1 D TiO 2 Nanostructures Prepared from Seeds Presenting Tailored TiO 2 Crystalline Phases and Their Photocatalytic Activity for Escherichia coli in Water

Universidad Nacional de Ingeniería, Av. TúpacAmaru s/n, Rimac, Lima, Peru Instituto de Física, Universidad Nacional Autónoma de México, 20364 Ciudad de México, Mexico Instituto de Investigación e Ingeniería Ambiental, CONICET, Universidad Nacional de San Martín, Campus Miguelete, 25 de Mayo y Francia, 1650 San Martín, Provincia de Buenos Aires, Argentina Centro de Microscopias Avanzadas, FCEyN, Universidad de Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires, Argentina


Introduction
TiO 2 nanomaterials are well-studied and commonly used photocatalysts for the degradation of organics, water splitting, and solar cells, among others [1][2][3][4].In the last years, several approaches were explored to increase the photoefficiency of TiO 2 , with the modification of the particle morphology and dimensionality being one of the newest [5].One-dimensional (1D) nanostructures such as nanotubes, nanorods, nanowires, and nanobelts have attracted great attention because of their unique properties that may be beneficial for photocatalysis: (i) enhanced light absorption due to the high length/diameter ratio, (ii) rapid and long-distance electron transport capability, (iii) large specific surface area, and (iv) ion exchange ability [6].Hydrothermal treatment of TiO 2 particles in alkaline solutions is one of the simplest and cheapest techniques to produce 1D-layered titanate structures.The hydrothermal synthesis of TiO 2 nanotubes involves several steps where the structure of the TiO 2 precursor changes completely.
Results obtained in our laboratories show that 1D TiO 2 nanostructures display photocatalytic activity for dye degradation [7].Although its performance as a photocatalyst is not as good as other industrially produced TiO 2 , this form of TiO 2 can be easily recuperated from the solution.
Furthermore, since 1985 when Matsunaga et al. [8] published the first report of the photocatalytic biocide effects of TiO 2 under metal halide lamp irradiation, there has been increasing interest in photocatalytic disinfection.Use of TiO 2 nanoparticles in suspension is an efficient method for decontamination due to the large surface area of catalysts available to perform the reaction.It has, however, some drawbacks before its scaling at the industrial level; for example, the necessity of removing the catalyst from the solution after decontamination using filtration increases the cost and time of the cleaning process [9].
In this sense, considering the advantages that our 1D TiO 2 nanostructures are more easily filterable than nanoparticles and can be easily removed from solutions-in addition to the fact that the efficiency of 1D TiO 2 as bactericide under UV-A irradiation was only briefly explored-in this work, we assess the photocatalytic activity of 1D TiO 2 , obtained from nanoparticles with a controlled proportion of anatase and rutile made by the sol-gel method, for E. coli ATCC 25922 in water.
2.2.Synthesis of TiO 2 Nanostructures.TiO 2 nanoparticles (TiO 2 NPs) were synthesized by the sol-gel method (SG).Titanium isopropoxide was added drop by drop to vigorously stirred HNO 3 or HCl solutions at pH 0.5, 0.8, and 1.0.Suspensions were heated at 70 °C for 2 h, autocleaved in a stainless-steel chamber at 220 °C for 12 h, washed by centrifugation, and dried at 60 °C.
1D TiO 2 nanostructures were synthesized by hydrothermal treatment of 1 g TiO 2 NPs obtained by the sol-gel method in 40 mL of 10 M NaOH at 130 °C for 24 h.After hydrothermal treatment, the obtained white powder was vacuum filtered, washed with HCl solution for ionic exchange, and then washed with distilled water until a neutral pH was reached.Finally, the samples were annealed at 400 °C for 2 h to crystallize the material.
The obtained nanostructures were characterized by X-ray diffraction (XRD) in a Rigaku diffractometer using Cu Kα radiation (λ = 1 54056 Ãƒâ€¦).The morphology was studied by field emission scanning electron microscopy (FE-SEM SUPRA 40, Carl Zeiss) and high-resolution transmission electron microscopy (HRTEM) using a JEOL JEM-2010F transmission electron microscope operating at 200 kV.TEM samples were prepared by dispersing a small amount of the sample in ethanol with the help of an ultrasonic bath.Small droplets of the freshly prepared dispersion were placed onto a copper grid covered with carbon to improve the conduction of the electrons.

