Nanocomposites and nanomaterials

The Nanosized Zn-Al hydrotalcite as potential photocatalyst  
V.Sydorchuk*, S.Levytska, L.Kotynska, L.Kuznetsova

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

The layered double hydroxides (LDHs), particularly hydrotalcites, are versatile nano-sized materials used including the solution of environmental problems, for example as photocatalysts. They must have certain properties in order to be effective photocatalysts: first of all, an optimal porous and crystalline structure, electronic characteristics. We applied the hydrothermal and mechanochemical treatments (HTT and MChT, respectively) of Zn/Al hydroxide with Zn/Al ratio 3 to regulate these parameters for the precipitated samples.      According to XRD data, as-precipitated sample has hydrotalcite structure with basal spacing 0.865 nm (in literature – 0.895 nm) which is also confirmed by DTA-TG and FTIR results. MChT in air partially destroys hydrotalcite structure without formation of other phases while milling in water only modifies it: basal spacing decreases to 0.786 nm. The latter is accompanied by development of porous structure, obviously, due to formation of additional mesoporosity: the specific surface area increases by 3 times, reaching 106 m2/g, and the pore volume is almost 4 times (to 0.27 cm3/g). Under HTT, 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. Samples hydrothermally modified in the form wet gel have high value of mesopores – about 0.70-0.75 cm3/g and specific surface area 50-67 m2/g.
All modified samples are characterized by stronger absorption in the visible region compared to initial sample. Besides, the narrowing the band gap Eg is observed: from 3.2 eV (initial sample) to 2.8-2.9 eV (hydrothermally treated samples). Both these effects should enhance the photocatalytic activity of modified samples under visible irradiation. Initial sample is practically non-active in process of photocatalytic degradation of safranin T. At the same time, the sample, modified by HTT in the form of gel at 200°C, possesses the maximal activity (rate constant of degradation Kd = 3.4 10-5 s-1) since it simultaneously has Eg = 2.9 eV and high value of specific surface area - 67 m2/g.
Therefore, mechanochemical and hydrothermal treatments allow to vary the physical-chemical characteristics and photocatalytic activity of hydrotalcite.