Impact of Single-Pulse, Low-Intensity Laser Post-Processing on Structure and Activity of Mesostructured Cobalt Oxide for the Oxygen Evolution Reaction

Co3O4 (111) surfaces in contact with water: molecular dynamics study of the surface chemistry and structure at room temperature

In this work, we have used ab initio molecular dynamics at room temperature to study the adsorption and dissociation of a thin water film on Co3O4 (111) surfaces, considering the O-rich and Co-rich terminations named as the A-type and B-type surface terminations, respectively. We investigate the occupation of active sites, the hydrogen bond network at the interface and the structural response of the surfaces to water adsorption. On both terminations, water adsorbs via a partial dissociative mode. The contact layer is populated by molecular water as well as OH groups and surface OH resulting from proton transfer to the surface. The B-termination is more reactive, with a higher degree of dissociation in the contact layer with water (46%). On the B-terminated surface, water barely adsorbs on the Co2+ sites and almost exclusively binds and dissociates on the Co3+ sites. The interaction with the surface consists mostly of Co3+-Ow bonds and proton transfer exclusively to the 3-fold unsaturated surface Os1. Hydrogen bonds between water molecules in the aqueous film dominate the hydrogen bond network and no hydrogen bonds between water and the surface are observed. The A-terminated surface is less reactive. Water binds covalently on Co2+ sites, with a dissociation degree of 13%. Proton transfer occurs mostly on the 3-fold unsaturated surface oxygens Os1. Besides, short-lived surface OH arising from proton transfer to 3-fold unsaturated surface oxygens Os2 is observed. H-bonding to surface Os1 and Os2 constitutes 12.7% and 19.8% of the H-bond network, respectively, and the largest contribution is found among the water molecules (67.4%). On both surfaces, the coordination number of the active sites drives the relaxations of the outermost atom positions to the their bulk counterparts. The occupation of active sites on B-termination could reach up to 3 adsorbates per Co3+ leading to a binding motif in which the Co is octahedrally coordinated and which was observed experimentally.

Surface Chemistry and Specific Surface Area Rule the Efficiency of Gold Nanoparticle Sensitizers in Proton Therapy

Gold nanoparticles (AuNPs) are currently the most studied radiosensitizers in proton therapy (PT) applicable for the treatment of solid tumors, where they amplify production of reactive oxygen species (ROS). However, it is underexplored how this amplification is correlated with the AuNPs' surface chemistry. To clarify this issue, we fabricated ligand-free AuNPs of different mean diameters by laser ablation in liquids (LAL) and laser fragmentation in liquids (LFL) and irradiated them with clinically relevant proton fields by using water phantoms. ROS generation was monitored by the fluorescent dye 7-OH-coumarin. Our findings reveal an enhancement of ROS production driven by I) increased total particle surface area, II) utilization of ligand-free AuNPs avoiding sodium citrate as a radical quencher ligands, and III) a higher density of structural defects generated by LFL synthesis, indicated by surface charge density. Based on these findings it may be concluded that the surface chemistry is a major and underexplored contributor to ROS generation and sensitizing effects of AuNPs in PT. We further highlight the applicability of AuNPs in vitro in human medulloblastoma cells.

Differentiating between Acidic and Basic Surface Hydroxyls on Metal Oxides by Fluoride Substitution: A Case Study on Blue TiO2 from Laser Defect Engineering

Both oxygen vacancies and surface hydroxyls play a crucial role in catalysis. Yet, their relationship is not often explored. Herein, we prepare two series of TiO₂ (rutile and P25) with increasing oxygen deficiency and Ti³⁺ concentration by pulsed laser defect engineering in liquid (PUDEL), and selectively quantify the acidic and basic surface OH by fluoride substitution. As indicated by EPR spectroscopy, the laser-generated Ti³⁺ exist near the surface of rutile, but appear to be deeper in the bulk for P25. Fluoride substitution shows that extra acidic bridging OH are selectively created on rutile, while the surface OH density remains constant for P25. These observations suggest near-surface Ti³⁺ are highly related to surface bridging OH, presumably the former increasing the electron density of the bridging oxygen to form more of the latter. We anticipate that fluoride substitution will enable better characterization of surface OH and its correlation with defects in metal oxides.

Photomechanical Laser Fragmentation of IrO2 Microparticles for the Synthesis of Active and Redox-Sensitive Colloidal Nanoclusters

Pulsed laser fragmentation of microparticles (MPs) in liquid is a synthesis method for producing high-purity nanoparticles (NPs) from virtually any material. Compared with laser ablation in liquids (LAL), the use of MPs enables a fully continuous, single-step synthesis of colloidal NPs. Although having been employed in several studies, neither the fragmentation mechanism nor the efficiency or scalability have been described. Starting from time-resolved investigations of the single-pulse fragmentation of single IrO₂ MPs in water, the contribution of stress-mediated processes to the fragmentation mechanism is highlighted. Single-pulse, multiparticle fragmentation is then performed in a continuously operated liquid jet. Here, 2 nm-sized nanoclusters (NCs) accompanied by larger fragments with sizes ranging between several ten nm and several µm are generated. For the nanosized product, an unprecedented efficiency of up to 18 µg J^-1 is reached, which exceeds comparable values reported for high-power LAL by one order of magnitude. The generated NCs exhibit high catalytic activity and stability in oxygen evolution reactions while simultaneously expressing a redox-sensitive fluorescence, thus rendering them promising candidates in electrocatalytic sensing. The provided insights will pave the way for laser fragmentation of MPs to become a versatile, scalable yet simple technique for nanomaterial design and development.

Dynamics of Reactive Oxygen Species on Cobalt-Containing Spinel Oxides in Cyclic CO Oxidation

Reactive oxygen species (ROS) are considered to be responsible for the high catalytic activity of transition metal oxides like Co3-xFexO4 in oxidation reactions, but the detailed influences of catalyst composition and morphology on the formation of these reactive oxygen species are not fully understood. In the presented study, Co3O4 spinels of different mesostructures, i.e., particle size, crystallinity, and specific surface area, are characterized by powder X-ray diffraction, scanning electron microscopy, and physisorption. The materials were tested in CO oxidation performed in consecutive runs and compared to a Co3-xFexO4 composition series with a similar mesostructure to study the effects of catalyst morphology and composition on ROS formation. In the first run, the CO conversion was observed to be dominated by the exposed surface area for the pure Co-spinels, while a negative effect of Fe content in the spinels was seen. In the following oxidation run, a U-shaped conversion curve was observed for materials with high surface area, which indicated the in situ formation of ROS on those materials that were responsible for the new activity at low temperature. This activation was not stable at the higher reaction temperature but was confirmed after temperature-programmed oxidation (TPO). However, no activation after the first run was observed for low-surface-area and highly crystalline materials, and the lowest surface-area material was not even activated after TPO. Among the catalyst series studied here, a correlation of small particle size and large surface area with the ability for ROS formation is presented, and the benefit of a nanoscaled catalyst is discussed. Despite the generally negative effect of Fe, the highest relative activation was observed at intermediate Fe contents suggesting that Fe may be involved in ROS formation.