Noble-metal-free amorphous CoMoSx modified CdS core-shell nanowires for dramatically enhanced photocatalytic hydrogen evolution under visible light irradiation

Applied Surface Science 498 (2019) 143863

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Noble-metal-free amorphous CoMoSx modified CdS core-shell nanowires for dramatically enhanced photocatalytic hydrogen evolution under visible light irradiation

T

Qiuhao Li, Xiu-Qing Qiao⁎, Yanlin Jia, Dongfang Hou, Dong-Sheng Li⁎ College of Materials and Chemical Engineering, Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang 443002, PR China.

ARTICLE INFO

ABSTRACT

Keywords: CoMoSx CdS Water splitting Amorphous Visible light H2 evolution

Design and construction of highly efficient and stable co-catalysts are critical for the photocatalytic H2 generation from water. In this work, we report the novel CdS/CoMoSx core-shell nanowires (CCMS NWs) prepared by a facile in situ synthesis method. The in situ growth of amorphous CoMoSx shell on the surface of CdS NWs core efficiently promoted the photocatalytic H2 evolution under visible light irradiation. The optimized CCMS sample containing 15 mol% CoMoSx (CCMS-15) achieves a H2 evolution rate as high as 477.96 μmol·h−1 under visible light irradiation, up to 26.3 times over bare CdS (18.2 μmol·h−1). Moreover, the constructed core-shell CdS/CoMoSx photocatalyst shows a long-term stability without noticeable activity degradation. Based on the detailed analyses of PL, UV–vis DRS, transient photocurrent responses, EIS, Mott-Schottky plots and ESR, the significantly enhanced activity is ascribed to the synergistic effect of increased visible light absorption, suppressed photo-generated charge carries recombination and the amorphous defective structure of CoMoSx cocatalyst. In consideration of its facile synthesis, low cost, and superior performance, the amorphous CoMoSx appears to be promising co-catalyst for photocatalytic water splitting.

1. Introduction Photocatalytic water splitting into H2 has been considered as one of the promising strategies for renewable energy evolution to address the energy crisis and environmental pollution problems [1]. Developing low cost, efficient and stable photocatalysts has been the research spot in the past several decades. Till now, series of superior visible-light driven photocatalysts including C3N4 [2], ZnIn2S4 [3], Bi2WO6 [4,5], graphene-based materials [6,7] and so on [8,9] have been developed. Cadmium sulfide (CdS) semiconductor is widely regarded as the most promising visible light photocatalyst for its suitable bandgap (~2.4 eV), efficient electron-hole pairs generation, and appropriate photo-redox potentials [10–12]. However, the fast recombination of charge carriers and severe photo-corrosion caused by the oxidation of S2− result in low quantum efficiency and limit its large-scale application [13,14]. Therefore, various strategies including heterojunctions construction [15,16], co-catalyst modification [17,18], and multi-component solid solutions fabrication [19] have been employed to improve the photocatalytic activity and stability of CdS-based photocatalysts. In particular, modifying CdS with appropriate co-catalyst has been proven to

⁎

be an effective method to greatly enhance the photocatalytic activity because the charge separation could be promoted [20–22]. Rational selection of co-catalyst is important for the construction of efficient photocatalyst. It has been reported that a promising co-catalyst should possess approaching hydrogen binding energy with Pt-group metal and large work function for electron sinking. Over the years, noble metal nanoparticles have been demonstrated to be unbeatable co-catalyst for photocatalytic H2 production from water splitting. However, the scarcity and high cost are detrimental for its large-scale application [23]. Therefore, it is highly desired to develop earth-abundant, efficient and stable alternative for noble metal co-catalyst. In the past decades, an increasing number of noble-metal-free cocatalyst including transition metals [24,25], transition-metal sulfides [17,26–28], and transition-metal phosphide [29] have been developed. Especially, MoS2 nanostructures composed of three atom layers (S-MoS) via week van der Waals interaction have been demonstrated to be promising co-catalyst for H2 production [30,31]. Extensive studies indicate that the activity of MoS2 is limited to its edge sites and the intrinsic low electrical conductivity restricted its activity for catalytic HER [32]. Thus, developing structures with more exposed edges should

Corresponding authors. E-mail addresses: [email protected] (X.-Q. Qiao), [email protected] (D.-S. Li).

