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May 13, 2013 - Dual Cocatalysts Loaded Type I CdS/ZnS Core/Shell Nanocrystals as Effective and Stable Photocatalysts for H2 Evolution. Lei Huang, Xiul...
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Dual Cocatalysts Loaded Type I CdS/ZnS Core/Shell Nanocrystals as Effective and Stable Photocatalysts for H2 Evolution Lei Huang, Xiuli Wang, Jinhui Yang, Gang Liu, Jingfeng Han, and Can Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, P.O. Box 110, Dalian 116023, China S Supporting Information *

ABSTRACT: The photocatalytic activity and stability of nanocrystals (NCs) are two important issues for their application in photocatalysis. In this work, we report a combined method to promote the photocatalytic activity as well as the stability of CdS NCs for photocatalytic H2 evolution. ZnS shell was grown on CdS NCs forming type I core/shell CdS/ZnS nanocrystals (NCs) to restrain the photocorrosion and passivate the trap states on CdS NCs. Thereafter, dual cocatalysts were loaded to promote the separation and transfer of electrons and holes from the core to outer surface for photocatalytic reactions. It is found that Pt and Ni are effective reduction cocatalysts (RCs) while PdS and PbS are effective oxidation cocatalysts (OCs) for promoting the charge separation and transfer. This strategy was also proved to be effective for other type I core/shell NCs, such as CdTe/ CdS/ZnS NCs.

1. INTRODUCTION Photocatalytic production of H2 from water splitting is a potential way to convert solar energy into chemical energy and has been attracting extensive attention recently.1−7 Unfortunately, the quantum efficiency (QE) for overall water splitting is still far below the requirement of commercialization. A key reason is that the photogenerated electrons/holes are easily consumed via recombination. Besides, stability is another important issue that restricts the application of photocatslysts. Therefore, it is absolutely necessary to design and prepare photocatalysts with high quantum efficiency and long lifetime. Semiconductor-based nanocrystals (NCs) have recently proven to be a kind of effective material for photocatalytic and photovoltaic applications due to their unique properties, such as high extinction coefficient of light absorption, tunable bandgap, and carrier multiplication effect, etc.8−15 However, the QEs of photoelectron conversion are easily reduced by both the radiative and nonradiative recombination due to the existence of abundant defects on the nanocrystal surface. On the other hand, the mostly reported NCs or NCs composites, such as chalcogenide compounds, are not stable under light irradiation mainly due to photocorrosion. Therefore, the key issues for the application of NCs in photocatalysis as well as in photovoltaic cells are to improve the efficiency of photoelectron conversion and the stability during photocatalytic reactions. The growth of protective shell on the core of NCs has demonstrated to be an effective solution to eliminate the surface defects by reducing the number of dangling bonds on surface and improve the photostability by physically separating the core surface from its surrounding medium.16−23 According to the differences in band alignments of the core and shell semiconductor materials, the core/shell heterojunction NCs © XXXX American Chemical Society

can be classified as three types, namely type I, reverse type I, and type II. Among them, the type I core/shell NCs are formed by overgrowing a wider bandgap semiconductor on the core NCs with relatively narrow bandgap. The conduction band (CB) and valence band (VB) of the shell lie above and below those of the core, respectively. This combination can effectively passivate the nonirradiation related surface states of core NCs, resulting in very high fluorescence QEs.24−26 Interestingly, a recent research reported that the type I CdSe/CdS NCs showed higher activity for H2 evolution than CdSe core due to the passivation of surface-deep trap via the outer CdS shell.27 Meanwhile, the shell semiconductor with relatively wider bandgap can effectively protect the core from photocorrosion, especially in the case that the shell is not excited by the incident light. However, due to the type I heterojunction nature, the photogenerated charge carriers are confined in the core and not available for photocatalytic reactions on the outer surface. On the other hand, the shell as a physical barrier may hinder the transfer of photogenerated electrons/holes from core to the outer surface for photocatalytic reactions. Therefore, the effective separation of the electrons and holes and transfer of the electron/hole pairs from the core to the outer surface are still challenging issues for the application of type I core/shell NCs as photocatalytic materials. Our previous work has demonstrated that the loading of cocatalysts is an effective strategy for the separation and transfer of photogenerated carriers and thereby promote the photocatalytic activity for semiconductor based photocatalysts.28−30 Received: January 1, 2013 Revised: May 11, 2013

