Light-Induced Efficient Molecular Oxygen Activation on a Cu(II

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Light-induced Efficient Molecular Oxygen Activation on Cu(II) Grafted TiO2/graphene Photocatalyst for Phenol Degradation Hui Zhang, Liang-Hong Guo, Dabin Wang, Lixia Zhao, and Bin Wan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 03 Jan 2015 Downloaded from http://pubs.acs.org on January 4, 2015

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ACS Applied Materials & Interfaces

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Light-induced Efficient Molecular Oxygen

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Activation on Cu(II) Grafted TiO2/graphene

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Photocatalyst for Phenol Degradation

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Hui Zhang, Liang-Hong Guo*, Dabin Wang, Lixia Zhao, Bin Wan

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State Key Laboratory of Environmental Chemistry and Eco-toxicology, Research Centre for

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Eco-environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Road, P.O. Box

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2871, Beijing 100085, China

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ACS Paragon Plus Environment

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ABSTRACT: An efficient photocatalytic process involves two closely-related steps: charge

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separation and the subsequent surface redox reaction. Herein, a ternary hybrid photocatalytic

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system was designed and fabricated by anchoring Cu(II) cluster onto TiO2/reduced graphene

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oxide (RGO) composite. Microscopic and spectroscopic characterization revealed that both TiO2

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nanoparticles and Cu(II) clusters were highly dispersed on graphene sheet with intimate

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interfacial contact. Compared with pristine TiO2, TiO2/RGO/Cu(II) composite yielded an almost

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3-fold enhancement in the photodegradation rate toward phenol degradation under UV

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irradiation. ESR spectra and electrochemical measurements demonstrated that the improved

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photocatalytic activity of this ternary system benefited from the synergetic effect between RGO

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and Cu(II), which facilitates the interfacial charge transfer and simultaneously achieves in-situ

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generation of H2O2 via two-electron reduction of O2. These results highlight the importance to

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harmonize the charge separation and surface reaction process in achieving high photocatalytic

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efficiency for practical application.

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KEYWORDS: TiO2, photocatalysis, oxygen reduction, phenol degradation

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INTRODUCTION

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Photo-induced oxidation/reduction reactions at semiconductor surface present a promising

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approach to address the emerging environmental issues.1-3 In a typical photocatalytic reaction,

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molecular oxygen is activated by photogenerated electrons to produce ·O2－ or H2O2, while holes

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are trapped by surface absorbed hydroxyls (OH － ) to generate ·OH.4 These reactive oxygen

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species (ROS) are identified as one of the most active initiators of photocatalytic degradation of

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organic contaminants. Previous studies demonstrate that the photocatalytic efficiency is critically

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dependent upon light adsorption, electron/hole separation and surface chemical reactions.5

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Unfortunately, optimizing all these processes in a single photocatalyst has remained a great

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challenge. Although UV light excited wide-band-gap photocatalysts (e.g., TiO2) usually exhibit

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more efficiency in initiating or accelerating photodegradation reaction because of the high redox

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potential of photogenerated electron/hole in the conduction/valance band,6 the quantum

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efficiency of molecular oxygen activation is often hindered due to the charge recombination in

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photoexcited semiconductor and the lack of active sites on the catalyst surface.

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To overcome these problems, heterostructured photocatalysts have been constructed to

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improve the quantum efficiency of TiO2.7,8 For example, TiO2/carbon nanomaterial (e.g., C60,

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carbon nanotube) hybrids greatly promoted the charge separation across their interface.9,10 Metal

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nanoparticles (e.g., Au, Ag and Pt) were used to quickly capture electrons from photoexcited

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TiO2.11,12 In recent years, to avoid the high cost of noble-metal nanoparticles, surface

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modification of TiO2 with transition metal ions (such as Cu(II) and Fe(III)) has been employed to

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promote multielectron reduction reaction of oxygen and subsequently enhance the photocatalytic

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performance.13-17 However, the increased quantum efficiency of Cu(II)- or Fe(III)-grafted TiO2 is

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still limited due to the weak charge separation ability in the hybrid system. Indeed, both charge


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separation and surface catalytic reaction are equally important, and the slower one will be the

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rate determining step for the photocatalytic reaction. The challenge in achieving a high-

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performance photocatalyst lies in ensuring a harmony of charge separation and the subsequent

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surface catalytic reaction for molecular oxygen activation, which requires deliberate control of

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the interactions among the photocatalyst components in a systematic and rational fashion.

