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Critical Review

Environmental Applications of 2D Molybdenum Disulfide (MoS2) Nanosheets Zhongying Wang, and Baoxia Mi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01466 • Publication Date (Web): 29 Jun 2017 Downloaded from http://pubs.acs.org on June 30, 2017

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Environmental Science & Technology

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Environmental Applications of 2D Molybdenum

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Disulfide (MoS2) Nanosheets

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Critical Review

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Revision Submitted to

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June 26, 2017

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Zhongying Wang, Baoxia Mi*

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Department of Civil and Environmental Engineering,

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University of California, Berkeley, California 94720, United States

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*

The author to whom correspondence should be addressed.

e-mail: [email protected]; tel.: +1-510-664-7446, fax: +1-510-643-5264

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ABSTRACT

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In an era of graphene-based nanomaterials as the most widely studied two-dimensional (2D)

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materials for enhanced performance of devices and systems in numerous environmental

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applications, molybdenum disulfide (MoS2) nanosheets stand out as a promising alternative 2D

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material with many excellent physicochemical, biological, and mechanical properties that differ

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significantly from those of graphene-based nanomaterials, potentially leading to new

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environmental phenomena and novel applications.

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advances in the use of MoS2 nanosheets for important water-related environmental applications

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such as contaminant adsorption, photocatalysis, membrane-based separation, sensing, and

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disinfection. Various methods for MoS2 nanosheet synthesis are examined and their suitability

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for different environmental applications is discussed. The unique structure and properties of

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MoS2 nanosheets enabling exceptional environmental capabilities are compared with those of

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graphene-based nanomaterials. The environmental implications of MoS2 nanosheets are

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emphasized, and research needs for future environmental applications of MoS2 nanosheets are

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identified.

This critical review presents the latest

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TOC/Abstract Art

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INTRODUCTION

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Nanomaterials have been extensively researched in the past two decades to offer new solutions

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or enhance the existing solutions to many of the pressing environmental problems from water

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shortage to air pollution.1-3 In particular, two-dimensional (2D) graphene-based nanomaterials,

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including pristine graphene and graphene oxide (GO) as well as reduced GO, have attracted

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enormous attention due to their high specific surface area and intriguing properties that are

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typically unavailable in their bulk forms.4,5

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themselves in myriad environmental applications such as adsorptive removal of contaminants,

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photocatalytic oxidation, sensing, and membrane-based separation.6-10

So far, graphene-based nanomaterials have found

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While graphene-based nanomaterials have been widely studied, other types of 2D

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nanomaterials, in particular the recently emerging molybdenum disulfide (MoS2) nanosheets,

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have also received increasing attention due to their significantly different electrical,11-13

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physicochemical,14 biological,15

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applications in electronics,11,12 catalysis,13,14 biomedical,15,16 and energy-related fields,17,18 MoS2

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nanosheets are expected to have novel applications in the environmental fields. In fact, the bulk

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MoS2, which naturally occurs as abundant mineral molybdenite, has long been considered for

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use as environmental catalysts13,19 and adsorbents.20 However, methods for isolating MoS2

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mono-/few-layers from bulks and producing large quantities of MoS2 nanosheets, which possess

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unique properties that are specific to nanosized materials, have been available only recently.21-23

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Since then, the synthesis,24 properties,25 functionalization,26 and tuning27 of MoS2 nanosheets

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have been reported, and their promise in various environmental fields has been revealed.28-33

and mechanical properties.13

With already demonstrated

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This review article aims to summarize the use of MoS2 nanosheets for potential

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environmental applications, and also cover other transition metal dichalcogenide (TMD)

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nanosheets wherever applicable. This choice is made because MoS2 has been the most intensely

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investigated 2D material beyond graphene, and findings about the prototype MoS2 can readily

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benefit the research on the other TMD materials that possess similar structural and

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physiochemical properties. First, the structure, properties, and synthesis methods of MoS2

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nanosheets are introduced. Then, the latest advances in the various environmental applications

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of MoS2 nanosheets are critically discussed, focusing on water-related applications such as

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contaminant adsorption, photocatalysis, membrane-based separation, sensing, and antibacterial 4 ACS Paragon Plus Environment

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treatment.

Special attention is given to the novel properties and distinct environmental

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performance of MoS2 compared to those of graphene-based nanomaterials. Finally, the

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implications of MoS2 nanosheets as they are discharged into the environment are highlighted,

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and future research needs for their enhanced environmental performance are pointed out.

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STRUCTURE AND PROPERTIES

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Structure. The bulk MoS2 exists in the form of black powder/particle and is composed of

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monolayers, with strong covalent Mo-S bonds within each monolayer and weak van der Waals

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forces between neighboring monolayers. As shown in Figure 1a, a MoS2 monolayer consists of a

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Mo atom layer sandwiched between two S atom layers, and the interlayer spacing of MoS2

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monolayers is 0.62 nm with a free spacing of 0.30 nm.32 The saturated S atoms on the basal

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plane (except at the edges) chemically stabilize the bulk MoS2 and also attain individual MoS2

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monolayers,34 which can form one of the two crystal structures (i.e., trigonal prismatic 2H phase

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and octahedral 1T phase) depending on atom-stacking configurations (Figure 1b-c).34

The

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natural

and

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thermodynamically stable 2H phase, while 1T polymorph is metallic and metastable and does not

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exist in natural environment.35 The weak interlayer interaction allows the use of relatively

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simple physicochemical exfoliation methods to produce monolayer or few-layer MoS2

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nanosheets, with a typical a thickness of < 10 nm and lateral dimensions ranging from 50 nm to

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~ 10 µm.21,36-40 Depending on the exfoliation methods used, MoS2 nanosheets may form either

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2H or 1T phase, which can be transformed from one to the other via intralayer atomic sliding

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induced by Li intercalation (2H to 1T)37 or annealing (1T to 2H).39

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physicochemical (see below) characterization of MoS2 nanosheets has been conducted using

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various techniques such as atomic force microscopy, scanning electron microscopy (SEM),

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transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy, X-ray

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photoelectron spectroscopy (XPS), and Raman and photoluminescence spectroscopy. Details

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about such characterization can be found in previous literature.41-43

molybdenite

of

bulk

MoS2

is

commonly

found

in

semiconducting

The structural and

94

FIGURE 1

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Properties. Primarily due to the confinement of charge carriers in their basal plane directions,34

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the exfoliated MoS2 nanosheets possess physicochemical properties that are dramatically

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different from those of the bulk MoS2. For instance, bulk MoS2 as a semiconductor has an 5 ACS Paragon Plus Environment


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indirect bandgap of 1.29 eV, which shifts to a direct bandgap of 1.9 eV for monolayer 2H-MoS2

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due to the decrease in overall thickness.21 Accordingly, a strong photoluminescence (PL), which

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is absent in the bulk form, emerges in monolayer 2H-MoS2.21 This direct bandgap finds great

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potential in electronic and photonic applications such as the fabrication of mono- or few-layer-

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MoS2-based field effect transistor (FET) sensors for detecting environmental contaminants.44,45

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The relatively small bandgap of MoS2 nanosheets makes it possible to use visible light for

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photocatalytic reactions.28,29 Compared with graphene, which is semi-metallic and has a zero

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bandgap, 2H-MoS2 nanosheets are more competitive for applications such as transistors,

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optoelectronics, and energy harvesting as well as for potential environmental applications such

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as photocatalytic degradation and disinfection.

