High Efficiency Photoelectrocatalytic Methanol Oxidation on CdS

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High Efficiency Photoelectrocatalytic Methanol Oxidation on CdS Quantum Dots Sensitized Pt Electrode Chunyang Zhai, Mingshan Zhu, Fenzhi Pang, Duan Bin, Cheng Lu, M. Cynthia Goh, Ping Yang, and Yukou Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10234 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on February 19, 2016

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ACS Applied Materials & Interfaces

High Efficiency Photoelectrocatalytic Methanol Oxidation on CdS Quantum Dots Sensitized Pt Electrode Chunyang Zhai†, Mingshan Zhu†‡*, Fenzhi Pang§, Duan Binǁ, Cheng Lu‡*, M. Cynthia Goh‡, Ping Yangǁ, and Yukou Du†ǁ* †

School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China ‡

ǁ

Department of Chemistry, University of Toronto, Toronto M5S 3H6, Canada

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China

§

College of Pharmacy, Chemistry Teaching & Research, Suzhou Health College, Suzhou 215009, China

ABSTRACT: A cadmium sulfide quantum dots sensitized Pt (Pt–CdS) composite was synthesized using a solvothermal method and characterized by transmission electron microscopy (TEM), X–ray diffraction (XRD), X–ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectroscopy. The catalytic properties of the as–prepared electrode for methanol oxidation were evaluated by cyclic voltammetry (CV), chronoamperometry, electrochemical impedance spectrum (EIS) and photocurrent responses. The as–prepared Pt–CdS electrode displayed a significant enhancement in the electrocatalytic activity and stability for methanol oxidation in the presence of visible light irradiation. The synergistic effect of both the electro– and photo– catalytic reaction contributes to this enhanced catalytic performance. Our result suggests a new paradigm to 1

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construct photoelectrocatalysts with high performance and good stability for direct methanol fuel cells with the assistance of visible–light illumination. KEYWORDS: direct methanol fuel cells; quantum dots; cadmium sulfide; visible light; photoelectrocatalysts INTRODUCTION Nowadays, the growing energy demand couple with severe environmental issues show the need to develop sustainable clean energy sources. Direct methanol fuel cells (DMFCs) are recognized as one of the most promising future power sources owing to their high energy density, rapid recharging rates, low operation temperatures and environmental benignity.1–5 At present, the barrier for the production of efficient DMFCs is to develop highly efficient and cost–effective electrocatalysts for fuel anodic oxidation. Pt–based electrocatalysts have demonstrated the excellent catalytic efficiency; however, the pure Pt electrocatalysts suffer from poor durability due to catalyst poisoning, which hampers the commercialization of DMFCs. 1–7 To maximize the utilization of Pt and to obtain both high catalytic performance and low cost, Pt nanoparticles are often dispersed on a selected support.5–7 Semiconductor materials such as TiO2, WO3, and ZnO have been certified as promising candidates for Pt–based electrocatalysts supports because of their excellent chemical and thermal stability, strong support/metal interactions and co–catalytic activity.5–22 Meanwhile, upon excitation by light with energy higher than the band gap, semiconductor nanostructures can generate electron–hole pairs which may further participate in redox reactions.8–25 Therefore, some researchers have utilized the semiconductor as a support in fuel cells to boost their electrocatalytic performance towards small organic molecules oxidation.8–25 2