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With its monolayer structure and superior electron mobility, graphene is highly desirable as a

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flexible two-dimensional (2D) catalyst support.18-20 Recently, graphene based photocatalysts

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have been demonstrated to manipulate the charge transfer across semiconductor-graphene

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interface.21,22 Upon excitation of TiO2 by light, electrons are transferred into RGO, which then

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participate in red-/ox- reaction or lead to their storage within the C-C network in the absence of

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additional electron acceptor.23,24 These elaborate works provide a hint to interface the light

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harvesting semoconductor and surface reaction sites onto an individual RGO sheet to accelerate

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charge transfer process and surface catalytic reaction simultaneously. Herein, we report for the

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first time anchoring Cu(II) on TiO2/RGO mat to achieve an efficient photocatalytic process for

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phenol degradation. The charge transfer and molecular oxygen activation process in the system

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were investigated to explore the possible reaction mechanism. Our work demonstrates a

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promising way to simultaneously tune charge separation and molecular oxygen activation,

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consequently the overall photocatalytic performance.

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EXPERIMENTAL SECTION

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Chemicals and materials. Graphite powder was purchased from Bodi Chemical Co. Ltd

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(Tianjin, China). Ti(OCH4)4, phenol and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were

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purchased from Sigma-Aldrich (St.Louis, MO, USA). HPLC-grade methanol was obtained from

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Fisher Scientific (Pittsburgh, PA, USA). All other chemicals were of analytical grade and used as


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received. All solutions were prepared with deionized (DI) water obtained using a Millipore Milli-

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Q water purification system (Bedford, MA, USA).

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Synthesis of Photocatalysts. Graphene oxide (GO) was synthesized by oxidation-exfoliation

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of graphite powder according to a modified Hummers’ method.25 TiO2/RGO composites were

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prepared by a hydrothermal method. Typically, GO (3.3 mg) was dispersed in a mixed solution

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containing ethanol (15 ml) and water (30 ml) by ultrasonic treatment. Then, a solution of

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Ti(OC4H9)4 (2.9 mL) and ethanol (15mL) was added dropwise to the GO suspension under

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magnetic stirring. After stirring for 2 h, a homogeneous suspension with a light gray color was

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obtained and then transferred into a 100 mL Teflon-lined autoclave and maintained at 180 0C for

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10 h. The resultant TiO2/RGO composite was collected by centrifugation, washed repeatedly by

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water and ethanol, and dried under vacuum at room temperature. Blank TiO2 sample was

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prepared following a similar method without the addition of GO.

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The grafting of Cu(II) ions onto TiO2/RGO composites was performed using an impregnation

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method. In a typical preparation, TiO2/RGO sample (0.5 g) was dispersed into CuCl2 solution

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(20 mL). The weight fraction of Cu2+ ions relative to TiO2/RGO was set to be 1%. Then, the

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suspension was heated at 90 0C for 1 h using a water bath under stirring. The resultant

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suspension was collected by centrifugation, washed by distilled water, and dried at 90 0C for 24

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h. The final products were ground into a powder using an agate mortar. TiO2/Cu(II) sample was

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prepared following the same method.

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Characterizations. Morphology of the synthesized photocatalysts was investigated by

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transmission electron microscopy (TEM) using JEOL JEM 2010 instrument (Tokyo, Japan)

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operated at 120 kV. The elemental mapping images were acquired using a scanning TEM

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attachment by equipping with an Oxford energy-dispersive X-ray detector (EDX) (Bucks, U.K.).


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For TEM measurements, a droplet of the sample-ethanol solution was put onto a Mo grid.

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Structural characteration of the prepared samples was performed by powder X-ray diffraction

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(XRD) on a X′Pert Pro MPD (Eindhoven, Netherlands) with Cu Kα radiation (λ = 0.15418 nm)

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and Raman spectra on a Renishaw InVia Raman spectrometer (Wotton-under-Edge, UK) with

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exciting wavelength at 532 nm. The ionic characterstics were investigated by X-ray

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photoelectron spectroscopy (XPS) on a Thermo VG ESCALAB 250 spectrometer (East Grinsted,

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UK) with Al Kα radiation at 1486.6 eV. Elemental analyses of the samples were performed using

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an inductively coupled plasma/optical emission spectrometer (ICP-OES) on a Perkin-Elmer

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Optima 8300 model instrument (Norwalk, CT, USA). Brunauer−Emmett−Teller (BET) surface

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area was obtained at 77 K on a QuadraSorb SI analyzer (Quantachrome, USA). Fluorescence

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spectra were measured on a Horiba Fluoromax-4 spectrofluorimeter (Edison, NJ, USA). Electron

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spin resonance (ESR) signal of the paramagnetic radicals trapped by 5,5-dimethyl-1-pyrroline-N-

