Alloying Behavior of Self-Assembled Noble Metal Nanoparticles

Structural Regulation of Au-Pt Bimetallic Aerogels for Catalyzing the Glucose Cascade Reaction

Bimetallic nanostructures are promising candidates for the development of enzyme-mimics, yet the deciphering of the structural impact on their catalytic properties poses significant challenges. By leveraging the structural versatility of nanocrystal aerogels, this study reports a precise control of Au-Pt bimetallic structures in three representative structural configurations, including segregated, alloy, and core-shell structures. Benefiting from a synergistic effect, these bimetallic aerogels demonstrate improved peroxidase- and glucose oxidase-like catalytic performances compared to their monometallic counterparts, unleashing tremendous potential in catalyzing the glucose cascade reaction. Notably, the segregated Au-Pt aerogel shows optimal catalytic activity, which is 2.80 and 3.35 times higher than that of the alloy and core-shell variants, respectively. This enhanced activity is attributed to the high-density Au-Pt interface boundaries within the segregated structure, which foster greater substrate affinity and superior catalytic efficiency. This work not only sheds light on the structure-property relationship of bimetallic catalysts but also broadens the application scope of aerogels in biosensing and biological detections.

Inorganic Hydrogel Based on Low-Dimensional Nanomaterials

Composed of three-dimensional (3D) nanoscale inorganic bones and up to 99% water, inorganic hydrogels have attracted much attention and undergone significant growth in recent years. The basic units of inorganic hydrogels could be metal nanoparticles, metal nanowires, SiO2 nanowires, graphene nanosheets, and MXene nanosheets, which are then assembled into the special porous structures by the sol-gel process or gelation via either covalent or noncovalent interactions. The high electrical and thermal conductivity, resistance to corrosion, stability across various temperatures, and high surface area make them promising candidates for diverse applications, such as energy storage, catalysis, adsorption, sensing, and solar steam generation. Besides, some interesting derivatives, such as inorganic aerogels and xerogels, can be produced through further processing, diversifying their functionalities and application domains greatly. In this context, we primarily provide a comprehensive overview of the current status of inorganic hydrogels and their derivatives, including the structures of inorganic hydrogels with various compositions, their gelation mechanisms, and their exceptional practical performance in fields related to energy and environmental applications.

Element Distributions in Bimetallic Aerogels

ConspectusMetal aerogels assembled from nanoparticles have captured grand attention because they combine the virtues of metals and aerogels and are regarded as ideal materials to address current environmental and energy issues. Among these aerogels, those composed of two metals not only display combinations (superpositions) of the properties of their individual metal components but also feature novel properties distinctly different from those of their monometallic relatives. Therefore, quite some effort has been invested in refining the synthetic methods, compositions, and structures of such bimetallic aerogels as to boost their performance for the envisaged application(s). One such use would be in the field of electrocatalysis, whereby it is also of utmost interest to unravel the element distributions of the (multi)metallic catalysts to achieve a ratio of their bottom-to-up design. Regarding the element distributions in bimetallic aerogels, advanced characterization techniques have identified alloys, core-shells, and structures in which the two metal particles are segregated (i.e., adjacent but without alloy or core-shell structure formation). While an almost infinite number of metal combinations to form bimetallic aerogels can be envisaged, the knowledge of their formation mechanisms and the corresponding element distributions is still in its infancy. The evolution of the observed musters is all but well understood, not to mention the positional changes of the elements observed in operando or in beginning- vs end-of-life comparisons (e.g., in fuel cell applications).With this motivation, in this Account we summarize the endeavors made in element distribution monitoring in bimetallic aerogels in terms of synthetic methods, expected structures, and their evolution during electrocatalysis. After an introductory chapter, we first describe briefly the two most important characterization techniques used for this, namely, scanning transmission electron microscopy (STEM) combined with element mapping (e.g., energy-dispersive X-ray spectroscopy (EDXS)) and X-ray absorption spectroscopy (XAS). We then explain the universal methods used to prepare bimetallic aerogels with different compositions. Those are divided into one-step methods in which gels formed from mixtures of the respective metal salts are coreduced and two-step approaches in which monometallic nanoparticles are mixed and gelated. Subsequently, we summarize the current state-of-knowledge on the element distributions unraveled using diverse characterization methods. This is extended to investigations of the element distributions being altered during electrochemical cycling or other loads. So far, a theoretical understanding of these processes is sparse, not to mention predictions of element distributions. The Account concludes with a series of remarks on current challenges in the field and an outlook on the gains that the field would earn from a solid understanding of the underlying processes and a predictive theoretical backing.

High Performance 3D Self-Supporting Cu-Bi Aerogels for Electrocatalytic Reduction of CO2 to Formate

The electrocatalytic reduction of CO₂ (CO₂ RR) to CO, formate, methane, and other high-value compounds is a promising technique. However, current electrocatalysts suffer from drawbacks such as few active catalytic sites, poor selectivity and low stability, etc, which restrict the practical application. Although monatomic metal catalysts have been widely reported in recent years, high performance non-noble metal aerogels were rarely investigated for electrocatalytic CO₂ RR. Herein, Cu-Bi aerogels with boosted CO₂ RR activity were well constructed by a simple one-step self-assembly method. The resultant Cu₁ Bi₂ exhibits excellent CO₂ RR activity with high faradaic efficiency (FE) of 96.57 % towards HCOOH at a potential of -0.9 V vs. RHE, and the FE_HCOOH remains over 80.18 % in a wide potential window (-0.8 V to -1.2 V vs. RHE). It demonstrated that the enhanced CO₂ RR activity of Cu-Bi aerogels could be attributed to the 3D self-supporting structure of the catalysis, synergistic effect, and low interfacial charge transfer resistance.

