the solvent role on the hydrolysis-condensation processes ... · de] during the sol – gel...

Journal of Chemical Technology and Metallurgy, 54, 2, 2019

292

The solvenT role on The hydrolysis-condensaTion processes and obTaining of Tio2 nanopowders

Albena Bachvarova-Nedelcheva1, Stancho Yordanov2, Reni Iordanova1, Irina Stambolova1

absTracT

This study is aiming to verify the influence of different solvents (ethylene glycol and isopropanol) on the degree of hydrolysis – condensation reactions of two Ti(IV) alkoxides [titanium(IV) butoxide and titanium(IV) isopropoxi-de] during the sol – gel processing and obtaining of titaniа powders. During the sol-gel synthesis the titanium(IV) alkoxide/solvent ratio was 1:1 and no water was added. It was established by XRD that the samples prepared wit-hout addition of solvents are amorphous up to 300°C. The addition of alcohols (ethylene glycol or isopropanol) to Ti(IV) isopropoxide retained the amorphous state up to 300°C. In the sample obtained by ethylene glycol and Ti(IV) butoxide, the amorphous state is stable up to 400°C. It is evident that TiO2 (anatase) is a dominating crystalline phase in the temperature range 400 - 600°C while at 700°C, TiO2 (rutile) appeared. It was established by DTA that decomposition of the organics accompanied by strong weight loss occurred in the temperature range 200-300°C. The IR and UV-Vis analyses revealed a higher degree of hydrolysis – condensation reactions in the gels obtained from Ti(IV) alkoxides and etylene glycol.

Keywords: sol-gel, phase transformations, IR spectra, UV-Vis spectra.

Received 04 October 2018Accepted 03 December 2018

Journal of Chemical Technology and Metallurgy, 54, 2, 2019, 292-302

1 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, “Acad. G. Bonchev” str. bl. 11, 1113 Sofia, Bulgaria E-mail: [email protected] of Metal Science, equipment, and technologies “Acad. A. Balevski” with Center for Hydro- and Aerodynamics at the Bulgarian Academy of Sciences, 67 “Shipchenski prohod” str., 1574 Sofia, Bulgaria

inTroducTion

Previously, titanium oxide (TiO2) as a semiconduc-tor material received special attention due to its unique optical, electrical and chemical properties. Over the past decades, it has found applications in many promising ar-eas ranging from photovoltaics and photocatalysis [1-3]. Tremendous interest has been shown also in studies of TiO2 nanomaterial structures for sensor applications [4]. Up to now, TiO2 nanostructures have been obtained by different methods such as hydrothermal, solvothermal, direct oxidation method, chemical vapor deposition, electrodeposition, etc. [5]. Among all methods, the

sol – gel technique is very useful for the preparation of amorphous and crystalline materials. It offers advantages such as the possibility of obtaining homogeneous hybrid materials at low temperature.

The preparation of new materials by the sol-gel method consists of the following steps: (i) preparation of a gel from an organic and/or inorganic sol-gel pre-cursor; (ii) gel ageing (drying under strictly controlled conditions); (iii) thermal treatment of dried gels. Gen-erally, nanosized TiO2 can be prepared by using Ti(IV) alkoxides as precursors [6, 7], although the hydrolysis and condensation reactions are not easy to control.

Alkoxides are the most common sol-gel precursor

Albena Bachvarova-Nedelcheva, Stancho Yordanov, Reni Iordanova, Irina Stambolova

293

and most work in the sol-gel field has been performed using these. They provide a convenient source of “in-organic” monomers, which in most cases are soluble in common solvents, especially alcohols. The use of alcohols allows sufficient addition of water to ensure the achieving of hydrolysis-condensation reactions [8].

Ethylene glycol was chosen for its ability to act as a bridging-chelating ligand [9-11]. Primarily, organic and inorganic condensation reactions are expected with ethylene glycol. Such reactions are desirable to maintain heterometallic units during the hydrolysis reactions, along with being a good way to promote homogeneity at the molecular level.

According to the literature data [12, 13], the use of diols as solvents is preferable due to their ability to modify the metal alkoxides and to act as chelating ligands forming bridges with other alkoxide groups. In this way, the solutions obtained become sufficiently stable. Another advantage of the alkoxide route is the possibility to control the rates by controlling hydrolysis and condensation [6, 7]. The mechanisms of these reac-tions have been extensively studied in the case of silicon alkoxides, whereas much less data is available for transi-tion – metal oxide precursors [14]. It was found that the titania sol – gel process essentially follows a different pathway from the silicon based one [15].

