Ultrathin Defective Nanosheet Subunit ZnIn2S4 Hollow Nanoflowers for Efficient Photocatalytic Hydrogen Evolution

Constructing hierarchical and ultrathin‐structured metal sulfides is beneficial for achieving high‐efficiency hydrogen evolution catalysts. Herein, ZnIn2S4 (ZIS) hollow nanoflowers (HNFs) composed of ultrathin nanosheets are creatively synthesized via a facile trisodium citrate‐mediated and stirring‐assisted solvothermal method. Experimental results reveal that the synergy effect of ethanol, trisodium citrate, and continuous stirring during solvothermal synthesis process play a significant role in optimizing microstructure as well as physicochemical properties of as‐prepared ZIS samples. Importantly, the fabricated ZIS HNFs with the thinnest nanosheets (2.28 nm) manifest the highest average photocatalytic hydrogen generation rate of 301.5 μmol h−1, which is 2.3 times higher than that of the pristine ZIS microspheres composed of nanoparticles with Pt as the cocatalyst and triethanolamine (TEOA) as the sacrificial agent and outperforms most reported ZnIn2S4‐based materials under similar testing conditions. Moreover, the optimized sample also shows a hydrogen generation rate of 0.53 μmol h−1.in pure water without any cocatalyst. This controllable agitation of the reaction mixture during the hydro/solvothermal synthesis process offers an eco‐friendly and scalable approach for tuning the microstructure of nanomaterials with enhanced performance for various applications.


Introduction
The rapid industrial development and continuous growing population have led to the excessive consumption of fossil fuels.Energy shortage and environmental pollution issues are becoming more and more severe. [1]As a clean, renewable, storable, and transportable energy source, hydrogen (H 2 ) has been intensely studied as a possible substitute for traditional fossil feedstocks.However, due to the high cost, low efficiency, and high energy consumption of current production routes, the large-scale application of H 2 is constrained.Photocatalytic splitting of water into H 2 directly driven by solar energy has been considered one of the most green, sustainable, and promising techniques for H 2 production. [2]Up to date, the overall photoconversion efficiency is still very low, far from meeting the requirements of commercialization.The efficiency of photocatalytic water splitting largely depends on the selection of appropriate photocatalytic materials.Therefore, to achieve efficient photochemical conversion of solar energy, designing and developing of superior photocatalysts with broad spectral response and high charge separation efficiency are preconditions. [3]ver the past decade, ZnIn 2 S 4 (ZIS) is considered to be a promising photocatalyst candidate because of its unique layered structure, easy availability, low cost, good physicochemical stability, and durability, prominent activity, and particularly appropriate bandgap (%2.42 eV) corresponding to the visible-light absorption. [4]These distinctive textural properties endow ZIS with great potential in photocatalytic applications, such as contaminate remediation, water splitting, CO 2 conversion, and selective organic transformations. [5,6]However, its applications are still limited due to the fast recombination, short lifetime of charge carriers, and poor light absorption ability.Therefore, to overcome these barriers, various strategies have been proposed to improve the photocatalytic performance of ZIS, such as impurity doping, [7] metal loading, [8][9][10][11] surface modification, [12] defect engineering, [13] morphology and structure regulation, [5] and constructing heterojunctions. [14]Nevertheless, there remains great space for further optimizing the performance of ZIS photocatalyst by facilitating the mass transfer and enhancing the separation of charge carriers.
