g -C 3 N 4 -Based Direct Z-Scheme Photocatalysts for Environmental Applications

: Photocatalysis represents a promising technology that might alleviate the current environmental crisis. One of the most representative photocatalysts is graphitic carbon nitride ( g -C 3 N 4 ) due to its stability, cost-effectiveness, facile synthesis procedure, and absorption properties in visible light. Nevertheless, pristine g -C 3 N 4 still exhibits low photoactivity due to the rapid recombination of photo-induced electron-hole (e − -h + ) pairs. To solve this drawback, Z-scheme photocatalysts based on g -C 3 N 4 are superior alternatives since these systems present the same band conﬁguration but follow a different charge carrier recombination mechanism. To contextualize the topic, the main drawbacks of using g -C 3 N 4 as a photocatalyst in environmental applications are mentioned in this review. Then, the basic concepts of the Z-scheme and the synthesis and characterization of the Z-scheme based on g -C 3 N 4 are addressed to obtain novel systems with suitable photocatalytic activity in environmental applications (pollutant abatement, H 2 production, and CO 2 reduction). Focusing on the applications of the Z-scheme based on g -C 3 N 4 , the most representative examples of these systems are referred to, analyzed, and commented on in the main text. To conclude this review, an outlook of the future challenges and prospects of g -C 3 N 4 -based Z-scheme photocatalysts is addressed.


Introduction
Environmental protection and energy generation are two of the great challenges that mankind is presently facing, since they have a large negative impact on our society, affecting, among others, air pollution, climate change, water pollution, thermal pollution, and solid waste disposal [1][2][3].Such problems have been caused by rapid industrialization, uncontrolled environmental pollution, and current energy scenario based on fossil fuels [4].Aiming to solve these issues, current policymakers are passing regulations to reach full environmental sustainability and avoid a prolonged energy crisis and environmental deterioration.One interesting approach to solve this issue is to replace the use of carbonbased energy with solar energy [5,6].Indeed, solar energy is a green, inexhaustible, and clean resource [6,7].Another essential approach to achieve the sustainability of our planet is the degradation of pollutants to less toxic products and the reduction of CO 2 to obtain value-added products, which can be achieved under the irradiation of solar light [8,9].
Photocatalysis represents a promising technology that might alleviate the current environmental crisis [10].Semiconductor photocatalysts may generate H 2 by water splitting, degrade organic pollutants, reduce CO 2 into high value-added products, etc. using solar energy.Moreover, heterogeneous photocatalysis can be performed under very mild conditions (room temperature and atmospheric pressure) [11,12].Although photocatalytic technology presents relevant characteristics, such as being clean, safe, and renewable, it is still far from commercial implementation, especially in solar-to-fuel conversion [13,14].Titanium dioxide (TiO 2 ) has been widely studied in the field of solar energy conversion after the pioneering work reported by Fujishima and Honda in 1972 based on photoelectrochemical water splitting on a TiO 2 electrode [15].Furthermore, this semiconductor has received great interest from the scientific community for its use in pollutant abatement due to its advantages, e.g., low price, high durability, and abundance [16].To understand the relevance of photocatalysis, it is important to briefly describe the reaction mechanism [17].Initially, a photon with energy equal to or greater than the energy band gap (Eg) of the semiconductor is absorbed by the semiconductor.Then, a photoexcited electron (e − ) is promoted from the valence band (VB) to the conduction band (CB).This effect leads to the formation of a hole (h + ) in the VB and, consequentially, to the generation of an electron-hole pair (e − -h + ).The produced e − and h + can recombine on the surface or within the photocatalyst rapidly and thus decrease the photocatalytic activity or they can also migrate to the surface of the semiconductor and initiate the redox reaction(s) with the species adsorbed on the surface of the photocatalyst.The generated holes can induce oxidation processes, which occurs in organic molecules abatement, and the electrons can promote reduction processes such as hydrogen evolution in water splitting and CO 2 reduction.To develop efficient photocatalysts, light absorption is an essential step in the manufacture of the material [18].Pristine TiO 2 is active only under UV light with a wavelength below 387 nm due to its wide bandgap (3.2 eV).This fact is the main drawback of TiO 2 since solar energy is mainly concentrated (between 95 and 97%) in the IR and visible light regions [16].Consequently, the exploration of visible-light-responsive photocatalysts with high efficiency is a highly interesting topic for the scientific community [19,20].
Graphitic carbon nitride (g-C 3 N 4 ) is a very promising material due to its potential application in photocatalytic pollutant degradation, photocatalytic H 2 production, and CO 2 reduction.Its bandgap of 2.7 eV makes it a good candidate as a visible-light-responsive photocatalyst.In addition, g-C 3 N 4 presents a unique electronic band structure, low cost, and easy preparation, allowing it to be a possible alternative to TiO 2 in solar energy applications [21,22].However, pristine g-C 3 N 4 still exhibits low photoactivity in several applications such as water splitting and CO 2 reduction due to the rapid recombination of photo-induced e − -h + pairs [23].In recent years, several strategies have been developed by the scientific community to improve the photocatalytic efficiency of g-C 3 N 4 , i.e., morphological control, doping elements, deposition of noble metals, and construction of heterojunctions [24,25].
The latest advancements on g-C 3 N 4 -related photocatalytic systems are based on g-C 3 N 4 -based heterostructures due to the effective separation of photogenerated e − -h + pairs [26,27].Traditional heterojunctions are categorized into type-I, type-II, and type-III, as shown in Figure 1.A different pathway of the traditional type-II is the Z-scheme family (Z-schemes and S-schemes) [28,29], as shown in Figure 1 [30,31].Since the present review is focused on direct Z-scheme heterojunctions, only type II heterojunctions will be described, as shown in Figure 1b.Type II heterojunction formation using g-C 3 N 4 is a noteworthy method used to increase the photocatalytic efficiency by suppressing the recombination of charge carriers [24].However, type II heterostructures limit the oxidation and reduction efficiency of the heterojunction material from the parent materials [27].To solve this drawback, the synthesis and study of Z-scheme photocatalysts may be a good compromise since Z-scheme-based photocatalysts present the same band configuration but follow a different charge carrier recombination mechanism.Thus, in the Z-scheme, the strongest reduction and oxidation potentials of each semiconductor are preserved similarly to what happens in a natural process such as photosynthesis [32,33].Reproduced from [30].Copyright 2022, Elsevier.
There are two types of Z-scheme semiconductor heterojunctions [34,35].The first one is the indirect Z-scheme (redox-mediated and all solid state), where the transport route of photogenerated charge carriers is not achieved directly but by the addition of an electron mediator.The second type is the direct Z-scheme, which consists of two semiconductors in close contact, eliminating the demand for an electron mediator.These novel direct Zscheme photocatalysts have attracted the attention of the scientific community, increasing the use of these photocatalysts.Indeed, the absence of mediators eliminates the backward reaction and light-shielding effects [36,37].The layered structure of g-C3N4 is a suitable building block for Z-scheme photocatalyst construction due to the possible high surface area, provided that a layered material is obtained, and its ability to perform photocatalytic reduction reactions (water splitting and CO2 reduction), making it possible to couple g-C3N4 with a wide range of oxidation-type photocatalysts to fabricate Z-scheme photocatalysts [21,38,39].This fact has boosted the study of Z-scheme photocatalysts based on g-C3N4 for their use in environmental applications, as evidenced by the increase in the number of scientific publications on this topic during the last decade (Figure 2).Therefore, it becomes timely to carry out a bibliographic review in which the most relevant works of direct Z-scheme photocatalysts based on g-C3N4 and their environmental applications are compiled and summarized.
In this review, we will briefly describe pristine g-C3N4, although it still exhibits low photoactivity in several applications such as water splitting and CO2 reduction due to the rapid recombination of photo-induced e --h + pair.Then, a new alternative to improve the photocatalytic activity of g-C3N4 based on the design of novel Z-schemes will be widely described.Although Z-schemes are applied in relevant photocatalytic reactions, such as environmental applications [34,40], energetic applications [40], transformation of organic compounds to added-value products [41][42][43], and photocatalytic biorefineries [44], among others, this review will cover most relevant works on g-C3N4-based Z-scheme photocatalysts and their use in environmental applications such as pollutant abatement and CO2 reduction due to the high impact in our society, as was mentioned previously.In addition, energy aspects, e.g., water splitting, are also briefly reviewed.There are two types of Z-scheme semiconductor heterojunctions [34,35].The first one is the indirect Z-scheme (redox-mediated and all solid state), where the transport route of photogenerated charge carriers is not achieved directly but by the addition of an electron mediator.The second type is the direct Z-scheme, which consists of two semiconductors in close contact, eliminating the demand for an electron mediator.These novel direct Z-scheme photocatalysts have attracted the attention of the scientific community, increasing the use of these photocatalysts.Indeed, the absence of mediators eliminates the backward reaction and light-shielding effects [36,37].The layered structure of g-C 3 N 4 is a suitable building block for Z-scheme photocatalyst construction due to the possible high surface area, provided that a layered material is obtained, and its ability to perform photocatalytic reduction reactions (water splitting and CO 2 reduction), making it possible to couple g-C 3 N 4 with a wide range of oxidation-type photocatalysts to fabricate Z-scheme photocatalysts [21,38,39].This fact has boosted the study of Z-scheme photocatalysts based on g-C 3 N 4 for their use in environmental applications, as evidenced by the increase in the number of scientific publications on this topic during the last decade (Figure 2).Therefore, it becomes timely to carry out a bibliographic review in which the most relevant works of direct Z-scheme photocatalysts based on g-C 3 N 4 and their environmental applications are compiled and summarized.