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In such a photo–assisted fuel cell system, the small organic molecules are oxidized at the surface of a noble metal/semiconductor anode through synergistic electrocatalytic and photocatalytic routes under UV light illumination. Because positively charged holes are highly oxidative, photo–illumination on the noble metal/semiconductor electrodes can enhance the electrocatalytic performance for methanol oxidation,10–23 and improve the stability of fuel cells by the photocatalytic self–cleaning ability.23–25 Despite the great efforts devoted to studying UV light assisted fuel cells, there are only a few reports exploring the photo–assisted fuel cell system with visible–light illumination. It is known that UV light accounts for no more than 5% of the total solar energy, which is a small amount compared to 45% of visible light.10,26 Significant efforts are being geared towards developing more efficient visible light driven photoelectrocatalysts. Cadmium sulfide (CdS) is a promising photocatalytic materials for the conversion of solar energy into chemical energy under visible–light irradiation.27–29 Nowadays, various CdS–based nanoarchitectures, such as CdS quantum dots (QDs), have been used in visible–light–driven solar energy conversion areas, including optical devices, solar cells, water splitting, photodetectors, pollutant degradation, etc.27–34 The above pioneering work inspired us to explore the potential application of CdS QDs decorated Pt as efficient visible–light–driven photoelectrocatalysts for methanol oxidation. Herein, we report a novel and simple method by using dimethyl sulfoxide (DMSO) as both a solvent and a source of sulfur to synthesize CdS QDs by the solvothermal method. Furthermore, for the first time, the CdS nanostructure was then used to support Pt nanoclusters to obtain a Pt–CdS composite electrode for visible–light assisted methanol oxidation in an alkaline medium. Irradiated by visible light, the as–prepared Pt–CdS composites displayed 2.4

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times higher electrocatalytic activity for methanol oxidation than without light irradiation. We believe the synergy between the electrocatalytic and photocatalytic effect contributed to the enhancement of the catalytic performance and the stability of Pt–CdS photoelectrocatalysts.

The

outstanding

catalytic

performance

suggests

that

visible–light–driven semiconductors hybridized with a noble metal could act as a promising electrocatalyst for the application in fuel cells. EXPERIMENTAL SECTION Materials. Cadmium acetate dihydrate (C4H6CdO4⋅2H2O,), dimethyl sulfoxide (DMSO, C2H6OS) and H2PtCl6⋅6H2O, CH3OH and KOH were of analytical reagent (AR) grade and purchased from Sinopharm Chemical Reagent Co., Ltd. without further purification before use. Double deionized water was used throughout the experiments. Synthesis of CdS QDs nanostructures. The CdS nanoparticles were synthesized by a solvothermal method. Typically, 0.2665 g C4H6CdO4⋅2H2O (1 mmol) was dispersed in 60 mL DMSO under ultrasonication for 0.5 h. Then, the solution was transferred into a 100 mL Teflon autoclave and held at 180 °C for 12 h. The DMSO was acted as both the S source and the solvent in here. After cooling to room temperature naturally, the precipitates were collected by centrifugation at 6000 rpm and washed with water and ethanol three times each. After that, the powder was dried in vacuum oven at 35 °C for 12 h, resulting in yellow CdS nanoparticles. Synthesis of Pt–CdS nanostructures and Pt–CdS modified electrode. In a typical experiment, 200 mg of the as–prepared CdS nanoparticles were added to a mixture containing 20 mL ethanol and 57 mL water. The pH of the solution was adjusted to 10 by using 0.1 M NaOH solution. 2.92 mL H2PtCl6 (3.8×10–2 M) was dispersed in above

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solution under ultrasonication for 1 h to give a homogeneous solution. Then the solution was transferred into a 100 mL Teflon autoclave and held at 140 °C for 4 h. After cooling to room temperature naturally, the precipitates were collected by centrifugation at 6000 rpm and washed with water and ethanol three times, respectively, followed by drying in vacuum oven at 35 °C overnight to obtain Pt–CdS composite with Pt weight ratio 10%. The Pt–CdS modified electrode was obtained by coating the as–synthesized Pt–CdS nanocomposite on F–doped tin oxide (FTO, 7 mm × 15 mm in size). Typically, 20 mg of the as–prepared Pt–CdS samples were dispersed in 1 mL water–ethanol mixture (Vwater:Vethanol = 1:1) and 10 µL Nafion (DuPond, USA) under ultrasonication for 30 min. Then, 33 µL mixture was deposited onto the surface of FTO (keeping the electrode geometric area to 0.56 cm2) and dried at room temperature, resulting in a Pt–CdS modified FTO electrode. Photoelectrochemical measurements and electrocatalytic or photo–electrocatalytic oxidation of methanol. The photoelectrochemical measurements were performed in quartz beaker by using an electrochemical workstation (CHI 760E) in a standard three–electrode configuration with Pt wire, saturated calomel electrode (SCE), and the modified FTO electrode as the counter, reference and working electrodes, respectively. To estimate the electrochemically active surface areas (ECSAs) of Pt–CdS electrode, cyclic voltammetry (CV) measurements were performed in H2SO4 (0.5 M) solution at a scan rate of 50 mV s−1. The mixture of 1.0 M CH3OH + 1.0 M KOH solution was used to evaluate electrocatalytic methanol oxidation in the photoelectrochemical measurements. CV measurements of the working electrodes were monitored in a 1.0 M CH3OH + 1.0 M KOH solution in the range of 0.151 to 1.251 V (vs. RHE). The chronoamperometry (CA)

