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Preparation of a New Polyaniline/CdO Nanocomposite and Investigation of Its Photocatalytic Activity: Comparative Study under UV Light and Natural Sunlight Irradiation Handan Gülce,*,† Volkan Eskizeybek,‡ Bircan Haspulat,† Fahriye Sarı,† Ahmet Gülce,† and Ahmet Avcı§ †

Department of Chemical Engineering and §Department of Mechanical Engineering, Selçuk University, Konya 42079, Turkey ‡ Department of Materials Science and Engineering, Ç anakkale Onsekiz Mart University, Ç anakkale, 17100, Turkey S Supporting Information *

ABSTRACT: Polyaniline (PANI)/CdO nanocomposite was prepared for the first time in aqueous diethylene glycol solution medium, by chemical oxidative polymerization, and its photocatalytical activity was studied. Optical analysis of the new PANI/ CdO nanocomposite revealed that electron densities and bond energies of the PANI homopolymer decreased after modifying with CdO nanoparticles, due to interactions between PANI chains and CdO nanoparticles. The prepared PANI/CdO nanocomposite exhibits excellent photocatalytic activity under both UV light and natural sunlight irradiation. The photocatalytic decolorization rate was increased up to 7 times after CdO addition, compared to the decolorization rate of PANI homopolymer under UV light irradiation. During the photocatalytic activity investigations, methylene blue and malachite green dyes were photocatalytically decolorized under natural sunlight irradiation with 99% efficiency by the use of 0.4 mg/mL PANI/CdO nanocomposite as photocatalyst. Furthermore, the PANI/CdO photocatalyst retains its efficiency with slight decreases upon being reused up to five times. et al.22 and Saravanan et al.19 Therefore, researchers have focused on increasing the degradation rates of pollutants by combining nanoparticles with conductive polymers to achieve synergetic and complementary behaviors.23−25 These conductive polymers act as stabilizers or surface capping agents when combined with semiconductor nanoparticles. 26−31 Polyaniline (PANI) is one of the most preferred polymers for synthesizing nanocomposites due to its high conductivity, simple synthesis procedure, good environmental stability, and reversible acid−base chemistry in aqueous solutions.32,33 Hence, it has a wide range of application areas such as electrochromic devices, light emitting diodes, corrosionprotecting paints, and electrostatic discharge protection.34−36 In addition to these applications, PANI has been combined with some inorganic nanoparticles to enhance their photocatalytic efficiencies.37−41 The goal of this paper is to prepare a new PANI/CdO nanocomposite and to determine its photocatalytic activity under UV light and natural sunlight irradiations for the degradation of MB and MG dyes. The PANI/CdO nanocomposite was synthesized for the first time in aqueous diethylene glycol solution medium by oxidative polymerization. The photocatalytic activity of PANI/CdO nanocomposite was studied experimentally under different conditions. Effects of organic dye type, photocatalyst amount, light source, irradiation time, and photocatalytic stability were also investigated.

1. INTRODUCTION Organic dyes have a wide range of application areas in the textile and food industries. On the other hand, they are important sources of environmental contamination due to their nonbiodegradability, high toxicity to aquatic creatures, and carcinogenic effects on humans. Malachite green (MG) and methylene blue (MB) are highly toxic organic dyes which are being used in many different fields such as biology and chemistry.1−4 Nowadays, the degradation of organic dyes in contaminated water or the conversion of them into harmless chemicals has been extensively studied in order to decrease the damage caused by organic dye pollution to the environment and humans. For this purpose, semiconductor metal oxide nanoparticles such as ZnO5,6 and TiO27 have been widely examined to degrade nonbiodegradable dyes via photocatalytical methods, particularly as emulsion powders. CdO is an important n-type metal oxide semiconductor with a direct band gap of 2.5 eV and an indirect band gap of 1.98 eV which has promising applications in the field of optoelectronic devices such as solar cells, phototransistors, photodiodes, transparent electrodes, catalysts, and gas sensors.8−17 Also, CdO is used as a photocatalyst for wastewater treatment,10,18,19 even though Cd is toxic and poisonous to humans and the environment. CdO material requires high temperatures for Cd to evaporate.20 In addition, there are some works in the literature which reported a positive effect on the decolorization efficiency when CdO nanostructures are used as catalysts, especially when they are combined with other semiconductor materials. Suarez-Parra et al.21 reported that combination of TiO2 nanoporous thin films with CdO nanostructures increased the photocatalytic performance of the films during degradation of blue azo dye under white-light irradiation. Similar results were reported by Kanjwal © XXXX American Chemical Society

