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Aggregation Behavior of Inorganic 2D Nanomaterials Beyond Graphene: Insights from Molecular Simulations and Modified DLVO Theory Tashfia M. Mohona, Anusha Gupta, Arvid Masud, Szu-Chia Chien, Li-Chiang Lin, Prathima C. Nalam, and Nirupam Aich Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05180 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Environmental Science & Technology

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Aggregation Behavior of Inorganic 2D Nanomaterials Beyond Graphene: Insights from Molecular Modeling and Modified DLVO Theory by

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Tashfia M. Mohona,1 Anusha Gupta,2 Arvid Masud,1 Szu-Chia Chien,3 Li-Chiang Lin,4 Prathima Nalam,5 and Nirupam Aich1,*

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1Department

of Civil, Structural and Environmental Engineering, University at Buffalo, The State University of New York, Buffalo, NY, USA

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2Department

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3Department

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4William

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5Department

of Civil Engineering, Indian Institute of Technology, Gandhinagar, Gujarat, India of Materials Science and Engineering, The Ohio State University, Columbus, OH, USA G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH, USA of Materials Design and Innovation, University at Buffalo, The State University of New York, Buffalo, NY, USA

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Submitted to Environmental Science & Technology (March 15, 2019)

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*Corresponding

Author: Nirupam Aich, Phone: 716-645-0977, Email: [email protected]

ACS Paragon Plus Environment


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Abstract:

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We report the comparative aggregation behavior of three emerging inorganic 2D nanomaterials

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(NMs): MoS2, WS2, and h-BN in aquatic media. Their aqueous dispersions were subjected to

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aggregation under varying concentrations of monovalent (NaCl) and divalent (CaCl2)

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electrolytes. Moreover, Suwanee River Natural Organic Matter (SRNOM) has been used to

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analyze the effect of natural macromolecules on 2D NM aggregation. An increase in electrolyte

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concentration resulted in electrical double-layer compression of the negatively charged 2D NMs,

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thus displaying classical Derjaguin-Landau-Verwey-Overbeek (DLVO) type interaction. The

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critical coagulation concentrations (CCC) have been estimated as 37, 60, and 19 mM NaCl and

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3, 7.2, and 1.3 mM CaCl2 for MoS2, WS2, and h-BN, respectively. Theoretical predictions of

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CCC by modified DLVO theory have been found comparable to the experimental values when

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dimensionality of the materials is taken into account and a molecular modeling approach was

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used for calculating molecular level interaction energies between individual 2D NM nanosheets.

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Electrostatic repulsion has been found to govern colloidal stability of MoS2 and WS2 while the

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van der Waals attraction has been found to govern that of h-BN. SRNOM stabilizes the 2D NMs

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significantly possibly by electrosteric repulsion. The presence of SRNOM completely stabilized

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MoS2 and WS2 at both low and high ionic strengths. While h-BN still showed appreciable

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aggregation in the presence of SRNOM, the aggregation rates were decreased by 2.6- and 3.7-

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fold at low and high ionic strengths, respectively. Overall, h-BN nanosheets will have higher

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aggregation potential and thus limited mobility in the natural aquatic environment when

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compared to MoS2 and WS2. These results can also be used to mechanistically explain fate,

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transport, transformation, organismal uptake, and toxicity of inorganic 2D NMs in the natural

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ecosystems.


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Keywords: Aggregation, 2D Nanomaterials, DLVO, MoS2, WS2, h-BN.

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1. Introduction

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The discovery of the unprecedented properties of planar atomically thin carbon allotrope

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graphene has invoked a keen interest in other two dimensional nanomaterials (2D NMs).1

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Molybdenum disulfide (MoS2), tungsten disulfide (WS2), and hexagonal boron nitride (h-BN), in

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particular, have received much attention as inorganic analogues of graphene, owing to their

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unique properties such as atomic thinness, tunable band-gap, high electron mobility, and thermal

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conductivity.2, 3 Bulk MoS2, WS2, and h-BN have been historically used as solid lubricants,4, 5

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while their mono-layered nanoscale counterparts are promising for optoelectronic,6,

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biomedical,8, 9 energy,10-12 and environmental1, 13, 14 applications. MoS2, WS2, and h-BN are non-

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carbonaceous and physicochemically distinct from graphene or its derivatives.15 MoS2 and WS2

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are transition metal dichalcogenides (TMDs) having a triatomic layer structure: one layer of

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transition metal atoms sandwiched between two layers of sulfur.3 h-BN, on the contrary, is a

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structural analogue of graphene with honeycomb lattice but having ionic B-N bond as opposed to

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covalent C-C bond of graphene.13 Unlike superconductive graphene with zero band gap, single

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layer MoS2, WS2, and h-BN are semiconductors and have direct bandgaps7,

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quantum confinement.17 This direct band gap imparts features such as silicon like electron

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mobility,18 excellent current on/off ratio,18 and strong photoluminescence19 enabling use for field

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effect transistors, photodetectors, photovoltaics, and energy storage and conversion. Moreover,

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superior optical absorption, photoluminescence, high lubricity, flexibility, and catalytic activity

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make them suitable for drug delivery, orthodontics, endoscopy, optogenetics, bioimaging, and

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biosensing.20

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photocatalysis,21 disinfection,22 and high performance water purification membrane.1 Such rise

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arising due to

Their use has recently been extrapolated for contaminant adsorption,13



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in research and potential commercialization point towards inevitable release and exposure of

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these inorganic 2D NMs to the environment.

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The increasing manufacture and use of 2D NMs can lead to their release, like any NMs, to

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different environmental matrices i.e., air, soil, and water.23 Release into ambient or indoor air

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may occur during their production and handling, and pose occupational risk to workers.24

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Landfills are expected to receive a considerable amount of NM-laden products, and thus,

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leaching of the NMs into surrounding soil and water is highly likely.25 Additionally, NMs not

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captured by wastewater treatment processes are released into the water bodies with effluent.26 It

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thus becomes imperative to study the environmental fate and toxicity of these emerging

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inorganic 2D NMs.

