Solid-State Conversion Chemistry of Multicomponent Nanocrystals Cast in a Hollow Silica Nanosphere: Morphology-Controlled Syntheses of Hybrid Nanocrystals

Hydrogen Spillover-Driven Dynamic Evolution and Migration of Iron Oxide for Structure Regulation of Versatile Magnetic Nanocatalysts

Magnetic nanocatalysts with properties of easy recovery, induced heating, or magnetic levitation play a crucial role in advancing intelligent techniques. Herein, we report a method for the synthesis of versatile core-shell-type magnetic nanocatalysts through "noncontact" hydrogen spillover-driven reduction and migration of iron oxide with the assistance of Pd. In situ analysis techniques were applied to visualize the dynamic evolution of the magnetic nanocatalysts. Pd facilitates the dissociation of hydrogen molecules into activated H*, which then spills and thus drives the iron oxide reduction, gradual outward split, and migration through the carbonaceous shell. By controlling the evolution stage, nanocatalysts having diverse architectures including core-shell, split core-shell, or hollow type, each featuring Pd or PdFe loaded on the carbon shell, can be obtained. As a showcase, a magnetic nanocatalyst (Pd-loaded split core-shell) can hydrogenate crotonaldehyde to butanal (26 624 h-1 in TOF, ∼100% selectivity), outperforming reported Pd-based catalysts. This is due to the synergy of the enhanced local magnetothermal effect and the preferential adsorption of -C═C on Pd with a small d bandwidth. Another catalyst (PdFe-loaded split core-shell) also delivers a robust performance in phenylacetylene semihydrogenation (100% conversion, 97.5% selectivity) as PdFe may inhibit the overhydrogenation of -C═C. Importantly, not only Pd, other noble metals (e.g., Pt, Ru, and Au) also showed a similar property, revealing a general rule that hydrogen spillover drives the dynamic reduction, splitting, and migration of encapsulated nanosized iron oxide, resulting in diverse structures. This study would offer a structure-controllable fabrication of high-performance magnetic nanocatalysts for various applications.

Co-encapsulation of paclitaxel and 5-fluorouracil in folic acid-modified, lipid-encapsulated hollow mesoporous silica nanoparticles for synergistic breast cancer treatment

A dual-loaded multi-targeted drug delivery nanosystem was constructed to simultaneously load paclitaxel (PTX) and 5-fluorouracil (5-FU) for targeted delivery and sustained release at tumor sites. Hollow mesoporous silica nanoparticles (HMSNs) were prepared by the inverse microemulsion method, then modified with folic acid and pH- and temperature-responsive materials, co-loaded with PTX and 5-FU, and finally encapsulated into lipid membranes. The obtained nanosystem was selectively internalized by human breast cancer MCF-7 cells that overexpress folate receptors through an energy-dependent process, and it released both drugs in vitro in a simulated tumor microenvironment. Moreover, the inhibitory effect of the dual-loaded nanoparticles was significantly better than that of the free drugs, suggesting that the composite nanosystem has the potential to selectively target tumor sites and perform the synergistic effect of PTX and 5-FU, while reducing their toxic effects on normal tissues.

Solid-State Reaction Synthesis of Nanoscale Materials: Strategies and Applications

Nanomaterials (NMs) with unique structures and compositions can give rise to exotic physicochemical properties and applications. Despite the advancement in solution-based methods, scalable access to a wide range of crystal phases and intricate compositions is still challenging. Solid-state reaction (SSR) syntheses have high potential owing to their flexibility toward multielemental phases under feasibly high temperatures and solvent-free conditions as well as their scalability and simplicity. Controlling the nanoscale features through SSRs demands a strategic nanospace-confinement approach due to the risk of heat-induced reshaping and sintering. Here, we describe advanced SSR strategies for NM synthesis, focusing on mechanistic insights, novel nanoscale phenomena, and underlying principles using a series of examples under different categories. After introducing the history of classical SSRs, key theories, and definitions central to the topic, we categorize various modern SSR strategies based on the surrounding solid-state media used for nanostructure growth, conversion, and migration under nanospace or dimensional confinement. This comprehensive review will advance the quest for new materials design, synthesis, and applications.

