High-Yield Preparation of Exfoliated 1T-MoS2 with SERS Activity

Article abstract of DOI:10.1021/acs.chemmater.9b01698

Molybdenum disulfide (MoS₂), a promising two-dimensional transition-metal dichalcogenide, presents a challenge in the tuning of its optoelectronic and chemical properties. Herein, we demonstrate an efficient route to alter the crystalline structure of MoS₂ by chemical exfoliation. Using NaK metal alloys, exfoliated and covalently functionalized MoS₂ derivatives were obtained with a high metallic (1T) phase ratio, up to 94.5%. Consequently, exfoliated MoS₂ showed a significant surface-enhanced Raman scattering activity toward rhodamine 6G (R6G) and crystal violet, with low detection limits. The versatility of this approach allows the covalent functionalization of MoS₂ without relying on edge or basal-plane defects of the structure and preserving the high-ratio 1T phase.

Full text of DOI:10.1021/acs.chemmater.9b01698

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Article
High yield preparation of exfoliated 1T-MoS2 with SERS activity
Engin Er, Hui-Lei Hou, Alejandro Criado, Judith Langer, Marco
Möller, Nevin Erk, Luis M. Liz-Marzán, and Maurizio Prato
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01698 • Publication Date (Web): 17 Jul 2019
Downloaded from pubs.acs.org on July 19, 2019
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High yield preparation of exfoliated 1T-MoS2
with SERS activity
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Engin Er,†,‡ Hui-Lei Hou,† Alejandro Criado,*,† Judith Langer,† Marco Möller,† Nevin Erk,‡
Luis M. Liz-Marzán,†,§ Maurizio Prato*,†, ,
§ ‖
† CIC biomaGUNE, Paseo de Miramón, 182, Donostia-San Sebastián 20014, Spain
‡ Department of Analytical Chemistry, Faculty of Pharmacy, Ankara University, Ankara
06560, Turkey
§ Ikerbasque, Basque Foundation for Science, Bilbao 48013, Spain
‖ Department of Chemical and Pharmaceutical Sciences, Università degli Studi di Trieste, Via
Licio Giorgieri 1,Trieste 34127, Italy
ABSTRACT: Molybdenum disulfide (MoS2), a promising two-dimensional transition metal
dichalcogenide (TMD), presents a challenge in the tuning of its optoelectronic and chemical
properties. Herein, we demonstrate an efficient route to alter the crystalline structure of MoS2
by chemical exfoliation. Using NaK metal alloys, exfoliated and covalently functionalized
MoS2 derivatives were obtained with high metallic (1T) phase ratio, up to 94.5%.
Consequently, exfoliated MoS2 showed significant surface enhanced Raman scattering
(SERS) activity towards Rhodamine 6G (R6G) and Crystal violet (CV), with low detection
limits. The versatility of this approach allows the covalent functionalization of MoS2 without
relying on edge or basal-plane defects of the structure and preserving the high-ratio 1T phase.
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INTRODUCTION
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Since the discovery of graphene, two-dimensional (2D) layered transition metal
dichalcogenides (TMDs) have attracted great attention due to their interesting properties in
various application fields, such as electronic and sensor devices, energy-storage, catalysis and
composite materials.1–5 Molybdenum disulfide (MoS2), one of the most interesting 2D
materials, features properties such as a direct band gap (1.9 eV) and high carrier mobility
(200–500 cm2 V-1 s-1), as well as interesting optical properties, such as surface enhanced
Raman scattering (SERS) and high thermal stability.6,7
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MoS2 is commonly found as a polytype involving the 2H phase (2H-MoS2) with
semiconducting properties and the 1T phase (1T-MoS2) with metallic properties.8 1T-MoS2
features a conductivity up to 107 times higher than 2H-MoS2, as well as superior charge
transfer ability. Therefore, the metallic phase of MoS2 may find applications including
supercapacitors, hydrogen evolution reaction (HER) and SERS.9–12
SERS is one of the most selective and ultrasensitive analytical techniques in bioimaging
and biosensing,13 due to the unique vibrational fingerprints of the target molecules and low
detection limits. The common SERS phenomenon mainly relies on the so-called
electromagnetic enhancement (EE) via localized surface plasmon resonances induced at the
surface of metal nanostructures to improve the sensitivity. On the other hand, chemical
enhancement (CE) in SERS, which is based on interface dipole-dipole interactions at the
substrate as well as charge-transfer (CT) resonances between substrate and analyte, is
considered as the dominant enhancement mechanism in 2D materials such as graphene and
TMDs.14,15 In this context, MoS2 has been shown to display unusually efficient SERS
enhancing properties,16 the dominant contribution being supported by charge-transfer
resonances. The charge-transfer mechanism for different dye molecules (copper
phthalocyanine, CuPc, rhodamine 6G, R6G, and crystal violet, CV) has been described as a
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two-step process where: (I) electrons are excited from the HOMO into the LUMO of the dye,
leaving holes in the HOMO level, and subsequently (II) electrons migrate from the valence
band edge of the MoS2 material into the HOMO, thus recombining with holes.17 Thereby, the
SERS response significantly depends on the phase state in which the material is presented.
