Materials Research Bulletin 48 (2013) 1913–1919

Photocatalytic degradation of pentachlorophenol in aqueous solution by visiblelight 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, Indiab Water Chemistry Lab, Water Institute, Karunya University, Coimbatore 641 114, Indiac Department of Energy (Chemistry-Interdisciplinary), University of Madras, Guindy Campus, Chennai 600025, India

A R T I C L E I N F O

Article history:

Received 12 October 2012

Received in revised form 3 January 2013

Accepted 23 January 2013

Available online 8 February 2013

Keywords:

N–F-TiO2 photocatalyst

Material characterization

Catalytic properties

Common oxidants

PCP degradation

A B S T R A C T

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.

� 2013 Elsevier Ltd. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Pentachlorophenol (PCP) is highly chlorinated hydrocarbon andit is extensively used as bactericide, insecticide, herbicide andwood preservative. In spite, the production and use of chlorinatedphenols are banned in many of the developed countries, whichcauses problems including mutations in animal and human cells[1,2]. TiO2 mediated photocatalytic degradation of PCP have foundmore effective treatment compare to other semiconductor such asSnO, WO3, CdS and ZnO [2–4].

Over the past two decades, titanium dioxide (TiO2) has beenan excellent photocatalyst and widely used in decomposition ofvarious organic pollutants because of its higher oxidative power,nontoxic, photo-stability, inexpensiveness and favourable op-toelectronic properties. A wide band gap of 3.2 eV for anatasephases, it can be active only ultraviolet light irradiation, which isapproximately 4% of solar energy on the earth surface. In orderto improve the photophysical properties of the TiO2, recentlydoping is a promising way to change the photoabsorptionproperties [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: govindanmu@gmail.com (K. Govindan).

0025-5408/$ – see front matter � 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 inthe carrier recombination centre [8]. A last few years,researchers have been extensively studied non-metal dopedTiO2 for photochemical applications [9,10]. There are manyreports 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 exhibitsimproved activity under visible irradiation [18,19]. Hence, thegreat interest has been focused on the research to N doped TiO2,because the doping of N atoms can effectively narrow the bandgap of TiO2.

N-doped TiO2 gives improvement in visible light absorption,and creation of surface oxygen vacancies. F-doped TiO2 providesseveral beneficial effects including the creation of surface oxygenvacancies and the increasing the surface acidity due to theformation of Ti3+ ions [22,23]. Moreover, the nonmetal codopedTiO2 with B–N [14,15], C–N [16] and N–F [22–30] could furtherincrease photocatalytic activity. N–F-codoped TiO2 shows betterphotocatalytic activity in visible light irradiation due to synergeticeffect induced by N and F.

Numerous researches have been reported about the photo-catalytic degradation of PCP under ultraviolet irradiation. Eventhough, there is no much attention in photodegradation of PCPunder visible light irradiation. Hence, in this present work, visiblelight sensitive N–F-codoped TiO2 is synthesis and the photo-catalytic degradation of PCP is examined under visible lightirradiation.

10 20 30 40 50 60 70 80

(211 )(105 )

(200 )

(004 )

(101 )

NFTO

TiO2

Inte

nsi

ty (

a.u

)

2 Theta (degree )

(204 )

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

K. Govindan et al. / Materials Research Bulletin 48 (2013) 1913–19191914

2. Experimental

2.1. Materials and methods

Tetrabutyl titanate was used as a precursor to prepare N–F-codoped TiO2. Pentachlorophenol (99%) was purchased fromAldrich Chemicals (India). Hydrogen peroxide, potassium perox-omonosulphate (PMS), a triple salt with the composition of2KHSO5�KHSO4�K2SO4 and peroxodisulphate (PDS) from Merck(India) were used. Solutions were prepared using double distilledwater and the chemicals were used as analytical grade reagents asreceived.

