Highlights from Faraday Discussion on Nanoalloys: recent developments and future perspectives, London, UK

Abstract

In this article, we present a summary of the Faraday Discussion that took place on September 21–23, 2022 in London, UK. The primary goal of this event was to promote and discuss the recent developments in the field of nanoalloys. Here we briefly outline each scientific session as well as other conference events.

For more than a hundred years, Faraday Discussion meetings have been a unique occasion for scientists of the chemical sciences community, to expose and discuss in depth the most cutting-edge advances of many different topics. In particular, these meeting are amongst the only conferences to give each delegate the opportunity to read in advance the speakers' research papers that will be presented. Moreover, the rather uncommon format of these meetings is such that each session is composed of a few quick presentations followed by a very rich, open discussion. All these aspects contribute to make this a very special event, in which scientific networking is at the top.

In September 2022, a Faraday Discussion meeting was held in London at the prestigious Burlington House (Fig. 1), as a hybrid event, in the historical headquarters of the Royal Society of Chemistry. It was fifteen years ago when the last Faraday Discussion about nanoalloys took place. Indeed, during this period of time a lot of experimental and theoretical works and developments have helped to better understand the nature of these complex systems. Nevertheless, a lot of questions, both practical and theoretical, have never been answered. This meeting was organized in four sessions, in order to cover as much as possible all the areas in which these developments have been made, and in which the most prominent future perspectives have emerged. In the first session, dedicated to nanoalloys structures, the different aspects of the structural characterization were the focus of speakers' presentations: experimental and computational works (such as TEM based techniques, density functional theory calculations, global optimizations and molecular dynamics) were in the spotlight. The second session covered catalysis. Again, experimental and computational studies were shown to demonstrate how more complex and realistic systems can now be tackled with surprising precision. Closely connected to the first, the third session was devoted to discussing magnetic and optical properties of nanoalloys. Finally, the fourth and last session revealed to the audience the most recent discoveries regarding technological applications, including for example biomedicine and electronic devices.

Fig. 1 The participants in front of the entrance of the historical Royal Society of Chemistry.

At the very beginning of the meeting, Prof. Ewald Janssens from KU Leuven warmly welcomed all delegates outlining the program of this Faraday Discussion. Shortly after, Prof. Miguel J. Yacaman from the Northern Arizona University gave a wonderful introductory lecture (https://doi.org/10.1039/D2FD00137C) about the future theoretical and experimental challenges of multi-metallic nanoalloys, as well as their applications, showing in particular how entropy and twin boundaries play a dominant role in the determination of the structural properties of these relatively unknown systems.

Here below, we would like to highlight and summarise all the events that took place in these three days. Starting from the abovementioned four sessions, we will then report on the poster discussions as well as on social events. Finally we will give a perspective from the online audience and some concluding remarks.

Session 1: Nanoalloy structures

The first session was divided in three parts, each with three presentations. In the first part, Prof. Marcelo M. Mariscal from the University of Córdoba discussed a work (https://doi.org/10.1039/D2FD00111J) on the nanoindentation of NiCo nanoalloys. With the help of Molecular Dynamics (MD) simulations, he and co-workers showed that under compression these bimetallic nanoparticles undergo a morphological transformation which is characterized by the formation and propagation of Shockley partial dislocations from the surface to the inner part. The following talk was given by Dr Georg D. Förster, from the University of Orléans. In his work (https://doi.org/10.1039/D2FD00114D) he and co-authors explored the amorphous carbon substrate effects on AgCo nanoalloys by a combined experimental–computational approach. It turned out that for the AgCo system, which is known to have a quite strong tendency towards segregation, atom-by-atom growth of Ag on Co in experiments is a little different from MD growth simulations. In particular, X-ray scattering spectra revealed a Janus structure which is not achieved in simulations, where a core–shell arrangement is instead obtained. This effect was ascribed to a possible underestimation of carbon–metal interactions strength (with respect to metal–metal ones), as well as the lack of angular bonding descriptions that may be crucial for nanoparticles stabilization on the substrate. The last presentation of the first part of the first session was given by Dr Diana Nelli, from the University of Genoa. Her work (https://doi.org/10.1039/D2FD00113F) was dedicated to the computational study of the evolution pathways of different nanoalloys structures, from out-of-equilibrium core–shell arrangements to intermixed ones. Very often in experiments, nanoparticles are produced in kinetically-trapped, out-of-equilibrium structures that may at some point revert to more stable isomers. The process involved in this evolution is often unclear, so that a theoretical explanation is of primary importance. With the help of both MD and Monte Carlo simulations, she and co-workers analyzed different binary systems showing that a complex interplay between size mismatch, cohesive energy differences, interdiffusion of atoms and shape rearrangements is at the origin of these evolution pathways.