https://doi.org/10.1016/j.apsusc.2019.143863 Received 22 May 2019; Received in revised form 22 August 2019; Accepted 3 September 2019 Available online 04 September 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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be a feasible strategy for efficient MoS2 co-catalyst. Moreover, it has been reported that heteroatom doping MoS2 to expose more active sites and enhance the conductivity is as an effective approach to enhance the electrocatalytic hydrogen production activity of MoS2 electrocatalyst [33–35]. For example, Xie and the coauthors reported that through the incorporation of oxygen atom into MoS2, the electronic structure can be effectively regulated and therefore further improve the conductivity and electrocatalytic hydrogen production activity [33]. It has been concluded that the introduction of heteroatoms into MoS2 could significantly decrease the binding energy with hydrogen atom and hence enhance the electrocatalytic hydrogen production activity [35]. Despite great efforts have been devoted to the effect of heteroatom-doped MoS2 on the electro catalytic activity [36,37], the research on photocatalytic H2 evolution is relatively new [38]. Recently, Yu group reported that the photo-induced deposit molybdenum-based bimetallic sulfide (aCoMoSx) on CdS, TiO2 and C3N4 could promote the H2-production rate, proving CoMoSx is a prospective co-catalyst for photocatalytic water splitting [18]. Moreover, single-crystalline CdS with fewer recombination centers is beneficial for the separation of photo-excited electronhole pairs, thus enhancing the photocatalytic activity. Inspired by these results, decorating heteroatom-doped MoS2 co-catalyst with singlecrystalline CdS to expose more active sites and increase the carriers transport will greatly accelerate the photocatalytic H2-production activity. Herein, amorphous CoMoSx nanoparticles (NPs) coated singlecrystalline CdS nanowires (NWs) with core-shell structure were fabricated by a facile in situ hydrothermal method. The obtained amorphous CoMoSx shell act as co-catalyst to accept the photo generated electrons from CdS core and accelerate the photocatalytic H2 evolution. Due to the unique core−shell structure and intimate contact between CoMoSx and CdS, the photocatalytic H2 evolution of CdS/CoMoSx NWs is up to 477.96 μmol·h−1, which is 26.3 times greater than that of pure CdS. Moreover, the core-shell structure can avoid the severe photo corrosion within 12 h and achieve much more stable than the physical mixture of CdS and CoMoSx.

2.2. Preparation of CdS nanowires (NWs) CdS NWs were prepared by a modified solvothermal method [39]. Briefly, 5 mmol of Cd(NO3)2·4H2O and 15 mmol of CH4N2S were dispersed in 35 mL of ethylenediamine under vigorous stirring for 10 min. Then the solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h. After cooled to room temperature, the product was collected and washed with deionized water and absolute ethanol. The resultant yellow-colored powders were dried at 80 °C overnight. 2.3. Synthesis of (NH4)2MoS4 precursor (NH4)2MoS4 precursor was obtained via a facile precipitation method. Typically, 1.5 g of (NH4)6Mo7O24·4H2O was added into 20 mL of (NH4)2S solution (16 wt%) and magnetically stirred for 2 h. Then the suspension was filtered and washed with distilled water and ethanol. After that, the obtained brown red crystals were dried at 80 °C in vacuum drying oven, resulting in (NH4)2MoS4 precursor. 2.4. Preparation of core-shell CdS/CoMoSx photocatalysts Core-shell CdS/CoMoSx photocatalysts were synthesized through an in situ hydrothermal process. In a typical procedure, x mmol of aforementioned (NH4)2MoS4 precursor, 1.5 g of Na3C6H5O7·2H2O and 1 mmol of above-mentioned CdS NWs were dispersed in 15 mL of water. After ultrasonication for 1 h, 15 mL of water solution containing x mmol of CoCl2·6H2O and 1.5 g of Na3C6H5O7·2H2O was mixed with above solution. Then the solution was transferred into the autoclave and kept at 160 °C for 12 h. After cooled, the precipitate was washed, dried and CdS/CoMoSx photocatalyst was collected. Moreover, the other photocatalysts with theoretical mole ratio of 5%, 10%, 15%, 20% and 25% CoMoSx were fabricated by changing the amounts of (NH4)2MoS4 precursor and CoCl2·6H2O.These samples were denoted as CCMS-5, CCMS10, CCMS-15, CCMS-20 and CCMS-25 respectively. Moreover, bare CoMoSx sample was obtained through the similar process without CdS NWs addition. For comparison, theoretical 15 at.% Pt-modified CdS (Pt/CdS) photocatalyst was synthesized via the photo reduction method. In addition, physical mixed 15 at.% CdS/CoMoSx photocatalyst (CCMS-m) was prepared to illustrate the advantages of the unique core-shell structure in this work. Furthermore, to elucidate the effect of Co amounts on the photocatalytic H2 evolution activity of CdS/CoMoSx, samples with different Co amounts (0 at.%, 5 at.%, 10 at.%, 15 at.% and 20 at.% Co) have been prepared by changing the CoCl2·6H2O content under the optimized synthesis conditions. The corresponding samples were labeled as C-MS, C-5C-MS, C-10C-MS, C-15C-MS, C-20C-MS,

2. Experimental 2.1. Materials Ethylenediamine (C2H4(NH2)2), cadmium nitrate tetrahydrate (Cd (NO3)2·4H2O), thiourea (CH4N2S), cobalt chloride hexahydrate ammonium molybdate tetrahydrate (CoCl2·6H2O), ((NH4)6Mo7O24·4H2O), sodium citrate (Na3C6H5O7·2H2O), ammonium sulfide solution ((NH4)2S) and ethanol (C2H5OH) were purchased from Aladdin (P. R. China). All reagents were used without any further purification.

