Electrocatalytic Study of the Oxygen Reduction Reaction at Gold Nanoparticles in the Absence and Presence of Interactions with SnOx Supports

Nanoclusters Embedded in Phthalocyanine Covalent Organic Polymer Enhance the Cycloaddition Reaction of Carbon Dioxide

Photocatalytic cycloaddition of carbon dioxide and epoxides under mild conditions is important for green chemistry, while metal clusters loaded on a variety of carriers are widely used in energy and biology. However, under long-term illumination, the light stability of metal clusters at the carrier interface is poor, which usually leads to the loss of catalytic performance. Covalent organic polymers (COPs) with periodic and ultrasmall pore structures are ideal carriers for dispersing and stabilizing metal clusters. Through a simple nitrogen purge process, embedding Mn2Dy2 clusters on a hydroxy-containing COP was successfully achieved, which makes it an effective photocatalyst for CO2 cycloaddition. By introducing adjacent hydroxyl and imine bonds in the COP unit, periodic dispersed adsorption sites of metal clusters were constructed, generating a large number of active photogenerated electrons. The photoelectric measurement results indicate that the catalyst is excellent, with good charge separation efficiency. The Mn2Dy2 clusters widely presented on COP expose large active surfaces, greatly promoting electron transfer. The highest cycloaddition conversion rate at room temperature and normal pressure is 99%, and the TON value is 2352. This is of great significance for the study of novel photocatalytic cycloaddition of carbon dioxide.

Development of Monolithic Hyper-Cross-Linked Polystyrene-Supported Ultrasmall Nano-Ag Catalysts for Enhanced NaBH4-Mediated Dye Degradation

Synthesizing catalyst supports with appropriate compositions and structures is crucial for reducing the sizes of metal nanoparticles and enhancing their catalytic activities. In this work, a series of monolithic hyper-cross-linked supports (HCP-CC) with hierarchical pores were synthesized. The monolithic structure facilitated easy operation in catalytic reactions, while the composition and structure of HCP-CC could be tailored simultaneously by utilizing the functional cross-linking agent cyanogen chloride. Furthermore, in situ loading of nano-Ag into HCP-CC resulted in the hybrid catalyst HCP-CC-Ag. The synergy of confinement and coordination effect controlled and limited the size of nano-Ag to approximately 3 nm, classifying them as ultrasmall nanoparticles, which ensured outstanding catalytic activity. This hybrid catalyst could improve the reaction rate constant to 0.423 min-1; it efficiently promoted the degradation of organic dye and exhibited great universality and recyclability, making it a potential heterogeneous catalyst for dye wastewater treatment.

Advances in stimuli-responsive gold nanorods for drug-delivery and targeted therapy systems

In recent years, the use of gold nanorods (AuNRs) has garnered considerable attention in biomedical applications due to their unique optical and physicochemical properties. They have been considered as potential tools for the advanced treatment of diseases by various stimuli such as magnetic fields, pH, temperature and light in the fields of targeted therapy, imaging and drug delivery. Their biocompatibility and tunable plasmonic properties make them a versatile platform for a range of biomedical applications. While endogenous stimuli have limited cargo delivery control at specific sites, exogenous stimuli are a more favored approach despite their circumscribed penetration depth for releasing the cargo at the specific target. Dual/multi-stimuli responsive AuNTs can be triggered by multiple stimuli for enhanced control and specificity in biomedical applications. This review provides to provide a summary of the biomedical applications of stimuli-responsive AuNRs, including their endogenous and exogenous properties, as well as their dual/multi-functionality and potential for clinical delivery. This review provides a comprehensive review on the improvement of therapeutic efficacy and the effective control of drug release with AuNRs, highlights AuNRs design strategies in recent years, discusses the advantages or challenges so far in the field of AuNRs. Finally, we have addressed the clinical translation bio-integrated nanoassemblies (CTBNs) in this field.

Single-Particle Spectroelectrochemistry: Promoting the Electrocatalytic Activity of Gold Nanorods via Oxygen Plasma Treatment without Structural Deformation

Understanding of the electrocatalytic activity enhancement in gold nanoparticles is still limited. Herein, we present the effect of the oxygen plasma treatment on the electrochemical activity of gold nanorods (AuNRs). Oxygen plasma treatment resulted in the blueshift and line width narrowing of the localized surface plasmon resonance (LSPR) spectra obtained from individual AuNRs immobilized on an indium tin oxide (ITO) surface. These changes can be attributed to increases in the surface charges of the AuNRs. The formation of a Au-ITO heterojunction provided structural stability to the immobilized AuNRs regardless of the duration of oxygen plasma exposure. The electrocatalytic oxidation of hydrogen peroxide (H2O2) was induced by increases in the free-electron densities on the surfaces of these AuNRs owing to oxygen plasma treatment, and Au did not dissolve under the experimental conditions. However, the potential-dependent LSPR spectra of the individual AuNRs showed similar patterns of LSPR behavior, irrespective of the duration of oxygen plasma treatment and the concentration of H2O2. Therefore, this study based on single-particle spectroelectrochemistry and cyclic voltammetry improves the understanding of the role of oxygen plasma treatment in promoting the catalytic activity of structurally stable AuNRs immobilized on an ITO surface.

