Layered photocatalytic nanomaterials for environmental applications

Citation: Fang Chen, Yihe Zhang, Hongwei Huang. Layered photocatalytic nanomaterials for environmental applications[J]. Chinese Chemical Letters, 2023, 34(3): 107523. doi: 10.1016/j.cclet.2022.05.037 shu

Layered photocatalytic nanomaterials for environmental applications

English

Layered photocatalytic nanomaterials for environmental applications

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China
^* Corresponding author.
E-mail address: hhw@cugb.edu.cn (H. Huang).
Received Date: 20 December 2021
Accepted Date: 11 May 2022
Revised Date: 09 February 2022
Available Online: 15 March 2023

Abstract: The increasing pollution and human demand for a cleaner environment have made achieving the environmental sustainability a current research focus. As a "green" technology, semiconductor photocatalysis is of great significance to the environmental purification. Benefiting from the unique anisotropic crystal structure and electronic properties, layered photocatalytic nanomaterials show great potential for efficient photocatalytic environmental treatment. This review comprehensively summarizes the recent progress on layered photocatalytic nanomaterials for oxidation or reduction of pollutants in water and air along with the basic understanding of related mechanisms and developments in this field. First, the existing diversified layered photocatalysts are classified, and their different synthesis and modification strategies are discussed in detail to provide a comprehensive view of the material design that affects their photocatalytic performance. Subsequently, the extensive applications of the above-mentioned layered photocatalytic nanomaterials in environmental fields are systematically summarized, including photooxidation of water and air pollutants, and photoreduction of heavy metal pollutants, NO₃^-, BrO₃^- and CO₂. Finally, based on the current research achievements in layered photocatalysts for environmental remediation, the future development direction and challenges are proposed.

Key words:

1. Introduction

Environmental deterioration (mainly of water and air) has always been an urgent issue in modern society. The current researches on wastewater treatment mostly focus on physical absorption, biodegradable organic matter and biological compounds removal [1,2]. However, due to the extremely low biodegradability and insufficient removal rate of traditional wastewater treatment technologies, the treatment of micropollutants that endanger the health of organisms, such as drugs, dyes, phenols, and heavy metals, are still far from fulfilling the requirements. In addition, air pollutants such as CO₂, SO_x and NO_x from the combustion of fossil fuels have been considered to cause the greenhouse effect, photochemical smog and even lung diseases [3,4]. Therefore, water remediation and air purification hold great urgency and broad prospects for the development of a clean and sustainable environment [5]. Since the 1980s, photocatalytic technique has developed rapidly due to its low cost, mild reaction conditions and low energy consumption [6–8]. The environmental purification technology based on photocatalysis pioneers the endeavors for sewage and air treatment. However, conventional metal oxides based semiconducting photocatalysts suffer from low conversion efficiency of solar to chemical energy and cannot be applied on an industrial scale. The TiO₂-based photocatalytic materials with the good chemical stability and environmental compatibility have been the most exhaustively researched photocatalysts [9,10]. Despite considerable development of the band structure regulation and surface chemical modification, due to its wide band gap (3.2 eV) that only respond to UV light and low quantum yield, the application of TiO₂ to large-scale environmental treatment is still completely uneconomical methods [11]. Materials scientists have been striving to find and develop more efficient semiconductor photocatalysts [12–14]. Among them, layered photocatalytic nanomaterials have gained extensive attention of the researchers due to their unique layered structure with large surface area and remarkable physiochemical properties, providing a new way for the design, optimization and mechanism research of photocatalysts in environmental purification [15].

In this review, we systematically summarize the recent reports on layered photocatalytic nanomaterials, and classify their different types into metal containing and metal-free layered photocatalytic materials for the degradation of pharmaceuticals and industrial micropollutants degradation, NO_x removal, CO₂ reduction and Cr(Ⅵ) reduction reactions bridging the similarities and differences between them based on the photocatalytic oxidation and reduction reaction mechanism. Therein, we focused on the morphology engineering, surface modification and heterostructure construction strategies based on the unique layered structure to improve the photocatalytic activity of layered nanomaterials. Finally, the various environmental applications and the future development of layered photocatalytic nanomaterials have been prospected. As far as we know, this will be the first comprehensive review of the synthesis, modification and application of a full range of layered photocatalytic nanomaterials. The different parts covered in this review are shown in Fig. 1.

