Shell Thickness Engineering Significantly Boosts the Photocatalytic

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Shell Thickness Engineering Significantly Boost the Photocatalytic H2 Evolution Efficiency of CdS/CdSe Core/Shell Quantum Dot Ping Wang, Minmin Wang, Jie Zhang, Chuanping Li, Xiaolong Xu, and Yongdong Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07211 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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ACS Applied Materials & Interfaces

Shell Thickness Engineering Significantly Boost the Photocatalytic H2 Evolution Efficiency of CdS/CdSe Core/Shell Quantum Dot Ping Wang,*,† Minmin Wang, †,‡ Jie Zhang,† Chuanping Li,†,‡ Xiaolong Xu,† and Yongdong Jin*,† †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Changchun 130022, Jilin, P. R. China. ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

KEYWORDS catalysis, semiconductor quantum dot, core/shell, hydrogen photogeneration, cadmium

ACS Paragon Plus Environment

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ABSTRACT Colloidal semiconductor quantum dot (QD) has recently emerged as a good candidate for the photocatalytic hydrogen (H2) evolution in water. A further understanding of the factors that can affect and boost the catalytic activity of the QD based H2-generating system is of great importance for the future design of such system for practical use. Here, we report on the fine shell-thickness engineering of the colloidal CdS/CdSe core/shell QDs and its effect on the photocatalytic H 2 production in water. Our results show that, with the proper shell thickness, the H2 photogeneration quantum yield (ΦH2) of CdS/CdSe core/shell QDs could reach 30.9 % under the illumination of 420 nm light, which is 49 % larger than that of CdS core. Furthermore, the underlying mechanism has also been tentatively proposed and discussed.


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1. INTRODUCTION The obtaining of Hydrogen (H2), a clean and highly efficient energy source, by utilizing the solar energy is one of the most active research fields for decades.1-7 Among various routes, the colloidal semiconductor quantum dot (QD)-based multicomponent H2-producing artificial (HPA) system has gained significant interests due to the merits of high efficiency, relatively low-cost and good stability.1-2, 6-20 As the light absorber of the system, semiconductor QD is responsible for absorbing and converting the incident light into the charge carriers (electron-hole pairs) which are further used to participate the proton reduction related reactions. The charge carrier recombination and transfer in and between the QD and co-catalyst or sacrificial electron donor are considered to have great impact on H2 evolution efficiency of the system. Investigations on quasi-type II CdSe/CdS dot-in-rod nanorods based systems revealed that by tuning the CdSe seed size and aspect ratio of the nanorods, the heterostructure exhibits the high activity for H2 photogeneration due to the efficient charge carrier separation.12-13 Further selective etching treatment results in 3-4 times enhancement of the catalytic activity, which is attributed to the fast hole transfer by direct contacting of the CdSe domain, where the photo-induced holes are trapped, with the hole scanvenger.9 Despite these advances, more efforts are still needed to develop such QD-based HPA system and gain more insights into it. Recently, we reported a new type of QD, the reverse type I CdS/CdSe core/shell QD, that exhibits ~ 5 times higher activity for H2 production than that of CdSe/CdS with the similar core and overall sizes.18 And this distinct difference in H2 production efficiency is attributed to the variation in charge carrier location, differing in the shell and core region, for the CdS/CdSe and CdSe/CdS core/shell QDs, respectively. Our preliminary results revealed that besides the increase


