Structural, optical properties and photocatalytic activity of Fe3+ doped TiO2 thin films deposited by sol-gel spin coating

Surfaces and Interfaces 17 (2019) 100368

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Structural, optical properties and photocatalytic activity of Fe3+ doped TiO2 thin films deposited by sol-gel spin coating

T

⁎

Durgam Komaraiah , Eppa Radha, J. Sivakumar, M.V. Ramana Reddy, R. Sayanna Department of Physics, University College of Science, Osmania University, Hyderabad- 500007, Telangana, India

A R T I C LE I N FO

A B S T R A C T

Keywords: TiO2:Fe3+ thin films Surface area HR-TEM Raman modes Bandgap Photoluminescence Photocatalytic activity

Nanocrystalline TiO2: Fe3+ films were deposited using a simple spin coating technique. The effect of Fe3+ on spectroscopic features and photocatalytic activity of films were studied using X-ray diffractometer (XRD), scanning electron microscope (SEM), Energy dispersive X-ray (EDX), high-resolution transmission electron microscope (HR-TEM), Raman spectrometer, UV–Vis and fluorescence spectrophotometer. The XRD of all the TiO2: Fe3+ films exhibits a decrease in the crystallite size with an increase in Fe3+ concentration. The Fe3+ doped TiO2 nanocrystals exhibited a high specific surface area. The HR-TEM images of 1% and 7% Fe doped TiO2 thin films confirmed the average particle size is about 14 and 8 nm respectively. The Raman modes of vibrations present in the TiO2:Fe3+ films also confirms the formation of the anatase phase. The optical bandgap energy of titania decreases from 3.32 to 2.57 eV. The luminescent emission intensity of TiO2: Fe3+ films decreased compare to pure titania film. Moreover, the photocatalytic activity (PCA) of TiO2:x%Fe3+ (x = 0, 1, 3, 5, 7 and 10) catalysts were determined by the decomposition of methylene blue (MB) under irradiation of visible light. The observed results revealed that the optimized Fe3+ doped TiO2 film displayed high PCA. The enhancement of PCA of the films is due to the effect of Fe doping.

1. Introduction Photocatalytic activity of TiO2 (Titania) has been attracting enormous attention in the view of its numerous applications [1-13]. Nanostructured TiO2 is considered as an excellent photocatalyst for the degradation of organic dye molecules in wastewater emissions from various industries, because of its high photosensitivity, strong oxidizing power, great stability, water-insoluble, chemical inertness, biocompatibility, relatively low-price, nontoxic and environmentally friendly features [2-4], but there are two serious disadvantages that limit the efficiency of photocatalytic activity of TiO2 using the light source in the solar spectrum (with 3–5% UV light) they are: (1) the large bandgap (3.0 eV of rutile, 3.2 eV of anatase) which suppress its photo-response in visible range due to this it is only active in the ultraviolet range, and (2) the fast recombination rate of photoinduced electron (e−)/hole (h+) pairs [14, 15]. When the visible light irradiated the synthesized novel catalysts exhibit maximum photocatalytic efficiency and minimum e−/ h+ pair recombination rates are one of the toughest challenges in photocatalysis. Therefore, it is of great interest to find ways to extend the optical absorption of TiO2 without decreasing the ability of catalyst and prevent the e−/h+ pair recombination rate. Numerous approaches were explored such as surface modification, ⁎

doping of metals and non-metal elements, Core/Shell nanostructurs, heterojunction nanostructures and heterogeneous composition [15–21]. Doping with the elements has been observed as a feasible process to reduce the bandgap with improved PCA. So far, many experts (scientists, researchers) reported that doping with non-metals including B, C, N, and S, metals such as V, Cr, Fe, Co, Ni, Cu, Zn, Zr, Mo, W, Ag, Pt, Au, and Sn [17, 21-35] could reduce the bandgap of titania nanoparticles. By doping the material with different kinds of elements decline the bandgap and improve the PCA of TiO2. Among all kinds of dopants, Fe3+ was chosen as a suitable dopant for titania because, it is easy to incorporate into titania due to the well matching of ionic radii with Ti4+ ions [36, 37]. Moreover, Fe3+ ions can act as shallow charge trapping centers in TiO2 lattice and expand the photocatalytic response of titania into the solar region. Zhu et al. [36] synthesized Fe-TiO2 nanoparticles by hydrothermal method with enhanced visible PCA for the degradation of XRG dye. Khan and Swati [37] prepared Fe3+ doped anatase TiO2 nanoparticles, which showed enhanced visible PCA for degradation of MB and 4-chlorophenol. Wang et al. [34] successfully synthesized nanotube arrays of Fe doped TiO2 with an improved photoelectrochemical response for MB degradation. Zhu et al. [38] synthesized nanocrystalline Fe doped TiO2, which exhibited increased visible PCA. Komaraiah et al. [39] synthesized Iron doped TiO2 nanoparticles,

