Size-induced structural transitions in the Cu-O and Ce-O systems

Heterostructure of ultrafine FeOOH nanodots supported on CoAl-layered double hydroxide nanosheets as highly efficient electrocatalyst for water oxidation

The supported and dispersed ultrafine active species for electrocatalytic water oxidation are quite promising for the high intrinsic activity. A novel heterostructure of ultrafine FeOOH nanodots with an average size of 2.3 nm supported on CoAl-LDH nanosheets, is constructed by a facile method under ambient conditions. The as-prepared FeOOH@CoAl-LDH shows a strong interfacial interaction upon the formation of heterostructure, and is demonstrated as a highly efficient and stable electrocatalyst that demands 272 mV to attain 50 mA cm^-2 and exhibits a Tafel slope of 40 mV dec^-1. Moreover, density functional theory calculations manifest the coupling of FeOOH with CoAl-LDH can effectively decrease the energy barrier during the water oxidation process by optimizing the adsorption free energy of intermediates in the reaction pathway. The successful development of FeOOH@CoAl-LDH can shed light on the design of novel electrocatalysts that can fully take advantages of small size, heterostructure and synergistic effect.

A density functional theory study on reduction-induced structural transformation of copper-oxide-based oxygen carrier

A clear understanding of the structural transformation of copper-oxide-based oxygen carriers accompanying their reduction by fuels helps to design more efficient oxygen carriers for chemical looping combustion. Herein, density functional theory calculations have been performed on the bulk CuO, CuO(111) surface, and (CuO)₃₂ cluster models with the same number of CuO molecular units to investigate structural transformation accompanying the reduction. The results showed that the averaged reaction energies of desorbing an oxygen molecule from the bulk and surface models are roughly the same [246.2 kJ/(mol O₂) and 245.9 kJ/(mol O₂), respectively]. The slab model does not significantly lower the overall reaction energy compared to the bulk model. In contrast, the averaged reaction energy using the cluster model is significantly lower [127.5 kJ/(mol O₂)] than that of bulk and slab models. The key structural difference is the obvious Cu-Cu bond formation in the cluster model, which would result in nucleation of a metallic Cu phase. The results also showed that different states can be reached by desorbing different number oxygen atoms in a single step, corresponding to different reaction rates, when the system reaches the same level of reduction. These results demonstrate the complexity of reactions involving solid state materials and are consistent with the structural diversity observed experimentally. This study illustrates the importance of particle sizes and reaction conditions in the formation of suboxides during CuO reduction.

Atomic Layer Deposition of Photoconductive Cu2O Thin Films

Herein, we report an atomic layer deposition (ALD) process for Cu₂O thin films using copper(II) acetate [Cu(OAc)₂] and water vapor as precursors. This precursor combination enables the deposition of phase-pure, polycrystalline, and impurity-free Cu₂O thin films at temperatures of 180-220 °C. The deposition of Cu(I) oxide films from a Cu(II) precursor without the use of a reducing agent is explained by the thermally induced reduction of Cu(OAc)₂ to the volatile copper(I) acetate, CuOAc. In addition to the optimization of ALD process parameters and characterization of film properties, we studied the Cu₂O films in the fabrication of photoconductor devices. Our proof-of-concept devices show that approximately 20 nm thick Cu₂O films can be used for photodetection in the visible wavelength range and that the thin film photoconductors exhibit improved device characteristics in comparison to bulk Cu₂O crystals.

A Reduction in Particle Size Generally Causes Body-Centered-Cubic Metals to Expand but Face-Centered-Cubic Metals to Contract

From a careful analysis of existing data as well as new measurements, we show that the size dependence of the lattice parameters in metal nanoparticles with face-centered cubic (fcc) and body-centered cubic (bcc) symmetries display opposite trends: nanoparticles with fcc structure generally contract with decreasing particle size, while those with bcc structure expand. We present a microscopic explanation for this apparently puzzling behavior based on first-principles simulations. Our results, obtained from a comparison of density functional theory calculations with experimental data, indicate that the nanoparticles are capped by a surface monolayer of oxygen atoms, which is routinely detected by surface-sensitive techniques. The bcc- and fcc-based nanoparticles respond in contrasting fashion to the presence of the oxygen capping layer, and this dictates whether the corresponding lattice parameter would increase or decrease with size reduction. The metal-oxygen bonds at the surface, being shorter and stronger than typical metal-metal bonds, pull the surface metal atoms outward. This outward movement of surface atoms influences the core regions to a larger extent in the relatively open bcc geometry, producing a rather large overall expansion of the cluster, compared to the bulk. In case of fcc clusters, on the other hand, the outward movement of surface metal atoms does not percolate too far inside, resulting in either a smaller net expansion or contraction of the cluster depending on the extent of surface oxygen coverage. Our study therefore provides a convincing physicochemical basis for the correlation between the underlying geometry and the nature of change of the lattice parameters under size reduction.

