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ZnIn2S4 architectures with MoS2 quantum dots as solid-state electron mediator
Applied Surface Science 504 (2020) 144406
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Full Length Article
Highly efficient visible-light-driven photocatalytic hydrogen evolution by all-solid-state Z-scheme CdS/QDs/ZnIn2S4 architectures with MoS2 quantum dots as solid-state electron mediator Wei Chena, Rui-Qiang Yana, Jian-Qun Zhua, Guo-Bo Huanga, , Zhong Chenb, ⁎
a b
T
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School of Pharmaceutical and Materials Engineering, Taizhou University, Taizhou 318000, Zhejiang, PR China School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
ARTICLE INFO
ABSTRACT
Keywords: CdS MoS2 ZnIn2S4 Water splitting Z-scheme
All-solid-state Z-scheme CdS/QDs/ZnIn2S4 architectures with MoS2 quantum dots as solid-state electron mediator were successfully designed and constructed by optimally combining one-dimensional CdS nanorods, zerodimensional MoS2 quantum dots (QDs) and two-dimensional ZnIn2S4 nanosheets. The photocatalytic water splitting for hydrogen evolution demonstrated that such structural design can synergistically trigger remarkably improved visible-light-driven photocatalytic activity. Photocatalytic H2-evolution at 2107.5 μmol g−1 h−1 was achieved on this CdS/QDs/ZnIn2S4 architectures under visible light irradiation, exceeding those of bare CdS nanorods and pure ZnIn2S4 nanosheets by a factor of 26 and 62, respectively. This highly efficient photocatalytic activity arises from the effective charge separation and favourable electron mediator of MoS2 QDs. The Z-scheme charge separation mechanism was verified by the ESR, PL-TA and organic electron acceptor test.
1. Introduction With the rapid development of industrialization, recent years have witnessed an indisputable fact that traditional fossil fuels are increasingly exhausted because of a rapid growing demand, giving rise to the serious environmental pollutions, such as acid rain, green house effect and photochemical smog [1–6]. Therefore, it is urgent for human beings to actively explore and develop new energy sources while effectively using fossil fuels, so as to gradually replace the existing fossil energy sources. As an important energy carrier, hydrogen energy, with the characteristics of high energy density, safety, only produces water after combustion. It does not pollute the environment, and therefore is considered as an ideal green energy source [7,8]. Photocatalysis technology provide a feasible tactics to produce hydrogen energy since Fujishima and Honda demonstrated photoelectrochemical reaction to prepare hydrogen from water splitting by using TiO2 photoelectrode in 1972 [9]. Till now, although great efforts have been made, a large number of photocatalysts are only excellently active in the ultraviolet (UV) light region. Their limited UV-driven activity largely inhibits their overall efficiency because solar spectrum consists of only 5% UV (300–400 nm), and the rest comprises 43% visible light (400–700 nm) and 52% infrared (700–2500 nm) [10]. Therefore, the researches on the design and construction of visible-light-responsive photocatalysts with
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the characteristics of high efficiency and excellently stable is crucial for hydrogen production on photocatalysts by transforming the solar energy into chemical energy. In the search of visible-light-driven photocatalysts, cadmium sulfide (CdS) had attracted constant attention in the fields of photocatalysis researches due to the narrow band gaps corresponding to visible light absorption and suitable band edges covering the potentials for the generation of hydrogen and oxygen [11,12]. However, the practical application was undoubtedly limited by the strong photocorrosion during photocatalytic process, leading to short lifespan of CdS photocatalysts. Also, the photogenerated holes and electrons generated by single CdS component often tend to easily recombine near or at its surface, thus leading to a low photocatalytic activity. Taking these bottlenecks into account, many attempts, including elemental doping [13], metal loading [14] and heterogeneous coupling [15,16], were utilized to boost the overall performance of CdS. Recently, Shi et al. [13] successfully prepared P doped CdS (P-CdS) with S vacancies using NaH2PO2 as the P source via a thermal phosphorization reaction. This structure has exhibited remarkably improved photocatalytic activity towards hydrogen evolution from pure water splitting in the absence of any sacrificial agents under LED irradiation because of the construction of effective electron trap level to prolong the lifetimes of photogenerated electrons. Chai et al. [14] utilized photodeposition routes by
Corresponding authors. E-mail addresses: [email protected] (G.-B. Huang), [email protected] (Z. Chen).
https://doi.org/10.1016/j.apsusc.2019.144406 Received 18 July 2019; Received in revised form 9 October 2019; Accepted 14 October 2019 Available online 08 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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in-situ loading Ni onto CdS nanoparticles to synthesize Ni/CdS nanocomposites, which were employed for photocatalytic splitting of alcohols into corresponding carbonyl compounds and hydrogen induced by visible light irradiation. Li et al. [17] reported the design and syntheses of heterogeneous phase photocatalysts with hexagonal@cubic CdS core@shell structure. Benefiting from the special structural design promoting the effective separation of charge pairs, the resultant heterogeneous phase photocatalysts possessed unprecedented photo-stability for highly efficient photocatalytic hydrogen evolution. Although the enhancement on photocatalytic activity and stability has been realized by above strategies, the overall performance of CdS or CdS based heterogeneous photocatalysts remains unsatisfactory to meet the practical requirements for more extensive industrial applications. Among different morphologies of CdS photocatalysts, one-dimensional (1D) CdS based architectures demonstrated a higher photocatalytic activity as comparison to their counterparts with other morphologies because of the shorter transportation distance, higher electron mobility and optical excitation originated from high length-to-diameter ratios [18,19]. Based on the above analyses, to optimize structural design and manipulate the transfer of charge carriers on the 1D CdS based nanocomposites probably are a critical means for the improvement of their overall performance toward practical photocatalytic applications. Ternary metal chalcogenides ZnIn2S4, with excellent properties of electrons confinement effect in their unique two-dimensional (2D) atomic layers and relatively high stability during photocatalytic reactions, has gained great amount of attention in the construction of ZnIn2S4 based hybrid photocatalysts [6,20–22]. Particularly worth mentioning is the widespread coupling between these 2D ZnIn2S4 and 1D photocatalysts in order to mitigate the drawbacks and to integrate the above-mentioned merits of single component. Dong′s group [23] prepared ZnIn2S4/TiO2 nanocomposites by decorating 2D ZnIn2S4 nanosheets onto the surface of 1D TiO2 nanorods at hydrothermal environment. The prepared ZnIn2S4/TiO2 nanocomposites exhibited highly efficient photocatalytic activity for hydrogen production compared with pure 1D TiO2 nanorods and 2D ZnIn2S4 nanosheets under solar light irradiation because of the advantages in synergistically promoting the photogenerated charge-carrier separation. Wang et al. [24] designed and fabricated the 1D hierarchical ZnIn2S4-In2O3 tubular heterostructures by growing ZnIn2S4 nanosheets on both outer and inner surfaces of In2O3 microtubes for efficient visible-light-driven photocatalytic CO2 reduction. Inspired by above investigations, we hold a view that it is a viable option to combine the merits of 2D ZnIn2S4 nanosheets and 1D CdS nanorods. It is well known that small size of MoS2 quantum dots (QDs) can not only minimize the charge recombination in MoS2, but also effectively modulate the charge transfer direction. Taking these effects together, we developed CdS/QDs nanorods by in-situ decoration of QDs on CdS surface at solvothermal environment. Then the CdS/QDs/ ZnIn2S4 hybrid photocatalyst were prepared by tightly wrapped 2D ZnIn2S4 nanosheets on the surface of 1D CdS/QDs nanorods. The photocatalytic water splitting for hydrogen evolution was selected to estimate the photocatalytic activity of as-obtained CdS/QDs/ZnIn2S4 ternary photocatalysts. We hope that our current research could provide some typical sights in the design, syntheses and applications of highly efficient visible-light-responsive photocatalysts.
