Hollow NiCo2S4 Nanospheres as a Cocatalyst to Support ZnIn2S4 Nanosheets for Visible-Light-Driven Hydrogen Production

Zhuang Xiong , Yidong Hou , Rusheng Yuan , Zhengxin Ding ,＊, Wee-Jun Ong , Sibo Wang ,＊

1 State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University,Fuzhou 350116, China.

2 School of Energy and Chemical Engineering, Xiamen University Malaysia, Selangor Darul Ehsan 43900, Malaysia.

3 College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian Province, China.

Abstract: The rational interface tailoring of nanosheets on hollow spheres is a promising strategy to develop efficient photocatalysts for hydrogen production with solar energy. Among the various photocatalyst materials,metal sulfides have been extensively researched because of their relatively narrow band gap and superior visible-light response. ZnIn2S4 is a layered ternary chalcogenide semiconductor photocatalyst with a tunable band gap energy (approximately 2.4 eV). Among various metal sulfide photocatalysts,ZnIn2S4 has gained considerable attention. However, intrinsic ZnIn2S4 only exhibits a relatively moderate photocatalytic activity, which is mainly owing to the high recombination and low migration rate of photocarriers. Loading cocatalysts onto semiconductor photocatalysts is an effective way to improve the performance of photocatalysts, because it can not only facilitate the separation of electron-hole pairs, but also reduce the activation energy for proton reduction. As a ternary transition metal sulfide, NiCo2S4 features a high electrical conductivity, low electronegativity, excellent redox properties, and outstanding electrocatalytic activity. Such favorable characteristics suggest that NiCo2S4 can expedite charge separation and transfer, thereby promoting photocatalytic H2 production by serving as a cocatalyst. Moreover, both NiCo2S4 and ZnIn2S4 possess the ternary spinel crystal structure, which may facilitate the construction of NiCo2S4/ZnIn2S4 hybrids with tight interfacial contact for an enhanced photocatalytic performance. Herein, ultrathin ZnIn2S4 nanosheets were grown in situ on a non-noble-metal cocatalyst, namely NiCo2S4 hollow spheres, to form hierarchical NiCo2S4@ZnIn2S4 hollow heterostructured photocatalysts with an intimately coupled interface and strong visible light absorption extending to ca. 583 nm. The optimized NiCo2S4@ZnIn2S4 hybrid with a NiCo2S4 content of ca. 3.1% exhibited a high hydrogen evolution rate of 78 μmol·h-1, which was approximately 9 times higher than that of bare ZnIn2S4 and 3 times higher than that of 1%(w, mass fraction) Pt/ZnIn2S4. Additionally, the hybrid photocatalysts displayed good stability in the reaction.Photoluminescence and electrochemical analysis results indicated that NiCo2S4 hollow spheres served as an efficient cocatalyst for facilitating the separation and transport of light-induced charge carriers as well as reducing the hydrogen evolution reaction barrier. Finally, a possible reaction mechanism for the photocatalytic hydrogen evolution was proposed.In the NiCo2S4@ZnIn2S4 composite photocatalyst, the NiCo2S4 cocatalyst with high electrical conductivity favorably accepts the photoinduced electrons transferred from ZnIn2S4 and then employs the electrons to reduce protons for H2 production on the reactive sites. Concurrently, the photogenerated holes are trapped by TEOA that acts as a hole scavenger to accomplish the photoredox cycle. This study provides guidance for the fabrication of hierarchical hollow heterostructures based on nanosheet semiconductor subunits as remarkable photocatalysts for hydrogen production.

Key Words: Photocatalysis; H2 production; Cocatalyst; Metal sulfides

1 Introduction

Due to the growing consumption of fossil fuels and consequent energy crisis and climate changes, considerable efforts have been exerted to seek alternatives for renewable energy. Hydrogen (H2) has been regarded as environmentfriendly and clean energy, which has motivated the exploration of various H2-producing technologies during the past decades1-5.Photocatalytic H2evolution with semiconductor materials by virtue of abundant sunlight has become a promising strategy for renewable energy production without the release of hazardous substances6-10. However, the traditional semiconductor photocatalysts, such as TiO2and ZnO, suffer from low sunlight utilization efficiency owing to the wide band gap, which seriously hampers their practical application11,12. Alternatively,photocatalysts with suitable band gap energies capable of harvesting sufficient visible-light have been intensively explored with some candidates showing great potential from the view point of solar energy utilization13-19.

