Atomically Dispersed Catalytic Sites: A New Frontier for Cocatalyst/Photocatalyst Composites toward Sustainable Fuel and Chemical Production

: Photocatalysis delivers a promising pathway toward the clean and sustainable energy supply of the future. However, the inefﬁciency of photon absorption, rapid recombination of photogenerated electron-hole pairs, and especially the limited active sites for catalytic reactions result in unsatisfactory performances of the photocatalytic materials. Single-atom photocatalysts (SAPCs), in which metal atoms are individually isolated and stably anchored on support materials, allow for maximum atom utilization and possess distinct photocatalytic properties due to the unique geometric and electronic features of the unsaturated catalytic sites. Very recently, constructing SAPCs has emerged as a new avenue for promoting the efﬁciency of sustainable production of fuels and chemicals via photocatalysis. In this review, we summarize the recent development of SAPCs as a new frontier for cocatalyst/photocatalyst composites in photocatalytic water splitting. This begins with an introduction on the typical structures of SAPCs, followed by a detailed discussion on the synthetic strategies that are applicable to SAPCs. Thereafter, the promising applications of SAPCs to boost photocatalytic water splitting are outlined. Finally, the challenges and prospects for the future development of SAPCs are summarized.


Introduction
Since the pioneering discovery of water photolysis on TiO 2 electrodes in 1972, solardriven water splitting has been recognized as one of the most promising ways to transform solar energy into renewable and clean hydrogen energy [1]. The photocatalysts are the key component of photocatalytic processes [2]. Over the past few decades, researchers have developed various photocatalysts toward the application of water splitting [3][4][5]. A typical photocatalytic water splitting reaction on semiconductor photocatalysts mainly involves three primary steps: (1) absorption of light to generate electron-hole pairs; (2) separation of the photogenerated electron-hole pairs and their migration to semiconductor surfaces; and (3) consumption of the photogenerated electrons and holes via surface redox reactions. Specifically, only the photogenerated electrons and holes that successfully reach the surface of the catalyst have the possibility to react with water. Therein, the water molecules can be reduced by photogenerated electrons to generate H 2 (hydrogen evolution reaction, HER), and at the same time oxidized by photogenerated holes to generate oxygen (Oxygen evolution reaction, OER), hydrogen peroxide, superoxide radicals, etc. [6]. The problem is that the single-component photocatalyst always suffers from poor photocatalytic activities because of its narrow light harvesting range, serious electron-hole recombination, and weak adsorption and activation abilities for reactant molecules. To overcome these bottlenecks, strategies aiming to expand the light absorption range, construct interface heterojunctions, and engineer surface reaction sites by rational design of multicomponent photocatalysts 2.1. 1
For metal oxide supports, individual metal atoms can be firmly anchored by forming chemical bonds with O atoms or superseding the sites of cations in supports [27,28]. TiO 2 is the earliest discovered photocatalytic active materials and possesses the merits of low price, good thermal and photochemical stability, and nontoxicity [29,30]. TiO 2 supports used in SAPCs usually are two-dimensional nanosheets [31], or have nanoporous [32,33] or mesoporous structures [34], which all possess a large specific surface area and expose abundant active sites. The bandgap of TiO 2 can be changed by morphology tuning. Batzill et al. reported that the bandgap of TiO 2 was narrowed from 3.2 eV for bulk TiO 2 to 2.1 eV for TiO 2 nanosheet, which closely matches the energy range of visible light [35]. Ishihara et al. [36] prepared an Rh-doped TiO 2 nanosheet (Figure 1a). According to the results of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and extended X-ray absorption fine structure (EXAFS) (Figure 1b-e), the isolated singleatom Rh dopants substitute Ti 4+ in two-dimensional TiO 2 crystals. The single Rh atom in crystal acted as an efficient reaction center in photocatalytic water splitting process and significantly improved the photocatalytic H 2 evolution activity. The photocatalytic activity for HER on this single-atom Rh-doped nanosheet was 10 times higher than that of undoped nanosheet. In addition, by controlling the morphology of TiO 2 , the exposed crystal planes can be changed to obtain different geometry and electronic structure [37]. Liu et al. successfully synthesized single-atom Pt on nanosized TiO 2 single crystals with highly exposed (001) facets (TiO 2 -A) through photo-deposition method [22]. In their study, the atomically dispersed Pt atoms were selectively dispersed on the (101) facets of TiO 2 -A while Pt nanoparticles were formed on commercial anatase nanoparticles (TiO 2 -0) under the same synthesized condition (As shown in Figure 1f). The isolated Pt atoms increased the number of photocatalytic active sites and inhibited the recombination of photogenerated electrons and holes. The Pt 1 /TiO 2 -A displayed significantly higher photocatalytic activity (0.6Pt/TiO 2 -A, 84.5 umol/h) as well as considerable stability for H 2 evolution compared to Pt/TiO 2 -0 (0.6Pt/TiO 2 -0, 18.8 umol/h) (Figure 1g).
Metal sulfides, such as CdS, MoS 2 , GaS, and so on, with smaller bandgaps compared to metal oxides, have also been widely used as supports for SAPCs [19,[38][39][40]. The abundant S atoms in metal sulfides can provide strong coordination with single atoms [41]. Zhang et al. decorated the surface step edges of CdS nanowires (NWs) with isolated Pt atoms to promote photocatalytic HER (Figure 2a,b) [42]. According to EXAFS curve fitting results, the isolated Pt atoms were strongly anchored on CdS surface by bonding with four unsaturated S atoms ( Figure 2c). As such, the utilization efficiency of noble Pt co-catalysts was significantly increased. Moreover, the HER activity of single Pt atom decorated-CdS NWs was greatly enhanced, which was 7.69 and 63.77 times higher than that of Pt nanoparticles decorated-CdS NWs and bare CdS NWs, respectively. Besides S atoms, the surface hydroxyl groups bonded to metal sulfide can also serve as anchoring sites. Zhong et al. reported an catalyst composed of atomically dispersed Ni on CdS nanorods (denoted as Ni 1 /CdS NRs, Figure 2d) prepared by a simple photochemical method [43]. X-ray absorption near edge structure (XANES) spectra indicated that the chemical state of Ni in Ni 1 /CdS NRs was similar to Ni(OH) 2 (Figure 2e). The EXAFS curve of Ni 1 /CdS NRs in Figure 3f clearly revealed that there was a Ni-O bond instead of a Ni-Ni or Ni-S bond in Ni 1 /CdS, strongly demonstrating the atomic dispersion of Ni on CdS NRs by bonding with O. The synthesis mechanism of Ni 1 /CdS by photochemical method was shown in Figure 2g. The atomically dispersed Ni ions were bonded with the surface hydroxyl groups generated during photochemical synthesis and the loading amount of Ni was up to 2.85%.
Apart from the studies mentioned above, other inorganic semiconductors, such as Bi 3 O 4 Br, SrTiO 3 and Cs2SnI6, have also been investigated as the support materials for constructing SAPCs [44,45]. Liu