2.2. Catalyst syntheses Syntheses of CdS nanorods and MoS2 quantum dots decorated CdS nanorods (CdS/QDs): Well-proportioned CdS nanorods and MoS2 quantum dots decorated CdS nanorods were synthesized by a simple one-pot solvothermal reaction using ethylenediamine as solvent with slight modification [25]. Typically, Cd(NO3)2·4H2O (1 mmol, 308 mg), CS(NH2)2 (5 mmol, 381 mg) and a certain amount of Na2MoO4·2H2O were added into 40 mL ethylenediamine and then transferred to a 50 mL polyphenylene-lined stainless steel autoclave, which was sealed and heated in an air-circulating oven at 220 °C for 24 h. After naturally cooling down to room temperature, the yellow CdS/QDs nanorods with 2 wt% QDs loading were collected, washed with distilled water and absolute ethanol to remove the residues of organic solvent and dried at 60 °C for 3 h. The pure CdS was also prepared using same route but without the addition of any Na2MoO4·2H2O. Syntheses of CdS/ZnIn2S4, CdS/QDs/ZnIn2S4 architectures and individual ZnIn2S4 nanosheets: The resultant CdS/QDs (300 mg) was completely dispersed into 150 mL deionized water to obtain uniform suspension. Then, ZnCl2 (0.5 mmol, 68.2 mg), InCl3·4H2O (1 mmol, 293.2 mg) and L-Cys (4 mmol, 484.6 mg) were successively dissolved into suspension. After vigorously stirred for 1 h, the above suspension was transferred into 200 mL polyphenylene-lined stainless steel autoclave and reacted at an air-circulating oven at 180 °C for 12 h. The resultant CdS/QDs/ZnIn2S4 architectures were gathered by centrifugation, washing and drying after autoclave was cooled to room temperature. The CdS/ZnIn2S4 architectures were obtained by replacing CdS/QDs with CdS nanorods and individual ZnIn2S4 was synthesized without adding CdS nanorods or CdS/QDs nanorods using same reaction condition. 2.3. Catalyst characterization Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance diffractometer using a Cu Kα source, with a scan range from 10° to 80°. Microstructure was observed on a JEOL JEM-2100F highresolution transmission electron microscope (TEM) at an accelerating voltage of 200 kV. UV–vis diffuse reflectance spectra (UV–vis DRS) were acquired on a Hitachi UV-4100 spectrometer equipped with BaSO4 as the reference and an integrating sphere attachment at room temperature in air. X-ray photoelectron spectroscopy (XPS) measurements were carried out with an ESCALab 250Xi spectrometer by using an unmonochromated Al Kα (1486.6 eV) as X-ray source. The photoluminescence (PL) spectra and the time-resolved photoluminescence (TRPL) spectra for solid-state samples were investigated through HORIBA iHR320. The data of photoluminescence terephthalic acid (PLTA) were obtained by using a Cary Eclipse type fluorescence spectrophotometer at the excitation wavelength of 315 nm. The electron spin resonance (ESR) spectra were measured on an ESR spectrometer (JES X320). Photoelectrochemical measurements were performed on a CHI760E electrochemical work station (Chenhua Instrument, Shanghai) in a standard three-electrode system with an Ag/AgCl as a reference electrode, Pt foil as the counter electrode and the powder-coated FTO conducting glass as the working electrode at 0.5 M Na2SO4 solution. The Mott-Schottky curves at different frequencies of samples were also recorded on a 0.5 M Na2SO4 solution.
2. Experimental
2.4. Photocatalytic activity
2.1. Chemicals
A Pyrex glass photoreactor connected to a glass closed gas circulation system, was used to analyze the photocatalytic hydrogen evolution. A photocatalyst sample of 50 mg was dispersed in 50 mL aqueous solution including 40 mL ultrapure water and 10 mL lactic acid. Prior to photocatalytic reactions, the mixed liquor was bubbled by nitrogen bubbler for 30 min. During photocatalytic reaction, the temperature was maintained at 25 °C by cyclic water and the irradiated source
Cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O), sodium molybdate dehydrate(Na2MoO4·2H2O), thiourea(CH4N2S), zinc chloride(ZnCl2), indium chloride tetrahydrate (InCl3·4H2O), L-Cysteine (C3H7NO2S, LCys), ethylenediamine (C2H8N2) and lactic acid (C3H6O3) were provided by Shanghai Aladdin biochemical technology co., LTD. All chemical reagents are used as provided without any purification. 2
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(EDS) revealed the coexistence of Cd, Zn, In, Mo, S, C and Cu, which indicats the as-prepared CdS/QDs/ZnIn2S4 hybrid architectures are composed of CdS, MoS2 and ZnIn2S4. The impurity elements including C and Cu are originated from holey carbon-copper grids for TEM characterization. The HRTEM image of CdS/QDs/ZnIn2S4 hybrid architectures exhibited three different lattice fringes. The clear lattice spacing with the value of 0.61 nm, representing the (0 0 3) plane of MoS2, while the lattice fringes with d = 0.36 nm and d = 0.41 nm belong to the (1 0 0) plane of hexagonal CdS and (0 0 6) plane of hexagonal ZnIn2S4, respectively [29]. Moreover, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding EDS mapping were further employed to investigate the chemical compositions of different components in CdS/QDs/ ZnIn2S4 hybrid architectures (Fig. 3h). The results indicate that ternary architectures indeed consist of Cd, Zn, In, Mo and S elements, which is in well agreement with above result [30]. The optical properties of bare CdS, CdS/QDs, CdS/ZnIn2S4, CdS/ QDs/ZnIn2S4 as well as pure ZnIn2S4 were measured by UV–vis DRS. As shown in Fig. 4, it is clearly seen that pure ZnIn2S4 possesses strong light absorption during measured spectrum range. It should be noted that bare CdS possesses obvious absorption towards UV irradiation and its absorption edge is located at visible light region with wavelength of about 530 nm, which is arisen from intrinsic band gap absorption [31]. More importantly, in-situ decoration of QDs and/or ZnIn2S4 nanosheets onto the surface of CdS nanorods can actively increase the visible light absorption in spectral range from 530 to 800 nm of as-synthetic CdSbased hybrid samples. Also note that, among of all CdS-based nanocomposites, no differences originated from inherent band structure can be observed after the hybrid reactions, indicating QDs and/or ZnIn2S4 nanosheets were immobilized onto the CdS surface rather than entering internal crystalline structure of CdS nanorods. The bandgap of CdS nanorods was not changed after coupling with QDs and/or ZnIn2S4 nanosheets [32]. The elemental compositions, valence states and chemical interactions between different constituent in hybrid samples were systematically investigated by XPS. The result from survey XPS spectrum (Fig. 5a) reveals that the ternary CdS/QDs/ZnIn2S4 hybrid architecture consists of Cd, S, Mo, Zn, In and O elements, which further verifies the coexistence of CdS, MoS2 and ZnIn2S4 in CdS/QDs/ZnIn2S4 hybrid architecture. The negligible O element is resulted from the absorbed molecules including H2O and CO2 on the surface of photocatalysts. The binding energies of fine-scanning Cd 3d signals are observed at 404.3 eV and 411.0 eV for CdS sample, indicative of Cd2+ in CdS (Fig. 5b) [33]. The Zn 2p region at CdS/QDs/ZnIn2S4 hybrid sample could be deconvoluted into four characteristic peaks at 1021.3 eV, 1022.5 eV, 1044.5 eV and 1045.7 eV by peak fitting(Fig. 5c) [34]. The binding energies of Zn 2p peak at 1021.3 eV and 1044.5 eV could be attributed to Zn 2p3/2 and Zn 2p1/2, representing chemical bonding between CdS and ZnIn2S4 nanosheets, whereas the peak at 1022.5 eV and 1044.5 eV is related to chemical interactions of ZnIn2S4 nanosheets in connection with the QDs on surface of CdS nanorods [35,36]. In Fig. 5d, two binding energies at location of about 444.5 and 452.1 eV independently belong to In 3d5/2 and In 3d3/2, demonstrating that the presence of In3+ in ZnIn2S4 nanosheets [37]. As for pure QDs based nanocomposites, the Mo 3d region (Fig. 5e) in CdS/QDs sample exhibited two weak peaks located at 227.7 eV and 231.1 eV, corresponding to Mo 3d5/2 and Mo 3d3/2, while CdS/QDs/ZnIn2S4 hybrid architectures showed slight shift in Mo 3d region at 228.1 and 231.5 eV, which is an indication of the existence of Mo4+ in these two samples [38]. The fine-scanning XPS spectrum in S 2p regions (Fig. 5f) present two binding energies at 160.6 and 161.8 eV for CdS nanorods, 161.3 and 162.5 eV for ZnIn2S4 nanosheets. The split energy of 1.2 eV between S 2p1/2 and S 2p3/2 in CdS nanorods and ZnIn2S4 nanosheets indicate that the S species exist as −2 [39]. The positive shifts of Cd 3d, Mo 3d and S 2p, and the negative shift of Zn 2p and In 3d indicate the charge transfer from CdS to QDs and then to ZnIn2S4 via tight
Fig. 1. XRD patterns of bare CdS nanorods, ZnIn2S4 nanosheets, CdS/QDs, CdS/ ZnIn2S4 and CdS/QDs/ZnIn2S4 hybrid samples.
