Photocatalytic degradation of pentachlorophenol in aqueous solution by visible light sensitive NF-codoped TiO2 photocatalyst

Materials Research Bulletin 48 (2013) 1913–1919

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Photocatalytic degradation of pentachlorophenol in aqueous solution by visible light sensitive N–F-codoped TiO2 photocatalyst Kadarkarai Govindan a,b,*, Sepperumal Murugesan a, Pitchai Maruthamuthu c a

Department of Inorganic Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai 625021, India Water Chemistry Lab, Water Institute, Karunya University, Coimbatore 641 114, India c Department of Energy (Chemistry-Interdisciplinary), University of Madras, Guindy Campus, Chennai 600025, India b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 12 October 2012 Received in revised form 3 January 2013 Accepted 23 January 2013 Available online 8 February 2013

In this present study, N–F-codoped titanium dioxide nanocatalyst (NFTO) has been synthesized by simple sol–gel assisted solvothermal method for the effective utilization of visible light in photocatalytic reactions. Structural characterization of the photocatalyst is analyzed by XRD, UV–vis diffuse reflectance spectra (DRS), SEM and TEM. Moreover the chemical statuses of NFTO are gathered by X-ray photoelectron spectroscopy (XPS). The results show that a high surface area with photoactive anatase phase crystalline is obtained. In addition, nitrogen and fluorine atoms are doped into TiO2 crystal lattice to extend the visible light absorption and higher photocatalytic activity. The photocatalytic degradation of pentachlorophenol in aqueous solution is examined under visible light irradiation, the addition of oxidants such as PMS, PDS and H2O2 is analyzed in detail. The rate of photocatalytic degradation of pentachlorophenol is obtained in the following order: PMS > PDS > H2O2. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: N–F-TiO2 photocatalyst Material characterization Catalytic properties Common oxidants PCP degradation

1. Introduction Pentachlorophenol (PCP) is highly chlorinated hydrocarbon and it is extensively used as bactericide, insecticide, herbicide and wood preservative. In spite, the production and use of chlorinated phenols are banned in many of the developed countries, which causes problems including mutations in animal and human cells [1,2]. TiO2 mediated photocatalytic degradation of PCP have found more effective treatment compare to other semiconductor such as SnO, WO3, CdS and ZnO [2–4]. Over the past two decades, titanium dioxide (TiO2) has been an excellent photocatalyst and widely used in decomposition of various organic pollutants because of its higher oxidative power, nontoxic, photo-stability, inexpensiveness and favourable optoelectronic properties. A wide band gap of 3.2 eV for anatase phases, it can be active only ultraviolet light irradiation, which is approximately 4% of solar energy on the earth surface. In order to improve the photophysical properties of the TiO2, recently doping is a promising way to change the photoabsorption properties [5,6]. Metal ions can be used as a dopant with TiO2 [7]. Nevertheless metal ion doped TiO2 materials exhibits lesser

* Corresponding author at: Water Chemistry lab, Water Institute, Karunya University, Coimbatore-641 114, India. Tel.: +91 97517 03442; fax: +91 97517 03442. E-mail address: [email protected] (K. Govindan). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.01.047

photocatalytic activity due to thermal instability or increase in the carrier recombination centre [8]. A last few years, researchers have been extensively studied non-metal doped TiO2 for photochemical applications [9,10]. There are many reports available for non-metal doping TiO2 especially boron [11–15], carbon [16], sulfur [17], nitrogen [18,19] and fluorine [20,21]. Among them nitrogen doped TiO2 materials exhibits improved activity under visible irradiation [18,19]. Hence, the great interest has been focused on the research to N doped TiO2, because the doping of N atoms can effectively narrow the band gap of TiO2. N-doped TiO2 gives improvement in visible light absorption, and creation of surface oxygen vacancies. F-doped TiO2 provides several beneficial effects including the creation of surface oxygen vacancies and the increasing the surface acidity due to the formation of Ti3+ ions [22,23]. Moreover, the nonmetal codoped TiO2 with B–N [14,15], C–N [16] and N–F [22–30] could further increase photocatalytic activity. N–F-codoped TiO2 shows better photocatalytic activity in visible light irradiation due to synergetic effect induced by N and F. Numerous researches have been reported about the photocatalytic degradation of PCP under ultraviolet irradiation. Even though, there is no much attention in photodegradation of PCP under visible light irradiation. Hence, in this present work, visible light sensitive N–F-codoped TiO2 is synthesis and the photocatalytic degradation of PCP is examined under visible light irradiation.

