Stefano Bianco

Silicon resonant microcantilevers for absolute pressure measurement

This work is focused on the developing of silicon resonant microcantilevers for the measurement of the absolute pressure. The microcantilevers have been fabricated with a two-mask bulk micromachining process. The variation in resonance response of microcantilevers was investigated as a function of pressure (10−1–105Pa), both in terms of resonance frequency and quality factor. A theoretical description of the resonating microstructure is given according to different molecular and viscous regimes. Also a brief discussion on the different quality factors contributions is presented. Theoretical and experimental data show a very satisfying agreement. The microstructure behavior demonstrates a certain sensitivity over a six decade range and the potential evolution of an absolute pressure sensor working in the same range.

A Chemometric Approach for the Sensitization Procedure of ZnO Flowerlike Microstructures for Dye-Sensitized Solar Cells

In this paper, a methodology for the streamlining of the sensitization procedure of flowerlike ZnO nanostructures for dye-sensitized solar cells (DSCs) is reported. The sensitization of ZnO surface with ruthenium-based complexes is a particularly critical process, since one has to minimize the dissolution of surface Zn atoms by the protons released from the dye molecules, leading to the formation of Zn(2+)/dye complexes. The fine-tuning of the experimental parameters, such as the dye loading time, the dye concentration, and the pH of the sensitizing solution, performed through a multivariate optimization by means of a chemometric approach, is here reported. The dye loading procedure was optimized using ZnO microparticles with nanostructured protrusions, synthesized by a simple and low-cost hydrothermal process. Mild reaction conditions were used, and wurtzite-like crystalline structure with a relatively high surface area was obtained once the reaction process was completed. After dispersion of ZnO flowerlike particles in an acetic acid-based solution, a 14 μm-thick ZnO layer acting as DSC photoanode was fabricated. The optimized sensitization procedure allowed minimizing the instability of ZnO surface in contact with acidic dyes, avoiding the formation of molecular agglomerates unable to inject electrons in the ZnO conduction band, achieving good results in the photoconversion efficiency. Moreover, the photoharvesting properties were further enhanced by adding N-methylbenzimidazole into the iodine-based liquid electrolyte. Such an additive was proposed here for the first time in combination with a ZnO photoelectrode, helping to reduce an undesired recombination between the photoinjected electrons and the oxidized redox mediator.

Toward Totally Flexible Dye-Sensitized Solar Cells Based on Titanium Grids and Polymeric Electrolyte

In this work, we present a novelty in the dye-sensitized solar cell scenario: a quasi-solid and completely flexible configuration based on plastic substrates and metallic meshes as support. The aim is to obtain a portable efficient device that can be competitive in the solar market due to the low cost and easy-to-prepare materials used for its fabrication. To fulfill this purpose, three different typologies of devices are proposed and tested in order to move from a rigid to a completely flexible setup in a gradual way. Materials and cells have been thoroughly characterized and tested by means of physicochemical, electrical, and electrochemical measurements to investigate the observed performances and the results that are reported in this paper.

In situ MoS2 Decoration of Laser-Induced Graphene as Flexible Supercapacitor Electrodes

Herein, we are reporting a rapid one-pot synthesis of MoS2-decorated laser-induced graphene (MoS2-LIG) by direct writing of polyimide foils. By covering the polymer surface with a layer of MoS2 dispersion before processing, it is possible to obtain an in situ decoration of a porous graphene network during laser writing. The resulting material is a three-dimensional arrangement of agglomerated and wrinkled graphene flakes decorated by MoS2 nanosheets with good electrical properties and high surface area, suitable to be employed as electrodes for supercapacitors, enabling both electric double-layer and pseudo-capacitance behaviors. A deep investigation of the material properties has been performed to understand the chemical and physical characteristics of the hybrid MoS2-graphene-like material. Symmetric supercapacitors have been assembled in planar configuration exploiting the polymeric electrolyte; the resulting performances of the here-proposed material allow the prediction of the enormous potentialities of these flexible energy-storage devices for industrial-scale production.

Electrodes/Electrolyte Interfaces in the Presence of a Surface-Modified Photopolymer Electrolyte: Application in Dye-Sensitized Solar Cells

Since hundreds of studies on photoanodes and cathodes show that the electrode/electrolyte interfaces represent a key aspect at the base of dye-sensitized solar cell (DSSC) performances, it is reported here that these interfaces can be managed by a smart design of the spatial composition of quasi-solid electrolytes. By means of a cheap, rapid, and green process of photoinduced polymerization, composition-tailored polymer electrolyte membranes (PEMs) with siloxane-enriched surfaces are prepared, and their properties are thoroughly described. When assembled in DSSCs, the interfacial action promoted by the composition-tailored PEMs enhances the photocurrent and fill factor values, thus increasing the global photovoltaic conversion efficiency with respect to the non-modified PEMs. Moreover, the presence of the siloxane-chain-enriched surface increases the hydrophobicity and reduces the water vapor permeation into the device, thus enhancing the cells durability.