It is well known that the titanium alkoxides are quite sensitive to water and it is difficult to achieve control of the hydrolysis-condensation reactions [6]. It is also known that the hydrolysis reaction is influenced by a number of factors such as the nature of the alkyl (R) group, the nature of the solvent, the concentration of the species present in solution, the water to alkoxide molar ratio and the temperature [16, 17].

Our previous investigations reported on the sol-gel obtaining of powders in different binary and multicomponent systems where Ti(IV) butoxide is used as a main precursor [9 - 11, 18]. Moreover, in our recent investigations the influence of only one solvent (ethylene glycol – EG) on the hydrolysis - condensa-tion behavior of Ti(IV) n-butoxide during the sol – gel process was established. Additionally a comparison of the phase formation and short range order of the gels obtained with and without the presence of a solvent was made. During the structural characterization of samples some questions arose related to the completeness of the

hydrolysis-condensation processes and their dependence on the nature of the alkoxide, dissolvent and temperature. This motivated our research in this direction.

This study is aiming to establish and compare the role of different solvents (ethylene glycol and isopro-panol) on the degree of hydrolysis – condensation reac-tions of two Ti(IV) alkoxides [titanium(IV) butoxide and titanium(IV) isopropoxide] during the sol – gel process-ing and obtaining of titaniа powders. All experiments have been performed without the addition of water in order to prevent the non – equilibrium fast hydrolysis of Ti(IV) alkoxides.

experimenTal

preparation of gelsTwo Ti(IV) alkoxides [titanium(IV) butoxide –

TBT (> 98 %, Merck) and titanium(IV) isopropoxide – TTIP (> 98 %, Merck)] were used for obtaining titaniа powders. During the sol-gel synthesis no ad-ditional water was added. The sol-gel hydrolysis reaction was accomplished only in the presence of air moisture. Some gels were obtained without using solvents [ethylene glycol (EG, C2H6O2, > 99.5 %, Merck) or isopropanol (i-PrOH, > 99.5 %, Merck)] as a dissolvent. The TBT/EG and TTIP/EG gels were prepared by dissolving the Ti(IV) alkoxides in EG in 1:1 molar ratio. The sample TTIP/i-PrOH was prepared from TTIP following a procedure described in detail [19] with a TTIP/C3H7OH/HNO3 molar ratio of 1:20:1. The acetylacetone (AcAc, Sigma-Aldrich) was used as a chelating agent to form stable complexes with TTIP. During the procedure the following molar ratio AcAc/TTIP > 2 was kept. The obtained sol was translucent orange in color typical of the formed chelate complex. The pH of as-prepared solutions was measured 4-5. Very fast gelation occurred after mixing the prepared solutions but for the sample containing TTIP, i-PrOH and chelating agent a slower gelation was observed. The ageing of gels was performed in air for several days in order to allow further hydrolysis. Aiming to verify the phase transformations, all gels were subjected to stepwise heating in air from 200 to 700°C for one hour exposure time for each temperature. The investigated samples were denoted as follow: TBT, TTIP, TBT/EG, TTIP/EG and TTIP/i-PrOH.


294

samples characterizationPowder XRD patterns were registered at room

temperature with a Bruker D8 Advance diffractometer using Cu-Kα radiation. The average crystallite size of the powders was calculated using Sherrer’s equa-tion. The thermal behavior of the gels dried at room temperature was determined by differential thermal analysis (DTA) (LABSYSТМ EVO apparatus) with Pt-Pt/Rh thermocouple at a heating rate of 10 K/min in air flow, using Al2O3 as a reference material. The accuracy of the temperature maintenance was deter-mined ± 5°C. Gases evolved (EGA) during the thermal treatments were analyzed by mass spectrometry (MS) with a Pfeiffer OmniStarTM mass spectrometer. Mass spectra recorded for TTIP and TTIP/i-PrOH show the m/z = 14, 15, 18 and 44 signals, being ascribed to CH2, CH3, H2O and CO2, respectively. The hydrolysis process was studied by means of infrared spectroscopy. The IR spectra were registered in the range 1600-400 cm-1 using the KBr pellet technique on a Nicolet-320 FTIR spectrometer with 64 scans and a resolution of ±1 cm-1.