The crystalline nature of the prepared catalyst was analyzed bypowder X-ray diffraction (XPERT-PRO diffractometer). Surfacemorphology was evaluated by scanning electron microscope (JOEL,JSM-6390 SEM) and transmission electron microscopy (PhilipsCM12 TEM). The photophysical properties of the catalyst wasevaluated using diffused reflectance UV–vis spectra (ShimadzuUV-vis 2550 spectrophotometer). The chemical nature of thesample was analyzed by X-ray photoelectron spectroscopy (XPS)measurements were performed with the PHI1600 Quantum ESCAMicroprobe System, using the Mg K line of a 300W Mg X-ray tubeas a radiation source at 15 kV voltages. All the binding energieswere referenced to the C 1s peak at 284.8 eV of the surfaceadventitious carbon. The photocatalytic test was conducted underambient atmospheric conditions in a pyrex glass bottle. Thereaction was stirred magnetically at a constant rate and thenirradiated with 250W tungsten-halogen lamp (Philips, India).Samples were collected at regular interval of time and filter withPVDF membrane filter. Then the filtered samples were analyzedusing the UV–vis spectrophotometer by following the absorbanceof the pentachlorophenol at its lmax 220 nm. The total organic

Fig. 2. SEM (a) and (b), and TEM (c) and (

carbon (TOC) of the samples was analyzed by Shimadzu TotalOrganic Carbon analyzer (Shimadzu TOC-VCPH model).

2.2. Preparation of visible light responsive NFTO

The N–F-codoped TiO2 (NFTO) was prepared from solvothermalmethod through sol–gel technique. 20 mL of tetrabutyl titanateand 10 mL of ethanol was taken together in 200 mL flask and kept

d) images of NFTO prepared sample.

200 30 0 40 0 50 0 60 0 70 0 80 0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

b

a

Ab

s

Wavel ength ( nm)

Fig. 3. UV–vis diffuse reflectance spectra of (a) TiO2 (Degussa P25); (b) NFTO and

inset figure shows Tauc plot of NFTO.

1000 80 0 60 0 40 0 20 0 0

0.0

2.0x1 05

4.0x1 05

6.0x1 05

8.0x1 05

1.0x1 06

1.2x1 06

1.4x1 06

Ti -

MN

N

OK

LL

O 1

s

F 1

s

Ti 2s Ti 2p3

N 1

s

C 1

s

Ti 3s

Ti 3p

Co

un

tsc/s

Bind ing Energy (eV)

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

K. Govindan et al. / Materials Research Bulletin 48 (2013) 1913–1919 1915

for stirring. The solution A was prepared by drop wise addition of5 mL acetic acid into above mixture and stirred 30 min continu-ously. 0.15 g ammonium fluoride, 6 mL ultra pure deionized water,4 mL trimethylamine, 3 mL nitric acid and 80 mL ethanol were

450 45 5 46 0 46 5 47 0

(a)

Ti 2p1/2

Ti 2p3/2

464.14

458.43

Inte

nsi

ty (

a.u

)

Bindi ng energy ( eV)

390 39 5 40 0 40 5 41 0

(c)

396.03

N 1s401.34

Inte

nsi

ty (

a.u

)

Binding energy (eV)

Fig. 5. High resolution XPS spectra of (a)

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

524 52 8 53 2 53 6 54 0

(b)O 1s

529.85

Inte

nsi

ty (

a.u

)

Bindi ng ener gy ( eV)

678 68 1 68 4 68 7 69 0 69 3

(d)687.6

688.5

F 1s

Inte

nsi

ty (

a.u

)

Binding ener gy ( eV)

Ti 2p; (b) O 1s; (c) N 1s and (d) F 1s.

K. Govindan et al. / Materials Research Bulletin 48 (2013) 1913–19191916

and placed in a 300 cm3 stainless steel autoclave and kept at 160 8Cfor 24 h. The resultant pale yellow precipitate was dried at 320 8Cfor 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 NFTOpowder. It confirms formation of anatase phase TiO2 with highcrystalline 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: 21-1272). However, there is no shift in peak positions caused by N andF 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 mightbe too less, though N has a larger ion radius (0.171 nm) [23].

