Coal derived carbon nanomaterials – Recent advances in synthesis and applications

Applied Materials Today 12 (2018) 342–358

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Applied Materials Today journal homepage: www.elsevier.com/locate/apmt

Review

Coal derived carbon nanomaterials – Recent advances in synthesis and applications Van Chinh Hoang, Mahbub Hassan, Vincent G. Gomes ∗ The University of Sydney, School of Chemical and Biomolecular Engineering, NSW 2006, Australia

a r t i c l e

i n f o

Article history: Received 23 March 2018 Received in revised form 20 June 2018 Accepted 20 June 2018 Keywords: Coal Nanomaterials Nanotubes Graphene Quantum dots

a b s t r a c t Carbon based nanomaterials have attracted attention due to their exceptional physical, chemical and biological properties, which make them promising candidates in transforming the future of various critical applications. The intense demand to develop carbon nanomaterials via environment-friendly and inexpensive synthesis strategies have been driving a significant proportion of efforts globally. Precursors utilized for the growth of carbon based nanomaterials play a crucial role toward the success of the evolving technology. Most conventional synthesis techniques employ expensive carbon feedstocks such as hydrocarbons and graphite which lead to high cost of production and hence limit their commercialization. Coal, an abundant and cheap natural resource, is considered a green and viable alternative to effectively generate carbon nanomaterials such as carbon nanotubes, nanofibers/particles/spheres, graphene, graphene oxide, graphene quantum dots and carbon dots. Our review includes recent advances in the production of various carbon based nanomaterials from various types of coal (lignite, bituminous, anthracite among others) as raw materials and outlines their potential applications in the energy, environmental and biomedical sectors. © 2018 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 Synthesis & properties of coal-derived carbon nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 2.1. Carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 2.2. Graphene oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 2.3. Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346 2.4. Graphene quantum dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346 2.5. Carbon dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349 2.6. Carbon nanoparticles/nanospheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350 2.7. Carbon nanofibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350 Applications of coal-derived carbon nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 3.1. Energy related applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 3.2. Environmental applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 3.2.1. Catalytic applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 3.2.2. Separation applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 3.2.3. Sensing applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 3.3. Biomedical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 Conclusions, prospects & challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356

∗ Corresponding author. E-mail address: [email protected] (V.G. Gomes). https://doi.org/10.1016/j.apmt.2018.06.007 2352-9407/© 2018 Elsevier Ltd. All rights reserved.

V.C. Hoang et al. / Applied Materials Today 12 (2018) 342–358

1. Introduction Carbon based nanomaterials with their exceptional characteristics are promising candidates poised to transform the future of various areas of applications. For instance, carbon nanotubes (CNTs) first reported in 1991 [1] and graphene discovered in 2004 [2] exhibit extraordinary mechanical strength and electrical conductivity, remarkable physical and chemical properties which make them attractive for use as key components in catalysts, biosensors, fuel cells, batteries and electronic devices [3–9]. Recently, carbon dots (CDs) and graphene quantum dots (GQDs) have been receiving attention for a wide range of applications such as bioimaging, photovoltaics, optical sensing and drug delivery because of their attractive properties of chemical inertness, tunable photoluminescence, nontoxicity and excellent biocompatibility [10–14]. Despite the development of numerous fabrication strategies, the mass production of environmentfriendly carbon based nanomaterials at low cost is still a major challenge. Precursors for synthesizing carbon based nanomaterials play a crucial role for a viable production technology. Most conventional techniques employ expensive carbon feedstocks, for example, graphite and hydrocarbons (acetylene, xylene, methane, etc.) for graphene and CNT synthesis [15–19] while CNTs, graphite, fullerenes, carbon fibers, carbon black, graphene and graphene oxide have been employed as carbonaceous precursors to synthesize GQDs and CDs [20–25]. The high cost of production has limited the commercialization of these materials. Hence, green fabrication routes to prepare carbon based nanomaterials via cost-effective approaches is of great interest. Coal is a heterogeneous material with a three dimensionally cross-linked network which consists of aromatic and hydroaromatic units connected by short aliphatic and ether linkages [20,26,27]. Such nanometer-sized crystalline regions or clusters with sp2 carbon allotropes vary with coalification degree with the carbon concentration rising from 70 wt% in lignite to 75 and 85 wt% in sub-bituminous and bituminous coal, respectively, and reaching 94 wt% in anthracites [26–28]. In addition, coal is a lowcost resource and abundant in nature [27,29,30]. These advantages make coal a promising carbon source for the preparation of nanomaterials. Since the first report by Pang et al. in 1991 [31] on the feasible fabrication of fullerenes from Australian coke, there have been several studies on the transformation of coal into diverse classes of carbon materials, for example, graphite [32–39], activated carbon [40–47], CNTs [3,48–66], graphene oxide [67–71], graphene [7,48,72–80], carbon fibers [81,82], carbon nanoparticles or nanospheres [83–86], porous carbon materials [82,87–89], and very recently GQDs and CDs [14,20,29,30,89–95]. However, metal contaminants in coal may have affect the properties of carbon-based nanomaterials, including CNTs and graphene, especially the redox capacity [96]. For example, Chua et al. [97] found that six metals Fe, Cr, Mo, Ni, Cu and Mn during ballmilling process, contaminated graphene naomaterials, causing a prominent catalytic peak reduction at −0.65 V and lower onset oxidation potential. In another case, Mn impurities were found to induce 10% lower onset potential of reduced graphene oxide [98]. Similarly, Wang et al. [99] demonstrated that trace amounts of Fe, Co, Ni and Mn metallic impurities in the starting graphite, remarkably influenced the oxygen reduction reaction (ORR) potentials of the synthesized graphene materials. In this review, we emphasize the affordable and facile fabrication of carbon based nanomaterials from different types of coal, nanomaterial structures and properties (in Section 2) and outline their potential use in energy, environment and biomedical fields (in Section 3).

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2. Synthesis & properties of coal-derived carbon nanomaterials Fig. 1 shows an overview of diverse classes of carbon based nanomaterials synthesized from coal as a raw carbon source. The detailed synthetic routes and corresponding properties are discussed below. 2.1. Carbon nanotubes The predominant techniques employed currently to synthesize CNTs from coal are based on thermal plasma, chemical vapor deposition (CVD) and arc discharge methods. Tian et al. [62] first developed the thermal plasma method for the production of multiwalled CNTs (MWCNTs) by injecting Baode coal fine powders with 5–25 ␮m size directly into an arc plasma jet. Arc sputtering the copper anode at high temperature ∼3700 K led to the generation of copper metal particles, which worked as a catalyst for the growth of CNTs. The authors obtained MWCNTs with interlayer spacing of 0.343 nm and length of 7 ␮m. Moreover, the Raman spectra showed 2 peaks at 1600 and 1291 cm−1 , corresponding to G (graphitic order) and D (defect signal) peaks, respectively. The intensity ratio of G to D peak (IG /ID ) evaluates the quality of carbon-based materials with respect to the presence of defect levels [100–103]. However, in this work, IG /ID was rather low, illustrating the low-graphited MWCNTs. The results showed that the addition of 5 wt% Cu powders to coal promoted the yield of CNTs. In addition, the effect of three catalysts (Cu, Fe and Co) on the formation of CNTs was investigated [63]. It was found that the diameter of MWCNTs was in the range of 50–70 nm and the catalytic activity of Cu was higher than that of Fe and Co, resulting in a higher yield of CNTs with Cu catalyst (5%) compared to Fe or Co (1%). Coal has shown great potential as a substitute for graphite electrode in arc discharge technique due to tenfold reduction in raw material cost [65]. For example, Kumar et al. [49] utilized this method to produce high quality single-walled CNTs (SWCNTs) from bituminous coal with the support of Zr/Ni catalyst. The diameter of the CNTs varied from 1 to 2 nm with bundle length and diameter of 50–140 nm and 4–10 nm, respectively. Also, a small ratio ID /IG of 0.325 indicated that SWCNTs had few defects and no amorphous carbon. They observed that the Zr/Ni ratio affected the growth of SWCNTs and the highest yield of the product was obtained at the Zr/Ni ratio of 3:1. Similarly, Awasthi et al. [48] achieved SWCNTs with diameter 1.7 nm and 1.2 nm using annealed bituminous coal electrode with Fe and Ni Y catalysts, respectively. The average length of these SWCNTs were 0.2 ␮m. Without adopting any catalyst, they obtained MWCNTs with diameter and length distributions of 8–20 nm and 5–10 ␮m, respectively. The Raman results showed low intensity ratio ID /IG of 0.29 for SWCNTs and 0.32 for MWCNTs, indicating high quality of the products. Recently, Li et al. [50] demonstrated the synthesis of bamboostructure CNTs (B-CNTs) from Bitumite via an arc discharge process in the presence of Ni-Sm2 O3 catalyst. The results showed that B-CNTs had a BET surface area of ∼23 m2 g−1 and consisted of hollow compartments which were separated at 50–100 nm distance by graphite layers. The stronger intensity of G to D band (IG /ID = 1.72) revealed the highly graphitized structure of B-CNTs. Table 1 summarizes different types of CNTs synthesized by arc discharge process using coal as a carbon precursor. Chemical vapor deposition (CVD) has been widely employed at large-scales to produce CNTs owing to its relatively low price and high yield potential [3]. Previous reports only used commercial coal gas to prepare SWCNTs [55] and ion carbide-oxide filled MWCNTs [56] via CVD method in the presence of ferrocene as a catalyst. In 2015, Moothi et al. [54] performed CVD to directly produce CNTs