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oxide (DMPO) was recorded on a Bruker ER073 spectrometer (Karlsruhe, Germany) during

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irradiation of the suspension (catalyst samples, 0.05mg/mL; DMPO, 100 mM) with a 500 W

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mercury lamp. The settings were the following: center field 3503.95 G, microwave frequency

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9.84 GHz and power 20 mW. Electrochemical experiments were performed in a three-electrode

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cell on CHI 630B workstation (Shanghai, China) with a platinum plate as the counter electrode, a

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saturated KCl Ag/AgCl electrode as the reference electrode. The working electrodes were

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prepared by coating photocatalyst-ethanol slurry onto FTO glass and a small amount of Nafion

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solution (0.5%) was added to paste the slurry. Current-potential curves and cyclic

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voltammograms of the prepared electrode were measured in a 0.1 M NaClO4 electrolyte solution.

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High-purity O2 gas was used to bubble the electrolyte during the electrochemical O2 reduction

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experiments. The normal hydrogen electrode (NHE) potential was converted from the Ag/AgCl


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electrode using: ENHE = E(Ag/AgCl) + 0.197 V. Electrochemical impedance spectra (EIS) were

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measured in 0.1 M KCl solution containing 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) by applying

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5 mV alternative signal versus the reference electrode over the frequency range of 1MHz to 100

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mHz.

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Evaluation of Photocatalytic Properties. Photodegradation of a phenol solution in ambient

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condition was performed to evaluate the photocatalytic activity of the prepared catalysts.

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Typically, an aqueous solution containing phenol (50 mL, 10 mg/L) and a photocatalyst (50 mg)

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was placed in a cylindrical Pyrex glass vessel and kept in dark for 30 min to ensure

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adsorption/desorption equilibrium. Then the photoreaction vessel was irradiated by a 500 W

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tubelike high-pressure mercury lamp with an irradiation intensity of 1.0 mW/cm2 and the main

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emission wavelength at 365 nm. The suspension was continuously stirred by a Teflon-coated

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magnetic stirrer bar during the irradiation. The temperature of the reaction solution was

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maintained at 20 oC by recirculating a cooling water system during the reaction. At given time

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intervals, aliquots of the irradiated solution were collected, centrifuged and analyzed by high-

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performance liquid chromatograph (HPLC) on Aligent 1260 (Palo Alto, CA, USA) with a

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Poroshell 120 EC-C18 column. The mobile phase was methanol and water (v:v = 0.55:0.45) at a

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flow rate of 0.3 mL min-1, and the detection wavelength was set at 280 nm.

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H2O2 generation during UV irradiation of the photocatalyst solution was determined by

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dimerization of p-hydrophenylacetic acid (POPHA) in the presence of H2O2 and horseradish

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peroxidase to yield a fluorescence product 5,5′-dicarboxymethyl-2,2-dihydroxybiphenyl (λex =

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315 nm, λem = 406 nm).26 Aliquots (0.5 mL) of irradiated catalyst solution was taken out and

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kept in dark for 30 min. Subsequently, 0.5 mL of the fluorometric reagent (8 mg of POPHA and

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2 mg of horseradish peroxidase dissolved in 50 mL Tris buffer (0.1 M, pH 8.8) solution) was


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added and allowed to react for 30 min. Then, the above solution was centrifugated and the

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supernatant was taken for fluorescence measurement. The calibration curves were obtained by

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standard additions of H2O2 to fluorometric reagents.

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RESULS AND DISCUSSION

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Characterization of TiO2/RGO/Cu(II) composite. Cu(II) grafted TiO2/RGO composite was

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synthesized by a two-step process: (i) anchoring TiO2 nanoparticles (NPs) on RGO sheet as a 2-

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D scaffold and (ii) adsorption of Cu(II) on TiO2/RGO. AFM image shows GO exists as

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fragments with a thickness of ca. 1.1 nm (Figure S1 in Supporting Formation (SI)), which

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confirms the structure of one or few layered GO sheet with oxygen-functional groups upon

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chemical exfoliation. These oxygen groups on GO sheet would facilitate in situ growth of TiO2

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nanocrystals.27 Meanwhile, the negative potential of GO given by these oxygen-functional

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groups could induce Cu2+ adsorption through electrostatic interactions.28 Figure 1a shows typical

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TEM images of the resulting TiO2/RGO/Cu(II) composites. It can be seen that spherically

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structured TiO2 NPs are uniformly anchored on 2-D RGO sheet with a crystallite size of ca. 8 nm

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(Figure S2). Both TiO2 NPs and the edge of RGO sheet are evident. Such decorated TiO2 NPs

