DU Zheng,SHEN Shu-ling,TANG Zhi-hong,YANG Jun-he

（School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China）

Abstract：Graphene quantum dots (GQDs),as a unique member of the nanocarbon family,have become important catalysts for overall water splitting and metal-air batteries because of their high specific surface area,abundant surface chemical reaction sites and high electron mobility.Understanding the fundamental catalytic mechanism of GQDs in heterogeneous catalysis is conducive to the rational design of high performance GQD-based catalysts.This article summarizes current research progress in the synthesis,modification and applications of GQD-based heterogeneous catalysts in overall water splitting,metal-air batteries and other fields.The issues related to the use of GQD-based catalysts in these fields are discussed together with their future development.

Key words:Graphene quantum dots；Catalysts；Overall water splitting；Metal-air batteries

1 Introduction

With the development of human society,the issue of energy sustainability has been taken seriously.The development of new energy technology has aroused widespread attention,and many new kinds of energy technologies,such as hydrogen energy from water splitting and metal-air batteries,have been developed.These technologies are restricted by the activity of catalysts.Noble metals have excellent catalytic activity,but their high price and rarity hinder their wide applications.Carbon-based catalysts have been regarded as alternative candidates of noble metal catalysts owing to their rich resources,low price,high electrical conductivity and environmentally friendliness[1,2].

Graphene quantum dots (GQDs),a kind of carbon-based quantum dots[3–5],are small pieces of graphene sheets with lateral sizes of less than 20 nm and thicknesses of about 0.4-2.0 nm.Graphene-based structural units give GQDs with peculiar properties,such as high electron mobility and high specific surface area.In addition,the quantum confinement and edge effects endow GQDs many unique physical and chemical properties,such as non-zero energy gap and photoluminescence,which greatly promotes the applications of graphene in the field of microelectronics[6],photoelectric conversion[7,8],sensors[9–11],biological imaging[12–15]and catalysis[16].The functions of GQDs are different depending on the applications.For example,in photovoltaic cells,GQDs are commonly used as an electron-acceptor and a buffer layer.In addition,GQDs are excellent hole transport and extraction layer material[17,18].When GQDs are used in light emitting diodes (LEDs),they usually serve as a phosphor and an active layer.Wang[19]prepared white light emitting GQDs that were used as the light-emitting phosphor for white light emitting diodes (WLEDs).It shows the best white electroluminescence performance among all WLED devices based on carbon nanomaterials due to their stability,high conductivity and nontoxicity.Very recently,Li et al.[20]demonstrated that GQDs can enhance the magnetic relaxivity of magnetic resonance imaging (MRI) contrast agents(GPG) by providing a localized superacid microenvironment to accelerate proton exchange.Additionally,folic-acid-modified GPG can be applied as the MRIfluorescence dual-modality for tumor targeting imaging in animals with a high specific cellular uptake rate of ＞ 98.3%.

Since the term of“graphene quantum dots”first appearance in 2008[21,22],concerns on GQDs have gradually increased,especially in recent years (Fig.1).During the past decade,the research on GQDs mainly focus on the synthetic methods,photoluminescence mechanism and their applications based on their photoluminescence properties,which have been summarized in several excellent reviews[23–26].As a new kind of carbon-based materials,GQDs have demonstrated huge potential in catalysis.This review will focus on current research progress in the synthesis and modification of GQD-based heterogeneous catalysts for overall water splitting,metal-air batteries and other fields.

Fig.1 Articles about GQD-based materials indexed in the web of science core database.

2 Synthesis and modification of GQDs

2.1 Synthesis of GQDs

The methods for preparation GQDs can be classified into two categories:the top-down and bottom-up methods[27](Fig.2).For the top-down methods,large graphene sheets or graphite-related materials are usually used as precursors,such as graphene oxide(GO)[28,29],carbon fibers[30],carbon nanotubes[31],C60[32],and carbon nano-onions[33],which are cut into nanodots by a series of physical and chemical processes such as the hydrothermal/solvothermal method[34,35],acid oxidation[30],electrochemical oxidation[36,37],the microwave-assisted method[38]and physical grinding[39,40].Such methods have the advantages of relatively simple operation and high yield.However,due to the randomness of the cutting sites,it is difficult to accurately control the size and uniformity of GQDs.In addition,cutting processes usually cause various atomic-scale defects in GQDs.The bottom-up methods are mainly based on chemical synthesis,in which GQDs are synthesized by aryl oxidative polymerization or carbonization of organic molecules[41–45].Comparatively,the bottom-up methods can precisely control the size and uniformity of GQDs.But the yield of GQDs is low and the purification and separation of obtained GQDs are difficult.More recently,many advanced methods and new precursors were developed to balance the quality and yield of GQDs.For example,Shen and coworkers[46]reported a new precursor of aphanitic graphite (AG),which is much cheaper than flake graphite.AG contains many immature graphite nanocrystals with the size of about 10 nm.So high quality GQDs can be obtained by directly exfoliating AG without applying high strength cutting.Lee et al.[47]recently proposed a dry synthesis route of hydrogen-assisted pyrolysis of SiC followed by sonication in ethanol.The obtained GQDs exhibited high crystallinity and clean surface,which displayed UV/blue light emission with a high quantum yield of 30.9%.Obtaining high quality GQDs with high yield and low cost is a critical issue for deeper research and practical applications in catalysis.

