Synthesis of TiO 2 Tubes Via Dissolution of TiOSO 4 Rod Using H 2 O 2

We demonstrate here for the first time an original and facile approach to preparing titanium oxide (TiO 2 ) tubes using hydrogen peroxide (H 2 O 2 ) and titanyl (IV) oxysulfate (TiOSO 4 ) as sacrificial templates. The method involves dissolving TiOSO 4 powder in H 2 O 2 solution and subsequently applying thermal treatment on the resulting amorphous precipitate. Prepared samples were characterized by X-ray diffraction, scanning electron microscopy (SEM) images, thermogravimetric (TG) analysis, and UV-vis analysis. Experimental results indicated that the samples exhibit tube-like structures with lengths of approximately 1.9 m m in diameter and 11.5 m m in length. Anatase and rutile phases were formed after annealing at different temperatures in air. The crystallite size estimated by Rietveld refinement showed an increase from 34.69 to 354.28 nm with an increase in the annealing temperature from 600 to 1000 °C. At the same time, the band gap values decreased from 3.14 to 2.97 eV. The electron density maps were calculated, and the electronic density was observed to increase from 497.56 e - Å -2 (600 C) to 533.10e - Å -2 (740 ° C) and decrease with the annealing temperature of samples. A possible mechanism for the formation of TiO 2 tubes was proposed.


Introduction
After the discovery by Fujishima and Honda that TiO 2 could be used as an electrode to split water into hydrogen 1 , its photocatalysis began to attract much more attention. The photocatalysis of TiO 2 is of great interest owing to its chemical stability, non-toxic characteristics, low cost, UV light absorption, photo-corrosion resistance, and oxidative properties.
TiO 2 exists mainly in three different crystalline habits: rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic). The photocatalytic performance of anatase is considered superior when compared to the other phases; this is attributed to a higher density of localized states and slower charge carrier recombination 2,3 . The anatase phase possesses a considerable bandgap energy of 3.2 eV; thus, it can only be excited under UV light irradiation. When excitation occurs, electrons from the valence band jump to the conduction band, leaving behind positive holes (h + ). The electrons and holes diffuse to the particle surface of the TiO 2 . When the holes react with water, free-radical oxidation can occur, which is capable of degrading organic dyes.
It is a well-known fact that the factors that affect the photocatalytic properties of TiO 2 include crystallinity, microstructure, surface area, phase composition, light harvesting, separation rate of photogenerated charges, and energy band structure. Hence, morphological control is a strategy to be considered for improving TiO 2 photocatalytic performance 4 . TiO 2 nanoparticles are widely applied in water and wastewater treatment because of their stronger catalytic activity. This increase in catalytic activity has been attributed to their smaller size, which has permitted a larger surface area per unit mass.
However, TiO 2 nanoparticles also have disadvantages, with the most important and critical being toxicity [5][6][7] . A large number of published studies have revealed that TiO 2 nanoparticles could be translocated through the pulmonary route to the brain or the systemic circulation through which other organs such as the kidney or liver could be reached.
Presently, TiO 2 tubes have attracted considerable interest because of their improved photocatalysis performance and lower toxicity than nanoparticles. TiO 2 tubes have many useful features, such as a high surface-to-volume ratio and multi-channel structure, making them convenient for mass transfer. The high surface area allows adsorbing many chemicals, and the unique structure makes better use of light through multiple reflections within its space 8 .
In this present study, we report an original and facile synthetic approach to the preparation of anatase TiO 2 tubes, using rod-like TiOSO 4 powder as a sacrificial template in the presence of H 2 O 2 . Different from most methods that use high-temperature processes, expensive precursors, or preparation instruments, our approach is eco-friendly and *e-mail: depaula@dfq.feis.unesp.br does not require expensive precursor reagents, exhibits reduced reaction time, and is template-free.

Materials characterization
The structures of the as-prepared samples, as well as the heat-treated sample, were studied by XRD at room temperature (25 °C) with a SHIMADZU XRD-6000 model, which provides Cu Kα radiation (λ = 1.544 Å). Measurements were taken in θ-2θ configurations from 20° to 65°, with a step size of 0.2° min −1 . To obtain the microstructural data of the TiO 2 tubes, a Rietveld refinement 10 was performed using the General Structure Analysis System program 11 suite with the EXPGUI interface 12 . The original Rietveld formulation and many of its successors 13 consider the diffraction line width as a smooth function of the d-spacing of the diffraction angle (2θ, whereas many peaks of interest near 2θ have very different widths). Hence, in this study, the peak profile function developed by Stephens 14 was used to model the experimental data. In this method, considerations are made for diffraction widths that are not a smooth function of d, which might arise from anisotropic sample-size broadening or from a particular pattern of defects (e.g., stacking faults). Finally, the bi-dimensional model for the crystallite size described by Larson and Von Dreele 11 was used to account for the anisotropy in the half-width of the reflection.
The electron density maps were calculated following the method previously described in detail 15 . In brief, a point (x, y, z) of the crystallite cell with volume (V) was calculated by Fourier series using the structural factors F(h, k, l): where (x, y, z) represents vector (r) of real space, with one vector space (a, b, c) and another vector (h, k, l), which represent the coordinates of one vector from the reciprocal space with base (a*, b*, c*), i.e., they are the coordinates from the diffraction plane, which is given by Bragg's Law. The electron density distribution in the plane {slice 1 with x = y = z = 1 (i.e., face a b c), with set (h k l) as the projection plane (101)} was calculated. A UV-Vis spectrophotometer (Varian Cary 100) with an integrating sphere attachment DRA-CA-30I for diffuse reflectance measurements was used to establish the optical band gap. Optical absorption was measured in the 250-1000 nm range. SEM images were obtained with a ZEISS model EVO LS15 with an acceleration voltage of 0.1-20.0 kV coupled with an EDX analyzer for stoichiometry. TG was performed using the TA Instruments Q60 under a nitrogen atmosphere from 30 °C to 1000 °C at a heating rate of 3 °C/min.

