General Synthesis of Multi‐Shelled Mixed Metal Oxide Hollow Spheres with Superior Lithium Storage Properties

Complex hollow structures of transition metal oxides, especially mixed metal oxides, could be promising for different applications such as lithium ion batteries. However, it remains a great challenge to fabricate well‐defined hollow spheres with multiple shells for mixed transition metal oxides. Herein, we have developed a new “penetration–solidification–annealing” strategy which can realize the synthesis of various mixed metal oxide multi‐shelled hollow spheres. Importantly, it is found that multi‐shelled hollow spheres possess impressive lithium storage properties with both high specific capacity and excellent cycling stability. Specifically, the carbon‐coated CoMn₂O₄ triple‐shelled hollow spheres exhibit a specific capacity of 726.7 mA h g^?1 and a nearly 100 % capacity retention after 200 cycles. The present general strategy could represent a milestone in design and synthesis of mixed metal oxide complex hollow spheres and their promising uses in different areas.     

Formation of Triple‐Shelled Molybdenum–Polydopamine Hollow Spheres and Their Conversion into MoO2/Carbon Composite Hollow Spheres for Lithium‐Ion Batteries

Unique triple‐shelled Mo‐polydopamine (Mo‐PDA) hollow spheres are synthesized through a facile solvothermal process. A sequential self‐templating mechanism for the multi‐shell formation is proposed, and the number of shells can be adjusted by tuning the size of the Mo‐glycerate templates. These triple‐shelled Mo‐PDA hollow spheres can be converted to triple‐shelled MoO₂/carbon composite hollow spheres by thermal treatment. Owing to the unique multi‐shells and hollow interior, the as‐prepared MoO₂/carbon composite hollow spheres exhibit appealing performance as an anode material for lithium‐ion batteries, delivering a high capacity of ca. 580 mAh g^?1 at 0.5 A g^?1 with good rate capability and long cycle life.     

Self‐Templated Formation of Uniform NiCo2O4 Hollow Spheres with Complex Interior Structures for Lithium‐Ion Batteries and Supercapacitors

Despite the significant advancement in preparing metal oxide hollow structures, most approaches rely on template‐based multistep procedures for tailoring the interior structure. In this work, we develop a new generally applicable strategy toward the synthesis of mixed‐metal‐oxide complex hollow spheres. Starting with metal glycerate solid spheres, we show that subsequent thermal annealing in air leads to the formation of complex hollow spheres of the resulting metal oxide. We demonstrate the concept by synthesizing highly uniform NiCo₂O₄ hollow spheres with a complex interior structure. With the small primary building nanoparticles, high structural integrity, complex interior architectures, and enlarged surface area, these unique NiCo₂O₄ hollow spheres exhibit superior electrochemical performances as advanced electrode materials for both lithium‐ion batteries and supercapacitors. This approach can be an efficient self‐templated strategy for the preparation of mixed‐metal‐oxide hollow spheres with complex interior structures and functionalities.     

Hydrothermal Synthesis of Unique Hollow Hexagonal Prismatic Pencils of Co3V2O8⋅n H2O: A New Anode Material for Lithium‐Ion Batteries

Hollow structures of transition‐metal oxides, particularly mixed‐metal oxides, could be promising for various applications such as lithium‐ion batteries (LIBs). Compared to the synthesis of metal oxide hollow spheres by the template method, non‐spherical metal oxide hollow hexagonal polyhedra have not been developed to date. Herein, we report the controlled hydrothermal synthesis of a new phase of Co₃V₂O₈?n H₂O hollow hexagonal prismatic pencils (HHPPs), which is composed of uniform structural units. By varying the amount of NaOH in the presence of NH₄⁺ and without any template or organic surfactant, the hexagonal prismatic pencils gradually transform from solid into hollow structures, with sizes varying from 5 to 20 μm. The structure of pencils can be preserved only in a limited range of the molar ratio of OH^?/NH₄⁺. As a new anode material for LIBs, such hollow pencils exhibit impressive lithium storage properties with high capacity, good cycling stability, and superior rate capability.     

General and Facile Syntheses of Metal Silicate Porous Hollow Nanostructures

Porous hollow nanostructures have attracted intensive interest owing to their unique structure and promising applications in various fields. A facile hydrothermal synthesis has been developed to prepare porous hollow nanostructures of silicate materials through a sacrificial‐templating process. The key factors, such as the concentration of the free metal cation and the alkalinity of the solution, are discussed. Porous hollow nanostructures of magnesium silicate, nickel silicate, and iron silicate have been successfully prepared by using SiO₂ spheres as the template, as well as a silicon source. Several yolk–shell structures have also been fabricated by a similar process that uses silica‐coated composite particles as a template. As‐prepared mesoporous magnesium silicate hollow spheres showed an excellent ability to remove Pb²⁺ ions in water treatment owing to their large specific surface and unique structures.     

