Towards High-Performance Photo-Fenton Degradation of Organic Pollutants with Magnetite-Silver Composites: Synthesis, Catalytic Reactions and In Situ Insights.

In this study, Fe3O4/Ag magnetite-silver (MSx) nanocomposites were investigated as catalysts for advanced oxidation processes by coupling the plasmonic effect of silver nanoparticles and the ferromagnetism of iron oxide species. A surfactant-free co-precipitation synthesis method yielded pure Fe3O4 magnetite and four types of MSx nanocomposites. Their characterisation included structural, compositional, morphological and optical analyses, revealing Fe3O4 magnetite and Ag silver phases with particle sizes ranging from 15 to 40 nm, increasing with the silver content. The heterostructures with silver reduced magnetite particle aggregation, as confirmed by dynamic light scattering. The UV-Vis spectra showed that the Fe:Ag ratio strongly influenced the absorbance, with a strong absorption band around 400 nm due to the silver phase. The oxidation kinetics of organic pollutants, monitored by in situ luminescence measurements using rhodamine B as a model system, demonstrated the higher performance of the developed catalysts with increasing Ag content. The specific surface area measurements highlighted the importance of active sites in the synergistic catalytic activity of Fe3O4/Ag nanocomposites in the photo-Fenton reaction. Finally, the straightforward fabrication of diverse Fe3O4/Ag heterostructures combining magnetism and plasmonic effects opens up promising possibilities for heterogeneous catalysis and environmental remediation.

A novel method for the synthesis of core-shell nanoparticles for functional applications based on long-term confinement in a radio frequency plasma.

A novel combined setup of a Haberland type gas aggregation source and a secondary radio frequency discharge is used to generate, confine, and coat nanoparticles over much longer time scales than traditional in-flight treatment. The process is precisely monitored using localized surface plasmon resonance and Fourier-transform infrared spectroscopy as in situ diagnostics. They indicate that both untreated and treated particles can be confined for extended time periods (at least one hour) with minimal losses. During the entire confinement time, the particle sizes do not show considerable alterations, enabling multiple well-defined modifications of the seed nanoparticles in this synthesis approach. The approach is demonstrated by generating Ag@SiO2 nanoparticles with a well-defined surface coating. The in situ diagnostics provide insights into the growth kinetics of the applied coating and are linked to the coating properties by using ex situ transmission electron microscopy and energy dispersive X-ray spectroscopy. Surface coating is shown to occur in two phases: first, singular seeds appear on the particle surface which then grow to cover the entire particle surface over 3 to 5 minutes. Afterwards, deposition occurs via surface growth which coincides with lower deposition rates. Our setup offers full control for various treatment options, which is demonstrated by coating the nanoparticles with a SiO2 layer followed by the etching of the part of the applied coating using hydrogen. Thus, complex multi-step nanofabrication, e.g., using different monomers, as well as very large coating thicknesses is possible.

Brain-like critical dynamics and long-range temporal correlations in percolating networks of silver nanoparticles and functionality preservation after integration of insulating matrix.

Random networks of nanoparticle-based memristive switches enable pathways for emulating highly complex and self-organized synaptic connectivity together with their emergent functional behavior known from biological neuronal networks. They therefore embody a distinct class of neuromorphic hardware architectures and provide an alternative to highly regular arrays of memristors. Especially, networks of memristive nanoparticles (NPs) poised at the percolation threshold are promising due to their capabilities of showing brain-like activity such as critical dynamics or long-range temporal correlation (LRTC), which are closely connected to the computational capabilities in biological neuronal networks. Here, we adapt this concept to networks of Ag-NPs poised at the electrical percolation threshold, where the memristive properties are governed by electro-chemical metallization. We show that critical dynamics and LRTC are preserved although the nature of individual memristive gaps throughout the network is fundamentally changed by filling the gaps with an insulating matrix. The results in this work generate important contributions towards the practical applicability of critical dynamics and LRTC in percolating NP networks by elucidating the consequences of NP network encapsulation, which is considered as an important step towards device integration.

Nanosensors Based on a Single ZnO:Eu Nanowire for Hydrogen Gas Sensing.

