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Physica B 405 (2010) 221–226

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Preparation of Fe3+-doped TiO2 nanoparticles and its photocatalytic activityunder UV light

Kashif Naeem, Feng Ouyang �

Research Center for Environmental Science and Engineering, Shenzhen Graduate School of Harbin Institute of Technology, Shenzhen 518055, China

a r t i c l e i n f o

Article history:

Received 10 February 2009

Received in revised form

23 July 2009

Accepted 3 August 2009

Keywords:

Phenol

Fe-doped TiO2

Nanoparticles

Photocatalytic degradation

Partition coefficient

26/$ - see front matter & 2009 Elsevier B.V. A

016/j.physb.2009.08.060

esponding author. Tel.: +86 755 26033472 601

ail address: ouyangfh@hit.edu.cn (F. Ouyang).

a b s t r a c t

The Fe3+-doped and undoped TiO2 nanoparticles have been prepared by sol–gel route using ferric nitrate

aqueous solution and tetrabutyl titanate. The nanoparticles were characterized by XRD, SEM, EDX and

UV–vis DRS techniques. The results showed that the Fe3+-doped TiO2 possess the anatase structures,

which were composed of 8–11 nm of crystal sizes. The photocatalytic activity of the nanoparticles under

UV light was investigated by measuring the photodegradation of phenol in aqueous dispersion. The

0.5 mol% Fe doping exhibited enhanced photocatalytic activity in this study. An attempt has been made

to correlate the degradation of phenol with partition coefficient of phenol in sodium dodecyl sulfate

micelles and water as a function of irradiation time.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Over the past two decades, semiconductor mediated photo-catalytic degradation has become an attractive technology forenvironmental pollution remediation because of great potential totreat a wide range of pollutants in water [1–4]. The reaction-taking place on the UV-illuminated semiconductor is the produc-tion of electron/hole ðe�CB=hþVBÞ pairs. If charge separation ismaintained, these charge carriers may migrate to the surfacewhere they are captured by a suitable electron donor andacceptor, participating in redox reactions, and/or they arerecombined, dissipating the input light energy onto heat. Inparticular, hþVB may react with H2O or OH� to yield hydroxylradical ( �OH) and e�CB is responsible for the formation of super-oxide radical anions (O2

� �). Among these radicals, hþVB and �OHradical play important roles in photocatalytic oxidation process.

The titanium dioxide (TiO2) is an effective photocatalyst owingto its inertness, strong oxidizing power, nontoxicity and stabilitywithin a wide range of pH, and inexpensive photosensitizedmaterial [1,3,5–7]. The photocatalyst, TiO2 can be activated underultraviolet (UV) light of wavelength o400 nm due to its largeband gap (3.2 eV). The photocatalytic activity of TiO2 generallydepends on a competition between the transfer rate of surfacecharge carriers from interior to the surface and the recombinationof photo-generated electrons and holes [8]. High recombination

ll rights reserved.

; fax: +86 755 26033472 602.

rate of e�CB=hþVB pairs results in low efficiency in utilizing photonand slow down the photocatalytic degradation process. In order toincrease the photocatalytic efficiency, some methods were used.The most promising method to do so is surface modification ofTiO2. The surface modification of TiO2 can be achieved by metaldoping into TiO2. Among various metal ions, Fe3+ is considered aninteresting dopant due to its half-filed electronic configuration[1,9]. Introduction of ferric ion in titania has resulted incontroversial results under different experimental conditionswith respect to the efficiency of the materials. The differentoptimum amount of Fe3+, ca. 0.3% [10], 0.5% [9,11], and 1.0% [4] isreported in the literature.

The presence of organic pollutants in aquatic environment hascaused several environmental problems since couple of decades.Phenol and its derivatives are extensively used in industrialprocesses (manufacture of dyes, food processing, pesticides,polymers, etc.) and as a result, phenol appears in surface waterand industrial water. This is considered potentially carcinogenicand mutagenic to mammalian and aquatic life [12]. An increasedischarge of this compound into environment has caused diverseproblems in water and wastewater treatment.

