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CdS(C): Enhanced photocatalytic hydrogen production under visible light
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Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light Liangfeng Luo a, Yidi Wang a, Siping Huo b, Peng Lv b, Jun Fang b,*, Yang Yang b, Bin Fei a,** a
Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China b State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No. 5 Xinmofan Road, Nanjing, 210009, China
highlights
graphical abstract
Cu-MOF acts as structural modifier leading to formation of CdS(H)/ CdS(C) phase junction. CdS(H)/CdS(C) with phase junction has narrower band gap than pure CdS(H). Deposited CuS enhances the light absorption in the region between 550 nm and 800 nm.
article info
abstract
Article history:
CuS/CdS(H)/CdS(C) photocatalysts were synthesized via the hydrothermal method by
Received 18 June 2019
employing thiourea, Cd(CH3COO)2$3H2O and copper 1,4-benzenedicarboxylate MOF (CuBDC).
Received in revised form
The photocatalysts were characterized by XRD, XPS, BET, TEM and UVevis diffuse reflectance
8 September 2019
spectra. Interestingly, hexagonal CdS (CdS(H)) and cubic CdS (CdS(C)) were formed with phase
Accepted 16 September 2019
junctions in one step when CuBDC was introduced in the synthesis process, in addition, CuS
Available online xxx
nanoparticles were deposited on CdS. However, only hexagonal CdS was obtained without CuBDC. It demonstrated that CuBDC was not only the precursor of CuS but also the structural
Keywords:
modifier for CdS. With the reduction of re-combination of photo-induced electrons and holes
H2 production
caused by phase junctions and the enhancement of visible-light absorptions due to the loading
CdS
of CuS, all CuS/CdS(H)/CdS(C) photocatalysts had higher photocurrent densities under visible-
Phase junction
light irradiation, and consequently the higher rates of H2 production than pure CdS(H). Typi-
CuS
cally, the catalyst with 2.89 wt% of Cu showed a highest rate of H2 evolution at 2042 mmol/g/h.
Cu-MOF
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Luo), [email protected] (J. Fang), [email protected] (B. Fei). https://doi.org/10.1016/j.ijhydene.2019.09.136 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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Introduction Photocatalytic water splitting to produce H2, ultimate clean energy and important chemical in industries, has been considered as one of the most promising approaches to solve the energy and environmental issues the world is facing [1e3]. Since n-type semiconductor TiO2 was found being able to produce H2 from water under UV light irradiation in the 1970s [4], numerous semiconductors for photocatalytic H2 evolution were widely investigated [5e8]. Generally, photocatalytic water splitting by semiconductor involves generation, separation, transportation and surface reaction of the photoinduced charge carriers (electrons and holes) [9]. Electrons in the conduction band and holes in the valence band are generated when the semiconductor is excited by photons with energy equivalent to or higher than its band gap, then photogenerated electrons and holes migrate to the surface of the semiconductor to reduce and oxidize the absorbed H2O, respectively. In the case of H2 evolution under visible light, semiconductor should have a band gap with energy less than 3.0 eV so that it can absorb the vast majority of visible light [10]. Meanwhile, the potential of the conduction-band edge should be more negative than the redox potential of H2/H2O and the potential of the valence-band edge should be more positive than the redox potential of O2/H2O [11]. Cadmium Sulfide (CdS), with a band gap of 2.4 eV and proper potentials of conduction band edge and valence band edge [12], is one of the prominent semiconductor photocatalysts satisfying the above-mentioned requirements for photocatalytic water splitting under visible light. However, CdS itself exhibits poor photocatalytic activity due to the rapid recombination of photogenerated chargecarries [13] and anodic photo-corrosion [14]. Many efforts have been made to overcome these limitations. Loading cocatalysts on CdS is one of the effective approaches, the cocatalysts can act as electron or hole acceptors to accelerate the migration of electrons or holes. Furthermore, phase junctions between cocatalysts and CdS help to reduce the recombination of charge carries. Thus, a variety of CdS-based [15e25] catalysts have been developed. Very recently, Cu based catalysts [26e31] have been wildly investigated. Among them, CuS is supposed as a costeffective and sustainable catalyst or co-catalyst for photocatalytic hydrogen production. Metal-organic frameworks (MOFs), a group of Zeolite-like materials with uniform pore and cavity size and large internal surface area, have been wildly investigated in heterogeneous catalysis in the latest two decades [32e34]. Recently, utilizing MOFs as precursors or templates to manufacture advanced catalytic materials has been attracting growing interest [35e37]. One of the CuBDC is one of the representatives of Cu-MOFs made up of Cu nodes with 1, 4benzenetricarboxylic acid (BDC) [38]. Mondal et al. reported a CuBDC-derived Cu/CuO@TiO2 by adding Titanium isopropoxide to CuBDC in ethanol/water mixture followed by calcinating at 350 C for 5 h in a N2 atmosphere [39]. The obtained Cu/CuO@TiO2 showed higher H2 evolution rate than conventional CuO loaded TiO2, which could be attributed to the formation of small size hetero-junction between CuO and TiO2. In this paper, we synthesized CuS deposited CdS photocatalysts with phase junctions between hexagonal and cubic CdS (CuS/CdS(H)/CdS(C)) in one step via a simple
hydrothermal method by introducing Cu-MOF (CuBDC). The CuBDC was suggested to be the structural modifier as well as the precursor of CuS. Moreover, the variation of the amount of CuBDC influenced the morphologies of CdS and the surface dispersion of the CuS, and consequently the activities of H2 evolution of CuS/CdS(H)/CdS(C) photocatalysts.
Experimental Materials 1, 4-benzenedicarboxylic acid (>99%) was purchased from J&K Scientific Ltd. Copper nitrate trihydrate (Cu(NO3)2$3H2O, >99%) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Cadmium acetate dehydrate (Cd(CH3OO)2$3H2O, >98%) was purchased from Strem Chemicals Inc. Thiourea (>99%) was purchased from Acros Organics. N,Ndimethylformamide (DMF, >99.8%) and Chloroform (CHCl3, >99%) were purchased from Sigma-Aldrich.
Synthesis of CuBDC MOF The copper 1,4-benzenedicarboxylate (CuBDC) MOF was synthesized according to the reported method [38]. Typically, 1.053 g of Cu(NO3)2$3H2O and 724 mg of 1,4benzenedicarboxylic acid and 87 mL of N,Ndimethylformamide (DMF) were mixed in a 250 mL round bottom flask and refluxed at 100 C during 24 h. The resulting powder was collected by centrifugation at 6000 rpm and the solid was consecutively washed 3 times with DMF (20 mL each step) followed by 3 times washing with CHCl3 (20 mL each step), and finally was dried at 60 C under vacuum condition.
