TECHNICAL FIELD
The present invention relates to method of preparing of N-doped graphene using
solid graphene oxide and a nitrogen source through ambient synthesis conditions.
BACKGROUND AND PRIOR ART
Graphene is well known as ‘Wonder material’ since its discovery. The twodimensional hexagonal array of carbon atoms of graphite is termed as graphene.
The extended electron cloud enhancing the electronic conductivity, lubrication
property, etc., is much prominent than that of the parent material graphite. The
quantum confinement of the structure in 1D makes the material more promising
owing to its high thermal conductivity, barrier properties, transmittance, water, and
air purification methods. The increased demand for reducing green-house gas
emissions and moving to a greener world, demand for greener materials like
graphene is promising. Nitrogen doped graphene or N-doped graphene has found
wide scale application in variety of areas including supercapacitors, batteries, fuel
cells etc. In commercial scale of operation, chemical exfoliation is getting
established faster. However, the inherent presence of oxygen reduces conductivity
of graphene derived through such processes compared to processes such as CVD
growth, electrochemical and mechanical exfoliation. The replacement of lower
conductive oxygen functional groups of GO with more conductive nitrogen
containing functional group enables the extended conjugation of electron cloud
thereby facilitating the enhanced conduction of the system.
The graphene oxide, being intermediate material during chemical exfoliation
process of graphite, contains oxygen containing functional groups such as hydroxyl
group, carbonyl group, carboxyl group, epoxy group, etc. These groups do not
facilitate the conduction of electron as being in different planes, hence replacing the
oxygen atoms with lower sized nitrogen atom favours the formation of pyrrolic and
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pyridinic bonds with carbon. These nitrogen containing groups enable the ease of
conduction of electron through extended delocalisation.
Several reports of nitrogen doped graphene synthesis are being conducted at
ammonia atmosphere with inert gas purging at elevated temperatures through
combustion method. For example, literature documents like ‘Phosphorous-doped
porous graphene via laser induction, RSC Advances 2020, 10(40), 23953-
23958’ and ‘Boron and nitrogen doping of graphene via thermal exfoliation of
graphite oxide in a BF3 and NH3 atmosphere: contrasting properties; Issue 42,
2013, Journal of material chemistry A’ teach preparation of doped graphene using
laser induction. However, the method taught by these documents uses inert
atmosphere for synthesis making the synthesis tedious and difficult to scale.
Several other types of synthesis of nitrogen doped graphene oxide are also been
reported. For example, ‘Chemical Modification of Graphene oxide by
Nitrogenation: An X-ray Absorption and Emission Spectroscopy Study by Abhijit
Ganguly et al’ which discloses describes synthesis of N-doped graphene by reacting
equal amounts of graphene oxide and urea in a closed container like microwave at
700W for 100-400 seconds. No inert gas is used. Graphene oxide is obtained by
Hummers Method. Another literature document, ‘N-doped reduced graphene oxide
for room-temperature NO gas sensors by Yu-Sung Chang et al’ discloses that
graphene oxide is dispersed in water and reacted with ammonia solution in an
autoclave at 200°C for 2 hours. Graphene oxide is obtained by modified Hummer’s
method. However, the difficulty of synthesis method inhibits escalation of
commercial production to bulk.
Another prior art, Nitrogen-Doped Graphene Oxide as Efficient Metal-Free by
Adriana Marinoiu et al, teaches reacting graphene oxide with reduction agent and
nitrogen reagent in a microwave at 60-80°C at 800W for 15 mins without
maintaining any inert atmosphere. Graphene oxide powder is sourced directly and
nitrogen reagents are urea, ammonia, and nitric acid. The ratio of GO: urea is 1:10.
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However, high amount of chemicals and usage of microwave does not render the
method environment friendly and scalable. Yet another prior art, ‘Nitrogen-doped
graphene with high nitrogen level via a one-step hydrothermal reaction of graphene
oxide with urea for superior capacitive energy storage by Li Sun et al’ discloses a
method in which graphene oxide is dispersed in water and reacted with urea in an
autoclave at 180°C for 12 hours. Graphene oxide is obtained by modified
Hummer’s method and ratio of urea: GO is 300:1. However, use of high amount of
chemicals does not make the process environment friendly. The disclosure also
teaches usage of autoclave which does not make it easy to escalate to bulk
production.
Therefore, it is an object of the present invention to provide for a method which is
environment friendly by using significantly lesser amount of chemicals as
compared to prior art. It is another object of the present invention to provide for a
method of synthesis of nitrogen doped graphene which is less cumbersome and does
not use inert conditions for synthesis. It is a further object of the invention to provide
for a method of synthesis of nitrogen doped graphene which is economical and
easier to escalate to bulk production.
