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Fe3+-doped anatase TiO2 with d-d transition, oxygen vacancies and Ti3+ centres: synthesis, characterization, UV/vis photocatalytic and mechanistic studies Hayat Khan, and Imran Khan Swati Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01104 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 27, 2016

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Industrial & Engineering Chemistry Research

Fe3+-doped anatase TiO2 with d-d transition, oxygen vacancies and Ti3+ centres: synthesis, characterization, UV/vis photocatalytic and mechanistic studies Hayat Khan*a,b, Imran Khan Swatib a

Department of Chemical Engineering, McGill University, 3610 University street,

Montreal, Quebec, Canada H3A 2B2 b

Department of Chemical Engineering, University of Engineering and Technology,

Peshawar, P.O Box 814, University Campus, Peshawar, Pakistan 25120

ACS Paragon Plus Environment


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ABSTRACT: The current work emphasis on the preparation, characterization, recyclability, stability and mechanistic study of nanosized Fe3+-doped TiO2 photocatalyst. The structural, optical and photocatalytic properties of undoped and doped TiO2 were studied by XRD, FTIR, N2 absorption-desorption, UV/vis DRS, PL, ESR, XPS, Raman and UV-visible spectroscopy. XRD analysis showed that prepared powders with different iron content (200, 100, 50 and 25 molar ratios) were consist of only anatase phase. FTIR study confirmed the chelation of acetate with titanium precursor through bidentate bridge mode; as a result, the condensation pathways are effectively altered by the acetate ligands favoring the formation of anatase phase, this result give further confirmation to the XRD analysis. A decrease in charge carrier recombination rate and the presence of oxygen vacancies and related Ti3+ centers in the prepared photocatalysts were confirmed through photoluminescence (PL) and EPR spectroscopic studies. XPS results have indicated the presence of dopant electronic states (Fe3+, Fe2+ and Fe4+), which could be due to substitution of Fe3+ ions in-place of Ti4+ in the crystal lattice. UV/vis DRS spectrum showed that undoped TiO2 exhibits an absorption edge in the UV region, the position of which was shifted towards the visible region on incorporation of Fe3+ into it. This red shift of the optical absorption in doped TiO2 was the outcome of d-d transition of Fe3+ (2T2g → 2A2g, 2T1g) and the charge transfer transition between interacting iron ions (Fe3+ + Fe3+ → Fe4+ + Fe2+). These Fe3+ 3d states in addition to oxygen vacancies and Ti3+ centers create band states, thereby favoring the electronic transition to these levels and resulting in narrowing of TiO2 band gap. A direct confirmation is the increase in the magnitude of Urbach energy with the lowering in the band gap of Fe3+-TiO2. The production of hydroxyl radicals (OH- + h+ → OH•) which are the main scavengers for the photogenerated holes (h+) were monitored by a PL technique using terephthalic acid (TA). The observed trend was TFe50 > TFe100 > TFe25 > TFe200 > TiO2, implying that the TFe50 powder produced enhanced amount of OH• radicals under light irradiation, which helps in its highest photocatalytic activity against the degradation of methylene blue and 4-chlorophenol under UV and visible light irradiations. Keywords: TiO2, Band gap, Recombination, UV and visible light, Photocatalytic activity


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1. INTRODUCTION Titanium dioxide (TiO2) mediated photocatalytic degradation is a promising technology in the near future especially for industrial wastewater treatment. However, there are still some disadvantages in using TiO2 as a photocatalyst which limit its practical applications such as; (i) anatase-TiO2 requires only UV light for excitation due to the wide band gap (~ 3.2eV) value, implying it is inactive under visible light irradiations, (ii) high recombination rate of photoinduced electron-hole pairs due to the short charge separation distances within the particle, resulting in low efficiency in utilizing the photons and slowing down the photocatalytic degradation process and (iii) TiO2 also possess low decomposition rate for many pollutants, because the photocatalyst particles surface coverage is typically very low

1, 2

. To address the

above stated problems, modification of TiO2, which is achieved by metal doping in TiO2, is one of the methods that not only enhances the TiO2 visible light absorption, but, also retards the carriers recombination process. Amongst a variety of metals ions, Fe3+ has been considered an interesting and appropriate dopant for TiO2 modification due to its several advantages such as; (i) half-filled (3d5) electronic configuration, (ii) the ionic radius of Fe3+ (0.064nm) is very close of that of Ti4+ (0.068nm), implying Fe3+ ions may insert into the TiO2 structure to occupy lattice sites in substitutional doping or located at interstices positions in interstitial doping, (iii) Fe3+ cations can acts as shallow charge trapping centers in TiO2 lattice, because the energy level of Fe2+/Fe3+ lies close to that of Ti3+/Ti4+, favoring the decrease in the carriers recombination rate and extension in photo response of TiO2 into the visible region

1, 3

and (iv) the incorporation of metal dopant in TiO2

weakens the bonding of neighboring oxygen atoms at the surface as a result oxygen atoms are readily released from the lattice causing an oxygen vacancy. Gaseous phase oxygen can easily be adsorbed at the oxygen vacancies site as a result consume the conduction band electrons and leads to increased photocatalytic activity of doped TiO2 compared to undoped TiO2 4. Reviewing the photocatalytic activity reports of iron (Fe3+) doped TiO2 powder, literature showed considerable controversial results, for example, Litter et al

5

reported that Fe-TiO2 has

enhanced photocatalytic activity in reducing N2 to NH3 than TiO2 doped with other transition metals. Nguyen et al

6

stated that Fe-TiO2 synthesized by hydrothermal method result into

increased degradation of methyl orange under visible light, because Fe3+ ions leads to narrowing



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of the band gap. Similarly, Yalcin et al 2 prepared Fe3+ doped TiO2 by wet impregnation method, the doped catalyst showed enhanced visible photocatalytic activity for the degradation of 4nitrophenol compared to undoped TiO2. In contrast, Sclafani et al

7

observed that in the

3+

photodegradation of phenol, Fe doping of TiO2 has little effect on relative photoreactivity. Li et al 8 reported that Fe-TiO2 catalyst result into decrease photocatalytic activity of methylene blue because the dopant ions acts as recombination centers for the photogenerated charge carriers. Paola et al

