Synthesis of Fe3+ doped TiO2 photocatalysts for the visible assisted degradation of an azo dye

Colloids and Surfaces A: Physicochem. Eng. Aspects 375 (2011) 231–236

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Synthesis of Fe3+ doped TiO2 photocatalysts for the visible assisted degradation of an azo dye Panneerselvam Sathishkumar a , Sambandam Anandan a , Pitchai Maruthamuthu b,∗∗ , T. Swaminathan c , Meifang Zhou d , Muthupandian Ashokkumar d,∗ a

Nanomaterials & Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India Department of Energy [Chemistry-Interdisciplinary], University of Madras, Guindy Campus, Chennai 600 025, India Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India d School of Chemistry, University of Melbourne, Vic 3010, Australia b c

a r t i c l e

i n f o

Article history: Received 19 October 2010 Received in revised form 2 December 2010 Accepted 10 December 2010 Available online 23 December 2010 Keywords: Photocatalytic degradation Fe–TiO2 Acid red 88 Electron acceptors

a b s t r a c t The nano-sized, Fe3+ doped TiO2 photocatalyst was successfully prepared by a simple wet impregnation method with an attempt to extend the light absorption of TiO2 into the visible region and reduce the rapid recombination of electrons and holes. The transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS) and X-ray diffraction (XRD) results showed that the crystallite size of the as prepared Fe–TiO2 particles is in the nano regime and indicated that Fe3+ was substituted for Ti4+ in the lattice of TiO2 . The photocatalytic activities of the samples were evaluated for the degradation of an azo dye, acid red 88 in aqueous solutions under visible light irradiation. It was observed that the photocatalytic degradation of acid red under visible light irradiation in the presence of Fe3+ doped TiO2 (1.8 g/L) followed pseudo-first order reaction kinetics with a rate constant of 5.24 × 10−4 s−1 . Significant enhancement in the photodegradation rate was observed upon the addition of electron acceptors (peroxomonosulphate (PMS), peroxodisulphate (PDS) and hydrogen peroxide (H2 O2 )) to the reaction medium. The extent of photocatalytic mineralization of the target pollutant and the degradation products were analyzed by total organic carbon (TOC) analyzer and electrospray mass spectrometry (ESMS). © 2010 Elsevier B.V. All rights reserved.

1. Introduction The synthesis of new visible light active photocatalysts for the degradation of organic pollutants in aqueous solutions has attracted considerable attention [1–4]. To achieve this, researchers have used metal doped semiconductors because such metal deposition on semiconductor particles significantly reduces e−/h+ recombination in addition to the substantial enhancement of the electron transfer rate constant owing to the electrocatalytic effect of metal dopants [5–11]. Overall the benefits of surface modification of semiconductors by metal doping include (i) the inhibition of e−/h+ recombination by enhancing the charge separation; (ii) increasing the wavelength response range (i.e., the photocatalyst can be excited in the visible light region), and (iii) changing the selectivity or yield of a particular product [12]. Hence, the mechanism and kinetics of the process of small metal particles deposition

onto the semiconductor surfaces have been extensively studied by many researchers [12–15]. Amongst a variety of transitional metals, iron has been considered to be an appropriate candidate due ˚ is similar to that of Ti4+ to the fact that the radius of Fe3+ (0.69 A) 3+ ˚ (0.745 A) and hence Fe can be easily incorporated into the crystal lattice of TiO2 [12,16–20]. Furthermore, Fe3+ can provide a shallow trap for photo-generated electron and hole because the energy level of Fe2+ /Fe3+ lies close to that of Ti3+ /Ti4+ , favoring the separation of photogenerated electron–hole pair, and consequently resulting in an improved quantum efficiency [12,16]. In this study, we have investigated the potential use of Fe–TiO2 for the photodegradation of an azo dye, acid red 88 in the presence of visible light. The semiconductor photocatalyst (Fe–TiO2 ) was prepared by simple wet impregnation method. 2. Experimental 2.1. Materials

∗ Corresponding author. Tel.: +61 3 93475180; fax: +61 3 83447090. ∗∗ Corresponding author. Tel.: +91 44 22351269; fax: +91 44 22352494. E-mail addresses: [email protected] (P. Maruthamuthu), [email protected] (M. Ashokkumar). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.12.022

Titanium dioxide (Aeroxide TiO2 , P25) was a gift sample received from Evonik Degussa India Pvt. Ltd. with a specific surface area of 50 m2 g−1 and particle size 21 nm was used

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as received. Ferric nitrate nanohydrate (FeNO3 ·9H2 O) and acid red 88 (AR88) was purchased from Sigma–Aldrich and used without further purification to prepare Fe–TiO2 photocatalyst. Potassium peroxomonosulphate, a triple salt with the composition of 2KHSO5 ·KHSO4 ·K2 SO4 (commercially known as “Oxone”) from Janssen Chimica (Belgium) was used as received. Unless otherwise specified, all the reagents used were of analytical grade and the solutions were prepared using double distilled water.

