ARTICLE pubs.acs.org/IECR
Enhanced and Selective Degradation of Pollutants over Cyclodextrin/TiO2 under Visible Light Irradiation Xu Zhang,* Xuankun Li, and Nansheng Deng School of Resources and Environmental Science, Wuhan University, Wuhan, 430079, P.R. China
bS Supporting Information ABSTRACT: The photocatalytic performance of cyclodextrin modified TiO2 (TiO2/β-CD) on the degradation of rhodamine B and bisphenol A (BPA) was studied in this work. The interactions between β-CD and TiO2 varied as the synthesis method changes. When applying TiO2/β-CD synthesized from photoinduced self-assembly method as photocatalyst, the initial visible light (λ g 420 nm) degradation rates R0 of rhodamine B and BPA increased by 4.6 and 3.2 times respectively. The TiO2/β-CD hybrid material synthesized from adsorption method only showed enhancement in dye pollutants degradation. The reactivity of TiO2/β-CD under visible light was correlated with the specific surface area of unmodified semiconductor. The mechanism for the enhanced and selective degradation of pollutants over TiO2/β-CD was also proposed in this work.
1. INTRODUCTION It is well-known that dye pollutants can be bleached on semiconductors’ surface through photosensitized degradation under visible irradiation.15 Only the adsorbed excited dye molecule can inject its charge to the semiconductor’s conduction band,4,6 thus the photosensitized degradation is usually inefficient. Recently, it has been found that the overall conversion efficiency of dye sensitized solar cells (DSSC) can be enhanced through the modification of TiO2 electrode by host molecules.710 Therefore, it is reasonable to propose that the photosensitized degradation of dye pollutants could also be enhanced over host modified semiconductor because of the enhanced electron injection kinetics. Unlike dye pollutants, the colorless aromatic compounds have no absorption in the visible light region. Therefore, they were not supposed to undergo a visible-light sensitized degradation. However, visible-light induced degradation of some colorless aromatic compounds do occur over TiO2 because of the formation of charge transfer complex (CTC).1116 In light of the aforesaid researches, it is also quite worthwhile to study the visible-light induced degradation of colorless compounds over host modified TiO2, since most guest molecules could form inclusion complexes with the linked host. Physical adsorption method and photoinduced self-assembly method are the two different ways to synthesize host modified metal oxide. For instance, β-cyclodextrin modified TiO2 electrode also can be obtained by simple adsorption method.8 Carboxymethyl-β-cyclodextrin modified mesoporous TiO2 film was synthesized by immersing TiO2 film in corresponding cyclodextrin solution.17 Calixarenes and metallocalixarenes can be grafted to the surface of metal oxide by the physically adsorption process.18,19 As for photoinduced self-assembly methods, it has been reported that β-CD can coat the surface of anatase TiO2 particles irreversibly under UV irradiation to form wire-like composites.20 It was also reported that the TiO2β-cyclodextrinMWCNT composite wires can be synthesized by a solar-induced self-assembly process.9,21 To the best of our knowledge, there was no report about comparing the r 2011 American Chemical Society
photoactivity of the hybrid materials synthesized by these two different methods. In this work, TiO2/β-CD composite nanopowder were synthesized by physical adsorption and photo induced selfassembly method, and their structures were characterized. To compare the photoactivity of these hybrid materials, the degradation of rhodamine B and bisphenol A (Scheme 1) over theses two TiO2/β-CD hybrid materials were carried out. The effects of TiO2 surface area on the formation and photoactivity of TiO2/ β-CD were also investigated in this study.
2. EXPERIMENTAL SECTION Chemicals and Materials. Rhodamine B was purchased from Shanghai SSS Regent Co., Ltd. Biphenol A (99+%) was obtained from Sigma-Aldrich. β-CD (g97%) was purchased from Wako Pure Chemical Inc. Two different anatase TiO2 (named as T1 and T2) were purchased from High Technology Nano Co. Ltd. (Nanjing, China). Degussa P25 was obtained from Degussa AGGermany. Milli-Q water (resistivity >18.0 MΩ cm) was used as the solvent. All other reagents were analytical grade and used as received. TiO2/β-CD Synthesis. In a typical synthesis of TiO2/β-CD, a suspension containing 2.0 g/L TiO2 and 10.0 g/L β-CD was irradiated for 24 h under a UV disinfection lamp and then centrifuged, later the solid phase was collected and redispersed in a fresh 10.0 g/L β-CD solution which was irradiated for another 24 h. After being centrifuged, the solid phase of the suspension was carefully washed with ultrapure water until no β-CD was detected in the supernatant liquid by phenolphthalein colorimetry. Eventually, the TiO2/β-CD hybrid nanoparticles were dried under dynamic vacuum for 12 h at 50 °C. The samples prepared in this way were Received: August 1, 2011 Accepted: December 4, 2011 Revised: October 30, 2011 Published: December 05, 2011 704
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Scheme 1. Structures for β-Cyclodextrin, Bisphenol A, and Rhodamine B
Figure 1. UVvis DRS spectra of different catalysts, in the arrow direct: T2, T1, P25, P-T2-CD, and P-T1-CD. Inset: T1, A-T1-CD, P-T1-CD (pH 3.5), P-T1-CD (pH 10.0) and P-T1-CD.
