CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201400028

Photocatalytic Hydrogen Evolution from Glycerol and Water over Nickel-Hybrid Cadmium Sulfide Quantum Dots under Visible-Light Irradiation Jiu-Ju Wang, Zhi-Jun Li, Xu-Bing Li, Xiang-Bing Fan, Qing-Yuan Meng, Shan Yu, Cheng-Bo Li, Jia-Xin Li, Chen-Ho Tung, and Li-Zhu Wu*[a] Natural photosynthesis offers the concept of storing sunlight in chemical form as hydrogen (H2), using biomass and water. Herein we describe a robust artificial photocatalyst, nickelhybrid CdS quantum dots (Nih-CdS QDs) made in situ from nickel salts and CdS QDs stabilized by 3-mercaptopropionic acid, for visible-light-driven H2 evolution from glycerol and water. With visible light irradiation for 20 h, 403.2 mmol of H2 was obtained with a high H2 evolution rate of approximately 74.6 mmol h1 mg1 and a high turnover number of 38 405 compared to MPA-CdS QDs (mercaptopropionic-acid-stabilized CdS quantum dots). Compared to CdTe QDs and CdSe QDs, the

modified CdS QDs show the greatest affinity toward Ni2 + ions and the highest activity for H2 evolution. X-ray photoelectron spectroscopy (XPS), inductively-coupled plasma atomic emission spectrometry (ICP-AES), and photophysical studies reveal the chemical nature of the Nih-CdS QDs. Electron paramagnetic resonance (EPR) and terephthalate fluorescence measurements clearly demonstrate water splitting to generate ·OH radicals. The detection of DMPO-H and DMPO-C radicals adduct in EPR also indicate that ·H radicals and ·C radicals are the active species in the catalytic cycle.

Introduction The utilization of hydrogen (H2) as a future energy carrier requires its cost-effective, sustainable, and efficient production.[1] Because solar energy is abundant, clean, and economic, using sunlight to produce H2 from renewable resources such as water or biomass is a topic of great interest.[2] Although platinum and other precious metals loaded onto semiconductors have been demonstrated to be excellent catalysts for H2 photoproduction,[3] replacing these rare and/or expensive catalysts with catalysts using earth-abundant materials[4] would be a significant step toward making H2 a competitive alternative energy source, and would facilitate the transition to a hydrogen economy. Semiconductor nanocrystals (quantum dots; QDs) have recently appeared at the forefront of H2 photoproduction, owing to their low cost and unique characteristics amenable to applications in light-harvesting and charge-separation.[5] Unlike bulk semiconductors, in which electrons and holes can undergo trap-mediated recombination that prevents diffusion to the surface, the quantum confinement and large surface-to[a] J.-J. Wang,+ Dr. Z.-J. Li,+ X.-B. Li, X.-B. Fan, Dr. Q.-Y. Meng, S. Yu, C.-B. Li, J.-X. Li, Prof. Dr. C.-H. Tung, Prof. Dr. L.-Z. Wu Key Laboratory of Photochemical Conversion and Optoelectronic Materials Technical Institute of Physics and Chemistry & University of Chinese Academy of Sciences The Chinese Academy of Sciences Beijing, 100190 (PR China) Fax: (+ 86) 10-8254-3580 E-mail: [email protected] [+] These authors contributed equally to this work. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201400028.

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volume ratios of QDs significantly enhance the surface amplitude of electrons and holes, which facilitates interfacial catalysis for H2 evolution. Compared to molecular photocatalysts, QDs exhibit size-dependent spectroscopic properties and have large extinction coefficients over a broad spectral range, which should lead to improved light-harvesting performance. Importantly, the surface of QDs can be readily modified with co-catalysts by using appropriate functional groups. For example, Pickett et al. introduced the molecular catalyst Fe2S2(CO)6 onto InP nanocrystals to construct a photoelectrochemical cell for H2 evolution in 2010.[5g] Chen et al. anchored cobaloxime molecular catalysts onto the surface of CdSe/ZnS QDs via a phosphonate linkage.[5l] Eisenberg, Holland, and Krauss et al. used CdSe QDs capped with dihydrolipoic acid (DHLA) as the light absorber and a soluble Ni2 + -DHLA catalyst with ascorbic acid as an electron donor at pH 4.5 for the light-driven generation of H2.[6] Our group prepared an assembly of CdSe QDs/ Fe2S2(CO)6 by using an interface-directed approach in aqueous/ organic solution for photocatalytic H2 evolution in water.[7] Particularly, we found that earth-abundant inorganic salts can be directly incorporated into CdTe QDs[8] or CdSe QDs[9] to form robust photocatalysts in situ for H2 evolution. The astonishing activity and stability of these photocatalysts stimulated us to design and synthesize new catalysts using QDs and metal salts for photocatalytic H2 production from glycerol aqueous solution. Glycerol is used as a typical biomass in this work, because photocatalytic H2 production from glycerol (among various biomasses and/or derivatives) has been studied with limited success.[2b, 10] As a byproduct of biodiesel production from vegetaChemSusChem 2014, 7, 1468 – 1475

