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Materials Today Energy 11 (2019) 1e23

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Materials Today Energy

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Recent advances in emerging single atom confined two-dimensionalmaterials for water splitting applications

Abeer Alarawi a, 1, Vinoth Ramalingam b, 1, Jr-Hau He b, *

a Material Science and Engineering (MSE) Division, King Abdullah University of Science & Technology (KAUST), Thuwal 23955-6900, Saudi Arabiab Computer, Electrical, and Mathematical Sciences and Engineering (CEMSE) Division, King Abdullah University of Science & Technology (KAUST), Thuwal23955-6900, Saudi Arabia

a r t i c l e i n f o

Article history:Received 4 September 2018Received in revised form7 October 2018Accepted 25 October 2018

Keywords:Single atom catalystTwo-dimensional (2D) materialsWater splittingHydrogen evolution reactionOxygen evolution reaction

* Corresponding author.E-mail address: [email protected] (J.-H. He).

1 These authors contributed equally.

https://doi.org/10.1016/j.mtener.2018.10.0142468-6069/© 2018 Elsevier Ltd. All rights reserved.

a b s t r a c t

Single metal atoms confined in two dimensional (2D) materials have gained substantial attention aspotential heterogeneous catalysts for various electrochemical applications. Single-atom catalysts (SACs)can be defined as a class of isolated metal atoms that are either atomically dispersed or coordinated withneighboring surface atoms of an appropriate support. Unlike nanoparticles or bulk materials, SACs offerunique characteristics which turn them as superior candidate for various catalytic applications. Thisreview aims to summarize recent advances in various synthetic approaches and characterization tech-niques used to design different SACs. After this overview, we focus our discussion on single atom-2Dsupport interactions, followed by recent progress in single atom incorporated 2D catalysts for watersplitting applications, which includes both electrocatalytic and photocatalytic hydrogen production.Finally, we summarize the current challenges and the future outlook exists for the rational design ofsingle atom based new catalyst with high catalytic activity, better stability and selectivity for varioussustainable energy conversion applications.

© 2018 Elsevier Ltd. All rights reserved.

1. Introduction

Tremendous efforts have been devoted to developing hetero-geneous catalysts based on noble metals, non-noble metals, non-metals, and metal oxides to improve the performance of variouselectrochemical reactions [1]. Among them, noble-metal-basedcatalysts have been extensively investigated due to their uniqueelectronic and catalytic properties [2e5]. However, the high costand scarcity of precious metals greatly hampers their usage forvarious catalytic applications. As a result, significant efforts havebeen taken to replace noble metals with different inexpensive non-noble metals and metal-free catalysts [6]. Unfortunately, thesematerials tend to suffer from poor conductivity and diminishedcatalytic properties compared to that of noble metals [7]. Therefore,increasing studies have been devoted toward the development ofcatalysts that utilize a minimal amount of noble metal resources. Itis well known that the heterogeneous catalytic process usuallyoccurs at the surface of the catalyst [8]. In the case of metal-

nanoparticle-based catalysts, only the small portion of the metalatoms that are exposed to the reactants can act as catalyticallyactive centers while the remaining material is not involved in thereaction and thereby simply wasted [7]. In addition, metal catalystswith irregular morphologies, non-uniform distribution, anddifferent particle sizes can produce multiple active sites that affectthe selectivity toward specific products [9].

Reducing the size of the metal nanoparticles to sub-nanometer oratomic-scale can address these issues by significantly increasing thenumber of catalytic active sites on the surface of the catalyst wherevirtually all isolated metal atoms are ideally accessible to reactants[10]. Atomic-sized metal catalysts have the following interestingproperties, including: i) atomically dispersed metal centers [11]; ii)coordinatively unsaturated metal active sites [12e14]; iii) electronconfinement/quantum size effects [15e18]; and iv) metalesolid sup-port interactions [19,20]. These interesting properties of metal atomcatalysts could simultaneously improve both the catalytic activity andselectivity of the materials for various electrochemical reactions.

Zhang et al. first proposed the concept of single atom catalysts(SACs) in 2011 through the development of individual Pt atomsincorporated in iron oxide (Pt/FeOx) to form a catalyst for COoxidation [21]. SACs consist of isolated single atoms that are

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uniformly dispersed on a solid support and are prepared usingdifferent techniques, such as atomic layer deposition (ALD), massselected soft landing, high temperature thermal annealing, wetchemistry, etc., [22,23]. For example, Pt single atoms on nitrogen-doped graphene nanosheets were synthesized using ALD [24], inwhich Pt atoms size were controlled by limiting the number of ALDcycles. SACs have shown extraordinary catalytic activity comparedto their nanoparticle or bulk-metal counterparts toward variouselectrochemical reactions, including the hydrogen evolution reac-tion (HER) [25], oxygen evolution reaction (OER) [26,27], oxygenreduction reaction (ORR) [28], and CO2 reduction [29], etc. More-over, density functional theory (DFT) simulations have revealed thepotential and unique catalytic nature of SACs. Such theoreticalsimulation studies can further help to design efficient catalystswith tunable catalytic activity and selectivity [30].

In SACs, the type of solid support plays a significant role in sta-bilizing the metal atoms, preventing their aggregation, minimizingthe metal usage, and enhancing the catalytic activity [31]. In fact,different types of materials, such as graphene, transition metaldichalcogenides (TMDs), metal oxides, zeolites, and metal-organicframeworks (MOFs) have been widely used as supports for singleatoms [25,32,33]. It is important to choose the suitable support forthe specific SAC to achieve the desired catalytic activity and selec-tivity. The metal species can chemically interact or bond with theneighboring atoms in the support, which in turn not only stabilizesthe single atom catalyst but also enhances its intrinsic catalytic ac-tivity [34]. In addition, the chemical interaction between the singleatom and solid support can enhance charge transfer. Moreover, theuse of different supports can provide increasedmetal-support activesites or improve the redox properties, which are generally respon-sible for attaining better catalytic performance [35].

Recently, two dimensional (2D) materials with interesting phys-ical and chemical properties have gained substantial attention as anew class of promising supports for SACs. In particular, the uniqueproperties of graphenemake it an ideal material for this application.Graphene is composed of a one-atom-thick layer of sp2-hybridizedcarbon atoms that are closely arranged in a hexagonal crystal latticestructure with alternating CC and C]C bonds [36]. Moreover, gra-phene has a large specific surface area (~2630 m2/g), which helps tosupport high catalyst loading and provide good stability and highelectrical conductivity. It also has the potential for lowmanufacturing costs [37]. Due to its attractive properties, graphenehas been widely utilized as a support material for water splittingapplications [36,38]. 2D graphene nanosheets with large specificsurface area offer more anchoring sites and defects to trap singlemetal atoms on the surface. The doping of various heteroatoms, suchas nitrogen (N), sulfur (S), boron (B), and phosphorous (P) onto gra-phene can also alter the electronic properties of the material [39].Most importantly, the heteroatoms in graphene, especially N and S,can act as binding sites for single metal atoms because of their ten-dency to coordinate with metals [40]. Single metal atoms incorpo-rated in graphene can also effectively alter the electronic structure ofadjacent carbon atoms, which can then serve as additional activesites for different catalytic applications [41]. In addition to graphene,the catalytic activity of the inert basal plane surface of MoS2 can beactivated byconfining Pt single atoms on the2D surface, inwhichMoatoms are replaced by Pt atoms [42]. Because of their promisingcatalytic activity and stability, SACs incorporated 2D materials havebeen extensively investigated for water splitting applications.

In this review, we aim to focus on different types of single atomsconfined on 2D supports for electrocatalytic (HER and OER) andphotocatalytic water splitting processes. We highlight the recentprogress on the synthesis and characterization of SACs and SAC-2Dsupport interactions, and also investigate the water splittingmechanism of single metal atoms confined to 2D supports.

2. Synthesis of SACs

SACs on a variety of supports have been synthesized usingdifferent experimental preparation methods, such as ALD, variouswet chemistries, high temperature pyrolysis, metal/acid leaching,and mass-selected soft-landing methods [43]. The fabrication ofSACs is a challenging task due to the tendency of metal atoms toaggregate and metal clusters to form. Most SAC synthesis methodsare not scalable and tend to be quite challenging and complex [32].In this section, we briefly discuss the different synthesis methodsadopted for SAC preparation.

2.1. Atomic layer deposition (ALD) of SACs

ALD can be used to deposit catalytic materials on large surfacearea supportswithprecise, atomic-scale control in termsof thicknessand composition [44]. In this process, the support material is alter-nately exposed to pulsed vapors of different reactive precursors. Theimportant characteristic of the ALDmethod is its self-limiting natureby depositing materials in an atomic layer-by-layer. Most recently,the ALD method has been used to deposit SACs on graphene andother solid supports [24]. Sun and his co-workers have synthesizedisolated single Pt-atom doped graphene using the ALD process,which is shown schematically in Fig.1 [45]. The existence of oxygen-rich functional groups on the surface of the graphene sheets reactswith the MeCpPtMe3 precursor, causing a Pt-containing monolayerto be formed on the graphene surface (Fig.1(a&b)). Furthermore, thePt-containing monolayer reacts with the subsequent oxygen pulseand forms new adsorbed oxygen molecules on the Pt surface(Fig.1(c&d)). These two steps form the complete ALD cycle. Similarly,Cheng et al. adopted the same ALD technique to prepare Pt singleatoms on N-doped graphene for HER application [24].

