F. Di Fonzo

Hierarchical TiO2 Photoanode for Dye-Sensitized Solar Cells

Hierarchical or one-dimensional architectures are among the most exciting developments in material science these recent years. We present a nanostructured TiO(2) assembly combining these two concepts and resembling a forest composed of individual, high aspect-ratio, treelike nanostructures. We propose to use these structures for the photoanode in dye-sensitized solar cells, and we achieved 4.9% conversion efficiency in combination with C101 dye. We demonstrate this morphology beneficial to hamper the electron recombination and also mass transport control in the mesopores when solvent-free ionic liquid electrolyte is used.

Hierarchically organized nanostructured TiO2 for photocatalysis applications

A template-free process for the synthesis of nanocrystalline TiO2 hierarchical microstructures by reactive pulsed laser deposition (PLD) is here presented. By a proper choice of deposition parameters a fine control over the morphology of TiO2 microstructures is demonstrated, going from classical compact/columnar films to a dense forest of distinct hierarchical assemblies of ultrafine nanoparticles (<10 nm), up to a more disordered, aerogel-type structure. Correspondingly, the film density varies with respect to bulk TiO2 anatase, with a degree of porosity going from 48% to over 90%. These structures are stable with respect to heat treatment at 400 degrees C, which results in crystalline ordering but not in morphological changes down to the nanoscale. Both as deposited and annealed films exhibit very promising photocatalytic properties, even superior to standard Degussa-P25 powder, as demonstrated by the degradation of stearic acid as a model molecule. The observed kinetics are correlated to the peculiar morphology of the PLD grown material. We show that the 3D multiscale hierarchical morphology enhances reaction kinetics and creates an ideal environment for mass transport and photon absorption, maximizing the surface area-to-volume ratio while at the same time providing readily accessible porosity through the large inter-tree spaces that act as distributing channels. The reported strategy provides a versatile technique to fabricate high aspect ratio 3D titania microstructures through a hierarchical assembly of ultrafine nanoparticles. Beyond photocatalytic and catalytic applications, this kind of material could be of interest for those applications where high surface-to-volume and efficient mass transport are required at the same time.

Structure-dependent optical and electrical transport properties of nanostructured Al-doped ZnO

The structure-property relation of nanostructured Al-doped ZnO thin films has been investigated in detail through a systematic variation of structure and morphology, with particular emphasis on how they affect optical and electrical properties. A variety of structures, ranging from compact polycrystalline films to mesoporous, hierarchically organized cluster assemblies, are grown by pulsed laser deposition at room temperature at different oxygen pressures. We investigate the dependence of functional properties on structure and morphology and show how the correlation between electrical and optical properties can be studied to evaluate energy gap, conduction band effective mass and transport mechanisms. Understanding these properties opens up opportunities for specific applications in photovoltaic devices, where optimized combinations of conductivity, transparency and light scattering are required.

Precision and accuracy in film stiffness measurement by Brillouin spectroscopy.

The interest in the measurement of the elastic properties of thin films is witnessed by a number of new techniques being proposed. However, the precision of results is seldom assessed in detail. Brillouin spectroscopy (BS) is an established optical, contactless, non-destructive technique, which provides a full elastic characterization of bulk materials and thin films. In the present work, the whole process of measurement of the elastic moduli by BS is critically analyzed: experimental setup, data recording, calibration, and calculation of the elastic moduli. It is shown that combining BS with ellipsometry a fully optical characterization can be obtained. The key factors affecting uncertainty of the results are identified and discussed. A procedure is proposed to discriminate factors affecting the precision from those affecting the accuracy. By the characterization of a model transparent material, silica in bulk and film form, it is demonstrated that both precision and accuracy of the elastic moduli measured by BS can reach 1% range, qualifying BS as a reference technique.

Island organization of TiO2 hierarchical nanostructures induced by surface wetting and drying.

We report on the reorganization and bundling of titanium oxide nanostructured layers, induced by wetting with different solvents and subsequent drying. TiO(2) layers are deposited by pulsed laser deposition and are characterized by vertically oriented, columnar-like structures resulting from assembling of nanosized particles; capillary forces acting during evaporation induce bundling of these structures and lead to a micrometer-size patterning with statistically uniform islands separated by channels. The resulting surface is characterized by a hierarchical, multiscale morphology over the nanometer-micrometer length range. The structural features of the pattern, i.e., characteristic length, island size, and channel width, are shown to depend on properties of the liquid (i.e., surface tension) and thickness and density of the TiO(2) layers. The studied phenomenon permits the controlled production of multiscale hierarchically patterned surfaces of nanostructured TiO(2) with large porosity and large surface area, characterized by superhydrophilic wetting behavior without need for UV irradiation.

