Synthesis of Graphene-Based Nanocomposites for Environmental Remediation Applications

Synthesis of Graphene-Based Nanocomposites for Environmental Remediation Applications: History

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Subjects: Materials Science, Composites

Contributor:

Yajvinder Saharan

The term graphene was coined using the prefix “graph” taken from graphite and the suffix “-ene” for the C=C bond, by Boehm et al. in 1986. The synthesis of graphene can be done using various methods. The synthesized graphene was further oxidized to graphene oxide (GO) using different methods, to enhance its multitude of applications. Graphene oxide (GO) is the oxidized analogy of graphene, familiar as the only intermediate or precursor for obtaining the latter at a large scale. Graphene oxide has recently obtained enormous popularity in the energy, environment, sensor, and biomedical fields and has been handsomely exploited for water purification membranes. GO is a unique class of mechanically robust, ultrathin, high flux, high-selectivity, and fouling-resistant separation membranes that provide opportunities to advance water desalination technologies. The facile synthesis of GO membranes opens the doors for ideal next-generation membranes as cost-effective and sustainable alternative to long existing thin-film composite membranes for water purification applications.

graphene
synthesis process
polymeric membranes
environmental remediation
composites

1. Introduction

Graphene is a purified form of graphite that recently gained enormous popularity in the energy [1,2,3], environment [4,5,6,7,8], membranes [1,7], sensor [9,10,11,12], and biomedical fields [13,14,15,16,17,18,19,20,21,22,23,24,25,26]. It is a sp² hybridized, hexagonally arranged, chain of polycyclic aromatic hydrocarbon with a honeycomb crystal lattice [27]. It is the most recent element of carbon allotropes and is actually the basic building block of other important carbon allotropes, including 3D graphite, 1D carbon nanotubes (CNTs), and 0D fullerene (C60), as shown in Figure 1.

Figure 1. Structural representation of 2D graphene with different dimensions. [Reprinted with permission from ref. [28], Wan, X., Huang, Y., & Chen, Y. (2012). Focusing on energy and optoelectronic applications: a journey for graphene and graphene oxide at large scale. Accounts of chemical research, 45(4), 598–607. Copyright © American Chemical Society].

The name graphene was coined by Boehm in 1986 [1], taking the prefix “graph” from graphite and the suffix “-ene” for sp² hybridized carbon, and was finally accepted by the International Union for Pure and Applied Chemistry in 1997 [29,30,31,32,33]. Furthermore, it became famous worldwide in 2004 when Geim and Novoselov obtained a single sheet of graphene on solid support, for which they were honored with the Nobel Prize in Physics in 2010 [34]. The main achievements of graphene in a timeline of history from 1840 to 2018 are shown in Figure 2.

Figure 2. Schematic representation of a graphene timeline.

2. General Methods of Graphene Synthesis

Generally, graphene can be synthesized using two different routes, viz, bottom-up and top-down [33,35,36], as depicted in Figure 3.

Figure 3. Schematic representation of the general methods for graphene synthesis.

3. Graphene Oxide (GO)

In comparison to graphene, graphene oxide is considered a more versatile and advanced material. GO has a broad range of oxygen containing functional groups such as carboxyl, hydroxyl, epoxy, carbonyl, and keto groups on its surface.

GO has shown great potential in a variety of fields by virtue of its high surface area [88], unique mechanical strength [89], and excellent optical and magnetic properties [90]. In comparison to other carbon-based nanomaterials, GO is considered a green oxidant, as it is enriched with oxygen-containing functional groups [91,92]. Further, GO has an aromatic scaffold, which acts as a template to anchor active species behaving as an organo-catalyst [93,94].

3.1. Synthesis of GO

In 1840, German scientist Schafhacutl was given the first report on the synthesis of graphene oxide and graphite intercalated compounds [95]. For the very first time, he attempted to exfoliate graphite and tried to purify impure graphite “kish” from iron smelters [27]. To date, several methods, as shown in Table 2, have been proposed.

Table 2. List of different methods used to synthesize graphene oxide.

