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Review

Design Principles and Material Engineering of ZnS for Optoelectronic Devices and Catalysis

Xiaojie Xu, Siyuan Li, Jiaxin Chen, Sa Cai, Zhenghao Long, and Xiaosheng Fang*

ZnS, as one of the first semiconductors discovered and a rising mate-rial star, has embraced exciting breakthroughs in the past few years. To shed light on the design principles and engineering techniques of ZnS for improved/novel optoelectronic properties, the fundamental mecha-nisms and commonly employed strategies are proposed in this review. Recent progress on modifications of ZnS allows it to be extensively and effectively used in versatile applications, including transparent conduc-tors, UV photodetectors, luminescent devices, and catalysis, which are clearly and comprehensively summarized in this work. Novel functional devices springing up from the newly developed ZnS-based materials are highlighted as well. This review not only provides a scientific insight into the advances of ZnS-based materials, but also touches on the future opportunities in this inspiring field.

DOI: 10.1002/adfm.201802029

Dr. X. J. Xu, S. Y. Li, J. X. Chen, S. Cai, Z. H. Long, Prof. X. S. FangDepartment of Materials ScienceFudan UniversityShanghai 200433, P. R. ChinaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201802029.

relatively high charge recombination rate and low charge transport process inhibit the optoelectronic properties of ZnS as a high-performance photodetector. As a window layer of optoelectronic devices, a lack of high electrical conductivity typically hinders the practical use of ZnS in solar cells, photodiodes, light-emitting devices, etc. As a photocatalyst, the transparency of ZnS becomes a drawback as it suffers from a poor utilization of sunlight.

There is not a perfect material, but the potential of developing a material is beyond limitation. The past few years have witnessed extensive designing and engi-neering of ZnS to explore its potentials in specific applications.[6–20] This review summarizes the recent breakthroughs on ZnS-based materials as excellent can-

didates in optoelectronic devices, photocatalysis, and other novel functional devices. More importantly, it sheds light on the design principles and fundamental mechanisms of the improved performance of ZnS through strategies such as devel-oping novel nanostructures, bandgap engineering, alloying with other materials, etc. To the best of our knowledge, it is the first comprehensive review on the design principles and material engineering of ZnS for novel/improved properties and intended applications. Briefly, current literature on a variety of ZnS-based structures was first summarized, ranging from 0D to 2D nanostructures. Then, the representative applications of ZnS-based materials are described, which are mainly consisted of four parts, as shown in Figure 1: transparent conductors, UV photodetectors, luminescent devices, and catalysis. Each part contains the latest design strategies of ZnS for the intended applications, fabrication techniques, representative examples, and the state-of-the-art devices. Finally, it outlines the chal-lenges and opportunities in each field. In particular, we provide our perspectives on the future research directions of ZnS in energy and environment.

2. ZnS Nanostructures

In the past few years, a rich variety of ZnS composite nano-structures have been developed. In general, it can be classified into three groups on the basis of morphologies, which are 0D, 1D, and 2D nanostructures, as shown in Figure 1 and 2. In this section, we highlight the representative nanostructures of ZnS-based materials in recent publications. A more comprehensive summary can be seen in Table 1.

Photodetectors

1. Introduction

ZnS, a wide bandgap semiconductor (≈3.72 eV for cubic zinc blende (ZB) and ≈3.77 eV for hexagonal wurtzite (WZ)), is con-sidered as a developing material star by researchers due to its remarkable chemical and physical properties, including polar surfaces, a high optical transmittance toward visible light, good electron mobility, decent charge transport properties, thermal stability, etc.[1] The unique fundamental properties endow ZnS with great potential in diverse applications, such as display technologies, luminescent devices, sensors, solar cells, biolog-ical devices, etc.[2–4]

ZnS has a large direct bandgap corresponding to the UV-light region, suggesting it is a UV-responsive and visible–blind semiconductor.[5,6] Thus, ZnS has been extensively studied as a UV photodetector (PD). Meanwhile, its high transmittance in the visible light range makes it a good window layer for opto-electronic devices. However, for plain ZnS, there remain critical issues to address to maximize its performance in intended appli-cations. For example, the photo/dark current ratio and response speed are key parameters for a photodetector. However, the

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2.1. 0D Nanostructures

0D nanostructures have all three dimensions at nanoscale (<10 nm). Diverse methods have been reported to synthesize different kinds of 0D ZnS composite nanostructures, especially core–shell quantum dots (QDs).

Quantum dots are so small in size that their optical and electronic properties differ from bulk materials due to the quantum effect. A variety of QDs show strong luminescence once applying light or electricity. These emission wavelength can be tuned by controlling the materials, core–shell structures, sizes, etc., which gives rise to applications in versatile fields.[24] Therefore, researchers are highly motivated to develop various methods to synthesize QDs.

Wang et al. fabricated CdSe/CdS/ZnS core-multishell QDs by a facile one-pot method at 300 °C. The CdSe/CdS/ZnS QDs (≈6.57 ± 0.63 nm) showed a strong PL intensity and great poten-tials in coherent random lasing.[14] Benayas et al. synthesized high-quality water-dispersible core/shell/shell PbS/CdS/ZnS QDs (≈5.6 ± 0.4 nm) through an injection procedure. The emission wavelength of the as-obtained PbS/CdS/ZnS QDs lied within the second biological window (1000–1350 nm), which is promising in biological applications.[21] Ko et al. used a multistep hot injec-tion method with a highly concentrated zinc acetate dihydrate precursor to synthesize highly efficient bright green-emitting Zn–Ag–In–S (ZAIS)/Zn–In–S (ZIS)/ZnS alloy core/innershell/shell QDs. The ZAIS/ZIS/ZnS QDs, with an average size of ≈3.35 nm, showed a high PLQY of 87% at a wavelength of 501 nm.[22]

2.2. 1D Nanostructures

1D nanostructures have attracted great attention from researchers because of their diversity in morphology. Nano-tubes, nanowires, nanobelts, and nanorods have been widely reported in ZnS systems, especially ZnS/ZnO core–shell nano-structures. Two main methods used in preparations of 1D ZnS nanostructures are chemical vapor deposition (CVD) and thermal evaporation.

2.2.1. Nanotubes

Nanotubes have a hollow structure, which brings a high spe-cific surface area that favors optical absorption and unique elec-tronic properties.

Chang synthesized ZnO/ZnS nanocable and nanotube arrays via aqueous chemical growth method.[26] Tarish et al. removed the anodic aluminum oxide (AAO) template by a simple sulfi-dation process and converted ZnO/AAO nanotube to ZnO/ZnS nanotube.[25]

2.2.2. Nanorods

Nanorods have solid structures with mostly hexagonal cross-sec-tions, among which core–shell nanorods are widely investigated.

Wang et al. used a two-step solution immersion and sub-sequent sulfidation method to produce FeS2-sensitized ZnO/

ZnS nanorod arrays.[16] Facile thermal evaporation route was employed by Liu et al. for growing both ZnS nanorods and nanobelts.[27]

2.2.3. Nanowires

Nanowires also show solid structures like nanorods. However, nanowires are longer, thinner, and more flexible with round cross-sections.

Siyuan Li received his B.S. degree in materials science from Qiushi Honors College, Tianjin University, China. After that he joined Prof. Xiaosheng Fang’s group and now a graduate student in Department of Materials Science, Fudan University. His current research inter-ests include the synthesis of inorganic, wide bandgap

semiconductors, and the fabrication of low-dimensional ultraviolet photodetectors.

Xiaosheng Fang is currently a professor in the Department of Materials Science, Fudan University, China. He obtained his Ph.D. degree from the Institute of Solid State Physics (ISSP), the Chinese Academy of Sciences in 2006. After then, he was a Japan Society for the Promotion of Science (JSPS) postdoctoral fellow at the National Institute for Materials

Science (NIMS), Japan, as well as a research scientist at the International Center for Young Scientists (ICYS)–International Center for Materials Nanoarchitectonics (MANA). He started working at ZnS materials since 2001.

Xiaojie Xu received her Ph.D. degree in materials physics and chemistry, Fudan University, in 2017. She is cur-rently a postdoc researcher at State Key Laboratory of Molecular Engineering of Polymers and Laboratory of Advanced Materials, Fudan University. Her current research interests include investigations of novel p-type

transparent conductors for optoelectronic devices and developing wearable electronic devices.

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Liu et al. prepared GaP/ZnS core–shell nanowires via a typical CVD method.[30] Cao et al. used atomic layer deposition (ALD) to synthesize ZnO–ZnS core–shell nanowires.[31]

2.2.4. Nanobelt

Nanobelts are flexible with rectangular cross sections.CVD is also extensively used in synthesis of ZnS nanobelts.

Hu et al. obtained GaP/ZnS biaxial nanostructures via CVD.[4] Thermal evaporation is another technique to prepare ZnS nanobelts. Huang et al. used this technique to fabricate ZnO/ZnS core–shell nanobelts.[33]

2.3. 2D and Other Nanostructures

2D nanostructures, including films, nanoplates, 1D nanostruc-ture arrays, etc., are reported in ZnS-based materials as well.

Tian et al. reported a nanofilm network formed of branched ZnS/ZnO nanostructures prepared via a simple thermal- evaporation method, which held great potentials as a flexible UV photodetector.[32] Ye et al. synthesized 2D nanobowl arrays[9] and Su et al. prepared a ZnS polyhedral nanocrystal array via a self-assembly process.[34]

Further, 2D nanostructures are also explored in alloying with other materials with different shapes.[36] Sun et al. reported ZnS dielectric matrix by an alternate deposition process, composited with PbS quantum dots.[20]

Further, ZnS microspheres composited with micro flower-shaped MoS2 were reported by Gomathi et al.[3] Our group presented controllable synthesis from ZnS nanoparticles and ZnS–CdS nanoparticle hybrids with a facile two-step solution-phase method. By changing the starting molar ratios, the size and morphology of the nanoparticles can easily be tuned.[24]

In a short summary, the richness of ZnS composite nano-structures springing up recently allows for a wider design space for efficient optoelectronic devices and new emerging

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Figure 1. Recent progresses on versatile applications springing up from various ZnS-based materials, ranging from 0D to 2D nanostructures in applications including transparent conductors, photodetectors, light-emitting devices, and catalysis. Reproduced with permission.[6] Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA; Reproduced with permission.[9] Copyright 2013, Royal Society of Chemistry; Reproduced with permission.[10] Copyright 2015, Elsevier B.V.; Reproduced with permission.[7] Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA; Reproduced with permission.[11] Copyright 2014, Wiley-VCH Verlag GmbH & Co. KGaA; Reproduced with permission.[8] Copyright 2014, Royal Society of Chemistry; Reproduced with permission.[12] Copyright 2010, Elsevier B.V.



functional applications, which is discussed in detail in the following sections.

3. Applications

3.1. Transparent Conductors

Transparent conductors that play an indispensable role in a variety of optoelectronic devices can be seen everywhere in our daily life such as light-emitting diodes (LEDs), smartphones, solar cells, display technologies, etc.[37–40] Thus, extensive efforts and progress have been made on developing high-performance transparent conductive materials (TCMs) for next-generation functional electronic devices.[41–44]

3.1.1. P-Type ZnS Transparent Conductors

To date, most of the commercial TCMs, such as tin doped indium oxide (ITO),[45] aluminum-doped zinc oxide (AZO),[46] and fluorine-doped tin oxide (FTO), are n-type.[47,48] The con-ductivities of p-type TCMs (1–250 S cm−1) are almost an order of magnitude lower than that of most conducting n-type TCMs (1000–10 000 S cm−1).[41] Thus, high-performance p-type TCMs

are highly desired for the developments of efficient optoelec-tronic devices. The difficulties in realizing highly conductive p-type TCMs mainly stem from the specific electronic struc-tures of most wide bandgap oxide semiconductors. The highly localized oxygen 2p valence band and the large hole effective mass generally lead to a very low hole mobility.[49] In addition, most oxides have a high ionization potential, making it very hard to achieve a high hole concentration through doping.[50]

In the past few years, researchers have looked beyond oxide semiconductors in search of p-type TCMs. Chalcogenide semi-conductors with large bandgaps have become attractive as their valence bands are more delocalized in comparison with oxides, resulting in a lower hole effective mass.[51] In this regard, ZnS, with a direct bandgap of ≈3.7 eV, has been explored and dem-onstrated as an excellent candidate for developing p-type TCMs.