Radiation endurance in Al 2 O 3 nanoceramics

The lack of suitable materials solutions stands as a major challenge for the development of advanced nuclear systems. Most issues are related to the simultaneous action of high temperatures, corrosive environments and radiation damage. Oxide nanoceramics are a promising class of materials which may benefit from the radiation tolerance of nanomaterials and the chemical compatibility of ceramics with many highly corrosive environments. Here, using thin films as a model system, we provide new insights into the radiation tolerance of oxide nanoceramics exposed to increasing damage levels at 600 °C –namely 20, 40 and 150 displacements per atom. Specifically, we investigate the evolution of the structural features, the mechanical properties, and the response to impact loading of Al2O3 thin films. Initially, the thin films contain a homogeneous dispersion of nanocrystals in an amorphous matrix. Irradiation induces crystallization of the amorphous phase, followed by grain growth. Crystallization brings along an enhancement of hardness, while grain growth induces softening according to the Hall-Petch effect. During grain growth, the excess mechanical energy is dissipated by twinning. The main energy dissipation mechanisms available upon impact loading are lattice plasticity and localized amorphization. These mechanisms are available in the irradiated material, but not in the as-deposited films.

Few-layer graphene improves silicon performance in Li-ion battery anodes

We demonstrate that few-layer graphene (FLG) flakes combined with ultra-small (below 10 nm) amorphous silicon nanoparticles (SiNPs) improve the performance of Li-ion battery anodes compared to both amorphous carbon and graphene oxide additives. The FLG flakes are produced by liquid phase exfoliation of pristine graphite, while the SiNPs are synthesized by means of a plasma-assisted aerosol synthesis technique. These novel hybrid electrodes are realized by drop casting, onto a copper current collector, a slurry paste with a 1 : 1 : 1 mass ratio of FLG, SiNPs and a polyacrylic acid (PAA) binder followed by annealing in a H2 atmosphere. The as-produced anode displays a capacity loss of only 8% over 300 cycles, reaching a maximum specific capacity of 1500 mA h gSi−1 and a coulombic efficiency exceeding 99% and 99.8% in the 20th and 300th cycles, respectively. The obtained results highlight the optimal synergy between FLG flakes and ultra-small SiNPs, allowing the best capacity retention to be achieved upon cycling. The observed stability coupled with the scalability of both the FLG and SiNP production methods offers a viable approach for the development of next generation Li-ion battery anodes based on nano-engineered hybrid materials.

Multiscale simulation of solid state dye sensitized solar cells including morphology effects

In this work we present a multiscale simulation of a solid state dye sensitized solar cell including the real morphology of the active layer. In order to include the real morphology the device domain is split into two different regions: one treated using an effective material approximation and another one using the real structure of the blend. The real morphology has been measured using electron tomography to reconstruct the mesoporous TiO2. The geometry was inserted into a mesher and used to solve a drift-diffusion model using finite element method. The simulation is used to cast light over morphology effects in solid state dye solar cells.

Periodic nanostructures for tunable thin optics

We report the realization and characterization of porous nanostructures where a periodic refractive index modulation is achieved by stacking layers with different nano-architectures. One multilayer photonic crystal has been fabricated starting from colloidal dispersion of silicon dioxide and zirconium dioxide using spin coating technique. Improved efficiency of Bragg reflectivity (up to 85%) has been obtained by a new bottom-up fabrication technique of photonic hierarchical nanostructures based on self-assembly from the gas-phase at low temperature whit a very thin (≈ 1 μm) photonic crystal devices. Due to the high porosity, these systems can be infiltrated with nematic liquid crystals leading to tuning of the Bragg reflection band by applying low voltages to the structure.

Quantitative electron tomography investigation of a TiO2 based solar cell photoanode

The development of efficient thin film solar cells requires a deep knowledge of the nanoscale morphology of the active layers. While conventional investigation is usually limited to 2D information, here we use electron tomography to unravel a complex particle network in a non-ambiguous, 3D reconstruction. We present our study of a dye sensitised solar cell, based on a nanostructured TiO2 photoanode produced by pulsed laser deposition (PLD) and displaying a hierarchical, quasi-1D arrangement. We prepare the sample for electron tomography using focused ion beam (FIB) milling to obtain a micro-pillar, instead of a conventional TEM lamella. This approach has the advantage of allowing higher quality tomographic reconstructions of complex morphologies due to the increased tilt range available and the constant thickness of the section. We analyse the resulting reconstruction to quantitatively investigate the geometry of the TiO2 network. We compare the findings with a photoanode based on a conventional TiO2 paste, determining the anisotropy of the PLD-grown film. To complement our nanoscale TEM characterization, we also employ FIB tomography, to obtain a complete structural characterisation of the photoanode at different length scales.