Journal of Thermal Analysis and Calorimetry

, Volume 112, Issue 2, pp 621–628

The effect of thermal treatment in TiO2 photocatalytic activity

Authors

    • Department of ChemistryUniversity of Ioannina
  • Tiverios C. Vaimakis
    • Department of ChemistryUniversity of Ioannina
Article

DOI: 10.1007/s10973-012-2631-9

Cite this article as:
Mitsionis, A.I. & Vaimakis, T.C. J Therm Anal Calorim (2013) 112: 621. doi:10.1007/s10973-012-2631-9

Abstract

Thermal analysis (TA) techniques were applied in order to predict the influence of thermal treatment, on the photocatalytic performance of TiO2 materials prepared via sol–gel method in various temperatures between 250 and 600 °C in different alcohols (methanol/ethanol). Calorimetric results showed that the formation of TiO2 is faster in methanol than in ethanol. TA patterns showed that slight differences observed in the thermal behavior of the material can affect both its textural and photocatalytic properties. The appearance of the endothermic peaks in the area of 250–450 °C refer to crystallization of amorphous to crystalline phases or to the transformation of the less active rutile to the more active anatase phase. The results obtained from TG/DSC are in accordance to XRD results and SEM images. Thermal treatment affects the photocatalytic properties of the materials. Samples prepared in methanol showed better photocatalytic behavior than those in ethanol while the increase in temperature decreases the effectiveness of the materials.

Keywords

Titanium dioxideThermal analysisTerephthalic acidSol–gel

Introduction

Titanium dioxide (TiO2) has been, lately, one of the most commonly studied materials due to its large range of applications. TiO2 powders have notable optical, dielectric, and catalytic properties and in the nano-scale have found many applications as pigments, catalyst supporters, fillers, and photo-catalysts [15]. The later use is of great interest nowadays in harmful pollutants treatment from industrial or domestic wastes. TiO2 has been successfully applied in dye and aromatic hydrocarbon degradation via photocatalytic oxidation process using heterogeneous catalysis in order to decompose harmful pollutants to less or non-toxic final products. TiO2 powders are considered to be very efficient photo-catalysts due to their stability, non-toxicity, and low cost of preparation [6, 7].

The photocatalytic properties of TiO2 depend on its intrinsic properties, such as crystal phases, crystallinity, surface area, the presence of heteroatoms, and the particle size [810]. The efficiency of the photocatalyst is also influenced by the presence of active species like hydroxyl radical (·OH), superoxide radical (·O2), and hydrogen peroxide (H2O2) on the surface of TiO2 [1116]. Generally, it is accepted that TiO2 efficiency is in accordance to the active radical production capability of the material. The matrix of those radicals production is the electrons (e) that are neglected to the covalence band, from the material surface after UV irradiation, as well as the positive holes (h+) that are left in the valance bands afterwards. The valance band holes have oxidative character, while conduction band electrons have reductive character [17, 18].

The lifetime of those charged species is considered to be the time period between their initial production under UV illumination and their final recombination. It is obvious that greater lifetime leads to an advanced efficiency. In order to arrange great species lifetime, it is necessary to overcome some obstacles such as high energy gap, limited irradiation UV range, and inappropriate crystallite size. Many methods have been applied in order to achieve the desirable results among them the most simple and inexpensive is the sol–gel method using titanium alkoxides dispersed in alcohols. In low temperatures sol–gel synthetic procedure via hydrolysis/condensation reactions can produce a great variety of materials with high surfaces and purity [1921].

Thermal analysis (TA) is one of the most commonly used analysis technique in the area of material science. Pulisova et al. [22] used TA results to determine temperature intervals of the degradation and microstructure development during heating of the samples. Through a wide database of analysis techniques, TA can be involved in the characterization procedure, providing useful information regarding phase transition, stability, and crystallinity of the material. It is well accepted that all the above mentioned factors can be linked to TiO2 photocatalytic performance. In this study, we apply TA techniques in order to predict the photocatalytic behavior of TiO2 materials prepared via alkoxide hydrolysis in two different alcohols. Heat flow measurements obtained from calorimetry studies can contribute to the estimation of the reaction rate of the procedure which leads to totally different results.