Due to unique physical as well as chemical properties, ultrathin two-dimensional (2D) nanomaterials have attracted considerable attention. [15,16]As previously reported, the ultrathin 2D nanosheet structure of ZIS not only has a large specific surface area but also shortens the migration path of charge carriers to plentiful exposed active sites. [13,17]However, the self-assembly and excessive aggregation of 2D ultrathin ZIS nanosheets greatly reduce the number of active sites and hinders the separation of photoexcited carrier, severely lowering their photocatalytic activities. [18]egarding of this, the introduction of hollow structure can greatly suppress the aggregation of subunits nanosheets and enhance the utilization of solar energy and harvest capability via internal multilight reflection and scattering owing to the presence of large inside cavities. [4,20]Moreover, hollow structures can not only endow a large surface area, superior mass transfer properties, and a considerably higher number of active sites but also decrease the transfer distance of charge carriers owing to the thin-shelled configuration of hollow scaffolds to boost photocatalytic performance. [21]21] Generally, for the preparation of hollow structured materials, the hard template method is a conventional strategy and has been widely used.However, template removal requires special handling, such as acid or alkaline solution etching or hightemperature annealing, which is time-consuming and uneconomical, and may limit the further application of certain materials that are less stable under these conditions. [22]Therefore, finding suitable strategies and understanding their formation are key factors for the synthesis of hollow-structured ZIS.Trisodium citrate (C 6 H 5 O 7 Na 3 ) is an effective structure-directing agent, which has been widely used to regulate the structure of nanomaterials before. [23]For example, Al 2 O 3 hollow microspheres were obtained via trisodium citrate-mediated synthesis. [24]Zhou's group reported trisodium citrate was used to control the morphology of ZnO crystals during the hydrothermal process. [25]Li et al. demonstrated a citrateassisted solution approach for the shape-selective synthesis of Bi 2 O 3 nanostructures with controllable morphology and bandgap. [26]n contrast, introducing sulfur vacancies (V s ) into the lattices of metal sulfides has been turned out to be an effective strategy to tune their electronic as well as band structures. [13]The V s can introduce an intermediate energy level to increase the charge carrier density and trap electrons to facilitate the separation of photogenerated carriers, thereby enhancing the activity. [9,13]Thus, a lot of synthetic approaches have been established to generate sulfur vacancies in ZIS, and it has been reported that solvothermal method by using ethanol as the cosolvent is a simple and effective way to introduce V s into ZIS. [13,15]rthermore, optimization of synthetic strategy has been proved to be an efficient and versatile way to improve material properties.The hydrothermal method has been widely applied for synthesis of ZIS. [27]However, to meet the growing demand for ZIS, a deeper insight into the role of synthesis parameters on product formation is required.In previous work, it was demonstrated that stirring the reaction mixture during the hydrothermal synthesis affects the structural characteristics of the formed material.For instance, Zaghib et al. reported the synthesis of nano C-LiFePO 4 with enhanced electrochemical properties for lithium batteries via stirring in the hydrothermal process. [28]hen proposed the stirring-assisted hydrothermal synthesis of ultralong α-MnO 2 nanowires, which exhibited higher catalytic activity and stability for oxygen reduction reaction compared with commercial MnO 2 powders. [29]Shi et al. presented the in-situ stirring-assisted hydrothermal fabrication of W-doped VO 2 (M) nanorods with improved doping efficiency and mid-infrared switching properties. [30]Therefore, reactors enabling stirring during material synthesis are generally preferred, as agitation helps to obtain homogeneous mixing, which allows for better control of a reproducible final product. [28]To the best of our knowledge, in most previously reported ZIS fabrication work, the precursor mixture in the autoclave was not agitated.
Here, a simple solvothermal route was established for the synthesis of ZIS hollow nanoflowers composed of ultrathin nanosheet subunits in ethanol-H 2 O binary solution, where trisodium citrate acts as the structure-directing agent.A Teflon autoclave and a high-temperature resistant magnetic agitator were put into the oven as the solvothermal reactor to synthesize ZIS, as shown in Figure 1.Aiming to optimize the speed of stirring, the mother solution of the precursor of ZIS was stirred with a rotation rate varying from 300 to 1200 rpm during the synthesis process.The influence of trisodium citrate, ethanol, and stirring rate on the phase composition, morphology, microstructure, and physicochemical properties of resulting products was systematically investigated by various comprehensive characterizations, and the energy band of layered-structured ZIS was verified via simulated density functional theory (DFT) calculation.In accordance with the experimental results, a possible formation mechanism of nanosheet and hollow structures was proposed, and the stirring speed of the evolution of ZIS hierarchical architecture was also deduced.Moreover, the structural effect-enhanced photocatalytic activities of ZIS samples were compared through photocatalytic hydrogen evolution.Finally, the migration and separation of photogenerated charges in ZIS samples before and after adjusting structure were measured by photoelectrical characterizations.

