Visible light improved, photocatalytic activity of magnetically separable titania nanocomposite

Chemical Engineering Journal 183 (2012) 349–356

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Visible light improved, photocatalytic activity of magnetically separable titania nanocomposite Azrina Abd Aziz a , Chee Kaan Cheng a , Shaliza Ibrahim a , Manickam Matheswaran b , Pichiah Saravanan a,∗ a b

Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Chemical Engineering, National Institute of Technology, 620015 Tiruchirappalli, India

a r t i c l e

i n f o

Article history: Received 8 September 2011 Received in revised form 3 January 2012 Accepted 3 January 2012 Keywords: TiO2 TiO2 /SiO2 /NiFe2 O4 nanocomposite Visible light Magnetically separable 2,4-DCP Degradation

a b s t r a c t A visible light improved, magnetically separable TiO2 nanocomposite was successfully synthesized with silicon dioxide (SiO2 ) as coating and supported on a permanent magnet Viz., nickel ferrite (NiFe2 O4 ). Thus synthesized photocatalysts was further characterized for its crystalline phase, particle size, surface morphology, inorganic composition, adsorption–desorption hysteresis, BET surface area, pore size distribution, magnetic hysteresis, saturation magnetization, coercivity, elemental composition, chemical state, electronic state and visible light absorption spectra analysis with respective techniques. The crystallographic peak and inorganic elemental composition revealed the structure and composition of pure and nanocomposite TiO2 . The prepared titania nanocomposite resulted in lower band gap energy (2.26 eV) and higher visible light absorption between 400 and 800 nm than that of pure TiO2 (2.76 eV). The photocatalytic activity was investigated with a recalcitrant phenolic compound namely 2,4-dichlorophenol (2,4-DCP) as a model pollutant under direct bright and diffused sunlight irradiation. An almost complete degradation of 2,4-DCP was achieved with an initial concentration of 50 mg/L for TiO2 nanocomposite in 90 min and 5 h under bright and diffused sunlight conditions. Similarly pure TiO2 resulted in a nearly complete degradation in 180 min under bright and ≈90% in 5 h under diffused conditions. Further the TiO2 nanocomposite was recovered under a magnetic field with a mass recovery ≈95%. The nanocomposite also exhibited improved remanence, saturation magnetization and coercivity property along with good stability against magnetic property losses for reuse. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The titanium dioxide (TiO2 ) semiconductor photocatalyst has been widely applied to various environmental applications including water, wastewater and air [1–3]. Various types of application specific photocatalytic reactor configurations have been proposed till date to degrade undesirable organics present in water and wastewater [2]. In general, immobilized reactor is preferred over slurry type reactor because the removal of nano-size titania particles requires an additional filtration adding up the treatment cost. Unfortunately, the immobilization leads to reduced reaction surface area per unit volume of the reactor [4–6]. In recent times, titania with magnetic property have been reported to ensure the recovery and reuse of the catalyst for the wastewater treatment [7]. Generally, photocatalytic reaction is conducted in a suspension of submicrometer-sized TiO2 [8]. Removing such fine particles from huge volume is not economical.

∗ Corresponding author. Tel.: +60 79677678; fax: +60 79675318. E-mail address: [email protected] (P. Saravanan). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.006

