MA Xio-Wei LIN Hi-Feng LI Yn-Yn② WANG Lei PU Xi-Peng② YI Xiu-Jie②

a (College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China) b (Key Laboratory of Eco-chemical Engineering, Taishan Scholar Advantage and Characteristic Discipline Team of Eco-chemical Process and Technology, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China)

ABSTRACT Photocatalytic hydrogen generation from water-splitting holds huge promise for resolving the current energy shortage and environmental issues. Nevertheless, it is still challenging so far to develop non-noble-metal photocatalysts which are efficient toward solar-powered hydrogen evolution reaction (HER). In this work, through an ultrasonic water-bath strategy combined with solvothermal and electrostatic assembly processes, we obtain homogeneous Cd1-xZnxS-Ni2P-MoS2 hybrid nano-spheres consisting of Cd1-xZnxS solid solutions decorated by Ni2P and 1T/2H MoS2 cocatalysts, which demonstrate excellent activity and stability for visible-light-responsive (λ > 420 nm) H2 production. Specifically, the Cd1-xZnxS-Ni2P-MoS2 nano-spheres with 2 wt% Ni2P and 0.2 wt% MoS2 (CZ0.7S-2N-0.2M) exhibit the optimal HER activity of 55.77 mmol·g-1·h-1, about 47 and 32 times more than that of CZ0.7S and Pt-CZ0.7S, respectively. The outstanding HER performance of Cd1-xZnxS-Ni2P-MoS2 can be ascribed to the presence of abundant HER active sites in Ni2P nanoparticles and 1T/2H MoS2 nanosheets as well as the effective transfer and separation of charge carriers. Moreover, the coupling sequence of cocatalysts in Cd1-xZnxS-Ni2P-MoS2 is found to be critical in the regulation of charge transfer pathways and thus the resultant photocatalytic efficiency. The results displayed here could facilitate the engineering of high-performance photocatalysts employing multi-component cocatalysts for sustainable solar-to-fuel conversion.

Keywords: uniform Cd1-xZnxS-Ni2P-MoS2 nano-spheres, MoS2 with 1T/2H mixed-phases, photocatalytic H2 evolution, charge transfer pathways, active sites; DOI: 10.14102/j.cnki.0254-5861.2011-2752

1 INTRODUCTION

Nowadays, it is urgently needed to search for a clean and alternative energy to fossil fuel due to the growing energy shortage and environmental crisises[1]. Hydrogen (H2) as an emerging energy carrier has recently received considerable attention because of its higher energy density and environ- mental friendliness[2]. Among various technologies for H2production, photocatalytic water-splitting using renewable solar energy is deemed as one of the most promising approaches[3]. For photocatalytic H2evolution reaction (HER), the exploitation of high-performance visible-light- responsive photocatalysts is of great importance, which could benefit the efficient utilization of solar energy to speed up the development of H2energy. Compared with other semiconducting photocatalysts, CdS has been extensively studied for HER owing to its effective visible-light- absorption and suitable conduction band edge for proton reduction[4]. However, pristine CdS suffers from the fast recombination of charge carriers and photocorrosion problems[5], which decrease its HER performance significantly. Lately, thanks to their tunable band gap and band structures, the Cd1-xZnxS solid solutions have demonstrated enhanced photocatalytic activity and stability toward HER[6]. Therefore, the Cd1-xZnxS solid solutions show great promise for the construction of novel photocatalysts with exceptional HER activity.

