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Fabrication and enhanced visible-light photocatalytic activity of Pt-deposited TiO2 hollow nanospheres
Chemical Engineering Journal 223 (2013) 592–603
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Fabrication and enhanced visible-light photocatalytic activity of Pt-deposited TiO2 hollow nanospheres Bing Wang a, Chuang Li a, Hao Cui a, Jin Zhang b, Jianping Zhai a, Qin Li a,⇑ a b
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210046, PR China School of Biochemical and Environmental Engineering, Nanjing Xiaozhuang University, Nanjing 211171, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel Pt-modified hollow TiO2
(HPT) spheres were synthesized. HPT exhibits good photocatalytic
ability under visible light irradiation. The concept is expected to be
applicable to fabricate other hollow materials. Reaction mechanism for wastewater treatment is proposed.
a r t i c l e
i n f o
Article history: Received 22 December 2012 Received in revised form 4 March 2013 Accepted 12 March 2013 Available online 21 March 2013 Keywords: Pt-deposition Hollow TiO2 Visible light photocatalyst EPR
a b s t r a c t To use visible light more effectively in photocatalytic reactions, well-defined Pt-modified hollow TiO2 (HPT) spheres were prepared through a hydrothermal-synthesis process using carbon spheres as templates, followed by calcinations and photochemical reduction. The photocatalysts were characterized by a number of techniques, including X-ray diffraction, transmission electron microscopy, UV–vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy and photoluminescence measurements. The photocatalytic activity of the fabricated HPT photocatalyst for the degradation of methylene blue under visible-light irradiation was determined. The HPT photocatalyst with a Pt/TiO2 mass ratio of 0.75% exhibits the optimal photocatalytic ability at a catalyst amount of 2 g L1. The pseudo-first-order rate constant kapp for 0.75% HPT is five times that for solid TiO2 nanoparticles (P25) and three times that for hollow TiO2. Hydroxyl and superoxide radical signals were detected by means of electron paramagnetic resonance spectroscopy; these radicals are most probably responsible for the effectiveness of the HPT photocatalyst. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, photocatalytic reactions of semiconductors have received great attention around the world [1,2]. TiO2, is one of the most important photocatalysts, and has attracted much attention because it shows relatively high reactivity, low toxicity, long-term stability, and is easy to prepare and recycle [3–5]. Ismail et al. [6] and De et al. [7] prepared porous TiO2 nanoparticles for photocatalytic applications. Molinari et al. [8] fabricated suspended ⇑ Corresponding author. Tel./fax: +86 25 83592903. E-mail address: [email protected] (Q. Li). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.03.052
polycrystalline TiO2 which exhibited good photoactivity for degradation of gemfibrozil and tamoxifen. Nevertheless, agglomeration of these small modified TiO2 particles decreased the surface area of the photocatalyst, and made phase separation after reaction difficult. These problems have motivated the development and fabrication of TiO2 composites. The formation of particles with welldefined shapes, for example hollow nanoparticles, nanotubes, and nanosheets, has also received tremendous attention in recent years, because these shapes offer many advantages over their solid counterparts, such as low density, good light scattering, excellent dispersion and high surface-to-volume ratios [9,10]. Various synthetic strategies using hard- and soft-templates, such as silica
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Scheme 1. Schematic illustration of the formation of HPT photocatalyst.
TiO2 photocatalyst can only absorb wavelengths in the near-UV region (k > 397 nm) because of its wide band gap (3.2 eV); this region is only 5% of the solar spectrum. However, visible light covers as much as 45% of the total solar energy. Therefore, one approach taken by researchers to improve the photocatalytic activity of TiO2 has been to extend its optical absorption edge towards the visible light range [18,19]. Recently, the degradation of waste water under visible light by catalysts made by loading noble metals onto semiconductors has been reported [20]. Finetti et al. [21] reported well-ordered ultrathin TiOx layers on Pt(1 1 1) surface; their material displays lower binding energy compared with that of pure TiO2. Ishibai et al. [22] modified TiO2 with Pt complexes, by using H3PO2 as a reductant to reduce H2PtCl66H2O. However, the methods mentioned in these studies contained complex operations. Instead, in this paper, we have synthesized visible-lightresponsive Pt-modified hollow TiO2 (HPT) by a simple method involving synthesis of hollow anatase-phase TiO2 and photoreduction to deposit Pt. The principal of the fabrication of Pt metal is based on the redox ability of photoirradiated TiO2 nanoparticles to induce simultaneous photoreduction of Pt4+ ions [23,24]. This operation is conducted in the absence of reducing agents. In this work, recent progress in producing hollow titanium materials and depositing Pt nanoparticles on their surfaces is highlighted. Fabrication approaches will be discussed individually. The photocatalytic activity of as-prepared samples was evaluated by photocatalytic decomposition of methylene blue (MB) at ambient temperature under visible-light irradiation. 2. Experimental 2.1. Materials Chemicals and materials that were used as received in this study include: methylene blue, glucose, titanium tetrabutoxide (Ti(OBu)4, AR grade), ethanol (C2H6O, >99.7%), (hydro)chloroplatinic acid (H2PtCl66H2O, AR grade), and commercial TiO2 particles (Degussa P25). The sample solution was prepared using ultrapure water, which was obtained by passing deionized water through a Nanopure II deionization system.
Fig. 1. XRD patterns of HT, 0.75% HPT and 2% HPT samples.
and polystyrene particles, were found to be effective for preparing hollow titanium spheres (HT) [11–13], and the desired hollow interiors were generated upon the removal of the templates by calcination or dissolution processes [14–16]. However, surface modification of these templates is necessary before titanium is deposited. Zeng et al. [17] used colloidal carbon spheres as templates to prepare hollow structures. Because the surface of carbon spheres is hydrophilic, with distributed AOH and AC@O groups, no further modification is needed, and it is easy to load Ti(OH)4 onto carbon sphere templates. Carbon sphere templates provide uniform size, as well as adjustable granularity, by controlling the hydrothermal-synthesis time. Calcination can then be used to not only remove the carbon spheres and form hollow structures, but also to transform the Ti(OH)4 shell into anatase phase TiO2, which is beneficial for photoactivity.
2.2. Preparation of Pt-loaded hollow TiO2 (HPT) photocatalyst HT spheres were prepared using colloidal carbon spheres as templates. Carbon spheres were prepared by first dissolving 6 g of glucose in 40 mL of water to form a clear solution. The solution was then sealed in a 50 mL Teflon-lined autoclave and maintained at 180 °C for 8 h. The obtained particles were cooled to ambient temperature, centrifuged, washed, and redispersed in water for several cycles. The carbon spheres were then dried at 80 °C for 5 h in a vacuum oven. 0.12 g of the obtained carbon spheres were then added to 24 mL of ethanol at room temperature, followed by ultrasonication for 20 min. 1.2 mL of Ti(OBu)4 was added into the mixture, which was continuously stirred for 24 h. The product was centrifuged, washed and redispersed in ethanol for four cycles
Table 1 XRD parameters of different samples. Catalysts
2-Theta
Area
d (Å)
Crystallinity (%)
FWHM
Grain size (nm)
HT 0.1% HPT 0.3% HPT 0.5% HPT 0.75% HPT 1% HPT 2% HPT
25.315 25.371 25.251 25.258 25.318 25.299 25.337
17,598 21,987 20,881 17,041 15,937 21,163 22,397
3.5153 3.5076 3.5241 3.5231 3.5149 3.5175 3.5123
45.33 61.17 62.51 65.39 65.41 62.94 62.57
0.477 0.364 0.339 0.338 0.336 0.338 0.344
17.3 20.1 21.5 21.6 21.7 21.1 20.3
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Fig. 2. (a) TEM images of colloidal carbon spheres, (b–d) HRTEM images of HT sample. The inset displays the distribution of titanium anatase phase d-spacings of the HT samples.
Fig. 3. (a) TEM image of 0.75% HPT, (b) HRTEM image of 0.75% HPT; distributions of d-spacings for (c) titanium and (d) Pt particles in the 0.75% HPT sample.
to obtain Ti(OH)4/C spheres, which were then aged at 40 °C for 12 h. The preparation of HT photocatalyst was carried out by calcination at 450 °C for 2 h.
HPT spheres were prepared by a photochemical reduction process: 1 g of HT was suspended by stirring in 200 mL of aqueous methanol solution containing a selected amount of
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(hydro)chloroplatinic acid. The mixture was irradiated with UV light using a Hg lamp (2 mW cm2) for 15 h, followed by centrifuging, washing and drying at 80 °C for 4 h under vacuum. For convenient comparison, in this experiment, seven final samples of HPT with different ratios of Pt to TiO2 (0, 0.1, 0.3, 0.5, 0.75, 1.0, and 2.0 wt.%) were prepared. The steps used to prepare the samples are illustrated in Scheme 1. As well as this, Pt-deposited P25 nanoparticles were prepared by dispersing 1 g of P25 powder in 200 mL of aqueous methanol solution a selected amount of (hydro)chloroplatinic acid. The following steps were the same as those used for the preparation of HPT. 2.3. Evolution of photocatalytic activity The resulting HPT photocatalysts were evaluated for photodecomposition of methylene blue. The reactor was composed of a 350 W Xe arc lamp with the main emission wavelength at 420 nm (Nanjing JYZCPST Co., Ltd.) and a magnetic stirrer. The Xe
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arc lamp was surrounded by a quartz jacket and placed within the inner part of a quartz reactor vessel (5.8 cm in diameter and 68 cm in length), through which a suspension of MB and the photocatalyst was circulated. An outer recycling water glass jacket maintained a constant reaction temperature (24 °C). Throughout the experiment, the solution was continuously stirred. Aqueous suspensions of the photocatalysts were prepared by suspending the synthesized nanoparticles that had been previously sonicated at high frequency for 30 min. A selected amount of catalyst and 50 mL of contaminated solution (20 mg L1 for MB) were stirred in the dark for half an hour to reach dissolved oxygen saturation and absorption equilibrium, and then irradiated for 300 min. Aliquots were withdrawn from the irradiated solutions at appropriate intervals and analyzed by UV–vis spectroscopy (Shimadzu UV-160A) at 664 nm for MB. Photolysis experiments were performed, without photocatalyst, using the same experimental setup previously described for the photocatalytic system.
