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Rapid Removal and Mineralization of Bisphenol A by Heterosupramolecular Plasmonic Photocatalyst Consisting of Gold Nanoparticle-Loaded Titanium(IV) Oxide and Surfactant Admicelle Shin-ichi Naya, Junpei Yamauchi, Takashi Okubo, and Hiroaki Tada Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02396 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017
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Langmuir
Rapid Removal and Mineralization of Bisphenol A by Heterosupramolecular Plasmonic Photocatalyst Consisting of Gold NanoparticleLoaded Titanium(IV) Oxide and Surfactant Admicelle Shin-ichi Naya,a Junpei Yamauchi,b Takashi Okubo,c and Hiroaki Tada* a,b a Environmental Research Laboratory, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. b Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. c
Department of Chemistry, Faculty of Science and Engineering, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan.
KEYWORDS: plasmon photocatalyst, Au nanoparticle, TiO2, surfactant admicelle, bisphenol A, water purification
Supporting Information Placeholder ABSTRACT: The establishment of the technology for rapidly and completely remove and mineralize harmful phenolic compounds from water is of great importance for environmental conservation. Visible-light irradiation (λ > 430 nm, light intensity integrated from 420 to 485 nm = 6.0 mW cm-2) of Au nanoparticle (NP)-loaded TiO2 (Au/TiO2) in dilute aqueous solutions of bisphenol A (BPA) and p-cresol (PC) causes degradation of the phenols. The addition of trimethylstearylammonium chloride (C18TAC) enhances the adsorption of BPA on Au/TiO2 to greatly increase the rate of reaction. Consequently, 10 µM phenols are completely removed from the solutions within 2.5 h irradiation, and prolonging irradiation time to 24 h quantitatively oxidizes BPA to CO2. Dynamic light scattering ζ-potential measurements indicate that a C18TAC bilayer or admicelle is formed on the Au/TiO2 particle surface at C18TAC concentration > 50 µM. The action spectrum for reaction shows that this reaction is driven by the Au NP localized surface plasmon resonance excitation-induced interfacial electron transfer from Au to TiO2. We propose a possible reaction scheme on the basis of the experimental results including intermediate analysis.
benzene to phenol,17,18 and amines to imines.19 Meanwhile, the Au/TiO2 plasmonic photocatalyst decomposes phenol under visible-light irradiation.20,21 However, the study on the application of the Au/TiO2 plasmonic photocatalyst to environmental purification is only limited.20-25 Here we report rapid removal and mineralization of BPA by the Au/TiO2 plasmonic photocatalyst with particular emphasis placed on a drastic enhancing effect by the addition of cationic surfactant. This is the first study on the application of a heterosupramolecular plasmonic photocatalyst consisting of Au/TiO2 and surfactant admicelle to environmental purification.
INTRODUCTION Phenol derivatives are useful starting materials for many industrial chemical products, e.g., bisphenol A (BPA) for polycarbonate and p-cresol (PC) for antiseptic. While the annual production amount of BPA attains to 4.6 × 105 tons in only Japan, it is an endocrine disruptor with the concentration in ambient water severely restricted < 0.1 µM so as not to exert a baneful effect on living life.1-3 Thus, the energy-saving technology should be developed for completely removing BPA from the wastewater. Although there have been many reports on the BPA decomposition by TiO2-based photocatalysts,4,5 the complete removal and mineralization need long-term irradiation of intense light irradiation on the order of several hundreds watt.6-12 In terms of effective utilization of sunlight as the energy source, the development of visible-light-driven photocatalyst is crucial. On the other hand, gold nanoparticle (NP)-loaded TiO2 (Au/TiO2) has emerged as a new type visible-light photocatalyst. The so-called Au/TiO2 plasmonic photocatalyst possesses strong absorption due to the localized surface plasmon resonance (LSPR) of Au NP in the visible region. Thus far, the plasmonic photocatalyst has mainly been applied for organic synthesis, i.e., the selective oxidations of alcohols to carbonyl compounds,13-15 thiols to disulfides,16
