This work outlines the recent significant implication of the Z‐scheme system in photocatalytic water splitting, particularly in the role of electron mediator and the key factors that improve the photocatalytic performance. The Review begins with the fundamental rationales in Z‐scheme water splitting, followed by a survey on the development roadmap of three different generations of Z‐scheme system: 1) PS‐A/D‐PS (first generation), 2) PS‐C‐PS (second generation), and 3) PS‐PS (third generation). Focus is also placed on the scaling up of the “leaf‐to‐tree” challenge of Z‐scheme water splitting system, which is also known as Z‐scheme photocatalyst sheet. A detailed investigation of the Z‐scheme system for achieving H2 evolution from past to present accompanied with in‐depth discussion on the key challenges in the area of Z‐scheme photocatalytic water splitting are provided.As the world decides on the next giant step for the renewable energy revolution, scientists have begun to reinforce their headlong dives into the exploitation of solar energy. Hitherto, numerous attempts are made to imitate the natural photosynthesis of plants by converting solar energy into chemical fuels which resembles the “Z‐scheme” process. A recreation of this system is witnessed in artificial Z‐scheme photocatalytic water splitting to generate hydrogen (H The significant implication of Z‐scheme systems of different generations in photocatalytic water splitting, from past to present, is outlined and reviewed. The rationales and mechanisms in Z‐scheme systems, followed by a survey on the development roadmap of Z‐scheme systems from the first to third generation are presented. Lastly, the rapid emergence of particulate Z‐scheme photocatalyst sheets is outlined. In this context, solar fuels bestow the advantages of sustainable mobility as compared to direct solar‐to‐electricity approach, attributed to the storage of solar energy in a medium carrier. Thus, solar‐to‐chemical conversion confers an intriguing route to harvest energy from the Sun to solve the on‐going severe energy deficit.Owing to the global energy consumption continues to escalate and the incapability of energy replenishment from finite sources of fossil fuels to fulfill such needs, a diffusion of energy to more promising renewable carriers is anticipated in the near future. Presently, renewable energy shares an ≈3.6% of the global primary energy mix with a projected rising trend.2 is worth noting as a potential candidate to become one of the future front runners in the energy mix ascribed to its nonpolluting nature and high energy density.2 photoelectrodes by Fujishima and Honda in 1972 marked the dawn of new era in solar H2 harvesting using artificial photosynthesis.2O) into oxygen (O2) and the desirable H2, as shown in Equation 2 evolution reaction (HER) and its O2 evolution reaction (OER) counterpart, respectively. To date, several techniques for solar H2 production are being explored including photovoltaic‐electrolysis (PV‐EC), photoelectrochemical (PEC) and photocatalysis, as delineated in Figure2 evolution catalyst (OEC) as anode and H2 evolution catalyst (HEC) as cathode to split water via the independent power generated from solar PV (light absorber). In a conventional wired PV‐EC system, the PV cell and electrolyzers are constructed independently where solar PV is free from water to eliminate the concern of stability against corrosion in aqueous medium. Thus, the configuration of the PV unit and electrolyzers can be freely modulated. However, the disadvantage of wired PV‐EC system is the complexity of the design. As of recent, the development of monolithic system of PV‐EC by compact assembly of PV light absorber and electrolyzers confers an intriguing approach to construct wireless device  with less complexity and omit the need of an external connection.Among the solar fuels, H2 due to its simplicity and low cost since the system only requires light and water. Complicated setup and the application of external applied bias can be exempted which in turn offers an energy‐efficient strategy to generate H2 fuel. From past to present, particulate photocatalysis is conducted in powder suspension form. As of recent, the development of particulate photocatalyst sheets with localized photocatalyst powder on a substrate demonstrates an appealing strategy for potential scalability of photocatalysis.2 production is outweighed by their efficiency. However, practical mass production might be realized in the future if revolutionary research studies on photocatalytic H2 production continue to grow.In view of PEC, the system is generally constructed in the photoelectrode–electrode configuration: 1) n‐type photoanode‐HEC cathode or 2) p‐type photocathode‐OEC anode. Different from PV‐EC, PEC photoelectrode is responsible for both light absorption and redox reaction. However, PEC setup with single light absorber requires external bias voltage to offset the overvoltages and other losses. An unassisted PEC water splitting reaction can be realized in a tandem cell which comprises of a photoanode for OER and a photocathode for HER. As compared to PV‐EC and PEC, particulate photocatalysis shows a more appealing route to generate H1.2Eg).−) will be accommodated in higher energy state CB, in turn leaving an empty hole (h+) in the lower energy state VB. After the initial photoexcitation process, there is a lifetime in nanosecond regime for the photoinduced electrons and holes to migrate to the surface of photocatalysts and eventually participate in oxidation–reduction reaction with the adsorbed reactants. However, electron–hole pairs recombination is in competition with the charge transfer process which impedes the supply of photogenerated electrons and holes.−9 s. Since the time for chemical reaction of photocatalyst with the surface adsorbed molecules is in the range of 10−8 to 10−3 s, de‐excitation is much slower than excitation process which makes the recombination of charge inevitable.Photocatalysts are generally made from semiconductors. Unlike the continuum electronic states of metal, the unique electronic structure of semiconductor which consists of an energy void region extended from the top of filled valence band (VB) to the bottom of vacant conduction band (CB) enables the promotion of ground state electron in VB to higher energy CB upon light irradiation. This phenomena, which is also known as bandgap photoexcitation, only occurs when the photocatalyst is induced by a photon absorption with energy equal or greater than the energy difference between the two energy levels or bandgap energy  at pH 7. at pH 7) while VB potential has to be more positive than water oxidation level, O2/H2O (+0.82 V vs NHE at pH 7).FigureFurthermore, thermodynamic law places a constraint for the redox reaction on the surface of photocatalysts. The law indicates that for a reduction reaction to occur, the potential level of CB must be more negative or analogously higher than the redox potential of lowest unoccupied molecular orbital (LUMO) of the acceptor molecule. Whereas for an oxidation reaction to proceed, the VB potential must be more positive or analogously lower than the redox potential of highest occupied molecular orbital (HOMO) of the donor molecule. This governs the tendency of acceptor or donor molecules to be reduced or oxidized which directly imply the probability of HIn this context, semiconductor plays a significant role in three major process steps of the heterogeneous photocatalysis: 1) bandgap photoexcitation to induce formation of electron–hole pairs, 2) migration of delocalized charge carriers to the surface of photocatalysts, and 3) provide active sites for the subsequent oxidation and reduction process.1.3FigureTo date, there are two primary approaches for photocatalytic overall water splitting. First approach is to split water using single‐component photocatalysts through one‐step photoexcitation. Based on the aforementioned rationales, it is desirable to have a narrow bandgap photocatalyst to ease the photoactivation but at the same time retaining a considerable large overpotential level. This leads to better reduction and oxidation of the reactants owing to a more negative level of CB potential and a more positive level of VB potential are being applied thermodynamically. Thus, single‐component photocatalysts should have sufficiently small bandgap to be activated by light and possess suitable band‐edge potentials to achieve overall water splitting. However, it is clear that the two aspects above are mutually exclusive. Generally, it is extremely difficult and challenging for single‐component photocatalysts to fulfill such stringent requirements and hence, this greatly limits the potential photocatalyst candidates for one‐step water splitting. Thus, it is not feasible for single‐component photocatalysts to simultaneously dominate the properties of small bandgap and strong redox ability. Besides, single‐component photocatalysts also suffer from rapid recombination of photogenerated electron–hole pairs.2 evolution photocatalysts (HEP) and OER on O2 evolution photocatalysts (OEP). Despite the competency of heterojunction‐type semiconductor nanocomposites in facilitating charge separation, redox ability of the photocatalytic system is weakened due to the migration of electrons and holes to more electropositive CB and electronegative VB potential ascribed to the nature of charge transfer, as depicted in Figure Owing to the shortcoming in single‐component photocatalytic system, extensive research efforts have been devoted over the years in tailoring the configuration of photocatalysts, for instance, forming type‐II heterojunction nanocomposites using two semiconductors in order to improve their charge separation efficiency and enhance the photocatalytic performance. Designing