CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201402885
Vectorial Electron Transfer for Improved Hydrogen Evolution by Mercaptopropionic-Acid-Regulated CdSe Quantum-Dots–TiO2–Ni(OH)2 Assembly Shan Yu,[a] Zhi-Jun Li,[a] Xiang-Bing Fan,[a] Jia-Xin Li,[a] Fei Zhan,[b] Xu-Bing Li,[a] Ye Tao,[b] Chen-Ho Tung,[a] and Li-Zhu Wu*[a] A visible-light-induced hydrogen evolution system based on a CdSe quantum dots (QDs)–TiO2–Ni(OH)2 ternary assembly has been constructed under an ambient environment, and a bifunctional molecular linker, mercaptopropionic acid, is used to facilitate the interaction between CdSe QDs and TiO2. This hydrogen evolution system works effectively in a basic aqueous solution (pH 11.0) to achieve a hydrogen evolution rate of 10.1 mmol g1 h1 for the assembly and a turnover frequency of 5140 h1 with respect to CdSe QDs (10 h); the latter is comparable with the highest value reported for QD systems in an acidic environment. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and control experiments demonstrate
that Ni(OH)2 is an efficient hydrogen evolution catalyst. In addition, inductively coupled plasma optical emission spectroscopy and the emission decay of the assembly combined with the hydrogen evolution experiments show that TiO2 functions mainly as the electron mediator; the vectorial electron transfer from CdSe QDs to TiO2 and then from TiO2 to Ni(OH)2 enhances the efficiency for hydrogen evolution. The assembly comprises light antenna CdSe QDs, electron mediator TiO2, and catalytic Ni(OH)2, which mimics the strategy of photosynthesis exploited in nature and takes us a step further towards artificial photosynthesis.
Introduction With the depletion of traditional fossil fuels reserves and the concern for environmental problems such as global warming caused by the continuous burning of fossil fuels, the development of renewable, clean energy sources is becoming an urgent and important subject to pursue. Hydrogen, as a sustainable, clean energy carrier, is particularly attractive because of its specific enthalpy of combustion and benign combustion product (water). In an effort to make hydrogen a competitive alternative energy source and facilitate the transition to a hydrogen economy, cost-effective, sustainable, and efficient hydrogen production is required.[1] In this regard, photosynthesis provides a paradigm for conversion from a renewable resource such as water or biomass to hydrogen by solar energy.[2] Nature long ago figured out how to use photosynthetic com[a] Dr. S. Yu,+ Dr. Z.-J. Li,+ X.-B. Fan, J.-X. Li, X.-B. Li, Prof. C.-H. Tung, Prof. L.-Z. Wu Key Laboratory of Photochemical Conversion and Optoelectronic Materials Technical Institute of Physics and Chemistry the Chinese Academy of Sciences Beijing 100190 (P.R. China) Fax: (+ 86) 10-8254-3580 E-mail: [email protected] [b] F. Zhan, Prof. Y. Tao Beijing Synchrotron Radiation Facility Institute of High Energy Physics the Chinese Academy of Sciences Beijing 100049 (P.R. China) [+] 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.201402885.
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plexes to capture sunlight. With precise control of three indispensible processes in photosynthesis, that is, light harvesting, charge separation, and catalysis, solar energy is converted by a chain of photoinduced electron transfer processes and stored in the form of hydrogen. These highly regulated photosynthetic systems have inspired scientists to develop similar functional integrated artificial systems for hydrogen evolution. Indeed, a variety of artificial systems, which include molecular systems or hybrid systems that comprise semiconductors and molecules have been reported.[3] The directional assembly of an antenna for light harvesting, an electron mediator for fast forward charge separation but slow back charge recombination, and a catalyst for hydrogen evolution in these systems is of great importance to achieve high efficiency. In this work, we report a new ternary assembly of CdSe quantum dots (QDs)– TiO2–Ni(OH)2 for hydrogen production and show that the traditional semiconductor TiO2 functions efficiently as the electron mediator to facilitate vectorial electron transfer from CdSe QDs to TiO2 and then from TiO2 to Ni(OH)2, which is responsible for the efficient hydrogen evolution of the integrated artificial system. TiO2 has benefits of cost effectiveness, stability against photochemical corrosion, and nontoxicity and has been studied widely in photocatalysis since pioneering work by Fujishima and Honda in 1972.[4] However, its wide band gap limits its response to the UV light range, which only accounts for 5 % of the full solar photon flux.[5] To achieve visible-light-responsive composites, the introduction of heteroatoms into TiO2 to decrease its band gap or the introduction of chromophores or ChemSusChem 0000, 00, 1 – 9
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CHEMSUSCHEM FULL PAPERS semiconductors onto TiO2 for sensitization have been suggested.[6] Recently, QDs have emerged as a choice to sensitize TiO2.[7] Compared to traditional molecular photosensitizers, QDs exhibit size-dependent spectroscopic properties and have a large extinction coefficient over a broad spectral range, which should lead to an improved light-harvesting performance.[8] The quantum confinement and the large surfaceto-volume ratio in QDs enhance the surface amplitude of the electrons and holes significantly.[9] Therefore, QDs attached TiO2 not only respond to visible light but also benefit from vectorial electron transfer to obtain charge carriers. Electron transfer from CdSe QDs to TiO2 has been studied thoroughly by Kamat et al. and usually occurs on the timescale of 107– 1010 s1.[10] However, attention has been paid mainly to CdSe QDs–TiO2 systems for the enhancement of light conversion efficiency in solar cells (both Grtzel cells and photoelectrochemical cells).[11] Few examples of light-driven hydrogen evolution from QD-modified TiO2 systems have been reported. In addition, although several QDs systems with or without hydrogenevolving catalysts were shown to function as photocatalysts for hydrogen production,[12] systems for highly effective photochemical hydrogen production were performed under acidic conditions almost exclusively. In this contribution, the new ternary assembly of CdSe QDs– TiO2–Ni(OH)2 for hydrogen evolution works in basic aqueous solution (pH 11.0; Scheme 1). CdSe QDs, TiO2, and Ni(OH)2 in
Scheme 1. MPA-regulated CdSe QDs–TiO2–Ni(OH)2 ternary assembly for hydrogen evolution.
this assembly perform their duties as an antenna, an electron mediator, and a catalytic site, respectively, which mimics the strategy of photosynthesis to a large extent. To facilitate the vectorial electron transfer from CdSe QDs to TiO2, the bifunctional molecular linker mercaptopropionic acid (MPA) was used to assemble CdSe QDs and TiO2 because of the different pKa values of the thiol group and carboxylic acid, respectively.[13] More strikingly, Ni(OH)2 formed in situ upon the addition of earth-abundant Ni salts into the MPA-assembled CdSe QDs– TiO2 has been demonstrated to be an effective catalyst distributed on the surface of TiO2 for hydrogen evolution. Upon irradiation by visible light (l > 400 nm) for 10 h, the CdSe QD– TiO2–Ni(OH)2 assembly exhibits a turnover number (TON) of 51 400 with respect to CdSe QDs in a solution of isopropanol 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org (IPA) and water (1:1 v/v) at pH 11.0, which corresponds to a turnover frequency (TOF) of 5140 h1; this is much higher than that of other QDs systems that work under basic conditions[14] and comparable to the highest value of QDs systems reported under acidic conditions.[15] In addition, the internal quantum yield of the system and the rate of hydrogen production for the CdSe QD–TiO2–Ni(OH)2 assembly reach 27.4 % (at 410 nm) and 10.1 mmol g1 h1, respectively.
