Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2024 Jan 29;58(5):2490–2501. doi: 10.1021/acs.est.3c08278

Influence of Inorganic Anions on the Chemical Stability of Molybdenum Disulfide Nanosheets in the Aqueous Environment

Ting-Wei Lee 1, Chiaying Chen 1,*
PMCID: PMC10851429  PMID: 38284181

Abstract

graphic file with name es3c08278_0008.jpg

Chemical stability is closely associated with the transformations and bioavailabilities of engineered nanomaterials and is a key factor that governs broader and long-term application. With the growing utilization of molybdenum disulfide (MoS2) nanosheets in water treatment and purification processes, it is crucial to evaluate the stability of MoS2 nanosheets in aquatic environments. Nonetheless, the effects of anionic species on MoS2 remain largely unexplored. Herein, the stability of chemically exfoliated MoS2 nanosheets (ceMoS2) was assessed in the presence of inorganic anions. The results showed that the chemical stability of ceMoS2 was regulated by the nucleophilicities and the resultant charging effects of the anions in aquatic systems. The anions promote the dissolution of ceMoS2 by triggering a shift in the chemical potential of the ceMoS2 surface as a function of the anion nucleophilicity (i.e., charging effect). Fast charging with HCO3– and HPO42–/H2PO4– was validated by a phase transition from 1T to 2H and the emergence of MoV, and it promoted oxidative dissolution of the ceMoS2. Additionally, under sunlight, ceMoS2 dissolution was accelerated by NO3–. These findings provide insight into the ion-induced fate of ceMoS2 and the durability and risks of MoS2 nanosheets in environmental applications.

Keywords: transition metal dichalcogenides, inorganic anions, oxidative dissolution, nucleophilicity

Short abstract

Oxidative dissolution of MoS2 in aquatic environments is affected by the nucleophilicity of anions present in the solution, which reveals the effects of inorganic anions on the fate and potential use of MoS2 nanosheets.

1. Introduction

With their remarkable attributes, two-dimensional (2D) nanosheets of layered transition metal dichalcogenides (TMDCs) have received considerable interest in both industrial and biomedical applications. Typically, the formula of a TMDC is MX2, where M is transition metal from groups IV, V, or VI, and X is a chalcogen such as S, Se, or Te. Among TMDCs, molybdenum disulfide (MoS2) is the most important 2D nanomaterial due to its tunable band gap1 and high electron mobility,2 which make MoS2 a promising photocatalyst, electrocatalyst, biosensor, etc. In general, MoS2 nanosheets exhibit three main structural phases: the tetragonal (1T) phase, hexagonal (2H) phase, and rhombohedral (3R) phase, where the 2H and 3R phases are semiconducting, and the 1T phase is metallic.3 Despite the utility of MoS2 nanosheets in practical applications, the chemical stability of the MoS2 nanosheets will determine their long-term applications. Prior studies probed the interaction between MoS2 nanosheets and environmental media, which affected the fate and transport of the MoS2 nanosheets. For instance, MoS2 nanosheets were oxidatively dissolved under alkaline4 and aerobic5 conditions. Our previous work demonstrated that the dissolution of chemically exfoliated MoS2 nanosheets (ceMoS2) was slowed in the presence of natural organic matter (NOM), including Suwannee River natural organic matter (SRNOM) and Aldrich humic acid (ALHA), in dark ambient conditions, while aging of MoS2 with co-occurring ALHA was accelerated by exposure to sunlight.6 These findings indicated the stability and behavior of MoS2 nanosheets in aqueous environments and were used to evaluate their persistence in the intended applications.

The interactions of MoS2 nanosheets with ionic species alter the characteristics and fate of the MoS2. For example, Li et al.7 showed that aggregation of MoS2 nanosheets dispersed by sodium cholate followed the 2D Schulze–Hardy rule, and the critical coagulation concentration (CCC) of MoS2 nanosheets was smaller in the presence of higher valence cations. Similarly, the CCC of MoS2 exposed to various cations decreased in the order K+ > Mg2+ > Al3+.8 In the presence of a natural macromolecule (e.g., NOM), the aggregation rate of MoS2 nanosheets was drastically reduced even with high ionic strengths (ISs).9 Liu et al.10 suggested that higher valence cations suppressed the electrical double layer for MoS2 nanosheets, and aggregation of the MoS2 nanosheets was accelerated by Ca2+ under visible light irradiation.10 In the chemisorption of cations, the sulfur atoms on MoS2 are soft Lewis base sites with strong affinities for soft Lewis acids (e.g., Hg2+ and Ag+).11 Mi and coauthors demonstrated that MoS2 nanosheets reduced heavy metal ions that had higher reduction potentials than MoO42– and SO42–/MoS2 pair (0.429 V) (e.g., Ag+ (0.7996 V), Hg2+ (0.920 V), and Cr(VI) (1.232 V)) and released soluble MoO42– and SO42–.12,13 These findings indicated that charge transfer between MoS2 and the cationic species altered the physical and chemical properties of MoS2. Nevertheless, the transformations undergone by MoS2 nanosheets during interaction with anionic environmental species remain largely unexplored thus far. Marks et al.14 illustrated the negative impact on the stability of MoS2 in the presence of model oxidants NO2– and BrO3–, while dissolved oxygen is a prerequisite for the reaction. The prevalence of anions and their potential interplay with MoS2 have prompted exploration of the impacts of anionic species on MoS2 in aquatic systems.

Inorganic anionic species are ubiquitous in aqueous environments, and the concentration profiles of these anions vary with the surrounding geology, ecology, and human activities. For example, the chloride (Cl–) concentration in shallow groundwater has increased from 0.02 to 0.34 mM to several to tens of mM due to human activities.15 Fertigated water contains 1.36–4.43 mM nitrate (NO3–),16 and sulfate (SO42–) concentrations of 12.09 mM originating from mining activities were found in rivers.17 The bicarbonate (HCO3–) present in natural water and wastewater comes from dissolved carbon dioxide in the atmosphere, with concentrations ranging from 1 to 5 mM.18,19 The concentration of dissolved phosphate is typically low due to its low mobility,20 but in swine wastewater, the total phosphorus concentration can be as high as 19.37–45.21 mM.21 The effluent from wastewater treatment plants (WWTPs) contains 1.30–3.36 mM Cl– and 0.23–1.01 mM NO3–,16 and the SO42– concentrations in industrial effluents are 2.60–5.21 mM in most countries.22,23 Therefore, the engineered nanomaterials (ENMs) released or applied in water treatment facilities inevitably contact anionic species, and the efficacy of their performance could be altered by the presence of anions. Jeong et al.24 demonstrated that the removal of As(V) by Fe2O3 and Al2O3 was not affected by Cl– or NO3– but was inhibited by HPO42– due to the structural similarity of arsenate and phosphate. The removal of contaminants was minimally affected by Cl–, NO3–, SO42–, and Br– but was restrained by F–, CO32–, PO43–, HPO42–, and H2PO4– in graphene oxide (GO)-based composites.25−27 Additionally, the photocatalytic degradation of contaminants has been affected by anions. In a study by Lien et al.28, bromide ion (Br–) promoted the photocatalytic degradation of sulfamethoxazole with CaCu3Ti4O7 perovskite due to the formation of highly reactive radicals, while Cl– decreased the degradation rate. Another study by Chen and Liu showed that higher valence anions suppressed the photodegradation efficiency, and the effect decreased in the order PO43– > SO42– > NO3–.29 Evidently, the effects of anions in aqueous solutions cannot be overlooked, and further research is required to fully comprehend the role of anions on ENMs. Furthermore, anions may also alter the surface characteristics and chemical stabilities of ENMs. For instance, the surfaces of silver nanoparticles (AgNPs) were chlorinated by chloride or sulfided by sulfide.30 Levard et al.31 reported that AgNPs formed solid AgCl(s) at low Cl/Ag ratios (≤2675), whereas at Cl/Ag ratios ≥2675, the formation of soluble AgClx(x–1) led to dissolution of the AgNPs.31 Liu et al.32 demonstrated that the sulfidation of AgNPs requires O2. They also illustrated that direct sulfidation occurred at high sulfide levels (≥0.025 mg/L), whereas at low sulfide levels (≤0.025 mg/L), intermediate Ag+ species were the predominant oxidized form. A study of environmental anions by Guo et al. showed that among environmental anions, only sulfide inhibited the dissolution of AgNPs and alleviated their toxicities, but the release of Ag+ was not affected by phosphate or chloride.33 Overall, the impacts of anions on the transformations and intended applications of ENMs have been recognized; therefore, the effects of anionic species on MoS2 nanosheets need to be further elucidated.

Given that MoS2-based nanosheets are promising membrane materials for water treatment and purification, including heavy metal removal,34 dye rejection,35 and desalination,36 it is crucial to assess the chemical stabilities of MoS2 in aquatic environments with coexisting species, including anions. However, the effects of anions on MoS2 remain largely unknown. Thus, the aims of the present study were to elucidate the chemical stability of MoS2 nanosheets with coexisting environmental anions (e.g., Cl–, NO3–, SO42–, HCO3–, and HPO42–/H2PO4–) under dark ambient conditions and during irradiation with sunlight in aqueous environments. The effects of the anions on MoS2 were determined by electrochemical analyses and X-ray photoelectron spectroscopy (XPS). Our findings suggest that the presence of anionic species affected the chemical stability of the MoS2 nanosheets, which will enable evaluations of the roles of inorganic anions in the environmental transformations of MoS2.

