Titanium dioxide is a versatile material with a range of applications associated with its unique optoelectronic and photochemical properties such as high refractive index, high dielectric constant, and excellent optical transmittance in the visible and near IR regions as well as its high performance as a photocatalyst  . Since TiO2 is chemically and biologically inert, photocatalytically stable, easy to produce and use without risks to environment and human, it is the well-researched material for wastewater treatment from industries, factories, laboratories, degradation of textile dyes, pesticides and antimicrobial studies    .
It exists in three crystalline modifications anatase, rutile and brookite. Anatase and rutile are well-known photocatalysts with anatase generally showing higher photocatalytic activity  . Rutile form of TiO2 has some advantages over the anatase form in terms of higher chemical stability, higher refractive index and is also comparable to anatase in application to dye-sensitized solar cells  . Applications of brookite are not much investigated probably because it is rarely obtained in a pure form.
Anatase TiO2 has been synthesized by different precursors (Ti(OnBu)4, Ti(OiPr)4, TiCl4, TiCl3, Ti(SO4)2 and by different synthesis routes such as hydrothermal, thermohydrolysis and sol-gel process. Among the different methods of synthesis, the sol-gel method is considered advantageous and reproducible one as it leads to nanoparticles of high surface area  .
Rutile is generally obtained by calcination of the first crystallized anatase at temperatures higher than 500˚C. Direct formation of rutile is possible through long term aging of sols or exposure to high relative humidity or by direct hydrolysis of inorganic salts such as titanium tetrachloride (TiCl4) in aqueous solutions under hydrothermal or moderate conditions  .
Anatase TiO2 was mostly studied for photocatalytic degradation but lately Rutile TiO2 has also shown promising results in photocatalytic studies. The high photocatalytic activity of rutile TiO2 for decomposition of rhodamine-B in water is reported under artificial solar light irradiation  . Basca and Kiwi found that the presence of rutile TiO2 showed enhanced catalytic activity compared to pure anatase TiO2 during degradation of p-coumaric acid  . Rutile phase has also seen to be more active than anatase in photodecomposition of H2S and photodecomposition of H2O   . The photovoltaic characteristics of rutile TiO2 based DSSCs were found to be comparable to those of anatase TiO2-based solar cells   .
The present investigation is aimed to synthesize pure rutile phase photoactive Titania by a simplified procedure. Rutile is generally obtained by calcination of the first crystallized anatase at temperatures higher than 500˚C or by doping it with cations such as (Li+, Al3+, Zn2+, K+, Na+)   . Direct formation of rutile is possible through long term aging of sols or exposure to high relative humidity or by direct hydrolysis of inorganic salts such as titanium tetrachloride (TiCl4) in aqueous solutions under hydrothermal or moderate conditions or by using nano rutile as the seed   . Photoactive rutile TiO2 nanorods have been synthesized using titanium bis (ammonium lactate) dihydroxide as precursor by hydrothermal method and by decorating with anatase nanoparticles  . Controlled synthesis of rutile TiO2 microspheres, nanoflowers, nanotubes, nanobelts and single crystal rutile TiO2 nanotube/nanorod arrays is reported, via acid-hydrothermal synthesis  .
We have earlier reported that TiCl3 is a convenient precursor to obtain TiO2 through a solid state decomposition route  . Recently TiCl3 hydrolysis method has been investigated in detail for the synthesis of TiO2 owing to simplicity of TiCl3 hydrolysis method. TiO2 nanoparticles have been synthesized by hydrolysis and oxidation of TiCl3 at 60˚C in aqueous medium through mixed phenomena of precipitation and oxidation by varying the pH of the precursor between 0.5 pH and 6.5 pH  . 1d nano rutile TiO2 has been synthesized by interfacial reaction using TiCl3 and benzoyl peroxide by hydrothermal treatment at 80˚C for 6 hours  . Rutile TiO2 was also obtained by direct oxidation of TiCl3  . The synthesis of rutile Titania by hydrothermal method using titanium isopropoxide has also been reported  . Flower-like rutile Titania nanocrystals were obtained via aqueous phase stirring of TiCl4 and HCl at low temperature  . However, TiCl4 is highly hazardous to handle as it fumes easily and hence not considered here.
The research to produce one step synthesis of anatase or rutile nanocrystalline powder at low temperature is of considerable significance. We hereby report for the first time rapid synthesis of photoactive pure rutile phase TiO2 by hydrolysis of TiCl3 in HNO3 medium at 25˚C without using any harsh experimental conditions.