As a two-dimensional binary metal sulfide semiconductor with an appropriate band gap of ～2.4 eV, ZnIn2S4has been applied to catalyze various photoredox reactions, especially for H2production20-24. Nevertheless, pure ZnIn2S4exhibits a relatively moderate photocatalytic activity, which is mainly due to the inefficient separation and transfer efficiency of photoinduced charges and the scarcity of abundant reactive sites for H2evolution. As such, this has motivated the development of strategies (e.g., energy band engineering, elemental doping,cocatalyst decoration,etc.) to modify ZnIn2S4for achieving enhanced H2evolution performance25-29.

Loading cocatalysts over semiconductor photocatalysts can not only facilitate the separation of electron-hole pairs, but also reduce the activation energy for proton reduction30-33. Noblemetal cocatalysts (e.g., Pt) have been proved to be highly efficient for promoting H2production, but their practical applications are severely restricted by their scarcity and high price34. Therefore, the exploitation of cost-affordable cocatalysts for H2evolution is highly desirable. Several attractive non-noble-metal cocatalysts (e.g., WS2, NiS, and carbon) have recently exploited and applied to photocatalytic H2production reaction35. Being a ternary transition metal sulfide,NiCo2S4features high electrical conductivity, low electronegativity, rich redox properties, and outstanding electrocatalytic activity36-38. Such favorable characteristics suggest that NiCo2S4can serve as a good electron storage medium to expedite charge separation and transfer, endowing the promotional effect as a cocatalyst for photocatalytic H2production39. Moreover, both NiCo2S4and ZnIn2S4possess the ternary spinel crystal structure, which may facilitate the construction of NiCo2S4/ZnIn2S4hybrids with tight interfacial contact for an enhanced photocatalytic performance.

Also, it is of great importance to design suitable architectures for hybrid photocatalysts with specified composition to advance their performance40. In comparison to conventional bulk materials, hollow structured materials hold intrinsic advantages for heterogeneous photocatalysis, such as shortened charge transfer distance to facilitate separation of electron-hole pairs41,42,profuse reactive sites on both surfaces of the shell to promote surface-dependent reactions, and strong light scattering/reflection in the cavity to strength photon utilization43-45. Meanwhile,ultrathin nanosheet structure can benefit charge carrier separation by reducing the diffusion length from bulk to surface as well as expose plentiful active sites for redox reaction46,47. In addition, the delicate assembly of nanosheet substructures on hollow scaffolds to establish hierarchical hollow heterostructures may combine and reinforce the structural merits of the two functional materials, which is desirable to achieve highefficiency photocatalysis, particularly when strong heterojunctions are formed to push the directed migration of photogenerated electrons and holes by the built-in electric field48,49.

On the basis of the above-mentioned concerns, here, ZnIn2S4nanosheets werein situgrown on the surface of NiCo2S4hollow spheres through a facile solvothermal reaction, forming the hierarchical NiCo2S4@ZnIn2S4hollow hybrids with intimately coupled interfaces. The NiCo2S4@ZnIn2S4composites were fully characterized by various physicochemical techniques,including powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM),energy-dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy(DRS) and N2-sorption. The optimized NiCo2S4@ZnIn2S4hybrid exhibited a high H2-evolution rate of 78 μmol·h-1, which was 9 times higher than that of pristine ZnIn2S4counterpart. The characterization results illustrated that NiCo2S4hollow spheres not only served as an active cocatalyst for proton reduction reaction, but also facilitated the separation and transfer of photogenerated electron-hole pairs. Finally, we proposed a possible reaction mechanism for the photocatalytic H2evolution reaction.

2 Experimental

2.1 Materials

All of the reagents in the experiments were analytical grade and used without further purification. Cobalt nitrate hexahydrate Co(NO3)2·6H2O (99.999%), Indium chloride InCl3(99.999%),zinc chloride ZnCl2(99.999%), nickel nitrate hexahydrate Ni(NO3)2·6H2O (99.999%) were purchased from Aladdin.N,NDimethylformamide (DMF), isopropyl alcohol, thiourea,ethanol, glycerol and thioacetamide (TAA) were bought from China Sinopharm Chemical Reagent Co. Ltd. (Guarantee Reagent).