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of the charge carrier separation efficiency, the trend to move the charge carrier to the surface region of QD also plays a key role for enhancing the catalytic activity of the QD system. Here, we investigate the effect of the finely tuned shell thickness of CdS/CdSe core/shell QD on the H2 photogeneration efficiency of the system. A strong correlation between the thickness of CdSe shell and the catalytic H2 production activity of the engineered core/shell QD has been revealed, and the possible mechanism has also been tentatively discussed. By using the Ni-{3mercaptopropionic acid, (3-MPA)}* as the co-catalyst,18 under the proper conditions, the CdS/CdSe core/shell QD based HPA system gives the H2 photogeneration quantum yield (ΦH2) as high as 30.9 % under the illumination of 420 nm light. 2. MATERIALS AND METHODS 2.1. Chemicals. Cadmium oxide (CdO, 99.998%), Sulfur (99.5%, powder, 100 mesh), and NiCl2·6H2O (99.9998%) were purchased from Alfa Aesar. Octadecene (ODE, tech. 90%), Octadecylamine (ODA, 90%), Oleic acid (OA, tech. 90%), Selenium (99.5%, 100 mesh), Stearic acid (HSt, reagent grade, 95%) and 3-Mercaptopropionic acid (3-MPA, ≥ 99%) were purchased from Aldrich. L-Ascorbic acid (AA, reagent grade, 99%) was obtained from Vetec. Sodium sulfide nonahydrate (Na2S·9H2O, 99.99%) and n-Butylamine (≥ 99.5%) were obtained from Aladdin. Formamide (FA, ≥ 99.0%) were purchased from Xilong Chemicals. Hydrochloric acid (HCl, 36 ~ 38%), Acetonitrile (≥ 99.0%) and Sodium hydroxide (NaOH, ≥ 96%) were purchased from Beijing Chemicals. All chemicals were used as received without any further purification. 2.2. Synthesis of CdS core. The synthesis was performed according to the previous works.18, 21 For a typical synthesis, 0.1 mmol of CdO, 1.8 mmol of OA and 4.4 mL ODE were loaded into a 25 mL three-neck flask, then the solution was pumped for ~10 min to eliminate the oxygen. After that, the solution was heated to 260 ℃ under the flow of argon until CdO is completely dissolved.


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At this point, 1 mL of sulfur ODE solution containing 0.05 mmol sulfur was injected to initiate the reaction. After keeping the solution at 230 ℃ at ~ 7 min, the reaction was stopped by cooling the solution to the room temperature. Then, the stock solution was subjected to the procedure22 to remove the side products and unreacted precursors. After the purification, the CdS core was dispersed into hexane for the further CdSe shell growth. The concentration of CdS core was determined by the previous reported method.22 2.3. Synthesis of CdS/CdSe core/shell QD with different shell thickness. 50 nmol of purified CdS core in hexane was loaded into a 50 mL three-neck flask containing 10 mL of ODE and 1.0 g of ODA. The solution was pumped at room temperature and at 120 ℃ for 1 h and 30 min, respectively, to remove the hexane and oxygen in the solution. Then, the solution was switched into the argon flow and heated to 230 ℃ for the shell growth. Certain amount of Cd (0.2 M) and Se (0.2 M) precursor ODE solution was alternatively injected into the solution, and each growth were completed at 260 ℃ for ~ 20 min. The CdSe shell thickness was controlled by the amount of the precursor added. The Cd precursor solution was prepared by dissolving 0.62 g of CdO in 14 mL of OA and 11 mL of ODE at 230 ℃. The Se precursor solution was prepared by dispersing the selenium powder in ODE at room temperature, and before the injection, the solution was sonicated for ~1 min. After the reaction solution was cooled to room temperature, it was subjected to the standard procedure to remove the side products, unreacted precursors and excess ligand.22 After the purification process, the as-synthesized CdS/CdSe core/shell QDs were dispersed into 20 mL of toluene for further usage. 2.4. Synthesis of CdSe with different sizes. The synthesis was according the reported method.23 For the synthesis of 2.9 nm CdSe, 0.2 mmol of CdO, 1 mmol of HSt and 4mL of QDE were loaded into a 25 mL three-neck flask. After the solution was pumped for ~10 min, the solution was


