ARTICLE pubs.acs.org/JPCC
Enhanced Photocatalytic and Photoelectrochemical Activity in the Ternary Hybrid of CdS/TiO2/WO3 through the Cascadal Electron Transfer Hyoung-il Kim, Jungwon Kim, Wooyul Kim, and Wonyong Choi* School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea ABSTRACT: The composite of different semiconductor nanoparticles may facilitate the charge separation and transfer because the difference in the band edge positions creates the potential gradient at the composite interface. For this purpose, the CdSTiO2WO3 ternary hybrid was successfully synthesized and characterized for the structural, optical, and morphological properties by X-ray diffraction, diffuse reflectance UV/visible absorption spectroscopy, high-resolution transmission electron micrography, and energy-dispersive X-ray analysis. The photocatalytic activity was tested by monitoring the photoreduction of polyoxometalate (POM: PMo12O403) spectrophotometrically. The photoelectrochemical (PEC) property of the ternary hybrid electrode was also characterized by the linear sweep voltammetry, and the incident photon-to-current conversion efficiency was measured as a function of wavelength. The results of both the POM reduction and photocurrent tests indicated that the photocatalytic and PEC activities of the CdSTiO2WO3 ternary hybrid are much higher than those of bare CdS and any binary hybrids. The enhanced activity could be attributed to the cascadal electron transfer from CdS to TiO2 to WO3 through the interfacial potential gradient in the ternary hybrid conduction bands. Such a cascadal electron transfer in the hybrid structure facilitated the charge separation and retarded the charge pair recombination. As a result, the CdSTiO2WO3 showed the maximum photocurrent density of 1.6 mA/cm2 (at 0 VAg/ AgCl) under visible light irradiation (λ > 495 nm), which is about 5 times larger than that of bare CdS and about 23 times larger than that of binary composites. The enhanced electron transfer within the CdSTiO2WO3 composite was also confirmed by the electrochemical impedance spectroscopy.
’ INTRODUCTION Visible-light-active photocatalysts have been widely investigated for the degradation of various pollutants,13 water splitting,4,5 and photoelectrochemical (PEC) cells.6,7 Among various visible-light-active materials, cadmium sulfide (CdS) has received much attention because of the ideal band gap (2.25 eV)8 and band position for the photoinduced redox conversions under solar visible light. However, the photocatalytic activities of CdS are low because of its fast recombination of charge carriers and CdS suffers from photocorrosion.9,10 Thus, there have been various efforts to solve these problems by making a composite with conducting carbon materials,11,12 surface loading of noble metals,13 or coating with insulating material, such as silica.14 Recently, many studies have highlighted the way of coupling a narrow band-gap semiconductor with a wide band-gap semiconductor (with the proper band positions) to enhance the charge separation efficiency.1517 By coupling CdS with wider band-gap semiconductors (e.g., TiO2, ZnO, and WO3), enhanced activities in photocatalysis and PEC cells have been demonstrated.1821 This is because the composite of semiconductors with different band gaps and positions has a built-in potential gradient at the interface, which facilitates the separation of electron and hole pairs and reduces the chance of recombination. Many binary semiconductor composites have been studied for this purpose, and the overall activity can be even more r 2011 American Chemical Society
enhanced if a ternary hybrid system is employed. Although some studies investigated the ternary hybrid for solar cell applications22,23 and hydrogen generation,24,25 the photocatalytic and electron-transfer processes occurring in the ternary hybrid have not received much attention. Here, we report the CdSTiO2WO3 ternary hybrid as a new photoactive composite. To have a proper band structure for efficient charge separation and transfer, CdS, TiO2, and WO3 were chosen as components in the ternary hybrid. TiO2 has the conduction band (CB) edge (ECB = 0.1 VNHE) that lies below that of CdS (ECB = 0.4 VNHE) and above that of WO3 (ECB = þ0.4 VNHE).16 Therefore, the CB positions in the ternary hybrid take the form of a cascadal structure. When the electronhole pairs are generated in CdS, electrons can be separated from holes by migrating to TiO2 and then WO3 in a cascadal way along the potential gradient as Scheme 1 illustrates. To test the photocatalytic and PEC activity of the as-prepared ternary hybrid, the photocatalytic reduction of polyoxometalates and the PEC measurements were carried out using visible light irradiation (λ > 495 nm; 2.51 eV equivalent) under which only CdS can be
Received: December 27, 2010 Revised: March 20, 2011 Published: April 22, 2011 9797
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Scheme 1. Schematic Illustration of the Electron-Transfer Processes on the CdSTiO2WO3 Ternary Hybrid
excited. The characterization and the enhanced photochemical behaviors of the ternary hybrid are discussed in detail.
