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Sequential Process Combination of Photocatalytic Oxidation and Dark Reduction for the Removal of Organic Pollutants and Cr(VI) using Ag/TiO
2
Yeoseon Choi, Min Seok Koo, Alok D. Bokare, Dong-hyo Kim, Detlef W. Bahnemann, and Wonyong Choi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06303 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017
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Sequential Process Combination of Photocatalytic Oxidation and
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Dark Reduction for the Removal of Organic Pollutants and Cr(VI)
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using Ag/TiO2
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Yeoseon Choi,† Min Seok Koo,† Alok D. Bokare,† Dong-hyo Kim,† Detlef W. Bahnemann,‡
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and Wonyong Choi*,†
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†
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‡
Department of Chemical Engineering and Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Korea 37673 Laboratory “Photoactive Nanocomposite Materials” Saint-Petersburg State University, SaintPetersburg, Russia and “Photocatalysis and Nanotechnology”, Institut fuer Technische Chemie, Gottfried Wilhelm Leibniz Universitaet Hannover, Hannover, Germany
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Submitted to
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Environmental Science & Technology
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(Revised)
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*Corresponding author.
Phone: +82-54-279-2283
E-mail:
[email protected] 18
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ABSTRACT
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We investigated a sequential photocatalysis-dark reaction, wherein organic pollutants were
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degraded on Ag/TiO2 under UV irradiation and the dark reduction of hexavalent chromium
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(Cr(VI)) was subsequently followed. The photocatalytic oxidation of 4-chlorophenol (4-CP),
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a test organic substrate, induced the generation of degradation intermediates and the storage
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of electrons in Ag/TiO2 which were then utilized for reducing Cr(VI) in the post-irradiation
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period. The dark reduction efficiency of Cr(VI) was much higher with Ag/TiO2 (87%),
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compared with bare TiO2 (27%) and Pt/TiO2 (22%). The Cr(VI) removal by Ag/TiO2 (87%)
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was contributed by adsorption (31%), chemical reduction by intermediates of 4-CP
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degradation (26%), and reduction by electrons stored in Ag (30%). When formic acid, humic
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acid or ethanol was used as an alternative organic substrate, the electron storage effect was
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also observed. The post-irradiation removal of Cr(VI) on Ag/TiO2 continued for hours, which
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is consistent with the observation that a residual potential persisted on the Ag/TiO2 electrode
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in the dark whereas little residual potential was observed on bare TiO2 and Pt/TiO2 electrodes.
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The stored electrons in Ag/TiO2 and their transfer to Cr(VI) were also indicated by the UV-
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visible absorption spectral change. Moreover, the electrons stored in the pre-irradiated
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Ag/TiO2 reacted with O2 with showing a sign of low-level OH radical generation in the dark
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period.
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Keywords: post-irradiation effect, advanced oxidation processes, aquatic pollutant transformation, hexavalent chromium, electron storage in silver
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INTRODUCTION
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The deposition of noble metals on semiconductor surface is an efficient way to enhance the
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photocatalytic activity by creating a Schottky barrier at the semiconductor-metal interface,
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which helps separating the photo-excited electrons in the conduction band (CB) of
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semiconductor into the metal nanoparticles (NPs).1-3 Such separation of charge pairs through
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the Schottky barrier facilitates the subsequent charge transfer at the semiconductor interface
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with enhancing the overall photocatalysis and reactive oxygen species (ROS) generation.4-6
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Various noble metals such as Pt, Au, and Ag have been frequently immobilized on TiO2 and
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other semiconductors to increase the efficiency of photocatalytic oxidation (PCO) of organic
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pollutants.5-8
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The photocatalytic activities of these TiO2-metal composites show different behaviors
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depending on the kind of the loaded metal. Although the electron transfer from the
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semiconductor CB to noble metal NPs is very fast (~10 ps),5 the electron entrapment and
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subsequent interfacial transfer to the aqueous electrolytes critically depends on the type of
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noble metal. While the capacitive nature of Au and Ag impedes the charge transfer of
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entrapped electrons out of the metal surface, the existence of an ohmic contact in the case of
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Pt and Pd renders quick discharge of electrons at the solid/liquid interface.9-11 Wood et al.10
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demonstrated that the electron transfer from photocatalyst (ZnO) to aqueous electrolytes was
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delayed by the presence of Ag NPs, which serve as an electron storage that keeps electrons
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for a longer time. The transfer rate of accumulated electrons was much slower for Ag/ZnO
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than Au/ZnO, whereas Pt/ZnO did not exhibit any electron storage property at all. To
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compare the electron storage properties depending on the kind of noble metals, Takai and
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Kamat11 investigated the electron storage and discharge properties of M/TiO2 (M = Ag, Au
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and Pt) colloid in deaerated ethanol under UV irradiation and concluded that the electron 3
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storage is most efficient in Ag/TiO2 compared to Au/TiO2 and Pt/TiO2. To be specific, they
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estimated that Ag/TiO2 can store up to 51 electrons per an Ag atom under UV irradiation.