Organic Materials
In recent years, organic polymer semiconductors, especially g-C 3 N 4 , have attracted extensive attention in photocatalysis owing to their fascinating advantages such as appropriate energy band structure, good stability, nontoxicity, low cost, etc. [47][48][49]. It is worth noting that the bandgap of pristine g-C 3 N 4 is about 2.7 eV, with the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) well fitted for driving water splitting from the aspect of thermodynamics [50]. At the same time, g-C 3 N 4 possesses abundant N atoms with lone-pair electrons, which are ideal coordinating sites for anchoring single metal atoms [51]. It has been found that the isolated metal atoms anchored on g-C 3 N 4 can serve as active centers for boosting photocatalytic HER and ameliorate the low separation efficiency of photogenerated charge carriers for pristine g-C 3 N 4 [52,53].
Hence, g-C 3 N 4 has been intensively explored as the support materials for hosting single atoms, giving birth to various SAPCs.
Xiong et al. successfully incorporated Pt 2+ into the pores of g-C 3 N 4 by introducing Pt precursor prior to the polymerization of g-C 3 N 4 (Figure 3a-c) [54]. The complete coordination of Pt 2+ with N sites was formed before the formation of g-C 3 N 4 . They demonstrated that the metal-to-ligand charge transfer between Pt 2+ and g-C 3 N 4 facilitated photocatalytic HER. Further modification of g-C 3 N 4 support with other coordinating atoms such as O and P can provide stronger anchoring ability for single atoms and improve the photocatalytic performance [55,56]. Au ions can be anchored on g-C 3 N 4 by Au-N bond (referred as CNUAu) [57]. However, in the photocatalytic process, photogenerated electrons are inclined to transfer to Au(I) via Au-N bonds and reduce Au(I) to Au(0) atoms. The interaction between Au(0) and N atoms was too weak to prevent the agglomerate of free Au(0) atoms (Figure 3e). To deal with this problem, Xue et al. introduced active C-OH groups to g-C 3 N 4 by pretreatment with H 2 O 2 (referred as CNUH) prior to the impregnation synthesis of single-atom Au [55]. This enabled the immobilization of single Au(I) in g-C 3 N 4 matrix through strong Au(0)-O bonds (referred as CNUHAu, Figure 3d). The stable and isolated Au(0) can act as efficient active sites for photocatalytic HER, allowing the oxygen-stabilized single-atom Au/g-C 3 N 4 to exhibit excellent activities and stabilities.  [54]; (b) k 3 -weighted Fourier-transform Pt L 3 -edge EXAFS spectra for g-C 3 N 4 -Pt 2+ (0.18 wt%) in reference to Pt foil (no phase corrected) [54]; (c) XPS Pt 4f spectrum of g-C 3 N 4 -Pt 2+ (0.18 wt%) [54]; (d) Schematic of the structural evolution of CNUHAu during photocatalytic reactions [55]; (e) schematic of the structural evolution of CNUAu during photocatalytic reactions [55].
MOFs are one class of porous crystalline materials in which the metal ions/clusters and organic bridging molecules are combined by coordination bonds [58,59]. The porous structure of MOFs brings a large surface area, which is beneficial for adsorption of reactants [60,61]. Additionally, the skeleton structure and component of MOFs can be flexibly designed and modified. Introducing photosensitizers or other functional materials with single-site catalysts into MOF matrix is an efficient strategy for constructing SAPCs [62][63][64]. Ye et al. synthesized the porphyrin-based MOFs by integrating porphyrin-based molecular units (one kind of photosensitizers) and Zr 6 clusters into a three-dimensional network [65]. The resulting material exhibited excellent performance in visible-light absorption and CO 2 capture. On this basis, coordinatively unsaturated single-atom Co sites were further implanted into the porphyrin units (Figure 4a,b) to yield the single-atom Co/MOF photocatalyst (referred as MOF-525-Co), which exhibited significantly enhanced activity in photocatalytic CO 2 conversion. An advantage associated with the MOF support is that the individual photocatalytic active atoms can be stably anchored in the MOF matrix via applicable coordination bonds; this can lead to robust coordination and achieve ultrahigh loading of single atoms. For example, Zhou et al. firstly synthesized Pt single atoms coordinated on ultrathin 2D MOF nanosheets (denoted as PtSA-MNSs) (Figure 4c,d) [66]. The Pt atoms were anchored within the porphyrin rings before the formation of MOF, thus resulting in well-dispersed Pt single atoms with an ultrahigh loading of 12.0 wt% in the MOF nanosheets. Benefiting from the sufficient Pt single-atom active sites, the PtSA-MNSs exhibited record-high photocatalytic HER activity of 11320 umol g −1 h −1 under visible light irradiation.
Besides incorporating the single atoms to the MOF matrix, COFs composed of periodic organic building blocks showing abundant binding groups as well as a porous structure are also ideal for confining single atoms, by forming coordination between the isolated metal atoms and the heteroatoms in COFs framework [67,68]. Zhou et al. reported a photocatalyst constituted by a 2,2 -bipyridine-based COF bearing single Ni sites (Ni-TpBpy) (Figure 4e,f) [69]. The electrons can be transferred from photosensitizer to Ni sites and reduce CO 2 to CO under visible-light irradiation. The excellent activity (4057 umol g −1 , 5 h) was attributed to the synergistic effects of TpBpy favoring the adsorption and activation of CO 2 molecules (thus inhibiting the competitive H 2 evolution), and the single Ni catalytic sites that can facilitate the selective conversion of CO 2 (96%).