Results and Discussion Construction of the CdSe QDs–TiO2–Ni(OH)2 ternary assembly The ternary assembly of CdSe QDs–TiO2–Ni(OH)2 for photocatalytic hydrogen evolution in this work is obtained by the initial construction of CdSe QDs–TiO2. Here, the bifunctional molecular linker MPA is used to assist the assembly of CdSe QDs with TiO2, in which the thiolate group has a good affinity with Cd from the QDs and the carboxylate group binds Ti from TiO2 effectively. In contrast to other linker-assisted adsorption (LAA) methods,[16] the LAA experiment was performed in aqueous solution. Given that the thiolate group dissociates from the thiol group (pKa = 10–11[13]) and the carboxylate group from carboxylic acid (pKa = 4–5[13]) in MPA at different pH values in water, we adjusted the pH of the aqueous solution to optimize the assembly process. To simplify the MPA-assisted assembly of CdSe QDs onto TiO2 in aqueous solution, CdSe QDs capped with MPA were synthesized directly from water.[12e] This not only avoids the introduction of other components in the system[17] but also omits a ligand-exchange step, which is usually necessary for oil-soluble QDs capped with organic ligands.[18] The as-prepared water-soluble CdSe QDs have a concentration of 2.0 105 m with a molar extinction coefficient of 2.9 104 cm1 m1 at the first excitonic band (433 nm) and their average size was determined to be around 1.9 nm (Figure S1). After the addition of commercial TiO2 powder (P25) with stirring, the amount of CdSe QDs adsorbed on TiO2 was determined by monitoring the absorption changes of the supernatant in the suspension. A larger absorbance of the supernatant suggests fewer CdSe QDs on TiO2. The increased concentration of MPA added to the system led to a larger amount of CdSe QDs anchored on TiO2 (Figure 1), which indicates the effectiveness of MPA as the linker for the assembly process. If 30 mL MPA is used, the maximal volume of CdSe QDs stock solution (2.0 105 m) that could anchor on 5.0 mg TiO2 was 1.5 mL (Figure 2). Typically, the assembly process reaches saturation within 30 min, which is significantly faster than that described in other reports.[10c, 11c, 19] As expected, an optimal pH exists for the assembly of TiO2 with CdSe QDs. To avoid the aggregation of MPA-capped CdSe QDs under acid conditions,[18, 20] the pH of the adsorption system was adjusted to be higher than 7.0. Similar to that in an early report,[13a] the optimal pH for the adsorption process is 8.0 over a wide pH range from 8.0–13.0 (Figure 3). Under these conditions, the surface of TiO2 is negatively charged beChemSusChem 0000, 00, 1 – 9
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Figure 1. Absorption spectra of the supernatant from the assembly systems that contain 1.5 mL of CdSe QDs stock solution (2.0 105 m) and 5.0 mg of TiO2 with a total volume of 5.0 mL at pH 8.0 in water after stirring for 30 min. Different amounts of MPA were added into the systems to investigate the effect of MPA on the assembly process.
Figure 2. Absorption spectra of the supernatant from the assembly systems that contain different amounts of CdSe QDs stock solution (2.0 105 m), 5.0 mg of TiO2, and 30 mL MPA with a total volume of 5.0 mL at pH 8.0 in water after stirring for 30 min. The dashed line corresponds to absorption spectrum of CdSe QDs (6.0 106 m).
Figure 3. Absorbance (at 418 nm) of the supernatant from the adsorption systems that contain 2.5 mL of CdSe QDs stock solution (2.0 105 m), 5.0 mg TiO2, and 30 mL MPA with a total volume of 5.0 mL at different pH values in water after stirring for 30 min. Error bars are based on three repeated experiments.
cause the point of zero charge of TiO2 (P25) is 6–7.[21] The surface of CdSe QDs capped with MPA should also be negatively charged under basic conditions as the pKa of the carboxylic acid group in MPA is 4–5.[13] A relatively less basic environment 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org at pH 8.0 would minimize the repulsion between TiO2 and CdSe QDs to some extent, which facilitates the interaction of TiO2 and CdSe QDs for assembly. In addition, anther optimal pH for assembly was noted at 12.0. This is because the dominant sulfur species in MPA is a thiol group at pH 8.0 as its pKa is 10–11.[13] A higher pH could benefit the dissociation of the thiol group into thiolate, which has a binding energy to Cd (1283 kJ mol1) that is approximately 40 times higher than that for the thiol group to Cd (34.7 kJ mol1).[13b, 22] Therefore, at pH 12.0, MPA with the main form of thiolate could bind to CdSe QDs more efficiently to promote the assembly process. Following centrifugation and the abandonment of the supernatant from the assembly suspension with 0.5 mL CdSe QDs (2.0 105 m) assembled on TiO2 (5.0 mg), we examined the UV/Vis diffuse reflectance spectra (DRS) of the precipitates. The characteristic absorption of CdSe QDs suggested the possible assembly of CdSe QDs with TiO2 (Figure 4). High-resolu-
Figure 4. UV/Vis DRS spectra of blank TiO2 (P25), CdSe QDs–TiO2, and CdSe QDs–TiO2–Ni(OH)2. Inset: magnified view.
tion transmission electron microscopy (HRTEM) images show that small particles with a size of 2.0 nm are distributed on the surface of TiO2 uniformly after assembly (Figure 5). The crystal lattice of CdSe QDs with an interplanar spacing of 2.12 [zinc blend, (2 2 0)] could be observed clearly, which confirms the assembly of CdSe QDs with TiO2. Next, we introduced Ni salts into the system to construct the ternary CdSe QDs–TiO2–Ni(OH)2 assembly. Specifically, earth-abundant NiCl2·6 H2O was introduced into CdSe QDs– TiO2 suspension of H2O/IPA (1:1 v/v) at pH 11.0. UV/Vis DRS spectra of the precipitates obtained from this suspension shows the characteristic absorption of CdSe QDs (Figure 4) with the appearance of a weak and broad absorption that ranges from l = 600–900 nm, which could be assigned to the d–d transition of Ni2+ ions.[23] Clearly, Ni2+ ions are deposited on the surface of CdSe QDs–TiO2. Furthermore, energy dispersive X-ray spectroscopy (EDX) and the related elemental mapping confirm the presence of Ni on the surface of TiO2 (Figure S3). On the basis of the concentration of Ni salts (1.7 104 m) and OH anions (pH 11.0) in our system as well as the solubility product of Ni(OH)2 (pKsp = 15.26[24]), we consider that most of the Ni2+ ions should precipitate from the suspension in the form of hydroxide. Yu and coworkers have demonstratChemSusChem 0000, 00, 1 – 9
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Figure 5. HRTEM images of TiO2 (P25) a) before and b), c), and d) after assembly with CdSe QDs.
ed that pre-precipitated Ni(OH)2 could combine with TiO2, CdS, or even graphitic-C3N4 for hydrogen evolution.[23] X-ray photoelectron spectroscopy (XPS) suggests the possible existence of Ni(OH)2 from the two peaks of Ni 2p at binding energies (BEs) of 855.61 and 873.46 eV separated by 17.85 eV (Figure 6), which resembles the spectrum of commer-
Figure 6. XPS spectra of CdSe QDs–TiO2–Ni(OH)2 and commercial Ni(OH)2 powder.
cial Ni(OH)2 powder (ref. Ni(OH)2), with peaks of Ni 2p3/2 at BE = 855.14 eV and Ni 2p1/2 at BE = 872.92 eV separated by 17.78 eV. The small shift of the Ni 2p peak to a higher BE in our sample may be caused by the hydration of Ni(OH)2 in aqueous solution. Furthermore, we investigated the samples by X-ray absorption spectroscopy (XAS), which can provide information such as the oxidation state, local coordination numbers, and identity of the neighbors of the absorbing atom. The Ni K-edge X-ray absorption near-edge astructure (XANES) of the XAS spectra shows that the edge absorption of our sample overlaps well with that of commercial reference Ni(OH)2 (Figure 7 a), which demonstrates the existence of Ni(OH)2 in our sample strongly. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 7. a) Comparison of Ni K-edge XANES spectra of CdSe QDs–TiO2-– Ni(OH)2 with reference samples. b) The k3-weighted Fourier-transformed Ni K-edge EXAFS data (exp.) and the fitting for CdSe QDs–TiO2--Ni(OH)2. Inset: k-space fitting of Ni(OH)2 to our ternary assembly.