’ EXPERIMENTAL SECTION Preparation of Photocatalysts. The CdSTiO2WO3 ternary hybrid was synthesized following a two-step synthesis. First, the TiO2WO3 binary hybrid was synthesized by a solgel method with a tungstic acid (H2WO4) solution. Tungstic acid solution was prepared by a cation-exchange method.26,27 An aqueous solution of 0.3 M Na2WO4 (Aldrich) was passed through the column loaded with a proton-exchange resin (Dowex 50 WX2-200) and eluted by distilled water. The solution was then collected in a 0.3 M nitric acid/ethanol mixture (1/1 volume ratio). The concentration of the final tungstic acid solution was determined to be 0.1 M. The tungstic acid solution was added into the stoichiometric amount of TiO2 (Degussa P25 aqueous suspension) with varying the molar ratio (TiO2/WO3 = 1:1, 1:3, and 2:1). The resulting mixture was aged for 1 h at room temperature and then dried under reduced pressure. After drying, the resulting solid was washed, dried, and calcined at 450 °C for 4 h. Bare WO3 was synthesized by the same method without TiO2. CdS was precipitated by hydrolysis13 on the optimized TiO2 WO3 hybrid (1:1 molar ratio) with varying the molar ratio (CdS/ TiO2/WO3 = 4:1:1, 1:1:1, 0.5:1:1, 0.25:1:1, and 0:1:1). A solution of sodium sulfide (4 mM Na2S, Aldrich) was added dropwise to an aqueous suspension of the TiO2WO3 hybrid with cadmium acetate dissolved (4 mM Cd(CH3COO)2 3 2H2O, Aldrich). The molar ratio of sodium sulfide and cadmium acetate was adjusted to 1:1. The mixture solution was aged for 1 h with continuous stirring, filtered, washed with distilled water several times, and dried. Bare CdS and the binary hybrids (CdSTiO2 and CdSWO3) were synthesized by the same precipitation method in the presence of TiO2 or WO3. Characterization of Photocatalysts. The CdSTiO2WO3 ternary composite with a Cd/W/Ti atom ratio of 1:1:1 was found to be the most active photocatalyst among the ternary hybrids of various ratios. Accordingly, the CdSTiO2WO3 ternary hybrid (Cd/W/Ti = 1:1:1) was characterized in most cases. Phase identification of the synthesized photocatalysts was characterized by X-ray-powder diffraction (XRD) using Cu KR radiation (Mac Science Co. M18XHF). Diffuse reflectance UV/visible absorption spectra (DRUVS) were recorded using a spectrophotometer
(Shimadzu UV-2401PC) with an integrating sphere attachment, and BaSO4 was used as the reference. The high-resolution transmission electron micrographs (HRTEM), energy-dispersive X-ray (EDX) analysis, and selected area electron diffraction (SAED) of the CdSTiO2WO3 were obtained using a JEM-2100F microscope with Cs-corrected. Photocatalytic Activity Test. The visible light activities of the synthesized composite photocatalysts were tested for the photoreduction of phosphomolybdate (PMo12O403, Fluka; abbreviated as PMo123). In a control test, tungstosilicate (SiW12O404, Fluka; abbreviated as SiW124) was also used. They are polyoxometalates (POMs) that can be easily reduced by accepting an electron from the CB of photocatalysts (POM þ ecb f reduced POM). Upon the photoreduction, the POM changes to blue color with a broad absorption band appearance around 730 nm (reduced PMo123 with ε730 = 2300 M1 cm1; reduced SiW124 with ε730 = 2100 M1 cm1).2830 The reduced POM can be easily determined spectrophotometrically. The photocatalyst powder was suspended in an aqueous solution of methanol (MeOH, 10 vol %) or t-butyl alcohol (TBA, 10 vol %) under ultrasonication. The suspension concentrations of all hybrid photocatalysts were adjusted to contain the same amount of CdS equivalent (0.1 g/L) regardless of the varied composition among the different hybrids. Because CdS is the only light-absorbing component, this makes the total number of absorbed photons comparable among the tested hybrid photocatalytic systems. An aliquot of PMo123 stock solution was added to the suspension to give a desired initial POM concentration, and then the pH of the