Metal Oxide-Supported Metal Catalysts for Electrocatalytic Oxygen Reduction Reaction: Characterization Methods, Modulation Strategies, and Recent Progress

The sluggish kinetics of the oxygen reduction reaction (ORR) with complex multielectron transfer steps significantly limits the large-scale application of electrochemical energy devices, including metal-air batteries and fuel cells. Recent years witnessed the development of metal oxide-supported metal catalysts (MOSMCs), covering single atoms, clusters, and nanoparticles. As alternatives to conventional carbon-dispersed metal catalysts, MOSMCs are gaining increasing interest due to their unique electronic configuration and potentially high corrosion resistance. By engineering the metal oxide substrate, supported metal, and their interactions, MOSMCs can be facilely modulated. Significant progress has been made in advancing MOSMCs for ORR, and their further development warrants advanced characterization methods to better understand MOSMCs and precise modulation strategies to boost their functionalities. In this regard, a comprehensive review of MOSMCs for ORR is still lacking despite this fast-developing field. To eliminate this gap, advanced characterization methods are introduced for clarifying MOSMCs experimentally and theoretically, discuss critical methods of boosting their intrinsic activities and number of active sites, and systematically overview the status of MOSMCs based on different metal oxide substrates for ORR. By conveying methods, research status, critical challenges, and perspectives, this review will rationally promote the design of MOSMCs for electrochemical energy devices.

Learning from the Heterogeneity at Electrochemical Interfaces

Understanding the structure-activity relationship at electrochemical interfaces is crucial in improving the performance of practical electrochemical devices, ranging from fuel cells, electrolyzers, and batteries to electrochemical sensors. However, functional electrochemical interfaces are often complex and contain various surface structures, creating heterogeneity in electrochemical activity. In this Perspective, we highlight the role of heterogeneity in electrochemistry, especially in the context of electrocatalysis. Current methods for revealing the heterogeneity at electrochemical interfaces, including nanoelectrochemistry tools and single-entity approaches, are discussed. Lastly, we provide perspectives on what one can learn by studying heterogeneity and how one can use heterogeneity to design more efficient electrochemical devices.

Controlled Synthesis of Au25 Superatom Using a Dendrimer Template

Superatoms are promising materials for their potential in elemental substitution and as new building blocks. Thus far, various synthesis methods of thiol-protected Au clusters including an Au₂₅ superatom have been investigated. However, previously reported methods were mainly depending on the thermodynamic stability of the aimed clusters. In this report, a synthesis method for thiol-protected Au clusters using a dendrimers template is proposed. In this method, the number of Au atoms was controlled by the stepwise complexation feature of a phenylazomethine dendrimer. Therefore, synthesis speed was increased compared with the case without the dendrimer template. Hybridization for the Au₂₅ superatoms was also achieved using the complexation control of metals.

Effect of Intermediate Semiconducting TiOx Thin Films on Nanoparticle-Mediated Electron Transfer: Electrooxidation of CO

The concept of nanoparticle-mediated electron transfer (eT) across insulating thin films was elucidated theoretically by Allongue and Chazalviel in 2011. In their model, metal nanoparticles (NPs) are immobilized atop passivating, self-assembled monolayers (SAMs). They found that under certain conditions, related to the thickness of the SAM and the size of the NPs, efficient faradaic oxidation and reduction reactions could proceed at the NP surface. In the absence of NPs, however, eT was suppressed by the insulating SAM thin films. Allongue and Chazalviel concluded that, within certain bounds, eT is mediated by fast tunneling between the conductive electrode and the metal NPs, while the kinetics of the redox reaction are controlled by the NPs. This understanding has been confirmed using a variety of experimental models. The theory is based on electron tunneling; therefore, the nature of the intervening medium (the insulator in prior studies) should not affect the eT rate. In the present manuscript, however, we show that the theory breaks down under certain electrochemical conditions when the medium between conductors is an n-type semiconductor. Specifically, we find that in the presence of either Au or Pt NPs immobilized on a thin film of TiOx, CO electrooxidation does not proceed. In contrast, the exact same systems lead to the efficient reduction of oxygen. At present, we are unable to explain this finding within the context of the model of Allongue and Chazalviel.

