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Applied Catalysis A: General 401 (2011) 98– 105

Contents lists available at ScienceDirect

Applied Catalysis A: General

jo u r n al hom epage: www.elsev ier .com/ locate /apcata

xperimental and theoretical studies of Fe-doped TiO2 filmsrepared by peroxo sol–gel method

iana V. Welliaa, Qing Chi Xua, Mahasin Alam Ska, Kok Hwa Lima,uti Mariana Limb, Timothy Thatt Yang Tana,∗

School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, SingaporeSchool of Life Sciences and Chemical Technology, Ngee Ann Polytechnic, Singapore 599489, Singapore

r t i c l e i n f o

rticle history:eceived 22 February 2011eceived in revised form 30 April 2011ccepted 3 May 2011vailable online 11 May 2011

eywords:

a b s t r a c t

Transparent, uniform, crack-free and visible light activated Fe-doped TiO2 thin films have been preparedby an organic-free approach using titanium tetrachloride (TiCl4), Fe3+ and ammonium hydroxide solutionsto form yellow aqueous peroxo titanic acid (PTA) solution. Pure glass was made superhydrophilic afterheating at 550 ◦C for 1 h and was used as the coating substrate. The PTA solution dispersed uniformly onthe superhydrophilic glass by forming a thin film, resulting in a crack-free Fe-doped TiO2 film as observedunder FESEM. XRD results confirmed the presence of only anatase phase for all samples after calcination

◦

eroxo titanic acid (PTA)isible lighte-doped TiO2

ensity functional theory (DFT)

at 550 C. The Fe-doped TiO2 films exhibited slight red-shift in absorbance and enhanced absorbance inthe visible-light region compared with undoped TiO2, attributed to bandgap narrowing by successful Fe3+

doping into TiO2, which is supported by DOS calculations. Photodegradation of stearic acid (SA) revealedthat Fe3+ doping increased visible light photocatalytic activity four-fold compared to undoped TiO2 atoptimal Fe-doped TiO2 (1 wt% Fe3+) film. At higher Fe ions concentrations, the existence of increased

es an

charge recombination sit

. Introduction

Titanium dioxide (TiO2) as a photocatalyst offers an inexpensive,ontoxic, stable, highly oxidizing and reducing power in the degra-ation of organic pollutants [1,2]. The application of TiO2 in thinlm (immobilized form) was superior to the suspension system ashe latter required expensive and complex post-operation separa-ion [3]. However, TiO2 can only be activated under UV irradiationue to its wide bandgap (3.2 eV) [4]. Numerous efforts have beenedicated to modify the photocatalyst so that it can be photoac-ivated under visible light. These include depressing the bandgapy doping TiO2 with metal ions (Fe [5,6], Cr [7,8], Co [9], Cu [10], V11]). Iron-doped TiO2 has gained attention due to the fact that Fe3+

adius (78.5 pm) is similar to that of Ti4+ (74.5 pm) [12] resulting inasier insertion of Fe3+ into the crystal structure of TiO2 [13]. As

dopant, the role of Fe3+ is still controversial [13]. Some authorsuggested detrimental effect of Fe3+ as a promoter that increaseshe rate of charge recombination [14] while other authors reportedhe beneficial effect of Fe3+ in enhancing electron/hole separation

nd thus increasing the photocatalytic activity [6,9,15,16].

The sol–gel method is one of the most widely used techniqueso prepare TiO2 coatings as it presents many advantages such

∗ Corresponding author. Tel.: +65 63168829/64608526.E-mail addresses: ltm3@np.edu.sg (T.M. Lim), tytan@ntu.edu.sg (T.T.Y. Tan).