This presents a major drawback to the application of the photocatalyst for treating water and wastewaters. Hence TiO2 photocatalyst with ferromagnetic property has evolved as a mean to resolve such difficulty of separation from the treated water, simply by applying an external magnetic field [9–14]. Till date few reports have been published. But most of them focused on soft ferromagnetic materials, like magnetite (Fe3 O4 ) and maghemite (␥-Fe2 O3 ) [12,13,15–17]. Regrettably, the nano-sized core magnetic materials are easily oxidized and transform rapidly when the temperature is raised beyond 400 ◦ C [9,18]. Therefore, it is intrinsically difficult to produce titania photocatalyst without losses of magnetic property. Recently many extensive efforts have been made in the development of TiO2 photocatalyst that can efficiently utilise visible light [19–22], since the conventional TiO2 is photo active only under ultraviolet (UV) radiation and cannot excite under visible light spectrum [23]. In addition, a low quantum yield is also observed due to relatively high recombination rate of photo-generated electron–hole (e− /h+ ) pairs from TiO2 itself [24,25]. Only a very few electrons (e− ) in the conduction band (CB) have high enough energy that can overcome the potential barrier to reach the surface [26]. The bulk property of TiO2 resulted in increased recombination possibility of electron–hole (e− /h+ ) pairs during the photocatalytic

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reaction. To overcome both the issue, Wang et al. [27] suggested that doping can lower the energy below the conduction band (CB) of TiO2 . Thus the photo-generated electrons (e− ) in the conduction band (CB) can transfer to the surface via this doping energy level, without crossing over the potential barrier, and take part in photocatalytic process [27]. As a result, more photo-generated electrons (e− ) and holes (h+ ) can contribute to the photocatalytic reaction, resulting in enhancement of photocatalytic activity of the catalyst under visible light [27]. Furthermore, numerous efforts have been made such as surface modification [28–31], doping TiO2 with metal ions [6,32,33] and combining TiO2 with other semiconductors [34–36]. Sikong et al. [6] in their work prepared Fe doped TiO2 /SnO2 by sol–gel method and photoactivity was investigated for E. coli killing. The E. coli was completely killed within 90 min under UV radiation and almost 100% under visible light exposure [6]. Metal coating has proved to improve photocatalytic efficiency of a semiconductor under visible light spectrum. The deposition of noble metals (NM) like Pt, Au, Ag, Pd and Rh on photocatalyst surface will also improve the photocatalytic efficiency [37,38]. As the photo-generated holes (h+ ) react with adsorbed species, the electrons (e− ) might be accumulated on the semiconductor particles, leading to an increase of the recombination process. Oxygen (O2 ) is habitually used as electron scavenger and equal role could be fulfilled by metal deposits acting as electrons (e− ) trapping [39,40]. Hence the major objectives of the present work are to prepare visible light improved Titania nanocomposite photocatalyst with additional ferromagnetic property for reuse. The objectives were achieved by supporting a transition ferromagnetic oxide material namely nickel ferrite (NiFe2 O4 ), by simple modified sol–gel technique along with hydro-thermal method. The photocatalytic activity of the prepared catalysts was investigated by degrading a bio-recalcitrant and toxic pesticide pollutant viz., 2,4-dichlorophenol (2,4-DCP) under bright and diffused sunlight irradiation condition.

2. Materials and methods 2.1. Synthesis of photocatalysts All chemicals were of analytical grade. Milli-Q water (>18.2 M  cm) was used for all experiments. Titanium tetrabutoxide (Ti(OC4 H9 )4 ) (97%) was used as a precursor to synthesize TiO2 sol by partial hydrolysis and poly-condensation with water (H2 O). Nitric acid (HNO3 ) was used as catalyst and isopropyl alcohol (IPA) as a solvent. The titanium tetra-butoxide (Ti(OC4 H9 )4 ): water (H2 O) in a mole ratio of 1:2 was prepared approximately. The mixture then was stirred vigorously with a magnetic stirrer for more than an hour. Finally, it resulted in a coloured transparent solution and calcined in atmospheric air for an hour at 500 ◦ C. Nickel ferrite (NiFe2 O4 ) nanoparticle was synthesized by mixing iron (III) nitrate nanohydrate (Fe(NO3 )3 ·9H2 O) solution (3 M, 60 mL) and nickel (II) nitrate hexahydrate (Ni (NO3 )2 ·6H2 O) solution (1.25 M, 60 mL) in 1:2 (mole) for Ni2+ /Fe3+ . Potassium hydroxide (KOH) solution (6–7 M) was added slowly into the mixture solution until pH 9.5–10, followed by ferrous (II) chloride tetrahydrate (FeCl2 ·4H2 O) solution (1.25 M) under vigorous stirring. The pH value of it was adjusted to ca. 10.0 by the drop wise addition of prepared KOH solution. Then it was kept boiling and refluxing for 2 h under vigorous stirring. Finally, the synthesized NiFe2 O4 nanoparticles dispersion was repeatedly washed by centrifugation to prevent agglomeration of the nanoparticle. Silica (SiO2 ) coating was provided to NiFe2 O4 in order to avoid the influence of magnetization onto the photocatalytic property and vice versa. This was achieved by adding aqueous sodium