Scheme 1. Schematic illustration for the preparation of core-shell CdS/CoMoSx photocatalyst. 2

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respectively. The whole preparation process is shown in Scheme 1. 2.5. Materials characterization The crystalline properties of the samples were investigated by X-ray powder diffraction (XRD) technology on Rigaku Ultima IV powder Xray diffractometer (Cu Kα radiation, λ = 1.54 Å). The morphologies of the samples were examined by field emission scanning electron microscopy (FESEM, JSM-7500F) and transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN). An energy-dispersive X- ray (EDX) spectroscope attached to the TEM was applied to examine the element analysis. The composition and valence state of the samples were measured from X-ray photoelectron spectroscopy (XPS) on ESCALAB 250 Xray instrument with Al Kα radiation. Inductively coupled plasma emission spectrometer (ICP-OES, Agilent ICPOES 730) was used to determine the relative contents of various elements in the photocatalyst. Nitrogen adsorption/desorption isotherms were measured on Microtrac BEL BELSORP-max. The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method and pore size distributions were analyzed using Barret-Joyner-Halender (BJH) method. The UV–vis diffuse reflection spectroscopy (UV-DRS) were recorded on the Shimadzu UV–vis spectrophotometer (2550, Japan). The photoluminescence (PL) spectra were recorded using F97 Pro fluorescence spectrometer at room temperature. Transient photocurrents measurements, electrochemical impedance spectroscopy (EIS) and MottSchottky (M-S) analyses were all performed on an electrochemical workstation (CHI 660e) under a 300 W Xe lamp with a 420 nm cut off filter. The electrolyte was 0.5 M Na2SO4 aqueous solution. In addition, active species were measured by the electronic spin resonance (ESR) trapping experiment on A300-10/12 (Bruker) under visible-light irradiation, using 5, 5-dimethyl-pyrroline-N-oxide (DMPO) as the radical trapping agent.

Fig. 1. XRD patterns of pure CdS, CoMoSx and CdS/CoMoSx photocatalysts.

[41–43]. The CdS/CoMoSx photocatalysts exhibit almost identical XRD diffraction peaks to that of bare CdS and no peaks of impurities were observed. This fact suggests that the amorphous CoMoSx co-catalyst will not affect the crystal structure of CdS. However, the characteristic diffraction peaks of CoMoSx cannot be observed due to the weak crystallinity and low content of CoMoSx NPs [44,45]. It has been suggested that amorphous structure will expose more unsaturated sites and active centers than that of crystalline structures [46]. Therefore, the amorphous CoMoSx NPs are expected to qualify the CdS with excellent catalytic properties. X-ray photoelectron spectroscopy (XPS) was employed to analyze the surface element composition and valence states of the samples. Fig. 2 shows the high resolution XPS spectra of Cd, Mo, Co and S elements calibrated by the C1s peak. As displayed in Fig. 2a, the binding energies at 405.1 and 411.8 eV can be assigned to the Cd 3d5/2 and Cd 3d3/2 respectively, indicative of Cd2+ in CdS [14]. Note that the binding energies of Cd 3d for the CdS/CoMoSx sample are higher than that of pure CdS, which is attributed to change of surface electron density after the growth of CoMoSx shell. It has been reported that electron will transfer from semiconductor with high Fermi energy (EF) to semiconductor with lower EF. Thus, electrons will transfer from CdS core to the CoMoSx shell through the intimate interface. The high resolution XPS spectrum of Mo 3d was shown in Fig. 2b. For CoMoSx, two well defined peaks with binding energies at 232.5 and 229.0 eV are attributed to Mo 3d3/2 and Mo 3d5/2, respectively, demonstrating the existence of Mo4+ in the photocatalyst [47]. The peak located at 235.7 eV can be attributed to the molybdenum (VI) in an octahedral configuration [48]. The binding energy at 226.6 eV is the characteristic peak of S 2s [49]. The resulting binding energies of Mo in the CdS/ CoMoSx sample are lower than that of CoMoSx because the electrons transfer from CdS to CoMoSx across the heterojunction interface. The high resolution XPS spectrum of Co 2p of the CoMoSx can be deconvoluted into six peaks at 779.4, 782.7, 787.0, 794.5, 799.0 and 804.1 eV, respectively, as illustrated in Fig. 2c. The two prominent peaks at binding energies of 779.4 and 794.5 eV can be attributed to the Co 2p3/2 and Co 2p1/2 of Co2+, respectively. The two prominent peaks at binding energies of 782.7 and 799.0 eV can be attributed to the Co 2p3/2 and Co 2p1/2 of Co3+, respectively [41,50,51]. The other two peaks at 787.0 and 804.1 eV with two shakeup satellite peaks (labeled as “Sat.”) indicate the coexistence of Co2+ and Co3+ in the CoMoSx sample [52]. Obviously, compared with CoMoSx, the binding energies of Co elements in CdS/CoMoSx sample shifted to the lower energy, which is similar with the Mo element. Fig. 2d shows the S 2p spectra of CdS, CdS/CoMoSx and CoMoSx samples. The S 2p peaks of CdS/CoMoSx located at 162.7 and 161.4 eV are in accordance with the S 2p1/2 and S 2p3/2 orbits of S2− [53]. The peak at 169.0 eV could be assigned to the