Synthesis of CuS/CdS(H)/CdS(C) photocatalysts 0.425 g of Cd(CH3OO)2$3H2O, 0.42 g of thiourea and a certain amount of CuBDC were added in 35 mL de-ionized water. The mixture was maintained under the ultrasonic condition for 10 min. After that, the mixture was transferred into an autoclave of 50 mL and kept at 180 C for 12 h. The obtained powder was washed and filtered with de-ionized water and dried at 60 C for 12 h. Five CuS/CdS(H)/CdS(C) photocatalysts were synthesized, along with the increasing amount of CuBDC (0.012, 0.023, 0.046, 0.12 and 0.23 g), they were denoted as CC-1, CC-2, CC-3, CC-4 and CC-5, respectively. The control sample without employing CuBDC was also prepared and denoted as CC-0.
Characterizations X-ray diffraction (XRD) patterns were acquired on a SmartLab diffractometer (Rigaku Corporation). The morphologies and structures of the samples were examined by scanning electron microscope (SEM, TESCAN VEGA3). Transmission electron microscope (TEM) images were obtained by using a JEOL 2100F microscope at an accelerating voltage of 200 kV. UVevis diffuse reflectance spectra (UVevis DRS) were recorded on a Cary 3000 spectrophotometer equipped with an integrating sphere with BaSO4 as the reference. X-ray photoelectron
Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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spectroscopy (XPS) measurements were recorded on an ESCALAB 250 high-performance electron spectrometer with monochromatized Al Ka (hn ¼ 1486.7 eV) source; the likely charging of all the catalysts was calibrated by setting the binding energy of adventitious carbon (C 1s) to 284.6 eV. The contents of Cu in the catalysts were determined using an inductively coupled plasma atomic emission spectroscopy (ICP-AES; Perkin Elmer, Optima 7000 DV). BrunauereEmmetteTeller (BET) surface areas were characterized in BET equipment (Bel MicrotracBel, Belsorp-max, Osaka, Japan). Photoluminescence (PL) measurements were performed at room temperature on a RF-5301PC fluorescence spectrophotometer with an excitation wavelength at 460 nm.
Photocatalytic H2 evolution Photocatalytic water-splitting reactions were carried out at room temperature using a top-irradiation quartz reactor. In a typical run, 20 mg photocatalysts were dispersed into 20 mL aqueous solution containing 0.1M Na2S and 0.1M Na2SO3. In control experiments, 3 wt% Pt was loaded on CC-0, 1, 2, 3, 4 and 5 via in-situ photo-deposition technique by using H2PtCl6. Before irradiation, the solution was purged with nitrogen gas to remove oxygen. Then the suspension was exposed under the 300 W Xe lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd.) coupling with a UV cutoff filter (l > 420 nm) to evaluate the photocatalytic efficiencies under visible light. The output light intensity was controlled at 180 mW/cm2. The evolved gases were analyzed by gas chromatography (Shimadzu GC-2014 equipped with a thermal conductive detector (TCD) and a 5 A molecular sieve column; N2 carrier).
Electrochemical measurement The photoelectrochemical properties were investigated on an electrochemical workstation (CHI660E, Shanghai Chenhua Co. Ltd.) using a standard three-electrode cell with a working electrode, a Pt wire counter electrode, and a saturated Ag/AgCl reference electrode. 0.5 M Na2SO4 was used as the electrolyte. The working electrodes were prepared as follows: 5 mg of the sample was suspended in 500 mL ethanol containing 50 mL nafion to produce a slurry, and then dip-coated on a 10 mm 15 mm fluorine-doped tin oxide (FTO) glass, which was cleaned by sonication in acetone, ethanol and water for 30 min, respectively. The electrodes were dried in air at 60 C. For transient photocurrent response (I-T) measurements, a 300 W Xe lamp coupling with a UV cutoff filter (l > 420 nm) was used as the light source. The electrochemical impedance spectroscopy (EIS) Nyquist plots were carried out at 0 V (vs. Ag/AgCl) in the frequency range of 1 to 1M Hz. Mott-Schottky (MS) plots were measured by using frequency of 3 k Hz.
Results and discussion Structural characterizations of CuBDC MOF Bulk-type copper 1, 4-benzenedicarboxylate (CuBDC) MOF was synthesized following a modified method described by Carson
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et al. [38], and its structure was analyzed by XRD (Fig. S1). According to Carson et al.’s results, all the diffraction peaks could be assigned to CuBDC. The morphology of CuBDC was also measured by SEM (Fig. S2) and it had a large range of size distribution but good crystallinity.
XRD analysis for CuS/CdS(H)/CdS(C) The as-prepared CuBDC was employed to synthesize CuS/ CdS(H)/CdS(C) composites via a hydrothermal process. As shown in Fig. S3, CC-0 was light orange, while other composites changed from red-brown to brown with the increasing amount of CuBDC. The crystallinity of the products was verified by powder X-ray diffraction (XRD) (Fig. 1). All the diffraction peaks of CC-0 belonged to hexagonal CdS (JCPDS 80-0006), with main peaks at 25.05 (H 100), 26.63 (H 002), 28.32 (H 101), 43.95 (H 110), 48.12 (H 103) and 52.10 (H 112). The diffraction patterns of CC-1, 2, 3, 4 and 5 were different from that of CC-0. For CC-1, peaks at 25.05 (H 100) and 28.32 (H 101) became much weaker and the peak at 48.12 (H 103) almost disappeared comparing with CC-0. These changes also occurred for CC-2, 3, 4 and 5, in addition, peaks belonging to cubic CdS (JCPDS 89-0440) arised at 30.68 (C 200). Moreover, the strongest CdS peak of CC-2, 3, 4 and 5 shifted from 26.63 to 26.52 . These results indicated the increases of cubic phase of CdS since the (C 111) peak of cubic CdS locates at 26.46 (JCPDS 890440). Thus, it demonstrated that the addition of CuBDC led to the formation of both hexagonal and cubic phases of CdS. Besides, peaks of hexagonal CuS could be observed at 29.25 (102) 31.84 (103) 32.87 (006) and 47.96 (110) (JCPDS 78-0877) for CC-3, 4 and 5, and their intensities increased with increasing Cu-MOF. This indicates that Cu-MOF was not only the structural modifier of CdS but also the precursor of CuS. However, peaks of CuS were hardly detected for CC-1 and 2, due to the low content of Cu species in the composites (Table 1) and consequently the high dispersion of Cu species [30,31]. In addition, no peak corresponding to CuBDC was observed for all the samples, which indicated the decomposition of CuBDC during the hydrothermal process. The released Cu2þ ions might further react with S2 to form CuS crystals while the BDC ligands were dissolved into water.
