Enhanced Photosensitized Degradation of Organic Pollutants under

ARTICLE pubs.acs.org/IECR

Enhanced Photosensitized Degradation of Organic Pollutants under Visible Radiation by (I2)n-Encapsulated TiO2 Films Xin Li,† Dong-Ting Wang,† Jian-Feng Chen,†,‡ and Xia Tao*,† † ‡

State Key Laboratory of OrganicInorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China Research Center of the Ministry of Education for High Gravity Engineering & Technology, Beijing University of Chemical Technology, Beijing 100029, China

bS Supporting Information ABSTRACT: We report on the preparation of (I2)n-encapsulated TiO2 (I2/TiO2) films with high photosensitized activity via a modified doctor blade technique, in which TiO2 nanocrystal particles and poly ethylene glycol (PEG) were introduced to improve specific surface area of films and adhesion between the films and the substrate. The as-prepared films were characterized by SEM, XRD, EDS, BET, and diffuse reflectance spectra. The visible-light-induced photoactivity of the films was evaluated by the degradation of organic pollutants containing rhodamine B (RhB), alizarin red S, fluorescein sodium, and 2,4-dichlorophenol in neutral water at ambient conditions. Compared to P25 film and homemade pure TiO2 film, the I2/TiO2 films exhibited higher photoreaction activity. Furthermore, under an optimized experimental condition i.e. ten wt% I/Ti doping ratio and 10 wt % PEG content, the degradation rate of the I2/TiO2 film was significantly better, with over 95% conversion ratio of RhB after 360 min of visible radiation. Subsequently, cycling degradation assay was taken, and the result showed that the film maintained an effective activity after 3 successive cycling experiments. This work may provide new insights into the design of flat and mechanical stable photocatalyst film with efficient visible light activity toward dyes and small organics degradation.

1. INTRODUCTION The utilization of renewable energy sources and the remediation of environmental contaminants are rapidly becoming mankind’s greatest challenges of modern society. Solar or visible light-induced photocatalytic technology can help to alleviate both issues by making dye-sensitized solar cells for future energy source supplies, splitting water for hydrogen generation and destroying toxic pollutants.13 TiO2, as a widely used heterogeneous photocatalyst for environmental cleanup, is only active in the UV range (λ < 385 nm).4 To utilize visible light that occupies 43% of the whole solar light energy, many attempts by researchers have been paid to modify the surface or bulk properties of TiO2 such as doping with various metal ions or nonmetals58 and mixing of two semiconductors.9 Dye sensitization is another effective method to extend the photoresponse of TiO2 into the visible range.10 The principle of TiO2 photosensitization is that the visible light excites the dye molecules adsorbed on TiO2 and subsequently injects electrons to conduction band (CB) of TiO2, leading to the formation of dye cationic radicals. The injected electrons then react with the dioxygen adsorbed on the surface of TiO2 to generate a series of reactive oxygen species such as O2•, H2O2, and 3 OH that can degrade target pollutants. In the photosensitized dye/TiO2/visible light system applied to the degradation of pollutants, various types of dyes have been used.1113 In most cases, organic dyes may serve as both a visiblelight harvester and a substrate to be degraded. But no further photoinduced transformation occurs when dye molecules are photobleached, thereby leading to the incomplete degradation and mineralization of dyes.14 Besides, some ruthenium complexes have also been used to sensitize TiO2 for the degradation of halogen-containing organic compounds and nitrogen oxides.11 r 2011 American Chemical Society

However, these costly ruthenium complexes are commonly unstable in aquatic environment under visible irradiation and readily experience a self-photodegradation process that makes them unsuitable for durable remediation of pollutants.15,16 To overcome the inefficiency and instability of the above-mentioned organic dye species in photosensitized treatment of pollutants, it is highly desirable to seek for a new dye based system capable of maintaining good visible activity and at one time avoiding external disturbance to customized sensitizer molecules. Recently, Usseglio et al.4 reported an iodine-encapsulated nanovoid-structured TiO2 material, in which encapsulated iodine species was protected in the interior of TiO2 framework walls and employed as a dye sensitizer to tune its photoactivity in the visible region. Though this material under sunlight exhibited high activity in the degradation of methylene blue, its relatively low specific surface area still remained a problem. In fact, a large specific surface area is believed to be indispensable for a highactivity photocatalyst. But to date, no further work associated with this has been done to improve favorable transport and consequent reaction of guest molecules. Generally, the visible light-driven degradation of organic pollutants is carried out in suspensions mediated by powder species.1719 In practical applications, the photoactive film is more viable due to its easy separation recovery and simple posttreatment process.20,21 As for photoactive coating layers, large specific surface area, sufficient mechanical/thermal stability, and Received: November 17, 2011 Accepted: December 18, 2011 Revised: December 12, 2011 Published: December 19, 2011 1110

