This study involves the investigation of altering the photocatalytic activity of TiO2 using composite materials. Three different forms of modified TiO2, namely, TiO2/activated carbon (AC), TiO2/carbon (C), and TiO2/PANi, were compared. The TiO2/carbon composite was obtained by pyrolysis of TiO2/PANi prepared by in situ polymerization method, while the TiO2/activated carbon (TiO2/AC) was obtained after treating TiO2/carbon with 1.0 M KOH solution, followed by calcination at a temperature of 450°C. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), thermogravimetric analysis (TG-DTA), Brunauer-Emmet-Teller (BET), and UV-Vis spectroscopy were used to characterize and evaluate the prepared samples. The specific surface area was determined to be in the following order: TiO2/AC > TiO2/C > TiO2/PANi > TiO2 (179 > 134 > 54 > 9 m2 g−1). The evaluation of photocatalytic performance for the degradation of methylene blue under UV light irradiation was also of the same order, with 98 > 84.7 > 69% conversion rate, which is likely to be attributed to the porosity and synergistic effect in the prepared samples.
Currently, the rapid industrialization in developing countries has begun to introduce harmful organic pollutants into the water supply. These effluents are also sourced from the textile industry that consumes a large quantity of water in the process of dyeing and washing of fabrics and the release of huge quantities of dyes [
Current literature shows that activated carbon is gaining attention as it is capable of modifying TiO2 photocatalyst. The work in [
The work in [
TiO2/polyaniline (TiO2/PANi) was synthesized using TiO2 (Sigma Aldrich, 99.9% purity) and aniline hydrochloride (Sigma Aldrich, 99.95%). In order to synthesize TiO2/PANi, 0.2 mol cm−3 aniline hydrochloride solution was prepared by adding 259 mg aniline hydrochloride to 5 mL deionized water and was vigorously stirred for 10 minutes at a temperature of 60°C. TiO2 powder, with various percentages (5%, 10% and 15%), is added to aniline hydrochloride solution being prepared previously. 0.25 mol cm−3 ammonium peroxydisulphate solution was prepared by adding 571 mg ammonium peroxydisulphate (Sigma Aldrich, 98% reagent grade) in 5 mL deionized water. While the mixture above was being stirred, the prepared ammonium peroxydisulphate solution was added to the mixture and was stirred for an additional 10 minutes. The polymerization was completed in about 10 minutes. The solution was then left to dry at room temperature for 48 hours. The precipitate powder was then centrifuged and washed with absolute ethanol, followed by distilled water in order to remove unreacted aniline monomer and its corresponding by-products. The product (TiO2/PANi) was dried at 65°C for 24 h. The composite that was the result of the process was labeled as TiO2/PANi. The product was then pyrolyzed at a temperature of 450°C for 1 hour in nitrogen flow at a heating rate of 10°C min−1 to produce titania/carbon (TiO2/C).
In order to produce porous carbon or the so-called activated carbon, the TiO2/C was treated with 1.0 M KOH solution. About 0.3 g TiO2/C was mixed with 2.5 mL 1.0 M KOH solution. The mixture was then heated at a temperature of 450°C under nitrogen gas flow for 1 hour. Then the treated sample was washed with deionized water until it was neutral and dried overnight at a temperature of 100°C and was labeled TiO2/AC. Since the early photocatalytic performance tests demonstrated better result for the TiO2/AC prepared from 15% TiO2/PANi, the characterization was done for 15% TiO2/PANi and the resulting TiO2/C and TiO2/AC (hereafter denoted as TiO2/PANi, TiO2/C, and TiO2/AC, resp.).
The functionality groups of the samples were determined using FTIR spectra. The morphology and structure of the samples were determined by field emission scanning electron microscope (FESEM, ZEISS Supra VP55) and transmission electron microscope (JEOL JEM-2100). The specific surface area, pore size, pore volume, and pore diameter of the samples were determined by Brunauer-Emmet-Teller (BET) method [
For investigation of prepared samples recyclability, the used catalyst was separated from reaction mixture by filtration. After that, it is washed with distilled water several times and dried in oven to be reused for next reaction cycle. Then they were used for subsequent cycles under similar reaction conditions as carried out by fresh catalyst.
The photocatalytic activities of the samples were evaluated via the photocatalytic oxidation of methylene blue (MB) under UV light irradiation. A 15 watt UV bench lamp was used as a light source. Since the photocatalytic test took into account different MB concentrations (0.05, 0.1, and 0.15 mM) and different irradiation times, it showed better results for 0.05 mM and 90 minutes, respectively, and was selected as the test condition (see Section
Prior to illumination, 20 mg photocatalyst was added to the MB solution (20 mL, 0.05 mM). The solution was stirred in the dark for 30 minutes in order to reach MB absorption-desorption equilibrium, which will then allow for the commencement of the photocatalytic reaction. The photocatalyst will then be exposed to the UV lamp for 90 minutes in room temperature. The results after this time period will be evaluated after 90 minutes.