926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2011.05.003

d segregated iron oxide phase suppressed the photoactivity.© 2011 Elsevier B.V. All rights reserved.

as the use of very simple equipment, low cost, the possibility ofusing different substrates, and the ability to control the microstruc-ture, homogeneity and density of thin films [17–20]. However, thistechnique suffers from several disadvantages such as the use oforganometallic precursors that are expensive and easily hydratedin air [21]. Besides, this method also requires acid or base to sta-bilize the prepared sol, rendering it difficulty in the application oncorrosive substrate [22]. The peroxo sol–gel method is a promisingapproach to overcome these issues in the preparation of TiO2 thinfilm [23]. The peroxo sol–gel method also offers other advantagessuch as neutral pH condition, low material cost and is environmen-tally friendly as it uses water instead of organic solvents [22–24].Moreover, peroxo titanic acid (PTA) is stable in air [23,25] and itspreparation is both simple and cost-efficient [25]. PTA solution canbe prepared from titanium alkoxide [22,26] or inorganic salt such asTiCl4 [26] or TiCl3 [27]. Ge and Xu added polyethylene glycol (PEG)to a PTA sol to develop porous TiO2 structure [21] while Murakamiet al. controlled the shape of TiO2 by adding polyvinyl alcohol [28].Sonawane et al. used PEG as a stabilizer in the preparation of Fe-doped TiO2 thin films [29]. Due to a significant amount of workon Fe-doped TiO2, theoretical work using density functional theory(DFT) has become important to investigate the electronic properties

of Fe-doped TiO2. Recent study on Fe-doped TiO2 by general-ized gradient approximation Perdew–Burke–Eznerhof (GGA-PBE)method showed that the Fe doping “split” the bandgap of pureanatase TiO2 by the introduction of mid-gap state. The predicted

D.V. Wellia et al. / Applied Catalysis A: General 401 (2011) 98– 105 99

TiO2 s

bau

NspFaaF(coae

2

2

TTw2a

Fa

Fig. 1. Scheme for the preparation of Fe-doped

andgap difference (�Egap) of Fe-doped TiO2 is 0.9 eV [30], whilenother study reported the �Egap of Fe-doped TiO2 to be 0.69 eVsing GGA-FP-LAPW method [31].

In our previous work, we have demonstrated the preparation of-doped [32] and C–N-codoped [33] TiO2 thin films by a peroxo

ol–gel method. In the present work, we use a similar approach torepare transparent, uniform, crack-free and visible-light activatede-doped TiO2 thin films without the addition of a stabilizer. Welso performed DFT (GGA-PBE) calculations to study the structuralnd electronic properties of pure anatase TiO2, Fe-doped TiO2, ande–TiO2 with elemental Fe in the lattice cage. The density of stateDOS) was determined to investigate the effect of Fe3+ dopant con-entration on the bandgap. The visible light photocatalytic activityf the Fe-doped TiO2 thin films is also investigated using steariccid (SA) as the model organic compound and the photodegradationfficiency is compared with undoped TiO2.

. Experimental

.1. Chemicals

All chemicals were used directly without further purification.itanium (IV) chloride (TiCl4), purchased from Merck was used as

iO2 source; iron (III) chloride hexahydrate (FeCl3·6H2O, Alfa Aesar)as used as the source of Fe dopant; ammonia solution (NH3·H2O,

5%) and hydrogen peroxide (H2O2, 30%) were obtained from Sigmand VWR BDH Prolabo, respectively.

ig. 2. 2 × 2 × 1 Supercell model of (a) anatase TiO2 with the dopant sites, (b) with elemtoms; blue spheres, doping sites. (For interpretation of the references to color in this fig

amples from peroxo titanic acid (PTA) solution.

2.2. Preparation of Fe-doped TiO2 Coatings

The schematic of Fe-doped TiO2 preparation is depicted inFig. 1. Typically, 3.6 mL of TiCl4 was added drop wise into 300 mLFe3+ solution containing 0.5 wt% of Fe3+ in an ice-water bath andstirred with a magnetic stirrer. Ammonia solution was then addedinto the solution, resulting in the formation of white precipitate[Ti(OH)4]. After stirring for 24 h, the obtained precipitates werewashed thoroughly with DI water repeatedly until no Cl− ionswere detected. 86 mL distilled water was added to disperse theprecipitate and then 20 mL H2O2 was added drop-wise into themixture under continuous magnetic stirring until a transparentyellow solution was obtained. This was used as the coating solu-tion. The same procedure was applied for 1.0, 1.5 and 5.0 wt%of Fe3+.