Figure 1

Figure 1. Schematic illustration of layered photocatalytic nanomaterials for environmental applications.

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2. Metal-containing layered photocatalytic nanomaterials

As an important part of photocatalytic materials, metal-containing layered photocatalytic nanomaterials have received the most extensive research in environmental applications. They can be typically divided into the following types: layered perovskite oxides, layered bismuth oxyhalides, layered double hydroxides (LDH), MXene and transition metal dichalcogenides (TMDs, Table S1 in Supporting information).

2.1 Layered perovskite oxides

Layered perovskite oxides are widely used in the environmental field due to their semiconducting properties. The structure consists of alternate layered stacks of discontinuous perovskite slabs separated by the adjacent layers by weak electrostatic interactions [16]. These distorted perovskite-like structures are similar to the general ABO₃ type and have the ordered corner-sharing BO₆ units [17]. According to the stacking orientation of the ideal cubic perovskite structure layers, the layered perovskite oxides can be classified as (100), (111) and (110) types [18]. For the (100) type, Aurivillius phase with the general formula (Bi₂O₂)(A_n−1B_nO_3n+1) that are constructed by the alternating layers of [Bi₂O₂]²⁺ and (A_n−1B_nO_3n+1) perovskite-like layers as the most well-known layered perovskite oxides is expected to exhibit efficient photocatalytic performance in environmental treatment (Fig. 2a), Bi₂WO₆ with a moderate bandgap of approximately 2.6 eV is a typical Aurivillius phase perovskite oxide and one of the most intensively researched materials in photocatalytic applications. The Bi₂WO₆ nanoplates incline to assemble into flower-like hierarchical structure. Wang et al. prepared the reduced graphene oxide wrapped Bi₂WO₆ composite, and the core-shell Bi₂WO₆@rGO exhibited outstanding photocatalytic activity in the degradation of Rhodamine B by forming strong interaction and efficient charge separation between Bi₂WO₆ and rGO (Fig. 2b) [19]. As a ferroelectric Aurivillius phase compound, BiMoO₆ has also been extensively and intensively studied. Our group synthesized the ultra-thin Bi₂MoO₆ nanosheets with strong ferroelectricity through combined CTAB-assisted hydrothermal method and corona poling post-treatment (Fig. 2c). The increase of catalytic sites and the enhancement of internal electric field synergistically increased the photocatalytic CO₂ reduction activity with a CO evolution rate of 14.38 µmol g⁻¹ h⁻¹ of the polarized Bi₂MoO₆ ultrathin nanosheets in the gas-solid system (Fig. 2d) [20]. As an effective route to promote the charge separation of particle ferroelectric photocatalysts, the corona poling has been successfully applied to another typical Aurivillius phase layered niobate perovskite oxide based photocatalyst, SrBi₂Nb₂O₉ (Fig. 2e). Similarly, our group prepared the ferroelectric SrBi₂Nb₂O₉ nanosheets by an one-pot hydrothermal route. Due to its outstanding ferroelectric polarization and anisotropic charge transfer characteristics, SrBi₂Nb₂O₉ nanosheets possess highly efficient photocatalytic activity. Further, the ferroelectric polarization was enhanced through the 30 kV/cm electric poling or 400 ℃ annealing treatment, and the SrBi₂Nb₂O₉ nanosheets revealed an outstanding CH₄ evolution rate of 25.91 µmol g⁻¹ h⁻¹ with the apparent quantum efficiency (AQE) of 1.96% at 365 nm (Fig. 2f) [21].