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In contrast to the semiconducting 2H-MoS2, the metallic 1T-MoS2 exhibits a significantly

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improved electrocatalytic activity and presents a promising low-cost catalyst for the hydrogen

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evolution reaction.14 1T-MoS2 nanosheets are primarily generated via phase transition, when

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defects and electron-rich atoms are formed, in chemical exfoliation of bulk MoS2.46 These active

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sites provide a facile functionalization route to surface modification. The defects have a higher

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molecular affinity for thiol groups and thus allow easy conjugation of 1T-MoS2 nanosheets by

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thiol-terminated ligands.47 Therefore, the thiol-containing molecules can be physically adsorbed

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on MoS2 and easily removed.48 More stable functionalization of MoS2 can be achieved by

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covalent bonds via chemical reactions between electron-rich 1T phase and an electrophile (e.g.,

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organohalide, aryl diazonium).26

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The exfoliated monolayer MoS2 nanosheets have all sulfur atoms exposed on their

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surfaces and to the surrounding environment. Because sulfur is a soft Lewis base showing a high

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affinity for heavy metal ions (e.g., Hg2+, Ag+) that act as soft Lewis acids, MoS2 nanosheets

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demonstrate both a high adsorption capacity due to abundant sulfur adsorption sites and fast

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kinetics because of easy access to these sites.33,46 Therefore, monolayer MoS2 is among the most

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effective adsorbents for the removal of heavy metal ions.32,49 In comparison, the widely studied

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2D adsorbent GO mainly adsorbs metal ions by its negatively charged oxygenated functional

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groups via electrostatic interaction or by loaded functional materials, leading to relatively low

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selectivity and adsorption capacity.10,50


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MoS2 nanosheets obtained from chemical exfoliation, which generates electron-rich

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atoms, are highly hydrophilic (thus dispersible in water) and negatively charged.51 The charge is

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distributed over the chemically exfoliated MoS2 nanosheet and abundant with an excess of ~0.25

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electron per Mo atom or an empirical formula of (MoS2)-0.25.51 Because chemical exfoliation

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does not change the morphology of MoS2, the resulting monolayer nanosheets maintain their

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original 2D structure and smooth surface.

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negative charge from chemical oxidation, which generates oxygenated functional groups

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extruding from the carbon plane and thus sacrifices the original surface smoothness, uniformity,

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and integrity of graphene. Such differences in morphology and charge distribution of MoS2 and

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GO nanosheets could greatly affect their performance in applications such as layer-stacked

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membrane-based separation.

In contrast, GO obtains its hydrophilicity and

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Monolayer MoS2 has an in-plane Young’s modulus of about 200 to 300 GPa,52-54

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comparable to that of GO but less than that of graphene.55 However, because monolayer MoS2

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contains three atomic layers while graphene has only one atomic layer, the out-of-plane bending

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modulus of monolayer MoS2 is 9.61 eV,56 much larger than 1.4 eV for graphene.57 Although the

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high out-of-plane rigidity of MoS2 limits its application as flexible electronics, such a property

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may enable MoS2 to be a suitable 2D building block for making separation membranes with

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relatively fixed-sized nanochannels, which are critical for long-term stable performance.

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SYNTHESIS METHODS

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Various methods have been developed to synthesize monolayer and few-layer MoS2 nanosheets

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and tailored to suit different applications. The synthesis methods can be categorized into two

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general types: (i) the top-down method, which mechanically, ultrasonically, or chemically

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exfoliate bulk materials by overcoming the weak interlayer binding force (e.g., van der Waals)

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and (ii) the bottom-up method, which assembles MoS2 nanosheets using individual atoms.

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Representative methods are discussed as follows and summarized in Table 1.

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TABLE 1

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Top-down Method. Mechanical exfoliation by scotch tape can generate large, defect-limited,

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and electronic-grade MoS2 nanosheets for fundamental studies (e.g., intriguing properties 7 ACS Paragon Plus Environment


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associated with the PL and FET performance) and electronic device demonstrations.12,21

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However, this low-yield, uncontrollable approach has limited usefulness in research and practical

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applications, which usually require large quantity of nanomaterials with good quality.

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In contrast, liquid-based ultrasonic exfoliation can easily produce bulk dispersion of

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single, few-layer MoS2 nanosheets, or their mixtures. For example, sonication of bulk MoS2

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materials in organic solvents with intermediate polarity (e.g., N-methyl-pyrrolidone or NMP)

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offers an energy-efficient exfoliation approach, which also works for other layered materials

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(e.g., graphite, TMD, and hexagonal boron nitride or h-BN), that are bonded by van der Waals

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force.36

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containing surfactants (e.g., sodium cholate, bovine serum albumin), which can bind to the

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exfoliated nanosheets via van der Waals force to stabilize the suspension.22,38 The exfoliation of

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MoS2 in aqueous solution not only provides a scalable production process but also allows a wide

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range of water-based processing techniques (e.g., spin-coating, deposition) for environmental

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applications.38 Another advantage is that ultrasonic exfoliation does not induce any structural

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distortion and thus maintains the semiconducting 2H phase, unlike the phase conversion during

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chemical exfoliation. However, the major limitation of ultrasonic exfoliation is that the raw

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product is mostly in multilayer form.36,58 In addition, the surface-bound organic solvent and

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surfactants sometimes need to be removed due to toxicity concerns (e.g., NMP) and/or technical

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requirements in certain applications.59,60

Exfoliation of MoS2 via sonication can also be carried out in aqueous solution

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In order to further increase the yield, chemical exfoliation has been used to produce MoS2

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monolayers via lithium ion intercalation.39 Specifically, incubation of bulk MoS2 in Li-

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containing organic solvent (e.g., n-butyllithium in hexane) generates Li-intercalated MoS2 with

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weakened interlayer attractions. Then, by reacting LixMoS2 with water under the assistance of

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ultrasonication, a colloidally stable dispersion of MoS2 nanosheets is produced.

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corresponding zeta potential of -45 to -50 mV at neutral pH is attributed to the charge transfer to

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MoS2 nanosheets during Li intercalation.37,61

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typically have a lateral dimension of 200 to 800 nm and a thickness of 1 to 1.2 nm, indicating the

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attainment of atom-thin monolayer MoS2 as majority,21 with a yield rate of nearly 100%.34

The

The chemically exfoliated MoS2 nanosheets

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The chemical exfoliation of MoS2 results in the partly loss of semiconducting 2H phase

185

due to structural deformation by Li intercalation, as evidenced by the characterization of 8 ACS Paragon Plus Environment

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chemically exfoliated MoS2 sample by UV–vis spectroscopy showing a nearly featureless

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spectrum without characteristic peaks of the 2H phase.61 Furthermore, the XPS spectra of

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chemically exfoliated MoS2 revealed a dominant (60 to 70%) 1T phase peak with a binding

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energy of ∼0.9 eV lower than that of the 2H phase peak.61,62 However, such intercalation-

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induced phase transformation can be fully reversed to restore the thermodynamically stable

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semiconducting 2H phase via mild annealing,39,62 laser irradiation,38 or microwave treatment.63

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Bottom-up Method. MoS2 nanosheets can also be produced by first decomposing Mo- and S-

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containing precursors and then assembling Mo and S atoms. Chemical vapor deposition (CVD)

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is a widely used technique for growing 2D nanomaterials in a controllable manner. Depending

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on the variations of precursors and substrates, the CVD-based synthesis of MoS2 nanosheets can

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be classified into three options: (i) vaporization and decomposition of Mo and S precursors, and

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subsequent formation of MoS2 layers on a growth substrate,26,64 (ii) direct sulfurization of Mo-

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based films (e.g., Mo metal or MoO3),23,65 and (iii) thermolysis of precursors containing Mo and

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S atoms.66 These options are discussed in detail as follows.

