Objectif
- Diamond has unique electrical and chemical properties that make it a particularly favourable material for electrodes withstanding harsh environments. Diamond, and diamond-based materials (such as t-aC), when made electrically conductive, are thus ideal chemically stable electrodes. The objectives of this collaborative project is the development of new materials such as diamond or "diamond-like-carbon" (DLC) films capable of forming electrodes for electrochemical reduction of compounds at very negative potentials or very cathodic reactions such as ozone fixation, reduction of nitrite, C02 and the electrodeposition of very negative metals.
- Throughout the project the boron doping procedure has been developed and continuously improved, hence good highly conductive diamond coated samples could be produced for electrochemical experiments It has now reach the stage where good control on B doping has been achieved on diamond thin films. The diamond thin films were deposited on various substrates such like silicon, molybdenum or tungsten.
- In order to obtain conductive DLC thin films, the BAM group had to realise the boron doping of hydrogenfree amorphous carbon (a-C) films. In analogy to successful doping experiments with crystalline diamond films produced by CVD, doping of the amorphous counterpart during the deposition process had to be demonstrated. The BAM group established a new UHV-PLD chamber. Mixing of carbon and boron was done by scanrling the two splitted beams of the XeCl excimer laser along the edges of two coplanar target sections (carbon and boron carbide). It can be concluded from the density and elasticity measurements that 308 nm excimer radiation with fluences of the order of 20 J cm-2 and intensities of the order of 1 GW cm-2 can only produce amorphous carbon films with sp3 (diamond) contents of less than 50%. The boron was homogeneously distributed throughout the film with an atomic concentration of 20%.
- In this work we correlated the ability to reduce nitrates and deposit metal on different diamond doped thin films with the physical properties of these films that depend on the growth process. The diamond thin films were grown by (HF)CVD (Hot Filament Chemical Vapour Deposition) and uWCVD (Microwave Plasma CVD) on silicon and tungsten substrates and they were boron doped in situ. Two kinds of films were made by laser ablation (one undoped and the other containing Boron).
- To characterise the structure of the films we carried out Near Edge X-ray Absorption Spectroscopy (NEXAFS). The nature of the surface was analysed by XPS and its morphology by SEM. (HF)CVD and uWCVD methods gave well-crystallised films, while those made by laser ablation were amorphous.
- The electrochemical behaviour of boron doped diamond films was investigated, in a number of neutral and alkaline solutions with and without nitrate ions. Two kinds of diamond electrodes were studied : self supported films (100 um) and diamond films supported on a silicon substrate. It was found that water oxidation and reduction appear at much larger polarisations for diamond electrodes, as compared to platinum and platinized platinum electrodes. In particular, the higher (cathodic) overpotential for hydrogen reduction permits efficient nitrate to ammonia reduction. The underlying Si substrate is shown to take part in the electrochemistry of the diamond electrodes. In the case of the Si supported electrode, the reaction with the Si substrate was imminent. For the free standing diamond electrode, various impurities in the grain boundaries and at the back of the electrode, including back metallic contact, intervened with the electrochemistry of the diamond electrode, but to a much lesser extent than with the supported sample. Meticulous cleaning and a careful working practice permitted this interference to be excluded altogether in the self supported diamond film. Because this kind of film is very long and expensive to produce CVD diamond films were grown on molybdenum and tungsten substrates. Their reflectance properties were studied using FTIR measurements, whereas their electrochemical properties are now under investigation.
- The reduction of nitrates in alkaline (KOH lM) and neutral solutions (KCl 0.1 M) were performed at - 2 V/SCE. At this potential there was a clear competition between the reduction of water and nitrates. Ammonia was detected as one product of the reaction. Under potential deposition (UPD) of copper in 0. lM sulphuric acid was observed at 0.1 V before the reversible Nernst potential. But the extent of UPD was surprisingly weak as it was calculated that only a few percentages of the electrode surface could be covered by one monolayer. However the UPD phenomenon is more important on well-crystallised films compared to amorphous ones and it is enhanced by an electrochemical activation of the electrodes at negative potentials.
- Currently, the reduction of bicarbonate solutions, which are a model system for C02 reduction, using diamond electrodes is investigated. Although, only one (formaldehyde) organic species was being analysed, the results of the work were quite astonishing. Exceedingly high Faradaic efficiencies of 2-3 were found for this process, i.e. for each electron delivered by the diamond electrode, two electrons were produced spontaneously by the redox reaction and were injected to the diamond electrodes.
Follow up
- We will try to carry-out a few studies using surface modified diamond electrodes. Thus, shallow (superficial) ion implantation of diamond samples by transition metal atoms, such as Ni, Fe, Cu etc., will be attempted, and the influence of this "surface" modification, on the rate of reduction of effluent gases, will be investigated. Even more superficial modification, like binding metal atoms to the surface, chemically, will be attempted;
- Doping of the CVD diamond with Ni will also be tried;
- Better boron doped DLC films will be grown by optimising the parameters of deposition and using a completely new contacting procedure. Also titanium doping will be tested. Because of the extreme thinness of the films spectroscopic ellipsometry measurements will be carried out to study the electronic properties of the thin films;
- A comparative study of the electrochemical properties of various p-type boron doped CVD diamond films deposited on silicon and non silicon substrates such like molybdenum and tungsten and insulating substrates such as quartz or on undoped CVD diamond films will be undertaken. The purpose is to evaluate the effect of the deposition pararneters on the electrochemical properties of the various eleckodes. This study will include the DLC thin films;
- The investigation of the reduction of bicarbonate solutions (which are a model system for C02 reduction) using diamond electrodes will continue during the next period of research, in order to produce some basic understanding of the reaction mechanism of formaldehyde species. Formation of other products, such as formate ion, or even methanol, will also be studied. Furthermore, this work is being extended to other products, using mixtures of precursors (nitrate and bicarbonate) with the aim to study the formation of urea.
- Development of small size experimental hot filament chemical vapour deposition (HFCVD) and microwave chemical vapour deposition (uwCVD) reactors;
- Growth of polycrystalline diamond films;
- Development of a turbo-pumped vacuum pulse laser deposition (PLD) chamber with a XeCl eximer laser, equipped with on-line control for optical emission diagnostics of laser plasmas;
- Growth of carbon-like-diamond thin films;
- Assess quality films (morphology and phase purity) using optical and electronic microscopies, Raman spectroscopy, electrical measurements of film resistance as function of temperature;
- Growth of high conductivity CVD diamond films by in situ boron doping and ion-implantation related damage to the diamond;
- Development of dimensionally stable electrodes for application in very corrosive media;
- Investigation of electrochemical properties of electrically conductive diamond thin films and DCL thin films;
- Electrosynthesis of ammonia (hydrazine, hydroxyl amine) from nitrates and nitrogen with CVD-diamond and PLD-DCL deposited on silicon substrates. Check these electrodes as electrocatalysts for the reduction of effluent gasses (N02, NO, S02);
- Investigations by Fourier transform infrared (FTIR) spectroscopy about dopants and phonons in the diamond films. Analysis of the chemical nature of adsorbed species as well a their bonding mode with the diamond surface after nitrate and nitrite reduction. Modification of the diamond surface through binding of catalytic metal atoms, such as Ni and Cu and study of their effect on the electroreduction of effluent gas.
Thème(s)
Data not availableAppel à propositions
Data not availableRégime de financement
CSC - Cost-sharing contractsCoordinateur
92195 Meudon
France