The glass substrates were first cleaned by ultrasonication in abath with distilled water for 30 min and then washed with iso-propanol, ethanol and acetone sequentially. The glass slides werethen heated in a furnace at 550 ◦C for 1 h with a heating rate of10 ◦C min−1. The glass slides were then cooled to room tempera-ture and the contact angle measured was 2.9◦ indicating the glassslides were superhydrophilic. The prepared glass slide was dipcoated with the PTA solution using the KSV Dip Coater at a speed

of 0.2 cm/s. The coated glass slide was then heated in the furnaceat 550 ◦C for 1 h.

To prepare the TiO2 powder, 30 mL of the yellow coating solutionwas put into a flask in water evaporator for 1 h followed by oven

ental Fe (blue sphere) in the lattice cage. Grey spheres, Ti atoms; red spheres, Oure legend, the reader is referred to the web version of the article.)

1 lysis A: General 401 (2011) 98– 105

d1

2

wl(towTEwUsfippc(u

2

do(cgcffgdcaot

2

i(malPial

3

pboTctap

Fig. 3. FTIR spectra (i) before calcinations, (ii) after calcinations, of Fe-doped TiO2

samples with different concentrations of Fe3+ after calcination at 550 ◦C for 1 h: (a)0 wt%, (b) 0.5 wt%, (c) 1.0 wt%, (d) 1.5 wt%, and (e) 5.0 wt%.

00 D.V. Wellia et al. / Applied Cata

rying at 70 ◦C for 12 h. The powder was then calcined at 550 ◦C for h.

.3. Characterization

The crystalline structures of the synthesized Fe-doped TiO2ere analyzed by X-ray diffraction (XRD). XRD pattern was col-

ected using Rigaku X-ray diffractometer with Cu K� radiation� = 1.54060 A) with a scan step of 0.01 min−1 in the range from 20◦

o 70◦. Bruker AXS TOPAS v.3 was used to analyze the compositionf the crystalline phase. The chemical structures of the samplesere determined by FTIR spectrophotometer (Digilab FTS3100).

he thermal property of the samples was studied by TG/DTA (Perkinlmer) in the temperature ranges from room temperature to 800 ◦Cith air flow rate of 20 mL/min and heating rate of 10 ◦C min−1.V–Vis spectra of the thin films were obtained using a UV-Visible

pectrophotometer (Shimadzu). The morphology of Fe-doped TiO2lms were evaluated by FESEM (JEOL JSM-6700F SEM). EDX sup-orted in the FESEM was used to determine the amount of Feresent in the Fe-doped TiO2 samples. Water contact angles ofoated glasses were measured on a commercial contact angle meterFTA200 Dynamic Contact Angle Analyzer) at ambient temperaturesing a sessile drop method.

.4. Photocatalytic activity test

The visible-light photocatalytic activity of the prepared Fe-oped TiO2 coating was investigated using stearic acid as a modelrganic compound [32,33]. Stearic acid was dissolved in methanol0.035 mol in 100 mL methanol). 300 �L of this solution was spun-oated by WS-400B-6NPPL/LITE for 2 min at 2000 rpm onto thelass substrate and dried in an oven at 70 ◦C. Prior to stearic acidoating, the samples were UV irradiated for 2 h to clean the sur-ace from any organic contaminants that might have been adsorbedrom atmosphere. The visible light source used was a 300 W halo-en lamp held at 15 cm from the sample with 420 nm UV filter. Theegradation of stearic acid was determined by monitoring the con-entration of stearic acid using FTIR (Digilab FTS3100), where thebsorbance at 2917 cm−1 was converted to a thickness on the basisf an earlier observation that an absorbance of 0.01 correspondedo a thickness of 12.5 nm [32,33].