Assessment of Photocatalytic Activity of TiO 2 Nanoparticles and 1D TiO 2 Nanostructures against Escherichia coli in
Water.The photocatalytic activity for water disinfection was tested using E. coli ATCC 25922.Experiments were performed in a batch reactor, with illumination from above using an Ultra-Vitalux 300 W lamp (30 W/m 2 ) and, under dark conditions, containing 100 mL aqueous solution with 10 7 CFU/mL bacteria.1.0 mL aliquots were collected after 0, 20, 40, and 60 min irradiation.Aliquots were diluted 1 : 10 with sterile water to fit in the range 10-500 CFU/mL.1.0 mL samples of the final dilutions were vacuum filtered through a sterile filter; this results in all bacteria present in the water being retained on the filter.Finally, the filters were placed onto a paper pad soaked in "membrane lauryl sulphate broth" (Oxoid MM0615), which feeds E. coli bacteria but inhibits the growth of any other bacteria.The bacterial concentration was determined by counting after 18 h incubation at 37 °C.
TiO 2 nanoparticle samples were codified with C or N (for samples made with HCl and HNO 3 , resp.) accompanied by 0.5, 0.8, and 1.0, depending on the pH used in the sol-gel synthesis.In a similar way, 1D TiO 2 nanostructures were codified adding 1D to the nanoparticle code (e.g., N0.5 refers to TiO 2 nanoparticles obtained with HNO 3 in pH 0.5, and N0.5-1D refers to a one-dimensional TiO 2 nanostructure obtained for N0.5) resulting in twelve samples.

Assessment of Stirring in Bacteria
Viability.In order to evaluate the mechanical stirring effect in bacteria viability, E. coli ATCC 25922 were tested in the dark under stirring (100 rpm) and without stirring at room temperature (20 °C).1.0 mL aliquots were collected after 0, 20, 40, and 60 min stirring.

Results and Discussion
3.1.1D TiO 2 Nanostructures Prepared from Seeds Presenting Tailored TiO 2 Crystalline Phases.Figure 1 shows FE-SEM images of TiO 2 nanoparticles obtained by the sol-gel method (SG-TiO 2 NPs) using HNO 3 and HCl as catalysts in the acid hydrolysis reaction of titanium isopropoxide (pH = 0 5, 0.8, and 1).At pH 1, regardless of the acid, the images show spherical nanoparticles with average diameters of about 15 nm and 13 nm, with HNO 3 and HCl, respectively.Polyhedral structures (60-100 nm) were observed when the pH decreased to 0.8, and octahedral structures with edges of about 140 nm were obtained with HCl at pH 0.5.
XRD patterns (Figures 2(a) and 2(b)) show that the crystalline structures correspond mainly to anatase when acidic solutions with pH 1 were used with both catalysts.Both anatase and rutile were observed with acidic solutions at pH 0.8 and 0.5; a small amount of brookite was detected in most cases.The amount of rutile increased as the pH decreased, and it was the dominant phase when HCl at pH 0.5 was used.The small peak for brookite disappears in this case.
The average crystallite size for anatase and rutile (D anatase and D rutile , resp.), the anatase content, A p , estimated with the Spurr-Myers equation [10] from the main diffraction peaks, and the pH of the acidic solution (HNO 3 and HCl) are shown in Table 1.
Figure 3 shows the morphology, by FE-SEM, of the 1D nanostructures obtained after alkaline hydrothermal treatment of the sol-gel TiO 2 nanoparticles presented in Figure 1 and Table 1.In the case of SG-TiO 2 synthesized with HNO 3 , the particles displayed a tube-like shape, with an average diameter of 11 ± 1 nm in different anatase contents within 18% to 100%, respectively.On the other hand, using as precursor SG-TiO 2 synthesized with HCl acid, flake-like International Journal of Photoenergy particles were identified together with tube-like structures.
The proportion of flake-shaped particles increased as the pH decreased.
After the annealing process at 400 °C for 2 h, TEM images (Figure 4) show nanotube structures in all the samples; depending on the seed material, some spherical and rodshaped structures were also present.It can be seen that a sintering-like process took place during the annealing and that, as a consequence, bundles of tube-like structures and cracked structures were produced.
Tube-like structures seemed to be best conserved when obtained from TiO 2 nanoparticles with 56% of anatase, synthesized with HNO 3 .When seed material with lower anatase content (~18%) was employed, large and irregular particles measuring about 80 nm were accompanying the nanotube structures.These might be rutile seed aggregates that could not react in the hydrothermal treatment because of their large particle size.In contrast, needle-like shapes and nanotubes turning to nanorods were observed when anataserutile TiO 2 nanoparticles synthesized with HCl were used   The XRD analysis of the samples after hydrothermal treatment (Figure 5(a)) shows that the crystalline structure of the seed material changed and displayed peaks around 2θ = 10, 24.5, 28.4, and 48.3 °.These peaks represent the diffraction of sodium titanate with the chemical formula Na 2 Ti n O 2n+1 (n = 3, 6, and 9).This is observed for samples where the anatase content in the seed material was higher than 55%.In other cases, rutile was also present as shown by the reflection peaks around 2θ = 27 5, 36.1, 41.5, 54, and 56 °, corresponding to the (110), ( 101), ( 111), (211), and (220) planes in agreement with JCPDS No. 21-1276.This confirms that part of the rutile seeds could remain unreacted after the hydrothermal treatment.After the acid treatment, the features corresponding to titanates almost disappeared, leaving those of the rutile TiO 2 polymorph (not shown).
After the annealing process (Figure 5(b)), a mix of anatase and rutile was observed for samples whose seed had a rutile content larger than 60%.Only peaks corresponding to anatase TiO 2 were observed for samples with anatase higher than 56% in seed.This suggests that when rutile was the dominant phase in the seed material, a portion of it remained unreacted, probably because of the large crystallite size of rutile (~28 nm), compared with the anatase crystallite size (~9 nm).The conditions of the hydrothermal treatment seem insufficient to carry out the dissolution-precipitation process that would be involved in the transformation of TiO 2 to sodium titanate, followed by proton exchange to produce hydrogen titanate and, finally, crystallization to anatase after thermal treatment.On the other hand, it can 4 International Journal of Photoenergy be noted that the seed material obtained with HCl produced samples with the best crystallinity, as the X-ray reflections were well defined, compared to those obtained with HNO 3 .

