1 Introduction

Graphene is an one atom thick 2-D sheet of carbon isotope, recently attracted the researchers due to their discrete chemical, physical, mechanical, and electrical properties like high thermal stability, high electric conductivity with low power dissipation, high-electron mobility of 15,000 cm2 V−1 s−1, large surface area, superior flexibility, tunable band gap, quantum confinement effect, and edge effect [1,2,3,4,5]. Due to above properties, graphene is a suitable material for energy applications like lithium-ion battery, hybrid super capacitors, solar cells, and fuel cells [6,7,8].

In graphene, valance band and conduction band are slightly overlap, which makes it as a zero band-gap semi-conductor [4, 9]. Observation of photoluminescence is difficult in graphene sheets, which limits its application in organic LEDs, photodetectors, and photodiodes [2, 10]. However, infinite exciton Bohr radius phenomenon in 0-D graphene quantum dots (GQDs) and 1-D nanoribbon overcomes these limitations and enables their use in above-mentioned applications [11]. The comparison between them is mentioned in Table 1.

Table 1 Comparison between 2-D, 1-D and 0-D Graphene structure
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Einollahzadeh et al. (2016) reported the simulation study on penta-graphene band structure and energy band gap with Discrete Fourier Transform (DFT) and G0W0 approximation were calculated as 2.21–2.336 eV and 4.1–4.3 eV, respectively [12]. RugeQuhe et al. (2013) reported the experimental study on bilayer graphene (BLG) and concluded that the band gap opens to 0.255 eV on bilayer graphene (BLG) and can be fabricated as a Field-Effect Transistor (FET) [13]. The scholars and researchers around the globe have been working on graphene like controlling the size & shape and doping with functional groups to modify the properties of graphene for various applications [3].

Recently, researchers have been focusing on GQDs due to their unique size-dependent luminescence properties which makes it distinct from other carbon derivatives [14, 15]. GQDs also exhibit other excellent properties like high photostability, biocompatibility, low toxicity, tunable fluorescence, high quantum efficiency, and chemical inertness compared to other semiconducting quantum dots [16,17,18,19]. GQDs have high potential to be applied in optoelectronics, organic photovoltaics, electronic devices, catalyst, electrochemical application, ion sensing, and bioimaging [16, 17, 20]. In this perspective, we focused on strategic method to synthesis GQDs related with the energy storage and conversion application.

Wen-Wen Liu et al. (2013) reported the study of GQD-based micro-super capacitor prepared by electrochemical deposition. GQDs used in the study have 1.0–5.4 nm size distribution and were synthesized from graphene oxide. GQDs were deposited on gold electrode and the test was carried out in both aqueous electrolyte and ionic liquid electrolyte. The GQD-based micro-super capacitor exhibited excellent rate capability up to 1000 V s−1, and short relaxation time constant of 103.6 μs in aqueous electrolyte and 53.8 μs in ionic liquid electrolyte, and also has a stable life cycle [21]. Zonglong Zhu et al. (2014) reported their study on role of GQDs in perovskite solar cell for faster electron extraction. The GQDs were synthesized by facile electrochemical method having an average diameter of 5–10 nm and were coated as an ultrathin layer between perovskite and Titanium dioxide (TiO2). The GQDs have improved the power conversion efficiency of solar cell from 8.81% to 10.15% and fast electron extraction time from 260–307 to 90–106 ps [22]. Li Ruiyi et al. (2015) studied the electrochemical performance of lithium titanate/nitrogen and sulfur co-doped graphene quantum dot hybrid to enhance the anode for lithium-ion battery, and they reported that the specific discharge of hybrid lithium-ion battery is 254.2 mAh g−1at 0.1°C and 126.5 mAh g−1 at 10°C. The capacity remains at 96.9% for at least 2000 cycles at 2°C [23]. This shows that the GQDs can be used in energy conversion devices, electronics, and optoelectronics for faster electron extraction [22].

2 Synthesis routes

Synthesis of graphene quantum dots (GQDs) is broadly classified into two categories; they are Top-Down and Bottom-Up approach. Top-Down approach is nothing but breaking down from bulk scale to nanoscale. In case of GQDs, the Top-Down approach involves decomposition and exfoliation of carbon derivative precursors like graphene sheet, graphite powder, carbon nanotubes, carbon black, and carbon fiber shown in Fig. 1 [4, 24]. The different types of methods available are hydrothermal or solvothermal cutting, ultrasonicated exfoliation, acidic or plasma oxidation, nanolithography technique, and electrochemical and photo-Fenton reaction.