Figure 2

Figure 2. (a) Crystal structure of Aurivillius layered perovskite oxides. (b) SEM images and formation of C-Bi bond of Bi₂WO₆@rGO. Reprinted with permission [19]. Copyright 2017, Elsevier. (c) The schematic illustration for the charge separation mechanism and CO₂ reduction reaction. (d) CO production rate over polarized ultrathin Bi₂MoO₆. Reprinted with permission [20]. Copyright 2020, Royal Society of Chemistry. (e) Crystal structure of SrBi₂Nb₂O₉. (f) Photocatalytic CH₄ production curves over SrBi₂Nb₂O₉ series samples annealed at 400 ℃. Reprinted with permission [21]. Copyright 2021, Elsevier.

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The other two types of (100) series layered perovskite oxides are Ruddlesden-Popper (RP) phase and Dion-Jacobsen (DJ) phase with general formula A_n+1B_nO_3n+1/A₂′A_n-1B_nO_3n+1 (Fig. 3a) and A′[A_n-1B_nO_3n+1] (Fig. 3b), respectively [22]. In Ruddlesden-Popper layered perovskite structure, the A and A′ are either alkali, alkaline-earth or rare-earth cations and there are the corners sharing BO₆ octahedra layers [23]. Sr₂TiO₄ and La₂NiO₄ are representative examples of RP layered perovskites and exhibit the strong photocatalytic ability to degrade pollutants. Sorkh-Kaman-Zadeh et al. synthesized the Sr₂TiO₄ nanostructures by a sono-chemical method with a good photocatalytic performance for degradation of toxic dyes (76%) in the presence of UV light [24]. A new nanostructure of La₂NiO₄ with hollow spherical morphology synthesized by the glycerol-assisted solvothermal approach was reported by Tao et al. Compared with the La₂NiO₄ nanosheets, the hollow-structured La₂NiO₄ spheres demonstrated a strong oxidation ability for organic pollutants (phenols and anionic dyes), and particularly a 87% removal rate of phenol in the dark within 12 h was achieved (Fig. 3c) [25]. La₂Ti₂O₇ is a characteristic DJ phase perovskite used for photocatalytic degradation of organic pollutants. Wang et al. reported that the Cr₂O₃/La₂Ti₂O₇ (14 wt% Cr₂O₃) p-n heterojunction exhibited high degradation activity for different pollutants under visible light irradiation, such as Rhodamine B and reactive brilliant red X-3B, which was due to the enhanced photoexcited charge separation and transfer [26]. The (111) and (110) series of layered perovskites oxide photocatalysts are represented by the general formulas (A_n+1B_nO_3n+3) and (A_nB_nO_3n+2), respectively, where n is the number of spanned corner-shared BO₆ octahedra in a layer (Figs. 3d and e) [27]. The weaker interlayer interactions in this type of structures allow facile exfoliation and heterocation substitution, which leads to increased surface area, wider light absorption range and faster charge transport. Iizuka et al. studied Ag loading on (111) layered perovskite ALa₄Ti₄O₁₅ (A = Ca, Sr and Ba) for photocatalytic CO₂ reduction into CO and HCOOH. The anisotropic structure of BaLa₄Ti₄O₁₅ contributed to the spatial separation of the oxidation and reduction reaction sites and the existence of the Ag cocatalyst specifically loading on the catalyst' edge, which led to its excellent photocatalytic activity (Fig. 3f) [28].

Figure 3

Figure 3. (a) Crystal structures of Ruddlesden Popper, (b) Dion-Jacobaen, (d) (111) layered and (e) (110) layered perovskite oxides. (c) Degradation of various organic pollutants (4-CP, phenol, MO, RhB) by using hollow spheres La₂NiO₄ and aggregated nanoplate La₂NiO₄ as the catalyst under visible-light irradiation and in the dark. Reprinted with permission [25]. Copyright 2019, American Chemical Society. (f) Mechanism of photocatalytic CO₂ reduction over BaLa₄Ti₄O₁₅ with Ag cocatalysts loaded by several methods. Reprinted with permission [28]. Copyright 2011, American Chemical Society.