Earlier, we have investigated the parameters’ effects onphotodegradation of phenol under the influence of TiO2 (anatase)[13]. In this paper, Fe3+-doped TiO2 nanoparticles were preparedemploying sol–gel method via hydrolysis mechanism withprecursors of ferric nitrate salt and tetrabutyl titanate. Thestructural characteristics, morphology, percentage of concentra-tions were achieved by employing X-ray diffraction (XRD),scanning electron microscopy (SEM), energy dispersive X-ray

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K. Naeem, F. Ouyang / Physica B 405 (2010) 221–226222

spectroscopy (EDX) and ultraviolet–visible spectroscopy (UV–visible) diffuse reflectance spectra (DRS). The photocatalyticactivities of the Fe3+-doped TiO2 with different doping amountswere evaluated for degradation of phenol in UV light. Photo-catalytic degradation was also discussed in terms of partitioncoefficient of phenol between sodium dodecyl sulfate (SDS)micelles and water. To the best of our knowledge, no study hasbeen made to correlate the micelle–water partition coefficient ofpollutant with degradation phenomenon. Thus, the objective ofpresent work is to attempt to correlate the photocatalyticdegradation of phenol with micelle–water partition coefficientin aqueous dispersion of iron-doped TiO2.

2. Experimental

Tetrabutyl titanate (TBOT, 98%) was the product of FuchenChemical Reagent Co., (Tianjin). Absolute ethanol was of analyticalreagent grade and supplied by Yong Da Chemical Reagent Co.,(Tianjin). Acetic acid and hydrochloric acid was obtained fromGuangzhou Chemical Reagent Company. Analytical reagent gradephenol, ferric nitrate and sodium hydroxide were purchased fromSinopharm Chemical Reagent Co., Ltd. (Shanghai). Sodium dodecylsulfate was obtained from Damao Chemical Reagent Co., (Tianjin).All chemicals were used as such without further purification. Allsolutions were prepared with de-ionized distilled water obtainedfrom a Milli-Q apparatus (Millipore Co.) with a resistivity betterthan 18 MO cm.

Fe-doped TiO2 nanoparticles were prepared by slowly added0.1 mol tetrabutyl titanate in a mixture of 2 mol% ethanol andwater under constant stirring. 1–2% acetic acid was added to themixture. The solution was vigorously stirred for 2 h to undergo ahydrolysis at room temperature to yield a milky, white dispersion.Acetic acid was used to increase the stability of the mixture. Thedopant chemistry of Fe-doped TiO2 nanoparticles were controlledby dissolving the different amounts Fe(NO3)3 �9H2O in deionizedwater prior to the dropwise addition to above solution undervigorous stirring. Stirring was continued until a light brownsolution (sol) formed. The mixture was then transferred to anautoclave, maintained at 140 1C for 10 h, and then air cooled toroom temperature. The Fe-doped TiO2 nanoparticles were col-lected by suction filtration and washed with little volume ofethanol and water. The synthesized particles were dried fromroom temperature to 100 1C in an oven and pulverized with pestleand mortar, followed by passing through a 120 mesh. The sampleswere calcined under air for 3 h at 450 1C, otherwise specified. Thenwhite (undoped) or light yellow (Fe-doped) synthesized TiO2

nanoparticles were then subjected to further characterization.Powder X-ray diffraction (XRD) patterns for Fe3+-doped TiO2

nanoparticles were carried out in a MSAL XD-2 diffractometerusing nickel filtered Cu Ka radiation (l ¼ 1.5406 A) operating at36 kV and 20 mA. The patterns were recorded over the range of10–501 (2y) with a step size of 0.021 and at the speed of 41min�1.Scanning electron microscopy (SEM) images of Fe-doped TiO2

nanoparticles were collected on a Hitachi S-4700 with theacceleration voltage of 15 kV. Prior to the SEM measurements,the samples were gold coated to make the sample electricallyconductive. SEM images were used to observe the surfacemorphology of particles. The energy dispersive X-ray spectroscopy(EDX) was measured with an EDAX32 Genesis XM2 and used tocharacterize the elemental composition of the sol–gel synthesizedparticles. The UV–visible diffuse reflectance spectra (DRS) wereobtained in the wavelength range from 200 to 800 nm using a UV–visible spectrophotometer (Shimadzu UV-2501PC). UV–visibleDRS scans were performed at a speed of 37 nm min�1 with a

bandwidth of 1.0 nm. The calibrated sample of BaSO4 was used asbaseline correction in the UV–visible DRS experiments.