2.2 Synthesis of materials

The NiCo-glycerate solid spheres were synthesized according to a previously reported method50. In a typical synthesis,Ni(NO3)2·6H2O (0.125 mmol), Co(NO3)2·6H2O (0.25 mmol),and glycerol (8 mL) were dissolved into isopropyl alcohol (40 mL), and stirred for 30 min to get a transparent pink solution.Then the pink solution was transferred into a Teflon-lined stainless-steel autoclave, and kept at 180 °C for 6 h. After the temperature was naturally cooled to room temperature, the brown precipitate was collected and washed with ethanol several times, and dried at 60 °C overnight.

The NiCo2S4hollow spheres were fabricated by anion exchange. 30 mg of the obtained NiCo-glycerate was added into 20 mL of ethanol solution containing 50 mg of thioacetamide(TAA), and stirred for 30 min. Then, the resultant mixture was transferred into a Teflon-lined autoclave (50 mL in capacity)preheated at 200 °C and maintained for 6 h. After the reaction,the black precipitate was washed with ethanol for several times.An annealing treatment (300 °C, 30 min) in a nitrogen flow was conducted to improve the crystallinity of the NiCo2S4hollow spheres.

The NiCo2S4@ZnIn2S4composites were synthesized by a low temperature hydrothermal method51. In a typical procedure, 5 mg NiCo2S4sample was placed in the flask, followed by 34 mL deionized water, 4 mL dilute hydrochloric acid (pH = 2.5) and 2 mL glycerol. Then the solution was ultrasonicated for 30 min after introducing 54.4 mg Zinc chloride, 177 mg Indium chloride and 120 mg thioacetamide into the above solution under vigorous stirring. Then the mixture was put into an oil bath at 80 °C for 6 h and NiCo2S4@ZnIn2S4core-shell structures were obtained by washing the precipitate with ethanol several times and drying at 60 °C. The obtained sample was denoted as 5-NiCo2S4@ZnIn2S4. Likewise, several other X-NiCo2S4@ZnIn2S4samples with varied NiCo2S4contents were also prepared, where X represents the added amount of NiCo2S4during the synthesis.Without specific denotation, the samples used for characterizations and photocatalytic evaluations were the 5-NiCo2S4@ZnIn2S4. The preparation of nanoparticulate NiCo2S4@ZnIn2S4sample was similar to 5-NiCo2S4@ZnIn2S4,but with the same amount of NiCo2S4nanoparticles to replace NiCo2S4hollow spheres.

2.3 Photocatalytic H2 evolution

The photocatalytic H2evolution reactions were performed using an on-line photo-catalytic analysis system (Labsolar-ⅢAG, Beijing Perfectlight) at 10 °C. Typically, 20 mg green powder of the NiCo2S4@ZnIn2S4material was placed in the hydrogen evolution reactor, followed by 90 mL deionized water and 10 mL triethanolamine (TEOA). During the whole photocatalytic reactions, a Xe lamp (300 W) with a cutoff filter(λ＞ 420 nm) was used as the light source. Every specific period later, an online gas chromatograph (GC-8A) equipped with a thermal conductivity detector (TCD) was used to analyze the amount of generated hydrogen.