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can act as spacer to partially prevent exfoliated graphene sheet from stacking. High-resolution

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TEM images (Figure 1b) exhibit a lattice fringes spacing of 0.35 nm, which can be assigned to

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anatase (101) planes of TiO2, and a layered structure with interlayer spacing of 0.34 nm

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corresponding to RGO.29 It needs to note that, compared with TiO2/RGO hybrids (Figure S3),

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surface modification with Cu(II) does not change their morphology and size in the ternary

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composite. Cu(II) on TiO2/RGO samples is hardly observed by TEM, probably due to the limited

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amount of Cu(II) and an amorphous form at the low-grafting temperature.13 However, EDX

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spectra confirm the presence of Cu(II). Figure 1c-e displays the representative element mapping


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images of Ti and Cu in the hybrids, demonstrating an homogeneous distribution of TiO2 NPs and

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Cu(II) on the RGO sheet. Thus, an intimate interfacial contact among TiO2, RGO, and Cu(II)

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components was achieved.

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The structure character of the TiO2/RGO/Cu(II) composites was further determined by XRD

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patterns (Figure S4). TiO2 in all the samples crystallize in an anatase phase structure (JCPDS No.

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21-1272). Notably, a characteristic (002) peak of GO at 2θ = 7.76o (corresponding to an

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interlayer distance of ~1.1 nm) disappeared completely after hydrothermal process, suggesting

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the great reduction of GO and the successful intercalation of TiO2 NPs and Cu(II) to the GO

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stacks in the final composites. No peaks associated with Cu-like compounds were detected,

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implying Cu(II) exists as an amorphous form. Raman spectra provide addition insights into the

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structure information of the composites (Figure 2). The Raman bands at 144 cm−1 (Eg), 399 cm−1

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(B1g), 513 cm−1 (A1g) and 638 cm−1 (Eg) match well with TiO2 anatase structure.30 Two bands at

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1326 cm-1 (D band) and 1597 cm-1 (G band) confirm the presence of RGO in the composites.31

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Significantly, a characteristic 2D band (at ~ 2665 cm-1) corresponding to sp2 bounded carbon

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atoms is also observed in the TiO2/RGO/Cu(II) hybrid. It is noted that the intensity ratio (I2D/IG)

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usually reflects the recovery degree of sp2 hybridized graphitic structure.32 The increase of I2D/IG

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ratio to 0.14 after the solvothermal process demonstrates an efficient restoration of sp2 carbon in

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GO. This would favor the formation of 2-D catalyst assembly with high quality.

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Further evidence for the interactions of components in the TiO2/RGO/Cu(II) composites come

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from XPS spectra (Figure 3). A successful reduction of GO to RGO by solvothermal process is

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verified by C 1s signal, according to the diminished C-O, C=O and O-C=O groups intensity. It is

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worth noting that the remaining C-O group but with much lower intensity (blue dashed line) in

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the composites provides the adsorption sites for Cu(II) anchoring. Moreover, a characteristic


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peak at 288.7 eV can be assigned to Ti-O-C bond, suggesting TiO2 NPs interact with RGO sheet

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through an esterification reaction.27,30 The successful loading of Cu(II) on the TiO2/RGO is

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further demonstrated by Cu 2p3/2 core-level XPS signal at 923.6 eV, which is consistent with

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previous results.13,14 By analogy with the reported Cu(II)-TiO2 system, Cu (II) is grafted on the

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TiO2/RGO composites in a distorted, amorphous CuO structure with a five-coordinated square

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pyramidal form. Although it is difficult to measure the location of Cu(II) in the composites, the

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Cu(II) cluster may prefer to attach on RGO sheet due to the great complex ability of oxygen-

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containing groups toward Cu2+ ions. ICP-OES measurements displayed that the loading amount

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of Cu(II) was calculated to be ~0.92 wt% in the TiO2/RGO/Cu(II) composites, nearly equal to

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the starting ratios used in the preparation. The cyclic voltammograms of the prepared

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TiO2/RGO/Cu(II) electrode provide additional evidence for the loading of Cu (II) (Figure S5). A

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couple of redox peaks were obviously observed, which are similar to the redox peaks of a Cu2+

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solution on a bare TiO2 electrode. The reduction peak located at about 0.10 V is assigned to the

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reduction of Cu(II) to Cu(I).33 In the reverse scan, a major oxidation peak at 0.24 V is detected

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and attributed to Cu(I) oxidation. These results indicate Cu(II) clusters are successfully grafted

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on TiO2/RGO composites.