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CHEMSUSCHEM FULL PAPERS ble oil and animal fat, glycerol is produced in large amounts (10 wt % of biodiesel obtained).[2b, 10b] However, its market price is low and it rapidly becomes a waste product, harmful to the environment. Because the degree of oxidation of glycerol is typically low at low temperature, glycerol itself is not an effective fuel.[10b, 11] Traditional methods for H2 production from glycerol, such as steam reforming, gasification, autothermal reforming, aqueous-phase reforming, electrochemical reforming, and supercritical water reforming, have disadvantages of either high temperatures or catalyst poisoning.[10] Photocatalytic H2 production from glycerol is a promising alternative from the point of view of green and sustainable chemistry.[1c, 2a,b, 10] In this regard, titania (TiO2) materials modified with gold, palladium, or platinum have been widely used as photocatalysts under irradiation with UV light.[2b, 10] Dyes, semiconductors,[10a] or heteropolyblue[12] have been anchored onto TiO2 to achieve visible-light-responsive composites. Bulk hexagonal phase CdS (hex-CdS) doped by platinum is also active, with a H2 production rate of ca. 0.10 mmol h1 mg1 from glycerol in aqueous solution.[13] ZnO/ZnS core/shell nanorod catalysts prepared by a low-temperature water bath route show H2 production rates of ca. 2.61 mmol h1 mg1 (under UV irradiation) and ca. 0.38 mmol h1 mg1 (under solar light irradiation),[14] while ZnO/ZnS-Ag2S nanorod catalysts show even higher H2 production rates under UV (ca. 4.94 mmol h1 mg1) and simulated solar (ca. 0.65 mmol h1 mg1) irradiation (all rates discussed here are vs. catalyst).[15] To date, most photocatalytic H2 production processes from glycerol and water are performed under UV light, and the maximal rate of photocatalytic H2 production from glycerol aqueous solution is ca. 3.75 mmol h1 mg1 (under visible light)[16] and ca. 1 1 [17] 37 mmol h mg (under UV light). These values are far behind those achieved using biomass derivatives.[9, 18] Herein, we report a simple and robust photocatalyst: nickelhybrid cadmium sulfide QDs (Nih-CdS QDs). The Nih-CdS QDs are made from CdS QDs stabilized by 3-mercaptopropionic acid (MPA) and nickel salts in situ under visible-light irradiation at room temperature. Herein, water-soluble MPA-CdS QDs were used because the more positive valence band of CdS,[19] as compared to CdTe or CdSe, is anticipated to oxidize the relatively inert species glycerol effectively, thereby resulting in higher activity for H2 evolution. This expectation was found to be indeed the case. With visible-light irradiation for 20 h (l > 400 nm), the Nih-CdS QD photocatalyst formed in situ produced H2 from glycerol and water at a high rate of up to ca. 74.6 mmol h1 mg1, and with a turnover number (TON) of 38 405 with respect to MPA-CdS QDs. Although the coupling of CdS nanoparticles and nickel species (such as nickel clusters,[20] NiO,[21] NiS[22] and Ni(OH)2[23]) for photocatalytic H2 evolution from biomass derivatives and water have been realized, few examples involving the use of glycerol, which is more difficult to oxidize than other biomass derivatives such as methanol or ethanol, have been reported. In contrast to findings reported in the literature using semiconductors,[20, 21, 23] the interaction between CdS QDs and nickel species in the present work is unique: by X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectrometry (ICP-AES), and  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org steady-state and time-resolved spectroscopy, we consider that Ni2 + -centered complex is formed by coordination of Ni2 + to “hanging” S2 bonds on the surface of MPA-CdS QDs. These complexes form the active sites for H2 evolution. The performance of the Nih-CdS QDs materials is, to the best of our knowledge, the highest activity for H2 production from glycerol and water under light irradiation known to date. Electron paramagnetic resonance (EPR) and terephthalate (TANa) experiments clearly demonstrate that H2O splitting takes place to generate ·OH radicals and ·H radicals. These radicals are responsible for the effective glycerol oxidation and H2 production. The effects of pH, glycerol concentration, and the concentration of metal salts (NiCl2·6 H2O) were investigated in detail, not only to optimize the catalytic activity for H2 evolution but also to understand the reaction mechanism (Scheme 1).