Received: May 1, 2013 Revised: July 15, 2013 Accepted: July 23, 2013

A

dx.doi.org/10.1021/ie401389e | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were of analytical purity. Monomer aniline (Merck) was purchased from Merck and distilled under reduced pressure and stored in the dark, below 4 °C. High purity Cd rods (Alfa Aesar, 99.99%) were used for the synthesis of CdO nanoparticles. All other reagents, including ammonium peroxydisulfate (APS) as oxidant, diethylene glycol, MB, and MG were supplied by Merck. 2.2. Synthesis of CdO Nanoparticles. CdO nanoparticles were synthesized by the calcination of Cd(OH)2 nanoparticles which acted as precursors produced via an arc discharge method submerged in deionized water as described in our previous studies.42,43 In detail, the arc discharge apparatus used to synthesize the Cd(OH)2 nanostructures consisted of two cadmium rods as anode and cathode electrodes. Both electrodes were placed vertically into the beaker containing 3000 cm3 of deionized water. The arc discharge was generated between the two Cd electrodes, and the gap between the electrodes was controlled manually to be about 1 mm. The discharge voltage between electrodes was measured and kept constant (34 V) by controlling the gap to provide a stable arc during the synthesis. A dc arc current (50 A) was applied by a dc power supply for the whole experiment. The arc discharge was continued until the anode electrode was consumed. The solution was kept at room temperature for several days to provide settling of the synthesized particles in deionized water. Following that, the settled products were separated from the solution by decantation. The products were dried under vacuum at about 80 °C and then calcined at 400 °C for 4 h under open atmosphere to produce CdO nanoparticles due to the dehydration reaction of the Cd(OH)2 nanoparticles. 2.3. Synthesis of PANI Polymer and PANI/CdO Nanocomposite. PANI/CdO nanocomposites were prepared by the chemical oxidative polymerization of aniline in the presence of CdO nanoparticles according to the procedure from the authors’ previous work.37 The 0.4 M aniline and 0.4 M APS solutions were prepared with 1 M diethylene glycol. Meanwhile, 0.1 mmol of CdO nanoparticles was dispersed into 50 mL of aqueous 1 M diethylene glycol solution by tip sonication for 10 min to obtain a uniform suspension. Twentyfive milliliters of aniline and 25 mL of APS solutions were added dropwise into the CdO/diethylene glycol suspension simultaneously. The resulting solution was stirred for 1 min by use of a magnetic stirrer. Then the mixture was allowed to polymerize under room temperature without stirring for 2 h. Finally, the PANI/CdO nanocomposites were filtered and washed with a large amount of ethanol and deionized water. Finally, the filtered nanocomposite was dried at 40 °C under vacuum for 24 h. PANI homopolymer was also prepared by a chemical oxidative polymerization technique, following the same procedure above without the use of CdO nanoparticles. 2.4. Characterization. X-ray diffraction (XRD) analysis was carried out by a Shimadzu XRD-6000 X-ray diffractometer using Cu Kα radiation (λ = 0.154 18 nm). The operating conditions were 40 kV and 30 mA in the scanning range 20− 70° at rate of 2 deg/min. The morphological analysis of the synthesized products was carried out with a ZEISS Evo LS 10 scanning electron microscope (SEM). The transmission electron microcopic (TEM) images of the CdO nanoparticles were taken by use of a JEOL 2100 HRTEM at 300 kV. The optical absorption spectra of all the samples were obtained by

use of an Ocean Optics HR4000 UV−visible spectrophotometer, and the Fourier transform infrared (FTIR) spectra of all materials were recorded by a Perkin-Elmer 1725 instrument. Electrical conductivity measurements were taken via the four probe method, by use of an ENTEK Elk. FPP 460 with Pt probes. Differential scanning calorimetric (DSC) investigations were carried out by use of a Perkin-Elmer DSC 4000 instrument. The total organic carbon (TOC) measurements were taken by use of a Teledyne Tekmar Torch Combustion TOC/TN Analyzer. 2.5. Measurement of Photocatalytic Activities. The photocatalytic degradation of MB and MG dyes was performed in quartz tubes under the irradiation of UV light or natural sunlight in the presence of the PANI homopolymer, the CdO nanoparticles, or the PANI/CdO nanocomposite as catalyst. The UV-C tube lamp (15 W, length 41 cm, diameter 2.5 cm), Model G15T8 (Philips, Holland), was used as a UV irradiation source (λ = 254 nm) located in a filtrated chamber. In the photocatalytic treatment of the dyes, a known concentration (samples were prepared in molar concentration, 1.5 × 10−5 M) of the dye solution was taken in a quartz tube. A desired amount of catalysts was added to the dye solution. Before irradiation of the dye solution, the suspension was stirred for 30 min in the dark to realize adsorption−desorption equilibrium in the presence of the catalyst. This procedure was applied to both MG and MB dye solutions. After that, the suspensions were irradiated without stirring. In order to determine the photocatalytic activity of the catalyst under natural sunlight irradiation, all experiments were done inside the laboratory under open atmosphere and clear sky between 10:00 a.m. and 3:00 p.m. when the solar intensity fluctuations were minimal, in the months of June and July 2012 as given before in the literature.37,44 The intensity of solar radiation was measured by use of a Global pyranometer (LI-COR LI-200) as 490 ± 30 W/ m2 during the course of the photooxidation under clear sky. The photocatalytic degradation of organic dyes was investigated at room temperature in the presence/absence of different catalysts, under irradiation or in the dark for given times. The concentrations of MB and MG organic dyes were analyzed using the UV−vis spectroscopy method. The degradation efficiency of dye is calculated by the following equation:37 degradation (%) =