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Photocatalytic performance of TiO2/RGO/Cu(II) catalyst. Phenol, a typical toxic aromatic

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compound with low degradability by conventional decomposition method, was selected as the

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target molecule to evaluate the photocatalytic activity of the prepared composites. Figure 4a

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shows the phenol degradation curves over various photocatalysts under UV light irradiation.

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Both the direct photolysis of phenol and the adsorption of phenol on catalysts in the dark are

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negligible, suggesting the observed phenol degradation is initiated by semiconductor

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photocatalysis. Under UV light irradiation, the unmodified TiO2 sample shows a low


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photocatalytic activity. It is evident that the introduction of RGO and Cu(II) resulted in a

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significant improvement in the photocatalytic phenol degradation. The temporal evolutions of

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phenol concentration are well fitted with a pseudo-first-order rate equation. The rate constants (k)

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for TiO2/Cu(II), TiO2/RGO and TiO2/RGO/Cu(II) composites are calculated to be 0.017 min-1,

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0.014 min-1 and 0.023 min-1, respectively, which is up to almost three-fold enhancement than the

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bare TiO2 (0.006 min-1) (Figure S6). Both TiO2/Cu(II) and TiO2/RGO exhibit a decent

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photocatalytic activity, which might arise from efficient molecular oxygen activation by Cu(II)

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clusters17 and enhanced charge separation by RGO,21,22 respectively. Importantly, the highest

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photocatalytic activity of TiO2/RGO/Cu(II) clearly demonstrates Cu(II) and RGO act a

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synergistic effect in boosting the photocatalytic performance of TiO2.

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A separate experiment of TiO2/RGO/Cu(II) in the N2 purged suspension under UV light shows

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a negligible phenol degradation, indicating that O2 participates in the photocatalytic degradation

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reaction of organic pollutants. Once O2 is consumed by photogenerated electrons for phenol

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degradation, it would be immediately supplied from air. Dissolved oxygen analysis displayed

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that its concentration was maintained at 8.50 mg/L during the photocatalysis process. Indeed, as

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the only molecular that can be reduced in ambient atmosphere, molecular O2 activation during

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photocatalysis can not only inhibit the recombination of electron-hole pairs by capturing

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electrons but also yield ·O2- or H2O2, which would participate in the subsequent photocatalytic

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reaction. To further investigate the improved photocatalytic performance of TiO2/RGO/Cu(II)

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composite, the generation of reactive oxygen species (ROS) during UV light irradiation was

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probed. A DMPO spin-trapping ESR technique was employed to detect ·O2- and ·OH generation

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(Figure S7). No signal was observed under the dark. Upon UV light irradiation, a characteristic

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quartet peak of the DMPO-·OH adduct with intensity ratio of 1:2:2:1 was clearly observed.·O2-


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was also captured to form DMPO-·O2- adduct with a quartet peaks in methanol suspension,

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which usually accompany with DMPO-·CH2OH adduct signal due to the attack of ·OH radicals

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onto the CH3OH solvent molecules.34 These results reveal that both ·O2- and ·OH were

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generated from the irradiated photocatalyst samples.

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To further assess ROS generation, temporal evolutions of ESR signal were displayed in Figure

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5a and 5b, which were performed under the same conditions for comparison. It displays that the

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rate of ·OH generation increases in the order of TiO2/RGO/Cu(II) > TiO2/RGO ≈ TiO2/Cu(II) >

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TiO2, the trend of which is in good agreement with the observed phenol degradation. In general,

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·OH radicals can generate from the photogenerated holes oxidation (h+ + OH- = ·OH), O2

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reduction by electrons, and/or both. Figure 5b displays that, after Cu(II) modification, the rate of

243

·O2- generation over TiO2/RGO/Cu(II) is inhibited compared with that over TiO2/RGO. Indeed,

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photogenerated electrons in the catalyst system could reduce O2 to ·O2- or H2O2 via one- or two-

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electron reduction process, respectively (O2 + e− =·O2-, −0.046 V vs NHE; O2 + 2H+ + 2e− =

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H2O2, 0.682 V vs NHE).35 Thus, a POPHA fluorescence method was employed to determine

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H2O2 generation amount from different photocatalysts and further check the pathway of

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molecular oxygen activation (Figure 5c). It was found that significantly more H2O2 were

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generated over TiO2/RGO/Cu(II) than TiO2/RGO, which is contrary to the trend of ·O2-

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generation. The rapid formation of H2O2 and slow growth of ·O2- indicate that one two-electron

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reduction pathway (O2 → H2O2) is governed over TiO2/RGO/Cu(II), while an one-electron

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reduction process for ·O2- generation is the main pathway over TiO2/RGO. Although the

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generated ·O2- can further convert into H2O2 by the subsequent reaction, the amount of generated

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H2O2 became steady after 40 min irradiation, indicating H2O2 was produced independent of ·O2-.