suspension was adjusted to 2.5 with HClO4 or NaOH standard solution. The solutions were equilibrated in the dark for 30 min prior to visible light illumination. Photoirradiation employed a 300 W Xe arc lamp (Oriel) combined with a 10 cm IR water filter and a cutoff filter (λ > 495 nm) as a light source, and then the filtered light was focused onto a 30 mL Pyrex reactor with a quartz window. The generation of the reduced POM could be visibly detected by the appearance of blue coloration of the irradiated solution. Sample aliquots were withdrawn from the reactor at a given time interval during the illumination and filtered through a 0.45 μm PTFE syringe filter (Millipore) to remove the photocatalyst particles prior to the spectrophotometric absorbance measurements. The reoxidation of the reduced POM by the ambient oxygen was slow enough not to interfere with the absorbance measurements. Multiple photolysis experiments were carried out under the 9798
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Figure 1. XRD patterns of the TiO2, WO3, CdS, the binary TiO2WO3 hybrid, and the ternary CdSTiO2WO3 hybrid (Cd/ Ti/W = 1:1:1).
identical condition to confirm the reproducibility. An incident light intensity was measured using an optical power meter (1815C, Newport) with a photodiode detector (818-UV, Newport) and determined to be about 180 mW/cm2 in the wavelength range of 420550 nm. Photoelectrochemical Measurements. PEC tests were carried out in a conventional three-electrode system in a stirred solution bubbled with nitrogen using a potentiostat (Gamry, Reference 600). The photocurrents were collected and measured via electron shuttles on an inert Pt electrode (with an applied potential of þ0.7 VAg/AgCl) immersed in the aqueous suspension of photocatalysts under visible light irradiation (λ > 495 nm) as described previously.31 FeCl3 was added as an electron shuttle (Fe3þ/Fe2þ) that transports the electron from the photocatalyst particles to the Pt electrode. A Pt plate (1 1 cm2), a graphite rod, and a Ag/AgCl electrode were used as a working, a counter and a reference electrode, respectively. The PEC tests were also performed using the electrodes on which the photocatalyst composites were deposited. To make sure to deposit the same quantity of CdS on each electrode, FTO glass (Pilkington, TEC8) was fabricated by the drop-casting method.32,33 An aliquot of each photocatalyst suspension containing 1.6 g/L CdS was deposited on the same electrode area (0.9 0.4 cm2). The deposited electrodes were dried for 12 h at room temperature and then were heated in a quartz tube at 450 °C for 30 min under a N2 atmosphere. Linear sweep voltammograms (LSVs) were collected under visible light irradiation (λ > 495 nm) at a scan rate of 10 mV/s. The incident photon-to-current conversion efficiencies (IPCEs) were measured at the wavelengths from 320 to 600 nm at the potential of 0 VAg/AgCl. The incident light from a 300 W xenon lamp was passed through a SAP301 grating monochromator (Newport, Oriel 77250). Electrochemical impedance spectra (EIS) were measured in potentiostatic mode with the ac voltage amplitude of 10 mV and the frequency range of 0.5500 Hz. LSV, IPCE, and EIS measurements were carried out in a three-electrode electrochemical cell, with a coiled Pt wire as a counter electrode and a Ag/ AgCl reference electrode. An aqueous solution of 50 mM Na2S
Figure 2. UVvisible absorption spectra of various hybrid photocatalysts (having a molar ratio of Cd/Ti/W = 1:1:1 for binary or ternary) along with the transmittance of the cutoff filter (λ > 495 nm) used for irradiation. Note that only the CdS-containing hybrid can absorb the light under the filtered irradiation. The inset shows the optical band gap (Eg) of CdSTiO2WO3 calculated from Tauc’s formula. The spectra are shown in two panels to avoid complexity.
was used as an electrolyte. The solution was magnetically stirred with nitrogen bubbling.