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Colloidal stability of 2D NMs in aquatic environment influences their interactions with

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microorganisms.27 Bioavailability, and thus toxicity, will be governed by their mobility and

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aggregation behavior in the corresponding environment.28 Among 2D NMs, graphene has been

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most widely studied and its toxicity has been most thoroughly scrutinized.29, 30 To date, only few

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studies have investigated the interactions of inorganic 2D NMs with microorganisms.31 Owing to

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its higher electron conductivity than bulk MoS2, exfoliated MoS2 is hypothesized to generate

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more reactive oxygen species (ROS) that lead to enhanced cytotoxicity.31 Membrane stress

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induced by atomically thin MoS2 nanosheets and their increased surface area have also been

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accounted for their cytotoxicity.32

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antibacterial activity with E. coli,22 planktonic cells,33 and biofilms33 compared to their bulk

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counterparts, suggesting that the degree of exfoliation influences their cytotoxicity. Similarly,

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WS2 nanosheets demonstrated bactericidal activity toward E. coli and S. aureus mediated by

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oxidative and membrane stress.34 The enhanced toxicity of h-BN nanosheets compared to its

MoS2 nanosheets have been found to show higher


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bulk is attributed to ROS generation by unsaturated boron atoms, acting as radicals, at the

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nanosheet edges.35 Aggregation of 2D NMs reverses the exfoliation mediated features, including

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thinness, surface area, and exposed edges, and thereby affects toxicity and its mechanisms.

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Knowledge of the aggregation behavior of 2D NMs is thus important for their environmental

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implication studies.

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The classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory is able to explain the

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aggregation behavior of charged colloidal particles.36 The assumption of spherical particles has

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also been made for sheet-like planar 2D NMs like graphene oxide (GO).37-39 However, the van

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der Waal (vdW) interaction energy scales inversely to the sixth and fourth powers of distance for

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spherical particles and 2D NMs respectively.40

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physicochemical properties of 2D NMs warrant the use of appropriate force laws for properly

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capturing their aggregation behavior. A recent study on GO has demonstrated agreement of the

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2D NM’s colloidal stability with DLVO theory;41 however, only when Hamaker constants and

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vdW forces for GO have been modified to account for the effect of its dimensionality. GO

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contains carboxyl and hydroxyl functional groups which provide them both hydrophilicity and

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hig negative surface charge. Hence, electrostatic repulsion is hypothesized to be the governing

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DLVO force behind GO’s excellent colloidal stability.42

Such difference coupled with the distinct

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On the contrary, inorganic 2D NMs i.e., MoS2, WS2, and h-BN are considered hydrophobic.

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However, MoS2 and WS2,have lone pair of electrons on the chalcogen atoms rendering them

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active for a wide range of chemical reactions.43 Although they are considered hydrophobic, their

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exfoliation in aqueous media through ultrasonic (i.e., by ultrasound mediated shear-forces)44 or

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chemical (i.e., intercalation by lithium ions)45 routes impart high negative charges on MoS2 and

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WS2 nanosheets.46, 47 In case of h-BN, the interplanar bonding is a composite of vdW forces and



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ionic attraction, the latter arising due to charge localization on N atom.48 The electron-deficient

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boron atoms act as Lewis acids, thereby enriching the h-BN chemistry.49 Thus, despite being

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considered hydrophobic in bulk form, h-BN is prone to both solvent polarity effect and

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hydrolysis.50 Sonication-assisted aqueous exfoliation of h-BN causes large sheets to break off

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along defect sites into smaller flakes. It is hypothesized that the boron edges of these flakes have

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dangling hydroxyl groups resulting from thermodynamically favorable release of ammonia.50

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Thus, exfoliated h-BN nanosheets become hydrophilic and colloidally stable in aqueous

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dispersions. It is anticipated that the interplay of these distinct physicochemical properties may

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affect the colloidal stability of inorganic 2D NMs differently from each other as well as from

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GO. The critical knowledge gap exists in understanding the applicability and validity of the

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modified DLVO theory for inorganic 2D NMs beyond GO.

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The objective of this paper is to assess and compare the aggregation behavior of these three

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emerging and important inorganic 2D NMs i.e., MoS2, WS2, and h-BN in aqueous media. An

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ethanol-water mixture is used to exfoliate and disperse these 2D NMs. The chemical identity,

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physical morphology, and electrokinetic properties of the 2D NMs are then characterized by UV-

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vis spectroscopy, high resolution transmission electron microscopy (HRTEM), scanning

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transmission electron microscopy (STEM), atomic force microscopy (AFM), and electrophoretic

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measurements.

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dynamic light scattering (TRDLS) under a broad spectrum of mono- and divalent electrolytic

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conditions and also in the presence of Suwanee River Natural Organic Matter (SRNOM). The

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mechanisms underlying the aggregation behavior of the inorganic 2D NMs are determined using

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modified DLVO theory and interactions at the molecular level are elucidated by computing the

Aggregation kinetics of the 2D NMs are determined using time resolved


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van der Waal (vdW) energies between two layers of 2D NMs via summing all pair-wise inter-

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atomic interactions with the aid from molecular modeling.

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2. Materials and Methods

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2.1

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exfoliated and dispersed in aqueous media in the same method which was already established in

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the literature and described elsewhere.15 Briefly, 30 mg of MoS2, WS2, or h-BN ultrafine

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powder (99%, Graphene Supermarket, Calverton, NY) was taken in 25 mL beaker and 10 mL of

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45:55 mixture of ethanol and deionized (DI) water (by volume) was added into it as dispersant.

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The beaker was then placed in an ice bath and the mixture was sonicated using a probe tip

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sonicator Q700 (Qsonica, Newtown, CT) for 60 min. The amplitude of the sonicator was set to

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50 corresponding to a power input of 12-15 W, and the pulse on and pulse off times were set to 8

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s and 2 s, respectively. After sonication, the contents of the beaker were poured into a 15 mL

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centrifuge tube and centrifuged at 3000 rpm for 20 minutes with Centrifuge 5810 R (Eppendorf

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AG, Hauppauge, NY). Next, the supernatant was collected from the centrifuge tube and stored

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in 20 mL scintillation vial, wrapped in aluminum foil, at 34 ºF until further use and between

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experiments. A flow-diagram of this experimental protocol for preparing aqueous dispersion of

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inorganic 2D NM has been provided in the supporting information (Figure S1).

