Ruthenium-functionalized nanodiamonds generate powerful solvated electrons under visible light, bypassing the deep UV requirement that has limited diamond photocatalysis to laboratory settings.
(Nanowerk Spotlight) Of all the materials physicists and chemists have probed for unusual electronic behavior, diamond may be the most surprising. It is one of nature’s best electrical insulators. Yet when its surface is coated with hydrogen atoms and struck by ultraviolet light, diamond does something unexpected: it ejects electrons directly into surrounding water. Those ejected electrons become “solvated,” meaning they are cushioned and stabilized by a cage of water molecules.
In this state, they are among the most powerful reducing agents chemistry can produce, capable of forcing electrons onto molecules that ordinarily refuse to accept them.
Carbon dioxide is one such molecule. Its bonds are strong, its geometry rigid, and the energy needed to push the first electron onto it is extreme: −1.90 V versus the normal hydrogen electrode. Diamond’s solvated electrons, at −2.9 V, exceed that threshold considerably. Once free in water, they travel roughly 1 µm and survive for 250–300 nanoseconds before recombining, giving them both reach and time to find a CO₂ molecule and react.
This property has drawn sustained interest from researchers exploring light-driven routes to CO₂ conversion, an area where most semiconductor materials struggle with either insufficient reducing power or rapid loss of excited charges. But a severe practical limitation has kept diamond photocatalysis confined to laboratories.
The material’s bandgap, the minimum energy needed to knock an electron into a reactive state, sits at approximately 5.5 eV. Only deep ultraviolet light below 225 nm carries enough energy, and generating such short-wavelength radiation is costly. Worse, deep UV attacks the hydrogen-terminated surface within about 2 hours, destroying the very condition that enables electron emission.
Researchers have tried lowering the energy threshold by doping diamond with nitrogen or phosphorus, but these atoms incorporate poorly into the lattice. Pushing defect density high enough to absorb lower-energy light distorts the crystal so severely that the beneficial surface properties collapse.
These intermediate states allow visible and near-ultraviolet light to trigger solvated-electron emission, the same process that previously demanded impractical deep UV. As a proof of concept, the team used this engineered material to reduce CO₂ to formate under simulated sunlight.
Synthesis and structure of surface functionalized nanodiamond carrying ruthenium complexes as surface bound sensitizers. (a) attrition milling of detonation nanodiamond in water leads to fully dispersed diamond nanoparticles; (b) Diazonium coupling using amyl nitrite in water yields the linker functionalized nanodiamond materials DND-L1 and DND-L2; (c) CuAAC chemistry using Ru1-C≡CH, Ph3PCuI, sodium ascorbate in DMF/H2O; (d) TEM image of DND-L2-Ru1, scale bar 5 nm. (Image: Reproduced from DOI:10.1002/adfm.202523545, CC BY)
The strategy starts with detonation nanodiamond, tiny diamond particles produced by controlled explosive synthesis. After breaking apart the tightly clumped particles through mechanical milling in water, the researchers bonded a ruthenium tris-bipyridine derivative to the diamond surface.
This type of ruthenium complex is widely used as a photosensitizer, a molecule that absorbs light and transfers the resulting energy or charge to another material. The bonding relied on copper-catalyzed click chemistry, a well-established method for joining complementary molecular building blocks.
Two different molecular bridges were tested: a non-conjugated linker (L1) and a conjugated one (L2). Conjugation means the linker contains alternating single and double bonds that allow electrons to flow more freely between the dye and the diamond.
The attachment preserved both the light-absorbing behavior of the ruthenium complex and the stability of the diamond particles in water. Spectroscopy confirmed the characteristic absorption of the dye shifted only slightly, by about 17 nm, after bonding. The functionalized particles stayed well dispersed, with typical sizes between 60 nm and 80 nm.
To map the energy levels of the hybrid material, the team combined spectroscopic techniques that separately reveal occupied and unoccupied electronic states. In vacuum, the highest occupied energy level of the ruthenium dye sat 1.9 eV below a gold reference level when attached through the conjugated linker, shifted by 0.2 eV compared to the free dye. The overall alignment produced a slight negative electron affinity of −0.3 eV, meaning the conduction band sits just above the vacuum level, a condition that favors electron emission.
In water, the electronic structure changed markedly. Measurements performed directly in aqueous conditions revealed new unoccupied states within the diamond’s valence band, which the researchers interpret as electrons migrating from the diamond to the dye. This charge transfer fills a long-lived excited state of the ruthenium complex called the triplet metal-to-ligand charge transfer state (³MLCT).
The ³MLCT state functions as an energy staging point: electrons settle there through charge transfer from the diamond, and can then be promoted further when the system absorbs an additional photon of visible light. This is not a simultaneous two-photon process but a sequential one, with charge transfer first populating the state and a separate photon later exciting it.
Ultrafast spectroscopy confirmed this mechanism. Under deep UV excitation at 225 nm, the functionalized diamond produced a distinct spectral signal at 585 nm absent in both bare diamond and the free ruthenium complex. This signal decayed within roughly 2 picoseconds, far faster than the hundreds-of-nanosecond lifetimes typical for isolated ruthenium dyes, pointing to rapid charge transfer between dye and diamond.
Photocurrent measurements delivered the central finding. Electrodes coated with the functionalized nanoparticles showed a photocurrent onset at approximately 385 nm, corresponding to 3.2 eV. This sits well within the near-UV/visible range, far below diamond’s native 5.5 eV threshold. The photocurrent scaled with the square root of light intensity, consistent with a single-photon excitation process. The 3.2 eV energy matches what is needed to promote electrons from the populated ³MLCT state into diamond surface states positioned above the solvated-electron redox potential.
For catalysis tests, the team deposited the functionalized nanoparticles onto electrodes made from boron-doped diamond, a form of diamond rendered electrically conductive by the incorporation of boron atoms. The boron-doped substrate provides the electrical connection to the external circuit, while the functionalized nanodiamond coating on its surface performs the photocatalytic work.
In aqueous potassium bicarbonate under UV light, the ruthenium-modified electrodes produced slightly more formate than bare nanodiamond controls. Performance improved substantially in ionic liquid electrolytes, which dissolve much more CO₂ than water. Under simulated solar illumination at −1.7 V in the ionic liquid N₁₁₁₄BTA, functionalized electrodes with both linker types produced formate at rates of 0.74–0.76 µmol cm⁻² h⁻¹, roughly double the rate from unfunctionalized nanodiamond and comparable to the best published values for diamond-based CO₂ reduction.
Neither spectroscopy nor elemental analysis detected degradation of the ruthenium complex after the experiments.
The formate yields remain modest in absolute terms, and the team acknowledges that the system is unoptimized. Future directions include exploring alternative sensitizers that avoid precious metals, and reactor designs such as thin falling-film systems or pressurized CO₂ environments that could improve performance in water-based electrolytes.
What the study establishes is a working principle: that surface chemistry alone can reshape diamond’s electronic structure enough to harvest visible light for the emission of exceptionally reactive electrons. That demonstration opens a route toward using diamond, a material already produced at industrial scale, as a platform for solar-driven reductive photocatalysis.
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ORCID information
Tristan Petit (Helmholtz-Zentrum Berlin für Materialien und Energie)
, 0000-0002-6504-072X corresponding author
Anke Krueger (Julius-Maximilians University Würzburg)
, 0000-0003-3082-4935 corresponding author
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