This is also in good agreement with the XPS data. Moreover, extended X-ray absorption fine structure (EXAFS) analysis ( Figure 7 b and Table S1) gives us information about the coordination environment of Ni2+. The nearest-neighbor NiO and Ni Ni distances in our sample are 2.04 and 3.03 , respectively, which match those of a-Ni(OH)2. On the basis of the above XPS and XAS data, we believe that Ni indeed exists in the form of Ni(OH)2 in our sample. Inductively coupled plasma optical emission spectroscopy (ICP-OES) of CdSe QDs–TiO2–Ni(OH)2 was performed to determine the amount of Cd and Ni (relative to TiO2) in the assembly, which was 1.14 and 1.75 wt %, respectively. This corresponds to 88 % of the Ni2+ ions added into the system that precipitate on the assembly. The large Ni-to-Cd molar ratio of 3.0 suggests clearly that Ni(OH)2 is in excess with respect to CdSe QDs. The small molar ratio of Cd to TiO2 (0.81 %) indicates that TiO2 has a much larger surface area to contact with Ni2+ ions from solution than CdSe QDs, which could also be conjectured from the HRTEM images (Figure 5 b). In this situation, the majority of Ni(OH)2 should be distributed on the surface of TiO2. This is further confirmed by a HRTEM study of CdSe QDs– TiO2–Ni(OH)2. The lattice distances of 2.12 and 2.33 , which correspond to the (220) and (101) faces of CdSe QDs and Ni(OH)2,[25] respectively, together with their specific distributions in different TiO2 surface regions were observed clearly (Figure 8).
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Figure 9. Amount of hydrogen evolved from CdSe QDs–TiO2–Ni(OH)2 obtained from 0.4 mg NiCl2·6 H2O and 5.0 mg TiO2 preassembled with 0.5 mL of CdSe QDs stock solution (2.0 105 m) in 10.0 mL of IPS/H2O (1:1 v/v) under irradiation (l > 400 nm) at different pH values. Error bars are based on three repeated experiments.
Figure 8. HRTEM images of a) and b) CdSe QDs–TiO2–Ni(OH)2 and c) and d) magnified areas of a) and b), respectively.
Photocatalytic hydrogen evolution by CdSe QDs–TiO2– Ni(OH)2 ternary assembly Photocatalytic experiments for hydrogen evolution were performed directly in the IPA/H2O solution (1:1 v/v) that contained the CdSe QDs–TiO2–Ni(OH)2 assembly. After 10 h illumination under visible light, (514 15) mmol [(11.5 0.3) mL] hydrogen was detected (average of 20 experimental results) with 0.5 mL CdSe QDs (2.0 105 m) preassembled on TiO2 (5.0 mg), which corresponds to a hydrogen evolution rate of 10.1 mmol g1 h1 for the CdSe QDs–TiO2–Ni(OH)2 assembly and a TON of 51 400 with respect to CdSe QDs; the latter yields a TOF of 5140 h1. If we used a light-emitting diode (LED; 410 nm) as the light source, we calculated an internal quantum efficiency of 27.4 % without the consideration of scattered light. After the first illumination of 10 h, the solid sample in the suspension was collected and redispersed in fresh IPA/H2O solution (1:1 v/v, pH 11.0) to use in a second and even third round of irradiation in which the efficiency of the system was almost unchanged (Figure S8), which confirms that the catalyst for hydrogen evolution is from the assembly itself and it is stable under a long irradiation time. Notably, the present hydrogen evolution system works most efficiently under relatively basic conditions (pH 11.0). The efficiency of hydrogen evolution is improved if the pH of the system is increased from 7.0 to 11.0 and then it declined at pH 12.0 (Figure 9). Several factors could contribute to the result. First, a basic environment is more beneficial for the formation of the catalytic Ni(OH)2 sites than acidic or neutral conditions. Second, at pH 11.0, MPA, the capping ligand of QDs, is bound strongly to CdSe QDs in the form of thiolate, which decreases the departure of the protecting group from the surface of CdSe QDs and hence improves the stability of CdSe QDs.[26] Third, as MPA is also the linker between CdSe QDs and 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
TiO2, a basic environment could help CdSe QDs assemble with TiO2 more strongly to increase the electron coupling between TiO2 and CdSe QDs, which could be beneficial for the electron transfer from CdSe QDs to TiO2.[13b] However, if we take into account the decrease of the concentration of protons for hydrogen evolution with the increasing pH of the system, moderately basic conditions (pH 11.0) are superb. To further confirm the hydrogen evolution ability of the CdSe QDs–TiO2–Ni(OH)2 assembly under basic conditions, IPA was replaced with triethylamine (TEA), an electron donor used widely for hydrogen evolution in a basic environment, and the catalytic activity of the assembly was maintained (Figure S7). Additionally, the efficiency of the CdSe QDs–TiO2–Ni(OH)2 assembly for hydrogen production also depends on the intensity of the visible light used (Figure S9). If a specific light intensity of incident visible light from an LED (410 nm LED, 160, 80, and 40 mW cm2) is monitored, the photocatalytic activity is increased proportionally to light intensity. Control experiments confirm that the omission of any component in the assembly decreases the efficiency of the system for hydrogen evolution remarkably (Figure 10). No hydrogen could be detected without CdSe QDs, which is consistent with
Figure 10. Amount of hydrogen evolved under illumination at l > 400 nm under different conditions with the omission of certain components or operations in comparison to CdSe QDs–TiO2–Ni(OH)2 obtained from 0.4 mg NiCl2·6 H2O and 5.0 mg TiO2 preassembled with 0.5 mL of CdSe QDs stock solution (2.0 105 m) in 10.0 mL of IPA/H2O (1:1 v/v, pH 11.0). Error bars are based on three repeated experiments.
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the fact that TiO2 itself is inactive under visible-light illumination because of its large band gap (3.2 eV for anatase and 3.0 eV for rutile).[27] If Ni(OH)2 was absent, the efficiency decreased significantly. Moreover, a comparison of the hydrogen evolution efficiency of TiO2 and TiO2–Ni(OH)2 revealed that the introduction of Ni(OH)2 enhanced the hydrogen evolution efficiency of TiO2 from 0.26 to 88.9 mmol under the illumination of a high-pressure mercury lamp for 5 h (Figure S10). This suggests that Ni(OH)2 can function as an efficient hydrogen evolution catalyst. However, if no TiO2 was present in the system, only (84.8 1) mmol hydrogen was generated, which suggests that TiO2 plays an important role in the system for hydrogen evolution. Strikingly, if CdSe QDs were not preassembled onto TiO2, the efficiency of the system for hydrogen evolution decreased remarkably. Simply mixing CdSe QDs and TiO2 at pH 11.0 could hardly afford the binary assembly of CdSe QDs– TiO2 (Figure S2). These results indicate that close contact and regulated assembly between CdSe QDs and TiO2 by MPA are necessary for highly efficient hydrogen evolution, which probably results from the enhanced efficiency of electron transfer between CdSe QDs and TiO2. Mechanistic insight into the photocatalytic hydrogen evolution If we consider the importance of TiO2 for efficient hydrogen evolution, we speculated that TiO2 acts as an electron mediator between CdSe QDs and Ni(OH)2 to facilitate vectorial electron transfer. To validate this speculation, we examined the energy levels of CdSe QDs and TiO2 in the assembly. The conduction band (CB) edge of the CdSe QDs is 1.6 V versus the normal hydrogen electrode (NHE; Supporting Information), and that of TiO2 (anatase) was calculated to be 0.26 V vs. NHE.[28] Therefore, electron transfer from the CB of CdSe QDs to that of TiO2 is thermodynamically feasible. Electron transfer from TiO2 (P25) to Ni(OH)2 has been demonstrated previously.[23a] Furthermore, the emission decay measurements of CdSe QDs (Figure 11 a) were investigated. According to the methods developed by Kamat et al.(see Supporting Information),[19a] the average lifetime of CdSe QDs decreased from 9.2 to 3.4 ns after assembly with TiO2. This demonstrates the electron transfer from CdSe QDs to TiO2. To prove the existence of electron transfer from TiO2 to Ni(OH)2, we measured the emission spectra of TiO2 and TiO2–Ni(OH)2 (Figure S11). After the introduction of Ni(OH)2 into the system, the band–band emission of TiO2 (380 ~ 420 nm) was quenched, which suggests possible electron transfer from TiO2 to Ni(OH)2.[23a, 29] Moreover, the lifetime of TiO2 at 410 nm decreased from 3.8 to 2.0 ns after the introduction of Ni(OH)2 (Figure 11 b), which confirms the existence of electron transfer from TiO2 to Ni(OH)2. On the basis of these results, we believe that vectorial electron transfer from CdSe QDs to TiO2 and then to Ni(OH)2 occurs in the CdSe QDs–TiO2– Ni(OH)2 ternary assembly. On the basis of the above studies, we suggest the mechanism for hydrogen evolution of the system (Scheme 2). Under visible-light illumination of the CdSe QDs–TiO2–Ni(OH)2 ternary assembly, electrons in CdSe QDs are first excited to their CB. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 11. a) Time-resolved emission decay of CdSe QDs (at 498 nm) deposited on a glass slide and in the CdSe QDs–TiO2 assembly. Excitation wavelength: 406 nm. b) Time-resolved emission decay of TiO2 (P25) at 410 nm before and after the introduction of Ni(OH)2 in the system. TiO2–Ni(OH)2 was prepared by the addition of NiCl2·6 H2O (0.4 mg) to a suspension of TiO2 (5.0 mg) in IPA/H2O (1:1 v/v, pH 11.0) followed by centrifugation and drying. Excitation wavelength: 375 nm.