2.2. Wet chemistry synthesis

Wet chemistry is the most widely used technique to synthesizenanomaterials because of its relatively facile and sustainable nature[31]. Wet chemistry approaches include co-precipitation and wetimpregnation processes, which have been successfully employed toprepare SACs [31]. For example, an incipient wetness method wasadopted to synthesize Pt single atoms anchored to titanium nitride(TiN) and titanium carbide (TiC) catalysts [46]. In this technique, Ptprecursor and supports are mixed together in ethanol solution anddried in a vacuum oven. The obtained powders are then heated at100 �C under H2 atmosphere to produce Pt single atoms on the TiNand TiC supports. The final Pt loading was calculated to be less than1 wt% and clearly showed the formation of Pt SACs [46]. Likewise,isolated Pt single atoms anchored on a graphitic carbon nitride (g-C3N4) support were prepared using a wet impregnation method[47]. Recently, Kim et al. have developed single atomic Pt catalystsupported on antimony doped tin oxide (ATO) using incipientwetness impregnation method [48]. Different weight content of Pt(1, 4, and 8 wt%) were mixed with ATO support and subsequentlyreduced at different temperatures under N2/H2 gas mixture. Thetemperature reduction process not only helps to form atomic Pt butalso leads to the substitution of Pt with the antimony sites of ATOsupport. Interestingly, a maximum Pt loading of 8 wt% was ach-ieved though incipient wetness impregnation method.

2.3. High temperature pyrolysis

Single atom Co, Ru, Fe, Mo, and Ni on heteroatom-doped gra-phene sheets have been synthesized using high temperature py-rolysis methods [34,49e52]. For example, Zhang et al. synthesizedsingle Fe atoms confined on nitrogen-doped graphene (NG) using a

Fig. 1. Schematic illustration of the ALD deposition of Pt single atoms on graphene nanosheets [45].

A. Alarawi et al. / Materials Today Energy 11 (2019) 1e23 3

simple high temperature thermal annealing process [53]. Fig. 2shows the schematic illustration for the synthesis of isolated sin-gle atom Fe/NG catalyst, in which graphene oxide (GO) and Feprecursor are homogenously mixed together using ultra-sonication. The oxygen-rich functional groups, such as epoxy, hy-droxyl and carbonyl, present on the surface of GO exhibit stronginteractions and adsorption towards Fe ions in the solution phase,which stabilize the Fe ions on the GO surface. Subsequent lyophi-lization is carried out on the homogenous solution to remove thewater molecules and prevent the GO nanosheets from restacking,which further results in the formation of Fe ions distributedthroughout a GO foam. Subsequent thermal annealing is performedon the Fe ion-GO foam at high temperature (750 �C) under Ar/NH3

Fig. 2. Schematic illustration for the synthesis of the F

gas flow. As a result, N is doped with graphene (forming NG) whileisolated single Fe metal atoms are simultaneously anchored to thesurface of the NG material. The isolated single Fe atoms are furtherstabilized via the formation of coordination bondswith the N atomsof the NG sheets [53]. Recently, Pan and his coworkers designed asingle Co atom-N-doped carbon sphere catalyst using a similar hightemperature pyrolysis method under Ar atmosphere [54].

2.4. Metal organic framework (MOF)-derived SACs

SACs derived from MOFs have gained considerable attention indifferent electrochemical applications. Very recently, Chen et al.synthesized single tungsten atoms (W-SAC) anchored on MOF-

e/NG catalyst by high temperature pyrolysis [53].


derived N-doped graphene for electrochemical HER application(Fig. 3) [55]. In this work, tungsten chloride (WCl5) was encapsu-lated with a UiO-66-NH2 MOF, followed by pyrolysis at high tem-perature (950 �C) under inert atmosphere. Finally, the pyrolyzedsample was treated with HF to remove the zirconia metal to obtainthe resulting W-SAC. Likewise, Jiao et al. employed a similarapproach to prepare single Fe atoms anchored on N-doped porouscarbon catalyst (FeSAeNC) derived from MOF-545 via pyrolysis toachieve a maximum single Fe atom loading of ~1.76 wt% [56].

2.5. Metal/acid leaching

Flytzani-Stephanopoulos's group first demonstrated the use of ametal leaching technique for the preparation of SACs. Using anestablished metal leaching process originally used to recover Aufrom Au-containing ores, the authors were able to produce Ausingle atom-metal oxide [57]. Similarly, Fig. 4 depicts a schematicillustration for the synthesis of single Sn atoms on N-doped carbonfibers (Sn-CFs) using the acid leaching method [58]. The synthesisof single atom Sn on CFs involves electrospinning a precursor so-lution containing SnCl2, polyacrylonitrile (PAN), and poly(methylmethacrylate) (PMMA), followed by pyrolysis. Finally, the pyro-lyzed sample undergoes the acid leaching process, in whichunreacted metal species are removed to obtain the single Sn atom-doped CFs. Though the metal leaching method is fast and selective,it is not environmentally friendly, nor scalable for industrialapplications.

2.6. Mass-selected soft-landing

The mass-selected soft-landing method is to deposit complexions from the gas phase onto the surface of a substrate with accu-rate control of the different parameters such as material composi-tion, charge state, kinetic energy, and coverage, thereby producinguniform multicomponent films on surfaces with tailored proper-ties. This soft-landing method is based on the physical depositionmethod, in which different types of flat substrates can be used toproduce any metal clusters or single atoms at low kinetic energiesand with an accurate number of atoms dispersed onto the surface.

Fig. 3. Schematic of the synthetic proce

This method is helpful for performing fundamental studies, such asdetermining the structureefunction relationships of depositedclusters and structure, and reactivity of the supported ions [59e61].

3. Characterization of SACs

After SAC fabrication, it is necessary to characterize the materialin order to confirm the formation of isolated single metal atoms, aswell as to study the chemical oxidation states and coordinationenvironment of the single atoms with their support and determinetheir spatial distribution. However, characterizing SACs is a chal-lenging task because it requires powerful tools with high spatialresolution to probe single atoms and atomic scale morphologies.Currently, highly sophisticated characterization tools are employedto study SACs, including high angle aberration corrected dark field-scanning transmission electron microscopy (HAADF-STEM). Inaddition to microscopic techniques, spectroscopic methods such asX-ray absorption near edge structure (XANES), X-ray absorptionfine structure (XAFS), and extended X-ray absorption fine structure(EXAFS) have been used to study themorphological, electronic, andchemical properties of SACs. In this section, we briefly discuss thecharacterization techniques used to analyze these materials.

3.1. HAADF-STEM for SACs

HAADF-STEM can be used to confirm the existence of singleatoms on various solid supports. Additionally, it helps to identifythe location of single atoms with respect to the surface structure ofthe support [19]. HAADF-STEM takes advantage of Rutherfordscattering of electrons to detect heavy metal atoms on low atomicnumber-based supports [19]. Fig. 5(aef) shows HAADF-STEM im-ages of single atoms, including Ni [52], Co [49], Mo [51], Fe [53], Pt[24], and Ru [34] incorporated onto graphene supports, whileFig. 5(g) displays Pt single atoms on MoS2 [42]. The small brightdots indicate the existence of single atoms, which are uniformlydispersed on the support. In addition, electron energy loss spec-troscopy (EELS) can be used to study SACs. As shown in Fig. 5(h&i),EELS atomic spectra of the bright dot indicates the presence of Feand N, which further suggests the formation of FeeNx bonding [62].

dure for MOF-derived W-SACs [55].

Fig. 4. Schematic illustration for the fabrication of single Sn atom-doped N-doped CFs [58].


However, HAADF-STEM cannot distinguish single atoms from thesupport when the materials' atomic numbers are similar. The maindrawback of this approach is the inability to detect electronic statesof single atoms within a reaction environment or the oscillation ofcatalysts. Additionally, the sample can be damaged under the high-energy beam or probe current [63].

Likewise, Chung et al. have designed carbon-embedded nitro-gen-coordinated iron (FeN4) catalyst for ORR application [64]. TheFeN4 sites were directly visualized using HAADF-STEM technique.Fig. 6(a) shows the HAADF-STEM image of FeN4 in which the smallbright dots correspond to atomic Fe. The individual Fe atoms(bright dots) are clearly observed on graphene sheet which aremarked as 1 and 2, while graphene support is marked as 3(Fig. 6(b)). As shown in Fig. 6(c), EELS spectrawere obtained aroundthe Fe atom to study the surrounding bonding environment ofsingle Fe atom. The peaks observed in EELS spectra correspond toFe and N indicates the successful coordination of Fe atom with N.