Fast detection of hydrogen gas leakage or its release in different environments, especially in large electric vehicle batteries, is a major challenge for sensing applications. In this study, the morphological, structural, chemical, optical, and electronic characterizations of ZnO:Eu nanowire arrays are reported and discussed in detail. In particular, the influence of different Eu concentrations during electrochemical deposition was investigated together with the sensing properties and mechanism. Surprisingly, by using only 10 µM Eu ions during deposition, the value of the gas response increased by a factor of nearly 130 compared to an undoped ZnO nanowire and we found an H2 gas response of ∼7860 for a single ZnO:Eu nanowire device. Further, the synthesized nanowire sensors were tested with ultraviolet (UV) light and a range of test gases, showing a UV responsiveness of ∼12.8 and a good selectivity to 100 ppm H2 gas. A dual-mode nanosensor is shown to detect UV/H2 gas simultaneously for selective detection of H2 during UV irradiation and its effect on the sensing mechanism. The nanowire sensing approach here demonstrates the feasibility of using such small devices to detect hydrogen leaks in harsh, small-scale environments, for example, stacked battery packs in mobile applications. In addition, the results obtained are supported through density functional theory-based simulations, which highlight the importance of rare earth nanoparticles on the oxide surface for improved sensitivity and selectivity of gas sensors, even at room temperature, thereby allowing, for instance, lower power consumption and denser deployment.

Al2O3/ZnO Heterostructure-Based Sensors for Volatile Organic Compounds in Safety Applications.

Monitoring volatile organic compounds (VOCs) in harsh environments, especially for safety applications, is a growing field that requires specialized sensor structures. In this work, we demonstrate the sensing properties toward the most common VOCs of columnar Al2O3/ZnO heterolayer-based sensors. We have also developed an approach to tune the sensor selectivity by changing the thickness of the exposed amorphous Al2O3 layer from 5 to 18 nm. Columnar ZnO films are prepared by a chemical solution method, where the exposed surface is decorated with an Al2O3 nanolayer via thermal atomic layer deposition at 75 °C. We have investigated the structure and morphology as well as the vibrational, chemical, electronic, and sensor properties of the Al2O3/ZnO heterostructures. Transmission electron microscopy (TEM) studies show that the upper layers consist of amorphous Al2O3 films. The heterostructures showed selectivity to 2-propanol vapors only within the range of 12-15 nm thicknesses of Al2O3, with the highest response value of ∼2000% reported for a thickness of 15 nm at the optimal working temperature of 350 °C. Density functional theory (DFT) calculations of the Al2O3/ZnO(1010) interface and its interaction with 2-propanol (2-C3H7OH), n-butanol (n-C4H9OH), ethanol (C2H5OH), acetone (CH3COCH3), hydrogen (H2), and ammonia (NH3) show that the molecular affinity for the Al2O3/ZnO(1010) interface decreases from 2-propanol (2-C3H7OH) ≈ n-butanol (n-C4H9OH) > ethanol (C2H5OH) > acetone (CH3COCH3) > hydrogen (H2), which is consistent with our gas response experiments for the VOCs. Charge transfers between the surface and the adsorbates, and local densities of states of the interacting atoms, support the calculated strength of the molecular preferences. Our findings are highly important for the development of 2-propanol sensors and to our understanding of the effect of the heterojunction and the thickness of the top nanolayer on the gas response, which thus far have not been reported in the literature.

Sparse CNT networks with implanted AgAu nanoparticles: A novel memristor with short-term memory bordering between diffusive and bipolar switching.

With this work we introduce a novel memristor in a lateral geometry whose resistive switching behaviour unifies the capabilities of bipolar switching with decelerated diffusive switching showing a biologically plausible short-term memory. A new fabrication route is presented for achieving lateral nano-scaled distances by depositing a sparse network of carbon nanotubes (CNTs) via spin-coating of a custom-made CNT dispersion. Electrochemical metallization-type (ECM) resistive switching is obtained by implanting AgAu nanoparticles with a Haberland-type gas aggregation cluster source into the nanogaps between the CNTs and shows a hybrid behaviour of both diffusive and bipolar switching. The resistance state resets to a high resistive state (HRS) either if the voltage is removed with a retention time in the second- to sub-minute scale (diffusive) or by applying a reverse voltage (bipolar). Furthermore, the retention time is positively correlated to the duration of the Set voltage pulse. The potential for low-voltage operation makes this approach a promising candidate for short-term memory applications in neuromorphic circuits. In addition, the lateral fabrication approach opens the pathway towards integrating sensor-functionality and offers a general starting point for the scalable fabrication of nanoscaled devices.