The first talk of the second part was given by Dr Damien Alloyeau from the Paris Cité University. In this work about high-entropy nanoalloys (https://doi.org/10.1039/D2FD00118G), he and co-authors discussed two original synthesis methods that allow a fabrication close to the equiatomic composition. The need of established procedures for precisely tuning multi-metallic nanoparticles is in fact a hot topic since it has been only recently in the scope of the community. In this work, CoNiPt, CoNiPtCu, CoNiPtAu and CoNiPtCuAu in the size range 3–10 nm were produced and analyzed by aberration corrected transmission electron microscopy. The two successful routes proposed here for synthesis are (i) a physical synthesis based on pulsed laser deposition with a precise control over composition given by alternating irradiation of the different pure metal targets and (ii) a chemical synthesis based on the hot injection method, in which metal precursors are directly placed in a solution heated up to 300 °C to make the growth process as homogeneous as possible, avoiding segregation effects due to unbalanced reduction rates. The second talk was given by Dr Alexis Front from the Paris-Saclay University. He and co-workers studied the solid–liquid transition of Ag_xPt_1−x nanoalloys by means of atomistic Monte Carlo simulations (https://doi.org/10.1039/D2FD00116K). This system is of particular interest because of its catalytic properties combined with a high melting point. For this, it could be used in the carbon nanotube production process, which is usually driven at high temperatures. The importance of this potential application comes from the fact that these objects have been the focus of extensive research studies in the past years, due to their exceptional mechanical, optical, thermal and electronic properties. Monte Carlo simulations showed in his presentation demonstrated that indeed the presence of Pt delays the melting point of the nanoalloy. However, it was also found that before the complete melting, the silver surface layer becomes liquid, so that the idea of having solid facets at high temperature has to be questioned. The last talk of this part was given by Prof. Wolfgang E. Ernst, from the Graz University of Technology. In this contribution, he and co-authors discussed the effect of temperature on the dynamics of deposited nanoparticles made of gold and vanadium oxide (https://doi.org/10.1039/D2FD00089J). Due to their potential applications in catalysis, gold nanoparticles based on vanadium oxide have been of particular interest. However, the preparation of such systems in a specific state is far from trivial and it is a problem that has been addressed in the past years. In this work, such nanoparticles were produced in the size range of a few nanometers with the unique procedure of sequential doping of cold helium droplets, deposited on a carbon substrate and finally analyzed with different techniques such as Scanning Transmission Electron Microscopy (STEM), Energy Dispersive X-ray Spectroscopy (EDXS) and Electron Energy Loss Spectroscopy (EELS). It was observed that Au–VO nanoparticles initially exhibited Janus-like configurations with the metal oxide V₂O₅ grown in an epitaxial-like fashion on one side, and then evolved into a hexagonal shape in which gold is encapsulated by the metal oxide that reduced to V₂O₃, after heating the substrate above 650 °C.