Scheme 1. Visible-light-driven H2 production from glycerol and water by a Nih-CdS quantum dot photocatalyst.

Results and Discussion Preparation and characterization of the photocatalyst. A typical visible-light-driven H2 production experiment was conducted in aqueous glycerol solution at room temperature. MPA-CdS QDs was prepared according to a literature procedure, with slight modification (see details in the Experimental Section).[24] The average diameter of the MPA-CdS QDs was determined as ca. 2.8 nm, based on absorption at 380 nm using the equation developed by Peng and co-workers (Supporting Information, Figure S1).[25] This value agreed well with high-resolution transmission electron microscopy (HRTEM) observations (Supporting Information, Figure S2). Figure 1 shows the time course of H2 evolution in the reaction system containing NiCl2·6 H2O (1.2  104 m), MPA-CdS QDs (1.05  106 m, 0.036 mg mL1), and glycerol (4.5 m) in 10 mL of water at pH 6. Under visible-light illumination (l > 400 nm), the reaction mixture produced H2 at a constant rate during almost 10 h. Irrespective of H2 dissolution in the solvent, the amount of H2 reached 403.2 mmol within 20 h of irradiation, corresponding to a TON of 38 405 with respect to the MPA-CdS QDs. The rate of H2 production and internal quantum yield (at 410 nm) were determined to be ca. 74.6 mmol h1 mg1 and 12.2 %, respectively (see Experimental Section). ChemSusChem 2014, 7, 1468 – 1475

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Figure 1. Time course of H2 production under optimal conditions: NiCl2·6 H2O (1.2  104 m), MPA-CdS QDs (1.05  106 m, 0.036 mg mL1), and glycerol (4.5 m) in 10 mL of water solution at pH 6, visible light irradiation (l > 400 nm). The error bars represent the mean  s.d. of three independent experiments.

Control experiments proved that the components MPA-CdS QDs, NiCl2·6 H2O, glycerol, and light were all essential for efficient H2 evolution. The absence of NiCl2·6 H2O led to a dramatic drop in the rate of H2 evolution. No H2 could be detected when MPA-CdS QDs were absent from the reaction system (Supporting Information, Figure S3). After irradiation, the reaction solution was isolated by centrifugation to obtain precipitates and a colorless solution. When the precipitates and the colorless solution were put into a solution of glycerol and water, photocatalytic activity towards H2 evolution was only observed from the precipitates, and not from the colorless solution (Supporting Information, Figure S4). To characterize the precipitates, we used XPS to determine the composition before and after irradiation. The characteristic Ni2 + 2p3/2 peak at ca. 855.3 eV and Ni2 + 2p1/2 peak at 872.8 eV, with two satellite peaks (labelled as “Sat.”) at about 860.4 eV and 879.5 eV, were clearly detected in the spectrum of the precipitate after irradiation (Figure 2 a). This result implies that the catalytic entity in the precipitate may be formed by interaction of Ni2 + ions and MPA-CdS QDs (as proposed in Scheme 1).[8, 9, 26] To examine this possibility further, we prepared a sample by centrifugation of a solution of NiCl2·6 H2O (1.2  104 m), MPACdS QDs (1.05  106 m, 0.036 mg mL1), and glycerol (4.5 m) at pH 6 before irradiation. The precipitates before and after irradiation gave similar spectra (Figure 2 b) suggesting that divalent Ni2 + exist in both precipitates without change of its oxidation state.[27] By using EPR, X-ray absorption near-edge structure (XANES), as well as extended X-ray absorption fine structure (EXAFS) techniques, Talapin and co-workers[26a] proposed that Mn2 + ions could primarily bind to the electron-rich S2 ligands at the surface of CdSe/S nanocrystals without change of oxidation state. Hence, we think that the Ni2 + -centered complex is formed by coordination of Ni2 + to hanging S2 groups on the surface of MPA-CdS QDs, and that the as-formed complex is the active H2 evolution site in the Nih-CdS QDs. The hypothesis about the active sites was further confirmed by examining the S 2p signals in the XPS spectrum of the precipitate. As compared to the spectrum of MPA-CdS QDs (with the surface S atoms having binding higher energies than those in the bulk/interior[9, 28]), the S 2p signals of the precipitate  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 2. XPS of Ni 2p from the precipitates of Nih-CdS QDs (a) after irradiation; and (b) before irradiation. (c) XPS of S 2p from MPA-CdS QDs (black line), and Nih-CdS QDs before irradiation (gray line).