Composition analysis (ICP-AES, XPS) The surface chemical environments of the samples were analyzed by XPS (Fig. 2). Peaks in high-resolution spectra of S 2p (Fig. 2a) and Cd 3d (Fig. 2b) for all the samples were typical S2 and Cd2þ in CdS, respectively [40]. No signal was observed in Cu 2p region for CC-0 (Fig. 2c), while for CC-1, 2, 3, 4 and 5, the XPS spectra of Cu 2p showed the binding energies of Cu 2p1/2 and Cu 2p3/2 peaks at 952.6 and 932.6 eV, respectively, which were typical values for Cu2þ in CuS [41e43]. Furthermore, symmetrical shapes of the two Cu 2p peaks implied the presence of pure CuS. Thus, XRD and XPS results revealed the compositions for all samples. CC-0 was pure hexagonal CdS while others consisted of CuS and CdS. The total contents of Cu, measured by ICP-AES, increased from 0.89 wt% in CC-1 to 8.61 wt% in CC-5 (See Table 1). The surface Cu content (atom%) in CC-1, CC-2 and CC-3, derived from
Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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XPS results, increased with the corresponding total Cu content. However, the surface Cu contents of CC-4 and CC-5 were close to that of CC-3, although their total Cu contents were much higher. This might be attributed to the larger particle of CuS in CC-4 and 5 than that in CC-3 [44].
Structural characterizations (TEM, SEM, BET) TEM was employed to further analyze the structures of all the photocatalysts. Fig. 3a showed that the size distribution of the nanoparticles of CC-0 was narrow with an average value of ~40 nm. The corresponding HRTEM image (Fig. 3b) exhibited
Table 1 e Composition, surface chemical environments and band gap of CC-0, 1, 2, 3, 4 and 5. sample total content surface content band BET of Cu (wt%)[a] of Cu (atom%)[b] gap surface (eV)[c] area (m2/g) CC-0 CC-1 CC-2 CC-3 CC-4 CC-5 a b c
Fig. 1 e XRD patterns of CC-0, 1, 2, 3, 4 and 5 (C ¼ peaks of hexagonal CdS; B ¼ peaks of cubic CdS; * ¼ peaks of hexagonal CuS).
0 0.89 1.64 2.89 5.96 8.61
0 2.95 3.58 4.08 4.28 4.08
2.31 2.10 1.98 2.19 2.15 2.23
34.14 3.76 3.03 3.46 2.02 3.67
Total contents of Cu were measured by ICP-AES. Surface contents of Cu were measured by XPS. Band gaps were derived from UVevis DRS, see Fig. 4.
fringes with lattice spacing of 0.353 and 0.192 nm which correspond to (H 100) and (H 103) plane of hexagonal CdS, respectively. No fringe corresponding to cubic CdS was found and this was consistent with the XRD results (Fig. 1). The morphologies of CC-1, 2, 3, 4 and 5 (Fig. S4, Fig. 3c and e) indicated the aggregation of the nanoparticles after introducing the CuBDC. The aggregations were also confirmed by BET surface areas (Table 1). The pure CdS(H) had a BET area of 34.14 m2/g, however, the areas of the composites decreased dramatically and were 3.76, 3.03, 3.46, 2.02 and 3.67 m2/g for CC-1, 2, 3, 4 and 5, respectively. In addition to fringes of 0.193 nm, the corresponding (H 103) plane of hexagonal CdS, CC-3 (Fig. 3d) also exhibited two groups of fringes with lattice spacing of 0.337 nm. Furthermore, the cross angle of those two lattice fringes was 70.5 , which demonstrated the corresponding fringes belonged to (C 111) planes of cubic CdS. Moreover, CuS nanoparticles less than 10 nm were deposited on CdS, indicated by the lattice fringes of 0.326 and 0.258 nm corresponding to (100) plane and (104) plane of hexagonal CuS, respectively. Moreover, EDX mapping measurement (Fig. S5) for CC-3 showed that CuS nanoparticles were well dispersed on the surface of CdS. When increasing CuBDC, the aggregation of the particles in CC-4 (Fig. 3e) was severer than that in CC-3, which was consistent with the BET results. Fig. 3f also clearly showed lattice fringes of both CdS(H) and CdS(C). In addition, the size of deposited CuS particles (the area circled by the yellow dash line in Fig. 3f) were much larger than those in CC-3. This might be the reason why the total Cu content of CC-4 was more than twice that of CC-3 but there was only a little increase in surface content of Cu derived from XPS: the percentage of surface Cu atom decreased due to the larger CuS particles and lower BET surface. From Fig. 3d and f, the bonding area between CdS(H) and CdS(C) could be assigned to phase junction. Li et al. reported a facile method to fabricate CdS nanorod phase junctions composed of hexagonal core and cubic shell by adjusting the ratio of Cd to S [45]. With low Cd/S ratio (Cd/S at 1:8), pure CdS(H) was formed via kineticsdominated process. While the Cd/S ratio increased, CdS(H)/ CdS(C) phase junction was obtained due to the decrease of nucleation rate and the reaction changed from kineticsdominated to thermodynamics-dominated process. In our synthetic process, the Cd/S ratio was kept at 1:3.37. Without introduction of CuBDC, pure CdS(H) was formed. However, as adding and increasing CuBDC, the Cu(II) would also consume
Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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Fig. 2 e XPS spectra of CC-0, 1, 2, 3, 4, 5 and 6 with high resolution for (a) S 2p, (b) Cd 3d and (c) Cu 2p.
the S released from thiourea, leading to the increase of Cd/S ratio. Thus, CdS(H)/CdS(C) phase junctions were formed under thermodynamics-dominated process.
