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Materials and Interfaces for Energy Conversion and Storages

Periodic Report Summary - MATCON (Materials and interfaces for energy conversion and storages)

There is today an increasing demand of well-educated young scientists knowledgeable in materials science for energy conversion and storage. This is particularly the case for solar based renewable energy or other sources with intermittent character. In this broader context, this MATCON network will concentrate on
i) Photo-electrochemical generation of hydrogen by water splitting,
ii) Bio-inspired and bio mimetic energy conversion, and
iii) Thermoelectric and thermo ionic heat conversion. For all of these topics, alternative or new materials and materials combinations are necessary to improve the efficiency of energy conversion or to overcome existing problems with stability. Below is a condensed presentation of results obtained during the first two years of MATCON (for each of the work packages), followed by examples of Highlights. The web page for MATCON is

Work package (WP) A: Growth and development of materials for energy applications

Different dimensions of B- and non-doped diamond were grown; heteroepitaxial single crystalline (SC), nanocrystalline and ultra-nanocrystalline diamond (NCD vs. UNCD) diamond. The SC films allow very systematic studies on diamond as electrode material, and it is planned to apply them for the preparation of p/n (hetero)-junction devices. The NCD films were applied for the fabrication of electrochemical sensors and electrode arrays. The UNCD samples were prepared for the preparation of nanocomposites for thermoelectric studies within WP D. Theoretical DFT calculations were used to aid in the explanation of experimental findings, and for the prediction of new setups.

Post-treatment was tailored to achieve well defined surface properties. As an example, a method was developed for optimizing the hydrophilicity and surface dipole of boron-doped diamond in order to mimic the natural binding partner of Cytochrome C.

The basic charge transfer mechanism across the diamond/silicon interfaces was studied for the p-Si/p-diamond heterostructures. In addition, the charge transfer between biological photo systems, immobilized on the semiconductor surface, and the semiconductor substrate upon illumination, was especially studied.

Some part of the training was also focused on the deposition of NCD layers on different metal layers that might play a role as electrochemical electrodes.

WP B. Photo-electrochemical generation of hydrogen by water splitting

Functionalization of a wide range of electrode materials (NCD, SC diamond, diamond nanowires, GaN (planar) and GaN nanowires) with a range of nanoparticles (NPs) comprising Pt, PtRu, Rh, Ir, Co, Ni has been achieved and the resulting structures characterised.

The electrochemical behaviour of Co NPs with regard to the oxygen reduction reaction was examined and compared to the behaviour of Pt. Electrochemical characterisation of the hydrogen evolution reaction has been performed to understand how the behaviour of Pt NPs on GaN substrates compares to the behaviour of bulk Pt electrodes. Fundamental studies to understand how water-based electrolytes interact with diamond surfaces have been completed.

Protocols for the modification of the electrode materials with Pt, PtRu, Rh, Ir, Co, Ni have been developed. Of particular focus has been the investigation and optimisation of the stability of these particles under electrolysis conditions.

WP C: Bio inspired and bio mimetic energy conversion

Nano-textured surfaces have been studied which show geometrical properties of typical protein dimensions. For OH-terminated diamond nano-textured surfaces, the electron transfer process and the peroxidase activity of native and denatured Cytochrome c were especially investigated.

Various solar cell structure modifications were designed, prepared and studied. There was a focus on designs suitable for thin Si substrates, and modification of the diagnostic method for the bulk carrier lifetime measurement in solar cell structures. For the application of hybrid cells for bio inspired conversion, p-type Si substrates with various surface morphologies were deposited with undoped or boron doped NCD.

Theoretical modelling has pointed at the possibility to transfer upper surface diamond to graphene at elevated temperatures and in the presence of H radicals.

WP D: Nanocomposite materials for thermoelectric and thermoionic heat conversion

Optimization of undoped and B-doped NCD growth has been carried out with the focus on specific parameters (e.g. growth chemistry, pulse frequency, temperature) with their effect on NCD layer quality, such as sp3/sp2, growth rate and crystal quality).

The thermoelectric properties of thin films of silicon and germanium nanocrystals have been explored, which will be extended in the future to silicon/diamond composites. The availability of undoped and doped Si and Ge nanocrystals, as well as the experience on the preparation of thin films based on these nanocrystals, makes this study a preliminary step necessary towards our final goal of studying diamond/Si nanocomposites.

The effect of the NCD surface properties by modification of the nanoparticles’s surface chemistry was investigated. This is an important step in order to enhance the electrical conductivity of the films, which is strongly reduced by the presence of the insulating oxide shell.

A systematic study of the morphology and electronic properties of thin heavily boron-doped NCD films was carried out in order to construct a transport model for high (metallic) doping.

An investigation of the thermal properties of 1 microm thin undoped NCD layers, deposited on silicon substrates, has been started. The diamond layer was found to enhance the lateral thermal diffusion, offering promising perspectives.

Scientific highlights.
a) Control of diamond nanoparticle seeding by varying the surface charges of particles;
b) Development of low temperature growth for the successful diamond coating of Si solar cell test structures;
c) Control of the sp²/sp³ ratio of NCD films by using continuous bias during growth or by changing the used methane-to-hydrogen ratios;
d) Development of a model describing transport in heavily B-doped NCD films above the metal-insulator transition;
e) Detailed protocols and process optimization for the deposition of metal nanoparticles on electrode materials;
f) Detailed modeling of the behavior of aqueous electrolytes at diamond interfaces;
g) New functionalization based on the short linker molecule aminocaproic acid (ACA);
h) Construct of diamond based biomimetic interface for fast electron transfer of redox proteins;
i) Preparation of epitaxial diamond nanocrystals of different sizes for thermionic electron emission studies.