EXPERIMENTAL SECTION Catalyst Preparation and Characterization. Au particles were loaded on rutile TiO2 particles with a specific surface area of 17.5 m2 g-1 (TAYCA, MT-700B, mean particle size = 80 nm) by the deposition–p recipitation method using HAuCl4 as a raw material.26 The post-heating was carried out at 400 °C for 4 h and 550 °C for 4 h to form Au particles with the mean Au particle sizes of 2.6 nm and 6.7 nm, respectively, on TiO2. The mean size of the Au NPs was determined by transmission electron microscopy at an applied
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Langmuir voltage of 200 kV (JEM-2100F, JEOL). Unless otherwise noted, Au/TiO2 with a mean Au particle size (d) of 2.6 nm was used in this study. The Au NPs loaded on the TiO2 surface was dissolved using aqua regia to be quantified inductively coupled plasma spectroscopy (ICPS-7500, Shimadzu). Diffuse reflectance UV-Vis-NIR spectra of the samples were recorded on a Hitachi U-4000 spectrometer mounted with an integrating sphere at room temperature. The reflectance (R∞) was recorded with respect to a reference of BaSO4, and the Kubelka-Munk function [F(R∞)] expressing the relative absorption coefficient was calculated by the equation F(R∞) = (1 - R∞)2/2R∞. Degradation of phenolic compounds. Au/TiO2 (100 mg) was dispersed into aqueous solution of BPA or PC (10 µM, 100 mL) containing 1% acetonitrile with or without trimethylstearylammonium chloride (C18TAC, 100 µM). Acetonitrile was added to completely dissolve BPA and PC into the solution. After the suspension was stirred with a magnetic stirring bar in the dark at 298 K for 30 min, irradiation was started using a 300 W Xe lamp (HX-500, Wacom) with a cut off filter Y-45 (AGC TECHNO GLASS)
ζ-potential measurements were performed using a Zetasizer Nano ZS (Malvern Instruments). Samples were prepared by dispersing of Au/TiO2 (20 mg) into an aqueous solution (100 mL) of C18TAS (0 - 200 µM) or NH4Cl (0 - 200 µM), and ultrasonicated for 30 min. The samples were transferred to a disposable folded capillary cell (DTS1060) for measurement. Electrophoretic mobilities measured at 25oC using the Malvern M3-PALS method were converted to ζ-potential by the Henry’s equation.
RESULTS AND DISCUSSION Au NPs were loaded on rutile TiO2 by the depositionprecipitation method (Au/TiO2). Figure 1a shows TEM image of Au/TiO2. Au NPs with a mean size (d) of 2.6 ± 0.7 nm were highly dispersed on the TiO2 surface. Figure 1b shows UV-visible absorption spectra of Au/TiO2, and unmodified TiO2 for comparison. While rutile TiO2 only has absorption at λ < 413 nm due to the interband transition, Au/TiO2 has broad and strong LSPR of Au NP in the visible region around 560 nm. The absorption of Au/TiO2 fairly matches the solar spectrum, which allows us to expect effective utilization of the sunlight as the energy source for the photocatalytic water purification. Photocatalytic activities of Au/TiO2 for degradation of BPA and PC (initial concentration C0 = 10 µM) was studied in the absence and presence of trimethylstearylammonium chloride (C18TAC) under visible-light irradiation (λ > 430 nm). Figure 2a shows time courses for the BPA and PC degradatoin under aerobic conditions at irradiation time (tp) ≤ 12 h. Without C18TAC, the concentrations of BPA and PC gradually decrease with irradiation. However, in each system, a significant amount of substrate (BPA or PC) still remains even at tp = 12 h. Surprisingly, the addition of C18TAC (100 µM) gives rise to drastic enhancement in the photocatalytic activity of Au/TiO2 (d = 2.6 nm) for the degradation of BPA and PC. Figure 2 b shows time courses for the BPA degradation in the presence of C18TAC under controlled conditions at tp ≤ 2.5 h. In the TiO2 system, the degradation is sluggish. In the Au/TiO2 (d = 2.6 nm) system, the degradation is completed at tp = 2.5 h, whereas no degradation occurs in the dark, and the deaeration
4 0.1 2
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Figure 1. (a) TEM image of Au/TiO2 (d = 2.6 nm). (b) UV-Vis absorption spectra of Au/TiO2 (d = 2.6 nm) and TiO2 and solar spectrum.