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a 100 mL three-neck round-bottomed flask under magnetic stirring. The mixture was degassed and recharged with Ar for 3 times at 120 °C and then was heated to 280 °C until the solution became clear. The solution was finally cooled to 70 °C and protected by Ar for further use. The growth of ZnS is as follows. Typically, the concentration of the purified CdS NCs (1.2 × 10−7 mol) was estimated based on the previous report for the growth of the ZnS shell.35 5 mL of the CdS NCs solution and 10 mL of oleylamine/OA (1:8 in volume) was added to a 100 mL flask. The mixture was degassed and recharged Ar for 3 times at 120 °C and Zn precursor solutions (0.48 mL) was injected. After that, the temperature was heated to 220 °C; S precursor solution (0.48 mL) was then injected for growing the first monolayer of ZnS. Zn and S precursors were then alternatively added for growing different thickness of ZnS shells. For the second, third, and fourth monolayer, the precursors were 0.83 mL (third) and 1.03 mL (fourth). The interval of 10 min was kept between each injection. Aliquots (about 0.5 mL) were taken in every step to monitor the reaction. After the injections, the solution was heated to 260 °C and kept for 10 min for further crystallization. Finally, the solution was cooled to room temperature and washed by hexane/methanol (1:1 in volume) extraction solvent for several times. The solution was then precipitated by acetone and centrifuged at 5000 rpm for 3 min, and the obtained precipitation was redispersed in chloroform for further use. 2.4. Phase Transfer. The ligand exchange process refers to the reference elsewhere.34,36 First, MPA (0.5 mL) was added to 50 mL of methanol, and then TMAH was added to adjust the pH of the solution to about 11.0. This solution was mixed with the purified CdS NCs (5 mL) or CdS/ZnS core/shell NCs (5 mL) in chloroform and stirred vigorously in the dark for 8 h at room temperature. The above solution was precipitated by acetone, followed by centrifugation. The obtained precipitant was then further washed with acetone and finally dispersed in aqueous solution. 2.5. Preparation of CdTe/CdS/ZnS Nanocrystals. The CdTe/CdS/ZnS NCs were prepared according to the reported protocol.37 First, a CdTe precursor solution was synthesized by adding freshly prepared NaHTe solution to an Ar-saturated CdCl2 solution (pH 8.4) with MPA as the surfactant. The concentrations of Cd, MPA, and Te precursor were 1.25, 3.0, and 0.625 mM, respectively. The CdTe precursor solution (50 mL) was then injected into a 100 mL Teflon vessel and irradiated in microwave for 1 min at 100 °C. The obtained CdTe NCs solution was then precipitated with acetone and collected via centrifugation. The colloidal precipitate was redissolved in ultrapure water (3 mL) and kept at 4 °C for further use. The CdTe/CdS precursor solution was prepared by adding the as-prepared CdTe NCs to an Ar-saturated solution containing 1.25 mM CdCl2, 1.0 mM Na2S, and 6.0 mM MPA (pH 8.4). The CdTe/CdS precursor solution (50 mL) was injected into a vitreous vessel. CdTe/CdS core−shell NCs were obtained after 5 min microwave irradiation of the precursor solution at 100 °C. The obtained CdTe/CdS NCs solution then was precipitated with acetone and collected via centrifugation. The colloidal precipitate was redissolved in ultrapure water (3 mL) and kept at 4 °C for further use. The CdTe/CdS/ZnS precursor solution was prepared by adding the as-prepared CdTe/CdS NCs to an Ar-saturated solution containing 1.25 mM ZnCl2, 1.0 mM Na2S, and 6.0