The first talk of the third and last part of the first session was given by Robert Jones from King's College London. His presentation was devoted to the introduction of a new open-source library, Sapphire (https://doi.org/10.1039/D2FD00097K), publicly available at https://github.com/kcl-tscm/Sapphire. Thanks to this module, a wealth of different quantities can be calculated for the characterization of nanoalloys. Pair distance and radial distribution functions, inertia tensor, adjacency matrices (and quantities related to this, such as coordination number and collectivity mechanisms parameters), common neighbor analysis for surface atoms classification and electrocatalytic properties are just some examples. The second talk was given by Prof. Catherine Amiens from CNRS, France. In this work (https://doi.org/10.1039/D2FD00095D), she and co-workers investigated the effect of temperature as well as air or hydrogen exposure on the chemical and structural properties of NiFe nanoalloys in the very small size range between 1.6 and 3.5 nm. This system is important to study since it shows catalytic capabilities with the benefit of using an abundant and cheap metal such as iron. As often happens though, experimental conditions play the role of a non-negligible perturbation. It is therefore of primary importance to deeply understand what are the effect of such perturbations, since the catalytic performance of these promising systems may be heavily altered. For this study, NiFe nanoalloys at different compositions were prepared following an organometallic approach and then deposited on silica. Analyses were made thanks to different techniques, such as wide angle X-ray scattering (WAXS) and X-ray absorption spectroscopy (XAS) in in situ conditions, transmission electron microscopy and inductively coupled plasma-optical emission spectroscopy (ICP-OES). It was found that iron, mostly placed in shells, gets easily oxidized under air exposure. During this process however, it was also found that H₂ exposure together with a temperature increment leads to iron reduction, thus providing a restoration of the initial state. Despite this result, it was also shown that such a transformation is followed by a shape rearrangement of the nanoalloy towards a more stable fcc-alloyed configuration. Finally, the last talk was given by Dr Hazar Guesmi from CNRS-ICGM-Montpellier, France. Guesmi and co-authors contributed to this Faraday Discussion by presenting a combined experimental–computational study of small (<4 nm) AuCu nanoparticles for the determination of their evolution under H₂ exposure and variable temperature conditions (https://doi.org/10.1039/D2FD00130F). This system has been the object of several studies since it was demonstrated that its catalytic performance is better than that of the pure metal counterparts in a number of reactions. Despite the interest, the structural and chemical evolution of small AuCu nanoalloys under hydrogenation was never reported. This work aimed at filling this gap, since the potential observation and investigation of segregation or alloying is crucial information for the deep understanding of the stability and catalytic performance of this system. Nanoalloys were synthesized by a deposition–precipitation with urea method and then deposited on TiO₂. The sample was then immersed in H₂ and its evolution for decreasing temperature from 400 °C to 200 °C was observed by in situ environmental transmission electron microscopy (ETEM). During this examination, AuCu nanoalloys were discovered to maintain their fcc symmetry. Density functional theory (DFT) and ab initio Molecular Dynamics (AIMD) were used to bring some atomic-level insights for the H₂ exposure process. It was revealed that Cu atoms segregate from subsurface sites towards surface sites and it was also noticed that the core evidences a chemical ordering in which Au atoms are surrounded by Cu ones.

At the end of every part of the first session, a very rich discussion followed. During this time, delegates had the unique opportunity to ask questions, debate and spread knowledge to solve right at the moment some issues that arose within the discussion. The possibility of reading in advance the papers related to each presentation, a remarkable feature of Faraday Discussions, was crucial to increase participants' awareness of all the material that was presented, thus making the above-mentioned discussion incredibly deep and always spot-on. During these discussions, some questions and comments related to the future of nanoalloys structure analysis emerged quite naturally. For example, Prof. Ewald Janssens asked how good and effective the replacement of time by temperature can be in numerical simulations. The large discrepancy between the time scales achieved by both experiments and simulations is in fact well known. A first comment was given by Dr Georg D. Förster, who answered that at the moment there is not a really good solution to the problem, since the longest MD simulations for medium size systems achieve tens of μs in a reasonable amount of time; a time scale which is far from those of experiments. In that regard, Prof. Marcelo M. Mariscal added that some Monte Carlo approaches could represent a possible path to tackle this problem. A comment was also given by Prof. Riccardo Ferrando from the University of Genoa, who suggested that on-the-fly Monte Carlo simulations could be used, but also that these are very difficult techniques to implement since they involve the calculations of many barriers for a meaningful simulation. A different perspective was taken instead by Prof. Francesca Baletto from King's College London, who proposed that a possible way to reconcile simulations and experiments is for the latter to try to detect nanoparticles' evolutions that happen at those shorter time scales that are indeed available for numerical simulations. During another general discussion of the first session, Prof. Miguel J. Yacaman suggested that the community should begin to study systems like NiCr in the near future for their prominent applications. Shortly after, Robert Jones asked to those present what will be the next step for strengthening the collaboration between experimental and theoretical groups. Dr Alexis Front answered that it would be important for simulations to include environmental effects as much as possible, especially for those systems and experimental conditions in which these effects may influence the structural properties of nanoparticles. For the same question, Dr Damien Alloyeau also answered that more theoretical results are needed in the field of high-entropy nanoalloys, for which a lot of experimental results are being obtained.

