Recent progress on Z- and S-scheme photocatalysis: mechanistic understanding toward green applications

Shahnaz Ghasemi, Ali Parastesh, Mohsen Padervand, Haitao Ren, Ximing Li, Abdelkader Labidi, Michela Signoretto, Elmuez A Dawi, Tayebeh Hamzehlouyan, Eric Lichtfouse, et al.

► To cite this version:

Shahnaz Ghasemi, Ali Parastesh, Mohsen Padervand, Haitao Ren, Ximing Li, et al. Recent progress on Z- and S-scheme photocatalysis: mechanistic understanding toward green applications. Current Opinion in Chemical Engineering, 2025, 47, pp.101059. 10.1016/j.coche.2024.101059. hal-04862745

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# Recent progress on Z- and S-scheme photocatalysis: mechanistic understanding toward green applications

Shahnaz Ghasemi<fnref n="1" />, Ali Parastesh<fnref n="2" />, Mohsen Padervand<fnref n="3" />,<fnref n="4" />,<fnref n="5" />,<fnref n="6" />,
Haitao Ren<fnref n="7" />, Ximing Li<fnref n="7" />, Abdelkader Labidi<fnref n="7" />,

Michela Signoretto<fnref n="6" />, Elmuez A Dawi<fnref n="8" />, Tayebeh Hamzehlouyan<fnref n="2" />,
Eric Lichtfouse<fnref n="9" /> and Chuanyi Wang<fnref n="4" />,<fnref n="5" />

Photocatalysis has been widely used to address the environmental issues and energy crises that threaten the future of planet Earth. One of the main drawbacks to developing photocatalysts for practical applications is the electron-hole recombination concept, which seriously hinders the photoreaction rate. To resolve this, heterojunctions with different patterns, including Z and S schemes, showed great potential to enhance photoactivity and thus attracted increasing attention. Herein, we concisely reviewed recent progress in various types of such systems, focusing on the mechanistic understanding of clean energy and environmental applications. The principles of constructions based on optoelectronic properties and semiconducting behavior are comprehensively discussed. The advantages and disadvantages of each system are also considered to make a logical conclusion and inspirational perspectives.

## Addresses

## Introduction

The recent global industrialization accompanied by a population boom has caused severe energy crises and environmental issues worldwide [1–3]; therefore, proper steps are needed to address these energy and environmental problems. Photocatalysis is a desirable approach for these purposes due to its eco-friendly nature, versatility, energy efficiency, and effectiveness [4]. However, an obstacle causing a diminished photocatalytic performance is the rapid recombination of electron-hole pairs, as they play a crucial role in the reduction–oxidation processes on the surface of photocatalysts. Several strategies, including creating heterojunctions, elemental doping, and defect engineering, were implemented to minimize the recombination rate [5]. However, single-component photocatalysts usually possess low light adsorption capacity and, therefore, low quantum efficiency, which is undesirable for the purposes we look forward to.

Heterojunctions, which are combinations of two or more components, however, have enhanced light adsorption

<footnote n="1">¹Sharif Energy, Water, and Environment Institute, Sharif University of Technology, Azadi Avenue, PO Box 11365-8639, Tehran, Iran</footnote>

<footnote n="2">²Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran</footnote>

<footnote n="3">³Department of Chemistry, Faculty of Science, University of Maragheh, PO Box 55181-83111, Maragheh, Iran</footnote>

<footnote n="4">⁴Guangzhou Institute of Industrial Intelligence, Guangzhou 511400, China</footnote>

<footnote n="5">⁵Guangdong Engineering and Technology Research Center for Environmental Purification and Functional Materials, Guangzhou 511400, China</footnote>

<footnote n="6">⁶CATMAT Lab, Department of Molecular Sciences and Nanosystems, Ca' Foscari University of Venice, Via Torino 155, Venice 30172, Italy</footnote>

<footnote n="7">⁷School of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China</footnote>

<footnote n="8">⁸College of Humanities and Sciences, Department of Mathematics and Science, Ajman University, PO Box 346, Ajman, United Arab Emirates</footnote>

<footnote n="9">⁹State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, PR China</footnote>
capacity and higher electron-hole pair separation com-
pared to single-component materials. These properties
suggest that developing heterojunctions is a more ad-
vantageous strategy for achieving our clean energy goal
and reducing environmental contaminants. Researchers
have recently developed different heterojunction types,
including types I, II, III, and Z-scheme photocatalysts
[6]. However, these conventional junctions suffer a se-
vere drawback, which is the reduced redox capability of
the photocatalyst despite their enhanced electron-hole
pairs separation. However, among these types of het-
erojunction, Z-scheme photocatalysts have been more
widely studied as they offer a higher redox capability.