Novel synthesis of a Cu2O–graphene nanoplatelet composite through a two-step electrodeposition method for selective detection of hydrogen peroxide

Optimized electrodeposition technique for the synthesis of Cu₂O–graphene composite for H₂O₂sensing.

Reversible Redox Cycling of Well-Defined, Ultrasmall Cu/Cu2O Nanoparticles

Exceptionally small and well-defined copper (Cu) and cuprite (Cu₂O) nanoparticles (NPs) are synthesized by the reaction of mesitylcopper(I) with either H₂ or air, respectively. In the presence of substoichiometric quantities of ligands, namely, stearic or di(octyl)phosphinic acid (0.1-0.2 equiv vs Cu), ultrasmall nanoparticles are prepared with diameters as low as ∼2 nm, soluble in a range of solvents. The solutions of Cu NPs undergo quantitative oxidation, on exposure to air, to form Cu₂O NPs. The Cu₂O NPs can be reduced back to Cu(0) NPs using accessible temperatures and low pressures of hydrogen (135 °C, 3 bar H₂). This striking reversible redox cycling of the discrete, solubilized Cu/Cu(I) colloids was successfully repeated over 10 cycles, representing 19 separate reactions. The ligands influence the evolution of both composition and size of the nanoparticles, during synthesis and redox cycling, as explored in detail using vacuum-transfer aberration-corrected transmission electron microscopy, X-ray photoelectron spectroscopy, and visible spectroscopy.

Calcination temperature as a probe to tune the non-enzymatic glucose sensing activity of Cu–Ni bimetallic nanocomposites

A seven-fold increase in the glucose sensing activity of CuO–NiO bimetallic nanocomposites was induced via calcination.

Functional up-converting SrTiO3:Er(3+)/Yb(3+) nanoparticles: structural features, particle size, colour tuning and in vitro RBC cytotoxicity

SrTiO3 nanoparticles co-doped with a broad concentration range of Er(3+) and Yb(3+) ions were fabricated using the citric route as a function of annealing temperatures of 500-1000 °C. The effect of a broad co-dopant concentration range and sintering temperature on structural and up-conversion properties was investigated in detail by X-ray diffraction techniques and optical spectroscopy. The TEM technique was used to estimate the mean particle size, which was around 30 nm for the inorganic product annealed at 600 °C. Up-conversion emission color tuning was achieved by particle size control. Power dependence of the green and red emissions was found to be a result of temperature determination in the operating range of SrTiO3 nanoparticles and a candidate for the fast and local microscopic heating and heat release induced by IR irradiation. The color changed from white-red-yellow-green upon an increase of sintering temperature, inducing changes in the surface-to-volume ratio and the number of optically active ions in particle surface regions. The cytotoxic activity of nanoparticles on human red blood cells was investigated, showing no harmful effects up to a particle concentration of 0.1 mg ml(-1). The cytotoxic response of a colloidal suspension of nanoparticles to RBC cells was connected with the strong affinity of SrTiO3 particles to the cell membranes, blocking the transport of important biological solutes.

An up-converting HAP@β-TCP nanocomposite activated with Er3+/Yb3+ ion pairs for bio-related applications

A HAP@β-TCP nanocomposite doped with Er³⁺/Yb³⁺ ions was prepared and its cytotoxicity was tested on canine osteosarcoma and murine macrophage cells. Metronidazole release from the nanocomposite was studied and its up-conversion properties measured.

Grain size effect on magnetic and phase transition features in one-dimensional S = 1/2 Heisenberg spin chain molecular crystals

Magnetic and thermal behaviors and the phase transition nature are strongly influenced by grain size in one-dimensional S = 1/2 molecular spin systems.

Structural and spectroscopic properties of Yb3+-doped MgAl2O4 nanocrystalline spinel

Pechini's sol–gel method was successfully applied to obtain an Yb³⁺-doped MgAl₂O₄ spinel nanopowder. The regularity between the observed structural and spectroscopic measurements was described in detail.