Whereas step (II) in the semiconductor phase requires an extra energy to transfer electrons
from the low-lying valence band edge, in the metallic 1T phase the higher-lying Fermi
electrons could migrate without extra energy into the HOMO. The resulting higher electron
transition probability leads to a higher SERS enhancement.10,17
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The preparation of the MoS2 1T-phase with high purity is thus crucial toward improving
applications that require high conductivity and effective charge transfer. However, current
methods do not allow the production of a pure 1T phase MoS2 material by any simple
approach, due to stability issues and moderately effective processes.8,18,19 Ding et al. produced
a pure MoS2 1T phase by a hydrothermal process in the presence of a magnetic field, using
harsh conditions.20 In this approach, the considerably high magnetic field (up to 10T) and
temperature used in the synthesis of 1T-MoS2 hinder the applicability of this method and the
large-scale production of 1T-MoS2. On the other hand, the 1T phase (octahedral), which is
metastable, readily converts into the stable 2H phase (trigonal prismatic). This limitation has
hindered progress regarding the preparation of 1T-MoS2. Existing approaches include
chemical exfoliation, electron-beam irradiation, electron transfer of plasma hot electrons,
mechanical strain, colloidal synthesis and hydrothermal reaction.8,18 Nevertheless, most of the
reported methods were shown to produce only mixtures of 1T and 2H phase MoS2
nanomaterials.
In general, the most useful and effective methodology to prepare 1T-MoS2 on a large scale
is chemical exfoliation.19,21,22 In this approach, metal ions (M: Na, K or Li) are intercalated
between MoS2 layers, and then isolated by ultrasound-assisted hydration to achieve the 1T
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phase in high yield. The intercalating agents lead to partial conversion of the crystal structure
from the 2H to the 1T phase, due to electron transfer from the metal ions.23 The highest
reported 1T phase ratio of MoS2 exfoliated by n-butyllithium (n-BuLi), the most common
intercalating agent for TMD exfoliation, has been ~60-65%.24,25 Recently, the electrochemical
exfoliation of MoS2 has also performed by lithium ion intercalation under ambient conditions,
and the corresponding 1T phase ratio was only 40%.26 Interestingly, even though metal alloys
comprising Na and K are rich electron sources, they remain unexplored in terms of phase
engineering for TMDs.27
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We report herein a highly efficient and simple route to obtain 1T-MoS2 by metal intercalation
with NaK alloys, followed by surface functionalization without using structural defects. This
reducing agent, which has excessive negative charges, facilitates MoS2 exfoliation, with a
significantly higher ratio of 1T phase compared to previous reports.19,24,25 Consequently, the
exfoliated 1T-MoS2 material was used as a SERS platform for the detection of various probe
molecules. Our SERS results demonstrate that 1T-MoS2 significantly increases the Raman
signal of probe molecules through efficient charge transfer.10 Furthermore, the obtained
negatively charged 1T-MoS2 layers feature the versatility to chemically modify the MoS2
structures for potential sensing applications.
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Scheme 1. Schematic representation of chemical exfoliation and functionalization for MoS2
with high 1T phase ratio.
EXPERIMENTAL SECTION
General. MoS2 (~6 µm) powder, iodine (I2; 99.8%), n-butyllithium (2.5 M in hexane), 4-
fluoroaniline (99%), 4-choloroaniline (98%), 4-bromobenzenediazonium tetrafluoroborate
(96%), 4-nitrobenzenediazonium tetrafluoroborate (97%) were purchased from Sigma-
Aldrich. All chemicals including organic solvents used in the reaction and purification were
purchased from either Sigma-Aldrich or Alfa-Aesar and used as received. Bulk-MoS2 was
prepared from a suspension of MoS2 powder (5 mg) in MilliQ water (5 mL). The resulting
suspension was then sonicated (Ultrasons-H Selecta) for 1h. Centrifugation was carried out
(Heraeus Megafuge 16R) at 500 rpm to precipitate the larger flakes. After centrifugation, the
top two-thirds of the dispersion were extracted by a pipet. The obtained suspension was
filtered (PTFE Millipore filter, 0.45 m pore size) and dried under vacuum to obtain Bulk-
MoS2 (1 mg).
Chemical exfoliation. MoS2 powder was dried at 150 °C under vacuum overnight prior to
use. Our chemical exfoliating 1T-MoS2 preparation was a modification of the method
reported by Feng et a.l27. Briefly, Na (35 mg, 1.5 mmol) and K (59 mg, 1.5 mmol) metals
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were mixed 1,2-dimethoxyethane (DME) solution (20 mL) and vigorously stirred for 2h at
room temperature to obtain a liquid NaK alloy dispersion with a blue color. Then, MoS2
powder (160 mg, 1 mmol) is added to this solution and the mixture is allowed to stir for 24 h.
The reaction is then quenched by the addition of I2 (761 mg; 3 mmol) to the stirring solution.
The mixture is washed three times with cyclohexane:water (1:1, v/v) solution and then filtered
thoroughly using PTFE (0.45 μm) membrane filters with DMF (4 x 30 mL), ethanol (2 x 20
mL), acetone (2 x 20 mL) and water (2 x 20 mL).