Scheme 2. Proposed photocatalytic hydrogen evolution mechanism of the CdSe QDs–TiO2–Ni(OH)2 system.
Then, the majority of electrons shift to the CB of TiO2 for initial charge separation, and the holes left in the valence band (VB) of CdSe QDs are quenched quickly by IPA. Electrons in the CB of TiO2 then transfer to Ni(OH)2, which catalyzes protons into hydrogen effectively. Here, TiO2 acts as the electron mediator and facilitates vectorial electron transfer from the light-harvesting antenna CdSe QDs to the earth-abundant metal catalyst Ni(OH)2.
Conclusions We used mercaptopropionic acid as the linker and the in situ preparation of Ni(OH)2 to construct the ternary assembly of ChemSusChem 0000, 00, 1 – 9
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CHEMSUSCHEM FULL PAPERS CdSe quantum dots (QDs)–TiO2–Ni(OH)2 successfully for efficient hydrogen evolution. The CdSe QDs–TiO2–Ni(OH)2 assembly works effectively in a basic aqueous solution. Here, CdSe QDs function as the light-harvesting antenna, and TiO2 functions as the electron mediator to facilitate vectorial electron transfer from CdSe QDs to the non-noble-metal catalyst Ni(OH)2. Under optimal conditions for hydrogen evolution, the system yields an internal quantum yield of 27.4 % (410 nm) and a hydrogen evolution rate of 10.1 mmol g1 h1. Notably, the turnover frequency (5140 h1) with respect to CdSe QDs in the assembly is comparable to the best result for QDs systems obtained in an acidic environment. We believe that the design of such a ternary assembly could be applied to other systems in future.
Experimental Section
www.chemsuschem.org light illumination. For the experiments on internal quantum efficiency and visible light intensity, the sample was irradiated under monochromated light from an LED (160 mW cm2) at 410 nm with varied light intensity (monochromated light from a 410 nm LED, 160, 80, and 40 mW cm2). To calculate the internal quantum yield of the system, light-driven H2 production was performed in a standard spectro-cell with a total volume of ~ 4 mL and a path length of 1 cm that contained a suspension of CdSe QDs–TiO2–Ni(OH)2 (3.0 mL) obtained from NiCl2·6 H2O (0.12 mg) and TiO2 (1.5 mg) preassembled with CdSe QDs stock solution (0.15 mL, 2.0 105 m) in a mixed solution of isopropanol and water (1:1 v/v, pH 11.0). The system, which was under constant magnetic stirring, was then illuminated with a monochromated LED light source (l = 410 nm, light intensity 160 mW cm2 at the spectro-cell surface). The internal quantum yield (IQE) was calculated by using Equation (1): IQEð%Þ ¼
2 Number of photogenerated H2 molecules 100 Number of absorbed photons
Synthesis of CdSe QDs CdSe QDs were synthesized according to the procedures in our early work.[12e] In detail, CdCl2·2.5 H2O (46.0 mg, 0.20 mmol) and MPA (26 mL, 0.30 mmol) were dissolved in deionized H2O (190 mL). The pH value was adjusted to 11.0 by the addition of NaOH solution. After deoxygenation for 30 min, Na2SeSO3 prepared freshly (10.0 mL 5.0 mm) was injected into the solution. After further deoxygenation for 10 min, the reaction system was heated to reflux for 3.5 h to yield a transparent yellowish green solution. This solution was then cooled to RT and used as a stock solution.
Assembly of CdSe QDs with TiO2 (P25) To avoid QDs aggregation under acidic conditions, the assembly experiments were performed under basic conditions. Typically, MPA (30 mL) was added to a suspension that contained TiO2 (5.0 mg), NaOH (0.4 mL, 1 m), and CdSe QDs stock solution (1.5 mL). The volume was adjusted to 5.0 mL, and the pH value of the system was adjusted to 8.0 by the addition of HCl and NaOH aqueous solution. The suspension was stirred under ambient conditions for 30 min. After centrifugation, the supernatant was discarded to leave the light yellow CdSe QDs–TiO2 binary assembly.
ð1Þ in which the number of photogenerated H2 molecules was obtained from GC analysis and the number of absorbed photons was calculated from the illumination power and absorbance of the reaction solution. Specifically, the illumination power was measured at the front of the reaction cuvette by using a digital photodiode power meter. The amount of absorbed light was determined from the absorbance of CdSe QDs at the illumination wavelength. The measured power and estimated reflection/scattering loss of the cuvette front window is neglected as the average particle diameter is smaller than the wavelength of the light.
Acknowledgements This work was supported financially by the Ministry of Science and Technology of China (2014CB239402, 2013CB834505, and 2013CB834804), the National Science Foundation of China (21090343, 91027041, 21390404, 21403260, and 51373193), and the Chinese Academy of Sciences. We especially thank the Beijing Synchrotron Radiation Facility (BSRF) for kindly providing beam time for XAS experiments.
Formation of CdSe QDs–TiO2–Ni(OH)2 Typically, the CdSe QDs–TiO2 assembly prepared above was redispersed in a mixed solution of water and IPA (9.6 mL, 1:1 v/v). An aqueous solution of NiCl2·6 H2O (0.4 mL, 1.0 mg mL1) was added under stirring, and the pH value of the system was adjusted to 11.0. The ternary assembly was formed in situ.
Photocatalytic hydrogen evolution After the formation of CdSe QDs–TiO2–Ni(OH)2 assembly, the solution was used directly for photocatalytic hydrogen evolution. Before illumination, the solution was deoxygenated with Ar for 20 min. If TEA was used as the electron donor, TEA (1.0 mL) was added into H2O (9.0 mL) that contained NiCl2·6 H2O (0.4 mg) and CdSe QDs–TiO2 (5.0 mg) without adjustment of the pH value. Without specific notation, photocatalytic hydrogen evolution was performed in a Pyrex tube illuminated with a high-pressure mercury lamp (Hanovia, 500 W, 100 mW cm2). A glass filter was used to cut off light with a wavelength shorter than 400 nm to ensure visible 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Keywords: electron transfer photosynthesis · quantum dots
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hydrogen
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nickel
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Received: August 23, 2014 Revised: October 9, 2014 Published online on && &&, 0000
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FULL PAPERS Dotty about hydrogen: If mercaptopropionic acid is used as the linker, a turnover frequency of 5140 h1 based on CdSe quantum dots (QDs) for photocatalytic hydrogen evolution is achieved by a CdSe QDs–TiO2–Ni(OH)2 ternary assembly at pH 11. Vectorial electron transfer from CdSe QDs to TiO2 and then to Ni(OH)2 is crucial for this high efficiency.