Materials and methods

The reactants used, were Titanium isopropoxide (TIP, C12H28O4Ti, Aldrich, MW: 284.26), methanol (MetOH, Fluka), ethanol (EtOH, Sigma-Aldrich), HCl (Riedel-de-Haen), and distilled water. In 100 ml solution of each alcohol, 0.1 mol of TIP was immersed and stirred in room temperature for 24 h. The resultant alkoxide solution was kept under stirring at room temperature for 24 h. The hydrolysis of the alkoxide solution was carried out, under acidic conditions, using 0.01 M HCl (pH < 2) resulting in the TiO2 sol. After hydrolysis each sample was dried in 90 °C overnight and heat treated in different temperatures for 3 h with heating rate of 2 °C min−1. The final material names and abbreviations are depicted in Table 1.
Table 1

Sample names and analysis results

Abbreviation

Solvent

Calcination temperature/°C

Crystal phases

Met250

Methanol

250

Amorphous TiO2/anatase

Met450

Methanol

450

100 % Anatase

Met600

Methanol

600

40 % Anatase/60 % rutile

Et250

Ethanol

250

Amorphous TiO2/anatase

Et450

Ethanol

450

100 % Anatase

Et600

Ethanol

600

61 % Anatase/39 % rutile

Hydrolysis process simulation was carried out under the same conditions in a heat flow twin calorimeter (Setaram, C80II) using two identical membrane vessels. 3 ml of TIP dispersed in each alcohol were added in the lower compartment of the sample vessel while the same amount of pure alcohol was added in the lower compartment of the reference vessel. In the upper compartment, 0.02 ml of 0.01 M HCl solution was added in both vessels. Both compartments were divided from each other by a parafilm membrane. As the experiment reaches the desirable temperature both membranes are broken simultaneously by movable rods and a plot of heat flow versus time is being recorded. The process enthalpy is then calculated by peak area intergration using Setsoft software package. The experimental rate constant of the reaction was determined by plotting the ln([Q∞ − Q(t)]/Q∞) versus time for the accumulated heat after the addition of HCl solution was complete. Q(t) is the accumulated heat over time and Q∞ is the final value of the accumulated heat when the reaction was finished.

Simultaneously, thermogravimetry/differential scanning calorimetry (TG/DSC) measurements were carried out by a STA 449C (Netzch-Gerätebau, GmbH, Germany) equipment. The heating range was from room temperature up to 1,000 °C, with a heating rate of 10 °C min−1 under synthetic air flow rate of 30 cm3 min−1. Al2O3 powder was used as reference. The identification of crystal phases was carried out by X-ray diffraction (XRD) technique using a Bruker P8 advance apparatus with 2θ range of 10–70° and step of 0.02°. The results were then compared to the cards of the International Centre of Diffraction Data (ICDD). Phase analysis was carried out according to Zhang and Banfiled relation [23] while the crystallite size was estimated according to Debye–Scherrer relation. In both cases, the (101) plane diffraction peak for anatase and (110) peak for rutile are used for the evaluation.

The textural properties of the solids were studied by N2 adsorption–desorption porosimetry, using a Quantachrome Autosorb automated gas sorption system. Before N2 adsorption–desorption measurement, each sample was degassed at 180 °C and pressure of 10–30 Torr for 6 h. Scanning electron microscopy (SEM), images were obtained using a JEOL JSM-6300 instrument.

The samples were tested according to their intrinsic photocatalytic properties in OH radical production, the width of their energy gap, UV/Vis absorbance and crystallite size. OH radical production was estimated according to the Terephthalic acid (TPA) fluorescence probe method.

