Space charge governs the kinetics of metal exsolution.

Nanostructured composite electrode materials play a major role in the fields of catalysis and electrochemistry. The self-assembly of metallic nanoparticles on oxide supports via metal exsolution relies on the transport of reducible dopants towards the perovskite surface to provide accessible catalytic centres at the solid-gas interface. At surfaces and interfaces, however, strong electrostatic gradients and space charges typically control the properties of oxides. Here we reveal that the nature of the surface-dopant interaction is the main determining factor for the exsolution kinetics of nickel in SrTi0.9Nb0.05Ni0.05O3-δ. The electrostatic interaction of dopants with surface space charge regions forming upon thermal oxidation results in strong surface passivation, which manifests in a retarded exsolution response. We furthermore demonstrate the controllability of the exsolution response via engineering of the perovskite surface chemistry. Our findings indicate that tailoring the electrostatic gradients at the perovskite surface is an essential step to improve exsolution-type materials in catalytic converters.

Restructuring of titanium oxide overlayers over nickel nanoparticles during catalysis.

Reducible supports can affect the performance of metal catalysts by the formation of suboxide overlayers upon reduction, a process referred to as the strong metal-support interaction (SMSI). A combination of operando electron microscopy and vibrational spectroscopy revealed that thin TiOx overlayers formed on nickel/titanium dioxide catalysts during 400°C reduction were completely removed under carbon dioxide hydrogenation conditions. Conversely, after 600°C reduction, exposure to carbon dioxide hydrogenation reaction conditions led to only partial reexposure of nickel, forming interfacial sites in contact with TiOx and favoring carbon-carbon coupling by providing a carbon species reservoir. Our findings challenge the conventional understanding of SMSIs and call for more-detailed operando investigations of nanocatalysts at the single-particle level to revisit static models of structure-activity relationships.

Correction: Poly(acrylic acid)-mediated synthesis of cerium oxide nanoparticles with variable oxidation states and their effect on regulating the intracellular ROS level.

Correction for 'Poly(acrylic acid)-mediated synthesis of cerium oxide nanoparticles with variable oxidation states and their effect on regulating the intracellular ROS level' by Xiaohui Ju et al., J. Mater. Chem. B, 2021, 9, 7386-7400, DOI: 10.1039/D1TB00706H.

Poly(acrylic acid)-mediated synthesis of cerium oxide nanoparticles with variable oxidation states and their effect on regulating the intracellular ROS level.

Cerium oxide nanoparticles (CeNPs) possess multiple redox enzyme mimetic activities in scavenging reactive oxygen species (ROS) as a potential biomedicine. These enzymatic activities of CeNPs are closely related to their surface oxidation state. Here we have reported a synthetic method to modify CeNPs' surface oxidation state by changing the conformation of the poly(acrylic acid) (PAA) polymers adsorbed onto the CeNP surface. The synthesized PAA-CeNPs exhibited the same core size, morphology, crystal structure, and colloidal stability, with the only variation being their surface oxidation state (Ce3+ percentage). The modification mechanism can be attributed to the polymers chemisorbed onto the metal oxide surface forming a metal complexation structure. Such adsorption further modified CeNPs' surface oxidation state in a temperature-dependent manner. The series of PAA-CeNPs exhibited multiple redox enzyme mimetic activities (superoxide dismutase, catalase, peroxidase, and oxidase) directly related to their surface oxidation state. In vitro experiments showed no cytotoxic effect of these PAA-CeNPs on the osteoblastic cell line SAOS-2 at high loadings. Microscopic images confirmed the internalization of PAA-CeNPs in the cells. All tested PAA-CeNPs can reduce the basal and hydrogen peroxide-induced intracellular ROS level in the cells, indicating their effective intracellular ROS scavenging effect. However, we did not observe a positive correlation between the CeNP surface oxidation state and their capacities to reduce the intracellular ROS levels. We propose that CeNPs can maintain a dynamic state of Ce3+/Ce4+ during their catalytic activities, exhibiting a non-linear correlation between the CeNP surface oxidation state and their effect on regulating the intracellular ROS level.

Identifying Ionic and Electronic Charge Transfer at Oxide Heterointerfaces.