Pristine g-C3N4
Graphitic carbon nitride is a stable polymeric and layered semiconductor formed by the polymerization of abundant nitrogen-containing precursors, such as melamine or urea [45,46].The first report about g-C3N4 was written in the 1830s by Berzelius and Liebig [47].Since then, many research studies have been carried out to investigate the structure of this material and to develop different synthetic routes [45].The structure of g-C3N4 is based on triazine or heptazine units polymerized in a layered structure (Figure 3a) [48,49].In the last decades, g-C3N4 has been presented as a promising material for its use in several applications such as sensors, energy storage, and photovoltaic cells, among others, since it presents facile, low-cost, and environmentally friendly preparation methods with prom- In this review, we will briefly describe pristine g-C 3 N 4 , although it still exhibits low photoactivity in several applications such as water splitting and CO 2 reduction due to the rapid recombination of photo-induced e − -h + pair.Then, a new alternative to improve the photocatalytic activity of g-C 3 N 4 based on the design of novel Z-schemes will be widely described.Although Z-schemes are applied in relevant photocatalytic reactions, such as environmental applications [34,40], energetic applications [40], transformation of organic compounds to added-value products [41][42][43], and photocatalytic biorefineries [44], among others, this review will cover most relevant works on g-C 3 N 4 -based Z-scheme photocatalysts and their use in environmental applications such as pollutant abatement and CO 2 reduction due to the high impact in our society, as was mentioned previously.In addition, energy aspects, e.g., water splitting, are also briefly reviewed.

Pristine g-C 3 N 4
Graphitic carbon nitride is a stable polymeric and layered semiconductor formed by the polymerization of abundant nitrogen-containing precursors, such as melamine or urea [45,46].The first report about g-C 3 N 4 was written in the 1830s by Berzelius and Liebig [47].Since then, many research studies have been carried out to investigate the structure of this material and to develop different synthetic routes [45].The structure of g-C 3 N 4 is based on triazine or heptazine units polymerized in a layered structure (Figure 3a) [48,49].In the last decades, g-C 3 N 4 has been presented as a promising material for its use in several applications such as sensors, energy storage, and photovoltaic cells, among others, since it presents facile, low-cost, and environmentally friendly preparation methods with promising stability and good physicochemical properties [45,50].Focusing on the application of this review, g-C 3 N 4 is a good candidate as a photocatalyst because it is a visible-light-active photocatalyst with a bandgap of ~2.7 eV (~460 nm) [51,52].In contrast to other semiconductors, g-C 3 N 4 does not contain metal ions, which may be leached into the ecosystem and cause unfavorable impacts therein.Instead, it is composed of Earthabundant carbon and nitrogen elements and can be easily synthesized using nitrogen-rich organic precursors by various methods.The obtained g-C 3 N 4 is endowed with desirable electronic structures and unique morphologies, and high thermal stability up to 600 • C in air [46].

