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Biomolecule-assisted hydrothermal synthesis of ZnxCd1−xS nanocrystals and their outstanding photocatalytic performance for hydrogen production
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Biomolecule-assisted hydrothermal synthesis of ZnxCd1¡xS nanocrystals and their outstanding photocatalytic performance for hydrogen production Meiying Liu a,*, Yunfei He a, Hong Chen a, Hongmei Zhao a, Wansheng You a, Jingying Shi b,**, Lancui Zhang a, Jiansheng Li a a
School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian, Liaoning 116029, PR China State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian, Liaoning 116023, PR China
b
article info
abstract
Article history:
To achieve scalable applications in solar hydrogen production, it is necessary to develop
Received 6 February 2017
visible-light-responsive photocatalysts that are highly efficient, cost-effective, stable and
Received in revised form
environmentally-benign. Here narrow bandgap ZneCdeS solid solution photocatalysts
21 June 2017
(Eg ¼ 2.11e2.53 eV) were prepared via a facile and green hydrothermal strategy under mild
Accepted 23 June 2017
conditions. Amazingly, over the naked Zn0.5Cd0.5S photocatalyst, an extraordinarily high
Available online 22 July 2017
H2 production activity in Na2SeNa2SO3 aqueous solution is achieved up to 18.3 mmol h1 g1 with an apparent quantum efficiency of 73.8% per 50 mg under 420 nm light irradiation,
Keywords:
which, to our knowledge, outperforms cocatalyst-free metal sulfide photocatalysts previ-
ZnxCd1xS
ously reported to date. Such super high performance arises from the enhanced visible-
Hydrogen production
light-absorption capacity, suitable conduction and valence band potential together with
Water splitting
the facilitated charge transport in ZneCdeS solid solutions. This work may open an avenue
L-cystine
for the green preparation of inexpensive photocatalysts for solar H2 production.
Green synthesis
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solar-driven photocatalytic water splitting over semiconductor catalysts is an essential and challenging subject of research which has received immense attention thanks to its capability of producing clean hydrogen fuel from renewable resources to address global energy and environmental problems [1]. In the past few decades, numerous metal oxides such
as TiO2 [2,3], SrTiO3 [4], NaTaO3 [5] and KNb6O17 [6], have been extensively investigated. However, these oxide photocatalysts cannot harness visible light (major proportion of the solar spectrum) due to their large band gaps (>3 eV), which strongly renders their commercial applications. In view of the efficient utilization of solar light, it remains highly desirable to develop visible-light-responsive photocatalysts for hydrogen production.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Liu), [email protected] (J. Shi). http://dx.doi.org/10.1016/j.ijhydene.2017.06.196 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Unlike the above-mentioned oxides, metal chalcogenides have been regarded as promising candidates for visible-lightresponsive photocatalysts because of their narrow band gap and negative valence edge position [7,8]. Considering the suitable bandgap and sufficiently negative conduction band position to thermodynamically drive the water reduction reaction, CdS is an ideal photocatalyst which has been a focus of huge interest [9]. Unfortunately, excessive recombination of photogenerated eehþ pairs, severe photocorrosion as well as the requirement of noble metals cocatalysts restricts its commercial application [10e12]. An effective strategy to overcome these drawbacks is to fabricate ZnxCd1xS solid solution photocatalysts based on CdS and wide bandgap semiconductor materials of ZnS (Eg ¼ 3.6 eV) to facilitate the separation and transport of the photogenerated charge carriers [13]. Up to date, tremendous efforts have been devoted to the synthesis of ZnxCd1xS solid solutions with a variety of sizes and morphologies to achieve high photocatalytic performance [14e22]. However, these techniques usually require high temperatures over 160 C together with expensive and poisonous sulfur sources such as Na2S [14], sulfur powder [15], thioactamide [16], thiourea [17e19], dimethyl sulfoxide [20], thioglycolic acid [21] as well as thiols [22], resulting in the liberation of nauseous scent H2S. Besides, some toxic chemicals including organic solvents [23,24], surfactants [25], template [26] and reducing agents [27] are used as additive. Consequently, it is still a great challenge for the high-through synthesis of robust ZnxCd1xS solid solutions by a facile, inexpensive and environment-friendly approach under mild conditions. Owing to special structures and fascinating self-assembly functions of biomolecules, the biomolecule-directed synthesis is very attractive to be a green and cost-effective route for synthesis of the various metal sulfide nanostructures [28e31]. L-cystine is such a biomolecule that contains hydrosulphonyl (eSH), amino (eNH2) and carboxyl (eCOOH) functional groups in its molecular structure, which enables it to combine with the most metal cations by ligand bonds and thus it is an alternative and versatile sulfur source directing for the synthesis of various metal sulphides [32e34]. Previously we reported the L-cystine-assisted synthesis of alloyed MneCdeS photocatalysts with high performance for solar H2 production from water decomposition [35]. In this work, we report an L-cystine-assisted hydrothermal route at moderate temperature to prepare nanosized ZnxCd1xS solid solutions with hexagonal wurtzite structure, which, to our best knowledge, has seldom been reported so far. Herein, L-cystine functions not only the sulfur source but also the coordinating agent, no additional ligand or template is needed for the synthesis. Thus, the compositions, sizes, shapes and properties of ZnxCd1xS solid solutions are easily controlled through adjusting the molar ratios of the precursors. In addition, homininoxious sulfur precursors and solvents are completely avoided. More significantly, the asprepared ZnxCd1xS solid solution photocatalysts are highly efficient towards the visible-light photocatalytic H2-production in Na2SeNa2SO3 aqueous solution. The optimal quantum efficiency reaches 73.8% per 50 mg at 420 nm, which, to our best knowledge, outperforms previously-reported cocatalyst-
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free metal sulfide photocatalysts [16e21,26]. The origin of the super high performance is investigated by calculating the electronic structure together with the EIS and TPC techniques.
Experimental procedure Synthesis A series of ZnxCd1xS photocatalysts were synthesized through the following procedure. Firstly, Zn(OAc)2$2H2O and Cd(OAc)2$2H2O with a specified x value in ZnxCd1xS (x ¼ 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) and a total molar amount of 3 mmol were added into 16 mL of deionized water to form a clear solution. Simultaneously, L-cystine (12 mmol) was dissolved into another 16 mL of deionized water with magnetic stirring, in which the pH of the mixed solution was adjusted to 10e11 using 6 M NaOH aqueous solution. Secondly, the above two solutions were slowly mixed together with vigorously stirring to form a milky suspension under ambient condition. Finally the as-generated mixed slurry was transferred into Teflon-lined stainless steel vessel of 45 mL capacity and maintained at 140 C for 10 h under autogenous pressure. After being cooled naturally to room temperature, the products were collected, rinsed with deionized water and ethanol for several centrifugation cycles, and then dried in an oven at 60 C overnight prior to use.
Characterization The crystallographic structure and phase of the synthesized ZnxCd1xS samples were identified by powder X-ray diffraction (XRD) analysis on a Bruker D8 Advance diffractometer with monochromatized Cu-Ka irradiation in Debye-Scherrer geometry. The sample was scanned in the 2q range from 20 to 70 at a step of 0.05 S1. The operating voltage and the applied current were 40 kV and 40 mA, respectively. The average crystallite size of the photocatalysts was calculated from the broadening of X-ray line by the Scherrer formula as follows: D ¼ 0.89l/b cosq, where l is the wavelength of Cu-Ka irradiation and equals to 1.54178 A, b is the full-width at halfmaximum and q is the diffraction angle, respectively. UVvisible spectra in reflection mode were recorded on a PerkinElmer Lambda 3 spectrophotometer equipped with an integrating sphere by adopting BaSO4 as a 100% reference standard and were converted from reflectance to absorbance by the Kubelka-Munk function. The morphology, composition and particle size of the samples were revealed by transmission electron microscopy (TEM, FEI Tecnai Spirit) and a high resolution transmission electron microscope (HRTEM, FEI Tecnai F30) together with an energy-dispersive X-ray spectrometer (EDS) performed at 300 kV, respectively. The samples were grounded, ultrasonically dispersed in ethanol, and then sticked to a Cu grid. The surface electronic state of the samples was analyzed with an ESCALAB250 X-ray photoelectron spectroscopy with a monochromatic Al-Ka (1486.6 eV) radiation source and a charge neutralizer under ultrahigh vacuum (<2 109 Torr). All the binding energies were calibrated to C1s peak at 284.6 eV with an accuracy of ±0.2 eV.The powdered samples were pressed into small
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aluminum cylinders and then mounted on a sample rod placed in a pretreatment chamber and heated under vacuum at 373 K for 1 h prior to being moved into the analysis chamber. FTIR spectra were performed on a Bruker TENSOR 27 Fourier transform infrared spectrometer. The thick film of the sample was prepared by employing a mixture of KBr and the sample. The BrunauereEmmett-Teller (BET) specific surface area and pore-size distributions of the samples were evaluated by nitrogen adsorption-desorption isotherms at 77 K in a Beckman Coulter SA3100 gas adsorption apparatus. Prior to the measurement, the samples were outgassed at 373 K for 2 h under vacuum (<105 Torr). The pore size distributions were obtained from the adsorption branches of the isotherms according to the BarretteJoynereHalenda (BJH) model. Elemental analysis of Cd and Zn in the samples was determined with an inductively coupled plasma atomic emission spectrometer (ICP-AES, Plasam-Spec-II). Specifically, the powder samples were firstly dissolved in HCl/HNO3 at volume ratio of 3:1 (aqua regia), boiled for 10 min, and then diluted to concentrations within a detection limit of the instrument.