Meanwhile, the timely transfer of photogenerated carriers, especially the holes, can also restrain the photocorrosion of the photocatalysts, since most of the photocorrosion are driven by the photogenerated carriers.28,29,31 However, this idea was seldom reported to apply for the core/shell NCs in photocatalytic H2 evolution. In this work, we combined the advantages of type I core/ shell NCs and cocatalysts to enhance both the efficiency and stability of NCs in photocatalytic H2 evolution. CdS NCs was used as a model material. ZnS was first grown as shell on the CdS NCs cores to passivate the nonirradiative surface states and meanwhile to improve the stability. Thereafter, dual cocatalysts were loaded to help the separation and transfer of photogenerated electrons/holes in the formed type I core/shell CdS/ZnS NCs. We found that this combination showed very good stability and high activity for photocataltyic H2 evolution. Steady-state and time-resolved photoluminescence (PL) spectroscopy further demonstrated that the cocatalysts played crucial roles in the separation and transfer of photogenerated charge carriers.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Cadmium oxide (CdO, ∼1 μm, 99.5%, Aldrich), 1-octadecene (ODE, tech., 90%, Aldrich), oleylamine (tech., 70%, Aladdin), sulfur (99.9%, Guanghua Chemistry Factory Co. Ltd. in Guangzhou), tetramethylammonium hydroxide pentahydrate (TMAH, 99%, Aldrich), 3-mercaptopropionic acid (MPA, ≥99%, Aldrich), oleic acid (OA, tech., 90%, Sinopharm Chemical Reagent Co., Ltd.). ZnO (99%), chloroform (99%), hexane (97%), methanol (99.5%), ethanol (95%), acetone (99.5%), Na2S, PdCl2, H2PtCl6, Ni(NO3)2, Pb(NO3)2, and AgNO3 were obtained from Sinopharm Chemical Reagent Co., Ltd. All these reagents were used without any further purification. All reactions were carried out using standard air-free techniques unless stated otherwise. 2.2. Preparation of CdS NCs. The CdS cores were prepared according to the procedure reported by Yu et al.32 Typically, a mixture (40.0 g in total) of CdO (0.128 g, 1.00 mmol), oleic acid (0.30−21.2 mmol), and ODE was first degassed and recharged with Ar for 3 times at 120 °C and then was heated to 280 °C. A solution of sulfur (0.016 g, 0.5 mmol) in ODE (5 mL) was swiftly injected into this hot solution, and the reaction mixture was allowed to cool to 250 °C for the growth of CdS nanocrystals (NCs) for different duration. After the reaction, the mixture was washed by hexane/methanol (1:1 in volume) extraction solvent for several times and was precipitated by acetone. Finally, the solution was centrifuged at 5000 rpm for 3 min, and the obtained precipitation was redispersed in chloroform for further use. The particle sizes could be adjusted by the growth duration and OA concentration. In this work, 3.0 and 4.0 nm CdS NCs were synthesized with OA concentrations at 0.1 and 0.25 mmol/kg, respectively. The 3.0 nm cores were used in this work without specific notes. 2.3. Preparation of CdS/ZnS NCs. The CdS/ZnS core/ shell NCs were prepared according to the typical ionic layer adsorption and reaction (SILAR) method reported elsewhere.26,33,34 First, the S and Zn precursors were first prepared for shell growth. The sulfur precursor solution (0.1 mol/L) was prepared by dispersing sulfur powder (0.032 g, 1 mmol) in 20 mL of ODE in ultrasonic irradiation. The Zn precursor solution (0.1 mol/L) was prepared by adding ZnO (0.164 g) and oleic acid (5.022 g) in a 1:8 molar ratio with ODE (11.362 g) inside B

