Alkali/alkaline-earth metal intercalated g-C3N4 induced charge redistribution and optimized photocatalysis: status and challenges

Limited by the intrinsic graphitic sp2-hybridized array of tri-s-triazine repeating units and inert stack of layers, the insertion of intercalant between layers of graphitic carbon nitride (CN) could be an effective way to strengthen the van der Waals force between adjoining layers and induce the charge redistribution in CN for the improvement of photocatalysis. This review summarizes the latest progress related to the design and construction of alkali/alkaline-earth metal intercalated CN, including (a) single alkali/alkaline-earth metal intercalated CN, (b) alkali/alkaline-earth metal and non-metallic species co-intercalated CN, (c) alkali/alkaline-earth metal intercalated and surface modification co-functionalized CN. The promotion mechanism of each classification will be critically discussed, namely alkali/alkaline-earth metal intercalated CN-induced charge redistribution facilitates the adsorption and activation of reactants, accelerates the separation efficiency of photogenerated carriers, and optimizes the reaction pathway. Also, the influence on band structure and optical property has been discussed with the intercalation of alkali/alkaline-earth metal. Finally, this mini-review highlights crucial issues that should be addressed in future research.


Introduction
Ever since the 1970s, the increasing challenges in energy demands and environmental concerns due to the consumption of fossil fuels have raised awareness of a potential global crisis. Thus, developing cutting-edge science and technology has been pursued to overcome the obstacle for effective energy conversion and environmental protection [1][2][3][4]. Among various renewable energy projects, semiconductor photocatalysis as a feasible technology that can be harvested by the inexhaustible and clean solar energy has gained considerable interdisciplinary attention for its diverse potential in energy and environmental applications [5][6][7][8][9][10][11].
Since the landmark event of photocatalytic water splitting using TiO 2 electrodes under ultraviolet light was ignited by Fujishima and Honda in 1972 [12], extensive research has been carried out on traditional semiconductor photocatalysts to address the energy shortages and environmental threats. Recently, great efforts have been made to design visible-light-responsive novel photocatalysts for effective utilization of the solar spectrum that comprises a large fraction of visible light (ca. 43%) [13][14][15][16][17][18]. In 2009, a metal-free polymeric photocatalyst graphitic carbon nitride (CN) was first reported by Wang et al for photocatalytic H 2 evolution, which potentially shifted the research exploration from inorganic compound to artificial conjugated polymer semiconductors [19]. Afterwards, CN became an alternative and attractive photocatalyst in view of its facile synthesis, appealing electronic structure, high physicochemical stability, and 'earth-abundant' nature [19][20][21][22][23][24].

Enhanced photocatalysis on alkali/alkaline-earth metal intercalated g-C 3 N 4
Even though several literatures reported that the introduction of alkali/alkaline-earth metal in CN could improve the photocatalytic performance, the position of doped atoms, promotion mechanism, and the influence of other congener alkaline elements with similar electronic structure on photocatalytic performance have not been revealed clearly [31,[44][45][46][47]. Recent studies on alkali/alkaline-earth metal intercalated CN are summarized in table 1. The first comprehensive study with theoretical and experimental proofs was reported by Xiong et al in 2016 [27]. Further, Li et al developed alkalis (K, Rb, Cs) intercalated in CN, and unravelled the key role of interlayer electron transfer direction and particular promotion mechanism [48]. The alkali species could suppress random charge transfer in the planes of CN and enable the electrons to directionally migrate between adjacent layers. The intercalation of alkali/alkaline-earth metal induces the charge redistribution in CN, which is beneficial to accelerate the adsorption and activation of  Li-intercalated CN Degradation of RhB dye 0.028 min −1 (photocatalytic efficiency, 2.8 times higher than CN) [53] 0.007 min −1 (photocatalytic efficiency, 1.8 times higher than CN) K-intercalated CN Hydrogen production 919.5 mol h −1 g −1 (13.1 times higher than CN) [54] Li/Na/K-intercalated CN Hydrogen production Vacuum condition: AQY = 37%, air condition: AQY = 4.9% [55] Ba-intercalated CN Tetracycline elimination 91.94% (degradation efficiency) [56] K-intercalated CN Degradation of RhB dye 0.03698 min −1 (photocatalytic rate constant, 8.3 times higher than CN) [57] reactants, the separation of charge carriers, and even the optimization of reaction pathways for the enhancement of photocatalysis.