Fig.2 Synthesis of GQDs:(a-e) the top-down methods,(a) solvothermal of dimethylformamide (Reproduced with permission[34],Copyright 2013,American Chemical Society),(b) acid oxidation of carbon fibers (Reproduced with permission[30],Copyright 2012,American Chemical Society),(c) electrochemical oxidation of graphite (Reproduced with permission[36],Copyright 2009,American Chemical Society),(d) microwave-assisted oxidation cutting of graphene oxide(Reproduced with permission[38],Copyright 2012,Royal Society of Chemistry),(e) physical grinding of graphite with tartrate tetrahydrate (Reproduced with permission[39],Copyright 2016,Wiley-VCH),f-h) the bottom-up methods,(f) carbonization of rice powder (Reproduced with permission[41],Copyright 2016,Royal Society of Chemistry),(g) hydrothermal treatment of citric acid (Reproduced with permission[44],Copyright 2012,Elsevier) and (h) microwave-assisted hydrothermal treatment of catecholamines (Reproduced with permission[45],Copyright 2016,Wiley-VCH).

2.2 Modification of GQDs

Modification of the surface structure or groups by chemical methods endows GQDs specific properties or improved performance.So,the physicochemical properties of GQD catalysts are tunable by chemical modification.The modification methods mainly include doping heteroatoms,surface modification and constructing heterostructure materials.

2.2.1 Doping heteroatoms

It has been demonstrated that the electronic properties of graphene and carbon nanotubes can be tuned by substitutional doping with heteroatoms,which expand their applications[48–52].Doping heteroatoms in GQDs also have great effect on the electronic characteristics,the surface and local chemistry of GQDs[53–55].Most of previous research on doped GQDs focus on the control of their optical properties,and the doped GQDs with tunable photoluminescence are mainly used in bioimaging and sensing.In the last decade,the catalytic activity of doped-GQDs has gradually attracted attention.Nitrogen (N) is the most typical atom doped in GQDs for tuning their optical properties in the literature[56–61].Inspired by the highly efficient catalytic activity,the significant quantum confinement and edge effect of GQDs induced by Ndoping,nitrogen-doped GQDs (N-GQDs) were successfully synthesized.N-GQDs not only exhibited excellent luminescence characteristic but also high electrocatalytic activity for the oxygen reduction reaction(ORR).Li[62]and coworkers[63]found that N-doping led to the size-dependent electrocatalytic activity of GQDs for ORR.Furthermore,the configurations of the nitrogen dopants within the lattice of GQDs(pyridinic,pyrrolic,graphitic,and oxidized N) and N content are generally believed to confer the selectivity and catalytic activity of N-GQD catalysts.Saidi[64]proposed that pyridinic and graphitic nitrogen in NGQD catalysts are the most active sites for ORR.In addition to the configuration,the location of N dopants also affects the catalytic activity of N-GQDs.Wu and co-workers[65]found that the pyridinic N at the edge sites of GQDs possesses particularly high catalytic activity for hydrogenation of CO2.The pyridinic N at the edge sites of N-GQDs not only lead to the decrease of initial reaction temperature for CO2reduction but also promote the CO2conversion and selectivity (Fig.3).In a very recent report,it is further proved that a large number of pyridinic N located at the edge sites of amidated GQDs provided additional active sites to adsorb protons and store hydrogen atoms.So,these amidated GQDs exhibited excellent electrocatalytic performance for ORR in both acidic and alkaline electrolytes[1].

Fig.3 (a) Schematic diagram of NGQD catalyst for CO2 hydrogenation at moderate reaction temperatures,(b) X-ray absorption near edge spectrum of N Kedge for NGQDs (the inset is a scheme of different N−C bonds (pyridinic (black),pyrrolic (blue),and graphitic N (pink))),(c) dependence of CO2 conversion on temperature over NGQDs/Al2O3 with three different loadings (0.8 wt %,1 wt % and 3 wt %) and (d) dependence of CO and CH4 selectivity on temperature over NGQDs/Al2O3 with three different loadings (Reproduced with permission[65],Copyright 2012,American Chemical Society).

