Rapid laser-induced low temperature crystallization of thermochromic VO2 sol-gel thin films

The thermochromic properties of vanadium dioxide (VO2) offer great advantages for energy-saving smart windows, memory devices, and transistors. However, the crystallization of solution-based thin films at temperatures lower than 400°C remains a challenge. Photonic annealing has recently been exploited to crystallize metal oxides, with minimal thermal damage to the substrate and reduced manufacturing time. Here, VO2 thin films, obtained via a green sol–gel process, were crystallized by pulsed excimer laser annealing. The influence of increasing laser fluence and pulse number on the film properties was systematically studied through optical, structural, morphological, and chemical characterizations. From temperature profile simulations, the temperature rise was confirmed to be confined within the film during the laser pulses, with negligible substrate heating. Threshold laser parameters to induce VO2 crystallization without surface melting were found. With respect to furnace annealing, both the crystallization temperature and the annealing time were substantially reduced, with VO2 crystallization being achieved within only 60 s of laser exposure. The laser processing was performed at room temperature in air, without the need of a controlled atmosphere. The thermochromic properties of the lasered thin films were comparable with the reference furnace-treated samples

Artificial photosynthesis: photoanodes based on polyquinoid dyes onto mesoporous tin oxide surface

Dye-sensitized photoelectrochemical cells represent an appealing solution for artificial photosynthesis, aimed at the conversion of solar light into fuels or commodity chemicals. Extensive efforts have been directed towards the development of photoelectrodes combining semiconductor materials and organic dyes; the use of molecular components allows to tune the absorption and redox properties of the material. Recently, we have reported the use of a class of pentacyclic quinoid organic dyes (KuQuinone) chemisorbed onto semiconducting tin oxide as photoanodes for water oxidation. In this work, we investigate the effect of the SnO2 semiconductor thickness and morphology and of the dye-anchoring group on the photoelectrochemical performance of the electrodes. The optimized materials are mesoporous SnO2 layers with 2.5 mu m film thickness combined with a KuQuinone dye with a 3-carboxylpropyl-anchoring chain: these electrodes achieve light-harvesting efficiency of 93% at the maximum absorption wavelength of 533 nm, and photocurrent density J up to 350 mu A/cm(2) in the photoelectrochemical oxidation of ascorbate, although with a limited incident photon-to-current efficiency of 0.075%. Calculations based on the density functional theory (DFT) support the role of the reduced species of the KuQuinone dye via a proton-coupled electron transfer as the competent species involved in the electron transfer to the tin oxide semiconductor. Finally, a preliminary investigation of the photoelectrodes towards benzyl alcohol oxidation is presented, achieving photocurrent density up to 90 mu A/cm(2) in acetonitrile in the presence of N-hydroxysuccinimide and pyridine as redox mediator and base, respectively. These results support the possibility of using molecular-based materials in synthetic photoelectrochemistry.[GRAPHICS]

Waste-derived glass as a precursor for inorganic polymers: From foams to photocatalytic destructors for dye removal

Synthetic alumino-silicate glasses may yield inorganic polymers, through activation with alkali hydroxide solutions. In this framework, we formulated a glass prepared by the melting of red mud from bauxite refinement, combined with coal combustion fly ash, discarded pharmaceutical glass and a minor addition of sodium carbonate. The activation with 6 M NaOH aqueous solution allowed for the manufacturing of highly porous foams, by gas generation at the early stages of gelation. These foams featured an extensive formation of zeolite at cell walls which, combined with the presence of magnetite formed upon cooling of the melt, favoured the application of the foams as sorbents for dye removal from contaminated water. The powders prepared by crushing the highly porous foams showed an excellent water purification ability documented by efficient removal of methylene blue used as a model contaminant. The specific iron oxide polymorph facilitated both magnetic recovery of dispersed powders and photocatalytic destruction of the dye under UV irradiation

Photoluminescence properties of silk–carbon quantum dots composites

In this paper, we report silk fibroin (SF) and carbon quantum dots (CQDs) nanocomposites obtained through a facile solution casting approach. The optical properties of the nanocomposites have been characterised by UV–vis absorption and photoluminescence spectroscopy. Crosslinking of SF and chemical interactions with the CQDs have been investigated by FTIR spectroscopy. In addition, water stability and degradability of the prepared composites have been investigated in terms of mass loss, important for applications in a real scenario. We observed that for a concentration of CQDs above 1%wt aggregation of nanoparticles occurs, affecting the photoluminescence of the material. The results show that the best composition in terms of photoluminescence intensity and water stability is 0.5%wt CQDs. [Figure not available: see fulltext.