Recent studies have indicated that[7,8]hybridizing semi- conductor photocatalyst with cocatalysts is an effective means to improve its photocatalytic capability, as a result of the introduction of fruitful active sites as well as the promoted transfer and separation of charge carriers. Commonly, the cocatalysts for CdS-based photocatalysts include noble metals[9], metal oxides and sulfides[10,11], C3N4[12], graphene[13], and so on. Of late, the layered transition-metal chalcogenides (TMCs, such as MoS2and WS2)[14]and transition-metal phosphides (TMPs, like Ni2P and CoP)[15,16]have been employed as the photocatalytic cocatalysts to display superior HER performance. It is well known that[17]the MoS2possesses two crystal structures, i.e., semiconducting 2H phase and metallic 1T phase, which are formed with the trigonal prismatic and octahedral coordination of Mo and S atoms, respectively. The HER active sites of 2H MoS2are only related to its edge sites, while both the basal planes and edge sites of 1T MoS2are catalytically active toward H2evolution[18]. Hence, 1T MoS2has attracted keen interest for HER study in recent years. In addition, TMPs such as Ni2P exhibits similar electronic structure and HER mechanism as compared with the hydrogenase or metal complex catalysts[19], which endows it with an excellent activity toward H2generation. Therefore, the 1T MoS2and Ni2P could serve as ideal cocatalysts for Cd1-xZnxS solid solutions to achieve efficient H2evolution from solar-powered water-splitting. Nevertheless, the 1T MoS2is usually obtained through the ultrasonic exfoliation of lithium-intercalated MoS2formed in n-butyllithium solution[17,20]. Meanwhile, the preparation of Ni2P requires the use of yellow phosphorus or phosphorization treatment under calcining condition[15,21]. Consequently, the involved dangerous reagents and cumbersome fabrication processes have restricted the wide application of these two cocatalysts to a large extent. Thus, it is highly necessary to develop safe and facile methods to synthesize the 1T MoS2and Ni2P nanocrystals. On the other hand, it’s worth mentioning that for the photocatalyst with multi-component cocatalysts, the coupling sequence of cocatalysts may influence the charge transfer pathways and finally the photocatalytic properties, however, which has still rarely been concerned up to now.

In this work, uniform Cd1-xZnxS solid solution nano- spheres were prepared by an ultrasonic water-bath method, which were then coupled by Ni2P and MoS2to form unique Cd1-xZnxS-Ni2P-MoS2hybrid nano-spheres. The Ni2P nanocrystals were obtained with an ethylenediamine- assisted solvothermal reaction using nontoxic red phos- phorus as P source. Meanwhile, the MoS2nanosheets synthesized under solvothermal conditions were composed of 1T and 2H mixed-phases. Under visible-light irradiation, the Cd1-xZnxS-Ni2P-MoS2demonstrates excellent activity and stability toward photocatalytic H2evolution, much superior to that of the Pt-Cd1-xZnxS, Cd1-xZnxS-MoS2and Cd1-xZnxS-Ni2P counterparts, as well as most CdS-based photocatalysts reported previously. In addition, comparative experimental results indicate that the coupling sequence of cocatalysts in Cd1-xZnxS-Ni2P-MoS2has a significant influence on the migration and separation of charge carriers, which can be rationally optimized to achieve an outstanding HER performance.

2 EXPERIMENTAL

2. 1 Materials

All the chemicals with high purity of analytical grade were purchased from Shanghai Adamas-Beta Reagent Co., Ltd. (China) and were used without further purification.

2. 2 Synthesis of Cd1-xZnxS nano-spheres

The Cd1-xZnxS nano-spheres were prepared by an ultrasound-assisted water-bath method. Typically, take Cd0.3Zn0.7S for an example, 7 mL 0.1 M C6H5Na3O7·2H2O, 2.5 mL 0.03 M CdCl2·2.5H2O, 2.5 mL 0.07 M ZnCl2, and 10 mL 0.1 M CH4N2S aqueous solution were added to 100 mL deionized water to form a homogeneous solution. After that 2 mL NH3·H2O was mixed into the above solution, which was sealed in a beaker and heated at 50 ℃ for 6 h under ultrasonic water-bath condition. After natural sedimentation for several hours, the product was washed with deionized water and ethanol for several times and then dried in a vacuum oven at 70 ℃ for 6 h.

2. 3 Synthesis of 1T/2H MoS2 nanosheets

MoS2nanosheets with 1T/2H mixed phases were synthesized by a solvothermal method. Specifically, 1 mmol of MoCl5and 10 mmol of C2H5NS were dissolved in 30 mL N,N-dimethylformamide via stirring for 1 h. The solution was then transferred into a 50 mL Teflon-lined stainless-steel autoclave and then heated at 200 ℃ for 24 h. After being naturally cooled to room temperature, the resulting preci- pitate was washed with ethanol for three times and dried in a vacuum oven at 70 ℃ for 6 h.