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Layered perovskite oxide photocatalysts are typically synthesized by solid state reaction or hydrothermal methods. The use of tetra(n-butyl) ammonium hydroxide as the intercalation agent can easily exfoliate bulk layered perovskite oxides to yield ultra-thin nanosheet structures with enlarged specific surface area and abundant reactive sites [29]. The two-dimensional (2D) structure of layered perovskite oxides has unique structural characteristics, which can provide the effective anisotropic charge transfer. In addition, most of the layered perovskites with a relatively wide band gap require band gap engineering to improve their light absorption properties. For example, uniform bulk nitrogen doping can redshift the band edge position and narrow the intrinsic wide band gap [30,31]. The layered crystal structure and anisotropic charge transfer endow the layered perovskite oxide photocatalysts with special photoelectronic and exciton transfer characteristics, and also enable a variety of possible modifications and performance adjustment strategies, making them interesting and showing great potential for various photocatalytic applications.

2.2 Sillén-phase bismuth-based photocatalysts

Bismuth oxyhalides, BiOX (X = Cl, Br and I) a typical Sillén-phase layered structure photocatalytic materials for environmental remediation, which is built by alternately stacked [Bi₂O₂]²⁺ slabs and double halogen slabs with nonbonding (weak der Waals) interaction (Fig. 4a) [32,33]. The unique layered structure allows them suitable band structure and promising physicochemical properties such as chemical stability, non-toxicity, and corrosion resistance. Strong intralayer covalent bonds and weak interlayer van der Waals interactions favor the formation of the anisotropic structures of BiOX with (110) lateral facet and (001) top facet [34,35]. In addition, the exposure of anisotropic facets of BiOX can be easily controlled by regulating the synthetic environment and additives, which provides the possibility for crystal facet engineering to improve photocatalytic activity [36–38]. Our group prepared the eighteen-faceted BiOCl with (001) facets and (112) and (102) oblique facets through the one-pot hydrothermal method under long reaction time (100 h, Fig. 4b) [39]. Owing to the matchable band structure among the above three facets, a (001)/(102)/(112) ternary facet junction was formed, which provided a more effective cascade path of charge flow for spacial charge separation (Fig. 4c). Compared with the conventional (001)/(110) binary junction, the eighteen-faceted BiOCl displayed highly enhanced production of H₂ and hydroxyl radicals (^•OH). As another feature of BiOX offered by the unique layered structure, the internal electric field along the crystal orientation perpendicular to the [Bi₂O₂] and [X] layers can provide a strong driving force for the separation of photogenerated electron-hole pairs [40]. Li et al. proved the effectiveness of carbon doping strategy for enhancing internal electric field by joint theoretical and experimental routes (Fig. 4d) [41]. The introduction of carbon increased the bulk charge separation efficiency to 80%, which strongly indicated that heteroatom doping is an effective way to control charge flow and improve photocatalytic performance (Fig. 4e). Compared with the introduction of foreign atoms, vacancy engineering provides another more convenient method to modulate the electronic properties and increase the charge separation. As an important type of lattice defects of BiOX photocatalysts, oxygen vacancies are qualitatively revealed through in-depth DFT calculations. Wang et al. analyzed the disordered redistribution of the band-edge charge states and the changes of localized valence states at different distances around the oxygen vacancy (N_cN_v)^1/2 (Fig. 4f), and first demonstrated that the introduced oxygen vacancies can significantly accelerate the dissociation of excitons, thereby enhancing the separation of charge carriers to efficiently participate in subsequent photocatalytic reactions (generation of superoxide radicals (^•O₂⁻) and synthesis of organic compounds) [42].

