Marie-Isabelle Baraton

Surface chemistry of TiO2 nanoparticles: influence on electrical and gas sensing properties

The modification of the surface chemistry of semiconducting nanoparticles is often required for optimising their performance. For example, surface modifications of semiconductor-based sensors can be envisaged to tailor the device selectivity. However, surface chemical modifications should deteriorate neither the bulk characteristics nor the electrical properties of the material. This becomes critical for nanoparticles due to their high surface-to-bulk ratio. In this work, surface modifications of titanium oxide nanoparticles by grafting hexamethyldisilazane (HMDS) are monitored in situ by Fourier transform infrared spectroscopy. The HMDS grafting decreases the density of the hydroxyl groups at the titanium oxide surface and, therefore modifies the surface affinity to water molecules. The consequences of these surface modifications on the gas sensing properties of the nanomaterial are discussed. In particular, it is shown how moisture adsorption subsequently alters these new grafted chemical species, resulting in a decrease of the cross-sensitivity to humidity. The variations of the infrared background absorption versus gas exposures are demonstrated to follow a λ2 dependence in agreement with the Drude–Zener theory, thus indicating that they are essentially due to the free carrier absorption. Therefore, the variations of the infrared absorption versus gas exposures can be directly correlated to the electrical conductivity variations.

Advances in Air Quality Monitoring via Nanotechnology

Urban air pollution has become an inescapable issue due to its serious consequences on public health and, therefore, needs more accurate tracking through denser networks of air quality monitoring (AQM) stations. A higher density of these networks can be afforded by cities only if the costs of future individual AQM stations decrease. We review here the outcome of two European projects where our objective was to provide an alternative approach consisting in the development of cost-effective mobile microstations based on semiconductor sensors and capable of complementing the expensive and bulky current AQM stations. Improvement of the sensor sensitivity to detect very low levels of pollutants (CO, NO, NO2, O3) in air was the major challenge to take up. This was achieved by using metal oxide nanosized particles with both controlled size and surface chemistry, and by adapting the screen-printing process to the nanometer size specificity. The detection thresholds for NO2, NO and O3 of our nanoparticles-based sensors have been decreased by a factor of 3–5 compared to currently commercialized sensors. The lowest detectable concentration of CO has been reduced from 5 to 3 ppm without affecting the selectivity. In terms of sensitivity performance, our sensor prototypes can now meet the criteria for outdoor AQM whereas the commercial semiconductor and electrochemical sensors still cannot. As for the implementation of the network as a whole, our technological approach is outlined.

Nano-TiO2 for Solar Cells and Photocatalytic Water Splitting:Scientific and Technological Challenges for Commercialization

Nanosized titanium dioxide (nano-TiO2) particles are used in diverse products and devices, including photocatalytic water splitting and solar cells whose successful commercialization is still facing scientific and technologi- cal challenges. Particularly, dye-sensitized solar cells (DSSCs) have recently attracted a lot of attention. The number of papers and patents published in this area has grown exponentially over the last ten years. However, at the present, commercial devices are of small sizes and produced in limited quantities, thus addressing niche markets. Research efforts have largely focused on the optimization of the dye, but the TiO2 nanocrystalline electrode itself also needs to be optimized. It has been shown that particle size and shape, crystallinity, surface morphology and chemistry of the TiO2 material are key parameters to be controlled for enhanced performance of the cell. This article gives an overview of the state-of-the-art on nano-TiO2 for applications in photocatalytic water splitting and, more specifically, in DSSCs. After a brief analysis of the commercial perspectives of DSSCs and of the remaining challenges for their successful commercialization, our approach to the control of the nano-TiO2 surface chemistry for improvement of the DSSC performance is briefly introduced.

Mechanistic investigation of the preparation of polymer/ceramic nanocomposites

Abstract Polyimide/aluminum nitride (AlN) nanocomposites with very high packing density were obtained through an efficient solution mixing method. The homogeneous composites showed a trend of decreasing thermal expansion, and increasing hardness and Youngs modulus, as the ceramic loading increased. A detailed investigation of the preparation process, via in-situ FT-IR analysis of a series of probe molecule reactions on the particle surface, suggested a strong chemisorption of amide groups on the nanoparticle surface. Therefore, a reaction mechanism for the chemisorption is proposed. Considering the structural similarity of the probe molecules and poly(amic acid), a precursor to polyimide, indications are that (1)chemisorption plays an important role for mixing processing of a nanoparticle/organic medium; and (2) amide group adsorption is one of the most efficient interaction during the suspension processing for the chemically synthesized nanostructured AlN powders.

