Research Article pubs.acs.org/journal/ascecg
CdTe/CdS Core/Shell Quantum Dots Cocatalyzed by Sulfur Tolerant [Mo3S13]2− Nanoclusters for Efficient Visible-Light-Driven Hydrogen Evolution Dongting Yue, Xufang Qian, Zichen Zhang, Miao Kan, Meng Ren, and Yixin Zhao* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China S Supporting Information *
ABSTRACT: CdTe quantum dots (QDs) have an extended absorbance region compared to that of CdSe or CdS QDs for solar utilization; however, their low activities, especially the chemical stability, limit their applications in photocatalytic hydrogen evolution. We report on enhanced visible-light-driven hydrogen evolution based on CdTe QDs via forming CdTe/CdS core/shell and using sulfur tolerant catalysts of the [Mo3S13]2− nanocluster. The aqueous synthesized CdTe/CdS QDs exhibit much better photocorrosion resistance than regular CdTe QDs for photocatalytic hydrogen generation. The sulfur compound covered CdTe/CdS QDs are facilely decorated with the low cost sulfur tolerant [Mo3S13]2− nanoclusters to exhibit enhanced visible-light photocatalytic H2 generation than the CdTe QDs catalyzed with classical cocatalysts of Pt. In all, the combination of sulfur tolerant [Mo3S13]2− nanoclusters and CdTe/CdS core/shell structure significantly enhance the activity and stability of CdTe QDs for visible-light photocatalytic hydrogen evolution. KEYWORDS: Core/shell, Quantum dots, [Mo3S13]2− nanoclusters, Hydrogen evolution
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evolution.5,6 However, these noble metal cocatalysts are very sensitive to the sulfur compound while most reported stable shell material are based on a sulfur composition based semiconductor such as CdS or ZnS for CdSe or CdTe QDs. Non-noble-metal alternatives especially molecular catalysts or biocatalysts with active centers of Co, Fe, and Ni, which can be directly in contact with QDs, have been demonstrated to be the robust photocatalysts in situ for H2 evolution.6,21−23 Recently, molybdenum sulfide materials (MoSx) have emerged as a promising cocatalyst due to its high activity, earth-abundant composition, low cost, and especially sulfur resistance.24−27 Herein, we combined the stable CdTe/CdS core/shell QDs with active [Mo3S13]2− nanoclusters to facilely assemble the [Mo3S13]2−-CdTe/CdS photocatalyst for photocatalytic H2 evolution with enhanced efficiency and photostability. CdTe/ CdS core/shell QDs aqueous solution was formed by growing a controllable CdS shell on CdTe cores with a facile aqueous synthesis. With the help of a stable CdS shell on the CdTe cores, the CdTe/CdS core/shell QDs exhibited better charge carrier separation and photostability due to the passivated surface. With the loading of highly active [Mo 3 S 13 ] 2− nanoclusters to CdTe/CdS QDs, the resultant [Mo3S13]2−CdTe/CdS QDs exhibited an improved photostability and
INTRODUCTION With the looming energy crisis and environmental pollution, solar based renewable energy has been heavily studied. Photocatalytic water splitting for hydrogen generation is one of the potential approaches to convert solar energy into chemical energy.1−3 So far, numerous photocatalysts have been investigated. The semiconductor quantum dots (QDs) with tunable band gaps such as CdX (X = S, Se, Te) have become one of the promising candidates for efficient visible-light-driven H2.4−6 Unfortunately, the regular CdX (X = Se, Te), especially CdTe QDs, with a suitable absorbance region has two main problems: low photostability due to low photocorrosion resistance and poor photocatalytic activity related to inefficient charge separation during the photocatalysis process.7−9 It is necessary to improve both the efficiency of photoelectron conversion and the photostability in CdX QD photocatalytic hydrogen evolution reactions. Accordingly, most straightforward strategies to overcome these drawbacks are to construct the core/shell heterostructures to improve the photostability10−13 and load cocatalysts to facilitate charge separation.14−16 For example, CdTe/CdSe and CdSe/ZnS core/shell photocatalysts exhibited high activity for H2 evolution due to the effective separation of the photoexcited charge carriers in core/ shell particles, as well as the passivation of a surface-deep trap via the outer shell.17−20 Usually, noble metal cocatalysts such as platinum have been incorporated into QDs including CdTe QDs or CdSe QDs to form excellent photocatalysts for H2 © XXXX American Chemical Society