In principle, there are two ways to obtain hole conductivity in ZnS while keeping it fairly transparent in visible light range: doping and forming nanocomposites.

Doping: It is proposed that by doping ZnS with Cu+, a p-type behavior could be obtained, as illustrated in Figure 3a, which is mainly ascribed to two factors: 1) the hole concentration is increased with Cu+ substituting Zn2+; 2) the hole mobility can also be improved as the effective mass will be lowered when Cu 3d10 hybridizes with S 3p states at the valence band max-imum (VBM).[52]

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Figure 2. Morphologies of 0D-3D ZnS-based nanostructures. a) TEM image of CuInS2/ZnS quantum dots. b) TEM image of multishell-coated CdSe/ZnS quantum dots. c) CdSe/CdS/ZnS quantum dots. d) Low-magnification TEM image of ZnO/ZnS core–shell nanorod heterojunction. e) SEM image of ZnO/ZnS nanowire arrays. f) SEM image of heterostructured ZnS/InP nanowires. g) SEM image of CGO/ZnS heterostructured film. h) SEM images of ZnS nanobowl arrays. i) Field emission scanning electron microscopy (FESEM) cross-sectional views of PbS/ZnS. a) Reproduced with permission.[13] Copyright 2017, American Chemical Society. b) Reproduced with permission.[14] Copyright 2014, Wiley-VCH Verlag GmbH & Co. KGaA. c) Reproduced with permission.[15] Copyright 2013, American Chemical Society. d) Reproduced with permission.[16] Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA. e) Reproduced with permission.[17] Copyright 2016, American Chemical Society. f) Reproduced with permission.[18] Copyright 2017, Royal Society of Chemistry. g) Reproduced with permission.[19] Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA. h) Reproduced with permission.[9] Copyright 2013, Royal Society of Chemistry. i) Reproduced with permission.[20] Copyright 2013, Wiley-VCH Verlag GmbH & Co. KGaA.



Later, Ager group fabricated Cu-alloyed-ZnS thin films via pulsed laser deposition (PLD) at 550 °C and demonstrated their p-type conductivities and transparency. The optimal film exhibited a hole conductivity of ≈54 S cm−1 and an optical transmission of 65% at 550 nm.[52] With the aim of reducing synthesis temperature, Woods-Robinson et al. reported room-temperature PLD deposition of Cu-alloyed-ZnS films.[53] The as-prepared CuxZn1−xS films showed high p-type conductivities, up to 42 S cm−1 at x ≈ 0.3, with an optical bandgap tunable from 3.0 to 3.3 eV. To explore other possible deposition techniques in preparation of Cu–Zn–S films, Maurya et al. investigated the electrical and optical properties of Cu-alloyed-ZnS thin films

deposited via radio frequency sputtering.[54] Interestingly, greatly enhanced hole conductivities (≥400 S cm−1) were obtained for the films with 30% and 40% of Cu, with an average trans-parency of ≥75% for film with 40% Cu as the corresponding crystal structure showed the presence of crystalline Cu2S phase.

Nanocomposite: We recently reported novel approach to achieve both high hole conductivity and transparency to make nanocomposite structures.[55] CuS, which is known as an intrinsic hole conducting material with a small bandgap of ≈2.1 eV, is generally not considered as an ideal candidate for p-type TCMs due to its high light absorption in the visible light spectrum. However, we demonstrated an interesting strategy by making ZnS–CuS nanocomposite films via a simple chemical bath deposition.[55] In such films, CuS nanocrystals served as the hole-conducting network while ZnS nanocrystals provided the optical transmittance as the transparent fillers, as illus-trated in Figure 3b. More importantly, the homogeneously dis-tributed ZnS and CuS nanocrystals were very small (less than 5 nm), which aids to open the bandgap of the films due to the quantum effect. The hole conductivity and transparency of the nanocomposite films could be tuned by adjusting the composi-tion ratio of ZnS to CuS nanocrystals. The optimal conductivity reached >1000 S cm−1, comparable to n-type TCMs.

To demonstrate the great potentials of this state-of-the-art p-type TCM, various efficient optoelectronic devices employing ZnS–CuS nanocomposite film as the hole selective contact were presented,[55] such as p-ZnS–CuS/n-Si heterojunction solar cells (Figure 4a). These solar cells exhibited a maximum open-circuit voltage of ≈530 mV, comparable to that of the best-in-class carbon nanotube/Si and graphene/Si photovoltaics. A fully

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Figure 3. Schematic illustrations of two proposed mechanisms for developing p-type ZnS transparent conductors. a) Cu-doped ZnS struc-ture with Cu+ replacing Zn2+, a higher hole concentration, and a lower effective mass will lead to a higher p-type conductivity. b) ZnS–CuS nanocomposite structure with tiny CuS nanocrystals working as the hole-conducting network and ZnS nanocrystals as the transparent fillers, both nanocrystals are ≈5 nm.

Table 1. ZnS nanostructures of various dimensions.

Dimension Structure Composition Synthesis method Size [nm] Application Ref.

0D Quantum dot CdSe/CdS/ZnS (shell) One-pot method 4.96 ± 0.32 Luminescence [14]

PbS/CdS/ZnS (shell) Injection 5.6 ± 0.4 Luminescence [21]

Zn–Ag–In–S (core)/Zn–In–S (inner

shell)/ZnS (shell)

Injection 3.35 Luminescence [22]

CdZnS/ZnS (shell) Injection 8.5 Luminescence [23]

1D Nanotube ZnO (core)/ZnS (shell) ALD 28 Optoelectronics [25]

ZnO (nanocable)/ZnS (nanotube) Aqueous chemical growth method 96.6 ± 20.7/22.6 Photocatalysis [26]

Nanorod ZnO(core)/ZnS(shell) Hydrothermal 50–80/8.5 Luminescence [16]

ZnS nanorods and nanobelts Thermal evaporation 200–2000 Luminescence [27]

Ultranarrow ZnS nanorods Thermal decomposition 3.25 ± 0.2 Optoelectronics [28]

Ce2S3–ZnO/ZnS core–shell nanorods Aqueous solution growth 30-30 Photocatalysis [29]

Nanowire GaP/ZnS CVD 50–200 Luminescence [30]

ZnO (core)/ZnS (shell) ALD 85.4 Electrode [31]

ZnS/InP CVD 207 Photodetector [18]

ZnS (nanowires)–ZnO branched

heterostructures

Thermal evaporation 30 Photodetector [32]

Nanobelt GaP/ZnS CVD 480 Photodetector [4]

ZnO (core)/ZnS (shell) Thermal evaporation 40–350 Optoelectronics [33]

2D Thin film ZnS/CuS Chemical bath deposition 50 Optoelectronics [55]

Nanobowl array ZnS Solution deposition 400, 1000 Photonic crystal [9]

Embedded matrix PbS (QD)/ZnS (matrix) In situ solution method – Optoelectronics [20]



transparent photodiode with the structure of p-ZnS–CuS/n-ZnO was also reported,[6] as shown in Figure 4b. This device not only showed a high rectification ratio, but also worked as a self-powered UV photodetector due to the photovoltaic effect from the p–n junction.

3.1.2. N-Type ZnS Transparent Conductor

ZnS, as an earth-abundant, nontoxic and relatively inexpensive wide bandgap semiconductor, has shown great potentials as a high-performance p-type TCM. However, it remains quite a challenge to synthesize an excellent n-type ZnS TCMs as the intrinsic Zn vacancies (hole defects), which are near the con-duction band minimum, would compete with n-type dopants as the thermodynamically stable defects.[56]

Faghaninia et al. employed hybrid density functional theory and AMSET (ab initio model for mobility and Seebeck coeffi-cient using the Boltzmann transport equation) to analyze the physical stability, optical transparency, and electrical conduc-tivity of ZnS with different cations (B, Al, Ga, and In) and anion dopants (F, Cl, Br, and I).[56] According to their calcula-tions, Al-doped ZnS (AZS) is the most promising n-type ZnS TCM as it exhibits the highest dopant solubility, largest elec-tronic bandgap, and highest electrical conductivity (up to ≈3830 S cm−1) at the optimal dopant level of 6.25%.

Liao et al. reported Al-doped ZnS thin films synthe-sized via chemical bath deposition.[57] The films showed an n-type behavior. After a fast annealing process, the optimal conductivity was 1.1 × 10−5 S cm−1. Further, Atwater and co-workers reported Al-doped zinc-blende ZnS films pre-pared via molecular beam epitaxy (MBE) as an n-type TCM. They have reported the maximum electron conductivity of ≈330 S cm−1.[58] The corresponding carrier concentra-tion and mobility were 4.5 × 1019 cm−3 and 46 cm2 V−1 s−1, respectively. Even though the current experimental values have not reached the ideal electron conductivity of ZnS:Al, as suggested by Faghaninia et al.,[56] the progress in developing ZnS-based TCM is quite exciting. It can be envisioned that the electrical conductivity of Al-doped ZnS will be further enhanced via fine tuning of dopant composition, deposition technique, annealing process, etc.

There have also been some demonstrations of n-type ZnS as the buffer layers for solar cells, which showed preliminary interesting

results.[59,60] Higher electron conductivity and better transparency will lead to a further improvement of the device efficiency.

3.2. UV Photodetectors

UV light is an electromagnetic radiation with the wavelengths ranging from 10 to 400 nm. UV-A light (≈320–400 nm) and UV-B light (≈290–320 nm) can reach the surface of the Earth and is believed to be a major risk factor for most skin cancers and diseases.[2,62] Thus, photodetectors which convert light to electrical signals are of great importance for UV radiation mon-itor. In addition, UV sensors play an important role in ozone hole monitoring, optical imaging, flame detection, communi-cation, air/water purification, etc.[10,63,64] So, it is desirable to develop high-performance UV photodetectors.

ZnS, with a wide bandgap corresponding to the UV region, is a suitable candidate for UV detection. Spectral selectivity, response speed, photocurrent to dark current ratio, responsivity (electrical output per optical input), and external quantum effi-ciency (EQE, the number of electron–hole pairs excited by one absorbed photo) are the key parameters to evaluate the perfor-mance of a PD.[62,65] In general, it is hard to realize both high responsivity and fast response speed in plain ZnS PDs due to the high charge recombination rate. Thus, extensive efforts have been made to improve its optoelectronic properties. Common ways to obtain high-performance ZnS UV PDs are by using suitable materials to composite with ZnS to enhance the UV-light absorption and to facilitate the charge separation of photogenerated electron–hole pairs, as illustrated in Figure 5.

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Figure 4. Demonstrations of the applications of transparent conductors in various efficient optoelectronic devices. a) p-Type ZnS–CuS transparent conductor as a hole selective contact in a Si solar cell. b) p-Type ZnS–CuS transparent conductor as a hole transport layer in a transparent photodiode. a) Reproduced with permission.[55] Copyright 2017, IOP Publishing Ltd. b) Reproduced with permission.[6] Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 5. Schematic illustrations of two main strategies employed for improving the performance of ZnS-based UV photodetectors. a) Two UV-sensitive materials (A and B) composited with each other for a higher UV-light absorption. b) Two semiconductors with a suitable band alignment that favors effective charge separation and transport.



3.2.1. ZnS–Metal Oxide Composite UV Photodetectors

Metal oxides are intensively explored to composite with ZnS in the field of UV photodetection due to their unique optical and electronic properties.

