Enhanced Photoelectrocatalytic Reduction of Oxygen Using Au@TiO2

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Enhanced Photoelectrocatalytic Reduction of Oxygen using Au@TiO2 Plasmonic Film Limin Guo, Kun Liang, Kyle Marcus, Zhao Li, Le Zhou, Prabhu Doss Mani, Hao Chen, Chen Shen, Yajie Dong, Lei Zhai, Kevin R. Coffey, Nina Orlovskaya, Yong-Ho Sohn, and Yang Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14586 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 14, 2016

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ACS Applied Materials & Interfaces

Enhanced Photoelectrocatalytic Reduction of Oxygen using Au@TiO2 Plasmonic Film †

†

‡

‡

Limin Guo†,#, Kun Liang ,#, Kyle Marcus , Zhao Li‡, Le Zhou , Prabhu Doss Mani , Hao Chen§, Chen Shen‡, Yajie Dong†,‡,§,﹠, Lei Zhai†,‡,♀, Kevin R. Coffey‡, Nina Orlovskaya♂, ‡

†‡

Yong-Ho Sohn , Yang Yang , ,* †

‡

NanoScience Technology Center, Department of Materials Science & Engineering,

College of Optics & Photonics (CREOL), ﹠College of Engineering, Chemistry,

♂

♀

§

Department of

Department of Mechanical and Aerospace Engineering, University of

Central Florida, 4000 Central Florida Blvd. Orlando, Florida, 32816, United States #

These authors contributed equally

*

Corresponding Email: [email protected]

ABSTRACT: Novel Au@TiO2 plasmonic films were fabricated by individually placing Au nanoparticles into TiO2 nanocavity arrays through a sputtering and dewetting process. These discrete Au nanoparticles in TiO2 nanocavities showed strong visible-light absorption due to the plasmonic resonance. Photoelectrochemical studies demonstrated that the developed Au@TiO2 plasmonic films exhibited significantly enhanced catalytic activities towards oxygen reduction reactions with an onset potential of 0.92 V (vs reversible hydrogen electrode), electron transfer number of 3.94, and limiting current density of 5.2 mA cm-2. A superior ORR activity of 310 mA mg-1 is achieved using low 1 ACS Paragon Plus Environment


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Au loading mass. The isolated Au nanoparticle size remarkably affected the catalytic activities of Au@TiO2, and TiO2 coated with 5 nm Au (Au5@TiO2) exhibited the best catalytic function to reduce oxygen. The plasmon-enhanced reductive activity is attributed to the surface plasmonic resonance of isolated Au nanoparticles in TiO2 nanocavities and suppressed electron recombination. This work provides comprehensive understanding of a novel plasmonic system using isolated noble metals into nanostructured semiconductor films as a potential alternative catalyst for oxygen reduction reaction.

KEYWORDS: plasmonics, photoelectrocatalysis, oxygen reduction reaction, Au nanoparticle, TiO2 nanocavity array

Complex four-electron transfer process and sluggish kinetics of oxygen reduction reaction (ORR) have been recognized as barriers to realize high-performance fuel cells and metal-air batteries.1,2 Conventional platinum-based catalysts are widely used for ORR, however, the scarcity and high cost of these precious metals hinder their large scale utilization.3,4 Some narrow band-gap metal oxides (semiconductors, Fe3O4, Co3O4, and etc.) have therefore been developed as low cost alternative catalysts.5,6 But these oxide semiconductors have unfavorable activities for ORR compared with Pt. Furthermore, the usage of solar energy, a clean power source, to produce fuels has motivated the study of using semiconductor photocatalysts in photoelectrocatalytic ORR.7 However, the 2 ACS Paragon Plus Environment

