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Functional ordered NANomaterials via ELectrochemical routes in non-aqueous electrolytes

Final Report Summary - NANEL (Functional ordered NANomaterials via ELectrochemical routes in non-aqueous electrolytes)

The NANEL joint exchange project aimed in establishment of long-lasting research cooperation between Portuguese, Bulgarian, Belgian, Belarusian and Russian scientists in the field of electrochemical synthesis of advanced nanostructured materials. The collaborative consortium joined together a critical mass of expertise available in the involved groups. Mutually beneficial transfer of knowledge was implemented through an intensive exchange program between five partner organizations involved in the consortium.
The main technical objective of the project is the development of novel functional nanomaterials for sensors and solar cell applications on the basis of ordered nanoporous anodic oxides. The main scientific novelty of the project is the functionalization of the porous anodic oxides, such as alumina or titania, via electrochemical or electrophoretic ways using non-aqueous electrolytes. Ionic liquids and near room-temperature molten salts are be used as prospective candidates of such electrolytes.
The electrochemical synthesis of nanomaterials has several important advantages because of its relatively low cost and fine tuning of the process parameters. The suggested approach confers creation of new ordered functional nanomaterials via electrochemical routes which have not been possible in water-based electrolytes. Use of non-aqueous solutions presents significant advantages for specific materials which are not stable in presence of water or can not be electrodeposited because of the relatively narrow electrochemical window of water.
The work program of NANEL project has united the specific complementary expertise of the involved partners in a synergistic way. These experiences cover all the needed steps from formation of the nanoporous ordered templates and deep knowledge on the mechanisms of electrochemical processes to electrodeposition from non-aqueous electrolytes and testing of the developed materials towards applications in photovoltaics and novel sensors.
The main focus of the project was directed towards proof of scientific concept, namely a possibility of electrochemical deposition of various functional materials in porous templates using non-aqueous electrolytes. The mechanistic understanding of the formation of ordered porous templates as well as mechanisms of electrodeposition processes were investigated in detail. For the first time, the electrodeposition from an ionic liquid is performed in a porous alumina template, anodically grown on the aluminium substrate without complete removal of the barrier layer.
In one of the initial working packages of NANEL project the preparation of ordered porous anodic oxide templates on Ti and Al was performed and mechanistically studied. Emphasis was put on the film formation efficiency as a deciding factor for the formation of self-organized nanoporous/nanotubular structures, and thus attempts were made to calculate this parameter already from the very first stages of anodic oxide growth in film-dissolving electrolytes. A calculation procedure for the estimation of the kinetic and transport parameters of the individual steps that are detectable by means of the combination of methods used was developed and validated. A kinetic model is proposed taking into account the processes of growth, hydroxylation and dissolution of the oxide via inward oxygen transport mediated by anion vacancies, the dissolution of Al through the oxide mediated by cation vacancies, as well as the reaction of hydrogen evolution occurring at potentials lower than the equilibrium potential of this reaction. . Additionally two different approaches were used to prepare porous templates on the surface of silicon. The porous silicon was obtained by the etching of heavy ion tracks and by using electrochemical etching techniques with silver ions. The mechanistic details of the latest process were also investigated in detail demonstrating an important role of silver silicate formation on the self-limitation of the pore growth.
The next important stage, which also was the main objective of the project, is electrodeposition of functional materials using non-aqueous electrolytes into porous templates. For the first time, the electrodeposition from an ionic liquid is performed in a porous alumina template, anodically grown on the aluminium substrate without complete removal of the barrier layer. The two-step process was applied with first AC-pulse nucleation of zinc nanoparticles on the bottom of pores followed by DC growth of the rods. The electrolyte composed of 0.5M ZnCl2 in a choline chloride and ethylene glycol in molar ratio 1:2 system was used. The resulting zinc nanorods were about 3 µm in length and 70 nm in diameter. The achieved fill-factor of pores is in the range of 70-80% as can be seen in illustration. Further experiments have revealed that a zinc deposition on barrier layers is possible when frequency of the alternating component of the applied potential is within the range of 1 Hz - 1 kHz regardless of the barrier layer thickness unless it exceeds ~60 nm. It was suggested that application of an alternating component over the constant potential and temperature which disturb the blocking layer, are independent on the barrier layer thickness. The difficulty in depositing zinc on a thicker barrier layer is caused by its high impedance.