Carbon-Based Materials
Graphene-based materials such as GO and NG with excellent conductivity can be used as electron-transporting layers to accelerate the separation of photogenerated electron-hole pairs [70][71][72]. Through structural modification, the O atoms in GO and N atoms in NG can provide sufficient coordination sites for anchoring single atoms [73]. These support materials are often used as a bridge to promote the transfer of photogenerated electrons from photosensitizer to single-atom reaction sites [74]. For example, Xiong et al. immobilized isolated single Co atoms on GO nanosheets (referred as Co 1 -G, Figure 5a-d) [75]. With the assistance of [Ru(bpy) 3 ]Cl 2 light absorber, the single Co sites in Co 1 -G achieved unprecedented turnover frequencies of 3.77 min −1 in photocatalytic CO 2 conversion reactions. The authors further demonstrated that the C/O functional groups in GO served as robust ligands to anchor Co atoms, and the high conductivity of GO strongly facilitated the transfer of photoexcited electrons from [Ru(bpy) 3 ]Cl 2 to catalytic Co sites ( Figure 5e). Xu et al. anchored single-atom Co on NG as a highly effective and durable cocatalyst for photocatalytic HER [76]. The Co-NG/CdS composite powder was further prepared by loading Co-NG on CdS. At a loading amount of 0.25 wt % for Co-NG, the hydrogen production rate of Co-NG/CdS was 3.42 and 1.3 times enhanced compared to NG-loaded CdS and Pt nanoparticle (1.5 wt %)-loaded CdS, respectively. (b) Fourier transform magnitudes of the experimental Co K-edge EXAFS spectra of samples (not corrected for phase shift). Key: Co foil (blue), Co@C (orange), MOF-525-Co (green) [65]; (c) Illustration of the synthetic route towards Pt single-atom coordinated ultrathin MOF nanosheets (PtSA-MNSs) through a surfactant-stabilized coordination strategy for photocatalytic hydrogen production [66]; (d) Pt L 3 -edge EXAFS spectra for PtSA-MNSs, PtCl 2 , and Pt foil [66]; (e) Schematic diagram of photocatalytic selective reduction of CO 2 over Ni-TpBpy [69]; (f) aberration-corrected HAADF-STEM image of Ni-TpBpy [69].
Carbon dots (CDs) are another attractive support material for hosting single atoms. Their bandgaps can be tuned by nitrogen doping to function as a good light-harvesting antenna [77]. Run Li et al. synthesized the CDs anchored with single Co atoms [78], in which the single Co atoms with a high loading content of 3.27% were stabilized through the Co−N 4 coordination structure. The authors unraveled that CDs played double functions in photocatalysis: the support for single-atom Co and the light-harvesting antenna. Meanwhile, the isolated Co atoms can boost the separation and directed migration of photogenerated charge carriers, and, more significantly, can gather the photogenerated holes for promoting oxidation reactions. As a result, the obtained photocatalyst exhibited excellent activity and stability in photocatalytic water oxidation (245 umol/g, 4 h), photocatalytic oxidative coupling of aromatic amines, and photodegradation of organic dye.  [75]; (b) HAADF-STEM image of the Co 1 -G catalyst. The atomically dispersed Co atoms are highlighted by the yellow circles [75]. (c) Normalized Co K-edge XANES spectra of the Co 1 -G sample in reference to Co foil and CoO [75]; (d) k 3 -weighted Fourier-transform Co K-edge EXAFS spectra [75]; (e) Schematic illustration of the process for photocatalytic reduction of CO 2 to CO using [Ru(bpy) 3 ]Cl 2 as a light absorber and Co 1 -G nanosheets as a catalyst [75].

