Hollow micro-/nanostructures are of great interest in many current and emerging areas of technology. Perhaps the best-known example of the former is the use of fly-ash hollow particles generated from coal power plants as partial replacement for Portland cement, to produce concrete with enhanced strength and durability. This review is devoted to the progress made in the last decade in synthesis and applications of hollow micro-/nanostructures. We present a comprehensive overview of synthetic strategies for hollow structures. These strategies are broadly categorized into four themes, which include well-established approaches, such as conventional hard-templating and soft-templating methods, as well as newly emerging methods based on sacrificial templating and template-free synthesis. Success in each has inspired multiple variations that continue to drive the rapid evolution of the field. The Review therefore focuses on the fundamentals of each process, pointing out advantages and disadvantages where appropriate. Strategies for generating more complex hollow structures, such as rattle-type and nonspherical hollow structures, are also discussed. Applications of hollow structures in lithium batteries, catalysis and sensing, and biomedical applications are reviewed.

Hollow Micro-/Nanostructures: Synthesis and Applications**

Applications in Theranostics Guanying Chen,*,†,‡ Hailong Qiu,†,‡ Paras N. Prasad,*,‡,§ and Xiaoyuan Chen* †School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China ‡Department of Chemistry and the Institute for Lasers, Photonics, and Biophotonics, University at Buffalo, State University of New York, Buffalo, New York 14260, United States Department of Chemistry, Korea University, Seoul 136-701, Korea Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health, Bethesda, Maryland 20892-2281, United States

Upconversion Nanoparticles: Design, Nanochemistry, and Applications in Theranostics

Hybrid porous nanowire arrays composed of strongly interacting Co3O4 and carbon were prepared by a facile carbonization of the metal–organic framework grown on Cu foil. The resulting material, possessing a high surface area of 251 m2 g–1 and a large carbon content of 52.1 wt %, can be directly used as the working electrode for oxygen evolution reaction without employing extra substrates or binders. This novel oxygen evolution electrode can smoothly operate in alkaline solutions (e.g., 0.1 and 1.0 M KOH), affording a low onset potential of 1.47 V (vs reversible hydrogen electrode) and a stable current density of 10.0 mA cm–2 at 1.52 V in 0.1 M KOH solution for at least 30 h, associated with a high Faradaic efficiency of 99.3%. The achieved ultrahigh oxygen evolution activity and strong durability, with superior performance in comparison to the state-of-the-art noble-metal/transition-metal and nonmetal catalysts, originate from the unique nanowire array electrode configuration and in situ carbon incorporati...

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Metal–Organic Framework Derived Hybrid Co3O4-Carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes

Template‐Free Synthesis of SnO2 Hollow Nanostructures with High Lithium Storage Capacity

Rare-earth upconversion nanophosphors (UCNPs), when excited by continuous-wave near-infrared light, exhibit a unique narrow photoluminescence with higher energy. Such special upconversion luminescence makes UCNPs promising as bioimaging probes with attractive features, such as no auto-fluorescence from biological samples and a large penetration depth. As a result, UCNPs have emerged as novel imaging agents for small animals. In this critical review, recent reports regarding the synthesis of water-soluble UCNPs and their surface modification and bioconjugation chemistry are summarized. The applications of UCNPs for small-animal imaging, including tumor-targeted imaging, lymphatic imaging, vascular imaging and cell tracking are reviewed in detail. The exploration of UCNPs as multifunctional nanoscale carriers for integrated imaging and therapy is also presented. The biodistribution and toxicology of UCNPs are further described. Finally, we discuss the challenges and opportunities in the development of UCNP-based nanoplatforms for small-animal imaging (276 references).

Upconversion nanophosphors for small-animal imaging

The well-known physical phenomenon Ostwald ripening in crystal growth has been widely employed in template-free fabrication of hollow inorganic nanostructures in recent years. Nevertheless, all reported works so far are limited only to stoichiometric phase-pure solids. In this work we describe the first investigation of doped (nonstoichiometric) materials using Ostwald ripening as a means of creating interior space. In particular, we chose the xSnO2−(1 − x)TiO2 binary system to establish preparative principles for this approach in synthesis of structurally and compositionally complex nanomaterials. In this study, uniform Sn-doped TiO2 nanospheres with hollow interiors in 100% morphological yield have been prepared with an aqueous inorganic route under hydrothermal conditions. Furthermore, our structural and surface analyses indicate that Sn4+ ions can be introduced linearly into TiO2, and preferred structural phase(s) can also be attained (e.g., either anatase or rutile, or their mixtures). Fluoride anion...