Among those metal oxides, ZnO has attracted great atten-tion for its direct wide bandgap of ≈3.37 eV at room tem-perature and excellent optoelectronic properties.[65,66] More importantly, the staggered band structure of ZnS and ZnO favors an effective charge separation process that is desirable for a high-performance UV PD. To date, various ZnO–ZnS composite structures have been fabricated, such as core–shell nanowires, biaxial nanobelts, nanobranches, heterostructured nanofilms, etc., see details in Figure 6. An overall improved performance (compared to plain ZnS PDs) of various ZnO–ZnS nanostructured PDs is observed; however, different con-figurations of the nanocomposites may contribute to the key parameters of a PD differently.

As shown in Figure 6a, Tian et al. reported nanobranch-shaped ZnO–ZnS heterostructures prepared via thermal evaporation and hydrothermal growth.[32] After a facile con-tact printing process, flexible ZnO–ZnS nanobranch PDs on PET substrate were obtained. This device exhibited excellent operational characteristics: a high responsivity toward UV light (≈2.0 × 10−23 from 300 to 360 nm) and a short response time (the average rise time and decay time are 0.77 and 0.73 s, respectively). It is also found that the photocurrent and dark current in vacuum are higher than that in air due to reduced photocarrier traps from the surface oxygen absorption.

ZnS–ZnO core–shell nanorod is another structure employed in fabricating high-performance UV PDs, as indicated in Figure 6b. A self-powered photoelectrochemical UV PD was fabricated with ZnS–ZnO core–shell nanorod array as the photoanode and deionized water as electrolyte.[67] As fabri-cated device showed a high responsivity and a fast response speed (decay time and rise time <0.02 s). This photodetector was integrated by placing Ag/polyester zigzag electrode on the top of ZnS–ZnO core–shell nanorod arrays. By applying dif-ferent pressure on the device, its responsivity and photocurrent also changed. Another interesting composite structure was side-to-side nanobelt, as can be seen in Figure 6c; the single-crystalline ZnS/ZnO biaxial nanobelts, which were grown on SiO2/Si substrate via thermal evaporation using Au as catalyst, showed a high responsivity of ≈5.0 × 105 A W−1 and an EQE of ≈2.0 × 108% under the illumination of 320 nm at a bias of 5 V.[68,69] Both the rise time and decay time of the device were about 1 s. The superior performance (compared to plain ZnS nanobelt PD) is attributed to the high-quality heterocrystal-line interface of ZnS and ZnO that facilitates the charge sepa-ration and transport process. Other than 1D nanocomposite structures, 2D nanostructures were also investigated in ZnS PDs. Hu et al. presented a ZnS/ZnO bilayer nanofilm device prepared via an oil–water interfacial self-assembly technique, as displayed in Figure 6d. This PD was fabricated by stacking one nanofilm assembled from nanoparticles on the other nano-film and using Cr/Au as electrodes.[70] The optimal properties of the bilayer UV PD were significantly improved compared to monolayer UV PDs. This device had a good EQE and spectrum

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Figure 6. Nanostructures, I–V curves, and on–off responses of various ZnS/ZnO nanocomposite UV photodetectors. a) ZnS/ZnO nanobranches, b) ZnO/ZnS core–shell nanorod arrays, c) ZnS/ZnO biaxial nanobelts, and d) ZnS/ZnO bilayer film. a,e,i) Reproduced with permission.[32] Copyright 2014, Wiley-VCH Verlag GmbH & Co. KGaA. b,f,j) Reproduced with permission.[67] Copyright 2016, Springer. c,d,g,h,k,l) Reproduced with permission.[69,70] Copyright 2012, Wiley-VCH Verlag GmbH & Co. KGaA.



responsivity as its current varied drastically between dark and illumination states at 370 nm, where the photocurrent was 4 orders of magnitude higher than that in dark. It is also found that the different stacking orders could lead to different opto-electronic properties. The maximum reported photocurrent of the ZnS (upper layer)/ZnO (lower layer) bilayer device was 6 times to that of the ZnO (upper layer)/ZnS (lower layer) device. The difference relying on stacking order may result from the whispering gallery mode (WGM) resonances in the device. These WGM resonances give rise to the improved light trapping and absorption in the device. Due to the different refractive index of ZnS and ZnO, different stacking orders show WGM resonances of different intensities, leading to varied optoelectronic properties.[70]

SnO2, with a direct bandgap of 3.6 eV, is another good candidate to hybrid with ZnS to achieve high-performance PD devices.[71,72] Similar to ZnS/ZnO, ZnS/SnO2 was also proposed with a variety of nanocomposite structures, ranging from 0D to 2D. Huang et al. reported ZnS/SnO2 core–shell ribbon-shaped field-effect transistor with ITO as electrode.[71] This device displayed a high sensitivity near 250 nm. Although the rise time and decay time were esti-mated to be 8 and 61 s, respectively, which were not as good as that of the devices made from intrinsic ZnS nanoribbons. They were better than that of n-doped ZnS PDs. The spec-tral responsivity and EQE were 6.2 × 104 A W−1 and 2.4 × 107%, respectively, indicating an effective charge separation and transport at the interface of ZnS and SnO2. Zhang et al. fabricated ZnS/SnO2 core–shell nanoparticles and integrated them into a thin film PD on polyimide substrate with gra-phene as the lateral electrodes.[72] This device exhibited a high sensitivity at 330 nm and a fair response speed. It is worth mentioning that its dark current and photocurrent before and after bending test remained stable, indicating its potential as a flexible wearable device.

In addition to ZnO and SnO2, CuGaO2 is another oxide candidate to composite with ZnS for improved optoelectronic performance.[19] CuGaO2 nanoplate/ZnS microsphere hetero-structure arrays were synthesized by oil–water interfacial self-assembly.[19] The superior photocurrent in contrast to that of plain ZnS was attributed to the higher UV-light absorption and lower charge recombination rate due to the synergistic effect of p-type CuGaO2 and n-type ZnS.

3.2.2. ZnS–Chalcogenide Composite UV Photodetectors

Chalcogenide semiconductors, including CdS and CdSe, have also been demonstrated as effective candidates to hybrid with ZnS for high-performance PDs.[73–75] Exemplified here by ZnS/CdS, Lou et al. synthesized flexible ZnS/CdS PD via thermal evaporation process and standard lithography.[76] Similar to the work mentioned above on ZnS/ZnO back-bone-branched PD, ZnS/CdS heterostructures, where CdS nanorods grew on ZnS backbones, had a large specific sur-face area, multiple light scattering, and high light absorption. Thus, it exhibited very high photo to dark current ratio (up to 105), which is higher than that of the PDs based on pure ZnS or CdS structures.

3.2.3. Others

Other than the materials summarized above, narrow bandgap semiconductors, including GaP, GaAs, and InP, have also been studied for compositing with ZnS due to their high light absorp-tion, matching band structure, and high carrier mobility.[18,77,78] ZnS nanowire arrays grown on GaAs substrate using Ga as cata-lyst were reported by Liang et al.[79] The device was ultrasensitive to UV light while almost blind to visible light, indicating its excel-lent spectral selectivity as a UV sensor. It also showed a linear rela-tionship between current and power density. The rise time and decay time at 325 nm with 10 Hz were as short as 5 and 40 ms, respectively. GaP/ZnS coaxial nanocable UV PD was fabricated via chemical vapor deposition using Cr/Au electrodes.[4] This device exhibited a high responsivity near 250 nm and a relatively low responsivity at wavelength longer than 400 nm. The enhanced optoelectronic performance was speculated from the straddling band alignment of ZnS and GaP which helps in effective charge separation.

2D materials that show outstanding charge mobility and transport properties have also been demonstrated as good can-didates to hybrid with ZnS for enhanced optoelectronic proper-ties. It has been reported that by combining ZnS with MoS2 on paper substrate, a new kind of flexible UV PD with a respon-sivity of 9.4 µA W−1 at 19.1 mW cm−2 intensity was obtained.[3] The absorption range of MoS2 could be tuned from the vis-ible–near IR spectrum to the UV–near IR spectrum, resulting in a controllable spectral selectivity of the device. Besides, a PD device with the structure of ZnS sandwiched by two graphene films was reported by Kim et al.[80] The single sandwiched ZnS device showed a responsivity of 1.9 × 103 A W−1 and an EQE of 8.0 × 105% at an applied bias of 1.0 V. This sensor also displayed a decent response speed, with a rise and a decay time of ≈2.8 and 7.5 s, respectively. Single sandwiched ZnS device was superior to the double-layer ZnS/graphene structure due to its enlarged effective junction region. The photocurrent of this device was about 10 times to that of the double-layer ZnS/graphene UV sensor. It is also noteworthy that by multiplying the sandwiched structure, the photocurrent could be further enhanced (from 37 to 115 µA at 1.0 V) due to higher UV absorption.

Recent publications on ZnS nanocomposite UV PDs are com-prehensively summarized in Table 2. The superior optoelectronic performance, in contrast to plain ZnS PDs, is mainly ascribed to the enhanced UV-light absorption as a result of the integration of two materials and more effective charge separation from the staggered band structures. Unique nanostructure such as branch-backbone shape is desirable for high-performance UV PDs to gain more light absorption. We believe that materials with a suitable band structure that will aid to facilitate electron–hole pair separa-tion, charge-carrier extraction, and suppress carrier recombination are good candidates to composite with ZnS for high-performance UV PDs. Further, materials including perovskites may also be com-posited with ZnS for developments of dual-color photodetectors.

3.3. Luminescence

ZnS has been extensively studied in luminescent applica-tions due to its unique electronic structure and excellent

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optoelectronic properties, which has been well summarized in our previous review.[231]

Herein, we highlight the recent breakthroughs on the lumi-nescent applications springing from ZnS. According to the luminescence mechanisms, ZnS-based materials can be catego-rized into two groups: metal-ion-doped ZnS (e.g., Cu2+, Mg2+, and Al3+) and QDs containing ZnS shells.

As shown in Figure 7a, it depicts the energy level diagram of ZnS:Mn2+/ZnS/ZnS:Cu2+/ZnS core/multishell QDs.[83] Reduced ZnS conduction band comes from Cu2+ doping. In codoped ZnS, the Mn2+ 6A1 states are placed above the Cu2+ d-states thus forming the upper level ground state of QDs. The orange emis-sion is resulted from the 4T1−6A1 transition of Mn2+ while the blue emission comes from the recombination of the d-orbital hole of Cu2+ with an excited electron of ZnS host. Excited by a near UV LED chip, the QDs exhibited a warm white light.

Much work so far has focused on the luminescence of QDs for the sake of a stable and efficient emission.[84,85] Research hotpots gravitate to quantum dots that typically contain ZnS shells for several reasons. i) Zn and S elements are inherently nontoxic for human body.[86] ii) Some QD cores embodying Cd/Pb need ZnS shells to realize complete isolation from the outer biological tissues.[21] iii) ZnS shells ensure the chemical and optical stability of the cores.[87] iv) To increase photolumi-nescence (PL) quantum yields (QY) of the naked cores and to enhance its luminescence.[84,87–89] v) To suppress fluorescence quenching[84] and reduce nonradiative relaxation of QDs.[90] For example, Figure 7b reveals the electronic states of tetrahedral shaped CuInS2/ZnS QDs to describe the emission mecha-nism involving a localized recombination center at a single dot level.[13] The single-particle PL line width (60 meV) is narrower than that of the ones with thick shells (352 meV), as presented in the bottom left corner of Figure 7b. This study demonstrated

that a thick ZnS shell would improve QDs’ photostability and greatly reduce PL blinking compared to core-only or thin-shell samples.

In this section, we emphasize on the recent luminescent applications of ZnS according to different excitation methods, which include photoluminescence, electroluminescence (EL), mechanoluminescence (ML), piezophotonic luminescence (PPL), and other luminescence.