2. 4 Synthesis of Cd1-xZnxS-Ni2P hybrid nano-spheres

In a typical procedure, 0.2 g Cd1-xZnxS was first dispersed into 20 mL ethylenediamine by ultrasonication for 5 mins. Meanwhile, calculated amounts of Ni(NO3)2·6H2O and red P with the molar ratio of Ni to P being 1:5 were dispersed into 10 mL ethylenediamine after ultrasonication for 30 mins, which were then added into the above suspension and agitated for 1 h. Finally, the mixed suspension was sealed into a 50 mL autoclave and kept at 200 ℃ for 24 h. After reaction, the product was rinsed thrice with ethanol and dried in a vacuum oven at 70 ℃ for 6 h. For the preparation of Ni2P nanocrystals, the process was similar to that for Cd1-xZnxS-Ni2P but without the addition of Cd1-xZnxS.

2. 5 Synthesis of Cd1-xZnxS-Ni2P-MoS2 hybrid nano-spheres

The Cd1-xZnxS-Ni2P-MoS2hybrid nano-spheres were prepared through the ultrasonic exfoliation of MoS2nanosheets followed by an assembling procedure under agitation. Generally, a certain amount of MoS2nanosheets were added into 35 mL N,N-dimethylformamide (filled in a round bottom flask) and sonicated for 3 h to get a homogeneous suspension of exfoliated MoS2. After this step, 0.2 g Cd1-xZnxS-Ni2P was mixed into the MoS2suspension through ultrasonication for 10 mins, which was then sealed and stirred for 12 h to complete the adsorption and assembling processes. The Cd1-xZnxS-Ni2P-MoS2can be obtained after the product was washed with deionized water and ethanol and dried in a vacuum oven at 70 ℃ for 6 h. For comparison, based on the above strategy, the Cd1-xZnxS-MoS2hybrid nano-spheres were fabricated by using Cd1-xZnxS to replace Cd1-xZnxS-Ni2P. In addition, when Cd1-xZnxS-MoS2was employed as growth substrate, the Cd1-xZnxS-MoS2-Ni2P hybrid nano-spheres were prepared according to the solvothermal method for Cd1-xZnxS-Ni2P synthesis.

2. 6 Characterization

The crystal structures of synthesized photocatalysts were determined by powder X-ray diffraction (XRD) using a Bruker Diffractometer (Bruker D8 Advance) with mono- chromatic Cu-Kα radiation (λ = 0.15406 nm). The scanning range was 5 ～85º and the scanning rate was 0.02º. Ultraviolet-visible (UV-vis) absorption spectra were recorded using a Shimadzu UV 3600 spectrometer. Raman signals were measured on the LabRAM HR Evolution Raman spectrometer with a laser wavelength of 532 nm. The surface morphology of the sample was observed employing a ZEISS MERLIN Compact scanning electron microscope (SEM). The internal structure, crystal lattice, and chemical composition of the catalyst were investigated by the transmission electron microscopy (TEM), high-resolution TEM (HRTEM), dark-field scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectro- scopy (EDX) element mapping measurements carried out by a JEOL JEM-2100F transmission electron microscope. Specific surface area data were obtained from a JW-BK222 auto-adsorption system according to the Brunauer-Emmet- Teller (BET) method. The surface composition and chemical state of photocatalyst were detected by X-ray photoelectron spectroscopy (XPS) using a Thermo Fisher ESCALAB 250 Xi spectrometer with monochromatized Al-Kα radiation. The photoluminescence (PL) spectra were collected by a Varian Cary Eclipse Fluorescence spectrophotometer utilizing an excitation wavelength of 365 nm.