of those suspensions were withdrawn to determine the concentration C0. During all degradation reactions, 5 mL aliquots were collected at selected time intervals, centrifuged and used for the determination of Ct .The degradation of rhodamine B was monitored by a Shimadzu 1601 UVVis spectrophotometer with 10 mm cuvette. The concentration of BPA was determined by HPLC method. The initial photodegradation rate R0 was discovered by extrapolating the tangent (based on the linear fit of the points) of the concentration profile back to initial conditions. The data were presented as means of duplicate or triplicate experiments.
Table 1. Physical Properties of the Catalysts Used in This Study crystalline form anatase
rutile
diameter/nma
BET (m2/g)
T1
100
0
1012
205
A-T1-CD
100
0
1012
198
P-T1-CD
100
0
1012
186
T2 A-T2-CD
100 100
0 0
1517 1517
91 89
P-T2-CD
100
0
1517
96
80
20
2325
47
catalysts
P25
3. RESULTS AND DISCUSSION UVvis DRS Spectra. It can be seen that T1 and T2 had almost the same absorption characteristics although the BET surface area of T1 is two times larger than that of T2 (Figure 1). P25 extended its absorption to 410 nm because of the existence of rutile phase. After modification with β-CD, all the hybrid materials synthesized by photo induced method had higher absorption intensity in the visible light region compared to the unmodified TiO2, especially for T1, whereas those prepared by physical adsorption method only improved slightly. As shown in the inset figure, P-T1-CD displayed relatively higher absorption in the visible light region than A-T1-CD . Since pH showed great influence on the adsorption of β-CD onto TiO2,23 the effect of synthesis pH on the corresponding hybrid material’s optical property were investigated. The spectra for P-T1-CD prepared at acidic pH (3.5) and alkaline pH (10.0) showed a relatively smaller absorbance at λ g 400 nm, which was probably because that the adsorption amount of β-CD onto TiO2 was larger at neutral pH than that at other pH values.23 XPS Spectra Analysis. XPS analysis was investigated to attain further information about the surface electronic structure and the chemical valence of both bare TiO2 and TiO2/β-CD hybrid catalysts. The high resolution XPS spectra of the O1s region were shown in Figure 2. The O1s region of pure TiO2 was decomposed into two contributions: the TiO bond in TiO2 and the surface hydroxyl. After modification, apart from the aforementioned inorganic oxygen species in pure TiO2, A-T1-CD had an additional organic oxygen contribution to the O1s region. However, this organic oxygen species was not discovered in A-T2-CD, probably because of its relative smaller specific surface area which results
The diameter was recovered from the fit of the experimental XRD data to Scherrer-equation. a
described as P-T1-CD and P-T2-CD for the later use (Table 1), and those prepared through adsorption method (The same processes as described above only omitting UV irradiation) were named as A-T1-CD and A-T2-CD. Characterization of Hybrid Materials. UVVis diffuse reflectance was recorded on a Shimadzu 2550 UVvis spectrophotometer between the wavelength of 200 and 800 nm with BaSO4 as the background. The BrunauerEmmettTeller surface area was determined by using a Gemini V 2380 setup. The X-ray diffraction (XRD) patterns of the prepared products were recorded by a DmaxrA powder diffractometer (Rigaku, Japan) with Cu Kα as a radiation and a step width of 2θ = 0.02°. X-ray photoelectron spectroscopy (XPS) measurements were performed on a XSAM 800 XPS system (Kratos, UK) with a monochromatic Al Kα source. High resolution transmission electron microscopy images were obtained on a JEOL JEM 2010FEF microscope (Japan Electronics, Japan) at an accelerating voltage of 200 kV. Photocatalytic Degradation Experiments. The photodegradation of rhodamine B and BPA were carried out by using a self-made photoreactor.22 Cutoff filters at 400 and 420 nm (Shimadzu) were used to shield off different wavelengths of light. Pollutant aqueous solutions (200 mL) with different catalysts were placed in 250 mL Pyrex beakers and well mixed by magnetic stirrers. The suspended solutions were equilibrated in the dark for half an hour before irradiation, then 5 mL aliquots 705
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Figure 2. High resolution XPS spectra of O1s region for (a) T1, (b) T2, (c) A-T1-CD, (d) A-T2-CD, (e) P-T1-CD, and (f) P-T2-CD.
