Extraction–Pyrolytic Method for TiO2 Polymorphs Production

The unique properties and numerous applications of nanocrystalline titanium dioxide (TiO2) are stimulating research on improving the existing and developing new titanium dioxide synthesis methods. In this work, we demonstrate for the first time the possibilities of the extraction–pyrolytic method (EPM) for the production of nanocrystalline TiO2 powders. A titanium-containing precursor (extract) was prepared by liquid–liquid extraction using valeric acid C4H9COOH without diluent as an extractant. Simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA–DSC), as well as the Fourier-transform infrared (FTIR) spectroscopy were used to determine the temperature conditions to fabricate TiO2 powders free of organic impurities. The produced materials were also characterized by X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM). The results showed the possibility of the fabrication of storage-stable liquid titanium (IV)-containing precursor, which provided nanocrystalline TiO2 powders. It was established that the EPM permits the production of both monophase (anatase polymorph or rutile polymorph) and biphase (mixed anatase–rutile polymorphs), impurity-free nanocrystalline TiO2 powders. For comparison, TiO2 powders were also produced by the precipitation method. The results presented in this study could serve as a solid basis for further developing the EPM for the cheap and simple production of nanocrystalline TiO2-based materials in the form of doped nanocrystalline powders, thin films, and composite materials.

where the subscripts w and o denote the aqueous and organic phases, respectively. Alkali is added to the extraction system to increase the efficiency of target metal extraction since monocarboxylic acids themselves (with or without a diluent) are usually ineffective extractants [49].
To date, the EPM has already been applied for producing photoactive titanium dioxide films [45]. As the initial components for preparing the Ti-containing extract, the authors used an aqueous solution of titanium (IV) oxysulfate TiOSO 4 and α-branched monocarboxylic acids of C 5 -C 9 fractions as an extractant.
The aim of this work is to develop the EPM for the production of nanocrystalline TiO 2 powders using valeric acid-based extracts; and to study the effect of pyrolysis conditions on the phase composition, the mean crystallite size, and morphology of the fabricated materials. In addition, the results acquired by the EPM are compared with those related to the simplest and widely known production method-precipitation. In both approaches, the initial components are a freshly prepared aqueous solution of titanium (III) chloride as a titanium source and an aqueous solution of sodium hydroxide as an alkaline agent. An aqueous solution of TiCl 3 in diluted hydrochloric acid HCl with a metal concentration of 0.1 M was used as a titanium source. It was prepared immediately before both extraction and precipitation. For this, 1.200 g of titanium powder (particle size d = 63-100 µm) was dissolved in 60 mL of HCl solution (1:1) during heating until the metal was completely dissolved. Thereafter, the solution was cooled down and diluted with distilled water to a volume of 250 mL.

Preparation of Titanium-Containing Precursors (E) via Liquid-Liquid Extraction
Valeric acid C 4 H 9 COOH without diluent was used as an extractant. During preparing the precursor E1, the initial ratio of the volumes of the aqueous (V w ) and organic (V o ) phases in the extraction system was 3:1. For the extraction, the extractant and TiCl 3 solution (pH~0.65) were placed in a separatory funnel, and 1 M NaOH solution was added stepby-step. When the organic phase (extract) turned deep blue, the addition of alkali was stopped. After a clear phase separation (~10 min), the aqueous phase was removed from the funnel, and its pH value was around 1.15. The organic phase was filtered through a cotton filter to remove water droplets.
To increase the titanium content in the organic phase for preparing the precursor E2, the initial V w :V o ratio was taken as 5:1. The metal was extracted from TiCl 3 solution with a pH value of~0.74. Moreover, the addition of an alkaline solution was continued until a saturated solution of titanium valerate Ti(C 4 H 9 COO) 3 in valeric acid was obtained, i.e., a finely dispersed precipitate appeared in the organic phase. As a result, the achieved pH value of the aqueous phase after extraction was about 1.23. To separate a small amount of the formed precipitate and to obtain a true solution, the organic phase was filtered through a double thick paper filter.