2.6. Evaluation of photocatalytic activity Photocatalytic water splitting reaction was carried out in a Pyrex reaction cell at room temperature. Typically, 50 mg of the photocatalyst was dispersed in 80 mL of aqueous solution containing lactic acid (10 vol%) as sacrificial reagent. The resultant mixture was then thoroughly vacuumed and irradiated by visible light. A 300 W Xe lamp (MICROSOLAR300) equipped with a 420 nm cut-off filter was used. The amount of evolved H2 was periodically extracted from the system and determined by the gas chromatograph (China; GC-9790II) equipped with thermal conductivity detector (TCD) detector and a molecular sieve 5 Å column using N2 carrier as the carrier gas. The apparent quantum efficiency (QE %) of photocatalyst was measured using monochromic light (420 nm). The AQY value was calculated by the following equation:

QE% =

2 × number of evolved H2 molecules × 100% number of incident photons

3. Results and discussion 3.1. Structure analysis The crystal structures of CdS, CoMoSx and CdS/CoMoSx photocatalysts with varying CoMoSx contents (0, 5, 10, 15, 20 and 25 at.%) were characterized by X-ray diffraction (XRD) analysis and the results are shown in Fig. 1. We can see that the bare CdS exhibits obvious diffraction peaks at 2θ = 24.9°, 26.7°, 28.3°, 36.8°, 44.1°, 48.1° and 52.2°, which can be well-indexed to (100), (002), (101), (102), (110), (103) and (112) planes of hexagonal CdS (PDF# 41-1049) [40]. For pure CoMoSx sample, no obvious characteristic diffraction peak can be observed, illustrating the amorphous structure of the CoMoSx sample. Similar results have been reported in other reported literatures 3

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Fig. 2. High resolution XPS spectra of (a) Cd 3d, (b) Mo 3d, (c) Co 3d and (d) S 2p for bare CdS, CoMoSx and CCMS-15 photocatalysts.

oxidation state of S ions in SO32− or SO42−, respectively [54]. The binding energies of S in CdS/CoMoSx are higher than that of CdS and lower than CoMoSx because of the transfer of electrons. The XPS results suggest the formation of intimate contact of CdS and CoMoSx and this is consistent with the SEM and TEM observations. Therefore, the interfacial charge transfer should be significantly increased due to the close contact between CdS and CoMoSx [45], which would greatly enhance the photocatalytic performance [55]. Moreover, the quantitative XPS elemental analysis (Table S1) confirmed that the mole ratio of Co:Mo is about 1:1. Fig. 3 shows the Raman spectra of CdS and CCMS-15. The three main peaks at 208.85, 297.56 cm−1 and 600.33 cm−1 are the

characteristic peaks of CdS. The Raman peaks at 297.56 cm−1 and 600.33 cm−1 are correspond to the characteristic longitudinal optical (LO) phonon vibration modes of 1-LO and 2-LO of CdS, respectively [56,57]. Compared with CdS, the 1-LO and 2-LO of CCMS-15 slightly red shift to 295.63 cm−1 and 599.36 cm−1. This shift may be caused by the significant electron transfer between CdS and CoMoSx [58] or the presence of strain in the CCMS-15 sample, illustrating the intimate contact between CdS and CoMoSx. The characteristic Raman peaks of CoMoSx in the CCMS-15 cannot be detected for the low content of CoMoSx and the strong Raman signal of CdS. In addition, the relative weaker intensity of CCMS-15 composite than that of CdS indicates the structure is not perfect and rich in defect. The morphologies of the as-prepared CdS and series CdS/CoMoSx were studied by SEM. As shown in Fig. 4a, the pure CdS exhibits nanowire (NW) structure with the diameter of 50–100 nm. After the decoration of CoMoSx co-catalyst, the CdS/CoMoSx photocatalysts show similar morphologies with pure CdS (Fig. 4b–f), except that the average diameters of CdS/CoMoSx photocatalysts are larger than that of bare CdS due to the deposition of amorphous CoMoSx on the surface of CdS NWs to form core-shell structure. Energy-dispersive X-ray (EDX) spectrum in Fig. S3 gives an atomic ratio of about 0.98:1:14.07:16.23 for Co/Mo/Cd/S in the CCMS-15 photocatalyst, which is close to the results from ICP-OES and XPS (Table S1). TEM image has been employed to further determine the features and microstructures of the samples. As shown in Fig. 5a, pure CdS exhibits highly uniform 1D structure with smooth surface. In the HRTEM image (Fig. 5b), obvious lattice fringes with spacing of 0.35 nm and 0.65 nm match well with the (100) and (001) planes of CdS. The distinct lattice fringes demonstrate the high crystallinity of CdS NWs. The selected area electron diffraction (SAED) pattern (Fig. 5c) with obvious diffraction spots indicates the single-crystal characteristics of CdS,