In situ determination of the grafting sites on nanosized ceramic powders by FT-IR spectrometry

Abstract The grafting of hexamethyldisilazane (HMDS) on the surface of alumina and titania nanosized powders has been followed in situ by Fourier transform infrared surface spectrometry. It is demonstrated that the reaction of HMDS with the different hydroxyl groups on the alumina and titania surfaces is selective and that Si-OH groups can be generated by oxidation of the grafted surfaces. Then the addition to these grafted surfaces of acetic acid which has a strong affinity for OH groups resulted in a partial or total displacement of the grafted HMDS and brought the evidence of the selectivity.

Chemical Phenomena at the Surface of Nanoparticles

In principle, the precise knowledge of the composition and the structure of a material allows the determination of the bulk properties. In a reverse way, the reproducibility of the composition and structure of the material should ensure the reproducibility of the material properties. However, when the size of a solid is decreased down to the nanometer scale, the overall properties of the nanomaterial are no longer controlled by the bulk structure but by the surface properties. Surface characterization then becomes a necessary prerequisite for the control of nanopowders.

The Surface Characterization of Nanosized Powders: Relevance of the FTIR Surface Spectrometry

Even though the study of the bulk properties of crystalline solids is facilitated by the periodicity existing in the lattice, the control of the surface properties and of the interface behavior still represents a challenge to scientists. Techniques to investigate the specific structure and composition of the first atomic layers are very often derived from bulk analysis methods. As a consequence, the minimum depth that can be analyzed, although adequate for traditional materials, may be too large for nanosized materials in which crystal sizes can be smaller than the depth resolution of the characterization technique. Fourier transform infrared (FTIR) spectrometry, widely used for bulk analyses, is, however, a powerful tool to characterize the very first atomic layer provided specific setups are attached to the spectrometer. Several examples will be discussed in the following showing the specific nature of the surface and the relevance of the FTIR spectrometry for obtaining detailed information on the chemical species and the atoms constituting the first atomic layer as well as the coordination number of the surface atoms. Moreover, because of the important role played by the nanomaterial surface in many industrial applications, the surface modifications are a key issue to tailor the surface properties. To this end, FTIR surface spectrometry is also a performant technique to follow the modification of the surface chemical species and to study in situ the selectivity and the behavior of the modifications under various treatments.

The ν(Si-H) absorption band as a sensor of the oxidation of a silicon carbide surface: Spectrometric and Ab initio studies

Abstract Silicon carbide powders show Si-H groups on their surface whose ν(Si-H) stretching frequency is known to be very sensitive to the silicon environment. In this work, we correlated the oxygen content on the first atomic layer to the ν(Si-H) absorption frequency. A SiC nanophase powder was analyzed in situ by Fourier transform infrared spectrometry while performing a controlled oxidation. The evolutions of the ν(Si-H) stretching bands were followed versus temperature, oxygen pressure and time. The ν(Si-H) absorption range was then resolved into subbands corresponding to the possible H-SiO x C 3 − x (x = 0, 1, 2, 3) species. The relative intensities of these subbands allow one to discuss their respective probability. Ab initio calculations based on self-consistent molecular orbital theory showed charge transfer in the SiH group when oxygen replaces carbon. The ν(Si-H) absorption frequency is proven to be a good sensor of the oxidation degree of the SiC surface. Moreover, our IR technique for surface analysis proves to be extremely relevant when a precise knowledge of the first atomic layer on a nanophase ceramic powder is required.

Surface Analysis of Semiconducting Nanoparticles by FTIR Spectroscopy

In this chapter, we have presented results proving that Fourier transform infrared spectroscopy is particularly relevant not only for the surface analysis of nanosized particles but also for the study of the electrical properties of semiconducting nanoparticles. A direct correlation of the chemical reactions taking place at the very surface with the resulting variations of the electrical conductivity can be established in real time, which is an important asset for the fundamental understanding of the gas detection mechanism and consequently for optimization of chemical gas sensors based on semiconductors. In addition, the evolution of the newly formed surface chemical species versus time can give information on the stability and reversibility of the gas sensors, thus allowing the determination of the optimum operating conditions.

Ion Projection Direct-Structuring For Nanotechnology Applications

Large-field ion-optics has been developed for reduction printing. Sub-100nm ion projection direct-structuring (IPDS) of patterned magnetic media discs has been demonstrated, extending over 17mm diameter exposure fields, in a single exposure. First results of IPDS patterning of nanocomposite resist material are presented. Information about a novel 200x reduction projection focused ion multi-beam (PROFIB) tool development is provided. Further IPDS nanotechnology applications are discussed.