9

described in his work that Fe-TiO2 prepared by wet impregnation method showed

decreased photocatalytic activity for the degradation of 4-nitrophenol compared to undoped TiO2 due to the increased recombination rate. Yang et al

10

used a hydrothermal process to prepare

iron doped TiO2 nanoparticles; the dopant ions played a detrimental role by accelerating the electron-hole recombination rate, which is manifested by the high PL intensity and low hydroxyl radical productivity. Similarly, the work of Mu et al

11

showed that TiO2 doping with trivalent

metal ions was detrimental for the UV photocatalytic degradation of liquid cyclohexane, because the trivalent dopant ions acts as recombination centers. Nagaveni et al

12

also observed the

negative effect of Fe3+ in TiO2 synthesized by the solution combustion method, the dopant ions increases the density of electron-hole recombination centers and deteriorate the photocatalytic power. In addition to the effect of Fe3+ ions on TiO2 photocatalytic activity, the amount of Fedopant addition in TiO2 is also controversial; different optimum amounts of Fe3+, ca. 0.3%, 0.5% and 1% are reported in literature

1, 2, 13

. Low addition of dopant content in TiO2 leads to low

visible light absorption, which prevent catalyst activity performance under visible light from being improved. In addition, it is also reported that optimal Fe-doped concentration is particle size dependent. Decrease in optimal concentration of dopant salt occurs with increase in particle size, because more Fe ions that enter the bulk for larger particles can increase the recombination of electrons and holes

14

. Therefore, it is desirable to develop and explore a new method or to

modify an existing method to synthesize Fe-doped TiO2 in which an increase amount of dopant ions can be added for the purpose of increase visible light absorption and decrease in the electron-hole recombination rate. Also, it is of interest to explore the tunable range of TiO2 band gap and the correlation among charge carriers recombination, band gap and photocatalytic activity. In this context, the current work aims to achieve the following objectives.


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•

To enhance the photocatalytic activity of TiO2 by increasing the spectral sensitivity of the photocatalyst to visible light and decreasing the charge carrier recombination rate at the same time.

•

To develop a facile, rapid and convenient experimental procedure for photocatalyst synthesis.

•

To

elucidate

the

structural

characteristics,

morphology,

optical,

photoluminescence and photocatalytic properties of Fe3+-TiO2 in order to study the effect of increase in dopant molar ratio on the UV and visible activity of the prepared photocatalyst powders. •

To elucidate a systematic and comprehensive mechanism and to conduct corresponding experiment to support it.

•

Finally, reusability, recyclability and stability of prepared photocatalyst were also studied.

To achieve these goals, the properties of the TiO2 material was modified by selective surface treatment such as surface chelation, surface modification and selective doping of the crystalline matrix. Fe3+-doped TiO2 nanoparticles were prepared by employing sol-gel method via hydrolysis mechanism using dopant salt ferric nitrate and titanium isopropoxide as titanium precursor chelated with acetic acid. To our best knowledge, no such comprehensive study has been performed; the present study results demonstrated an enhanced photodegradation activity could be obtained for the Fe3+-TiO2 nanocomposites of the model pollutants, methylene blue and 4-chlorophenol under UV and visible light, respectively. The synthesis procedure adopted in this study for producing iron doped TiO2 might be a cost effective strategy for industrial organic pollutants remediation. Moreover, the stability and recyclability results of the most active doped photocatalyst are also promising towards improved photocatalytic activity. 2. MATERIALS AND METHODS 2.1. Catalyst Synthesis. Titanium (IV) isopropoxide (TTIP, ≥97% Aldrich), glacial acetic acid (CH3COOH, ACS Fisher Scientific) and iron nitrate nanohydrate (Fe(NO3)3.9H2O, 99.996% trace metal basis Aldrich) were used as received without any further purification process. Fe-TiO2 nanoparticles were synthesized by a facile sol-gel method, a molar ratio of 1:10:200:x among TTIP, CH3COOH, H2O and Fe(NO3)3.9H2O was maintained. Glacial acetic acid (35.8ml) was added to TTIP (18.5ml) at 0oC under strong mechanical stirring. This was



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followed by dropwise addition of deionized H2O until a clear solution is obtained, then measured amount of Fe(NO3)3.9H2O (x = 200, 100, 50 and 25) dissolved in water (65ml) is added to the clear solution followed by further addition of balance water (100ml) remaining. The solution was stirred for 6h, and after this time the stabilized sol was aged for 12h at room temperature followed by placing the solution in water bath set at a temperature of 70oC for 12h. The gel was dried in an oven for overnight at 100oC and finally, the dried gel pellets were pulverized to get an amorphous doped TiO2 nanopowder. The nanopowder was then annealed in air at 300 and 500oC for 1h, respectively, to obtain two sets of iron doped TiO2 (TFex, where x is mole ratio of the dopant cation). Undoped TiO2 is prepared by the same process, except that no Fe(NO3)3.9H2O is added. 2.2. Catalyst Characterization. The X-ray diffraction patterns were recorded on a Phillips PW 1710 diffractometer using monochromatic high intensity Cu Kα radiations (λ = 0.15418nm) to determine the crystalline phase and crystallite size of undoped and Fe-TiO2. Transmission electron microscopy (TEM) images of doped sample were collected on a Phillips Technai G2 20TEM (FET, US) operated at 200kV and attached energy dispersive X-ray (EDX) detector was used to confirm the dopant cation in the prepared catalyst powder. The BET (Brunauer-EmmettTeller) surface area (SBET) was measured by nitrogen adsorption-desorption isotherm measured at 77K and Barret-Joyner-Halender (BJH) method was applied to obtained the pore size distribution. The FTIR spectra of the samples mixed with the reference potassium bromide (KBr) were obtained on a Bruker Tensor 27 with OPUS data collection program (V 1.1) in the energy range of 5000-400 cm-1. Diffuse reflectance spectrum (DRS) in the 190-600nm range was recorded on Carry 5000 NIR UV/vis spectrophotometer (Varian Inc. US) with attached Praying MantisTM diffuse reflection (DRIFTS) accessory using potassium bromide (KBr) as a reference. The obtained spectrum was used to calculate the energy value of the photocatalyst following the Kubulka-Munk (K-M) function calculations. The room temperature photoluminescence spectra of the samples were recorded with FluroMax-2 spectrofluorometer using the 320nm line of Xe lamp as the excitation source. The electron paramagnetic resonance (EPR) spectra were collected at room temperature by Bruker ER 200D-SRC electron spin resonance. Raman spectra were acquired at room temperature using a SENTERRA confocal Raman microscope (Bruker, US) equipped with a 532nm laser and X-ray photoelectron spectroscopy (Thermo Scientific K-alpha (USA) equipped with X-ray source of AlKα (1486.6eV) was used to investigate the oxidation