.5. Theoretical calculation

The structural and electronic properties of pure anatase, andron doped-TiO2, TiO2 with elemental iron in the lattice cagesee Fig. 2) was studied using density functional theory (DFT)

ethod. The interaction of elemental Fe with TiO2 crystal waslso studied by placing one Fe atom in TiO2 cage. All the calcu-ations were performed using generalized gradient approximationerdew–Burke–Eznerhof (GGA-PBE) method [34] as implementedn Vienna ab initio simulation package (VASP) [35]. For TiO2 modelnd computational parameters used in our present study, we wouldike to refer to our previous work [36].

. Results and discussion

The isoelectric point of TiO2 is in the range of pH 4.5–6.8. AtH values far from the isoelectric point, the TiO2 particles wouldear an electric charge and hence need to be stabilized in the formf sol. Sasirekha et al. reported that at neutral and basic pH, theiO2 particles are stabilized by the mutual repulsion of the negative

harges present at the surface of the TiO2 particle and maintainheir particle size [23]. Xu et al. reported N-doped TiO2 was formedt alkaline pH [32]. In this work, pH 7 was chosen to avoid theresence of nitrogen in the Fe-doped TiO2 photocatalyst.

Fig. 4. TG-DTA of Fe-doped TiO2 with 1.0 wt% Fe3+.

D.V. Wellia et al. / Applied Catalysis A: General 401 (2011) 98– 105 101

F oped

T

oitin

ig. 5. (a) Image of transparent Fe-doped TiO2 coated glass; (b) FESEM image of Fe-diO2 (1.0 wt% Fe3+).

In this work, Fe ion was added as the dopant together withther precursors at the first stage to obtain stable PTA coat-

ng solution. It was noted that if the Fe ions were added intohe PTA at a final stage, the viscosity of solution immediatelyncreased with Fe ion being precipitated out. The precipitateever reverted to the original transparent PTA solution even if

TiO2 thin films (1.0 wt% Fe3+); (c) EDX pattern of the undoped TiO2 and (d) Fe-doped

ammonia solution or hydrogen peroxide solution was added. Thiswas probably due to the formation of TiO(OH)(OOH) immedi-

ate species when Fe3+ polyvalent ion was added, resulting in theprecipitation [26].

The FTIR spectra of the yellow peroxo titanic acid (PTA)before and after heat treatment are presented in Fig. 3(i) and

102 D.V. Wellia et al. / Applied Catalysis A: General 401 (2011) 98– 105

F 3+

a(

(tTbtTtaPpdpt

1oitmorwts(p

cdTPaipwaFa0t

dTd

2 5showed that 0.5, 1.0 and 1.5 wt% Fe3+ samples contain 100% rutile,while 5.0 wt% Fe3+ sample contain 0.84% Fe2TiO5 in addition to TiO2rutile phase. These data suggest that almost all of the Fe3+ in 0.5, 1.0

ig. 6. XRD patterns of Fe-doped TiO2 samples with different concentrations of Fefter calcination at 550 ◦C for 1 h: (a) 0 wt%, (b) 0.5 wt%, (c) 1.0 wt%, (d) 1.5 wt%, ande) 5.0 wt%.

ii) respectively. In Fig. 3(i), a peak at 900 cm−1 corresponding tohe stretching vibration of the O–O bond (peroxo group) in thei–O–O–H bond of the peroxo titanic acid was observed. The wideands at 3100–3700 cm−1 are attributed to the stretching vibra-ion of the hydrogen-bonded OH groups of the adsorbed water.he absorption around 1630 cm−1 is assigned to the bending vibra-ion of adsorbed H2O molecules. The peaks around 1400 cm−1 aressigned to the stretching vibrations of the N–H bonds in NH4

+.TA has unstable chained atoms of hydrogen, which will decom-ose and release oxygen molecules to form stable Fe-doped TiO2uring heating. In this process, NH4

+ also decomposes. The disap-earance of 900 cm−1 and 1400 cm−1 bands in Fig. 3(ii) evidenceshe decomposition of PTA and NH4+, respectively [21,26].

