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 carbon-based 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
3N
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
3N
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
3N
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
3N
4, i.e., morphological control, doping elements, deposition of noble metals, and construction of heterojunctions [
24,
25].
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
3N
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
3N
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
3N
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.
2. Direct Z-Scheme Photocatalysts Based on g-C3N4
Inspired by natural photosynthesis, 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 [
34]. The traditional Z-scheme introduced by Bard [
63] needs a shuttle redox mediator (electron acceptor/donor pair) to form a liquid-phase 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 Z-scheme 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
3N
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].
3. 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 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
3N
4, there are several methods [
58,
64], such as varying the texture of
g-C
3N
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.
Solid-state synthesis. This methodology is broadly used in the synthesis of pristine
g-C
3N
4 materials [
47,
70] and it is most employed to synthesize Z-scheme photocatalysts based on
g-C
3N
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
3N
4 in the presence of pre-synthesized WO
3 to obtain a direct Z-scheme
g-C
3N
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
3N
4. Although this methodology is widely used in the synthesis of the
g-C
3N
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
3N
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
3N
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
3N
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 controlled 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
3N
4-based Z-scheme photocatalysts. Jo et al. reported the synthesis of Z-scheme
g-C
3N
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
3N
4, synthesizing a C
3N
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
3N
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
3N
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 photo-deposition 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
2O
3/
g-C
3N
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 [
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.
In conclusion, g-C3N4-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 for their use in environmental applications, being the synthesis and characterization methodology of material a crucial aspect in this topic.