The photocatalytic activity of Fe3+-doped TiO2 particles for thephotocatalytic degradation of phenol, as model pollutant of water,were carried out at atmospheric pressure and room temperature(ca. 2571 1C). Phenol photodegradation were done with an openPyrex-glass cell with 500 mL capacity. The walls of the cell werecovered by aluminum foil to avoid release of radiation. Anaqueous dispersion was prepared by required amount of Fe3+-doped TiO2 nanoparticles to a 200 mL solution containing thephenol at appropriate concentration (ca. 3.56 �10�4 M). Amagnetic stirrer was used to ensure uniform mixing of solutionin vessel. An air pump (BOYU model ACQ-001) was used toprovide oxygen to maintain an aerobic condition. Prior toirradiation, the dispersions were sonicated for 60 s, and thenmagnetically stirred in the dark for ca. 15 min to ensure theestablishment of adsorption/desorption equilibrium. The irradia-tion was carried out using high-pressure 125W UVA bulb(Shenzhen Leijian Special Light Source Ltd., Co.), with the strongemission at 365 nm.

At any given irradiation time interval, aliquots of 7 mL of thesolution were drawn. The liquid samples were centrifuged at3000 rpm for 10 min and subsequently filtered to separate TiO2

particles. The filtered samples were stored at 4 1C prior to analysis.The concentrations of phenol in the filtrates were measuredquantitatively through the Helios Gamma UV–vis spectrophot-ometer. The UV–vis spectrophotometer was set at a wavelength of270 nm for analysis of phenol. The degradation efficiency (DE) ofphotodegradation was calculated from the equation given below:

DE ¼A0 � At

A0� 100 ð1Þ

where A0 is the absorbance at zero time and At is the absorbanceat time t. For partition coefficient studies, differential absorbance(DA) of irradiated phenol in SDS micellar solution was recorded insuch a way that a cuvette filled with aqueous phenol solution wasset on the reference side and cuvette filled with phenol and SDSmicellar solution was set on sample side of the instrument.

3. Results and discussion

3.1. Characterization of Fe-doped TiO2

X-ray diffractograms were used to investigate the effect of Fedoping on the phase structure and the phase composition of Fe-doped TiO2 nanoparticles. Fig. 1a displays the XRD patterns ofdifferent amounts of Fe-doped TiO2 nanoparticles after calcinationat 450 1C for 3 h. There is no significant difference between theXRD pattern of undoped TiO2 and that of Fe-doped TiO2. Asshown, the peaks at 25.31, 37.81 and 48.01 elucidate thediffractions of the (10 1), (0 0 4) and (2 0 0) anatase-type TiO2.This indicates that the obtained nanoparticles exist in the anatasestructure with dominated phase at 25.31. The obtained pattern isdifferent from some researchers as Fe accelerates thetransformation of anatase to rutile [14] having poorphotocatalytic efficiency. It is inferred from XRD pattern that therelative intensity of (10 1) peak first increases and then decreaseswith the increase of Fe3+ doping. A maximum is observed at0.5 mol% doping level. In the XRD pattern, no characteristic peaksof rutile TiO2 (27.51) were found. This indicates that the presenceof Fe3+ can stabilize the anatase phase of TiO2. It can be seen thatas the dopant concentration increases, diffraction peaks areslightly broadened. Fig. 1b depicts the XRD pattern of 0.5 mol%Fe–TiO2 at various calcinations temperature. An increase in XRDintensity of (10 1) anatase peaks indicates the conversion from

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3.0 mol% Fe-TiO2

2.0 mol% Fe-TiO2

1.0 mol% Fe-TiO2

0.5 mol% Fe-TiO2

0.0 mol% Fe-TiO2

2θ/(°)

2θ/(°)

0.2 mol% Fe-TiO2

(101)

Inte

nsity

(a.u

)In

tens

ity (a

.u)

as-prepared

150 °C

300 °C

450 °C

(101)

(004)

10 20 30 40 50

10 20 30 40 50 60

(200)

(004) (200)

Fig. 1. XRD pattern of (a) Fe3+-doped TiO2 nanoparticles calcined at 450 1C and (b)

0.5 mol% Fe3+-doped TiO2 nanoparticles calcined at different temperatures.

