Synthesis and electrochemical investigation of cluster assembled carbon thin films

Nanostructured materials grown by the deposition of clusters with low kinetic energy are unique systems that preserve the original size dependent features of their building blocks. As porous materials with high surface to volume ratio they have a tremendous technological potential toward the development of green, cheap and efficient energy storage and harvesting systems, including carbon based supercapacitors. This thesis is devoted to the synthesis and electrochemical investigation of nanostructured carbon (ns-C) thin films and composites grown by the Supersonic Cluster Beam Deposition (SCBD) of clusters formed in a Pulsed Microplasma Cluster Source (PMCS). The electrochemical properties of cluster assembled thin films are assessed by the study of the electric double layer (EDL) formed at the interface between ns-C based electrodes and a liquid electrolyte. Ns-C behavior as electrode material has been characterized as function of thickness, post deposition thermal treatment, metal nanoparticles embedding and electrolyte type. This study is carried out by means of atomic force microscopy (AFM), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and electrochemical techniques, such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Our results establish the feasibility of the SCBD technique for the synthesis of nanostructured carbon and metal:carbon nanocomposite thin films with promising potential as porous material for thin film electrochemical energy storage

Immobilization of TiO2 nanopowders in multilayer fluorinated coatings for highly efficient clear and turbid wastewater remediation

The use of nanosized photocatalytic TiO2 in suspension or slurry type reactors is well reported. However, the industrial feasibility of such systems is limited, on account of the low quantum efficiency reported for slurry processes and of the need for a post-treatment catalyst recovery stage, which may present poor efficiency, with the double drawback of dispersing nanoparticles in the environment and losing precious photocatalyst. In this work, the photodegradative activity of titanium dioxide immobilized into a multilayered transparent fluoropolymeric matrix has been studied. Several TiO2 nanostructured powders featuring different primary particle size, crystalline phase and specific surface area have been produced by the flame spray pyrolysis (FSP) of organic solutions containing titanium (IV) isopropoxide. The activity of such powders has been tested and compared to that of TiO2 P25 by Evonik Degussa, which is the reference nanopowder commonly employed in the evaluation of the photocatalytic activity of non-commercial TiO2. A multilayer ionomeric-perfiuorinated matrix has been used to immobilize the powders, having high chemical resistance and transparency towards UV light, good permeability to oxygen and good wettability to favor interactions with the polluted aqueous solutions

WO3–TiO2 vs. TiO2 photocatalysts: effect of the W precursor and amount on the photocatalytic activity of mixed oxides

Aiming at producing TiO2-based photocatalytic materials with reduced charge carriers recombination, WO3\u2013TiO2 mixed oxides were synthesized by a sol\u2013gel method employing either an inorganic salt, Na2WO4, or an organic alkoxide, W(OC2H5)6, as tungsten precursor, with different W/Ti ratios. The so-obtained materials were characterized by XRPD, BET, UV\u2013vis reflectance, XPS and EDX analyses and their photoactivity was tested under UV\u2013visible irradiation in both the mineralization of formic acid in aqueous suspension and the gas phase oxidation of acetaldehyde. Both photoactivity results and photocurrent measurements point to a superior performance of photocatalysts obtained from the organic precursor with an optimal tungsten content (3%). The formation of an intimately mixed oxide, as revealed by XRPD analysis, results in photoactivity higher than that of pure TiO2, and also of benchmark P25 TiO2, consequent to a better charge separation due to the migration of photoproduced holes from WO3 domains to TiO2 and of photopromoted electrons in the opposite direction. The persistence of pure anatase phase in W-containing photocatalysts also after calcination at 700 \ub0C and their higher surface area with respect to pure TiO2 also contribute in increasing the photocatalytic activity of the WO3\u2013TiO2 mixed oxides

Hydrogen production by photocatalytic water splitting on TiO2 nanotubes

The photocatalytic activity of a series of TiO2 nanotube arrays obtained under different anodization conditions is discussed in relation to the morphology and phase composition changes occurring during anodization. TiO2 nanotubes of different lenghts were prepared by anodization in NH4F-H2O-formamide solutions and characterized mainly by SEM, XRD and photocurrent measurements. Their photoactivity in hydrogen production by water splitting was investigated by evaluating the amount of hydrogen evolved in a two compartment PEC cell for separate H2 and O2 production. Phase compsition and nanotubes length were strictly related to the anodization time. After the formation of a well grown porous morphology, an ordered tubular structure was produced, followed by the formation of a compact TiO2 layer on the nanotubes top at long anodization time. An increase of the anatase phase content occurred in parallel. During 60 s-long lightdark cycles, a fast raise and decay of photocurrent was observed with fully open nanotube arrays with well defined, regular architecture, wheres slower photocurrent raise and decay were observed in the presence of a compact top oxide surface layer. In all photocatalytic tests water photosplitting into hydrogen and oxygen occurred at constant rate, with no applied external bias. Highest photoactivity was achieved with ordered nanotube arrays obtained under optimized anodization conditions and composed of mixed anatase and rutile phases

Supersonic cluster beam printing of carbon microsupercapacitors on paper

Paper is a cheap, lightweight and renewable material with increasing applicative interest as a substrate for disposable and flexible electronics. The integration of planar energy storage devices on paper is a necessary and challenging step for the development of smart and autonomous flexible electronic platforms. Here we report the one-step, room temperature, fabrication of planar microsupercapacitors where nanostructured current collectors and carbon electrodes are deposited by supersonic cluster beam deposition (SCBD) on plain paper sheets and 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ([Emim][NTf2]) is used as ionic liquid electrolyte. As-prepared microsupercapacitors showed a high capacitance density of about 7 F cm 123 and good capacity retention upon prolonged cycling. The encapsulation of SCBD-made microsupercapacitors by means of a polydimethylsiloxane layer and their usability in driving a low power temperature sensor are demonstrate

