Influence of calcination temperatures on the formation of anatase TiO₂ nano rods with a polyol-mediated solvothermal method

Abstract

A simple and convenient method has been demonstrated for large-scale synthesis of titanium dioxide (TiO₂). Nano-rod TiO₂ with anatase structure had been prepared by a polyol-mediated solvothermal process of titanium tetra-isopropoxide (TTIP) and ethylene glycol (EG) followed by powder calcination at high temperature. The growth mechanism of the TiO₂ nano-rods was discussed and supported by X-ray diffraction (XRD), Fourier transform infra-red (FT-IR) spectroscopy, Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and scanning electron microscopy (SEM). Anatase TiO₂ nano-rods (NRs) were synthesized by adjusting the preparation parameters such as precursor concentration, autoclaving temperature and time duration, and calcination temperature. SEM showed TiO₂ NRs of 100 to 250 nm that were produced due to the reaction of 0.7 mL of TTIP with 25 mL EG, autoclaving temperature of 205 °C for a time duration of 12 h, and calcination in air at 600 °C for 1 h. Calcination temperature had a great effect on the production of TiO₂ nano-rods, where well-defined hexagonal NRs were produced at a high calcination temperature of 600 °C for 1 h.

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1. Introduction

TiO₂ represents one of the most studied and widely used materials due to its low cost, enhanced stability, and easy preparation.¹ TiO₂ has a variety of promising applications, for example in dye sensitized solar cells,² photocatalysis,^3,4 lithium batteries,⁵ and photochromic devices,⁶ due to its attractive optical, electrical, chemical, and catalytic properties. The size, shape and crystalline structure of TiO₂ greatly influence the suitability of the material for device fabrication.⁷ The synthesis of one-dimensional TiO₂ materials, such as nano-tubes, nano-wires, nano-fibers, and nano-rods has attracted particular interest due to their unique microstructure and promising functions.⁸

Nano-rods (NRs) are synthesized from the necking of truncated nano-particles (NPs) applying the synthesis of an oriented attachment approach. The driving force for the oriented attachment between truncated spherical units is the joining of each unit by eliminating impure molecules from the spherical titanium dioxide surface.⁹

In the 1980s, Fievet et al. used ethylene glycol as a solvent and reducing agent for the preparation of submicrometer particles of the transition metals as this method was known as polyol process.¹⁰ The solvent (i.e.: ethylene glycol) plays an important role in determining the crystal morphology.¹¹ Solvents with different physical and chemical properties can influence the solubility, reactivity, and diffusion behavior of the reactants. Specifically, the polarity and coordinating ability of the solvents can influence the crystal morphology of the final product. The polyol process or the use of polyol or diol as the reducing reagent, had been widely applied to reduce metal salts to metal nanoparticles and was typically used in combination with a surfactant to control particle morphology.¹² However, it had also been established that inorganic species may provide a means as powerful as organic surfactants and polymers for controlling the shape of metallic and metal oxide nanoparticles.¹³

In this article, the researchers reported for a facile solution phase synthesis of anatase 1-D single crystalline TiO₂ NRs arrays by polyol-mediated solvothermal process using titanium tetra-isopropoxide and ethylene glycol without any other additives, followed by powder calcination, which may provide a more promising approach in terms of cost and efficiency. The morphology of the TiO₂ nano-crystals was changed dramatically by using various solvothermal parameters and different calcination temperatures. Through the present experiments, the researchers systemically studied the influence of precursor concentration, autoclaving temperature and time duration, and calcination temperature on the morphology of the TiO₂ NPs.

2. Experimental work

Synthesis of TiO₂ NRs by solvothermal reaction in polyol medium

TiO₂ NRs were synthesized via a solvothermal route of titanium tetra-isopropoxide (TTIP) in ethylene glycol (EG) followed by thermal treatment. The preparation reaction mechanism for the anatase TiO₂ NRs was illustrated in Fig. 1.