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For comparison, the chemical exfoliation of MoS2 was also performed by n-BuLi, in a similar
method as reported by Knirsch et al.24 3 mL of n-BuLi solution (2.5 M in hexane) is added to
the 200 mg MoS2 suspension in dry hexane (10 mL) and allowed to stir under an inert
atmosphere for 24 h. The black precipitate was filtered thoroughly with dry hexane to remove
the unreacted n-BuLi and organic impurities and followed by quenching of the reaction with
100 mL of Milli-Q water. The resulting MoS2 suspension was centrifuged at 750 rpm for 1.5h
to remove the non-exfoliated material, and then filtered thoroughly using PTFE (0.45 μm)
membrane filters with Milli-Q water.
Diazonium salt synthesis. 4-fluorobenzenediazonium tetrafluoroborate and 4-
chlorobenzenediazonium tetrafluoroborate were synthesized from precursors such as 4-
fluoroaniline and 4-choloroaniline following published diazotization procedure (Scheme
S1).28 Firstly, 4-fluoroaniline (1 mmol) was added to into HBF4 aqueous solution (0.4 mL).
Then, 40 mL acetic acid was added and isoamlynitrite (0.4 mL; 3 mmol) solution dissolved in
20 mL acetic acid was slowly dropped to the mixture. The reaction was stirred for 15 min at
room temperature. The mixture was then quenched by adding of 40 mL diethyl ether solution
and stored at -22 °C overnight. The resultant product was filtered via PTFE filter (0.45 µM)
and then washed several times with diethyl ether. The obtained 4-fluorobenzenediazonium
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tetrafluoroborate was dried under vacuum at room temperature. The 4-
chlorobenzenediazonium tetrafluoroborate was prepared by same diazotization procedure as
presented above. The FTIR spectra of 4-fluorobenzenediazonium tetrafluoroborate and 4-
chlorobenzenediazonium tetrafluoroborate show the characteristic bands of diazonium salt at
around 2270 cm-1 as presented in Figure S1.
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Covalent functionalization. For covalent functionalization of MoS2, MoS2 intercalated with
NaK alloy as described in the chemical exfoliation paragraph was reacted with 4-
fluorobenzenediazonium tetrafluoroborate (630 mg; 3 mmol), 4-chlorobenzenediazonium
tetrafluoroborate (680 mg; 3 mmol) 4-bromobenzenediazonium tetrafluoroborate (812 mg; 3
mmol) and 4-nitrobenzenediazonium tetrafluoroborate (711 mg; 3 mmol), respectively. After
the reaction, each of the resulting precipitates were filtered thoroughly using PTFE (0.45 μm)
membrane filters with DMF (4 x 30 mL), ethanol (2 x 20 mL), acetone (2 x 20 mL) and water
(2 x 20 mL). The exfoliated and functionalized MoS2 materials were then dried under vacuum
for the structural and morphological characterizations.
Characterization of exfoliated materials. To evaluate the exfoliation of bulk crystal of
layered MoS2 precursor (Bulk-MoS2) the exfoliated materials where measured by X-ray
diffraction (XRD) and transmission electron microscopy (TEM). XRD analyses were carried
out in a Bruker D8 Advance diffractometer with X-ray source of CuKalpha (1600W). The
diffractometer presents Bragg-Brentano theta-theta configuration for powder samples. TEM
images were obtained using a JEOL JEM-1400PLUS electron microscope at 120 kV equipped
with a GATAN US1000 CCD camera. High-resolution TEM (HRTEM) images and selected
area electron diffraction patterns (SAED) were acquired with a JEOL JEM-2100F UHR
electron microscope at 200 kV equipped with aTVIPS F-216 CMOS camera. The TEM
samples were prepared by drop-casting sample solutions on the ultra-thin carbon film coated
Cu-grids (Ted Pella Inc., USA) and dessicated under ambient conditions.
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The change of crystallinity was detected by Raman spectroscopy, X-ray photoelectron
spectroscopy (XPS) and UV-Vis spectroscopy (UV). Raman spectra were recorded with a
Renishaw Invia Raman microscope equipped with laser wavelengths λ= 532, 633 and 785 nm,
a lens-based spectrometer with two gratings (1800 gr/mm for vis and 1200 gr/mm for NIR
and a Peltier-cooled front-illuminated CCD (1024 px x 532 px). XPS measurements were
performed in a SPECS Sage HR 100 spectrometer with a nonmonochromatic X ray source of
Aluminium with a Kα line of 1486.6 eV energy and 300 W. XPS measurements were
performed in a SPECS Sage HR 100 spectrometer with a nonmonochromatic X ray source of
Aluminium with a Kα line of 1486.6 eV energy and 300 W. The fitting of the XPS data were
applied using CasaXPS software. The quantification of 1T phase was calculated by the
deconvolution of Mo 3s core levels. UV-visible spectra were performed in a Jasco V-630 BIO
or Varian Cary 5000 spectrophotometer.
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The chemical modification of MoS2 was characterized by Raman spectroscopy, XPS,
thermogravimetric analysis (TGA) and Fourier-transform IR spectroscopy (FTIR). TGA
measurements were performed on a TA Instruments Discovery system under air starting from
100 ºC with ramp of 10 ºC /min up to 800 ºC after an isotherm at 100 ºC for 20 min. The
functionalization degree (FD) for all functionalized samples was calculated by considering the
decomposition of MoS2 at 650 °C in the corresponding TGA profiles (Eq.1). FD is expressed
in terms of number of MoS2 molecules per introduced functional group. R and L are
corresponded to the weight loss (%) and residue (%) observed at 650 °C, after having
subtracted the analogous loss from the pristine MoS2. The expected moiety on the MoS2
surface is expressed as (Mw), and the molecular weight of MoS2 determined as 160.07 g/mol.