K. Naeem, F. Ouyang / Physica B 405 (2010) 221–226 223

amorphous to crystal phase at 300–450 1C, which results in betterphotocatalytic activity. According to the XRD results of iron-dopedTiO2 (Figs. 1a and b), increase of anatase phase was attributed to aconversion from amorphous to anatase at low Fe3+ doping. Thiscould be happen due to Fe3+ dispersed in TiO2 lattice forming solidsolution because of comparable radii of Fe3+ (0.64 A) and Ti4+

(0.68 A) without causing a significant anarchy in crystallineanatase phase and Fe3+ may occupy some of the titanium latticesites [15]. Moreover, it is energetically favorable for Fe3+ ions tooccupy Ti4+ sites [16]. Nevertheless, the increase in dopingamount above 0.5% resulted in worse crystalline and peak (10 1)broadening.

The crystallite sizes of catalysts were estimated by applyingthe Scherrer equation:

D ¼Kl

b cosyð2Þ

where D is the average crystallite size of the catalyst (nm), l is thewavelength of the Cu Ka X-ray radiation (l ¼ 1.5406 A), K is acoefficient usually taken as �0.94, b is the full width at half-maximum (FWHM) intensity of the peak observed at 2y (radian),and y is the diffracting angle. The average crystallite size of theFe3+-doped TiO2 are smaller to some extent than undoped TiO2,demonstrating that Fe3+ doping leads to slower the growth of theTiO2 catalyst [17]. For the anatase crystal system, the latticeparameters (in anatase form, a ¼ bac) were determined usingX-ray diffraction peaks of crystal plane (10 1) and (2 0 0) by usingfollowing equations:

d�2ðh k lÞ ¼ h2a�2 þ k2b�2 þ l2c�2 ð3Þ

The value of d(h k l), for an XRD peak can be determined fromBragg’s law:

dðh k lÞ ¼ l=2 siny ð4Þ

The results of lattice parameters and crystallite size of Fe3+-dopedTiO2 are presented in Table 1.

SEM and EDX spectroscopy was used to characterize theinformations on structure and morphology, and elementalcomposition of the Fe-doped TiO2 nanoparticles, respectively.Fig. 2 shows the SEM images of the Fe3+-doped TiO2 nanoparticlesat certain concentration of dopant are nearly round in shape. Asseen from the SEM images, the nanoparticles are agglomerated tosome extent and the rough measurements of images show thatthe average size of particles is in the nanoscale range. To validatethe existence of dopant on TiO2, EDX analysis was carried out. TheEDX is a chemical microanalysis technique used together withSEM. Table 1 represents the Fe contents obtained from EDXmeasurements. It can be seen that Fe contents on the samples arein good agreement with the nominal and observed values. It isworthwhile to note that nanoparticles are mainly composed of Tiand O with a little Fe contents.

UV–visible diffuse reflectance spectra of TiO2 with different Fecontents calcined at 450 1C are shown in Fig. 3. The experimentalresults indicated that the undoped TiO2 powder shows strongphotoabsorption only at wavelengths shorter than 400 nm. WhileFe3+-doped TiO2 nanoparticles show photoabsorption in visibleregion and the absorption edge shifts to a longer wavelength. Theshift of the reflectance spectrum is due to increasing Fe3+

contents. This is escorted with the change in color from white(undoped TiO2) to light yellow (Fe-doped TiO2). This indicates adecrease in the band gap of TiO2. Ohno et al. [18] considered thatthe formation of defects in the TiO2 lattice by the addition ofmetal ions is most probable explanation. Similar report with TiO2

nanoparticles doped with other metal ions is also available in theliterature [19].

3.2. Photocatalytic activity

The photocatalytic activity of Fe3+-doped TiO2 catalysts wereevaluated in terms of degradation of phenol at pH 5 underirradiation of UV light. Phenol was selected as model pollutantbecause of its extensive use in industrial processes. The degrada-tion kinetics was computed by the change in phenol concentrationemploying UV–visible spectrometry as a function of irradiationtime. Fig. 4 summarizes the effect of Fe3+ dopant concentration ondegradation efficiencies of phenol solution as a function ofirradiation time. The experimental results revealed that thephotodegradation efficiency initially increases with increase indoping amount, but decreases while the Fe3+ amount reaches ahigher level. It can be found that the TiO2 catalyst containing 0.5%Fe3+ enhance the photocatalytic activity and is optimal amount of

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Table 1Crystallite size, lattice parameters, cell volume and Fe contents in Fe-doped TiO2.