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The electrons accumulated on Ag metal can be in equilibrium with TiO2 where the electrons
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can be trapped as Ti(III) or in the conduction band with an elevated Fermi level, which may
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explain how an Ag atom can hold such many electrons. Although the actual number of
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electrons that can be stored in M/TiO2 should depend on many parameters such as the size of
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metal particles, the light irradiation intensity, and the kind of electron donors and solvents,
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the large electron storage capacity in Ag may enable its application for post-irradiation
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reactions. On the other hand, Pt/TiO2 has an ohmic contact at the interface and the
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photogenerated electrons on Pt/TiO2 are rapidly transferred to the aqueous electrolyte with
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little chance of electron accumulation on Pt. The much smaller interfacial charge transfer
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resistance of Pt/TiO2 than Ag/TiO2 was also confirmed in a recent study.12
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Ag/TiO2 has been employed for the photocatalytic removal of water contaminants on
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the basis of various properties which include bactericidal properties of silver species,13-15
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electron accumulation in silver metal to improve the reactivity of the photo-generated holes16
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and the plasmonic effect of silver nanoparticles to utilize the visible light spectrum.17, 18
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Although the electron storage properties of Ag/TiO2 have also been the subject of many
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fundamental investigations,10, 19, 20 the possible utilization of these stored electrons for the
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transformation of water pollutants has been largely unexplored. Gunawan et al.21 used pre-
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irradiated Ag/TiO2 as antimicrobial agent, but they focused on the effect of Ag+ dissolution
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by pre-irradiation not on the stored electrons in Ag NPs. This study aims to utilize and
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combine the characteristics of TiO2 photocatalyst and the silver NPs loaded on TiO2 for the
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redox transformation of aquatic pollutants in a sequential light-dark process. Although the
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photocatalytic activity of TiO2 and the electron storage capacity of Ag NPs are well known, 4
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the serial combination of those properties has not been proposed for the removal of water
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contaminants. The electrons stored in Ag/TiO2 (capacitive charging) during UV illumination
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might be utilized to activate redox conversions in the subsequent dark cycle (thermal
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discharging). Considering great interests in making solar water treatment technologies more
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viable, the idea of utilizing not only the irradiation period (daytime) but also the post-
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irradiation period (night time) for remediation purpose can be a conceptually interesting
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strategy.
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Herein, we investigated a sequential treatment of oxidation (light) and reduction (dark)
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using Ag/TiO2. For this purpose, 4-chlorophenol (4-CP) and Cr(VI) (hexavalent chromium)
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were employed as model pollutants for PCO and dark thermal reduction, respectively. Since
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both 4-CP and Cr(VI) are ubiquitous contaminants and have been extensively investigated for
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their photocatalytic removal,22-28 they are good model substrates that can be employed for the
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demonstration of this sequential process. During the UV-irradiation period, photo-generated
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electrons are charged in Ag while 4-CP is degraded by hole-mediated oxidation with
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generating intermediates. In the following dark period, the stored electrons in Ag and the
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degradation intermediates are used to reduce Cr(VI). Sequential removal of 4-CP and Cr(VI)
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was monitored and the dual activities of Ag/TiO2 were compared with those of bare TiO2 and
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Pt/TiO2 to understand the mechanism of the sequential process.
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EXPERIMENTAL SECTION
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Materials. Chemicals and materials used in this work include: A commercial TiO2 (P25),
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which is a mixture of anatase and rutile (8:2) with primary particle sizes of 20-30 nm was
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used as the substrate TiO2 sample, Na2Cr2O7·2H2O (Cr(VI), Aldrich), 4-chlorophenol (4-CP,
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Sigma), ethanol (EtOH, J. T. Baker), formic acid (Aldrich), hydroxyhydroquinone (HHQ, 5
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Aldrich), 4-chlorocatechol (4-CC, Kasei), 1,4-benzoquinone (BQ, Aldrich), perchloric acid
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(HClO4, Sigma-Aldrich), acetone (Samchun), sulfuric acid (H2SO4, Sigma-Aldrich),
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diphenyl-carbazide (DPC, Aldrich). Humic acid (HA) was purchased from the International
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Humic Substances Society (http://www.ihss.gatech.edu). Deionized water (D. I. water) used
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was ultrapure (18 MΩ-cm) and prepared by a Barnstead purification system.