The degradation efficiency of MB was analyzed using UV-Vis spectrometer. Peaks were observed to be present between 600 and 700 nm and were assigned as the absorption of the
The existence of polyaniline on the TiO2 surface particle was verified by IR spectra. The characteristic bands of TiO2, polyaniline (made in the same way for comparison purpose only), TiO2/PANi, and TiO2/AC are shown in Figures
FTIR spectra of (a) TiO2, (b) Polyaniline, (c) TiO2/PANi, and (d) TiO2/AC, respectively.
The morphologies and size of the prepared nanoparticles were studied by variable pressure scanning electron microscope (VPSEM) and transmission electron microscopy (TEM) (Figures
FESEM photographs of (a) TiO2 particles, (b) TiO2/PANi, (c) TiO2/C, and (d) TiO2/AC, respectively.
TEM photographs of (a) TiO2 particles, (b) TiO2/PANi, (c) TiO2/C, and (d) TiO2/AC, respectively (magnification 45000x).
From the TEM image of Figure
The XRD patterns of the prepared samples were shown in Figure
Texture properties of the TiO2, TiO2/PANi, TiO2/C, and TiO2/AC composites.
Physical properties | TiO2 | TiO2/PANi | TiO2/C | TiO2/AC |
---|---|---|---|---|
Surface area (m2/g) | 8.55 | 53.73 | 134 | 178.57 |
Pore size (nm) | 9.48 | 13.8 | 9.26 | 6.79 |
Micropore area (m2/g) | 3.34 | 8.22 | 29.33 | 76.62 |
Micropore volume (cm3/g) | 0.001 | 0.004 | 0.015 | 0.037 |
Crystallite sizea (nm) anatase | 25 | 14 | — | 11 |
Crystallite sizea (nm) rutile | 48 | 19 | — | 20 |
|
0.135 | 0.061 | 0.087 | 0.092 |
bAnatase content calculated using (
XRD patterns of samples. (a) TiO2 particles, (b) TiO2/PANi, (c) TiO2/C, and (d) TiO2/AC, respectively.
The thermal stability of the prepared TiO2/PANi and TiO2/AC was investigated and the TG-DTA analysis results are shown in Figure
TGA thermogram analysis of (a) TiO2/PANi and (b) TiO2/AC composite.
The degradation of methylene blue (MB) was used in this work in order to evaluate the photocatalytic activity of the prepared samples. The effect of different loading percentages of TiO2 in activated carbon on photocatalytic activity has been investigated as well (Figure
Effect of TiO2 loading (%) on degradation of MB (under 90 minutes of UV light irradiation in room temperature).
It is known that the concentration of methylene blue influences the photocatalytic performance of the samples [
Effect of MB concentration on photocatalytic activity for TiO2/PANi and TiO2/AC.
The photocatalytic activity was also evaluated using irradiation time acting as its parameter (Figure
Photocatalytic activity: effect of irradiation time in degradation of MB.
Figure
Photocatalytic degradation of methylene blue for bare TiO2, TiO2/PANi, TiO2/C, and TiO2/AC (0.05 mM MB and 90 min irradiation time).
The results can be explained using texture properties of the prepared samples, as shown in Table
On top of the synergistic effect for the mixture of TiO2 with activated carbon and highly porous structures, the high photocatalytic performance could be the result of the mixture of anatase and rutile phase of TiO2 in this work (Table
Recyclability test was carried out for TiO2/AC to up to five cycles. The results are depicted in Figure
Recyclability of TiO2/AC was carried out for 5 cycles used.
Regarding the possibility of photocatalytic improvement using a composite of TiO2, a different form of modified TiO2, namely, TiO2/activated carbon (AC), TiO2/Carbon (C), and TiO2/PANi, were synthesized and characterized, and their catalytic performance have been studied. The effect of TiO2 loading, irradiation time, and MB concentration has been studied. TiO2/AC showed the highest porosity and photocatalytic performance compared to other composites. On top of considering the synergistic effect for the mixture of TiO2 with activated carbon and its high porosity, this high performance can also be attributed to the anatase and rutile mixture. Meanwhile, PANi demonstrated better AC source compared to previous works, and the recyclability test demonstrated excellent performance for the synthesized composite.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to acknowledge the financial support of the work by the Universiti Kebangsaan Malaysia for funding this project under the research Grants of Dana Impak Perdana (DLP-2013-015 and FRGS/1/2012/TK07/UKM/3/4) from Ministry of Higher Education (MOHE), Malaysia, and Centre of Research and Innovation Management (CRIM), UKM.