proper heterojunction‐type photocatalytic system can induce the isolation of electrons and holes in two separated locations, which in turn bolstering the lifetime of photogenerated carriers.To overcome these bottlenecks, the second approach of photocatalytic system introduces an anisotropic configuration of two photocatalysts and an electron mediator known as the Z‐scheme photocatalytic system to perform water splitting through two‐step photoexcitation process. Inspired by natural photosynthesis in green plants, biomimetic artificial Z‐scheme photocatalytic system can simultaneously exhibit the following three requirements, which are lacking from both single‐component photocatalysts and heterojunction‐type nanocomposites: 1) small bandgap, 2) suitable band‐edge position with considerable large overpotential, and 3) suppression in electron–hole pairs recombination.22.12 reduction into carbohydrate and H2O oxidation into O2 with quantum yield of approximate unity.3−/I− shuttle redox couple (electron acceptor/donor pair or known as A/D pair) was implemented as an ionic mediator to govern the charge transfer between two semiconductors as photosystems.Beginning with the pioneering discussion on two light reactions and two pigment systems by Rabinowitch on the statement of James Frank in 1945, numerous interests have peaked on mimicking the natural photosynthesis of green plants in harvesting solar energy.With that in mind, an all‐solid‐state Z‐scheme system with Au as electron mediator was then introduced in 2006 which opened a new horizon for low resistance electron transfer via Ohmic contact.3:Rh (PS I) and BiVO4 (PS II) using pH adjustment.One step forward in Z‐scheme photocatalysis can be witnessed in a mediator‐free two‐step photoexcitation system. As reported by Wang et al. in 2009, Z‐schematic vectorial electron transfer profile was found in coupling of ZnO and CdS via chemical formation method.+ and OH− concentration overpotentials and pH gradient which in turn compensate for the Ohmic drop (IR drop) between PS I and II as a contrast to conventional powder system.FigureMore recently, scalable Z‐scheme photocatalyst sheets system was proposed by Wang et al. which confers potential scalability for the augmentation of solar water splitting.2.2Figure2 and H2O into carbohydrate and O2 with quantum efficiency close to 100%. First, initial photoexcitation occurs when reaction center chlorophylls in PS I (λ ≤ 700 nm) and II (λ ≤ 680 nm) absorb photons from sunlight, leading to delocalization of charges to form electron–hole pairs.2O into O2 happens in manganese calcium oxide cluster of PS II ascribed to the accumulated holes in HOMO of this unit, labelled as oxygen‐evolving complex. Whereas LUMO of PS I is full of electrons which in turn reduce nicotinamide adenine dinucleotide phosphate (NADP+) into NADPH, labelled as ferrodoxin‐NADP reductase unit.2 into carbohydrate.Inspired by natural photosynthesis, biomimetic artificial Z‐scheme photocatalytic system shows similar vectorial charge transport profile that features strong reducibility and oxidizability of PS I and II connected by an electron mediator. Besides, the nature of electron‐rich region in PS I can efficiently suppress photo‐degradation from self‐oxidation.33.13.1.1Figure2 and O2. The cascade type of vectorial electron transfer from PS II to PS I is governed by A/D pair with no physical contact between PS I and II. Initially, after generation of electron–hole pairs due to photoexcitation in both PS I and II, the electron acceptors will be reduced by the electrons from CB of PS II into electron donor, leaving photogenerated holes accumulated in VB of PS II 3]3+/2+, [Co(phen)3]3+/2+, and VO2+/VO2+.However, electron A/D pair in PS‐A/D‐PS system can also react with the photogenerated electrons and holes in CB of PS I and VB of PS II, which cause inevitable decrease in effective number of charge carriers, as disclosed by the red dotted lines in Figure 3.1.23−/I− redox couple as an ionic electron mediator to interface two different types of TiO2, namely anatase and rutile as corresponding HEP and OEP.(0/1−) and I(1−/5+) are more stable as redox cycles in aqueous medium as shown in Equations 3−/I− is ordinarily implemented in dye‐sensitized solar cell, attributed to its facile reversible redox reaction and exhibits a low degree of photoabsorption which is only up to ≈500 nm.3−/I− redox cycle is very limited due to the low reactivity of I3− as electron acceptor and the restricted selection of photosystems.3−/I− redox pair possesses standard potential that is close to I3−/I− couple. Thus, the incorporation of IO3−/I− as ionic redox mediator is feasible due to its electron natureEarly work of photocatalytic Z‐scheme system was demonstrated by Abe et al. using IO3−/I− redox cycle. This ionic pair exhibits larger contribution under increasing pH due to the shifting in standard potential according to Nernst equation.2 (Pt‐TiO2‐A1) as PS I and rutile TiO2 (TiO2‐R2) as PS II was conducted under UV illumination.3− as the main redox product rather than IO3−, which is incompetent to serve as an efficient electron acceptor in photocatalysis as mentioned earlier. On the other hand, basic solution of pH 9 is the optimum condition for IO3−/I− redox system to work efficiently ascribed to the presence of favorable IO3−/I− ionic pairs rather than I3−/I−. The gradual increment in activities from pH 5 to 9 is believed to be the induction period of IO3− production with certain amount of I3−, which leads to the nonstoichiometric evolution of H2 and O2. Besides, the accumulation of ineffective I3− will also cause some shielding effect due to their large extent of light absorption. Thus, it can be concluded that IO3−/I− redox cycle can function efficiently when pH is higher than 9 owing to the absence of by‐product of I3−, resulting in stoichiometric evolution of H2 and O2 with high activities. Besides, the rate of gas evolution in such a system is also strongly dependent on the ratio of IO3− to I−. The higher concentration of I− will cause competitive oxidation known as backward reaction. Hence, suitable pH and concentration of NaI are crucial to operate PS‐A/D‐PS system with IO3−/I− redox mediator. The investigation of IO3−/I− redox system with other classes of semiconductor will be discussed henceforth.Even so, pH of the medium plays a significant role on the performance of IO3 co‐doped with Cr and Ta  as HEP and Pt‐WO3 as OEP to govern IO3−/I− Z‐scheme redox system.3 (3.1 eV), many endeavors such as cation doping and loading of co‐catalysts have been devoted to enhancing the photoabsorption of this material in water splitting.3 and tune the electronic band structure.3  is another conventional A/D pair used in the early stage of PS‐A/D‐PS system.[qv: 37c] Interestingly, Fe3+/Fe2+ redox couple confers numerous advantages as compared to the ionic forebear of IO3−/I− pair, attributed to the facile reduction process of Fe3+ to Fe2+. In this sense, Fe3+ is often being employed as an effective sacrificial reagent (electron acceptor) in photocatalytic water oxidation due to its one‐electron transfer nature.3+/Fe2+ ionic pair is shown in Equation Similar to IO3+/Fe2+ redox pair enable bolstering effect on the photocatalytic performance. As shown in Figure3+ and Fe2+ on suspension containing rutile TiO2 were tested.3+ is preferably adsorbed on TiO2 nanoparticles even with the presence of Fe2+ ions. Thus, Fe3+ will be reduced into Fe2+ by the photoexcited electrons from TiO2, while the photogenerated holes preferably oxidize water rather than Fe2+. This can be validated by the concurrent evolution of O2 gas and detection of Fe2+ from photocatalytic reaction of TiO2 under aqueous Fe3+ as shown in Figure 3+/Fe2+ ionic pair only applicable to certain materials.3−/I− ionic pair, Fe3+/Fe2+ redox couples are only chemically stable in environment with pH less than 2.5. Precipitation of Fe(OH)3 from Fe3+ occurs under higher pH due to hydrolysis which constraint the application of this redox mediator. Besides, aqueous Fe3+ exhibits strong light absorption up to ≈450 nm which poses shielding effect to the system. Though FeCl2 only displays absorption of wavelength less than 250 nm, the photochemical oxidation of Fe2+ to Fe3+ might happen under UV irradiation.3+/Fe2+ redox system in order to exploit its function.Besides, the unique adsorption properties of Fe3+/Fe2+ redox system can be witnessed in the incorporation of BiVO4 as PS II with Ru/SrTiO3:Rh as PS I.[qv: 37c] BiVO4 is one of the well‐documented OEPs in photocatalytic water splitting attributed to its strong hybridization between Bi 6s and O 2p orbitals, which leads to a more narrow bandgap (2.4 eV) as compared to WO3 (2.8 eV).3 as mediator, BiVO4 can readily oxidize water into O2 and reduce Fe3+ into Fe2+ for the corresponding redox cycle to uptake holes from Ru/SrTiO3:Rh. As a result, the photocatalytic overall water splitting of such a system demonstrated an AQY of 0.3%, along with a long term stability up to 70 h.