Solar light driven dye degradation using novel organo–inorganic (6,13-pentacenequinone/TiO₂) nanocomposite

Abstract

The coupled semiconductor photocatalysts of 6,13-pentacenequinone/TiO₂ (PQ/Ti) with different weight ratios were prepared. The PQ/TiO₂ nanocomposite was prepared by a simple wet impregnation method. The prepared catalyst was characterized with various spectroscopic methods. XRD confirms the monoclinic and anatase phase of PQ and TiO₂, respectively. FE-SEM showed the sheet like morphology of PQ, whereas the coupled catalyst depicts the formation of a homogeneous layer of TiO₂ on the PQ surface in all three compositions. Optical studies by diffuse reflectance UV-Visible absorbance spectroscopy (DRS) indicate the band gap of PQ and TiO₂ around 2.8 and 3.2 eV respectively. The absorbance in the range of 450 nm was also observed in the case of the prepared coupled catalyst confirming the loading of PQ on TiO₂. The photocatalytic activity of the coupled catalyst was studied by observing the degradation of methylene blue (MB) dye under visible light irradiation. As compare to the individual components, the coupled organo–inorganic catalyst showed higher photocatalytic activity towards MB degradation. The PQ/TiO₂ nanocomposite having 0.2 wt% PQ showed the highest rate of MB degradation i.e. K_app = 5.2 × 10⁻² min⁻¹.

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1. Introduction

Nowadays, industries are growing tremendously in order to fulfill human requirements. But, on the other hand this growth results in an increase in air and water pollution.¹ Day by day the problem of pollution shows a serious impact on the ecosystem as well as on human beings.² Primarily, water pollution shows a severe impact on the aquatic and terrestrial organisms.^3,4 Industries are unable to design pollution free processes so one has to develop an eco-friendly method for water treatment.⁵ Conventional physical, chemical and biological methods have been utilized for water treatment.⁶ These methods include biological digestion, coagulation, chemical oxidation, clay minerals, flocculation, membrane separation, ozonation and various adsorbents.^7–14 From these, adsorption is a very effective and economical method for the purification of waste water.¹⁵ All these techniques require a long time span, use of organic solvents and some special types of reactors. Overall, these techniques have certain merits as well as demerits. The scientific community is in search of suitable techniques for pollution abatement.¹⁶ Among the various industries, the textile industry is a major source of water pollution.¹⁷ Various dyes like methylene blue, methyl orange, methyl red, Rhodamine 6B, Rose Bengal, crystal violet and Congo red have been regularly utilized in the textile industry and the colored waste water is directly release into the eco-system.^18–25 Some of the dyes are carcinogenic, mutagenic and toxic for human beings when they come into contact with mankind.²⁶ To avoid such serious hazards, the development of an effective method is needed to treat the polluted water.^27,28

Recently, heterogeneous photocatalysis have been exploited effectively for environmental purification.²⁹ Photocatalysis involves the use of semiconductor oxide/sulphide in presence of light.³⁰ Upon illumination with appropriate wavelength of light, the electron from valence band goes in to the conduction band creating electron–hole pairs.³¹ The formed electron and hole react with surface adsorbed oxygen and water molecule generating superoxide anion and OH radical, respectively. These OH and superoxide radicals have higher oxidizing and reducing potentials respectively and have capability of reducing as well as oxidizing large number of compounds. Till today, various oxide/sulphide semiconductors have been used viz. TiO₂, ZnO, WO₃, SnO₂, Fe₂O₃, CdS, ZnS, SnS₂, CdIn₂S₄ etc.^32–40 Among the studied catalyst, TiO₂ gained enormous attention due to its higher chemical and biological stability, cost-effective and non-toxic nature.^16,34 However the main drawback associated with TiO₂ for photocatalytic process is its wide band gap (3.0 eV for rutile and 3.2 eV for anatase phase, respectively).^37,39 TiO₂ requires UV light (<390 nm) for generating electron–hole pairs. Many researchers have reported the various methods for enhancing the photocatalytic activity of TiO₂.⁴¹ It includes transition metal doping, non-metal doping, loading of noble metals and changing the synthesis methods.^16,17,32,35 Recently, uses of coupled semiconductor have been utilized to enhance the photocatalytic activity; it involves use of CdS, WS₂, MoS₂, Ta₂O₅, WO₃ coupled with ZnO and TiO₂ respectively.^34,42–45 Of late, use of carbon nano tubes and graphene coupled with semiconductor oxides have been reported, it showed enhanced photocatalytic activity.⁴⁶ Recently our group synthesized some photocatalyst showing the higher photocatalytic activity towards dye degradation and Hydrogen generation via H₂O and H₂S splitting. Also few reports are available mentioning the effective use of organic semiconductors coupled with inorganic materials; Shang et al. reported the poly-(3-thiopheneacetic acid) coated Fe₃O₄ as photocatalyst for the efficient photocatalytic disinfection of pathogenic bacteria under solar light irradiation.^47–59 Herein, we have reported synthesis of coupled organo–inorganic semiconductor photocatalyst of 6,13-pentacenequinone/TiO₂ (PQ/Ti). We have prepared PQ/TiO₂ nanocomposite with different weight ratios of PQ ranging from 0.01, 0.1, and 0.2 wt% by simple wet impregnation method and studied its photocatalytic activity towards aqueous methylene blue degradation under visible light irradiation. The nanocomposite obtained is characterized thoroughly for its structural and optical study.

