Modulating Surface Redox Reactions and Solvated Electron Emission on Boron-Doped Diamond by (Photo)electrochemistry

At a Glance

Metadata	Details
Publication Date	2025-08-28
Journal	PRX Energy
Authors	Arsène Chemin, Louis Godeffroy, Marin Rusu, Michael Drisch, Maik Finze
Institutions	University of Würzburg, Helmholtz-Zentrum Berlin für Materialien und Energie
Analysis	Full AI Review Included

Technical Documentation & Analysis: Boron-Doped Diamond Photoelectrochemistry

Executive Summary

This research successfully demonstrates the precise control of charge transfer mechanisms at the boron-doped diamond (BDD)/water interface using combined photoelectrochemical methods. The findings are critical for advancing solar fuel generation and energy storage applications.

Material Focus: Nanostructured Polycrystalline Boron-Doped Diamond (BDD) electrodes with Hydrogen (H), Oxygen (O), and Fluorine (F) surface terminations were analyzed.
Mechanism Deciphered: The study distinguished between sub-band-gap photocurrent (onset at 3.5 eV), driven by surface redox reactions involving C-H and C-OH states, and solvated electron emission (onset > 5.47 eV), driven by direct band-to-band excitation.
Surface Control: Electrochemical tuning of the surface band bending and termination (H, O) allows for the modulation of electron affinity (EA) and Flat Band Potential (FBP), enabling control over photoreduction or photooxidation reactions.
Quantified Recombination: A high Schottky ideality factor (n = 43 ± 5) was measured, indicating significant charge carrier recombination, particularly at grain boundaries and defects inherent to polycrystalline nanostructures.
6CCVD Value Proposition: The high recombination rate explicitly highlights the need for high-quality, defect-free Single Crystal Diamond (SCD) materials, which 6CCVD specializes in, to maximize Incident-Photon-to-Current Efficiency (IPCE).
Methodology: Photocurrent Cyclic Voltammetry (PC-CV) was introduced as a robust technique to monitor surface oxidation/reduction operando by tracking FBP hysteresis.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Band Gap (E_g)	5.47	eV	Bulk diamond
BDD Doping Concentration	5700 (10²¹ cm^-3)	ppm	Nanostructured Polycrystalline BDD
Photocurrent Onset Energy	3.5	eV	Sub-band-gap excitation (Surface States)
Solvated Electron Emission Onset	> 5.47	eV	Above band gap excitation
Saturation Current IPCE	0.10 ± 0.02	%	Above band gap excitation efficiency
Schottky Ideality Factor (n)	43 ± 5	-	Indicates high charge carrier recombination
Flat Band Potential (FBP) H-BDD	0.37 ± 0.03	V	vs Ag
Flat Band Potential (FBP) O-BDD	0.40 ± 0.04	V	vs Ag
Flat Band Potential (FBP) F-BDD	0.93 ± 0.04	V	vs Ag
PC-CV Excitation Energy	3.22	eV	High-intensity LED source
Working Electrode Area	0.8	mm²	Small-scale PEC cell testing
Electrolyte Concentration (PC-CV)	3	M	KCl (High ionic conductivity)

Key Methodologies

The experimental approach combined advanced material synthesis and surface functionalization with highly sensitive electrochemical and spectroscopic characterization.

Material Synthesis: Polycrystalline Boron-Doped Diamond (BDD) wafers were grown using Microwave Plasma Chemical Vapor Deposition (MPCVD).
Nanostructuring: The BDD wafers were nanostructured via Reactive Ion Etching (RIE) following the dewetting of a metal mask, resulting in needlelike structures (~100 nm).
Surface Termination:
- H-BDD: Achieved by exposure to hydrogen plasma treatment at 700 °C.
- O-BDD: Achieved by wet chemical treatment in a concentrated sulfuric acid/nitric acid mixture (3:1 ratio) for 1.5 h at 250 °C.
- F-BDD: Achieved by fluorination in liquid anhydrous HF containing 30% elemental fluorine at 50 °C for 5 days.
Photocurrent Spectroscopy: Measurements were conducted in a three-electrode PEC cell (3M KCl electrolyte) using a potentiostat and modulated IR-UV illumination (1.8 Hz) coupled with lock-in detection for picoampere sensitivity.
Solvated Electron Quantification: Fluorimetric titration was performed using NO₃^- ions as electron scavengers, followed by reaction with 2,3-diaminonaphthalene (2,3-DAN) to form a fluorescent 1H-naphthotriazole probe.
Surface State Characterization: X-ray Absorption Spectroscopy (XAS) and Photoelectron Yield Spectroscopy (PYS) were used to map the position of C-H, C-OH, and C=O surface states within the diamond band gap and determine the surface electron affinity (EA).

6CCVD Solutions & Capabilities

This research demonstrates the critical role of material quality and precise surface engineering in achieving high-efficiency photoelectrochemical conversion. 6CCVD is uniquely positioned to supply the advanced diamond materials required to replicate and significantly extend these findings, particularly by mitigating the high charge recombination observed (n = 43 ± 5).

