Generalized Synthesis of Ternary Sulfide Hollow Structures with

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Generalized Synthesis of Ternary Sulfide Hollow Structures with Enhanced Photocatalytic Performance for Degradation and Hydrogen Evolution Shuoping Ding, Xiufan Liu, Yiqiu Shi, Ye Liu, Tengfei Zhou, Zaiping Guo, and Juncheng Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02955 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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ACS Applied Materials & Interfaces

Generalized Synthesis of Ternary Sulfide Hollow Structures with Enhanced Photocatalytic Performance for Degradation and Hydrogen Evolution

Shuoping Ding, † Xiufan Liu, † Yiqiu Shi, † Ye Liu, † Tengfei Zhou, †, ‡, § Zaiping Guo, ‡ Juncheng Hu, *, † †

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of

Education, South-Central University for Nationalities, Wuhan, 430074, P. R. China. ‡

Institute for Superconducting and Electronic Materials, School of Mechanical, Materials and Mechatronics

Engineering, Faculty of Engineering and Information Science. University of Wollongong, North Wollongong, New South Wales, 2500, Australia. §

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin

300071, P. R. China.

*Corresponding author Tel: +86 27 67841302; E-mail: [email protected] (Juncheng Hu).

.

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ABSTRACT

A series of ternary sulfide hollow structures have been successfully prepared by a facile glutathione (GSH)-assisted one step hydrothermal route, in which GSH acts as the source of sulfur and bubble template. We demonstrate the feasibility and versatility of this in situ gas-bubble template strategy by the fabrication of novel hollow structures of MIn2S4 (M= Cd, Zn, Ca, Mg, and Mn). Interestingly, with the reaction time varying, the hierarchical CdIn2S4 microspheres with controlled internal structures can be regulated from yolk-shell, smaller yolk-shell (yolk-shell with shrunk yolk), hollow, to solid. Under visible light irradiation, all of our prepared CdIn2S4 samples with different morphologies were photo-activated. In virtue of the appealing hierarchical hollow structure, the yolk-shell structured CdIn2S4 microspheres exhibited the optimal photocatalytic activity and excellent durability for both the X3B degradation and H2 evolution, which can be ascribed to the synergistic promoting effect of the small crystallite size together with the unique structural advantages of the yolk-shell structure. Thus, we hypothesize that this proof-of-concept strategy paves an example of rational design of hollow structured ternary or multinary sulfides with superior photochemical performance, holding great potential for future multifunctional applications.

KEYWORDS: ternary sulfide, hollow structures, gas bubble-template, yolk-shell, photocatalytic activities.


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1. INTRODUCTION Micro-/nano materials of hollow structures have aroused tremendous attention for both fundamental research and practical applications owing to their superior characteristics of distinguishable interior voids, well-defined morphology, uniform size, low density, high specific surface area, reduced transport length for both mass and charge transport.

1

These unique structural features endow hollow structures with potential applications in

nanoreactors, gas sensors, drug delivery, energy storage and conversion, photonics, electronics, and biomedical applications, catalysis and so on. 2 Up to now, various hollow spheres including carbons, 3 polymers, 4 metals 5 and inorganic materials 6 have been extensively investigated. At present, a variety of methods have been employed to fabricate hollow structures of inorganic materials with controllable composition, tailored structure, and unique properties via various procedures, including liquids droplets, and micro-emulsion droplets,

11, 12

7, 8

and inorganic nanoparticles.

latex templates, 9 polymer templates, 13, 14

10

emulsion

However, these methods require templates,

which usually need to be removed by tedious, time-consuming and uneconomic ways like calcination (or chemical etching). What’s more, the yield of hollow structures using these methods is low, and the structures are merely intact or uniform, accompanied by crystal defects or unwanted impurities which lowers the mechanical performance. Therefore, in the case of no additional template removal step, the development of a facile, economic, and versatile approach to fabricate various hollow structures, which is of great importance, for material scientists remains a great challenge. A more efficient without sacrificing template approach is to use gas bubble. An emerging approach that toward one-step synthesize hollow structures is the strategy of in situ gas bubble template, which may effectively tackle most of the above barriers during the template removal step in hollow nanomaterials synthesis.

15

Comparing to the templated synthetic approaches, this novel method is much more

feasible, convenient and is away from introducing impurities. For instance, ZnSe hollow spheres were fabricated by using N2 as the soft template, during which N2 bubbles were released from the reducing agent hydrazine. 16 Li-Zhu Wu had reported Nih‑QD hollow nanospheres via in situ produced hydrogen bubbles. 17 Gu et al. synthesized ZnS hollow nanospheres by aggregation of primary nanoparticles around the H2S bubbles liberated from decomposition of thioacetamide. 18 Promising as the materials are, semiconductor ternary chalcogenide compounds ABmCn (A= Zn, Cd, Cu, Ag, etc.; B= Al, Ga, In; C= S, Se, Te) with chalcopyrite structures have been receiving increasing attentions owing to their potential applications in charge storage, 19, 20 thermo-electricity, 21 photocatalysis 22-25 and so on. Over the past decades, considerable efforts have been devoted to the preparation of these ternary chalcogenide materials with



various morphologies. Gou et al 26 have synthesized ZnIn2S4 nanomicrostructures with controllable size, and Baeg et al. 27 have prefabricated CdIn2S4 “marigold-like” nanostructures and nanotubes. Specifically, hierarchical hollow structures with well-defined interior porous structures are highly desirable for photocatalysts due to the merits of large specific surface area, enhanced light absorption capabilities, superior mass transfer properties, and considerable reactive sites. Owing to the virtues of efficient, economical and environmentally friendly, the bubbleassisted approach may open up a great promising and potential prospect for synthetic hollow material with one reagent. However, conventional bubble-induced synthesis of metal sulfides normally involves the addition of pressure gas supply,