TiO2/Cu2O/CuO Multi-Nanolayers as Sensors for H2 and Volatile Organic Compounds: An Experimental and Theoretical Investigation.

TiO2/Cu2O/CuO multi-nanolayers highly sensitive toward volatile organic compounds (VOCs) and H2 have been grown in various thicknesses by a cost-effective and reproducible combined spray-sputtering-annealing approach. The ultrathin TiO2 films were deposited by spray pyrolysis on top of sputtered-annealed Cu2O/CuO nanolayers to enhance their gas sensing performance and improve their protection against corrosion at high operating temperatures. The prepared heterostructures were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and ultraviolet visible (UV-vis) and micro-Raman spectroscopy. The gas sensing properties were measured at several operating temperatures, where the nanolayered sensors with oxide thicknesses between 20 and 30 nm (Cu2O/CuO nanolayers) exhibited a high response and an excellent selectivity to ethanol vapor after thermal annealing the samples at 420 °C. The results obtained at an operating temperature of 350 °C demonstrate that the CuO/Cu2O nanolayers with thicknesses between 20 and 30 nm are sensitive mainly to ethanol vapor, with a response of ∼150. The response changes from ethanol vapors to hydrogen gas as the thickness of the CuO/Cu2O nanolayers changes from 50 to 20 nm. Density functional theory-based calculations were carried out for the geometries of the CuO(1̅11)/Cu2O(111) and TiO2(111)/CuO(1̅11)/Cu2O(111) heterostructures and their sensing mechanism toward alcohols of different chain lengths and molecular hydrogen. The reconstructed hexagonal Cu2O(111) surface and the reconstructed monoclinic CuO(1̅11) and TiO2(111) facets, all of which terminate in an O layer, lead to the lowest surface energies for each isolated material. We studied the formation of the binary and ternary heteroepitaxial interfaces for the surface planes with the best-matching lattices. Despite the impact of the Cu2O(111) substrate in lowering the atomic charges of the CuO(1̅11) adlayer in the binary sensor, we found that it is the different surface structures of the CuO(1̅11)/Cu2O(111) and TiO2(111)/CuO(1̅11)/Cu2O(111) devices that are fundamental in driving the change in the sensitivity response observed experimentally. The experimental data, supported by the computational results, are important in understanding the use of the multi-nanolayered films tested in this work as reliable, accurate, and selective sensor structures for the tracking of gases at low concentrations.

Enhancing Reliability of Studies on Single Filament Memristive Switching via an Unconventional cAFM Approach.

Memristive devices are highly promising for implementing neuromorphic functionalities in future electronic hardware, and direct insights into memristive phenomena on the nanoscale are of fundamental importance to reaching this. Conductive atomic force microscopy (cAFM) has proven to be an essential tool for probing memristive action locally on the nanoscale, but the significance of the acquired data frequently suffers from the nonlocality associated with the thermal drift of the tip in ambient conditions. Furthermore, comparative studies of different configurations of filamentary devices have proven to be difficult, because of an immanent variability of the filament properties between different devices. Herein, these problems are addressed by constraining the memristive action directly at the apex of the probe through functionalization of a cAFM tip with an archetypical memristive stack, which is comprised of Ag/Si3N4. The design of such functionalized cantilevers (entitled here as "memtips") allowed the capture of the long-term intrinsic current response, identifying temporal correlations between switching events, and observing emerging spiking dynamics directly at the nanoscale. Utilization of an identical memtip for measurements on different counter electrodes made it possible to directly compare the impact of different device configurations on the switching behavior of the same filament. Such an analytical approach in ambient conditions will pave the way towards a deeper understanding of filamentary switching phenomena on the nanoscale.

Single CuO/Cu2O/Cu Microwire Covered by a Nanowire Network as a Gas Sensor for the Detection of Battery Hazards.