in the smaller attachment of β-CD (Figure S1, Supporting Information). Another oxygen functionality at 532.5 eV (assigned as carbonyl oxygen) was found in the materials of P-T1-CD and P-T2CD. The formation of CdC and CdO bonds had already been confirmed by FTIR in our previous study.24 It was suggested that the formation of CdO double bonds was generated from the irreversible hole oxidation of the CO bond in cyclodextrin. Because the probability of desorption decreases roughly exponentially with the number of glucose units in the cyclodextrins,25 desorption of the attached cyclodextrin was quite difficult to achieve even though a few transient CdC and CdO bonds were formed during the self-assembly process. It was suggested that the specific surface area and the particle size of TiO2 played a key role in forming charge transfer complex. The ability of forming charge-transfer complex between 4-chlorophenol and TiO2 was found to be correlated with the surface area rather than the crystalline form.13 Thiolactic acid formed bidentate binding complex to the surface of small-sized titanium dioxide colloid, while only a monodentate binding via carboxyl group was detected on large-sized particle colloids’ surface.26 Although the adsorption of β-CD onto both TiO2 followed Langmuir isotherm model (Figure S1, Supporting Information), the amount of attached cyclodextrin on T1 was much larger than that of T2 due to its relative larger specific surface area. Therefore, the UVvis spectra change for T1 after modification was
larger than that of T2. The optical spectra change of the modified hybrid materials were similar with the carboxymethyl-β-CD modified TiO2 colloids.27 Therefore, the visible light absorbance for P-T1-CD and P-T2-CD was generated mainly due to the ligand to metal charge transfer (LMCT) between β-CD and TiIV located in an octahedral under-coordination environment.27,28 Unfortunately, based on the results in this work and those in the previous references, it was still not clear whether it was the primary rim or the secondary rim that attached to the surface of TiO2. High Resolution Transmission Electron Microscopy (HRTEM). HRTEM images were obtained on a JEOL JEM 2010FEF microscope operating at an accelerating voltage of 200 kV. As presented in Figure 3, there were no differences in the lattice structure between P-T2-CD and T2. The results were consistent with that of P-T1-CD and T1.24 Photodegradation of Rhodamine B. Photosensitized degradations of rhodamine B were first carried out to investigate the photoactivity of the cyclodextrin modified hybrid materials (Figure 4). The degradation efficiency followed the order by P25 > T1 > T2. This was probably because the rutile crystallites in P25 can enhance the usage of visible light and slow down the e/h+ recombination.29 The degradation of rhodamine B was drastically enhanced on A-T1-CD, P-T1-CD, A-T2-CD and P-T2-CD. It was found that the dynamic (electron from TiO2 conduction band to dye cation) for dye-cyclodextrin inclusion complex was almost two 706
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Table 2. Photodegradation of BPA in the Presence of 1 g/L Catalysts, BPA = 20 μM, λ g 420 nm catalyst
T1
T2
P25
P-T1-CD
P-T2-CD
R0 (μM/min)
0.045
0.026
0.016
0.145
0.065
Figure 3. HRTEM images of pure T2 (left) and P-T2-CD hybrid powders (right).
Figure 6. Mode of photocatalytic degradation of pollutants over TiO2/ β-CD hybrid nanopowder under visible light irradiation.
