Synthesis of TiO 2 by the Pechini Method and Photocatalytic Degradation of Methyl Red

Department of Materials Engineering, Federal University of Campina Grande – UFCG, Av. Aprígio Veloso, 882, Bodocongó, CEP 58429-140, Campina Grande, PB, Brazil Department of Materials Engineering, Federal University of São Carlos – UFSCar, Rod. Washington Luis, Km 235, CEP 13565-905, São Carlos, SP, Brazil Department of Physics, Federal University of Ceará – UFC, CEP 60455-970, Fortaleza, CE, Brazil


Introduction
Titanium dioxide is one of the engineered nanomaterials most frequently used because it can serve two distinct functions: absorption and scattering of ultraviolet (UV) radiation and/or semiconductor photocatalysis activated by UV radiation 1,2 .As a UV reflecting agent it is found in a variety of applications, including sunscreens, paints, and coatings, where it serves in a protective role 3 .The photocatalytic ability of nano-TiO 2 is used in photovoltaic devices and self-cleaning or self-sterilizing product coatings 2 .Additionally, as a photocatalytic agent, nano-TiO 2 could be useful for degrading environmental pathogencontaminated drinking water 4,5 .
TiO 2 has been the material most widely studied and used in many photocatalysis applications because of its strong oxidizing abilities for the decomposition of organic pollutants, superhydrophilicity, chemical stability, long durability, non-toxicity, and transparency to visible light 6 .
The photocatalytic properties of TiO 2 derive from the photogeneration of charge carriers (electrons and holes) that occurs upon the absorption of ultraviolet (UV) light in the band gap of 3.2 eV 7 .The photogenerated holes in the valence band diffuse on the TiO 2 surface and react with adsorbed water molecules, forming hydroxyl radicals (•OH) 8 .The photogenerated holes and hydroxyl radicals oxidize adjacent to organic molecules on the TiO 2 surface.Meanwhile, electrons in the conduction band participate in reduction processes, typically reacting with molecular oxygen in the air to produce superoxide radical anions (O 2 •-).
Titanium oxide presents three different crystallographic forms: anatase, rutile and brookite.The anatase and rutile phases are used in heterogeneous photocatalysis.The literature about the photocatalytic activity of TiO 2 is controversial due to the polymorphousness of materials and the conditions and type of synthesis employed in the preparation of TiO 2 , which can promote one or more phases.Some researchers assert that the anatase phase is more efficient in the photocatalytic degradation process 9 .However, the activation of anatase phase requires high energy and is achieved by UV irradiation 4 .On the other hand, other researchers report that small amounts of rutile present with anatase in TiO 2 heighten its photocatalytic efficiency, which, allied to other factors such as high surface area, contributes to increase the effectiveness of the photocatalytic degradation process 10,11 .
It is therefore important to choose a suitable synthesis method to ensure the formation of the desired phase with specific characteristics such as high surface area, nanometric particle size and mesoporosity, which can activate photocatalysis.
The Pechini method stands out among several chemical synthesis methods because it allows for the use of different temperatures and proportions of citric acid and metallic cations, enabling controlled particle and/or agglomerate stoichiometry and morphology, compositional homogeneity, and low toxicity to produce a monophasic nanometric powder [12][13][14] .In the Pechini method the reaction is hydrolytic and produces a polymer which, after calcination, forms an oxide.The Pechini process involves two basic chemical reactions: the formation of a chelate complex consisting of carboxylic acid, chelating agent, and metallic matrix, followed by its polyesterification with an excess of polyalcohol 15 .The reactants usually employed in this application are citric acid and glycol ethylene 16 .
In this work, TiO 2 photocatalyst was prepared by the Pechini method, with varying molar ratios of citric acid to metallic cations of 2:1, 3:1 and 4:1.The samples were characterized and subjected to a photocatalytic degradation reaction of methyl red.