Methods	Year	Starting Material	Different Oxidants Used	Reaction Time for GO Synthesis	Temperature °C	Features	References
Brodie	1859	Graphite	KclO₃, HNO₃	3–4 days	60	First attempt to synthesize GO	[96]
Staudenmaier	1898	Graphite	KclO₃, H₂SO₄, HNO₃	96 h	Room temperature	Improved efficiency	[97]
Hummers	1958	Graphite	KmnO₄, H₂SO₄, NaNO₃	<2 h	<20–35–98	Water-free, less than 2 h of reaction time	[98]
Fu	2005	Graphite	KmnO₄, H₂SO₄, NaNO₃	<2 h	35	Validation of NaNO₃	[99]
Shen	2009	Graphite	Benzoyl peroxide	10 min	110	Fast and non-acidic	[100]
Su	2009	Graphite	KmnO₄, H₂SO₄	4 h	Room temperature	Large-size GO sheets formed	[101]
Marcano and Tour	2010 & 2018	Graphite	KmnO₄, H₃PO₄, H₂SO₄	12 h	50	Eco-friendly resulting in a high yield	[102]
Sun	2013	Graphite	KmnO₄, H₂SO₄	1.5 h	Room temperature-90	High-yield and safe method	[103]
Eigler	2013	Graphite	KmnO₄, NaNO₃, H₂SO₄	16 h	10	High-quality GO produced	[104]
Chen	2015	Graphite	KmnO₄, H₂SO₄	<1 h	40–95	High-yield product	[105]
Panwar	2015	Graphite	H₂SO₄, H3PO₄, KmnO₄, HNO₃	3 h	50	Three component acids and high-yield product	[106]
Peng	2015	Graphite	K₂FeO₄, H₂SO₄	1 h	Room temperature	Results in a high-yield and eco-friendly method	[107]
Rosillo-Lopez	2016	Graphite	HNO₃	20 h	Room temperature	Nano-sized GO obtained	[108]
Yu	2016	Graphite	K₂FeO₄, KmnO₄ H₂SO₄, H₃BO₃ (NH₄)₂S₂O₈	5 h	<5–35–95	Low manganite impurities and high yield obtained	[109]
Dimiev	2016	Graphite	98% H₂SO₄, fuming H₂SO₄	3–4 h	Room temperature	25 nm thick and ~100%conversion rate	[110]
Pei	2018	Graphite foil	H₂SO₄	<5 min	Room temperature	High efficiency	[111]
Ranjan	2018	Graphite	H₂SO₄, H₃PO₄, KmnO₄	<24 h	<RT-35–95	Cooled exothermal reaction to make the process safe	[112]

The most preferred methods are Brodie [96], Staudenmaier [97], and Hummers [98], as shown in Figure 11. From these familiar methods, a number of variations have been derived to improve the overall yield and quality of the GO. In 1859, Brodie used graphite as the starting material for the synthesis of graphene oxide (GO). In his experimental work, he used KclO₄ (strong oxidizing agent) along with nitric acid and heated the content at 60 °C for 3–4 days [96]. The GO obtained was soluble in pure or basic water. The chemical composition showed mainly carbon, oxygen, and hydrogen with the general formula C₁₁H₄O₅. After nearly four decades, in 1898, Staudenmaier and Hoffmann modified Brodie’s method and trimmed down the reaction time of graphene oxide synthesis from 4 days to 2 days [97]. The nitric acid used in Brodie method was also replaced with sulfuric acid, which further reduced the liberation of toxic gases such as NO₂ or N₂O₄.

Figure 11. Schematic representation of the synthesis of graphene oxide with different methods.

In 1958, Hummer reduced the reaction time from 2 days to 12 h by using KmnO₄ as the oxidizing agent instead of KclO₄, followed by the addition of sodium nitrate, but the problem of toxic gases still remains a challenge [98]. Further, in 2010, at Rice University, Tour’s group [102] replaced sodium nitrate with phosphoric acid and increased the amount of KmnO₄. This improvement made the process eco-friendly, as it completely stops the release of toxic gases such as NO₂, N₂O₄ or ClO₂, along with easy temperature control and better yield [102]. In addition to this, the GO suspension obtained was treated with hydrogen peroxide (H₂O₂) to eliminate all impurities due to permanganate and manganese dioxide.

Furthermore, the final color of the product GO varies from army green to light yellow, depending on the carbon-to-oxygen ratios [113], as depicted in Table 3.