Direct Z-Scheme Photocatalysts
To compensate for the drawbacks of pristine g-C3N4 for its use in photocatalysis, as described in Section 2, a novel alternative is the design of heterojunctions, defined as the interface between two different semiconductors, which can result in suitable band alignments [59,60].The design of type II heterojunctions using g-C3N4 is an interesting method to increase the photocatalytic efficiency by suppressing the recombination possibilities of the generated e − -h + pairs.In type-II heterojunction photocatalysts, the CB and the VB levels of semiconductor 1 are higher than the corresponding levels of semiconductor 2. Electron and hole pairs are separately generated in 1 and 2 subjected to light irradiation.The photogenerated electrons will transfer to semiconductor 2 while the photogenerated holes will migrate to semiconductor 1.Consequently, type II heterostructures diminish the oxidation and reduction efficiency of the isolated materials, which is the main drawback of this heterostructure (Figure 4c) [21].
Inspired by natural photosynthesis (Figure 4a), the artificial Z-scheme heterojunction Nowadays, there are several innovative and relevant synthesis methods of g-C 3 N 4 , that may be classified as solvothermal, chemical vapor deposition (CVD), plasma sputtering reaction deposition, and polycondensation [21,53].Among them, thermal polycondensation is the most attractive method due to its simplicity and low cost.Nitrogen-rich small molecules can polymerize into g-C 3 N 4 following a calcination process at 450-650 • C (Figure 3b) [54].The choice of the precursor exhibits effects on the electronic band structures and the textural properties of the obtained pristine bulk g-C 3 N 4 [55].In this sense, Wang et al. demonstrated in 2009 that g-C 3 N 4 was a promising visible-light photocatalyst for H 2 evolution.The as-prepared g-C 3 N 4 achieved steady H 2 production from water containing triethanolamine as a sacrificial electron donor on light illumination (λ > 420 nm) even in the absence of noble metal co-catalysts such as Pt (4 µmol/h), opening a new approach for the search of materials for H 2 production [56].After this pioneering work, the scientific community has carried out a lot of work to synthesize g-C 3 N 4 by thermal treatment of nitrogen-rich precursors such as urea, thiourea, melamine, cyanamide, and dicyandiamide, among others, to obtain g-C 3 N 4 in different morphologies such as nanosheets or nanotubes [57,58].However, pristine g-C 3 N 4 presents several drawbacks for its use in photocatalysis, including a low specific surface area, insufficient visible light utilization, and, the most relevant, rapid recombination of photogenerated charge carriers [23].To compensate for the drawbacks of pristine g-C 3 N 4 for its use in photocatalysis, as described in Section 2, a novel alternative is the design of heterojunctions, defined as the interface between two different semiconductors, which can result in suitable band alignments [59,60].The design of type II heterojunctions using g-C 3 N 4 is an interesting method to increase the photocatalytic efficiency by suppressing the recombination possibilities of the generated e − -h + pairs.In type-II heterojunction photocatalysts, the CB and the VB levels of semiconductor 1 are higher than the corresponding levels of semiconductor 2. Electron and hole pairs are separately generated in 1 and 2 subjected to light irradiation.The photogenerated electrons will transfer to semiconductor 2 while the photogenerated holes will migrate to semiconductor 1.Consequently, type II heterostructures diminish the oxidation and reduction efficiency of the isolated materials, which is the main drawback of this heterostructure (Figure 4c) [21].
Inspired by natural photosynthesis (Figure 4a), the artificial Z-scheme heterojunction design is a great alternative for the development and manufacture of novel photocatalysts, with outstanding results in numerous applications [61,62].The Z-scheme photocatalytic system concept was proposed by Bard in 1979 for the first time [63].Similar to what happens in photosynthesis, under light excitation, the electrons generated in a semiconductor with low reduction potential are recombined with the holes with low oxidation potential, i.e., those with the highest potential.This phenomenon generates electrons and holes isolated in the Z-scheme system with maximum redox abilities, making it the greatest advantage of these materials (Figure 4b) [34].The traditional Z-scheme introduced by Bard [63] needs a shuttle redox mediator (electron acceptor/donor pair) to form a liquidphase Z-scheme.Thus, Z-scheme photocatalysts suffer from the limitations of redox mediator reversibility and their specific applications, e.g., CO 2 reduction, can only be applied in the liquid phase [34].The second generation of Z-scheme (all solid-state Zscheme photocatalysts) was discovered in 2006 by Tada et al. [64].These are composed of two different semiconductors with a solid-phase electron mediator as a noble metal nanoparticle (NP) or carbon material (graphene and carbon nanotubes) [65].To solve the inconvenience of using electron mediation, Yu et al. [66] constructed a direct Z-scheme photocatalyst by combining g-C 3 N 4 and TiO 2 in 2013.The interfacial contact between the semiconductors facilitates the direct electron transfer without the help of an electron mediator.This novel direct Z-scheme system presented the advantage of significantly reducing the construction cost [37].
In fact, g-C 3 N 4 has received significant attention due to its potential use as a reductant semiconductor in direct Z-scheme photocatalysts given its superior chemical and physical features [26,33].In addition, the great advantage of its use as part of Z-scheme photocatalysts is that the VB and CB positions are located at approximately +1.6 and −1.1 eV, respectively [28].This redox potential converts g-C 3 N 4 into an interesting reduction semiconductor for its use in several applications, such as H 2 production and CO 2 reduction (Figure 5) [67].Nevertheless, the formation of the g-C 3 N 4 /semiconductor interface is a challenge for a Z-scheme heterostructure [33,68].Therefore, it is necessary to design and modify novel routes of synthesis to tackle the drawbacks of these systems.
tively [28].This redox potential converts g-C3N4 into an interesting reduction semiconductor for its use in several applications, such as H2 production and CO2 reduction (Figure 5) [67].Nevertheless, the formation of the g-C3N4/semiconductor interface is a challenge for a Z-scheme heterostructure [33,68].Therefore, it is necessary to design and modify novel routes of synthesis to tackle the drawbacks of these systems.

Synthesis and Characterization of Z-Scheme Photocatalysts Based on g-C3N4
Generally, the synthesis of Z-scheme photocatalysts is crucial to obtain systems with high efficiency in photocatalysis.In the literature, there are several methods to synthesize catalysts or composites such as deposition, solid-state synthesis, and hydrothermal synthesis, among others [22,37,69].In the synthesis of Z-scheme photocatalysts, two semicon-

