10.5 Top-Down Electrochemical Synthesis of Nanosheets
347
the production of single but solvent-encapsulated graphene layers, although standalone graphene in only 0.335 nm thick. The appearance of layers with thickness much
larger than 1 nm definitely means that a few layers exfoliated together [79, 82–84]
(although the folding of the layers may produce artefacts when the samples are posttreated for thickness measurements either by AFM or TEM). When the dependence
of the quality of the exfoliated products was studied, it was observed that higher cell
voltages are preferred for the specific solution used [85].
10.5.4 Electrochemistry-Based Transfer Methods
of Graphene and Graphene Oxide Nanosheets
Almost 15 years after the isolation of a single graphene sheet, in spite of the extraordinary electron mobility coupled with transparency, the adoption of the graphene
technology in industry proceeds very slowly. One of the main bottlenecks using
chemical vapour deposition (CVD) for graphene production is the lack of a fully
reliable graphene transfer technique which would serve as an interface between the
CVD process taking place on one substrate and the application demanding another
substrate of different type. The delamination of graphene sheets, especially with a
surface area of industrial scale, is still challenging. Among others, electrochemical
techniques have been developed for delaminating single graphene sheets produced
on transition metal catalysts.
The major electrochemical technique used for peeling off a CVD-produced single
graphene sheet from its metal substrate is the so-called electrochemical bubbling
transfer. Before the electrochemical treatment, the graphene layer is covered with
a support layer [typically poly(methylmethacrylate)] in a manner that a part of the
uncovered metal substrate is free and is available for contacting an electrolyte solution. Then, this workpiece is immersed in an electrochemical cell and is connected as
cathode, and hydrogen generation is started. The hydrogen bubble formation delaminates the plastic cover layer from the metal together with the graphene sheet, i.e.,
the attack point of the stress is between the graphene sheet and the metal catalyst
used for graphene growth. It is evident that the metal surface and its contact with the
electrolyte solution play a key role in the bubble transfer process. This technique was
reported to work with various metal substrates like Cu [89–92], Pt [93, 94] and Ni
[95, 96], preventing at the same time the substrate damage and allowing its repeated
application as graphene growth substrate in the CVD process. Beside the cathodic
exfoliation, anodic treatment with sacrificial Cu substrate was also elaborated [97].
A simple version of the anodic exfoliation method was shown to work in the same
beaker as the cathodic one, hence doubling the current efficiency of the process [98].
Cathodic exfoliation with a parallel reduction and the elimination of the majority of
the oxygen-containing groups works in an analogous way for removing graphene
oxide nanosheets from Au or Al substrate [99].
347
the production of single but solvent-encapsulated graphene layers, although standalone graphene in only 0.335 nm thick. The appearance of layers with thickness much
larger than 1 nm definitely means that a few layers exfoliated together [79, 82–84]
(although the folding of the layers may produce artefacts when the samples are posttreated for thickness measurements either by AFM or TEM). When the dependence
of the quality of the exfoliated products was studied, it was observed that higher cell
voltages are preferred for the specific solution used [85].
10.5.4 Electrochemistry-Based Transfer Methods
of Graphene and Graphene Oxide Nanosheets
Almost 15 years after the isolation of a single graphene sheet, in spite of the extraordinary electron mobility coupled with transparency, the adoption of the graphene
technology in industry proceeds very slowly. One of the main bottlenecks using
chemical vapour deposition (CVD) for graphene production is the lack of a fully
reliable graphene transfer technique which would serve as an interface between the
CVD process taking place on one substrate and the application demanding another
substrate of different type. The delamination of graphene sheets, especially with a
surface area of industrial scale, is still challenging. Among others, electrochemical
techniques have been developed for delaminating single graphene sheets produced
on transition metal catalysts.
The major electrochemical technique used for peeling off a CVD-produced single
graphene sheet from its metal substrate is the so-called electrochemical bubbling
transfer. Before the electrochemical treatment, the graphene layer is covered with
a support layer [typically poly(methylmethacrylate)] in a manner that a part of the
uncovered metal substrate is free and is available for contacting an electrolyte solution. Then, this workpiece is immersed in an electrochemical cell and is connected as
cathode, and hydrogen generation is started. The hydrogen bubble formation delaminates the plastic cover layer from the metal together with the graphene sheet, i.e.,
the attack point of the stress is between the graphene sheet and the metal catalyst
used for graphene growth. It is evident that the metal surface and its contact with the
electrolyte solution play a key role in the bubble transfer process. This technique was
reported to work with various metal substrates like Cu [89–92], Pt [93, 94] and Ni
[95, 96], preventing at the same time the substrate damage and allowing its repeated
application as graphene growth substrate in the CVD process. Beside the cathodic
exfoliation, anodic treatment with sacrificial Cu substrate was also elaborated [97].
A simple version of the anodic exfoliation method was shown to work in the same
beaker as the cathodic one, hence doubling the current efficiency of the process [98].
Cathodic exfoliation with a parallel reduction and the elimination of the majority of
the oxygen-containing groups works in an analogous way for removing graphene
oxide nanosheets from Au or Al substrate [99].