equipped with a cutoff filter to remove the wavelength less than 420 nm was provided by CEL-HXF-300 typed 300 W Xe-lamp (Ceaulight, Beijing). The produced hydrogen amount was measured on gas chromatography (GC-7920) equipped with a 5 Å molecular sieve column and a thermal conductivity detector (TCD). Ar gas was used as the carrier. 3. Results and discussion The phase structure of resultant samples were characterized by XRD, as shown in Fig. 1. The as-obtained CdS nanorods are highly crystalline, which is well ascribed to pure hexagonal phase of CdS (JCPDS No. 65-3414) [19]. The XRD patterns of pristine ZnIn2S4 nanosheets match well with the structure of hexagonal ZnIn2S4 as evidenced from the PXRD database (JCPDS: 65-2023) [20,26]. For the XRD pattern of CdS/ZnIn2S4, the characteristic diffraction peaks for both hexagonal CdS and hexagonal ZnIn2S4 are observed in same diffraction position as individual components, which indicate that synthetic CdS/ZnIn2S4 architecture is indeed composed of pure CdS nanorods and pristine ZnIn2S4 nanosheets. Moreover, no diffraction peaks belonging to MoS2 QDs can be found in CdS/QDs and CdS/QDs/ZnIn2S4 architectures, which are probably attributed to relatively low diffraction intensity of MoS2 QDs and/or the small amount beyond the detection limit of XRD. Actually, the existence of QDs could be well confirmed by microstructure observation and elemental analyses. The morphology and microstructure of the as-obtained CdS/QDs/ ZnIn2S4 hybrid architecture is observed by FESEM, TEM and HRTEM, as presented in Figs. 2 and 3. Fig. 2 displayed FESEM images of as-prepared samples, from which we noticed that 1D structure of CdS nanorods with high aspect ratio and three-dimensional (3D) ZnIn2S4 assembly from 2D nanosheets were successfully synthesized. The asobtained CdS/QDs nanocomposites by in-situ loading of MoS2 QDs onto CdS surface does not result in the structural change and the resultant CdS/QDs samples still maintained rod-like structure(Figs. 2c and 3b). From HRTEM image, it is obvious that some MoS2 QDs were tightly decorated on the surface of CdS nanorods as evidenced by lattice fringes. The lattice spacing of 0.61 nm and 0.27 nm separately corresponded to (0 0 3), (1 0 1) planes of MoS2, and 0.36 nm (1 0 0) plane for hexagonal CdS [27]. When precursor of ZnIn2S4 were added during hydrothermal environment, the as-prepared CdS/ZnIn2S4 and CdS/ QDs/ZnIn2S4 hybrid architectures were characteristic of helical structure with the entanglement of ZnIn2S4 nanosheets onto CdS surface (Fig. 2(d–f) and Fig. 3(d and e)). Such structural design is expected to be able to expose its entire ZnIn2S4 surface, thus improving the photocatalytic activity [19,28]. The energy-dispersive X-ray spectrometer 3
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Fig. 2. FESEM image of different samples: (a) bare CdS nanorods, (b) pure ZnIn2S4 microspheres assembled from nanosheets, (c) CdS/QDs nanorods, (d) CdS/ZnIn2S4 and (e and f) CdS/QDs/ZnIn2S4 hybrid architectures.
Fig. 3. TEM and HRTEM analyses: TEM images of (a) bare CdS, (b) CdS/QDs, (d) CdS/ZnIn2S4 and (e) CdS/QDs/ZnIn2S4; HRTEM image of (c) CdS/QDs and (g) CdS/ QDs/ZnIn2S4; (f) EDS and (h) HAADF-STEM and corresponding elemental mapping of CdS/QDs/ZnIn2S4 hybrid architectures.
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interfacial contacts [40]. Therefore, we could draw a conclusion, from the joint results of TEM, UV–vis DRS and XPS, that the CdS/QDs/ ZnIn2S4 hybrid architectures have been successfully prepared by the combination of solvothermal and hydrothermal method. The photocatalytic activity of resultant samples was evaluated by visible-light-driven photocatalytic hydrogen evolution with addition of lactic acid as sacrificial agent, as shown in Fig. 6. Control experiments revealed that no hydrogen was detected when irradiation or photocatalyst were withdrew during photocatalytic reactions, indicating that light irradiation and photocatalyst are inextricably intertwined for photocatalytic reaction towards hydrogen evolution. Clearly, the pure ZnIn2S4 displays a relatively low photoactivity with a produced H2 rate of 33.9 μmol g−1 h−1. Compared with ZnIn2S4, the bare CdS showed relatively decent photo activity (78.5 μmol g−1 h−1) towards water splitting for hydrogen evolution due to the limited separation efficiency of charge pairs. After coverage of ZnIn2S4 nanosheets onto CdS surface, there is an enhancement of photoactivity (168.7 μmol g−1 h−1), which revealed that hydrothermal process can boost the construction of heterogeneous interface in the CdS/ZnIn2S4 hybrid system to effectively
Fig. 4. UV–vis diffuse reflectance spectra of bare CdS nanorods, ZnIn2S4 nanosheets, CdS/QDs, CdS/ZnIn2S4 and CdS/QDs/ZnIn2S4 hybrid samples.
Fig. 5. XPS analyses of as-obtained samples: (a) survey spectra, (b) Cd 3d, (c) Zn 2p, (d) In 3d, (e) Mo 3d and (f) S 2p regions. 5
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Fig. 6. (a) Photocatalytic activity for hydrogen evolution of as-prepared samples from water splitting with addition of lactic acid as sacrificial agent under visible light irradiation, (b) corresponding to XRD patterns of fresh and used CdS/QDs/ZnIn2S4 hybrid sample.
facilitate the separation and transfer of photoinduced charge. Note that, after single decorating of QDs, the as-prepared CdS/QDs hybrid sample displays an obvious photoactivity enhancement towards the hydrogen evolution (685.3 μmol g−1 h−1), which is assigned to the fact that the in-situ decoration of QDs can effectively separate the photogenerated electrons from CdS nanorods to QDs to weaken faster recombination between photogenerated holes and electrons. When the CdS nanorods were collaboratively decorated by QDs and ZnIn2S4 nanosheets, a significant enhancement in photocatalytic activity can be observed (2107.5 μmol g−1 h−1) and its hydrogen evolution rate exceeded those produced on bare CdS nanorods and pure ZnIn2S4 nanosheets by more than 26 times and 62 times, respectively. Given that the activity increment by construction of heterogeneous interfaces between CdS nanorods and ZnIn2S4 nanosheets was relatively minor, the remarkably improved photocatalytic activity of this CdS/QDs/ZnIn2S4 hybrid sample can be primarily attributed to solid-state electron mediator of QDs. They not only favor the inhibition of carrier recombination but also modulate the flow direction of photogenerated electron, thereby improving the photocatalytic activity for hydrogen generation. Furthermore, the photocatalytic durability of CdS/QDs/ZnIn2S4 was tested by recycling reactions for successive 24 h. After recycles, the CdS/QDs/ ZnIn2S4 hybrid photocatalysts only had a slight loss of photoactivity (~7.79%) for hydrogen evolution, implying highly catalytic stability during the photocatalytic reaction. This result is further confirmed by XRD analysis as shown in Fig. 6b. From the comparison of crystal phase structure of fresh and used samples, it is clearly seen that no difference can be observed from XRD patterns, which confirms the phase structure of CdS/QDs/ZnIn2S4 hybrid sample remains intact. Above results confirm that CdS/QDs/ZnIn2S4 hybrid sample are sufficiently stable during photocatalytic process. To consolidate the above hypothesis that the speedy charge separation process occurs on the CdS/QDs/ZnIn2S4 architectures, we have performed systematical characterizations including steady-state photoluminescence (PL) spectra, time-resolved photoluminescence (TRPL) spectra, transient photocurrent (I-t) and electrochemical impedance spectra (EIS). Photoluminescence (PL) spectra have been thought as a powerful tool to investigate separation efficiency of the interfacial electron transfer because the PL emission is caused by free charge carriers recombination [41]. The results from steady-state PL spectra (Fig. 7a) indicate that PL emission intensity of CdS nanorods were remarkably quenched after decorating QDs to form the heterogeneous contacts, suggesting the recombination between photogenerated holes and electrons is inhibited owing to the occurrence of efficient charge separation. Further decoration of ZnIn2S4 nanosheets on QDs loaded CdS nanorods led to a much more significant quenching in the resultant PL emission, indicating that the immobilized QDs promoted the charge separation by regulating the interfacial charge transfer between CdS nanorods and ZnIn2S4 nanosheets [42]. The effective transfer of charge
carriers of these samples has also been verified by the TRPL spectra (Fig. 7b). The fluorescence lifetime of CdS/QDs/ZnIn2S4 hybrid architectures is longer than the others, which also revealed the higher separation and transfer efficiency of photogenerated charge pairs in CdS/ QDs/ZnIn2S4 hybrid architectures [43]. This result corroborates that QDs have served as an effective solid-state electron mediator for CdS/ QDs/ZnIn2S4 hybrid sample by readily accepting photoexcited electrons. This gives rise to the increasingly pronounced charge separation for CdS/QDs/ZnIn2S4 hybrid architectures as revealed in the depressed PL intensity and increased fluorescent life-time as well as the photoelectrochemical measurements [44]. Fig. 7c compared the transient photocurrent responses during periodic on-off cycles of different photoelectrodes prepared by spin-coating samples on FTO under visible light irradiation. Apparently, surface decoration of QDs on CdS nanorods dramatically strengthens photocurrent, which revealed the effective transfer of photogenerated charge pairs by QDs [45,46]. Moreover, the effective separation of electron-hole pairs was further demonstrated by EIS analysis as illustrated in Fig. 7d. It was clear that the semicircle radius follows the same order as the photocatalytic performance. The semicircle radius of CdS/QDs/ZnIn2S4 hybrid architectures was smallest than those of counterparts, demonstrating the lower recombination rate of photo-generated charge and the enhanced interface charge transfer ability [47,48]. To acquire further insights into the charge separation-transfer mechanism in CdS/QDs/ZnIn2S4 hybrid architectures, the band potentials of samples were determined by a combination of Mott-Schottky plots and UV–vis DRS. Fig. 8(a and b) illustrates Mott-Schottky plots of bare CdS nanorods and pure ZnIn2S4 nanosheets, from which we could notice that flat-band potentials (Efb) of these two samples can be computed to be −0.696 V vs. Ag/AgCl (equivalent to −0.144 V vs. RHE) for bare CdS nanorods and −1.07 V vs. Ag/AgCl (equivalent to −0.545 V vs. RHE) for pure ZnIn2S4 nanosheets according to the computational formula (E (RHE) = E(Ag/AgCl) + 0.197 V + 0.0591pH), respectively. It is wellknown the Efb in n-type semiconductors is more positive by about 0.1 V than conduction band minimum (CBM) [49]. As results, the CBM of CdS nanorods and ZnIn2S4 nanosheets were −0.244 V and −0.645 V vs. RHE, respectively. Moreover, the results from UV–vis DRS (Fig. 8c) revealed the band gaps difference between single components with value of 2.36 eV for bare CdS nanorods and 2.51 eV for ZnIn2S4 nanosheets. Based on above analyses, the valence band maximum (VBM) of CdS nanorods and ZnIn2S4 nanosheets were about 2.116 V and 1.865 V vs. RHE, respectively. Therefore, the band alignment of bare CdS nanorods and pure ZnIn2S4 nanosheets in CdS/QDs/ZnIn2S4 hybrid architectures were proposed and shown as in Fig. 8d. Evidently, the staggered band edge potentials are observed between CdS nanorods and ZnIn2S4 nanosheets, leading to the buildup of band bending due to the presence of chemical potentials between these two samples. Such energy band structure significantly strengthened the separation-transfer efficiency 6
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Fig. 7. (a) Solid state room-temperature PL emission spectra, (b) time-resolved photoluminescence spectra, (c) transient photocurrent and (d) electrochemical impedance spectroscopy of bare CdS nanorods, ZnIn2S4 nanosheets, CdS/QDs, CdS/ZnIn2S4 and CdS/QDs/ZnIn2S4 hybrid samples.