dx.doi.org/10.1021/ie202641w | Ind. Eng. Chem. Res. 2012, 51, 1110–1117

Industrial & Engineering Chemistry Research good adhesion to the substrates are the basic requirements. Currently, the most commonly used coating methods are dipcoating, spraying, solgel, and evaporation-induced selfassembly (EISA),1922 among which EISA, pioneered by Brinker et al.23 and adapted by many other groups,24,25 is effective for preparation of highly organized mesostructured thin films with enlarged specific surface area and multiple scattering.8,2325 Another effective method of fabricating a flat and well-dispersed coating film is doctor blade technique, which has been widely used in the fabrication of electrode films of solar cells.26 The interest in the use of this method suitable for the remediation of organic compounds is due to several advantages: i) controlled large specific surface area able to enhance the interfacemediated photoreaction; ii) continuous channels and pores among particles apt to the mass transfer between reactant molecules and the active sites; and iii) strong adhesion and mechanical stability for coating layers and substrates capable of being repeatedly utilized in the photodegradation process. Such structural features have attractive bulk-chemistry applications, especially for heterogeneous photoreaction.5 However, to date, investigation on visible light-induced TiO2-based photocatalytic films synthesized by this technique has not been decumented. In this paper, we fabricated visible light-driven (I2)n-encapsulated TiO2 (I2/TiO2) films via a simple doctor blade method, in which (I2)n as a stable photosensitizer can exist in the nanocavities responsible for light harvesting. In the fabrication process of films, TNC particles and PEG were added so as to increase the surface area and the adhesion between the film and substrate. It was observed that the as-prepared I2/TiO2 films exhibited strong absorption in the 400750 nm range with a red shift in the band gap transition. The photosensitized activity of the films was assayed by monitoring the destruction of different kinds of substrates (Rhodamine B, Alizarin Red S, fluorescein sodium, and dichlorophenol). The degradation rate of the I2/TiO2 film under an optimized condition was obviously higher than that of pure TiO2 film or P25 film. The mechanism behind the improvement is also discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. Iodine crystal, titanium(IV) isopropoxide (TTIO), and polyethylene glycol M-20000 (PEG20000) were received from Sigma. Sodium hydroxide (NaOH), acetic acid (HAc), nitric acid (HNO3), ethanol (C2H5OH), and all other chemicals used in this study were purchased from commercial sources of analytical grade and used without further purification. Rhodamine B (RhB), Alizarin Red S (AR), fluorescein sodium (FL), and 2,4-dichlorophenol (2,4-DCP) were Acros products. Milli-pore water with a resistivity of 18.2 MΩ cm1 was used throughout the study. 2.2. Preparation and Characterization of (I2)n-Encapsulated TiO2 and TiO2 Nanocrystal. (I2)n-encapsulated TiO2 powders with different doping ratios were synthesized by a solgel route according to an analogous procedure as reported by Usseglio and Damin.4,21 Typically, 10 wt % (I2)n-encapsulated TiO2 with an iodine/TTIP weight ratio of 10% was obtained as follows: 3 g of TTIP, dissolving 0.3 g of iodine crystals, was hydrolyzed in air under continuous stirring. After 48 h, the obtained powder was dried at 373 K in an oven and then calcined in air at 773 K for 3 h. In parallel, other samples with different I2-doping ratios (I2/TTIP weight ratios of 0, 5%, 15%) were also

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synthesized by following the same procedure as mentioned above. To improve the adhesion between the fabricated film and glass substrate, TiO2 nanocrystals with average crystallite size around 16 nm were also prepared according to the reported procedure,4 based on hydrolysis of TTIP and peptization under hydrothermal condition of 180 °C for 12 h. The TiO2 nanocrystal powder shows a highly crystallized anatase structure (Figure S1 in the Supporting Information) and a large surface area of 180 m2/g (Table 2). The crystalline structures of the assynthesized (I2)n-encapsulated TiO2 were identified by an X-ray diffractometer (XRD) (X’ Pert PRO MPD, Panalytical) using Cu Kα radiation. The BrunauerEmmettTeller (BET) data were obtained using an ASAP 2010 surface area analyzer. High resolution transmission electron microscope (HRTEM, Jeol, JEM3010; 300 kV) was used to study the morphology of the (I2)nencapsulated TiO2 particles. 2.3. Preparation and Characterization of (I2)n-Encapsulated TiO2 (I2/TiO2) Films. The flat I2/TiO2 films were coated on glass substrate by using a simple doctor blade method. Prior to fabrication, the glass substrate (2 4 cm2) was ultrasonically cleaned for 10 min in alkali, diluted HCl, water, and alcohol, respectively. Afterward, the treated glass substrate was immersed in 0.4 M TiCl4 solution and then heated in a water bath at 70 °C for 30 min. 0.2 g of pure TiO2 or (I2)n-encapsulated TiO2, 2.0 g of TiO2 nanocrystal, and various contents of PEG (Mw 20000) were mixed in a mortar and ground for 1 h to produce a homogeneous mixture. The viscous suspension was then spread on the glass substrate with two tracks of Scotch tape (with a nominal thickness of 40 μm) as a mold to obtain a unique film thickness. After the tape was removed, the glass plate was first dried in air for 2 h and finally calcined at 500 °C for 1 h to burn off organics and consequently bind TiO2 onto the glass substrate. P25 film was also coated according to the abovementioned procedure, except the change of precursor to the mixture of 0.4 g of P25, 1.6 g of water, and 0.1 g of PEG (Mw 20000). The surface morphologies of I2/TiO2 films were observed by scanning electron microscope (SEM) images on a Hitachi S-4700 microscope. 2.4. Measurements of Visible Light-Sensitized Activity. The visible light source was a 500-W halogen lamp positioned inside a cylindrical Pyrex glass vessel that was surrounded by a recycling water glass jacket (Pyrex) to cool the lamp; meanwhile a cutoff filter was placed outside the water jacket to completely remove radiation below 420 nm, thereby ensuring illumination by visible light only. The above-mentioned films were placed in an 80 mL Pyrex vessel, immersing in 40 mL of aqueous solution containing given concentrations of target organics. Prior to irradiation, the reaction solution was magnetically stirred in the dark for 30 min to ensure the establishment of an adsorption/ desorption equilibrium. After irradiation, 3 mL aliquots at given time intervals were sampled and filtered through a Millipore filter (with a pore size of 0.2 μm). The concentration variations of the bulk solution in the process of photoreaction were monitored with a UVvis spectrophotometer (Shimadzu UV 2501 spectrometer) and high performance liquid chromatograph (HPLC, Waters 2965 and Waters 2996 photodiode array detector). In HPLC, a reverse-phase column (Waters SunfireTM, C18) with a particle size of 5 μm, the length of 150 mm, and an inner diameter of 4.6 mm was used, and the concentration signals were detected at 270 nm by using a mobile phase of methanol:water:phosphoric acid (70:30:0.1, V:V:V) with a flow rate of 1 mL/min. 1111