Photocatalytic hydrogen production The experiments on photocatalytic H2 production were conducted in a closed glass gas circulation and evacuation system with a top irradiation Pyrex vessel. A 300 W Xenon arc lamp (CEL-HXF300) equipped with an optical UV cut-off filter (l 420 nm) was used to provide visible light to trigger the photocatalytic reaction. A shutter window filled with water was placed between the lamp and the reaction pool to eliminate infrared irradiation. Another circulating water jacket was set around the reaction cell to ensure the thermostatted temperature at (283 ± 5 K) of the working solution. In a typical photocatalytic experiment, 50 mg of the ZnxCd1xS photocatalyst was dispersed with intensive magnetic stirring into 100 mL of aqueous solution containing 0.5 M Na2S and 0.5 M Na2SO3 as hole scavengers. Prior to the reaction, the system was evacuated several times to purge air and remove dissolved oxygen, and finally filled with argon of approximately 30 Torr. During the photocatalytic reaction, a continuous magnetic stirrer was applied at the bottom of the reaction cell to ensure the ZnxCd1xS particles in suspension status in the reaction solution. The amount of hydrogen evolved was quantified online by gas chromatography (GC) equipped with a thermal couple detector (TCD), argon as carrier gas and a molecular sieve 5 Å column. The photocatalytic performance was evaluated by the average rate of H2 evolution in the initial 4 h. The apparent quantum efficiency (4) was estimated by the formula: 4(%) ¼ (2NH2 /I) 100, where N is the rate of H2 evolution (molecules h1) and I is the rate of absorption of incident photons, which is typically 1.49 1021 photons h1 at l ¼ 420 ± 10 nm assuming all incident photons are absorbed by the catalyst [36].
Photocatalytic degradation of methylene blue The photocatalytic activities of ZnxCd1xS photocatalysts were also evaluated by degradation of methylene blue (MB) solution at room temperature under visible light irradiation, which was provided by the above-described 300 W Xenon
lamp equipped with a 420 nm cut-off filter. Typically, 20 mg of photocatalyst powder was dispersed in 100 mL of MB aqueous solution (3 105 mol/l) and stirred magnetically in the dark for 2 h to establish the adsorption-desorption equilibrium of MB on catalysts. At certain time intervals, 3 mL of reaction solution was continually withdrawn and centrifuged to remove the catalyst particles. The residual MB in the supernatant was analyzed by monitoring the characteristic absorption peak at 664 nm of MB using a UV-vis Perkin-Elmer spectrophotometer. The degradation efficiency (h) is evaluated as follows: h(%) ¼ (1 C/C0) 100%, where C0 and C are the initial concentration after the adsorption equilibrium and temporal concentration of MB at different times, respectively.
Photoelectrochemical measurements The transient photocurrent responses and electrochemical impedance spectroscopy (EIS) analysis were measured with a CHI604B electrochemical workstation (Shanghai Chenhua Instrument Corp., China) over a frequency range of 1 MHze100 mHz with an AC amplitude of 10 mV, using a standard three-compartment system in a quartz cell including the as-obtained samples coated on F-doped SnO2 (FTO) conductive glass s as the working electrode, a Pt foil as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode, respectively. The working electrode was fabricated according to the following procedure: firstly, FTO conductive glass was ultrasonically in turn cleaned with a detergent solution, deionized water, acetone and ethanol. Secondly, 0.05 g powder of photocatalyst was dispersed evenly in the mixture of polyethylene glycol (PEG; molecular weight: 20000; 0.2 g) and 0.5 mL of ethanol by ultrasonication to make a homogeneous slurry. The slurry was then casted onto the surface of 0.36 cm2 FTO glass substrate by the screen-printing technique. After drying by evaporation under ambient conditions, the resultant electrode was finally sintered under a flow of nitrogen gas at 450 C for 30 min. All electrodes used in the experiments had a similar film thickness (10e11 mm). 0.5 M Na2Se0.5 M Na2SO3 aqueous solution was used as the electrolyte. A 300 W Xe amp with a cut-off filter (l 420 nm) was used to provide visible light.
Results and discussion ZnxCd1xS nanoparticles were fabricated by an L-cystineassisted one-pot hydrothermal reaction. As reported in previous work [32e35], the coordination interaction between Zn2þ, Cd2þ ions and L-cystine plays a key role. Initially, Lcystine interacts with Zn2þ and Cd2þ ions in strong alkali medium through the combination of metal cation and the thiol chain to form complexes at room temperature. Then at elevated temperature and autogenerated vapor pressure, the complexes undergo thermal decomposition to release S2 ions slowly, which recombine with Zn2þ/Cd2þ ions to generate ZnxCd1xS nanocrystals. Powder X-ray diffraction technique was employed to analyze the crystal structure and crystallinity of the ZnxCd1xS photocatalysts prepared by the hydrothermal method. The XRD patterns of a series of ZnxCd1xS (x ¼ 0, 0.1, 0.3, 0.5, 0.7, 0.9,
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and 1.0) samples together with the standard diffraction peaks of hexagonal wurtzite CdS and ZnS reported by the JCPDS, are shown in Fig. 1. All samples exhibit similar characteristic diffraction profiles of hexagonal wurtzite structure CdS (JCPDS Card No. 41-1049, space group: P63mc) and ZnS (JCPDS Card No. 36-1450). The absence of any peaks assignable to cubic phase or other impurities indicates the high purity of the samples. With increasing amount of Zn content, the ionic radius of A) relative to that of Cd2þ (0.97 A) [37]. The successive Zn2þ(0.74 shifts of the XRD patterns reveals the as-obtained samples are not a merely physical mixture of hexagonal CdS and ZnS but the homogeneous ZnxCd1xS solid solutions in wurtzite structure by introducing Zn2þ into the lattice of hexagonal CdS. Moreover, the electronegativity for Cd (1.69) diffraction peak positions of CdS are appreciably shifted to higher angles, which can be attributed to the fringe lattice constriction resulting from Zn2þ incorporating owing to the smaller is almost equal to that of Zn (1.65, Pauling scale), which is favorable to the nucleation of alloyed ZneCdeS solid solutions [38]. Meanwhile, the structural similarity between ZnS and CdS proved by XRD data and the relatively small discrepancy (8%) in the bondlength between CdS and ZnS also facilitates the formation of homogeneous ZneCdeS alloyed structure [38]. The a-axis and c-axis lengths for hexagonal ZnxCd1xS solid solutions are calculated by fitting the XRD patterns (Fig. 1) according to the MDI Jade5.0 software, as listed in Table 1. Meanwhile, the unit cell volumes of hexagonal ZnxCd1xS solid solutions can be estimated by using the equation pffiffiffi V ¼ 3=2a2 c and the results are also shown in Table 1. Both the lattice constants (a, c) and the unit cell volumes of hexagonal ZnxCd1xS solid solutions are plotted as a function of Zn mole fractions (x), as displayed in Fig. 2aec, respectively. It is found that both the unit cell constants (a and c) and the unit cell volumes decrease almost linearly as the Zn mole fractions increase. This observation quite obeys the well-known Vegard's law [39,40], which rules out the phase separation and separated nucleation of hexagonal CdS or ZnS nanocrystals by the present hydrothermal process in the presence of L-cystine as the S source. In addition, the mean crystallite sizes of the ZnxCd1xS samples were determined from the
Fig. 1 e XRD patterns of ZnxCd1¡xS solid solutions with various x values: (a) 0, (b) 0.1, (c) 0.3, (d) 0.5, (e) 0.7, (f) 0.9 and (g) 1.