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2.2

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99+%, Acros Organics) salts of a wide range of concentrations were used to replicate natural

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aquatic environment.

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dissolving required amount of their powders in DI water and then passing the solutions through

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0.22 µm cellulose acetate membrane filters. The stock solutions were then diluted with DI water

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to produce electrolyte solutions of required cation concentrations. In order to determine the

Aqueous 2D NM Suspension Preparation.

All three inorganic 2D NMs were

Solution Chemistry. Mono- (NaCl, 99.8%, Fischer Scientific) and di-valent (CaCl2,

Stock solutions of NaCl (5 M) and CaCl2 (1 M) were prepared by



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effect of natural organic matter on 2D NM aggregation, two different electrolyte conditions were

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used in the presence and absence of SRNOM. These two electrolyte conditions were (1) a

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mixture 7 mM NaCl and 1 mM CaCl2 which gives an equivalent ionic strength of 10 mM and is

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referred to as low ionic strength; and (2) a salt solution of 100 mM NaCl with equivalent ionic

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strength of 100 mM which is referred to as high ionic strength. SRNOM powder was purchased

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from International Humic Substances Society (St. Paul, Mn) and first dispersed in DI water

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under magnetic stirring for 6 h and then was filtered through 0.22 µm cellulose acetate

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membrane filters to obtain a 400 mg/L SRNOM stock. This was further diluted to prepare 2.052

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mg/L TOC solution when added to the 2D NM suspension for aggregation experiments at both

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low and high ionic strength conditions.

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2.3

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and electrophoresis techniques were used to characterize the physicochemical properties of

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exfoliated 2D NM suspensions. First, a Cary 60 UV-visible spectroscope (Agilent Technologies,

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Santa Clara, CA) was used to characterize all three 2D NMs in aqueous dispersions for their

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respective unique UV-vis spectroscopic signatures. Baseline was established with 45:55 ethanol-

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water mixture contained in a cleaned quartz cuvette.

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introduced individually in the cuvette to obtain spectral scans (three scans per 2D NM

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suspension) for a spectral range of 100 to 900 nm.

Aqueous 2D NM Characterization. UV-visible spectroscopy, HRTEM, STEM, AFM,

Each 2D NM suspension was then

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For performing HRTEM and STEM, carbon-coated copper TEM grids (Tedpella Inc,

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Redding, CA) were prepared by adding two to three drops of each 2D NM suspension and air-

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drying for few minutes. Electron micrographs were obtained at an accelerating voltage of 200

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kV and a point resolution of 0.19 nm with a JEOL JEM 2010 HRTEM (JEOL USA, Inc.,

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Peabody, MA), located at the Integrated Nanostructured Systems Instrumentation Facilities


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(INSIF) at the University at Buffalo to characterize the physical morphology of exfoliated 2D

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NMs. Furthermore, STEM imaging was performed at an accelerating voltage of 20 kV using a

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Hitachi SU70 field emission scanning electron microscope (FESEM). For each 2D NM type, 60

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individual flakes were analyzed using imageJ software to estimate their lateral dimensions i.e.,

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planar flake area distributions.

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AFM imaging was performed to measure the thickness of exfoliated 2D NMs. For this, few

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microliters of as prepared 2D NM aqueous dispersion was diluted with ethanol and drop casted

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on silicon wafer which were than imaged in tapping mode using a Bruker Dimension Icon AFM.

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The electrokinetic behavior of the 2D NMs was determined by measuring the electrophoretic

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mobility (EPM) using a Malvern ZetaSizer Nano ZS (Malvern Instruments Ltd., Westborough,

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MA). pH of the suspensions was kept unchanged to avoid introduction of additional ionic

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species and was measured to be between 4.6 and 4.9. Then, appropriate dilutions of NaCl or

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CaCl2 salts were added to 2D NM suspensions to get desired salt strength in the final volume of

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2 mL. Four replicate measurements were taken for each different salt concentration and for each

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sample.

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2.4

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used to measure aggregation kinetics of the 2D NM suspensions. The Malvern ZetaSizer Nano

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ZS containing a 4 mW He-Ne 633 nm laser was used to perform the TRDLS and to measure the

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hydrodynamic diameter of the 2D NM dispersions. 2 mL of MoS2, WS2 or h-BN suspensions,

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each diluted to 100 times of their stock suspension concentrations, with added salt or SRNOM

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was mixed in Malvern Zetasizer cuvettes, vortexed, and placed inside the ZetaSizer chamber.

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Continual measurements at a time interval of 15 seconds were taken for 30 min to allow for at

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least 30% increase in the initial average hydrodynamic radius.

Aggregation Kinetics Studies. Time resolved dynamic light scattering (TRDLS) was



The initial aggregation rates, k, were determined from the slope of the aggregation history

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i.e., time vs. hydrodynamic diameter (Dh) plots.

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𝑘∝

1 𝑑𝐷ℎ(𝑡) 𝑁𝑜 𝑑𝑡

[

]

(1) 𝑡→0

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The initial slope was calculated by linear regression of the hydrodynamic diameter values up

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to 1.3 times their initial values measured by the instrument at t = 0. For comparatively low ionic

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strengths, i.e., up to 8.5 - 30 mM NaCl or 0.55 – 1 mM CaCl2, 30% increase in initial diameter

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was not achieved within 30 min. In such scenario, the slope of the initial linear region of the

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hydrodynamic diameter versus time plot was estimated to determine the aggregation rate for

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each ionic strength. Attachment efficiency, α, is then determined by normalizing the initial rate

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of change of hydrodynamic diameters (or the aggregation rate) at each solution condition by that

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in the favorable regime, following equation (2).