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Figure 1. XRD patterns of (a) pure TiO2; (b) 5 wt % (I2)n-encapsulated TiO2; (c) 10 wt % (I2)n-encapsulated TiO2; and (d) 15 wt % (I2)nencapsulated TiO2.

Figure 3. (A) Optical absorption spectra of (a) pure TiO2 film; (b) 5 wt % I2/TiO2 film; (c) 10 wt % I2/TiO2 film; and (d) 15 wt % I2/TiO2 film. (B) Plots of the square root of the KubelkaMunk function (F(R)) versus the photon energy (Eph).

Figure 2. HRTEM micrograph of 10 wt % (I2)n-encapsulated TiO2.

3. RESULTS AND DISCUSSION 3.1. XRD and HRTEM Measurements of (I2)n-Encapsulated TiO2 Nanoparticles. Structural characterization of (I2)n-

encapsulated TiO2 powders was performed by XRD. Figure 1 shows XRD patterns of pure TiO2 and (I2)n-encapsulated TiO2 with various doping contents after annealing at 500 °C. A series of characteristic peaks of 25.3°, 37.9°, 48.0°, 55.1°, and 62.7° were observed, corresponding to anatase (101), (004), (200), (211), and (204) crystal planes (JCPDS 21-1272). No peak from other allotropes of TiO2 (rutile and brookite) or shift caused by iodine doping was detected, and all as-prepared (I2)nencapsulated TiO2 nanoparticles exhibited pure anatase structure. This result is slightly different from that reported in the literature,4 among which the mixed structure containing anatase and rutile coexists. Since high quality anatase crystalline TiO2 can ensure a low density of charge trap rates inside the bulk of TiO2,27,28 the homemade (I2)n-encapsulated TiO2 with anatase structure is supposed to have high photoreaction activity.28,29 Besides, the average crystal size of samples was calculated from the full width at half-maximum (fwhm) of the (101) peak based on the Scherrer equation (d = 0.9 λ/β1/2 cosθ), corresponding to 31 nm, 35 nm, 35 nm, and 32 nm for pure TiO2 and 5 wt %, 10 wt %, and 15 wt % (I2)n-encapsulated TiO2, respectively. A typical HRTEM image of 10 wt % (I2)n-encapsulated TiO2 is shown in Figure 2. The regions of significant contrast, due to an important difference of material crossed by the beam, were observed, indicating that nanovoids exist in TiO2 particles.4,21 The color changes from slight brown to brown and fawn with increasing doping ratios were visible to the naked-eye. Moreover,

the visible-responsive (I2)n species as the dye are more likely to be trapped and protected in TiO2-based nanovoids on account of several aspects of reasons:12 i) that iodine is soluble in the titanium isopropoxide used as precursor allowing a straightforward synthesis, ii) that Ti-OR formed from the combustion of Ti precursor afforded a reducing character against the further oxidation of iodine, and iii) that all prepared samples were calcined at 500 °C. For comparison, undoped TiO2 as well as 5 wt % and 15 wt % (I2)n-encapsulated TiO2 were also synthesized by following the same procedure as 10 wt % sample, and the results indicated that all these particles maintained analogous nanovoid-structure (not shown here). 3.2. Diffuse Reflectance Spectra and Optical Micrographs of I2/TiO2 Films. The monolayer pure TiO2 film and I2/TiO2 films were formed using the doctor-blade method onto glass substrates and subsequently followed by thermal treatment. The steady-state diffuse reflectance spectra (DRUVS) of the as-prepared films were demonstrated in Figure 3A. Compared with pure TiO2 film, the absorption spectra of I2/TiO2 films had add-on shoulders onto the cutoff edge, extended the absorption from 400 nm to the visible light region. According to the energy band structure of TiO2, the optical absorption at the wavelength range of shorter than 400 nm is mainly attributed to electron transfer from the valence band to the conduction band, while the absorption in the visible region is induced by electron transfer from the ground state to the lowest excited states, giving rise to broad bands.29,30 These absorption bands centered around 500 nm are assigned to the iodine absorption.3133 Specifically, upon introducing iodine contents from 0 to 10 wt %, the absorption spectra of I2/TiO2 films markedly shifted toward longer wavelengths. Afterward, upon increasing the doping ratio up to 15 wt %, the absorption exhibited an obvious decrease in the visible range. A possible reason for the absorption decrease is that the excess amount of iodine present in the precursor might cause the aggregation of iodine molecules,12 leaving more iodine (its boiling point around 184 °C) on the surface of nanovoidstructured TiO2 particles. As a result, iodine nonencapsulated 1112