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full-width-at-half-maximum of (111) diffraction plane by the Scherrer equation and the results were also listed in Table 1. It can be seen that all samples were nanosized. The crystalline sizes of the ZnxCd1xS solid solutions are between 39 nm for CdS and 29 nm for ZnS. Fig. 3 presents the UV-vis diffuse-reflectance absorption spectra and corresponding colors of the as-synthesized ZnxCd1xS (0 x 1.0) solid solutions. All ZnxCd1xS samples show an intense absorption band with a steep absorption edge, demonstrating the absorption is assigned to an intrinsic band-gap transition from the valence band to the conduction band rather than the transition from impurity levels. The absorption edges of ZnxCd1xS solid solutions were located between those of CdS and ZnS phases, and gradually blueshifted to shorter wavelengths with increasing Zn content in the solid solutions, which provides another strong evidence for the homogeneous alloying of ZnxCd1xS not a simple physical mixture of CdS and ZnS. The observation is also consistent with the consecutive shift of XRD diffraction peaks to higher angles shown in Fig. 1 and the gradually changed colors of the solid solutions from yellow to white between CdS and ZnS. According to the empirical function ahn ¼ A(hn Eg)n [41], the band gaps (Eg) of the ternary alloyed ZnxCd1xS nanomaterials can be determined from the Tauc plots of (ahn)2 versus hn (the incident photon energy) by linearly extrapolating to zero cross the X axis, where a is the absorption coefficient (the absorbance in Fig. 3), h is Planck's constant and A is the probability parameter for the transition, respectively. As shown in the inset of Fig. 3, the band gaps of alloyed ZnxCd1xS can be adjusted between 2.11 and 3.21 eV. These results implied that the band gaps of the ZnxCd1xS solid solutions can be precisely controlled by modifying the molar ratios of Zn/Cd precursors. The morphology and size of the as-synthesized ZnxCd1xS solid solutions were investigated by transmission electron microscopy (TEM). Fig. 4 displays the TEM images of ZnS, CdS and Zn0.5Cd0.5S samples, respectively. It can be seen that all samples were composed of numerous tiny nanoparticles. The average diameter of nanoparticles was 44 nm for CdS, 25 nm for Zn0.5Cd0.5S and 31 nm for ZnS, respectively, which are in line with the data calculated from XRD peaks. The welldefined lattice fringe of HRTEM of Zn0.5Cd0.5S demonstrates its highly crystalline nature, as displayed in Fig. S1. The lattice spacing is about 0.344 nm, corresponding to the (100) plane of hexagonal CdS. The surface compositions and electronic structures of Zn0.5Cd0.5S sample were ascertained by X-ray photoelectron spectra (XPS) using the photo energy of 1486.6 eV, as shown in Fig. 5. The spectrum of survey scan clearly demonstrates the presence of Zn, Cd and S as well as C and O elements. The appearance of a very small amount of C and O signals is due to the carbon tape for measurement and the adsorbed oxygen, respectively. In addition, the doublet peaks for Zn 2p (1022.4 and 1045.5 eV), Cd 3d (411.6 and 405.2 eV), and S2p (161.1 and 162.3 eV) in the high-resolution XPS spectra of Fig. 5bed are attributed to the spin-orbit separation components. The bind energies are in well consistent with the data reported in the literature [42]. Additionally, the peak of N1s at 405.2 eV is also observed (Fig. 5e), which is ascribed to the amine group (the nitrogen from eNHe of L-cystine) [43], implying a very small
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Table 1 e Summary of materials properties of various ZnxCd1¡xS samples. Sample
CdS Zn0.1Cd0.9S Zn0.3Cd0.7S Zn0.5Cd0.5S Zn0.7Cd0.3S Zn0.9Cd0.1S ZnS
Atomic ratio of Zn:Cd
Lattice constants ( A)
Cell volume ( A3)
In precursor
ICP
EDS
a
c
v
0:1 0.1:0.9 0.3:0.7 0.5:0.5 0.7:0.3 0.9:0.1 1:0
0:1 0.09:0.91 0.32:0.68 0.51:0.49 0.70:0.30 0.88:0.12 1:0
0:1 0.10:0.90 0.32:0.68 0.46:0.54 0.69:0.31 0.89:0.11 1:0
4.1370 4.1070 4.0320 3.9750 3.9220 3.8430 3.8240
6.7180 6.6900 6.5800 6.5570 6.4050 6.3370 6.2590
99.5730 97.6955 92.6518 88.8516 85.3054 81.0566 79.2457
XS (nm)
TEM (nm)
Specific surface area (m2 g1)
Band gap (eV)
39.0 25.7 25.2 19.1 21.4 26.0 28.9
43.8 37.5 31.3 25.0 18.8 25.0 31.3
33.2 40.6 37.8 42.1 53.3 50.2 48.9
2.11 2.16 2.21 2.32 2.42 2.53 3.21
Fig. 2 e Dependence of lattice constants and the unit cell volumes on the Zn content (x) of ZnxCd1¡xS solid solutions.
Fig. 3 e UV-Vis diffuse reflection absorption spectra, digital photograph (inset: bottom-left) and the plots of (ahn)2 vs hn (inset: top-right) of ZnxCd1¡xS solid solutions with various x values: (a) 0, (b) 0.1, (c) 0.3, (d) 0.5, (e) 0.7, (f) 0.9 and (g) 1.
amount of L-cystine molecules adsorbed on the surface of Zn0.5Cd0.5S sample during the synthesis period. This is further proved by FT-IR technique, as displayed in Fig. S2. The peaks at 1630 and 1406 cm1 are assigned to the NeH deformation vibration and eCN bonding [44,45], respectively, indicating the existence of L-cystine on the surface of Zn0.5Cd0.5S.Besides, the metal chemical compositions from the surface and the bulk of ZnxCd1xS photocatalyst were further checked by EDS and ICP analyses, respectively. The results are also summarized in Table 1. It can be found that the metal element compositions of ZnxCd1xS solid solutions fabricated by the present hydrothermal route are very close to those designed in the synthesis
procedure. Thus for the sake of convenience, the samples are addressed with their stoichiometric compositions. To identify the porous nature of the as-synthesized nanoparticles, the typical N2 adsorption-desorption isotherms and the corresponding BJH (BarreteJoynereHalenda) pore-size distribution curves (inset) of the as-obtained CdS, Zn0.5Cd0.5S and ZnS samples are displayed in Fig. 6. Obviously, all the isotherms can be classified as type IV with apparent H3 hysteresis loops with high adsorption in the high relative pressure (P/P0) range from 0.8 to 1.0, indicating the presence of large mesopores and macropores. As shown in the inset of Fig. 6, a wide pore size distribution of 2e300 nm is observed, further confirming the existence of mesoporous and macroporous structure in the samples. The macropores (50e300 nm) probably arise from the interspaces formed between the nanoparticles. The photocatalytic activities of H2-production over ZnxCd1xS solid solutions were systematically evaluated under visible light irradiation (l > 420 nm) in an aqueous solution containing S2 and SO2 3 ions as hole scavengers without the presence of any co-catalysts. As presented in Fig. 7a, the H2evolution activity is strongly dependent on the compositions of ZnxCd1xS solid solutions. All ZnxCd1xS solid solutions exhibit much better performance for photocatalytic water reduction than that of bulk CdS as well as ZnS. No H2 is detected over ZnS alone because ZnS with a large band gap (3.21 eV) is only active under UV light [46]. Naked CdS demonstrates poor photocatalytic activity for H2 evolution (576 mmol h1 g1), due to the rapid recombination of photoinduced eehþ pairs. With the introduction of Zn2þ into CdS, the rate of H2 evolution correspondingly increases. A remarkable improvement in the H2evolution activity is observed as the amount of Zn2þ is above 0.1. Surprisingly, a super high H2-production performance of
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Fig. 4 e TEM images of CdS, Zn0.5Cd0.5S and ZnS samples.
Fig. 5 e XPS data of Zn0.5Cd0.5S solid solution: (a) survey spectrum, and high-resolution XPS spectra of: (b) Zn2p, (c) Cd3d, (d) S2p and (e) N1s.