C0 − C ·100 C0

(1)

where C0 is the initial concentration of dye before irradiation and C is the concentration of dye after a certain irradiation time. In order to investigate the photocatalytic stability of the photocatalyst, the PANI/CdO nanocomposite was used for several photocatalytic runs under UV light or natural sunlight irradiation for both dyes. For this, 1.6 mg/mL PANI/CdO nanocomposite was added into (1.5 × 10−5 M) MB or MG aqueous solutions. The photocatalytic activity test procedures were applied as mentioned above. After the measurements, the photocatalyst was separated from the solution by centrifugation and following decantation. The separated catalyst was washed with deionized water many times and dried at 100 °C under open air in order to maintain the concentration of the dye solutions. Following that, it was analyzed by FTIR spectroscopy to determine its structure. After FTIR spectroscopy, the catalyst was added into a new MB or MG solution to reuse it for B



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photocatalysis experiments. The same experimental procedure was repeated five times, and the FTIR spectra of catalyst were recorded for each experiment.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. The XRD patterns of CdO nanoparticles, PANI homopolymer, and PANI/CdO

Figure 1. XRD patterns of (a) CdO nanoparticles, (b) PANI homopolymer, and (c) PANI/CdO nanocomposite.

nanocomposite are shown in Figure 1. It can be seen from Figure 1a that all of the peaks can be indexed to the cubic CdO crystal (JCPDS Card No. 05-0640). The XRD curve of the CdO nanoparticles reveals that the relatively strong diffraction peaks are (111) and (200). The sharp peaks of the XRD patterns indicate that the fabricated CdO nanostructures possess good crystallinity. In Figure 1b,c, the XRD patterns of PANI homopolymer and PANI/CdO nanocomposite are represented. Four peaks can be observed in the region 2θ = 15−30°. For the XRD pattern of PANI, the maximum peak around 2θ = 18.5° can be ascribed to periodically parallel and perpendicular polymer (PANI) chains. The peak at 2θ = 20° is evidence of the characteristic distance between the ring planes of benzene rings in adjacent chains or close contact interchains.45 The peak centered at 2θ = 25° can be assigned to the scattering from PANI chains at interplanar spacing46,47 and indicates that PANI homopolymer also has some degree of crystallinity (Figure 1b). As can be seen in Figure 1c, two phases are identified in the PANI/CdO nanocomposite. One is the cubic structure of CdO and the other is PANI with characteristic peaks. A typical TEM image of CdO nanoparticles is shown in Figure 2a. As seen in the figure, the particles are spherical, elliptical, or irregularly shaped, and the sizes are up to 50 nm. Parts b and c of Figure 2 show the SEM images of the PANI homopolymer and the PANI/CdO nanocomposite, respectively. The PANI homopolymer displays micrometer sized irregular sheetlike morphology as seen in Figure 2b. CdO nanoparticles can be seen on PANI sheets as shown in Figure 2c. It is observed that the distribution of CdO nanoparticles in the PANI matrix is homogeneous and the sizes of the CdO nanoparticles are measured as 52 and 67 nm as indicated by the rectangle, which are compatible with the sizes of the CdO nanoparticles observed during TEM analysis. FTIR spectroscopic analyses were carried out to characterize the prepared PANI homopolymer, CdO nanoparticles, and PANI/CdO nanocomposite. The FTIR spectrum of CdO is represented in Figure 3a. The bands at 833, 686, and 631 cm−1 are related to the stretching vibration of Cd−O bonds. As seen

Figure 2. (a) TEM image of bare CdO nanoparticles, (b) SEM image of PANI homopolymer, and (c) SEM image of PANI/CdO nanocomposite.

in Figure 3b, the FTIR spectrum of the PANI homopolymer exhibits a characteristic peak around 3263 cm−1 which is attributed to the N−H stretching mode.48 The peaks of the CN and CC stretching vibrations of quinoid and benzenoid units are observed at 1578 and 1490 cm−1, respectively. The band at 1297 cm−1 is assigned to the C−N stretching of the benzenoid, while the band at 1041 cm−1 is due to the quinoid unit of PANI. The presence of the benzenoid and quinoid units is evidence of the emeraldine form of PANI. C