Mechanism for the formation of TiO 2 tubes
The process of the morphology evolution to obtain TiO 2 tubes is summarized in Figure 1. The tubular structure is obtained via an in-situ reaction and the dissolution of TiOSO 4 . In the process of synthesis, when the TiOSO 4 reacts with H 2 O 2 , a yellow-colored solution is immediately obtained, indicating the formation of peroxo complexes of titanium. This yellow-colored solution yields a water-insoluble yellow gel, titanium oxyhydroxide, that is maintained at room temperature. The formation of titanium oxyhydroxide occurs preferentially in the surface of the rod where the precursor has full contact with the reaction solution. As the reaction time increases, the TiOSO 4 in the central part of the rod that has not reacted with H 2 O 2 begins to be dissolved by the water present in the solution, resulting in the formation of a void space. According to Tian et al. 16 the center part of the rod has considerable surface energy and shows a greater tendency to dissolve.

SEM images
The obtaining the crystalline TiO 2 tubes and the morphological evolution process of tube formation as a function of reaction time is showed in Figure 2. As seen in Figure 2a, the initial precursor, titanium oxysulfate, exhibits a rod structure of approximately 2.3 µm in diameter and 13.8 µm in length. After 6 h (2b) and 12 h (2c) reactions, those rods evolved, for the most part, into a tubular structure, presenting a lot of tiny particles on the surface. Increasing the reaction time to 24 h (2d) is observed only tubular structure with a uniform diameter of 1.9 µm and walls characterized by a smooth surface, with a reduction in the number of particles on the surface with average size 460 nm.
After the synthesis of the precursors, we investigated the effect of calcination on the precursors' morphology. After calcination at a temperature of 740 e 800 °C (Figures 2e, 2f) respectively, the hollow tube structure is maintained, suggesting the excellent thermal stability of the tubes.

TG analysis
The TG of the TiO 2 precursor after drying at 60 °C for 24 h is shown in Figure 3. The weight loss is 11.6% as the temperature increase from 20 to 155 ºC, with the loss of free water and bound moisture, as well as some surface hydroxide radicals. A mass loss of about 5% occurs in the temperature range of 155 to 390 °C; this is attributed to the thermal decomposition of the peroxo-titanium complex and is due to the removal of sulfur. In the range of 390 to 700 °C, the  phase conversion from amorphous to anatase occurs, and in the range of 700 to 1000 °C the phase conversion from anatase to rutile phase take places.

