Stefano Polizzi

Photogenerated Defects in Shape-Controlled TiO2 Anatase Nanocrystals: A Probe To Evaluate the Role of Crystal Facets in Photocatalytic Processes

The promising properties of anatase TiO(2) nanocrystals exposing specific surfaces have been investigated in depth both theoretically and experimentally. However, a clear assessment of the role of the crystal faces in photocatalytic processes is still under debate. In order to clarify this issue, we have comprehensively explored the properties of the photogenerated defects and in particular their dependence on the exposed crystal faces in shape-controlled anatase. Nanocrystals were synthesized by solvothermal reaction of titanium butoxide in the presence of oleic acid and oleylamine as morphology-directing agents, and their photocatalytic performances were evaluated in the phenol mineralization in aqueous media, using O(2) as the oxidizing agent. The charge-trapping centers, Ti(3+), O(-), and O(2)(-), formed by UV irradiation of the catalyst were detected by electron spin resonance, and their abundance and reactivity were related to the exposed crystal faces and to the photoefficiency of the nanocrystals. In vacuum conditions, the concentration of trapped holes (O(-) centers) increases with increasing {001} surface area and photoactivity, while the amount of Ti(3+) centers increases with the specific surface area of {101} facets, and the highest value occurs for the sample with the worst photooxidative efficacy. These results suggest that {001} surfaces can be considered essentially as oxidation sites with a key role in the photoxidation, while {101} surfaces provide reductive sites which do not directly assist the oxidative processes. Photoexcitation experiments in O(2) atmosphere led to the formation of Ti(4+)-O(2)(-) oxidant species mainly located on {101} faces, confirming the indirect contribution of these surfaces to the photooxidative processes. Although this work focuses on the properties of TiO(2), we expect that the presented quantitative investigation may provide a new methodological tool for a more effective evaluation of the role of metal oxide crystal faces in photocatalytic processes.

Macroporous WO3 Thin Films Active in NH3 Sensing: Role of the Hosted Cr Isolated Centers and Pt Nanoclusters

Macroporous WO(3) films with inverted opal structure were synthesized by one-step procedure, which involves the self-assembly of the spherical templating agents and the simultaneous sol-gel condensation of the semiconductor alkoxide precursor. Transition metal doping, aimed to enhance the WO(3) electrical response, was carried out by including Cr(III) and Pt(IV) centers in the oxide matrix. It turned out that Cr remains as homogeneously dispersed Cr(III) centers inside the WO(3) host, while Pt undergoes reduction and aggregation to form nanoclusters located at the oxide surface. Upon interaction with NH(3), the electrical conductivity of transition metal doped-WO(3) increases, especially in the presence of Pt dopant, resulting in outstanding sensing properties (S = 110 ± 15 at T = 225 °C and [NH(3)] = 74 ppm). A mechanism was suggested to explain the excellent electrical response of Pt-doped films with respect to the Cr-doped ones. This associates the easy chemisorption of ammonia on the WO(3) nanocrystals, promoted by the inverted opal structure, with the catalytic action exerted by the surface Pt nanoclusters on the N-H bond dissociation. The overall results indicate that in Pt-doped WO(3) films the effects of the macroporosity positively combine with the electrical sensitization promoted by the metal nanoclusters, thus providing very lightweight materials which display high functionality even at relatively low temperatures. We expect that this synergistic effect can be exploited to realize other functional hierarchical metal oxide structures to be used as gas sensors or catalysts.