The optical absorption spectra of the powdered samples in the wavelength range 200-800 nm were recorded at room temperature using a UV-Vis diffused reflectance spectrophotometer “Evolution 300” using a magnesium oxide reflectance standard as the baseline at wavelength within the range of 200 – 1000 nm.

resulTs and discussion

Phase transformations and thermal stabilityTransparent gels were obtained from pure TBT,

TTIP as well in the systems TBT/EG, TTIP/EG and TTIP/i-PrOH. No gels were obtained in the systems Ti(IV) butoxide – isopropanol (1:1) and Ti(IV) isopro-poxide – isopropanol (1:1) with the applied experimental conditions. The XRD patterns of pure TBT and TTIP gels heat treated in the temperature range 200-700°C are shown in Fig. 1.

The phase transformations of pure TBT as well as TBT/EG gels were already discussed in our previous paper [20]. As a result, our attention will be focused on

Fig. 1. XRD patterns of Ti(IV) n-butoxide (TBT) and Ti (IV) isopropoxide (TTIP) at different temperatures: (A) TiO2-anatase, (R) TiO2-rutile.


295

the comparison of the phase formation of the gels (TBT, TTIP, TBT/EG, TTIP/EG and TTIP/i-PrOH) along with the obtained powders as well. As is seen in both sam-ples (TBT and TTIP) the amorphous phase is dominant up to 300°C. In both samples the first crystals of TiO2 (anatase) (JCPDS 78-2486) are registered at 400°C and it is a dominant phase at higher temperatures. The increase in temperature (600°C) led to the appearance of TiO2 (rutile) (JCPDS 21-1276) in the TTIP sample. The difference is that TiO2 (rutile) did not appeared at this temperature in the XRD pattern of the TBT sample. For TBT, the anatase to rutile phase transformation occurred at the higher temperature of 700°C. The XRD patterns of the samples obtained with addition of solvent (EG or i-PrOH) are shown in Fig. 2. As is evident all samples are amorphous up to 300°C, but the sample TBT/EG is amorphous even at 400°C. Possibly, the presence of organic groups from the dissolvent led to stabilization of the amorphous state at higher temperatures. There are some differences in the samples behavior at higher temperatures. At 600°C, the TiO2 (rutile) is detected only in the XRD pattern of sample TTIP/EG, while in the other two samples (TBT/EG and TTIP/i-PrOH) it appeared at 700°C. Moreover, it is a dominant phase in the sample TBT/EG. Obviously, EG as a solvent only in combination with Ti(IV) butoxide led to a more complete(d) transformation of TiO2 (anatase) to rutile at higher temperatures. At 500°C the average crystallite size

(calculated using Sherrer’s equation) of TiO2 (anatase) in all samples is about 5-10 nm (Figs. 1 and 2). At higher temperature (600°C) the crystallite size increased and it is about 15-20 nm.

The thermal stability of gels aged at room tem-perature was investigated by simultaneous thermogravi-metric (TG) and differential thermal analysis (DTA). The DTA/TG curves are presented in Fig. 3a,b. In our previous study the thermal stability of pure TBT gel was verified and the results obtained have been discussed and published elsewhere [20, 21]. So, in this paper the obtained data for TTIP (Fig. 3a) and TTIP/i-PrOH (Fig. 3b) will be compared with those for TBT. Several stages could be marked on the DTA/TG curves in both sam-ples by analogy with our previous results. The common feature is the presence of a weak endothermic effect at about 100°C which is the first decomposition step of the gels (Fig. 3a, b). This step could be associated with the evaporation of physically adsorbed water and/or organic solvent (isopropanol) in the sample TTIP/i-PrOH. The average mass loss after dehydration is about 10 %. The first exothermic peak in the TTIP sample (Fig. 3a) is about 235°C and it is accompanied by the mass loss of ~ 23 % which could be related to the combustion of alkox-ide groups bonded to the Ti-atom. The next exothermic effects are about 400, 580 and 680°C. The first one at about 400°C could be assigned to the crystallization of TiO2 (anatase), while those at 580°C (is) are related to the

Fig. 2. XRD patterns of TBT/EG, TTIP/EG and TTIP/i-PrOH at different temperatures: (A) TiO2-anatase, (R) TiO2-rutile.