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state of the doping metal ion. Data spectrum processing was performed using the Avantage (Thermo Scientific) software. The hydroxyl radicals (OH•) formation on the surface of prepared powder in water under illuminated light was measured by a terephthalic acid (TA) fluorescence probe technique

10

. The PL spectra of the produced 2-hydroxyterephthalic acid (TAOH) were

measured by using the same fluorescence spectrophotometer as stated above. 2.3. UV/vis Degradation Experiments. The model pollutants, methylene blue (MB) and 4chlorophenol (4-CP) were used as supplied by Sigma Aldrich. The photocatalytic activity of FeTiO2 particles for the UV photocatalytic degradation of MB was carried out at atmospheric pressure and room temperature (ca. 25oC ± 1). Before each experiment required amount of MB (7.5mgL-1) and synthesized powder of undoped or Fe-doped TiO2 (100mgL-1) were sonicated for 30min in separate flasks. The final uniform pollutant solution and catalyst suspension were poured into the cylindrical reactor (final reactor volume was 2.4L). The walls of the reactor were covered by aluminum foil to avoid any release of light radiation. Oxygen was supplied continuously by bubbling filtered air throughout the experiment. Prior to irradiation the reactor solution was magnetically stirred for 30min to ensure uniform mixing of MB solution and TiO2 suspension and the establishment of adsorption/desorption equilibrium. The irradiation was carried out for 1h experimental duration using a single UV light (TUV 11W 4P-SE, UV-Technik Speziallampen Wolfsberg Germany) having strong emission at 254nm, located in the centre of the reactor and protected in a quartz sleeve. An aliquots of 5ml solution is withdrawn from the reactor at regular intervals and is filtered (Millipore syringe filters, porosity 0.22µm) to remove the photocatalyst particles and then analyzed with UV/vis spectrophotometer. The photocatalytic decomposition of MB was measured by following the decrease in its concentration with time and the pseudo first order apparent reaction rate constant (kapp) was calculated assuming first order kinetics by plotting ln(C/Co) versus time, where C is the reaction concentration and Co is the initial concentration of model pollutant. For all the activity analysis, the maximum absorbance of MB at 662nm was measured on Carry 5000 NIR UV/vis spectrophotometer (Varian Inc. US). The pH of the reactor suspension was measured with accumetTM AB15 pH meter equipped with a glass pH electrode and a constant value of 5.7 ~ 5.8 was measured throughout the photocatalytic experiments. In the visible photocatalytic experiments, the reactor solution (final reactor volume is 0.85L) containing 35mgL-1 of 4-CP and 100mgL-1 of doped TiO2 was exposed to light in the



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wavelength range of 420-660nm for 10min experimental duration, using a mercury lamp (180W Hg-medium pressure lamp, Heraeus Nobel Light, Germany), a UV blocking film (UVPS, USA) was used to eliminate any UV radiations in the emitted light. To maintain the reactor solution temperature at 25oC, cold water was circulated at the outer jacked of the reactor. 4-CP concentration was determined by an Agilent 1100 series high performance liquid chromatography (HPLC) equipped with Zorbax Eclipse XDB C-18 column (5µm, 4.6mm x 250mm). Eluents consists of water and acetonitrile, using an eluent gradient program profile 40% acetonitrile in water was injected for the first 5min, then from 40% to 70% acetonitrile in water over the next 15min at a constant flow rate of 0.8mLmin-1. Detection wavelength of 228nm was applied to record the concentration of 4-CP. 3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction. Figure 1 displays the XRD patterns of selected undoped (T) and doped (TF50) TiO2 powders calcined at 300 and 500oC (insert), respectively. The diffraction peaks of the samples evidence the presence of essentially the anatase form of titania (tetragonal D4h, I4I/amd) without any hint for rutile or iron oxide containing phases, regardless of the amount of Fe dopant. The sample structural phase was further confirmed by Raman analysis (S1.1), showing only the anatase peaks (1-Eg, 2-Eg, A1g, B1g, and 3-Eg). The XRD results are in contradiction to that of Ghosh et al

15

work reporting that Fe accelerates the transformation of

anatase to rutile as a result the sample possess poor catalytic efficiency. In the prepared catalyst, the absence of rutile phase indicates that the presence of Fe3+ can stabilize the anatase phase of TiO2, also the addition of Fe dopant causes the decrease in TiO2 crystallite size (Table 1), which may also be the reason for rutile phase retardation because large anatase grains can easily be converted to rutile phase 16, 17. Moreover, no presence of the crystalline phase containing Fe even at highest Fe concentration indicates that the dopant ions were successfully incorporated into the frame work of anatase TiO2 because XRD is sensitive enough to record such minor changes to TiO2. This result also revealed that may be due to the low calcination temperature (300 or 500oC) applied in the catalyst synthesis procedure, Fe3+ ions do not react with TiO2 to form new crystalline phases such as α-Fe2O3 or Fe2TiO5. The existence of these metal oxides are disadvantageous, because of their poor photocatalytic activity and they may also block the TiO2 surface active sites resulting in a decrease in the number of OH• radicals 2.


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a a = anatase

a

Intensity (a.u.)

a

Intensity (a.u.)

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a

a a

a a a a

TFe50

T 10

20

30

40

50 Position (2θ)

a

60

70

80

a a a a

a

TFe50

T

10

20

30

40 Position (2θ)

50

60

70

80

Figure. 1. XRD diffraction spectra of the indicated samples calcined at 300 and (insert) 500oC. The crystallite sizes of the prepared catalysts calcined at 300 and 500oC were estimated by using the Scherrer equation

13

and the values are presented in Table 1, showing that on Fe

doping the crystallite size increases compared to undoped TiO2. But an increase in dopant concentration result in the decrease of crystallite size for the doped samples, which may be due to the possible formation of Fe-O-Ti bonds, indicating that Fe3+ doping restrains the growth rate of the TiO2 catalyst.