One of the important tasks was to model and simulate the growth of single nuclei and validate our simulation result by comparing with the experimental result obtained in literature as well as by the existing theory of nucleation and growth. This allowed us within the project to provide some information how modelling can support the electrochemical nucleation and growth of different systems. In order to do these, we chose silver deposition in an acetonitrilel solution with lithium perchlorate electrolyte. To obtain parameters for our simulation such as diffusion coefficient, rate constant and transfer coefficient for the silver reduction, we fitted experimental LSV data with analytical equations. After we obtained valuable parameters, the next step was to prepare a model for the growth of single Ag nuclei to provide some detailed information about the growth mechanism as function of overpotential and Ag concentration. In order to simplify the problem and fasten the computation we prepared a 2D axis-symmetric model with different domain sizes and imposed boundary conditions. The set of species considered in the model consists of the active ion Ag+ and electrolyte ions (Li+, ClO4- and NO3- . In order to describe the phenomena that take place during deposition, an in house developed model based on Multi-Ion Transport and Reaction Model (MITReM) was applied. This particular model takes into account transport of all species in the electrolyte together with their production/consumption in the electrode. Finite Element Method (FEM) is used to solve the balance equations for the concentration of all the active species, Ci, and the electrolyte potential U. Within this approach successful fittings were done and we are currently preparing a first paper showing the growth of Ag particles including charge and mass transfer.
The arrays of metallic nanowires are considered as promising precursors for 1D semiconductor nanostructures after appropriate treatment at temperatures close to the melting point. Therefore the melting behaviour of the metallic structures in oxide templates is a key parameter for the subsequent conversion process. We have focused in the project on understanding of the effect of mechanical stress generated during heating on the melting point of the metal nanowires deposited into the pores of anodic alumina. Extremely high local compressive stress appears due to the difference in the thermal coefficients of the oxide template and nanowires inside the pores. The effect of the composite structural parameter that may be related to the concentration of nanowires on the melting temperature has been investigated. A numerical model predicting the melting point has been developed for composites with indium, tin, and zinc nanowires. The simulation results obtained using the suggested model were compared with the experimental data.
The activities on creation of novel solar cells and sensors on the basis of the developed materials were also actively performed. The magnetic oxide nanoparticles were synthesized as well as mixed sulfide compounds for sensors and solar cells respectively. The main approach in this case is to utilize the electrophoretic deposition from non-aqueous electrolytes in order to form nanostructured layers on different substrates.
A new approach to create metal-supported Sr2FeMoO6 (SFMO)-based layers was suggested. The SFMO films were formed on metallic substrates by electrophoretic deposition (EPD) method. Ethyl alcohol with phosphate ester as a dispersant and isopropyl alcohol with I2-acetone mixture as a charge additive were considered as an effective medium for EPD of SFMO particles. The synthesis of SFMO powder as well as suspension preparation and deposition kinetics were systematically studied. The effect of applied voltage on the thickness and morphology of SFMO films was established. The thickness, morphology and porosity of the SFMO layers can be fine-tuned by varying solvent, charging additives, deposition time, and applied voltage.
The magnetic properties as well as photovoltaic response were studied for the new materials developed in the frame of NANEL project.

Most of the project objectives were achieved during the project execution from both the staff exchange and the scientific points of view. The consortium has executed over 90% of planned secondments between EU and third-country institutions. 25 early stage and experienced researchers have benefited from the mobility programs in terms of training, knowledge exchange and scientific collaboration. The exchange of staff was accompanied by seminars and workshops which allowed better dissemination of knowledge in host institutions including one joint workshop organized together with two other IRSES projects. The results of the project were published in 16 journal articles including papers in high-impact factor journals. This allowed establishing even deeper networking between the researchers from 5 different countries.

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