Nevertheless, to address this downside, in 2019, Xu et al. [6] proposed a new type of heterojunction called Step-scheme or, in short, S-scheme. This heterojunction offers the heightened separation efficiency of conventional heterojunctions and an enhanced redox capability simultaneously. Many researchers, therefore, have dedicated their work to S-scheme photocatalysts and their environmental applications [3,7–9]. This is not to say that heterojunction formation is the ultimate strategy, and even S-schemes have their shortcoming. That is why most recent works focus on enhancing the light adsorption and redox capability of Z-scheme and S-scheme instead of solely developing heterojunctions [3,10,11]. The modification methods, which are the same as those suggested for single-component photocatalysts, such as elemental doping, can be implemented in Z- or S-scheme heterojunctions to maximize the light adsorption and redox capacity.

This review aims to offer an overview of different het-
erojunctions and their photocatalytic mechanism by
methodically classifying the existing types of hetero-
junctions. The main focus, however, is on the recent
development of Z- and S-scheme heterojunctions and
the strategies to enhance their photocatalytic activity
and redox capability in environmental applications such
as H₂ production, CO₂ photoreduction, and pollutant
degradation. This comprehensive approach provides a
novel perspective by integrating an in-depth under-
standing of photocatalytic mechanisms with practical
applications to highlight the knowledge needed to boost
the desired performance of traditional photocatalysts.
Overall, the novelty of the study lies in overcoming the
limitations of existing heterojunctions, the strategic
focus on optimizing photocatalytic performance, and the
comprehensive review linking scientific mechanisms to
environmental applications.

**Types of photocatalytic constructions**

**Conventional heterojunctions**

Based on the pathways of charge carriers mobility, three
categories of heterojunction photocatalysts (PS) have

been identified, including the type I or ‘straddling gap’,
type II, or ‘staggered gap’, and type III or ‘broken gap’
PS (Figure 1). In type I, both electrons and holes are
accumulated on the conduction band (CB) and valence
band (VB) of PS I, respectively [12]. Since charge car-
riers are confined within the same PS, the spatial charge
separation is insufficient due to the rapid recombination
rate (Figure 1a). Therefore, type I is less capable of
harnessing the energy of separated charge carriers for
photocatalytic reactions. In type II, both the CB and VB
of PS I exhibit higher energy levels than those of PS II
(Figure 1b), which means electrons are at a more nega-
tive potential in PS I (reductive), while holes are at a
more positive potential in PS II (oxidative). Therefore,
electrons tend to move to the CB of PS II, while holes
accumulate on the VB of PS I. Consequently, improved
charge carrier separation, reduced recombination prob-
ability, and increased charge carrier lifetimes can be
expected for type II [13]. Meanwhile, type II hetero-
junctions also demonstrate a reduced redox ability si-
milar to that of heterojunction I since the oxidation and
reduction both take place on the CB and VB with lower
potentials. Type III is comparable to type II, with the
distinction that the staggered gap is too broad, pre-
venting the CB and VB of two PSs from contacting
(Figure 1c). Hence, due to the band position in type III
junctions, charge transfer does not even occur [14].

Therefore, it is evident that type II heterojunctions
demonstrate the most advantageous potential for spatial
charge separation among these three heterojunction
types. On this basis, a large number of researchers have
worked on this type of heterojunction, an example of
which is magnetically recoverable Sn₃O₄/SnFe₂O₄ for
degradation of ciprofloxacin (CIP) [15] (Figure 2). This
type II heterojunction was found to have a 4.87 and 3.02
times faster degradation efficiency for CIP compared to
pure Sn₃O₄ and SnFe₂O₄, respectively. Additionally,
WO₃/Bi₂WO₆ for reduction of Cr (VI) and rhodamine B
(RhB) degradation is also another successful example of
type II heterojunction development [16]. In this work, it
was shown that by establishing a type II heterojunction
consisting of Bi₂WO₆ and WO₃, the degradation effi-
ciency of RhB and reduction efficiency of Cr (VI) was
1.63 and 1.95 times higher than those for pure Bi₂WO₆
reaching 98.82% and 53.85% in 60 min, respectively.
These examples are just a few recent works indicating
the higher photocatalytic activity of type II photo-
catalysts caused by the heightened spatial charge se-
paration in this heterojunction type.

Z-scheme photocatalysts

Traditional Z-scheme

The Z-scheme heterojunction was introduced by Bard to
overcome the restricted redox capability of conventional
heterojunctions [17]. Within this context, two
<figure><img src="image_1.png" /><figcaption>Figure 1</figcaption></figure>

Type I (a), II (b), and III (c) heterojunctions.

photocatalysts directly interact through appropriate in-
termediate redox couples, such as Fe³⁺/Fe²⁺, IO³⁻/I⁻, and
I³⁻/I⁺. The photogenerated holes in the VB of PS I in-
teract with donor (D) species, producing the corre-
sponding acceptor (A) species. Meanwhile, the
photoexcited electrons in the CB of PS II interact with

species A, forming the respective donor. Moreover, the
holes on VB of PS II participate in oxidation pathways,
while the electrons on CB of PS I take part in reduction
(Figure 3a). Implementing this particular scheme is
constrained by its reliance on intermediate pairs, limiting
its applicability to solutions and side reactions which

<figure><img src="image_2.png" /><figcaption>Figure 2</figcaption></figure>

Type II heterojunction photocatalyst for CIP degradation [15].
occur in practical scenarios and interfere with the charge transfer events.