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This result further suggests that H2O2 is generated via a two-electron reduction of O2 under the


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catalytic action of the grafted Cu(II) cluster. Indeed, considering the work function of RGO (-

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4.42 eV) and the Fermi level of RGO at about 0 V (vs NHE), which is lower than that of

258

Cu2+/Cu+ (0.16 V vs NHE),28,36 Cu2+ clusters could notably capture the accumulated electrons on

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RGO sheet. In addition, the potential of Cu2+/Cu+ is more positive than the one-electron

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reduction of O2 →·O2-. Thus, a multielectron reduction pathway for H2O2 is more favored over

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TiO2/RGO/Cu(II). This result is consistent with the multielectron reduction of O2 over the

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reported Cu(II)/TiO2, Fe(III)/TiO2, and Rh(III)/TiO2 system.15,16,37 BET surface area analysis

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displayed that the introduction of RGO and Cu(II) didn’t alter the surface area of TiO2 (Table

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S1), indicating that the enhanced photoreactivity of the TiO2/RGO/Cu(II) was not ascribed to the

265

difference of surface area. Thus, it can be concluded that the grafted Cu(II) clusters on

266

TiO2/RGO work as efficient reactive sites to transfer the photogenerated electrons to O2

267

molecule, resulting in a highly efficient multielectron reduction of O2 to H2O2. The greatly

268

enhanced ROS generation demonstrate the synergic effect of RGO and Cu(II) in promoting

269

charge carrier separation.

270

Based on the above results, a schematic illustration of the photocatalytic reaction mechanism

271

over TiO2/RGO/Cu(II) is shown in Figure 6. RGO was demonstrated to store and shuttle

272

electrons to the surface adsorbed species.23 Upon UV light irradiation, a stepwise transfer of

273

electrons from photoexcited TiO2 to Cu(II) cluster via RGO (which acts as a “highway” for

274

electron shuttling) occurs and subsequently achieves O2 multielectron reduction. This process

275

makes phtogenerated holes and electrons to carry out selective catalytic reaction at separate sites.

276

Owing to the efficient consumption of electrons, more remaining holes participate in the

277

subsequent photocatalytic degradation reaction. Since Cu+/Cu2+ redox system provides only one

278

electron, twice amount of Cu species are required to achieve two-electron reduction of oxygen


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

279

molecules (O2 + 2Cu+ + 2H+ = H2O2 + 2Cu2+). This structure has been confirmed by the

280

aforementioned XPS results. The generated H2O2 could be further converted into ·OH radicals

281

through a Fenton-like reaction (Cu+ +H2O2 = Cu2+ + ·OH + OH-)28 or photocatalytic reactions

282

(H2O2 + UV light = 2·OH and H2O2 + e- =·OH + OH-).38 This indicates the synergetic effect

283

between RGO and Cu(II) not only suppresses the charge recombination by improving the

284

interfacial charge transfer, but also facilitates the surface catalytic reaction by providing reactive

285

centers. In addition, some photogenerated electrons can also be transferred directly to Cu(II) on

286

TiO2 surface or to RGO, and subsequently participate in photocatalytic reaction, which is

287

consistent with the decent photocatalytic activity over TiO2/Cu(II), TiO2/RGO composites.

288

The photocatalytic performance of the prepared composites under visible light irradiation (λ >

289

400 nm) was further investigated (Figure S8). The bare TiO2 showed rather poor photocatalytic

290

activity under visible light, while TiO2/Cu(II), TiO2/RGO and TiO2/RGO/Cu(II) composites

291

exhibited much improvements in the photocatalytic degradation of phenol. These results indicate

292

the narrowing of the band gap of TiO2 after Cu(II) and RGO incorporation, which can be

293

attributed the photoinduced interfacial charge transfer between TiO2 and the adsorbed Cu(II)

294

species13,14,36 and the formation of Ti-O-C band in the TiO2/RGO composites,39,40 respectively.

295

However, the photocatalytic efficiency of these composites under visible light is still low.

296

To evaluate the stability of the TiO2/RGO/Cu(II) photocatalyst, a recycling photocatalytic

297

experiment was carried out. As shown in Figure 4b, the high performance of TiO2/RGO/Cu(II)

298

was well maintained during the repeated light irradiation cycles. Moreover, the amount of Cu(II)

299

species in the five-cycle repeated TiO2/RGO/Cu(II) was about 0.87 wt%, which is comparable to

300

the as-prepared TiO2/RGO/Cu(II) composites. Few Cu species in the photocatalytic reaction

301

solution also indicated the good stability of Cu(II) in the composites (Figure S9). Thus, the high


14

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


302

photocatalytic activity and stability of TiO2/RGO/Cu(II) composites suggest its great potential in

303

practical applications.