400 nm filter and simulated solar irradiation, respectively. The same trends can be observed in the photodegradation of Orange II.24 The R0 for rhodamine B degradation over T1 surface under λ g 365 nm was 7.1 times faster than that under the λ g 420 nm, indicating the photocatalysis degradation rather than photosensitized degradation was the predominant path for rhodamine B degradation under solar irradiation. When the catalyst was irradiated under λ g 365 nm, the linked cyclodextrin can scavenge the photogenerated holes,33 which depressed the electronhole recombination so that the degradation of rhodamine B over P-T1CD was enhanced. Photodegradation of BPA. The R0 for BPA degradation over different catalysts was listed in Table 2. Contrary to the degradation of rhodamine B, the degradation of BPA was not accelerated over TiO2/β-CD hybrid materials synthesized by physical adsorption method although it has a much larger inclusion constant with β-CD than that of rhodamine B.34 The rate constant for BPA degradation was 3.2 times higher in P-T1-CD suspension than that in T1 suspension. Compared with the BPA degradation over T2, a 2.5 times higher R0 was obtained over P-T2-CD. For bisphenol E and bisphenol F, which have a high inclusion constant with β-CD,34 their degradation were also promoted by using TiO2/β-CD as catalysts (Figure S2, Supporting Information). Preliminary Mechanism for the Visible Photoactivity of TiO2/β-CD. On the basis of the above results, it was found that TiO2/β-CD synthesized by photoinduced self-assembly method showed enhancement for both rhodamine B and BPA, while the TiO2/β-CD synthesized by adsorption method only showed high enhancement for rhodamine B degradation. The preliminary mechanism for the selective degradation of pollutants over TiO2/β-CD was presented in Figure 6. As for the structure of TiO2/β-CD that synthesized by the adsorption method, the binding of cyclodextrin to TiO2 surface occurred due to the adhesion of the hydroxyl functional groups, which
Figure 4. Photodegradation of rhodamine B in 1 g/L catalysts suspension. rhodamine B = 5 mg/L, λ g 400 nm.
Figure 5. Photodegradation of rhodamine B aqueous solutions under different irradiation source: rhodamine B = 5 mg/L, catalysts =1 g/L.
magnitude slower than that for the free dye molecules.7,30 Besides, host molecules like cyclodextrin and cucurbituril can efficiently reduce the formation of dye aggregate on metal oxide surface.6,31 Since the crystalline form did not change and the specific surface area only changed slightly after modification, it was reasonable to assume that the enhanced degradation was mainly due to the attached cyclodextrin. Rhodamine B can form 1:1 inclusion complex with β-CD (with a constant of 4240 M1),32 the electron injection from excited state of dye molecules to the conduction band of TiO2 can be facilitated because of the formation of inclusion complex. The results of rhodamine B photodegradation in different irradiation conditions were shown in Figure 5. The hybrid catalyst P-T1-CD effectively bleached rhodamine B under simulated solar (without any filter, λ g 365 nm) and visible light irradiation. After being modified by β-CD, the initial rate R0 increased by 4.6, 2.4, and 1.5 times when using a 420 nm filter, a 707
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bisphenol F. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: 86-27-68772910. Fax: 86-27-68778893. Email: xuzhangwhu@ gmail.com.
Figure 7. Cyclic photocatalytic degradation of rhodamine B aqueous solution under λ g 400 nm irradiation: rhodamine B 5 mg/L each run.
probably generated by physisorption and/or H-bonding interactions. No reactive oxygen species was generated in irradiated TiO2/ β-CD suspension leads to no degradation of colorless aromatic compounds like BPA in this work. The attached β-CD played a role as a “channel” or “bridge” between the dyes and TiO2 powders, which facilitated the electron injection from excited dyes to the conduction band of TiO2 and finally led to a faster degradation of dye pollutants. The sequential photodegradation experiments were performed to test the catalyst stability. As shown in Figure 7, in the primary stage, 200 mL of 5.0 mg/L rhodamine B aqueous solution was almost completely decomposed after 5 h of irradiation. The decrease trend in the final degradation efficiency was 6% after 5 repetitive experiments. Besides the above-mentioned interaction between β-CD and TiO2, charge transfer complex (CTC) between cyclodextrin and TiO2 was formed during the photo induced self-assembly process, superoxide ion radical generated when the CTC was irradiated under visible light,.13,16 which was found to be the predominant reactive oxygen species in the irritated P-T1-CD suspension.24 Cyclodextrin derivates were used to improve the efficiency of O2• trapping using various nitrones as scavengers3537 and the reaction between β-CD and diffusion mediated O2• can be neglected. Therefore, the overall visible light degradation of BPA over TiO2/ β-CD was faster than that over the correspondingly unmodified titanium dioxide.