Preparation of Titanium-Containing Precursor (P) via Precipitation
As the first step, alkaline hydrolysis of TiCl 3 solution was carried out at room temperature. 0.5 M NaOH solution was added dropwise (at a rate of~3 mL/min) under vigorous stirring until the pH of the aqueous phase reached~6.0. Then, the mixture was left to stay for a day. This was followed by filtration, multiply washing of the resulting precipitate with distilled water (the presence of chloride ions in the decanted solution was controlled with an AgNO 3 solution) and, after all, with ethanol. The precipitate was dried at room temperature for 36 hours, ground in an agate mortar and used as a precursor (P).

Thermal Treatment of Precursors
The resulting precursors E1 and E2, as solutions, and precursor P as powder were heated from room temperature to 350-750 • C at a heating rate of 10 • /min, annealed for an hour and rapidly cooled down under ambient conditions. Such thermal treatment was performed in laboratory furnace SNOL 8.2/1100. Thereafter, the produced samples were ground by pestle in an agate mortar and collected. For further investigations, only as-prepared powders without any additional posttreatment were used.

Characterization Methods
The metal concentration in the resulting precursors E was determined by the gravimetric method [50]. The thermal behavior of all the produced precursors was studied by simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) using the STA PT1600 (LINSEIS). The samples under test were heated from room temperature to 700 • C or 1000 • C at a rate of 10 • /min in the static air atmosphere.
The phase composition of the produced materials was investigated by the X-ray diffraction (XRD) method (diffractometer D8 Advance, Bruker Corporation) with CuKα radiation (λ = 1.5418 Å). The XRD patterns were referenced to the PDF ICDD 00-021-1272 for anatase phase of TiO 2 , PDF ICDD 00-021-1276 for rutile phase of TiO 2 , and PDF ICDD 00-014-0277 for sodium polytitanate (Na 2 Ti 6 O 13 ) identification. The mean crystallite size (d) of the titanium dioxide was defined from the half-width of the diffraction peaks (101) of anatase (d A ) and (110) one of rutile (d R ) by the Scherrer method (EVA software). The weight fraction of the rutile phase (W R ) was determined by Gribb and Banfield [34] using integrated intensities A (areas) of the most intense diffractions peaks as follows (Equation (2)).
IR spectra were recorded at room temperature using Bruker Tensor II FTIR spectrometer at a resolution of 4 cm −1 and 36 scans for each spectrum. TiO 2 powder was mixed with KBr, and the pellets with a 7 mm diameter were prepared using Specac Mini-Pellet press under a load of 2000 kg. The morphology of the samples was examined by transmission electron microscopy (TEM) (FEI Tecnai G2 F20 operating at 200 kV).

Precursors Characterization
Titanium-containing precursors (E) During extraction, Ti 3+ cations are transferred from the aqueous phase into the organic phase as Ti(C 4 H 9 COO) 3 , and the organic solution gradually turns deep blue. As a result of the storage of the produced titanium-containing extract E1 in a glass flask, the organic solution underwent discoloration, first, gradually and after~60 minutes complete. The process was likely associated with the oxidation of Ti (III) to Ti (IV) by atmospheric oxygen. To our knowledge, there is no data on the composition of the final titanium (IV) carboxylate formed this way in an organic solution. However, it can be assumed that this compound may have the following composition: Ti(C 4 H 9 COO) 4 and/or (C 4 H 9 COO) 3 TiOTi(OOCC 4 H 9 ) 3 .
Note that the discoloration of the organic titanium-containing extract E2 was observed already during filtration. According to the results of the gravimetric analysis, the titanium concentration in the precursors E1 and E2 was 0.14 M and 0.50 M, respectively.
Thus, colorless transparent organic solutions with different titanium concentrations were prepared. Upon storage of E1 and E2 precursors in glass flasks with ground-glass stoppers at room temperature, no changes in color and transparency (homogeneity) were observed.
Titanium-containing precursor (P) As a result of the produced precipitate (gel) storage during the day, its color changed from deep gray-blue to white because of the oxidation of titanium (III) hydroxide by atmospheric oxygen and the formation of hydrated titanium dioxide (titanium oxyhydrate) TiO 2 ·nH 2 O [51].