2. Experimental

2.1. Synthesis of PQ/TiO₂ nanocomposite

For the synthesis of PQ/TiO₂ nanocomposite, anatase TiO₂ Aldrich make was used as such, and 6,13 pentacenequinone was prepared in our laboratory. The PQ was prepared by simple mixing of 1,4-cyclohexadione and ortho-pthalaldehyde in mortar and pestle with 1 : 2 ratio for 30 min. without any solvent. A stock solution of PQ was prepared in 100 mL chloroform containing 5 mg of PQ. Appropriate amount of this solution was stirred with TiO₂ dispersed in chloroform in order to get the 0.01, 0.1 and 0.2 wt% PQ respectively. Moreover, this mixture was stirred for 10 h, the excess solvent was naturally evaporated, the obtained powder containing PQ/TiO₂ organo–inorganic nanocomposites was dried at 50 °C for 4 h followed by grinding, and used as such for further analysis.

3. Photocatalytic activity measurement

For the photocatalytic activity measurement of the prepared nanocomposites, the 100 mL aqueous methylene blue (MB) having 10 ppm MB was stirred in 150 mL conical flask. To this solution 50 mg of nanocomposite was added and stirred under 400 W mercury vapour lamp irradiation. The mercury vapour lamp was kept vertically in the quartz tube, provided with the water circulation arrangement in order to minimize the heating effect due to IR radiation. At regular intervals of time, fixed amount of aqueous solution were taken from the flask, centrifuged and the UV-visible absorption spectrum of the clear solution recorded using a double beam spectrophotometer. The decrease in the absorbance value at 664 nm wavelength, corresponding to the typical peak for the absorption spectra of MB, was utilized to determine the extent of degradation of MB and the photocatalytic activity of the sample with respect to irradiation time.

4. Results and discussion

4.1. X-ray analysis

The formed nanocomposite was characterized by powder X-ray diffractometry (XRD) to study the crystalline formed phases PQ/TiO₂ catalyst. The powder XRD patterns of the PQ/TiO₂ nanocomposites are plotted in Fig. 1. The diffraction peaks at 2θ = 25.3, 37.8, 48, 53.9, 55° can be indexed to the (101), (004), (200), (105) and (211) crystal planes of anatase TiO₂ (JCPDS no. 21-1272). XRD pattern of pure PQ indicates the formation of crystalline monoclinic phase, the diffraction peaks at 14.7, 23.6 and 27.7° matches with (012), (112) and (104) reflection planes, respectively (JCPDS no. 47-2123). The monoclinic phase of PQ was observed in all three prepared compositions with varying the PQ wt. ratios (0.01 to 0.2 wt%). Even though, the amount of PQ used is too small but due to its higher crystalline nature, PQ showed slight hump in the XRD pattern of PQ/TiO₂ nanocomposites. This confirms the formation of PQ/TiO₂ nanocomposite. The crystallite size of TiO₂ in composite was observed to be 23 nm which is smaller than pristine TiO₂ (46 nm).

Fig. 1 XRD pattern of photocatalyst (6,13-pentacenequinone/TiO₂) where (♦) and (□) indicates peaks due to TiO₂ and PQ respectively.