2. 7 Electrochemical and photoelectrochemical measurements

The photocurrent response and electrochemical impe- dance spectroscopy (EIS) tests were performed with a CHI 660E electrochemical workstation, in which the sample, Ag/AgCl, and Pt plate were the working electrode, reference electrode, and counter electrode, respectively. The working electrode was prepared according to the following pro- cedures: firstly, the sample powder was dispersed into an aqueous polyethylene glycol solution by ultrasonication for 30 mins, which was subsequently coated onto a fluorinated tin oxide (FTO) glass with a coating area of 2 cm2. After that the coated FTO glass was dried naturally and then roasted at 450 ℃ for 1 h under N2atmosphere so as to counter the peeling of sample during test. To perform the transient photocurrent measurement, the Na2S (0.35 M)/Na2SO3(0.25 M) aqueous solution was used as electrolyte and the bias potential was set as 0.5 V. The illumination was furnished by using a 300-W Xe lamp equipped with a 420-nm cutoff filter. On the other hand, the EIS test was carried out using a 0.5 M Na2SO4aqueous solution as the electrolyte, and the EIS spectra were collected with the amplitude of 0.005 V and the frequency of 0.1～105Hz under open circuit potential conditions.

2. 8 Evaluation of photocatalytic hydrogen generation activity

The photocatalytic H2evolution reaction was measured by a Labsolar-6A online H2evolution and detecting apparatus (Beijing Perfectlight Technology Co., Ltd.). To provide visible-light irradiation, a 300-W Xe lamp equipped with a 420-nm cutoff filter was employed as the light source. For each test, 100 mL aqueous solution containing 0.35 M Na2S/0.25 M Na2SO3was prepared beforehand, into which 10 mg photocatalyst was then dispersed after ultrasonication for 3 mins. Following this step, the photocatalyst suspension was evacuated to high-vacuum state. The reaction temperature for the H2evolution reaction was controlled at 6 ℃ by a thermostatic circulating water pump. In order to study the effect of sacrificial agent on the H2-evolving activity of photocatalyst, 0.375 M ascorbic acid or 10 vol% lactic acid was used to replace the 0.35 M Na2S/0.25 M Na2SO3solution. At an interval of 60 mins, the generated H2was monitored by a GC-7806 gas chromatograph with a TCD detector. The apparent quantum yield (AQY) for the H2evolution reaction can be determined by the following expression:

In this formula, 2Hn and pn denote the number of produced H2molecules and photons, respectively; P is the light power; t = 3600 s represents the irradiation time; λ=420 nm means the wavelength of incident light; h = 6.63 × 10-34J·s stands for the Planck constant; and c = 3.0 × 108m·s-1is the light speed.

3 RESULTS AND DISCUSSION

3. 1 Photocatalyst characterization

The preparation procedure of Cd1-xZnxS-Ni2P-MoS2is schematically illustrated in Fig. 1. Firstly, homogeneous nano-spheres assembled by ultra-small Cd1-xZnxS nanocrys- tals were produced through an ultrasound-assisted water- bath method, where the high-energy chemistry caused by ultrasound was critical to the nucleation, growth, as well as subsequent assembling processes of the product[22,23]. In the second step, the Ni2P nanoparticles were anchored onto the Cd1-xZnxS nano-sphere surface to form Cd1-xZnxS-Ni2P by a solvothermal reaction in ethylenediamine. Finally, the MoS2nanosheets possessing 1T/2H mixed-phases synthesized under solvothermal conditions were ultrasonically exfoliated, and then combined with Cd1-xZnxS-Ni2P via electrostatic adsorption to form the unique Cd1-xZnxS-Ni2P-MoS2hybrid nano-spheres.

XRD measurement was carried out to study the crystal structure of photocatalyst. Fig. 2a shows that as the Zn-doping content of Cd1-xZnxS solid solutions increases from 0.3 to 0.7, the diffraction patterns of Cd0.7Zn0.3S (CZ0.3S), Cd0.5Zn0.5S (CZ0.5S), and Cd0.3Zn0.7S (CZ0.7S) nano-spheres are all in good consistence with that of the hexagonal phase of CdS (JCPDS Card No. 02-0549). Nevertheless, when the Zn-doping ratio was further raised to 0.9, a small amount of ZnO (JCPDS Card No. 65-3411) was generated in Cd0.1Zn0.9S (CZ0.9S) product. According to the solubility diagram reported by Hubert et al.[24], the CdS and ZnS possess much lower solubility as compared with ZnO in ammonia solution. Therefore, in an ammonia solution containing Cd2+, Zn2+, and S2-ions, the Cd1-xZnxS solid solutions can be produced more easily than ZnO, while the formation of ZnO might be possible only at a higher Zn2+concentration, matching well with our experimental results. On the other hand, the XRD patterns of individual CZ0.7S, MoS2and Ni2P, as well as their composites are also tested as presented in Fig. 2b. Obviously, the diffraction peaks of pure Ni2P nanocrystals could be assigned to the structure of hexagonal Ni2P (JCPDS Card No. 65-3544). In comparison with bulk counterpart (JCPDS Card No. 77-1716), the (002) and (004) peaks of synthesized MoS2nanosheets were moved toward lower 2θ direction, suggesting that layer-spacing of the latter was expanded[25]. Moreover, we can see after careful comparison that the CZ0.7S-0.2 wt%MoS2(CZ0.7S-0.2M), CZ0.7S-2 wt%Ni2P (CZ0.7S-2N), and CZ0.7S-2 wt%Ni2P-0.2 wt%MoS2(CZ0.7S-2N-0.2M) hybrids display the diffraction peaks belonging to the CZ0.7S component, while the signals of MoS2and Ni2P cannot be found from the spectra due to their lower loading amounts (below the XRD detection limit of 5 wt%)[26].