Synthesis and Characterization of Z-Scheme Photocatalysts Based on g-C 3 N 4
Generally, the synthesis of Z-scheme photocatalysts is crucial to obtain systems with high efficiency in photocatalysis.In the literature, there are several methods to synthesize catalysts or composites such as deposition, solid-state synthesis, and hydrothermal synthesis, among others [22,37,69].In the synthesis of Z-scheme photocatalysts, two semiconductors are combined to optimize the oxidation and reduction potential by the recombination of e − -h + pairs, which makes intimate contact between both semiconductors crucial [63].To increase the activity of photocatalysts based on g-C 3 N 4 , there are several methods [58,64], such as varying the texture of g-C 3 N 4 by means of template synthesis or deposition of metal cocatalysts.However, this section of the manuscript will focus on the synthesis of the direct Z-scheme without any mediator.
The most relevant synthetic methodologies used by the scientific community to design direct Z-scheme systems based on g-C 3 N 4 are (Figure 6): Solid-state synthesis.This methodology is broadly used in the synthesis of pristine g-C 3 N 4 materials [47,70] and it is most employed to synthesize Z-scheme photocatalysts based on g-C 3 N 4 [71].This methodology is based on the calcination of one or a mixture of precursors in air or an inert gas atmosphere at high temperatures [72].There are some crucial experimental parameters in the synthesis of materials using solid-state methodologies such as the heating rate, calcination temperature, and calcination time to control the crystallinity, morphology, surface properties, and phase structure of the composite [73,74].This methodology is very interesting for the design of Z-scheme materials, as reported by W. Yu et al. [75], who synthesized g-C 3 N 4 in the presence of pre-synthesized WO 3 to obtain a direct Z-scheme g-C 3 N 4 /WO 3 photocatalyst.Another noticeable work that uses this methodology was reported by L. Lu et al. [74].In this work, the authors studied the effects of the calcination temperature on the photocatalytic activity of direct Z-scheme TiO 2 /g-C 3 N 4 .Although this methodology is widely used in the synthesis of the g-C 3 N 4 -based direct Z-scheme, the high temperatures used and the low control of the composition are the main drawbacks.
Deposition precipitation method.This methodology is commonly used for the synthesis of photocatalysts when one of the precursors is cationic and the other is anionic.A uniform precipitated composite is formed [76,77].The deposition precipitation technique is based on the formation of a precipitate on the surface of another component by slow addition or in situ growth of a substance, following the addition of a precipitating agent at a low temperature [78,79].Although this methodology is not widely used in direct Z-schemes, it is an interesting methodology due to the easy fabrication of these systems [80].
Impregnation.Impregnation is another popular method for the synthesis of catalysts [81,82] and it has been used for the fabrication of Z-scheme photocatalysts [83,84].In this approach, a solid precursor or material is in contact with a solution containing the precursor to be deposited on the solid surface.There are two methods of impregnation: (1) the wet impregnation method, in which the solid precursor is introduced with an excess volume of the second precursor solution, and (2) incipient wetness impregnation, in which the volume of the second precursor solution used is equal or less than the pore volume of the solid [82].Due to the advantages of this methodology, Feng et al. [85] reported a composite synthesized using a simple impregnation-heating method in which MoO 3 nanoparticles were in situ supported on g-C 3 N 4 sheets.This Z-scheme photocatalyst had photocatalytic activity in CO 2 reduction to fuels under simulated sunlight radiation.Zhou et al. [86] reported a simple impregnation method to synthesize Z-scheme g-C 3 N 4 decorated with TiO 2 nanotubes with improved visible-light photocatalytic activity in pollutant abatement.Another example is the work of Jin et al. [84], where the authors reported the use of a one-step impregnation method to prepare direct Z-scheme LaCoO 3 /g-C 3 N 4 photocatalysts.
Hydrothermal synthesis.In the 21st century, hydrothermal technology is one of the preferred methods for the synthesis of materials in various interdisciplinary fields such as advanced materials technology, nanotechnology, biotechnology, etc. due to the ease of processing particles with high purity, high crystallinity, controlled stoichiometry, and con-trolled chemical and physical characteristics, and to the environmental friendliness [87,88].Hydrothermal processing is defined as a heterogeneous reaction performed under high temperatures and pressure in the presence of an aqueous solvent to dissolve and recrystallize substances that are relatively insoluble under ambient conditions [89].It became one of the mostly used synthetic methodologies for the synthesis of g-C 3 N 4 -based Z-scheme photocatalysts.Jo et al. reported the synthesis of Z-scheme g-C 3 N 4 /TiO 2 photocatalysts for isoniazid degradation.Moreover, they studied the effect of the TiO 2 morphology in the synthesis of the Z-scheme photocatalysts and their catalytic activity [90].Di et al. [91] reported the synthesis of a direct Z-scheme based on g-C 3 N 4 , synthesizing a C 3 N 4 /SnS 2 photocatalyst with an in situ hydrothermal method at 140 • C, and the photocatalysts had a superior visible-light CO 2 reduction performance.Recently, Lu et al. [92] reported a 2D/2D g-C 3 N 4 /BiVO 4 Z-scheme heterojunction using the hydrothermal methodology with remarkable photocatalytic activity enhancement of CO 2 conversion promoted by efficient interfacial charge transfer.In this sense, Wu et al. [93] synthesized 2D g-C 3 N 4 -supported nanoflower-like NaBiO 3 using a facile hydrothermal synthesis.
Photo-deposition methodology.Photo-deposition is a common technique for loading a cocatalyst (such as Pt NPs) onto a photocatalyst via photoreduction [94].Photo-deposition is the phenomenon through which a cocatalyst is deposited on the surface of a semiconductor, upon illumination of a solution containing the cocatalyst precursor and the support [95].In the last years, the scientific community has developed a great interest in the photodeposition method to obtain Z-schemes [17,95].One representative example is the work reported by Jiang et al. [96], where two routes for constructing the Fe 2 O 3 /g-C 3 N 4 direct Z-scheme through photo-deposition were demonstrated.
The characterization of direct Z-scheme photocatalysts is crucial to identify Z-scheme heterojunctions because type II heterojunctions and Z-scheme photocatalysts have similar structures [37].The main difference between both heterojunctions is the charge carrier mechanism, as described in Section 3.1.In the last years, researchers have studied and developed several experimental and theoretical simulation methods to characterize these novel heterostructures (Figure 6) [33,37].The most interesting and widely used methodology to characterize and understand the mechanism of Z-scheme systems is the radical species trapping methodology [97,98].Indeed, this methodology can be applicable because Z-scheme semiconductor 1 with a high oxidizing capacity can produce • OH while semiconductor 2 with a sufficient reduction potential is capable of generating O 2 •− species.However, type-II heterojunction photocatalysts with a low redox (reduction or oxidation) ability can only generate one type of radicals (either O 2 •− or • OH).To elucidate this effect, the radical scavenging methodology is applied.In radical scavenging experiments, a chemical agent is introduced in the photocatalytic medium system to quench a radical, consequently decreasing the activity of the studied reaction [99,100].Common scavengers used in the literature are tert-butyl alcohol (TBA) and isopropanol (IPA) for • OH, and N 2 gas and p-benzoquinone (BQ) for O 2 •− while ammonium oxalate (AO), triethanolamine (TEA), and disodium EDTA are used for holes (h + ) [100].Other characterization techniques used to study Z-scheme systems are photoluminescence (PL) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy [101][102][103].During the last years, there has been an increase in the application of other spectroscopic characterization techniques such as ultraviolet photoelectron spectroscopy (UPS), transient absorption spectroscopy (TAS), surface photovoltage spectroscopy (SPS), and in situ irradiated X-ray photoelectron spectroscopy (ISI-XPS) to verify the Z-scheme charge transfer mechanism [26,104].Another methodology used to verify Z-schemes in heterojunctions is photocatalytic reduction testing because the photogenerated electrons with adequate reduction potential in a semiconductor can be used to produce some selective products that a semiconductor with a lower reduction potential is unable to generate [33,105].To experimentally verify the proposed system, this methodology usually needs the aid of computational calculations based on density functional theory [106][107][108].In the last years, this methodology has been used to calculate the Fermi level and to interpret the charge transfer mechanism.
Finally, it is relevant to indicate that there are several methodologies used to synthesize direct Z-scheme photocatalysts, as described in this section.However, it is essential to choose and develop easy and sustainable methodologies focusing on the intimate contact between both semiconductors to obtain direct Z-scheme photocatalysts.It is also necessary to use and develop powerful characterization tools to investigate the charge-transfer mechanism to elucidate whether the heterojunctions synthesized and studied are Z-scheme photocatalyst or another heterostructure, such as type II.
toelectron spectroscopy (ISI-XPS) to verify the Z-scheme charge transfer mechanism [26,104].Another methodology used to verify Z-schemes in heterojunctions is photocatalytic reduction testing because the photogenerated electrons with adequate reduction potential in a semiconductor can be used to produce some selective products that a semiconductor with a lower reduction potential is unable to generate [33,105].To experimentally verify the proposed system, this methodology usually needs the aid of computational calculations based on density functional theory [106][107][108].In the last years, this methodology has been used to calculate the Fermi level and to interpret the charge transfer mechanism.
Finally, it is relevant to indicate that there are several methodologies used to synthesize direct Z-scheme photocatalysts, as described in this section.However, it is essential to choose and develop easy and sustainable methodologies focusing on the intimate contact between both semiconductors to obtain direct Z-scheme photocatalysts.It is also necessary to use and develop powerful characterization tools to investigate the charge-transfer mechanism to elucidate whether the heterojunctions synthesized and studied are Zscheme photocatalyst or another heterostructure, such as type II.