296

phase transition TiO2 (anatase) to TiO2 (rutile). Accord-ing to the XRD analysis (Fig. 1), the TiO2 (anatase) is a dominant phase up to 500°C. The exothermic effect at 680°C is connected to the crystallization of TiO2 (rutile).

In the other sample TTIP/i-PrOH (Fig. 3b) the first exothermic effect is shifted to a higher temperature (~ 270°C) in comparison to the previous sample and it is also accompanied by the mass loss of about 10 %. Obviously, the presence of chelating agent (AcAc) and solvent (i-PrOH) increased the thermal stability of the sample. This exothermic effect (~ 270°C) could be related to the beginning of combustion of organic groups building up the AcAc and i-PrOH. The second exothermic effect is observed about 340°C and is connected to a stronger combustion process related to decomposition of organic

compounds, residual hydroxyl groups and AcAc ligands. The total mass loss of these two steps is ~ 24 %. Prob-ably, the higher amount of organics due to presence of the solvent and chelating agent led to the stepwise release of the organics. Looking at the DTA curve, one small exo-thermic effect around 440°C could be also seen and no mass loss is observed. It is assigned to the beginning of the TiO2 (anatase) crystallization as it was confirmed by XRD (Fig. 2). The last exothermic effect (530°C) is very strong and it could be associated with the intensive crys-tallization of anatase as well as the oxidation of residual carbon and release of CO2 accompanied by a weight loss about 9 % (Fig. 3b). The total mass loss in the sample TTIP/i-PrOH is about 43 % which is higher compared to pure TTIP (about 23 %). The obtained by DTA results

Fig. 3. DTA/TG curves of obtained TTIP (a) and TTIP/i-PrOH (b) gels.


297

compare well with the already indicated XRD data as well as our previous DTA/TG results concerning the thermal stability of pure TBT sample [20, 21].

IR spectroscopyТhe rate and degree of hydrolysis and condensation

processes were verified by IR spectroscopy. This method was applied also to monitoring the phase transformations in the gels during the heat treatment (in the temperature range 200-700°C). The IR spectra of both alkoxides (TBT and TTIP) obtained by us as well as of solvents used are shown in Figs. 4 and 5. The IR spectra of investigated samples are presented in Fig. 6a,b. With the aim of evaluating the degree of hydrolysis and condensation reactions it is necessary to compare the vibrational spectra of pure alkoxides (TBT, TTIP) (Fig. 4) with those of the corresponding alcohols (n-butanol and 2-propanol) (Fig. 5). Any differences between these can be correlated with structural differences between the molecules to help in understanding of the vibra-tional spectrum of the alkoxides. The assignments of the vibrational bands of separate structural units are made on the basis of well - known spectral data for the precursors

[Ti(IV) n-butoxide, Ti(IV) isopropoxide], etylen glycol and isopropanol (solvents), n-butanol and 2-propanol (products of the hydrolysis-condensation reactions), crystalline TiO2 (anatase) and TiO2 (rutile). According to the literature data [22-25] both Ti(IV) alkoxides precursors are characterized by bands located between 1500-1300 cm-1 assigned to the bending vibrations of CH3 and CH2 groups. For Ti(IV) butoxide, the band at 1130 cm-1 is characteristic for the stretching vibrations of Ti-O-C, while those at 1100 and 1040 cm-1 are assigned to the vibrations of terminal and bridging C-O bonds in butoxy ligands. For Ti(IV) isopropoxide, the characteristic bands at 1160, 1130, 1115, 990 and 850 cm-1 are very intensive in the IR spectrum and could be assigned to the vibrations of CH3, C-O and C-C-C stretching modes of the isopropoxy ligands. All these bands could also be related to the stretching vibrations of Ti-O modes [25]. Generally, the absorption bands below 1000 cm-1 in both alkoxides correspond to C-H, C-O and deformation Ti-O-C vibrations [22, 26]. On the other hand, the ethylene glycol possesses asymmetric and symmetric stretching vibrations of C-O bonds in CH2-OH groups at 1090 and 1040 cm-1 while in the

Fig. 4. IR spectra of the used alkoxides: (a) Ti(IV) isopropoxide and (b) Ti(IV) butoxide.


