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TiO2 magnetic spheres for enhanced photocatalytic activity towards the degradation of azo dye
Applied Surface Science 357 (2015) 433–438
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Synthesis of Ni nanoparticles decorated SiO2 /TiO2 magnetic spheres for enhanced photocatalytic activity towards the degradation of azo dye K.P.O. Mahesh ∗ , Dong-Hau Kuo ∗ Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
a r t i c l e
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Article history: Received 6 August 2015 Received in revised form 28 August 2015 Accepted 31 August 2015 Available online 2 September 2015 Keywords: Ni magnetic nanoparticle SiO2 sphere TiO2 photocatalyst Ni-SiO2 /TiO2 magnetic photocatalyst Photocatalytic activity AB1 dye
a b s t r a c t Highly photocatalytic active Ni magnetic nanoparticles-decorated SiO2 core/TiO2 shell (Ni-SiO2 /TiO2 ) particles have been prepared by the simultaneous hydrolysis and condensation of titanium tetraisopropoxide on SiO2 sphere of ∼300 nm in size followed by the reduction of nickel chloride using hydrazine hydrate as a reducing agent. The crystalline nature, surface morphology, electrochemical impedance spectra and UV–vis diffuse reflectance spectra of the Ni-SiO2 /TiO2 magnetic spheres were characterized by PXRD, FE-SEM, TEM, EIS and UV–vis DRS. The Ni-SiO2 /TiO2 magnetic photocatalyst was used for the degradation of Acid Black 1 (AB 1) dye under UV irradiation. The effects of different concentrations of the Ni nanoparticles deposited on the SiO2 /TiO2 composite spheres for the photo-mineralization of AB 1 dye were analyzed. The results showed the Ni-SiO2 /TiO2 magnetic photocatalyst to be efficient and reusable. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The environment is contaminated by various organic compounds that are commonly used in industrial, agricultural and domestic applications. Among these most of the organic compounds are non-biodegradable and some of them are biodegradable at low concentration and they produce non-biodegradable intermediates. The conventional treatment methods could not mineralize the pollutants to harmless products by flocculation, adsorption and membrane separation. The heterogeneous semiconductor photocatalysts are capable of mineralizing organic pollutants into harmless products like CO2 , H2 O and mineral acids [1–5]. Semiconductor photocatalysis is a topic of current interest mainly in view of its potential application in wastewater purification [6–9]. In our previous work, we have used Ag-SiO2 /TiO2 spheres for the photodegradation of AB 1 dye [10–12]. In this study, the sub-micrometer-sized SiO2 is used as a substrate and TiO2 nanoparticles were coated on it to increase the active surface area. The Ag nanoparticles helped to suppress the recombination of electron–hole pairs. After photodegradation experiment, it is
∗ Corresponding authors. E-mail addresses: [email protected] (K.P.O. Mahesh), [email protected] (D.-H. Kuo). http://dx.doi.org/10.1016/j.apsusc.2015.08.264 0169-4332/© 2015 Elsevier B.V. All rights reserved.
very difficult to remove the photocatalyst because those are in the nanopowder form. Separation and purification of catalyst consumes money and time. The catalyst needs long time to settle or needs centrifugation to be removed from the solution. In order to overcome the above-mentioned difficulties, the magnetism is introduced into the SiO2 /TiO2 composite spheres with the help of magnetic nanoparticles. Once the catalyst gets magnetic behavior, it could be easily separated by magnet or the magnetic particles join together and settle at the bottom without using separation technique. The metal oxide catalysts are modified with the transition-metal nanoparticle that increases their catalytic efficiency and reduces the recombination of electron–hole pair as well as easy separation of catalyst after photocatalytic experiment [13–15]. Recently, the magnetic nanoparticles have attracted great attention for easy separation of catalysts. TiO2 -Ni magnetic nanoparticles for several applications have been investigated. For example, Guo et al. introduced the magnetic nickel particles to the porous silica core by using electroless plating followed by coating of titania on the outer layer and the multilayer-coated spheres were investigated under external electric and magnetic fields [16]. Zhang et al. prepared the Ni/TiO2 -SiO2 catalyst by simultaneous mixing of nickel, titania and silica precursors followed by calcination at different temperatures and then tested the catalyst for the reaction of CO2 reforming of methane to synthesis gas [17]. Stefanov et al. modified the titanium dioxide (Degussa P25) with Ni nanoparticles which
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were dip coated on microscope slides for the degradation of methylene [18]. Zhu et al. synthesized the Ni/TiO2 core/shell nanorod arrays by one-step electrodeposition technique [19]. Pang et al. synthesized TiO2 film-coated one-dimensional Ni nanostructures by hydrothermal method and their photocatalytic and biocompatibility results showed the outstanding photocatalytic activities for the decomposition of organic pollutants and great biocompatibility [20]. Mohamed and Aazam reported the nano-sized Ni particles deposited on TiO2 -SiO2 by two methods of photo-assisted deposition and impregnation for enhanced H2 production [21]. Rodriguez et al. synthesized the Ni/TiO2 catalysts that were used for the degradation of the herbicide 2,4-dichlorophenoxy acetic acid in aqueous solution in the presence of ozone and the initial activity of Ni/TiO2 catalysts was 26% higher when comparing the conventional ozonation [22]. Olya et al. reported the mixing of TiO2 sol with nickel sol to get the Ni/TiO2 nanocomposite photocatalyst for the degradation of Acid Red 88 in aqueous solution [23]. Casados et al. reported the different concentrations of Ni nanoparticles incorporated into the TiO2 then coated on the glass substrate to form catalytic thin films for the degradation of Malachite Green solutions [24]. Cui and Lu investigated the surface electron transfer and the enhancement on hydrogen evolution under visible light irradiation using Ni@NiO core/shell cluster on TiO2 surface (Ni@NiO/TiO2) [25]. The reported Ni nanoparticles were only deposited on the agglomerated SiO2 /TiO2 nanocomposite or Ni-SiO2 /TiO2 thin films. None of the above studies had synthesized the core–shell structure of Ni-SiO2 /TiO2 composite spheres with uniformity in shape and size. To the best of our knowledge, we report for the first time the synthesis of Ni magnetic nanoparticle-deposited SiO2 /TiO2 (core–shell) composite spheres with uniform in size and shape. This was done without the assistance of template and surfactant. Transmission electron microscopy (TEM) and field-emission scanning microscopy (FE-SEM) were used to study the surface morphologies of the Ni-SiO2 /TiO2 . Its ability in the degradation of AB 1 dye under UV light illumination was also demonstrated. 2. Materials and methods 2.1. Materials For this study, analytical grade chemicals were used as received without further purification. 2.2. Synthesis of SiO2 spheres and SiO2 /TiO2 composite spheres The synthesis procedures for the SiO2 spheres and SiO2 /TiO2 composite spheres have been described in our previous study [26]. The SiO2 spheres were prepared by mixing 2 mL TEOS, 0.6 mL dodecane, 10 mL anhydrous ethanol, 2 mL water, 0.4 mL ammonia solution, and 6 mL anhydrous ethanol in sequence and the mixture was allowed to stir at room temperature for 2 h. The SiO2 spheres were separated by centrifugation and washed three times with ethanol. The SiO2 spheres in ethanol were rotovaped to remove ethanol and dried in a vacuum oven at 60 ◦ C for 6 h. The SiO2 spheres (1 g) were uniformly dispersed in anhydrous 2-propanol (50 mL) by ultrasonication for 30 min, followed by the addition of 1 mL titanium tetra-isobutoxide (Ti(iOBu)4 ), 0.2 mL dodecane, and 0.1 mL water. The mixture was allowed to stir at room temperature for 4 h. The TiO2 -coated SiO2 spheres were separated by centrifugation and washed three times with ethanol and dried in a vacuum oven at 60 ◦ C for 6 h. To obtain the anatase TiO2 , the prepared SiO2 /TiO2 composite spheres were calcined at 450 ◦ C for 4 h. The total amount of TiO2 nanoparticles coated on the SiO2 spheres, analyzed by energy dispersion spectrometer (EDS) under a FE-SEM microscope, was 4 at%.
