Recent developments in ZnS photocatalysts from synthesis to photocatalytic applications — A review

    Recent developments in ZnS photocatalysts from synthesis to photocatalytic applications — A review Gang-Juan Lee, Jerry J. Wu PII: DOI: Reference:

S0032-5910(17)30395-9 doi:10.1016/j.powtec.2017.05.022 PTEC 12548

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Please cite this article as: Gang-Juan Lee, Jerry J. Wu, Recent developments in ZnS photocatalysts from synthesis to photocatalytic applications — A review, Powder Technology (2017), doi:10.1016/j.powtec.2017.05.022

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ACCEPTED MANUSCRIPT Recent Developments in ZnS Photocatalysts from Synthesis to Photocatalytic

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Gang-Juan Lee and Jerry J. Wu*

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Applications – A Review

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Department of Environmental Engineering and Science, Feng Chia University, Taichung 407, Taiwan

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* To whom correspondence should be addressed:

Abstract

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E-mail: [email protected], Tel.: +886-4-24517250 Ext. 5206, Fax: +886-4-24517686.

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This review article comprehensively discusses the recent development of band engineering ways, synthetic methods, and photocatalytic applications using ZnS nanocrystalline semiconductors. The

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band engineering is just the first step in the design of visible-light-driven photocatalysts. Particle size, shape, surface area, crystal structure, and degree of crystallinity also affect photocatalytic activity. The reason we chose ZnS as a target due to its remarkably chemical stability against oxidation and hydrolysis when the particle size steps down to just a few nanometers. In addition, photocatalytic water splitting technology driven by ZnS photocatalyst has great potential for low-cost and environmentally friendly solar-hydrogen production to support the future hydrogen economy. Therefore, the ZnS assisted photocatalytic degradation of pollutants and water splitting under various conditions have been summarized in this review article. The possible reaction mechanisms for organic

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ACCEPTED MANUSCRIPT pollutants degradation and the photocatalytic hydrogen evolution using the metal-doped ZnS

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photocatalysts have been also included and compared.

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Keywords: Zinc sulfide, Hydrothermal and solvothermal method, Ultrasound and microwave irradiation, Photocatalytic degradation, Hydrogen production

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Table of Contents

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1. Introduction

2. Development of visible-light-driven ZnS photocatalysts

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2.2 Nonmetal ion doping

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2.1 Metal ion doping

2.3 Dye sensitization

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2.4 Composite semiconductors 2.5 Surface defects

2.6 Summary of ZnS photocatalyst modification techniques 3. Synthesis of ZnS nanostructures 3.1 Hydrothermal and solvothermal method 3.2 Ultrasonic irradiation method 3.3 Microwave irradiation method 3.4 Summary of ZnS photocatalyst synthesis methods

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ACCEPTED MANUSCRIPT 4. Applications 4.1 Photocatalytic degradation of organic contaminants

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4.2 Photocatalytic hydrogen evolution from water splitting

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5. Conclusion Acknowledgments

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Reference

1. Introduction

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Zinc sulfide (ZnS) is an important II–VI group semiconductor photocatalyst and it has been

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intensively studied due to its rich morphologies at the nanoscale, excellent physical properties, and

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unique photocatalytic properties.[1,2,3] The morphology of ZnS at the nanoscale has been demonstrated to be one of the richest types among all inorganic semiconductor photocatalysts.[1] ZnS

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has traditionally shown a remarkable versatility and promise for novel fundamental properties and diverse applications, including light-emitting diodes (LEDs), electroluminescence, flat panel displays, infrared windows, sensors, lasers, and biodevices, etc.[1] It has been found that ZnS nanostructures have high photocatalytic activity, such as photoreductive dehalogenation of halogenated benzene derivatives, photoreduction of CO2, photocatalytic degradation of organic pollutants, and photocatalytic splitting water of producing H2.[2,4,5] In addition, ZnS owns a lot of advantages, such as excellent transport properties (reduction of the carriers scattering and recombination), an intrinsically n-type semiconductor, good thermal stability, high electronic mobility, nontoxicity, water 3

ACCEPTED MANUSCRIPT insolubility, and comparatively inexpensive cost.[1,6] ZnS exists in two main crystalline forms. One is the cubic (sphalerite) and the other is the

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hexagonal (wurtzite).[1] In both crystalline forms, the coordination geometry at Zn and S are of

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tetrahedral of crystal system and their band gaps of 3.72 eV and 3.77 eV belong to cubic and hexagonal ZnS, respectively.[1] Hence, ZnS is just only responsive to the UV light absorption (λ <

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340 nm) for electron-hole separation upon illumination. Furthermore, it undergoes photochemical

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decomposition into the components when irradiated in the absence of sacrificial electron donors.[4] Therefore, considerable attempts have been attempted on developing visible-light-driven ZnS

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photocatalysts by using less energy but more abundant visible light (λ≥420 nm) of solar spectrum.[7]

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In order to harvest solar light efficiently, photocatalysts exhibiting an absorption band at longer

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wavelengths are highly desirable.[8] Moreover, a visible-light-driven photocatalyst must be not only visible light active, but stable under sunlight irradiation. Many modifications have been performed in

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the development of visible-light-active photocatalysts that can efficiently utilize the maximum of solar light.[9] Then, some researches focused on widely used methods, namely, metal ion loading, transition metal doping, anion doping, dye sensitization, composite semiconductor, and metal ion implantation.[10] Moreover, particle size, shape, surface area, crystal structure, and degree of crystallinity also affect the charge separation and surface reactions.[11,12] Accordingly, the photocatalytic activity of semiconductor materials can be controlled by three key factors: (1) light absorption property, (2) rate of reduction and oxidation of reaction substrates by, respectively, electron and hole, and (3) rate of electron/hole recombination.[13] Therefore, ZnS nanostructure with 4

ACCEPTED MANUSCRIPT various morphologies are receiving a significant consideration from both fundamental research and practical applications.

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In this review, we focused on ZnS photocatalysts from their synthesis to photocatalytic

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applications. The synthesis of ZnS nanostructures, including (a) bare ZnS nanoparticles with different morphologies, such as nanospheres, nanorods, nanotubes, and nanoflowers, etc., (b) nanocomposites

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containing ZnS nanoparticles with metal, nonmetal, dye, and composite semiconductor, and (c) ZnS

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nanoparticles were discussed using the different synthetic methods available in the literatures. Furthermore, this review has also discussed the utilization of these ZnS materials in the field of

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photocatalytic degradation of organic pollutants and photocatalytic hydrogen evolution.

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2. Development of visible-light-driven ZnS photocatalysts ZnS is a direct wide-gap semiconductor photocatalyst with remarkable chemical stability against

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oxidation and hydrolysis when the particle size steps down to just a few nanometers.[1] However, its photocatalytic efficiency is extremely low due to a high charge recombination rate, and it is not stable under irradiation due to photocorrosion.[6,14] In response to these deficiencies, a suitable band engineering is necessitated to prohibit rapid recombination of electron/hole pairs and backward reactions.[10,15] There are some ways to improve the shortages of ZnS photocatalyst as mentioned below. 2.1 Metal ion doping The doping of foreign elements into UV-active non-oxide photocatalysts is a conventional 5

ACCEPTED MANUSCRIPT method employed for the preparation of efficient photocatalytic and visible-light-responsive photocatalysts.[16] Doping often means replacement with a foreign element at a crystal lattice point

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of the host material.[11] In the forbidden band through metal ion doping, either a donor level above

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the original valence band or an acceptor level below the original conduction band is created to make the photocatalysts respond to visible light as shown in Fig. 1.[17] The extent of the red shift depends

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on the amount and type of metal ions implanted.[17] Dopants in the photocatalyst act not only as

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visible-light absorption centers, but also as recombination-avoiding sites by trapping electrons or holes, which in return promoted the charge separation required for the photocatalytic reaction.[16,17]

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For example, doping ZnS leads to a tunable emission covering the entire visible spectrum. With the

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Mn2+ ion as a dopant, the photoluminescence of the ZnS nanoparticles is red-shifted from the blue

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region of electromagnetic spectrum to the orange region.[3] However, in some cases, it was found that the metal ion dopants could also serve as the recombination sites for photoinduced charges and not a