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𝛼=

[ [

𝑑𝐷ℎ(𝑡) 𝑑𝑡

] ]

𝑡→0

(2)

𝑑𝐷ℎ(𝑡) 𝑑𝑡

𝑡→0, 𝑓𝑎𝑣

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2.5

Molecular Modeling. To probe the vdW interaction energies between layered 2D NMs,

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the molecular-level energies of a system composed by two layers of MoS2, WS2 or hBN

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nanostructures as a function of separation distance were computed using the open-source

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LAMMPS package.51 For this, we built a number of initial configurations of two parallel AB-

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stacking MoS2 (or WS2 or h-BN) sheets at separation distances (i.e., the shortest distance

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between the two layers) ranging from 2.0 Å to 12.0 Å at an interval of 0.1 Å. These MoS2 (or

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WS2 or h-BN) sheets have a surface area of approximately 70 x 70 Å2 and are assumed to be

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rigid. Periodic boundary conditions were applied along the directions parallel to the sheets. The


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total vdW energy of each of the systems was computed via summing all pair-wise inter-atomic

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vdW interactions between layers. In these calculations, the 6-12 Lennard-Jones (L-J) potential,

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with parameters taken from the Universal force field (UFF),52 was used to describe the

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interactions between MoS2 – MoS2 (or WS2 – WS2 or h-BN – h-BN), and the Lorentz-Berthelot

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mixing rule was applied to estimate the L-J parameters between dissimilar atoms. The potential

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was truncated and shifted to zero at a cutoff radius of 12 Å.

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2.6

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the 2D NMs and determine if the classical DLVO theory can capture their aggregation behavior,

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the following procedure adopted from a recent article explaining GO aggregation behavior using

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a modified DLVO theory was followed.41 The total interaction energy was obtained from the

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sum of van der Waals interaction energy (WvdW) and electrostatic double layer interaction energy

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(WEL). The net interaction energy for the 2D TMDs i.e., MoS2 and WS2 will be discussed first as

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they follow similar calculation procedure.

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Net Interaction (DLVO) Energy Calculation. In order to account for the geometry of

WvdW between two MoS2 nanosheets was calculated using the potential law for two thin slabs:

𝑊𝑣𝑑𝑊 = ―

𝐴𝐻 (12𝜋 ){(𝑑1 ) + ((𝑑 + 𝑡1+ 𝑡 ) ) ― ((𝑑 +1𝑡 ) ) ― (𝑑 +1𝑡 ) } 2

1

2

2

1

2

2

2

(3)

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where, t1 and t2 are the thicknesses of interacting slabs and are taken as 0.65 nm each for

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MoS2,53 d is the sheet separation distance, A is the surface area of the sheets, and H is the

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Hamaker constant. The Hamaker constant between two media contained in a third medium can

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be determined from the following equation:40

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𝐻𝑡𝑜𝑡𝑎𝑙 ≈

3ℎ𝑣𝑒 8 2

×

(𝑛21 ― 𝑛23)(𝑛22 ― 𝑛23) (𝑛21 + 𝑛23)0.5(𝑛22 + 𝑛23)0.5{((𝑛21 + 𝑛23)0.5(𝑛22 + 𝑛23)0.5)}


(4)


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257

where, n1 = n2 = n is the refractive index of MoS2 in visible regime, n3 is the refractive index

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of water (1.33), h is the Planck’s constant (6.626 × 10 ―34 Js), ve is the main absorption frequency

259

in the UV region. The refractive index of MoS2 depends on its number of layers.54, 55 For this

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study, the upper and lower boundaries of the refractive index i.e., for bulk and monolayer MoS2

261

respectively, were found using the following an empirical equation which is strictly valid for

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covalent oxides, nitrides, and sulfides:56

263

𝑛=

(

1 16 2.8 10

8.8 × 𝜔𝑈𝑉

)

(5)

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where, 𝜔𝑈𝑉 is absorption frequency corresponding to band gap. For bulk and monolayer

265

MoS2, 𝜔𝑈𝑉 is 1.29 eV and 1.9 eV, respectively.1 The n for monolayer and bulk MoS2 are 3.4 and

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3.9, respectively. For our calculation, an intermediate value of 3.7 is taken for n since the

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number of sheets formed from our exfoliation method is between 3 to 4.15

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The main electronic absorption frequency in the ultraviolet (UV) region, ve, is found from the

269

following equation40

270

𝑣𝑒 = 𝑣𝐼

3

(6)

𝑛2 + 2

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where, 𝑣𝐼 = absorption frequency of a Bohr atom (3.3 × 1015 s ―1). For n = 3.7, we obtained a

272

𝑣𝑒 value of 1.443 × 1015 s ―1. Using the aforementioned parameters, we obtained a Hamaker

273

constant value of 296 × 10 ―21 J for MoS2. The parameters used for WS2 and h-BN are tabulated

274

in the supporting information, SI (Table S1).

275

constant for WS2 was found to be 320 × 10 ―21 J. For h-BN, the Hamaker constant was estimated

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using the ratio of favorable interaction energies of h-BN to MoS2, obtained from the energy

277

minimum values computed for molecular systems of two layered 2D NMs of each type as

Applying the same approach, the Hamaker


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278

described above (Section 2.5 Molecular modeling). The vdW interaction energies between

279

layered MoS2 and between layered WS2 were found to be similar, however, the vdW interaction

280

energies between layered h-BN were found substantially higher. The ratio obtained, 2.58,

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indicates the substantially stronger vdW interaction energy between layered h-BN relative to that

282

between layered MoS2 (and also relative to that between layered WS2). By multiplying the

283

Hamaker constant of MoS2 with this ratio, a Hamaker constant value of 764 × 10 ―21 J was

284

calculated for h-BN, which was then used to calculate the vdW contributions between 2D h-BN

285

nanosheets in the DLVO theory.

286 287 288

The electrostatic double layer (EL) interaction energy between two identical surfaces can be obtained from the following equation:57 𝑊𝐸𝐿 ≈ 2𝐴𝜀𝑟𝜀0𝜅𝜉2𝑒 ―𝜅𝑑

(7)

289

where, 𝜀𝑟= dielectric constant (78.54 at 25 °C), 𝜀0 = permittivity of vacuum (8.854 × 10 ―12 F/m),

290

𝜉 = zeta potential (in Volt), 𝜅 = inverse of debye length (m-1).