Except N atoms,S and B atoms are also used as dopants to tune the optical and electrical properties of GQDs[66].N-S[67–69]or N-B[70,71]co-doping strategy is usually adopted due to the emergence of more active sites and the significant synergistic effect of doped heteroatoms.Favaro and coworkers[72]reported that the co-doping of N and B atoms in GQDs (N,BGQDs) can lead to a much lower overpotential for ORR than that of single element doped GQDs (B-or N-GQDs).With the help of photoemission spectroscopy,scanning tunneling microscopy and DFT calculation results,it is found that the better performance of N,B-GQDs should be attribute to the doped N atoms with a high electron-withdrawing power in GQDs,which not only improve the activity of adjacent C atoms by the donation and back-donation processes but also enhance positive charge on B.However,due to the uncontrollable relative distance of doped heteroatoms,it is difficult to quantitatively study the synergistic effect of doped atoms in experiments.How to“anchor”the doping sites and determine the distance between the doped atoms is of great significance to understand the role of the doping atoms and the synergistic effect between doped heteroatoms,which improve the catalytic activity of GQD-based catalysts.

2.2.2 Surface modification

Surface modification by introducing functional groups is an important way to adjust the properties of GQDs.GQDs synthesized by the top-down or bottomup routes usually contain oxygenated functional groups,such as hydroxyl,carboxyl and epoxy groups.Previous researches mainly focused on the controlling of optical properties of GQDs by changing these surface groups[73,74].Because these oxygen-rich groups are non-radiative fluorescent emission sites,removing of these groups by chemical reduction or grafting other functional groups is beneficial to obtain GQDs with tunable fluorescence or high fluorescence quantum yield.Yang et al.[75]tuned the fluorescence performance of GQDs by surface chemical modification and reduction.In general,grafting electron donating groups to GQDs can prevent non-radiative recombination,thus significantly increasing the fluorescence quantum yield[76].Deka and coworkers[77]tuned the wettability and photoluminescence of GQDs by covalent modification of oxygenated groups with dodecyl amine (DDA) and simultaneously reduced them with glycine (Fig.4).Although there is controversial in the photoluminescence mechanism on GQDs based on current researches,the unique properties of GQDs have been realized.More and more potential applications of GQDs were explored.For example,GQDs functionalized with hydroxyl group (OH-GQDs) were used as ammonia sensors.OH-GQDs displayed high response and selectivity to ammonia at room temperature due to the strong interaction between N in NH3and OH at the edge of OH-GQDs[78].Gao et al.[79]tuned the fluorescence properties of GQDs by controlling the amount of carboxyl and hydroxyl groups on the surfaces of GQDs.These obtained fluorescence GQDs showed excellent reversible and repeatable linear response to temperature,which could be applied as efficient temperature probes in living cells.Kwon et al.[80]functionalized GQDs with various aniline derivatives.Due to the interaction between the intrinsic energy levels of GQDs and aniline derivatives,new extrinsic energy levels are formed,which makes the photoluminescence linewidths of functionalized GQDs narrow dramatically.The electroluminescent diodes (LEDs) based on these functionalized GQDs exhibited the highest maximum current efficiency and external quantum efficiency than those of LEDs based on carbon-nanoparticle phosphors in previous reports.

Fig.4 (a) Schematic of the synthesis of hydrophilic and hydrophobic GQDs (C12-GQDs) from commercially available graphite nanopowder,(b) stacked photoluminescence emission spectra of GQDs,C12-GQDsHexane and C12-GQDsToluene (at an excitation wavelength of 360 nm) and (c) proposed photoluminescence mechanism of the GQDs and functionalized GQDs (C12-GQDs) (Reproduced with permission[77],Copyright 2018,Royal Society of Chemistry).

More recently,the catalytic activity of surface modified GQDs has come into focus.Yan et al.[81]demonstrated the continuously tuning of GQD bandgap from 2.4 to 1.88 eV by functionalizing GQDs with polyaromatic molecules or introducing intermediate n-orbital via conjugating with electron-donating functional groups,which all induced the expansion of the π-conjugated system.The precisely functionalized GQDs with narrow bandgap (broadening light adsorption) or a ‘Z scheme’ structure (promoting the separation of photo-induced electrons and holes) exhibited excellent photocatalytic activity for water splitting and carbon dioxide reduction under visible light.Another recent work reported a Rhodamine dye sensitized GQDs by covalently attaching Rhodamine 123 to the surface of GQDs (Fig.5)[82].The dye-sensitized GQDs displays a high photocatalytic activity for hydrogen evolution reaction (HER) due to efficient photoexcited electron transfer and the transformation of the ptype GQDs to the n-p ambipolar semiconductor both boosted by the covalent bonding.It can be found that up to now the related studies on surface-modified GQDs mainly focus on the tuning of photoluminescence properties and the applications mainly based on their optical properties.It is expected that the properties and application fields of GQDs can be expanded by surface modification,especially in the field of electrocatalysis.

