Influence of pH on the Formulation of TiO2 Nanocrystalline Powders

TiO2 nanoparticles were prepared by Sol-Gel method at different pH values (4, 3.5, 3, 2.5, and 2). All samples were heated at 500°C for 18 h. The optical, morphological, and structural properties of the samples have been investigated using XRD, SEM, and UV-vis spectrophotometer techniques. The results indicated the formation of TiO2 nanoparticles with pure Anatase phase at pH= 4 and 3.5, while further decreasing of pH, the Rutile phase start to appear intensively compare to Anatase phase. For pH=3, the ratio Rutile phase to Anatase phase was found to be around 76%.

Titania nanoparticles are typically synthesized in three different crystalline structures; anatase, rutile, or brookite [6], the first two types are mainly used as industrial materials [7]. Anatase and rutile structures are based on distorted TiO 6 octahedra, and due to slightly different bonding lengths and angles between titanium and oxygen atoms, they have different space groups and cell parameters. Anatase has I 41/amd space group with cell parameters of a=3.784 and c=9.515, whereas rutile has P42/mnm space group with cell parameters of a=4.5936 and c=2.9587 [8,9]. Both structures are tetragonal. Brookite, on the other hand, has a more complicated orthorhombic structure with Pbca space group and cell parameters of a=9.184, b=5.447, and c=5.145 [8,10]. Brookite has been considered to be brittle, and generally it is less studied than the other two crystal structures, even experimentally. Although brookite is usually considered to be industrially uninteresting there are, however, some reasonable recent studies on brookite nanorods as highly active photocatalysts [11]. The experimental band gap of anatase TiO 2 is 3.2 eV, and the band gap is indirect. Rutile is thermodynamicly the most stable form of TiO 2 , and it can withstand high temperatures, whereas anatase and brookite are converted to rutile when heated. Rutile has quite high measured melting point of 1840 ± 10°C [12]. Rutile has experimentally measured direct band gap of 3.14 eV, where anatase has band gap of 3.55 eV [13].
In case of all the crystalline forms, each Ti 4+ ions are surrounded by an irregular octahedron of oxide ions. In the rutile structure each octahedron is in contact with 10 neighbour octahedrons (two sharing edge oxygen pairs and eight sharing corner oxygen atoms) while in anatase structure each octahedron is in contact with eight neighbours (4 sharing an edge and four sharing a corner). The octahedral linkage in brookite is such that three edges are shared per octahedron [14].
Anatase (A) phase is less stable and undergoes transition into rutile (R) phase because of its low density. This transformation takes place between temperatures 450 and 1200°C [15]. The transformation is dependent on several parameters such as initial particle size, initial phase, dopant concentration, reaction atmosphere and calcination temperature, etc. [16][17][18][19]. The control of A to R phase transitions, which is an irreversible process, is necessary for the performance of this material in many applications such as sensors, photocatalysis, pigments and electronics [20]. From the mechanism of A to R transformation, it seems that phase transformation is a nucleation and growth process [21,22].
In this work, an effect of pH level on AR transformation will be studied. Pure and mixed phase of TiO 2 nanoparticles have been successfully grown by controlling in level of pH value during growth processing.

Experimental
Titanium (i v) iso-propoxide [C 12 H 28 O 4 Ti, purity 97%, Sigma-Aldrich] and 2-propanol [(CH 3 ) 2 CHOH] (purity 99% Sigma-Aldrich) are mixed with each other in volume ratio 1:4 respectively, then the resulted mixture is stirred for one hour. Another mixture of deionized water and 2-propanol with volume ratio 1:1 is added, the mixture is stirred for one hour more. pH value of the solution was adjusted at 2, 2.5, 3, 3.5, and 4 by adding HCl acid (concentration 98%, Sigma-Aldrich). The final solution is stirred for one hour at room temperature. Final mixture was put in water path at 80°C to evaporate any liquid in mixture. A resulted white powder TiO 2 is heated for all samples with different pH values at 500°C for 18 h.
XRD measurements of all samples carried out with Shimadzu diffractometer using CuKα1 radiation with a wavelength of 0.154 nm. The XRD data were collected at scan step of 0.02°, scan speed of 8.0°/min, 40 kV, and 30 mA. Absorption spectrums of our samples have been measured by UV-vis spectrophotometer model (JASCO V-630). All morphology images have been taken by SEM system model (JEOL JSM-651OLV).   [24]. From these results, it is clear that Anatase Rutile Transformation (ART) occurs at pH value lower than 3.5. This result has good agreement with previous reports on ART of TiO 2 nanoparticles [25]. From the mechanism of A to R transformation, it seems that phase transformation is a nucleation and growth process [22,26]. Also it is stated that in the synthesis of TiO 2 by various methods, the initial crystalline TiO 2 phase formed is generally anatase [27,28]. From a structural perspective, this could be due to the greater ease of the short-range ordered TiO 6 octahedra in arranging into long-range ordered anatase structure owing to the less-constrained molecular construction of anatase relative to rutile [29].

Results and Discussion
The relative weight fractions of anatase phase, W A , and rutile phase, W R , are calculated using the relative intensity of the (1 0 1) peak of anatase and the (1 1 0) peak of rutile and the relationship W R = 1/ 1 + 0.8I A /I R ) and W A = 1/(1 + 1.26I R /I A ) as used by Spurr and Myers [30]. These calculations have been done for mixed phase samples (e. g. pH value equal 3, 2.5, and 2). Results are shown in table 1. From table 1, a sample that prepared at pH 3 has mixed phase where 76.8% of sample have rutile phase structure while only 23.05% of sample have anatase phase. By decreasing pH value to lower levels, a percentage of rutile phase more increase while a percentage of anatase phase more decrease. It is reported that ART is irreversible process [25]. Furthermore, it is reported frequently that a mixed anatase/rutile or anatase/brookite/rutile phase composition is advantageous for photocatalytic and optoelectronic applications owing to improved charge carrier separation [31][32][33].
The crystal size for all samples was calculated by using Scherrer equation. Results are shown in figure 2. It is clear that the average crystal size of TiO 2 nanoparticles is decreased with increasing in pH value during synthesis method. This occurs because of the changing in phase of TiO 2 nanoparticles.   Figure 3 shows the morphology of TiO 2 nanoparticles prepared at different pH value. All samples were annealed at 500°C. It is clear from figure 3 that the particle size of TiO 2 is decreasing by the increasing in pH value where all images were taken at magnification equal 1 µm. these results have a good agreement with XRD results.
From XRD analysis and SEM images of these samples, the sample which was prepared at 3 level of pH value is the most mixed phase sample according to calculations that were shown in table 1. Mixed phase TiO 2 nanoparticles are preferred for photocatalyst and optoelectronic applications.

Conclusion
In this work, the pure and mixed phases of TiO 2 nanoparticles have been successfully grown by Sol-gel processing. XRD analyses of our samples show that AR transformation taken place at pH value lower than 3.5 levels.
SEM images of all samples show that grain size of TiO 2 decrease with increasing of pH value. This has a good agreement with XRD results. The calculated band gap of TiO 2 in this work is found to be about 3.14 eV.