before irradiation (Figure 2 c) are wider and extend to higher binding energy levels. This may be due to the difference in electronegativity between Ni2 + (1.91) and Cd2 + (1.69) ions.[29] Upon coordination between Ni2 + ions and S2 at the surface, the electron density of the S orbital would decrease, leading to a higher binding energy of surface S 2p (the higher range of the S 2p signals in Figure 2 c).[30] As compared to reports on NiS in the literature,[20] the extended S 2p signal also indicates the formation of NiS bonds in the precipitate. However, the binding energy of S 2p in the lower energy range was unchanged, suggesting that the interior of the QDs is not affected by the Ni2 + ions.[23] Based on these results, we infer that the precipitate, namely Nih-CdS QDs, is formed in situ upon the addition of Ni2 + ions to MPA-CdS QDs aqueous solution. The amount of Ni2 + in the precipitates before and after irradiation was determined as 3.27 wt % and 3.25 wt % (Table 1), respectively, by ICP-AES, which corresponds to 20 active nickel sites on one QD (see the Supporting Information for details). After 10 h of irradiation, the XPS spectrum of S 2p from Nih-CdS QDs showed a tiny peak at a binding energy ca. 168.9 eV, indicating ChemSusChem 2014, 7, 1468 – 1475

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Table 1. Composition of Nih-CdS QDs photocatalyst, as determined by ICP-AES. Sample[a] Nih-CdS QDs, before irradiation Nih-CdS QDs, after irradiation

Elements [wt %] Cd S

Ni

56.78 59.65

3.27 3.25

11.85 14.60

[a] With NiCl2·6 H2O. See the Supporting Information for further details.

oxidized S species (Supporting Information, Figure S5).[28b, 31] This oxidation could be partially responsible for the decreasing activity of the photocatalysts with longer irradiation times. The recycling study suggests that the reactivity of the Nih-CdS QDs photocatalyst toward glycerol (4.5 m) in aqueous solution (pH 6) cannot be fully restored (Figure S4), possibly due to mild photocorrosion. Photocatalytic activity for H2 production from glycerol and water The activity of Nih-CdS QDs photocatalysts, for H2 production was evaluated by adjusting the concentrations of MPA-CdS QDs, NiCl2·6 H2O, and glycerol, as well as the solution pH value. Increasing the concentration of MPA-CdS QDs from 0.35  106 m to 1.05  106 m results in a substantial increase of the amount of H2 evolved (Supporting Information, Figure S6). A further increase of the MPA-CdS QD concentration, however, led to only a slight increase. Therefore, we conducted subsequent experiments with 1.05  106 m of MPA-CdS QDs. Figure 3 a shows the amount of H2 from a system containing MPA-CdS QDs and glycerol with varied concentrations of NiCl2·6 H2O in water. The rate of H2 formation reaches a peak value at a NiCl2·6 H2O concentration of 1.2  104 m (approximately equal to 3.25 wt % nickel as determined by ICP-AES), while a substantial decrease in the amount of H2 at both higher and lower concentrations. The optimal pH of the initial solution for H2 evolution ranges from 5 to 9, but the rate of H2 evolution decreased sharply at lower or higher pH values (Figure 3 b). This is closely related to