Fig. 1
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Top–down approach of GQDs’ process

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The Bottom-Up approach is building up from atomic scale to nanoscale. In case of GQDs, this approach uses aromatic compounds like benzene and C60 (fullerene) as precursors, since the shape and size distribution of GQDs can be controlled during their growth. The techniques involves microwave-assisted method, cage opening, and stepwise organic synthesis, as shown in Fig. 2 [4, 24,25,26,27].

Fig. 2
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Bottom–up approach of GQDs’ process

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2.1 Top-down approach

2.1.1 Hydrothermal and solvothermal method

These processes are more effective to synthesize two dimensional quantum dots and also simple and cost effective when compared to other processes [28]. In hydrothermal method, water is used as a solvent, whereas in solvothermal method, organic solution like DimethylFormamide (DMF) is used as a solvent [4, 24, 25]. Pan et al. (2010) synthesized blue luminescent GQDs having an average diameter of 9.6 nm by oxidization of graphene sheets (GSs) and deoxidization of oxidized GSs at low-temperature range of about 200–300 °C [29]. Initially, the precursors were treated with strong oxidizing agents like concentrated sulphuric acid (H2SO4) and concentrated nitric acid (HNO3) at high temperature. The oxidization steps involves creating epoxy (C–O–C), carbonyl (C=O), and carboxylic function group (− COOH) at edge, which rupture and break C–C bond. Furthermore, alkaline hydrothermal reaction tends to remove the epoxy and carbonyl compounds. The carboxylic function groups remain stable at the edge, which is responsible for dispersing GQDs in water. The oxygen atom is removed by further thermal treatment at high temperature of the solution. The selection of temperature in thermal reduction steps has large impact in breaking down the precursor [4, 29]. A group of researchers have reported about the synthesis of green luminescent GQDs of 3.6 nm average diameter from the graphene oxide sheets at high temperature using thermal reduction method [30].

Wang et al. (2014) reported preparation of functionalized GQDs with amine group at average lateral distribution of 3.5 ± 0.6 nm with thickness of 1.47 ± 0.86 nm by facile molecular fusion route under mild and green hydrothermal conditions. The synthesis of amine functionalized GQDs involves in nitration of pyrene using nitric acid in alkaline aqueous solution. In this process, alkaline plays an important role in functionalization of GQDs with (−OH, −NH2, and − NHNH2), which is done by intermolecular dehydrolysis in graphene framework [31].

Zhu et al. (2011) synthesized GQDs using DMF as a solvent by one-step solvothermal method. The GQDs have 5.3 nm average diameter with thickness of 1.2 nm. Due to the presence of chemical groups like – OH, epoxy/ether, C=O, and –CO–NR2, it exhibits excellent solubility of 15 mg/ml in acetone, tetrahydrofuran (THF), DMF, ethanol, dimethyl sulfoxide (DMSO), and water. The synthesized GQDs produced strong luminescence of quantum yield about 11.4% [32]. Liu et al. (2013) synthesized nitrogen-doped GQDs having average diameter of 2.5 nm by solvothermal method. They used graphene oxide (GO) as precursor and DMF as solvent, where DMF undergo decomposition at 200 °C to obtain dimethylamine for nitration of GO for 4–5 h at 200 °C [33]. Even though the hydrothermal and solvothermal methods are very efficient, control of distribution of size and morphology of the GQDs is a drawback in these methods.

2.1.2 Ultrasonic waves assisted

Ultrasonic wave-assisted liquid-phase exfoliation generates altering low- and high-pressure regions in the liquid which induces cavitation that creates hydrodynamic shear forces and high-speed impinging liquid jets into the solution. The force created by the gradient pressure utilized to exfoliate the bulk graphite layers and for the synthesis of GQDs from glucose precursor needs carbonization and polymerization with the same force mentioned. This will help to synthesize GQDs with increase in reaction rate and to reduce edge defects on graphite layers [34,35,36,37].

Kumar et al. (2014) synthesized of amine functionalized GQDs by passing ultrasonic waves at 250 W power for 30 min. The size distribution of GQDs was approximately between 5–7 nm and 20–30 nm when using quantum graphene flakes as precursor with the same parameters applied before. Due to p orbital hybridization of C–N atoms at the edge sites make additional inter-band energy of 3.28 eV. Photoluminescence quantum yield was found to be 29% for GQDs and 16% for QGFs, with spectrum excitation at 310 nm. The energy transfer efficiency of GQDs and QGFs is 17% and 15%, respectively [38]. Zhu et al. (2014) synthesized high quantum yield GQDs by one-step ultrasonication synthesis for alkaline phosphatase sensing. The average diameter of GQDs is 3.0 nm with thickness range between 0.7 and 3 nm. The excitation achieved at 380 nm along with emission at 470 nm. The synthesized GQDs have about 27% of quantum yield at pH 7 [39]. Ali et al. (2016) reported GQDs synthesized from graphene flakes through grinding-assisted co-solvent ultrasonication operating at 100 W for 1 h. This involved chemo-mechanical techniques which was mortar grinding and ultrasonication where energy transfer is used to breakdown the graphene nanoplatelets. This method produces GQDs at a diameter range between 2 and 4 nm with lattice space of 2.4 nm. The GQDs gave a strong luminescence at 350 nm with band-gap energy of 2.6 eV [40].