3.3.1. Photoluminescence

Imaging Nanoprobes: PL is commonly used for emitting light of different wavelengths. For example, Montanarella et al. fabri-cated superparticles composed of Cd(Se, ZnS) core/(Cd, Zn)S shell nanocrystals with varied colors by controlling the ratio of mixed particles.[91] Here, we would like to show its more prac-tical use as a means of image analysis. Fluorescence imaging is a crucial technique in clinical theranostic for its low-cost and fast acquisition speed.[87,92]

Near-infrared (NIR) imaging refers to fluorescence imaging using NIR and is often used as an in vivo means for bioim-aging.[93] The second biological window (corresponding to 1000–1350 nm, and the first biological window lies in 650–950 nm) enables deep tissue imaging with a higher signal-to-noise ratio.[94] For instance, Benayas et al. demonstrated a device outperforming other similar work for they paid more attention to the selection of fluorescence wavelength. Low-dose PbS/CdS/ZnS QDs emitting in the second biological window (II-BW) gain real-time in vivo imaging with an improved detec-tion depth and resolution.[21]

Besides, multiphoton imaging using nonlinear optical pro-cesses excited by NIR pulsed lasers has been exploited for

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Table 2. Photodetectors based on ZnS heterojunctions.

Material Eg [eV] Device structure Rise time/decay time [s] Responsivity (at bias) [A W−1] Ion/Ioff (at bias) EQE (at bias) [%]a) Ref.

ZnO ≈3.37 Branched ZnO/ZnS nanofilms 0.77/0.73 – 6.2 (at 10.0 V) – [32]

ZnO ≈3.4 ZnO/ZnS core–shell nanorods Both <0.04 0.056 (at 0 V) – – [67]

ZnO ≈3.37 ZnO/ZnS biaxial nanobelts <0.3/1.7 5 × 105 (at 5.0 V) 6.9 (at 5.0 V) 2.0 × 108 (at 5.0 V) [69]

ZnO ≈3.4 ZnO/ZnS hollow microspheres – 94.5 (at 5.0 V) 1.2 × 104 (at 5.0 V) 3.34 × 104 (at 5.0 V) [70]

SnO2 3.6 SnO2/ZnS core–shell ribbons 8/61 ≈6.2 × 104 (at 1.0 V) ≈103 (at 5.0 V) ≈2.4 × 107 (at 1.0 V) [71]

SnO2 3.6 SnO2/ZnS nanoparticle films 4.2/1.5 ≈1 × 103 (at 1.0 V) 13.5 (at 10.0 V) – [72]

CuGaO2 ≈3.6 CuGaO2/ZnS microsphere films – – 2.6 (at 5.0 V) – [19]

PS – PS/ZnS nanospheres – – ≈10 (at 10.0 V) – [75]

CdS 2.4 Branched CdS nanorods/ZnS back-

bone nanobelts<10−2 – 105 (at 2.0 V) – [76]

MoS2 1.2–1.8 MoS2 nanoflowers/ZnS spheres 22/– 9.4 × 10−6 – – [3]

Cl – Chlorine-doped n-type ZnS

nanoribbons

46/30 9.7 × 104 (at 1.0 V) ≈103 (at 1.0 V) – [81]

C – ZnS sandwiched between two

graphene layers

2.8/7.5 1.9 × 103 (at 1.0 V) – 8.0 × 105 (at 1.0 V) [80]

Ag – Ag nanowires/ZnS nanotubes 0.09/0.07 2.56 (at 0 V) ≈1.9 × 104 (at 0 V) – [82]

GaP 2.26 GaP/ZnS core–shell nanocables – – – – [4]

a)EQE: external quantum efficiency.



tumor imaging as well.[86,87] One groundbreaking work success-fully achieved NIR-stimulated visible emission from ZnS:Mn nanocrystals via three-photon excitation, resulting in cellular imaging with a high spatial resolution and in vivo tumor imaging.[86]

Though fluorescence imaging shows a high sensitivity, its low spatial resolution hinders the further applications. Hence, magnetic resonance imaging (MRI), which displays high-resolution 3D images but lacking in a satisfactory sensitivity, can be a complement to fluorescence imaging.[89]

Dual-modal imaging using ZnS-based QDs or rods has aroused recent interest.[84,85,88,89,95,96] Notably, combining fluo-rescent and paramagnetic properties, Mn2+-doped QDs not only gain host PL quenching through a fast energy transfer, but also own long-lived luminescence and thermal stability.[90]

The future looks bright in use of multimodal imaging.[87,92] An interesting example was described by Lv et al. As Figure 8a shows, they constructed “all-in-one” theranostic nanomedicines on the basis of ternary CuInS/ZnS QDs. What is surprising is that they combined multispectral optical tomography (MSOT) imaging, photodynamic therapy (PDT), photothermal therapy (PTT), and fluorescence imaging together as versa-tile nanomedicines. It provides a viable source for mediating photoinduced tumor ablation.[92] It is noteworthy that the pow-erful structures as all-in-one nanomedicines meet the trend of integration of multifunctional devices. Moreover, in vivo imaging nanoprobing with a high resolution should concen-trate on enhancing Stokes shift and suppressing fluorescence quenching for a more accurate diagnosis and efficient therapy.

Other Applications: With regard to other biological applica-tions, high PL QY, NIR emitting CdSeTe/CdS/ZnS core/shell/shell QDs encoded microbeads were suggested for immunoas-says. Interestingly, wide NIR photoluminescence full width at half-maximum (FWHM) (55–75 nm) was exploited for “single

wavelength” encoding method to acquire optical codes.[97] Sim-ilar devices were obtained in studies where another group chose CdSe/CdS/ZnS QDs emitting around 605 nm as an acceptor in time-resolved Förster resonance energy transfer (FRET) immunoassays.[98] Both works valued QDs’ special performance including tunable absorption and emission wavelength, a high PL QY, and large surface.

Next, we turn our attention to two real-time thermal sen-sors in the second biological window.[7,99] Both devices utilized PbS/CdS/ZnS QDs to show emissions in the II-BW when stimulated in the I-BW for a large penetration depth in tissues and autofluorescence-free thermal sensing. One of them was further applied for PTT.[7] Though compared to emission in other spectral range, photoluminescence in II-BW for thermal sensing owns a deeper penetration for in vivo uses, and these sensors’ thermal sensitivity needs to be further promoted.

ZnS PL materials have been explored for other applications as well. For instance, CuInS2/ZnS core–shell QDs deposited on a solar cell have brought an increased conversion efficiency as a result of more light absorption.[100] CdSe/ZnS QDs are mostly picked up for their high PL QY, stability, and superiority in quantitative probing of Cu2+ for selective PL quenching by Cu2+ ions.[101] Li et al. illustrated a tunable, transparent, and stretchable luminescent film for anticounterfeiting applications through employing colloidal CdS/ZnS/ZnS:Mn2+/ZnS core/multishell QDs embedded in polydimethylsiloxane (PDMS).[102]

3.3.2. Electroluminescence

Light-Emitting Diodes: The recent publications on EL stand as a testimony to the fast developments of LEDs. A spurt of pro-gress in LEDs has recoursed to the research about QDs. QDs possess a superior color gamut property,[103] a narrow spectral

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Figure 7. Two main luminescent mechanisms of ZnS-based materials: 1) dopant states that lie in the bandgap of ZnS serve as the emission center and 2) quantum dots with ZnS as the shell. a) Energy level diagram of ZnS:Mn2+/ZnS/ZnS:Cu2+/ZnS core/multishell QDs. The CIE chromaticity diagram, CRI, and CCT values of white LEDs formed by as-mentioned QDs with a UV LED chip, and TEM image of QDs is shown below. b) Schematic depiction of electronic states of CuInS2/ZnSe QDs. The lower left shows PL for the ensemble sample (gray shading) and the single-dot spectrum (red shading) of the thick-shell CuInS2/ZnS core–shell QDs for illustration of the dramatic difference in their PL line widths. The right part refers to the tetrahedral structure and TEM image of the final thick-shell CuInS2/ZnS QDs with a narrow QD size dispersion. Reproduced with permission.[13,83] Copyright 2017, American Chemical Society.



bandwidth,[11] a clear band-edge emission,[104] etc. Applied as light sources, the main trend of recently reported ways for prep-arations of LEDs is all-solution method.[103,105–107] Moreover, other specific fabrication processes have been reported, such as electrophoretic deposition,[108] nucleation at low tempera-ture/shell growth at high temperature,[109] electrospray deposi-tion,[110] and inkjet printing.[111]

There is a general agreement that the suppression of non-radiative Auger recombination (AR) and FRET into QD processes is a necessary target for supercharging LED appli-cations.[11,103,104,106,112] Most QDs prefer core–shell structures and ZnS is an excellent shell candidate. Core–shell QDs with thicker ZnS shells find their way into the functions containing restraining Auger-type recombination,[104] enhancing external quantum efficiency,[106] improving charge injection rates,[15] and minimizing the FRET process.[107] According to Lee et al.’s results, ZnS might keep the electron–hole wave functions of core domain from leaking into QDs’ surface trap sites. As a physical barrier, ZnS shells will defend the excitonic recombi-nation against the environmental change of QDs’ surface.[103] Except for increasing shell thickness, adding a small amount of polymer to emission layer (EML) as an additive for a bal-anced charge injection[113] and replacing oleic acid ligands with a shorter one to increase the electron mobility[114] and carrier transfer properties[115] are also demonstrated as effective ways to improve the QY of QDs.

LEDs emitting light of different colors are listed in Table 3. Blue LEDs with a high PL QY[106,109] and EQE[116] are an impor-tant component for full color luminescence. Additionally, UV LEDs suffer from the impurity emission with visible light. To solve the problem, Kwak et al. introduced CdZnS ternary core with a thick ZnS shell for reducing undesired PL redshift.[104] Figure 8b illustrates that by reducing the effective core size via atomic diffusion of interior Cd atoms to the outer ZnS shell, a high performance UV LED was achieved by this method. As a full visible range emission could be excited by such device, it can be widely used in revealing hidden information.

It is vital to note the recent publications on several white LEDs. As extremely promising and urgently needed artificial lighting devices, white light-emitting diodes (WLEDs) possess numerous advantages, such as a long operational life span, low energy consumption, and high luminance.[121,122] To realize white emitting, there are three frequently adopted methods. First, combining EML emission with parasitic emission stimu-lated by adjoining charge transport layer (CTL). Second, mixing EMLs with different colors, such as red, blue, and green. Third, using single QD emitters.[105] It is important to highlight that efforts are now under way to apply QDs as color converters for WLEDs. In these circumstances, luminescent materials with an emission wavelength longer than blue (e.g., red, green, and yellow) are motivated by a blue LED chip.[22,121,122,124,132] Here, a representative example based on CuGaS/ZnS core–shell QDs

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Figure 8. Demonstrations of versatile applications of ZnS-based luminescent materials. a) CuInS/ZnS QDs used as theranostic nanomedicines with intrinsic fluorescence/MSOT imaging and PTT/PDT therapy abilities. The below images are in vitro confocal fluorescence images of cells treated with laser and show the penetration of CuInS/ZnS nanomedicines (NMs)-25 and CuInS/ZnS NMs-80 into 4T1 multicellular spheroids. b) The above images are the diagram of Cd atoms diffusion in CdZnS/ZnS QDs and schematic representation as well as the cross-sectional TEM image of CdZnS/ZnS UV LEDs. Below are the process of UV LEDs exciting visible emission and manifesting the hidden color patterns on a bill for counterfeit detection. c) Five-layer structure of HLEC and the devices in various states of deformation and illumination. d) The CIE coordinate (x, y) values of the wind-driven ML device are in the top left corner. The photographs of ML device operation under continuous N2 gas flow are shown in the top right corner. At the bottom, photographs of the wind-driven blue, green, and orange ML are shown. a) Reproduced with permission.[92] Copyright 2016, American Chemical Society. b) Reproduced with permission.[104] Copyright 2015, American Chemical Society. c) Reproduced with permission.[140] Copyright 2016, American Association for the Advancement of Science. d) Reproduced with permission.[147] Copyright 2014, Royal Society of Chemistry.



as near-UV-to-white downconverters was shown. Such Cd-free WLEDs were truly accomplished only-from-EML luminescence and the PL quantum yield reached up to 76%.[105] Furthermore, added to the blue LEDs in a tripackage, ZnAgInS/ZnInS/ZnS QDs WLEDs exhibited an even higher PL QY (87%).[22]

Similar to WLED based on downconversion, mixing red, green, and blue QDs in an active layer as EML is easier to achieve white LEDs with multiple brightness and corre-lated color temperature (CCT),[11] but at least three QDs are needed. By changing the ratio of orange CuInS2/ZnS QDs and blue ZnCdSe/ZnS QDs, Wepfer et al. demonstrated a stable WLED device with a tunable CCT. The color rendering indices (CRI) reached 78 but the maximum EQE was only 0.37%.[120] Researchers should hold fast to propelling QDs with a high

PL QY as well as stability and improving the charge balance of QDs and CTLs. Controlling FRET for a higher charge injec-tion and a lower charge recombination is the key to developing highly efficient LEDs.