012

024

Intensity (a.u)

232

(b)

2.2. Preparation of the photocatalyst (a)

Nano-size Fe–TiO2 was prepared by the impregnation method as reported by Bickley et al. [21]. Calculated amount of previously calcined TiO2 (at 400 ◦ C for 5 h) was mixed with an aqueous solution of FeNO3 ·9H2 O (2 at.%). The mixture was stirred for 48 h to allow the impregnation of iron into the titanium dioxide crystal matrix and the supernatant was evaporated by heating the mixture at 100 ◦ C over a time of 24 h. The resultant product was calcined at 550 ◦ C for 5 h. 2.3. Characterization techniques The surface morphology, particle size, and the various parameters of the photocatalyst powders were analyzed by XRD (Philips PW1710 diffractometer, Cuk␣ radiation, Holland) and TEM (recorded using TECNAI G2 model). The diffuse reflectance spectra of the samples were recorded in the wavelength range 200–800 nm using a UV–visible spectrophotometer (T90+, PG instruments, UK) equipped with an integrating sphere accessory. BaSO4 was used as a reference. The surface area of the samples was measured with the assistance of Flowsorb II 2300 of Micrometrics, Inc. The TOC was analyzed using a Shimadzu Total Organic Carbon Analyzer (Shimadzu TOC-VCPH model). Prior to analysis, the instrument was calibrated with potassium hydrogen phthalate for TC analysis, and sodium carbonate/bicarbonate of different concentrations was used as standards to get the reproducible results for TIC. TOC0 is the TOC measured after the equilibrium adsorption of the dye on the Fe–TiO2 surface and TOC obtained at various irradiation times is denoted as TOCt . The degradation products were analyzed using ESMS. The mass spectrometer used was a Micromass QUATTRO 11 coupled to a Hewlett Packard series 1100 degasser. The instrument was calibrated using the automatic tuning procedure with respect to the parent compound as the standard. The mass range scanned was m/z 50–500 and several spectra were obtained across each chromatographic peak. All mass spectra were the averages of 3–4 spectra obtained across the top of each chromatographic peak with background noise subtracted. The mobile phase consisted of 50/50 (acetonitrile/water). The flow rate of the solvent was 0.03 mL/min and the capillary voltage was set at 3.5 kV. 2.4. Evaluation of Photocatalytic activity The photocatalytic experiments were conducted under ambient atmospheric conditions and at natural pH (∼6.0) using 150 W tungsten halogen lamp ( ≥ 400 nm; intensity of incident radiation is 80600 ± 10 Lux measured using Extec, USA as the light source. In order to ensure the adsorption/desorption equilibrium, the solution was stirred for about 45 min in dark, prior to irradiation. The apparent kinetics of disappearance of the substrate, AR88, was determined by following the concentration of the substrate (max = 506 nm) using a UV–visible spectrophotometer (PG Instruments, UK) after a certain period of irradiation of the photocatalyst suspension and then filtered with a 0.45 ␮m polyvinylidene fluoride (PVDF) filter.

10

20

30

40

50

60

70

80

2(θ) degree Fig. 1. XRD spectra: (a) TiO2 and (b) Fe–TiO2 .