[qv: 37c] Owing to the facile reduction of Fe3+, the employment of co‐catalyst on OEP is not necessary. Recently, innovative research focuses on constructing Sillén‐Aurivillius class of bismuth oxyhalides, Bi4MO8X  as efficient OEP. This class of material comprised of alternative stacking of halogen anionic blocks in between (Bi2O2)2+ layers.4NbO8Cl as the center of discussion, the strong orbital hybridization of Bi 6s with O 2p rather than Cl 3p bestows a more electronegative VB as compared to conventional bismuth oxyhalides.4NbO8Cl possesses bandgap that is much smaller than its forebear BiOCl. As shown in Figure4NbO8Cl and Bi4NbO8Br confer more redshifted light absorption compared to BiOCl and BiOBr, which offers a more favorable band structure for water oxidation. Consequently, visible‐light‐driven overall water splitting of Fe3+/Fe2+ redox system consisting of Bi4NbO8Cl and Ru/SrTiO3:Rh shows stoichiometric evolution of H2 (10.4 µmol h−1) and O2 gas (5.2 µmol h−1), respectively.Early work of Fe2 has been regarded as one of the most established semiconductors for photocatalytic application owing to its stability and suitable VB for water oxidation.2 is greatly limited by its wide bandgap in various polymorphs, i.e., rutile (3.0 eV) and anatase (3.2 eV).2, forming titania with a narrow bandgap of 2.3 eV.Figure2  and Ru/SrTiO3 (HEP) work well under both Fe3+/Fe2+ and IO3−/I− redox systems, resulting in stoichiometric evolution of H2 and O2. Thus, this finding marks the endeavors in enhancing the function of TiO2 in Z‐scheme water spitting under visible light.TiO2 Figure . Consequ3.1.33]3+/2+ and [Co(phen)3]3+/2+ show promising potential in driving PS‐A/D‐PS system attributed to their tunable standard potential and facile reversibility over a wide range of pH.3]3+/2+ and [Co(phen)3]3+/2+ redox system due to their capability of generating Co2+ ions. A series of [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ aqueous systems containing Pt/TiO2‐anatase, Pt/SrTiO3, Pt/SnNb2O6 and Pt/SrTiO3:Rh as HEPs were investigated by Sasaki et al. for their corresponding H2 evolution, as depicted in Table3:Rh loaded with either Pt or Ru as co‐catalysts is the sole photocatalyst from the list that can produce H2 under aqueous solution of [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+. These findings elucidate the incompetency of TiO2 and undoped SrTiO3 in driving the oxidation of Co2+ ions into Co3+, resulting in the negligible photocatalytic gas evolution. Besides, it can be entrenched that the interstitial doping of Rh plays an important role in facilitating the oxidation of Co2+ according to Equation 3]3+/2+ and [Co(phen)3]3+/2+ redox cycles. As shown in Table2:Cr,Sb and BiVO4 demonstrate stoichiometric evolution of H2 and O2 gas, whereas WO3 renders much sluggish O2 rate which is well below stoichiometric value of 1:2 to H2. This phenomenon is well explained by the stronger oxidation potential of O 2p (WO3) orbital as compared to Bi 6s (BiVO4) and Cr 2p  which induce the decomposition of Co‐complex from Co3+ rather than being oxidized into its redox counterpart Co2+. Thus, the more electropositive VB potential of WO3 reflects the shortcoming of cobalt‐based mediator ascribed to its insufficient stability under strong oxidationOther than the conventional simple ion redox couples, transition metal complexes such as cobalt‐based mediator 3+/2+ redox system is displayed in Figure4 via Co3+‐complex. Thus, the cascade electron transfer pathway within the Z‐scheme system is facilitated, culminating to stoichiometric evolution of H2 and O2 with large turnover number.In a recent study of Ir and La ions‐doped BaTa2+/VO2+ ionic couple, is employed as an electron mediator to govern PS‐A/D‐PS Z‐schematic reaction.[qv: 37b] It is worth noting that the redox cycle between V5+ and V4+ is competent in relaying electron according to Equation 5+/4+ complex. In this regard, the redox potential of V5+/4+ complex was measured to be +0.7 and +1.0 V versus NHE at pH 3.8 and 1.7, respectively. According to Table2 evolution of TiO2 in a solution containing V5+ ions decreased when the pH increased from pH 1.7 to pH 6.5 and 10.9. The enhanced performance of water oxidation by TiO2 at pH 1.7 is attributed to presence of VO2+ ions at lower pH, which serves as an effective reductant for semiconductors in photocatalytic reaction. In whole, this section provides a discerning understanding on the properties of various redox couples in constructing PS‐A/D‐PS systemAs of recent, vanadate redox system, which comprises of VO2 fuel. The summary of PS‐A/D‐PS Z‐scheme heretofore was tabulated in Table3−/I− can only work efficiently at a more basic condition with pH more than 9 ascribed to the formation of inactive I3− under lower pH. Whereas for Fe3+/Fe2+ redox cycle, the system is only stable at pH lower than 2.5 due to the precipitation of Fe(OH)3 from Fe3+ under higher pH. This greatly restricts the application of redox mediator at a wide pH range. On top of that, the presence of redox mediator will absorb light to a certain extent which reduces the available photon for the photocatalysis reaction. With this in mind, development of redox mediator‐free Z‐scheme is imperative to surmount the problems from PS‐A/D‐PS system. Nevertheless, PS‐A/D‐PS system also provides a prominent platform to mimic natural photosynthesis and demonstrates remarkable results for overall water splitting. In addition, separation of produced H2 and O2 gas on site can be achieved in PS‐A/D‐PS system with a suitable reactor as shown in Figure2 and O2 gases on separated cells can be accomplished, which poses their upper hand as compared to other powder suspension system.With the capability of ionic redox pairs in driving reversible cycles for PS‐A/D‐PS Z‐scheme, incessant studies have been carried out to construct overall water splitting system for harvesting H3.23.2.1FigureThe inevitable drawbacks from PS‐A/D‐PS Z‐scheme, such as backward reaction and shielding effect, have impeded further application of such redox‐mediated system. This has prompted the investigation on the development of redox‐mediator‐free system. In fact, the two‐step photoexcitation system can be devised in all‐solid‐state without the need of ionic pairs. A recreation of the system can be witnessed in all‐solid‐state PS‐C‐PS in which A/D pair is substituted by a conductor (C) as the electron mediator, which is shown in 3.2.22 ternary structure by Tada et al. marked the beginning of all‐solid‐state Z‐scheme system in achieving photocatalytic water splitting.2 nanojunction that is synthesized through photochemical deposition–precipitation method is able to seize the large work function of Au nanoparticles to transfer electron. The unique type of Z‐scheme structure highlights the importance of ingenious arrangement of semiconductors in nanoscale. As depicted in Figure2 which resembles an intimate Z‐scheme relationship between CdS (101)||Au (111)||TiO2 (101) orientated nanostructure. The ternary structure demonstrated a higher photocatalytic activity in stark contrast to the two‐component systems of either Au/CdS or TiO2/CdS, attributed to the facilitated cascade electron transfer between CdS and TiO2 via Au electron mediator  was studied by Yun et al. to replace TiO2 in the Z‐scheme system of CdS and Au, as displayed in Figure2 with large band, TiO1.96C0.04 displayed a larger extend of light absorption close to 440 nm, which is corresponded to bandgap of 2.6 eV. It is worth noting that the presence of two shoulders in UV–vis DRS of CdS/Au/ TiO1.96C0.04  and CdS (region I), which corroborate the co‐existence of both semiconductors in the Z‐scheme system. As a result, CdS/Au/TiO1.96C0.04 exhibited an exceptional high H2 evolution rate of 433.2 µmol h−1 in the presence of Na2S/Na2SO3 sacrificial condition, which is about 4.7‐fold enhancement as compared to CdS/Au/TiO2. Besides, the AQY of H2 production was measured to be 23.6% under monochromatic light of 420 nm. Even though the water splitting reaction occurs with the aids of sacrificial reagents, the striking idea of all‐solid‐state Z‐scheme has evoked considerable attention toward its development in water splitting.In 2006, the construction of CdS/Au/TiOr Figure . Later o4 Figure  matches Figure2 evolution under Na2S/Na2SO3 solution, which is ≈4.5 times higher than the corresponding CdS/ZnO without Au as the mediator. Interestingly, wing‐architecture TiO2 (WA‐TiO2) prepared by immersion–calcination route resembles the antireflection properties of butterfly wings, as shown in Figure2 using a similar two‐step deposition method, forming a Z‐scheme heterostructure  to Ag2S (VB) via the interconnected Ag, as displayed in Figure Apart from controlling the geometry of Z‐scheme architecture, PS‐C‐PS system can allow in situ generation of conductor from the photocatalysts if the material of the mediator is the same as PS I or II. AgFigure2 evolution rate of 1.92 mmol h−1 under 300 W Xe lamp irradiation, which is ≈5.1 times enhancement over CdS/ZnO nanostructure without a Cd core. The development of such transition metal‐aided Z‐scheme system is very similar to the in situ formation of conductor as discussed previously in the Ag‐mediated PS‐C‐PS system. This highlights the importance of metallic core in assisting facile charge transport and improving the photocatalytic performance.Other than rare and expensive noble metal, low cost transition metal is amenable to serve as a conductor in PS‐C‐PS Z‐scheme system. In a study employing Cd as an electron mediator, it was found that the presence of this metallic core can efficiently facilitate the charge transfer between CdS and ZnO as PS I and II.3:La,Rh as PS I and CoOx/Ta3N5 as PS II mediated by an Ir conductor.Figure3N5 (CB) was relayed to SrTiO3:La,Rh (VB) via Ohmic contact at Ir‐semiconductors interface. The presence of co‐doping of La and Rh ions into the lattice of SrTiO3 can substitute parts of the Ti4+ sites in SrTiO3 by inducing oxygen vacancies to compensate the higher valence states. As aforementioned, the existence of vacancy sites can lead to the formation of midgap states which reduce the bandgap of the semiconductor. Consequently, SrTiO3:La,Rh exhibited light absorption up to 700 nm. Visible‐light‐driven photocatalytic water splitting reaction of the Z‐schematic Ru/SrTiO3:La,Rh‐Ir‐CoOx/Ta3N5 system demonstrated remarkable activities of H2 and O2 evolution with measured AQY and STH of 1.1% and 0.037%, respectively.Iridium (Ir), being another transition metal that confers similar electronic properties as Pt, is competent for driving electron transfer within PS‐C‐PS Z‐scheme system. Wang et al. reported a