Mechanochemistry is a promising alternative to solution-based protocols across the chemical sciences, enabling different types of chemistries in solvent-free and environmentally benign conditions. ...The use of mechanical energy to promote physical and chemical transformations has reached a high level of refinement, allowing for the design of sophisticated molecules and nanostructured materials. Among them, the synthesis of noble metal nanoparticles deserves special attention due to their catalytic applications. In this review, we discuss the recent progress on the development of mechanochemical strategies for the controlled synthesis of noble metal nanostructures. We start by covering the fundamentals of different preparation routes, namely top-down and bottom-up approaches. Next, we focus on the key examples of the mechanochemical synthesis of non-supported and supported metal nanoparticles as well as hybrid nanomaterials containing noble metals. In these examples, in addition to the principles and synthesis mechanisms, their performances in catalysis are discussed. Finally, a perspective of the field is given, where we discuss the opportunities for future work and the challenges of mechanochemical synthesis to produce well-defined noble metal nanoparticles.
This review covers the fundamentals and recent developments on the mechanochemical synthesis of noble metal nanoparticles.
Plasmonic photocatalysis has emerged as a prominent and growing field. It enables the efficient use of sunlight as an abundant and renewable energy source to drive a myriad of chemical reactions. For ...instance, plasmonic photocatalysis in materials comprising TiO2 and plasmonic nanoparticles (NPs) enables effective charge carrier separation and the tuning of optical response to longer wavelength regions (visible and near infrared). In fact, TiO2‐based materials and plasmonic effects are at the forefront of heterogeneous photocatalysis, having applications in energy conversion, production of liquid fuels, wastewater treatment, nitrogen fixation, and organic synthesis. This review aims to comprehensively summarize the fundamentals and to provide the guidelines for future work in the field of TiO2‐based plasmonic photocatalysis comprising the above‐mentioned applications. The concepts and state‐of‐the‐art description of important parameters including the formation of Schottky junctions, hot electron generation and transfer, near field electromagnetic enhancement, plasmon resonance energy transfer, scattering, and photothermal heating effects have been covered in this review. Synthetic approaches and the effect of various physicochemical parameters in plasmon‐mediated TiO2‐based materials on performances are discussed. It is envisioned that this review may inspire and provide insights into the rational development of the next generation of TiO2‐based plasmonic photocatalysts with target performances and enhanced selectivities.
While photocatalysis has been focused on TiO2‐based materials, the harvesting of plasmonic effects has emerged as an efficient strategy to enhance photocatalytic performances. Therefore, the combination of TiO2 with plasmonic nanoparticles has attracted massive attention. This review covers the fundamentals, recent advances, and representative examples, and provides future guidelines in the field of TiO2‐based plasmonic photocatalysis toward various important molecular transformations.
Noble-metal nanoparticles have been widely employed in catalysis. As catalytic properties are dependent on their physical/chemical parameters, strategies for their controlled synthesis and the ...understanding of performance relationships have gained attention. In this review, we discuss developments on well-defined noble-metal nanoparticles focusing on relationships between performance and physical/chemical features. We begin with the control over shape, composition, and size. Then, we focus on nanoparticles with hollow interiors, which provide further possibilities for performance optimization. We provide a case-by-case discussion to illustrate how, in addition to the hollow interiors, the control over their composition, size, and surface morphology relate to catalytic performance.
This review discusses the importance of physical/chemical control in metallic NPs for the optimization and understanding of catalytic performances.
Metallic nanoparticles have been extensively studied towards applications in catalysis. Among the several methods for their controlled synthesis, galvanic replacement is particularly attractive as it ...enables the production of bimetallic and hollow nanomaterials displaying ultrathin walls in a single reaction step. This procedure is versatile, but final morphologies are often limited to shapes that represent the hollow analogues of the starting template nanocrystals. For catalytic applications, it is highly desirable to broaden the scope of physicochemical control that can be achieved by this method. This feature article discusses recent strategies developed in our group for the synthesis of hollow bimetallic nanomaterials by galvanic replacement that enable a further level of control over surface morphologies and composition. We begin by briefly explaining the fundamentals of the conventional galvanic replacement reaction between Ag and AuCl
4
−
. This is one of the most characteristic galvanic replacement reactions, and it can be tuned to create a huge variety of nanoparticle morphologies. We will discuss how advanced electron microscopy characterization enables us to uncover surface-segregation behavior as a function of compositions, and relate this to the detected catalytic performance. We will also discuss how galvanic replacement can be extended to trimetallic compositions, leading to improvements in catalytic activities compared to mono or bimetallic counterparts. Furthermore, we will show how surface morphology, size, and anisotropic growth can be controlled by tuning the temperature during the synthesis and by combining galvanic replacement reaction with co-reduction. Finally, we will demonstrate how these approaches are promising for large-scale synthesis of controlled hollow nanostructures and their incorporation into supports to produce catalysts at the gram-scale. We believe the developments described herein shed important insights and may inspire the development of sophisticated and controlled nanomaterials at relatively larger scales for catalytic applications.