Table 3. Effect of acid concentration, reaction temperature, reaction time, and the quantity of the oxidizing agent on the oxidation of graphene [113].

S. No.	Source of Carbon	H₂SO₄ (in mL)	Other Ingredients	Temp. (in °C)	Time (in h)	C:O	Colour of GO Obtained
1	Graphite	15.0	1.0 g Na₂Cr₂O₇	30	72	16:1	Black
2	Graphite	15.0	4.0 g Na₂Cr₂O₇	30	72	3.4:1	Black
3	Graphite	15.0	15.0 mL 70% HNO₃ 3.0 g KmnO₄,	30	24	--	Black
4	Graphite	20.0	11.0 g KclO₃, 10.0 mL 70% HNO₃	0–60	33	3.1:1	Midnight green
5	Graphite	30.0	3.0 g KmnO₄,1.0 g NaNO₃	30	2	3.0:1	Bluish green
6	Graphite	30.0	3.0 g KmnO₄,1.0 g NaNO₃	45	1	--	Green
7	Graphite	22.5	3.0 g KmnO₄,1.0 g NaNO₃	45	1	--	Brittle yellow
8	Graphite	22.5	3.0 g KmnO₄,0.5 g NaNO₃	45	1	--	Yellow
9	Graphite	22.5	3.0 g KmnO₄,0.5 g NaNO₃	45	0.5	2.3:1	Yellow
10	Graphite	22.5	3.0 g KmnO₄,0.5 g NaNO₃	35	0.5	2.05:1	Bright yellow
11	Graphite	22.5	3.0 g KmnO₄, 1.0 g fuming HNO₃	35	1	--	Bright yellow
12	Graphite	22.5	3.0 g KmnO₄, 1.0 g BaNO₃	45	2	--	Light green

3.2. Structural Aspects of GO

Various structural models of GO, as shown in Figure 12, have been proposed and were refined over the years by the advancement of characterization techniques and technologies. The structural history of GO started in 1936, when Hofmann and Rudolf [135] proposed the first structure of GO in which epoxy groups were unsystematically spotted over the graphene sheets, and then in 1946, Ruess [136] restructured the Hofmann model by introducing hydroxyl moieties and the alternation of the basal plane structure from an sp² to an sp³ hybridized carbon system.

Figure 12. Schematic representation of the year-wise progress in proposed structures of graphene oxide [135,136,137,138,139,140,141,142].

Scholz and Boehm in 1969 [137] proposed a GO structure that was less ordered, having C=C and periodically cleaved C-C bonds within the channeled carbon layers labeled with carbonyl and hydroxyl groups. Further, in 1994, Nakajima and Matsuo [138] presented a graphite intercalation compound (GIC) to look like a lattice framework. Adding to the history, in 1998, Lerf and Klinowski et al. (L–K model) [139,140] proposed a uniform carbon lattice framework GO structure with randomly distributed benzene rings having attached epoxides, carboxyl, and hydroxyl groups. Thereafter, in 2006, Szabó and coworkers [141] put forward a carboxylic-acid-free model comprising two distinct domains: a trans-linked cyclohexyl species interspersed with tertiary alcohols, 1,3-ethers, and a keto/quinoidal species corrugated network. Even closer to the present time, in 2018, Liu et al. [142] experimentally noticed oxygen bonding and evidenced the C=O bonds on the edge and plane of GO, confirming parts of earlier proposed models, especially the L–K model.

Among the above-discussed models from 1936 to 2018, the L–K model has been accepted the most, due to good interpretability over the majority of experimental observations and the ease of further adaption and modification.

3.3. Characterization of GO

In order to authenticate the synthesis of GO and to analyze its chemical configuration, a range of characterization techniques have been employed by numerous research groups. For example, in order to achieve the information of size and surface morphology of graphene oxide, SEM, TEM, and AFM were used abroad [143,144,145,146,147]. With respect to the elemental analysis of graphene oxide, quantitative XPS, EDX, and inductively coupled plasma mass spectrometry (ICP-MS) were utilized generally [148,149,150,151,152,153,154,155,156]. Additionally, Raman spectra, XRD, and FTIR spectra are widely used to point out the graphene oxide chemical structure [156,157,158,159,160].

This entry is adapted from the peer-reviewed paper 10.3390/molecules27196433

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