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Experimental procedure. Ag/TiO2 and Pt/TiO2 were prepared by using a typical photo-
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deposition method.12 TiO2 was suspended in the aqueous solution containing methanol (4%,
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v/v) and noble metal precursor (H2PtCl6 or AgNO3) and then stirred and sonicated for 30 min
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before irradiation. After UV irradiation (200-W mercury lamp) for 30 min, the metal-loaded
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TiO2 particles were filtered, sufficiently washed with D. I. water, and dried at 80 °C. The
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prepared photocatalyst was suspended in an aqueous solution (0.5 g/L) containing target
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organic species (300 µM) in a pyrex reactor. The suspension was dispersed well by ultra-
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sonication and the pH was adjusted to 3.0 with 1 N HClO4. The suspension was stirred for 30
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min in the dark to allow equilibrium adsorption of the model organic substrate. A 300-W Xe
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arc lamp (Oriel) was used as a solar simulating light source. The incident light was filtered
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through a 10-cm IR water filter and a UV cutoff filter (λ > 320 nm) that blocks the light of
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wavelengths shorter than 320 nm to avoid the direct photolysis of organic substrates and the
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incident light intensity was 1.7 W/cm2 (optical power meter, Newport 1918-R). After a
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certain time of UV irradiation, the reactor was placed in the dark and then Cr(VI) was
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subsequently spiked into the pre-irradiated suspension that was well stirred during the entire
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dark reaction period. The sample aliquots were intermittently withdrawn and filtered through
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a 0.45-µm PTFE filter (Millipore) for the analysis of Cr(VI). For experiments studying the
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effect of oxidation intermediates, the photocatalyst was first filtered, and Cr(VI) was then
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added in the filtrate solution. 6
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Analysis. The concentration of Cr(VI) was measured spectrophotometrically using the DPC
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(diphenylcarbazide) method.27 An aliquot of 0.5 mL sample solution was added to a vial
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containing 2.5 mL of prepared DPC reagent (100 mL of D.I. water and 4 mL of DPC reagent
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that contains 5 mL of acetone, 50 µL of H2SO4, and 0.01 g of DPC). The vial was mixed
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vigorously and kept standing for 30 min before the analysis. The absorbance measurements at
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540 nm (ε = 6850 M-1cm-1) were done using a UV/visible spectrophotometer (Libra S22,
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Biochrom). Quantitative analysis of organic substrates was done using a high performance
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liquid chromatograph (HPLC Agilent 1100) equipped with a C-18 column (Nova-Pak C18)
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and a photodiode-array detector (Waters 996). The eluent was composed of 0.1% phosphoric
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acid aqueous solution and acetonitrile (75:25 v/v). The residual organic carbon content in the
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irradiated organic substrate solution was monitored by a total organic carbon (TOC) analyzer
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(TOC-VCSH, Shimadzu).
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The generation of OH radicals in the irradiated or pre-irradiated suspension of bare TiO2,
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Ag/TiO2, and Pt/TiO2 was indirectly monitored by the fluorescence emission of 7-
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hydroxycoumarin (7-HC) that was produced from the reaction of OH radical and coumarin (a
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probe reagent for OH radical).29 The fluorescence emission intensity of 7-HC was monitored
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at 460 nm under the excitation at 332 nm using a spectrofluorometer (FluoroMax 4, HORIBA
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Jobin Yvon Inc.) The appearance of Ag plasmon band with stored electrons was observed by
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using a UV/visible spectrophotometer (Shimadzu UV-2401PC).
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Characterization of photocatalysts. The BET surface area of photocatalyst samples was
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measured by using nitrogen adsorption/desorption isotherms (Micromeritics Tristar 3000
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analyzer at 77K). JEOL JEM-2200FS with image Cs-corrector (200 KeV, 0.1nm) was used
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for scanning transmission electron microscopy (STEM) image of bright and dark field.
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Crystalline phases were determined by X-ray-powder diffraction (XRD) using Cu-Kα 7
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radiation (Mac Science Co. M18XHF). The chromium species deposited on the photocatalyst
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surface after reaction with Cr(VI) was analyzed by X-ray photoelectron spectroscopy (XPS)
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(Theta probe AR-XPS, Thermo Fisher Scientific) using Al-Kα (1486.6 eV) as the X-ray
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source.
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RESULTS AND DISCUSSION
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Three different catalysts (bare TiO2, Pt/TiO2 and Ag/TiO2) degraded 300 µM of 4-CP
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within 60 min (Figure 1a), but the efficiency of 4-CP removal was retarded by the
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coexistence of Cr(VI) (see Figure S1). A previous study also reported that the photocatalytic
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degradation of organic pollutants can be hindered by the co-existence of Cr(VI).30 We carried
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out a control experiment which employed bare TiO2 photocatalyst that was pre-adsorbed with
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Cr(VI) and observed that its photocatalytic activity for 4-CP degradation was markedly
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inhibited. This implies that the presence of Cr(VI) on the surface of TiO2 is an inhibitor of
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PCO activity although Cr(VI) is a good electron acceptor which may facilitate the hole-
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induced PCO. The most plausible explanation is that Cr(VI) serves as an external charge
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recombination center where Cr(VI) reacts serially with electrons and holes to make null
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photocatalytic cycles. Such role of Cr(VI) should block the pathways of ROS generation
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(e.g., O2−, H2O2, •OH) with retarding the PCO process. To test the inhibitory role of Cr(VI),
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the generation of OH radical was indirectly monitored by detecting 7-hydroxycoumarin (7-
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HC) which is produced from the PCO reaction between OH radical and coumarin (coumarin
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+ •OH → 7-HC). In the presence of Cr(VI), the generation of OH radical was indeed
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retarded for all three photocatalysts (see Figure S2). Since Cr(VI) is an intrinsic inhibitor of
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PCO, it is a better approach to remove 4-CP and Cr(VI) in a sequential process (as shown in
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Figure 1a) than in one batch reactor. 8
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Among the photocatalysts, the PCO degradation efficiency was in the order of Pt/TiO2
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> Ag/TiO2 > TiO2. Noble metal-loaded TiO2 was prepared by a typical photo-deposition
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method, which did not change the BET surface area and the crystal structure of the substrate
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TiO2 (P25) (see Figure S3). The average size of noble metal NPs of Pt/TiO2 and Ag/TiO2 was
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1.5 nm and 4 nm, respectively (see Figure S4). The positive effects of noble metals deposited
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on TiO2 on the PCO of organic compounds are well known and Pt is more efficient than Ag
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in most reported cases.8, 31, 32 The role of Pt is markedly prominent in the photocatalytic
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mineralization ability as shown in Fig. 1b. Ag/TiO2 was only slightly more efficient than bare
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TiO2 for 4-CP removal and no better than bare TiO2 for the mineralization. When the parent
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4-CP was completely removed (after 60 min of UV irradiation), the TOC removal was only
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partial with bare TiO2 and Ag/TiO2 but was almost complete with Pt/TiO2.