Fig. 4. (a) UV–vis diffuse reflectance spectra of samples. (b) Band gap evaluation for linear dependence of (ahv)2 versus photon energy for the samples. The inset shows the plot for photon energies of 3.2–3.5 eV.
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Fig. 5. XPS spectra of (a) HT and (b) 0.75% HPT samples.
Table 2 Atomic concentrations based on C 1s, O 1s, Ti 2p, and Pt 4f XPS peaks. Catalysts
C 1s
O 1s
Ti 2p
Pt 4f
HT HPT
28.32 29.38
49.66 50.39
22.02 18.68
0.00 1.55
2.4. Characterization Phase structure was obtained using an X’TRA X-ray diffractometer ((XRD) ARL, Switzerland) and Cu Ka radiation. High-resolution transmission electron micrographs (HRTEM) of HT were obtained using a Jeol JEM-200CX microscope. UV–vis diffuse reflectance spectra were recorded on a UV-2450PC spectrometer (Shimadzu, Japan) with barium sulfate as the reference sample. X-ray photoelectron spectra (XPS) were measured on an ESCALAB 250 spectrometer (Thermo Fisher Scientific, Waltham, MA) with an Al Ka X-ray source (1486.6 eV). Total Organic Carbon (TOC) was determined using a TOC analyzer (TOC-5000, Japan). Photoluminescence spectra (PL) were recorded with a fluorescence spectrometer
(Fluorolog-3-Tau, France) using a Xe lamp as the excitation light source, and a 420 nm UV cut-off filter (to ensure only visible light reached the sample). A Bruker spectrometer (Bruker instrument, Inc.) equipped with a Xe lamp was used to measure the electron paramagnetic resonance (EPR) signals of radicals trapped by 5,5dimethyl-1-pyrroline N-oxide (DMPO). Magnetic parameters of the radicals detected were obtained from direct measurements of magnetic field and microwave frequency. 3. Results and discussion 3.1. Structures and properties of HPT nanospheres The phases of samples were investigated by XRD, and the patterns from the HT, 0.75% HPT and 2% HPT spheres are shown in Fig. 1. HT spheres exhibit peaks characteristic of anatase phase titania (major peaks: 25.3°, 38.0°, 48.0°, and 54.7° 2h). The deposition of Pt cannot change the anatase phase. Table 1 displays the XRD parameters of different samples. The average crystallite size of HT spheres is estimated to be 18 nm. It is worth noting that
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Fig. 6. High-resolution XPS spectra of HT and 0.75% HPT samples. Core levels of O 1s, Ti 2p and Pt 4f.
appropriate addition of Pt can increase the size of the anatase crystals, and that the 0.75% HPT sample exhibits the best crystallinity. The interplanar crystal spacing of anatase is about 0.35 nm for the samples. Fig. 2 depicts the morphology of the as-synthesized colloidal carbon spheres and HT spheres, along with a high magnification image of the HT spheres. The as-synthesized colloidal carbon spheres are monodisperse with an average diameter of about 350 nm. Fig. 2b and c shows that the TiO2 layer in HT is about 65 nm thick, while the anatase crystal size phase is about 20 ± 2 nm. The inset of Fig. 2d displays the distribution of titanium anatase phase d-spacings; the spacing value is about 0.35 nm. These results are consistent with the XRD measurements. TEM images of the as-synthesized HPT are shown in Fig. 3. There are no obvious differences in morphology between HT and HPT. The layer of TiO2 in Fig. 3b encapsulates dispersed Pt nanoparticles that appear as black dots, about 3 nm in size in this image (marked with arrows). Fig. 3c and d shows the distributions of d-spacings in HPT as around 0.34 nm for titania and 0.22 nm for Pt particles, respectively, measured from the rectangles on the TEM image in Fig. 3b. Fig. 4a shows the UV–vis diffuse reflectance spectra of HT and HPT photocatalysts with different Pt/TiO2 mass ratios. The spectra show exhibit a broad and very strong absorption in the range from 200 to 350 nm. The HPT photocatalysts show huge optical absorption in the visible light region, and the absorption intensity increases with increasing Pt content in the samples. The
mechanism of visible light photosensitization can be explained in two ways. First, the deposited Pt nanoparticles can absorb visible light. Second, the Pt particles can change the original energy equilibrium, and produce some lower energy levels. Mizushima et al. [25] have confirmed the existence of defect energy levels attributable to Ti3+, which forms as a result of the deposition of noble metal; these levels are positioned near the TiO2 valence band. The energy band gaps of these samples can be calculated using an equation that had been widely adopted for crystalline semiconductors:
ahv ¼ Aðhv EgÞn=2 where a, h, v, A, Eg and n are the absorption coefficient, Planck constant, incident light frequency, a constant, the band gap and an integer, respectively. An n value of 1 was chosen for all the samples, indicating that the observed optical transition should be direct [26–28]. Fig. 4b shows plots of (ahv)2 versus photon energy. The band gap energies (Eg) of HT, 0.1% HPT, 0.3% HPT, 0.5% HPT, 0.75% HPT and 1% HPT are estimated to be 3.05, 2.94, 2.93, 2.90, 2.82 and 2.84 eV, respectively. Fig. 5 shows typical XPS survey spectra of HT and 0.75% HPT samples. Peaks from C 1s, Pt 4f, O 1s and Ti 2p orbitals can be identified. The C 1s line originates from surface impurity carbons, such as adsorbed CO2, as well as carbon pollution from XPS instrument. The atomic concentrations of all the elements in HT and 0.75% HPT
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samples are listed in Table 2. The high-resolution XPS spectra and relevant data about the O 1s, Ti 2p, and Pt 4f regions are shown in Fig. 6, respectively. The O 1s region can be deconvoluted into two peaks. The peaks at 530.5 eV and 529.5 eV for HT are assigned to the adsorbed hydroxyl oxygen (Oad) and crystal lattice oxygen
(Ola), respectively. However, the values are slightly shifted to lower binding energies compared with the data for HPT (Oad: 529.9 eV, Ola: 528.9 eV), which can be indicative of the effect of the platinum. The atom ratio of Ti to Ola is 1:1 for HT, whilst that to total oxygen is 1:2. This indicates that there are oxygen deficiencies present in
Fig. 7. (a) UV–vis spectra of MB after 0.5 h dark absorption onto various samples; (b) UV–vis spectra of MB with 0.75% HPT under visible light for times up to 5 h. Photocatalytic degradation efficiencies of MB after particular irradiation times: (c) using different amounts of 0.75% HPT; (d) using samples with different Pt/TiO2 mass ratios (2 g L1). (e) Pseudo-first-order kinetic rate plot and (f) the corresponding degradation rate constants for different samples.
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Fig. 8. Evolution of total organic carbon during the degradation of MB (a) versus amount of 0.75% HPT catalyst and (b) versus Pt/TiO2 mass ratio of catalyst.
Fig. 9. PL spectra of HT and HPT samples under 430 nm visible light.
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Scheme 2. (a) Schematic drawing of photocatalytic degradation of MB by HPT photocatalyst under visible light; (b) energy level variation at the HT–Pt interface before and after contact.
the near-surface region of HT. These can then lead to adsorption of large amounts of oxygen onto the surface of HT during the photochemical reduction process [29]. The surface Oad can play an important role in the photocatalytic reactions, because the photoinduced holes can attack the surface hydroxyls and yield surfacebound OH radicals with high oxidation capability [30–32]. The content of crystal lattice oxygen decreases as Pt is added, with the atom ratio of Ti to Ola being 2.6:1 in the 0.75% HPT sample, indicating that Pt is mainly deposited on the crystal lattice oxygen sites [33]. Binding energies for Ti(2p3/2) peaks at 458.6 eV clearly correspond to Ti4+ in the HT structure. No amount of Ti3+ was detected in the HT sample. However, the XPS spectra of the 0.75% HPT sample showed the presence of two peaks corresponding to the trivalent and tetravalent states of Ti. It is clear that Ti3+ arose from the deposition of Pt onto the surface of TiO2, which implies that interaction between Pt and the TiO2 matrix occurred during the photochemical reduction process. This result is consistent with that of Li and Li [34]. In the platinum region, the HPT sample shows an XPS profile in which peaks at 70.4 eV, 73.8 eV and 75.4 eV can be identified. The valence states of Pt are Pt0 and Pt4+. The formation of Pt0 is because H2PtCl6 is reduced by the photogenerated electrons in TiO2 when the aqueous suspension containing TiO2 and H2PtCl6 is irradiated by light with energy greater than the band gap of TiO2.