Assessment of the Photocatalytic Activity of TiO 2
Nanostructures for Escherichia coli in Water.Bacteria viability under the stirring process was determined by colony counting after 24 h of incubation.The results showed that the stirring process affects in 1, 3, and 5% (gradually for 20, 40, and 60 minutes, resp.).The assays without stirring were not performed because it was not possible to obtain a homogeneous bacteria distribution.The effect of stirring in the presence of SG-TiO 2 nanoparticles and their corresponding 1D TiO 2 nanostructures against E. coli was evaluated in the dark and under UV-A/B irradiation.As shown in Figure 6, considering that the initial E. coli concentration was 1 × 10 7 CFU/mL, the presence of TiO 2 nanoparticles and nanotubes under stirring 5 International Journal of Photoenergy conditions in the dark produced a diminution of bacteria viability of around two orders of magnitude (10 5 CFU/mL).It is ascribed to mechanical stress produced by the stirring process.
Also, as is reported in other studies [11], the photolysis is present in our experiments.It contributes to a decrease in bacteria viability at three orders of magnitude.The bactericidal activity of TiO 2 nanostructures is similar to the photolysis in consequence; the catalyst plus irradiation can decrease bacteria viability until five orders of magnitude.However, no major difference was observed for the bactericidal effect of nanoparticles and one-dimensional TiO 2 nanostructures.The latter have an important advantage since 1D TiO 2 nanostructures can be easily removed from solutions.

Conclusions
In summary, TiO 2 anatase 1D nanostructures, with different shapes such as tube-and rod-like shapes, were synthesized by hydrothermal treatment of seeds controlling the anataserutile proportion.The synthesized 1D TiO 2 nanostructure was effectively used for photocatalytic abatement of E. coli in water.Although the 1D TiO 2 nanostructures have a similar photocatalytic activity than the nanoparticles have, the use of one-dimensional TiO 2 nanostructures has an important advantage since the 1D TiO 2 nanostructure can be easily removed from solutions and could be reusable avoiding the necessity of use filtration that increases the cost and time of the cleaning process.

Figure 1 :
Figure1: FE-SEM images of TiO 2 nanoparticles obtained from the sol-gel method using HNO 3 and HCl as catalysts of the titanium isopropoxide hydrolysis reaction (pH = 0 5, 0.8, and 1.0, from left to right, resp.).

Table 1 :
Crystallite size, D anatase or D rutile , and anatase proportion, A p , of the SG-TiO 2 powders prepared at the indicated pH using HCl or HNO 3 acid solutions.