18.3 mmol h1 g1 is achieved over naked Zn0.5Cd0.5S catalyst, exceeding that of sole CdS by a factor of over 32 times. The apparent quantum yield is calculated to be as high as of 73.8% at 420 nm, which, to our knowledge, also outperforms previously reported cocatalyst-free metal chalcogenide photocatalysts [14e19,24]. A further increase of Zn2þ content in the synthesis solution leads to a dramatic deterioration of the photocatalytic performance under the identical experimental conditions. For instance, in the case of x ¼ 0.9, the H2-
generation rate is decreased to only 294 mmol h1 g1 with a Q. E. of 1.2%. Thus, a suitable molar ratio of Zn/Cd is crucial for optimizing the photocatalytic performance of alloyed hZneCdeS solid solutions. The stability is a vital factor that affects the practical applications of photocatalysts in solar conversion. In our experiments, the stability of the photocatalyst with the maximum H2-production rate, Zn0.5Cd0.5S, was further assessed by monitoring the H2-evolution from aqueous
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Fig. 6 e Nitrogen adsorption-desorption isotherms and corresponding BJH pore-size distribution curves (inset) of CdS, Zn0.5Cd0.5S and ZnS samples.
solutions containing S2 and SO2 ions during consecutive 3 recycle experiments under visible light irradiation, as displayed in Fig. 7b. It is found that there is a slight decrease of H2 production activity in the second cycle, which possibly comes from the consumption of the sacrificial reagents. After the catalyst was collected by centrifugation and re-dispersed into the fresh mixed solution of S2 and SO2 3 , the H2-evolution activity is recovered in the third cycle. Moreover, no obvious differences can be detected from both XRD patterns and TEM images of the catalyst before and after the photocatalytic H2production reactions (Fig. S3 and S4). These results indicate that the as-synthesized Zn0.5Cd0.5S solid solution is quite stable against photocorrosion in the presence of sacrificial agents during the photocatalytic reaction under visible light. In this case, L-cystine, as an organic S source, is prone to be dissolved into water medium under strong alkali condition (pH ¼ 10e11). The catalyst crystallizing in the water medium of hydrothermal conditions over a prolonged period is beneficial to achieving the high stability for photocatalytic water reduction under visible light irradiation. The excellent photocatalytic stability of ZnxCd1xS solid solutions implies their promising potential in practical applications. The photocatalytic performance of as-prepared ZnxCd1xS nanoparticles was also tested by measuring the decolorization
of MB in aqueous solution under visible light irradiation (l > 420 nm). The photodegradation efficiency of ZnxCd1xS catalysts is shown in Fig. S5. Similarly to the photoreduction of H2O to H2 mentioned as above, Zn0.5Cd0.5S sample is the most efficient in the photocatalytic degradation of MB. The stability test of representative Zn0.5Cd0.5S powder by performing consecutive recycle experiments under the same reaction conditions is also demonstrated in Fig. S5. The photodegradation efficiency after each run is calculated to be 95%, 92%, and 91%, respectively. The slightly decrease in the photocatalytic activity for MB degradation can be mainly ascribed to the loss of the photocatalyst powder during the process of washing and centrifugation. These results also demonstrate the excellent photocatalytic performance and good stability of ZnxCd1xS nanocrystals. To investigate the origin of the drastically enhanced performance of ZnxCd1xS solid solutions compared to that of CdS, the conduction band (CB) edge position can be evaluated empirically according to Butler and Ginley's approach [27,47], as shown in the following equation: ECB ¼ X Ee e 0.5Eg. Here, ECB is the conduction band potential, Ee equals to a constant as 4.5 eV for the energy of free electrons on the hydrogen scale, Eg is the band gap energy of the semiconductor, estimated from Tauc Plot from UV-visible spectra shown in Fig. 3, and X represents the geometric mean of the absolute electronegativity of the constituent, and is denoted as the arithmetic mean of the atomic electron affinity and the first ionization energy. Accordingly, the valence band (VB) potential of the semiconductor is determined by the equation: EVB ¼ ECB þ Eg. On basis of the calculated results above, a schematic band structure is proposed for ZnxCd1xS solid solutions, as shown in Fig. 8. The conduction and valence band potentials of ZnxCd1xS solid solutions are located between those of CdS and ZnS as well as their band gap energies. With increasing Zn content, the levels of the conduction and valence band of ZnxCd1xS solid solutions are shifted toward more negative potential and more positive potential, which results in an obvious increase in the band energy of ZnxCd1xS solid solutions. Generally, more negative potential of the conduction band provides a larger thermodynamic driving force to reduce water to H2 and more positive potential of the valence band makes holes owning a much stronger ability of
Fig. 7 e Comparisons of the H2-evolution rate over various ZnxCd1¡xS solid solution samples under visible light irradiation. Reaction conditions: 50 mg catalyst, 100 mL aqueous solution containing 0.5 M Na2S and 0.5 M Na2SO3, 300 W Xe lamp with an optical filter (l > 420 nm). (b) Stability test by measuring the time courses of H2 evolution over Zn0.5Cd0.5S solid solution under visible light irradiation.
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Fig. 8 e Schematic energy level diagram of ZnxCd1¡xS solid solutions, CdS as well as ZnS.
Fig. 9 e (a) Transient photocurrent responses and (b) electrochemical impedance spectra (EIS) Nyquist plots of CdS, Zn0.5Cd0.5S and ZnS electrodes in an aqueous solution of 0.5 M Na2S and 0.5 M Na2SO3 under visible light illumination (l > 420 nm).
oxidation. Besides, a suitable narrow band gap allows for efficient utilization of incident light. These parameters are beneficial to improve the photocatalytic performance. Here, the best photocatalytic activity is achieved over Zn0.5Cd0.5S solid solution under visible light, suggesting that the catalyst possesses an optimum band gap with a moderate edge potential of the conduction and valence band. In addition, the intrinsic electronic and charge transport behavior of photoinduced electrons and holes play a crucial role in the photocatalytic process [14,21,48e50]. Here both the transient photocurrent (TPC) response and electrochemical impedance spectroscopy (EIS) analysis are used to investigate the surface and bulk charge-transfer process occurring in the three-electrode system. Fig. 9a compares the photocurrentetime (Iet) curves for as-fabricated CdS, Zn0.5Cd0.5S and ZnS electrodes with five on-off cycles of intermittent visible light illumination. As expected, all the electrodes show obvious photoresponse as the light turns on, which decreases quickly to a steady state once the light is switched off. The onoff cycles of the photocurrent are reproducible. Impressively, the photocurrent density of Zn0.5Cd0.5S electrode is clearly enhanced as compared to those of ZnS and CdS electrodes, indicating the improvement of the charge transfer and the suppression of the charge recombination [51e53].
Correspondingly, the representative EIS Nyquist plots for CdS, Zn0.5Cd0.5S and ZnS electrodes are displayed in Fig. 9b. Generally, a semicircular arc reflects a lower charge-transfer resistance [14,53e55]. Among the three samples, Zn0.5Cd0.5S solid solution electrode exhibits the smallest radius of arc, implying much easily charge transfer in Zn0.5Cd0.5S solid solution, which is in consistent with its higher photocurrent transient response. Based on the results described as above, it is reasonable that the interfacial charge separation and transfer of photoinduced electronehole pairs is greatly facilitated due to the formation of alloyed Zn5Cd0.5S solid solutions, which in turn remarkably boosts the photocatalytic performance for water decomposition and MB degradation under visible light.
Conclusions In summary, we demonstrated for the first time a green and facile route for the preparation of visible-light-responsive ZnxCd1xS solid solution photocatalyst with hexagonal phase by L-cystine-assisted hydrothermal approach, which is very promising for large-scale production. The as-obtained ZnxCd1xS nanocrystals exhibit the enhanced activity and excellent stability for photocatalytic hydrogen evolution and
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dye degradation under visible light irradiation without loading any cocatalyst. Specially, cocatalyst-free Zn0.5Cd0.5S shows a super high H2-production activity of 18.3 mmol h1 g1, exceeding that of CdS by a factor of 32. Besides, the optimal Q.E. of 73.8% is achieved at 420 nm, outperforming the previously-reported cocatalyst-free metal sulfide photocatalysts. The unusual photocatalytic activity can be attributed to the strong absorption of visible light photons and appropriate edge potential of conduction and valence band along with the highly efficient charge-transport due to the formation of alloyed ZneCdeS solid solution. This work represents a significant advance in the green synthesis of metal chalcogenides, which can be used directly (cocatalyst-free) for highly-efficient solar H2 production.
Acknowledgments This work was financially supported by the projects of the National Natural Science Foundation of China (No. 21573100). We acknowledge the support from the Open Project of State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences (No. N-14-04).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.06.196.