Applicable Materials for Replication and Extension

Research Requirement	6CCVD Solution	Technical Advantage
High Doping (10²¹ cm^-3)	Heavy Boron-Doped PCD or SCD (BDD)	Provides the necessary conductivity for electrochemical applications and high carrier concentration for photoexcitation.
Low Defect Density	Optical Grade Single Crystal Diamond (SCD)	Directly addresses the high ideality factor (n=43) observed. SCD minimizes grain boundaries and defects, drastically reducing charge recombination and maximizing IPCE.
Nanostructured/High Surface Area	Polycrystalline Diamond (PCD) Substrates	Offers robust, scalable PCD wafers (up to 125mm) suitable for subsequent nanostructuring (e.g., RIE) or use as highly conductive substrates (up to 10mm thick).
Surface Termination Studies	Standard H- and O-Terminated Wafers	Provides baseline materials with standard terminations. 6CCVD supports custom functionalization protocols (e.g., Fluorination) for advanced surface chemistry research.
Baseline Polishing	SCD Polishing (Ra < 1nm)	Provides atomically smooth surfaces for fundamental studies, allowing researchers to isolate the effects of surface chemistry from structural defects.

Customization Potential for PEC Research

To move this research from small-scale (0.8 mm²) laboratory experiments toward scalable, high-efficiency devices, 6CCVD offers comprehensive customization services:

Custom Dimensions: We provide BDD plates and wafers in custom sizes up to 125 mm (PCD) and various thicknesses (SCD/PCD from 0.1 µm to 500 µm), enabling scale-up studies for solar fuel generation.
Precision Metalization: The creation of Ohmic contacts is crucial for efficient charge extraction. 6CCVD offers in-house metalization services, including deposition of Au, Pt, Pd, Ti, W, and Cu, tailored to specific electrode geometries and contact requirements.
Advanced Polishing: While the paper used nanostructured surfaces, studies requiring pristine interfaces benefit from our ultra-low roughness polishing (Ra < 1 nm for SCD; Ra < 5 nm for inch-size PCD).
Substrate Engineering: We can supply thick, highly conductive BDD substrates (up to 10 mm) for robust electrode fabrication and integration into complex PEC flow cells.

Engineering Support

6CCVD’s in-house PhD team specializes in the electronic and chemical properties of MPCVD diamond. We offer authoritative professional consultation to assist engineers and scientists in:

Material Selection: Guiding the choice between SCD and PCD based on the trade-off between surface area (PCD) and defect mitigation (SCD) for specific photoelectrochemical solar fuel generation projects.
Interface Optimization: Advising on optimal doping levels, termination methods, and metal contact strategies to minimize the large charge recombination observed in this study and maximize quantum efficiency.
Global Logistics: Ensuring reliable global shipping (DDU default, DDP available) for time-sensitive research projects worldwide.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

The interplay between photochemical and electrochemical reactions fundamentally influences charge transfer processes at solid-liquid interfaces. Nevertheless, chemical processes at semiconductor surfaces triggered by light excitation under an applied potential remain poorly explored. This work deciphers the synergistic effect of potential and light excitation on boron-doped diamond electrodes in producing either surface redox reactions or emission of solvated electrons in water. The effect of diamond surface termination on electron affinity, band bending, and charge extraction is identified in a photoelectrochemical cell. While photocurrent is observed for excitation as low as 3.5 eV, we show that it is induced mostly by surface redox reactions, whereas solvated electrons are detected only for excitation above the band gap (5.47 eV). Solvated electrons are generated irrespective of band bending, which affects only the emission yield. Depending on the surface band bending, photoreduction of the hydroxylated surface groups and photooxidation of the C—<a:math xmlns:a=“http://www.w3.org/1998/Math/MathML” display=“inline”><a:mrow><a:mrow><a:mi mathvariant=“normal”>H</a:mi></a:mrow></a:mrow></a:math> surface groups can be induced by direct photoexcitation in the range of 4.2-4.8 eV. The surface of the diamond can be electrochemically reduced when the Fermi level of the oxidized surface decreases below the <d:math xmlns:d=“http://www.w3.org/1998/Math/MathML” display=“inline”><d:msup><d:mrow><d:mrow><d:mi mathvariant=“normal”>H</d:mi></d:mrow></d:mrow><d:mo>+</d:mo></d:msup></d:math>/<g:math xmlns:g=“http://www.w3.org/1998/Math/MathML” display=“inline”><g:msub><g:mrow><g:mrow><g:mi mathvariant=“normal”>H</g:mi></g:mrow></g:mrow><g:mn>2</g:mn></g:msub></g:math> redox potential. On the other hand, the hydrogenated surface oxidizes spontaneously for potentials at which the Fermi level drops below the occupied <j:math xmlns:j=“http://www.w3.org/1998/Math/MathML” display=“inline”><j:mstyle displaystyle=“false” scriptlevel=“0”><j:mtext>C---H</j:mtext></j:mstyle></j:math> surface states, depending on both the pH and the electron affinity of the surface. This work provides insights into (photo)redox processes on diamond materials, which may find applications in photoelectrochemical solar fuel generation or energy storage.

Tech Support

Original Source

DOI: https://doi.org/10.1103/zhcb-13pn

References

2016 - Part 1 Fundamental Aspects of Photocatalysis [Crossref]
2016 - Photoelectrochemical Solar Fuel Production: From Basic Principles to Advanced Devices