Metals
Aside from semiconductor materials, plasmonic metals such as Cu and Au nanocrystals can also be used as support materials for SAPCs. The single atoms are anchored within the metal support via metallic bond [79,80]. The strong interaction between light and plasmonic metals via LSPR effect can efficiently absorb light and generate a lot of hot electrons [81]. At the same time, the excellent electrical conductivity of metal and the metallic bonds can rapidly transfer the generated hot electrons to single-atom active sites for promoting photocatalytic reaction. Halas and colleagues prepared atomically dispersed Ru reactive sites on Cu antenna nanoparticles (Ru content below 0.2%, Figure 6a) for photocatalytic dry methane reforming [82]. The results of CO probe molecule diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS) coupled with density functional theory (DFT) calculations with van der Waals 'D3 correction (Figure 6b) unambiguously uncovered the surface structure of Cu-Ru catalyst with atomically dispersed Ru. As shown in Figure 6c,d, compared to pure Cu and the Cu-Ru catalyst with high Ru loading (Cu 19.5 Ru 0.5 ), the sparsely distributed single-atom Ru made the Cu-Ru catalyst present exceptionally high selectivity (>99%) and long-term stability (50 h) with greatly suppressed coking.

Composites
In addition to single-component support, composite support, combining the advantages of multiple materials, is also promising for constructing SAPCs. An et al. anchored Pt single atoms on composites constituted by CDs and ultrathin CdS nanosheets (referred as CdS@CDs/Pt-SAs, Figure 7) [83]. The formation of Pt-S bonds between single Pt atoms and unsaturated S atoms of CdS accounted for the high stability of the catalysts. The presence of single Pt atoms promoted the directional migration of photogenerated electrons to Pt reaction sites for boosting HER. The addition of CDs slightly decreased the bandgap of CdS and was also favorable for charge separation. More importantly, it was found CDs were the key to facilitating the isolated dispersion of Pt on the surface of CdS. Experimental results showed that with the absence of CDs, Pt nanoparticles would be formed on the surface of CdS at a Pt loading amount of 1.15%. The CdS@CDs/Pt-SAs afforded an outstanding H 2 production rate (45.5 mmol g −1 h −1 ) in photocatalytic reaction, 133 times higher than that of Cd@CDs.

Single-Atom Cocatalysts
It is widely recognized that the presence of cocatalysts can effectively promote the kinetic process of photocatalysis and is significantly important for achieving high overall photocatalytic efficiency [84]. In general, loading of metal cocatalysts can bring about the following two functions: (1) the metal cocatalysts can serve as electron traps and induce the directional migration of photogenerated electrons from semiconductor to metal, thus suppressing the recombination of photogenerated electrons and holes and prolonging the lifetime of these charge carriers; and (2) the cocatalyst can reduce the adsorption and activation energies toward reactants and serve as the reaction centers for surface photocatalytic reaction [85,86]. Considering that only the surface atoms of cocatalyst particles participate in the catalytic reaction, the atomic dispersion of cocatalyst in SAPCs can realize maximized atomic utilization and lower the cost of the noble catalyst [87]. In addition to these advantages, single-atom cocatalysts will exhibit unexpected photocatalytic activity and selectivity due to the unsaturated coordination condition and tunable electronic structures derived from the interaction with supports [88]. The strong interaction between the single metal atoms and the support in SAPCs are also effective means to modulate the charge kinetics, which is critical for the overall conversion efficiency of photocatalysis [89,90]. In addition, the interaction between single metal atoms and supports can also contribute to adjusting the energy band structure and expanding the spectrum of light absorption of supports [91,92]. Hence, SAPCs as a new paradigm of cocatalyst/photocatalsyt composite promise to exhibit excellent photocatalytic activity, selectivity, and durability [93]. In the following, the various single metal atoms applied as cocatalysts in SAPCs are discussed in terms of noble metals and non-noble metals.