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On the other hand, the reactivity in the dark period that immediately followed the UV
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PCO process was significantly different among the three photocatalysts. After 60 min of UV
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irradiation (when all 4-CP was removed), the reduction of Cr(VI) under the dark condition
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(right after UV light turn-off) was investigated by spiking 100 µM Cr(VI) into the pre-
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irradiated catalyst suspension. The right panel of Figure 1a compares the time-dependent
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profiles of [Cr(VI)] which exhibit two distinct regions. An initial immediate decrease in
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[Cr(VI)] (regime I) was followed by a slow and gradual decrease (regime II). It is noted that
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Ag/TiO2 shows an outstanding activity for Cr(VI) removal compared with bare TiO2 and
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Pt/TiO2 systems. Although Pt/TiO2 was the most active for the PCO of 4-CP, it was the least
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active for the following dark thermal reduction of Cr(VI). The followings can be proposed as
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mechanisms that are responsible for the removal of Cr(VI) in the dark period: (1) adsorption
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on the catalyst surface, (2) chemical reduction by remaining intermediates of 4-CP
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degradation such as quinone derivatives,33-35 and (3) reduction by electrons in the noble metal 9
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phase (Pt or Ag), which were stored during the pre-irradiation period. Control experiments
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were carried out to estimate the contribution from each mechanism.
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First, the removal of Cr(VI) by adsorption was estimated for bare TiO2, Ag/TiO2, and
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Pt/TiO2 (see Figure 2a). In the case of bare TiO2 and Pt/TiO2, the total amount of Cr(VI)
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removed by the adsorption is almost the same as observed in pre-irradiation-dark cycle
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experiments (Figure 1a). Second, the chemical reduction by 4-CP oxidation intermediates
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was negligible with bare TiO2 and Pt/TiO2 as shown in Figure 2b. This indicates the
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adsorption mechanism is mainly responsible for Cr(VI) removal for both bare TiO2 and
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Pt/TiO2 in the dark period. In contrast, both adsorption and chemical reduction mechanisms
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contribute to initial rapid Cr(VI) removal in the case of Ag/TiO2. The photocatalytic
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degradation of 4-CP is known to generate intermediates such as benzoquinone (BQ),
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hydroquinone
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hydroxybenzoquinone (HBQ), and other minor intermediates.36 We also confirmed the
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generation of those intermediates during PCO of 4-CP in the presence of bare TiO2, Ag/TiO2
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and Pt/TiO2 by HPLC analysis (Figure S5). To confirm that these intermediates of 4-CP
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degradation reduce Cr(VI), four different quinone species were tested. As shown in Figure 3,
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all 4 compounds (BQ, 4-CC, HQ and HHQ) exhibited Cr(VI) reduction capacity and HHQ
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showed the highest activity. Since 4-CP was almost completely mineralized after 1 h PCO in
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the case of Pt/TiO2, the Cr(VI) reduction by intermediates was negligible for Pt/TiO2 as
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shown in Figure 2b. On the other hand, it is interesting to note that the reduction by
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intermediates was significantly higher for Ag/TiO2 than bare TiO2 despite the fact that the
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residual TOC values after 1 h PCO were similar for both Ag/TiO2 and bare TiO2 (see Figure
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1b). This implies that intermediates generated from Ag/TiO2 PCO are more reactive towards
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Cr(VI) than those from bare TiO2 PCO.
(HQ),
4-chlorocatechol
(4-CC),
hydroxyhydroquinone
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However, they cannot be responsible for the gradual decrease in [Cr(VI)] observed at a
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longer time scale (Regime II in Figure 1a). These observations suggest that stored electrons
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in the noble metal are mainly responsible for dark Cr(VI) removal (in regime II) in the
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Ag/TiO2 system. It has been previously reported that the electrons trapped in bare TiO2 NPs
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during UV irradiation can be used for reducing metal ions such as Cu(II),37 Ag(I),38 and
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Cr(VI)39 immediately after turning off the light. However, the post-irradiation electron
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transfer rate on bare TiO2 was fast enough to be completed in a sub-second time scale, which
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could be monitored only by using the stopped flow technique. The presence of Ag NPs not
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only increases the electron storage capacity significantly but also enables the slow release of
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the stored electrons for a longer time. Based on the results of the aforementioned control
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experiments, Figure 4a quantifies each contribution of the adsorption, the chemical reduction
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by intermediates, and the reduction by stored electrons towards the total Cr(VI) removal. It
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can be clearly seen that the contribution of the reduction by the stored electrons is
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significantly higher in Ag/TiO2 (30%) compared to bare TiO2 (2.8%) and Pt/TiO2 (1.8%).