Fabrication and characterization of double‐shelled CeO2‐La2O3/Au/Fe3O4 hollow architecture as a recyclable and highly thermal stability nanocatalyst

A novel magnetic binary‐metal‐oxide‐coated nanocataly composing of a hollow Fe₃O₄ core and CeO₂‐La₂O₃ shells with Au nanoparticles encapsulated has been created in this work. The structural features of catalysts were characterized by several techniques, including SEM, TEM, UV‐vis, FTIR, XRD, XPS and TGA analyses. After the coating of CeO₂‐La₂O₃ layer, CeO₂‐La₂O₃/Au/C/Fe₃O₄ microspheres showed a superior thermal stability and catalytic reactivity compared with a pure CeO₂ or La₂O₃ layer. Accompanied by the burning of carbon layer, the specific surface could be increased by the formation of double‐shelled structure. Besides, the desired samples could be separated by magnet, implying the superior recycle performance. Using the reduction of 4‐nitrophenol by NaBH₄ as a model reaction, the microspheres exhibited highly reusability, superior catalytic activity, thermal stability, which are attributed to the unique double‐shelled structure of the support, uniform distribution of Au nanoparticles, the highly thermal stability of CeO₂‐La₂O₃ layer and mixed oxide synergistic effect. As a consequence, the unique nanocatalyst will open a promising way in the fabrication of the double‐shelled hollow binary‐metal‐oxide materials for future research and has great potential in other applications.     

Template‐Free Synthesis of Nanorod‐Assembled Hierarchical Zn1−xMnxS Hollow Nanostructures with Enhanced Pseudocapacitive Properties

Mixed‐metal sulfide Zn_1?xMn_xS nanorod‐assembled hierarchical hollow spheres were synthesized by a template‐free solvothermal process based on Ostwald ripening. In the reaction system, glycerol plays a key role in the formation of Zn_xMn_1?xS hierarchical hollow structures by a quasi‐microemulsion‐template mechanism. When applied as capacitor electrode material, the hierarchical Zn_1?xMn_xS hollow spheres show excellent electrochemical performance. Specifically, Zn_0.25Mn_0.75S hollow spheres can deliver a high specific capacitance of 664 F g^?1 at a current rate of 1 A g^?1, which is almost five times of that of MnS under the same conditions and higher than those of previously reported single Mn‐based compounds.     

Novel synthesis of Fe2O3–Pt ellipsoids coated by double‐shelled La2O3 as a catalyst for the reduction of 4‐nitrophenol

A facile strategy is reported for the fabrication of Pt‐loaded core–shell nanocomposite ellipsoids (Fe₂O₃‐Pt@DSL) consisting of ellipsoidal Fe₂O₃ cores, double‐layered La₂O₃ shells and deposited Pt nanoparticles (NPs). The formation of the doubled‐shelled structure uses Fe₂O₃‐Pt@mSiO₂ as template sacrificial agent and it involves the re‐deposition of silica and self‐assembly of metal oxide units. The preparation methods of double‐shelled metal oxides avoid repeated coating and etching and could be utilized to fabricate other shaped double‐shelled composites. Characterization results indicated that the Fe₂O₃‐Pt@DSL nanocomposites possessed mesoporous structure and tunable shell thickness. Moreover, due to the formation of Fe₂O₃ and La₂O₃ composites, Pt NPs can also be stabilized via deposition on chemically active oxides with a synergistic effect. Therefore, as a catalyst for the reduction of 4‐nitrophenol, Fe₂O₃‐Pt@DSL showed superior catalytic activity and reusability due to structural superiority and enhanced composite synergy. Finally, well‐dispersed Pt NPs were encapsulated into the void between the shell layers to construct the Fe₂O₃‐Pt@DSL‐Pt catalyst.     