in a double jacket type reaction cell. The cell was kept at 298 K by circulating thermostated water through an outer jacket around the cell. The light intensity integrated from 420 to 485 nm (I420–485) was adjusted to 6.0 mW cm-2. The concentration of phenol analogs was determined by high-performance liquid chromatography (LC-6 AD, SPD-6 A, C-R8A (Shimadzu)) [measurement conditions : column = Shim-pack CLC-ODS (4.6 mm×150 mm) (Shimadzu); mobile phase MeOH : H2O = 7 : 3; flow rate = 1.0 mL min-1; λ = 277 nm]. . Action spectrum analysis. According to our previous report, Au/mp-TiO2/FTO electrode was prepared. Slurry of TiO2 particles (0.5 g/H2O 1 mL) was coated on fluorine-doped SnO2 film-coated glass substrates (FTO, sheet resistance = 12 Ω/square), and the sample was heated in air at 573 K for 1 h to form mp-TiO2/FTO electrodes. Subsequently, Au NPs were loaded on the surface by the DP method to form Au/mp-TiO2/FTO. The electrode potential (E) was measured in a deaerated 10 µM BPA solution containing 0.1 M Na2SO4 electrolyte and 100 µM C18TAC for a regular three-electrode electrochemical cell using a galvanostat/potentiostat (HZ-7000, Hokuto Denko). Photoirradiation by using a xenon lamp with a monochromator (fwhm, 10 nm) (HM-5, JASCO) led to a shift of E in the cathodic direction.
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Figure 2. (a) Time courses for BPA and PC degradation by Au/TiO2 (d = 2.6 nm) under visible light (λ > 430 nm) illumination with and without C18TAC (100 µM) at irradiation time (tp) < 12 h. (b) Time courses for BPA degradation with C18TAC (100 µM) at tp < 2.5 h in the Au/TiO2 (d =2.6 nm), Au/TiO2 (d =6.7 nm), and TiO2 systems.
ζ-potential measurements.
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of the reaction solution by Ar gas bubbling retards the reaction under irradiation. To examine the Au particle size effect on the photocatalytic activity, the reaction was further carried out using Au/TiO2 (d = 6.7 nm). The activity is much smaller than that of Au/TiO2 (d = 2.6 nm). This finding indicates that the Au particle size is also a key factor for the plasmonic activity in this reaction. Importantly, gas chromatgraphy analysis confirmed that BPA is quantitatively oxidized to CO2 with > 99% conversion at tp = 24 h, whereas CO2 is hardly detected without BPA. Also, the oxidation of PC affords CO2 with > 87% conversion at tp = 24 h. The strong resistance to oxidation of C18TAC can be attributed to the low energy level of HOMO due to the positive charge on the head group.27 Clearly, the rapid removal and complete mineralization of BPA can be achieved by the visible-light irradiation of Au/TiO2 in the presence of C18TAC.
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Figure 4. (a) Action spectrum of (dE/dt)0 and UV-Vis absorption spectra of Au/TiO2 (d = 2.6 nm) and TiO2. (b) Plots of k versus oxidation potential of the phenol analogs (Eox).
