Characterization and Photoluminescent, Photocatalytic and Antimicrobial Properties of Boron-Doped TiO2 Nanoparticles Obtained by Microwave-Assisted Solvothermic Method

Boron doped TiO2:xB (x = 0 mol.%, 1 mol.%, 2 mol.%, 4 mol.% and 8 mol.%) was quickly synthesized by a microwave-assisted solvothermic method at 140°C for 10 min. The nanoparticles obtained were characterized by x-ray diffraction (XRD), Fourier transform infrared spectroscopy, Raman spectroscopy, photoluminescence, field emission scanning electron microscopy, electron microscopy and diffuse optical reflectance. The photocatalytic properties were estimated against methylene blue dye. The antimicrobial activity was measured by the disc diffusion technique against S. aureus and E. coli bacteria. The XRD patterns show that there was no formation of secondary phases and that all the peaks correspond to the anatase phase of TiO2. Rietveld’s refinement showed that the addition of B3+ in the TiO2 lattice promotes a reduction in the size of the crystallites and this reduction it effectively increases the degradation capacity of the methylene blue dye, which after 50 min the 8%B sample degraded completely, while the pure TiO2 sample reduced its concentration by 95%. Boron-doped TiO2 was effective when reused and after the third cycle the photocatalytic activity of the powders was maintained. In addition, the incorporation of 8%B in the TiO2 lattice resulted in an increase from 8.66 mm to 15.61 mm and 9.04 mm to 13.65 mm in the inhibition halos of the S. aureus and E. coli bacteria, respectively.


INTRODUCTION
Increasing population growth in parallel with the industrial sector generates a great concern about the pollution of water, soil and air. 1 Industries such as textiles, generate a high amount of toxic waste with the presence of complex chemical compounds, salts and dissolved solids and dyes. 2 The treatment of synthetic dyes is difficult due to the presence of aromatic rings. 3Heterogeneous photocatalysts are very efficient for the treatment of dyes because they possess strong oxidative capacity and have CO 2 and H 2 O 4 as a product.
In addition, health problems related to microbes have been gaining much attention because of their increased resistance to conventional treatments. 5everal papers report different types of food and environmental sources that harbor bacteria or microbes resistant to one or more antibiotics. 6,7hus, the search for materials with antimicrobial (Received December 1, 2018; accepted February 14, 2019)  properties that act more efficiently than common antibiotics only tends to grow.Some semiconductor materials have proved to be efficient for photocatalytic and antimicrobial applications. 83][14][15] The principles of their performance in the degradation of organic and  antimicrobial compounds are in the generation of reactive oxygen species ( AE O 2 À , H 2 O 2 and AE OH). 16The introduction of nonmetals in the TiO 2 lattice has been the object of study of several research groups in order to increase their properties.Compounds based on boron, such as boric acid, have fungicidal properties and have already been used in conventional treatments, because the boron atoms are supposed to replace the oxygen atoms in the TiO 2 lattice, where these elements become acceptor impurities generating a p-type material. 12,17n this work, TiO 2 nanoparticles doped with different amounts of boron (0 mol.%, 1 mol.%, 2 mol.%, 4 mol.% and 8 mol.%) were obtained by the microwave assisted solvothermic method.The photocatalytic properties of the nanoparticles were estimated by varying the concentration of the methylene blue dye by the test time when

EXPERIMENTAL Synthesis
Titanium isopropoxide, boric acid and deionized water were used as received to prepare boron-doped titanium dioxide nanoparticles.The procedure was as follows: 0.25 mmol of titanium isopropoxide was dissolved into 20 mL of ethyl alcohol (99%) and kept under stirring in parallel; the different concentrations of boric acid (1%, 2%, 4% and 8%) were dissolved in 40 mL of deionized water, also under stirring.The two solutions were mixed and kept under stirring for 20 min.The pH of the solution remained at 7. The mixture was transferred into a Teflon autoclave, which was sealed, and the reaction system was heated under solvothermic temperature conditions at 140°C for 10 min using microwave radiation (2.45 GHz and a maximum power of 800 W).The pressure in the autoclave was stabilized at 3 atm.The white product obtained by the MAH treatment was centrifuged, washed with distilled water and ethanol and finally dried at 100°C.