Sample Crystallite sizea (nm) a ¼ b (A) c (A) Cell volume (A3) Feb (%)

Nominal Observed

0.0 mol% Fe–TiO2 11.4 3.8011 9.6509 139.44 0.0 0.0

0.2 mol% Fe–TiO2 9.9 3.8011 9.3297 134.80 0.2 0.16

0.5 mol% Fe–TiO2 9.9 3.7744 9.2191 131.34 0.5 0.45

1.0 mol% Fe–TiO2 9.8 3.8041 9.6581 139.76 1.0 1.02

2.0 mol% Fe–TiO2 8.7 3.7922 9.9827 143.56 2.0 1.88

3.0 mol% Fe–TiO2 8.3 3.7759 10.296 146.79 3.0 2.89

a Estimated from Scherrer equation.b Measured by EDX analysis.

Fig. 2. SEM images of (a) 0.5 mol% Fe3+ and (b) 3.0 mol% Fe3+ doped TiO2

nanoparticles calcined at 450 1C.

0

20

40

60

80

R %

Wavelength (nm)

0.0mol%Fe-TiO2

0.2mol%Fe-TiO2

0.5mol%Fe-TiO2

1.0mol%Fe-TiO2

2.0mol%Fe-TiO2

3.0mol%Fe-TiO2

200 400 600 800

Fig. 3. UV–visible DRS of Fe-doped TiO2 samples calcined at 450 1C.

0

10

20

30

40

50

60

70

Irradiation Time (min)

Deg

rada

tion

Effi

cien

cy (%

)

0.0mol%Fe3+

0.2mol%Fe3+

0.5mol%Fe3+

1.0mol%Fe3+

2.0mol%Fe3+

3.0mol%Fe3+

0 100 200 300 400 500

Fig. 4. Degradation efficiencies of aqueous solution of phenol using Fe3+-doped

TiO2 nanoparticles as a function of irradiation time.

K. Naeem, F. Ouyang / Physica B 405 (2010) 221–226224

doping for degradation of phenol in the experimental conditions.The appreciation in the photodegradation efficiency of Fe-dopedTiO2 is in good agreement with the literature [9,11,20]. Hence,photocatalytic activity of TiO2 can be improved at appropriatedoping amount. In our study, the photocatalytic efficiencyincreases from 55.2% to 65.68% when the reaction time is 8 h.Beyond 0.5 mol% Fe3+, the photocatalytic activity is lower thanundoped TiO2. Yan et al. [21] has reported only 13.5% increase ofphotocatalytic activity as compared to the undoped TiO2 for asimilar system. As particle size is in comparable range, doping ofTiO2 seems to be important factor affecting photocatalytic

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Table 2Partition coefficient of phenol at different irradiation time.

Irradiation time (h) Kc (dm3 mol�1) Kx �DG3

p (kJ mol�1)

Without TiO2 12.42 689.31 16.18

0 11.79 654.34 16.05

2 11.16 619.38 15.92

4 9.95 552.22 15.64

6 9.41 522.26 15.50

8 8.68 481.74 15.30

K. Naeem, F. Ouyang / Physica B 405 (2010) 221–226 225

efficiency in our experimental set up. The Langmuir–Hinshelwood(L–H) kinetic equation is one of the most useful models todescribe this type of reaction. The photocatalytic reaction rate isgiven by

r ¼ �dCs

dt¼

krKCs

ð1þ KCsÞð5Þ

here r is the rate of reaction, CS is substrate (phenol)concentration, t is time of the reaction, kr is the reaction rateconstant and K is adsorption constants associated with thesubstrate. When the pollutant concentration is very low, theterm KCS is negligible and using krK ¼ kapp, the apparent reactionrate becomes a pseudo first order reaction.

�lnCt

Co

� �¼ kappt ð6Þ

where kapp is the apparent first order rate constant (min�1) and Co

and Ct are the concentrations of phenol at initial and a givenreaction time (min), respectively. The slope of the plot gave thevalue of kapp. The resulting first order rate constant has been usedto calculate degradation rate for phenol and were used for acomparison of the efficiency of photocatalytic process.