X-ray analysis
Techniques of structural refinement, such as XRD, are robust in revealing qualitative and quantitative information of phase composition and lattice parameters. In this sense, we used Rietveld 11,17 refinement for all TiO 2 tubes obtained in this study, with the data presented in the Supplementary Information (SI). As can be observed, Figure 4 displays the XRD patterns of the peroxo-titanium complex obtained and of the starting material TiOSO 4 . There are differences between TiOSO 4 and the peroxo-titanium complex. The XRD pattern of TiOSO 4 (Figure 4a) shows two sharp peaks at about 10.7° and 18.8°; however, these peaks did not appear in the XRD patterns of the peroxo-titanium complex (as-prepared), indicating the occurrence of a chemical reaction and the formation of the amorphous titanium precursor. The XRD patterns of the samples annealed at 600, 700, 740, 800, and 1000 °C for 4 h are also shown in Figure 4b. The position of the peaks associated with anatase and rutile polymorphs was taken from ICDD 00-021-1272 and ICDD 01-076-0317, respectively. At a sample annealing temperature of 600 °C, the XRD pattern shows only the anatase phase; meanwhile, at 700 °C and 740 °C, the main content is the anatase phase with a modest amount of rutile, 6% and 12% respectively. With increasing annealing temperature, the presence of rutile phase increases; thus, for sample annealing at 800 °C, the amount of rutile is 50% and, finally for the samples at 1000 °C, only the rutile phase is present. The full-width at half-maximum (FWHM) of the 101 diffraction peak of pure-phase anatase decreased with increasing annealing temperature, suggesting that the average crystallite size increased through thermal treatment. In fact, the crystallite size estimated by Rietveld refinement, reported in Table 1 for the pure-phase anatase annealed at 600 °C was 34.69 nm, and for the pure-phase rutile obtained at 1000 °C, the average crystallite size was 354.28 nm.
By analyzing the Rietveld refinement obtained for all TiO 2 tubes, it is possible to obtain fundamental information regarding the electronic structure of such samples as Fourier electron density maps. These electronic density maps can be observed in Figure 5.
Several papers in the literature [18][19][20][21][22] have described physicochemical properties tuned with the Fourier electron density maps obtained by Rietveld refinement 10,11 . Many of those papers 23-27 also observed similar Fourier electron density maps obtained by Rietveld refinement 10,11 of TiO 2 with several types of morphology. Freitas et al. 22 observed an increase in dielectric relaxation 26 , electronic lifetime 27 , Debye length screening 27 and yield of water splittingfor BaBi 4 Ti 4 O 15 nanoparticles, CdSe@TiO 2 nanotubes, and TiO 2 nanotubes, respectively. In this study, we observed similar results, whereas it is possible to observe that the electronic density increases from 497.56 e -Å -2 (600 °C) to 533.10 e -Å -2 (740 °C). In addition, it is observed that by increasing the rutile phase amount, as observed in Table 1 for TiO 2 tubes obtained at 800 °C, the electronic density decreases to 513.90 e -Å -2 , as observed in Figure 5. It is important to stress that all electronic density obtained herein for TiO 2 tubes were obtained along the anatase (101) plane.
The tunning of electronic and structural properties can be observed not only for nanotubes, but also for several other morphologies such as nanocube 28 , nanocone 29 , nanopillar 30 , nanoplate 31 , nanopyramid 32 , nanorod 33,34 , nanosheet 35 , nanosphere 36,37 and nanowire 38,39 . He et al. 33 studied leadfree (K,Na)NbO 3 nanorods arrays and observed that both, the charge density map and deformation charge density map exhibit asymmetric charge distribution along the [110] orientation, which should be due to the displacement of Nb atom from the center position toward the [110] direction. Botelho et al. 37 studied structural and electronic properties of CaMoO 4 nanospheres using electron density maps. According to the authors 37 , the color scale of each map represents zones of high and low electronic densities, i.e., the blue and red regions have a high electronic density and no electronic charge, respectively. Therefore, the authors 37 obtain in a fundamental point of view the position of electron and hole along the semiconductor. Nanowires morphology is also an important nano-morphology for several devices' studies. In this sense, Degtyarev et al. 39 studied the effect of geometry, doping and surface states in InAs nanowires. The authors observed that electron space distribution and the electron density in the intrinsic InAs nanowires are strongly affected by the charged surface states, nanowire width and doping. The dominating type of the electron space distribution is tubular with additional electron accumulation regions at the nanowire corners coupled to each other. Therefore, electron density map is pivotal for the fundamental comprehension and tuning of electronic and structural effects as consequence of a-priori morphology.

UV-vis analysis
The bandgaps of the synthesized TiO 2 tubes were investigated by UV-Vis diffuse reflectance spectra following the Kubelka-Munk (K-M) formula 40 : where R ∞ is the absolute diffuse reflectance for an infinitely thick sample, and K and S are the sample absorption and scattering coefficients, respectively. The energy of the band gap is calculated by extrapolating a straight line to the abscissa axis. The plots of / K S versus E g from UV-Vis spectrum data of the samples annealed at 600 °C, and 1000 °C were estimated to be 3.14 eV and 2.97 eV for the phasepure anatase and rutile, respectively. When the calcination temperature increases, the band gap decreases. These values are consistent with those obtained by Landmann et al. 41 . All dates obtained are reported in Table 1.

Conclusion
In this study, we described a novel method with a simple approach, efficient and low cost, for the preparation of large-diameter TiO 2 tubes via dissolution of rod-like TiOSO 4 using H 2 O 2 . Anatase and rutile phase was formed after annealing at different temperatures in air. TiO 2 tubes obtained after annealing exhibited a tubular structure with a uniform diameter of 1.9 μm and walls characterized by a smooth surface and small particles on the surface. XRD analysis showed that annealing at temperatures higher than 600 °C resulted in the evident transformation of the anatase phase into the rutile phase. Pure rutile phase was obtained by annealing nanostructured anatase at 1000 °C. The crystallite size estimated by Rietveld refinement increased from 34.69 to 354.28 nm with an increase in the annealing temperature from 600 to 1000 °C. At the same time, the band gap values decreased from 3.14 to 2.97 eV. Fourier electron density maps obtained by Rietveld refinement was possible to observe that the electronic density increases from 497.56 e -Å -2 (600 o C) to 533.10 e -Å -2 (740 o C) and for TiO 2 tubes obtained at 800 o C, the electronic density decreases to 513.90 e -Å -2 .
In summary, this study provides a novel pathway to the synthesis of TiO 2 tubes with an eco-friendly approach that does not require an expensive precursor, exhibits reduced reaction time, and is template-free. This novel pathway offers a new material platform for catalysis, photovoltaic cell, and other applications.