2.1.3 Electrochemical method

This electrochemical method involves use of carbon derivatives such as graphite, CNTs, carbon nanofibres as electrodes, and platinum as the counter electrode. This method lacks to prises the size and surface morphology, and further undergoes filtration and chromatography for separation to obtain monodispersed GQDs. Major drawback of electrochemical synthesis methods is that they have low photoluminescence efficiency on GQDs [41,42,43,44].

Zhou et al. (2006) studied blue luminescent nanocrystals (NCs) from multiwalled carbon nanotubes (MWCNTs) by produced by electrochemical method. The size distribution of NCs being 2.8 nm in diameter with spacing distance from 3.1 to 3.4 Å and the quantum yield about 0.064, excited at 340 nm [45]. Zhang et al. (2012) studied highly fluorescent water soluble GQDs from graphite rod through electrochemical method for biological labeling of stem cells. Uniform graphene sheets were observed with size distribution range between 5 and 10 nm providing the interspacing distance about 0.3 nm. By following this method, the strong yellow luminescence could obtain with 14% yield, at the excitation of 540 nm. From this experiment, the GQDs were used to penetrate stem cells for studying the metabolism activity and self-renewability of stem cells without affecting their viability, proliferation, strong photoluminescence, and photostability with low cytotoxicity [46]. Tan et al. (2014) reported small-sized red fluorescent graphene quantum dots as a bioimaging platform synthesized through electrochemical method with uniform size distribution about 3 nm of diameter with 0.24 nm as interspacing distance. The fluorescence decay of GQDs is observed with initial emission of red fluorescence at 600 nm and then emission of yellow fluorescence at 550 nm after 3 h. Six hours later, the emission is converted to green fluorescence at 500 nm. At last after about 12 h, the GQD solution emitted blue fluorescence at 420 nm [47]. Luo et al. synthesized GQDs through electrochemical oxidation of graphene with uniform distribution range of 3–5 nm and thickness of 0.93 nm for single layer graphene sheet. This study reported the synthesis of large size GQDs of about 52 nm diameter, which is purified with dialyzing solution using dialysis bag. The GQDs solution possessed blue emission under 365 nm irradiation [48].

2.2 Comparison between top–down approaches

Form the studies between different top–down processes are analyzed and verified. The deoxidization mechanism is the basic process under-grows in every cutting top–down method of the bulk carbon precursor. The main difference in source involved in the process. Comparing them the hydrothermal process attracted many people due to simple process in the deoxidization. The hydrothermal can further enhanced by addition ultrasonication and electrostatic force. Few more example are given in Table 2.

Table 2 Top–down synthesis method for GQDs
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2.3 Bottom–up approach

2.3.1 Microwave assisted

The microwave-assisted method synthesizes GQDs at a faster rate and homogenous heating is employed with precursors with uniform size distribution. The main advantage is that the GQDs are synthesized without any passivizing agent. The time period of passing microwaves is an important parameter, since increase in time period increases heating time which in turn increases the size of the GQDs, respectively [49,50,51].

Zhang et al. (2015) produced GQDs using aspartic acid and ammonium bicarbonate NH4HCO3 by one-pot microwave-assisted synthesis at 560 W operating power for detection of iron ions and pH. These GQDs have 2.1 average diameter and 14% of quantum yield exhibiting blue luminescence [52]. Zheo et al. reported green fluorescent GQDs derived from deoiled asphalt using one-step microwave-assisted method at 600 W power. The GQDs’ size distribution was in the range 1–6 nm, producing green luminescence at 365 nm excitation and high quantum yield up to 14% [53]. Kumawat et al. (2018) reported one-pot microwave-assisted green synthesis route at 900 W for the synthesis of red-luminescent GQDs from ethanol extracts obtained from Mangifera indiia (mango) leaves. The size distribution of GQDs synthesized from the extract ranges from 2 to 8 nm and the fluorescence emission was found to be between 650 and 750 nm [54].