Session 2: Nanoalloy catalysis

The second session of the Faraday Discussion continued with the theme of nanoalloy catalysis. This session began with the talk given by Prof. Štefan Vajda from J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences, on the catalytic activity of monometallic Cu and Pd clusters towards oxidative dehydrogenation of cyclohexene to benzene selectively via dehydrogenation to produce water as the byproduct (https://doi.org/10.1039/D2FD00108J). Vajda and co-authors prepared a TiO₂ supported tetramers catalyst which can also produce benzene selectively from cyclohexene without any combustion to CO₂. The use of zirconia supported mixed CuPd clusters and monometallic Cu clusters exhibited radically distinct activities. The ZrO₂ supported clusters produced cyclohexadiene as the major product and the selectivity was found to increase with an increasing number of Cu atoms. For instance, in the case of Cu₃Pt, the selectivity of cyclohexadiene reached 50%, whereas the selectivity of ZrO₂-supported Cu tetramers improved to 70%. These findings show the effect of Cu on the stability of mixed tetramers and potential new ways to fine tune the catalyst performance by controlling the compositions of the active site. Focusing on the CO oxidation reaction, Dr Noelia Barrabés from TU Wien presented their work on synthesis of Co doped Au₂₅ nanoclusters supported on CeO₂. The Co doped nanoclusters were synthesized in different solvents such as acetonitrile, acetone and dichloromethane and were used to study the catalysis of the Co oxidation reaction. The catalytic activity was found to be different in three different solvent conditions and was caused by the formation of Co–Au active sites as a result of mobility of atoms within the cluster structure. The evolution of Co–Au alloy from the Au₂₅ cluster was identified through techniques such as X-ray absorption fine structure (XAFS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS) (https://doi.org/10.1039/D2FD00120A). These findings shed light on the structural dynamics of bimetallic nanoclusters that evolve into nanoalloys. On the second day, following a brief coffee break, the nanoalloy catalysis session continued with a presentation by Prof. Alessandro Fortunelli from CNR–Istituto di Chimica dei Composti Organometallici, whose research aimed at developing a computational study of the energetics and mechanisms of the oxidation of Pt–Mn systems by combining DFT, neural network potentials and metadynamics simulations (https://doi.org/10.1039/D2FD00107A). Fortunelli and co-authors used slab models of Pt₂Mn with the (111) facet to simulate the oxidation process. The Mn atoms, present at the surface of Pt₂Mn, dissociates O₂ to generate MnO_x species. The transformation of Mn atoms to the surface oxide structures (MnO_x) creates one or more vacancy sites on the metallic framework which successively promote further oxidation of O₂ on the Pt₂Mn surface. This provides a tool for comprehending the complex phenomenon of the oxidation in bimetallic nanoparticles. The subsequent talk was given by Prof. Michael Bowker on behalf of Prof. Graham Hutchings from Cardiff University. In this work, Hutchings and co-authors synthesized different supported monometallic and bimetallic materials consisting of Pd and PdZn, respectively (https://doi.org/10.1039/D2FD00119E). For the synthesis, Al₂O₃, TiO₂, Ga₂O₃, and ZnO were utilised as supports, and the catalytic activity of the supported materials for the hydrogenation of CO₂ to methanol was evaluated. The team's research revealed that the selectivity towards methanol synthesis was much higher for the bimetallic materials compared to their monometallic counterparts. Characterizing the catalyst before and after reaction, the authors revealed the formation of β-PdZn 1 : 1 alloy formation due to their strong heat of mixing. On the third day, the nanoalloy catalysis session began with the presentation by Prof. Daojian Cheng from Beijing University of Chemical Technology who presented their research work on the use of ternary alloys for catalysis. Here, Cheng and co-authors synthesized a PdAgCu ternary alloy catalyst for selective hydrogenation of diolefins to mono-olefins (https://doi.org/10.1039/D2FD00074A). It was observed through their experiments that addition of moderate amounts of Ag and Cu to the Pd catalyst significantly improved the performance of the catalyst also decreasing the formation of undesired over hydrogenated products. The authors probed the catalyst using techniques like temperature-programmed reduction of hydrogen (H₂-TPR), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and XPS to understand its activity and revealed that the surface Pd composition ratio between the metallic and oxidized states played a major role in mono-olefin formation. This work provides guidelines for the development of other Pd-based ternary nanoalloy catalysts for diolefin hydrogenation reactions. Prof. Lichang Wang from the Southern Illinois University was the penultimate speaker of the session. She presented research targeted at exploring one of the three pillars of catalysis research, the durability of the catalyst, utilising DFT computation. In this study, the authors employ a trimetallic PtPdCu catalyst and demonstrated its superior activity and enhanced durability towards the oxygen reduction reaction (ORR) (https://doi.org/10.1039/D2FD00101B). In this work, the process of Cu species dissolution/deposition, and diffusion between the surface and inner layer were studied on a modeled Pt₂₀Pd₂₀Cu₆₀ catalyst to gain understanding of their roles towards the stabilization of the catalyst. The results of their research revealed that the range of dynamic Cu surface composition in the catalyst was dictated by the interplay of the four processes: dissolution, deposition, diffusion from the surface to inner layer and diffusion from the inner layer to the surface of the Cu species which determined the stability in the Cu-rich PtPdCu nanoalloy catalyst.