An example of the traditional Z-scheme photocatalysts is the co-doped $SrTiO_3$ with rhodium (Rh) and antimony (Sb) developed by Li et al. [18]. This ionic Z-scheme photocatalyst was used for an efficient water-splitting reaction with $Fe^{+2}/Fe^{+3}$ as the redox couple. Fundamentally, $IrO_2/SrTiO_3$:Rh/Sb (PC-A) and Ru/SrTiO_3:Rh (PC-B) form a Z-scheme photocatalyst, and $Fe^{+2}$ and $Fe^{+3}$ as acceptor and donor interact with the electrons in the CB of PC-A and holes in the VB of PC-B. This way, the electrons in the CB of PC-B and holes in the VB of PC-A can participate in the redox and oxidation reactions, respectively. This mechanism offers the double benefit of a high spatial charge separation and redox capability.

As discussed, in practical applications, the traditional Z-scheme has lost its appeal as the mechanism described for this type of heterojunction is prone to undesirable side reactions, resulting in a lower redox capability and a lower charge separation. Furthermore, traditional Z-scheme photocatalysts require a solution phase for their redox couples to be involved. These issues have tipped the researchers toward developing solid z-scheme photocatalysts.

### All-solid Z-scheme

The ‘All-solid Z-scheme’ addresses the constraints in traditional Z-scheme junctions [19] by employing a conductive solid (Ag, Pt, and Au) for charge transfer instead of ion pairs. Despite the alterations in the charge mediator, the fundamental principle remains unchanged, in which both electrons and holes follow the same trajectory. As illustrated in Figure 3b, they engage with the metal instead of interacting with a donor or acceptor. This design offers the dual benefit of having a substantial redox capacity, promoting efficient separation of electron-hole pairs. However, specific issues persist, including the challenging synthesis processes, the light-shielding properties of the solid materials, as well as the expensive nature of solid mediators, which is a hurdle in the way of large-scale industrialization of this type of heterojunction.

An example of an all-solid Z-scheme is $g-C_3N_4$/Pt/TiO$_2$ for efficient $CO_2$ to $CH_4$ conversion [20]. In this work, platinum is used as the electron mediator, where it engages with electrons in the CB of TiO$_2$ and holes in the VB of $g-C_3N_4$. This way, the electron in CB of $g-C_3N_4$ with the more negative electro-potential will engage in reduction, while the holes in VB of TiO$_2$ with a more positive electro-potential engage in oxidation. This mechanism offers a high spatial charge separation as well as a high redox capacity, yielding 6.56 µmol h$^{-1}$ of methane in 60 min under visible light. The problem,

however, persists due to the expensive nature of the mediators, which prevents researchers from the large-scale fabrication of All-solid Z-scheme photocatalysts.

### Direct Z-scheme

The latest progress in the Z-scheme junction family was called ‘Direct Z-scheme’ [21]. The attraction of this specific junction lies in its inherent feature that eliminates the requirement for a charge mediator to facilitate the directed flow of electrons and holes. In this regard, the two PSs must possess different work functions. Electrons are transferred from PS I, which has a lower work function, to PS II with a higher work function. Consequently, an excess of negative charge is present on the surface of PC II, while the surface of PS I keeps a positive charge. The accumulation of charges leads to the generation of an internal electric field, facilitating the interaction between the electrons of PS II’s CB and PS I’s VB, establishing the vital charge pathway (Figure 3c).

Compared to type II heterojunction, direct Z-scheme is more physically possible as the electrostatic attraction between the holes and electrons favors the proposed mechanism for this type of heterojunction. In the case of type II, though, there is electrostatic repulsion between electrons and holes, which makes the mechanism less favorable. This is why direct Z-scheme heterojunctions have been the subject of much research.

An example of this type of heterojunction is 3D/2D direct Z-scheme $Zn_2SnO_4$/CdS for methyl orange and Cr (VI) removal under visible light [22]. In this work, the direct contact of $Zn_2SnO_4$ and CdS results in the formation of a direct Z-scheme heterojunction with enhanced redox capacity and heightened charge separation without needing an electron mediator. Fundamentally, the electron in the CB of $Zn_2SnO_4$ directly interacts with the holes in the VB of CdS, which leaves the electron in the CB of CdS and the holes in the VB of $Zn_2SnO_4$ free to participate in reduction and oxidation reactions, respectively.

