1,721,063 research outputs found
Size sorting of citrate reduced gold nanoparticles by sedimentation field-flow fractionation
No full textGold nanoparticles (GNPs) have been synthesized through the citrate reduction method; the citrate/gold(III) ratio was changed from 1:1 up to 10:1 and the size of the resulting nanoparticles was
measured by sedimentation field-flow fractionation (SdFFF). Experimental data showed that the GNPs size decreases in the ratio range 1:1–3:1 and then increases from 5:1 to 10:1 passing through a plateau region in between, and is almost independent of the precursor solution concentrations. In the zone of minimum diameters the synthetic process does not produce monodispersed GNPs but often multiple distributions, very close in size, are observed as evidenced by the particle size distributions (PSDs) derived from the SdFFF fractograms. UV–vis spectrophotometry, being the most common technique employed in the optical characterization of nanoparticles suspensions, was used throughout this work. A confirmation of the nucleation–aggregation–fragmentation mechanism was inferred from the cross-correlation between UV–vis and SdFFF results
WO3 colloids for photoelectrochemical uses studied through SdFFF and FlFFF.
No full textTungsten trioxide (WO3) is an indirect band gap semiconductor with interesting photoelectrochemical properties. Nanocrystalline WO3 thin films are used as high performance photoanodes, whose current–voltage behavior is strongly affected by the crystal size. For this reason the nanoparticle formation, evolution and aggregation are aspects which call for a carefully investigation.
In this work, colloidal suspensions of about 20-30 nm WO3 particles were obtained through a simple sol-gel. The WO3 colloidal suspensions were subjected to a controlled temperature aging process whose time evolution in terms of particle size distribution was followed by Sedimentation Field-Flow Fractionation (SdFFF) and Flow Field-Flow Fractionation (FlFFF). The experiments performed at 60 °C showed that in a few hours the initially transparent sol of WO3 particles undergo a progressive increase in mass, giving after 5 hours particles and/or aggregates of about 120 nm, expressed in terms of equivalent spherical diameter. The observed shift in particle size distribution SdFFF maxima, the broadening of the FlFFF fractograms and the SEM observations seem to suggest that primary WO3 nanocrystals formed during the initial stage tend to change their shape spontaneously becoming well-defined square particles which aggregate to form large clusters at a rate determined by the temperature set during the aging process.
The particle size distribution of the suspensions were related to the photoelectrochemical properties of the WO3 particles, preparing thin films on a transparent conductive glass from the aged suspensions sampled at regular intervals. The current-voltage polarization curves recorded in a biased photoelectrolytic cell in the potential range 0-1.8 V (vs SCE) showed a diminution of the maximum photocurrent from 3.7 mA cm-2 to 2.8 mA cm-2 with aging times of 1h and 5h respectively. This loss of performance was mainly attributed to the reduction of the electroactive surface area
Sedimentation field flow fractionation and flow field flow fractionation as tools for studying the aging effects of WO3 colloids for photoelectrochemical uses
No full textWO3 colloidal suspensions obtained through a simple sol-gel procedure were subjected to a controlled temperature aging process whose time evolution in terms of particle mass and size distribution was followed by Sedimentation Field Flow Fractionation (SdFFF) and Flow Field Flow Fractionation (FlFFF). The experiments performed at a temperature of 60 °C showed that in a few hours the initially transparent sol of WO3 particles, whose size was less than 25 nm, undergoes a progressive size increase allowing nanoparticles to reach a maximum equivalent spherical size of about 130 nm after 5 hours. The observed shift in particle size distribution maxima (SdFFF), the broadening of the curves (FlFFF) and the SEM-TEM observations suggest a mixed mechanism of growth-aggregation of initial nanocrystals to form larger particles. The photoelectrochemical properties of thin WO3 films obtained from the aged suspensions at regular intervals, were tested in a biased photoelectrocatalytic cell with 1M H2SO4 under solar simulated irradiation. The current-voltage polarization curves recorded in the potential range 0-1.8 V (vs SCE) showed a diminution of the maximum photocurrent from 3.7 mA cm-2 to 2.8 mA cm-2 with aging times of 1 h and 5 h respectively. This loss of performance was mainly attributed to the reduction of the electroactive surface area of the sintered particles as suggested by the satisfactory linear correlation between the integrated photocurrent and the cyclic voltammetry cathodic wave area of the W(VI) ->W(V) process measured in the dark
Sedimentation field flow fractionation and optical absorption spectroscopy for a quantitative size characterization of silver nanoparticles
No full textMany advanced industrial and biomedical applications that use silver nanoparticles (AgNPs), require that particles are not only nano-sized, but also well dispersed, not aggregated and not agglomerated. This study presents two methods able to give rapidly sizes of monodispersed AgNPs suspensions in the dimensional range of 20-100nm. The first method, based on the application of Mie's theory, determines the particle sizes from the values of the surface plasmon resonance wavelength (SPRMAX), read from the optical absorption spectra, recorded between 190nm and 800nm. The computed sizes were compared with those determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS) and resulted in agreement with the nominal values in a range between 13% (for 20nm NPs) and 1% (for 100nm NPs), The second method is based on the masterly combination of the Sedimentation Field Flow Fractionation (SdFFF - now sold as Centrifugal FFF-CFFF) and the Optical Absorption Spectroscopy (OAS) techniques to accomplish sizes and quantitative particle size distributions for monodispersed, non-aggregated AgNPs suspensions. The SdFFF separation abilities, well exploited to size NPs, greatly benefits from the application of Mie's theory to the UV-vis signal elaboration, producing quantitative mass-based particle size distributions, from which trusted number-sized particle size distributions can be derived. The silver mass distributions were verified and supported by detecting off-line the Ag concentration with the graphite furnace atomic absorption spectrometry (GF-AAS)