’ RESULTS AND DISCUSSION Characterization of the CdSTiO2WO3 Ternary Hybrid. Crystalline phases of CdSTiO2WO3 were analyzed by powder XRD using CuKR radiation and are shown in Figure 1 along with those of TiO2WO3, CdS, WO3, and TiO2. The WO3TiO2 hybrid was prepared by the solgel synthesis on the base of TiO2 nanoparticles (Degussa P25, a mixed phase of anatase and rutile). Diffraction peaks of WO3 in the WO3TiO2 hybrid correspond to the monoclinic phase of WO3 (JCPDS 431035). The XRD pattern of the as-obtained CdS by hydrolysis precipitation is identical to the cubic phase (JCPDS 10-0454) with broad and weak peaks at 2θ = 28, 44, and 52°. It is known that CdS synthesized by precipitation methods without thermal treatment exhibits a low crystalline cubic phase.34 The XRD patterns of the CdSTiO2WO3 ternary hybrid indicated the presence of cubic CdS and monoclinic WO3 along with anatase and rutile TiO2 (intrinsic to P25). The DRUVS spectra of CdSTiO2WO3, TiO2WO3, and TiO2 are compared in Figure 2. Bare TiO2 can absorb the wavelengths shorter than 390 nm due to its large band gap (3.2 eV). 9799
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Figure 3. TEM images and energy-dispersive X-ray images of the CdSTiO2WO3 ternary composite: (a) a low-resolution and (b) a high-resolution bright-field TEM image and the elemental mapping of (c) Cd, (d) W, and (e) Ti on the CdSTiO2WO3 ternary hybrid (Cd/Ti/W = 1:1:1).
The absorption edge was red shifted from 390 to 440 nm after hybridizing with WO3, whereas the ternary CdSTiO2WO3 showed a further red shift due to the incorporation of CdS with a narrower band gap. The absorption edge of CdSTiO2WO3 was located at 530 nm, and the inset in Figure 2 shows that the optical band gap (Eg) of CdSTiO2WO3 is estimated to be 2.41 eV (calculated by Tauc’s formula35). This value is larger than the literature value of the bulk CdS band gap (2.25 eV),8 which indicates that the size of CdS on the ternary hybrid is within the region of quantum confinement. The mean size of CdS nanoparticles can be estimated by eq 1.36,37 The mean diameter of CdS was calculated to be ca. 5.5 nm, which is in close agreement with HRTEM analysis. ! ! π 2 p2 1 1 1:8e2 ð1Þ þ ΔEg ¼ 2R 2 4πε0 εR me mh where ΔEg indicates a shift compared to the bulk Eg; R represents the radius of the CdS particle; m*e (=0.19 me) and m*h (=0.80 me) are the effective masses of the electron and hole in CdS, respectively; ε0 is the vacuum permittivity; and ε is the relative permittivity of CdS (5.7).37,38 Figure 3a shows the low-resolution TEM image of the asprepared CdSTiO2WO3 ternary hybrid. Figure 3ce displays the elemental EDX images of Cd, W, and Ti spots within the ternary hybrid in the local region that Figure 3b shows. TEM and EDX images clearly reveal that CdS, WO3, and TiO2 have the well-mixed heterojunctions in the ternary hybrid. To get more detailed information of the crystalline structure of the ternary hybrid, HRTEM and the selected area electron diffraction (SAED) were carried out. In Figure 4a, the lattice spacing of