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MoS2 nanosheets can be directly grown on SiO2/Si substrates using MoO3 and S powders

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as precursors.64 Pretreatment of the substrate using graphene-like coating (e.g., reduced GO) is a

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critical step to provide the seeding sites for growing MoS2 thin films. Besides the indirect

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sulfurization of Mo on a separated substrate, MoS2 thin films can be easily obtained by the direct

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sulfurization of Mo-based thin films (Mo, MoO3).23,65 The size and thickness of the resulting

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MoS2 film is determined by the deposited Mo-based thin film, making this approach highly

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controllable and scalable. In contrast to the production of horizontal MoS2 film via in-plane

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growth, production of vertically aligned MoS2 nanosheets can be achieved through a rapid

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sulfurization process at 550 °C.67 The vertical growth is a kinetic-driven process at higher

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heating temperature leading to faster sulfurization than sulfur diffusion. Because sulfur diffuses

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much faster along van der Waals gaps than across MoS2 nanosheets, vertically aligned MoS2

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nanosheets are favorably grown exposing van der Waals gaps.67 The maximum exposure of

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high-energy edge sites facilitates a variety of catalytic reactions such as the photocatalytic

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production of reactive oxygen species (ROS) for disinfection.28 In addition, a two-step process

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involving precursor thermolysis and sulfur vapor healing can generate highly crystalline and

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large-area MoS2 thin films on insulating substrates.66 First, a (NH4)2MoS4 thin film was dip9 ACS Paragon Plus Environment


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coated and annealed in Ar/H2 flow below 500 °C to undergo thermolysis. Then, a second

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annealing was conducted at 1000 °C with sulfur vapor to remove the oxygen-containing defects

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and improve the crystallinity of MoS2 thin films. However, the produced MoS2 is restricted in

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size due to the difficulty in obtaining a uniform, large (NH4)2MoS4 thin film.

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Another widely explored bottom-up method is to use hydrothermal/solvothermal reaction,

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which occurs in a sealed autoclave at high temperature and pressure. Typically, Mo precursors

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(e.g., MoO3 or ammonium molybdate) and S precursors (e.g., sulfur powder, KSCN, or thiourea)

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are dispersed or dissolved in water (and/or other solvents).17,18,68-72 Under elevated temperature

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and pressure, MoS2 nanosheets are initially created and then aggregated to form structures such

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as nanoflowers69,71 and nanotubes.18,72 In particular, the defects on MoS2 nanosheet surfaces,

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offering the active sites crucial for various catalytic applications, can be increased by employing

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excess thiourea as precursor to form a defect-rich structure.68 Hydrothermal reactions usually

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produce hybrid 1T/2H phase in the as-prepared MoS2 nanosheets, while pure 2H-MoS2 can be

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obtained by post-annealing.18

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hydrothermal/solvothermal approach is very attractive owing to its facile hybridization with

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other functional nanomaterials such as magnetic nano-Fe3O4 for quick separation of MoS2-based

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adsorbents.73

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hydrothermal/solvothermal reaction, well-dispersed monolayer or few-layer MoS2 nanosheets

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are difficult to obtain.

However,

Besides its simplicity and wide applicability, the

because

of

the

restacking

nature

of

2D

MoS2

during

235 236

CONTAMINANT ADSORPTION CAPABILITY

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Adsorption of Heavy Metal Ions. Adsorption is considered as a versatile water treatment

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technique with wide applicability and low cost. Given the abundance of exposed sulfur atoms on

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its surface, MoS2 could be applied as an efficient adsorbent for the removal of heavy metal ions

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(e.g., Hg2+, Ag+) due to the strong Lewis acid/base soft-soft interactions. By assuming a

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stoichiometric Hg(or Ag)/S ratio of 1:1, the maximum theoretical adsorption capacity of MoS2

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can reach 2506 and 1348 mg/g for Hg2+ and Ag+, respectively.32 Besides, MoS2 is also

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potentially used as an adsorbent for the removal of Hg0 vapor and other ionic species such as

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Pb2+, Cd2+, Zn2+ and Co2+.33,49,74,75


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The mechanism of the interactions between MoS2 and adsorbed heavy metal ions is

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summarized in Figure 2a. Because of redox reactions or defect formation during the synthesis

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process, the resulting MoS2 nanosheets usually exhibit negative surface charge with H+ or Li+ as

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counterion.32,47,49 Thus, the ion exchange to form metal-sulfur bonding could be a primary

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adsorption mechanism.76,77 Hg2+ has been reported to replace H+ ions to complex with two sulfur

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atoms at a low Hg/MoS2 ratio and with one sulfur atom when Hg2+ is in excess.32 Furthermore, a

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multilayer adsorption scheme has been proposed, including an inner-layer complex formation

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between the adsorbed Hg2+ and the surface sulfur atoms, and outer layer interactions due to the

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electrostatic forces between Hg2+ and negatively charge of MoS2 surfaces.77 The electrostatic

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attraction has been widely acknowledged, though its contribution might be minor compared to

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metal-sulfur chemical complexation.74,76,77 The complexation mechanism is also applicable to

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Pb2+,75 while electrostatic attraction is dominant in the adsorption of Co, which is not a typical

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soft acid.74

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The direct use of naturally occurring bulk MoS2 as an adsorbent is limited by the free

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spacing of 0.30 nm between two neighboring layers (Fig. 1a),32 too narrow to allow the hydrated

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metal ions to access the interior sulfur atoms. To address this issue, various MoS2-based

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architectures and composites have been synthesized to maximize the number of sulfur sites

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exposed for metal ion binding while offering convenient separation.32,75,76 As an example, a

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one-step hydrothermal method can be used to prepare multilayer MoS2 nanosheets by widening

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their interlayer spacing to 0.94 nm (Figure 2b) from the original interlayer spacing of 0.62 nm to

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fully expose the interior sulfur atoms. The enlarged interlayer spacing fully exposes interior

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sulfur atoms and thus enables an extremely high mercury uptake capacity that closely matches

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the theoretical maximum level of 2506 mg/g for adsorption as well as fast adsorption kinetics

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and excellent mercury selectivity.32 The Hg2+ uptake capacity of MoS2 nanosheets exceeds those

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of various sulfur-based advanced adsorbents (200-2100 mg/g),32,78-81 as well as the widely

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studied pristine GO nanosheets (187 mg/g)82 and GO-based composites (270-1000 mg/g)82-85.

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In addition, the high specific surface area (164.6 m2/g) of MoS2 can be achieved by

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forming a 3D interconnected macroporous framework (Figure 2c) using a solvothermal method,

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thus generating a high adsorption capacity (e.g., ∼1527 mg/g for Hg2+).76 Another benefit is that

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other materials can be simultaneously loaded to enable multifunctionality of the macroporous 11 ACS Paragon Plus Environment


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framework.

For example, loading Au nanoparticles into the 3D structure by solvothermal

276

treatment leads to sensitive detection of Hg2+,76 and simultaneous loading of carbon dots and

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magnetic nanoparticles provides additional binding sites for Pb sequestration and convenient

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adsorbent recycling by using a magnetic field.75

279

Due to the specific soft-soft interactions, MoS2-based adsorbents are highly selective

280

towards heavy metal ions and resistant to interfered cations in complicated systems.49,74,75 The

281

order of selectivity of Li-intercalated MoS2 for heavy metal ions is Hg2+ > Pb2+ > Cd2+ > Zn2+,

282

consistent with the trend of softness.49 MoS2-based composites exhibit a high adsorption capacity

283

(~ 600 mg/g) of Pb2+ even in the presence of a high concentration of interfering cations, for

284

example, Ca2+ and Mg2+ with the ratio of Ca2+ or Mg2+/Pb2+ up to 100.75 Co2+ is a borderline acid

285

and shows moderate interactions with sulfur. Consequently, the adsorption capacity for Co2+ on

286

MoS2 nanosheets is only 80 mg/g, much less than that for Hg2+ and Pb2+.74 However, the

287

presence of hard base Na+ (up to 50 g/L) has no significant effect on the adsorption of Co2+.74

288

FIGURE 2

289

Adsorption of Organic Contaminants. MoS2 nanomaterials can also be used as adsorbents to

290

remove organic contaminants such as dye, oil, and antibiotics.73,81,86-90 MoS2-based adsorbents

291

show a good adsorption capacity on a wide range of organic dye molecules, including methylene

292

blue, malachite green, rhodamine B, fuchsin acid, and congo red.86,89,90 The maximum adsorption

293

capacity is in the range of 50 to 200 mg/g, comparable to that of graphene-based adsorbents.91

294

The mechanism of dye adsorption is most likely contributed to van der Waals force and

295

electrostatic attraction.86,90 Since MoS2 is negatively charged, the removal of cationic species is

296

much more efficient than that of anionic dyes (e.g., methyl orange), indicating the significance of

297

electrostatic attractions.90

298

Hydrophobic interaction is primarily responsible for the removal of oil and organic

299

solvents by MoS2-based adsorbents.88

300

superhydrophobic adsorbent (with a water contact angle of 150°) by combining the hydrophobic

301

nature of pristine MoS2 nanosheets (with a water contact angle of ~85°)92 and micro/nano-

302

textured surface, which provides sufficient roughness for superhydrophobicity.