Fig. 8. Mott-Schottky plot of (a) bare CdS nanorods and (b) pure ZnIn2S4 nanosheets; (c) band gap width and (d) energy band structure of bare CdS nanorods and pure ZnIn2S4 nanosheets. 7
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Fig. 9. Schematic illustrations of two possible photocatalytic systemsin CdS/QDs/ZnIn2S4 hybrid nanocomposites: (a) heterojunction-type and (b) Z-scheme type.
between photogenerated holes and electrons towards opposite direction to suppress the charge recombination, thereby improving the photocatalytic activity. Actually, the photocatalytic mechanism based on the charge separation-transfer between CdS nanorods and ZnIn2S4 nanosheets in CdS/QDs/ZnIn2S4 hybrid architectures can be classified into two models, named as heterojunction-type and Z-scheme. The first model involves acceptor-donor charge separation towards opposite directions in CdS/QDs/ZnIn2S4 hybrid architectures, as displayed in Fig. 9a, in which photogenerated holes jump from the VB of CdS to that of ZnIn2S4; simultaneously conduction band (CB) electrons of ZnIn2S4 transferred to the CdS and finally enriched on the MoS2 QDs or directly migrate to MoS2 QDs, giving rise to the decreased redox ability of photogenerated charge pairs. If the charge transfer followed first model, the photogenerated electrons were not able to reduce dissolved oxygen to produce superoxide radical because the photogenerated electrons on QDs was more positive than standard redox potential of E(O2/%O2–) = −0.284 V vs. RHE [50,51]. Moreover, if charge separation was carried out as first model, the photogenerated holes concentrated on VB of ZnIn2S4 nanosheets with the potential of 1.865 V vs. RHE also cannot be used in the production of hydroxyl radical (%OH) when CdS/QDs/ZnIn2S4 hybrid architectures used as photocatalyst because %OH generation from H2O by photogenerated hole oxidation path required a larger potential of 1.99 V vs. RHE [52]. For further verification, the ESR spectra were employed to detect the existence of %O2– and %OH during photocatalytic reactions, as shown in Fig. 10 (a, b). It is clearly seen that CdS/QDs/ZnIn2S4 hybrid architectures exhibited several characteristic peaks of DMPO-%O2– (Fig. 10a) in methanol dispersion with the exposure of visible light irradiation, revealing the generation of %O2– during photocatalytic process [53]. Furthermore, considering multiple pathways of hydroxyl radical formation, an appropriate amount of AgNO3 was added to rule out the effect of photogenerated electrons on the assessment of oxidation ability of photogenerated holes by directly oxidizing hydroxyl ion. By introducing 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the trapping agent, the quadruple peaks in CdS/QDs/ZnIn2S4 hybrid system, belonging to the signal of DMPO-%OH (Fig. 10b), can be obviously observed upon irradiation by visible light, representing the photogenerated holes in CdS/ QDs/ZnIn2S4 hybrid system has strong enough oxidation ability to produce %OH [54]. The results obtained from ESR spectra clearly indicated the heterojunction-type charge separation theory is not operating in the current case. Instead, Z-scheme charge transfer is more likely to be the transfer mechanism of photogenerated charge pairs and responsible for the improved photocatalytic activity in hydrogen evolution (Fig. 9b). In this model, the photogenerated electrons injection occurred from CB of CdS to QDs, and then migrated to VB of ZnIn2S4 and recombine with photogenerated holes on VB of ZnIn2S4, leaving the photogenerated holes on CdS nanorods and photogenerated electrons on ZnIn2S4 nanosheets. Such Z-scheme charge separation not only promotes the effective separation but also guarantees the strong redox ability of photogenerated holes and electrons, which is responsible for the generation of %O2– and %OH and remarkable enhancement of photocatalytic activity.
The strong redox ability of photogenerated holes and electrons were further verified by organic electron acceptor method and PL-TA. The reducing power of photogenerated electrons in CdS/QDs/ZnIn2S4 hybrid system was investigated by probing the photoreduction of methyl viologen dication (MV2+, E(MV2+/MV+) = −0.45 V vs. RHE). When a mixed solution, consisting of Na2SO3 (0.875 M), Na2S (0.625 M), MV2+ (50 μM) and a suitable amount of CdS/QDs/ZnIn2S4 hybrid photocatalysts was irradiated by visible light, the solution color rapidly changed to blue and the UV–vis spectra showed two new absorption peaks as the characteristic absorption of 395 and 603 nm, corresponding to the reduced viologen species (MV+) (Fig. 10c) [55,56]. The generation of %OH was further confirmed with PL-TA method by the detection of luminescent probe molecule (2-hydroxyterephthalic acid, TAOH) generated from chemical reaction between TA and %OH as displayed in Fig. 10d [57,58]. It is obvious that the intensity of luminescent substance is gradually increasing with prolonged irradiation time, which is an indirect evidence of Z-scheme charge separation due to successful generation of %OH. The above results have verified that CdS/QDs/ZnIn2S4 hybrid photocatalyst has sufficient reducing potential (E < −0.45 V vs. RHE) for and strong oxidative potential (E > +1.99 V vs. RHE), which is a solid evidence for the Z-scheme charge transfer in CdS/QDs/ZnIn2S4 hybrid system during photocatalytic reactions [59–63]. 4. Conclusion We have successfully designed and constructed an all-solid-state Zscheme CdS/QDs/ZnIn2S4 architecture with MoS2 quantum dots as solid-state electron mediator by optimally combining one-dimensional CdS nanorods, zero-dimensional MoS2 quantum dots and two-dimensional ZnIn2S4 nanosheets. The photocatalytic experiments towards water splitting have demonstrated that the coupling between CdS nanorods, MoS2 quantum dots and ZnIn2S4 nanosheets can lead to remarkably improved visible-light-driven photocatalytic activity towards hydrogen evolution. Using the CdS/QDs/ZnIn2S4 architectures, an average hydrogen evolution rate of 2107.5 μmol g−1 h−1 has been achieved, which is more than 26 times and 62 times higher than those of bare CdS nanorods and pure ZnIn2S4 nanosheets, respectively. Also, the recycling experiments for hydrogen generation revealed the high stability of resultant CdS/QDs/ZnIn2S4 architectures. The enhanced photocatalytic activity was attributed to the realization of the Z-scheme charge separation originated from the insertion of MoS2 quantum dots, which have not only facilitated the effective electron-hole separation between CdS and ZnIn2S4, but also maintained the high redox ability. Our current work has provided an insight into the design and application of artificial all-solid-state Z-scheme photocatalysts for solar hydrogen generation. Declaration of Competing Interest The authors declared that there is no conflict of interest. 8
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Fig. 10. ESR spectra of (a) DMPO-%O2− and (b) DMPO-%OH adducts with addition of AgNO3; (c) UV–vis spectra obtained from mixed solution composed of methyl viologen dichloride (MV2+, 50 μM), Na2SO3 (0.875 M), Na2S (0.625 M) and CdS/QDs/ZnIn2S4(0.25 mg/mL) before and after visible light irradiation with removal of photocatalyst; (d) PL spectral variations in 2 mM NaOH solution with addition of 0.5 mM terephthalic acid using CdS/QDs/ZnIn2S4 hybrid sample as photocatalyst.