Controlled Synthesis of Hollow PbS‐TiO2 Hybrid Structures through an Ion Adsorption–Heating Process and their Photocatalytic Activity

Hollow hybrid nanostructures have received significant attention because of their unique structural features. This study reports a facile ion adsorption–heating method to fabricate hollow PbS‐TiO₂ hybrid particles. In this method, the TiO₂ spheres used as a substrate material to grow PbS are aggregates of many small amorphous TiO₂ particles, and each small particle is covered with thioglycolic acid ligands through Ti⁴⁺–carboxyl coordination. When Pb²⁺ ions are added to a colloidal solution of these TiO₂ spheres, these ions are adsorbed by sulfhydryl (‐SH) groups to form metal thiolates, and the C−S bond is dissociated by heating to release S²⁻. The S²⁻ ions react with Pb²⁺ ions to form PbS without additive sulfur sources. Additionally, the amorphous TiO₂ spheres are transformed into the anatase phase during the heating process. As a result, the crystallization of TiO₂ spheres along with the formation of PbS is simultaneously carried out by heating. During the heating process, owing to the Kirkendall effect of S²⁻ diffusion and the Ostwald ripening effect of the crystallization of amorphous TiO₂ spheres, PbS‐TiO₂ hollow hybrid structures can be obtained. The XRD and XPS characterizations proved the formation of anatase TiO₂ and PbS. The TEM characterization confirmed the formation of hollow structures in the PbS‐TiO₂ hybrid sample. The photocatalytic activity of the hollow PbS‐TiO₂ hybrid spheres have been investigated for the degradation of Cr⁶⁺ under visible light. The results show that hollow PbS‐TiO₂ hybrid spheres exhibited the highest photocatalytic activity, in which almost all the Cr⁶⁺ was degraded after 140 min.     

Synthesis of Copper‐Substituted CoS2@CuxS Double‐Shelled Nanoboxes by Sequential Ion Exchange for Efficient Sodium Storage

The construction of hybrid architectures for electrode materials has been demonstrated as an efficient strategy to boost sodium‐storage properties because of the synergetic effect of each component. However, the fabrication of hybrid nanostructures with a rational structure and desired composition for effective sodium storage is still challenging. In this study, an integrated nanostructure composed of copper‐substituted CoS₂@Cu_xS double‐shelled nanoboxes (denoted as Cu‐CoS₂@Cu_xS DSNBs) was synthesized through a rational metal–organic framework (MOF)‐based templating strategy. The unique shell configuration and complex composition endow the Cu‐CoS₂@Cu_xS DSNBs with enhanced electrochemical performance in terms of superior rate capability and stable cyclability.     

A Hollow Multi‐Shelled Structure for Charge Transport and Active Sites in Lithium‐Ion Capacitors

The lithium‐ion capacitor (LIC) has attracted tremendous research interest because it meets both the requirement on high energy and power densities. The balance between effective surface areas and mass transport is highly desired to fabricate the optimized electrode material for LIC. Now, triple‐shelled (3S) Nb₂O₅ hollow multi‐shelled structures (HoMSs) were synthesized for the first time through the sequential templating approach and then applied for the anode of LIC. The unique structure of HoMSs, such as large efficient surface area, hierarchical pores, and multiple shells, provides abundant reaction sites, decreases the electron transport resistance, and increases the diffusion rate for ion transport. In this case, the best combination performance has been achieved among all the reported Nb₂O₅‐based materials, which delivered an excellent energy and power densities simultaneously, and superb cycling stability.     

Microwave‐Hydrothermal Preparation and Visible‐Light Photoactivity of Plasmonic Photocatalyst Ag‐TiO2 Nanocomposite Hollow Spheres

Visible‐light‐driven plasmonic photocatalyst Ag‐TiO₂ nanocomposite hollow spheres are prepared by a template‐free chemically‐induced self‐transformation strategy under microwave‐hydrothermal conditions, followed by a photochemical reduction process under xenon lamp irradiation. The prepared samples are characterized by using scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, N₂ adsorption‐desorption isotherms, X‐ray photoelectron spectroscopy, UV/Vis and Raman spectroscopy. Production of ?OH radicals on the surface of visible‐light illuminated TiO₂ was detected by using a photoluminescence method with terephthalic acid as the probe molecule. The photocatalytic activity of as‐prepared samples was evaluated by photocatalytic decolorization of Rhodamine B (RhB) aqueous solution at ambient temperature under visible‐light irradiation. The results show that the surface plasmon absorption band of the silver nanoparticles supported on the TiO₂ hollow spheres was red shifted, and a strong surface enhanced Raman scattering effect for the Ag‐TiO₂ nanocomposite sample was observed. The prepared nanocomposite hollow spheres exhibits a highly visible‐light photocatalytic activity for photocatalytic degradation of RhB in water, and their photocatalytic activity is higher than that of pure TiO₂ and commercial Degussa P25 (P25) powders. Especially, the as‐prepared Ag‐TiO₂ nanocomposite hollow spheres at the nominal atomic ratio of silver to titanium ( R ) of 2 showed the highest photocatalytic activity, which exceeds that of P25 by a factor of more than 2.     