2. Materials and Methods

2.1. Characterization and Electrochemical Measurements of ceMoS2

The chemicals and synthesis of ceMoS2 are described in the Supporting Information (Text S1 and Figure S1). The preparation of the chemically exfoliated MoS2 nanosheet solutions (ceMoS2) followed previous studies with minor adjustments.6 The morphology of ceMoS2 was surveyed by high-resolution transmission electron microscopy (TEM) (JEOL JEM-1200). The optical absorption spectrum of ceMoS2 was determined with a spectrophotometer (HITACHI U-3900). The concentration of the as-prepared ceMoS2 was determined from the absorbance at 450 nm with a mass extinction coefficient of 5010 L m–1 g–1 (Figure S2).6 The surfaces of ceMoS2 were analyzed with XPS (ULVAC-PHI, PHI 5000 VersaProbe/Scanning ESCA Microprobe). The binding energies were calibrated with the C 1s peak at 284.6 eV. The Mo 3d and S 2p core-level XPS data were analyzed with XPSPEAK41 software by using Gaussian–Lorentzian components after Shirley background subtraction. The hydrodynamic radius (Rh) and zeta potential of ceMoS2 suspensions were measured by a ZetaSizer Nano ZS (Malvern Instrument, Worcestershire, U.K.) with a monochromatic coherent 633 nm He–Ne laser. Electron paramagnetic resonance (EPR) spectroscopy (Bruker EMX-10/12 EPR spectrometer) was applied to monitor radicals (e.g., ·OH and NO2·) generated from ceMoS2 or/and anions under a light source within the solar range (MORITEX, Hg lamp, 150 W). 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) was adopted as the spin-trapping agent. EPR signals were recorded at 298 K with a microwave power of 40 mW, power attenuation of 7 dB, modulation frequency of 100.0 kHz, and modulation amplitude of 1.0 G. The electrochemical investigations were conducted with an electrochemical workstation (CH Instruments, Inc.) with a three-electrode system. The glassy carbon working electrode was modified by drip-casting 10 μL of 100 mg/L ceMoS2 and drying at room temperature. The reference electrodes were Ag/AgCl or saturated calomel electrodes (SCE), and the counter electrode was a platinum wire. The measured potentials were converted to the reversible hydrogen electrode (RHE) potential. The probed anions (Cl–, NO3–, SO42–, HCO3–, and HPO42–/H2PO4–) at 100 and 10 mM were utilized as the electrolytes for the open circuit potential (OCP) and cyclic voltammetry (CV) measurements, respectively. The oxygen reduction reaction (ORR) was probed with linear scan voltammetry conducted from 0.2 to −1 V at a scan rate of 5 mV s–1 in O2-saturated electrolytes with 100 mM of the tested anions as the electrolytes.

2.2. Influence of Anionic Species on the Stability of ceMoS2

The effects of anions on ceMoS2 were assessed by mixing 10.5 mg/L ceMoS2 and varying concentrations (1, 10, and 100 mM) of different anions (Cl–, NO3–, SO42–, HCO3–, and HPO42–/H2PO4–) in borosilicate glass tubes and keeping them in the dark or under irradiation in an Atlas SunTest CPS+ sunlight simulator (Atlas Materials Testing Technology, Chicago, IL, USA) equipped with a 1-kW xenon arc lamp with an incident irradiance of 0.065 W/cm2 (Figure S3a). Throughout irradiation, the irradiated samples were maintained at 25 °C in a recirculating water bath. To probe the wavelength dependency, the effects of irradiated nitrate ions were characterized with visible wavelength illumination. ceMoS2 (10.5 mg/L) mixed with 10 mM NaNO3 was irradiated in a sunlit simulator equipped with a UV-cutoff filter that blocked irradiation with wavelengths below 420 nm (Figure S3b). For a primary assessment, the metrics of the reaction were monitored, including the absorbance at 450 nm (Abs450) and the pH. To further determine the effects of anions on ceMoS2 dissolution, the dissolved Mo species were gathered by filtering and centrifuging with ultrafiltration tubes (Amicon Ultra15 3 kDa, Millipore, USA). The concentrations of dissolved Mo species in the filtrate were digested and then determined with inductively coupled plasma–optical emission spectrometry (ICP–OES) (PerkinElmer Optima 8000). A control test indicated that there is no adsorption loss of Mo ionic species to the utilized 3 kDa MWCO membranes by comparing the concentration of sodium molybdate solution and its filtrate under different pH values and in the presence of anionic species (Figure S4). The IS effect on the dissolution of ceMoS2 was examined with 1, 10, and 100 mM anionic species (Cl–, NO3–, SO42–, HCO3–, and HPO42–/H2PO4–). It is worth noting that, except for the sets with 100 mM anions, the IS of the ionic species concentrations used in this study (Table S1) were lower than the CCC (50 mM of KCl)37 of ceMoS2. Additionally, these concentrations were also lower than the minimum level (31.6 mM of KCl)38 known to affect the transport of ceMoS2.

2.3. ceMoS2 Dissolution under Environmentally Relevant Conditions

In natural water, the concentrations of anionic species (Cl–, NO3–, SO42–, HCO3–, and HPO42–/H2PO4–) vary from a few μM to hundreds of mM.15,17,39 Although environmentally relevant concentrations of MoS2 are unknown, the rapid development and widespread use of MoS2 nanosheets has prompted the need to evaluate the transformations of MoS2 nanosheets. According to a prior study by Surette et al.40, the environmental concentrations of ENMs ranged from a few ng/L to a few mg/L, with lower concentrations considered to be more realistic. Thus, the experimental concentrations of the anions and ceMoS2 were 1 mM and 100 μg/L, respectively. The dissolved Mo species were collected and measured with high-resolution inductively coupled plasma–mass spectrometry (HR-ICP–MS) (Thermo Scientific Element 2), which can quantify ultralow concentrations (ng/L).

3. Results and Discussion

3.1. Characterization of ceMoS2

The as-prepared ceMoS2 was characterized with TEM, UV–vis spectrophotometry, and XPS (Figure 1). The TEM image of ceMoS2 (Figure 1a) showed that ceMoS2 had a sheet-like appearance with lateral sizes of approximately 200–300 nm, which was consistent with the general morphology of ceMoS2.4 In the UV–vis spectrum (Figure 1b), ceMoS2 exhibited no peaks in the visible region, which was attributed to the metallic nature of the 1T phase,41 the dominant phase in the chemically exfoliated (i.e., lithium-intercalated) MoS2 nanosheets. The orbital configuration and phase composition of ceMoS2 were analyzed by XPS. In Figure 1c, the Mo 3d doublets of ceMoS2 were located at approximately 229.0 eV (Mo 3d5/2). After peak fitting, the Mo 3d peaks of ceMoS2 were deconvoluted into the 1T phase (3d5/2: 228.2 eV, 3d3/2: 231.3 eV) and 2H phase (3d5/2: 229.1 eV, 3d3/2: 232.2 eV), which indicated the different binding energies (0.7–0.9 eV) of the 1T and 2H phases.42 The 1T phase content in the Mo–S bond was 64.7%, confirming the predominance of the 1T phase in ceMoS2. In the S 2p core-level spectrum (Figure 1d), the broad peak was partitioned into four peaks corresponding to the 1T phase (2p3/2: 161.1 eV, 2p1/2: 162.8 eV) and the 2H phase (2p3/2: 162.0 eV, 2p1/2: 163.5 eV). The content of the 1T phase indicated by the S 2p XPS data was 64.1%, which was similar to the result from the Mo 3d XPS data. The laminate structure and the presence of the 1T phase in the as-prepared ceMoS2 implied a successful synthesis of chemically exfoliated MoS2.

Figure 1.

Figure 1

Open in a new tab

Characterization of as-prepared ceMoS2. (a) TEM image and (b) UV–vis absorption spectrum of ceMoS2. (c) Mo 3d and (d) S 2p X-ray photoelectron spectra of ceMoS2.

3.2. Effect of Anions on the Suspension Stability and Dissolution of ceMoS2

The effects of anions on the chemical stability of ceMoS2 were probed in the presence of 1–20 mM NaCl, considering that Cl– is one of the most common anions in both natural water43 and wastewater.44 Given the correlation between the absorbance at 450 nm and the concentration of suspended MoS2 (Figure S2), the changes in the concentration of MoS2 nanosheets were tracked with the normalized absorbance At/A0, where At and A0 are the 450 nm absorbance of the ceMoS2 dispersion at time t and the initial point, respectively. As shown in Figure S5, a slight decrease in At/A0 was observed upon increasing the Cl– concentration from 1 to 20 mM under dark conditions. During exposure to sunlight, the decrease in At/A0 for ceMoS2 became pronounced at higher chloride concentrations (10 and 20 mM), which were 2.0 and 2.8 times greater than that of the control at 48 h, indicating light-accelerated destabilization of the ceMoS2 dispersion. The light-induced destabilization of MoS2 near the surface of the sunlit water was ascribed to the surface plasmon oscillations enhanced by the cations (e.g., Na+),10 which decreased the energy barrier for aggregation.

