Antireflective Coatings with Adjustable Refractive Index and Porosity Synthesized by Micelle-Templated Deposition of MgF₂ Sol Particles

Minimizing efficiency losses caused by unwanted light reflection at the interface between lenses, optical instruments and solar cells with the surrounding medium requires antireflective coatings with adequate refractive index and coating thickness. We describe a new type of antireflective coating material with easily and independently tailorable refractive index and coating thickness based on the deposition of colloidal MgF₂ nanoparticles. The material synthesis employs micelles of amphiphilic block copolymers as structure directing agent to introduce controlled mesoporosity into MgF₂ film. The coatings thickness can be easily adjusted by the applied coating conditions. The coatings refractive index is determined by the materials porosity, which is controlled by the amount of employed pore template. The refractive index can be precisely tuned between 1.23 and 1.11, i.e., in a range that is not accessible to nonporous inorganic materials. Hence, zero reflectance conditions can be established for a wide range of substrate materials

Highly Active Binder-Free Catalytic Coatings for Heterogeneous Catalysis and Electrocatalysis: Pd on Mesoporous Carbon and Its Application in Butadiene Hydrogenation and Hydrogen Evolution

Heterogeneous catalysis performed in wall-coated reactors and electrocatalysis require homogeneous catalytic coatings with high surface area and good accessibility of the active sites. Conventional coating methods necessitate the use of binder components that often block pores and active sites, which limits catalytic efficiency, and utilization of expensive active metals. We report an approach for the direct and binder-free synthesis of chemically, mechanically, and thermally stable catalytic coatings based on ordered mesoporous carbon films employed as catalyst support. The synthesis relies on the codeposition of a structure-directing agent and small clusters of polymeric carbon precursors along with ionic metal species on a substrate. A sequence of thermal treatments converts the polymer into partly graphitized carbon, decomposes the structure-directing agent, and converts the metal precursor into highly active nanoparticles. Syntheses and catalytic applications are exemplarily demonstrated for palladium on carbon, a system widely used in heterogeneous catalysis and electrocatalysis. The obtained catalysts provide significantly higher space–time yields in the selective gas-phase hydrogenation of butadiene than all reported Pd/C catalysts while at the same time retaining isothermal reactor conditions. Moreover, when they were tested in the electrocatalytic hydrogen evolution reaction (HER), the catalysts outperformed reported Pd/C catalysts by a factor of 3, which underlines the benefits of the developed binder-free catalyst system

New Approach on Quantification of Porosity of Thin Films via Electron-Excited X‑ray Spectra

One of the crucial characteristics of functionalized thin films is their porosity (i.e., the ratio between the pore volume and the volume of the whole film). Due to the very low amount of material per coated area corresponding to thin films, it is a challenge for analytics to measure the film porosity. In this work, we present an approach to determine the porosity of thin films by means of electron probe microanalysis (EPMA) either by wavelength-dispersive X-ray spectrometry (WDX) or by energy-dispersive X-ray spectrometry (EDX) with a scanning electron microscope (SEM). The procedure is based on the calculation of the film mass deposition from electron-excited X-ray spectra. The mass deposition is converted into film density by division of measured film thickness. Finally, the film porosity is calculated from the measured film density and the density of bulk, nonporous film material. The general applicability of the procedure to determine the porosity is demonstrated on thin templated mesoporous TiO₂ films, dip-coated on silicon wafer, with controlled porosity in the range of 15 to 50%. The high accuracy of the mass deposition as determined from X-ray spectra was validated with independent methods (ICP-OES and weighing). Furthermore, for the validation of the porosity results, ellipsometry, interference fringes method (IFM), and focused ion beam (FIB) cross sectioning were employed as independent techniques. Hence, the approach proposed in the present study is proven to be suited as a new analytical tool for accurate and relatively fast determination of the porosity of thin films

Formation Mechanism of Colloidal Silver Nanoparticles: Analogies and Differences to the Growth of Gold Nanoparticles

The formation mechanisms of silver nanoparticles using aqueous silver perchlorate solutions as precursors and sodium borohydride as reducing agent were investigated based on time-resolved <i>in situ</i> experiments. This contribution addresses two important issues in colloidal science: (i) differences and analogies between growth processes of different metals such as gold and silver and (ii) the influence of a steric stabilizing agent on the growth process. The results reveal that a growth due to coalescence is a fundamental growth principle if the monomer-supplying chemical reaction is faster than the actual particle formation

Missing Piece of the Mechanism of the Turkevich Method: The Critical Role of Citrate Protonation