UVevis diffuse reflectance spectra The UVevis diffuse reflectance spectra (DRS) of the samples were shown in Fig. 4a. CC-0 had a typical UVevis DRS
absorption curve of hexagonal CdS with an absorption edge at ~525 nm [24]. Compared with CC-0, all the CuS/ CuS(H)/CdS(C) possessed similar red-shifted visible light absorption edges and showed increasing absorption between 550 and 800 nm with the increase of CuS content. This could be attributed to CuS, which had an obvious absorption in the region between 550 and 800 nm [46]. The curves of (ɑhʋ)2 vs hʋ for CC-0, 1, 2, 3, 4 and 5 were
Fig. 3 e TEM images of (a) CC-0, (c) CC-3, (e) CC-4. High resolution TEM images of (b) CC-0, (d) CC-3 and (f) CC-4. Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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catalyst to reduce the overpotential in the production of H2 from water and suppress the fast backward reaction as well (photocatalytic H2 production for CC-3 without Pt was showed in Fig. S6) [48]. All CuS/CdS(H)/CdS(C) samples exhibited enhanced catalytic performance in comparison to pure CdS(H) (Fig. 5a). Typically, CC-3 showed the best H2 production rate of 2042 mmol/g/h, almost 14 times higher than that of CC-0. Although the photo-corrosion was inevitable, which was indicated by the XRD of CC-3 after 3-cycle photocatalytic test (Fig. S7), CC-3 still had a rate of H2 production at 1413 mmol/g/h (Fig. 5b). To further understand the mechanisms of improvement of H2 production by CuS/CdS(H)/CdS(C), transient photocurrent measurements were carried out. As shown in Fig. 5c, CC-0 had the lowest photocurrent density, while all the composite samples showed the improved photocurrent responses. Particularly, CC-3 exhibited much higher photocurrent density than other samples. Photoluminescence (PL) spectra (Fig. 5d) also showed that, all the composite samples had the lower PL intensities than that of pure CdS(H). Therefore, formation of phase junction between CdS(H) and CdS(C) and the loading of CuS did improve the reduction of re-combination of photo-generated electrons and holes so that the composite samples had higher photocatalytic activities.
Inferred mechanism
Fig. 4 e (a) UVevis diffuse reflectance spectra of CC-0, 1, 2, 3, 4 and 5. (b) Tauc plots, i.e. the curves of (ɑhʋ)2 vs hʋ for CC-0, 1, 2, 3, 4 and 5, where ɑ, h and ʋ are the absorption coefficient, Plank’s constant and light frequency, respectively.
revealed in Fig. 4b. The corresponding band gaps were aslo obtained and listed in Table 1. CC-0 had a band gap of 2.31 eV, while those composites, CC-1, 2, 3, 4 and 5, had lower band gap energies. Recently, Ai et al. reported the red-shift of the absorption edge and hence the reduction of the band gap for hexagonal/cubic CdS composites due to the phase junction between hexagonal and cubic CdS [47]. Our HRTEM (Fig. 3d, f) results confirm the formation of phase junction as discussed above, which could be contributed to the reduction of the band gap of the composites compared to pure hexagonal CdS. Thus, our UVevis DRS results demonstrated that the light absorptions for CC-1, 2, 3, 4 and 5 are extended and enhanced in the visible-light region compared with CC-0.
Photocatalytic H2 productions and electrochemical measurements Photocatalytic H2 evolutions of all the samples were evaluated under visible-light irradiation (>420 nm), using Pt as co-
Among all the composite samples, CC-3 showed much higher photocatalytic activity. Besides the photocurrent responses and PL results, EIS Nyquist plots (Fig. S8) also indicated that CC-3 had a much lower interfacial charge-carrier transfer resistance than that of CC-0 under visible-light irradiation, suggesting the apparent enhancement of interfacial chargecarrier transfer on the surface of CC-3. The other composite samples also showed the similar structure to CC-3 (indicated by XRD, XPS, BET and UVevis DRS results), however, their photocurrents were much lower than that of CC-3. In the cases of CC-1 and CC-2, both the total amounts and surface amounts of CuS, were less than that of CC-3 (Table 1) and lead to the lower rates of H2 production. On the other hand, CC-4 and CC-5 had more total amounts of CuS, however the particle sizes were much larger (Fig. 3f), which might increase the difficulty for charge-carriers transferring to surface [49] (CC-4 and 5 had lower photocurrent densities than that of CC-3) and thus the poorer photocatalytic performance than CC-3. Furthermore, as indicated by TEM result and photoelectrochemical measurement, it was reasonable to infer that the well dispersed CuS nanoparticles could form heterojunctions with CdS, which also accelerated the transportation of photo-induced electrons from CuS to CdS. To illustrate this mechanism, Mott-schottky measurement was employed for CC-3 (Fig. S9) and the estimated CB edge for CC-3 was 0.34 V. Combined with the estimated band gap of CC-3 (Table 1), the schematic illustration for the mechanism of H2 production via water splitting by CC-3 was showed in Fig. 6. CuS/CdS(H)/ CdS(C) was obtained in one step via hydrothermal treatment. With the phase junction between CdS(H) and CdS(C), recombination of the photo-generated electrons and holes was suppressed. In addition, CuS derived from CuBDC helped to
Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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Fig. 5 e (a) Rates of H2 evolution by CC-0, 1, 2, 3, 4 and 5 under visible light. (b) Recycling test of H2 evolution for CC-3 catalyst. (c) Transient photocurrent responses for CC-0, 1, 2, 3, 4 and 5. (d) Photoluminescence spectra of CC-0, 1, 2, 3, 4 and 5.
Fig. 6 e Schematic illustration for the mechanism of H2 production via water splitting by CC-3.
enhance the light absorption in the visible region. Moreover, heterojunctions were formed between the well dispersed CuS nanoparticles and CdS of CC-3, therefore, the transportation of electrons photo-generated from CuS to CdS was accelerated. Consequently, CC-3 showed the highest photocurrent density and the highest rate of H2 evolution.
Conclusions In this work, CuS/CdS(H)/CdS(C) composites with various contents of CuS were synthesized via hydrothermal method with the introduction of CuBDC. According to XRD, XPS, BET,
Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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SEM and TEM results, CuBDC was suggested as the structrual modifier to CdS, which led to the formation of phase junction between hexagonal and cubic CdS for CuS/CdS(H)/CdS(C) composites. Therefore, CuS/CdS(H)/CdS(C) composites had narrower band gap than that of pure hexagonal CdS and consequently the enhancement of light absorption. Moreover, CuS particles derived from CuBDC deposited on CdS also enhanced the absorption in the visible region. Thus, all the CuS/CdS(H)/CdS(C) composites showed higher photocurrent densities so that the higher rates of H2 production than pure hexagonal CdS under visible-light irradiation. Particularly, CC-3 with proper heterojunction between CuS and CdS gave the highest rate of H2 evolution.
Acknowledgements This work was supported by PolyU fund (1-ZE27). This work was also supported by the National Natural Science Foundation of China (No. 21706130), the Jiangsu Natural Science Funds (BK20150943).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.136.