Thermal Behavior of Precursors E1, E2, and P
The main thermal decomposition products of the salts of many carboxylic acids are ketones and the corresponding metal oxides, while the temperature of their decomposition is characteristic for each certain compound [52]. This is why the study of the thermal behavior of the extracts produced during the EPM (solutions of metal carboxylates in carboxylic acid or diluent) is rather important and necessary for determining the minimal pyrolysis temperature for the production of organic impurity-free oxide materials.
The results of TG-DSC analysis of liquid precursors E1 and E2 with different titanium concentrations are shown in Figure 1. According to the data presented, studied precursors demonstrate similar thermal behavior during the heating process. At the same time, the thermal effects are more pronounced for the precursor with higher titanium concentration (E2) and just these results (see Figure 1B) will be discussed in detail.
simultaneously with the removal of volatile organic decomposition products. The increase in sample weight observed on the TG curve ( Figure 1B, curve 2) shows that TiO oxidation is the dominant process. At the same time, the predominance of the endothermic evaporation process is observed as well (see curve 1 in Figure 1B). A weak exothermic peak at ~318 °C (fragment IV) on the DSC curve is caused by the combustion of gaseous organic residue. In the temperature range, ~433-561 °С (fragment V), an intense asymmetric exothermic peak assumes the superposition of several thermal effects: crystallization of an amorphous phase, anatase-to-rutile polymorphic transformation and pyrocarbon burnout. Thus, according to the analysis of the obtained results, it could be assumed that upon heating, the organic decomposition product (most likely, ketone) is removed after forming TiO and its oxidation to TiO2. For comparison, the thermal behavior of a solid precursor P (titanium oxyhydrate sample) was also studied. The presented thermogram ( Figure 2, curve 1) shows two en- The precursor E2 is thermally stable up to a temperature of~30 • C. The first endothermic peak on the DSC curve at~153 • C is accompanied by active sample weight loss. In the temperature range from~30 • C to~153 • C (fragment I), this weight loss is mainly associated with the removal of free extractant (valeric acid) and co-extracted water, while at a further temperature increase (fragment II)-with the decomposition of the titanium (IV) carboxylate. The second broad asymmetric endothermic peak is observed at~180-306 • C (fragment III). In the region of this peak, the decomposition of titanium carboxylate still continues and is followed by active evaporation of the organic decomposition product (probably, dibutyl ketone C 4 H 9 COC 4 H 9 with T boiling = 182-187 • C). According to the TG curve, the weight loss reaches~82% at~210 • C and stops. A further temperature rise to 700 • C is accompanied by a gradual increase in the sample weight by~4%. That is most likely associated with the gradual oxidation of titanium monoxide TiO to TiO 2 using the proposed in Ref. [53] decomposition mechanism of the metal (IV) carboxylate via the formation of metal monoxide as intermediate. Moreover, at~210-300 • C, this process occurs simultaneously with the removal of volatile organic decomposition products. The increase in sample weight observed on the TG curve ( Figure 1B, curve 2) shows that TiO oxidation is the dominant process. At the same time, the predominance of the endothermic evaporation process is observed as well (see curve 1 in Figure 1B). A weak exothermic peak at~318 • C (fragment IV) on the DSC curve is caused by the combustion of gaseous organic residue. In the temperature range,~433-561 • C (fragment V), an intense asymmetric exothermic peak assumes the superposition of several thermal effects: crystallization of an amorphous phase, anatase-to-rutile polymorphic transformation and pyrocarbon burnout. Thus, according to the analysis of the obtained results, it could be assumed that upon heating, the organic decomposition product (most likely, ketone) is removed after forming TiO and its oxidation to TiO 2 .
For comparison, the thermal behavior of a solid precursor P (titanium oxyhydrate sample) was also studied. The presented thermogram ( Figure 2, curve 1) shows two endothermic and one exothermic peak. The endothermic effect observed at~56-150 • C (fragment I) is accompanied by active weight loss (~20%) of the dried precipitate due to the removal of adsorbed water. With a further increase in temperature, the sample loses crystallization water. This process is accompanied by a wide endothermic peak at~208-308 • C (fragment II) and a small weight loss (~5%). The intense exothermic peak at~776 • C (fragment III) is associated with the crystallization of titanium dioxide and polymorphic anatase-to-rutile transformation. Ongoing slight loss of sample weight is probably due to the continuation of the dehydration process.
dothermic and one exothermic peak. The endothermic effect observed at ~56-150 °C (fragment I) is accompanied by active weight loss (⁓20%) of the dried precipitate due to the removal of adsorbed water. With a further increase in temperature, the sample loses crystallization water. This process is accompanied by a wide endothermic peak at ~208-308 °C (fragment II) and a small weight loss (⁓5%). The intense exothermic peak at ~776 °C (fragment III) is associated with the crystallization of titanium dioxide and polymorphic anatase-to-rutile transformation. Ongoing slight loss of sample weight is probably due to the continuation of the dehydration process. According to Figures 1 and 2, due to the different chemical compositions, the thermal behavior of the studied precursors differs significantly. Thus, the observed weight loss of the precursor P upon heating is associated with successive dehydration processes. At the same time, thermal transformations in precursors E are associated with the complex decomposition of titanium carboxylate, which is preceded by the evaporation processes of the solvent (valeric acid) and co-extracted water being the parts of the extracts.