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The pH dependence of the photocatalytic removal of 4-CP and the subsequent removal of
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Cr(VI) in the post-irradiation (dark) period was compared (see Table S1). Although the PCO
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activity was little influenced by the pH change (pH 3-10), the dark removal efficiency of
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Cr(VI) was markedly influenced by pH, which dramatically decreased with increasing pH 3
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to 10. Being an oxyanion, chromate should be electrostatically attracted to the positively
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charged TiO2 surface (pHzpc ~ 6) at acidic condition and repelled from the negatively charged
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TiO2 surface at alkaline condition. As a result, the reductive conversion of chromate anions
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on Ag/TiO2 is favored at acidic condition and hindered at alkaline condition.
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On the other hand, the post-irradiation (dark) reduction of Cr(VI) was also carried out
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in the absence of dissolved O2 to investigate the effect of O2 on the Cr(VI) reduction 11
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pathways, which is shown in Figure 4b. The most outstanding feature of the anoxic system is
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the increased contribution of the reduction by stored electrons for all catalysts. In particular,
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the contribution of stored electrons was markedly enhanced for Pt/TiO2 from 2% (in the
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presence of O2) to 55% (in the absence of O2). This indicates that the storage of electrons and
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their subsequent transfer in the dark condition are enabled only in the absence of dissolved O2
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for Pt/TiO2 since platinum is a good catalyst for O2 reduction.40 Therefore, electrons in the Pt
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metal are immediately scavenged by dioxygen without leaving any electrons that would
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discharge slowly in the following dark period.5, 7, 10 On the other hand, for Ag/TiO2, the
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contribution by stored electrons was only moderately enhanced in the absence of O2 (Figure
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4a vs 4b). This supports that the capacitive electron storage and discharge from Ag is little
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affected by the presence of O2. That is, the storage of electrons in metals during the
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irradiation period and the subsequent discharge of them in the following dark period are
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enabled only with Ag/TiO2 regardless of the presence of O2.
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As for the chemical reduction by intermediates in the anoxic condition, its contribution
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was reduced with Ag/TiO2 but enhanced with Pt/TiO2 compared to that in the oxic condition.
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This implies that the concentrations of 4-CP degradation intermediates in the anoxic
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condition were reduced with Ag/TiO2 but enhanced with Pt/TiO2, compared with those in the
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oxic condition. To investigate this effect, TOC measurements were compared between the
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oxic and anoxic systems. After 1 h PCO of 4-CP, the residual TOC values were 36% and 0%
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in the oxic condition and 95% and 88% in the anoxic condition for Ag/TiO2 and Pt/TiO2,
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respectively. (see Figures 1b and S6) For Ag/TiO2, the PCO of 4-CP was significantly
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retarded in the anoxic condition and hence lower concentration of intermediates should be
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generated. As for Pt/TiO2, the PCO was also drastically retarded in the absence of O2, but
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about 50% of 4-CP was removed after 1 h irradiation. Considering that the residual TOC was 12
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88% after 1 h PCO of Pt/TiO2, a significant fraction of removed 4-CP should be converted
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into intermediates, which can reduce Cr(VI) in the dark phase (see Figure 4b). Since Pt/TiO2
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can completely mineralize 4-CP after 1 h PCO in the presence of O2 with leaving little TOC,
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the contribution of chemical reduction by intermediates was absent in the Pt/TiO2/oxic
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system.
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To further confirm the electron storage properties of Ag/TiO2, an additional control
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experiment was carried out to exclude the competitive effect of the PCO intermediates. The
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post-irradiation removal of Cr(VI) was carried out after 1 h UV irradiation of Ag/TiO2
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suspension containing no organic pollutants (D.I. water only, aerated). Since the production
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of organic intermediates should be absent in this case, the effect of stored electrons on Cr(VI)
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reduction can be easily demonstrated as shown in Figure 4c. Even in the absence of organic
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substrates, the intrinsic electron storage capacity of Ag/TiO2 is outstanding compared to
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Pt/TiO2. The effects of other organic substrates on the post-irradiation removal of Cr(VI)
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were also investigated with employing ethanol, formic acid, and humic acid. As seen in
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Figure 5, intermediates generated during the PCO of these alternative substrates showed very
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small or negligible Cr(VI) reduction capacity compared to the case of 4-CP. However, the
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amount of Cr(VI) reduced by stored electrons remained almost the same (20-30 µM)
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regardless of the kind of substrate organic compounds (even in their absence). The reduction
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mechanism by stored electrons in Ag seems to operate by itself, not depending on the PCO of
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organic substrates.
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The observation that the removal of Cr(VI) on Ag/TiO2 continued even after hours in
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the post-irradiation period implies that the stored electrons are very long-lived even in the
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aerated solution. To monitor this slow discharge of electrons directly, the decay of the
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photovoltage of the irradiated electrodes (bare TiO2, Pt/TiO2, and Ag/TiO2) upon turning off 13
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the light was recorded as a function of time as shown in Figure 6. The post-irradiation
308
potential decay on the Ag/TiO2 electrode is significantly retarded compared with bare TiO2
309
and Pt/TiO2 electrodes. Although the potential rapidly decayed within a minute, small
310
residual potential on Ag/TiO2 persisted for a much longer time (far beyond 30 minutes after
311
UV turn-off), which is consistent with the fact that the removal of Cr(VI) on Ag/TiO2
312
continued for hours in the post-irradiation period (see Figure 1a). Another evidence for the
313
stored electrons was obtained from UV/visible absorption spectra of Ag/TiO2 (see Figure 7).