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Table 1. EDS Analysis of I2/TiO2 Film 5 wt %

10 wt %

15 wt %

samples

I2/TiO2 film

I2/TiO2 film

I2/TiO2 film

I2/Ti (g/g)

2.1%

3.1%

0.9%

Table 2. BET of TiO2 Nanocrystal Particles, 10 wt % (I2)nEncapsulated TiO2 Nanoparticles and I2/TiO2 Films with Different PEG Percentagesa BET/

BET/

m2/g

samples

TiO2 nanocrystal NPs

180

TiO2 film * with 10% PEG

69

10 wt % (I2)n-encapsulated

8


46

41


55

samples

m2/g

TiO2 NPs TiO2 film * no PEG a

TiO2 film * here is 10 wt % I2/TiO2 film.

inside the nanovoids, followed by the easy elimination at a high annealing temperature of 500 °C, would inevitably lead to the decrease of the actual amount of I2 encapsulated in the film. A similar variation trend toward the loading amount of I2 in the films was also evidenced by EDS data (Table 1), that is, the actual wt% of encapsulated I2 was 2.1% for 5 wt % I2/TiO2 film, 3.1% for 10 wt % I2/TiO2 film, and 0.9% for 15 wt % I2/TiO2 film, respectively. To further gain insight into the absorption red-shift of materials originated from I2-encapsuling, the band gap energies (Eg) of samples were estimated. As shown in Figure 3B, the plots of the KubelkaMunk functions (F(R))34,35 against the photon energy (Eg) exhibit that the band gap of pure TiO2 film was about 3.0 eV. After introducing iodine into the TiO2, the absorption threshold of I2/TiO2 films markedly shifted to low energy region, corresponding to 1.5 eV for 5 wt % I2/TiO2 film, 1.3 eV for 10 wt % I2/TiO2 film, and 2.2 eV for 15 wt % I2/TiO2 film, respectively. 3.3. Effect of PEG Content on I2/TiO2 Films. PEG as binder and pore forming agent has been widely employed to fabricate the electrode film with porous channel structure.1921 The morphology and the specific surface area of the film strongly depends on the addition amount of PEG.21 To successfully conduct photodegradation test by using I2/TiO2 films, well dispersed (I2)n-encapsulated TiO2 particles based films with flat surface and no cracking are required. Figure 4 shows the SEM images of I2/TiO2 films with different PEG (Mw 20000) additions. It can be seen that with increasing the wt% of PEG from 0 to 10%, the channels in the smooth film appeared and gradually increased (see Figure 4A-C). However, upon a further increase to 15%, particle agglomerates arose from the separated phase of excess PEG via a self-assembling occur (see Figure 4D).19 Additionally, the dependence of PEG on specific surface area of I2/TiO2 films was also corroborated by N2 adsorption measurement as summarized in Table 2, that is, 41 m2/g, 46 m2/g, 69 m2/g, and 55 m2/g at no PEG, 5% PEG, 10% PEG, and 15% PEG, respectively. Compared with (I2)nencapsulated TiO2 particles (SBET = 4 m2/g), as-prepared I2/TiO2 films exhibited higher specific surface area allowing the enhanced mass transfer between active states and organic targets and better photoactivity consequences. Hence, under our current experimental conditions, an optimal I2/TiO2 film

Figure 4. SEM micrographs of the surface morphology 10 wt % I2/ TiO2 films prepared from the precursor solution: (A) without PEG and with (B) 5 wt % PEG; (C) 10 wt % PEG; and (D) 20 wt % PEG.

Figure 5. (A) UVvis spectra changes of RhB (initial concentration: 5 106 M) as a function of irradiation time with 10 wt % I2/TiO2 film. (B) Comparison of the photocatalytic degradation of RhB (initial concentration: 5 106 M) in the presence of different films: (a) blank; (b) pure TiO2 film; (c) P25 film; (d) 5 wt % I2/TiO2 film; (e) 10 wt % I2/TiO2 film; and (f) 15 wt % I2/TiO2 film. Note: PEG content of all films above is 10 wt %.