28

surfactants,

29, 30

or toxic reagents,

31

while feasible one-step bubble-templated method for

the synthesis of hollow structured chalcogenide materials without high pressure gas supply or surfactant addition remains a major challenge. Herein, we demonstrate a facile and general hydrothermal approach to synthesis of hierarchical ternary sulfide hollow structures with the assistant of GSH. In this synthetic route, GSH is not only the direct sulfur source of the ternary sulfide nanocrystals, but also generates NH3 and CO2 bubbles, which are critical for the hollow interior formation during the reaction. The formation mechanism of the hollow structures was investigated in detail, and a sequential gas-templating mechanism was deduced. Furthermore, to survey the growth mechanism of these ternary sulfide hollow structures, the case of the growth process of CdIn2S4 have been investigated systematically by varying the reaction time and analyzing the corresponding samples obtained in different stages of their growth. Moreover, the structural effects on photocatalytic performance of hierarchical CdIn2S4 microspheres with different internal structures (hollow and solid structures) were also surveyed systematically by degradation of X3B and H2 evolution. Due to the synergistic effect of the small crystallite size together with well-defined interior, porous structure, large surface area, and improved light harvesting via multiple reflections of light in yolk-shell architecture, the yolk-shell structured CdIn2S4 microspheres achieved the optimized photocatalytic activity for both the X3B degradation and H2 generation. Given this, the generality of our strategy provides a new synthetic approach toward ternary or multinary sulfides with hollow structures, and is expected to be extendable to design other similar materials with enhanced properties for various applications.

2. EXPERIMENTIONAL SECTION 2.1 Materials


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All the reagents were analytical grade and can be used without further purification. L-Glutathione reduced (GSH) and indium nitrate were purchased from Aladdin, and other reagents were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2 Synthesis of Ternary Sulfide Hollow Structures In a typical route, 0.2 mmol Cd(CH3COO)2·2H2O, 0.8 mmol In(NO3)3·4H2O, and 1.6 mmol glutathione (C10H17N3O6S, GSH) were dissolved in 80 mL deionized water to form a clear solution and stirred vigorously at room temperature for 1h. The clear solution then was transferred to a Teflon-lined autoclave with a capacity of 100 mL, heated and maintained at 140 ℃ for 5-10h. Upon cooling down to room temperature naturally, the obtained yellow CdIn2S4 precipitate was collected and washed several times with deionized water and ethanol, and finally dried in an oven at 60 ℃ overnight to for further use. Similar syntheses were also carried out for the preparation of ZnIn2S4, CaIn2S4, MgIn2S4, and MnIn2S4 hollow spheres, where zinc acetate, calcium acetate and magnesium acetate, manganese acetate were used, respectively. The hierarchical CdIn2S4 microspheres with different internal structures from yolk-shell, smaller yolk-shell, hollow, to solid were marked as CYS, CSYS, CHS, and CSS, respectively.

2.3 Characterization of Photocatalysts X-ray diffraction (XRD) was performed on a Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation (λ= 1.5406Å). X-ray photoelectron spectroscopy (XPS) was conducted on a VG Multilab 2000 instrument (VG Inc.) using Al Kα radiation as the excitation source. The morphologies and microstructures and of the resultant samples were observed by a SU8010 field emission scanning electron microscope (FESEM, Hitachi, Japan) and a Tecnai G20 transmission electron microscopy (TEM, FEI Co., Holland, 200kV). Brunauer-Emmett-Teller (BET) specific surface areas and porosities of the samples were evaluated by a micromeritics ASAP2020 instrument (USA). UV-vis absorption spectra and diffuse reflectance spectrum (DRS) were recorded on a spectrophotometer (Agilent Cary-5000), and BaSO4 was used as the background. Photoluminescence (PL) experiments were performed at 240 nm excitation wavelength in a F-7000 (Hitachi). Electron paramagnetic resonance (EPR) spectra were recorded at room temperature using JES FA-200 spectrometer (JEOL, Japan) equipped with a 350 W Xe lamp and a cut-off filter (λ≥420 nm), and 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) was used as the spin-trapped reagent.



2.4 Photoelectrochemical Measurements Photoelectrochemical measurements were performed on a conventional three-electrode cell on an electrochemical workstation (IviumStat. h, Ivium Holland, Inc.) using 0.5 M Na2SO4 solution (pH=7.0) as the electrolyte. The CdIn2S4 samples coated on FTO, platinum wire and saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. All the photoelectrochemical tests were carried out at a LED (420nm, 3W, LAMPLIC Science Co. Ltd., Shenzhen, China) as the visible light source.

