4 research outputs found
In Situ Determination of Colloidal Gold Concentrations with UV–Vis Spectroscopy: Limitations and Perspectives
This paper studies the UV–vis
absorbance of colloidal gold
nanoparticles at 400 nm and validates it as a method to determine
Au(0) concentrations in colloidal gold solutions. The method is shown
to be valid with restrictions depending on the investigated system.
The uncertainty of the determined Au(0) concentration can be up to
30%. This deviation is the result of the combined influence of parameters
such as particle size, surface modification, or oxidation state. However,
quantifying the influence of these parameters enables a much more
precise Au(0) determination for specific systems. As an example, the
reduction process of the well-known Turkevich method was monitored
and the Au(0) concentration was determined with a deviation of less
than 5%. Hence, a simple, fast, easy, and cheap in situ method for
Au(0) determination is demonstrated that has in the presence of other
gold species such as AuÂ(III) an unprecedented accuracy
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