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recombination inhibitor.[17] Since the doped materials might suffer from thermal instability and the metal centers act as electron traps and the charge becomes unbalanced,[18] this would result in relatively low photocatalytic activity even under UV irradiation. 2.2 Nonmetal ion doping Nonmetal ion doping is another approach used to modify visible-light-driven photocatalysts. It is unlike metal ion dopants to form donor levels in the forbidden band, but nonmetal ion dopants shift the valence band edge upward.[17] This would result in a narrowing of band gap as shown in Fig. 2.[17] Doping of nonmetal ions, such as B, C, N, S, F, Cl, and Br, was found to be most effective 6

ACCEPTED MANUSCRIPT because the impurity states of anions are near the valence band edge.[17] For N-doped ZnS, there have two complex defect models. One is denoted as NS-Znint, where NS means an N atom replacing an

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S atom in the structure. And the other is presented as NS-Zn-VS, where NS and VS atoms are bonded

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through a Zn atom.[19] The reason is that Zn atoms easily deviate from the lattice sites to become interstitial Zn (Znint) atoms, which may combine with dopants to form complex defects.[19] When

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nitrogen with one less valence electrons replaces sulfur in forming the structure, the 4p orbital of Zn

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atom gets unoccupied, resulting in the enhancement of good photocatalytic activity in the degradation of Orange II dye under visible light irradiation.[20] For example, the band gap of nitrogen-doped ZnS

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composite catalysts is between 2.58 and 2.74 eV. Therefore, N-doping ZnS substantially shows the

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shifted photoabsorption properties, which implies the potential absorption to effectively harvest

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visible light.[21] 2.3 Dye sensitization

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Dye sensitization is another approach to improve the visible-light harvesting of UV-active non-oxide photocatalysts.[17] It extends the range of excitation energies of the photocatalyst into visible region, making more complete use of solar energy.[22] The basic principle of dye-sensitized photocatalytic reaction is shown in Fig. 3.[17] Photoexcitation of the dye adsorbed onto the photocatalyst results in the injection of electrons into the conduction band of the photocatalyst.[17] The electron undergoes two further reactions (1 and 2). Over the catalyst surface, the electrons reduce the adsorbed oxygen species to produce superoxide anion radical (‧O2－). Subsequently, organic compounds are degraded by the active oxygen species (‧O2－).[22] Reaction 2 is a typical hydrogen 7

ACCEPTED MANUSCRIPT evolution reaction. The electrons are consumed to reduce water to produce hydrogen.[17] Therefore, dye modified photocatalytic system could be workable for the degradation of colorless organics in the

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photocatalysts for water splitting under visible-light irradiation.

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presence of oxygen and visible light. Moreover, dye-sensitized semiconductors could also function as

2.4 Composite semiconductors

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In addition to expanding the light absorption edge of the photocatalyst, preventing the

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recombination of photoexcited electron/hole pairs is another key strategy to improve photocatalytic efficiency. By combining two different semiconductors with suitable conduction and valence bands of

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the photocatalyst could induce better collection of the photogenerated electrons and holes on the

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different semiconductor surfaces, and enhance the redox reactions of the electrons and holes,

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respectively.[23] The reason is the photogenerated charge carrier separation with a formation of a heterojunction structure, thus improving the photocatalytic activity.[17] For example, the prepared

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CuS/ZnS porous nanosheet photocatalysts show especially high visible light photocatalytic H2 production activity (H2 production rate is 4,147 μmol h-1 g-1 with quantum efficiency of 20% at 420 nm.).[24] Since the electrons are photoexcitated from valence band of ZnS directly to CuS (belonging to interfacial charge transfer, IFCT), they would enhance the charge separation and photocatalytic efficiency.[24] The possible mechanism of charge transfer in CuS/ZnS composite system was proposed as shown in Fig. 4. 2.5 Surface defects Surface defects are another strategy to improve light harvesting in photocatalytic materials. The 8

ACCEPTED MANUSCRIPT reason is that surface defects can serve as adsorption sites where a charge transfer to the adsorbed species can prevent the recombination of photogenerated electrons and holes.[25] This is because

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vacancy defect could affect the geometric and electronic structures of the crystalline materials.[25,26]

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Vacancy defects are easily formed in quasi-two-dimensional materials because the exposed atoms on their surface can escape from the lattice hence influencing their physical and chemical properties,

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such as conductivity, optical and field emission properties, and reactivity crystal.[25,26] S vacancies

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can decrease the band gap of ZnS and conversely, Zn vacancies lead to the increase of the band gap of ZnS.[27] However, when presence in excessive amount, defects can act as traps for charge carriers,

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resulting in the recombination of photogenerated electrons and holes and hence decreasing the

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photocatalytic activity.[25] For ZnS, each S2- ion is surrounded by four Zn2+ ions to form a SZn4

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tetrahedron, and each Zn2+ ion surrounded by four S2- ions to form a ZnS4 tetrahedron. When an S atom is removed from a SZn4 tetrahedron, each of the four Zn atoms surrounding the S vacancy

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becomes a ZnS3 pyramid with one sp3 dangling bond. When the structure around the vacancy site is relaxed, the energy gap between the 1a and 1t levels becomes large with the 1t level splitting due to the symmetry lowering as shown in Fig. 5. As a consequence, the 1a level comes close to the valence band maximum and becomes doubly filled, while the three empty levels come close to the conduction band minimum. In the relaxed structure around an S vacancy, one ZnS3 pyramid that becomes strongly pyramidal has its dangling bond doubly filled to become a lone pair. Then the optical excitations associated with the filled defect level are close to the valence band maximum and the empty defect levels below the conduction band minimum are responsible for the visible-light 9

ACCEPTED MANUSCRIPT absorption of the ZnS samples with S vacancies. Furthermore, trapping of photogenerated electrons and holes by these defect states helps slow down their recombination, thereby enhancing the

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photocatalytic activities of ZnS samples with S vacancies.[25] In addition, S vacancy and Zn vacancy

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could decrease the cell volume and induce slight deformation of the perfect ZnS.[26] Since the volume occupied by vacancy S-atom is bigger than that of vacancy Zn-atom, the cell volume decrease

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due to S-vacancy is more obvious than that due to Zn-vacancy after geometric relaxations.[26] The

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introduction of S vacancies is harder than Zn vacancies in ZnS crystals. The reason is the energy for the formation of S vacancies (7.05 eV) is higher than that of Zn vacancies (5.99 eV). Thus, more Zn

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vacancies are expected in ZnS crystals than S vacancies.[27]

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2.6 Summary of ZnS photocatalyst modification techniques

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On the basis of aforementioned discussion, we could know that the ability to adjust and fine-tune band gap and conduction band/valence band edge positions is extremely important in developing

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visible-light-driven photocatalysts. Metal ion or nonmetal ion doping could create an electron donor level or a new valence band, which results in a band gap narrower. The other strategy could also improve the photocatalytic activity, such as dye sensitization, where dye can absorb in the visible region and transfer electron directly to the conduction band, resulting in the enhancement of the charge separation. In addition, composite semiconductors would be able to inhibit the recombination reaction of photoexcited electron/hole pairs. The charge carriers can transfer to surface defects, resulting in promoting the charge separation and avoiding the recombination reaction. Therefore, the modifications of visible-light-driven photocatalysts can efficiently utilize the maximum of solar light 10

ACCEPTED MANUSCRIPT by these methods as mentione and the visible-light-driven ZnS photocatalysts can be successfully

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synthesized to improve the photocatalytic activity.