2.5 Evaluation of Photocatalytic Activity 2.5.1Photocatalytic Degradation of X3B The photocatalytic performances of as prepared photocatalyst for decomposition of X3B were evaluated under visible light irradiation using a 300 W Xe lamp (with a 420nm cut-off filter, PLS-SXE300/300UV, Perfect light, Beijing) as light source. In a typical test, 50 mg catalyst was placed in 50 mL X3B aqueous solution (70 mg·L-1) and ultrasonically dispersed for 10 min. Before irradiation, the suspension was vigorously stirring in the dark for 1h to ensure the establishment of the desorption-adsorption equilibrium between the dye and photocatalyst. Afterward, light was turned on. 4 mL of the suspension was sampled at regular time intervals of 3 min and centrifuged for 40 min. Finally, the concentration of X3B was analyzed according to the characteristic absorbance at 530 nm on a UV-vis spectrophotometer. (Cary-5000, Agilent).

2.5.2 Photocatalytic of Hydrogen Generation The photocatalytic H2 evolution experiments were performed in an outer Pyrex glass reaction reactor. In a typical process, 50 mg photocatalyst was suspended in 100 mL of aqueous solution containing 10 mL of triethanolamine (sacrificial electron donor), and subsequently sonicated for 10 min to get a homogenous solution. Prior to irradiation, the reaction solution was deaerated by the bubbled treatment with nitrogen for 30 min to completely remove the dissolved oxygen and create an anaerobic condition. Then the reaction was carried out under the illumination of a 300 W Xenon lamp equipped with a 420nm cut-off filter, and hydrogen generated from the photocatalytic reaction was analyzed using a gas chromatography (Fuli 9790) with TCD detector and a 5Å molecular sieve column. During the measurement, the oven, injection and detector temperature parameters were set as 80, 100, and 120 ℃, respectively. Nitrogen was used as the carrier gas at a flow rate of 30 mL min−1. For each


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evaluation of hydrogen generation, 400 µL gas sample of the headspace was intermittently injected into the GC and quantified by a calibration plot to the internal hydrogen standard.

3. RESULTS AND DISCUSSION

3.1 Characterization of Structure and Morphology Exhibited in the Figure 1a are the XRD patterns of CdIn2S4 and ZnIn2S4 samples. All the diffraction peaks of the as-prepared CdIn2S4 could be indexed to the cubic phase of CdIn2S4 (JCPDS card NO.27-0060) with the lattice constant a=10.845Å. Similar results are also observed in other CdIn2S4 samples prepared at different time (Figure S1). In addition, all of the peaks could be assigned to hexagonal ZnIn2S4 with the lattice constants of a=3.85Å, c= 24.68 Å, which are well in accordance with the standard values (JCPDS card NO. 65-2023). No other impurity peaks are observed in any of the samples, indicating high purity of the products. The morphologies and internal structures of the as-prepared samples were characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Figure 1b and c show numerous microspheres are packed with fine nanosheets (NSs), the average diameter is about 300-400 nm for CdIn2S4, and that is 900-1200 nm for ZnIn2S4. Meanwhile, the broken spheres on the wall can be observed clearly from the SEM images inset of Figure 1b and c, implying that these spheres may have a hollow interior and providing a porous nature. In addition, the NSs as building blocks for the microspheres with the size of 10-20 nm for CdIn2S4 and 60-70nm for ZnIn2S4, respectively, which could be further observed distinctly on the magnified SEM images (Figure S2a-b). Furthermore, the TEM and HRTEM investigations reveal more detailed hollow structural features. Figure 1d-e and 1h-j show the TEM images of the CdIn2S4 and ZnIn2S4, which disclose that the products consist of well-defined yolk-shell and hollow sphere structures. As can be clearly seen from the TEM images (Figure 1d, g and inset) the diameter of the inside sphere is about 100 nm on average for CdIn2S4 yolk-shell spheres with the shell thickness estimated to be 50 nm, and the hollow interiors owns an average diameter of about 800 nm for ZnIn2S4 hollow spheres with the shell thickness of 70 nm approximately. The HRTEM images from the selected area of a single CdIn2S4 yolk-shell sphere and ZnIn2S4 hollow sphere display that lattice fringe spacings of 0.383 nm and 0.411nm, corresponding to the interplanar distances of (220) and (006) atomic planes of cubic CdIn2S4 and hexagonal ZnIn2S4, respectively, which are well agreement with the results from XRD. In addition, the element mapping (Figure 1f and i) of CdIn2S4 and ZnIn2S4 show the uniform distribution Cd, In, S elements across



