Effect of pH on the Formation of Amorphous TiO2 Complexes and TiO2 Anatase during Pyrolysis of TiCl4 Aqueous Solution

The TiO2 nanostructures resulted by pyrolysis of TiCl4 at low temperature of 80 oC are found to be a mixture of amorphous TiO2 complexes and anatase nanostructure that depends on the pH of the pyrolysis medium. Anatase nanostructure is predominant at low pH level and gradually converts to amorphous TiO2 complexes with increasing pH level. By means of heat treatment, the TiO2 nanostructures can be recovered from amorphous TiO2 complexes. Amongst of the TiO2 nanostructure recovered from amorphous TiO2 complexes, the anatase nanostructure shows to be the strongest photocatalyst in decomposition of methylene blue.


Introduction
Titanium dioxide (TiO2), a typical metal oxide with high refractive index, chemical stability, long durability, and nontoxicity has been widely used for many applications such as white pigments, textiles, papers, cosmetics, medicines, ceramics, etc. As a n-type wide bandgap semiconductor, TiO2 exhibits a unique photoinduced effect that involves photogenerated charge carriers on the material surface that initiate strong redox reaction of adsorbed substances and hydrophilic conversion of itself [1,2]. The effect offers more potential applications such as splitting hydrogen from water, ultraviolet fillers, photoconductor, photocatalyst, environment cleaning, antibacterial purpose, chemical sensors, dye-sensitized solar cells (DSSC) and so forth [3 -5].
Under normal condition, TiO2 exists in three main structures: stable rutile, metastable anatase, and brookite phase. For pure phase it is generally accepted that anatase exhibits a higher photocatalytic activity compared to that of rutile despite of its larger band gap (3.2 eV for anatase vs. 3.0 eV for rutile). Longer lifetime for photo-excited electrons and holes in the indirect band gap of TiO2 anatase semiconductor is accounted for the feature [6]. On the other hand, TiO2 in the microstructure have been considered as a poor photocatalyst but in the nanostructured form, due to the quantum confinement the material shows stronger photocatalytic activity when compared to that of the microstructure [7]. The unique photocatalyst of TiO2 nanostructures are size dependent. Therefore, clarification out the synthesis condition to achieve desirable nanostructures of TiO2 is of important to diverse photocatalystic applications. electrodeposition, sonochemical, and microwave method has been used [8]. Pyrolysis offers a simple route to synthesize well-crystalline TiO2 using inexpensive precursors such as TiCl4, TiCl3, amorphous TiO2, P25, etc. In addition, the pyrolysis modest medium of low temperature and adjustable time can provide an effective reaction environment for the synthesis of TiO2 with high purity, good dispersion and controllable crystalline. In a series of experiments made on TiO2 synthesis in acidic medium, say HCl, we have found that the pyrolysis of TiCl4 results in a nanocrystalline mixture of both the anatase and rutile phase. The TiO2 anatase mainly suspends in the aqueous solution while the TiO2 rutile predominantly deposits in the sedimentation [9,10]. Higher HCl concentration enables the agglomeration of small anatase particles forming uniform cluster and enhances the anatase to rutile transition due to the compensation of Clfor the positive charge of polyhedral complexes [9]. In addition to the involvement of Cl -, the pH level of pyrolysis medium is considered to be a critical parameter regulating the anatase fraction and variety of intermediate type of Ti(OH)4 in the form of [Ti(OH)4-n(H2O)2+n] n+ or Ti-complexes. The Ti-complexes then can decompose into amorphous TiO2 nanostructure, hence hereafter termed as amorphous TiO2 complexes. Based on that consumption, in this attempt NH4OH is used as an additive to adjust pH of the reaction medium and then clarify pH level's effects on the formation of amorphous TiO2 complexes as well as of anatase nanostructure during pyrolysis of TiCl4 aqueous solution.