the surface charge of the MPA-CdS QDs [the pH for the zeropoint of charge (pHzpc) for CdS is 7.5 or lower[32]]. As the surface charge is neutral at pHzpc, both protons and glycerol can easily reach the surface of the CdS QDs. At acidic pH (pH < 7), the CdS surface is positively charged. Electrostatic repulsion between the positively charged surface and the protons would hinder the absorption of protons, and decrease the rate of H2 production. At alkaline pH values (> 7), on the other hand, an electrostatic repulsion between the negatively charged surface of CdS and the lone-pair electron of oxygen in glycerol inhibits the adsorption of the glycerol, to scavenge the valence band holes. As a result, pH changes can influence the adsorption of species onto the surface of CdS QDs; an important step governing the overall process of H2 production. Increasing the concentration of glycerol from 0.02 m to 4.5 m in aqueous solution resulted in a remarkable enhancement of the H2 production rate (Figure 3 c). The initial rate of H2 generation was found to obey a Langmuir-type reaction model as a function of glycerol concentration,[3b, 9] where the rate constant of H2 evolution and adsorption constant of glycerol were 285.7 mmol h1 and 9.8  107 L mmol1, respectively (Figure 4). When the glycerol concentration was higher than 6 m, the rate and the amount of H2 evolution decreased.

Figure 4. Fitting plot of 1/rH2 versus 1/C0, based on Figure 3 c using the Langmuir-type reaction model (rH2 refers to the initial rate of photocatalytic H2 generation, and C0 refers to the concentration of glycerol in H2O).

Figure 3. Optimization of experimental conditions for the photocatalysis of glycerol to H2 (l > 400 nm) in a 10 mL of aqueous solution. Factors studied: (a) concentration of NiCl2·6 H2O; (b) pH; and (c) concentration of glycerol. The parameters of factors other than the one studied were fixed as follows: NiCl2·6 H2O (1.2  104 m), MPA-CdS QDs (1.05  106 m, 0.036 mg mL1), glycerol (4.5 m) at pH 6. The error bars represent the mean  s.d. of three independent experiments.

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CHEMSUSCHEM FULL PAPERS The role of water in the photocatalytic system Water is crucial for photocatalytic H2 evolution. Trace amounts of H2 were obtained when water was absent from the reaction solution containing NiCl2·6 H2O (1.2  104 m), MPA-CdS QDs (1.05  106 m, 0.036 mg mL1) in 10 mL of pure glycerol at about pH 6 (Figure S7). To gain insight into this observation, EPR measurements and TANa fluorescence probing technique was employed.[9, 33] EPR spectra (Exp) of adducts generated in the system containing MPA-CdS QDs and NiCl2·6 H2O in water upon irradiation for 140 s are shown in Figure 5 a. Typical signals of ·H radicals and ·OH radicals were detected with the help of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The percentage of the two components was simulated by Simfonia software: the signal from the DMPO-H adduct (88.81 %) shows nine peaks with an approximate intensity ratio of 1:1:2:1:2:1:2:1:1 (aN = 16.44 G and aHb = 22.40 G)[34] and that of the DMPO-OH radical adduct (11.19 %) shows a quartet of peaks (1:2:2:1, aN = aH = 15.4 G) (Figure 5 a).[35] These results demonstrate that surface-adsorbed H2O (OH) could be oxidized to ·OH radicals by photo-generated holes, consistent literature reports (surface-adsorbed OH groups could be oxidized to ·OH at more negative redox potential of Eox = + 1.5 V vs. NHE than the free OH groups in solution, which is indeed more negative than that the valence band of CdS, Evb = 1.6 V).[9, 36] Simultaneously, the protons could be reduced to ·H

Figure 5. Experimental (Exp) and simulated (Sim) EPR spectra of adducts, generated in (a) the absence, and (b) the presence of glycerol (0.45 m). Conditions: MPA-CdS QDs (5.25  106 m, 0.18 mg mL1), NiCl2·6 H2O (1.2  104 m), and DMPO (2.0  102 m) in water under laser light (l = 355 nm) for 140 s.