the yolk-shell structured CdIn2S4 spheres and Zn, In, S element across the hollow structured ZnIn2S4 spheres, respectively. Given the above results, we can preliminary conclude that we have successfully prepared yolk-shell structured CdIn2S4 and hollow structured ZnIn2S4 microspheres via this facile one step GSH-assisted approach. In order to verify the availability and generality of this proposed protocol, this simple and effective method has been extended to fabricate other ternary sulfide microspheres with a wide range. CaIn2S4, MgIn2S4 and MnIn2S4 single-shelled hollow microspheres have been obtained. SEM and TEM images in Figure 2 show the single-shelled CaIn2S4, MnIn2S4, MgIn2S4 hollow microspheres with an average diameter of about 2.5µm. Their detailed microstructures were further reveled by the higher magnification SEM images in Figure S3a-c and TEM images in Figure 2 (b, e, h inset), these spheres are also assembled by NSs with the sizes between 60 and 70 nm and the shell thickness of them are estimated to be 70nm. In addition, the element mapping of MIn2S4 (M= Ca, Mg, Mn) in Figure 2 (c, f, i) show the uniform distribution of Ca, Mg, Mn, In, S elements across the hollow structured CaIn2S4, MgIn2S4, and MnIn2S4 microspheres, respectively. The XRD patterns of the MIn2S4 (M=Ca, Mg, Mn) hollow spheres are shown in Figure S4. Notably, the diffraction peaks of all these samples could be well indexed to cubic CaIn2S4 (JCPDS NO. 31-0272), MgIn2S4 (JCPDS NO. 70-2893), and MnIn2S4 (JCPDS NO. 65-7474). In addition, XPS analysis were further performed to identify the chemical state of the elements in the prepared samples. The survey spectrum of MIn2S4 (M=Cd, Zn, Ca, Mg, Mn) and their XPS spectra of Cd 2p, Zn 2p, Ca 2p, Mg 2p, Mn 2p, In 3d and S 2p are presented in Figure S5-S8. Broad scan of the MIn2S4 confirm the existence of Cd, Zn, Ca, Mg, Mn, In and C elements without any other impurities, implying the surface of MIn2S4 with high purity. On the basis of the above experimental results, we can conclusion that this synthetic strategy is available and versatility for fabricating of hollow structured ternary sulfides without the addition of any other templates. Notably, this protocol could also be extended to other similar system that sufficient gases are generated in the reaction.

3.2 The Formation Mechanism of Hollow Structures To gain insightful understanding the formation mechanism of hollow structures, and identify the critical role of GSH played in the reaction system, a possible mechanism was proposed. According to the previous literature, 32, 33

the biomolecule glutathione (GSH) has a strong propensity to coordinate with metal ions, and owing to the

presence of -SH, -COOH, and -NH2 groups, the GSH could act as both the sulfur source and bubble source. The formation process of these hollow structures can be conceptualized in several steps based on the aggregation of


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primary MIn2S4 crystalline nanoparticles on the in situ formed gas bubble templates: as proposed in Scheme 1, first, M2+ (M = Cd, Zn, Ca, Mg, Mn) and In3+ can coordinate with GSH, resulting in the formation of metallic complex M-GSH (M = Cd, Zn, Ca, Mg, Mn, In). Due to the relative stability of these complexes, the thermolysis proceeds slowly, resulting in the formation of a number of M and In sulfide nucleus upon heating the solution. These newly formed nuclei in solution are unstable and contain large numbers of dangling bonds, defects or traps on the surface of the nucleus, 34 which may contribute to the formation of MIn2S4 crystals. With increasing the temperature, owing to the hydrolytic and thermal stability of the metal-sulfur bonds and GSH decomposition, the primary metal sulfide nanocrystallites containing In-S-M are formed with simultaneous release of CO2 and NH3 bubbles, 28 which serves as the assembly centers for the randomly moving metal sulfides monomers. Due to the high surface energy, small MIn2S4 nanocrystallites tend to attach and further aggregate at the gas/liquid interface of the in situ formed gas bubbles and water, driven by the minimization of interfacial energy. The continuous aggregation process and the subsequent growth process as proposed, which is responsible for the formation of MIn2S4 yolk-shell or hollow spheres.

35

Alternatively, the gas bubbles generated in this process may provide an aggregation center, and these

bubbles serve as soft templates for forming a hollow structure. According to the literature, this process has been named as “gas-liquid interface aggregation mechanism”. 16 Nevertheless, we must realize that the mechanisms behind the bubble template method is actually very complex. From SEM and TEM images (Figure 1and Figure 2), it is evident that the average size of the MIn2S4 (M=Ca, Mn, Mg) hollow spheres is 2.5 µm, which owns greater size than that of CdIn2S4 (300-400 nm) and ZnIn2S4 (900-1200 nm) microspheres. In addition, the morphology of CdIn2S4 also differs obviously from other samples with a yolk-shell structure, while others are hollow. This major difference can be attributed to the following two aspects. On the one hand, this may originate from the bubble life (Te) generated in the reaction system is different. As given by the Rayleigh formula:

36

= 1.83

, where is the vapor pressure, is the ambient

pressure, is the density of the liquid, and is the maximum radius of the bubble, we found that the lifetime is positively correlated with the maximum radius of the bubble. Speculations on the equation reveal that the liquid density ρ and vapor pressure Pv differ in different reaction systems concerning the preparation of MIn2S4 (M=Cd, Zn, Ca, Mg, Mn), therefore the sizes of MIn2S4 hollow spheres distinguish with each other due to the different diameters of bubbles. On the other hand, considering hydrodynamic conditions, electrostatic interactions, particle



surface properties, particle size, the morphology of primary nucleus, and some other factors, the attachment of solid particles onto the gas bubble is a multi-factor driven process, which plays a crucial role in forming the resulted morphology. 37, 38. The particle size of MIn2S4 nanocrystallites is also different from each other, the particle size of CdIn2S4 is about 10-20nm, while others are about 60-70nm. (Figure S2-3), which main due to the radius of metal ions differs from each other, and the coordination ability of metal ions with GSH is also different. From the periodic table we can deduce that the ionic radius of Cd2+ is very close to that of In3+, while other metal ions differ greatly with it, which may result in the different coordination ability with GSH and the smaller size of CdIn2S4 nanocrystal. It has been generally acknowledged that smaller sized single crystals are easier to assemble into spheres under the high surface tension. 39 The fact that the interior of CdIn2S4 differs from others is mainly attributed to the smaller particle size of CdIn2S4 single crystal and the bubble generation time lag behind some single crystal assembly. As illustrated in Scheme 1, right after the initial nucleation, the monomers will convert into yolk with the aggregation tendency prior to the gas bubble generation. Upon releasing gaseous CO2 and NH3 from GSH during the solvothermal reaction, the gases would transform into bubbles and act as aggregation centers for the crystalline of yolk surface and those residual unassembled single crystals, in other words, they tend to attach with each other and further aggregate at the gas/liquid interface of bubbles and water. Finally, CdIn2S4 yolk-shell spheres formed with fairly different sizes and morphologies compared to other hollow sphere analogues.

3.3 The Evolution Process and Formation Mechanism of CdIn2S4 Flowerlike Microspheres In order to investigate the evolution of the CdIn2S4 hierarchical microspheres, the time-dependent experiments were conducted. The detailed morphologies of the CdIn2S4 products prepared at different time are shown in Figure 3a-h. When the initial reaction time was 5h, CdIn2S4 crystalline aggregated around the gas/liquid interface between bubbles and water, the yolk-shell CdIn2S4 spheres (CYS) with the shell thickness of 50 nm were obtained, as shown in Figure 3a and e. Upon the reaction time was increased to 10h, the aggregation proceeded continuously, the inner yolk shrunk, and the size of the yolk was significantly smaller (CSYS), presenting the trend of converting to the hollow structure, compared with the sample prepared at 5h. Meanwhile, a relatively larger nanosheets appeared on the surface of the shell (Figure 3b and f). Interestingly, when the reaction time was prolonged to 24h, the number of larger crystalline nanosheets on the surface of the shell further increased, the shell surface was packed with great quantities larger nanosheets, and the internal core completely disappeared at the same time, thus a hollow structure


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formed absolutely (CHS), as shown in Figure 3c and g. And finally, when the reaction time was further extended to 50h or even longer, the flower-like CdIn2S4 microspheres with well-arranged nanosheets formed (CSS), and the hollow structure vanished completely (Figure 3d and h). The above phenomenon demonstrate that time is the key factor for shape control, there is an apparent change in the size of the nanosheets, their surface morphologies and interior structures with the increased reaction time. Alternatively, the prolonged reaction time brought about more nanosheets in larger size and the shrinkage of internal sphere which would disappear finally. The morphology evolution process of the yolk-shell, smaller yolk-shell, hollow and flower-like spheres is schematically illustrated in Scheme 2. In view of the above evolutionary experiments, it is deduced that the formation of the flowerlike CdIn2S4 microspheres might be reasonably explained by the Ostwald ripening effect. Ostwald ripening is a thermodynamically driven spontaneous process, which involves the growth of larger crystal from smaller sized one, since larger crystal is more essentially immobile stable than smaller one. During Ostwald ripening, smaller sized crystallites would eventually dissolve into solution and regrow on the larger ones to lower the total energy of a system.

40

During this process, the tiny CdIn2S4 nanosheets assembled into yolk-shell spheres first. With the

extending the reaction time, the loose crystallites packed on the outermost surface would act as the nucleation seeds for the recrystallization process, and the inhomogeneous tiny core within the yolk-shell spheres gradually dissolves and recrystallizes to form new crystallites. Duo to the intrinsic crystal orientation, these aggregates have priority to grow toward one direction, which grow along the wall of the spheres and removed from the inner space eventually. Alternatively, with prolonging the reaction time, the tiny CdIn2S4 nanosheets would recrystallize and assemble to form larger sheets, result in flowerlike CdIn2S4 microsphere forms and the increased sizes of the microspheres.

3.4 Nitrogen Adsorption Analysis The microstructures of the synthesized CdIn2S4 samples were further appreciated by N2 sorption isotherms. Figure 4 and Figure S10-12 show that the isotherm is a typical IV-type curve with a typical H3-type hysteretic loop (0.5CHS >CSS, while the order



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of pore volume changes in reverse. In the insets of Figure 4, the pore size distribution curves from the desorption branches of the isotherms show that the pores are less than 10nm in diameter. It is generally considered that the porous structures with high specific surface areas could absorb more active species and reactants on its surface, provide efficient transport pathways for the reactants and products during the photocatalytic process,

41

thus

contributing to accelerate the photocatalytic reaction rate. In this regard, we assume that the yolk-shell CdIn2S4 spheres (CYS) with higher BET surface area might have the improved photocatalytic activity.