Figure 4

Figure 4. (a) Crystal structure of BiOX (X = Cl, Br, I). (b) SEM images and the corresponding schematic representation for BiOCl-100. (c) The schematic representations for photodeposited BiOCl samples with Pt and MnO_x. Reprinted with permission [39]. Copyright 2019, Wiley-VCH. (d) Schematic illustration of the separation and (e) migration of electrons and holes in the bulk of C-doped Bi₃O₄Cl. Reprinted with permission [41]. Copyright 2016, Wiley-VCH. (f) The conduction charge density neat the band edges of the BiOBr model with oxygen vacancy and (N_c N_v)^1/2 value of atomic sites at different distances around the oxygen vacancy. Reprinted with permission [42]. Copyright 2018, American Chemical Society.

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BiOIO₃ has a Sillén-related layered crystal structure consisting of [Bi₂O₂]²⁺ layers interleaved with [IO₃]⁻ trigonal pyramids (Fig. 5a). Due to the alignment of lone-pair containing polar IO₃ polyhedra, BiOIO₃ has a strong macroscopic polarity along the [001] direction [43]. Based on the above structural characteristics, our group synthesized single crystal BiOIO₃ nanostrips oriented along the [001] direction by controlling the pH during crystal growth process [44]. The morphology evolution of BiOIO₃ from nanoparticles (BIO-S) to nanostrips (BIO-L) allowed the catalyst an enhanced macroscopic polarization electric field, which provided a strong driving force for the separation of bulk charges (Figs. 5b and c). Further, by introducing oxygen vacancies on the surface, the photoinduced charges are driven to quickly migrate to the active sites on the surface of the BiOIO₃. Benefiting from the aforementioned bulk and surface co-polarization strategies, the efficient CO₂ reduction system was constructed, realizing a CO yield of 17.33 µmol g⁻¹ h⁻¹. As another typical Sillén-related bismuth-based photocatalysts, Bi₂O₂CO₃ has a crystal structure similar to BiOIO₃ with alternative stacking of (Bi₂O₂)²⁻ sheets interleaved by CO₃²⁻ groups [45]. The studies in recent years have shown that the exposure of (001) facets of Bi₂O₂CO₃ is beneficial to the improvement of photocatalytic activity, because the compression of Bi-O square anti-prism with 8-coordination along c-axis provides more oxygen defects to generate the electrons and holes [46,47].

Figure 5

Figure 5. (a) Crystal structure of BiOIO₃. (b) The morphology evolution and the corresponding schematic illustration for BIO-S and BIO-L. (c) The dipole moment (z-axis) of IO₃ polyhedra and the BiOIO₃ unit cells with different c-lattice lengths. Reprinted with permission [44]. Copyright 2020, Wiley-VCH. (d) Crystal structure of Bi₄MO₈X (M = Ta, Nb; X = Cl, Br). (e) Diagram of band bending of Bi₄NbO₈Br under different conditions. (f) The concentration of ^•O₂⁻ over Bi₄NbO₈X (X = Cl, Br) under light, ultrasonic and simultaneous light and ultrasonic irradiation within 2 h. Reprinted with permission [51]. Copyright 2019, Wiley-VCH.

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The alternating layer structure composed of the Aurivilius phase and the Sillén phase consisting of the [Bi₂O₂]²⁺ layers intergrown with the halide slabs is named the Sillén–Aurivilius structure. Bi₄MO₈X (M = Ta, Nb; X = Cl, Br), as the most representative Sillén–Aurivillius structured perovskite oxyhalides, has received extensive attention since the first discovery of Fujito et al. and our group (Fig. 5d) [48,49]. The Bi₄MO₈X plates with (001) exposing facets are usually prepared by a flux method [50]. For example, our group synthesized Bi₄NbO₈X (X = Cl, Br) monocrystalline nanosheets by the flux method using NaCl and KCl as the molten salt [51]. In view of their noncentrosymmetric crystal structure and strong light absorption in the visible light region, they are remarkable candidates for piezoelectric photocatalysis. The ultrasound-induced [001] oriented piezoelectric field and related band bending not only promoted the separation of photogenerated charges, but also enriched the reduction reaction sites on the surface of Bi₄NbO₈X to achieve high-efficiency evolution of reactive oxygen species by coupling piezocatalysis and photocatalysis (Figs. 5e and f).