4.2. Morphological study by FE-SEM

FE-SEM micrographs of PQ/TiO₂ nanocomposites are depicted in Fig. 2. The FESEM shows the uniform loading of TiO₂ on PQ surfaces, particularly 0.01 and 0.1 wt% loading depicts the homogeneous distribution of PQ and TiO₂ (Fig. 2a and d). Here, we expect loading of PQ on TiO₂ surface but instead of PQ loading on TiO₂ we obtained TiO₂ loaded PQ nanocomposites.

Fig. 2 FESEM micrographs of coupled semiconductor photocatalyst (6,13-pentacenequinone/TiO₂) where (a and b) 0.01 PQ; (c and d) 0.1 PQ and (e and f) 0.2 wt% PQ on TiO₂ respectively.

This might be because of large flat surface (micron size) of PQ formation due to solid-state reaction technique. The dipole–dipole interaction among the individual PQ molecules lead the formation of micron sized plates. The existence of spherical nano sized TiO₂ particles (10 to 25 nm) at the surface of PQ matrix is observed for higher PQ loading (Fig. 2e and f).

4.3. TEM analysis

The TEM analysis of PQ/TiO₂ nanocomposite with 0.2 wt% PQ is depicted in Fig. 3.

Fig. 3 TEM micrograph of coupled semiconductor photocatalyst with 0.2 wt% PQ/TiO₂ (6,13-pentacenequinone/TiO₂), and SAED pattern PQ/TiO₂ is shown in inset of (b).

The spherical shaped TiO₂ particles having size in the range of 10–25 nm and sheet like PQ plates are clearly visible. The size of PQ plates is in the range of submicron size, having diameter around 200 to 500 nm and length around 2 to 3 μm. The homogeneous distributions of TiO₂ particles on PQ plates are clearly visible. The high resolution image depicts the lattice spacing of 0.34 nm which corresponds to the (101) plane of anatase TiO₂ (inset of Fig. 3b). The SAED pattern clearly indicates the (101) and (103) planes confirming presence of anatase TiO₂ and this result supports the XRD analysis (inset of Fig. 3b).

4.4. UV-visible absorbance study

The diffuse reflectance UV-Visible absorbance spectra of PQ/TiO₂ nanocompositeare shown in Fig. 4. The pristine PQ showed broad absorbance centered at 420 nm, and absorbance edge around 450 nm indicating band gap of 2.8 eV. Anatase TiO₂ is having band gap of 3.2 eV. The prepared nanocomposites showed absorbance profiles same as anatase TiO₂ however additional absorbance was observed around 425 nm. The presence of hump at 425 nm indicates the existence of PQ on TiO₂ surface. Intensity of the PQ peak at 425 nm in the prepared nanocomposites increases with raise of PQ concentration (shown in inset of Fig. 4). Further, the PQ/TiO₂ composite indicate blue shift in the absorbance profile as compare to pristine PQ and TiO₂. This blue shift might be because of lowering of particle size of TiO₂ (crystallite size 23 nm) during the synthesis of PQ/TiO₂ nanocomposites due to vigorous stirring.

Fig. 4 DRS spectra of coupled semiconductor photocatalyst (6,13-pentacenequinone/TiO₂).

To validate the possible interaction between PQ and TiO₂, we have analysed the PQ/TiO₂ composites with FTIR. The C=O stretching frequency in PQ was observed around 1672 cm⁻¹.⁴⁷ In case of prepared PQ/TiO₂ composites the same C=O stretching frequency observed at 1697 cm⁻¹. This shift in frequency clearly indicates the formation of PQ/TiO₂ composites. This suggests the possible interaction of carbonyl group of PQ with TiO₂.

4.5. Photocatalytic MB degradation

After completing the structural analysis of PQ/TiO₂ organo–inorganic nanocomposites the photocatalytic activity of samples was studied by degrading the 100 mL aqueous solution of 10 ppm MB under illumination of 400 W mercury vapour lamp using 50 mg of catalyst. The graph of % MB remained as functions of irradiation time are depicted in Fig. 5.

Fig. 5 Graph of percent MB remained vs. irradiation time over PQ/TiO₂ coupled photocatalyst.