Noble Metals
Due to their suitable Fermi level for trapping electrons and a low potential for H 2 evolution, noble metal Pt is an efficient cocatalyst for H 2 evolution and has been intensively studied in SAPCs [94][95][96][97]. Wu et al. reported a photocatalyst with significantly enhanced photocatalytic activity and maximized utilization of Pt [98], in which the isolated Pt atoms were incorporate into the sub-nanoporosity of 2D g-C 3 N 4 (referred as Pt-C 3 N 4 , Figure 8b) and acted as co-catalyst for photocatalytic HER. The intrinsic change of the surface trap states induced by single-atom Pt contributed to a longer lifetime of photogenerated electrons, which was the primary factor for the prominently improved photocatalytic performance. Bi et al. further investigated the in situ charge transfer and chemical bond evolution between the single-atom Pt and g-C 3 N 4 during photocatalytic HER (Figure 8a) [99]. The results showed that different from the metallic Pt decorated-g-C 3 N 4 catalyst, within single-atom Pt/C 3 N 4 , the Pt-N bond broke to form Pt 0 species and the C-N bond transformed into C=N bond of g-C 3 N 4 under visible light irradiation. Such dynamic changes designated Pt 0 and g-C 3 N 4 as the reaction sites for reduction and oxidation reactions respectively, and this contributed to the remarkably enhanced H 2 evolution activity. Furthermore, it has been demonstrated that a higher loaded content of Pt single atoms can give much higher photocatalytic activity. For example, Zhou et al. developed a surfactant-stabilized coordination strategy to realize ultrahigh single-atom Pt loading of 12% within MOFs [66]. The resulting SAPCs exhibited an exceptionally high activity of 11320 umol g −1 h −1 for photocatalytic HER under visible light. Apart from the bonding of single-atom Pt with the coordinating atoms within supports, isolated Pt complex can also be anchored on the support and act as reaction sites. Jean-Marie Basset et al. prepared Pt single atoms for photocatalytic water splitting via the surface organometallic chemistry (SOMC) technique [100]. The Pt complex (1,5-cyclooctadiene)dimethylplatinum(II) ((CH 3 ) 2 Pt(COD)) was grafted on anatase TiO 2 with highly exposed (001) facets (referred as Pt GR :TiO 2 ) (Figure 8c,d). Compared to the photocatalyst synthesized by impregnation method with the same Pt loading amount (referred as 0.5%Pt NP :TiO 2 , Figure 8e), Pt in 0.5%Pt GR :TiO 2 was atomically distributed, leading to excellent activity for photocatalytic HER and effectively suppressing side reaction between H 2 and O 2 forming H 2 O in dark conditions. However, the problem is that in this system the anchoring effect for the single SOMC-grafted Pt atoms was weak, and as a result the Pt atoms rapidly aggregated and formed nanoparticles under photocatalytic conditions.
In addition, Pd, Ru, Rh, and Au have also been applied in the study of SAPCs for photocatalytic water splitting [101][102][103]. Single-atom Pd catalysts have shown good potential for photocatalytic reduction reactions. In the study carried out by Chen [104], single Pd atoms were intercalated into the space among the adjacent layers of g-C 3 N 4 and anchored on the surface of g-C 3 N 4 simultaneously through a photo-deposition method (referred as Pd-g-C 3 N 4 , Figure 9a). This distinct structure facilitated the charge transfer from the bulk interior to the outermost surface where single Pd atoms acted as reactive sites for sequential reactions. The Pd/g-C 3 N 4 photocatalyst exhibited more efficient photocatalytic hydrogen evolution than that of the optimal Pt loaded Pt/g-C 3 N 4 benchmark (synthesized by the same photo-deposition method). Yan et al. developed a photocatalyst with high-valence single-atom Ru(IV) intercalated in a two-dimensional GaS nanosheet for photocatalytic HER (Figure 9b,c) [42]. The intercalated single-atom Ru possessed a hexagonal structural configuration of Ru 1 -S 6 , which served as a fast electron channel for rapid segregation and transfer of photogenerated carriers in photocatalytic process. As a result, the photocatalytic activity of H 2 production under visible light was 30 times higher than that of the pure GaS nanosheet. Ishihara et al. introduced isolated single-atom Rh dopants in TiO 2 nanosheet [27], and this led to 10 times higher photocatalytic activity for H 2 evolution than that of undoped TiO 2 nanosheet. More interestingly, developing bifunctional single-atom redox sites in one photocatalyst would significantly enhance the dynamics of photocatalytic water splitting [105,106]. Liu et al. introduced Pt and Au dual single atoms within g-C 3 N 4 in order to construct bifunctional redox sites [107]. The resulting catalysts, referred to as Pt-Au/g-C 3 N 4 , realized spontaneous overall water splitting, opening a new prospect for the application of SAPCs (Figure 9d).