Accelerated separation efficiency of photogenerated carriers
During a photocatalytic reaction, the successful migration of charge carriers to the reactant molecules determines the subsequent processes. Once spatial separation is realized, the ignited charge carriers are migrated to the surface of the photocatalyst for initiating the reduction and oxidation processes, aiming to the photocatalytic conversion of reactants. However, it is noteworthy that the photocatalyst is chemically active only when the photogenerated electron-hole pair is consumed simultaneously before the recombination occurs in a fraction of nanoseconds. Limited by the intrinsic character, random charge transfer in the planes of CN andthe inert stack of layers gives rise to a high recombination rate of charge carriers. The intercalation of heteroatom could be an efficient way to induce the charge redistribution in CN and also strengthen the vdW force between adjoining layers for the spatial separation of charge carriers and thus the improvement of photocatalysis. Xiong et al developed CN doped with Na and K atoms, and demonstrated that the photocatalytic performance of K-doped CN (CN-K) was superior to Na-doped CN (CN-Na) [27]. According to the DFT calculation, there was a static coulomb interaction between Na/K and N nearest to them after the introduction of heteroatoms. K atoms tended to intercalate into the CN interlayer and the charge of upper layer N (C) atoms and lower layer C (N) atoms close to the intercalated K atoms increased, forming a funnel-like three-dimensional structure (figure 2(a)). K atoms could chemically bond with atoms at the adjacent layers to form charge delivery channels and bridge the layers, which induced the charge redistribution and extended the π conjugated system to accelerate the charge carrier transfer between neighbouring layers and thus enhance the photocatalytic performance. However, Na atoms existing in the caves of the CN plane resulted in the increase of in-planar electron density (figure 2(b)), which led to a high recombination rate of the charge carrier and thus a decrease of performance. The quenched PL intensity (figure 2(e)), increased photocurrent response (figure 2(f)), and prolonged lifetime of charge carriers (figure 2(g)) also demonstrated that intercalated K atoms promoted the charge transfer and separation. Thus, the introduction of heteroatoms between adjacent layers of CN steers the random carriers and then induces the interlayer delivery of carriers to promote the separation efficiency.
Besides, Zhou et al modified CN using multi-site Sr-doping with simultaneous N atom replacement (CNSr0.05), cavity padding, and intercalation, and further demonstrated that intercalation was the best doping site to improve the separation efficiency of electron and hole pairs [52]. The PL intensity of CNSr0.05 was obviously lower than that of CN (figure 3(a)), the circular arc radius on the EIS Nyquist plot of CNSr0.05 was significantly smaller than that of pure CN (figure 3(b)), and the intensity of the photocurrent was enhanced (figure 3(c)), suggesting that the introduction of Sr atoms could improve the separation efficiency of electron and hole pairs. Further, the promotion mechanism was confirmed by theoretical simulation. According to the electron density distribution of CN, it could be found that the replacement of N atoms and cavity padding had no effect on the electron transfer (figures 3(e)-(g)), however, the intercalation of Sr could form an interlayer electron channel (figure 3(h)). The intercalary Sr atom induced channel links two adjacent layers of CN and then facilitated the migration of carriers from one layer to another layer, which was beneficial to the separation of photogenerated carriers for the optimization of performance [52].
Several similar research results that intercalated alkali/alkaline-earth metal induced charge redistribution in CN and then accelerated the separation efficiency of photogenerated carriers have also been reflected [53,55,57]. Liu et al indicated that the separation efficiency of charge carriers was improved by the introduction of K, which was attributed to the static electricity deriving from intercalated K atoms controlling the directional flow of charge carriers [57]. Zeng et al developed alkali-metal-atoms intercalated CN (K, Na, Li co-intercalated CN) for efficient overall water splitting and verified that intercalated alkali-metal-atoms bridged the adjacent layers of CN, which boosted the interlayer transportation of charge and then accelerated the separation of the charge carrier [55]. Even though alkali/alkaline-earth metal intercalated CN induced the charge redistribution and promoted the separation efficiency of charge carriers to optimize the photocatalytic performance, the difference of congener elements has been neglected and the promotion mechanism of photocatalytic efficiency should be further revealed to conduct the effective design and fabrication of 2D photocatalysts.
Li et al found that Rb intercalated CN (CN-Rb) improved the photocatalytic efficiency of CN, significantly exceeding that of bare and the other alkali K/Cs-intercalated CNs (CN-K/CN-Cs). The transfer direction of electrons was firstly confirmed and also the difference of congener alkaline elements was further clarified, which advanced the explanation of the promotion mechanism. As shown in figure 4(a), the electrostatic potential of the second layer (L2) was significantly increased after the introduction of alkalis and (c) charge difference distribution between metal atoms and CN layers (charge accumulation is in blue and depletion is in yellow, the isosurfaces are set to 0.005 eV Å −3 , brown, blue, purple, red and green spheres depict C, N, K, Rb and Cs atoms, respectively). Reproduced from [48] with permission of The Royal Society of Chemistry.