Interfacial properties of a carbyne-rich nanostructured carbon thin film in ionic liquid

Nanostructured carbon sp2 (ns-C) thin films with up to 30% of sp-coordinated atoms (carbynes) were produced in a high vacuum by the low kinetic energy deposition of carbon clusters produced in the gas phase and accelerated by a supersonic expansion. Immediately after deposition the ns-C films were immersed in situ in an ionic liquid electrolyte. The interfacial properties of ns-C films in the ionic liquid electrolyte were characterized by electrochemical impedance spectroscopy and cyclic voltammetry (CV). The so-prepared carbyne-rich electrodes showed superior electric double layer (EDL) capacitance and electric conductivity compared to ns-C electrodes containing only sp2 carbon, showing the substantial influence of carbynes on the electrochemical properties of nanostructured carbon electrodes

H2 and O2 photocatalytic production on TiO2 nanotube arrays : effect of the anodization time on structural features and photoactivity

TiO2 nanotube (NT) arrays of different lengths were prepared by electrochemical anodization of ca. 10 cm2 area titanium disks in NH4F-H2O-formamide solution for different times, followed by annealing at 450\ub0C. After Pt deposition on the opposite side of the disk, the so obtained Ti-supported crystalline anodic oxides were employed as photoactive electrodes in a two compartments cell for separate H2 and O2 production through water photosplitting and characterized by SEM, XRD analysis and photocurrent measurements. The anodization time affected the phase composition and morphology of the growing NTs, which strictly influenced their photocatalytic activity. Short anodization times (40-60 min) resulted in well aligned short tubes composed of an anatase-rutile mixed phase. Longer anodization (> 2.5 h) yielded thicker NT arrays covered on top by a preferentially oriented anatase layer that limited their photoactivity. Photocurrent intensity measurements perfectly paralleled the water splitting activity results obtained with the different NT arrays. In particular, a square-shaped fast photoresponse was recorded with ordered and fully top-open nanotubular structures. On the other hand, clogged tubes not only yielded low current densities, but also showed delayed photocurrent transient signals due to the reduced mobility of the charge carriers within the preferentially oriented anatase layer. NT arrays obtained under optimized conditions had a ca. 80:20 anatase:rutile composition and ensured a hydrogen production rate of 83 mmol h-1 m-2 (i.e. 1.9 NL h-1 m-2) in the absence of any hole scavenger or external bias

Electrochemical impedance spectroscopy on nanostructured carbon electrodes grown by supersonic cluster beam deposition

Nanostructured porous films of carbon with density of about 0.5 g/cm3 and 200 nm thickness were deposited at room temperature by supersonic cluster beam deposition (SCBD) from carbon clusters formed in the gas phase. Carbon film surface topography, determined by atomic force microscopy, reveals a surface roughness of 16 nm and a granular morphology arising from the low kinetic energy ballistic deposition regime. The material is characterized by a highly disordered carbon structure with predominant sp2 hybridization as evidenced by Raman spectroscopy. The interface properties of nanostructured carbon electrodes were investigated by cyclic voltammetry and electrochemical impedance spectroscopy employing KOH 1 M solution as aqueous electrolyte. An increase of the double layer capacitance is observed when the electrodes are heat treated in air or when a nanostructured nickel layer deposited by SCBD on top of a sputter deposited film of the same metal is employed as a current collector instead of a plain metallic film. This enhancement is consistent with an improved charge injection in the active material and is ascribed to the modification of the electrical contact at the interface between the carbon and the metal current collector. Specific capacitance values up to 120 F/g have been measured for the electrodes with nanostructured metal/carbon interface

Immobilized TiO2 nanoparticles produced by flame spray for photocatalytic water remediation

Anatase/rutile mixed-phase titanium dioxide (TiO2) photocatalysts in the form of nanostructured powders with different primary particle size, specific surface area, and rutile content were produced from the gas-phase by flame spray pyrolysis (FSP) starting from an organic solution containing titanium (IV) isopropoxide as Ti precursor. Flame spray-produced TiO2 powders were characterized by means of X-ray diffraction, Raman spectroscopy, and BET measurements. As-prepared powders were mainly composed of anatase crystallites with size ranging from 7 to 15\ua0nm according to the synthesis conditions. TiO2 powders were embedded in a multilayered fluoropolymeric matrix to immobilize the nanoparticles into freestanding photocatalytic membranes. The photocatalytic activity of the TiO2-embedded membranes toward the abatement of hydrosoluble organic pollutants was evaluated employing the photodegradation of rhodamine B in aqueous solution as test reaction. The photoabatement rate of best performing membranes significantly overcomes that of membranes produced by the same method and incorporating commercial P25-TiO2

Highly functional titania nanoparticles produced by flame spray pyrolysis : photoelectrochemical and solar cell applications

Nanoparticulate titania was synthesized by flame spray pyrolysis and was used to construct photoanodes for photoelectrochemical cells and quantum dot sensitized solar cells. Powders obtained by flame spray pyrolysis were composed of smaller nanoparticles and had higher specific surface areas than common commercial types of titania and this was carried over to the structure of the photoanodes and reflected on the photoelectrocatalytic and solar cell behavior of the photoanodes. The highest specific surface area and the smallest nanoparticle size produced in this work were 249 m2g-1 and 7 nm, respectively. CdS-sensitized photoanodes were affected by the amount of the deposited sensitizer, which was the largest in the case of the powder with the highest specific surface area. When, however, the photoanodes were loaded with a relatively large amount of CdSe sensitizer, the role of the latter increased and the differences between the different forms of titania diminished