### S-scheme

The step-scheme or S-scheme heterojunction consists of two photocatalysts; reduction photocatalyst (RP) and oxidation photocatalyst (OP) [23]. The RP possesses an effective CB but a noneffective VB for the movement of holes. In contrast, the scenario is reversed in the case of OP, resembling the characteristics of a type II photocatalyst. On the other hand, the charge-carrier path for the S-scheme is fundamentally different. In the S-scheme, electrons located on the CB of OP interact with the holes positioned on the VB of RP. This enables electrons on the CB of RP to participate in reduction processes, while holes on the VB of OP engage in oxidation reactions (Figure 4). This mechanism facilitates
<figure><img src="image_3.png" /><figcaption>**Figure 3**</figcaption></figure>

Traditional Z-scheme (a), All-solid Z-scheme (b), and direct Z-scheme heterojunctions.

efficient charge separation and maximizes redox cap-
abilities.

It is important to note that both the CB and fermi levels
of the reduction photocatalyst must be higher than that
of the oxidative photocatalyst for the S-scheme hetero-
junction to form. If this condition is met, an S-scheme
heterojunction can be constructed of either n- or p-type
photocatalysts such as p-p, n-n, and n-p S-scheme het-
erojunction.

In principle, the direct Z-scheme and S-scheme share a
similar mechanism, but Xu et al. suggested using the
new S-scheme term since the Z-scheme heterojunctions
have fundamental shortcomings and the S-scheme de-
finition they proposed was clearer and did not have the
inherent downsides of Z-scheme heterojunction fa-
mily [6].

An example of S-scheme heterojunction photocatalysts is the core–shell Co₃O₄@ZnIn₂S₄ for efficient hydrogen production under natural sunlight irradiation [7]. The result indicates 18.9 mmol h⁻¹ hydrogen production in this operational condition. To find the mechanism of heterojunction, Kebulka–Munk calculations were used, showing the band gaps of 1.68 and 2.47 eV for Co₃O₄ and ZnIn₂S₄, respectively. Furthermore, Mott–Schottky figures were also drawn, and with additional calculations, the E<sub>CB</sub> and E<sub>VB</sub> of Co₃O₄ were calculated to be 0.16 and 1.84 eV versus NHE, respectively. The same calculations for ZnIn₂S₄ were also performed, and the E<sub>CB</sub> and E<sub>VB</sub> of 1.84 and –0.93 eV versus NHE were shown, respectively. Finally, to find the work function, ultraviolet photoelectron spectroscopy was performed, showing work functions of 5.94 and 4.39 for Co₃O₄ and ZnIn₂S₄, respectively. The higher work function means

a lower Fermi level, and based on this fact, Co₃O₄ not
only has a lower Fermi level but also a lower CB com-
pared with ZnIn₂S₄. Along with the scavenger tests, this
lower Fermi level and CB suggest the formation of the
S-scheme heterojunction of Co₃O₄ and ZnIn₂S₄.

**Green energy and environmental applications**

Clean energy (hydrogen) production

Clean energy production has been a significant concern
in recent years. After introducing photocatalytic water-
splitting [24], the production process of clean hydrogen
using only water and light appeared as an attractive
means of clean energy production. As discussed, het-
erojunctions benefit from broader light adsorption
spectra and lower electron–hole pairs recombination in
the field of photocatalysts. These advantages mean that
heterojunctions can maximize light harvesting and pro-
duce higher amounts of hydrogens than single-compo-
nent photocatalysts. In section *Types of photocatalytic
constructions*, different types of heterojunction were in-
troduced; among them, the Z- and S-schemes are the
most attractive as they offer a lower recombination rate
and a higher redox ability simultaneously. In the case of
the Z-scheme, three types were introduced, two of
which are constrained by their need for electron medi-
iators, limiting their application. The third type, known
as direct Z-scheme heterojunction, shares a mechanism
similar to that of S-scheme heterojunctions, although
their fundamental definition is different. An S-scheme
heterojunction consists of two photocatalysts, one of
which must be reductive, while the other one is oxida-
tive, and the CB and the Fermi levels of the oxidative
photocatalysts must be lower than that of reductive
photocatalysts. Meanwhile, the Z-scheme heterojunc-
tion definition is less clear, and the only distinction it has
to other Z-schemes is its mechanism, which is similar to
that of S-schemes.

Nevertheless, researchers have taken further steps to
maximize H₂ production in recent years. For instance,
surface engineering, precious metal deposition, and
elemental doping are the main methods applied to en-
hance the performance of heterojunctions even further.
Table 1 presents a compilation of some recent hetero-
junctions modified for water splitting.

