The effect of thermal treatment in TiO2 photocatalytic activity
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- Mitsionis, A.I. & Vaimakis, T.C. J Therm Anal Calorim (2013) 112: 621. doi:10.1007/s10973-012-2631-9
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.
KeywordsTitanium dioxideThermal analysisTerephthalic acidSol–gel
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 [1–5]. 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 [8–10]. 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 [11–16]. 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 [19–21].
Thermal analysis (TA) is one of the most commonly used analysis technique in the area of material science. Pulisova et al.  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
Sample names and analysis results
100 % Anatase
40 % Anatase/60 % rutile
100 % Anatase
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  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, 24–26]. 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 . Diffuse reflectance UV–Vis spectra were recorded at room temperature using Shimadzu UV-2401PC equipment with a BaSO4-coated integration sphere.
Results and discussion
Heat flow results summary
Calorimetry Results/25 °C
Total mass loss/%
Experimental rate constant/s−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.
Intrinsic properties of samples
Surface area/m2 g−1
TPAOH fluorescence intensity peak/a.u.
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.