Non-Noble Metals
Non-noble metal cocatalysts with lower prices and more abundant reserves can dramatically reduce the cost of the widely studied photocatalysts that utilize precious metals as cocatalysts [108]. In particular, the non-noble transition metals with vacant orbitals can be used as electron traps to receive electrons [109,110]. In some cases, the non-noble metal cocatalyst exhibited even superior performance than that of noble metal cocatalysts.
Co is a frequently explored non-noble cocatalyst in photocatalysis. The single-atom Co photocatalysts with isolated Co sites cooperated with photoactive supports enabled photocatalytic water splitting and CO 2 fixation reactions. Wei et al. developed a Co 1 -N 4 singlesite photocatalyst via atomic layer deposition (ALD), in which the single-atom Co was anchored on phosphorus-doped g-C 3 N 4 support (referred as Co 1 /PCN, Figure 10a-c) [111]. The phosphidation treatment of g-C 3 N 4 support improved the separation and transfer of photogenerated carriers. Atomically dispersed Co atoms were stably immobilized on g-C 3 N 4 nanosheets by forming covalently bonded Co 1 -N 4 structure. The coordinated donor N increased the electron density of Co and lowered the formation barrier of the key Co-H intermediate, consequently boosting H-H coupling and H 2 evolution in photocatalytic reaction. On this basis, the authors further developed another photocatalyst with single site Co 1 -phosphide anchored on phosphorus-doped g-C 3 N 4 support through Co-P 4 bonds (referred as Co 1 -phosphide/PCN, Figure 10d) [56]. Compared to Co 1 -N 4 , the coordinatively-unsaturated Co 1 -P 4 single sites were more favorable for water molecular adsorption and activation in oxygen evolution reaction. Therefore, Co 1 -phosphide/PC 3 N 4 afforded a more stable and higher photocatalytic activity. Huang et al. developed a singlesite MOF photocatalyst, in which single-atom Co and Ru-based photosensitizers were incorporated into an MOF matrix (referred as Co-Ru-UIO-67(bpy)) [112]. They demonstrated the exceptional catalytic activities of Co-Ru-UIO-67(bpy) and established the fundamental structure−function relationships of this system for HER. Li et al. employed g-C 3 N 4 to anchor the Co 2+ sites and harvest visible light [113]. The photocatalyst was synthesized by a simple photo-deposition method and exhibited excellent activity (0.128 umol/mg) and high selectivity for CO in photocatalytic CO 2 reduction.
Other transition metals such as Cu, Ni, Fe, etc. have also been applied in SAPCs and exhibited excellent photocatalytic performance. For example, Hyeon et al. demonstrated that a reversible and cooperative photoactivation process occurred on anatase TiO 2 incorporated with single Cu atoms (referred as Cu/TiO 2 ) [114]. During the photocatalytic process, the photogenerated electron from TiO 2 was localized into the Cu d z 2 state and changed the valence state of single-atom Cu in Cu/TiO 2 . Because of the axial antibonding character of Cu d z 2 state, valence state change of Cu would leaded to a lattice distortion in the adjacent TiO 2 , which activated the adjacent TiO 2 and significantly enhanced photocatalytic performance. The localized electrons were consumed after reduction reactions and the single Cu atoms return to the Cu II state. Similarly, through tuning the oxidation state of Ni, the photocatalytic performance of SAPCs can be efficiently improved [115]. Shi et al. constructed partially oxidized Ni single-atom sites on g-C 3 N 4 (referred as g-C 3 N 4 -0.2Ni-HO) and achieved over 30 times enhancement for photocatalytic HER performance [116]. They revealed that the unpaired d-electrons in 3d orbital of partially oxidized Ni single atoms can be excited under irradiation, leading to enhanced light absorption, charge separation and transportation. Therefore, the photocatalytic HER performance of g-C 3 N 4 -0.2Ni-HO was significantly improved. Fu and colleagues fabricated a photocatalyst with single-atom Fe anchored on crimped g-C 3 N 4 (referred as Fe@g-C 3 N 4 ) for photocatalytic HER [117]. The appropriate d-band position and negatively shifted Fermi level of Fe@g-C 3 N 4 effectively enhanced photocatalytic HER rate (3390 mol h −1 g −1 , λ > 420 nm), giving an apparent quantum efficiency (AQE) value of 6.89% at 420 nm.

Synthesis of SAPCs
A simple and effective synthesis method is one critical factor for the practical application of SAPCs [118]. It is essential to construct strong coordination interactions between single metal atoms and support materials to avoid the aggregation of these isolated atoms with high surface free energy during synthesis and reaction processes [119]. This can be achieved through various strategies including defect engineering, coordinated design, and spatial confinement [120]. In the following section, the commonly used synthesis methods for SAPCs are summarized.

Impregnation Method
The impregnation method is a typical and flexible method for the synthesis of singleatom catalysts. The support materials are immersed in a solution containing appropriate metal precursors. In this condition, the metal ions could be anchored or absorbed on the support surface through coordination, electrostatic adsorption, or spatial confinement. This is followed by drying and activation treatment, which can make the single metal atoms stably anchored on the supports. Here, the bonding with coordination atoms in supports such as, O, N, P, S, etc. is the key to keep the metal atoms spatially isolated with restricted migration.
Supports with high specific surface areas and sufficient coordination sites, such as g-C 3 N 4 , GO, NG, MOF, TiO 2 , etc., are suitable supports for SAPCs synthesized via the impregnation method. Wang et al. explored zirconium-porphyrinic MOF hollow nanotubes (HNTM) as the support materials [121]. Benefitting from the well-defined square-planar anchoring sites of porphyrin units, a series of noble-metal (Ir, Pt, Ru, Au, and Pd) single atoms were successfully immobilized on HNTM structures by the simple impregnation method (Figure 11a). In some cases, when the coordination between single metal atoms and supports cannot be established under room temperature, appropriate heat treating is required to promote the bond formation. Xu et al. prepared atomically dispersed Ni atoms on nitrogen-doped graphene (referred as Ni-NG) by the impregnation method [122]. In a typical synthesis, the GO support was dispersed in NiCl 2 aqueous solution by sonication treatment to make Ni 2+ uniformly and atomically adsorbed on the support. Thereafter, the sample was collected and freeze-dried, followed by treatment in NH 3 atmosphere for doping N into GO and establishing a strong interaction between Ni and the N-doped GO support. The prepared Ni-NG sample was further coupled with CdS and employed as an active non-noble-metal cocatalyst for photocatalytic HER. The results showed that Ni-NG/CdS exhibited much improved stability and activity (1351.1 umol/h) compared to noble metal Pt loaded-CdS, and this can be attributed to the beneficial effects associated with the single Ni atoms anchored on NG.

