Modulating Surface Redox Reactions and Solvated Electron Emission on Boron-Doped Diamond by (Photo)electrochemistry
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-08-28 |
| Journal | PRX Energy |
| Authors | ArsĂšne Chemin, Louis Godeffroy, Marin Rusu, Michael Drisch, Maik Finze |
| Institutions | Université Claude Bernard Lyon 1, Fraunhofer Institute for Applied Solid State Physics |
| Analysis | Full AI Review Included |
Modulating Surface Redox Reactions and Solvated Electron Emission on Boron-Doped Diamond
Section titled âModulating Surface Redox Reactions and Solvated Electron Emission on Boron-Doped DiamondâExecutive Summary
Section titled âExecutive SummaryâThis study provides a comprehensive analysis of the boron-doped diamond (BDD)/water interface, demonstrating precise control over charge transfer mechanisms using combined potential and light excitation.
- Dual Mechanism Deciphered: Photocurrent (PC) observed below the diamond band gap (as low as 3.5 eV) is attributed solely to surface redox reactions (C-H and C-OH groups), while the emission of solvated electrons (e-(aq)) occurs only upon excitation above the 5.47 eV band gap.
- Band Bending Control: The applied potential (band bending) dictates the type of surface photochemistry: downward band bending favors photoreduction (C-OH to C-H), and upward band bending favors photooxidation (C-H).
- Surface Stability Mapping: Photocurrent Cyclic Voltammetry (PC-CV) reveals significant hysteresis in the Flat Band Potential (FBP), directly correlating FBP shifts with the electrochemical oxidation (FBPO = +0.52 V) and reduction (FBPH = -0.11 V) of the diamond surface.
- Instability of H-BDD: The hydrogenated (H-BDD) surface is unstable in water, spontaneously oxidizing until the Fermi level aligns with the occupied C-H surface states, demonstrating self-limited oxidation.
- Efficiency Limitation: A very high ideality factor (n = 43 ± 5) was measured, indicating severe charge carrier recombination, which significantly limits the overall Incident-Photon-to-Current Efficiency (IPCE) to about 0.1% above the band gap.
- Future Engineering: The insights gained enable rational design strategies, such as surface passivation and interface engineering, to mitigate recombination and improve diamond photoelectrode efficiency for solar fuel and energy storage applications.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Band Gap (Eg) | 5.47 | eV | Bulk BDD material |
| Boron Doping Level | 5700 (1021) | ppm (cm-3) | Nanostructured BDD bulk |
| Nanostructure Size | ~100 | nm | Needlelike structures (cross-section) |
| Solvated Electron Threshold | > 5.47 | eV | Minimum excitation energy required for e-(aq) emission |
| Sub-Band-Gap PC Onset | 3.5 | eV | Photocurrent onset for H-BDD and O-BDD |
| Saturation IPCE (above Eg) | 0.10 ± 0.02 | % | Incident-Photon-to-Current Efficiency |
| Ideality Factor (n) | 43 ± 5 | N/A | Schottky barrier fit, indicating high charge recombination |
| FBP (Oxidized Surface) | +0.52 ± 0.05 | V vs Ag | AgCl |
| FBP (Reduced Surface) | -0.11 ± 0.01 | V vs Ag | AgCl |
| Electrolyte Concentration | 3 | M | KCl (for PEC measurements) |
| C-H Oxidation Onset | -0.6 | V vs Ag | AgCl |
| Photoredox Excitation (C-OH) | 4.19 ± 0.1, 4.83 ± 0.1 | eV | Photoreduction energies (downward band bending) |
| Photooxidation Excitation (C-H) | 4.8 ± 0.2 | eV | Photooxidation energy (upward band bending) |
| XAS Energy Resolution | ~75 | meV | C 1s X-ray absorption edges |
Key Methodologies
Section titled âKey MethodologiesâThe study employed a combination of advanced electrochemical and spectroscopic techniques to characterize the BDD/water interface in operando.
-
Electrode Preparation:
- Material: Polycrystalline BDD wafer (5700 ppm B doping) nanostructured via reactive ion etching (RIE) after dewetting a metal mask.
- H-BDD Termination: Exposure to hydrogen plasma treatment at 700 °C.