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450, and 740−950 nm.51 The first absorption band arises from π−π*, the electron transition between the benzenoid segments.52 The second and third absorption bands are related to the doping level and the formation of polarons (quinoid segments), respectively. For the absorption spectrum of PANI homopolymer, as shown in Figure 4a, a shoulder centered at 300 nm and a broad band centered at 450 nm are observed. Other characteristic bands which are related to the doping level of the PANI homopolymer cannot be observed. This result may be attributed to the synthesis medium, in which diethylene glycol used as solvent during the chemical polymerization of aniline. In the literature, the absorption bands of PANI homopolymer between 700 and 800 nm with high intensities are related to the presence of acidic solvents used as a medium during the polymerization of aniline.53 The absorption spectrum of the CdO nanoparticles (Figure 4b) presents a broad band between 200 and 400 nm in the UV range. The absorption spectrum of the PANI/CdO nanocomposite is shown in Figure 4c. As can be seen from Figure 4c, the absorption spectrum of the PANI/CdO nanocomposite represents the same characteristic bands as the PANI homopolymer; however, slight shifts to lower wavelengths are observed in the corresponding spectrum. In addition, the absorption intensity of the PANI/CdO nanocomposite was increased compared to PANI homopolymer. Therefore, these results prove the existence of strong interactions between PANI and CdO nanoparticles which affect the conjugation and electron density. The optical band gap can be determined using the fundamental absorption, which corresponds to the electron excitation from the valence band to the conduction band. Direct absorption band gaps of the CdO nanoparticles and PANI homopolymer can be obtained by putting the absorption data into the equation54 αhν = B(hν − Eg)n, where α is the absorption coefficient, hν is the photon energy, Eg is the optical band gap of the material, B is the material constant, and n is either 2 for direct transition or 1/2 for an indirect transition. Therefore, the optical direct band gaps of the CdO nanoparticles, the PANI homopolymer, and the PANI/CdO nanocomposite for the absorption edge can be determined by extrapolating the straight portion of the curve (αhν)2 versus hν when α = 0. The calculated Eg values were 2.6, 2.81, and 2.79 eV for the CdO nanoparticles, the PANI homopolymer, and the PANI/CdO nanocomposite, respectively. 3.3. Electrical Conductivity. Electrical conductivity measurements of the PANI homopolymer and the PANI/ CdO nanocomposite were taken using the four probe method, and the conductivity values were measured as 4.45 × 10−5 and 3.06 × 10−7 S/cm, respectively. These results show that the electrical conductivity of the PANI decreases after CdO modification. In addition, the conductivity values are supported by FTIR and UV−vis analyses in which it is observed that the interactions between the polymer and the nanoparticles lead to decreased conjugation and electron density. 3.4. DSC Analysis. The DSC thermograms of the PANI homopolymer and PANI/CdO nanocomposite are represented in curves a and b, respectively, of Figure 5. Glass transition temperature (Tg) values were measured as 82 and 143 °C for the PANI homopolymer and the PANI/CdO nanocomposite, respectively.55 The measured Tg values indicate that the PANI/ CdO nanocomposite is more thermally stable than the PANI homopolymer. In addition, the endothermic peaks related to the thermal degradation of the PANI homopolymer and the

Figure 3. FTIR spectra of (a) CdO nanoparticle, (b) PANI homopolymer, and (c) PANI/CdO nanocomposite.

Figure 4. UV−vis spectra of (a) PANI homopolymer, (b) CdO nanoparticles, and (c) PANI/CdO nanocomposite.

Figure 5. Differential scanning calorimetric analysis (DSC) of (a) PANI and (b) PANI/CdO nanocomposite.

In Figure 3c, the FTIR spectrum of the PANI/CdO nanocomposite is represented. The corresponding spectrum has the same characteristic bands as the PANI homopolymer, but some of the bands shifted to higher wavenumbers after the addition of CdO nanoparticles. The bands at 1578, 1490, and 1041 cm−1 shifted to 1587, 1499, and 1049 cm−1, respectively. These shifts of characteristic bands can be attributed to the interactions between PANI homopolymer chains and CdO nanoparticles which decrease the electron density and bond energy of the PANI after the addition of CdO nanoparticles.48−50 3.2. Optical Properties. The UV−vis spectra of CdO nanoparticles, PANI homopolymer, and PANI/CdO nanocomposite are shown in Figure 4. As reported previously by some researchers, doped forms of PANI homopolymer usually show three characteristic absorption bands at 320−360, 400− D



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Figure 6. Decomposition comparison of MB and MG dyes with respect to time intervals over PANI homopolymer and PANI/CdO nanocomposite under (a) UV light irradiation of MB, (b) natural sunlight irradiation of MB, (c) UV light irradiation of MG, and (d) natural sunlight irradiation of MG. Catalyst concentration, 0.4 mg/mL; initial concentration of dyes, 1.5 × 10−5 M.