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Fig. 1. Various types of coal as precursors for synthesizing carbon based nanomaterials.

Table 1 CNTs synthesized from coal by arc discharge method. Product

Coal

Catalyst

Yield

Properties

Ref.

SWCNTs SWCNTs SWCNTs MWCNTs Bamboo-structure CNTs

Bituminous Bituminous Bituminous

45% 46 mg

Bamboo-shaped CNTs

Anthracite

Fe

MWCNTs

Anthracite

SWCNTs MWCNTs

Anthracite Bituminous

Double-walled CNTs

Anthracite

Single and double-walled CNTs Branched CNTs

Bituminous Anthracite

Diameter ∼1 nm Diameter 1–2 nm, ID /IG = 0.325 Diameter 1.2–1.7 nm, length 0.2 ␮m, ID /IG = 0.29 Diameter 2–80 nm, length 5–10 ␮m, ID /IG = 0.32 Length several micrometers, hollow compartments 50–100 nm, interlayer spacing 0.3452 nm; IG /ID = 1.72 Outer diameter 40–60 nm, length 5 ␮m, hollow compartment size 100 nm Diameter 2–15 nm, length 4–70 ␮m, interlayer spacing 0.34 nm Diameter 1.24–2.19 nm, IG /ID = 22 Diameter 2–15 nm, length 5–60 ␮m, interlayer spacing 0.34–0.36 nm Outer diameter 1.0–5.0 nm, interlayer spacing 0.41 nm, IG /ID = 16 Diameter 20–30 nm Inner diameter 40–50 nm, outer diameter 50–60 nm, interlayer separation 0.34 nm, purity 20–90%

[65] [49] [48]

Bitumite

Ni Y Zr/Ni Fe, Ni Y None Ni Sm2 O3

8.17% Fe 62.2%

None CuO

<1%

from the Bank Colliery-Witbank coal, South Africa. The pyrolysis of coal was first conducted at a temperature of 400–750 ◦ C. The resulting gases mainly consisting of CH4 and CO were then fed into a vertical reactor tube at 900 ◦ C for CNT synthesis with ferrocene as a catalyst. Using the optimized pyrolysis temperature of 400–550 ◦ C, the diameter and length of CNTs obtained were 60–130 nm and 250–550 nm, respectively (Fig. 2), whereas an outer diameter of 75 nm was obtained at a temperature of 600 ◦ C. When the coal pyrolysis temperature was increased to 650 ◦ C, the outer and inner diameters of the product reduced to 24–30 nm and 6–10 nm, respectively. The Raman results indicated that the nanotubes had multi-walls. The yield of the MWCNTs obtained was between 4.0 and 6.1 g h−1 with the highest production rate at the pyrolysis temperature of 400 ◦ C. They found that increased

[50] [51] [57] [58] [60] [59] [53] [64]

pyrolysis temperature decreased the concentrations of CH4 and CO in the product gases, leading to low yield of the CNTs. Similarly, extracted coal tar containing small quantities of N and S (0.62 and 0.3 wt%, respectively) was adopted as nitrogen and sulfur precursors to prepare N,S-codoped CNTs with diameter range of 75–91 nm, catalyzed by CoCl2 with the support of dicyandiamide as an evocating agent in a quartz tube reactor at 900 ◦ C [52]. The dicyandiamide was found to reduce the sticking behavior of extracted coal tar onto the CoCl2 surface, effectively promoting the nucleation of N,S-codoped CNTs and also worked as a nitrogen precursor to increase the doping level of nitrogen in the final product. Song et al. [61] demonstrated the fabrication of MWCNTs from coal tar pitch in an alumina tube furnace with Co(NO3 )2 ·6H2 O catalyst precursor. Cobalt was found to accelerate C2 H4 dissociation

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Fig. 2. TEM images of CNTs produced at a temperature of 900 ◦ C in the CVD reactor with coal pyrolysis reactor temperatures of (a) 400, (b) 450, (c) 500, and (d) 550 ◦ C [54]. Reprinted with permission from the publisher.

via electron donation effect, followed by the formation of straight MWCNTs through diffusion-precipitation mechanism. Ultimately, the optimized quality CNTs (IG /ID = 1.19) with diameter of 40 nm and length of 200 ␮m were achieved at the pyrolysis temperature of 1173 K with 0.75 wt% Co clusters. 2.2. Graphene oxide Graphene oxide (GO), an amphiphilic macromolecule, has received extensive attention in recent years, especially with respect to its potential for large-scale synthesis of graphene at relatively low prices [6,7,104–107]. The nanometer-sized crystalline regions or clusters of sp2 carbon allotropes vary with degree of coalification and carbon concentration, rising from 70 wt% in lignite to 75 and 85 wt% in sub-bituminous and bituminous coal, respectively, and reaching 94 wt% in anthracites [26–28]. The higher ranked coal provides more graphitized carbon framework of the product, whereas the lower ranked coal creates more oxygen functional groups on carbon based nanomaterials. Pakhira et al. [69] developed a strategy to extract GO by leaching low grade coal with HNO3 . Large GO sheets were obtained after thermal removal of excess HNO3 . These sheets were not stable, and they rapidly converted into small spherical morphologies. Further leaching with NaOH resulted in their fragmentation into smaller GO sheets with sizes between 40 and 200 nm with a broad (002) peak at 2 = 25.92◦ . Interestingly, in an acidic medium (pH ≤ 6.8), such flat GO sheets transformed into closed spheres (clenched fists), which could revert back to

their initial shapes in the exposure to NH3 vapor. They named this phenomenon a “close and open sesame” behavior. The modified Hummers method was utilized for preparing GO using coke from coal tar as a carbon source [71]. The yield of coke oxide was between 60 and 70 wt%. The GO obtained after sonication of 2–8 h with 10–30 wt% exfoliation yield showed C/O atomic ratio of 3.5 with C O, C O and COO functional groups and ID /IG of 0.93. It was found that 24% GO was mono-layer with average height and lateral size of 1.1–1.4 nm and 300–400 nm, respectively. A similar approach to chemically oxidize four types of coal including brown coal, low-volatile bituminous, bituminous and anthracite coal was reported by Savitskii [70]. The results demonstrated that only anthracite coal could produce a GO like colloidal dispersion with a size distribution of 122–190 nm, interlayer distance of 0.945 nm and the presence of oxygen functional groups such as OH (3420 cm−1 ), C O (1053 and 1226 cm−1 ) and C O (1293 and 1720 cm−1 ). Fernández-García et al. [67] described the production of GO from industrial coal liquids including an impregnation tar, binder tar and anthracene oil via a two-step strategy. First, graphite was synthesized by carbonization at 650 and 1000 ◦ C under N2 gas, followed by graphitization at 2800 ◦ C under Ar flow. The next step was to chemically oxidize synthetic graphite via modified Hummers method. The results illustrated that all GO had an intensity ratio of D to G less ≤0.9 with interlayer spacing of 0.814–0.949 nm and average height of 1.2 nm, indicating that the GO obtained was monolayer. In addition, a broad lateral size distribution was seen in all samples and