2.3. Deposition of magnetic Ni nanoparticles on SiO2 /TiO2 composite spheres The Ni nanoparticles deposited on SiO2 /TiO2 composite spheres with magnetic behavior were obtained by the reduction of nickel chloride using hydrazine hydrate as a reducing agent [27]. The synthesis process of Ni-deposited SiO2 /TiO2 magnetic spheres is as follows: 0.1 g nickel chloride was dissolved in a mixture of 60 mL ethylene glycol and 0.1 mL hydrazine hydrate. The SiO2 /TiO2 composite spheres (0.5 g) were ultrasonically dispersed in the above mixture for 30 min followed by the addition of 0.4 mL of 1 M sodium hydroxide solution and stirred at 60 ◦ C for 1 h. Ni-coated SiO2 /TiO2 magnetic spheres were separated using magnet and washed three times by ethanol and dried in a vacuum oven at 60 ◦ C for 6 h. 2.4. Measurements Powder X-ray diffraction (PXRD) data was obtained on a Bruker ˚ The D2-phaser diffractometer using CuK␣ radiation ( = 1.5418 A). morphology of the magnetic Ni-SiO2 /TiO2 spheres was observed by FE-SEM (JSM 6500F, JEOL, Tokyo, Japan) and TEM (H-7000, equipped with a CCD camera, Hitachi, Tokyo, Japan). The UV–vis diffuse reflectance spectra (DRS) were evaluated by a Jasco V-670 UV spectrophotometer. Electrochemical impedance spectroscopy (EIS) was carried out using an IM6ex Zahner (Kroanch, Germany). A glassy carbon electrode (GCE) was used as a working electrode and an Ag/AgCl electrode (Sat. KCl) and platinum wire were used as reference and counter electrodes, respectively. 2.5. Photodegradation experiments A specially designed apparatus was used for the photocatalytic reaction under identical conditions. A 550 W Hg-Xe lamp with a cut-off filter used for blocking light (infrared and visible) with the wavelength above 420 nm. The distance between the top of the sample bottle and light source was kept at 1 m. In each experiment, 10 mg of prepared SiO2 /TiO2 and Ni-SiO2 /TiO2 photocatalysts were charged into 50 mL of 5 ppm AB 1 dye aqueous solution. To ensure the adsorption and desorption equilibrium between the photocatalyst and AB 1 dye, the reaction solutions were kept in the dark and stirred for 30 min. Then the reaction solution was kept under UV illumination for complete degradation of dye. At certain intervals of time, 5 mL aliquots were sampled out and the photocatalyst was separated from the aqueous solution by centrifugation. Further, the concentration of AB 1 was monitored by UV absorbance intensity at 615 nm that corresponds to the maximum absorption wavelength of AB 1 dye. 3. Results and discussion Powder X-ray diffraction patterns of the prepared SiO2 sphere, TiO2 nanoparticle, SiO2 /TiO2 composite spheres, and Ni-SiO2 /TiO2 magnetic spheres are shown in Fig. 1. There was no diffraction peaks observed for SiO2 spheres, which were in an amorphous state. The TiO2 appears to be in an anatase phase after calcination of the SiO2 /TiO2 composite spheres at 450 ◦ C for 2 h. The PXRD pattern of the prepared SiO2 /TiO2 composite spheres showed the diffraction peaks in good agreement with the reference profile, PDF #21-1272, for TiO2 of the anatase phase [28]. After depositing Ni on SiO2 /TiO2 , new characteristic peaks were appeared at 44.5◦ and 51.8◦ that were contributed from the (111) and (200) diffraction planes of pure face-centered cubic Ni with the reference profile of PDF #040850 [27]. There is no Ni oxide peaks found in the PXRD patterns of Ni-SiO2 /TiO2 magnetic spheres. In our previous work, we reported the FE-SEM images of the SiO2 spheres and SiO2 /TiO2 composite spheres and their sizes were 310
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(d)
*
*
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(c) (b)
TiO2 Ni * (a)
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30
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2θ (deg) Fig. 1. XRD patterns of (a) SiO2 nanosphere, (b) TiO2 nanoparticle, (c) SiO2 /TiO2 composite sphere and (d) 3 wt% Ni-SiO2 /TiO2 magnetic spheres.
