Galina Marzun

Ligand-free Gold Nanoparticles as a Reference Material for Kinetic Modelling of Catalytic Reduction of 4-Nitrophenol

The reduction of 4-nitrophenol by sodium borohydride is a common model reaction to test the catalytic activity of metal nanoparticles. As all reaction steps proceed solely on the surface of the metal nanoparticles (Langmuir–Hinshelwood model), ligand-coverage of metal nanoparticles impedes the merging of theory and experiment. Therefore we analyzed the catalytic activity of bare gold nanoparticles prepared by laser ablation in liquid without any stabilizers or ligands. The catalytic reaction is characterized by a full kinetic analysis including 4-hydroxylaminophenol as an intermediate species. Excellent agreement between theory and experiment is found. Moreover, the suspension of the nanoparticles remains stable. Hence, ligand-free nanoparticles can be used as a reference material for mechanistic studies of catalytic reactions. In addition, the analysis shows that gold nanoparticles synthesized by laser ablation are among the most active catalysts for this reaction.Graphical Abstract

Adsorption of colloidal platinum nanoparticles to supports: charge transfer and effects of electrostatic and steric interactions.

Adsorption of colloidal nanoparticles to surfaces and supports is a convenient approach to heterogeneous catalysts, polymer additives, or wastewater treatment. We investigated the adsorption efficiency of laser-generated and initially ligand-free platinum nanoparticles to TiO2 supports as a function of pH, ionic strength, and ligand surface coverage. The nanoparticle adsorption is dominantly controlled by electrostatic interactions: if the pH of the suspension is between the isoelectric point of the nanoparticles and the support, nanoparticles are adsorbed and transfer a net charge to the support. This charge-driven adsorption is not affected by steric repulsion due to various ligands attached to the nanoparticle surface. In addition to electrostatic interactions, colloidal stability given by moderate ionic strengths and pH values above the isoelectric point of nanoparticles are prerequisites for colloidal deposition.

Laser synthesis, structure and chemical properties of colloidal nickel-molybdenum nanoparticles for the substitution of noble metals in heterogeneous catalysis

Platinum and iridium are rare and expensive noble metals that are used as catalysts for different sectors including in heterogeneous chemical automotive emission catalysis and electrochemical energy conversion. Nickel and its alloys are promising materials to substitute noble metals. Nickel based materials are cost-effective with good availability and show comparable catalytic performances. The nickel-molybdenum system is a very interesting alternative to platinum in water electrolysis. We produced ligand-free nickel-molybdenum nanoparticles by laser ablation in water and acetone. Our results show that segregated particles were formed in water due to the oxidation of the metals. X-ray diffraction shows a significant change in the lattice parameter due to a diffusion of molybdenum atoms into the nickel lattice with increasing activity in the electrochemical oxygen evolution reaction. Even though the solubility of molecular oxygen in acetone is higher than in water, there were no oxides and a more homogeneous metal distribution in the particles in acetone as seen by TEM-EDX. This showed that dissolved molecular oxygen does not control oxide formation. Overall, the laser ablation of pressed micro particulate mixtures in liquids offers a combinational synthesis approach that allows the screening of alloy nanoparticles for catalytic testing and can convert micro-mixtures into nano-alloys.

Cysteine-containing oligopeptide β-sheets as redispersants for agglomerated metal nanoparticles

Oligopeptide β-sheets comprising a fluorenyl methoxy carbonyl (Fmoc) group on its N-terminus and five amino acid residues of cysteine, lysine and valine displays redispersive properties with respect to agglomerated metal nanoparticles (MNPs, M = Au, Cu, Pt and Pd). The ligand-free MNPs prepared by a laser ablation technique in liquid maintain a high dispersion state due to the inherent surface charges delivered by anionic species present in solution, but may agglomerate after the preparation depending on concentration or salinity. We show how the agglomerated MNPs can be returned to the dispersed state by adding the Fmoc-oligopeptide β-sheets in methanol, as characterized by photoabsorption spectroscopy and transmission electron microscopy. Systematic studies in which we vary the concentration, the amino acid sequences and the secondary structures of a series of the oligopeptides clarify that the β-sheet structure is essential for the redispersion of the MNPs, where metal-binding thiol groups are integrated on one side and positively charged amino groups are located on the other side of the β-sheet. A possible mechanism for the redispersion may be that the agglomerated MNPs are subsequently enwrapped by the flexible β-sheets and gradually separated due to the reconstruction of peptide β-sheets under the assembly/disassembly equilibrium.