Layered Na0.71CoO2: a powerful candidate for viable and high performance Na-batteries

The present study reports on the synthesis and the electrochemical behavior of Na(0.71)CoO(2), a promising candidate as cathode for Na-based batteries. The material was obtained in two different morphologies by a double-step route, which is cheap and easy to scale up: the hydrothermal synthesis to produce Co(3)O(4) with tailored and nanometric morphology, followed by the solid-state reaction with NaOH, or alternatively with Na(2)CO(3), to promote Na intercalation. Both products are highly crystalline and have the P2-Na(0.71)CoO(2) crystal phase, but differ in the respective morphologies. The material obtained from Na(2)CO(3) have a narrow particle length (edge to edge) distribution and 2D platelet morphology, while those from NaOH exhibit large microcrystals, irregular in shape, with broad particle length distribution and undefined exposed surfaces. Electrochemical analysis shows the good performances of these materials as a positive electrode for Na-ion half cells. In particular, Na(0.71)CoO(2) thin microplatelets exhibit the best behavior with stable discharge specific capacities of 120 and 80 mAh g(-1) at 5 and 40 mA g(-1), respectively, in the range 2.0-3.9 V vs. Na(+)/Na. These outstanding properties make this material a promising candidate to construct viable and high-performance Na-based batteries.

Phosphate Diester and DNA Hydrolysis by a Multivalent, Nanoparticle-Based Catalyst

2-nm gold nanoclusters coated with Zn(II) complexes bearing auxiliary hydrogen bond donors act as multivalent catalysts capable of promoting the hydrolysis of model phosphate diesters with exceptional activity and inducing DNA double strand cleavage.

Nucleation and crystallization behavior of glass-ceramic materials in the Li2O-Al2O3-SiO2 system of interest for their transparency properties

Abstract We investigated the nucleation and crystallization behavior of multi-phase glass-ceramic materials (in the Li2O–Al2O3–SiO2 system to which low amounts of oxides such as TiO2, ZrO2, P2O5, BaO, Sb2O3 and ZnO were added), with interest in their transparency properties as glazes for industrial tiles with high scratch resistance. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used. XRD showed that the nucleating oxides produce, during the nucleation stage (at 953 K for 20 h), an orthorhombic ZrTiO4 phase, which maintains a similar crystallite size (about 4 nm) during the subsequent crystallization processes. We succeeded in determining the concentration of nuclei by using XRD data only. This quantity resulted close enough to the corresponding one, determined using the classical TEM method. The formation of each crystalline phase, developed during the crystallization stage, as a function of the thermal treatment, was quantitatively measured using the Rietveld method. In particular, the isothermal transformations at 1003 and 1023 K of the characteristic main phase, i.e., β-eucryptite s.s., which is richer in silica with respect to the relevant stoichiometric phase, into β-spodumene s.s., also richer in silica, were found to be consecutive solid state reactions. Moreover, this preliminary investigation aims at improving knowledge of the mechanisms leading to the development of transparent/opaque materials during the thermal treatment of Li2O–Al2O3–SiO2 glass-ceramics.

Lanthanide doped upconverting colloidal CaF2 nanoparticles prepared by a single-step hydrothermal method: toward efficient materials with near infrared-to-near infrared upconversion emission

Colloidal Er(3+)/Yb(3+), Tm(3+)/Yb(3+) and Ho(3+)/Yb(3+) doped CaF(2) nanoparticles have been prepared by a one-pot hydrothermal procedure and their upconversion properties have been investigated.

Shape-Controlled TiO2 Nanocrystals for Na-Ion Battery Electrodes: The Role of Different Exposed Crystal Facets on the Electrochemical Properties

Rechargeable sodium-ion batteries are becoming a viable alternative to lithium-based technology in energy storage strategies, due to the wide abundance of sodium raw material. In the past decade, this has generated a boom of research interest in such systems. Notwithstanding the large number of research papers concerning sodium-ion battery electrodes, the development of a low-cost, well-performing anode material remains the largest obstacle to overcome. Although the well-known anatase, one of the allotropic forms of natural TiO2, was recently proposed for such applications, the material generally suffers from reduced cyclability and limited power, due to kinetic drawbacks and to its poor charge transport properties. A systematic approach in the morphological tuning of the anatase nanocrystals is needed, to optimize its structural features toward the electrochemical properties and to promote the material interaction with the conductive network and the electrolyte. Aiming to face with these issues, we were able to obtain a fine tuning of the nanoparticle morphology and to expose the most favorable nanocrystal facets to the electrolyte and to the conductive wrapping agent (graphene), thus overcoming the intrinsic limits of anatase transport properties. The result is a TiO2-based composite electrode able to deliver an outstandingly stability over cycles (150 mA h g-1 for more than 600 cycles in the 1.5-0.1 V potential range) never achieved with such a low content of carbonaceous substrate (5%). Moreover, it has been demonstrated for the first time than these outstanding performances are not simply related to the overall surface area of the different morphologies but have to be directly related to the peculiar surface characteristics of the crystals.