2.3.2 Cage opening process

The cage opening process basically uses fullerene C60 to produce GQDs by breaking done the covalent bond between carbon atoms by the host molecules.

Chua et al. (2014) reported the study of strong fluorescent GQDs by cage opening Buckminster fullerene. This process undergoes oxidation, cage opening, and fragmentation on C60 (fullerene) by strong acid, which produce the GQDs in the sized range of approximate 2–3 nm with 0.6–1 nm height. Show the intensity at 460 under 340 nm excitation wavelength [55].

2.3.3 Stepwise organic synthesis

The solution chemistry synthesis processes provide identical molecular structures to the grapheme which leads to large quantity of production. These molecular modification leads to interact with other complex materials [56,57,58]. Even though it can produce monodispersed GQDs with uniform size distribution, it cannot prevent GQDs from aggregation caused by the π–π-bond interaction [59].

Yan et al. (2012) reported the study of GQDs synthesized by stepwise organic synthesis method and its mechanism. The polyphenylene dendritic precursor oxidized and make of 168, 132, and 137 conjugated carbons. To make GQDs as more stable due to the 2ˊ, 4ˊ, 6ˊ-triakyl phenyl groups are attached covalently to the ten positions of the graphene moieties [60].

2.4 Comparison between bottom–up approaches

Comparison between bottom–up microwave processes is the attracted method, which involves hydrothermal process irradiated with microwave to form GQDs from carbon derivatives like glucose and starch, however, which lack in size control. To control the size cage opening and step organic synthesis are complimentary to microwave-assisted process. Its lack in low quantum yield and complicated process comparing to microwave-assisted. Due to that, the many researcher scholars are following microwave-assisted and more related reference are given in Table 3.

Table 3 Bottom-up approach for GQDs
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3 Energy-related applications of GQDs

3.1 Capacitors

Generally, capacitors are classified into two types based on the adsorption/desorption and surface redox reactions. The need for best capacitor material is satisfied with the GQDs, because of the existence properties like high surface area, conductivity, long life stability, rapid adsorption and desorption rate, etc., [74]. Super capacitors which possess specific energy storage and high surface area along persistent cyclic behavior with the help of the charge carriers present which could be applied for on-chip operations in micro-power systems [75]. Integrated power sources for displays, FETs, etc., with transparent look attracted the researchers to move on to GQDS [76, 77]. The highlighted properties of GQDs include optical transparency with nanoscale size, high-electron mobility, and elevated strength along with additional micro pores expanded its capacitance power [78,79,80,81].

Complete discussion of Table 4. Keunsik et al. (2016) reported the high capacitance performance of GQD-based supercapacitors fabricated by simple electrophoretic method about 9.09 µF cm−2 with optical transparency of 92.97% and 550 nm [75]. Sanjoy et al. (2015) reported, in his study, about the high capacitance retention of 80% over 3000 cycles which had been obtained through the introduction of 6.0 nm-sized GQDs with a capacitance range of 1044 Fg−1 [82]. Wen-wen liu et al. (2013) mentioned in his work about GQD-based micro-capacitors synthesized by one-step solvo thermal method which exhibits capacitance and density of energy in 1107.4 µF cm−2 and 0. 154 μWh cm−2. [83]. GQDs fabricated via a new method of top–down strategy which has shown the density of energy value as 41.2 W h kg−1 with high capacitance of 307.6 F g−1 and outstanding retention capacitance even after 5000 cycles had been reported by Zhang Shuo et al. (2018) [84]. Jiahuan Luo et al. (2019) demonstrated the excellent specific capacitance of tremella-shaped NiCo2O4 encapsulated by graphene quantum dots (GQDs) and specific energy density of asymmetric supercapacitor. They have been synthesized GQDs by solvothermal assisted by ultrasonicated and confirm the uniform size distribution of 2—4.5 nm with 0.21 nm interlayer spacing by HR-TEM. The GQDs encapsulate NiCo2O4 by annealing at 300 ℃ for 24 h. They have compared the specific capacitance of pristine NiCo2O4 and NiCo2O4@GQDs. NiCo2O4@GQDs exhibit 1242 F g−1 at 30 A g−1 in 2.0 M KOH, which show 57% enhanced specific capacitance compared to pristine NiCo2O4 (790 F g−1 at 30 A g−1) and 99% retention capacitance after 4000 cycles. Asymmetric supercapacitor (ASC) is fabricated by pairing NiCo2O4@GQDs and activated carbon (AC) as positive electrode and negative electrode. Its shows enhanced specific energy density is as high as 38 W h kg−1 at a power density of 800 W kg−1 [85].