Table 1. Degradation (%) of MB and MG Dyes in the Presence of PANI/CdO Nanocomposite and PANI and CdO Photocatalysts after 4 h Illumination under Different Light Source Irradiationsa PANI/CdO MB MG a

PANI

CdO

UV

sunlight

dark

UV

sunlight

dark

UV

sunlight

dark

92 97

98 99

15 13

27 33

82 79

10 9

47 55

48 9

28 55

Catalyst concentration, 0.4 mg/mL; initial concentration of dyes, 1.5 × 10

−5

M.

Figure 7. Comparison of apparent rate constants of MB and MG dyes in the presence of PANI homopolymer and PANI/CdO nanocomposite photocatalysts under UV light and natural sunlight irradiation: (a) MB dye and (b) MG dye. Catalyst concentration, 0.4 mg/mL; initial concentration of dyes, 1.5 × 10−5 M.

Sunlight Irradiation. The photocatalytic degradation of MB dye and that of MG dye (initial concentration of both dyes, 1.5 × 10−5 M) in the presence of the PANI homopolymer, the CdO nanoparticles, or the PANI/CdO nanocomposite catalyst under UV light or natural sunlight irradiation were investigated. The changes in the optical absorption spectra of MB and MG

PANI/CdO nanocomposite are slightly different for each structure. The degradation temperature is about 281 °C for the PANI homopolymer, while it is 293 °C for the PANI/CdO nanocomposite. 3.5. Photocatalytic Activity. 3.5.1. Photocatalytic Degradation of MB and MG under UV Light and Natural E



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Table 2. Apparent Rate Constants (kapp) of Dye Degradation and Linear Regression Coefficients from Plot of ln(C0/Ct) = kappta PANI/CdO

PANI

UV R MB MG a

sunlight −1

2

0.99 0.98

−1

2

kapp (min )

R

0.016 69 0.019 01

0.99 0.99

Concentration of dyes, 1.5 × 10

−5

−1

2

kapp (min )

R

0.019 73 0.021 31

0.99 0.98

sunlight

MB MG

2

UV −1

2

kapp (min )

R

kapp (min )

R

0.002 37 0.002 8

0.99 0.99

0.018 14 0.011 73

0.98 0.99

sunlight −1

2

kapp (min )

R

0.006 09 0.006 46

0.98 0.98

kapp (min−1) 0.005 14 0.006 09

M; catalyst concentration, 0.4 mg/mL.

to investigate the effect of irradiation on the catalyst/dye interaction, three sets of experiments were carried out. The first set involved the degradation of the dyes under UV light or natural sunlight without catalyst. No significant changes were observed in the initial concentrations of the dyes; the decolorization efficiency was less than 2% for MB, while it was 5% for MG after 4 h of irradiation under UV light or natural sunlight (Figure 6). For the first set of experiments, the photolysis of the dyes was negligible. For the second set, the experiments were carried out in the dark with the catalyst material to understand the effect of the adsorption mechanism on the degradation of dyes. As seen, the decolorization efficiencies for PANI/CdO composite as catalyst were 15 and 16% for MB and MG dyes, respectively, after 4 h due to the adsorption mechanism. Similarly, the decolorization efficiency was 10% for both dyes after 4 h, when the PANI homopolymer was used as a catalyst. When the CdO nanoparticles were used as catalyst, the decolorization efficiencies were 28 and 9% for MB and MG, respectively, under dark conditions. The third sets of experiments were performed to understand the effect of UV light and natural sunlight irradiations on the catalyst/dye interaction. As can be seen in Figure 6, the concentration decrease of MB dye under UV light and natural sunlight irradiation indicates that MB has been degraded almost completely by the PANI/ CdO catalyst after 4 h (Figure 6a,b). In addition, the decolorization efficiencies of MB dye solution after 30 min exposure time are 45 and 52% under UV light and natural sunlight irradiation, respectively (Figure 6a,b). The decolorization of MG dye in the presence of the PANI/CdO catalyst under UV light and natural sunlight irradiation corresponding to different time intervals was also investigated. In Figure 6c,d, the concentration decrease of the MG dye under UV light and natural sunlight irradiation indicates that MG dye has been degraded almost completely by the PANI/CdO nanocomposite after 4 h. Furthermore, the decolorization efficiencies of the MG dye solution after 30 min of exposure time are 50 and 54% under UV light and natural sunlight irradiation, respectively (Figure 6c,d). The experimental results are summarized in Table 1. It is clear that almost complete photodegradation of MB and MG dyes has been achieved after 4 h under natural sunlight irradiation using the PANI/CdO nanocomposite as a photocatalyst. The decolorization efficiencies of the PANI homopolymer and CdO nanoparticles are lower than that of the corresponding PANI/CdO catalyst in the same conditions. Especially the decolorization efficiency of the PANI homopolymer increased almost up to 3 times under UV light irradiation for both dyes. It is clear that the CdO nanoparticles play an important role to increase the photocatalytic activity of the PANI matrix especially under UV light irradiation. The decolorization efficiencies of the dyes are much higher under irradiation than the corresponding ones in the dark, in the