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S. Yu, Z.-J. Li, X.-B. Fan, J.-X. Li, F. Zhan, X.-B. Li, Y. Tao, C.-H. Tung, L.-Z. Wu* && – && Vectorial Electron Transfer for Improved Hydrogen Evolution by Mercaptopropionic-Acid-Regulated CdSe Quantum-Dots–TiO2–Ni(OH)2 Assembly
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Vectorial Electron Transfer for Improved Hydrogen Evolution by Mercaptopropionic-Acid-Regulated CdSe Quantum-Dots–TiO2–Ni(OH)2 Assembly Shan Yu,[a] Zhi-Jun Li,[a] Xiang-Bing Fan,[a] Jia-Xin Li,[a] Fei Zhan,[b] Xu-Bing Li,[a] Ye Tao,[b] Chen-Ho Tung,[a] and Li-Zhu Wu*[a] A visible-light-induced hydrogen evolution system based on a CdSe quantum dots (QDs)–TiO2–Ni(OH)2 ternary assembly has been constructed under an ambient environment, and a bifunctional molecular linker, mercaptopropionic acid, is used to facilitate the interaction between CdSe QDs and TiO2. This hydrogen evolution system works effectively in a basic aqueous solution (pH 11.0) to achieve a hydrogen evolution rate of 10.1 mmol g1 h1 for the assembly and a turnover frequency of 5140 h1 with respect to CdSe QDs (10 h); the latter is comparable with the highest value reported for QD systems in an acidic environment. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and control experiments demonstrate
that Ni(OH)2 is an efficient hydrogen evolution catalyst. In addition, inductively coupled plasma optical emission spectroscopy and the emission decay of the assembly combined with the hydrogen evolution experiments show that TiO2 functions mainly as the electron mediator; the vectorial electron transfer from CdSe QDs to TiO2 and then from TiO2 to Ni(OH)2 enhances the efficiency for hydrogen evolution. The assembly comprises light antenna CdSe QDs, electron mediator TiO2, and catalytic Ni(OH)2, which mimics the strategy of photosynthesis exploited in nature and takes us a step further towards artificial photosynthesis.
Introduction With the depletion of traditional fossil fuels reserves and the concern for environmental problems such as global warming caused by the continuous burning of fossil fuels, the development of renewable, clean energy sources is becoming an urgent and important subject to pursue. Hydrogen, as a sustainable, clean energy carrier, is particularly attractive because of its specific enthalpy of combustion and benign combustion product (water). In an effort to make hydrogen a competitive alternative energy source and facilitate the transition to a hydrogen economy, cost-effective, sustainable, and efficient hydrogen production is required.[1] In this regard, photosynthesis provides a paradigm for conversion from a renewable resource such as water or biomass to hydrogen by solar energy.[2] Nature long ago figured out how to use photosynthetic com[a] Dr. S. Yu,+ Dr. Z.-J. Li,+ X.-B. Fan, J.-X. Li, X.-B. Li, Prof. C.-H. Tung, Prof. L.-Z. Wu Key Laboratory of Photochemical Conversion and Optoelectronic Materials Technical Institute of Physics and Chemistry the Chinese Academy of Sciences Beijing 100190 (P.R. China) Fax: (+ 86) 10-8254-3580 E-mail: [email protected] [b] F. Zhan, Prof. Y. Tao Beijing Synchrotron Radiation Facility Institute of High Energy Physics the Chinese Academy of Sciences Beijing 100049 (P.R. China) [+] 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.201402885.
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plexes to capture sunlight. With precise control of three indispensible processes in photosynthesis, that is, light harvesting, charge separation, and catalysis, solar energy is converted by a chain of photoinduced electron transfer processes and stored in the form of hydrogen. These highly regulated photosynthetic systems have inspired scientists to develop similar functional integrated artificial systems for hydrogen evolution. Indeed, a variety of artificial systems, which include molecular systems or hybrid systems that comprise semiconductors and molecules have been reported.[3] The directional assembly of an antenna for light harvesting, an electron mediator for fast forward charge separation but slow back charge recombination, and a catalyst for hydrogen evolution in these systems is of great importance to achieve high efficiency. In this work, we report a new ternary assembly of CdSe quantum dots (QDs)– TiO2–Ni(OH)2 for hydrogen production and show that the traditional semiconductor TiO2 functions efficiently as the electron mediator to facilitate vectorial electron transfer from CdSe QDs to TiO2 and then from TiO2 to Ni(OH)2, which is responsible for the efficient hydrogen evolution of the integrated artificial system. TiO2 has benefits of cost effectiveness, stability against photochemical corrosion, and nontoxicity and has been studied widely in photocatalysis since pioneering work by Fujishima and Honda in 1972.[4] However, its wide band gap limits its response to the UV light range, which only accounts for 5 % of the full solar photon flux.[5] To achieve visible-light-responsive composites, the introduction of heteroatoms into TiO2 to decrease its band gap or the introduction of chromophores or ChemSusChem 0000, 00, 1 – 9
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CHEMSUSCHEM FULL PAPERS semiconductors onto TiO2 for sensitization have been suggested.[6] Recently, QDs have emerged as a choice to sensitize TiO2.[7] Compared to traditional molecular photosensitizers, QDs exhibit size-dependent spectroscopic properties and have a large extinction coefficient over a broad spectral range, which should lead to an improved light-harvesting performance.[8] The quantum confinement and the large surfaceto-volume ratio in QDs enhance the surface amplitude of the electrons and holes significantly.[9] Therefore, QDs attached TiO2 not only respond to visible light but also benefit from vectorial electron transfer to obtain charge carriers. Electron transfer from CdSe QDs to TiO2 has been studied thoroughly by Kamat et al. and usually occurs on the timescale of 107– 1010 s1.[10] However, attention has been paid mainly to CdSe QDs–TiO2 systems for the enhancement of light conversion efficiency in solar cells (both Grtzel cells and photoelectrochemical cells).[11] Few examples of light-driven hydrogen evolution from QD-modified TiO2 systems have been reported. In addition, although several QDs systems with or without hydrogenevolving catalysts were shown to function as photocatalysts for hydrogen production,[12] systems for highly effective photochemical hydrogen production were performed under acidic conditions almost exclusively. In this contribution, the new ternary assembly of CdSe QDs– TiO2–Ni(OH)2 for hydrogen evolution works in basic aqueous solution (pH 11.0; Scheme 1). CdSe QDs, TiO2, and Ni(OH)2 in
Scheme 1. MPA-regulated CdSe QDs–TiO2–Ni(OH)2 ternary assembly for hydrogen evolution.
this assembly perform their duties as an antenna, an electron mediator, and a catalytic site, respectively, which mimics the strategy of photosynthesis to a large extent. To facilitate the vectorial electron transfer from CdSe QDs to TiO2, the bifunctional molecular linker mercaptopropionic acid (MPA) was used to assemble CdSe QDs and TiO2 because of the different pKa values of the thiol group and carboxylic acid, respectively.[13] More strikingly, Ni(OH)2 formed in situ upon the addition of earth-abundant Ni salts into the MPA-assembled CdSe QDs– TiO2 has been demonstrated to be an effective catalyst distributed on the surface of TiO2 for hydrogen evolution. Upon irradiation by visible light (l > 400 nm) for 10 h, the CdSe QD– TiO2–Ni(OH)2 assembly exhibits a turnover number (TON) of 51 400 with respect to CdSe QDs in a solution of isopropanol 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org (IPA) and water (1:1 v/v) at pH 11.0, which corresponds to a turnover frequency (TOF) of 5140 h1; this is much higher than that of other QDs systems that work under basic conditions[14] and comparable to the highest value of QDs systems reported under acidic conditions.[15] In addition, the internal quantum yield of the system and the rate of hydrogen production for the CdSe QD–TiO2–Ni(OH)2 assembly reach 27.4 % (at 410 nm) and 10.1 mmol g1 h1, respectively.