Co-Precipitation Method
As for the co-precipitation method, single atoms are immobilized on the supports during their formation process. This method is suitable for support materials such as g-C 3 N 4 , MOF, metal sulfides, and metal [123][124][125]. For example, Lu et al. prepared a series of single-atom Mo catalysts by calcining low-cost primary material of urea with various amounts of Mo(VI) salts [126]. The isolated Mo atoms were incorporated into g-C 3 N 4 through MoN 2 species in the polymerization process. The coordinatively unsaturated Mo sites acted as reaction centers for N 2 chemisorption and activation during photocatalysis. With regard to the metal supports, the atomic ratio of metal precursor to support is a key factor that should be precisely controlled for preparation of single atoms in metal supports. For example, Yang et al. synthesized a single-atom Pt 1 Ag 24 alloy cocatalyst on g-C 3 N 4 , and the resulting composite (referred as Pt 1 Ag 24 / g-C 3 N 4 ) was applied in photocatalytic HER [127]. The H 2 evolution rate of Pt 1 Ag 24 /g-C 3 N 4 reached 39.7 umol/h, which was four times higer than that of Ag 25 /g-C 3 N 4. The precise composition of single-atom Pt 1 Ag 24 alloy cocatalyst was tuned by carefully tuning the molar ratio of Ag and Pt precursors during the nucleation and growth process.

Photo-Deposition Method
Similar to the impregnation method, the photo-deposition approach is also carried out in solution, but uses the energy of photons to reduce and anchor metal ions on the support. It is a mild and economical preparation method [128]. Moreover, this method is favorable for increasing the loading content of single metal atoms. Guo et al. anchored single-atom Pt on N-vacancy-riched g-C 3 N 4 by a icing-assisted in-situ photocatalytic reduction method at sub-zero temperature [129]. The schematic illustration of photoreduction of H 2 PtCl 6 on g-C 3 N 4 is shown in Figure 11b. Under illumination, the [PtCl 6 ] 2adsorbed on g-C 3 N 4 was reduced to metal Pt species by photogenerated electrons from excited g-C 3 N 4 . Because of the facile diffusion and nucleation of Pt species, the Pt nanoparticles was readily generated in solution. However, only the [PtCl 6 ] 2species absorbed at N vacancies could be reduced into single-atom Pt under icing conditions. Moreover, a strong reactive metal-support interaction was formed between Pt single atoms and g-C 3 N 4 due to the rich N vacancies, and this contributed to an ultrahigh average covering density of PtSAs (0.35 mg m −2 ) on the electron-deficient g-C 3 N 4 . In addition, it has also been demonstrated that the metal ions could be precisely deposited on the electronic enrichment area in photochemical reduction process, which is expected to enable the position-controlled deposition of single atoms [130,131].

Atomic Layer Deposition Method
Atomic layer deposition (ALD) is a precise technology in which the steam of a metal precursor is controlled deposited on the support in the reaction chamber. By altering the reaction temperature, deposition time, and deposition cycles, single-atom photocatalysts have been successfully synthesized via the ALD method [132,133]. For example, Sun et al. utilized ALD to produce isolated Pt single atoms and clusters on NG nanosheets. [134] The typical process for ALD is shown in Figure 11c. The Pt precursors (MeCpPtMe 3 ) were firstly reacted with NG nanosheets. Subsequently, they were completely oxidized in the presence of O 2 to result in the single Pt atoms. These two steps formed a complete cycle of ALD. By exactly adjusting the cycle number, the size and density of Pt on NG can be intentionally tuned from single atoms, sub-nanometre clusters, to NPs. However, it should be noted that ALD is relatively more complex and expensive compared to the other commonly used methods for constructing SAPCs [135].

Other Methods
In addition to the above widely explored methods, some distinct novel synthetic strategies have also been developed by researchers to create SAPCs. For instance, Zhang et al. fabricated SAPCs with an ultrahigh Pt loading content of 8.7 wt % via ion exchange between Pt 2+ and the alkali metal ions K + that were intercalated into carbon nitrides (referred as SA-Pt/g-C 3 N 4 -8.7, Figure 11d) [136]. The spatial confinement within adjacent layers of support was important factor for achieving atomic dispersion of Pt with an ultrahigh loading content. Moreover, the well-confined interlayer environment alerted the charge distribution around the Pt atoms, and this favored proton adsorption as well as lowered the energy barrier of HER. Hence, SA-Pt/g-C 3 N 4 -8.7 exhibited an excellent activity of 22.5 mmol g −1 h −1 and an apparent quantum yield of 22.5% for photocatalytic HER, which are record-breaking for the reported g-C 3 N 4 -based photocatalysts. Figure 11. (a) Formation of the hollow nanotube MOF and the single atom-immobilized hollow nanotube MOF, which are denoted as HNTM and HNTM-M, respectively [121]; (b) Schematic illustration of the icing-assisted in-situ photocatalytic reduction method for preparing the suprahigh-density PtSAs-loaded g-C 3 N 4 . Scale bar: 2 nm [129]; (c) Schematic illustration of the ALD mechanism for creating Pt single atoms on NGNs [134]; and (d) Illustration of the synthetic process of single-atom Pt photocatalysts [136].

