Synthesis of hcp-Co Nanodisks

A low-cost process for complete utilization of bauxite residue

Cost-effective treatment or even valorization of the bauxite residue (red mud) from the alumina industry is in demand to improve their environmental and economic liabilities. This study proposes a strategy that provides a near-complete conversion of bauxite residue to valuable products. The first step involves dilute acid leaching, which allowed the fractionation of raw residues into (1) an aqueous fraction rich in silica and aluminium and (2) a solid residue rich in iron, titanium and rare earth elements. For the proposed process, 91% of the original silicon, 67% of the aluminium, 78% of the scandium and 69% of the cerium were recovered. The initial cost evaluation suggested that this approach is profitable with a gross margin of 167 $US per tonne. This "Residue2Product" approach should be considered for large-scale practices as one of the most economical and sustainable solutions to this environmental and economic liability for the alumina industry.

Magnetically Induced Anisotropic Interaction in Colloidal Assembly

The wide accessibility to nanostructures with high uniformity and controllable sizes and morphologies provides great opportunities for creating complex superstructures with unique functionalities. Employing anisotropic nanostructures as the building blocks significantly enriches the superstructural phases, while their orientational control for obtaining long-range orders has remained a significant challenge. One solution is to introduce magnetic components into the anisotropic nanostructures to enable precise control of their orientations and positions in the superstructures by manipulating magnetic interactions. Recognizing the importance of magnetic anisotropy in colloidal assembly, we provide here an overview of magnetic field-guided self-assembly of magnetic nanoparticles with typical anisotropic shapes, including rods, cubes, plates, and peanuts. The Review starts with discussing the magnetic energy of nanoparticles, appreciating the vital roles of magneto-crystalline and shape anisotropies in determining the easy magnetization direction of the anisotropic nanostructures. It then introduces superstructures assembled from various magnetic building blocks and summarizes their unique properties and intriguing applications. It concludes with a discussion of remaining challenges and an outlook of future research opportunities that the magnetic assembly strategy may offer for colloidal assembly.

Simulation and Synthesis of Cobalt (Co) Nanoparticles by Gamma Radiation Technique

Cobalt nanoparticles were synthesized using the gamma radiolytic technique, and the particle size was found to be reduced from 12±1 to 7±1 nm by increasing the dose from 10 to 60 kGy. The UV-visible absorption spectra were measured and exhibited a steady absorption maxima at 517 nm in the UV region, which blue-shifted toward a lower wavelength with a decrease in particle size. By taking the conduction electrons of an isolated particle that are not entirely free but are instead bound to their respective quantum levels, the optical absorption of the cobalt nanoparticles can be calculated and simulated via intra-band quantum excitation for particle sizes comparable to the measured ones. We found that the simulated absorption maxima of electronic excitations corresponded to the measured absorption maxima. Moreover, the structural characterizations were performed utilizing dynamic light scattering (DLS), transmission electron microscopy (TEM), and X-ray diffraction (XRD).

Ligand Chemistry in Antitumor Theranostic Nanoparticles

ConspectusTheranostic nanoparticles' potential in tumor treatment has been widely acknowledged thanks to their capability of integrating multifaceted functionalities into a single nanosystem. Theranostic nanoparticles are typically equipped with an inorganic core with exploitable physical properties for imaging and therapeutic functions, bioinert coatings for improved biocompatibility and immunological stealth, controlled drug-loading-release modules, and the ability to recognize specific cell type for uptake. Integrating multiple functionalities in a single nanosized construct require sophisticated molecular design and precise execution of assembly procedures. Underlying the multifunctionality of theranostic nanoparticles, ligand chemistry plays a decisive role in translating theoretical designs into fully functionalized theranostic nanoparticles. The ligand hierarchy in theranostic nanoparticles is usually threefold. As they serve to passivate the nanoparticle's surface, capping ligands form the first layer directly interfacing with the crystalline lattice of the inorganic core. The size and shape of nanoparticles are largely determined by the molecular property of capping ligands so that they have profound influences on the nanoparticles' surface chemistry and physical properties. Capping ligands are mostly chemically inert, which necessitates the presence of additional ligands for drug loading and tumor targeting. The second layer is commonly utilized for drug loading. Therapeutic drugs can either be covalently conjugated onto the capping layer or noncovalently loaded onto nanoparticles via drug-loading ligands. Drug-loading ligands need to be equally versatile in properties to accommodate the diversity of drugs. Biodegradable moieties are often incorporated into drug-loading ligands to enable smart drug release. With the aid of targeting ligands which usually stand the tallest on the nanoparticle surface to seek and bind to their corresponding receptors on the target, theranostic nanoparticles can preferentially accumulate at the tumor site to attain a higher precision and quantity for drug delivery. In this Account, the properties and utilities of representative capping ligands, drug-loading ligands, and targeting ligands are reviewed. Since these types of ligands are often assembled in close vicinity to each other, it is essential for them to be chemically compatible and able to function in tandem with each other. Relevant conjugation strategies and critical factors with a significant impact on ligands' performance on nanoparticles are discussed. Representative theranostic nanoparticles are presented to showcase how different types of ligands function synergistically from a single nanosystem. Finally, the technological outlook of evolving ligand chemistry on theranostic nanoparticles is provided.

