Micaela Castellino

Effects of Functionalization on Thermal Properties of Single-Wall and Multi-Wall Carbon Nanotube–Polymer Nanocomposites

Carboxylic functionalization (-COOH groups) of carbon nanotubes is known to improve their dispersion properties and increase the electrical conductivity of carbon-nanotube-polymer nanocomposites. We have studied experimentally the effects of this type of functionalization on the thermal conductivity of the nanocomposites. It was found that while even small quantities of carbon nanotubes (~1 wt %) can increase the electrical conductivity, a larger loading fraction (~3 wt %) is required to enhance the thermal conductivity of nanocomposites. Functionalized multi-wall carbon nanotubes performed the best as filler material leading to a simultaneous improvement of the electrical and thermal properties of the composites. Functionalization of the single-wall carbon nanotubes reduced the thermal conductivity enhancement. The observed trends were explained by the fact that while surface functionalization increases the coupling between carbon nanotube and polymer matrix, it also leads to formation of defects, which impede the acoustic phonon transport in the single-wall carbon nanotubes. The obtained results are important for applications of carbon nanotubes and graphene flakes as fillers for improving thermal, electrical and mechanical properties of composites.

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.

Microwave-Assisted Synthesis of Reduced Graphene Oxide/SnO2 Nanocomposite for Oxygen Reduction Reaction in Microbial Fuel Cells

We report on an easy, fast, eco-friendly, and reliable method for the synthesis of reduced graphene oxide/SnO2 nanocomposite as cathode material for application in microbial fuel cells (MFCs). The material was prepared starting from graphene oxide that has been reduced to graphene during the hydrothermal synthesis of the nanocomposite, carried out in a microwave system. Structural and morphological characterizations evidenced the formation of nanocomposite sheets, with SnO2 crystals of few nanometers integrated in the graphene matrix. Physico-chemical analysis revealed the formation of SnO2 nanoparticles, as well as the functionalization of the graphene by the presence of nitrogen atoms. Electrochemical characterizations put in evidence the ability of such composite to exploit a cocatalysis mechanism for the oxygen reduction reaction, provided by the presence of both SnO2 and nitrogen. In addition, the novel composite catalyst was successfully employed as cathode in seawater-based MFCs, giving electrical performances comparable to those of reference devices employing Pt as catalyst.

Nanobranched ZnO Structure: p‐Type Doping Induces Piezoelectric Voltage Generation and Ferroelectric–Photovoltaic Effect

Dr. M. Laurenti, Dr. G. Canavese, Dr. A. Sacco, Dr. M. Fontana, Dr. K. Bejtka, Dr. M. Castellino, Prof. C. F. Pirri, Dr. V. Cauda Center for Space Human Robotics@PoliTo Istituto Italiano di Tecnologia C.so Trento 21 , 10129 Turin , Italy E-mail: valentina.cauda@iit.it Dr. G. Canavese, Dr. M. Fontana, Prof. C. F. Pirri Department of Applied Science and Technology Politecnico di Torino C.so Duca degli Abruzzi 24 , 10129 Turin , Italy

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.

Ionic liquid-enhanced soft resistive switching devices

Resistive switching phenomena are of paramount importance in the area of memory devices. In the present study, we have fabricated a simple resistive switching device using a solution processable nanocomposite based on silver nitrate and poly(vinylidene fluoride-hexafluoropropylene). The change in resistance is ascribed to an initial ionic conduction, followed by a non-continuous field induced filament formation. The switching device fabricated with the above-mentioned active matrix displayed a volatile switching behavior. The addition of room temperature ionic liquid plays a fundamental role in triggering permanent memory and reducing the set voltage range up to ten-fold. The change in switching behavior with respect to the applied voltage bias and compliance level set during electrical characterization was studied thoroughly. The present work also gives a glimpse into the importance of device architecture on resistive switching phenomena.

Study of carbon nanotubes based Polydimethylsiloxane composite films

Thanks to their remarkable characteristics, carbon nanotubes (CNTs) have fields of applications which are growing every day. Among them, the use of CNTs as filler for polymers is one of the most promising. In this work we report on Polydimethylsiloxane (PDMS) composites with different weight percentages (0.0% to 3.0%) of multiwall carbon nanotubes (MWCNTs) having diameter 10?30 nm and length 20?30 ?m. To achieve optimum dispersion of CNTs in PDMS matrix, high speed mechanical stirring and ultrasonication were performed. By using the doctor blade technique, 70 ?m thick uniform films were produced on glass. They were subsequently thermally cured and detached from the glass to get flexible and self standing films. The surface morphological study done by FESEM, shows that CNTs are well dispersed in the PDMS. Raman spectroscopy and FTIR were used to investigate the possible structural changes in the polymer composite. To examine the optical behavior UV-VIS spectroscopy was employed in both specular and diffused modes. A linear increase in absorption coefficient is found with the increasing percentage of CNTs while the transmittance decreases exponentially. The results confirm the dependence of optical limiting effect on the quantity of MWCNTs. Based on optical study, MWCNTs/PDMS composite films can be a promising material to extend performances of optical limiters against laser pulses, which is often required in lasing systems.