298

spectrum of butanol and 2-propanol they appeared about 1130-1110, 1050-1030 cm-1 along with a very intensive band at 1070 cm-1 [27, 28]. As supporting information, the spectra measured by us of ethylene glycol, n-butanol, and 2 – propanol are shown in Fig. 5. In our previous investigations [20, 21] it was found that the absorp-tion region 1100 - 1020 cm-1 is very complex due to the overlapping of the vibrations of different structural units from Ti(IV) butoxide, EG and n-butanol. In spite of that, many authors [18, 35, 36] use these bands for the interpretation of hydrolysis-condensation processes. The IR spectrum of TiO2 (anatase) is characterized by bands at 620 - 610 and 480 - 470 cm-1 [35, 36] connected with the vibrations of TiO6 units.

Generally, looking at the IR spectra of the gels obtained by us TBT and TTIP (25°C) (Fig. 6a) a differ-ence could be seen in the bands intensity in the region 1130-1040 cm-1 as well as of the band about 620 cm-1. These bands are related to the stretching vibrations of non-hydrolyzed alkoxy groups which remain bound to titanium [29]. It has to be noted that in the spectrum of TBT these bands are well defined compared to those of the TTIP sample. As is known, the hydrolysis and condensation rates decreased in the row: Ti(OEt)4 > Ti(OPri)4 > Ti(OBun)4 [6, 7, 30]. Bearing in mind that the band about 620 cm-1 is weaker in the spectra of TTIP gel it could be suggested that there is a higher degree of hydrolysis – condensation reactions in the TTIP

sample. In the spectrum of TTIP above 1000 cm-1, only one very weak band at 1120 cm-1 is observed which is an additional proof for the more completed hydrolysis – condensation reactions. Heating the gels up to 250°C (Fig. 6a) led to a decrease in the bands intensity in the ranges 1130-1040 cm-1 and 1500-1300 cm-1 in both sam-ples as a consequence of the decrease in the number of non-hydrolyzed alkoxy groups. The spectra above 300°C showed several broad bands in the region 700 - 400 cm-1 that are characteristic of the formation of a Ti-O-Ti network (Fig. 6a) [27-29]. It has to be mentioned that at 500°C the IR spectra of both samples exhibited bands around 1400 and 1200 cm-1 that could be related to the presence of residual carbon, participating in different organic groups which is in accordance with the obtained DTA data (Fig. 3).

The IR spectra of compositions obtained by the ad-dition of solvents (TBT/EG, TTIP/EG and TTIP/i-PrOH) are shown in Fig. 6b. Looking at the spectra it is seen that in the presence of solvents the IR bands are more inten-sive. Another peculiarity of the spectra is the similarity in the behavior of samples TBT/EG and TTIP/EG in the absorption ranges 1130-1040 cm-1 and 910-790 cm-1. It is obvious that the band at 1080 cm-1 is very intensive due to the separation of butanol or 2-propanol as a result of the greater degree of hydrolysis-condensation reactions as compared to the pure alkoxides (Fig. 6a). The spectral behavior of the third sample TTIP/i-PrOH in the region

Fig. 5. IR spectra of the used solvents: ethylene glycol, butanol and isopropanol.


299

1200 – 800 cm-1 is characterized by very weak bands (1190, 1160, 1130 cm-1) as a result of the more completed hydrolysis reactions. Moreover, the strong band around 620 cm-1 related to the non-hydrolyzed alkoxy groups is missing (Fig. 6b). The bands below 1100 cm-1 are con-nected to the vibrations of different organic groups from the solvent, chelating agent and Ti(IV) isopropoxide [29, 31]. The strong band at 660 cm-1 is assigned to the vibrations of CH3-CO-C groups from the AcAc [29, 31].

The intensive band at 440 cm-1 is related to the vibra-tions of AcAc groups bound to titanium. The chelation between AcAc and TTIP is also proved by the presence of bands at 1590 and 1360 cm-1 [32]. The gelation for sample TTIP/i-PrOH was slower in comparison to the samples obtained with EG as a result of the chelating complex formation. The heating of all samples in the range 200-300°C, led to a decrease in the intensity of or-ganic groups and their subsequent disappearance above

Fig. 6a. IR spectra of the pure Ti(IV) alkoxide gels.

Fig. 6b. IR spectra of the investigated samples obtained from Ti(IV) alkoxides with ethylene glycol or isopropanol.