and 320 nm, respectively. The TiO2 nanoparticles are deposited on the surface of the SiO2 spheres and there was no large and isolated TiO2 particles formed separately, as evidenced from the FE-SEM images. The EDX results showed that the atomic ratio of Si/Ti is 96/4 [26]. FE-SEM images of 3 and 6 wt% Ni-SiO2 /TiO2 magnetic spheres are shown in Fig. 2. The presence of Ni nanoparticles was confirmed by EDX, as shown in Fig. 2(c). The lower Ni content of 3 wt% on the SiO2 /TiO2 did not change the surface morphology whereas the higher Ni content of 6 wt% led to the large Ni aggregates separately formed. Ni needs to be kept at an optimal content in order to have its
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nanoparticles uniformly cover on the SiO2 /TiO2 composite spheres. If Ni is over the optimal content, the excessive Ni will not uniformly cover SiO2 /TiO2 but precipitates out. Fig. 3(a) and (b) show the TEM images of 3 and 6 wt% NiSiO2 /TiO2 magnetic spheres with their uniformity in size. For the 3 wt% Ni-SiO2 /TiO2 , the Ni nanoparticles are uniformly deposited along the outer surface of the SiO2 /TiO2 composite spheres with the sizes between 20 and 40 nm, as shown in Fig. 3(a). In the case of 6 wt% Ni, the excessive Ni nanoparticles showed as the large aggregates that formed separately and/or connected to SiO2 /TiO2 composite spheres (Fig. 3(b)). Fig. 4 shows the results of UV–vis diffuse reflectance spectra for the SiO2 spheres, SiO2 /TiO2 composite spheres, and 3 wt% NiSiO2 /TiO2 magnetic spheres. There is no absorption observed in either the UV or visible regions for the SiO2 spheres. The SiO2 /TiO2 composite spheres show a strong absorption observed only in the ultraviolet region of 200–400 nm which indicates greater photocatalytic activity in the UV region, whereas in the case of 3 wt% Ni-SiO2 /TiO2 magnetic spheres, the absorption is observed in both the UV and visible regions. The UV absorption is contributed from TiO2 and the visible absorption is related to the surface plasmon resonance of Ni nanoparticles. Electrochemical impedance spectroscopy (EIS) was performed to further evaluate the electron transport and interfacial properties of the various catalysts. The fitted Nyquist plots of the EIS results of SiO2 and SiO2 /TiO2 composite spheres and 3 wt% Ni-SiO2 /TiO2 magnetic spheres with modified GCEs in 0.1 M KCl containing 5 mM of Fe(CN)6 3−/4− are shown in Fig. 5. The Randles equivalent circuit model (see the inset to Fig. 5) was used to fit the experimental data. Here, Rs is electrolyte resistance, Ret the charge transfer resistance, Cdl the double layer capacitance, and Zw the Warburg impedance. The EIS measurements are represented as Nyquist
Fig. 2. FE-SEM images of (a) 3 wt% Ni-SiO2 /TiO2 , (b) 6 wt% Ni-SiO2 /TiO2 magnetic spheres and (c) EDX spectrum of 3 wt% Ni-SiO2 /TiO2 magnetic spheres.
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Fig. 3. TEM images of (a) 3 wt% Ni-SiO2 /TiO2 and (b) 6 wt% Ni-SiO2 /TiO2 , magnetic spheres and inset shows higher magnification of Ni-SiO2 /TiO2 .
plots, in which the semicircles indicate the parallel combination of electron transfer resistance (Ret ) and double layer capacitance (Cdl ) at the electrode surface resulting from electrode impedance, while the linear portion represents the diffusion-limited process. The EIS for the SiO2 spheres exhibited the largest semicircles due to the insulating behavior of the SiO2 sphere. The EIS results for the SiO2 /TiO2 composite spheres exhibited comparatively small semicircles compared to SiO2 due to the semiconducting behavior of the TiO2 . The EIS of the Ni-SiO2 /TiO2 magnetic spheres exhibited the smallest semicircles with the smallest diameters, indicating very low resistance for these spheres. The electron transfer resistance observed for the SiO2 and SiO2 /TiO2 composite spheres and 3 wt% Ni-SiO2 /TiO2 magnetic spheres are 11,000, 4600 and 1425 −1 , respectively. The low resistance of the magnetic spheres may be ascribed to the assembly of highly conductive Ni with semiconducting TiO2 . The Ni-SiO2 /TiO2 magnetic spheres possess higher conductivity than the SiO2 spheres and SiO2 /TiO2 composite spheres as confirmed by EIS results.
Absorbance
3 wt% Ni-SiO2/TiO2
SiO2/TiO2
SiO2 300
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Wavelength (nm)
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Fig. 4. UV–vis DRS of SiO2 sphere, SiO2 /TiO2 composite sphere and 3 wt% NiSiO2 /TiO2 magnetic spheres.
-4000
-Z''im/Ω
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Fig. 5. Electrochemical impedance spectra (EIS) of (a) SiO2 , (b) SiO2 @TiO2 composite sphere and (c) 3 wt% Ni-SiO2 @TiO2 composite with modified GCEs.
There are three reasons affects the concentration of AB 1 dye in the solution. Physical adsorption of AB 1 by catalyst in the absence of UV irradiation and degradation of AB 1 dye under UV irradiation with or without catalyst. The photocatalytic activity of 3 wt% Ni nanoparticles deposited SiO2 /TiO2 was estimated by examining degradation of AB 1 dye under UV light irradiation. Fig. 6 shows the illumination time profiles of UV–vis spectra of the AB 1 dye. The intensities of peaks observed at 320 nm and 615 nm decreased gradually with increasing the irradiation time, which confirms the degradation of AB 1 dye. The 100% degradation of AB 1 aqueous solution with 3 wt% Ni-SiO2 /TiO2 catalyst was achieved at 90 min. The inset figure shows the settling of catalyst after UV illumination of dye solution. The rate of settling of SiO2 /TiO2 composite spheres with 6 wt% Ni is faster than the 3 wt% Ni due to the magnetic behavior of the Ni-SiO2 /TiO2 magnetic spheres. It is one of the advantages of using magnetic semiconductor catalyst for the degradation of pollutants without using any separation technique. 3.2. Effect of Ni content in SiO2 /TiO2 composite sphere Fig. 7 shows the photocatalytic degradation of AB 1 using SiO2 /TiO2 and 3 and 6 wt% Ni-SiO2 /TiO2 magnetic catalysts. There was a negligible degradation of dye observed when the dye solution
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% of AB 1 dye degradation
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0 Cycle 1
Cycle 2
Cycle 3
Fig. 8. Reusability of the 3 wt% Ni-SiO2 /TiO2 composite spheres catalyst under UV irradiation for 90 min.
Fig. 6. Changes in UV–vis spectra of AB 1 dye after UV irradiation in the presence 3 wt% Ni-SiO2 /TiO2 magnetic spheres with pH 6.5 and inset figures showed the settling of 3 and 6 wt% Ni-SiO2 /TiO2 magnetic catalyst without centrifugation.
% AB 1 dye remaining
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SiO2/TiO2 3 wt% Ni-SiO2/TiO2 6 wt % Ni-SiO2/TiO2
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powder [33]. The energy level of TiO2 conduction band is −4.4 eV, so that the energy level of Ni nanoparticles should be less than the energy level of TiO2 to effectively accept the photo-induced electron [33]. From the TEM images, the size of the Ni nanoparticles is smaller for the 3 wt% Ni-SiO2 /TiO2 spheres when compared to the 6 wt% Ni on SiO2 /TiO2 spheres. It is noted that the energy level of Ni nanoparticles is related to the size of the Ni particles and its value varies from −1.156 eV for Ni atom (n = 1) to −5.15 for Ni bulk material that might be the reason for the negligible improvement in the degradation of AB 1 in the presence of 6 wt% Ni-SiO2 /TiO2 magnetic sphere catalyst [33].