CdS was 0.336 nm, which corresponds to the (111) plane of cubic CdS, and that of WO3 and TiO2 was 0.386 and 0.354 nm, which matched with the (002) plane of monoclinic WO3 and the (101) plane of anatase TiO2, respectively. Figure 4bd exhibits SAED patterns of the CdS, WO3, and TiO2 regions from Figure 4a. The ring pattern shown in Figure 4b corresponds to the polycrystalline cubic CdS, and the clear spots of Figure 4c,d represent the monoclinic WO3 and anatase TiO2, respectively, which are in good agreement with the result of HRTEM. Photocatalytic Activity. To assess the photocatalytic activity of CdSTiO2WO3, we used the photoreduction of POM (PMo123) as a test reaction. POM is a metaloxygen cluster anion that has been widely used as an electron acceptor and carrier because it can be readily reduced and reoxidized reversibly without undergoing the structural decomposition.39,40 Phosphomolybdate (PMo123) with the reduction potential of E0 = 0.65 VNHE can be easily reduced by the CB electrons of CdS, TiO2, and WO3 (eqs 2 and 3). S:C: ðsemiconductorÞ þ hν f S:C: ðecb þ hvb þ Þ ecb þ PMo12 3 f PMo12 4 ðblue coloredÞ
ð2Þ ð3Þ
The time profiles of the photoreduction of PMo123 were measured under visible light irradiation with various hybrid composites. Under the irradiation of visible light, the solution containing POM gradually turned blue and showed the appearance of a broad absorption peak centered at 730 nm (see the inset of Figure 6). The absorbance at 730 nm is proportional to the concentration of the reduced POM. To find the optimal composition of the CdSTiO2WO3 hybrid for the photocatalytic 9800
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Figure 4. HRTEM analysis of the CdSTiO2WO3 ternary composite. (a) A high-resolution TEM image of the CdSTiO2WO3 composite showing the arrangement of CdS and WO3 around a TiO2 crystallite. SAED images of the (b) CdS region, (c) WO3 region, and (d) TiO2 region and (e) a low-magnification TEM image of the CdSTiO2WO3 ternary composite.
Figure 5. Time profiles of POM reduction (monitored by the absorbance at 730 nm) in the presence of the TiO2WO3 composite (with varied ratios) under the irradiation of λ > 420 nm. Other experimental conditions were [catalyst] = 0.5 g/L, [PMo123] = 1 mM, [methanol]0 = 10% (v/v), pHi = 2.5, and air-equilibrated.
activity, the TiO2WO3 hybrids with different ratios were first tested for POM reduction under the irradiation of λ > 420 nm, which can excite WO3 only (not TiO2). Figure 5 compares the POM reduction profiles obtained with TiO2WO3 hybrids of three different molar ratios (2:1, 1:1, and 1:3), among which the 1:1 hybrid shows the highest activity. Therefore, the TiO2WO3 ratio of 1:1 was employed in the preparation of CdS TiO2WO3 composites. With the increase of CdS content on TiO2WO3 (1:1), the rate of POM reduction increased and then decreased, as Figure 6 shows. The optimal ternary composition was determined to be 1:1:1. Thus, all experiments of the ternary hybrid of CdSTiO2WO3 were carried out using the hybrid composite of a 1:1:1 molar ratio. Among various photocatalyst composites, it is clearly shown in Figure 7a that the CdSTiO2WO3 exhibits the highest photocatalytic activity compared with bare CdS and any of the binary hybrids. The photocatalytic activity increased in the order of TiO2WO3 < CdS < CdSWO3 ≈ CdSTiO2
495 nm, which can excite CdS only. Experimental conditions were [CdS] = 0.1 g/L (only for the CdS case), [catalyst] = 0.5 g/L, [PMo123] = 1 mM, [methanol]0 = 10% (v/v), pHi = 2.5, and air-equilibrated. The inset shows the UVvisible absorption spectrum of the reduced PMo123 solution after 16 min of irradiation.