303

superhydrophobic MoS2 exhibits excellent adsorption towards a wide range of oils and organic

MoS2 nanosheets can be engineered into a


Such

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solvents with a capacity of 82 to 159 times its own weight, outperforming many commercial and

305

previously reported high-performance adsorbents.93,94

306

Similar to metal ion adsorption, the adsorption capacity of organics on MoS2 depends on

307

surface area. Thus, it is critical to increase the specific surface area of MoS2 by exposing

308

individual nanosheets and minimizing their restacking. One popular MoS2-based adsorbent is

309

the flower-like 3D nanostructure prepared via hydrothermal reactions.89,90,95 The specific surface

310

area of MoS2 nanoflowers can be as high as 107 m2/g.90 The self-assembly strategy via

311

hydrothermal reaction is also applicable to MoSe2, which is analogous to MoS2, similarly leading

312

to flower-like MoSe2 microspheres with good adsorption performance for organic dyes.95 In

313

addition, MoS2 can be easily hybridized with other functional nanomaterials during hydrothermal

314

reaction. For instance, MoS2 nanosheets can be coupled with magnetic Fe3O4 nanoparticles as

315

functional heterostructures, which can be quickly separated from the suspension by applying an

316

external magnetic field.73

317

Another strategy for maintaining the high surface area of MoS2 nanosheets is to anchor

318

them onto a 3D frame as hierarchical structures. Ultrasonically exfoliated MoS2 nanosheets can

319

be firmly adhered to sponge skeletons by strong van der Waals interactions via a facile dipping-

320

and-drying process.88

321

such MoS2-coated sponges typically exhibit fast adsorption kinetics and excellent capacity for a

322

wide range of organic solvents. Interestingly, the organics-containing adsorbent can be

323

regenerated by burning without much degradation of its adsorption capacity, a feature attributed

324

to the chemical stability of MoS2.88

Due to its high porosity, capillary action, and superhydrophobic nature,

325

MoS2 is a more efficient 2D adsorbent for metal ion removal than graphene-based 2D

326

materials. The functionalized GO has a high content of oxygen-containing groups available for

327

interactions with metal ions, thus making it the primary graphene-based adsorbent.10

328

primary mechanism of metal ion adsorption on GO is electrostatic interaction, and consequently

329

GO adsorbents usually show poor selectivity, low adsorption capacity, and strong dependence on

330

pH, ionic strength, and natural organic matters (NOMs).10,50 Compared to GO adsorbents, MoS2

331

nanosheet-based adsorbents provide the following advantages. First, MoS2 monolayer exposes

332

every sulfur atom, which is a soft Lewis acid having a high affinity for soft-acid heavy metal

333

ions, thus generating an extremely high adsorption capacity. Second, the selectivity of MoS2 is 13 ACS Paragon Plus Environment

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334

much higher than GO. This is because the soft-soft interaction renders MoS2 adsorbents highly

335

selective for a variety of heavy metal ions while being resistant to the interfering cations, even in

336

high ionic strength and low pH conditions. For molecular species adsorption, the primary

337

interactions for graphene-based and MoS2 materials are different — π-π interaction for graphene,

338

hydrogen bonding for GO, and van der Waals force for MoS2. Combined with photocatalytic

339

property, MoS2 adsorbents can be regenerated by photodegradation.

340 341

PHOTOCATALYTIC CAPABILITY

342

The appropriate band structure makes MoS2 nanosheets one of the most promising photocatalyst

343

candidates in environmental fields. A semiconducting photocatalyst can be excited by the

344

photon whose energy exceeds its bandgap energy, and consequently electrons on the valence

345

band are excited to the conduction band leaving behind holes.96 These electrons and holes can

346

react with dissolved oxygen and water in separated reactions, forming reactive oxygen species

347

(ROSs) that effectively destroy or mineralize organic contaminants.

348

degradation has many advantages, including complete mineralization, low cost, and mild

349

reaction conditions.96

Such photocatalytic

350

The minimum energy for light to excite a specific semiconductor is determined by the

351

bandgap energy E, which corresponds to a cut-off wavelength of ߣ = ℎܿ/‫ܧ‬, where h and c are

352

Planks constant and speed of light, respectively. The cut-off wavelengths of widely used

353

photocatalysts in environmental fields are shown in the solar spectrum (Figure 3a). TiO2

354

(anatase), for instance, has a bandgap of ~ 3.2 eV and can only be excited by UV light (with a

355

wavelength < ~ 390 nm), which approximately accounts for 3% of energy in the solar

356

spectrum.97 In contrast, bulk MoS2 has a smaller bandgap of 1.3 eV, allowing most of the solar

357

spectrum to be harvested. As the number of MoS2 layers decreases, the bandgap gradually

358

increases and reaches 1.9 eV when MoS2 is exfoliated into monolayer,40 thus allowing the use of

359

visible light (with a wavelength < 660 nm).

360

An effective photocatalyst also relies on its band edge positions (i.e., conduction band

361

minimum and valence band maximum), which determine the redox potentials of photogenerated

362

electrons and holes. In general, as the potential of conduction band (or valent band) becomes


Page 15 of 42


363

smaller (or larger), the photogenerated electrons (or holes) have a stronger reductive (oxidation)

364

capability. Though mono- or few-layer MoS2 nanosheets use narrow portions of the solar

365

spectrum for photocatalysis compared to bulk MoS2, their larger bandgaps shift the band edge

366

positions to yield the redox potentials that favor the ROS generations.97 The band structures of

367

few-layer vertically aligned (FLV) MoS2 and TiO2 (as a reference photocatalyst), are compared

368

in Figure 3b in terms of ROS formation potentials,98 indicating that both FLV-MoS2 and TiO2

369

are capable of catalytically generating a variety of ROS. Besides, the band structure of MoS2

370

nanosheets can be modulated by controlling their lateral dimensions and by doping

371

them as versatile photocatalysts that utilize visible light.

99

to enable

372

FIGURE 3

373

The use of MoS2-based photocatalysts to directly degrade organic contaminants or

374

conduct reductive removal of heavy metal ions have been investigated.29,88,97 The most widely

375

used MoS2 nanosheets for photocatalytic activities are prepared via a scalable hydrothermal

376

approach.29 The as-prepared MoS2 is in the semiconducting 2H phase and photocatalytically

377

active towards degradation of rhodamine B under visible light.29 The photocatalytic performance

378

can be further enhanced by hybridization with reduced GO, which is able to adsorb the dye to the

379

catalyst surface and inhibit photogenerated hole+/e- recombination. In addition, 3D flower-like

380

MoS2 has been prepared in a hydrothermal reaction and further composited with polyaniline

381

(PANI) to make an organic-inorganic hybrid material.88 Under UV irradiation, the maximum

382

Cr(VI) removal capacity of by PANI/MoS2 composite is ~600 mg/g in acidic environment, due

383

to the photocatalytic reduction of Cr(VI) to Cr(III) and subsequent complexation with amine and

384

imine groups on PANI.88

385

The size of MoS2 plays a critical role in the photocatalytic degradation of organic

386

contaminants. Because of the quantum size effect, the decrease of MoS2 size not only increases

387

the band gap but also shifts the redox potential of the conduction and valence bands toward the

388

directions that are favorable for the ROS catalytic generation. The positions of valence band

389

edges of variously sized MoS2 were compared with the redox potential of hydroxyl radical

390

generation.97 The oxidation potential of bulk and ~10-nm-thick MoS2 nanoclusters were found

391

not sufficiently large to allow the oxidation of water to hydroxyl radicals by photo-generated

392

holes, consistent with the experimental results that the two studied forms of MoS2 failed to 15 ACS Paragon Plus Environment


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393

photooxidize phenol.97

394

bandgap and shifted valence band edge with a more positive potential enabling the hydroxyl

395

radical generation and phenol degradation.97

The 4.5-nm-thick MoS2 nanocluster, however, showed a widened

396

An advantage of MoS2 compared to other chalcogenides (e.g., CdS) is the photostability

397

against oxidation. Defect-free bulk MoS2 is very stable during photocatalytic oxidation of water.