The final talk of the nanoalloy catalysis session was delivered by Dr Valérie Caps from CNRS, ICPEES, Université de Strasbourg, who presented their work which aimed at showing the formation of supported Au–Pd sub-nanometer clusters and Au–Pd Janus type nanoparticles under two different reduction conditions, namely dimethylformamide (DMF) and NaBH₄ reduction, respectively (https://doi.org/10.1039/D2FD00094F). Here, the TiO₂ surface served as the support. The work presented demonstrates CO₂ reduction with water using the synthesized catalysts. Caps and coauthors suggest that the use of Au–Pd Janus NPs as catalyst resulted in the formation of reduced CO₂ products such as CH₄ while the use of Au–Pd sub-nanometer clusters simply resulted in the reduction of water to H₂. This revealed the role of plasmonic nanoparticles in the catalytic system. Increasing the Au loading led to the formation of Au/Pd core–shell type nanoparticles, while structures with the highest Pt loading exhibited an increase in CH₄ conversion.

After every three presentations, there was a lively panel discussion, in which attendees have the opportunity to discuss and remark on the work presented. The chair of the session also allocated time to involve the questions raised by the online attendees, which made the entire conference beneficial for both the in-person and online participants. The important takeaway from the conference was the necessity for the development of the nanoalloy catalyst and the need for additional research to comprehend the mechanism of the catalysis process. One of the intriguing aspects discussed during the panel discussion was the role of the catalyst on substrate activation. For instance, in the research reported by Dr Valérie Caps, where plasmonic gold nanoparticles were shown to catalyse CO₂ reduction, the significance of water in the CO₂ reduction process was brought into question. In response, the presenter described how the water utilised in the reaction medium served as both a solvent for catalysis and a reductant for CO₂ to CH₄ conversion, which is the primary constituent of natural gas. While there was a great deal of interest in determining how the nanoalloy contributed to the advancement of the catalysis reaction, it was evident that the identification of the surface process, in particular the identification of surface intermediates, posed the greatest challenge in comprehending the entire catalysis process and improving catalyst design. Thus, efforts to enhance in operando and in situ characterisation techniques were called for to be deepened. The area of nanoalloy catalysis is still in its infancy and requires much research before challenging reactions can be attempted. During the discussion session, the audience, both in-person attendees and online participants, were enthusiastic to challenge each other to contemplate breaking through the current conceptual limitations. Nanoalloy catalysis being a rich and unexplored area of current research, several groups have made contributions in the past few years to this developing field by providing convincing evidence for the process of nanoalloy catalysis.