polyphosphate ((NaPO3 )6 ) solution (5%) using 5 g (NaPO3 )6 and 100 g of Milli-Q water. The aqueous solution was then added into 150 mL of NiFe2 O4 dispersion. As to get the mixed dispersion with 16 wt.% (NaPO3 )6 for NiFe2 O4 , 3.62 g of NiFe2 O4 was added into distilled water and made up to 150 mL dispersion. It was followed by the addition of 35 mL of sodium trisilicate (Na2 O·3SiO2 ) solution into the dispersion. The dispersion was sonicated for 15–20 min in ultrasonic water bath and followed by heating at 90–100 ◦ C on a magnetic stirrer provided with heater. The pH value of the dispersion was adjusted to ca. 10.0 by titrating sulphuric acid (H2 SO4 ) solution (5%) under vigorous stirring. Further stirring was carried out at 90–100 ◦ C to obtain a viscous dispersion. A thin silica layer was deposited on the NiFe2 O4 nanoparticles dispersion. The silica-coated NiFe2 O4 dispersion was washed by centrifugation and redispersed repeatedly with distilled water to prevent them from agglomeration. Finally TiO2 supported NiFe2 O4 magnetic nanoparticle was synthesized by dispersing 0.15 g of NiFe2 O4 dispersion into 1.0 g of TiO2 in Milli-Q water. The mixture was sonicated for 20 min, dried, grinded and calcined for 30 min at 500 ◦ C. 2.2. Characterization of synthesized photocatalysts The X-ray diffraction (XRD) analysis was performed with Bruker ˚ radiation, to D8 Advance diffractometer using CuK␣ ( = 1.5406 A) study the crystal structure and crystallinity of the photocatalyst. The average crystallite size was obtained using the Scherrer’s equation (D = k/ˇ cos ). A transmission electron microscope (TEM) (Philips CM-12) was employed to obtain catalyst size and structure at the nanoscale. The samples in ethanol were dispersed using an ultrasonicator (Starsonic, 35) for 15 min and fixed on carbon-coated copper grid. Samples were analysed with FESEM and energy dispersive X-ray spectroscopy (EDS) (Zeiss Auriga® ) to detect the surface morphology and characteristic X-ray excited by incident electrons. Brunauer–Emmett–Teller (BET) surface area, pore volume, and Barret–Joyner–Halenda (BJH) pore size distribution (based on nitrogen adsorption and desorption isotherms) were determined by Quantachrome Autosorb Automated Gas Sorption. Prior to determination the samples were degassed for 5 h at 150 ◦ C with nitrogen. The fine elemental composition and electronic structure was determined with an X-ray photoelectron spectroscope (KRATOS XPS, Axis Ultra DLD). The binding energies were calibrated with respect to C1s core level peak at 284.6 eV. The magnetization with applied magnetic field was measured by vibrating sample magnetometer (VSM, Lakeshore 7410) at room temperature. It reveals the magnetic properties like coercivity, saturation magnetization and remanence of the synthesized supported photocatalyst. The visible light absorption spectrum (350–800 nm) was obtained with a UV-Vis spectrophotometer (Merck, Spectroquant Pharo 100) and quartz cell (10 mm path length). The prepared catalysts were dispersed in distilled water, and their spectral were examined. The band gap energy (E) was calculated as per the literature report [41] using the following equation: Bandgap energy (E) =