Results and Discussion
Experiments show that the additive NH4OH significantly affects the appearance and properties of TiO2 nanostructures in the resulting materials. As clearly seen in Figure 1, the resulting aqueous solution is transparent at low pH level but gradually changes to slightly opalescent and then separates into transparent and milky parts when the pH level exceeding 2.40. The separated milky part increases with NH4OH and becomes unchanged as pH level exceeding 7.34. Depending on the transparent or milky state, the surface morphology of the resulting materials transforms from grain to gelation structure as shown in the SEM images in Figure 2. In the sample with pH of 0.98, the resulting material grains are uniform granular with mean size of around 50-70 nm, that in turn has found to be clusters of anatase nanoparticles of 4-5 nm in the mean size [9]. However, when the pH level of the reactive medium increases, the resulting material grain increasingly inflates to coagulated clusters of 150-200 nm in size and gradually become jellylike or amorphous structures as shown in Fig. 2    X-ray diffraction spectra in Figure 3 show the evolution of the resulting materials depending on the pH level, i.e, on NH4OH additive. In the sample with pH level of 0.98, the XRD pattern contains principal peak around 25.29° and the other peaks around 37.80°, 48.05°, 53.89°, 62.68° respectively asigning for the diffraction of anatase structure at (101) 211), and (220) planes, respectively from NH4Cl crystal [12]. The disappearance of TiO2 diffraction in XRD pattern is account for the gradual formation of amorphous TiO2 or generally amorphous TiO2 complexes [13]. Using Scherrer equation, i.e., D = kλ/βcosθ, where k = 0.94, λ = 0.154 nm and β is FWHM at diffraction angle θ according to (101) peak to calculate the mean size D of anatase particles, it has found that the mean sizes of anatase particle is almost unchanged around 4.5 nm as shown in Table 1. This size is considered to be the limitation of anatase size in the conversion to amorphous TiO2 complexes.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 2 August 2020 doi:10.20944/preprints202008.0037.v1 Raman spectra also confirm the present of anatase and NH4Cl in the resulting materials. As shown in Figure 4, in the starting materials, namely, the sample with pH level of 0,98, the spectrum exhibits vibrational mode around 155 cm -1 , 399 cm -1 , 513 cm -1 and 634 cm -1 respectively representing the Eg, B1g, A1g + B1g and Eg modes of anatase structure [14]. The present of NH4Cl in the gives rise to a broad saddle spectrum of two vibration modes around 168 cm -1 and 144 cm -1 that is asumed to be the supposition of Eg vibration mode of anatase and ν2, ν3 and ν4 vibration modes of NH4Cl oscilating against Cl along [100] direction and oscilating along three orthogonal directions [15].   diffraction in the XRD spectra. When the pH level exceeding 7.34, the milky part is unchnged, in the transparent part no trace of Ti present but in the milky as shown in the EDS spectra in Figure 5. This indirectly indicates the fact that the milky trace is an indicator of amorphous TiO2 complexes and the anatase fraction is completely converted into amorphous TiO2 complexes at the pH level of 7.34. Due to the armophous nature, no diffraction partten of amorphous TiO2 complexes is observed in XRD spectra. Figure 6. The appearance of anatase nanoparticle scatteringly embedded in the amorphous TiO2 complexes.