Table 3. TOC Removal after 4 h Irradiation under UV Light and Natural Sunlight Irradiationa PANI/CdO

CdO

UV

PANI

CdO

UV

sunlight

UV

sunlight

UV

sunlight

59 66

71 82

21 15

52 48

17 26

17 26

Concentration of dyes, 1.5 × 10−5 M; catalyst concentration, 0.4 mg/ mL. a

Figure 8. Effect of photocatalyst concentration on degradation of MB and MG dyes under UV light irradiation after 5 h (initial concentration of dyes, 1.5 × 10−5 M).

dyes for different time intervals are shown in Figure S1 in the Supporting Information. As can be seen in Figure S1, no absorption band was observed in the absorption spectra which were recorded after treatment. These results demonstrated that the lifetime of intermediate products which emerged as a result of decomposition is short, or that their concentrations are not within detection limits. The effect of the amount of CdO nanoparticles in the polymerization medium on the photocatalytic activity of nanocomposite was investigated. For this purpose, different amounts of CdO nanoparticles (0.01, 0.04, and 0.1 mmol) were added to the polymerization medium and photocatalytic activities of the synthesized PANI/CdO nanocomposites were investigated for decolorization of the MB under UV light irradiation. The optimum amount of CdO nanoparticles was found to be 0.1 mmol. Hereafter, all experiments were carried out in the presence of 0.1 mmol of CdO. When 0.1 mmol of CdO nanoparticles was used in the polymerization medium, MB dye solution was decolorized 97% under UV light irradiation after 4 h of exposure time. The photocatalytic activity of the PANI/CdO nanocomposite was decreased for 0.01 and 0.04 mmol of CdO. Under UV light irradiation, the decolorization efficiencies of MB dye solution after 4 h of exposure time were 39 and 73% for 0.01 and 0.04 mmol of CdO, respectively. The variation of decomposition versus irradiation time of MB and MG dye solutions is represented in Figure 6. In order F



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Figure 9. Effect of number of runs on the degradation of dyes in the presence of PANI/CdO nanocomposite: (a) UV light irradiation of MB, (b) natural sunlight irradiation of MB, (c) UV light irradiation of MG, and (d) natural sunlight irradiation of MG. Catalyst concentration, 0.4 mg/mL; initial concentration of dyes, 1.5 × 10−5 M.

natural sunlight irradiation is higher than that under UV light irradiation. However, the kapp values of the PANI/CdO catalyst for UV light and natural sunlight irradiation are higher than those of the PANI homopolymer and CdO nanoparticle catalyst. To the best of our knowledge, there are no theoretical and experimental studies on the photocatalytical behavior of PANI/ CdO nanocomposites. In this manner, comparisons of the obtained decolorization efficiencies and the apparent rate constants with the literature are not available. However, these results may be compared with a similar study that was reported recently by the authors.37 Degradation of MB and MG dyes in the presence of the PANI/ZnO nanocomposite under UV light and natural sunlight irradiation was investigated with the corresponding study. Our studies indicated that the synthesized PANI/CdO nanocomposite is more effective on decolorization of MB and MG dyes under the same conditions compared to our previous work. In addition, the PANI/CdO nanocomposite has been compared with other types of photocatalysts such as PANI/TiO24,47,56 and results indicate that the PANI/CdO nanocomposite system is more effective compared to those of the PANI/TiO2 photocatalysts under natural sunlight irradiation. To understand whether the result is decolorization or degradation, TOC values were measured after 4 h of the process. The TOC values are given in Table 3. As can be seen from Table 3, the PANI/CdO composite provided organic carbon removal as 66% in UV irradiation and 82% in natural sunlight for MG and as 59% in UV irradiation and 71% in natural sunlight for MB. The TOC removal carried out by

Figure 10. FTIR spectra of PANI/CdO nanocomposite after photocatalytic reaction in (a) methylene blue and (b) malachite green. Catalyst concentration, 0.4 mg/mL; initial concentration of dyes, 1.5 × 10−5 M.