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(Figure 1a−e) and absorption spectra (Figure 2a) show that the size, shape, and size/shape distribution are controlled quite

mM MPA (pH 8.4). The CdTe/CdS/ZnS precursor solution (3 mL) was injected into a vitreous vessel and irradiated with microwave for 5 min at 60 °C. The obtained CdTe/CdS/ZnS NCs solution was then precipitated with acetone and collected via centrifugation. The colloidal precipitate was redissolved in ultrapure water (3 mL) and kept at 4 °C for further use. 2.6. Characterization. The morphologies of the CdS NCs were characterized by TEM (Tecnai G2 Spirit, FEI Co.) with the accelerating voltage of 120 kV and HRTEM (Tecnai G2 Spirit, FEI Co.) with the accelerating voltage of 300 kV. X-ray diffraction (XRD) analysis was carried out on an X-ray diffractometer (Rigaku; MiniFlex diffractometer) using Cu Kα as radiation source, and the applied current and voltage were 30 mA and 40 kV, respectively. The UV−vis absorption spectra were recorded on a Cary 5000 spectrometer (Varian Inc.). Steady-state photoluminescence (PL) spectra and timeresolved PL spectra were obtained on a FLS920 fluorescence spectrometer (Edinburgh Instruments) in air at room temperature. A 450 W Xe lamp and a picosecond pulsed diode laser (406.8 nm) with pulse width of 64.2 ps were used as the excitation source, respectively. ICP (ICPS-8100, Shimadzu) was conducted to determine the concentration of CdS for photocatalytic reaction and PL measurment. 2.7. In Situ Cocatalysts Loading for Photocatalytic Reaction and PL Measurement. The reduction and oxidation cocatalysts were in situ deposited as described elsewhere.28,29 H2PtCl6 and Ni(NO3)2 are the precursors for Pt and Ni (reduction cocatalysts) and were just added in the NCs solution before irradiation. The reduction potential of H2PtCl6 and Ni(NO3)2 lie around 0.4 and 0.1 V (NHE, pH = 7.0) which are below the conduction band of CdS NCs. The oxidation cocatalysts (PdS and PbS) were in situ loaded by adding PdCl2 and Pb(NO3)2 into the vigorously stirred NCs solution before characterization and photocatalytic reactions. Since the solubility constants of these semiconductors are extremely low (PdS: 2.0 × 10−58; PdS: 1.0 × 10−28), they are very easy to be absorbed on the NCs to form sulfide. 2.8. Photocatalytic Reaction. The photocatalytic reactions were carried out in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. Typically, 2.5 mL (depending on the CdS concentration determined by ICP) of purified CdS NCs was diluted by 47.5 mL of H2O to obtain a 50 mL of aqueous solution. Sacrificial reagent (Na2S, 0.1 M) and cocatalysts precursors were added (if needed) and followed by adjusting the pH value to around 12 by NaOH (1.0 M). The solution was then degassed for 30 min, followed by irradiation with a 300 W Xe lamp (ILC Technology; CERAMAX LX-300) equipped with a 380 nm filter. The reactant solution was stirred and maintained at room temperature by a flow of cooling water during the photocatalytic reaction. The amount of evolved H2 was determined with online gas chromatography (Tianmei; GC 7890, MS-5 A column, TCD, Ar carrier). The photocatalytic reactions of the NCs were run five times to test the stability. Na2S (100 μmol) was used as the sacrificial reagent and was properly supplied after each run according to the consumed Na2S amount.

Figure 1. TEM images of the CdS NCs and CdS/ZnS NCs with different numbers of ZnS monolayers. (a−e) are the NCs dispersed in chloroform, while (f) is the CdS/ZnS-4 ML NCs in H2O after phase transfer.