3 Results and discussion

Firstly, uniform NiCo-glycerate spheres were synthesized as a precursor according to the reported recipe (Fig. S1). Powder X-ray diffraction (XRD) and energy-dispersive X-ray (EDX)measurements were carried out to confirm the formation of the NiCo-glycerate precursor (Fig. S2). As can be seen in the XRD pattern, the peak at 12° can be indexed to the metal alkoxides.The obtained NiCo-glycerate precursor was then transformed into NiCo2S4hollow spheres through a liquid phase sulfidation reaction. The use of NiCo-glycerate as the precursor will be available to facilely tune the chemical composition of the final NiCo2S4product, and more importantly, to generate a welldefined hollow structure for the NiCo2S4materialviaan ionexchange reaction. Specifically, during the sulfidation reaction,the sulfide ions released from the decomposition of thioacetamide, and then reacted with the cobalt and nickel ions on the surface of NiCo-glycerate solid spheres to form the outermost layer of NiCo2S4sample. The inward diffusing sulfide ions continue to react with outward diffusing metal ions to promote the generation of the NiCo2S4hollow spheres. The FESEM images revealed that the pristine ZnIn2S4presents an aggregate structure assembled form nanosheet randomly (Fig.1a,b), while the NiCo2S4material is composed of uniform hollow spheres with an average size of about 500 nm (Fig. 1c,d),and the hollow structure of NiCo2S4is verified by transmission electron microscopy (TEM) images (Fig. 1e,f). No impurity peaks are found in the XRD spectrum of the NiCo2S4sample(Fig. S3), suggesting the successful fabrication of high-purity NiCo2S4hollow spheres52.

Fig. 1 SEM images of (a, b) nanosheet-assembled ZnIn2S4 particles and (c, d) NiCo2S4 hollow spheres. (e, f) TEM images of NiCo2S4 hollow spheres.

Next, ultrathin ZnIn2S4nanosheets were grown on the surface of the as-prepared NiCo2S4hollow spheres to form hierarchical NiCo2S4@ZnIn2S4heterostructured photocatalyst. As illustrated in the XRD patterns (Fig. 2a), the diffraction peaks located at 21.6°, 27.7° and 47.2° are assignable to the (006), (102) and(110) crystal planes of hexagonal ZnIn2S4(JCPDS card No. 65-2023), while the diffraction peaks at 32.5°, 38.1° and 56.11° are indexed to the (311), (400) and (440) crystal planes of cubic NiCo2S4(JCPDS card No. 43-1477), respectively. Meanwhile,EDX test is used to check chemical composition of the NiCo2S4@ZnIn2S4, and only S, Co, Ni, Zn and In elements are observed in the spectrum (Fig. 2b). The mass ratio of NiCo2S4in the sample isca.3.1%, approaching to the theoretical value of 3.0%. The results indicate the successful preparation of NiCo2S4@ZnIn2S4hybrids.

The magnified FESEM images reveal that the ZnIn2S4nanosheets were uniformly anchored on the surface of NiCo2S4hollow spheres (Fig. 3a,b), thus constructing a hierarchical coreshell hollow structure for the NiCo2S4@ZnIn2S4material. The structural features of the hierarchical NiCo2S4@ZnIn2S4hollow spheres are further confirmed by TEM tests (Fig. 3c,d). Fig. 3e shows the HRTEM image of NiCo2S4@ZnIn2S4, in which the lattice fringes of 0.283 and 0.321 nm correspond to (311) crystal planes of cubic NiCo2S4and (102) crystal planes of hexagonal ZnIn2S4, respectively. In the selected area electron diffraction(SAED) pattern (Fig. 3f), two sets of diffraction fringes are observed visibly, ascribing to (440) crystal planes of NiCo2S4and (110), (102) crystal planes of ZnIn2S4, respectively. In the elemental mapping images (Fig. 3g), Co and Ni elements are homogeneously distributed on the surface of NiCo2S4hollow spheres, and Zn, In, S elements are well-dispersed on the ZnIn2S4nanosheets that are covered on the surface of NiCo2S4hollow spheres. This finding reflects the formation of cross linkages between the NiCo2S4core and ZnIn2S4shell. In addition, the solid NiCo2S4@ZnIn2S4nanoparticles (NPs) are also synthesized under the similar conditions for comparison (Fig.S4).

Fig. 2 (a) XRD patterns of NiCo2S4, ZnIn2S4, and NiCo2S4@ZnIn2S4 samples, (b) EDX spectrum of 5-NiCo2S4@ZnIn2S4 sample.

Fig. 3 (a, b) SEM images, (c, d) TEM images, (e) HRTEM image,(f) SAED pattern, (g) elemental mappings of NiCo2S4@ZnIn2S4 sample.

