Operando SAXS/WAXS on the a‑P/C as the Anode for Na-Ion Batteries

A complete chemical and morphological analysis of the evolution of battery electrode materials can be achieved combining different and complementary techniques. Operando small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) were combined to investigate structural and electrochemical performances of an Na-ion battery, with amorphous red phosphorus in a carbon matrix (a-P/C) as the active anode material in a Swagelok-type cell. The charging process results in the formation of crystalline Na₃P, while during discharging, the anode material returns to the initial a-P/C. From the analysis of the WAXS curves, the formation of crystalline phases appears only at the end of charging. However, SAXS data show that partial reorganization of the material during charging occurs at length scales nonaccessible with conventional X-ray diffraction, corresponding to a real space ordering distance of 4.6 nm. Furthermore, the analysis of the SAXS data shows that the electrode remains dense during charging, while it develops some porosity during the discharge phase. The presented results indicate that the combination of SAXS/WAXS adopted simultaneously, and nondestructively, on a working electrochemical cell can highlight new mechanisms of reactions otherwise undetected. This method can be applied for the study of any other solid electrode material for batteries

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 <i>c</i>-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

Generalized One-Pot Synthesis of Copper Sulfide, Selenide-Sulfide, and Telluride-Sulfide Nanoparticles

Here we report a facile approach to synthesize copper chalcogenide (Cu_2–<i>x</i>S, Cu_2–<i>x</i>Se_<i>y</i>S_1–<i>y</i> and Cu_2–<i>x</i>Te_<i>y</i>S_1–<i>y</i>) nanocrystals without employing hot-injection, at moderate reaction temperatures (200–220 °C) and free of phosphines. Scaling up of the synthesis yields monodisperse nanoparticles without variations in their morphology. We have observed the formation of alloyed copper selenide-sulfide and telluride-sulfide nanocrystals due to the incorporation of sulfur by using 1-dodecanethiol as a ligand along with oleic acid. The materials obtained possess localized surface plasmon resonances in the near-infrared region, which are demonstrated to be widely tunable via a controlled oxidation generating copper vacancies. Copper sulfide nanoparticles with well-defined initial chalcocite crystal phase were subjected to oxidation followed by structural characterization. Structural rearrangement of the oxidized chalcocite Cu_2–<i>x</i>S crystal lattice to roxbyite by aging is proven to release the copper vacancies. Further oxidation again can create new copper vacancies in the roxbyite lattice, however its structure does not evolve into covellite CuS. These findings suggest that besides nonstoichiometry (i.e., the value of <i>x</i>) induced by oxidation, crystal structure is an important factor responsible for plasmonic properties of copper chalcogenide nanocrystals. Furthermore, successful water solubilization of Cu_2–<i>x</i>Te_<i>y</i>S_1–<i>y</i> nanoparticles with preservation of their plasmon band has been realized via a ligand exchange approach employing a mPEG-SH stabilizer

Colloidal CdSe/Cu₃P/CdSe Nanocrystal Heterostructures and Their Evolution upon Thermal Annealing

We report the synthesis of colloidal CdSe/Cu₃P/CdSe nanocrystal heterostructures grown from hexagonal Cu₃P 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 Cu₃P 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 Cu₃P 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 <i>in situ</i> HR- and EFTEM analyses. These processes transformed the starting sandwiches into Cu₂Se nanoplatelets. Under the same conditions, both the pristine (uncoated) Cu₃P 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 Cu₃P 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 Cu₂Se nanocrystals. A similar fate was followed by the coral-like structures. These CdSe/Cu₃P/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 CdCl₂, 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 CdCl₂ 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 CdCl₂, 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 Cd²⁺ ions in solution, most likely <i>via</i> 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 CdCl₂ 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 CdCl₂ 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

Atomic Ligand Passivation of Colloidal Nanocrystal Films via their Reaction with Propyltrichlorosilane

Colloidal nanocrystal films of different materials (semiconductors, metals) and shapes (spheres and rods) were dipped in solutions of propyltrichlorosilane (PTCS) in acetonitrile. This process removed most of the surfactants covering the surface of the tested nanocrystals, leaving their surface either unpassivated or passivated with chlorine atoms, depending on their composition. PTCS was reactive toward most of the surfactants used in nanocrystal synthesis and therefore such a procedure could be applied to a large variety of materials. All samples were characterized with FTIR, XRD, and XPS measurements. In nanocrystal films, the reduction of the separation between the nanocrystals resulting from the removal of surfactants led to an enhancement in both dark and photocurrent. The surface of Au nanocrystals is left unpassivated by the reaction with PTCS, which makes the process potentially useful for applications in catalysis and plasmonics