Characterization of ZIS Samples
The effects of trisodium citrate and stirring on the morphology and microstructure of ZIS samples were investigated by SEM and TEM and the thicknesses of single ZIS nanosheets were measured by AFM.Specifically, solid microspheres (MSs), hollow microflowers (HMFs), hollow nanoflowers (HNFs), and nanoflowers (NFs) can be readily obtained by the simple addition of the surfactant and varying stirring rate.In the absence of trisodium citrate and without stirring, the as-prepared ZIS are solid microspheres with an average diameter of 2.6 μm, which were composed of small and irregular nanoparticles (Figure 2a-c).With the addition of trisodium citrate, the sample was converted to well-defined hollow microspheres with a size of 1.6 μm, and its shells were composed of loosely connected nanoflakes with a thickness of 4.77 nm (Figure 2d-f ).When the reaction mixture was additionally stirred at a low rate (300 rpm), smaller-sized 350 nm hollow spheres assembled from numerous interconnected ultrathin nanosheets with a thickness of about 2.28 nm were obtained (Figure 3a,e, 5a-c, S1a, and S3a, Supporting Information), implying that this sample has one-unit-cell structure based on with a thickness of 2.468 nm along the c axis. [13,31,32]As the stirring rate was 600 rpm, the hierarchical nanospheres still exhibited distinct hollow structures, while the nanosheet subunits of these hierarchical nanospheres became thicker and larger.The sheet thickness increased to 3.23 nm and the size of the microsphere to about 350 nm (Figure 3b and S4c-d, Supporting Information).At a further increase of stirring rate (900 rpm), fewer assembled hollow structures were formed, besides smaller-sized structures, but the thickness of the single nanosheets was increased (Figure 3c).When the stirring rate was further raised (1200 rpm), the morphology was like the sample ZIS-NFs-900, and again formed a lot of small structures with tiny and relatively thick connected nanosheets (Figure 3a).
Based on all the experimental results, it can be concluded that the addition of trisodium citrate is the key parameter for the formation of nanosheets and of the hollow structure.The stirring rate controls the thickness of nanosheets as well as the whole structure of ZIS nanoflowers.The morphology evolution of the solid microspheres, hollow microflowers, hollow nanoflowers, and nanoflowers is schematically displayed in Figure 4.
Representative HRTEM images of ZIS-HNFs-300 also illustrate the layered crystal lattice structure (Figure 5a,b), and lattice fringes of 0.165 nm correspond to the interlayer distance of the (022) planes of hexagonal ZnIn 2 S 4 (Figure 5e).35] STEM-energy-dispersive X-ray (EDS) mapping reveals the uniform distribution of Zn, In, and S elements (Figure 5g-i), and the relative EDX is shown in Figure S4, Supporting Information.Moreover, an in-depth investigation of the elemental composition and surface chemical states of ZIS-HNFs-300 by means of XPS was performed.As can be seen from Figure S5, Supporting Information, the peaks at 1,024.0 and 1,047.1 eV correspond to Zn 2p 3/2 and Zn 2p 1/2 , respectively, and the binding energies at 444.9 and 452.4 eV belong to the In 3d 5/2 and 3d 3/2 , respectively, and the S2p peak is split into 2p 3/2 (161.8 eV) and 2p 1/2 (163.0 eV) peaks, which are in agreement with the previous report on ZnIn 2 S 4 [30]   .The phase composition and crystallinity of the samples were surveyed by XRD.As shown in Figure 6a, all the diffraction reflections can be indexed as hexagonal ZnIn 2 S 4 (JCPDS card no.65-2023).No other reflections were observed in any of the samples, indicating the high purity of the crystalline ZIS products.Further observation implies that the width of the diffraction peaks becomes slightly wider with increasing stirring rate, implying the formation of smaller ZnIn 2 S 4 crystallites.
The Raman spectra of all samples are shown in Figure 6b.Two characteristic peaks centered at about 243 and 354 cm À1 are seen, which are typical vibration peaks that are assigned to the longitudinal optical modes LO 1 and LO 2 , [36,37] respectively.The peak located at around 563 cm À1 is attributed to the typical stretching modes of ZIS. [38]The absence of any further peak supports the above conclusion that the levels of crystalline impurities in all samples are very low.