Received: July 2, 2016 Revised: August 29, 2016
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DOI: 10.1021/acssuschemeng.6b01520 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
and CdTe/CdS has been listed in Figure 1C. The PL lifetime of CdTe QDs increases from 11.2 to 36.3 ns after coating the CdS shell, which is attributed to the fast separation of carriers on core/shell QDs. These results reveal that the deposition of the CdS shell on the CdTe core is a benefit to broaden the absorbance and enhance the PL intensity by passivating the surface defects of CdTe QDs.30 The XRD diffraction pattern of the CdTe QDs presents the zinc blende planes of (111), (220), and (311) at 23.9°, 40.2°, and 46.8° (Figure 1D). The CdTe/ CdS core/shell shows a diffraction pattern with 2θ peaks at 24.5°, 41.1°, and 47.6°, corresponding to the (111), (220), and (311) reflections. By comparison, the diffraction peaks of CdTe/CdS gradually move progressively toward values corresponding to the CdS phase, which supports the formation of the CdTe/CdS core/shell structure.30 All of these results demonstrate that the CdS shell has been successfully coated on the CdTe core. In addition, the diffraction pattern of (NH4)2Mo3S13 is shown in Figure S1, and the cubic lattice orientation of CdTe/CdS is maintained with the addition of [Mo3S13]2− clusters, which demonstrates that the [Mo3S13]2− clusters are coupled with CdTe/CdS without any crystal regrowth (Figure 1D). Figure 2 shows the morphologies and crystal structure of the CdTe QDs, CdTe/CdS QDs, [Mo3S13]2− nanoclusters, and
higher photocatalytic H2 evolution comparable to those of CdTe QDs systems.
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EXPERIMENTAL SECTION
Synthesis of CdTe QDs. CdTe QDs stabilized by 3-MPA were synthesized by introducing freshly prepared NaHTe (formed upon the reaction between NaBH4 and tellurium powder) into Ar-saturated solution containing Cd 2+ and MPA at pH 11.0. 28−30 The concentration of Cd2+ was set as 0.02 M, and the feed ratios of Cd:Te:MPA were 1:1:2.23. The pH was adjusted by 1 M NaOH solution under vigorous stirring. The resulting reaction mixture was immediately refluxed under open-air conditions to generate CdTe QDs of desired sizes controlled by the refluxing time. The CdTe QDs solution with emission peak at 504 nm was kept at 4 °C for further use. Synthesis of CdTe/CdS Core/Shell QDs. A stepwise synthesis of CdTe/CdS core/shell QDs using GSH as both stabilizer and sulfur source is described as follows.29,30 The 5 mL portion of as-prepared CdTe QDs was added into 45 mL of an Ar-saturated solution containing 1.63 mM CdCl2 (0.015 g) and 3.9 mM GSH (0.060 g) at pH 9.8. Then, the QD solutions were heated and refluxed under openair conditions. The size of core/shell QDs was controlled at different times. The CdTe/CdS core/shell QD solution with emission peak at 540 nm was kept at 4 °C for further use. Cocatalyst Loading for Photocatalytic Reaction. Pt as the cocatalyst was deposited as described elsewhere.31 H2PtCl6 as the precursor was added to the QD solution before irradiation. Similarly, the thiomolybdate (NH4)2Mo3S13·nH2O clusters (n = 0−2) prepared previously27 were also added to the QDs solution and then sonicated for 10 min before irradiation.
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RESULTS AND DISCUSSION The absorption and PL spectra of CdTe and CdTe/CdS QDs are shown in Figure 1A,B. The CdTe QDs exhibit a typical excitonic peak at 504 nm with an emission peak at around 540 nm. After formation of the CdS shell, the CdTe/CdS QDs’ absorbance peak shifts to 544 nm, and the corresponding PL peak also shifts to around 580 nm. Along with these shifts, the PL intensity of CdTe/CdS QDs is about 5 times higher than that of the CdTe QD. Simultaneously, the PL lifetime of CdTe
Figure 2. TEM images and high resolution (HR) TEM images of (A, a) CdTe, (B, b) CdTe/CdS, (C, c) [Mo3S13]2− clusters, and (D, d) [Mo3S13]2−-CdTe/CdS.