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3. Synthesis of ZnS nanostructures

Among the various synthetic techniques for preparing ZnS nanostructures, the major difference

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is the energy source. The properties of a specific energy source determines the course of a chemical

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reaction.[28] Chemical synthesis has several advantages such for the synthesis of nanoparticles, such as producing size-controlled, un-agglomerated nanoparticles, easy handling and large scale

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production potential.[3] Therefore, the photocatalytic materials mainly depend on the preparation

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method, which controls the morphology, particle size, and cation order. The growth of novel ZnS

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nanostructure not only depends on their applications, but also on the better understanding of the synthetic methods which modulate the structural morphology, particle size distributions, and

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composition. ZnS catalysts are synthesized by various methods, including chemical precipitation method, hydrothermal method, ultrasonic irradiation method, microwave irradiation method, sol–gel method, thermal decomposition method, and chemical vapor deposition method, etc. This section reviews the common and simple synthetic methods recently developed to synthesize various ZnS nanostructures for tuning the morphology, size, and crystallinity. 3.1 Hydrothermal and solvothermal method The hydrothermal and solvothermal process could be defined as purely heterogeneous reactions at temperatures above the boiling point of mineralizers in a closed system at 1 atm pressure.[29] In the 11

ACCEPTED MANUSCRIPT hydrothermal process, water is used as the main mineralizer and in the solvothermal process organic solvents are used as main mineralizers.[29] The hydrothermal and solvothermal processes have a lot

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of advantages, including energy efficient (low temperature synthesis), environmentally friendly

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process (closed system conditions), no precipitants, hydroxylated and hydrated materials (clays and zeolites), high crystallinity, phase purity, and high yield products.[29,30] The hydrothermal and

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solvothermal processes are only in low temperature post-calcinations, and they are cost effective,

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environmental friendly and easily scalable.[29] There are also a few disadvantages to hydrothermal and solvothermal processes, such as the need for expensive stainless-steel autoclaves and Teflon

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liners, possible safety issues during reaction processes, and the impossibility of studying in-situ

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reactions due to their closed systems.[29] Even though various synthetic methods are available,

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hydrothermal and solvothermal methods are solution-based methods capable of converting metal oxides into desired nanostructures known for their unique electronic and electrochemical

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applications.[29] Hydrothermal preparaiton has been extensively studied for the synthesis of simple oxides such as those of TiO2[30], ZnO[31,32], Bi2O3[33,34], InOOH[35], and Fe2O3[36] etc., and today many of the key parameters have been understood. In addition, solvothermal processes refer to the heterogeneous reaction involving the thermal decomposition of the metal complexes using the solvent mineralizer either by boiling the contents in an inert atmosphere or in a sealed vessel (autoclave) with a suitable capping agent, stabilizer, surfactant to hinder the nanoparticles growth. These stabilizers provide stabilization against agglomeration as well as help in dissolution of the particles in different solvents and this process results in highly crystalline phase pure homogeneous 12

ACCEPTED MANUSCRIPT particles with narrow size distribution.[37] Although the conventional hydrothermal and solvothermal process has been in use for a long time, the hydrothermal and solvothermal process in

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combination with electric, microwave, and ultrasonic fields is rather recent. Some significant

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advantages are: (a) it can increase the reaction rate by one to two orders of magnitude, (b) it can lead to novel phases, (c) it can eliminate metastable phases, and (d) it can lead to a rapid heating.[30] The

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microwave-hydrothermal process could lead to higher yields and crystallization in a shorter time at all

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temperatures compared to the conventional-hydrothermal process.[30] There are a series of ZnS nanostructures with controllable crystal phase and morphology via a

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simple solvothermal method using different sources of zinc as a precursor, solvents, and surfactant. In

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addition, the morphology of the ZnS synthesized under similar experimental conditions is also

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controlled at different reaction time or reaction temperature. The different crystal phases and morphologies were obtained at various solvents and reaction conditions as shown in Table 1 and Fig.

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6. It has been known that the polyol plays a key role in forming hexagonal ZnS nanocrystals at low temperatures, and the polyol probably forms some intermediates with ZnS, which can decomposes into wurtzite ZnS at lower temperatures.[38] The OH group may bond with zinc atoms on the surface of ZnS. The formation of ligation between the surface zinc atoms of ZnS and the OH groups of solvent may lead to a change of the surface energy of ZnS nanocrystals. The ligation of OH groups with Zn atoms on the surface of ZnS is responsible for the phase evolution of ZnS in polyols.[38] This results in a low surface energy, which implies that smaller ZnS nanoparticles in a vacuum are more thermodynamically stable in wurtzite phase than sphalerite phase. Moreover, it is interesting to note 13

ACCEPTED MANUSCRIPT the morphology of the ZnS synthesized under similar experimental conditions in water or solvents at different reaction time. According to the reference, increasing the reaction time, the microsphere

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starts to degrade and eventually generate various nanoparticles/plates from the microsphere surface as

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shown in Fig. 6 (4-3~4-4).[42] Furthermore, the surface area of the ZnS products decreases with increasing the reaction time, which may be due to the increased particle size on the microsphere

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surface as the reaction time increases.[42] The different types of by-productions in water and in

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water-ethanol solvent could facilitate different surface structured ZnS. The reason is the presence of alcohol as co-solvent may favor the formation and solubility of diethyl disulfide and diethyl sulfide

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and plays an indirect role on the formation of hierarchical surface structure.[42] The formed diethyl

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sulfides and diethyl disulfide could act as capping agents with ZnS surface, which may induce thiol

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type bonding (-S-H-) to facilitate hierarchical surface structure.[42] Therefore, ethanol is playing a very important role of co-solvent to control the surface structures of ZnS. Meanwhile, the influence of

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initial concentration of the precursors, various zinc precursors, solvents, and surfactants could change the morphology of the products.[38-56] 3.2 Ultrasonic irradiation method Compared to conventional techniques, ultrasonic irradiation differs from traditional energy sources (such as heat, light, or ionizing radiation) in duration, pressure, and energy per molecule.[28] The reason is the use of the immense temperatures and pressures and the extraordinary heating and cooling rates generated by cavitation bubble collapse. Ultrasound is a unique means of interaction energy and matter because the chemical effects of ultrasound arising from acoustic cavitation include 14

ACCEPTED MANUSCRIPT the formation, growth, oscillation, and implosive collapse of gas-bubbles in a liquid under proper conditions.[28,57] The collapse of cavitation bubbles can generate localized hot spots with ephemeral

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temperature and pressures of about 10,000 K and 1,000 atm, respectively, or more and cooling rates in

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excess of 109 K/s.[28,57,58,59,60] Therefore, bubble collapse caused by cavitation produces intense local heating, high pressures, and very short lifetimes. Thus, acoustic cavitation serves as a means of

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concentrating the diffuse energy of sound. Under such extreme environments, it can obviously

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accelerate the condensation reaction or hydrolysis reaction.[60] In heterogeneous systems (liquid–solid), the dynamics of cavity collapse changes dramatically.

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Since its surroundings are not uniform, the asymmetry of the environment near the interface between

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liquid and solid induces a deformation of the cavity during its collapse. This deformation is

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self-reinforcing and it sends a high-speed jet of liquid through the cavity at the surface with velocities of roughly 100 m/s. Such a great effect of high-speed jets and shock waves on the surface of cavity

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creates the localized erosion responsible for many of the sonochemical effects on heterogeneous reactions. The high-velocity interparticle collisions formed in such slurries cause smoothing of individual particles and agglomeration of particles into extended aggregates. Then, various nanostructured materials can be effectively synthesized with required particle size distribution. The ultrasonic irradiation method can create highly reactive surfaces and thereby increase their catalytic activity.[28,57,58,59,60] The chemical and physical reactions produced by acoustic cavitation can be affected the properties of doped materials.[59] According to results of the references as shown in Table 2 and Fig. 7, the particle sizes of the 15

ACCEPTED MANUSCRIPT as-synthesized ZnS products are all in the morphology with diameters below 1 m. The reason is that the ultrasonic waves cause the breakup of the agglomerates. The deagglomeration by ultrasonication

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is a result of ultrasonic cavitation. Such jets press liquid at high pressure between the particles and

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separate them from each other. Furthermore, very small ZnS nanoparticles could be easily prepared via sonochemical method using ionic liquid.[64] In addition, bacterium as a sacrificial template have

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ever been used to produce the hollow-sphere-like ZnS structures with the assist of sonochemistry for

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the in situ one-step synthesis.[62] 3.3 Microwave irradiation method

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Microwave-assisted synthesis has the advantages of short reaction time, rapid volumetric

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heating, small particle size, narrow particle size distribution, high purity, low cost, simple, and energy

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saving.[68,69] Microwave energy is delivered directly into materials through molecular interaction with electromagnetic energy, which could be transferred into thermal energy.[70] Heat can be

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generated throughout the volume of the material because microwaves can penetrate materials and deposit energy. Thus, it is possible to achieve rapid and uniform heating of relatively thicker materials.[70] In addition, microwaves are electromagnetic waves including electric and magnetic field components. The charged particles start to migrate or rotate due to the electric field force. Meanwhile, polarization of polar particles takes place. The concerted forces are rapidly changing in direction which creates friction and collisions of the molecules.[71,72] Furthermore, the molecular structure affects the ability of the microwaves to interact with materials and transfer energy. When materials contact with different dielectric properties, microwaves will selectively couple with the 16

