9 research outputs found
Antireflective Coatings with Adjustable Refractive Index and Porosity Synthesized by Micelle-Templated Deposition of MgF<sub>2</sub> 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 MgF<sub>2</sub> nanoparticles.
The material synthesis employs micelles of amphiphilic block copolymers
as structure directing agent to introduce controlled mesoporosity
into MgF<sub>2</sub> 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 TiO<sub>2</sub> 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 HAuCl<sub>4</sub>, 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 AgClO<sub>4</sub> with NaBH<sub>4</sub>. 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 (H<sub>2</sub>O<sub>2</sub>)
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
H<sub>2</sub>O<sub>2</sub> production of a number of distinct nitrogen-doped
mesoporous carbon catalysts and report a previously unachieved H<sub>2</sub>O<sub>2</sub> 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 H<sub>2</sub>O<sub>2</sub> product yield and the surface area and interfacial
zeta potential. Nitrogen doping was found to sharply boost H<sub>2</sub>O<sub>2</sub> activity and selectivity. Chronoamperometric H<sub>2</sub>O<sub>2</sub> electrolysis confirms the exceptionally high
H<sub>2</sub>O<sub>2</sub> production rate and large H<sub>2</sub>O<sub>2</sub> faradaic selectivity for the optimal nitrogen-doped
CMK-3 sample in acidic, neutral, and alkaline solutions. In alkaline
solution, the catalytic H<sub>2</sub>O<sub>2</sub> yield increases
further, where the production rate of the HO<sub>2</sub><sup>–</sup> anion reaches a value as high as 561.7 mmol g<sub>catalyst</sub><sup>–1</sup> h<sup>–1</sup> with H<sub>2</sub>O<sub>2</sub> faradaic selectivity above 70%.
Our work provides a guide for the design, synthesis, and mechanistic
investigation of advanced carbon-based electrocatalysts for H<sub>2</sub>O<sub>2</sub> production