxidation process, revealing hydroxyl species to be dominant on the electrode surface. Morphological tests using scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed that prolonged durations of high-temperature treatment lead not only to surface oxidation but also to irreversible structural defects in the form of etch pits. Our results show that even subsequent electrode rehydrogenation in plasma is not sufficient to reverse this surface oxidation in terms of electrochemical and physico-chemical properties, and the nature of high-temperature corrosion of BDD electrodes should be considered irreversible.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma13040964

References

2019 - Boron-doped diamond: Current progress and challenges in view of electroanalytical applications [Crossref]
2019 - Environmental Applications of Boron-Doped Diamond Electrodes: 1. Applications in Water and Wastewater Treatment [Crossref]
2018 - Nanocrystalline Boron-Doped Diamond as a Corrosion-Resistant Anode for Water Oxidation via Si Photoelectrodes [Crossref]
2020 - Effect of reactor configuration on the kinetics and nitrogen byproduct selectivity of urea electrolysis using a boron doped diamond electrode [Crossref]
2018 - The Dependence of Oxidation Parameters and Dyes’ Molecular Structures on Microstructure of Boron-Doped Diamond in Electrochemical Oxidation Process of Dye Wastewater [Crossref]
2016 - Beyond Thermal Management: Incorporating p-Diamond Back-Barriers and Cap Layers Into AlGaN/GaN HEMTs [Crossref]