ACCEPTED MANUSCRIPT higher lossy material.[70] Thus, it may be possible to creat materials with new or unique microstructures by selectively heating distinct phases. Microwaves may also be able to initiate

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chemical reactions that are impossible to be implemented using conventional processing via selective

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heating of reactants. Therefore, microwave irradiation method has been widely used in various applications, including molecular sieve preparation, inorganic complexes and oxide preparation,

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radiopharmaceuticals, organic reactions, plasma chemistry, analytical chemistry, and catalysis.[68,69]

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There are a series of ZnS nanostructures with controllable crystal phase and morphology via a microwave irradiation method as shown in Table 3 and Fig. 8. It can form a hierarchical structure

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without template agents by microwave irradiation method and the particles are round/rod in shape

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with a good uniform size distribution.[74] From the preliminary investigations, it has been shown that

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the morphology and particle size of ZnS nanostructures could be controlled by synthetic method, zinc precursor, initial Zn2+ concentration, solvent, surfactant, and reaction temperature and time. For the

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microwave irradiation method, it could produce the most quantities, hierarchical structure, and uniform size distribution of ZnS powders. 3.4 Summary of ZnS photocatalyst synthesis methods According to the references, the crystal phase and morphology of ZnS nanostructures could be controlled by the zinc precursors, solvents, surfactants, and synthetic methods. The zinc precursor and initial concentration can influence the formation of surface structure. It may be due to the different activity of the zinc precursors. The functional groups of the solvent and surfactant might lead to the change of the surface energy of ZnS nanocrystals by forming chemical bonds between the surface Zn 17

ACCEPTED MANUSCRIPT atoms of ZnS nanocrystals and the functional groups, which may be responsible for the formation of the various ZnS nanostructures. Therefore, the solvent and surfactant could serve as a

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surface-modifying reagent. In addition, the properties of the mixed solvent can be controlled by

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adjusting the various solvents. Thus, the difference of polarity, viscosity, and solvency might cause different crystal growth habits. Surfactant also has the hydrophilic end and a long molecular chain to

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control the structural morphology and particle size. Among the three synthetic techniques for

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preparing ZnS nanostructures, each of them has its own advantages and disadvantages. Hydrothermal and solvothermal method can create the most various morphologies, however it needs a longer

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reaction time to complete the synthetic reaction. Ultrasonic irradiation method can produce the

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nanoscale particle size without templates. Microwave irradiation method has the advantages of

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narrow particle size distribution, high purity, low cost, simple, and energy saving. Therefore, the synthetic method and experimental conditions to prepare ZnS photocatalyst need to be adopted for

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various applications and considerations.

4. Applications ZnS has many applications, such as light-emitting diodes (LEDs), electroluminescence, flat panel displays, field emission device, infrared windows, window layers in photovoltaic cells, electrooptic modulators, lasers, solar cells, chemical/biological sensors, phosphors, biodevices, photoluminescent tags for bioassays and bioimaging, photodetectors, photoconductors, gas storage, energy transformation, and catalysts.[1,5,41,78,79,80] In this review paper, we mainly focuses on the 18

ACCEPTED MANUSCRIPT applications of photocatalytic degradation and hydrogen evolution using bare ZnS or ZnS composite catalysts.

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4.1 Photocatalytic degradation of organic contaminants

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With the rapid industrialization of the world, organic contaminants have become a major pollutant contributing to the deterioration of the environment.[81] The photoinduced degradation of

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several toxic inorganic and organic compounds by photochemically active semiconductors is one of

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the most challenging and interesting topics of global energy and environment management.[5,82] The process of photocatalysis is a sort of advanced oxidative processes by which a semiconductor material

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absorbs light of energy greater than or equal to its band gap, causing excitations of valence band

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electrons in the conduction band.[12,83] The electron/hole pairs can further generate free radicals,

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such as hydroxyl (•OH) in the system to redox the compounds absorbed on the surface of a photocatalyst.[12,82,83] One of the potential solutions for effective utilization is to shift the

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absorption of the semiconductor from the UV region into the visible region by allowing for more photons to be absorbed and utilized in decomposing the pollutants.[82] Table 4 collects a brief summary on photocatalytic degradation of ZnS catalysts with organic pollutants. Based on the references, the proposed mechanism for the degradation of organic pollutants using the transition metal-doped ZnS photocatalysts (denoted as M/ZnS) are suggested as follows:[24,81,84,85,96,97] step 1: When photon energy (hν) equals or exceeds the M/ZnS’s band gap, it excites an electron (e−CB) from the valence band to the conduction band. It simultaneously generates an electron vacancy, a positive charge called a hole (h+VB), in the valence band. 19

ACCEPTED MANUSCRIPT M/ZnS + hν → M/ZnS(h + VB ) + M/ZnS(e − CB )

(1)

step 2: After an electron and hole are separated by the excitation, it may undergo undesired

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(2)

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M/ZnS(h + VB ) + M/ZnS(e − CB ) → M/ZnS + heat

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recombination, including surface and volume recombination.

step 3: The transferred electrons from the valence band of ZnS to Mn+ cause the partial reduction of

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Mn+ to M(n−1)+. The holes from the valence band of ZnS to Mn+ cause the partial reduction of Mn+ to M(n+1)+, where M is the transition metal ion dopant and n is the valency of dopant ion. The energy level

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of Mn+/M(n−1)+ lies below the conduction band edge and the energy level of Mn+/M(n+1)+ lies above the

(3.2)

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M n+ + h + → M (n+1)+

(3.1)

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M n + + e − → M (n−1 )+

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valence band edge.

step 4: The e− and h+ may migrate to the surface and react with the adsorbed reactants in the desired

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process. The electrons reduce the adsorbed oxygen species to superoxide anion radical (O2•−) and the hole oxidizes the adsorbed hydroxide ion to hydroxyl radical (OH•). Reduction reaction: M(n−1)+ + O2ads → M(n−1+1)+ + O2•−

(4.1)

Oxidation reaction: M(n+1)+ + OH−ads → M(n+1−1)+ + OH•

(4.2)

step 5: The active oxygen species, HO2•, H2O2 and OH•, could be further formed from O2•−. O 2 • − + H + → HO 2 •

(5.1.1)

2HO 2 • → O 2 + H 2 O 2

(5.1.2)

O 2 • − + H 2 O → OH• + OH − + O 2

(5.2) 20

ACCEPTED MANUSCRIPT (5.3.1)

H 2 O 2 + e − → OH• + OH −

(5.3.2)

H 2 O 2 + OH• → HO 2 • + H 2 O

(5.3.3)

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O 2 • − + e − + 2H + → H 2 O 2

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step 6: The active oxygen species, O2•−, HO2•, H2O2 and OH•, lead to the partial or complete mineralization of several organic pollutants.

(6)

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O2•− + HO2• + H2O2 + OH• + organic pollutant → degradation of the organic pollutant

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The complexities of the role of transition metal dopant ion are that it can participate in all of these processes. Dopant acts as electron and/or hole traps, which reduces the recombination rate of

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electron/hole pairs, and consequently increases the lifetime of charge carriers. Thus, the energy level

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of transition metal ion affects the trapping efficiency.[81] The trapping of electrons makes it easy for

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holes to transfer onto the surface of semiconductor and react with OH− in aqueous solution and form active oxygen species to participate the destruction of organic pollutants.