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and photocurrent responses of the working electrode with or without visible light irradiation in 1.0 M CH3OH + 1.0 M KOH solution were recorded at a potential of 0.6 V (vs. RHE) at a scan rate of 50 mV s–1. For the pulsed potential cycle measurements, 30 s scan alternatively at 0.3 and 0.9 V (vs. RHE) were employed on the working electrodes with or without visible light irradiation in 1.0 M CH3OH + 1.0 M KOH solution. Electrochemical impedance spectroscopy (EIS) measurements were performed by using a 2.5 mM K3[Fe(CN)6/K4[Fe(CN)6] (1:1) mixture as a redox probe in a 0.1 M KCl aqueous solution. The EIS spectra were recorded under an AC perturbation signal of 5.0 mV over the frequency range from 100 kHz to 0.1 Hz at a potential of 1.301 V (vs. RHE). The working electrode was irradiated by using a Xe arc lamp (150 W) during the photoelectrochemical experiment. Characterization. Scanning electron microscope images were obtained on a Hitachi S–4700

SEM.

Transmission

and

scanning

transmission

electron

microscopy

(TEM/STEM) images were recorded on TecnaiG220 (FEI America) with an energy dispersive X–ray (EDX) attachment. X–ray diffraction (XRD) measurements were measured on a PANalytical X' Pert PRO MRD system with Cu Ka radiation (k =1.54056 Å) operated at 40 kV and 30 mA. UV–vis diffuse reflectance spectra were obtained on a spectrophotometer (UV–VIS–NIR Shimadzu UV3150, Japan). X–ray photoelectron spectroscopy (XPS) was measured on an ESCALab220i–XL electron spectrometer. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. RESULTS AND DISCUSSION

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Figure 1. (A) The SEM image of CdS nanoclusters. TEM and HRTEM images of (B and C) CdS and (D and E) Pt–CdS nanostructures. (F) The EDX spectrum of the Pt–CdS nanostructures. DMSO is a solvent often used in various organic syntheses. Recently, some researchers have discovered that DMSO can slowly release S−2 ions into solution for the synthesis of S–based inorganic materials at a controlled temperature.35–37 For example, Cao and coworkers showed that DMSO released H2S at 180 °C for the synthesis of CdS QDs.35 Wang et al. also found that at 150 °C DMSO was easily reduced to CH3SH that would further break to release H2S.36 Herein, the CdS QDs were first synthesized by a solvothermal method, where DMSO served both as the solvent and as the source of sulfur. The morphology of the CdS nanoparticles was analyzed by SEM and TEM, as shown in Figure 1A~1C. The size of obtained CdS nanoparticles was around 5 nm in diameter. The lattice fringes with d–spacing at ca. 0.33 nm in HRTEM image (Figure 1C) was assigned to the CdS (111) plane.31–33 When the Pt was deposited on CdS nanoparticles, an average size of 5.42 ± 0.7 nm for Pt nanoclusters was observed, as