Role of Dissolved and Molecular Oxygen on Cu and PtCu Alloy Particle Structure during Laser Ablation Synthesis in Liquids

The role of molecular oxygen dissolved in the solvent is often discussed as being an influential factor on particle oxidation during pulsed laser ablation in liquids. However, the formation of the particles during laser synthesis takes place under extreme conditions that enable the decomposition of the liquid medium. Reactive species of the solvent may then affect particle formation due to a chemical reaction in the reactive plasma. Experimental results show a difference between the role of dissolved molecular oxygen and the contribution from the oxygen in water molecules. Using a metallic Cu target in air-saturated water, laser ablation led to 20.5 wt % Cu, 11.5 wt % Cu2 O, and 68 wt % CuO nanoparticles, according to X-ray diffraction results. In contrast to particles obtained in air-saturated water, no CuO was observed in the colloid synthesized in a Schlenk ablation chamber in completely oxygen-free water. Under these conditions, less-oxidized nanoparticles (25 wt % Cu and 75 wt % Cu2 O) were synthesized. The results show that nanoparticle oxidation during laser synthesis is mainly caused by reactive oxygen species from the decomposition of water molecules. However, the addition of molecular oxygen promotes particle oxidation. Storage of the Cu colloid in the presence of dissolved oxygen leads, due to aging, to nanostructures with a higher oxidation state than the freshly prepared colloid. The XRD pattern of the sample prepared in air-saturated acetone showed no crystalline phases, which is possibly due to small crystallites or low particle concentration. Concentration of the particles by centrifugation showed that in the large fraction (>20 nm), even less oxidized nanoparticles (46 wt % Cu and 54 wt % Cu2 O) were present, although the solubility of molecular oxygen is higher in acetone than in water. The nanoparticles in acetone were stable due to a Cu-catalyzed graphite layer formed on their surfaces. The influence of the solvent on alloy synthesis is also crucial. Laser ablation of PtCu3 in air-saturated water led to separated large CuO and Pt-rich spherical nanoparticles, whereas homogeneous PtCu3 alloy nanoparticles were formed in acetone.

Peptide Cross-linkers: Immobilization of Platinum Nanoparticles Highly Dispersed on Graphene Oxide Nanosheets with Enhanced Photocatalytic Activities

For exerting potential catalytic and photocatalytic activities of metal nanoparticles (MNPs), immobilization of MNPs on a support medium in highly dispersed state is desired. In this Research Article, we demonstrated that surfactant-free platinum nanoparticles (PtNPs) were efficiently immobilized on graphene oxide (GO) nanosheets in a highly dispersed state by utilizing oligopeptide β-sheets as a cross-linker. The fluorenyl-substituted peptides were designed to form β-sheets, where metal-binding thiol groups and protonated and positively charged amino groups are integrated on the opposite sides of the surface of a β-sheet, which efficiently bridge PtNPs and GO nanosheet. In comparison to PtNP/GO composite without the peptide linker, the PtNP/peptide/GO ternary complex exhibited excellent photocatalytic dye degradation activity via electron transfer from GO to PtNP and simultaneous hole transfer from oxidized GO to the dye. Furthermore, the ternary complex showed photoinduced hydrogen evolution upon visible light irradiation using a hole scavenger. This research provides a new methodology for the development of photocatalytic materials by a bottom-up strategy on the basis of self-assembling features of biomolecules.

Catalyst Support Effect on the Activity and Durability of Magnetic Nanoparticles: toward Design of Advanced Electrocatalyst for Full Water Splitting