hc 

(1)

where h is the Planks constant, 6.626 × 10−34 J s, c is the speed of light, 3.0 × 108 m/s;  is the wavelength (nm) of cut-off absorbance (lowest absorbance). 2.3. Photocatalytic activity experiment The photocatalytic activity of prepared photocatalyst was evaluated for the degradation of recalcitrant pesticide namely 2,4dichlorophenol (2,4-DCP). The experiments were carried out in a batch reactor of 500 mL capacity, with a working volume of 250 mL

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500 450

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2 Theta (Degree) Fig. 1. XRD patterns of (a) TiO2 ; (b) NiFe2 O4 coated SiO2 ; (c) TiO2 nanocomposite; (d) uncalcined TiO2 .

under continuous stirring. The visible light spectrum of sunlight was used as a source of excitation. The experiments were carried out under two different irradiation conditions i.e. bright and diffused sunlight irradiation. These two conditions give variation in number of available photons for the reaction. The initial 2,4DCP concentration was fixed at 50 mg/L with a catalyst dosage of 1 g each. Samples were drawn at regular interval and analysed for residual 2,4-DCP concentration using High Performance Liquid Chromatography (HPLC, Perkin Elmer Series 200, UV Detector) after separating the catalyst by centrifugation at 10,000 × g. The column employed was C18 (Supelco) column (150 mm × 4.6 mm, 5 ␮m particle size) with Acetonitrile/Water (60/40) as the mobile phase at a flow rate of 1 mL min−1 and the injection volume was 20 ␮L with UV absorption wavelength being 275 nm. The detection limit of the HPLC was 30 pg mL−1 . The experiments were performed until nearly complete degradation was ensured. The TiO2 supported NiFe2 O4 was recovered under a permanent magnetic field and used for further experimental runs. 3. Results and discussion 3.1. Structural and morphology characteristics of synthesized photocatalysts Crystallography characterization is an important analysis for the prepared photocatalysts, because it supplies proof on visible changes on crystalline structure during the calcination. The XRD patterns of synthesized nanocomposite photocatalysts are shown in Fig. 1. Fig. 1a depicts the crystallographic peaks of pure calcined TiO2 and designated to anatase crystal phase (most active phase) without any indication of other crystalline phases (Fig. 1S), such as rutile except for the uncalcined TiO2 (Fig. 1d). This phase transition was achieved with a calcination temperature at 500 ◦ C, thus indicates that the synthesized photocatalyst is also stable under such high temperature. It is obvious that anatase is more stable than that of rutile with higher enthalpy of formation [45]. Moreover, it is also responsible for increased photoexcitation processes. The pattern of the NiFe2 O4 supported on TiO2 (Fig. 1c) resembles the combinations of the individual peaks of TiO2 (Fig. 1a) and NiFe2 O4 coated SiO2 (Fig. 1b). It is also evident that after supporting with SiO2 and NiFe2 O4 , the pattern showed the presence of anatase, rutile and spinel NiFe2 O4 in an equal ratio (Fig. 2S). Furthermore, it is proved that the phase transition is caused by the possibility of structural nickel (Ni) and iron (Fe) doping, i.e. the substitution of

Fig. 2. TEM image of (a) TiO2 photocatalyst; (b) TiO2 nanocomposite photocatalyst.

