Electrocatalytic oxygen reduction on functionalized gold nanoparticles incorporated in a hydrophobic environment

Fundamentals and Challenges of Ligand Modification in Heterogeneous Electrocatalysis

The development of efficient catalytic materials in the energy field could promote the structural transformation from traditional fossil fuels to sustainable energy. In heterogeneous catalytic reactions, ligand modification is an effective way to regulate both electronic and steric structures of catalytic sites, thus paving a prospective avenue to design the interfacial structures of heterogeneous catalysts for energy conversion. Although great achievements have been obtained for the study and applications of heterogeneous ligand-modified catalysts, the systematical refinements of ligand modification strategies are still lacking. Here, we reviewed the ligand modification strategy from both the mechanistic and applicable scenarios by focusing on heterogeneous electrocatalysis. We elucidated the ligand-modified catalysts in detail from the perspectives of basic concepts, preparation, regulation of physicochemical properties of catalytic sites, and applications in different electrocatalysis. Notably, we bridged the electrocatalytic performance with the electronic/steric effects induced by ligand modification to gain intrinsic structure-performance relations. We also discussed the challenges and future perspectives of ligand modification strategies in heterogeneous catalysis.

Conceptual Developments of Aryldiazonium Salts as Modifiers for Gold Colloids and Surfaces

Modified colloids and flat surfaces occupy an important place in materials science research due to their widespread applications. Interest in the development of modifiers that adhere strongly to surfaces relates to the need for stability under ambient conditions in many applications. Diazonium salts have evolved as the primary choice for the modification of surfaces. The term "diazonics" has been introduced in the literature to describe "the science and technology of aryldiazonium salt-derived materials". The facile reduction of diazonium salts via chemical or electrochemical processes, irradiation stimuli, or spontaneously results in the efficient modification of gold surfaces. Robust gold-aryl nanoparticles, where gold is connected to the aryl ring through bonding to carbon and films modified by using diazonium salts, are critical in electronics, sensors, medical implants, and materials for power sources. Experimental and theoretical studies suggest that gold-carbon interactions constructed via chemical reactions with diazonium salts are stronger than nondiazonium surface modifiers. This invited feature article summarizes the conceptual development of recent studies of diazonium salts in our laboratories and others with a focus on the surface modification of gold nanostructures, flat surfaces and gratings, and their applications in nanomedicine engineering, sensors, energy, forensic science, and catalysis.

Active Sites and Mechanism of Oxygen Reduction Reaction Electrocatalysis on Nitrogen-Doped Carbon Materials

The oxygen reduction reaction (ORR) is a core reaction for electrochemical energy technologies such as fuel cells and metal-air batteries. ORR catalysts have been limited to platinum, which meets the requirements of high activity and durability. Over the last few decades, a variety of materials have been tested as non-Pt catalysts, from metal-organic complex molecules to metal-free catalysts. In particular, nitrogen-doped graphitic carbon materials, including N-doped graphene and N-doped carbon nanotubes, have been extensively studied. However, due to the lack of understanding of the reaction mechanism and conflicting knowledge of the catalytic active sites, carbon-based catalysts are still under the development stage of achieving a performance similar to Pt-based catalysts. In addition to the catalytic viewpoint, designing mass transport pathways is required for O₂ . Recently, the importance of pyridinic N for the creation of active sites for ORR and the requirement of hydrophobicity near the active sites have been reported. Based on the increased knowledge in controlling ORR performances, bottom-up preparation of N-doped carbon catalysts, using N-containing conjugative molecules as the assemblies of the catalysts, is promising. Here, the recent understanding of the active sites and the mechanism of ORRs on N-doped carbon catalysts are reviewed.