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4.2 Photocatalytic hydrogen evolution from water splitting Conversion of solar energy into hydrogen is one of the most promising renewable energy technologies.[93] Photocatalytic production of hydrogen from water, H2S, and organic wastes using semiconductors is one of the potential strategies for converting the sunlight energy into chemical energy.[93] Hydrogen, a clean and renewable energy source, has become even more attractive with the depletion of fossil fuel reserves and the deterioration of the global environment.[49] Development of visible-light-driven photocatalysts for solar hydrogen production from water has been studied extensively.[107] This is a renewable process which is one of the ways to solve both the environment 21

ACCEPTED MANUSCRIPT and energy problems in the future. Visible light responsive photocatalyst systems driven by one-step and Z-schematic excitation have been developed for water splitting.[107] Till now, semiconductor

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photocatalysts, such as ZnS, have been widely used to solve the problems of environment

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pollution.[108] However, sulfide photocatalysts are not stable for water splitting, because photocorrosion could be induced when photogenerated holes oxidize the photocatalyst itself.[51] In

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the absence of sacrificial reagents, the photocorrosion reaction of chalcogenide in water would happen:[109,110]

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MS + hν → h + + e −

(9)

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2H 2 O + 2e − → H 2 + 2OH −

(8)

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MS + 2h + → M n+ + S

(7)

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Here M represents the electropositive element and n is the valency of metal ion. Chalcogenide is initiated by the reaction of photogenerated holes with active sites at the crystal surface. The final

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products of the two holes oxidation process are Mn+ and elemental sulfur. Therefore, it is necessary to design and synthesize stable chalcogenide particles which can respond to visible light even in the absence of any expensive noble metal cocatalyst. Researchers have made enormous efforts to improve the efficiency of H2 production by modifying ZnS nanostructures, as well as developing new photocatalysts. Recently, a large number of studies have been devoted to the synthesis of metal-doped ZnS photocatalysts, such as Mn/ZnS,[50] Cu/ZnS,[67,76] and Pb/ZnS,[111]

or

ternary

semiconductors,

such

as

ZnmIn2S3+m,[51,52,55]

Zn1-XCdXS,[99,109,116,117,122,123] Zn1-XCuXS,[100] Zn1-XNiXS,[75,100] Zn1-XCrXS,[124], etc. 22

ACCEPTED MANUSCRIPT The reason is multicomponent sulfides have been reported to show high photocatalytic efficiency, representing that they may be a new class of efficient visible-light-driven photocatalysts. For sulfide

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metals in the presence of aqueous sulfide/sulfite solution (Na2S and Na2SO3) as reducing reagents,

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they have been reported as promising photocatalytic solids for H2 production. It is important to take account that the electron-hole recombination process causes a reduction in the efficiency of the

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photocatalytic activity by absorbed protons.[108] The possible reaction mechanisms for the photocatalytic hydrogen evolution over the metal-doped ZnS photocatalysts (denoted as M/ZnS) in

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the presence of S2− and SO32−, which act as sacrificial reagents, are suggested as follows:[7,76,109]

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step 1 and step 2 are the same with section 4.1 as shown in equations (1) and (2).

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step 3: The 2e−CB reduce the water to form hydrogen. 2e − CB + 2H 2 O → H 2 + 2OH −

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(10)

step 4: At the same time, the h+VB oxidize SO32− and S2− to form SO42− and S22− directly. S2− ions are

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very efficient hole acceptors, therefore allowing a good separation of charge carriers. SO 3 2− + 2OH − + 2 h+ VB → SO4 2− + 2H +

(11)

2S 2− + 2 h + VB → S 2 2− (yellow)

(12)

step 5: The production of S22− ions acts as an optical filter, electron acceptor capturing the photogenerated electrons, and compete with the reduction of protons. Thus, it may prevent the water from being reduced. However, this process can be efficiently suppressed by mixing solution with SO32− ions. Furthermore, S2O32− is inactive in the reduction process, which has little negative effect on the reaction. S2O32− ions are colorless so they could avoid the decrease of the light absorption. 23

ACCEPTED MANUSCRIPT S 2 2− + SO 3 2− → S 2− + S 2 O 3 2−

(13)

step 6: The presence of excess S2− ions in the reaction solution can react with SO32− and h+VB to form

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S2O32−. Then, the hydrogen formation would drop due to the excess consumption of SO32− and 2 h+VB. (14)

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S 2− (excess) + SO3 2− + 2 h+ VB → S 2 O3 2−

In addition, optimized photocatalytic conditions for the highest hydrogen evolution have been

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investigated, considering the factors such as kinds of sacrificial reagents, the pH value of sacrificial

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reagents, the concentration of sacrificial reagents, the amount of photocatalyst, the content of doped metal, the kinds of doped metal, and the stability of photocatalyst, etc. Table 5 lists the results of ZnS

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photocatalysts used in photocatalytic hydrogen evolution from aqueous solutions containing

5. Conclusion

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sacrificial reagents under visible light irradiation.

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In this article, we review recent papers of ZnS nanocrystalline semiconductors from the synthetic processes of ZnS based nanomaterials along with the various methods to photocatalytic applications. A number of modification techniques and chemical additives have been developed in recent years to improve photocatalytic activity of ZnS under visible light irradiation. The morphology of ZnS at the nanoscale has been demonstrated to be one of the richest types among all inorganic semiconductors. Therefore, we would further cover the common and simple synthetic methods, including hydrothermal method, ultrasonic irradiation method, and microwave irradiation method, and recently developed strategies to synthesize various ZnS nanostructures for tuning the morphology, size, and 24

ACCEPTED MANUSCRIPT crystallinity. Furthermore, their utilization in the field of photocatalytic degradation of organic pollutants and photocatalytic water splitting for hydrogen evolution are well discussed. Here we also

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propose the possible reaction mechanisms for the degradation of organic pollutants and the

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photocatalytic hydrogen evolution using the metal-doped ZnS photocatalysts as an example. Although there have been many achievements in this field, several key challenges have also emerged

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that need to be overcomed for the purpose of practical applications. The efficiency of the hydrogen

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production from water splitting is too low to achieve better conversion from solar energy into hydrogen energy. Therefore, the photoelectrochemically splitting water into H2 and O2 using an

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efficient and sustainable photocatalyst should be a potential route. Obviously, the development of the

Acknowledgments

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ZnS photocatalysts is still a challenge for the renewable energy and environmental remediation.

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The research was financially supported by the Ministry of Science and Technology (MOST) in Taiwan under the contract number of MOST-104-2221-E-035-004-MY3. The support in providing the fabrication and measurement facilities from the Precision Instrument Support Center of Feng Chia University is also acknowledged.

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D

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CE P

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AC

degradation of orange II dye, Ind. Eng. Chem. Res. 52 (2013) 11904−11912. 85. R.M. Mohamed, D.L. McKinney, W.M. Sigmund, Enhanced nanocatalysts, Mater. Sci. Eng. R 73 (2012) 1–13. 86. F. Chen, Y. Cao, D. Jia, A facile route for the synthesis of ZnS rods with excellent photocatalytic activity, Chem. Eng. J. 234 (2013) 223–231. 87. L.M. Devi, D.P.S. Negi, Effect of starting pH and stabilizer/metal ion ratio on the photocatalytic activity of ZnS nanoparticles, Mater. Chem. Phys. 141 (2013) 797–803. 88. L. Yu, H. Ruan, Y. Zheng, D. Li, A facile solvothermal method to produce ZnS quantum 36

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IP

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ZnS as a visible light-responsive photocatalyst, J. Phys. Chem. C 113 (2009) 16144–16150. 91. W. Li, G. Song, F. Xie, M. Chen, Y. Zhao, Preparation of spherical ZnO/ZnS core/shell particles

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92. L. Yu, W. Chen, D. Li, J. Wang, Y. Shao, M. He, P. Wang, X. Zheng, Inhibition of photocorrosion

CE P

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AC

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NU

98. H. Labiadh, T.B. Chaabane, L. Balan, N. Becheik, S. Corbel, G. Medjahdi, R. Schneider,

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MA

Preparation of Cu-doped ZnS QDs/TiO2 nanocomposites with high photocatalytic activity, Appl.

D

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TE

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CE P

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AC

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IP

T

104. J. Chen, H. Zhang, P. Liu, Y. Li, X. Liu, G. Li, P.K. Wong, T. An, H. Zhao, Cross-linked

SC R

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105. X. Wang, H. Tian, W. Zheng, Y. Liu, Visible photocatalytic activity enhancement of Zn0.8Cd0.2S

MA

by hybridization of reduced graphene oxide, Mater. Lett. 109 (2013) 100–103. 106. H. Chen, G. Jiang, W. Yu, D. Liu, Y. Liu, L. Li, Q. Huang, Z. Tong, W. Chen, Preparation of

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electrospun ZnS-loaded hybrid carbon nanofiberic membranes for photocatalytic applications,

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TE

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evolution kinetics for visible light water splitting over the Ru/(CuAg)0.15In0.3Zn1.4S2 photocatalyst, Int. J. Hydrogen Energ. 38 (2013) 11727–11736.