with flat morphology and high surface area can be achieved at the 10 wt % PEG addition. 3.4. Dye Degradation by I2/TiO2 Films. The visible lightinduced activity of I2/TiO2 films were evaluated by the degradation of RhB solution (initial concentration is 5 106 M) under visible light irradiation (400 nm < λ < 800 nm). The temporal evolution of the spectra changes taking place during the decomposition of RhB is shown in Figure 5A. It can be seen that the main absorbance of RhB at ca. 554 nm markedly decreased with concomitant wavelength shift of the band to shorter wavelengths. As reported previously,22,36,37 the blue shift is caused by deethylation of RhB mediated by TiO2-based catalysts under visible light irradiation. When the de-ethylation reaction was complete 1113



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Figure 6. Profiles of cyclic tests of 10 wt % I2/TiO2 film (10 wt % PEG) in degradation of the RhB solution (initial concentration: 5 106 M) by visible light irradiation.

after 360 min of irradiation, the absorption band shifted to 495 nm and RhB turned to rhodamine. Rhodamine was then gradually decomposed with further irradiation up to 420 min due to the further destruction of the conjugated structure.36,37 In addition, it is also seen that not only the main absorbance in visible region but also the peaks in the UV region reduced with irradiation, indicating that the dye chromophores and aromatic ring was destroyed.26 To investigate the photoactivity of I2/TiO2 films with different iodine doping contents, a comparative experiment for the photodegradation of RhB was performed as shown in Figure 5B. The blank reaction (curve a) indicated that the RhB degraded scarcely in the absence of photocatalyst under visible light irradiation. Compared to pure TiO2 film (curve b) and P25 film (curve c), the degradation rate of RhB for all I2/TiO2 films (curves df) was markedly accelerated. Furthermore, the order of photodegradation activity of films displayed an increased trend in a sequence of 10 wt % I2/TiO2 film > 5 wt % I2/TiO2 film > 15 wt % I2/TiO2 film > P25 film > pure TiO2 film, consistent with their reflectance spectra in Figure 3A. The 10 wt % I2/TiO2 film (curve e) appeared to be the most efficient. Specifically, over 95% of RhB was degraded over the film after the irradiation for 360 min. Moreover, the stability of the 10 wt % I2/TiO2 film was also tested through the detection of the degradation rate of cycling runs for the photodegradation of RhB. As shown in Figure 6, there was no significant change in the conversion ratio of three cycling runs for the photodegradation implying the stability and reusability of the films. Other organic dyes such as fluorescein sodium (FL) and alizarin red (AR) (initial concentrations both are 2 105 M) were also chosen as target contaminants to evaluate the photodegradation activity of the 10 wt % I2/TiO2 film. The temporal absorption spectra for FL are shown in Figure 7A. Similar to RhB, the optical density at the maximum wavelength (λmax = 481 nm) decreased rapidly with increasing exposure time. As for the AR dye, the maximal absorption peak at 420 nm rapidly diminished when irradiated for 60 min; concomitantly, the absorbance peak at ∼260 nm attributed to the benzene ring decreased at a slow rate with illumination. Such a relatively slow decrease of AR at ∼260 nm can be related to the formation of intermediates from the degradation of the anthraquinone dye still containing a benzene ring.38 All these results indicate that under visible light irradiation the 10 wt % I2/TiO2 film can decompose the target dyes including some small molecular organic intermediates. 3.5. Degradation of 2,4-DCP by I2/TiO2 Film. 2,4-DCP with no absorbing photo with λ > 420 nm in aqueous solution13 as a target compound was used to assay small molecule pollutant degradation by the 10 wt % I2/TiO2 film. As shown in Figure 8,

Figure 7. UVvis spectra changes of dyes as a function of irradiation time with 10 wt % I2/TiO2 film (10 wt % PEG): (A) FL (2 105 M) and (B) AR (2 105 M).

Figure 8. Comparison of the photocatalytic degradation of 2,4-DCP (initial concentration: 1 104 M) in the presence of (a) blank; (b) pure TiO2 film; and (c) 10 wt % I2/TiO2 film.

upon irradiating 420 min with visible light, the concentration of 2,4-DCP remained unchanged in the absence of film (curve a), ca. 10% of 2,4-DCP was degraded by pure TiO2 film (curve b) and approximate 75% of 2,4-DCP was degraded by the I2/TiO2 film (curve c). In the reported literature,34 2,4-DCP formed a surface complex on TiO2 nanoparticles responsible for weak visible light absorption and subsequent visible-light-induced electron transfer. To further verify whether the visible lightactivated complex still exists in the fabricated TiO2-based film system, the DRUV spectra were also examined in this work (see Figure S2). As for pure TiO2 film, the absorption threshold of film has a slightly red shift after absorbing 2,4-DCP, which is in accord with the literature reports34,39 and responsible for the ca. 10% degradation of 2,4-DCP as above-mentioned. In contrast, the DRUV spectrum of the I2/TiO2 film shifted markedly toward longer wavelengths. Furthermore, the spectra profiles of the I2/TiO2 film before and after DCP absorbing retain almost identical. This means that the effect of surface complex formed between DCP and TiO2 on visible light response is trivial and ignored. Thus, it can also be reasonably deduced that the enhancement of photocatalytic activity of the I2/TiO2 film in the 2,4-DCP photodegradation under visible radiation is more likely to be assigned to the (I2)n dyes encapsulated and protected inside the nanovoid TiO2. 3.6. Proposed Mechanism of Degradation. According to the above overall experimental results and analysis, the I2/TiO2 1114