Graphene paper supported MoS2 nanocrystals monolayer with Cu submicron-buds: High-performance flexible platform for sensing in sweat

Flexible sweat biosensors are of considerable current interest for the development of wearable smart miniature devices. In this work, we report a novel type of flexible and electrochemical sweat platform fabricated by depositing Cu submicron buds on freestanding graphene paper (GP) carrying MoS₂ nanocrystals monolayer for bio-functional detection of glucose and lactate. Quantitative analysis of glucose and lactate was carried out by using amperometric i-t method. Linear ranges were obtained between 5 and 1775 μM for glucose and 0.01-18.4 mM for lactate, and their corresponding limits of detection were 500 nM and 0.1 μM, respectively. The platform demonstrates fast response, good selectivity, superb reproducibility and outstanding flexibility, which enable its use for monitoring glucose and lactate in human perspiration. The strategy of structurally integrating 3D transition metal, 0D transition metal sulfide and 2D graphene will provide new insight into the design of flexible electrodes for sweat glucose and lactate monitoring and a wider range of applications in biosensing, bioelectronics, and lab-on-a-chip devices.

Citrate-Capped Hybrid Au-TiO2 Nanomaterial for Facile and Enhanced Electrochemical Hydrazine Oxidation

Effective and facile electrochemical oxidation of chemical fuels is pivotal for fuel cell applications. Herein, we report the electrocatalytic oxidation of hydrazine on a citrate-capped Au-TiO₂-modified glassy carbon electrode, which follows two different oxidation paths. These two pathways of hydrazine oxidation are ascribed to occur on Au and the activated TiO₂ surface of the Au-TiO₂ hybrid electrocatalyst. This activation was achieved through molecular capping of the Au-TiO₂ surface by citrate, which leads to favorable hydrazine oxidation with a lower Tafel slope compared to that of the clean surface of the respective materials, that is, Au and TiO₂.

Impacts of interfacial charge transfer on nanoparticle electrocatalytic activity towards oxygen reduction

Interfacial electron transfer within platinum and non-platinum-based nanocatalysts plays a significant role in the manipulation of the electronic interactions between oxygen species and the catalyst surfaces, which may be exploited as an effective mechanism to enhance and optimize the activity towards oxygen reduction.

Gold core@silver semishell Janus nanoparticles prepared by interfacial etching

Gold core@silver semishell Janus nanoparticles were prepared by chemical etching of Au@Ag core-shell nanoparticles at the air/water interface. Au@Ag core-shell nanoparticles were synthesized by chemical deposition of a silver shell onto gold seed colloids followed by the self-assembly of 1-dodecanethiol onto the nanoparticle surface. The nanoparticles then formed a monolayer on the water surface of a Langmuir-Blodgett trough, and part of the silver shell was selectively etched away by the mixture of hydrogen peroxide and ammonia in the water subphase, where the etching was limited to the side of the nanoparticles that was in direct contact with water. The resulting Janus nanoparticles exhibited an asymmetrical distribution of silver on the surface of the gold cores, as manifested in transmission electron microscopy, UV-vis absorption, and X-ray photoelectron spectroscopy measurements. Interestingly, the Au@Ag semishell Janus nanoparticles exhibited enhanced electrocatalytic activity in oxygen reduction reactions, as compared to their Au@Ag and Ag@Au core-shell counterparts, likely due to a synergistic effect between the gold cores and silver semishells that optimized oxygen binding to the nanoparticle surface.

Functionalization of nanomaterials with aryldiazonium salts

This paper reviews the surface modification strategies of a wide range of nanomaterials using aryldiazonium salts. After a brief history of diazonium salts since their discovery by Peter Griess in 1858, we will tackle the surface chemistry using these compounds since the first trials in the 1950s. We will then focus on the modern surface chemistry of aryldiazonium salts for the modification of materials, particularly metallic, semiconductors, metal oxide nanoparticles, carbon-based nanostructures, diamond and clays. The successful modification of sp(2) carbon materials and metals by aryldiazonium salts paved the way to innovative strategies for the attachment of aryl layers to metal oxide nanoparticles and nanodiamonds, and intercalation of clays. Interestingly, diazotized surfaces can easily trap nanoparticles and nanotubes while diazotized nanoparticles can be (electro)chemically reduced on electrode/materials surfaces as molecular compounds. Both strategies provided organized 2D surface assembled nanoparticles. In this review, aryldiazonium salts are highlighted as efficient coupling agents for many types of molecular, macromolecular and nanoparticulate species, therefore ensuring stability to colloids on the one hand, and the construction of composite materials and hybrid systems with robust and durable interfaces/interphases, on the other hand. The last section is dedicated to a selection of patents and industrial products based on aryldiazonium-modified nanomaterials. After nearly 160 years of organic chemistry, diazonium salts have entered a new, long and thriving era for the benefit of materials, colloids, and surface scientists. This tempts us to introduce the terminology of "diazonics" we define as the science and technology of aryldiazonium salt-derived materials.