Environmental Applications of Direct Z-Schemes Based on g-C3N4
As mentioned in previous sections, the use of semiconductors capable of utilizing sunlight has received great interest in environmental remediation and in contributing to the solution of the energy crisis [10,109].g-C3N4 is a semiconductor with optimal properties (absorption in visible light) for its use in this field, although it has several drawbacks [54,110], as shown in Section 2. Therefore, an alternative for g-C3N4 to be a key material in the current scenario is the use of the direct Z-scheme based on g-C3N4 [33,67].Hence, g-C3N4-based direct Z-schemes have been widely studied by scientists for their application in photocatalytic pollutant remediation [111][112][113], photocatalytic H2 production [114,115],

Environmental Applications of Direct Z-Schemes Based on g-C 3 N 4
As mentioned in previous sections, the use of semiconductors capable of utilizing sunlight has received great interest in environmental remediation and in contributing to the solution of the energy crisis [10,109].g-C 3 N 4 is a semiconductor with optimal properties (absorption in visible light) for its use in this field, although it has several drawbacks [54,110], as shown in Section 2. Therefore, an alternative for g-C 3 N 4 to be a key material in the current scenario is the use of the direct Z-scheme based on g-C 3 N 4 [33,67].Hence, g-C 3 N 4based direct Z-schemes have been widely studied by scientists for their application in photocatalytic pollutant remediation [111][112][113], photocatalytic H 2 production [114,115], and photocatalytic CO 2 reduction [85,116,117].In this section, the most representative direct Z-schemes based on g-C 3 N 4 for their use in environmental applications focusing on pollutants remediation, H 2 production, and CO 2 photoreduction will be presented.Additionally, in the main text, the most relevant aspects of the developed direct Z-schemes based on g-C 3 N 4 for these three relevant environmental applications are described.

Pollutant Remediation
During the last decades, photocatalytic pollutant remediation has been intensively studied, leading to the synthesis of a plethora of photocatalysts active in the degradation of different pollutants.Many reports were also focused on the study of the so-called advanced oxidation processes (AOPs), which are fundamental for pollutant remediation.Indeed, AOPs include by definition the formation of highly reactive oxygen species (ROS), e.g., O 2 •− , OH • , and more recently SO 4 2•− radicals, which can generate cascade processes during pollutant degradation.Indeed, these radicals have the capability of rapidly reacting with most common pollutants.This will generate radicals on the organic skeleton of the molecule and bring about defragmentation, which might lead to a complete mineralization of the contaminant in the form of CO 2 [118].The use of heterogeneous photocatalysts can positively influence the formation of these species.As shown in Equation ( 1), the promotion of electrons from the VB to the CB origins holes, which might react with water or SO 2 O 8 2− for the generation of OH • (see Equations ( 2) and ( 3)) or SO 4 2•− radicals (Equation (4)), respectively, which participate in the subsequent radical reaction pathway for pollutant degradation [119,120].On the other hand, the electrons accumulated at the CB might react with O 2 to generate O 2 •− radicals (Equation (5)), fundamental for the following cascade reactions [121]: E 0 = 2.8 V (V vs. NHE, pH 7, 25 • C, 1 atm) As mentioned in the text, the use of heterojunctions plays a key role in overcoming the main drawbacks of photocatalysts.Thus, the oxidation processes involved in the formation of ROS might be more favored by the composite than its isolated components [122].The coupling of g-C 3 N 4 with different semi-conductors gives birth to an extended family of direct Z-scheme heterojunctions, active in AOPs (Figure 7) and, subsequently, applied in pollutant remediation [67].
In the following pages, we will analyze some of the most important examples of pollutant photodegradation by direct Z-schemes based on g-C 3 N 4 .Mainly, we will focus on some of the most representative pollutants used in photodegradation studies and some of the most illustrative results are summarized in Table 1.
Among the different pollutants, rhodamine B (RhB) is one of the most widely studied in recent reports related to this topic.Indeed, this water-soluble molecule is used as a red dye in the textile industry, thus representing one of the most concentrated wastes in water.Being classified as carcinogenic, RhB must be removed from the residual water, and this can be achieved by the use of photocatalysis [123].Metal-free g-C 3 N 4 -based direct Z-scheme photocatalysts have been extensively applied in the degradation of RhB with highly remarkable results.M. Tang et al. [124] developed a ternary system composed of AgI/Ag 3 PO 4 /g-C 3 N 4 with high activity in the degradation of RhB and a neonicotinoid pesticide due to the low recombination rate assured by the presence of the three compounds, as shown in Figure 8a.Indeed, the use of isolated components or even binary systems containing these materials did not lead to the same high yield in degradation.This finding was also demonstrated by Y.He et al. [116] using SnO 2-x /g-C 3 N 4 composite in RhB degradation.Figure 8b depicts another advantage of this heterostructure, i.e., an increase in the surface area, which is known to be one of the main drawbacks of the application of g-C 3 N 4 in catalysis [116].Another noteworthy example is the 2D/2D SnS 2 /g-C 3 N 4 heterostructure developed by H. Che et al. [125], where the efficiency of the charge carrier separation is enhanced by the high interfacial contact area generated from the contact between the 2D materials.In addition, the photocatalytic activity of this catalyst is boosted because both OH • and O 2 •− radicals might be generated, as presented in Figure 8c [126].Considering lower dimensionality systems, 0D materials have found applicability in the generation of novel composites.For example, Y. Fu et al. [111] developed the MoS 2 QD/g-C 3 N 4 heterostructure and demonstrated that the interaction between the 0D QD and 2D moieties can stabilize the QDs.Photocurrent measurements (Figure 8d) highlight that in the composites, the interfacial charge transfer is faster and the recombination of photoexcited charges is inhibited, thus boosting the photocatalytic activity in RhB degradation.
Tetracycline (TC) is a common antibiotic but with many side effects to live bodies.Its wide usage in husbandry (cattle, swine, poultry, and fishery) leads to an increase in its concentration in water and poses a threat to the natural balance.Among other degradation techniques, photocatalysis has found application in the degradation of this molecule [127][128][129], and g-C 3 N 4 -based direct Z-scheme photocatalysts have been extensively applied for this purpose.One recent report developed by J. Kang et al. [130] shows how the construction of the heterostructure g-C 3 N 4 /Na-BiVO 4 can boost the photodegradation of TC, promoted in this case by the formation of peroxymonosulfate radical.Indeed, the calculated CB and VB presented in Figure 9a demonstrate that the use of g-C 3 N 4 can overcome one of the main drawbacks of the promising photocatalyst BiVO 4 , i.e., the low CB potential.Similarly, peroxymonosulfate-mediated TC photodegradation was demonstrated by C. Jin et al. [131] to be successful when using the 2D/2D Co 3 O 4 /g-C 3 N 4 heterostructure.In this case, the two-dimensionality allows a better separation and transfer of the charges, as demonstrated by the photoluminescence measurements in Figure 9b RhB and TC represent two of the most common pollutants that have been studied in photodegradation processes, but, certainly, according to the latest publications, there are other dangerous pollutants, which are present worldwide in high concentrations in water [133].Consequently, photocatalysts based on g-C 3 N 4 have also been used for the degradation of different pharmaceutical products or persistent organic pollutants (POPs), e.g., fluoroquinolone antibiotics, benzodiazepines, or parabens.Y. Zhou et al. [134] successfully used a 3D/2D MOF-derived CoCeO x /g-C 3 N 4 Z-scheme heterojunction in the photocatalytic degradation of carbamazepine.The presence of the MOF-derived CoCeO x increased the surface area, overcoming one of the main drawbacks of g-C 3 N 4 .Additionally, J. Huang et al. [135] showed that the combination of g-C 3 N 4 and TiO 2 originated an active photocatalyst for the visible-light elimination of enrofloxacin in water, as indicated in Figure 9c.In this case, the presence of g-C 3 N 4 favors the absorbance of TiO 2 under visible light, giving more applicability to the well-known photocatalyst TiO 2 .Furthermore, J. Meng et al. [136] synthesized the WO 3 /g-C 3 N 4 heterojunction for the degradation of methylparaben.In this case, the heterostructure avoids the fast recombination typical in both materials and the presence of g-C 3 N 4 enhances the formation of O 2 •− radicals, which are involved in the degradation process (Figure 9d).
In summary, direct Z-schemes are a great alternative to improve the catalytic activity of bare photocatalysts in pollutant abatement, obtaining total conversion in a few hours.However, it is necessary to test the photocatalysts under more realistic conditions since most of the catalytic tests were performed with isolated pollutants in pure water.In summary, direct Z-schemes are a great alternative to improve the catalytic activity of bare photocatalysts in pollutant abatement, obtaining total conversion in a few hours.However, it is necessary to test the photocatalysts under more realistic conditions since most of the catalytic tests were performed with isolated pollutants in pure water.