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switched to argon flow and heated to 240 ℃ to dissolve CdO. At this point, 1 mL of Se precursor solution (0.1 M) was injected to initiate the reaction. The solution was kept at 240 ℃ for further ~ 10 min. to complete the reaction. For the synthesis of CdSe with larger size, 0.2 mmol of CdO, 1 mmol of HSt and 4 mL of ODE were loaded into a 25 mL three-neck flask. The solution was then pumped for ~10 min and switched into the argon flow. After the solution was heated to 250 ℃, 0.5 mL of 0.1 M Se precursor solution was injected to initiate the reaction. After ~10 min, 0.1 mL of Se precursor solution was injected to further grow the CdSe. After ~ 4 min, another 0.05 mL of Se precursor solution was added. After that, multiple injections with 0.03 mL of Se precursor solution were repeated every ~ 4 min to get different sizes of CdSe, which was monitored by its first absorption peak. The stock CdSe solution was purified according to the reported procedure.23 2.5. Ligand exchange process to make the CdS/CdSe core/shell QD water-soluble. The ligand exchange process was performed according to the previous works.18, 24 1.25 nmol of purified CdS/CdSe core/shell QD in toluene was mixed with 2 mL of Na2S FA solution (10 mg mL-1). The mixed solution was stirred for ~ 15 min at room temperature to complete the ligand transfer process. Then the FA solution containing QD was separated and followed by adding 4 mL of acetonitrile to precipitate the QD. Next, the QD was dispersed into deionized water, and 0.2 mL of 6 M NaOH aqueous solution was added to precipitate QD again, which was repeated three times to remove the excess amount Na2S. 2.6. Characterizations. TEM, high-resolution TEM (HRTEM), and elemental mapping measurements were carried out by using a FEI TECNAI F20 EM with an accelerating voltage of 200 kV equipped with an energy dispersive spectrometer. TEM samples were prepared by placing a drop of the QD solution on a carbon-coated copper grid and drying under ambient conditions. The UV-Vis extinction spectra were obtained on a Shimazu UV-2600 UV-Vis spectrometer. The


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fluorescence emission spectra were performed on a Fluoromax-4 Spectrofluorometer (HORIBA Jobin YVON, INc NJ, USA) at room temperature. X-ray diffraction (XRD) patterns were recorded by a D8 ADVANCE (BRUKER) diffractometer with Cu Kα radiation (λ=1.54056 Å) in the range of 15-60º(2). Infrared spectra were carried out on a VERTEX 70 Fourier transform infrared (FTIR) spectrometer (Bruker). 2.7. Hydrogen evolution studies. The experiments were carried out on an airtight inner gas circulation system connected with a gas chromatography (Shimadzu GC-2014c, with a 5 Å molecular sieve column and a TCD detector) that could realize the online quantitative detection of H2. The experimental solution containing 0.025 M QDs (calculated based on the mole amount of CdS core), 100 M Ni-{3-MPA}* and 0.8 M AA was fixed to 50 mL and kept in the dark before use. The Ni-{3-MPA}* was obtained by introducing 100 M Ni2+ and 200 M 3-MPA into the solution. The pH of the solution was adjusted by adding HCl or NaOH solution and measured using a pH meter (PB-10, Sartorius). The temperature of the solution was maintained at 8 ℃ during the experiments. The solution was firstly degassed ~20 min to remove the residual air and then illuminated from the top of the reaction cell by using a 300 W Xeon lamp equipped with the filter of λ≥ 420 nm or single band pass filter (420, 450, 475, 500, 520, 550, 600, 650 and 700 nm, band width is 10 nm). All the measurements were performed according to the above conditions unless specified in this work. 2.8. Calculation of the quantum yield (ΦH2) and apparent quantum yield (ΦH2-app) of the HPA system. ΦH2 (orΦH2-app) is defined as the number of the electrons involved in the reduction reaction divided by the number of photons absorbed (or incident photons). The number of the electrons was determined by the mole amount of H2 generated, and the number of photons absorbed (incident photons) was calculated by the light power intensity difference between the solution without and


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with QDs (light intensity) and illumination time, while keeping other conditions the same. The light power intensity was determined a Thorlabs PM200 energy meter equipped with a S120VC probe. ΦH2 (orΦH2-app) is obtained by the following equations:2, 6-7

𝑝=

𝑐×ℎ×𝑛

𝑞𝑝 =

𝜆×𝑡 𝑛 𝑡

𝛷H2 (𝑜𝑟 𝛷H2−app ) =

(1) (2)

2×𝑘 𝒒𝒑

× 100% (3)

