TiO2- based ceramics, from basic science to applications

Nanomaterials and nanoparticles are nowadays used in a wide range of applications; from sunscreens, paint and food industry to catalytic processes, lighting, TV's and displays, their presence is percolating in our daily life. New applications rise every day, pushing forward the technology limits with increasing efforts and funding for nanomaterial science and applications. Among different nanomaterials, titanium dioxide (TiO2) continues to attract considerable theoretical and experimental attention because of its technological relevance in applications such as environmental purification, solar energy harvesting, memristors, catalysis, advanced ceramic materials and more. Different allotropes and morphologies of the compound are being examined, in particular nanoparticles and nanofilaments (nanowires and nanotubes) in titanate (H2Ti3O7), anatase or rutile phase. In this dissertation, we have deeply investigated the behavior of titanium-oxide based nanowires (TiO2 NWs), which, thanks to their larger specific surface area than bulk or polycrystalline counterparts, are very promising for photocatalysis. The TiO2 NWs are very suitable for such nanofilamentary applications since they have an ideal form for assembling themselves into nanoporous aerogel membranes. Our nanowires can be used directly in both 2D and 3D architecture, and they can serve as ideal support (reducing Ostwald ripening and coarsening in general) for various metal nanoparticles. H2Ti3O7 NWs are equally important, indeed, both as active components, for example in case of very sensitive humidity sensors and efficient catalysts and as reinforcement fibers in composites or as metal nanoparticles support for advanced CO oxidation catalysis. The NW behavior in titanate, anatase and rutile phase has been studied and the phase transition temperature determined, finding out that the strong surface effects, in the 2D system, result in unexpected stabilization of the metastable anatase phase and the absence of rutile phase above 900 °C. However, the classical Ostwald ripening particle coarsening is observed in sintered pellets, accompanied by a typical anatase to rutile phase transition at 900 °C. Mechanical measurements and nanoscopic observations both on bulk and on individual nanowire have allowed determining the mechanical moduli and the hardness. In particular, for the first time, the shear modulus of individual nanowire in titanate phase has been directly measured and its relatively low value of 1.5±0.8 GPa confirms the layered structure with a relatively weak bond between layers. The elastic modulus is within the expected range of about 70 GPa for titanate and above 100 GPa for anatase. Functionality and potential impact of TiO2 NWs for a few specific catalytical applications have been explored. The photocatalytic effectiveness of titania nanowires for destroying DNA chains after a suitable exposition to UV light (elimination of pollutants) was established. Finally, I investigated the application of our TiO2 NWs in the form of paper filters for viruses and bacteria, like the current SARS-CoV-2. I demonstrated the concept of novel personal protective equipment (PPE), that TiO2 NWs based-facial mask is may be very effective stopping the virus or bacteria, but also that it is easily regenerated with simple UV exposition, cutting down the economical and environmental cost of a future product.

Novel catalytic applications of carbon nanofibers on sintered metal fibers filters as structured supports

