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Bare Cd1−xZnxS ZB/WZ Heterophase Nanojunctions for Visible Light Photocatalytic Hydrogen Production with High Efficiency Hong Du,†,‡ Kuang Liang,† Cheng-Zong Yuan,† Hong-Li Guo,† Xiao Zhou,† Yi-Fan Jiang,† and An-Wu Xu*,† †
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China ‡ College of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi 830054, China S Supporting Information *
ABSTRACT: In this work, we report the synthesis of Cd1−xZnxS zinc blende/wurtzite (ZB/WZ) heterophase nanojunctions with highly efficient charge separation by a solvothermal method in a mixed solution of diethylenetriamine (DETA) and distilled water. L-Cysteine was selected as a sulfur source and a protecting ligand for stabilization of the ZB/WZ homojunction. The optimal ternary chalcogenide Cd0.7Zn0.3S elongated nanocrystals (NCs) without any cocatalyst loading show very high visible light photocatalytic activity with H2 production efficiency of 3.13 mmol h−1 and an apparent quantum efficiency of 65.7% at 420 nm. This is one of the best visible light photocatalysts ever reported for photocatalytic hydrogen production without any cocatalysts. The charge separation efficiency, having a critical role in enhancing photocatalytic activity for hydrogen production, was significantly improved. Highly efficient charge separation with a prolonged carrier lifetime is driven by the internal electrostatic field originating from the type-II staggered band alignment at the ZB/WZ junctions, as confirmed by steady and time-resolved photoluminescence spectra. Further, the strong binding between the L-cysteine ligand and Cd1−xZnxS elongated nanocrystals protects and stabilizes NCs; the L-cysteine ligand at the interface could trap holes from Cd1−xZnxS NCs, while photogenerated electrons transfer to Cd1−xZnxS catalytic sites for proton reduction. Our results demonstrate that Cd1−xZnxS ZB/ WZ heterophase junctions stabilized by L-cysteine molecules can effectively separate charge carriers and achieve highly visible light photocatalytic hydrogen production. The present study provides a new insight into the design and fabrication of advanced materials with homojunction structures for photocatalytic applications and optoelectronic devices. KEYWORDS: ZB/WZ heterophase junctions, L-cysteine, noble-free cocatalyst, photocatalytic H2 production, visible light
1. INTRODUCTION
semiconductor photocatalysts such as sulfide, nitride, metal/ metallic oxide, and other materials have been investigated.10−13 Over the few past decades, although significant efforts have been made to develop new catalysts for improving the catalytic efficiency, further commercial development and mass production of catalysts are still restricted by the high cost of catalysts with low efficiency.14,15 To overcome the above challenges, some methods have been used to enhance photocatalytic activity by increasing charge separation efficiency, such as chemical doping to tune the band gap of a photocatalyst,16 surface modification of photocatalysts
Due to the growing global energy crisis and environmental problems, photocatalytic splitting of water into hydrogen, a clean, highly efficient, and renewable energy substitution for the fossil fuels, has aroused wide public concern.1−4 Among various hydrogen production technologies, hydrogen generation from fossil fuels or through electrolysis is neither green nor economical. Photocatalytic hydrogen production powered by renewable energy such as solar light is one of the most promising ways to achieve hydrogen economy.5−7 The TiO2 photocatalyst has good activity and stability in a variety of semiconductors but needs UV irradiation for effective photocatalysis, yet UV light accounts for only a small fraction (4%) of the solar energy compared to visible light (43%), and such a disadvantage constrains its practical application.8,9 So far, many © XXXX American Chemical Society
Received: May 24, 2016 Accepted: August 26, 2016
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DOI: 10.1021/acsami.6b06182 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces Scheme 1. Structural Configuration of ZB/WZ Heterophase Nanojunctionsa
a
(a) Schematic illustration of twinning in a zinc blende fcc nanocrystal. (b) The nanocrystal view from the [−110] direction. (c) Three-dimensional view of periodically twinned nanocrystals with wurtzite cross section. The red layers in panels b and c represent twinned planes.