TG-DTA profile (Fig. 4) shows an endothermic minimum at00 ◦C and exothermic peak maximum at 260 ◦C. The first stagef weight loss (endothermic) is attributed to the removal of phys-cally adsorbed water. The second stage (exothermic) is attributedo the decomposition of peroxo group [37]. Other broad exother-

ic peak at about 320 ◦C is probably due to the decompositionf remaining NH4Cl [26]. There is no further weight loss in theange of 400–420 ◦C, but a broad exothermic peak was observedhich is attributed to the slow conversion of amorphous phase

o anatase [37]. Our observation is consistent with the 3 distincttages reported during the annealing process: (1) removal of water,2) decomposition of a peroxo group, and (3) amorphous-anatasehase transformation [37].

A prepared Fe-doped TiO2 coated glass is shown in Fig. 5a. Theoated glass is visually transparent and uniform with a yellow tintue to Fe doping. FESEM image (Fig. 5b) reveals that the Fe-dopediO2 thin films have no cracks, attributed to uniform dispersion ofTA on the superhydrophilic pure glass. EDX measurement (Fig. 5cnd d) further confirmed the presence and amount of iron presentn the Fe-doped TiO2 samples. Fe element was observed in the EDXattern in the Fe-doped TiO2 (Fig. 5d), while no trace of Fe elementas found in the EDX pattern of the undoped TiO2 (Fig. 5c). The

mounts of Fe element in the undoped TiO2, Fe-doped TiO2 (0.5 wt%e3+), Fe-doped TiO2 (1.0 wt% Fe3+), Fe-doped TiO2 (1.5 wt% Fe3+)nd Fe-doped TiO2 (5.0 wt% Fe3+) were determined to be 0, 0.60,.96, 1.74 and 4.96 wt%, respectively. The results are consistent withhe original amounts of Fe in the samples.

Fig. 6 shows the XRD patterns of Fe-doped TiO2 samples withifferent concentrations of Fe3+ after calcination at 550 ◦C for 1 h.he XRD spectra show the presence of only anatase phase. No peaksue to haematite (Fe2O3) are observed in any of the samples. This

Fig. 7. XRD patterns of samples after calcination at 800 ◦C for 1 h with differentconcentrations of Fe3+: (a) 0.5 wt%, (b) 1.0 wt%, (c) 1.5 wt%, and (d) 5.0 wt% (r = rutile,o = iron titanium oxide peak).

may due to Fe3+ ions substituting Ti4+ ions in the TiO2 lattice sincethe radii of Ti4+ and Fe3+ ions are similar (78.5 pm and 74.5 pmfor Ti4+ and Fe3+ radii [12] respectively) and the electronegativityof both ions are reasonably close (Fe3+: 1.96 [12], Ti4+: 1.5 [38]).Based on Hume–Rothery rule, if the % difference of atomic radii isless than 15% and the electronegativity of the elements are simi-lar, a substitutional solid solution is most likely to be formed. Thephenomenon of Fe3+ ion doping to TiO2 crystal satisfies the third ofHume–Rothery rule as well which states that a lower-valent metalwill be soluble in a higher-valent host [39]. Hence the Fe ions maybe inserted into the lattice site of Ti4+ [13].

To detect possible segregated iron oxide phase in the Fe-dopedTiO2, the samples were calcined at 800 ◦C for 1 h. The XRD spectra(Fig. 7) show the presence of rutile for all samples. For Fe-dopedTiO2 samples with 0.5, 1.0 and 1.5 wt% Fe3+, no other peak wasobserved. However, for the sample with 5.0 wt% Fe3+, two peaksat 2� = 33◦ and 60.5◦ were identified as perovskite structure of irontitanium oxide (Fe TiO ) [13]. Analysis using Bruker AXS TOPAS v.3

Fig. 8. Absorbance spectra of Fe-doped TiO2 with different concentrations of Fe3+:(a) 0 wt% (undoped TiO2), (b) 0.5 wt%, (c) 1.0 wt%, (d) 1.5 wt% and (e) 5.0 wt%.