This contribution investigates the growth mechanism of the Turkevich method. The experimental results provide the missing piece of the mechanistic puzzle which enables the actual control of particle growth in the commonly used Turkevich method. Applying the gained knowledge, the boundary conditions for a successful Turkevich synthesis are deduced. Moreover, the conditions under which the Turkevich method is highly reproducible are derived. Following these conditions, the Turkevich synthesis is modified to reveal small monodisperse particles with an unprecedented reproducibility of ±0.1 nm

Unifying Concepts in Room-Temperature CO Oxidation with Gold Catalysts

The oxidation of CO is a fundamental model reaction in heterogeneous catalysis. This contribution presents an uncommon approach to investigate a catalytic gas-phase reaction by using colloidal gold and provides a unified picture of the CO oxidation of supported gold nanoparticles at room temperature. Our experiments on ligand-free colloidal gold nanoparticles prove that gold activates molecular oxygen independently from the presence of any support. Isotope experiments along with studies on colloidal stability reveal that the active oxygen species is a stable surface oxide that can be protonated. The role of the support is to provide water for protonation steps. Therefore, the hydrophilicity is the main property of the support which determines the catalytic activity and not, as is often assumed, its acidity or reducibility. The deduced model provides explanations for experimental results described in the literature for various gold catalysts and reaction conditions

Turkevich in New Robes: Key Questions Answered for the Most Common Gold Nanoparticle Synthesis

This contribution provides a comprehensive mechanistic picture of the gold nanoparticle synthesis by citrate reduction of HAuCl₄, known as Turkevich method, by addressing five key questions. The synthesis leads to monodisperse final particles as a result of a seed-mediated growth mechanism. In the initial phase of the synthesis, seed particles are formed onto which the residual gold is distributed during the course of reaction. It is shown that this mechanism is a fortunate coincidence created by a favorable interplay of several chemical and physicochemical processes which initiate but also terminate the formation of seed particles and prevent the formation of further particles at later stages of reaction. Since no further particles are formed after seed particle formation, the number of seeds defines the final total particle number and therefore the final size. The gained understanding allows illustrating the influence of reaction conditions on the growth process and thus the final size distribution

Size-Controlled Synthesis of Colloidal Silver Nanoparticles Based on Mechanistic Understanding

Metal nanoparticles have attracted much attention due to their unique properties. Size control provides an effective key to an accurate adjustment of colloidal properties. The common approach to size control is testing different sets of parameters via trial and error. The actual particle growth mechanisms, and in particular the influences of synthesis parameters on the growth process, remain a black box. As a result, precise size control is rarely achieved for most metal nanoparticles. This contribution presents an approach to size control that is based on mechanistic knowledge. It is exemplified for a common silver nanoparticle synthesis, namely, the reduction of AgClO₄ with NaBH₄. Conducting this approach allowed a well-directed modification of this synthesis that enables, for the first time, the size-controlled production of silver nanoparticles 4–8 nm in radius without addition of any stabilization agent

Efficient Electrochemical Hydrogen Peroxide Production from Molecular Oxygen on Nitrogen-Doped Mesoporous Carbon Catalysts

Electrochemical hydrogen peroxide (H₂O₂) production by two-electron oxygen reduction is a promising alternative process to the established industrial anthraquinone process. Current challenges relate to finding cost-effective electrocatalysts with high electrocatalytic activity, stability, and product selectivity. Here, we explore the electrocatalytic activity and selectivity toward H₂O₂ production of a number of distinct nitrogen-doped mesoporous carbon catalysts and report a previously unachieved H₂O₂ selectivity of ∼95–98% in acidic solution. To explain our observations, we correlate their structural, compositional, and other physicochemical properties with their electrocatalytic performance and uncover a close correlation between the H₂O₂ product yield and the surface area and interfacial zeta potential. Nitrogen doping was found to sharply boost H₂O₂ activity and selectivity. Chronoamperometric H₂O₂ electrolysis confirms the exceptionally high H₂O₂ production rate and large H₂O₂ faradaic selectivity for the optimal nitrogen-doped CMK-3 sample in acidic, neutral, and alkaline solutions. In alkaline solution, the catalytic H₂O₂ yield increases further, where the production rate of the HO₂^– anion reaches a value as high as 561.7 mmol g_catalyst^–1 h^–1 with H₂O₂ faradaic selectivity above 70%. Our work provides a guide for the design, synthesis, and mechanistic investigation of advanced carbon-based electrocatalysts for H₂O₂ production