with cocatalysts, and developing artificial Z-scheme systems.17 The semiconductor nanojunctions have been designed to improve interfacial charge transport and separation of electron/ hole (e/h) pairs driven by an internal electrostatic field formed in the type-II junction region.18,19 However, these junctions are usually obtained by several synthetic processes, leading to the stochastic and chaotic distributions of junctions which impede the migration and separation of charge carriers. Hence, it is highly desirable to design nanostructures of materials that not only enhance the effective e/h separation but also facilitate their transport pathway.20 Highly ordered heterophase homojunctions can be fabricated within the same chemical composition of one semiconductor which has several crystalline phases.21 The band alignment at the junction can boost interfacial charge separation. The ordered type-II heterophase homojunctions in one semiconductor not only retain the transfer characteristics of charge carriers just as perfect nanocrystals do as a result of its highly ordered structures but also effectively separate photoinduced e/h pairs to hinder their recombination. In our pervious study, the band gap of Cd1−xZnxS solid solutions can be tuned by adjusting the value of x to meet the premise of the photocatalytic reactions.22 Even though the conduction band (CB) position and the band gap of Cd1−xZnxS can be tuned to improve photocatalytic performance, the rapid recombination of photogenerated e/h pairs is still a key problem that restricts further improvement of photocatalytic efficiency. The popular method to improve the performance of Cd1−xZnxS solid solution is mainly achieved by loading noble metal cocatalysts such as Pt and Pd. However, it is still challenging to expand the application of Pt-based cocatalysts because of its rareness and the high cost of noble metals. Thus, it is urgent to develop precious metal-free photocatalysts that achieve high photocatalytic H2 production quantum efficiency. Regretfully, only limited bare photocatalysts show good performance for hydrogen production.23,24 An alternative arrangement of zinc blende (ZB) and wurtzite (WZ) section heterostructures in III−V nanowires has been
studied in recent years.25,26 Nevertheless, Cd1−xZnxS solid solutions with ZB/WZ heterophase homojunctions have been rarely reported.27 Generally, the type-II staggered band alignment generated between ZB and WZ has a periodic arrangement forming a twinning superlattice, which is in favor of efficient charge separation and high photocatalytic H2 efficiency.26 To investigate the relationships between periodically alternating structures and the excellent performance, the structural configuration of ZB/WZ heterophase nanojunctions was elucidated (Scheme 1). In general, the ZB phase has a facecentered cubic (fcc) structure, while the WZ phase belongs to hexagonal closed-packing structure. Normally, the perfect stacking sequences of ZB and WZ phase are ···AaBbCcAaBbCc··· and ···AaBbAaBb···, where one layer can be considered as a double-layer consisting of paired atoms of its ingredients. For example, Aa represents a double-layer, as do Bb and Cc. Because of the same type of position of the projection normal to the (111) plane, the ···AaBbCcAaBbCc··· stacking sequence can be described by the ···ABCABC··· sequence by dropping the double-index notation where each letter represents bilayer. However, stacking faults break the wellordered structures. The twinned plane in the ZB structure (stacking fault in WZ structure) could be regarded as a single layer of the WZ structure.28 The ZB structure, as expected for the bulk phase, possesses rotational twin planes which can be formed through twinning on {111} planes by rotating along the ⟨111⟩ direction. It is known that the twins and stacking faults in the cubic lattice are the same, which means that this rotation results in a symmetry structure from ···ABCABCA··· to ··· ABCACBA··· with mirror plane Aa as a twinning plane or twinning boundary.26,29 Thus, the WZ structure (stacking sequence CcAaCc) is created locally within the fcc structure. Accordingly, the continuity of the ZB lattice structure is abruptly broken by the twinned planes, producing innumerable WZ segments in the ZB lattice (Scheme 1a). It is assumed that periodic twinning may significantly affect the electronic properties of the nanocrystals and form the electronic miniB