D.V. Wellia et al. / Applied Catalysis A: General 401 (2011) 98– 105 103

h elem

awa

scsc

E

wnO

TCm

Fig. 9. Density of state (DOS) of pure and Fe-doped TiO2, TiO2 wit

nd 1.5 wt% Fe3+ samples were trapped in the crystal lattice of TiO2,hereas the 5.0 wt% Fe3+ sample contains segregated iron oxide in

ddition to Fe3+ doped TiO2.DFT calculation was used to prove the existence of doping and

egregated oxide or other phase. In our present study, we haveonsidered 4.3 and 8.6 wt% Fe doping in TiO2. To investigate thetability of pure and Fe-doped TiO2, the formation energy (Ef) isalculated according to Eq. (1) as follows:

f = Ecrystal − ni�i (1)

here, Ecrystal is the total energy of the crystal; ni and �i are theumber of atoms and chemical potential of various elements (Ti, Fe,, etc.), respectively. The chemical potential of Ti and Fe is derived

able 1alculated formation energies difference �Ef of Fe-doped TiO2 and TiO2 with ele-ental Fe in the lattice cage. Please refer to Fig. 2 for the definition of doping.

No Systems �Efa

1 4.3 wt% Fe–TiO2b 0.13

2 8.6 wt% Fe–TiO2c 0.25

3 Fe–TiO2–Ed 0.10

a Pure TiO2 is taken as reference formation energy.b 4.3 wt% Fe–TiO2: Ti1 is replaced by Fe.c 8.6 wt% Fe–TiO2: both Ti1 and Ti2 are replaced by Fe.d TiO2 with atomic Fe in the lattice cage.

ental Fe in the lattice cage. The dotted line is Fermi Energy level.

from bulk crystal and the chemical potential of O is derived frommolecular O2.

The calculated formation energy difference (�Ef) is listed inTable 1 with the formation energy of TiO2 (−3.39 eV/atom) as areference. We also observed that with increasing dopant concen-tration, the stability of Fe-doped TiO2 decreases compared to TiO2.Thus, our results suggest that at higher dopant concentration, seg-regated Fe2O3 phase may be formed, which is consistent with ourexperimental observation. The formation energy of TiO2 with ele-mental Fe in the lattice cage is similar to Fe-doped TiO2, whichsuggests the possibility of entrapment of elemental Fe in TiO2 cage.

In the doping process of Fe3+ into TiO2, wherein Fe3+ ions replace

Ti4+, Fe3+ will act as an electron donor and form donor level closeto the conduction band, resulting in a smaller energy transition,which may lead to visible light photoactivation [39,40]. In Fig. 8, the

Table 2Calculated bandgap differences (�Egap) of Fe-doped TiO2 and TiO2 with elementalFe in the lattice cage.

No Systems �Egapa

1 4.3 wt% Fe–TiO2 −0.532 8.6 wt% Fe–TiO2 −0.613 Fe–TiO2–E −1.34

a Pure TiO2 is taken as the reference bandgap.

104 D.V. Wellia et al. / Applied Catalysis A

Fig. 10. (a) Photocatalytic activities of undoped TiO2 and Fe-doped TiO2 films undervd

UoctuoadAwgTbFiqdt

oeomb

[10] T. Morikawa, Y. Irokawa, T. Ohwaki, Appl. Catal. A: Gen. 314 (2006) 123–127.

isible light illumination for 24 h; (b) evolution of the IR absorbance spectra of Fe-oped TiO2 (1.0 wt%).