Fig. 3. Raman spectra of CdS, CCMS-15 and CoMoSx. 4

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Fig. 4. SEM images of (a) CdS, (b) CCMS-5, (c) CCMS-10, (d) CCMS-15, (e) CCMS-20, (f) CCMS-25.

which will contribute to the charge transfer and lower carrier recombination [40]. Fig. 5d–e show the TEM and HRTEM images of CCMS-15 photocatalyst. Obviously, uniform CoMoSx NPs are successfully coated on the surface of CdS NWs. As can be seen from the HRTEM image of CCMS-15, the lattice fringes of CoMoSx NPs are discontinuous in long-range order, indicating the amorphous structure of CoMoSx NPs. It has been reported that amorphous nanostructure with a plenty of exposed sites on the edge will favor the HER activity. Therefore, it is anticipated that the as obtained CdS/CoMoSx photocatalyst may exhibit significantly enhanced activity [54]. The existence of Mo, Co, S, and Cd elements was also certified by the energy-dispersive X-ray spectrometry (EDX). The EDX results in Fig. 5f confirmed the existence of Cd, Mo, Co and S elements in the CdS/CoMoSx-15sample. Element mapping results in Fig. 5g–l further illustrate the uniform distribution of CoMoSx shell on CdS core. Fig. 6 shows the nitrogen adsorption–desorption isotherms of CdS, CoMoSx and core-shell CdS/CoMoSx. It can be found that CdS and CdS/ CoMoSx samples show similar type IV isotherms with H3 hysteresis loops at high P/P0, indicating the mesoporous structure (Fig. 6a). The pore size distribution curves in Fig. 6b exhibit wide pore size distributions from 2 to 100 nm, illustrating the presence of mesoporous and microporous structure. The Brunauer–Emmett–Teller (BET) specific surface areas, total pore volume and mean pore diameter of the samples are summarized in Table 1. Notably, CCMS-15 sample shows the highest BET surface area and total pore volume, which will be helpful for the increase of active sites and transfer of ions as they would facilitate the improvement of photocatalytic H2 evolution [45].

fact indicates that the amorphous CoMoSx NPs could effectively enhance the photocatalytic H2 evolution activity of CdS NWs. Moreover, the H2 evolution rates increase remarkably with the increase of CoMoSx amount in the composite. The highest photocatalytic H2 evolution rate (477.96 μmol∙h−1) was observed for the CCMS-15 sample, corresponding to an AQE of ca. 2.8% at 420 nm monochromatic light. Such an excellent photocatalytic H2 evolution activity is about 26.3 times higher than that of pure CdS. However, further increase the amount of CoMoSx NPs will decrease the H2 evolution rate, which is probably due to the “shielding effect” of the CoMoSx [39,55]. That is, excess CoMoSx amount on the surface of CdS NWs will inhibit the light absorption of CdS, thus decrease the photo-generated electron-hole pairs. Moreover, the excess CoMoSx would also prevent the effective contact between active sites and reactants, yet hindering the reduction of H+ into H2. Pt has been proved to be extraordinary co-catalyst for photocatalytic H2 evolution. For comparison, herein, we prepared 15 at.% Ptmodified CdS (CdS-Pt) and evaluated the H2 evolution activity. Photocatalytic H2 evolution rate of core-shell CCMS-15 is much larger than that of CdS-Pt (93.75 μmol∙h−1), clearly revealing that the CoMoSx NPs can be a good candidate for Pt as co-catalyst in photocatalytic H2 evolution reaction. In addition, the photocatalytic H2 evolution rate of pure MoSx modified CdS (C-MS) is only 90.65 μmol∙h−1, which is much lower than that of CCMS-15. This strong evidence indicates the CoMoSx co-catalyst is more effective than MoSx for the photocatalytic H2 evolution activity of CdS. To further illustrate the advantages of the coreshell structure in this work, the photocatalytic activity of CdS and CoMoSx physical mixture (CCMS-m) were also compared. The result shows that the CCMS-m sample merely shows a H2 evolution rate of 196.79 μmol·h−1 under the identical conditions. The much inferior photocatalytic activity may be caused by the fact that the weak contact between CoMoSx and CdS hinders the fast transfer of electrons across the interface. This result indicates the importance of the intimate contact and strong interactions in the heterojunctions for the reaction and further illustrates the advantages of the developed core-shell structure in this work. In terms of CCMS-15 photocatalyst, the effect of different Co amounts in the CoMoSx on the H2 evolution rates was also investigated, as shown in Fig. S5. It can be seen that with the increase of Co amounts in CoMoSx, the photocatalytic H2 evolution rate gradually increases, which are 90.65, 291.99, 349.99, 451.84 and 450.60 μmol·h−1 for CMS, C-5C-MS, C-10C-MS, C-15C-MS and C-20C-MS, respectively. In particular, C-15C-MS shows the highest H2 evolution rate, which is up