Characterization
TiO 2 :xB (x = 0 mol.%, 1 mol.%, 2 mol.%, 4 mol.% and 8 mol.%) powders were investigated in a Shimadzu (XRD-6000) diffractometer using CuKa radiation (1.5418A ˚).For a better verification of the changes promoted by doping, Rietveld refinement was performed using the General Structure Analysis System (GSAS) program with the graphic interface EXPGUI. 18A scanning electron microscope (SEM) was used to observe its morphology.The transmission electron microscopy (TEM) was used to better observe the nanoparticles.The UV-Vis spectroscopy was performed on a Shimadzu equipment (UV-2550), with a wavelength range of 200-900 nm and programmed for the diffuse reflectance mode, where from these results, the energy of the optical band gap (Egap) of these materials was determined using the Wood and Tauc Equation. 19he Fourier Transform Infrared (FTIR) technique was performed using the IRTracer-100, Shimadzu, from 500 cm À1 to 4000 cm À1 scanning.Raman scattering spectroscopy was used with 532 nm laser and spectral range between 100 cm À1 and 1000 cm À1 , with acquisition time of 10 s.The photoluminescence (PL) measurements were obtained using a 375 nm laser at room temperature in a Mosospec 27 monochromator (Thermal Jarrel Ash, USA) coupled to an argon laser photomultiplier (Coherent innova 90 K, USA).

Photocatalytic Properties
The photocatalytic properties of the samples were tested (as a catalyst agent) for the degradation of the methylene blue dye (MB), at pH 5, with the following molecular formula [C 16 H 18 ClN 3 S] (99.5% purity, Mallinckrodt) in an aqueous solution under UV-light illumination.The sample was placed in a cylindrical quartz reactor, containing 50 mL of MB dye solution (concentration 1 9 10 À5 mol L À1 ).Then, a cylindrical quartz reactor was placed into a photo-reactor at controlled temperature (25°C) and, illuminated by six UVC lamps (TUV Philips 15 W, with maximum intensity of 254 nm = 4.9 eV).In 10-min intervals, the 3 mL aliquot of the dye solution was taken and analyzed by its variations of the maximum absorption band of MB dye solutions by UV-Vis absorbance spectra measurements using a Shimadzu spectrophotometer (model UV-2600).

Antimicrobial Properties
The antimicrobial properties of TiO 2 :xB powders were evaluated by in vitro tests using agar diffusion against gram negative E. coli (ATCC-25922) and gram positive S. aureus bacteria (ATCC-25923), according to standard procedures stipulated by the Institute of Clinical and Laboratory Standards (CLSI), similarly to a study carried out by Arau ´jo et al. 20 The inoculum was prepared by a direct suspension of colonies in phosphate-buffered saline (PBS) in water plate and maintained for 24 h at 35°C.The solution turbidity was then adjusted according to the McFarland scale (bacterial density of 1 9 10 8 CFU/mL).The test was performed using disc diffusion with 6 mm disc and adding 30 lL sample aliquots.

RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns for the TiO 2 :xB (x = 0 mol.%, 1 mol.%, 2 mol.%, 4 mol.% and 8 mol.%) samples.XRD patterns were indexed according to the tetragonal crystalline structure of the spatial group l41/amd (No. 141) with parameters a = 3.789 and c 9.537, according to the ICSD 9853 card.The broad peaks indicate the formation of small particles and the absence of deleterious phases indicates the formation of a pure anatase phase of TiO 2 and the success of doping. 21ietveld refinement 22 was employed for a better analysis of the possible structural changes introduced by the addition of B 3+ in TiO 2 .The General Structure Analysis System (GSAS) with graphical interface EXPGUI 18 was used for the refinement.The card used in the refinement was the ICSD 9853.The parameters used in the refinement were: fact of scale and phase fraction; background, which was modeled by the displaced Chebysshev polynomial function; peak shape, which was modeled using Thomson-Cox-Hasting pseudo-Voigt; changes in lattice constants; fractional atomic coordinates; and isotropic thermal parameters.The results of the Rietveld refinement are shown in Fig. 2 and Table I.
The patterns of the samples are well adapted to ICSD 9853.Note that the differences between the experimentally observed XRD patterns and the theoretically calculated data are close to zero, as shown in the obs-calc line.Reliability parameters v 2 , Rwp and Rp show low values, indicating good quality of structural refinements and numerical results.These data confirm that the crystals were well matched to the tetragonal structure with spatial group l41/amd (No. 141).A decrease in the size of the crystallite with the addition of the dopant is observed with the refinement, a fact that is associated with the difference between the ionic radius (0.60 A ˚of Ti 4+ versus 0.23 A ˚of B 3+ ).Quin ˜ones et al. 23 showed that the presence of boron reduces the crystal size of the anatase phase of TiO 2 particles.Figure 3 shows the unit cells of the samples, obtained from the Rietveld refinement.The positions x, y and z occupied by the atoms remained the same, maintaining the positions of the cations Ti 4+ /B 3+ being x = 0.00, y = 0.25 and z = 0.375 and of the anions O 2À being x = 0.00, y = 0.25 and z = 0.162.
Scanning electron microscopy (SEM) was used to observe the morphology of boron doped TiO 2 nanoparticles.Figure 4 shows the SEM images and the EDX spectra for samples of pure TiO 2 (4ac) and doped with 8%B (4b-d).In Fig. 4e and f presents the histograms of the nanoparticle diameters of pure TiO 2 and with 8%B. Figure 4 shows that the nanoparticles have semi-spherical morphology, with a mean diameter ranging from 7.796 nm to 9.316 nm for 8%B and pure samples, indicating that the addition of boron provides a reduction in the nanoparticles size.The reduction in the size of TiO 2 particles by doping was also observed by Barmeh et al., 24 which reduced the  size of the TiO 2 grains from nm to 19 nm on the surface of thin films by adding nickel.Figure 4 shows the agglomeration between the TiO nanoparticles, which is possibly related to the high surface energy coming from its small size. 16ecause of the small diameters, the isolated characterization of the nanoparticles is made impossible by SEM. Figure 5 shows TEM images for samples of pure TiO 2 and doped with 8%B.Through the images of MET, it is observed that the doping with boron results in the reduction of the interplanar spacing, in which a reduction is seen from 0.402 nm to 0.384 nm of the sample of pure TiO 2 for the doped with 8%B.As previously seen, the addition of boron reduces the lattice parameter a of TiO 2 and, according to Bragg's law, the reduction in the lattice parameter proportionally reduces the interplanar spacing.FTIR analyses was used to determine the functional groups on the surface of the FTIR spectra of these samples were studied at 500-4000 cm À1 region.Figure 6 shows the FTIR spectrum obtained for samples of pure and boron doped TiO 2 .The bands found around 600 cm À1 are related to the anatase phase of TiO 2 , which is called Ti-O-Ti and Ti-O vibration. 26,27The bands present in all samples at 1440 cm À1 and 1640 cm À1 are due to the mode of vibration of the OH group relative to H 2 O adsorbed on the surface of TiO 2 powders. 28The increase in intensity in 1440 cm À1 with the addition of boron was ascribed to asymmetric B-O stretching on the surface. 26Boric acid species possibly ascribed to the presence of tricoordinated interstitial boron (Ti-O-B, in the form of B 3p), as well as outersphere boric acid, were potential sources between 1300 cm À1 and 1400 cm À1 . 