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41

ACCEPTED MANUSCRIPT

2

ZnS

ZnS

Zn(NO3)2

ZnCl2

H2O H2O:EDA=50:1 H2O:EDA=10:1 H2O:EOA=50:1 H2O:EOA=10:1 VHM/VEN=0.5 VHM/VEN=1 VHM/VEN=3

H2O H2O H2O H2O H2O:ethanol=1:1 H2O:ethanol=1:1 H2O:ethanol=1:1 H2O:ethanol=3:1 H2O:ethanol=1:3

4

ZnS

ZnCl2

5

ZnS

Zn(NO3)2

H2O EN H2O+SDS H2O+PEG

6

ZnS

ZnSO4 (6 g) Zn(NO3)2 (6 g)

H2O

IP

CR

US

Zn(NCS)2(C5H5N)2

EG EN EN + 0.1 mL HNO3 EN + 0.3 mL HNO3 EN + 0.6 mL HNO3 EN + 0.8 mL HNO3 EN + 1.2 mL HNO3 1-hexanol

Reaction condition Temp. (℃) Time (hrs) 200 12 4

220

12

MA N

solvent* or surfactant#

200

180

Morphology

6

Crystallinity from XRD

Figure 6

Ref.

1-1 1-2 1-3 1-4 1-5 1-6 1-7 1-8

[38]

2-1 2-2 2-3 2-4 2-5

[40]

hexagonal

3-1 3-2 3-3

[41]

cubic

4-1 4-2 4-3 4-4 4-5 4-6 4-7 4-8 4-9

[42]

hexagonal

5-1 5-2 5-3 5-4

[43]

cubic

6-1 6-2

[44]

quasi-spheres microflowers radiate bundle nanorods interlinked nanorods grains microspheres spheres

24

nanorods spheres assembled from nanorods nanorods nanosheets

hexagonal

cubic cubic cubic + hexagonal hexagonal

nanobelts 10 20 32 48 20 48 90 20 20

AC

3

ZnS

Precursor of Zn

TE D

1

catalyst

CE P

No.

T

Table 1. A series of ZnS nanostructures with controllable crystal phase and morphology have been synthesized under different experimental conditions

195

microspheres nanoplates microspheres nanoplates microspheres

180

12

polyhedron fan-shaped hexagonal rectangles missing angle rectangle

195

10

microspheres

42

ACCEPTED MANUSCRIPT Zn(CH₃COO)₂ (6 g) Zn(CH₃COO)₂ (0.6 g)

microflowers flowerlike

N2H4•H2O

100 150 200 180

30

Zn(NO3)2 ZnCl2

H2O H2O

200 180

5 12

ZnS ZnS/CdS

Zn(CH₃COO)₂ ZnS

H2O:EN=2.4:1 H2O

170 80

12 13

Au-loaded ZnS Mn-doped ZnS

ZnO Zn(CH₃COO)₂

NaOH EOA+EN

230 160

14

ZnIn2S4

ZnSO4

H2O

ZnCl2

9 10

ZnS ZnS

11

ZnIn2S4

16

ZnS(17 mol%)–ZnIn2S4

ZnSO4

17 18

ZnIn2S4/CdIn2S4 Zn2In2S5

Zn(CH₃COO)₂ ZnSO4

H2O CTAB

19

ZnS ZnS/ZnO

Zn(CH₃COO)₂ ZnS

H2O -

hexagonal

hollow spheres microspheres

hexagonal cubic

9 10

[46] [47]

24 1

urchin-like

hexagonal

11-1 11-2

[48]

12 24

flowers sea urchin-like

hexagonal hexagonal

12 13

[49] [50]

160

1 48

microspheres microclusters and microflowers

hexagonal

160

24

microspheres

hexagonal

160

11

rod-like grains with microspheres

hexagonal

14-1 14-2 15-1 15-2 15-3 15-4 16

110 160

12 12

spherical microspheres

hexagonal/ cubic hexagonal

17 18

[54] [55]

180 500

10 12

microspheres

hexagonal

19-1 19-2

[56]

AC

ZnSO4

CE P

15

CTAB CPBr SDS methanol

nanowire

7-1 7-2 7-3 8

T

ZnS

hexagonal

IP

8

microspheres

4

CR

H2O

US

ZnSO4

MA N

ZnS

TE D

7

6-3 6-4



solvent*: EG: ethylene glycol, EN: ethylenediamine, EDA: ethanediamine, EOA: ethanolamine.



surfactant#:CTAB: cetyltrimethylammoniumbromide, CPBr: cetylpyridinium bromide, SDS: sodium dodecyl sulfate, PEG: polyethyleneglycol.

43

[45] [39]

[51]

[52]

[53]

ACCEPTED MANUSCRIPT Table 2. A series of ZnS nanostructures with controllable crystal phase and morphology have been synthesized vis ultrasonic irradiation method

Precursor of Zn

solvent* or surfactant/template#

1

ZnS

Zn/ZnO/Na2S

2

ZnS

-

3

ZnS

Zn(CH₃COO)₂

H2O Str. theromophilus L. bulgaricus H2O+DDA

6

7

ZnO

ZnO/ZnS ZnS

ZnO+Zn(NO3)2 600 mg Zn(CH₃COO)₂ 600 mg Zn(CH₃COO)₂ 300 mg Zn(CH₃COO)₂ 100 mg Zn(CH₃COO)₂

ZnS-coated SiO2 ZnS 2.0Cu/ZnS

90% EtOH+TAA 90% EtOH+TU 90% EtOH+TAA H2O

CR

US

ZnS

20 KHz

MA N

Zn(CH₃COO)₂

20 KHz/ 60 W cm2-

TE D

5

ZnS

50C 20 KHz/ 100 W cm2-

1h

30 min 3h

H2O+SiO2

Zn(CH₃COO)₂

20 h 6h 2h 3h

80C 2h

CE P

4

H2O+TAA H2O+[C4mim][NTf2]+TAA H2O+[C6mim][NTf2]+TAA H2O+[C8mim][NTf2]+TAA

Reaction condition Power or Temp. Time

T

catalyst

IP

No.

H2O+EtOH

20 kHz

0.5 h

Morphology

Crystallinity from XRD

Figure 7

Ref.

spherical hollow spheres nanotubes spherical

cubic cubic

1 2-1 2-2 3

[61]

cubic

4-1 4-2 4-3 -

quantum dots

hollow spheres solid spheres core/shell irregularly shaped fine particles microspheres spherical clusters hollow nanostructures

cubic

cubic

cubic

5-1 5-2 5-3 6-1 6-2 6-3 6-4 7-1 7-2

[62] [63]

[64]

[65]

[66]

[67]

solvent*: EtOH: ethanol.



surfactant/template#: Str. theromophilus: Streptococcus thermophiles, L. bulgaricus: Lactobacillus bulgaricus, DDA: dodecylamine, TAA: thioacetamide, TU: thiourea.



ionic liquid: [C4mim][NTf2]: 1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide, [C6mim][NTf2]: 1-hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide, [C8mim][NTf2]:

AC



1-octyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide.

44

ACCEPTED MANUSCRIPT

T

Table 3. A series of ZnS nanostructures with controllable crystal phase and morphology have been synthesized vis microwave irradiation method

catalyst

Precursor of Zn

solvent* or surfactant/capping agent#

Power

Reaction condition Temperature

Time

1 2

ZnS ZnS

Zn(NO3)2 Zn(CH₃COO)₂

H2O H2O+PVP

300 W 700 W

150C -

3

ZnS

H2O+[C4mim][NTf2]+TAA

1,000 W

4

ZnS

H2O

5

ZnS

EG

6

ZnS:Ni

Zn(CH₃COO)₂ Zn(CH₃COO)₂ ZnSO4 Zn(NO3)2 Zn(NO3)2 ZnCl2 Zn(CH₃COO)₂ Zn(NO3)2

9

Cu2ZnSnS4

Figure 8

Ref.

15 min 10 min

spheres nanorods

hexagonal hexagonal

1 2

[45] [73]

-

-

cubic

-

15 min

2.45 GHz, 800 W

140C

1 min

irregular-shaped

hexagonal

720 W

-

1 min

nanoparticles

cubic

3 4-1 4-2 4-3 5-1 5-2 5-3 6

[64]

280 W

quantum dots nanoballs nanoparticles agglomeration

US

CR

IP

Crystallinity from XRD

TE D

8

H2O+TG

Zn(CH₃COO)₂

H2O+EtOH

100 W

140C

10 min

Zn(CH₃COO)₂

H2O

600 W

-

Zn(CH₃COO)₂

CE P

7

ZnS 2.0Cu/ZnS 2% Ag – 0.4% In ZnS

Morphology

MA N

No.