Marina Ruta

Supported metal catalysts are important from both an industrial and a scientific point of view. They are used, amongst others, in large-scale processes such as catalytic reforming, hydrotreating, polymerization reactions and hydrogenations. Often, these catalysts consist of nanosized metal particles deposited on a suitable support, which acts as an anchor for the active phase and, in several cases, contributes to improve the overall catalyst performances. The growth of carbon nanofibers on sintered metal fibers filters (CNF/SMF) by hydrocarbons catalytic decomposition results in a structured composite material presenting all the suitable characteristics for its application as catalyst support. The main objective of this thesis was to develop novel catalytic systems based on CNF/SMF as catalytic support for continuous gas-phase hydrogenations. At first, the synthesis of CNF/SMF was optimized to tailor the properties of the material for further applications as catalytic support. The use of ethylene as carbon precursor and high synthesis temperature (973K) provided high yields of well-ordered CNF with increased specific surface area (SSA), up to 516 m2/g. On the other hand, the synthesis of CNF by ethane decomposition, which is less reactive than ethylene, resulted in a support with higher permeability, minimizing the pressure drop and being more suitable in a continuous-flow reactor. After the synthesis, the CNF surface was activated by treatment with O3 or H2O2. A novel technique, which consist of XPS analysis after step-wise TPD in UHV conditions, was developed and applied for the surface characterization. It allowed to assess the nature of the functional groups created on the CNF surface. The O3-activation was found the most effective for increasing the acidity of the CNF/SMF surface, yielding a relatively high amount of carboxylic functional groups. These groups are important because involved in the chemical anchoring and stabilization of the active metal deposited on the support. Industrially, many hydrogenations are performed over supported metal catalyst. The majority of these reactions are known to be structure sensitive reactions, so the control of the metal particle size is crucial in order to study the catalyst activity and selectivity. We prepared monodisperse-sized Pd nanoparticles (8, 11 and 13 nm) via the reverse microemulsion method and deposited on CNF/SMF and activated-CNF/SMF for a two-fold study: the effect of the nanoparticles size and the effect of the support nature on the selective hydrogenation of acetylene. The antipathetic size dependence of the TOF disappeared at particle size bigger than 11 nm. The initial selectivity to ethylene (∼60%) was found size-independent. The structure-sensitivity relations have been discussed in terms of "geometric" and "electronic" nature of the size-effect and rationalized assuming a Pd-Cx phase formation which is known to be size-dependent. CNF/SMF supports with increased acidity diminished the formation of coke and changed the by-products distribution, diminishing the catalyst deactivation. Subsequently, CNF/SMF was applied to develop a new catalytic system: the Structured Supported Ionic Liquid Phase (SSILP) catalyst, embedding an active Rh complex. The selective gas-phase hydrogenation of 1,3-cyclohexadiene to cyclohexene was used as model reaction. The SSILP based on CNF/SMF supports showed a high turnover frequency, up to 250 h-1 and selectivity > 96%. Multiple advantages of supporting a homogeneous active phase, dissolved in IL, on CNF/SMF have been demonstrated: the thin layer of IL was homogeneously distributed over the mesoporous support, so mass transfer limitations could be avoided, furthermore the chemical inertness of the CNF prevented any interaction between the active phase dissolved in IL ensuring an efficient use of the Rh complex. The SSILP concept can be applied not only for the "heterogenization" of a transition metal catalyst, but also to obtain metal nanoparticles with monodispersed size via the reduction of a metal precursor dissolved in IL. In this case, IL provides a suitable environment for the nucleation and growth of the nanoparticles. By applying the SSILP concept to the selective hydrogenation of acetylene, the influence of IL on the stability of the nanoparticles was investigated. Moreover, the different solubilities of reactants and products in the IL phase allowed to improve the selectivity toward ethylene. Monodispersed Pd nanoparticles of 5 and 10 nm in SILP on CNF/SMF showed excellent long-term stability. The lower solubility of ethylene than acetylene in the IL had the beneficial effect of allowing high selectivity to ethylene up to 85%, due to the inhibition of its consecutive hydrogenation to ethane. In conclusion, a novel catalytic system for continuous-flow, gas-phase hydrogenations was conceived, taking advantage of the suitable characteristics of CNF/SMF supports. The SSILP concept, involving both homogeneous and heterogeneous active phases has been demonstrated, indicating a great potential and flexibility for continuous-flow industrial applications.