DOI: 10.1021/acsami.6b06182 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces bands,30 which could be beneficial not only for the bandgap engineering but also for direct intersubband and optical transitions.27,31 Schemes 1b and 1c present three-dimensional (3D) models of a ZB/WZ heterophase nanojunction. In this research, we report the synthesis of Cd1−xZnxS elongated nanocrystals with highly ordered heterophase junctions consisting of alternative ZB and WZ segments by a facile biomolecule-assisted solvothermal route. In the reaction system, L-cysteine (cys) acts as the sulfur source and also as complexing molecules with metal cations to control nanocrystal (NC) growth. It is noted that L-cysteine has a strong affinity to metal cations on the surface of NCs, thus stabilizing NCs against oxidation. The band gaps of Cd1−xZnxS solid solutions can be readily tuned by changing the Cd/Zn molar ratios. The type-II band alignment organized at the ZB/WZ homojunctions not only restrains the stochastic and chaotic distribution of electrons and holes but also affords an internal electrostatic field to prevent their recombination. Further investigations prove that the photocatalytic hydrogen production from water over bare Cd0.7Zn0.3S ZB/WZ nanojunctions is dramatically enhanced with the H2 production efficiency of 3.13 mmol h−1 and an apparent quantum yield (AQY) of 65.7% at 420 nm. These findings offer a new and widespread method with easy preparation and environmental friendliness for the development of novel efficient visible light photocatalysts for water reduction reactions. AQYs were determined by employing a 425 nm band-pass filter and an irradiation meter and were calculated by the following formula:
(Photon Technology International (Canada) Inc.) with a USHIO xenon lamp source. The UV−vis absorption spectra were recorded on a UV-2510 spectrophotometer (Shimadzu). The Fourier transform infrared (FT-IR) spectra were recorded on a MAGNA-IR 750 (Nicolet Instrument Co., United States) with samples embedded in KBr pellets (with a frequency range of 4000−450 cm−1 and resolution of 4 cm−1). The chemical states and compositions of samples were performed at the Photoemission Endstation in the National Synchrotron Radiation Laboratory (NSRL, Hefei, P. R. China). The UV−vis absorption spectra were measured with a Shimadzu UV-2510 spectrophotometer in the region of 300−800 nm. Brunauer−Emmett−Teller (BET) surface areas were measured at 77 K by nitrogen sorption with a Micromeritics ASAP 2020 nitrogen adsorption apparatus (United States). Steady PL spectra for solid samples were recorded on Acton Sp2500 spectrofluorometer (Princeton Instruments) with a liquid nitrogen cooled CCD. Photocatalytic Hydrogen Production. Photocatalytic water reduction reactions were performed in a gas-closed circulation system with outer irradiation. Photocatalyst powder (0.1 g) was dispersed by ultrasonication for 20 min in an aqueous solution (100 mL) containing sodium sulfide (Na2S, 0.3 M) and sodium sulphite (Na2SO3, 0.3 M) in a 500-ml Pyrex glass reactor. The suspension was irradiated with light from a 300 W Xe lamp with a cutoff filter (λ ≥ 420 nm) to remove ultraviolet light. The amount of H2 was recorded by an online gas chromatography (Agilent 6820, TCD detector, N2 carrier).
