Order and Disorder in Calcium Oxalate Monohydrate: Insights from First-Principles Calculations

Calcium oxalate minerals are broadly present in nature. They form through biogenic, geogenic, and pathogenic processes that lead to different pseudopolymorphs. Being the most common solid phase in human nephrolithiasis, calcium oxalate monohydrate (COM) in particular has been the focus of much investigation. It exists in several crystalline forms, two of which appear to be of biological and medical relevance: the low- and high-temperature forms (COM-LT and COM-HT, respectively). While there is broad consensus on the ordered structure of COM-LT, which possesses the P21/n space group symmetry, for COM-HT controversy remains. Experimental results suggest that there is a certain degree of structural disorder in the high-temperature form. However, the exact character of disorder in COM-HT is yet an open question. Here, we examine the effect of the disorder of water molecules on the structure of COM using first-principles calculations based on dispersion-augmented density functional theory. Such calculations allow for controlled examination of specific disorder features and their effect on crystal structure and stability. On the basis of our first-principles analysis, we suggest that in COM-HT each water dimer site is randomly occupied by any of four water dimer arrangements present in COM-LT, leading to statistical 2/m point symmetry at each site and a statistical I2/m space group symmetry

Chemical Deposition of Cu₂O Nanocrystals with Precise Morphology Control

Copper(I) oxide nanoparticles (NPs) are emerging as a technologically important material, with applications ranging from antibacterial and fungicidal agents to photocatalysis. It is well established that the activity of Cu₂O NPs is dependent on their crystalline morphology. Here we describe direct preparation of Cu₂O nanocrystals (NCs) on various substrates by chemical deposition (CD), without the need of additives, achieving precise control over the NC morphology. The substrates are preactivated by gold seeding and treated with deposition solutions comprising copper sulfate, formaldehyde, NaOH, and citrate as a complexant. Production of NC deposits ranging from complete cubes to complete octahedra is demonstrated, as well as a full set of intermediate morphologies, <i>i</i>.<i>e</i>., truncated octahedra, cuboctahedra, and truncated cubes. The NC morphology is defined by the NaOH and complexant concentrations in the deposition solution, attributed to competitive adsorption of citrate and hydroxide anions on the Cu₂O {100} and {111} crystal faces and selective stabilization of these faces. A sequential deposition scheme, <i>i.e.</i>, Cu₂O deposition on pregrown Cu₂O NCs of a different morphology, is also presented. The full range of morphologies can be produced by controlling the deposition times in the two solutions, promoting the cubic and octahedral crystal habits. Growth rates in the ⟨100⟩ and ⟨111⟩ directions for the two solutions are estimated. The Cu₂O NCs are characterized by SEM, TEM, GI-XRD, and UV–vis spectroscopy. It is concluded that CD furnishes a simple, effective, generally applicable, and scalable route to the synthesis of morphologically controlled Cu₂O NCs on a variety of conductive and nonconductive surfaces

Chemical Deposition and Stabilization of Plasmonic Copper Nanoparticle Films on Transparent Substrates

Preparation of supported copper nanostructures has been scarce, compared to the more noble metals Ag and Au, mainly due to the lower stability of Cu toward corrosion in aqueous solutions and oxidation in air, either during or after preparation. Still, as a markedly inexpensive metal, Cu might present an attractive substance, if suitable Cu nanoparticle (NP) deposition and stabilization methods could be developed. Here, we present the first case of glass substrates coated with Cu or Cu2O NPs using wet chemical deposition (CD), performed under well-defined conditions optimized for obtaining each of the two nanoparticulate deposits. Cu NP films were also obtained by chemical reduction of the Cu2O NP films, thereby achieving improved size uniformity. The Cu NP films display a prominent surface plasmon (SP) band in the visible range. The dependence of the SP absorbance on the local dielectric environment is shown to provide a useful tool for monitoring Cu NP corrosion processes and their inhibition. Stabilization of the Cu NP films by treatment with the corrosion inhibitor benzotriazole (BTAH), shown here for the first time, enabled study of the films’ plasmonic properties, such as their refractive index sensitivity (RIS), a basic property in sensing applications. The measured RIS values are similar to those of typical gold NP films. Introduction of an effective, low-cost, and scalable method for the preparation of stable supported Cu and Cu2O NP films may open the way to a variety of plasmonic and other applications