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Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light Liangfeng Luo a, Yidi Wang a, Siping Huo b, Peng Lv b, Jun Fang b,*, Yang Yang b, Bin Fei a,** a
Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China b State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No. 5 Xinmofan Road, Nanjing, 210009, China
highlights
graphical abstract
Cu-MOF acts as structural modifier leading to formation of CdS(H)/ CdS(C) phase junction. CdS(H)/CdS(C) with phase junction has narrower band gap than pure CdS(H). Deposited CuS enhances the light absorption in the region between 550 nm and 800 nm.
article info
abstract
Article history:
CuS/CdS(H)/CdS(C) photocatalysts were synthesized via the hydrothermal method by
Received 18 June 2019
employing thiourea, Cd(CH3COO)2$3H2O and copper 1,4-benzenedicarboxylate MOF (CuBDC).
Received in revised form
The photocatalysts were characterized by XRD, XPS, BET, TEM and UVevis diffuse reflectance
8 September 2019
spectra. Interestingly, hexagonal CdS (CdS(H)) and cubic CdS (CdS(C)) were formed with phase
Accepted 16 September 2019
junctions in one step when CuBDC was introduced in the synthesis process, in addition, CuS
Available online xxx
nanoparticles were deposited on CdS. However, only hexagonal CdS was obtained without CuBDC. It demonstrated that CuBDC was not only the precursor of CuS but also the structural
Keywords:
modifier for CdS. With the reduction of re-combination of photo-induced electrons and holes
H2 production
caused by phase junctions and the enhancement of visible-light absorptions due to the loading
CdS
of CuS, all CuS/CdS(H)/CdS(C) photocatalysts had higher photocurrent densities under visible-
Phase junction
light irradiation, and consequently the higher rates of H2 production than pure CdS(H). Typi-
CuS
cally, the catalyst with 2.89 wt% of Cu showed a highest rate of H2 evolution at 2042 mmol/g/h.
Cu-MOF
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Luo), [email protected] (J. Fang), [email protected] (B. Fei). https://doi.org/10.1016/j.ijhydene.2019.09.136 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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Introduction Photocatalytic water splitting to produce H2, ultimate clean energy and important chemical in industries, has been considered as one of the most promising approaches to solve the energy and environmental issues the world is facing [1e3]. Since n-type semiconductor TiO2 was found being able to produce H2 from water under UV light irradiation in the 1970s [4], numerous semiconductors for photocatalytic H2 evolution were widely investigated [5e8]. Generally, photocatalytic water splitting by semiconductor involves generation, separation, transportation and surface reaction of the photoinduced charge carriers (electrons and holes) [9]. Electrons in the conduction band and holes in the valence band are generated when the semiconductor is excited by photons with energy equivalent to or higher than its band gap, then photogenerated electrons and holes migrate to the surface of the semiconductor to reduce and oxidize the absorbed H2O, respectively. In the case of H2 evolution under visible light, semiconductor should have a band gap with energy less than 3.0 eV so that it can absorb the vast majority of visible light [10]. Meanwhile, the potential of the conduction-band edge should be more negative than the redox potential of H2/H2O and the potential of the valence-band edge should be more positive than the redox potential of O2/H2O [11]. Cadmium Sulfide (CdS), with a band gap of 2.4 eV and proper potentials of conduction band edge and valence band edge [12], is one of the prominent semiconductor photocatalysts satisfying the above-mentioned requirements for photocatalytic water splitting under visible light. However, CdS itself exhibits poor photocatalytic activity due to the rapid recombination of photogenerated chargecarries [13] and anodic photo-corrosion [14]. Many efforts have been made to overcome these limitations. Loading cocatalysts on CdS is one of the effective approaches, the cocatalysts can act as electron or hole acceptors to accelerate the migration of electrons or holes. Furthermore, phase junctions between cocatalysts and CdS help to reduce the recombination of charge carries. Thus, a variety of CdS-based [15e25] catalysts have been developed. Very recently, Cu based catalysts [26e31] have been wildly investigated. Among them, CuS is supposed as a costeffective and sustainable catalyst or co-catalyst for photocatalytic hydrogen production. Metal-organic frameworks (MOFs), a group of Zeolite-like materials with uniform pore and cavity size and large internal surface area, have been wildly investigated in heterogeneous catalysis in the latest two decades [32e34]. Recently, utilizing MOFs as precursors or templates to manufacture advanced catalytic materials has been attracting growing interest [35e37]. One of the CuBDC is one of the representatives of Cu-MOFs made up of Cu nodes with 1, 4benzenetricarboxylic acid (BDC) [38]. Mondal et al. reported a CuBDC-derived Cu/CuO@TiO2 by adding Titanium isopropoxide to CuBDC in ethanol/water mixture followed by calcinating at 350 C for 5 h in a N2 atmosphere [39]. The obtained Cu/CuO@TiO2 showed higher H2 evolution rate than conventional CuO loaded TiO2, which could be attributed to the formation of small size hetero-junction between CuO and TiO2. In this paper, we synthesized CuS deposited CdS photocatalysts with phase junctions between hexagonal and cubic CdS (CuS/CdS(H)/CdS(C)) in one step via a simple
hydrothermal method by introducing Cu-MOF (CuBDC). The CuBDC was suggested to be the structural modifier as well as the precursor of CuS. Moreover, the variation of the amount of CuBDC influenced the morphologies of CdS and the surface dispersion of the CuS, and consequently the activities of H2 evolution of CuS/CdS(H)/CdS(C) photocatalysts.
Experimental Materials 1, 4-benzenedicarboxylic acid (>99%) was purchased from J&K Scientific Ltd. Copper nitrate trihydrate (Cu(NO3)2$3H2O, >99%) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Cadmium acetate dehydrate (Cd(CH3OO)2$3H2O, >98%) was purchased from Strem Chemicals Inc. Thiourea (>99%) was purchased from Acros Organics. N,Ndimethylformamide (DMF, >99.8%) and Chloroform (CHCl3, >99%) were purchased from Sigma-Aldrich.
Synthesis of CuBDC MOF The copper 1,4-benzenedicarboxylate (CuBDC) MOF was synthesized according to the reported method [38]. Typically, 1.053 g of Cu(NO3)2$3H2O and 724 mg of 1,4benzenedicarboxylic acid and 87 mL of N,Ndimethylformamide (DMF) were mixed in a 250 mL round bottom flask and refluxed at 100 C during 24 h. The resulting powder was collected by centrifugation at 6000 rpm and the solid was consecutively washed 3 times with DMF (20 mL each step) followed by 3 times washing with CHCl3 (20 mL each step), and finally was dried at 60 C under vacuum condition.