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www.chemsuschem.org radical by photogenerated electrons. The formation of TAOH fluorescence at 425 nm, captured by nonfluorescent TANa (Figure S8), further confirms the generation of ·OH radicals[33] from the system containing MPA-CdS QDs and NiCl2·6 H2O after 10 min irradiation. When glycerol was added into the solution, the fluorescence of TAOH was quenched dramatically. The finding indicated that the photogenerated ·OH radical is one of the possible active species to react with glycerol. Upon irradiation of the system containing MPA-CdS QDs, NiCl2·6 H2O, water and glycerol (Figure 5 b), carbon radicals (denoted as ·C) were also detected. The presence of ·C radicals in addition to ·OH and ·H radicals implied either the abstraction of H atoms from glycerol or carbon–carbon cleavage.[3b, 9] All of the experimental signals were simulated to resolve DMPO–C, DMPO–H and DMPO–OH adducts. We conclude that the generated ·OH radicals are not only able to react with glycerol but also accelerate the dehydration of glycerol to accomplish the whole catalytic reaction.[2b]

Mechanism of H2 evolution by the photocatalyst To shed more light on the photocatalytic process, we examined the photophysical properties of the MPA-CdS QDs with the addition of NiCl2·6 H2O in aqueous solution. Excitation of the MPA-CdS QDs at 406 nm resulted in band-edge emission and trap emission centered at 470 nm and 610 nm, respectively (Figure 6 a). When Ni2 + ions were added to the MPA-CdS QDs aqueous solution the emission was quenched dramatical-

Figure 6. (a) Emission spectra of the MPA-CdS QDs (1.05  106 m, 0.036 mg mL1) with the addition of NiCl2·6 H2O (from 0 to 1.2  105 m) in water (excitation wavelength: 406 nm). (b) Plot of log[(I0/I)1] as a function of the concentration of NiCl2·6 H2O, where I0 and I refer to the emission intensity of MPA-CdS QDs in the absence and presence, respectively, of NiCl2·6 H2O in water.

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CHEMSUSCHEM FULL PAPERS ly, indicating that strong coordination interaction between the S2 on MPA-CdS QDs and Ni2 + ions[8, 9, 37] facilitates the nonradiative pathways.[28a, 37a] The blue-shift of the trap emission center may also result from the as-formed Ni2 + -centered complex between Ni2 + ions and the hanging S2 bonds on the surface of QDs,[9, 26, 38] which could passivate the surface trap sites and quench the defect emission at lower energy.[39] Analysis of the emission decay revealed that the average emission lifetime was constant in the presence of Ni2 + ions, and the quenching kinetics obeyed a static interaction model.[37b] With the increase of the concentration of Ni2 + ions, the emission intensity was gradually quenched to give a straight line (Figure 6 b),[40] yielding a binding constant of 6.82  105 m1. Because of quantum confinement effects, the MPA-CdS QDs used in this study have a larger band gap (Eg = 2.69 eV)[7] than the bulk material (Eg = 2.4 eV; conduction band position Ecb = 0.8 V, valence band position Evb = 1.6 V;[19b, 41] all potentials vs. NHE). The relative positions of QD valence bands are likely to be close to the bulk band gap, and most of the increase of band-gap is due to the shift of the conduction band.[19b] Thus the Ecb for MPA-CdS QDs is more negative than the redox potential of the Ni2 + /Ni0 couple (0.25 V vs. NHE),[42] rendering electron transfer from the conduction band of MPA-CdS QDs to Ni2 + active sites thermodynamically favorable. Upon irradiation by visible light, the photoexcited electrons in the conduction band transfer to the surface bound Ni2 + -centered complexes that are the active sites for photocatalytic H2 production.[8, 9, 26a] On the other hand, the hole-trapping process is considered crucial for photocatalytic H2 evolution. To this end, MPA-CdSe QDs and MPA-CdTe QDs were introduced to test the photocatalytic H2 evolution activity, where the absorbance of these QDs at 410 nm was kept identical (Supporting Information, Figure S9). Among them, MPA-CdS QDs with NiCl2·6 H2O showed the highest rate of H2 evolution. There are two possibilities for the activity: First, the valence band of CdS (Evb = 1.6 V) is more positive[19b] than those of both CdSe (Evb = 1.1 V)[43] and CdTe (Evb = 0.54 V).[19b] Thus, CdS would have a larger driving force to oxidize surface-adsorbed H2O (OH) or glycerol to ·OH, which eventually promotes the H2 evolution process. Second, the nickel content (from ICP-AES results) was 0.16 wt % for NihCdTe QDs, 0.5 wt % for Nih-CdSe QDs, but 3.25 wt % for Nih-CdS QDs, thus there are fewer S2 ligands (directly related to the amount of Ni2 + -centered complex) on the CdTe QDs and CdSe QDs than on the CdS QDs. Based on the above results, we propose a photocatalytic process for H2 production from glycerol (Scheme 2). Ni2 + ions coordinate to S2 bonds on the surface of MPA-CdS QDs to form the photocatalyst, Nih-CdS QD, in situ under visible light irradiation. Photoinduced electron transfer from the conduction band (CB) of the CdS QDs to the Ni2 + -centered complex active sites results in the capture of H + to form H2. The photogenerated holes on the valence band, on the other hand, can be trapped by H2O molecules or OH ions on the surface of Nih-CdS QDs to form ·OH radicals,[9, 44] which react with glycerol to form carbon radicals (·C), such as CH2OHCHOHCH(OH)· or CH2OHC(HO)·CHOH2, by the abstraction of ·H. The generated  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Scheme 2. Schematic of the mechanism for photocatalysis of glycerol to H2 by the Nih-CdS QDs photocatalyst.