Fig.5 (a) Energy profile diagram of dye-sensitized Rho-GQDs system for H2 evolution under visible light irradiation,(b) comparison of H2 evolution efficiency between covalent-bonded dye-sensitized Rho-GQDs system and Rho/GQDs system attached via electrostatic interactions along with virgin GQDs and Rhodamine 123 dye,(c) photocatalytic HER efficiency of covalent-bonded Rho-GQDs with different weight percentages of Rho after 4 h of visible light (λ＞400 nm) irradiation and (d) photocatalytic HER performance with Rho-GQDs in presence of 2 wt%-Pt as a co-catalyst during photocatalytic water splitting under visible light irradiation using 10 vol% triethanolamine as a sacrificial electron donor (Reproduced with permission[82],Copyright 2020,Elsevier).

2.2.3 Constructing heterostructures in materials

Constructing heterostructures in materials is conducive to combining the superior properties of each component to meet the requirements of different applications,which also results in a synergistic effect.Due to the excellent properties such as large specific surface area,high electron mobility,tunable optical properties,abundant active sites,nontoxicity,high chemical stability and oxygen-rich functional groups,GQDs have become a research focus to form heterostructures in materials,especially in the field of catalysis[83–85].In the photocatalysis field,noble metal nanocrystals are usually used as co-catalysts to enhance the photocatalytic performance of photocatalysts by promoting the separation of photogenerated carriers.However,noble metals are rare and expensive,which hinders their applications.Giri et al.[86]replaced noble metal co-catalysts with GQDs and fabricated TiO2/GQD heterostructures.The heterostructured materials show excellent photocatalytic performance under visible light irradiation.The enhanced photocatalytic performance is due to the enhanced interfacial charge transfer rate between GQDs and TiO2.Wu[87]composited metal-free g-C3N4with GQDs to form heterostructures and used the heterostructured materials for high efficiency photocatalytic solar water splitting.Similarly,in the electrocatalytic field,GQDs are applied as alternatives of noble metals.Zhou et al.[88]prepared GQD-decorated multiwalled carbon nanotubes (GQD-MWCNTs) as an allcarbon electrocatalyst for ORR.Compared with commercial Pt/C catalysts,GQD-MWCNTs exhibited higher current density,electrocatalytic selectivity and durability.As conductive supports,MWCNTs not only enhance the charge transfer rate,but also achieve the high dispersion of GQDs.In addition,efficient interface charge transfer is achieved through the π-π conjugation between GQDs and MWCNTs.This work clearly demonstrates that the synergistic effect of GQDs with another nanocarbon material by π-π conjugation can significantly improve the catalytic activity.This is proved by Wang and co-workers[89],they replaced MWCNTs with the graphene hydrogel (GH)as conductive supports to enhance the electron transfer rate.GQDs embedded in the framework of the graphene hydrogel are monodispersed and provide a large amount of active sites.Due to the synergistic effect of GQDs and GH,the GH-GQDs heterostructured material displayed excellent electrocatalytic activity and durability in alkaline solutions for ORR.The materials that can generate these synergistic effects with GQDs are not limited to carbon materials.Tian et al.[90]recently designed and synthesized heterostructured nanosheets (NSs) by engineering nickelcobalt phosphide (NiCo2P2) with GQDs (NCP/G NSs).The fabricated NCP/G NSs as a bifunctional catalyst for overall water splitting exhibit extremely low overpotential,low cell voltage,high stability and cost effectiveness.The impressive electrocatalytic performance of NCP/G NSs is attributed to the key roles of GQDs in controlling the morphology of NiCo2P2and in facilitating charge transfer and transport rates.Constructing heterostructured catalysts containing GQDs has the advantages of replacing noble metal catalysts with high cost,poor durability and availability,the synergistic effect and maximizing the superiority of GQDs by maintaining the monodispersity to fully expose active sites[91,92].

Although recent progress has been made in the modification and applications of GQDs,the study on precisely controlling the properties of GQDs to meet specific application requirements is still in its early stage.The physical and chemical properties of GQDs should be explored in deeper,especially the optical and electrical properties.