3.2. Photodegradation activity Fig. 7a shows UV–vis spectra of MB in contact with different catalyst samples after 0.5 h dark adsorption. The adsorption ability of MB improves as the deposition of Pt increases. The MB dye and phenol molecules are stable under direct photolysis, indicating that both catalyst and light are essential for degradation of contaminates. Fig. 7b shows UV–vis spectra of MB with 0.75% HPT after particular irradiation times. The intensity of the characteristic absorption peak at 664 nm decreases substantially and undergoes hypsochromic shift as irradiation time increases, indicating that direct cleavage of the MB chromophore structure and N-demethylation occurred simultaneously during photoreaction in the presence of 0.75% HPT photocatalyst. Fig. 7c displays the efficiency of photocatalytic degradation of MB after particular irradiation times, using
different amounts of 0.75% HPT. As the catalyst amount increases from 0.1 g L1 to 4 g L1, the photocatalytic efficiency at first increases and then becomes stable. 2 g L1 of 0.75% HPT is the dose required for optimal degradation efficiency for MB. The reason for this is that as the number of HPT spheres increases, the number of active surface sites also increases. However, light scattering and reduction of light penetration at relatively high catalyst loadings have a negative impact on efficiency, especially when the catalyst concentration is over 2 g L1 [35,36]. Fig. 7d demonstrates the effect of Pt deposition on the degradation efficiency after particular illumination times at the optimized catalyst amount (2 g L1). The degradation ratio of MB increases with an increasing Pt loading until a Pt/TiO2 mass ratio of 0.75% is reached. At higher Pt concentrations, the degradation efficiency begins to drop off and then becomes steady. Fig. 7e shows that the degradation of MB can be described as a first-order reaction; the time-dependence of the concentration can be fitted to ln(c0/c) = kappt. For comparison, the photocatalytic activities of P25 and Pt–P25 were also studied; these catalysts gave kapp values of 0.0025 and 0.0105 min1, respectively. The enhanced photocatalytic performance for HPT may be because of its high dispersion and light scattering. In contrast, small P25 particles tend to agglomerate, leading to a decrease in surface area, and reducing the photodecomposition of MB. The apparent photocatalytic degradation rate constants (kapp) for samples with different Pt contents are plotted in Fig. 7f, with 0.75% HPT showing the highest kapp. To study the mineralization of the dye, TOC removal for MB was investigated. This technique is another way of evaluating removal of pollutants, and is complementary to examining the decolorization level. The removal efficiency of TOC for MB as a function of catalyst amount for 0.75% HPT, and as a function of Pt/TiO2 mass ratio in the catalyst, are shown in Fig. 8a and b, respectively. It is worth noting that some TOC is still found in all systems, even after the suspension is completely discolored, implying that some reaction intermediates form which are more resistant than MB to total oxidation, thus leading to partial mineralization. The stability of 0.75% HPT photocatalysts was also studied by monitoring the photocatalytic activity during four cycles of use (Fig. S1). After each run of 5 h, samples were recovered, and dried at 100 °C for 2 h and were reused for photocatalytic study. It is found that the photocatalytic activity of the recycled photocatalyst decreased indistinctively after three recycles, and the removal of MB can still reach 85%, which reveals that the HPT photocatalyst prepared in this study is stable and effective for the removal of organic pollutants in water. 3.3. Photodegradation mechanism In this section we propose photoreaction processes for the photoinduced degradation of MB over Pt-deposited hollow TiO2 nanospheres, based on the data presented above. PL emission spectra are often used to reveal the efficiency of charge carrier trapping, migration and transfer, and to understand the fate of electron–hole pairs in semiconductor particles [37,38]. In this study, the PL emission spectra of HT and 0.75% HPT samples were examined, as shown in Fig. 9. All the HPT samples display a broad-band emission from 530 to 580 nm, while no PL signal is observed in this range for HT photocatalyst. The energy of the visible light used for excitation is not sufficient to promote electronic transitions from the valence band (VB) to conduction band (CB) for HT, according to the UV–vis diffuse reflectance spectra. Therefore, no electron/hole pairs can be generated to form PL signals. However, for the HPT samples, it is generally believed that a lower stimulated PL intensity indicates an enhanced separation and transfer of photoinduced electrons trapped in the photocatalyst. The PL spectrum under visible-light excitation is usually attributed to a range of origins: oxygen
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vacancies, defect sites and surface states [39,40]. The presence of Pt on the surface of the HT favors the migration of photo-produced electrons to the metal, thus improving the electron–hole separation. Notably, the drastic quenching of PL intensity in 0.75% HPT suggests a markedly enhanced charge separation, compared with that observed in other HPT samples. This promotes visible-light photocatalytic activity, which will be confirmed in the following section. Under visible light illumination, HT is not excited, as its absorption threshold is 406 nm; only the MB molecule is excited to produce the excited-state MB. MB can be reduced by conduction band electrons into MB (1.08 V) or oxidized by valence band holes into MB+ (0.23 V) by light absorbed by MB rather than by catalyst, followed by decomposing into leuco-MB (0 V) (see Scheme 2) [41]. The energy level of free electrons for Pt is higher than that for TiO2. The Pt particles deposited on the surface of HT can change the original equilibrium by drawing the electrons out of the bulk HT through the HPT interface until the Fermi levels are equal. The electrons react with adsorbed O2 to produce super oxide O 2 radical anions and then OH radicals (see Scheme 2a). The HT layer accumulates positive charges because of the loss of electrons, and thus the band curves upward (see Scheme 2b). This elec-
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tron transfer leads to efficient electron/hole separation and suppresses somewhat the recombination rate since the rate depends on the densities of the carriers within the HT particles, and these densities have decreased due to the introduction of Pt [42]. The electrical potential gradient also becomes lower because of the decrease in the electron density gradient. It is well known that the potential gradient and the carrier concentration gradient are the two main ‘‘driving forces’’ for carriers to drift and diffuse during photocatalysis. When the two gradients are too small to increase the electron flux through the HPT interface further, a new equilibrium is reached; at this point, further platinum deposition cannot increase the charge separation further and thus cannot further increase the decomposition rate. Therefore, there should be an optimal Pt amount. The deductions are supported by the EPR spectra in Fig. 10. No EPR signals were detected when the reaction was performed in the dark (Fig. 10a). However, after irradiation, both the samples containing HT and 0.75% HPT clearly show the classical 1:2:2:1 spectral signature of trapped OH radicals. The intensity for 0.75% HPT is obviously stronger than that for HT, which means that more OH radicals are produced in photoreaction with HPT photocatalyst, or, in other words, the 0.75% HPT system is photocatalytically
Fig. 10. DMPO trapping EPR spectra (a) for HT and 0.75% HPT samples in an aqueous dispersion (to evaluate DMPO-OH), and (b) for HT, and 0.75% HPT photocatalysts and pure MB without catalyst in methanol dispersion (to evaluate DMPO- O 2 ).
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more active than the HT system. This further confirms the mechanism of the enhancement of the photocatalytic activity of HPT. Fig. 10b shows the EPR spectra for 0.75% HPT and HT in methanol dispersion (used to detect DMPO- O 2 ). Similarly, no EPR signals were observed when the reaction was performed in dark in the presence of the catalysts. However, weak EPR signals appeared without photocatalyst, indicating that MB can be photosensitized under visible light. Under illumination in the presence of photocatalyst, EPR signals from DMPO- O 2 adducts (identified by their characteristic six peaks) are observed in both 0.75% HPT and HT methanolic dispersions. The EPR signals from O 2 were stronger in the presence of HPT than in the presence of HT, implying that more O 2 is produced when HPT is present. The EPR results provide a strong indication that the photogenerated charge carriers in 0.75% HPT photocatalyst can not only be efficiently excited by visible light to create electron–hole pairs but are also long-lived enough to react with the surface adsorbed O2 or H2O to produce OH radicals. These generated OH radicals play a pivotal role, and are recognized as the main reactive species responsible for the degradation of pollutants [28,43]. 4. Conclusion In summary, well-defined Pt-deposited hollow TiO2 nanospheres were successfully prepared through a simple method involving synthesis of hollow anatase-phase TiO2 and photoreduction to deposit Pt. The Pt nanoparticles were dispersed on the layer of HT spheres; the crystal sizes of anatase-phase TiO2 and Pt nanoparticles were about 20 and 3 nm, respectively. The deposition of Pt nanoparticles effectively narrowed the band gap of HT photocatalyst, improving the visible-light responsive ability of the photocatalyst. XPS spectra imply that, compared with the Pt-free HT spheres, the HPT photocatalyst can yield more surface-bound OH radicals with high oxidation capability. Interaction between Pt and the TiO2 matrix occurred during photoreduction, and the presence of both Pt0 and Pt4+ on the surface of the HPT spheres was confirmed. PL emission spectra indicate that 0.75% HPT shows markedly enhanced charge separation compared with HT and other HPT samples. The active species, OH and O 2 radicals, were detected by EPR – these species are believed to be responsible for the photodegradation of organic pollutants in the presence of HPT photocatalyst. The photocatalytic activity results indicate that 0.75% HPT photocatalyst exhibits the best photocatalytic activity, showing a kapp value five times that of P25 and three times that of HT for decomposition of MB under visible light. Thus Pt-deposited hollow TiO2 photocatalysts are promising, effective photocatalysts in the visible region. The concept provided in the present paper should be applicable to fabrication of other hollow semiconductor photocatalysts, and deposition of other metal nanoparticles. Acknowledgements The authors gratefully acknowledge financial support from the Foundation of State Key Laboratory of Pollution Control and Resource Reuse of China, the Natural Science Foundation of China (No. 51008154), the Jiangsu Natural Science Foundation (No. SBK201022682), the Research Fund for the Doctoral Program of Higher Education of China (No. 20090091120007), the Fundamental Research Funds of the Central University (No. 1112021101), the Jiangsu Cultivation of Innovative Engineering for Graduate Students (CXZZ12_0063), the Scientific Research Foundation of the Graduate School of Nanjing University (No. 2012CL10), and the China Postdoctoral Science Foundation (funded Project No. 2012M511254).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2013.03.052.