Stacking two-dimensional (2D) materials precisely cre-
ates a Van der Waals heterostructure. Van der Waals
forces typically hold these 2D materials together rather
than strong covalent or ionic bonds [31]. A metal-free
2D/2D g-C₃N₄/C-BN/Pt is a practical example of a Z-
scheme Van der Waals heterostructure [25]. By mea-
suring the photocatalytic activity of 2D/2D g-C₃N₄/C-
BN/Pt, the sample with a 0.5% BCN to CN weight ratio
exhibited the highest rate of H₂ evolution. This rate was
2.58 times higher than that observed for a single CN
<table><caption>**Table 1**</caption><caption>Recent S- and Z-scheme heterojunctions for green H<sub>2</sub> evolution.</caption><thead><tr><th>Sample</th><th>Heterojunction type</th><th>H<sub>2</sub> yield (mmol/g.h)</th><th>Ref.</th></tr></thead><tbody><tr><td>g-C<sub>3</sub>N<sub>4</sub>/C-BN/Pt</td><td>Z-scheme</td><td>3.35</td><td>[25]</td></tr><tr><td>SnIn<sub>4</sub>S<sub>8</sub>/CeO<sub>2</sub></td><td>Z-scheme</td><td>0.61</td><td>[26]</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub>/Bi<sub>3</sub>TaO<sub>7</sub></td><td>S-scheme</td><td>2.45</td><td>[27]</td></tr><tr><td>Ag-C<sub>3</sub>N<sub>4</sub>/SnS<sub>2</sub></td><td>Z-scheme</td><td>1.10</td><td>[28]</td></tr><tr><td>Au-ZnIn<sub>2</sub>S<sub>3</sub>/NaTaO<sub>3</sub></td><td>S-scheme</td><td>11.4</td><td>[11]</td></tr><tr><td>Mo<sub>2</sub>C@ZnIn<sub>2</sub>S<sub>4</sub></td><td>S-scheme</td><td>26.1</td><td>[29]</td></tr><tr><td>CuI-GDY/ZnAl-LDH</td><td>S-scheme</td><td>0.03</td><td>[10]</td></tr><tr><td>FeS<sub>2</sub>@ZnIn<sub>2</sub>S<sub>4</sub></td><td>S-scheme</td><td>5.05</td><td>[30]</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Figure 4**</figcaption></figure>

S-scheme heterojunction construction.

sample (3.35 mmol g⁻¹.h⁻¹ vs 1.29 mmol g⁻¹.h⁻¹). As shown in Figure 5, the heterostructure design has notably improved the separation of electrons and holes, as confirmed by the results from both transient photo-current responses and Nyquist plots.

Another study reported the S-scheme Au-ZnIn₂S₃/NaTaO₃ prepared by a solvothermal approach [11]. According to the H₂ evolution tests, the sample containing 1% Au and 25% NaTaO₃ exhibited the highest production yield, which was 58 times higher than the reaction yield over pure NaTaO₃ and 10 times greater than that of ZnIn₂S₃. The charge carrier separation capability and interfacial charge transfer resistance of the prepared sample were assessed by photoelectrochemical tests such as transient photoresponse (TPR) and electron spin resonance (EIS), which supported the aforementioned results.

While the high recombination rates of single-component semiconductors remain a significant challenge, composite semiconductors with S- and Z-scheme mechanisms offer a considerable potential to efficiently separate the electron-hole pairs, leading to inspirational efforts to industrialize the photocatalytic H₂ generation approaches.

## H₂O₂ production

Being in the top 100 most important chemicals, the production of hydrogen peroxide has attracted much attention ever since its first synthesis. Its applications vary from energy to the environment and even the medical industry. H₂O₂ has significant amounts of active oxygen, and for this reason, it is a highly efficient oxidant for environmental remediation purposes. It is also a green oxidant with safe reaction products, such as water (H₂O) and oxygen (O₂). An example of the practical application of hydrogen peroxide is its role as an oxidizing agent in the advanced oxidation process, producing hydroxyl radicals after being irradiated with UV light. Additionally, hydrogen peroxide can be directly used for water disinfection, and it is considered a cleaner approach as it does not produce any hazardous by-products [32].

Self-fenton red mud (RM)/CdS S-scheme for degradation of amoxicillin (AMX) is an example of H₂O₂ application in the literature [33]. In this work, hydrogen peroxide is considered to produce hydroxyl radicals, which, in turn, can effectively degrade AMX. It was found that 2% RM/CdS generated 1.05 mg L⁻¹.h⁻¹ of H₂O₂ in 2 hours. One of the main advantages of this work was the photocatalytic production of H₂O₂ without the need for any external oxidants. This source of H₂O₂, which transforms into hydroxyl radicals on top of the hydroxyl radical of the S-scheme photocatalyst itself, could maximize AMX degradation. Meanwhile, the formation of the S-scheme was confirmed with work function calculations using Kubelka-Munk equations and Mott-Schottky plots, as well as photoelectrochemical tests such as TPR and EIS.