EG+PVP

800 W

surfactant/capping agent#: PVP: polyvinylpyrrolidone, TG: thioglycerol.



solvent*: EG: ethylene glycol, EtOH: ethanol.



ionic liquid: [C4mim][NTf2]: 1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide.

AC



45

cubic

microspheres

cubic

30 min

regular shapes

cubic

5 min 10 min 15 min

lamellar particles ball cactus nanosheets+ spherical

kesterite

7-1 7-2 8-1 8-2 8-3

[68]

[74] [75] [76] [69] [77]

ACCEPTED MANUSCRIPT Table 4. Photocatalytic degradation of organic pollutants in the presence of ZnS catalysts under different experimental conditions

Method

notes*

Band gap (eV)

1

ZnS

solvothermal decomposition

EN TOP+HDA

3.78 4.06

2

ZnS

thermodynamic recycling

dilute acetic acid as neutralize the photocarrier quenching center

-

3

ZnS

wet chemistry

thioglycollic acid as a capping agent

3.97

organic pollutant#

T

catalyst

IP

No.

Photocatalytic degradation light

removal rate

incandescent tungsten halogen lamp (200 W)

93%/225 min 99%/225 min

[5]

5.0×10-5 M eosin B

Hg lamp (354 nm, 125 W)

100%/1 h

[2]

10 mg/L MO 10 mg/L 2,4-NP

Hg lamp (365 nm, 300 W)

95%/20 min 54%/20 min

[86]

-

1.0×10-5 M Rh.B 1.0×10-5 M BCG 1.0×10-5 M BCB

UV lamp (254 nm, 15 W)

57%/6 min 57%/6 min 54%/6 min

[3]

95%/30 min

[87]

99%/1 h

[43]

MA N

US

CR

1.6×10-5 M RB

ZnS

wet chemistry

PVP as the capping agent

5

ZnS

-

amino acid histidine as a stabilizing agent

-

1.0×10-5 M MO

6

ZnS

hydrothermal

-

-

5 mg/L MB

7

ZnS

hydrothermal

3.19

10 mg/L MO

low pressure Hg lamp (254 nm)

8

ZnS–GR

hydrothermal

3.54

10 mg/L MB

UV lamp (254 nm, 4 W)

77%/30 min 63.1%/30 min 58%/30 min 96.7%/80 min

hydrothermal hydrothermal

11 12

PVP-ZnS ZnS-RGO

microwave hydrothermal

13

N,C-codoped ZnS

14

ZnO/ZnS

15

ZnO/ZnS 1 wt%Au-ZnS

16 1 wt% Pt-ZnS 17

15 wt% ZnS- MCM-41

18

2.0 at.% Fe-loaded ZnS

CE P

ZnS ZnS

-5

Hg (Xe) arc lamp (200 W) high pressure Hg lamp (365 nm, 400 W)

[44] [88]

-

2.90

1.0×10 M Rh.B 1.14×10-4 M X-3B

Xe lamp (λ>420 nm, 350 W) Xe lamp (λ>420 nm, 350 W)

100%/30 min ~100%/110 min

[46] [89]

PVP as the capping agent 1.5 wt% graphene oxide (RGO)

4.07 4.02

10 mg/L MB 6.25×10-5 M MB

halogen lamp (400-800 nm, 500 W) Xe lamp (λ>400 nm, 150 W)

31%//6 hr 79%/1 h

[83] [47]

-

2.60

2.5 mg/L AO7

Xe lamp (λ>400 nm, 500 W)

~100%/10 h

[90]

ZnO core/ZnS shell structure

-

20 mg/L MO

low-pressure Hg tube (365 nm)

95%/2 h

[91]

microwave-assisted hydrothermal

hollow ZnO core/ZnS shell structure

-

3*10-5 M MO

UV lamp (254 nm, 4 W)

93.7%/1 hr

[92]

refluxing under an alkaline medium

Metal photodeposition/ UV light (125 W Hg arc, 10.4 mW/cm2) for 2 h

3.81

10 μM 4-nitrophenol

UV

room temperature method hydrothermal

hexadecyltrimethylammonium bromide as surfactant template -

thermal decomposition hydrothermal

AC

9 10

TE D

4

precursor of Zn: ZnSO4 precursor of Zn: Zn(CH₃COO)₂ precursor of Zn: Zn(NO3)2 graphene (GR) as an ideal platform

Ref.

72%/4 h

3.03

[94] 78%/4 h

4.04

0.32 mg/L MB

UV

99%/15 min

[95]

3.24

500 g C of oxalic acid

NEC T10 black light blue (20 W)

100%/20 min

[96]

46

ACCEPTED MANUSCRIPT 500 g C of formic acid hydrothermal

20

TiO2/Cu:ZnS

ultrasound

ZnS as both a precursor and a sacrificial template 3-mercaptopropionic acid (MPA) as surface ligand

-

1.0×10-5 M MO

Xe lamp (350 nm, 300 W)

91%/36 min

[6]

3.12

10 mg/L SA

fluorescent lamp (365 nm)

90%/1 h

[98]

1.0×10-5 M MO 1.0×10-5 M PB 1.0×10-5 M Rh.B 1.0×10-5 M MB

UV (40 W)

77%/75 min 66%/75 min 84%/75 min 85%/75 min

[48]

10 mg/L MB

Xe lamp (300 W)

96%/1 h

[99]

10 mg/L MB

halogen lamp (1000 W)

100%/3 h

[81]

5 mg/L Congo red

mercury vapor lamp (332 nm, 400 W)

T

ZnS–Ag2S

ZnS/CdS

hydrothermal

-

3.65

22

Zn0.2Cd0.8S

-

-

23

Zn0.97Cu0.03S

hydrothermal chemical precipitation hydrothermal

Zn0.94Ni0.06S chemical precipitation

-

-

MA N

Zn0.90Cu0.10S 24

US

21

2-Mercaptoethanol (2-hydroxyethanthiol, HOCH2CH2SH): capping agent

4.75

-

-

10 mg/L MB

halogen lamp (1000 W)

100%/2 h

[101]

2-Mercaptoethanol: capping agent

3.68

10 mg/L MV

mercury lamp (40 W)

98.74%/2 h

[102]

-

-

20 mg/L VBR

mercury lamp (40 W)

99.63%/1 h

[103]

1.92 2.58

10 mg/L 4-nitrophenol 0.01 mM MB

Xe lamp (λ>400 nm, 500 W) Xe lamp (λ>420 nm, 300 W)

97.8%/6 hr 96%/2 h

[104] [105]

3.41

2.0×10-3 M 4AT

metal halide lamp (300-400 nm; 400 W)

90%/1 h

[106]

Zn0.97Fe0.03S

26

Zn0.95Fe0.05S

27

Zn0.95Fe0.05S

28 29

ZnIn2S4/rGO-1.5% Zn0.8Cd0.2S/rGO-5 wt%

hydrothermal hydrothermal

rGO: reduced graphene oxide rGO: reduced graphene oxide

30

GO/ZnS-CNFs

electrospinning

CNFs: carbon nanofibrous membranes

notes*:

CE P

AC



TE D

25

chemical co-precipitation chemical precipitation

CR

IP

19

100%/25 min

99%/2 h

4.41

[100] 92%/2 h



solvent: EN: ethylenediamine, TOP: trioctylphosphine, HDA: hexadecylamine, EG: ethylene glycol, EDA: ethanediamine, EOA: ethanolamine.



surfactant:CTAB: cetyltrimethylammoniumbromide, CPBr: cetylpyridinium bromide, SDS: sodium dodecyl sulfate, PEG: polyethyleneglycol.



organic pollutant#: RB: Rose Bengal, MO: Methyl Orange, 2,4-NP: 2,4-dinitrophenol, Rh.B: Rhodimine B, BCG: Bromocresol Green, BCB: Bromochlorophenol Blue, MB: Methylene Blue, X-3B: Reactive Brilliant X-3B, AO7: Acid Orange 7, SA: Salicylic Acid, PB: Pyronine B, MV: Methyl Violet, VBR: Victoria Blue R dye, 4AT: 4-aminotoluene.