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Biomolecule-assisted hydrothermal synthesis of ZnxCd1¡xS nanocrystals and their outstanding photocatalytic performance for hydrogen production Meiying Liu a,*, Yunfei He a, Hong Chen a, Hongmei Zhao a, Wansheng You a, Jingying Shi b,**, Lancui Zhang a, Jiansheng Li a a
School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian, Liaoning 116029, PR China State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian, Liaoning 116023, PR China
b
article info
abstract
Article history:
To achieve scalable applications in solar hydrogen production, it is necessary to develop
Received 6 February 2017
visible-light-responsive photocatalysts that are highly efficient, cost-effective, stable and
Received in revised form
environmentally-benign. Here narrow bandgap ZneCdeS solid solution photocatalysts
21 June 2017
(Eg ¼ 2.11e2.53 eV) were prepared via a facile and green hydrothermal strategy under mild
Accepted 23 June 2017
conditions. Amazingly, over the naked Zn0.5Cd0.5S photocatalyst, an extraordinarily high
Available online 22 July 2017
H2 production activity in Na2SeNa2SO3 aqueous solution is achieved up to 18.3 mmol h1 g1 with an apparent quantum efficiency of 73.8% per 50 mg under 420 nm light irradiation,
Keywords:
which, to our knowledge, outperforms cocatalyst-free metal sulfide photocatalysts previ-
ZnxCd1xS
ously reported to date. Such super high performance arises from the enhanced visible-
Hydrogen production
light-absorption capacity, suitable conduction and valence band potential together with
Water splitting
the facilitated charge transport in ZneCdeS solid solutions. This work may open an avenue
L-cystine
for the green preparation of inexpensive photocatalysts for solar H2 production.
Green synthesis
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solar-driven photocatalytic water splitting over semiconductor catalysts is an essential and challenging subject of research which has received immense attention thanks to its capability of producing clean hydrogen fuel from renewable resources to address global energy and environmental problems [1]. In the past few decades, numerous metal oxides such
as TiO2 [2,3], SrTiO3 [4], NaTaO3 [5] and KNb6O17 [6], have been extensively investigated. However, these oxide photocatalysts cannot harness visible light (major proportion of the solar spectrum) due to their large band gaps (>3 eV), which strongly renders their commercial applications. In view of the efficient utilization of solar light, it remains highly desirable to develop visible-light-responsive photocatalysts for hydrogen production.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Liu), [email protected] (J. Shi). http://dx.doi.org/10.1016/j.ijhydene.2017.06.196 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Unlike the above-mentioned oxides, metal chalcogenides have been regarded as promising candidates for visible-lightresponsive photocatalysts because of their narrow band gap and negative valence edge position [7,8]. Considering the suitable bandgap and sufficiently negative conduction band position to thermodynamically drive the water reduction reaction, CdS is an ideal photocatalyst which has been a focus of huge interest [9]. Unfortunately, excessive recombination of photogenerated eehþ pairs, severe photocorrosion as well as the requirement of noble metals cocatalysts restricts its commercial application [10e12]. An effective strategy to overcome these drawbacks is to fabricate ZnxCd1xS solid solution photocatalysts based on CdS and wide bandgap semiconductor materials of ZnS (Eg ¼ 3.6 eV) to facilitate the separation and transport of the photogenerated charge carriers [13]. Up to date, tremendous efforts have been devoted to the synthesis of ZnxCd1xS solid solutions with a variety of sizes and morphologies to achieve high photocatalytic performance [14e22]. However, these techniques usually require high temperatures over 160 C together with expensive and poisonous sulfur sources such as Na2S [14], sulfur powder [15], thioactamide [16], thiourea [17e19], dimethyl sulfoxide [20], thioglycolic acid [21] as well as thiols [22], resulting in the liberation of nauseous scent H2S. Besides, some toxic chemicals including organic solvents [23,24], surfactants [25], template [26] and reducing agents [27] are used as additive. Consequently, it is still a great challenge for the high-through synthesis of robust ZnxCd1xS solid solutions by a facile, inexpensive and environment-friendly approach under mild conditions. Owing to special structures and fascinating self-assembly functions of biomolecules, the biomolecule-directed synthesis is very attractive to be a green and cost-effective route for synthesis of the various metal sulfide nanostructures [28e31]. L-cystine is such a biomolecule that contains hydrosulphonyl (eSH), amino (eNH2) and carboxyl (eCOOH) functional groups in its molecular structure, which enables it to combine with the most metal cations by ligand bonds and thus it is an alternative and versatile sulfur source directing for the synthesis of various metal sulphides [32e34]. Previously we reported the L-cystine-assisted synthesis of alloyed MneCdeS photocatalysts with high performance for solar H2 production from water decomposition [35]. In this work, we report an L-cystine-assisted hydrothermal route at moderate temperature to prepare nanosized ZnxCd1xS solid solutions with hexagonal wurtzite structure, which, to our best knowledge, has seldom been reported so far. Herein, L-cystine functions not only the sulfur source but also the coordinating agent, no additional ligand or template is needed for the synthesis. Thus, the compositions, sizes, shapes and properties of ZnxCd1xS solid solutions are easily controlled through adjusting the molar ratios of the precursors. In addition, homininoxious sulfur precursors and solvents are completely avoided. More significantly, the asprepared ZnxCd1xS solid solution photocatalysts are highly efficient towards the visible-light photocatalytic H2-production in Na2SeNa2SO3 aqueous solution. The optimal quantum efficiency reaches 73.8% per 50 mg at 420 nm, which, to our best knowledge, outperforms previously-reported cocatalyst-
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free metal sulfide photocatalysts [16e21,26]. The origin of the super high performance is investigated by calculating the electronic structure together with the EIS and TPC techniques.
Experimental procedure Synthesis A series of ZnxCd1xS photocatalysts were synthesized through the following procedure. Firstly, Zn(OAc)2$2H2O and Cd(OAc)2$2H2O with a specified x value in ZnxCd1xS (x ¼ 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) and a total molar amount of 3 mmol were added into 16 mL of deionized water to form a clear solution. Simultaneously, L-cystine (12 mmol) was dissolved into another 16 mL of deionized water with magnetic stirring, in which the pH of the mixed solution was adjusted to 10e11 using 6 M NaOH aqueous solution. Secondly, the above two solutions were slowly mixed together with vigorously stirring to form a milky suspension under ambient condition. Finally the as-generated mixed slurry was transferred into Teflon-lined stainless steel vessel of 45 mL capacity and maintained at 140 C for 10 h under autogenous pressure. After being cooled naturally to room temperature, the products were collected, rinsed with deionized water and ethanol for several centrifugation cycles, and then dried in an oven at 60 C overnight prior to use.
Characterization The crystallographic structure and phase of the synthesized ZnxCd1xS samples were identified by powder X-ray diffraction (XRD) analysis on a Bruker D8 Advance diffractometer with monochromatized Cu-Ka irradiation in Debye-Scherrer geometry. The sample was scanned in the 2q range from 20 to 70 at a step of 0.05 S1. The operating voltage and the applied current were 40 kV and 40 mA, respectively. The average crystallite size of the photocatalysts was calculated from the broadening of X-ray line by the Scherrer formula as follows: D ¼ 0.89l/b cosq, where l is the wavelength of Cu-Ka irradiation and equals to 1.54178 A, b is the full-width at halfmaximum and q is the diffraction angle, respectively. UVvisible spectra in reflection mode were recorded on a PerkinElmer Lambda 3 spectrophotometer equipped with an integrating sphere by adopting BaSO4 as a 100% reference standard and were converted from reflectance to absorbance by the Kubelka-Munk function. The morphology, composition and particle size of the samples were revealed by transmission electron microscopy (TEM, FEI Tecnai Spirit) and a high resolution transmission electron microscope (HRTEM, FEI Tecnai F30) together with an energy-dispersive X-ray spectrometer (EDS) performed at 300 kV, respectively. The samples were grounded, ultrasonically dispersed in ethanol, and then sticked to a Cu grid. The surface electronic state of the samples was analyzed with an ESCALAB250 X-ray photoelectron spectroscopy with a monochromatic Al-Ka (1486.6 eV) radiation source and a charge neutralizer under ultrahigh vacuum (<2 109 Torr). All the binding energies were calibrated to C1s peak at 284.6 eV with an accuracy of ±0.2 eV.The powdered samples were pressed into small
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aluminum cylinders and then mounted on a sample rod placed in a pretreatment chamber and heated under vacuum at 373 K for 1 h prior to being moved into the analysis chamber. FTIR spectra were performed on a Bruker TENSOR 27 Fourier transform infrared spectrometer. The thick film of the sample was prepared by employing a mixture of KBr and the sample. The BrunauereEmmett-Teller (BET) specific surface area and pore-size distributions of the samples were evaluated by nitrogen adsorption-desorption isotherms at 77 K in a Beckman Coulter SA3100 gas adsorption apparatus. Prior to the measurement, the samples were outgassed at 373 K for 2 h under vacuum (<105 Torr). The pore size distributions were obtained from the adsorption branches of the isotherms according to the BarretteJoynereHalenda (BJH) model. Elemental analysis of Cd and Zn in the samples was determined with an inductively coupled plasma atomic emission spectrometer (ICP-AES, Plasam-Spec-II). Specifically, the powder samples were firstly dissolved in HCl/HNO3 at volume ratio of 3:1 (aqua regia), boiled for 10 min, and then diluted to concentrations within a detection limit of the instrument.