H 2 Production
One of the greatest current challenges of our society from an environmental and energy point of view is the production and use of clean fuel from renewable energies [2,145].During the last decades, H 2 has become a highly relevant alternative to fossil fuels because it has clean combustion and high energy density [145].Therefore, it is essential to develop technologies to allow the production of H 2 without using fossil fuels [146].This subsection is placed here, as the water splitting uses similar starting radicals to pollutant abatement in aqueous ambience.The generation of H 2 by water splitting using sunlight has received great interest from the scientific community and in society [147,148].The water splitting reaction that generates H 2 and O 2 (Equation ( 6)) is a non-spontaneous endergonic reaction (∆G = +237 KJmol −1 ), which means that energy is needed for the reaction to occur [148]: One alternative to the generation of H 2 from water splitting is the use of photocatalysis, which would require the use of sunlight and a semiconductor (photocatalysis).For the reaction to occur, the CB potential of the semiconductor should be more negative than the reduction potential of H 2 O (0 V vs. NHE) for H 2 generation, and the VB potential of the semiconductor should be more positive than the oxidation potential of H 2 O (1.23 V vs. NHE) for O 2 generation [148,149].An interesting semiconductor for this reaction is g-C 3 N 4 since the VB and CB positions are located at approximately +1.6 and −1.1 eV, respectively, with a band gap of ~2.7 eV, making the reaction of water splitting possible using sunlight [56,150].However, the g-C 3 N 4 photocatalyst presents low activity in water splitting [23], as mentioned in Section 2. To solve this low activity of g-C 3 N 4 in water splitting, the scientific community has focused on studying heterojunctions based on g-C 3 N 4 [26,60].A heterojunction that has aroused the interest of the scientific community is the direct Z-scheme because the latter is present in the systems responsible for the water splitting reaction in photosynthesis due to the decrease in the e − -h + pair recombination rate [38,98].This makes the direct Z-schemes promising systems, which may lead to adequate quantum efficiency values for their commercialization [38].In this subsection, several direct Z-scheme photocatalysts based on g-C 3 N 4 in different conditions used in water splitting are summarized in Table 2.In addition, a detailed discussion of the most relevant Z-scheme photocatalysts from the point of view of the catalytic activity and reaction mechanism will be described in the main text.R. Ye et al. [151] described a CoTiO 3 /g-C 3 N 4 novel Z-scheme heterostructure photocatalyst for H 2 evolution (858 µmol g −1 h −1 ) under visible light with effectively intimate interfaces linked through the Co-O-N or Ti-O-N bonds, as shown in Figure 10a.The authors showed that this system presents a direct Z-scheme mechanism by PL, PEC, and ESR analyses.The work described by R. Ye et al. focusing on Z-scheme CoTiO 3 /g-C 3 N 4 was useful for further development of novel heterojunction photocatalytic systems based on perovskite, polymer, and composite photocatalysts for photocatalytic H 2 production.X.She et al. [152] showed a novel strategy for the synthesis of an α-Fe 2 O 3 /2D g-C 3 N 4 Z-scheme hybrid structure containing ultrathin g-C 3 N 4 platelets.In the synthesis methodology used in this study, α-Fe 2 O 3 promotes the formation of g-C 3 N 4 nanosheets.Moreover, the authors claimed that the Fe 2 O 3 /2D g-C 3 N 4 direct Z-scheme photocatalyst exhibits unprecedented photocatalytic activity toward H 2 evolution (31,400 µmol g −1 h −1 ), with a remarkably high quantum efficiency of 44.35% at a wavelength of 420 nm, as shown in Figure 10b.The enhanced performance of H 2 evolution in α-Fe 2 O 3 /2D g-C 3 N 4 was attributed to the 2D structure of g-C 3 N 4 , allowing an efficient transfer of the photoexcited electron to the reactant.Another factor relevant to improving the photocatalytic activity of this system is the Z-scheme structure due to the low electron-hole recombination in both α-Fe 2 O 3 and 2D g-C 3 N 4 and the close interface contact between α-Fe 2 O 3 and 2D g-C 3 N 4 , enabling fast electron transfer from the conduction band of α-Fe 2 O 3 to the valence band of 2D g-C 3 N 4 .This work shows that the dimensionality (2D) and the porous hierarchy of the systems present a great effect on the photocatalytic activity of Z-schemes.Moreover, Fe 2 O 3 /2D g-C 3 N 4 direct Z-scheme photocatalysts exhibit great values in H 2 production, indicating the effectivity and interest of these artificial Z-schemes for their use in this application.Considering the effect of the 2D materials, Y. Yang, et al. [153] designed and constructed a direct Z-scheme van der Waals heterojunction composed of ultrathin WO 3 •H 2 O and g-C 3 N 4 nanosheets using simple hydrothermal synthesis to achieve efficient water splitting without the addition of any sacrificial agents (482 µmol g −1 h −1 ).This WO 3 •H 2 O/g-C 3 N 4 system could efficiently transport the electrons, decreasing the e − -h + recombination rates, as shown in Figure 10c, which led to a considerable improvement in the photocatalytic performance.Another relevant parameter in the study of Z-schemes is the presence of interfacial defects since they might be beneficial to the recombination of interfacial photogenerated e − /h + and could serve as Z-scheme pathways for charge transfer.L. Kong et al. [154] fabricated a direct Z-scheme g-C 3 N 4 /Ti 3+ -doped TiO 2 to show this effect.The authors observed that the Ti 3+ defects were introduced under CB of TiO 2 , which improved the interfacial charge transfer channels for Z-scheme g-C 3 N 4 /Ti 3+ -doped TiO 2 composite and improved the visible light absorption.These properties were studied using a radical species trapping methodology, UV-Vis spectroscopy, and EPR analysis.Due to the enhanced properties, this system has high activity (1938 µmol•h −1 •g −1 ) under simulated solar light and good stability in photocatalytic H 2 evolution.S. Qin et al. [155] showed a promising way to construct Z-scheme heterostructures based on metal-organic cages (MOCs) and semiconductors for their use in photocatalytic H 2 generation.The authors synthesized an MOC, integrating four organic photosensitized ligands M 4− and two Pd 2+ catalytic centers.Then, the MOC was successfully immobilized by hydrogen bonds to obtain a robust heterogeneous direct Z-scheme g-C 3 N 4 /MOC-Q1 (Figure 10e).This innovative system presents high H 2 evolution activity (4495 µmol g -1 h -1 ).In this manuscript, it was described that Z-scheme systems improved the photocatalytic H 2 production rate, but, on the other hand, the photoconversion efficiency is still far from the requirement for practical applications.
One novel methodology based on direct Z-scheme system photocatalysts is the design of dual-Z-scheme systems that ensure the efficient transmission of photogenerated carriers and further optimize the structure of the Z-scheme system.Y. Yang et al. [114] successfully constructed an efficient dual-Z-scheme heterojunction, CdS/W 18 O 49 /g-C 3 N 4 (CWOCN), by introducing CdS nanoparticles in the W 18 O 49 /g-C 3 N 4 (WOCN) composite.This dual-Z-scheme system exhibits extraordinary photocatalytic activity and stability.The TRPL measurement results demonstrated that the formation of a dual-Z-scheme system facilitates the transmission of photogenerated electrons and prolonged the electrons' lifetime, thus boosting the photocatalytic activity.It led to a prominent photocatalytic H 2 evolution rate (11,658 µmol h −1 g −1 ).This study offers guidance for exploiting and designing efficient and practical photocatalysts.
To conclude this section, there are several methodologies based on direct Z-scheme system photocatalysts used to obtain photocatalysts that are active in H 2 production.However, this technology is far from commercialization due to the low reproducibility of the materials and low quantum efficiency.ing efficient and practical photocatalysts.
To conclude this section, there are several methodologies based on direct Z-scheme system photocatalysts used to obtain photocatalysts that are active in H2 production.However, this technology is far from commercialization due to the low reproducibility of the materials and low quantum efficiency.