Z‐scheme system comprising of Ru/SrTiO3.2.3While the research on metallic conductor in PS‐C‐PS system is becoming more prevalent, nanocarbon‐based electron mediator is a relatively new addition to the family of Z‐scheme owing to the versatile tailoring and extraordinary electronic properties.3:Rh and BiVO4 were employed as PS I and II, respectively.Figure3:Rh or BiVO4. Owing to the more electronegative nature of Ru/SrTiO3:Rh, the reduction of GO on Ru/SrTiO3:Rh is stronger than that on BiVO4. Hence, the oxygen‐bound carbon content of PRGO/BiVO4 (28%) was found to be higher than that of PRGO/SrTiO3:Rh (10%), which indicates a lower restoration of graphitic structure. According to the overall water splitting activities in Table4 (entry 8) demonstrated the highest photocatalytic gas evolution. Imputed to the strong hydrophobicity of RGO produced by hydrazine and photoreduction on Ru/SrTiO3:Rh, the photocatalysts were found to be immiscible in water. Thus, it is crucial to obtain a balance between the degree of reduction of GO and the hydrophilic nature of the composites in water. The presence of PRGO can facilitate the cascade electron transfer from BiVO4 to Ru/SrTiO3:Rh, as shown in Figure The first graphene‐mediated Z‐scheme system for photocatalytic water splitting was studied by Iwase et al., in which Ru/SrTiO2 production.2 as OEP and RGO as mediator.Table2 and CuInS2) loaded with Pt demonstrated stoichiometric evolution of H2 and O2 when coupled with TiO2 in RGO‐mediated Z‐scheme system. As a stark contrast, Zn‐ and Ag‐based sulfides (entry 1–5) only show sluggish H2 evolution without O2 imputed to the gradual photocorrosion. Besides, the presence of RGO would propel the vectorial electron transfer between CuInS2 and TiO2. As shown in entry 10 of Table 2 production, in which the performance of Z‐scheme system without Pt was severely reduced with nonstoichiometric evolution of H2 and O2 gas as observed (entry 11).For years, metal chalcogenides have gained explosive popularity as efficient HEPs attributed to their narrow bandgap and suitable relative band edge for H2‐RGO‐BiVO4 Z‐scheme system by Iwase et al., p‐type and n‐type semiconductors were connected through an electron mediator and that resembles the electron flow in PEC cell  and O2 (1.7 µmol h−1) gas as shown in Figure In another investigation of Pt/CuGaSll Figure25b.78 IFigure2 region, which function as an internal conductor that facilitates vectorial electron transfer via Ohmic contact. The p‐type domain contains more electronegative band structure while n‐type domain is more electropositive. Hence, each domain can function effectively as HEP and OEP, respectively. Consequently, NGO‐QDs demonstrated efficient overall water splitting performance under visible light with stoichiometric evolution of H2 and O2 gas.Heteroatoms  doping into lattice of graphene is a relatively new introduction to the nanocarbon family.2 bonded carbon allotrope that features versatile tailoring and excellent intrinsic properties, are worth noting as potential candidate for hosting Z‐scheme composites.5 cm2 V−1 s−1 mobility at room temperature.0.5Cd0.5S and TiO2 were employed as PS I and II, respectively.[qv: 17a] The ternary Zn0.5Cd0.5S‐CNTs‐TiO2 nanocomposites synthesized via two‐step facile coating and hydrothermal route demonstrated ≈4.5‐ and 2.8‐fold increment in photocatalytic H2 evolution over Zn0.5Cd0.5S and CNTs‐Zn0.5Cd0.5S, respectively. As delineated in FigureIn addition to graphene discussed earlier, other nanocarbon materials such as CNTs and CQDs are also competent to drive vectorial electron transfer within Z‐scheme system. CNTs, a dimensionally confined sp4‐CQDs‐CdS was developed by Wu et al. for photocatalytic overall water splitting under visible light, achieving a stoichiometric evolution of H2 (1.24 µmol h−1) and O2 (0.61 µmol h−1) when the mass ratio of PS I to II was 1:1.Figure4 (CB) to CdS (VB) through CQDs as the electron shuttling channel. As a result, this system demonstrates suitable band position for overall water splitting with strong reduction and oxidation abilities. In another similar study, a thin carbon layer was employed to interface 2D g‐C3N4 nanosheets and ZnInS4 (ZIS) to form a ternary nanocomposite with Z‐scheme electronic configuration.2 evolution up to 50.32 µmol h−1 under Na2S/Na2SO3 sacrificial condition. As is known, pristine g‐C3N4 commonly demonstrates sluggish H2 evolution in the absence of co‐catalysts, Pt. However, the incorporation of g‐C3N4 into the Z‐scheme system is able to render high H2 evolution even without Pt, which implies the advantages of facilitated charge transfer and bestow suitable dynamics for photocatalytic reaction.CQDs are amenable to function as electron mediator in PS‐C‐PS system attributed to their excellent charge relaying properties. A ternary system which comprises of BiVO2 production were shown by nanocarbon‐mediated Z‐scheme system, extra attention should be devoted to the synthesis of such nanocomposites. An intimate contact interface between photocatalysts and mediator is of paramount significance to provide a low‐resistance pathway for continuous vectorial electron flow from PS II to PS I. Besides, the electron relaying ability of nanocarbon is commonly plagued by the formation of defects during the surface modification. The extent of these defects could be diminished via post reduction of oxygenated graphitic structure. However, excessive reduction of oxygenated groups will turn the nanocarbon materials into hydrophobic behavior, which make their dispersion in water harder. Nevertheless, the intriguing properties of all‐solid‐state Z‐scheme system have raised considerable attention worldwide to develop highly efficient H2 production devices that resemble natural photosynthesis. TableAttributed to the competency of various metals and nanocarbons in driving electron transfer, PS‐C‐PS system has been highlighted as an alternative to govern Z‐scheme photocatalysis than its forebear PS‐A/D‐PS system. Detrimental backward reaction and shielding effect can be perfectly evited due to the absence of redox ionic pairs. Besides, nanocarbon‐based mediator is a relatively new addition to the family of PS‐C‐PS system. Albeit still not prevalent as the employment of metallic mediator, there is a growing interest into the exploration of nanocarbon materials such as graphene, CNTs and CQDs in driving Z‐scheme system. Despite promising results on both sacrificial aided and pure water splitting for H3.33.3.1FigureApart from the compelling electron transfer ability displayed by metallic and nanocarbon‐based materials as conductor, solid–solid contact interface also renders similar energy profile which is quasi‐continuous.3:Rh‐BiVO4 composite was developed in aqueous suspension at pH 3.5.Figure3:Rh‐BiVO4 at pH 3.5 in contrast to other pH values . This reveals that the strongest physical interaction between Ru/SrTiO3:Rh and BiVO4 happens at this pH, which is further corroborated by their high activity in photocatalytic water splitting with measured AQY of 1.7% at 420 nm.3:Rh and BiVO4 simultaneously exhibited opposite charges. The zeta potential of BiVO4 is negative across a wide range of pH from 2 to 9, whereas Ru/SrTiO3:Rh confers isoelectric point of ≈4. As a result, the opposite charges of both Ru/SrTiO3:Rh and BiVO4 at pH 3.5 deliver a strong interaction which leads to the formation of solid–solid contact, as delineated in Figure The formation of solid–solid contact in direct Z‐scheme is generally divided into two main categories, namely physical and chemical methods. Physical contact mode of direct Z‐scheme can be realized when the two photosystems possess opposite charges in an aqueous medium, and hence, the strong attraction will induce an interparticle electrostatic adsorption to joint PS I and II. The first demonstration of PS‐PS system formed by physical means was witnessed in 2009 by Sasaki et al., in which Ru/SrTiOFigure2 from its precursor Zn(CH3COO)2.2H2O when the pH of the suspension was adjusted to 7 using aqueous ammonia. Subsequently, CdS was formed on top of ZnO as a facile template to outline a two‐component system. Attributed to the unique vectorial transfer between ZnO and CdS, the two‐component system renders a prolonged lifetime of photogenerated carriers as compared to the single‐component ZnO and CdS. The charge transport properties can be elucidated from the decay lifetime of charge dynamics shown in fluorescence emission spectra . In all, the facilitated charge transport and enhanced photocatalytic performance in PS‐PS system are credited to the formation of low resistance interface between solid–solid contact. This substantiates the feasibility of direct Z‐scheme in achieving two‐step photoexcitation without any mediator. Verification of Z‐scheme electron transfer profile in PS‐PS system will be discussed henceforth.Contrary to the physical construction method, chemical contact mode of PS‐PS system emphasizes on the chemical bond formation between PS I and II. In typical, one of the photosystems will be firstly synthesized and it serves as the potent platform for the subsequent loading of another photosystem. Consequently, the conjugation of surface will confer strong chemical bond, which is more stable than the surface interaction formed by physical mean. For instance, ZnO/CdS system was developed using wet chemistry method which displayed Z‐schematic cascade electron profile as shown in a Figure . In addi3.3.2As aforementioned, the band structures of both direct Z‐scheme and type‐II heterojunction are of utmost similar due to the arrangement of semiconductors. However, both systems confer totally reverse electron flow profiles ascribed to the properties of solid–solid interfaces. Thus, it is imperative to investigate the charge transfer mechanism of the direct Z‐scheme system