light having various wavelength was measured. Figure 4a compares the action spectrum for the initial rate of the potential change ((dE/dt)0) with irradiation and the UV-visible absorption spectrum of Au/TiO2. The good resemblance between the profiles indicates that the LSPR-induced interfacial electron transfer (IFET) from Au to TiO2 is the driving force for this reaction. Also, as shown in Figure 4b, a negative correlation is observed in the plots of k versus the oxidation potential (Eox) of BPA, PC, and 2-naphthol (2-NAP). These results indicate that the LSPR-induced IFET from Au to TiO2 lowers the Fermi energy to oxidize the phenols on the Au "nanoelectrode".14 Secondly, the ζ-potential of Au/TiO2 in the C18TAC solution was measured. Figure 5a shows the ζ-potential as function of concentrations of C18TAC (Csurf) and NH4Cl (CNH4Cl) added. The ζ-potential of Au/TiO2 has a negative value of -23 mV in pure water. Interestingly, the sign of ζpotential is inversed by the addition of C18TAC at Csurf > 25 µM to reach +35 ± 5 mV at Csurf > 50 µM, whereas the addition of NH4Cl hardly affects the ζ-potential. Our previous study on the adsorption of C18TAC on TiO2 showed that in the saturated adsorption state, one C18TAC molecule occupies a cross-sectional area of 0.23 nm2 molecule-1, which is approximately half the value of dodecyltrimethylammonium bromide with the same head group in a close-packed monnolayer (0.49 nm2 molecule-1).27 Evidently, a C18TAC bilayer of C18TAC or admicelle is formed on the surface of Au/TiO2 at Csurf ≈ 50 µM. Once the admicelle formation is completed, the adsorption amount of C18TAC does not increase with a further increase in Csurf probably because of the electrostatic repulsion between C18TAC molecules. Figure 5b shows plots of the adsorption amount of BPA (Y / mmol g-1) as funcitons of Csurf and CNH4Cl. Whereas the adsorption amount of BPA is invariant with the NH4Cl addition, the C18TAC addition sharply increases Y at Csurf < 50 µM, gradually increasing it at Csurf > 50 µM. Thus, the enhancement of the BPA adsorption is derived from not the ammonium moiety and chloride ions but the tail of C18TAC molecule self-assembling to form a hydrophobic space. Since BPA is a highly hydrophobic molecule, it would be spontaneously incorporated and concentrated into the hydrophobic nanospace in the admicelle. Consequently, the photocatalytic activity of Au/TiO2 for the BPA degradation is greatly enhanced by the addition of C18TAC.
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p-cresol
Figure 3. (a) Plots of lnC0/C versus tp for the Au/TiO2 (d = 2.6 nm)-photocatalyzed degradation of BPA and PC in the absence and presence of C18TAC. (b) Rate constant for the reaction, and the ratio of the rate constant with respect to the value without C18TAC.
Kinetic analyses were carried out for the Au/TiO2photocatalyed degradation of BPA and PC. As shown in Figure 3a, each reaction apparently obeys the first-order kinetics, where C0 and C express the substrate concentrations at tp = 0 and tp = t, respectively. Figure 3b compares the firstorder rate constant (k) for the degradation of BPA and PC in the absence and presence of C18TAC. In each system, a remarkable accelerating effect by the C18TAC addition is observed. The C18TAC addition increases the k values for the degradation of BPA and PC by a factor of 5.2 and 10.4, respectively. Thus, the addition of C18TAC is valid for the Au/TiO2-photocatalyzed degradation of phenol analogs although the enhancement factor is dependent on the molecular structure. To clarify the basic reaction mechanism and action mechanism of the surfactant, several experiments were carried out. Firstly, a mesoporous TiO2 nanocrystalline film was coated on fluorine-doped tin oxide electrode (mp-TiO2/FTO), and further, Au NPs were loaded on the surface (Au/mpTiO2/FTO) by the deposition-precipitation method. Photoelectrochemical (PEC) measurements were performed for a three-electrode cell with the structure of Au/mpTiO2/FTO (working electrode) ∣ 0.1 M NaClO4 containing 10 µM BPA ∣ Ag/AgCl (reference electrode) ∣ glassy carbon (counter electrode). The change in the Au/mp-TiO2/FTO electrode potential (E) with illumination of monochromatic