two putative CH radicals can recombine to form molecular H2.[9, 13, 45] The CH2OHCHOHCH(OH)· or CH2OHC(OH)·CH2OH is ultimately transformed to oxidized species (Supporting Information, Figure S10). The structures of these oxidized products are postulated based on compound fragments detected by ESI-MS and GC-MAS analyses (see Supporting Information). Simultaneously, the holes can react directly with the surface-adsorbed glycerol to form oxidized species by releasing H + or ·H, which are eventually transformed to molecular H2 over the Nih-CdS QD photocatalyst.

Conclusions We report a cheap, simple, and efficient artificial photocatalyst, Nih-CdS QDs, for photocatalytic conversion of glycerol to H2 under visible-light irradiation. The photocatalyst, formed in situ from MPA-CdS QDs and Ni2 + ions, is capable of producing H2 at pH values ranging from 5 to 9, with a turnover number (TON) of up to 38 405 and an internal quantum yield of 12.2 % (410 nm). The H2 evolution rate of ca. 74.6 mmol h1 mg1 is, to the best of our knowledge, the highest one reported for photocatalysis from aqueous solutions of glycerol. The exceptional performance for H2 evolution from the relatively inert compound glycerol, with a low market price, stimulates us to develop more efficient photocatalytic systems by using earthabundant metal ions and QDs in the future.

Experimental Section Synthesis and purification of water-soluble MPA-CdS QDs A mixture of CdCl2·2.5 H2O (228.4 mg) and MPA (1 mL) in ultrapure water (190 mL) was placed in a 500 mL flask, and then de-aerated by Ar bubbling for 30 min. 10 m NaOH aqueous solution was added with vigorous stirring; during this process, the system changed from clear and transparent to a blue white turbid liquid, and then changed to clear and colorless again. 1 m NaOH aqueous solution was used to adjust the pH to 7. After that, 10 mL of Na2S·9 H2O (240.18 mg) aqueous solution was added to the above mixture. Finally, the bright-yellow, transparent solution was stirred for about 3.5 h to promote the growth of MPA-CdS QDs. The prepared MPA-CdS QDs were purified before use in H2 evolution experiments. First, the prepared MPA-CdS QDs solution was ChemSusChem 2014, 7, 1468 – 1475

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concentrated to ca. 1/3 the volume of the stock solution. Then, addition of isopropyl alcohol (4 times the volume of the MPA-CdS QDs solution) resulted in precipitation. After centrifugation at a rate of 8000 r min1 and decantation of the supernatant, the remaining precipitates were dissolved into water for further purification; this operation was repeated 3 times. The average weight of MPA-CdS QDs was ca. 3.43  104 g mol1 (see Supporting Information).

emits fluorescence at around 425 nm upon excitation of its 312 nm absorption band. Tests were performed in 10 mL of water (TANa is soluble in water). After removing residual air through nitrogen refill cycles, an aqueous solution of MPA-CdS QDs (1.05  106 m, 0.036 mg mL1), NiCl2·6 H2O (1.2  104 m), and TANa (5.0  103 m) at pH 6 was irradiated under visible light (l > 400 nm) for 10 min; then a 3.0 mL aliquot of the reaction solution was sampled and centrifuged to remove the photocatalyst for the determination of photoluminescence.