Corresponding author. E-mail address: [email protected] (D. Komaraiah).

https://doi.org/10.1016/j.surfin.2019.100368 Received 17 April 2019; Received in revised form 25 July 2019; Accepted 31 July 2019 Available online 01 August 2019 2468-0230/ © 2019 Published by Elsevier B.V.

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deposited films were dried at 150 °C for 30 min. The spin coating process was repeated several times to obtain the desired film thickness. Finally, un-doped and Fe doped TiO2 films were annealed at 550 °C for 4 h in the closed furnace. The TiO2:x%Fe3+ (x = 0, 1, 3, 5, 7 and 10) thin films were named as TiO2, FTO1, FTO3, FTO5, FTO7, and FTO10 respectively. The Fe3+-doped titania films were characterized structurally by PANanalytical Xpert pro Phillips powder X-ray diffractometer with CuKα radiation (λ = 1.5406 Å). The particle size was measured from the HRTEM (JEM-2100). The morphological and compositional analysis of the films were studied using ZEISS EVO-18 SEM together with EDX. The vibrational characterization (Raman spectra) of TiO2:Fe3+ nanoparticles were carried out using confocal Raman microscopy (WITEC ALPHA300 RA) with an excitation of 532 nm. The optical spectra of TiO2:Fe3+ films were noted by UV–VIS spectrometer (Shimazu, UV3092) in the range of 200–900 nm. TiO2:Fe3+ films were characterized by a fluorescence spectrometer (Horiba Jobin Yvon Fluorlog-3). The xenon lamp (150 W) is used as the excitation source in photoluminescence (PL) emission. Shimazu UV-3092 UV–VIS- spectrophotometer was used to record the absorption spectrum of the photocatalytic degradation of MB for both initial concentrations and for the irradiated sample.

which showed enhanced visible PCA for degradation of MB. However, the earlier reports mainly focused on the effects of Fe-doping on the PCA of TiO2 powders, and studies referring to Fe doped TiO2 films are rare. Wu et al. [40] and Singh et al. [41] synthesized TiO2 films and studied the effect of thickness on the PCA of TiO2 films, Komaraiah et al. [42] reported the effect of Ti-precursor concentration on the PCA of spin coated anatase TiO2 films, Kongsong et al. [43] fabricated nanorod array of TiO2 films through hydrothermal method and studied the effect annealing temperature on PCA of TiO2 films. Recently some efforts have been done on doped TiO2 films [23, 27, 44, 45] which showed enhanced visible PCA. Generally, the photocatalytic reactions takes place at the surface of the photocatalysts. Thus, nanosized titania particles which have a high surface-to-volume ratio and large specific surface area accessible for photocatalysis. As such, titania photocatalysts synthesized in different forms including mobilized nanostructures [15-21] and immobilized nanostructures like films deposited onto substrates [23, 27, 40-50], with each possessing its own merits and demerits. For example, TiO2 nanopowder exhibits a high catalytic response due to its large surface area accessible for catalysis [45]. However, the separation of nanopowder catalysts completely from suspension was difficult for recycling of photocatalytic process; this causes secondary contamination and catalyst loss. This, rises the health risks because of its nanotoxicity, as well as decreasing its photocatalytic activity in the recyclability and reusability examinations. On the contrary, titania films display great stability in an aqueous solution, resistance to photocorrosion, as well as excellent surface possessions [50] and also TiO2 film develop the surface with photo-induced hydrophilicity, which is a vital property of titania associated with photoinduced e−/h+ pairs. Owing to the above properties, immobilized TiO2 photocatalysts as thin films have attracted much attention and it improves cost-effectiveness due to lesser catalyst loss during cycling test for waste water treatment, as well as it improves photo-response in the visible light. For easy recovery, the catalysts needed to immobilize like films deposited onto substrates. The main objective of this paper is to enhance the optical absorption and the photocatalytic activity of titania films with Fe doping. The TiO2:Fe3+ thin films were synthesized using the spin coating technique and characterized the nanostructured TiO2: Fe3+ films by XRD, SEM, EDX, HR-TEM, Raman spectrometer, UV–Vis and a fluoroscence spectrophotometer.