Fig. 1. Schematic illustration for the preparation of Cd1-xZnxS-Ni2P-MoS2 hybrid nano-spheres

Fig. 2. (a) XRD patterns of Cd1-xZnxS solid solutions with different Zn/Cd ratios, (b) XRD patterns of CZ0.7S, MoS2, Ni2P, CZ0.7S-0.2M, CZ0.7S-2N, and CZ0.7S-2N-0.2M

Fig. 3. (a) UV-vis absorption and (b) calculated bandgaps of the CZ0.3S, CZ0.5S, and CZ0.7S assembled nano-spheres

Fig. 4. (a) UV-vis absorption spectra of different samples, (b) Determined bandgap of pure MoS2 nanosheets

The morphologies of CZ0.5S, CZ0.7S, and CZ0.9S were disclosed by the scanning electron microscopy (SEM) measurements. As displayed in Fig. 5a～5c, both CZ0.5S and CZ0.7S are featured by uniform nano-sphere architecture, whilst the CZ0.9S sample appears as irregular nanoparticles. According to the size histograms shown in Fig. 5d～5f, the major diameters of CZ0.5S and CZ0.7S are found to be ～85 and 115 nm, respectively. In comparison with CZ0.5S and CZ0.7S, the CZ0.9S is provided with a much wider size distribution. On the other hand, the broadened XRD reflections of CZ0.5S and CZ0.7S (Fig. 2a) are indicative of their smaller particle sizes[29]. Correspondingly, the average grain size was calculated using the Scherrer’s equation[30]to be merely ～11.5 nm for CZ0.5S and ～13.0 nm for CZ0.7S, respectively. Hence, it can be inferred from the above results that the CZ0.5S and CZ0.7S nano-spheres were formed through the assembly of ultra-small primary crystalline grains. Differing from the reported CdS nano-spheres with bigger constituent particles[8,31], these homogeneous nano-spheres assembled by ultra-small Cd1-xZnxS nanocrys- tals are endowed with more abundant compositions and energy band structures, which enable us more flexibly to mediate their photocatalytic capabilities. Moreover, the rougher surfaces of Cd1-xZnxS assembled nano-spheres could furnish more active sites for photocatalytic reactions and more unsaturated atoms to anchor cocatalysts. Therefore, the Cd1-xZnxS assembled nano-spheres are anticipated to serve as a desirable photocatalytic material for efficient solar conversion and utilization.

Fig. 5. (a～c) SEM images and (d～f) Corresponding particle-size distributions of (a, d) CZ0.5S, (b, e) CZ0.7S, and (c, f) CZ0.9S assembled nano-spheres