3.2. Photocatalytic H2 production and stability tests The photocatalytic H2 evolution rate of all the samples were evaluated under visible light irradiation using lactic acid as the sacrificial reagent. For comparison, MoSx modified CdS (C-MS), Pt coated CdS (CdS-Pt) and physical mixed CdS/CoMoSx (CCMS-m) samples were also evaluated the photocatalytic H2 evolution under the same reaction conditions. As shown in Fig. 7, pure CoMoSx exhibit no photocatalytic activity for H2 evolution, revealing that the CoMoSx can only act as a co-catalyst. It can be seen that, the photocatalytic H2 evolution rate of bare CdS is rather low (18.20 μmol h−1) due to the rapid recombination of photo-generated electron-hole pairs. After the loading of amorphous CoMoSx co-catalyst, the core-shell CdS/CoMoSx photocatalysts perform significantly enhanced photocatalytic H2 evolution rate than CdS. This 5

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Fig. 5. TEM and HRTEM images of (a–c) CdS and (d–e) CCMS-15. (f) EDS, (g–l) STEM image and element mapping of Cd, Co, Mo, and S in the CCMS-15 sample.

Fig. 6. (a) N2 adsorption-desorption isotherms of bare CdS, CoMoSx and core-shell CdS/CoMoSx and (b) the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution curves. 6

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composite shows a much more reduced activity by nearly ca. 45% compared with that of ca. 19% for core-shell CCMS-15, indicating that the core-shell CCMS-15 photocatalyst has better anti-photocorrosion ability and stability than the physical mixture CCMS-m. Therefore, the obtained core-shell CCMS-15 sample indeed exhibits outstanding photocatalytic activity and stability due to the unique core-shell structure. This result was caused by the fact the strong interaction between CdS core and CoMoSx shell formed during the in situ hydrothermal process facilitates the transfer of charge carriers. Moreover, the high resolution XPS spectra of the core-shell CCMS15 photocatalyst before and after successive 12 h reaction were carried out and the results are shown in Fig. S6. Obviously, XPS peaks of both Cd 3d and S 2p remain unchanged after a consecutive 12 h visible light irradiation, illustrating the good stability of the CdS in the core-shell photocatalyst for the protective effect of CoMoSx shell. However, the XPS peaks of Co 2p and Mo 3d are slightly different in the intensity and position after durability experiment. The decreased intensity and binding energy may be caused by the partially decomposition or redox reaction of the CoMoSx shell. However, further studies are needed to provide atomic insights into the reason. These results indicate the CoMoSx shell could improve the stability of CdS photocatalysts, boding for its promising applications as co-catalyst.

Table 1 The BET specific surface areas, mean pore diameters and total pore volumes of all the samples. Samples

SBET (m2 g-) 1

Total pore volume (cm3 g−1)

Mean pore diameter (nm)

CdS CCMS-5 CCMS-10 CCMS-15 CCMS-20 CCMS-25 CMS

25.728 23.626 24.891 43.327 35.631 27.534 2.0312

0.1417 0.1140 0.1351 0.2288 0.1832 0.1738 0.0115

22.034 19.297 21.711 21.125 20.568 25.254 22.651

3.3. Understanding of the enhanced photocatalytic H2 evolution over coreshell CdS/CoMoSx photocatalyst To investigate the mechanism of the enhanced photocatalytic activity for core-shell CdS/CoMoSx photocatalysts, UV–vis DRS, PL spectra, transient photocurrent responses, EIS and ESR of the samples were studied. The optical property of the samples was characterized using UV–vis DRS. As depicted in Fig. 9a, the absorption band edge of pristine CdS is at ~525 nm, which can be assigned to the intrinsic band gap absorption of CdS [59,60]. The CoMoSx sample shows obvious absorption in the region of 200–800 nm, indicating the narrow band gap and strong visible light absorption of CoMoSx NPs. After decorated with CoMoSx, the visible light absorption ability of CdS was significantly enhanced. It can be observed that the visible light absorption ability of CdS increase with the increase of CoMoSx content. The remarkable impact on optical properties of core-shell CdS/CoMoSx samples also agrees well with the color change from yellow to yellow-green. Interestingly, the similar absorption band edges of all the CdS/CoMoSx samples with CdS further suggests the loading of CoMoSx did not affect the crystal structure of CdS. The decoration of CoMoSx NPs could effectively increase the visible light absorption and create more available photo-generated carriers, thus enhancing the photocatalytic H2 evolution. It has been generally accepted that the optical absorption property of photocatalyst is closely correlated with its band gap. The band gap (Eg) of the samples can be simply estimated by extrapolating the straight portion of plots to α = 0 based on the formula [51]:

Fig. 7. Photocatalytic H2 production over CdS NWs, core-shell CdS/CoMoSx (CCMS) photocatalysts, pure CoMoSx, CdS/MoSx (C-MS), CdS-Pt and physical mixed CdS/CoMoSx (CCMS-m) composites.

to 5 times higher than that over C-MS alone. However, further increase the Co content will decrease the H2 evolution activity of the C-CMS sample, but is still higher than that of C-MS sample. Therefore, the incorporation of Co atoms into MoSx co-catalyst could enhance the H2 evolution activity for the binding energy of co-catalyst with hydrogen atom decrease and the conductivity increase. As is well known, the stability of a photocatalyst is extremely vital for its practical application. The core-shell CCMS-15 photocatalyst was examined by the long-term photocatalytic H2 evolution to examine the stability. As displayed in Fig. 8, the CCMS-15 did not degrade significantly during the photocatalytic H2 evolution process in 12 h. Moreover, the stability of CCMS-m was also detected and compared with the CCMS-15 photocatalyst. By comparison, the CCMS-m

h = A(h

Eg) n/2

Here, α, h, ν and A are absorption coefficient, Planck constant, frequency of the incident light and proportionality constant, respectively. Eg is the estimated band energy. The value of n is 4 for indirect transition while it is 1 for direct transition semiconductor. For CdS, n = 4. The corresponding Tauc plots of the UV–vis DRS spectra are shown in Fig. 9b, where the band gaps for bare CdS and CoMoSx are calculated to be 2.41 and 1.08 eV, respectively. Obviously, the small band gap and large visible light absorption coefficient of CoMoSx could greatly increase the photo generated electrons, making it an attractive co-catalyst for H2 evolution. In order to further explore the reason for the high photocatalytic performance of the core-shell CdS/CoMoSx, transient photocurrent response of the samples were conducted under visible light irradiation by a 300 W Xe lamp equipped with a λ > 420 nm cutoff-filter. Fig. 10a

Fig. 8. Cycling test of core-shell CCMS-15 and CCMS-m samples for photocatalytic H2 generation under visible light irradiation. 7

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Fig. 9. (a) The UV–visible diffuses reflectance spectroscopy of the bare CdS, CoMoSx and core-shell CdS/CoMoSx photocatalysts. (b) The corresponding Tauc plots for the determination of Eg.

shows the photocurrent responses of bare CdS, CoMoSx and CdS/CoMoSx photocatalysts. Obviously, the loading of CoMoSx on the surface of CdS can significantly enhance the photocurrent density. Among all the CdS/CoMoSx photocatalysts, the CCMS-15 exhibits the highest photocurrent density of 8 × 10−5 A cm−2, which is about 4 times higher than that of CdS. The highest photocurrent density of CCMS-15 indicates the lowest recombination rate and improved separation efficiency of photo-generated electron–hole pairs [61]. Consequently, more electrons could be transferred from CdS core to CoMoSx shell. The interfacial charge transfer dynamics between the interfacial regions of CdS and CoMoSx were evaluated by the electrochemical impedance spectrum (EIS). Generally speaking, smaller arc in an EIS Nyquist plot indicates a smaller charge-transfer resistance on the electrode surface [62]. In the Nyquist plot of Fig. 10b, the arc radius of CCMS-15 is smaller than CdS and other CdS/CoMoSx samples, indicating the lowest charge transfer resistance and fastest interfacial charge separation. Thereby, the core-shell CdS/CoMoSx samples permit fast transportation and separation of photo-generated electron–hole pairs, which will accelerate the photocatalytic H2 evolution. It is well known that the photoluminescence (PL) mainly results from the recombination of photo-generated charge carriers. Therefore, a lower PL intensity indicates a lower recombination rate of excited electrons and holes [63]. Here, the steady-state photoluminescence (PL) spectroscopy was measured to investigate the separation/recombination rate of electron–hole pairs. Fig. 11 shows the comparison of PL emission spectra of CdS, CoMoSx and core-shell CdS/CoMoSx photocatalysts. The CdS sample exhibits strong emission peaks at approximately 532 nm, which can be assigned to the band gap transition of CdS. Apparently, the emission intensities of core-shell CdS/CoMoSx

Fig. 11. Room-temperature steady state PL spectra of bare CdS, CoMoSx and core-shell CdS/CoMoSx photocatalysts.

decreased greatly, implying that the recombination of electron–hole pairs in core-shell CdS/CoMoSx was forcefully suppressed. Therefore, the excited electrons in CdS core will more easily migrated to the CoMoSx co-catalyst shell for water reduction reaction. All these results demonstrate that the constructed core-shell CdS/CoMoSx photocatalyst can enhance the light absorption, migration and separation of charge carriers, and optimize the kinetic for water reduction, thus resulting in efficient photocatalyst for water reduction. All characterization results have shown that the generation and

Fig. 10. (a) Transient photocurrent responses and (b) electrochemical impedance spectra (EIS) Nyquist plots of bare CdS, CoMoSx and core-shell CdS/CoMoSx photocatalysts. 8

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Fig. 12. The Mott-Schottky plots of (a) CdS and (b) CoMoSx for determining the flat-band potentials.