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shown in Figure 1D. The corresponding Pt nanoparticles size distribution in the Pt–CdS displayed in Figure S1. Moreover, the d–spacing of 0.22 nm in Figure 1E, which agreed well with the lattice space of Pt (111) plane,30,31 confirmed the generation of Pt nanoclusters. The composition of the Pt–CdS composites was also analyzed by EDX, as shown in Figure 1F. The Pt, Cd, and S elements were observed in the as–prepared samples, suggesting the formation of Pt–CdS composite in our samples. The interface between metallic Pt nanoparticle and CdS nanoparticle was observed from the above HRTEM image (Figure 1E). The different observed d–spacing suggests the corresponding nanoparticles were assigned to CdS and Pt clusters, respectively. The yellow dots curve showed a clear interface between the Pt and CdS nanoparticles. We used STEM images and EDX elemental line scans to further reveal the interface formation in Pt–CdS nanocomposites, and the results were shown in Figure S2. The STEM images showed sphere–like nanoparticles dispersed on the surface of carbon film. From the EDX elemental line scans (Figure S2B), we first saw that all Pt, Cd, and S elements were detected in the scanned lines. More importantly, a different count trend for the Pt elemental line scan compared to that for Cd and S was observed, suggesting the presence of an interface between Pt and CdS regions. This reported experimental approach produced a highly dispersed Pt–CdS composite with a clear hybrid interface that will benefit the photogenerated charges transfer. This feature was not observed in the Pt–CdS nanocomposites fabricated by other methods such as the one–pot solvothermal method, where the Pt precursors, C4H6CdO4⋅2H2O and DMSO were used together during a solvothermal process (details seen in Figure S3).

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Figure 2. (A) XRD patterns of (a) CdS and (b) Pt–CdS nanostructures. XPS spectra of (B) S 2p and (C) Cd 3d of the (a) CdS and (b) Pt–CdS nanostructures. (D) XPS spectrum of the Pt 4f in the Pt–CdS nanocomposites. To further confirm the atomic constituents and the crystal structures of the as–prepared samples, the XRD patterns of the CdS QDs and Pt–CdS nanocomposites were recorded (Figure 2A). The peaks centered at 26.5°, 44.0°, and 51.9° corresponded to the diffraction of the (111), (220) and (311) planes of cubic CdS (JCPDS 80–0019), respectively.32,33 The diffraction peaks were broadened due to the small crystallite size of CdS nanoparticles in the samples. This small size due to the DMSO regulating the nucleation rate of CdS particles by slowly releasing S2– ions into the solution, resulting in a small and uniform crystallite size.32,35–37 Moreover, the peak centered at 40.1° in the Pt–CdS nanocomposite was assigned to the diffraction of the (111) plane of Pt (JCPDS

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NO. 04−0802).38,39 This assignment was also consistent with the above HRTEM observation, further suggesting the formation of CdS and metallic Pt in Pt–CdS species. Additionally, estimated from the Scherrer equation (D = 0.9λ/βcosθ),37 the average crystallite sizes of CdS (26.5°) and Pt (40.1°) clusters were 4.5 nm and 5.5 nm, respectively, which slight made sense within error range to the observations from the TEM images. XPS was applied to further reveal the chemical compositions of the CdS and Pt–CdS nanostructures, as shown in Figure 2B~2D. First, two characteristic peaks at ca. 161.5 eV and 162.6 eV (Figure 2B) in the bare CdS sample were attributed to the doublet of S 2p3/2 and S 2p1/2, respectively,34,39 indicating that the valance state of element S is –2.40 The characteristic peaks attributed to the binding energy for Cd 3d5/2 and Cd 3d3/2 were also observed at ca. 405.2 eV and 411.9 eV, respectively,34 as shown in Figure 2C. These results confirm the existing of CdS in the samples. A doublet peaks originating from the spin orbital splitting of the 4f7/2 and 4f5/2 states for Pt centred at 72.2 eV and 75.5 eV,38,39 futher reveals the generation of metallic Pt in the Pt–CdS nanocomposites. Interestingly, when the metallic Pt nanoparticles were deposited on the CdS nanostructures, the binding energy of Cd 3d and S 2p both shifted to slightly lower values by 0.2 eV compared to those of the bare CdS. This was attributed to the chemical environment change because of the introduction of Pt nanoparticles in the composite.41