Plate-Like Colloidal Metal Nanoparticles

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

Controlled CO labilization of tungsten carbonyl precursors for the low-temperature synthesis of tungsten diselenide nanocrystals

We report a low-temperature colloidal synthesis of WSe2 nanocrystals from tungsten hexacarbonyl and diphenyl diselenide in trioctylphosphine oxide (TOPO). We identify TOPO-substituted intermediates, W(CO)5TOPO and cis-W(CO)4(TOPO)2 by infrared spectroscopy. To confirm these assignments, we synthesize aryl analogues of phosphine-oxide-substituted intermediates, W(CO)5TPPO (synthesized previously, TPPO = triphenylphosphine oxide) and cis-W(CO)4(TPPO)2 and fac-W(CO)3(TPPO)3 (new structures reported herein). Ligation of the tungsten carbonyl by either the alkyl or aryl phosphine oxides results in facile labilization of the remaining CO, enabling low-temperature decomposition to nucleate WSe2 nanocrystals. The reactivity in phosphine oxides is contrasted with syntheses containing phosphine ligands, where substitution results in decreased CO labilization and higher temperatures are required to induce nanocrystal nucleation.

Necklace-Like Nanostructures: From Fabrication, Properties to Applications

The shape-controlled synthesis of nanocrystals remains a hot research topic in nanotechnology. Particularly, the fabrication of 1D structures such as wires, rods, belts, and tubes has been an interesting and important subject within nanoscience in the last few decades. 1D necklace-like micro/nanostructures are a sophisticated geometry that has attracted increasing attention due to their anisotropic and periodic structure, intrinsic high surface area, abundant transport channels, exposure of each component to the surface, and multiscale roughness of the surface. These characteristics enable their unique electrical, optical, and catalytic properties. This review provides a comprehensive summary of the advanced research progress on the fabrication strategies, novel properties, and various applications of necklace-like structures. It begins with the main fabrication methods of necklace-like structures and subsequently details a variety of their properties and applications. It concludes with the authors' perspectives on future research and development of the necklace-like structures.

Dynamic Observation of Anisotropic Chainlike Structures during Nonclassical Two-Step Nucleation in Solid-State Iron Oxide Crystallization

The demonstration of the self-crystallization nucleation process from an amorphous precursor in a solid is crucial for understanding of interactions between atoms. We report a study of dynamic crystallization process of iron oxides by virtue of in situ measurement of transmission electron microscopy. At first, semiordered chainlike structures are observed with the increase of concentration, and when sufficient chains form, the crystalline lattice begins to grow. The two-step nucleation pathway has also been confirmed by performing a molecular dynamics simulation, where Lennard-Jones and magnetic dipole-dipole interaction potentials are both taken into account and take effect individually predominantly in different ranges of distance between atoms. Furthermore, the total free energy profile in the crystallization nucleation process is calculated to evidence the stabilization of intermediate state. This work advances our understanding of nonclassical nucleation theory.

1D Colloidal chains: recent progress from formation to emergent properties and applications

Integrating nanoscale building blocks of low dimensionality (0D; i.e., spheres) into higher dimensional structures endows them and their corresponding materials with emergent properties non-existent or only weakly existent in the individual building blocks. Constructing 1D chains, 2D arrays and 3D superlattices using nanoparticles and colloids therefore continues to be one of the grand goals in colloid and nanomaterial science. Amongst these higher order structures, 1D colloidal chains are of particular interest, as they possess unique anisotropic properties. In recent years, the most relevant advances in 1D colloidal chain research have been made in novel synthetic methodologies and applications. In this review, we first address a comprehensive description of the research progress concerning various synthetic strategies developed to construct 1D colloidal chains. Following this, we highlight the amplified and emergent properties of the resulting materials, originating from the assembly of the individual building blocks and their collective behavior, and discuss relevant applications in advanced materials. In the discussion of synthetic strategies, properties, and applications, particular attention will be paid to overarching concepts, fresh trends, and potential areas of future research. We believe that this comprehensive review will be a driver to guide the interdisciplinary field of 1D colloidal chains, where nanomaterial synthesis, self-assembly, physical property studies, and material applications meet, to a higher level, and open up new research opportunities at the interface of classical disciplines.