3. RESULTS AND DISCUSSION Cd1−xZnxS nanocrystals (NCs) with different nominal x values were prepared by a solvothermal route (Supporting Informa-
⎛ 2 × no. of evolved H 2 molecules ⎞ AQY (%) = ⎜ ⎟ × 100 no. of incident protons ⎠ ⎝ (1)
2. EXPERIMENTAL SECTION Photocatalyst Synthesis. The optimal Cd0.7Zn0.3S (nominal molar ratio of Cd/Zn = 0.7/0.3) nanocrystals were prepared as follows: 4.2 mmol of Cd(CH3COO)2·2H2O and 1.8 mmol of Zn(CH3COO)2·2H2O were dissolved into 15 mL of mixed solvent composed of distilled water (6.25 mL) and 8.75 mL of DETA. After being stirred for 5 min, 25 mmol of L-cysteine was mixed into the above liquor. The mixture liquor was further stirred for 30 min to form stable L-cysteine−Cd2+/Zn2+ complexes. Then, the mixture was transferred into a Teflon autoclave and heated to 160 °C for 24 h. The product was cooled to room temperature naturally and then centrifuged and washed with double-distilled water and ethanol three times. The obtained product was dried at 60 °C for 12 h in a vacuum oven. Other Cd1−xZnxS nanocrystals with different nominal Cd/Zn molar ratios (the total molar amount was kept to be 6 mmol), DETA/ H2O volume ratios, and L-cysteine dosages in the starting materials were synthesized under the same conditions to study the influence of the composition and concentration of DETA and L-cysteine on photocatalytic performance. Characterization. X-ray powder diffraction (XRD) patterns of the prepared photocatalysts were collected with Cu Kα radiation (λ = 0.154178 nm, Rigaku/Max-3A X-ray diffractometer), with the operating voltage 40 kV and current 200 mA. The photocatalyst morphologies and structures were characterized by field-emission scanning electron microscopy (SEM, JSM-6701F, JEOL) with an accelerating voltage of 5 kV, transmission electron microscopy (TEM, Hitachi 7650), and high-resolution transmission electron microscopic images (HRTEM, Hitachi 7650) with an accelerating voltage of 200 kV. Elemental mapping in the desired regions of the photocatalysts was taken by an attached energy-dispersive X-ray spectrometer (EDS). The time-resolved photoluminescence measurements (TRPL) were performed on a LaserStrobe time-resolved spectrofluorometer
Figure 1. XRD patterns of Cd1−xZnxS solid solutions with different x values: (a) 0, (b) 0.2, (c) 0.3, (d) 0.4, (e) 0.5, (f) 0.7, (g) 0.8, and (h) 1.0. The vertical lines at the top and bottom indicate ZnS zinc blende structure (PDF #77-2100) and CdS wurtzite structure (PDF #772306).
tion). X-ray diffraction (XRD) patterns of Cd1−xZnxS solid solution NCs are displayed in Figure 1. XRD patterns of Cd1−xZnxS show that the diffraction peaks gradually shift to a high angle with an increase in x value, indicating a decrease in lattice spacing for Cd1−xZnxS NCs. In addition, the successive peak shifts imply that the obtained samples are not mixtures of ZnS and CdS but rather Cd1−xZnxS solid solutions, which is due to not only the fact that Zn2+ cations are incorporated in CdS lattices or its interstitial sites with a smaller ionic radius (Zn2+: 0.74 nm, Cd2+: 0.97 nm) but also that the electronegativity of Zn2+ (1.65) is approximately equal to that of Cd2+ (1.70).32 The widening of the diffraction peaks results from the small crystal size of obtained Cd1−xZnxS NCs. The diffuse reflectance spectra of obtained samples are shown in Figure S1a. CdS and ZnS have absorption edges determined to be about 540 and 350 nm, respectively, while the C
DOI: 10.1021/acsami.6b06182 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. TEM image (a) of synthesized Cd0.7Zn0.3S elongated NCs. HRTEM image (b) of ZB/WZ heterophase zigzag structure. Selected area electron diffraction pattern (c) implying the twins in each grain are parallel to each other in {111} planes. STEM image (d) of a single Cd0.7Zn0.3S nanorod and the corresponding elemental mapping images (e−g) of the Cd0.7Zn0.3S nanorod. Inset in panel a shows the single elongated nanorod with higher resolution.
Figure 3. XPS survey spectrum of Cd0.7Zn0.3S (a) and high-resolution XPS spectra of Zn 2p (b), Cd 3d (c), and S 2p (d).
absorption edges of Cd1−xZnxS solid solutions systematically blue-shift as the Zn2+ nominal content is increased, which is attributed to the incorporation of Zn2+ into the CdS lattice. A continuous shift of the absorption edges implies that the band gaps of the solid solutions formed between ZnS and CdS can be precisely tuned by varying the Cd/Zn atomic ratios in Cd1−xZnxS NCs. The band gaps of Cd1−xZnxS photocatalysts
can be readily adjusted between 2.32 and 3.58 eV (x = 0−1), as calculated from the onsets of the absorption edges (Figure S1b). The SEM image (Figure S2) clearly reveals that the overall morphology of Cd0.7Zn0.3S is elongated NCs with an average diameter of around 70 nm. The TEM and HRTEM images shown in Figures 2a and b clearly present the anisotropic D
DOI: 10.1021/acsami.6b06182 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. Nitrogen adsorption−desorption isotherms (a) and corresponding pore size distribution curves (b) of the Cd0.7Zn0.3S sample.