Recent developments to achieve further physicochemical control in metallic nanomaterials by galvanic replacement are discussed towards applications in catalysis.
Controlling the morphology of Pt nanostructures can provide a great opportunity to improve their catalytic properties and increase their activity on a mass basis. We synthesized Pd-Pt bimetallic ...nanodendrites consisting of a dense array of Pt branches on a Pd core by reducing K₂PtCl₄ with L-ascorbic acid in the presence of uniform Pd nanocrystal seeds in an aqueous solution. The Pt branches supported on faceted Pd nanocrystals exhibited relatively large surface areas and particularly active facets toward the oxygen reduction reaction (ORR), the rate-determining step in a proton-exchange membrane fuel cell. The Pd-Pt nanodendrites were two and a half times more active on the basis of equivalent Pt mass for the ORR than the state-of-the-art Pt/C catalyst and five times more active than the first-generation supportless Pt-black catalyst.
The use of sunlight to drive chemical reactions
via
photocatalysis is of paramount importance towards a sustainable future. Among several photocatalysts, earth-abundant polymeric carbon nitride (PCN, ...often wrongly named g-C
3
N
4
) has emerged as an attractive candidate due to its ability to absorb light efficiently in the visible and near-infrared ranges, chemical stability, non-toxicity, straightforward synthesis, and versatility as a platform for constructing hybrid materials. Especially, hybrids with metal nanoparticles offer the unique possibility of combining the catalytic, electronic, and optical properties of metal nanoparticles with PCN. Here, we provide a comprehensive overview of PCN materials and their hybrids, emphasizing heterostructures with metal nanoparticles. We focus on recent advances encompassing synthetic strategies, design principles, photocatalytic applications, and charge-transfer mechanisms. We also discuss how the localized surface plasmon resonance (LSPR) effect of some noble metals NPs (
e.g.
Au, Ag, and Cu), bimetallic compositions, and even non-noble metals NPs (
e.g.
, Bi) synergistically contribute with PCN in light-driven transformations. Finally, we provide a perspective on the field, in which the understanding of the enhancement mechanisms combined with truly controlled synthesis can act as a powerful tool to the establishment of the design principles needed to take the field of photocatalysis with PCN to a new level, where the desired properties and performances can be planned in advance, and the target material synthesized accordingly.
This review discusses synthetic strategies, photocatalytic applications, charge-transfer mechanisms, and design principles in hybrids composed of g-C
3
N
4
and metal nanoparticles.
Although catalytic processes mediated by surface plasmon resonance (SPR) excitation have emerged as a new frontier in catalysis, the selectivity of these processes remains poorly understood. Here, ...the selectivity of the SPR‐mediated oxidation of p‐aminothiophenol (PATP) employing Au NPs as catalysts was controlled by the choice of catalysts (Au or TiO2‐Au NPs) and by the modulation of the charge transfer from UV‐excited TiO2 to Au. When Au NPs were employed as catalyst, the SPR‐mediated oxidation of PATP yielded p,p‐dimercaptobenzene (DMAB). When TiO2‐Au NPs were employed as catalysts under both UV illumination and SPR excitation, p‐nitrophenol (PNTP) was formed from PATP in a single step. Interestingly, PNTP molecules were further reduced to DMAB after the UV illumination was removed. Our data show that control over charge‐transfer processes may play an important role to tune activity, product formation, and selectivity in SPR‐mediated catalytic processes.
Controlling selectivity: The selectivity of the surface plasmon resonance (SPR) mediated oxidation of p‐aminothiophenol was controlled by the choice of catalysts (Au or TiO2–Au nanoparticles; NPs) and by the modulation of the charge transfer from UV‐excited TiO2 to Au. While p,p‐dimercaptobenzene was obtained using Au NPs as catalyst, the use of TiO2–Au NPs under both UV illumination and SPR excitation led to the formation of p‐nitrophenol.