푅(%) 푀 (푔/푚표푙)
‧
푤
(Eq. 1)
퐹퐷 = 퐿(%) 160.07(푔/푚표푙)
‧
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The FTIR characterization was carried out in transmission mode by means of a Nicolet
6700 spectrometer. For this purpose, solid MoS2 powders (2 mg) were mixed with 200 mg
KBr matrix stored in the oven at 80 ºC, pestled and pressed to discs (diameter 13 mm) under
154 MPa using KBr pellet kit and hydraulic press (Specac).
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Elemental analysis was performed using a TRUSPEC CHNS microanalyzer, which is able
to determinate the contents of C, H, N and S by combustion of the sample at 1100 °C under
oxygen atmosphere and with the quantitative detection of the combustion products. Before
sample analysis, the determination of blanks were performed (in absence of samples) to check
the detection levels of the device. The maximum values of the verified blanks are the
following: 0.06 % for C, 0.16% of H, 0.02% for N, 0.35% for S.
SERS measurements. The SERS substrates were prepared on 300 nm SiO2/Si surfaces
after cleaning procedure with ethanol and Milli-Q water. NaK-MoS2 and Li-MoS2 were
dispersed in Milli-Q water for 1 h, and then MoS2 dispersions (1 mg mL-1) were covered by
drop casting onto the cleaned SiO2/Si (~25 mm2) surfaces. The substrates were dried at room
temperature and then incubated in the different concentrations of R6G and CV solutions for
2h. The R6G and CV deposited MoS2 substrates were washed with Milli-Q water to remove
the free R6G or CV and dried under nitrogen atmosphere. Bulk-MoS2 was also exfoliated by
sonication for 1h to obtain few-layered material for comparison of SERS activity with NaK-
MoS2 and Li-MoS2. Afterwards, Bulk-MoS2 substrate was also prepared according to the
same preparation procedure mentioned above for SERS measurements. The SERS spectra of
so-prepared Bulk-MoS2, NaK-MoS2 and Li-MoS2 substrates were recorded using a 100x
objective (NA=0.85) at excitation wavelengths of 532 nm (0.9 mW power laser, 30s of
integration time), 633 nm (0.9 mW power laser, 30s of integration time) and 785 nm (1.7 mW
power laser, 10s of integration time in scanning mode). The presented spectra were baseline
corrected and normalized to the same laser power and integration time. The enhancement
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factor (EF) value of the fabricated MoS2 substrates were estimated according to the Equation
S1.29
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RESULTS AND DISCUSSIONS
Phase engineering by chemical exfoliation. Exfoliated and functionalized MoS2 sheets were
synthesized using NaK alloy intercalation (Scheme 1), as detailed in the Experimental
section. Briefly, the exfoliation mechanism of Bulk-MoS2, which is a 2H polytype, can be
explained by the intercalation of cations between layers of MoS2.27 Firstly, Bulk-MoS2 is
reduced with solvated electrons, which are generated from the very potent electron source
NaK alloy dissolved in DME at room temperature. The initially formed solvated electrons
would be consumed by the Bulk-MoS2, and further reduction proceeds until the maximum
ratio of cations to MoS2 is reached. Thus, the exfoliation process is produced by Coulombic
repulsion between layers of reduced MoS2. The negative charges on MoS2 are balanced by
solvated and intercalated Na+ or/and K+ cations. In addition, the phase of TMDs depends
strongly on the d orbital electron density of the transition metal.8 As a consequence of the
reduction process of Bulk-MoS2, the electron density in the d orbital is increased, leading to a
strong structural reorientation, from the thermodynamically more stable semiconducting 2H
polytype to the metallic 1T polytype. Finally, the negatively charged MoS2 is quenched by I2
as electrophile to obtain an exfoliated material NaK-MoS2 with a high proportion of 1T phase.