XRD Analysis
To obtain a solid final product from precursors E1 and E2, based on the TG-DSC results (see Figure 1), the minimal temperature of pyrolysis was chosen as 350 °C. To study the effect of the precursor preparation method on the phase composition, anatase-to-rutile transformation temperature, and the mean crystallite size of TiO2, heat treatment of precursors E and P was carried out in the range from 450 °C to 750 °C with a temperature step of 100 °C. Table 1 summarizes the results of the XRD analysis of all the produced materials.
The study of the regularities of phase formation during the pyrolysis of the precursor E1 testifies ( Figure 3, Table 1, samples E1-1-E1-6) that amorphous powders are produced at temperatures of 350 °C and 400 °C. The crystallization of anatase polymorph begins at 450 °C, while the polymorphic anatase-to-rutile transformation starts at 650 °C. TiO2 powder produced at 750 °C contains rutile polymorph with only a small anatase admixture (WA = 1.1%).
According to the XRD analysis ( Figure 4, Table 1, samples E2-1-E2-6), the heat treatment of a more concentrated precursor E2 at 400 °C corresponds to the beginning of the anatase phase crystallization. Pyrolysis of the precursor at 550 °C and 650 °C leads to the gradual polymorphic transformation of anatase to rutile with a simultaneous increase in the mean crystallite size of anatase from ~20 nm to ⁓35 nm and of rutile from ⁓30 nm to ~45 nm, respectively. As a result of heat treatment at 750 °C, a monophase product consisting of a rutile polymorph with dR ~53 nm is formed. According to Figures 1 and 2, due to the different chemical compositions, the thermal behavior of the studied precursors differs significantly. Thus, the observed weight loss of the precursor P upon heating is associated with successive dehydration processes. At the same time, thermal transformations in precursors E are associated with the complex decomposition of titanium carboxylate, which is preceded by the evaporation processes of the solvent (valeric acid) and co-extracted water being the parts of the extracts.