3.5 Optial Properties Figure 5a exhibits the UV-vis DRS spectra of the CdIn2S4 samples with different morphologies, a steep absorption edges near 550 nm were observed, implying a similar visible-light response range of these samples. The intrinsic band gap is calculated to be 2.48 eV, corresponding to the characteristic of CdIn2S4. 42 It is notable that the light absorbance varied in the order of CSS > CHS> CSYS > CYS in the range of 200-550 nm, which is coincident with the color variations (Figure S13). However, CYS, CSYS and CHS show obvious absorption peaks around 600-650 nm, and vanish as the inner sphere shrinks. (Figure S14) This phenomenon could be explained as follows: (1) Larger surface contact between light and photocatalysts is more favorable for light harvesting, which is evidenced by the surface area in the order of CYS > CSYS >CHS >CSS; 43 (2) The pore volume of the shell varied in the order of CSS< CHS< CSYS< CYS, large pore volume is beneficial for light contact between light and photocatalysts, but hinders the inner multiple reflection, thus limiting the light utilization; (3) Light absorption capacities vary with the sizes of samples. As is revealed in the above SEM and TEM images (Figure 3), the sizes of single crystal nanosheets assembling the corresponding microspheres are different. Specifically, nanosheets in yolk-shell CdIn2S4 microspheres are relatively smaller than that of other samples (Table 1), including smaller yolk-shell, hollow and flower-like microspheres. The larger size of the single crystal nanosheet may result in more light to be reflected and absorbed less; (4) Light only reflects among the interconnected nanosheets in samples with a solid interior, but hollow structures allow for additional multiple reflections between the shell and hollow chambers, especially for the sphere-in-sphere structure of the yolk-shell microspheres,

44, 45

which also greatly

enhanced the light harvesting, thereby improving the light utilization, as illustrated in Figure S15. Moreover, the PL spectrum in Figure 5b displays the PL emission peaks intensities of about 390 nm weakened in order of CYS> CSYS> CHS> CSS, which corresponds to a decrease in recombination rate of photogenerated electrons and holes.

46, 47

This can be ascribed to the size variations among the samples, briefly, smaller crystallite


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size give rise to shortened transfer distance of photoelectrons and favored for more rapid transfer rate, thus inhibiting their recombination with holes.

44

In consequence, the recombination of photoelectrons and holes are

effectively inhibited due to the smaller crystallite size and unique yolk-shell structure, thus more photocharges would be involved for the photoreactions. The photoinduced charge separation and transfer of CdIn2S4 electrodes was also surveyed by the electrochemical impedance spectroscopy (EIS). In general, a smaller EIS arc radius demonstrates a higher charge mobility. 48, 49 As the EIS Nyquist plots shown in Figure 5c, the smaller diameters of the arc radius of CYS compared to the pristine CSS indicates the higher charge transfer rate of CYS, for which the yolk-shell structure is more capable of separation and transmission the photogenerated e-h+ pairs from bulk to the surface, resembling with the results in PL tests. Figure 5d demonstrates all CdIn2S4 samples produced a photocurrent under the condition of visible light irradiation. When the light was turned off, all these photocurrents rapidly decayed, and the photocurrent intensity varied in the order of CYS> CSYS> CHS> CSS. These results reveal the fact that the unique yolk-shell structure favors more intensive light harvesting, more efficient e-h+ pairs separation and faster transportation of carriers. 50, 51

3.6 Photocatalytic Activity The photocatalytic performances of the as-prepared CdIn2S4 samples with different morphologies were evaluated by the degradation of X3B and H2 generation under visible light irradiation (λ≥420 nm). Figure 6a shows the degradation curves of X3B over different CdIn2S4 samples. The X3B photolysis without photocatalyst is negligible during experiment under visible light. Notably, CYS shows the best photocatalytic activity, where X3B became colorless in less than 12 min. After 6 minutes of irradiation, the degraded percentages of X3B were estimated to be 91.7, 86.3, 68.4, 52.6, and 16.6, corresponding to CYS, CSYS, CHS, CSS and commercial CdS (Figure 6b), respectively. Apparently, the degradation rate of CYS is 6 times that of commercial CdS in the case of X3B degradation. Time-dependent photocatalytic H2 evolution for different CdIn2S4 samples have been performed using triethanolamine as the sacrificial electron donor without the additive Pt co-catalyst (Figure 6d). Figure 6e presents a comparison of photocatalytic H2 generation rates of samples, CYS, CSYS, CHS, CSS and commercial CdS. Significantly, it is obviously that the yolk-shell structured CdIn2S4 exhibited the highest photocatalytic activity with a hydrogen evolution rate of about 762.3 µmolg-1 h−1, which is 35.6 folds higher than commercial CdS. The superior photocatalytic performance of CYS might benefit from the synergistic effect of various factors, as



follows: first, owing to the smaller crystal size, CYS naturally reduced the photoelectrons transfer distance, restrain their recombination with holes.