Lasers: Lasers are a typical sort of EL applications. Similar to LEDs, outstanding optoelectronic properties, such as the tun-ability of the emission wavelength, have brought QDs to the fore, for example, CdSe/ZnS QDs lasing in the red.[133] Among numerous lasers, blue QD lasers remain essential for full-vis-ible-range lasers. Here, we introduce a blue laser from solu-tion containing CdZnS/ZnS alloy QDs. Because of the wide bandgap of the components, these QDs reduced the emitting wavelength down to the blue range without a significant non-radiative AR. Besides, a smaller defect density, a larger gain

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Table 3. Performance of various ZnS-based LEDs.

QDs EQE [%]a) PL QY [%]b) LED color Efficiency CCT [K]c) CRId) CIEe) Ref.

CdSe/ZnS/ZnS 12.6 71 Green 46.4 cd A−1 – – (0.075–0.079, 0.776–0.782) [103]

CdZnS/ZnS 7.1 98 Blue 2.2 cd A−1 – – (0.153, 0.027) [106]

CdZnS/ZnS 0.23 20–68 UV – – – – [104]

ZnCdS/ZnS 12.2 90 Blue-violet 1.2–4.0 cd A−1 – – (0.14–0.17, 0.02–0.03) [114]

ZnxCd1−xS/ZnS 3.8 ≈100 Violet-blue 1.13 cd A−1 – – – [109]

CuInS/ZnS 7.3 89 Yellow 18.2 cd A−1 – – – [107]

CdSe/ZnS 19.8 87 Blue 14.1 cd A−1 – – (0.136, 0.078) [116]

CdSe/CdS/ZnS 16.8 85 Blue, green, red 19.0 cd A−1 – – (0.69, 0.31), (0.21, 0.74), (0.14, 0.05) [113]

CdSe/ZnS – 75 Red 15 cd A−1 – – (0.67,0.33) [117]

AgIn5S8/ZnS 1.52 26, 29 Amber, red – – (0.5663,0.3976) [115]

CdSe/ZnS/ZnS – 38 Green 4.5 cd A−1 – – (0.20, 0.74) [111]

CdSe/CdS/ZnS 3.7, 4.1 78, 95 Orange-red, green 12, 16.4 cd A−1 – – – [15]

ZnxCd1−xS1−ySey/ZnS 0.8 75 Blue-green – – – – [118]

ZnSe/ZnS 7.83 83 Violet 1.38 cd A−1 – – (0.169, 0.023) [119]

CdZnS/ZnS, CdSe/ZnS,

CdSe/CdS/ZnS

0.8–1.3 – White 1.3–2.4 lm W−1 3982–8642 5.3–92.8 – [11]

CuInS2/ZnS, ZnCdSe/ZnS 0.37 53, 80 White 0.002–0.4 lm W−1 2200– 7200 48–65 – [120]

CuGaS/ZnS 1.9 76 White 1.9 lm W−1 7494–8234; 4700 83–88 (0.286−0.294, 0.330−0.334) [105]

ZnAgInS/ZnInS/ZnS 28–38 87 White 71–98 lm W−1 2700–10 000 89–95

R9 = 93–97

– [22]

CdS/ZnS:Cu – 40 White 46.5 lm W−1 6591 90 (0.3155, 0.3041) [121]

Cu:InP/ZnS/InP/ZnS – ≈35 White – 5000−5300 89 (0.349, 0.335), (0.335, 0.322), (0.332,

0.330)

[122]

CuGaS:Mn/ZnS – 75−78 White 21.7–29.3 lm W−1 3651−5351 85–87 – [123]

AgInS2/ZnS – 50.5, 57, 52 White 39.8 lm W−1 2634 71 – [124]

CdSe/ZnS – 38.5–42.6 White 341 lm W−1 2720 91.1 (0.4557, 0.4056) [125]

(Mn,Cu):ZnInS/ZnS – 75 White 73.2 lm W−1 2716−7700 95 (0.29–0.38, 0.270.36) [126]

CuInS2/ZnS/ZnS – 80 White 80.3 lm W−1 5497−6375 73 (0.3229, 0.2879) [127]

CuInS2/ZnS – 44–81 White 36.7 lm W−1 6552 90 (0.3163, 0.2988) [128]

CuInS2/ZnS – 60 White 3.72 lm W−1 – 91 (0.285–0.374, 0.273–0.397) [129]

CuInS2/ZnS – 50 545, 667 30.6 lm W−1 2784 84 (0.2966−0.2835, 0.3309−0.2970) [130]

CdSe/ZnS – 83 655 33.1–63.9 lm W−1 1930–3112 80.7–82.8 – [131]

a)EQE: external quantum efficiency; b)PL QY: photoluminescence quantum yields; c)CCT: correlated color temperature; d)CRI: color rendering indices; e)CIE: Commission Internationale de l’Eclairage.



cross-section and high Q-factor resonators sowed the seeds for the blue laser.[23] Feber et al. first made efforts to inspire lasing from both the core and the shell with switching ability from CdSe/CdS/ZnS QDs. The assumption came true that the core emitted red lasing while the shell lighting was green. The switching was prompted by the competition model between the core’s exciton localization and exciton-induced shell emit-ting.[134] Efforts are now under way to achieve full color lasers with lower thresholds, a higher color purity, a better stability, and multifunction.

Alternating Current Electroluminescence (ACEL): As a crit-ical part of EL, after decades of developments, ACEL devices still catch everyone’s eyes for the low cost and extensive use in lighting, display, large-scale decorations, etc.[135] With the booming of portable and wearable electronics, stretchable and flexible ACEL with a higher brightness is generally in demand. Innovations are mainly promoted by integrating polydimethyl-siloxane matrix with ZnS:Cu particles as well as transparent Ag nanowire (NW) networks. The intrinsically stretchable device presented by Wang et al. remained a perfect emission at a stretching strain of 100%, and self-deformation was fulfilled via the driving force of dielectric elastomer actuators (DEAs).[136] Afterward, You et al. presented another stretchable device that outperformed Wang’s device. They pointed out the issue of damage to the Ag NW electrodes during stretching and added another polymer polyurethane urea (PUU) to strengthen the mechanical stability of the device. At 150% stretching strain, it still worked as virgin state and managed 5000 stretching cycles at a strain of ≈100%.[137] However, both findings laid no stress on the luminance. Nocturnal animal eyes enlightened retrore-flective structures of Shim et al. They advanced the research by enhancing the brightness to 1017 cd m−2 (6.67 V µm−1 at 10 kHz) while keeping its flexibility and stretchability. More-over, the luminance was more than 4 times compared conven-tional EL devices.[138] Likewise, doped ZnS particles and CdSe/ZnS quantum dots have also been fully appreciated in ACEL, especially for color mixing of white emission devices.[139]

In the following, attention will be paid to noticeable research on other novel EL applications. Shepherd and co-workers pub-lished their creative work about a hyperelastic light-emitting capacitor (HLEC) consisted of a ZnS phosphor-doped dielectric elastomer layer and hydrogel electrodes (see Figure 8c). Under deformation, the changed capacitance and luminance could be a recipe for applications in optical signaling and tactile sensing. Replica molding technique made it possible that arrays of inde-pendent pixels translated into the robot skin which could be stretched up to ≈480%.[140] Furthermore, multicolor-HLEC sur-viving biaxial stretching up to ≈200% was fabricated via pho-topatterning and transfer printing for wider applications,[141] although the capacitors still had the shortage of poor lumi-nance. These newly born ACEL devices offer great potentials for smart and cost-effective lighting and display applications, but compared with EL devices using direct current, its oper-ating voltage is still much higher.

Triboelectrification-Induced Electroluminescence (TIEL): Except for EL directly excited by electric field, TIEL should be men-tioned either. The reported self-powered TIEL goes further than conventional mechanoluminescence, which will be discussed later. Wei et al. perceived an opportunity to transfer extremely

weak stimuli (less than 10 kPa) into nondestructive light emis-sion with high sensitivity (0.03 kPa−1).[142] Nevertheless, the same as ACEL, the problem over brightness still exists. Next-generation TIEL may be focused on a lower applied voltage and a greater brightness.

3.3.3. Mechanoluminescence and Piezophotonic Luminescence

Mechanoluminescence: Mechanoluminescence refers to mechan-ical stress driving light emission on a solid.[143] Since Francis Bacon found the phenomenon of mechanoluminescence in 1605,[144] many materials have been used in this field. Zinc sulfide is one of the most studied ML materials these days. After decades of research, ZnS has become a top priority for nondestructive, flexible, durable, and easy fabrication methods in ML applications, especially for its excellent optoelectronic performances.[145,146]

To realize intense, reliable ML with durability in light sources, Jeong et al. have published a series of articles about elastico-ML comprising ZnS and soft PDMS matrix.[144,147–149] This kind of materials can emit luminescence during elastic deformation without fracture.[146] Using PDMS as substrates with high elastic moduli not only transfers mechanical stress effectively, but also ensures reproducible and durable devices, which used to be the limitation of rigid matrix ML materials.[150] As can be seen from Figure 8d, they reported a wind-driven ML device made of ZnS microparticles embedded in PDMS, which produced a significant brightness and emitted white light over a wide range of colors. By adopting strong vibration conditions, the composites formed a blueshift in the green luminescence perfectly improving the blue emission and thus realizing white ML. The device also showed an increased brightness by regu-lating the simulated wind flow, which paved the way to eco-friendly luminescent appliance.[147] Furthermore, color tuning by using only two ML materials via regulating the weight ratio of ZnS:Cu,Mn, and ZnS:Cu composites was realized.[148]

ML could also be applied in mechanics and energy-related applications, such as strain sensors.[150] A novel personalized handwriting mapping was achieved by designing a flexible pressure sensor matrix (PSM). Strain-induced polarization charges in ZnS particles essentially tilted the band structure and resulted in the recombination of holes and detrapping electrons. The excited dopant Mn2+ turned to ground state and emitted yellow light. The fast, flexible device recorded single-point dynamic pressure and 2D planar pressure mapping in the range of 0.6–50 MPa without any external energy supplies.[151] As for stretchable device, Fang et al. took the lead in achieving such dual-mode energy converters. Stretchable, transparent nanogenerators that transfer mechanical energy to either elec-tric or light by integrating ZnS ML powders and single-elec-trode triboelectric nanogenerator (S-TENG) presented a simple and effective method for application of wearable devices and artificial e-skins.[145]

Piezophotonic Luminescence: Since Wang introduced the new concept of piezophotonic effect, the coupling effect between piezoelectricity and photoexcitation, the publication has inspired a new wave of research in doped ZnS lumines-cence.[152] The effect modulates the charge transport behavior

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and controls the piezoelectric phosphors’ performance.[153] Initiation of the ML process via the PPL offers series of original light emission. Hao and co-workers developed such integrated devices. Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) was combined with doped ZnS and the thin film triggered piezophotonic luminescence for dual-modal source[154] as well as addressable and color-tunable applications.[153] For further broadening the applications of ZnS-based phosphors, PDMS was adopted for flexible piezophotonic applications. Both the applied voltage upon PMN-PT and the strain changing rate worked together to regulate the luminescence intensity. The mechanism behind the white light emissions of this facile device was discussed.[146] Then, they went deeper on this issue by modulating the mag-netic field. Simply by controlling the frequency of magnetic-field excitation, a real-time light emission was realized via piezophotonic effect. The group brought up the idea of tuning phosphor’s energy band structure and the trap depth of charge carriers for temporal and remote luminescence controlling.[155] These results may offer more opportunities for red–green–blue full-color displays in the future.