Where c is the speed of the light (m/s); h is the Planck’s constant (J·s); n is the number of photons absorbed (or incident photons); 𝜆 is the wavelength of the incident light; t is the time (s); 𝑞𝑝 is the photo-flux; k is the average generation rate of hydrogen (mol/s). 3. RESULTS AND DISCUSSION 3.1. CdS/CdSe core/shell QD characterization. The CdS/CdSe core/shell QD with different shell thickness was synthesized according to the previous works with a slight modification (see details in the materials and methods).18, 21-22 With the increase of the CdSe shell thickness in our synthesis, the first exciton absorption peak and photoluminescence (PL) emission peak are gradually redshifted from 417 and 432 nm to 608 and 632 nm, respectively (Figure 1 & Figure S1, Supporting Information). As shown in the inset of Figure 1, under the illumination of UV light (365 nm), the colloidal CdS/CdSe core/shell QD with different shell thickness exhibits PL emission at different colors, from blue to red. The existence of fine structure of UV-Vis spectra and relatively narrow FWHM (22 and 37 nm for CdS core and final CdS/CdSe core/shell QD, respectively) indicates the good monodispersity of the QDs (Figure 1). This is further confirmed by the transmission electron microscopy (TEM) observation (Figure 2), which shows that the mean size of CdS core is


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gradually increased from 2.9 to 5.2 nm after the CdSe shell coating (~ 0.55 monolayer (ML) for each shell growth), which equals to ~ 3.30 ML thickness, and for convenience, the corresponding QD is denoted as CdS/CdSe(3.30ML) thereafter. High-resolution TEM (HRTEM) images (Figure 2) demonstrate the good crystallinity of the QDs. X-ray diffraction (XRD) patterns (Figure 3) show that both CdS core and CdS/CdSe (3.30ML) core/shell QD are in the zinc-blend structure, and the increase of the intensity of the diffraction peak further confirms the increase of QD size. Only CdSe diffraction pattern was observed for the CdS/CdSe(3.30ML) QD is attributed to the thick shell of CdSe.25

Figure 1. UV-Vis and PL spectra of the organic ligand capped CdS core and CdS/CdSe(3.30ML) in toluene, the inset is the photograph of toluene solution of CdS/CdSe with different shell thickness from 0 to 3.30ML under the illumination of UV light (365nm).

Here, it should be noted that further increase the CdSe shell thickness of the QD was also attempted, and the corresponding first exciton absorption peak and PL emission peak can be further red-shifted to 657 and 676 nm, respectively (Figure S1, Supporting Information). However, as


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revealed by the TEM results (Figure S2, Supporting Information), the obtained QDs with much thicker CdSe shell showed relatively poor monodispersity as compared with the QD with shell thickness below 3.30 ML. This is assumed to be caused by the lower growth efficiency and possible independent nucleation and growth of pure CdSe QD during the growth of thick shell thickness. Therefore, to eliminate this interference on revealing the true correlation between the shell thickness and the catalytic activity of QD, we employed the CdS/CdSe QD with the shell thickness below 3.30 ML unless specified in this work.

Figure 2. Typical TEM, HRTEM images and size distribution histogram of CdS core (a, d and g), CdS/CdSe(1.65ML) (b, e and h) and CdS/CdSe(3.30ML) (c, f and i), respectively. The scale bar of TEM and HRTEM images is 20 and 5 nm, respectively. The size distribution histogram for each is obtained by counting more than 200 single nanoparticles. The insets of (a), (b) and (c) are the toluene solution containing the corresponding colloidal QD, respectively. The


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insets of (d), (e) and (f) are the enlarged HRTEM images of CdS core, CdS/CdSe(1.65ML), and CdS/CdSe(3.30ML), respectively. The scale bar is 5 nm.

Figure 3. XRD patterns of CdS core, CdS/CdSe(3.30ML) and 5.1 nm CdSe, respectively. The broad low signal at ~ 20°for CdS core pattern is attributed to the glass substrate. The standard diffraction patterns of bulk zinc blende CdS (JCPDS no. 65-2887, vertical lines at the bottom) and CdSe (JCPDS no. 65-2991, vertical lines at the top) are indicated as references.