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conventional powder catalysts cannot maintain stable activity because of catalyst deactivation, poisoning, and un-accessible for recycling.8,9 It is therefore crucial to develop inexpensive and stable semiconductor thin-film catalysts that adsorb most of sunlight and deliver high activities towards ORR. TiO2 has been widely explored as a low cost, non-toxic and stable photocatalyst,10 however, its wide band-gap (3.2 eV) and the requirement of ultraviolet (UV) light excitation limit its practical applications.11 A promising strategy to overcome the large energy barrier of TiO2 is to form plasmonic heterogeneous nanostructures by integrating noble metal (Pt, Au or Ag) nanoparticles (NPs) with dielectrics (semiconductors). The catalytic activities will be drastically enhanced in visible region due to the localized surface plasmon resonance (LSPR) effects generated in the plasmonic nanostructures.12-14 Basically, three major energy-transfer pathways, including resonant photon scattering, near-field electromagnetic enhancement and “hot” electron transfer, are involved in plasmon-enhanced catalytic reactions.14 For instance, plasmonic Au-TiO2 systems show much higher photo-response than non-plasmonic Au-TiO2 composites owing to the LSPR-facilitated reduction of oxygen.13-15 LSPR is the predominant feature of plasmonic photocatalysis, which induces electric field oscillation under sunlight excitation, while a significant plasmonic absorption arises and high-energy (“hot”) electrons are generated at noble metals/dielectrics interfaces for catalytic reactions.8,16 However, conventional fabrication techniques, including complicated cleanroom-based patterning processes and high-temperature annealing of noble metals on Si wafer, fail to make significant progress 3 ACS Paragon Plus Environment


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in producing plasmonic nanostructures.17,18 In addition, the most frequently used dielectric substrates in the plasmonic systems have flat surfaces, which provide less capability to control NPs geometry, size and distribution than nanostructured dielectrics. Noble metal dewetting on the nanostructured dielectric films is still rarely investigated due to the lack of facile techniques (out of cleanroom) to generate dielectric nanostructures with highly ordered patterned surfaces. Highly ordered porous TiO2 films are therefore considered as ideal platforms to develop novel plasmonic nanostructures with controllable geometry of noble metal particles because of the facilitated metal film splitting into small islands on top of periodic porous films.19-21 In this work, isolated Au NPs in TiO2 nanocavity arrays (NCAs) were utilized as promising plasmonic films, which were proposed to be beneficial for extending majority carrier lifetime and improve electron transfer kinetics. To this end, Au films with different thickness were deposited on TiO2 NCAs, which were then converted to Au@TiO2 NCAs plasmonic films after thermal dewetting treatments. A plasmon-enhanced ORR activity was then achieved by forming 20 nm Au NPs in the plasmonic films. Resonance wavelength (in visible or near-UV range) can be further tailored by controlling noble metal particle size. These studies provide experimental evidences that wide band-gap semiconductors can be applied for photoelectrocatalytic ORR in visible light by taking advantages of plasmonic resonance. A typical fabrication route for Au@TiO2 NCAs plasmonic films is illustrated in Figure 1a, which is much more facile than conventional cleanroom-based photolithographic 4 ACS Paragon Plus Environment

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processing of plasmonic films. More specifically, three steps are involved. Step 1: Highly ordered TiO2 NCAs fabricated via anodic treatment of Ti foils are used as dielectric host materials to load Au NPs and form plasmonic structures. These TiO2 NCAs show a highly porous structure (Supplementary Figure S1a) with an average pore size of 50 nm and film thickness of 70 nm (Supplementary Figure S1b). These 70 nm thick TiO2 NCAs are rationally chosen in this work because the maximum penetration depth of incident light in TiO2 is less than 1 µm.22 Step 2: Au thin layers with different thickness (2-8 nm, abbreviated to Au2@TiO2, Au5@TiO2, Au8@TiO2, respectively) are conformally deposited onto TiO2 NCAs by sputtering (Supplementary Figure S1c). Step 3: Thermal dewetting at 500 oC is executed to form isolated Au NPs in TiO2 NCAs, which exhibit LSPR effect for plasmon-enhanced catalytic reactions. The thickness of deposited Au films has significant effect on the dewetting kinetics based on scanning electron microscopy (SEM) studies (Figure 1b-e). Au NPs with an average diameter of 10 nm are thermal dewetted from a deposited 2 nm Au layer and form “crown” at the tubular edges of TiO2 NCAs walls (Au2@TiO2, Figure 1b and e). This is due to the lack of dewetting nucleation centers (the thickest metastable places in the deposited films) in the deposited 2 nm Au layer. When the thickness of the deposited Au layers increases to 5 and 8 nm (Au5@TiO2 and Au8@TiO2), highly ordered discrete Au NPs formed inside TiO2 NCAs after thermal dewetting (Figure 1c-d). The Au particle sizes obtained from 5 and 8 nm Au layers dewetting are about 20 nm and 30 nm, respectively, which are further confirmed by transmission electron microscope (TEM) (Figure 1f-g). Lattice fringes of 0.35 nm and 5 ACS Paragon Plus Environment