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into the hydrophobic nanospace in the admicelle. Upon excitation of the LSPR of Au NP in Au/TiO2 in this state, the IFET from Au to the conduction band (CB) of TiO2 occurs. The CB-electrons are consumed by the oxygen reduction. On the other hand, the lowering in the Fermi level of Au NP causes the oxidation of BPA on the surface. The reactive oxygen species generated in the oxygen reduction process may also act as the oxidant.20 The successive oxidation of BPA finally yields CO2 via the intermediates such as benzoquinone and benzylalcohol derivative. The basic mechanism would also be valid for the degradation of the other phenol analogs. The regulation value for C18TAC is 50 µM, the moclecules self-assemble to form the admicelle on the surface of Au/TiO2. Hydrophobic BPA molecules are spontaneously incorporated and concentrated
WATER POLLUTION CONTROL LAW
Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Au/TiO2-photocatalyzed C18TAC decomposition (Figure S1)
AUTHOR INFORMATION Corresponding Author TEL: +81-6-6721-2332, FAX: +81-6-6727-2024
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Langmuir (17) Ide, Y.; Matsuoka, M.; Ogawa, M. Efficient Visible-LightInduced Photocatalytic Activity on Gold-Nanoparticle-Supported Layered Titanate. J. Am. Chem. Soc. 2010, 132, 16762-16764. (18) Zheng, Z.; Huang, B.; Qin, X.; Zhang, X.; Dai, Y.; Whangbo, M.-H. Facile in Situ Synthesis of Visible-Light Plasmonic Photocatalysts M@TiO2 (M = Au, Pt, Ag) and Evaluation of Their Photocatalytic Oxidation of Benzene to Phenol. J. Mater. Chem. 2011, 21, 9079-9087. (19) Naya, S.; Kimura, K.; Tada, H. One-Step Selective Aerobic Oxidation of Amines to Imines by Gold Nanoparticle-Loaded Rutile Titanium(IV) Oxide Plasmon Photocatalyst. ACS Catal. 2013, 3, 1013. (20) Zielińska-Jurek, A.; Kowalska, E.; Sobczak, J.; Lisowski, W.; Ohtani, B.; Zaleska, A. Preparation and Characterization of Monometallic (Au) and Bimetallic (Ag/Au) Modified Titania Photocatalysts Activated by Visible Light. Appl. Catal. B: Environ. 2011, 101, 504-514. (21) Wei, Z.; Rosa, L.; Wang, K.; Endo, M.; Juodkazis, S.; Ohtani, B.; Kowalska, E. Size-Controlled Gold Nanoparticles on Octahedral Anatase Particles as Efficient Plasmonic Photocatalyst. Appl. Catal. B: Environ. 2017, 206, 393-405. (22) Rodríguez-González, V.; Zanella, R.; del Angel, G.; Gómez, R. MTBE Visible-Light Photocatalytic Decomposition over Au/TiO2 and Au/TiO2–Al2O3 Sol–Gel Prepared Catalysts. J. Mol. Catal. A 2008, 281, 93-98. (23) Neaţu, Ş.; Cojocaru, B.; Pârvulescu, V. I.; Somoghi, V.; Alvaro, M.; García, H. Visible-Light C–Heteroatom Bond Cleavage and Detoxification of Chemical Warfare Agents using TitaniaSupported Gold Nanoparticles as Photocatalyst. J. Mater. Chem. 2010, 20, 4050-4054. (24) Naya, S.; Nikawa, T.; Kimura, K.; Tada, H. Rapid and Complete Removal of Nonylphenol by Gold Nanoparticle/Rutile Titanium(IV) Oxide Plasmon Photocatalyst. ACS Catal. 2013, 3, 903907. (25) Wang, C.; Wu, Y.; Lu, J.; Zhao, J.; Cui, J.; Wu, X.; Yan, Y.; Huo, P. Bioinspired Synthesis of Photocatalytic Nanocomposite Membranes Based on Synergy of Au-TiO2 and Polydopamine for Degradation of Tetracycline under Visible Light. ACS Appl. Mater. Interfaces 2017, 9, 23687-23697. (26) Tsubota, S.; Haruta, M.; Kobayashi, T.; Ueda, A.; Nakahara, Y. Preparation of Catalysis V. Elsevier, Amsterdam, 1991. (27) Tada, H.; Matsui, H.; Shiota, F.; Nomura, M.; Ito, S.; Yoshihara, M.; Esumi, K. Heterosupramolecular Photocatalysis: Oxidation of Organic Compounds in Nanospaces between Surfactant Bilayers Formed on TiO2. Chem. Commun. 2002, 1678-1679.
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ACKNOWLEDGMENT The authors acknowledge TAYCA Co. for the kindly gift of rutile TiO2. This work was partially supported by a Grant-in-Aid for Scientific Research (C) No. 15K05654, and MEXT-Supported Program for the Strategic Research Foundation at Private Universities.