Earth-abundant element-based inorganic–organic hybrid materials are attractive alternatives for electrocatalyzing energy conversion reactions. Such material structures do not only increase the surface area and stability of metal nanoparticles (NPs) but also modify the electrocatalytic performance. Here, we introduce, for the first time, multiwall carbon nanotubes (MWNTs) functionalized with nitrogen-rich emeraldine salt (ES) (denoted as ES-MWNT) as a promising catalyst support to boost the electrocatalytic activity of magnetic maghemite (γ-Fe2O3) NPs. The latter component has been synthesized by a simple and upscalable one-step pulsed laser ablation method on Ni core forming the core–shell Ni@γ-Fe2O3 NPs. The catalyst (Ni@γ-Fe2O3/ES-MWNT) is formed via self-assembly as strong interaction between ES-MWNT and Ni@γ-Fe2O3 results in NPs’ encapsulation in a thin C–N shell. We further show that Ni does not directly function as an active site in the electrocatalyst but it has a crucial role in synthesizing the maghemite shell. The strong interaction between the NPs and the support improves notably the NPs’ catalytic activity toward oxygen evolution reaction (OER) in terms of both onset potential and current density, ranking it among the most active catalysts reported so far. Furthermore, this material shows a superior durability to most of the current excellent OER electrocatalysts as the activity, and the structure, remains almost intact after 5000 OER stability cycles. On further characterization, the same trend has been observed for hydrogen evolution reaction, the other half-reaction of water splitting.

Development of A Low-Cost FPGA-Based Measurement System for Real-Time Processing of Acoustic Emission Data: Proof of Concept Using Control of Pulsed Laser Ablation in Liquids

Today, the demand for continuous monitoring of valuable or safety critical equipment is increasing in many industrial applications due to safety and economical requirements. Therefore, reliable in-situ measurement techniques are required for instance in Structural Health Monitoring (SHM) as well as process monitoring and control. Here, current challenges are related to the processing of sensor data with a high data rate and low latency. In particular, measurement and analyses of Acoustic Emission (AE) are widely used for passive, in-situ inspection. Advantages of AE are related to its sensitivity to different micro-mechanical mechanisms on the material level. However, online processing of AE waveforms is computationally demanding. The related equipment is typically bulky, expensive, and not well suited for permanent installation. The contribution of this paper is the development of a Field Programmable Gate Array (FPGA)-based measurement system using ZedBoard devlopment kit with Zynq-7000 system on chip for embedded implementation of suitable online processing algorithms. This platform comprises a dual-core Advanced Reduced Instruction Set Computer Machine (ARM) architecture running a Linux operating system and FPGA fabric. A FPGA-based hardware implementation of the discrete wavelet transform is realized to accelerate processing the AE measurements. Key features of the system are low cost, small form factor, and low energy consumption, which makes it suitable to serve as field-deployed measurement and control device. For verification of the functionality, a novel automatically realized adjustment of the working distance during pulsed laser ablation in liquids is established as an example. A sample rate of 5 MHz is achieved at 16 bit resolution.

Colloidal nanoparticles for heterogeneous catalysis

For many industrial processes, catalytic conversion is indispensible for reducing energy consumption, pollution, and unwanted side products. The majority of technical processes are heterogeneously catalyzed, and heterogeneous catalysts are important components in systems for exhaust gas cleaning and energy conversion or storage. Recently, nanomaterials, which often exhibit different properties from the bulk material, have attracted attention as potentially cheaper and more efficient catalysts. We have prepared a variety of heterogeneous nanoparticle catalysts on carrier particles. We used pulsed laser ablation in liquid and investigated the influence of factors such as ligands, pH, ionic strength, and ligand surface coverage on adsorption efficiency to titanium dioxide supports.1, 2 Colloidal nanoparticles offer fascinating possibilities for investigating new functional heterogeneous catalysts. The surfaces of nanomaterials such as gold or platinum nanoparticles, which are synthesized by reducing precursors, are usually covered by surfactants. However, these can reduce or even prevent catalytic activity. As a result, extensive and expensive purification methods (such as thermal treatment, centrifugation, and solvent extraction) are required.3–5 However, quantitative removal of the surfactants is very challenging. Furthermore, sintering or agglomeration, which modify the nanoparticle properties, often take place, limiting follow-up treatments. In contrast, highly pure nanoparticles can be fabricated by pulsed laser ablation of a bulk target in liquid (see Figure 1). This permits rapid nanomaterial design and can be applied to a variety of materials and solvents. In addition, such plasma-induced nanoparticle formation positively charges the nanoparticles’ surfaces, stabilizing them electrostatically without surfactants. The effective charge of nanoparticles in colloidal solution is measured by the zeta potential. Using a saline solution during the laser ablation also delivers a surface charge from anions, enabling control of the particle size.6 The increase in surface charge Figure 1. Laser ablation of a gold target in a solvent (left) and colloidal solutions with gold, silver, and nickel nanoparticles (right).