CdSTiO2WO3. This indicates that photogenerated electrons in CdS efficiently transfer to WO3 through the CB of TiO2 because of the cascadal positioning of the CBs of three semiconductors. This composite band structure facilitates the separation of electron and hole pairs and reduces the chance of the recombination. TiO2 WO3 showed no photocatalytic activity under the irradiation of λ > 495 nm because the TiO2WO3 hybrid cannot absorb the wavelengths above 450 nm, as shown in Figure 2. In the photocatalytic systems where both electrons and holes should transfer concurrently to maintain the electroneutrality condition, the activities of photoreduction are coupled with those of photooxidation. The photoreduction of POM on CdS TiO2WO3 should be accompanied by the simultaneous photooxidation of electron donors (methanol in this study). Therefore, the higher activity of POM photoreduction with the CdS TiO2WO3 hybrid implies the higher activity of photooxidation. However, it should be noted that the photooxidation power of 9801
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In the above POM reduction experiments, methanol was used as an electron donor. However, the photooxidation of methanol generates a highly reducing radical intermediate (•CH2OH, E0 = 0.97 VNHE),41 which may reduce POM directly. To eliminate such an interfering effect of methanol, t-butyl alcohol (TBA) that cannot generate the reducing radical was also used as an alternative electron donor in the POM reduction experiment, and the results are shown in Figure 7b. The general trend remains the same as that of Figure 7a. This confirms that the photocatalytic reduction of POM is indeed induced by the CB electron transfer, not by the reducing radical species generated from the electron donor. Incidentally, the low crystallinity of CdS in the ternary hybrid may lower the overall photoactivity. The photocatalytic activity of CdS is highly dependent on the crystallization level and is generally higher with the higher crystallinity.42 To check the crystalline effect on the photocatalytic activity, the ternary hybrid that was heat-treated at 450 °C was also compared in Figure 7b. The photocatalytic activity of the calcined hybrid was indeed higher than the uncalcined one. The photocatalytic activity of the semiconductor hybrids was also tested with using an alternative POM, tungstosilicate (SiW124) with the reduction potential of E0 = 0.054 VNHE.29 Because the reduction potential is more negative than the CB edge of WO3 (E0 = þ0.4 VNHE), its reduction on WO3 should not be favored. In such a case, the reduction of SiW124 should occur on CdS, and therefore, the cascadal electron transfer from CdS to WO3 in the ternary hybrid does not contribute to the enhanced photoreduction of SiW124. Figure 7c compares the photocatalytic reduction of SiW124 among various semiconductor hybrids. As expected, the photoreduction activities of the binary and ternary hybrids (measured with SiW124) were little different from that of bare CdS, unlike the case of PMo123 (Figure 7a,b). This clearly confirms that the enhanced photocatalytic activity of the ternary hybrid (measured with PMo123) is a result of the cascadal electron transfer from CdS to WO3. Incidentally, the data of Figure 7c also imply that the activities of CdS in different samples are similar, although the size, dispersion, and surface area of the CdS component can be different among bare CdS and binary and ternary hybrids. Photoelectrochemical Characteristics. To demonstrate the enhanced electron transfer in the CdSTiO2WO3 ternary hybrid system, some PEC tests were carried out. Figure 8a shows the photocurrent collected in the aqueous catalyst suspension using Fe3þ/Fe2þ (as an electron shuttle in reactions 4 and 5) as a function of irradiation time under visible light (λ > 495 nm). (PMo123)
Figure 7. Time profiles of POM reduction in the presence of (a) methanol and (b) TBA and (c) time profiles of POM (SiW124) reduction in the presence of methanol with CdS, binary hybrids, or the ternary hybrid under the irradiation of λ > 495 nm, which can excite CdS only. Experimental conditions were [CdS] = 0.1 g/L (only for the CdS case), [catalyst] = 0.5 g/L, [PMo123] = 1 mM, [SiW124] = 1 mM, [methanol]0 = 10% (v/v), [TBA]0 = 10% (v/v), pHi = 2.5, and airequilibrated (N2-purged in the case of (c) SiW124). HT_CdS TiO2WO3 in (b) denotes that the sample was “heat-treated at 450 °C”.