314
Stored electrons in silver NPs induce the silver plasmon band.10, 11, 41 While Ag/TiO2 before
315
light irradiation exhibits no absorption peak in the range 300-900 nm, a peak at 450 nm was
316
observed only after UV illumination but not after visible light (λ > 420 nm) irradiation. Since
317
this peak cannot be ascribed to the plasmonic absorption peak of Ag NPs, it should be related
318
to the accumulation of electrons in the Ag metal (see Figure 7a). The UV-induced band at
319
450 nm did not change for 1 h even after the UV illumination was switched off, but decreased
320
in intensity upon spiking Cr(VI) (see Figure 7b). This confirms that the absorbance band at
321
450 nm is indicative of photo-induced electron accumulation, which disappears upon the
322
subsequent discharge on Ag/TiO2.
323
The post-irradiation catalytic effect of Ag/TiO2 might not be limited to Cr(VI)
324
reduction only. We tested several other chemical conversion reactions in the dark period
325
using the pre-irradiated Ag/TiO2. For example, the reductive conversion of nitrate (NO3-) by
326
electrons stored in Ag/TiO2 was negligible. The reductive discoloration of methylene blue in
327
the post-irradiation period, which had been demonstrated with the pre-irradiated Ag/TiO2 in
328
deaerated ethanol,11 was not observed in the present experimental condition (aerated aqueous
329
solution). However, we observed a sign of OH radical generation on Ag/TiO2 in the post-
330
irradiation period whereas bare TiO2 and Pt/TiO2 exhibited no such sign (see Figure 8). This 14
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implies that electrons stored in Ag/TiO2 reacted with dioxygen sequentially in the dark to
332
generate OH radicals (i.e., O2 → HO2• → H2O2 → •OH). This observation is consistent with a
333
previous report that PdO/TiON photocatalyst exhibited a bactericidal activity up to 8 hours in
334
the post-irradiation period.42 Small but persisting generation of OH radicals (or other ROS)
335
through the reaction of stored electrons should be responsible for the dark bactericidal
336
activity. Although the electrons stored in Ag/TiO2 are not energetic enough to be utilized for
337
a wide variety of contaminants removal, they have long-lasting reactive effects for selected
338
substrates.
339
Finally, the stability and reusability of catalysts need to be mentioned since this is of
340
high practical importance. The catalyst performance of bare TiO2, Pt/TiO2 and Ag/TiO2 was
341
tested through multiple cycles for PCO removal of 4-CP, PCO removal of 4-CP in the
342
presence of Cr(VI), and PCO removal of 4-CP followed by dark reaction of Cr(VI) (see
343
Figure S7). Regardless of whether Cr(VI) was co-present with 4-CP during the irradiation
344
period or added in the post-irradiation period, the PCO activity of the tested catalysts (bare
345
TiO2, Pt/TiO2, and Ag/TiO2) was irreversibly deactivated. The dark activity of Ag/TiO2 for
346
Cr(VI) removal in the post-irradiation period was also markedly reduced with repeating the
347
irradiation-dark cycles (see Figure S8). This implies that the catalyst surface was deactivated
348
by the reductive deposition of Cr(III) species. The XPS analysis of the catalyst surface after
349
the post-irradiation reaction with Cr(VI) clearly showed that the Cr(III) species is present on
350
Ag/TiO2, but not on bare TiO2 and Pt/TiO2. (see Figure S9).
351 352
ENVIRONMENTAL IMPLICATIONS
353
Most photocatalytic water treatment applications have been focused on pollutants removal
354
under continuous illumination (UV or visible light) conditions. Excess charges can be 15
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accumulated on the photocatalyst due to the difference between the interfacial charge transfer
356
rates of electron and hole to the aquatic redox species.43, 44 When electrons are accumulated
357
in the metal NPs during illumination, they can be subsequently discharged to suitable electron
358
acceptors in the absence of light. The utilization of such stored charges in the post-irradiation
359
process was applied to anticorrosion system.44 On the basis of this principle, this study
360
utilized the electron storage capacity of Ag NPs loaded on TiO2 surface to reduce Cr(VI) in
361
the following dark period. While the surface of Pt has ohmic contact for rapid discharge of
362
trapped electrons and exhibits a high electrochemical activity for O2 reduction,40 the
363
capacitive property of Ag and its lower catalytic activity for O2 reduction allows electron
364
accumulation and temporary storage even in the presence of dissolved O2. By coupling the
365
PCO of 4-CP with dark thermal reduction (Cr(VI)) reaction, the effect of metal loading on
366
TiO2 was examined in the presence of different organic substrates. The post-irradiation
367
removal of Cr(VI) was contributed by adsorption on catalyst, chemical reduction by organic
368
degradation intermediates, and reduction by electrons stored in noble metal NPs. In
369
particular, the reduction mechanism by stored electrons in Ag seems to operate for itself, not
370
depending on the PCO of organic substrates and the presence of dissolved O2.