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it varied with the starting coal liquid used. The authors also studied the effect of quinoline on the fabrication of GO from coal tars [68]. Quinolone was found to decrease the oxidation yield due to its greater oxidizing potential, producing GO with lower interlayer distance and hindering the exfoliation of GO in aqueous solvents. 2.3. Graphene Several techniques have been developed for fabricating graphene from coal, such as arc discharge [48], CVD [7,78], oxidation-cum-extraction (OCE) [72], chemical leaching [76], templating [74,75], chemical oxidation-reduction by hydrazine [73], thermal treatment [77,79] or dielectric barrier discharge (DBD) plasma [80]. Awasthi et al. [48] observed the formation of graphene like nanosheets using annealed bituminous coal as an anode in the arc discharge set-up under H2 gas. It was found that the (002) peak shifted to a lower position of 25.8◦ with an increase in the interlayer spacing of 0.344 nm in comparison with graphite (26.3◦ and 0.334 nm, respectively). The hexagonal pattern in TEM images, together with the evidence of D (1346 cm−1 ), G (1573 cm−1 ) and 2D (2688 cm−1 ) in Raman spectra confirmed the existence of graphene in the product. In 2014, Xu et al. [7] reported a fabricating procedure of graphene nanosheets by pyrolysis of coal tar pitch at 1700 ◦ C using Al powder as a catalyst. The graphene with a high C/O molar ratio of 60/1 and sizes up to several ␮m2 possessed single and few layers with thicknesses of 0.6 nm and 3.2 nm, respectively. The Raman spectrum exhibited a weak D band at 1351 cm−1 and strong G band at 1583 cm−1 with ID /IG of 0.26 and a symmetric 2D peak at 2695 cm−1 with an intensity ratio of 2D to G (I2D /IG ) bands of 0.82, illustrating that the product comprised few-layer graphene sheets with small amount of defects. Recently, the mechanism of graphene film formation on Cu foils via CVD using sub-bituminous coal as a carbon source was investigated by Vijapur et al. [78], which underwent two key steps: Cu-based catalysis of hydrocarbon gases generated during the pyrolysis of coal to form an amorphous C film within the first 6 min, followed by graphitization under H2 to nucleate graphene domains. The films revealed Raman peaks at 1350, 1575 and 2700 cm−1 , corresponding with D, G and 2D bands, respectively with I2D /IG ratio of ∼1 and the full width at half maximum (FWHM) of the 2D band of 31 cm−1 . These indicated the growth of two-layer graphene film, which was further confirmed by TEM images with a hexagonal diffraction pattern. Das et al. [72] explored an oxidation-cum-extraction (OCE) strategy to obtain graphene-like nanosheets form sub-bituminous coal. The coal was first oxidized with nitric acid, followed by the alkaline extraction by NaOH solution. The results showed that nanosheets might be single-layer graphene and non-crystallite due to high quantity of oxygen with a broad (002) band at 26.7◦ , d-spacing of 0.378–0.383 nm and ID /IG of 1.81–2.79. A chemical leaching was also employed to treat sub-bituminous coal [76]. It was found that the (002) interlayer spacing for the samples treated with HF acid and ethylenediamine (EDA) were 0.352 and 0.376 nm, respectively. The product with ID /IG of 0.86 had a lateral size of 4.19 nm and height of 2.3 nm, corresponding to eight-layer graphene. He at al. [75] demonstrated the preparation of 3D hollow porous graphene balls (HPGB) with high yield (up to 67.3%) from coal tar pitch using magnesium oxide template and alkaline activation. The graphene balls showed diameters of 85–100 nm (Fig. 3) and consisted of micro-, meso- and macro-pore structures with micropore accounting for ∼58% of the pore volume. The BET surface area was 1871–1947 m2 g−1 and the total pore volume was 1.08–1.16 cm3 g−1 . The Raman spectra revealed D and G peaks at 1320 and 1590 cm−1 , respectively. The same group also reported the use of coal tar pitch to synthesize 3D interconnected graphene

nanocapsules (GNC) (>50% yield) via a ZnO template coupled with KOH activation. The results indicated that the GNC consisted of 4 layer graphene with shell thickness of 1–4 nm and size in the range of 115–150 nm, having a micro/mesopore structure with specific surface area and total pore volume of 1862–1985 m2 g−1 and 0.9–1.19 cm3 g−1 , respectively. D, G and 2D bands were observed at 1320, 1590 and 2650 cm−1 in Raman spectra with ID /IG of 1.0, indicating the modest degree of graphitization of the product. A large-scale synthesis of graphene at low price could be obtained via the reduction of coal-derived graphene oxide. A variety of reducing methods such as hydrazine [73], thermal treatment [77,79] and H2 DBD plasma [80] have been used to reduce coal-derived GO. For instance, Zhou et al. [80] demonstrated the fabrication of high quality graphene sheets (GS) and their composites with noble metal (Pt, Ru and Pt/Ru) nanoparticles via H2 plasma in a DBD reactor. The GS with layer structures showed wrinkled and thin sheets with many ripples and folded areas, which was then uniformly decorated by <2 nm metal nanoparticles. It was found that hydrogen DBD plasma remarkably reduced the peak intensity of oxygen functional groups with a complete removal of O C O group and decreased ID /IG ratio compared to those of anthracite-derived GO. Moreover, the graphene sheets revealed a typical mesoporous structure with specific surface area of 306 m2 g−1 and a pore size of 4.02 nm. Thermal treatment was carried out at 1100 ◦ C under Ar gas to foster a graphene film from coal-derived GO [79]. They observed that G band of graphene film shifted to higher position of 1606 cm−1 than that of GO (1590 cm−1 ) and an increase in the intensity ratio of D to G peaks (ID /IG ), rising from 0.63 in GO to 0.87 in graphene. Due to the removal of oxygen functional group via heat treatment, the film showed a good conductivity of 2.5 × 105 S m−1 , indicating that graphene possessed a perfect structure. Recently, N-doped few layer graphene was prepared by thermally treated bituminous-derived GO with the presence of ammonia [77]. The product showed an absorption peak at ∼280 nm in UV–visible spectra, reduced peak intensity of oxygenated functional groups and (002) peak at 2 = 25.01◦ , corresponding to d-interlayer spacing of 0.43 nm. In addition, the Raman spectrum demonstrated D and G bands at 1348 and 1584 cm−1 , respectively with ID /IG = 1.3, lower than that of bituminous-derived GO (ID /IG = 1.73), indicating an increased graphitization level of graphene. Feng et al. [73] proposed an eco-friendly process for the production of graphene/Mn3 O4 nanocomposites from coal-derived GO using hydrazine as a reducing agent. It was found that the intensity ratio between D and G peak ID /IG of the composite was 1.15, suggesting defect abundance in the graphene and the presence of peak at 651 cm−1 corresponded to the Raman scattering of Mn3 O4 . Fig. 4 illustrates the SEM and TEM images of the graphene/Mn3 O4 nanocomposites, which clearly displays Mn3 O4 spheres with size distribution of 20–30 nm and interlayer spacing of (211) plane of 0.25 nm evenly deposited on the surface of graphene nanosheets. The XPS analysis revealed 2 strong Mn 2p3/2 and Mn 2p1/2 peaks at 641.5 and 653.3 eV, respectively, along with N 1s peak which could be deconvoluted into pyrrolic and pyridinic with C/N ratio of 20.4 wt%. 2.4. Graphene quantum dots Chemical oxidation, a top-down route, was the most widely implemented technique to fabricate GQDs from coal. Interestingly, the size of GQDs could be easily tuned by the use of different coal precursors and temperatures, leading to changes in fluorescence. A brief summary of the synthesis of GQDs from coal is illustrated in Table 2. A facile one-step fabrication of GQDs from 3 different coals including bituminous, coke and anthracite was first reported by