(K, Rb, Cs), and a further increase of potential in L1 was acquired in CN-Rb and CN-Cs. Compared with CN-K, the electron migration in the direction of L1 → L2 → L3 by the one-way transmission manner after the intercalation of Rb and Cs. The unspecific electrostatic potential difference between L1 and L2 of CN-K resulted in a random interlayer electron delivery, thus increasing the potential possibility of charge carrier recombination. Besides, stronger covalent interactions between Rb/Cs and adjacent N atoms were confirmed by the electronic location function and the charge difference distribution (figures 4(b) and (c)), which indicated that the covalence mainly exists between the alkali and L2 and thus provided a more solid interlayer channel for directional electron transfer in CN-Rb and CN-Cs than in CN-K. The directional electron delivery via a one-way transmission manner could further boost charge separation, thus contributing to much higher photocatalytic performance of CN-Rb/CN-Cs than CN-K [48].
The intercalary heteroatoms steer the random intralayer carriers and also strengthen the vdW force between adjoining layers, thus realizing the directional electron delivery via a vertical channel between CN layers. Therefore, the intercalation of alkali/alkaline-earth metal is an effective way to facilitate the migration of carriers from one layer to another layer, which is beneficial to the separation of photogenerated electrons and holes. Nevertheless, the ignited charge carriers initiate subsequent reduction and oxidation processes and need to be further explored for the sake of revealing the promotion mechanism, which could conduct the fabrication of highly efficient 2D photocatalysts.

Optimized reactant activation and reaction pathway
The heterogeneous photocatalysis involves successive procedures of reactant adsorption, photoactivated reaction, and product desorption occurring at the catalyst surface [58][59][60]. Thus, the adsorption and activation of reactants is the prerequisite condition for subsequent photogenerated carrier-initiated reactions and then the separation efficiency of electron and hole pairs has a great effect on the formation of radicals participating in redox reaction. Generally, the introduction of heteroatoms could induce the charge redistribution, contributing to the transform of electrons from a delocalized state to a localized state for the formation of localized excess electrons (e − ex ). The generation of e − ex is beneficial to accelerate the adsorption and activation of reactants and facilitate subsequent chemical reactions, thus elevating the photocatalytic performance.
Dong et al developed Sr-intercalated CN (CN-5Sr) to realize the electron localization, which promoted the activation of reactants and intermediates, as well as the charge separation and transfer for the optimization of photocatalytic activity and selectivity in oxidizing NO into target products (NO 2 − and NO 3 − ) [49]. According to the charge difference distribution between Sr atoms and CN layers, randomly distributed electrons on the surface layer were localized to Sr atom and formed a local region with high electron density, and then e − ex could directionally transfer to the sublayer via the Sr-mediated electronic channel (figures 5(a) and (b)). The localized electrons provided more possibility for the adsorption and activation of reactants and intermediates. Besides, prolonged lifetime of charge carriers in CN-5Sr verified that the formation of electron localization and the interlayered electrons transfer channel effectively inhibited the recombination of carriers (figure 5(e)). Therefore, the facilitated activation of reactants and the accelerated separation of charge carriers jointly promoted the formation of reactive oxygen species (ROS) participating in photocatalytic pollutant removal, which was well reflected by the enlarged O-O bond length, increased adsorption energy, and stronger signals of light-induced DMPO-·O 2 − on CN-5Sr (figures 5(f) and (g)).
Also, Li et al constructed an interlayer channel between CN layers by the intercalation of Ca to induce the formation of e − ex , and besides, further demonstrated the function of e − ex [50]. The e − ex prefer to be captured by  reactants [61,62], which dominantly contributes to reinforced reactant activation and ROS generation [63].
Then it directly initiates the photocatalytic reaction, which reduces the reaction activation energies and increases the reaction rates of elementary reactions, hence overcoming the rate-determining step for more efficient pollutant conversion and target product generation [5,64]. According to the calculation of adsorption and activation of one O 2 molecule and multiple O 2 molecules (figures 6(a)-(d)), more intense charge transfer was observed between O 2 (one O 2 molecule and multiple O 2 molecules) and CN-Ca than between O 2 (one O 2 molecule and multiple O 2 molecules) and CN [50]. Besides, O 2 molecules on CN-Ca were more easily activated to form ROS-H (ROS with high oxidative capability, ROS-L: ROS with low activity), which could directly participate in the photooxidation reaction and overcome the rate-determining step of elementary reactions for the improvement of photocatalysis (figure 5(h)). The calculation of ROS-driven reaction pathways show that activation Energy (∆E a ) and reaction energies (∆E r ) of CN-Ca were much lower than that of pristine CN (figures 6(e) and (f)), which indicated that promoted activation of reactants could reduce the energy barrier for a chemical reaction. Similarly, adsorbed intermediates and products and corresponding adsorption energies have been monitored and calculated by combined in situ DRIFTS and DFT simulation, revealing the function on the intercalation of Sr in CN layers [49]. As shown in the tendency of species evolution (figure 7(a)), the adsorption and transformation of NO, intermediates cis-N 2 O 2 and trans-N 2 O 2 were all greatly boosted on CN-5Sr in the NO adsorption process, indicating that doped Sr promoted the adsorption and activation of NO. During the irradiation process (figure 7(b)), two target products NO 2 − and NO 3 − were greatly increased as well, which implied that the promoted activation of reactants significantly improved the selectivity during photocatalytic oxidation of NO into target products. According to DFT calculations (figure 7(c)), the energy profile clearly demonstrated that the whole NO→cis- − processes were exothermic reactions and all energy were favourable in comparison with pristine CN, indicating that the intermediates could be transformed into the target products more smoothly.