Solid-State Thermal Dewetting of Just-Percolated Gold Films Evaporated on Glass: Development of the Morphology and Optical Properties

Solid-state thermal dewetting of just-percolated gold films of nominal thicknesses in the range 10–16 nm, prepared by evaporation on glass slides and annealing, was systematically studied. The kinetics of thermal dewetting and transition from a percolated film to isolated islands were monitored using <i>in situ</i> transmission localized surface plasmon resonance (LSPR) spectroscopy combined with <i>ex situ</i> high-resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and selected-area electron diffraction (SAED) to correlate between evolution of the film morphology and development of the optical properties. Annealing at 550 °C results in transformation of the as-evaporated, percolated polycrystalline films, with mean crystallite dimensions close to the film nominal thickness, to (111) textured films comprising large separated single-crystalline islands. The dewetting scenario depends on the initial morphology of the unannealed, just-percolated Au film, in particular on the structure of the voids at the metal–ambient and metal–glass interfaces. Dewetting of films of <13 nm (nominal thickness), the latter exhibiting a majority of voids which are open at both interfaces (denoted type I films), shows faster kinetics than in-plane grain growth. In films of >13 nm (nominal thickness), in which the majority of voids do not protrude through the entire film and are closed at the metal–glass interface (denoted type II films), grain growth presents faster kinetics than dewetting. The annealed films display discrete single-crystalline Au islands with flat, (111) textured top surfaces. Island diameters range from <100 nm to submicrometer, while the surface plasmon extinction band varies over >300 nm for different average island sizes

Solid-State Crystal-to-Crystal Phase Transitions and Reversible Structure–Temperature Behavior of Phosphovanadomolybdic Acid, H₅PV₂Mo₁₀O₄₀

The crystal packing and secondary structure of H₅PV₂Mo₁₂O₄₀ was followed by careful X-ray diffraction studies that revealed four unique structures and three solid phase transitions at temperatures between 25 and 55 °C, with loss of solvated water and concomitant contraction of the volume and increase of the packing density. Above 60 °C H₅PV₂Mo₁₂O₄₀ becomes amorphous and then anhydrous although the polyoxometalate cluster is stable indefinitely up to 300 °C. Above this temperature, combined IR, Raman, XRD, and XPS measurements show the decomposition of H₅PV₂Mo₁₂O₄₀ to crystalline MoO₃ and probably amorphous vanadium oxide and vanadylphosphate, the latter appearing to cover the surface of MoO₃. Importantly, H₅PV₂Mo₁₂O₄₀ can be easily recovered by dissolution in water at 80 °C

Solid-State Crystal-to-Crystal Phase Transitions and Reversible Structure–Temperature Behavior of Phosphovanadomolybdic Acid, H₅PV₂Mo₁₀O₄₀

The crystal packing and secondary structure of H₅PV₂Mo₁₂O₄₀ was followed by careful X-ray diffraction studies that revealed four unique structures and three solid phase transitions at temperatures between 25 and 55 °C, with loss of solvated water and concomitant contraction of the volume and increase of the packing density. Above 60 °C H₅PV₂Mo₁₂O₄₀ becomes amorphous and then anhydrous although the polyoxometalate cluster is stable indefinitely up to 300 °C. Above this temperature, combined IR, Raman, XRD, and XPS measurements show the decomposition of H₅PV₂Mo₁₂O₄₀ to crystalline MoO₃ and probably amorphous vanadium oxide and vanadylphosphate, the latter appearing to cover the surface of MoO₃. Importantly, H₅PV₂Mo₁₂O₄₀ can be easily recovered by dissolution in water at 80 °C

Cooperative Doping in Ultrasmall BaF₂ Nanocrystals for Multimodal ¹⁹F‑MRI and CT Applications