3.2. Hydrogen photogeneration measurements. For H2 photogeneration in water, the assynthesized core/shell QDs were firstly needed to be transferred into water by using the inorganic ligand (S2- ions, note: although the S2- ions can also trap holes, however, due to the extremely low amount of S2- ions on the surface of QDs compared with that of AA in solution, AA plays the major role for acting as the sacrificial electron donor in our H2 photogeneration system, Figure S3, Supporting Information), which also facilitates the charge carriers transfer (see details in the materials and methods).18, 24, 26 A significant decrease in the intensities of the band at 2923 and 2852 cm-1 associated with C-H stretching vibrations27 (Figure 4) indicates the success of ligand


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exchange, and no obvious change of the UV-Vis spectra of the QDs (Figure S4, Supporting Information) demonstrates the negligible influence of the S2- ions, coating on the electronic band structure of the QD. The PL emission of QD after the ligand exchange process is negligible, which is ascribed to the strong PL quenching property of S2- ions.24 The catalytic activity of the QDs was then evaluated in an airtight inner gas circulation system connected to a gas chromatography instrument (see details in the materials and methods). AA and Ni-{3-MPA}* is used as the sacrificial electron donor and co-catalyst in our system. Here, it should be noted that the role of Ni-{3-MPA}* is to receive the photogenerated electrons from QDs and further use them to reduce protons to hydrogen. As we have reported recently, Ni-{3-MPA}* is a highly active and relatively stable co-catalyst for the photocatalytic hydrogen production, and it has been demonstrated to be a homogeneous and independently existing co-catalyst in the solution. 16

Figure 4. FTIR spectra of OA capped QD, CdS core and CdS/CdSe core/shell QD with different shell thickness after the ligand exchange process, respectively.

Figure 5a presents the variation of H2 generation rate of the CdS/CdSe QD-based HPA systems with the CdSe shell thickness varied from 0 to 3.30 ML under the illumination of visible


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light (≥ 420 nm). As shown in Figure 5a & Figure S5a, Supporting Information, with the increase of the shell thickness, the catalytic activity of the QD system is gradually increased. For the CdS core-based HPA system, the rate of H2 generation is only 0.16 mL h-1; while for that of CdS/CdSe (3.30 ML), it is significantly increased to 4.8 mL h-1, which accords for ~ 30 times enhancement. For the case of core/shell QD with much thicker CdSe shell thickness, its catalytic activity is further increased (Figure S5a, Supporting Information), and the rate of H2 generation of CdS/CdSe(8.70ML) reaches 9.8 mL h-1. However, it is clearly seen that the increasing trend of the H2 evolution efficiency of core/shell QD with the shell thickness beyond 3.30ML is decreased (Figure S5b, Supporting Information), which is assumed to be due to the monodispersity deterioration of QD. Alternative using of Pt as co-catalyst (see details in the materials and methods and Figure S6, Supporting Information) instead of Ni-{3-MPA}* also gives the similar trend, which further confirms that the enhancement is attributed to the increase of the CdSe shell thickness. 3.3 Mechanism analyses. As seen from Figure 1 & Figure S1, Supporting Information, the first exciton absorption peak of CdS/CdSe core/shell QDs is red-shifted with the increase of the shell thickness, which reflects the change of the band alignment of the core/shell QDs and hence the catalytic activity of the QDs. Figure 5b outlines the band edge variation of the QDs, estimated by Tauc plots,28 as a function of CdSe shell thickness. As shown, for the CdS core in our system, the band gap energy (Eg = 2.84 eV) is larger than the bulk value (Eg bulk = 2.4 eV), which is attributed to the quantum confinement effect.29 With the increase of the CdSe shell thickness, the band edge of QDs is gradually decreased from 2.84 to 1.92 eV (for detail data see Table S1 in the Supporting Information). Similar band edge transition has also been observed for the CdSe/CdS core/shell QDs with varying shell thickness.30 Here, it should be noted that the