Additionally, the concentration of dissolved Mo species produced after 72 h was determined by passing the suspensions through 3 kDa membranes. The dissolution rate of ceMoS2 increased from 29.1% (0 mM Cl–) to 35.2% (1 mM Cl–), 40.1% (5 mM Cl–), 42.9% (10 mM Cl–), and 54.3% (20 mM Cl–) in the dark, and the dissolution rates were increased to 47.4% (0 mM Cl–) and 68.0% (20 mM Cl–) with light exposure. The concurrent dissolution of Mo species and the decrease in At/A0 suggested that the destabilization of ceMoS2 with Cl– and light was due to not only light- and cation-induced aggregation but also oxidative dissolution of the ceMoS2. Furthermore, the effects of different anions on the transformations of ceMoS2 and the role of sunlight have not been well-explored previously. Therefore, the transformations of ceMoS2 dispersions were examined with various anions (i.e., Cl–, SO42–, NO3–, HCO3–, and HPO42–/H2PO4–) found in aquatic systems.

The stabilities of the ceMoS2 dispersions were probed in the presence of various anions with concentrations of 1, 10, and 100 mM. As shown in Figure 2a, a faster decrease in At/A0 over time was found in the presence of 1 mM HCO3– and HPO42–/H2PO4– under both dark and irradiation conditions. Notably, with the elimination of O2, the stabilities of ceMoS2 suspensions in the presence of the probed anions exhibited no significant difference to the blank control (Figure S6), illustrating the essential role of O2 in the promoted decrease in At/A0 of MoS2 by HCO3– and HPO42–/H2PO4–. During irradiation with sunlight (solid symbols), the At/A0 ratios for ceMoS2 declined to 0.62 and 0.38 in the presence of 1 mM HPO42–/H2PO4– and HCO3–, respectively. To determine the oxidative transformation of ceMoS2 caused by anions, the products from ceMoS2 were identified by filtering the dispersions through 3 kDa MWCO membranes, and the dissolved Mo species from ceMoS2 were monitored after incubation with anions. As shown in Figure 2b, the amount of ionic Mo species produced (Ct/C0) was computed from the normalized concentration of ionic Mo species at a predetermined time (Ct) and the initial concentration of dissolved Mo species (C0) (i.e., 2.65 mg/L). The produced ionic Mo species displayed a similar trend for At/A0 and illustrated that HCO3– and HPO42–/H2PO4– facilitated the formation of dissolved ionic species from ceMoS2 under both dark and irradiated conditions. Note that the MoS2 nanosheets can produce photogenerated free radicals (e.g., ·O2– and ·OH), leading to facilitated oxidative dissolution of MoS2 with light exposure.5 Furthermore, bicarbonate and phosphate promoted the detachment of photogenerated reactive radicals from the surfaces of materials, thereby increasing the mobility of radicals and enhancing the oxidation reaction.45,46 As shown in Figure S7, photogenerated ·OH was identified in the irradiated ceMoS2 suspension in the presence of HCO3– and HPO42–/H2PO4–, suggesting that the further oxidative degradation of ceMoS2 promoted by HCO3– and HPO42–/H2PO4– under irradiation was attributed to the enhanced availability of the photogenerated active species.

Figure 2.

Figure 2

Open in a new tab

Stabilities of ceMoS2 suspensions (10.5 mg/L) determined by measuring the normalized absorbance at 450 nm (At/A0) and the normalized concentration of dissolved Mo species (passed through 3 kDa membranes) (Ct/C0) in the presence of (a,b) 1 mM, (c,d) 10 mM, and (e,f) 100 mM anionic species under both dark and solar irradiation. Error bars are sample standard deviations from triplicate measurements.

In the presence of a higher concentration of anions (10 mM in Figure 2c), At/A0 showed a greater decline rate compared to those seen at 1 mM, particularly under irradiation, which was due to the destabilization of ceMoS2 during irradiation in concentrated electrolyte solutions. The light-induced destabilization of ceMoS2 was further verified by the zeta potential of ceMoS2 suspensions (Figure S8), which illustrates that the zeta potential of irradiated ceMoS2 with anionic species became less negative (i.e., destabilization) as a function of time, while the zeta potential remained relatively unchanged under dark condition. In Table 1, the amount of ionic Mo species produced (Ct/C0) was further computed from the normalized concentration of ionic Mo species at longer term for 72 h (Ct) and the initial concentration of dissolved Mo species (C0). In the presence of HCO3– and HPO42–/H2PO4–, the production of ionic Mo species from ceMoS2 was significantly elevated under both dark and irradiated conditions. Additionally, greater dissolution of ceMoS2 was found with NO3– and irradiation. The findings were consistent with observed At/A0, demonstrating that the chemical stability of the MoS2 nanosheets was affected by the different anions in aquatic systems.

Table 1. Concentrations of Ionic Mo Species (Obtained by Passage through 3 kDa Membranes) in ceMoS2 (10.5 mg/L) Incubated with 10 mM Anions for 72 h.

coexisting anions ionic Mo species at 0 h ionic Mo species at 72 h in dark ionic Mo species at 72 h under sunlight irradiation
C0 (mg/L) Ct (mg/L) Ct/C0 Ct (mg/L) Ct/C0
blank 2.65 ± 0.11 3.35 ± 0.13 1.26 ± 0.08 3.86 ± 0.14 1.46 ± 0.09
blank (visible light)       3.77 ± 0.13 1.42 ± 0.08
Cl–   3.82 ± 0.20 1.44 ± 0.11 4.31 ± 0.22 1.63 ± 0.12
SO42–   3.85 ± 0.17 1.45 ± 0.10 4.65 ± 0.13 1.75 ± 0.10
NO3–   3.60 ± 0.16 1.36 ± 0.09 6.29 ± 0.19 2.37 ± 0.14
NO3– (visible light)       4.10 ± 0.12 1.55 ± 0.09
HCO3–   6.19 ± 0.27 2.33 ± 0.16 10.49 ± 0.34 3.96 ± 0.24
HPO42–/H2PO4–   5.82 ± 0.18 2.19 ± 0.13 7.81 ± 0.21 2.95 ± 0.16
Open in a new tab

With 100 mM anion concentrations (Figure 2e), the At/A0 for ceMoS2 plummeted during the first 12 h, indicating that the suspension stability was disrupted at high anion concentrations. The suspension stability was monitored using time-resolved dynamic light scattering (DLS) to determine the time-dependent change in the hydrodynamic radius (Rh). Although DLS is not ideal for determining nonspherical particles, the intensity averaged Rh could be used to obtain a general index of the size population of ceMoS2 suspensions.10,47 As shown in Figure S9, the significant increase in Rh with the presence of 100 mM anionic species confirms the disrupted suspension stability at higher anion concentrations. In the dissolved Mo species measurement, an elevated release of Mo ionic species was seen with 100 mM HCO3– and HPO42–/H2PO4– and was increased by 129.3% with NO3– and irradiation for 48 h (Figure 2f). The nitrate-enhanced transformations of ceMoS2 seen during solar irradiation are discussed later. Collectively, the findings revealed that oxidative transformation of the ceMoS2 was promoted to different degrees by environmental anions; HCO3– and HPO42–/H2PO4– promoted the degradation of ceMoS2 into dissolved Mo species under both dark and irradiated conditions, while NO3– exhibited light-accelerated dissolution of ceMoS2 at higher concentrations.

It is worth noting that the trend for oxidative dissolution was clear in the dissolved Mo species measurements, while At/A0 determined the concentration of the MoS2 dispersion, which was affected by aggregation and sedimentation of the ceMoS2 nanosheets. As listed in Table S2, the time-dependent At/A0 in 1, 10, and 100 mM anionic species in Figure 2 was fit by the first-order decay equation, which illustrates an increase in rate constants under higher anion concentrations (e.g., 100 mM) and light exposure. Given that the aggregation and sedimentation of the ceMoS2 nanosheets contribute to the normalized absorbance At/A0,10 the results, in addition to the zeta potential (Figure S8) and DLS measurement (Figure S9), clearly indicated a light- and higher-anion-concentration-induced destabilization of ceMoS2. The suspension stability is known to be one of the factors affecting the dissolution of ENM particles.48−50 As shown in Figure 2b,d, the dissolved Mo species produced in ceMoS2 were comparable with 1 and 10 mM anionic species, while ceMoS2 with 100 mM anionic species demonstrated a higher initial dissolution rate (Figure 2f). Along with the potential influence of aggregation on ceMoS2 dissolution at higher anion concentrations, the promoted oxidative dissolution of ceMoS2 by HCO3– and HPO42–/H2PO4–, as well as irradiated NO3–, was consistent in the probed anion concentration range (i.e., 1–100 mM).