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mechanism, the generated O2 may form a series of active oxygen species (HO, O2•, 3 OH, and H2O2),39,41 thereby resulting in the degradation of organic pollutants. Based on the all above experimental results and analysis, it was found that the flat and well-dispersed coating I2/TiO2 film exhibited expanded visible-light absorption and narrowed band gap comparing with P25 and homemade pure TiO2 film. Furthermore, upon illumination with visible light, the degradation activity of dyes and small organics by I2/TiO2 film was significantly improved, mainly because the (I2)n adducts as photosensitizer may also be excited by visible light and consequently inject the photogenerated electrons into the CB of TiO2. We also found that I2/TiO2 film maintained efficient catalytic activity in cycling experiments, suggesting substantial potential in practical application.

Scheme 1. Schematic of Photosensitized Degradation of Organics on I2/TiO2 Film under Visible Radiation

Figure 9. Photodegradation of RhB (initial concentration: 5 106 M) over 10 wt % I2/TiO2 film (10 wt % PEG) under (a) N2-bubbled and (b) air-equilibrated condition with exposure to visible light.

films are found to have efficient photoactivity to both dyes and small molecule organic substances under visible light irradiation. According to the report of Usseglio et al.,4 the enhanced photoactivity of I2/TiO2 films may be attributed to the presence of (I2)n encapsulated in TiO2 nanoviodes. The (I2)n dye can work as a photosensitizer in the degradation process. As illustrated in Scheme 1, the (I2)n promotes into an excited electronic state (I2)n* via visible light irradiation, from which an electron can be injected into the conduction band (CB) of TiO2 according to eqs 1and 2 ðI2 Þn þ hν f ðI2 Þn

ð1Þ

ðI2 Þn þ TiO2 f ½ðI2 Þn þ þ TiO2 ðe Þ

ð2Þ

The efficiency of reaction 1 is high, as we use the (I2)n dye able to absorb visible photons. The efficiency of reaction 2 depends upon the quantum yield of the (I2)n dye/TiO2 redox process, which is determined by the match between the energy position of the (I2)n* level and the bottom of the TiO2 CB.12,35 The degradation results imply that the energy level of the excited electron in the (I2)n* molecule should be very close, or slightly above the bottom of the TiO2, thus guaranteeing a good quantum efficiency of reaction 2. Once the electron reaches the CB of TiO2, it is subsequently captured by O2 to form O2.3942 To examine the role of dissolved dioxygen in the photoreaction system, the photosensitized reactivity in the presence of dissolved oxygen and in a stricter anoxic condition by N2-sparging was examined by RhB photolysis using 10 wt % I2/TiO2 film, and the results were shown in Figure 9. Compared with air-saturated conditions, the photosensitized degradation rate of RhB was evidently suppressed under the anoxic suspension. Apparently, the presence of oxygen is responsible for the significant reduction. Following the standard degradation

’ CONCLUSION Visible-light-activated I2/TiO2 films with various molar ratios of I/Ti have been successfully synthesized via a simple doctor blade technique. With addition of TiO2 nanocrystalline particles and PEG, the fabricated I2/TiO2 films exhibit high specific surface area and strong adhesion to the substrate. The presence of iodine expands the photoresponse in visible light region. Under an optimized experimental conditions i.e. ten wt% I/Ti doping ratio and 10 wt % PEG content, the film exhibits an enhanced photosensitized degradation activity to dye and small molecule species under visible illumination. The film maintained high catalytic activity in 3 cycling experiments for RhB degradation. Therefore, this study could provide a new perspective on designing high-performance visible light photocatalyst films for environmental pollution control. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +86-10-6445-3680. Fax: +86-10-6443-4784. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by NSFC (Nos. 20906004, 20977005, 20821004). ’ REFERENCES (1) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature 2001, 414, 625–627. (2) Chen, C. C.; Ma, W. H.; Zhao, J. C. Semiconductor-mediated photodegradation of pollutants under visible-light irradiation. Chem. Soc. Rev. 2010, 39, 4206–4219. (3) Pan, L.; Zou, J. J.; Zhang, X. W.; Wang, L. Water-mediated promotion of dye sensitization of TiO2 under visible light. J. Am. Chem. Soc. 2011, 133, 10000–10002. (4) Usseglio, S.; Damin, A.; Scarano, D.; Bordiga, S.; Zcchina, A.; Lamberti, C. (I2)n Encapsulation inside TiO2: A way to tune photoactivity in the visible region. J. Am. Chem. Soc. 2007, 129, 2822–2828. 1115