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Fig. 3. TEM images of HPGB under (a & b) Ar and (c & d) N2 [75]. Reprinted with permission from the publisher.

Fig. 4. (a) SEM, (b) TEM and (c) HRTEM images of graphene/Mn3 O4 composite [73]. Reprinted with permission from the publisher. Table 2 GQDs synthesized from coal by chemical oxidation. Oxidizers

H2 SO4 and HNO3

H2 SO4 and HNO3

H2 SO4 and HNO3 HNO3 H2 SO4 and HNO3 H2 SO4 and HNO3

Precursor

Temperature, ◦ C

Diameter, nm

Anthracite Bituminous coal

100 100

29 ± 11 2.96 ± 0.96

Coke Anthracite

120 100 50 100

2.30 ± 0.78 5.8 ± 1.7 54 ± 7.2 4.5 ± 1.2 16 ± 1.33 41 ± 6.4 70 ± 15 27 ± 3.8 25 ± 5.0 7.6 ± 1.8

Bituminous Bituminous Coal, China Bituminous Bituminous

110 130 150 120 100 130 70 80

15–50 10 2–4 2–3.2

Length, nm

∼1.5–4 1.5–3

Yield, %

10–20 10–20

Quantum yield, %

Relative QY ∼ 0.2 at 3 mg mL−1 , pH 6

10–20 10–20

0.3–0.9, average: 0.5

1.6 6 10 2

1.1 0.89 0.65 0.38

14.66–56.3

1.8

Optical properties em , max, nm

Color under UV light

530 500

Yellow Green

460 480 ∼580 ∼520 ∼560 ∼580 ∼620 ∼520 ∼500 ∼420 ∼450 ∼420–540 420

Blue Green Orange red Green Green-yellow Yellow Orange-red Yellow Yellow Blue-green Blue Bright white Blue

Ref.

[20]

[94]

[91] [29] [89] [95]

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Fig. 5. (a) Schematic illustration of the synthesis of b-GQDs. Oxygenated sites are shown in red. (b) TEM image of b-GQDs showing a regular size and shape distribution (c) HRTEM image of representative b-GQDs from (b); the inset is the 2D FFT image that shows the crystalline hexagonal structure of these quantum dots. (d) AFM image of b-GQDs showing height of 1.5–3 nm [20]. Reprinted with permission from the publisher.

Ye et al. [20] (Fig. 5a). Briefly, bituminous was chemically oxidized in a mixture of concentrated H2 SO4 and HNO3 acids at 100 and 120 ◦ C for a day, followed by neutralization to pH 7 and dialysis for 5 days to obtain b-GQDs which were 2.96 ± 0.96 and 2.30 ± 0.78 nm in size with thickness of 1.5–3 nm (Fig. 5b–d) and ID /IG of 1.55 ± 0.19, and revealed green and strong blue fluorescence with emission maxima at 500 and 460 nm, respectively under 345 nm UV irradiation. Moreover, by choosing anthracite and coke as starting materials, the size of GQDs could be easily adjusted to 29 ± 11 and 5.8 ± 1.7 nm with the intensity of D to G peaks ID /IG of 1.90 ± 0.22 and 1.28 ± 0.18, which emitted yellow and green fluorescence with maximum emission wavelength at 530 and 480 nm, respectively. The same group also controlled the bandgaps of anthracite-derived GQDs by either using a cross-flow ultrafiltration membrane with various sizes or varying the temperature of the oxidation reaction [94]. When the pore size of the membrane changed from 1000 to 30,000 Dalton, the average size of obtained GQDs increased from 4.5 ± 1.2 to 70 ± 15 nm, emitting visible light from green (2.4 eV) to orange-red (1.9 eV) (Fig. 6) with emission peak red-shifting from 520 to 620 nm due to the quantum confinement effect. In addition, increased temperature of oxidation process between 50 and 150 ◦ C resulted in smaller sizes of GQDs with average diameter reducing from 54 ± 7.2 to 7.6 ± 1.8 nm, blue-shifting the maximum emission from 580 to 420 nm, emitting orange-red to blue-green luminescence. It was found that higher temperature induced a higher percentage of oxygen functional groups in GQDs with COOH content rising from 4% at 50 ◦ C to 22% at 100 ◦ C. Therefore, they included that both size and functionality effects might contribute to tunable bandgaps of GQDs at different oxidation temperatures. Later, bituminous-derived GQDs with a size distribution

Fig. 6. TEM images of GQDs with size of (a) 4.5 ± 1.2, (b) 16 ± 1.33, (c) 41 ± 6.4 and (d) 70 ± 15 nm; (e) solution of GQDs under 365 nm excitation UV lamp. The left-most vial is the GQDs solution with size of 4.5 ± 1.2 nm, then 16 ± 1.33, 41 ± 6.4 nm and the right-most vial is the GQDs solution with size of 70 ± 15 nm [94]. Reprinted with permission from the publisher.