For comparison purposes, MoS2 was also exfoliated by n-BuLi (Li-MoS2) as a conventional
intercalating agent for MoS2 exfoliation.21
Chemically exfoliated MoS2 was studied by XRD, Raman spectroscopy, XPS, UV, TGA,
and TEM. The chemical exfoliation of MoS2 was confirmed by XRD. Figure S2 indicates
typical 2θ values of bulk MoS2 and NaK-MoS2, with the characteristic sharp (002) diffraction
peak of Bulk-MoS2 at 14.5°, shifted to 14.2° and broadened (Figure 1A), indicating the
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expansion of the interlayer distance and successful exfoliation of MoS2.11,30 In addition, TEM
images of NaK-MoS2 demonstrated few-layers of MoS2 with relatively small lateral
dimensions, in the nanometer size range (Figure S24B). HRTEM images showed well-
ordered and well-oriented crystalline structuring in the sample, which allows the acquisition
of SAED patterns from individual sheets (Figures S25). The crystal structure of NaK-MoS2
was determined by selected area electron diffraction (Figure S25B), confirming the lattice
spacing for our structure to coincide accurately with published reference data for MoS2.31–33
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Raman spectroscopy was used to characterize the change in crystallinity. Figure 1B shows
the Raman spectra of Bulk-MoS2 and NaK-MoS2 excited at 633 nm. In the spectrum of Bulk-
MoS2, we observed the two main phonon peaks located at ~380 cm-1 and ~405 cm-1, arising
from the E2g (in-plane) and A1g (out-of-plane) vibration modes of MoS2, respectively. In
addition, a typical second-order longitudinal acoustic phonon peak (2LA(M)) around ~450
cm-1 was also detected. After chemical exfoliation, new peaks were observed at 187, 224, 289
cm-1, which correspond to the J1-J3 phonon modes of NaK-MoS2, respectively. These low-
frequency phonon modes confirm that NaK-MoS2 has 1T phase features, and is in line with
the characterization of Li-MoS2 (Figure S3) and with previously reported values.24,34
In the UV spectrum of Bulk-MoS2 (Figure S4), the two excitonic A and B bands, which
are related to the presence of a highly ordered structure, were observed.35,36 These
characteristic bands disappeared after exfoliation of MoS2 by NaK alloy, confirming a change
in the crystallinity from 2H to 1T MoS2.17
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Figure 1. (A) Magnified XRD patterns of the (002) peaks of Bulk-MoS2 and NaK-MoS2
crystals; (B) Raman spectra of Bulk-MoS2 and NaK-MoS2 obtained by excitation at 633 nm;
(C-D) XPS core level spectra of Mo 3d (~25 meV shift) and S 2p (~24 meV shift) for Bulk-
MoS2 and NaK-MoS2.
XPS analysis was employed to obtain the MoS2 1T/2H ratio, by fitting Mo 3d and S 2p
core levels. Sharp Mo 3d signals at 233.05 (Mo 3d3/2) and 229.92 (Mo 3d5/2) were observed in
Bulk-MoS2; however, the corresponding signals after exfoliation were shifted (~25 meV) to
lower binding energies, suggesting the existence of both 1T and 2H phases after exfoliation
(Figure 1C-D). The 1T phase was identified by a shift (~0.8 eV) with respect to that for 2H
phase in the fitting of Mo 3d for NaK-MoS2. A similar XPS result was obtained for Li-MoS2
(Figure S5); but deconvolution of the Mo 3d peak shows two new peaks for MoO3 and MoO2
in Li-MoS2, whereas NaK-MoS2 does not display any significant peak from oxidized products
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of MoS2. We hypothesize that this may be due to the quenching agent (H2O) used in the
preparation of Li-MoS2.24,34,37
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Table 1. Ratio of 1T phase for exfoliated and functionalized samples, based on integrated
areas of Mo 3d core level spectra. [a] Bibliographic data from reference 21. Li-MoS2
additionally presents 22% of MoO3 and MoO2.
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Sample
2H (%)
100
1T (%)
-
Bulk-MoS2
NaK-MoS2
5.5
94.5
59.0
88.8
86.1
88.9
79.3
[a]
Li-MoS2
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13.9
11.1
20.7
MoS2/p-(F)Ph
MoS2/p-(Cl)Ph
MoS2/p-(Br)Ph
MoS2/p-(NO2)Ph
Upon deconvolution of the corresponding Mo 3d core level, the 1T phase ratio was found
to be ~94.5% for NaK-MoS2 (Table 1). Such a high value indicates that NaK alloy is a
suitable intercalating agent, more efficient than Li, and single Na or K.19,22 Several factors are
crucial for successful exfoliation and phase conversion of TMDs. For instance, Loh et al.
reported the high influence of the cation species of alkali metal naphtalenides in the
exfoliation efficiency of TMDs, Na+ being more efficient than Li+ and K+.19 More recently,
Sofer et al. also reported that the cation size of intercalating agents is a dominating factor for
phase engineering, smaller cation species showing a better exfoliation and 1T phase
conversion for electrochemically exfoliated TMDs.22 However, other parameters such as
dispersibility of intercalating agents must be considered for an efficient chemical exfoliation
and phase conversion. Comparing NaK alloy with single Na or K, the alkali alloy (which is a
liquid) is more easily dispersed in solution than Na or K solid metals at room temperature
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under stirring conditions, thus leading to higher intercalation and conversion efficiencies than
individual Na or K.