HRTEM image taken from milky sample with pH level of 10.0 as in Figure 6 shows the appearance of tiny nanocrystallites scatteringly embedded in amorphous medium. A lattice spacing of the tiny nanocrystallite around 0.346 nm is identical as the lattice spacing of the (101) plane of TiO2 anatase. The amorphous surrounding materials is considered to be amorphous TiO2 complexes. The estimated sizes of TiO2 anatase particle are the same as of those calculated from XRD pattern, around 4.5 nm that is considered to be the size limitation of anatasse in equilibrium with amorphous TiO2 complexes. The present of anatase nanoparticles embedded in the amorphous TiO2 complexes is accounted for the appearance of Eg vibration mode of anatase in Raman spectra in Figure 4.  Under heat treatment at elevated temperature, the XRD and Raman spectra indicate the conversion process from amorphous TiO2 complexes to anatase and then the transition from anatase to rutile. At heating temperature bellow 200 °C, the XRD pattern in Figure 7 shows only the trace of NH4Cl but not amorphous TiO2 complexes nor anatase. However, when heating temperature exceeds 200 °C the anatase diffraction is gradually emerging while NH4Cl diffraction is gradually disappearing in the XRD patterns. At high temperature, the NH4Cl is decomposed and vaporized in the form of NH3 and HCl gases while the amorphous TiO2 complexes is partly decomposed and recrystallized in the form of TiO2 anatase. When the heating temperature exceeds 300 °C, the NH4Cl is completely decomposed the materials is totally converted into TiO2 nanostructure with predominant anatase. When the heating temperature exceeds 450 °C the appearance of rutile diffraction in the XRD patterns indicates the onset of the anatase-ruttile transition. Using the Scherer equation, we could calculate the mean size of anatase incresing from arouns 4.5 nm at heating temperature of 200 °C to 8.9 nm at heating temperature of 600 °C as shown in Table 2. For the sample with pH level of 0.98, the anatase is predominant over amorphous TiO2 complexes, the heat treatment is merely the mean to enable the separation of the anatase nanoparticle from the cluster. The Raman spectra also confirm the conversion of anatase from the amorphous TiO2 complexes (the milky part) when it is heat up as can be seen in Figure 8. With baking temperature bellow 200 °C the Raman spectrum is a composition of NH4Cl vibration mode centered around 168 cm -1 and 144 cm -1 and the Eg vibration mode TiO2 anatase at 147 cm -1 [14]. As the heating temperature increases from 200 °C to 600 °C the Eg vibration mode shows a shift in frequency from 154 cm -1 to 145 cm -1 and an increase in FWHM (see in Table 2). The feature accounts for the increase in size of the TiO2 nanocrystallites.
The formation of TiO2 nanostructures by pyrolysis of TiCl4 in elevated pH medium is explained by the decomposition, dissolution mechanism [16]. At elevated temperature exceeding 80 °C, TiCl4 is decomposed into Ti(OH)4 and then into amorphous TiO2 complexes, TiO2 anatase and HCl in the pyrolysis medium. The component ratio of TiO2 anatase and amorphous TiO2 complexes then is established by an equilibrium between H + , OHand Clin the medium. In the acidic medium, TiO2 predominantly exists in the form of tiny anatase crystallites whose mean size bellow the size limitation for the anatase to rutile transition [9,10]. On the other hand, the weaker acidic, stronger basic medium enhances the formation of Ti(OH)4 and then amorphous TiO2 complexes. By addition of NH4OH to reduce the activity of HCl and increase of basic level in the reaction medium, the anatase portion is decreased and the amorphous TiO2 complexes are increased in the medium. At low pH level, anatase fraction is dominant while at high pH level the amorphous TiO2 complexes is dominant.  The amorphous TiO2 complexes can be converted back to the TiO2 anatase nanoparticles by heat treatment at temperature around 300 °C. Experiments show that amorphous TiO2 complexes and anatase nanoparticles exhibit strong photocatalytic activity upon exposure to UV light radiation. Quatitatively, a mixture of 50 ml of 0.25 µmol methylene blue (MB) aqueous solution and 50. 0 mg of amorphous TiO2 complexes or TiO2 anatase nanoparticles is stirred magnetically under dark conditions for 30 min before exposed upon a UV mercury vapor lamp. After a fixed exposure duration, 1.0 ml of the aqueous solution is taken out for UV-Vis characterization. The percentage of MB left in the solution standing for the photocatalytic activity of amorphous TiO2 complexes or TiO2 anatase nanoparticles is calculated by comparison of the relative intensity of MB principal adsorption peak in the UV-Vis spectrum. As shown in Figure 9, the photocatalytic activity of the materials is affected by synthesis condition, i.e, pH level of synthesized medium and the heat treatment following. The rates of photocatalytic oxidation of MB over UV illuminated TiO2 show to be well fitted to the Langmuir-Hinshelwood (L-H) kinetics model [17] as seen in the plots. rate) of the materials is weak as heating temperature bellow ~200 °C but rapid increases with elevated temperature beond that point. The anatase appears in the materials is accounted for the decomposion of amorphous TiO2 complexes and then recrystallization. At heating temperature around 300 °C, NH4Cl and amorphous TiO2 complexes are totally decomposed the materials completely turn into pure anatase as a result the photocatalytic activity of the materials is found to be maximum. Futher increasing the heat temperature, the photocatalytic is reduced due to the growth in size of anatase particle and the appearance rutile fraction from the anatase-rutile transition. As a comparison with the sample of predominant anatase that is synthesized in the medium of pH = 0.98, a lower heating temperature of around 200 °C is adequate to completely convert the materials to totally pure anatase.