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of 15–50 nm and poly (vinyl alcohol) (PVA) composite film was synthesized [91]. The thickness of the film was around 10 ␮m with high optical transparency (maximum of 91% with 3 wt% GQDs) and GQDs could be well dispersed in PVA matrix up to 5 wt%. It was found that GQDs reduced the crystallinity level of the PVA polymer due to strong hydrogen bonding between GQDs and PVA. Interestingly, the GQDs/PVA composite film emitted white photoluminescence (PL) under UV light with a QGDs concentration-dependent PL intensity, which reached a peak at 10 wt% loading of GQDs. The quantum yield of the composite was estimated to be 0.5%. In another case, Dong et al. [29] reported the preparation of single-layer GQDs with a size of ∼10 nm and thickness falling in the range of 0.3–0.9 nm (average thickness of 0.5 nm) from 6 coal samples with different coalification. The GQDs solution exhibited a peak and a shoulder at 280 and 400 nm, respectively in UV–vis absorption spectrum and an excitation-dependent PL property with blue luminescence under 365 nm UV light and PL quantum yield of 1.8%. In addition, a significant enhancement in electrochemiluminescence (ECL) activity was observed as the potential swept between −1.5 and +1.8 V with the aid of S2 O8 2− ions. Recently, Zhang et al. [95] employed bituminous-derived GQDs with a size distribution of 2–3.2 nm to assemble hierarchical porous carbon nanosheets (HPCNs) using a Mg(OH)2 template coupled with KOH activation. The activated HPCNs consisted of thin layer nanosheets which were stacked loosely with each other. It was found that the material possessed low crystallinity with the intensity ratio between D and G peak ID /IG of 0.83–0.91 and contained a micro/mesoporous structure with a large specific surface area of 1450–1882 m2 g−1 and a total pore volume of 3.397–5.222 cm3 g−1 and the volume ratio between micropores and mesopores Vmicro /Vmeso of 0.160–0.215. Very recently, the same group enhanced the strength and flexibility of electrospun carbon nanofiber fabrics by the introduction of bituminous-derived GQDs with size of 2–4 nm and ID /IG of 2.01 to a polyacrylonitrile (PAN) solution, followed by electrospinning and carbonization at 1000 ◦ C under N2 gas [89]. They found that the diameter and water contact angle of nanofibers steadily rose from 141.5 to 773.3 nm and 129 to 142.6◦ with increased concentration of GQDs, respectively. Moreover, all carbon nanofibers exhibited a smooth surface. A highest tensile strength of ∼2.2 MPa and Young’s modulus of ∼70 MPa were obtained at the ratio of GQDs to PAN = 1:1 (w:w) with ID /IG of 1.01 due to enhanced viscosity of the electrospinning solution caused by a strong interaction between oxygen functional groups of GQDs and PAN matrix. 2.5. Carbon dots Similar to GQDs, carbon dots (CDs) were also synthesized from coal by a top-down strategy (listed in Table 3). For example, after coal was refluxed with nitric acid at 130 ◦ C for 12 h, the deposit was further vacuum dried to remove the excess acid and redispersed in distilled water, followed by the neutralization with an ammonia solution before the supernatant was collected by centrifugation [29]. They observed some monodispersed spherical CDs with size in the range of 3–5 nm with a (100) graphite lattice spacing of 0.212 nm. In the same year, Hu et al. [90] reported the preparation of size-controlled carbon dots from anthracite coal via carbonization at temperature in the range of 0–1500 ◦ C, followed by chemical oxidation in HNO3 at 140 ◦ C for one day. They found that the temperature of the carbonization process strongly affected the size of CDs, yet the size variation almost did not tune their optical properties. As the temperature increased from 0 to 1500 ◦ C, the average size and height of anthracite-derived CDs also rose from 1.96 ± 0.73 to 3.10 ± 0.80 nm (Fig. 7) and 1.04 to 1.42 nm, respectively. Besides, all carbon dots inhibited a crystalline structure with a lattice spacing of ∼0.21 nm, which corresponded to the (100) graphite facet.

349

The CDs solution prepared from the coal carbonized at 900 ◦ C (CDs900) showed an absorption peak at ∼ 230 nm which was assigned to ␲–␲* transition of aromatic C C bonds and a PL emission spectra with a broad peak and a shoulder at ∼505 and 420 nm, respectively under 280 nm excitation light. Moreover, its PL emission intensity could be remarkably improved by NaBH4 reduction with sevenfold enhancement in quantum yield (∼8.8%). The reduced CDs-900 exhibited an excitation wavelength-dependent PL behavior and emitted strong blue fluorescence under 365 nm UV irradiation. Nitrogen-doping was found to significantly improve the optical properties of CDs with enhanced UV–vis absorption and red-shift PL, leading to increased QY. Various nitrogen forms, for instance, pyridinic, pyrrolic or graphitic nitrogen and their concentration levels affect the absorption and emission characteristics of the CDs [108,109]. In 2017, Li et al. [14] reported the high yield (25.6%) preparation of N-doped CDs with an average size of 4.7 nm from anthracite via a facile one-pot solvothermal method in DMF solvent at 180 ◦ C for 12 h. The N-doped CDs solution exhibited an absorption peak at 270 nm due to n–␲* transition of C O bond and an excitation-dependent PL behavior with cyan luminescence under 365 nm UV excitation and PL QY of 47%. The PL spectra of N-doped CDs showed the excitation and emission maxima at 310 and 445 nm, respectively. They found that the solvothermal method produced carbon dots with fewer oxygen functional groups and more sp2 conjugated domains, which could effectively inhibit the non-radiative electron-hole recombination, leading to high QY. In a similar experiment, coal tar, after refluxed with nitric acid at 80 ◦ C for a day, was solvothermally treated in toluene at 200 ◦ C for 12 h [110]. The obtained CDs had a size distribution of 1.5–4.5 nm and high crystallinity with (100) lattice spacing of 0.21 nm. The XRD results exhibited a broad (002) peak at 2 = 24◦ with its interlayer spacing of 0.364 nm. The CDs with G band at 1582 and D band at 1372 cm−1 and ID /IG of 1.029 emitted orange fluorescence under 365 nm UV light with emission and excitation PL maxima at 605 and 535 nm, respectively. As the excitation wavelength rose from 495 to 575 nm, the PL emission peak redshifted between 598 and 612 nm. In addition, to improve the water-solubility of CDs, the liposome/CDs composite in a size range of 50–100 nm were fabricated. The results showed that the composite could be dispersed in water stably, and emitted white fluorescence in aqueous solution with emission PL maxima shifting to 640 nm and QY of 10.7%, together with a stable PL against varied pH between 2 and 10. Chemical oxidation is simple and effective in large-scale production of CDs from coal. However, such approach has some drawbacks due to the use of strong oxidizing agent (e.g. HNO3 ), releasing toxic gases during the experiment, and costly removal of excess acid. Therefore, green and safe strategies were developed to solve the aforementioned obstacles. For instance, Hu et al. [30] employed H2 O2 to oxidize anthracite at 80 ◦ C for only 3 h to produce CDs. The excess hydrogen peroxide could be easily removed by simple solution boiling. The results demonstrated that CDs with size in a range of 1–3 nm and crystalline structure with (100) lattice spacing of 0.21 nm exhibited a wavelength-dependent PL property and emitted cyan fluorescence under excitation at 365 nm. The CDs were further functionalized with EDA, leading to a red-shift in the PL emission with emission maxima at ∼510 nm and emitting green fluorescence under 375 nm UV irradiation. In another case, Thiyagarajan et al. [92] reported the synthesis of CDs with tunable size and surface functionalization from lignite by three different methods: refluxing, microwave irradiation and laser ablation using an ethylenediamine (EDA) solution. The obtained CDs which were 35–90, 20–50 and 2.5–5.5 nm in size all showed excitation-dependent PL behavior with maximum emission wavelength at ∼468, 435 and 403 nm, respectively. Furthermore, the CDs obtained by refluxing and microwave irradiation methods exhibited an absorption peak at ∼260 nm due to ␲–␲*

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Table 3 CDs synthesized from different types of coal. Precursor

Product

Fabrication route

Diameter, nm

Yield, %

Quantum yield, %

Optical properties em , max, nm

Coal Lignite

Coal tar

Anthracite Anthracite

CDs CDs

CDs Liposome/ CDs composite N-doped CDs CDs

Reduced CDs Anthracite

CDs

Chemical oxidation Refluxing Microwave irradiation Laser ablation Laser ablation and dispersion in Na2 SO4 Chemical oxidation and solvothermal

Solvothermal Chemical oxidation Calcination at 900 ◦ C and chemical Oxidation Calcination at 1500 ◦ C and chemical oxidation NaBH4 reduction of CDs-900 Chemical oxidation

3–5 35–90 20–50, average 35

4.5 5.1

468 435

2.5–5.5, average 3.5

6 34.5

403 493

1.5–4.5

29.7

605

50–100

10.7

640

Ref. Color under UV light [29] [92]

Orange (toluene) Blue (toluene), white (H2 O)

[110]

Cyan

[14] [90]