398

97

399

ions, the process is quite slow because of the covalent nature of Mo-S bond.97 On the contrary,

400

CdS is an ionic semiconductor and readily undergoes photodegradation, releasing toxic cadmium

401

ions upon receiving irradiation.100

Even if the photocorrosion occurs at the defect sites where edge sulfurs are dissolved as sulfate

402

The photocatalytic property of MoS2 nanosheets can also be used to disinfect drinking

403

water under visible light.28 The CVD-grown vertically aligned MoS2 nanosheets are able to

404

directly expose the highly catalytically active edge sites, a unique morphology that provides

405

more reaction sites, decreases diffusion distances for electrons and holes to the surface, and thus

406

promotes pair separation and the overall catalytic reaction rates.

407

confinement, the bandgap (1.55 eV) of as-prepared MoS2 allows the activation in most of the

408

visible-light range of solar irradiation (Figure 3b). The potential of conduction band in MoS2 is

409

lower than the redox potential of selective ROS formation reactions, indicating O2 could be

410

reduced by photogenerated e- in conduction band generating superoxide anion and hydrogen

411

peroxide. Experimental results showed that, under visible light, MoS2 nanofilms achieved rapid

412

water disinfection with 5 log inactivation of E. coli within 2 h, with overall performance much

413

better than that of widely used TiO2 under visible light or sunlight.28 Four reactive species were

414

found in the system with concentrations of H2O2>O2-·>1O2>OH·; the low concentration of

415

hydroxyl radicals could be attributed to the lower energy of valence band relative to the

416

formation potential of H2O/OH· couple, and thus the photo-generated holes were incapable of

417

reacting with H2O to form hydroxyl radicals.28

Because of the size

418

Other TMDs have also been applied in the photocatalytic reactions. MoSe2 has a narrow

419

bandgap (1.33 to 1.72 eV) and is thus considered as a promising visible-light driven

420

photocatalyst. Vertically aligned MoSe2 nanosheets have been grown on graphene via a facile

421

hydrothermal method and demonstrated the excellent performance of photocatalytically

422

degrading methylene blue, rhodamine B, and methyl orange under visible light.8 The observed 16 ACS Paragon Plus Environment

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423

strong photocatalytic activities can be attributed to the fully exposed active edges of vertically

424

aligned nanosheets and the reduced electron/hole pair recombination by graphene hybridization.

425

Due to its unique band structure, MoS2 is a more promising visible-light driven

426

photocatalyst than graphene-based materials. Pristine graphene displays a semi-metallic

427

character and cannot initialize any photocatalytic treatment alone. Thus, graphene-based

428

materials are primarily used to combine with a photoactive semiconductor (e.g., TiO2) to

429

suppress the recombination of photogenerated electron–hole pairs.7,101 In comparison, MoS2

430

nanosheets, especially monolayer MoS2, are direct bandgap semiconductors with enhanced

431

photo-absorption efficiency. Furthermore, the bandgap structure of MoS2 nanosheets, depending

432

on their thickness, lateral dimension, and doping, can be further tuned for photocatalytic

433

applications.40,97,99,102

434 435

MEMBRANE-BASED SEPARATION

436

Similar to graphene-based nanomaterials, MoS2 nanosheets have recently received much

437

attention for potential applications in membrane-based separation.31,103,104

438

mechanical and thermal stability, facile and scalable synthesis process, and many excellent

439

properties (e.g., photocatalytic and antibacterial properties), MoS2 holds great promise for being

440

used as 2D building blocks to fabricate novel membranes with exceptional separation capability

441

and enable multifunctional and anti-fouling properties.

442

membranes can be made using MoS2 nanosheets — nanoporous membrane and layer-stacked

443

membrane (Figure 4).

444

Due to its high

Two general types of separation

FIGURE 4

445

Nanoporous Membrane. This type of membrane can be made ultrathin by using as few as just

446

a single monolayer MoS2. With appropriately sized nanopores drilled through the monolayer,

447

water molecules are allowed to pass through the pores while unwanted species (e.g., salt and

448

various water contaminates) are blocked (Figure 4a1). Recent molecular dynamics simulations

449

have revealed that the MoS2 nanopore exhibits a very small energy barrier for water molecules to

450

overcome,105 and a single chain of hydrogen bonds that connect water molecules inside and

451

outside the nanopore is formed, thus enhancing water transport through the MoS2 nanopore.104

452

The nanoporous MoS2 membrane (Figure 4a2) demonstrates excellent separation performance, 17 ACS Paragon Plus Environment


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453

with a water flux up to 70% higher than that of graphene nanopores.30,104,105 Besides, it is found

454

that water molecules are unable to permeate through the MoS2 nanopore with a diameter of 0.23

455

nm or smaller (excluding the space occupied by edge Mo and S atoms), while they can freely

456

pass through the nanopore with a diameter of 0.44 nm or larger.104 However, as the nanopore

457

diameter increases to 1.05 nm, salt starts to pass through the nanopore, decreasing the salt

458

rejection rate of the membrane. Therefore, in order to achieve optimal water flux and salt

459

rejection of a nanoporous MoS2 membrane targeting desalination, the nanopore size should be

460

controlled in the range of 0.44 to 1.05 nm.

461

Similar to that for nanoporous graphene membranes,99 water permeance for a nanoporous

462

MoS2 membrane increases linearly with the increasing hydrostatic pressure difference.104,106 For

463

a nanopore with a diameter of no more than 0.44 nm, salt can be completely rejected regardless

464

of the applied pressure (up to 300 MPa); for a larger nanopore, however, the ion rejection

465

capability decreases at a higher pressure, which overcomes the energy barrier of stripping

466

hydration shells and thus enables poorly hydrated ions to pass through the nanopores.104 Besides,

467

the MoS2 nanopore can be enlarged by applying an in-plane tensile strain.30 Because of the

468

relatively low in-plane Young’s modulus of monolayer MoS2 (~270 GPa) compared to that of

469

graphene (1 TPa), MoS2 monolayers are more deformable when subjected to mechanical strains.

470

The local pore chemistry of nanoporous MoS2 plays a significant role in regulating the

471

water flux and ion rejection.30,105 Compared with graphene composed of neutral carbon atoms,

472

MoS2 has electron redistribution between Mo and S atoms, giving rise to charged edge atoms

473

once nanopores are created. Mo-terminated nanopores are positively charged and create a high

474

Coulombic barrier for cationic species,30 implying a very high energy barrier for Na+ transport

475

but not for water or Cl-. Therefore, electrostatic repulsion is an important rejection mechanism

476

of the nanoporous MoS2 membrane (Figure 4a1). In addition, the performance of Mo-only, S-

477

only, Mo/S mixed, and graphene nanopores with similar pore sizes have been compared, and the

478

Mo-only nanopore exhibits the highest water permeance due to the hydrophilic nature of Mo

479

sites at the pore edge, leading to a denser packing and higher velocity of water molecules

480

through the Mo-only nanopore.105

481

So far, the separation capability of nanoporous MoS2 membrane has only been

482

numerically simulated but not yet experimentally verified. One challenge is to create nanopores 18 ACS Paragon Plus Environment

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483

on monolayer MoS2. Nevertheless, the feasibility of forming MoS2 nanopores in a controllable

484

manner has already been demonstrated by using various approaches such as electron beam,107,108

485

ion bombardment,109 and defect engineering110,111 (Figure 4a3), although the pores made at the

486

present stage are still too large (a few nanometers) for the porous MoS2 membrane to be

487

qualified as desalination

488

successively remove individual atoms around defects or single atom vacancy, provides a

489

convenient and scalable route for generating a large number of nanopores with relatively uniform

490

sizes.112 Similar to making porous graphene membranes, fabricating the porous MoS2 membrane

491

also faces a daunting challenge of preparing large-area MoS2 monolayers that are needed for the

492

membrane scale-up.