Fig. 13. ESR spectra of (a) DMPO-·OH and (b) DMPO -·O2– for CCMS-15 photocatalyst under visible-light irradiation.

Fig. 14. The schematic diagram of photocatalytic H2 evolution over core-shell CdS/CoMoSx photocatalyst under visible light irritation.

separation of electron-hole pairs can be efficiently promoted by the core-shell CdS/CoMoSx photocatalyst under visible light irradiation. In this research, Mott-Schottky (M–S) plots were employed to investigate the semiconductor types and their electronic structures for CdS and CoMoSx. The electrode potential versus reversible hydrogen electrode (ERHE) was obtained based on the Nernst (Eq. (1)) and Mott–Schottky equations (Eq. (2)) [55]:

0 ERHE = ESCE + 0.059pH + ESCE

c

2

=

2 e

0 Nd

[E

Efb

KT/e]

(1) (2)

where ESCE is the measured potential against Hg/Hg2Cl2/saturated KCl reference electrode; ESCE0 is 0.24 V at room temperature; c is the capacitance of the sample; ε is the dielectric constant; ε0 is the 9

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permittivity of free space; Nd is the concentration of carriers; Efb and E represent the flat band potential and applied potential, respectively; K is the Boltzmann constant while T is the absolute temperature (K) and e is the electron charge (1.602 × 10−19C). As shown in Fig. 12, the positive slopes of liner C−2/V plots indicate that both CdS and CoMoSx have n-type semiconductor characteristic. The flat band potentials (Efb) calculated from the interception of the linear regions of these plots are −1.38 and - 0.83 V (vs. Hg/Hg2Cl2) for CdS and CoMoSx, respectively. In general, the conduction band bottom of n-type semiconductor is approximately equal the flat band potential [64,65]. Thus, the CB levels of CdS and CoMoSx are estimated to be −1.14 and −0.59 V versus a normal hydrogen electrode (NHE), respectively. The valence band (VB) potentials of CdS and CoMoSx can be calculated by the equation: EVB = ECB + Eg and the values are 1.27 and 0.49 V, respectively. In addition, the room-temperature electron spin resonance (ESR) trapping experiment with DMPO was conducted to study the specific mechanism of the photocatalytic progress during light illumination. From Fig. 13, we can observe that no corresponding signals of DMPO·OH and DMPO-·O2– can be observed for CCMS-15 photocatalyst in the dark. After visible light irradiation, characteristic peaks with intensity of 1:2:2:1 for DMPO-•OH confirms the formation of •OH radical while the characteristic four peaks of DMPO-•O2– species with intensity ratio of 1:1:1:1 indicates the generation of •O2−. Moreover, the intensities of the peaks increase with the irradiation time. Consequently, we can conclude that both •OH and •O2−.radicals are generated during the photocatalytic process. Based on the above results and characterization analysis, a reaction mechanism for photocatalytic overall water splitting over core-shell CdS/CoMoSx catalyst is proposed and the schematic diagram is displayed in Fig. 14. Under visible light irradiation, the photo generated electrons in CdS core will transfer to the CoMoSx shell with the aid of intimate heterojunction. In here, amorphous CoMoSx NPs can act as active sites and electron collectors to prolong the lifetime of the charge carriers, thus enhancing the photocatalytic activity. Moreover, the intimate contact between CdS core and CoMoSx shell, as well as the fast transfer ability for electrons of CoMoSx contribute to the photo activity enhancement toward H2 evolution. Therefore, core-shell CdS/CoMoSx catalyst could realize the efficient water splitting performance.

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4. Conclusions In summary, a highly efficient noble-metal-free CdS/CoMoSx photocatalyst with core-shell structure were prepared by an in situ hydrothermal synthesis method. The amorphous CoMoSx NPs with defect-rich structure are dispersed uniformly on the surface of CdS single crystalline NWs and form intimate contact interfaces. The highest H2 amount of 477.96 μmol·h−1 was obtained in CdS/CoMoSx-15 sample, which is up to 26.3 times than that for pure CdS and 5.1 times higher than that for Pt loaded CdS. In addition, the core-shell CdS/CoMoSx sample exhibits good stability in recycle experiments. The observed greatly enhanced H2 evolution activity is caused by the synergistic effect of abundant unsaturated active sites in the amorphous structure, promoted charge transfer and suppressed charge recombination, as well as the enhanced visible light absorption. Therefore, the results illustrate that amorphous CoMoSx is a promising co-catalyst for CdS, which can be also be potentially used in photocatalytic H2 production process for other photocatalysts. Acknowledgements This work was financially supported by the NSF of China (Nos. 51502155, 51572152, 21673127 and 21671119) and the 111 Project of Hubei Province (2018-19-1). 10

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