## CO₂ reduction

The world's energy demands are rising significantly, with the majority of this energy still being sourced from fossil fuels. Buring these fuels to produce energy releases a significant amount of pollution into the air, mainly in the form of carbon dioxide. Different strategies for CO₂ capturing and sequestration have been suggested, including photocatalytic CO₂ reduction or CO₂ photoreduction. The main appeal of this approach lies in its low energy consumption and the valuable byproducts it yields [34]. In practical terms, CO₂ photoreduction serves the dual purpose of mitigating pollution and harnessing solar energy to produce hydrocarbon fuels such as methane, formic acid, and methanol. These products can be categorized into liquid fuels (CH₃OH and HCOOH) and low calorific gaseous fuels (CH₄ and CO) [35]. The basic steps involved in this process are as follows: first, CO₂ is adsorbed on the catalyst's surface, then the light source causes excitation and electron-hole pairs generation. Then, charge transfer and separation occur, and while a certain amount of this charge is recombined on the surface of the
<caption>**Table 2**</caption>

**Recent S- and Z-scheme heterojunctions for CO₂ reduction.**

<table><thead><tr><th>Sample</th><th>Heterojunction type</th><th>Conversion yield (µmol/g.h)</th><th>Ref.</th></tr></thead><tbody><tr><td>CPDs/Bi<sub>4</sub>O<sub>5</sub>Br<sub>2</sub></td><td>Z-scheme</td><td>CO: 134.42</td><td>[37]</td></tr><tr><td>ZnFe<sub>2</sub>O<sub>4</sub>/BiMoO<sub>6</sub></td><td>S-scheme</td><td>CO: 32.58<br>CH<sub>4</sub>: 14.52</td><td>[38]</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub>/CQD/BiOIO<sub>3</sub></td><td>Z-scheme</td><td>CO: 117.88</td><td>[39]</td></tr><tr><td>BP:La/InVO<sub>4</sub>:La</td><td>Z-scheme</td><td>CO: 11.96</td><td>[40]</td></tr><tr><td>Bi<sub>2</sub>S<sub>3</sub>/BiVO<sub>4</sub>/Mn<sub>0.5</sub>Cd<sub>0.5</sub>S</td><td>S-scheme</td><td>CO: 44.74</td><td>[41]</td></tr><tr><td>ZnS/ZnIn<sub>2</sub>S<sub>4</sub></td><td>Z-scheme</td><td>CH<sub>3</sub>OH: 1.37<br>C<sub>2</sub>H<sub>4</sub>O: 376.63</td><td>[42]</td></tr><tr><td>Cs<sub>2</sub>AgBiBr<sub>6</sub>/Bi<sub>2</sub>WO<sub>6</sub></td><td>Z-scheme</td><td>CO: 42.19<br>CH<sub>4</sub>: 0.415</td><td>[43]</td></tr><tr><td>S-Bi<sub>2</sub>WO<sub>6</sub>/NiO</td><td>S-scheme</td><td>CO: 46.90<br>CH<sub>4</sub>: 12.10</td><td>[44]</td></tr><tr><td>Bi<sub>5</sub>O<sub>7</sub>I-Ovs/Cd<sub>0.5</sub>ZnO<sub>0.5</sub>S</td><td>S-scheme</td><td>CO: 58.60</td><td>[8]</td></tr></tbody></table>

catalyst, the other part interacts with the CO₂ and reduces it to specific products with different electro-potential levels.

Numerous approaches can actively and selectively convert CO₂ to CH₄ and other hydrocarbon fuels [36]. Nevertheless, carbon dioxide photoreduction is accompanied by some significant thermodynamic challenges, requiring a substantial redox potential. In this regard, S and Z-scheme heterojunctions exhibit promising potential in developing composite materials with high redox capability and efficient solar light absorption that can effectively address the challenging process of CO₂ photoreduction. Additionally, the conversion of carbon dioxide into each of the above-mentioned products occurs at a different electro-potential (CO₂ to COOH at -1, CO₂ to CO and CH₃OH at -0.5, and CO₂ to CH₄ at 0 eV vs NHE). Therefore, heterojunctions, which usually cover a higher range of electro-potential, could be of extra use as they offer a higher range of photo-reduction routes. Table 2 compiles some recently

reported heterojunctions for CO₂ photoreduction purposes.