References [1] M.A. Rauf, S.S. Ashraf, Fundamental principles and application of heterogeneous photocatalytic degradation of dyes in solution, Chem. Eng. J. 151 (2009) 10–18. [2] M. Khraisheh, L. Wu, A.H. Al-Muhtaseb, A.B. Albadarin, G.M. Walker, Phenol degradation by powdered metal ion modified titanium dioxide photocatalysts, Chem. Eng. J. 213 (2012) 125–134. [3] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 36 (2008) 515–582. [4] Y. Liu, J. Li, B. Zhou, H. Chen, Z. Wang, W. Cai, A TiO2-nanotube-array-based photocatalytic fuel cell using refractory organic compounds as substrates for electricity generation, Chem. Commun. 47 (2011) 10314–10316. [5] S. Cao, K.L. Yeung, P.L. Yue, An investigation of trichloroethylene photocatalytic oxidation on mesoporous titania–silica aerogel catalysts, Appl. Catal. B 76 (2007) 64–72. [6] A.A. Ismail, D.W. Bahnemann, Mesoporous titania photocatalysts: preparation, characterization and reaction mechanisms, J. Mater. Chem. 21 (2011) 11686– 11707. [7] S. De, S. Dutta, A.K. Patra, A. Bhaumi, B. Saha, Self-assembly of mesoporous TiO2 nanospheres via aspartic acid templating pathway and its catalytic application for 5-hydeoxymethyl-furfural synthesis, J. Mater. Chem. 21 (2011) 17505– 17510. [8] R. Molinari, A. Caruso, P. Argurio, T. Poerio, Degradation of the drugs gemfibrozil and tamoxifen in pressurized and de-pressurized membrane photoreactors using suspended polycrystalline TiO2 as catalyst, J. Membr. Sci. 319 (2008) 54–63. [9] M. Sasidharan, K. Nakashima, N. Gunawardhana, T. Yokoi, M. Inoue, S. Yusa, M. Yoshio, T. Tatsumi, Novel titania hollow nanospheres of size 28 ± 1 nm using soft-templates and their application for lithium-ion rechargeable batteries, Chem. Commun. 47 (2011) 6921–6923. [10] X. Ding, X. Song, P. Li, Efficient visible light driven photocatalytic removal of NO with aerosol flow synthesized B, N-codoped TiO2 hollow spheres, J. Hazard. Mater. 190 (2011) 604–612. [11] J. Choi, O. Kim, Y. Kwak, Suppression of dioxin emission in co-incineration of poly(vinyl chloride) with TiO2-encapsulating polystyrene, Environ. Sci. Technol. 41 (2007) 5833–5838. [12] T. Sreethawong, S. Yoshikawa, Impact of photochemically deposited monometallic Pt and bimetallic Pt–Au nanoparticles on photocatalytic dyesensitized H2 production activity of mesoporous-assembled TiO2–SiO2 mixed oxide nanocrystal, Chem. Eng. J. 197 (2012) 272–282. [13] N. Yao, S. Cao, K.L. Yeung, Mesoporous TiO2–SiO2 aerogels with hierarchal pore structures, Microporous Mesoporous Mater. 221 (2009) 570–579. [14] H. Strohm, P. Lobmann, Porous TiO2 hollow spheres by liquid phase deposition on polystyrene latex-stabilised pickering emulsions, J. Mater. Chem. 14 (2004) 2667–2673. [15] S. Ikeda, H. Kobayashi, T. Sugita, Y. Ikoma, T. Harada, M. Matsumura, Efficient photodecomposition of gaseous organics catalyzed by titanium(IV) oxide encapsulated in a hollow silica shell with high porosity, Appl. Catal. A 363 (2009) 216–220. [16] Y. Wang, X. Su, S. Lu, Shape-controlled synthesis of TiO2 hollow structures and their application in lithium batteries, J. Mater. Chem. 22 (2012) 1969–1976. [17] H.B. Zeng, W.P. Cai, P.S. Liu, X.X. Xu, H.J. Zhou, C. Klingshirn, H. Kalt, ZnO-based hollow nanoparticles by selective etching: elimination and reconstruction of metal-semiconductor interface, improvement of blue emission and photocatalysis, ACS Nano 2 (2008) 1661–1670. [18] R. Asahi, T. Morikawa, Nitrogen complex species and its chemical nature in TiO2 for visible-light sensitized photocatalysis, Chem. Phys. 339 (2007) 57–63. [19] M.H.H. Mahmoud, A.A. Ismail, M.M.S. Sanad, Developing a cost-effective synthesis of active iron oxide doped titania photocatalysts loaded with palladium, platinum or silver nanoparticles, Chem. Eng. J. 187 (2012) 96–103. [20] O. Rosseler, M.V. Shankar, M.K. Du, L. Schmidlin, N. Keller, V. Keller, Solar light photocatalytic hydrogen production from water over Pt and Au/TiO2(anatase/ rutile) photocatalysts: influence of noble metal and porogen promotion, J. Catal. 269 (2010) 179–190. [21] P. Finetti, F. Sedona, G.A. Rizzi, U. Mick, F. Sutara, M. Svec, V. Matolin, K. Schierbaum, G. Granozzi, Core and valence band photoemission spectroscopy of well-ordered ultrathin TiOx films on Pt(1 1 1), J. Phys. Chem. C 111 (2007) 869–876. [22] Y. Ishibai, J. Sato, T. Nishikawa, S. Miyagishi, Synthesis of visible-light active TiO2 photocatalyst with Pt-modification: role of TiO2 substrate for high photocatalytic activity, Appl. Catal. B 79 (2008) 117–121. [23] Y.H. Ng, S. Ikeda, T. Harada, S. Park, T. Sakata, H. Mori, M. Matsumura, Photocatalytic route for synthesis of hollow porous carbon/Pt nanocomposites with controllable density and porosity, Chem. Mater. 20 (2008) 1154–1160. [24] C.H. Li, Y.H. Hsieh, W.T. Chiu, Study on preparation and photocatalytic performance of Ag/TiO2 and Pt/TiO2 photocatalysts, Sep. Purif. Technol. 57 (2007) 148–151.
B. Wang et al. / Chemical Engineering Journal 223 (2013) 592–603 [25] K. Mizushima, M. Tanaka, A. Asai, S. Iida, Impurity levels of iron-group ions in TiO2(II), J. Phys. Chem. Solids 40 (1979) 1129–1140. [26] X. Li, S. Ouyang, N. Kikugawa, J. Ye, Novel Ag2ZnGeO4 photocatalyst for dye degradation under visible light irradiation, Appl. Catal. A 334 (2008) 51–58. [27] F. Dong, H. Wang, Z. Wu, One-step ‘‘Green’’ synthetic approach for mesoporous C-doped titanium dioxide with efficient visible light photocatalytic activity, J. Phys. Chem. C 113 (2009) 16717–16723. [28] W. Zhao, Y. Wang, Y. Yang, J. Tang, Y. Yang, Carbon spheres supported visiblelight-driven CuO–BiVO4 heterojunction: preparation, characterization, and photocatalytic properties, Appl. Catal. B 115–116 (2012) 90–99. [29] L. Jing, B. Wang, B. Xin, S. Li, K. Shi, W. Cai, H. Fu, Investigations on the surface modification of ZnO nanoparticle photocatalyst by depositing Pd, J. Solid State Chem. 177 (2004) 4221–4227. [30] L. Jing, X. Sun, W. Cai, Z. Xu, Y. Du, H. Fu, The preparation and characterization of nanoparticle TiO2/Ti films and their photocatalytic activity, J. Phys. Chem. Solids 64 (2003) 615–623. [31] H. Chen, S.W. Lee, T.H. Kim, B.Y. Hur, Photocatalytic decomposition of benzene with plasma sprayed TiO2-based coatings on foamed aluminum, J. Eur. Ceram. Soc. 26 (2006) 2231–2239. [32] J. Li, J. Zeng, L. Jia, W. Fang, Investigations on the effect of Cu2+/Cu1+ redox couples and oxygen vacancies on photocatalytic activity of treated LaNi1xCuxO3 (x = 0.1, 0.4, 0.5), Int. J. Hydrogen Energy 35 (2010) 12733– 12740. [33] M.A. Aramendı´a, J.C. Colmenares, A. Marinas, J.M. Marinas, J.M. Moreno, J.A. Navı´o, F.J. Urbano, Effect of the redox treatment of Pt/TiO2 system on its photocatalytic behaviour in the gas phase selective photooxidation of propan2-ol, Catal. Today 128 (2007) 235–244. [34] F.B. Li, X.Z. Li, The enhancement of photodegradation efficiency using Pt–TiO2 catalyst, Chemosphere 48 (2002) 1103–1111.
603
[35] A.L. Giraldo, G.A. Penuela, R.A. Torres-Palma, N.J. Pino, R.A. Palominos, H.D. Mansilla, Degradation of the antibiotic oxolinic acid by photocatalysis with TiO2 in suspension, Water Res. 44 (2010) 5158–5167. [36] B. Lopez-Alvarez, R.A. Torres-Palma, G. Penuela, Solar photocatalitycal treatment of carbofuran at lab and pilot scale: effect of classical parameters, evaluation of the toxicity and analysis of organic by-products, J. Hazard. Mater. 191 (2011) 196–203. [37] Y. Yu, J. Yu, C. Chan, Y. Chen, J. Zhao, L. Ding, W. Ge, P. Wong, Enhancement of adsorption and photocatalytic activity of TiO2 by using carbon nanotubes for the treatment of azo dye, Appl. Catal. B 61 (2005) 1–11. [38] Y. Wu, J. Zhang, L. Xiao, F. Chen, Preparation and characterization of TiO2 photocatalysts by Fe3+ doping together with Au deposition for the degradation of organic pollutants, Appl. Catal. B 88 (2009) 525–532. [39] Y. Yu, J. Yu, J. Yu, Y. Kwok, Y. Che, J. Zhao, L. Ding, W. Ge, P. Wong, Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes, Appl. Catal. A 289 (2005) 186–196. [40] Y. Zhao, B. Yang, J. Xu, Z. Fu, M. Wu, F. Li, Facile synthesis of Ag nanoparticles supported on TiO2 inverse opal with enhanced visible-light photocatalytic activity, Thin Solid Films 520 (2012) 3515–3522. [41] G.K.R. Senadeera, T. Kitamura, Y. Wada, S. Yanagida, Deposition of polyaniline via molecular self-assembly on TiO2 and its uses as a sensitiser in solid-state solar cells, J. Photochem. Photobiol. A 164 (2004) 61–66. [42] B. Sun, A.V. Vorontsov, P.G. Smirniotis, Role of platinum deposited on TiO2 in phenol photocatalytic oxidation, Langmuir 19 (2003) 3151–3156. [43] Z. Wang, W. Ma, C. Chen, H. Ji, J. Zhao, Probing paramagnetic species in titaniabased heterogeneous photocatalysis by electron spin resonance (ESR) spectroscopy–a mini review, Chem. Eng. J. 170 (2011) 353–362.