300

300°C. By analogy with the previous IR spectra of pure alkoxides, all spectra above 400°C showed broad bands in the region 700 - 400 cm-1 that are characteristic for the formation of a Ti-O-Ti network (Fig. 6b) [29, 31-34]. In the IR spectra of TTIP/EG and TTIP/i-PrOH heated at 500°C, a weak broad band about 800 cm-1 is distin-guished that is related to the residual organics, proved also by DTA (Fig. 3). The results obtained compare well with the XRD data already discussed above.

UV - Vis spectroscopyThe UV-Vis spectroscopy is used in order to obtain

additional structural information for the investigated samples. The optical absorption spectra of gels (25°C) (TBT, TTIP, TBT/EG and TTIP/EG) and heated at 200°C samples are presented in Fig. 7. The interpretation of the UV-Vis spectra is made on the basis of some litera-ture data [35, 36] as well as our previous experimental results in various systems containing TiO2 [20, 21, 37-38]. Generally, the UV–Vis spectra exhibit two maxima about 240 – 260 nm and 310 – 340 nm. According to the

literature data [35] for the isolated TiO4 units, the ligand to metal charge transfer band is in the region 200 – 260 nm, while in a titania network (anatase) the charge transfer in TiO6 groups is above 300 nm. As was confirmed in our previous investigations, during the hydrolysis – condensa-tion processes the coordination geometry is changed from TiO4 to TiO6 as a result of polymerized Ti species (Ti–O–Ti links between TiO6 units) [36]. As can be seen from the figure, in the pure TBT and TTIP gels the intensity of UV peaks at 240-250 nm and that near 320 nm are comparable (Fig. 7a,b), which is an indication of the negligible differ-ence in the TiO4 and TiO6 amount in the gel network. The same is also observed in TBT and TTIP samples heated at 200°C (Fig. 7a-e). The addition of solvent (EG) to both alkoxides did not cause any difference in the spectra, however, in the sample simultaneously containing TTIP, i-PrOH, AcAc (Fig. 7e) it can be seen that the amount of TiO6 units is higher than those of TiO4. Accordingly, it may be concluded that this combination of precursors is more appropriate for achieving a greater degree of polyconden-sation processes (Fig. 7e). Looking at all UV-Vis spectra

Fig. 7. UV-Vis spectra of pure Ti alkoxides gels and heated at 200°C samples: (a, b) – pure Ti(IV) alkoxides, (c, d, e) – TBT/EG, TTIP/EG and TTIP/i-PrOH/AcAc.


301

of heated samples (pure and with solvents) an increase of the absorption above 400 nm could be seen which may be explained by the presence of carbon. It is known that carbon is found to be a strong factor responsible for visible light absorption above 400 nm [39].

conclusionsPure titania gels are prepared from two different

Ti(IV) alkoxides with and without the addition of different solvents (ethylene glycol and isopropanol) in the presence of only air moisture. Both solvents are suitable for obtaining transparent and homogeneous gels. The presence of ethylene glycol preserved the mixed organic–inorganic amorphous structure at higher temperatures (400°C). In compositions containing Ti(IV) isopropoxide TiO2 (anatase) crystallized about 400°C. In all compositions the phase transition TiO2 (anatase) to TiO2 (rutile) is registered above 600°C, but in samples TBT/EG it is not detected even at 700°C. It was sug-gested that the nanosized powders obtained would exhibit improved photocatalytic properties which could be useful for environmental applications.

AcknowledgementsThe authors are grateful to the financial support of

Bulgarian National Science Fund at the Ministry of Edu-cation and Science, Contract No DN07/2 14.12.2016.

references

1. O. Carp, C. Z. Huisman, A. Reller, Progress in solid state chemistry, 32, 2004, 33-177.

2. S. Sakka, Processing, characterization and applications, vol. I, ed. H. Kozuka, Kluwer Acad. Publishers, 2005, Boston-Dordrecht-London.

3. A. Zaleska, Doped-TiO2: a review, Recent Patents on Engineering, 2, 2008, 157-164.

4. J. Bai and B. Zhou, Titanium dioxide nanomaterials for sensor applications, Chem. Rev., 114, 19, 2014, 10131-10176.

5. M. Malekshahi Byranvand, A. Nemati Kharat, L. Fatholahi, Z. Malekshahi Beiranvan, A review on synthesis of nano-TiO2 via different methods, J. Nanostructures, 3, 2013, 1-9.