Terephthalic acid-fluorescence (TPA-FL) probe method was used in order to evaluate the ability of the materials on producing active OH radicals. According to this method, the ability of the photocatalyst to produce the OH radical is calculated by measuring the fluorescence intensity of the produced 2-hydroxyterephthalic acid (TPAOH) [17, 2426]. For that purpose aqueous solution containing 0.01 M NaOH and 3 mM TPA (Sigma-Aldrich) was prepared as stock solution. 15 mg of each powder was suspended into 3.5 cm3 of the stock solution placed in a Pyrex glass cell. The cell was placed in a dark box and the suspension was stirred by magnetic stirrer for 10 min prior to the UV irradiation in order to obtain the adsorption/desorption equilibrium in the materials’ surface. The light source for the excitation of the materials was a 150 W Xe lamp (Hamamatsu Photonics, C2499). The excitation wavelength was confined to 387 nm and light intensity was 40 mW cm2. The irradiation period was 5 min. The photoluminescence spectra of the samples studied were recorded at room temperature in air using a SHIMADZU RF-5301 spectrofluorophotometer equipped with a 150 W xenon lamp, a red sensitive photomultiplier and reflection grating monochromators with fixed slits of 0.5 nm. The wavelength accuracy was ±1.5 nm. A long-wavelength passing filter (UV-35) was used on the emission monochromator side to cut off the scattered and the second order lights.

Energy gaps were estimated via UV/DRS spectroscopy after applying the Kubelka–Munk theory of reflectance [27]. Diffuse reflectance UV–Vis spectra were recorded at room temperature using Shimadzu UV-2401PC equipment with a BaSO4-coated integration sphere.

Results and discussion

The curves of heat flow versus time are shown in Fig. 1a. After mixing of the TIP with the HCl solution (time 0 min) a sharp exothermic peak is observed in both experiments. The calculated enthalpies of the mixing procedures are depicted in Table 2. As it can be seen when ethanol is used as solvent the procedure enthalpy is almost twice higher than in methanol. As the procedure processes no other phenomenon is observed. The later is due to the formation of TiO2 which in this temperature is stable and no other transformations take place. In an earlier calorimetric study on the hydrolysis and condensation reactions of titanium alkoxides by Blanchard et al. [28], the exothermic behavior of the hydrolysis and condensation sequence was attributed to an increase in the coordination number. The authors assumed that hydrolysis, oxolation, and alkoxylation are substitution reactions and do not involve coordination expansion. The increase of the coordination number was proposed to take place in the early stages of hydrolysis, i.e., the authors assumed that hydrolysis proceeds according to a nucleophilic addition reaction (SN2 mechanism), which in fact is not true for metal alkoxides [29]. However, recently has been proved that sol–gel chemistry for metal alkoxides is clearly different from what it has been earlier assumed. Kessler et al. [30] proposed that titanium alkoxides are stronger Lewis bases than Lewis acids and hydrolysis proceeds for them through a proton-assisted SN1 mechanism, rather than a SN2 mechanism. The high reactivity of the modified precursors causes hydrolysis and also the condensation reactions to proceed for metal alkoxides almost instantaneously upon the addition of water. The modifying ligands are forming the particles of sols through their interaction with the solvents and can stabilize dispersions in essentially the same way as surfactants. The formed hydrated oxides are hydrophilic, which means that polar solvents are favoring the direct and non-polar ones—inverted micellar self-assembly. Acidic catalysis exploits a different reaction pathway. Addition of acids leads at the first step to the protonation of an oxygen atom in the alkoxide ligand, leading to the formation of the reactive cationic species (see Reaction 1).
https://static-content.springer.com/image/art%3A10.1007%2Fs10973-012-2631-9/MediaObjects/10973_2012_2631_Fig1_HTML.gif
Fig. 1

(a) Calorimetric curves for samples prepared at 25 °C in methanol (MetOH) and ethanol (EtOH) and (b) Plot of the ln([Q∞ − Q(t)]/Q∞) versus time for the accumulated heat of each reaction