[Mo3S13]2−-CdTe/CdS. Structural variations resulting from the progressive deposition of CdS shells are studied by TEM and HRTEM. The morphology of CdTe/CdS QDs (Figure 2B) does not change dramatically, compared with the CdTe QDs (Figure 2A). However, the HRTEM reveals that the interplanar spacing across the CdTe/CdS QDs particle lattice progressively reduces with the growth of the CdS shell (Figure 2a,b). The interplanar distance of (111) crystal lattice planes is 0.370 nm for CdTe, which is consistent with bulk zinc blende CdTe. The (111) interplanar spacing subsequently decreases to 0.342 nm,
Figure 1. (A) Absorption and (B) PL emission spectra of CdTe and CdTe/CdS QDs. (C) Emission decays of CdTe and CdTe/CdS QDs in water (excitation wavelength: 410 nm). (D) XRD patterns of asprepared CdTe (black), CdTe/CdS (red), and [Mo3S13]2−-CdTe/CdS (blue). The vertical solid lines shown below the pattern are standard bulk XRD data for CdTe (JCPDS No. 75-2086, black line) and CdS (JCPDS No. 80-0019, orange line). B
DOI: 10.1021/acssuschemeng.6b01520 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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the change in its coordination situation with sulfur or tellurium, supports the formation of a CdS shell on the surface of CdTe QDs.29 As shown in Figure 3b, the S 2p region of [Mo3S13]2− clusters is fitted with two distinct doublets (2p1/2, 2p3/2): (1) one doublet at (163.2 eV, 162.0 eV), which arises from the terminal S22− ligands, (2) one doublet at (164.5 eV, 163.3 eV), which reflects the bridging S22− ligands and the apical S22− ligand of the [Mo3S13]2− clusters.27,35 According to Figure 3c, the assembly of the [Mo3S13]2− clusters onto CdTe/CdS QDs results in the overlap of S 2p peaks.36 In addition, the binding energies of Cd 3d do not change with the addition of [Mo3S13]2− clusters (Figure 3d), which indicates that the adsorption of [Mo3S13]2− clusters on the QD surface would not affect the chemical state of Cd. It is difficult to observe the Mo signal in XPS due to the small amount of [Mo3S13]2−, while the ICP results provide evidence for the existence of Mo (1.046 mg/L) on [Mo3S13]2−-CdTe/CdS QDs as listed in Table S1. The above results demonstrate the successful coupling between CdTe/CdS QDs and [Mo3S13]2− nanoclusters. Figure 4a shows the pH effect on the H2 evolution under the same concentrations of [Mo3S13]2− clusters, CdTe/CdS QDs,
suggesting the formation of the CdS shell on the CdTe core surface is consistent with previous reports.30 The [Mo3S13]2− cluster particle size is less than 10 nm, and its interplanar spacing is 0.259 nm (Figure 2C,c). After loading the [Mo3S13]2− clusters with CdTe/CdS QDs, the resultant [Mo3S13]2−-CdTe/CdS has excellent dispersivity (Figure 2D). Simultaneously, the measured lattice spacings of 0.342 and 0.259 nm between the interface of the two crystals are consistent with the (111) crystal planes of CdTe/CdS QDs and the [Mo3S13]2− clusters, respectively (Figure 2d). The results demonstrate that [Mo3S13]2− clusters have been successfully deposited on the CdTe/CdS QDs surface. XPS characterizations are taken for analyzing surface properties of the CdTe QDs, CdTe/CdS QDs, [Mo3S13]2− clusters, and [Mo3S13]2−-CdTe/CdS QDs. The S 2p spectra of CdTe QDs and CdTe/CdS QDs are shown in Figure 3a. The S
Figure 3. High resolution XPS spectra and fitted peaks of samples: (a) sulfur 2p of CdTe and CdTe/CdS; (b) [Mo3S13]2− clusters; (c) [Mo3S13]2−-CdTe/CdS; (d) cadmium 3d of CdTe, CdTe/CdS, and [Mo3S13]2−-CdTe/CdS. The sulfur 2p region can be fitted with three district doublets (2p3/2, 2p1/2): one doublet (in blue) that reflects the terminal S22− ligands, one doublet (in green) that arises from the bridging S22− ligands and the apical S22− ligand of the [Mo3S13]2− clusters, and a doublet (in red) accounts for CdTe or CdTe/CdS.