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SERS performance. To show the effect of the phase transition from 2H to 1T on the
SERS efficiency, a NaK-MoS2 substrate was prepared on 300 nm SiO2/Si surface and
compared to MoS2 exfoliated by n-BuLi (Li-MoS2),21 and to Bulk-MoS2, in the detection of
rhodamine 6G (R6G) and crystal violet (CV) as target analytes. Figure 2A,B displays the
SERS spectra of CV and R6G on three MoS2 modifications excited at 532 nm. The main
vibrations of CV were recorded at 1618, 1583, 1378, 1175 (symmetric and asymmetric in-
plane CC ring stretch modes), and 914 cm-1 (out-of-plane CC ring bending mode). The most
intense R6G vibrations were found at 1647, 1575, 1363 (in-plane CC ring stretch modes),
775, 611 cm-1 (out-of-plane CC ring bending modes). The results indicate that exfoliated
MoS2 samples provided a significantly higher SERS enhancement than Bulk-MoS2 for both
probe molecules (Table 2). The enhancement factor (EF) of the prepared substrates was
calculated to evaluate their SERS efficiency using 10 µM Raman dye solutions, according to
the Equation S1 (details are given in Supporting information). NaK-MoS2 showed an EF one
order of magnitude higher than Bulk-MoS2 and two times higher than Li-MoS2, for both R6G
and CV. Interestingly, the difference between the EFs of NaK-MoS2 and Li-MoS2 practically
coincides with the difference between their 1T ratios, indicating a higher conductivity which
corresponds to a higher-lying Fermi level and provides more efficient charge transfer from the
substrate to probe molecules.38,39
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Figure 2. SERS spectra of 10 µM R6G (A) and CV (B) on Bulk-MoS2 (a), Li-MoS2 (b) and
NaK-MoS2 (c) substrates, excited by 532 nm laser; SERS spectra of various concentrations of
R6G (C) and CV (D), deposited on NaK-MoS2.
The SERS performance of NaK-MoS2 substrate was also estimated by evaluating the
detection limits for R6G and CV. As shown in Figure 2C-D, the SERS intensity is directly
correlated to the concentrations of Raman probes. Although Yin et al. and Anbazhangan et al.
reported the use of MoS2 with high 1T-phase ratio, as SERS substrates for R6G and CV
detection,10,17 we demonstrate here superior SERS sensitivities for both R6G and CV, using
NaK-MoS2. The lowest detected concentrations for R6G and CV on NaK-MoS2 were as low
as 10-9 M and 10-8 M, respectively, which compare favorably with previously reported
studies,10,17,40,41 and are even comparable to efficient metallic substrates.42,43
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Table 2. Calculated enhancement factors (EFs) for Bulk-MoS2, Li-MoS2 and NaK-MoS2
substrates on different Raman dyes.
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Dyes Raman Shift (cm-1)
Bulk-MoS2
Li-MoS2
NaK-MoS2
R6G
CV
611
2.04 x 103
2.12 x 103
1.73 x 104
1.39 x 104
2.92 x 104
2.87 x 104
1618
Trying to understand in more depth the origin of such an unusually strong SERS effect for
NaK-MoS2, UV, photoluminescence (PL) and Raman scattering were measured. As
mentioned above, contributions to SERS may be of both electromagnetic nature (EM), due to
excitation of localized surface plasmons, and of chemical nature (CE), due to the probe-
substrate interactions providing charge-transfer and molecular resonances. The EM
mechanism can be nearly excluded because of the absence of strong plasmons or excitons in
the UV-vis spectrum (Figure 3 and S4). The quenching of the intrinsic absorbance and
photoluminescence (PL) of CV and R6G with increasing NaK-MoS2 concentration indicates a
strong coupling between probe and substrate, leading to effective (non-radiative) charge-
transfer resonances (Figure S10-13). Measurements of R6G and CV using 532, 633 and 785
nm excitation wavelengths revealed that only strong SERS signals were recorded when the
laser was in resonance with an electronically excited state of the probe, or close to it. In
addition, we note that no Raman signals were detected when using other probe molecules
such as mercaptobenzoic acid (data not shown), which can be readily measured on metal
nanostructures.44–46 In conclusion, all these experimental data suggest as the major cause of
SERS is the CE mechanism. The molecular resonances were identified by UV-vis
spectroscopy to be around 527 nm in R6G (Figure 3A) and around 595 nm, with a prominent
shoulder around 545 nm, in CV (Figure 3B). The difference between the UV-Vis spectra of
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NaK-MoS2 in presence and absence of the dyes did not display new resonances close to the
excitation wavelengths, but in the case of CV the 7 nm (blue) shift of the 545 nm shoulder
indicated significant binding of the molecule to the surface concomitantly pushing the
resonance maximum close towards the 532 nm excitation wavelength. This, in turn, could
explain the relatively high SERS intensity of CV at 532 nm.
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Figure 3. UV-vis absorbance spectra of dye-NaK-MoS2 (red line) and pure NaK-MoS2
(black line) suspensions compared to pure dye solutions (green line) containing 1 M R6G
and 20 mgml-1 NaK-MoS2 (A) and 1 M CV and 20 mgml-1 NaK-MoS2 (B). Dashed, vertical
lines (grey) highlight available laser excitation wavelengths. The difference spectrum between
the dye-NaK-MoS2 and NaK-MoS2 (purple lines) reveal that the excitation of the molecular
resonances of adsorbed dyes is partially quenched by the presence of NaK-MoS2 indicating
effective interaction with the MoS2 substrate.