Two-dimensional biomaterials: material science, biological effect and biomedical engineering applications

To date, nanotechnology has increasingly been identified as a promising and efficient means to address a number of challenges associated with public health. In the past decade, two-dimensional (2D) biomaterials, as a unique nanoplatform with planar topology, have attracted explosive interest in various fields such as biomedicine due to their unique morphology, physicochemical properties and biological effect. Motivated by the progress of graphene in biomedicine, dozens of types of ultrathin 2D biomaterials have found versatile bio-applications, including biosensing, biomedical imaging, delivery of therapeutic agents, cancer theranostics, tissue engineering, as well as others. The effective utilization of 2D biomaterials stems from the in-depth knowledge of structure-property-bioactivity-biosafety-application-performance relationships. A comprehensive summary of 2D biomaterials for biomedicine is still lacking. In this comprehensive review, we aim to concentrate on the state-of-the-art 2D biomaterials with a particular focus on their versatile biomedical applications. In particular, we discuss the design, fabrication and functionalization of 2D biomaterials used for diverse biomedical applications based on the up-to-date progress. Furthermore, the interactions between 2D biomaterials and biological systems on the spatial-temporal scale are highlighted, which will deepen the understanding of the underlying action mechanism of 2D biomaterials aiding their design with improved functionalities. Finally, taking the bench-to-bedside as a focus, we conclude this review by proposing the current crucial issues/challenges and presenting the future development directions to advance the clinical translation of these emerging 2D biomaterials.

Unconventional-Phase Crystalline Materials Constructed from Multiscale Building Blocks

Crystal phase, an intrinsic characteristic of crystalline materials, is one of the key parameters to determine their physicochemical properties. Recently, great progress has been made in the synthesis of nanomaterials with unconventional phases that are different from their thermodynamically stable bulk counterparts via various synthetic methods. A nanocrystalline material can also be viewed as an assembly of atoms with long-range order. When larger entities, such as nanoclusters, nanoparticles, and microparticles, are used as building blocks, supercrystalline materials with rich phases are obtained, some of which even have no analogues in the atomic and molecular crystals. The unconventional phases of nanocrystalline and supercrystalline materials endow them with distinctive properties as compared to their conventional counterparts. This Review highlights the state-of-the-art progress of nanocrystalline and supercrystalline materials with unconventional phases constructed from multiscale building blocks, including atoms, nanoclusters, spherical and anisotropic nanoparticles, and microparticles. Emerging strategies for engineering their crystal phases are introduced, with highlights on the governing parameters that are essential for the formation of unconventional phases. Phase-dependent properties and applications of nanocrystalline and supercrystalline materials are summarized. Finally, major challenges and opportunities in future research directions are proposed.

Phase behavior of polymer-nanorod composites: A comparative study using PRISM theory and molecular dynamics simulations

Well-dispersed composites of polymer and nanorods have many emerging applications and, therefore, are an important area of research. Polymer reference interaction site model (PRISM) theory and molecular dynamics simulations have become powerful tools in the study of the structure and phase behavior of polymer nanocomposites. In this work, we employ both PRISM theory and molecular dynamics simulations to determine the structure and spinodal phase diagram of 1% volume fraction of nanorods in a polymer melt. We make quantitative comparisons between the phase diagrams, which are reported as a function of nanorod aspect ratio and polymer-nanorod interactions. We find that both PRISM theory and molecular dynamics simulations predict the formation of contact aggregates at low polymer-nanorod attraction strength (γ) and bridged aggregates at high polymer-nanorod attraction strength. They predict an entropic depletion-driven phase separation at low γ and a bridging-driven spinodal phase separation at high γ. The polymer and nanorods are found to form stable composites at intermediate values of the polymer-nanorod attraction strength. The fall of the bridging boundary and the gradual rise of the depletion boundary with the nanorod aspect ratio are predicted by both PRISM theory and molecular dynamics simulations. Hence, the miscible region narrows with increasing aspect ratio. The depletion boundaries predicted by theory and simulation are quite close. However, the respective bridging boundaries present a significant quantitative difference. Therefore, we find that theory and simulations qualitatively complement each other and display quantitative differences.

Nitrogen-Doped Mixed-Phase Cobalt Nanocatalyst Derived from a Trinuclear Mixed-Valence Cobalt(III)/Cobalt(II) Complex for High-Performance Oxygen Evolution Reaction

Because of a continuous increase in energy demands and environmental concerns, a focus has been on the design and construction of a highly efficient, low-cost, environmentally friendly, and noble-metal free electrocatalyst for energy technology. Herein we report facile synthesis of the mixed-valence trinuclear cobalt complex 1 by the reaction of 2-amino-1-phenylethanol and CoCl₂·6H₂O in methanol as the solvent at room temperature. Further, 1 was reduced by using aqueous N₂H₄ as a simple reducing agent, followed by calcination at 300 °C for 3 h, yielding a nitrogen-doped mixed phase cobalt [β-Co(OH)₂ and CoO] nanocatalyst (N@MPCoNC). Both 1 and N@MPCoNC were characterized by various physicochemical techniques. Moreover, 1 was authenticated by single-crystal X-ray diffraction studies. The hybrid N@MPCoNC reveals a unique electronic and morphological structure, offering a low overpotential of 390 mV for a stable current density of 10 mA cm^-2 with high durability. This N@MPCoNC showed excellent electrocatalytic as well as photocatalytic activity for oxygen evolution reaction compared to 1.