Most of the concentrations of ionic Mo species were higher in the sunlight-irradiated samples than in the dark samples (Table 1), while irradiation with NO3– produced more dissolved Mo species (Ct/C0 = 2.37). The enhancement of ceMoS2 dissolution by NO3– and sunlight was attributed to photolysis of the nitrate ions and the generation of hydroxyl radicals:51,52

3.2. 1
3.2. 2
3.2. 3

EPR analysis was employed to determine the photoproduced radicals. In Figure S10a, the photogenerated ·OH and NO2· in NO3– were detected by distinguishable EPR signals of 2-hydroxy-5,5-dimethyl-1-pyrrolidinyloxy (DMPO–OH)53 and 5,5-dimethyl-2-oxopyrroline-1-oxyl (DMPOX)54 (Figure S10b) adducts, respectively. Notably, the oxidation of ceMoS2 by nitrate was more pronounced under sunlight irradiation. In a comparison experiment, the Ct/C0 of Mo ionic species with NO3– and visible light irradiation (with UV-cutoff filter (<420 nm)) was 1.55, which was much lower than that seen for full-spectrum sunlight irradiation (i.e., Ct/C0 = 2.37). This was consistent with the fact that NO3– absorbs light below 350 nm (Figure S11) in the solar irradiation,51 while other anions absorbed no UV and were optically transparent. Scheme 1 depicts the UV-accelerated oxidation and dissolution of ceMoS2 with nitrate anions.

Scheme 1. UV-Accelerated Oxidative Dissolution of ceMoS2 with Nitrate Anions.

Scheme 1

Open in a new tab

Given that the greatest oxidative dissolution of ceMoS2 was found in the presence of HCO3–, the ionic Mo species produced in the oxidative dissolution process were examined by ion chromatography. As illustrated in Figure S12, the MoO42– and SO42– produced indicated that aging of the ceMoS2 in the presence of HCO3– occurred according to the reported dissolution reaction of MoS2 (eq 4),4 which suggested that oxidative dissolution, rather than the complexation of anion by the ionic Mo species, was likely the dominant reaction pathway.

3.2. 4

Previously, NO2– and BrO3– acted as the oxidants for promoting the dissolution of MoS2 in the presence of dissolved oxygen, owing to their relative ease of reduction. Given that the reduction potential of the probed anions in the current work is generally more negative (see Table S3) than that of MoO42– and SO42–/MoS2 pair (0.429 V),12 the probed anions herein were likely not acting as oxidants. While the essential role of O2 in this oxidative dissolution has been illustrated (Figure S6), the promoted oxidative dissolution of ceMoS2 by HCO3– and HPO42–/H2PO4– is surprising since they are unlikely to act as oxidants, thereby further investigation through electrochemical characterization will be conducted later to unveil the origin of the enhanced oxidation. Note that the ionic radius of SO42– and H2PO4– are larger than Cl–, NO3–, HPO4–, and HCO3– (Table S4), which results in no identifiable correlation with the observed trend in the oxidative dissolution of ceMoS2 and thus suggests that the ionic radius did not likely play a decisive role in affecting the oxidative dissolution of MoS2. Additionally, along with the dissolved Mo species, the protons released throughout the aging process were monitored. As demonstrated in Figure S13, the pH of the ceMoS2 solution decreased from neutral to acidic in the presence of Cl–, NO3–, and SO42–, but the pH was relatively stable in the presence of HCO3– and HPO42–/H2PO4–, which was likely due to their buffering capacities. Notably, a pH dependence was previously illustrated for oxidative dissolution of ceMoS2, and the dissolution rates were accelerated at higher pH.4 To examine whether the oxidative dissolution of ceMoS2, particularly in the presence of HCO3–, resulted solely from pH-induced reactions, the At/A0 values for ceMoS2 with HCO3– and ceMoS2 in the control medium (i.e., no anions) were compared at pH 8.5. In Figure S14, the pronounced At/A0 declines seen under both dark and light conditions in the presence of HCO3– compared to those in the pH 8.5 control medium, clearly indicated that other factors, in addition to the pH dependency, regulated the oxidative dissolution of MoS2. The mechanism for dissolution of ceMoS2 in the presence of anions is discussed later.

3.3. Morphology and Phase Transitions of ceMoS2 Triggered by Anions

The morphological and phase transitions of ceMoS2 triggered by anions were analyzed by TEM and XPS. As shown in Figure 3a, no change in the as-prepared nanosheets (Figure 1a) was observed after 72 h of dark incubation. In the presence of anions under dark conditions, the structure of the MoS2 nanosheets deteriorated, and cracks appeared on the surface and edges, particularly in the presence of HCO3– (Figure 3h) and HPO42–/H2PO4– (Figure 3i). The anion-induced morphological deterioration was consistent with the ceMoS2 dissolution profiles, both of which indicated the decay of ceMoS2 stability. During irradiation, no significant difference was observed with Cl– (Figure 3e) and SO42– (Figure 3f) relative to their dark controls. In contrast, the sheet edges of ceMoS2 were destroyed by the presence of NO3– (Figure 3j), indicating photoenhanced destruction by nitrate. In the presence of HCO3– (Figure 3k), ceMoS2 was utterly fragmented, which was consistent with the enhanced dissolution of ceMoS2 during sunlight exposure (Figure 2).

Figure 3.

Figure 3

Open in a new tab

TEM images of ceMoS2 (10.5 mg/L) incubated for 72 h in 10 mM anion solutions. Under dark conditions: (a) blank, (b) Cl–, (c) SO42–, (g) NO3–, (h) HCO3–, and (i) HPO42–/H2PO4–. Under irradiation: (d) blank, (e) Cl–, (f) SO42–, (j) NO3–, (k) HCO3–, and (l) HPO42–/H2PO4–.

Prior studies showed phase-dependent oxidative dissolution of ceMoS2 during the aging process; the 1T phase selectively underwent oxidative dissolution, leading to conversion of the 1T phase to the 2H phase.4,6 Herein, the phase transition of ceMoS2 was monitored in the presence of various anions with high-resolution Mo 3d core-level XPS under both dark and light conditions (Figures 4 and S15). After a 72-h incubation in the dark (Figure 4a), the 1T content of ceMoS2 exhibited a slight decline from 64.7% (as-prepared, Figure 1c) to 58.7% in the dark blank and 55.4% under sunlight. In addition to the phase transition, an increase in the MoVI–O content of ceMoS2 was observed from 19.8% in the dark to 30.9% under irradiation. In the presence of Cl–, NO3–, and SO42–, ceMoS2 displayed 1T:2H ratios similar to that in the blank, which was consistent with the relatively low dissolution rate of ceMoS2 in the presence of these three anions. On the other hand, for HCO3– and HPO42–/H2PO4–, the 1T proportions declined to 38.0 and 51.4%, respectively, indicating that HCO3– and HPO42–/H2PO4– promoted the phase transition (1T to 2H) of ceMoS2. Additionally, the hexavalent Mo (MoVI–O) in ceMoS2 disappeared with HCO3– and HPO42–/H2PO4–, and new peaks were found at approximately 230 eV (3d5/2) and 233 eV (3d3/2), for pentavalent Mo (MoV).55 The origin of the emergence of MoV with HCO3– and HPO42–/H2PO4– will be discussed later. Under sunlight irradiation for 72 h, the proportion of the 1T phase in ceMoS2 exposed to all anions was decreased to a lower level compared to that in the dark. In particular, the MoVI–O proportion reached its highest value (62.0%) in the presence of NO3– and sunlight irradiation, suggesting that a photoreaction of NO3– with ceMoS2 caused a greater oxidation of the 1T phase. The photooxidation of ceMoS2 by NO3– was likely due to the formation of reactive radicals that oxidized the MoS2 nanosheets (Scheme 1). The changes in morphology and phase transition indicated that the stability of ceMoS2 varied as a function of anion species and light exposure, and detailed mechanisms are discussed in the following section.

Figure 4.

Figure 4

Open in a new tab

Mo 3d core-level XPS data for ceMoS2 (10.5 mg/L) incubated with 10 mM anions under dark conditions: (a) blank, (b) Cl–, (c) SO42–, (d) NO3–, (e) HCO3–, and (f) HPO42–/H2PO4–. The Mo 3d core-level spectra of the sunlight-illuminated samples and the fitting data in the dark or under sunlight are shown in Figure S15.

3.4. Dissolution Mechanism for ceMoS2 in the Presence of Anions

To elucidate the interactions between ceMoS2 and anions, the electrochemistry of ceMoS2 was probed with various approaches, including OCP, CV, and ORR. As described by Lee et al.56, during the OCP process, the electrons transferred from the electrolyte charged the electrode surface. Thus, with a shorter time required to achieve a stable potential, electron transfer from the anions to the electrode surface is accelerated. In Figure 5a, the OCP of ceMoS2 in a 100-mM anion solution was recorded for 2 h to reach a stable potential. As shown in Figure 5b, the OCP stabilization rate of ceMoS2 was faster with HCO3– and HPO42–/H2PO4–, while the rates with Cl–, SO42–, and NO3– were relatively slow. Given that there was no current flowing during OCP measurements (i.e., equilibrium was achieved between the working electrode and the reference electrode in the electrolyte),57 the OCP change indicated the electron transfer rate from the electrolyte to the electrode surface. Therefore, HCO3– and HPO42–/H2PO4– exhibited faster charging rates with ceMoS2, while Cl–, SO42–, and NO3– exhibited slower charging rates. Fast electron charging with the HCO3– and HPO42–/H2PO4– was confirmed by the emergence of MoV (Figure 4e,f), rather than the transition to MoVI, in the presence of other anions.

Figure 5.