3. Results and discussion In order to investigate the changes in the crystal structure due to Fe doping, XRD measurements were carried out in the range of 2 = 10–80◦ for the Fe–TiO2 and bare TiO2 photocatalysts. The detected major peaks for the modified and unmodified samples appeared to be the same, but the intensities of the peaks were found to be significantly reduced in the case of Fe–TiO2 . In addition, peak broadening was noticed, which might be related to grain refinement due to doping. It is believed that part of Fe3+ penetrated into the semiconductor lattice [22] and Fe3+ ions were distributed uniformly in the interstices of semiconductor crystalline structure ˚ and Fe3+ (0.69 A) ˚ [23]. Based on the ionic radius of Ti4+ (0.745 A) it is reasonable to assume that Ti4+ can be replaced by Fe3+ without any significant alteration of the crystal structure. Hence, there is a possibility for formation of FeTiO3 by the intervalence charge transfer between Fe3+ and Ti4+ which is identified by a diffraction peak at 2 value 48.93◦ (JCPDS 75-1211) [24–26], in addition to the Fe deposits observed in TEM (discussed below). Further the XRD spectra of both the bare TiO2 and Fe–TiO2 samples showed a peak at 2 value 25.3◦ (JCPDS 21-1272) confirming the predominantly anatase structure (Fig. 1). Scherer formula was used to calculate the particle size of the as prepared photocatalyst and it was found to be ∼30 nm. Finally, no Fe2 O3 phase formation was detected in the present study which may be due to the low loading of iron (2 at.%) [27]. To obtain further insight into the nature of the metal dopant, a detailed TEM analysis was carried out for the prepared Fe–TiO2 nanocatalyst which showed direct information about the distribution of the dopant on the semiconductor surface (Fig. 2). The bright grey particles in the micrographs are obviously titania and the iron dopants are observed as the dark patches on the grey surface of titania. Thus, the XRD and TEM data indicate that the dopant is present as metal particles on the surface as well as Fe3+ in the interstitial sites. TEM images also reveal that the crystallinity of the photocatalysts was increased as indicated by the increased crystal size with well developed faces of Fe–TiO2 (Fig. 2a). Further, upon clear examination at higher magnification, the crystal lattice fringes were clearly visible for Fe–TiO2 (Fig. 2b and c). The average particle size of iron doped TiO2 formed is found to be ∼30 nm. The TEM micrograph results are consistent with the crystal size calculated by the Scherrer equation from the XRD pattern. The BET surface area for the Fe–TiO2 sample (38 m2 g−1 ) is significantly lower compared to bare TiO2 (57 m2 g−1 ). Fig. 3 shows the diffuse reflectance UV–visible spectra of the prepared photocatalysts Fe–TiO2 and bare TiO2 . Bare TiO2 shows absorption edge cut off around 400 nm suggesting that the band gap is ∼3.2 eV. While Fe doped TiO2 shows the absorption edge cut

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Fig. 2. (a) Representative TEM image of Fe–TiO2 nanoparticles, and (b) the corresponding high resolution TEM images.

(b)

Absorbance (a.u.)

1

(a)

0.5

0 200

300

400

500

600

700

800

Wavelength (nm) Fig. 3. . Solid state diffuse reflectance UV–visible spectra of (a) TiO2 and (b) Fe–TiO2 .

off around 600 nm suggesting the incorporation of the dopants at interstitial sites creating new energy levels between the valence and conduction bands. Thus, such red shift endorsed that incorporation of the iron dopant in the prepared nano photocatalyst (Fe–TiO2 ). In addition, the strong absorption of the Fe-doped samples in the visible range may also be due to an increase in the crystallinity of the photocatalyst, as evidenced by the intense reflections belonging to the change in the crystal phase of the semiconductor [28,29]. The photocatalytic activity of the nano-sized Fe–TiO2 (1.8 g/L) in decomposing acid red 88 (5 × 10−5 M) were evaluated under visible light irradiation by monitoring the decrease in absorption at  = 506 nm (Fig. 4). Fig. 5 describes the kinetics of decomposition of acid red 88 with various concentrations of Fe–TiO2 photocatalysts (0.6–2.6 g/L). Specifically, Fe–TiO2 nanocomposite at 1.8 g/L exhibits the highest decomposition rate constant (5.24 × 10−4 s−1 ) which is significantly higher than that of bare TiO2 (1.72 × 10−4 s−1 ) under similar experimental conditions. This may be due to the higher number of photons absorbed by the Fe–TiO2 nanocomposite. Thus, nano-sized Fe–TiO2 exhibited notably higher photocatalytic activity (about 3 fold increase) compared to pure TiO2 . As discussed earlier, Fe–TiO2 shows profound absorption over the entire visi-

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0.6

Fe-TiO2 with PMS 12

-4

-1

Rate constant (10 s )

Absorbance

0 - 60 min

0.3

8

Fe-TiO2 with PDS

Fe-TiO2 4

PMS alone

0.0 200

300

400

500

600

700

Fig. 4. UV–visible spectra acid red 88 at different intervals of time (0–60 min). [AR 88] = 5 × 10−5 M; [Fe–TiO2 ] = 1.8 g/L.

ble light range because of the introduction of new energy levels between its valence band and its conduction band by the dopant. To enhance the decomposition rate further, experiments were performed with external electron acceptors such as peroxomonosulfate (PMS), peroxodisulfate (PDS) and hydrogen peroxide (H2 O2 ) and the observed rate constants are in the following increasing order, i.e., PDS (k = 5.7 × 10−4 s−1 ) < H2 O2 (k = 7.3 × 10−4 s−1 ) < PMS (k = 13.6 × 10−4 s−1 ) (experimental conditions: Dye [5 × 10−5 M]; Fe–TiO2 = 1.8 g/L and [Electron acceptors] = 3 × 10−4 M) (Fig. 6). The reason for such enhanced rate constant is the generation of enhanced number of surface active radicals by the reaction between electron acceptors and the redox species formed during the visible light irradiation of Fe–TiO2 . The electron acceptor (PMS) may react with both e− CB and Fe-h+ VB whereas PDS and H2 O2 may react only with e− CB [30,31]. The removal of TOC usually takes longer time after the disappearance of the substrate color. Hence TOC removal was monitored even after color removal at 1 h by extending the experiment up to 9 h (Fig. 7). With Fe–TiO2 and PMS, about 82% of the TOC was removed. In contrast, with bare Fe–TiO2 discoloration and mineralization of acid red 88 took place at a much slower rate and TOC was reduced only by 27%. This also supports the generation of enhanced number of surface active radicals by reaction between electron acceptors and Fe–TiO2 .

