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Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

The ternary mixed metal oxides with high specific surface area are industrially important for sensors, catalysis, energy storage, and optoelectronics. However, synthesis of such metal oxides is alw...

Ultrathin bimetallic nanowires are of importance and interest for applications in electronic devices such as sensors and heterogeneous catalysts. In this work, we have designed a new, highly reproducible and generalized wet chemical method to synthesize uniform and monodispersed Au-based alloy (AuCu, AuPd, and AuPt) nanowires with tunable composition using microwave-assisted reduction at the liquid-liquid interface. These ultrathin alloy nanowires are below 4 nm in diameter and about 2 μm long. Detailed microstructural characterization shows that the wires have an face centred cubic (FCC) crystal structure, and they have low-energy twin-boundary and stacking-fault defects along the growth direction. The wires exhibit remarkable thermal and mechanical stability that is critical for important applications. The alloy wires exhibit excellent electrocatalytic activity for methanol oxidation in an alkaline medium.

The development and industrial application of advanced lithium based energy-storage materials are directly related to the innovative production techniques and the usage of inexpensive precursor materials. Flame spray pyrolysis (FSP) is a promising technique that overcomes the challenges in the production processes such as scalability, process control, material versatility, and cost. In the present study, phase pure anode material Li4Ti5O12 (LTO) was designed using FSP via extensive systematic screening of lithium and titanium precursors dissolved in five different organic solvents. The effect of precursor and solvent parameters such as chemical reactivity, boiling point, and combustion enthalpy on the particle formation either via gas-to-particle (evaporation/nucleation/growth) or via droplet-to-particle (precipitation/incomplete evaporation) is discussed. The presence of carboxylic acid in the precursor solution resulted in pure (>95 mass %) and homogeneous LTO nanoparticles of size 4–9 nm, attributed to...

Mullite-type Bi2Fe4O9 was synthesized using a polyol-mediated method. X-ray powder diffraction (XRD) revealed that the as-synthesized sample is nanocrystalline. It transformed into a rhombohedral perovskite-type BiFeO3 followed by a second transformation into mullite-type Bi2Fe4O9 during heating. Each structural feature, from as-synthesized into crystalline phase, was monitored through temperature-dependent XRD in situ. The locally resolved high resolution transmission electron micrographs revealed that the surface of some heated samples is covered by 4–13 nm sized particles which were identified from the lattice fringes as crystalline Bi2Fe4O9. XRD and Raman spectra demonstrate that the nucleation of both BiFeO3 and Bi2Fe4O9 might simultaneously commence; however, their growth and ratios are dependent on temperature. The diffuse UV/vis reflectance spectra showed fundamental absorption edges between 1.80(1) and 2.75(3) eV. A comparative study between the “derivation of absorption spectrum fitting method” ...

Single crystalline magnetron sputter-deposited Ru(0001) epitaxial thin films on c-plane sapphire were prepared and used as a template for reactive CeO2 growth. Low-energy electron microscopy and diffraction, as well as transmission electron microscopy and atomic force microscopy, experiments were performed to investigate the crystallinity and morphology of the prepared films. Multiple cycles of Ar+ sputtering and high-temperature annealing produces films of exceptional surface quality. High-temperature reactive ceria growth leads to perfectly aligned triangular single-crystalline CeO2(111) islands of extraordinary morphological and structural homogeneity. At the chosen growth conditions, ceria nucleation takes place only at V-shaped surface defects on the otherwise atomically flat Ru terraces, opening up the possibility to influence the nucleation by introducing artificial surface defects using standard etching techniques. Due to their high crystallinity and extraordinary surface quality, these substrates...

A detailed understanding of structure and stability of nanowires is critical for applications. Atomic resolution imaging of ultrathin single crystalline Au nanowires using aberration-corrected microscopy reveals an intriguing relaxation whereby the atoms in the close-packed atomic planes normal to the growth direction are displaced in the axial direction leading to wrinkling of the (111) atomic plane normal to the wire axis. First-principles calculations of the structure of such nanowires confirm this wrinkling phenomenon, whereby the close-packed planes relax to form saddle-like surfaces. Molecular dynamics studies of wires with varying diameters and different bounding surfaces point to the key role of surface stress on the relaxation process. Using continuum mechanics arguments, we show that the wrinkling arises due to anisotropy in the surface stresses and in the elastic response, along with the divergence of surface-induced bulk stress near the edges of a faceted structure. The observations provide new understanding on the equilibrium structure of nanoscale systems and could have important implications for applications in sensing and actuation.