493

Layer-stacked Membrane. An alternative approach to making MoS2-based membrane is to

494

restack the exfoliated MoS2 monolayers through vacuum filtration technique.6 As discussed

495

earlier (Table 1), the chemically exfoliated MoS2 nanosheets are mostly monolayers and with

496

large lateral dimensions, while the liquid exfoliated ones are usually small in size and coated

497

with surfactants. Therefore, chemically exfoliated MoS2 nanosheets are more suitably used as 2D

498

building blocks to make high-performance membranes. The 1.7 µm-thick lamellar MoS2

499

membrane from a previous study had a water flux of 245 L h-1 m-2 bar-1(LMH/bar), significantly

500

higher than that of a reduced GO membrane (~ 45 LMH/bar) with a comparable thickness, while

501

the rate of rejection for small molecules (Evans blue and cytochrome c) was maintained at

502

~90%.113 The high water flux can be attributed to low hydraulic resistance of the smooth

503

channel surface, since MoS2 nanosheets do not have any functional groups. In comparison, GO

504

contains many oxygenated functional groups sticking out from its carbon plane, generating

505

hydraulic resistance to water flow. Meanwhile, the smooth MoS2 channel also allows the

506

continuous transport of light organic vapors, to which a GO membrane is impermeable because

507

of the blocked pathway by its oxygenated groups.114 In addition, the water permeance of a layer-

508

stacked MoS2 membrane can be further increased by templating ultrathin nanowires between

509

MoS2 layers, a strategy that has been demonstrated in the membrane made of WS2, an analogue

510

of MoS2.6

membrane. Especially, electrochemical reaction, which can

511

An exceptional property of the layer-stacked MoS2 (and its analogue WS2) membrane is

512

its excellent stability in water. It was reported that an as-prepared layer-stacked MoS2 membrane 19 ACS Paragon Plus Environment


Page 20 of 42

513

exhibited steady water permeance and molecule rejection during a week-long testing,6 without

514

any stabilization treatment (e.g., crosslinking). This property has also been recently demonstrated

515

by the outstanding integrity of plain MoS2 membranes in water under various pH conditions.114

516

In contrast, the layer-stacked GO membrane is hydrophilic and thus instable in water, typically

517

demanding stabilization steps such as reduction,8 intentional crosslinking,9 unintentional

518

crosslinking,115 or layer-by-layer assembly.116 MoS2 does not have any hydrophilic groups on its

519

surface, and the van der Waals force between MoS2 nanosheets may provide the necessary

520

stability against the redispersion of layer-stacked MoS2 nanosheets in water.114 In addition, the

521

relatively high rigidity (due to the existence of a three atomic layers) and high surface

522

smoothness (due to the lack of crosslinkers or functional groups) of MoS2 nanosheets may lead

523

to a more neatly packed membrane structure that is less likely to be disturbed. In comparison,

524

MoS2 does not have any hydrophilic groups on its surface, potentially making the layer-stacked

525

MoS2 membrane highly stable in water.

526

The layer-stacked MoS2 membrane exhibits a linear relationship between water

527

permeance and applied pressure,103 a flux behavior that is quite different from that of a layer-

528

stacked GO membrane, which typically shows a saturated water flux under a high pressure.6,9

529

This is possibly because the relatively low out-of-plane rigidity of GO nanosheets leads to the

530

elastic deformation of GO nanochannels under high pressure while the high rigidity of MoS2

531

nanosheets helps maintain the original size of their nanochannels. Such an interesting flux

532

behavior along with the excellent stability of MoS2 nanosheets suggests that they can be

533

advantageously used as a novel material to make high-performance membranes by conveniently

534

manipulating the interlayer spacing of the layer-stacked MoS2 nanosheets and thus fine-tuning

535

the membrane separation capability.

536

Finally, the absence of conjugated structure in MoS2 could help avoid the problem of

537

scaling and organic fouling, which are common for graphene-based membraned because of the

538

cation-π and π -π interactions. So far, research on the separation performance and the underlying

539

mechanisms of layer-stacked MoS2 membranes is at its very early stage (e.g., organic dye

540

removal 6).

541


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542

ENVIRONMENTAL SENSING

543

Unlike graphene with zero bandgap, 2D layered semiconductors with bandgaps, particularly

544

MoS2 monolayers with a direct bandgap and excellent capacitance, are potential materials for

545

FET-based sensors. In fact, MoS2-based FET sensors have exhibited exceptional charge carrier

546

mobility and high on/off ratios with excellent sensitivity in the detection of biomolecules, heavy

547

metal ions, and toxic gas.44,45,117-119 In addition, the fluorescence quenching ability of MoS2 can

548

also be applied for sensing.120 Here, only the environmentally relevant sensing applications are

549

reviewed.

550

MoS2-based FET devices use pristine or functionalized MoS2 nanosheets as a dielectric

551

layer for selectively capturing the desired target and then transducing target concentration signal

552

to the change of current or channel conductance. Because of the high surface-to-volume ratio, 2D

553

MoS2 nanosheets provide active sites for the adsorption of analyte molecules. For instance,

554

electron-withdrawing NO can be adsorbed on MoS2, increasing the resistance and decreasing the

555

current.44 Due to a high binding affinity between Hg2+ ions and sulfur sites, Hg2+ ions can also be

556

strongly adsorbed on pristine MoS2 surface in the FET sensor, resulting in the highly sensitive,

557

selective detection of Hg2+ ions, compared to the interfering chemicals (e.g., Na+, K+, Ca2+) at

558

the same concentrations.119 Furthermore, MoS2 channel can be conjugated with other

559

components (e.g., thiolated ligand) to increase the sensitivity and detection diversity. The

560

primitive and thiolated ligand-functionalized MoS2 channels exhibit very different sensing

561

behavior for various volatile organic compounds (VOCs) due to the different molecular

562

interactions between VOC molecules and the MoS2 surface,111 highlighting the importance of

563

surface functionalization in MoS2 sensor versatility. Both primitive and functionalized sensors

564

displayed high selectivity toward representative VOC groups (e.g., toluene, hexane).111

565

addition to the target species discussed, the MoS2–based FET device has been applied in the

566

detection of proteins117 and gases (e.g., H2, NH3, NO2).45,121

In

567

The fluorescence quenching ability of MoS2 nanosheets is primarily used in the detection

568

of biomolecules and metal ion contaminants.120,122 Dye-labelled single-stranded DNA can be

569

almost entirely quenched by MoS2 nanosheets, while the addition of complementary target DNA

570

forms double-stranded DNA, which is detached from the MoS2 surface due to the weaker

571

interaction, resulting in the recovery of fluorescence for quantitative detection of the target 21 ACS Paragon Plus Environment


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572

DNA.122 Furthermore, loading the rhodamine B isothiocyanate onto MoS2 nanosheets enables

573

sensitive and selective detection of soluble Ag ions.120 Ag+ can be reduced to Ag nanoparticles

574

on MoS2 nanosheets, replacing the fluorescent molecules and thus recovering the fluorescence.