XRD Analysis
To obtain a solid final product from precursors E1 and E2, based on the TG-DSC results (see Figure 1), the minimal temperature of pyrolysis was chosen as 350 • C. To study the effect of the precursor preparation method on the phase composition, anatase-torutile transformation temperature, and the mean crystallite size of TiO 2 , heat treatment of precursors E and P was carried out in the range from 450 • C to 750 • C with a temperature step of 100 • C. Table 1 summarizes the results of the XRD analysis of all the produced materials.
The study of the regularities of phase formation during the pyrolysis of the precursor E1 testifies ( Figure 3, Table 1, samples E1-1-E1-6) that amorphous powders are produced at temperatures of 350 • C and 400 • C. The crystallization of anatase polymorph begins at 450 • C, while the polymorphic anatase-to-rutile transformation starts at 650 • C. TiO 2 powder produced at 750 • C contains rutile polymorph with only a small anatase admixture (W A = 1.1%).
According to the XRD analysis ( Figure 4, Table 1, samples E2-1-E2-6), the heat treatment of a more concentrated precursor E2 at 400 • C corresponds to the beginning of the anatase phase crystallization. Pyrolysis of the precursor at 550 • C and 650 • C leads to the gradual polymorphic transformation of anatase to rutile with a simultaneous increase in the mean crystallite size of anatase from~20 nm to~35 nm and of rutile from~30 nm to~45 nm, respectively. As a result of heat treatment at 750 • C, a monophase product consisting of a rutile polymorph with d R~5 3 nm is formed.
Thus, an increase in titanium concentration in the precursor solution from 0.14 M to 0.50 M decreases the temperature of anatase-to-rutile transformation by~100 • C (see Figures 3 and 4, Table 1). Thus, an increase in titanium concentration in the precursor solution from 0.14 M to 0.50 M decreases the temperature of anatase-to-rutile transformation by ⁓100 °C (see Figures 3 and 4, Table 1).    According to XRD analysis, precursor P is amorphous ( Figure 5, Table 1). The anatase phase, produced as a result of the heat treatment of precursor P, is similar to these at the precursor E1 treatment at 450 °C and 550 °C. The increase of the processing temperature to 650 °C or 750 °C leads to the formation of two phases of TiO2. Moreover, depending on the heat treatment temperature, either anatase or rutile is a dominating phase ( Figure 5, samples P-3 and P-4). It was also found two processes occur simultaneously at 750 °C: the polymorphic transformation of anatase into rutile and the crystallization of the admixture phase, Na2Ti6O13. This phase is a product of the interaction of NaOH with TiO2 at high temperatures, i.e., during the preparation of a precursor P, it is impossible to completely remove the residual amounts of NaOH by washing the precipitate (gel). In the case of the EPM, a system of two immiscible liquids is used, and the target product (titanium carboxylate) is dissolved in the organic phase, while water-soluble reaction components, in the aqueous phase. Hence, the presence of impurity phases in the TiO2 samples was not established (see Figures 3 and 4).  According to XRD analysis, precursor P is amorphous ( Figure 5, Table 1). The anatase phase, produced as a result of the heat treatment of precursor P, is similar to these at the precursor E1 treatment at 450 • C and 550 • C. The increase of the processing temperature to 650 • C or 750 • C leads to the formation of two phases of TiO 2 . Moreover, depending on the heat treatment temperature, either anatase or rutile is a dominating phase ( Figure 5, samples P-3 and P-4). It was also found two processes occur simultaneously at 750 • C: the polymorphic transformation of anatase into rutile and the crystallization of the admixture phase, Na 2 Ti 6 O 13 . This phase is a product of the interaction of NaOH with TiO 2 at high temperatures, i.e., during the preparation of a precursor P, it is impossible to completely remove the residual amounts of NaOH by washing the precipitate (gel). In the case of the EPM, a system of two immiscible liquids is used, and the target product (titanium carboxylate) is dissolved in the organic phase, while water-soluble reaction components, in the aqueous phase. Hence, the presence of impurity phases in the TiO 2 samples was not established (see Figures 3 and 4).  According to XRD analysis, precursor P is amorphous ( Figure 5, Table 1). The anatase phase, produced as a result of the heat treatment of precursor P, is similar to these at the precursor E1 treatment at 450 °C and 550 °C. The increase of the processing temperature to 650 °C or 750 °C leads to the formation of two phases of TiO2. Moreover, depending on the heat treatment temperature, either anatase or rutile is a dominating phase ( Figure 5, samples P-3 and P-4). It was also found two processes occur simultaneously at 750 °C: the polymorphic transformation of anatase into rutile and the crystallization of the admixture phase, Na2Ti6O13. This phase is a product of the interaction of NaOH with TiO2 at high temperatures, i.e., during the preparation of a precursor P, it is impossible to completely remove the residual amounts of NaOH by washing the precipitate (gel). In the case of the EPM, a system of two immiscible liquids is used, and the target product (titanium carboxylate) is dissolved in the organic phase, while water-soluble reaction components, in the aqueous phase. Hence, the presence of impurity phases in the TiO2 samples was not established (see Figures 3 and 4).