Nitrogen-sorption tests are carried out to investigate surface properties and porous characteristics of the NiCo2S4@ZnIn2S4hybrids. The results reveal that this material presents the typical type IV isotherms with a H3 hysteresis loop for N2sorption,pointing to the mesoporous feature of the material (Fig. S5).Obviously, compared with ZnIn2S4(83.6 m2·g-1) and NiCo2S4(22.8 m2·g-1), NiCo2S4@ZnIn2S4sample displays a greatly increased BET surface area of 108 m2·g-1. The enlarged surface area stemmed from the unique hierarchical hollow structure with ultrathin nanosheet subunits. The porous feature and high surface area of the NiCo2S4@ZnIn2S4composite are considered to be beneficial to provide rich catalytically active sites for heterogeneous photocatalysis53,54.

To study the chemical states of elements in the NiCo2S4@ZnIn2S4sample, X-ray photoelectron spectroscopy(XPS) measurements are conducted. Fig. 4a shows that the sample consists of Zn, In, Ni, Co, S elements. The presence of C and O element is mainly resulted from the adventitious hydrocarbon and absorbed oxygen in the air, which is inevitable for XPS tests. The high resolution XPS spectrum of S 2pof NiCo2S4@ZnIn2S4shows two peaks located at 161.85 and 163.05 eV (Fig. 4b), corresponding to S 2p3/2and S 2p1/2of S2-,respectively. The peaks of NiCo2S4@ZnIn2S4shift towards higher binding energy by 0.34 and 0.37 eV compared to that of ZnIn2S4, respectively. For NiCo2S4@ZnIn2S4, the peaks located at 445.3 and 452.85 eV are indexed to In 3d5/2and In 3d3/2of In3+(Fig. 4c), while two peaks of Zn 2pat 1022.45 and 1045.53 eV are assignable to Zn 2p3/2and Zn 2p1/2spin-orbital splitting photoelectrons, indicating the existence of Zn2+(Fig. 4d).Moreover, when ZnIn2S4nanosheets are loaded onto surface of NiCo2S4, the peaks in In 3dand Zn 2pcores shift towards higher binding energy by 0.44 and 0.5 eV, respectively. The shift of binding energy may indicate the existence of a robust electronic interaction between NiCo2S4and ZnIn2S4materials55. The highresolution XPS spectra of Co 2pand Ni 2pcan be well divided into two spin-orbit doublets and two shakeup satellites (denoted as ‘‘Sat.’’). In Fig. 4e, two peaks of NiCo2S4@ZnIn2S4located at 853.13 and 870.66 eV belong to Ni 2p3/2and Ni 2p1/2,respectively, which point to the existence of both Ni2+and Ni3+.Compared to Ni 2pof NiCo2S4, lower binding energies of 0.04 and 0.06 eV are found35. For the high-resolution XPS spectrum of Co 2pin NiCo2S4@ZnIn2S4(Fig. 4f), two strong peaks at 778.38 and 793.28 eV match well with the Co 2p3/2and Co 2p1/2of the NiCo2S4sample, which suggests the coexistence of Co3+and Co2+in the material. Likewise, the peaks in Co 2pof NiCo2S4@ZnIn2S4shift towards lower binding energy by 0.34 and 0.05 eV than that of NiCo2S4in Fig. 4f. The binding energies of S, Co, Ni of bare NiCo2S4differ from that of the hybrid, which further confirms the existence of the strong interaction between NiCo2S4and ZnIn2S4in NiCo2S4@ZnIn2S4nanocomposite.

Fig. 4 (a) XPS survey spectrum and high-resolution XPS spectra of (b) S 2p (c) In 3d, (d) Zn 2p, (e) Ni 2p, (f) Co 2p XPS spectra of NiCo2S4@ZnIn2S4.

Fig. 5 (a) DRS spectra of NiCo2S4, ZnIn2S4 and NiCo2S4@ZnIn2S4, (b) Tauc plots of NiCo2S4 and ZnIn2S4. UPS spectra of (c) NiCo2S4 and(d) ZnIn2S4. The inset shows the onset values for the valence band.