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transferring of photogenerated electrons from QDs to the co-catalyst is possible. This is because although the band edge for CdS/CdSe(3.30ML) is 1.9 eV, it is still higher than that of bulk CdSe (Eg bulk = 1.74 eV, ECB at -0.6 V versus NHE at pH=6.817, 31). And, this difference would induce the negative shift of ECB of CdSe in the CdS/CdSe core/shell QDs, therefore, facilitating the transfer of photogenerated electrons from QDs to the co-catalyst (Ni-{3-MPA}*, -0.87 V versus NHE at pH = 5.2).18 Further experiments were performed to elucidate the underlying mechanism of the shell thickness-dependent catalytic performance of CdS/CdSe core/shell QDs. Firstly, the effect of light absorption change with the increase of the shell thickness was investigated. It is known that the increase of the size would result in the enhancement of QD’s light absorption, such as CdSe,22 and hence more charge carriers would be photogenerated. This is considered to have positive effect on the catalytic activity of CdS/CdSe core/shell QDs. To identify this, a series of CdSe QDs with varied sizes from ~ 2.9 to 5.1 nm (See details in the materials and methods, Figure S7, Supporting Information),23 similar to the sizes spanning from the CdS core to CdS/CdSe(3.30ML) were synthesized. The CdSe QDs of 2.9 and 5.1 nm (considered as 2.9 nm CdSe core with ~ 3.14 ML of CdSe shell) are used to represent for the CdS core (2.9 nm) and CdS/CdSe(3.30ML), respectively. XRD pattern indicates that the synthesized CdSe QDs is also in zinc-blend structure (Figure 3), which is the same as that of CdS core and CdS/CdSe core/shell QDs. The band edge is nearly linear decreased from ~ 2.20 to 1.93 eV with the increase of the CdSe QDs size (Figure 5b & Table S1, Supporting Information), indicating the similar shift of ECB and EVB as that of CdS/CdSe core/shell QDs in response to the change in shell thickness. As clearly seen, with the increase of the CdSe size, the catalytic activity is gradually increased (Figure 5c). This increase is assumed to be attributed to the size increase-induced light absorption enhancement of QDs, since


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the catalytic activity change for the CdSe of varied sizes is well fit with that of coefficient ( )22 (Figure 5c). The results indicate that the increase of the light absorption with the increasing of the CdSe shell thickness of CdS/CdSe core/shell QDs is beneficial for improving its catalytic activity.

Figure 5. a) H2 photogeneration rate of the CdS/CdSe QDs based HPA system versus the shell thickness under the illumination of visible light (≥ 420 nm); b) The band edge of CdS/CdSe and CdSe (considered 2.9 nm CdSe as the core) versus the shell thickness; c) The rate of H2 photogeneration and the coefficient () of CdSe (considered 2.9 nm CdSe as the core) versus the CdSe shell thickness under the illumination of visible light (≥ 420 nm).

In addition, due to the fixed core size during the CdSe shell coating, it is reasonable to assume that the energy level of conduction band (CB) and valence band (VB), referred as ECB and EVB, and Eg (2.84 eV) of CdS core will not be changed,32 and therefore, the gradually band edge decrease of QDs should be attributed to the increase of the CdSe shell thickness. The result


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demonstrates that the increase of shell thickness would induce the shift of ECB and EVB of CdSe shell toward more positive and negative potentials (versus NHE), respectively. This will result in the increase of the band energy difference between the CdS core and CdSe shell, and thus the driving force (ΔEdf) to move the photogenerated charge carriers (electrons and holes) from the core to shell region is increased. As a result, under the same illumination condition, the amount of the charge carriers in the shell region of core/shell QDs would be increased, and this is expected to have two aspects on affecting the catalytic activity of the QDs based HPA system: one is the increase of the reaction probability of charge carriers for the proton reduction related reactions, which would be beneficial for increasing the catalytic activity of the QDs system; while, the other is the decrease of the charge separation efficiency due to the accumulation of the electrons and holes at the shell region, which is not beneficial for increasing the catalytic performance of QDs. Obviously, these two processes are competing on affecting the H2 photogeneration performance of the QDs based HPA system. To investigate the exact effect of these two aspects on the catalytic performance of QDs, the influence of the absorption change of QDs with different shell thickness should be excluded. To realize this, the optical density of the reaction solutions with different QDs should be kept at the same value. Based on the UV-Vis spectra of different samples (Figure S1, Supporting Information), 420 nm light that can excite all the samples was employed. Before the H2 photogeneration measurements, the optical density of the reaction solutions with different QDs was normalized to 0.15, while other conditions were kept consistent. As shown in Figure 6, it is interestingly found that, under the illumination of 420 nm light, the H2 photogeneration rate of QDs with different shell thickness exhibits a distinct variation trend as compared with that of QDs with the same mole amount (Figure 5a). For the CdS core, the H2 photogeneration rate is 1.68 mL