An analysis of carbon nanotube structure wettability before and after oxidation treatment

In this work contact angle measurements have been carried out on different carbon nanotube-based materials. Thin layers of entangled carbon nanotubes have been considered, together with thick mats of vertically aligned carbon nanotubes. Pressed single-walled carbon nanotubes and graphite have also been considered, for comparison. In particular, the properties of the as-grown thick carpets have been compared to those of the oxidized material. Oxidation treatments have been performed either by chemical oxidation with a nitric/sulfuric acid mixture or by applying a bias voltage of 10 V on the material. Tests have been performed using three liquids with differing polarity (water, dimethylsulfoxide and diiodomethane). Very high hydrophobicity was observed for the top surface of the aligned carbon nanotube mats, but after oxidation a dramatic reduction in the contact angle was observed, bringing about a hydrophilic behaviour. Theoretical calculations of the bulk pore distribution of the material have been made to estimate the liquid percolation through the oxidized carpets surface.

Ultrafast room-temperature crystallization of TiO2 nanotubes exploiting water-vapor treatment.

In this manuscript a near-room temperature crystallization process of anodic nanotubes from amorphous TiO2 to anatase phase with a fast 30 minutes treatment is reported for the first time. This method involves the exposure of as-grown TiO2 nanotubes to water vapor flow in ambient atmosphere. The water vapor-crystallized samples are deeply investigated in order to gain a whole understanding of their structural, physical and chemical properties. The photocatalytic activity of the converted material is tested by dye degradation experiment and the obtained performance confirms the highly promising properties of this low-temperature processed material.

Toward Plastic Smart Windows: Optimization of Indium Tin Oxide Electrodes for the Synthesis of Electrochromic Devices on Polycarbonate Substrates

Plastic smart windows are becoming one of the key elements in view of the fabrication of inexpensive, lightweight electrochromic (EC) devices to be integrated in the new generation of high-energy-efficiency buildings and automotive applications. However, fabricating electrochromic devices on polymer substrates requires a reduction of process temperature, so in this work we focus on the development of a completely room-temperature deposition process aimed at the preparation of ITO-coated polycarbonate (PC) structures acting as transparent and conductive plastic supports. Without providing any substrate heating or surface activation pretreatments of the polymer, different deposition conditions are used for growing indium tin oxide (ITO) thin films by the radiofrequency magnetron sputtering technique. According to the characterization results, the set of optimal deposition parameters is selected to deposit ITO electrodes having high optical transmittance in the visible range (∼90%) together with low sheet resistance (∼8 ohm/sq). The as-prepared ITO/PC structures are then successfully tested as conductive supports for the fabrication of plastic smart windows. To this purpose, tungsten trioxide thin films are deposited by the reactive sputtering technique on the ITO/PC structures, and the resulting single electrode EC devices are characterized by chronoamperometric experiments and cyclic voltammetry. The fast switching response between colored and bleached states, together with the stability and reversibility of their electrochromic behavior after several cycling tests, are considered to be representative of the high quality of the EC film but especially of the ITO electrode. Indeed, even if no adhesion promoters, additional surface activation pretreatments, or substrate heating were used to promote the mechanical adhesion among the electrode and the PC surface, the observed EC response confirmed that the developed materials can be successfully employed for the fabrication of lightweight and inexpensive plastic EC devices.

Synthesis of ferroelectric BaTiO3 tube-like arrays by hydrothermal conversion of a vertically aligned TiO2 nanotube carpet

In this study, the conversion of TiO2 nanotubes (NTs) into barium titanate (BaTiO3) exploiting a two-step low-temperature synthesis method is reported. Vertically oriented TiO2 NT arrays were grown by ultra-fast anodic oxidation of titanium foils in ammonium fluoride-based electrolytic solution and then converted into BaTiO3 exploiting a shape-preserving hydrothermal treatment. A parametric study was carried out taking into account the effect of alkalinity, temperature and reaction time on the converted materials. The crystalline phase and morphology of the nanostructured material were investigated by means of X-ray diffraction, scanning and transmission electron microscopy, evidencing the formation of ordered polycrystalline BaTiO3 arrays. The tetragonal phase of the obtained material was revealed by Raman spectroscopy and its ferroelectric behavior was confirmed by domain switching observation during electrical characterization. The tube-like nanostructured arrays of BaTiO3 could open the way for new applications of energy harvesting materials where easy and low temperature processing, controlled morphology and functional properties are required.

Electric Characterization and Modeling of Microfluidic-Based Dye-Sensitized Solar Cell

The electric response to an external periodic voltage of small amplitude of dye-sensitized solar cells (DSCs) made up with an alternative architecture has been investigated. DSCs have been fabricated with a reversible sealing structure, based on microfluidic concepts, with a precise control on the geometric parameters of the active chamber. Cells with different electrolyte thicknesses have been characterized, without varying the thickness of the TiO2 layer, both under illumination and in dark conditions. Measurements of the electric impedance have been performed in the presence of an external bias ranging from 0 V to 0.8 V. The experimental data have been analyzed in terms of a transmission line model, with two transport channels. The results show that the photovoltaic performances of the microfluidic cell are comparable with those obtained in irreversibly sealed structures, actually demonstrating the reliability of the proposed device.