Fig. 1 Schematic pathway for TiO₂ nano-powder production.

TTIP and EG were sufficiently well reacted to give the titanium glycolate (Ti(OCH₂CH₂O)₂) in good yields. Performing the reaction in an autoclaving allowed to obtain single crystals.¹⁴ The titanium glycolated had high stability not only in alcohol but also in water and in a humid atmosphere.¹⁵ Because of this advantage, only very few groups had reported the synthesis of TiO₂ from these precursor by hydrolysis and condensation reactions. But in the study under investigation, the researchers focused on solvothermal process and the calcination at higher temperatures where each titanium glycolate precursor could be transformed into the corresponding TiO₂ one-dimensional rod-like structure morphology. Basic alkoxide (TTIP with EG) oxygen tended to repel the nucleophilic, –OH under solvothermal condition. However, once the initial hydrolysis had occurred, reactions proceed stepwise with each subsequent an alkoxide group. Therefore, more highly hydrolyzed titanium was more prone to attack. Titanium glycolate was formed as intermediate compound (as shown in Fig. 1). TiO₂ nano-particles were formed with little organic residue after solvothermal step. In calcination process in air, the oxidation and desorption of carbon-containing species occurred, eventually giving carbon-free TiO₂ nano-crystals without any impurities.¹⁶

Table 1 illustrated the different parameters which were used in the TiO₂ NRs preparation. Different volumes of TTIP solution (0.35, 0.7 and 1.4 mL) were added to 25 mL EG and stirred for 1 h. The solutions were loaded into a 80 mL polytetrafluoroethylene (PTFE) lined stainless steel autoclaves and then solvothermally treated at different temperatures (205 °C, 225 °C and 250 °C) and autoclaving time durations (12, 24 and 36 h). Finally, the colloids were then condensed to a final TiO₂ powder using a rotary evaporator where after which it was naturally cooled. Following precipitate isolation using rotary evaporator and drying, the samples were calcinated in air for 1 h at different temperatures (400 °C, 500 °C, and 600 °C).

Table 1 Different parameters used for the preparation of TiO₂ nano-powders using polyol-mediated solvothermal process

Sample code	TTIP (mL)	EG (mL)	Autoclaving temp. (°C)	Autoclaving time duration (h)	Calcination temp. °C for 1 h
S1	0.7	25	205	12	—
S2	0.7		205	12	400
S3	0.7		205	12	500
S4	0.7		205	12	600
S5	0.35		205	12	600
S6	1.4		205	12	600
S7	0.7		—	—	600
S8	0.7		225	12	600
S9	0.7		245	12	600
S10	0.7		205	24	600
S11	0.7		205	36	600

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Characterization of TiO₂ nano-powder

X-ray diffraction patterns of the TiO₂ nano-powders were measured using (Schimadzu 7000, Japan) diffractometer, operating with Cu Kα radiation (λ = 0.15406 nm) generated at 30 kV and 30 mA. Scans were done at 2° min⁻¹ for 2θ values between 20° and 80°. The crystallite size can be determined from the broadening of corresponding X-ray spectral peaks by Scherrer formula (eqn (3))¹¹

L = Kλ/(β cos θ) (3)

where L is the crystallite size, λ is the wavelength of the X-ray radiation (Cu Kα = 0.15418 nm), K is usually taken as 0.89, and β is the line width at half-maximum height, after subtraction of equipment broadening. Infrared absorption spectra (IR) were recorded on Shimadzu FTIR-8400 S spectroscope using transparent KBr pellets: the sample was crushed and mixed with 300 mg of KBr. Raman spectra were recorded at different locations of the sample using a SENTERRA (spectrometer-Bruker, Germany) with a 532 nm Ar laser. Thermal gravimetric analyzer (Shimadzu TGA – 50, Japan) was used for studying the thermal behavior of samples with a temperature range from ambient to 800 °C. The morphology of the produced power had been investigated with high-resolution transmission electron microscopy operating at 200 kV (JEM-HR- 2100) and scanning electron microscope (JEOL JSM 6360LA, Japan).