18

This result is supported by the work reported elsewhere

contradicts to that of Guo et al 1, Adnan et al

19

and Ma et al

20

13

, while

, illustrating that Fe-TiO2

crystallite size increases with increase in Fe content up to certain value 3.0%, 1.5% and 4.0 at.%, respectively, after words the crystallite size decreases with further increase in dopant amount. The decrease in crystallite size is confirmed from the anatase [101] peak shift to low 2ϴ direction (not shown here) as the Fe concentration increases, which indicates that the FWHM (full width at half maximum) of the anatase [101] plane increases with the increase in Fe3+ ions into the TiO2 crystal lattice. This slight distortion in the crystal structure is further confirmed by calculating the anatase lattice parameters (a = b) and (c) from the two appropriate reflection [101] and [200], the parameters values are given in Table 1. The value of the parameter "a" remains essentially unchanged while the parameter "c" decreases with Fe3+ ions addition. This



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means that anatase lattice deform predominantly along the c-axis to accommodate substitutionaly incorporated Fe3+ ions, because the effective ionic radii of Fe3+ (0.064nm) is slightly smaller than Ti4+ (0.068nm). The noticeable change in the parameter "c" in comparison to "a", suggest that Fe3+ substitute Ti4+ preferentially on body centered and face centered lattice sites in the anatase structure

18

. Such a substitution not only lowers the crystallization of TiO2 (illustrated by the

weak diffraction intensity in the XRD pattern of the doped samples), but also hinders the growth of the TiO2 crystallite. Table 1. Crystallite size, lattice parameters, band gap and Urbach energy values of undoped and Fe3+-doped TiO2 powders calcined at 300 and 500oC. Catalyst sample

Temperature (oC)

Anatase crystallite size (nm) T 9.0 (8 – 11) TFe200 300 11.0 (10 - 13) 9.0 TFe100 TFe50 7.0 (6 – 8) TFe25 6.0 T 15.0 (13 - 16) TFe200 500 17.0 (15 – 18) TFe100 15.0 13.0 TFe50 (12-14) TFe25 7.0 Values in the parenthesis show the range of

Lattice parameters (Å) a=b c 3.79 9.50

Band gap (eV)

Urbach energy (meV)

3.15

120.0

3.79

9.47

3.07

165.0

3.79 3.79

9.34 9.31

2.90 2.80

223.0 263.0

3.79 3.78

9.24 9.57

2.70 3.12

303.0 68.0

3.79

9.56

3.00

142.0

3.79 3.79

9.50 9.46

2.87 2.75

184.0 221.0

3.79 9.42 2.60 245.0 the photocatalyst crystallite size, the experiments

were performed in several replicates (between 3 and 5). Table 1 also shows that crystallite size of undoped and doped powders increases with increase in calcination temperature. The width of all the XRD diffraction peaks are decreased with increase in calcination temperature without phase transformation, the peaks [211] and [220] observed at (54.96o) and (69.74o) on 2ϴ axis in the powders calcined at 300oC are individually fragmented into two peaks (insert of Figure 1); (53.95o [105], 55.10o) and (68.89o [116], 70.30o), such changes are attributed to the enhancement in crystallinity with increasing calcination temperature.


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TEM images and EDX spectrum of TF50 powder calcined at 500oC shown in Figure 2 were used to characterize the information on morphology, crystalline phase, particle size and elemental composition. The average particle size (9.3nm) obtained from the TEM analysis (Figure 2i-ii) is in the nano scale range, which are close to the value calculated from the XRD analysis. To validate the existence of the Fe dopant in TiO2, EDX analysis was carried out (Figure 2iii), EDX is a chemical micro analysis technique used together with TEM. The EDX spectra clearly indicated the presence of Fe in the sample along with the main constituents of Ti and O, further no impurities were observed in the sample, the signals observed for Cu and Si were due to the sample holder. The structural information given by the SAED (Figure 2iv) pattern indicated that of anatase phase, which is also in good agreement with X-ray diffraction data, at least eight diffraction rings are clear to tell, indicating that the particles are well crystalline. Moreover, the TEM images fairly showed the absence of amorphous domains which is an important prerequisite for enhanced photocatalytic activity.

Figure 2. HR-TEM images (i, ii), EDX spectra (iii) and SAED diffraction pattern (iv) of TFe50 powder calcined at 500oC.



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3.2. Fourier Transform Infrared Spectroscopy Analysis. The fourier transform infrared spectroscopy (FTIR) spectra of undoped and Fe-doped TiO2 powders calcined at 300 and 500oC shown in Figure 3 were compared to examine the difference in their surface functional groups. In Figure 3i, all the spectra has a broad band in the region 2800-3600cm-1 centered at around 3148cm-1 and a minor peak at 1656cm-1 attributed to the characteristic O-H bridging mode of OH group and H-O-H bending mode of the adsorbed water molecule, respectively. In the region below 1000cm-1 several peaks can be observed which are ascribed to the absorption band of TiO, Ti-O-Ti stretching and O-Ti-O bending vibrations. In TFex powders any band corresponding to iron phases were not observed, in addition the O-H broad band (2800-3600cm-1) intensity is still maintained independent of increase in dopant concentration. This result indicates that may be Fe ions are linked with the surface OH groups that hinder the loss of OH groups upon calcination. As reported elsewhere

21

under visible light irradiation Fe3+ ions can improve the

photocatalytic activity of TiO2, Fe3+ ions can form complexation with hydroxyl groups bound to TiO2 surface, upon illumination such complexation favor the formation of hydroxyl radicals (OH•). The weak peak at 1340cm-1 is attributed to the stretching vibration of C=O group. The peak at 2366cm-1 is assigned to physisorbed CO2 from the atmosphere, the intensity of this peak (2366cm-1) decreases with increase in Fe content indicating that powder ability to absorb atmospheric CO2 or any carbonate formation at the surface decreases which may be useful during photocatalysis, because the amount of catalyst impurity decreases. The peak at 1452 and 1548cm-1 in the spectrum of undoped TiO2 (Figure 3i) is attributed to symmetric (υ(COO-)s) carboxylate stretch and asymmetric (υ(COO-)as) carboxylate stretch, respectively. No observable change in both the carboxylate peak intensity were recorded in the doped samples, which means that Fe dopant is not interacting with the titanium precursor or carboxylic acid, the condensation pathways are not altered in the sol-gel process, thus resulting in a strong TiO2 oligomer network and this may be the reason that Fe3+ doping favors anatase phase formation. The difference in vibrational frequencies, ∆υ = υ(COO-)as – υ(COO-)s, observed in transitional metal carboxylate compared to ∆υionic, gives the guidelines regarding the binding mode of carboxylate groups with metal oxide surfaces.

