Photocatalytic degradation of methylene blue under UV light irradiation on prepared carbonaceous TiO2.

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 m(2) 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.

1. Introduction

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 n class="Chemical">water in the process of dyeing and washing of fabrics and the release of huge quantities of dyes [1]. Therefore, the development of inexpensive and green methods to treat and purify contaminated water has been the focal subject in technological developments. Among many strategies, photocatalysis is regarded as the most viable one, especially for treatment of contaminants, due to its usage of sunlight to decompose organic pollutants [2-4]. Titanium dioxide or titania (TiO2) acts as an agent in this process [5, 6]. However, TiO2 is not without its drawback in the context of photocatalysis, such as its high band gap energy 3.2 eV for anatase [7], the limitation of photo response only under UV light region, and its high rate of electron and hole recombination. Attempts to address these problems involve doping, nonmetallic elements doping, surface modification, forming composites with narrow semiconductors, using semiconductor polymer as support, and coating with carbon layers to improve and enhance the photocatalytic activity of titanium dioxide.

Current literature shows that activated n class="Chemical">carbon is gaining attention as it is capable of modifying TiO2 photocatalyst. The work in [8-12] showed that this enhancement is caused by porous structure of activated carbon, which provides a large surface area for photoactive TiO2 particle.

The work in [8, 13–15] found the synergistic effect of the mixture of n class="Chemical">TiO2 with activated carbon (AC). The improvement of TiO2/AC composite was explained by the high adsorption of the impurities on the surface of activated carbon and their transfer to the TiO2 surface. In this study, carbonaceous TiO2, namely, TiO2/activated carbon (AC), TiO2/carbon (C), and TiO2/PANi were prepared and evaluated for photocatalytic performance for degradation of methylene blue under UV light irradiation. The carbon and activated carbon/TiO2 were synthesized using polyaniline (PANi) as its carbon source.

2. Experimental and Characterizations

2.1. Preparation of TiO2/Polyaniline and TiO2/Carbon

TiO2/n class="Chemical">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).

2.2. Chemical Activation of TiO2/Carbon

In order to produce porous carbon or the so-called n class="Chemical">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.).

2.3. Characterizations

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 (n class="Chemical">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 [16] using a nitrogen adsorption instrument (Micrometics ASAP 2010). The samples were degassed at 100°C for 24 hours prior to the analysis. Pore size distribution was calculated from the adsorption desorption of the isotherms using the Barret-Joyner-Halenda (BJH) model [17], while the crystallinity of the samples was analyzed using Brucker DB-advance X-ray diffractometer (XRD). The analyses were done under the setting of Cu kα radiation at 2 theta, ranging from 10° to 80° for a 1-gram sample. The particle size of the sample was calculated by applying Scherrer's equation. Thermal gravimetric analysis (TGA) with a simultaneous TGA-DTG system (model: Mettler Toledo) has been used for investigation of thermal stability of samples. To reduce the influence of the sample quantity on the analyses, 5 ± 0.2 mg of each sample was used in each analysis with a constant nitrogen (N2) flow 50.0 mL min−1 maintained throughout the entire process. To minimize possible differences in the moisture content between samples, all TGA samples were equilibrated at 50°C for 5 min before being heated to 700°C at a ramping rate of 5°C min−1.

For investigation of prepared samples recyclability, the used catalyst was separated from reaction mixture by filtration. After that, it is washed with distilled n class="Chemical">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.

2.4. Photocatalytic Experimental

The photocatalytic activities of the samples were evaluated via the photocatalytic oxidation of n class="Chemical">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 3.2).

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 ren class="Chemical">ach 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 π-system [18], which was indicative of the degradation of n class="Chemical">MB. According to Beer-Lambert Law, MB's concentration is directly proportional to its absorbance. This makes it possible to determine the oxidation efficiency of MB using the following equation: where C is the initial concentration of MB solution and C is concentration of MB during irradiation.

3. Results and Discussions

The existence of polyaniline on the n class="Chemical">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 1(a), 1(b), 1(c), and 1(d), respectively. The FTIR spectrum of polyaniline was clearly observed in TiO2/PANi sample. The spectra display the two bands at 1490 and 1585 cm−1, which were assigned to benzenoid and quinoide rings. The band at 1300 cm−1 is related to C–N stretching of a secondary aromatic amine, while the band at 1140 cm−1 is attributed to the vibration mode of the polymer. These bands reveal the existence of PANi in the synthesized TiO2/PANi sample. In TiO2 and TiO2/AC spectra, peaks around 3400 cm−1 were assigned to –OH stretching, while those at 1600 cm−1 were assigned to –OH vibration. These peaks can be attributed to the adsorbed water from the environment.

Figure 1

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 2 and 3).

Figure 2

FESEM photographs of (a) TiO2 particles, (b) TiO2/PANi, (c) TiO2/C, and (d) TiO2/AC, respectively.

Figure 3

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 3(a), it was shown that bare TiO2 particles are uniform in shape, with sizes ranging from 25 to 30 nm. Figure 3(b) shows n class="Chemical">TiO2/PANi composite, where the sizes range from 30 to 40 nm. Both TEM and FESEM photographs show that there is no significant difference of particle size of the samples TiO2/C and TiO2/AC (before and after chemical activation of KOH solution) (40–80 nm). This agrees with previous studies, which reported that the sintering effect did not occur for TiO2/carbon after being heated at temperatures below 700°C [19].