via different testing and characterization in order to differentiate them from the conventional heterojunction‐typed nanocomposites. Up to now, various methods have been employed to study and verify the electronic structure of direct Z‐scheme, as outlined in the following sections. This includes metal loading, sacrificial reagent testing, radical trapping experiment, and X‐ray photoelectron spectroscopy (XPS) testing for the validation of direct Z‐scheme. However, a single experiment is out of precision to corroborate the vectorial charge transfer profile in direct Z‐scheme and hence, a series of comprehensive testing should be conducted to confirm its electronic structure.Figure2 nanocomposites.2 which serves as PS I for the direct Z‐scheme system. Thus, the accumulation of photogenerated electrons on PS I is validated, which indicates the migration of electrons from anatase to rutile TiO2.Photodeposition of noble metal as co‐catalysts is one of the typical methods to verify the charge transfer profile in direct Z‐scheme. As opposed to type‐II heterojunction, the interaction in solid–solid contact within direct Z‐scheme causes the redistribution of charge carriers ascribed to the vectorial electron transfer via Ohmic contact from PS II (CB) to PS I (VB). Thus, PS I is an electron‐rich region while PS II is aggregated with holes, as displayed in 2 evolution under sacrificial electron donors, 2) PS II can only perform O2 evolution under sacrificial electron acceptors, and 3) the two‐component PS I‐PS II system is able to perform overall water splitting in pure water, leading to simultaneous evolution of H2 and O2. In a more recent work, Zhu et al. performed sacrificial reagent testing to verify the cascade electron transfer profile within black phosphorus (BP)‐BiVO4 nanocomposites.Table3O4‐loaded BP and BiVO4 can render H2 and O2 activities under EDTA and AgNO4, respectively (entries 1–8). This proves the capabilities of individual PS I and II to perform their corresponding half reaction via reduction and oxidation reaction. The coupling of Co3O4/BP‐Co3O4/BiVO4 was tested under the aforementioned three conditions: 1) EDTA electron donors, 2) AgNO4 electron acceptors, and 3) pure water. As shown from entry 9, the two‐component system was able to demonstrate photocatalytic H2 evolution under EDTA without any O2 being detected. Similarly, Co3O4/BP‐Co3O4/BiVO4 shows O2 evolution under AgNO4 with no H2 product, which consolidates the electron cloud was isolated at BP while BiVO4 was aggregated with photogenerated holes, as delineated in Figure2 and O2 gas with close to stoichiometric value (entry 12). Thus, sacrificial reagent testing is feasible to be utilized for determining the plausible charge transfer mechanism in direct Z‐scheme. However, this application is only suitable for coupling of semiconductors with band structure that is solely responsible for H2 or O2 evolution. The electron transfer within a semiconductor with bandgap that can fulfill both H2 and O2 activities will be hard to be determined since it can respond to both electron donor and acceptors in the half reaction testing.Similar to metal loading experiment, the employment of sacrificial reagent testing can determine the region of aggregated electrons and holes within the Z‐scheme system. In order to be validated as direct Z‐scheme, the two‐component system will have to fulfill three conditions: 1) PS I can only perform H•OH) will be produced from oxidation of water when the semiconductor possesses more electropositive potential than 2.4 versus NHE.•O2−) are the reduction products from water if the semiconductor fulfills sufficient potential of −0.33 V versus NHE.3N4/TiO2 nanocomposites, radical testing of •OH and •O2− was investigated using terephthalic acid (TA) and 1,4‐benzoquinone (BQ) as probe molecules.•OH radicals are readily to react with TA and form a fluorescence product, namely, 2‐hydroxyterephthalic acid. These products can be detected using photoluminescence (PL) measurements, in which a corresponding peak will be observed at wavelength ≈430 nm.Figure•OH radical from g‐C3N4/TiO2 nanocomposites. This finding elucidates the strong oxidation ability of the two‐component system imputed to the isolation of holes at VB of TiO2. Besides, several scavengers were used to quench the presence of radicals during degradation of isoniazid (ISN), which includes isopropyl alcohol (IPA), ammonium oxalate (AO) and BQ associated to scavenge •OH, holes and •O2. As shown in Figure •OH and •O2 as main radicals during photocatalytic reaction of g‐C3N4/TiO2 nanocomposites can be confirmed. This gives a solid proof on the plausible accumulation of photogenerated electrons and holes at g‐C3N4 and TiO2 separately, as depicted in Figure According to kinetic theory of heterogeneous photocatalysis, hydroxyl radicals  will preferably transport to g‐C3N4 (VB) via the low resistance interface, accomplishing the Z‐schematic electronic configuration. Hence, XPS analysis is an efficient tool to differentiate direct Z‐scheme from type‐II heterojunction.XPS analysis has been widely employed to determine the chemical composition and surface states of materials. As of late, the application of XPS has been extended to gain further insights on the changes in electron density via examination in the shifting of binding energies of constituent elements with reference to C 1s as the calibration peak. In an investigation on the direct Z‐scheme mechanism of g‐CN4 Figure35b, whe3.3.33N4), there is a huge interest into the exploration of g‐C3N4 based Z‐scheme system in water splitting. As stated, the successful coupling of direct Z‐scheme relies on the formation of internal electric field within the solid–solid contact interface. In other words, the Fermi levels of the two photocatalysts that are about to be coupled will play a significant role on the final electronic configuration. A recent intrinsic insight on the interfacial properties of g‐C3N4/W18O49 nanocomposites was found to render switchable charge transfer behaviors via the tuning of band bending.Figure3N4 has more electronegative band structure which is suitable to serve as PS I while W18O49 with a lower band configuration can bestow strong oxidation ability as PS II. However, the Fermi level of g‐C3N4 is lower than W18O49 which induces electron injection from W18O49 to g‐C3N4 during the Fermi level alignment. As a result, contact interface of g‐C3N4 will be negatively charged, whereas W18O49 will be positively charged. In this regard, the energy bands of g‐C3N4 will be bending downward in contrary to the upward band bending of W18O49. Thus, g‐C3N4/W18O49 nanocomposites confer type‐II heterojunction electronic configuration in normal circumstances. Due to the charge isolation profile of type‐II heterojunction shown in Figure 3N4 was upshifted and accommodated in a more electronegative position than that of W18O49. The calculated work function (WF) of g‐C3N4 was found to have decreased from 5.714 to 4.607 eV, which implies the upshifting of Fermi level after the addition of TEOA . This elucidates the transfer of electrons occur from W18O49 to g‐C3N4 ascribed to the increase in electron density of g‐C3N4, accomplishing a cascade electron transfer within the system that resembles direct Z‐scheme. In whole, it is important to modulate the Fermi level of semiconductors to form a direct Z‐scheme nanocomposites rather than a type‐II heterojunction in order to govern a considerable large overpotential for highly efficient water splitting reaction.Additionally, XPS was employed by Huang et al. to determine the migration of charge carrier in g‐C2O3 and g‐C3N4 in assembling a 2D/2D structured direct Z‐scheme system.Figure2O3 and g‐C3N4 with uniform deposition of Pt can be clearly visualized. The intrinsic insight on the contact interface was calculated using DFT, which revealed g‐C3N4 has a lower WF and a higher Fermi level than Fe2O3. Consequently, a band configuration of direct Z‐scheme is formed within the nanocomposites, in which photogenerated electron was accumulated in g‐C3N4 (CB) and holes being isolated in Fe2O3 (VB). Besides, g‐C3N4/Fe2O3 exhibited photocatalytic H2 evolution rate of 19.9 µmol h−1 under TEOA, which is ≈13‐fold enhancement over pristine g‐C3N4. Interestingly, the formation of p–n interface can induce similar space charged region as the internal electric field of direct Z‐scheme. In a study on the coupling of g‐C3N4 and Bi4Ti3O12, a type‐II heterojunction with p–n interface was formed ascribed to the p‐type semiconductor nature of g‐C3N4 and n‐type of Bi4Ti3O12.3N4/Fe2O3, the formation of space charged region within g‐C3N4/Bi4Ti3O12 is attributed to the diffusion of electron across p–n interface, as depicted in Figure3N4) is lower than PS II (Bi4Ti3O12). As a result, the interface was negatively charged at PS I while positive at PS II. Hence, electron will be driven from PS I (CB) to PS II (CB). Whereas for direct Z‐scheme, Fermi level of PS I is higher than PS II, which results in the formation of positively charged interface at PS I and negative at PS II. Thus, cascade electron transfer profile can be witnessed in direct Z‐scheme system. Even though the presence of internal electric field in p–n junction nanocomposites can facilitate electron–hole pairs separation, redox ability of the system can be greatly reduced. In all, the formation of direct Z‐scheme can be realized when the Fermi level of PS I is higher than PS II as it provides synergy in band alignment for vectorial electron flow.Similarly, Xu et al. utilized the conjugation between FeTableIn conclusion, PS‐PS system offers a new dimension for the construction of a two‐step photoexcitation system without any electron mediator. The summary of PS‐PS system or direct Z‐scheme is presented in TableAs previously mentioned, each Z‐scheme