22, 23

In most of the transition metal carboxylate, monodentate mode of

binding is observed, ∆υ > ∆υionic while bridge type of coordination is formed, ∆υ < ∆υionic. Similar observation is confirmed for acetate group absorbed on TiO2, for acetic acid, ∆υionic is found to be 137cm-1

23

. In the present analysis for undoped and Fe-doped TiO2, ∆υ = 96cm-1


Page 13 of 33

indicating that acetate group is chelated with titanium precursor in bidentate bridge mode

24

.

When acetic acid reacts with titanium isopropoxide, the OPri (OR) groups are preferentially hydrolyzed and result into the formation of titanium alkoxo-acetate, the bridging acetate ligands are not hydrolyzed and remains coordinated with titanium throughout much of the condensation process. Thus, the CH3COO- ligands effectively alter the condensation pathway favoring the formation of linear polymer composed of edge-shared octahedra, means anatase formation 25. TFe25 TFe50 TFe100 TFe200 T

(ii)

1000

1630

3148

1600 2200 2800 Wavenumber (cm-1)

3700

2366

1548 1340 1452 1656

400

TFe25 TFe50 TFe100 TFe200 T

% Transmittance

(i)

% Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3400

4000

400

1000

1600 2200 2800 Wavenumber (cm-1)

3400

4000

Figure 3. FTIR spectra of the indicated samples calcined at (a) 300 and (b) 500oC. The FTIR spectra of the samples calcined at 500oC are shown in Figure 3ii, three major changes were recorded with increase in calcination temperature, such as; (a) major decrease in the intensity of carboxylate stretches due to vaporization of the acetate group from catalyst surface has occurred, (b) the weak peak at 1656 cm-1 (Figure 3i) is shifted to 1630cm-1 attributed to OH stretching and deformation mode of H2O indicating the adsorbed hydroxyl group to catalyst surface is promoted. This may be due to the decrease in acetate linkage with titanium atoms. (c) A weak peak appears at 3700cm-1, which represents terminal hydroxyl (bound to one Ti4+ cation) group

26

. The broad peak in the range (2800-3600cm-1) attributed to bridging



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 33

hydroxyl (bound to two Ti4+ cations) group is still maintained even at 500oC. Thus, an enhanced photocatalytic activity from the prepared powders is expected, because of increased presence of OH groups at the catalyst surface. The OH groups are main scavenger of the photogenerated holes resulting in the formation of hydroxyl radicals (OH•) required for the decomposition of organic pollutants. 3.3. UV/vis diffuse reflectance spectroscopy. UV/vis spectra of undoped and doped TiO2 with different iron content calcined at 300 and 500oC are displayed in Figure S1. The UV/vis spectra showed an increase absorption red shift with the increase in amount of Fe dopant, in consistency with the changes in the color of the powders from white to yellow to deep brown, by simple visual inspection. The incorporation of Fe3+ dopant into the TiO2 crystal lattice result in a lower band gap, the higher the concentration of Fe3+ incorporated, the more number of photons will be absorbed in the longer wavelength range. Thus, the utility rage of light will be widened, which in turn may considerably enhance the TiO2 photocatalytic activity under visible light illumination. The reflectance data of DRS spectrum in combination with the modified Kubelka-Munk (K-M) function was used to measure the indirect band transition of the prepared samples. The KM plots ((αhυ)1/2 is plotted versus photon energy, hυ (eV)) of the powders calcined at 300 and 500oC are shown in Figure 4i-ii, respectively. The optical band gap was determined by extrapolating the linear portion of the spectra until it intersect hυ axis (x-axis) at Eg, the band gap values are given in Table 1. Undoped TiO2 calcined at 300oC has an absorption edge at 400nm (Figure S1), this wavelength corresponds to the band gap of 3.1eV (Figure 4i). When TiO2 is doped with Fe the absorption edge smear out into the visible region, with increase in Fe concentration the absorption edge become more apparent and shifts further into the visible region. The absorption edge corresponds to the electron transfer from the valance band (VB) to the conduction band (CB). Since in Fe3+ the 3d orbital is half filled, when doped with TiO2, the empty Eg state is near the bottom of the conduction band, while the occupied t2g state of Fe locates at the top of valance band 1. As observed Fe doping leads to band gap narrowing, this may be due to the upward shift in the VB edge or downward shift in TiO2 CB edge or both depending on Fe3+ content. Compared to undoped TiO2, Fe doped powders (for example TF25, Figure S1) showed two broad absorption bands; one ranges from 350-455nm attributed to the excitation of 3d electrons of Fe3+ to the TiO2 CB (charge transfer transition), second ranges from 480-530nm ascribed to the d-d


Page 15 of 33

transition of Fe3+ (2T2g → 2A2g, 2T1g) or to the charge transfer transition between interacting iron ions via conduction band (Fe3+ + Fe3+ → Fe4+ + Fe2+). 19, 27 This implies that Fe3+ doping in TiO2 by the current synthesis method induces electronic states (Fe4+ and Fe2+) that are spread across the band gap of TiO2. (i)

(ii) 4

3

(αhv)1/2

3 (αhυ)1/2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 (a) (b) (c) (d) (e)

(a) (b) (c) (d) (e)