Session 3: Magnetic and optical properties of nanoalloys

The third session of the Faraday Discussion focused on the magnetic and optical properties of nanoalloys. The first speaker of the session was Prof. Rolf Schäfer from the Technical University of Darmstadt in Germany. In his presentation, Schäfer discussed a study on the magnetic properties of MSn₁₂ clusters, where M represents elements such as aluminum, gallium, and indium (https://doi.org/10.1039/D2FD00091A). Using electric and magnetic beam deflection, Schäfer and his co-authors studied the relationship between the structure and magnetic properties of these clusters. They found that each cluster had two structural isomers, one polar and one nonpolar. Additionally, they observed superatomic behavior and significant spin–orbit coupling. They also demonstrated the connection between superatomic and polar fractions by combining electric and magnetic beam deflection, showing the correlation between the Stark and Zeeman effects. Finally, they compared their experimental results to computational studies of all cluster species.

In the second presentation, Prof. Joost M. Bakker from Radboud University in the Netherlands discussed a study on the activity of carbon-doped Cu_n⁻ clusters with CO₂ (https://doi.org/10.1039/D2FD00128D). Bakker and his co-authors used IR multiple-photon dissociation spectroscopy to probe the degree of CO₂ activation, and they verified their results using density functional theory calculations. They found that the size of the Cu_nC⁻ clusters determined the nature of the bonding with CO₂. For clusters with a single carbon atom, activated adsorption of CO₂ was observed for n = 6 and 10, and dissociative adsorption of CO₂ was observed for n = 7–9. For clusters with two carbon atoms, Cu_nC₂⁻, molecular adsorption of CO₂ was observed for n = 38, with the exception of Cu₅C₂⁻, which showed dissociative adsorption of CO₂.

In the third presentation, Dr Piero Ferrari from KU Leuven in Belgium discussed a study on the cooling of gold clusters using recurrent fluorescence. Ferrari and his co-authors used laser excitation to study pure gold clusters as well as gold clusters doped with silver and palladium (https://doi.org/10.1039/D2FD00090C). They observed an odd–even oscillation in the rates of radiation magnitude for the excited clusters, with the highest rates observed for clusters with closed subshells. They also found that the odd–even pattern was the same for Au_n⁺ and AgAu_n−1⁺ clusters, but it was shifted by one atom for PdAu_n−1⁺ clusters. Using linear-response time-dependent density functional theory calculations, they showed how the dopant influenced the low-lying electronic transitions and photon emission rates of the cluster.

One of the topics discussed in the discussion was the idea of super atom behavior for clusters and nanoparticles. It was noted that in order to observe atom-like properties in a cluster, the cluster must be atomic precise, which means that its atoms must be arranged in a specific and well-defined way. However, these atomic precise clusters are typically synthesized using ligands, which can influence the properties of the cluster. Therefore, it is important to clean the cluster to remove these ligands, but this process may also alter the structure of the cluster.

In the first presentation during the second part of the third session, Prof. Christine Aikens from Kansas State University in the United States discussed a study on the influence of platinum dopants on thiolate-protected gold clusters (https://doi.org/10.1039/D2FD00110A). Aikens and her co-authors used time-dependent density functional theory calculations to study the nanoclusters Au₂₄Pt(SR)₁₈ and [Au₂₅(SR)₁₈]¹⁻, where SR represents a thiolate group. They found that the presence of platinum distorted the geometry of the cluster from an icosahedron to an ellipsoid, redshifted the adsorption energy of the main peak, and quenched the photoluminescence. Additionally, they discovered that replacing the hydrogen groups in the thiolate with propyl groups resulted in even more distortion in the core/shell structure and a further redshift in the absorption energy.

