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81
Synthesis of Nanocrystalline Titania via Microwave-Assisted Homogeneous Hydrolysis Under Hydrothermal Conditions Egor A. Moskalenko1, Alexey A. Sadovnikov2, Alexander E. Baranchikov2*, Anastasia E. Goldt1,2, Vladimir V. Kozik3 and Vladimir K. Ivanov1,2 1
Materials Science Department, Moscow State University, 119991, Moscow, Russia; 2Kurnakov Institute of General and Inorganic Chemistry, 119991, Moscow, Russia; 3Tomsk State University, 634050, Tomsk, Russia Abstract: In this paper, we report on a new procedure for nanocrystalline titania synthesis using microwave-assisted homogeneous hydrolysis under hydrothermal conditions. We show that this synthetic procedure promotes formation of low-aggregated TiO2 powders with very high specific surface area (up to 240 m2/g) and small particle size (5-7 nm), and that it hinders SO42– ions’ sorption on the surface of nanoparticles.
Keywords: Hexamethylenetetramine, homogeneous hydrolysis, nanocrystalline materials, microwave-hydrothermal synthesis, titania. INTRODUCTION Microwave heating is widely used for the synthesis of a variety of advanced inorganic materials such as sorbents, catalysts, magnetic materials, dense ceramics etc. [1, 2]. As a rule, microwave-assisted synthesis is time and energy saving, in comparison with traditional methods [3]. Moreover, in some cases, microwave treatment provides results that otherwise cannot be achieved by other methods [4, 5]. For instance, microwave treatment facilitates formation of some phases, like layered titanium phosphates [6] and yttrium ferrite [7]. This is due to a number of advantages of microwave heating over the usual methods, in particular, the high speed, high efficiency and low inertia of heating, the absence of contact between the heated body and the heater, the uniformity of heating of the reaction media, and the possibility of selective heating of reaction mixture components [4, 8-10]. On a wide variety of examples, it was shown that numerous advantages of microwave heating are indeed applicable to the synthesis of nanomaterials [11-13]. Recently, microwave-assisted hydrothermal (MW-HT) synthesis was proposed, to synthesize individual oxides, including ZnO [14], γ-Ga2O3 [15], Co3O4 [16] and CeO2 [17, 18], as well as complex oxides such as Bi12TiO20 [19], Bi2 MoO6 [20] and SrTiO3 [21]. MW-HT synthesis was also used to obtain nanocrystalline titania [22, 23] - an efficient photocatalytic material which is promising for solar energy conversion applications [24]. Recently, we have suggested a novel modification of microwave-assisted hydrothermal synthesis, so-called microwave-assisted homogeneous hydrolysis under hydrothermal conditions, which allows additional control over the microstructure and properties of nanocrystalline oxide materials [25]. *Address correspondence to this author at the Kurnakov Institute of General and Inorganic Chemistry, Leninsky, 31, Moscow, 119991, Russia; Tel: +7(495)6338534; Fax: +7(495)9541279; E-mail:
[email protected] 2213-3364/14 $58.00+.00
It is well known that, at temperatures below 100°C, hexamethylenetetramine (HMT) hydrolyzes slowly, according to the reaction [26, 27]: C6H12N4 + 6H2O → 6H2 CO + 4NH3 Ammonia that is produced during the HMT hydrolysis precipitates metal ions in the form of metal hydroxocompounds or oxides. As the HMT hydrolysis is a homogeneous process, high local supersaturations of the solution are absent, so the newly formed nanoparticles are characterized by narrow particle size distributions and chemical homogeneity. However, the rather slow hydrolysis rate results in a considerable duration (up to 10-100 h) of synthesis. We have established that, under hydrothermal conditions, the rate of HMT hydrolysis rises dramatically [25], thus increasing the rate of hydrolysis of metal salts, and promoting the formation of nanocrystalline materials, in comparison with conventional hydrothermal or hydrothermal-microwave synthesis. The present work aims at the synthesis of nanocrystalline titania using microwave-assisted hydrothermal hydrolysis of titanium oxysulfate in the presence of HMT. We have shown that such a method allows obtaining nanocrystalline titania with very small particle size (5-7 nm) and high specific surface area (up to 240 m2 /g), in a wide range of synthesis conditions. MATERIALS AND METHODS Nanocrystalline titania was prepared from aqueous solutions containing titanium oxysulfate sulfuric acid hydrate (Aldrich, synthesis grade), and HMT (puriss. p.a.). Molar ratio TiOSO4: HMT was fixed at 1.0:0.2, with TiOSO4 concentration equal to 0.3 M. Blank specimens were synthesized without the addition of HMT into the reaction mixture. Starting solutions were placed in DAP-100 100 mL polytetrafluoroethylene autoclaves (with a filling coefficient of © 2014 Bentham Science Publishers