Based on these in-depth research, problems hindering ML devices such as rigidity, weak intensities, and lack of dura-bility are solved gradually; however, there is still large room for improvements. Compared with other light-emitting devices of PL and EL, the brightness and color purity are barely satisfac-tory. In addition, more smart ML applications are required for convenience in daily life.

3.3.4. Other Luminescence Methods

In this part, some luminescence materials based on other lighting principles are elaborated. For ease of understanding, the basic theories and novel applications in recent years will be involved. Some of these methods are completely new while others are quite mature after years of development. In spite of limited publications on these theories, all of the following applications exhibit prominent properties and unique functions of ZnS luminescence.

Magnetic-Induced luminescence (MIL): The first introduced new strategy was proposed by Wong et al.[156] Different from magnetoluminescence, MIL means direct luminescence under magnetic stimulation instead of PL or EL. Though the magnetic–luminescence coupling is not common, it serves as a cause for new efforts in magnetic sensing. On the basis of aforesaid research in piezophotonic,[146,154] Wong and co-workers introduced the concept of MIL by combining phosphor materials with magnetic actuator. Phosphor composites (metal-ion-doped ZnS + PDMS) and magnetic elastomer (Fe–Ni–Co alloy + PDMS) formed the laminates, resulting in a modulated light emission controlled by a time-varying magnetic field. The self-powered device successfully converted magnetic actions into green and white optical signals. Even so, the poor lumi-nance and efficiency of the devices still cannot compete with PL or EL light sources. The creative MIL has pioneered in the field of magnetic–luminescence coupling and future work should bring in improvements in brightness as well as efficiency.

Radioluminescence (RL): Aside from PL bioimaging described in Section 3.3.1, doped ZnS shows potentials in X-ray

and high-energy particles excited luminescence, i.e., radiolumi-nescence as well. X-ray-excited ZnS:Cu,Co that produced long afterglow was able to serve as a continuous light source. Sol-berg and co-workers pioneered the work in cell imaging and photodynamic therapy of cancer cells.[157] ZnS-based self-illumi-nating QDs also play a significant role in biological imaging. Emission can be excited under the stimulation of high-energy particles such as alpha particles, beta particles, or gamma rays. Both 64Cu-doped CdSe/ZnS QDs[158] and 64CuInS/ZnS QDs[159] taking advantages of Cerenkov resonance energy transfer (CeRET) displayed efficient Cerenkov luminescence (CeL) for in vivo tumor imaging. The fabrication process was chelator free. CeL was produced when β- or α-particles went through a dielectric medium with a velocity that surpassed the speed of light in that medium, which had a low autofluorescence back-ground.[158] All above-mentioned are techniques of significance in cancer therapy. Yet the dose should be in careful manage-ment considering practical applications.

Chemiluminescence: Chemiluminescence (ChL) resonance energy transfer (ChRET), involving nonradiative (dipole–dipole) energy transfer from a chemiluminescent donor to a suitable acceptor molecule, is a powerful technique for molecular detec-tions by the oxidation of a luminescent substrate without an excitation source.[160] Willner and co-workers used CdSe/ZnS semiconductor QDs powered by a ChRET process as biosen-sors for the detection of DNA,[161] metal ions, and aptamer.[162] Except for the above sensors, a DNA/QDs switchable lumines-cent device was reported.[163] The aggregation and deaggrega-tion led to the ON–OFF states of ChRET-induced luminescence of QDs by controlling the K+-ions’ elimination from G-quadru-plex units, which was linked to the CdSe/ZnS QDs. The find-ings put forward a new external trigger for bioswitches.

Cathodoluminescence: Cathodoluminescence (CaL) is now often used as a common means to study the optical perfor-mance of different materials. A simple thermal evaporation route was exploited by Liu et al. for the synthesis of high-quality single-crystalline ZnS nanobelts and nanorods.[27] This study fills the gap of changing CaL properties of 1D ZnS nanomate-rials that are dependent on its morphology, doping, and tem-perature. Besides, further CaL results about Fe/Mn codoping effects and UV near band-edge emission of 1D ZnS were explored for future discussions. There are other papers car-rying on intensive study over a local defect-induced redshift[8] and optical emission properties[30] with the assistance of CaL spectroscopy.

In this section, we summarized diverse luminescent appli-cations of ZnS-based materials based on different excitation principles. Binary or ternary core–shell semiconductor QDs are particularly attractive due to their high PL QY, size-dependent emission colors, and narrow emission band. Metal ions are usually introduced in ZnS QDs for unique performance such as superior brightness, large Stokes shift, and long-lived luminescence. The applications presented here include light sources, imaging nanoprobes, various sensors, and many novel devices. Extension of ZnS luminescent applications will be built on better performance such as tunability over full color range with a higher efficiency and stability. It is predictable that the trend for next-generation luminescent devices is nontoxic, eco-friendly, flexible, stretchable, and even self-powered.

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3.4. Photocatalysis

Energy crisis and environmental concerns are huge challenges today. Earth-abundant, robust, and nontoxic photocatalysts based on semiconductors have attracted increasing interest in both research and industry.[164,165] Particularly, ZnS, as one of the most studied chalcogenide semiconductors, is considered as a promising catalytic material because of its highly nega-tive potentials of excited electrons and rapid photoelectron generation.[166,167] Moreover, ZnS is nontoxic and abundant which meets the requirements of green chemistry. Thanks to the advantages of ZnS, it is nowadays a hot material that draws great attention in solar energy utilization and environ-mental remediation, including water splitting for hydrogen generation and degradation of organic pollutants. In addition, ZnS plays an important role in organic synthesis as an active catalyst.[168,169] Tables 4 and 5 summarized the recent publica-tions on ZnS-based catalysts toward water splitting and other photocatalytic applications, respectively.

For pristine ZnS, the relatively high charge recombination rate and insufficient utilization of solar energy in the visible light range due to its wide bandgap (≈3.7 eV) limit its photo-catalytic activities and set limitations for its practical applica-tions.[219] To address these obstacles, great efforts have been made in recent years to improve the catalytic performance of ZnS. On the other hand, in the case of photocatalytic water splitting in aqueous solution containing sacrificial reagents, noble metals, including Pt and Au, are generally used as the cocatalysts to improve the overall efficiency. As it is not eco-nomically viable to use rare metals, many attempts have been made to replace the highly expensive cocatalysts with other economical and eco-friendly materials in the past decade. And ZnS has shown its promising potentials as a cocatalyst and photosentisizer.

3.4.1. Band Engineering

Doping/Solid Solutions: It is known that the band structure of a semiconductor can be tuned by doping with other elements and forming solid solutions. From the perspective of band engi-neering, the method of doping foreign atoms or ions in ZnS introduces discrete dopant states within the bandgap of ZnS. And the formation of solid solutions is also a viable strategy for bandgap engineering where two or more semiconductors share similar crystal structures and lattice constants are typi-cally selected. Figure 9a,b shows the schematic illustrations of doping and solid solutions for enhanced catalytic performance toward water splitting, respectively. In recent years, an emer-gence of a variety of doped ZnS-based materials and solid solu-tions for widespread catalytic applications has been witnessed.

Surface and bulk Cu2+-doped ZnxCd1−xS solid solutions were prepared by Zhang et al. with the method of coprecipitation for bulk modification and cation-exchange reactions for surface modification, respectively.[172] Experimental results indicated that surface-doped ZnxCd1−xS had a higher hydrogen production rate than the bulk-doped one due to the new pathways for elec-tron transfer and massive surface active sites. Another practical strategy for bandgap modification of ZnS is to develop solid solu-

tions with narrow bandgap semiconductors, such as Zn1−xCdxS, ZnS–AgInS2, and ZnS–In2S3–Ag2S, whose bandgap could be modified by chemical composition.[173,174,176,178,179] The above-mentioned solid solutions have already shown great potentials in various applications, including protein probing, hydrogen generation, and organic pollutant decomposition.[176,211,220]

Defects: Though defects existed in the lattice of semiconduc-tors are generally considered as photogenerated electron–hole recombination centers, which leads to a poor photocatalytic performance, there have been reports about enhanced photo-catalytic activities of ZnS-based catalysts via defect engineering. Through carefully introduced defects, whose energy level lies within the bandgap of ZnS, the photoexcited electrons could be injected into the conduction band (CB) more easily after reaching the defect states, as shown in Figure 9c.[204,221] Thus, even under visible light illumination, ZnS could exhibit photo-catalytic activity toward multiple applications.

For instance, the ultrasonication technique was employed for the fabrication of cocatalyst-free ZnS nanoparticles with interstitial zinc atoms and sulfur vacancies for highly efficient decomposition of reactive black 5 (RB5).[210] Hao et al. investi-gated the vacancy-dependent band structure and photocatalytic activity in ZnS (Zn-deficient) prepared by using Na2S as the sulfur source during hydrothermal reaction.[171] Kurnia et al. reported the ZnS thin film with a bandgap of 2.4 eV prepared via gas-assisted pulsed laser deposition.[170] And a much higher photocurrent density (up to ≈1.5 mA cm−2) under visible light irradiation during photoelectrochemical water splitting was observed due to a better light absorption. Interestingly, defect engineering provides exceptional advantages of a higher sta-bility for ZnS against photocorrosion. It is reported that by the introduction of defects in ZnS nanocrystals, the oxidative capability of holes is suppressed due to the raised position of valence band (VB), which inhibits ZnS from being decomposed into Zn2+ and elemental S.[171,210]

3.4.2. Composites

Semiconductors: To enhance the photocatalytic performance of semiconductor-based catalysts, one of the most widely used strategies is the combination of two or more semiconductors with proper band structures to build heterojunctions for broad-ened absorption range and efficient charge separation.[167,193]

Cadmium sulfide (CdS) is one of the semiconductors with the highest catalytic performance toward sunlight-to-hydrogen conversion, thanks to its proper direct bandgap and satisfac-tory flat-band potential.[222–224] Nevertheless, its vulnerability to photocorrosion restricts its applications as a stable photocata-lyst.[173] Therefore, it is a reasonable approach to design and fabricate heterojunctions with ZnS to form staggered band alignments in order to overcome above shortcomings. ZnS-porous CdS core–shell nanostructures were prepared by Xie et al. with a hydrothermal method.[195] The porous ZnS shell covered on CdS core showed multiple effects, such as col-lecting photoexcited holes to suppress the photocorrosion of CdS, working as a protective shell to passivate surface traps, and allowing reactants/products to pass through effectively, as shown in Figure 10a.

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Table 4. ZnS-based photocatalysts for hydrogen generation through water splitting.

Mechanism Catalyst Synthesis method Phase Light source Sacrificial agent SBET [m2 g−1]a)

H2 generation activity [µmol h−1 g−1]

Highest AQY [%]b)

Year Ref.