Transmission electron microscopy (TEM) images of the CZ0.7S nano-spheres are shown in Fig. 6a and 6b, which display clearly that the nano-spheres are assembled by numerous tiny nanoparticles, agreeing well with the inference based on the SEM observation and particle-size calculation (Fig. 5). Moreover, the high-resolution TEM (HRTEM) graph in Fig. 6c evidences that the CZ0.7S nano-spheres are composed of lots of smaller lattice domains with different orientations, further certifying their assembled architecture. In addition, the dark-field scanning transmis- sion electron microscopy (STEM) photo and corresponding energy-dispersive X-ray spectroscopy (EDX) elemental mapping results of the CZ0.7S nano-sphere are presented in Fig. 6d, which demonstrates the uniform distribution of Zn, Cd and S elements all over the entire nano-sphere. Besides CZ0.7S, the CZ0.7S-2N-0.2M was also characterized by TEM and HRTEM measurements. It can be seen from Fig. 6e that the CZ0.7S-2N-0.2M hybrid possesses homogeneous nano-sphere morphology similar to that of individual CZ0.7S. On the other hand, as indicated by the HRTEM result in Fig. 7, the synthesized MoS2nanosheets are characteristic of lower crystallinity or even amorphous structure. Thus, the amorphous layers coating on the surface of CZ0.7S-2N-0.2M nano-spheres (Fig. 6f, denoted by the yellow dotted ellipses) could be related to the MoS2component. Meanwhile, the CZ0.7S-2N-0.2M hybrid exhibits the lattice fringes with spacings of 0.22 and 0.35 nm (Fig. 6g), which are attributed to the (111) plane of Ni2P and (100) plane of CZ0.7S, respectively. What’s more, the existence of Zn, Cd, Ni, Mo, P, and S elements in CZ0.7S-2N-0.2M nano-spheres was confirmed by the STEM and corresponding EDX elemental mapping results in Fig. 6h.

Fig. 6. (a, b) TEM and (c) HRTEM images, as well as (d) Dark-field STEM photo and corresponding EDX elemental mapping results of CZ0.7S nano-spheres, (e) TEM and (f, g) HRTEM graphs, as well as (h) Dark-field STEM image and corresponding EDX elemental mapping results of CZ0.7S-2N-0.2M hybrid nano-spheres

Fig. 7. (a) TEM and (b) HRTEM images of MoS2 nanosheets

X-ray photoelectron spectroscopy (XPS) measurement was employed to analyze the chemical composition and element valence state of Cd1-xZnxS-Ni2P-MoS2composite (Here, CZ0.7S-2N-5M was taken for an example). As shown in Fig. 8a, for CZ0.7S-2N-5M, the Cd 3d5/2and 3d3/2peaks locate at ～403.7 and ～410.4 eV, respectively, which are associated with the Cd2+species[32]. The Zn 2p3/2and 2p1/2signals are found to be around 1020.6 and 1043.7 eV (Fig. 8b), respectively, confirming the formation of Zn2+ions[33]. Compared with CZ0.7S-2N-5M, the CZ0.7S possesses higher binding energies of Zn 2p and Cd 3d (1021.5～1044.6 eV and 404.7～411.4 eV) (Fig. 9a and 9b), which suggests that the electron cloud densities of Zn and Cd atoms were increased after CZ0.7S was hybridized by Ni2P and MoS2to produce Cd1-xZnxS-Ni2P-MoS2, indicating the transfer of electrons and formation of heterostructure[34]. Besides, the P 2p XPS spectrum is displayed in Fig. 8c, the signal at ～129.5 eV can be attributed to P in Ni2P, while the other peak situating at ～133.1 eV belongs to a small amount of oxidized P species formed on the surface of catalyst due to exposure to air[35]. We can see from Fig. 8d that Ni 2p spectrum exhibits the binding energies of 854.1 (2p3/2) and 871.6 (2p1/2) eV, which are consistent with that of Ni-P species[36]. Presented in Fig. 8e is the Mo 3d XPS spectrum of Cd1-xZnxS-Ni2P-MoS2hybrid, which comprises three sets of doublet peaks, i.e., 228.0～231.2, 229.2～232.5 and 231.7～234.8 eV, corresponding to the Mo in 1T MoS2[37], 2H MoS2[18], and Mo-O species[38], respectively. The Mo signals of individual MoS2(Fig. 9c) shift toward higher binding energies when compared to that of Cd1-xZnxS- Ni2P-MoS2(Fig. 8e), which also verifies the occurrence of electron transfer in the hybrid. On the other hand, in addition to XPS spectrum, the Raman measurement was also carried out to study the phase structure of MoS2nanosheets. According to the Raman spectrum in Fig. 8f, the vibration signals appearing at about 281 and 375 cm-1are related to the 2H phase of MoS2[39], while the signals of 235 and 335 cm-1demonstrate the existence of 1T phase MoS2[40,41], in good agreement with the XPS results.