2.1. Catalyst Synthesis
A titanium trichloride solution in HCl (15%) was taken in a beaker and HNO3 was added to it, till the colour of TiCl3 changed from violet to clourless. To this distilled water was added such that the ratio of TiCl3 and water was 1:9 (V/V). The resulting mixture was then kept on a magnetic stirrer for hydrolysis at room temperature. Hydrolysis of TiCl3 in presence of nitric acid was complete in 10 hours. The obtained precipitate was washed free of chloride ions and calcined at 400˚C for 3 h. This sample was named as A1. A blank was kept with the same ratio as TiCl3 and water (1:9) but without the addition of HNO3. It took 48 hours for TiCl3 hydrolysis in the absence of nitric acid medium.
2.2. Catalytic Activity Test
The photocatalytic activity of synthesized sample was investigated for the degradation of methylene blue dye (10−4 M) and congo red dye (3.5 × 10−6 M) using synthesized catalyst A1 and Degussa (Evonik) P-25 as a standard reference catalyst as per the procedure reported earlier  . The experiments were carried out simultaneously for both the catalysts in bright sunlight between 10.00 a.m. to 12.00 noon. Experiments were repeated simultaneously on three different days in order to confirm the consistency of results. In a typical run 50 mL of aqueous dye solution and 0.1 g of the activated catalyst was exposed to sunlight for the duration of the experiment. The solutions thus exposed to sunlight were stirred intermittently. After every 30 minutes, 2 mL aliquots were pipetted out, centrifuged and the absorbance of the clear supernatants was determined at 660 nm and 400 nm wavelength for methylene blue and congo red respectively against appropriate blanks using spectrophotometer.
Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of precursors were carried out on Shimadzu DTG-60 thermal analyzer. The X-ray powder diffraction patterns (XRD) have been recorded on a Rigaku powder X-Ray diffractometer (Mini Flex II) with Cu Kα radiation at a scanning speed of 2˚/min. The crystallite size was determined by Scherrer’s formula, t = (0.9 λ/β cosθ) where λ is the wavelength characteristic of the Cu Kα radiation, β is the full width at half maximum (in radians) and θ is the angle at which 100 intensity peak appears. The BET surface area was measured by N2 adsorption-desorption method on a Micromeritics Tristar 3000. FEI (model Tecnai F30) high resolution transmission electron microscope (HRTEM) operating at 300 kV was used for TEM measurements. The absorption edges and band gaps were determined from the onset of diffuse reflectance spectra of the samples measured using UV–VIS spectrophotometer (Shimadzu UV-2450).
3. Results and Discussions
3.1. Characterization of Materials
3.1.1. Thermogravimetric Analysis
TG-DTA thermogram of the white precursor obtained by hydrolysis reaction of TiCl3 with nitric acid is given in Figure 1. It is clear from the TG profile that the decomposition of the precursor is completed between 300˚C - 400˚C. The weight loss at 120˚C is indicative of loss of water owing to partial dehydroxylation of the OH groups.
3.1.2. X-Ray Analysis
The XRD pattern of the synthesized sample is shown in Figure 2. The diffraction
Figure 1. Thermogravimetric and differential thermal analysis curves of the precursor for A1.
Figure 2. XRD pattern of the synthesized catalyst A1.
peaks in this pattern can be indexed to rutile phase [JCPDS No. 76-1940]. Thus, sample A1obtained by precipitation of TiCl3 in presence of nitric acid respectively, showed pure rutile phase.
Table 1 describes the synthesis of rutile phase Titania and its structural features. As evident from Table 1, hydrolysis of TiCl3 in presence of nitric acid was complete in 10 hours (A1) as compared to the blank that took 48 hours. It is noted from literature  that relatively fast hydrolysis of TiCl3 in presence of H2O is possible only at elevated temperatures. It has been observed that the ratio inorganic precursor: Water:HNO3 is an important factor for controlling the rate of hydrolysis and phase formation. In TiO2 rutile structure, (110) plane is the most thermodynamically stable and the growth rate of the plane (110) occurs faster compared to other planes  . Thus rutile TiO2 is formed in the stable Ti4+ acidic solution. In the present investigation a simple and rapid method is reported, to obtain TiO2 from aqueous solutions of TiCl3 at room temperature. The surface area of A1 is also significantly higher than Degussa (Evonik) P-25 (Table 1).
3.1.3. HRTEM Analysis
HRTEM micrographs of the synthesized sample are shown in Figure 3. HRTEM images of A1 (synthesized in presence of HNO3) show the formation of flower-like structures. It reveals that these structures are a unique assembly of nanoflowers like structures with ~40 nm length and 10 - 15 nm diameter. It is observed that the reported synthesis process has a capability to produce self organized nanostructures with flower-like morphology without use of any templating agent.
Table 1. Synthesis and structural features of titania samples.
Figure 3. TEM image of A1 synthesized by hydrolysis of TiCl3 in presence of HNO3 (Scale represents (a) 500 nm, (b) 200 nm).