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Figure 3. The UV–vis diffuse reflectance spectra of the samples of (a) CdS and (b) Pt–CdS nanocomposites. Inset: the photograph of as–prepared CdS and Pt–CdS samples. The UV–vis diffuse reflectance spectra were used to analyze the optical properties of the samples. Figure 3 showed the results from the reflectance measurements of CdS and Pt–CdS nanocomposites. The as–prepared CdS QDs displayed an orange yellow color, and the corresponding UV–vis diffuse reflectance spectrum (Figure 3, curve a) showed the absorption edge at ca. 550 nm, revealing the semiconductor CdS with band gap of 2.25eV.31–33 When the Pt nanoparticles were introduced, a small red–shift was observed in the absorption edge,31 accompanied with a visual color change to yellowish brown, likely because the presence of metallic Pt nanoparticles contributed to the enhanced absorption in the visible light region.31 This distinct absorption in the visible light range suggests that the Pt–CdS composite was more efficient at utilizing visible light, in the solar spectrum, a potential advantage to the photoelectrocatalytic efficiency for the catalytic oxidation methanol. To evaluate the electrochemical performance of the as–prepared samples, the electrochemically active surface areas (ECSAs) of the Pt–CdS electrode under both dark

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and visible light conditions were compared. Although the ECSA accounts for the electrocatalyst surface available for charge transfer, it also includes the access of conductive paths to transfer the electrons to and from the electrode surface.42 The ECSA of the as–prepared electrode were calculated by the hydrogen desorption area from the cyclic voltammagram (CV) using the following equation:43 ECSA =

QH 0.21× m

(1)

where m represents the mass of Pt (mg cm−2) loading in the electrode, QH represents the charge for hydrogen adsorption and desorption (mC cm−2) and 0.21 mC cm−2 is the Pt crystalline activity surface area transfer coefficient. Derived from Figure S4, the ECSAs for Pt–CdS electrode under visible light irradiation (0.947 cm2 mg−1) was around 5 times larger than that of in the dark (0.194 cm2 mg−1). The large ECSA indicated that the Pt–CdS electrode displayed superior charge–storage property under visible light irradiation compared to in the dark. The synergistic effect of the electro– and photo– catalytic processes results in an enhanced electron and ion transport for the electrode, generating a large ECSA upon visible irradiation. The similar observation was often discovered by other reports.44 The linear sweep voltammetric (LSV) behavior of Pt/FTO and Pt–CdS/FTO electrodes in a 1.0 M CH3OH + 1.0 M KOH solution was also investigated. As shown from Figure 4, the onset potentials on the Pt–CdS/FTO electrode with and without light irradiation are negatively shift compared to that on the bare Pt modified electrode. Moreover, at a given potential of 0.8 V, the current density of Pt–CdS/FTO electrode was 4.08 mA cm–2 with light irradiation, which is ca. 3.1 and ca. 8.3 times higher than that of Pt–CdS/FTO without light irradiation (1.33 mA cm–2) and of Pt/FTO electrode (0.49 mA 12


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cm–2), respectively. Both the negative shift onset potential and enhanced current density were owing to the presence of semiconductor supporter CdS, which not only increased the active Pt dispersion for high active surface area, but also worked as photocatalysts to produce the photogenerated charges and to improve the charges mobility at the surface of Pt–CdS/FTO electrode.

Figure 4. Linear sweep voltammetry performed of (a) Pt/FTO electrode, Pt–CdS/FTO electrode (b) without and (c) with visible–light irradiation in 1.0 M CH3OH + 1.0 M KOH solution at a scan rate of 50 mV s–1.

Figure 5. (A) The 150th CVs of (a) FTO, (b) CdS/FTO, (d) Pt/FTO and (e) Pt–CdS/FTO electrodes without light illumination; (c) CdS/FTO and (f) Pt–CdS/FTO electrode with 13