Figure 4. Photocatalytic H2 evolution from systems containing CdTe/ CdS QDs (CdTe, 6.5 × 10−6 M) and [Mo3S13]2− clusters (4.6 × 10−6 M) under visible light (λ > 420 nm) at different conditions: (a) pH range 2−5 in the presence of 20 mg/mL H2A; (b) emission spectra at various pH values, containing CdTe/CdS QDs (CdTe, 6.5 × 10−6 M), H2A (20 mg/mL), and [Mo3S13]2− clusters (4.6 × 10−6 M); (c) different amount of H2A at pH 2.5; (d) concentration effect of [Mo3S13]2− clusters on H2 evolution in the presence of 20 mg/mL H2A at pH 2.5. Error bars represent the mean ± SD of three independent experiments.
2p spectrum of the CdTe QDs can be fitted as a single doublet with S 2p1/2 peaking at 164.17 eV and S 2p3/2 at 162.15 eV, respectively. The binding energies demonstrate the existence of free thiol on the surface of the CdTe QDs.32,33 During the deposition reaction, S2− liberated from the GSH accesses the particle surface and then combines with the Cd2+ ions. After growing a CdS shell on the surface of the CdTe core, the binding energy is shifted to 163.8 eV (S 2p1/2) and 162.0 eV (S 2p3/2), respectively.34 In addition, the doublet at 159.0 eV in both CdTe and CdTe/CdS is attributed to thiols from 3mercaptopropionic acid. The Cd 3d1/2 and Cd 3d5/2 peaks of CdTe QD locate at 411.8 and 405.1 eV with doublet peaks, respectively (Figure 3d).29 The doublet peak of Cd at 414.0 eV is due to the oxidized CdO, which reveals the relative low stability of CdTe QDs under ambient conditions. After being coated with CdS, the doublet peak related to CdO disappears, suggesting the increased stability of CdTe QDs. Additionally, the peaks of Cd 3d1/2 and Cd 3d5/2 are shifted to 411.6 and 404.8 eV, respectively, which can be indexed to the Cd 3d of a surface CdS shell. The shift of the Cd binding energy, caused by
and H2A. The optimized pH value for H2 production is observed at pH 2.5, while the amounts of H2 decrease sharply at lower or higher pH values.5 As shown in Figure 4b and Figure S2, the decrease of pH leads to a remarkable decrease in the emission intensity and lifetime, as well as aggregation of CdTe/CdS QDs. The stability of CdTe/CdS QDs and the interaction between CdTe/CdS QDs and [Mo3S13]2− clusters are dramatically affected by the pH value. At a too low pH value, ligands would dissociate from the surface of CdTe/CdS QDs, resulting in precipitation and defects that could capture the excited electrons on the surface of the CdTe/CdS species. A high pH value would decrease the interaction between the [Mo3S13]2− clusters and the binding sites of the CdTe/CdS QD surface including Cd-terminated faces and the surface defects such as cadmium adatoms and sulfur vacancies. The H2 C
DOI: 10.1021/acssuschemeng.6b01520 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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However, the activity of Pt could be easily poisoned by S2− on the surface of CdTe/CdS QDs, and thus results in a serious loss of photocatalytic activities.37 In contrast, low cost [Mo3S13]2− nanoclusters based on earth-abundant elements exhibit high sulfur resistance and high catalytic activities. The sulfur ligand covered CdTe/CdS QDs are facilely decorated with the low cost sulfur tolerant [Mo3S13]2− nanoclusters to exhibit enhanced visible-light photocatalytic H2 generation as compared to that of the CdTe/CdS QDs catalyzed with classical cocatalysts of Pt. The sulfur tolerant [Mo3S13]2− nanoclusters, as the cocatalyst, could efficiently separate photogenerated e− and h+ and function as an active site for H2 generation.35 The photocatalytic H2 evolutions of Pt-CdTe/ CdS and [Mo3S13]2−-CdTe/CdS are enhanced, compared with the optimal system of Pt-CdTe and [Mo3S13]2−-CdTe. More than 1176.5 μmol of H2 (TON, 12788 for [Mo3S13]2− clusters) is generated over 20 h under visible-light irradiation (Figure S5). The CdS shell on the surface of CdTe could effectively separate the photoexcited charge carriers, passivate the surface states of QDs, and then improve its stability for photocatalytic H2 evolution.