In this study, a strategy to prepare CuO/Cu2O/Cu microwires that are fully covered by a nanowire (NW) network using a simple thermal-oxidation process is developed. The CuO/Cu2O/Cu microwires are fixed on Au/Cr pads with Cu microparticles. After thermal annealing at 425 °C, these CuO/Cu2O/Cu microwires are used as room-temperature 2-propanol sensors. These sensors show different dominating gas responses with operating temperatures, e.g., higher sensitivity to ethanol at 175 °C, higher sensitivity to 2-propanol at room temperature and 225 °C, and higher sensitivity to hydrogen gas at ∼300 °C. In this context, we propose the sensing mechanism of this three-in-one sensor based on CuO/Cu2O/Cu. X-ray diffraction (XRD) studies reveal that the annealing time during oxidation affects the chemical appearance of the sensor, while the intensity of reflections proves that for samples oxidized at 425 °C for 1 h the dominating phase is Cu2O, whereas upon further increasing the annealing duration up to 5 h, the CuO phase becomes dominant. The crystal structures of the Cu2O-shell/Cu-core and the CuO NW networks on the surface were confirmed with a transmission electron microscope (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED), where (HR)TEM micrographs reveal the monoclinic CuO phase. Density functional theory (DFT) calculations bring valuable inputs to the interactions of the different gas molecules with the most stable top surface of CuO, revealing strong binding, electronic band-gap changes, and charge transfer due to the gas molecule interactions with the top surface. This research shows the importance of the nonplanar CuO/Cu2O layered heterostructure as a bright nanomaterial for the detection of various gases, controlled by the working temperature, and the insight presented here will be of significant value in the fabrication of new p-type sensing devices through simple nanotechnology.

Pd-Functionalized ZnO:Eu Columnar Films for Room-Temperature Hydrogen Gas Sensing: A Combined Experimental and Computational Approach.

Reducing the operating temperature to room temperature is a serious obstacle on long-life sensitivity with long-term stability performances of gas sensors based on semiconducting oxides, and this should be overcome by new nanotechnological approaches. In this work, we report the structural, morphological, chemical, optical, and gas detection characteristics of Eu-doped ZnO (ZnO:Eu) columnar films as a function of Eu content. The scanning electron microscopy (SEM) investigations showed that columnar films, grown via synthesis from a chemical solutions (SCS) approach, are composed of densely packed columnar type grains. The sample sets with contents of ∼0.05, 0.1, 0.15, and 0.2 at% Eu in ZnO:Eu columnar films were studied. Surface functionalization was achieved using PdCl2 aqueous solution with additional thermal annealing in air at 650 °C. The temperature-dependent gas-detection characteristics of Pd-functionalized ZnO:Eu columnar films were measured in detail, showing a good selectivity toward H2 gas at operating OPT temperatures of 200-300 °C among several test gases and volatile organic compound vapors, such as methane, ammonia, acetone, ethanol, n-butanol, and 2-propanol. At an operating temperature OPT of 250 °C, a high gas response Igas/Iair of ∼115 for 100 ppm H2 was obtained. Experimental results indicate that Eu doping with an optimal content of about 0.05-0.1 at% along with Pd functionalization of ZnO columns leads to a reduction of the operating temperature of the H2 gas sensor. DFT-based computations provide mechanistic insights into the gas-sensing mechanism by investigating interactions between the Pd-functionalized ZnO:Eu surface and H2 gas molecules supporting the experimentally observed results. The proposed columnar materials and gas sensor structures would provide a special advantage in the fields of fundamental research, applied physics studies, and ecological and industrial applications.

Diffusive Memristive Switching on the Nanoscale, from Individual Nanoparticles towards Scalable Nanocomposite Devices.

Nanoscale memristive phenomena are of great interest not only to miniaturize devices and improve their performance but also to understand the details of the underlying mechanism. Herein, we utilize conductive atomic force microscopy (C-AFM) as a non-invasive method to examine the nanoscale memristive properties of individual noble metal alloy nanoparticles that are sparsely encapsulated in a thin SiO2 dielectric matrix. The measurement of current-voltage hysteresis loops at the level of individual nanoparticles, enabled by the nanoscopic contact area of the C-AFM tip, indicates reliable memristive switching for several hours of continuous operations. Alongside the electrical characterization on the nanoscale, the method of C-AFM offers the potential for in situ monitoring of long term operation induced morphological alterations and device failure, which is demonstrated at the example of nanoparticle-based devices with additional Cr wetting layer. The application of alloy nanoparticles as reservoir for mobile silver species effectively limits the formation of stable metallic filaments and results in reproducible diffusive switching characteristics. Notably, similar behaviour is encountered on macroscopic nanocomposite devices, which incorporate multiple stacks of nanoparticles and offer a high design versatility to tune switching properties and engineer scalable memristive devices with diffusive switching characteristics. No additional forming step is required for the operation of the presented alloy nanoparticle based memristive devices, which renders them very attractive for applications.