We present a synthesis route to fabricate submicrometer-sized colloidosomes at ambient conditions and mild pH with tailorable nanopore sizes and porosity. The capsules are formed via self-assembly of metal oxide nanoparticles on emulsion droplets in a water-in-oil emulsion. Through the adsorption of oil-soluble surfactants on emulsion droplets, which carry the same sign of charge as the colloidosome-forming particles in aqueous media, colloidosomes with positive and negative zeta potentials were synthesized. The hollow capsules are inherently rigid and necessitated no further stabilization. By varying the sizes and shapes of the nanoparticles, we were able to tailor the pore diameters and pore size distributions on the surface of the capsules. Particles of high spherical uniformity with narrow size range generated colloidosomes with a mainly hexagonal close packed surface structure and narrow pore size, while particles of elliptical and uneven shape created colloidosomes with no surface order and an incon...

ZnO nanowire arrays were functionalized with colloidal CdSe quantum dots stabilized by 3-mercaptopropionic acid to form hybrid devices. The photoconductivity of the nanowire/quantum-dot devices was studied under selective photoexcitation of the quantum dots, and it was found that the dynamics strongly depend on the gas environment. Desorption of surface oxygen from both the ZnO nanowires and the CdSe quantum dots, activated by electron tunnelling between the nanowires and the quantum dots, is found to be the dominating process that determines the dynamics of the photoconductivity in the hybrid nanowire/quantum-dot devices.

The rational design of composite nanoparticles with desired optical and electronic properties requires the detailed analysis of surface and bulk contributions to the respective overall function. We use flame spray pyrolysis (FSP) to generate nanoparticles of the ternary Zn-Mg-O system the compositions of which range from solid solutions of Zn(2+) ions in periclase MgO to phase separated particle mixtures which consist of periclase (cubic) MgO and wurtzite (hexagonal) ZnO phases. The structure and composition of the composite Zn(x)Mg(1-x)O (0 ≤ x ≤ 0.3) particles are investigated using X-ray diffraction and high-resolution transmission electron microscopy, whereas UV diffuse reflectance and photoluminescence (PL) spectroscopy are used for the investigation of their optical properties. Vacuum annealing has been carried out to track the effects of stepwise elimination of surface adsorbates on the photoexcitation and PL emission properties. We demonstrate that for Zn(0.1)Mg(0.9)O particles, the admixed ZnO suppresses the MgO specific surface excitons and produces a PL emission band at 470 nm. Although gaseous oxygen partially reduces the emission intensity of hydroxylated particles, it leads to entire quenching in completely dehydroxylated samples after vacuum annealing at 1173 K. Consequently, surface hydroxyls at the solid-gas interface play a significant role as protecting groups against the PL-quenching effects of O(2). The obtained results are relevant for the characterization of ZnO-based devices as well as for other metal oxide materials where the impact of the surface composition on the photoelectronic properties is usually neglected.

The stability and electronic structure of bare [001] nanowires of anatase and rutile have been investigated using ab initio density functional calculations. It was found that symmetry plays an important role in both properties below a critical diameter. Up to 2.1 nm in anatase and 3.7 nm in rutile, those {100}-walled anatase and {110}-walled rutile wires are most stable, which retain the nonsymmorphic character of the bulk space group. Our results explain the observed properties of atomic size anatase nanowires. The wires with screw symmetry also show a consistently larger gap in their electronic structures compared to similarly walled wires without it. Additionally, in rutile the indirect or direct character of the band structure is coupled to the presence or absence of the screw axis.

During the growth process of the nacre layer in abalone shells, aragonite platelets are built into a preformed organic matrix. This matrix consists of chitin and different proteins that may affect ...

Flame spray pyrolysis (FSP) has been employed for the syntheses of single crystalline WO3 nanoparticles using tungsten precursors with tungsten in usual (+6) and unusual (+4 and 0) oxidation states. The WO3 nanoparticles have been characterized by powder X-ray diffraction and the patterns have been refined using the Rietveld and the Le Bail method (space group P21/n, monoclinic system; the averaged lattice parameters of the three WO3: a = 7.3913(4) A, b = 7.5630(5) A, c = 7.6615(6) A, β = 90.78(3)°, Z = 8, V = 423.19 A3). Because of the very small crystallite sizes, non-Bragg scattering is observed. The crystallite sizes derived from Rietveld or Le Bail analyses were in the range of 6−8 nm, reasonably agreeing with the crystallite sizes (7−10 nm) determined from Brunauer−Emmett−Teller and microscopic analysis where each particle is basically a small single crystal. Well-developed lattice fringes of 3.860−3.994 A were illustrated by high resolution transmission electron microscopy (HRTEM). Indexing of sele...