SUMMARY OF THE INVENTION
The present invention provides for a method of preparing N-doped graphene by
mixing a nitrogen source with solid state graphene oxide and reducing the same by
induction assisted thermal treatment.
DETAILED DESCRIPTION OF ACCOMPANYING DRAWINGS
The various aspects of the present invention will be apparent and more readily
appreciated from the following description of the example embodiment taken in
conjunction with the accompanying drawings in which:
Figure 1 is the scanning electron microscope image of undoped graphene.
Figure 2 is the scanning electron microscope image of the nitrogen-doped graphene.
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Figure 3 is the scanning electron microscope image of nitrogen-doped graphene in
presence of urea;
Figure 4 is the EDX spectrum of un-doped graphene;
Figure 5 is the EDX spectrum of nitrogen doped graphene in presence of ammonia
solution;
Figure 6 is the EDX spectrum of nitrogen doped graphene in the presence of urea;
Figure 7 is the FTIR spectrum of undoped graphene;
Figure 8 is the FTIR spectrum of nitrogen-doped graphene in presence of ammonia
solution;
Figure 9 is the FTIR spectrum of nitrogen doped graphene in the presence of urea;
Figure 10 is the conductivity of undoped graphene;
Figure 11 is the conductivity of nitrogen doped graphene
Figure 12 is the TEM images of undoped graphene
Figure 13 is TEM image of nitrogen doped graphene in presence of ammonia
solution.
Figure 14 is the representation of band diagram for samples
DETAILED DESCRIPTION OF INVENTION
For the purposes of the following detailed description, it is to be understood that the
invention may assume various alternative variations and step sequences, except
where expressly specified to the contrary. Moreover, other than in any operating
examples, or where otherwise indicated, all numbers expressing, for example,
quantities of ingredients used in the specification are to be understood as being
modified in all instances by the term "about". It is noted that, unless otherwise
stated, all percentages given in this specification and appended claims refer to
percentages by weight of the total composition.
The use of examples anywhere in this specification including examples of any terms
discussed herein is illustrative only, and in no way limits the scope and meaning of
the invention or of any exemplified term. Likewise, the invention is not limited to
various embodiments given in this specification.
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Unless otherwise defined, all technical and scientific terms used herein have the
same meaning as commonly understood by one of ordinary skill in the art to which
this invention pertains. In the case of conflict, the present document, including
definitions will control. It must be noted that, as used in this specification and the
appended claims, the singular forms “a,” “an” and “the” include plural referents
unless the content clearly dictates otherwise. Thus, for example, reference to a
“solvent” may include two or more such solvents. The terms “preferred” and
“preferably” refer to embodiments of the invention that may afford certain benefits,
under certain circumstances. However, other embodiments may also be preferred,
under the same or other circumstances. Furthermore, the recitation of one or more
preferred embodiments does not imply that other embodiments are not useful, and
is not intended to exclude other embodiments from the scope of the invention.
As used herein, the terms “comprising,” “including,” “having,” “containing,”
“involving,” and the like are to be understood to be open-ended, i.e., to mean
including but not limited to.
Unless defined otherwise, all technical and scientific terms used herein have the
same meaning as commonly understood by one of ordinary skill in the art to which
the invention pertains. It is also to be understood that the terminology used herein
is for the purpose of describing a particular embodiment of the invention only, and
is not intended to limit the scope of the invention in any manner.
Thus, before describing the present invention in detail, it is to be understood that
this invention is not limited to the following embodiment. The description of a
particular embodiment and examples provided herein are only for the purpose of
illustration and does not limit the scope of the invention to a particular embodiment.
In one aspect, the present invention provides for a method of preparing N-doped
graphene. Graphene oxide is mixed with at least one nitrogen containing precursor
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to form a premix. Graphene oxide is in solid state, preferably in dry powdered form
obtained by chemical exfoliation of natural or synthetic graphite through known
methods such as Hummer’s method. The washed and dried graphene contains
carbon, oxygen, nitrogen, and sulphur atoms with preferably at least 18% of oxygen
and more preferably up to 35% of oxygen. The graphene oxide used for forming
the premix are multilayered, preferably between 3 to 15 layers.