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Tetrabutyl titanate was used as a precursor to prepare N–Fcodoped TiO2. Pentachlorophenol (99%) was purchased from Aldrich Chemicals (India). Hydrogen peroxide, potassium peroxomonosulphate (PMS), a triple salt with the composition of 2KHSO5KHSO4K2SO4 and peroxodisulphate (PDS) from Merck (India) were used. Solutions were prepared using double distilled water and the chemicals were used as analytical grade reagents as received. The crystalline nature of the prepared catalyst was analyzed by powder X-ray diffraction (XPERT-PRO diffractometer). Surface morphology was evaluated by scanning electron microscope (JOEL, JSM-6390 SEM) and transmission electron microscopy (Philips CM12 TEM). The photophysical properties of the catalyst was evaluated using diffused reflectance UV–vis spectra (Shimadzu UV-vis 2550 spectrophotometer). The chemical nature of the sample was analyzed by X-ray photoelectron spectroscopy (XPS) measurements were performed with the PHI1600 Quantum ESCA Microprobe System, using the Mg K line of a 300W Mg X-ray tube as a radiation source at 15 kV voltages. All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. The photocatalytic test was conducted under ambient atmospheric conditions in a pyrex glass bottle. The reaction was stirred magnetically at a constant rate and then irradiated with 250W tungsten-halogen lamp (Philips, India). Samples were collected at regular interval of time and filter with PVDF membrane filter. Then the filtered samples were analyzed using the UV–vis spectrophotometer by following the absorbance of the pentachlorophenol at its lmax 220 nm. The total organic

Intensity (a.u)

2.1. Materials and methods

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NFTO

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TiO2 10

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2 Theta (degree) Fig. 1. Powder X-ray diffraction pattern for TiO2 (Degussa P25) and NFTO.

carbon (TOC) of the samples was analyzed by Shimadzu Total Organic Carbon analyzer (Shimadzu TOC-VCPH model). 2.2. Preparation of visible light responsive NFTO The N–F-codoped TiO2 (NFTO) was prepared from solvothermal method through sol–gel technique. 20 mL of tetrabutyl titanate and 10 mL of ethanol was taken together in 200 mL flask and kept

Fig. 2. SEM (a) and (b), and TEM (c) and (d) images of NFTO prepared sample.

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Wavelength (nm) Fig. 3. UV–vis diffuse reflectance spectra of (a) TiO2 (Degussa P25); (b) NFTO and inset figure shows Tauc plot of NFTO.

Fig. 4. X-ray photoelectron spectroscopy spectrum of NFTO sample.

for stirring. The solution A was prepared by drop wise addition of 5 mL acetic acid into above mixture and stirred 30 min continuously. 0.15 g ammonium fluoride, 6 mL ultra pure deionized water, 4 mL trimethylamine, 3 mL nitric acid and 80 mL ethanol were

mixed and stirred for 10 min to form solution B. Solution B was added drop wise into solution A under vigorous stirring. The above solution was stirred slowly till transparent immobile gel formation. Finally, the sol–gel mixture was transferred to a Teflon beaker

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Fig. 5. High resolution XPS spectra of (a) Ti 2p; (b) O 1s; (c) N 1s and (d) F 1s.

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and placed in a 300 cm3 stainless steel autoclave and kept at 160 8C for 24 h. The resultant pale yellow precipitate was dried at 320 8C for 24 h and it was converted to powder through grinding. 3. Results and discussion 3.1. Physical characterizations of NFTO Fig. 1 exhibits the crystalline nature of the prepared NFTO powder. It confirms formation of anatase phase TiO2 with high crystalline in which the intense peak at 2u = 25.15, 37.73, 47.89, 54.00, 54.90, and 62.468 are corresponding to the anatase (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1) and (2 0 4) crystal planes (JCPDS: 211272). However, there is no shift in peak positions caused by N and F atoms codoped into TiO2. This is because ion radius of F atom (0.133 nm) is almost same as that of replaced oxygen atom (0.132 nm). In addition the concentration of doped N atom might be too less, though N has a larger ion radius (0.171 nm) [23]. The SEM images of prepared NFTO sample are shown in Fig. 2a and b. It clearly shows that the NFTO forms as nanoparticles. In further higher resolution microscopy i.e., TEM shows that the prepared NFTO forms as nanorods. Fig. 2c and d shows the

uniformly distributed nanorods of NFTO materials and the average particle size is about 20–30 nm. The obtained nanorods of NFTO has smooth surface, which may help to increase the photocatalytic performances due to higher absorption ability towards reactant. The optical characteristic of the NFTO is analyzed by UV–vis diffuse reflectance spectra studies as shown in Fig. 3. The optical absorption of Degussa P25 TiO2 (3.2 eV) has no ability to respond to visible light, whereas the N and F codoped TiO2 powder extends the absorption edges to the visible light region. It is due to the fact that, visible light responses by nitrogen doping into a TiO2 lattice and the formation of isolated levels that consists of N 2p orbitals in the band gap of TiO2. The isolated N 2p narrow band above the O 2p valence band is responsible for the visible light absorption of nitrogen doped TiO2 [22,24,25]. Moreover, due to F doping, the surface acidity increases by charge compensation between F and Ti4+ and it creates surface oxygen vacancies. This would increase the photocatalytic activity due to higher absorption ability towards reactant. The band gap energy is calculated by plotting (Ahy)1/2 and photon energy (hy) [28]. The NFTO nanorods show broad absorption spectrum between 405–600 nm which indicates the band gap energy (3.14 eV) of NFTO decreases. The creation of surface oxygen