Table 4 GQDs for capacitor
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3.2 Solar cells

From the conventional Si wafers to metal oxide, lead-based composite wafers had been used in solar cells with low power conversion efficiency which moves forward the solar cells for GQD-based wafers [89]. Silicon, perovskite, and TiO2-based solar cells are moving in a broad way of research due to its biocompatible nature, chemical stability, and low cost [90,91,92]. The GQDs indulged in solar cells applications exhibited the properties like hole/electron transport, additive active layer in the donar and acceptor blends, and as a sensitizer too [93, 94]. While comparing with TiO2 and CdSe-based quantum dots, GQDs enhances the solar cell efficiency by the existence of quantum confinement along size specified band gap, hot-electron lifetime, and ultrafast electron extraction with distinctive optical and electrical transport properties [95,96,97].

Briefing of Table 5,  Travis G. Noval et al. (2016) mentioned in his study about the PEG functionalized GQDs application on solar cells with an improved efficacy of 36% along the conversion energy of 6.63% [98]. Zonglong Zhu et al. (2014) reported in his study about the ultrathin GQDs layer which is sandwiched between the perovskite solar cell and mesoporous titanium dioxide increases its efficiency of greater than 10%, where before it was only 8.81% without the presence of GQDs [99]. Senlin Diao et al. (2017) reported the GQDs as an excellent heterojunction solar cell with optimal efficiency of 12.35% about 20 nm thickness along with the remarkable physical and chemical stability [89]. Liu et al. (2017) mentioned in his study about the electrochemical stability and efficiency of SrRuO3 encoded GQDs of about 8.05% than before encoded [100].

Table 5 GQDs for solar cell
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3.2.1 Lithium-ion batteries

Lithium-ion-based batteries are having the specific capacity and a remarkable energy source which could be applied in electric vehicles, defense, smart electric grids, and other electronic devices [105, 106]. The foremost properties of Li-ion batteries like long cycle usage without memory effect and high power density made it as a noticeable chemical energy storage device [107]. Li-ion batteries almost reached the capacity level up to 2.6 mAhg−1 than the conventional one and also possesses high-quality lithium-ion cells for consumer use also [108].

Briefing of Table 6, Chao et al. (2015) reported in his study about the GQDs anchored VO2 electrode arrays exhibits the retention percentage of 94 for 1500 cycles with 36% capacitance and 389 mAhg−1 capacity, while GQDs was used as a anode material [109]. Li Ruiyi et al. (2015) mentioned in his study about the electrochemical performance of sulfur co-doped GQDs, with a specific discharge capacity of 254 2mAhg−1, involves 2000 cycles at 2C along with retention percentage of 96.9 [110].

Table 6 GQDs for lithium-ion battery
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4 Conclusion

In this review, our perspective have discussed various synthesis techniques and its relevant application of GQDs. Top-down and bottom-up approaches utilized for the synthesis, provides optimized and unique quantum yield for the GQDs, have been demonstrated through this work. Comparing with the bottom-up approaches, hydrothermal-assisted techniques under top-down approach possessed the highest quantum yield of 78% and particle-size range of 2–3 nm. The factor affecting GQDs such as shape, size, surface effect, doped material, etc., have still in an unclear condition in accordance with theoretical evaluation. When it comes to large production of GQDs, uniformity has been a great challenging, because synthesis of GQDs involves multiple steps. The developments in synthesis process have great concern.

10% of references have taken from past 2 years, which enlighten the GQDs current energy-based application. The multi-tasking ability of GQDs such as quantum efficacy with inert chemical nature, etc., which could made it applicable in different fields of electronics and energy applications. Electrophoretic method fabricated supercapacitors using GQDs provides the highest capacitance and optical transparency of 92.97%. SnO2 helps perovskite solar cell for enhance energy conversion efficiency up to 21.1% due to the incorporated GQDs were reported [102]. The current research moves onto a another leap, which plastic solar made up of polyaniline–GQDs nanocomposite with energy conversion up to 0.86%, reported by Firoz Khan et al. (2020) [101]. GQDs applied lithium-ion batteries exhibited the highest retention efficacy of 94% and 97% and possessed stability over 1500 cycles and above. In the other hand, Xiaojie Yin fabricated new type battery with NiO@Co3O4@GQDs composite material which exhibit excellent specific capacitance 1158 mAhg-1 with 76% retention over 3000 cycles. Thus, GQDs are the best and novelized way for building energy-based applications.

By the obtained literature reviews, GQDs an exceptional material gained with ease of synthesis provoked a tremendous change in energy-based application. Being a 0-D nanomaterial, it has been put forth into organic semiconducting QDs. Moreover, GQDs started an ear in the advance material and applied technologies for the further development in science.