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3.3. Resusability test 20 0 0
20
40
60
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Time (min) Fig. 7. The percentage of AB 1 dye remaining in the solutions using the SiO2 /TiO2 composite sphere, 3 wt% Ni-SiO2 /TiO2 and 6 wt% Ni-SiO2 /TiO2 magnetic sphere catalysts after UV irradiation for different periods.
was irradiated without catalyst under UV light because AB 1 dye is resist to self-photolysis [29–32]. The photodegradation experiments of the 3 and 6 wt% Ni-SiO2 /TiO2 magnetic catalysts were also tested in the absence of UV light. There was a slight decrease i.e., 4% in the dye concentration for the 3% Ni-SiO2 /TiO2 catalyst and it was 11% decrease in the 6% Ni-SiO2 /TiO2 catalyst with the absence of UV light for 30 min. This is due to the dark adsorption of dye on Ni-SiO2 /TiO2 . In the presence of SiO2 sphere, only 20% of AB 1 dye was degraded after 60 min UV irradiation. Because SiO2 is an insulating material that could not degrade the dye under UV irradiation, the 20% degradation is due to the adsorption of AB 1 on the surface of SiO2 spheres. The dye undergoes almost complete degradation in the presence of 3 and 6 wt% Ni-SiO2 /TiO2 catalysts at 90 min whereas SiO2 /TiO2 composite spheres took 140 min to achieve the 100% degradation of AB 1 dye. Ni nanoparticle on SiO2 /TiO2 plays a major role for the higher photocatalytic activity than SiO2 /TiO2 composite sphere that is due to the suppression of recombination of photoinduced electrons and holes on the surface of the TiO2 catalyst decorated with metallic Ni nanoparticles. Further, there is not much difference in the photodegradation of AB 1 when increasing the concentration of Ni nanoparticle on the surfaces of SiO2 /TiO2 composite spheres. Liu et al. reported the relative vacuum energy levels of TiO2, Ni and molecular oxygen absorbed on TiO2
To study the reusability and stability of the 3 wt% Ni-SiO2 /TiO2 magnetic spheres, the photocatalytic experiments were repeated for three times with the same catalyst. After each experiment, the catalyst was centrifuged, washed, and recycled. The 3 wt% Ni-SiO2 /TiO2 magnetic sphere catalyst remained a high photocatalytic activity after three times cycles, and the degradation of AB 1 reached 97% for each test as shown in Fig. 8. The slight decrease of photocatalytic activity could be due to some Ni magnetic nanoparticles detached from the SiO2 /TiO2 composite spheres during the recycle of the magnetic spheres. This result indicates that the 3 wt% Ni-SiO2 /TiO2 magnetic sphere catalysts are fairly photo-stable and possess the potential of practical application. 4. Mechanism We propose a mechanism to explain the enhanced photocatalytic activity of the Ni-SiO2 /TiO2 magnetic spheres as shown in Scheme 1. In this work, we used two photocatalysts i.e., SiO2 /TiO2 and Ni-SiO2 /TiO2 magnetic spheres for the degradation of AB 1 dye. The SiO2 spheres act as substrate as well as adsorbent and the TiO2 is a photoactive center which generates the electron–hole pair. In general, the semiconductor materials are irradiated under UV light, the electrons alone from the valence band goes to the conductance band, and still the hole is in the valence band. When the recombination rate of these electron–hole pair is slow, the semiconductor catalyst showed very good photocatalytic activity. In this work, Ni nanoparticles on the SiO2 /TiO2 catalyst help to trap the electron from conductance band of TiO2 that leads to suppress the recombination of electron–hole pair and enhanced the photocatalytic activity of Ni-SiO2 /TiO2 magnetic photocatalyst. The trapping ability of Ni nanoparticle generated more and more superoxide radical anion, and at the same time the valence-band holes of TiO2 react
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Scheme 1. Mechanism of photocatalytic degradation.
with water to form the large number of reactive hydroxyl (• OH) radical. The superoxide radical anion and hydroxyl radical are used for degradation of AB 1 dye. 5. Conclusions The Ni-SiO2 /TiO2 magnetic spheres were prepared by a simple sol–gel method followed by the reduction of NiCl2 using hydrazine hydrate as a reducing agent. The different concentrations of Ni magnetic particles were deposited on SiO2 /TiO2 composite spheres. The 6 wt% Ni-SiO2 /TiO2 spheres could be magnetically separated with a faster rate than the 3 wt% Ni-SiO2 /TiO2 due to the higher magnetic behavior. For the photocatalytic degradation of AB 1 dye under UV illumination, Ni-SiO2 /TiO2 magnetic spheres showed better photocatalytic activity than SiO2 /TiO2 composite spheres. Furthermore, 3 wt% Ni-SiO2 /TiO2 magnetic spheres showed the highest photodegradation efficiency for complete degradation of AB 1. The Ni-SiO2 /TiO2 magnetic sphere photocatalyst was found to be stable and reusable without loss of catalytic activity for up to three runs and the catalyst could be easily separated from the dye solution after photocatalytic test. As compared to SiO2 /TiO2 composite spheres, the outstanding photocatalytic performance of Ni-SiO2 /TiO2 is attributed to the high separation efficiency of the photo-generated electron–hole pairs and the advanced absorption of light due to surface plasmon effect of Ni nanoparticles. Acknowledgment This work was supported by the National Taiwan University of Science and Technology through grant number 102H451201. References [1] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [2] D.S. Muggli, J.L. Falconer, Parallel pathways for photocatalytic decomposition of acetic acid on TiO2 , J. Catal. 187 (1999) 230–237. [3] J. Theurich, D.W. Bahnemann, R. Vogel, F.E. Ehamed, G. Alhakimi, I. Rajab, Photocatalytic degradation of naphthalene and anthracene: GC–MS analysis of the degradation pathway, Res. Chem. Intermed. 23 (1997) 247–274. [4] B. Fujihara, T. Ohno, M. Matsumura, Splitting of water by electrochemical combination of two photocatalytic reactions on TiO2 particles, J. Chem. Soc. Faraday Trans. 94 (1998) 3705–3709. [5] K. Sayama, H. Arakawa, Photocatalytic decomposition of water and photocatalytic reduction of carbon dioxide over zirconia catalyst, J. Phys. Chem. 97 (1993) 531–533. [6] A. Sobczynski, A. Dobosz, Water purification by photocatalysis on semiconductors, Pol. J. Environ. Stud. 10 (2001) 195–205. [7] M. Lazar, S. Varghese, S. Nair, Photocatalytic water treatment by titanium dioxide: recent updates, Catalysts 2 (2012) 572–601.