4.7 1.96 ± 0.73 2.27 ± 0.74

25.6 7 30

47 Relative QY ∼0.67 Relative QY ∼ 0.7

445 420 420

3.1 ± 0.8

5

Relative QY ∼ 1

505

8.8

400

Blue

455

Cyan

1–3

transition of C C bonds and shoulder at ∼325 nm assigned to n–␲* transition of the surface functional groups whereas the former peak shifted to ∼220 nm in the case of CDs prepared by laser ablation strategy. However, low PL quantum yields were achieved (≤6% for all samples). The author further dispersed CDs synthesized by laser irradiation in solid sodium sulfate (referred as CD3 M) and the QY was enhanced significantly to 34.6% due to the inhibition of non-radiative deactivation routes and PL quenching. Interestingly, CD3M also revealed phosphorescence at room temperature with a peak maxima at 515 nm and lifetime of 290 ms due to intersystem crossing caused by a closer gap between singlet and triplet state of aromatic C O bonds. 2.6. Carbon nanoparticles/nanospheres Keller et al. [85] used 4 types of coal (lignite, high volatile bituminous, low volatile bituminous and anthracite) as carbon source for the preparation of thin film of carbon nanoparticles. After ballmilling with isopropyl alcohol (IPA) for 106 h and centrifugation, coal particles with diameters less than 100 nm was spin-coated onto a silicon wafer or a quartz substrate to fabricate thin films. It was found that the most uniform thickness with little cracking of the film was achieved from high volatile bituminous coal. Finally, thermal treatment of the film led to a change in sp2 allotropes and aromatic unit size, allowing the variation of its electrical conductivity in over seven magnitude orders (conductivity >∼10−6 S m−1 ). The Raman spectra indicated that ID /IG of high volatile bituminous and anthracite films steadily increased as the annealing temperature rose from 450 ◦ C to 950 ◦ C, indicating a remarkable rise in carbon disorder. In addition, the optical gaps associated with ␲–␲* transition were calculated to be 1.8 and 0.68 eV for high volatile bituminous and anthracite films, respectively. Interestingly, the gap was reduced to 0 eV at annealing temperature 800 ◦ C, which illustrated the tunability of optical band gaps of the thin films. Das et al. [83] explored molten caustic leaching (MCL) strategy to produce carbon nanomaterials using low grade coal in a quartz reactor. The product showed G band at 1590 and D band at 1340 cm−1 with relatively high ID /IG of 1.61, indicating low level of

50–60

[30]

graphitization. They observed the formation of carbon nanoballs in the product with size varying from 5 to 10 nm, together with CNTs and branched CNTs. Therefore, an efficient post-processing technique needs to be developed for the purification of these carbon nanoballs. Recently, Zhang et al. [86] converted coal tar into hollow porous carbon nanospheres utilizing zinc acetate template together with KOH activation. The nanospheres possessed a size range of 70–80 nm and contained a micro/mesoporous structure with a large specific surface area of 1374 m2 g−1 , total pore volume of 2.54 cm3 g−1 and average pore size of 7.41 nm. Another study described synthesis of hierarchical porous carbon spheres from coal via facile ultrasonic spray pyrolysis [84]. The raw coal was chemically oxidized by a mixture of HNO3 and H2 SO4 , and was dispersed in water with sodium dodecyl benzene sulfonate before spray pyrolysis in a quartz reactor at variable temperatures. The increased temperature was found to weaken functional groups formed and to promote the formation of lamellar graphite crystallites, leading to an increase in (002) interlayer spacing and degree of graphitization. The rough carbon spheres with holes had sizes of 200–300 nm (Fig. 8) and a strong peak G band at 1593 cm−1 with intensity ratio of D to G peak (ID /IG ) rising from 0.79 (700 ◦ C) to 0.91 (900 ◦ C), indicating the gradual collapse of carbon crystal structure with increased temperature. Micro/mesoporous structures were observed in these carbon spheres with specific surface area and total pore volume of the samples rising rapidly from 547.22 to 948.55 m2 g−1 and 0.23 to 0.52 cm3 g−1 , respectively as the temperature increased from 700 and 1000 ◦ C. 2.7. Carbon nanofibers Few reports have been published on the use of coal as an additive for the fabrication of carbon nanofibers. Zhao et al. [82] fabricated activated carbon nanofibers based on coal from Xinjiang, China by electrospinning. The coal, after treated with a mixture of HNO3 and H2 SO4 (1:3, v:v) was added into a polyacrylonitrile (PAN) solution, followed by an electrospinning process. The as-prepared sample was further carbonized and steam activated at 800 ◦ C to obtain

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Fig. 7. TEM images and size distribution of CDs at the carbonization temperature of (a, d) 0 ◦ C (CDs-900), (c, d) 900 ◦ C (CDs-900) and (e, f) 1500 ◦ C (CDs-1500) [90]. Reprinted with permission from the publisher.

activated carbon nanofibers. They reported that the acid treated coal could be dispersed well in N,N-dimethylformamide (DMF) with the solubility reaching 6.6 wt% due to the introduction of N and O groups, which was characterized by FTIR with peaks at 1540 and 1630–1700 cm−1 , respectively. The nanofibers having diameters of 50–80 nm showed micropore structure with a pore size range between 0.7 and 1.5 nm, and BET surface area of 902 m2 g−1 . However, the coal based carbon nanofibers showed a lower IG /ID ratio of 1.15 in comparison with the raw coal (IG /ID = 1.59), which might stem from the disordered graphitic framework during the acid treatment and activation process. A similar approach was applied for synthesis of Pt/Co carbon nanofiber from coal (Pt/Cocoal-CF) [81]. Typically, acid treated coal and cobalt acetate were mixed with PAN in DMF before electrospinning and carbonization to foster cobalt embedded carbon nanofiber (Co-coal-CF). Following that, small size Pt nanoparticles were homogeneously deposited on the Co-coal-CF via an impregnation-reduction process. Co-coal-CF with a lower ID /IG of 2.78 than CF (ID /IG = 2.78) indicated higher degree of graphitization and synergistic effect of Co and coal in the formation of well-graphitized CF. From XRD

results, the average crystal size of Pt nanoparticles in Pt/Co-coal-CF sample was calculated to be 3.6 nm. 3. Applications of coal-derived carbon nanomaterials 3.1. Energy related applications Several reports are available on the use of coal-derived carbon nanomaterials for energy applications in Li-ion batteries (LIBs), electrocatalysts and supercapacitors. For instance, Li et al. [50] synthesized bamboo like CNTs (B-CNTs) from bitumite coal as an anode material in LIBs, which exhibited initial discharge capacity of 628.7 mAh g−1 and a high reversible specific capacity of 450.1 mAh g−1 with coulombic efficiency of ∼99% over 100 cycles at a current density of 25 mA g−1 . After 60 charge/discharge cycles at current densities of 70–1260 mA g−1 , the current density was returned to 70 mA g−1 with specific capacity recovered at >250 mAh g−1 , implying good reversibility of the B-CNTs electrode. The excellent rate capability and capacity retention stemmed from the special structure of B-CNTs having defects and exposed

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Fig. 8. FE-SEM and TEM images of porous carbon sphere with variable pyrolysis temperatures: (a, b) 700 ◦ C, (c, d) 800 ◦ C, (e, f) 900 ◦ C, (g, h) 1000 ◦ C [84]. Reprinted with permission from the publisher.