General procedure for photocatalytic H2 production from glycerol The system for the preparation of photocatalyst, Nih-CdS QDs, and the subsequent H2 evolution contained purified MPA-CdS QDs (1.05  106 m, 0.036 mg mL1), NiCl2·6 H2O (1.2  104 m), and glycerol (4.5 m) aqueous solution in a Schlenk tube. The total volume of the sample was 10 mL. With the addition of NaOH or HCl aqueous solution, the pH of the system was adjusted to 6. The sample was degassed by nitrogen for 15 min. Then CH4 (1000 mL in control experiments and optimized condition experiments to ensure sufficient CH4 was extracted from the samples) was injected as the internal standard for quantitative GC analysis. The sample was irradiated under a high-pressure Hanovia mercury lamp (500 W) with a cut-off filter (l > 400 nm, 120  10 mW cm2 at flask surface). Then 1000 mL of mixed gas was extracted from the sample tube and injected into the GC immediately (Tianmei 7890-II). The response factor for H2/CH4 was about 5.1 under the experimental conditions.

Fluorescence signal fitting The equilibrium between free and bound molecules is given by the Equation (1):[40] h  i log I0=I  1 ¼ log KA þ n log½QðNi2þ Þ

ð1Þ

Herein, the term log[(I0/I)1] was calculated and plotted against the logarithm of the concentration of Ni2 + ions, where KA is the binding constant.

Langmuir-type reaction model According to the Langmuir-type reaction model,[3b, 9] the initial rate for H2 production can be expressed as Equation (2): rH2 ¼

dnH2 kH KC0 ¼ 2 dt 1 þ KC0

ð2Þ

Inversion of this rate expression provides a linear plot:   1 1 1 1 ¼ þ rH2 kH2 kH2 K C0

A mixture containing 100 mL of water in the presence of MPA-CdS QDs (5.25  106 m, 0.18 mg mL1), NiCl2·6 H2O (1.2  104 m), and DMPO (2.0  102 m) was saturated by nitrogen gas to eliminate oxygen in the EPR flat cell. Then the EPR spectra were recorded at room temperature using a Bruker ESP-300E spectrometer at 9.8 GHz, X-band, with 100 Hz field modulation. Samples were illuminated directly in the cavity of the EPR spectrometer with a laser (355 nm) for 140 s. When glycerol was involved, the concentration of glycerol was 0.45 m.

Internal quantum efficiency[9] Light-driven H2 evolution was performed in a standard spectrocell with a total volume of 4 mL and a path-length of 1 cm. The cuvette was filled with 2.0 mL reaction solutions (at pH 6.0  0.3) of purified MPA-CdS QDs (1.05  106 m, 0.036 mg mL1), NiCl2·6 H2O (1.2  104 m) and glycerol (4.5 m) aqueous solution). An LED light source (l = 410 nm; light intensity 100 mW cm2 at the surface of the spectrocell) was used with constant stirring by a magnetic stirrer. The internal quantum efficiency (F) was calculated using Equation (4): ¼

rH2 0 Iabs

ð4Þ

where rH2is the H2 evolution rate and I0abs is the photon absorption rate by the reaction solution. The H2 generation rate D(H2) was obtained by GC. The photon absorption rate I0abs was calculated from the illumination power and absorbance of the reaction solution. The amount of absorbed light was determined from the absorbance (abs) of the reaction solution at l = 410 m. The measured power and estimated reflection/scattering loss of the cuvette front window was negligible for a colloid suspension that is transparent. From the combined measurements of H2 mass production rate, and the specific light absorption rate, the quantum yield for the H2 reaction system was determined to be 12.2 %.

ð3Þ

where C0 is the initial concentration of glycerol, kH2 and K are the rate constant of H2 evolution and adsorption constant of glycerol, respectively. Figure 4 shows that kH2 and K are about 285.7 mmol h1 and 9.8  107 L mmol1, respectively.

Fluorescent experiment for detection of ·OH radicals Hydroxyl radicals (·OH) reacted with terephthalate (TANa) to produce highly fluorescent 2-hydroxyterephthalic acid (TAOH), which  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Electron paramagnetic resonance experiments

Acknowledgements We are grateful for financial support from the Ministry of Science and Technology of China (2014CB239402, 2013CB834804, and 2013CB834505), the National Science Foundation of China (2109343, 21390404, 91027041, and 51373193), the Solar Energy Initiative of the Knowledge Innovation Program of the Chinese Academy of Sciences, and the Bureau for Basic Research of the Chinese Academy of Sciences. ChemSusChem 2014, 7, 1468 – 1475

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Received: January 8, 2014 Published online on April 1, 2014

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