well. Both the CdS cores and CdS/ZnS core/shell NCs are crystallized and in hexagonal phase (Figure SI 1). The PL spectra (Figure 2b) indicate that the band edge emission is dominant while the S2− related deep trap emission (∼600 nm) is ignorable. The intensities of the band edge emissions obviously increase with the increase of ZnS thickness, attributed to the passivation of the defects for the nonirradiative process on the CdS surface. According to the reported results, the increase of both the particle size and the intensity of the band edge emission may indicate that most part of the CdS surface is well enclosed by the ZnS shell.26,38 However, it is hard to confirm the continuous coverage of all CdS particles by the ZnS shell based on the present results. The CdS/ZnS core/shell NCs with 4 monolayers of ZnS (CdS/ZnS-4 ML NCs) were chosen as the model catalyst for exploring the application of core/shell NCs in photocatalytic reactions because of their good performance in PL emission. Hereafter, CdS/ZnS NCs refer to CdS/ZnS-4 ML NCs in the text. Since the photocatalytic reactions are performed in aqueous solution, we transferred the CdS/ZnS core/shell NCs from chloroform to an aqueous solution through ligand exchange.34,36 3-Mercaptopropionic acid (MPA) was used as the surfactant in this process. No obvious change of particle sizes occurs during the ligand exchange process as indicated by the TEM images (Figure 1e,f) and absorption spectra (Figure 2a). However, the PL properties are dramatically changed during this process (Figure 2c). For CdS cores, the band edge emission is almost fully quenched. Although a deep trap emission at around 580 nm is observed (Figure 2c), the intensity is quite weak (Figure 2c). The weak emission intensities indicate that most of the photogenerated electron/ hole pairs are quenched through nonirradiative recombination in this case. For the CdS/ZnS core/shell NCs, the PL intensity is also decreased after ligand exchange. However, a part of the band edge emission intensity is still preserved and the deep trap emission is still ignorable (Figure 2b). The quench of PL intensities is mainly attributed to the modification of surface defects and coordination condition.39,40 Noticeably, the preserved band edge emission in CdS/ZnS NCs indicates that the corresponding photogenerated electrons or holes could

3. RESULTS AND DISCUSSION 3.1. Morphologies and Optical Properties of CdS NCs and CdS/ZnS Core/Shell NCs. The CdS cores were prepared according to the reported procedure.32 The CdS/ZnS core/ shell NCs were prepared using a successive ion layer adsorption and reaction (SILAR) method.26,32−34 The TEM images C

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Figure 3. Photocatalytic activities of the CdS (3.0 nm) NCs and CdS (3.0 nm)/ZnS NCs with and without cocatalysts. The amounts of CdS (3.0 nm) NCs and CdS (3.0 nm)/ZnS NCs are 2.4 and 1.5 mg, respectively. The loading amounts of Pt and PdS are 3.0 and 3.7 wt % (3.7 wt % when Pt was coloaded) for CdS (3.0 nm) NCs and 5.0 and 5.9 wt % (0.39 wt % when Pt was coloaded) for CdS (3.0 nm)/ZnS NCs, respectively. For all the photocatalytic reaction experiments, the amount of Na2S is 100 μmol through adding 1.0 mL of Na2S (0.1 M), and the light source is a 300 W Xe lamp equipped with a 380 nm filter.

According to the type I heterojunction nature, most of the photogenerated electrons and holes are confined in the core of CdS/ZnS NCs; thus, the CdS/ZnS NCs might not be active for photocatalytic reactions. However, the CdS/ZnS NCs show higher activities than that of CdS NCs here. This result can be explained as follows (Scheme 1). For the CdS NCs, some of Scheme 1. Schematic Description of the Relative Positions of the Conduction Band (CB) and Valence Band (VB) of CdS NCs and CdS/ZnS NCs and the Redox Potentials of Na2S

Figure 2. UV−vis absorption spectra (a) and PL spectra (b) of the CdS NCs and CdS/ZnS NCs with different numbers of ZnS monolayers in chloroform and H2O. (c) PL spectra of the CdS NCs and CdS/ZnS-4 ML NCs in aqueous solution.