Photocatalytic hydrogen production The experiments on photocatalytic H2 production were conducted in a closed glass gas circulation and evacuation system with a top irradiation Pyrex vessel. A 300 W Xenon arc lamp (CEL-HXF300) equipped with an optical UV cut-off filter (l 420 nm) was used to provide visible light to trigger the photocatalytic reaction. A shutter window filled with water was placed between the lamp and the reaction pool to eliminate infrared irradiation. Another circulating water jacket was set around the reaction cell to ensure the thermostatted temperature at (283 ± 5 K) of the working solution. In a typical photocatalytic experiment, 50 mg of the ZnxCd1xS photocatalyst was dispersed with intensive magnetic stirring into 100 mL of aqueous solution containing 0.5 M Na2S and 0.5 M Na2SO3 as hole scavengers. Prior to the reaction, the system was evacuated several times to purge air and remove dissolved oxygen, and finally filled with argon of approximately 30 Torr. During the photocatalytic reaction, a continuous magnetic stirrer was applied at the bottom of the reaction cell to ensure the ZnxCd1xS particles in suspension status in the reaction solution. The amount of hydrogen evolved was quantified online by gas chromatography (GC) equipped with a thermal couple detector (TCD), argon as carrier gas and a molecular sieve 5 Å column. The photocatalytic performance was evaluated by the average rate of H2 evolution in the initial 4 h. The apparent quantum efficiency (4) was estimated by the formula: 4(%) ¼ (2NH2 /I) 100, where N is the rate of H2 evolution (molecules h1) and I is the rate of absorption of incident photons, which is typically 1.49 1021 photons h1 at l ¼ 420 ± 10 nm assuming all incident photons are absorbed by the catalyst [36].
Photocatalytic degradation of methylene blue The photocatalytic activities of ZnxCd1xS photocatalysts were also evaluated by degradation of methylene blue (MB) solution at room temperature under visible light irradiation, which was provided by the above-described 300 W Xenon
lamp equipped with a 420 nm cut-off filter. Typically, 20 mg of photocatalyst powder was dispersed in 100 mL of MB aqueous solution (3 105 mol/l) and stirred magnetically in the dark for 2 h to establish the adsorption-desorption equilibrium of MB on catalysts. At certain time intervals, 3 mL of reaction solution was continually withdrawn and centrifuged to remove the catalyst particles. The residual MB in the supernatant was analyzed by monitoring the characteristic absorption peak at 664 nm of MB using a UV-vis Perkin-Elmer spectrophotometer. The degradation efficiency (h) is evaluated as follows: h(%) ¼ (1 C/C0) 100%, where C0 and C are the initial concentration after the adsorption equilibrium and temporal concentration of MB at different times, respectively.
Photoelectrochemical measurements The transient photocurrent responses and electrochemical impedance spectroscopy (EIS) analysis were measured with a CHI604B electrochemical workstation (Shanghai Chenhua Instrument Corp., China) over a frequency range of 1 MHze100 mHz with an AC amplitude of 10 mV, using a standard three-compartment system in a quartz cell including the as-obtained samples coated on F-doped SnO2 (FTO) conductive glass s as the working electrode, a Pt foil as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode, respectively. The working electrode was fabricated according to the following procedure: firstly, FTO conductive glass was ultrasonically in turn cleaned with a detergent solution, deionized water, acetone and ethanol. Secondly, 0.05 g powder of photocatalyst was dispersed evenly in the mixture of polyethylene glycol (PEG; molecular weight: 20000; 0.2 g) and 0.5 mL of ethanol by ultrasonication to make a homogeneous slurry. The slurry was then casted onto the surface of 0.36 cm2 FTO glass substrate by the screen-printing technique. After drying by evaporation under ambient conditions, the resultant electrode was finally sintered under a flow of nitrogen gas at 450 C for 30 min. All electrodes used in the experiments had a similar film thickness (10e11 mm). 0.5 M Na2Se0.5 M Na2SO3 aqueous solution was used as the electrolyte. A 300 W Xe amp with a cut-off filter (l 420 nm) was used to provide visible light.
Results and discussion ZnxCd1xS nanoparticles were fabricated by an L-cystineassisted one-pot hydrothermal reaction. As reported in previous work [32e35], the coordination interaction between Zn2þ, Cd2þ ions and L-cystine plays a key role. Initially, Lcystine interacts with Zn2þ and Cd2þ ions in strong alkali medium through the combination of metal cation and the thiol chain to form complexes at room temperature. Then at elevated temperature and autogenerated vapor pressure, the complexes undergo thermal decomposition to release S2 ions slowly, which recombine with Zn2þ/Cd2þ ions to generate ZnxCd1xS nanocrystals. Powder X-ray diffraction technique was employed to analyze the crystal structure and crystallinity of the ZnxCd1xS photocatalysts prepared by the hydrothermal method. The XRD patterns of a series of ZnxCd1xS (x ¼ 0, 0.1, 0.3, 0.5, 0.7, 0.9,
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and 1.0) samples together with the standard diffraction peaks of hexagonal wurtzite CdS and ZnS reported by the JCPDS, are shown in Fig. 1. All samples exhibit similar characteristic diffraction profiles of hexagonal wurtzite structure CdS (JCPDS Card No. 41-1049, space group: P63mc) and ZnS (JCPDS Card No. 36-1450). The absence of any peaks assignable to cubic phase or other impurities indicates the high purity of the samples. With increasing amount of Zn content, the ionic radius of A) relative to that of Cd2þ (0.97 A) [37]. The successive Zn2þ(0.74 shifts of the XRD patterns reveals the as-obtained samples are not a merely physical mixture of hexagonal CdS and ZnS but the homogeneous ZnxCd1xS solid solutions in wurtzite structure by introducing Zn2þ into the lattice of hexagonal CdS. Moreover, the electronegativity for Cd (1.69) diffraction peak positions of CdS are appreciably shifted to higher angles, which can be attributed to the fringe lattice constriction resulting from Zn2þ incorporating owing to the smaller is almost equal to that of Zn (1.65, Pauling scale), which is favorable to the nucleation of alloyed ZneCdeS solid solutions [38]. Meanwhile, the structural similarity between ZnS and CdS proved by XRD data and the relatively small discrepancy (8%) in the bondlength between CdS and ZnS also facilitates the formation of homogeneous ZneCdeS alloyed structure [38]. The a-axis and c-axis lengths for hexagonal ZnxCd1xS solid solutions are calculated by fitting the XRD patterns (Fig. 1) according to the MDI Jade5.0 software, as listed in Table 1. Meanwhile, the unit cell volumes of hexagonal ZnxCd1xS solid solutions can be estimated by using the equation pffiffiffi V ¼ 3=2a2 c and the results are also shown in Table 1. Both the lattice constants (a, c) and the unit cell volumes of hexagonal ZnxCd1xS solid solutions are plotted as a function of Zn mole fractions (x), as displayed in Fig. 2aec, respectively. It is found that both the unit cell constants (a and c) and the unit cell volumes decrease almost linearly as the Zn mole fractions increase. This observation quite obeys the well-known Vegard's law [39,40], which rules out the phase separation and separated nucleation of hexagonal CdS or ZnS nanocrystals by the present hydrothermal process in the presence of L-cystine as the S source. In addition, the mean crystallite sizes of the ZnxCd1xS samples were determined from the
Fig. 1 e XRD patterns of ZnxCd1¡xS solid solutions with various x values: (a) 0, (b) 0.1, (c) 0.3, (d) 0.5, (e) 0.7, (f) 0.9 and (g) 1.
