Skip to content
6CCVD Research

Visible-Light Activation of Photocatalytic for Reduction of Nitrogen to Ammonia by Introducing Impurity Defect Levels into Nanocrystalline Diamond

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2020-10-14
JournalMaterials
AuthorsRui Su, Zhangcheng Liu, Haris Naeem Abbasi, Jinjia Wei, Hongxing Wang
InstitutionsXi’an Jiaotong University
Citations5
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”
  • Core Achievement: Demonstrated visible-light activation of photocatalytic Nitrogen Reduction Reaction (NRR) to synthesize ammonia (NH3) using nanocrystalline diamond (NDD) doped with nitrogen impurities.
  • Mechanism: Nitrogen doping creates Nitrogen-Vacancy (NV) centers (NV0 and NV- states) which introduce intermediate energy levels within the diamond’s wide bandgap (5.45 eV).
  • Activation: These defect levels enable internal photoemission, allowing sub-bandgap photons (wavelengths > 225 nm, including visible light) to excite electrons into the conduction band, overcoming the high energy barrier of intrinsic diamond.
  • Performance Gain: Nitrogen-doped diamond (NDD) exhibited significantly enhanced catalytic activity, achieving an average ammonia yield rate of 6.27 ± 1.48 nmol/cm2·h, substantially higher than undoped polycrystalline diamond (PD) and single crystal diamond (SCD).
  • Electron Source: The material utilizes the Negative Electron Affinity (NEA) property of hydrogen-terminated diamond to efficiently generate solvated electrons (e-aq) in liquid, which are essential for initiating the high-energy N2 reduction step (N2 + e- + H+ → N2H).
  • Implication: This approach provides a low-cost, ambient-condition alternative to the energy-intensive Haber-Bosch process, extending diamond’s utility in solar-driven chemical synthesis.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Diamond Bandgap Energy5.45eVIntrinsic excitation threshold
Intrinsic Excitation Wavelength< 225nmDeep UV light required for intrinsic excitation
NV- Zero Phonon Line (ZPL)1.945 (637)eV (nm)Optical transition energy of the negatively charged state
NV0 Zero Phonon Line (ZPL)2.156 (575)eV (nm)Optical transition energy of the neutral state
NDD Average Grain Diameter> 10nmNanocrystalline structure confirmed by SEM
NDD Layer Thickness200nmThickness of the top nitrogen-doped layer
NDD Ammonia Yield Rate (Avg)6.27 ± 1.48nmol/cm2·hBest performance under full spectrum illumination
PD Ammonia Yield Rate (Avg)3.16 ± 0.19nmol/cm2·hUndoped polycrystalline diamond performance
SCD Ammonia Yield Rate (Avg)2.53 ± 0.16nmol/cm2·hSingle crystal diamond performance
Solvated Electron Reduction Potential-3.2eV vs NHEPotential required for the critical N2H formation step
Raman D-Band Peak1332cm-1Characteristic peak for sp3 diamond crystals
Raman G-Band Peak1520cm-1Characteristic peak for graphite/amorphous carbon
NDD Surface Resistance Change30 kΩ to 100 MΩΩChange observed after photocatalysis due to surface oxidation (C-O bonds)

Key Methodologies

Section titled “Key Methodologies”

The nitrogen-doped nanocrystalline diamond (NDD) films were synthesized using Microwave Plasma Chemical Vapor Deposition (MPCVD) on silicon substrates.

  1. Substrate Preparation:

    • 2-inch Si substrates were cleaned via sonication in acetone, alcohol, and water (15 min each).
    • Nucleation enhancement was achieved by seeding the substrates via sonication in a mixture of 0.3 g nanodiamond particles (5-8 nm diameter) and 20 mL ethanol (15 min).

  2. Polycrystalline Diamond (PD) Base Layer Growth:

    • A 3 ”m thick PD layer was grown using MPCVD (modified AsTex system).
    • Gas Flow: 500 sccm total flow.
    • Gas Ratio: CH4/H2 ratio of 4%.
    • Pressure: 100 Torr.
    • Temperature: 1000 °C.
    • Microwave Power: 2000 W.

  3. Nitrogen-Doped Diamond (NDD) Top Layer Growth:

    • A 200 nm thick NDD layer was grown on the PD base.
    • Doping Gas: 1% N2/H2 ratio was introduced.
    • Other Parameters: Remained consistent with the PD base layer growth (100 Torr, 1000 °C, 2000 W).

  4. Single Crystal Diamond (SCD) Contrast Growth:

    • Ib-type HPHT diamond was used as the substrate.
    • Homoepitaxial Layer: 500 nm undoped layer grown first.
    • Growth Parameters: CH4/H2 ratio of 5%, 85 Torr pressure, 1000 °C, and 1000 W microwave power.
    • NDD Layer: A subsequent 200 nm nitrogen-doped layer was grown using the NDD parameters (1% N2/H2).