The ability to tailor oxide heterointerfaces has led to novel properties in low-dimensional oxide systems. A fundamental understanding of these properties is based on the concept of electronic charge transfer. However, the electronic properties of oxide heterointerfaces crucially depend on their ionic constitution and defect structure: ionic charges contribute to charge transfer and screening at oxide interfaces, triggering a thermodynamic balance of ionic and electronic structures. Quantitative understanding of the electronic and ionic roles regarding charge-transfer phenomena poses a central challenge. Here, the electronic and ionic structure is simultaneously investigated at the prototypical charge-transfer heterointerface, LaAlO3 /SrTiO3 . Applying in situ photoemission spectroscopy under oxygen ambient, ionic and electronic charge transfer is deconvoluted in response to the oxygen atmosphere at elevated temperatures. In this way, both the rich and variable chemistry of complex oxides and the associated electronic properties are equally embraced. The interfacial electron gas is depleted through an ionic rearrangement in the strontium cation sublattice when oxygen is applied, resulting in an inverse and reversible balance between cation vacancies and electrons, while the mobility of ionic species is found to be considerably enhanced as compared to the bulk. Triggered by these ionic phenomena, the electronic transport and magnetic signature of the heterointerface are significantly altered.

Synergy between Metallic and Oxidized Pt Sites Unravelled during Room Temperature CO†Oxidation on Pt/Ceria.

Pt-based materials are widely used as heterogeneous catalysts, in particular for pollutant removal applications. The state of Pt has often been proposed to differ depending on experimental conditions, for example, metallic Pt poisoned with CO being present at lower temperature before light-off, while an oxidized Pt surface prevails above light-off temperature. In stark contrast to all previous reports, we show herein that both metallic and oxidized Pt are present in similar proportions under reaction conditions at the surface of ca. 1 nm nanoparticles showing high activity at 30 °C. The simultaneous presence of metallic and oxidized Pt enables a synergy between these phases. The main role of the metallic Pt phase is to provide strong adsorption sites for CO, while that of oxidized Pt supposedly supplies reactive oxygen. Our results emphasize the complex dual oxidic-metallic nature of supported Pt catalysts and platinum's evolving nature under reaction conditions.

Mobility and versatility of the liquid bismuth promoter in the working iron catalysts for light olefin synthesis from syngas.

Liquid metals are a new emerging and rapidly growing class of materials and can be considered as efficient promoters and active phases for heterogeneous catalysts for sustainable processes. Because of low cost, high selectivity and flexibility, iron-based catalysts are the catalysts of choice for light olefin synthesis via Fischer-Tropsch reaction. Promotion of iron catalysts supported by carbon nanotubes with bismuth, which is liquid under the reaction conditions, results in a several fold increase in the reaction rate and in a much higher light olefin selectivity. In order to elucidate the spectacular enhancement of the catalytic performance, we conducted extensive in-depth characterization of the bismuth-promoted iron catalysts under the reacting gas and reaction temperatures by a combination of cutting-edge in situ techniques: in situ scanning transmission electron microscopy, near-atmospheric pressure X-ray photoelectron spectroscopy and in situ X-ray adsorption near edge structure. In situ scanning transmission electron microscopy conducted under atmospheric pressure of carbon monoxide at the temperature of catalyst activation showed iron sintering proceeding via the particle migration and coalescence mechanism. Catalyst activation in carbon monoxide and in syngas leads to liquid bismuth metallic species, which readily migrate over the catalyst surface with the formation of larger spherical bismuth droplets and iron-bismuth core-shell structures. In the working catalysts, during Fischer-Tropsch synthesis, metallic bismuth located at the interface of iron species undergoes continuous oxidation and reduction cycles, which facilitate carbon monoxide dissociation and result in the substantial increase in the reaction rate.

Gram-scale synthesis of alkoxide-derived nitrogen-doped carbon foam as a support for Fe-N-C electrocatalysts.

Non-platinum group metal (non-PGM) catalysts for the oxygen reduction reaction (ORR) are set to reduce the cost of polymer electrolyte membrane fuel cells (PEFCs) by replacing platinum at the cathode. We previously developed unique nitrogen-doped carbon foams by template-free pyrolysis of alkoxide powders synthesized using a high temperature and high pressure solvothermal reaction. These were shown to be effective ORR electrocatalysts in alkaline media. Here, we present a new optimised synthesis protocol which is carried out at ambient temperature and pressure, enabling us to safely increase the batch size to 2 g, increase the yield by 60%, increase the specific surface area to 1866 m2 g-1, and control the nitrogen content (between 1.0 and 5.2 at%). These optimized nitrogen-doped carbon foams are then utilized as effective supports for Fe-N-C catalysts for the ORR in acid media, whilst multiphysics modelling is used to gain insight into the electrochemical performance. This work highlights the importance of the properties of the carbon support in the design of Pt-free electrocatalysts.

Colloidal stability and catalytic activity of cerium oxide nanoparticles in cell culture media.