3.2. XANES and EXAFS for SACs

XANES and EXAFS can evaluate the active sites, local atomicstructure, chemical state, and coordination structure of SACs[21,65,66]. In particular, XANES is sensitive to the oxidation stateand coordination chemistry of single metal atoms. In contrast,EXAFS can provide information about the coordination number andchemical bonding between the single metal atom and the neigh-boring atoms of the support. For example, using XANES and EXAFS,Yang et al. confirmed the local coordination environment andoxidation state of single Ni atoms doped on a N and S-co-dopedgraphene catalyst (AeNieNSG; Fig. 7) [52]. The oxidation state of Niin AeNieNSG was identified to be þ1 with the electronic config-uration of 3d9 and S ¼ 1/2 (Fig. 7(a)). Furthermore, the chemicalinteraction between the single Ni atom and the NSG support wasstudied using Fourier transform-EXAFS spectroscopy, as displayedin Fig. 7(b). A high intense peak at 1.45 Å corresponds to the NieNbond, while a small peak at 1.81 Å is assigned to the NieS bond,indicating the successful chemical coordination of single atom Niwith the neighboring N and S atoms in the graphene sheets.Similarly Jiao et al. have studied the structure of single-atom (SA)iron-implanted N-doped porous carbon (FeSAeNeC) using XANESand EXAFS (Fig. 7(c&d)) [56]. As shown in Fig. 7(c), the normalizedFe K-edge XANES spectra of FeSAeNeC is entirely different fromthose of Fe foil and Fe2O3, indicating the different oxidation sate ofFeSAeNeC than the reference samples. However, the XANES spectraof FeSAeNeC is much closer to that of 5, 10, 15, 20-tetraphe-nylporphine iron(III) chloride (FeTPPCl) and iron phthalocyanine(FePc), suggesting that the Fe atoms are stabilized by N atoms inFeSAeNeC. As shown in Fig. 7(d), FT-EXAFS spectra of FeSAeNeCshows a main peak at 1.44 Åwhich corresponds to RueN scatteringpair, confirming the coordination of isolated Fe atom with N.

Moreover, the absence of FeeFe and FeeO scattering paths in FeS-AeNeC reveals the successful incorporation of isolated FeSA on NeCsupport. EXAFS curve fitting was further performed to study thecoordination structure of Fe atoms. The best fitting results showedthat each Fe atom was coordinated with 4 N atoms i.e., FeeN4structure.

4. Single atom-2D support interactions

The catalytic performance of SACs not only depends on thenature of the single atom but also on the support and the metal-support interactions [67]. 2D carbon-based supports have gainedsignificant attention due to the materials' high stability, large sur-face area, and high electrical conductivity [68]. For example, het-eroatom such as N- and S-doped graphene nanosheets areconsidered especially promising supports for SACs as the hetero-atoms can act as binding sites for single atom metals, which helpsto stabilize the atoms to serve as catalytic active sites for variouscatalytic applications [52,69].

To take advantage of this synergistic reactivity, various researchgroups have studied the effects of depositing SACs on doped carbonsupports. For example, Chen and his co-workers designed Mo SACsanchored to an N-doped carbon catalyst for HER [51]. As can beclearly seen from the FT-EXAFS spectra, the Mo1N1C2 exhibits acoordination peak at 1.3 Å that corresponds to MoeN bond(Fig. 8(a)). Similarly, Fei et al. confined atomic Co metal onto N-graphene (CoeNG) and demonstrated it as an efficient catalyst forHER applications [49]. As shown in Fig. 8(b), a binding energy peakcentered at 398.4 eV is assigned to Co-pyridinic N-bonding.Furthermore, a small peak identified at 3.5 Å in the FT-EXAFSspectrum confirms the formation of the CoeN bond (Fig. 8(c)).Similarly, single Ni atoms were coordinated with N and S sites ofgraphene, which further acted as active sites for electrocatalyticCO2 reduction [52].

In addition to graphene, SACs can also be incorporated ontoother 2D supports, such as MoS2, g-C3N4, and layered double hy-droxides, etc. Recently, isolated single Co atoms were doped inmonolayer MoS2 (CoeSMoS2) [70]. EXAFS shows the formation ofCoeS with a coordination peak of 2.21 Å (Fig. 8(d)). As shown inFig. 8(e), the HAADF-STEM image of the CoeSMoS2 material clearlyreveals the presence of single atoms of Co, which are located on theMo side. Based on the experimental results, the HAADF image wassimulated using a DFT-optimized atomic structure (Fig. 8(f&g)). Thesimulated HAADF image shows good agreement with the experi-mental result in Fig. 8(e).

Very recently Li and his co-workers investigated the chemicalinteraction between isolated Co single atom and ultrathin 2Dporous N-doped carbon nanosheets i.e., Co-ISA/CNS using X-rayabsorption fine structure analysis (XAFS) [71]. Fig. 9 shows theXAFS results of Co-ISA/CNS. Fig. 9(a) depicts the normalized Co-k

Fig. 5. (aef) HAADF-STEM images of single Ni [52], Co [49], Mo [51], Fe [53], Pt [24] and Ru [34] atoms anchored on graphene-based supports. (g) HAADF-STEM image of single atomPt on MoS2 [42]. (h&i) HAADF STEM image and EELS elemental analysis of single Fe atom doped graphene [62].


edge XANES spectra of Co-ISA/CNS in comparison with Co foil, CoOand Co3O4 reference samples. The XANES curve of Co-ISA/CNS isdifferent than the XANES spectra of reference samples. The ab-sorption edge of Co-ISA/CNS is observed between CoO and Co3O4indicating the positive valence state of Codþ, which further revealsthe chemical interaction of Co atoms with N. As shown in Fig. 9(b),FT-EXAFS spectra of Co-ISA/CNS exhibits a main peak at 1.41 Å,which is assigned to CoeN/C scattering pair. This result confirmsthat Co atoms in Co-ISA/CNS are coordinated with N atoms of CNSsupport. Besides, wavelet transforms (WT) contour plot of Co-ISA/CNS shows an intensity maximum at 4.0 Å indicates the presenceof CoeN/C and no intensity maximum related to CoeCo and CoeOare observed (Fig. 9(c)). Furthermore, quantitative EXAFS curvefitting on both k-space and r-space was performed to collect morestructural information (Fig. 9(d&e)). The best fitting data confirmed

that Co atoms are coordinated with four N atoms and thereby formsCoeN4. Based on the both experimental and curve fitting results,atomicmodel for Co-ISA/CNSwas proposed as shown in the inset ofFig. 9(e).

5. SACs confined on 2D supports for water splittingapplications

5.1. Electrocatalytic HER applications of SACs confined on 2Dmaterials

Hydrogen is a next-generation clean energy fuel that could bethe alternative for the fast exhausting fossil fuels. One of the mostpromising ways to produce hydrogen is by water splitting usingphoto/electrochemical methods. Thewater splitting reaction can be

Fig. 6. (a) HAADF-STEM image of Fe atoms distributed on the carbon surface, (b) HAAD-STEM image of individual Fe single atom (marked as 1, 2) on few layer graphene support(marked as 3) and (c) EELS spectra of the N k-edge and Fe L-edge acquired from marked area 1, 2 and 3 [64].


divided into two half-reactions, as represented in Eqs. (1) and (2).HER takes place at the cathode while OER occurs at the anode. Thewater molecules are dissociated into hydrogen and oxygen at theirrespective electrodes when an external potential is applied. Theoverall electrochemical water splitting reaction is described in Eq.(3).

Half reactions:

Cathode: 2Hþ þ 2e� / H2 (1)

Anode: H2O / 2Hþ þ ½O2 þ 2e� (2)

Fig. 7. (a) Normalized Ni-k edge XANES spectra and (b) FT-EXAFS spectra of AeNieNG, AeNspectra of Fe foil, Fe2O3, FeSAeNeC, FeTPPCl and FePc [56].

Overall reaction:

H2O / H2 þ ½O2 (3)

The minimum thermodynamic potential that is required toproduce H2 and O2 from water is 1.23 V. In reality, to performelectrochemical water splitting we need to overcome the intrinsicactivation barriers that exist on both the anode (ha) and cathode(hc) in addition to other resistances (hother) by applying the voltageeven higher than the thermodynamic potential. This excess po-tential is known as the overpotential (h) and is equal to the sum ofthe resistances, which includes the electrolyte resistance and

ieNSG, and NiPc catalysts [52], (c) normalized Fe-K edge XANES spectra and (d) EXAFS

Fig. 8. (a) EXAFS spectra of Mo1N1C2 [51]. (b&c) High resolution N1s XPS and EXAFS spectra of Co atoms confined on an N-doped graphene (CoeNG) catalyst [49]. (d) EXAFS spectraof CoeSMoS2 [70]. (e) HAADF-STEM images of CoeSMoS2 [70]. (f&g) Simulated HAADF image and atomic model of CoeSMoS2 [70].


electrode contact resistance [72]. Therefore, the practical opera-tional voltage (Eop) for water splitting can be described as:

Eop ¼ 1:23 Vþ ha þ hc þ hother (4)

The HER mechanism in alkaline media is still unclear due to theharsh reaction environment. However, in acidic media HER in-volves two reaction steps. The first step is the Volmer step, inwhichthe electron reacts with a proton and produces an adsorbedhydrogen atom (Hads) on the electrode surface (Hþ þ e� / Hads).The second step after Hads is the HER process, which can beaccomplished either by the Tafel step (2Hads / H2), the Heyrovskystep (Hads þ Hþ þ e� / H2), or both. In both paths, adsorbedhydrogen atoms at the electrode surface control the HER processregardless of which route is taking place [72].