29,30The absorption band at 3330 cm À1 are associated with the vibrations of free water molecules and to the OH group adsorbed on the surface of the oxide, where the band increases with the amount of boron added to it.This is due to Boron, when incorporated into the TiO 2 lattice, to behave as a Lewis acid, facilitating the adsorption of water on its surface, a fact also seen in the 1440 cm À1 band. 31,32aman scattering spectroscopy was used for a better structural characterization of the powders of pure TiO 2 and powders doped with boron.Figure 7 shows the Raman scattering peaks obtained for all samples from this study.4][35] There was no formation of peaks related to other TiO 2 phases or from boron doping.Momose et al. 36 observed that the addition of boron atoms as charge carriers on silicon substrates with a concentration of less than 10 19 cm À3 did not promote changes in the Raman spectra as compared to pure silicon.The addition of boron did not promote the appearance of peaks.However, the distortions generated in the crystalline structure promoted the displacement in the main peaks of TiO 2 .Volodin et al. 37 showed that boron atoms generate clutter in the crystal lattice of the diamond, shifting and extending its maximum peak.
Diffuse reflectance spectroscopy was used in the visible ultraviolet region to estimate the band gap of the TiO 2 :xB powders.The Kubelka-Munk function 38 was used to convert the reflectance data to absorbance data.The Wood and Tauc method 19 was used to estimate the value of the optical band gap (Egap).In the Wood and Tauc method, the optical gap energy is given by F(R)hv / (hv À Egap) k , where h is the Planck constant, v is the frequency, F(R) is the absorbance and k is indicated for the different transitions (k = 1/2, 2, 3/2 or 3 for permissible direct, indirect permissible, direct prohibited and indirect prohibited, respectively).For the TiO 2 :xB powders, we used k = 1/2, that is, direct permissible.Plot of absorbance versus photon energy (eV) extrapolated the linear portion of the curve for zero absorption to estimate Egap. Figure 8 illustrates how Egap stipulation procedures were performed for the permissible direct modes.
Figure 8 shows that the pure TiO 2 presents the lowest Egap, as the boron concentration increases.The Egap of the compounds is directly related to the creation of defects in the material lattice.Wang et al. 39 showed that boron doping at concentrations  of up to 1% promoted a reduction of Egap, whereas for higher values, Egap tended to increase.This is related to mixture of orbital related to boron (1s) and oxygen (2p).The increase in Egap may still be related to the Burstein-Moss effect, 40 indicating that the concentration of electron carriers exceeded the density of the conduction band states, corresponding to a degenerate doping.Photoluminescence spectra were obtained by excitation of the powders using a 375-nm laser.Figure 9 shows the spectra obtained for the TiO 2 :xB (x = 08 mol.%, 18 mol.%,28 mol.%,48 mol.% and 8 mol.%) samples.It is noted that increasing the amount of boron decreases the photoluminescent intensity of the material.The photoluminescence of the material is directly related to the generation and recombination capacity of the electron/hole pairs (e À /h + ), the higher this recombination rate, the better the photoluminescence. 12Boron doping promotes the appearance of intermediate levels in the conduction band of TiO 2 and due to the valence difference (Ti 4+ and B 3+ ) generates oxygen vacancies, which act to prevent the recombination of the e À /h + . 41Furthermore, boron ions act to convert Ti 4+ in Ti 3+ , which can act as photogenerated electron traps and thus facilitate the charge separation. 42In addition, after boron doping, 2p orbits of oxygen may be overlapped with 2p orbits of boron. 43ccording to CIE, the increase in amount of boron shift the emission band from the blue to the yellow region.The increase in the recombination rate of the e À /h + pairs reduces the emission intensity in the blue region, increasing the contribution at higher wavelengths, consequently emitting in yellow for the sample with 8%B. 44igure 10a shows the variation of the methylene blue (MB) concentration by time under UV-Vis radiation for 60 min for the TiO 2 :xB (x = 0 mol.%, 1 mol.%, 2 mol.%, 4 mol.% and 8 mol.%) samples.The photocatalytic process can be described by a first order kinetic model with respect to the absorbance of methylene blue, 45 as shown in Eq. 1.