Nanostructured metal fluorides (nanofluorides) are an emerging type of inorganic nanocrystals (NCs) with unique physiochemical properties for advanced applications. One recent demonstration used water-dispersed ultrasmall CaF2 nanofluorides as imaging agents that combined the advantages of inorganic NCs with the benefit of background-free 19F-magnetic resonance imaging (19F-MRI). Nevertheless, obtaining small nanofluorides with a face-centered cubic crystal structure, where all fluorides are magnetically equivalent to result in a single 19F NMR signal, is challenging for other types of nanofluorides, preventing their use in 19F-MRI. Here, we show the development of ultrasmall, water-dispersed, barium fluoride (BaF2) NCs for bioimaging applications. By doping BaF2 with two types of lanthanides, diamagnetic-La3+ and paramagnetic-Sm3+, we were able to control the morphology and 19F-MR properties of the final La,Sm:BaF2 (termed LaSamBa) formulation. The fine-tuning of the La3+ content enabled us to obtain monodispersed 4.5 nm NCs, and control over the Sm3+ content provided LaSamBa with very short T1 relaxation properties (ca. 100 ms) needed for enhanced 19F-MRI sensitivity. This type of nanofluorides was examined in two different imaging modalities (i.e., 19F-MRI and CT), benefiting from their single 19F-NMR signal and the high atomic number of barium atoms, respectively. As their 19F chemical shift significantly differs from that of other nanofluorides (e.g., CaF2 and SrF2), LaSamBa expanded the nanofluoride library for future multitarget 19F-MRI studies

Molecular Length, Monolayer Density, and Charge Transport: Lessons from Al–AlOx/Alkyl–Phosphonate/Hg Junctions

A combined electronic transport–structure characterization of self-assembled monolayers (MLs) of alkyl–phosphonate (AP) chains on Al–AlOx substrates indicates a strong molecular structural effect on charge transport. On the basis of X-ray reflectivity, XPS, and FTIR data, we conclude that “long” APs (C14 and C16) form much denser MLs than do “short” APs (C8, C10, C12). While current through all junctions showed a tunneling-like exponential length-attenuation, junctions with sparsely packed “short” AP MLs attenuate the current relatively more efficiently than those with densely packed, “long” ones. Furthermore, “long” AP ML junctions showed strong bias variation of the length decay coefficient, β, while for “short” AP ML junctions β is nearly independent of bias. Therefore, even for these simple molecular systems made up of what are considered to be inert molecules, the tunneling distance cannot be varied independently of other electrical properties, as is commonly assumed

Fullerene-Like (IF) Nb<i>_x</i>Mo₁_-<i>_x</i>S₂ Nanoparticles

IF-Mo1-xNbxS2 nanoparticles have been synthesized by a vapor-phase reaction involving the respective metal halides with H2S. The IF-Mo1-xNbxS2 nanoparticles, containing up to 25% Nb, were characterized by a variety of experimental techniques. Analysis of the powder X-ray powder diffraction, X-ray photoelectron spectroscopy, and different electron microscopy techniques shows that the majority of the Nb atoms are organized as nanosheets of NbS2 within the MoS2 host lattice. Most of the remaining Nb atoms (3%) are interspersed individually and randomly in the MoS2 host lattice. Very few Nb atoms, if any, are intercalated between the MoS2 layers. A sub-nanometer film of niobium oxide seems to encoat the majority of the nanoparticles. X-ray photoelectron spectroscopy in the chemically resolved electrical measurement mode (CREM) and scanning probe microscopy measurements of individual nanoparticles show that the mixed IF nanoparticles are metallic independent of the substitution pattern of the Nb atoms in the lattice of MoS2 (whereas unsubstituted IF-MoS2 nanoparticles are semiconducting). Furthermore the IF-Mo1-xNbxS2 nanoparticles are found to exhibit interesting single electron tunneling effects at low temperatures

Guanine Crystallization in Aqueous Solutions Enables Control over Crystal Size and Polymorphism

Anhydrous guanine crystals are among the most widespread organic crystals used by organisms to produce structural colors. The main advantage of guanine is its exceptionally high refractive index in the reflecting direction (∼1.8). For the same reason, guanine is a promising candidate material for a variety of different optical applications. Crystallization of guanine is challenging and usually involves using polar aprotic organic solvents such as dimethyl sulfoxide (DMSO). Here, we show that the crystallization of guanine from aqueous solutions is possible under conditions that provide control over crystal polymorphism and size. Using this approach we were able produce large crystals of the elusive guanine monohydrate phase. We were also able to rationalize the formation of the different phases obtained as a function of which tautomer of guanine is stable in solutions of varying pH