304

Electrochemical measurements of TiO2/RGO/Cu(II) catalyst. To further examine the

305

oxygen reduction process, electrochemical O2 reduction was investigated on the prepared

306

catalyst film electrode at bias potential to avoid the complicated redox process under light

307

irradiation conditions. Figure 7a shows the current-potential curves of the prepared composite

308

electrodes in Ar- or O2-saturated electrolyte. Little current was observed in Ar-bubbled solution

309

regardless of the electrodes, while in O2-saturated solution, a reduction current was observed,

310

indicating that O2 reduction occurred on the composite electrodes. Compared with the bare TiO2

311

electrode, a larger O2 reduction current is observed over TiO2/RGO and TiO2/Cu(II) electrodes

312

due to the important roles of RGO and Cu(II) in promoting charge transfer and facilitating O2

313

reduction, respectively. Under the synergetic effect of RGO and Cu(II), TiO2/RGO/Cu(II)

314

ternary system exhibited the highest O2 reduction current. Another interesting observation is that

315

the onset potential for oxygen reduction gradually shifted from below 0 V on TiO2 and

316

TiO2/RGO electrode, which corresponds to one-electron reduction of O2, to the positive region

317

(about 0.15 V) in the case of Cu(II) grafted electrode, in good agreement with the redox potential

318

of Cu2+/Cu+. At this potential, multielectron reduction of O2 probably occurred for H2O2

319

generation. Thus, the electrochemical oxygen reduction behavior of the electrodes further

320

demonstrate that the grafted Cu2+ accept electrons from TiO2 via RGO, and then transfer

321

electrons to O2 via multielectron reduction reaction, which is consistent with the observed ROS

322

generation during photocatalysis process.

323

The charge-carrier migration behavior in the TiO2/RGO/Cu(II) ternary system was further

324

characterized by EIS spectra (Figure 7b). Compared with the pure TiO2 sample, both TiO2/Cu(II)


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

325

and TiO2/RGO show depressed semicircles at high frequencies, suggesting a decrease in the

326

contact resistance at solid state interface layer and the charge transfer resistance at the electrode-

327

electrolyte solution contact interface.41 Notably, the TiO2/RGO/Cu(II) ternary system exhibited

328

the shortest circle among all three hybrid samples, indicating a faster interfacial charge transfer

329

and more efficient charge separation due to the excellent electron shuttling feature of RGO and

330

effective Cu(II) cluster reactive sites for electron capture. The efficient charge transport behavior

331

was further confirmed by the cyclic voltammograms (Figure S4). The anodic and cathodic

332

current density of Cu(II) on the TiO2/RGO/Cu(II) composite electrode is larger than that of TiO2

333

/Cu(II), implying enhanced electron transfer due to the introduction of RGO sheet as a 2-D

334

conducting substrate. These results demonstrate the significant role of RGO and Cu(II) in

335

improving charge separation and subsequent O2 activation for photocatalytic application.

336

Conclusion

337

In summary, Cu(II) grafted TiO2/reduced graphene oxide (RGO) composite was prepared for

338

developing high-performance photocatalyst. Under the synergetic effect between RGO and

339

Cu(II), this composition engineering not only optimizes charge transfer pathways for improved

340

charge separation, but also provide abundant photocatalytic reactive sites to reduce oxygen

341

effectively, resulting in enhanced photocatalytic performance for phenol degradation. In

342

addition, Cu(II) cluster modified TiO2/RGO preferentially reduce O2 to H2O2 via two-electron

343

transfer. The modulation of charge separation and molecule oxygen activation pathway described

344

here can be potentially applied to other photocatalysis system for environmental cleanup.

345 346

ASSOCIATED CONTENT


16

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


347

Supporting Information. AFM image of GO, size distribution of TiO2 NPs in TiO2/RGO/Cu(II)

348

composites, TEM images of TiO2/RGO, XRD patterns, cyclic voltammograms, ESR spectra of

349

OH and·O2− species, BET surface area and visible light photoactivity of the prepared

350

photocatalysts, Cu-species concentration in the TiO2/RGO/Cu(II) composites suspension. This

351

material is available free of charge via the Internet at http://pubs.acs.org.

352

AUTHOR INFORMATION

353

Corresponding Author

354

*Dr. Liang-Hong Guo, phone: 86 10-62849685, email: [email protected].

355

Notes

356

The authors declare no competing financial support.