this metal ions into titanium matrix [9]. However, a reduction in intensity was observed, which might be due to supporting NiFe2 O4 onto TiO2 . The shell formation of TiO2 on the magnetic core lowered the crystallographic peak intensity. The crystallite sizes of prepared photocatalysts are found to be 25.4 nm for TiO2 and 25.5 nm for TiO2 nanocomposite respectively. The high photocatalytic activity generally depends on the crystallite sizes because nano-size will result in the quantum effect, where the electronic properties of solids changed with the reduction of crystal size. The absorption spectra shift to a higher photon energy and develop a discrete character, when the size of particles is smaller [42–44]. In general the quantum effect will become dominant by altering physical properties if the particle size of material reaches 100 nm or below [42–44]. In addition, it also increases the surface to volume ratio which can contribute to improved thermal, mechanical and photocatalytic properties of the photocatalyst. The wider peaks of TiO2 and nanocomposite proved that the smallest crystallite size caused the XRD peak broadening. The pure TiO2 and TiO2 nanocomposite were investigated with TEM to explore the size and interactions amongst the components in the synthesized photocatalysts. TiO2 micrograph (Fig. 2a) under bright field image indicates irregular discoidal porous structure and particle shapes of the nanoparticles. However, Fig. 2b shows the agglomeration of irregular porous structure with discoidal and spherical particle shapes. The bright contrast (grey in colour) of micrographs clearly shows the particle of TiO2 which are

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Fig. 4. Adsorption–desorption isotherm and pore size distribution curve of TiO2 photocatalyst.

the nitric acid (HNO3 ) but it just act only as a catalyst. However, in the present study it involved in the reaction pathway and contributed for the improved visible light absorption property of the photocatalysts. 3.2. BET surface area analysis

Fig. 3. EDS spectrum of (a) TiO2 photocatalyst; (b) TiO2 nanocomposite photocatalyst.

enwrapped on the surface of NiFe2 O4 . Meanwhile darker contrast shows the agglomeration of NiFe2 O4 . This is due to the high ability of NiFe2 O4 electron scattering and larger surface energy compared to TiO2 . The particle sizes range for both synthesized photocatalysts from TEM were less than 30 nm. The surface morphology of the synthesized TiO2 and TiO2 nanocomposite photocatalyst as shown in Figs. 3S and 4S (supporting figures) distinctly differentiate between them. Fig. 4S clearly shows the presence of NiFe2 O4 as core and TiO2 as a shell i.e. onto the surface of NiFe2 O4 . Hence the FESEM images from the supporting files confirmed the formation and existence of TiO2 /SiO2 /NiFe2 O4 nanocomposite in the prepared magnetic photocatalyts. The EDS spectrum of synthesized photocatalysts is illustrated in Fig. 3a and b. It is specifically observed that nanocomposite TiO2 (Fig. 3b) consists of Ti, O, N, Ni, Si, Fe and C. Each peak is particular to an atom, or resemble to an element. They were distributed due to calcination at high temperature. The photocatalyst consists of Ti (30.87 wt.%) and O (47.04 wt.%) as major constituents. Meanwhile minority dopants includes N, Si, Ni, Fe and C which are less than 10 wt.%. The significant amount of Fe encourages the ferromagnetic property of photocatalyst for better recovery under magnetic field. Similarly the presence of transition metal (Ni) enhances the visible light absorption of the prepared nanocomposite titania [9]. The EDS also shows the presence of nitrogen (N) in its spectrogram. It is due to incomplete removal of anions (NO3 − ) from nitric acid (HNO3 ) which was used as catalyst during preparation of TiO2 . Moreover, the occurrences of N can also contribute to the better visible light absorption [23]. Many researchers have utilised

Figs. 4 and 5 show the adsorption–desorption isotherm of the TiO2 photocatalyst and TiO2 nanocomposite photocatalyst respectively along with their pore size distribution (PSD). Both the sorption hysteresis indicates that the synthesized photocatalysts were categorized in Type IV (IUPAC Classification), commonly associated with the presence of mesoporosity. The capillary condensation in mesopores gives rise to a hysteresis loop and exhibited a limited uptake at high relative pressure (P/P0 ). Based on adsorption–desorption isotherm, TiO2 contained strong adsorbate (nitrogen gas-adsorbent) interactions compared to TiO2 nanocomposite. In addition, a rounded knee effect was occurred in the isotherms indicates approximate location of monolayer formation and confirms the non occurrences of micropores. The adsorption hysteresis patterns showed that the synthesized photocatalysts had cylindrical pore geometry. The average pore width was found to be 12.56 nm and 10.23 nm for TiO2 and TiO2 nanocomposite respectively, which proved the both photocatalysts are in mesopores class. These nanosize particles occur due to sol–gel and hydrothermal methods of preparation. The higher value of pore width of TiO2 shows that precursor used during preparation is thermally less