system has its own advantages and disadvantages. PS‐A/D‐PS system confers the flexibility of PS development ascribed to its isolated units, but comes at the expense of shielding effect and backward reaction. PS‐C‐PS system renders an all‐solid‐state approach for Z‐schematic water splitting; however, costly metallic and nanocarbon electron mediators are required. On the other hand, the formation of direct Z‐scheme is highly dependent on the nature of PS units which limits the flexibility in material selection. The charge transfer mechanisms, representative systems, and key issues of each Z‐scheme mode are summarized in 44.1Figure2 production by immobilizing Pt‐loaded mesoporous g‐C3N4 (Pt/g‐C3N4) onto stainless steel plates using Nafion as the polymeric binder.Figure2 was setup by Schröder et al. for H2 production under natural sunlight. With the usage of plexiglass plate, the reactor allows UV transmission with wavelength above 300 nm. This designed reactor produced a total of 18.2 L of H2 within 30 days with measured STH of 0.12%.2 production using photocatalyst opens a new research interest to scale‐up this technology.With the progressive research efforts in developing efficient photocatalytic system for durable solar water splitting, it is timely to deliberate on scaling up of this technology for practical application. As delineated in x/Al:SrTiO3 for large‐scale evolution of H2 gas.2 each) that are arranged in 3 × 3 configuration. In the study by Goto et al., the importance of continuous gas evolution from the surface of photocatalyst sheets is highlighted, which is associated to the depth of water layer. The proposed 1 mm deep layer of water above the photocatalyst sheets allows the uninterrupted release of gas without the need of forced convection. On top of that, the water splitting panel was tilted at 10°–20° to warrant efficient release of gas and at the same time to ensure effective acceptance of sunlight. As a result, this water splitting panel demonstrated pure water splitting under natural sunlight irradiation with measured STH of 0.4%.In 2018, Goto et al. reported the concept of readily extensible water splitting panel with immobilized RhCrO4.22 production is still low which mainly imputed to the limitation of single‐component photocatalytic configuration. On the other hand, Z‐scheme photocatalytic system which mimics the natural photosynthesis process of green plant has undoubtedly paved a giant leap toward solar H2 harvesting via two‐step photoexcitation configuration. As a matter of fact, these photocatalysts are mostly available in the form of powder suspension, which greatly constraints the viability of large‐scale application. As solar energy augmentation becomes a global ambition for renewable energy revolution, a serious devotion to construct Z‐scheme system with scalability is imperative. More recently, Wang et al. demonstrated the concept of particulate Z‐scheme photocatalyst sheets by constructing a two‐layer structured film: 1) top layer of photocatalysts (PS I and II) and 2) bottom layer of conductor (mediator).Figure3:La,Rh (PS I/HEP) and BiVO4:Mo (PS II/OEP) were rationally assembled on top of Au conductor to form a two‐layered photocatalyst sheets using particle transfer method . Consequently, Ru/Cr2O3/SrTiO3:La,Rh‐Au‐BiVO4:Mo photocatalyst sheets demonstrated STH efficiency of 1.1% with background pressure of 10 kPa, which is almost one‐order magnitude higher than ever reported Z‐scheme systems, and it serves as a benchmark for photocatalytic water splitting research.Despite numerous efforts have been attempted for scaling up of particulate photocatalyst, the efficiency of the solar H4.33:La,Rh‐C‐BiVO4:Mo photocatalyst sheets.FigureFigure3:La,Rh‐C‐BiVO4:Mo photocatalyst sheets.Similar to Au, nanocarbon is another intriguing option to serve as an electron mediator. The employment of carbon conducting layer was firstly implemented on particulate SrTiO4.42 film.Figure2 resembles a tree‐like structure in nanoscale. Si nanowires bear a resemblance to a trunk while TiO2 nanowires are the branches. Coupling between semiconductors with PS I of higher Fermi level will lead to the formation of direct Z‐scheme. In this case, internal electric field is generated at the interface of Si and TiO2 due to band bending, which works as an electron‐transporting channel via Ohmic contact as shown in Figure2 bestows the advantages of large surface area and shorter charge diffusion pathway. Besides, the nanotree architecture also endows strong light harvesting ability to the system. Consequently, Si/TiO2 was loaded with co‐catalysts and realized overall water splitting with stoichiometric H2 and O2 evolution. In this case, Pt was loaded as the co‐catalyst for HER (Pt/Si) while IrO2 was employed to promote OER (IrO2/TiO2). STH efficiency of this system was measured to be 0.12% under simulated solar light.2 nanowires to accomplish on‐site H2 and O2 separation. In whole, this finding suggests the concept of direct Z‐scheme in macroscopic scale, which postulates a pathway for augmentation of photocatalytic water splitting other than Z‐scheme photocatalyst sheets with conductor.Apart from Z‐scheme photocatalyst sheets that utilize conductor as electron mediator, a scale‐up version of direct Z‐scheme can be witnessed on Si/TiO+ and OH− concentration overpotential and pH gradient. So far, Au and carbon film have been employed as the conductive layer to govern Z‐schematic water splitting using particulate sheets. Besides, the enhancement of electron relaying between semiconductors and mediator can be realized using post calcination and surface modification, i.e., loading of co‐catalysts with protective shell. This arouses new interest in the development of this new generation Z‐scheme system attributed to its scalability and commercial potential. A summary of Z‐scheme photocatalyst sheets for overall water splitting is tabulated in TableParticulate Z‐scheme photocatalyst sheets have posed as a viable option for solar water splitting augmentation attributed to the ingenious arrangement of PS I/PS II layers on top of conductive film in macroscale. In stark contrast to powder suspension, water splitting reaction with photocatalyst sheets render the alleviation on the effect of H2 production to be commercially feasible. Strategic approach to improve the photocatalytic performance of Z‐scheme system will be outlined henceforth in Despite the potential of Z‐scheme photocatalyt sheets in realizing large‐scale solar water splitting, numerous challenges are faced for instance: 1) improved photocatalytic efficiency of materials, 2) enhanced interfacial contact between photocatalysts layer and electron mediator, 3) reduced internal resistance loss due to increase in grain boundaries and 4) minimized overall efficiency loss when scaling up. All the requirements are essential for reducing the cost of solar H52 produced from PEC and photocatalysis systems by the United States Department of Energy, a STH value of 10% with 10 years of lifetime is required to meet the target H2 price of 1.6 USD kg−1.FigureAs aforementioned in the previous sections, Z‐scheme photocatalytic system confers unique band configuration with cascade electron transport profile which demonstrated water splitting advancement as compared to single‐component and heterojunction‐typed photocatalysts. Even so, the efficiency of current Z‐scheme systems is still far from being economically feasible for commercialization. According to techno‐economical evaluation of solar H0‐based or d10‐based oxides is one of the many research efforts in bandgap reduction of materials.Figure3 is used as HEP.3 is redshifted as compared to pristine SrTiO3 attributed to transition of photogenerated electrons from the hybridized state of Rh3+ to the CB which mainly constituted of Ti 3d orbital. However, the presence of Rh4+ species is detrimental due to its inactivity for H2 production and serves as charge recombination center for the photogenerated carriers. Meanwhile, the introduction of codoping of La atoms can suppress the formation of oxygen defects due to the partial substitution of Ti4+ sites. In 2016, Kobayashi reported an Ag‐mediated Z‐scheme system that can be operated up to 740 nm.2O4 (Eg: 1.2 eV) and Aurivilluis‐typed Bi4V2O11 as small bandgap HEP and OEP can harness nearly the whole visible light spectrum for overall water splitting. Hence, this result offers a new approach for the devise of Z‐scheme system to be operated under higher wavelength of the solar spectrum.The interstitial doping of transition metals with partially filled d orbitals as foreign heteroatoms into d3 has been widely documented as an efficient OEP. Sayama et al. firstly implemented Fe3+/Fe2+ ionic Z‐scheme system by using RuO2‐WO3 as OEP.3− in IO3−/I− redox cycle will significantly suppress the reduction process from OEP. One of the prominent ways is to control the pH of solution to be more than 9 which in turn hinders the production of I3−. However, this will greatly limit the application of IO3−/I− redox system in a wide pH range. In an effort to overcome this problem, surface treatment of WO3 by Cs+ salt and H+ exchange reaction yielded the formation of H‐Cs‐WO3 with ion‐exchangeable sites.