Pb-free 0D/2D Cs₂AgBiBr₆/Bi₂WO₆ was implemented to reduce CO₂ to carbon monoxide and methane [43]. The results revealed that the Z-scheme composite exhibited a much higher photocatalytic activity than the single Cs₂AgBiBr₆, with a seven-fold reaction rate increase. The synthesized composite had a production rate of 42.19 µmol g⁻¹·h⁻¹ for carbon monoxide and 0.41 µmol g⁻¹·h⁻¹ for methane. *In situ* X-ray photoelectron spectroscopy and ESR confirmed the electron transport pathway of the Z-scheme heterostructure. Electrochemical techniques (EIS and TPR) also confirmed the enhanced separation efficiency of the Z-scheme heterojunction.

**Pathogenic microorganisms disinfection**

Various methods are established for disinfecting bacteria, such as ozonation, chlorination, and UV disinfection. However, each method has its downsides, such as hazardous byproducts of ozonation and chlorination and the fatality of exposure to UV in the long term. Photocatalysis, however, is an affordable and environmentally friendly approach without any hazardous by-products for bacterial disinfection purposes [45].

In detail, photocatalysts are found to be very effective for bacteria disinfection for various kinds of bacteria such as fungi, algae, and viruses. This effectiveness, however, varies depending on whether the bacteria is Gram-positive or Gram-negative. The difference between these types of bacteria is in their cell wall structure, which for the Gram-negative type consists of an outer membrane (OM), a peptidoglycan layer (PG), and an inner membrane (IM). Meanwhile, the Gram-positive bacteria have no OM and a thicker PG. This difference indicates that photocatalytic Gram-positive bacteria disinfection is less effective than Gram-negative bacteria [46]. The mechanism for bacteria disinfection involves cell wall and cytoplasmic membrane destruction by reactive radicals, such as hydroxyl and superoxide. This causes cellular

<caption>**Table 3**</caption>

**Recent S- and Z-scheme heterojunctions for the pathogenic micro-organism's disinfection.**

<table><thead><tr><th>Sample</th><th>Heterojunction type</th><th>Bacteria</th><th>Inactivation efficiency</th><th>Ref.</th></tr></thead><tbody><tr><td>BiO<sub>2</sub>-x/BiOBr</td><td>Z-scheme</td><td>E. coli<br>S. aureus</td><td>100% in 20 min<br>100% in 20 min</td><td>[47]</td></tr><tr><td>Agl/MIL-100 (Fe)</td><td>Z-scheme</td><td>E. coli<br>S. aureus</td><td>100% in 45 min<br>100% in 45 min</td><td>[48]</td></tr><tr><td>CuBi<sub>2</sub>O<sub>4</sub>/Bi<sub>4</sub>O<sub>5</sub>I<sub>2</sub></td><td>S-scheme</td><td>E. coli<br>S. aureus</td><td>100% in 20 min<br>100% in 40 min</td><td>[9]</td></tr><tr><td>Ag@AgCl/PDI</td><td>Z-scheme</td><td>Sulfonamide ARB</td><td>99.6% in 2 h</td><td>[49]</td></tr><tr><td>MoO<sub>3</sub>-x/S-CN</td><td>S-scheme</td><td>E. coli<br>S. aureus</td><td>100% in 50 min<br>100% in 75 min</td><td>[50]</td></tr><tr><td>P-g-C<sub>3</sub>N<sub>4</sub>/Fe<sub>3</sub>O<sub>4</sub> QDs/BiOI</td><td>S-scheme</td><td>E. coli<br>S. aureus</td><td>100% in 20 min<br>100% in 40 min</td><td>[51]</td></tr></tbody></table>
<table><caption>**Table 4**

Recent S- and Z-scheme heterojunctions for pollutant degradation.</caption><thead><tr><td>Sample</td><td>Heterojunction type</td><td>Pollutant</td><td>Inactivation efficiency</td><td>Ref.</td></tr></thead><tbody><tr><td>PPy/NH<sub>2</sub>-UiO-66</td><td>Z-scheme</td><td>TC</td><td>90% in 180 min</td><td>[53]</td></tr><tr><td>BiVO<sub>4</sub>@ZnIn<sub>2</sub>S<sub>4</sub>/Bi<sub>2</sub>n<sub>2</sub>O<sub>7</sub></td><td>Z-scheme</td><td>RhB</td><td>81% in 60 min</td><td>[52]</td></tr><tr><td>BiOCl/CuBi<sub>2</sub>O<sub>4</sub></td><td>S-scheme</td><td>DFC</td><td>90% in 60 min</td><td>[54]</td></tr><tr><td>MoS<sub>2</sub>/Bi<sub>2</sub>S<sub>3</sub>/BiVO<sub>4</sub></td><td>S-scheme</td><td>FQs</td><td>92.8% in 60 min</td><td>[55]</td></tr><tr><td>ZnIn<sub>2</sub>S<sub>4</sub>/Bi<sub>4</sub>Ti<sub>3</sub>O<sub>12</sub></td><td>Z-scheme</td><td>TC</td><td>82.1% in 60 min</td><td>[56]</td></tr><tr><td>MoS<sub>2</sub>/Ag-ZnFe<sub>2</sub>O<sub>4</sub></td><td>Z-scheme</td><td>TC</td><td>94.8% in 60 min</td><td>[57]</td></tr><tr><td>Co<sub>9</sub>S<sub>8</sub>@ZnS/g-C<sub>3</sub>N<sub>4</sub></td><td>Z-scheme</td><td>CIP</td><td>98.07% in 140 min</td><td>[58]</td></tr><tr><td>Ag-AgI/FeVO<sub>4</sub>-C<sub>3</sub>N<sub>4</sub></td><td>Z-scheme</td><td>Cr (VI)</td><td>100% in 60 min</td><td>[59]</td></tr></tbody></table>