Fig. 5. Adsorption–desorption isotherm and pore size distribution curve of TiO2 nanocomposite photocatalyst.

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Fig. 6. XPS spectra of TiO2 nanocomposite photocatalyst: (a) Ti 2p ; (b) Si 2p ; (c) Ni 2p .

stable. The BET specific surface area of TiO2 was found to be 49.45 m2 /g with a total pore volume of 0.1553 cm3 /g compared to 30.52 m2 /g and 0.0781 cm3 /g for TiO2 nanocomposite. The reduced surface area and pore volume is due to doping of Ni2+ ion onto Ti2+ ion. This dopant has reduced the pore volume by decreasing the pores that can fit into the solid. It is possible since the electrons from Ni2+ can occupy the d shell of the Titanium ion orbital. Hence, it is evident that there could be a chance of partial doping of these metals into TiO2 . In addition doping can also reduce the surface area of TiO2 nanocomposite photocatalyst along with reduced pores. This is proved by the reduction in the value of surface area for TiO2 nanocomposite photocatalyst.

their study on tin doping into TiO2 [45]. However, the present study resulted with an increased binding energy of TiO2 and thus finally contributing enhanced visible light activity of the photocatalyst. 3.4. VSM analysis Fig. 7 shows clear ferromagnetic hysteresis behaviour of TiO2 supported on NiFe2 O4 . The intensity of applied magnetic field

3.3. XPS analysis The chemical state of synthesized photocatalyst was analysed by XPS consecutively to determine the absorption of nickel into TiO2 after heat treatment. High resolution XPS spectra of Ti 2p , Si 2p and Ni 2p are shown in Fig. 6a, b and c respectively. The spectra of TiO2 nanocomposite contain principally Ti, O, Ni, Fe and Si elements and a low amount of carbon with a peak at 284.6 eV. The Ti 2p spectra had a narrow peak at about 458 eV and a broad peak at about 463 eV, characteristic feature for TiO2 species. It also resulted in higher intensity and gives a peak shifted to higher binding energy. The two peaks considered as the active sites for photocatalytic activity under the visible light irradiation. Similar kind of peaks with different binding energy level was observed by Wang et al. [45] in

Fig. 7. Ferromagnetic property of TiO2 nanocomposite photocatalyst.

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Fig. 8. Visible light absorption spectra of TiO2 photocatalyst and TiO2 nanocomposite photocatalyst.