The SERS mechanism can thus be explained by the excitation of a molecular resonance
coupled with electron transfer from the Fermi level of the substrate into the molecule. As the
Fermi level EF of NaK-MoS2 (EF = -5.0 eV) lies energetically above the HOMO of both dyes
(EHOMO, CV = -6.0 eV and EHOMO, R6G = -5.7eV),17 the subsequent charge transfer from EF
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to the HOMO does not require extra energy and thus is strongly favored. This eventually
results in a greatly enhanced electron transition probability, leading to high signal intensities
for R6G and CV at 532 nm. At 633 nm excitation, a pre-resonance condition originates strong
CV intensities whereas no R6G signal can be measured because it is off-resonance with the
molecular excitation. The absence of electronically excited states in both dye molecules at
785 nm and the consequently missing SERS signals are in line with this interpretation (Figure
S6). Further, the extraordinarily large enhancement of the vibrational out-of-plane modes at
611 and 775 cm-1 in R6G and the mode at 914 cm-1 in CV, compared to the in-plane modes,
are related to vibrionic coupling between a molecular resonance and charge-transfer state.47,48
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The reproducibility and signal homogeneity are also important parameters for SERS
substrates in routine analytical applications. The homogeneity of the SERS signals for R6G at
611 cm-1 and CV at 1620 cm-1 on NaK-MoS2 was studied by Raman mapping (Figures S7-9).
To evaluate the reproducibility for R6G and CV, we randomly collected SERS spectra from
20 different positions on the same NaK-MoS2 substrate (Figure S8-9). The results indicate
uniform and reproducible SERS signals over the whole surface, which is likely related to
homogenous and efficient capture of the probe molecules.
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Figure 4. (A) Raman spectra of Bulk-MoS2 MoS2/p-(F)Ph, MoS2/p-(Cl)Ph, MoS2/p-(Br)Ph
and MoS2/p-(NO2)Ph at 633 nm; (B) TGA profiles of Bulk-MoS2, NaK-MoS2, MoS2/p-(F)Ph,
MoS2/p-(Cl)Ph, MoS2/p-(Br)Ph and MoS2/p-(NO2)Ph from 100 °C to 650 °C under air; Core
level spectra of (C) F 1s, (D) Cl 2p (E) Br 3d and (F) N 1s for MoS2/p-(F)Ph, MoS2/p-(Cl)Ph,
MoS2/p-(Br)Ph and MoS2/p-(NO2)Ph.
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Chemical modification. In addition, chemical exfoliation by intercalation of metal ions is
highly versatile because it allows carrying out chemical modifications on the exfoliated MoS2
structure.7 The chemical modification of 2D-based materials is of high interest, as it allows
modifying chemical and physical properties such as conductivity, stability and reactivity.49,50
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In general, the functionalization of MoS2 is typically achieved by exploiting sulfur
vacancies or defects occurring on the surface during chemical exfoliation. Recently, E. M.
Pérez et al. reported a versatile mild covalent functionalization at basal planes of exfoliated
2H-MoS2 and WS2 through the nucleophilic attack of sulfur atoms from the metal transition
dichalcogenide structure on maleimide derivatives.51 M. Pumera et al. reported a highly
efficient modification of MoS2 in water with thiobarbituric acid by nucleophilic attack of its
sulfur on the exfoliated MoS2 material.52 1T-MoS2 presents a higher chemical reactivity
towards functionalization, especially at basal sites, as compared to 2H-MoS2.53 Voiery et al.
successfully functionalized the surface of 1T-MoS2 via the covalent attachment of
organohalide functional groups.25 Chemically exfoliated MoS2 can also be covalently
functionalized by aryl diazonium compounds. Knirsch et al. and Benson et al. reported a well-
known chemical approach, which does not rely on structural defects, to functionalize the
surface of chemically exfoliated 1T-MoS2 with substituted phenyl diazonium salts.24,34 Thus,
the negatively charged 1T phase MoS2 nanolayers obtained by treatment with NaK alloy were
functionalized with different para (p)-substituted aryl diazonium salts, thereby demonstrating
the versatility of the proposed reaction on the MoS2 surface. For this purpose, we selected
four para-substituted (R: F, Cl, Br and NO2) aryl diazonium salts with heteroatoms because
they can be readily analyzed by XPS. We refer to MoS2 functionalized with para F, Cl, Br or
NO2-phenyl rings as MoS2/p-(F)Ph, MoS2/p-(Cl)Ph, MoS2/p-(Br)Ph or MoS2/p-(NO2)Ph,
respectively. It is worth mentioning that, in principle, substituents of phenyl groups would not
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be reduced by the NaK alloy during the reaction, since the same amount of diazonium
compounds and NaK alloy were used, and the diazonium group is more readily reduced.34,53
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Chemically functionalized MoS2 derivatives were characterized by Raman spectroscopy,
XPS, FTIR, TGA and TEM. In Raman spectroscopy, all functionalized MoS2 samples were
found to display the expected transitions around at 376 cm-1 (E2g) and 402 cm-1 (A1g) and also
to have weak Raman peaks for J1, J2 and J3 modes, indicating the presence of the 1T-phase
after functionalization (Figure 4A).34 The covalent functionalization of MoS2 layers was also
confirmed by FTIR (Figure S14). All functionalized samples showed the characteristic C=C
stretching bands of benzene rings located around 1640 cm-1 and the weak C-S stretching
bands between 620 and 690 cm-1, suggesting that the benzene moieties were covalently
attached to the MoS2 surface via C-S bonds. Additionally, MoS2/p-(NO2)Ph presented two
main bands at 1504 cm-1 and 1330 cm-1, which were attributed to characteristic asymmetric
and symmetric stretching of a nitro group attached to the phenyl moiety.