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0

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[PMS] mM Fig. 7. (a) Log (OD) vs time plot for the photodegradation of PCP at various concentrations of PMS (0.02–0.12 mM) with [NFTO] = 60 mg/L and [PCP] = 5  105 M and (b) plot of photodegradation rate of PCP for various concentrations of PMS.

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vacancies is responsible for the enhancement of photocatalytic activity, in which the oxygen vacancy can act as active sites to provide oxidizing species. Therefore, the visible light photocatalytic activity is improved for NFTO by the synergetic effects induced due to N and F codoping.

0.4 0

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[H 2O 2] mM Fig. 9. (a) Log (OD) vs time plot for the photodegradation of PCP for various concentrations of H2O2 (0.02–0.12 mM) with [NFTO] = 60 mg/L and [PCP] = 5  105 M and (b) plot of photodegradation rate of PCP for various concentrations of PMS.

nucleophilic substitution of F ions into bulk TiO2 [15], which act as hole-trapping species during the photocatalytic reaction and leads to the best degradation performances. 3.3. Photocatalytic degradation studies

3.2. Chemical status of NFTO photocatalyst Chemical composition of NFTO powder is analyzed by X-ray photoelectron spectroscopy (XPS), in order to elucidate the nitrogen and fluorine codoped into TiO2. The survey spectrum of NFTO sample is shown in Fig. 4. Predominantly, the NFTO sample contains Ti, O, N and F with a trace quantity of carbon peak at 284.9 eV. The high resolution XPS spectra of Ti 2p, O 1s, N 1s and F 1s peaks are shown in Fig. 5a, b, c and d respectively. In Fig. 5a, the Ti 2p peaks are 458.4 eV and 464.2 eV with split of 5.6  0.1 eV which indicates the existence of Ti in the form of Ti4+ [29]. Fig. 5b illustrates O 1s peak at 529.8 pertain to lattice oxygen of TiO2. Fig. 5c shows N 1s peaks at 396.0 eV and 403.1 eV. The peak at 396.0 eV is reflected which is the evidence for the presence of Ti–N bond formation. When N atom replaced the oxygen in TiO2 crystal lattice and then another peak at 401.3 eV may be due to physical adsorption of N2 or NH3 on TiO2 surface [31]. In Fig. 5d, F 1s peaks are observed at 687.6 eV and 688.5 eV. This is due to lattice F from O–Ti–F moieties in TiO2xFx solid solutions, originated by

The effect of concentration of NFTO on the photocatalytic degradation of pentachlorophenol is investigated at neutral pH. Also the amount of photocatalyst is optimized for the efficient photocatalytic degradation of PCP. The amount of NFTO is varied between 20 to 100 mg/L in the interval of 20 mg/L. Fig. 6a shows linear relationship for the plots 2 + log(OD) between irradiation time. Fig. 6b shows that the photodegradation rate increases with increasing concentration of catalyst up to 60 mg/L due to the number of active site increases. Which means the magnitude of photon absorption by active site increases, its may leads photodegradation process. However, beyond which the rate constant decreases due to scattering of light by the excess of semiconductor particles. Hence, the 60 mg/L is considered as an optimum concentration for further analysis. PMS (commercially called oxone) is a strong oxidant, that can be explained for the following key reactions (reaction (1)–(3)) and it is an irreversible electron acceptor [32]. The aqueous solution of oxone provides electron acceptor species as HSO5 and it can

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degradation demonstrates at high concentration of H2O2 are already reported [32,33].

TOCt /TOC0

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H2 O2 þ e CB !  OH þ OH

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H2 O2 þ hþ VB ! HO2  þ Hþ

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3.4. Effect of oxidants on photocatalytic degradation of PCP

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Time (h) Fig. 10. Comparison of photocatalytic degradation rate of PCP under optimum concentrations are maintained as follows: [PCP] = 5  105 M, [PMS] = [PDS] = [H2O2] = 0.1 mM, and [NFTO] = 1 g/L.

accept electrons from conduction band and dissociate into different ways as below. HSO5  þ e CB !  OH þ SO4 2

(1)

HSO5  þ e CB ! OH þ SO4 

(2)

HSO5  þ hþ VB ! SO5  þ Hþ

(3)

