Although nanocrystalline diamond powders have been produced in industrial quantities, mainly by detonation synthesis, for many decades their use in applications other than traditional polishing and grinding have been limited, until recently. This paper presents the wide-ranging applications of nanodiamond particles to date and discusses future research directions in this field. Owing to the recent commercial availability of these powders and the present interest in nanotechnology, one can predict a huge increase in research with these materials in the very near future. However, to fully exploit these materials, fundamental as well as applied research is required to understand the transition between bulk and surface properties as the size of particles decreases.

Reactions of Fe₂(CO)₆(μ-pdt) (pdt = SCH₂CH₂CH₂S) with aminodiphosphines Ph₂PN(R)PPh₂ (R = allyl, ⁱPr, ⁱBu, p-tolyl, H) have been carried out under different conditions. At room temperature in MeCN with added Me₃NO·2H₂O, dibasal chelate complexes Fe₂(CO)₄{κ²-Ph₂PN(R)PPh₂}(μ-pdt) are formed, while in refluxing toluene bridge isomers Fe₂(CO)₄{μ-Ph₂PN(R)PPh₂}(μ-pdt) are the major products. Separate studies have shown that chelate complexes convert to the bridge isomers at higher temperatures. Two pairs of bridge and chelate isomers (R = allyl, ⁱPr) have been crystallographically characterised together with Fe₂(CO)₄{μ-Ph₂PN(H)PPh₂}(μ-pdt). Chelate complexes adopt the dibasal diphosphine arrangement in the solid state and exhibit very small P–Fe–P bite-angles, while the bridge complexes adopt the expected cisoid dibasal geometry. Density functional calculations have been carried out on the chelate and bridge isomers of the model compound Fe₂(CO)₄{Ph₂PN(Me)PPh₂}(μ-pdt) and reveal that the bridge isomer is thermodynamically favourable relative to the chelate isomers that are isoenergetic. The HOMO in each of the three isomers exhibits significant metal–metal bonding character, supporting a site-specific protonation of the iron–iron bond upon treatment with acid. Addition of HBF₄·Et₂O to the Fe₂(CO)₄{κ²-Ph₂PN(allyl)PPh₂}(μ-pdt) results in the clean formation of the corresponding dibasal hydride complex [Fe₂(CO)₄{κ²-Ph₂PN(allyl)PPh₂}(μ-H)(μ-pdt)][BF₄], with spectroscopic measurements revealing the intermediate formation of a basal–apical isomer. A crystallographic study reveals that there are only very small metric changes upon protonation. In contrast, the bridge isomers react more slowly to form unstable species that cannot be isolated. Electrochemical and electrocatalysis studies have been carried out on the isomers of Fe₂(CO)₄{Ph₂PN(allyl)PPh₂}(μ-pdt). Electron accession is predicted to occur at an orbital that is anti-bonding with respect to the two metal centres based on the DFT calculations. The LUMO in the isomeric model compounds is similar in nature and is best described as an antibonding Fe–Fe interaction that contains differing amounts of aryl π* contributions from the ancillary PNP ligand. The proton reduction catalysis observed under electrochemical conditions at ca. −1.55 V is discussed as a function of the initial isomer and a mechanism that involves an initial protonation step involving the iron–iron bond. The measured CV currents were higher at this potential for the chelating complex, indicating faster turnover. Digital simulations showed that the faster rate of catalysis of the chelating complex can be traced to its greater propensity for protonation. This supports the theory that asymmetric distribution of electron density along the iron–iron bond leads to faster catalysis for models of the Fe–Fe hydrogenase active site.

This paper demonstrates the promoting effects of 5 nm undoped detonation diamond nanoparticles on redox reactions in solution. An enhancement in faradaic current for the redox couples Ru(NH₃)₆ ^3+/2+ and Fe(CN)₆ ^4–/3– was observed for a gold electrode modified with a drop-coated layer of nanodiamond (ND), in comparison to the bare gold electrode. The ND layer was also found to promote oxygen reduction. Surface modification of the ND powders by heating in air or in a hydrogen flow resulted in oxygenated and hydrogenated forms of the ND, respectively. Oxygenated ND was found to exhibit the greatest electrochemical activity and hydrogenated ND the least. Differential pulse voltammetry of electrode-immobilised ND layers in the absence of solution redox species revealed oxidation and reduction peaks that could be attributed to direct electron transfer (ET) reactions of the ND particles themselves. It is hypothesised that ND consists of an insulating sp³diamond core with a surface that has significant delocalised π character due to unsatisfied surface atoms and CO bond formation. At the nanoscale surface properties of the particles dominate over those of the bulk, allowing ET to occur between these essentially insulating particles and a redox species in solution or an underlying electrode. We speculate that reversible reduction of the ND may occur via electron injection into available surface states at well-defined reduction potentials and allow the ND particles to act as a source and sink of electrons for the promotion of solution redox reactions.

This article discusses some of our recent work on the origins of redox activity of undoped nanodiamond (ND) powders, as well as reviewing some properties and applications of this material. The electrochemical activity is attributed to unsaturated bonding at the ND particle surface; hence the most recent understanding of the surface chemistry of these materials is discussed. The implications of the observed redox activity, especially for use in biological applications, are highlighted, along with future avenues of research.

Copper oxide modified electrodes were investigated as a function of applied electrode potential using in situ infrared spectroscopy and ex situ Raman and X-ray photoelectron spectroscopy. In deoxygenated KHCO₃ electrolyte bicarbonate and carbonate species were found to adsorb to the electrode during reduction and the CuO was reduced to Cu(i) or Cu(0) species. Carbonate was incorporated into the structure and the CuO starting material was not regenerated on cycling to positive potentials. In contrast, in CO₂ saturated KHCO₃ solution, surface adsorption of bicarbonate and carbonate was not observed and adsorption of a carbonato-species was observed with in situ infrared spectroscopy. This species is believed to be activated, bent CO₂. On cycling to negative potentials, larger reduction currents were observed in the presence of CO₂; however, less of the charge could be attributed to the reduction of CuO. In the presence of CO₂ CuO underwent reduction to Cu₂O and potentially Cu, with no incorporation of carbonate. Under these conditions the CuO starting material could be regenerated by cycling to positive potentials.