N‐Doped Yellow TiO2 Hollow Sphere‐Mediated Visible‐Light‐Driven Efficient Esterification of Alcohol and N‐Hydroxyimides to Active Esters

Herein we report a simple synthetic protocol for N‐doped yellow TiO₂ (N‐TiO₂) hollow spheres as an efficient visible‐light‐active photocatalyst using aqueous titanium peroxocarbonate complex (TPCC) solution as precursor and NH₄OH. In the developed strategy, the ammonium ion of TPCC and NH₄OH acts as nitrogen source and structure‐directing agent. The synthesized N‐TiO₂ hollow spheres are capable of promoting the synthesis of active esters of N‐hydroxyimide and alcohol through simultaneous selective oxidation of alcohol to aldehyde followed by cross‐dehydrogenative coupling (CDC) under ambient conditions upon irradiation of visible light. It is possible to develop a novel and cost‐effective one‐pot strategy for the synthesis of important esters and amides on gram scale using the developed strategy. The catalytic activity of N‐TiO₂ hollow spheres is much superior to that of other reported N‐TiO₂ samples as well as TiO₂ with varying morphology.     

Porous ZnO/Co3O4/N‐doped carbon nanocages synthesized via pyrolysis of complex metal–organic framework (MOF) hybrids as an advanced lithium‐ion battery anode

Metal oxides have a large storage capacity when employed as anode materials for lithium‐ion batteries (LIBs). However, they often suffer from poor capacity retention due to their low electrical conductivity and huge volume variation during the charge–discharge process. To overcome these limitations, fabrication of metal oxides/carbon hybrids with hollow structures can be expected to further improve their electrochemical properties. Herein, ZnO‐Co₃O₄ nanocomposites embedded in N‐doped carbon (ZnO‐Co₃O₄@N‐C) nanocages with hollow dodecahedral shapes have been prepared successfully by the simple carbonizing and oxidizing of metal–organic frameworks (MOFs). Benefiting from the advantages of the structural features, i.e. the conductive N‐doped carbon coating, the porous structure of the nanocages and the synergistic effects of different components, the as‐prepared ZnO‐Co₃O₄@N‐C not only avoids particle aggregation and nanostructure cracking but also facilitates the transport of ions and electrons. As a result, the resultant ZnO‐Co₃O₄@N‐C shows a discharge capacity of 2373 mAh g^?1 at the first cycle and exhibits a retention capacity of 1305 mAh g^?1 even after 300 cycles at 0.1 A g^?1. In addition, a reversible capacity of 948 mAh g^?1 is obtained at a current density of 2 A g^?1, which delivers an excellent high‐rate cycle ability.     

Microwave‐Assisted Solvothermal Synthesis of VO2 Hollow Spheres and Their Conversion into V2O5 Hollow Spheres with Improved Lithium Storage Capability

Monodispersed hierarchically structured V₂O₅ hollow spheres were successfully obtained from orthorhombic VO₂ hollow spheres, which are in turn synthesized by a simple template‐free microwave‐assisted solvothermal method. The structural evolution of VO₂ hollow spheres has been studied and explained by a chemically induced self‐transformation process. The reaction time and water content in the reaction solution have a great influence on the morphology and phase structure of the resulting products in the solvothermal reaction. The diameter of the VO₂ hollow spheres can be regulated simply by changing vanadium ion content in the reaction solution. The VO₂ hollow spheres can be transformed into V₂O₅ hollow spheres with nearly no morphological change by annealing in air. The nanorods composed of V₂O₅ hollow spheres have an average length of about 70 nm and width of about 19 nm. When used as a cathode material for lithium‐ion batteries, the V₂O₅ hollow spheres display a diameter‐dependent electrochemical performance, and the 440 nm hollow spheres show the highest specific discharge capacity of 377.5 mAhg^?1 at a current density of 50 mAg^?1, and are better than the corresponding solid spheres and nanorod assemblies.     

