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nanoelectronic devices. [21,23] Bimetallic nanoparticles are of specific interest because of their versatility and tunability mainly for (photo)catalytic applications. Especially the gold-silver combination has been thoroughly investigated due to its optical surface plasmon resonance (SPR) absorbance that can be tuned over the entire visible range of the electromagnetic spectrum, merely depending on the composition of both metals. [2,13,14,24] Various methods have been applied to synthesize Au-Ag bimetallic nanoparticles, of which the most widely used is the Turkevich method. [25] This method is based on the coreduction of metal precursors in the presence of a stabilizing agent, usually sodium citrate, in boiling water. Most studies report on the formation of alloy nanoparticles, [26][27][28][29][30][31][32] although some researchers have also indicated the possible formation of core-shell structures. [29,33] The assumption of a full alloy composition is mostly based on UV-vis absorption data, that only show one single plasmon band in the visible light range. [29][30][31][32]34,35] Indeed, two separate plasmon bands are usually observed in the case of a core-shell structure. [27,35,36] Hereby, it is important to note that according to Mie theory, the total extinction cross-section scales with the volume of the nanoparticles. [37] The observation of only one plasmon band in the UV-vis spectrum therefore not excludes a core-shell structure, since such a morphology with a sufficiently thick shell also leads to a sole plasmon band. Other indirect evidence of alloy formation is provided by 2D energy-dispersive X-ray (EDX) projection images. However, 2D TEM projection images of 3D objects can be very misleading and are not necessarily representative for the real 3D composition. [38] In this work, we aim to accurately elucidate the real structure and synthesis pathway of gold-silver bimetallic nanoparticles, synthesized according to the universally applied Turkevich method, by analyzing samples taken at several time intervals during the synthesis. An experimental wet-chemical study is linked to both an advanced electron microscopy analysis, including EDX tomography, and extensive modeling. In this way a complete analysis is provided to once and for all unravel the structural enigma surrounding the Turkevich synthesis of gold-silver bimetallic nanoparticles.For the synthesis of gold-silver bimetallic nanoparticles, the Turkevich method has been the state-of-the-art method for several decades. It is presumed that this procedure results in a homogeneous alloy, although this has been debatable for many years. In this work, it is shown that neither a full alloy, nor a perfect core-shell particle is formed but rather a core-shelllike particle with altering metal composition along the radial direction. In-depth wet-chemical experiments are performed in combination with advanced transmission electron microscopy, including energy-dispersive X-ray tomography, and finite element method modeling to support the observations. From th...

Silver nanoparticles are widely used in the field of plasmonics because of their unique optical properties. The wavelength-dependent surface plasmon resonance gives rise to a strongly enhanced electromagnetic field, especially at so-called hot spots located in the nanogap in-between metal nanoparticle assemblies. Therefore, the interparticle distance is a decisive factor in plasmonic applications, such as surface-enhanced Raman spectroscopy (SERS). In this study, the aim is to engineer this interparticle distance for silver nanospheres using a convenient wet-chemical approach and to predict and quantify the corresponding enhancement factor using both theoretical and experimental tools. This was done by building a tunable ultrathin polymer shell around the nanoparticles using the layer-by-layer method, in which the polymer shell acts as the separating interparticle spacer layer. Comparison of different theoretical approaches and corroborating the results with SERS analytical experiments using silver and silver-polymer core-shell nanoparticle clusters as SERS substrates was also done. Herewith, an approach is provided to estimate the extent of plasmonic near-field enhancement both theoretically as well as experimentally.

Photocatalytic reactors for the degradation of gaseous organic pollutants often suffer from major limitations such as small reaction area, sub-optimal irradiation conditions and thus limited reaction rate. In this work, an alternative solution is presented that involves a glass tube coated on the inside with (silver-modified) TiO 2 and spiraled around a UVA lamp. First, the spiral reactor is coated from the inside with TiO 2 using an experimentally verified procedure that is optimized toward UV light transmission. This procedure is kept as simple as possible and involves a single casting step of a 1 wt% suspension of TiO 2 in ethanol through the spiral. This results in a coated tube that absorbs nearly all incident UV light under the experimental conditions used. The optimized coated spiral reactor is then benchmarked to a conventional annular photoreactor of the same outer dimensions and total catalyst loading over a broad range of experimental conditions. Although residence time distribution experiments indicate slightly longer dwelling of molecules in the spiral reactor, no significant difference in by-passing of gas between the spiral reactor and the annular reactor can be claimed. Acetaldehyde degradation efficiency of 100% is obtained with the spiral reactor for a residence time as low as 60 s, whereas the annular reactor could not achieve full degradation even at 1000 s residence time. In a final case study, addition of long-term stable silver nanoparticles, protected by an ultra-thin polymer shell applied via the layer-by-layer (LbL) method, to the spiral reactor coating is shown to double the degradation efficiency and provides an interesting strategy to cope with higher pollutant concentrations without changing the overall dimensions.

Direct photocatalytic reduction of CO2 has become an highly active field of research. It is thus of utmost importance to maintain an overview of the various materials used to sustain this process, find common trends, and, in this way, eventually improve the current conversions and selectivities. In particular, CO2 photoreduction using plasmonic photocatalysts under solar light has gained tremendous attention, and a wide variety of materials has been developed to reduce CO2 towards more practical gases or liquid fuels (CH4, CO, CH3OH/CH3CH2OH) in this manner. This Review therefore aims at providing insights in current developments of photocatalysts consisting of only plasmonic nanoparticles and semiconductor materials. By classifying recent studies based on product selectivity, this Review aims to unravel common trends that can provide effective information on ways to improve the photoreduction yield or possible means to shift the selectivity towards desired products, thus generating new ideas for the way forward.