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Imae, Chemically modified polyurethane-SiO2 /TiO2 hybrid composite film and its reusability for photocatalytic degradation of Acid Black 1 (AB 1) under UV light, Appl. Catal. A: Gen. 475 (2014) 235–241. [27] X. Wu, W. Xing, L. Zhang, S. Zhuo, J. Zhou, G. Wang, S. Qiao, Nickel nanoparticles prepared by hydrazine hydrate reduction and their application in supercapacitor, Powder Technol. 224 (2012) 162–167. [28] C. Liu, D. Yang, Y. Jiao, Y. Tian, Y. Wang, Z. Jiang, Biomimetic synthesis of TiO2 –SiO2 –Ag nanocomposites with enhanced visible-light photocatalytic activity, ACS Appl. Mater. Interfaces 5 (2013) 3824–3832. [29] B. Krishnakumar, B. Subash, M. Swaminathan, AgBr-ZnO – an efficient nano-photocatalyst for the mineralization of Acid Black 1 with UV light, Sep. Purif. Technol. 85 (2012) 35–44. [30] B. Subash, B. Krishnakumar, R. Velmurugan, M. Swaminathan, M. 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Synthesis of Ni nanoparticles decorated SiO2 /TiO2 magnetic spheres for enhanced photocatalytic activity towards the degradation of azo dye K.P.O. Mahesh ∗ , Dong-Hau Kuo ∗ Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
a r t i c l e
i n f o
Article history: Received 6 August 2015 Received in revised form 28 August 2015 Accepted 31 August 2015 Available online 2 September 2015 Keywords: Ni magnetic nanoparticle SiO2 sphere TiO2 photocatalyst Ni-SiO2 /TiO2 magnetic photocatalyst Photocatalytic activity AB1 dye
a b s t r a c t Highly photocatalytic active Ni magnetic nanoparticles-decorated SiO2 core/TiO2 shell (Ni-SiO2 /TiO2 ) particles have been prepared by the simultaneous hydrolysis and condensation of titanium tetraisopropoxide on SiO2 sphere of ∼300 nm in size followed by the reduction of nickel chloride using hydrazine hydrate as a reducing agent. The crystalline nature, surface morphology, electrochemical impedance spectra and UV–vis diffuse reflectance spectra of the Ni-SiO2 /TiO2 magnetic spheres were characterized by PXRD, FE-SEM, TEM, EIS and UV–vis DRS. The Ni-SiO2 /TiO2 magnetic photocatalyst was used for the degradation of Acid Black 1 (AB 1) dye under UV irradiation. The effects of different concentrations of the Ni nanoparticles deposited on the SiO2 /TiO2 composite spheres for the photo-mineralization of AB 1 dye were analyzed. The results showed the Ni-SiO2 /TiO2 magnetic photocatalyst to be efficient and reusable. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The environment is contaminated by various organic compounds that are commonly used in industrial, agricultural and domestic applications. Among these most of the organic compounds are non-biodegradable and some of them are biodegradable at low concentration and they produce non-biodegradable intermediates. The conventional treatment methods could not mineralize the pollutants to harmless products by flocculation, adsorption and membrane separation. The heterogeneous semiconductor photocatalysts are capable of mineralizing organic pollutants into harmless products like CO2 , H2 O and mineral acids [1–5]. Semiconductor photocatalysis is a topic of current interest mainly in view of its potential application in wastewater purification [6–9]. In our previous work, we have used Ag-SiO2 /TiO2 spheres for the photodegradation of AB 1 dye [10–12]. In this study, the sub-micrometer-sized SiO2 is used as a substrate and TiO2 nanoparticles were coated on it to increase the active surface area. The Ag nanoparticles helped to suppress the recombination of electron–hole pairs. After photodegradation experiment, it is
∗ Corresponding authors. E-mail addresses: [email protected] (K.P.O. Mahesh), [email protected] (D.-H. Kuo). http://dx.doi.org/10.1016/j.apsusc.2015.08.264 0169-4332/© 2015 Elsevier B.V. All rights reserved.
very difficult to remove the photocatalyst because those are in the nanopowder form. Separation and purification of catalyst consumes money and time. The catalyst needs long time to settle or needs centrifugation to be removed from the solution. In order to overcome the above-mentioned difficulties, the magnetism is introduced into the SiO2 /TiO2 composite spheres with the help of magnetic nanoparticles. Once the catalyst gets magnetic behavior, it could be easily separated by magnet or the magnetic particles join together and settle at the bottom without using separation technique. The metal oxide catalysts are modified with the transition-metal nanoparticle that increases their catalytic efficiency and reduces the recombination of electron–hole pair as well as easy separation of catalyst after photocatalytic experiment [13–15]. Recently, the magnetic nanoparticles have attracted great attention for easy separation of catalysts. TiO2 -Ni magnetic nanoparticles for several applications have been investigated. For example, Guo et al. introduced the magnetic nickel particles to the porous silica core by using electroless plating followed by coating of titania on the outer layer and the multilayer-coated spheres were investigated under external electric and magnetic fields [16]. Zhang et al. prepared the Ni/TiO2 -SiO2 catalyst by simultaneous mixing of nickel, titania and silica precursors followed by calcination at different temperatures and then tested the catalyst for the reaction of CO2 reforming of methane to synthesis gas [17]. Stefanov et al. modified the titanium dioxide (Degussa P25) with Ni nanoparticles which
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were dip coated on microscope slides for the degradation of methylene [18]. Zhu et al. synthesized the Ni/TiO2 core/shell nanorod arrays by one-step electrodeposition technique [19]. Pang et al. synthesized TiO2 film-coated one-dimensional Ni nanostructures by hydrothermal method and their photocatalytic and biocompatibility results showed the outstanding photocatalytic activities for the decomposition of organic pollutants and great biocompatibility [20]. Mohamed and Aazam reported the nano-sized Ni particles deposited on TiO2 -SiO2 by two methods of photo-assisted deposition and impregnation for enhanced H2 production [21]. Rodriguez et al. synthesized the Ni/TiO2 catalysts that were used for the degradation of the herbicide 2,4-dichlorophenoxy acetic acid in aqueous solution in the presence of ozone and the initial activity of Ni/TiO2 catalysts was 26% higher when comparing the conventional ozonation [22]. Olya et al. reported the mixing of TiO2 sol with nickel sol to get the Ni/TiO2 nanocomposite photocatalyst for the degradation of Acid Red 88 in aqueous solution [23]. Casados et al. reported the different concentrations of Ni nanoparticles incorporated into the TiO2 then coated on the glass substrate to form catalytic thin films for the degradation of Malachite Green solutions [24]. Cui and Lu investigated the surface electron transfer and the enhancement on hydrogen evolution under visible light irradiation using Ni@NiO core/shell cluster on TiO2 surface (Ni@NiO/TiO2) [25]. The reported Ni nanoparticles were only deposited on the agglomerated SiO2 /TiO2 nanocomposite or Ni-SiO2 /TiO2 thin films. None of the above studies had synthesized the core–shell structure of Ni-SiO2 /TiO2 composite spheres with uniformity in shape and size. To the best of our knowledge, we report for the first time the synthesis of Ni magnetic nanoparticle-deposited SiO2 /TiO2 (core–shell) composite spheres with uniform in size and shape. This was done without the assistance of template and surfactant. Transmission electron microscopy (TEM) and field-emission scanning microscopy (FE-SEM) were used to study the surface morphologies of the Ni-SiO2 /TiO2 . Its ability in the degradation of AB 1 dye under UV light illumination was also demonstrated. 2. Materials and methods 2.1. Materials For this study, analytical grade chemicals were used as received without further purification. 2.2. Synthesis of SiO2 spheres and SiO2 /TiO2 composite spheres The synthesis procedures for the SiO2 spheres and SiO2 /TiO2 composite spheres have been described in our previous study [26]. The SiO2 spheres were prepared by mixing 2 mL TEOS, 0.6 mL dodecane, 10 mL anhydrous ethanol, 2 mL water, 0.4 mL ammonia solution, and 6 mL anhydrous ethanol in sequence and the mixture was allowed to stir at room temperature for 2 h. The SiO2 spheres were separated by centrifugation and washed three times with ethanol. The SiO2 spheres in ethanol were rotovaped to remove ethanol and dried in a vacuum oven at 60 ◦ C for 6 h. The SiO2 spheres (1 g) were uniformly dispersed in anhydrous 2-propanol (50 mL) by ultrasonication for 30 min, followed by the addition of 1 mL titanium tetra-isobutoxide (Ti(iOBu)4 ), 0.2 mL dodecane, and 0.1 mL water. The mixture was allowed to stir at room temperature for 4 h. The TiO2 -coated SiO2 spheres were separated by centrifugation and washed three times with ethanol and dried in a vacuum oven at 60 ◦ C for 6 h. To obtain the anatase TiO2 , the prepared SiO2 /TiO2 composite spheres were calcined at 450 ◦ C for 4 h. The total amount of TiO2 nanoparticles coated on the SiO2 spheres, analyzed by energy dispersion spectrometer (EDS) under a FE-SEM microscope, was 4 at%.