weakens to zero when the remanence magnetism (Mr ) of the photocatalyst is 0.24 emu/g. The coercivity (Hci ) of the photocatalyst is 264 G and it was believed to be affected by modifications i.e. doping or coating with other material as coercivity represents the intensity of the magnetic field needed to reduce the magnetization of ferromagnetic material to zero after it has reached saturation. As shown in Fig. 7, the magnetization (Ms ) and coercivity (Hc ) value are lower due to the coating of ferrite particle which lead to the decreasing of the interparticle interactions which arise from dipolar coupling. Furthermore, it will contribute to magnetic anisotrophy, thus change the magnetic properties. The SiO2 protective coating in the nanocomposite photocatalyst has also avoided the influence of magnetization onto the photoactivity of the TiO2 . Nevertheless the protective coating has not reduced the ferromagnetic property of the nanocomposite. Besides, the paramagnetic TiO2 was shifted to a ferromagnetic material and could be recovered under a permanent magnetic field, reused for many cycles with better photocatalytic activity. 3.5. UV–vis absorption spectra and band gap energy The UV–vis absorption spectra of both synthesized photocatalysts are depicted in Fig. 8. It is well known and proven that unmodified commercialized TiO2 (Degussa P25 ) has almost zero absorption towards visible light (can absorb only UV light). However, the presence of non-metal ion, nitrogen (that was occur due to HNO3 catalyst used on preparation) on TiO2 indicates that there is a clear extension of absorption range in visible light region and high visible absorbance (0.25) was observed at 550 nm wavelength. The nitrogen as dopant enhanced potential of visible light absorption. The N atom isolated with O atom to form impurity energy level and easy to absorb visible light. Besides, it formed oxygen deficient sites that are proved to only emerge visible light and act as blocker for reoxidation. The high consistency in the absorption contributed to the increasing of visible light photocatalytic activity of synthesized photocatalysts. Furthermore, the absorption in the visible light region (0.45) increased upon supporting on the silicacoated NiFe2 O4 core at 600 nm wavelength. This is an imperative finding as the electron of the nickel ion diffused into TiO2 orbital that enhanced the absorption in this region. There is clear reduction in the band gap energy for both TiO2 (2.76 eV) and TiO2 nanocomposite (2.26 eV) drastically. These values of band gap energy are much lower than that of well established and well studied commercial TiO2 photocatalyst, Degussa P25 (3.20 eV). This proves that the photocatalyst had lowered the band gap energy between the

Fig. 9. Photocatalytic activity of TiO2 photocatalyst and TiO2 nanocomposite photocatalyst in degrading 2,4-DCP under sunlight irradiation (A) TiO2 nanocomposite run 1 under bright sunlight; (B) TiO2 nanocomposite run 2 under diffused sunlight; (C) TiO2 nanocomposite run 3 under bright sunlight; (D) pure TiO2 under diffused sunlight; (E) pure TiO2 under bright sunlight.

conduction and valance band, means it can excite even at very lower photon energy obtained from the visible spectrum of electromagnetic radiation. Typically, completely filled O 2p orbital and the empty Ti 3d orbital contribute in the formation of the valence and conduction bands of TiO2 [45]. During N-doping, the 2p orbital of the dopant N atom interacts with the O 2p orbital. The interactions create charge transfer from the supporter (NiFe2 O4 ) to conduction or valence band of titania, thus gives rise to the shift of absorption [45]. In addition, it can contribute much onto improve photoactivity of the prepared catalysts. It also implies that the synthesized TiO2 nanocomposite photocatalyst can excite even with diffused sunlight irradiation, generally obtained in monsoon season. 3.6. Photocatalytic activity Fig. 9 illustrates the percentage degradation of 2,4dicholorophenol (2,4-DCP) with pure and composite TiO2 , at irradiation conditions. The supported catalyst showed an excellent photoactivity with almost complete degradation (100%) of 2,4-DCP in 90 min under bright sunlight. Similarly it took nearly 300 min under the diffused light irradiation with a complete degradation of 2,4-DCP. The experimental runs were carried out by separating the catalyst under a strong magnetic field. A mass recovery of ≈95% was achieved with a negligible loss and it was due to its particle size. A maximum of two runs were carried out by separating the catalyst under a magnetic field. All of them showed photoactivity similar to the virgin catalyst i.e. similar to that of run 1. The last run resulted in 100% degradation of 2,4-DCP in same 90 min as observed in run 1. Similar experiments were also carried out with the prepared pure titania catalyst. It also resulted in better photoactivity, whilst it took a longer duration (180 min), under bright sunlight irradiation. Meanwhile ≈90% degradation was achieved under diffused sunlight irradiation in 5 h. The anatase crystalline phase in nanocomposite titania effectively reduced the recombination of photogenerated charge carriers and enhance the photocatalytic activity by reducing the band gap to 2.26 eV. This band gap is highly responsible for such excellent photoactivity even under diffused light irradiation i.e. with minimum no. of photons. The photocatalytic activity with pure TiO2 catalyst also showed a comparatively better performance, due to its reduced band gap energy i.e. 2.76 eV. The photogenerated electrons from conduction band get transferred to trapping sites of anatase phase in the presence of sunlight irradiation. Such subsequent transfer of electrons to lattice trapping sites of anatase helps