Thermally stable surfactant-free ceria nanocubes in silica aerogel

Surfactant-mediated chemical routes allow one to synthesize highly engineered shape- and size-controlled nanocrystals. However, the occurrence of capping agents on the surface of the nanocrystals is undesirable for selected applications. Here, a novel approach to the production of shape-controlled nanocrystals which exhibit high thermal stability is demonstrated. Ceria nanocubes obtained by surfactant-mediated synthesis are embedded inside a highly porous silica aerogel and thermally treated to remove the capping agent. Powder X-ray Diffraction and Scanning Transmission Electron Microscopy show the homogeneous dispersion of the nanocubes within the aerogel matrix. Remarkably, both the size and the shape of the ceria nanocubes are retained not only throughout the aerogel syntheses but also upon thermal treatments up to 900 °C, while avoiding their agglomeration. The reactivity of ceria is measured by in situ High-Energy Resolution Fluorescence Detected - X-ray Absorption Near Edge Spectroscopy at the Ce L₃ edge, and shows the reversibility of redox cycles of ceria nanocubes when they are embedded in the aerogel. This demonstrates that the enhanced reactivity due to their prominent {100} crystal facets is preserved. In contrast, unsupported ceria nanocubes begin to agglomerate as soon as the capping agent decomposes, leading to a degradation of their reactivity already at 275 °C.

Cobalt-Containing Dispersion Catalysts for Three-Phase Fischer-Tropsch Synthesis

Nanosized catalyst dispersions have significant potential for improving hydrocarbon production from carbon monoxide and hydrogen via Fischer–Tropsch synthesis, an essential alternative to the use of petroleum as a raw material. New dispersed cobalt catalysts and dispersed-phase cobalt-based catalysts with Pd, Al₂O₃, or ZrO₂ additives for the Fischer–Tropsch synthesis were synthesized in the present work. A dispersed cobalt phase was prepared in a heavy paraffin medium using ex situ and in situ approaches through thermal decomposition of a nitrate precursor at various temperatures. Analyses showed that an increase in the temperature for catalytic suspension formation from 215 to 260°C enlarged the particles in the dispersed phase from 190 to 264 nm, which was probably due to increased agglomeration at elevated temperatures. The rheological properties of the obtained catalytic suspensions can be described by the Bingham equation. Furthermore, the concentration of the dispersed phase had a direct impact on the structure of the entire catalytic system. Ultrafine suspensions of palladium-promoted catalytic systems were tested for the Fischer–Tropsch synthesis. The overall yield of C₅₊ hydrocarbons was as high as 50 g/m³, and the productivity of the Pd-promoted catalytic systems reached 270–290 g/(kg_Co · h).

A Critical Review on Crystal Growth Techniques for Scalable Deposition of Photovoltaic Perovskite Thin Films

Although the efficiency of small-size perovskite solar cells (PSCs) has reached an incredible level of 25.25%, there is still a substantial loss in performance when switching from small size devices to large-scale solar modules. The large efficiency deficit is primarily associated with the big challenge of coating homogeneous, large-area, high-quality thin films via scalable processes. Here, we provide a comprehensive understanding of the nucleation and crystal growth kinetics, which are the key steps for perovskite film formation. Several thin-film crystallization techniques, including antisolvent, hot-casting, vacuum quenching, and gas blowing, are then summarized to distinguish their applications for scalable fabrication of perovskite thin films. In viewing the essential importance of the film morphology on device performance, several strategies including additive engineering, Lewis acid-based approach, solvent annealing, etc., which are capable of modulating the crystal morphology of perovskite film, are discussed. Finally, we summarize the recent progress in the scalable deposition of large-scale perovskite thin film for high-performance devices.

Shaping non-noble metal nanocrystals via colloidal chemistry

Non-noble metal nanocrystals with well-defined shapes have been attracting increasingly more attention in the last decade as potential alternatives to noble metals, by virtue of their earth abundance combined with intriguing physical and chemical properties relevant for both fundamental studies and technological applications. Nevertheless, their synthesis is still primitive when compared to noble metals. In this contribution, we focus on third row transition metals Mn, Fe, Co, Ni and Cu that are recently gaining interest because of their catalytic properties. Along with providing an overview on the state-of-the-art, we discuss current synthetic strategies and challenges. Finally, we propose future directions to advance the synthetic development of shape-controlled non-noble metal nanocrystals in the upcoming years.