Defect

engineering

ZnS thin film Pulsed laser

deposition

ZBc) 300 W Xe lamp

(>435 nm)

0.24 m Na2S

and 0.35 m Na2SO3

– 1.6 mA/cm2 – 2016 [170]

Defect

engineering

ZnS nanoparticles Hydrothermal ZB 300 W Xe lamp

(>420 nm)

0.35 m Na2S

and 0.25 m Na2SO3

– 337.7 – 2017 [171]

Doping/solid

solutionZnxCd1−xS: Cu2+ Coprecipitation, cation

exchange

WZd) 350 W Xe lamp

(>420 nm)

0.35 m Na2S

and 0.25 m Na2SO3

25.1 4638.5 20.9% at

420 nm

2013 [172]

Solid solution Zn1−xCdxS Coprecipitation ZB 300 W Xe lamp

(>420 nm)

0.1 m Na2S

and 0.1 m Na2SO3

156 2290 (0.25 wt% Pt) – 2012 [173]

Solid solution CdS QD/Zn1−xCdxS Hydrothermal, cation

exchange

ZB 350 W Xe lamp

(>400 nm)

0.1 m Na2S

and 0.04 m Na2SO3

27.4 2128 6.3% at

420 nm

2010 [174]

Solid solution ZnxCd1−xS Solvothermal (EDA) ZB 300 W Xe lamp

(>420 nm)

0.7 m Na2S

and 0.5 m Na2SO3

27.2 10970 30.4% at

420 nm

2011 [175]

Solid solution (AgIn)xZn2(1−x)S2 Thermal reaction WZ 300 W Xe lamp

(>350 nm)

Water/2-propanol (1:1)

and 50 × 10−3 m Na2S

– ≈68 5.9% at

400 nm

2016 [176]

Solid solution/

photosensitizer(AgIn)xZn2(1−x)S2 Thermal reaction ZB 300 W Xe lamp

(>420 nm)

0.1 m ascorbic acid – 276000 8.2% at

450 nm

2017 [177]

Solid solution/

heterojunction

CdS/ZnS/In2S3 Sonochemical method ZB 300 W Xe lamp

(>400 nm)

0.1 m Na2S

and 1.4 m Na2SO3

7.57 8100 40.9% at

420 nm

2012 [178]

Solid solution/

heterojunction

ZnS–In2S3–Ag2S@

TiO2−xSx

Electrochemical anodic

oxidation, solvothermal

ZB 500 W Xe lamp

(300–800 nm)

0.1 m Na2S

and 0.02 m Na2SO3

– 25.02 mmol h−1 – 2012 [179]

Heterojunction ZnO/ZnS–Ag2S Hydrothermal,

cation exchange

ZB 500 W Xe lamp 5% glycerol – 650.4 – 2013 [180]

Heterojunction Few-layer

phosphorene/ZnS

Hydrothermal,

mechanical mixing

WZ 300 W Xe lamp 18 vol% lactic acid – 1699 – 2017 [181]

Heterojunction CuInS2/ZnS Thermal reaction ZB 85 mW LED

(405 nm)

0.2 m ascorbic acid – 289 – 2017 [182]

Heterojunction ZnO/ZnS core–shell Water bath WZ 500 W Xe lamp 7% glycerol 34 2608.7 (UV) 22% at

365 nm

2012 [183]

Heterojunction CdS–ZnS/ZTPe) Ion exchange, thermal

treatment, sulfurization

both 125 W Hg lamp

(>420 nm)

0.02 m Na2S 49 35711 9.60% 2011 [184]

Heterojunction Ag2S–ZnO@ZnS

core−shell

Hydrothermal,

sulfurization

ZB Hg lamp 0.1 m Na2S and 0.04 m

Na2SO3 and 3 m NaCl

– 5870 (UV) – 2016 [185]

Heterojunction CuGaS2–ZnS Thermal reaction WZ 300 W Xe lamp

(>420 nm)

0.01 m Na2S

and 0.03 m Na2SO3

– 131 – 2016 [186]

Heterojunction ZnS/C Hydrothermal WZ 300 W Xe lamp 10 vol% lactic acid – 969.6 (C shell) – 2014 [187]

Heterojunction ZnS–(CdS/Au) Cation exchange,

modification

ZB 300 W Xe lamp 0.35 m Na2S

and 0.25 m Na2SO3

– 5050 – 2015 [188]

Heterojunction CdS/ZnS Solvothermal ZB 300 W Xe lamp

(>420 nm)

1.0 m Na2S

and 1.4 m Na2SO3

– 239000 16.8% at

420 nm

2016 [189]

Heterojunction/

solid solution

Pt–RuS2–Cd0.5Zn0.5S Coprecipitation, wet

impregnation

WZ Daylight

fluorescent lamp

0.6 m Na2S

and 0.8 m Na2SO3

28 724 4% 2016 [190]

Heterojunction/

solid solutionZnxCd1−xS/RGOf) Coprecipitation,

hydrothermal

ZB Simulated

sunlight (AM 1.5,

100 mW cm−2)

0.35 m Na2S

and 0.25 m Na2SO3

81 1824 23.4% at

420 nm

2012 [191]

Heterojunction/

cocatalyst

Ti2C3–ZnS Hydrothermal ZB 300 W Xe lamp 18 vol% lactic acid – 718 40.1% at

420 nm

2017 [192]

Heterojunction/

cocatalyst

CdS/ZnS/nickel-

salen complex

Precipitation, mixing ZB 300 W Xe lamp

(>420 nm)

0.75 m Na2S

and 1.05 m Na2SO3

– 815000 58.3% at

420 nm

2016 [193

Heterojunction/

cocatalyst

CdIn2S4/ZnS/RGO Hydrothermal ZB 300 W Xe lamp

(>420 nm)

17 vol%

triethanolamine

79.77 6820 19.34% at

>430 nm

2017 [194]

Heterojunction/

porous structure

CdS/mesoporous

ZnS core–shell

Hydrothermal ZB 300 W Xe lamp

(>400 nm)

0.1 m Na2S

and 0.1 m Na2SO3

35 792 – 2014 [195]



Considering the toxicity and photocorrosion of CdS, others tried to replace CdS with nontoxic and robust semiconductors with suitable band structures. Zhao et al. reported a thermal reaction method for the fabrication of matchstick-like CuGaS2 and ZnS p–n heterojunction.[186] Due to the efficient charge

separation and transfer process, the p–n heterojunction system showed 15 times higher catalytic performance than pure CuGaS2 under visible light. Ag2S[180,205] and TiO2

[216] are also good semi-conductor candidates to form heterojunctions with ZnS, where the suitable band alignment favors effective charge separations.

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Table 5. ZnS-based catalysts in other applications.

Catalyst Synthesis method Catalytic application Phase Light source Year Ref.

Zn0.4Cd0.6S/Al2O3, ZnS/Al2O3 Impregnation, gas–solid

sulfurization

Photocatalytic decomposition of H2S ZB – 2013 [198]

ZnS-montmorillonite Precipitation Photocatalytic reduction of CO2 – 8 W Hg lamp (254 nm) 2014 [199]

ZnS particles Precipitation Photocatalytic reduction of CO2 – 1000 W high-pressure Hg

(Xe) arc lamp

2014 [200]

CuS–ZnS/clinoptilolite Cation exchange Photodegradation of benzophenone WZ 35 W Hg lamp 2016 [201]

ZnS/CQD Coprecipitation Photodegradation of organic dye ZB 150 W (400–520 nm) 2016 [202]

ZnO–ZnS/C Carbothermal reduction Photodegradation of organic dye WZ 500 W Xe lamp (>420 nm) 2011 [203]

Thioglycerol (TG) capped

ZnS

Precipitation Photodegradation of organic dye ZB 200 W Xe lamp (>300 nm) 2012 [204]

Ag2S/ZnS Hydrothermal, cation exchange Photodegradation of organic dye ZB 500 W Xe lamp (>420 nm) 2016 [205]

ZnS–Ag2S Cation exchange Photodegradation of organic dye WZ 300 W Xe lamp (<350 nm) 2013 [206]

CoFe2O4–ZnS Ultrasonication-assisted

coprecipitation

Photodegradation of organic dye ZB 12 mW UV lamp (main wave-

length of 400 nm)

2012 [207]

ZnS QD/chitosan Precipitation Photodegradation of organic dye ZB Radiation source (6 W,

254 nm)

2014 [208]

CdZnS@Fe3S4 core–shell Precipitation Photodegradation of organic dye ZB 300 W Xe lamp 2018 [209]

ZnS with defect sites Ultrasonication-assisted

precipitation

Photodegradation of organic dye ZB 400 W UV arc lamp (365 nm) 2017 [210]

TiO2/(ZnS)x(CuInS2)1−x Hydrothermal, thermal reaction Photodegradation of organic dye – Simulated sunlight

(AM1.5, 100 mW cm−2)

2012 [211]

SnO2 QD/ZnS Hydrothermal Photodegradation of organic dye WZ 300 W Xe lamp (>400 nm) 2017 [212]

g-C3N4/ZnS/CuS Solvothermal, microwave-assisted

precipitation

Photodegradation of organic dye ZB 500 W Xe lamp (>420 nm) 2018 [213]

ZnS hollow spheres Gas-bubble template

hydrothermal

Photodegradation of organic

dye and salicylic acid

WZ 350 W Xe lamp (365 nm) 2012 [214]

ZnS/RGO Hydrothermal Photodegradation of norfloxacin WZ 300 W Hg vapor lamp 2017 [215]

ZnS–Ag2S/TiO2 Precipitation Photodegradation of phenol – 450 W Hg vapor lamp 2017 [216]

Au/ZnS core–shell Hydrothermal Direct methanol fuel cells WZ – 2013 [165]

ZnO/ZnS·(HDA)0.5 Hydrothermal, sulfurization Electrochemical catalysis of hydrazine WZ – 2014 [217]

ZnS/C Precipitation Electrochemical oxidation of ethanol – – 2010 [218]

Mesoporous ZnS Pyrolysis Synthesis of organic compounds WZ – 2010 [168]

ZnS nanoparticles Precipitation Synthesis of organic compounds ZB – 2011 [169]

Mechanism Catalyst Synthesis method Phase Light source Sacrificial agent SBET [m2 g−1]a)

H2 generation activity [µmol h−1 g−1]

Highest AQY [%]b)

Year Ref.

Porous structure/

heterojunction

Pt–ZnS–CoS Sulfurization ZB 300 W

Xe lamp

10 vol%

of methanol

185 8210 – 2017 [196]

Photosensitizer ZnS–[Fe2S2]

hydrogenase mimic

Hydrothermal ZB 300 W Xe lamp 85.2 × 10−3 m

ascorbic acid

– 195 2.5% at

325 nm

2012 [197]

a)SBET: Brunauer–Emmett–Teller surface area; b)AQY: apparent quantum yield; c)ZB: zinc blende; d)WZ: wurtzite; e)ZTP: zirconium titanium phosphate; f)RGO: reduced graphene oxide.

Table 4. Continued.