Nanomechanical and tribological characterization of silk and silk-titanate composite coatings

This paper investigates the tribological and mechanical properties of silk-based nanocomposite coatings which are finding applications in optics, biomedicine and dentistry, thanks to the exceptional mechanical/optical properties and associated biocompatibility of silk. Three different nanocomposite formulations were synthesized, and thin films were prepared by spin coating at different thicknesses and with different post-deposition annealing processes. Ellipsometry, FTIR spectroscopy, AFM, nanoindentation, scratch testing, continuous/reciprocating wear testing, confocal microscopy and SEM were used to characterize the coatings. The results reveal that their hardness and elastic modulus are in the range 0.561.30 GPa and 23.655.4 GPa, respectively, which are much higher than those reported for other silk films in literature. Incorporation of titanate nanosheets also improved coatings scratch resistance

Enhancing Tungsten Oxide Gasochromism with Noble Metal Nanoparticles: The Importance of the Interface

Crystalline tungsten trioxide (WO3) thin films covered by noble metal (gold and platinum) nanoparticles are synthesized via wet chemistry and used as optical sensors for gaseous hydrogen. Sensing performances are strongly influenced by the catalyst used, with platinum (Pt) resulting as best. Surprisingly, it is found that gold (Au) can provide remarkable sensing activity that tuned out to be strongly dependent on the nanoparticle size: devices sensitized with smaller nanoparticles display better H-2 sensing performance. Computational insight based on density functional theory calculations suggested that this can be related to processes occurring specifically at the Au nanoparticle-WO3 interface (whose extent is in fact dependent on the nanoparticle size), where the hydrogen dissociative adsorption turns out to be possible. While both experiments and calculations single out Pt as better than Au for sensing, the present work reveals how an exquisitely nanoscopic effect can yield unexpected sensing performance for Au on WO3, and how these performances can be tuned by controlling the nanoscale features of the system

Nanomechanical and tribological characterization of silk and silk-titanate composite coatings

This paper investigates the tribological and mechanical properties of silk-based nanocomposite coatings which are finding applications in optics, biomedicine and dentistry, thanks to the exceptional mechanical/optical properties and associated biocompatibility of silk. Three different nanocomposite formulations were synthesized, and thin films were prepared by spin coating at different thicknesses and with different post-deposition annealing processes. Ellipsometry, FTIR spectroscopy, AFM, nanoindentation, scratch testing, continuous/reciprocating wear testing, confocal microscopy and SEM were used to characterize the coatings. The results reveal that their hardness and elastic modulus are in the range 0.56\u20131.30 GPa and 23.6\u201355.4 GPa, respectively, which are much higher than those reported for other silk films in literature. Incorporation of titanate nanosheets also improved coatings\u2019 scratch resistance

Designing the Iridescences of Biopolymers by Assembly of Photonic Crystal Superlattices

Structural color in naturally occurring systems is generally constituted by nanoscale lattices of biopolymers that generate beautiful iridescences through their regular structures, often interspersed with defects or period mismatch. Taking inspiration from both formats and materials found in Nature, a series of large-scale, highly reflective biopolymer-based photonic crystal superlattices constituted by stacking layers of 3D nanoscale lattices with different periodicity is presented. These silk photonic crystal superlattices (SPCSs) are fully composed of naturally derived structural proteins (silk fibroin) and exhibit brilliant structural color while being mechanically flexible. Multi-stopbands over broad wavelength ranges or single-stopbands with narrowband spectral responses can be readily realized and precisely controlled by manipulating the hierarchy of the lattice stacks or the repetition periods of the assembled colloidal monolayers. The unique ability to vary the silk protein conformation allows to vary the lattice and controllably \u201cdesign\u201d the iridescences of the SPCSs with water vapor adding versatility to this biopolymer-based photonic structure

Dropwise condensation mechanisms when varying vapor velocity

The promotion of dropwise condensation (DWC) has been identified as an effective strategy to significantly improve the heat transfer coefficient (HTC) as compared to filmwise condensation (FWC). Understanding the mechanisms governing dropwise condensation on modified wettability surfaces is crucial for a wide range of energy applications. In the literature, most of the experimental data are collected during DWC with quiescent vapor. On the other hand, in industrial applications, the vapor to be condensed can have a non-negligible velocity which is expected to affect the droplet population on the condensing surface and the heat transfer. However, measurements of heat transfer coefficient and droplet population with flowing steam are rare and the effect of vapor velocity on the drop-size distribution, which is a key parameter in DWC modeling, needs to be investigated. In the present work, the effect of steam velocity during DWC is experimentally studied on two different specimens: a sol–gel coated aluminum sample and a reduced graphene oxide coated copper sample. HTC, droplet departing radius and drop-size distribution measurements are performed at constant saturation temperature and heat flux, while varying the inlet vapor velocity in the range between 3 and 15.5 m s-1. Due to the increase of the vapor drag force on the droplets, a reduction of the droplet departing radius is observed along with an increase of the condensation HTC. The vapor flow is found to affect the droplet population and, in particular for the largest droplets radii, the drop-size distribution function is lowered when increasing vapor velocity. The experimental data are used to assess a model for the estimation of the droplet departing radius in presence of vapor velocity previously proposed by the present authors. As a second step, the equation for the calculation of the droplet departing radius is coupled with available models for droplet population and heat transfer through a single droplet to model the whole DWC process. The proposed calculation method is able to predict the effect of vapor velocity on the DWC heat transfer coefficient