3. Results and discussion Fig. 1 shows the XRD results of TiO2:x%Fe3+ (x = 0, 1, 3,5, 7 and 10) thin films. The XRD results confirmed that the Fe3+-doped TiO2 exhibits the tetragonal crystal structure with a single anatase phase and are agreed with the standard JCPDS No: 894921. The resolved diffraction peaks are labeled with the Miller indices and shown in Fig. 1. The (101) plane is preferential crystal growth orientation. The crystallite size (D) of TiO2: Fe3+ films was measured using Scherrer's formula [39, 46]. The average crystallite size decreased from 21.5 to 8.1 nm for increasing doping concentration (Table 1). Decrease of peak (at 2θ = 25.355°) intensity and widening of the peak width indicates that the decrease of crystallite size may be due to the incorporation of Fe3+ ions into cation sites of TiO2 lattice. The lattice parameters a = b and c of anatase TiO2: Fe3+ was calculated using the given relation 1 dhkl2

h2 + k2

l2

= + 2 [39,46], where dhkl = λ/2 sin θ is the interplanar a2 c spacing. The ionic radius of Fe3+ is 0.064 nm, which is very close to that of Ti4+ (0.068 nm), thus it is easy for Fe3+ to enter into the lattice of TiO2. From XRD results (Table 1), it is seen that there is no difference of the lattice parameter between undoped and Fe-doped TiO2 nanoparticles, which also implies that Fe3+ can enter into the lattice of TiO2 and substitute for the Ti4+ ion. The X-ray density (ρ = zM/NV) and the

2. Experimental TiO2:x%Fe3+ (x = 0, 1, 3, 5, 7 and 10) thin films were coated on ultrasonically cleaned glass substrates using a simple sol-gel spin coating technique. Titanium (IV) propoxide (Ti (OC3H7)4, 98% SigmaAldrich), Acetylacetone (AcAc, 99.9% Sigma-Aldrich), HNO3 (S. D. Fine Chem. Limited), and ethonol were used as a Titania (TiO2) pre-cursor, chemical modifier, catalyst, and solvent respectively. Fe(NO3)3. 9H2O (99.0%, Sigma Aldrich) used as a Fe3+ dopant. The concentration of Fe3+ ions (atomic percent) can be defined as x=[Fe/(Fe+Ti)]. Initially, 0.5 ml acetylacetone (AcAc) was added to Titania (TiO2) pre-cursor and then added 20 ml ethanol according to our earlier report [46]. The stoichiometric amounts of dopant and 0.25 ml HNO3 were added above mixer and vigorous stirring at 60 °C temperature for 1 h. The transparent solutions were aged for one day at room temperature in the covered beakers to obtain the hydrolysis and polycondensation reaction. Before film deposition, the microscopic glass slides were washed with a liquid detergent, diluted HCl (HCl+DI water) and deionized (DI) water. Further, substrates were washed in an ultrasonic bath and acetone [51]. After ultrasonication substrates were dried at RT and heated them at 100 °C for 10 min and then cooled them naturally. The precursor solutions were used to deposit the films on substrates by SPIN COATING UNIT (SCU 2007), APEX INSTRUMENTATIONS CO. In the film deposition process, few drops of prepared solution were injected on to the substrate and set the fixed spin speed of 3000 rpm for 30 s. The

Fig. 1. XRD patterns of the Fe-doped TiO2 thin films. 2

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Table 1 Structural parameters of undoped and Fe doped TiO2 nanoparticles. Sample

TiO2 FTO1 FTO3 FTO5 FTO7 FTO10

2θ

25.355 25.345 25.340 25.320 25.297 25.279

(FWHM) (°)

0.4520 0.5507 0.5780 0.6175 0.9657 0.9835

D (nm)

18.0 14.8 14.1 13.0 8.3 8.1

Lattice parameter (Å) a=b

c

3.777 3.778 3.780 3.783 3.786 3.786

9.502 9.502 9.504 9.512 9.518 9.522

Strain ε × 10−3

Dislocation density (δ) Lines/ nm2

X-ray density ρ (g/ cm3)

specific surface area Sa × 105 (cm2/g)