2D Materials: Recently, 2D materials have attracted widespread interest for their relatively large surface area and high charge carrier mobility.[181] Graphene-like 2D materials, such as g-C3N4, reduced graphene oxide (RGO), phosphorene, 1T MoS2, and their combinations, have been investigated as photoexcited electron

reservoirs or macromolecular photosensitizers to improve the catalytic performance of ZnS-based catalysts.[181,191,194,213,225–228] Their unique properties endorse new pathways in improving catalytic performance by promoting charge separation and pro-viding a large amount of active surface sites.[191] Figure 10b,e,h

Adv. Funct. Mater. 2018, 28, 1802029

Figure 10. Schematic illustration, TEM images, band alignments, and catalytic performance toward water splitting of a,d,g) CdS–ZnS core–shell structure; b,e,h) graphene oxide–ZnxCd1−xS and c,f,i) Au–CdS–ZnS ternary hetero-nanorods. a,d,g) Reproduced with permission.[195] Copyright 2014, Royal Society of Chemistry. b,e,h) Reproduced with permission.[191] Copyright 2012, American Chemical Society. c,f,i) Reproduced with permission.[188] Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 9. Schematic illustrations of band engineering strategies for enhanced catalytic performance of ZnS-based catalysts: a) doping of metallic ions, b) forming of solid solutions, and c) defect engineering. The typical materials and preparation techniques are listed below the images.



shows enhanced catalytic performance by the formation of het-erojunction between RGO and ZnxCd1−xS.[191]

Ran et al. reported the design and fabrication of a MXene-type 2D metal carbide Ti3C2 as a highly efficient cocatalyst with ZnS, instead of highly expensive noble metal cocatalysts, for photocatalytic water splitting.[192] By mechanical mixing hydro-thermal-produced ZnS nanoparticles with 1 wt% as-synthesized Ti3C2, the ZnS/Ti3C2 catalyst exhibited 2 times higher the photo-catalytic performance than pure ZnS catalyst. The improved catalytic performance is attributed to the better utilization of visible light, numerous surface hydrophilic functionalities, and the excellent metallic conductivity of O-terminated Ti3C2. Moreover, Ti3C2 nanoparticles showed a strong synergetic effect on efficiently promoting the separation and transfer of light-induced charges, which made it a promising hydrogen evolution reaction (HER) cocatalyst on ZnS. In addition, Ran et al. successfully obtained 4 times higher the photocatalytic hydrogen generation activity by building the strong electronic coupling between few-layer phosphorene nanosheets and ZnS when compared with pure ZnS.[181] Therefore, the progress in the combination between 2D materials and ZnS-based catalysts gives us a new insight toward various applications in catalysis.

Nanostructures: Numerous novel nanostructures of ZnS for enhanced catalytic performance and stability have been reported by researchers in recent years. For efficient photo-catalytic hydrogen evolution, Zhuang et al. reported the con-trolled growth of Au-particle-modified CdS node sheaths on ZnS nanorods (Figure 10c).[188] Due to compatible lattice matching between the metal and semiconductor, Au particles tended to grow preferentially on the surface of CdS instead of ZnS. The 1D nanoarchitecture, proved by HAADF-STEM images and EDS mapping, was successfully formed a type-II heterojunction, facilitating fast electron transfer from CdS nodes to metal and ZnS rods (Figure 10f). Furthermore, the decoration of Au particles on the surface of CdS nodes led to the formation of staggered gap alignment of the ternary system, which would greatly facilitate the photoinduced charge separation and promote the photocatalytic hydrogen generation (5050 µmol h−1 g−1), as shown in Figure 10i. Wang et al. tailored the morphology of carbon, from 0D carbon quantum dots to ZnS–carbon core–shell nanostructure, on the surface of ZnS nanoparticles by modifying glucose concentration during the hydrothermal process. And the enlarged specific surface area also helped improve the photocatalytic performance.[187]

Porous structures are beneficial for the reactant transfer process and large catalytic active surface area, thus would dra-matically facilitate the reaction process. Zhao et al. prepared ZnS catalyst on γ-Al2O3 and observed improved catalytic per-formance toward H2S decomposition.[198] Esmaili-Hafshejani and Nezamzadeh-Ejhieh grew CuS–ZnS onto porous clinop-tilolite nanoparticles for the degradation of benzophenone.[201] Lan et al. fabricated porous Pt-doped Pt–ZnS–CoS hetero-junction using the method of sulfurization of ZnCo–zeolitic-imidazolate-framework (ZnCo–ZIF) template for an efficient visible-light-to-hydrogen conversion.[196] The porous nature of the bimetallic MOF derivative was inherited from its ZnCo–ZIF precursor, which therefore greatly improved its photocatalytic performance due to the enhanced light absorption in the visible light range and strong electron coupling.

Furthermore, there are also reports about the synthesis of catalytic composites consisted of ZnS and magnetic materials for the purpose of catalyst recycling. Typical magnetic materials such as Fe3S4, CoFe2O4, and Fe2O3 were successfully applied for the preparation of various nanostructures toward magneti-cally separable ZnS-based catalysts.[207,209,229]

Others in the field have searched for semiconductor photo-sensitizers to replace expensive rare noble metals. According to previous literature, ZnS is proven to be a promising candidate as a photosensitizer. Wen et al. reported the use of ZnS nano-particles as a highly efficient photosensitizer on a hydrogenase mimic [(µ-SPh-4-NH2)2Fe2(CO)6] toward water splitting.[197] The photocatalytic system exhibited a high hydrogen genera-tion activity as well as high stability. Efficient charge separa-tion and transfer process from ZnS, the light harvester, to the molecular complex was contributed to the outstanding catalytic performance. Lian et al. proposed CuInS2/ZnS quantum dots as an efficient photosensitizer on a meso-tetraphenylporphyrin iron(III) chloride (FeTPP) catalyst for CO2 reduction to CO under visible light illumination.[230] In the system, the forma-tion of QD/FeTPP complexes had the ability to enhance ultra-fast electron transfer between the QD and FeTPP, resulting in a higher reduction reaction efficiency.

In summary, we have summarized several widely employed strategies in literature toward improved catalytic performance for ZnS-based catalysts. Generally, typical methods could be categorized into two major groups: band engineering and formation of heterojunctions. Apart from the common aims of modifying bandgap for a broader light absorption and sup-pressing the recombination of photoinduced electrons and holes, some other intriguing strategies have been proposed by recent researchers. And the richness of morphologies in ZnS catalysts, such as nanoparticles, nanorods, core–shell construc-tions, and porous structures, has proven the feasibility of con-trollable nanoscale fabrication of novel catalysts with higher catalytic performance. However, there are still many challenges needed to be addressed in preparing ZnS-based catalysts with precisely controlled composition and nanostructures. In addi-tion, more detailed theoretical and experimental exploration about the enhanced catalytic mechanism of ZnS-based mate-rials is still desired by academia and industry to guide the design and fabrication of future catalysts. There is still a long way to go for the development of eco-friendly, scalable, and efficient catalysts.

4. Conclusion and Outlook

ZnS, a rising material star owing to its rich morphologies and unique optical and electrical properties, holds great potentials in efficient optoelectronic devices and catalysts.[231,232] Notable progress on design and engineering of ZnS for novel/greatly improved optoelectronic properties and versatile applications has been witnessed in the past few years. The design principles and fundamental mechanisms employed in modifying ZnS in terms of its morphology, optical, electrical, charge separation, and transport properties have been comprehensively summa-rized in this review. Novel properties and applications of ZnS-based materials in recent publications are also highlighted.

Adv. Funct. Mater. 2018, 28, 1802029



More importantly, critical issues that still remain and need to be addressed are listed below, which may serve as the roadmap to discover more exciting treasures in the “ZnS World.”

1. The developments in various ZnS nanocomposite struc-tures are intriguing, exemplified by ZnO/ZnS, with in-teresting nanostructures such as core–shell nanowires, side-to-side nanowires, branched nanowires, and so on. However, the thermodynamics and growth kinetics of such nanocomposite structures have yet to be demonstrated. More importantly, how the different configurations of the composite materials will affect their fundamental chemi-cal and physical properties need to be further investigated. Only a more clear understanding of the charge separation and transport process at different interfaces (e.g., side-to-side/core–shell) will lead to a better design and control of the composite materials for intended applications and functionalities.

2. Great achievements have been made in exploring ZnS as an earth-abundant, nontoxic, high-performance transparent conductor, especially the work on p-type transparent con-ducting ZnS–CuS nanocomposite films prepared by a low-cost solution bath method. Not only the state-of-the-art hole conductivity (>1000 S cm−1) is achieved, approaching that of n-type ITO and AZO, a new insight into developing TCMs is provided via creating electron/hole-conducting networks filled with tiny transparent nanocrystals (wide bandgap semiconductors like ZnS). Even though ZnS is expected to be a good candidate in replacement of expensive n-ITO, the progress in improving its n-type conductivity is not satis-factory. According to theoretical calculations, Al-doped-ZnS will be the most promising n-type TCM compared with other dopants, with an optimal electrical conductivity of ≈3830 S cm−1 at a doping level of 6.25%. To date, the experi-mental values are almost an order of magnitude lower than those of the theoretically predicted conductivities. Depo-sition techniques, synthesis temperature, and annealing process need to be further investigated to realize the ideal n-type conductivity of Al-doped ZnS. On the other hand, it will be interesting to explore the possibilities of enhancing the n-type conductivity of ZnS via employing the composite nanostructure in p-type ZnS–CuS nanocomposite TCM. If high-performance p-type and n-type ZnS TCMs are made available, it will be an enormous benefit for developing next-generation optoelectronic devices, including transpar-ent electronics, solar cells, light-emitting devices, display technologies, etc.

3. Notable progress has been made in the past few years on de-veloping high-performance ZnS UV photodetectors with a high responsivity, fast response speed, and self-powered prop-erties. The spectrum selectivity can also be tuned via doping or being composited with other semiconductors. However, a low-cost, scalable, high out-put fabrication technique is ur-gently desired to bring those nanostructured photodetectors out of lab and put them into use in real life. Furthermore, intelligent architectures and assembly techniques that en-dow the rigid inorganic electronic devices with flexibility and stretchability is another direction to purse to meet the trend of wearable consumer electronics.

4. ZnS plays an extremely important role in luminescence. It can not only work as the light-emitting material due to the introduced mid-gap dopant states, but also help improve the PL QY of core–shell semiconductor QDs as the shell. Recent developments on the diverse luminescent applica-tions of ZnS-based materials are very infusive, including biological imaging, light sources, various sensors, and many novel devices. However, further extensions of lumi-nescent applications need to be built on materials with bet-ter performance, which means tunability over a full color range with a higher efficiency and stability, a lower power consumption, and a more sensitive stimuli response (such as ML and CL).

5. Artificial photosynthesis is one of the most promising approaches to address energy crisis and environmental concerns faced by the 21st century. The recent progress on developing ZnS as an active photocatalyst for water split-ting hydrogen generation and photodegradation of organic pollutants is appealing. Band engineering and nanostruc-ture design are employed, as the wide bandgap of ZnS is considered as an advantage in transparent conductors and UV photo detectors but inhibits the efficient utilization of sunlight. Narrowing the bandgap (solid solutions, e.g., ZnxCd1−xS), introducing interband defect/dopant states, and creating nanostructures with large surface to volume ratios are the three main strategies to improve the photocat-alytic performance of ZnS. To date, the hydrogen generation rates of ZnS-based photocatalysts have been pushed into the best-in-class in this field. Meanwhile, CO2 reduction, as an indispensable part of artificial photosynthesis, is becoming an emerging and rapid growing topic. However, the work on studying the role of ZnS in CO2 reduction is just a begin-ning. Preliminary study has shown a high product rate of formate (HCOO−) from CO2 with the presence of sphalerite ZnS. The design principles and fabrication techniques in exploring this new scientific territory can be adopted from the research of ZnS photocatalytic water splitting. The high selectivity of photoreduction products and thermodynamic mechanisms are highly desired and extremely important in future study. We do hope this review will motivate more researchers to join our journey in exploring “ZnS” and un-locking those mysteries.

AcknowledgementsX.J.X., S.Y.L., J.X.C., S.C., and Z.H.L. contributed equally to this work. The authors would like to thank Dr. Lingxia Zheng, Sandeep Kumar Maurya, and Dr. Longxing Su for their kind support and helpful discussions. This work was supported by the National Natural Science Foundation of China (Grant Nos. 51721002, 11674061, 51471051, and 11811530065), Science and Technology Commission of Shanghai Municipality (18520710800, 17520742400, and 15520720700), the Programs for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and National Program for Support of Top-notch Young Professionals.

Conflict of InterestThe authors declare no conflict of interest.



Keywordscatalysis, luminescent devices, transparent conductors, UV photo detectors, ZnS nanostructures

Received: March 21, 2018Revised: May 14, 2018

Published online: June 25, 2018

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