Photocatalysis with Na4W10O32 in water system: Formation and reactivity of OH center dot radicals
No full textPhotoexcitation of Na4W10O32 dissolved in water leads to the formation of hydroxyl radicals. These species originate both from H2O oxidation and H2O2 reduction. EPR-spin trapping investigation and laser flash photolysis experiments contribute to clarify their formation mechanism and their involvement in the oxidation of propan-2-ol. Continuous irradiation of water solutions of Na4W 10O32 leads to the overoxidation of propan-2-ol to CO 2 with high yield. This result may be of interest for the development of photocatalytic systems aimed to pollutants degradation. Entrapment of Na4W10O32 into a microporous silica matrix gives a rather robust photocatalyst where the decatungstate structure is preserved. This material is able to catalyze the photooxidation of propan-2-ol to acetone with appreciable chemoselectivity. In particular, it is seen that the solid photocatalyst yields acetone as main product inhibiting its over-oxidation to carbon dioxide. © 2013 Elsevier B.V
Polynuclear metal complexes as supramolecular species. intercomponent energy- and electron-transfer processes in homogeneous and heterogeneous phases
No full textA series of cyano-bridged polynuclear complexes have been prepared in the context of intramolecular energy and
electron-transfer studies. Experimental results are discussed to emphasize that (i) a valence localized description is generally
appropriate for such complexes, (ii) time-resolved resonance Raman and infrared spectroscopies can be successfully applied to the study of the excited-state intercomponent processes, (iii) polynuclear species capable of supporting efficient intramolecular energy transfer can be used for the visible sensitization of wide-band gap semiconductors
Size sorting of citrate reduced gold nanoparticles by SdFFF
No full textIn the past few decades, gold nanoparticles (GNPs) synthesis has attracted a considerable interest in the field of nanoscience because of their remarkable physical and chemical properties. Useful properties can be incorporated into the design of GNPs for manipulation or detection of biological structures and systems. The new materials which derive from the combination of GNPs and biological molecules have great potential applications in the area of electronics, optics as well as in genomics, proteomics. GNPs are currently used in the fields of catalysis, nanophotonics, and gene and drug delivery.
In the present study, the preparation of GNPs with the citrate reduction method has been investigated in the citrate/gold(III) ratio 1:1 up to 10:1 and the size of the resulting nanoparticles was measured by Sedimentation Field-Flow Fractionation (SdFFF). Experimental data, in agreement with those presented in a previous study [1], showed that the GNPs size decreases in the citrate/gold(III) ratio range 1:1 to 3:1 and then increases from 5:1 to 10:1 passing through a plateau region in between, and is almost independent of the precursor solutions concentrations. In the zone of minimum diameters the synthetic process does not produce monodispersed GNPs but often multiple distributions, very close in size, are observed as evidenced by the particle size distributions (PSD) derived from the SdFFF fractograms. It has been also proven that GNPs suspensions tend to agglomerate over the time and the agglomerates cannot be destroyed by a mechanical action. UV-Vis spectrophotometry was used throughout this work being the most common technique employed in the optical characterization of nanoparticles suspensions and SEM has been used to check data consistency. A confirmation of the nucleation-aggregation-fragmentation mechanism was inferred from the cross-correlation between UV-Vis and SdFFF results
A METHOD FOR PREPARING NANOCRYSTALLINE TRANSPARENT FILMS OF TUNGSTEN OXIDE.
No full textThe object of the present invention is the preparation, through a sol-gel method, of a tungsten oxide (WO3)-based colloidal paste that allows providing transparent films on conductive glasses in an easy and cost-effective manner. Particularly, the present invention relates to a method for preparing a substrate coated with at least one thick layer of tungsten oxide, comprising at least one step of coating said substrate with a colloidal solution of tungsten oxide obtained with the sol-gel method, said colloidal solution being additioned with a thickener and a surfactant
Solvatochromic Dye Sensitized Nanocrystalline Solar Cells
No full textThe coordination compound TBA(4)[Ru(CN)(4)(dcb)], where TBA is tetrabutylammonium and dcb is 4,4'-(CO2-)(2)-2,2'-bipyridine, was synthesized and attached to optically transparent nanocrystalline (anatase) TiO2 films, abbreviated [Ru(CN)(4)(dcb)]/TiO2. The metal-to-ligand-charge-transfer (MLCT) absorption and emission bands were found to shift in wavelength with solvent. The absorption maximum of the low energy MLCT band was observed at 517 nm in acetonitrile and 535 nm in dimethylformamide for TBA(4)[Ru(CN)(4)(dcb)] and at 470 and 500 for [Ru(CN)(4)(dcb)]/TiO2, respectively. Pulsed light excitation of TBA(4)[Ru(CN)(4)(dcb)] in acetonitrile produces a long-lived emissive MLCT excited state, tau = 30 ns. Pulsed light excitation of [Ru(CN)(4)(dcb)]/TiO2 yields an absorption difference spectrum attributed to an interfacial charge separated state, [Ru-III(CN)(4)(dcb)]/TiO2(e(-)). This state forms within 10 ns and returns cleanly to ground-state product within milliseconds. Regenerative solar cells based on [Ru(CN)(4)(dcb)]/TiO2 were prepared whose spectral sensitivity and efficiency were a function of the solvent used with 0.5 M Lil and 0.05 M I-2. The maximum incident photon-to-current efficiency (IPCE) was measured at 480 nm (25%) in acetonitrile and 510 nm (5%) in dimethylformamide. This work reports a new approach for controlling the spectral sensitivity of solar cells and for probing the solvation of molecules anchored to nanocrystalline semiconductor surfaces
- …