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0.23 nm observed in high resolution TEM image (Figure 1g) are corresponding to TiO2 (101) and Au (111), respectively. Furthermore, the phase composition and morphology of TiO2 NCAs significantly affect the dewetting kinetics of Au layers (Supplementary Figure S1d-f). Nonuniformly distributed Au particles are observed on the amorphous TiO2 NCAs and compact TiO2, which are ascribed to the poor hydrophilicity of amorphous TiO2 and the lack of the nanoscale confinement effect of highly ordered porous structure in Au film dewetting process. Further increasing the deposited Au layer thickness to 10 nm generates excessive Au particles on the surface of TiO2 NCAs, which may not be beneficial for enhancing plasmonic effect in the films.

Figure 1. (a) Schematic illustration of fabrication steps for plasmonic Au@TiO2 NCAs: Step 1, anodic formation of TiO2 nanocavity arrays, Step 2, Au films deposition, Step 3, thermal dewetting treatment. (b-d) Top-view SEM images of Au2@TiO2, Au5@TiO2 and Au8@TiO2, respectively. (e) Cross-sectional view SEM image of Au2@TiO2 NCAs. (f-g) 6 ACS Paragon Plus Environment

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TEM images of isolated Au NPs inside TiO2 nanocavities. The phase and chemical compositions of the plasmonic Au@TiO2 NCAs were characterized using X-ray diffraction (XRD, Figure 2a) and X-ray photoelectron spectroscopy (XPS, Figure 2b-c, more XPS analysis is included in Supplementary Figure S2). A rutile phase TiO2 (PDF Card No.: 73-1765) and a typical cubic Au (PDF Card No.: 4-787) XRD pattern appeared after dewetting treatment at 500 oC. XRD peak at 38.2° corresponding to Au (111) intensifies with increased Au thickness. XPS Au 4f peaks at 85 eV and 88.6 eV shift to higher binding energy compared with the ground state Au metal, suggesting a slight surface oxidization of the thermally dewetted Au NPs.23 XPS Au 4f binding energy shift is also attributed to the size-dependent electrostatic interaction between the ionized cluster (OH·) with Au surface.23 UV-visible diffuse reflectance spectroscopy was used to analyze the photo-response of Au@TiO2 NCAs (Figure 2d). A slight red shift of intrinsic adsorption of TiO2 at 360 nm can be found in the Au8@TiO2 sample because of more oxygen deficiency in TiO2 introduced by more Au loading. A broad band centered at 480 nm is also observed in blue-green light regions owing to the LSPR of embedded Au NPs. A slight peak red-shift with considerably increased absorption cross-section are found with increased Au particle sizes from 10 to 30 nm, which are ascribed to the LSPR effect and electric field polarization in the plasmonic Au@TiO2 NCAs. LSPR derived absorption can be significantly affected by the dipole shaking frequency and intensity nearby the Au surface (enhanced electromagnetic field).17 According to Mie theory and related electrodynamics 7 ACS Paragon Plus Environment


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description (equation 1),24 a Au particle with size D (D 8 nm), suggesting that the Au particle only plays an essentially role in ORR catalysis below a critical size. Interestingly, Au5@TiO2 NCAs has a higher IPCE than Au8@TiO2, which is due to the higher “hot” electron recombination in Au8@TiO2 with higher Au loading amount. Confirmed by photo-absorption spectra and SERS analysis, the amount of “hot” electrons increases with increasing Au loading amount (from 2 to 8 nm). But when the Au loading amount increases to 8 nm (Au8@TiO2 NCAs), the dewetted Au particles with bigger size than Au2@TiO2 and Au5@TiO2 NCAs have the possibility to contact with Ti substrate. The "hot" electrons may therefore be partially recombined at Au/Ti substrate interface and fail to participate in ORR processes. Generally, three reaction steps are involved in four-electron transfer processes of the ORR, including O2 molecules transport and adsorption on the surface of catalysts, reduction reactions at the catalyst/electrolyte interface and desorption of product (or intermediates) from catalyst. A space-charge region formed at the Au/TiO2 interface builds up an internal electric field (from TiO2 to Au), which prevents the electron recombination.30 However, the internal electric field can also suppress the desorption of the produced OH- and slow down the reaction rate. Such negative impact of internal electric field on the ORR will become dominant when the 13 ACS Paragon Plus Environment