The SEM images of prepared NFTO sample are shown in Fig. 2aand b. It clearly shows that the NFTO forms as nanoparticles. Infurther higher resolution microscopy i.e., TEM shows that theprepared NFTO forms as nanorods. Fig. 2c and d shows the

2.1

2.15

2.2

2.25

2.3

2.35

2.4

0 30 60 90 12 0 15 0 18 0 21 0

2 +

lo

g (

OD

)

Time (min)

20 mg/L 40 mg/L

60 mg/L 80 mg/L

100 mg/L

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 10 0 12 0

k1x

10

-4S

-1

Amoun t of Catalyst (mg/L)

(a)

(b)

Fig. 6. (a) Log (OD) vs irradiation time plot and (b) plot of photodegradation rate of

PCP for the photodegradation of PCP at various concentrations of NFTO (20–100 mg/

L) with [PCP] = 5 � 10�5 M.

uniformly distributed nanorods of NFTO materials and the averageparticle size is about 20–30 nm. The obtained nanorods of NFTOhas smooth surface, which may help to increase the photocatalyticperformances due to higher absorption ability towards reactant.

The optical characteristic of the NFTO is analyzed by UV–visdiffuse reflectance spectra studies as shown in Fig. 3. The opticalabsorption of Degussa P25 TiO2 (3.2 eV) has no ability to respond tovisible light, whereas the N and F codoped TiO2 powder extends theabsorption edges to the visible light region. It is due to the fact that,visible light responses by nitrogen doping into a TiO2 lattice andthe formation of isolated levels that consists of N 2p orbitals in theband gap of TiO2. The isolated N 2p narrow band above the O 2pvalence band is responsible for the visible light absorption ofnitrogen doped TiO2 [22,24,25].

Moreover, due to F� doping, the surface acidity increases bycharge compensation between F� and Ti4+ and it creates surfaceoxygen vacancies. This would increase the photocatalytic activitydue to higher absorption ability towards reactant. The band gapenergy is calculated by plotting (Ahy)1/2 and photon energy (hy)[28]. The NFTO nanorods show broad absorption spectrumbetween 405–600 nm which indicates the band gap energy(3.14 eV) of NFTO decreases. The creation of surface oxygen

1

1.2

1.4

1.6

1.8

2

2.2

2.4

0 10 20 30 40 50 60 70

2 +

lo

g (

OD

)

Time (min)

0.02 mM 0. 04 mM

0.06 mM 0. 80 mM

0.10 mM 0. 12 mM

0

1

2

3

4

5

6

0 0.0 2 0.0 4 0.06 0.0 8 0. 1 0.1 2 0.14

k1x

10-4

S-1

[PMS] mM

(a)

(b)

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 � 10�5 M

and (b) plot of photodegradation rate of PCP for various concentrations of PMS.

2

2.05

2.1

2.15

2.2

2.25

2.3

0 10 20 30 40 50 60 70

2+

log

(OD

)

Time (min)

0.02 mM 0.04 mM

0.06 mM 0. 08 mM

0.10 mM 0. 12 mM

1.1

1.2

1.3

1.4

1.5

1.6

0 0.0 2 0.0 4 0.0 6 0.0 8 0. 1 0.12 0.14

k1x

10

-4S

- 1

[PDS] mM

(b)

(a)

Fig. 8. (a) Log (OD) vs time plot for the photodegradation of PCP for various

concentrations of PDS (0.02–0.12 mM) with [NFTO] = 60 mg/L and [PCP] = 5 � 10�5 M

and (b) plot of photodegradation rate of PCP for various concentrations of PDS.

0.4

0.5

0.6

0.7

0.8

0.9

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

[H2O2] mM

k1x

10-4

S-1

2.15

2.2

2.25

2.3

2.35

0 10 20 30 40 50 60 70

2 +

log (

OD

)

Time (min)

0.02 mM 0.04 mM

0.06 mM 0.08 mM

0.10 mM 0.12 mM

(b)

(a)

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 � 10�5 M

and (b) plot of photodegradation rate of PCP for various concentrations of PMS.