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Fabrication and enhanced visible-light photocatalytic activity of Pt-deposited TiO2 hollow nanospheres Bing Wang a, Chuang Li a, Hao Cui a, Jin Zhang b, Jianping Zhai a, Qin Li a,⇑ a b
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210046, PR China School of Biochemical and Environmental Engineering, Nanjing Xiaozhuang University, Nanjing 211171, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel Pt-modified hollow TiO2
(HPT) spheres were synthesized. HPT exhibits good photocatalytic
ability under visible light irradiation. The concept is expected to be
applicable to fabricate other hollow materials. Reaction mechanism for wastewater treatment is proposed.
a r t i c l e
i n f o
Article history: Received 22 December 2012 Received in revised form 4 March 2013 Accepted 12 March 2013 Available online 21 March 2013 Keywords: Pt-deposition Hollow TiO2 Visible light photocatalyst EPR
a b s t r a c t To use visible light more effectively in photocatalytic reactions, well-defined Pt-modified hollow TiO2 (HPT) spheres were prepared through a hydrothermal-synthesis process using carbon spheres as templates, followed by calcinations and photochemical reduction. The photocatalysts were characterized by a number of techniques, including X-ray diffraction, transmission electron microscopy, UV–vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy and photoluminescence measurements. The photocatalytic activity of the fabricated HPT photocatalyst for the degradation of methylene blue under visible-light irradiation was determined. The HPT photocatalyst with a Pt/TiO2 mass ratio of 0.75% exhibits the optimal photocatalytic ability at a catalyst amount of 2 g L1. The pseudo-first-order rate constant kapp for 0.75% HPT is five times that for solid TiO2 nanoparticles (P25) and three times that for hollow TiO2. Hydroxyl and superoxide radical signals were detected by means of electron paramagnetic resonance spectroscopy; these radicals are most probably responsible for the effectiveness of the HPT photocatalyst. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, photocatalytic reactions of semiconductors have received great attention around the world [1,2]. TiO2, is one of the most important photocatalysts, and has attracted much attention because it shows relatively high reactivity, low toxicity, long-term stability, and is easy to prepare and recycle [3–5]. Ismail et al. [6] and De et al. [7] prepared porous TiO2 nanoparticles for photocatalytic applications. Molinari et al. [8] fabricated suspended ⇑ Corresponding author. Tel./fax: +86 25 83592903. E-mail address: [email protected] (Q. Li). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.03.052
polycrystalline TiO2 which exhibited good photoactivity for degradation of gemfibrozil and tamoxifen. Nevertheless, agglomeration of these small modified TiO2 particles decreased the surface area of the photocatalyst, and made phase separation after reaction difficult. These problems have motivated the development and fabrication of TiO2 composites. The formation of particles with welldefined shapes, for example hollow nanoparticles, nanotubes, and nanosheets, has also received tremendous attention in recent years, because these shapes offer many advantages over their solid counterparts, such as low density, good light scattering, excellent dispersion and high surface-to-volume ratios [9,10]. Various synthetic strategies using hard- and soft-templates, such as silica
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Scheme 1. Schematic illustration of the formation of HPT photocatalyst.
TiO2 photocatalyst can only absorb wavelengths in the near-UV region (k > 397 nm) because of its wide band gap (3.2 eV); this region is only 5% of the solar spectrum. However, visible light covers as much as 45% of the total solar energy. Therefore, one approach taken by researchers to improve the photocatalytic activity of TiO2 has been to extend its optical absorption edge towards the visible light range [18,19]. Recently, the degradation of waste water under visible light by catalysts made by loading noble metals onto semiconductors has been reported [20]. Finetti et al. [21] reported well-ordered ultrathin TiOx layers on Pt(1 1 1) surface; their material displays lower binding energy compared with that of pure TiO2. Ishibai et al. [22] modified TiO2 with Pt complexes, by using H3PO2 as a reductant to reduce H2PtCl66H2O. However, the methods mentioned in these studies contained complex operations. Instead, in this paper, we have synthesized visible-lightresponsive Pt-modified hollow TiO2 (HPT) by a simple method involving synthesis of hollow anatase-phase TiO2 and photoreduction to deposit Pt. The principal of the fabrication of Pt metal is based on the redox ability of photoirradiated TiO2 nanoparticles to induce simultaneous photoreduction of Pt4+ ions [23,24]. This operation is conducted in the absence of reducing agents. In this work, recent progress in producing hollow titanium materials and depositing Pt nanoparticles on their surfaces is highlighted. Fabrication approaches will be discussed individually. The photocatalytic activity of as-prepared samples was evaluated by photocatalytic decomposition of methylene blue (MB) at ambient temperature under visible-light irradiation. 2. Experimental 2.1. Materials Chemicals and materials that were used as received in this study include: methylene blue, glucose, titanium tetrabutoxide (Ti(OBu)4, AR grade), ethanol (C2H6O, >99.7%), (hydro)chloroplatinic acid (H2PtCl66H2O, AR grade), and commercial TiO2 particles (Degussa P25). The sample solution was prepared using ultrapure water, which was obtained by passing deionized water through a Nanopure II deionization system.
Fig. 1. XRD patterns of HT, 0.75% HPT and 2% HPT samples.
and polystyrene particles, were found to be effective for preparing hollow titanium spheres (HT) [11–13], and the desired hollow interiors were generated upon the removal of the templates by calcination or dissolution processes [14–16]. However, surface modification of these templates is necessary before titanium is deposited. Zeng et al. [17] used colloidal carbon spheres as templates to prepare hollow structures. Because the surface of carbon spheres is hydrophilic, with distributed AOH and AC@O groups, no further modification is needed, and it is easy to load Ti(OH)4 onto carbon sphere templates. Carbon sphere templates provide uniform size, as well as adjustable granularity, by controlling the hydrothermal-synthesis time. Calcination can then be used to not only remove the carbon spheres and form hollow structures, but also to transform the Ti(OH)4 shell into anatase phase TiO2, which is beneficial for photoactivity.
2.2. Preparation of Pt-loaded hollow TiO2 (HPT) photocatalyst HT spheres were prepared using colloidal carbon spheres as templates. Carbon spheres were prepared by first dissolving 6 g of glucose in 40 mL of water to form a clear solution. The solution was then sealed in a 50 mL Teflon-lined autoclave and maintained at 180 °C for 8 h. The obtained particles were cooled to ambient temperature, centrifuged, washed, and redispersed in water for several cycles. The carbon spheres were then dried at 80 °C for 5 h in a vacuum oven. 0.12 g of the obtained carbon spheres were then added to 24 mL of ethanol at room temperature, followed by ultrasonication for 20 min. 1.2 mL of Ti(OBu)4 was added into the mixture, which was continuously stirred for 24 h. The product was centrifuged, washed and redispersed in ethanol for four cycles
Table 1 XRD parameters of different samples. Catalysts
2-Theta
Area
d (Å)
Crystallinity (%)
FWHM
Grain size (nm)
HT 0.1% HPT 0.3% HPT 0.5% HPT 0.75% HPT 1% HPT 2% HPT
25.315 25.371 25.251 25.258 25.318 25.299 25.337
17,598 21,987 20,881 17,041 15,937 21,163 22,397
3.5153 3.5076 3.5241 3.5231 3.5149 3.5175 3.5123
45.33 61.17 62.51 65.39 65.41 62.94 62.57
0.477 0.364 0.339 0.338 0.336 0.338 0.344
17.3 20.1 21.5 21.6 21.7 21.1 20.3
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Fig. 2. (a) TEM images of colloidal carbon spheres, (b–d) HRTEM images of HT sample. The inset displays the distribution of titanium anatase phase d-spacings of the HT samples.
Fig. 3. (a) TEM image of 0.75% HPT, (b) HRTEM image of 0.75% HPT; distributions of d-spacings for (c) titanium and (d) Pt particles in the 0.75% HPT sample.
to obtain Ti(OH)4/C spheres, which were then aged at 40 °C for 12 h. The preparation of HT photocatalyst was carried out by calcination at 450 °C for 2 h.
HPT spheres were prepared by a photochemical reduction process: 1 g of HT was suspended by stirring in 200 mL of aqueous methanol solution containing a selected amount of
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(hydro)chloroplatinic acid. The mixture was irradiated with UV light using a Hg lamp (2 mW cm2) for 15 h, followed by centrifuging, washing and drying at 80 °C for 4 h under vacuum. For convenient comparison, in this experiment, seven final samples of HPT with different ratios of Pt to TiO2 (0, 0.1, 0.3, 0.5, 0.75, 1.0, and 2.0 wt.%) were prepared. The steps used to prepare the samples are illustrated in Scheme 1. As well as this, Pt-deposited P25 nanoparticles were prepared by dispersing 1 g of P25 powder in 200 mL of aqueous methanol solution a selected amount of (hydro)chloroplatinic acid. The following steps were the same as those used for the preparation of HPT. 2.3. Evolution of photocatalytic activity The resulting HPT photocatalysts were evaluated for photodecomposition of methylene blue. The reactor was composed of a 350 W Xe arc lamp with the main emission wavelength at 420 nm (Nanjing JYZCPST Co., Ltd.) and a magnetic stirrer. The Xe
595
arc lamp was surrounded by a quartz jacket and placed within the inner part of a quartz reactor vessel (5.8 cm in diameter and 68 cm in length), through which a suspension of MB and the photocatalyst was circulated. An outer recycling water glass jacket maintained a constant reaction temperature (24 °C). Throughout the experiment, the solution was continuously stirred. Aqueous suspensions of the photocatalysts were prepared by suspending the synthesized nanoparticles that had been previously sonicated at high frequency for 30 min. A selected amount of catalyst and 50 mL of contaminated solution (20 mg L1 for MB) were stirred in the dark for half an hour to reach dissolved oxygen saturation and absorption equilibrium, and then irradiated for 300 min. Aliquots were withdrawn from the irradiated solutions at appropriate intervals and analyzed by UV–vis spectroscopy (Shimadzu UV-160A) at 664 nm for MB. Photolysis experiments were performed, without photocatalyst, using the same experimental setup previously described for the photocatalytic system.