6. C. J. Brinker, G. W. Sherer, Sol-Gel Science, Academic Press, 1990, p. 69.

7. H. Schmidt, Chemistry of material preparation by the

sol-gel process, J. Non – Cryst. Sol., 100, 1988, 51-64.8. M. Guglielmi, G. Carturan, Precursors for sol-gel

preparations, J. Non-Cryst. Sol., 100, 1988, 16-30.9. M. Gonzales, A. Wu, P. Vilarinho, Influence of Solvents

on the Microstructure and Dielectric Properties of Ba0.5Sr0.5TiO3 Thin Films Prepared by a Diol-Based Sol-Gel Process, Chem. Mater., 18. 2006, 1737-1744.

10. C. Livage, A. Safari, L. Klein, Glycol-based sol-gel process for the fabrication of ferroelectric PZT thin films, J. Sol-Gel Sci. Technol., 2, 1-3, 1994, 605-609.

11. S. Cristoni, L. Armelao, S. Gross, E. Tondello, P. Traldi, Electrospray ionization in the study of the polycondensation of Ti(O-i-C3H7)4 and Ti(O-n-C4H9)4, Rapid Commun. Mass Spectrom., 14, 2000, 662-668.

12. M. Gotic, M. Ivanda, A. Sekulic, S. Music, S. Popovic, A. Turkovic, K. Furic, Microstructure of nanosized TiO2 obtained by sol-gel synthesis, Mater. Lett., 28 1996, 225.

13. K. Yu, J. Zhao, Y. Guo, X. Ding, H. Bala, Y. Liu, Z. Wang, Sol–gel synthesis and hydrothermal processing of anatase nanocrystals from titanium n-butoxide, Mater. Lett., 59, 2005, 2515-2518.

14. M. E. Simonsen, E. G. Sogaard, Sol–gel reactions of titanium alkoxides and water: influence of pH and alkoxy group on cluster formation and properties of the resulting products, J. Sol-Gel Sci. Technol., 53, 2010, 485-497.

15. A. Shalaby, Y. Dimitriev, R. Iordanova, A. Bachvarova-Nedelcheva and Tz. Iliev, Modified sol-gel synthesis of submicron powders in the system ZnO-TiO2, J. Univ. Chem. Technol. Metall., 46, 2, 2011, 137 - 142.

16. A. Stoyanova, Y. Dimitriev, A. Shalaby, A. Bachvarova-Nedelcheva, R. Iordanova, M. Sredkova, Antibacterial properties of ZnTiO3 synthesized by sol-gel method, J. Optoel. Biomed. Mater., 3, 2011, 24-29.

17. A. Stoyanova, A. Bachvarova-Nedelcheva, R. Iordanova, N. Ivanova, H. Hitkova, M. Sredkova, Synthesis, photocatalytic and antibacterial properties of nanosized ZnTiO3 powders obtained by different sol–gel methods, Digest J. Nanomater. Biostr., 7, 2012, 777-784.

18. A. Bachvarova-Nedelcheva, R. Gegova, A. Stoyanova, R. Iordanova, V. E. Copcia, N. Ivanova, I. Sandu, Synthesis, characterization and properties of ZnO/TiO2 powders obtained by combustion gel method, Bulg. Chem. Commun., 46, 2014, 585-593.

19. V. Blaskov, I. Ninova, L. Znaidi, I. Stambolova, SEM characterization of spin-coated nanocrystalline


302

TiO2 thin film influenced by the presence of acetyl acetone during the sol preparation, Nanoscience & Nanotechnology, 4, eds. E. Balabanova, I. Dragieva, Heron Press, Sofia, 2004.

20. St. I. Yordanov, A. D. Bachvarova-Nedelcheva, R. S. Iordanova, Influence of ethylene glycol on the hydrolysis-condensation behavior of Ti(IV) butoxide, Bulg. Chem. Commun., 49 (special issue D), 2017, 265-270.

21. R. Iordanova, A. Bachvarova-Nedelcheva, R. Gegova, Y. Dimitriev, Sol-gel synthesis of composite powders in the TiO2-TeO2-SeO2 system, J. Sol-Gel Sci. Technol., 79, 1, 2016, 12-28.

22. S. Doeuff, M. Henry, C. Sanchez, J. Livage, Hydrolysis of titanium alkoxides: Modification of the molecular precursor by acetic acid, J. Non-Cryst. Sol., 89, 1987, 206-216.