Two-dimensional multimetallic alloy nanocrystals: recent progress and challenges

In this highlight article, the recent progress on the preparation and application of multimetallic alloy nanocrystals with 2D nanostructures is systematically reviewed, as well as perspectives on future challenges and opportunities.

Noble Metal Aerogels

Noble metal-based nanomaterials have been a hot research topic during the past few decades. Particularly, self-assembled porous architectures have triggered tremendous interest. At the forefront of porous nanostructures, there exists a research endeavor of noble metal aerogels (NMAs), which are unique in terms of macroscopic assembly systems and three-dimensional (3D) porous network nanostructures. Combining excellent features of noble metals and the unique structural traits of porous nanostructures, NMAs are of high interest in diverse fields, such as catalysis, sensors, and self-propulsion devices. Regardless of these achievements, it is still challenging to rationally design well-tailored NMAs in terms of ligament sizes, morphologies, and compositions and profoundly investigate the underlying gelation mechanisms. Herein, an elaborate overview of the recent progress on NMAs is given. First, a simple description of typical synthetic methods and some advanced design engineering are provided, and then, the gelation mechanism models of NMAs are discussed in detail. Furthermore, promising applications particularly focusing on electrocatalysis and biosensors are highlighted. In the final section, brief conclusions and an outlook on the existing challenges and future chances of NMAs are also proposed.

Preparation and Electrocatalysis Application of Pure Metallic Aerogel: A Review

Nanomaterials are widely used in electrocatalysts due to their quantum size effect and high utilization efficiency. There are two ways to improve the activity of nanoelectrocatalysts: increasing the number of active sites and improving the inherent activity of each catalytic site. The structure of the catalyst itself can be improved by increasing the number of exposed active sites per unit mass. The high porosity and three-dimensional network structure enable aerogels to have the characteristics of a large specific surface area, exposing many active sites and bringing structural stability through the self-supporting nature of aerogels. Thus, by adjusting the compositions of aerogels, the synergetic effect introduced by alloy elements can be utilized to further improve the single-site activity. In this review, we summarized the basic preparation strategy of aerogels and extended it to the preparation of alloys and special structure aerogels. Moreover, through the eight electrocatalysis cases, the outstanding catalytic performances and broad applicability of aerogel electrocatalysts are emphasized. Finally, we predict the future development of pure metallic aerogel electrocatalysts from the perspective of preparation to application.

Promoting the Electrocatalytic Performance of Noble Metal Aerogels by Ligand-Directed Modulation

Noble metal aerogels (NMAs) are an emerging class of porous materials. Embracing nano-sized highly-active noble metals and porous structures, they display unprecedented performance in diverse electrocatalytic processes. However, various impurities, particularly organic ligands, are often involved in the synthesis and remain in the corresponding products, hindering the investigation of the intrinsic electrocatalytic properties of NMAs. Here, starting from laser-generated inorganic-salt-stabilized metal nanoparticles, various impurity-free NMAs (Au, Pd, and Au-Pd aerogels) were fabricated. In this light, we demonstrate not only the intrinsic electrocatalytic properties of NMAs, but also the prominent roles played by ligands in tuning electrocatalysis through modulating the electron density of catalysts. These findings may offer a new dimension to engineer and optimize the electrocatalytic performance for various NMAs and beyond.

Diatoms – A “Green” Way to Biosynthesize Gold-Silica Nanocomposites?

Biosynthesis by diatoms provides a green approach for nanoparticle (NP) production. However, reproducible and homogeneous shapes are essential for their application. To improve these characteristics during biosynthesis, the underlying synthesis mechanisms as well as involved substances need to be understood. The first essential step for suitable analyses is the purification of Au-silica-nanocomposites from organic biomass. Succesfully cleaned nanocomposites could, for example, be useful as catalysts. In combination with the biosynthesized NPs, this material presents a “green” catalyst and could contribute to the currently thriving green nanochemistry. In this work, we compare different purification agents with respect to their ability to purify cells of the diatom Stephanopyxis turris without separating the biosynthesized Au-silica-nanocomposites from the diatom cell walls. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) are used to localize and identify Au-silica-nanocomposites around the cells. The amount of remaining organic compounds on the purified cell is detected by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Furthermore, inductively coupled plasma optical emission spectrometry (ICP-OES) is used to track the “gold path” during cell growth and the different purifications steps.

Emerging Hierarchical Aerogels: Self-Assembly of Metal and Semiconductor Nanocrystals

Aerogels assembled from colloidal metal or semiconductor nanocrystals (NCs) feature large surface area, ultralow density, and high porosity, thus rendering them attractive in various applications, such as catalysis, sensors, energy storage, and electronic devices. Morphological and structural modification of the aerogel backbones while maintaining the aerogel properties enables a second stage of the aerogel research, which is defined as hierarchical aerogels. Different from the conventional aerogels with nanowire-like backbones, those hierarchical aerogels are generally comprised of at least two levels of architectures, i.e., an interconnected porous structure on the macroscale and a specially designed configuration at local backbones at the nanoscale. This combination "locks in" the inherent properties of the NCs, so that the beneficial genes obtained by nanoengineering are retained in the resulting monolithic hierarchical aerogels. Herein, groundbreaking advances in the design, synthesis, and physicochemical properties of the hierarchical aerogels are reviewed and organized in three sections: i) pure metallic hierarchical aerogels, ii) semiconductor hierarchical aerogels, and iii) metal/semiconductor hybrid hierarchical aerogels. This report aims to define and demonstrate the concept, potential, and challenges of the hierarchical aerogels, thereby providing a perspective on the further development of these materials.