Silver Molybdate and Silver Tungstate Nanocomposites with Enhanced Photoluminescence

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Processo FAPESP: 13/07296-2

Processo FAPESP: 08/57872-1

Silver molybdate (Ag2MoO4) and silver tungstate (Ag2WO4) nanomaterials were prepared using two complementary methods, microwave assisted hydrothermal synthesis (MAH) (pH 7, 140 degrees C) and co-precipitation (pH 4, 70 degrees C), and were then used to prepare two core/shell composites, namely alpha-Ag2WO4/beta-Ag2MoO4 (MAH, pH 4, 140 degrees C) and beta-Ag2MoO4/beta-Ag2WO4 (co-precipitation, pH 4, 70 degrees C). The shape and size of the microcrystals were observed by field emission scanning electron microscopy (FE-SEM), different morphologies such as balls and nanorods. These powders were characterized by X-ray powder diffraction and UV-vis (diffuse reflectance and photoluminescence). X-ray diffraction patterns showed that the Ag2MoO4 samples obtained by the two methods were single-phased and belonged to the beta-Ag2MoO4 structure (spinel type). In contrast, the Ag2WO4 obtained in the two syntheses were structurally different: MAH exhibited the well-known tetrameric stable structure alpha-Ag2WO4, while co-precipitation afforded the metastable beta-Ag2WO4 allotrope, coexisting with a weak amount of the alpha-phase. The optical gap of beta-Ag2WO4 (3.3 eV) was evaluated for the first time. In contrast to beta-Ag2MoO4/beta-Ag2WO4, the alpha-Ag2WO4/beta-Ag2MoO4 exhibited strongly-enhanced photoluminescence in the low-energy band (650 nm), tentatively explained by the creation of a large density of local defects (distortions) at the core-shell interface, due to the presence of two different types of MOx polyhedra in the two structures.