Disk-Shaped Cobalt Nanocrystals as Fischer–Tropsch Synthesis Catalysts Under Industrially Relevant Conditions

Colloidal synthesis of metal nanocrystals (NC) offers control over size, crystal structure and shape of nanoparticles, making it a promising method to synthesize model catalysts to investigate structure-performance relationships. Here, we investigated the synthesis of disk-shaped Co-NC, their deposition on a support and performance in the Fischer–Tropsch (FT) synthesis under industrially relevant conditions. From the NC synthesis, either spheres only or a mixture of disk-shaped and spherical Co-NC was obtained. The disks had an average diameter of 15 nm, a thickness of 4 nm and consisted of hcp Co exposing (0001) on the base planes. The spheres were 11 nm on average and consisted of ε-Co. After mild oxidation, the CoO-NC were deposited on SiO2 with numerically 66% of the NC being disk-shaped. After reduction, the catalyst with spherical plus disk-shaped Co-NC had 50% lower intrinsic activity for FT synthesis (20 bar, 220 °C, H2/CO = 2 v/v) than the catalyst with spherical NC only, while C5+-selectivity was similar. Surprisingly, the Co-NC morphology was unchanged after catalysis. Using XPS it was established that nitrogen-containing ligands were largely removed and in situ XRD revealed that both catalysts consisted of 65% hcp Co and 21 or 32% fcc Co during FT. Furthermore, 3–5 nm polycrystalline domains were observed. Through exclusion of several phenomena, we tentatively conclude that the high fraction of (0001) facets in disk-shaped Co-NC decrease FT activity and, although very challenging to pursue, that metal nanoparticle shape effects can be studied at industrially relevant conditions.

Two-Dimensional van der Waals Nanoplatelets with Robust Ferromagnetism

We have synthesized unique colloidal nanoplatelets of the two-dimensional (2D) van der Waals ferromagnet CrI₃ and have characterized these nanoplatelets structurally, magnetically, and by magnetic circular dichroism spectroscopy. The CrI₃ nanoplatelets have lateral dimensions of ∼25 nm and thicknesses of only ∼4 nm, corresponding to just a few CrI₃ monolayers. Magnetic and magneto-optical measurements demonstrate robust 2D ferromagnetic ordering with Curie temperatures similar to bulk CrI₃, despite their small size. These data also show magnetization steps akin to those observed in micron-sized few-layer 2D sheets associated with concerted spin-reversal of individual CrI₃ layers within few-layer van der Waals stacks. Similar data have also been obtained for CrBr₃ and anion-alloyed Cr(I_1-xBr_x)₃ nanoplatelets. These results represent the first example of lateral nanostructures of 2D van der Waals ferromagnets of any composition. The demonstration of robust ferromagnetism at nanometer lateral dimensions opens new doors for miniaturization in spintronics devices based on van der Waals ferromagnets.

Ultrathin Pd-based nanosheets: syntheses, properties and applications

Two-dimensional (2D) noble metal-based nanosheets (NSs) have received considerable interest in recent years due to their unique properties and widespread applications. Pd-based NSs, as a typical member of 2D noble metal-based NSs, have been most extensively studied. In this review, we first summarize the research progress on the synthesis of Pd-based NSs, including pure Pd NSs, Pd-based alloy NSs, Pd-based core-shell NSs and Pd-based hybrid NSs. The synthetic strategy and growth mechanism are systematically discussed. Then their properties and applications in catalysis, biotherapy, gas sensing and so on are introduced in detail. Finally, the challenges and opportunities towards the rational design and controlled synthesis of Pd-based NSs are proposed.

Ultra-small intermetallic NiZn nanoparticles: a non-precious metal catalyst for efficient electrocatalysis

Intermetallics are long-range-ordered alloys traditionally synthesized by annealing nanoparticles of a random alloy, which results in the sintering of the nanoparticles and leads to the formation of polydispersed samples. It thus remains a challenge to achieve a monodispersion of tiny intermetallics. In the current work, ultra-small monodisperse intermetallic NiZn nanoparticles were synthesized based on a low-temperature solution chemistry route involving the chemical conversion of metal nanoparticles into an ordered alloy using an organometallic zinc precursor. During the transformation of single metal nanoparticles into the corresponding alloy, the particles retained their morphology. The resulting ordered alloy made up of earth-abundant materials demonstrated high electrocatalytic performance for the oxygen evolution reaction (OER) with a low overpotential of 283 mV at a current density of 10 mA cm^-2 and a small Tafel slope of 73 mV dec^-1, along with excellent stability and durability. The prepared intermetallic NiZn exhibited outstanding OER efficacy, better than those of a Ni_0.7Zn_0.3 alloy, pure Ni nanoparticles and even state-of-the art RuO₂. The atomic ordering as well as the modification of the electronic structure of Ni upon becoming alloyed with Zn, together with an atomic-scale synergistic effect produced from Ni and Zn, led to the enhanced intrinsic catalytic activity. The present findings point to a general route to produce nanoscale tiny alloys and also provide excellent electrocatalysts having exceptional energy conversion efficiency.