291 292 293

The total interaction energy, 𝑊𝐷𝐿𝑉𝑂, is found by adding 𝑊𝑣𝑑𝑊 from equation (3) to twice the value of 𝑊𝐸𝐿 from equation (7) to account for both sides of the sheets being charged.57 𝑊𝐷𝐿𝑉𝑂 = 𝑊𝑣𝑑𝑊 +2𝑊𝐸𝐿

(8)

294 295

3. Results and Discussion

296

3.1 Chemical Identity and Morphological Characteristics. The sonication of inorganic 2D

297

NM in ethanol-water mixture followed by centrifugation resulted in stable aqueous dispersions

298

of exfoliated 2D NMs (supernatants) with different colors i.e., dark green, medium green, and

299

milky white dispersion for exfoliated MoS2, WS2, and h-BN, respectively. This is consistent

300

with previous literature reports.15, 58 The chemical identities of these exfoliated 2D NMs, i.e.,



301

MoS2, WS2, and h-BN nanosheets in aqueous dispersions, were further confirmed by obtaining

302

their UV-vis spectra as presented in Figure 1 (a), (b), and (c), respectively. The spectrum

303

obtained for MoS2 dispersed in 45:55 ethanol-water mixture shows two absorbance peaks at 627

304

nm and 672 nm for exfoliated MoS2 (Figure 1a) that can be attributed to characteristic A1 and B1

305

direct excitonic transitions.15, 59

306

638 nm, which is the characteristic of excitonic A band, while a shoulder associated with indirect

307

B excitonic transition at 525 nm is not prominent (Figure 1b).60 The peaks indicate that the

308

exfoliated 2D TMD nanosheets are dispersed in the aqueous dispersion as 2H – phase.61 These

309

absorption peaks arise due to energy split of valence-band and spin-orbital coupling.61 Both

310

these results are consistent with values reported in literature.59, 60 The absorption spectrum of

311

aqueous h-BN dispersions does not show any characteristic peak due to its large band gap

312

(Figure 1c).15

313

transparent and transmits more than 99% of the light in the wavelength range of 250 to 900 nm.62

314

The physical morphology of the exfoliated 2D NMs have been determined by HRTEM,

315

STEM, and AFM. Figure S2 presents with the HRTEM images of bulk and exfoliated MoS2

316

where contrast differences between the images help to distinguish between hundreds of 2D

317

layers in the bulk form (high contrast) vs only few layers in the exfoliated form (low contrast).

318

The HRTEM images of exfoliated 2D NMs as shown in (Figure 1 (d), (e), and (f)) show low

319

contrast of MoS2, WS2, and h-BN nanosheets which implies that each of these exfoliated

320

samples has few layers of 2D NM sheets.63 These images also depict irregular sheet-like

321

structures with lateral sizes which range from nanoscale to several microns as observed

322

elsewhere.64 Exfoliated h-BN have much smaller particle sizes than MoS2 and WS2 and this

323

observation is consistent with literature.15, 58

The spectrum from exfoliated WS2 shows prominent peak at

However, the spectrum decays exponentially as h-BN nanosheet is highly


Page 14 of 43

Page 15 of 43


324

Size distribution for the 2D nanomaterials were calculated by identifying 60 separate sheets

325

for each type of 2D NMs from their respective STEM images using imageJ. This method has

326

been established in literature for estimating and reporting 2D NM particle size distribution.65-67

327

Figure S3(a-c) show the frequency distribution of flake area for exfoliated MoS2, WS2, and h-

328

BN, respectively. For MoS2 and WS2, the dominant range of flake area was below 1 µm2

329

representing more than 75% and 60% of the distribution respectively. However, the flake areas

330

for h-BN were significantly smaller, all the flakes having areas below 0.1 µm2. Such differences

331

in lateral sizes between exfoliated TMD and h-BN nanosheets have been evidenced in the

332

literature.15, 58

333

AFM imaging of the exfoliated 2D NMs provide with their thickness information. Figure

334

S4(a-c) present AFM images of exfoliated MoS2, WS2, and h-BN, respectively; while Figure

335

S4(d-f) present with their corresponding height profiles. All the three types of 2D NMs had

336

thicknesses between 4-6 nm which indicate towards the presence of 3-4 layers of 2D nanosheets.

337

This is consistent with the literature reported values for the thickness of inorganic 2D NMs

338

exfoliated by the same method.15 All these different physicochemical characterization results

339

confirm the successful exfoliation of the inorganic 2D NMs in aqueous dispersions.

340

Figure 1. UV-vis spectra of aqueous dispersions of exfoliated (a) MoS2, (b) WS2, and (c) h-BN

341

nanosheets. Representative HRTEM micrographs of exfoliated (d) MoS2 (e) WS2, and (f) h-BN

342

nanosheets in aqueous dispersions.

343 344

3.2

Electrokinetic Properties. As mentioned above, the stable aqueous dispersions of 2D

345

NMs were prepared by their exfoliation in a 45:55 ethanol-water mixture – a procedure that was

346

established for the utilization of a mixed solvent system (i.e., the ethanol—water mixture) that

347

was selected based on the solvents’ Hansen solubility parameters.15 The ethanol molecules act



Page 16 of 43

348

like surfactant molecules as their non-polar –CH3 groups cluster together and are absorbed on the

349

hydrophobic 2D NMs.68 The hydrophilic –OH groups protrude from the nanosheets and are

350

most likely responsible for the surface charge that keeps the dispersion stable.50 A colloidal

351

dispersion is generally considered stable if its zeta potential is more positive than +30 mV or

352

more negative than -30 mV.69 The as synthesized aqueous 2D NMs were all negatively charged

353

with zeta potentials of -37.22.4, -40.30.9, and -44.31.5 mV for MoS2, WS2, and h-BN

354

respectively, indicating good colloidal stability in aqueous dispersion in the absence of ionic

355

species. The zeta potential and electrophoretic mobility (EPM) values for all salt concentrations

356

tested are given in Table S2, while figures 2 (a-b) show the EPM values of MoS2, WS2, and h-

357

BN over a broad range of mono- and di-valent cations, however within a pH range of 4.6-4.9.

358

For each salt concentration, WS2 has higher EPM than MoS2 indicating higher electrostatic (or

359

zeta) potential for WS2.

360

difference in their electron affinities with WS2 having a higher electron affinity of 4.5 eV than

361

MoS2 which has an electron affinity of 3.56 eV.70, 71 For all three 2D NMs at low ionic strength

362

of only 0.001 M NaCl the zeta potential values slightly increased from their as synthesized zeta

363

potential. Such behavior is not uncommon among colloids which show decreasing absolute

364

value of zeta potential only at higher electrolyte conditions.72

365

concentrations, the absolute values of zeta potentials decreased consistently in accordance to the

366

Gouy-Chapmann double layer model.73 Electrokinetic measurements obtained in this study

367

show that for electrolyte concentrations of up to 0.01 M NaCl and 0.001 M CaCl2, h-BN

368

possesses the highest EPM, followed by WS2 and MoS2.