The intercalation of alkalis induces the charge redistribution and fabricates a vertical channel between adjacent layers for directional electron delivery, contributing to promote the formation of e − ex and suppress the random transfer of charge carriers. Correspondingly, the accelerated activation of reactants and the facilitated separation efficiency of photogenerated carriers has been realized to promote the generation of ROS, which could optimize the reaction pathway to elevate the photocatalytic performance. Meanwhile, considering the remarkable effects on the activation of reactants and the separation of carriers after the introduction of alkali/alkaline-earth metal, the controllable doping content and co-intercalation should be taken into account to further enhance the overall photocatalysis efficiency.

Enhanced photocatalysis on alkali/alkaline-earth metal and non-metallic species co-intercalated g-C 3 N 4
Even though the fabrication of the directional electron delivery channel by intercalating alkali/alkaline-earth metal in CN promotes the photocatalytic performance, the accelerated accumulation of charge carriers in one layer would inevitably lead to the recombination of carriers. Alkali/alkaline-earth metal and non-metallic species co-intercalated CN have been proposed to address this challenge and further improve the photocatalytic performance. It is found that co-doped alkali/alkaline-earth metal and non-metallic species could function as interlayer dual channels to relieve the accumulation of carriers in one layer, and thus inhibit the recombination of carriers.
Correctly, the intercalated alkali/alkaline-earth metal and non-metallic species can not only bridge the adjacent layers but also be served as the cations and anions to regulate the separate transfer of electrons and holes. Xiong et al used KCl as the doping precursor to introduce K and Cl ions into the interlayer of CN (CN-KCl) [65]. As shown in the charge redistribution of CN ( figure 8(b)), K atoms donated electrons to the  upper N atoms and were then ionically bonded with them, and in contrast the upper and lower atoms donated electrons to the Cl atoms and were chemically bonded to the Cl atoms. The K atoms were accommodated as cations, while the Cl atoms served as anions, indicating that K and Cl ions could separately behave as transport paths for electrons and holes. Therefore, K and Cl ions coexisting in the interlayer of CN might function as a dual channel for electrons and holes transfer respectively, which induced the charge redistribution and simultaneously inhibited the inevitable recombination of carriers in single-channel transportation. Also, the average electrostatic potential of the CN layers from L3 to L1 showed a stepwise increase by the co-doping of K and Cl ( figure 8(c)). The interlayer electrical field drove excited electrons to transfer from L1 to L2, then to L3, and inversely, holes transferred in the opposite direction, from L3 to L2, and then to L1. The introduction of KCl lowered the barrier of charge transfer between the adjacent layers, and thus contributed to the highly efficient separation of charge carriers. As shown in the experimental characterization, increased photocurrent density ( figure 8(d)), smaller arc radius in the electrochemical impedance spectrum (figure 8(e)), prolonged lifetime (figure 8(f)) and lower PL intensities ( figure 8(g)) also reflected the enhanced separation efficiency of photogenerated electrons and holes.
Furthermore, apart from the construction of interlayer dual channels for the separate transfer of electron and hole, interlayer circuit by co-doping alkali/alkaline-earth metal and non-metallic species also could be an efficient way to inhibit the recombination of carriers in single-channel transportation. As reported by Cui et al, an interlayer bioriented electron transportation channel in CN by co-doping K and NO 3 − species (CN-KN) between neighbouring layers was crafted to balance the charge distribution, which significantly decreased the recombination of charge carrier [66]. As shown in charge difference distribution ( figure 9(b)), K atoms could chemically bond with N (C) atoms to bridge the neighbouring layers and form interlayer electron delivery channels (the electron delivery direction was from L1 to L2), also, N (C) atoms (in L2) close to the interbedded NO 3 − could donate electrons to NO 3 − , resulting in the formation of another vertical electron transportation channel via NO 3 − species (the electron delivery direction was from L2 to L1). The interlayer bioriented electron transportation channels significantly reduced the energy barriers for electron transfer between adjacent layers. Thus, the inevitable recombination of carriers caused by the accumulation of electrons in one layer through single-channel transportation was precluded, and thus the separation and transportation efficiency of carriers was boosted, as evidenced by the prolongation of radiative lifetime and the quenching of PL peaks. Therefore, the intercalated alkali/alkaline-earth metal and non-metallic species as interlayer dual channels could effectively relieve the accumulation of carriers in one layer. Further, Zhang et al constructed a modified CN (K and Cl co-interlayered and OH group decorated CN, AKCN), and demonstrated that the charge balance can be reached in co-intercalated CN by calculating the absolute value about the difference of electron distribution [67]. The presence of doped K atoms induced the anisotropic electron density redistribution, and a relatively large number of electrons were accumulated on the first layer (−2.34 e of layer charge) than on the second one (−0.99 e of layer charge) ( figure 10(a)). Further, the additional doping of Cl atoms made the electron distribution more balanced between the layers ( figure 10(b)). Therefore, the K-induced electron density polarization could be counterbalanced by Cl and thus the |∆q| (|∆q| represents the absolute value of the difference of the electron distribution between the first and second layer) were decreased from 1.35 e of K-doped CN to 0.16 e of KCl-doped CN. When the hydroxyl (OH) group was additionally introduced in AKCN, the OH group was preferably bonded to the surface carbon, inducing an outstanding electron depletion region around the OH group on the first layer ( figure 10(c)). The introduction of OH groups further decreased the |∆q| from 0.16 e to 0.06 e, contributing to the charge transfer between adjacent layers, simultaneously addressing the accumulation of charge in one layer and alleviating the recombination of carriers.
