The development of materials with radically new properties is inextricably related with obtaining of nanoscale systems. One of the most promising materials in this area is TiO₂. Its nano-modifications are unique due to the formation of self-generating surface OH groups with a high reaction activity. From the 13 known stable and metastable modifications only one (η-TiO₂) was obtained just in the nanoscale form [1,2]. According to [1] this polymorph and anatase exhibit properties that distinguishes them from the another of nanoscale modifications. The purpose of the investigation is to optimize of sulfate method of samples preparation with nano-anatase and η-TiO₂ and establish the relationship between synthesis conditions, characteristics of the samples and their adsorption and photocatalytic properties. Samples with η-TiO₂ differ from samples with anatase by the presence of two distinct diffraction reflections at 2θ ~ 4÷5° and 2θ ~ 33° (CuKα). Based on the X-ray data of anatase and η-TiO₂, it is likely that the structure of η-TiO₂ should be derived from the structure of anatase, or rather, the superstructure to the structure of anatase. Nano-anatase is formed with the practical yield of > 90% in long-term (<4 h) hydrolysis of TiOSO₄ at 70°C to 98°C, therefore, in order to optimize its production process we searched the terms of its uniform formation and precipitation. In the results the synthesis parameters (the rate of dissolution of the starting reagent in the water, its amount, method of heating and the pH of the reaction mixture at different stages of the synthesis) were defined and their optimal values ​​for laboratory obtaining of the product were found. Due to the metastability of the phase with η-TiO₂ we have to get it in a strong suppression of precursor hydrolysis, resulting in extremely low practical yield. Modification of the method of obtaining η-TiO₂, described in [1], is to replace the coagulant (solution of HCl [1]) to the more active solution of alkali metal - potassium and cesium halogenides which led to increasing of the practical yield from ~ 30% to ~ 90% [2]. Samples with different modifications differ in their characteristics: specific surface area (30 ÷ 170 m²/g for anatase and 3 ÷ 18 m²/g for η-TiO₂), a volume of nano-and ultrananopores (0.04÷0.48 and 0.0005÷0.06 cm³/g for anatase, 0.0009÷0.004 and 0.008÷0.04 cm³/g for η-TiO₂), the size of coherent scattering (5÷10 and 2÷4 nm for anatase and η-TiO₂, respectively), the size of crystallites (7÷10 nm), the size of the nanoparticles (~15÷~40 nm for anatase, 10÷~ 30 nm for η-TiO₂), aggregates (120÷150 nm for anatase and 400÷500 nm for η-TiO₂) and agglomerates (aglomerization of the particles with different sizes depending on the synthesis conditions was found in all samples, but mostly it is expressed in samples with η-TiO₂). It was established by IR spectroscopy that the intensity of the absorption bands of the deformation vibrations of the Ti-O-H bonds (about 1050-1150 cm^-1) and H-O-H bonds (about 1600-1650 cm^-1) in the spectrum of the sample with η-TiO₂ more than in the anatase one. This fact indicates greater content of OH groups on the surface of the particles with η-TiO₂ in comparison with anatase. Photocatalytic activity (PA) of the obtained samples (pure - in the UV, and sensitized by the cold impregnation of diazo- and thiazole dyes under visible light range) is lower than that of the not aggregated commercial Degussa (Evonik) P25 (a mixture of anatase and rutile) and Hombifine N (anatase). However, non-destructive conditions of synthesis for the dyes used (in contrast to pyrolysis that is used in obtaining of commercial analogs) allow sensitizing colloidal intermediate product and obtaining samples with greater PA in comparison with the commercial analogs after impregnation. In the UV range the samples with η-TiO₂ modified by VO(NO₃)₂ show larger PA in the reaction of photodegradation of dye Rhodamine B in comparison with Degussa P25, but lower in comparison with Hombifine N. However, when irradiated with visible light (λ= 450÷490 nm) PA of the sample with η-TiO₂ exceeds Hombifine N (Fig. 1).