The nitrogen containing precursor is selected from any primary amino containing
reagents that are capable of reaction so that ammonia can be released on reacting
with graphene oxide to form the doped system. It can be an organic nitrogen source,
an inorganic nitrogen source or a combination of both. In particular, the nitrogen
precursor is selected from at least one of hydroxyl amine, NH2-NH2, primary
amines, semicarbazide, liquid ammonia, thiourea, and urea. The mass mixing ratio
of graphene oxide to nitrogen containing precursor is preferably over 1 and more
preferably between 1:1.5 to 1:4.
The said premix is reduced by induction assisted thermal exfoliation to obtain Ndoped graphene. The method employs use of induction-based heating to carry out
the reaction. Any known induction equipment can be used for this purpose even an
induction cooktop. Any containers suitable for induction equipment may be
employed for example SS304, SS316 etc. The reaction takes place at lower
temperatures of 300-400°C in atmospheric conditions for 5-15 mins. The
instantaneous injection is carried out at a rate of 1g/sec and the same is proceeded
via a continuous processing route. As the induction works on the power utilized, in
terms of wattage, the induction temperature is around 2000W for the given feed rate
and residence time.
Unlike prior arts, the thermal exfoliation takes place in ambient atmosphere without
use of any inert gases. The reaction takes place in a closed chamber which favours
the expansion and chemical reaction replacing the oxygen function group with
nitrogen at lower temperature and time.
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Graphene oxide containing 18-35% of oxygen is thermally reduced on almost
instantaneous injection. Introduction of nitrogen containing precursors in a closed
chamber creates favourable nitrogenising atmosphere which induces replacement
of oxygen species with nitrogen forming pyrrolic and pyridinic linkages with
graphene chain. The heterogenous functionalisation in graphene causes extension
in Ω – electron cloud extension through extended conjugation and donation of lone
pair of electrons for the delocalisation enhances the conductive nature of the
material. The introduction on nitrogen in the graphene by the invented method is
further confirmed by comparative studies of EDS measurements conducted in the
presence and absence of nitrogenising medium (Figures 4-6). The induction based
spontaneous heating of graphene oxide in presence of nitrogen containing reagent
in higher composition facilitates replacement of oxygen atoms with nitrogen in the
range of 43 to 230%. The conductivity of graphene oxide prepared by this invented
method has shown enhancement in the electronic conductivity on introduction of
nitrogen (Figures 10 and 11).
EXEMPLARY EMBODIMENTS
The present invention is hereinafter illustrated by way of an exemplary
embodiments read along with enclosed diagrams for better understanding but is not
intended to limit the scope of the disclosure. While they are typical of those that
might be used, other procedures, methodologies or techniques known to those
skilled in the art may alternatively be used. In this regard, the present example
embodiment may have different forms and should not be construed as being limited
to the descriptions set forth herein. Accordingly, the example embodiments are
merely described below for the purpose of explanation only.
We claim:
1. A method of preparing N-doped graphene comprising:
mixing at least one nitrogen containing precursor with solid state graphene
oxide to form a premix; and
reducing said premix by induction assisted thermal exfoliation to obtain
nitrogen doped graphene.
2. The method of preparing N-doped graphene as claimed in claim 1 wherein,
graphene oxide is multilayered, preferably between 3 to 15 layers.
3. The method of preparing N-doped graphene as claimed in any of the
preceding claims wherein nitrogen containing precursor is a compound
containing primary amino group capable of releasing ammonia on reacting
with graphene oxide to form the doped system.
4. The method of preparing nitrogen doped graphene as claimed in any of the
preceding claims wherein said nitrogen precursor is an organic nitrogen
source, an inorganic nitrogen source or a combination of both and selected
from at least one of hydroxyl amine, NH2-NH2, primary amines,
semicarbazide, liquid ammonia, thiourea, and urea.
5. The method of preparing N-doped graphene as claimed in any of the
preceding claims wherein the mass mixing ratio of graphene oxide to
nitrogen containing precursor is preferably over 1.
6. The method of preparing N-doped graphene as claimed in any of the
preceding claims wherein the mass mixing ratio of graphene oxide to
nitrogen containing precursor is preferably between 1:1.5 to 1:4.
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7. The method of preparing N-doped graphene as claimed in any of the
preceding claims wherein graphene oxide contains at least 18% of oxygen
and more preferably up to 35% of oxygen.
8. The method of preparing N-doped graphene as claimed in any of the
preceding claims wherein the thermal exfoliation is conducted under
temperatures 300-400°C in atmospheric conditions.
9. The method of preparing N-doped graphene as claimed in any of the
preceding claims wherein the thermal exfoliation occurs for 5-15 mins.
10. The method of preparing N-doped graphene as claimed in any of the
preceding claims wherein content of nitrogen in N-doped graphene was at
least 8%.