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in separating the charge carriers effectively. These trapping sites also benefits by preventing the recombination to a large extent and facilitating the charge separation thereby activating the catalyst. These effects could be observed in both the prepared catalysts, but much largely reflected TiO2 nanocomposite photocatalyst. Most similar kind of observation was reported by Wang et al. [45], where they obtained by using a tin as a dopant. A complete degradation of 4-Cholorophenol (4-CP) was achieved by them in 90 min of irradiation with an initial 4-CP concentration of 5 × 10−5 moles/L under artificial visible light irradiation [45]. However, in the present study sunlight was employed as a source of visible light illumination. In spite the result showed an excellent photocatalytic activity than that of later. It can also address the issues related to sustainable development and green engineering. Over all the present study revealed the potential of improved visible light photoactivity of the prepared photocatalyst, than that of the literature reports along with recovery. 4. Conclusions The photocatalytic oxidation of 2,4-DCP was successfully studied with the prepared photocatalysts. The NiFe2 O4 supported photocatalysts exhibited an improved photocatalytic activity under visible light spectrum along with ferromagnetic properties. This synergistic effect was induced by partial doping of a transition metal ion (Ni) into the d shell orbit of TiO2 . A highest degradation of 2,4-DCP concentration was achieved under both bright and diffused sunlight for both photocatalysts. The results of the present prepared nanocomposite photocatalyst showed a promising approach for the suitability of such catalysts for practical applications, owing to its better visible light absorption and better recovery under magnetic field. Thus prepared NiFe2 O4 nanocomposite could be a new generation photocatalyst for treating toxic wastewater completely and economically. Acknowledgements This work was supported by University of Malaya Research Grant, UMRG (RG091/10SUS) and Postgraduate Research Grant, PPP (PV092/2011A). The authors are grateful to Centre for Research & Instrumentation Management (CRIM) and Electron Microscopy Unit (EMU), Universiti Kebangsaan Malaysia (UKM) for the XPS and TEM analysis and NANOCEN/COMBICAT for BET, XRD and FESEM analysis respectively. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cej.2012.01.006. References [1] Y. Zhang, J.C. Crittenden, D.W. Hand, D.L. Perram, Fixed-bed photocatalysts for solar decontamination of water, Environ. Sci. Technol. 28 (1994) 435–442. [2] M. Hoffmann, S. Martin, W. Choi, D. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [3] J. Zhao, T. Wu, K. Wu, K. Oikawa, H. Hidaka, N. Serpone, Photoassisted degradation of dye pollutants. 8. Irreversible degradation of alizarin red under visible light radiation in air-equilibrated aqueous TiO2 dispersions, Environ. Sci. Technol. 32 (1998) 2394–2400. [4] P. Mukherjee, A. Ray, Major challenges in the design of a large scale photocatalytic reactor for water treatment, Chem. Eng. Technol. 22 (1999) 253–260. [5] I. Arslan, I.A. Balcioglu, D. Bahnemann, Heterogeneous photocatalytic treatment of simulated dyehouse effluents using novel TiO2 -photocatalysts, Appl. Catal. B – Environ. 26 (2000) 193–206. [6] L. Sikong, B. Kongreong, D. Kantachote, W. Sutthisripok, Photocatalytic activity and antibacterial behavior of Fe3+ doped TiO2 /SnO2 nanoparticles, Energy Res. J. 1 (2010) 120–125.

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