10.5 Top-Down Electrochemical Synthesis of Nanosheets
349
Molybdenum disulfide, MoS 2 , gained a considerable interest in electrochemical
exfoliation procedures because of possible application of its nanosheets in fieldeffect transistors [106, 107] or in supercapacitors [108]. The thickness of nanosheets
that could be produced with electrochemical exfoliation varies from a single to a
trilayer, even if the preparation methods were fundamentally different [106, 108].
“Single layer” means here a formula unit containing one Mo atomic layer in the
middle plane of a nanosheet with S-termination along both sides. Anodic exfoliation from aqueous solution proceeds through the uptake of sulphate and hydroxide
ions which can later be oxidized to SO 2 and O 2 , respectively [106, 107], similarly
to the processes taking place during the anodic exfoliation of graphene. Despite the
anodization up to a 10 V cell voltage, the surfaces of the S–Mo–S nanosheets were
not oxidized. For cathodic exfoliation of MoS 2 , the first step models the electrochemical lithium uptake process in lithium ion batteries, and the second step involves the
decomposition of the lithiated bulk MoS 2 in water. This process can be extended
to related materials like MoS 2x Se 2(1–x) and Mo x W 1–x S 2 [109]. The exfoliation can
be performed with either a pellet containing conducting carbon black powder and
polyvinylidene fluoride binder [109] or by simple pressing a binder-free pellet [106].
Electrochemical exfoliation of the MoS 2 compound family has a multifold advantage
as compared to the solution-based chemical exfoliation methods: (i) The time scale
of the experiment is at least an order of magnitude smaller in the case of the electrochemical exfoliation. (ii) The control of the extent of lithium uptake can be simply
regulated by the charge passed during the electrolysis, while chemical modifications
are rather cumbersome to control. (iii) Electrochemical lithium intercalation can be
performed in ambient conditions; hence, glove box environment is not necessary. (iv)
From the various phases that can be produced, electrochemical intercalation-based
exfoliation leads to a larger ratio of the desired 1T-MoS 2 phase. It is interesting
to note that anodic exfoliation in a water–ionic liquid mixture with dilute lithium
bis(trifluoromethylsulphonyl) imide as solvent leads to nanodots instead of large
surface area nanosheets [110]. The mechanistic background for such a difference in
the exfoliation process is yet to be clarified.
Exfoliation from Na 2 SO 4 solution was adapted to produce Bi 2 X 3 nanosheets (X:
Se or Te) [111]. In both calcogenides, the typical exfoliation product was a sheet
composed of 5 covalently bonded atomic layers (quintuple layer) with calcogenideterminated surfaces. In the exfoliation process, the precursor bulk material was a
cathode connected against a Pt counter electrode, and the potential program applied
for the cell voltage included several steps between 2 and 10 V. The structure and
the composition of the nanosheets produced were confirmed by TEM, and XPS,
respectively, indicating no damage as a result of the exfoliation.
349
Molybdenum disulfide, MoS 2 , gained a considerable interest in electrochemical
exfoliation procedures because of possible application of its nanosheets in fieldeffect transistors [106, 107] or in supercapacitors [108]. The thickness of nanosheets
that could be produced with electrochemical exfoliation varies from a single to a
trilayer, even if the preparation methods were fundamentally different [106, 108].
“Single layer” means here a formula unit containing one Mo atomic layer in the
middle plane of a nanosheet with S-termination along both sides. Anodic exfoliation from aqueous solution proceeds through the uptake of sulphate and hydroxide
ions which can later be oxidized to SO 2 and O 2 , respectively [106, 107], similarly
to the processes taking place during the anodic exfoliation of graphene. Despite the
anodization up to a 10 V cell voltage, the surfaces of the S–Mo–S nanosheets were
not oxidized. For cathodic exfoliation of MoS 2 , the first step models the electrochemical lithium uptake process in lithium ion batteries, and the second step involves the
decomposition of the lithiated bulk MoS 2 in water. This process can be extended
to related materials like MoS 2x Se 2(1–x) and Mo x W 1–x S 2 [109]. The exfoliation can
be performed with either a pellet containing conducting carbon black powder and
polyvinylidene fluoride binder [109] or by simple pressing a binder-free pellet [106].
Electrochemical exfoliation of the MoS 2 compound family has a multifold advantage
as compared to the solution-based chemical exfoliation methods: (i) The time scale
of the experiment is at least an order of magnitude smaller in the case of the electrochemical exfoliation. (ii) The control of the extent of lithium uptake can be simply
regulated by the charge passed during the electrolysis, while chemical modifications
are rather cumbersome to control. (iii) Electrochemical lithium intercalation can be
performed in ambient conditions; hence, glove box environment is not necessary. (iv)
From the various phases that can be produced, electrochemical intercalation-based
exfoliation leads to a larger ratio of the desired 1T-MoS 2 phase. It is interesting
to note that anodic exfoliation in a water–ionic liquid mixture with dilute lithium
bis(trifluoromethylsulphonyl) imide as solvent leads to nanodots instead of large
surface area nanosheets [110]. The mechanistic background for such a difference in
the exfoliation process is yet to be clarified.
Exfoliation from Na 2 SO 4 solution was adapted to produce Bi 2 X 3 nanosheets (X:
Se or Te) [111]. In both calcogenides, the typical exfoliation product was a sheet
composed of 5 covalently bonded atomic layers (quintuple layer) with calcogenideterminated surfaces. In the exfoliation process, the precursor bulk material was a
cathode connected against a Pt counter electrode, and the potential program applied
for the cell voltage included several steps between 2 and 10 V. The structure and
the composition of the nanosheets produced were confirmed by TEM, and XPS,
respectively, indicating no damage as a result of the exfoliation.