The effects of trisodium citrate and stirring rate on the Brunauer-Emmett-Teller (BET) surface area and pore structure of ZIS samples were surveyed by nitrogen adsorption-desorption analysis.The isotherms are shown in Figure 6c, and the results are summarized in Table 1.With the addition of trisodium citrate, the BET surface area increased considerably from 1.5 to 76.7 m 2 g À1 in comparison to that without trisodium citrate.The significant increase in BET surface area is attributed to the formation of 2D nanosheets instead of irregular-formed 3D nanoparticles.The highest BET surface area (113.7 m 2 g À1 ) was obtained with the lowest stirring rate (300 rpm).This sample contains ZnIn 2 S 4 hollow nanoflowers assembled by ultrathin nanosheets.Relatively high BET surface areas were also obtained with stirring rates of 900 rpm (96.4 m 2 g À1 ) and 1200 rpm (104.9 m 2 g À1 ).Porosity seems to be affected both by the size of the cavity and the thickness of the nanosheets.The results demonstrate that textural properties like surface area and porosity can be influenced by trisodium citrate as well as agitation.UV-vis diffuse reflectance spectra (DRS) are presented in Figure 6d.Compared with pristine ZIS (ZIS-MSs), the modified samples show obvious blue shifts of light absorption in the range of 450-800 nm, probably due to the presence of sulfide defect states in their band structure and the quantum size effect. [39,40]eanwhile, the optical bandgaps (E g ) of ZIS-MSs, ZIS-HMFs, ZIS-HNFs-300, ZIS-HNFs-600, ZIS-NFs-900, and ZIS-NFs-1200 were calculated using Tauc's plot, and their corresponding values of Eg (Table 1) are estimated to be 2.44, 2.88, 2.92, 2.95, 2.81, and 2.82 eV, respectively, which is coincident with the color variations, as shown in Figure S6, Supporting Information.Moreover, the band structure and density of states of ZIS-HNFs-300 were calculated by DFT (Figure 7a,b), demonstrating that the theoretical E g value of layer-structured ZnIn 2 S 4 is close to the experimental result.

Effects of Synthesis Conditions on the Formation of ZIS Hierarchical Structures
The effects of trisodium citrate, ethanol, and stirring rate on the synthesis of ZIS hierarchical structures were investigated.Without trisodium citrate using only water as the solvent, no pure phase ZIS sample was obtained (Figure S7, Supporting Information).When only ethanol was added to the water solution, solid microspheres composed of ZIS nanoparticles were formed.Likewise, the addition of trisodium citrate in the absence of ethanol leads to the formation of bulk and larger-sized ZIS structures (Figure S8, Supporting Information).Therefore, the cooperative effect between trisodium citrate and ethanol plays a significant role in the formation of nanosheet and hollow structures (Figure 2).
Trisodium citrate has one hydroxyl group and three carboxylate groups, and it is a chelating ligand with a strong coordinating ability.To obtain insight into the role of trisodium citrate, the FTIR spectrum of the product synthesized in the presence of trisodium citrate (ZIS-HMFs) was recorded (Figure S9, Supporting Information).The broad absorption band around 3400 cm À1 corresponds to the -OH stretching vibration of adsorbed water.The absorption peaks at 1614 and 1395 cm À1 appeared in the FTIR spectra of citrate-treated magnetite nanoparticles, [41] and are characteristic of carboxylate groups.The bands at 1558 and 1416 cm À1 are attributed to the carbonyl asymmetric and symmetric stretching vibration, respectively, the band appearing at around 1092 cm À1 is assigned to the stretching vibrations of the C-OH species. [42]These results verified the complexation between the citrate groups and metal ions on the surface of ZIS nanoparticles.Therefore, it can provide coordinating sites to form stable coordination with metal ions and selectively bind to specific crystallographic surfaces of minerals to significantly alter the surface properties and crystal growth behavior. [24][45] Generally, during the crystal growth process, the facet with higher surface atomic density is easily blocked by the adsorption of surfactants, so the growth along this facet subject to considerable restrictions. [43]uring the synthesis of ZIS, it was observed that the mother solution became milky after the addition of trisodium citrate, the pH of the solution also changed from 2.68 to 4.46, and numerous visible particles were observed.These particles could not be homogeneously dispersed in the ethanol-H 2 O solution even under intense ultrasonic vibration, as shown in Figure S10, Supporting Information.This might occur due to the citrate ions being coordinated with Zn 2þ and In 