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h-1, with which theΦH2 is at 20.7 %. After coating a thin CdSe shell with the thickness up to 1.10 ML, the H2 photogeneration rate of the core/shell QDs is significantly increased to 2.51 mL h-1 corresponding toΦH2 at 30.9 %, which accounts for 49 % enhancement as comparing with that of CdS core. However, once the CdSe shell thickness is beyond 1.10 ML, the H2 photogeneration rate is gradually decreased and reached 1.34 mL h-1 for the CdS/CdSe(3.30 ML) QDs based HPA system. Previous work has reported that the elimination of surface traps of CdSe by coating CdS shell could results in the increase of the catalytic activity of QDs,33 however, no surface traps related PL emission was observed for CdS core in our case (Figure S1, Supporting Information), therefore, we consider the enhancement of the catalytic activity of QDs with the thin shell thickness is not due to the elimination of surface traps of CdS core. Therefore, it is reasonable to propose that the interesting catalytic activity variation of QDs with different shell thickness should be the results of the interaction between the two above mentioned competing processes: that is, with the CdSe shell thickness below 1.10 ML, the increase of the reaction probability of charge carriers for the proton reduction related reactions plays a major role, and thus the catalytic activity of QDs is increased; while for the CdSe shell thickness beyond 1.10 ML, the decrease of the charge separation efficiency would gradually dominate the influence on the catalytic activity of core/shell QDs, and therefore, results in the decrease of the catalytic performance of core/shell QDs.


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Figure 6. a) H2 photogeneration rate of CdS/CdSe QDs based HPA system versus the shell thickness under the illumination of 420 nm light, and the correspondingΦH2; b) Comparison of the H2 photogeneration rate of CdS/CdSe (3.30 ML) and similar sized CdSe QDs based HPA system under the illumination of 420 nm light. (the insets of a and b is the schematic structure illustration of CdS, CdSe and CdS/CdSe core/shell QDs with different shell thickness. the optical density of the solutions was all adjusted to 0.15 at 420 nm light).

Furthermore, similar sized CdSe (5.1 nm) QDs (Figure S8, Supporting Information) were employed to comparing its catalytic activity with that of CdS/CdSe (3.30 ML). As is shown in Figure 6b, under the illumination of 420 nm light, the H2 photogeneration rate of CdS/CdSe (3.30 ML) is 1.34 mL h-1, which is higher than that of similar sized CdSe (0.47 mL h-1). Since the reaction conditions, including the optical density of the reaction solutions, are the same, our results strongly indicate that the band structure induced increase of the amount of surface charge carriers in core/shell QDs is beneficial for increasing its catalytic activity.


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Scheme 1. Schematic illustration of the variations of the relative energy band diagram and the probable charge carriers transfer process of CdS/CdSe QD based HPA system with the increase of the shell thickness. dHA represents the dehydroascorbic acid. (Note: the exact energy level of CB and VB of CdS core and CdSe shell are not known)

Besides, hole scavenging is another possible affecting factor that has been identified to be a limiting factor for H2 photogeneration.34-36 However, due to the reverse type-I band structure of CdS/CdSe core/shell QDs, holes are expected to always easily transfer to the CdSe shell and directly contacted with the hole scavenger (AA). And, it has been reported that once the holetrapping domain are directly contacted with the hole scavenger, the rate of hole transfer would be significantly increased.9 In addition, the surface status of QDs and hole scavenger used in our work are the same. Therefore, we consider that the hole scavenging process should not be the main factor for affecting the H2 photogeneration in our case. Based on above discussions, a probable mechanism (Scheme 1) is tentatively proposed: with the increasing of the shell thickness of the CdS/CdSe core/shell QDs, both the light absorption and ΔEdf to move the charge carriers transfer from the core to the shell region are increased. These changes will likely result in the increase of the amount of charge carriers (q) in the CdSe shell region, and further improve the catalytic performance of core/shell QDs. For the light absorption,