In the second presentation, Prof. Emmanuel Cottancin from the University Claude Bernard Lyon 1 in France discussed a study on the optical and structural properties of nanoalloys combining gold or silver with aluminum or indium. Cottancin and his coauthors used localized surface plasmon resonance, transmission electron microscopy, and X-ray-based techniques to study the nanoparticles (https://doi.org/10.1039/D2FD00109H). They found that silver-based nanoparticles formed alloyed cores rich in metallic silver and shells of indium or aluminum oxide. In contrast, they observed that gold-based nanoparticles formed ordered nanoalloys with indium and aluminum after exposure to air, but these alloys demixed during oxidation.

In the third virtual presentation, Dr Junpeng Wang from the Northwestern Polytechnical University in China discussed a study on PtAg nanoalloys as catalysts for the methanol oxidation reaction (MOR) (https://doi.org/10.1039/D2FD00102K). Wang and his co-authors synthesized PtAg nanoparticles using pulsed laser deposition, and they observed that the PtAg nanoalloy had both the catalytic activity of Pt for MOR and the plasmonic response of Ag in the visible spectrum. They found that the mass catalytic activity of the PtAg nanoalloy was 4.5 times higher than that of the commercial Pt/C catalyst, and it had a catalytic activity 16% higher due to the surface plasmon resonance.

During the discussion, an interesting comparison was made between experiments and calculations with respect to the detection of single electronic transitions and plasmonic excitations. It was noted that computationally, it is possible to observe single electronic excitations and to derive plasmon-like transitions from linear combinations of multiple excitations. In contrast, high resolution spectroscopy can experimentally distinguish between single electronic transitions and plasmonic excitations by looking at the width of the corresponding peaks. This allows researchers to identify whether a particular peak corresponds to a single electronic transition or a plasmonic excitation.

Session 4: Applications of nanoalloys

In this session the discussion was focused on the applications of, especially, Au-based alloy nanoparticles. The session contained two talks. The first one was given by Prof. Vincenzo Amendola from the University of Padova. His presentation was on structural evolution under physical and chemical stimuli of metastable Au–Fe nanoalloys obtained by laser ablation in liquid (https://doi.org/10.1039/D2FD00087C). In this work, the authors tracked the dynamics process of Au–Fe nanoparticles at high temperature in vacuum and ambient atmosphere using different experimental characterization techniques. TEM and Mössbauer analysis showed that the initial sample contains structural heterogeneities such as ultrafine Au–Fe nanoparticles (few nm) and an Fe(III) component with a high crystalline disorder. In addition, by means of XRD and Rietveld refinement, the authors monitored the structural evolution of the AuFe sample at temperatures from 30 to 700 °C in vacuum and air environment. The exposition of Au–Fe nanoparticles to thermal annealing in vacuum or in air induced the transformation of ultrafine NPs to larger ones with different structures: Au–Fe alloys in vacuum and Au–Fe oxide in air environment. In both vacuum and air environment, the degree of crystallinity exhibited a sharp increase when the temperature increased, reaching 100% at 700 °C and remaining 100 wt% even after cooling to 30 °C. At 400 °C, they showed that the size of the crystalline domains is small in air environment, suggesting that oxidation is hampering the grain coalescence which is possible in vacuum at the same temperature.

Next, Prof. Stephan Barcikowski from the University of Duisburg-Essen presented results about gold–silver alloy nanoparticles and their implementation in catalysis and biomedicine applications (https://doi.org/10.1039/D2FD00092J). In this work, Barcikowski and co-workers focused their attention on the characterization of surface chemistry and surface reactions of AgAu alloy NPs in correlation with particle composition. They used a broad range of gold molar fractions: Ag₉₀Au₁₀, Ag₈₀Au₂₀, Ag₇₀Au₃₀, Ag₅₀Au₅₀, Ag₄₀Au₆₀, and Ag₂₀Au₈₀. The surface composition and surface oxidation of AuAg NPs have been understood by means of XPS measurements. Their findings showed that the surface of the nanoparticles is more enriched in the minority component – Au-rich AuAg NPs have a surface enriched with silver and vice versa. However, they observed an increase in the catalytic activity of AuAg NPs during the oxygen reduction reaction with increasing surface gold molar fraction. This indicated that gold surface atoms are the catalytically most active species for this specific reaction. In addition, they showed that the equimolar composition of AuAg NPs exhibited interesting results such as low surface reactivity and high structural stability.