The XRD patterns of the prepared samples were shown in Figure 4. The main diffraction peaks were at 25.3°, 37.8°, 48°, 54°, and 62.6°, assigned to the diffrn class="Chemical">action planes of (1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 0 4) for anatase (JCPDS number 021-1272), respectively. Other crystal phases corresponding to the peaks at 27.4 and 36.1° were assigned to the diffraction peaks of (1 1 0) and (1 0 1) of rutile (JCPDS number 021-1276), respectively. The XRD patterns revealed the diffraction peaks of a mixture of anatase and rutile in the prepared samples, which is favorable for photocatalytic reactions [20-23]. XRD also showed that the preparation of TiO2 composites did not affect the crystal structure of TiO2, but the peaks intensity became lower and its shape got wider, especially in the case of TiO2/PANi. This can be attributed to certain crystalline structure in PANi overlapping TiO2 peaks, but this effect is less pronounced in TiO2/C and TiO2/AC. The anatase content (f(A)) was determined from the integrated intensity of the anatase diffraction line (1  0  1),  I , and that of the rutile diffraction line (1  1  0), I , using the following equation [24] and the results are listed in Table 1: The f(A) values showed that the rutile phase is dominant in all of the prepared samples. Besides that, the crystallite size of the samples was calculated by applying the Scherrer equation on the plane (101) diffraction peak of anatase and (110) diffraction peak of rutile, as listed in Table 1. The BET surface area, pore size, micropore area, and volumes are listed in Table 1 as well. The results showed that the specific surface area adhered to the following order: TiO2/AC > TiO2/C > TiO2/PANi >TiO2 (179 >134 > 54 > 9 m2 g−1). This suggests that the characteristics of the composited catalyst were enhanced, as it provided more active sites for photocatalytic reactions to take place.

Figure 4

XRD patterns of samples. (a) TiO2 particles, (b) TiO2/PANi, (c) TiO2/C, and (d) TiO2/AC, respectively.

Table 1

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
f(A)b	0.135	0.061	0.087	0.092

aCalculated by applying Scherrer's equation.

bAnatase content calculated using (2).

The thermal stability of the prepared TiO2/n class="Chemical">PANi and TiO2/AC was investigated and the TG-DTA analysis results are shown in Figure 5. The first weight loss took place in the range of temperature 50–120°C for TiO2/PANi, which is 6.5% decrease from the original weight. The weight loss is due to the loss of moisture or water content, the evaporation of solvents such as ethanol and small molecules from the polymer matrix [25]. The second loss was determined at temperatures between 310 and 700°C. During this step, the weight loss was attributed to the degradation of polyaniline main chain (about 54%) [26-28]. At a temperature of about 700°C, the PANi molecule was completely decomposed to carbon. The result showed that TiO2/AC has significantly improved thermal stabilities compared to TiO2/PANi.

Figure 5

TGA thermogram analysis of (a) TiO2/PANi and (b) TiO2/AC composite.

3.1. Evaluation of Photocatalytic Performance

The degradation of methylene blue (n class="Chemical">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 6). The results showed that increasing the content of TiO2 improves photocatalytic performances. From the tests, it was determined that the 15% TiO2 loading was to be further tested.

Figure 6

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 [29]. Therefore, in this study, the photocatalytic n class="Chemical">activity of TiO2/PANi and TiO2/AC (0.05 mM) using different concentrations of MB within 90 minutes irradiation time was studied. It can be seen that TiO2/activated carbon demonstrated superior photocatalytic activity for all different concentrations of MB (Figure 7).

Figure 7

Effect of MB concentration on photocatalytic activity for TiO2/PANi and TiO2/AC.

The photocatalytic activity was also evaluated using irradiation time n class="Chemical">acting as its parameter (Figure 8). The result show rapid increase in MB degradation from 0 to 30 minutes which becomes slower after 30 minutes for both samples. The much higher MB degradation rate for TiO2/AC proves its better performance compared to TiO2/PANi. TiO2/AC showed almost complete degradation of MB after 90 minutes of irradiation.

Figure 8

Photocatalytic activity: effect of irradiation time in degradation of MB.

Figure 9 presents the result of the degradation of MB for bare n class="Chemical">TiO2, TiO2/PANi, TiO2/C, and TiO2/AC. The experiment was carried out in 90-minute irradiation, and the concentration of methylene blue was 0.05 mM for all samples. The degradation of MB without catalyst was used as a reference.

Figure 9

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 1. The specific surface area, micropore area, and volume adhered to the trend n class="Chemical">TiO2/AC > TiO2/C > TiO2/PANi >TiO2, which is similar to photocatalytic performance. This is attributed to the higher porosity of the activated carbon compered to others, which provide high adsorption capacity and more active sites for the reacting species during chemical reaction, which agrees with previous studies [8, 9, 19, 30–32]. It was noted that the surface area of TiO2/AC is larger compared to the ones reported in the literature, which used polystyrene (PS) as a carbon source [33]. It also shows that PANi is a better source for the preparation of highly porous activated carbon.

On top of the synergistic effect for the mixture of n class="Chemical">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 1). This can be explained by the synergic effect between anatase and rutile in TiO2 particles that are approximately close to each other, thus enhancing the activity [34]. This finding is in agreement with other studies reporting that the photocatalytic activity was enhanced when anatase and rutile were mixed [34, 35].

3.2. Recyclability of TiO2/AC

Recyclability test was carried out for TiO2/n class="Chemical">AC to up to five cycles. The results are depicted in Figure 10, demonstrating good recyclability for the prepared composite.

Figure 10

Recyclability of TiO2/AC was carried out for 5 cycles used.

4. Conclusion

Regarding the possibility of photocatalytic improvement using a composite of TiO2, a difn class="Chemical">ferent 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.