Figure 5

Open in a new tab

Electrochemical analyses of ceMoS2 in the presence of anionic electrolytes (Cl–, NO3–, SO42–, HCO3– and HPO42–/H2PO4–); (a) OCP evolution of ceMoS2 in the presence of anions and (b) the OCP kinetics of ceMoS2 in the presence of anions (electrolyte: 100 mM of anions). (c) CV curves for ceMoS2 in the presence of anions (electrolyte: 10 mM of anions). (d) ORR polarization curves and (e) corresponding Tafel slopes for ceMoS2 in the presence of different anions (electrolyte: 100 mM of anions).

Next, the extent of ceMoS2 oxidation by the different anions was assessed by CV. Figure 5c shows the oxidation sweep starting at 0 V followed by the reduction sweep. The oxidation peaks indicated oxidation of the ceMoS2 nanosheets, while the reduction peaks were attributed to reduction of the oxidized MoS2.58,59 Among the anionic electrolytes, HCO3– and HPO42–/H2PO4– generated higher oxidation peak currents for ceMoS2, implying oxidation of more ceMoS2. Additionally, the reduction peak currents for ceMoS2 were negligible in the presence of HCO3– and HPO42–/H2PO4–, which was ascribed to oxidative dissolution of the ceMoS2. This led to deterioration of the electrode material (i.e., ceMoS2) and explained the low reduction peak current in the subsequent cathodic scan.60 Lower current intensities for ceMoS2 were observed in the presence of NO3–, SO42–, and Cl–, suggesting less ceMoS2 was oxidized. The electrochemical data were consistent with the observed trend for the dissolution of ceMoS2 in anion solutions under dark conditions (Table 1).

The results of the OCP and CV suggested different charging rates and oxidative dissolution of ceMoS2 in the presence of the examined anions, which was ascribed to the chemical potential of the ceMoS2 surface. In a prior study, Lenhart and coauthors61 indicated that the oxidative dissolution of AgNPs by electron acceptors (e.g., O2) was catalyzed by nucleophilic reagents owing to alteration of the chemical potential of the AgNPs by the nucleophiles. In the absence of nucleophiles, oxidation of the AgNPs shifted the chemical potential of the particle surfaces to more positive values, which decreased the difference in the chemical potential between the AgNPs and the electron acceptors (e.g., O2) and slowed the oxidation. In contrast, in the presence of absorbed nucleophiles, the nucleophilic reagents generated excess negative charge62 and shifted the chemical potential of the AgNPs to a more negative value, enabling oxidation of the AgNPs. A nucleophile is an electron-rich molecule or ion with a lone pair of electrons, and it reacts with electron-deficient compounds;63 the anions used in this study are nucleophiles with different nucleophilicities. The nucleophilic constants of HCO3–, HPO42–, Cl–, SO42–, and NO3– are 3.8, 3.8, 2.7, 2.5, and 1.0,64,65 respectively, indicating that HCO3– and HPO42– are the strongest nucleophiles among the probed anions. As shown in Figure 5c, the anodic peak potential and current of ceMoS2 (highlighted zone) in HCO3– and HPO42– illustrated an increase in the oxidation propensity of ceMoS2. The compiled results suggest that the enhanced oxidative dissolution of ceMoS2 in HCO3– and HPO42– was likely similar to that of the AgNPs, as the presence of nucleophiles shifted the chemical potential and facilitated oxidation. Therefore, oxidation of ceMoS2 by O2 was enhanced by strong nucleophiles.

Additionally, the Mo atom was an electron donor, and the S atom was as an electron acceptor in the electronic structure of MoS2,66 which indicated covalent Mo–S bonding and an electron-deficient Mo center. Moreover, the chemical exfoliation and lithiation process induced sulfur vacancies on the MoS2 nanosheets,67 thereby rendering the surrounding Mo atoms more electrophilic.68,69 The MoVI–O peak in the Mo 3d core-level spectrum of ceMoS2 (Figure 1c) indicated the presence of sulfur vacancies.70 When potent nucleophiles (HCO3– and HPO42–/H2PO4–) were present, the formation of MoV in the ceMoS2 (Figure 4e,f) was the result of reduction of MoS2 by the nucleophilic anions. The MoV altered the chemical potential of ceMoS2, which was more susceptible to oxidation and subsequent dissolution.

Given that oxidation dissolution of ceMoS2 with the electron acceptor O2 was catalyzed by nucleophilic anions and involved the electrophilic Mo center, oxygen reduction during the oxidation of ceMoS2 was examined by studying the ORR in the presence of different anionic electrolytes. In the ORR polarization curves (Figure 5d), ceMoS2 exhibited a more positive onset potential (Eonset) (listed in Table S5) and lower Tafel slope (Figure 5e) in HCO3– and HPO42–/H2PO4– compared with other anions. The results indicated that the presence of strong nucleophiles enhanced electron transfer from ceMoS2 to oxygen (i.e., faster ORR kinetic process), which was consistent with the higher oxidation current seen for ceMoS2 in the presence of HCO3– and HPO42–/H2PO4–. The proposed mechanism for the effects of anions on ceMoS2 is shown in Scheme 2. The presence of anions facilitated the oxidation of ceMoS2 by oxygen by triggering changes in the chemical potential of the ceMoS2 surface as a function of the nucleophilicities (i.e., charging effect) of the various anions.

Scheme 2. Proposed Mechanism for Oxidative Dissolution of ceMoS2 Accelerated by Various Anions.

Scheme 2

Open in a new tab

3.5. Dissolution of ceMoS2 under Environmentally Relevant Conditions

To confirm the effects of anions on the MoS2 nanosheets, the dissolution of ceMoS2 in the presence of anions was surveyed under more environmentally relevant conditions. As listed in Table 2, the initial concentration of dissolved Mo species from the ceMoS2 was 12.21 μg/L. After 72 h of incubation in the dark, the Ct/C0 of ceMoS2 increased to 2.62, 2.76, and 2.57 for Cl–, SO42–, and NO3–, respectively, indicating mild enhancement of ceMoS2 dissolution by these three ions. On the other hand, the Ct/C0 for ceMoS2 increased substantially to 3.97 and 4.02 in the presence of HCO3– and HPO42–/H2PO4–, respectively, confirming enhanced promotion of ceMoS2 oxidative dissolution. For sunlight irradiation with NO3–, the Ct/C0 of ceMoS2 was 4.94, higher than those seen with Cl– and SO42–. These results demonstrated that the impacts of the anions on the stability of the MoS2 nanosheets were valid at lower concentrations.

Table 2. Concentrations of the Ionic Mo Species (Obtained by Passing ceMoS2 Suspensions through 3 kDa Membranes) Produced by ceMoS2 in the Presence of Anions under Environmentally Relevant Conditions for 72 h (Initial Concentration: 100 μg/L ceMoS2 and 1 mM Anions).

ceMoS2 anionic samples ionic Mo species at 0 h ionic Mo species at 72 h in dark ionic Mo species at 72 h under sunlight irradiation
C0 (μg/L) Ct (μg/L) Ct/C0 Ct (μg/L) Ct/C0
blank 12.21 ± 0.31 26.76 ± 2.07 2.19 ± 0.22 43.09 ± 1.38 3.53 ± 0.20
Cl–   32.05 ± 1.96 2.62 ± 0.23 43.29 ± 2.16 3.55 ± 0.33
SO42–   33.71 ± 0.78 2.76 ± 0.13 51.82 ± 1.76 4.24 ± 0.25
NO3–   31.42 ± 0.28 2.57 ± 0.09 60.28 ± 1.39 4.94 ± 0.24
HCO3–   48.44 ± 1.02 3.97 ± 0.18 82.88 ± 3.98 6.79 ± 0.50
HPO42–/H2PO4–   49.08 ± 1.37 4.02 ± 0.21 69.32 ± 0.83 5.68 ± 0.21
Open in a new tab

4. Environmental Implications

The chemical stability of the MoS2 nanosheets has a considerable impact on the potential uses of MoS2, especially in aqueous environments. Our findings demonstrated that the oxidative dissolution of ceMoS2 was affected by the nucleophilicities of coexisting anionic species. The ceMoS2 charging effects of the nucleophilic anions (i.e., HCO3– and HPO42–/H2PO4–) shifted the chemical potential of ceMoS2, promoting oxidative dissolution of the ceMoS2. With the fast charging by HCO3– and HPO42–/H2PO4–, the oxidative transformation of the MoS2 nanosheets accelerated the morphology and phase transitions, leading to deteriorated sheets, a 1T to 2H transition, and the emergence of pentavalent Mo (MoV). It is noteworthy that over the past two decades, the average atmospheric CO2 concentration has increased from 372.59 ppm in 2002 to 418.64 ppm in 2023, with an annual growth rate of approximately 2 ppm.71 Most of the dissolved inorganic carbon (DIC) will likely generate HCO3– (>90%) and elevate the concentrations of HCO3– in aqueous environments.72 Between 2004 and 2019, the DIC level in the global ocean increased from 16 to 38% at depths shallower than 1500 m.73 Thus, the reactions of HCO3– with released ENMs, including MoS2, in aquatic environments hold increasing importance. The effects of anions could also provide insights into the environmental implications of MoS2 nanosheets, including their toxicity and bioavailability. It has been shown that the transformed ENMs throughout environmental processes exhibit different toxicity profiles from those of as-prepared materials.74 Our prior study demonstrated that acidic ionic Mo species released during the aging process of MoS2 nanosheets were detrimental to aquatic organisms.75 Given the dependence of ceMoS2 oxidative dissolution on the anionic species studied in the present work, the ecological risks of the transformed MoS2 nanosheets could be shifted by these species. Additionally, our findings demonstrated the accelerated dissolution of ceMoS2 with anions, particularly NO3–, HCO3–, and HPO42–/H2PO4–, under sunlight exposure. Therefore, with the numerous photocatalytic applications of MoS2 in aquatic environments,76,77 it is important to consider the presence of anionic species when assessing the durability of MoS2 for photocatalytic water treatment.