As in the previous sessions, these talks were followed by an extensive discussion between the presenters and audience. The questions and comments mainly revolved around the topics that had already been central in the presentations. During the discussion, the audience wondered on the ability of laser-based synthesis to control the nanoparticle size and the composition effect of the studied nanoalloys on catalysis and biomedicine applications. A very worthwhile question emerged from the discussion regarding the environmental impact of nanoparticles production.

One point of discussion concerning the current understanding of the nanoalloys was raised by Christian Kuttner from Nature Communications to the nanoalloy community. Synthesizing nanoalloys with a record number of distinct metals makes compositional characterization exceedingly challenging. He wanted to know what the scientific community felt about increasing the total number of metals in a nanoalloy (now reported as 50) and the instrumental limitations associated with their analysis. According to Prof. Miguel J. Yacaman from the Northern Arizona University, the maximum number of metals should be six. As a result of electron interactions, an increase in the number of metals may trigger a change in the electronic structure from metallic to an insulating Mott state. Dr Damien Alloyeau from CNRS, commenting on the difficulty in controlling the size, composition, and atomic structure of nanoalloys, suggested using no more than ten metals to properly characterize their atomic structure by TEM.

Poster session and conference dinner

At the end of the first day, the poster session took place. More than 15 posters were lively discussed and shared by all participants, who actively interacted with each presenter. A wide range of topics were covered, including catalysis, structural and physical properties, machine learning techniques for nanoalloys and much more. On this occasion, all delegates had the opportunity to network with each other, thus strengthening the scientific connection already built-up during the discussions. The Poster Prize was awarded to Swathi Swaminathan (Fig. 2) and Riccardo Farris (Fig. 3).

Fig. 2 Catalysis Science & Technology poster prize winner Swathi Swaminathan together with Edward Gardner.

Fig. 3 Nanoscale journal family poster prize winner Riccardo Farris together with Edward Gardner.

Other important discussions took place during the conference dinner (Fig. 4), another occasion for delegates to have some further exchanges in the context of a beautiful dining room. Here, everybody could also participate in the ancient Loving Cup Ceremony.

Fig. 4 Prof. Ewald Janssens welcoming all delegates to the dinner.

Online perspective

Unlike previous years, this edition of the Faraday Discussion meeting was taken in two different formats. The online platform allowed many delegates to network remotely with other attendees. All the virtual participants had opportunities to attend, debate and discuss all the topics of the meeting. Besides, they were able to present and discuss their results with the community.

Concluding remarks

We would like to remark on the incredibly delightful and entertaining mood that persisted during the whole conference. Within this Faraday Discussion, young scientists could discuss, debate and collaborate with more experienced scientists of the community. The genuine and spontaneous interest for this field of research led all delegates to undertake a continuous open discussion that went on far beyond its dedicated time, lasting instead throughout all three days.

The concluding remarks lecture on Nanoalloys: recent developments and future perspectives of the Faraday Discussion was presented by Prof. Florent Calvo from Université Grenoble Alpes, CNRS, France (https://doi.org/10.1039/D2FD00139J). In comparison to the 2007 Faraday Discussion on Nanoalloy: from theory to application, Florent provided an excellent overview on the evolution of the field over the years. With a particular emphasis on developing new synthesis techniques for constructing well-defined nanostructures for applications such as catalysis. The dearth of advanced characterization techniques, including microscopy, diffraction, scattering, and spectroscopy, was also notable 15 years ago. Compared to the past, the search for a better catalyst has reached scales as small as four atoms. There is also a better understanding of catalysts under light excitation, leading to the creation of a new field of plasmon catalysis, which was absent 15 years ago. Florent added, with reference to the 2007 Faraday Discussion, that technological advancements have unarguably altered how both experiments and theory are undertaken today

Conflicts of interest

There are no conflicts to declare.

Footnote

† All authors contributed equally to this work.

---