Correlating Surface Structures and Electrochemical Activity Using Shape-Controlled Single-Pt Nanoparticles

We report a method for synthesizing and studying shape-controlled, single Pt nanoparticles (NPs) supported on carbon nanoelectrodes. The key advance is that the synthetic method makes it possible to produce single, electrochemically active NPs with a vast range of crystal structures and sizes. Equally important, the NPs can be fully characterized, and, therefore, the electrochemical properties of the NPs can be directly correlated to the size and structure of a single shape. This makes it possible to directly correlate experimental results to first-principles theory. Because just one well-characterized NP is analyzed at a time, the difficulty of applying a theoretical analysis to an ensemble of NPs having different sizes and structures is avoided. In this article, we report on two specific Pt NP shapes having sizes on the order of 200 nm: concave hexoctahedral (HOH) and concave trapezohedral (TPH). The former has {15 6 1} facets and the latter {10 1 1} facets. The electrochemical properties of these single NPs for the formic acid oxidation (FAO) reaction are compared to those of a single, spherical polycrystalline Pt NP of the same size. Finally, density functional theory, performed prior to the electrochemical studies, were used to interpret the experimental results of the FAO experiments.

Role of topological surface states and mirror symmetry in topological crystalline insulator SnTe as an efficient electrocatalyst

The surface orientation dependence on the hydrogen evolution reaction (HER) performance of topological crystalline insulator (TCI) SnTe thin films is studied. Their intrinsic activities are determined by linear sweep voltammetry and cyclic voltammetry measurements. It is found that SnTe (001) and (111) surfaces exhibit intrinsic activities significantly larger than the (211) surface. Density functional theory calculations reveal that pure (001) and (111) surfaces are not good electrocatalysts, while those with Sn vacancies or partially oxidized surfaces, with the latter as evidenced by X-ray photoelectron spectroscopy, have high activity. The calculated overall performance of the (001) and (111) surfaces with robust topological surface states (TSSs) is better than that of the lowly symmetric (211) surface with fragile or without TSSs, which is further supported by their measured weak antilocalization strength. The high HER activity of SnTe (001) and (111) is attributed to the enhanced charge transfer between H atoms and TSSs. We also address the effect of possible surface facets and the contrast of the HER activity of the available active sites among the three samples. Our study demonstrates that the TSSs and mirror symmetry of TCIs expedite their HER activity.

An Electrocatalytic Activity of AuCeO2/Carbon Catalyst in Fuel Cell Reactions: Oxidation of Borohydride and Reduction of Oxygen

This paper describes the investigation of electrocatalytic activity of the AuCeO2/C catalyst, prepared using the microwave irradiation method, towards the oxidation of sodium borohydride and oxygen reduction reactions in an alkaline medium. It was found that the obtained AuCeO2/C catalyst with Au loading and electrochemically active surface area of Au nanoparticles (AuNPs) equal to 71 µg cm−2 and 0.05 cm2, respectively, showed an enhanced electrocatalytic activity towards investigated reactions, compared with the Au/C catalyst with an Au loading and electrochemically active surface area of AuNPs equal to 78 µg cm−2 and 0.19 cm2, respectively. The AuCeO2/C catalyst demonstrated ca. 4.5 times higher current density values for the oxidation of sodium borohydride compared with those of the bare Au/C catalyst. Moreover, the onset potential of the oxygen reduction reaction (0.96 V) on the AuCeO2/C catalyst was similar to the commercial Pt/C (0.98 V).

Turning on electrocatalytic oxygen reduction by creating robust Fe-Nx species in hollow carbon frameworks via in situ growth of Fe doped ZIFs on g-C3N4

Iron-nitrogen-carbon (Fe-N-C) electrocatalysts have been demonstrated to be promising candidates to substitute conventional Pt/C electrocatalysts in the oxygen reduction reaction (ORR) due to the benefits of high efficiency and affordable price. Unfortunately, Fe is prone to aggregation upon high-temperature treatment, which may cover the active sites of the Fe-Nx species and further affect the ORR performance. Thus, the key issue is to avoid Fe aggregation and keep it uniformly dispersed as much as possible. In this work, Fe-N-C catalysts with robust Fe-Nx species in hollow carbon frameworks were created via in situ growth of Fe doped Zn based zeolitic imidazolate frameworks (ZIFs) on g-C3N4 with the subsequent pyrolysis treatment. The developed catalysts demonstrate superb ORR activity, high resistance to methanol and ultralong stability as compared with traditional Pt/C catalysts in alkaline solution. The brilliant performance benefits from the firm connection and robust structure of the optimal Fe-Nx species that are homogeneously dispersed in the hollow carbon frameworks. This work presents a facile and reasonable strategy for the development of excellent ORR electrocatalysts.

