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

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

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

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

Decoration of WS₂ Nanotubes and Fullerene-Like MoS₂ with Gold Nanoparticles

A new technique of gold nanoparticle (AuNP) growth on the sidewalls of WS₂ inorganic nanotubes (INT-WS₂) and the surface of MoS₂ fullerene-like nanoparticles (IF-MoS₂) is developed to produce metal–semiconductor nanocomposites. The coverage density and mean size of the nanoparticles are dependent on the HAuCl₄/MS₂ (M = W, Mo) molar ratio. AuNPs formation mechanism seems to involve spatially divided reactions of AuCl₄^– reduction and WS₂/MoS₂ oxidation taking place on the surface defects of the disulfide nanostructures rather than directly at the AuNP-INT/IF interface. A strong epitaxial matching between the lattices of the gold nanoparticles and the INT-WS₂ or IF-MoS₂ seems to suppress plasmon resonance in the nanocomposites with small (<10 nm mean size) AuNPs