82 Current Microwave Chemistry, 2014, Vol. 1, No. 2
~30%), and subjected to microwave–hydrothermal (MWHT) treatment in a Berghof Speedwave MWS four setup at 130°С, 170°С, and 200°С, for 15 and 60 minutes. The heating rate in all cases was 30°C/min. After the synthesis was complete, the autoclaves were cooled in air; the solid products were isolated by centrifugation (10 000 min–1), repeatedly washed with distilled water and then dried in air for 24 h at 50°С. X-ray powder diffraction analysis was carried out using a Rigaku D/MAX 2500 diffractometer (CuKα radiation) with a goniometer rotation speed of 2°2θ/min. Particle sizes were estimated using the Scherrer equation: K $# , D101 = "101 (2! )$ cos(! 0 )
where θ0 is the peak position, λ is the CuKα radiation wavelength and β101(2θ) is the physical broadening of the (101) diffraction peak. The Scherrer constant K was set equal to unity. The physical broadening was calculated from:
" hkl = " ! s , where β is the full width at half maximum of the X-ray diffraction peak and s is instrumental broadening (0.09 ± 0.01°2θ). Contributions from microstresses in diffraction peak broadening were ignored. The reference used to determine instrumental broadening was a sapphire single crystal.
Moskalenko et al.
RESULTS AND DISCUSSION According to our previously reported calorimetric data [25], HMT hydrolysis, which occurs upon heating of aqueous HMT solutions, is an endothermal process, and starts at ~45°C - 50°C. At temperatures up to 120°C - 125°С, the hydrolysis rate is comparatively low, and at 130°C - 140°С it increases abruptly, to reach a peak at 158°С. On the basis of these results we have chosen temperatures for MW-HT treatment of the starting titanium oxysulfate solutions - equal to, and higher than the temperature corresponding to the maximal rate of HMT hydrolysis. According to X-ray diffraction data, all the samples obtained using MW-HT treatment of titanium oxysulfate solutions in the presence of HMT, as well as the corresponding blanks (prepared without addition of HMT), correspond to single phase crystalline anatase (I41/amd space group, PDF2 #21-1272). Several representative diffraction patterns are given in Figure 1. No impurities, and no significant amounts of amorphous phases were found. Anatase is a metastable TiO2 polymorph which readily forms under hydrothermal treatment of titanium salts or hydrated amorphous titania [29]. The most probable reason for its formation relates to size effects, since, thermodynamically, anatase is more stable than rutile for titania particles smaller than ~13-15 nm [30, 31]. Anatase stabilization may also be due to the presence of sulfate species in the reaction media, which are sorbed on the surface of crystalline titania in the course of the crystallization process, thus retarding the transformation from anatase to rutile [32].
For determining β, after subtracting the background, the (101) peak profile in the range 20°–30°2θ was fitted by the pseudo-Voigt function: V () ) =
& 4 ln 2() ' ) 0 )2 # , # 2(1 ' c )A ln 2 2cA & (L + exp $' ! $ ! 2 * % 4() ' ) 0 ) + (L2 " (G * (G2 % "
where ωL and ωG are Lorentzian and Gaussian parameters, respectively (ωL = ωG = β), A is a normalizing factor and c is the relative contribution from the Lorentzian to the overall reflection intensity. Low temperature nitrogen adsorption measurements were conducted using an ATX-06 analyzer (KATAKON, Russia) in a 0.05–0.25 range of N2 partial pressures. Before measurement, the samples were outgassed at 200°C for 30 min under dry helium flow. Determination of the surface area was carried out using the 5-point Brunauer-Emmett-Teller (BET) method. Particle size (DBET, nm) of the powders was independently estimated from low temperature nitrogen adsorption data, as follows:
DBET =
6000 , !S
where ρ is the density of the crystalline phase (3.9 g/cm3 for anatase), S - specific surface area (m2/g) [28]. Thermal analysis was performed in air, using an SDT Q600 TGA/DSC/DTA analyzer. The heating rate was equal to 10°C/min.