2.965 2.390 2.509 2.684 4.201 4.282

0.00309 0.00456 0.00503 0.00601 0.01417 0.01452

3.914 3.915 3.918 3.918 3.919 3.926

8.516 10.36 10.86 11.78 18.45 18.67

Fig. 2. HR-TEM images of Fe-doped TiO2 films: FTO1 (a, b) and FTO7 (d, e). Selected Area Electron Diffraction (SAED) pattern of FTO1 (c), and FTO7 (f) thin films.

Raman mode at frequency 144 cm−1 indicates the anatase phase [54, 55]. There is no evidence in the Raman spectra related to impurity, rutile and brookite phase modes was found. These results are in good agreement with the HRTEM and XRD results. The intensity of the band at 144 cm−1 decreases and full width at half maximum [FWHM] increases with Fe doping, it may be due to the lattice distortion and decrease in the crystallite size. The phonon lifetime estimated for the Raman band at 144 cm−1 of Fe doped TiO2 thin films [23, 39] and were placed in Table 2. It is seen that the phonon lifetime reducing with an increase in the Fe amount due to the decreasing degree of crystalline nature and the formation of defect levels in the lattice of TiO2 due to the addition of Fe3+ ions [36, 39]. The optical transmission spectra (Fig. 5(a)) of TiO2:x%Fe3+ films revealed all the films showed the high transmittance. The variations in transmission spectra and absorption edge wavelength of TiO2 film with Fe doping can be ascribed to variances in grain size, a specific surface area, microstructure, and absorption of light. The band edge of TiO2 film with different Fe doping concentrations showing a significant red shift. Oscillations were created in the transmittance due to interference between air-film and film-substrate interface [46]. The thickness of films was measured using the Swanepoel envelope method [56]. The measured film thickness and the average refractive index was shown in Table 2. The direct and indirect bandgap of TiO2: Fe3+ films were determined from the linearity of the Tauc's plots [33, 46]. Fig. 5(b) displays the Tauc's plots were plotted with (αhυ)1/2 versus hυ of TiO2:Fe3+

specific surface area (Sa = 6/D) of TiO2: Fe3+ nanocrystals were determined [23, 39]. Evaluated specific surface area, and the density of titania nanocrystals were shown in Table 1. These results indicate that the surface area of TiO2 nanocrystals increases with Fe doping concentrations. The HR-TEM has been used for the determination of the pore size and shape of TiO2: Fe3+ films. Fig. 2 shows the HRTEM and SAED images of FTO1 and FTO7 films. As observed from the HRTEM pictures of FTO1 and FTO7 consists of spherical like shape particles with the average particle size of 14 nm and 8 nm respectively. The selected area electron diffraction (SAED) pattern of these HR-TEM images of FTO1 and FTO7 confirms that the grain walls are made up of nanocrystalline that demonstrate typical electron diffraction concentric circles. The definite diffuse diffraction circles of the SAED images are labeled with the Miller indices of anatase TiO2 nanocrystals [52, 53]. These definite diffuse diffraction circles are ascribed to the polycrystalline nanoparticles. The SEM pictures of iron-doped TiO2 films are shown in Fig. 3(a, b, c). The SEM micrographs revealed the formation of crack free films with good adherence and uniform throughout the substrate. The elemental analysis of the TiO2: Fe3+ films carried out using EDX. Fig. 3(a′, b′, c′) shows the EDX pattern of the iron doped TiO2 films. The presence of oxygen, Ti and Fe elements indicate the formation of TiO2: Fe3+ films. Fig. 4 illustrates the Raman spectra of TiO2: Fe3+ thin films and the characteristic Raman vibrational modes centered at 144 (Eg), 196 (Eg), 394 (B1g), 516 (A1g + B1g), and 638 cm−1 (Eg). The strong and sharp 3

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Fig. 3. SEM micrographs and EDX of the FTO1 (a, a′), FTO5 (b, b′), and FTO7 (c, c′) thin films. Table 2 Refractive index (n), Thickness (nm), direct, indirect band gap energy, FWHM (Γ), and phonon life time (along 144 cm−1) of Fe doped TiO2 thin films. Sample

n

Thickness (nm)