Conspectus Colloidal particles with a spherical shape and diameters in the range of 0.01–1 μm have been a subject of extensive research, with applications in areas such as photonics, electronics, ...catalysis, drug delivery, and medicine. For most of these applications, it is of critical importance to achieve monodispersity for the size while expanding the diversity in terms of structure and composition. The uniformity in size allows one to establish rigorous correlations between this parameter and the physicochemical properties of the colloidal particles while ensuring experimental repeatability and measurement accuracy. On the other hand, the diversity in structure and composition offers additional handles for tailoring the properties. By switching from the conventional plain, solid structure to a core–shell, hollow, porous, or Janus structure, it offers immediate advantages and creates new opportunities, especially in the context of self-assembly, encapsulation, and controlled release. As for composition, monodispersed colloidal spheres were traditionally limited to amorphous materials such as polystyrene and silica. For metals and semiconducting materials, which are more valuable to applications in photonics, electronics, and catalysis, they tend to crystallize and thus grow anisotropically into nonspherical shapes, especially when their sizes pass 0.1 μm. Taken together, it is no wonder why chemical synthesis of monodispersed colloidal spheres has been a constant theme of research in areas such as colloidal science, materials chemistry, materials science, and soft matter. In this Account, we summarize our efforts over the past two decades in developing solution-phase methods for the facile synthesis of colloidal spheres that are uniform in size, together with a broad range of compositions (including metals and semiconductors) and structures (e.g., solid, core–shell, hollow, porous, and Janus, among others). We start with the synthesis of monodispersed colloidal spheres made of semiconductors, metals with low melting points, and precious metals. Through chemical reactions, these colloidal spheres can be transformed into core–shell or hollow structures with new compositions and properties. Next, we discuss the synthesis of colloidal spheres with a Janus structure while taking a pseudospherical shape. Specifically, metal–polymer hybrid particles composed of one metal nanoparticle partially embedded in the surface of a polymer sphere can be produced through precipitation polymerization in the presence of metal seed. With these Janus particles serving as templates, other types of Janus structures such as hollow spheres with a single hole in the surface can be obtained via site-selected deposition. Alternatively, such hollow spheres can be fabricated through a physical transformation process that involves swelling of polymer spheres, followed by freeze-drying. All these synthesis and transformation processes are solution-based, offering flexibility and potential for large-scale production. At the end, we highlight some of the applications enabled by these colloidal spheres, including fabrication of photonic devices, encapsulation, and controlled release for nanomedicine.
Sodium (Na)- and potassium (K)-doped δ-MnO2, which presented different band gaps, were synthesized by a hydrothermal method. Then, uniform Au nanoparticles (NPs) were deposited on MnO2 to form ...metal–semiconductor nano-heterojunctions (MnO2–Au). By comparing their temperature-dependent thermal catalytic performances, p-aminothiophenol to p,p′-dimercaptoazobenzene conversion was used as proof-of-concept transformations. MnO2–Au hybrid materials demonstrated better thermal catalytic performances relative to individual Au NPs. Meanwhile, K-doped MnO2–Au, with a MnO2 support displaying a narrower bandgap, displayed superior catalytic activities relative to Na-doped MnO2–Au. To get the same catalytic performance by individual Au NPs, it can be ∼50 K less by Na-doped MnO2–Au and ∼100 K less by K-doped MnO2–Au. The enhancement is mainly attributed to the thermally excited electrons in MnO2, which were transferred to Au NPs. The additional electrons in Au NPs increase the electron density and thus contribute to the improvement of thermal catalysis. Our findings show that the establishment of a nano-heterojunction formed by metal NPs on a semiconductor support has a significant impact on thermal catalysis, where a narrower band gap can facilitate thermally excited carriers and thus bring about better catalytic performances. Thus, the results presented here shed light on the design of a nano-heterojunction catalyst to approach reactions with superior performance under moderate conditions.
This article describes a systematic study of the galvanic replacement reaction between PtCl6 2− ions and Pd nanocrystals with different shapes, including cubes, cuboctahedrons, and octahedrons. It ...was found that Br− ions played an important role in initiating, facilitating, and directing the replacement reaction. The presence of Br− ions led to the selective initiation of galvanic replacement from the {100} facets of Pd nanocrystals, likely due to the preferential adsorption of Br− ions on this crystallographic plane. The site-selective galvanic replacement resulted in the formation of Pd−Pt bimetallic nanocrystals with a concave structure owing to simultaneous dissolution of Pd atoms from the {100} facets and deposition of the resultant Pt atoms on the {111} facets. The Pd−Pt concave nanocubes with different weight percentages of Pt at 3.4, 10.4, 19.9, and 34.4 were also evaluated as electrocatalysts for the oxygen reduction reaction (ORR). Significantly, the sample with a 3.4 wt.% of Pt exhibited the largest specific electrochemical surface area and was found to be four times as active as the commercial Pt/C catalyst for the ORR in terms of equivalent Pt mass.
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