Figure 7. Fluorescence decay traces measured at room temperature for Cd0.7Zn0.3S solid solutions prepared using different volumes of DETA in a DETA/water binary solution (excitation at 420 nm and probe at 590 nm).
Figure 5. Comparison of H2 production rates of Cd1−xZnxS samples with different x values under visible light irradiation (λ ≥ 420 nm).
Figure 6. Comparison of H2 production rates of Cd0.7Zn0.3S solid solutions prepared using different volumes of DETA in a DETA/H2O binary solution: (a) 0, (b) 5, (c) 7.5, (d) 8.75, (e) 10, and (f) 15 mL.
Figure 8. Comparison of H2 production rates over Cd0.7Zn0.3S samples obtained with different dosages of L-cysteine: (a) 9, (b) 12.5, (c) 25, (d) 30, and (e) 36 mmol.
geometry of the as-prepared sample. It can be seen that Cd0.7Zn0.3S NCs are composed of a high-density parallel distributed stacking faults (see inset in Figure 2a). The fringes with a lattice spacing of d = 0.33 nm are close to the lattice spacing of the {111} planes of the ZB phase (Figure 2b). The HRTEM image exhibits that each nanoparticle has a regular strip-like structure, and the particle surface is not as smooth as that found in the SEM image (Figure S2). Nanorod-like NCs are composed of nonuniform zigzag structures. This is due to the fact that ZB/WZ boundaries suddenly destroy the lattice continuity, leading to dislocations residing in coherent twinned boundaries. Figure 2c shows the selected area electron diffraction (SAED) pattern of the Cd0.7Zn0.3S sample, indicating the nature of quasi-single crystals, and the strips existing on the surface of each NC contain a high density of grown twins of the {111}/[112] type. Scanning TEM (STEM)
images (Figure 2d), elemental mapping (Figures 2e−g), and energy dispersive X-ray spectrometry (EDS) (Figure S3) analysis prove that Cd1−xZnxS NCs are homogeneously composed of Zn, Cd, and S elements; these results provide clear evidence for the formation of Cd0.7Zn0.3S solid solutions and preclude the possibility of phase separation between CdS and ZnS, in agreement with XRD results. X-ray photoelectron spectroscopy (XPS) was carried out to determine the surface chemical compositions and electronic states of Cd0.7Zn0.3S sample (Figure 3). The XPS survey spectrum reveals the existence of Zn, Cd, and S elements (Figure 3a), indicating the successful synthesis of solid solution. Figures 3b−d show the high-resolution XPS spectra of Zn 2p3/ E
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among the highest AQY attained for a single metal sulfide photocatalyst without any noble metal cocatalyst. It is noted that our study is of great importance in view of practical applications due to high quantum efficiency. It was reported that the Pt-PdS/CdS photocatalyst can achieve higher efficiency in photocatalytic H2 production;38 however, it contains a noble metal cocatalyst. From Figure 3 it can be seen that for Cd1−xZnxS samples (x < 0.3) with decreasing Zn2+ content, the rate of photocatalytic H2 evolution decreases, which is likely due to their lower conduction band (CB) position and less overpotential.27 However, for Cd1−xZnxS solid solutions (x > 0.3) with increasing Zn2+ content, the decrease in visible light absorption leads to the decline of photocatalytic activity. The influence of other important experimental factors on the activities of the photocatalysts was explored. Figure 6 shows photocatalytic H2 production rates over Cd0.7Zn0.3S prepared using different DETA volumes in a DETA/H2O solution, which further provides clear evidence of the key role of the ZB/ WZ heterophase junction in boosting visible light photocatalytic efficiency. Without using DETA solvent, the obtained sample exhibits lower photocatalytic activity (0.31 mmol h−1); however, photocatalytic hydrogen evolution rates increase with an increase in the amount of DETA used. The optimal amount of DETA solvent is about 8.75 mL, achieving the highest H2 evolution rate of 3.13 mmol h−1, which is 10 times higher than that of the phase pure Cd0.7Zn0.3S sample. Further increasing the content of DETA in a DETA/H2O mixed solvent decreases photocatalytic efficiency. Excess DETA molecules may block the surface active sites, leading