3þ to form stable complex ligands.Another reason could be that the solubility of sodium citrate in an ethanol-water mixture is lower than that in pure water. [46]ue to the higher solubility of trisodium citrate in water compared to ethanol, the addition of the citrate salt to the aqueous ethanol solution led to the migration of water molecules away from the alcohol to the citrate ions which are mainly located in the aqueous phase. [46,47]This process already occurs at room temperature, resulting in the formation of a continuous phase with high water content containing the citrate anions and the adsorbed metal cations, and a dispersed phase with high ethanol content.The dispersed ethanol-rich phase will have a nearly spherical shape, since spheres have the lowest surface area at constant volume. [25]When the temperature of the reaction mixture achieved a certain level, nucleation of ZIS starts in the aqueous-rich phase.Because the continuous migration of H 2 O molecules also can coordinate metal ions.Surface sites of the formed ZIS nucleus were blocked owing to the selective adsorption of the citric acid anions on preferred crystal surfaces, which hindered the crystal growth of the nanoparticles in this direction.Therefore, the growth of the crystals on the unblocked surface sites leads to the formation of nanosheets.The ZIS sheets formed in the water-rich phase agglomerate at the interface of the water and ethanol-rich phases, which reduces their surface energy.
Moreover, the electrostatic repulsion between the carboxyl and hydroxyl groups on the surface of ZIS monocrystal makes it disperse uniformly in water. [48]The hierarchical architectures formation consists of the self-assembly of nanosheets with monocrystal growth, and many other reports about this kind of method for the synthesis of hierarchical architectures consisting of a shell composed of nanosheets and a cavity within the shell. [24,49,50]Alternatively, due to the constant migration of water molecules during this process, ethanol molecules may provide aggregation centers and serve as soft templates to form hollow structures.
Additionally, in accordance with above-mentioned results, stirring has an influence on the structures of the obtained ZIS samples, which might be due to stirring can accelerate the crystallization and growth rate of ZIS crystals as well as the mass transfer and the adsorption of citrate ions on the surface of nuclei of ZnIn 2 S 4 .This changes the thickness of nanosheets.It can also affect the external migration rate of water molecules and the size and shape of the dispersed ethanol-rich phase, thereby affecting the sizes of hollow caves of ZIS samples.When ZIS was prepared at a stirring rate of 300 rpm, the size of the dispersed structures was reduced compared to the unstirred sample due to the mechanical interaction of the dispersed phase with the stirrer.This might happen directly after starting to stir or through the interaction of the stirrer with already-formed ZIS structures.Due to the relatively low stirring rate, the spherical shape of the dispersed phase remained largely intact, and only a small part of the sphere was destroyed which led to the observed hollow structure.
Furthermore, stirring also increased the mass transfer within the reaction solution, causing the ZIS crystallization rate to increase.This led to the formation of thinner sheets in the water-rich phase, which in turn aggregated at the interface between water-rich and ethanol-rich phases.As a result of these processes, the synthesized ZIS structures exhibit a cavity and a thin solid shell around the cavity which is composed of agglomerated thin ZIS sheets.When the stirring rate was increased to 600 rpm, the higher energy input caused individual spheres to agglomerate together, resulting in larger structural units with larger cavities.Furthermore, the layer thickness of the formed sheets increases as the surface area of the dispersed phase decreases.When the stirring rate was increased to 900 or 1200 rpm, the spheres were destroyed, and irregularly sized small structures of the dispersed phase were formed directly after starting to stir.These ZIS sheets formed in the aqueous phase were comparably thicker than those formed at the low agitation rates and were again assembled around the interphase between aqueous and ethanolrich phases.Because of the higher number of the small, dispersed structures compared to lower stirring rates, the finally obtained irregularly shaped nanoflowers were generally smaller and formed with or without small cavities.The possible formation process of ZIS hierarchical structures is schematically illustrated in Figure 8.