Stable Cu₂O nanocrystals grown on functionalized graphene sheets and room temperature H₂S gas sensing with ultrahigh sensitivity

Stable Cu(2)O nanocrystals of around 3 nm were uniformly and densely grown on functionalized graphene sheets (FGS), which act as molecular templates instead of surfactants for controlled nucleation; the distribution density of nanocrystals can be easily controlled by FGS with different C/O ratios. The nanocomposite displays improved stability of the crystalline phase in wet air, which is attributed to finite-size effects that the high-symmetry crystalline phase is to be more stable at smaller size. Meanwhile, we conjecture that the oxygen adsorbed on the interfacial surface prefers to extract electrons from FGS, thus the interfacial bonding also makes a contribution in alleviating the process of corrosion to some extent. More importantly, the Cu(2)O-FGS nanocomposite based sensor realizes room temperature sensing to H(2)S with fantastic sensitivity (11%); even at the exposed concentration of 5 ppb, the relative resistance changes show good linearity with the logarithm of the concentration. The enhancement of sensitivity is attributed to the synergistic effect of Cu(2)O and FGS; on the one hand, surfactant-free capped Cu(2)O nanocrystals display higher surface activity to adsorb gas molecules, and on the other hand, FGS acting as conducting network presents greater electron transfer efficiency. These observations show that the Cu(2)O-FGS nanocomposite based sensors have potential applications for monitoring air pollution at room temperature with low cost and power consumption.

Photodegradation of methyl orange under visible light by micro-nano hierarchical Cu2O structure fabricated by hybrid laser processing and chemical dealloying

Micro-nano hierarchical structure on the substrate was fabricated by a hybrid approach including laser deposition, laser ablation and chemical dealloying. The structure consists of micro bumps with a width of 50 μm and a height of 100 μm, and nanoporous structures with a size of 70-150 nm on the micro bumps. XRD and XPS results confirm that these hierarchical structures were made of Cu(2)O. For use in comparison, three additional structures with feature size in milliscale, microscale, and nanoscale were also prepared respectively by the proposed methods. Under visible light, the micro-nano structure exhibited the best performance of photodegradation. It is the result of the large specific surface and the catalytic reaction driven by the cuprous oxides.

Following charge separation on the nanoscale in Cu₂O-Au nanoframe hollow nanoparticles

Cu(2)O-Au nanoframes with different nanolayer thicknesses of Cu(2)O were prepared, and their photocatalytic properties in aqueous solutions were studied. Cu(2)O semiconductor excitation leads to electron-hole separation. In aqueous solution, the hole is known to oxidize water to produce hydroxyl radicals whose concentration (and that of the holes) can be monitored by the rate of the degradation of dissolved methylene blue dye. The exciton lifetime is determined by femtosecond techniques and is determined by electron-hole recombination which depends on the rates of a number of competing processes such as, electron or hole transfer to an acceptor such as a gold nanoframe and/or the electron or hole trapping processes at the Cu(2)O-Au nanoframe interface. We measured the exciton lifetime as a function of the average Cu(2)O-Au layer separation. A good correlation was found between the rate of the photocatalytic degradation of methylene blue and the exciton lifetime. The exciton lifetime is found to increase as the Cu(2)O thickness is increased. This leads to an increase in the electron-hole separation time and thus an increase in the hole (and so the hydroxyl radical) concentration leading to an observed enhanced rate of the dye degradation.

Effect of focusing conditions on laser-induced shock waves at titanium-water interface

The spatial and temporal evolution of laser-induced shock waves at a titanium-water interface was analyzed using a beam deflection setup. The focusing conditions of the source laser were varied, and its effect onto the dynamics of shock waves was elucidated. For a tightly focused condition, the speed of the shock wave was ~6.4 Km/s, whereas for a defocused condition the velocities reduced to <3 km/s at the vicinity of the titanium-water interface. When the laser is focused a few millimeters above the target, i.e., within the water, the emission of dual shock waves was observed toward the rear side of the focal volume. These shock waves originate from the titanium-water interface as well as from the pure water breakdown region, respectively. The shock wave pressure is estimated from the shock wave velocity using the Newton's second law across a shock wave discontinuity. The shock wave pressure for a tightly focused condition was 18 GPa, whereas under a defocused condition the pressure experienced was ≤1 GPa in the proximity of target.

