Hollow micro-/nano-structured materials are now playing an important role in cutting edge innovations for energy conversion and storage technologies such as solar cells, fuel cells, lithium ion batteries and super capacitors. These materials show great promise in addressing growing environmental concerns for cleaner power sources at a time of increasing global demand for energy. In this perspective, we show that complex multi-shelled micro-/nano-materials show significant material advantages in many applications over conventional simple hollow structures. We also summarize the vast array of synthetic strategies used to create multi-shelled hollow structures, and discuss the possible application of these novel materials for power generation and storage. Finally, the emergent challenges and future developments of multi-shelled hollow structures are further discussed.

Hierarchically ordered macro-mesoporous titania films have been produced through a confinement self-assembly method within the regular voids of a colloidal crystal with three-dimensional periodicity. Furthermore, graphene as an excellent electron-accepting and electron-transporting material has been incorporated into the hierarchically ordered macro-mesoporous titania frameworks by in situ reduction of graphene oxide added in the self-assembly system. Incorporation of interconnected macropores in mesoporous films improves the mass transport through the film, reduces the length of the mesopore channel, and increases the accessible surface area of the thin film, whereas the introduction of graphene effectively suppresses the charge recombination. Therefore, the significant enhancement of photocatalytic activity for degrading the methyl blue has been achieved. The apparent rate constants for macro-mesoporous titania films without and with graphene are up to 0.045 and 0.071 min(-1), respectively, almost 11 and 17 times higher than that for pure mesoporous titania films (0.0041 min(-1)).

A series of multishelled ZnO hollow microspheres with controlled shell number and inter-shell spacing have been successfully prepared by a simple carbonaceous microsphere templating method, whose large surface area and complex multishelled hollow structure enable them load sufficient dyes and multi-reflect the light for enhancing light harvesting and realize a high conversion efficiency of up to 5.6% when used in dye-sensitized solar cells.

Great progress has been made in the preparation and application of multi-shelled hollow micro-/nanostructures during the past decade. However, the synthetic methodologies and potential applications of these novel and interesting materials have not been reviewed comprehensively in the literature. In the current review we first describe different synthetic methodologies for multi-shelled hollow micro-/nanostructures as well as their compositional and geometric manipulation and then review their applications in energy conversion and storage, sensors, photocatalysis, and drug delivery. The correlation between the geometric properties of multi-shelled hollow micro-/nanostructures and their specific performance in relevant applications are highlighted. These results demonstrate that the geometry has a direct impact on the properties and potential applications of such materials. Finally, the emerging challenges and future development of multi-shelled hollow micro-/nanostructures are further discussed.

Hollow spheres with nanometer-to-micrometer dimensions, controlled internal structure, and shell composition have attracted tremendous attention because of their potential application in catalysis, drug delivery, nanoreactors, energy conversion and storage systems, photonic devices, chemical sensors, and biotechnology.[1] Single-shell and double-shell hollow spheres of various compositions have been synthesized by a number of methods, such as vesicles, emulsions, micelles, gas-bubble, and hard-templating methods.[2] More recently, efforts have focused on the fabrication of hollow spheres with multiple shells, as these materials are expected to have better properties for applications such as drug release with prolonged release time, heterogeneous catalysis, lithiumion batteries, and photocatalysis.[3] For example, multipleshell hollow microspheres of Cu 2 O have been prepared by vesicle templating and an intermediate-templating phasetransformation process. [3a,b] Multiple-shell azithromycin hollow microspheres were fabricated by hierarchical assembly.[3c] Cao and co-workers reported the synthesis of tripleshelled SnO 2 hollow microspheres by chemically induced selfassembly in the hydrothermal environment which exhibited enhanced electrochemical performance.[3d] Yao and co-workers reported excellent cycle performance and enhanced lithium storage capacity of multiple-shell Co 3 O 4 hollow microspheres synthesized by oriented self-assembly.[4] These preparative methods, however, are suited for each specific material and cannot be applied generally to a wide range of materials. Currently, there is no general synthetic approach for fabricating multiple-shell hollow nanostructures of any desired material.Herein, we present a straightforward and general strategy to prepare metal oxide hollow microspheres with a controlled number of shells. Carbonaceous microspheres were used as sacrificial templates. The microspheres were saturated with a desired metal salt solution and then heated in air; the carbonaceous template evaporates and templates the formation of metal oxide shells. The number of shells is controlled by the metal ion loading and the process is general for a wide range of metal oxide materials.Scheme 1 illustrates the general process of fabricating multiple-shell hollow metal oxide microspheres. The key to this process is the use of carbonaceous particles rich with surface functional groups available for metal ion adsorption. [5] Multiple shells are generated by supplying enough shell precursor material to the sacrificial carbonaceous spheres. Recently, our group reported the synthesis of hollow coreshell ferrite microspheres, [5b] demonstrating that carbonaceous particles can absorb a significant amount of metal ions within the interior of the particle ( Figure S1 in the Supporting Information). In this work, we extend these methods and demonstrate the general and facile synthesis of multiple-shell hollow microspheres of a wide range of different metal oxides.Figure 1 a,b shows a transmission electron micro...