Acknowledgments

The authors gratefully acknowledge financial support from the National Science and Technology Council of Taiwan (NSTC 111-2628-E-005-003-MY3) and the use of ESCA, TEM, EPR, and HR-ICP-MS belonging to the Instrument Center of NCHU.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.3c08278.

  • Supplementary experimental section; residual molar ratio of Li to Mo and pH values of ceMoS2 suspension; correlation between ceMoS2 concentrations and absorbance at 450 nm; spectra of natural sunlight and light emitted in the CPS+ solar simulator and transmittance spectrum of the UV-cutoff filter; passing ratio of Na2MoO4 through the 3 kDa MWCO membranes; IS calculation of ionic species used in this study; effect of Cl– on the normalized absorbance at 450 nm of ceMoS2; stabilities of ceMoS2 suspensions in the presence of 1 mM anionic species with and without elimination of O2; EPR spectra for DMPO adducts in ceMoS2 with HCO3– and HPO42–/H2PO4–; zeta potential measurement of ceMoS2 with sodium anionic species; aggregation profiles of ceMoS2 suspensions with sodium anionic species; rate constants of absorbance in ceMoS2 with coexisting anionic species; EPR spectra for DMPO adducts in NO3– under irradiation; UV–vis absorption spectra of sodium anionic species and the spectrum of light emitted by the CPS+ solar simulator; ion chromatograms of ceMoS2 incubated with HCO3–; standard reduction potential of anionic species; ionic radius of anionic species used in this study; pH variation of ceMoS2 incubated with anionic species; pH effects on the stability of ceMoS2; Mo 3d XPS spectra of ceMoS2 in anions; and the ORR onset potential of ceMoS2 in anions (PDF)

The authors declare no competing financial interest.

Supplementary Material

es3c08278_si_001.pdf (3.6MB, pdf)