Acknowledgments
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Highly efficient visible-light-driven photocatalytic hydrogen evolution by all-solid-state Z-scheme CdS/QDs/ZnIn2S4 architectures with MoS2 quantum dots as solid-state electron mediator Wei Chena, Rui-Qiang Yana, Jian-Qun Zhua, Guo-Bo Huanga, , Zhong Chenb, ⁎
a b
T
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School of Pharmaceutical and Materials Engineering, Taizhou University, Taizhou 318000, Zhejiang, PR China School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
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ABSTRACT
Keywords: CdS MoS2 ZnIn2S4 Water splitting Z-scheme
All-solid-state Z-scheme CdS/QDs/ZnIn2S4 architectures with MoS2 quantum dots as solid-state electron mediator were successfully designed and constructed by optimally combining one-dimensional CdS nanorods, zerodimensional MoS2 quantum dots (QDs) and two-dimensional ZnIn2S4 nanosheets. The photocatalytic water splitting for hydrogen evolution demonstrated that such structural design can synergistically trigger remarkably improved visible-light-driven photocatalytic activity. Photocatalytic H2-evolution at 2107.5 μmol g−1 h−1 was achieved on this CdS/QDs/ZnIn2S4 architectures under visible light irradiation, exceeding those of bare CdS nanorods and pure ZnIn2S4 nanosheets by a factor of 26 and 62, respectively. This highly efficient photocatalytic activity arises from the effective charge separation and favourable electron mediator of MoS2 QDs. The Z-scheme charge separation mechanism was verified by the ESR, PL-TA and organic electron acceptor test.
1. Introduction With the rapid development of industrialization, recent years have witnessed an indisputable fact that traditional fossil fuels are increasingly exhausted because of a rapid growing demand, giving rise to the serious environmental pollutions, such as acid rain, green house effect and photochemical smog [1–6]. Therefore, it is urgent for human beings to actively explore and develop new energy sources while effectively using fossil fuels, so as to gradually replace the existing fossil energy sources. As an important energy carrier, hydrogen energy, with the characteristics of high energy density, safety, only produces water after combustion. It does not pollute the environment, and therefore is considered as an ideal green energy source [7,8]. Photocatalysis technology provide a feasible tactics to produce hydrogen energy since Fujishima and Honda demonstrated photoelectrochemical reaction to prepare hydrogen from water splitting by using TiO2 photoelectrode in 1972 [9]. Till now, although great efforts have been made, a large number of photocatalysts are only excellently active in the ultraviolet (UV) light region. Their limited UV-driven activity largely inhibits their overall efficiency because solar spectrum consists of only 5% UV (300–400 nm), and the rest comprises 43% visible light (400–700 nm) and 52% infrared (700–2500 nm) [10]. Therefore, the researches on the design and construction of visible-light-responsive photocatalysts with
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the characteristics of high efficiency and excellently stable is crucial for hydrogen production on photocatalysts by transforming the solar energy into chemical energy. In the search of visible-light-driven photocatalysts, cadmium sulfide (CdS) had attracted constant attention in the fields of photocatalysis researches due to the narrow band gaps corresponding to visible light absorption and suitable band edges covering the potentials for the generation of hydrogen and oxygen [11,12]. However, the practical application was undoubtedly limited by the strong photocorrosion during photocatalytic process, leading to short lifespan of CdS photocatalysts. Also, the photogenerated holes and electrons generated by single CdS component often tend to easily recombine near or at its surface, thus leading to a low photocatalytic activity. Taking these bottlenecks into account, many attempts, including elemental doping [13], metal loading [14] and heterogeneous coupling [15,16], were utilized to boost the overall performance of CdS. Recently, Shi et al. [13] successfully prepared P doped CdS (P-CdS) with S vacancies using NaH2PO2 as the P source via a thermal phosphorization reaction. This structure has exhibited remarkably improved photocatalytic activity towards hydrogen evolution from pure water splitting in the absence of any sacrificial agents under LED irradiation because of the construction of effective electron trap level to prolong the lifetimes of photogenerated electrons. Chai et al. [14] utilized photodeposition routes by
Corresponding authors. E-mail addresses: [email protected] (G.-B. Huang), [email protected] (Z. Chen).
https://doi.org/10.1016/j.apsusc.2019.144406 Received 18 July 2019; Received in revised form 9 October 2019; Accepted 14 October 2019 Available online 08 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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in-situ loading Ni onto CdS nanoparticles to synthesize Ni/CdS nanocomposites, which were employed for photocatalytic splitting of alcohols into corresponding carbonyl compounds and hydrogen induced by visible light irradiation. Li et al. [17] reported the design and syntheses of heterogeneous phase photocatalysts with hexagonal@cubic CdS core@shell structure. Benefiting from the special structural design promoting the effective separation of charge pairs, the resultant heterogeneous phase photocatalysts possessed unprecedented photo-stability for highly efficient photocatalytic hydrogen evolution. Although the enhancement on photocatalytic activity and stability has been realized by above strategies, the overall performance of CdS or CdS based heterogeneous photocatalysts remains unsatisfactory to meet the practical requirements for more extensive industrial applications. Among different morphologies of CdS photocatalysts, one-dimensional (1D) CdS based architectures demonstrated a higher photocatalytic activity as comparison to their counterparts with other morphologies because of the shorter transportation distance, higher electron mobility and optical excitation originated from high length-to-diameter ratios [18,19]. Based on the above analyses, to optimize structural design and manipulate the transfer of charge carriers on the 1D CdS based nanocomposites probably are a critical means for the improvement of their overall performance toward practical photocatalytic applications. Ternary metal chalcogenides ZnIn2S4, with excellent properties of electrons confinement effect in their unique two-dimensional (2D) atomic layers and relatively high stability during photocatalytic reactions, has gained great amount of attention in the construction of ZnIn2S4 based hybrid photocatalysts [6,20–22]. Particularly worth mentioning is the widespread coupling between these 2D ZnIn2S4 and 1D photocatalysts in order to mitigate the drawbacks and to integrate the above-mentioned merits of single component. Dong′s group [23] prepared ZnIn2S4/TiO2 nanocomposites by decorating 2D ZnIn2S4 nanosheets onto the surface of 1D TiO2 nanorods at hydrothermal environment. The prepared ZnIn2S4/TiO2 nanocomposites exhibited highly efficient photocatalytic activity for hydrogen production compared with pure 1D TiO2 nanorods and 2D ZnIn2S4 nanosheets under solar light irradiation because of the advantages in synergistically promoting the photogenerated charge-carrier separation. Wang et al. [24] designed and fabricated the 1D hierarchical ZnIn2S4-In2O3 tubular heterostructures by growing ZnIn2S4 nanosheets on both outer and inner surfaces of In2O3 microtubes for efficient visible-light-driven photocatalytic CO2 reduction. Inspired by above investigations, we hold a view that it is a viable option to combine the merits of 2D ZnIn2S4 nanosheets and 1D CdS nanorods. It is well known that small size of MoS2 quantum dots (QDs) can not only minimize the charge recombination in MoS2, but also effectively modulate the charge transfer direction. Taking these effects together, we developed CdS/QDs nanorods by in-situ decoration of QDs on CdS surface at solvothermal environment. Then the CdS/QDs/ ZnIn2S4 hybrid photocatalyst were prepared by tightly wrapped 2D ZnIn2S4 nanosheets on the surface of 1D CdS/QDs nanorods. The photocatalytic water splitting for hydrogen evolution was selected to estimate the photocatalytic activity of as-obtained CdS/QDs/ZnIn2S4 ternary photocatalysts. We hope that our current research could provide some typical sights in the design, syntheses and applications of highly efficient visible-light-responsive photocatalysts.