One of the biggest challenges for the biomedical applications of cerium oxide nanoparticles (CeNPs) is to maintain their colloidal stability and catalytic activity as enzyme mimetics after nanoparticles enter the human cellular environment. This work examines the influences of CeNP surface properties on their colloidal stability and catalytic activity in cell culture media (CCM). Near-spherical CeNPs stabilized via different hydrophilic polymers were prepared through a wet-chemical precipitation method. CeNPs were stabilized via either electrostatic forces, steric forces, or a combination of both, generated by surface functionalization. CeNPs with electrostatic stabilization adsorb more proteins compared to CeNPs with only steric stabilization. The protein coverage further improves CeNPs colloidal stability in CCM. CeNPs with steric polymer stabilizations exhibited better resistance against agglomeration caused by the high ionic strength in CCM. These results suggest a strong correlation between CeNPs intrinsic surface properties and the extrinsic influences of the environment. The most stabilized sample in CCM is poly(acrylic acid) coated CeNPs (PAA-CeNPs), with a combination of both electrostatic and steric forces on the surface. It shows a hydrodynamic diameter of 15 nm while preserving 90% of its antioxidant activity in CCM. PAA-CeNPs are non-toxic to the osteoblastic cell line SAOS-2 and exhibit promising potential as a therapeutic alternative.

Spectroscopic Understanding of SnO2 and WO3 Metal Oxide Surfaces with Advanced Synchrotron Based; XPS-UPS and Near Ambient Pressure (NAP) XPS Surface Sensitive Techniques for Gas Sensor Applications under Operational Conditions.

The most promising and utilized chemical sensing materials, WO3 and SnO2 were characterized by means advanced synchrotron based XPS, UPS, NAP-XPS techniques. The complementary electrical resistance and sensor testing experiments were also completed. A comparison and evaluation of some of the prominent and newly employed spectroscopic characterization techniques for chemical sensors were provided. The chemical nature and oxidation state of the WO3 and SnO2 thin films were explored at different depths from imminent surface to a maximum of 1.5 nm depth from the surface with non-destructive depth profiling. The adsorption and amount of chemisorbed oxygen species were precisely analyzed and quantified as a function of temperature between 25-400 °C under realistic operating conditions for chemical sensors employing 1-5 mbar pressures of oxygen (O2) and carbon monoxide (CO). The effect of realistic CO and O2 gas pressures on adsorbed water (H2O), OH- groups and chemisorbed oxygen species ( O 2 ( a d s ) - ,   O ( a d s ) ,   - O 2 ( a d s ) 2 - ) and chemical stability of metal oxide surfaces were evaluated and quantified.

Electrifying model catalysts for understanding electrocatalytic reactions in liquid electrolytes.

Electrocatalysis is at the heart of our future transition to a renewable energy system. Most energy storage and conversion technologies for renewables rely on electrocatalytic processes and, with increasing availability of cheap electrical energy from renewables, chemical production will witness electrification in the near future1-3. However, our fundamental understanding of electrocatalysis lags behind the field of classical heterogeneous catalysis that has been the dominating chemical technology for a long time. Here, we describe a new strategy to advance fundamental studies on electrocatalytic materials. We propose to 'electrify' complex oxide-based model catalysts made by surface science methods to explore electrocatalytic reactions in liquid electrolytes. We demonstrate the feasibility of this concept by transferring an atomically defined platinum/cobalt oxide model catalyst into the electrochemical environment while preserving its atomic surface structure. Using this approach, we explore particle size effects and identify hitherto unknown metal-support interactions that stabilize oxidized platinum at the nanoparticle interface. The metal-support interactions open a new synergistic reaction pathway that involves both metallic and oxidized platinum. Our results illustrate the potential of the concept, which makes available a systematic approach to build atomically defined model electrodes for fundamental electrocatalytic studies.

A Nitrogen-Doped Carbon Catalyst for Electrochemical CO2 Conversion to CO with High Selectivity and Current Density.

We report characterization of a non-precious metal-free catalyst for the electrochemical reduction of CO2 to CO; namely, a pyrolyzed carbon nitride and multiwall carbon nanotube composite. This catalyst exhibits a high selectivity for production of CO over H2 (approximately 98 % CO and 2 % H2 ), as well as high activity in an electrochemical flow cell. The CO partial current density at intermediate cathode potentials (V=-1.46 V vs. Ag/AgCl) is up to 3.5× higher than state-of-the-art Ag nanoparticle-based catalysts, and the maximum current density is 90 mA cm-2 . The mass activity and energy efficiency (up to 48 %) were also higher than the Ag nanoparticle reference. Moving away from precious metal catalysts without sacrificing activity or selectivity may significantly enhance the prospects of electrochemical CO2 reduction as an approach to reduce atmospheric CO2 emissions or as a method for load-leveling in relation to the use of intermittent renewable energy sources.