Results and Discussion Construction of the CdSe QDs–TiO2–Ni(OH)2 ternary assembly The ternary assembly of CdSe QDs–TiO2–Ni(OH)2 for photocatalytic hydrogen evolution in this work is obtained by the initial construction of CdSe QDs–TiO2. Here, the bifunctional molecular linker MPA is used to assist the assembly of CdSe QDs with TiO2, in which the thiolate group has a good affinity with Cd from the QDs and the carboxylate group binds Ti from TiO2 effectively. In contrast to other linker-assisted adsorption (LAA) methods,[16] the LAA experiment was performed in aqueous solution. Given that the thiolate group dissociates from the thiol group (pKa = 10–11[13]) and the carboxylate group from carboxylic acid (pKa = 4–5[13]) in MPA at different pH values in water, we adjusted the pH of the aqueous solution to optimize the assembly process. To simplify the MPA-assisted assembly of CdSe QDs onto TiO2 in aqueous solution, CdSe QDs capped with MPA were synthesized directly from water.[12e] This not only avoids the introduction of other components in the system[17] but also omits a ligand-exchange step, which is usually necessary for oil-soluble QDs capped with organic ligands.[18] The as-prepared water-soluble CdSe QDs have a concentration of 2.0 105 m with a molar extinction coefficient of 2.9 104 cm1 m1 at the first excitonic band (433 nm) and their average size was determined to be around 1.9 nm (Figure S1). After the addition of commercial TiO2 powder (P25) with stirring, the amount of CdSe QDs adsorbed on TiO2 was determined by monitoring the absorption changes of the supernatant in the suspension. A larger absorbance of the supernatant suggests fewer CdSe QDs on TiO2. The increased concentration of MPA added to the system led to a larger amount of CdSe QDs anchored on TiO2 (Figure 1), which indicates the effectiveness of MPA as the linker for the assembly process. If 30 mL MPA is used, the maximal volume of CdSe QDs stock solution (2.0 105 m) that could anchor on 5.0 mg TiO2 was 1.5 mL (Figure 2). Typically, the assembly process reaches saturation within 30 min, which is significantly faster than that described in other reports.[10c, 11c, 19] As expected, an optimal pH exists for the assembly of TiO2 with CdSe QDs. To avoid the aggregation of MPA-capped CdSe QDs under acid conditions,[18, 20] the pH of the adsorption system was adjusted to be higher than 7.0. Similar to that in an early report,[13a] the optimal pH for the adsorption process is 8.0 over a wide pH range from 8.0–13.0 (Figure 3). Under these conditions, the surface of TiO2 is negatively charged beChemSusChem 0000, 00, 1 – 9
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Figure 1. Absorption spectra of the supernatant from the assembly systems that contain 1.5 mL of CdSe QDs stock solution (2.0 105 m) and 5.0 mg of TiO2 with a total volume of 5.0 mL at pH 8.0 in water after stirring for 30 min. Different amounts of MPA were added into the systems to investigate the effect of MPA on the assembly process.
Figure 2. Absorption spectra of the supernatant from the assembly systems that contain different amounts of CdSe QDs stock solution (2.0 105 m), 5.0 mg of TiO2, and 30 mL MPA with a total volume of 5.0 mL at pH 8.0 in water after stirring for 30 min. The dashed line corresponds to absorption spectrum of CdSe QDs (6.0 106 m).
Figure 3. Absorbance (at 418 nm) of the supernatant from the adsorption systems that contain 2.5 mL of CdSe QDs stock solution (2.0 105 m), 5.0 mg TiO2, and 30 mL MPA with a total volume of 5.0 mL at different pH values in water after stirring for 30 min. Error bars are based on three repeated experiments.
cause the point of zero charge of TiO2 (P25) is 6–7.[21] The surface of CdSe QDs capped with MPA should also be negatively charged under basic conditions as the pKa of the carboxylic acid group in MPA is 4–5.[13] A relatively less basic environment 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org at pH 8.0 would minimize the repulsion between TiO2 and CdSe QDs to some extent, which facilitates the interaction of TiO2 and CdSe QDs for assembly. In addition, anther optimal pH for assembly was noted at 12.0. This is because the dominant sulfur species in MPA is a thiol group at pH 8.0 as its pKa is 10–11.[13] A higher pH could benefit the dissociation of the thiol group into thiolate, which has a binding energy to Cd (1283 kJ mol1) that is approximately 40 times higher than that for the thiol group to Cd (34.7 kJ mol1).[13b, 22] Therefore, at pH 12.0, MPA with the main form of thiolate could bind to CdSe QDs more efficiently to promote the assembly process. Following centrifugation and the abandonment of the supernatant from the assembly suspension with 0.5 mL CdSe QDs (2.0 105 m) assembled on TiO2 (5.0 mg), we examined the UV/Vis diffuse reflectance spectra (DRS) of the precipitates. The characteristic absorption of CdSe QDs suggested the possible assembly of CdSe QDs with TiO2 (Figure 4). High-resolu-
Figure 4. UV/Vis DRS spectra of blank TiO2 (P25), CdSe QDs–TiO2, and CdSe QDs–TiO2–Ni(OH)2. Inset: magnified view.
tion transmission electron microscopy (HRTEM) images show that small particles with a size of 2.0 nm are distributed on the surface of TiO2 uniformly after assembly (Figure 5). The crystal lattice of CdSe QDs with an interplanar spacing of 2.12 [zinc blend, (2 2 0)] could be observed clearly, which confirms the assembly of CdSe QDs with TiO2. Next, we introduced Ni salts into the system to construct the ternary CdSe QDs–TiO2–Ni(OH)2 assembly. Specifically, earth-abundant NiCl2·6 H2O was introduced into CdSe QDs– TiO2 suspension of H2O/IPA (1:1 v/v) at pH 11.0. UV/Vis DRS spectra of the precipitates obtained from this suspension shows the characteristic absorption of CdSe QDs (Figure 4) with the appearance of a weak and broad absorption that ranges from l = 600–900 nm, which could be assigned to the d–d transition of Ni2+ ions.[23] Clearly, Ni2+ ions are deposited on the surface of CdSe QDs–TiO2. Furthermore, energy dispersive X-ray spectroscopy (EDX) and the related elemental mapping confirm the presence of Ni on the surface of TiO2 (Figure S3). On the basis of the concentration of Ni salts (1.7 104 m) and OH anions (pH 11.0) in our system as well as the solubility product of Ni(OH)2 (pKsp = 15.26[24]), we consider that most of the Ni2+ ions should precipitate from the suspension in the form of hydroxide. Yu and coworkers have demonstratChemSusChem 0000, 00, 1 – 9
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Figure 5. HRTEM images of TiO2 (P25) a) before and b), c), and d) after assembly with CdSe QDs.
ed that pre-precipitated Ni(OH)2 could combine with TiO2, CdS, or even graphitic-C3N4 for hydrogen evolution.[23] X-ray photoelectron spectroscopy (XPS) suggests the possible existence of Ni(OH)2 from the two peaks of Ni 2p at binding energies (BEs) of 855.61 and 873.46 eV separated by 17.85 eV (Figure 6), which resembles the spectrum of commer-
Figure 6. XPS spectra of CdSe QDs–TiO2–Ni(OH)2 and commercial Ni(OH)2 powder.
cial Ni(OH)2 powder (ref. Ni(OH)2), with peaks of Ni 2p3/2 at BE = 855.14 eV and Ni 2p1/2 at BE = 872.92 eV separated by 17.78 eV. The small shift of the Ni 2p peak to a higher BE in our sample may be caused by the hydration of Ni(OH)2 in aqueous solution. Furthermore, we investigated the samples by X-ray absorption spectroscopy (XAS), which can provide information such as the oxidation state, local coordination numbers, and identity of the neighbors of the absorbing atom. The Ni K-edge X-ray absorption near-edge astructure (XANES) of the XAS spectra shows that the edge absorption of our sample overlaps well with that of commercial reference Ni(OH)2 (Figure 7 a), which demonstrates the existence of Ni(OH)2 in our sample strongly. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 7. a) Comparison of Ni K-edge XANES spectra of CdSe QDs–TiO2-– Ni(OH)2 with reference samples. b) The k3-weighted Fourier-transformed Ni K-edge EXAFS data (exp.) and the fitting for CdSe QDs–TiO2--Ni(OH)2. Inset: k-space fitting of Ni(OH)2 to our ternary assembly.