Photocatalytic Performance
Photocatalytic hydrogen evolution is applied to study the structure-activity relationship of the as-prepared materials.The evolved gas volume upon irradiation with a 10 vol% TEOA solution is shown in Figure S11, Supporting Information, these data were used to calculate the hydrogen production profile (Figure 9a) and the hydrogen evolution rate (Figure 9b) for different samples.Compared with pristine ZIS microspheres (ZIS-MSs) composed of 3D nanoparticles, the evolved hydrogen amount of the ZIS sample synthesized by adding trisodium citrate (ZIS-HMFs) exhibited a little decline, which might be due to the narrow optical absorption range, as presented in the DRS spectrum (Figure 6d).When the reaction mixture was stirred during the solvothermal process, the average hydrogen generation rate of the synthesized samples increased (Figure 9b).The order of the hydrogen production rate was ZIS-HNFs-300 > ZIS-NFs-900% ZIS-NFs-1200 > ZIS-HNFs-600, and a maximum of 301.5 μmol h À1 was observed for ZIS-HNFs-300 (Figure 9b), outperforming the previously reported ZnIn 2 S 4 related materials under similar testing conditions, as shown in Table S1, Supporting Information.Moreover, the solar-to-hydrogen energy conversion efficiency (STH) and the apparent quantum efficiency (AQE) of ZIS-HNFs-300 were also measured.The STH was estimated to be 0.023%, and the AQE was calculated to be 6.62% at 450 nm.
The good photocatalytic HER performance of the synthesized ZIS materials can be ascribed to the following factors: 1) the 3D-hierarchical structure provides a large surface area and a high number of active sites to facilitate adsorption and photochemical

reactions. 2)
The highly open and hierarchical hollow structures with large number of open channels and mesopores not only make full use of light in view of the multiple reflections but also enlarge the contact surface between light and the catalyst.3) The 2D-ultrathin structure shortens the diffusion distance of photogenerated charge carriers to facilitate the fast separation and transfer and minimize their interior recombination during the photochemical process.4) The superior robustness of hierarchical structures ensures good stability of the photocatalyst during the long-term experiment.Among all the synthesized samples, the sample prepared at a stirring speed of 300 rpm in the presence of trisodium citrate showed the highest photocatalytic activity.ZIS-HNFs-300 has the largest BET surface area and the smallest thickness of 2D nanosheets thereby increasing the number of reaction sites as well as the number of charge carriers reaching the surface compared to the other materials.
Moreover, to further confirm the correlation between structure and activity, the performance of as-synthesized samples was compared with photocatalytic oxidation of benzyl alcohol (BA) without Pt as the cocatalyst.As illustrated in Figure S12, Supporting Information, the result also implied that ZIS-HNFs-300 exhibited the best activity among all samples, and a significant improvement of conversion of BA was observed compared with pristine ZIS (ZIS-MSs).
Furthermore, ZIS-HNFs-300 also showed H 2 generation activity even without any cocatalyst and sacrificial agent (Figure 9c,d).Its stability was investigated by performing pure water splitting over 25 h.As displayed in Figure 9d, ZIS-HNFs-300 almost maintained the hydrogen generation rate of 0.53 μmol h À1 over the whole time studied.The comparison of XRD and SEM results of ZIS-HNFs-300 before and after 25 h irradiation (Figure S13, Supporting Information) manifested only very minor changes in phase composition and morphology, which can be considered as an indication of the good photocatalytic stability of ZIS-HNFs-300.In the future, we will continue to investigate robust oxygen evolution cocatalysts and hybrids for overall water splitting.

Influence Factors of Photocatalytic Activity
Previously, it was already reported that S vacancies (V s ) can affect the photocatalytic performance of ZIS because sulfide defects can act as electron traps. [13,39]Figure S14, Supporting Information, compares the EPR spectra of both samples.The EPR signal at g = 2.004 in ZIS was attributed to V s . [13]The higher EPR signal intensity of ZIS-MSs indicated that the V s concentration in ZIS-MSs was much higher than in ZIS-HNFs-300.This difference could be attributed to trisodium citrate since its addition released more H 2 S from TAA, which was dissolved in the water-rich phase of the reaction solution to react almost stoichiometrically with Zn 2þ and In 3þ to form ZnIn 2 S 4 .Further reasons for the lower V s concentration in ZIS-HNFs-300 might be the poorer solubility of metal ions in ethanol, and the strong chelating effect of citrate ions.Due to the presence of trisodium citrate, the reaction proceeded mainly in a water-rich phase instead of a homogeneous water/ethanol phase, therefore, fewer ethanol molecules reached the ZnIn 2 S 4 crystals.The formation of V s in homogeneous water/ethanol mixtures has been described previously. [13]Another reason for the high V s concentration might be the larger thickness of the nanoparticles compared to the ultrathin films, resulting in a high V s concentration in the bulk.