2.3. Deposition of magnetic Ni nanoparticles on SiO2 /TiO2 composite spheres The Ni nanoparticles deposited on SiO2 /TiO2 composite spheres with magnetic behavior were obtained by the reduction of nickel chloride using hydrazine hydrate as a reducing agent [27]. The synthesis process of Ni-deposited SiO2 /TiO2 magnetic spheres is as follows: 0.1 g nickel chloride was dissolved in a mixture of 60 mL ethylene glycol and 0.1 mL hydrazine hydrate. The SiO2 /TiO2 composite spheres (0.5 g) were ultrasonically dispersed in the above mixture for 30 min followed by the addition of 0.4 mL of 1 M sodium hydroxide solution and stirred at 60 ◦ C for 1 h. Ni-coated SiO2 /TiO2 magnetic spheres were separated using magnet and washed three times by ethanol and dried in a vacuum oven at 60 ◦ C for 6 h. 2.4. Measurements Powder X-ray diffraction (PXRD) data was obtained on a Bruker ˚ The D2-phaser diffractometer using CuK␣ radiation ( = 1.5418 A). morphology of the magnetic Ni-SiO2 /TiO2 spheres was observed by FE-SEM (JSM 6500F, JEOL, Tokyo, Japan) and TEM (H-7000, equipped with a CCD camera, Hitachi, Tokyo, Japan). The UV–vis diffuse reflectance spectra (DRS) were evaluated by a Jasco V-670 UV spectrophotometer. Electrochemical impedance spectroscopy (EIS) was carried out using an IM6ex Zahner (Kroanch, Germany). A glassy carbon electrode (GCE) was used as a working electrode and an Ag/AgCl electrode (Sat. KCl) and platinum wire were used as reference and counter electrodes, respectively. 2.5. Photodegradation experiments A specially designed apparatus was used for the photocatalytic reaction under identical conditions. A 550 W Hg-Xe lamp with a cut-off filter used for blocking light (infrared and visible) with the wavelength above 420 nm. The distance between the top of the sample bottle and light source was kept at 1 m. In each experiment, 10 mg of prepared SiO2 /TiO2 and Ni-SiO2 /TiO2 photocatalysts were charged into 50 mL of 5 ppm AB 1 dye aqueous solution. To ensure the adsorption and desorption equilibrium between the photocatalyst and AB 1 dye, the reaction solutions were kept in the dark and stirred for 30 min. Then the reaction solution was kept under UV illumination for complete degradation of dye. At certain intervals of time, 5 mL aliquots were sampled out and the photocatalyst was separated from the aqueous solution by centrifugation. Further, the concentration of AB 1 was monitored by UV absorbance intensity at 615 nm that corresponds to the maximum absorption wavelength of AB 1 dye. 3. Results and discussion Powder X-ray diffraction patterns of the prepared SiO2 sphere, TiO2 nanoparticle, SiO2 /TiO2 composite spheres, and Ni-SiO2 /TiO2 magnetic spheres are shown in Fig. 1. There was no diffraction peaks observed for SiO2 spheres, which were in an amorphous state. The TiO2 appears to be in an anatase phase after calcination of the SiO2 /TiO2 composite spheres at 450 ◦ C for 2 h. The PXRD pattern of the prepared SiO2 /TiO2 composite spheres showed the diffraction peaks in good agreement with the reference profile, PDF #21-1272, for TiO2 of the anatase phase [28]. After depositing Ni on SiO2 /TiO2 , new characteristic peaks were appeared at 44.5◦ and 51.8◦ that were contributed from the (111) and (200) diffraction planes of pure face-centered cubic Ni with the reference profile of PDF #040850 [27]. There is no Ni oxide peaks found in the PXRD patterns of Ni-SiO2 /TiO2 magnetic spheres. In our previous work, we reported the FE-SEM images of the SiO2 spheres and SiO2 /TiO2 composite spheres and their sizes were 310
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(d)
*
*
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(c) (b)
TiO2 Ni * (a)
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30
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2θ (deg) Fig. 1. XRD patterns of (a) SiO2 nanosphere, (b) TiO2 nanoparticle, (c) SiO2 /TiO2 composite sphere and (d) 3 wt% Ni-SiO2 /TiO2 magnetic spheres.