369

aforementioned values, however, EPM values of h-BN decreased faster and are mostly lower

370

than those for the TMDs.

This discrepancy between these 2D TMDs could arise from the


For higher electrolyte

For concentrations above the

Page 17 of 43


371

The absolute values of EPM decreased more with the increase in CaCl2 concentration than

372

with the increase in NaCl concentration as the divalent counter ions (i.e., Ca2+) screen more of

373

the electric double layer and induce a smaller Debye length than the monovalent counter ions

374

(i.e., Na+).74 Furthermore, specific adsorption of Ca2+ to surfaces and short-ranged attractive

375

non-DLVO forces originating from ion-ion correlations charge fluctuations, surface charge

376

heterogeneities, and depletion forces have also been considered as probable causes behind the

377

marked decrease in diffuse layer potential.75

378 379

Figure 2. Electrophoretic mobility (EPM) values of aqueous MoS2, WS2, and h-BN suspensions

380

as a function of (a) monovalent NaCl and (b) divalent CaCl2 electrolytes. Measurements were

381

carried out at pH 4.6 to 4.9 with no additional buffers and at 25 ºC.

382 383

3.3

Aggregation Kinetics. Aggregation kinetics parameters of MoS2, WS2, and h-BN have

384

been determined from their respective aggregation history profiles as shown in Figure S5. The

385

initial hydrodynamic diameters of MoS2, WS2, and h-BN, as obtained from TRDLS, are 195.3

386

± 4.8, 142.5 ± 2.6, and 211.3 ± 3.9 nm respectively. Without any salt addition, the

387

dispersions remained stable i.e., the average hydrodynamic diameters did not change over the

388

duration of DLS measurement and even up to several months after synthesis.

389

increasing mono- and divalent (NaCl and CaCl2) salt concentrations, the surface charge on the

390

2D NMs gets screened thereby expediting aggregation.

391

hydrodynamic diameters (i.e., the aggregation rates) with electrolytic concentration also

392

increases. Attachment efficiency (α) has been estimated by normalizing initial aggregation rate

393

at any condition with that at the very fast, favorable regime. Figure 3 shows a log-log plot of

394

attachment efficiency values for MoS2, WS2, and h-BN dispersions as functions of


With the

Thus, the rate of increase of


Page 18 of 43

395

concentrations of mono- and divalent (NaCl and CaCl2) salts. These plots of the 2D NMs,

396

otherwise known as stability plots, display distinct unfavorable or reaction-limited (RLCA) and

397

favorable or diffusion-limited (DLCA) regimes implying agreement with classical DLVO-type

398

interaction. It is observed that the attachment efficiency increases as the ionic strength of the

399

salts is increased. This is due to the suppression of range and magnitude of the electric double

400

layer, and thereby the height of the energy barrier.76

401

The transition between the two regimes, i.e. from RLCA to DLCA, is used to estimate

402

critical coagulation concentration (CCC). The CCC values have been approximated as 37 mM,

403

60 mM, and 19 mM NaCl, and 3 mM, 7.2 mM, and 1.3 mM CaCl2 for MoS2, WS2, and h-BN

404

respectively. This is the first report of CCC values for WS2 and h-BN dispersions. However,

405

there has been one study that determined the CCC of MoS2 from salt-initiated aggregation.77

406

According to that study, the estimated CCC values for MoS2 were 4.4 mM NaCl and 0.036 mM

407

CaCl2, which are significantly different from those obtained in our study. That previous study

408

was performed using sodium cholate assisted aqueous dispersion of MoS2, while in our case, the

409

MoS2 (or all 2D NMs) was exfoliated in ethanol-water mixture and the difference possibly

410

contributed to different surface properties and aggregation behavior.

411

The lower CCC values for the divalent salt compared to the monovalent salt is in

412

accordance to the Schulze-Hardy rule, even though the 2D NMs are not spherical.

413

phenomenon has also been observed for the 2D NM GO.78 Quantitatively, the Schulze-Hardy

414

rule translates to proportionality of CCC to Z-n , where Z is the counter ion valence (Z equals to

415

1 for Na+ and 2 for Ca2+) and n is 2 or 6 for low and high zeta potentials, respectively.79 For our

416

study, the exponents have been found to be 3.62, 3.06, and 3.87 for MoS2, WS2, and h-BN

417

respectively, which lie within the 2 to 6 range dictated by the Schulze-Hardy rule even though


This

Page 19 of 43


418

the 2D NMs are not spherical. The n values for GO have also been found to vary between 5 to 6,

419

conforming to the Schulze-Hardy rule for spherical particles.37, 80, 81

420 421

Figure 3. Stability plots of aqueous MoS2, WS2, and h-BN suspension as a function of (a)

422

monovalent NaCl and (b) divalent CaCl2 electrolytes. Measurements were carried out at pH 4.6

423

to 4.9 with no additional buffers and at 25 ºC.

424 425

3.4

DLVO Prediction for Aggregation.

DLVO theory can quantitatively predict

426

aggregation behavior by estimating the CCC.41 It also identifies which of the DLVO forces,

427

electrostatic repulsion or vdW attraction, contributes to CCC for a particular material. First, we

428

made an effort to predict the CCC values for MoS2, WS2, and h-BN by solving equations (1) to

429

(6) for various electrolyte concentrations. The input parameters in these equations include zeta

430

potential values and thickness of the nanosheets. Performing calculations using these equations

431

resulted in theoretically predicted CCC values as 55 mM, 85 mM, and 100 mM NaCl, and 5.5

432

mM, 10 mM, and 10 mM CaCl2 for MoS2, WS2, and h-BN, respectively.