production at pH 2.5 would be greatly enhanced with an increase in the H2A concentration from 5 to 20 mg/mL. However, a further increased H2A concentration (40 mg/mL) results in decreased H2 production (Figure 4c). The addition of H2A would result in the pH decrease and a large generation of HA−. As mentioned before, at a lower pH, CdTe/CdS QDs would seriously aggregate. As the pH was maintained at 2.5, excessive H2A results in a large amount of HA−, which could inhibit the adsorption of [Mo3S13]2− clusters and then cause a deterioration of photocatalytic performances. As demonstrated in Figure 4d, the H2 production is very low without the [Mo3S13]2− nanoclusters. After [Mo3S13]2− nanoclusters coupled with the QDs, the H2 production is dramatically increased with [Mo3S13]2− nanocluster concentration from 0.92 × 10−6 to 4.60 × 10−6 M. [Mo3S13]2− nanoclusters on QDs possess plenty of active sites, which would act as the photogenerated electron capture center and promote the charge separation. A further increase of the [Mo3S13]2− nanoclusters from 4.60 × 10−6 to 9.20 × 10−6 M gives rise to a slight reduction of the amount of H2 production. As listed in Figure S3, the yellowish [Mo3S13]2− nanoclusters exhibit an absorbance edge up to 650 nm which could block some of the light once they covered CdTe/CdS QDs to limit photogenerated electrons for H2 production.37 The interaction of [Mo3S13]2− clusters and CdTe/CdS QDs was explored by the CdTe/CdS QDs’ PL variation with the addition of different molar ratio [Mo3S13]2− as shown in Figure S4. The PL intensity of CdTe/CdS QDs is gradually quenched with the increase of the concentration of [Mo3S13]2− nanoclusters, suggesting an effective electron transfer between CdTe/CdS QDs and [Mo3S13]2− clusters. Such effective electron transfer could be ascribed to the efficient adsorption of [Mo3S13]2− clusters via the QD’s two kinds of binding sites, such as Cd-terminated faces and the surface defects including cadmium adatoms and sulfur vacancies, as in refs 5, 6, and 36. According to the above results, a photocatalytic H2 evolution experiment is carried out under optimal conditions, with 20 mL of aqueous solution containing CdTe/CdS QDs (CdTe, 6.5 × 10−6 M), [Mo3S13]2− nanoclusters (4.6 × 10−6 M), and H2A (20 mg/mL) present at pH 2.5. Figure 5 shows the H2
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CONCLUSION In summary, a highly active nanocomposite photocatalyst of [Mo3S13]2−-CdTe/CdS with improved stability has been successfully developed by protecting CdTe QD cores with CdS shells and then facilely assembled with [Mo3S13]2− nanoclusters via facile aqueous chemistry. The [Mo3S13]2− nanocluster-CdTe/CdS photocatalysts containing CdTe/CdS QDs (CdTe, 6.5 × 10−6 M), with [Mo3S13]2− nanocluster concentration of 4.6 × 10−6 M, shows the highest photocatalytic activities for H2 generation (12788 TON for [Mo3S13]2− nanoclusters) in the presence of H2A (20 mg/ mL) at pH 2.5 under visible-light irradiation for 20 h. The CdS shell on the surface of the CdTe core not only helps to separate the photoexcited charge carriers effectively but also to passivate the surface states of QDs with improved stability. Low cost and earth-abundant [Mo3S13]2− nanoclusters work as an effecient cocatalyst for hydrogen evolution with excellent sulfur resistance. In all, the core/shell QDs using CdS as a shell layer decorated with [Mo3S13]2− nanoclusters is a promising strategy for preparing highly efficient and durable photocatalysts based on CdTe QDs for hydrogen generation with enhanced stability and more efficient solar utilization.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01520. Additional experimental details and experimental results (PDF)
Figure 5. H2 evolution of Pt-CdTe, Pt-CdTe/CdS, [Mo3S13]2−-CdTe, and [Mo3S13]2−-CdTe/CdS under the optimized conditions.
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AUTHOR INFORMATION
Corresponding Author
evolution with time dependent H2 production under visible light (λ > 420 nm). The H2 production of [Mo3S13]2−-CdTe/ CdS increases dramatically within 10 h. [Mo3S13]2−-CdTe/CdS shows a 25 times higher average H2 evolution amount (992.4 μmol) than that of Pt-CdTe/CdS (38.4 μmol), respectively, after 10 h irradiation. Platinum (Pt), as the cocatalyst, is widely employed to enhance photocatalysis efficiency due to its high activities with low overpotential for hydrogen evolution.
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the Recruitment Program of Global Experts in China, the start-up funds from Shanghai D
DOI: 10.1021/acssuschemeng.6b01520 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Jiao Tong University, NSFC (Grants 51372151, 21303103), and the Foundation of Shanghai Government (15PJ1404000).
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DOI: 10.1021/acssuschemeng.6b01520 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