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visible–light illumination in a 1.0 M CH3OH + 1.0 M KOH solution at a scan rate of 50 mV s–1. (B) Photocurrent responses of Pt–CdS/FTO electrode under visible light irradiation in 1.0 M CH3OH + 1.0 M KOH solution recorded at a potential of 0.8 V. The irradiation from a Xe lamp was interrupted every 30 s. The photoelectrocatalytic activity of the as–prepared CdS QDs sensitized Pt electrode was evaluated by methanol electro–oxidation. Figure 5A presents the CV curves for the bare FTO and modified FTO electrodes in a 1.0 M CH3OH + 1.0 M KOH solution with and without visible light irradiation. A typical CV profile for electrocatalytic oxidation of methanol between 0.151 V and 1.251 V (vs. RHE) was featured by two strong oxidation peaks defined as the forward (ca. 0.80 V) and backward (ca. 0.70 V) scan peaks for the Pt/FTO and Pt–CdS/FTO electrodes. The bare FTO and CdS/FTO electrodes displayed negligible electrocatalytic activities toward methanol oxidation. The loading of Pt nanoparticles significantly improved the methanol oxidation efficiency. Figure 5A and Figure S5 (the current densities of Pt–based electrodes were normalized with respect to the noble metal mass) show the forward peak current density of methanol oxidation on Pt/FTO and Pt–CdS/FTO electrodes were improved to 1.5 mA cm–2 (13.4 mA mg–1) and 3.61 mA cm–2 (30.7 mA mg–1) at the 150th cycle, respectively. This enhanced electrocatalytic activity was due to the fact that CdS acted as a cooperative catalyst for improving electrocatalytic performance. In addition, the supports also can enhance the catalytic performance by increasing the active Pt dispersion on the support surface, thereby resulting in a large active surface area.15 The peak current density of the Pt–CdS/FTO electrode was further enhanced to 8.64 mA cm–2 (73.5 mA mg–1) at the 150th cycle under visible light illumination, which was ca. 5.7 and 2.4 times higher than that

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from the Pt/FTO and Pt–CdS/FTO electrodes without light irradiation, respectively. Moreover,

we

measured

the

responsive

photocurrent

to

evaluate

the

photoelectrocatalytic performance of the electrode. Figure 5B shows the recorded photocurrent–time (I–t) curve on the Pt–CdS electrode. There was a responsive photocurrents at intensity ca. 0.3 mA cm–2 when visible light was illuminated on the Pt–CdS electrode; and the photocurrent response was closed promptly once the light irradiation was turned off. This behavior was repeatable during the on/off cycles upon light irradiation. The above enhanced electrocatalytic performance and photocurrent responses suggested effective charge transfer in the as–synthesized Pt–CdS electrode upon light illumination. A K3[Fe(CN)6/K4[Fe(CN)6] (1:1) mixture was used as a redox probe to investigate the enhanced charge transfer efficiency during the photoassisted electrocatalytic reaction. Figure 6A illustrates the response of the reversible redox couple (Fe(CN)64−/Fe(CN)63−) on the Pt/CdS electrode under both dark and visible light irradiation. Compared to the Pt/CdS electrode under dark, the Pt/CdS electrode under visible light irradiation exhibited two distinct changes: first, the peaks of redox potential were shifted to lower potentials; second, the redox peak current was enhanced. These results indicate a significant enhancement of the interfacial electron transfer at the Pt–CdS electrode surface with the assistance of visible light irradiation.45,46

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Figure 6. (A) CVs and (B) Nyquist plots of the Pt–CdS/FTO electrode in a 2.5 mM K3[Fe(CN)6/K4[Fe(CN)6] and 0.1 M KCl solution at a potential of 1.301 V (a) without and (b) with visible–light irradiation. In addition, the EIS data were analyzed by using Nyquist plots to further verify the internal resistances and interfacial charge transfers of the samples in the range of 0.1 Hz–100 kHz under both dark and visible light condition. Figure 6B shows that the diameter of the semicircle arc of the Pt–CdS electrode under visible light irradiation was much smaller than that without light irradiation, suggesting an enhanced interfacial charger transfer in the photoilluminated electrode. This phenomenon further confirms that visible light illumination promoted the charges mobility at the surface of electrode, and as a result, increasd the electrocatalytic reaction rate.47 We also investigated the corresponding EIS of the Pt–CdS electrode with and without visible light irradiation in a 1.0 M CH3OH + 1.0 M KOH solution. Figure S6 shows that the results of EIS in the presence of methanol were similar to the above results, which further confirmed that an enhanced interfacial charger transfer in the photo irradiated electrode.