3 GQDs-based catalysts

3.1 Overall water splitting

In the development of new energy,hydrogen has attracted much attention owing to its high calorific value,environmentally friendly and diversified types of utilization.Electrochemical water splitting is an ideal method to realize the industrialization and cheap production of hydrogen.Two reactions occur simultaneously in the electrochemical water splitting,the oxygen evolution reaction (OER) on the anode and HER on the cathode.The thermodynamic electrical potential for a reversible electrolysis cell is 1.23 V[93].Theoretically,when the applied potential exceeds 1.23 V,water will be decomposed,and hydrogen and oxygen will be generated at two electrodes.GQDs have been applied as catalysts in electrochemical water splitting in recent years.Its application in this field began with Luo’s preliminary report[91]on the utilization of GQDs in HER in 2015,and then in 2017 Guo et al.[94]reported MoS2-decorated with GQDs for the use as a highly active HER electrocatalyst.The above researches further inspire the research of GQDs-based materials as bifunctional catalysts for overall water splitting.

In 2017,Zhu and co-workers[95]first applied GQDs in overall water splitting.They designed a novel noble metal-free nanocomposite material,which is composed of nitrogen-doped GQDs (NGQDs) and Ni3S2nanosheets on the surface of the Ni foam (Ni3S2-NGQDs/NF).The GQDs were synthesized by the microwave synthesis technology with watermelon as the raw material (Fig.6a).Using biomass as precursors for GQDs has the advantages of wide sources and low cost.In the electrochemical test,Ni3S2-NGQDs/NF is used as the electrode material in both cathode and anode.The results indicate that Ni3S2-NGQDs/NF shows the best catalytic activity both with the lowest onset potential and highest catalytic current density (Fig.6b).At the current density of 10 mA·cm−2,the overpotentials of Ni3S2-NGQDs/NF for OER and HER in an alkaline medium are 216 and 218 mV,respectively.It has the highest OER activity of all noble-metal-free electrocatalysts and HER activity is approaching that of Pt-C/NF.Simultaneously,it shows excellent stability that the degradation can be ignored over a period of 12 h of constant current electrolysis and the applied potential maintains at a low cell voltage of 1.58 V versus RHE.NGQDs endows the composite with excellent conductivity and abundant edge active sites,which leads to the enhanced catalytic performance of Ni3S2-NGQDs.

Fig.6 (a) Schematic illustration of the preparation of Ni3S2-NGQDs/NF electrodes,and its utilization as OER and HER electrocatalysts for alkaline water splitting,(b) polarization curves of Ni3S2-NGQDs/NF,Ni3S2/NF,RuO2-Pt-C/NF and NF and (c) chronoamperometric curves obtained in a constant current(J=10 mA·cm−2) bulk water electrolysis with Ni3S2-NGQDs/NF,Ni3S2/NF,and RuO2-Pt-C/NF electrodes (Reproduced with permission[95],Copyright 2017,Wiley-VCH).

Based on this,Wei and Chu[96]prepared GQDs-Mo-Ni3S2composites.GQDs were synthesized from citric acid by a bottom-up method.And then the obtained GQDs were coupled with Mo-doped Ni3S2on Ni foam to synthesize GQDs-Mo-Ni3S2porous composites (Fig.7a-c).The GQDs-Mo-Ni3S2composites as the electrode materials show a higher HER activity(68 mV at 10 mA·cm−2) than Mo-Ni3S2.The OER activity of GQDs-Mo-Ni3S2(326 mV at 20 mA·cm−2)is near to the commercial RuO2.At the current density of 10 mA·cm−2,the operating voltage of the GQDs-Mo-Ni3S2couple (1.58 V versus RHE) is lower than that of the other electrode for overall water splitting(Fig.7d).Importantly,after a 50 h durability test at 1.65 V,the electrode activity sustains without an obvious decay,manifesting its excellent stability as shown in the long-term durability test of GQDs-Mo-Ni3S2in Fig.7e.The generation of gas bubbles on both electrodes can be observed by naked eyes.The loading of GQDs inside Mo-Ni3S2and then anchoring them on external NF greatly facilitate the electron transport.Moreover,the doping of Mo further changes the electronic structure and introduce more active sites.

Fig.7 (a) The synthesis processes of Mo-Ni3S2/NF and G-Mo-Ni3S2-2/NF,(b,c) HRTEM images of G-Mo-Ni3S2, (d) polarization curves and (e) the long-term durability test at 1.65 V of GQDs-Mo-Ni3S2 for overall water splitting,the inset shows the photograph of generated gas bubbles on both electrodes (Reproduced with permission[96],Copyright 2019,Elsevier).