Synthesis of CuS/CdS(H)/CdS(C) photocatalysts 0.425 g of Cd(CH3OO)2$3H2O, 0.42 g of thiourea and a certain amount of CuBDC were added in 35 mL de-ionized water. The mixture was maintained under the ultrasonic condition for 10 min. After that, the mixture was transferred into an autoclave of 50 mL and kept at 180 C for 12 h. The obtained powder was washed and filtered with de-ionized water and dried at 60 C for 12 h. Five CuS/CdS(H)/CdS(C) photocatalysts were synthesized, along with the increasing amount of CuBDC (0.012, 0.023, 0.046, 0.12 and 0.23 g), they were denoted as CC-1, CC-2, CC-3, CC-4 and CC-5, respectively. The control sample without employing CuBDC was also prepared and denoted as CC-0.
Characterizations X-ray diffraction (XRD) patterns were acquired on a SmartLab diffractometer (Rigaku Corporation). The morphologies and structures of the samples were examined by scanning electron microscope (SEM, TESCAN VEGA3). Transmission electron microscope (TEM) images were obtained by using a JEOL 2100F microscope at an accelerating voltage of 200 kV. UVevis diffuse reflectance spectra (UVevis DRS) were recorded on a Cary 3000 spectrophotometer equipped with an integrating sphere with BaSO4 as the reference. X-ray photoelectron
Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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spectroscopy (XPS) measurements were recorded on an ESCALAB 250 high-performance electron spectrometer with monochromatized Al Ka (hn ¼ 1486.7 eV) source; the likely charging of all the catalysts was calibrated by setting the binding energy of adventitious carbon (C 1s) to 284.6 eV. The contents of Cu in the catalysts were determined using an inductively coupled plasma atomic emission spectroscopy (ICP-AES; Perkin Elmer, Optima 7000 DV). BrunauereEmmetteTeller (BET) surface areas were characterized in BET equipment (Bel MicrotracBel, Belsorp-max, Osaka, Japan). Photoluminescence (PL) measurements were performed at room temperature on a RF-5301PC fluorescence spectrophotometer with an excitation wavelength at 460 nm.
Photocatalytic H2 evolution Photocatalytic water-splitting reactions were carried out at room temperature using a top-irradiation quartz reactor. In a typical run, 20 mg photocatalysts were dispersed into 20 mL aqueous solution containing 0.1M Na2S and 0.1M Na2SO3. In control experiments, 3 wt% Pt was loaded on CC-0, 1, 2, 3, 4 and 5 via in-situ photo-deposition technique by using H2PtCl6. Before irradiation, the solution was purged with nitrogen gas to remove oxygen. Then the suspension was exposed under the 300 W Xe lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd.) coupling with a UV cutoff filter (l > 420 nm) to evaluate the photocatalytic efficiencies under visible light. The output light intensity was controlled at 180 mW/cm2. The evolved gases were analyzed by gas chromatography (Shimadzu GC-2014 equipped with a thermal conductive detector (TCD) and a 5 A molecular sieve column; N2 carrier).
Electrochemical measurement The photoelectrochemical properties were investigated on an electrochemical workstation (CHI660E, Shanghai Chenhua Co. Ltd.) using a standard three-electrode cell with a working electrode, a Pt wire counter electrode, and a saturated Ag/AgCl reference electrode. 0.5 M Na2SO4 was used as the electrolyte. The working electrodes were prepared as follows: 5 mg of the sample was suspended in 500 mL ethanol containing 50 mL nafion to produce a slurry, and then dip-coated on a 10 mm 15 mm fluorine-doped tin oxide (FTO) glass, which was cleaned by sonication in acetone, ethanol and water for 30 min, respectively. The electrodes were dried in air at 60 C. For transient photocurrent response (I-T) measurements, a 300 W Xe lamp coupling with a UV cutoff filter (l > 420 nm) was used as the light source. The electrochemical impedance spectroscopy (EIS) Nyquist plots were carried out at 0 V (vs. Ag/AgCl) in the frequency range of 1 to 1M Hz. Mott-Schottky (MS) plots were measured by using frequency of 3 k Hz.
Results and discussion Structural characterizations of CuBDC MOF Bulk-type copper 1, 4-benzenedicarboxylate (CuBDC) MOF was synthesized following a modified method described by Carson
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et al. [38], and its structure was analyzed by XRD (Fig. S1). According to Carson et al.’s results, all the diffraction peaks could be assigned to CuBDC. The morphology of CuBDC was also measured by SEM (Fig. S2) and it had a large range of size distribution but good crystallinity.
XRD analysis for CuS/CdS(H)/CdS(C) The as-prepared CuBDC was employed to synthesize CuS/ CdS(H)/CdS(C) composites via a hydrothermal process. As shown in Fig. S3, CC-0 was light orange, while other composites changed from red-brown to brown with the increasing amount of CuBDC. The crystallinity of the products was verified by powder X-ray diffraction (XRD) (Fig. 1). All the diffraction peaks of CC-0 belonged to hexagonal CdS (JCPDS 80-0006), with main peaks at 25.05 (H 100), 26.63 (H 002), 28.32 (H 101), 43.95 (H 110), 48.12 (H 103) and 52.10 (H 112). The diffraction patterns of CC-1, 2, 3, 4 and 5 were different from that of CC-0. For CC-1, peaks at 25.05 (H 100) and 28.32 (H 101) became much weaker and the peak at 48.12 (H 103) almost disappeared comparing with CC-0. These changes also occurred for CC-2, 3, 4 and 5, in addition, peaks belonging to cubic CdS (JCPDS 89-0440) arised at 30.68 (C 200). Moreover, the strongest CdS peak of CC-2, 3, 4 and 5 shifted from 26.63 to 26.52 . These results indicated the increases of cubic phase of CdS since the (C 111) peak of cubic CdS locates at 26.46 (JCPDS 890440). Thus, it demonstrated that the addition of CuBDC led to the formation of both hexagonal and cubic phases of CdS. Besides, peaks of hexagonal CuS could be observed at 29.25 (102) 31.84 (103) 32.87 (006) and 47.96 (110) (JCPDS 78-0877) for CC-3, 4 and 5, and their intensities increased with increasing Cu-MOF. This indicates that Cu-MOF was not only the structural modifier of CdS but also the precursor of CuS. However, peaks of CuS were hardly detected for CC-1 and 2, due to the low content of Cu species in the composites (Table 1) and consequently the high dispersion of Cu species [30,31]. In addition, no peak corresponding to CuBDC was observed for all the samples, which indicated the decomposition of CuBDC during the hydrothermal process. The released Cu2þ ions might further react with S2 to form CuS crystals while the BDC ligands were dissolved into water.
