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Recebido em 28/07/2015; aceito em 22/12/2015; publicado na web em 11/03/2016ZnO and Ag-ZnO were synthesized in a simple and efficient manner by thermal decomposition of zinc oxalate and silver/zinc mixed oxalate. The influence of the addition of metallic silver on ZnO particles and the effect of temperature in the thermal treatment were investigated. The samples were characterized by thermogravimetric analysis, Raman, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, specific surface area (Brunauer-Emmett-Teller) and diffuse reflectance spectroscopy. The photocatalytic activity of these materials in the decolorization of direct red 23 diazo dye was studied. The complete conversion into oxides from oxalates at lower temperatures was determinant in the photocatalytic efficiency of both the oxides. The presence of silver in zinc oxide, treated at 400 °C, more than doubled the rate constant of diazo dye decolorization (6.87×10-3 min -1) with respect to ZnO, treated at 600 °C, resulting in 3.07×10 -3 min -1 under UV irradiation at 30 °C.

ResumoO óxido de bismuto (Bi 2 O 3 ), semicondutor do tipo-n, tem sido satisfatoriamente investigado para a remediação fotocatalítica de contaminantes orgânicos. O Bi 2 O 3 foi preparado por síntese de combustão em solução (SCS) utilizando como oxidante o nitrato de bismuto, em meio ácido, e a ureia como combustível. A influência da condição de síntese sobre as propriedades fotocatalíticas do óxido formado foi investigada por DRX. A partir dos difratogramas foi verificado que os materiais obtidos são predominantemente de cristais de Bi 2 O 3 , sendo possível identificar uma amostra com duas fases cristalinas, monoclínica (α-Bi 2 O 3 ) e tetragonal (β-Bi2O3), e a outra com apenas a fase monoclínica (α-Bi 2 O 3 ). O óxido com duas fases apresentou maior atividade fotocatalítica para a descoloração do azul de metileno tanto sob irradiação UV (60,59%) quanto sob luz solar (61,64%), em 664 nm (λ máx ), seguindo lei cinética de pseudo-primeira ordem. Palavras-chave: Fotocatálise. Bi 2 O 3 . Semicondutor. AbstractBismuth oxide (Bi 2 O 3 ), an n-type semiconductor has been satisfactorily investigated for photocatalytic organic contaminant remediation. The Bi 2 O 3 was prepared by solution combustion synthesis (SCS) using as the oxidizing bismuth nitrate in acidic medium and urea as fuel. The influence of the type of synthesis on the photocatalytic properties of the oxide formed was investigated by XRD. From the diffractograms was verified that the materials obtained are predominantly of Bi 2 O 3 crystals, it is possible to identify a sample with two crystalline phases, monoclinic (α-Bi 2 O 3 ) and tetragonal (β-Bi 2 O 3 ), and the other with only the monoclinic (α-Bi 2 O 3 ). The two-phase oxide showed higher photocatalytic activity for discoloration of methylene blue under UV irradiation (60.59%) and under sunlight (61.64%) in 664 nm, followed kinetic law of pseudo-first order.