photogenerated carriers are easily transferred to the abundant surface states on which the electrons or the holes with lower energy are not available for the photocatalytic reactions. While for the CdS/ZnS NCs, most of the surface states are passivated, making the electrons and holes energetically favorable for photocatalytic reactions. This is indicated by the band edge emission in Figure 2c mentioned above. Here, the confined electrons and holes with high energy in the core might tunnel through the ZnS shell to outside or leak from the uncovered CdS NCs surface to the out surface for reactions. The CdS/ ZnS NCs therefore exhibit higher activity for H2 production than CdS NCs. A similar phenomenon was also observed on the type I heterojunction of CdSe/CdS NCs.27 3.3. Influence of Cocatalysts on Photocatalyitc Behaviors of CdS NCs and CdS/ZnS NCs. Although the growth of ZnS shell on CdS NCs can effectively promote the photocatalytic H2 evolution through passivating the surface states, the activities are still quite low. As indicated in Figure 4a, the CdS/ZnS NCs possess very high PL intensity originated by the recombination of photogenerated electrons and holes in the

be maintained on the CB or VB, instead of transferring to the surface states with relatively lower energy. 3.2. Photocatalytic Behaviors of the CdS NCs and CdS/ZnS NCs without Cocatalysts. We first studied the photocatalytic behaviors of the CdS NCs and CdS/ZnS NCs without any cocatalysts. The CdS (3.0 nm) NCs show the capability of H2 evolution, but the activity is lower than that of CdS(3.0 nm)/ZnS NCs (Figure 3). The results indicate that the activity of H2 evolution is significantly promoted after the growth of ZnS shell on CdS NCs. It is noticed that the absorption band of CdS (3.0 nm) NCs is blue-shifted compared with that of CdS(3.0 nm)/ZnS NCs (Figure SI 2a). To exclude the influence of absorption ability on photocatalytic activity, we also checked the activities of CdS (4.0 nm) NCs and CdS(4.0 nm)/ZnS NCs with more similar absorption (Figure SI 2b). It is found out that the CdS(4.0 nm)/ZnS NCs also show higher activity than CdS (4.0 nm) NCs (Figure SI 3). This implies that the main reason for the enhancement of photocatalytic activity is due to the growth of ZnS shell instead of the difference in light absorbance. D

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Figure 5. Comparison of the PL decay curves for CdS/ZnS NCs loaded with different cocatalysts. A picosecond pulsed diode laser (406.8 nm) with pulse width of 64.2 ps was used as the excitation source.

Table 1. Summary of the Fluorescence Lifetimes Obtained from the Time-Resolved Fluorescence Spectra upon Loading Different Cocatalystsa

Figure 4. (a) Evolution of the PL spectra with successively adding different amount of Na2S and PdCl2 in the CdS/ZnS NCs solution. (b) PL spectra of Pt loaded CdS/ZnS NCs in the presence of Na2S. The excitation wavelength is 365 nm. The amount of CdS in CdS/ZnS NCs solution is about 2.1 × 10−7 mol, estimated from the volume (2.0 mL) and CdS concentration (determined by ICP) of the CdS/ZnS NCs solution for PL measurement.

with different cocatalysts

lifetimeb (ns)

with different cocatalysts

lifetimeb (ns)

none Pt PdS

70.6 33.3 3.9

Ni PbS

24.8 11.2

The fluorescence lifetime of the CdS/ZnS NCs is 32.8 ns in the presence of Na2S. bThe lifetimes are the average lifetimes calculated from the simulated lifetimes.