Composition analysis (ICP-AES, XPS) The surface chemical environments of the samples were analyzed by XPS (Fig. 2). Peaks in high-resolution spectra of S 2p (Fig. 2a) and Cd 3d (Fig. 2b) for all the samples were typical S2 and Cd2þ in CdS, respectively [40]. No signal was observed in Cu 2p region for CC-0 (Fig. 2c), while for CC-1, 2, 3, 4 and 5, the XPS spectra of Cu 2p showed the binding energies of Cu 2p1/2 and Cu 2p3/2 peaks at 952.6 and 932.6 eV, respectively, which were typical values for Cu2þ in CuS [41e43]. Furthermore, symmetrical shapes of the two Cu 2p peaks implied the presence of pure CuS. Thus, XRD and XPS results revealed the compositions for all samples. CC-0 was pure hexagonal CdS while others consisted of CuS and CdS. The total contents of Cu, measured by ICP-AES, increased from 0.89 wt% in CC-1 to 8.61 wt% in CC-5 (See Table 1). The surface Cu content (atom%) in CC-1, CC-2 and CC-3, derived from
Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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XPS results, increased with the corresponding total Cu content. However, the surface Cu contents of CC-4 and CC-5 were close to that of CC-3, although their total Cu contents were much higher. This might be attributed to the larger particle of CuS in CC-4 and 5 than that in CC-3 [44].
Structural characterizations (TEM, SEM, BET) TEM was employed to further analyze the structures of all the photocatalysts. Fig. 3a showed that the size distribution of the nanoparticles of CC-0 was narrow with an average value of ~40 nm. The corresponding HRTEM image (Fig. 3b) exhibited
Table 1 e Composition, surface chemical environments and band gap of CC-0, 1, 2, 3, 4 and 5. sample total content surface content band BET of Cu (wt%)[a] of Cu (atom%)[b] gap surface (eV)[c] area (m2/g) CC-0 CC-1 CC-2 CC-3 CC-4 CC-5 a b c
Fig. 1 e XRD patterns of CC-0, 1, 2, 3, 4 and 5 (C ¼ peaks of hexagonal CdS; B ¼ peaks of cubic CdS; * ¼ peaks of hexagonal CuS).
0 0.89 1.64 2.89 5.96 8.61
0 2.95 3.58 4.08 4.28 4.08
2.31 2.10 1.98 2.19 2.15 2.23
34.14 3.76 3.03 3.46 2.02 3.67
Total contents of Cu were measured by ICP-AES. Surface contents of Cu were measured by XPS. Band gaps were derived from UVevis DRS, see Fig. 4.
fringes with lattice spacing of 0.353 and 0.192 nm which correspond to (H 100) and (H 103) plane of hexagonal CdS, respectively. No fringe corresponding to cubic CdS was found and this was consistent with the XRD results (Fig. 1). The morphologies of CC-1, 2, 3, 4 and 5 (Fig. S4, Fig. 3c and e) indicated the aggregation of the nanoparticles after introducing the CuBDC. The aggregations were also confirmed by BET surface areas (Table 1). The pure CdS(H) had a BET area of 34.14 m2/g, however, the areas of the composites decreased dramatically and were 3.76, 3.03, 3.46, 2.02 and 3.67 m2/g for CC-1, 2, 3, 4 and 5, respectively. In addition to fringes of 0.193 nm, the corresponding (H 103) plane of hexagonal CdS, CC-3 (Fig. 3d) also exhibited two groups of fringes with lattice spacing of 0.337 nm. Furthermore, the cross angle of those two lattice fringes was 70.5 , which demonstrated the corresponding fringes belonged to (C 111) planes of cubic CdS. Moreover, CuS nanoparticles less than 10 nm were deposited on CdS, indicated by the lattice fringes of 0.326 and 0.258 nm corresponding to (100) plane and (104) plane of hexagonal CuS, respectively. Moreover, EDX mapping measurement (Fig. S5) for CC-3 showed that CuS nanoparticles were well dispersed on the surface of CdS. When increasing CuBDC, the aggregation of the particles in CC-4 (Fig. 3e) was severer than that in CC-3, which was consistent with the BET results. Fig. 3f also clearly showed lattice fringes of both CdS(H) and CdS(C). In addition, the size of deposited CuS particles (the area circled by the yellow dash line in Fig. 3f) were much larger than those in CC-3. This might be the reason why the total Cu content of CC-4 was more than twice that of CC-3 but there was only a little increase in surface content of Cu derived from XPS: the percentage of surface Cu atom decreased due to the larger CuS particles and lower BET surface. From Fig. 3d and f, the bonding area between CdS(H) and CdS(C) could be assigned to phase junction. Li et al. reported a facile method to fabricate CdS nanorod phase junctions composed of hexagonal core and cubic shell by adjusting the ratio of Cd to S [45]. With low Cd/S ratio (Cd/S at 1:8), pure CdS(H) was formed via kineticsdominated process. While the Cd/S ratio increased, CdS(H)/ CdS(C) phase junction was obtained due to the decrease of nucleation rate and the reaction changed from kineticsdominated to thermodynamics-dominated process. In our synthetic process, the Cd/S ratio was kept at 1:3.37. Without introduction of CuBDC, pure CdS(H) was formed. However, as adding and increasing CuBDC, the Cu(II) would also consume
Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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Fig. 2 e XPS spectra of CC-0, 1, 2, 3, 4, 5 and 6 with high resolution for (a) S 2p, (b) Cd 3d and (c) Cu 2p.
the S released from thiourea, leading to the increase of Cd/S ratio. Thus, CdS(H)/CdS(C) phase junctions were formed under thermodynamics-dominated process.