Time-resolved PL (TRPL) decay measurements were carried out to survey the migration process of photoinduced charge carriers (Figure 10a).The photoluminescence spectra were fitted to a biexponential function according to the following equation And the average lifetime was calculated by using the following equation A 1 and A 2 are amplitudes, and τ 1 and τ 2 represent the fluorescent lifetime.
Table 2 summarizes the results obtained from TPRL decay curve fitting.The time constants for the decay τ 1 = 0.36 ns (A 1 = 78.7%)and τ 2 = 2.69 ns (A 2 = 21.3%) of ZIS-HNFs-300 were significantly larger than those of ZIS-MSs (τ 1 = 0.20 ns, A 1 = 87.0%,τ 2 = 1.22 ns, A 2 = 13.0%)indicating a slower decay of the PL in this material.The appearance of two relaxation processes (τ 1 and τ 2 ) could be understood according to the bandgap trap states scheme in Figure 10f.The fast decay component τ 1 (0.37, 0.20 ns) reflected the trapping of electrons from the conduction band (CB) into trap states within the bandgap, while the much slower decay component τ 2 (2.69 ns, 1.22 ns) corresponded to the recombination of trapped electrons and the holes in value band (VB). [13]By comparison, the higher V s density of ZIS-MSs brought about a %1.9-fold increase for τ 1 and 1.3 times for τ 2 .It could be assumed that the obvious changes in τ 1 were related to the V s concentration and τ 2 were associated with the structural difference. [32]ue to the presence of V s , photogenerated electrons at the bottom of the conduction band (CB) can be nonradiatively captured by V s and then perform trap-to-trap hopping for donor-acceptor recombination, instead of jumping directly to the valence band (VB) for recombination, thus the charge recombination of carriers was restrained. [13]The higher V s concentration resulted in more photoexcited electrons being captured by the V s traps, while more long-lived electrons helped to enhance the electron-hole separation efficiency, and longer lifetimes of charge represent more opportunities to participate in surface reactions [32] .Therefore, this favors higher photocatalytic activity.
ZIS-MSs have higher sulfide vacancy concentration, thus, more photoexcited electrons were captured by trap states induced by V s .While the longer-lived electron density is higher in ZIS-HNFs-300 contributed to improve the efficiency of electron-hole separation.Because of the ultrathin sheet thickness of ZIS-HNFs-300 connected with a reduced diffusion length, this can accelerate the separation and migration of photoinduced carriers from bulk to the surface, thus, reducing their fast recombination.As a result, longer-lived electrons will have time to reach the interface surface of the catalyst where they are captured by the reactant molecules, hence, ZIS-HNFs-300 has more opportunities for photocatalytic H 2 generation.
The photoelectrochemical measurements were also performed to find further reasons for the enhanced performance of ZIS-HNFs-300.Figure 10b shows the linear sweep voltammetry curves of ZIS-HNFs-300 and ZIS-MSs, revealing a higher cathodic current density of ZIS-HNFs-300 that is attributed to the reduction of water to H 2 .Moreover, the electrochemical impedance spectra (EIS) show a smaller semicircle in the Nyquist plot (Figure 10d), indicating the reduced charge transport resistance of ZIS-HNFs-300 compared to ZIS-MSs.The transient photocurrent generation of ZIS-HNFs-300 was also greatly enhanced (Figure 10c), indicating increased charge transfer in ZIS-HNFs-300.The presented results demonstrate the suppressed recombination of charge carriers and the improved charge carrier transfer in ZIS-HNFs-300, thus ensuring higher H 2 generation efficiency.Based on these results, it can be concluded that charge separation and transfer efficiency can be enhanced by the construction of ultrathin and hollow structures.From Mott-Schottky (M-S) plots (Figure 10e), these two flat bands show a 0.1 V difference, and ZIS-HNFs-300 displays a substantially smaller slope of M-S plots than that of pristine ZIS-MSs (Figure S15, Supporting Information), suggesting that charge carrier density is increased after treatment with trisodium citrate and stirring. [51]Finally, the band structures of ZIS-HNFs-300 and ZIS-MSs were estimated by M-S and DRS results.The calculations show that the CB edge of ZIS-HNFs-300 is À0.80 V and that of ZIS-MSs is À0.70 V (vs NHE, pH = 7).The high conduction band edge position indicates the sufficient reduction ability for hydrogen production.