presence of PANI homopolymer, CdO nanoparticles, and PANI/CdO nanocomposite. In general, the kinetics of photocatalytic degradation of organic pollutants on semiconducting oxides have been determined and can be described well by the apparent first order reaction ln(C0/Ct) = kappt, where kapp is the apparent rate constant, C0 is the concentrations of dyes after adsorption in darkness for 30 min, and Ct is the concentration of dyes at the given time t. Figure 7 shows the relationship between the irradiation time (1 h) and ln(C0/Ct). The activity of synthesized nanocomposite catalysts under UV light and natural sunlight irradiation can be evaluated by comparing kapp values listed in Table 2. As seen in Table 2, it is obvious that the photocatalytic activity of the PANI catalyst under G



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transfer of photoexcited electrons and positive holes between interconnecting PANI and CdO nanoparticles may enhance the charge separation, hence increase the efficiency of utilization of electron−hole pairs. On the other hand, the CdO nanoparticles in the PANI/CdO nanocomposite may be acting as electron sinks which inhibit the recombination to accumulate the electrons leading to fast degradation compared with the pure PANI.46 The results show that the decolorization of dye takes place with both the PANI homopolymer and the PANI/CdO under natural sunlight irradiation, but it has been observed that the degradation rates of the PANI/CdO catalyst were much higher than that of the PANI homopolymer (Figure 6 and Table 2). A sensitized photocatalytic process may be able to be operated in the presence of a colored organic compound; in this case the adsorbed dye molecules are excited by visible light and thus act as photosensitizers.57,58 The excited dye molecule subsequently transfers electrons into the conduction band of the PANI homopolymer or the PANI/CdO nanocomposite, while the dye itself is converted to its cationic radical (eqs 7−9).

PANI/CdO is more than the sum of the TOC removal carried out by PANI and CdO. 3.5.2. Possible Mechanism. The general understanding of the photocatalysis mechanism is that the photoabsorption of a semiconducting material causes the electrons to excite from the valence band (VB) to the conduction band (CB), leaving positive holes in the VB resulting in the electron−hole pair eCB−/hVB+ generation. It is known that PANI homopolymer is a conducting polymer, and the conductivities of the synthesized PANI homopolymer and the PANI/CdO nanocomposite were measured as 4.45 × 10−5 and 3.06 × 10−7 S/cm, respectively. These values are at the level of semiconductivity. Photons are absorbed by the PANI homopolymer or the PANI/CdO catalyst when the energy (hν) is equal to or greater than the semiconductor band gap, and electron−hole pairs are generated in these semiconducting materials (SCM) (eq 2). SCM (CdO, PANI, PANI/CdO) + hν → (eCB−)SCM + (hVB+)SCM

(2)

Then, the photogenerated electron−hole pairs migrate to the surface of the catalyst and react with the species adsorbed on the surface (eqs 3−6). O2 + (eCB−)SCM → O2•−

(3)

O2 + 2(eCB−)SCM + 2H+ → H 2O2

(4)

(eCB−)SCM + O2•− + 2H+ → HO• + HO−

(5)

H 2O + (hVB+)SCM → HO• + H+

(6)

dyeads + hν → dyeads*

(7)

dyeads* + SCM → dyeads•+ + (eCB−)SCM

(8)

dyeads + (hVB+)SCM → dyeads•+

(9)

These reactive species produced in the above manner can then react with the dye to form the degradation products and thus are responsible for the discoloration of MB and MG (eqs 10 and 11).

These reactions prevent the electron−hole pairs from recombining which reduces the efficiency of photocatalytic activity. As can be seen in Figure 6 and Table 2, the photocatalytic activity of the PANI homopolymer under UV light irradiation was lower than that under natural sunlight irradiation. This result can be attributed to the presence of benzenoid and quinoid segments of the emeraldine form of the PANI homopolymer. The photoexcitation of the PANI homopolymer in the UV range arises from π−π* electron transition within the benzenoid segments, while it occurs within the quinoid segments in visible light (about 440 nm).51,52 It is known that the benzenoid segments have lower energy than the quinoid segments and the possibility of the combination of electron−hole pairs is more likely in benzenoid segments, which reduces the efficiency of photocatalytic activity. The degradation rates of MB and MG in the presence of the PANI/CdO catalyst are higher those than in the presence of the PANI homopolymer for both UV light and natural sunlight irradiation (Figure 6 and Table 2) which indicates that the CdO nanoparticles increase the photocatalytic activity of the PANI matrix. As can be seen in Figure 4, the PANI/CdO nanocomposite has high absorption intensity in the UV range and in the visible light region compared to the PANI homopolymer. Therefore, the PANI/CdO nanocomposite can be excited to produce more electron−hole pairs under UV light and visible light irradiation, which could result in an efficient photocatalytic activity. The synergetic effect between the CdO nanoparticles and the PANI matrix on the photocatalytic degradation of dyes also exists clearly for the PANI/CdO nanocomposite. The copresence of PANI and CdO nanoparticles induces a high level of photocatalytic activity; the

dye + radical species (O2•− , HO•)/H 2O2 → degradation product dyeads•+ → degradation product

