The sol-gel process was employed in the preparation of titania-doped spherical nanosilica for application in photocatalysis. To this end, the silica matrix was doped with 1 and 10% titania, and the catalytic activity of the resulting solids in the degradation of rhodamine was tested. The synthesized materials were thermally treated at 120, 400, and 800°C. Differential thermal analysis did not evidence the titania-phase transition from anatase to rutile. Scanning electron microscopy revealed the formation of monodisperse spherical nanoparticles with sizes varying between 400 and 500 nm. The UV-Vis absorption spectra showed that the silica doped with 10% titania promoted 86% rhodamine degradation within 90 minutes, as compared to 40% in the case of the silica containing 1% titania. The silica matrix was demonstrated to affect the titania-phase transformation.
Advances in nanotechnology have enabled production of nanosized silica via the sol-gel process, and this material has been widely employed in scientific research as well as in engineering development. Nowadays, the sol-gel technique is the method that is most commonly employed for the synthesis of silica nanoparticles. This route involves simultaneous hydrolysis and condensation of alkoxides, so that silica nanoparticles with several characteristics can be achieved [
Titanium dioxide exists in two main forms, more specifically anatase and rutile. Countless applications have been described for these different phases, but titanium dioxide is mainly used in catalysis (photocatalysis) [
Photocatalytic reactions are well established in the literature. In the specific case of titanium oxide, water adsorbed onto the titania surface is homolytically cleaved (
In this work the hydrolytic sol-gel route was utilized for preparation of silica doped with 1 and 10% titania, and the obtained material was employed in photocatalysis. The synthesized samples were characterized by thermal analysis, scanning electron microscopy, infrared spectroscopy, and X-ray diffraction. The photocatalytic activity of the prepared materials for the degradation of the dye rhodamine was evaluated by means of UV/Vis absorption spectra.
Tetraethyl orthosilicate (Si-(OC2H5)4, TEOS, 98%), titanium isopropoxide (Ti(OC3H7)4, isoTi, 99%), and Rhodamine B (95%) were purchased from Sigma-Aldrich. Isopropyl alcohol and ammonium hydroxide were acquired from Merck. Stock solutions of dye were prepared via the dissolution of appropriate amounts of the dye in distilled water.
The silica nanoparticles were obtained by basic catalysis, using 0.11 mol of isopropyl alcohol (solvent), 0.03 mol of ammonium hydroxide in aqueous medium. 3.23 mmol of the alkoxide precursor tetraethyl orthosilicate (TEOS) was added after 3 minutes of magnetic stirring at 40°C. Titanium IV isopropoxide (isoTi) was introduced into the reaction mixture after ten minutes, under continuous magnetic stirring. The samples were centrifuged, washed with ethanol, and dried at 120°C. The silica matrix was then doped with 1 or 10% titania in relation to silica, and the resulting xerogels were calcined at 400 or 800°C.
The photocatalytic activity of these materials was evaluated through suspension of 50 mg of catalyst in 10 mL of aqueous solution of rhodamine (10−5 mol
Thermal Analyses were carried out using a Thermal Analyst TA Instrument SDT Q600 Simultaneous TGA/DTA/ DSC, in nitrogen, at a heating rate of 20°C/min, from 25 to 1000°C, under a N2 flow of 100 mL/min.
X-ray diffraction (XRD) patterns of the powdered samples were acquired on a Shimadzu model XRD 6000 diffractometer.
Scanning electron microscopy (SEM) of the materials was performed in a Carl Zeiss Model EVO 50 Cambridge (UK) microscope. The samples were coated with a thin gold layer using a sputtering method.
The infrared absorption spectra (FTIR) were acquired on a Bomem MB 100 spectrophotometer with Fourier transform, using the KBr pellet technique, with a sample/KBr ratio of 1 : 300.
The thermogravimetric curves (TG) evidenced occurrence of mass losses up to 600°C, corresponding to approximately 13% of the sample mass. The maximum mass loss took place between 40 and 173°C, which can be ascribed to loss of water and solvent employed in the syntheses. The residual mass loss was detected between 200 and 600°C, which can be ascribed to pyrolysis of organic matter remaining from the alkoxide precursors.
Figures
X-ray diffraction patterns of the SiO2 matrix doped with 1% TiO2 and treated at (a) 120, (b) 400, and (c) 800°C.
X-ray diffraction patterns of the SiO2 matrix doped with 10% TiO2 and treated at (a) 120, (b) 400, and (c) 800°C.
The XRD patterns of all the samples showed the presence of an amorphous phase (Figures
The supplementary information (See Supplementary Material available online at doi:10.5402/2012/304546) show the SEM images and the corresponding size distributions for the samples containing 1% and 10% TiO2 doped into the SiO2 and treated at different calcination temperatures, namely, 120, 400, and 800°C. Monodispersed spherical particles (100–600 nm) were observed for all the samples, and the mean sizes are listed in Table
Average particle size for the samples with 1 and 10% TiO2 doped into SiO2 matrix and treated at 120, 400, and 800°C.