Self-assembling diacetylene molecules on atomically flat insulators.

Single crystal sapphire and diamond surfaces are used as planar, atomically flat insulating surfaces, for the deposition of the diacetylene compound 10,12-nonacosadiynoic acid. The surface assembly is compared with results on hexagonal boron nitride (h-BN), highly oriented pyrolytic graphite (HOPG) and MoS2 surfaces. A perfectly flat-lying monolayer of 10,12-nonacosadiynoic acid self-assembles on h-BN like on HOPG and MoS2. On sapphire and oxidized diamond surfaces, we observed assemblies of standing-up molecular layers. Surface assembly is driven by surface electrostatic dipoles. Surface polarity is partially controlled using a hydrogenated diamond surface or totally screened by the deposition of a graphene layer on the sapphire surface. This results in a perfectly flat and organized SAM on graphene, which is ready for on-surface polymerization of long and isolated molecular wires under ambient conditions.

Alcohol Dehydration on Monooxo W═O and Dioxo O═W═O Species.

The dehydration of 1-propanol on nanoporous WO3 films prepared via ballistic deposition at ∼20 K has been investigated using temperature-programmed desorption, infrared reflection absorption spectroscopy, and density functional theory. The as-deposited films are extremely efficient in 1-propanol dehydration to propene. This activity is correlated with the presence of dioxo O═W═O groups, whereas monooxo W═O species are shown to be inactive. Annealing of the films induces densification that results in the loss of catalytic activity due to the annihilation of O═W═O species.

Influence of nitric acid treatment of carbon nanotubes on their physico-chemical properties.

We report a systematic evaluation of the influence of the use of nitric acid at elevated temperature (80 degrees C) as purification and functionalization treatment for four different types of multi wall carbon nanotubes (MWCNTs) and for carbon microparticles upon their physico-chemical properties. We used BET, Raman spectroscopy, high resolution X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy measurements for MWCNTs characterization. We found that nitric acid treatment significantly changes physico-chemical properties of MWCNTs and that these changes vary significantly between different MWCNTs. Therefore we urge researchers to be careful when relying on the physico-chemical properties measured on different MWCNTs samples.

Ultrathin organically modified silica layer coated carbon nanotubes: fabrication, characterization and electrical insulating properties.

This paper describes the use of organically modified silica (ormosil) for the ultrathin nanoprecise coating of individual multiwall carbon nanotubes using a soft-chemistry approach. Hybrid organic/inorganic ormosil nanocoated carbon nanotubes were successfully prepared by in-situ deposition of 3-aminopropyltrimethoxysilane or N-methylaminopropyltrimethoxysilane in an aqueous suspension by means of their electrostatic interactions with carboxylic group functionalized multiwall carbon nanotubes. The coating layer was found to have a uniform thickness of about 3 nm. The products were characterized by high-angle annular dark field scanning transmission electron microscopy, high-resolution transmission electron microscopy (HR-TEM), TEM/energy-dispersive X-ray spectroscopy, TEM/electron energy loss spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and current-voltage measurements. The ormosil coatings demonstrated the favorable electrical insulating properties of individual multiwall carbon nanotubes. We also show that the resistance of the insulating thin layer can be tuned by altering the substituents of alkylmethoxysilanes.

Spontaneous coating of carbon nanotubes with an ultrathin polypyrrole layer.

A novel protocol for precisely coating individual multiwall carbon nanotubes (MWCNTs) with an ultrathin layer of polypyrrole was developed. The nanocoated MWCNTs were successfully prepared by in situ chemical deposition of polypyrrole in an aqueous suspension of MWCNTs. The coating layer was very uniform and the thickness of the layer was determined by controlling the monomer concentration used, which gave nanometer precision. The products were characterized by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and conductivity and current-voltage measurements. The ultrathin polypyrrole layer could electrically insulate individual MWCNTs.

Redox protein noncovalent functionalization of double-wall carbon nanotubes: electrochemical binder-less glucose biosensor.

Double wall carbon nanotubes are noncovalently functionalized with redox protein and such assembly is used for construction of electrochemical binder-less glucose biosensor. Redox protein glucose oxidase performs as biorecognition element and double wall carbon nanotubes act both as immobilization platform for redox enzyme and as signal transducer. The double carbon nanotubes are characterized by cyclic voltammetry and specific surface area measurements; the redox protein noncovalently functionalized double wall carbon nanotubes are characterized in detail by X-ray photoelectron spectroscopy, cyclic voltammetry, amperometry, and transmission electron microscopy.