Direct band gap (eV)

Indirect band gap (eV)

Γ cm−1

Phonon life time τ ( × 10−13 s)

TiO2 FTO1 FTO3 FTO5 FTO7 FTO10

2.450 2.460 2.461 2.465 2.472 2.478

352 356 354 356 357 358

3.55 3.38 3.29 3.09 2.98 2.86

3.32 3.15 2.98 2.83 2.72 2.57

12.673 15.047 17.382 17.623 22.553 15.841

4.19 3.52 3.05 3.01 2.35 3.35

to be 3.55 eV, which is comparable to the values (3.33–4.03 eV) reported in the literature for anatase titania films [57, 63-67]. Moreover, indirect bandgap (Eind g ) for pure titania thin film was found to be 3.32 eV, which is comparable to the earlier reports [58-60, 67]. Generally pure TiO2 films showed higther bandgap compared to TiO2 powder nanoparticls. In this investigation, pure titania film shows higher babd gap compared to anatase titania (Eind g = 3.2 eV) [36, 39]. The indirect bandgap of Fe3+ doped titania films decreases from 3.32 to 2.57 eV. The redshift of band gap observed in Fe3+ doped films. The redshift of bandgap energy may be due to the doping of Fe3+ ions can form a Fe3+ dopant level above the valence band (VB) of TiO2 [39, 67].

Fig. 4. Raman spectra of pure and Fe-doped TiO2 thin films.

films. The resultant bandgap of the films shown in Table 2. It is seen the bandgap decline with an increasing Fe3+ concentration compared to pure titania and similar behavior also reported in the literature [57-62]. The calculated direct bandgap for the pure titania thin film was found 4

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Fig. 6. PL excitation spectra of Fe-doped TiO2 thin films monitored at the emission peaks of 425 nm.

Fe3+ ions in TiO2 lattice effectively suppressed the undesirable e−/h+ pair recombination resulting in the more free electrons and holes participate in the photocatalysis, hence enhance the PCA. Photocatalytic activity (PCA) of TiO2: Fe3+films was evaluated by photodecomposition of MB dye in an aqueous solution of a concentration of 3 mg/L and 5 mg/L as a model pollutant. The visible light photocatalytic experiment was done according to our previous works [39, 46, 47, 75]. Fig. 8 shows the absorption spectrum of MB (5 mg/L, 25 mL) solution catalyzed over FTO7 film. The results are displayed in Fig. 9(a, b). It can be clearly shown that the FTO7 film exhibits more degradation efficiency than the other films. After 4 h light irradiation, about 96.7% of MB (3 mg/L) molecules are decolorized over the FTO7 film, while only 80%, 83%, 85%, 87% and 89% are decolorized over TiO2, FTO1, FTO3, FTO5, and FTO10 films respectively. It was clearly seen that the activity is increased with Fe-doping concentration, getting more efficient at 7%Fe and then decreased gradually due to at a higher concentration the distance between trapping sites decreases and thus increases the probability of recombination rate of the charge carriers [76]. H.J Lin et al. [58, 77] and Mathews et al. [60] prepared Fe3+ -doped TiO2 thin film catalysts, which showed the PCA increased with an increase in the doping concentration. Lin et al. [78] reported, the photocatalytic activity increases with increasing Fe3+ content and then decreases at high concentration. The first order kinetics in terms of the C Langmuir-Hinshelwood mechanism can be given as ln( C ) = − kapp t o [39,51], where Co and C denote the initial and photodegraded concentration of MB respectively, t, and kapp denote the light irradiation time, and reaction rate or apparent kinetic constant respectively. The C plot of − ln( C ) versus reaction time gives the linear plots as displayed o in Fig. 9(b). The kappvalue for TiO2: Fe3+ films was determined from the slope of Fig. 9(b) and placed in Table 3. It is seen that the decreasing decomposition reaction rate with increase in the MB doses due to the slower photo-oxidation rate as compared to rate of adsorption on catalyst surface, resulting in the increase of more adsorption sites on the photocatalyst. Fig. 10 shows the schematic illustration for the visible light photocatalytic process over TiO2:Fe3+ films. The possible mechanism for enhanced PCA is proposed as follows. Due to the Fe doping, the band edge shifted into the visible light, which would be favorable for the PCA under visible light. When the semiconductor TiO2:Fe3+ films were exposed with suitable light, electrons (e−) are excited to the CB from valance band (VB), thus electron (e−)/(h+) hole pairs were generated (Eq. (1)) [77]. The ionic radius of Fe3+ (0.64 Å) being slightly lesser than the ionic radii of the Ti4+ (0.68 Å) [39, 52, 57, 73], it is possible for the Fe3+ to substitute and occupy Ti4+ sites. From Table 1, it is

Fig. 5. Transmittance spectra (a), and plots of (αhυ)1/2 versus photon energy (b) of Fe doped TiO2 thin films.