2

1 1

Eg

0 1.6

2.1

2.6

3.1

3.6 hυ

Eg

0 4.1

4.6

5.1

5.6

1.6

2.1

2.6

3.1

3.6 hυ

4.1

4.6

5.1

5.6

Figure 4 Kubulka-Munk (K-M) spectra of the powders calcined at 300 (i) and 500oC (ii), the samples are (a) T, (b) TFe200, (c) TFe100, (d) TFe50 and (e) TFe25. The K-M spectra of the doped samples calcined at 500oC as depicted in Figure 4ii are further shifted towards the longer wavelength with increase in calcination temperature; this may be due to the decrease in the amorphous content and increase in the crystallinity of the doped samples powders. In the K-M plots (Figure 4i-ii), the straight portion of the spectra which is extrapolated to intersect hυ axis (x-axis) at Eg is known as absorption tail or Urbach tail and the energy associated with it is known as Urbach energy and is given by α* = αo exp((hυ - Eo)/Eu)), where α* is the absorption coefficient, αo and Eo are material properties, hυ is photon energy and Eu is the Urbach energy 28. Urbach energy values are calculated by plotting lnα* versus E (hυ) as shown in Figure 5; the reciprocal of the slope of the linear portion gives the value of Eu. As



shown in Table 1, the Urbach energy value increases with increase in Fe3+ concentration in TiO2 lattice, this clearly supports the argument that defect energy levels are formed in between the TiO2 VB and CB bands that result in the narrowing of the band gap. With doping level, the number of impurity levels below the CB and above the VB increase to such an extent that the band gap is shifted deep into the forbidden gap, thereby decreasing the effective band gap of TiO2. Moreover, the Urbach energy values of the samples are minimized with increase in calcination temperature and in effect decrease in crystal deformation through increase in crystallinity with more order structure, stability and particle size occurs. T R2

TFe50

-1.2 (ii)

= 0.994

-1.5

R2 = 0.995

lnα*

lnα*

-1.7

-2.5

-2.2

-0.1

TFe200

(iii) -0.6

R2 = 0.995

-1.1

lnα*

-0.5 (i)

-1.6 -2.1 -3.5 3.15 -0.5 (iv) -2

3.25

3.35 hυ

T R2

= 0.995

3.45

-2.7 2.89

2.99

hυ 3.09

TFe50

0.3 (v)

R2 = 0.999

3.17

3.27 hυ

3.37

3.47

TFe200 R2 = 0.998

-1.5

lnα*

lnα*

-3.5

-2.6 3.19 3.07 0 (vi)

-0.2

lnα*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

-0.7 -3 -5

-6.5 2.95

-1.2

3.15 hυ

-1.7 3.35 2.89

3.09

hυ

3.29

-4.5 3.49 2.66

2.86

3.06 hυ

3.26

3.46

Figure 5 Urbach energy curves of the indicated samples calcined at 300 (i,ii and iii) and 500oC (iv, v and vi). 3.4.

Photoluminescence

and

Electron

Paramagnetic

Resonance

Spectroscopy.

Photoluminescence (PL) spectroscopy is a non-destructive technique used to supply information about lattice defects and oxygen vacancies, charge trapping and diffusion and recombination of photogenerated charge carriers. In addition to the physicochemical properties (particle size, surface area, phase structure and composition, band gap, crystallinity, etc.) surface and lattice


Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


defects (oxygen vacancies, surface states, etc.) also plays a significant role in the electronic band structure and photocatalytic activity of anatase Fe-TiO2. Figure 6 display the room temperature PL emission spectra of undoped and Fe-doped TiO2 samples calcined at 500oC and excited at 320nm. The following peak were resolved; a weak UV shoulder at 396nm and a small violet peak at 410nm are attributed to band edge luminescence which is related to photoinduced electron-hole recombination in conduction and valance band of TiO2 1. The other three violet peaks appear at; 418nm is ascribed to the phonon assisted indirect transition from the edge to the center of the Brillouin zone (X1a to Г1b) 29, 437nm is attributed to self-trapped excitons localized on TiO6 octahedra and 449nm is related to band edge free excitons. Six blue emission at; 456nm is related to surface states, 460 and 466nm are ascribed to oxygen vacancy (F+ color center) associated with Ti3+ 31

30

centers and oxygen vacancy with two trapped electrons (F color centre)

, 470nm is ascribed to band edge emission or related to crystal defects such as oxygen

vacancies or interstitial sites in TiO2 or to the free O2- → Ti4+ charge transfer transition, 480nm is attributed to charge transfer transition from Ti3+ to oxygen anion in a TiO6-8 complex associated with surface oxygen vacancies and 491nm is due to bound excitons 32. Two green emission peaks at; 542nm may appears due to structural defects, which may be attributed to native defects and impurities such as Ti interstitials and oxygen vacancies or from the recombination process of the excited holes and electrons in a self-trapped excited states

31

and 558nm peak might be

associated with transitions of electrons from the conduction band edge to deep trap holes owing to oxygen vacancies. An orange emission peak at 611nm is associated with the electron transition from the F+ center to the acceptor level just above the valance band, hydroxyl (OH) species form an acceptor level just above the valance band and may be responsible for the observed 611nm PL emission peak 30. Figure 6ii shows the PL spectra excited at 400nm, it was noticed that the position of the blue emission peak at 491nm, green emission peak at 542nm and orange emission peak at 611nm is not effected by the excitation wavelength but the green emission peak at 558nm disappear, implying that due to the lower number of absorbed photons back transferring of electrons from the conduction band to deep oxygen vacancies are hindered. On, further investigation the peak positions in the PL spectra of TFex powders are in agreement with the undoped TiO2, but the peaks intensities as well the PL intensity remarkably decreases with increase in Fe content in doped powders compared to undoped TiO2. This result shows that Fe doping has a significant effect on TiO2 electronic structure. Since PL signal is the



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Page 18 of 33

outcome of charge carrier recombination, lower PL intensity indicates lower recombination rate, this means that Fe3+ ions and its electronic states (Fe4+ and Fe2+) increase the probability of interfacial transfer of photoinduced electrons and holes. Moreover, the weak UV shoulder at 396nm and the peaks at 410, 418 and 437nm in undoped TiO2 are diminished with the addition of Fe dopant in TiO2 and an increase in its concentration, which further supports the shifting of the absorption spectra from the UV region to the visible region, and, also confirm that Fe3+ can trap charge carriers and prevent them from direct recombination. Electron paramagnetic resonance (EPR) spectroscopy, which is highly sensitive to detect paramagnetic species containing unpaired electrons, has been widely employed to confirm the existence of Ti3+ and oxygen vacancies in the bulk or at the surface

30

. As indicated in Figure

6iii, undoped TiO2 presents a weak EPR signals at a g-value of 2.03 and 1.99 which are attributed to O- anion radicals and or oxygen vacancies and Ti3+ cations, respectively. Under UV irradiations of TiO2 electron and holes are generated, the electron can be trapped and tend to reduce Ti4+ cations to Ti3+ state, and the holes oxidizes O2- anions for the generation of Otrapped holes 30. Another process for the formation of Ti3+ from Ti4+ is usually accompanied by the loss of oxygen from the surface of TiO2 due to calcination at elevated temperature (generally >400oC)