52

Second, the porous structure of interconnected nanosheet self-assembled CYS,

and its relatively high surface area not only expand the contact area between the light and photocatalyst, ultimately facilitating light harvesting, but also promoted the reactant molecules adsorption and diffusion. 53 Third, the unique structural features of the yolk-shell structure, for instance, the reactant molecules and products could diffuse rapidly through the shells, and the hollow interior void between the shell and core not only provide a large surface area, but also a perfect and confined homogenous microenvironment for photocatalytic reaction, which contribute to increase the contact time between reactant molecules and photocatalysts.

46, 54, 55

Significantly, compared with the interior

cavity of hollow and solid microspheres, the existence of inner spheres in the yolk-shell structure favored more multiple light reflections, allowing the light source to be utilized more effectively, and thereby resulting in a remarkably enhanced photocatalytic activity. Thus, optimum photoactivities of CYS were mainly ascribed to the synergistic effect of the small crystallite size together with well-defined interior, porous structure, the high surface area, and increased light absorption induced by the multiple light reflections in the yolk-shell structure. Additionally, the photocatalytic stability of CYS for photolysis of X3B and H2 evolution were examined by repeating the experiment five times. Apparently, the cycling experiments disclose that the CYS could still degrade the X3B completely, and continuous H2 evolution was observed with no noticeable degradation in the subsequent runs with the same irradiation time (Figure 6c and f). The slight decrease of the photocatalytic performance during the recycling reactions is attributed to loss of catalysts during the centrifugation process. The almost unchanged photoactivities indicated the prominent photostability of CYS, which might be a durable photocatalyst for both pollutant degradation and H2 evolution. XRD and SEM analysis of CYS after the recycle experiments complementarily illustrated the photostability of CYS, as depicted in Figure S16 that no obvious phase and morphological changes were observed.

3.7 Photocatalytic Mechanism To deduce the photocatalytic mechanism, the radical trapping experiments were conducted by addition of ethylenediamine tetraaceticacid disodium (EDTA-2Na), 4-benzoquinone (BZQ), and isopropanol (IPA) to the reaction system as the hole (h+), superoxide (•O2−), and hydroxyl radical (•OH) scavengers, respectively. As shown in Figure 7a and b, the photodegradation efficiency of CYS is 99.6% in absence of scavengers, and there is a small decrease in the degradation efficiency (94.1%) of with adding IPA. When EDTA-2Na and BZQ were added, the


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photocatalytic efficiency of X3B reduces dramatically to 21.7% and 41.7%, respectively. The above results clearly illustrated that the photoinduced h+ and •O2− are the main active species for the decomposing of X3B in the photocatalytic reaction. To further confirm the reactive oxygen species formed during the photocatalytic process, the electron spin resonance (ESR) technique was utilized. As depicted in Figure 7c and d, the characteristic peaks of both •O2− and •OH over the CdIn2S4 sample can be clearly detected under visible light illumination, meanwhile, the intensities of those peaks are obviously strong along with prolonging the irradiation time. This observation clearly confirms that the •O2− and •OH are both the existed oxidizing species for contaminants elimination. To further understand the interfacial charge separation and transfer in the yolk-shelled CdIn2S4, the VB edge positions was determined by XPS VB spectra, as the maximum VB energy of 1.65eV displayed in Figure 7e. Based on the above experimental and theoretical results, a mechanism illustrating the outstanding photocatalytic activity of the yolk-shelled CdIn2S4 is proposed in Figure 7f and Scheme 3. The energy band diagram of CYS, the reduction potentials of O2/ •O2-, •OH/ OH-, H+/ H2 and the generation and transfer of photocarriers throughout the indirect semiconductor photoexcitation channels are depicted in Figure 7f. The potential of H+/H2 is 0V vs. NHE, O2/ •O2- is -0.33V vs. NHE, 56 while the ECB of CYS (-0.83V) is not less negative, therefore the electrons from photogenerated from CB can not only easily split water into H2 with TEOA electron donor, but also react with oxygen to produce •O2-. Simultaneously, the VB position (1.65V vs. NHE) of CYS is less positive enough to generate •OH in its VB via h+ direct oxidation reaction, compared with the potential of •OH/ OH- (2.38V vs. NHE) 57, implying that •OH is converted from •O2− via the following two pathways: 2•O2- +2H+ → H2O2 + O2 and H2O2 + e−→ OH− + •OH. 58 The above explanations were in good agreement with the ESR results that DMPO-•O2- and DMPO-•OH adducts were detected for CYS under visible light irradiation for 12 min (Figure 7c and d). Therefore, a powerful photoreduction ability is enabled for CYS owing to the suitable position of conduction band. Moreover, the robust •O2−, •OH radicals, along with the oxidative h+ play synergic roles in the photocatalytic degradation of X3B.