[qv: 36c] Remarkably, the presence of these ion‐exchangeable sites can promote the reduction of IO3− and at the same time enhance the water oxidation performance of WO3. The photocatalytic O2 evolution of H‐Cs‐WO3 in IO3− solution resulted in AQY of 20% under monochromatic 420 nm irradiation. Besides, PtOx‐loaded H‐Cs‐WO3 exhibited high O2 evolution in solution containing I3− ions in stark contrast to the negligible activity from untreated PtOx/WO3. It can be speculated that the presence of ion‐exchangeable sites induced by the surface treatment can promote the reduction of I3− ions as well. Consequently, overall water splitting with stoichiometric evolution of H2 and O2 was demonstrated using NaI solution containing PtOx/H‐Cs‐WO3 as OEP and Pt/SrTiO3:Cr,Ta as HEP over a wider pH range (H2O)5]2+, the water oxidation performance of the system is greatly reduced.The loading of co‐catalysts is indispensable to promote photocatalytic overall water splitting associated to the decrease in kinetic barrier and facilitate forward reaction. Besides, the presence of uniformly dispersed co‐catalysts in sub‐nanoscale can provide surface active sites and maximize the atom‐utilization efficiency ascribed to the minimum agglomeration of nanoparticles.ng Figure53. Eve2O3 co‐catalyst to promote forward reaction. The early work of Maeda et al. in utilizing Rh/Cr2O3 as co‐catalyst on (Ga1−xZnx)(N1−xOx) solid solution effectively suppressed ORR and achieved overall water splitting.Figure2O3 as protective shell, water can be diffused into the metal core and the H2 product is freely to be released. At the same time, the protective shell prohibits O2 produced from water oxidation to permeate through the layer, which in turn hinders the tendency of ORR process. In a recent work by Wang et al., carbon conductive film was incorporated as electron mediator to interface SrTiO3:La,Rh, and BiVO4:Mo as Z‐scheme photocatalyst sheet.2O3 was chosen as the nanostructured co‐catalyst to prevent ORR effect. As a result, the particulate Z‐scheme photocatalyst sheets demonstrated a high STH value of 1.0% under ambient pressure.3−/I− Z‐scheme system with Ta3N5 as HEP and TaON as OEP.2O3 shell onto Pt metal can serve as an efficient co‐catalyst in driving overall water splitting with negligible competitive IO3− reduction. This technology presents a viable approach to incorporate co‐catalyst that is suitable to drive overall water splitting in Z‐scheme system.In an effort to suppress the undesirable backward reaction from ORR and redox couples, Cr species is introduced to form nanostructured core/shell metal/Crt on Ga1−xZnx PS‐A/D‐PS, 2) PS‐C‐PS, and 3) PS‐PS or direct Z‐scheme system. The pioneering work of Z‐scheme photocatalytic water splitting focuses on utilizing redox mediator for cascade electron flow, which is also known as the PS‐A/D‐PS (first generation) system. The nature of ionic pairs in liquid form allows PS I and PS II to be constructed independently in PS‐A/D‐PS system. Thus, the configuration of PS I and PS II can be freely modified and modulated as two isolated units. This feature of PS‐A/D‐PS system enables high degree of freedom in fabrication of photosystems. However, the strong pH dependency, shielding effect, and reverse reaction associated to redox pairs greatly limit the application of PS‐A/D‐PS system. With regard to this problem, surface modification of photocatalysts has been introduced to minimize the effect of reverse process imputed to the reversibility of ionic pairs.Recently, the employment of all‐solid‐state Z‐scheme system becomes more prominent due to the suppression of backward reaction and shielding effect with the absence of redox pairs. PS‐C‐PS and PS‐PS Z‐scheme are the two commonly studied systems in photocatalytic water splitting. In view of PS‐C‐PS (second‐generation) system, an external conductor is required for the electron relaying. The competency of metal and nanocarbon materials in driving electron transfer has made them the ideal candidate as a conductor. The path for electron transfer is also shortened for all‐solid‐state Z‐scheme system ascribed to the electron flow via low resistance contact which is the Ohmic contact. For years, PS‐C‐PS photocatalytic water splitting system has been widely conducted by employing metals as the electron mediator. As of recent, carbonaceous materials such as graphene and CNTs are relative new addition to the family of electron mediator for Z‐scheme. Constructing PS‐C‐PS system with conductive carbon as mediator, albeit still not as prevalence as the conventional metallic conductor, have increased exponentially attributed to the competency of carbonaceous materials in shuttling cascade electron flow. However, attention should be given on the fabrication of conductive carbon‐driven Z‐scheme to avoid formation of defects on the graphitic structure during the synthesis process.2 generation.For the PS‐PS (third generation) or direct Z‐scheme system, the formation of internal electric field is highly dependent on the nature of semiconductors. Coupling of two semiconductors with PS I of higher Fermi level will induce negatively charged interface, which eventually leads to the formation of direct Z‐scheme. Conversely, when PS I has a lower Fermi level, a type‐II heterojunction will be formed. The quasi‐continuous energy state at the solid–solid interface of PS‐PS system can bestow internal electric field for vectorial electron flow with low resistance. In addition, verification testing such as metal loading, radical trapping experiment, and XPS methods are important to distinguish the presence of direct Z‐scheme configuration rather than a conventional type‐II heterojunction. Since the turn of a new decade, attention has been drawn into the augmentation of technologies. Particulate Z‐scheme photocatalyst sheets are the new promising system with potential scalability in overall water splitting. Even so, there is still much vagueness in the charge transfer dynamics of Z‐scheme system. Besides, mediator should be examined on not only their electrical performance but also their work function with the coupled semiconductors. In the hopeful future, highly efficient Z‐scheme system can be methodically realized in driving large‐scale photocatalytic water splitting for HThe authors declare no conflict of interest.


RSi2 and R2TSi3 compounds are only interested in one specific compound or in a series of compounds with varying T elements while keeping R fixed (or vice versa). Here, the focus lies on the complete space of 2:1:3 and 1:2 silicides. In addition further variations of superstructures are revealed and focus on crystallographic properties.Most articles dealing with  RSi2 and R2TSi3 compounds  are summarized. The parameters comprising composition, lattice parameters a and c, ratio c/a, formula unit per unit cell and structure type are tabulated. The relationships between the underlying structure types are presented within a group–subgroup scheme (Bärnighausen diagram). Additionally, unexpectedly missing compounds within the R2TSi3 compounds were examined with density functional theory and compounds that are promising candidates for synthesis are listed. Furthermore, a correlation was detected between the orthorhombic AlB2-like lattices of, for example, Ca2AgSi3 and the divalence of R and the monovalence of T. Finally, a potential tetragonal structure with ordered Si/T sites is proposed.Here, structural parameters of various structure reports on  Some of the formed structures are considered as prototypes for further R2TSi3 compounds.The rare earth disilicides et al., 2013RSi2 and R2TSi3 compounds crystallize with the hexagonal AlB2 and the tetragonal ThSi2 type and derivative structure types  and its unit cell incorporates only the two Wyckoff sites 1a and 2d , but the Si sublattice forms a more complex 3D network . Appendix BR crystals. The more negative the formation energy, the more thermodynamically favorable is the formation of that compound. We considered a formation energy of up to −25 meV per atom as potentially stable at room temperature. However, this assumption does not take into account potential energy barriers which might kinetically hinder the formation of the ground state. The projector-augmented wave (PAW) method  to predict the stability of not yet reported 3.R2TSi3 compounds as a distinct phase with a fixed composition and not as a solid solution. As ternary phase diagrams are scarce for these compounds, we checked all available data, in particular the thermodynamic assessment of Bodak & Gladyshevskii , the change of the lattice (direction and distance), the characteristics of the structure  as well as the absolute occurrence of the structure types in the literature. Additionally, Fig. 2AOur diagram provides information about the type of transition .First, we will present the relationships of 2-related compounds. Only three of Hoffmann’s Bärnighausen branches are applicable for the stoichiometries addressed here (RSi2 and R2TSi3). We identify further structure types not discussed by Hoffmann & Pöttgen  type .Compared to their ideal crystallographic positions, the ErThe second branch only includes the Ho2PdSi3 structure type . Its Si/T atoms are partially ordered and only shifted along the b direction. These shifts induce a break in translational symmetry and a klassengleiche reduction. The Ho2PdSi3, Ba4Li2Si6 and Ca2AgSi3 structure types  and inserted it to the software FINDSYM . We changed the setting to P2mm (No. 25) for a better comparability to its supergroup Pmmm (No. 47). Thus, the triplication causes a translationengleiche and a klassengleiche symmetry reduction, which is accompanied with potential shifts of all atoms within the a,b plane.A second structure type is Utgen 2001 have alrtgen 2001 did not The fourth branch comprises the ordered R3□Si5 structures, which are not related to the disordered Er3□Si5 type within the Bärnighausen diagram.et al. , see gray box of Fig. 2at ≈ ah, ct ≈ 13.4–14.4 Å). The Si/T atoms form a complex 3D network, in contrast to the 2D honeycombs in AlB2. So far, the only reported variation of the ThSi2 type is the GdSi2 structure .Compounds of the ThSi2 or GdSi2 type structures exhibit Si vacancies, these do not order regularly and only cause partially occupied Wyckoff positions. The proportion of vacancies is generally 10% (RSi1.8), thus almost one Si ion per tetragonal or orthorhombic unit cell is vacant. The