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full-width-at-half-maximum of (111) diffraction plane by the Scherrer equation and the results were also listed in Table 1. It can be seen that all samples were nanosized. The crystalline sizes of the ZnxCd1xS solid solutions are between 39 nm for CdS and 29 nm for ZnS. Fig. 3 presents the UV-vis diffuse-reflectance absorption spectra and corresponding colors of the as-synthesized ZnxCd1xS (0 x 1.0) solid solutions. All ZnxCd1xS samples show an intense absorption band with a steep absorption edge, demonstrating the absorption is assigned to an intrinsic band-gap transition from the valence band to the conduction band rather than the transition from impurity levels. The absorption edges of ZnxCd1xS solid solutions were located between those of CdS and ZnS phases, and gradually blueshifted to shorter wavelengths with increasing Zn content in the solid solutions, which provides another strong evidence for the homogeneous alloying of ZnxCd1xS not a simple physical mixture of CdS and ZnS. The observation is also consistent with the consecutive shift of XRD diffraction peaks to higher angles shown in Fig. 1 and the gradually changed colors of the solid solutions from yellow to white between CdS and ZnS. According to the empirical function ahn ¼ A(hn Eg)n [41], the band gaps (Eg) of the ternary alloyed ZnxCd1xS nanomaterials can be determined from the Tauc plots of (ahn)2 versus hn (the incident photon energy) by linearly extrapolating to zero cross the X axis, where a is the absorption coefficient (the absorbance in Fig. 3), h is Planck's constant and A is the probability parameter for the transition, respectively. As shown in the inset of Fig. 3, the band gaps of alloyed ZnxCd1xS can be adjusted between 2.11 and 3.21 eV. These results implied that the band gaps of the ZnxCd1xS solid solutions can be precisely controlled by modifying the molar ratios of Zn/Cd precursors. The morphology and size of the as-synthesized ZnxCd1xS solid solutions were investigated by transmission electron microscopy (TEM). Fig. 4 displays the TEM images of ZnS, CdS and Zn0.5Cd0.5S samples, respectively. It can be seen that all samples were composed of numerous tiny nanoparticles. The average diameter of nanoparticles was 44 nm for CdS, 25 nm for Zn0.5Cd0.5S and 31 nm for ZnS, respectively, which are in line with the data calculated from XRD peaks. The welldefined lattice fringe of HRTEM of Zn0.5Cd0.5S demonstrates its highly crystalline nature, as displayed in Fig. S1. The lattice spacing is about 0.344 nm, corresponding to the (100) plane of hexagonal CdS. The surface compositions and electronic structures of Zn0.5Cd0.5S sample were ascertained by X-ray photoelectron spectra (XPS) using the photo energy of 1486.6 eV, as shown in Fig. 5. The spectrum of survey scan clearly demonstrates the presence of Zn, Cd and S as well as C and O elements. The appearance of a very small amount of C and O signals is due to the carbon tape for measurement and the adsorbed oxygen, respectively. In addition, the doublet peaks for Zn 2p (1022.4 and 1045.5 eV), Cd 3d (411.6 and 405.2 eV), and S2p (161.1 and 162.3 eV) in the high-resolution XPS spectra of Fig. 5bed are attributed to the spin-orbit separation components. The bind energies are in well consistent with the data reported in the literature [42]. Additionally, the peak of N1s at 405.2 eV is also observed (Fig. 5e), which is ascribed to the amine group (the nitrogen from eNHe of L-cystine) [43], implying a very small
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Table 1 e Summary of materials properties of various ZnxCd1¡xS samples. Sample
CdS Zn0.1Cd0.9S Zn0.3Cd0.7S Zn0.5Cd0.5S Zn0.7Cd0.3S Zn0.9Cd0.1S ZnS
Atomic ratio of Zn:Cd
Lattice constants ( A)
Cell volume ( A3)
In precursor
ICP
EDS
a
c
v
0:1 0.1:0.9 0.3:0.7 0.5:0.5 0.7:0.3 0.9:0.1 1:0
0:1 0.09:0.91 0.32:0.68 0.51:0.49 0.70:0.30 0.88:0.12 1:0
0:1 0.10:0.90 0.32:0.68 0.46:0.54 0.69:0.31 0.89:0.11 1:0
4.1370 4.1070 4.0320 3.9750 3.9220 3.8430 3.8240
6.7180 6.6900 6.5800 6.5570 6.4050 6.3370 6.2590
99.5730 97.6955 92.6518 88.8516 85.3054 81.0566 79.2457
XS (nm)
TEM (nm)
Specific surface area (m2 g1)
Band gap (eV)
39.0 25.7 25.2 19.1 21.4 26.0 28.9
43.8 37.5 31.3 25.0 18.8 25.0 31.3
33.2 40.6 37.8 42.1 53.3 50.2 48.9
2.11 2.16 2.21 2.32 2.42 2.53 3.21
Fig. 2 e Dependence of lattice constants and the unit cell volumes on the Zn content (x) of ZnxCd1¡xS solid solutions.
Fig. 3 e UV-Vis diffuse reflection absorption spectra, digital photograph (inset: bottom-left) and the plots of (ahn)2 vs hn (inset: top-right) of ZnxCd1¡xS solid solutions with various x values: (a) 0, (b) 0.1, (c) 0.3, (d) 0.5, (e) 0.7, (f) 0.9 and (g) 1.
amount of L-cystine molecules adsorbed on the surface of Zn0.5Cd0.5S sample during the synthesis period. This is further proved by FT-IR technique, as displayed in Fig. S2. The peaks at 1630 and 1406 cm1 are assigned to the NeH deformation vibration and eCN bonding [44,45], respectively, indicating the existence of L-cystine on the surface of Zn0.5Cd0.5S.Besides, the metal chemical compositions from the surface and the bulk of ZnxCd1xS photocatalyst were further checked by EDS and ICP analyses, respectively. The results are also summarized in Table 1. It can be found that the metal element compositions of ZnxCd1xS solid solutions fabricated by the present hydrothermal route are very close to those designed in the synthesis
procedure. Thus for the sake of convenience, the samples are addressed with their stoichiometric compositions. To identify the porous nature of the as-synthesized nanoparticles, the typical N2 adsorption-desorption isotherms and the corresponding BJH (BarreteJoynereHalenda) pore-size distribution curves (inset) of the as-obtained CdS, Zn0.5Cd0.5S and ZnS samples are displayed in Fig. 6. Obviously, all the isotherms can be classified as type IV with apparent H3 hysteresis loops with high adsorption in the high relative pressure (P/P0) range from 0.8 to 1.0, indicating the presence of large mesopores and macropores. As shown in the inset of Fig. 6, a wide pore size distribution of 2e300 nm is observed, further confirming the existence of mesoporous and macroporous structure in the samples. The macropores (50e300 nm) probably arise from the interspaces formed between the nanoparticles. The photocatalytic activities of H2-production over ZnxCd1xS solid solutions were systematically evaluated under visible light irradiation (l > 420 nm) in an aqueous solution containing S2 and SO2 3 ions as hole scavengers without the presence of any co-catalysts. As presented in Fig. 7a, the H2evolution activity is strongly dependent on the compositions of ZnxCd1xS solid solutions. All ZnxCd1xS solid solutions exhibit much better performance for photocatalytic water reduction than that of bulk CdS as well as ZnS. No H2 is detected over ZnS alone because ZnS with a large band gap (3.21 eV) is only active under UV light [46]. Naked CdS demonstrates poor photocatalytic activity for H2 evolution (576 mmol h1 g1), due to the rapid recombination of photoinduced eehþ pairs. With the introduction of Zn2þ into CdS, the rate of H2 evolution correspondingly increases. A remarkable improvement in the H2evolution activity is observed as the amount of Zn2þ is above 0.1. Surprisingly, a super high H2-production performance of
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Fig. 4 e TEM images of CdS, Zn0.5Cd0.5S and ZnS samples.
Fig. 5 e XPS data of Zn0.5Cd0.5S solid solution: (a) survey spectrum, and high-resolution XPS spectra of: (b) Zn2p, (c) Cd3d, (d) S2p and (e) N1s.