and 320 nm, respectively. The TiO2 nanoparticles are deposited on the surface of the SiO2 spheres and there was no large and isolated TiO2 particles formed separately, as evidenced from the FE-SEM images. The EDX results showed that the atomic ratio of Si/Ti is 96/4 [26]. FE-SEM images of 3 and 6 wt% Ni-SiO2 /TiO2 magnetic spheres are shown in Fig. 2. The presence of Ni nanoparticles was confirmed by EDX, as shown in Fig. 2(c). The lower Ni content of 3 wt% on the SiO2 /TiO2 did not change the surface morphology whereas the higher Ni content of 6 wt% led to the large Ni aggregates separately formed. Ni needs to be kept at an optimal content in order to have its
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nanoparticles uniformly cover on the SiO2 /TiO2 composite spheres. If Ni is over the optimal content, the excessive Ni will not uniformly cover SiO2 /TiO2 but precipitates out. Fig. 3(a) and (b) show the TEM images of 3 and 6 wt% NiSiO2 /TiO2 magnetic spheres with their uniformity in size. For the 3 wt% Ni-SiO2 /TiO2 , the Ni nanoparticles are uniformly deposited along the outer surface of the SiO2 /TiO2 composite spheres with the sizes between 20 and 40 nm, as shown in Fig. 3(a). In the case of 6 wt% Ni, the excessive Ni nanoparticles showed as the large aggregates that formed separately and/or connected to SiO2 /TiO2 composite spheres (Fig. 3(b)). Fig. 4 shows the results of UV–vis diffuse reflectance spectra for the SiO2 spheres, SiO2 /TiO2 composite spheres, and 3 wt% NiSiO2 /TiO2 magnetic spheres. There is no absorption observed in either the UV or visible regions for the SiO2 spheres. The SiO2 /TiO2 composite spheres show a strong absorption observed only in the ultraviolet region of 200–400 nm which indicates greater photocatalytic activity in the UV region, whereas in the case of 3 wt% Ni-SiO2 /TiO2 magnetic spheres, the absorption is observed in both the UV and visible regions. The UV absorption is contributed from TiO2 and the visible absorption is related to the surface plasmon resonance of Ni nanoparticles. Electrochemical impedance spectroscopy (EIS) was performed to further evaluate the electron transport and interfacial properties of the various catalysts. The fitted Nyquist plots of the EIS results of SiO2 and SiO2 /TiO2 composite spheres and 3 wt% Ni-SiO2 /TiO2 magnetic spheres with modified GCEs in 0.1 M KCl containing 5 mM of Fe(CN)6 3−/4− are shown in Fig. 5. The Randles equivalent circuit model (see the inset to Fig. 5) was used to fit the experimental data. Here, Rs is electrolyte resistance, Ret the charge transfer resistance, Cdl the double layer capacitance, and Zw the Warburg impedance. The EIS measurements are represented as Nyquist
Fig. 2. FE-SEM images of (a) 3 wt% Ni-SiO2 /TiO2 , (b) 6 wt% Ni-SiO2 /TiO2 magnetic spheres and (c) EDX spectrum of 3 wt% Ni-SiO2 /TiO2 magnetic spheres.
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Fig. 3. TEM images of (a) 3 wt% Ni-SiO2 /TiO2 and (b) 6 wt% Ni-SiO2 /TiO2 , magnetic spheres and inset shows higher magnification of Ni-SiO2 /TiO2 .
plots, in which the semicircles indicate the parallel combination of electron transfer resistance (Ret ) and double layer capacitance (Cdl ) at the electrode surface resulting from electrode impedance, while the linear portion represents the diffusion-limited process. The EIS for the SiO2 spheres exhibited the largest semicircles due to the insulating behavior of the SiO2 sphere. The EIS results for the SiO2 /TiO2 composite spheres exhibited comparatively small semicircles compared to SiO2 due to the semiconducting behavior of the TiO2 . The EIS of the Ni-SiO2 /TiO2 magnetic spheres exhibited the smallest semicircles with the smallest diameters, indicating very low resistance for these spheres. The electron transfer resistance observed for the SiO2 and SiO2 /TiO2 composite spheres and 3 wt% Ni-SiO2 /TiO2 magnetic spheres are 11,000, 4600 and 1425 −1 , respectively. The low resistance of the magnetic spheres may be ascribed to the assembly of highly conductive Ni with semiconducting TiO2 . The Ni-SiO2 /TiO2 magnetic spheres possess higher conductivity than the SiO2 spheres and SiO2 /TiO2 composite spheres as confirmed by EIS results.
Absorbance
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Fig. 4. UV–vis DRS of SiO2 sphere, SiO2 /TiO2 composite sphere and 3 wt% NiSiO2 /TiO2 magnetic spheres.
-4000
-Z''im/Ω
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Fig. 5. Electrochemical impedance spectra (EIS) of (a) SiO2 , (b) SiO2 @TiO2 composite sphere and (c) 3 wt% Ni-SiO2 @TiO2 composite with modified GCEs.
There are three reasons affects the concentration of AB 1 dye in the solution. Physical adsorption of AB 1 by catalyst in the absence of UV irradiation and degradation of AB 1 dye under UV irradiation with or without catalyst. The photocatalytic activity of 3 wt% Ni nanoparticles deposited SiO2 /TiO2 was estimated by examining degradation of AB 1 dye under UV light irradiation. Fig. 6 shows the illumination time profiles of UV–vis spectra of the AB 1 dye. The intensities of peaks observed at 320 nm and 615 nm decreased gradually with increasing the irradiation time, which confirms the degradation of AB 1 dye. The 100% degradation of AB 1 aqueous solution with 3 wt% Ni-SiO2 /TiO2 catalyst was achieved at 90 min. The inset figure shows the settling of catalyst after UV illumination of dye solution. The rate of settling of SiO2 /TiO2 composite spheres with 6 wt% Ni is faster than the 3 wt% Ni due to the magnetic behavior of the Ni-SiO2 /TiO2 magnetic spheres. It is one of the advantages of using magnetic semiconductor catalyst for the degradation of pollutants without using any separation technique. 3.2. Effect of Ni content in SiO2 /TiO2 composite sphere Fig. 7 shows the photocatalytic degradation of AB 1 using SiO2 /TiO2 and 3 and 6 wt% Ni-SiO2 /TiO2 magnetic catalysts. There was a negligible degradation of dye observed when the dye solution
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Fig. 8. Reusability of the 3 wt% Ni-SiO2 /TiO2 composite spheres catalyst under UV irradiation for 90 min.
Fig. 6. Changes in UV–vis spectra of AB 1 dye after UV irradiation in the presence 3 wt% Ni-SiO2 /TiO2 magnetic spheres with pH 6.5 and inset figures showed the settling of 3 and 6 wt% Ni-SiO2 /TiO2 magnetic catalyst without centrifugation.
% AB 1 dye remaining
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SiO2/TiO2 3 wt% Ni-SiO2/TiO2 6 wt % Ni-SiO2/TiO2
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powder [33]. The energy level of TiO2 conduction band is −4.4 eV, so that the energy level of Ni nanoparticles should be less than the energy level of TiO2 to effectively accept the photo-induced electron [33]. From the TEM images, the size of the Ni nanoparticles is smaller for the 3 wt% Ni-SiO2 /TiO2 spheres when compared to the 6 wt% Ni on SiO2 /TiO2 spheres. It is noted that the energy level of Ni nanoparticles is related to the size of the Ni particles and its value varies from −1.156 eV for Ni atom (n = 1) to −5.15 for Ni bulk material that might be the reason for the negligible improvement in the degradation of AB 1 in the presence of 6 wt% Ni-SiO2 /TiO2 magnetic sphere catalyst [33].