Fig. 4. (a) UV–vis diffuse reflectance spectra of samples. (b) Band gap evaluation for linear dependence of (ahv)2 versus photon energy for the samples. The inset shows the plot for photon energies of 3.2–3.5 eV.
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Fig. 5. XPS spectra of (a) HT and (b) 0.75% HPT samples.
Table 2 Atomic concentrations based on C 1s, O 1s, Ti 2p, and Pt 4f XPS peaks. Catalysts
C 1s
O 1s
Ti 2p
Pt 4f
HT HPT
28.32 29.38
49.66 50.39
22.02 18.68
0.00 1.55
2.4. Characterization Phase structure was obtained using an X’TRA X-ray diffractometer ((XRD) ARL, Switzerland) and Cu Ka radiation. High-resolution transmission electron micrographs (HRTEM) of HT were obtained using a Jeol JEM-200CX microscope. UV–vis diffuse reflectance spectra were recorded on a UV-2450PC spectrometer (Shimadzu, Japan) with barium sulfate as the reference sample. X-ray photoelectron spectra (XPS) were measured on an ESCALAB 250 spectrometer (Thermo Fisher Scientific, Waltham, MA) with an Al Ka X-ray source (1486.6 eV). Total Organic Carbon (TOC) was determined using a TOC analyzer (TOC-5000, Japan). Photoluminescence spectra (PL) were recorded with a fluorescence spectrometer
(Fluorolog-3-Tau, France) using a Xe lamp as the excitation light source, and a 420 nm UV cut-off filter (to ensure only visible light reached the sample). A Bruker spectrometer (Bruker instrument, Inc.) equipped with a Xe lamp was used to measure the electron paramagnetic resonance (EPR) signals of radicals trapped by 5,5dimethyl-1-pyrroline N-oxide (DMPO). Magnetic parameters of the radicals detected were obtained from direct measurements of magnetic field and microwave frequency. 3. Results and discussion 3.1. Structures and properties of HPT nanospheres The phases of samples were investigated by XRD, and the patterns from the HT, 0.75% HPT and 2% HPT spheres are shown in Fig. 1. HT spheres exhibit peaks characteristic of anatase phase titania (major peaks: 25.3°, 38.0°, 48.0°, and 54.7° 2h). The deposition of Pt cannot change the anatase phase. Table 1 displays the XRD parameters of different samples. The average crystallite size of HT spheres is estimated to be 18 nm. It is worth noting that
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Fig. 6. High-resolution XPS spectra of HT and 0.75% HPT samples. Core levels of O 1s, Ti 2p and Pt 4f.
appropriate addition of Pt can increase the size of the anatase crystals, and that the 0.75% HPT sample exhibits the best crystallinity. The interplanar crystal spacing of anatase is about 0.35 nm for the samples. Fig. 2 depicts the morphology of the as-synthesized colloidal carbon spheres and HT spheres, along with a high magnification image of the HT spheres. The as-synthesized colloidal carbon spheres are monodisperse with an average diameter of about 350 nm. Fig. 2b and c shows that the TiO2 layer in HT is about 65 nm thick, while the anatase crystal size phase is about 20 ± 2 nm. The inset of Fig. 2d displays the distribution of titanium anatase phase d-spacings; the spacing value is about 0.35 nm. These results are consistent with the XRD measurements. TEM images of the as-synthesized HPT are shown in Fig. 3. There are no obvious differences in morphology between HT and HPT. The layer of TiO2 in Fig. 3b encapsulates dispersed Pt nanoparticles that appear as black dots, about 3 nm in size in this image (marked with arrows). Fig. 3c and d shows the distributions of d-spacings in HPT as around 0.34 nm for titania and 0.22 nm for Pt particles, respectively, measured from the rectangles on the TEM image in Fig. 3b. Fig. 4a shows the UV–vis diffuse reflectance spectra of HT and HPT photocatalysts with different Pt/TiO2 mass ratios. The spectra show exhibit a broad and very strong absorption in the range from 200 to 350 nm. The HPT photocatalysts show huge optical absorption in the visible light region, and the absorption intensity increases with increasing Pt content in the samples. The
mechanism of visible light photosensitization can be explained in two ways. First, the deposited Pt nanoparticles can absorb visible light. Second, the Pt particles can change the original energy equilibrium, and produce some lower energy levels. Mizushima et al. [25] have confirmed the existence of defect energy levels attributable to Ti3+, which forms as a result of the deposition of noble metal; these levels are positioned near the TiO2 valence band. The energy band gaps of these samples can be calculated using an equation that had been widely adopted for crystalline semiconductors:
ahv ¼ Aðhv EgÞn=2 where a, h, v, A, Eg and n are the absorption coefficient, Planck constant, incident light frequency, a constant, the band gap and an integer, respectively. An n value of 1 was chosen for all the samples, indicating that the observed optical transition should be direct [26–28]. Fig. 4b shows plots of (ahv)2 versus photon energy. The band gap energies (Eg) of HT, 0.1% HPT, 0.3% HPT, 0.5% HPT, 0.75% HPT and 1% HPT are estimated to be 3.05, 2.94, 2.93, 2.90, 2.82 and 2.84 eV, respectively. Fig. 5 shows typical XPS survey spectra of HT and 0.75% HPT samples. Peaks from C 1s, Pt 4f, O 1s and Ti 2p orbitals can be identified. The C 1s line originates from surface impurity carbons, such as adsorbed CO2, as well as carbon pollution from XPS instrument. The atomic concentrations of all the elements in HT and 0.75% HPT
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samples are listed in Table 2. The high-resolution XPS spectra and relevant data about the O 1s, Ti 2p, and Pt 4f regions are shown in Fig. 6, respectively. The O 1s region can be deconvoluted into two peaks. The peaks at 530.5 eV and 529.5 eV for HT are assigned to the adsorbed hydroxyl oxygen (Oad) and crystal lattice oxygen
(Ola), respectively. However, the values are slightly shifted to lower binding energies compared with the data for HPT (Oad: 529.9 eV, Ola: 528.9 eV), which can be indicative of the effect of the platinum. The atom ratio of Ti to Ola is 1:1 for HT, whilst that to total oxygen is 1:2. This indicates that there are oxygen deficiencies present in
Fig. 7. (a) UV–vis spectra of MB after 0.5 h dark absorption onto various samples; (b) UV–vis spectra of MB with 0.75% HPT under visible light for times up to 5 h. Photocatalytic degradation efficiencies of MB after particular irradiation times: (c) using different amounts of 0.75% HPT; (d) using samples with different Pt/TiO2 mass ratios (2 g L1). (e) Pseudo-first-order kinetic rate plot and (f) the corresponding degradation rate constants for different samples.
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Fig. 8. Evolution of total organic carbon during the degradation of MB (a) versus amount of 0.75% HPT catalyst and (b) versus Pt/TiO2 mass ratio of catalyst.
Fig. 9. PL spectra of HT and HPT samples under 430 nm visible light.
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Scheme 2. (a) Schematic drawing of photocatalytic degradation of MB by HPT photocatalyst under visible light; (b) energy level variation at the HT–Pt interface before and after contact.
the near-surface region of HT. These can then lead to adsorption of large amounts of oxygen onto the surface of HT during the photochemical reduction process [29]. The surface Oad can play an important role in the photocatalytic reactions, because the photoinduced holes can attack the surface hydroxyls and yield surfacebound OH radicals with high oxidation capability [30–32]. The content of crystal lattice oxygen decreases as Pt is added, with the atom ratio of Ti to Ola being 2.6:1 in the 0.75% HPT sample, indicating that Pt is mainly deposited on the crystal lattice oxygen sites [33]. Binding energies for Ti(2p3/2) peaks at 458.6 eV clearly correspond to Ti4+ in the HT structure. No amount of Ti3+ was detected in the HT sample. However, the XPS spectra of the 0.75% HPT sample showed the presence of two peaks corresponding to the trivalent and tetravalent states of Ti. It is clear that Ti3+ arose from the deposition of Pt onto the surface of TiO2, which implies that interaction between Pt and the TiO2 matrix occurred during the photochemical reduction process. This result is consistent with that of Li and Li [34]. In the platinum region, the HPT sample shows an XPS profile in which peaks at 70.4 eV, 73.8 eV and 75.4 eV can be identified. The valence states of Pt are Pt0 and Pt4+. The formation of Pt0 is because H2PtCl6 is reduced by the photogenerated electrons in TiO2 when the aqueous suspension containing TiO2 and H2PtCl6 is irradiated by light with energy greater than the band gap of TiO2.