FTIR Spectroscopy
FTIR spectroscopy was used to determine the conditions for the thermal treatment of the precursor E2 that ensure complete removal of the organic component during TiO 2 production.
The FTIR spectra (see Figure 6) contain the peaks at 3449 cm −1 and 1622 cm −1, which correspond to the stretching and bending vibrations of-OH groups. Weak absorption bands at 2362 cm −1 and 2332 cm −1 in the samples are associated with the presence of carbon dioxide CO 2 absorbed from the atmosphere [54]. In the case of the samples produced at 350 • C or 400 • C ( Figure 6, samples E2-1 and E2-2), the spectra contain the absorption bands peaked at 1520 cm −1 and 1375 cm −1 , which indicate the presence of undecomposed organic residue in these materials [55,56]. The presence of TiO 2 in the studied materials is confirmed by a wide absorption band at~1000 cm −1 -400 cm −1 associated with the vibrations of Ti-O-Ti bonds in the TiO 2 lattice [57,58]. In the mentioned spectral region, a shift of the maximum from 515 cm −1 to 442 cm −1 is observed upon the decrease in the precursor pyrolysis temperature from 550 • C to 350 • C ( Figure 6, samples E2-4-E2-1). This fact may be related to the changes in the size of the produced TiO 2 particles, as described earlier in [41,59]. This is also consistent with the results of our XRD analysis (Table 1), under which a decrease in the pyrolysis temperature of the precursor E2 in this temperature range leads to a decrease in the mean crystallite size of anatase from 20 nm to 5 nm, and, finally, to amorphization. Thus, according to our results, to produce organic impurity-free TiO 2 powders via the EPM, the minimal pyrolysis temperature of the extracts (precursors E) should be 450 • C. The data obtained do not contradict the results of the TG-DSC analysis ( Figure 1) presented above. A similar picture was observed at the comparison of the infrared spectra for bulk and nanosized AlN and LaPO 4 [60][61][62].

FTIR Spectroscopy
FTIR spectroscopy was used to determine the conditions for the thermal treatment of the precursor E2 that ensure complete removal of the organic component during TiO2 production. The FTIR spectra (see Figure 6) contain the peaks at 3449 cm −1 and 1622 cm −1, which correspond to the stretching and bending vibrations of-OH groups. Weak absorption bands at 2362 cm −1 and 2332 cm −1 in the samples are associated with the presence of carbon dioxide CO2 absorbed from the atmosphere [54]. In the case of the samples produced at 350 °C or 400 °C ( Figure 6, samples E2-1 and E2-2), the spectra contain the absorption bands peaked at 1520 cm −1 and 1375 cm −1 , which indicate the presence of undecomposed organic residue in these materials [55,56]. The presence of TiO2 in the studied materials is confirmed by a wide absorption band at ⁓1000 cm −1 -400 cm −1 associated with the vibrations of Ti-O-Ti bonds in the TiO2 lattice [57,58]. In the mentioned spectral region, a shift −1 −1 Figure 6. FTIR spectra of the samples produced from the precursor E2 at different pyrolysis temperatures: E2-1-350 • C, E2-2-400 • C, E2-3-450 • C, E2-4-550 • C. Nanoparticles with a mean size of ∼8 nm can be observed that is in line with the XRD results (d A = 9 nm, Table 1).