Correspondingly, the ideal efficiency (about 100% quantum efficiency of H 2 O 2 generation) of AKCN implied that the photogenerated charge carriers in AKCN were efficiently separated to realize the selective reduction of O 2 to H 2 O 2 , as evidenced by the highest photocurrent, smallest arc radius in Nyquist plot analysis, the highly enhanced interfacial charge transfer on AKCN by measuring the Fe 3+/2+ shuttlemediated photocurrent, higher open-circuit voltage and slower photovoltage decay. Meanwhile, the presence of the OH group induced the charge distribution over different atoms and thus achieved the local polarization (figures 10(d) and (e)), which accelerated the adsorption and activation of reactants. The synergistic improvement of spatial charge separation and local polarization between the interlayers and in-plane was critical for the ideal efficiency of AKCN. Therefore, the improvement strategies of photocatalysis are to optimize the charge distribution for accelerated reactant activation and the spatial charge separation for the inhibition of excessive accumulation of charge carriers in one layer.

Enhanced photocatalysis on alkali/alkaline-earth metal intercalated and surface modification co-functionalized g-C 3 N 4
Recently, surface modification and alkali/alkaline-earth metal intercalated co-functionalized g-C 3 N 4 have been developed to further facilitate the charge redistribution and alleviate the inevitable recombination of carriers in one layer (table 2). The introduction of hetero atoms/species or vacancies in plane and the intercalation of alkali/alkaline-earth metal between adjacent layers induce the intralayer charge localization and bridge the adjacent layers, respectively, which supresses random migration of electrons in plane and strengthens the Coulomb interactions between layers. Therefore, the co-functionalized strategy could realize the charge redistribution, spatial charge separation and most importantly make the electron distribution more balanced in CN, thus improving the activation of reactants and the separation efficiency of charge carriers for the optimization of photocatalytic performance.

Alkali/alkaline-earth metal intercalated and surface decoration co-functionalized g-C 3 N 4
As shown in figure 10, the presence of OH group in-plane induced the charge redistribution over different atoms, which further balanced the distribution of electron in CN and thus alleviated the excessive accumulation of charge carriers in one layer. Also, the decoration of OH group induced the localization of charge to facilitate the adsorption and activation of reactants. Therefore, the intercalation of alkali/alkaline-earth metal and simultaneously the intralayer decoration of hetero atoms/species is an efficient way to balance the charge distribution, accelerating the spatial charge separation and optimizing the photocatalytic reaction. Similarly, Ran et al synthesized the phosphate/potassium co-functionalized carbon nitride via a one-step in situ co-pyrolysis of thiourea and potassium phosphate [71]. The surface modification with phosphate groups and the interlayer incorporation with potassium significantly promoted the activation of O 2 , NO and H 2 O on the catalyst surface and facilitated the separation of carriers, contributing to the enhancement of photocatalytic performance. Apart from the decoration of functional groups, the surface modification by heteroatoms-doped is another conventional approach. Considering the limitation of intrinsic graphitic sp 2 -hybridized array of tri-s-triazine repeating units and the inert stack of layers in CN, the construction of intralayer electronic trapping/converging districts and interlayer electronic mediators was proposed by Cui et al [68]. An O/Ba co-functionalized amorphous carbon nitride (O-ACN-Ba) was designed and synthesized. The intralayer Generally, the introduction of heteroatoms has an effect on the electronic structure and energy band configuration of photocatalyst, which influences the subsequent photocatalytic reaction. Thus, it is important to further explore the function of inner structure optimization and explain the promotion mechanism. Li et al further found that once O 'adjuster' atoms were introduced into a layer of CN, a van der Waals heterostructure was established between the O-modified CN layer (OCN) and adjacent CN sublayer, and meanwhile, interlayer charge flow could be expedited via intercalated elemental K, which served as a 'mediator' to strengthen the interlayer vdW interaction (scheme 1(a)) [70]. According to the calculated total density of states of CN and OCN layers, OCN band structure was adjusted by the O adjustor atoms, leading to a band-offset between the OCN layer and the CN sublayer and thus indicating the formation of internal van der Waals heterostructures (IVDWHs). Furthermore, the band-offset between CN and OCN layers allowed light-generated hole migration to OCN layer and electron transfer to CN sublayer to achieve a spatial charge carrier separation, which suppressed the accumulation of carriers in one layer and addressed the inevitable recombination (scheme 1(c)).