575

This sensing platform allows highly sensitive detection of Ag+ (down to 10 nM) with excellent

576

selectivity over interfering metal ions.120 More important, the biocompatible nature of MoS2

577

enables the monitoring of Ag+ ions level in/on living E. coli cells. The unique quenching

578

characteristic of MoS2 nanosheets can also be applied for sensing other species such as proteins

579

123

and H2O2.124

580 581

ANTIBACTERIAL CAPABILITY

582

Besides the disinfection capability enabled by their photocatalytic properties in semiconducting

583

phase as discussed earlier, MoS2 nanosheets have shown other biological properties leading to

584

antibacterial applications.61,125-127 Chemically exfoliated MoS2 nanosheets, which are primarily

585

metallic and conductive 1T phase, have shown antibacterial property in a three-step mechanism,

586

including direct bacterium-MoS2 contact, membrane damage by sharp MoS2 edges, and

587

disruption of microbial processes in redox reactions.125 ROS (e.g. superoxide) can be plausibly

588

generated due to the electron transfer from biological components to oxygen via the conductive

589

planar MoS2.128 Surface functionalization of chemically exfoliated MoS2 nanosheets reduces the

590

oxidative stress but increases the damage to cell membranes due to the stronger interactions

591

between the functionalized groups (alkane chain) and the cell membranes.126

592

toxicity of MoS2 nanosheets to E. coli. at the metabolomics level confirmed the antibacterial

593

mechanism for the induced damage to cell membranes and ROS accumulation.127 Another

594

antibacterial mechanism of MoS2 nanosheets is related to their peroxidase catalytic activity,

595

which can catalyze the decomposition of H2O2 to generate highly reactive hydroxyl radicals.15

596

Further combined with their photothermal property, a synergistic effect may be achieved towards

597

enhanced antibacterial performance.

598


Study on the

Page 23 of 42


599

ENVIRONMENTAL IMPLICATIONS

600

In parallel to the development of MoS2-based nanomaterials and devices, research is needed to

601

understand the fate and transport of MoS2 nanosheets after they are eventually released to the

602

environment.

603

surface chemistry, can undergo profound chemical and physical transformations (e.g., oxidation,

604

dissolution, sulfidation, aggregation, and deposition) in the natural biological and environmental

605

systems.129 The dissolution of nanomaterials is a particularly important process, which could

606

produce soluble species that are highly bioavailable and possibly toxic to aquatic animals and

607

human beings.130,131

608

persistent and non-degradable.128

Nanomaterials, depending on their specific chemical phase/composition and

Furthermore, nanomaterials may pose a long-term threat if they are

609

MoS2 is traditionally considered as a chemically stable material against environmental

610

stressors because of the absence of dangling bonds in the terminating S atoms.34 The solubility

611

of MoS2 is low under ambient conditions, indicating its long-term persistence in the

612

environment.128 In harsh conditions (e.g., high temperature, strong oxidation), however, bulk and

613

monolayer MoS2 materials can be oxidized to molybdenum oxide.81,132 The oxidation behavior

614

is less prominent in the case of bulk MoS2 because a passivating oxide layer is usually formed

615

first on the surface to prevent complete conversion to the oxide.133 The oxidation of MoS2

616

nanosheets has recently been shown to occur in aqueous solutions, leading to soluble, low-toxic

617

oxidation products according to the equation of MoS2 + 9/2 O2 + 3H2O = MoO42- + 2SO42-

618

+6H+.61,134 The oxidation kinetics is pH-dependent, with faster degradation of MoS2 nanosheets

619

in higher pH condition. In addition, it is also affected by the phase of MoS2: the metallic 1T-

620

MoS2 (e.g., that generated from Li-intercalation/exfoliation) is more easily oxidized than the

621

thermodynamically stable 2H-MoS2.61 Based on density functional theory, a recent study135

622

revealed that oxidation of MoS2 nanosheets in the metallic 1T phase proceeds by a two-site

623

corrosion mechanism, where electrons are generated at one site on MoS2 plane and conduct to

624

the second site to complete the redox reaction. The required internal electron transfer is enabled

625

by the conductive and metallic 1T phase but not the insulating semiconducting 2H phase.

626

The biopersistence and cytotoxicity of various types of MoS2 nanosheets, including

627

polyethylene glycol (PEG)-coated 2H-MoS2, metallic 1T-MoS2, and functionalized 1T-MoS2,

628

have also been studied.87,136-138 Despite the variations of toxicity depending on preparation 23 ACS Paragon Plus Environment


Page 24 of 42

629

methods, MoS2 nanosheets generally show high biocompatibility at concentrations up to ~ 100

630

ppm and certain cytotoxicity at high concentrations (a few hundred ppm).16,87,138-142 For example,

631

in one study, neither pristine nor PEG-functionalized MoS2 nanosheets caused any loss of

632

viability in Hela cells after 24-h incubation at the 160-ppm concentration of MoS2, and the

633

viabilities after 2 days were slightly reduced to 80% and 90% for pristine and functionalized

634

samples, respectively.16 In another study, low toxicity of exfoliated, well-dispersed MoS2

635

nanosheets was observed but aggregated samples were found to induce acute lung inflammation

636

in mice,142 raising concerns about the size effects on the toxicity of MoS2 nanosheets. In addition,

637

the toxicity of MoS2 nanosheets is lower than that of graphene-based nanomaterials, revealing

638

MoS2-based materials as a promising alternative in environmental applications.136,137

639

Furthermore, PEG-coated MoS2 nanosheets show fast degradation and complete excretion within

640

a month, in marked contrast to other TMD materials accumulated in the organs for months.87

641

These in vivo results are consistent with the results of enzyme-catalyzed MoS2 degradation138

642

and oxidative dissolution of MoS2 nanosheets in aqueous solutions.61 Moreover, the low toxicity

643

profile of the soluble products (e.g., MoO42-) of MoS2 oxidation accounts for, at least in part, the

644

high biocompatibility of MoS2-based materials, which readily undergo degradation and release

645

non-toxic soluble Mo species.61

646 647

OUTLOOK AND RESEARCH NEEDS

648

The fast-growing interest in using 2D MoS2 nanomaterials for environmental applications has

649

been inspired by the exciting discoveries of exceptional properties and performance of graphene-

650

based nanomaterials.4

651

properties that are expected to enable a number of environmental benefits unattainable by using

652

graphene-based nanomaterials.

653

nanomaterials has already demonstrated promising applications in, for example, the removal of

654

heavy metals, photodegradation of contaminants, membrane separation, sensing, and

655

antibacterial treatment. In order to maximize the advantages and avoid the disadvantages of

656

MoS2 nanomaterials in their environmental applications, many outstanding research questions,

657

regarding their unique adsorption, photodegradation, semiconducting, and separation capabilities

658

remain to be answered and the corresponding mechanisms need to be fundamentally understood.

As summarized in Figure 5, MoS2 nanosheets have many unique Still in its infancy, the research on the use of MoS2


Page 25 of 42


659

FIGURE 5

660

As an adsorbent, MoS2 faces challenges similar to those for typical adsorbents, that is,

661

fouling in real environment and separation of nanosized adsorbents from water for recycling.

662

Ideally, chemically exfoliated MoS2 monolayer is the most promising adsorbent for heavy metal

663

ion removal. However, separation of the MoS2 monolayer from water poses a huge challenge.

664

Potential strategies for adsorbent separation include conjugation with magnetic nanoparticles,

665

loading of nanosheets on the surface of other porous materials, and assembly into 3D

666

macroscopic architecture using nanosheets as building blocks. In addition, various synthesis

667

routes provide MoS2 with rich and different surface chemistry (e.g., negative charge, phase,

668

defects, saturated/unsaturated sulfur), the effects of which on the contaminant adsorption

669

activities should be thoroughly investigated to provide insights into the rational design of high-

670

performance MoS2-based adsorbents. The application of MoS2 for the removal of

671

environmentally relevant species other than heavy metal and organic contaminants, as discussed

672

in this review, is still lacking. Due to its natural negative charge, MoS2 holds promise for

673

adsorbing cationic radioactive species, yet extra efforts are needed to enable specific bonding of

674

anionic pollutants (e.g., phosphate, fluoride) with MoS2 surface. Various environmental factors,

675

such as pH, ionic strength, and presence of NOM, can influence the adsorption capacity and

676

kinetics of MoS2 adsorbents, and such effects should also be investigated in the future.