47

ACCEPTED MANUSCRIPT Table 5. Photocatalytic hydrogen production rates of ZnS catalysts suspended in aqueous solution Photocatalytic hydrogen evolution

catalyst

Method

notes*

Band gap (eV)

sacrificial reagents

1

ZnS

hydrothermal

NaBH4 added as reducing agent

3.3

0.25 M Na2SO3/0.35 M Na2S

2

ZnS-UV

precipitation

methanol as hole scavenger

3.6

1.4% Pb-doped ZnS

precipitation

SO3 as an electron donor

-

4

Au-loaded ZnS

hydrothermal

RAu = 4 wt %

3.43

5

5%Cu-doped ZnS

ultrasound

Cu= 5 molar ratio

IP

50% methanol

2.97

Ref.

light

H2(g)

Xe-arc lamp (λ>420 nm, 300 W)

232.7 μmol h-1

[25]

Hg lamp (λ=254 nm, Pen-Ray model)

4,825 μmol h-1g-1

[108] [111]

0.5 M K2SO3/0.005 M Na2S

Xe lamp (λ>420 nm, 300 W)

31 μmol h

0.25 M Na2SO3/0.35 M Na2S

Xe-arc lamp (λ>420 nm, 350 W)

3,306 μmol h-1g-1

[49]

0.1 M Na2S

Xe lamp (λ>400 nm, 350 W)

6.7 μmol h-1g-1

[67]

US

3

CR

2−

T

No.

-1

2%Cu-doped ZnS

microwave

Cu= 2 molar ratio

2.71

0.1 M Na2S

Xe lamp (λ>400 nm, 350 W)

973.1 μmol h g

[76]

7

ZnS1–x–0.5yOx(OH)y(1:1)

precipitation/calcination

ZnS solid solution

2.23

0.1 M Na2SO3/0.04 M Na2S

halide lamp (λ>420 nm, 400 W)

~16 μmol h−1

[112]

8

CdS/ZnS/1 wt% Ru

stepped chemical bath deposition

-

-

formic acid

Xe lamp (λ>420 nm, 300 W)

123 mmol h−1m2

Xe lamp (300 W)

135 mmol h−1m2

[113]

9

(CdS+ZnS)/Fe2O3

ultrasound

core/shell structure

-

0.2 M Na2SO3/0.2 M Na2S

pen type halogen lamp (219 W/m2)

1,451 μmol h−1g−1

[114]

10

5CdS-ZnS/ZTP

two-step sulfidation procedure

2.6

0.02 M Na2S

He lamp (λ>420 nm, 125 W)

2142.7 μmol

[115]

TE D

MA N

6

-

-1 -1

−1

−1

Cd0.62Zn0.16S

coprecipitation

solid solution

2.35

0.1 M Na2S/0.04 M Na2SO3

He lamp (λ>400 nm, 300 W)

Cd0.7Zn0.3S

coprecipitation

Na2S as precipitating agent

2.64

0.02 M Na2SO3/0.05 M Na2S

Xe-arc lamp (λ>420 nm, 300 W)

350 μmol h−1g−1

[117]

13

Cd0.44Zn0.56S

-

SDS as the surfactant

2.47

0.35 M Na2SO3/0.25 M Na2S

Xe lamp (λ>420 nm, 500 W)

2,640 μmol h−1g−1

[109]

14

ZnIn2S4

CTAB-assisted hydrothermal

Pt as a cocatalyst

2.43

0.25 M K2SO3/0.35 M Na2S

Xe lamp (λ>430 nm, 300 W)

112.45 μmol h−1

[51]

15

Zn2In2S5-CTAB

hydrothermal method

CTAB as surfactant

2.62

0.25 M Na2SO3/0.35 M Na2S

Xe lamp (λ>430 nm, 300 W)

31.9 μmol h−1

[55]

17

1 wt%Pt-loaded ZnIn2S4

0.025 mol% NiS/ Cd0.5Zn0.5S

hydrothermal

18

3 wt%Pt-loaded AgInZn10S18

19

0.5 wt% Pt-loaded ZnS(17 mol%)–ZnIn2S4

precipitation/photodeposition solvothermal

20

0.75 wt% PdS-loaded ZnIn2S4/CdIn2S4

21 22

16,320 mol h g

2.33

27.7 μmol h

CTAB

2.51

122.5 μmol h−1

CPBr

2.36

SDS

2.35

NiS as active sites

2.62

Pt as cocatalyst

[116]

−1

no surfactant

AC

16

surfactant-assisted hydrothermal (surfactant: CTAB, CPBr, SDS)

CE P

11 12

0.25 M Na2SO3/0.35 M Na2S

Xe lamp (λ>430 nm, 300 W)

9.0 μmol h−1

[52]

20.1 μmol h−1 0.25 M Na2SO3/0.35 M Na2S

Xe lamp (λ>430 nm, 300 W)

1.4 mmol h−1

[118]

−1

[119]

340 μmol h

2.37

0.25 M K2SO3/0.35 M Na2S

Xe lamp (300 W)

glucose as an electron donor

2.43

0.10 M glucose/0.10 M NaOH

halide lamp (λ>420 nm, 400 W)

~28 μmol h−1

[53]

hydrothermal

PdS as cocatalyst

2.29

0.8 M Na2SO3/0.7 M Na2S

Xe lamp (λ>400 nm, 300 W)

780 μmol h−1

[54]

Ru/(CuAg)0.15In0.3Zn1.4S2

ultrasound

I− as an electron donor

1.9

KI

Xe lamp (300 W)

~525 μmol h−1

[120]

Pd/Zn0.4(CuGa)0.3Ga2S4

solid state reaction/photodeposition

Pd as cocatalyst

2.59

0.5 M K2SO3/0.4 M Na2S

Xe-arc lamp (λ>420 nm, 300 W)

308 μmol h-1

[107]

*notes: ZIP: Zirconiumetitanium phosphate, CTAB: cetyltrimethylammonium bromide, CPBr: cetylpyridinium bromide, SDS: sodium dodecyl sulfate.

48

ACCEPTED MANUSCRIPT Figure Captions

IP

T

Figure 1. (a) Donor level and (b) acceptor level formed by metal ion doping

SC R

Figure 2. New valence band formation by doping of nonmetal ions

Figure 3. Schematic diagram of dye-sensitized photocatalytic mechanisms

NU

Figure 4. Proposed reaction mechanisms for hydrogen evolution using the CuS/ZnS composite

MA

photocatalyst

Figure 5. Schematic diagram showing the midgap defect states of ZnS created by an S vacancy

D

before (left) and after (right) relaxing the structures around the vacancy site.

TE

Figure 6. FE-SEM and HR-TEM images of ZnS products obtained at various solvents and reaction

CE P

conditions under hydrothermal condition

Figure 7. FE-SEM and HR-TEM images of ZnS products obtained at various solvents and reaction

AC

conditions by ultrasonic irradiation method Figure 8. FE-SEM and HR-TEM images of ZnS products obtained at various solvents and reaction conditions by microwave irradiation method

49

ACCEPTED MANUSCRIPT Figure 1 (b)

CB Vis

acceptor level

T

UV

CB

donor level

UV

VB

Vis

AC

CE P

TE

D

MA

NU

SC R

VB

IP

(a)

50

ACCEPTED MANUSCRIPT Figure 2

AC

CE P

TE

D

MA

NU

SC R

VB

IP

UV Vis new valence band

T

CB

51

ACCEPTED MANUSCRIPT Figure 3 degradation products

organic compounds

e-

e-

dye VB visible light

NU

CB recombination

electron donor regeneration

O2

AC

CE P

TE

D

MA

semiconductor

52

H+ reaction 2:

SC R

dye*

reaction 1: organic degradation

IP

‧O2-

T

electron injection

water reduction H2

ACCEPTED MANUSCRIPT Figure 4 CuS cluster H2

IP

T

H2 O

IFCT

S2－, SO32－ visible light

oxidation

SC R

ZnS

AC

CE P

TE

D

MA

NU

IFCT: interfacial charge transfer

53

ACCEPTED MANUSCRIPT Figure 5 CB

CB 2a

1e

T

1t

1a VB

AC

CE P

TE

D

MA

NU

SC R

VB

IP

relaxation

1a

54

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

Figure 6

55

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

Figure 7

56

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

SC R

IP

T

Figure 8

57

ACCEPTED MANUSCRIPT

Graphical Abstract

‧O2dye*

O2

-

e

NU

e-

dye VB visible light

MA

CB recombination

electron donor regeneration

reaction 1: organic degradation

SC R

electron injection

degradation products

T IP

organic compounds

AC

CE P

TE

D

semiconductor

58

H+ reaction 2:

water reduction H2