433

experimentally obtained values of CCC may vary from theoretical predictions by a factor of 2.73

434

The factor by which our modified DLVO prediction deviates from experimental observation

435

varies between 1.3 to 1.8 for the TMDs i.e., MoS2 and WS2. However, the factor varies by

436

almost an order of magnitude for h-BN. Thus, it is evident that the modified DLVO theory

437

captured the aggregation behavior of MoS2 and WS2 well but didn’t perform well for h-BN. A

438

close analysis into the apparently aberrant prediction of h-BN revealed that the Hamaker

439

constant (107 × 10 ―21 J ) obtained for h-BN using equations (4) to (6) does not reflect the fast

440

aggregation tendency of h-BN observed in experiments. Since the B-N bond in h-BN is ionic,

441

equation (5) cannot be used to calculate its refractive index and thus equation 4 cannot be used to


Typically,


442

obtain their Hamaker constant of h-BN. Also, no other Hamaker constant was found for h-BN in

443

the literature.

444

Figure 4. vdW energy profiles of bilayer (a) MoS2 and (b) WS2, and (c) h-BN structures as a

445

function of layer separation distance calculated by summing of all pair-wise inter-atomic

446

interactions between layers using molecular modeling.

447

between MoS2 and WS2.

Inset shows difference in energies

448 449

To address this discrepancy and also to further elucidate the aggregation behavior of 2D

450

NMs, we computed the molecular level interaction energies between layered MoS2, WS2, and h-

451

BN nanosheets individually using molecular modeling (as described above in Section 2.5).

452

Figure 4 shows the vdW energy profiles of bilayer MoS2, WS2, and h-BN structures as a function

453

of separation distance. It can be observed that the separation distance dependent energy profiles

454

and the first minima at a separation distance of approximately 3.5 Å for layered MoS2 and WS2

455

almost overlap with each other indicating similar vdW attraction forces (Figure 4 inset). Based

456

on the ratio of vdW interaction energies at the first minima for WS2 to MoS2 (which was slightly

457

more than 1) and the previously calculated Hamaker constant of 296 × 10 ―21 J for MoS2 (from

458

equation 4), we obtained a Hamaker constant value of 303 × 10 ―21 𝐽 for WS2, which is only

459

slightly less than its previously calculated Hamaker constant of 320 × 10 ―21 𝐽 and provide with the

460

same predicted CCC values for WS2. These two similar Hamaker constant values and the same

461

predicted CCC value of WS2 obtained from two independent computational approach (i.e., using

462

equation 4 and using molecular modeling) validate the use of molecular modeling technique for

463

predicting Hamaker constants for like 2D NMs such as h-BN. Figure 4 also presents that the

464

vdW interaction energy profiles and first minima values are significantly higher (for attraction)

465

for layered h-BN compared to layered TMDs indicating towards much stronger interactions


Page 20 of 43

Page 21 of 43


466

between h-BN nanosheets. Thus, using the ratio of vdW interaction energies of h-BN to MoS2 at

467

the first minima, we were able to determine a new Hamaker constant of 764 × 10 ―21 J. A

468

previous study used a similar approach to reasonably estimate the Hamaker constant values of

469

higher order fullerenes (i.e., for C70, C76, and C84) based on known Hamaker constant of fullerene

470

C60 and their molecular modeling.82 Using this new Hamaker constant and equations (3), (7),

471

and (8) we then computed the net interaction energy profiles of h-BN for a range of electrolytic

472

strengths to obtain its CCC value. Figure 5 shows the total interaction energy plots as a function

473

of separation distance predicted by the DLVO theory for the three 2D NMs for a range of NaCl

474

and CaCl2 concentrations. From Figure 5, this modified DLVO theory predicted the CCC of h-

475

BN to be 30 mM NaCl and 5.5 mM CaCl2, which are in much better agreement with the

476

experimentally obtained values than previously estimated using equation 4.

477

experimentally obtained CCC values and theoretically predicted CCC values in Table 1.

478

Figure 5. DLVO interaction energy between two 2D NM (MoS2, WS2 or h-BN) sheet in

479

presence of various concentrations of NaCl and CaCl2.

We listed the

480 481

Table 1. Experimentally obtained and modified DLVO theory predicted CCC values of MoS2,

482

WS2, and h-BN nanosheets.

483 484

According to our observed and predicted values of CCC, the colloidal stability of the studied

485

inorganic 2D NMs is in the following order: h-BN < MoS2 < WS2. Our calculated values of

486

Hamaker constants are 764 × 10 ―21 𝐽 , 296 × 10 ―21, and 320 × 10 ―21 𝐽 for h-BN, MoS2, and

487

WS2, respectively. These Hamaker constant values suggest that the vdW attraction force is the

488

highest for h-BN among these three 2D NMs and exceeds significantly compared to the TMDs

489

i.e., MoS2 and WS2. However, this was not the case for the relative colloidal stability of MoS2



Page 22 of 43

490

and WS2 i.e., WS2 being more colloidally stable than MoS2, although WS2’s Hamaker constant

491

was slightly higher than that of MoS2. This suggests that the electrostatic force of repulsion is

492

the governing DLVO force dictating the colloidal behavior of MOS2 and WS2. The force

493

calculations showed that the electrostatic repulsion forces of WS2 significantly exceeded the

494

repulsion forces for MoS2, at all separation distances and for different ionic strength conditions.

495 496

3.5

Role of Natural Organic Matter on Aggregation. The presence of 2.025 mg/L TOC

497

SRNOM significantly enhanced the stability of 2D NM dispersions and decreased their

498

aggregation rates at both low (7 mM NaCl and 1 mM CaCl2; Figure 6a) and high (100 mM NaCl;

499

Figure 6b) electrolyte concentrations. All the inorganic 2D NM dispersions were significantly

500

stabilized by the presence of SRNOM. Without SRNOM, MoS2 and WS2 had aggregation rates

501

of respectively 0.06 and 0.16 nm/s at low ionic strength and respectively 0.13 and 0.30 nm/s at

502

high ionic strength. However, the presence of SRNOM completely diminished their aggregation

503

tendency. The aggregation rates of h-BN were reduced to 2.6- and 3.7-fold of their original

504

values of 0.31 and 0.88 nm/s for low and high ionic strengths, respectively.

505

aggregation rate of h-BN compared to the TMDs even in presence of SRNOM is in accordance

506

with our experimental finding that h-BN has the least CCC value or the highest vdW interaction

507

energy amongst the three 2D NMs studied. NOM consists of humic substances which are known

508

to stabilize nanomaterials by steric or electrosteric repulsion82, 83 in the presence of electrolytes.