Metal ion doping influence the photoactivity of TiO2 by electronor hole traps. When trap can cause the formation of some activespecies that benefit degradation of phenol, dopant introductionis positive effect. If dopant introduction cannot decrease e�CB andhVB

+ recombination rate, the introduction is ineffective for thedegradation. Fe3+ ions have an intense absorption in the UV–visiblelight region and make a red shift in the band gap transition of theFe3+-doped TiO2 resulting in production of more photo-generatedelectrons and holes to participate in the photocatalytic reactions.However, since e�CB or hþVB recombination can occur quickly above0.5 mol% Fe3+, therefore, the degradation efficiency is low. At anappropriate doping concentration, Fe3+ ions may act a mediator ofthe transfer of interfacial charge. The experimental results show thatthe presence of a small amount of Fe3+ ions can improve thephotodegradation efficiency. This may be due to the followingreasons.

The electron scavenger effect of Fe3+, which prevents therecombination of e�CB and hþVB results in increase of the efficiencyof photodegradation process [22]. The possible reaction can berepresented as

Fe3þþ e�CB-Fe2þ

ð7Þ

According to crystal field theory, Fe2+ is relatively unstable ascompared to Fe3+ that has half-filled d orbital (d5). Therefore,release of trapped electron is easy to return to Fe3+. However, theFe2+/Fe3+ energy level lies close to Ti3+/Ti4+ level. Because of thisproximity, the trapped electron in Fe2+ can be easily transferred toa neighboring surficial Ti4+ and combines with oxygen molecule toinitiate the following reaction to form O�2 and finally �OH [9].

Fe2þþ O2-Fe3þ

þ O�2 ð8Þ

Moreover, in the photocatalytic system, H2O2 may also beproduced on TiO2 surface as well [23]. The coexistence of Fe2+ andH2O2 in acidic media (Fenton’s reaction) may produce �OH thatacts as a strong oxidizing agent. After that, the �OH is going toreact with the phenol to oxidize it.

Fe2þþH2O2-Fe3þ

þ �OHþ OH� ð9Þ

In conclusion, the introduction of Fe3+ ions in TiO2 is responsiblefor a reduction in the electron/hole pair recombination rate. Manyreports have shown that the photocatalytic activity of Fe-dopedTiO2 is strongly dependent on the dopant concentration [4,9–11,15].Thus, the Fe3+ ions can serve not only as a mediator of interfacialcharge transfer but also as a recombination center for the

photogenerated electrons and holes. In this study, the optimaldopant concentration is 0.5 mol% above which the photodegrada-tion efficiency gradually decreases. When the dopant concentrationreaches beyond optimum amount, Fe3+ ions mainly act asrecombination centers through quantum tunneling at high Fe3+

doping [9,24]. On the other hand, when the dopant concentration ishigh, the Fe3+ entered into the crystal lattice has become saturated,thus the excess Fe3+ combines with TiO2 as a separate phase [10].Therefore, the activity of the 3.00 mol% Fe–TiO2 becomes the lowestand only 46.18% of the undoped sample.

3.3. Partition coefficient

The presence of other compounds may influence the degrada-tion process when we deal with really contaminated water.Among other substances, the presence of surfactants in waste-water is expected due to their well-known domestic use andindustrial applications. The presence of surfactants has beenshown to influence the kinetics and mechanism of degradationprocess. Photodegradation of phenol was also assessed bypartitioning of phenol in micelles of SDS and aqueous bulk media.The affinity of water for the substrate is important in partitioning,due to water-dragging effect where the water is carried as a shellaround the substrate into the organic (micellar) phase [25]. Theinteractions between the substrate and water, between thesubstrate and micelles, and between water and micellar phaseplay a vital role in the partitioning process. The partitioncoefficient is dependent on the structure of substrate and thesurfactant that constitute the micelles. Based on the assumptionthat the Lambert–Beer law holds for both the solubilized and themonomeric form of the phenol in the micellar solution, thedifferential absorbance, DA, is given by DA ¼ Am�Aa. Am and Aa arethe absorbance of the phenol in micelles and aqueous solution,respectively. Kawamura et al. [26] proposed a quantitative modelfor determination of the micelle–water partition coefficient forsolubilized substrate.