By comparing the electronic structures of pristine CN and OCN-K-CN, the potential energies of the OCN layer and CN sublayer in OCN-K-CN were significantly increased after the incorporation of O and K, which provided the driving force for electron transfer from the OCN layer to the CN sublayer through the interlayer K channel (figures 12(a) and (b)). Correspondingly, the local charge distribution in the OCN layer was altered and thus induced the accumulation of electrons around the O adjuster atoms, and the interlayer electron transfer was strengthened with the introduction of O adjuster atoms, which reinforced the internal vdW force to effectively facilitate the interlayer charge flow and accelerate the separation of electron-hole pairs (figures 12(c)-(f)). The expedited spatial charge separation thus promoted the generation of abundant ROS for the highly efficient photocatalytic performance.
The surface decoration in layered materials not only induces the charge redistribution to facilitate the activation of reactants, but also makes a difference between surface layer and sublayer to establish an inner heterostructure for the directional charge transfer. Meanwhile, the intercalation of alkali/alkaline-earth metal served as a 'mediator' to strengthen the interlayer interaction and realize the spatial charge carrier separation, which greatly suppressed the accumulation of charge carriers in one layer and thus addressed the inevitable recombination. The intrinsic defect of CN could be well remedied by the co-functionalization of alkali/alkaline-earth metal intercalated and surface decoration.

Alkali/alkaline-earth metal intercalated and intralayer modification co-functionalized g-C 3 N 4
Considering the intrinsic graphitic sp 2 -hybridized array of tri-s-triazine repeating units and the inert stack of layers, multiple dopants (intralayer and interlayer co-doping) provides an approach to overcome the intrinsic shortcomings of CN. The intralayer and interlayer co-doping could realize the comprehensive modification of the electronic structure and thus make the charge distribution more balanced, achieving the spatial separation of carriers to optimize photocatalysis. Bi et al fabricated S/K co-doped CN photocatalysts for hydrogen evolution [73]. The binary-doped S and K atoms offered more electrons to the band gap in comparison with pure CN, and simultaneously the intercalation of the K atom bridged the adjacent layers for interlayer charge transfer, contributing to the separation and transport of photogenerated carries for the efficient photocatalytic hydrogen evolution. Also, halogen and potassium binary-doped graphitic carbon nitride (named as X-K-C 3 N 4 , X=F, Cl, Br, I) photocatalysts were synthetized via simply one pot thermal polymerization method [74]. F-K-C 3 N 4 showed the highest H 2 evolution rate and stability, which was attributed to the synergistic effect of the intralayer C-F bond, the C≡N triple bond and the interlayer K junction promoting the separation and transfer of carriers.
Elemental doping is one of the appealing strategies to modulate the physicochemical properties, but the fact that introduced heteroatoms would lead to the distortion of crystal lattice cannot be ignored. Wu et al developed a Zn and K co-doped water-soluble carbon nitride (MCN) as a PDT photosensitizer for cancer therapy and confirmed that the introduction of Zn and K atoms caused the distortion of CN layers and thus the lattices were tilted to a certain extent to balance the distortion, which enhanced the coupling in layer-to-layer and therefore regulated the electronic structure of CN [75]. Based on the DFT calculation, Zn atoms tended to occupy the vacancy of π-π conjugate planes while K atoms occupied the spaces between adjacent CN layers ( figure 14(b)). The sp 2 orbitals were partially opened due to the broken symmetry when Zn and K atoms were introduced, as shown in differential charge density distribution (figure 13(c)). Therefore, the electrons of the sp 2 orbital would transfer to the p z orbital for the expansion of π conjugate electron system, and also bond with the outer electrons of doped metal atoms to enhance the stability of metal atoms in the system. Besides, a stronger ionic bond between Zn atoms and CN atoms at the edge of holes and the covalent bond between K atoms and the adjacent layers were reflected by the electron localization function (figures 13(e) and (f)). The π-π conjugate electron system was immensely extended, and the electrons were rapidly shifted through the bridge of K atoms, which was beneficial to the transmission of the photogenerated carriers in different MCN layers. Correspondingly, the dramatically decreased PL peak intensity and the longer lifetime of charge carriers of MCN well demonstrated that the co-doping of Zn or K atoms could hinder the annihilation of carries (figures 14(g) and (h)). Also, the distinctly smaller semicircle radius of impedance and R ct reflected a remarkable promotion on interfacial transfer efficiency of carriers, which thus boosted the generation of ROS and enhanced the therapeutic effect of PDT (figures 14(i)-(l)).