’ ACKNOWLEDGMENT The author Xu Zhang thanks Dr. Huayi Yin for BET analysis. This work was financed by the Natural Science Foundation of P.R. of China (No. 20777057) and Scholarship Award for Excellent Doctoral Student granted by Ministry of Education. Figures showing the adsorption isotherm of cyclodextrin onto TiO2 and the degradation of other colorless aromatic compounds were given as the supplementary data. This information is available free of charge via the Internet at http://pubs.acs.org/. ’ REFERENCES (1) Vinodgopal, K.; Wynkoop, D. E.; Kamat, P. V. Environmental photochemistry on semiconductor surfaces: Photosensitized degradation of a textile azo dye, acid orange 7, on TiO2 particles using visible light. Environ. Sci. Technol. 1996, 30, 1660–1666. (2) Huang, S. Y.; Schlichthorl, G.; Nozik, A. J.; Gratzel, M.; Frank, A. J. Charge recombination in dye-sensitized nanocrystalline TiO2 solar cells. J. Phys. Chem. B 1997, 101, 2576–2582. (3) Zhang, F. L.; Zhao, J. C.; Shen, T.; Hidaka, H.; Pelizzetti, E.; Serpone, N. TiO2-assisted photodegradation of dye pollutants—II. Adsorption and degradation kinetics of eosin in TiO2 dispersions under visible light irradiation. Appl. Catal. B: Environ. 1998, 15, 147–156. (4) Zhao, J. C.; Wu, T. X.; Wu, K. Q.; Oikawa, K.; Hidaka, H.; Serpone, N. Photoassisted degradation of dye pollutants. 3. Degradation of the cationic dye rhodamine B in aqueous anionic surfactant/TiO2 dispersions under visible light irradiation: Evidence for the need of substrate adsorption on TiO2 particles. Environ. Sci. Technol. 1998, 32, 2394–2400. (5) Konstantinou, I. K.; Albanis, T. A. TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations—A review. Appl. Catal. B: Environ. 2004, 49, 1–14. (6) Willner, I.; Eichen, Y.; Willner, B. Supramolecular semiconductor receptor assemblies: Improved electron transfer at TiO2-β-cyclodextrin colloid interfaces. Res. Chem. Intermediat. 1994, 20, 681–700. (7) Haque, S. A.; Park, J. S.; Srinivasarao, M.; Durrant, J. R. Molecular-level insulation: An approach to controlling interfacial charge transfer. Adv. Mater. 2004, 16, 1177–1181. (8) Choi, H.; Kang, S. O.; Ko, J.; Gao, G.; Kang, H. S.; Kang, M.-S.; Nazeeruddin, M. K.; Gr€atzel, M. An efficient dye-sensitized solar cell with an organic sensitizer encapsulated in a cyclodextrin cavity. Angew. Chem., Int. Ed. 2009, 48, 5938–5941. (9) Zhou, W.; Pan, K.; Zhang, L.; Tian, C.; Fu, H. Solar-induced selfassembly of TiO2-β-cyclodextrin-MWCNT composite wires. Phys. Chem. Chem. Phys. 2009, 11, 1713–1718. (10) Faiz, J.; Philippopoulos, A. I.; Kontos, A. G.; Falaras, P.; Pikramenou, Z. Functional supramolecular ruthenium cyclodextrin dyes for nanocrystalline solar cells. Adv. Funct. Mater. 2007, 17, 54–58. (11) Agrios, A. G.; Gray, K. A.; Weitz, E. Photocatalytic transformation of 2,4,5-trichlorophenol on TiO2 under sub-band-gap illumination. Langmuir 2003, 19, 1402–1409. (12) Agrios, A. G.; Gray, K. A.; Weitz, E. Narrow-band irradiation of a homologous series of chlorophenols on TiO2: Charge-transfer complex formation and reactivity. Langmuir 2004, 20, 5911–5917. (13) Kim, S.; Choi, W. Visible-light-induced photocatalytic degradation of 4-chlorophenol and phenolic compounds in aqueous suspension
4. CONCLUSIONS Besides the application in DSSC and pollutants sensor, the results obtained in this work clearly showed that the host modified TiO2 could be an efficient catalyst for the visible light degradation of dye pollutants and colorless aromatic compounds. Cyclodextrin facilitated the charge transfer from the excited state of dye molecules to semiconductor conduction band, which enhanced the overall dye photosensitized degradation. The visible degradation of BPA was mainly caused by the O2•‑, which was generated through the charge transfer complex between cyclodextrin and TiO2. Synthesis method and TiO2 surface area showed great effect on the photoactivity of cyclodextrin modified TiO2. To enhance the adsorption of cyclodextrin on metal oxide surface, further work on cyclodextrin modification is necessary, which aims of achieving higher photoactive host modified catalysts in the long run. ’ ASSOCIATED CONTENT Supporting Information. Adsorption of β-CD onto TiO2 and photodegradation of bisphenol A, bisphenol E, and
bS
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