The Gibbs free energy of hydrogen adsorption (DGH*) is a gooddescriptor for studying the intrinsic activity of catalysts for HER[73]. Hydrogen surface bonding always controls the adsorption stepand affects the overall reaction rate. Fig. 10 displays a volcano plotfor the relationship between the exchange current density (ameasure of catalytic activity) and DGH*. Among different metals, Ptoccupies the top of the volcano plot with the highest activity andclose to zero hydrogen absorption energy [74]. The metals locatedon the left side of Pt in the volcano plot can strongly adsorb thehydrogen molecule, which can block the active sites and fail torelease hydrogen. This leads to a relatively easy Volmer step, butdifficult subsequent Tafel or Heyrovsky steps, and results in a smalland positive DGH*. In contrast, the metals on the right side of Ptpossess weak hydrogen adsorption, which fails to stabilize the in-termediate state and thereby significantly affects the HER process.As a result, a slow Volmer step will occur, which affects the overallturnover rate, causing DGH* to be large and negative [72,75]. Theperfect catalyst material must have a hydrogen adsorption energyclose to zero (DGH* ¼ 0), binding with hydrogen neither too weaknor too strong [76].

5.1.1. SACs anchored onto graphene-based supportsDue to the difference in size and electronegativity compared to

that of carbon, single heteroatom dopants such as nitrogen (N),

sulfur (S), boron (B) and phosphorous (P) can alter the internalelectronic properties and charge distribution of graphene, whichcan lead to enhanced catalytic activity [29]. In addition to manip-ulating the surface chemistry and modifying the elementalcomposition of the graphene substrate [77,78], doping the edges ofgraphene sheets with heteroatom dopants while keeping the car-bon basal plane undamaged plays a vital role in altering graphene'swork function and imparting improved solubility and catalytic ac-tivity while retaining the physicochemical properties of the pristinematerial [79]. In fact, the dangling bonds at the edge of a graphenesheet are more catalytically activity than the basal plane [80,81].

Heteroatoms can be doped on graphene in two different ways.The first is called surface transfer doping, in which the dopantsadsorb only onto the surface of graphene and do not cause sp3

defects in the lattice. The second is substitutional doping, in whichcarbon atoms in the graphene lattice are substituted by hetero-atoms, which disrupt the sp2 network and create sp3 defects [82].These type of heteroatoms, especially N or S dopants on graphenesheets, can be binding sites for SACs because of these elements'tendency to coordinate with metal atoms. Therefore, heteroatom-doped graphene sheets have been identified as potential supportsto confine different types of metal single atom catalysts for HERapplications [83].

Fei and his group studied the effect of Co SACs in N-dopedgraphene (CoeNG) sheets for HER applications [49]. In this work,the authors used a high temperature thermal annealing processunder NH3 atmosphere to synthesize the atomic CoeNG catalystand confirmed this structure using HAADF-STEM, as shown inFig. 11(a). The small bright dots of ~2e3 Å in size indicates thepresence of single Co atoms in the NGmatrix. Fig. 11(b) displays theelectrocatalytic HER performance of NG, CoeG, CoeNG, and Pt/Ccatalysts in acidic electrolyte. The CoeNG catalyst showed excellentelectrochemical HER activity, as evidenced by a very small onsetpotential of ~30 mV and small overpotential of ~147 mV in order toachieve a current density of 10 mA cm�2. As shown in Fig. 11(c), theTafel slope of the CoeNG catalyst was calculated to be 82 mV/dec,which is much lower than the Tafel values of NG and CoeG, indi-cating the superior HER activity of the CoeNG catalyst. The Co

Fig. 9. (a) Normalized Co-k edge XANES spectra, (b) FT-EXAFS spectra of Co-ISA/CNS, Co foil, CoO and Co3O4, (c) WT for the k3-weighted EXAFS signal of Co-ISA/CNS, Co foil, CoO andCo3O4, (d) EXAFS curve fitting in k-space and (e) EXAFS curve fitting in r-space, inset shows the local atomic structure of Co-ISA/C [71].


atoms coordinated with the N atoms on graphene are catalyticallyactive sites for enhanced HER activity. The stability of the CoeNGcatalyst was evaluated in both acidic and alkaline electrolytes(Fig. 11(d)). The resultant CoeNG catalyst was found to be stable foralmost 1000 cyclic voltammogram (CV) cycles in acidic and alkaline

Fig. 10. The volcano curve represents the relationship between the exchange currentdensity (jo) and the Gibbs free energy (DGH*) for different metals [74].

pH without any significant degradation in its activity, suggestingthe excellent stability of the catalyst. In addition to the CV cyclingtest, the authors studied the long-term stability of the CoeNGcatalyst and demonstrated it was stable for up to 10 h of reactiontime with only negligible degradation in activity (Fig. 11(e)).Moreover, it is interesting to note that CoeNG exhibits almost 100%Faradaic efficiency, as presented in Fig. 11(f).

Likewise, Chen and his group designed a single molybdenumatom (Mo-SAC)/N-doped carbon catalyst (Mo1N1C2) for HER ap-plications using Na2MoO4 and chitosan as precursors via a com-bined templating and pyrolysis technique [51]. To investigate thestructure at the atomic level, XANES and FT-EXAFS spectroscopywere used to confirm the bonding of the Mo SACs to one nitrogenatom and two carbon atoms of the N-doped carbon support. TheMo1N1C2 exhibited efficient activity for HER in alkaline conditions,with a low onset potential of 13 mV and an overpotential of 132mVat a current density of 10 mA cm�2 (Fig. 12(a&b)). In addition,Mo1N1C2 shows a small Tafel slope of 90 mV/dec and excellentstability of up to 1000 CV cycles (Fig. 12(c&d)). Furthermore, inorder to support the experimental results, DFT simulations wereperformed to calculate the Gibbs free energy of hydrogen adsorp-tion (DGH*) of the catalysts. As depicted in Fig. 12(e), DGH* on

Fig. 11. (a) HAADF-STEM image of atomic Co confined on NG. (b) HER polarization curves of NG, CoeG, CoeNG, and Pt/C in 0.5 M H2SO4 and (c) the corresponding Tafel slope values.(d) CV cycles of the CoeNG catalyst in 0.5 M H2SO4 and 1 M NaOH electrolytes. (e) Long term stability of CoeNG in 0.5 M H2SO4 and 1 M NaOH electrolytes at 10 mA/cm2. (f) H2

production rate with respect to time [49].


Mo1N1C2 was calculated to be 0.082 eV, while the DGH* values ofMo2C, MoN, and N-doped graphene were 0.260, 0.401, and0.672 eV, respectively. Thus, Mo1N1C2 has the lowest DGH* value,which is consistent with the experimental HER results. As displayedin Fig. 12(f), the density of states (DOS) calculations show that theFermi level of Mo1N1C2 is much higher than that of Mo2C and MoN,which leads to the high carrier density and fast charge carriertransformation during the HER process [84]. The low DGH* anddense charges around the Fermi level contribute to enhancing theHER performance of Mo1N1C2.

Recently, Cheng et al. synthesized Pt single atoms on N-dopedgraphene nanosheets (Pt/NGNs) as an efficient electrocatalyst forHER [24]. The authors' ultimate aim was to downsize Pt nano-particles to single atoms andmaximize the HER catalytic activity byutilizing nearly all Pt single atom catalysts. Pt atoms and clusters onNGNs were synthesized using ALD, in which the size and loadingdensity were precisely controlled by adjusting the number of ALDcycles. Fig. 13(a) shows the HER activity of Pt/NGN catalysts pre-pared using 50 (ALD50Pt/NGN) or 100 ALD cycles (ALD100Pt/NGN).Bare NGNs without Pt featured poor HER activity, whereas the ALDPt/NGNs displayed remarkable HER performance. At an over-potential of 50 mV, commercial Pt/C, ALD50Pt/NGN, and ALD100Pt/NGN catalysts exhibited current densities of 8.2, 16, and 12.9 mA/cm2, respectively. Compared to ALD50Pt/NGNs, the HER activity ofthe ALD100Pt/NGNs decreased, which the authors attributed to theformation of Pt clusters or NPs after 100 ALD cycles rather thansingle atoms. The ALD50Pt/NGNs also exhibited a smaller Tafelvalue (29 mV/dec) than the Pt/C catalyst (31 mV/dec), indicatingthe material's fast HER rate. The mass activity of the ALD50Pt/NGNcatalyst at 50 mV was 10.1 A mg�1, which is 37.4-times higher thanthe commercial Pt/C catalyst, suggesting the outstanding HER ac-tivity of the ALD50Pt/NGN material (Fig. 13(b)). As shown inFig. 13(c), ALD50Pt/NGN was found to be stable up to 1000 CV cy-cles. Fig. 13(def) depicts the XANES spectra of the Pt/NGN and Pt/Ccatalysts. The improvement in the HER activity of the Pt/NGNelectrode was attributed to the formation of unoccupied densitiesof 5d orbitals belonging to the Pt catalysts, which results in higherHER activity. Furthermore, the peaks noted at 1.7 Å and 2.7 Å of the

ALD deposited sample confirms the formation of PteC or PteO andPtePt bonds, respectively. The unoccupied d-orbitals of individualatoms enhance the activity of the catalysts, as confirmed by DFTcalculations. The N atoms introduced in the graphene lattice alsohelp to create more nucleation sites for metal atoms, leading toimproved interaction (i.e., bonding) between the Pt and C atoms,which has a positive impact in stabilizing the Pt SACs [85]. DFTcalculations on the bonding effect between the confined Pt singleatoms and the NG support indicate that the Pt single atoms preferto adsorb to the N-dopant sites of the NGNs, while in pure graphenethe Pt atoms tend to form clusters and nanoparticles due to theweak interaction with the substrate [24].