357

ACKNOWLEDGMENT

358

This work was supported by the National Basic Research Program of China (2011CB936001)

359

and National Nature Science Foundation of China (21207146, 21177138, and 21477146).

360 361


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362 363 364 365 366

Page 18 of 28

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473 474

Figure 1. (a) TEM and (b) High-resolution TEM image of the prepared TiO2/RGO/Cu(II)

475

composite. (c-e) EDX element mapping images of Ti and Cu in the TiO2/RGO/Cu(II)

476

composites.

477

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

B1g Eg Eg A 1g

D

G

2D D+G GO TiO2/RGO/Cu(II) TiO2/RGO TiO2/Cu(II) TiO2

500

478

1000 1500 2000 2500 3000 -1 Raman shift (cm )

479

Figure 2. Raman spectra of GO, TiO2, TiO2/Cu(II), TiO2/RGO and TiO2/RGO/Cu(II)

480

composites.


24

Page 25 of 28

(a)

(b)

Intensity (a.u.)

C-O C=O Ti-O-C

TiO2/RGO/Cu(II)

Cu 2p3/2

Intensity (a.u.)

C-C

TiO2/RGO/Cu(II)

O=C-O

TiO2

GO

280

284 288 292 Bingding Energy (eV)

481

930

940 950 Bingding Energy (eV)

960

482

Figure 3. (a) Deconvoluted peaks of C 1s spectra of GO and TiO2/RGO/Cu(II) samples. (b) Cu

483

2p core-level spectra of TiO2 and TiO2/RGO/Cu(II) samples.

484 485 (b)

(a) 1.2 Photolysis

Dark Light On

1.0

5th

4th

0.8

TiO2/RGO/Cu(II)

C/C0

0.6 0.4

0.6 0.4 0.2

0.2

486

3rd

2nd

TiO2/Cu(II) TiO2/RGO

0.0 -30

1.0 1st

TiO2

0.8 C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

30

60

90 120 150 180

0.0

0

3

Time (min)

6

9

12

15

Time (h)

487

Figure 4. (a) Phenol degradation curves under UV light irradiation over TiO2, TiO2/Cu(II),

488

TiO2/RGO and TiO2/RGO/Cu(II) composites. (b) Cycling runs of TiO2/RGO/Cu(II) for

489

photocatalytic degradation of phenol.

490


25

TiO2/Cu(II)

0.12

TiO2/RGO TiO2/RGO/Cu(II)

0.09

0.5

Page 26 of 28

TiO2 TiO2/Cu(II) TiO2/RGO TiO2/RGO/Cu(II)

0.4

(b)

0.3 0.2

-

0.06 0.03 0.00 0

491

(a)

DMPO-·O2 Signal Intensity (a.u.)

TiO2

0.15

4 8 12 Irradiation Time (min)

H2O2 concentration (mM)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

DMPO-·OH Signal Intensity (a.u.)


16

0.0 0

4

8

12

16

Irradiation Time (min)

TiO2 TiO2/Cu(II) TiO2/RGO TiO2/RGO/Cu(II)

(c)

1.6

0.1

1.2 0.8 0.4 0.0 0

492

20

40

60 80 Time (min)

100

120

493

Figure 5. (a, b) The temporal evolution curves of DMPO-·OH adduct and DMPO-·O2- adduct

494

obtained from the first peak intensity. (c) The generation curves of H2O2 over different samples

495

under UV light irradiation.

496 497 498 499 500


26

Page 27 of 28

501 502

Figure 6. Schematic illustration of the structure of TiO2/RGO/Cu(II) composites (a) and the

503

possible photocatalytic process (b).

504

0.0 (a)

-2

-0.2

TiO2/Cu(II) (N2) TiO2/RGO (N2) TiO2/RGO/Cu(II) (N2)

-0.4

TiO2 (O2) TiO2/Cu(II) (O2) TiO2/RGO/Cu(II) (O2)

-0.2 0.0 0.2 Potential (V vs. NHE )

TiO2/Cu(II)

5000

TiO2/RGO

4000

TiO2/RGO/Cu(II)

3000 2000

TiO2/RGO (O2)

-0.6 -0.4

505

TiO2 (N2)

TiO2

(b)

6000 -Z′′ (ohm)

Current density (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

1000 0 0

1000

2000 3000 Z′ (ohm)

4000

5000

506

Figure 7. (a) Current-potential curves of the prepared electrodes in Ar or O2-saturated NaClO4

507

solutions. (b) EIS Nyquist plots of the the prepared electrodes in 0.1 M KCl solution containing

508

2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1).

509


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The Table of Contents

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Light-Induced Efficient Molecular Oxygen Activation on a Cu(II

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