K. Govindan et al. / Materials Research Bulletin 48 (2013) 1913–1919 1917

vacancies is responsible for the enhancement of photocatalyticactivity, in which the oxygen vacancy can act as active sites toprovide oxidizing species. Therefore, the visible light photocata-lytic activity is improved for NFTO by the synergetic effectsinduced due to N and F codoping.

3.2. Chemical status of NFTO photocatalyst

Chemical composition of NFTO powder is analyzed by X-rayphotoelectron spectroscopy (XPS), in order to elucidate thenitrogen and fluorine codoped into TiO2. The survey spectrum ofNFTO sample is shown in Fig. 4. Predominantly, the NFTO samplecontains Ti, O, N and F with a trace quantity of carbon peak at284.9 eV. The high resolution XPS spectra of Ti 2p, O 1s, N 1s andF 1s peaks are shown in Fig. 5a, b, c and d respectively. In Fig. 5a, theTi 2p peaks are 458.4 eV and 464.2 eV with split of 5.6 � 0.1 eVwhich indicates the existence of Ti in the form of Ti4+ [29]. Fig. 5billustrates 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 at396.0 eV is reflected which is the evidence for the presence of Ti–Nbond formation. When N atom replaced the oxygen in TiO2 crystallattice and then another peak at 401.3 eV may be due to physicaladsorption of N2 or NH3 on TiO2 surface [31]. In Fig. 5d, F 1s peaksare observed at 687.6 eV and 688.5 eV. This is due to lattice F fromO–Ti–F moieties in TiO2�xFx solid solutions, originated by

nucleophilic substitution of F� ions into bulk TiO2 [15], whichact as hole-trapping species during the photocatalytic reaction andleads to the best degradation performances.

3.3. Photocatalytic degradation studies

The effect of concentration of NFTO on the photocatalyticdegradation of pentachlorophenol is investigated at neutral pH.Also the amount of photocatalyst is optimized for the efficientphotocatalytic degradation of PCP. The amount of NFTO is variedbetween 20 to 100 mg/L in the interval of 20 mg/L. Fig. 6a showslinear relationship for the plots 2 + log(OD) between irradiationtime. Fig. 6b shows that the photodegradation rate increases withincreasing concentration of catalyst up to 60 mg/L due to thenumber of active site increases. Which means the magnitude ofphoton absorption by active site increases, its may leadsphotodegradation process. However, beyond which the rateconstant decreases due to scattering of light by the excess ofsemiconductor particles. Hence, the 60 mg/L is considered as anoptimum concentration for further analysis.

PMS (commercially called oxone) is a strong oxidant, that canbe explained for the following key reactions (reaction (1)–(3)) andit is an irreversible electron acceptor [32]. The aqueous solution ofoxone provides electron acceptor species as HSO5

� and it can

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7

TiO2

NFTO

NFTO + PMS

NFTO + PDS

NFTO + H2O2

Time (h)

TO

Ct/

TO

C0

Fig. 10. Comparison of photocatalytic degradation rate of PCP under optimum

concentrations are maintained as follows: [PCP] = 5 � 10�5 M, [PMS] = [PDS] =

[H2O2] = 0.1 mM, and [NFTO] = 1 g/L.

K. Govindan et al. / Materials Research Bulletin 48 (2013) 1913–19191918

accept electrons from conduction band and dissociate intodifferent ways as below.

HSO5� þ e�CB ! �OH þ SO4

2� (1)

HSO5� þ e�CB ! OH� þ SO4

�� (2)

HSO5� þ hþVB ! SO5

�� þ Hþ (3)

The concentration of PMS on the photocatalytic degradation ofPCP, [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 rateconstant increases with increasing in the concentration of PMS asshown in Fig. 7b. The increase in degradation rate constant of PCP isdue to the involvement of both �OH and SO4

�� radicals producedfrom PMS [33].