Experiments
The sample TiO Three samples of TiO 2 were prepared by the Pechini method 12 in citric acid/metallic cation molar ratios of 2:1, 3:1 and 4:1.First, titanium citrate was prepared by reacting citric acid with titanium.The reactants were mixed under constant stirring for 24 hours at 80 °C until a homogeneous system was formed.Glycol ethylene was then slowly added to citric acid to reach a mixture of 40/60 w/w.This mixture was then heated to 100 °C, resulting in a resin which was pyrolyzed at 400 °C for 1 hour, at a heating rate of 10 °C/min.The pyrolyzed material was disagglomerated in a mortar, sifted through an ABNT 200 (74 µm) mesh sieve, and calcined at 500 °C for 1 hour, applying a heating rate of 10 °C/min.
The samples were characterized by X-ray diffraction in a Shimadzu XRD 6000 diffractometer using CuKα radiation with λ = 1.5418Å, operating at 40 kV and 30 mA.The measurements were taken in the range of 15° to 85° (2θ) with a step size of 0.02 o (2θ).Samples of 2 g of mass were pretreated under vacuum at 200 °C for 6 hours, followed by a textural analysis by nitrogen/helium adsorption developed by Brunauer, Emmett and Teller (BET).The 40-point nitrogen absorption isotherms were measured with a Quantachrome NOVA 3200e analyzer.Using this technique, the average particle size was also determined by Equation 1 17 . where, • D BET = equivalent spherical diameter (nm); • D = True density (g.cm -3 ); • S BET = surface area (m 2 .g - ).
The pore volume and pore diameter were determined by the theory developed by Brunauer, Joyner and Halenda (BJH).
The phases were identified using Shimadzu PMGR software and the JCPDF database.The structural parameters were refined by the Rietveld method 18 using the DBWS-9807 graphical interface to quantify the phases in the TiO 2 samples.The crystallite size of all the crystallographic planes was calculated by Scherrer's equation (Equation 2) 19 .
where k is the coefficient of the shape of the reciprocal lattice point (0.9 to 1.0), λ is the wavelength of the radiation to be utilized (1.54 Å), β is the width at half peak height (FWHM), and θ is the angle of diffraction.
The catalytic activity of all the samples was evaluated based on a photocatalytic degradation reaction using methyl red solution (20 mg.L -1 ).To this end, 1 g.L -1 of each sample was immersed in 200 mL of this solution under constant stirring and subjected to UV-C radiation (λ max = 280 nm and 60 W) using four 15 W low-pressure mercury-vapor lamps.The mixtures were placed at a distance of 12 cm from the UV-C source.The photocatalytic performance of the samples was evaluated in a reactor operating for 24 hours.