3.2. Photodegradation activity Fig. 7a shows UV–vis spectra of MB in contact with different catalyst samples after 0.5 h dark adsorption. The adsorption ability of MB improves as the deposition of Pt increases. The MB dye and phenol molecules are stable under direct photolysis, indicating that both catalyst and light are essential for degradation of contaminates. Fig. 7b shows UV–vis spectra of MB with 0.75% HPT after particular irradiation times. The intensity of the characteristic absorption peak at 664 nm decreases substantially and undergoes hypsochromic shift as irradiation time increases, indicating that direct cleavage of the MB chromophore structure and N-demethylation occurred simultaneously during photoreaction in the presence of 0.75% HPT photocatalyst. Fig. 7c displays the efficiency of photocatalytic degradation of MB after particular irradiation times, using
different amounts of 0.75% HPT. As the catalyst amount increases from 0.1 g L1 to 4 g L1, the photocatalytic efficiency at first increases and then becomes stable. 2 g L1 of 0.75% HPT is the dose required for optimal degradation efficiency for MB. The reason for this is that as the number of HPT spheres increases, the number of active surface sites also increases. However, light scattering and reduction of light penetration at relatively high catalyst loadings have a negative impact on efficiency, especially when the catalyst concentration is over 2 g L1 [35,36]. Fig. 7d demonstrates the effect of Pt deposition on the degradation efficiency after particular illumination times at the optimized catalyst amount (2 g L1). The degradation ratio of MB increases with an increasing Pt loading until a Pt/TiO2 mass ratio of 0.75% is reached. At higher Pt concentrations, the degradation efficiency begins to drop off and then becomes steady. Fig. 7e shows that the degradation of MB can be described as a first-order reaction; the time-dependence of the concentration can be fitted to ln(c0/c) = kappt. For comparison, the photocatalytic activities of P25 and Pt–P25 were also studied; these catalysts gave kapp values of 0.0025 and 0.0105 min1, respectively. The enhanced photocatalytic performance for HPT may be because of its high dispersion and light scattering. In contrast, small P25 particles tend to agglomerate, leading to a decrease in surface area, and reducing the photodecomposition of MB. The apparent photocatalytic degradation rate constants (kapp) for samples with different Pt contents are plotted in Fig. 7f, with 0.75% HPT showing the highest kapp. To study the mineralization of the dye, TOC removal for MB was investigated. This technique is another way of evaluating removal of pollutants, and is complementary to examining the decolorization level. The removal efficiency of TOC for MB as a function of catalyst amount for 0.75% HPT, and as a function of Pt/TiO2 mass ratio in the catalyst, are shown in Fig. 8a and b, respectively. It is worth noting that some TOC is still found in all systems, even after the suspension is completely discolored, implying that some reaction intermediates form which are more resistant than MB to total oxidation, thus leading to partial mineralization. The stability of 0.75% HPT photocatalysts was also studied by monitoring the photocatalytic activity during four cycles of use (Fig. S1). After each run of 5 h, samples were recovered, and dried at 100 °C for 2 h and were reused for photocatalytic study. It is found that the photocatalytic activity of the recycled photocatalyst decreased indistinctively after three recycles, and the removal of MB can still reach 85%, which reveals that the HPT photocatalyst prepared in this study is stable and effective for the removal of organic pollutants in water. 3.3. Photodegradation mechanism In this section we propose photoreaction processes for the photoinduced degradation of MB over Pt-deposited hollow TiO2 nanospheres, based on the data presented above. PL emission spectra are often used to reveal the efficiency of charge carrier trapping, migration and transfer, and to understand the fate of electron–hole pairs in semiconductor particles [37,38]. In this study, the PL emission spectra of HT and 0.75% HPT samples were examined, as shown in Fig. 9. All the HPT samples display a broad-band emission from 530 to 580 nm, while no PL signal is observed in this range for HT photocatalyst. The energy of the visible light used for excitation is not sufficient to promote electronic transitions from the valence band (VB) to conduction band (CB) for HT, according to the UV–vis diffuse reflectance spectra. Therefore, no electron/hole pairs can be generated to form PL signals. However, for the HPT samples, it is generally believed that a lower stimulated PL intensity indicates an enhanced separation and transfer of photoinduced electrons trapped in the photocatalyst. The PL spectrum under visible-light excitation is usually attributed to a range of origins: oxygen
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vacancies, defect sites and surface states [39,40]. The presence of Pt on the surface of the HT favors the migration of photo-produced electrons to the metal, thus improving the electron–hole separation. Notably, the drastic quenching of PL intensity in 0.75% HPT suggests a markedly enhanced charge separation, compared with that observed in other HPT samples. This promotes visible-light photocatalytic activity, which will be confirmed in the following section. Under visible light illumination, HT is not excited, as its absorption threshold is 406 nm; only the MB molecule is excited to produce the excited-state MB. MB can be reduced by conduction band electrons into MB (1.08 V) or oxidized by valence band holes into MB+ (0.23 V) by light absorbed by MB rather than by catalyst, followed by decomposing into leuco-MB (0 V) (see Scheme 2) [41]. The energy level of free electrons for Pt is higher than that for TiO2. The Pt particles deposited on the surface of HT can change the original equilibrium by drawing the electrons out of the bulk HT through the HPT interface until the Fermi levels are equal. The electrons react with adsorbed O2 to produce super oxide O 2 radical anions and then OH radicals (see Scheme 2a). The HT layer accumulates positive charges because of the loss of electrons, and thus the band curves upward (see Scheme 2b). This elec-
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tron transfer leads to efficient electron/hole separation and suppresses somewhat the recombination rate since the rate depends on the densities of the carriers within the HT particles, and these densities have decreased due to the introduction of Pt [42]. The electrical potential gradient also becomes lower because of the decrease in the electron density gradient. It is well known that the potential gradient and the carrier concentration gradient are the two main ‘‘driving forces’’ for carriers to drift and diffuse during photocatalysis. When the two gradients are too small to increase the electron flux through the HPT interface further, a new equilibrium is reached; at this point, further platinum deposition cannot increase the charge separation further and thus cannot further increase the decomposition rate. Therefore, there should be an optimal Pt amount. The deductions are supported by the EPR spectra in Fig. 10. No EPR signals were detected when the reaction was performed in the dark (Fig. 10a). However, after irradiation, both the samples containing HT and 0.75% HPT clearly show the classical 1:2:2:1 spectral signature of trapped OH radicals. The intensity for 0.75% HPT is obviously stronger than that for HT, which means that more OH radicals are produced in photoreaction with HPT photocatalyst, or, in other words, the 0.75% HPT system is photocatalytically
Fig. 10. DMPO trapping EPR spectra (a) for HT and 0.75% HPT samples in an aqueous dispersion (to evaluate DMPO-OH), and (b) for HT, and 0.75% HPT photocatalysts and pure MB without catalyst in methanol dispersion (to evaluate DMPO- O 2 ).
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more active than the HT system. This further confirms the mechanism of the enhancement of the photocatalytic activity of HPT. Fig. 10b shows the EPR spectra for 0.75% HPT and HT in methanol dispersion (used to detect DMPO- O 2 ). Similarly, no EPR signals were observed when the reaction was performed in dark in the presence of the catalysts. However, weak EPR signals appeared without photocatalyst, indicating that MB can be photosensitized under visible light. Under illumination in the presence of photocatalyst, EPR signals from DMPO- O 2 adducts (identified by their characteristic six peaks) are observed in both 0.75% HPT and HT methanolic dispersions. The EPR signals from O 2 were stronger in the presence of HPT than in the presence of HT, implying that more O 2 is produced when HPT is present. The EPR results provide a strong indication that the photogenerated charge carriers in 0.75% HPT photocatalyst can not only be efficiently excited by visible light to create electron–hole pairs but are also long-lived enough to react with the surface adsorbed O2 or H2O to produce OH radicals. These generated OH radicals play a pivotal role, and are recognized as the main reactive species responsible for the degradation of pollutants [28,43]. 4. Conclusion In summary, well-defined Pt-deposited hollow TiO2 nanospheres were successfully prepared through a simple method involving synthesis of hollow anatase-phase TiO2 and photoreduction to deposit Pt. The Pt nanoparticles were dispersed on the layer of HT spheres; the crystal sizes of anatase-phase TiO2 and Pt nanoparticles were about 20 and 3 nm, respectively. The deposition of Pt nanoparticles effectively narrowed the band gap of HT photocatalyst, improving the visible-light responsive ability of the photocatalyst. XPS spectra imply that, compared with the Pt-free HT spheres, the HPT photocatalyst can yield more surface-bound OH radicals with high oxidation capability. Interaction between Pt and the TiO2 matrix occurred during photoreduction, and the presence of both Pt0 and Pt4+ on the surface of the HPT spheres was confirmed. PL emission spectra indicate that 0.75% HPT shows markedly enhanced charge separation compared with HT and other HPT samples. The active species, OH and O 2 radicals, were detected by EPR – these species are believed to be responsible for the photodegradation of organic pollutants in the presence of HPT photocatalyst. The photocatalytic activity results indicate that 0.75% HPT photocatalyst exhibits the best photocatalytic activity, showing a kapp value five times that of P25 and three times that of HT for decomposition of MB under visible light. Thus Pt-deposited hollow TiO2 photocatalysts are promising, effective photocatalysts in the visible region. The concept provided in the present paper should be applicable to fabrication of other hollow semiconductor photocatalysts, and deposition of other metal nanoparticles. Acknowledgements The authors gratefully acknowledge financial support from the Foundation of State Key Laboratory of Pollution Control and Resource Reuse of China, the Natural Science Foundation of China (No. 51008154), the Jiangsu Natural Science Foundation (No. SBK201022682), the Research Fund for the Doctoral Program of Higher Education of China (No. 20090091120007), the Fundamental Research Funds of the Central University (No. 1112021101), the Jiangsu Cultivation of Innovative Engineering for Graduate Students (CXZZ12_0063), the Scientific Research Foundation of the Graduate School of Nanjing University (No. 2012CL10), and the China Postdoctoral Science Foundation (funded Project No. 2012M511254).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2013.03.052.