Table 2

Heat flow results summary

Solvent

Enthalpy/J g−1

Calorimetry Results/25 °C

TG/DSC/DTG results

Total mass loss/%

Firstst stage

Second stage

Third stage

Fourth stage

Experimental rate constant/s−1

Time yield/min

Mass loss/%

DTG peak/°C

Mass loss/%

DTG peak/°C

Mass loss/%

DTG peak/°C

Mass loss/%

DTG peak/°C

Ethanol

−311.7

0.00243

39.8

25.2

14.9

87.0

4.5

245.9

5.2

411.4

0.6

662.9

Methanol

−168.7

0.00274

28.1

25.8

20.0

64.7

4.3

231.1

1.3

409.4

0.2

661.1

https://static-content.springer.com/image/art%3A10.1007%2Fs10973-012-2631-9/MediaObjects/10973_2012_2631_Equ1_HTML.gif
(1)

This step is rate-determining, which results in an SN1-type mechanism. The cation thus formed is attacked by a water molecule, which leads to the formation of a Ti–OH bond, release of an alcohol molecule (better leaving group) and regeneration of a proton catalyst. After the initial hydrolysis, the product can react further either via another hydrolysis reaction or a condensation reaction. The differences in dielectric constants of EtOH (24.5 at 25 °C) and MetOH (32.7 at 25 °C) are significant. It is well known that polar solvents with high dielectric constant are SN1 reaction promoters. Hence, the procedure in MetOH is faster than in EtOH. This assumption is fortified by the linear plots depicted in Fig. 1b. The observed rate constants calculated from the slope of the graphs are given in Table 1. As it can be seen the rate constant in MetOH is higher than EtOH showing that the reaction takes place faster in MetOH.

Figure 2 shows the TG/DSC/DTG curves of the dried materials. Both of the samples have almost same amount of mass loss of about 25 %. However, they seem to decompose in different ways. In both samples, there is an initial endothermic stage of mass loss at the area of 20–180 °C which is attributed to the removal of the absorbed water and the moisture. Methanol sample has an almost vertical mass loss in this stage compared to the ethanol sample. The total mass loss in this stage (as is depicted in Table 2) is ~20 % and ~15 % for methanol and ethanol, correspondingly. The second stage occurs in temperature range between 180 and 250 °C and is depicted as an exothermic tension and a sharp peak in MetOH and EtOH samples, respectively. These phenomena are attributed to crystallization and the organic residue loss of both samples. The mass loss of the samples is almost the same (~4.5 %). The crystallization of amorphous TiO2 to (crystalline) anatase-TiO2 occurs in the third stage between 250 and 450 °C. This stage is denoted as a sharp exothermic peak in both samples. The mass loss of EtOH sample is almost three times bigger than the MetOH sample. This fact may denote that EtOH is involved in a higher amount in TIP hydrolysis and condensation. The last (fourth) stage between 500 and 750 °C, is attributed to the transformation of anatase-TiO2 to rutile-TiO2, denoted as a large exothermic tendency, in both samples. Both of the samples have almost the same mass loss close to zero. This stage is also the yielding of the thermal decomposition of each sample. The broader tendency in ethanol in this stage denotes that the procedure seems to be slower. The appearance of the exothermic transformation phenomena in relatively higher temperatures than the literature [31, 32] can be attributed to the high heating rate (10 °C min−1).
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Fig. 2

TG/DSC/DTG curves of the prepared samples in methanol (MetOH) and ethanol (EtOH)

In order to fortify our hypothesis about TA transformations, a portion of each material was calcined in 250, in 450, and in 600 °C for 3 h with heating rate of 5 °C min−1. Figure 3 shows the XRD patterns of each material while the results of phase analysis and crystallite size are depicted in Table 1.
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Fig. 3

XRD patterns of calcined samples, where A and R denote anatase and rutile phase, respectively