contents to leak and cell lysis; the subsequent step may involve completely mineralizing the bacteria [46]. S- and Z-scheme heterostructures offer a high spatial charge separation and a heightened redox ability that is suitable for effective bacteria disinfection. A series of recently developed heterojunctions for this purpose are listed in Table 3.

An excellent illustration for this application is the Z-scheme Ag@AgCl/PDI (perylene diimide) composite [49], where silver nanoparticles demonstrate a plasmonic effect, resulting in ameliorated light absorption. Furthermore, it was suggested that the photogenerated electrons located on the CB of PDI appear to be effectively trapped by Ag, reducing the recombination rate of electron-hole pairs within PDI. This effect is particularly significant due to the very small band gap of PDI (1.69 eV). Contrary to this, AgCl has a band gap of 3.15 eV, which prevents it from being stimulated by visible light. Nevertheless, the electrons trapped by Ag are subsequently transported to the CB of AgCl, providing an additional reduction site (O<sub>2</sub><sup>-</sup> generation) aside from the PDI's CB.

## Pollutant degradation

Rapid industrialization around the world has resulted in high amounts of pollution released into the atmosphere, which destroys the environment and endangers human species as well. A recent approach for removing various organic, inorganic, and biological contaminants from the environment is photocatalysis [52]. During this process, the hydroxyl, superoxide radicals, electrons, and holes (h<sup>+</sup>) are usually the active species responsible for eliminating the hazards. An effective photocatalyst requires a high redox capacity and a low recombination rate to enable significant pollution degradation. As such, S- and Z-scheme heterojunctions that possess enhanced redox ability and high spatial charge separation are the most favorable choices for the effective degradation of pollutants. Several examples listed in Table 4 demonstrate the simultaneous occurrence of S and Z heterojunctions.

As an instance, 2D/2D ZnIn<sub>2</sub>S<sub>4</sub>/Bi<sub>4</sub>Ti<sub>3</sub>O<sub>12</sub> Z-scheme heterostructure efficiently removed 82.1% of tetracycline (TC) within 60 min (Figure 6). Trapping experiments prove that superoxide is the dominant active species responsible for promoting the process, whereas hydroxyl

<figure><img src="image_5.png" /><figcaption>**Figure 5**</figcaption></figure>

Photocatalytic H<sub>2</sub> production mechanism over the 2D/2D g-C<sub>3</sub>N<sub>4</sub>/C-BN/Pt heterojunction [25].
<figure><img src="image_6.png" /><figcaption>Figure 6</figcaption></figure>

Degradation mechanism of 2D/2D ZnIn₂S₄/Bi₄Ti₃O₁₂ S-scheme heterojunction for efficient removal of TC [56].

and h⁺ exhibit relatively lower effective contribution in degradation. Moreover, this construction was observed to demonstrate excellent electron–hole separation and reduced interfacial charge transfer resistance through the electrochemical spectroscopic techniques (TPR and EIS), thus confirming the eminent efficacy of the heterojunction system [56].

## Conclusion and perspectives

Heterojunctions are crucial for creating an eminent photocatalyst with both high redox capability and efficient absorption of sunlight. Z- and S-scheme heterojunctions offer advantages in many applications, such as water splitting, pollutant degradation, and bacteria disinfection, due to effective charge separation and high redox potential. However, both heterojunctions have certain limitations. For instance, traditional and all-solid Z-schemes require an electron mediator, which limits their applications. While the conventional Z-scheme is primarily applicable to the solution phase, the challenge of phase restriction is effectively addressed by all-solid Z-schemes; however, the electron pathway remains a concern. The direct Z-scheme logically addresses this without suffering from the limitations of the other two types. Furthermore, the introduction of S-scheme heterojunctions drew a vivid line between Z- and S-schemes with a clear definition of these heterojunctions. Additionally, methods such as doping and surface engineering must be implemented to maximize direct Z-scheme and S-scheme photocatalytic activity. Further investigation is critically demanded to govern the charge transfer pathways in various multiphase systems.

## Data Availability

No data were used for the research described in the article.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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