3. Results and discussion

In the following section, the researchers indicated that the produced powder from solvothermal process was TiO₂ NPs mixed with Ti glycolate. After calcination in air at high temperature (600 °C), the titanium glycolate was transformed into TiO₂ NPs which bonded with each other and formed pure anatase TiO₂ NRs.

X-ray diffraction (XRD)

Fig. 2 showed the X-ray diffraction patterns of TiO₂ nano-powder before and after calcination. The produced powder from solvothermal step (S1 sample) was amorphous without showing any distinct diffraction peak.¹⁷ After calcination at 400 °C (S2) for 1 h, TiO₂ with an amorphous structure was obtained. Increasing the calcination temperature to 500 °C (S3) resulted into low-crystalline TiO₂ nano-powder. For sample S4 (calcination at 600 °C for 1 h), it can be seen that the peaks at 2θ of 25.65°, 37.6°, 38.95°, 48.37°, 54.23°, 55.6°, 63.2°, 68.99°, 70.78° and 75.37° were assigned to (101), (004), (112), (200), (105), (211), (204), (116), (220) and (215) lattice planes of anatase TiO₂ phase, respectively (according to JCPDS 21-1272). Through the crystallization process, the original bonding in the amorphous particles were broken to form new bonding for anatase structure, resulting in the deformation of particles and fine crystals structure would be produced.^2,17

Fig. 2 XRD spectra of produced powder samples; before (S1) and after calcination process at different temperatures; 400 (S2), 500 (S3), and 600 °C (S4).

Direct crystallization of anatase TiO₂ nanocrystals with exposed {001} facets was successfully demonstrated at 600 °C for 1 h in a polyol-mediated solvothermal process. This indicated that the calcination temperature was an important parameter controlling the TiO₂ architecture. This feature was also observed by SEM study (as shown below).

The XRD pattern of anatase became discernible at 400 °C. The crystallite size of the powder as a function of calcination temperature was calculated using Scherrer formula (eqn (3)). The crystallite size of S1, S2, S3, and S4 was 8.96, 10.69, 13.09, and 18.73 nm, respectively. It can be explained as follows:

The growth rate was given by:²

u = a₀ν₀[exp(−Q/KT)][1 − exp(−ΔFν/KT)] (4)

where a₀ is the particle diameter, ν₀ is the atomic jump frequency, Q is the activation energy for an atom to leave the matrix and attach itself to the growing phase, ΔFν is the molar free energy difference between the two phases.

For non-crystallization, ΔFν ≫ KT, so eqn (4) could be reduced to (5):

u = a₀ν₀[exp(−Q/KT)] (5)

The crystallite size increased with increasing the calcination temperature. It grows slowly at low calcination temperatures and then increased fast at high calcination temperatures.

FT-IR, Raman and reaction efficiency

Fig. 3 showed the FTIR spectra recorded from TiO₂ nano-powder before and after calcination at 600 °C for 1 h. For the produced powder from solvothermal step (S1), the bands corresponding to physically absorbed water or ethylene glycol (the O–H stretching mode at ∼3400 cm⁻¹ and the O–H bending mode at ∼1630 cm⁻¹) were observed. The band located at 2899 cm⁻¹ was assigned to the stretching vibrations of ν(C–H). This is consistent with the symmetric and anti-symmetric C–H stretching bands of ethylene glycol, roughly.¹⁸ The band at 2349 cm⁻¹ attributed to ν OH mode of interacting hydroxyl groups (i.e., involved in hydrogen bonds) and the symmetric and antisymmetric v OH modes of molecular water coordinated to Ti⁴⁺ cations.¹⁹ The presence of bands at 1086 cm⁻¹ was characterized as a C–C–O stretching vibration for ethylene glycol. At approximately 1465 cm⁻¹ a peak corresponding to the bending of CH₂ groups was appeared. Ti–O stretching bands appeared at ∼610 cm⁻¹ and 490 cm⁻¹ with very low intensity. The broad peak at 490 cm⁻¹ was attributed to ν Ti–O–Ti stretching vibration in the anatase phase that indicated characteristic strong absorbance of TiO₂ due to Ti–O stretching Ti–O–Ti bridge stretching vibration or bending vibration.¹⁹

Fig. 3 FTIR spectra of produced powder before (S1) and after calcination (S4).