H2O2 alone

Fig. 6. Photocatalytic degradation efficiency of Fe–TiO2 in the presence and absence of electron acceptors. [AR 88] = 5 × 10−5 M, [Fe–TiO2 ] = 1.8 g/L and [electron acceptors] = 3 × 10−5 M.

In order to identify the degradation products, ESMS analysis was employed. The ESMS spectrum was taken at the negative mode showed peaks at m/z values at 144, 182, 201, 223 and 378 and the possible structure to match the above mass values are presented in Table 1. The sharp peak identified at m/z = 378 correspond to the parent compound 4-(2-hydroxy-naphthalene1-ylazo)-naphthalene-1-sulphonic acid (I) with the loss of one sodium ion. The hydroxyl radicals generated at the surface of photocatalyst breaks the azo linkage first [32] and the resulting products were identified as 4-amino naphthalene sulfonic acid (II; m/z = 223) and napthalene-2-ol (III; m/z = 144). On further analysis of the photocatalytically treated samples revealed a peak at m/z = 201 corresponding to either 2-sulfobenzoic acid (IV) or 3-sulfobenzoic acid (V) since both have the possibility for the formation as an intermediate in the photocatalytic degradation of AR 88. On the other hand 4-hydroxy-phthalic acid (VI) was detected as another intermediate at m/z = 182. The final sample of photocatalytic degradation of acid red 88 (more than 6 h) did not show any such peaks and hence it is assumed that the compound was completely mineralized.

1.0

0.8

TOCt\TOC0

0.3

0.2 -ln (C/C0)

PDS alone

0

800

Wavelength (nm)

Fe-TiO2 with H2O2

0.6

0.4

Fe-TiO2 PMS+ Fe-TiO2

0.1

0.2

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0

0

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30 40 Time (min)

50

60

Fig. 5. Effect of Fe–TiO2 amount (() 0.6 g/L; () 1 g/L; () 1.4 g/L; (×) 1.8 g/L; (*) 2.2 g/L; () 2.6 g/L) and TiO2 () 1.8 g/L on degradation kinetics [AR] = 5 × 10−5 M.

3

6

9

Time (h) Fig. 7. Comparison of photocatalytic mineralization of acid red 88 in the presence of Fe–TiO2 nanocatalysts with/without the electron acceptor (PMS). [AR 88] = 5 × 10−5 M, [Fe–TiO2 ] = 1.8 g/L and [PMS] = 3 × 10−4 M.

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Table 1 The possible degradation products formed during the photocatalytic decomposition of AR 88 in the presence of Fe–TiO2 .

SO3H

SO3H OH

N N HO NH2

m/z = 223

m/z = 378 4-(2-Hydroxy-naphthalen-1-yla zo)-naphthalene-1-sulfonic acid

m/z = 144

4-Amino-naphthalene-1-sulfonic acid II

I

Naphthalen-2-ol III

O

SO3H

SO3H

COOH

HO OH OH

COOH O

m/z = 201 3-Sulfo-benzoic acid IV

m/z = 201 2-Sulfo-benzoic acid V

4. Conclusions Nano-sized Fe–TiO2 photocatalyst was synthesized through a wet impregnation method. XRD analysis showed that there is a possibility for formation of FeTiO3 . TEM pictures showed the deposition of Fe particles on the surface of the semiconductor particles. The absorption of Fe–TiO2 photocatalyst in the visible region of the spectrum was substantially increased due to iron doping, which was responsible for the notably higher photocatalytic activity of the Fe–TiO2 photocatalyst compared to bare TiO2 . Both the degradation and mineralization efficiencies were significantly increased in the presence of external oxidizing agents, such as PMS due to the efficient utilization of the redox species generated during the visible light irradiation of the Fe–TiO2 photocatalyst.

Acknowledgement The authors thank DST, India and DIISR, Australia for the financial support from Indian–Australian strategic research fund (INT/AUS/P-1/07 dated 19 September 2007).

m/z = 182 4-Hydroxy-phthalicacid VI

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