V–Vis absorption spectra of the Fe-doped TiO2 films show obvi-us enhanced absorption in the visible light region (400–600 nm)ompared to that of the undoped TiO2 film, indicating their poten-ial to absorb visible light and improve photocatalytic activitiesnder visible light illumination, even though no significant shiftf the absorption edge to visible light region is observed [41]. Thebsorption edges of undoped TiO2, Fe-doped TiO2 (1.0 wt%) and Fe-oped TiO2 (5.0 wt%) films are 340, 350 and 362 nm, respectively.ccording to the empirical formula, Eg = 1239/�edge (�edge = theavelength of the optical absorption edge) [42], the bandgap ener-

ies of the undoped TiO2, Fe-doped TiO2 (1.0 wt%) and Fe-dopediO2 (5.0 wt%) films are 3.64, 3.54 and 3.42 eV, respectively. Theandgap energies of the undoped TiO2, Fe-doped TiO2 (1.0 wt%) ande-doped TiO2 (5.0 wt%) films are higher than the usually reportedn literature (3.2 eV for anatase TiO2), which may be attributed touantum size effect of TiO2 film and the thermal stress in the filmsue to the difference in the thermal expansion coefficients betweenhe fused substrate and coating material [43].

The density of states (DOS) were determined to study the effectf doping on bandgap variation. We have also investigated theffect of elemental Fe doping on the changes of bandgap structure

f TiO2. The DOS plots of TiO2, Fe-doped TiO2 and TiO2 with ele-ental Fe in the lattice cage are shown in Fig. 9. The calculated

andgap difference (�Egap) of Fe-doped TiO2 with respect to TiO2

[[[

: General 401 (2011) 98– 105

is listed in Table 2. The calculated bandgap difference (�Egap) of4.3 and 8.6 wt% Fe-doped TiO2 is −0.53 and −0.61 eV, respectively.The experimental bandgap difference (�Egap = −0.69) of Fe-dopedTiO2 [44] is comparable with our calculated bandgap differenceof 8.6 wt% Fe-doped TiO2. Our results indicate that Fe dopingdecreases the bandgap of TiO2. Here, we observe that bandgapdecreases with increasing Fe doping which is consistent with thecurrent experimental observation of absorbance red shift (Fig. 8).Our results also show that the elemental Fe has strong effect on thebandgap of TiO2 as compared to doped Fe (�Egap = −1.34). Althoughour theoretical study shows that the entrapment of elemental Fein the cage of TiO2 is possible, the large bandgap difference, whichwill result in a very small bandgap for elemental Fe contained in thecage of TiO2, is not supported by current experimental data. There-fore, we suggest that all the iron in the current Fe-doped TiO2 filmswere doped in the crystal lattice with a small amount of segregatedphase for the case of 5.0 wt% Fe3+.

The photocatalytic activity of the Fe-doped TiO2 coated glasswas evaluated by photodegradation study of stearic acid (SA) undervisible light irradiation (Fig. 10). 1.0 wt% Fe3+ was found to be theoptimal dopant concentration and the associated photocatalyticactivity was 4 times higher than that of undoped TiO2 coated glass.Fe ions doping extends light absorption into the visible light region,supported by other studies [13,16] and our current results (Fig. 8).However, higher Fe ions concentration at 1.5 wt% may increase thenumber of trap sites [7,45], resulting in higher recombination rateand lower photocatalytic activity. At 5.0 wt% Fe ions concentration,the existence of segregated iron oxide phase further decreased thephotoactivity [45–48].

4. Conclusions

Visible light active Fe-doped TiO2 coated glass slides havebeen successfully prepared using a “green” aqueous PTA solutionapproach. The prepared thin films are transparent, uniform andcrack free. The visible light photocatalytic activity of the Fe-dopedTiO2 films is attributed to bandgap narrowing as observed fromabsorption red-shift and DOS results. The photocatalytic activity ofoptimal 1.0 wt% Fe3+ doped TiO2 was about 4 times higher than thatof undoped TiO2 coated glass. DFT results indicated that Fe3+ wasdoped into the TiO2 crystal lattice but at higher dopant concentra-tion, its stability may be lowered due the presence of segregatediron oxide phase.

Acknowledgments

Financial support from Nanyang Technological University AcRFTier 1 RG29/07 is gratefully acknowledged. D.V.W. acknowledges aresearch scholarship from Nanyang Technological University.

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