2.2. Catalyst syntheses Syntheses of CdS nanorods and MoS2 quantum dots decorated CdS nanorods (CdS/QDs): Well-proportioned CdS nanorods and MoS2 quantum dots decorated CdS nanorods were synthesized by a simple one-pot solvothermal reaction using ethylenediamine as solvent with slight modification [25]. Typically, Cd(NO3)2·4H2O (1 mmol, 308 mg), CS(NH2)2 (5 mmol, 381 mg) and a certain amount of Na2MoO4·2H2O were added into 40 mL ethylenediamine and then transferred to a 50 mL polyphenylene-lined stainless steel autoclave, which was sealed and heated in an air-circulating oven at 220 °C for 24 h. After naturally cooling down to room temperature, the yellow CdS/QDs nanorods with 2 wt% QDs loading were collected, washed with distilled water and absolute ethanol to remove the residues of organic solvent and dried at 60 °C for 3 h. The pure CdS was also prepared using same route but without the addition of any Na2MoO4·2H2O. Syntheses of CdS/ZnIn2S4, CdS/QDs/ZnIn2S4 architectures and individual ZnIn2S4 nanosheets: The resultant CdS/QDs (300 mg) was completely dispersed into 150 mL deionized water to obtain uniform suspension. Then, ZnCl2 (0.5 mmol, 68.2 mg), InCl3·4H2O (1 mmol, 293.2 mg) and L-Cys (4 mmol, 484.6 mg) were successively dissolved into suspension. After vigorously stirred for 1 h, the above suspension was transferred into 200 mL polyphenylene-lined stainless steel autoclave and reacted at an air-circulating oven at 180 °C for 12 h. The resultant CdS/QDs/ZnIn2S4 architectures were gathered by centrifugation, washing and drying after autoclave was cooled to room temperature. The CdS/ZnIn2S4 architectures were obtained by replacing CdS/QDs with CdS nanorods and individual ZnIn2S4 was synthesized without adding CdS nanorods or CdS/QDs nanorods using same reaction condition. 2.3. Catalyst characterization Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance diffractometer using a Cu Kα source, with a scan range from 10° to 80°. Microstructure was observed on a JEOL JEM-2100F highresolution transmission electron microscope (TEM) at an accelerating voltage of 200 kV. UV–vis diffuse reflectance spectra (UV–vis DRS) were acquired on a Hitachi UV-4100 spectrometer equipped with BaSO4 as the reference and an integrating sphere attachment at room temperature in air. X-ray photoelectron spectroscopy (XPS) measurements were carried out with an ESCALab 250Xi spectrometer by using an unmonochromated Al Kα (1486.6 eV) as X-ray source. The photoluminescence (PL) spectra and the time-resolved photoluminescence (TRPL) spectra for solid-state samples were investigated through HORIBA iHR320. The data of photoluminescence terephthalic acid (PLTA) were obtained by using a Cary Eclipse type fluorescence spectrophotometer at the excitation wavelength of 315 nm. The electron spin resonance (ESR) spectra were measured on an ESR spectrometer (JES X320). Photoelectrochemical measurements were performed on a CHI760E electrochemical work station (Chenhua Instrument, Shanghai) in a standard three-electrode system with an Ag/AgCl as a reference electrode, Pt foil as the counter electrode and the powder-coated FTO conducting glass as the working electrode at 0.5 M Na2SO4 solution. The Mott-Schottky curves at different frequencies of samples were also recorded on a 0.5 M Na2SO4 solution.
2. Experimental
2.4. Photocatalytic activity
2.1. Chemicals
A Pyrex glass photoreactor connected to a glass closed gas circulation system, was used to analyze the photocatalytic hydrogen evolution. A photocatalyst sample of 50 mg was dispersed in 50 mL aqueous solution including 40 mL ultrapure water and 10 mL lactic acid. Prior to photocatalytic reactions, the mixed liquor was bubbled by nitrogen bubbler for 30 min. During photocatalytic reaction, the temperature was maintained at 25 °C by cyclic water and the irradiated source
Cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O), sodium molybdate dehydrate(Na2MoO4·2H2O), thiourea(CH4N2S), zinc chloride(ZnCl2), indium chloride tetrahydrate (InCl3·4H2O), L-Cysteine (C3H7NO2S, LCys), ethylenediamine (C2H8N2) and lactic acid (C3H6O3) were provided by Shanghai Aladdin biochemical technology co., LTD. All chemical reagents are used as provided without any purification. 2
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(EDS) revealed the coexistence of Cd, Zn, In, Mo, S, C and Cu, which indicats the as-prepared CdS/QDs/ZnIn2S4 hybrid architectures are composed of CdS, MoS2 and ZnIn2S4. The impurity elements including C and Cu are originated from holey carbon-copper grids for TEM characterization. The HRTEM image of CdS/QDs/ZnIn2S4 hybrid architectures exhibited three different lattice fringes. The clear lattice spacing with the value of 0.61 nm, representing the (0 0 3) plane of MoS2, while the lattice fringes with d = 0.36 nm and d = 0.41 nm belong to the (1 0 0) plane of hexagonal CdS and (0 0 6) plane of hexagonal ZnIn2S4, respectively [29]. Moreover, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding EDS mapping were further employed to investigate the chemical compositions of different components in CdS/QDs/ ZnIn2S4 hybrid architectures (Fig. 3h). The results indicate that ternary architectures indeed consist of Cd, Zn, In, Mo and S elements, which is in well agreement with above result [30]. The optical properties of bare CdS, CdS/QDs, CdS/ZnIn2S4, CdS/ QDs/ZnIn2S4 as well as pure ZnIn2S4 were measured by UV–vis DRS. As shown in Fig. 4, it is clearly seen that pure ZnIn2S4 possesses strong light absorption during measured spectrum range. It should be noted that bare CdS possesses obvious absorption towards UV irradiation and its absorption edge is located at visible light region with wavelength of about 530 nm, which is arisen from intrinsic band gap absorption [31]. More importantly, in-situ decoration of QDs and/or ZnIn2S4 nanosheets onto the surface of CdS nanorods can actively increase the visible light absorption in spectral range from 530 to 800 nm of as-synthetic CdSbased hybrid samples. Also note that, among of all CdS-based nanocomposites, no differences originated from inherent band structure can be observed after the hybrid reactions, indicating QDs and/or ZnIn2S4 nanosheets were immobilized onto the CdS surface rather than entering internal crystalline structure of CdS nanorods. The bandgap of CdS nanorods was not changed after coupling with QDs and/or ZnIn2S4 nanosheets [32]. The elemental compositions, valence states and chemical interactions between different constituent in hybrid samples were systematically investigated by XPS. The result from survey XPS spectrum (Fig. 5a) reveals that the ternary CdS/QDs/ZnIn2S4 hybrid architecture consists of Cd, S, Mo, Zn, In and O elements, which further verifies the coexistence of CdS, MoS2 and ZnIn2S4 in CdS/QDs/ZnIn2S4 hybrid architecture. The negligible O element is resulted from the absorbed molecules including H2O and CO2 on the surface of photocatalysts. The binding energies of fine-scanning Cd 3d signals are observed at 404.3 eV and 411.0 eV for CdS sample, indicative of Cd2+ in CdS (Fig. 5b) [33]. The Zn 2p region at CdS/QDs/ZnIn2S4 hybrid sample could be deconvoluted into four characteristic peaks at 1021.3 eV, 1022.5 eV, 1044.5 eV and 1045.7 eV by peak fitting(Fig. 5c) [34]. The binding energies of Zn 2p peak at 1021.3 eV and 1044.5 eV could be attributed to Zn 2p3/2 and Zn 2p1/2, representing chemical bonding between CdS and ZnIn2S4 nanosheets, whereas the peak at 1022.5 eV and 1044.5 eV is related to chemical interactions of ZnIn2S4 nanosheets in connection with the QDs on surface of CdS nanorods [35,36]. In Fig. 5d, two binding energies at location of about 444.5 and 452.1 eV independently belong to In 3d5/2 and In 3d3/2, demonstrating that the presence of In3+ in ZnIn2S4 nanosheets [37]. As for pure QDs based nanocomposites, the Mo 3d region (Fig. 5e) in CdS/QDs sample exhibited two weak peaks located at 227.7 eV and 231.1 eV, corresponding to Mo 3d5/2 and Mo 3d3/2, while CdS/QDs/ZnIn2S4 hybrid architectures showed slight shift in Mo 3d region at 228.1 and 231.5 eV, which is an indication of the existence of Mo4+ in these two samples [38]. The fine-scanning XPS spectrum in S 2p regions (Fig. 5f) present two binding energies at 160.6 and 161.8 eV for CdS nanorods, 161.3 and 162.5 eV for ZnIn2S4 nanosheets. The split energy of 1.2 eV between S 2p1/2 and S 2p3/2 in CdS nanorods and ZnIn2S4 nanosheets indicate that the S species exist as −2 [39]. The positive shifts of Cd 3d, Mo 3d and S 2p, and the negative shift of Zn 2p and In 3d indicate the charge transfer from CdS to QDs and then to ZnIn2S4 via tight
Fig. 1. XRD patterns of bare CdS nanorods, ZnIn2S4 nanosheets, CdS/QDs, CdS/ ZnIn2S4 and CdS/QDs/ZnIn2S4 hybrid samples.