Results show that crystallinity is depended on the calcinations temperature and in fact increases with the increase of temperature. This can be attributed to the thermally promoted crystallite growth. XRD patterns confirm the TA proposals. Samples calcined in low temperature (250 °C) have low crystallinity and seem to be consisted of amorphous and anatase phases. The later can be attributed a relatively lower purity due to the inclusion of more organic impurities originated from the sol–gel process [33]. Higher temperature (450 °C) seems to increase crystallinity and favors the appearance of anatase phase in both samples. This is confirmed by the TG/DSC analyses in which an exothermic peak is observed at ~300 °C, which is associated with the mass loss within 250–450 °C owing to the oxidative elimination of the organic residue (Fig. 2). Further calcination (600 °C) leads to the formation of biphasic materials consisted of anatase and rutile. The anatase percentage in ethanol is significantly higher than in methanol. The TiO2 formation procedure from titanium alkoxides follows the way of hydrolysis (Reaction 1) and condensation (Reaction 2):
$$ {\text{Ti(OH)}}_{4} + 2{\text{H}}_{2} {\text{O}} \to {\text{TiO}}_{2} x{\text{H}}_{2} {\text{O}} + (2 - x){\text{H}}_{2} {\text{O}} $$
(2)
where R: (iso-propyl, ethyl-, n-butyl, etc.). TiO2xH2O is considered to be amorphous hydrous oxide which is formed according to Reaction 2. Mahshid et al. [34] proposed that the amount of amorphous TiO2 produced depends on the experimental conditions such as pH. Due to the positive partial charge of water molecules [34, 35], low experimental pH (~2) leads to the crystallization of all samples. However, condensation is very complex procedure and we have to consider three competitive mechanisms: alcoxolation, oxolation, and olation [30, 35]. Alcoxolation is a reaction by which a bridging oxo group is formed through elimination of an alcohol molecule. Oxolation follows the same mechanism, but a water molecule is eliminated. Olation can occur when bridging hydroxo groups can be formed through elimination of a solvent molecule. All these reactions are catalyzed by acidic medium. Since the only difference in the experimental procedure is the solvent, we can assume that olation mechanism is the most important. From the calculated procedure enthalpies, it is quite clear that methanol can promote the process. The increase of the crystal structures’ intensities seems to proceed faster in methanol than in ethanol. Taking into account that rutile particle growth starts right after its nucleation we can assume that methanol conducted experiments are one step further than those conducted in ethanol. The later can be also proposed if we take under consideration the higher mass loss percentage, which appears in the third stage of thermal analysis, of EtOH sample. This stage could be considered as the yielding of organic matter decomposition and removal which was incomplete for EtOH, in the second stage. The organic presence inhibits the nucleation of rutile thus the rutile amount in Et600 is significantly lower than that in Met600.
The textural properties of the samples were studied using SEM while the surface area of each material was estimated by BET method and are presented in Table 3. According to the results, the surface areas decrease as the calcination temperature increases. Samples prepared in lower temperatures present two or three times higher surfaces than samples prepared in 600 °C. The later could be attributed to the sintering procedure. Surface areas of the samples prepared in methanol are higher than those prepared in ethanol. Seems like the better dispersion of isopropoxide molecules in methanol leads to greater interparticle spaces in the yielding materials. The lower surface areas of samples heated in higher temperatures is a result of faster nucleation and crystalline growth leading to denser packing of these materials [36].
Table 3

Intrinsic properties of samples

Sample name

Crystallite size/nm

Surface area/m2 g−1

Energy gap/eV

TPAOH fluorescence intensity peak/a.u.

Anatase

Rutile

Met250

17.6

65.1

3.04

3.57

Met450

24.1

48.8

3.13

2.62

Met600

20.3

31.8

27.9

3.01

1.78

Et250

14.0

62.3

3.07

2.04

Et450

20.8

42.9

3.10

1.65

Et600

24.8

13.5

17.4

2.92

1.13

Figure 4 shows the SEM microphotographs of the prepared samples. As it can be seen samples prepared in ethanol are smooth and are consisted of large particles without any specific shape or size. Calcination seems to increase the particles size from less than 1 μm in sample Et450 to more than 1 μm in sample Et600. Samples produced in methanol have totally different structure except from sample Met250 which is almost similar to the Et250. However, at higher temperatures large particles of TiO2 are formed with characteristic needle-like morphology. Each particle is consisted of a bunch of TiO2 “needles” with smooth surface and size of ~2 μm. Sintering rearranges the surface modification and leads to the formation of rough particles. Furthermore in sample Met600, the number of “needles” in each particle increases while their size decreases. That can be attributed to a possible densification of the agglomerates. SEM images showed that small spherical particles like those expected from a coprecipitation method [37].
https://static-content.springer.com/image/art%3A10.1007%2Fs10973-012-2631-9/MediaObjects/10973_2012_2631_Fig4_HTML.jpg
Fig. 4