18.3 mmol h1 g1 is achieved over naked Zn0.5Cd0.5S catalyst, exceeding that of sole CdS by a factor of over 32 times. The apparent quantum yield is calculated to be as high as of 73.8% at 420 nm, which, to our knowledge, also outperforms previously reported cocatalyst-free metal chalcogenide photocatalysts [14e19,24]. A further increase of Zn2þ content in the synthesis solution leads to a dramatic deterioration of the photocatalytic performance under the identical experimental conditions. For instance, in the case of x ¼ 0.9, the H2-
generation rate is decreased to only 294 mmol h1 g1 with a Q. E. of 1.2%. Thus, a suitable molar ratio of Zn/Cd is crucial for optimizing the photocatalytic performance of alloyed hZneCdeS solid solutions. The stability is a vital factor that affects the practical applications of photocatalysts in solar conversion. In our experiments, the stability of the photocatalyst with the maximum H2-production rate, Zn0.5Cd0.5S, was further assessed by monitoring the H2-evolution from aqueous
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Fig. 6 e Nitrogen adsorption-desorption isotherms and corresponding BJH pore-size distribution curves (inset) of CdS, Zn0.5Cd0.5S and ZnS samples.
solutions containing S2 and SO2 ions during consecutive 3 recycle experiments under visible light irradiation, as displayed in Fig. 7b. It is found that there is a slight decrease of H2 production activity in the second cycle, which possibly comes from the consumption of the sacrificial reagents. After the catalyst was collected by centrifugation and re-dispersed into the fresh mixed solution of S2 and SO2 3 , the H2-evolution activity is recovered in the third cycle. Moreover, no obvious differences can be detected from both XRD patterns and TEM images of the catalyst before and after the photocatalytic H2production reactions (Fig. S3 and S4). These results indicate that the as-synthesized Zn0.5Cd0.5S solid solution is quite stable against photocorrosion in the presence of sacrificial agents during the photocatalytic reaction under visible light. In this case, L-cystine, as an organic S source, is prone to be dissolved into water medium under strong alkali condition (pH ¼ 10e11). The catalyst crystallizing in the water medium of hydrothermal conditions over a prolonged period is beneficial to achieving the high stability for photocatalytic water reduction under visible light irradiation. The excellent photocatalytic stability of ZnxCd1xS solid solutions implies their promising potential in practical applications. The photocatalytic performance of as-prepared ZnxCd1xS nanoparticles was also tested by measuring the decolorization
of MB in aqueous solution under visible light irradiation (l > 420 nm). The photodegradation efficiency of ZnxCd1xS catalysts is shown in Fig. S5. Similarly to the photoreduction of H2O to H2 mentioned as above, Zn0.5Cd0.5S sample is the most efficient in the photocatalytic degradation of MB. The stability test of representative Zn0.5Cd0.5S powder by performing consecutive recycle experiments under the same reaction conditions is also demonstrated in Fig. S5. The photodegradation efficiency after each run is calculated to be 95%, 92%, and 91%, respectively. The slightly decrease in the photocatalytic activity for MB degradation can be mainly ascribed to the loss of the photocatalyst powder during the process of washing and centrifugation. These results also demonstrate the excellent photocatalytic performance and good stability of ZnxCd1xS nanocrystals. To investigate the origin of the drastically enhanced performance of ZnxCd1xS solid solutions compared to that of CdS, the conduction band (CB) edge position can be evaluated empirically according to Butler and Ginley's approach [27,47], as shown in the following equation: ECB ¼ X Ee e 0.5Eg. Here, ECB is the conduction band potential, Ee equals to a constant as 4.5 eV for the energy of free electrons on the hydrogen scale, Eg is the band gap energy of the semiconductor, estimated from Tauc Plot from UV-visible spectra shown in Fig. 3, and X represents the geometric mean of the absolute electronegativity of the constituent, and is denoted as the arithmetic mean of the atomic electron affinity and the first ionization energy. Accordingly, the valence band (VB) potential of the semiconductor is determined by the equation: EVB ¼ ECB þ Eg. On basis of the calculated results above, a schematic band structure is proposed for ZnxCd1xS solid solutions, as shown in Fig. 8. The conduction and valence band potentials of ZnxCd1xS solid solutions are located between those of CdS and ZnS as well as their band gap energies. With increasing Zn content, the levels of the conduction and valence band of ZnxCd1xS solid solutions are shifted toward more negative potential and more positive potential, which results in an obvious increase in the band energy of ZnxCd1xS solid solutions. Generally, more negative potential of the conduction band provides a larger thermodynamic driving force to reduce water to H2 and more positive potential of the valence band makes holes owning a much stronger ability of
Fig. 7 e Comparisons of the H2-evolution rate over various ZnxCd1¡xS solid solution samples under visible light irradiation. Reaction conditions: 50 mg catalyst, 100 mL aqueous solution containing 0.5 M Na2S and 0.5 M Na2SO3, 300 W Xe lamp with an optical filter (l > 420 nm). (b) Stability test by measuring the time courses of H2 evolution over Zn0.5Cd0.5S solid solution under visible light irradiation.
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Fig. 8 e Schematic energy level diagram of ZnxCd1¡xS solid solutions, CdS as well as ZnS.
Fig. 9 e (a) Transient photocurrent responses and (b) electrochemical impedance spectra (EIS) Nyquist plots of CdS, Zn0.5Cd0.5S and ZnS electrodes in an aqueous solution of 0.5 M Na2S and 0.5 M Na2SO3 under visible light illumination (l > 420 nm).
oxidation. Besides, a suitable narrow band gap allows for efficient utilization of incident light. These parameters are beneficial to improve the photocatalytic performance. Here, the best photocatalytic activity is achieved over Zn0.5Cd0.5S solid solution under visible light, suggesting that the catalyst possesses an optimum band gap with a moderate edge potential of the conduction and valence band. In addition, the intrinsic electronic and charge transport behavior of photoinduced electrons and holes play a crucial role in the photocatalytic process [14,21,48e50]. Here both the transient photocurrent (TPC) response and electrochemical impedance spectroscopy (EIS) analysis are used to investigate the surface and bulk charge-transfer process occurring in the three-electrode system. Fig. 9a compares the photocurrentetime (Iet) curves for as-fabricated CdS, Zn0.5Cd0.5S and ZnS electrodes with five on-off cycles of intermittent visible light illumination. As expected, all the electrodes show obvious photoresponse as the light turns on, which decreases quickly to a steady state once the light is switched off. The onoff cycles of the photocurrent are reproducible. Impressively, the photocurrent density of Zn0.5Cd0.5S electrode is clearly enhanced as compared to those of ZnS and CdS electrodes, indicating the improvement of the charge transfer and the suppression of the charge recombination [51e53].
Correspondingly, the representative EIS Nyquist plots for CdS, Zn0.5Cd0.5S and ZnS electrodes are displayed in Fig. 9b. Generally, a semicircular arc reflects a lower charge-transfer resistance [14,53e55]. Among the three samples, Zn0.5Cd0.5S solid solution electrode exhibits the smallest radius of arc, implying much easily charge transfer in Zn0.5Cd0.5S solid solution, which is in consistent with its higher photocurrent transient response. Based on the results described as above, it is reasonable that the interfacial charge separation and transfer of photoinduced electronehole pairs is greatly facilitated due to the formation of alloyed Zn5Cd0.5S solid solutions, which in turn remarkably boosts the photocatalytic performance for water decomposition and MB degradation under visible light.
Conclusions In summary, we demonstrated for the first time a green and facile route for the preparation of visible-light-responsive ZnxCd1xS solid solution photocatalyst with hexagonal phase by L-cystine-assisted hydrothermal approach, which is very promising for large-scale production. The as-obtained ZnxCd1xS nanocrystals exhibit the enhanced activity and excellent stability for photocatalytic hydrogen evolution and
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dye degradation under visible light irradiation without loading any cocatalyst. Specially, cocatalyst-free Zn0.5Cd0.5S shows a super high H2-production activity of 18.3 mmol h1 g1, exceeding that of CdS by a factor of 32. Besides, the optimal Q.E. of 73.8% is achieved at 420 nm, outperforming the previously-reported cocatalyst-free metal sulfide photocatalysts. The unusual photocatalytic activity can be attributed to the strong absorption of visible light photons and appropriate edge potential of conduction and valence band along with the highly efficient charge-transport due to the formation of alloyed ZneCdeS solid solution. This work represents a significant advance in the green synthesis of metal chalcogenides, which can be used directly (cocatalyst-free) for highly-efficient solar H2 production.
Acknowledgments This work was financially supported by the projects of the National Natural Science Foundation of China (No. 21573100). We acknowledge the support from the Open Project of State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences (No. N-14-04).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.06.196.
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