Conclusion
In summary, a novel synthetic strategy was demonstrated to fabricate ZIS hierarchical hollow nanoflowers comprised of 2D ultrathin defective nanosheets as a superior photocatalyst for hydrogen evolution.Trisodium citrate and ethanol contribute to the formation of pure-phase ZIS as well as sheet and hollow structures.The size of the cavity and thickness of the single nanosheets for ZIS hierarchical architectures can be tailored by adjusting the stirring rate in the presence of trisodium citrate during the solvothermal process.The well-defined hollow structure and ultrathin nanosheet building blocks give rise to many appealing features, including sufficient light harvesting, larger surface area, abundant active sites, and boosted charge transport efficiency.Benefiting from the unique structural advantages, the as-prepared hierarchical ZIS hollow nanoflowers exhibited superior and  remarkably enhanced photocatalytic performance for hydrogen evolution reaction.The sample ZIS-HNFs-300 reached an average of hydrogen evolution rate of 301.5 μmol h À1 with Pt as the cocatalyst and TEOA as sacrificial agent, 2.3 times in comparison with the pristine ZIS composed of nanoparticles.Moreover, ZIS-HNFs-300 also showed good performance and stability for the hydrogen generation reaction even without cocatalyst and sacrificial agents.This facile synthetic method can be extended to design and synthesis better-performing versatile nanomaterials for multifunctional applications.

Experimental Section
Chemicals: Zinc acetate dihydrates (Zn(CH 3 COO)  O (4.0 mmol, 1.2 g) were dissolved in a mixture of 75 mL of H 2 O and 75 mL ethanol and stirred for 30 min.Then, the mixed solution was transferred into a 250 mL Teflon-lined stainlesssteel autoclave and maintained at 180 °C for 24 h without or with magnetic stirring.In different experiments, the stirring rate was varied between 300 and 1200 rpm in steps of 300 rpm each.After that time, the samples were collected by centrifugation at 8000 rpm and washed several times with water and ethanol.Finally, the photocatalysts were dried at 80 °C overnight.The obtained ZnIn 2 S 4 products with different morphologies prepared under various stirring rates were named, ZIS-HNFs-300 (hollow nanoflowers, r = 300 rpm), ZIS-HNFs-600 (hollow nanoflowers, r = 600 rpm), ZIS-NFs-900 (nanoflowers, r = 900 rpm), and ZIS-NFs-1200 (nanoflowers, r = 1200 rpm), respectively.The sample prepared without stirring was named ZIS-HMFs (hollow micro flowers).Additionally, a sample without the addition of Na 3 C 6 H 5 O 7 •2H 2 O and without stirring was also synthesized and named as ZIS-MSs (microspheres).
Further detailed characterizations and photochemical measurements can be found in the Supporting Information file.

Figure 1 .
Figure 1.Schematic diagram of synthesis procedure of using a stirring-assisted solvothermal method.

Figure 5 .
Figure 5. a,b) TEM images, c) AFM image (inserted the corresponding height profiles), d) HRTEM, e) The inverse fast Fourier transformation (IFFT) of the dotted square area of (d), f ) the corresponding line scans of the rectangular region in (e), g) HADDF, and h-j) elemental mapping of ZIS-HNFs-300.

Figure 8 .
Figure 8. Illustration of the assumed formation process of ZIS samples with different structures.

Figure 9 .
Figure 9. a) H 2 generation profile, b) comparison of average hydrogen evolution rates of different ZIS samples in the presence of TEOA with 1% Pt as the cocatalyst under white light irradiation for 3 h.c) Photocatalytic H 2 activity of ZIS-HNFs-300, and d) results of a long-term experiment under white light irradiation using ZIS-HNFs-300 in pure water without cocatalyst and sacrificial agents.

Table 1 .
Textural characteristics of as-synthesized ZIS samples.