CO 2 Photoreduction
The great use of fossil fuels in the last decades has led to significant CO 2 emissions into the atmosphere, resulting in global warming responsible for climate change, which represents one of the greatest challenges of the current society [161,162].An innovative alternative to reduce the emissions of CO 2 into the atmosphere would be to capture CO 2 [163] and transform it into high value-added products using sunlight, mimicking natural photosynthesis [164].However, CO 2 is a stable covalent molecule due to the C=O bond, resulting in high dissociation energy (531 kJ mol −1 ) [165].CO 2 photoreduction is an alternative method to break the C=O bond using solar energy.Nevertheless, this process is a multielectron transfer process, which may result in a great variety of products with different carbon oxidation states, including CH 4 , CH 3 OH, HCHO, HCOOH, and CO [166] (see Equations ( 7)-( 13)): E 0 = −1.90V (V vs. NHE, pH 7, 25 • C, 1 atm) E 0 = −0.61V (V vs. NHE, pH 7, 25 • C, 1 atm) E 0 = −0.53V (V vs. NHE, pH 7, 25 • C, 1 atm) E 0 = −0.48V (V vs. NHE, pH 7, 25 • C, 1 atm) E 0 = −0.38V (V vs. NHE, pH 7, 25 • C, 1 atm) E 0 = −0.24V (V vs. NHE, pH 7, 25 E 0 = −0.41V (V vs. NHE, pH 7, 25 • C, 1 atm) Another challenge in the photocatalytic reduction of CO 2 is that the reduction of H 2 O to H 2 (see Equation (1)) has a similar reduction potential to the reduction of CO 2 .This fact means that in the aqueous ambience, there is competition between both reactions [167].To overcome the challenges presented above in the reduction of CO 2 , the scientific community has focused on designing selective photocatalysts for the reduction of CO 2 to various high value-added products [168].Among the different photocatalysts developed, g-C 3 N 4 should be highlighted.G. Dong et al. reported that pristine g-C 3 N 4 without any cocatalyst presented selectivity in the photocatalytic reduction of CO 2 into CO under visible-light irradiation in the presence of water vapor [169].However, the efficiency of this photocatalyst was low due to the high e − -h + pair recombination rate [47].As mentioned throughout this review, an alternative used to reduce the e − -h + recombination ratio, which is one of the challenges of g-C 3 N 4 , is the design and synthesis of heterojunctions [26].Within the use of heterojunctions for the photoreduction of CO 2 , artificial direct Z-scheme photocatalysts have received great interest from the scientific community since these systems mimic the electronic processes that occur in photosynthesis to reduce CO 2 into hydrocarbons [33,34].Therefore, the study of these artificial systems for the reduction of CO 2 has grown noticeably in the last years, being a key approach to understanding the photocatalytic mechanism and to design efficient systems capable of being marketed [39,170,171].In this subsection, several direct Z-scheme photocatalysts based on g-C 3 N 4 used in CO 2 photoreduction are summarized in Table 3.In addition, a detailed discussion of the most relevant photocatalyst Z-scheme from a synthesis, catalytic activity, and reaction mechanism perspective will be described in the main text.
W. Yu et al. [172] described the coupling effect of ZnO for the improved photoactivity of g-C 3 N 4 and the authors also proposed a direct Z-scheme mechanism to explain the better performances of the g-C 3 N 4 /ZnO binary composite photocatalytic system.The g-C 3 N 4 /ZnO photocatalytic system exhibited enhanced photocatalytic activity for CO 2 reduction compared with pure g-C 3 N 4 , with selectivity towards CH 3 OH.This enhancement of photocatalytic CO 2 reduction activity is attributed to the highly efficient ZnO-to-g-C 3 N 4 electron transfer occurring at the intimate contact interface between the g-C 3 N 4 and ZnO phases.This work provided new insights into the rational construction of a g-C 3 N 4 -based photocatalytic system and the design of a direct Z-scheme system without an electron mediator for the photocatalytic CO 2 reduction reaction.To enhance the photocatalytic activity of the materials, Z. Jiang et al. [170] described that the Z-scheme hybrid material should present several characteristics to accelerate the reduction process, including a 3D hierarchical structure and preferably basic sites, which promote CO 2 adsorption.In addition, these systems should present an efficient separation of the e − -h + pairs and an enhancement in the reduction character of electrons in the conduction band of g-C 3 N 4 .In this work, a hierarchical direct Z-scheme hybrid was developed by combining 3D urchinlike α-Fe 2 O 3 and g-C 3 N 4 photocatalysts (Figure 11a) for the photocatalytic reduction of CO 2 without the use of any sacrificial agent or cocatalyst.The hierarchical system improved the photocatalytic reduction of CO 2 to form CO (27.2 µmol g −1 h −1 ), and the system was stable for four cycles (Figure 11a).N.T. Thanh Truc et al. [173] synthesized a Nb-TiO 2 /g-C 3 N 4 direct Z-scheme system, in which the photo-excited e − in the CB of the Nb-TiO 2 combines with the photo-excited h + in the VB of the g-C 3 N 4 , preserving the existence of e − in the CB of the g-C 3 N 4 and h + in the VB of Nb-TiO 2 .Thus, the established Nb-TiO 2 /g-C 3 N 4 direct Z-scheme system produced a huge amount of available e − /h + pairs for the reduction of CO 2 into various valuable fuels.Moreover, the authors observed that the photo-current response of the prepared Nb-TiO 2 /g-C 3 N 4 (~57.55µA/cm 2 ) was much higher than that of g-C 3 N 4 (~3.92µA/cm 2 ) and Nb-TiO 2 (~12.41µA/cm 2 ) under the full visible light spectrum (Figure 11b).This result showed that the photo-excited e − in the CB of the Nb-TiO 2 tends to combine with photo-excited h + in the VB of the g-C 3 N 4 , preserving the existence of e − in the CB of the g-C 3 N 4 and h + in the VB of the Nb-TiO 2 , indicating the power of this characterization technique to address the Z-scheme mechanism.M. Lu et al. [92] synthesized 2D/2D ultrathin nanosheets of g-C 3 N 4 /BiVO 4 Z-scheme heterojunction via thermal-polymerization and a subsequent hydrothermal method.The 2D/2D g-C 3 N 4 /BiVO 4 ultrathin nanosheets exhibited a great photocatalytic evolution rate for CO (145 µmol g −1 h −1 ) and CH 4 (133 µmol g −1 h −1 ) (Figure 11c).The study presented by Lu et.al. evidenced the relevance of using 2D materials.Indeed, the direct Z-scheme heterojunction and face-to-face interfacial contact between 2D g-C 3 N 4 and 2D BiVO 4 accelerated the charge transfer and separation, improving the catalytic activity of these systems in CO 2 reduction.B. Tahir et al. [117] designed 3D/2D WO 3 /g-C 3 N 4 microspheres with an effective interfacial contact using a facile single-step hydrothermal method.The direct growth of WO 3 microspheres with g-C 3 N 4 enables good interaction among both semiconductors, enabling proficient charge carrier separation (Figure 11d).The synergistic effect with the larger interfacial contact area and proficient charge carrier separation of this system improves the CO and CH 4 production over WO 3 /g-C 3 N 4 (145 and 133 µmol g −1 h −1 , respectively), which are 2-and 4-fold higher than the use of pristine g-C 3 N 4 .This work reveals that effective interfacial contact between both semiconductors present on the Z-scheme photocatalyst is a key factor to promoting the photocatalytic CO 2 conversion to solar fuels under visible light irradiation.J. Zhao et al. [174] showed the development of a novel direct Z-scheme Bi 19 S 27 Br 3 /g-C 3 N 4 composite using the ionic liquid-assisted solvent-thermal method.The Bi 19 S 27 Br 3 /g-C 3 N 4 composites showed enhanced CO 2 photoreduction activity for the production of CO (12.87 µmol g −1 h −1 ).The authors observed in this work that the C-S bond boosts the transfer of photogenerated charge between Bi 19 S 27 Br 3 and g-C 3 N 4, as shown in Figure 11e.Therefore, the construction of a chemical bond-bridged direct Z-scheme is a promising strategy for precisely tailoring the photogenerated charge separation direction of the photocatalyst.
In conclusion, although the synthesis of Z-schemes has been successfully achieved and they are active in CO 2 photoreduction, nowadays, the photocatalytic efficiencies of Z-schemes are still too low for their commercialization.