UVevis diffuse reflectance spectra The UVevis diffuse reflectance spectra (DRS) of the samples were shown in Fig. 4a. CC-0 had a typical UVevis DRS
absorption curve of hexagonal CdS with an absorption edge at ~525 nm [24]. Compared with CC-0, all the CuS/ CuS(H)/CdS(C) possessed similar red-shifted visible light absorption edges and showed increasing absorption between 550 and 800 nm with the increase of CuS content. This could be attributed to CuS, which had an obvious absorption in the region between 550 and 800 nm [46]. The curves of (ɑhʋ)2 vs hʋ for CC-0, 1, 2, 3, 4 and 5 were
Fig. 3 e TEM images of (a) CC-0, (c) CC-3, (e) CC-4. High resolution TEM images of (b) CC-0, (d) CC-3 and (f) CC-4. Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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catalyst to reduce the overpotential in the production of H2 from water and suppress the fast backward reaction as well (photocatalytic H2 production for CC-3 without Pt was showed in Fig. S6) [48]. All CuS/CdS(H)/CdS(C) samples exhibited enhanced catalytic performance in comparison to pure CdS(H) (Fig. 5a). Typically, CC-3 showed the best H2 production rate of 2042 mmol/g/h, almost 14 times higher than that of CC-0. Although the photo-corrosion was inevitable, which was indicated by the XRD of CC-3 after 3-cycle photocatalytic test (Fig. S7), CC-3 still had a rate of H2 production at 1413 mmol/g/h (Fig. 5b). To further understand the mechanisms of improvement of H2 production by CuS/CdS(H)/CdS(C), transient photocurrent measurements were carried out. As shown in Fig. 5c, CC-0 had the lowest photocurrent density, while all the composite samples showed the improved photocurrent responses. Particularly, CC-3 exhibited much higher photocurrent density than other samples. Photoluminescence (PL) spectra (Fig. 5d) also showed that, all the composite samples had the lower PL intensities than that of pure CdS(H). Therefore, formation of phase junction between CdS(H) and CdS(C) and the loading of CuS did improve the reduction of re-combination of photo-generated electrons and holes so that the composite samples had higher photocatalytic activities.
Inferred mechanism
Fig. 4 e (a) UVevis diffuse reflectance spectra of CC-0, 1, 2, 3, 4 and 5. (b) Tauc plots, i.e. the curves of (ɑhʋ)2 vs hʋ for CC-0, 1, 2, 3, 4 and 5, where ɑ, h and ʋ are the absorption coefficient, Plank’s constant and light frequency, respectively.
revealed in Fig. 4b. The corresponding band gaps were aslo obtained and listed in Table 1. CC-0 had a band gap of 2.31 eV, while those composites, CC-1, 2, 3, 4 and 5, had lower band gap energies. Recently, Ai et al. reported the red-shift of the absorption edge and hence the reduction of the band gap for hexagonal/cubic CdS composites due to the phase junction between hexagonal and cubic CdS [47]. Our HRTEM (Fig. 3d, f) results confirm the formation of phase junction as discussed above, which could be contributed to the reduction of the band gap of the composites compared to pure hexagonal CdS. Thus, our UVevis DRS results demonstrated that the light absorptions for CC-1, 2, 3, 4 and 5 are extended and enhanced in the visible-light region compared with CC-0.
Photocatalytic H2 productions and electrochemical measurements Photocatalytic H2 evolutions of all the samples were evaluated under visible-light irradiation (>420 nm), using Pt as co-
Among all the composite samples, CC-3 showed much higher photocatalytic activity. Besides the photocurrent responses and PL results, EIS Nyquist plots (Fig. S8) also indicated that CC-3 had a much lower interfacial charge-carrier transfer resistance than that of CC-0 under visible-light irradiation, suggesting the apparent enhancement of interfacial chargecarrier transfer on the surface of CC-3. The other composite samples also showed the similar structure to CC-3 (indicated by XRD, XPS, BET and UVevis DRS results), however, their photocurrents were much lower than that of CC-3. In the cases of CC-1 and CC-2, both the total amounts and surface amounts of CuS, were less than that of CC-3 (Table 1) and lead to the lower rates of H2 production. On the other hand, CC-4 and CC-5 had more total amounts of CuS, however the particle sizes were much larger (Fig. 3f), which might increase the difficulty for charge-carriers transferring to surface [49] (CC-4 and 5 had lower photocurrent densities than that of CC-3) and thus the poorer photocatalytic performance than CC-3. Furthermore, as indicated by TEM result and photoelectrochemical measurement, it was reasonable to infer that the well dispersed CuS nanoparticles could form heterojunctions with CdS, which also accelerated the transportation of photo-induced electrons from CuS to CdS. To illustrate this mechanism, Mott-schottky measurement was employed for CC-3 (Fig. S9) and the estimated CB edge for CC-3 was 0.34 V. Combined with the estimated band gap of CC-3 (Table 1), the schematic illustration for the mechanism of H2 production via water splitting by CC-3 was showed in Fig. 6. CuS/CdS(H)/ CdS(C) was obtained in one step via hydrothermal treatment. With the phase junction between CdS(H) and CdS(C), recombination of the photo-generated electrons and holes was suppressed. In addition, CuS derived from CuBDC helped to
Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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Fig. 5 e (a) Rates of H2 evolution by CC-0, 1, 2, 3, 4 and 5 under visible light. (b) Recycling test of H2 evolution for CC-3 catalyst. (c) Transient photocurrent responses for CC-0, 1, 2, 3, 4 and 5. (d) Photoluminescence spectra of CC-0, 1, 2, 3, 4 and 5.
Fig. 6 e Schematic illustration for the mechanism of H2 production via water splitting by CC-3.
enhance the light absorption in the visible region. Moreover, heterojunctions were formed between the well dispersed CuS nanoparticles and CdS of CC-3, therefore, the transportation of electrons photo-generated from CuS to CdS was accelerated. Consequently, CC-3 showed the highest photocurrent density and the highest rate of H2 evolution.
Conclusions In this work, CuS/CdS(H)/CdS(C) composites with various contents of CuS were synthesized via hydrothermal method with the introduction of CuBDC. According to XRD, XPS, BET,
Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136
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SEM and TEM results, CuBDC was suggested as the structrual modifier to CdS, which led to the formation of phase junction between hexagonal and cubic CdS for CuS/CdS(H)/CdS(C) composites. Therefore, CuS/CdS(H)/CdS(C) composites had narrower band gap than that of pure hexagonal CdS and consequently the enhancement of light absorption. Moreover, CuS particles derived from CuBDC deposited on CdS also enhanced the absorption in the visible region. Thus, all the CuS/CdS(H)/CdS(C) composites showed higher photocurrent densities so that the higher rates of H2 production than pure hexagonal CdS under visible-light irradiation. Particularly, CC-3 with proper heterojunction between CuS and CdS gave the highest rate of H2 evolution.
Acknowledgements This work was supported by PolyU fund (1-ZE27). This work was also supported by the National Natural Science Foundation of China (No. 21706130), the Jiangsu Natural Science Funds (BK20150943).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.136.
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Please cite this article as: Luo L et al., Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.136