REFERENCES (1) United States Environmental Protection Agency (U. S. EPA), Criterion Maximum Concentration in Clean Water Act Aquatic life criteria. (2) U. S. EPA, AQUIRE in ECOTOX database. (3) Eco-toxicity Tests of Chemicals Conducted by Ministry of the Environment in JAPAN Based on OECD Test Guideline, 2009. (4) Kim, J.; Choi, W. Hydrogen Producing Water Treatment through Solar Photocatalysis. Energy Environ. Sci. 2010, 3, 10421045. (5) Kim, J.; Choi, W. TiO2 Modified with Both Phosphate and Platinum and its Photocatalytic Activities. Appl. Catal. B: Environ. 2011, 106, 39-45. (6) Hou, D.; Goei, R.; Wang, X.; Wang, P.; Lim, T.-T. Preparation of Carbon-Sensitized and Fe-Er Codoped TiO2 with Response Surface Methodology for Bisphenol A Photocatalytic Degradation under Visible-Light Irradiation. App. Catal. B: Environ. 2012, 126, 121-133. (7) Katsumata, H.; Taniguchi, M.; Kaneco, S.; Suzuki, T. Photocatalytic Degradation of Bisphenol A by Ag3PO4 under Visible Light. Catal. Commun. 2013, 34, 30-34. (8) Tian, H.; Li, J.; Zhao, Y.; Liu, L. Removal of Bisphenol A by Mesoporous BiOBr under Simulated Solar Light Irradiation. Catal. Sci. Technol. 2012, 2, 2351-2355. (9) Repousi, V.; Petala, A.; Frontistis, Z.; Antonopoulou, M.; Konstantinou, I.; Kondarides, D. I. Mantzavinos, D. Photocatalytic Degradation of Bisphenol A over Rh/TiO2 Suspensions in Different Water Matrices. Catal. Today 2017, 284, 59-66. (10) Xiao, X.; Hao, R.; Zuo, X.; Nan, J.; Li, L.; Zhang, W. Microwave-Assisted Synthesis of Hierarchical Bi7O9I3 Microsheets for Efficient Photocatalytic Degradation of Bisphenol-A under Visible Light Irradiation. Chem. Engin. J. 2012, 209, 293-300. (11) Ali, M. B.; Hamdi, A.; Elhouichet, H.; Sieber, B.; Addad, A.; Coffinier, Y.; Boussekey, L.; Férid, M.; Szunerits, S.; Boukherroub, R. High Photocatalytic Activity of Plasmonic Ag@AgCl/Zn2SnO4 Nanocomposites Synthesized using Hydrothermal Method. RSC Adv. 2016, 6, 80310-80319. (12) Di, J.; Xia, J.; Ji, M.; Wang, B.; Yin, S.; Xu, H.; Chen, Z. Carbon Quantum Dots Induced Ultrasmall BiOI Nanosheets with Assembled Hollow Structures for Broad Spectrum Photocatalytic Activity and Mechanism Insight. Langmuir 2016, 32, 2075-2084. (13) Kowalska, E.; Abe, R.; Ohtani, B. Visible Light-Induced Photocatalytic Reaction of Gold-Modified Titanium(IV) Oxide Particles: Action Spectrum Analysis. Chem. Commun. 2009, 46, 241-243. (14) Naya, S.; Inoue, A.; Tada, H. Self-Assembled Heterosupramolecular Visible Light Photocatalyst Consisting of Gold Nanoparticle-Loaded Titanium (IV) Dioxide and Surfactant. J. Am. Chem. Soc. 2010, 132, 6292-6293. (15) Tsukamoto, D.; Shiraishi, Y.; Sunagano, Y.; Ichikawa, S.; Tanaka, S.; Hirai, T. Gold Nanoparticles Located at the Interface of Anatase/Rutile TiO2 Particles as Active Plasmonic Photocatalysts for Aerobic Oxidation. J. Am. Chem. Soc. 2012, 134, 6309-6315. (16) Naya, S.; Teranishi, M.; Isobe, T.; Tada, H. Light Wavelength-Switchable Photocatalytic Reaction by Gold NanoparticleLoaded Titanium(IV) Dioxide. Chem. Commun. 2010, 46, 815-817.
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