Pathways to Tailor Photocatalytic Performance of TiO2 Thin Films Deposited by Reactive Magnetron Sputtering.

TiO2 thin films are used extensively for a broad range of applications including environmental remediation, self-cleaning technologies (windows, building exteriors, and textiles), water splitting, antibacterial, and biomedical surfaces. While a broad range of methods such as wet-chemical synthesis techniques, chemical vapor deposition (CVD), and physical vapor deposition (PVD) have been developed for preparation of TiO2 thin films, PVD techniques allow a good control of the homogeneity and thickness as well as provide a good film adhesion. On the other hand, the choice of the PVD technique enormously influences the photocatalytic performance of the TiO2 layer to be deposited. Three important parameters play an important role on the photocatalytic performance of TiO2 thin films: first, the different pathways in crystallization (nucleation and growth); second, anatase/rutile formation; and third, surface area at the interface to the reactants. This study aims to provide a review regarding some strategies developed by our research group in recent years to improve the photocatalytic performance of TiO2 thin films. An innovative approach, which uses thermally induced nanocrack networks as an effective tool to enhance the photocatalytic performance of sputter deposited TiO2 thin films, is presented. Plasmonic and non-plasmonic enhancement of photocatalytic performance by decorating TiO2 thin films with metallic nanostructures are also briefly discussed by case studies. In addition to remediation applications, a new approach, which utilizes highly active photocatalytic TiO2 thin film for micro- and nanostructuring, is also presented.

Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization.

Nanomaterials for highly selective and sensitive sensors toward specific gas molecules of volatile organic compounds (VOCs) are most important in developing new-generation of detector devices, for example, for biomarkers of diseases as well as for continuous air quality monitoring. Here, we present an innovative preparation approach for engineering sensors, which allow for full control of the dopant concentrations and the nanoparticles functionalization of columnar material surfaces. The main outcome of this powerful design concept lies in fine-tuning the reactivity of the sensor surfaces toward the VOCs of interest. First, nanocolumnar and well-distributed Ag-doped zinc oxide (ZnO:Ag) thin films are synthesized from chemical solution, and, at a second stage, noble nanoparticles of the required size are deposited using a gas aggregation source, ensuring that no percolating paths are formed between them. Typical samples that were investigated are Ag-doped and Ag nanoparticle-functionalized ZnO:Ag nanocolumnar films. The highest responses to VOCs, in particular to (CH3)2CHOH, were obtained at a low operating temperature (250 °C) for the samples synergistically enhanced with dopants and nanoparticles simultaneously. In addition, the response times, particularly the recovery times, are greatly reduced for the fully modified nanocolumnar thin films for a wide range of operating temperatures. The adsorption of propanol, acetone, methane, and hydrogen at various surface sites of the Ag-doped Ag8/ZnO(0001) surface has been examined with the density functional theory (DFT) calculations to understand the preference for organic compounds and to confirm experimental results. The response of the synergistically enhanced sensors to gas molecules containing certain functional groups is in excellent agreement with density functional theory calculations performed in this work too. This new fabrication strategy can underpin the next generation of advanced materials for gas sensing applications and prevent VOC levels that are hazardous to human health and can cause environmental damages.

3D-Printed Chemiresistive Sensor Array on Nanowire CuO/Cu2O/Cu Heterojunction Nets.

In this work, the one-step three-dimensional (3D) printing of 20 nm nanowire (NW)-covered CuO/Cu2O/Cu microparticles (MPs) with diameters of 15-25 µm on the surface of the glass substrate forming an ordered net is successfully reported for the first time. 3D-printed Cu MP-based stripes formed nonplanar CuO/Cu2O/Cu heterojunctions after thermal annealing at 425 °C for 2 h in air and were fully covered with a 20 nm NW net bridging MPs with external Au contacts. The morphological, vibrational, chemical, and structural investigations were performed in detail, showing the high crystallinity of the NWs and 3D-printed CuO/Cu2O/Cu heterojunction lines, as well as the growth of CuO NWs on the surface of MPs. The gas-sensing measurements showed excellent selectivity to acetone vapor at an operating temperature of 350 °C with a high gas response about 150% to 100 ppm. The combination of the possibility of fast acetone vapor detection, low power consumption, and controllable size and geometry makes these 3D-printed devices ideal candidates for fast detection, as well as for acetone vapor monitoring (down to 100 ppm). This 3D-printing approach will pave a new way for many different devices through the simplicity and versatility of the fabrication method for the exact detection of acetone vapors in various atmospheres.