equipped with a cutoff filter to remove the wavelength less than 420 nm was provided by CEL-HXF-300 typed 300 W Xe-lamp (Ceaulight, Beijing). The produced hydrogen amount was measured on gas chromatography (GC-7920) equipped with a 5 Å molecular sieve column and a thermal conductivity detector (TCD). Ar gas was used as the carrier. 3. Results and discussion The phase structure of resultant samples were characterized by XRD, as shown in Fig. 1. The as-obtained CdS nanorods are highly crystalline, which is well ascribed to pure hexagonal phase of CdS (JCPDS No. 65-3414) [19]. The XRD patterns of pristine ZnIn2S4 nanosheets match well with the structure of hexagonal ZnIn2S4 as evidenced from the PXRD database (JCPDS: 65-2023) [20,26]. For the XRD pattern of CdS/ZnIn2S4, the characteristic diffraction peaks for both hexagonal CdS and hexagonal ZnIn2S4 are observed in same diffraction position as individual components, which indicate that synthetic CdS/ZnIn2S4 architecture is indeed composed of pure CdS nanorods and pristine ZnIn2S4 nanosheets. Moreover, no diffraction peaks belonging to MoS2 QDs can be found in CdS/QDs and CdS/QDs/ZnIn2S4 architectures, which are probably attributed to relatively low diffraction intensity of MoS2 QDs and/or the small amount beyond the detection limit of XRD. Actually, the existence of QDs could be well confirmed by microstructure observation and elemental analyses. The morphology and microstructure of the as-obtained CdS/QDs/ ZnIn2S4 hybrid architecture is observed by FESEM, TEM and HRTEM, as presented in Figs. 2 and 3. Fig. 2 displayed FESEM images of as-prepared samples, from which we noticed that 1D structure of CdS nanorods with high aspect ratio and three-dimensional (3D) ZnIn2S4 assembly from 2D nanosheets were successfully synthesized. The asobtained CdS/QDs nanocomposites by in-situ loading of MoS2 QDs onto CdS surface does not result in the structural change and the resultant CdS/QDs samples still maintained rod-like structure(Figs. 2c and 3b). From HRTEM image, it is obvious that some MoS2 QDs were tightly decorated on the surface of CdS nanorods as evidenced by lattice fringes. The lattice spacing of 0.61 nm and 0.27 nm separately corresponded to (0 0 3), (1 0 1) planes of MoS2, and 0.36 nm (1 0 0) plane for hexagonal CdS [27]. When precursor of ZnIn2S4 were added during hydrothermal environment, the as-prepared CdS/ZnIn2S4 and CdS/ QDs/ZnIn2S4 hybrid architectures were characteristic of helical structure with the entanglement of ZnIn2S4 nanosheets onto CdS surface (Fig. 2(d–f) and Fig. 3(d and e)). Such structural design is expected to be able to expose its entire ZnIn2S4 surface, thus improving the photocatalytic activity [19,28]. The energy-dispersive X-ray spectrometer 3
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Fig. 2. FESEM image of different samples: (a) bare CdS nanorods, (b) pure ZnIn2S4 microspheres assembled from nanosheets, (c) CdS/QDs nanorods, (d) CdS/ZnIn2S4 and (e and f) CdS/QDs/ZnIn2S4 hybrid architectures.
Fig. 3. TEM and HRTEM analyses: TEM images of (a) bare CdS, (b) CdS/QDs, (d) CdS/ZnIn2S4 and (e) CdS/QDs/ZnIn2S4; HRTEM image of (c) CdS/QDs and (g) CdS/ QDs/ZnIn2S4; (f) EDS and (h) HAADF-STEM and corresponding elemental mapping of CdS/QDs/ZnIn2S4 hybrid architectures.
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interfacial contacts [40]. Therefore, we could draw a conclusion, from the joint results of TEM, UV–vis DRS and XPS, that the CdS/QDs/ ZnIn2S4 hybrid architectures have been successfully prepared by the combination of solvothermal and hydrothermal method. The photocatalytic activity of resultant samples was evaluated by visible-light-driven photocatalytic hydrogen evolution with addition of lactic acid as sacrificial agent, as shown in Fig. 6. Control experiments revealed that no hydrogen was detected when irradiation or photocatalyst were withdrew during photocatalytic reactions, indicating that light irradiation and photocatalyst are inextricably intertwined for photocatalytic reaction towards hydrogen evolution. Clearly, the pure ZnIn2S4 displays a relatively low photoactivity with a produced H2 rate of 33.9 μmol g−1 h−1. Compared with ZnIn2S4, the bare CdS showed relatively decent photo activity (78.5 μmol g−1 h−1) towards water splitting for hydrogen evolution due to the limited separation efficiency of charge pairs. After coverage of ZnIn2S4 nanosheets onto CdS surface, there is an enhancement of photoactivity (168.7 μmol g−1 h−1), which revealed that hydrothermal process can boost the construction of heterogeneous interface in the CdS/ZnIn2S4 hybrid system to effectively
Fig. 4. UV–vis diffuse reflectance spectra of bare CdS nanorods, ZnIn2S4 nanosheets, CdS/QDs, CdS/ZnIn2S4 and CdS/QDs/ZnIn2S4 hybrid samples.
Fig. 5. XPS analyses of as-obtained samples: (a) survey spectra, (b) Cd 3d, (c) Zn 2p, (d) In 3d, (e) Mo 3d and (f) S 2p regions. 5
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Fig. 6. (a) Photocatalytic activity for hydrogen evolution of as-prepared samples from water splitting with addition of lactic acid as sacrificial agent under visible light irradiation, (b) corresponding to XRD patterns of fresh and used CdS/QDs/ZnIn2S4 hybrid sample.
facilitate the separation and transfer of photoinduced charge. Note that, after single decorating of QDs, the as-prepared CdS/QDs hybrid sample displays an obvious photoactivity enhancement towards the hydrogen evolution (685.3 μmol g−1 h−1), which is assigned to the fact that the in-situ decoration of QDs can effectively separate the photogenerated electrons from CdS nanorods to QDs to weaken faster recombination between photogenerated holes and electrons. When the CdS nanorods were collaboratively decorated by QDs and ZnIn2S4 nanosheets, a significant enhancement in photocatalytic activity can be observed (2107.5 μmol g−1 h−1) and its hydrogen evolution rate exceeded those produced on bare CdS nanorods and pure ZnIn2S4 nanosheets by more than 26 times and 62 times, respectively. Given that the activity increment by construction of heterogeneous interfaces between CdS nanorods and ZnIn2S4 nanosheets was relatively minor, the remarkably improved photocatalytic activity of this CdS/QDs/ZnIn2S4 hybrid sample can be primarily attributed to solid-state electron mediator of QDs. They not only favor the inhibition of carrier recombination but also modulate the flow direction of photogenerated electron, thereby improving the photocatalytic activity for hydrogen generation. Furthermore, the photocatalytic durability of CdS/QDs/ZnIn2S4 was tested by recycling reactions for successive 24 h. After recycles, the CdS/QDs/ ZnIn2S4 hybrid photocatalysts only had a slight loss of photoactivity (~7.79%) for hydrogen evolution, implying highly catalytic stability during the photocatalytic reaction. This result is further confirmed by XRD analysis as shown in Fig. 6b. From the comparison of crystal phase structure of fresh and used samples, it is clearly seen that no difference can be observed from XRD patterns, which confirms the phase structure of CdS/QDs/ZnIn2S4 hybrid sample remains intact. Above results confirm that CdS/QDs/ZnIn2S4 hybrid sample are sufficiently stable during photocatalytic process. To consolidate the above hypothesis that the speedy charge separation process occurs on the CdS/QDs/ZnIn2S4 architectures, we have performed systematical characterizations including steady-state photoluminescence (PL) spectra, time-resolved photoluminescence (TRPL) spectra, transient photocurrent (I-t) and electrochemical impedance spectra (EIS). Photoluminescence (PL) spectra have been thought as a powerful tool to investigate separation efficiency of the interfacial electron transfer because the PL emission is caused by free charge carriers recombination [41]. The results from steady-state PL spectra (Fig. 7a) indicate that PL emission intensity of CdS nanorods were remarkably quenched after decorating QDs to form the heterogeneous contacts, suggesting the recombination between photogenerated holes and electrons is inhibited owing to the occurrence of efficient charge separation. Further decoration of ZnIn2S4 nanosheets on QDs loaded CdS nanorods led to a much more significant quenching in the resultant PL emission, indicating that the immobilized QDs promoted the charge separation by regulating the interfacial charge transfer between CdS nanorods and ZnIn2S4 nanosheets [42]. The effective transfer of charge
carriers of these samples has also been verified by the TRPL spectra (Fig. 7b). The fluorescence lifetime of CdS/QDs/ZnIn2S4 hybrid architectures is longer than the others, which also revealed the higher separation and transfer efficiency of photogenerated charge pairs in CdS/ QDs/ZnIn2S4 hybrid architectures [43]. This result corroborates that QDs have served as an effective solid-state electron mediator for CdS/ QDs/ZnIn2S4 hybrid sample by readily accepting photoexcited electrons. This gives rise to the increasingly pronounced charge separation for CdS/QDs/ZnIn2S4 hybrid architectures as revealed in the depressed PL intensity and increased fluorescent life-time as well as the photoelectrochemical measurements [44]. Fig. 7c compared the transient photocurrent responses during periodic on-off cycles of different photoelectrodes prepared by spin-coating samples on FTO under visible light irradiation. Apparently, surface decoration of QDs on CdS nanorods dramatically strengthens photocurrent, which revealed the effective transfer of photogenerated charge pairs by QDs [45,46]. Moreover, the effective separation of electron-hole pairs was further demonstrated by EIS analysis as illustrated in Fig. 7d. It was clear that the semicircle radius follows the same order as the photocatalytic performance. The semicircle radius of CdS/QDs/ZnIn2S4 hybrid architectures was smallest than those of counterparts, demonstrating the lower recombination rate of photo-generated charge and the enhanced interface charge transfer ability [47,48]. To acquire further insights into the charge separation-transfer mechanism in CdS/QDs/ZnIn2S4 hybrid architectures, the band potentials of samples were determined by a combination of Mott-Schottky plots and UV–vis DRS. Fig. 8(a and b) illustrates Mott-Schottky plots of bare CdS nanorods and pure ZnIn2S4 nanosheets, from which we could notice that flat-band potentials (Efb) of these two samples can be computed to be −0.696 V vs. Ag/AgCl (equivalent to −0.144 V vs. RHE) for bare CdS nanorods and −1.07 V vs. Ag/AgCl (equivalent to −0.545 V vs. RHE) for pure ZnIn2S4 nanosheets according to the computational formula (E (RHE) = E(Ag/AgCl) + 0.197 V + 0.0591pH), respectively. It is wellknown the Efb in n-type semiconductors is more positive by about 0.1 V than conduction band minimum (CBM) [49]. As results, the CBM of CdS nanorods and ZnIn2S4 nanosheets were −0.244 V and −0.645 V vs. RHE, respectively. Moreover, the results from UV–vis DRS (Fig. 8c) revealed the band gaps difference between single components with value of 2.36 eV for bare CdS nanorods and 2.51 eV for ZnIn2S4 nanosheets. Based on above analyses, the valence band maximum (VBM) of CdS nanorods and ZnIn2S4 nanosheets were about 2.116 V and 1.865 V vs. RHE, respectively. Therefore, the band alignment of bare CdS nanorods and pure ZnIn2S4 nanosheets in CdS/QDs/ZnIn2S4 hybrid architectures were proposed and shown as in Fig. 8d. Evidently, the staggered band edge potentials are observed between CdS nanorods and ZnIn2S4 nanosheets, leading to the buildup of band bending due to the presence of chemical potentials between these two samples. Such energy band structure significantly strengthened the separation-transfer efficiency 6
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Fig. 7. (a) Solid state room-temperature PL emission spectra, (b) time-resolved photoluminescence spectra, (c) transient photocurrent and (d) electrochemical impedance spectroscopy of bare CdS nanorods, ZnIn2S4 nanosheets, CdS/QDs, CdS/ZnIn2S4 and CdS/QDs/ZnIn2S4 hybrid samples.