The concentration of PMS on the photocatalytic degradation of PCP, [PMS] is varied from 0.02 to 0.12 mM. The plots of 2 + log (OD) vs irradiation time is represented in Fig. 7a. The photocatalytic rate constant increases with increasing in the concentration of PMS as shown in Fig. 7b. The increase in degradation rate constant of PCP is due to the involvement of both OH and SO4 radicals produced from PMS [33]. The different concentration of PDS on the photocatalytic degradation of the PCP is analyzed by the concentration of PDS from 0.02 to 0.12 mM. Peroxydisulphate also act as an oxidizing agent in photocatalytical detoxification because SO4S forms from the oxidant by reaction (4) with the conduction band electrons (eCB). S2 O8 2 þ e CB ! SO4  þ SO4 2

(4) S

can directly Whereas, strongly oxidizing nature of SO4 participate in the degradation processes. Fig. 8a and b shows that when PCP is irradiated in the presence of NFTO with peroxydisulphate. This leads to significant increase in rate constant. It is possible due to strong accelerating effect of PDS even at modest concentrations. The effect of different concentration of H2O2 on the photocatalytic degradation of pentachlorophenol is studied by varying the [H2O2] from 0.02 to 0.14 mM. The plot of 2 + log (OD) vs irradiation time is presented in Fig. 9a, and the obtained k1 values are shown in Fig. 9b. It can be seen that the photodegradation rate increases by increasing concentration of H2O2 from 0.02 to 0.10 mM, due to the increased formation of hydroxyl radicals from the adsorbed H2O2 (reaction (5)). Further as increase in concentration of H2O2, rate constant k1 values decreases. This is because of the hydroxyl radicals scavenging reaction predominates (reaction (6) and (7)). Similar results for the photocatalytic

To evaluate the best oxidant for the NFTO mediated photocatalytic degradation of PCP, experiments are carried out under identical conditions are [PCP] = 5  105 M, [NFTO] = 60 mg/L, [PMS] = [PDS] = [H2O2] = 0.10 mM. The effect of NFTO with and without oxidants on photocatalytic degradation of PCP is analyzed. The result shows that the rate of degradation of PCP using NFTO with oxidant is more than that of NFTO without oxidant. Among these oxidants with NFTO the photocatalytic degradation rate of PCP is superior with PMS than that of PDS and H2O2. Hence, PMS is the most efficient oxidant for the photocatalytic degradation of PCP along with NFTO. This may be attributed to the involvement of both the eCB and h+VB in the photocatalytic degradation reaction [32]. But the PDS only reacts with eCB alone. The photocatalytic degradation (Fig. 10) of pentachlorophenol is obtained for 6 h of irradiation in the presence of NFTO. In the absence of oxidants, only 20% of photodegradation efficiency is observed but it increases up to about 60% as PMS is used as an oxidant. This can be rationalized by the fact that PMS gets decomposed through both eCB and h+VB of the semiconductor photocatalysts. Also, the addition of PDS is beneficial for the photocatalytic degradation of PCP. In the case of H2O2 as oxidant, photodegradation rate is not much. This may be due to the generation of excess hydroxyl radicals upon illumination by visible light, which may cause hole-scavenging effects. 4. Conclusion In summary, the NFTO photocatalyst is synthesized by modified solvothermal method. NFTO material exhibits excellent structural properties such as photoactive anatase phase, smaller crystalline size (<20 nm) and low degree of agglomeration, which demonstrates higher catalytic activity under visible light towards PCP degradation. Moreover, the photocatalytic performances are carried out under visible light irradiation with inorganic oxidizing agents. The enhancement of photocatalytic degradation of PCP is by the synergetic effect induced by nitrogen and fluorine codoping into TiO2. The highest photocatalytic degradation of PCP rate is achieved for NFTO with PMS as an oxidant, which indicates PMS is a more efficient oxidant than PDS and H2O2 for the photocatalyzed degradation of PCP. Acknowledgement The financial support received from Department of Science and Technology, India for the sanction of DST Indo – Australian Project (San No: INT/AUS/P-1/07) is gratefully acknowledged. References [1] M.P. Titus, V.G. Molina, M.A. Banos, J. Gime´neza, S. Esplugas, Appl. Catal. B: Environ. 47 (2004) 219–256. [2] S. Devipriya, S. Yesodharan, Sol. Energy Mater. Sol. Cells 86 (2005) 309–348. [3] J. Gunlazuardi, W.A. Lindu, J. Photochem. Photobiol. A: Chem. 173 (2005) 51–55. [4] J.K. Kim, K. Choi, I.H. Cho, H.S. Son, K.D. Zoh, J. Hazard. Mater. 148 (2007) 281–286. [5] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C: Photochem. Rev. 1 (2000) 1–21. [6] U.I. Gaya, A.H. Abdullah, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2008) 1–12.

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