Similar work has also been reported by Chen et al.[90]They decorated NiCo2P2nanosheets with GQDs and the decorated nanosheets were coated on Ti mesh.When the NiCo2P2/GQD composites were applied as bifunctional electrocatalysts for overall water splitting,the overpotentials for OER and HER in alkaline media are 340 mV and 52 mV at the current density of 10 mA·cm−2,respectively.The operating voltage of the NiCo2P2/GQD couple is lower than that of the commercial Pt/C-RuO2couple.In addition,both the HER activity at the current density of 100 mA·cm−2and the OER activity at the current density of 50 mA·cm−2maintain after 20 h continuous electrolysis without a remarkable decay,indicating the high stability of NiCo2P2/GQD bifunctional electrocatalysts.

Chung and Choi[97]embedded B-doped GQDs(BGQDs) in GH by a simply hydrothermal method from boric acid and GO (Fig.8a).The obtained GHBGQD has a 3D porous structure composed of interconnected graphene sheets decorated with BGQD(Fig.8b-d).When the GH-BGQD composite was applied for overall water splitting,the cell voltage of the GH-BGQD electrode (1.61 V) is lower than that of Pt/C and Ir/C electrodes at the current density of 10 mA·cm−2(Fig.8e).After a 70 h durability test at 10 mA·cm−2and 1.61 V,the electrode activity remains stable (Fig.8f).It should be noted that GHBGQD composites have an excellent trifunctional electrocatalytic activities for ORR,OER and HER.

Fig.8 (a) Schematic illustration of the synthesis procedure for GH-BGQD;(b) SEM (Inset of (b) is the photograph image of GH-BGQD),(c,d) TEM images of the GH-BGQD composite,(e) polarization curves of GH-BGQD//GH-BGQD and Pt/C//Ir/C for overall water splitting in 0.1 mol L−1 KOH,(f) chronopotentiometric curves of the GH-BGQD electrode for 70 h at 10 mA·cm−2 and 1.61 V (The inset shows the photograph of generated gas bubbles on both electrodes) (Reproduced with permission[97],Copyright 2019,Wiley-VCH).

The comparison of different GQDs-based electrocatalysts for overall water splitting performance in alkaline media reported in recent references are summarized in Table 1.In summary,the large specific surface area,abundant active sites,and high electron mobility of GQDs play the critical role in improving the catalytic performance.From the catalytic point of view,the porous structure of the catalyst is conducive to expose more active sites to improve the catalytic performance,but it also increases the difficulty of uniform dispersion of GQDs in porous composites.In addition,the agglomeration of GQDs in strong alkaline media[98]will induce the decrease of the number of active sites.

Table 1 The comparison of different GQDs-based electrocatalysts for overall water splitting.

3.2 Metal-air batteries

Rechargeable metal-air batteries have attracted extensive attention owing to their high theoretical energy and low cost[99,100].Obtaining high-efficiency,stable,and low-cost bifunctional catalytic materials is of great importance for the development of metal-air batteries.At present,there are mainly three types of bifunctional catalytic materials,noble metals and alloys,transition metals and their oxides,and carbonbased materials.Noble metals and alloys have high catalytic activity[101,102],but the scarcity and high cost hinder their practical applications.As noble metal-free catalysts,transition metals and their oxides are abund-ant,low-cost and environmentally friendly.But the oxides have the problem of poor electrical conductivity[103].Carbon-based materials have the advantages of large specific surface area,low cost,high electrical conductivity,excellent catalytic activity and chemical stability.There are a number of reports about the applications of carbon-based bifunctional catalysts in metal-air batteries,such as graphene[104–107]and carbon nanotubes[108–111].As a new type of carbon material,GQDs have not only the properties of graphene,but also the quantum confinement and edge effects,which may provide the catalytic materials with larger specific surface area,better charge transfer performance and more catalytic active sites.

Lai et al.[89]synthetized all-carbon composites with GQDs nested in GH,which were applied the GQDs-based electrocatalyst to the primary Zn-air batteries.Dispersing GQDs in the GH effectively inhibits the aggregation of GQDs.The monodispersed GQDs in the composites have significant influence on the exposed proportion of lattice oxygen,which has great importance to disrupt the electroneutrality of GH-GQD to create more positive charged carbon atoms.The GH-GQD containing 90 mg GQDs exhibits the highest lattice oxygen content and the lowest interfacial charge-transfer resistance.As a result,it demonstrates the highest ORR onset potential and excellent durability in an alkaline solution,indicating its excellent ORR activity.When it was applied as the cathode electrocatalyst for a primary zinc-air battery,the galvanodynamically discharging current density of the battery could reach to 100 mA cm−2and the electrochemical performance at the current density densities of 2 and 20 mA·cm−2could all remain stable,which are all comparable with Pt/C catalysts.