Industrial & Engineering Chemistry Research (5) Wu, Y. M.; Xing, M. Y.; Zhang, J. L. Gel-hydrothermal synthesis of carbon and boron co-doped TiO2 and evaluating its photocatalytic activity. J. Hazard. Mater. 2011, 192, 368–373. (6) Livraghi, S.; Paganini, M. C.; Giamello, E.; Selloni, A.; Valentin, C. D.; Pacchioni, G. Origin of photoactivity of nitrogen-doped titanium dioxide under visible light. J. Am. Chem. Soc. 2006, 128, 15666–15671. (7) In, S.; Orlov, A.; Berg, R.; Garcia, F.; Jimenez, S. P.; Tikhov, M. S.; Wright, D. S.; Lambert, R. M. Effective visible light-activated B-doped and B,N-codeped TiO2 photocatalysts. J. Am. Chem. Soc. 2007, 129, 13790–13791. (8) Zhang, W.; Lei, L. C.; Zhang, J. L; Chen, Q, X.; Bao, J. G.; Fang, B. A novel CdS/S-TiO2 nanotubes photocatalyst with high visible light activity. Sep. Purif. Technol. 2009, 66, 417–421. (9) Zhang, Y. G; Ma, L. L.; Li, J. L.; Yu, Y. In situ feton reagent generated from TiO2/Cu2O composite film: a new way to utilize TiO2 under visible light irradiation. Environ. Sci. Technol. 2007, 41, 6264–6269. (10) Qin, G.; Sun, Z.; Wu, Q.; Lin, L.; Liang, M.; Xue, S. Dyesensitized TiO2 film with bifunctionalized zones for photocatalytic degradation of 4-cholophenol. J. Hazard. Mater. 2011, 192, 599–604. (11) Bae, E.; Choi, W. Highly enhanced photoreductive degradation of perchlorinated compounds on dye-sensitized metal/TiO2 under visible light. J. Phys. Chem. B 2006, 110, 14792–14799. (12) Xu, P.; Lu, J.; Xu, T.; Gao, S.; Huang, B.; Dai, Y. I2-hydrososeeded growth of (I2)n-C-codoped meso/nanoporous TiO2 for visible light-driven photocatalysis. J. Phys. Chem. 2010, 144, 9510–9517. (13) Vilac€ua, G.; Barathieu, K.; Jousseaume, B.; Toupance, T. New perylene-substituted organotrialkynyltin compounds for the photosensitization of Tin dioxide. Organometallics 2003, 22, 4584–4592. (14) Hu, C.; Hu, X. X.; Wang, L. S.; Qu, J. H.; Wang, A. M. Visiblelight-induced photocatalytic degradation of azodyes in aqueous AgI/ TiO2 dispersion. Environ. Sci. Technol. 2006, 40, 7903–7907. (15) Fan, F. R. F.; Bard, A. Spectral sensitization of the heterogeneous photocatalytic oxidation of hydroquinone in aqueous solutions at phthalocyanine-coated TiO2 powders. J. Am. Chem. Soc. 1979, 101, 6139–6140. (16) Li, Y. Z.; Zhang, H.; Hu, X. L.; Zhao, X. J.; Han, M. Efficient visible-light-induced photocatalytic activity of a 3D-ordered titania hybrid photocatalyst with a core/shell structure of dye-containing polymer/titania. J. Phys. Chem. C 2008, 112, 14973–14979. (17) Su, W. Y; Zhang, Y. F; Li, Z. H; Wu, L.; Wang, X. X.; Li, J. Q; Fu, X. Z. Multivalency iodine doped TiO2: preparation, characterization, theoretical studies, and visible-light photocatalysis. Langmuir 2008, 24, 3422–3428. (18) Yang, K.; Dai, Y.; Huang, B.; Whangbo, M. H. Density functional characterization of the band edges, the band gap states, and the preferred doping sites of halogen-doped TiO2. Chem. Mater. 2008, 20, 6528–6534. (19) Guo, B.; Liu, Z. L.; Hong, L.; Jiang, H. X. Sol gel derived photocatalytic porous TiO2 thin films. Surf. Coat. Technol. 2005, 198, 24–29. (20) Liu, Z. F.; Jin, Z. G.; Li, W.; Qiu, J. J. Preparation of ZnO porousthin films by sol-gel method using PEG template. Mater. Lett. 2005, 59, 3620–3625. (21) Yu, H. B.; Chen, S.; Quan, X.; Zhao, H.; Zhang, Y. B. Fabrication of a TiO2-BDD heterojunction and its application as a photocatalyst for the simultaneous oxidation of an Azo dye and reduction of Cr (VI). Environ. Sci. Technol. 2008, 42, 3791–3796. (22) Zhuang, J. D.; Dai, W. X.; Tian, Q. F.; Li, Z. H.; Xie, L. Y.; Wang, J. X.; Liu, P. Photocatalytic degradation of RhB over TiO2 bilayer films: effect of defects and their location. Langmuir 2010, 26, 9686–9694. (23) (a) Lu, Y. F.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W. L.; Guo, Y. X.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Continuous formation of supported cubic and hexagonal mesoporous films by sol-gel dip-coating. Nature 1997, 389, 364. (b) Brinker, C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. Evaporation-induced self-assembly: nanostructures made easy. Adv. Mater. 1999, 11, 579.