References [1] M.A. Rauf, S.S. Ashraf, Fundamental principles and application of heterogeneous photocatalytic degradation of dyes in solution, Chem. Eng. J. 151 (2009) 10–18. [2] M. Khraisheh, L. Wu, A.H. Al-Muhtaseb, A.B. Albadarin, G.M. Walker, Phenol degradation by powdered metal ion modified titanium dioxide photocatalysts, Chem. Eng. J. 213 (2012) 125–134. [3] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 36 (2008) 515–582. [4] Y. Liu, J. Li, B. Zhou, H. Chen, Z. Wang, W. Cai, A TiO2-nanotube-array-based photocatalytic fuel cell using refractory organic compounds as substrates for electricity generation, Chem. Commun. 47 (2011) 10314–10316. [5] S. Cao, K.L. Yeung, P.L. Yue, An investigation of trichloroethylene photocatalytic oxidation on mesoporous titania–silica aerogel catalysts, Appl. Catal. B 76 (2007) 64–72. [6] A.A. Ismail, D.W. Bahnemann, Mesoporous titania photocatalysts: preparation, characterization and reaction mechanisms, J. Mater. Chem. 21 (2011) 11686– 11707. [7] S. De, S. Dutta, A.K. Patra, A. Bhaumi, B. Saha, Self-assembly of mesoporous TiO2 nanospheres via aspartic acid templating pathway and its catalytic application for 5-hydeoxymethyl-furfural synthesis, J. Mater. Chem. 21 (2011) 17505– 17510. [8] R. Molinari, A. Caruso, P. Argurio, T. Poerio, Degradation of the drugs gemfibrozil and tamoxifen in pressurized and de-pressurized membrane photoreactors using suspended polycrystalline TiO2 as catalyst, J. Membr. Sci. 319 (2008) 54–63. [9] M. Sasidharan, K. Nakashima, N. Gunawardhana, T. Yokoi, M. Inoue, S. Yusa, M. Yoshio, T. Tatsumi, Novel titania hollow nanospheres of size 28 ± 1 nm using soft-templates and their application for lithium-ion rechargeable batteries, Chem. Commun. 47 (2011) 6921–6923. [10] X. Ding, X. Song, P. Li, Efficient visible light driven photocatalytic removal of NO with aerosol flow synthesized B, N-codoped TiO2 hollow spheres, J. Hazard. Mater. 190 (2011) 604–612. [11] J. Choi, O. Kim, Y. Kwak, Suppression of dioxin emission in co-incineration of poly(vinyl chloride) with TiO2-encapsulating polystyrene, Environ. Sci. Technol. 41 (2007) 5833–5838. [12] T. Sreethawong, S. Yoshikawa, Impact of photochemically deposited monometallic Pt and bimetallic Pt–Au nanoparticles on photocatalytic dyesensitized H2 production activity of mesoporous-assembled TiO2–SiO2 mixed oxide nanocrystal, Chem. Eng. J. 197 (2012) 272–282. [13] N. Yao, S. Cao, K.L. Yeung, Mesoporous TiO2–SiO2 aerogels with hierarchal pore structures, Microporous Mesoporous Mater. 221 (2009) 570–579. [14] H. Strohm, P. Lobmann, Porous TiO2 hollow spheres by liquid phase deposition on polystyrene latex-stabilised pickering emulsions, J. Mater. Chem. 14 (2004) 2667–2673. [15] S. Ikeda, H. Kobayashi, T. Sugita, Y. Ikoma, T. Harada, M. Matsumura, Efficient photodecomposition of gaseous organics catalyzed by titanium(IV) oxide encapsulated in a hollow silica shell with high porosity, Appl. Catal. A 363 (2009) 216–220. [16] Y. Wang, X. Su, S. Lu, Shape-controlled synthesis of TiO2 hollow structures and their application in lithium batteries, J. Mater. Chem. 22 (2012) 1969–1976. [17] H.B. Zeng, W.P. Cai, P.S. Liu, X.X. Xu, H.J. Zhou, C. Klingshirn, H. Kalt, ZnO-based hollow nanoparticles by selective etching: elimination and reconstruction of metal-semiconductor interface, improvement of blue emission and photocatalysis, ACS Nano 2 (2008) 1661–1670. [18] R. Asahi, T. Morikawa, Nitrogen complex species and its chemical nature in TiO2 for visible-light sensitized photocatalysis, Chem. Phys. 339 (2007) 57–63. [19] M.H.H. Mahmoud, A.A. Ismail, M.M.S. Sanad, Developing a cost-effective synthesis of active iron oxide doped titania photocatalysts loaded with palladium, platinum or silver nanoparticles, Chem. Eng. J. 187 (2012) 96–103. [20] O. Rosseler, M.V. Shankar, M.K. Du, L. Schmidlin, N. Keller, V. Keller, Solar light photocatalytic hydrogen production from water over Pt and Au/TiO2(anatase/ rutile) photocatalysts: influence of noble metal and porogen promotion, J. Catal. 269 (2010) 179–190. [21] P. Finetti, F. Sedona, G.A. Rizzi, U. Mick, F. Sutara, M. Svec, V. Matolin, K. Schierbaum, G. Granozzi, Core and valence band photoemission spectroscopy of well-ordered ultrathin TiOx films on Pt(1 1 1), J. Phys. Chem. C 111 (2007) 869–876. [22] Y. Ishibai, J. Sato, T. Nishikawa, S. Miyagishi, Synthesis of visible-light active TiO2 photocatalyst with Pt-modification: role of TiO2 substrate for high photocatalytic activity, Appl. Catal. B 79 (2008) 117–121. [23] Y.H. Ng, S. Ikeda, T. Harada, S. Park, T. Sakata, H. Mori, M. Matsumura, Photocatalytic route for synthesis of hollow porous carbon/Pt nanocomposites with controllable density and porosity, Chem. Mater. 20 (2008) 1154–1160. [24] C.H. Li, Y.H. Hsieh, W.T. Chiu, Study on preparation and photocatalytic performance of Ag/TiO2 and Pt/TiO2 photocatalysts, Sep. Purif. Technol. 57 (2007) 148–151.
B. Wang et al. / Chemical Engineering Journal 223 (2013) 592–603 [25] K. Mizushima, M. Tanaka, A. Asai, S. Iida, Impurity levels of iron-group ions in TiO2(II), J. Phys. Chem. Solids 40 (1979) 1129–1140. [26] X. Li, S. Ouyang, N. Kikugawa, J. Ye, Novel Ag2ZnGeO4 photocatalyst for dye degradation under visible light irradiation, Appl. Catal. A 334 (2008) 51–58. [27] F. Dong, H. Wang, Z. Wu, One-step ‘‘Green’’ synthetic approach for mesoporous C-doped titanium dioxide with efficient visible light photocatalytic activity, J. Phys. Chem. C 113 (2009) 16717–16723. [28] W. Zhao, Y. Wang, Y. Yang, J. Tang, Y. Yang, Carbon spheres supported visiblelight-driven CuO–BiVO4 heterojunction: preparation, characterization, and photocatalytic properties, Appl. Catal. B 115–116 (2012) 90–99. [29] L. Jing, B. Wang, B. Xin, S. Li, K. Shi, W. Cai, H. Fu, Investigations on the surface modification of ZnO nanoparticle photocatalyst by depositing Pd, J. Solid State Chem. 177 (2004) 4221–4227. [30] L. Jing, X. Sun, W. Cai, Z. Xu, Y. Du, H. Fu, The preparation and characterization of nanoparticle TiO2/Ti films and their photocatalytic activity, J. Phys. Chem. Solids 64 (2003) 615–623. [31] H. Chen, S.W. Lee, T.H. Kim, B.Y. Hur, Photocatalytic decomposition of benzene with plasma sprayed TiO2-based coatings on foamed aluminum, J. Eur. Ceram. Soc. 26 (2006) 2231–2239. [32] J. Li, J. Zeng, L. Jia, W. Fang, Investigations on the effect of Cu2+/Cu1+ redox couples and oxygen vacancies on photocatalytic activity of treated LaNi1xCuxO3 (x = 0.1, 0.4, 0.5), Int. J. Hydrogen Energy 35 (2010) 12733– 12740. [33] M.A. Aramendı´a, J.C. Colmenares, A. Marinas, J.M. Marinas, J.M. Moreno, J.A. Navı´o, F.J. Urbano, Effect of the redox treatment of Pt/TiO2 system on its photocatalytic behaviour in the gas phase selective photooxidation of propan2-ol, Catal. Today 128 (2007) 235–244. [34] F.B. Li, X.Z. Li, The enhancement of photodegradation efficiency using Pt–TiO2 catalyst, Chemosphere 48 (2002) 1103–1111.
603
[35] A.L. Giraldo, G.A. Penuela, R.A. Torres-Palma, N.J. Pino, R.A. Palominos, H.D. Mansilla, Degradation of the antibiotic oxolinic acid by photocatalysis with TiO2 in suspension, Water Res. 44 (2010) 5158–5167. [36] B. Lopez-Alvarez, R.A. Torres-Palma, G. Penuela, Solar photocatalitycal treatment of carbofuran at lab and pilot scale: effect of classical parameters, evaluation of the toxicity and analysis of organic by-products, J. Hazard. Mater. 191 (2011) 196–203. [37] Y. Yu, J. Yu, C. Chan, Y. Chen, J. Zhao, L. Ding, W. Ge, P. Wong, Enhancement of adsorption and photocatalytic activity of TiO2 by using carbon nanotubes for the treatment of azo dye, Appl. Catal. B 61 (2005) 1–11. [38] Y. Wu, J. Zhang, L. Xiao, F. Chen, Preparation and characterization of TiO2 photocatalysts by Fe3+ doping together with Au deposition for the degradation of organic pollutants, Appl. Catal. B 88 (2009) 525–532. [39] Y. Yu, J. Yu, J. Yu, Y. Kwok, Y. Che, J. Zhao, L. Ding, W. Ge, P. Wong, Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes, Appl. Catal. A 289 (2005) 186–196. [40] Y. Zhao, B. Yang, J. Xu, Z. Fu, M. Wu, F. Li, Facile synthesis of Ag nanoparticles supported on TiO2 inverse opal with enhanced visible-light photocatalytic activity, Thin Solid Films 520 (2012) 3515–3522. [41] G.K.R. Senadeera, T. Kitamura, Y. Wada, S. Yanagida, Deposition of polyaniline via molecular self-assembly on TiO2 and its uses as a sensitiser in solid-state solar cells, J. Photochem. Photobiol. A 164 (2004) 61–66. [42] B. Sun, A.V. Vorontsov, P.G. Smirniotis, Role of platinum deposited on TiO2 in phenol photocatalytic oxidation, Langmuir 19 (2003) 3151–3156. [43] Z. Wang, W. Ma, C. Chen, H. Ji, J. Zhao, Probing paramagnetic species in titaniabased heterogeneous photocatalysis by electron spin resonance (ESR) spectroscopy–a mini review, Chem. Eng. J. 170 (2011) 353–362.