Colloidal synthesis of two-dimensional nanocrystals by the polyol route

The field of 2D nanomaterials is ever-growing with a myriad of synthetic advancements that have been used to obtain such materials. There are top-down, as well as bottom-up, fabrication methods for obtaining 2D nanomaterials; however, synthesis of 2D nanomaterials from solution offers a simple scalable way to control size, shape, and surface. This review outlines the recent advances in colloidal polyol synthesis of 2D nanomaterials and provides perspectives on the similarities and differences in various syntheses. Various materials classes are presented and discussed, including metals, oxides, chalcogenides, and halides, that can be synthesized as 2D nanomaterials via a polyol process. Throughout the literature, polyol media is demonstrated to be versatile not only as a solvent and reducing agent for metal precursors but also as a binding and shape-directing agent for many 2D nanomaterials. Polyols also offer the ability to dissolve various surfactants and additives that can further control the morphology and composition of various nanomaterials. In this review, we outline the various 2D materials that have been realized via the solution polyol route.

Hydroxyethylcellulose-Directed Synthesis of Gold Microplates with Hydrogen Peroxide in an Aqueous Phase

In this study, we successfully synthesized well-defined polygonal gold microplates for the first time in an aqueous phase using hydroxyethylcellulose (HEC) in the presence of hydrogen peroxide (H2O2). HEC played a pivotal role during the synthesis, acting not only as a biotemplate but also as an in situ reduction site for the nucleation and growth of gold (Au) microplates. H2O2 played a crucial role in accelerating the growth of Au microplates from the Au nucleus. This methodology is ecofriendly and easy to operate and has potential applications in various fields, such as electronics, photonics, and biotechnology.

Two-Dimensional Metal Nanostructures: From Theoretical Understanding to Experiment

This paper reviews recent studies on the preparation of two-dimensional (2D) metal nanostructures, particularly nanosheets. As metal often exists in the high-symmetry crystal phase, such as face centered cubic structures, reducing the symmetry is often needed for the formation of low-dimensional nanostructures. Recent advances in characterization and theory allow for a deeper understanding of the formation of 2D nanostructures. This Review firstly describes the relevant theoretical framework to help the experimentalists understand chemical driving forces for the synthesis of 2D metal nanostructures, followed by examples on the shape control of different metals. Recent applications of 2D metal nanostructures, including catalysis, bioimaging, plasmonics, and sensing, are discussed. We end the Review with a summary and outlook of the challenges and opportunities in the design, synthesis, and application of 2D metal nanostructures.

Valence, Size, and Shape Control of Gold Nanoparticles Synthesized by Electron-Assisted Reduction

An electron-assisted strategy was developed to prepare gold nanoparticles (AuNPs) at room temperature. Glow discharge plasma as electron source was successfully used to control the valence state, size, and shape of AuNPs. Stable Au(I) was obtained in 3 min by plasma, and Au(I) was reduced to zero valence with the increase in treatment time. An increase in the amount of Au did not induce an increase in particle size. A narrow size distribution was also achieved. The narrowest size distribution was observed at 9 min at 600 V. AuNPs grew slowly under glow discharge plasma, which slightly changed the mean size of AuNPs. Moreover, the average size of AuNPs was smaller under alkaline conditions. The initial pH of the solution can affect the nucleation and growth of AuNPs and further affect their particle size. Spherical AuNPs, hexagonal AuNPs, rectangular AuNPs, flower-shaped AuNPs, and Au nanorods were easily obtained within 30 min by adding different additives. The hexagonal AuNPs exhibited the largest current response toward caffeine and showed a good linear range (0.1-1000 μM) with a low detection limit (0.064 μM), because their high-energy planes can increase the electron transfer rate and improve electrocatalytic activity.

Spherical Sandwich Au@Pd@UIO-67/Pt@UIO- n ( n = 66, 67, 69) Core-Shell Catalysts: Zr-Based Metal-Organic Frameworks for Effectively Regulating the Reverse Water-Gas Shift Reaction

In this study, spherical sandwich Au@Pd@UIO-67/Pt@UIO- n ( n = 66, 67, 69) core-shell catalysts were assembled. Au nanoparticles (NPs) were used as the core for the epitaxial growth of Pd shells, and Au@Pd core-shell NPs were successfully encapsulated in the center of monodispersed Au@Pd@UIO-67 nanospheres. Pt NPs were fully fixed onto the nanosphere surfaces to obtain Au@Pd@UIO-67/Pt composites; further coating with UIO- n led to Au@Pd@UIO-67/Pt@UIO- n, in which Pt NPs are sandwiched between the Au@Pd@UIO-67 core and the UIO- n shell. The Au@Pd core-shell NPs efficiently controlled the morphology and structure of UIO-67 and enhanced the CO selectivity of the catalyst. Pt NPs increased the CO₂ conversion, and the UIO- n component effectively regulated the reverse water-gas shift reaction.

Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and Applications

As one unique group of two-dimensional (2D) nanomaterials, 2D metal nanomaterials have drawn increasing attention owing to their intriguing physiochemical properties and broad range of promising applications. In this Review, we briefly introduce the general synthetic strategies applied to 2D metal nanomaterials, followed by describing in detail the various synthetic methods classified in two categories, i.e. bottom-up methods and top-down methods. After introducing the unique physical and chemical properties of 2D metal nanomaterials, the potential applications of 2D metal nanomaterials in catalysis, surface enhanced Raman scattering, sensing, bioimaging, solar cells, and photothermal therapy are discussed in detail. Finally, the challenges and opportunities in this promising research area are proposed.

Differential role of PVP on the synthesis of plasmonic gold nanostructures and their catalytic and SERS properties

Differential role of PVP modified with halide ions has been meticulously studied for in situ tuning of Au nanoparticle growth utilizing XRD measurements together with FTIR data, thus quantifying their catalysis and SERS applications.

Characterization of Conventional One-Step Sodium Thiosulfate Facilitated Gold Nanoparticle Synthesis

Gold-gold sulfide nanoparticles are of interest for drug delivery, biomedical imaging, and photothermal therapy applications due to a facile synthesis method resulting in small particles with high near-infrared (NIR) absorption efficiency. Previous studies suggest that the NIR sensitivity of these nanoparticles was due to hexagonally shaped metal-coated dielectric nanoparticles that consist of a gold sulfide core and gold shell. Here, we illustrate that the conventional synthesis procedure results in the formation of polydisperse samples of icosahedral gold particles, gold nanoplates, and small gold spheres. Importantly, through compositional analysis, via UV/vis absorption spectrophotometry, transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS), we show that all of the nanoparticles exhibit identical face center cubic (FCC) gold crystalline structures, thus suggesting that sulfide is not present in the final fabricated nanoparticles. We show that icosahedrally shaped nanoparticles result in a blue-shifted absorbance, with a peak in the visible range. Alternatively, the nanoplate nanoparticles result in the characteristic NIR absorbance peak. Thus, we report that the NIR-contributing species in conventional gold-gold sulfide formulations are nanoplates that are comprised entirely of gold. Furthermore, polydisperse gold nanoparticle samples produced by the traditional one-step reduction of HAuCl4 by sodium thiosulfate show increased in vitro toxicity, compared to isolated and more homogeneous constituent samples. This result exemplifies the importance of developing monodisperse nanoparticle formulations that are well characterized in order to expedite the development of clinically beneficial nanomaterials.

Engineering noble metal nanomaterials for environmental applications

Besides being valuable assets in our daily lives, noble metals (namely, gold, silver, and platinum) also feature many intriguing physical and chemical properties when their sizes are reduced to the nano- or even subnano-scale; such assets may significantly increase the values of the noble metals as functional materials for tackling important societal issues related to human health and the environment. Among which, designing/engineering of noble metal nanomaterials (NMNs) to address challenging issues in the environment has attracted recent interest in the community. In general, the use of NMNs for environmental applications is highly dependent on the physical and chemical properties of NMNs. Such properties can be readily controlled by tailoring the attributes of NMNs, including their size, shape, composition, and surface. In this feature article, we discuss recent progress in the rational design and engineering of NMNs with particular focus on their applications in the field of environmental sensing and catalysis. The development of functional NMNs for environmental applications is highly interdisciplinary, which requires concerted efforts from the communities of materials science, chemistry, engineering, and environmental science.

Enhancing nanoparticle electrodynamics with gold nanoplate mirrors

Mirrors and optical cavities can modify and enhance matter-radiation interactions. Here we report that chemically synthesized Au nanoplates can serve as micrometer-size mirrors that enhance electrodynamic interactions. Because of their plasmonic properties, the Au nanoplates enhance the brightness of scattered light from Ag nanoparticles near the nanoplate surface in dark-field microscopy. More importantly, enhanced optical trapping and optical binding of Ag nanoparticles are demonstrated in interferometric optical traps created from a single laser beam and its reflection from individual Au nanoplates. The enhancement of the interparticle force constant is ≈20-fold more than expected from the increased intensity due to standing wave interference. We show that the additional stability for optical binding arises from the restricted axial thermal motion of the nanoparticles that couples to and reduces the fluctuations in the lateral plane. This new mechanism greatly advances the photonic synthesis of ultrastable nanoparticle arrays and investigation of their properties.

Controllable synthesis of three-dimensional branched gold nanocrystals assisted by cationic surfactant poly(diallyldimethylammonium) chloride in acidic aqueous solution

An effective strategy was used for the controllable synthesis of single crystalline branched gold nanocrystals with a three-dimensional hierarchical structure in aqueous solution.