Reverse micelles or microemulsion systems have been widely
used as ideal media to prepare nanoparticles [19–21]. Water-in-oil
(w/o) microemulsion is a transparent and isotropic liquid medium
with nanosized water pools dispersed in a continuous phase and
stabilized by surfactant and co-surfactant molecules at the water/
oil interface. These water pools offer ideal microreactors for the
formation of nanoparticles. Compared to other synthetic methods,
the microemulsion approach has distinct advantages in effective
control over the size and shape of the particles, such as relatively
low reaction temperature, less time consumption, high homogeneity, and well-crystallized products with definite composition.
A schematic diagram of the proposed growth process is shown in
Fig. 3. When the microemulsion solutions containing Ca2+ and
WO42, respectively, were mixed, CaWO4 nucleation and micellar
fusion may be concomitant. And nanoparticles with surfactant
coatings are formed in a very short time. The obtained
nanoparticles can be regarded as ‘‘spherical core–shell nanoparticles’’ with an inorganic core and an organic surfactant shell. Then
the dumbbell superstructures of CaWO4 were formed via a self