Qiu et al. studied the effect of anchoring single Ni atom dopantsonto nanoporous graphene (Ni-doped np-G) to serve as an HERcatalyst in acidic solution [86]. For growing the graphene layer,chemical vapor deposition was used while single Ni atoms weresynthesized via a chemical exfoliation method involving thedissolution of nonporous Ni templates in 2.0 M HCl solution.Electrochemical analysis of the Ni-doped np-G electrode revealedexcellent HER catalysis with a low overpotential of 50 mV and aTafel slope of 45 mV/dec, along with excellent cycling stability. DFTcalculations indicated that the improvement in the catalytic per-formance was due to the charge transfer between the Ni atoms andthe surrounding carbon atoms through CeNi sped hybrid orbitals.As a consequence of the resultant chemical bonds between the H*atom and the hybrid orbital, the CeNi orbitals become catalyticallyactive sites, significantly stabilizing the Ni SACs in acidic solutionand thus enabling exceptional HER catalysis.

In general, graphene-based supporting materials in SACsenhance the catalytic activity and stability of the catalyst due to thesubstrate's available sites for confining single atoms, includingdefective C, doped N and S [87]. The geometric structure of gra-phene and its excellent electrical properties can modulate thecatalytic performance of SACs towards HER.

5.1.2. SACs on 2D transition metal dichalcogenides (TMDs)In addition to graphene, TMDs have been used as a support to

SACs for HER applications. TMDs are a class of materials that feature

Fig. 12. (a) HER polarization curve of Mo1N1C2 in 0.1 M KOH. (b) Onset and overpotential of Mo1N1C2 compared with Mo2C, MoN, and 20% Pt/C catalysts. (c) Tafel slopes and (d) HERpolarization curves of Mo1N1C2 and 20% Pt/C catalysts before and after 1000 CV cycles. (e) DGH* of Mo1N1C2, MoN, Mo2C, and N-Graphene catalysts. (f) The density of state (DOS)calculations for Mo1N1C2 [51].


the formula MX2 and a layered structure (XeMeX), where M is atransition metal from group IV, group V or group VI and X is achalcogen element, such as S, Se, or Te, etc. These materials formstacking layers in which the chalcogen atoms are in two planesseparated by a plane of metal atoms, with each layer featuring athickness of ~6e7 Å, as shown in Fig. 14(aec) [88]. The intra-layerMeX bond is composed of a covalent bond while the differentlayers are coupled by weak van der Waals forces [88]. Moreover,TMDs possess a crystal coordination structure of either hexagonalor rhombohedral. Additionally, the metal atoms have octahedral ortrigonal prismatic coordination. Importantly, the electronic prop-erties of TMDs range from metallic (1T, tetragonal symmetry) tosemiconducting (2H, hexagonal symmetry; 3R, rhombohedralsymmetry), as illustrated in Fig. 14(e). The band gap can be modi-fied by changing the number of layers and therefore the thicknessof the material. For example, MoS2 in the bulk phase (Fig. 14(d)) hasan indirect band gap of 1.3 eV, which increases to a direct band gapof 1.8 eV in the single-layer form (Fig. 14(b&c)) [88].

MoS2 has been considered a promising electrocatalyst for HERdue to the following advantages. First, MoS2 has an optimal DGH*value, which is close to that of Pt, as illustrated from the Gibbs freeenergy volcano plot in Fig. 8 [74]. Second, 2D MoS2 has an ultra-large surface area that possesses a large number of active sites,

comparable to the total number of surface atoms. Third, the ultra-high surface area that arises from the 2D nanostructure allowsMoS2 to serve as an ideal platform to couple with other substrates.Finally, the unique surface of MoS2 can be easily activated andoptimized for achieving high HER activity by simple chemicalmodification methods, such as by introducing defects, strain, anddoping single atoms [89].

There are several strategies that can be applied to improve thecatalytic activity of MoS2 towards SACs for HER [89]. The strategiescan involve either increasing the active sites on the surface of thematerial or enhancing the MoS2 electrical conductivity. Obtainingmore active sites has been achieved by exposing more edges orcreating active defects using plasma [90], or thinning the layersusing an exfoliation process [91]. In addition to doping with singleatoms [42], accelerating the kinetics of charge transfer andenhancing the conductivity can be done by introducing strain in thebasal plane [92] or coupling with other conductive materials orsubstrates [93]. In this section, we will elaborate on single atomdoped MoS2 for HER applications.

The catalytic active sites of MoS2 occur at unsaturated S or Moatoms along the material's edges while the in-plane area is inert[94,95]. Therefore, introducing single atoms to the MoS2 lattice cantrigger the catalytic activity of in-plane S atoms. Based on this

Fig. 13. (a) HER polarizatsion curves of Pt/NGN and commercial Pt/C catalysts in 0.5 M H2SO4. (b) Mass activity of the Pt/NGNs and commercial Pt/C at an overpotential of 50 mV. (d)XANES spectra of the Pt L3-edge and (e) Pt L2-edge for ALD Pt/NGNs, Pt/C catalysts, and Pt foil. (f) FT-EXAFS spectra of the ALD Pt/NGNs and Pt/C catalysts [24].


strategy, Deng Jiao and co-workers confined Pt single atoms in few-layered MoS2 lattices (PteMoS2) using a direct chemical synthesismethod [42]. The characterization results demonstrated that pureMoS2 has a layer distance of 0.62 nm, as shown in the inset ofFig. 15(a). Additionally, HAADF-STEM images confirmed theconfinement of Pt single atoms in the pure MoS2 lattice, asdemonstrated in Fig. 15(bed). The Pt atoms tend to substitute forMo in the lattice and are uniformly dispersed in the MoS2 plane.

Fig. 14. (a) A scheme representing the typical 3D MX2 structure of TMDs (X-chalcogen atomsmonolayer MoS2 flake and a corresponding atomic force microscopy image. (d) A photograpsymmetry), 3R (rhombohedral symmetry), and 1T (tetragonal symmetry) TMDs. The chalco

Furthermore, EXAFS spectra confirmed the existence of Pt singleatoms by the fact that no PtePt bond peak was observed. Incontrast, the peak corresponding to PteS appeared at 2.2 Å(Fig.15(e,f)). The HAADF-STEM image and corresponding EDXmapsof PteMoS2 indicates the homogeneous distribution and existenceof Mo, S, and Pt elements (Fig. 15(gej)). As shown in Fig. 15(k), theHER electrochemical results demonstrated that the bulk MoS2 haspoor HER activity, while the pure few-layered MoS2 without Pt

(X) in yellow and M-metal atoms (M) in grey). (b & c), An optical microscopy image of ah of a bulk MoS2 crystal 1 cm in length. (e) The structural polytypes of 2H (hexagonalgen atoms (X) and metal atoms (M) are shown in yellow and grey, respectively [88].

Fig. 15. (a) TEM image of pure MoS2. (b, c) HAADF-STEM images of Pt single atoms (red circles) confined in a MoS2 plane. (d) Magnified view of the marked square region of (c),demonstrating that the Pt single atoms are substitutionally doped for Mo atoms (marked by red arrows). (e) EXAFS and (f) XANES spectra of PteMoS2, Pt foil, and commercial 40%Pt/C catalyst. (gej) HAADF-STEM image and the corresponding EDX mapping images of the PteMoS2 catalyst. (k) HER polarization curves of PteMoS2, bulk MoS2, few-layered MoS2(FL-MoS2), and commercial Pt/C catalysts. (l) Corresponding Tafel slopes and (m) HER polarization curves of PteMoS2 before and after 5000 CV cycles [42].


SACs showed improved activity but is still much inferior to the 40%Pt/C catalyst. In contrast, the PteMoS2 material displayed a distinctenhancement of HER activity, with an overpotential of 60 mV toreach a current density of 10 mA cm�2. The Tafel slope value ofPteMoS2 was 96 mV/dec, which is quite deviated from the Tafelslope value of the Pt/C electrocatalyst (32 mV/dec), indicating thatthe enhancement in the catalytic HER activity of PteMoS2 not onlyoriginates from Pt but also arises from the S atoms in MoS2(Fig. 15(l)). In addition, PteMoS2 was found to be stable up to 5000CV cycles without any significant degradation in activity(Fig. 15(m)).

DFT calculations were carried out to investigate the HER cata-lytic properties of PteMoS2. Pt-doped S-sites were found to be thecatalytically active sites and demonstrated a DGH* value of close to0 eV while the DGH* of the S edges of pure MoS2 was ~0.1 eV, whichis consistent with previous reports in the literature [96]. Moreover,the presence of Pt SACs in the lattice enhances the S in-plane cat-alytic activity towards HER compared to that of S atoms at the edgeof the material. As demonstrated by the calculated total DOS, pureMoS2 has a significant band gap while the position of the valencebandmoves downward for PteMoS2. Additionally, some hybridizedelectronic states occur near the Fermi level of the PteMoS2 struc-ture, which activates the S-edge atoms and thereby enhances theirHER activity. Consequently, introducing Pt single atoms to theMoS2lattice produces a significant change in the electronic propertiesand atomic structure of MoS2, resulting in the improved HER ac-tivity [42].