This is also in good agreement with the XPS data. Moreover, extended X-ray absorption fine structure (EXAFS) analysis ( Figure 7 b and Table S1) gives us information about the coordination environment of Ni2+. The nearest-neighbor NiO and Ni Ni distances in our sample are 2.04 and 3.03 , respectively, which match those of a-Ni(OH)2. On the basis of the above XPS and XAS data, we believe that Ni indeed exists in the form of Ni(OH)2 in our sample. Inductively coupled plasma optical emission spectroscopy (ICP-OES) of CdSe QDs–TiO2–Ni(OH)2 was performed to determine the amount of Cd and Ni (relative to TiO2) in the assembly, which was 1.14 and 1.75 wt %, respectively. This corresponds to 88 % of the Ni2+ ions added into the system that precipitate on the assembly. The large Ni-to-Cd molar ratio of 3.0 suggests clearly that Ni(OH)2 is in excess with respect to CdSe QDs. The small molar ratio of Cd to TiO2 (0.81 %) indicates that TiO2 has a much larger surface area to contact with Ni2+ ions from solution than CdSe QDs, which could also be conjectured from the HRTEM images (Figure 5 b). In this situation, the majority of Ni(OH)2 should be distributed on the surface of TiO2. This is further confirmed by a HRTEM study of CdSe QDs– TiO2–Ni(OH)2. The lattice distances of 2.12 and 2.33 , which correspond to the (220) and (101) faces of CdSe QDs and Ni(OH)2,[25] respectively, together with their specific distributions in different TiO2 surface regions were observed clearly (Figure 8).
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Figure 9. Amount of hydrogen evolved from CdSe QDs–TiO2–Ni(OH)2 obtained from 0.4 mg NiCl2·6 H2O and 5.0 mg TiO2 preassembled with 0.5 mL of CdSe QDs stock solution (2.0 105 m) in 10.0 mL of IPS/H2O (1:1 v/v) under irradiation (l > 400 nm) at different pH values. Error bars are based on three repeated experiments.
Figure 8. HRTEM images of a) and b) CdSe QDs–TiO2–Ni(OH)2 and c) and d) magnified areas of a) and b), respectively.
Photocatalytic hydrogen evolution by CdSe QDs–TiO2– Ni(OH)2 ternary assembly Photocatalytic experiments for hydrogen evolution were performed directly in the IPA/H2O solution (1:1 v/v) that contained the CdSe QDs–TiO2–Ni(OH)2 assembly. After 10 h illumination under visible light, (514 15) mmol [(11.5 0.3) mL] hydrogen was detected (average of 20 experimental results) with 0.5 mL CdSe QDs (2.0 105 m) preassembled on TiO2 (5.0 mg), which corresponds to a hydrogen evolution rate of 10.1 mmol g1 h1 for the CdSe QDs–TiO2–Ni(OH)2 assembly and a TON of 51 400 with respect to CdSe QDs; the latter yields a TOF of 5140 h1. If we used a light-emitting diode (LED; 410 nm) as the light source, we calculated an internal quantum efficiency of 27.4 % without the consideration of scattered light. After the first illumination of 10 h, the solid sample in the suspension was collected and redispersed in fresh IPA/H2O solution (1:1 v/v, pH 11.0) to use in a second and even third round of irradiation in which the efficiency of the system was almost unchanged (Figure S8), which confirms that the catalyst for hydrogen evolution is from the assembly itself and it is stable under a long irradiation time. Notably, the present hydrogen evolution system works most efficiently under relatively basic conditions (pH 11.0). The efficiency of hydrogen evolution is improved if the pH of the system is increased from 7.0 to 11.0 and then it declined at pH 12.0 (Figure 9). Several factors could contribute to the result. First, a basic environment is more beneficial for the formation of the catalytic Ni(OH)2 sites than acidic or neutral conditions. Second, at pH 11.0, MPA, the capping ligand of QDs, is bound strongly to CdSe QDs in the form of thiolate, which decreases the departure of the protecting group from the surface of CdSe QDs and hence improves the stability of CdSe QDs.[26] Third, as MPA is also the linker between CdSe QDs and 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
TiO2, a basic environment could help CdSe QDs assemble with TiO2 more strongly to increase the electron coupling between TiO2 and CdSe QDs, which could be beneficial for the electron transfer from CdSe QDs to TiO2.[13b] However, if we take into account the decrease of the concentration of protons for hydrogen evolution with the increasing pH of the system, moderately basic conditions (pH 11.0) are superb. To further confirm the hydrogen evolution ability of the CdSe QDs–TiO2–Ni(OH)2 assembly under basic conditions, IPA was replaced with triethylamine (TEA), an electron donor used widely for hydrogen evolution in a basic environment, and the catalytic activity of the assembly was maintained (Figure S7). Additionally, the efficiency of the CdSe QDs–TiO2–Ni(OH)2 assembly for hydrogen production also depends on the intensity of the visible light used (Figure S9). If a specific light intensity of incident visible light from an LED (410 nm LED, 160, 80, and 40 mW cm2) is monitored, the photocatalytic activity is increased proportionally to light intensity. Control experiments confirm that the omission of any component in the assembly decreases the efficiency of the system for hydrogen evolution remarkably (Figure 10). No hydrogen could be detected without CdSe QDs, which is consistent with
Figure 10. Amount of hydrogen evolved under illumination at l > 400 nm under different conditions with the omission of certain components or operations in comparison to CdSe QDs–TiO2–Ni(OH)2 obtained from 0.4 mg NiCl2·6 H2O and 5.0 mg TiO2 preassembled with 0.5 mL of CdSe QDs stock solution (2.0 105 m) in 10.0 mL of IPA/H2O (1:1 v/v, pH 11.0). Error bars are based on three repeated experiments.
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the fact that TiO2 itself is inactive under visible-light illumination because of its large band gap (3.2 eV for anatase and 3.0 eV for rutile).[27] If Ni(OH)2 was absent, the efficiency decreased significantly. Moreover, a comparison of the hydrogen evolution efficiency of TiO2 and TiO2–Ni(OH)2 revealed that the introduction of Ni(OH)2 enhanced the hydrogen evolution efficiency of TiO2 from 0.26 to 88.9 mmol under the illumination of a high-pressure mercury lamp for 5 h (Figure S10). This suggests that Ni(OH)2 can function as an efficient hydrogen evolution catalyst. However, if no TiO2 was present in the system, only (84.8 1) mmol hydrogen was generated, which suggests that TiO2 plays an important role in the system for hydrogen evolution. Strikingly, if CdSe QDs were not preassembled onto TiO2, the efficiency of the system for hydrogen evolution decreased remarkably. Simply mixing CdSe QDs and TiO2 at pH 11.0 could hardly afford the binary assembly of CdSe QDs– TiO2 (Figure S2). These results indicate that close contact and regulated assembly between CdSe QDs and TiO2 by MPA are necessary for highly efficient hydrogen evolution, which probably results from the enhanced efficiency of electron transfer between CdSe QDs and TiO2. Mechanistic insight into the photocatalytic hydrogen evolution If we consider the importance of TiO2 for efficient hydrogen evolution, we speculated that TiO2 acts as an electron mediator between CdSe QDs and Ni(OH)2 to facilitate vectorial electron transfer. To validate this speculation, we examined the energy levels of CdSe QDs and TiO2 in the assembly. The conduction band (CB) edge of the CdSe QDs is 1.6 V versus the normal hydrogen electrode (NHE; Supporting Information), and that of TiO2 (anatase) was calculated to be 0.26 V vs. NHE.[28] Therefore, electron transfer from the CB of CdSe QDs to that of TiO2 is thermodynamically feasible. Electron transfer from TiO2 (P25) to Ni(OH)2 has been demonstrated previously.[23a] Furthermore, the emission decay measurements of CdSe QDs (Figure 11 a) were investigated. According to the methods developed by Kamat et al.(see Supporting Information),[19a] the average lifetime of CdSe QDs decreased from 9.2 to 3.4 ns after assembly with TiO2. This demonstrates the electron transfer from CdSe QDs to TiO2. To prove the existence of electron transfer from TiO2 to Ni(OH)2, we measured the emission spectra of TiO2 and TiO2–Ni(OH)2 (Figure S11). After the introduction of Ni(OH)2 into the system, the band–band emission of TiO2 (380 ~ 420 nm) was quenched, which suggests possible electron transfer from TiO2 to Ni(OH)2.[23a, 29] Moreover, the lifetime of TiO2 at 410 nm decreased from 3.8 to 2.0 ns after the introduction of Ni(OH)2 (Figure 11 b), which confirms the existence of electron transfer from TiO2 to Ni(OH)2. On the basis of these results, we believe that vectorial electron transfer from CdSe QDs to TiO2 and then to Ni(OH)2 occurs in the CdSe QDs–TiO2– Ni(OH)2 ternary assembly. On the basis of the above studies, we suggest the mechanism for hydrogen evolution of the system (Scheme 2). Under visible-light illumination of the CdSe QDs–TiO2–Ni(OH)2 ternary assembly, electrons in CdSe QDs are first excited to their CB. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 11. a) Time-resolved emission decay of CdSe QDs (at 498 nm) deposited on a glass slide and in the CdSe QDs–TiO2 assembly. Excitation wavelength: 406 nm. b) Time-resolved emission decay of TiO2 (P25) at 410 nm before and after the introduction of Ni(OH)2 in the system. TiO2–Ni(OH)2 was prepared by the addition of NiCl2·6 H2O (0.4 mg) to a suspension of TiO2 (5.0 mg) in IPA/H2O (1:1 v/v, pH 11.0) followed by centrifugation and drying. Excitation wavelength: 375 nm.