to the decrease in photocatalytic activity. To unravel the origin of these differences, the structures of these samples were characterized by XRD and TEM measurements (Figure S4). In the absence of DETA solvent, the obtained Cd0.7Zn0.3S NCs crystallized into the form of ZB phase, and no WZ phase was observed. The proportion of WZ phase segments increased gradually from the phase pure ZB NCs to twinned ZB/WZ heterophase as the volume of DETA increased, as confirmed by XRD measurements (Figure S4f). Further increasing the volume of DETA results in the content of WZ segments remaining almost constant. DETA, providing an alkaline condition in the reaction system, is
Figure 9. Long-time stability test of hydrogen yield over Cd0.7Zn0.3S (DETA volume: 8.75 mL and L-cysteine dosage: 25 mmol) under visible light irradiation (λ ≥ 420 nm).
2, Cd 3d, and S 2p3, respectively. The Zn 2p and Cd 3d peaks at binding energies 1020.7 eV (Zn 2p3/2), 404.5 eV (Cd 3d5), and 411.2 eV (Cd 3d3) are attributed to Cd0.7Zn0.3S molecular environments, in agreement with the previously reported values with small variations.33,34 The S 2p peak located at 161.7 eV indicates the presence of the sulfur ions (Figure 3d).35 Figure 4a shows the N2 adsorption−desorption isotherms and the corresponding pore size distribution curve of the Cd0.7Zn0.3S sample, which has isotherms characterized as type IV with a H3 hysteresis loops in the P/P0 range of 0.8−1.0,36 indicating the presence of slit-like pores and mesopores (2−50 nm). Moreover, the observed hysteresis loop at high relative pressures suggests the presence of macropores (>50 nm). The pore size distribution curve (Figure 4b) indicates a wide size distribution ranging from 10 to 100 nm.37 Photocatalytic hydrogen production over Cd1−xZnxS NCs was measured under visible light illumination (λ ≥ 420 nm) using Na2S/Na2SO3 as hole scavengers. Among all synthesized samples, the best efficiency of photocatalytic H2 production was found for the optimal sample of Cd0.7Zn0.3S (x = 0.3; DETA: 8.75 mL; L-cysteine: 25 mmol) with an average rate of 3.13 mmol h−1, corresponding to an AQY of 65.7% at 420 nm (Figure 5 and Table S1). To the best of our knowledge, this is
Scheme 2. Schematic Diagram of the Charge Transfer Process for the Cd1−xZnxS ZB/WZ Heterophase Junction with SurfaceCoordinated L-Cysteine Moleculesa
a
(a) Sketch of the formation of the Cd1−xZnxS NCs ZB/WZ homojunction. (b) HRTEM image of a typical Cd1−xZnxS nanorod from the 10 nm diameter ensemble. The blue squares and green arrows index the segments of WZ and ZB structures, respectively. (c) The migration of charge carriers over corresponding Cd1−xZnxS ZB/WZ segmented nanojunctions stabilized by the L-cysteine ligand. F
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ACS Applied Materials & Interfaces favorable for the growth of ZB/WZ homojunction under hydrothermal processes.39 The unequal coordination effect between Zn2+ and Cd2+ with DETA molecules leads to the inequality in mass transport for the principal elements, which is critical to generate twinned structures. On the other hand, the hydrothermal conditions result in thermal fluctuations in the growth process, which likely produces twin-plane nucleation due to the low formation energy of the coherent twinned plane.40 In addition, twin planes in one-dimensional (1D) nanocrystals are conducive for release of the stored elastic energy.41 This is advantageous to the shape transition from nanospheres to elongated nanorods,42,43 as demonstrated by TEM observations. Taken together, high photocatalytic H2 production is attributed to the formation of Cd0.7Zn0.3S ZB/ WZ heterophase junctions. Steady PL spectra of Cd1−xZnxS samples with different x values were measured, which are able to reflect the migration and separation of photogenerated charge carriers. As displayed in Figure S5, an emission band around 530 nm is observed from CdS, which is attributed to a band−band PL emission where the energy of the light is approximately equal to the band gap energy of CdS. With an increase in Zn content, their PL emission peak gradually blue-shifts. In addition, it can be seen that the intensity of the PL emission peak from