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Figure 7. (A) Chronoamperometric curves measured at 0.6 V (vs. RHE) of (a) FTO, (b) CdS/FTO, (c) Pt/FTO and (d) Pt–CdS/FTO electrodes without light illumination, and (e) Pt–CdS/FTO electrode with visible light illumination in 1.0 M CH3OH + 1.0 M KOH solution at a scan rate of 50 mV s–1. (B) The peak current of methanol oxidation in the forward scan vs the CV cycle number of the Pt–CdS/FTO electrode (a) without light irradiation (300th cycles) and with light illumination from 300th to 600th cycles; (b) with continuous visible light illumination. The catalytic stability is another important parameter for the application of electrocatalys in fuel cells. Chronoamperometric curve and scan cycling experiments were often used to determine the stability of an electrode. Figure 7A shows the current density change in 5000s for the bare FTO, CdS/FTO, Pt/FTO and Pt–CdS/FTO electrodes at 0.6 V (vs. RHE) with and without visible light irradiation. The bare FTO (curve a) and CdS/FTO (curve b) electrodes had no reactivity for methanol oxidation. The Pt modified FTO electrode exhibited nice electrocatalytic activity, which however gradually degraded due to the Pt catalyst poisoning.5–7 By introducing CdS as a support for Pt nanoparticles, the Pt–CdS/FTO electrode showed an enhanced and stable oxidation current density under the same condition. Moreover, the initial and steady−state of oxidation current density of Pt–CdS/FTO electrode was further improved under visible light illumination, 17



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revealing that the photo–assistance was beneficial for the electrocatalytic performance and stability. Figure 7B shows the peak current of methanol oxidation in the forward scan vs the cycle number of the CV scan on Pt–CdS electrode. We observed the following two facts in the first stage (the first 300th scan cycles) on the Pt–CdS electrode under both dark and visible–light illumination. First, visible light illumination increased the oxidation peak current on the Pt–CdS electrode compared with that in the dark. Second, the catalytic acitivity of the Pt–CdS electrode was more stable under light illumination, because by comparing with the corresponding maximum value, the oxidation peak current densities at the 300th scan cycle only dropped 5.3 % while it dropped ca. 29.8 % in the dark. In the second stage (300th ~ 600th cycle scan), we observed a gradual drop in the oxidation peak current even with light illumination. However, we also noticed a sharp increase in peak current density when the Pt–CdS electrode was switched from dark to light illumination (Figure 7B, curve a). These results clearly support that the Pt–CdS electrode under visible–light illumination has higher catalytic performance for the methanol oxidation.

Figure 8. CVs of the Pt–CdS/FTO electrode before (a) and after (b) 1500 pulsed potential cycles without (A) and with (B) visible–light illuminationin a 1.0 M CH3OH + 1.0 M KOH solution. 18


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In order to further verify the long–term stability of the Pt–CdS/FTO electrode, the stability accelerated tests on the electrode were employed by using pulsed potential cycles, 30 s alternatively at 0.3 and 0.9 V (vs. RHE). Stepping between two limiting potentials with a long dwell time is considered to be a more precise and severe stability test than conventional chronoamperometric measurement.48,49 The forward peak current density value represents the activity of the catalyst for methanol oxidation. Figure 8 shows the pulsed potential cycle treatment lowered the forward peak current, which was recorded at about 0.8 V in the CVs curves on all electrodes. However, visible light illumination made a significant improve to the scale of forward peak current by causing it to drop. Without visible light illumination (Figure 8A) , the forward peak current decreased 48.7% after 1500 cycles by comparing with the initial current density (1.33 mA cm–2). In contrast, upon visible light illumination (Figure 8B), there was only a 25.6% decrease after 1500 cycles compared to the initial current density (5.15 mA cm–2). This suggests that photo–assistance can promote the electrocatalytic performance and stability of the Pt–CdS catalyst.50

Scheme 1. Schematic illustration for the synergistic photo– and electro– catalytic oxidation methanol process by using the Pt–CdS modified electrode under visible−light