edges planes, and reduced charge-transfer resistance caused by Ni particles. Liu et al. [52] investigated the electrocatalytic activity of N,Scodoped CNTs via an oxygen reduction reaction (ORR) in both acidic and alkaline environments. The N,S-codoped CNTs presented an O2 reduction peak at −0.27 V (vs Ag/AgCl) and peak current density of 5.6 mA cm−2 at a scan rate of 100 mV s−1 in O2 saturated 0.1 M KOH whereas these values were +0.33 V and 5.2 mA cm−2 in O2 saturated 1 M HClO4 , which indicate ORR activities comparable to those of commercial Pt/C catalysts. The linear sweep voltammetry (LSV) results exhibited onset potential of −0.09 V and a diffusion current of 7.22 mA cm−2 at −0.6 V in alkaline condition, which significantly surpassed those of a commercial Pt/C catalyst (47.6 wt%) while the onset potential at +0.47 V was observed in an acidic environment. The electron numbers transferred per oxygen molecule of the catalyst were calculated to be 4.0 and 3.2 in alkaline and acidic

medium, respectively. The high reaction current of the material was attributed to increased active sites induced by a synergistic effect of both nitrogen and sulfur. Similarly, a report by Mu et al. [81] exhibited that Pt/Co-coal-derived carbon nanofibers had a remarkably high methanol electro-oxidation behavior in an electrolyte of 0.5 M H2 SO4 and 0.5 M CH3 OH with onset potential of 0.1 V, mass and specific activity of 78.5 A g−1 and 3.3 A m−2 , respectively. The outstanding electrocatalytic performance and stability of the material were attributed to increased Pt dispersion caused by a synergistic effect of both coal and cobalt nanoparticles, and low charge transfer resistance. The coal-derived graphene/Mn3 O4 composite was employed as an active electrode material [73]. The composite with 86% Mn3 O4 showed excellent capacitive activity with a maximum value of 260 F g−1 at a current density of 50 mA g−1 and achieved good cycling stability after 1000 galvanostatic charge/discharge

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Fig. 9. (a) CV curves at different scan rates, (b) galvanostatic charge/discharge curves at different current densities, (c) ragone plot of the supercapacitor, and (d) capacitance retention vs the cycle number measured at 50 mA g−1 for symmetrical supercapacitor using the graphene/Mn3 O4 (86% Mn3 O4 ) as the electrode material [73]. Reprinted with permission from the publisher.

cycles. The capacitance retentions of 92% and 94% at 100 mA g−1 and 2 A g−1 , respectively with 97–99% Coulombic efficiency was attributed to low electronic resistance and pseudo-capacitance contributions. Further, a symmetrical supercapacitor based on graphene/Mn3 O4 (86% Mn3 O4 ) composite exhibited high specific capacitance of 63 F g−1 at 50 mA g−1 with specific energy density reaching 8.7 Wh kg−1 and excellent cycle stability with capacitance retention of 94% after 1000 charge/discharge cycles (Fig. 9). Similarly, anthracite-derived graphene sheets (GS) showed good electrochemical performance in a 6 M KOH electrolyte [80]. A quasi-rectangular shape of the cyclic voltammetry (CV) profiles and a triangular shape of galvanostatic charge/discharge curves were noted with an initial specific capacitance of ∼200 F g−1 at 50 mA g−1 , which remained at ∼140 F g−1 with 1 A g−1 and retained around 70% capacitance after 1000 cycles at 100 mA g−1 . He et al. [75] utilized 3D hollow porous graphene balls (HPGBs) synthesized from coal tar pitch as an electrode material for supercapacitors. In this unique porous structure, the graphene shells significantly enhanced the electronic conductivity while large specific surface area of 1871 m2 g−1 with micro/meso/macropores effectively promoted the electrolyte ion transport and reduced ion transfer resistance. As a result, the HPGBs exhibited a high specific capacitance of 321 F g−1 (17.2 ␮F cm−2 ) at a current density of 50 mA g−1 , superior rate capability of 244 F g−1 at a current density of 20 A g−1 and good cycling stability with 94.5% capacitance retention in 6 M KOH after 1000 charge/discharge cycles at 100 mA g−1 . The authors prepared 3D interconnected graphene nanocapsules (GNCs) from coal tar pitch and used them as active materials in supercapacitors [74]. The fabricated GNCs demonstrated

a capacitance of 277 F g−1 (127 F cm−3 ) with an energy density of 9.6 Wh kg−1 at 50 mA g−1 and a rate capability of 213 F g−1 (10.7 ␮F cm−2 ) and 194 F g−1 (9.8 ␮F cm−2 ) at 10 and 20 A g−1 , respectively. Moreover, 97.4% capacitance could be retained after 15,000 galvanostatic charge/discharge cycles at 1.5 A g−1 . The excellent electrochemical behavior of GNCs stemmed from their unique capsule structure with good electronic conduction, high surface area and quick ionic transportation. Zhao et al. fabricated a binder-free supercapacitor based on PAN and coal-derived carbon nanofibers (PAN-coal-CFs) [82]. Quasirectangular and quasi-triangular shapes of the CV plots at variable scan rates and galvanostatic charge/discharge curves, respectively revealed electric double layer capacitive response of the electrode whereas a hump in CV profiles and a slight distortion of galvanostatic charge/discharge plots was induced by pseudo-capacitance of oxygen functional groups. The PAN-coal-CFs demonstrated a specific capacitance of 230 F g−1 at 1 A g−1 and excellent cycle stability with an capacitance retention of 97% over 1000 cycles at 2 A g−1 (Fig. 10), which might come from good contact between the active material and graphite current collector, and an high surface area of 902 m2 g−1 with numerous cavities inside the carbon fibers. Zhang et al. [95] prepared a high performance supercapacitor electrode based on activated hierarchical porous carbon nanosheets (HPCNs) from bituminous coal-derived GQDs. It was observed that when the current density increased from 1 to 100 A g−1 , the specific capacitance decreased by only 26% from 230 to 170 F g−1 , implying a superior rate capability of the activated HPCNs. Moreover, the material demonstrated an excellent endurability with no obvious fading in specific capacitance after

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Fig. 10. Electrochemical performance of PAN–coal-CFs measured in a three-electrode system using 6 M KOH aqueous solution as the electrolyte: (a) CV curves at scan rates from 5 to 100 mV s−1 ; (b) plot of current density vs scan rate; (c) galvanostatic charge–discharge curves at different current densities ranging from 1 to 50 A g−1 ; (d) cycling performance of PAN–coal-CFs electrode at 2 A g−1 [82]. Reprinted with permission from the publisher.

10,000 galvanostatic charge/discharge cycles at a current density of 10 A g−1 . The outstanding electrochemical activity of activated HPCNs came from their good electronic conductivity, high surface area (1450 m2 g−1 ) and micro/mesoporous distribution, providing more active sites for energy storage and more channels for ionic transport. Meanwhile, a report by Guo et al. [84] adopted coal based hierarchical porous carbon spheres (PCS) as an electrode active material for supercapacitors. They found that CV plots were in a quasi-rectangular shape with a broad peak between 0.2 and 0.8 V, which was attributed to the surface oxygen pseudocapacitance. The material exhibited a high specific capacitance of 227 F g−1 at 1 A g−1 and superior cycle stability in 6 M KOH after 10,000 charge/discharge cycles at 2 A g−1 . In addition, a symmetrical device based PCS showed a specific capacitance of 180 F g−1 at 0.2 A g−1 , which remained at 112 F g−1 at 5 A g−1 , indicating an excellent rate capability. 3.2. Environmental applications 3.2.1. Catalytic applications The noble metal (Pt, Ru and PtRu) nanoparticles deposited on anthracite-derived graphene sheet (GS) was employed as a catalyst in selective catalytic reduction (SCR) of NOx (92% NO and 8% NO2 ) with NH3 in a quartz reactor [80] via the following reaction: 4NO + 4NH3 + O2 = 4N2 + 6H2 O It was found that the PtRu/GS composite exhibited the superior catalytic performance with over 90% NO conversion at 165 ◦ C, which may have resulted from a synergistic effect between Ru and Pt.

