On the Oxidation State of Cu2 O upon Electrochemical CO2 Reduction: An XPS Study.

The encouraging selectivity of copper oxides for the electroreduction of CO2 into ethylene and alcohols has led to a vivid debate on the possible relation between their operando (sub-)surface oxidation state (i. e. fully reduced or partially oxidized) and this distinct reactivity. The high roughness of the Cu oxides used in previous studies on this matter adds complexity to this controversy and motivated us to prepare quasi-planar Cu2 O thin films that displayed a CO2 reduction selectivity similar to that of oxide-derived copper catalysts reported in previous studies. Most importantly, when the post-mortem thin films were transferred for characterization in an air-free environment, X-ray photoelectron spectroscopy measurements confirmed their complete reduction in the course of the CO2 reduction reaction. Thus, our results indicate that the selectivity of the Cu oxides featured in previous studies stems from their enhanced roughness, highlighting the importance of controlled sample transfer upon post-mortem characterization with ex situ techniques.

Etched colloidal LiFePO4 nanoplatelets toward high-rate capable Li-ion battery electrodes.

LiFePO4 has been intensively investigated as a cathode material in Li-ion batteries, as it can in principle enable the development of high power electrodes. LiFePO4, on the other hand, is inherently "plagued" by poor electronic and ionic conductivity. While the problems with low electron conductivity are partially solved by carbon coating and further by doping or by downsizing the active particles to nanoscale dimensions, poor ionic conductivity is still an issue. To develop colloidally synthesized LiFePO4 nanocrystals (NCs) optimized for high rate applications, we propose here a surface treatment of the NCs. The particles as delivered from the synthesis have a surface passivated with long chain organic surfactants, and therefore can be dispersed only in aprotic solvents such as chloroform or toluene. Glucose that is commonly used as carbon source for carbon-coating procedure is not soluble in these solvents, but it can be dissolved in water. In order to make the NCs hydrophilic, we treated them with lithium hexafluorophosphate (LiPF6), which removes the surfactant ligand shell while preserving the structural and morphological properties of the NCs. Only a roughening of the edges of NCs was observed due to a partial etching of their surface. Electrodes prepared from these platelet NCs (after carbon coating) delivered a capacity of ∼ 155 mAh/g, ∼ 135 mAh/g, and ∼ 125 mAh/g, at 1 C, 5 C, and 10 C, respectively, with significant capacity retention and remarkable rate capability. For example, at 61 C (10.3 A/g), a capacity of ∼ 70 mAh/g was obtained, and at 122 C (20.7 A/g), the capacity was ∼ 30 mAh/g. The rate capability and the ease of scalability in the preparation of these surface-treated nanoplatelets make them highly suitable as electrodes in Li-ion batteries.

CO oxidation on colloidal Au(0.80)Pd(0.20)-Fe(x)O(y) dumbbell nanocrystals.

We report a colloidal synthesis of Au(0.80)Pd(0.20)-Fe(x)O(y) dumbbell nanocrystals (NCs) derived from Au(0.75)Pd(0.25) NCs by metal oxide overgrowth. We compared the catalytic activity of the two types of NCs in the CO oxidation reaction (CO + 1/2O(2) → CO(2)), after they had been dispersed on an alumina nanopowder support. In both cases, the surface active sites were identified by means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The enhanced catalytic performance of the dumbbell NCs (Au(0.80)Pd(0.20)-Fe(x)O(y)) catalyst over that of the initial Au(0.75)Pd(0.25) NCs could be correlated to the presence of the epitaxial connection between the Fe(x)O(y) and the Au(0.80)Pd(0.20) domains (as the main factor). Such connection should result in an electron flow from the metal oxide (Fe(x)O(y)) domain to the noble metal (Au(0.80)Pd(0.20)) domain and appears to influence favorably the nature and composition of the catalytically active surface sites of the dumbbells. Our experiments indicate indeed that, when the metal alloy domain is attached to the metal oxide domain (that is, in the dumbbell), surface Pd species are more active than in the case of the initial Au(0.75)Pd(0.25) NCs and also Au(δ-) sites are formed that were not present on the initial Au(0.75)Pd(0.25) NCs.