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space-charge region is increased in Au8@TiO2 NCAs. By controlling Au loading amount, a balance between LSPR and "hot" electron recombination is achieved in Au5@TiO2 NCAs which deliver an excellent plasmon-enhanced ORR activity. The Au particle size dependent charge transfer kinetics (transport time and lifetime) was further analyzed by intensity

modulated

photocurrent/photovoltage

spectroscopy

(IMPS/IMVS,

Supplementary Figure S7 and S8). The carrier lifetime on the surface of Au5@TiO2 NCAs is longer than that on Au2@TiO2 and Au8@TiO2 NCAs, indicating a facilitated charge transfer for ORR. Potential-dependent electron transfer number (n) and percentage of the produced intermediate hydrogen peroxide (χH2O2) (Figure 4d) were analyzed to determine the reaction pathway for ORR . The n was then calculated by:11 n=

4 jD j N

jD + R

and

χH

2 O2

=

j 2 R N

j N

( R )+jD

(5)

where jD and jR denote the disk and ring current densities, respectively, N is the collection efficiency (about 3.7).11 The LSV curves collected at 2500 rpm using RRDE were used for the n value calculation. In a dark environment, the n value of Au5@TiO2 NCAs is about 3.8, which is significantly enhanced to 3.94 by shining a simulated sunlight. The χH2O2 is also greatly reduced from 5% (in dark) to less than 1% (under simulated sunlight). These results suggest a distinguished plasmon-enhanced ORR and a four-electron transfer process using plasmonic Au5@TiO2 NCAs (The n and χH2O2 of other Au@TiO2 NCAs are provided in the Supplementary Figure S9.). 14 ACS Paragon Plus Environment

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Koutecky-Levich (K-L) plots (the inset in Figure 4e) derived from LSV measurements (Figure 4e) were also used to estimate the electron transfer numbers. K-L equation is given in the Supporting Information. From the gradual changes in the slopes of K-L curves, ORR is controlled by mixed kinetics and diffusion of O2 molecules at potentials higher than 0.6 V, whereas the mass diffusion dominates the ORR at potentials less than 0.6 V. Long-term durability of Au5@TiO2 NCAs for ORR was also examined by chronoamperometric study (Figure 4f, constant potential was set at 0.6 V for 10,000 s in an O2-saturated 0.1 M NaOH solution at 2500 rpm). Approximately 90% of current density can be maintained after 10,000 s. Moreover, the durability was also examined by repeating LSV measurements for 1,000 cycles (the insert in Figure 4f) where only a slight current density decay can be observed. These novel Au5@TiO2 NCAs are proposed for further applications in photoelectrocatalytic reduction of CO2 because of their merits when used as advanced catalysts including excellent visible-light absorption, suppressed charge carrier recombination, and long-term durability.

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Figure 4. (a) CV curves of Au5@TiO2 NCAs in N2- and O2-saturated 0.1 M NaOH solution. (b) LSV curves of Au5@TiO2 NCAs (disk current in the lower panel and ring current in the upper panel). (c) Onset potentials (left Y-axis) and limiting current densities (right Y-axis) of different samples in dark and simulated sunlight irradiation. (d) Variation of electron transfer number (n) and χH2O2 as functions of applied disk electrode potential during ORR for Au5@TiO2 NCAs at 2500 rpm. (e) LSV curves and derived 16 ACS Paragon Plus Environment