Hollow Multi‐Shelled Structural TiO2−x with Multiple Spatial Confinement for Long‐Life Lithium–Sulfur Batteries

TiO_2?x with well‐controlled hollow multi‐shelled structures (HoMSs) were designed and synthesized, via a sequential templating approach (STA), to act as sulfur carrier materials. They were explored as physico‐chemical encapsulation materials. Particularly, the sulfur cathode based on triple‐shelled TiO_2?x HoMSs delivered a specific capacity of 903 mAh g^?1 with a capacity retention of 79 % at 0.5 C and a Coulombic efficiency of 97.5 % over 1000 cycles. The outstanding electrochemical performance is attributed to better spatial confinement and integrated conductivity of the intact triple‐shell that combine the features of physico‐chemical adsorption, short charge transfer path along with mechanical strength.     

Highly active catalysts: Double‐shelled hollow nanospheres with tiny Pt NPs

A facile strategy was reported to fabricate a novel Pt‐based metal oxide double‐shelled hollow nanospheres (MDSHs), which avoided the traditional tedious procedures. It was attractive that the formation mechanism of DSHs involved redeposition of etch‐released silica species and self‐assembly of metal oxide units. To verify the successful synthesis and structure features of Pt‐LCDSHs catalyst, the as‐prepared samples were characterized by several techniques, such as SEM, N₂ adsorption–desorption isotherm analysis, TEM, EDX, XRD and XPS. Results indicated that all of MDSHs possessed double‐shelled structures with both the inner and outer shells composing of metal oxide units. Interestingly, the metal oxide of the DHSs could offer abundant active points for Pt NPs and the space between the double shells also could be filled with Pt NPs. What's more, compared with the pure samples, the Pt‐embedded La₂O₃‐CeO₂‐DSHs exhibited the highest catalytic performance (6.58 × 10^?3 min^?1) and good reusability with a conversion of 94% even after eight cycles, which were evaluated by means of the reduction of 4‐nitrophenol monitored by UV–vis spectra. Finally, a possible reaction mechanism for the reduction reaction on Pt‐based La₂O₃‐CeO₂‐DSHs was also proposed.     

Electrochemical Properties of a Boron‐Doped Diamond Electrode Modified with Gold/Polyelectrolyte Hollow Spheres

Oppositely charged polyelectrolyte (poly(allyamine hydrochloride) (PAH) and poly(sodium 4‐styrene‐sulfonate) (PSS)), and negatively charged gold nanoparticles (Au) were assembled alternately on polystyrene (PS) spheres via layer‐by‐layer technique, and the different PAH/(PSS/PAH)_n/(Au/PAH)_m/Au composite hollow spheres were derived by dissolving PS core. These hollow spheres were used to modify boron‐doped diamond (BDD) electrodes for electrochemical sensors. The cyclic voltammetric results for dopamine (DA) detection demonstrated that hollow‐sphere‐modified BDD exhibited better electrocatalytic activity than did bare BDD. Influence of the wall thickness and composition of hollow spheres on electrochemical properties were investigated. The results showed that the oxidative peak potential of DA and the peak current varied with different PSS/PAH and Au/PAH layers. The optimized wall structure of hollows spheres was PAH/(PSS/PAH)₇/(Au/PAH)₅/Au.     

Zirconium–Porphyrin‐Based Metal–Organic Framework Hollow Nanotubes for Immobilization of Noble‐Metal Single Atoms

Single atoms immobilized on metal–organic frameworks (MOFs) with unique nanostructures have drawn tremendous attention in the application of catalysis but remain a great challenge. Various single noble‐metal atoms have now been successfully anchored on the well‐defined anchoring sites of the zirconium porphyrin MOF hollow nanotubes, which are probed by aberration‐corrected scanning transmission electron microscopy and synchrotron‐radiation‐based X‐ray absorption fine‐structure spectroscopy. Owing to the hollow structure and excellent photoelectrochemical performance, the HNTM‐Ir/Pt exhibits outstanding catalytic activity in the visible‐light photocatalytic H₂ evolution via water splitting. The single atom immobilized on MOFs with hollow structures are expected to pave the way to expand the potential applications of MOFs.     

Multishelled Metal Oxide Hollow Spheres: Easy Synthesis and Formation Mechanism

Uniform multishelled NiO, Co₃O₄, ZnO, and Au@NiO hollow spheres were synthesized (NiO and Co₃O₄ hollow spheres for the first time) by a simple shell‐by‐shell self‐assembly allowing for tuning of the the size, thickness and shell numbers by controlling the heat treatment, glucose/metal salt molar ratio, and hydrothermal reaction time. These findings further the development of synthetic methodologies for multishelled hollow structures and could open up new opportunities for deeper understanding of the mechanisms of shell‐by‐shell self‐assembly. Moreover, the double‐shelled NiO hollow sphere exhibits a higher photocatalytic activity for degradation of methyl orange than its morphological counterparts.