Synthesis of uniform disk-shaped copper telluride nanocrystals and cation exchange to cadmium telluride quantum disks with stable red emission.

We present the synthesis of novel disk-shaped hexagonal Cu2Te nanocrystals with a well-defined stoichiometric composition and tunable diameter and thickness. Subsequent cation exchange of Cu to Cd at high temperature (180 °C) results in highly fluorescent CdTe nanocrystals, with less than 1 mol % of residual Cu remaining in the lattice. The procedure preserves the overall disk shape, but is accompanied by a substantial reconstruction of the anion sublattice, resulting in a reorientation of the c-axis from the surface normal in Cu2Te into the disk plane in CdTe nanodisks. The synthesized CdTe nanodisks show a continuously tunable photoluminescence (PL) peak position, scaling with the thickness of the disks. The PL lifetime further confirms that the CdTe PL arises from band-edge exciton recombination; that is, no Cu-related emission is observed. On average, the recombination rate is about 25-45% faster with respect to their spherical quantum dots counterparts, opening up the possibility to enhance the emission rate at a given wavelength by controlling the nanocrystal shape. Finally, with a PL quantum efficiency of up to 36% and an enhanced PL stability under ambient conditions due to a monolayer of CdS formed on the nanocrystal surface during cation exchange, these flat quantum disks form an interesting enrichment to the current family of highly fluorescent, shape-controlled nanocrystals.

Sequential cation exchange in nanocrystals: preservation of crystal phase and formation of metastable phases.

We demonstrate that it is possible to convert CdSe nanocrystals of a given size, shape (either spherical or rod shaped), and crystal structure (either hexagonal wurtzite, i.e., hexagonal close packed (hcp), or cubic sphalerite, i.e., face-centered cubic (fcc)), into ZnSe nanocrystals that preserve all these characteristics of the starting particles (i.e., size, shape, and crystal structure), via a sequence of two cation exchange reactions, namely, Cd(2+) ⇒Cu(+) ⇒Zn(2+). When starting from hexagonal wurtzite CdSe nanocrystals, the exchange of Cd(2+) with Cu(+) yields Cu(2)Se nanocrystals in a metastable hexagonal phase, of which we could follow the transformation to the more stable fcc phase for a single nanorod, under the electron microscope. Remarkably, these metastable hcp Cu(2)Se nanocrystals can be converted in solution into ZnSe nanocrystals, which yields ZnSe nanocrystals in a pure hcp phase.

Reversible tunability of the near-infrared valence band plasmon resonance in Cu(2-x)Se nanocrystals.

We demonstrate that colloidal Cu(2-x)Se nanocrystals exhibit a well-defined infrared absorption band due to the excitation of positive charge carrier oscillations (i.e., a valence band plasmon mode), which can be tuned reversibly in width and position by varying the copper stoichiometry. The value of x could be incrementally varied from 0 (no plasmon absorption, then a broad peak at 1700 nm) to 0.4 (narrow plasmon band at 1100 nm) by oxidizing Cu(2)Se nanocrystals (upon exposure either to oxygen or to a Ce(IV) complex), and it could be incrementally restored back to zero by the addition of a Cu(I) complex. The experimentally observed plasmonic behavior is in good agreement with calculations based on the electrostatic approximation.

Comparative Study of Loading of Anodic Porous Alumina with Silver Nanoparticles Using Different Methods.

Three different routes were used to infiltrate the pores of anodic porous alumina templates with silver nanoparticles, selected as an example of a bioactive agent. The three methods present a continuous grading from more physical to more chemical character, starting from ex situ filling of the pores with pre-existing particles, moving on to in situ formation of particles in the pores by bare calcination and ending with in situ calcination following specific chemical reactions. The resulting presence of silver inside the pores was assessed by means of energy dispersive X-ray spectroscopy and X-ray diffraction. The number and the size of nanoparticles were evaluated by scanning electron microscopy of functionalized alumina cross-sections, followed by image analysis. It appears that the best functionalization results are obtained with the in situ chemical procedure, based on the prior formation of silver ion complex by means of ammonia, followed by reduction with an excess amount of acetaldehyde. Elution of the silver content from the chemically functionalized alumina into phosphate buffer saline has also been examined, demonstrating a sustained release of silver over time, up to 15 h.