3. 2 Photocatalytic activity

In order to investigate the influence of sacrificial agent on the HER activity of CZ0.7S-2N-0.2M, lactic acid and ascorbic acid were utilized for the photocatalytic reaction. One can ascertain from Fig. 12a and 12b that both the photocatalytic activities of CZ0.7S-2N-0.2M in lactic acid and ascorbic acid aqueous solution were lowered as compared with that in Na2S/Na2SO3solution. The activity difference of CZ0.7S-2N-0.2M in the above three sacrificial agents could be related to their distinct reaction environ- ments. According to the literature[47], the electrochemical potentials of semiconductor conduction and valence bands, water reduction and oxidation, hydroxyl anion, and sacrificial agent oxidation have a linear relationship with the pH of reaction solution. Moreover, the valence band potential of semiconductor can be more positive than the ·OH/-OH potential in the solution with very high pH[48]. Consequently, in highly alkaline solution, ·OH/-OH redox couple will be produced through the oxidation of hydroxyl anion by photo-excited holes of photocatalyst. The small molecular shuttle ·OH/-OH owns higher mobility and oxidizing power, which can efficiently react with the sacrificial agent to inhibit the recombination of charge carriers, leading to a dramatically enhanced HER capability. However, when the photocatalytic reaction was performed in low-pH solution, in comparison with ·OH/-OH, the slow transfer and subsequent reaction with the scavenger of photogenerated holes results in high charge recombination and decreased photocatalytic activity. In our experiment, the alkaline Na2S/Na2SO3solution possesses a much higher pH value (～13.5) than that of acidic ascorbic acid (～2.3) and lactic acid (1.4) solutions. Therefore, in Na2S/Na2SO3solution, the CZ0.7S-2N-0.2M could perform photocatalytic HER through the aforementioned redox shuttle mechanism to exhibit an excellent H2evolution activity as compared with that in ascorbic acid and lactic acid solutions.

Fig. 8. (a) Cd 3d, (b) Zn 2p, (c) P 2p, (d) Ni 2p, and (e) Mo 3d XPS spectra of CZ0.7S-2N-5M, (f) Raman spectrum of pure MoS2

Fig. 9. (a) Cd 3d and (b) Zn 2p XPS spectra of CZ0.7S nano-spheres, (c) Mo 3d and (d) S 2p XPS spectra of MoS2

Fig. 10. (a) Photocatalytic H2 generation activities of Cd1-xZnxS solid solutions, (b) Their corresponding average rates, (c) Influence of MoS2 loading amount on the HER activity of CZ0.7S-2N-MoS2 composites, and (d) H2 generation curve for calculating the apparent quantum yield (AQY) of CZ0.7S-2N-0.2M

Fig. 11. (a) Visible-light-induced HER activities and (b) Corresponding average rates of different photocatalysts, (c) Long-term H2 production and (d) Cycling H2-evolving stability of CZ0.7S-2N-0.2M composite

Fig. 12. (a) Photocatalytic HER activities and (b) Corresponding H2 evolution rates of CZ0.7S-2N-0.2M measured using different hole scavengers

3. 3 Photocatalysis mechanism

It is widely acknowledged that[49,50]in photocatalytic reactions, the transfer and separation processes of charge carriers exert great influence on the overall photocatalytic efficiency. In order to assess the charge separation ability of photocatalyst, the photoluminescence (PL) spectroscopy measurements were carried out. As shown in Fig. 13a, the signals appearing at ～476.5 and 519.5 nm could be attributed to the PL emissions from excitonic and trap-state recombination, respectively[26]. Compared with CZ0.7S, the CZ0.7S-2N-0.2M hybrid exhibited a lowered PL intensity, which suggests that the separation of electron-hole pairs was promoted by the latter[51], responsible for its excellent HER activity. Moreover, the charge transfer process of photocatalyst was also studied by the electrochemical impedance spectroscopy (EIS) tests. Generally, for a photocatalyst electrode, the semicircular arc diameter of Nyquist curve is in direct proportion to its charge-transfer resistance[52]. Hence, one can see from Fig. 13b that the charge-transfer resistance of CZ0.7S-2N-0.2M is much smaller than that of the MoS2and CZ0.7S counterparts, which indicates the enhanced capability of CZ0.7S-2N-0.2M in the transfer and separation of charge carriers[53], agreeing well with the PL results. In addition, the photocurrent responses of MoS2, CZ0.7S and CZ0.7S-2N-0.2M were compared as displayed in Fig. 13c. In consistence with the PL and EIS analyses, the higher photocurrent density of CZ0.7S-2N-0.2M than MoS2and CZ0.7S suggests that the CZ0.7S-2N-0.2M possesses a better ability in decreasing the recombination of photogenerated charges[33].