the Cd1−xZnxS solid solutions appears to decline compared to that of CdS. This is due to formation of the heterophase junctions, which are beneficial for boosting the migration and separation of photogenerated charge carriers. Time-resolved fluorescence emission decay spectra are usually employed to assess the photogenerated carrier lifetime. Figure 7 shows a series of the observed lifetimes obtained from the time-resolved PL decay traces by fitting to a single exponential function. The photoexcited carriers in the Cd0.7Zn0.3S sample present a progressively longer lifetime as the volume of DETA is increased from 0 to 8.75 mL, which is in accordance with the results of photocatalytic hydrogen generation. The longest photoexcited carrier lifetime is 6.51 ns when the volume of DETA reached 8.75 mL, which is 9 times longer than that of the ZB phase pure Cd0.7Zn0.3S sample with the shortest lifetime of 0.72 ns. This is in line with photocatalytic H2 production activity (Figure 4). Prolonged carrier lifetime is ascribed to the efficient interface charge transfer between the ZB and WZ phase in Cd0.7Zn0.3S heterophase junction and the suppression of the electron− hole recombination. As the sulfur source and ligand, the content of cys has a crucial influence on photocatalytic activity. As shown in Figure 8, it was found that H2 production rates increase as the amount of cys is increased from 9 to 25 mmol and then decrease when the amount of cys increased from 25 to 36 mmol. The H2 evolution rate achieves a maximum of 3.13 mmol h−1 when the amount of cys reached 25 mmol. Further increasing the amount of cys leads to a gradual reduction in the hydrogen production efficiency. This is due to the fact that the active sites on the surface of catalysts are blocked by excess cys ligand molecules, hindering the valid contact of the active sites and the reactants. Burford and co-workers reported that metal ions could react with cys to form stable complexes.44 L-cysteine molecules, employed as the sulfur source, decompose slowly. At the early stage of the reaction process, it is reasonable to conclude that free Cd2+/Zn2+ can coordinate with L-cysteine to form cysCd2+/Zn2+ complex precursors under the alkaline conditions by adding DETA. The S−C bond is broken, and cys-Cd2+/Zn2+
precursor decomposes under the solvothermal conditions. Organometal complex causes metal ions and S2− to be released slowly, resulting in homogeneous nucleation and uniform growth. Metal ions react with S2− ions released from the precursor due to the low value of the solubility−product constant (Ksp) of Cd1−xZnxS. In addition, DETA molecules act as a template for directing 1D growth of Cd1−xZnxS ZB/WZ NCs based on the quantity of three-coordination N atoms (N3C) and the length of atomic chain of molecules.45 The Lcysteine ligand could passivate and protect Cd1−xZnxS nanorod surfaces against oxidation.46 Moreover, IR spectra (Figure S6) reveal that L-cysteine molecules bind onto Cd1−xZnxS nanorod surfaces through the sulfur atom due to the strong surface binding to metal cations, which can trap holes from Cd1−xZnxS nanorods at the interface.47 Meanwhile, photogenerated electrons transfer to the catalytic sites of Cd1−xZnxS; hence, charge carrier recombination is significantly reduced. This is favorable for long-lived electron−hole charge separation, which dramatically promotes the photocatalytic hydrogen production efficiency. Furthermore, the stability of the Cd0.7Zn0.3S catalysts was investigated over 20 h, and the photocatalysts show good photostability during photocatalytic H2 production (Figure 9). In general, stochastic defects are regarded as the trapping and recombination centers for photoinduced electron−hole pairs, which is detrimental for the photocatalytic performance because they have high and chaotic potential barrier.48 However, the twinned defects in our Cd1−xZnxS elongated NCs exhibit periodically alternating ZB and WZ segments along the ⟨111⟩ direction to form type-II staggered band alignment at the boundaries (Scheme 2a). This sequential ZB/ WZ homojunction is beneficial for effective charge separation and migration. In this case, the twin boundary is not a