Formation of two-dimensional transition metal oxide nanosheets with nanoparticles as intermediates

Two-dimensional (2D) materials have attracted significant interest because of their large surface-to-volume ratios and electron confinement. Compared to common 2D materials such as graphene or metal hydroxides, with their intrinsic layered atomic structures, the formation mechanisms of 2D metal oxides with a rocksalt structure are not well understood. Here, we report the formation process for 2D cobalt oxide and cobalt nickel oxide nanosheets, after analysis by in situ liquid-phase transmission electron microscopy. Our observations reveal that three-dimensional (3D) nanoparticles are initially formed from the molecular precursor solution and then transform into 2D nanosheets. Ab initio calculations show that a small nanocrystal is dominated by positive edge energy, but when it grows to a certain size, the negative surface energy becomes dominant, driving the transformation of the 3D nanocrystal into a 2D structure. Uncovering these growth pathways, including the 3D-to-2D transition, provides opportunities for future material design and synthesis in solution.

The Role of Ligands in the Chemical Synthesis and Applications of Inorganic Nanoparticles

The design of nanoparticles is critical for their efficient use in many applications ranging from biomedicine to sensing and energy. While shape and size are responsible for the properties of the inorganic nanoparticle core, the choice of ligands is of utmost importance for the colloidal stability and function of the nanoparticles. Moreover, the selection of ligands employed in nanoparticle synthesis can determine their final size and shape. Ligands added after nanoparticle synthesis infer both new properties as well as provide enhanced colloidal stability. In this article, we provide a comprehensive review on the role of the ligands with respect to the nanoparticle morphology, stability, and function. We analyze the interaction of nanoparticle surface and ligands with different chemical groups, the types of bonding, the final dispersibility of ligand-coated nanoparticles in complex media, their reactivity, and their performance in biomedicine, photodetectors, photovoltaic devices, light-emitting devices, sensors, memory devices, thermoelectric applications, and catalysis.

Phase Controlled Synthesis of Pt Doped Co Nanoparticle Composites Using a Metal-Organic Framework for Fischer–Tropsch Catalysis

Recently, metal nanoparticles embedded in porous carbon composite materials have been playing a significant role in a variety of fields as catalyst supports, sensors, absorbents, and in energy storage. Porous carbon composite materials can be prepared using various synthetic methods; recent efforts provide a facile way to prepare the composites from metal-organic frameworks (MOFs) by pyrolysis. However, it is usually difficult to control the phase of metal or metal oxides during the synthetic process. Among many types of MOF, recently, cobalt-based MOFs have attracted attention due to their unique catalytic and magnetic properties. Herein, we report the synthesis of a Pt doped cobalt based MOF, which is subsequently converted into cobalt nanoparticle-embedded porous carbon composites (Pt@Co/C) via pyrolysis. Interestingly, the phase of the cobalt metal nanoparticles (face centered cubic (FCC) or hexagonal closest packing (HCP)) can be controlled by tuning the synthetic conditions, including the temperature, duration time, and dosage of the reducing agent (NaBH4). The Pt doped Co/C was characterized using various techniques including PXRD (powder X-ray diffraction), XPS (X-ray photoelectron spectroscopy), gas sorption analysis, TEM (transmission electron microscopy), and SEM (scanning electron microscopy). The composite was applied as a phase transfer catalyst (PTC). The Fischer-Tropsch catalytic activity of the Pt@Co/C (10:1:2.4) composite shows 35% CO conversion under a very low pressure of syngas (1 MPa). This is one of the best reported conversion rates at low pressure. The 35% CO conversion leads to the generation of various hydrocarbons (C1, C2–C4, C5, and waxes). This catalyst may also prove useful for energy and environmental applications.

Sulfur-free synthesis of size tunable rickardite (Cu3-x Te2) spheroids and planar squares

We report a novel synthesis of monodisperse samples of copper telluride with crystallinity and stoichiometry corresponding to forms of rickardite, Cu_3-x Te₂ (x < 1). This synthesis makes use of a ligand balanced reaction to allow control over shape and size by varying the relative and absolute concentration of oleylamine to stearic acid. The rickardite samples presented here display size dependent plasmon peaks in the near infrared and direct energy band gaps between 1.7 and 2.3 eV. As such they may find utility in photovoltaic, thermoelectric or as novel optical materials for study of surface plasmons.

A glassware-free combinatorial synthesis of green quantum dots using bubble wrap

Here, we describe the use of commercially-available bubble wrap as the basis for the simple, cheap combinatorial exploration of the synthesis of brightly emitting core/shell quantum dots.

In this communication, we highlight the use of bubble wrap in the simple parallel synthesis of CuInS₂-based quantum dots with different optical properties, based on varying precursors concentrations.

