We describe a new approach to making ultrathin Ag nanoshells with a higher level of extinction in the infrared than in the visible. The combination of near-infrared active ultrathin nanoshells with ...their isotropic optical properties is of interest for energy-saving applications. For such applications, the morphology must be precisely controlled, since the optical response is sensitive to nanometer-scale variations. To achieve this precision, we use a multi-step, reproducible, colloidal chemical synthesis. It includes the reduction of Tollens' reactant onto Sn
-sensitized silica particles, followed by silver-nitrate reduction by formaldehyde and ammonia. The smooth shells are about 10 nm thick, on average, and have different morphologies: continuous, percolated, and patchy, depending on the quantity of the silver nitrate used. The shell-formation mechanism, studied by optical spectroscopy and high-resolution microscopy, seems to consist of two steps: the formation of very thin and flat patches, followed by their guided regrowth around the silica particle, which is favored by a high reaction rate. The optical and thermal properties of the core-shell particles, embedded in a transparent poly(vinylpyrrolidone) film on a glass substrate, were also investigated. We found that the Ag-nanoshell films can convert 30% of the power of incident near-infrared light into heat, making them very suitable in window glazing for radiative screening from solar light.
The mechanism of phosphorescence in SrAl2O4:Eu,Dy material is discussed on the basis of a thermo-activated release of electrons trapped at oxygen vacancy sites. The depths and concentration of the ...electron traps are strongly affected by codoping with trivalent lanthanide cations Ln3+. This finding is rationalized by comparing the ionization potentials of the dopant cation Eu2+, the codopant cation Ln3+ and the substituted cation Sr2+. Such a use of ionization potentials provides a practical guide with which the variation of the phosphorescent decay time in the codoped derivatives of SrAl2O4:Eu is predicted.
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•Reliable and reproducible chemical preparation of AgBi2I7 semiconductor layers.•Control of surface morphology and roughness.•Integration of the layers in perovskite-like solar ...cells.•The rough layers improve the charge extraction from the iodobismuthate absorber layer.
Silver iodobismuthates are semiconductor compounds potentially interesting for optoelectronics applications. In this work, a strategy is presented for the preparation by spin coating and at low temperature of well-crystallized, well-covering thin films with a AgBi2I7 global composition. The films are shown of high structural quality and the layer morphology has been controlled to produce either smooth or rough layers of similar thickness. We show that controlling the layer morphology and its interface with a hole transporting layer (Spiro-OMeTAD) plays an important role for the charge extraction and then the performances of devices. These films have been optimized and yielded perovskite -like solar cells with a maximum power conversion efficiency of 0.83%.
Octahedral molecular sieves (OMS) are built of transition metal-oxygen octahedra that delimit sub-nanoscale cavities. Compared to other microporous solids, OMS exhibit larger versatility in ...properties, provided by various redox states and magnetic behaviors of transition metals. Hence, OMS offer opportunities in electrochemical energy harnessing devices, including batteries, electrochemical capacitors and electrochromic systems, provided two conditions are met: fast exchange of ions in the micropores and stability upon exchange. Here we unveil a novel OMS hexagonal polymorph of tungsten oxide called h'-WO
, built of (WO
)
tunnel cavities. h'-WO
is prepared by a one-step soft chemistry aqueous route leading to the hydrogen bronze h'-H
WO
. Gentle heating results in h'-WO
with framework retention. The material exhibits an unusual combination of 1-dimensional crystal structure and 2-dimensional nanostructure that enhances and fastens proton (de)insertion for stable electrochromic devices. This discovery paves the way to a new family of mixed valence functional materials with tunable behaviors.
Ceria-type materials were investigated as reactive chemical intermediates, in view of solar thermochemical hydrogen production via two-step water-splitting. Ceria/zirconia mixed oxides and ceria ...doped with yttrium, lanthanum, praseodymium, or gadolinium were studied using a thermobalance to evaluate their thermal reduction capacity in inert atmosphere and their subsequent reactivity with water steam to generate hydrogen. Ceria/zirconia materials present the highest reduction yields with a noticeable linear increase as a function of the zirconium content (in the range 0–54% Zr), while the gravimetric amount of O2 released during reduction tends to level off for Zr atomic contents above 25%. Temperature-programmed reduction experiments demonstrate that the zirconium insertion favors the bulk reduction. The addition of different dopants (among Y, La, Pr, and Gd) did not affect the global materials reducibility, although it should favor the material thermal stability during repeated cycles. The marked effect of the synthesis method of the material and of the temperature of the reduction reaction on the reactivity of ceria/zirconia was highlighted. In addition, the beneficial influence of decreasing the system total pressure for improving the thermal reduction of ceria/zirconia was experimentally evidenced, offering new prospects for operating a solar thermochemical reactor.
Silver iodobismuthates are semiconductor compounds potentially interesting for optoelectronics applications. In this work, a strategy is presented for the preparation by spin coating and at low ...temperature of well-crystallized, well-covering thin films with a AgBi2I7 global composition. The films are shown of high structural quality and the layer morphology has been controlled to produce either smooth or rough layers of similar thickness. We show that controlling the layer morphology and its interface with a hole transporting layer (Spiro-OMeTAD) plays an important role for the charge extraction and then the performances of devices. These films have been optimized and yielded perovskite -like solar cells with a maximum power conversion efficiency of 0.83%.
We report the synthesis of colloidal EuS, La2S3, and LaS2 nanocrystals between 150 and 255 °C using rare-earth iodides in oleylamine. The sulfur source dictates phase selection between La2S3 and ...LaS2, which are stabilized for the first time as colloidal nanocrystals. The indirect bandgap absorption of LaS2 shifts from 635 nm for nanoellipsoids to 365 nm for square-based nanoplates. Er3+ photoluminescence in La2S3:Er3+ (10%) is sensitized by the semiconducting host in the 390–450 nm range. The synthetic route yields tunable compositions of rare-earth sulfide nanocrystals. Interaction of light with these novel semiconducting nanostructures hosting rare-earth emitters should be attractive for applications that require broadband sensitization of RE emitters.
The existing mechanisms proposed to explain the phosphorescence of SrAl2O4:Eu2+,Dy3+ and related phosphors were found to be inconsistent with a number of important experimental and theoretical ...observations. We formulated a new mechanism of phosphorescence on the basis of the facts that the d orbitals of Eu2+ are located near the conduction band bottom of SrAl2O4, that the Eu2+ concentration decreases during UV excitation, and that trace amounts of Eu3+ are always present in these phosphors. In our mechanism, some Eu2+ ions are oxidized to Eu3+ under UV, and the released electrons are trapped at the oxygen vacancy levels located in the vicinity of the photogenerated Eu3+ cations. The phosphorescence arises from the recombination of these trapped electrons around the photogenerated Eu3+ sites with emission at 520 nm. The codopant Dy3+ enhances the phosphorescence by increasing the number and the depth of electron traps, and the codopant B3+ enhances the phosphorescence by increasing the depth of electron traps. We also probed the origin of another emission at 450 nm of SrAl2O4:Eu2+ that occurs at low temperatures. Our analysis indicates that this emission is caused by a charge transfer from oxygen to Eu3+ cations and is associated with a hole trapping.
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