SEM microphotographs of the prepared TiO2 samples

According to the DRS spectra depicted in Fig. 5, samples prepared in ethanol showed higher absorbance in the UV range for preparation temperatures of 250 and 450 °C (blue-shift of the absorption edge). On the contrary samples prepared in methanol showed a totally different behavior. Sample Met600 has significant absorbance in the UV range instead of those prepared in lower temperatures. Generally, anatase is considered to be more photoactive than rutile [38, 39]. However, recently some researchers proposed that the combination of both rutile stability and anatase active species lifetime in composite enhances their photocatalytic activity [40, 41]. The band gap widths calculated, applying the Kubelka–Munk theory [27], are given in Table 3. The values for the samples that rutile phase is present (Et600, Met600), have low (2.90–3.0 eV) band gaps. Samples with pure anatase phase (Et450, Met450) have broader band gaps (>3.10 eV) while samples Et250 and Met250 with amorphous phase have little lower band gaps (3.04–3.07 eV). According to XRD and TG/DSC results calcinations in 450 and 600 °C favors anatase and rutile phases having as a result an increase (for Et450/Met450) and a decrease (samples Et600/Met600) correspondingly in band gaps. The calculated band gaps tend to reach those of pure anatase (~3.20 eV) and pure rutile (~3.0 eV) [7, 42].
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Fig. 5

UV DRS spectra of the prepared TiO2 samples

The fluorescence emission spectra of the prepared powders (Fig. 6) exhibit a peak at ~425 nm due to the formation of TPAOH [19, 43]. It can be observed that the intensity of the fluorescence peak for the sample prepared in 250 °C is high revealing OH production on the surface of the materials. As the preparation temperature increases, the intensity of the peak decreases. Moreover, composites prepared in highest temperature (samples Et600/Met600) demonstrate the lowest OH production. This effect can be attributed to the specific texture of the material which provides low specific surface area, high crystallite sizes and rutile ratios. Researches [44, 45] have shown that high crystallite sizes (>20 nm) are inappropriate in photocatalysis due to the increase of time spent by the active species to reach the material surface. This staling has as a result the recombination of charges and the decrease of their lifetime expectancy. Samples prepared in 250 °C have mediocre crystallite sizes (~14 nm) and higher surface areas compared with those of samples prepared in 450 °C and thus seem to promote their photocatalytic activity. Comparing sample prepared in MetOH to those prepared in EtOH, it is well observed that the later are less effective. Higher specific surface areas of MetOH samples are a major factor in their increased activity. In this way, higher amounts of TPA molecules can be adsorbed in the material surface and provide higher fluorescence intensity. Thermal treatment in higher temperatures seems to inhibit the effectiveness of the material in TPA to TPAOH transformation. The later is possibly due to phase transformations (anatase → rutile) and sintering effects which lead to surface area collapse and crystallite growth.
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Fig. 6

TPA fluorescence intensity of the prepared materials

Conclusions

Sol–gel method has been applied in order to prepare TiO2 using methanol and ethanol as solvents. Thermal treatment leads to significant changes in the composition, morphology, and photocatalytic properties of the materials. By studying the TG/DSC patterns, we are able to estimate any possible phase transformation that may occur and could affect the photocatalytic performance of our material. Heat flow measurements can predict the reaction rate of the experimental procedure providing useful results regarding the preparation of the samples. Methanol seems to promote the formation, the nucleation and the crystallization of TiO2 compared to ethanol leading to better photocatalytic results. Samples prepared in methanol seem to be more effective than those prepared in ethanol under the same conditions.

Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2012