Synthetic Approaches to Colloidal Nanocrystal Heterostructures Based on Metal and Metal-Oxide Materials

Composite inorganic nanoarchitectures, based on combinations of distinct materials, represent advanced solid-state constructs, where coexistence and synergistic interactions among nonhomologous optical, magnetic, chemical, and catalytic properties lay a basis for the engineering of enhanced or even unconventional functionalities. Such systems thus hold relevance for both theoretical and applied nanotechnology-based research in diverse areas, spanning optics, electronics, energy management, (photo)catalysis, biomedicine, and environmental remediation. Wet-chemical colloidal synthetic techniques have now been refined to the point of allowing the fabrication of solution free-standing and easily processable multicomponent nanocrystals with sophisticated modular heterostructure, built upon a programmed spatial distribution of the crystal phase, composition, and anchored surface moieties. Such last-generation breeds of nanocrystals are thus composed of nanoscale domains of different materials, assembled controllably into core/shell or heteromer-type configurations through bonding epitaxial heterojunctions. This review offers a critical overview of achievements made in the design and synthetic elaboration of colloidal nanocrystal heterostructures based on diverse associations of transition metals (with emphasis on plasmonic metals) and transition-metal oxides. Synthetic strategies, all leveraging on the basic seed-mediated approach, are described and discussed with reference to the most credited mechanisms underpinning regioselective heteroepitaxial deposition. The unique properties and advanced applications allowed by such brand-new nanomaterials are also mentioned.

Silica-Enveloped 2D-Sheet-to-Nanocrystals Conversion for Resilient Catalytic Dry Reforming of Methane

Here, lamellar confinement strategy is introduced for "sheet-to-nanocrystals (NCs)" conversion within a 2D-SiO₂ envelope, which constructs a catalytic nanocartridge holding a platoon of isolated and in-plane-aligned ultrasmall Ni-NCs, performing as a robust and coking-resistant catalytic system for dry reforming of methane. Overcoming the problem of unavoidable bulk crystal growth from multiple sheets-stack or sheet-on-open-support, silica bilayer-encasing tightly clamps the atomic-thin Ni(OH)₂ -nanosheet during thermal conversion and further hinders the migratory fusion of the resultant Ni-NCs. Upon heating-cooling cycle, the flapping silica envelope clutches the Ni-NCs like "eggs in a carton," subsequently, ensuring their thermal stability. Owing to the unique 2D-enveloped rigid architecture, Ni-NCs can circumvent sintering and coke deposition while tolerating the high temperatures (>700 °C) for long operation (>100 h), affording high conversions to syngas.

A scalable synthesis of ternary nanocatalysts for a high-efficiency electrooxidation catalysis by microfluidics

Microfluidic synthesis has attracted extensive attention due to the ability for the multistep precise control of the synthesis parameters, continuous and reproducible preparation, and its ease of integration. However, its commercial application is still affected by its low production efficiency. In this case, we report a high-throughput continuous flow synthesis of highly dispersed PtFeCu/C nanocatalysts using a metal microchip setup with four parallel channels. The high flow rate and integrated channels enabled improving the throughput, whereby 1.33 g h^-1 of catalysts could be achieved with the flow rate of 1200 mL h^-1 under the experimental conditions. The as-prepared PtFeCu/C exhibited excellent performance, 1.94 times higher than Pt/C for methanol oxidation. More importantly, the yield of the PtFeCu/C nanocatalysts could be further increased through designing numerous parallel channels, which might provide a promising approach for large-scale commercialization of the catalysts. Such a high-throughput fabrication pathway is significant for the large-scale industrial production of nanomaterials.

Diffusion of Nanoparticles with Activated Hopping in Crowded Polymer Solutions

A long-distance hop of diffusive nanoparticles (NPs) in crowded environments was commonly considered unlikely, and its characteristics remain unclear. In this work, we experimentally identify the occurrence of the intermittent hops of large NPs in crowded entangled poly(ethylene oxide) (PEO) solutions, which are attributed to thermally induced activated hopping. We show that the diffusion of NPs in crowded solutions is considered as a superposition of the activated hopping and the reptation of the polymer solution. Such activated hopping becomes significant when either the PEO molecular weight is large enough or the NP size is relatively small. We reveal that the time-dependent non-Gaussianity of the NP diffusion is determined by the competition of the short-time relaxation of a polymer entanglement strand, the activated hopping, and the long-time reptation. We propose an exponential scaling law τ_hop/τ_e ∼ exp(d/d_t) to characterize the hopping time scale, suggesting a linear dependence of the activated hopping energy barrier on the dimensionless NP size. The activated hopping motion can only be observed between the onset time scale of the short-time relaxation of local entanglement strands and the termination time scale of the long-time relaxation. Our findings on activated hopping provide new insights into long-distance transportation of NPs in crowded biological environments, which is essential to the delivery and targeting of nanomedicines.