Besides, aiming to modify the electronic structure, vacancy-engineering is proven to be a promising approach and shows high efficiency in forming active sites, which is beneficial to balance the charge distribution and thus facilitate the separation of the charge carriers for the improvement of photocatalytic performance. Sun et al synthesized an amino-rich carbon nitride photocatalyst with incorporated K and carbon defects (K-AUCN) for photocatalytic CO 2 conversion [69]. The enriched basic amino groups contributed to improved CO 2 fixation and activation, and importantly the incorporated potassium (K + ) among the disordered layers and the accompanied carbon defects as electron promoters facilitated the separation and transfer of photogenerated carriers. The optimized sample exhibited a more-than-five-fold enhancement in CO 2 photocatalytic conversion under simulated sunlight compared to pristine CN. It is worth noting that the synergetic effect of multiple modification greatly improves the performance but the respective roles also need to be further confirmed, which could direct the rational design of the inner structure for the targeted optimization. Wang et al proposed a new synthesis strategy of rational co-doping of B, K elements in line with the controllable introduction of N vacancies (N v ) into CN (KBH-C 3 N 4 ), aiming to compensate the drawback of single element doping [72]. The modified KBH-C 3 N 4 achieved 161% and 527% increases for the production of CH 4 and CO respectively, compared with pristine CN during photocatalytic CO 2 reduction. The enhancement of performance was attributed to the synergistic effects that the multiple modifications constructed the electron-rich surface and tailored the electronic structure to significantly facilitate the adsorption and activation of CO 2 and accelerate the separation charge carriers.
Experimental and theoretical characterization has been employed to demonstrate that co-doping of B, K elements in line with controllable introduction of N v play their primary role to elevate the performance. According to the CO 2 -TPD profiles ( figure 14(a)), CO 2 uptake ability increased with the introduction of B, K dopant and N v , and the promoting effects were in the order of N v > B > K. Also, as shown in the theoretical simulation of CDD, both N v and B induced charge redistribution (electron-rich environment) at their adjacent atoms and it was worth noting that the electron accumulation caused by N v was stronger than B doping (figures 14(b)-(d)), which explained the facts that N v exhibited superior enhancement on CO 2 uptake than B doping. After the calculation of CO 2 adsorption on the optimal site of KBH-C 3 N 4 , it reflected a spontaneous process with the adsorption energy of −0.361 eV, which was four times as much as the pristine one. Also, the total charge (∆q) further demonstrated that there were more electrons (0.23 e − ) transferred from KBH-C 3 N 4 to CO 2 than pristine CN (0.02 e − ). Therefore, the introduction of N v greatly induced the charge redistribution and promoted the accumulation of electrons to accelerate the activation of CO 2 to participate in following conversion reactions. Besides, the CDD plots of KBH-C 3 N 4 illustrated the existence of K and significantly accelerated the spatial electron distribution, especially for the adjacent layers, and the ∆q of intercalated K atom was −1.55. The charge analyses also illustrated that B dopant with a ∆q (electron transfer) of −2.10 which donated ∼0.8 more electron than the C atom in the pristine CN, and N atoms as the Lewis basic sites showing ∆q of 1.37 gained ∼0.3 more electron than the N at the same site in pristine CN. According to the orbital plots ( figure 14(g)), the HOMO and LUMO in KBH-C 3 N 4 were not adjacent but localized mainly at N and C atoms of different layers of CN, which was benefit to the electron/hole separation and ensured a relative long carrier lifetime. Therefore, intercalated K bridged the adjacent layers and facilitated the electron transfers between the layers; intralayer doped B and N v rearranged the electron and molecular orbitals spatial distributions, and correctly B helped to maintain a high reduction potential and compensated the drawbacks of the K, and also N v narrowed the bandgap and significantly enhanced the CO 2 adsorption. Therefore, the synergistic effect on the introduction of hetero atoms/species or vacancies in plane and the intercalation of alkali/alkaline-earth metal between adjacent layers could alleviate the intrinsic shortcomings of CN. Surface modification and alkali/alkaline-earth metal intercalated co-functionalized CN make the electron distribution more balanced in CN to realize the expedited spatial charge separation, which facilitates the adsorption and activation of reactants and improves the separation efficiency of charge carriers for the optimization of photocatalytic performance.