371
The utilization of stored photo-electrons to initiate reductive conversions may
372
provide a useful concept to achieve continuous pollutant removal using both day
373
(illumination) and night (dark) cycles. This sequential combination of PCO and dark reaction
374
cycles can be employed in the design of photocatalytic water treatment reactors in which the
375
irradiated and non-irradiated parts can be integrated into a single treatment system. By
376
utilizing both UV PCO and dark reaction period, the overall process can be more energy
377
efficient for the removal of various pollutants. For example, the co-presence of 4-CP and
378
Cr(VI) in the photocatalyst suspension significantly retarded the PCO of 4-CP (see Figure S1) 16
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but this present sequential combination of PCO-dark reactions successfully removed both
380
pollutants with consuming the same amount of photon energy. However, it should be
381
mentioned that the present study is at a proof-of-concept stage and the real application might
382
be hindered by many practical problems such as low reactivity of stored electrons and
383
catalyst deactivation. The present system of photocatalytic removal of 4-CP combined with
384
the dark reaction of Cr(VI) may not be of great environmental significance but the proposed
385
concept can be ideally suitable for target applications where the long-lasting reactivity of
386
stored electrons in Ag/TiO2 can be utilized for the removal of low-level contaminants.
387
Further investigations of the proposed system may find more suitable applications.
388
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ASSOCIATED CONTENT
390
Supporting Information. PCO degradation of 4-CP with and without Cr(VI) (Figure S1);
391
Time profiles of 7-HC generation (Figure S2); XRD patterns and BET surface area (Figure
392
S3); STEM images (Figure S4); HPLC chromatograms of 4-CP degradation intermediates
393
(Figure S5); photocatalytic removal of 4-CP and TOC in the absence of O2 (Figure S6);
394
repetition tests of 4-CP degradation and Cr(VI) removal (Figure S7-S8); XPS analysis of
395
Cr(III) deposited on catalysts (Figure S9). This material is available free of charge via the
396
Internet at http://pubs.acs.org.
397
ACKNOWLEDGMENT
398
This work was supported by Global Research Laboratory (GRL) Program (NRF-
399
2014K1A1A2041044) and KCAP (Sogang Univ.) (2009-0093880), which were funded by the
400
Korea Government (MSIP) through the National Research Foundation (NRF).
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References
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shell composite clusters under UV-irradiation. J. Am. Chem. Soc. 2005, 127 (11), 3928-3934.
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(42) Li, Q.; Li, Y. W.; Wu, P.; Xie, R.; Shang, J. K. Palladium oxide nanoparticles on
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(43) Kim, S.; Park, H. Sunlight-harnessing and storing heterojunction TiO2/Al2O3/WO3
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(44) Tatsuma, T.; Saitoh, S.; Ohko, Y.; Fujishima, A. TiO2-WO3 photoelectrochemical
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anticorrosion system with an energy storage ability. Chem. Mater. 2001, 13 (9), 2838-2842. 23
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521 522
Scheme 1. Conceptual illustration of photocatalytic degradation of 4-CP and the following
523
reductive removal of Cr(VI) in the dark. After the photocatalytic degradation reaction, the
524
reactor was placed in the dark and Cr(VI) was spiked. Cr(VI) can be removed by three
525
different mechanisms that include adsorption, reduction by 4-CP degradation intermediates,
526
and reduction by stored electrons.
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in the Dark
(a) UV irradiation
80
200 TiO2
150
Ag/TiO2
Pt/TiO2
regime I
TiO2
Pt/TiO2
100
Ag/TiO2
60 40 20
50 regime II
0 0
10
20
30
40
50
irradiation time (min)
0
60
120
[Cr(VI)] (µM)
[4-CP] (µM)
250
100
0 180
240
300
Dark reaction time (min)
100
[TOC]/[TOC0] x 100%
(b)
After 60 min UV irradiation
80 60 40 20 0
TiO2
Ag/TiO2
Pt/TiO2
528 529
Figure 1. (a) Time profiles of the photocatalytic degradation of 4-CP under UV irradiation
530
and the subsequent Cr(VI) removal in the dark (after turning off the light) and (b) the
531
resulting removal of total organic carbon (TOC) after 60 min UV irradiation. The
532
experimental conditions were [Cat] = 0.5 g/L, [noble metal (Pt or Ag) loading on TiO2] = 3.0
533
wt%, [4-CP]0 = 300 µM, [Cr(VI)]0 = 100 µM, pH0 = 3.0, air-equilibrated, and λ > 320 nm
534
irradiation.
535
25
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100
[Cr(VI)] (µ M)
80 60 Pt/TiO2 TiO2
40
Ag/TiO2
(a) dark adsorption
20 0 0
30
60
90
120
150
180
time (min) 100
[Cr(VI)] (µM)
80 60
Pt/TiO2 TiO2
40
Ag/TiO2
(b) dark reduction by 4-CP oxidation intermediates
20 0 0
536
30
60
90
120
150
180
time (min)
537 538
Figure 2. Time profiles of Cr(VI) removal by (a) physical adsorption on catalyst in the dark,
539
and (b) dark thermal reduction induced by the remaining intermediates of 4-CP oxidation that
540
were generated in the pre-PCO period for 60 min. After the pre-PCO period, the catalyst was
541
filtered out and Cr(VI) was then added into the filtrate solution. The experimental conditions
542
were [Cat] = 0.5 g/L, [noble metal (Pt or Ag) loading on TiO2] = 3.0 wt%, [4-CP]0 = 300 µM,
543
[Cr(VI)]0 = 100 µM, pH0 = 3.0, air-equilibrated.