The UV-Vis diffuse reflectance spectroscopy (DRS)measurements were used to examine the optical absorption of samples. As show in Fig. 5a, NiCo2S4has intense optical harvesting from ultraviolet to visible region, while ZnIn2S4affords a visible light absorption with the edge atca.564 nm.After growing ZnIn2S4nanosheets on hollow NiCo2S4spheres,the NiCo2S4@ZnIn2S4composite exhibits a strong visible light absorption extending toca.583 nm. The wider spectrum response of NiCo2S4@ZnIn2S4hybrid is consistent with the change of its color from bright yellow to green. In addition, two band gap energies are obtained according to the Tauc plot (Fig.5b). To figure out bandgap structures of the materials, ultraviolet photoelectron spectra (UPS) technique was employed (Fig.5c,d)56,57. Thus, the valence band maximum (EVB,vs. NHE) is estimated to be 0.63 and 1.42 V for NiCo2S4and ZnIn2S4,respectively. According toEg=EVB-ECB, the conduction band(CB) positions of NiCo2S4and ZnIn2S4are calculated to be at around -0.63 and -0.9 V (vs. NHE), respectively, which endows NiCo2S4@ZnIn2S4material with a proper redox ability for photocatalytic H2generation.

As shown in the Fig. 6a, the ZnIn2S4sample shows a low H2-genrating rate ofca.8 μmol·h-1, while the bare NiCo2S4is completely inactive for the H2evolution reaction. The inactivity of pure NiCo2S4may be due to the fact that its valence band position is not sufficient to drive the oxidation half-reaction58.In comparison, all the NiCo2S4@ZnIn2S4composite samples show obviously enhanced photocatalytic performance, and a maximum H2evolution rate of 78 μmol·h-1is achieved for the 5-NiCo2S4@ZnIn2S4sample, which is about 9 times higher than that of pure ZnIn2S4and 3 times higher than that of 1% (w)Pt/ZnIn2S4catalyst (Fig. 6b). The 5-NiCo2S4@ZnIn2S4photocatalyst is determined to afford an apparent quantum efficiency of 0.68% under monochromatic light irradiation of 400 nm. This value is comparable to the values of many reported works (Table S1). NiCo2S4@ZnIn2S4NPs only delivers an inferior H2evolution rate of 41.2 μmol·h-1, much smaller than that of the NiCo2S4@ZnIn2S4hybrid, which demonstrates the advantage of the hierarchical hollow heterostructure for the photocatalytic reaction. Fig. 6b shows the relationship between the amount of produced H2and reaction time over the different samples. Both NiCo2S4and ZnIn2S4samples alone have poor photocatalytic performance. Also, their physical mixture behaves only moderate activity toward the H2evolution reaction.Interestingly, 274.4 μmol of H2is produced after photoreaction of 5 h for NiCo2S4@ZnIn2S4samples. This underlines the advantage of the intimate contact between ZnIn2S4nanosheets and NiCo2S4hollow sphere achieved byin situsolution processed surface growth method. Fig. 6c shows that the H2generation under different wavelengths is consistent with the optical absorption spectrum of NiCo2S4@ZnIn2S4, indicating that the photocatalytic H2evolution is initiated by photoexcitation of the hybrid. Stability of photocatalytic H2production over NiCo2S4@ZnIn2S4sample is investigated (Fig.6d). Only a minor decline in H2production is observed after the successive tests for four runs, highlighting the robustness of the NiCo2S4@ZnIn2S4photocatalyst in the photoredox system.During the stability tests, the accumulated H2production is about 652 μmol, corresponding to catalytic turnover number (TON) of about 362 for the NiCo2S4@ZnIn2S4photocatalyst with respect to the NiCo2S4cocatalyst. The slight activity loss may be induced by the photocorrosion effect, which is typically encountered in metal sulfide photocatalysts. Moreover, XRD analysis was further conducted to examine the structure of the used sample after photocatalytic reactions (Fig. S6). No noticeable change is discerned in the XRD patterns of the fresh and used samples, further indicating the structural and chemical stability of the NiCo2S4@ZnIn2S4material.

Fig. 6 (a) Evolution of H2 over different samples, (b) Time-yield plots of over various catalysts, (c) H2 evolution of NiCo2S4@ZnIn2S4 under light irradiation of different wavelengths, (d) Stability tests of NiCo2S4@ZnIn2S4.