Fig. (1). X-ray diffraction patterns for TiO2 samples prepared by MW-HT treatment of pure TiOSO4 solution (1, 3), and TiOSO 4 solution in the presence of HMT (2, 4). Temperature and duration of synthesis: 130°C, 15 minutes (1, 2); 200°C, 1 hour (3, 4).
Synthesis of Nanocrystalline Titania via Microwave-assisted Homogeneous
Table 1.
Current Microwave Chemistry, 2014, Vol. 1, No. 2
Structural characteristics (particle size derived from XRD and low temperature nitrogen adsorption results, and specific surface area) of TiO2 samples prepared by hydrolysis of TiOSO 4 in the presence of HMT under MW–HT conditions.
Temperature, °C
Synthesis Duration, min
DXRD, nm
SBET, m2/g
DBET, nm
130
15
5
220
7
60
5
240
6
15
6
230
7
60
6
210
7
15
7
220
7
60
7
200
8
170
200
Table 2.
83
Structural characteristics (particle size derived from XRD and low temperature nitrogen adsorption results, and specific surface area) of TiO2 samples prepared by hydrolysis of TiOSO 4 under MW–HT conditions (blanks).
Temperature, °C
Synthesis Duration, min
DXRD, nm
SBET, m2/g
DBET, nm
130
15
7
130
12
60
8
210
7
15
8
190
8
60
10
180
9
15
8
170
9
60
13
120
13
170
200
In Tables 1 and 2, the results of particle size and specific surface area measurements for titania powders prepared by HMT-assisted hydrolysis of TiOSO4 under MW-HT conditions, as well as corresponding blanks, are given. In all cases, TiO2 particle size (5–13 nm) corresponds to the anatase stability region. Experimental data show that microwave-assisted homogeneous hydrolysis of titanium oxysulfate results in titania powders with smaller particles, in comparison with blank experiments carried out without HMT added to the starting solutions. For example, hydrothermal treatment of an HMTcontaining solution of TiOSO4 at 200°C and 170°C for 1 hour provides 7 nm and 6 nm TiO2 particles. MW-HT treatment of blank solutions under the same conditions produces 13 nm and 10 nm particles, respectively. Such a sharp decrease in particle size is probably due to higher supersaturation reached in HMT-containing solutions under microwave treatment. TEM data presented in Figure. 2 corroborate XRD results. It can be clearly seen that titania particles formed under hydrothermal treatment in the presence of HMT are much smaller than in the blank experiment. X-ray powder diffraction data and TEM micrographs show good consistency with low temperature nitrogen adsorption measurements. In all cases, surface area of the powders synthesized under MW-HT treatment in the presence of HMT is 30–90 m2/g (15%-40%) higher than that of blanks.
The results of particle size calculations from specific surface area values, for all the samples, are given in Table 2. One can see that, generally, particle size calculated from BET data is nearly equal to particle size obtained using the Scherrer equation. This indicates a low degree of particle aggregation, which is also confirmed by TEM data (see Fig. 2). Further analysis of data given in Tables 1, 2 leads us to the following conclusions. Particle size of TiO2 synthesized by hydrolysis of TiOSO4 in the absence of HMT depends on both the duration and the temperature of MW-HT treatment. Increase in these parameters results in notable particle growth. On the other hand, addition of HMT into the reaction mixture levels the effect of both duration and the temperature of MW-HT treatment on the resulting TiO2 particle size, because of the higher degree of supersaturation. In the latter case, synthesized titania powders consist of 5 nm - 7 nm crystals, and possess 200 m2/g - 240 m2/g surface area. Variation of MW-HT treatment conditions also affects the surface composition of TiO2 samples. It is well known that nanocrystalline titania synthesized by high temperature hydrolysis of titanium oxysulfate contains notable amounts of physisorbed and chemically bound water, as well as residual sulfate ions [33]. In order to estimate the composition of samples prepared by the MW-HT treatment of HMTcontaining titanium oxysulfate solutions, we have conducted thermal analysis of corresponding titania powders (see Fig. 3).
84 Current Microwave Chemistry, 2014, Vol. 1, No. 2
A
Moskalenko et al.