The electronic transition from the modified Fe-dopant level to the conduction band (CB) can successfully enhance the photocatalytic activity in visible light region. The plots of PL excitation (Fig. 6) of TiO2: Fe3+films monitored at the emission peak of 425 nm. The excitation sharp peaks centered at 270, 306, 340, 362, 381 and 393 nm. Among all the peaks, the broad peak at 270 nm, and most intense peak at 340 nm and a peak at 362 nm were used to record the PL-emission spectra. Fig. 7(a-c) shows the emission spectra of TiO2: Fe3+ films which are excited at different wavelengths. Photoluminescence emission is generally a surface phenomenon and is involving the e−/h+ recombination because the emission is due to the de-excitation of electrons from an excitation state to its ground state with releasing of light energy [39, 68]. The emission peak at lower wavelengths from 398 nm to 399 nm corresponds to an indirect transition X1b → Γ3 [69, 70]. PL peak at 425 nm is related to indirect transition Γ1b → X1a [63]. The emission peaks of titania films located at higher wavelengths may be due to the oxygen vacancies associated with Ti3+ in anatase TiO2 [69, 70]. Moreover, the Fig. 7 revealed that the presence of visible emission peaks at 440, 476, 530, 573, and 608 nm are due to the oxygen vacancies with trapped electrons [69-74]. The emission band located at 494 nm is an indication of the charge transfer transition from Ti3+ to TiO62 − octahedra [39, 73]. Moreover, the low intense PL emissions were found in the TiO2: Fe3+thin films compare to TiO2 film, which denoted that the doping of 5

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Fig. 8. UV–Vis absorption spectra of methylene solution catalyzed at room temperature over FTO7 film.

adsorbed H2O or OH− ions on the surface could trap the surface holes and produce highly reactive hydroxide radicals (OH•) [36, 79], which could not only suppress the photoinduced electron-hole pair recombination rate, but also oxidize and adsorb more reactive substrates, and then improved its photocatalytic performance efficiently. The doped Fe3+ions reduce photoinduced electron-hole pair recombination rate due to the energy level Fe3+/Fe4+ (2.20 V vs normal hydrogen electode (NHE)) [80] photo-oxidation process (Eq. (2)) above the VB of anatase TiO2, develops charge carrier separation [36, 77]. The trapped surface holes in the photooxidation can migrate to the surface adsorbed hydroxy ions to generate hydroxyl radicals (OH•) (Eq. (3)).

Fe:TiO2 + hυ → Fe: TiO2 (h+vb + e−cb)

(1)

Fe3 + + h+vb → Fe4 +

(2)

Fe4 +

+

OH− (ads)

→

Fe3 +

+

OH•

3+

(3) 2+

Fe traps the photoinduced electron to form Fe Eq. (4)), and the trapped electrons transfer to the surface adsorbed O2 (Eq. (5)) or a neighboring Ti4+ ions (Eq. (6), ((7)) and thus yield superoxide radicals (O2•−) [36, 79]. The energy level of Fe3+/Fe2+ (0.771 V vs NHE) below the CB of anatase TiO2 [80], supporting to enhance the charge carrier separation and resulting in the decline of the (e−)/(h+) pair recombination [39]. The subsequent reactions Eq. (3) and ((5)) demonstrated that Fe3+ could act as electron-hole trapper [36, 39, 57, 67, 79]. As a result, the doping of suitable Fe3+ ions is favorable for the decrease of the photoinduced e−/h+ pairs recombination rate and favors the improvement of photocatalytic activity.