Scheme 2. Proposed photocatalytic hydrogen evolution mechanism of the CdSe QDs–TiO2–Ni(OH)2 system.
Then, the majority of electrons shift to the CB of TiO2 for initial charge separation, and the holes left in the valence band (VB) of CdSe QDs are quenched quickly by IPA. Electrons in the CB of TiO2 then transfer to Ni(OH)2, which catalyzes protons into hydrogen effectively. Here, TiO2 acts as the electron mediator and facilitates vectorial electron transfer from the light-harvesting antenna CdSe QDs to the earth-abundant metal catalyst Ni(OH)2.
Conclusions We used mercaptopropionic acid as the linker and the in situ preparation of Ni(OH)2 to construct the ternary assembly of ChemSusChem 0000, 00, 1 – 9
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CHEMSUSCHEM FULL PAPERS CdSe quantum dots (QDs)–TiO2–Ni(OH)2 successfully for efficient hydrogen evolution. The CdSe QDs–TiO2–Ni(OH)2 assembly works effectively in a basic aqueous solution. Here, CdSe QDs function as the light-harvesting antenna, and TiO2 functions as the electron mediator to facilitate vectorial electron transfer from CdSe QDs to the non-noble-metal catalyst Ni(OH)2. Under optimal conditions for hydrogen evolution, the system yields an internal quantum yield of 27.4 % (410 nm) and a hydrogen evolution rate of 10.1 mmol g1 h1. Notably, the turnover frequency (5140 h1) with respect to CdSe QDs in the assembly is comparable to the best result for QDs systems obtained in an acidic environment. We believe that the design of such a ternary assembly could be applied to other systems in future.
Experimental Section
www.chemsuschem.org light illumination. For the experiments on internal quantum efficiency and visible light intensity, the sample was irradiated under monochromated light from an LED (160 mW cm2) at 410 nm with varied light intensity (monochromated light from a 410 nm LED, 160, 80, and 40 mW cm2). To calculate the internal quantum yield of the system, light-driven H2 production was performed in a standard spectro-cell with a total volume of ~ 4 mL and a path length of 1 cm that contained a suspension of CdSe QDs–TiO2–Ni(OH)2 (3.0 mL) obtained from NiCl2·6 H2O (0.12 mg) and TiO2 (1.5 mg) preassembled with CdSe QDs stock solution (0.15 mL, 2.0 105 m) in a mixed solution of isopropanol and water (1:1 v/v, pH 11.0). The system, which was under constant magnetic stirring, was then illuminated with a monochromated LED light source (l = 410 nm, light intensity 160 mW cm2 at the spectro-cell surface). The internal quantum yield (IQE) was calculated by using Equation (1): IQEð%Þ ¼
2 Number of photogenerated H2 molecules 100 Number of absorbed photons
Synthesis of CdSe QDs CdSe QDs were synthesized according to the procedures in our early work.[12e] In detail, CdCl2·2.5 H2O (46.0 mg, 0.20 mmol) and MPA (26 mL, 0.30 mmol) were dissolved in deionized H2O (190 mL). The pH value was adjusted to 11.0 by the addition of NaOH solution. After deoxygenation for 30 min, Na2SeSO3 prepared freshly (10.0 mL 5.0 mm) was injected into the solution. After further deoxygenation for 10 min, the reaction system was heated to reflux for 3.5 h to yield a transparent yellowish green solution. This solution was then cooled to RT and used as a stock solution.
Assembly of CdSe QDs with TiO2 (P25) To avoid QDs aggregation under acidic conditions, the assembly experiments were performed under basic conditions. Typically, MPA (30 mL) was added to a suspension that contained TiO2 (5.0 mg), NaOH (0.4 mL, 1 m), and CdSe QDs stock solution (1.5 mL). The volume was adjusted to 5.0 mL, and the pH value of the system was adjusted to 8.0 by the addition of HCl and NaOH aqueous solution. The suspension was stirred under ambient conditions for 30 min. After centrifugation, the supernatant was discarded to leave the light yellow CdSe QDs–TiO2 binary assembly.
ð1Þ in which the number of photogenerated H2 molecules was obtained from GC analysis and the number of absorbed photons was calculated from the illumination power and absorbance of the reaction solution. Specifically, the illumination power was measured at the front of the reaction cuvette by using a digital photodiode power meter. The amount of absorbed light was determined from the absorbance of CdSe QDs at the illumination wavelength. The measured power and estimated reflection/scattering loss of the cuvette front window is neglected as the average particle diameter is smaller than the wavelength of the light.
Acknowledgements This work was supported financially by the Ministry of Science and Technology of China (2014CB239402, 2013CB834505, and 2013CB834804), the National Science Foundation of China (21090343, 91027041, 21390404, 21403260, and 51373193), and the Chinese Academy of Sciences. We especially thank the Beijing Synchrotron Radiation Facility (BSRF) for kindly providing beam time for XAS experiments.
Formation of CdSe QDs–TiO2–Ni(OH)2 Typically, the CdSe QDs–TiO2 assembly prepared above was redispersed in a mixed solution of water and IPA (9.6 mL, 1:1 v/v). An aqueous solution of NiCl2·6 H2O (0.4 mL, 1.0 mg mL1) was added under stirring, and the pH value of the system was adjusted to 11.0. The ternary assembly was formed in situ.
Photocatalytic hydrogen evolution After the formation of CdSe QDs–TiO2–Ni(OH)2 assembly, the solution was used directly for photocatalytic hydrogen evolution. Before illumination, the solution was deoxygenated with Ar for 20 min. If TEA was used as the electron donor, TEA (1.0 mL) was added into H2O (9.0 mL) that contained NiCl2·6 H2O (0.4 mg) and CdSe QDs–TiO2 (5.0 mg) without adjustment of the pH value. Without specific notation, photocatalytic hydrogen evolution was performed in a Pyrex tube illuminated with a high-pressure mercury lamp (Hanovia, 500 W, 100 mW cm2). A glass filter was used to cut off light with a wavelength shorter than 400 nm to ensure visible 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Keywords: electron transfer photosynthesis · quantum dots
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hydrogen
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nickel
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Received: August 23, 2014 Revised: October 9, 2014 Published online on && &&, 0000
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FULL PAPERS Dotty about hydrogen: If mercaptopropionic acid is used as the linker, a turnover frequency of 5140 h1 based on CdSe quantum dots (QDs) for photocatalytic hydrogen evolution is achieved by a CdSe QDs–TiO2–Ni(OH)2 ternary assembly at pH 11. Vectorial electron transfer from CdSe QDs to TiO2 and then to Ni(OH)2 is crucial for this high efficiency.
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S. Yu, Z.-J. Li, X.-B. Fan, J.-X. Li, F. Zhan, X.-B. Li, Y. Tao, C.-H. Tung, L.-Z. Wu* && – && Vectorial Electron Transfer for Improved Hydrogen Evolution by Mercaptopropionic-Acid-Regulated CdSe Quantum-Dots–TiO2–Ni(OH)2 Assembly
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