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Interface design of metal nanocluster-carbon nanotube hybrids via control of structural and chemical defects in a plasma discharge

Final Report Summary - NANO2HYBRIDS (lnterface design of metal nanocluster-carbon nanotube hybrids via control of structural and chemical defects in a plasma discharge)

The NANO2HYBRIDS project related to the field of 'interfacial phenomena in materials', and more precisely, on the functionalisation of Carbon nanotubes (CNTs) in order to activate specific sites of the CNTs to allow gas detection. The key components that determine device functionality and efficiency, i.e. the inherently existing interfaces between dissimilar materials in devices, were addressed in this project.

At LISE, carbon nanotubes were functionalised, activated and metal-decorated using low pressure 'cold' plasmas at 13.56 MHz. Metals were evaporated in UHV onto the carbon nanotube surface (a new UHV chamber was constructed and a new triple evaporation gun was acquired). Fundamental study of the interaction between metals and carbon nanotubes using TEM, XPS and STXM has been performed. A new metal nanoparticles deposition method using organometallics precursors has been developed and patented. The mastering of the best nanoparticle size/size distribution for gas sensing has been optimised with feedback from the theoretical modelling (by IMN and PCPM) and from the sensorstesting (URV). Rh-Pd bimetallic nanoparticles (and thus NANO3HYBRIDS) have been successfully prepared. Study of defect density in individual CNTs using Scanning transmission X-ray microscopy (STXM, only available at synchrotron sources) is unique in the literature.

At CHANI, deposition of Pt, Rh, Ni, Au... particles onto MWCNTs at atmospheric pressure, using a plasma torch, has been developed through extensive tests and characterisation feedback. A new method, using pre-formed colloids has been patented. A decisive progress in the preparation of the nanohybrids consists in the addition of oxygen into the plasma, which could increase the amount of metal deposited and control the metal particle size.

At SAM, the optimisation of the treatment parameters for the new DBD atmospheric plasma has been performed (large chamber, for preparation of larger quantities of NANO2HYBRIDS). The deposition of Pt and Ni nanoparticles on carbon nanotubes and the characterisation (TEM, XPS, Nanosims) thereof has been particularly productive for the 'design' of the final benzene sensor.

At PCPM, the structural, electronic, magnetic and electron transport properties of CNTs with point defects, CNTs with oxygenated monovacancy in presence of various gas species (NOz, NHs, HzO, CO, COz), and pristine CNTs decorated with metallic nanoclusters have been studied. This very significantly highlighted that the quality and number of defects on the carbon nanotubes has to be tuned precisely, in order to promote interaction of external gases with the NANO2HYBRIDS.

At IMN, studies of single metal atoms on CNT surfaces (Au, Pd, Ni, Ti), both pristine and with oxygenated defects have been completed. Nanoparticle growth in gas phase and on carbon surfaces has been investigated theoretically. Studies on 1-,2- and 3- layer metal coverage, on ideal flat graphene (model for curved carbon nanotubes) for Au, Pd and Ti have been performed, what allowed calculations of benzene absorption on such surfaces.

With key integration between theory (PCPM and IMN) and experimental characterisation (LlSE, CHANI and SAM), the calculations of metal behaviour at defective sites of carbon nanotubes represent the current state of the art. The project partners published all together in 'Nanotechnology' an article detailing their combined theoretical and experimental approach.

At URV a set of deposition methods for the NANO2HYBRIDS materials has been employed and optimised. The sensors using materials sent by all partners involved in the production of NANO2HYBRIDS materials have been fabricated and tested. Using principal components analysis, the conditions to optimise a sensor for benzene detection have been determined.

Together with Sensotran, the optimisation of the components (microcontroller, display, AD converter and wireless modem) and of the prototype device has been realised. The so-obtained NANO2HYBRIDS sensor is beyond the state of the art in sensor technology, and this has been protected by a patent.

VEGA has been involved in the maintenance and update of the interactive website of the project, a crucial tool for the 'Science communication' work package: using video on the web and YouTube to communicate with the public, the website has received international awards, both for the videos themselves, and for the project as a whole. It is being used as a demonstration of best web practice by independent evaluators for the European Union.

The project was organised into six work packages (WP) as follows:

WP1: Interface design and optimisation
The oxygen plasma was particularly studied, showing the possibility to tune the occurrence of different types of functional groups (CO, OH, COOH...) at the CNT surface, and from that step, to control metal particle size and size distribution. The knowledge on plasma-CNT interaction has been studied in details for the following parameters: nature, composition and flow rate of the plasma gas, sample position (inside the plasma or in remote mode), and discharge power and treatment time.

WP2: Sample characterisation
Our group has shown during the NANO2HYBRIDS project that scanning transmission X-ray microscopy was able to performed spectroscopy on isolated multiwall carbon nanotubes. lt was important to develop this application, as all the other spectroscopies were sampling a large amount of materialsl a large surface (e.g. the analysed surface in XPS is at least a few mm2; all spectroscopic information is averaged over many CNTs). During our work performed principally at beamline 5.3.2. at the ALS (Berkeley, California) in collaboration with Professor A. P. Hitchcock and co-workers, we discovered that, via the use of the polarisation property of the synchrotron light, it was possible to study the plasma functionalisation of isolated CNTs and to characterise the density of defects (which is important for gas sensing) in isolated nanotubes.

WP3: modelling and theoretical calculations
The analysis of the interaction of NOz, NHs, CO, HzO, and COz molecules wlth oxygenated-defected CNTs reveals that NOz, NHg and CO can be detected since they exchange a significant fraction of electronic charge with the tube, thus modifying in a specific way the tube conductance. The interaction of COz with the tube is, instead, so weak that this gas could hardly be detected. Besides, the predicted temperature dependence of adsorption and desorption of NOz confirms that the CNT-based sensor can be used at room temperature and assess the reuse of the sensor after heating it up at higher temperature.

At last, within the combined experimental - theoretical work, we have demonstrated that the sensing of NOz and CO can be improved by functionalisation of CNTs with Au NPs, but that benzene detection cannot be achieved with Au-CNT systems. Hence, in order to detect benzene, CNT decoration with other metals (Rh, Ni and Pt) has been considered, finding that benzene interacts strongly with both Ni and Pt, a result consistent with the experimental findings reported by URV.

WP4: Gas sensor fabrication, characterisation and optimisation
The principal findings / tunings were:
-the sensor substrate consists of a silicon micromachined, four-element integrated micro-hotplate;
-sample deposition with the air brushing employing dimethylformamide as organic solvent has been found to results in coatings with excellent homogeneity, controllable thickness and highly reproducible results. The detection limit is below 20 ppb;
-humidity interference does not causes serious interference if humidity remains in the range 50 to 90 % R.H.

WP5: lndustrial applications and validation
A patent protects the benzene sensors and this 'product development' work is foreseen until a marketable product was reached. Considering that the existing benzene detectors (photo ionisation detectors) are only selective to benzene thanks to the use of specific diffusion tubes (disposable tubes that increase response time and make each single measurement rather expensive), the specifications obtained for the NANO2HYBRIDS benzene detector are very competitive.

WP6: Science communication
All the NANO2HYBRIDS partners have been involved on the production -supervised by VEGA Trust- of documentaries following the project, showing aims, key questions, scientific approach, potential applications, and relevance for Europe. These documents were made available to the public for free viewing over the internet using video streaming. Furthermore, we developed an online discussion forum to accompany the documentaries, allowing questions and discussion between the public and project partners throughout the project duration. DVDs containing the documentaries have been made available to the European Union for potential use in future nanoscience and nanotechnology video and TV projects.