Based on the improvement in the HER activity of PteMoS2, theauthors also studied the HER activity of different metallic single

atoms confined in MoS2 using DFT calculations [42]. As illustratedfrom the volcano plot in Fig. 16, when metal atoms are substitutedfor Mo in theMoS2 lattice, the metal atoms located on the right sideof the volcano plot, such as V, Ti, Fe, Mn, and Cr, tend to remain inthe middle of the lattice to bond with six S atoms. Keeping thesurrounding six S atoms occupied results in low HER activity due totheir weak binding ability with Hþ. Meanwhile, the metals on theleft side of the volcano plot (e.g., Pt, Ag, Pd, Co, and Ni) prefer to shifttowards one side of the MoS2 lattice and bond with only four Satoms, which helps to adsorb Hþ atoms and connect to theneighboring unsaturated S atoms [42].

Shi and his group [97] proposed a method to engineer theelectronic density of MoS2 for improving HER performance byintroducing a transition metal atom into the lattice, such as Zn. TheZn single atom confinement process was achieved using a sol-vothermal method to produce Zn-doped MoS2 (ZneMoS2), whichexhibited superior electrochemical HER activity, with an onsetpotential of 130 mV and a turnover frequency of 15.44 s�1 at300 mV. Compared to pure MoS2, ZneMoS2 XPS spectra showed anegative shift in the binding energies of Mo 3d and S 2p, indicatingthe increase in the electronic density of MoS2 after introducing ZnSACs, which the authors attributed to the good HER performance.Meanwhile for other transition metals, introducing Fe atoms toMoS2 significantly decreased the HER activity, while Ni atoms had anegligible effect [42].

To enhance the HER activity of theMoS2 basal plane, Luo and co-workers [98] introduced Pd (1 wt%) to the MoS2 lattice. They foundthat single Pd atoms substituted at Mo sites simultaneously createdsulfur vacancies in the basal plane. Consequently, a phase

Fig. 16. Volcano plot of different configurations of various SACs confined in MoS2 ascoordinated with four (left) or six (right) S atoms. In the ball and stick molecularstructures shown, the green balls refer to Mo atoms, yellow balls to S, and blue andpurple balls represents the doped SACs [42].


conversion from 2H MoS2 to the stabilized 1T structure wasobserved, enabling more electrons to access absorbed Hþ atoms atthe electrode/electrolyte interface. The theoretical calculationsindicated that S atoms located next to the Pd sites featured a DGH*value of approximately �0.02 eV, which was far better than thereported Gibbs energy value of most favorable edge sites [96,99].The resultant Pd-doped MoS2 showed an exchange current densityof 805 mA cm�2 and an overpotential of 78 mV to achieve a currentdensity of 10 mA cm�2. Additionally, the catalyst exhibited excel-lent long-term stability up to 100 h of continuous reaction time. Theelectrocatalytic HER performance of different SACs confined 2Dmaterials are summarized in Table 1.

The results obtained from the previously discussed studiesdemonstrate the influence of single atoms in enhancing MoS2 HERactivity by altering the intrinsic catalytic activity of the MoS2 sur-face. We believe that this mechanism can be expanded to other 2DTMD materials, such as WS2, MoSe2, WSe2, and MoTe2. Moreover,SACs confined onto the MoS2 support can be applied to other cat-alytic processes, such as hydride-sulfurization [100] and syngasconversion to higher alcohols [101].

5.2. Electrocatalytic OER application of SAC-confined 2D materials

OER is a half reaction which takes place at the anode during thewater splitting reactions (Eq. (5)). In contrast to HER, producingoxygen molecules in OER involved several proton/electron coupledsteps In fact, the OER reaction mechanism is different under acidicand alkaline conditions as represented in Eqs. (6)e(9) [102]. Forexample, the OER half-cell reactions at potentials ðE0a Þ at 1 atm and25 �C, are [103]:

2H2O / 2H2 þ O2 (5)

Acidic conditions:

Cathode reaction: 4Hþ þ 4e�/2H2; E0a ¼ 0 V (6)

Anode reaction: 2H2O/O2ðgÞþ4Hþþ4e�; E0a ¼1:23V (7)

Alkaline conditions:

Cathode reaction: 4H2Oþ4e�/2H2þ4OH�; E0a ¼0:83V (8)

Anode reaction: 4OH�/2H2 þ 2H2Oþ 4e�; E0a ¼ 0:40 V

(9)

The four proton-coupled electron-transfer steps of the OER re-action slows thekinetics,which leads tomore energy (overpotential)required to exceed the barrier to oxidize water. Therefore, researchaimed at finding a high activity catalyst ismore difficult for OER thanthat of HER [102]. However, the use of SACs on 2D supports mayprovide a solution to this problem. 2Dmaterials have been used as asupport for confining single metal atoms to do advanced OER [102].For example, NiFe layered double hydroxides (LDHs) are inexpensivematerials based on earth-abundant elements and could serve as analternative material for replacing noble metal oxides (e.g., RuO2 andIrO2) for OER [104]. To evaluate the OER activity and understand theorigin of the catalytic activity at the atomic level. Zhang et al. syn-thesized Au single atoms on NiFe LDH (sAu/NiFe LDH) using anelectrodeposition method [105]. HAADF-STEM imaging of theresulting material clearly shows the formation of Au single atomsthat are uniformly distributed on the NiFe LDH (Fig. 17(a)). Theo-retical results have revealed that the in situ formation of NiFe oxy-hydroxide during the OER process is the catalytically active speciesfor OER. Furthermore, the incorporation of single Au atoms on theNiFe LDH significantly reduces the overpotential to 0.21 V. The OERactivities of NiFe LDH and Au/NiFe LDH are shown in Fig. 17(b). TheAu/NiFe LDH and NiFe LDH catalysts display an overpotential of237 mV and 263 mV to reach a current density of 10 mA/cm2

(Fig. 17(c)). Au/NiFe LDH is stable up to 2000 CV cycles without anyobvious decay in its activity (Fig.17(d)). The transformationof surfacecharge of sAu/NiFeOOH-NiFe LDH with O* is theoretically investi-gated to understand the reaction mechanism (Fig. 17(e)). In case ofNiFe oxyhydroxide LDH, Fe atom is considered as active centers forOER. In contrast, Au atoms help to transfer the electrons to the LDH insAu/NiFeOOH-NiFe LDH, which not only alter the surface chargedistribution but also improve the OER catalytic performance.

Fei and his group reported a series of single atom catalysts(M¼ Fe, Co or Ni) embedded in the nitrogen-doped holey grapheneframeworks (MeNHGFs) for electrocatalytic OER [106]. They usedsol-gel method followed by thermal annealing process at hightemperature to prepare MeNHGFs catalysts. XAFS analysis wascarried out to identify the chemical state and the coordinationenvironment of MeNHGFs catalysts. Fig. 18(a) shows the FT-EXAFScurves of MeNHGFs catalysts in comparison with the referencesamples (NiO and bulk Ni). FT-EXAFS spectra of Ni-NHGF exhibits amain peak located at 1.44 Å, which is different from those ofreference samples indicating the backscattering between Ni andlight atoms from the support. In addition, there is a minor signalpresents at 2.01 Å and overlaps partially with the NieNi peak at2.18 Å, attributed to bulk Ni. The similar trend is also observed forFeeNHGF and CoeNHGF samples. Furthermore, quantitative EXAFScurve fitting results revealed the MN4C4 formation in all the threeMeNHGFs catalysts. DFT simulation was performed on the MN4C4moieties in the presence of the axial dioxygen molecule to dem-onstrates the catalytic activity trend remains as Ni > Co > Fe. Thecalculated energy diagrams ofMeNHGFs for OER and the suggestedreaction pathways are depicted in Fig. 18(b). The Ni-NHGF showsthe limiting barrier of 0.42 eV, which is smaller than that ofCoeNHGF and FeeNHGF, suggesting that NiN4C4 is most active OERcatalyst. The electrocatalytic OER results indicates that the NHGFelectrode demonstrates an overpotential of 494 mV to reach cur-rent density of 10 mA cm�2, which is significantly higher over-potential value than the MeNHGFs (Fig. 18(c)). Impressively,NieNHGF catalyst exhibits a low overpotential of 331 mV to attain10 mA cm�2, which is smaller than the overpotentials of CoeNHGF(402 mV) and FeeNHGF (488 mV) catalysts. Moreover, Ni-NHGFoffers a smallest Tafel value (63 mV decade�1) than CoeNHGF(80 mV decade�1), FeeNHGF (164 mV decade�1) and bare NHGF(175 mV decade�1), suggesting the promising role of Ni metal toaccelerate the OER reaction rate (Fig. 18(d)). The TOF values of

Table 1Electrocatalytic HER performance of different SACs confined 2D materials.