References

  1. Huang L.; Huo N.; Li Y.; Chen H.; Yang J.; Wei Z.; Li J.; Li S.-S. Electric-field tunable band offsets in black phosphorus and MoS2 van der Waals p-n heterostructure. J. Phys. Chem. Lett. 2015, 6 (13), 2483–2488. 10.1021/acs.jpclett.5b00976. [DOI] [PubMed] [Google Scholar]
  2. Kim J. H.; Kim T. H.; Lee H.; Park Y. R.; Choi W.; Lee C. J. Thickness-dependent electron mobility of single and few-layer MoS2 thin-film transistors. AIP Adv. 2016, 6, 065106 10.1063/1.4953809. [DOI] [Google Scholar]
  3. Chhowalla M.; Shin H. S.; Eda G.; Li L.-J.; Loh K. P.; Zhang H. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat. Chem. 2013, 5 (4), 263–275. 10.1038/nchem.1589. [DOI] [PubMed] [Google Scholar]
  4. Wang Z.; Bussche V. D. A.; Qiu Y.; Valentin T. M.; Gion K.; Kane A. B.; Hurt R. H. Chemical dissolution pathways of MoS2 nanosheets in biological and environmental media. Environ. Sci. Technol. 2016, 50 (13), 7208–7217. 10.1021/acs.est.6b01881. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Zou W.; Zhou Q.; Zhang X.; Hu X. Dissolved oxygen and visible light irradiation drive the structural alterations and phytotoxicity mitigation of single-layer molybdenum disulfide. Environ. Sci. Technol. 2019, 53 (13), 7759–7769. 10.1021/acs.est.9b00088. [DOI] [PubMed] [Google Scholar]
  6. Lee T.-W.; Chen C.-C.; Chen C. Chemical stability and transformation of molybdenum disulfide nanosheets in environmental media. Environ. Sci. Technol. 2019, 53 (11), 6282–6291. 10.1021/acs.est.9b00318. [DOI] [PubMed] [Google Scholar]
  7. Li B. L.; Zou H. L.; Lu L.; Yang Y.; Lei J. L.; Luo H. Q.; Li N. B. Size-dependent optical absorption of layered MoS2 and DNA oligonucleotides induced dispersion behavior for label-free detection of single-nucleotide polymorphism. Adv. Funct. Mater. 2015, 25 (23), 3541–3550. 10.1002/adfm.201500180. [DOI] [Google Scholar]
  8. Li J.; Liu C.; Wei J.; Yan Y.; Zhao X.; Yang X. Morphology mediation of MoS2 nanosheets with organic cations for fast sodium ion storage. Chin. Chem. Lett. 2021, 32 (2), 880–884. 10.1016/j.cclet.2020.06.036. [DOI] [Google Scholar]
  9. Mohona T. M.; Gupta A.; Masud A.; Chien S.-C.; Lin L.-C.; Nalam P. C.; Aich N. Aggregation behavior of inorganic 2D nanomaterials beyond graphene: Insights from molecular mmodeling and modified DLVO theory. Environ. Sci. Technol. 2019, 53 (8), 4161–4172. 10.1021/acs.est.8b05180. [DOI] [PubMed] [Google Scholar]
  10. Liu B.; Han Q.; Li L.; Zheng S.; Shu Y.; Pedersen J. A.; Wang Z. Synergistic effect of metal cations and visible light on 2D MoS2 nanosheet aggregation. Environ. Sci. Technol. 2021, 55 (24), 16379–16389. 10.1021/acs.est.1c03576. [DOI] [PubMed] [Google Scholar]
  11. Wang Z.; Mi B. Environmental applications of 2D molybdenum disulfide (MoS2) nanosheets. Environ. Sci. Technol. 2017, 51 (15), 8229–8244. 10.1021/acs.est.7b01466. [DOI] [PubMed] [Google Scholar]
  12. Wang Z.; Sim A.; Urban J. J.; Mi B. Removal and Recovery of Heavy metal ions by two-dimensional MoS2 nanosheets: Performance and mechanisms. Environ. Sci. Technol. 2018, 52 (17), 9741–9748. 10.1021/acs.est.8b01705. [DOI] [PubMed] [Google Scholar]
  13. Han Q.; Yu J.; Poon S.; Sun L.; Teli M.; Liu B.; Chen H.; Wang K.; Wang Z.; Mi B. Highly efficient removal and sequestration of Cr(VI) in confined MoS2 interlayer Nanochannels: Performance and mechanism. Sep. Purif. Technol. 2022, 293, 121104 10.1016/j.seppur.2022.121104. [DOI] [Google Scholar]
  14. Marks R.; Schranck A.; Stillwell R.; Doudrick K. Stability of 2H- and 1T-MoS2 in the presence of aqueous oxidants and its protection by a carbon shell. RSC Adv. 2020, 10 (16), 9324–9334. 10.1039/D0RA00788A. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Panno S. V.; Hackley K. C.; Hwang H. H.; Greenberg S. E.; Krapac I. G.; Landsberger S.; O’Kelly D. J. Characterization and identification of Na-Cl sources in ground water. Ground Water 2006, 44 (2), 176–187. 10.1111/j.1745-6584.2005.00127.x. [DOI] [PubMed] [Google Scholar]
  16. Lubello C.; Gori R.; Nicese F. P.; Ferrini F. Municipal-treated wastewater reuse for plant nurseries irrigation. Water Res. 2004, 38 (12), 2939–2947. 10.1016/j.watres.2004.03.037. [DOI] [PubMed] [Google Scholar]
  17. Moreno P.; Aral H.; Vecchio-Sadus A.. Environmental impact and toxicology of sulphate; Enviromine, Santiago, Chile, 2009. [Google Scholar]
  18. Cai C.; Liu Y.; Xu R.; Zhou J.; Zhang J.; Chen Y.; Liu L.; Zhang L.; Kang S.; Xie X. Bicarbonate enhanced heterogeneous activation of peroxymonosulfate by copper ferrite nanoparticles for the efficient degradation of refractory organic contaminants in water. Chemosphere 2023, 312, 137285 10.1016/j.chemosphere.2022.137285. [DOI] [PubMed] [Google Scholar]
  19. Stiff M. J. Copper/bicarbonate equilibria in solutions of bicarbonate ion at concentrations similar to those found in natural water. Water Res. 1971, 5 (5), 171–176. 10.1016/0043-1354(71)90049-2. [DOI] [Google Scholar]
  20. Kent R.; Johnson T. D.; Rosen M. R. Status and trends of orthophosphate concentrations in groundwater used for public supply in California. Environ. Monit. Assess. 2020, 192 (8), 550. 10.1007/s10661-020-08504-x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Villamar C. A.; Cañuta T.; Belmonte M.; Vidal G. Characterization of swine wastewater by toxicity identification evaluation methodology (TIE). Water, Air, & Soil Pollution 2012, 223 (1), 363–369. 10.1007/s11270-011-0864-z. [DOI] [Google Scholar]
  22. Zahedi R.; Mirmohammadi S. J. Sulfate removal from chemical industries’ wastewater using ettringite precipitation process with recovery of Al(OH)3. Appl. Water Sci. 2022, 12, 226. 10.1007/s13201-022-01748-7. [DOI] [Google Scholar]
  23. Tang W.; He D.; Zhang C.; Waite T. D. Optimization of sulfate removal from brackish water by membrane capacitive deionization (MCDI). Water Res. 2017, 121, 302–310. 10.1016/j.watres.2017.05.046. [DOI] [PubMed] [Google Scholar]
  24. Jeong Y.; Fan M.; Van Leeuwen J.; Belczyk J. F. Effect of competing solutes on arsenic(V) adsorption using iron and aluminum oxides. Journal of Environmental Sciences 2007, 19 (8), 910–919. 10.1016/S1001-0742(07)60151-X. [DOI] [PubMed] [Google Scholar]
  25. Sheng G.; Li Y.; Yang X.; Ren X.; Yang S.; Hu J.; Wang X. Efficient removal of arsenate by versatile magnetic graphene oxide composites. RSC Adv. 2012, 2 (32), 12400–12407. 10.1039/c2ra21623j. [DOI] [Google Scholar]
  26. Chen Y.; Zhang Q.; Chen L.; Bai H.; Li L. Basic aluminum sulfate@graphene hydrogel composites: preparation and application for removal of fluoride. Journal of Materials Chemistry A 2013, 1 (42), 13101–13110. 10.1039/c3ta13285d. [DOI] [Google Scholar]
  27. Li Y.; Zhao C.; Wen Y.; Wang Y.; Yang Y. Adsorption performance and mechanism of magnetic reduced graphene oxide in glyphosate contaminated water. Environmental Science and Pollution Research 2018, 25 (21), 21036–21048. 10.1007/s11356-018-2282-x. [DOI] [PubMed] [Google Scholar]
  28. Lien E.; Sahu R. S.; Chen W.-L.; Shih Y.-H. Effective photocatalytic degradation of sulfamethoxazole using tunable CaCu3Ti4O7 perovskite. Chemosphere 2022, 294, 133744 10.1016/j.chemosphere.2022.133744. [DOI] [PubMed] [Google Scholar]
  29. Chen Y.; Liu K. Preparation and characterization of nitrogen-doped TiO2/diatomite integrated photocatalytic pellet for the adsorption-degradation of tetracycline hydrochloride using visible light. Chemical Engineering Journal 2016, 302, 682–696. 10.1016/j.cej.2016.05.108. [DOI] [Google Scholar]
  30. Zhang W.; Xiao B.; Fang T. Chemical transformation of silver nanoparticles in aquatic environments: Mechanism, morphology and toxicity. Chemosphere 2018, 191, 324–334. 10.1016/j.chemosphere.2017.10.016. [DOI] [PubMed] [Google Scholar]
  31. Levard C.; Mitra S.; Yang T.; Jew A. D.; Badireddy A. R.; Lowry G. V.; Brown G. E. Jr. Effect of chloride on the dissolution rate of silver nanoparticles and toxicity to E. coli. Environ. Sci. Technol. 2013, 47 (11), 5738–5745. 10.1021/es400396f. [DOI] [PubMed] [Google Scholar]
  32. Liu J.; Pennell K. G.; Hurt R. H. Kinetics and mechanisms of nanosilver oxysulfidation. Environ. Sci. Technol. 2011, 45 (17), 7345–7353. 10.1021/es201539s. [DOI] [PMC free article] [PubMed] [Google Scholar]
  33. Guo Z.; Chen G.; Zeng G.; Yan M.; Huang Z.; Jiang L.; Peng C.; Wang J.; Xiao Z. Are silver nanoparticles always toxic in the presence of environmental anions?. Chemosphere 2017, 171, 318–323. 10.1016/j.chemosphere.2016.12.077. [DOI] [PubMed] [Google Scholar]
  34. Zhao F.; Peydayesh M.; Ying Y.; Mezzenga R.; Ping J. Transition metal dichalcogenide–silk nanofibril membrane for one-step water purification and precious metal recovery. ACS Appl. Mater. Interfaces 2020, 12 (21), 24521–24530. 10.1021/acsami.0c07846. [DOI] [PubMed] [Google Scholar]
  35. Wang Z.; Tu Q.; Zheng S.; Urban J. J.; Li S.; Mi B. Understanding the aqueous stability and filtration capability of MoS2 membranes. Nano Lett. 2017, 17 (12), 7289–7298. 10.1021/acs.nanolett.7b02804. [DOI] [PubMed] [Google Scholar]
  36. Sapkota B.; Liang W.; VahidMohammadi A.; Karnik R.; Noy A.; Wanunu M. High permeability sub-nanometre sieve composite MoS2 membranes. Nat. Commun. 2020, 11, 2747. 10.1038/s41467-020-16577-y. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Story S. D.; Boggs S.; Guiney L. M.; Ramesh M.; Hersam M. C.; Brinker C. J.; Walker S. L. Aggregation morphology of planar engineered nanomaterials. J. Colloid Interface Sci. 2020, 561, 849–853. 10.1016/j.jcis.2019.11.067. [DOI] [PubMed] [Google Scholar]
  38. Lanphere J. D.; Luth C. J.; Guiney L. M.; Mansukhani N. D.; Hersam M. C.; Walker S. L. Fate and transport of molybdenum disulfide nanomaterials in sand columns. Environ. Eng. Sci. 2015, 32 (2), 163–173. 10.1089/ees.2014.0335. [DOI] [PMC free article] [PubMed] [Google Scholar]
  39. Camargo J. A.; Alonso A.; Salamanca A. Nitrate toxicity to aquatic animals: A review with new data for freshwater invertebrates. Chemosphere 2005, 58 (9), 1255–1267. 10.1016/j.chemosphere.2004.10.044. [DOI] [PubMed] [Google Scholar]