677

Though there have been a few preliminary reports showing photocatalytic

678

decontamination or disinfection capabilities of MoS2 or other materials alike, more studies

679

should be conducted to better understand (i) the bandgap structures of MoS2 synthesized via

680

various approaches and types of catalytically generated ROS, (ii) the products of MoS2-catalyzed

681

decontamination with and without complete mineralization, respectively, and (iii) the

682

photostability of MoS2 nanosheets.

683

photocatalytic applications might be chemically exfoliated MoS2 nanosheets, in which 2H-1T

684

phase interfaces exist as unique electronic heterojunctions across a chemically homogeneous

685

layer. The metallic 1T can serve as an electron trapper to extend the lifetime of electron−hole

686

pairs, enhancing the photocatalytic activities of 2H semiconducting regions. Further research in

687

this field of MoS2 photocatalytic applications is definitely warranted.

In addition, a promising MoS2 nanomaterial for



Page 26 of 42

688

The use of MoS2 as membrane materials is only in its infancy. The major challenges

689

facing the nanoporous MoS2 membrane are the synthesis of large-area MoS2 nanosheets and

690

creation of uniformly and densely distributed nanosized pores on such nanosheets. As the other

691

alternative, the layer-stacked MoS2 membrane demonstrates great potential for high water flux,

692

finely tunable interlayer spacing, good structural stability against swelling or compression, and

693

excellent fouling resistance against organic substances. More research is needed to investigate

694

such interesting behaviors and understand their correlations with the unique MoS2 nanosheet

695

properties, which are very different from those of GO nanosheets, including rigid MoS2 vs.

696

flexible GO, smooth MoS2 vs. relatively rough GO with extruding oxygenated functional groups,

697

and weak MoS2-organics interactions vs. strong π-π interactions between organics and graphene.

698

Fundamental understanding of the interlayer spacing and free spacing of the MoS2 membrane,

699

molecular and ion transport within the 2D channels, the underlying separation mechanisms, and

700

anti-fouling behaviors is urgently needed through systematic experimental characterization

701

assisted with molecular dynamics simulation. 143,144

702

The fabrication cost and environmental implications of MoS2 nanomaterials are important

703

factors to consider in the development of MoS2-based technologies for environmental

704

applications. At present, the cost for making MoS2 nanosheets, especially high-grade samples by

705

CVD method, is higher than many widely studied/used materials in environmental fields.

706

However, the cost is expected to decrease after the synthesis method is improved, production is

707

scaled up, and supply chain is optimized. For example, the recently developed simple synthesis

708

methods, e.g., exfoliating low-cost, naturally occurring MoS2 mineral in water without the use of

709

any surfactants, may dramatically lower the cost of MoS2 nanosheets suitable as adsorbents and

710

membrane materials.145 In addition, the environmental fate and transformation as well as human

711

health risk of MoS2 nanosheets must be thoroughly investigated to minimize their potential

712

environmental impacts. Although preliminary data have indicated low toxicity of MoS2, the

713

variability of MoS2 nanosheets in, for example, thickness, lateral size, phase, and defects could

714

further complicate the toxicity effects and thus requires extensive future research to understand

715

the effects and corresponding mechanisms.

716

2D MoS2 nanosheets are graphene-inspired inorganic material but they have preliminarily

717

demonstrated apparently different and novel applications in many of the environmental fields. 26 ACS Paragon Plus Environment

Page 27 of 42


718

While the research is underway, it is anticipated that 2D MoS2 nanomaterials will bring more

719

exciting opportunities and outcomes in environmental applications, after ever-increasing efforts

720

have been devoted to resolve the scientific tasks identified in this review article.

721 722

ACKNOWLEDGEMENT

723

The material is based upon work supported by the U.S. National Science Foundation under

724

award no. CBET-1565452 and the U.S. Department of Energy under award no. DE-IA0000018.

725

The opinions expressed herein, however, are those of the authors and do not necessarily reflect

726

those of the sponsors.



Page 28 of 42

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1111 1112

Figure 1. MoS2 structure. (a) 3D illustration, adapted with permission from Ref. 12. Copyright 2011,

1113

Nature Publishing Group. Atomic positions in (b) the 2H phase with trigonal prismatic coordination and

1114

in (c) the 1T phase with octahedral coordination.

1115 1116



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1117 1118

Figure 2. Adsorption mechanism for heavy metal removal and fabrication strategies for maximizing

1119

nanosheet exposure. (a) The multilayer metal adsorption formed by chemical complexation and

1120

electrostatic attraction. (b) Characterization and schematic illustration of MoS2 with widened interlayer

1121

spacing, reproduced with permission from Ref. 32. Copyright 2016, John Wiley & Sons, Inc.

1122

Schematic illustration of 3D nanoporous MoS2 architecture and loaded functional Au nanoparticles for

1123

the removal and detection of Hg2+, reproduced with permission from Ref 76. Copyright 2016, American

1124

Chemical Society.

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(c)

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1126 1127

Figure 3. (a) Spectral irradiance of solar radiation, with the cut-off wavelengths of several well-

1128

known photocatalysts and MoS2 materials including bulk-, monolayer-, and few-layered vertically

1129

aligned (FLV) MoS2.28,146-148 (b) The band positions of FLV-MoS2 and TiO2 (reference photocatalyst)

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with respect to the ROS formation potential.98 (c) Schematic illustration of disinfection mechanism of

1131

FLV-MoS2, reprinted with permission from Ref 28. Copyright 2016, Nature Publishing Group.

1132 1133



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1135 1136

Figure 4. (a1) Side view of nanoporous MoS2 membrane and rejection mechanisms involving size

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exclusion and electrostatic repulsion. (a2) Top view of the modeled MoS2 nanopore and illustration of the

1138

simulation model, reproduced with permission from Ref. 30, Copyright 2016, American Chemical

1139

Society. (a3) TEM of MoS2 nanopore generated by e-beam drilling, reproduced with permission from Ref.

1140

108, Copyright 2016, Nature Publishing Group. (b1) Schematic illustration of layer-stacked MoS2

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membrane and its separation mechanism. (b2) SEM of layer-stacked MoS2 membrane prepared by

1142

vacuum filtration, reproduced with permission from Ref. 103, Copyright 2013, Royal Society of

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Chemistry.

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Figure 5. Comparison of characteristic properties of MoS2 nanosheets and graphene-based

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nanomaterials.



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Table 1. Different methods for synthesizing MoS2 nanosheets. Category

Synthesis method

Topdown

Mechanical exfoliation

Bottomup

MoS2 product Phase Morphology 2H Monolayer and few-layers

Advantages Mild conditions High-quality nanosheets

Low yield Random layer numbers

Use of toxic solvent or presence of surfactant residue Large distribution in size and thickness Use of flammable chemicals Phase conversion

Liquid exfoliation

2H

Few-layers

High yield Scalable

Chemical exfoliation

1T/2H hybrid

Primarily monolayers Lateral size ~ 200-800 nm

High yield Presence of defects for further facile functionalization

Chemical vapor deposition (CVD) Hydrothermal/ solvothermal reaction

2H

Mono- and fewlayers Lateral size up to several µm Nanoflowers, nanotubes

Crystalline and high-quality sample Controllable size and thickness Scalable High yield Facile hybridization with other functional materials

1T/2H hybrid

Disadvantages

High-temperature requirement Potential leaking of toxic vapor (sulfur, H2S) Harsh conditions Aggregation into microscale structures


Primary applications

References

Fundamental study and device performance demonstration Energy storage Electronic device

12,21,40

Hydrogen evolution reaction Membrane materials Optoelectronics Electronics Sensors Catalysis Energy storage Catalysis Environmental adsorbents

37,39,62,63,103

22,36,58

23,64,66,67,149

29,68-70,72

Environmental Applications of 2D Molybdenum ... - ACS Publications

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