After calcinations at 600 °C for 1 h (S4), the peaks corresponding to physically absorbed water or ethylene glycol at 3400 cm⁻¹ and the O–H bending mode at 1640 cm⁻¹ were disappeared. The two bands at 611 and 490 cm⁻¹ became more intense after calcination process. The band occurring at 611 cm⁻¹ was assigned to the ν(Ti–O) stretching frequency for the Ti–O band.²⁰ These two bands were indicating the formation of TiO₂ anatase before and after calcination.

Fig. 4 showed Raman spectra of produced nano-powder before and after calcination at 600 °C for 1 h. Before calcination (S1), six intense peaks were recorded at 71.53, 152.7, 201.57, 393.7, 510.45, and 627.56. The Ti–O–C stretching peaks characteristic of ethylene glycolate ligands linked to titanium appeared at 1091 and 1116 cm⁻¹ with very low intensities. After calcination (S4), all last peaks were obtained sharply but shifted into 142, 195.53, 395.69, 514.39, 638.36 cm⁻¹ and the peaks of ethylene glycolate ligands linked to titanium were disappeared. This result was match with previous reports of pure TiO₂ nanostructures. Where, the anatase phase of TiO₂ had five peaks for Raman-active modes (A_1g + 2B_1g + 3E_g); the most intense being the bands at (142, 195.53, 638.36 cm⁻¹) E_g, (395.69 cm⁻¹) B_1g, (514.39 cm⁻¹) A_1g.^21–23 The peaks at 627.56 and 638.56 cm⁻¹ were assigned to Ti–O stretching vibration. As the calcination occurred, the intensity of the frequency peak at 152.7 cm⁻¹ significantly increased, indicating the enhancement of crystallinity.^24,25

Fig. 4 Raman spectra of produced powder before (S1) and after calcination (S4).

It is clear that anatase nanoparticles were obtained by heating the titanium glycolate (solvothermal process), pure anatase TiO₂ nano-rods were produced after calcination at 500 °C for 1 h in air (which is in accordance with the XRD and FTIR results).

The reaction efficiency value of 90.65% (Ti produced in TiO₂/Ti in the precursor) was calculated for S4 sample (after calcination process) indicating transformation of the titanium precursor into pure TiO₂. So the polyol-mediated solvothermal method could consider as low cost and efficient process for production of pure anatase TiO₂ nano-powder.

TGA studies

Fig. 5 showed TGA of produced nano-powder before and after calcination at 600 °C for 1 h. For S1 sample; two endothermic peaks were obtained. First broad endothermic peak appeared around 90 °C due to the removal of adsorbed water (free or interstitial). The second endothermic peak and 150 °C was caused by desorption of coordinately bounded water or physically bounded residual ethylene glycol. The weight loss was estimated about 7.98% for adsorbed water and ethylene glycol. Two exothermic peaks at 280 °C to 340 °C and at 400 °C were owing to the removal of residual organic groups such as ethylene glycol units or alkyl groups chemically bonded to titanium and the induced weight loss was about 31.3%.²⁶ After calcination at 600 °C for 1 h (S4), the sample was thermally stable due to complete transformation into anatase TiO₂ and no adsorbed water or organic component were detected.

Fig. 5 TGA spectra of produced powder before (S1) and after calcination (S4).

Morphology studies

The morphology-controlled synthesis of TiO₂ NPs and NRs were of great interest for future TiO₂ nano-device application. By adjusting the precursor concentration, autoclaving temperature and time duration, and calcination at higher temperatures, different sizes of 1-D TiO₂ NPs had been prepared via polyol-mediated solvothermal route.