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this enhancement will result in the increase of the catalytic activity of core/shell QDs; while for that of ΔEdf, the improving effect can only be observed for the core/shell QDs with thin shell thickness (below 1.10 ML), and it will be offset by the decrease of the charge separation efficiency for the one with thicker shell thickness. 3.4 Stability and Origin of H2 production. We further checked the long-term H2-production performance of the CdS/CdSe(3.30ML) QD based HPA system during the successive light on/off cycles (each cycle is ~ 24 h) to simulate the practical H2 production application of the system under the sunrise and sunset conditions. As seen from Figure 7a, the H2 photogeneration rate of the QD system is significantly decreased from 4.8 mL h-1 to 0.8 mL h-1 under the illumination of visible light (≥ 420 nm) after the three-successive light on/off cycles (3 days). However, we found interestingly that the decrease only occurs in the light off condition; while in the light on condition, only a slight decrease of the activity of the system was observed. And the QDs can be recycled by precipitation and further washing three times with the de-ionized water, and consequently adding into the new solution for the H2 evolution measurement. As seen from the inset of Figure 6a, for the first one hour of illumination, the rate of H2 generation of the system is low (~ 0.8 mL h-1), and then quickly increased and maintained at 4.3 mL h-1 by prolonging the illumination time. This demonstrates that the decrease of the activity of the system is not by the decomposition but the surface passivation of QD occurred in the light off condition which is still under investigation in our lab. Figure 7b presents the change of the ΦH2-app of the CdS/CdSe(3.30ML) QD-based HPA system versus the incident light. It shows that the variations of Φ H2-app are consistent with the absorption profile of the QDs, indicating that the H2 production is indeed mediated by the light absorption of QDs.


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Figure 7. a) The long-term light on/off cycles of H2 production of the CdS/CdSe(3.30ML) core/shell QDs based HPA system under the illumination of visible light (≥ 420 nm); b) the ΦH2-app of CdS/CdSe(3.30ML) core/shell QDs based HPA system versus incident light and the UV-Vis spectrum of CdS/CdSe(3.30ML) core/shell QDs in toluene.

4. CONCLUSION In conclusion, the correlation between the H2 production activity of CdS/CdSe core/shell QDs and the shell thickness have been investigated in detail. The results show that both the light absorption enhancement and the band change with the increase of the shell thickness would increase the amount of surface charge carriers, which has been proven to be beneficial for increasing the catalytic activity of QDs. However, the enhancement induced by band change can only be observed for the core/shell QDs with thin shell thickness, and it will be offset for the ones with thicker shell thickness by the decrease of charge separation efficiency. Through optimizing of the shell thickness, theΦH2 of CdS/CdSe core/shell QDs with the shell thickness at 1.10 ML


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reaches 30.9 % under the illumination of 420 nm light, which is 49 % higher than that of CdS core (20.7%). Our findings are considered to provide the new information for understanding the influence factors for further improving the H2 photogeneration efficiency of QDs systems in water.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. UV-Vis & PL results of CdS/CdSe core/shell QDs with different shell thickness. TEM results of CdS/CdSe core/shell QDs with thick shell thickness. Comparison of the catalytic activity of CdS/CdSe(3.30ML) core/shell QDs with and without AA as the electron donor under the illumination of visible light. Hydrogen photogeneration activity of CdS/CdSe core/shell QDs with different shell thickness. UV-Vis results of CdSe QDs with different sizes. Band edge variations of CdS/CdSe and CdSe QDs with different sizes. (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (P. Wang) * E-mail: [email protected] (Y. Jin) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 51502286), the Hundred Talents Program of the Chinese Academy of Sciences, and the State Key Laboratory of Electroanalytical Chemistry (No. 110000R387).

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Table of Contents (TOC)


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Shell Thickness Engineering Significantly Boosts the Photocatalytic

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