(10) (11)

As can be seen in Table 3, the TOC values show that the amounts of organic materials decreased significantly. As a result, the degradation was also carried out beside decolorization. 3.5.3. Effect of Photocatalyst Dosages. In order to optimize the photocatalyst dosage in dye solution, the effect of photocatalyst dosages on the degradation of dyes in water was investigated under UV light irradiation with a fixed dye concentration (1.5 × 10−5 M). The degradation time was also fixed at 3 h, and the effect of the photocatalyst loading on percentage degradation of the dyes was examined by varying its concentration from 0.2 to 1.6 mg/mL. As seen in Figure 8, the degradation efficiency is increased with increasing concentration of the photocatalyst up to 0.4 mg/mL; however, for higher concentration values of the photocatalyst, the efficiency was almost stable for both dyes. The increase in the degradation efficiency seems to be due to the increase in the total surface area, namely the number of active sites, available for the photocatalytic reaction as the dosage of photocatalyst increases. However, the number of active sites on the photocatalyst surface may become almost constant because of the decreased light penetration, increased light scattering, and loss in the surface area occasioned by agglomeration (particle− particle interactions) at high solid concentration when the photocatalyst was overdosed.59 Similar trends were reported previously in other photocatalytic reactions over 0.4 mg/mL catalyst.60 Therefore, 0.4 mg/mL photocatalyst concentration H



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was selected as the optimal concentration of photocatalyst for the sequential experiment. 3.5.4. Photocatalytic Stability. Parts a and c of Figure 9 respectively represent the cyclic usage of the PANI/CdO catalyst for degradation of the MB and MG dye solutions under UV light irradiation. It has been confirmed that the PANI/CdO catalyst maintains its good photocatalytic activity even after five cycles. Otherwise, the photocatalytic activity of the PANI/CdO catalyst decreases slightly with the increasing number of cycles. For the first usage, the PANI/CdO catalyst degraded 99% of the MB dye solution, while the decolorization efficiency of the PANI/CdO catalyst was 93% after the fifth usage. In addition, the PANI/CdO catalyst degraded 98% of the MG dye solution for the first usage while the decolorization efficiency of the PANI/CdO catalyst was 93% after the fifth usage. Parts b and d of Figure 9 respectively show the cyclic usage of the PANI/CdO catalyst for the degradation of MB and MG dye solution under natural sunlight irradiation. It is obvious that the PANI/CdO catalyst also continues to maintain good photocatalytic activity after five cycles. The decrease of the concentration of MB dye solution is 99% in the first cycle in the presence of the PANI/CdO catalyst, while 82% of the MB dye solution degraded in the fifth cycle. Furthermore, the PANI/ CdO catalyst degraded 98% of MG dye solution for the first usage, while the decolorization efficiency of the PANI/CdO catalyst was 93% after the fifth usage. FTIR analyzes were carried out to confirm the photocatalytic stability of the PANI/CdO nanocomposite after photocatalytic reaction. As can be seen in Figure 10, the similarity between the PANI/CdO nanocomposite spectra before and after photocatalytic degradation of the MB and MG dyes is clear. It is clear that the structure of the PANI/CdO nanocomposite was not affected during the photocatalytic process or was not chemically transformed to other organic compounds.

Article

ASSOCIATED CONTENT

S Supporting Information *

UV−vis absorption spectra of dye solutions treated with PANI/ CdO nanocomposite for different exposure times under UV light and natural sunlight irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +90 332 2232071. Fax: +90 332 2410651. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS In summary, PANI homopolymer and PANI/CdO nanocomposites have been successfully prepared by a chemical polymerization method. The SEM observations revealed that PANI has irregular sheetlike morphology and the morphology remained stable after the combination with CdO nanoparticles. The FTIR and UV−vis spectroscopic analyses confirm that there is an interaction between PANI chains and CdO nanoparticles which affect electron density and bond energy. The nanocomposite catalyst exhibited a good photocatalytic activity on the degradation of MB and MG dyes under natural sunlight irradiation. The kapp values of the PANI/CdO nanocomposite under both UV light and natural sunlight irradiation are higher than those of PANI homopolymer. We propose that the mechanism behind the decomposing of MB and MG dyes in the presence of PANI homopolymer and PANI/CdO nanocomposites includes two collaborative processes: a photocatalytic process and a photosensitization process. The proposed method may be used for the synthesis of various nanocomposites. Overall, PANI/CdO nanocomposites present a promising material for addressing the environmental pollution caused by organic dyes. Furthermore, the high photocatalytic activity in the visible light will save the cost of UV irradiation and facilitate creating industrial applications. I



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