Titania nanoparticles (P25) are successfully chemically bonded with graphdiyne (GD) nanosheets by a facile hydrothermal treatment, to form a novel nanocomposite photocatalyst. The as-prepared P25-GD nanocomposite exhibits higher photocatalytic activity for degrading methylene blue under UV irradiation than not only P25 and P25-carbon nanotube composite but also the current well-known P25-graphene composite photocatalysts. Moreover, P25-GD also shows considerable visible-light-driven photocatalytic activity, since the formation of chemical bonds between P25 and GD effectively decreases the bandgap of P25 and extends its absorbable light range. The photocatalytic activity of P25-GD can be adjusted by changing the content of GD in composites and the optimized value is about 0.6 wt%. Such a nanocomposite photocatalyst might find potential application in a wide range of fields including air purification and waste water treatment.

A general method has been developed for the synthesis of homogeneous hollow core−shell microspheres of spinel ferrites (MFe2O4, M = Zn, Co, Ni, Cd) by using carbonaceous saccharide microspheres as template. The products were characterized by X-ray powder diffraction, inductively coupled plasma-atomic emission spectroscopy, scanning electronic microscopy, transmission electron microscopy, and nitrogen sorption measurement. The effects of the concentration of metal salts have been studied using ZnFe2O4 as an example. Increasing the concentration of metal salts could avoid the generation of impurity phase. The core size and shell thickness of hollow spheres obtained can be manipulated by changing the concentration of metal salts. Gas-sensor investigations revealed the ZnFe2O4 hollow spheres used as gas-sensor materials possess high sensitivity and quick responses to organic gases such as ethanol.

Hollow spheres with nanometer-to-micrometer dimensions, controlled internal structure, and shell composition have attracted tremendous attention because of their potential application in catalysis, drug delivery, nanoreactors, energy conversion and storage systems, photonic devices, chemical sensors, and biotechnology. [1] Single-shell and double-shell hollow spheres of various compositions have been synthesized by a number of methods, such as vesicles, emulsions, micelles, gas-bubble, and hard-templating methods. [2] More recently, efforts have focused on the fabrication of hollow spheres with multiple shells, as these materials are expected to have better properties for applications such as drug release with prolonged release time, heterogeneous catalysis, lithiumion batteries, and photocatalysis. [3] For example, multipleshell hollow microspheres of Cu 2 O have been prepared by vesicle templating and an intermediate-templating phasetransformation process. [3a,b] Multiple-shell azithromycin hollow microspheres were fabricated by hierarchical assembly. [3c] Cao and co-workers reported the synthesis of tripleshelled SnO 2 hollow microspheres by chemically induced selfassembly in the hydrothermal environment which exhibited enhanced electrochemical performance. [3d] Yao and co-workers reported excellent cycle performance and enhanced lithium storage capacity of multiple-shell Co 3 O 4 hollow microspheres synthesized by oriented self-assembly. [4] These preparative methods, however, are suited for each specific material and cannot be applied generally to a wide range of materials. Currently, there is no general synthetic approach for fabricating multiple-shell hollow nanostructures of any desired material.Herein, we present a straightforward and general strategy to prepare metal oxide hollow microspheres with a controlled number of shells. Carbonaceous microspheres were used as sacrificial templates. The microspheres were saturated with a desired metal salt solution and then heated in air; the carbonaceous template evaporates and templates the formation of metal oxide shells. The number of shells is controlled by the metal ion loading and the process is general for a wide range of metal oxide materials.Scheme 1 illustrates the general process of fabricating multiple-shell hollow metal oxide microspheres. The key to this process is the use of carbonaceous particles rich with surface functional groups available for metal ion adsorption. [5] Multiple shells are generated by supplying enough shell precursor material to the sacrificial carbonaceous spheres. Recently, our group reported the synthesis of hollow coreshell ferrite microspheres, [5b] demonstrating that carbonaceous particles can absorb a significant amount of metal ions within the interior of the particle ( Figure S1 in the Supporting Information). In this work, we extend these methods and demonstrate the general and facile synthesis of multiple-shell hollow microspheres of a wide range of different metal oxides. Figure 1 a,b shows a transmission elect...