  40. Surette M. C.; Nason J. A.; Harper S. L.; Mitrano D. M. What is “Environmentally Relevant”? A framework to advance research on the environmental fate and effects of engineered nanomaterials. Environmental Science: Nano 2021, 8 (9), 2414–2429. 10.1039/D1EN00162K. [DOI] [Google Scholar]
  41. Tong X.; Qi Y.; Chen J.; Wang N.; Xu Q. Supercritical CO2-assisted reverse-micelle-induced solution-phase fabrication of two-dimensional metallic 1T-MoS2 and 1T-WS2. ChemNanoMat 2017, 3 (7), 466–471. 10.1002/cnma.201700011. [DOI] [Google Scholar]
  42. Saber M. R.; Khabiri G.; Maarouf A. A.; Ulbricht M.; Khalil A. S. A comparative study on the photocatalytic degradation of organic dyes using hybridized 1T/2H, 1T/3R and 2H MoS2 nano-sheets. RSC Adv. 2018, 8 (46), 26364–26370. 10.1039/C8RA05387A. [DOI] [PMC free article] [PubMed] [Google Scholar]
  43. Wu D.; Hu Y.; Liu Y.; Zhang R. Review of chloride ion detection technology in water. Applied Sciences 2021, 11 (23), 11137. 10.3390/app112311137. [DOI] [Google Scholar]
  44. Li Y.; Yang Z.; Yang K.; Wei J.; Li Z.; Ma C.; Yang X.; Wang T.; Zeng G.; Yu G.; Yu Z.; Zhang C. Removal of chloride from water and wastewater: Removal mechanisms and recent trends. Sci. Total Environ. 2022, 821, 153174 10.1016/j.scitotenv.2022.153174. [DOI] [PubMed] [Google Scholar]
  45. Choi J.; Kim H. H.; Lee K. M.; Chen N.; Kim M. S.; Seo J.; Lee D.; Cho H.; Kim H.-I.; Lee J.; Lee H.; Lee C. Bicarbonate-enhanced generation of hydroxyl radical by visible light-induced photocatalysis of H2O2 over WO3: Alteration of electron transfer mechanism. Chemical Engineering Journal 2022, 432, 134401 10.1016/j.cej.2021.134401. [DOI] [Google Scholar]
  46. Kim J.; Choi W. TiO2 modified with both phosphate and platinum and its photocatalytic activities. Appl. Catal., B 2011, 106 (1), 39–45. 10.1016/j.apcatb.2011.05.002. [DOI] [Google Scholar]
  47. Liu B.; Han Z.; Han Q.; Shu Y.; Li L.; Chen B.; Wang Z.; Pedersen J. A. Redispersion behavior of 2D MoS2 nanosheets: Unique dependence on the intervention timing of natural organic matter. Environ. Sci. Technol. 2023, 57 (2), 939–950. 10.1021/acs.est.2c05282. [DOI] [PubMed] [Google Scholar]
  48. Lowry G. V.; Gregory K. B.; Apte S. C.; Lead J. R. Transformations of nanomaterials in the environment. Environ. Sci. Technol. 2012, 46 (13), 6893–6899. 10.1021/es300839e. [DOI] [PubMed] [Google Scholar]
  49. He D.; Bligh M. W.; Waite T. D. Effects of aggregate structure on the dissolution kinetics of citrate-stabilized silver nanoparticles. Environ. Sci. Technol. 2013, 47 (16), 9148–9156. 10.1021/es400391a. [DOI] [PubMed] [Google Scholar]
  50. Misra S. K.; Dybowska A.; Berhanu D.; Luoma S. N.; Valsami-Jones E. The complexity of nanoparticle dissolution and its importance in nanotoxicological studies. Sci. Total Environ. 2012, 438, 225–232. 10.1016/j.scitotenv.2012.08.066. [DOI] [PubMed] [Google Scholar]
  51. Machado F.; Boule P. Photonitration and photonitrosation of phenolic derivatives induced in aqueous solution by excitation of nitrite and nitrate ions. J. Photochem. Photobiol., A 1995, 86 (1), 73–80. 10.1016/1010-6030(94)03946-R. [DOI] [Google Scholar]
  52. Chen Y.; Zhang K.; Zuo Y. Direct and indirect photodegradation of estriol in the presence of humic acid, nitrate and iron complexes in water solutions. Sci. Total Environ. 2013, 463–464, 802–809. 10.1016/j.scitotenv.2013.06.026. [DOI] [PubMed] [Google Scholar]
  53. Fontmorin J. M.; Burgos Castillo R. C.; Tang W. Z.; Sillanpää M. Stability of 5,5-dimethyl-1-pyrroline-N-oxide as a spin-trap for quantification of hydroxyl radicals in processes based on Fenton reaction. Water Res. 2016, 99, 24–32. 10.1016/j.watres.2016.04.053. [DOI] [PubMed] [Google Scholar]
  54. Reszka K. J.; McCormick M. L.; Buettner G. R.; Michael Hart C.; Britigan B. E. Nitric oxide decreases the stability of DMPO spin adducts. Nitric Oxide 2006, 15 (2), 133–141. 10.1016/j.niox.2006.03.004. [DOI] [PubMed] [Google Scholar]
  55. Chang Y.-H.; Lin C.-T.; Chen T.-Y.; Hsu C.-L.; Lee Y.-H.; Zhang W.; Wei K.-H.; Li L.-J. Highly efficient electrocatalytic hydrogen production by MoSx grown on graphene-protected 3D Ni foams. Adv. Mater. 2013, 25 (5), 756–760. 10.1002/adma.201202920. [DOI] [PubMed] [Google Scholar]
  56. Lee I.; Loew N.; Tsugawa W.; Ikebukuro K.; Sode K. Development of a third-generation glucose sensor based on the open circuit potential for continuous glucose monitoring. Biosens. Bioelectron. 2019, 124–125, 216–223. 10.1016/j.bios.2018.09.099. [DOI] [PubMed] [Google Scholar]
  57. Wise C. F.; Agarwal R. G.; Mayer J. M. Determining proton-coupled standard potentials and X–H bond dissociation free energies in nonaqueous solvents using open-circuit potential measurements. J. Am. Chem. Soc. 2020, 142 (24), 10681–10691. 10.1021/jacs.0c01032. [DOI] [PubMed] [Google Scholar]
  58. Ambrosi A.; Sofer Z.; Pumera M. Lithium intercalation compound dramatically influences the electrochemical properties of exfoliated MoS2. Small 2015, 11 (5), 605–612. 10.1002/smll.201400401. [DOI] [PubMed] [Google Scholar]
  59. Eng A. Y. S.; Ambrosi A.; Sofer Z.; Šimek P.; Pumera M. Electrochemistry of transition metal dichalcogenides: Strong dependence on the metal-to-chalcogen composition and exfoliation method. ACS Nano 2014, 8 (12), 12185–12198. 10.1021/nn503832j. [DOI] [PubMed] [Google Scholar]
  60. Tan S. M.; Sofer Z.; Luxa J.; Pumera M. Aromatic-exfoliated transition metal dichalcogenides: Implications for inherent electrochemistry and hydrogen evolution. ACS Catal. 2016, 6 (7), 4594–4607. 10.1021/acscatal.6b00761. [DOI] [Google Scholar]
  61. Li X.; Lenhart J. J.; Walker H. W. Aggregation kinetics and dissolution of coated silver nanoparticles. Langmuir 2012, 28 (2), 1095–1104. 10.1021/la202328n. [DOI] [PubMed] [Google Scholar]
  62. Mulvaney P.; Linnert T.; Henglein A. Surface chemistry of colloidal silver in aqueous solution: Observations on chemisorption and reactivity. J. Phys. Chem. 1991, 95 (20), 7843–7846. 10.1021/j100173a053. [DOI] [Google Scholar]
  63. Liu C.; Zhang H.; Shi W.; Lei A. Bond formations between two nucleophiles: Transition metal catalyzed oxidative cross-coupling reactions. Chem. Rev. 2011, 111 (3), 1780–1824. 10.1021/cr100379j. [DOI] [PubMed] [Google Scholar]
  64. Swain C. G.; Scott C. B. Quantitative correlation of relative rates. Comparison of hydroxide ion with other nucleophilic reagents toward alkyl halides, esters, epoxides and acyl halides1. J. Am. Chem. Soc. 1953, 75 (1), 141–147. 10.1021/ja01097a041. [DOI] [Google Scholar]
  65. Wells P. R. Linear free energy relationships. Chem. Rev. 1963, 63 (2), 171–219. 10.1021/cr60222a005. [DOI] [Google Scholar]
  66. Yuan J.-N.; Cheng Y.; Zhang X.-Q.; Chen X.-R.; Cai L.-C. First-principles study of electronic and elastic properties of hexagonal layered crystal MoS2 under pressure. Zeitschrift für Naturforschung A 2015, 70 (7), 529–537. 10.1515/zna-2015-0102. [DOI] [Google Scholar]
  67. Wang L.; Xu Z.; Wang W.; Bai X. Atomic mechanism of dynamic electrochemical lithiation processes of MoS2 nanosheets. J. Am. Chem. Soc. 2014, 136 (18), 6693–6697. 10.1021/ja501686w. [DOI] [PubMed] [Google Scholar]
  68. Wang T.; Shang H.; Zhang Q. Adsorption behavior of thiophene on MoS2 under a microwave electric field via DFT and MD studies. Chem. Eng. Sci. 2020, 228, 115950 10.1016/j.ces.2020.115950. [DOI] [Google Scholar]
  69. Ling F.; Xia W.; Li L.; Zhou X.; Luo X.; Bu Q.; Huang J.; Liu X.; Kang W.; Zhou M. Single transition metal atom bound to the unconventional phase of the MoS2 monolayer for catalytic oxygen reduction reaction: A first-principles study. ACS Appl. Mater. Interfaces 2021, 13 (15), 17412–17419. 10.1021/acsami.0c21597. [DOI] [PubMed] [Google Scholar]
  70. Park M. J.; Gravelsins S.; Son J.; van der Zande A. M.; Dhirani A.-A. A scalable, solution-based approach to tuning the solubility and improving the photoluminescence of chemically exfoliated MoS2. ACS Nano 2019, 13 (6), 6469–6476. 10.1021/acsnano.8b09578. [DOI] [PubMed] [Google Scholar]
  71. The Global Monitoring Laboratory, National Oceanic & Atmospheric Administration, U.S. Department of Commerce . Trends in Atmospheric Carbon Dioxide. doi: 10.15138/9N0H-ZH07 (accessed January 11, 2024). [DOI]
  72. Cohen R.; Happer W.. Fundamentals of Ocean pH; 2015. https://co2coalition.org/publications/fundamentals-of-ocean-ph/ (accessed January 11, 2024).
  73. Keppler L.; Landschützer P.; Lauvset S. K.; Gruber N. Recent trends and variability in the oceanic storage of dissolved inorganic carbon. Global Biogeochem. Cycles 2023, 37 (5), e2022GB007677 10.1029/2022GB007677. [DOI] [Google Scholar]
  74. Das S.; Ghosh D.; Kerkar K.; Tiwari M.; Tiwari D. K.. Effects of the transformation of metallic nanoparticles in the environment and its toxicity on aquatic and terrestrial life forms. In Free Radical Biology and Environmental Toxicity, Kesari K. K.; Jha N. K., Eds.; Springer International Publishing: Cham, 2021; pp 43–71. [Google Scholar]
  75. Lee T.-W.; Lai Y.-H.; Chen J.-L.; Chen C. The role of transformation in the risks of chemically exfoliated molybdenum disulfide nanosheets to the aquatic environment. Journal of Environmental Management 2022, 324, 116278 10.1016/j.jenvman.2022.116278. [DOI] [PubMed] [Google Scholar]
  76. Li Z.; Meng X.; Zhang Z. Recent development on MoS2-based photocatalysis: A review. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2018, 35, 39–55. 10.1016/j.jphotochemrev.2017.12.002. [DOI] [Google Scholar]
  77. Fan J. J.; Li Y. F.; Nguyen H. N.; Yao Y.; Rodrigues D. F. Toxicity of exfoliated-MoS2 and annealed exfoliated-MoS2 towards planktonic cells, biofilms, and mammalian cells in the presence of electron donor. Environ.-Sci. Nano 2015, 2 (4), 370–379. 10.1039/C5EN00031A. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

es3c08278_si_001.pdf (3.6MB, pdf)

Articles from Environmental Science & Technology are provided here courtesy of American Chemical Society

RESOURCES