A facile synthesis of novel urchin-like NiCo2O4 hierarchical hollow microspheres has been developed based on a template-free solvothermal and subsequent calcination method. The growth process of NiCo2O4 hollow microsphere precursors has been investigated, and a plausible mechanism was proposed. Because of their unique structure and high specific surface area, these NiCo2O4 hollow microspheres displayed enhanced electrochemical properties in methanol electrooxidation and determination of heavy-metal ions compared with solid urchin-like NiCo2O4 microspheres, Co3O4, and NiO microspheres. The good electrochemical performances suggested that these unique hierarchical NiCo2O4 hollow microspheres could be promising materials for energy and environmentally related applications.

Monodispersed vanadium dioxide (VO 2 ) hollow microspheres aggregated by elliptical nanorods were synthesized via a onestep template-free method. The inside-out Ostwald ripening mechanism is responsible for the formation of the hollow microspheres.Hollow micro-and nano-structures which are hierarchically porous have attracted considerable attention recently because of their fantastic features, such as large interior spaces, low densities, high specific surface areas and high mass transfer rates with wide potential applications in the field, such as in energy conversion and storage devices, water treatment, drug-delivery carriers, gas sensing, and catalysis. [1][2][3][4][5][6][7][8][9] Much effort has been exerted on developing synthetic strategies for fabricating inorganic hollow micro-and nano-structures. Various removable or sacrificial templates, such as mono-dispersed silica or polymer latex spheres, emulsion droplets/micelles 10 and even gas bubbles, 11 have been used in preparing hollow structures. Generally, these methods usually encounter numerous obstacles, such as size limitation by the templates, damaged structures during template removal, high cost and tedious synthetic procedures, and thus are not suitable for large-scale applications.A number of template-free methods have been developed very recently in fabricating hollow micro-and nano-structures by using different mechanisms, such as the nanoscale Kirkendall effect, 12,13 corrosion-based inside-out evacuation, 14,15 oriented attachment and Ostwald ripening. 16,17 Amongst them, Ostwald ripening, a process by which larger particles grow at the expense of smaller ones has been proven to be a facile approach to generate hollow structures. 18 Different materials, such as SnO 2 , 16 TiO 2 , 17 CaF 2 , 19 ZnS 20 and Co 3 O 4 , 20 have been synthesized based on the Ostwald ripening mechanism. However, there are no reports on the synthesis of VO 2 hollow spheres based on the Ostwald ripening mechanism through a hydrothermal route in the literature.As a well-known transition-metal oxide, vanadium dioxide has been extensively studied. Various nanostructures, such as hollow spheres, nanowires, nanobelts, nanorings and nanorods, have been CrystEngComm