EPFL2008

Shape memory polymers with controlled time-dependent shape recovery

Charly David Azra

The aim of this thesis has been to investigate the use of heat-activated shape memory polymers (SMPs) as novel smart actuators for the controlled delivery of fluids and their suitability for applications such as low-cost drug delivery. On heating to temperatures in the vicinity of their glass transition temperature, Tg, amorphous SMPs are able to recover their equilibrium shape from a “programmed” temporary shape generated by mechanical deformation, and fixed by cooling to below Tg. This phenomenon is linked to the substantial increase in molecular mobility on heating through the glass transition, and the memory of an equilibrium shape associated a crosslinking network. While SMPs have received considerable attention in the literature, there has to date been relatively little focus on the time-dependence of shape recovery in these materials. There are nevertheless many potential biomedical applications, for example, in which it is necessary to control the actuation rate in SMP-based devices activated by natural heat sources such as the human body, whose temperature may be subject to significant fluctuations. The present work has therefore focused on the time-dependent shape memory effect under isothermal conditions and its sensitivity to temperature variations. The first part of the study involved commercial thermoplastic (physically cross-linked) and thermoset (chemically cross-linked) shape memory polyurethanes (SMPUs). This provided insight into the physical mechanisms underlying the time-dependence of the shape memory effect, which was found to correlate strongly with the viscoelastic behaviour of a given material and the thermomechanical variables associated with the shape memory cycle. It also revealed the importance of the width of the α-transition, ∆Tα , as determined by dynamic mechanical analysis (DMA), and its position on the temperature axis relative to the recovery temperature, Tr. In particular, it was shown that controlled recovery rates could be obtained if Tr corresponded to the onset of the α-transition, and a broad transition led to a reduction in the sensitivity of the shape recovery rate to thermal fluctuations about a given Tr. The next part of the study was devoted to the development of materials optimized for controlled shape recovery under well-defined conditions. The commercial formulations were found to be unsuitable for this purpose, because they offered relatively limited possibilities for tailoring Tg and ∆Tα . Thermoset SMPUs with tunable Tg and ∆Tα were therefore produced in house, based on polytetrahudrofuran (PTHF), 4,4’-diphenylmethane diisocyanate (MDI) and 1,1,1-trimethylolpropane (TMP). The starting point was a literature formulation, which was used to optimize a reactive injection molding (RIM) set-up designed to produce void-free parts with a molding time of less than 3 minutes. These SMPUs were characterized by a PTHF network linked by rigid domains made up of crosslinked MDI and TMP. This made it possible to vary Tg from -60 to 80 °C and ∆Tα from 35 to 100 °C, by varying the molecular weight of the PTHF from 2000 to 650 g/mol, and hence the cross-link density for a given stoichiometric ratio PTHF:MDI:TMP. In order to adjust ∆Tα independently of Tg, flexible segments were introduced to the rigid domains and to the junctions between the rigid domains and the PTHF, using a fourth component, 1,4-butanediol (BDO), or by replacing the TMP with ethoxylated TMP (eTMP). This provided a highly flexible platform for optimizing the shape memory performance. Moreover, the resulting SMPUs showed equilibrium moduli, Ee, of up to 13 MPa, which is relatively high for unreinforced thermosets, and clearly beneficial for fluid displacement where high recovery stresses may be important. To increase Ee further, alumina nanoparticles were dispersed in the SMPUs. This was achieved through ultrasonic, solvent-assisted dispersion of the alumina in PTHF and subsequent curing with the MDI and TMP. Reproducible, void-free, SMPU nanocomposite sheets with alumina contents as high as 15 wt% could be produced in this way within the 3 minute pot-life of the reactive mixtures. These showed a homogeneous dispersion of the alumina (aggregates less than 50 nm in diameter), as confirmed by transmission electron microscopy, leading to a modest increase in Ee to about 21 MPa. This indicated reinforcement to be essentially a geometrical effect (strain amplification), suggesting relatively limited matrix/filler interactions. On the other hand, Rr remained high in these nanocomposites (a decrease of only 5 % on addition of 15 wt% alumina), in contrast with reports in the literature of a significant degradation in shape memory performance for SMPUs containing highly interacting fillers. A linear thermo-viscoelastic model was used to simulate the small strain shape memory effect. Continuous relaxation and retardation spectra, H(τ) and L(τ) respectively, were used to model the time-dependence, and a shift factor, aT(T), was used to model the temperature dependence. To streamline the procedure, a novel characterization method was developed for the determination of aT (T ) from a constant frequency DMA temperature sweep, based on the establishment of an approximate relationship between the storage modulus, the loss modulus and aT (T ). It was hence possible to generate continuous data for aT (T ) from a single DMA run, avoiding the time-consuming and operator-dependent construction of a mastercurve from multiple experiments. Using the Havriliak-Negami parametric functions, H(τ) and L(τ) could be subsequently obtained over a large number of decades of time from the modulus and compliance data. Comparison with results from experimental shape memory data for different model SMPUs, showed this approach to provide a reliable indication of the long-term shape recovery rate. Moreover, this method has great potential for providing rapid insight into the temperature-dependent viscoelastic properties of thermorheologically simple polymers in general, regardless of whether they are being considered specifically for shape memory applications. In the final part of the study, a tailored SMPU nanocomposite formulation was used to produce demonstrators in the form of active shape memory reservoirs, capable of expelling a liquid content under isothermal conditions. The reservoirs consisted of circular membranes (the equilibrium shape), inflated at elevated temperature to form 5 ml blisters and subsequently stabilized in this form by cooling (the programmed shape), using a biaxial deformation set-up incorporating temperature control. This innovative programming procedure made it possible to generate active SMPU reservoirs that could be activated at the skin temperature (32 °C), resulting in low average flow rates ranging between 1 and 5 μL/min over several hours. Such as a device has important potential applications, e.g. skin-temperature activated, low-cost transdermal drug delivery patches.

EPFL2013

Contact angle and adsorption energies of nanoparticles at the air-liquid interface determined by neutron reflectivity and molecular dynamics

Mauro Moglianetti, Evgeniy Ponomarev, Javier Reguera Gomez, Francesco Stellacci

Understanding how nanomaterials interact with interfaces is essential to control their self-assembly as well as their optical, electronic, and catalytic properties. We present here an experimental approach based on neutron reflectivity (NR) that allows the in situ measurement of the contact angles of nanoparticles adsorbed at fluid interfaces. Because our method provides a route to quantify the adsorption and interfacial energies of the nanoparticles in situ, it circumvents problems associated with existing indirect methods, which rely on the transport of the monolayers to substrates for further analysis. We illustrate the method by measuring the contact angle of hydrophilic and hydrophobic gold nanoparticles, coated with perdeuterated octanethiol (d-OT) and with a mixture of d-OT and mercaptohexanol (MHol), respectively. The contact angles were also calculated via atomistic molecular dynamics (MD) computations, showing excellent agreement with the experimental data. Our method opens the route to quantify the adsorption of complex nanoparticle structures adsorbed at fluid interfaces featuring different chemical compositions.