Colloidal synthesis of cuprite (Cu2O) octahedral nanocrystals and their electrochemical lithiation.

We report a facile colloidal route to prepare octahedral-shaped cuprite (Cu2O) nanocrystals (NCs) of ∼40 nm in size that exploits a new reduction pathway, i.e., the controlled reduction of a cupric ion by acetylacetonate directly to cuprite. Detailed structural, morphological, and chemical analyses were carried on the cuprite NCs. We also tested their electrochemical lithiation, using a combination of techniques (cyclic voltammetry, galvanostatic, and impedance spectroscopy), in view of their potential application as anodes for Li ion batteries. Along with these characterizations, the morphological, structural, and chemical analyses (via high-resolution electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy) of the cycled Cu2O NCs (in the lithiated stage, after ∼50 cycles) demonstrate their partial conversion upon cycling. At this stage, most of the NCs had lost their octahedral shape and had evolved into multidomain particles and were eventually fragmented. Overall, the shape changes (upon cycling) did not appear to be concerted for all the NCs in the sample, suggesting that different subsets of NCs were characterized by different lithiation kinetics. We emphasize that a profound understanding of the lithiation reaction with NCs defined by a specific crystal habit is still essential to optimize nanoscale conversion reactions.

Colloidal CdSe/Cu3P/CdSe nanocrystal heterostructures and their evolution upon thermal annealing.

We report the synthesis of colloidal CdSe/Cu(3)P/CdSe nanocrystal heterostructures grown from hexagonal Cu(3)P platelets as templates. One type of heterostructure was a sort of "coral", formed by vertical pillars of CdSe grown preferentially on both basal facets of a Cu(3)P platelet and at its edges. Another type of heterostructure had a "sandwich" type of architecture, formed by two thick, epitaxial CdSe layers encasing the original Cu(3)P platelet. When the sandwiches were annealed under vacuum up to 450 °C, sublimation of P and Cd species with concomitant interdiffusion of Cu and Se species was observed by in situ HR- and EFTEM analyses. These processes transformed the starting sandwiches into Cu2Se nanoplatelets. Under the same conditions, both the pristine (uncoated) Cu(3)P platelets and a control sample made of isolated CdSe nanocrystals were stable. Therefore, the thermal instability of the sandwiches under vacuum might be explained by the diffusion of Cu species from Cu(3)P cores into CdSe domains, which triggered sublimation of Cd, as well as out-diffusion of P species and their partial sublimation, together with the overall transformation of the sandwiches into Cu(2)Se nanocrystals. A similar fate was followed by the coral-like structures. These CdSe/Cu(3)P/CdSe nanocrystals are therefore an example of a nanostructure that is thermally unstable, despite its separate components showing to be stable under the same conditions.

Influence of chloride ions on the synthesis of colloidal branched CdSe/CdS nanocrystals by seeded growth.

We studied the influence of chloride ions (Cl(-)), introduced as CdCl(2), on the seeded growth synthesis of colloidal branched CdSe(core)/CdS(pods) nanocrystals. This is carried out by growing wurtzite CdS pods on top of preformed octahedral sphalerite CdSe seeds. When no CdCl(2) is added, the synthesis of multipods has a low reproducibility, and the side nucleation of CdS nanorods is often observed. At a suitable concentration of CdCl(2), octapods are formed and they are stable in solution during the synthesis. Our experiments indicate that Cl(-) ions introduced in the reaction reduce the availability of Cd(2+) ions in solution, most likely via formation of strong complexes with both Cd and the various surfactants. This prevents homogeneous nucleation of CdS nanocrystals, so that the heterogeneous nucleation of CdS pods on top of the CdSe seeds is the preferred process. Once such optimal concentration of CdCl(2) is set for a stable growth of octapods, the pod lengths can be tuned by varying the relative ratios of the various alkyl phosphonic acids used. Furthermore, at higher concentrations of CdCl(2) added, octapods are initially formed, but many of them evolve into tetrapods over time. This transformation points to an additional role of Cl species in regulating the growth rate and stability of various crystal facets of the CdS pods.