Cauliflower-like CeO2-TiO2 hybrid nanostructures with extreme photocatalytic and self-cleaning properties.

In recent years, heterogeneous photocatalysis has gained enormous interest due to increasing concerns about environmental pollution. Here we propose a facile approach to synthesize cauliflower-like CeO2-TiO2 hybrid structures by magnetron reactive sputtering, exhibiting an extremely high photocatalytic activity. While heating and air-quenching of the sputter deposited TiO2 thin film (first layer) triggered the formation of a nanocrack network, the second heat-treatment led to transformation of the CeO2 film (second layer) into CeO2 nanoclusters (NCs). We attribute the resulting high photocatalytic activity to the confined structure of the CeO2 NCs and the CeO2-TiO2 interface, which allows Ce3+/Ce4+ dynamic shifting. In addition to high photocatalytic activity in an aqueous medium, the prepared CeO2-TiO2 hybrid structures exhibited significant self-cleaning properties in air (non-aqueous).

Electron Beam Effects on Oxide Thin Films-Structure and Electrical Property Correlations.

In situ transmission electron microscope (TEM) characterization techniques provide valuable information on structure-property correlations to understand the behavior of materials at the nanoscale. However, understanding nanoscale structures and their interaction with the electron beam is pivotal for the reliable interpretation of in situ/ex situ TEM studies. Here, we report that oxides commonly used in nanoelectronic applications, such as transistor gate oxides or memristive devices, are prone to electron beam induced damage that causes small structural changes even under very low dose conditions, eventually changing their electrical properties as examined via in situ measurements. In this work, silicon, titanium, and niobium oxide thin films are used for in situ TEM electrical characterization studies. The electron beam induced reduction of the oxides turns these insulators into conductors. The conductivity change is reversible by exposure to air, supporting the idea of electron beam reduction of oxides as primary damage mechanism. Through these measurements we propose a limit for the critical dose to be considered for in situ scanning electron microscopy and TEM characterization studies.

Concept and modelling of memsensors as two terminal devices with enhanced capabilities in neuromorphic engineering.

We report on memsensors, a class of two terminal devices that combines features of memristive and sensor devices. Apart from a pinched hysteresis (memristive property) and stimulus dependent electrical resistance (sensing property) further properties like dynamic adaptation to an external stimulus emerge. We propose a three component equivalent circuit to model the memsensor electrical behaviour. In this model we find stimulus dependent hysteresis, a delayed response to the sensory signal and adaptation. Stimulus dependent IV hysteresis as a fingerprint of a memsensor device is experimentally shown for memristive ZnO microrods. Adaptation in memsensor devices as found in our simulations resembles striking similarities to the biology. Especially the stimulus dependency of the IV hysteresis and the adaptation to external stimuli are superior features for application of memsensors in neuromorphic engineering. Based on the simulations and experimental findings we propose design rules for memsensors that will facilitate further research on memsensitive systems.

Single target sputter deposition of alloy nanoparticles with adjustable composition via a gas aggregation cluster source.

Alloy nanoparticles with variable compositions add a new dimension to nanoscience and have many applications. Here we suggest a novel approach for the fabrication of variable composition alloy nanoparticles that is based on a Haberland type gas aggregation cluster source with a custom-made multicomponent target for magnetron sputtering. The approach, which was demonstrated here for gold-rich AgAu nanoparticles, combines a narrow nanoparticle size distribution with in operando variation of composition via the gas pressure as well as highly efficient usage of target material. The latter is particularly attractive for precious metals. Varying argon pressure during deposition, we achieved in operando changes of AgAu alloy nanoparticle composition of more than 13 at%. The alloy nanoparticles were characterized by x-ray photoelectron spectroscopy and energy dispersive x-ray spectroscopy. The characteristic plasmon resonances of multilayer nanoparticle composites were analyzed by UV-vis spectroscopy. Tuning of the number of particles per unit area (particle densities) within individual layers showed an additional degree of freedom to tailor the optical properties of multilayer nanocomposites. By extension of this technique to more complex systems, the presented results are expected to encourage and simplify further research based on plasmonic multi-element nanoparticles. The present method is by no means restricted to plasmonics or nanoparticle based applications, but is also highly relevant for conventional magnetron sputtering of alloys and can be extended to in operando control of alloy concentration by magnetic field.