The sub-bituminous coal-derived carbon nanomaterials were also used to photo-catalytically degrade 2-nitrophenol (2-NP) under sunlight illumination [72]. It was found that over 92% of 2-NP was effectively degraded within 40 min with the adsorption efficiency above 21% at 2-NP concentration of 0.1 mM, catalyst loading of 0.5 g L−1 and pH 5. The photodegradation mechanism of 2-NP was proposed as follows: O2 + e− → O2 •− −

•−

−

•

e + O2

(1) •

+ H2 O → H2 O + OH +

e + H2 O + H → H2 O2 −

•

e + H2 O2 → OH + OH •

−

OH + 2-NP → Degradedproducts

(2) (3) (4) (5)

The 2-nitrophenol, a sensitizer, generated electrons under sunlight and were then transferred to the surface of the carbon nanomaterials. The adsorbed oxygen on the surface of catalysts trapped the electrons, producing O2 •− radicals which then reacted with water to form highly active H2 O• , followed by its reaction with H+ to generate hydroxyl radicals, fragmenting 2-NP molecules. Therefore, coal-derived carbon nanomaterials played a key role in promoting continuous electron transfer from 2-NP to their surfaces, which inhibited the electron-hole recombination process, leading to excellent photocatalytic performance. Hu et al. [30] indicated that anthracite-derived CDs themselves could be directly applied to the photodecomposition of dyes (methylene blue MB and methyl orange MO) under 300 W xenon lamp irradiation with  < 420 nm cut-off filters. It was observed that the CDs contributed

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Fig. 11. The dependence of F/F0 on the concentration of Cu2+ in a range of 0–50 ␮ M. Inset shows the corresponding fluorescence response of r-CDs dispersion upon addition of various concentration of Cu2+ [90]. Reprinted with permission from the publisher.

to ∼90 and ∼85% decomposition of MO and MB, respectively, within 15 min under visible light with degradation rates of 0.135 min−1 and 0.115 min−1 , respectively, which was remarkably faster than those of the commercial P25 catalyst (0.012 min−1 for MO and 0.045 min−1 for MB). This is due to greater number of hydroxyl radicals generated by CDs than that of P25 under visible light illumination and various energy trap levels on the CD surface. 3.2.2. Separation applications The hollow porous carbon nanospheres synthesized from coal tar exhibited excellent adsorption capability for direct black 38 dye (C34 H25 N9 Na2 O7 S2 ) due to their large surface area of 1374 m2 g−1 and mesoporous structure [86]. The adsorption results fitted well with the Langmuir adsorption isotherm with the maximum capacity of adsorption qmax 294 mg g−1 . Recently, hydrophobic electrospun carbon nanofiber fabrics (ECNFs) with the water contact angle (WCA) of 142.6 ± 1.1◦ could separate toluene and water with a separation efficiency of >99.5% within a few minutes with a high toluene flux of 324 L m−2 h−1 [89]. High fluxes (250–410 L m−2 h−1 ) were also observed with other organic solvents such as methylbenzene, ethyl acetate, tetrahydrofuran, petroleum ether, etc., demonstrating that ECNFs are promising candidates for efficient oil/water separation. 3.2.3. Sensing applications Kumar et al. [77] developed electrochemical biosensors by modifying electrodes using bitumen-derived graphene, which was applied for detecting caffeine in the concentration range of 0.2–120 ␮mol L−1 with a limit of detection of 90 nmol L−1 (signal/noise = 3). The efficient performance of the biosensors was attributed to good electrocatalytic property of a few layer graphene for caffeine oxidation. In another investigation, Hu et al. [90] reported that reduced CDs synthesized from anthracite coal was successful for Cu(II) analysis due to their selective PL quenching with Cu2+ ions. The favorable response was attributed to the thermodynamic affinity of Cu2+ toward nitrogen and oxygen functional groups of the reduced-CDs, and rapid copper-to-ligand bonding kinetics. Interestingly, the quenched PL could be recovered by the addition of ethylenediaminetetraacetic acid (EDTA), which worked

as a Cu2+ ion chelator. The results showed that the PL quenching response was linear in the range of 0–0.5 ␮M Cu2+ concentration with a limit of detection 2 nM and a signal/noise ratio of 3 (Fig. 11). The interaction between metal ions and reduced CDs was rapid and stabilized within 3 min, revealing that this PL sensing with fast response is a promising candidate for real-time Cu(II) tracking in environmental applications. In addition, the feasibility of the PL sensor was further confirmed via PL quenching by adding Cu2+ to tap water and seawater, providing a linear correlation in the range of 0–500 nM Cu2+ with a recovery of 102.6–97.9% and 104.8–96.8% for tap water and seawater, respectively, demonstrating accurate and reliable analysis of Cu(II) concentrations with environmentally derived samples.

3.3. Biomedical applications Graphene oxide extracted from low grade coal was shown to be a potentially suitable vehicle for drug delivery [69]. With this experiment, donepezil (DL), an acetylcholinesterase inhibitor utilized in the treatment of mild to moderate Alzheimer’s disease was encapsulated by GO to form a GO-DL composite. The composite was found to provide the open clenched fist form of GO under pH 7.4, triggering the release of DZ molecules whereas no electronic signal of DZ was detected under an acidic pH of 6.8, demonstrating that drug molecules were stably entrapped inside the clenched fist GO in an acidic environment. Thus, the DL drug could be effectively delivered by closed fist shapes of coal-derived GO with its release depending on pH. The metal contaminants in coal and the difficulty with the removal of acidic species from the synthesized carbon based nanomaterials lower their biocompatibility, which limits their potential application in biomedicine. However, Geng et al. [110] managed to improve the biocompatibility of coal-derived carbon quantum dots by encapsulating the CDs in liposomes for in vitro and in vivo bioimaging applications. The liposome-CD composite was shown to exhibit low cytotoxicity with HeLa cells having a viability above 92% after incubation over one day. A dose of 40 mg mL−1 (Fig. 12g) was able to label the cytoplasm of the cancer cells, which was observed by a confocal microscope [110]. The images of the HeLa

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Fig. 12. Confocal fluorescence image at 405, 488, and 546 nm excitation (a–c) and merged image (d) of HeLa cells using liposome-CQDs internalized into the cytoplasm of the cells. (e and f) In vivo fluorescence images of HeLa tumor-bearing nude mice obtained in tumor sites at 0.5 and 1 h after intratumor injection of 200 ␮L of an aqueous solution of liposome-CQDs. The color bars represent the fluorescence intensity. (g) Cytotoxicity assessment of liposome-CQDs at higher doses for incubation time varied from 24 to 48 h using HeLa cells [110]. Reprinted with permission from the publisher.

cells treated with the liposome-CDs composite (40 mg mL−1 ) for 30 min at 37 ◦ C with laser excitations of 405, 488 and 546 nm are shown in Fig. 12a–d, which clearly reveals an improved fluorescence around the nucleus of the cancer cells, indicating the emissive penetration of CDs into the HeLa cells. Moreover, the in vivo imaging was further studied by injecting 200 ␮L liposome-CDs 40 mg mL−1 into HeLa tumor-bearing nude mice. They observed a strong PL signal with an emission maxima at 620 nm after 30 min injection, and an accumulation of CDs in the tumor area after 1 h under 580 nm light irradiation (Fig. 12e, f).

4. Conclusions, prospects & challenges Our review describes the synthesis of several carbon nanomaterials derived from coal using a range of fabrication routes. These processes enable specific properties, morphologies and size which greatly affect their applications ranging from energy, environmental to biological systems. A key challenge is to produce high quality carbon nanomaterials at large scales via environmentally friendly and inexpensive strategies. Most conventional synthesis techniques utilize high-priced carbon precursors leading to high cost of production, thus limiting its industrial scale-up and application. Replacing the existing carbon feedstocks by abundant and lowprice coal will be of significant benefit. In addition, instead of coal combustion to run thermal power stations, coal based carbon nanomaterials could be harnessed for sustainable energy generation and environmental remediation via electrocatalysis, separation of components in mixtures through efficient adsorption, and in sensing applications. Recent advances in the production of various carbon based nanomaterials from different types of coal (lignite, bituminous, anthracite, etc.) as raw materials and their potential use in energy, environmental and biomedical applications have been discussed. Among the fabrication strategies reported, CVD has shown advantages in the mass production of high quality carbon based nanomaterials such as graphene and CNTs while chemical oxidation is a simple and effective technique in large-scale

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