Synthesis of Polystyrene-Coated Superparamagnetic and Ferromagnetic Cobalt Nanoparticles

Polystyrene-coated cobalt nanoparticles (NPs) were synthesized through a dual-stage thermolysis of cobalt carbonyl (Co₂(CO)₈). The amine end-functionalized polystyrene surfactants with varying molecular weight were prepared via atom-transfer radical polymerization technique. By changing the concentration of these polymeric surfactants, Co NPs with different size, size distribution, and magnetic properties were obtained. Transmission electron microscopy characterization showed that the size of Co NPs stabilized with lower molecular weight polystyrene surfactants (M_n = 2300 g/mol) varied from 12–22 nm, while the size of Co NPs coated with polystyrene of middle (M_n = 4500 g/mol) and higher molecular weight (M_n = 10,500 g/mol) showed little change around 20 nm. Magnetic measurements revealed that the small cobalt particles were superparamagnetic, while larger particles were ferromagnetic and self-assembled into 1-D chain structures. Thermogravimetric analysis revealed that the grafting density of polystyrene with lower molecular weight is high. To the best of our knowledge, this is the first study to obtain both superparamagnetic and ferromagnetic Co NPs by changing the molecular weight and concentration of polystyrene through the dual-stage decomposition method.

Synthesis of thermo-responsive nanocomposites of superparamagnetic cobalt nanoparticles/poly(N-isopropylacrylamide)

Novel nanocomposites of superparamagnetic cobalt nanoparticles (Co NPs) and poly(N-isopropylacrylamide) (PNIPAM) were fabricated through surface-initiated atom-transfer radical polymerization (SI-ATRP). We firstly synthesized a functional ATRP initiator, containing an amine (as anchoring group) and a 2-bromopropionate group (SI-ATRP initiator). Oleic acid- and trioctylphosphine oxide-coated Co NPs were then modified with the initiator via ligand exchange. The process is facile and rapid for efficient surface functionalization and afterwards the Co NPs can be dispersed into polar solvent DMF without aggregation. Transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and dynamic light scattering measurements confirmed the success of ligand exchange. The following polymerization of NIPAM was conducted on the surface of Co NPs. Temperature-dependent dynamic light scattering study showed the responsive behavior of PNIPAM-coated Co NPs. The combination of superparamagnetic and thermo-responsive properties in these hybrid nanoparticles is promising for future applications e.g. in biomedicine.

Binary Transition-Metal Oxide Hollow Nanoparticles for Oxygen Evolution Reaction

Low-cost transition metal oxides are actively explored as alternative materials to precious metal-based electrocatalysts for the challenging multistep oxygen evolution reaction (OER). We utilized the Kirkendall effect allowing the formation of hollow polycrystalline, highly disordered nanoparticles (NPs) to synthesize highly active binary metal oxide OER electrocatalysts in alkali media. Two synthetic strategies were applied to achieve compositional control in binary transition metal oxide hollow NPs. The first strategy is capitalized on the oxidation of transition-metal NP seeds in the presence of other transition-metal cations. Oxidation of Fe NPs treated with Ni (+2) cations allowed the synthesis of hollow oxide NPs with a 1-4.7 Ni-to-Fe ratio via an oxidation-induced doping mechanism. Hollow Fe-Ni oxide NPs also reached a current density of 10 mA/cm² at 0.30 V overpotential. The second strategy is based on the direct oxidation of iron-cobalt alloy NPs which allows the synthesis of hollow Fe _xCo_{100- x}-oxide NPs where x can be tuned in the range between 36 and 100. Hollow Fe₃₆Co₆₄-oxide NPs also revealed the current density of 10 mA/cm² at 0.30 V overpotential in 0.1 M KOH.

Solvent dependent morphology and 59Co internal field NMR study of Co-aggregates synthesized by a wet chemical method

Different shapes of Co-aggregates were synthesized via reduction of a Co salt (CoCl2·6H2O) by chemical precipitation using glycerol, ethylene glycol and ethanol as solvents. The effect of solvent on the morphology, fcc or hcp phase-content and the magnetic properties of the synthesized samples were investigated. The Co-aggregates synthesized using glycerol have a dense spherical shape and high saturation magnetization (MS), whereas ethylene glycol leads to formation of flower-shaped spherical aggregates through loose packing of smaller plate-like particles which have a moderate MS value. When ethanol was used as a solvent, a dendritic (leaf like)-shape of the aggregates with the lowest MS value was obtained. The formation of the obtained morphology of the aggregates was explained based on the size of the solvent molecule, the viscosity of the solvent and the number of polar groups (-OH) present in the solvent molecules. The magnetic domain state and domain wall dynamics of all the Co-samples were investigated using 59Co Internal Field Nuclear Magnetic Resonance (IFNMR) spectroscopy at RT and at 77 K. Through the IFNMR spectroscopy, the presence of gain boundaries, single domain particles and multi-domain particles/aggregates with domain walls associated with fcc and hcp phases were identified and quantified. We observed that the use of ethanol facilitates formation of a higher amount of hcp phase in the sample than the use of glycerol or ethylene glycol.

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.