544
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HHQ HQ 4CC BQ 4CP
Cr(VI) removal (%)
100 80 60 40 20 0 0
50
100
150
200
250
300
intermediates (µΜ) 545 546
Figure 3. Cr(VI) removal by various intermediates of 4-CP degradation in the dark.
547
Hydroxyhydroquinone
548
benzoquinone (BQ) were used as model intermediates. The Cr(VI) removal efficiency was
549
tested as a function of the intermediate concentration. The effect of 4-CP itself was also tested
550
as a control. The experimental conditions were [Cr(VI)]0 = 100 µM, pH0 = 3.0, air-
551
equilibrated. The reaction was finished within 10 min.
(HHQ),
hydroquinone
(HQ),
4-chlorocatechol
552
27
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(4-CC),
and
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100
-
Cr(VI) removal (%)
(a) Oxic
stored e interm. adsorption
80 60 40 20 0
TiO2
Ag/TiO2
Pt/TiO2
Ag/TiO2
Pt/TiO2
100
(b) Anoxic Cr(VI) removal (%)
-
80
Stored e interm. adsorption
60 40 20 0
TiO2 100
-
Cr(VI) removal (% )
(c) DW
stored e adsorption
80 60 40 20 0
TiO2
Ag/TiO2
Pt/TiO2
553 554 555 556 557 558 559
Figure 4. Removal of Cr(VI) in pre-irradiated aqueous solution of bare TiO2, Ag/TiO2, and Pt/TiO2 (a) in air-equilibration and (b) in anoxic condition. (c) Removal of Cr(VI) in preirradiated deionized water suspension (without 4-CP, aerated) of bare TiO2, Ag/TiO2, and Pt/TiO2 (uncertainty of ±2%). The experimental conditions were [Cat] = 0.5 g/L, [4-CP]0 = 300 µM, [Cr(VI)]0 = 100 µM, pH0 = 3.0, air-equilibrated or Ar-saturated, and λ > 320 nm irradiation.
28
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Cr(VI) removal (%)
100
-
stored e interm. adsorption
80 60 40 20 0
4-CP 560
EtOH Formic acid
HA
DW
561
Figure 5. Removal of Cr(VI) in the pre-irradiated aqueous suspension of Ag/TiO2 which
562
contained various organic substrates (4-CP, EtOH, Formic acid, or HA) or deionized water
563
only (DW, as a control) (uncertainty of ± 2%). The experimental conditions were [Ag(3.0
564
wt%)/TiO2] = 0.5 g/L, [4-CP, EtOH, or formic acid]0 = 300 µM, [HA]0 = 1 ppm, [Cr(VI)]0 =
565
100 µM, pH0 = 3.0, air-equilibrated, and λ > 320 nm irradiation.
566
29
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1.4 Light OFF
Light ON
1.2
(E-E0)/(Eph-E0)
1.0 0.8 0.6
TiO2
0.4
Pt/TiO2 Ag/TiO2
0.2 0.0 0
10
20
30
40
50
60
70
time (min) 567 568
Figure 6. Time profiles of the normalized open circuit potential (OCP) of the bare TiO2,
569
Pt/TiO2, and Ag/TiO2 electrode before, during, and after UV irradiation. E0 is the OCP
570
stationary value in the dark, and Eph is the stationary potential under UV irradiation.
571
Experimental conditions were [4-CP]0 = 300 µM, [NaClO4]0 = 0.1 M, pH0 = 3.0, air
572
equilibrated, and λ > 320 nm irradiation.
573
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1.0
(a)
Absorbance
0.8
0.6 Ag/TiO2 0.4
0.2
VL (10 min) UV (1 min) UV (10 min) UV (60 min)
1.0
(b)
Absorbance
0.8
0.6
0.4
0.2 350
574
Ag/TiO2 UV (60 min) UV (60 min) + Cr(VI) spiking in the dark (10 min) 400
450
500
550
600
650
Wavelength (nm)
575
Figure 7. UV/visible absorption spectral changes of Ag/TiO2 under UV-irradiation. (a) a
576
broad band around 450 nm appeared under UV irradiation, but not under visible light. (b) The
577
UV-induced broad band disappeared after spiking Cr(VI) in the dark. Experimental
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conditions were [Ag/TiO2] = 0.5 g/L (in D.I. water), pH0 = 3.0, [Cr(VI)]add = 100 µM, air-
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equilibrated, and λ > 320 nm irradiation.
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Emission intensity (a.u.)
350 Ag/TiO2
300
TiO2 Pt/TiO2
250
Ag/TiO2 (Dark)
200 150 100 50 0
20
40
60
80
100
120
time (min)
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Figure 8. Time profiles of 7-hydroxycoumarin (7-HC) production (monitored by its
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fluorescence emission intensity) in the dark in the pre-irradiated suspensions of bare TiO2,
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Pt/TiO2, and Ag/TiO2. Experimental conditions were [catalyst] = 0.5 g/L, [noble metal (Pt or
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Ag) loading on TiO2] = 3.0 wt%, [coumarin]0 = 1 mM, pH0 =3.0, air-equilibrated, and pre-
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irradiation (λ > 320 nm) for 1 hour.
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