Stable aqueous based Cu nanoparticle ink for printing well-defined highly conductive features on a plastic substrate

With the aim of inkjet printing highly conductive and well-defined Cu features on plastic substrates, aqueous based Cu ink is prepared for the first time using water-soluble Cu nanoparticles with a very thin surface oxide layer. Owing to the specific properties, high surface tension and low boiling point, of water, the aqueous based Cu ink endows a variety of advantages over conventional Cu inks based on organic solvents in printing narrow conductive patterns without irregular morphologies. It is demonstrated how the design of aqueous based ink affects the basic properties of printed conductive features such as surface morphology, microstructure, conductivity, and line width. The long-term stability of aqueous based Cu ink against oxidation is analyzed through an X-ray photoelectron spectroscopy (XPS) based investigation on the evolution of the surface oxide layer in the aqueous based ink.

Adsorption of HCN onto Copper@Copper-Oxide Core–Shell Nanoparticle Systems

Copper compounds are widely used as impregnants that enhance the removal of HCN by carbon-based filter media. The reaction mechanism involved is poorly understood. In this study, we have followed the reaction of HCN with pristine copper, copper oxide (CuO and Cu2O) and copper@copperoxide (Cu@Cu2O) core–shell nanoparticles of well-defined size and composition. We have established a cooperative reaction mechanism where both the copper oxide shell and copper core are required for the chemisorption of HCN onto copper nanoparticle impregnants. The suitability of copper@copperoxide nanoparticles as impregnants for the removal of HCN in respirator canisters is discussed.

High spatially resolved morphological, structural and spectroscopical studies on copper oxide nanocrystals

Copper oxide nanocrystals decorated on multi-wall carbon nanotubes have been prepared. Comprehensive morphological, structural and spectroscopical studies have been carried out on the nanometre/atomic scale by the combination of high-resolution transmission electron microscopy and electron energy-loss near-edge structure in electron energy-loss spectroscopy, which has a high spatially resolved capacity advantage over the normally used analytical techniques such as x-ray powder diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). The result reveals that highly crystalline cubic Cu(2)O nanocrystals with highly uniform dispersion, homogeneous size of about 5.3 nm and nearly spherical morphology are synthesized as the predominant phase, while rare individual monoclinic CuO nanocrystals with irregular shape are still present as the minor phase. The analysis based on the survey result and the structural symmetry difference between Cu(2)O and CuO demonstrates that XRD underestimates the presence of the CuO phase with much lower structural symmetry while XPS overestimates the proportion of CuO phase.

Copper Oxide Nanocrystals

It is well-known that inorganic nanocrystals are a benchmark model for nanotechnology, given that the tunability of optical properties and the stabilization of specific phases are uniquely possible at the nanoscale. Copper (I) oxide (Cu(2)O) is a metal oxide semiconductor with promising applications in solar energy conversion and catalysis. To understand the Cu/Cu(2)O/CuO system at the nanoscale, we have developed a method for preparing highly uniform monodisperse nanocrystals of Cu(2)O. The procedure also serves to demonstrate our development of a generalized method for the synthesis of transition metal oxide nanocrystals. Cu nanocrystals are initially formed and subsequently oxidized to form highly crystalline Cu(2)O. The volume change during phase transformation can induce crystal twinning. Absorption in the visible region of the spectrum gave evidence for the presence of a thin, epitaxial layer of CuO, which is blue-shifted, and appears to increase in energy as a function of decreasing particle size. XPS confirmed the thin layer of CuO, calculated to have a thickness of approximately 5 A. We note that the copper (I) oxide phase is surprisingly well-stabilized at this length scale.

Controlled reactions on a copper surface: synthesis and characterization of nanostructured copper compound films

We report the synthesis of nanostructured copper compound films on a copper surface under mild conditions. A series of low-dimensional structures including Cu(OH)(2) fibers and scrolls, CuO sheets and whiskers, and Cu(2)(OH)(2)CO(3) rods have been successfully grown on the copper surfaces at ambient temperature and pressure. Most of the structures are phase-pure single crystallites. The films were formed by the direct oxidation of copper in aqueous solutions of NaOH with an oxidant (NH(4))(2)S(2)O(8). The evolution of the ultrafine structures as a function of the reaction conditions has been revealed, from fibers of Cu(OH)(2) to scrolls of Cu(OH)(2) to sheets or whiskers of CuO. By replacing NaOH with NaHCO(3) in the synthesis, square/rectangular rod arrays of Cu(2)(OH)(2)CO(3) were obtained. The controlled reactions allow the large-scale, template-free, cost-effective synthesis of copper compound films with ordered, uniform, stable, ultrafine structures.