  5. Photocatalytic Testing (N2 to NH3):

    • Light Source: 450 W high-pressure Hg/Xe lamp (200-800 nm range).
    • Reaction Medium: 18.2 MΩ water and 0.01 M high-purity Na2SO4.
    • Procedure: N2 gas was slowly bubbled into the sealed quartz reaction vessel. Gas exported from the vessel was run through an NH3 absorption bottle.
    • Measurement: Ammonia yield was quantified using the indophenol blue method, measuring absorbance at 635 nm.

Commercial Applications

Section titled “Commercial Applications”

The research on NV-center activated diamond photocatalysis is highly relevant to several emerging industrial sectors, particularly those focused on sustainable chemistry and high-energy processes.

  • Sustainable Chemical Synthesis:

    • Green Ammonia Production: Enables the synthesis of NH3 (a key fertilizer and energy carrier) under ambient temperature and pressure using solar or visible light, drastically reducing the energy footprint compared to the Haber-Bosch process.
    • CO2 Reduction: The same mechanism (solvated electron generation via NV centers) can be applied to the photocatalytic reduction of CO2, which also binds weakly to surfaces and requires high-energy electrons.

  • Advanced Photocatalysis and Water Treatment:

    • Visible-Light Activated Catalysts: Provides a robust, chemically inert diamond material capable of operating efficiently under the visible spectrum, maximizing the use of solar energy for chemical reactions.
    • High-Energy Reduction: Utilizes the Negative Electron Affinity (NEA) of diamond to generate highly energetic solvated electrons (E = -3.2 eV vs NHE), suitable for initiating difficult reduction reactions in aqueous solutions.

  • Diamond Materials Manufacturing:

    • Doped Nanocrystalline Diamond (NCD) Films: Development of precise CVD recipes for introducing optically active defect centers (NV centers) into NCD films, which is critical for both catalysis and quantum applications.
    • Electrochemical Applications: The high stability and tunable surface chemistry of doped diamond electrodes are valuable for large electrochemical potential windows and control of surface termination.
View Original Abstract

Nitrogen impurity has been introduced in diamond film to produce a nitrogen vacancy center (NV center) toward the solvated electron-initiated reduction of N2 to NH3 in liquids, giving rise to extend the wavelength region beyond the diamond’s band. Scanning electron microscopy and X-ray diffraction demonstrate the formation of the nanocrystalline nitrogen-doped diamond with an average diameter of ten nanometers. Raman spectroscopy and PhotoLuminescence (PL) spectrum show characteristics of the NV0 and NV− charge states. Measurements of photocatalytic activity using supraband (λ < 225 nm) gap and sub-band gap (λ > 225 nm) excitation show the nitrogen-doped diamond significantly enhanced the ability to reduce N2 to NH3 compared to the polycrystalline diamond and single crystal diamond (SCD). Our results suggest an important process of internal photoemission, in which electrons are excited from negative charge states into conduction band edges, presenting remarkable photoinitiated electrons under ultraviolet and visible light. Other factors, including transitions between defect levels and processes of reaction, are also discussed. This approach can be especially advantageous to such as N2 and CO2 that bind only weakly to most surfaces and high energy conditions.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2017 - Unoccupied Surface State Induced by Ozone and Ammonia On H-Terminated Diamond Electrodes for Photocatalytic Ammonia Synthesis [Crossref]
  2. 2003 - Electrochemical and Related Processes at Surface Conductive Diamond-Solution Interfaces [Crossref]
  3. 2008 - Electrochemical Oxidation of Highly Oriented Pyrolytic Graphite During Potential Cycling in Sulfuric Acid Solution [Crossref]
  4. 2019 - Electrochemical Fabrication of Porous Au Film On Ni Foam for Nitrogen Reduction to Ammonia [Crossref]
  5. 2019 - Electrochemical Nitrogen Reduction Reaction on Ruthenium [Crossref]
  6. 2018 - Negative Electron Affinity from Aluminium on the Diamond (100) Surface: A Theoretical Study [Crossref]
  7. 2005 - Direct Observation of Negative Electron Affinity in Hydrogen-Terminated Diamond Surfaces [Crossref]
  8. 1998 - Electron Affinity and Schottky Barrier Height of Metal-Diamond (100), (111), (110) Interfaces [Crossref]
  9. 2019 - Efficient Electrocatalytic N2 Fixation with Mxene Under Ambient Conditions [Crossref]
  10. 2008 - Electrical and Photoelectrical Characterization of Undoped and S-Doped Nanocrystalline Diamond Films [Crossref]

Reads Similar to This

Measuring magnetic field texture in correlated electron systems under extreme conditions interview Revealing the Adsorption Behavior of Nitrogen Reduction Reaction on Strained Ti2CO2 by a Spin‐Polarized d‐band Center Model (Small 9/2024)