Hollowing Sn-Doped TiO2 Nanospheres via Ostwald Ripening

Atomically dispersed Zn-N-C nanomaterials are promising platinum-free catalysts for the oxygen reduction reaction (ORR). However, the fabrication of Zn-N-C catalysts with a high Zn loading remains a formidable challenge owing to the high volatility of the Zn precursor during high-temperature annealing. Herein, we report that an atomically dispersed Zn-N-C catalyst with an ultrahigh Zn loading of 9.33 wt % could be successfully prepared by simply adopting a very low annealing rate of 1° min-1 . The Zn-N-C catalyst exhibited comparable ORR activity to that of Fe-N-C catalysts, and significantly better ORR stability than Fe-N-C catalysts in both acidic and alkaline media. Further experiments and DFT calculations demonstrated that the Zn-N-C catalyst was less susceptible to protonation than the corresponding Fe-N-C catalyst in an acidic medium. DFT calculations revealed that the Zn-N4 structure is more electrochemically stable than the Fe-N4 structure during the ORR process.

Ultrahigh-Loading Zinc Single-Atom Catalyst for Highly Efficient Oxygen Reduction in Both Acidic and Alkaline Media

Electrochemical conversion of CO2 into valued products is one of the most important issues but remains a great challenge in chemistry. Herein, we report a novel synthetic approach involving prolonged thermal pyrolysis of hemin and melamine molecules on graphene for the fabrication of a robust and efficient single-iron-atom electrocatalyst for electrochemical CO2 reduction. The single-atom catalyst exhibits high Faradaic efficiency (ca. 97.0 %) for CO production at a low overpotential of 0.35 V, outperforming all Fe-N-C-based catalysts. The remarkable performance for CO2 -to-CO conversion can be attributed to the presence of highly efficient singly dispersed FeN5 active sites supported on N-doped graphene with an additional axial ligand coordinated to FeN4 . DFT calculations revealed that the axial pyrrolic nitrogen ligand of the FeN5 site further depletes the electron density of Fe 3d orbitals and thus reduces the Fe-CO π back-donation, thus enabling the rapid desorption of CO and high selectivity for CO production.

A Graphene‑Supported Single‑Atom FeN 5 Catalytic Site for Efficient Electrochemical CO 2 Reduction

Carbon nanotubes (CNTs)-based composites have attracted significant research interest in recent years, owing to their important applications in various technological fields. In this investigation, we describe a general approach to make CNTs-based nanocomposites via self-assembly. The method allows one to prepare binary composites as well as complex systems such as ternary or even quaternary composites where nanoparticles of active phases (e.g., metals and metal oxides) are used as primary building blocks. Six different kinds of binary, ternary, and quaternary nanocomposites, TiO2/CNTs, Co3O4/CNTs, Au/CNTs, Au/TiO2/CNTs, TiO2/Co3O4/CNTs, and Co/CoO/Co3O4/CNTs, have been reported herein in order to draw common features for various assembly schemes. To understand the interconnectivity between the active phases and CNTs, we have devised a range of experiments and examined the resultant samples with many instrumental techniques. On the basis of this work, we demonstrate that highly complex inorganic-organic nanohybrids with good controls in particle shape, size, and distribution can be fabricated from presynthesized nanobuilding units. Concerning their workability, we further show that self-assembled TiO2/CNTs are sufficiently robust and the electrochemical performance of TiO2 is significantly enhanced when it is used as a cathode material in Li-battery application.

Preparation of Nanocomposites of Metals, Metal Oxides, and Carbon Nanotubes via Self-Assembly

Efficient photocatalytic nanocrystals with high-ratio exposure of active facets have aroused a great number of research interests in recent years However, most preparations of such materials need the addition of special capping agents (like surfactants) or harsh reaction conditions (such as hydrothermal reactions) In this work, a controllable synthesis of BiOBr nanosheets with a thickness from 9 nm to 32 nm was easily achieved in a hydrolysis system through adjusting temperature and solvent, without adding any surfactant or capping agents As the thickness of the nanosheets decreases from 32 nm to 9 nm, the ratio of exposed {001} facets, the active photocatalysis facets in BiOBr crystals, increases from 83% to 94%, along with an increased photocatalytic efficiency over rhodamine B (RhB) under visible-light Various methods such as SEM, TEM, AFM, DRS and Raman spectroscopy were used to fully characterize the as-obtained BiOBr nanosheets More importantly, the obtained BiOBr nanosheets exhibit a selective visible-light photocatalytic behavior as the activity over RhB is much higher than that over Methyl Orange (MO) or Methylene Blue (MB) This phenomenon was studied with in situ electron paramagnetic resonance (EPR) measurements and the potential mechanism was explored

Jing Li

Papers

Hollowing Sn-Doped TiO2 Nanospheres via Ostwald Ripening

Ultrahigh-Loading Zinc Single-Atom Catalyst for Highly Efficient Oxygen Reduction in Both Acidic and Alkaline Media

A Graphene‑Supported Single‑Atom FeN 5 Catalytic Site for Efficient Electrochemical CO 2 Reduction

Preparation of Nanocomposites of Metals, Metal Oxides, and Carbon Nanotubes via Self-Assembly

High {001} facets dominated BiOBr lamellas: facile hydrolysis preparation and selective visible-light photocatalytic activity