Expeditious synthesis of water-soluble, monolayer-protected gold nanoparticles of controlled size and monolayer composition.

A protocol is reported for the preparation of water-soluble, thiol-protected Au nanoparticles (Au-MPC) where dioctylamine is used as a stabilizing agent when the gold cluster is formed using the two-phase Brust and Schiffrin procedure. The amount of amine controls the size of the nanoparticles in the 1.9-8.9 nm diameter range. The final stabilization of the gold clusters by addition of functionalized thiols is performed under very mild conditions compatible with most biomolecules. The procedure is suitable for a wide variety of functional groups present in the thiol and allows one to use thiol mixtures with a precise control of their composition in the monolayer. As a proof of principle, examples of nanoparticles protected with thiols comprising functional groups ranging from polyethers, saccharides, polyamines and ammonium ions are reported.

PEG-capped, lanthanide doped GdF3 nanoparticles: luminescent and T2 contrast agents for optical and MRI multimodal imaging

A facile method for the synthesis of water dispersible Er(3+)/Yb(3+) and Tm(3+)/Yb(3+) doped upconverting GdF(3) nanoparticles is reported. Strong upconversion emissions are observed in the red (for Er/Yb doped) and near-infrared (for Tm/Yb doped) regions upon laser excitation at 980 nm. The PEG coating ensures a good dispersion of the system in water and reduces the radiationless de-excitation of the excited states of the Er(3+) and Tm(3+) ions by water molecules. The r(2) relaxivity values are quite high with respect to the common T(2)-relaxing agents (22.6 ± 3.4 mM(-1) s(-1) and 15.8 ± 3.4 mM(-1) s(-1) for the Tm/Yb and Er/Yb doped samples, respectively), suggesting that the present NPs can be interesting as T(2) weighted contrast agents for proton MRI purpose. Preliminary experiments conducted in vitro, in stem cell cultures, and in vivo, after subcutaneous injection of the lanthanide-doped GdF(3) NPs, indicate scarce toxic effects. After an intravenous injection in mice, the GdF(3) NPs localize mainly in the liver. The present results indicate that the present Er(3+)/Yb(3+) and Tm(3+)/Yb(3+) doped GdF(3) NPs are suitable candidates to be efficiently used as bimodal probes for both in vitro and in vivo optical and magnetic resonance imaging.

Synthesis, characterization and properties of water-soluble gold nanoparticles with tunable core size

Robust, water-soluble gold clusters protected by monolayers of ligands containing a short alkyl chain (C7) close to the gold surface and a triethylene glycol monomethylether unit (TEG) to impart solubility in water and other polar solvents were prepared and characterized. Thiol 7 (N1-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}-8-sulfanyloctanamide) constitutes a good and versatile capping agent for the preparation of these nanoparticles. By tuning the Au/thiol ratio and sodium borohydride addition rate, nanoparticles with different core diameters ranging from 1.5 to 4.2 nm, as determined by TEM analysis, could be obtained. The size distribution of the gold cores appears to become broader as the Au/thiol ratio used to prepare the nanoparticles increases. Characterization of these nanoclusters also by NMR, UV-Vis and FTIR spectroscopies is reported. Solubility properties have been studied in a large variety of solvents and different solubility behaviors were observed for nanoparticles of different sizes. Exchange reactions were carried out successfully with small (1.9 nm) and large nanoparticles (4.2 nm) using dodecanethiol as the entering thiol. This demonstrates that these materials can be used for the preparation of nanoclusters with different functional groups soluble in polar solvents including water. The synthetic procedure described represents a facile route to tailoring the size and solubility properties of Au nanoparticles.