The different concentration of PDS on the photocatalyticdegradation of the PCP is analyzed by the concentration of PDSfrom 0.02 to 0.12 mM. Peroxydisulphate also act as an oxidizingagent in photocatalytical detoxification because SO4

�S forms fromthe oxidant by reaction (4) with the conduction band electrons(e�CB).

S2O82� þ e�CB ! SO4

�� þ SO42� (4)

Whereas, strongly oxidizing nature of SO4�S can directly

participate in the degradation processes. Fig. 8a and b shows thatwhen PCP is irradiated in the presence of NFTO with peroxydi-sulphate. This leads to significant increase in rate constant. It ispossible due to strong accelerating effect of PDS even at modestconcentrations.

The effect of different concentration of H2O2 on the photo-catalytic degradation of pentachlorophenol is studied by varyingthe [H2O2] from 0.02 to 0.14 mM. The plot of 2 + log (OD) vsirradiation time is presented in Fig. 9a, and the obtained k1 valuesare shown in Fig. 9b. It can be seen that the photodegradation rateincreases by increasing concentration of H2O2 from 0.02 to0.10 mM, due to the increased formation of hydroxyl radicalsfrom the adsorbed H2O2 (reaction (5)). Further as increase inconcentration of H2O2, rate constant k1 values decreases. This isbecause of the hydroxyl radicals scavenging reaction predominates(reaction (6) and (7)). Similar results for the photocatalytic

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

H2O2þ e�CB ! �OH þ OH� (5)

H2O2þ hþVB ! HO2� þ Hþ (6)

HO2� þ �OH ! H2O þ O2 (7)

3.4. Effect of oxidants on photocatalytic degradation of PCP

To evaluate the best oxidant for the NFTO mediated photo-catalytic degradation of PCP, experiments are carried out underidentical conditions are [PCP] = 5 � 10�5 M, [NFTO] = 60 mg/L,[PMS] = [PDS] = [H2O2] = 0.10 mM. The effect of NFTO with andwithout oxidants on photocatalytic degradation of PCP is analyzed.The result shows that the rate of degradation of PCP using NFTOwith oxidant is more than that of NFTO without oxidant. Amongthese oxidants with NFTO the photocatalytic degradation rate ofPCP is superior with PMS than that of PDS and H2O2. Hence, PMS isthe most efficient oxidant for the photocatalytic degradation of PCPalong with NFTO. This may be attributed to the involvement ofboth the e�CB and h+

VB in the photocatalytic degradation reaction[32]. But the PDS only reacts with e�CB alone.

The photocatalytic degradation (Fig. 10) of pentachlorophenolis obtained for 6 h of irradiation in the presence of NFTO. In theabsence of oxidants, only 20% of photodegradation efficiency isobserved but it increases up to about 60% as PMS is used as anoxidant. This can be rationalized by the fact that PMS getsdecomposed through both e�CB and h+

VB of the semiconductorphotocatalysts. Also, the addition of PDS is beneficial for thephotocatalytic degradation of PCP. In the case of H2O2 as oxidant,photodegradation rate is not much. This may be due to thegeneration of excess hydroxyl radicals upon illumination by visiblelight, which may cause hole-scavenging effects.

4. Conclusion

In summary, the NFTO photocatalyst is synthesized by modifiedsolvothermal method. NFTO material exhibits excellent structuralproperties such as photoactive anatase phase, smaller crystallinesize (<20 nm) and low degree of agglomeration, which demon-strates higher catalytic activity under visible light towards PCPdegradation. Moreover, the photocatalytic performances arecarried out under visible light irradiation with inorganic oxidizingagents. The enhancement of photocatalytic degradation of PCP isby the synergetic effect induced by nitrogen and fluorine codopinginto TiO2. The highest photocatalytic degradation of PCP rate isachieved for NFTO with PMS as an oxidant, which indicates PMS isa more efficient oxidant than PDS and H2O2 for the photocatalyzeddegradation of PCP.

Acknowledgement

The financial support received from Department of Science andTechnology, India for the sanction of DST Indo – Australian Project(San No: INT/AUS/P-1/07) is gratefully acknowledged.

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