Results and Discussion
Figure 1 depicts the X-ray curves of the TiO 2 samples prepared by the Pechini method.As can be seen, the 2:1 sample presented formation of primary phase anatase (ICDD 84-1286), while the 3:1 and 4:1 samples presented anatase phase and traces of rutile as secondary phase (ICDD 88-1172).According to the literature, rutile phase is formed at high temperatures (≈1000 °C), while anatase phase is formed at lower temperatures close to 450 °C20 .The formation of anatase phase was found to be favored by a low ratio of citric acid to metallic cation, and increasing this ratio favored the formation of rutile phase at temperatures lower than those reported in the literature.
The citric acid molecule can play the role of a polydentate ligand, which has an α-hydroxyl group, an α-carboxyl group and two β-carboxyl groups, adding seven donor sites that can coordinate metallic ions.This can favor the formation of several types of titanium coordination 21 .From the thermodynamic standpoint, the chelating effect is caused mainly by the increase in entropy associated with the formation of polydentate compounds.This occurs due to the formation reaction in which each polydentate ligand substitutes at least two monodentate ligands, increasing the number of ions, molecules or free radicals.Thus, increasing the citric acid concentration in the samples may have promoted the formation of intermediate structures that can favor the formation of rutile phase at low temperatures.
Table 1 describes the quantities and crystallite sizes (D c ) of the phases in TiO 2 samples prepared by the Pechini method in citric acid/metallic cation ratios of 2:1, 3:1 and 4:1.The crystallite sizes correspond to the main planes of the family of crystallographic planes {hkl}.
The crystallite size of sample 2:1 was smaller than 20 nm, indicating the formation of anatase nanoparticles.It was found that increasing the citric acid/metallic cation ratio generally caused the crystallite size of the anatase phase to increase.In Table 1, note that samples 3:1 and 4:1 present rutile and anatase phase, with the rutile phase not exceeding 20%.reported by Li al. 23 , who prepared TiO 2 by the alcohothermal method, and also to those reported by Hussain et al. 9 , who prepared TiO 2 nanoparticles for application as a photocatalyst for the photocatalytic degradation of ethylene.
Figure 2 shows the kinetic curves (C/C o vs. t) of the photocatalytic degradation of methyl red dye, with and without the presence of the TiO 2 catalyst prepared by the Pechini method.
In Figure 2, note the discoloration of the dye in the presence of all the TiO 2 samples after 24 hours of reaction.This is due to the radiation wavelength used in this work Table 2 lists the values of surface area (S BET ), pore radius (R P ), pore volume (V P ) and particle size (D BET ) of the TiO 2 samples prepared by the Pechini method.To calculate the particle size from the BET results required first estimating the density of the mixture of phases based on the results of the phase quantification by Rietveld refinement.
A comparison of the results in Table 2 for the 2:1 and 3:1 samples indicated that the surface area of these samples decreased as the citric acid/metallic cation ratio increased.This may have been due to the increase in the formation of rutile phase, which has a more tightly packed tetragonal crystalline structure, in which each octahedron is coordinated with 10 neighboring octahedral, while each octahedron in the anatase phase is with coordinated with 8 neighboring octahedral.On the other hand, among the samples with rutile as secondary phase, the 4:1 sample showed the largest estimated surface area, which is consistent with the low D BET values, indicating a low degree of particle agglomeration in this sample.The nitrogen adsorption-desorption isotherms indicated that all the samples were mesoporous, with ink-bottle pores and an average pore radius of 19 Å.This mesoporosity was also reported by Nguyen-Phan et al. 22 , who prepared TiO 2 by the sol-gel route.The results of our study are similar to those (280 nm), which corresponds approximately to the required activation energy of TiO 2 (anatase), with a band gap of 3.23 eV 7 .Consequently, the best reactions were obtained with the 2:1 sample, which contained only anatase as a pure crystalline phase.
The superior performance of anatase phase over that of rutile phase in the photocatalytic process was not clear.However, this behavior was tentatively attributed to three factors: i) the rutile phase cannot adsorb O 2 , which is important in capturing electrons in the conduction band, reducing the electron-hole pair recombination rate; ii) the band gap energy of the anatase phase (3.23 eV) is higher than that of the rutile phase (3.02 eV), contributing to increase the probability of electron-hole pair recombination; and iii) the anatase phase presents higher-order quantum results, favoring the high density of active sites on the surface 24,25 .
Among the TiO 2 samples containing a combination of anatase and rutile phases, the photocatalytic activity was found to be proportional to the anatase phase in the sample.Therefore, the 4:1 sample containing 89.5% of anatase phase showed a better performance than the 3:1 sample, which contained a lower percentage of this phase (84.4%) and hence lower catalytic activity.In addition, the high specific surface area of 84.03 m 2 .g - of the 4:1 sample may have favored the high charge separation, inhibiting electron-hole recombination and favoring its performance.
Yang et al. 10 prepared TiO 2 by the sol-gel method using different heat treatments, and investigated its performance as a photocatalyst of methyl orange degradation.These authors observed the presence of anatase as the single phase after 2 hours of treatment at a temperature of 500 °C, and anatase and rutile phase after 2 hours at 650 °C.The results of the photocatalysis indicated that the performance of biphasic TiO 2 was slightly lower than that of the monophasic TiO 2 , with the former showing an efficiency of 80% and the latter of 85%.A comparison of the results of our work with those reported by the aforementioned authors indicates that each method of synthesis produces different characteristics in TiO 2 , which likely explains the formation of one or more phases.Melian et al. 25 obtained TiO 2 by the sol-gel method, followed by hydrothermal treatment to evaluate its photocatalytic activity in the degradation of phenol compared to Degussa P25 TiO 2 .Their results indicated that the sample calcined at 600 °C and containing 84% anatase phase and 16% rutile phase with particle sizes of 29 and 39 nm, respectively, was more efficient in degrading phenol than the commercial TiO 2 Degussa P25.
In general, it was found that both monophasic and biphasic samples presented satisfactory photodegradation efficiency.It was assumed that photocatalytic performance is affected not only by the type of phase in the TiO 2 sample but also by the method of synthesis and the type of dye employed.

Conclusions
The Pechini method proved efficient for the preparation of nanometric and mesoporous TiO 2 samples.The TiO 2 catalyst presented good performance in the photocatalytic degradation of methyl red dye.However, the photocatalytic activity proved to be dependent on the anatase content in the samples, which explains the higher photocatalytic activity of the sample with a citric acid/metallic cation ratio of 2:1 containing pure anatase phase.As for the 4:1 sample, its high specific surface area of 84.03 m 2 .g - was probably the reason for its better photocatalytic performance than that of the 3:1 sample, which also contained a trace amount of rutile phase.

Table 1 .
Phase quantities and crystallite sizes (D c ).

Table 2 .
Textural properties of TiO 2 samples prepared by the Pechini method.
*Calculated from BET surface area.