Fig. 13. (a) PL spectra, (b) EIS Nyquist plots, (c) Photocurrent responses of MoS2, CZ0.7S, and CZ0.7S-2N-0.2M, and (d) Schematic energy band structures of MoS2, Ni2P, and CZ0.7S

The significant difference between the HER activities of CZ0.7S-2N-0.2M and CZ0.7S-0.2M-2N implies that there might exist distinct charge transfer mechanisms in these two composites prepared with opposite coupling sequence of the Ni2P and MoS2cocatalysts. To clarify the mechanism for charge transfer, the energy band structures of CZ0.7S (2.55 eV, Fig. 3b), Ni2P, and MoS2(1.41 eV, Fig. 4b) were schematically illustrated in Fig. 13d. For CZ0.7S-Ni2P-MoS2, under light-irradiation, the photogenerated electrons could first migrate from the conduction band (CB) of CZ0.7S to the Fermi level of metallic Ni2P[36,54], resulting in the spatial separation and remarkably inhibited recombination of charge carriers. Subsequently, the electrons captured by Ni2P will quickly move to the adjacent MoS2due to the higher conductivity of Ni2P[21]. What’s more, the Ni2P and defective 1T/2H MoS2cocatalysts are in possession of a large number of active sites for HER[55,56]. Hence, benefiting from the effective separation of charge carriers and presence of abundant active sites, the CZ0.7S-2N-0.2M is provided with an excellent activity toward photocatalytic H2evolution. In comparison with CZ0.7S-2N-0.2M, the charge transfer in CZ0.7S-0.2M-2N follows a quite different pattern. As MoS2was loaded onto CZ0.7S prior to Ni2P, the photo-excited electrons in CZ0.7S will transfer into the CB of MoS2before further flowing into Ni2P, while the holes left in CZ0.7S valence band (VB) could be injected spontaneously into the VB of MoS2[57]. Unfortunately, the type-I charge transfer between CZ0.7S and MoS2could cause the detrimental recombination of electron-hole pairs. Meanwhile, the oxidizing power of photogenerated holes is lowered after the migration process. As a result of the above two negative factors, the CZ0.7S-0.2M-2N is endowed with a dramatically weakened HER activity compared to CZ0.7S-2N-0.2M. Therefore, the results displayed in this work indicate that we can optimize the capability of photocatalyst with multi-component cocatalysts by rationally regulating the coupling sequence to achieve efficient migration and separation of charge carriers.

4 CONCLUSION

In summary, homogeneous Cd1-xZnxS-Ni2P-MoS2hybrid nano-spheres were obtained through the ultrasonic water- bath method combined with solvothermal and electrostatic assembly processes, which exhibit outstanding photoca- talytic activity and stability toward visible-light-driven H2evolution. Particularly, the optimized CZ0.7S-2N-0.2M composite with 2 wt% Ni2P and 0.2 wt% MoS2demonstrates the maximum HER rate of 55.77 mmol·g-1·h-1, exceeding that of the Pt-CZ0.7S, CZ0.7S-0.2M and CZ0.7S-2N counterparts, as well as most CdS-based photocatalysts ever reported. The superior HER performance of CZ0.7S-2N-0.2M can be attributed to the existence of abundant active sites in Ni2P and 1T/2H MoS2cocatalysts and the efficient transfer and separation of charge carriers. The findings presented in this work could facilitate the development of highly-active noble-metal-free photoca- talysts to achieve efficient solar conversion and utilization.

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