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K-L plots for Au5@TiO2 NCAs measured at different rotation speeds from 100 to 2500 rpm (5 mV s-1). (f) Long-term (10,000 s) durability test of Au5@TiO2 NCAs under AM 1.5 simulated sunlight illumination. The inset shows LSV curves of Au5@TiO2 NCAs before and after 1000 cycles ORR tests). In summary, a novel Au@TiO2 plasmonic structure was fabricated by individually placing Au nanoparticles into TiO2 nanocavities via a facile and cost-effective approach. These plasmonic Au@TiO2 NCAs exhibited a plasmon-enhanced catalytic activity towards ORR. An investigation of the effect of Au particle size on the plasmon-enhanced ORR demonstrated that 5 nm Au layer deposited on TiO2 NCAs delivered a superior ORR performance with an onset potential of 0.92 V vs RHE, limiting current density of 5.2 mA cm-2, electron transfer number of 3.94 under an AM 1.5 simulated sunlight illumination. This plasmonic film also possessed a long-term durability. The kinetics of plasmon-enhanced ORR was ascribed to the increased amount of "hot" electrons and suppressed electron recombination originated from the unique morphological merits of the developed plasmonic films, i.e. isolated Au nanoparticles in highly ordered dielectric TiO2 nanocavities.

EXPERIMENTAL SECTION Growth of TiO2 nanocavity arrays (NCAs). Titanium foils (0.05 mm thickness with a purity of 99.7%, Milliren Technologies, Inc.) were cleaned by ultrasonication in acetone, ethanol, and deionized water for 30 minutes, respectively, and dried in air. TiO2 NCAs 17 ACS Paragon Plus Environment


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were anodically fabricated in a solution of 3 M HF/H3PO4 (98%, Alfa Aesar, US) at 80 oC using a constant voltage of 10 V for 3 hours in a two electrode set-up. A Pt foil was used as counter electrode. After anodization, the samples were rinsed with ethanol and dried in air. Subsequently, the samples were annealed at 450 °C for 30 min in air using a muffle furnace (Thermo Scientific). Au film deposition. Magnetron sputtering system (AJA International, Inc., ATC 2200-V) was used to deposit Au films onto TiO2 NCAs. The Au film thickness was monitored by an automated quartz crystal. Thermal dewetting was carried out at 500 °C in air for 1 hour to form Au nanoparticles (NPs) over the TiO2 NCAs with heating rate of 5 °C min-1. Characterization. Morphologies of the samples were investigated by a field-emission scanning electron microscope (FE-SEM, ZEISS ultra 55) and a high resolution transmission electron microscope (HR-TEM, FEI Tecnai F30). The chemical composition was characterized by X-ray photoelectron spectroscopy (XPS, Physical Electronics 5400 ESCA). X-ray diffraction (XRD) analysis was performed by an X’pert Powder (PANalytical, equipped with a Panalytical X’celerator detector using Cu Kα radiation, λ = 1.54056 Å). Raman spectra were recorded with a Renishaw InVia Microscope using a 785 nm excitation laser. Absorption spectra of the samples were obtained on a Cary Win UV-visible

spectrometer

in

a

wavelength

range

from

200-800

nm.

The

photoluminescence spectra were collected by a NanoLog Spec fluorescence spectrometer excited by a helium-cadmium lamp. Photoelectrocatalytic experiments. The photoelectrocatalytic oxygen reduction 18 ACS Paragon Plus Environment

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reactions were carried out in 0.1 M NaOH aqueous solutions in a three-electrode configuration. Au@TiO2 photoanodes were used as the working electrodes, an Ag/AgCl (3 M KCl) electrode and a Pt wire were set as reference and counter electrode, respectively. Photoanodes were attached to a rotating ring-disk electrode (RRDE: 5.61 mm in diameter) using conductive silver paint (Spi supplies, Inc, US). Prior to use, RRDE was polished by alumina powder suspension on felt polishing pads. Before testing, electrolyte was bubbled by N2 or O2 flow for 30 minutes to reach a saturated state. Both the cyclic and linear sweep voltammetry (CV, LSV) experiments were performed at a scan rate of 5 mV s-1. RRDE measurements were acquired at various rotation speeds (100-2500 rpm). A solar light simulator (AM 1.5, 300 W Xe, 100 mW cm-2) was employed as light source. The light intensity was calibrated prior to the experiments using a Si photodiode. Photocurrent variation versus potential was examined using an electrochemical workstation (CHI 760E, scan rate of 5 mV s-1). Incident photon to electron conversion efficiency was obtained using a Zahner CIMPS-QEIPCE system with monochromator and light source from 365-800 nm. The short-circuit current was measured in a 0.1 M NaOH solution with a working area of 1 cm2. Calculation is given by the equation (6): IPCE %=

1240×J(A m-2 ) λ(nm)×P(W m-2 )