Optimization and characterization of a homogeneous carboxylic surface functionalization for silicon-based biosensing

A well-organized immobilization of bio-receptors is a crucial goal in biosensing, especially to achieve high reproducibility, sensitivity and specificity. These requirements are usually attained with a controlled chemical/biochemical functionalization that creates a stable layer on a sensor surface. In this work, a chemical modification protocol for silicon-based surfaces to be applied in biosensing devices is presented. An anhydrous silanization step through 3-aminopropylsilane (APTES), followed by a further derivatization with succinic anhydride (SA), is optimized to generate an ordered flat layer of carboxylic groups. The properties of APTES/SA modified surface were compared with a functionalization in which glutaraldehyde (GA) is used as crosslinker instead of SA, in order to have a comparison with an established and largely applied procedure. Moreover, a functionalization based on the controlled deposition of a plasma polymerized acrylic acid (PPAA) thin film was used as a reference for carboxylic reactivity. Advantages and drawbacks of the considered methods are highlighted, through physico-chemical characterizations (OCA, XPS, and AFM) and by means of a functional Protein G/Antibody immunoassay. These analyses reveal that the most homogeneous, reproducible and active surface is achieved by using the optimized APTES/SA coupling.

Self-standing polymer-functionalized reduced graphene oxide papers obtained via a UV-process

Graphene based materials are attracting great attention every day due to their outstanding properties. Widening their potentialities through synergic effects in conjunction with other materials represents an intriguing challenge in order to obtain lighter and multi-functional composites. In this paper, novel self-standing graphene-based paper-like sheets are investigated, obtained via a facile dual step UV-induced process. This method, employing graphene oxide as a starting material, allows the obtaining of polymeric functionalized reduced graphene oxide papers that could be easily handled, featuring improved mechanical and peculiar electrical properties. The mechanical and thermal properties were investigated as well as their electrical response under different stimuli, such as temperature and humidity, showing remarkable changes.

Electrical Properties of CNT-Based Polymeric Matrix Nanocomposites

NanoComposites (NCs) are a class of materials widely investigated in the last decade. The term NC material has broadened significantly to encompass a large variety of systems such as one-dimensional, two-dimensional, three-dimensional and amorphous materials, made of distinctly dissimilar components and mixed at the nanometer scale. The general class of organic/inorganic NC materials is a fast growing area of research. The properties of NCs depend not only on the properties of their individual elements but also on their morphology and interfacial characteristics. Large interface area between the matrix and the nano filler is a key issue for NCs. NCs’ fillers include Carbon nanomaterials. They are a large family of materials carbon based that include: fibers, nanotubes, fullerene, nano-diamonds, etc. Carbon Nanotubes (CNTs) are one of the most popular and intensively studied Carbon nanomaterials. Since their discovery in 1991 by Iijima (Iijima, 1991), they have attracted great interest as an innovative material in most fields of science and engineering. CNTs can be thought of as sheets of graphite rolled up to make a tube. They are divided in two large classes: Multi wall CNTs (MWCNTs) and Single wall CNTs (SWCNTs). This classification depends on the number of graphite walls: several in the case of MWCNTs and only one for SWCNTs. The dimensions are variable, from few nanometers for SWCNTs to tenths of nanometers for MWCNTs. CNTs have outstanding mechanical, thermal and electrical properties. For example as shown by Collins et al. (Collins et al., 2000) CNTs have electrical properties and electric-current-carrying capacity 1000 times higher than a copper wire. Young’s module for a single-walled carbon nanotube (SWCNTs) has been estimated by Yu et al. (Yu et al., 2000) in a range of 0.32-1.47 TPa and strengths between 10 and 52 GPa with a toughness of ~ 770 Jg-1. A room-temperature thermal conductivity of 1750–5800 Wm-1K-1 has been estimated by Hone et al. (Hone et al., 1999). These remarkable properties make them excellent candidates for a range of possible new classes of materials. Several studies demonstrate how just a small percentage of CNTs loading can improve the material properties (Breuer, 2004), while still maintaining the plasticity of the polymers.