where Ct = absorbance of methylene blue at time t; Co = initial absorbance; t = irradiation time; ki = kinetic constant.Because of the complete reduction of MB dye concentration after 50 min in the 8%B sample, the kinetic constant (K) was estimated after 40 min of test.The results obtained are shown in Fig. 10b.The graph shows the linear relationship with the irradiation time.The kinetic constant (K) and the correlation coefficient (R 2 ) are shown in Table II.With the obtained data, we can see that the correlation coefficient R 2 decreases with the increase of the amount of boron, indicating a nonlinear behavior in its degradation.As the K constant increases, there is an increase in the catalytic power of the material under methylene blue.
The concentration of MB increases the concentration of boron, favoring photocatalytic activity, where after 50 min the sample with 8%B completely degraded the solution, while the pure TiO 2 sample degraded 95%.Wang et al. 29 and Liang et al. 46 reported that the addition of boron to TiO 2 showed significant improvements in photocatalytic activity against rhodamine B dye under Xe lamps.The substitution of Ti 4+ cations by B 3+ generates oxygen vacancies in the TiO 2 lattice, due to differences in atomic and valence acting in the generation of electron/hole pairs (e À /h + ), which by reacting with the medium, produce hydroxyl radicals ( AE OH), activating the indirect oxidation mechanism, increasing the photocatalytic efficiency. 47eusing the photocatalyst deals with the ability of the material to remain active even after its use, avoiding beyond the need of exchange to continue the catalysis, the generation of another type of tailings.The evaluation of the catalyst is based on two criteria 48 : (I) the ability to catalyze its activity along the time of use and (II) an installation with the catalyst can be recycled from the solution.Through Fig. 11, it can be observed that both the TiO 2 and the TiO 2 :xB (x = 1%, 2%, 4% and 8%) retain their photocatalytic efficiency even when used in three cycles.This characterizes it for applications where more than one cycle is required for the degradation of the compound.Moreover, through the diffractograms shown in Fig. 11f, it is shown that the samples have good chemical stability, where even after the third cycle there was no formation of any secondary phases.
Through the mean diameters of the inhibition halos, it is observed that the addition of boron increased the bactericidal activity against S. aureus and E. coli bacteria.As previously seen, the incorporation of the B 3+ ions into the TiO 2 lattice promotes the reduction of the particle size.Smaller particles have larger specific surface areas and, consequently, higher energies associated with them, increasing their interaction with bacteria. 49Furthermore, as shown in the FTIR spectra, boron incorporation promotes the appearance of B-O bands, which according to Wang et al., 12 in contact with water molecules form boric acid and metabolic, which have great antimicrobial potential.The incorporation of the B 3+ ions into the TiO 2 lattice also acts to convert the Ti 4+ ions into Ti 3+ , creating separation centers of e À /h + pairs, providing more reactive oxygen species (ROS) that are highly toxic to bacteria. 50,51[54]

CONCLUSIONS
Nanoparticles of TiO 2 :xB (x = 0 mol.%, 1 mol.%, 2 mol.%, 4 mol.% and 8 mol.%) were quickly obtained by a microwave-assisted solvothermic method in 140°C for 10 min, obtaining anatase as a single phase.The defects generated by the introduction of boron into the TiO 2 lattice promoted a reduction in the crystallite and the particle size.The reduction in PL intensity and the increase in photocatalytic activity indicate that the substitution of Ti 4+ ions by B 3+ acts as the center of charge separators e À /h + , making it difficult to recombine them.The photocatalytic reuse tests indicate that the materials are indicated for continuous applications, where their photocatalytic activity and chemical stability are maintained after three cycles.The antimicrobial tests show that boron doped TiO 2 powders have a high bactericidal potential, where the 8%B samples formed inhibition halos of 15.61 mm and 13.65 mm against the S. aureus and E. coli bacteria, respectively.The results indicate that boron doped TiO 2 is a promising material both for the treatment of organic effluents and for antimicrobial applications.

Table 2 .
The apparent first-order rate constant, k of photocatalytic degradation and correlation coefficient, R 2 Sample k 3 10 22 (min 21 ) R 2