The steady-state and time-resolved photoluminescence spectra are used to investigate the light-induced charge carrier behavior of NiCo2S4@ZnIn2S4and ZnIn2S4. An evident lower peak centered at about 520 nm can be found for NiCo2S4@ZnIn2S4in the steady-state photoluminescence (PL)spectra (Fig. 7a), indicating that NiCo2S4as a supporter significantly quenches PL and declines recombination of photoinduced charges59-62. In Fig. 7b, the average PL lifetime of NiCo2S4@ZnIn2S4(0.28 ns) is shorter than that of ZnIn2S4(0.699 ns), suggesting that the charge carriers of ZnIn2S4quickly separate and migrate to NiCo2S4in the hybrid. Afterwards,electrochemical impedance spectroscopy (EIS) is used to investigate the charge transport regularity of ZnIn2S4and NiCo2S4@ZnIn2S4under visible light. As presented in Fig. 7c,the NiCo2S4@ZnIn2S4hybrid shows a smaller radius of Nyquist plot compared to that of pure ZnIn2S4, indicative of a lower charge transfer resistance63,64. Correspondingly, as indicated by the transient photocurrent spectra (Fig. 7d), the NiCo2S4@ZnIn2S4composite generates a greatly improved photocurrent than the bare ZnIn2S4counterpart. The cyclic voltammetry curves (CV) of pristine NiCo2S4, bare ZnIn2S4, and NiCo2S4@ZnIn2S4composites were collected. As depicted in Fig. 7e, the area of pristine NiCo2S4is extremely larger than others, representing the highest specific capacitance in the three materials, in line with the results of previous reports65.Compared to the nanosheet-constructed ZnIn2S4aggregate, the NiCo2S4@ZnIn2S4hybrid gives an increased specific capacitance. By combining with results of PL tests, this finding reveals that NiCo2S4can serve as an electron storage medium to suppress the recombination of electron-hole pairs66,67. In the linear sweep voltammetry (LSV) curves (Fig. 7f), a much lower overpotential is observed over the NiCo2S4@ZnIn2S4sample in relative to pristine ZnIn2S4, suggesting the lower barrier for H2production over the hybrid. All these results highlight that the NiCo2S4hollow spheres act as an effective cocatalyst in the

Fig. 7 (a) Steady-state PL spectra and (b) TPRL decay, (c) EIS spectra, (d) transient photocurrent spectra, (e) CV curves of different samples and(f) LSV curves of the bare NiCo2S4 and pristine ZnIn2S4.

Scheme 1 The possible mechanism for the photocatalytic hydrogen production over NiCo2S4@ZnIn2S4nanocomposite.photocatalytic H2evolution reaction.

A possible mechanism of the photocatalytic H2evolution over hierarchical NiCo2S4@ZnIn2S4hollow heterostructure is then proposed. As shown in Scheme 1, being illuminated with visible light, the electrons and holes are generated in the valence band(VB) and conduction band (CB) of ZnIn2S4, respectively. Since NiCo2S4has lower CB edge potential than ZnIn2S4, the photogenerated electrons will be thermodynamically promoted to transfer from ZnIn2S4to NiCo2S4. Besides, benefiting from the high electrical conductivity, NiCo2S4is favorable to accept the electrons transferred from ZnIn2S4and then employs the electrons to reduce the protons for H2production on the reactive sites. On the other hand, the holes are trapped by TEOA that acts as a hole scavenger to accomplish the photoredox cycle.

4 Conclusions

In summary, ultrathin ZnIn2S4nanosheets were successfully grown on the surface of NiCo2S4hollow spheres by a one-step solvothermal reaction to form hierarchical NiCo2S4@ZnIn2S4hollow heterostructures. The preparation method is advantageous to tune the structure and composition of the hybrids with a strongly coupled heterogeneous interface for fast charge transfers. The physicochemical and photo/electrochemical studies indicate that the NiCo2S4hollow spheres act as an effective cocatalyst for the hybrids, promoting the separation and transport of light-induced charge carriers as well as enabling the reduction of activation energy of H2evolution. As a result, the hierarchical NiCo2S4@ZnIn2S4hollow heterostructure with the optimized composition manifests enhanced photocatalytic activity and high stability for H2evolution with visible light. This work may arouse broad interest in the design of nanosheet-constructed hierarchical hybrids for solar-driven chemical fuel production by artificial photosynthesis.

Supporting Information:available free of chargeviathe internet at http://www.whxb.pku.edu.cn.