Conclusions and Outlook
In conclusion, g-C 3 N 4 -based direct Z-scheme photocatalysts are innovative alternatives for overcoming the main drawbacks of the parent material, i.e., high e − -h + recombination rate and inadequate redox potential.In this review, a comprehensive overview of the recent research on g-C 3 N 4 -based Z-scheme photocatalysts is described, focusing on the utilization of these systems in three environmental applications, such as pollutant abatement, H 2 evolution, and CO 2 reduction.There is undoubtedly a substantial amount of ground to cover by the scientific community since there are few reports where the synthesis and catalytic activity of novel g-C 3 N 4 -based direct Z-scheme photocatalysts have been studied.Especially, several synthetic methodologies (hydrothermal, solid-state, impregnation, etc.), characterization techniques to study the properties of Z-schemes (EPR, PL . . .), and the application of these materials in environmental reactions have been investigated, focusing on the effect of the catalysts and their catalytic activity to obtain efficient solar photocatalysts.Nevertheless, the photocatalytic efficiencies are still low, and many catalytic mechanisms remain unclear.These characteristics represent the main drawbacks hindering the commercialization of these systems.The challenges for new researchers in the design and applicability of g-C 3 N 4 -based direct Z-scheme photocatalysts might be in the synthetic methodology of this photocatalyst, focusing on the interaction between g-C 3 N 4 and another SC, since the intimate contact between both systems has a strong influence on the photocatalytic activity.
The effect of the dimensionality and surface chemistry of g-C 3 N 4 and the heterocounterpart present in the Z-scheme photocatalyst is a paradigm in heterostructure synthesis, and thus, in the improvement of the photocatalytic activity.Although the transfer directions of photogenerated charge carriers in Z-scheme systems are studied by EPR, photoluminescence spectroscopy, reactive species scavenging experiments, and theoretical calculations, it is necessary to develop more powerful characterization tools to investigate the charge-transfer mechanism.Subsequently, it is also mandatory to understand the electron photoexcitation, trapping, and migration at the interface through a series of operando characterization techniques and computational studies.
Regarding the applicability of Z-schemes in environmental reactions, it is essential to develop new techniques to elucidate the mechanisms of catalytic reactions in all the stages of these catalytic processes (adsorption, reaction, and desorption), considering that the monitoring of the reaction is sometimes complex due to the formation of by-products.This scenario opens the door for the design and engineering of new Z-schemes based on g-C 3 N 4 , and the development of characterization techniques and studies of new mechanisms to obtain active photocatalysts in environmental applications under visible light.
photocatalysis, including a low specific surface area, insufficient visible light utilization, and, the most relevant, rapid recombination of photogenerated charge carriers[23].

Figure 4 .
Figure 4. (a) Scheme of the charge separation and transformation process mechanism of natural photosynthesis.Adapted from [33].Copyright 2019, Elsevier.(b) Z-scheme schematic diagram and (c) type II schematic diagram.Adapted from [28].

Figure 6 .
Figure 6.Scheme of representative synthesis and characterization methodologies of g-C3N4-based direct Z-schemes.

Figure 6 .
Figure 6.Scheme of representative synthesis and characterization methodologies of g-C 3 N 4 -based direct Z-schemes.
. In addition, S. Wang [132] et al. showed that the use of α-Fe 2 O 3 /g-C 3 N 4 in TC degradation could improve the photocatalytic activity of the isolated components due to the formation of interfacial -NH/-NH 2 groups on the g-C 3 N 4 surface.The synergy between the two components is determined by the fact that the defect-rich g-C 3 N 4 improves the absorption of visible light on the material and α-Fe 2 O 3 diminishes the recombination rate.

Table 2 .
Summary of direct Z-scheme photocatalysts based on g-C 3 N 4 for water splitting.

Table 2 .
Summary of direct Z-scheme photocatalysts based on g-C3N4 for water splitting.

Table 3 .
Summary of direct Z-scheme photocatalysts based on g-C 3 N 4 for CO 2 photoreduction.