Chung et al.[97]reported an all carbon composite containing both GH and B-doped GQDs (BGQDs) in 2019.Compared to Lai’s work[89],the difference of this work lies in the introduction of B in GQDs,which could provide abundant catalytic active sites.As expected,the trifunctional electrocatalytic activities of GH-BGQD composites for ORR,OER and HER are comparable with Pt/C and Ir/C catalysts.After GHBGQD is loaded on the Ni foam,it can be directly used as the air electrode for the flexible solid-state Znair battery (Fig.9).An open circuit potential of 1.4 V,a discharge voltage of 1.23 V for 100 h,a specific capacity of 687 mAh·g−1,and a peak power density of 112 mW·cm−2are achieved.The wonderful catalytic performance is caused by the more active sites and high-rate charge transport of GQDs.Hetero elements doped GQDs have promising application potentials in the field of multifunctional catalysis,due to their abundant active sites and the reduction of reaction barrier.

Fig.9 (a) Schematic configuration of the solid-state Zn-air battery,(b) discharge curves of Zn-air battery with GH-BGQD as the air electrode at various current densities,(c) polarization curves and corresponding power density plots,(d) specific capacity of the GH-BGQD based Zn-air battery at a current density of 10 mA·cm−2,(e) discharge/charge cycling curves for the GH-BGQD based Zn-air battery at a current density of 5 mA·cm−2 (20 min per cycle) and(f) discharge/charge cycling curves for the GH-BGQD based Zn-air battery at a current density of 10 mA·cm−2 under different bending states(Reproduced with permission[97],Copyright 2019,Wiley-VCH).

In 2019,Yu and Chen[112]prepared nitrogendoped GQDs (N-GQDs) by a solvothermal method and then combined with sulfurized NiCo2S4by electrodeposition to form N-GQDs/NiCo2S4bifunctional catalysts.After the N-GQDs/NiCo2S4is loaded on carbon cloth (N-GQDs/NiCo2S4/CC),it was used as a flexible air cathode for the flexible Zn-air battery.Compared with the NiCo2S4/CC catalyst,the power density of N-GQDs/NiCo2S4/CC is higher (Fig.10).The smallest overpotential among these catalysts also confirm its excellent catalytic performance.Additionally,the flexible Zn-air battery has excellent charge and discharge cycle stability.The potential can maintain stable at a current density of 20 mA·cm−2for 200 h.For further study the flexibility of the device,the galvanostatic charge/discharge test was carried out at different bending angles.The results showed that there was no significant voltage decay for 12 h at different bending angles of 0°−90°.The density functional theory (DFT) calculation showed that the excellent catalytic performance of N-GQDs/NiCo2S4/CC was mainly due to the dissociation adsorption of OOH* at the interface between N-GQDs and NiCo2S4,resulting in the decrease of overpotentials for both ORR and OER.

Fig.10 (a) Discharge polarization curves of the Zn-air batteries based on NiCo2S4/CC,N-GQDs/NiCo2S4/CC,and Pt/C+Ir/C/CC catalysts and the corresponding power densities,(b) charge/discharge polarization curves of the Zn-air battery based on NiCo2S4/CC,N-GQDs/NiCo2S4/CC,and Pt/C+Ir/C/CC and (c) galvanostatic charge/discharge curve of the Zn-air battery based on the N-GQDs/NiCo2S4/CC catalyst at a current density of 20 mA·cm−2(Reproduced with permission[112],Copyright 2019,Wiley-VCH).

Based on above facts（Table 2）,the introduction of GQDs endows catalysts more active sites and fast electron transport ability.The surface/interface structure also plays an important role in the catalytic process,which can be further regulated by doping ions in GQDs.

Table 2 The comparison of different GQDs-based electrocatalysts for metal-air batteries.