resulting structures remain in the original space group and are called ThSi2-defect and Nd□xSix2−, respectively.If the ThSi2, we did not find evidence for a tetragonal superstructure induced by ordering. This absence may be partially due to the small number of reports concerning tetragonal R2TSi3 compounds . The formation energy of Eu2RhSi3 differs from the second highest formation energy of Ho2RhSi3 by 160 meV per atom which exceeds the limit of 25 meV per atom, see green markers in Fig. 82RhSi3 compound in Ce2CoSi3 or Er2RhSi3 structure type is significantly less stable.Another interesting compound is Eu2PtSi3. In the R2PtSi3 series only a few element combinations have not yet been experimentally confirmed. Nevertheless, we identified missing compounds for R between Nd and Gd. Due to the radioactivity and low abundance of Pm and the volatility of Sm, we chose the Eu compound for further investigation. In analogy to the Rh series, we additionally chose R = Gd, Tb, Dy as references for formation energy and structure. In addition we modeled the not-yet-reported compound Ho2PtSi3. We decided to calculate the compounds in the reported Er2RhSi3 (2CoSi3 as well as in the lowest possible symmetry P1 (No. 1) to evaluate the influence of the degrees of freedom onto the formation energies. The energies for the R2PtSi3 compounds range from −6.18 eV to −5.11 eV, see orange markers in Fig. 82RhSi3 structure types are always lower than those of the highly symmetric type Ce2CoSi3, due to the additional degrees of freedom in atomic positions. The spread is between 0 meV for Gd and 28 meV for Ho per atom and about additional 1 meV going down to P1 (No. 1). The energies of the low-symmetric versions of the R2PtSi3 compounds are even lower than that of existing Gd2PtSi3. The formation energy of the (still) hypothetical Ho2PtSi3 in Ce2CoSi3 type structure is 33 meV per atom higher than that of Gd2PtSi3, thus this high-symmetry type is certainly not stable. However, the lower symmetry types will very probably be stable. The formation energy of Eu2PtSi3 is 14 meV per atom higher than for Gd2PtSi3; therefore, the compound is in the two considered symmetries most probably accessible as the thermodynamically stable phase. On the one hand, these data show that in some cases  the formation energy hardly changes for different structure types. On the other hand, the formation energy of different structure types may change so strongly that our relative limit of 25 meV per atom is by far exceeded and only the lower symmetric variations may be stable. This is the case for Tb2PtSi3, Dy2PtSi3 and Ho2PtSi3.The third compound of interest is EuR series, we discovered further characteristics in the R–T diagram worth studying for different reasons. Compound La2PdSi3 attracted our attention because Chaika et al. . We recommend checking La2PdSi3 for indicators of an ordered Si/T site, e.g. satellite reflections.After analyzing those three  al. 2001 and Behr al. 2008 have alr2. Most reported space groups of BaSi2 are orthorhombic  and, thus, expected to be equally stable. Nevertheless, these data alone are not sufficient to convey the stability of BaSi2 to SrSi2 as the elements Ba and Sr are too different. Furthermore, given the degrees of freedom, the tetragonal model of BaSi2 relaxed into an orthorhombic lattice with differences in lattice parameters a and b in the order of 0.4%. It should be noted that the a parameters of hexagonal and tetragonal symmetry differ for both BaSi2 and SrSi2 compounds 2AgSi3 and the Ca2AgSi3 type are reasonable. We excluded other structure types as other chemically similar compounds only crystallize in those two structures. Here, chemically similar means a noble metal T and R preferring the +II oxidation state . For T = Ag, Sr2AgSi3 is the only alkaline earth compound that has not yet been synthesized.Subsequently, we use the chemical similarity of Ba and Sr to evaluate which orthorhombic structure type is more favorable for compound Sr2AgSi3, also in both structure types. For Ba2AgSi3, the respective formation energies exhibited a clear preference for the reported Ca2AgSi3 type structure. However, the formation energies for both Sr2AgSi3 models are almost identical with a value of −2.83 eV, therefore we conclude that both structure types are equally stable. The formation energy of Sr2AgSi3 is slightly lower than that of Ba2AgSi3, which supports a stable structure.As a reference, we used BaR2TSi3 superstructure as determined in Section 3.1et al., 1994et al., 1983et al., 1986et al., 20082CuSi3  by rotation and elongation by a factor of 2AgSi3 are approximately the same aThSi2 = 4.12 Å ≈ 4.21 Å = aPOTS/2CuSi3, we compared three different symmetries, the high symmetry Ce2CoSi3, experimentally confirmed Er2RhSi3P1 (No. 1). The lattice parameters of all three models are a = 8.06 Å and  c ≈ 4.24 Å, which is in good agreement with the experimental values [Er2RhSi3Please note that the lattice parameters of the POTS type  are related to those of the ThSi2AgSi3 stoichiometry are −3.69 eV for the Ce2CoSi3 type and −3.72 eV for the tetragonal superstructure. With an absolute formation energy which is lower by 0.30 eV per atom, the tetragonal type is clearly favored. In general, the superstructural order for tetragonal symmetries may be suppressed for further reasons. On the one hand, the 3D Si/T network itself may present kinetic barriers. On the other hand, the entropy of mixing may hinder structural ordering more severe for the degeneracies of the 3D Si/T network than for the planar stacking of hexagonal symmetries.The formation energies of Nd3.4.RSi2 and R2TSi3 compounds. This figure adapts the R–T grid of Fig. 4T atoms do not order and otherwise as the number of Si/T layers along c in the unit cell. The range of order is highlighted by the color of the marker. The symmetry is marked by shape: hexagon for hexagonal AlB2-like, open star for orthorhombic AlB2-like, diamond for tetragonal ThSi2, elongated diamond for orthorhombic GdSi2. For technical reasons, this diagram shows at most three reports of the same compound . Our algorithm chooses the datasets with the highest as well as the lowest a parameter and an additional dataset with a different structure type, to depict the most significant variations. Fig. 9R and T cations; it depicts the following trends:Fig. 92-like lattice. The other lattice types are mainly determined by the included R and T element. For example, the orthorhombic GdSi2 structure type arises exclusively for lanthanide disilicides. The tetragonal lattice is dominant for R = Th compounds as well as for the disilicides with light rare earth elements. Additional compounds with tetragonal lattice are Ce2AuSi3, Nd2AgSi3 and Er2CuSi3, all possessing a noble metal T element. Thus, the Fermi level of the T element affects the structural stability, see Nentwich et al. , followed by the orthorhombic GdSi2 type in the intermediate range and the hexagonal AlB2 type for the heavy rare earth elements .We present an extensive literature study of the 2FeSi3, La2CoSi3, La2NiSi3, Ce2CuSi3 and Ce2NiSi3 form a solid solution of structure type AlB2 (disordered Si/T sites). Nevertheless, as evident from the discussion, we conclude that superstructures are expected to be the thermodynamic equilibrium structures, although they may be hard to synthesize, as they require obtaining the exact chemical composition on the one hand and for a careful thermal treatment on the other hand.According to Bodak & Gladyshevskii 1985, compounR–T grid showed a special characteristic of the structure types Ca2AgSi3 and Ba4Li2Si6. These structure types only arise if R has the formal +II oxidation state and T is either Au or Ag. Additionally, these structures are reported to have ionic character, whereas all other compounds are reported to be metallic. The given R–T diagram also shows a transition from tetragonal ThSi2 to orthorhombic GdSi2 to hexagonal AlB2 type within the lanthanide disilicides with increasing atomic number of R. The structure types behave similarly with increasing temperature when respective crystals are heated.Comparison of the symmetry distribution within the RSi2 and R2TSi3 compounds. On the one hand, these systematic investigations reduce systematic errors. On the other hand, the author’s expectations may also have an impact on the evaluation (such as the structure type).Figs. 52PtSi3, Pr2CoSi3, Eu2PtSi3 and Nd2CoSi3 are suggested to be stable, whereas Eu2RhSi3 will be unstable. Due to the positive results for Pr2CoSi3 and Nd2CoSi3, we recommend reinvestigating the R2TSi3 compounds reported by Mayer & Tassa . To complete the crystal structure information of La2PdSi3, we predict the lattice parameters a = 8.34 Å and c = 4.38 Å in a Ce2CoSi3 type structure. With respect to the question whether Sr2AgSi3 prefers the Ca2AgSi3 or the Ba4Li2Si6 structure type, both models result in almost identical formation energies of −2.83 eV and are equally stable from a theoretical point of view. Likewise, BaSi2 may exhibit hexagonal as well as tetragonal symmetry, as the formation energy of both models is −1.03 eV. In comparison, the potential tetragonal superstructure is less favorable than a highly symmetric hexagonal structure. The results of this work do not exclude the existence of structures that are equally or more stable than the ones presented here. The solid solutions with disorder at the Si/T position may always present potential candidates for the ground state of a specific R2TSi3 compound.Concluding the DFT analysis, hypothetical compounds Hoassa 1969, with R et al., 2020At this point, the question of particular driving forces for a certain type of symmetry and the multiplicity of the superstructure symmetry types and structure types remains. This question will be addressed in the second part of this work (Nentwich