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

Combining SAXS and XAS To Study the <i>Operando</i> Degradation of Carbon-Supported Pt-Nanoparticle Fuel Cell Catalysts

In the last two decades, small-angle X-ray scattering (SAXS) and X-ray absorption spectroscopy (XAS) have evolved into two well-established techniques capable of providing complementary and <i>operando</i> information about a sample’s morphology and composition, respectively. Considering that operation conditions can often lead to simultaneous and related changes in a catalyst’s speciation and shape, herein we introduce a setup that combines SAXS and XAS in a configuration that allows optimum acquisition and corresponding data quality for both techniques. To determine the reliability of this setup, the latter was used to study the <i>operando</i> degradation of two carbon-supported Pt-nanoparticle (Pt/C) catalysts customarily used in polymer electrolyte fuel cells. The model used for the fitting of the SAXS curves unveiled the fractal nature of the Pt/C-electrodes and their evolution during the <i>operando</i> tests, and both X-ray techniques were complemented with control, ex situ transmission electron microscopy, and standard electrochemical measurements. Ultimately, the results obtained with this combined setup quantitatively agree with those reported in previous studies, successfully validating this apparatus and highlighting its potential to study the <i>operando</i> changes undergone by worse-understood (electro)­catalytic systems

IrO₂‑TiO₂: A High-Surface-Area, Active, and Stable Electrocatalyst for the Oxygen Evolution Reaction

The utilization and development of efficient water electrolyzers for hydrogen production is currently limited due to the sluggish kinetics of the anodic processthe oxygen evolution reaction (OER). Moreover, state of the art OER catalysts contain high amounts of expensive and low-abundance noble metals such as Ru and Ir, limiting their large-scale industrial utilization. Therefore, the development of low-cost, highly active, and stable OER catalysts is a key requirement toward the implementation of a hydrogen-based economy. We have developed a synthetic approach to high-surface-area chlorine-free iridium oxide nanoparticles dispersed in titania (IrO₂-TiO₂), which is a highly active and stable OER catalyst in acidic media. IrO₂-TiO₂ was prepared in one step in molten NaNO₃ (Adams fusion method) and consists of ca. 1–2 nm IrO₂ particles distributed in a matrix of titania nanoparticles with an overall surface area of 245 m² g^–1. This material contains 40 mol_M % of iridium and demonstrates improved OER activity and stability in comparison to the commercial benchmark catalyst and state of the art high-surface-area IrO₂. Ex situ characterization of the catalyst indicates the presence of iridium hydroxo surface species, which were previously associated with the high OER activity. Operando X-ray absorption studies demonstrate the evolution of the surface species as a function of the applied potential, suggesting the conversion of the initial hydroxo surface layer to the oxo-terminated surface via anodic oxidation (OER regime)

CuIn_<i>x</i>Ga_1–<i>x</i>S₂ Nanocrystals with Tunable Composition and Band Gap Synthesized via a Phosphine-Free and Scalable Procedure

We report a phosphine-free colloidal synthesis of CuIn_<i>x</i>Ga_1–<i>x</i>S₂ (CIGS) nanocrystals (NCs) by heating a mixture of metal salts, elemental sulfur, octadecene, and oleylamine. In contrast with the more commonly used hot injection, this procedure is highly suitable for large-scale NC production, which we tested by performing a gram-scale synthesis. The composition of the CIGS NCs could be tuned by varying the In and Ga precursor ratios, and the samples showed a composition-dependent band gap energy. The average particle size was scaled from 13 to 19 nm by increasing the reaction temperature from 230 to 270 °C. Two concomitant growth mechanisms took place: in one, covellite (CuS) NCs nucleated already at room temperature and then incorporated increasing amounts of In and Ga until they evolved into chalcopyrite CIGS NCs. In the second mechanism, CIGS NCs directly nucleated at intermediate temperatures. They were smaller than the NCs formed by the first mechanism, but richer in In and Ga. In the final sample, obtained by prolonged heating at 230–270 °C, all NCs were homogeneous in size and composition. Attempts to replace the native ligands on the surface of the NCs with sulfur ions (following literature procedures) resulted in only around 50% exchange. Films prepared using the partially ligand exchanged NCs exhibited good homogeneity and an ohmic dark conductivity and photoconductivity with a resistivity of about 50 Ω·cm