Controlled Synthesis of Manganese Oxide Nanoparticles Encaged in Hollow Mesoporous Silica Nanoreactors and Their Enhanced Dye Degradation Activity

In this study, controlled synthesis of hollow mesoporous silica nanoreactors with small manganese oxide nanoparticles in their cavities (Mn _x O _y @HMSNs) is reported, and the dye degradation performance in the presence of hydrogen peroxide over Mn _x O _y @HMSNs is investigated. Specifically, triple ligands (a compound with three dipicolinic acid groups) were used to coordinate manganese ions to form negatively charged coordination complex networks, which further combine with positively charged copolymers to obtain metal ion-containing polymer micelles. Following silica deposition onto micellar coronas and calcinations simultaneously result in hollow mesoporous silica nanoreactors and manganese oxide nanoparticles in their cavities. In this work, the influences of synthetic parameters on the structures are studied in detail. The obtained Mn _x O _y @HMSNs show greatly enhanced activity and stability for a series of dye degradations. The performance enhancement is ascribed to their unique nanostructures, where mesoporous silica walls provide protection to the inner Mn _x O _y nanoparticles and the small size of the manganese oxide nanoparticles greatly enhances the dye degradation activity.

Metal@SiO2 Core-Shells with Self-Arrested Migrating Core

Developing easy and customizable strategies for the directional structure modulation of multicomponent nanosystems to influence and optimize their properties are a paramount but challenging task in nanoscience. Here, we demonstrate highly controlled eccentric off-center positioning of metal-core in metal@silica core-shells by utilizing an in situ generated biphasic silica-based intraparticle solid-solid interface. In the synthetic strategy, by including Ca²⁺-ions in silica-shell and successive oxidative and reductive annealing at high temperature, a unique hairline-biphasic interface is evolved via the heat-induced concentric radial segregation of calcium silicate phase at the interior and normal silica phase at the exterior of core-shell, which can effectively arrest the outwardly migrating metal-core within rubbery calcium silicate phase, affording various eccentric core-shells, where core-positions are flexibly controlled by the annealing time and amounts of initially added Ca²⁺-ions. In the structure-property correlation study, the strategy allows fine-tuning of dipolar interaction-based blocking temperatures and magnetic anisotropies of different eccentric core-shells as the function of variable off-center distance of magnetic core without changing the overall size of nanoparticles. This work demonstrates the discovery and potential application of biphasic solid-solid media interface in controlling the heat-induced migration of metal nanocrystals and opens the avenues for exploiting the rarely studied high-temperature solid-state nanocrystal conversion chemistry and migratory behavior for directional nanostructure engineering.

Spatially Confined Formation and Transformation of Nanocrystals within Nanometer-Sized Reaction Media

The extensive research performed in the past two decades has enabled the production of a range of colloidal nanocrystals, mostly through solution-based procedures that generate and transform nanostructures in bulk-phase solutions containing precursors and surfactants. However, the understanding and control of each nanocrystal (trans)formation step during the synthesis are still complicated because of the high complexity of this process, in which multiple diverse events such as nucleation, subsequent growth, attachment, and ripening occur simultaneously in bulk suspensions. Unlike well-established solution-based methods, solid-state reactions, which had been at the forefront of traditional inorganic materials chemistry, are quite rarely utilized in the realm of nanomaterials because of the high temperatures required for most solid-state reactions, as a result of which the clusters and NCs are prone to migrate through the bulk reaction medium and sinter together uncontrollably into large particles. We have been pursuing the "nanospace-confined approach" to explore the use of a variety of solid and hollow silica nanoparticles as either solid-state or solution-phase reaction media to carry out the syntheses and transformations of nanocrystals in a unique microenvironment, partitioning the reactants, intermediates, and transition states from the rest of the bulk reaction medium. Such nanoconfined systems have the potential not only to enable efficient and selective nanocrystal conversion chemistries but also to provide fundamental understanding pertaining to the synthetic evolution of nanostructures and transient mechanistic steps. The unique spaces with sizes of a few tens of nanometers inside nanoconfined systems offer the opportunity to observe and elucidate novel deconvoluted chemical phenomena that are impossible to investigate in bulk systems, and comprehensive understanding of nanoconfined chemistry can be implicated in explaining and controlling the macroscopic chemical behaviors. This Account describes our focused research on developing spatially confined platforms for nanocrystal syntheses and transformations, highlighting our diversity-oriented strategy, namely, the "postdecoration approach", which results in the evolution of new nanocatalytic sites in a preformed cavity for diversifying and controlling their morphologies, number, density and combinations. We discuss key examples of the "nanoconfined solid-state conversion approach" that involve novel reactions of nanocrystals within thermally stable solid silica nanospheres to synthesize and transform complex hybrid nanocrystals with increased complexity and functionality. In addition, an enlightening discussion of the examples of nanocrystal syntheses and conversions in nanoconfined solutions inside enclosed and exposed cavities of silica nanospheres is included. Finally, the important applications of nanospace-confined systems in various fields are also briefly discussed.