Fe3 + + e−cb → Fe2 +

(4)

Fe2 + + O2 (ads) → Fe3 + + O−2

(5)

Fe2 +

(6)

+

Ti4 +

→

Fe3 +

+

Ti3 +

Ti3 + + O2 (ads) → Ti4 + +

O−2

(7)

The high PCA of this thin films can also be related to its surface which has photoinduced super-hydrophilic property. However, at a very high doping concentration, unfortunately Fe3+ ions can performance as a charge carrier (photoinduced electrons and holes) recombination centers Eqs. (5), and ((8)-(10)) due to decrase the distance between trapping sites at a high concentration of Fe3+ ions and resulting in decline of the PCA.

Fig. 7. (a-c). PL emission spectra of Fe-doped TiO2 thin films with different excitation wavelengths.

found that there is no significant difference of the lattice constants between pure TiO2 and Fe doped TiO2 films, which also implies that Fe3+ can easily enter into the lattice of TiO2 and substitute for the Ti4+ ion. As a result, the substitution of Fe3+ to Ti4+ ion could create a charge imbalance and then more H2O/hydroxide ions would be adsorbed onto the surface of TiO2 catalysts for charge balance. These

Fe4 + + e− → Fe3 +

(8)

h+vb

(9)

Fe2 + 6

+

→

Fe3 +

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Fig. 9. (a) Photocatalytic activity of Fe-doped TiO2 thin films under visible light irradiation. (b) The first order kinetic fit for the degradation of MB in the presence of Fe-doped TiO2 thin films under visible light irradiation. . Table 3 photocatalytic reacting constant kapp of Fe doped TiO2 thin films. Sample

kapp (min−1) 3 mg/L MB

kapp (min−1) 5 mg/L MB

TiO2 FTO1 FTO3 FTO5 FTO7 FTO10

0.00616 0.00840 0.01068 0.01106 0.01418 0.01264

0.00232 0.00425 0.00493 0.00569 0.00768 0.00679

Fe2 + + OH•ads) → Fe3 + + OH−

(10)

The reusability of FTO7 photocatalyst is necessary for practical applications. Fig. 11 shows the repeatability experiment for decolorization of MB solution over FTO7 film. For every experiment, FTO7 film was reused for the degradation of a fresh MB solution (5 mg/L MB) under the same conditions. The repetitive experiments were carried out for 10 times, initially, the degradation rate of MB was 83.8% whereas the degradation rate for last round was 83.5% These indicated that the photocatalytic activity of FTO7 film did not show any significant loss in PCA, which emphasize the outstanding photochemical stability of the

Fig. 10. Shows the schematic illustration for the visible light photocatalytic process over Fe-doped TiO2:Fe+3 thin films.

7

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Fig. 11. Cycling runs in the photodegradation of MB in the presence of FTO7 thin film under visible light irradiation. .

Fe doped TiO2 photocatalyst. These highly reliable results have shown the excellent stability of FTO7 film.

4. Conclusions High-quality nanocrystalline Fe doped TiO2 films were coated using a simple spin coating technique. The effect of Fe doping on structural, optical properties and PCA has been studied. The average crystallite size of TiO2 decreased with Fe doping concentrations. The average particle size of 1% and 7% Fe doped TiO2 films was found to be 14 and 8 nm respectively, using the HRTEM images. The Raman blue shift was attributed due to the decrease in the crystallite size with an increase in Fe doping concentration. The bandgap of the TiO2 decreases from 3.32 to 2.57 eV by increasing in Fe doping concentration. The PL emission intensity of Fe3+ doped TiO2 thin films decreases with an increase in the dopant quantity. The enhanced PCA of TiO2: Fe3+ films was ascribed due to the high specific surface area, lower bandgap energy, expanded absorption in the visible light. The maximum photodegradation of MB dye obtained over FTO7 film. The PCA of TiO2 thin films indicates that the photodegradation efficiencies decreased with an increase in the Fe-precursor concentration for the decomposition of MB under visible light illumination.

Declaration of Competing Interest None.

Acknowledgments The authors thank the Head, Dept. of Physics, OU and UGC-SAPDSA-III for providing experimental facilities. One of the authors DK, thank the UGC, New Delhi for awarding SRF (201213-BSR-10196-72) under the RFSMS scheme and also the author MVRR thanks to the DST (SERB), New Delhi, file No: EMR/2017/002651 for proving financial support to carry out this work. MVRR also acknowledges OU DST PURSE-III/37/2018 for providing necessary funding.

Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.surfin.2019.100368. 8

Surfaces and Interfaces 17 (2019) 100368

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