a

results were also obtained for CdS/ZnS NCs with larger CdS cores (Figure SI 3). Amazingly, the PL spectra show that the band edge emission could be fully quenched upon loading very small amount of PdS on CdS/ZnS NCs (Figure 4), indicating that the loaded PdS has even more influence on the PL property in comparison with Pt. The lifetime of the PL is also remarkably reduced upon loading PdS (Figure 5). Namely, the PL lifetime is 3.9 ns, which is much shorter than 33.3 ns for the Pt loaded case (Table 1). The PL decay traces in Figure 5 are in near-exponential shape upon loading PdS, consistent with a charge-transfer-induced quenching of emission.42 According to our previous work, PdS is very effective for the transfer of holes from CdS to PdS.28,29 It is then deduced that PdS is also helpful for the transfer of holes from core CdS of CdS/ZnS NCs to the outer surface. The CdS/ZnS NCs coloading of Pt and PdS exhibits the highest photocatalytic activities compared with that of individual cocatalyst (Figure 3 and Figure SI 4), mainly attributed to the simultaneous transfer of electrons and holes.28 Furthermore, coloaded with Pt and PdS, both the CdS(3.0 nm)/ZnS and CdS(4.0 nm)/ZnS NCs show better activities than that of the CdS (3.0 nm) and CdS (4.0 nm) NCs (Figure 3 and Figure SI 3), respectively. This result indicates that the growth of ZnS shell benefits for the enhancement of photocatalytic activity of CdS NCs, which is consistent with the above results (section 3.2) where cocatalysts are absent. More importantly, the Pt and PdS coloaded CdS/ZnS NCs are demonstrated to be very stable for H2 production using Na2S solution as sacrificial reagent (Figure 6a). The particle sizes hardly change during the five runs of reactions as indicated by the absorption spectra (Figure 6b). Differently, the particle sizes of CdS NCs increase obviously after the photocataltytic

core. Obviously, this part is not used for photocatalytic reactions. On the other hand, the addition of Na2S as the sacrificial reagent could quench the PL to some degree (Figure 4a); about 1/3 of the PL remained even though lots of Na2S is added. This indicates that the residual PL corresponded photogenerated electrons/holes are still not available for photocatalytic reactions. It is therefore necessary to effectively transfer the electrons and holes from the cores to the outer surface for photocatalytic reactions. In order to further improve the activity, we loaded Pt as the RC on CdS/ZnS NCs through an in situ photocatalytic reduction process. The activities are obviously enhanced upon loading Pt (Figure 3). This behavior is further confirmed by the CdS/ZnS NCs with larger CdS cores (Figure SI 3). Correspondingly, we find that the band edge emissions are remarkably quenched upon loading Pt in the presence of Na2S solution (Figure 4b), attributed to the transfer of electrons to Pt and the influence of Na2S. The time-resolved PL spectra further indicate that the PL lifetime is shortened from 70.6 to 33.3 ns (Figure 5 and Table 1). A recent work also reported a similar rapid exciton quenching resulted from the transfer of electrons from the core of type I core/shell CdZnS/ZnS to outer Pd nanoparticles.41 It is then concluded that the reduction of the PL lifetime with loading Pt corresponds to the charge transfer from core CdS to cocatalyst Pt on the outer surface of ZnS. We also deposited PdS as the OC through an in situ adsorption-reaction method to improve the activity.28 Interestingly, we find that the photocatalytic activities can also be significantly promoted upon loading PdS (Figure 3). Similar E

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Figure 7. Influence of cocatalysts loading on the photocatalytic activities (a) and PL properties (b) of CdTe/CdS/ZnS NCs.

core/shell NCs, significantly promoting the photocatalytic activity. On the basis of the above results, we summarize the roles of cocatalysts played in the separation and transfer of the carriers on type I core/shell NCs (Scheme 2). To favor the transfer of carriers thermodynamically, the Fermi energies of RCs should be more positive than the CB of the core NCs, while the VB or Fermi energies of OCs should be more negative than the VB of the core NCs. Therefore, the RCs help the transfer of electrons out from the cores and meanwhile to provide active sites for proton reduction. On the other hand, the OCs help the transfer of holes and also provide active sites for the oxidation reactions. Thus, the electrons and holes can be transferred out from the core NCs and spatially separated in different cocatalysts. In the transfer process, it should be noticed that since most part of the CdS cores are covered by ZnS shell on CdS/ZnS NCs, the photogenerated electrons and holes possibly transfer through the ZnS shell by tunneling effect which has been well studied by many groups.38,43 Besides, some part of the photogenerated charges may also leak directly to the loaded cocatalysts due to the incomplete cover of the CdS cores. In both cases, the loaded cocatalysts are very helpful to the separation and transfer of photogenerated carriers from the core to the outside. We suggest that the potential differences between the Fermi energies of RCs and CB of CdS NCs and the potential differences of VBs between OCs and CdS NCs are the main driving force. The exact mechanism needs further investigation.