Monofacet-Selective Cavitation within Solid-State Silica-Nanoconfinement toward Janus Iron Oxide Nanocube

Here, a highly selective solid-state nanocrystal conversion strategy is developed toward concave iron oxide (Fe₃O₄) nanocube with an open-mouthed cavity engraved exclusively on a single face. The strategy is based on a novel heat-induced nanospace-confined domino-type migration of Fe²⁺ ions from the SiO₂-Fe₃O₄ interface toward the surrounding silica shell and concomitant self-limiting nanoscale phase-transition to the Fe-silicate form. Equipped with the chemically unique cavity, the produced Janus-type concave iron oxide nanocube was further functionalized with controllable density of catalytic Pt-nanocrystals exclusively on concave sites and utilized as a highly diffusive catalytic Janus nanoswimmer for the efficient degradation of pollutant-dyes in water.

Colloids of Holey Gd2 O3 Nanosheets Converted from Exfoliated Gadolinium Hydroxide Layers

This paper proposes a confined solid-state conversion approach using layered metal-hydroxides for the production of a colloidal suspension of porous 2D crystalline metal oxide layers with superior electrochemical H₂ O₂ sensing performance. This study investigates the conversion chemistry of delaminated layers of gadolinium hydroxide (LGdH), [Gd₂ (OH)₅ ]⁺ , encapsulated in a silica nanoshell that provides an antistacking and antisintering environment during the phase-transition at high temperature. Thermal treatment of the LGdH layers within the protected environment results in a dimensionally confined phase-transition into crystalline Gd₂ O₃ nanosheets with an isomorphic 2D structure. Furthermore, annealing at higher temperatures leads to the evolution of in-plane mesoporous structure on the Gd₂ O₃ nanosheet. Based on insight acquired from in-depth investigation, the evolution of in-plane porosity proceeds through the in-plane dominant silicate-formation reaction at the interface with the surrounding silica shell. Their 2D-anisotropic and mesoporous morphological features are preserved, producing a colloidal suspension of holey nanosheets that can be used to fabricate a thin and porous film through wet-coating deposition. This study also demonstrates the superior electrochemical H₂ O₂ sensing ability of the resultant porous Gd₂ O₃ film, which represents a ≈1000- and 10-fold enhancement of the detection limit and sensitivity, respectively, in comparison to previously reported Gd₂ O₃ films.

Thermal degradation mechanism of triangular Ag@SiO2 nanoparticles

Triangular silver nanoparticles are promising materials for light harvesting applications because of their strong plasmon bands; these absorption bands are highly tunable, and can be varied over the entire visible range based on the particle size. A general concern with these materials is that they are unstable at elevated temperatures. When thermally annealed, they suffer from changes to the particle morphology, which in turn affects their optical properties. Because of this stability issue, these materials cannot be used in applications requiring elevated temperatures. In order to address this problem, it is important to first understand the degradation mechanism. Here, we measure the changes in particle morphology, oxidation state, and coordination environment of Ag@SiO2 nanotriangles caused by thermal annealing. UV-vis spectroscopy and TEM reveal that upon annealing the Ag@SiO2 nanotriangles in air, the triangular cores are truncated and smaller nanoparticles are formed. Ag K-edge X-ray absorption spectroscopy (XANES and EXAFS) shows that the small particles consist of Ag(0), and that there is a decrease in the Ag-Ag coordination number with an increase in the annealing temperature. We hypothesize that upon annealing Ag in air, it is first oxidized to AgxO, after which it subsequently decomposes back to well-dispersed Ag(0) nanoparticles. In contrast, when the Ag@SiO2 nanotriangles are annealed in N2, since there is no possibility of oxidation, no small particles are formed. Instead, the triangular core rearranges to form a disc-like shape.