×100

(6)

where J is the photocurrent density generated by monochromatic light with wavelength λ and P is monochromatic light flux. Intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) was 19 ACS Paragon Plus Environment


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performed using Zahner CIMPS-QEIPCE system in a 0.1 M NaOH solution with a Pt wire used as counter electrode. 1 mL of 31% H2O2 was added in 200 mL NaOH solution. A monochromatic light with a wavelength of 405 nm was illuminated from front side of the testing cell. Light flux intensity was set from 20-80 W m-2 with a 10% modulation, and frequency was swept from 10 kHz to 0.1 Hz. The measured potentials vs Ag/AgCl reference electrode was converted to the reversible hydrogen electrode (RHE) scale via the equation (7): ERHE =0.0592 pH+EoAg/AgCl +EAg/AgCl

(7)

where ERHE is the converted potential vs RHE, EAg/AgCl is the experimental potential measured against Ag/AgCl reference electrode, and EoAg/AgCl is the standard potential of 3 M KCl solution filled Ag/AgCl at 25 °C.

ASSOCIATED CONTENT Supporting Information

Detailed method and characterization for morphology, composition of the nanocavity arrays. Photo-response in visible light and photoelectrochemical ORR property of Au@TiO2 NCAs. Mechanistic study of electron transfer during ORR.

AUTHOR INFORMATION

Corresponding Author *E-mail [email protected] Author Contribution L.G. and K.L. contributed equally to this work. Author Contribution Y.Y., L.G., K.L., and K.M. designed the experiments, synthesized and characterized the 20 ACS Paragon Plus Environment

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nanocavity films. P.D.M. and K.R.C. deposited the Au coating. L.Z, C.S., N.O. and Y.H.S carried out the Raman spectra and TEM observation. Y.Y. and L.G. analyzed the data and wrote the manuscript. All authors discussed the results and commented on the manuscript. Notes All authors declare no competing financial interest.

ACKNOWLEDGMENT：：

This work was financially supported by the University of Central Florida through a startup grant (No. 20080741).

REFERENCE

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Cu, Co, Mn) Nanoparticles and Their Electrocatalysis for Oxygen Reduction Reaction. Nano lett. 2013, 13, 2947-2951. (6) Xu, J.; Gao, P.; Zhao, T. Non-Precious Co3O4 Nano-Rod Electrocatalyst for Oxygen Reduction Reaction in Anion-Exchange Membrane Fuel Cells. Energy Environ. Sci. 2012, 5, 5333-5339. (7) Wu, Z. S.; Chen, L.; Liu, J.; Parvez, K.; Liang, H.; Shu, J.; Sachdev, H.; Graf, R.; Feng, X.; Müllen, K. High-Performance Electrocatalysts for Oxygen Reduction Derived from Cobalt Porphyrin-Based Conjugated Mesoporous Polymers. Adv. Mater. 2014, 26, 1450-1455. (8) Zhang, J.; Jin, X.; Morales-Guzman, P. I.; Yu, X.; Liu, H.; Zhang, H.; Razzari, L.; Claverie, J. P. Engineering the Absorption and Field Enhancement Properties of Au-TiO2 Nanohybrids via Whispering Gallery Mode Resonances for Photocatalytic Water Splitting. ACS Nano 2016, 10, 4496-4503. (9) Liu, E.; Kang, L.; Wu, F.; Sun, T.; Hu, X.; Yang, Y.; Liu, H.; Fan, J. Plasmonic Ag Deposited TiO2 Nano-Sheet Film for Enhanced Photocatalytic Hydrogen Production by Water Splitting. Plasmonics 2013, 9, 61-70. (10) Pei, D.-N; Gong, L.; Zhang, A.-Y.; Zhang, X.; Chen, J.-J.; Mu, Y.; Yu, H.-Q. Defective Titanium Dioxide Single Crystals Exposed by High-Energy {001} Facets for Efficient Oxygen Reduction. Nat. Commun. 2015, 6. 8696. (11) Setvin, M.; Aschauer, U.; Hulva, J.; Simschitz, T.; Daniel, B.; Schmid, M.; Selloni, A.; Diebold, U. Following the Reduction of Oxygen on TiO2 Anatase (101) Step by Step. 22 ACS Paragon Plus Environment

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Enhanced Photoelectrocatalytic Reduction of Oxygen Using Au@TiO2

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