Embedding Ultrasmall Au Clusters into the Pores of a Covalent Organic Framework for Enhanced Photostability and Photocatalytic Performance

Gold clusters loaded on various supports have been widely used in the fields of energy and biology. However, the poor photostability of Au clusters on support interfaces under prolonged illumination usually results in loss of catalytic performance. Covalent organic frameworks (COFs) with periodic and ultrasmall pore structures are ideal supports for dispersing and stabilizing Au clusters, although it is difficult to encapsulate Au clusters in the ultrasmall pores. In this study, a two-dimensional (2D) COF modified with thiol chains in its pores was prepared. With -SH groups as nucleation sites, Au nanoclusters (NCs) could grow in situ within the COF. The ultrasmall pores of the COF and the strong S-Au binding energy combine to improve the dispersibility of Au NCs under prolonged light illumination. Interestingly, Au-S-COF bridging as observed in this artificial Z-scheme photocatalytic system is deemed to be an ideal means to increase charge-separation efficiency.

Contrasting Oxygen Reduction Reactions on Zero- and One-Dimensional Defects of MoS2 for Versatile Applications

Oxygen reduction reaction (ORR) is a key microscopic process in many electrochemical applications of materials, where the requirements of their ORR performances may vary strikingly, for example, during the uses of MoS₂ as an electrocatalyst and anticorrosion/lubricating coating in aqueous/humid environments, ORR should be activated and inhibited, respectively. To reveal a complete ORR profile of MoS₂, using first-principles calculations, we examine the stabilities of various possible zero-dimensional point defects on the surface and one-dimensional edge defects and comprehensively explore the ORR activities on pristine MoS₂ surface and those defects in acid/alkaline solutions. It is found that the ORRs on the pristine surface and surfaces with point defects always require large overpotentials (>1.0 V), indicating a defect-immune resistance of the planar MoS₂ surface against the ORR. However, the ORR overpotentials on edge defects can reach as low as 0.66 V, corresponding to a relatively high activity close to that of the prototypical catalyst Pt (overpotential ∼0.45 V). Such contrasting ORR behaviors of point and edge defects are also understood in depth by analyzing the underlying thermodynamic and electronic-structure mechanisms. This work not only quantitatively explains the performances of MoS₂ in both galvanic corrosion and electrochemical catalysis but also provides a useful structure-ORR map that can facilitate adapting the realistic MoS₂ to versatile electrochemical applications.

Isolated Au Atom Anchored on Porous Boron Nitride as a Promising Electrocatalyst for Oxygen Reduction Reaction (ORR): A DFT Study

The development of efficient, stable, and low-cost catalytic material for the oxygen reduction reaction (ORR) is currently highly desirable but challenging. In this work, based on first-principles calculation, the stabilities, catalytic activities and catalytic mechanisms of isolated Au atom supported on defective porous BN (p-BN) have been studied in detail. The results reveal that the defective p-BN anchor Au atom strongly to ensure the stability of Au/p-BN. Based on frontier molecular orbital and charge-density analysis, isolated Au atom supported on porous BN with V_N defect (Au/p-BN-V_N) is an effective ORR catalyst. Especially, the low barriers of the formation (0.38 eV) and dissociation (0.31 eV) of ^*OOH and the instability of H₂O₂ on Au/p-BN-V_N catalyst suggest that ORR proceeds via 4-electron pathway. Along the favorable pathway, the reduction of O₂ to ^*OOH is the rate-limiting step with the largest activation barrier of 0.38 eV and the maximum free energy change is 1.88 eV. Our results provide a useful guidance for the design and fabrication of new Au-base catalyst with high-efficiency and are beneficial for the developing of novel isolated metal atom catalysts for ORR.

Pd-Catalyzed Reaction-Producing Intermediate S on a Pd/In2O3 Surface: A Key To Achieve the Enhanced CS2-Sensing Performances

Although chemiresistive gas sensors, based on metal-oxide semiconductors, have exhibited particular promise for the monitoring of air pollution, they are often limited because of poor selectivity. In that case, to overcome this issue, according to the essence of the gas-sensing process, the method of reforming the surface reaction path on the surface of the sensing materials was used. Here, we report that Pd nanoparticles supported over the In₂O₃ composites, featured with a yolk-shell structure, enable the trace detection of carbon disulfide (CS₂) gas molecules, which are immensely dangerous to humans and animals. Moreover, the prominent enhancement of the gas response and the ultraselective CS₂-sensing characteristic were acquired in comparison with pristine In₂O₃ sensors. Significantly, density functional theory calculations revealed that the Pd supported on In₂O₃ greatly facilitates the adsorption capacity to CS₂, and the intermediate S, produced by Pd-catalyzed desulfurization reaction, on the Pd/In₂O₃ surface during the sensing process is a key to achieving a high CS₂ gas response as well as ultraselectivity, which is well in agreement with the X-ray photoelectron spectroscopy analysis results. On the basis of these results, a new sensing mechanism model for the CS₂-sensing process was put forward.