Figure 6. (a) Stability of photocatalytic activities of the CdS/ZnS NCs. The amount of CdS/ZnS NCs is 1.5 mg. The loading amount of Pt and PdS are 5.0 and 0.39 wt % for CdS/ZnS NCs, respectively. UV− vis absorption spectra of the CdS NCs (b) and CdS/ZnS NCs (c) after different runs of photocatalytic reactions.

reactions as indicated by the red-shift of absorption band (Figure 6c). This result indicates that the presence of ZnS shell can significantly improve the stability of CdS NCs during photocatalytic reactions by avoiding the photocorrosion and the agglomeration of the CdS NCs. To further confirm the versatility of the strategy developed in this work, we also checked the influence of PdS and Pt on other core/shell NCs and other cocatalysts on CdS/ZnS NCs. Interestingly, we found PdS and Pt also work as effective cocatalysts for CdTe/CdS/ZnS NCs. The influence of cocatalysts on photocatalytic reactions and PL properties is quite similar to that of CdS/ZnS NCs (Figure 7). Moreover, we find that Ni and PbS are also effective RC and OC on CdS/ZnS NCs, respectively. Corresponding photocatalytic and PL results demonstrate their ability in transfer of electrons/holes for photocatalytic reaction (Figure 8 and Table 1). These results demonstrate the fact that the cocatalysts are very helpful to the transfer of the carriers from the core to the outer surface of

4. CONCLUSIONS In summary, a strategy was developed to enhance the efficiency and stability of NCs for photocatalytic H2 evolution. It is demonstrated that the growth of ZnS shell on CdS NCs can F

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Scheme 2. Schematic Description of the Transfer and Separation of the Photogenerated Electrons and Holes from Type I Core/Shell NCsa

a

The loaded reduction cocatalysts (RCs) and oxidation cocatalysts (OCs) not only provide the driving force for the transfer of photogenerated electrons and holes but also supply active sites for proton reduction and sacrificial reagent (electron donor) oxidation, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

More characterization and photocatalytic activity results. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel 86-411-84379070; Fax 86-41184694447 (C.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by 973 National Basic Research Program of the Ministry of Science and Technology (Grant 2009CB220010), National Natural Science Foundation of China (No. 21061140361, 21090340), and the Knowledge Innovative Program of The Chinese Academy of Sciences (KGCX2-EW-310-1).

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Figure 8. (a) Photocatalytic activities of the CdS/ZnS NCs upon loading different RCs and OCs. The amount of CdS/ZnS NCs is 1.5 mg. The loading amounts of Ni and PbS are 1.1 and 10.2 wt %, respectively. (b) The corresponding PL spectra of the CdS/ZnS NCs upon in situ loading PbS. (c) PL decay curves for CdS/ZnS NCs loaded with different cocatalysts. A picosecond pulsed diode laser (406.8 nm) with pulse width of 64.2 ps was used as the excitation source.

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improve the stability in photocatalytic reactions and is also beneficial for the photocatalytic H2 evolution due to the passivation of surface states. Moreover, the loading of reduction and oxidation cocatalysts can help the separation and transfer of electrons and holes in type I core/shell CdS/ZnS NCs for photocatalytic reactions, respectively, significantly promoting the photocatalytic activity. The remarkable quench of the band edge emissions and the reduction of the photoluminescence lifetimes demonstrate the effective function of cocatalysts on the transfer of photogenerated electrons and holes from inside (core) to outside driven by the cocatalysts. We believe that this design will advance the utilization of different core/shell nanomaterials in the application of photocatalysis and photovoltaics. G

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