2D support Single atom Synthesis method Overpotential mVvs. RHE @ 10 mA cm�2

Tafel slope(mV dec�1)

TOF (S�1) Stability Electrolyte Ref

N-doped graphene Co Chemical synthesis ~147 82 0.022e1.189 10 h 0.5 M H2SO4 49N-doped graphene Pt ALD 50 29 NA 1000 CV cycles 0.5 M H2SO4 24NG/CdS Co Chemical synthesis 210 126 8.8 15 h 10 vol % lactic

acid aqueoussolution

210

Graphene Ni Chemical exfoliation 50 45 0.8 1000 CV cycles 0.5 M H2SO4 86CN Mo Combining templated &

pyrolysis methods132 90 0.05 1000 CV cycles 0.1 M KOH 51

Graphdiyne Ni Electrochemical deposition 88 45.8 1.59 5000 CV cycles 0.5 M H2SO4 25Fe 66 37.8 4.15

MoS2-Few layers Pt Chemical synthesis 60 96 NA 5000 CV cycles 0.1 M H2SO4 42MoS2 Zn Solvothermal methods NA 51 15.44 1000 CV cycles 0.5 M H2SO4 97MoS2 Pd Spontaneous interfacial

redox technique78 62e80 0.15e21.15 100 h & 5000

CV cycles0.5 M H2SO4 98


NieNHGF catalyst at different potential is shown in Fig. 18(e). Incomparison with non-precious reported OER catalyst, NieNHGFshows higher TOF value, which suggest the excellent OER activityof NieNHGF.

In another study, Li used first-principles calculations to evaluatethe OER catalytic performances of five different types of transitionmetal single atoms which includes Pt, Pd, Co, Ni, Cu supported by agraphitic carbon nitride (g-CN) monolayer [26]. In general, g-CN isporous structure in nature. Interestingly, it can support singleatoms due to available sites that are formed from the sp2-bondednitrogen atoms at the vacancy hole edges, in addition to the strongcovalent transition metaleN bonds that prohibit the transitionmetal atoms from drifting or aggregating. Using this strategy, theauthors observed that Co/NieCN and Co/OeCN electrocatalystsexhibited excellent OER activity at very low overpotentials.

5.3. Photocatalytic H2 production of SAC-confined 2D materials

Photocatalytic water splitting was first performed by Fujishimaand Honda in 1972 using a TiO2 electrode [107]. Since semi-conductor band gap plays a key role in exciting electrons/holes

Fig. 17. (a) HAADF-STEM image of Au/NiFe LDH. (b) CV curves of Au/NiFe LDH and NiFe LDHLDH and Au/NiFe LDH. (d) OER polarization curves of Au/NiFe LDH before and after 2000 CV[105].

upon illumination [108], different types of semiconductors, such asTiO2 [109], CdS [110], and BiVO4, have been widely used as photo-catalysts for different photocatalytic applications [111]. In fact,many noble metals have been combined with semiconductors towork as co-catalysts (e.g., Pt [112], Au [113], Ag [114], and Pd [115]).However, the high cost and scarcity of noble metals have greatlylimited their usage and photocatalytic water splitting in general.

In addition to graphene [116], other 2Dmaterials, including TMDs[117], TMD oxides [118], and graphitic carbon nitride (g-C3N4) [119],possess unique structural and electronic characteristics, which aresuitable for various photocatalytic applications. These materials canbe used as both light absorbers, in which the band gap can be tunedby changing the thickness of the material, as well as co-catalysts inthe reaction. Unlike TMDs, graphene is a semi-metal with excellentcharge carrier properties that can be used for fast electron transfer,while TMDs can accept electrons and function as active sites. Ingeneral, both graphene and TMDs have a positive influence onincreasing charge separation and enhancing photoactivity [120,121].

For photocatalytic hydrogen evolution, TMD materials partici-pate in improving the activity and stability via three approaches[121]: (1) TMDs are preferable supports for anchoring different

in 1 M KOH. (c) Overpotential at 10 mA/cm2 and Tafel slope comparison between NiFecycles. (e) Differential charge densities of NiFe LDH with and without Au single atoms

Fig. 18. (a) FT-EXAFS spectra of Ni-NHGF, FeeNHGF and CoeNHGF in comparison with NiO and bulk Ni reference samples, (b) Free energy diagram of FeeNHGF, CoeNHGF andNieNHGF for OER with a single-site mechanism, and NieNHGF with a dual-site mechanism, (c) OER polarization curves of Ni-NHGF, FeeNHGF and CoeNHGF in 1 M KOH, (d)corresponding Tafel plots and (e) TOF value comparison of NieNHGF with non-precious metal based OER catalysts at different overpotentials [106].


nanoparticles and single atoms due to their special 2D layeredstructure, which can increase the charge mobility and offer moreactive sites. Moreover, these materials can help prevent theagglomeration of the anchored species, thus preserving the activityand stability throughout the photocatalytic process [122,123]. (2)Combining TMDs (in the semiconductor phase) with a co-catalystleads to an improved junction interface. As a result, charge sepa-ration and transportation can be improved to facilitate the HERprocess [124,125]. (3) The rich exposed edge sites (catalytic activesites) in TMD materials can also reduce the activation barrier andenhance photocatalytic H2 evolution [75,126,127].

Moreover, 2D-materials combined with single metal atoms havebeen utilized as composites for photocatalytic H2 evolution. Webelieve that there is still a need for more research in this field tofully understand how SACs are involved in enhancing the solar tohydrogen conversion efficiency. Belowwe discuss several examplesof confining SACs (as co-catalysts) along with graphene and TMDmaterials for photocatalytic hydrogen production.

5.3.1. SACs in graphene-based supportsSingle Co atoms incorporated N-graphene (CoeNG) was used as

an effective co-catalyst for CdS based visible light driven photo-catalyst [128]. The high active proton reduction ability and fast carrierseparation properties of the CoeNG support enhances the photo-catalytic activity of the CoeNG/CdS photocatalyst, which exhibited amaximumH2 production rate of 1382 mmol/h, whereas the Pt-loadedCdS photocatalyst showed a H2 production rate of 1077 mmol/h [128].Moreover, the experimental results confirmed that the hydrogenproduction rate of the CoeNG/CdS photocatalyst mainly depends onthe amount of CoeNG loading. A maximum photocatalytic H2 pro-duction rate activity was achieved for a very narrow concentrationrange centered at 0.25wt%of CoeNG,which the authors attributed tothe minimal light blocking effect of the material.

5.3.2. SACs in graphitic carbon nitride (g-C3N4)Graphitic carbon nitride (g-C3N4) is a 2D material composed of

stacked planes of synthetic polymers [129] that are packed throughvan der Waals bonding via interlayer tri-s-triazine molecules [130].

In addition, it is a semiconductor with an sp2 p-conjugated system[83] and a wide range of applications, such as photocatalysis [131],electrocatalysis [132], andorganocatalysis [133]. Inparticular, g-C3N4has attracted significant attention for use as a photocatalyst due to itsunique characteristics, such as its high stability, non-toxicity, andvisible-light response [131]. g-C3N4 can also be used as a potentialsupport to anchor single-atom-based catalysts for photocatalytic H2evolution, particularly because g-C3N4 has a N/C-coordinatingframework that originates from the tri-s-triazine and which can beused as a scaffold to trap or confine single metal atoms [130].

Li and colleagues demonstrated this application through theintroduction of isolated single Pt atoms to a g-C3N4 (PteCN) matrixand showed that the composite was an efficient catalyst for pho-tocatalytic H2 production [47]. The PteCN catalyst was preparedusing a simple liquid-phase reaction of g-C3N4 and a Pt precursor,followed by annealing at low temperature (125 �C for 1 h in Aratmosphere). As shown in Fig. 19(a), the bright dots indicate theexistence of single Pt atoms on the CN support and the size of the Ptsingle atoms was calculated to be 0.1e0.2 nm. Compared to Pt foiland K2PtCl6, the PteCN showed a shift to low coordination values,which indicates the successful incorporation of single Pt atoms onthe g-C3N4 matrix (Fig. 19(b)). As shown in Fig. 19(c), the FT-EXAFSspectra demonstrates that PteCN has a coordination number of 5,distance of 2.03 Å, and disorder of 0.0031 Å2. These results confirmthat the Pt atoms are uniformly dispersed on top of the five-membered rings of the g-C3N4 network. A schematic model of asingle Pt atom incorporated in g-C3N4 is shown in Fig. 19(d). Thephotocatalytic H2 production rate of PteCN (with a Pt loadingamount of 0.16 wt%) was 318 mmol h�1, while the bare g-C3N4

demonstrated only 6.5 mmol h�1 (Fig. 19(e)). The PteCN showsnearly 50-fold higher H2 production rate than that of bare g-C3N4.Additionally, PteCN is stable up to almost 16 h of continuousoperation at a constant H2 production rate. Interestingly, as can beclearly seen from Fig. 19(f), the photocatalytic activity of PteCNincreases linearly with increasing Pt loading contents from 0.075 to0.16 wt% to generate average H2 production rates of162.8e318 mmol h�1. More experimental work and theoreticalstudies need to be continued on g-C3N4confined SACs to

Fig. 19. (a) HAADF-STEM image of the PteCN. The inset shows the size distribution of the bright dots corresponding to Pt. (b) FT-EXAFS spectra of PteCN, Pt foil, and K2PtCl6. (c) FT-EXAFS results comparison between the experimental data and the fitted curves of the PteCN. (d) Schematic structure of PteCN. (e) Photocatalytic H2 production rate of g-C3N4 andPteCN photocatalysts. (f) The photocatalytic H2 production rate of the PteCN photocatalyst with different amounts of Pt loading [47].


understand the catalytic mechanism [83]. Cao et al. have success-fully prepared the atomic Pd anchored graphitic carbon nitride (Pd/g-CN) photocatalyst for photocatalytic H2 production [134]. DFTcalculations were conducted to examine the electronic character-istics of single Pd atoms in Pd/g-CN photocatalyst. It was found thatthe incorporation of atomic Pd greatly enhanced the H2O adsorp-tion on Pd/g-CN surface because of its negative H2O adsorptionenergy (�1.93 eV) than bare g-CN (�0.68 eV). As a result, Pd/g-CNphotocatalyst can involve to drive the photocatalytic H2 productionefficiently. FT-EXAFS spectra confirmed the distribution of isolatedatomic Pd on g-CN support with two different coordination envi-ronments i.e., PdN and PdC scattering pairs. The Pd/g-CN photo-catalyst produced a maximum H2 production rate of6688 mmol h�1 g�1 which is 1.8 folder higher than the optimized Pt/g-CN benchmark (0.96 wt %) catalyst. A maximum apparentquantum efficiency of ~4% was achieved at 420 nm. Most impor-tantly, Pd/g-CN photocatalyst was found to be stable for long termphotocatalytic cycling test due to the strong interaction betweenatomic Pd atoms and g-CN support.