33

. This analysis verifies that TiO2 calcined at 500oC shows the existence of Ti3+ and

oxygen vacancies. Moreover, on Fe3+ doping the EPR intensity for both the peaks increases (the strongest value is obtained for TFE50, Figure 6iii), which means that Fe3+ is substituted for Ti4+ in the TiO2 lattice and as a result of charge compensation oxygen vacancies and Ti3+ centers are produced. Based on the above PL and EPR results, we conclude that Fe3+ dopant ions have been incorporated in TiO2 lattice, as a result introduces many defects states, including Ti3+ centers, oxygen vacancies and Ti interstitials into the TiO2. These defect states form donor levels in the electronic structure of TiO2 as a consequence enhances the visible light absorption, in addition they can also acts as charge trapping sites and in effect helps to reduce the electron-hole recombination process. Moreover, no additional peaks for iron impurities were observed in the modified powders, which eliminated the presence of surface adsorbed iron impurities. Furthermore, it can be expected that doped powder will have enhanced photocatalytic activity compared to undoped powders.


Page 19 of 33

(i)

(ii)

PL intensity (a.u.)

T TFe200

T TFe200 TFe100 TFe25 TFe50

PL intensity (a.u.)

TFe100 TFe25 TFe50

480

530

580 630 Wavelenght (nm)

(iii)

1.99

340

370

400

430

680

TFe50 TFe100 T

Intensity (a.u.)

437

410 418

2.03

396

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


460 490 520 Wavelength (nm)

550

580

610 2000

2500

3000

3500

4000

4500

5000

Magnetic field (gauss)

Figure 6. PL spectra of the indicated samples calcined at 500oC, excited at 320nm (i) and at 400nm (ii); (iii) EPR signals of O- anion radicals and or oxygen vacancies and Ti+3 cations measured at room temperature on TFe500, TFe100 and T samples. 3.6. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) study was performed to investigate the valencey state of the elements and surface defects in the prepared photocatalyst powders. The full XPS spectrum (Figure 7i) confirmed the existence of Ti, O and Fe elements in the TFe50 powder calcined at 500oC, for comparison the XPS spectrum of calcined undoped TiO2 is also given in Figure 7i. The relative atomic percentage of the elements in the doped powder (TFe50), estimated from the XPS analysis were determined to be about 38.3, 50.2, 6.5 and 5.0 at.% for titanium, oxygen, carbon and iron, respectively. To evaluate the chemical states of the iron dopant and the surface defect states (Ti3+, F+) Ti 2p, O 1s and Fe 2p core level spectra were measured as shown in Figure 7ii-iv. All the binding energies were measured in reference to the C 1s peak (284.8eV) which is attributed to the adventitious carbon from the XPS instrument itself. The Ti 2p core level yields two major characteristic doublets for Ti 2p3/2 and 2p1/2 at binding energies of 458.9eV and 464.7eV,



respectively. After the Ti 2p peak deconvolution the 463.0eV and 456.8eV peaks are attributed to +3 while the peaks at 464.3eV and 458.6eV are assigned to +4 valance state of Ti

32

. The

presence of Ti3+ species implies that that Ti4+ ions occupy electrons from nearby oxygen vacancies and transform to Ti3+ ions yielding F+ centers 30. This result is fully consistent with the photoluminescence (PL) and eelectron paramagnetic resonance (EPR) studies and provide rather direct evidence that Ti3+ accompanied with oxygen vacancies are the dominant defects in the prepared powders. The O 1s peak (Figure 7iii) can be deconvoluted into four peaks, the intense peak at about 529.9eV is attributed to the Ti-O band while the other three oxygen peaks are assigned to the absorbed hydroxyl groups (OH, 530.4eV), chemiadsorbed oxygen (O, 531.4eV) and surface hydroxyl groups (OH, 532.6eV). The Fe 2p spectrum (Figure 7iv) show the presence of two characteristics peaks at 710.9eV and 724.9eV which is for Fe 2p3/2 and 2p1/2, respectively 34

. After deconvolution both the peaks (Fe 2p3/2 and 2p1/2) can be resolved into three peaks at

binding energy position of 711.2eV and 725.1eV attributed to Fe3+, 709.1eV and 723.5eV corresponds to Fe2+ and 710.5eV and 725.6eV attributed to Fe4+

35

. In summary, based on the

XPS results it is obvious that iron in the prepared powder is in the form of Fe(+3) state that can substitute Ti4+ cations on incorporation in TiO2 lattice. O 1s

(i)

(ii)

Ti 2p3/2

Ti 2p

Intensity (a.u.)

Intensity (a.u.)

Ti 2p TFe50

Fe 2p

T

Ti +4 Ti 2p1/2 Ti +4 Ti +3

C 1s

(iii)

1000

O 1s

526

800 600 400 Binding energy (eV)

Ti +3

200

468

466

464

462 460 458 456 Binding energy (eV)

(iv) Fe 2p Ti-O

chemiadsorbed O surface OH

530 532 Binding energy (eV)

534

735

Fe+4

454

452

Fe 2p3/2 Fe 2p1/2

O-H

528

0

Intensity (a.u.)

1200

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

Fe+3

Fe+3 Fe+4 Fe+2

Fe+2

730

725

720 715 710 Binding energy (eV)

705

700

Figure7 XPS investigation of photocatalyst powder calcined at 500oC; (i) survey spectra of TFe50 and T, high resolution spectrum of (ii) Ti 2p, (iii) O 1s and (iv) Fe 2p in TFe50 powder.


