Nanocomposites and nanomaterials

Nano-dispersed Zn-Al double hydroxide and oxides for photocatalytic application
Sydorchuk V., Levytska S., Khalameida S., Kuznetsova L.

Institute for Sorption and Problems of Endoecology, NAS of Ukraine. 13 Naumova St., Kyiv-03164, Ukraine E-mail: bilychi@ukr.net

The layered double hydroxides LDHs and oxide compositions prepared from them are sorption and catalytically active materials used to purify polluted air and water, for example as photocatalysts. They should have certain physicochemical properties in order to be effective photocatalysts.
The hydrothermal and mechanochemical treatments (HTT and MChT) of coprecipitated Zn/Al hydroxide with Zn/Al ratio were used to regulate physicochemical parameters. These kinds of modification were performed in the form of wet gel and dried xerogel. Such approach has not previously been used to modification of Zn/Al hydroxide although was effective for other oxides
 XRD, DTA-TG and FTIR measurements show that MChT in air results in partially decomposition of LDHs structure without formation of other phases while only modification of its structure takes place after milling in water. At the same time, elevation in HTT temperature leads to gradual decomposition of LDHs: if its structure is improved at a temperature of 150°C, then it is partially destroyed at 200°C, and zinc oxide is formed at 250°C. Crystal structure of ZnO is also improved after next calcination of all modified LDHs. Also, significant changes in porous structure occurs during MChT and HTT. It is important that formation of additional mesoporosity and corresponding increase in specific surface area take place. Thus, the samples after have value of mesopores up to 0.70-0.75 cm3/g and specific surface area 50-67 m2/g and the latter for samples milled in water reaches 106 m2/g. The changes in crystal structure are accompanied by changing the electronic characteristics: the narrowing of band gap from 3.2 eV (initial sample) to 2.8-2.9 eV (hydrothermally treated samples) is observed. 
 All of these changes result in enhancement of the photocatalytic activity of modified samples under visible irradiation. Indeed, initial sample is practically non-active in process of photocatalytic degradation of safranin T (as model substrate) in aqueous medium under these conditions. At the same time, sample subjected to dry milling at 300 rpm which has the bang gap 3.1 eV becomes active: rate constant of degradation Kd = 1.8 10-5 s-1. The sample, modified by HTT in the form of gel at 200°C, possesses the maximal activity (Kd = 3.4 10-5 s-1) since it simultaneously has band gap 2.9 eV and high value of specific surface area - 67 m2/g.