4. CONCLUSIONS In summary, we have successfully fabricated a series of ternary sulfide hollow structures through a facile onestep GSH-assisted hydrothermal strategy without the addition of any high-pressure gas or surfactant. The mechanisms underneath the formation of these unique hollow structures can be attributed to the continuous



aggregation and growth of MIn2S4(M=Cd, Zn, Ca, Mg, Mn) nanocrystals on the liquid-gas interfaces, which is created by the gas bubble released from GSH. The morphologies of the samples can be readily manipulated by just simply vary the reaction time. Benefiting from the desirable structural advantages, the yolk-shell CdIn2S4 microspheres exhibited superior photoactivities toward both X3B degradation and H2 evolution under visible light, which is 6 times and 36.6 times higher than that of commercial CdS, respectively. The synergistic promoting effect of small crystallite size together with the unique advantages of the yolk-shell structure, including well-defined interior, porous structure, large surface area, and enhanced light harvesting capacity via multiple reflections in the yolk-shell chambers, make for the dramatically elevated photoactivities. We believe that our study may not only open up an alternative way optimizing the photoactivities of ternary sulfides, but also shed light on engineering other novel hollow structured ternary or multinary sulfides for multipurpose applications.

ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (21673300) and Hubei Provincial Natural Science Foundation of China (2018CFB237). Appendix A Supplementary data associated with this article can be found, in the online version, at XXXXXXXXX

Supporting Information XRD patterns of CdIn2S4, CaIn2S4, MgIn2S4, and MnIn2S4 samples; Magnified SEM images of CdIn2S4, ZnIn2S4, MgIn2S4 and MnIn2S4 samples; XPS spectra of MIn2S4(M=Cd, Zn, Ca, Mg, Mn); Nitrogen absorption-desorption isotherms, pore size distributions, image, magnified UV-vis DRS spectra, multiple light reflections illustration of different CdIn2S4 samples; XRD patterns and the corresponding SEM image of the CYS before and after the cycle experiment.

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Figure

Figure 1. (a) XRD patterns and (b, c) SEM images of (b) yolk-shell CdIn2S4 spheres and (c) ZnIn2S4 hollow spheres obtained at 140 ℃ for 5h. (d, g) TEM images, (e, h) HRTEM images and (f, i) elements mappings of (d-f) CdIn2S4 yolk-shell and (g-i) ZnIn2S4 hollow spheres.


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Figure 2. (a, d, g) SEM images of (a) CaIn2S4 hollow spheres (d) MgIn2S4 hollow spheres, and (d) MnIn2S4 hollow spheres obtained at 140 ℃ for 5h. (b, e, h) TEM images of (b) CaIn2S4 hollow spheres, (e) MgIn2S4 hollow spheres, and (h) MnIn2S4 hollow spheres. (c, f, i) elements mapping of (c) CaIn2S4 hollow spheres, (f) MgIn2S4 hollow spheres and (i) MnIn2S4 hollow spheres.



Scheme 1. Illustration of the ternary sulfide hollow and yolk-shell microspheres formation.


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Figure 3. SEM and TEM images of CdIn2S4 samples at different reaction time: (a, e) 5h, (b, f) 10h, (c, g) 24h, (d, h) 50h.



Scheme 2. Illustration of the time-dependent morphology evolution for CdIn2S4 samples.


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Figure 4. Nitrogen absorption-desorption isotherms of CYS and the corresponding pore size distribution (inset).



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Table 1 Physical structural parameters and photoactivity of different CdIn2S4 samples.

Sample

Crystallite size a (nm)

SBET (m2 g−1)

Vp (cm3 g−1)

X3B degradation percentage b

H2 production rate (mol h−1 g−1)

CYS

13

181.8

0.264

91.7

762.3

CSYS

22

162.3

0.280

86.3

367.5

CHS

53

154.3

0.346

68.4

142.4

CSS

89

148.6

0.389

52.6

78.3

a

Average crystallite sizes of all the CdIn2S4 samples were calculated by Scherrer formula using the (311) facet diffraction

peaks. b

X3B degradation percentage were calculated with the samples being irradiated for 6 minutes.


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Figure 5. (a) DRS spectra and the corresponding bandgap energies (Eg, inset), (b) PL spectra, (c) EIS Nyquist plots and (d) Transient photocurrent densities of the obtained CdIn2S4 samples.



Figure 6. (a, d) Photolysis of X3B and hydrogen evolution in the presence of different CdIn2S4 samples under visible light irradiation. (b, e) Comparison of photodegradation of X3B and hydrogen evolution rate over different CdIn2S4 samples. (e, f) Reuse of the CdIn2S4 (CYS).


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Figure 7. (a-b) Effects of different reactive species scavengers on the photodecomposition of X3B by CYS under visible light irradiation. ESR spectra of radical adducts trapped by DMPO in dispersion under visible light irradiation: (c) in methanol dispersion for DMPO-•O2−; (d) in aqueous dispersion for DMPO-•OH. (e) XPS VB spectra of CYS. (f) Band structure and photocatalytic mechanism of CYS system under visible light irradiation.



Scheme 3. Illustration of the photocatalytic mechanism of CYS system under visible light irradiation.


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TOC art


Generalized Synthesis of Ternary Sulfide Hollow Structures with

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