Transmission Electron Microscopy
are formed as a result of the low-temperature treatment (450°C) of both precursors ( Figure  7A,C). Nanoparticles with a mean size of ∼8 nm can be observed that is in line with the XRD results (dA = 9 nm, Table 1).
In the case of the EPM, the increase in the pyrolysis temperature up to 750 °C leads to the formation of layered aggregates consisting of the faceted particles with a mean size of ~11 nm ( Figure 7B). It is possible that the formation of such structures is associated with the thermal behavior of the precursor upon heating (see Section 3.2), in particular, with the effect of the pyrolysis products of the organic precursor on the nanoparticle surface. The average size of the aggregates is about 58 nm that is consistent with the XRD data (dR = 53 nm, Тable 1).

Conclusions
This study suggests an original two-stage approach for synthesizing nanocrystalline TiO2 powders-the extraction-pyrolytic method (EPM).
The conditions for preparing titanium-containing extracts (precursors) using valeric acid without a diluent as an extractant were determined. The minimum temperature of pyrolysis (450 °C) of the precursors for organic impurity-free nanocrystalline TiO2 production was established. We have shown that the phase composition of the resulting powders is affected by the pyrolysis temperature and titanium concentration in the precursor solution. According to the XRD results, depending on pyrolysis conditions, the produced TiO2 samples contain anatase (dA ~8-15 nm), mixed anatase-rutile or rutile (dR ~53 nm) polymorphs. We have shown that the decrease in titanium concentration in the precursor In the case of the EPM, the increase in the pyrolysis temperature up to 750 • C leads to the formation of layered aggregates consisting of the faceted particles with a mean size of 11 nm ( Figure 7B). It is possible that the formation of such structures is associated with the thermal behavior of the precursor upon heating (see Section 3.2), in particular, with the effect of the pyrolysis products of the organic precursor on the nanoparticle surface. The average size of the aggregates is about 58 nm that is consistent with the XRD data (d R = 53 nm, Table 1).

Conclusions
This study suggests an original two-stage approach for synthesizing nanocrystalline TiO 2 powders-the extraction-pyrolytic method (EPM).
The conditions for preparing titanium-containing extracts (precursors) using valeric acid without a diluent as an extractant were determined. The minimum temperature of pyrolysis (450 • C) of the precursors for organic impurity-free nanocrystalline TiO 2 production was established. We have shown that the phase composition of the resulting powders is affected by the pyrolysis temperature and titanium concentration in the precursor solution. According to the XRD results, depending on pyrolysis conditions, the produced TiO 2 samples contain anatase (d A~8 -15 nm), mixed anatase-rutile or rutile (d R~5 3 nm) polymorphs. We have shown that the decrease in titanium concentration in the precursor solution from 0.50 to 0.14 M leads to the increase of the temperature of anatase-to-rutile polymorphic transformation by~100 • C. Comparative analysis of the results for the materials produced by two methods-the EPM and the precipitation, revealed some differences. According to the XRD data, as a result of the heat treatment at 750 • C, impurity phases were not detected in the EPM-produced materials, while the Na 2 Ti 6 O 13 impurity phase was identified in the material produced by the precipitation method.
The results presented in this study could serve as a solid basis for further developing the EPM for the cheap and simple production of nanocrystalline TiO 2 -based materials in the form of doped nanocrystalline powders, thin films and composite materials.