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3.3. Resusability test 20 0 0
20
40
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Time (min) Fig. 7. The percentage of AB 1 dye remaining in the solutions using the SiO2 /TiO2 composite sphere, 3 wt% Ni-SiO2 /TiO2 and 6 wt% Ni-SiO2 /TiO2 magnetic sphere catalysts after UV irradiation for different periods.
was irradiated without catalyst under UV light because AB 1 dye is resist to self-photolysis [29–32]. The photodegradation experiments of the 3 and 6 wt% Ni-SiO2 /TiO2 magnetic catalysts were also tested in the absence of UV light. There was a slight decrease i.e., 4% in the dye concentration for the 3% Ni-SiO2 /TiO2 catalyst and it was 11% decrease in the 6% Ni-SiO2 /TiO2 catalyst with the absence of UV light for 30 min. This is due to the dark adsorption of dye on Ni-SiO2 /TiO2 . In the presence of SiO2 sphere, only 20% of AB 1 dye was degraded after 60 min UV irradiation. Because SiO2 is an insulating material that could not degrade the dye under UV irradiation, the 20% degradation is due to the adsorption of AB 1 on the surface of SiO2 spheres. The dye undergoes almost complete degradation in the presence of 3 and 6 wt% Ni-SiO2 /TiO2 catalysts at 90 min whereas SiO2 /TiO2 composite spheres took 140 min to achieve the 100% degradation of AB 1 dye. Ni nanoparticle on SiO2 /TiO2 plays a major role for the higher photocatalytic activity than SiO2 /TiO2 composite sphere that is due to the suppression of recombination of photoinduced electrons and holes on the surface of the TiO2 catalyst decorated with metallic Ni nanoparticles. Further, there is not much difference in the photodegradation of AB 1 when increasing the concentration of Ni nanoparticle on the surfaces of SiO2 /TiO2 composite spheres. Liu et al. reported the relative vacuum energy levels of TiO2, Ni and molecular oxygen absorbed on TiO2
To study the reusability and stability of the 3 wt% Ni-SiO2 /TiO2 magnetic spheres, the photocatalytic experiments were repeated for three times with the same catalyst. After each experiment, the catalyst was centrifuged, washed, and recycled. The 3 wt% Ni-SiO2 /TiO2 magnetic sphere catalyst remained a high photocatalytic activity after three times cycles, and the degradation of AB 1 reached 97% for each test as shown in Fig. 8. The slight decrease of photocatalytic activity could be due to some Ni magnetic nanoparticles detached from the SiO2 /TiO2 composite spheres during the recycle of the magnetic spheres. This result indicates that the 3 wt% Ni-SiO2 /TiO2 magnetic sphere catalysts are fairly photo-stable and possess the potential of practical application. 4. Mechanism We propose a mechanism to explain the enhanced photocatalytic activity of the Ni-SiO2 /TiO2 magnetic spheres as shown in Scheme 1. In this work, we used two photocatalysts i.e., SiO2 /TiO2 and Ni-SiO2 /TiO2 magnetic spheres for the degradation of AB 1 dye. The SiO2 spheres act as substrate as well as adsorbent and the TiO2 is a photoactive center which generates the electron–hole pair. In general, the semiconductor materials are irradiated under UV light, the electrons alone from the valence band goes to the conductance band, and still the hole is in the valence band. When the recombination rate of these electron–hole pair is slow, the semiconductor catalyst showed very good photocatalytic activity. In this work, Ni nanoparticles on the SiO2 /TiO2 catalyst help to trap the electron from conductance band of TiO2 that leads to suppress the recombination of electron–hole pair and enhanced the photocatalytic activity of Ni-SiO2 /TiO2 magnetic photocatalyst. The trapping ability of Ni nanoparticle generated more and more superoxide radical anion, and at the same time the valence-band holes of TiO2 react
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Scheme 1. Mechanism of photocatalytic degradation.
with water to form the large number of reactive hydroxyl (• OH) radical. The superoxide radical anion and hydroxyl radical are used for degradation of AB 1 dye. 5. Conclusions The Ni-SiO2 /TiO2 magnetic spheres were prepared by a simple sol–gel method followed by the reduction of NiCl2 using hydrazine hydrate as a reducing agent. The different concentrations of Ni magnetic particles were deposited on SiO2 /TiO2 composite spheres. The 6 wt% Ni-SiO2 /TiO2 spheres could be magnetically separated with a faster rate than the 3 wt% Ni-SiO2 /TiO2 due to the higher magnetic behavior. For the photocatalytic degradation of AB 1 dye under UV illumination, Ni-SiO2 /TiO2 magnetic spheres showed better photocatalytic activity than SiO2 /TiO2 composite spheres. Furthermore, 3 wt% Ni-SiO2 /TiO2 magnetic spheres showed the highest photodegradation efficiency for complete degradation of AB 1. The Ni-SiO2 /TiO2 magnetic sphere photocatalyst was found to be stable and reusable without loss of catalytic activity for up to three runs and the catalyst could be easily separated from the dye solution after photocatalytic test. As compared to SiO2 /TiO2 composite spheres, the outstanding photocatalytic performance of Ni-SiO2 /TiO2 is attributed to the high separation efficiency of the photo-generated electron–hole pairs and the advanced absorption of light due to surface plasmon effect of Ni nanoparticles. Acknowledgment This work was supported by the National Taiwan University of Science and Technology through grant number 102H451201. References [1] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [2] D.S. Muggli, J.L. Falconer, Parallel pathways for photocatalytic decomposition of acetic acid on TiO2 , J. Catal. 187 (1999) 230–237. [3] J. Theurich, D.W. Bahnemann, R. Vogel, F.E. Ehamed, G. Alhakimi, I. Rajab, Photocatalytic degradation of naphthalene and anthracene: GC–MS analysis of the degradation pathway, Res. Chem. Intermed. 23 (1997) 247–274. [4] B. Fujihara, T. Ohno, M. Matsumura, Splitting of water by electrochemical combination of two photocatalytic reactions on TiO2 particles, J. Chem. Soc. Faraday Trans. 94 (1998) 3705–3709. [5] K. Sayama, H. Arakawa, Photocatalytic decomposition of water and photocatalytic reduction of carbon dioxide over zirconia catalyst, J. Phys. Chem. 97 (1993) 531–533. [6] A. Sobczynski, A. Dobosz, Water purification by photocatalysis on semiconductors, Pol. J. Environ. Stud. 10 (2001) 195–205. [7] M. Lazar, S. Varghese, S. Nair, Photocatalytic water treatment by titanium dioxide: recent updates, Catalysts 2 (2012) 572–601.
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