recombination center for e/h pairs.49 The type-II ZB/WZ heterophase junctions can be clearly identified from the HRTEM image (Scheme 2b), and Cd0.7Zn0.3S heterophase junctions have an average band gap of about 2.41 eV, as shown in Figure S1b. Scheme 2c presents the band positions of ZB and WZ phase in Cd0.7Zn0.3S based on experimental results and theoretical calculations.39 It is proposed that the free charges can transfer through the twin boundaries (only a monolayer), usually by a tunneling effect caused by the formation of the lower boundary energy.50 In other words, the scattering free charges on the twinning boundaries can migrate through them without being trapped. Furthermore, it was reported that the WZ phase occupies a CB and VB position higher than that of its ZB counterpart;23 consequently, the photoinduced electrons transfer from the CB of WZ to the CB of ZB, while holes migrate from the VB of ZB to the VB of WZ, achieving highly effective separation of electron−hole pairs. Moreover, it is believed that the large distribution and atomic level interconnection of twin-induced heterophase junctions in Cd1−xZnxS NCs lead to the vectorial interfacial transfer of photoexcited charge carriers between the ZB and WZ phase.49,51 L-Cysteine molecules can bind to Cd1−xZnxS surfaces by coordination through the sulfur atoms and metal cations, as confirmed by IR analysis (Figure S6). The LCysteine ligand protects nanocrystals from oxidation and traps holes from NCs at the interface. Overall, as confirmed by transient PL spectra, charges carrier separation efficiency and lifetimes are significantly enhanced by the ZB/WZ heterophase nanojunctions and strong surface complexation between Cd1−xZnxS NCs and L-cysteine molecules. This results in an G
DOI: 10.1021/acsami.6b06182 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
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unexpected high photocatalytic hydrogen efficiency without a noble metal cocatalyst.
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CONCLUSIONS In summary, we synthesized noble metal-free Cd1−xZnxS ZB/ WZ heterophase nanojunctions stabilized by L-cysteine ligand via a mild and simple solvothermal method. The obtained bare Cd0.7Zn0.3S elongated nanocrystals exhibit highly efficient visible light hydrogen generation due to effective charge migration and separation. By varying experimental parameters such as the molar ratio of Cd2+/Zn2+, metal cations/L-cysteine, and the volume ratio of solvent DETA to water, a series of Cd1−xZnxS twin-induced ZB/WZ homojunctions can be controllably fabricated. The best photocatalytic hydrogen production rate of the optimal Cd0.7Zn0.3S sample is 3.13 mmol h−1, corresponding to an AQY of 65.7%. The possible mechanism of significantly boosted hydrogen evolution is proposed. On one hand, the internal electrostatic field originating from the type-II staggered band alignment at the ZB/WZ junctions facilitates charge separation and promotes charge migration rate. On the other hand, the L-cysteine ligand protects the nanocrystal surface against oxidation and can trap photogenerated holes enriched at the twinned boundaries of Cd1−xZnxS. Overall, the ability to generate such an ordered type-II homojunction in one semiconductor with coordinated L-cysteine protecting molecules provides an environmental friendly and effective strategy to develop highly efficient photocatalysts for hydrogen production without the use of noble metal cocatalysts.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06182. Details of characterization, XRD patterns, STEM and EDS analyses, TEM and HRTEM images, UV−vis diffuse reflection spectra, and infrared spectra (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support from the National Basic Research Program of China (Grant 2011CB933700), the National Natural Science Foundation of China (Grants 51561135011, 51572253, and 21271165) and a Scientific Research Grant for Hefei Science Center of CAS (Grant 2015SRG-HSC048) are gratefully acknowledged.
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ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.6b06182 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
