Hydrothermal synthesis and polarized micro-Raman spectroscopy of copper molybdates

Conventional and microwave-assisted hydrothermal methods were used to synthesize Cu3(MoO4)2(OH)2 ceramics. Different experimental conditions of temperature, time and precipitation rate were employed to explore the growth mechanisms, structural and morphological properties of the obtained copper molybdates. The use of microwave-assisted reactors at 150 °C/10-30 min produced a mixture of copper molybdate hydrates, while only Cu3(MoO4)2(OH)2 was obtained by using microwave processing at longer times (above 120 min) or conventional hydrothermal reactors (110-250 °C/24 h). Experiments conducted after fast and slow coprecipitation rates resulted in different crystallographic phases and morphologies. Flower-like and rod-like micrometer-sized ceramics were produced with high anisotropy and single-crystalline nature. A better understanding on the growth mechanisms, as well as on the structural and morphological characteristics of copper molybdates was attained. Based upon the relative enhancement of Raman bands in parallel or in perpendicular configuration, a symmetry assignment of 34 (17Ag + 17Bg) external modes for Cu3(MoO4)2(OH)2 ceramics (42 bands are expected) was possible. Besides, five (of six possible) internal OH-stretching modes were identified in the spectral region above 3200 cm−1.