B
Fig. (2). TEM images of titania powders synthesized from TiOSO4 solution containing HMT (A), and from pure TiOSO4 solution (B) at 200°C for 1 hour.
Fig. (3). Thermal analysis data for TiO2 powders synthesized by the MW–HT treatment of TiOSO4 in the presence of HMT (A) and pure TiOSO4 (B). 1 – 130°C, 15 min; 2, 5 – 130°C, 60 min; 3 – 200°C, 15 min; 4, 6 – 200°C, 60 min.
A noticeable loss of weight is detected upon heating of the titanium dioxide samples obtained by MW-HT treatment of TiOSO4 solutions. The value of the total weight loss depends on both the temperature and duration of the hydrothermal synthesis, and ranges between 9 and 18 wt%. In all cases, thermal analysis curves include two distinct stages of thermal decomposition. The first one starts at temperatures even below 100°C, and finishes at approximately 400°C. This stage is obviously connected with the loss of physically and chemically bonded water. The second decomposition
stage falls within the temperature range 550°C – 800°C, and corresponds to decomposition of surface-bound sulfate species. Chemical composition of samples, as estimated from thermal analysis data, is given in Table 3. These data indicate that HMT addition in the reaction mixtures results in a notable decrease of sulfur content in resulting powders. The possible reason for this effect is close to one observed in our previous work devoted to the study of
Synthesis of Nanocrystalline Titania via Microwave-assisted Homogeneous
Table 3.
t, °C
130
200
Current Microwave Chemistry, 2014, Vol. 1, No. 2
85
Thermal analysis data for samples prepared by the MW–HT treatment of TiOSO4 in the presence of HMT. HMT Added
Synthesis Duration, min
Weight Loss at 1 Stage, %
Weight Loss at 2 Stage, %
Total Weight Loss, %
Ti:S Molar Ratio
Yes
15
12.1
4.1
16.2
20
Yes
60
11.4
2.7
14.1
32
No
60
14.1
3.9
18.0
21
Yes
15
8.7
1.5
10.2
58
Yes
60
7.2
1.7
8.9
50
No
60
7.4
2.7
10.2
33
pH on the structure and composition of sulfated zirconia [34]. Precipitation pH has a great influence on the sulfur content in oxide materials prepared by the sol-gel route in the presence of sulfates. Decomposition of HMT in the reaction mixture raises the pH of the hydrothermal fluid, and decreases the amount of sulfate species sorbed on the surface of forming oxide material. It is well known that the high concentration of SO42– ions on the surface of nanocrystalline titania has a negative impact on its functional properties, e.g. photocatalytic activity [35, 36]. Thus, we hope that the method of synthesis that we introduce in this paper would be helpful in the production of TiO2-based photocatalysts of high activity.
ABBREVIATIONS
Data presented in Table 3 also demonstrates that MWHT treatment at higher temperatures, and for a longer time, results in a significant decrease of both SO42– and water content in resulting powders. Analysis of differential thermal decomposition curves has shown that SO42– content in samples correlates to a temperature corresponding to the highest rate of decomposition of sulfate species in the range of 550°C – 800°C. The lower the sulfur content is, the higher is the decomposition temperature.
[2]
CONCLUSION Here, we propose a facile time- and energy-saving method for nanocrystalline titania synthesis based on microwave-assisted homogeneous hydrolysis of titanium oxysulfate solutions under hydrothermal conditions. This method allows obtaining titania nanopowders consisting of low aggregated 5 nm - 7 nm particles which possess a very high specific surface area (up to 240 m2/g). Variation of synthetic conditions (temperature and duration of microwave-assisted hydrothermal treatment) does not result in notable changes in TiO2 particle size and specific surface area. The addition of HMT in the reaction mixtures results in a notable decrease of sulfur content in the resulting TiO2 powders.
BET
=
Brunauer-Emmett-Teller
HMT
=
Hexamethylenetetramine
MW-HT
=
Microwave-hydrothermal
TEM
=
Transmission Electron Microscopy
XRD
=
X-ray diffraction
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CONFLICT OF INTEREST
[13]
The authors confirm that this article content has no conflict of interest.
[14]
ACKNOWLEDGEMENTS This work was supported by RFBR (grant 13-03-12415).
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Accepted: April 21, 2014