Fig. 20. (a) HAADF-STEM image of Rhx¼0.026 confined in a Ti1.82�xRhxO4 nanosheet. (b) FT-Photocatalytic activity of the Ti1.82�xRhxO4 nanosheet with different concentration of Rh do

5.3.3. SACs anchored in 2D TMD-oxide supports2D nanosheet crystal oxides with a thickness of 1 nm can be ob-

tained by exfoliating layered oxide materials [135]. This class of com-pound can be used as a potential support for SACs, in which thedopants are confined near the surface due to the low nanosheetthickness andhigh surface area-to-volume ratio, allowing themajorityof the confined SACs to directly participate in the catalytic process.

For example, Ida and his group reported the fabrication of titaniasingle-crystal nanosheets featuring confined isolated Rh atoms(Ti1.82�xRhxO4) with a uniform thickness of 0.7 nm [136]. HAADF-STEM imaging confirmed that the Rh atoms tend to occupy Tisites in the titania nanosheet lattice, as shown in Fig. 20(a). The Rhatoms appear as the bright spots in the image while Ti atomsdemonstrate intermediate brightness. The FT-EXAFS spectra werealso consistent with the HAADF-STEM imaging, in which the Rhatoms localize in the nanosheet in the form of individual isolatedatoms (Fig. 20(b)). The authors showed that 1.4% Rh-doped titaniananosheets achieved a photocatalytic H2 production rate of51 mmol/h, which is 10-times higher than the H2 production rate

EXAFS of Rhx¼0.026 confined in Ti1.82�xRhxO4 and RhOx-loaded titania nanosheets. (c)ping [136].

Fig. 21. (a) HAADF-STEM image of A-Ni@DG, (b) zoom in HAADF-STEM image marked with yellow dashed frame at the bottom left of Fig. 21(a), (c) zoom in HAADF-STEM imagemarked with yellow dashed frame at the top left of Fig. 21(a), (d) HER polarization curves of Pt/C, defective graphene (DG), Ni@DG and A-Ni@DG catalyst in 0.5 M H2SO4, (e)Comparison of TOF of A-Ni@DG with reported non-precious metal catalyst at different overpotentials for HER, (f) OER polarization curves of DG, Ni@DG, A-Ni@DG and Ir/C in 1 MKOH and (g) TOF comparison of A-Ni@DG with the reported non-precious transition metal catalyst at various overpotentials for OER [137].



(5 mmol/h) of the undoped material. This enhancement in thephotocatalytic activity was attributed to the Rh concentration, inwhich Rh atoms act as active centers for H2 production, as illus-trated in Fig. 20(c).

5.4. SAC-confined 2D materials as bi-functional catalyst (HER andOER)

Yao and his coworkers has designed a graphene defect trappedatomic Ni species (A-Ni@DG) using incipient wetness impregnationmethod and subsequent acid leaching [137]. A-Ni@DG was suc-cessfully demonstrated as bifunctional electrocatalyst for both HERand OER. Fig. 21(aec) shows the HAADF-STEM images of [email protected] Ni single atom is clearly seen in the marked yellow dashesrepresented in the top left of Fig. 21(a), while the graphene lattice ismarked in the bottom left with yellow dashes. The zoom in image ofthe area marked with yellow dashes at the bottom shows thedefective graphene lattice (Fig. 21(b)). Interestingly, the zoom inimage of Ni sites clearly confirm that Ni single atom is trapped onthe defective site of graphene which is marked in red color dashedframe in Fig. 21(c). The A-Ni@DG exhibits superior HER perfor-mance compared to the commercial Pt/C catalyst (Fig. 21(d)). TheA-Ni@DG has a smallest overpotential of 70 mV to produce a cur-rent density of 10 mA cm�2 which is lower than the bare DG(155 mV) or Ni@DG (84 mV) catalysts. Moreover, Tafel slope valueof A-Ni@DG was found to be low as 31 mV/decade, suggesting aVolmereTafel reaction pathway. As shown in Fig. 21(e), A-Ni@DGexhibits much higher TOF values at different overpotentials incomparison with the reported non-precious HER catalysts, indi-cating the excellent HER activity of A-Ni@DG. Likewise, the A-Ni@DG electrode offers an overpotential of 270 mV to reach OERcurrent density of 10 mA cm�2. Thus, the achieved overpotentialvalue is smaller compared to that of DG (340 mV), Ni@DG (310 mV)and commercial Ir/C catalyst (320 mV) (Fig. 21(f)). Remarkably, TOFvalue of A-Ni@DG is significantly higher among the reportedtransistion metal based catalyst at different overpotentials for OER(Fig. 21(g)).

6. Summary and future outlook

SACs with unique properties, such as low coordination activesites, quantum confinement effects, andmetal-support interactionshave significantly improved the catalytic activity, stability, andselectivity of these materials for various catalytic applications.Unlike metal nanoparticles or bulk metal catalysts, SACs are iden-tical in size and uniformly dispersed on a solid support, enabling allthe isolated metal atoms to be actively involved in the catalyticreaction, and as a result SACs have shown improved catalytic ac-tivity compared to their metal nanoparticle counterparts.

In this review, we first emphasized the recent development ofSAC synthesis and characterization techniques. We briefly dis-cussed the unique characteristics of SACs, interaction between themetals and 2D supports. Moreover, we elaborated the electro-catalytic and photocatalytic water splitting performance ofdifferent single atoms confined in 2D materials and discussed theirpossible catalytic mechanisms. Among the different 2D materials,graphene and MoS2 are the most studied for the anchoring andstabilization of various types of SACs, including Co, P, Mo, Ni, andAu. In the case of graphene-based supports, most SACs are stabi-lized by neighboring heteroatom sites, which further act as cata-lytic active sites. In contrast, the Mo atoms in the basal plane of theMoS2 lattice can be replaced with either Pt or single Pd atoms,which leads an increased number of unsaturated S active sites(geometric activation) or the creation of additional electronic statesnear the Fermi level (enhancing the internal conductivity). The

reported mechanisms of this hybrid system reveal that the chem-ical interaction between the metal and the support synergisticallyenhance the catalytic activity.

Though SACs have been demonstrated as efficient catalysts fordifferent catalytic applications. However, the catalytic performanceof SACs is not yet superior to all conventional catalysts. There aremany challenges that remain and need to be addressed to develophighly efficient and robust SACs.

i) More scalable, controllable, and facile synthetic proceduresare particularly needed to achieve efficient SACs with highloading of metal atoms on the substrate, especially methodsthat are suitable for large scale applications.

ii) The stabilization of atomic single atoms on the support isanother critical challenge as single atoms have a very highsurface energy, which tends to promote aggregation and theformation of nanoparticles. Therefore, more attention shouldbe paid to developing a viable route to stabilizing SACs thatcan withstand prolonged reaction times under harshenvironments.

iii) Single metal atom-support interactions are one of the mostimportant parameters that play a vital role in achieving thedesired catalytic activity and selectivity. The conceptualmechanism underlying themetal-support interactions needsto be further supported with experimental results to un-derstand how single metal atoms interact with functionalsupports to enhance the catalytic properties. Moreover, realtime in situ analysis would be helpful to probe the structuraland dynamic changes that occur on SACs during catalyticreactions. In such studies, it would be ideal to have thecapability to identify the specific active sites responsible forthe catalytic reactions.

iv) Other than carbon supports, recently developed 2D mate-rials, such as MXene, 2D TMDs, and LDH, etc., may also beemployed as potential supports to anchor single atoms. Inaddition, dual single atom doping is a novel, developingapproach involving the addition of two different singleatoms onto two different heteroatom sites of the same sup-port, which could enhance the catalytic activity of SACs forwater splitting applications.

To date, SACs have been used either as HER or OER catalysts.Further studies should be focused on designing bi-functional SACsto drive both HER and OER simultaneously. Considering the inter-esting SACs properties, we also foresee that the development ofSACs confined on 2D supportsmay be further extended to advancedsustainable energy conversion and storage applications.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgment

This work was financially supported by the King Abdullah Uni-versity of Science and Technology (KAUST) Office of SponsoredResearch (OSR), KAUST Solar Center, KAUST Catalysis Center, andKAUST baseline funding.

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