The PQ degrades almost 60% of MB within 90 min. of irradiation while pure TiO₂ degrades 90% respectively. The prepared nanocomposite showed degradation of MB almost higher than 95%, especially, PQ loaded with 0.2 wt% showed 99.5% MB degradation within 90 min. higher among the prepared compositions.

The PQ/TiO₂ nanocomposite shows enhanced photocatalytic activity; this increase in activity was due to the effective electron–hole separation due to coupling of PQ and TiO₂ catalysts and also increases in the absorbance range of catalyst in the visible region. In order to study the kinetics of photocatalytic MB degradation, the graph of ln C_o/C_t vs. irradiation time are plotted and shown in Fig. 6. It is well known that the photocatalytic dye degradation reaction follows Langmuir–Hinshelwood model and slope of the graph ln C_o/C_t vs. irradiation time gives the apparent rate constant (K_app). In our case, we observed the highest apparent rate constant value for PQ/TiO₂ nanocomposite having 0.2 wt% PQ. The observed rate constant for the MB degradation is K_app = 5.2 × 10⁻² min⁻¹. The rate constant values are depicted in Table 1. To investigate the stability of the PQ/TiO₂ photocatalyst, we also carried out the recycle study and found almost same degradation even after 5 recycles. The detailed recycle study is given in ESI, Fig. S2.†

Fig. 6 Kinetic plot of photocatalytic MB degradation vs. irradiation time over PQ/TiO₂ coupled photocatalyst.

Table 1 Rate constant values of photocatalytic MB degradation reaction using PQ/TiO₂ coupled photocatalyst

Sr. no.	Name of the catalyst	Rate (Kapp) (min^−1)	Standard error
1	PQ	0.88 × 10^−2	0.00164
2	TiO2	2.30 × 10^−2	0.00222
3	0.01 wt% PQ/TiO2	3.50 × 10^−2	0.00174
4	0.10 wt% PQ/TiO2	4.40 × 10^−2	0.00302
5	0.20 wt% PQ/TiO2	5.20 × 10^−2	0.00149

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The detailed photocatalytic mechanism of MB degradation using PQ/TiO₂ nanocomposites is shown in Fig. 7.

Fig. 7 Mechanism of photocatalytic MB degradation over PQ/TiO₂ coupled photocatalyst.

Upon illumination with light the electron–hole pairs are produced on the semiconductor surface, both the catalyst able to generate the electron–hole pairs. The PQ absorbs visible light and generated conduction band electron transfer to the conduction band of TiO₂. Further, we studied the stability of PQ/TiO₂ catalyst. For this purpose, 50 mg of PQ/TiO₂ was stirred in 100 mL DI water for 12 h, then the catalyst separated using centrifugation and UV-Visible spectra of supernatant solution was taken to check the presence of PQ in the solution (ESI, Fig. S1†). The linear spectrum in the range of 250 to 800 nm confirms that the leaching of PQ was not occurring which confirms the stability of catalyst.

This process lowers the possibility of electron–hole pair recombination and enhances the rate of degradation. Due to this reason the nanocomposite catalyst shows higher activity as compare to pure catalyst.

5. Conclusions

The PQ/TiO₂ nanocomposite was successfully synthesized via simple wet-impregnation having PQ/Ti ratios (0.01, 0.1, 0.2 wt%). XRD indicates the presence of monoclinic phase of PQ in the prepared organo–inorganic nanocomposites with anatase TiO₂. DRS clearly show absorbance in the visible region because of PQ. FE-SEM of prepared nanocomposites indicates the homogeneous distribution of TiO₂ in PQ matrix. The photocatalytic activity was evaluated by following the degradation of MB and found to be more for PQ/TiO₂ nanocomposite having 0.2 wt% PQ. The highest apparent rate constant among the prepared catalyst is 5.2 × 10⁻² min⁻¹ using PQ/TiO₂ catalyst having 0.2 wt% PQ. The proposed method can also be used to synthesize other semiconductor–PQ nanocomposites.

Acknowledgements

One of the authors Mr Vikram U. Pandit gratefully acknowledges Council of Scientific and Industrial Research (CSIR), New Delhi, India for financial support. Authors are grateful to Dr D. P. Amalnerkar, Executive Director, C-MET, for providing the facilities and useful discussion.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1. Leaching study of PQ/TiO₂ coupled photocatalyst. See DOI: 10.1039/c4ra11920g

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