the ternary hybrid is largely limited by the less-positive VB position of CdS. On the other hand, the ternary hybrid was not very stable during the irradiation and its photocatalytic activity was gradually reduced with repeated uses, which is a common problem of CdS-based photocatalytic systems.
ecb þ Fe3þ f Fe2þ
ð4Þ
Fe2þ f Fe3þ þ Ptðe Þ
ð5Þ
The photocurrent time profiles are similarly compared with the POM reduction time profiles shown in Figure 7a,b. The photocurrent generation efficiency increased in the order of TiO2 WO3 < CdS < binary hybrid < ternary hybrid, which is also consistent with the order observed in POM reduction. Such similarities between the photocatalytic reduction of POM and the photocurrent collection in the suspension clearly indicate that the interfacial electron transfers are surely enhanced in the composite structure with the cascadal band positioning. To confirm the potential advantages of the CdSTiO2WO3 ternary hybrid as a photoanode, five types of photoanodes with CdSTiO2WO3, CdSTiO2, CdSWO3, TiO2WO3, or 9802
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Figure 9. IPCE values as a function of wavelength for the electrodes of CdS, CdSTiO2, CdSWO3, TiO2WO3, and CdSTiO2WO3 (at the applied voltage of 0 VAg/AgCl). The electrolyte solution contained [Na2S] = 50 mM and was continuously N2-purged.
Figure 8. (a) Time-dependent profiles of Fe3þ-mediated photocurrent collected on a Pt electrode in the visible-light-irradiated suspension of CdS, CdSTiO2, CdSWO3, TiO2WO3, and CdSTiO2WO3. The experimental conditions were [CdS in the composite] = 0.2 g/L, [WO3/ TiO2] = 1 g/L, [Fe3þ] = 0.1 mM, [LiClO4] = 0.1 M, pHi = 1.8, Pt electrode held at þ0.7 VAg/AgCl, continuously N2-purged, and λ > 495 nm irradiation. (b) Linear sweep voltammograms were performed at a scan rate of 10 mV/s at λ > 495 nm; [Na2S] = 50 mM.
CdS were fabricated on FTO glass for comparison. The linear sweep voltammograms (LSVs) for the five photoanodes are compared in Figure 8b. These measurements were carried out under visible light (λ > 495 nm) irradiation under which only CdS is excited. Whereas the TiO2WO3 photoanode showed negligible photocurrent generation during LSV measurement, other photoanodes containing CdS exhibited significant photocurrent generation under visible irradiation. This indicates that all photocurrents are attributed to electrons generated from CdS. Among them, the maximum photocurrent density of 1.6 mA/cm2 was obtained with CdSTiO2 WO3 at 0 VAg/AgCl, which is about 5 times larger than that of bare CdS and about 3 and 2 times larger than that of binary hybrids (CdSTiO2, CdSWO3), respectively. This supports that the efficient charge transfer occurs from CdS to WO3 via TiO2. Figure 9 shows the incident photon-to-current conversion efficiency (IPCE, eq 6) for five photoanodes as a function of the incident light wavelength. The TiO2WO3 photoanode showed the IPCE value up to only 450 nm, which corresponds to the band-gap energy (2.8 eV) of WO3. IPCEð%Þ ¼
1240 IðAÞ 100% λðnmÞ Pi ðWÞ
ð6Þ
On the other hand, the photoresponse range of CdS-containinig hybrids is extended up to 530 nm due to the smaller band gap of CdS, and the maximum IPCE was obtained with the CdS TiO2WO3 photoanode, which exhibited over 50% of IPCE at the incident wavelengths from 360 to 490 nm at a potential of 0 V Ag/AgCl. IPCE values of other photoanodes increase in the order of TiO2WO3 < CdS < CdSTiO2 < CdSWO3, which is consistent with the LSV data shown in Figure 8b. Binary systems (CdSTiO2, CdSWO3) show enhanced activity than bare CdS in both LSV and IPCE because of the efficient charge separation, as demonstrated in previous studies.18,20,21 The better performance of CdSWO3 than CdSTiO2 is mainly caused by the larger built-in potential at the interface between CdS and WO3 because WO3 has a more positive CB position than TiO2 (see Scheme 1). Incorporating TiO2 on CdSWO3 further enhanced the LSV and IPCE. This indicates that the presence of TiO2 in CdSTiO2WO3 facilitates the charge separation, in agreement with the photocatalytic (Figure 7) and the PEC (Figure 8) data. WO3, which has a more positive CB position and higher conductivity43,44 than TiO2,45 rapidly withdraws electrons from the CB of TiO2 as soon as they are injected into TiO2 from CdS. Once electrons are transferred to WO3, the TiO2 CB serves as a barrier that prevents the electrons in WO3 from recombining with the holes in CdS. A similar phenomenon was also observed in the case of dye-sensitized solar cells45,46 that employed a SnO2 electrode that has a more positive CB position and a higher conductivity than TiO2. The dye-sensitization efficiency of the dye/TiO2/SnO2 system was markedly higher than that of the dye/SnO2 system because the TiO2 layer between the dye and SnO2 served as a barrier layer that retarded the recombination.46 To better understand the enhanced electron-transfer property in the CdSTiO2WO3 photoanode, we also carried out the electrochemical impedance spectroscopy (EIS). Figure 10 shows the typical EIS Nyquist plots of four electrodes. The size of the arc for the CdSTiO2WO3 photoanode is smallest among all the CdS-containing hybrid electrodes. The smaller size of the arc in an EIS Nyquist plot indicates the smaller resistance at the interface and the smaller charge-transfer resistance on the electrode surface.47,48 This also confirms that the cascadal positioning of CBs in the CdSTiO2WO3 hybrid induced 9803
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’ ACKNOWLEDGMENT This work was supported by the KOSEF NRL program funded by the Korea government (MEST) (No. R0A-2008000-20068-0), the KOSEF EPB center (Grant No. R11-2008052-02002), and the Korea Center for Artificial Photosynthesis (KCAP: Sogang Univ.) funded by MEST through NRF (2009C1AAA001-2009-0093879). ’ REFERENCES
Figure 10. Electrochemical impedance spectroscopic Nyquist plots were obtained with various hybrid photoelectrodes at the applied voltage of 0 VAg/AgCl. The electrolyte solution contained [Na2S] = 50 mM, was continuously N2-purged, and was irradiated under λ > 495 nm.
better charge separation and efficient electron transfer within the hybrid structure compared with those of binary composites and bare CdS.
’ CONCLUSIONS The nanostructure and composition of the semiconductor hybrids can be carefully designed for higher solar conversion efficiency. In this work, the ternary hybrid of CdSTiO2WO3 was successfully synthesized as a model hybrid system and tested for its photocatalytic and PEC activities. The optimal ratio of the ternary hybrid was found to be a 1:1:1 molar ratio of CdS TiO2WO3. The ternary hybrid showed significantly enhanced activities in both the POM reduction and PEC tests compared with bare CdS and binary hybrids. All experimental evidence obtained in this work indicates that the cascadal CB positioning of CdSTiO2WO3 generates a built-in potential gradient. This induces better charge separation and retards the recombination of charge pairs, thereby facilitating the electron transfer within the hybrid structure. Thanks to the advantage, the CdS TiO2WO3 hybrid electrode showed the maximum photocurrent density of 1.6 mA/cm2 at 0 VAg/AgCl in LSV, which is much higher than that of bare CdS and binary hybrid electrodes. The measurements of IPCE and EIS also confirmed that the CdSTiO2WO3 ternary hybrid has a higher photoconversion efficiency and a lower charge-transfer resistance compared with the binary hybrids and bare CdS electrodes. Various kinds of semiconductors are being investigated as light-harvesting material, and one of the promising strategies for maximizing the solar conversion efficiency appears to be the design of multicomponent semiconductor hybrids. The present study demonstrates that this goal can be achieved by hybridizing semiconductor nanoparticles with different band gaps and band positions. The present ternary system should serve as a model for the development of semiconductor hybrids with multiple components. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: þ82-54-279-8299.
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