(A) Effect of calcination temperature. Fig. 6 showed the TEM image of as prepared TiO₂ nano-particles without calcination (S1-produced from solvothermal step). The powder was composed of quiet homogenous small particles, 8–18 nm in size, in reasonable agreement with the XRD data (see Fig. 2 sample S1).

Fig. 6 HRTEM and SAED of produced powder before calcination.

From the previous discussion mechanism, when TTIP reacted sufficiently well to give TiO₂ nano-particles after solvothermal process (see Fig. 1). It showed clear lattice fringes, which allows for the identification of crystallographic spacing and indicates that the prepared anatase TiO₂ powders were well crystalline. The fringes of 1.1 nm match that of the (101) crystallographic plane of anatase TiO₂.

Performing the reaction in an autoclave and the presence of ethylene glycol allowed obtaining single crystals. The structure was solved in the C-centred monoclinic space group C2/c and consists of an infinite one-dimensional chain structure with edge-sharing TiO₆ tetrahedra in the c-direction.²⁷

Fig. 7 showed the SEM images of TiO₂ nano-powder being calcinated at different temperatures for 1 h (400, 500, and 600 °C). By solvothermal and calcination at elevated temperatures, each glycolate precursor could be transformed into the corresponding TiO₂ without changing the particles morphology. It was shown in Fig. 7 that the TiO₂ NPs were produced after calcination at 400 °C (S2).

Fig. 7 SEM images of calcinated TiO₂ powder at different temperatures for 1 h; 400 (S2), 500 (S3), and 600 °C (S4).

Calcination in air at high temperature (>400 °C) led to serious particle agglomeration.²⁸ Using calcination temperature of 500 °C (S3) caused NPs aggregated to NRs geometry. Increasing the calcination temperature into 600 °C (S4), no NPs were observed and NRs with diameter of 100 nm were produced. In conclusion, thermal treatment higher than 400 °C allowed the formation of the 1D nano-rods.

(B) Effect of TiO₂ precursor concentration. The influence of the TiO₂ precursor concentration was examined. According to the reaction mechanism (as shown in Fig. 1), the ratio 0.35 mL of TTIP to 25 mL EG was not suitable for complete transformation of TiO₂ NPs to NRs (after calcinations) as shown in Fig. 8 (S5). Increasing the amount of TTIP into 0.7 mL in 25 mL EG (S4), the density of NRs increased due to all NPs which were stacked and connected to each other to form NPs architecture with diameter of 500 to 800 nm. These experiments suggested that the stability of the NRs formation was achieved only once they reached a given size. In other words, despite the low concentration of TTIP, the rods grew to fixed size, and once this stability was achieved, larger rods could be formed with increasing the TTIP concentration (S6) and NPs were observed.

Fig. 8 SEM images of calcinated TiO₂ nano-powders prepared using different TTIP volumes, 0.35 (S5), 0.7 (S4), and 1.4 mL (S6).

(C) Effect of autoclaving temperature and time duration. The effect of the autoclaving temperature was demonstrated in Fig. 9. An autoclaving was employed to obtain a high temperature and pressure for the nucleation and crystallization of nano-materials.²⁸ This method was favorable for the formation of better crystallization TiO₂ NRs. However, after autoclaving time duration, the NRs began to grow and continued growing at a decreasing rate until the titania NRs began to reform.²⁹ No growth took place at temperatures lower than 100 °C, and higher temperatures increased the growth rate. The morphology of nano-TiO₂ powder prepared without (S7) and with using different autoclaving temperatures 205 (S4), 225 (S8) and 245 °C (S9) was characterized by SEM as shown in Fig. 5.

Fig. 9 SEM images of calcinated TiO₂ powder without and with autoclaving temperatures; without (S7), 205 °C (S4), 225 °C (S8), and 245 °C (S9).

