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.

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 Au(III) an unprecedented accuracy.

This contribution investigates the growth mechanism of the Turkevich method. The experimental results provide the important missing piece of the mechanistic puzzle which enables the actual control of particle growth in the common 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.

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.

The authors note that the units of all values of determined extinction coefficients of Au(0) at a wavelength of 400 nm (ε 400 ) were given as L mol −1 cm −1 but should be replaced with L mmol −1 cm −1 . This affects ε 400 values on page 11121 (last paragraph), in the caption of Table 3 on page 11122, in the conclusions section on page 11122, and in the caption of Table S2 in the Supporting Information.A corrected version of Table 3 and Table S2 can be found in this Addition and Correction.However, none of the main findings or conclusions are changed. We deeply apologize for any confusion created by this mistake.