3.3 Others

In addition to the above-mentioned applications in overall water splitting (OER and HER),GQDsbased catalysts are also used for photocatalytic water splitting.For example,Jiang and Chen[81]synthesized narrow bandgap GQDs-based materials for both water splitting and CO2reduction.GQDs were made from anthracite coal by acid cutting,then 1,1’-bi(2-naphthalene) (BNPTL) was conjugated with the asprepared GQDs (GQDs-BNPTL).Both the red shift of photoluminescence emission peak and the high UVvis absorption spectra confirmed that the bandgap of GQDs-BNPTL (1.88 eV) was narrower than that of GQDs (2.40 eV).DFT calculation can further explain the reduction of bandgap.More importantly,as shown in Fig.11,GQDs-BNPTL have an intramolecular Zscheme structure,which can be indicated by the Mott-Schottky analysis that the electron-withdrawing oxygen-containing groups and electron-donating pyrazine N atoms formed p-type and n-type domains,respectively.This structure can effectively facilitate charge separation and inhibit charge recombination.When the GQDs-BNPTL was applied in photocatalytic water splitting and CO2reduction,it showed excellent bifunctional catalytic activity.The hydrogen yield(130 μmol·h−1) with GQD-BNPTL under the visible light was 9 times higher than that of GQDs.The yield of methanol (0.695 μmol·h−1·gcat−1) by GQD-BNPTL in visible light catalytic reduction of CO2was 3 times higher than that of GQDs.The catalytic activity was better than that of commercial TiO2.Due to the small size of GQDs,it is difficult to separate GQDs-based photocatalysts from reaction systems after photocatalytic reaction.Very recently,Tu and co-workers[114]found that the surface wettability of tertiary aminefunctionalized GQDs (GQD-DMA) can be changed by the reversible formation of hydrophilic bicarbonate salts during bubbling or removing CO2.Based on this property,they successfully separated GQD-DMA photocatalysts after photocatalytic reaction by a simple CO2bubbling method.The recovery efficiency can reach approximately 90%.

Fig.11 (a) Illustration of standard redox potentials and the energy-level of GQD,(b) time courses of H2 evolution and (c) CO2 reduction of all GQD types under visible light (420-800 nm) and (d) schematic illustration of GQD-BNPTL Z-scheme photocatalysis of CO2 reduction (Reproduced with permission[81],Copyright 2018,American Chemical Society).

In addition,the photosensitivity and capability of photocatalyzing the formation of reactive oxygen species (ROS) make it possible to use GQDs-based materials as antimicrobial agents.Rojas-Andrade et al.[115]recently presented that graphene oxide quantum dots (GOQDs) exhibit higher antimicrobial activity against Staphylococcus epidermidis cells than the chemically reduced counterparts.Because the reduced GOQDs lost the interaction with the electrontransport chain of the bacterial cells and did not generate ROS,which are known as the main contributive factors to eliminate bacteria.Kuo and co-workers[116]recently found that the GQD containing a high composition of N allows the generation of more ROS.100% bacterial elimination can be achieved by 1 mg·mL−1of N-GQDs (approximately 5.1% N) under a short photoexcitation time (3 min) and a low irradiation dosage (0.1 W·cm−2).Although great progress has been made in the applications of GQDsbased materials in various fields[117,118],the effects of size,composition and surface/interface properties on the catalytic performance still needs further systematic study.

4 Conclusion and prospects

GQDs demonstrate excellent potential applications as catalysts due to their large specific surface area,high electron mobility,abundant surface chemical reaction sites,and easy to be modified.In this review,the current development and challenges in the synthesis methods,modification means and applications of GQDs-based catalysts in overall water splitting,metal-air batteries and other fields have been addressed.Whether it is composited with other semiconductor catalysts or in all-carbon catalysts,the main function of GQDs include:(1) providing more active sites,(2) enhancing the transport of ion/electron by doping heteroatoms such as N and B in GQDs to further promote the charge redistribution and improve inplane electron transfer efficiency,(3) the synergistic effects between GQD and other components to promote the absorption and desorption of oxygenates in the electrocatalytic process.It is worth noting that GQDs have achieved rapid development and breakthrough in recent years.However,the research and applications of GQDs are still in their infancy,and there are some issues remaining to be settled.First,in order to satisfy the extensive applications,simple and scalable synthetic methods for high quality GQDs must be developed.Second,the optoelectronic performance of GQDs can be affected by many factors,but the mechanism is not clear.Third,the development of GQDsbased catalysts in overall water splitting mainly focuses on the study of electrocatalytic properties,and there are few researches on the photoelectric driven and light driven applications with low energy consumption.Finally,there are still some difficulties in the design,development and commercial applications of high-efficiency and low-cost GQDs-based devices,which need to coordinate the comprehensive performance of each component of devices.

Due to the limitations of synthesis methods for large scale and low-cost GQDs,the practical applications of GQDs-based materials are hindered.In the rapid development,how to find a green and sustainable road is the future direction.It is expected that more efficient,low-cost and stable GQDs-based materials as noble metal-free catalysts can be utilized in practical applications.

Acknowledgements

Shanghai Scientific and Technological Innovation Project (19JC1410402),Shanghai Natural Science Foundation (18ZR1426400),Shanghai Municipal Science and Technology Commission(18511110600) and the Innovation Program of Shanghai Municipal Education Commission (2019-01-07-00-07-E00015).