Optimization of light absorption and band structure of g-C 3 N 4 via alkali/alkaline-earth metal intercalation
Photocatalysis is essentially dependent on the band structure and optical property of photocatalyst, which enables the electron excitation for subsequent photocatalytic redox reaction. The intercalated alkali/alkaline-earth metal atoms are bonded with the adjacent C or N atoms and thus lead to the expansion of the crystal lattice, correspondingly, the band gap and the band edge position would be influenced [27,48,49,52,53,57].
As shown in figures 15(a) and (b), K-intercalated CN samples showed enhanced visible absorption and the absorption edges undergo red shifts, confirming that the bandgap could be narrowed by introducing K atoms into CN interlayers. Also, it is obvious that the valence band (VB) and conduction band (CB) position downshift over CN-K5, which indicated K-intercalated CN samples possess enhanced oxidization ability of VB holes relative to pristine CN. Li et al also revealed that K/Rb/Cs-intercalated CN exhibited red shifts both in UV-vis spectra and calculated adsorption spectra (figures 15(e)-(g)) [48], and as the total density of states suggested, alkali-doped CN displayed narrowed band gaps and the VB and CB edges of the alkali-doped CN downshifted in comparison with pristine CN ( figure 15(h)). According to the experimental characterization and theoretical simulation [27,[48][49][50], alkali-doped CN displays narrowed band gaps and theVB and CB edges of the alkali-doped CN downshift in comparison with pristine CN. The introduction of alkali metal atoms regulates the light adsorption and make photocatalysts absorb more photons, which is easier to ignite the photocatalytic reaction. Also, the downshifted band structure endowed a stronger oxidization ability to the valence hole participating in the photocatalytic oxidation reaction.
The intercalation of atoms led to the distortion of the tri-s-triazine CN layers and the expansion of the interlayer distance, and inducing the charge distributions to be changed from the relative uniform state to the un-uniform state [31]. The charge separation was thus featured with the electrons confined in the intercalated region while the holes are in the far intercalated region. As reported by Wang et al, the HOMO and LUMO of CN were quite localized and in coplanar relation, which was responsible for the high recombination rate [72]. However, the HOMO and LUMO in KBH-C 3 N 4 were not adjacent but localized mainly at N and C atoms of different layers of CN to realize the charge redistribution, contributing to the efficient separation of carriers. Also, the co-doping of K and Cl induced the charge redistribution and narrowed the bandgap [65]. According to the layer-projected density of states of KCl doped CN, the VBs of the first and second CN layers downshifted, and upshifted for the third CN layer ( figure 16(b)). Notably, the bandgap of the second CN layer was narrower than that of the first CN layer, meaning that the second CN layer could be more easily excited by light to yield charge carriers and simultaneously the enlarged bandgap of first layer was beneficial for enhancing the lifetime of the charge carriers. Furthermore, the Cl 3p and K 3s mainly contributed to the top of VB and the bottom of CB between L1 and L2, respectively, which could act as a local hub for hole acceptors/donors and electron transfer. Therefore, the intercalated K and Cl ions broaden the visible light absorption and also might function as a dual channel for electrons and holes transfer, respectively, realizing the optimization of photocatalytic performance.
The intercalation of heteroatoms induces the expansion of the crystal lattice to influence the electronic structure and energy band configuration of CN, which subsequently could optimize the charge distribution to facilitate reactant activation and adjust the light absorption range and electron excitation to improve subsequent photocatalytic redox reaction. Therefore, it is an efficient way to engineer the intrinsic structure of CN-based and even two-dimensional photocatalysts for the enhancement of overall photocatalysis efficiency.

Conclusion and perspectives
This mini review summarizes the latest research efforts on the construction strategy of alkali/alkaline-earth metal intercalated CN and the core principles of photocatalytic enhancement. The introduction of heteroatoms induces the charge redistribution in CN, contributing to (a) the adsorption and activation of the reactant, (b) the spatial charge separation, and (c) the optimization of the reaction pathway, and therefore realizing the highly efficient photocatalytic performance. This review inspires new concepts and presents a good reference to engineer CN-based photocatalysts and even 2D materials for energy and environmental applications.
Despite the impressive advancement made in the past few years, there are still some challenges that face the development of alkali/alkaline-earth metal intercalated CN. In previous research, a first-principle calculation based on density functional theory has been employed to investigate the structural, electronic, and thermodynamic properties of reactants and photocatalysts at atomic or unit-cell levels, however, (a) experimental techniques still need to be utilised to intuitively reveal the position of introduced hetero species and the information of coordination environment; (b) in situ observations are also highly desirable to obtain a true picture of photocatalytic processes, including the charge redistribution, the adsorption and activation of reactants, and the transfer of charge carriers. (c) Additionally, the exact reaction mechanism still remains doubtful and unresolved to date, and thus the experimental investigation of reaction pathways is vital to elucidate the fundamental enhancements and further optimization of the photoactivity in the future. With that successful accomplishment in years to come, it is expected the breakthrough can take place in the near future.