For S7, mixture of NPs and few NPs was obtained. Also, the NPs were bonded to form aggregated NPs which appeared as sheets on the same sample which explained the importance of pressure and autoclaving heat in the separation process of the aggregated rods and also on the formation of uniform NPs. While titanium glycolate tended to form 1-D nanostructures at 205 °C (S4). As a result, 1-, 2- and 3-D were produced by increasing the reaction temperature.

As the results of using different autoclaving temperature, the peeling off process was occurred. All TiO₂ samples had uniform rod-like particles with different aspect ratios and sizes after calcination. SEM image of S4 sample showed the formation of TiO₂ NRs-like with straight and flat faces and the diameters of this rod were relatively uniform, average around from 130 to 300 nm. TiO₂ NRs became much thinner, longer, and distinguishable for S4. When the temperature increased to 225 °C (S8) at the same autoclaving time interval of 12 h, 1-D NRs were still the predominant structure but thicker rods were observed compared with S4. Increasing the autoclaving temperature to 245 °C (S9), both 2-D and 3-D morphologies were observed, and NPs became thicker than S2 and S5 and produced more agglomerates. This might have had been formed because of the enhanced mobility and the increased solubility of TTIP in EG solution at the higher temperature. The latter reduced the amount of TTIP to feed the NRs growth. These results proved that there was great difference in the morphology of TiO₂ samples obtained at different autoclaving temperatures.

Fig. 10 showed the SEM images of TiO₂ nano-powder prepared with different autoclaving time duration (S4, S10, and S11). As shown before, the as-prepared titanium glycolate rods were comprised of NPs innano-porous geometry, when sintered at 600 °C for 1 h they turned into NRs consisting of pure anataseTiO₂ (S4 andS10). For S11, no NPs appeared and only NPs architecture of TiO₂ had been detected. The absence of NPs was consistent with the fact that the Ti–OH was acting as a reservoir. The SEM images demonstrated that the rod-shaped morphology with hexagonal structure of TiO₂ had appeared after 12 h of solvothermal at 245 °C. The as-formed TiO₂ rods had diameters of about 350 nm.

Fig. 10 SEM images of prepared TiO₂ using autoclaving temperature of 205 °C for different time durations; 12 (S4), 24 (S10), and 36 h (S11).

Increasing the autoclaving time duration to 24 h, agglomerated hexagonal faceted particles were formed (S10) and smooth rods of TiO₂ with diameters of 400 nm were obtained, indicating that the shapes of the TiO₂ NPs were hexagonal and were dependent of the solvothermal time. With increasing the autoclaving time duration to 36 h (S11), deformation of rods architecture had a resulted and only NPs were obtained. It was noticed that the shapes of the TiO₂ NRs were hexagonal and were dependent of the solvothermal time duration.

4. Conclusions

Polyol-mediated solvothermal process was carried out to prepare titanium dioxide crystals of various morphologies (nano-particles and nano-rods). The synthetic strategy began with production of NPs structure from a base system of TTIP and EG to form titanium dioxide mixed with impurities of glycolate. On the basis of the experimental results, a possible reaction mechanism of formation of TiO₂ NPs had been proposed. With controlling the preparation parameters, such as TTIP concentration, autoclaving temperature and time duration, and calcination temperature, the particles had been aggregated and formed hexagonal rod-shaped. The uniform TiO₂ nano-rods with diameter of 100–250 nm had been produced after calcination process for powder that was produced from solvothermal reaction of 0.7 mL TTIP with 25 mL EG at autoclaving temperature of 205 °C for 12 h. The calcinated powder exhibited pure anatase phase TiO₂ structure. This phase was stable with increasing the calcination temperature into 600 °C.

Acknowledgements

This work had been implemented under the project funded by the Science and Technology Development Fund (STDF), Ministry of Scientific Research, Project ID: 1414, “Quantum Dots Nanomaterials Dye Sensitized Solar Cells”.

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