509

NOM adsorbs on the surface of WS2 nanosheets probably by strong coordination of the carboxyl

510

groups with tungsten atoms.84 In case of h-BN, the aromatic fraction of the NOM possibly

511

interacts via Π-Π stacking forces.85


The higher

Page 23 of 43


The ubiquity of NOM in natural aquatic systems plays an important role in the aggregation

512 513

and thus mobility of the 2D NMs in aquatic environment.

Additionally, NOM influences

514

generation of ROS and photochemical dissolution processes of TMDs, thereby governing

515

bioavailability of the NMs. The relative colloidal stability of the different inorganic 2D NMs

516

imparted by NOM thus holds immense environmental significance.

517 518

Figure 6. Aggregation rates of MoS2, WS2, and h-BN without and with 2.025 mg/L TOC

519

SRNOM and in presence of (a) 7 mM NaCl and 1 mM CaCl2, (b) 100 mM NaCl.

520 521

3.6

Environmental Implications. This study provides valuable insight into the relative

522

colloidal stability of exfoliated MoS2, WS2, and h-BN nanosheets in aqueous media. The

523

inorganic 2D NMs have distinct physicochemical properties which dictate their aggregation

524

behavior. While EDL repulsion appears to govern stability of the TMDs, a combination of non-

525

DLVO and vdW attraction forces might dictate the instability of h-BN. The critical coagulation

526

concentration (CCC) values of the 2D NMs suggest that h-BN will be the least stable while WS2

527

will be the most stable in presence of either mono- or divalent salts. The salinity of fresh and

528

brackish waters are approximately 500 ppm and 35000 ppm respectively while that of ground

529

water may vary from 0 to greater than 13,000 ppm depending on its origin, seawater intrusion,

530

leaching of salt sediments and anthropogenic activities.86 This implies that in freshwater 2D

531

NMs will be carried farther downstream from their point of release, thereby having a greater

532

extent of exposure to aquatic organisms. In brackish waters, the 2D NMs are more likely to

533

aggregate and settle since their CCC values are about an order or magnitude lower than typical

534

seawater salinity. However, at concentrations present in natural aquatic systems, SRNOM will

535

stabilize the 2D NMs i.e., suppress their aggregation, even at systems with high electrolyte



Page 24 of 43

536

concentration such as marine and estuarine bodies. Thus, 2D NM mobility will depend on the

537

relative amount of NOM present in the brackish water. Similarly, the aggregation of 2D NMs in

538

ground water will depend on the salinity and NOM content with high salinity facilitating

539

aggregation and high NOM suppressing it. The complex mobility and transport of 2D NMs in

540

aquatic systems and their effect on native microorganisms mandate comprehensive evaluation of

541

their environmental risk.

542

Supporting Information

543

The Supporting Information is available free of charge on the ACS Publications website. The

544

supporting information includes figures showing experimental protocol for preparing aqueous

545

dispersions of inorganic 2D NMs; HRTEM images of bulk and exfoliated MoS2; particle size

546

distritbution for exfoliated 2D NMs; AFM images of exfoliated 2D NMs; table showing

547

parameters used for DLVO calculation; table for zeta potential and EPM values for 2D NM

548

dispersions with varied ionic strength; figure showing aggregation history profiles for 2D NM

549

dispersions at various electrolyte conditions.

550

Acknowledgements

551

We thank Prof. Yueling Qin from Department of Physics at the University at Buffalo (UB) for

552

his assistance with HRTEM imaging. We also thank Mr. Peter Bush, Director of South Campus

553

Instrument Center at UB, for his assistance with STEM imaging.

554

Department of Civil, Structural and Environmental Engineering (CSEE) at UB for supporting

555

Ms. Anusha Gupta’s travel to UB and research internship in Prof. Nirupam Aich’s laboratory.

556

References


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793 794 795 796

Figure 1. UV-vis spectra of aqueous dispersion of exfoliated (a) MoS2, (b) WS2, and (c) h-BN

797

nanosheets and representative HRTEM micrographs of exfoliated (d) MoS2 (e) WS2, and (f) h-

798

BN

799 800 801 802 803 804 805 806


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807 808

Figure 2. Electrophoretic mobility (EPM) values of aqueous MoS2, WS2, and h-BN suspensions

809

as a function of (a) monovalent NaCl and (b) divalent CaCl2 electrolytes. Measurements were

810

carried out at pH 4.6 to 4.9 with no additional buffers and at 25 ºC.

811 812 813



814 815

Figure 3. Stability plots of aqueous MoS2, WS2, and h-BN suspension as a function of (a)

816

monovalent NaCl and (b) divalent CaCl2 electrolytes. Measurements were carried out at pH 4.6

817

to 4.9 with no additional buffers and at 25 ºC.

818 819


Page 38 of 43

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820 821

Figure 4. vdW energy profiles of bilayer (a) MoS2 and (b) WS2, and (c) h-BN structures as a

822

function of layer separation distance calculated by summing of all pair-wise inter-atomic

823

interactions between layers using molecular modeling. Inset shows difference in energies

824

between MoS2 and WS2.

825 826 827 828 829 830 831 832 833 834 835



836

837 838

Figure 5. DLVO interaction energy between two 2D NM (MoS2, WS2 or h-BN) sheet in

839

presence of various concentrations of NaCl and CaCl2.


Page 40 of 43

Page 41 of 43


840 841

Figure 6. Aggregation rates of MoS2, WS2, and h-BN without and with 2.025 mg/L TOC

842

SRNOM and in presence of (a) 7 mM NaCl and 1 mM CaCl2, (b) 100 mM NaCl.

843 844 845 846 847 848 849 850 851 852 853 854 855



Page 42 of 43

856 857

Table 1. Experimentally obtained and modified DLVO theory predicted CCC values of MoS2,

858

WS2, and h-BN nanosheets.

2D NM MoS2 WS2 h-BN

Experimentally obtained CCC values

Modified DLVO theory predicted CCC values

mM NaCl

mM CaCl2

mM NaCl

mM CaCl2

37 60 19

3 7.2 1.3

55 85 30

5.5 10 5.5

859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878


Page 43 of 43

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