                                                                         a.

                                                                         b.

    Fig.1 Efficiency of photodegradation of Rhodamine B under UV (a) and visible irradiation (b) in the presence of various photocatalysts Degussa P25 (1), Hombifine N (2), η-TiO₂:V(3).

   We have examined adsorption ability of the nano-TiO₂ samples with anatase structure and anatase modified with peroxide (H₂O₂) to extract from water environments Nb (V) and Ta (V) ions. It was found that the sorption degree (R=100-(Cf)/(Ci),%, where Cf and Ci are finish and initial concentrations of analytes, respectively) depends on the sorption conditions and the type of analyte. So, the R,% increases with the increasing of the sorption duration, decreases with the increasing of the sample weight for Ta(V) and does not change for Nb(V), decreases with the use of the membrane filter with 0.45 μm pores and dramatically increases with the use of centrifugation. The R,% weakly depends on the adsorbent characteristics (size of nanoparticles, specific surface area, volume of ultra- and nanopores), and is determined by the nature of the analyte (the degree of Nb (V) extraction is larger than found for Ta (V) one). The results led to the conclusion that the maximal recovery efficiency depends on the sample composition and concentration of adsorption centers – ОН-groups – on the adsorbent surface. The amount of Nb (V) and Ta (V) ions adsorbed on the TiO₂ nanoparticles from aqueous solutions first decreases and then increases with increasing the temperature from 4°C to 10°C (Nb(V)) or 20°C (Ta(V)) and from 10°C or 20°C to 80°C, respectively (Fig.2a).

                                                                     a.

                                                                       b.

  Fig.2 The sorption degree (R,%) vs. the temperature (a) and the nature of the analyte (b).

   This behavior suggests exothermic nature of the process (in range of 4°C-10°C for Nb(V) or 4°C -20°C for Ta(V)) which can be caused by physical modification of the adsorbents. Surfaces of metal oxides usually have positive charge at low pH (our case: рН=0.35, i=0.43(1), i-ionic strength). Mo(VI) and W(VI) ions in hydrated associates of different compositions probably have a negative charge (a similar to As(V) ions [3]-[5]). At low temperatures, they are adsorbed at the expense of forces of electrostatic character. With temperature increasing from 4°C to 10°C (Nb(V)) or from 4°C to 20°C (Ta(V)), the attractive forces between TiO₂ and analytes are weakened and the adsorption decreases. Besides, at increasing temperature the thickness of the boundary layer decreases, due to the increasing tendency of the adsorbate to escape from the surface of the adsorbent to the solution phase, which results in a decrease in adsorption. So, it was found that in the temperature range from 10°C to 80°C (Nb(V)) or 20°C to 80°C (Ta(V)) the process is endothermic, since the adsorption increases with increasing temperature. Therefore, the rate of physical adsorption and the amount of ions adsorbed by this way decrease, meanwhile the rate of chemical adsorption increases. As a result, we have found that the samples containing anatase modified with peroxide and Degussa P25 shown the maximum adsorption ability for Ta (V) ions (R = 99.9%) and for Nb (V) ions (R = 99.9%), respectively (Fig. 2b). Note that the sorption process was carried out at pH = 0.35 and a change of this magnitude can have a significant influence on sorption properties. Thus the obtained samples with nano-TiO₂ have a higher photocatalytic activity and comparable adsorption ability in comparison with analogs used at present with increasing of practical output and simplification of the preparation.

[1] Dadachov M. US Patent Application Publication. US 2006/0171877

[2] Kuzmicheva G. M. et al., Patent RU 2463252 C1.

[3] E. Valencia-Trejo et al., J.Applied Sciences in Environmental Sanitation. 5(2) (2010) 171-184

[4] Nabi Deedar et al., J.Environmental Sciences. 21 (2009) 402–408.

[5] By Xiao et al., Water Environ. Res. 79 (2007) 1015.

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