Applications of SAPCs for Photocatalytic Water Splitting
H 2 produced from photocatalytic water splitting is an ideal clean energy to replace fossil fuels. However, great challenges remain for developing high-activity, durable, and economical photocatalysts that can realize practical applications toward photocatalytic H 2 generation. The creation of SAPCs provides a new clue for the development of water splitting photocatalysts with excellent activity, selectivity, and stability, as well as high atomic utilization of noble metals to lower the cost of the catalyst. Table 1 summarizes the recent progress in this research area. As discussed above, the single metal sites in SAPCs have been intensively used as efficient cocatalysts for promoting photocatalytic H 2 evolution.
In particular, benefitting from the high redox potential and work function, Pt in the form of atomically dispersed single atoms on TiO 2 , g-C 3 N 4 , and MOF have manifested excellent performance in photocatalytic H 2 evolution [137,138]. Li et al. reported an efficient photocatalyst (referred as Pt 1 /def-TiO 2 ) by dispersing single Pt atoms on defective TiO 2 [94]. Due to the hydrogen spillover effect, the atomically dispersed Pt atoms caused the formation of surface oxygen vacancies in neighboring TiO 2 units, giving birth to the Pt-O-Ti 3+ atomic interface. According to the experimental results and DFT calculations, the Pt-O-Ti 3+ atomic interface effectively facilitated the transfer of photogenerated electrons from Ti 3+ defective sites to single Pt atoms, thereby enhancing the separation of electron-hole pairs. This unique structure allowed Pt 1 /def-TiO 2 to exhibit an exceptionally high turnover frequency of 51,423 h −1 , which was 591 times higher than that of the Pt nanoparticlessupported TiO 2 photocatalyst.
SAPCs with non-noble metals such as Co, Ni, etc. have also been developed as cocatalysts to give excellent performance in photocatalytic water splitting. Importantly, apart from the benefits for H 2 evolution reaction, it has been reported that the cocatalyst of single-atom Co could facilitate the oxidation of water for O 2 evolution half reaction or even overall water splitting [139]. For example, Wei et al. developed a single-atom, Co-based photocatalyst (referred as Co/g-C 3 N 4 ) with single Co atoms confined within phosphide doped-g-C 3 N 4 nanosheets. [56] The Co 1 -phosphide sites with coordination unsaturated Co sites exhibited oxygen evolution activity by effectively promoting water molecular adsorption and activation, and also helped to suppress electron-hole recombination. Meanwhile the phosphide dopants accelerated the electric conductivity of Co/g-C 3 N 4 . As a result, in photocatalytic overall water splitting reaction, Co/g-C 3 N 4 afforded a H 2 evolution rate as high as 410.3 umol h −1 , with the carrier lifetime over 20 times prolonged compared to the pristine g-C 3 N 4 .

Summary and Prospect
SAPCs provide a new pathway for the construction of high performance cocatalyst/photocatalyst composites. For the past few years, the preparation method, characterization technology, theoretical modeling, and mechanism investigation of SAPCs have been developed rapidly, promoting the rational design and fabrication of more efficient SAPCs. As discussed above, benefitting from their low-coordination status, unique electric structures, and metal-support interactions, SAPCs can bring about unique advantages in comparison with the nanocluster-based, nanoparticle-based, and bulk catalysts. These include: (1) reduced usage amounts of metals (especially pertinent to noble metals) due to the ultrahigh atomic utilization; (2) distinguished catalytic activity, selectivity, and stability; and (3) simple model of the material system, facilitating the study of catalytic mechanism and photocatalyst design. Hence, SAPCs possess great potentials and good application prospects for photocatalytic water splitting.
Although SAPCs have shown excellent performance in photocatalysis, there remain several great challenges. The high surface free energy of isolated atoms makes it is intrinsically difficult to synthesize SAPCs, in consideration of the serious tendency for agglomeration of single atoms and the resultant deactivation during reaction. Further research should be undertaken to expand the library of supports, develop new synthesis methods, and improve the loading content and stability of single metal atoms. The newly supports such as BiOCl, perovskite, et al., provide more abundant coordination atoms different from the traditional N, O, C etc., which will bring new possibilities. Various synthesis methods, such as ion exchange, could be tried to synthesize SAPCs; Besides, the current studies exploiting single-atom photocatalysts for photocatalytic H 2 evolution mainly relied on the usage of sacrificial reagents. The design and creation of SAPCs capable of driving photocatalytic overall water splitting need to be further explored. Loading dual single metal atoms as bifunctional cocatalysts may be a promising strategy worthy to be investigated for improving the quantum efficiency of photocatalysis; Moreover, indepth study of structure-function relationship and photocatalytic mechanism in SAPCs are strongly required, which require the exploration of advanced characterization techniques for precisely determining the single atoms. By making progress in these directions, it is believed that we can rationally construct SAPCs with optimized catalytic properties to realize practical application in solar energy conversion in the feature.

Conflicts of Interest:
The authors declare no conflict of interest.