Syntheses and Properties of Metal Nanomaterials with Novel Crystal Phases

In recent decades, researchers have devoted tremendous effort into the rational design and controlled synthesis of metal nanomaterials with well-defined size, morphology, composition, and structure, and great achievements have been reached. However, the crystal-phase engineering of metal nanomaterials still remains a big challenge. Recent research has revealed that the crystal phase of metal nanomaterials can significantly alter their properties, arising from the distinct atomic arrangement and modified electronic structure. Until now, it has been relatively uncommon to synthesize metal nanomaterials with novel crystal phases in spite of the fact that these nanostructures would be promising for various applications. Here, the research progress regarding the fine control of noble metal (Au, Ag, Ru, Rh, Pd) and non-noble metal (Fe, Co, Ni) nanomaterials with novel crystal phases is reviewed. First, synthesis strategies and their phase transformations are summarized, while highlighting the peculiar characteristics of each element. The phase-dependent properties are then discussed by providing representative examples. Finally, the challenges and perspectives in this emerging field are proposed.

The structural and surface modification of zeolitic imidazolate frameworks towards reduction of encapsulated CO2

Experimental and theoretical studies reveal that the {112} facets of ZIF-8 efficiently reduce the stored CO₂ to fuel compared to the {011} facets.

Redox-responsive UCNPs-DPA conjugated NGO-PEG-BPEI-DOX for imaging-guided PTT and chemotherapy for cancer treatment

The disulfide bond (–S–S–) is an enormously valuable functional group in a variety of chemical and biological agents that display effective reactivity or biological activities (e.g., antitumor activities).

Nanoparticle Superlattices: The Roles of Soft Ligands

Nanoparticle superlattices are periodic arrays of nanoscale inorganic building blocks including metal nanoparticles, quantum dots and magnetic nanoparticles. Such assemblies can exhibit exciting new collective properties different from those of individual nanoparticle or corresponding bulk materials. However, fabrication of nanoparticle superlattices is nontrivial because nanoparticles are notoriously difficult to manipulate due to complex nanoscale forces among them. An effective way to manipulate these nanoscale forces is to use soft ligands, which can prevent nanoparticles from disordered aggregation, fine-tune the interparticle potential as well as program lattice structures and interparticle distances - the two key parameters governing superlattice properties. This article aims to review the up-to-date advances of superlattices from the viewpoint of soft ligands. We first describe the theories and design principles of soft-ligand-based approach and then thoroughly cover experimental techniques developed from soft ligands such as molecules, polymer and DNA. Finally, we discuss the remaining challenges and future perspectives in nanoparticle superlattices.

Anomalous Growth Rate of Ag Nanocrystals Revealed by in situ STEM

In situ microscopy of colloidal nanocrystal growth offers a unique opportunity to acquire direct and straightforward data for assessing classical growth models. Here, we observe the growth trajectories of individual Ag nanoparticles in solution using in situ scanning transmission electron microscopy. For the first time, we provide experimental evidence of growth rates of Ag nanoparticles in the presence of Pt in solution that are significantly faster than predicted by Lifshitz-Slyozov-Wagner theory. We attribute these observed anomalous growth rates to the synergistic effects of the catalytic properties of Pt and the electron beam itself. Transiently reduced Pt atoms serve as active sites for Ag ions to grow, thereby playing a key role in controlling the growth kinetics. Electron beam illumination greatly increases the local concentration of free radicals, thereby strongly influencing particle growth rate and the resulting particle morphology. Through a systematic investigation, we demonstrate the feasibility of utilizing these synergistic effects for controlling the growth rates and particle morphologies at the nanoscale. Our findings not only expand the current scope of crystal growth theory, but may also lead to a broader scientific application of nanocrystal synthesis.

Formation and shape-control of hierarchical cobalt nanostructures using quaternary ammonium salts in aqueous media

Aggregation and self-assembly are influenced by molecular interactions. With precise control of molecular interactions, in this study, a wide range of nanostructures ranging from zero-dimensional nanospheres to hierarchical nanoplates and spindles have been successfully synthesized at ambient temperature in aqueous solution. The nanostructures reported here are formed by aggregation of spherical seed particles (monomers) in presence of quaternary ammonium salts. Hydroxide ions and a magnetic moment of the monomers are essential to induce shape anisotropy in the nanostructures. The cobalt nanoplates are studied in detail, and a growth mechanism based on collision, aggregation, and crystal consolidation is proposed based on a electron microscopy studies. The growth mechanism is generalized for rods, spindles, and nearly spherical nanostructures, obtained by varying the cation group in the quaternary ammonium hydroxides. Electron diffraction shows different predominant lattice planes on the edge and on the surface of a nanoplate. The study explains, hereto unaddressed, the temporal evolution of complex magnetic nanostructures. These ferromagnetic nanostructures represent an interesting combination of shape anisotropy and magnetic characteristics.

Prediction of Co nanoparticle morphologies stabilized by ligands: towards a kinetic model

The morphology variation of hcp cobalt nanoparticles as a function of ligand concentration is predicted using a novel kinetic method.

High coercivity stellated cobalt metal multipods through solvothermal reduction of cobalt hydroxide nanosheets

Solvated 2D nanosheets of dodecylsulphate intercalated α-cobalt hydroxide in 1-butanol are solvothermally reduced to hexagonal close packed (hcp) Co metal multipods in the presence of oleylamine.