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illumination. In summary, upon visible light irradiation, CdS QDs sensitized Pt electrode significantly improved the electrocatalytic activity as well as the stability for methanol oxidation. This is owing to the synergistic effects of electro– and photo– catalytic processes during catalytic methanol oxidation, as shown in Scheme 1. It is known that methanol electro–oxidation on Pt–based electrodes is through parallel pathway mechanisms: the direct path way and the indirect pathway.51–55 The direct pathway proceeds via reactive intermediates such as formaldehyde or formate, while the indirect pathway proceeds via the formation of adsorbed carbon monoxide (COads). Both the pathways finally lead to CO2 during the electro–oxidation of methanol on Pt–based electrodes.54 The main steps were described in equation 2 to equation 7. When introducing CdS QDs in the electrode, there are two main factors to enhance the catalytic activity of methanol oxidation. First, CdS QDs enhance the catalytic performance by increasing the active Pt dispersion on the support surface, thereby resulting in a large active surface area. Second, CdS QDs absorb light energy under visible light illumination, resulting in electrons (eCB−) in the conduction band (CB) and holes (hVB+) in the valence band (VB) (equation 8).31,32 The holes can react with surface adsorbed OH−/H2O to form strong oxidative hydroxyl radicals (•OHs) on the surface of catalyst (equation 9).10,18–20 The •OHs then further oxidize the methanol adsorbed on the surface of the catalyst (equation 10), resulting in photoassisted oxidation of methanol at the anode.10,18–20 Usually, the intermediate carbonaceous species like COads will absorb on the surface of catalysts, which will poison the catalysts. The highly reactive free radicals (•OHs) might also oxidize the intermediate carbonaceous species, resulting in an efficiently poisoning

20


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suppression (equation 11).23–25 Simultaneously, the photoexcited electrons are transferred to the circuit by external an electric field, thus preventing the charges recombination which is the main reason for the low efficiency of a photocatalyst. Therein, the catalytic activity and stability toward methanol oxidation of QDs sensitized Pt–CdS electrode can be both improved efficiently with the assistance of visible light illumination. Pt + CH3OH + OH– → Pt–HCOads + 2H2O + e−

(2)

Pt–HCOads + 2OH– → Pt–HCOOads + H2O + 2e−

(3)

Pt–HCOOads → Pt + CO2 + H+ + e−

(4)

Pt–HCOads → Pt–COads + H+ + e−

(5)

Pt + OH– → Pt–OHads + e−

(6)

Pt–COads + Pt–OHads → CO2 + 2Pt + H+ + e−

(7)

CdS + hν → CdS + eCB− + hVB+

(8)

h+ + OH− → OH•

(9)

CH3OH + OH• + 5OH− → CO2 + 5H2O+ e−

(10)

Intermediates(COads) + OH• → CO2 + H+ + e−

(11)

CONCLUSIONS In this contribution, a high–performance visible–light–driven Pt–CdS nanocomposite was synthesized by the solvothermal method. The Pt–CdS composite showed improved charger–transfer efficiency upon visible light irradaition. Moreover, compared with the traditional ambient electrocatalytic oxidation, the developed QDs sensitized Pt–CdS electrode exhibited distinctly enhanced electrocatalytic performance and stability towards methanol oxidation upon visible light irradiation, due to the synergistic effect of both electro– and photo– catalytic processes during the catalytic oxidation of methanol. The present result provide a new insight into developing novel visible–light–driven

21



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photoelectrocatalysts for applications in fuel cells. ASSOCIATED CONTENT Supporting Information. STEM, EDX line scan and the Pt nanoparticles size distribution of Pt–CdS, the EDX, TEM and STEM images of Pt–CdS nanostructures by one–pot solvothermal method, the CVs of Pt–CdS electrode in H2SO4 (0.5 M) solution, the EIS of the Pt–CdS electrode in methanol solution. This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *E–mail: [email protected] (M.

Zhu); [email protected] (C.

Lu)

[email protected] (Y. Du). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes: The authors declare no competing financial interest. ACKNOWLEDGMENT This work was sponsored by K.C. Wang Magna Fund in Ningbo University and also supported by the NSFC (51073114, 51373111 and 20933007), Suzhou Nano–project (ZXG2012022), the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201207), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Natural Sciences and Engineering Research Council of Canada. 22


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Recent Fuel Cell Related Research. J. Electroanal. Chem. 1988, 257, 9–45.

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High Efficiency Photoelectrocatalytic Methanol Oxidation on CdS

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