Heat treatment | % TiO2 | Mean particle size (nm) |
---|---|---|
120°C | 1 | 477 |
10 | 513 | |
400°C | 1 | 474 |
10 | 505 | |
800°C | 1 | 415 |
10 | 457 |
All the samples were spherical. The micrographs of the samples with 1 and 10% TiO2 doped into SiO2 evidenced uniformly monodispersed spherical particles with an average size of 475 and 510 nm, respectively (supplementary information). All the samples prepared with 10% TiO2 had larger particle size as compared to the materials containing 1% TiO2, suggesting formation of a Si
The mechanism of TiO2 photocatalysis has been well discussed in the literature. Hanaor and Sorrel [
Absorption spectra of rhodamine as a function of irradiation time in the presence of SiO2 materials doped with 1% TiO2 treated at 800°C.
Rhodamine degradation results in the presence of SiO2 materials doped with 1% TiO2 treated at different calcination temperatures.
UV irradiation at 365 nm was employed during photocatalysis. The absorption maximum did not change during rhodamine degradation in the presence of the SiO2 materials doped with 1% TiO2, regardless of the calcination temperature. Degradation was faster during the first 15 min. of reaction, for all the samples. Degradation percentages of 11, 22, and 36% were achieved for the samples treated at 120, 400, and 800°C, respectively. At 90 min., dye degradation was 54% larger for the sample treated at 120°C as compared to the percentage obtained at 15 min. As for the sample calcined at 400°C, degradation at 90 min. was lower as compared to the result achieved at 15 min., but it remained stable. Concerning the sample treated at 800°C, degradation increased by 16% at 90 min. This fact can be ascribed to H2O and OH groups present on the surface of the TiO2/SiO2 matrix. The presence of adsorbed radicals is necessary for photocatalytic reactions. Adsorbed water produces free radicals contain a free unpaired electron and are photogenerated when TiO2 is exposed to radiation corresponding to its band gap [
The anatase phase is described as being a better photocatalyst as compared to the TiO2 brookite and rutile phases [
Figure
Absorption spectra of rhodamine as a function of irradiation time in the presence of SiO2 materials doped with 10% TiO2 treated at 800°C.
Rhodamine degradation results in the presence of SiO2 materials doped with 10% TiO2 treated at different calcination temperatures.
Rhodamine degradation in the presence of SiO2 materials doped with 10% TiO2 is accompanied by a shift in the absorbance band from 553 nm to 549 nm (after 45 min.) and then to 547 nm (after 60 min.). Zhang et al. [
The rhodamine degradation reaction in the presence of the material containing 10% TiO2 was faster within the first 15 min. for the sample treated at 800°C, furnishing 70% dye degradation, which had risen by 21% at 90 min. As for the sample treated at 120°C, the percentage of degradation was the same as that obtained for the sample containing 1% TiO2, that is, around 10%, at 15 min. At 90 min., degradation rose by 110%. Photocatalysis in the presence of the sample containing 10% titania and treated at 400°C afforded low degradation yields: only 6.5% rhodamine was degraded up to 90 min.
The samples with 1 and 10% titania treated at 400°C gave poor results. The former yielded initial degradation of 21%, but this value decreased as a function of time to 18%, and the degradation percentage was very low for the latter sample. As for the samples treated at 120 and 800°C, there was an increase in the degradation percentage as a function of time, but this rise was not directly proportional. It is noteworthy that a tenfold increase in TiO2 concentration elicited only a twofold rise in degradation.
These results show that the percentage of TiO2 is not the main factor influencing the rhodamine photodegradation reaction. There are many other parameters, such as TiO2 particle size and morphology, and the presence of H2O and OH groups, affecting this reaction.
Prior to the photocatalytic reactions, the infrared spectra of the prepared materials displayed a broad band at 3420 cm−1, ascribed to water molecules and hydroxyl groups, and a band at 1641 cm−1 confirmed the presence of these groups. The shoulder at 1220 cm−1 is attributed to Ti
The results of this work show the importance of temperature, reaction time, percentage of titania in the sample, and presence of water and OH groups for the effectiveness of rhodamine photodegradation. In industrial processes for degradation of polluting dyes, TiO2 can be diluted into the SiO2 matrix, thereby reducing costs while promoting good degradation. The samples treated at 800°C gave the best yields, which were achieved during the first 15 minutes of reaction. Thereafter, the percentage of degradation increased no more than 20% of the value obtained at 15 minutes. As for the samples treated at 120°C, degradation percentage increased by approximately 50 and 100% at 90 minutes as compared to 15 minutes for the materials with 1 and 10% TiO2, respectively. All these observations can be explained by the presence of the H2O and OH groups on the surface of the SiO2 nanoparticles doped with TiO2. In the case of the samples treated at 800°C, the amount of Si–OH and Ti–OH bonds on the surface of the nanoparticles is lower, as compared to the samples treated at 120°C. Moreover, for materials containing less TiO2 diluted into SiO2, pollutant degradation in solution is favored over pollutant deethylation on the matrix surface.
The authors acknowledge FAPESP, CNPq, and CAPES (Brazilian research funding agencies) for support of this work and the Microscopy Laboratory of the FFCLRP, São Paulo University, São Paulo, Brazil.