Page 21 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.7. Photocatalytic Activity. The photocatalytic activity of the prepared powders and commercially available Degussa P25 were determined against the model pollutants; methylene blue (MB) and 4-chlorophenol. Figure 8i-ii illustrate the UV degradation of MB with synthesized powders calcined at 300 and 500oC, while the visible light decomposition of 4-CP with TFex powders calcined at 500oC is shown in Figure 8iii. In general it was noticed that doped powders calcined at 500oC showed enhanced activity than undoped TiO2 and the catalyst activity increases with increase in dopant content in TiO2 up to the molar ratio of 50, implying that the powder TFe50 showed highest activity under both UV and visible light irradiations. Moreover, experimentally (Figure 8) it was noticed that the decomposition of both MB and 4-CP was not affected by applying direct UV and or visible light in the absence of photocatalyst (photolysis) and any change in the pollutant concentration can be ascribed only to the photocatalysis. The enhancement in photocatalytic activity for the doped powders prepared by the present method can be attributed to several factors, such as; small crystal size via increase in specific surface area, narrowing of band gap, increase in crystallinity and surface chemisorbed water and hydroxyl groups as well as the different defect structure (Ti3+ species and oxygen vacancies) and dopant electronic states (Fe2+, Fe4+). XRD analysis showed that the crystallite size of Fe-doped TiO2 is smaller than undoped TiO2, which can signify the fact that the presence of Fe3+ ion in the reaction media might be used to control the crystallite size of TiO2. Reduction in particle size with increase in Fe3+ content led to high surface area (S1.2), which is significant for their high photocatalytic performance. Larger surface area allows an increased adsorption of pollutant molecule on the active surface sites. Decrease in the band gap result in greater absorption of photons which is beneficial for the production of electrons and holes required for the photocatalytic reactions. The current preparation method also enables to synthesize doped TiO2 powder showing high degree of hydroxyl groups and chemisorbed or physisorbed water molecule at the surface, which are main scavengers for the photoinduced holes results in the formation of hydroxyl radicals (OH▪). To verify the production of major reactive species, hydroxyl radicals, was monitored by the PL method using terephthalic acid (TA) as the fluorescence probe into the aqueous solution 10. TA reacts easily with the hydroxyl radicals to generate fluorescence hydroxyl terephthalic acid (TAOH). Thus, to quantify the amount of hydroxyl radicals the fluorescence of TAOH is measured at 425nm, a higher fluorescence intensity of TAOH is consistent with the increased



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Page 22 of 33

generation of hydroxyl radicals. Figure 9 shows the TAOH fluorescence intensity in suspension solution with the prepared powders as a function of UV light illumination time. It can be observed that the TFe50 sample produced large amount of hydroxyl radicals compared to other doped and undoped ones, which is in agreement with the photocatalytic activity results shown in Figure 8. This result indicates that a gain in the light absorption due to the decrease in the band gap of Fe-doped powders helps the photogenerated carriers for producing hydroxyl radicals, which results in the enhanced decomposition of methylene blue and 4-chlorophenol. Moreover, as depicted in DRS spectra, the presence of impurity energy levels below the conduction band edge and above the valance band edge influence the photoreactivity of TiO2 since the metal (Fe3+, Fe2+ and Fe4+) ions can acts as electrons and holes trapping centers altering the electron-hole recombination rate 1. In addition, PL, XPS and EPR studies confirms the presence of Ti3+ species and oxygen vacancies formed as a result of charge compensation due to the substitution of Fe3+ ions for Ti4+. Ti3+ centers are chemically active, long lived and are considered to be an important reactive agent for many adsorbates (such as; O2 and water) because it influence the surface chemistry of TiO2, therefore many surface reactions are influenced by these point defects

36

. Moreover, the coupling of oxygen vacancies with Ti3+

centers has also the capability to inhibit the photoinduced charge carrier recombination as well as to improves the visible light absorbance in the wavelength range of 400-520nm shown that undoped TiO2 possess n-type semiconducting behaviour

37

33

. Also, it is

, the change in its

semiconducting behaviour to p-type by doping with Fe3+ will result to inhibit the photoinduced charge carrier recombination. Therefore, the observed enhanced photocatalytic activity of the doped powders may also be attributed to the hetero-unions formed between n-type TiO2 and ptype doped metal oxide. It was also noticed that further addition of dopant ions beyond the molar ratio (Ti:Fe = 1:50) result in the decrease of photocatalytic activity as observed with TFe25 powder (Figure 8iiii). This means that dopant concentration is in excess of the optimum level, the TiO2 crystal lattice become saturated with Fe3+ ions, thus the excess dopant combines with TiO2 as a separate phase that exhibit a detrimental effect on catalyst activity. Moreover, at high dopant concentrations, due to the decrease in the distance between trapping sites, the recombination rate of the electron-hole pair increases and competes with the redox reactions occurring at the surface of the photocatalyst causing the photocatalytic activity to decrease 27, 38.


Page 23 of 33

100

90

(ii)

80

90

70

80

% MB degradation

% MB degradation

(i)

60

50

40

70 60 50 40

30 30 20 20 10 10 0 0 T

TFe200 TFe100 TFe50 TFe25

3 (iv)

1 (iii)

2

ln(C/Co)

Photolysis TFe200 TFe100 TFe25 TFe50

0.8

P25

Fe-TiO2 photocatalyst

Fe-TiO2 photocatalyst

TF50

R2 = 0.995

TF25

R2 = 0.996

TF100

R2 = 0.998

TF200

R2 = 0.994

Photolysis

R2 = 0.922

1

0.6

C/Co

0 0

2

0.3

0.4

4 6 Irridiation time (min)

8

10

(v) 0.2

kapp (min-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

0.1

0

0 0

2

4

6

8

10

12

200

Irridiation time (min)

100 50 Fe content (x)

25

Figure 8 UV light degradation of MB in the absence (photolysis) and in the presence of photocatalyst calcined at (i) 300 and (ii) 500oC; (iii) visible light degradation curves of 4-CP in the absence (photolysis) and in the presence doped photocatalyst calcined at 500oC, (iv) plot of ln(C/Co) versus irradiation time and (v) the dependence of rate constant on iron (Fe3+) concentration in TiO2.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

Langmuir Hinshelwood (L-H) kinetic model is most commonly used to explain the kinectics of photocatalytic degradation of organic contaminants in solution

39, 40

. For an ideal

batch reactor, the expression for L-H model is as follow;

−

=

(1)

where C is the concentration (mgL-1) of degraded molecule in solution, k is the reaction rate constant (mgL-1min-1) and Ke is the equilibrium concentration (Lmg-1) of the adsorbed molecule on the catalyst surface at the reaction temperature. The above equation (1) can be rewritten as

−

=

(2)

kapp is the apparent rate constant (min-1) equal to kKe. At low concentration, KeC

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