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The thermal properties and functionalization degree of chemically exfoliated and
derivative MoS2 samples were investigated by TGA in air. Figure 4B shows that 2H-MoS2
started to lose weight at approximately 350 °C, which is attributed to partial oxidation of
MoS2 into MoO3. In addition, the surface functional groups were decomposed in the range of
200 to 500 °C, as expected. The second rapid weight loss (>50% wt.) in all samples was
attributed to the decomposition of MoS2 by sublimation of MoO3 (Figure S16).54,55 The FD
for all functionalized samples was calculated by considering the decomposition of MoS2 at
650 °C, according to Eq. 1.
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Table 3. Functionalization degree of 1T-MoS2 nanosheets obtained by TGA.a Determined
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Weight loss (%)a
FDb
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p-NO2-C6H4
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5.9
p-F-C6H4
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8.6
p-Cl-C6H4
p-Br-C6H4
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The results suggest that the reactivity of the diazonium compounds is related to the
substituted groups in the phenyl ring (Br < Cl < F < NO2). This trend can be explained by the
single-electron transfer reaction mechanism from negatively charged MoS2 to the diazonium
compound, being reduced to the corresponding aryl radical.44 The lower reduction potentials
of the employed diazonium salts are caused by the stronger electron-withdrawing substituted
groups in the phenyl ring (Br < Cl < F < NO2) improving the electron transfer process, which
is in agreement with previous results for carbon nanotubes.56
XPS analysis was used to study the atomic composition of MoS2 surfaces (Figure S17-22).
XPS was first employed to provide evidence for the functionalization of MoS2 derivatives, by
identifying and quantifying the functional groups anchored to the surface (Table S2). Bulk-
MoS2 is not expected to contain C and O in the bulk material; however, undesired C and O
content probably arises from CO and CO2 species in air, adsorbed on the substrates. Such C
and O contents were detected by XPS in the starting material and in the rest of functionalized
MoS2 samples, with the same proportion. The presence of F, Cl, Br and N was detected after
chemical modification in the atomic composition of MoS2/p-(F)Ph (1.0%), MoS2/p-(Cl)Ph
(0.9%), MoS2/p-(Br)Ph (0.7%) and MoS2/p-(NO2)Ph (3.8%), respectively. Besides, the
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deconvolution of F 1s, Cl 2p, Br 3d and N 1s for MoS2/p-(F)Ph, MoS2/p-(Cl)Ph, MoS2/p-
(Br)Ph and MoS2/p-(NO2)Ph, respectively, showed spectral components from C-F (687.4 eV),
C-Cl (200.9 eV), C-Br (71.0 eV) and C-NO2 (399.8 and 405.6 eV, Figure 4C-F). The
presence of these chemical bonds on the corresponding MoS2 surfaces confirmed
functionalization. The obtained XPS results are in agreement with related functionalization
approaches in the literature,24,25,34,53 confirming the presence of p-substituted benzene
functional groups on MoS2 layers. TEM images indicate that, functionalized MoS2 derivatives
keep similar dimensions to those of exfoliated material after chemical modification (Figures
S24C-F). Elemental analysis of the different modified MoS2 derivatives was performed to
calculate the functionalization degree with respect to C/S atom ratio (Table S3). The C/S
ratios of functionalized materials were higher than those for NaK-MoS2, due to the
introduction of phenyl groups on the MoS2 surfaces.
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CONCLUSIONS
In conclusion, we report a novel approach for exfoliation and covalent functionalization of
MoS2, which provides a remarkably efficient phase modification, by using NaK alloys. The
proposed exfoliation method leads to highly enriched 1T-phase MoS2 (94.5%). MoS2 was
chemically functionalized by radical reactions with different phenyl diazonium compounds,
leading to MoS2 enriched in metallic 1T phase. NaK-MoS2 revealed a remarkable SERS
enhancement on R6G and CV, due to an efficient CE mechanism, which may open the way
toward further sensing applications. Work is in progress to explore the limits of this approach,
in terms of reactivity, physical properties of the obtained enriched 1T phase MoS2 materials
(e.g. electrical conductivity, optical properties, etc.) and sensing applications.
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ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website
(http://pubs.acs.org) at DOI:
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Experimental details on the preparation and characterization of the different para-
substituted benzenediazonium tetrafluoroborates; XRD data; Raman spectra, UV-vis
adsorption spectra, XPS analysis of Bulk-MoS2, Li-MoS2 and NaK-MoS2; estimation of
the EF, SERS spectra, SERS reproducibility, UV-vis adsorption spectra, PL spectra of
NaK-MoS2 with R6G and CV; FTIR data, TGA data, Raman spectra, XPS analysis,
Combustion elemental analysis and TEM images of the different MoS2 derivatives
(PDF).
AUTHOR INFORMATION
Corresponding Authors
*E-mail: prato@units.it ; acriado@cicbiomagune.es
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
M. Prato is the recipient of the AXA Chair (2016-2023). A. Criado thanks MINECO to his
researcher grant (Juan de la Cierva - Incorporación). E. Er is grateful for a fellowship from the
Scientific and Technological Research Council of Turkey (TUBITAK). This work was
supported by the Graphene Flagship Core 2 grant agreement No. 785219. This work was
performed under the Maria de Maeztu Units of Excellence Program from the Spanish State
Research Agency – Grant No. MDM-2017-0720.
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