- O-BDD Termination: Wet chemical treatment using a 3:1 mixture of concentrated sulfuric acid and nitric acid for 1.5 h at 250 °C.
- F-BDD Termination: Fluorination in liquid anhydrous HF containing 30% elemental fluorine, stirred at 50 °C for 5 days.
-
Photoelectrochemical (PEC) Setup:
- Cell: Three-electrode PEC flow cell (SEC-3F).
- Electrodes: BDD Working Electrode (WE), stainless steel Counter Electrode (CE), Ag|AgCl Reference Electrode.
- Electrolyte: Aqueous 3M KCl solution (high ionic conductivity).
- Illumination: Modulated light (1.8 Hz) provided by a laser-driven light source and custom quartz prism monochromator (0.45 to 6.2 eV range).
- Photocurrent Detection: Lock-in amplifier (EG&G 5210) for high sensitivity (picoampere range).
-
Surface State Characterization:
- X-ray Absorption Spectroscopy (XAS): Performed at the BESSY II synchrotron (LiXEdrom beamline) at the C K-edge to determine the position of C-H, C-OH, C=O, and C=C surface states relative to the conduction band minimum (CBM).
- Photoelectron Yield Spectroscopy (PYS): Used to measure the ionization energy (Ei) and determine the surface electron affinity (EA) under atmospheric pressure with adsorbed water.
-
Redox and Band Bending Analysis:
- Photocurrent Cyclic Voltammetry (PC-CV): Used constant 3.22 eV LED illumination during CV scans to isolate redox reactions by plotting photocurrent vs. cell current (Ipc vs Icell), enabling precise determination of FBP hysteresis.
- Schottky Barrier Analysis: Used to fit the above-band-gap IPCE data as a function of applied potential to determine FBP and the ideality factor (n).
-
Solvated Electron Quantification:
- Fluorimetric Titration: Used 2 mM KNO3 as an electron scavenger (NO3- + 2e-(aq) â NO2-). The resulting NO2- concentration was quantified by reaction with 2,3-diaminonaphthalene (2,3-DAN) to form a highly fluorescent probe (1H-naphthotriazole).
Commercial Applications
Section titled âCommercial ApplicationsâThe ability to precisely control and understand charge transfer at the BDD/water interface is critical for several high-value engineering applications.
- Solar Fuel Generation:
- Direct photoelectrochemical (PEC) conversion of solar energy into chemical fuels (e.g., H2, CH3OH).
- Enhanced CO2 reduction (using solvated electrons) to valuable products like CO.
- Energy Storage Devices:
- Development of durable, chemically stable, aqueous-based photorechargeable energy storage devices and supercapacitors.
- Water Remediation and Decontamination:
- Utilizing the high oxidative potential of diamond and the generation of solvated electrons for efficient degradation of persistent organic pollutants.
- Advanced Sensor and Interfacing Technology:
- BDDâs stability and biocompatibility, combined with its photoelectrochemical response (including potential near-infrared response via nitrogen defects), make it suitable for neural interfacing and specialized biosensors.
- High-Performance Electrodes:
- BDD electrodes are ideal for harsh electrochemical environments due to their exceptional chemical stability, conductivity, and sustainability.
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 Câ<a:math xmlns:a=âhttp://www.w3.org/1998/Math/MathMLâ display=âinlineâ><a:mrow><a:mrow><a:mi mathvariant=ânormalâ>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=âhttp://www.w3.org/1998/Math/MathMLâ display=âinlineâ><d:msup><d:mrow><d:mrow><d:mi mathvariant=ânormalâ>H</d:mi></d:mrow></d:mrow><d:mo>+</d:mo></d:msup></d:math>/<g:math xmlns:g=âhttp://www.w3.org/1998/Math/MathMLâ display=âinlineâ><g:msub><g:mrow><g:mrow><g:mi mathvariant=ânormalâ>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=âhttp://www.w3.org/1998/Math/MathMLâ display=âinlineâ><j:mstyle displaystyle=âfalseâ scriptlevel=â0â><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
Section titled âTech SupportâOriginal Source
Section titled âOriginal SourceâReferences
Section titled âReferencesâ- 2016 - Part 1 Fundamental Aspects of Photocatalysis [Crossref]
- 2016 - Photoelectrochemical Solar Fuel Production: From Basic Principles to Advanced Devices