ARTICLE

(24) Zheng, C. C.; Xu, X. J.; He, F.; Li, L. D.; Wu, B.; Yu, G.; Liu, Y. Q. Preparation of high-quality organic semiconductor nanoparticle films by solvent-evaporation-induced self-assembly. Langmuir 2010, 26, 16730–16736. (25) Lu, Y. F.; Fan, H. Y.; Doke, N.; Loy, D. A.; Assink, R. A.; LaVan, D. A.; Brinker, C. J. Evaporation-induced self-assembly of hybrid bridged silsesquioxane film and particulate mesophases with entegral organic functionality. J. Am. Chem. Soc. 2000, 122, 5258–5261. (26) Yin, M. G.; Li, Z. S.; Kou, H. H.; Zou, Z. G. Mechanism investigation of visible light-induced degradation in a heterogeneous TiO2/Eosin Y/Rhodamine B system. Environ. Sci. Technol. 2009, 43, 8361–8366. (27) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Charvet, R.; Baker, R. H.; Gr€atzel, M. Enhance the performance of dye-sensitized solar cells by co-grafting amphiphilic sensitizer and hexadecylmalonic acid on TiO2 nanocrystals. J. Phys. Chem. B 2003, 107, 14336–14341. (28) Shen, L.; Bao, N.; Zheng, Y.; Gupta, A.; An, T.; Yanagisawa, K. Hydrothermal splitting of titanate fibers to single-crystalline TiO2 nanostructures with controllable crystalline phase, morphology, microstructure, and photocatalytic activity. J. Phys. Chem. C 2008, 112, 8809–8818. (29) Stoimenov, P. K.; Zaikovski, V.; Klabunde, K. J. Novel halogen and inter halogen adducts of nanoscale magnesium oxide. J. Am. Chem. Soc. 2003, 125, 12907–12913. (30) Li, G. S.; Zhang, D. Q.; Yu, J. C. A new visible-light photocatalyst: CdS quantum dots embedded mesoporous TiO2. Environ. Sci. Technol. 2009, 43, 7079–7085. (31) Hou, Q.; Zheng, Y. Z.; Chen, J. F.; Zhou, W. L.; Deng, J.; Tao, X. Visible-light-activated iodine-doped titanium dioxide nanocrystals for dye-sensitized solar cells. J. Mater. Chem. 2011, 21, 3877–3883. (32) Yamabi, S.; Imai, H. Crystal phase control for titanium dioxide films by direct deposition in aqueous solutions. Chem. Mater. 2002, 14, 609–614. (33) Liu, G.; Sun, C. H.; Yan, X. X.; Cheng, L. N.; Chen, Z. G.; Wang, X. W.; Wang, L. Z.; Smith, S. C.; Qi., Lu G.; Cheng, H. M. Iodine doped anatase TiO2 photocatalyst with ultra-long visible light response: correlation between geometric/electronic structures and mechanisms. J. Mater. Chem 2009, 19, 2822–2829. (34) Kim, S.; Choi, W. Visible-light-induced photocatalytic degradation of 4-chlorophenol and phenolic compounds in aqueous suspension of pure titania: demonstrating the existence of a surface-complexmediated path. J. Phys. Chem. B 2005, 109, 5143–5149. (35) Tojo, S.; Tachikawa, T.; Fujitsuka, M.; Majima, T. IodineDoped TiO2 Photocatalysts: Correlation between Band Structure and Mechanism. J. Phys. Chem. C 2008, 12, 14948–14954. (36) Dong, W. Y.; Lee, C. W.; Lu, X. C.; Sun, Y. J.; Hua, W. M.; Zhuang, G. H.; Zhang, S. C.; Chen, J. M.; Hou, H. Q.; Zhao, D. Y. Synchronousrole of coupled adsorption and photocatalytic oxidation on ordered mesoporous anatase TiO2SiO2 nanocomposites generating excellent degradation activity of RhB dye. Appl. Catal., B 2010, 95, 197–207. (37) Li, W. J.; Li, D. Z.; Meng, S.; Chen, W.; Fu, X. Z.; Shao, Y. Novel approach to enhance photosensitized degradation of rhodamine B under visible light irradiation by the ZnxCd1‑xS/TiO2 nanocomposites. Environ. Sci. Technol. 2011, 45, 2987–2993. (38) Faouzi, A. M.; Xasr, B.; Abdellatic, G. Electrochemical degradation of anthraquinone dye Alizarin Red S by anodic oxidation on borondoped diamond. Dyes Pigm. 2007, 73, 86–89. (39) Sun, Q.; Xu, Y. M. Sensitization of TiO2 with aluminum phthalocyanine: factors influencing the efficiency for chlorophenol degradation in water under visible light. J. Phys. Chem. C 2009, 113, 12387–12394. (40) (a) Tao, X.; Su, J. M.; Chen, J. F. Photooxidative degradation of dye pollutants accumulated in self-assembled natural polyelectrolyte micro-shells under visible radiation. Chem.—Eur. J. 2006, 12, 4164– 4169. (b) Tao, X.; Su, J. M. Confined photoreaction in nano-engineered multilayer microshells. Curr. Nanosci. 2008, 4, 308–313. 1116



ARTICLE

(41) Hu, X. L.; Li, G. S.; Yu, J. C. Design, fabrication, and modification of nanostructured semiconductor materials for environmental and energy applications. Langmuir 2010, 26, 3031–3039. (42) Sene, J. J.; Zeltner, W. A.; Anderson, M. A. Fundamental photoelectrocatalytic and electrophoretic mobility studies of TiO2 and V-doped TiO2 thin-film electrode materials. J. Phys. Chem. B 2003, 107, 1597–1603.

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