Materials and Methods
Titanium tetrachloride (TiCl4) of 99.9 % purity (Sigma Aldrich Chemical Co.) as Ti precursor was used as received. Ammonium hydroxide solution (NH4OH) of 28 % NH3 (Merck Corp.,) was used as basic agent to change the pH of pyrolysis medium. The TiO2 synthesis process was straightforward as follows: TiCl4 was added dropwise into DI water at 5 °C to a concentration of 0.04 M, the pH level of the solution then was dropped to around 0.98 (starting point). By adding a small amount of NH4OH into the solution the pH level was adjusted and preserved at a point in range of 0.98 to 10.04. The solution then was poured into test tubes and placed in an oven at 80 °C , the onset point of TiCl4 decomposition. The solution was gradually changed to opalescent suspension indicating that the TiCl4 was thermally decomposed and converted into Ti complexes [Ti(OH)4-n(H2O)2+n] n+ (n is number of protonated OH ¯ groups) and then TiO2 accompanying with the formation of HCl and NH4Cl [11]. The pyrolysis was carried on about 3.0 h and the power was shut down, the solution was slowly cooled to room temperature. Depending on pH level, the appearance of resulting solution shows either transparent, opalescent or clearly splits into transparent and milky part as seen in Figure 1. For characterization, these parts were separated and dried by vacuum evaporation then were thermally treated in an oven with baking temperature up to 600 °C.
The structure of the resulting materials was determined by D8 Advance Bruker diffractometer using CuKα radiation of 0.154 nm wavelength. The mean size, D of TiO2 crystallites was calculated using Scherrer equation, i.e., D = kλ/βcosθ, where k = 0.94, λ = 0.154 nm and β is full width at half maximum (FWHM) according to the principal diffracted angle θ, i.e., (101) peak for anatase. Raman spectra were obtained on a LabRAM HR800 (Horiba) using a 632.8 nm excitation laser at a resolution of 1.0 cm -1 . TEM images were obtained using a JEOL JEM-2100 Transmission Electron Microscope. SEM images were conducted on a JEOL JEM-7600F Field Emission Scanning Electron Microscope. The photocatalystic activity of TiO2 nanostrutures was determined by measuring the degradation rate of methylene blue (MB) under UV light radiation.

Conclusion
The pyrolysis of TiCl4 aqueous solution generally results in a mixture of TiO2 anatase nanostructure and amorphous TiO2 complexes. The ratio of TiO2 anatase nanostructure and amorphous TiO2 complexes can be controlled by changing the pH of the pyrolysis medium. The anatase fraction is predominant at low pH level and gradually declines and completely converts to the amorphous TiO2 complexes at high pH level. By addition of NH4OH to adjust the pH, the pyrolysis of a 0.04M TiCl4 aqueous solution results in a mixture of TiO2 anatase nanostructure and amorphous TiO2 complexes at pH bellow 7.34 and predominant amorphous TiO2 beyond that point.
The amorphous TiO2 complexes is found to be converted to TiO2 nanostructure by heat treatment.
With annealing temperature around 300 °C, the amorphous TiO2 is completely converted into anatase nanostructure and gradually transform into rutile at high temperature. Amongst of the TiO2 nanostructure recovered from amorphous TiO2 complexes, the anatase nanostructure shows to be the strongest photocatalyst in decomposition of methylene blue.