Synthesis of halogen-doped reduced graphene oxide nanosheets as highly efficient metal-free electrocatalyst for oxygen reduction reaction

We demonstrate F-, Cl-, Br- and I-doped reduced graphene oxide (XRGO) as metal-free graphene electro-catalysts for oxygen reduction reaction (ORR) in alkaline media. Reduced graphene oxide (GO) is prepared from graphite electrode using electrochemical exfoliation. In situ doping of halide in a graphene film has many problems. In this technique, different halides individually or all of them were mixed with the RGO and ionic liquids precursor at H2SO4 solution. Then we have evaluated the effectiveness of doping and performed electrochemical measurements of the ORR activity on XRGO. Fourier-transform infrared spectroscopy spectra show a variety of the halogen-containing functional groups. Energy-dispersive X-ray spectroscopy analysis confirmed the presence of doped halogens in RGO. Raman spectroscopy shows a high density of defects in the RGO layer. The electrochemical properties of the XRGO catalysts on carbon paper as a gas diffusion electrode (GDE) are investigated by several electrochemical methods in oxygen saturated alkaline solutions. The catalytic activity of the XRGO and Pt-C electrodes for ORR is 50 and 30mAcm(-2) at -1V in GDEs. This enhanced efficiency is the result of the influence of the nature and percentage of the halogen, especially fluorine presence in the graphene layer.

Ordered mesoporous carbons-supported gold nanoparticles as highly efficient electrocatalysts for oxygen reduction reaction

Mesoporous carbons-supported gold nanoparticles exhibit apparent electrocatalytic activity towards oxygen reduction reaction, due to their intimate interactions that facilitate fast electron transfer and rapid mass transport.

Enhanced catalytic electrochemical reduction of dissolved oxygen with ultraclean cucurbituril[7]-capped gold nanoparticles

Gold nanoparticles capped with cucurbituril[7] have been prepared in the absence of metallic cations and organic ligands. Remarkably, these nanohybrids encapsulate dissolved oxygen and are highly active in electrochemical reduction. The effect of the presence of sodium and ammonium salts on this catalysed process is also analysed.

Coating graphene paper with 2D-assembly of electrocatalytic nanoparticles: a modular approach toward high-performance flexible electrodes

The development of flexible electrodes is of considerable current interest because of the increasing demand for modern electronics, portable medical products, and compact energy devices. We report a modular approach to fabricating high-performance flexible electrodes by structurally integrating 2D-assemblies of nanoparticles with freestanding graphene paper. We have shown that the 2D array of gold nanoparticles at oil-water interfaces can be transferred on freestanding graphene oxide paper, leading to a monolayer of densely packed gold nanoparticles of uniform sizes loaded on graphene oxide paper. One major finding is that the postassembly electrochemical reduction of graphene oxide paper restores the ordered structure and electron-transport properties of graphene, and gives rise to robust and biocompatible freestanding electrodes with outstanding electrocatalytic activities, which have been manifested by the sensitive and selective detection of two model analytes: glucose and hydrogen peroxide (H(2)O(2)) secreted by live cells. The modular nature of this approach coupled with recent progress in nanocrystal synthesis and surface engineering opens new possibilities to systematically study the dependence of catalytic performance on the structural parameters and chemical compositions of the nanocrystals.