The formation of nanoflower structured rutile TiO2 may be related to the synthesis method applied in the procedure. The oxidation of the TiCl3 (in 15% HCl solution) with HNO3 produces Ti4+ ions in the aqueous acidic solution. The hydrolysis of Ti4+ ions in the acidic solutions results in the production of fewer TiO2 polycrystalline seeds. Thus the solution consists of supersaturated solution of Ti4+ ions and fewer polycrystalline TiO2 seeds. The crystal growth of seeds into nanorods occurs by the deposition of Ti4+ ions from the solution on the surface of this polycrystalline seeds. Subsequent crystal growth proceeds to form nanorods along specific directions and then self-assemble into nanoflowers through Ostwald ripening process  .
3.1.4. N2 Adsorption-Desorption Isotherm
The N2 adsorption-desorption isotherms and pore distribution analysis for sample A1 (inset) is given in Figure 4. The adsorption-desorption isotherm of the synthesized materials shows Type IV physisorption isotherm with H2 hysteresis loop, which is the characteristic of mesoporous materials  . In the isotherm the pore filling step occurs at low relative pressure (P/P0 = 0.44) and also the upper closure point of the hysteresis loop appeared at a relatively lower value of (P/P0 = 0.72) which indicates that the sample contains smaller pores  . From pore size distribution it is observed that the pores are small and narrow with uniform pore size of 3.6 nm diameter. Sharp hysteresis loop and narrow pore size distribution in the porosity analysis confirmed the presence of smaller pores which may have formed by the ordered arrangement of nanosize rods of individual nanoflowers.
3.1.5. UV-Vis Absorption Study
Figure 5 shows the UV-Vis absorbance spectra of the synthesized Titania sample. It is clear that the samples showed absorption edge in the visible region (Table 2). The band gap of the samples was determined by the equation Eg = 1239.8/λ, where Eg is the band gap energy (eV) and λ (nm) is the wavelength of the absorption edges in the spectra  .
Figure 4. N2 adsorption-desorption isotherm and BJH pore analysis of A1.
3.2. Photocatalysis Study
The percentage degradation profiles of methylene blue and congo red with time are shown in Figure 6(a) and Figure 7(a) respectively. A1 catalyst showed high photocatalytic activity almost equal as TiO2 Degussa (Evonik) P-25 catalyst.
The time dependant UV-Vis spectra of methylene blue and congo red dye in presence of catalyst A1 is given in Figure 6(b) and Figure 7(b) respectively. It is clear from both the figures that the decolourisation of dye precedes with increase in time. A1 obtained by room temperature hydrolysis of TiCl3 in presence of nitric acid medium showed a 100% rutile phase, yet it showed photocatalytic activity comparable to Degussa (Evonik) P-25 for photodegradation of methylene blue as well as congo red. This is probably the first report of a rutile phase showing such high photocatalytic activity for both cationic dye methylene blue and anionic dye congo red. This excellent photodegradation using A1 can be due to the presence of favorable parameters such as particle size, nanorod shape of the particles, crystalline phase and very high surface area. It also possesses small
Figure 5. UV-Vis absorbance spectra of the catalyst A1.
Table 2. The calculated band gaps of the Titania samples.
Figure 6. (a) Percentage degradation of methylene blue with time during photocatalysis using (i) A1, (ii) Degussa (Evonik) P-25; (b) Time dependent UV-Vis spectra of methylene blue in presence of catalyst A1.
Figure 7. (a) Percentage degradation of congo red with time during photocatalysis using (i) A1, (ii) Degussa (Evonik) P-25; (b) Time-dependent UV-Vis spectra of congo red in presence of catalyst A1.
band gap (2.88) in comparison to the reported band gap of Degussa (Evonik) P-25 (3.14). It is earlier reported that for each titanium dioxide phase, some optimum size and shape exists where each phase presents high photocatalytic activity  . Thus, this investigation resulted in rapid synthesis of a nano rutile catalyst having excellent efficiency for degradation of methylene blue as well as congo red. The photocatalytic process mainly occurs on the surface of the photocatalyst and adsorption precedes photodegradation  . As seen from Table 1 catalyst A1 had a high surface area and a smaller crystallite size. Other factors such as narrow pore size distribution and lower band gap could have facilitated the adsorption and degradation of the dye in the visible region with enhanced photocatalytic activity comparable to that of Degussa (Evonik) P-25.
· TiO2 can be rapidly synthesized at room temperature from aqueous solutions of TiCl3 in presence of nitric acid;
· Controlled hydrolysis of TiCl3 yields pure rutile phase TiO2 with high surface area having flower-like assembly of nanosized rods;
· Rutile TiO2 synthesized by a simple hydrolysis method showed excellent efficacy for degradation of cationic dye methylene blue as well as anionic dye congo red in the visible light;
· The ease of synthesis makes it an attractive and promising catalyst and its efficiency could further be explored for DSSC’s, water splitting, photocatalytic reduction of CO2, etc. in future studies.
The authors are grateful for the financial support received from DST Nano Mission program, SR/NM/NS-86/2009.