Fig. 8. Mott-Schottky plot of (a) bare CdS nanorods and (b) pure ZnIn2S4 nanosheets; (c) band gap width and (d) energy band structure of bare CdS nanorods and pure ZnIn2S4 nanosheets. 7
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Fig. 9. Schematic illustrations of two possible photocatalytic systemsin CdS/QDs/ZnIn2S4 hybrid nanocomposites: (a) heterojunction-type and (b) Z-scheme type.
between photogenerated holes and electrons towards opposite direction to suppress the charge recombination, thereby improving the photocatalytic activity. Actually, the photocatalytic mechanism based on the charge separation-transfer between CdS nanorods and ZnIn2S4 nanosheets in CdS/QDs/ZnIn2S4 hybrid architectures can be classified into two models, named as heterojunction-type and Z-scheme. The first model involves acceptor-donor charge separation towards opposite directions in CdS/QDs/ZnIn2S4 hybrid architectures, as displayed in Fig. 9a, in which photogenerated holes jump from the VB of CdS to that of ZnIn2S4; simultaneously conduction band (CB) electrons of ZnIn2S4 transferred to the CdS and finally enriched on the MoS2 QDs or directly migrate to MoS2 QDs, giving rise to the decreased redox ability of photogenerated charge pairs. If the charge transfer followed first model, the photogenerated electrons were not able to reduce dissolved oxygen to produce superoxide radical because the photogenerated electrons on QDs was more positive than standard redox potential of E(O2/%O2–) = −0.284 V vs. RHE [50,51]. Moreover, if charge separation was carried out as first model, the photogenerated holes concentrated on VB of ZnIn2S4 nanosheets with the potential of 1.865 V vs. RHE also cannot be used in the production of hydroxyl radical (%OH) when CdS/QDs/ZnIn2S4 hybrid architectures used as photocatalyst because %OH generation from H2O by photogenerated hole oxidation path required a larger potential of 1.99 V vs. RHE [52]. For further verification, the ESR spectra were employed to detect the existence of %O2– and %OH during photocatalytic reactions, as shown in Fig. 10 (a, b). It is clearly seen that CdS/QDs/ZnIn2S4 hybrid architectures exhibited several characteristic peaks of DMPO-%O2– (Fig. 10a) in methanol dispersion with the exposure of visible light irradiation, revealing the generation of %O2– during photocatalytic process [53]. Furthermore, considering multiple pathways of hydroxyl radical formation, an appropriate amount of AgNO3 was added to rule out the effect of photogenerated electrons on the assessment of oxidation ability of photogenerated holes by directly oxidizing hydroxyl ion. By introducing 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the trapping agent, the quadruple peaks in CdS/QDs/ZnIn2S4 hybrid system, belonging to the signal of DMPO-%OH (Fig. 10b), can be obviously observed upon irradiation by visible light, representing the photogenerated holes in CdS/ QDs/ZnIn2S4 hybrid system has strong enough oxidation ability to produce %OH [54]. The results obtained from ESR spectra clearly indicated the heterojunction-type charge separation theory is not operating in the current case. Instead, Z-scheme charge transfer is more likely to be the transfer mechanism of photogenerated charge pairs and responsible for the improved photocatalytic activity in hydrogen evolution (Fig. 9b). In this model, the photogenerated electrons injection occurred from CB of CdS to QDs, and then migrated to VB of ZnIn2S4 and recombine with photogenerated holes on VB of ZnIn2S4, leaving the photogenerated holes on CdS nanorods and photogenerated electrons on ZnIn2S4 nanosheets. Such Z-scheme charge separation not only promotes the effective separation but also guarantees the strong redox ability of photogenerated holes and electrons, which is responsible for the generation of %O2– and %OH and remarkable enhancement of photocatalytic activity.
The strong redox ability of photogenerated holes and electrons were further verified by organic electron acceptor method and PL-TA. The reducing power of photogenerated electrons in CdS/QDs/ZnIn2S4 hybrid system was investigated by probing the photoreduction of methyl viologen dication (MV2+, E(MV2+/MV+) = −0.45 V vs. RHE). When a mixed solution, consisting of Na2SO3 (0.875 M), Na2S (0.625 M), MV2+ (50 μM) and a suitable amount of CdS/QDs/ZnIn2S4 hybrid photocatalysts was irradiated by visible light, the solution color rapidly changed to blue and the UV–vis spectra showed two new absorption peaks as the characteristic absorption of 395 and 603 nm, corresponding to the reduced viologen species (MV+) (Fig. 10c) [55,56]. The generation of %OH was further confirmed with PL-TA method by the detection of luminescent probe molecule (2-hydroxyterephthalic acid, TAOH) generated from chemical reaction between TA and %OH as displayed in Fig. 10d [57,58]. It is obvious that the intensity of luminescent substance is gradually increasing with prolonged irradiation time, which is an indirect evidence of Z-scheme charge separation due to successful generation of %OH. The above results have verified that CdS/QDs/ZnIn2S4 hybrid photocatalyst has sufficient reducing potential (E < −0.45 V vs. RHE) for and strong oxidative potential (E > +1.99 V vs. RHE), which is a solid evidence for the Z-scheme charge transfer in CdS/QDs/ZnIn2S4 hybrid system during photocatalytic reactions [59–63]. 4. Conclusion We have successfully designed and constructed an all-solid-state Zscheme CdS/QDs/ZnIn2S4 architecture with MoS2 quantum dots as solid-state electron mediator by optimally combining one-dimensional CdS nanorods, zero-dimensional MoS2 quantum dots and two-dimensional ZnIn2S4 nanosheets. The photocatalytic experiments towards water splitting have demonstrated that the coupling between CdS nanorods, MoS2 quantum dots and ZnIn2S4 nanosheets can lead to remarkably improved visible-light-driven photocatalytic activity towards hydrogen evolution. Using the CdS/QDs/ZnIn2S4 architectures, an average hydrogen evolution rate of 2107.5 μmol g−1 h−1 has been achieved, which is more than 26 times and 62 times higher than those of bare CdS nanorods and pure ZnIn2S4 nanosheets, respectively. Also, the recycling experiments for hydrogen generation revealed the high stability of resultant CdS/QDs/ZnIn2S4 architectures. The enhanced photocatalytic activity was attributed to the realization of the Z-scheme charge separation originated from the insertion of MoS2 quantum dots, which have not only facilitated the effective electron-hole separation between CdS and ZnIn2S4, but also maintained the high redox ability. Our current work has provided an insight into the design and application of artificial all-solid-state Z-scheme photocatalysts for solar hydrogen generation. Declaration of Competing Interest The authors declared that there is no conflict of interest. 8
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Fig. 10. ESR spectra of (a) DMPO-%O2− and (b) DMPO-%OH adducts with addition of AgNO3; (c) UV–vis spectra obtained from mixed solution composed of methyl viologen dichloride (MV2+, 50 μM), Na2SO3 (0.875 M), Na2S (0.625 M) and CdS/QDs/ZnIn2S4(0.25 mg/mL) before and after visible light irradiation with removal of photocatalyst; (d) PL spectral variations in 2 mM NaOH solution with addition of 0.5 mM terephthalic acid using CdS/QDs/ZnIn2S4 hybrid sample as photocatalyst.
Acknowledgments
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