One-pot synthesis of Cu–carbon hybrid hollow spheres

Novel hybrid hollow spheres (HSs) are easily prepared by a combination of hydrothermal carbonization (HTC) and emulsion template method for the first time. The HSs have an average diameter of 3.5 μm and a shell thickness of about 70 nm. The HSs consist of a matrix of hydrothermal carbon, in which Cu nanoparticles of several nanometers in size are imbedded, and exhibit a well-defined shape, a relatively uniform size and high flexibility. With regard to the formation mechanism, trioctylamine (TOA) droplet in the oil-in-water (W/O) emulsion plays the role of a template. Under hydrothermal conditions, TOA in the droplet and Cu2+ outside the droplet form the Cu–amine complex, which aggregates at the interface and is then reduced to form Cu nanoparticles by ascorbic acid (VC), and HTC of VC to hydrothermal carbon occurs at the interface. By changing the carbon source into glucose the size of HSs can be tuned down to 1 μm and the bowl-like hollow structure can be obtained by increasing reaction time, indicating the flexibility of this approach. Due to the special structure of these HSs, they can be used as a new ultrasound contrast agent or a template for porous CuO HSs.