23. M. Henry, J. Leavage, C. Sanchez, Sol-gel chemistry of transition metal oxides, Progr. Sol. State Chem., 18, 1988, 259-341.

24. M. J. Velasco, F. Rubio, J. Rubio, J. Oteo, Hydrolysis of Titanium Tetrabutoxide. Study by FT-IR Spectroscopy, Spectr. Lett., 32, 1999, 289-304.

25. P. D. Moran, G. A. Bowmaker, R. P. Cooney, Vibrational spectra and molecular association of titanium tetraisopropoxide, Inorg. Chem., 37, 1998, 2741-2748.

26. S. Barboux-Doeuff, C. Sanchez, Synthesis and characterization of titanium oxide-based gels synthesized from acetate modified titanium butoxide precursors, Mater. Res. Bull., 29, 1994, 1-13.

27. O. Zubkova, A. Shabadash, Dimeric, and Trimeric Ethylene Glycol, Zh. Prikl. Spektr., 14, 5, 1971, 874.

28. K. Kato, A. Tsuge, K. Niihara, Microstructure and Crystallographic Orientation of Anatase Coatings Produced from Chemically Modified Titanium Tetraisopropoxide, J. Amer. Ceram. Soc., 79, 6, 1996, 1483-1488.

29. A. Leustic, F. Babonneau, J. Livage, Structural investigations of the hydrolysis-condensation process ot titanium alkoxides Ti(OR)4 (OR=OPri, OEt) modified by AcAc, 1. Study of the alkoxide

modification, Chem Mater, 1, 1989, 240-247.30. C. Sanchez, J. Livage, M. Henry, F. Babonneau,

Chemical modification of alkoxide precursor, J. Non-Cryst. Sol., 100, 1988, 65-76.

31. A. Leustic, F. Babonneau, J. Livage, Structural investigations of the hydrolysis-condensation process ot titanium alkoxides Ti(OR)4 (OR=OPri, OEt) modified by AcAc, 2. From the modified precursor to the colloids, Chem. Mater., 1, 1989, 248-252.

32. K. Siwinska-Stefanska, J. Zdarta, D. Paukszta, T. Jesionowski, The influence of addition of a catalyst and chelating agent on the properties of titanium dioxide synthesized via the sol-gel method, J. Sol-Gel Sci. Technol., 75, 2015, 264-278.

33. R. Iordanova, R. Gegova, A. Bachvarova-Nedelcheva, Y. Dimitriev, “Sol-gel synthesis of composites in the ternary TiO2-TeO2-B2O3 system, Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B, 56, 2015, 128-138.

34. A. Shalaby, A. Bachvarova-Nedelcheva, R. Iordanova, Y. Dimitriev, A. Stoyanova, H. Hitkova, N. Ivanova, Sol-gel synthesis and properties of nanocomposites in the Ag/TiO2/ZnO system”, J. Optoel. Adv. Mater., 17, 1-2, 2015, 248-256.

35. X. Gao, IE Wachs, Titania-silica as catalysts: molecular structural characteristics and physico-chemical properties, Cat Today, 51, 1999, 233-254.

36. V. Barlier, V. Bounor-Legare, G. Boiteux, J. Davenas, Hydrolysis–condensation reactions of titanium alkoxides in thin films: A study of the steric hindrance effect by X-ray photoelectron spectroscopy, Appl Surf Sci., 254, 2008, 5408-5412.

37. R. Gegova, A. Bachvarova-Nedelcheva, R. Iordanova, Y. Dimitriev, “Synthesis and crystallization of gels in the TiO2-TeO2-ZnO system”, Bulg. Chem. Commun., 47, 1, 2015, 378-386.

38. R. Gegova, R. Iordanova, A. Bachvarova-Nedelcheva, Y. Dimitriev, “Synthesis, structure and properties of TiO2-TeO2-MnOm (M= Zn, B) gels: A comparison”, J. Chem. Technol. Metall., 50, 4, 2015, 449-458.

39. C. Wang, B-Q Xu, X Wang, J. Zhao, Preparation and photocatalytic activity of ZnO/TiO2/SnO2 mixture, J Solid State Chem., 178, 2005, 3500-3506.

the solvent role on the hydrolysis-condensation processes ... · de] during the sol – gel...

Documents