1

DA¼

1

KcDA1ðCa þ Cmos Þþ

1

DA1ð10Þ

where Ca is the initial concentration of phenol and Cmos is the

analytical concentration of micellized surfactant, i.e., totalsurfactant concentration (Cs) less the critical micelle concentra-tion (CMC) of SDS in water. DAN is the differential absorbance atthe infinity of Cs, a concentration where all the phenol moleculesare assumed to have formed aggregates with the surfactant.

The dimension of Kc in Eq. is dm3 mol�1, which is related withKx as Kx ¼ Kcnw, where nw is the number of moles of water perdm3, i.e., 55.5 mol dm�3. Intercept and slope of the linear relationin Eq. (10) yields the value of DAN and Kc (plot not shown). Thevalues of Kx calculated from Kc are summarized in Table 2. The Kx

values decrease with the increase in irradiation time. This impliesa decrease in equilibrium distribution of the phenol moleculesthat are transferred into the micellar phase with the irradiationtime. Standard free energy change (DGo

p) of the transfer of the

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0

0.2

0.4

0.6

0.8

1

1.2

Irradiation Time (min)

C/C

o

without TiO2

undoped TiO2

0.5 mol% Fe-undoped TiO2

0.5 mol%Fe-doped TiO2

0 100 200 300 400 500

Fig. 5. Change in concentration as a function of irradiation time for aqueous

solution of phenol.

K. Naeem, F. Ouyang / Physica B 405 (2010) 221–226226

phenol from aqueous phase to micellar phase (Table 2) can beexpressed in the following form

DGop ¼ �RT ln Kx ð11Þ

Where R is the universal gas constant and T is the absolutetemperature.

3.4. Effect of Fe3+ on undoped TiO2

Surface modification of photocatalyst can also be concededthrough in situ by adding proper compounds into the solutionduring the photodegradation. The enhanced photocatalytic de-gradation of organic pollutants by the addition of metal ions toTiO2 dispersions is reported in the literature [27–30]. Controlexperiment showed that no appreciable direct photolysis wasobserved when the phenol solution was irradiated withoutcatalyst (Fig. 5). It can be neglected with less than 6.6%conversion within 8 h of UV-irradiation. It is clear from Fig. 5that the rate of phenol degradation with undoped TiO2 isincreased to some extent in the presence of 0.5 mol% Fe3+ ionsrather than the phenol degradation over undoped TiO2

nanoparticles alone. This enhancement of degradation is anindication of in situ surface modification of the undoped TiO2 bydissolved Fe3+ ion. However, this enhancing effect is rather weakas compared to 0.5 mol% Fe3+-doped TiO2. Thus, it can beaccomplished that metal doping of TiO2 enhances thephotodegradation than in situ process and undoped TiO2. Thisphenomenon supports that the presence of a small amount of Fe3+

as electron scavengers can also enhance the charge separation.The resultant Fe2+ initiates the Fenton’s reaction and consequentlyin increasing of hþVB and �OH radicals. Therefore, the phenoldegradation is slightly increased by the addition of a little amountof Fe3+ ions.

4. Conclusions

Iron doped TiO2 nanoparticles with different doping amountswere prepared using sol–gel method with hydrolysis mechanism

and calcined at temperature 450 1C. The resulting nanoparticleswere characterized by X-ray diffraction (XRD), scanning electronmicroscopy (SEM), energy dispersive X-ray spectroscopy (EDX)and ultraviolet–visible spectroscopy (UV–visible) diffuse reflec-tance spectra (DRS). XRD pattern showed that the nanoparticlesare dominated by anatase phase with main particle size innanoscale range. Due to same size of nanoparticles, dopantamount seems to have the effective role in photodegradationproperty. An appropriate doping at 0.5 mol% facilitates themaximum photocatalytic activity in our study. The presence ofFe3+ can help the separation of photogenerated electrons andholes due to its scavenger effect. Decrease in partition coefficientof phenol in SDS micelles with increase in irradiation time showedeffective way to monitor photodegradation.

Acknowledgments

First author (K.N) is grateful to the Higher Education Commis-sion of Pakistan for providing him a scholarship for pursuingdoctoral studies at Harbin Institute of Technology ShenzhenGraduate School, Shenzhen (China). The authors thank Ms. GuoMingxin for her help in arranging UV–visible DRS analysis. Fruitfulcomments and suggestions from reviewers are gratefully ac-knowledged.

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