With unique layered-structure mediated properties, such as pH-dependent crystal facet exposure, facet-dependent internal electric field intensity, etc., the Sillén-phase bismuth-based nanomaterials are considered as one of the most promising photocatalysts in the environmental fields.

2.3 Layered double hydroxides

Layered double hydroxides (LDHs) are a group of anionic clay materials with layered structure, also known as hydrotalcite [52]. The general formula is [M_1-x²⁺M_x³⁺(OH)₂]^x+(A_x/nⁿ⁻)·yH₂O, where M²⁺ is divalent metal ion (Zn²⁺, Mg²⁺, Cu²⁺); M³⁺ is trivalent metal ion (Al³⁺, Cr³⁺, Fe³⁺); each metal cation on the slabs is bonded with the OH⁻ groups to form the framework of LDHs. Aⁿ⁻ are charge balancing anions including organic, inorganic and heteropoly-anions [53,54]. The crystal structure of LDHs is shown in Fig. 6a, where the layered structure is composed of two or more divalent or trivalent metal hydroxides, and the anions located in the interlayer maintain the stability of LDH by keeping the charge neutrality. According to application requirements, LDHs-based materials can have unique structure and photocatalytic properties by adjusting components such as the type of metal ions and interlayer anions [55].

Figure 6

Figure 6. (a) Structure of LDHs. (b) Leu-LDHs catalyzed chemoselective O-methylation of substituted phenols and thiophenols with DMC and the schematic structural models of Mg-Al LDHs and Leu-LDHs. Reprinted with permission [57]. Copyright 2013, Elsevier. (c) Schematic illustration for synthesis of nanoPt intercalated Zn-Ti LDHs composite (Zn-Ti LDHs; subsequent exchange of PtCl₆²⁻ with anions of CO₃²⁻; followed by photoreduction to form the assembly of Pt/Zn–Ti LDHs. Reprinted with permission [58]. Copyright 2014, Elsevier.

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Due to the relatively weak interlayer bonding, LDHs exhibit remarkable ability to exchange interlayer anions. The high fluidity and exchangeability of the anions residing in the interlayer have important applications in improving photocatalytic activity [56]. The size and valence state of anions and the intensity of interlayer hydrogen bond network affect the interlayer spacing, electron density and band structure of LDHs, respectively, greatly affecting the photocatalytic performance. The interlayer spacing of LDHs can be adjusted by inserting amino acids with different physiochemical properties. Subramanian et al. intercalated sixteen different amino acids into Mg-Al LDHs by reconstruction methods with the increase in interlayer space of LDHs and found that the insertion of amino acids possessing a hydrophobic sidechain (Leu-LDHs) can effectively improve the photocatalytic degradation reactivity and selectivity of LDHs towards phenol degradation (Fig. 6b) [57]. As another typical example, Chen et al. inserted Pt containing organic ligands between the LDHs layer to synthesize Pt/ZnTi-LDHs photocatalyst through ion exchange, which showed a large specific surface area (111.46 m²/g) and pore volume (0.317 cm³/g, Fig. 6c) [58]. The modified LDHs displayed expanded photoresponse range and largely improved photoexcited charge separation and transport performance, thereby increasing the photodegradation of RhB.

Based on the layered configuration of LDHs, they can be exfoliated into positively-charged 2D nanosheets and oppositely-charged catalytically active species via layer-by-layer assembly [59]. So, Gunjakar et al. synthesized the mesoporous layer-by-layer ordered nano-hybrid with preeminent visible light photocatalytic activity through electrostatic self-assembly between positively-charged Zn-Cr-LDH nanosheets and negatively-charged layered titanate nanosheets (ZCT) [60]. The excellent photocatalytic O₂ production rate (~0.67 mmol h⁻¹ g⁻¹) and chemical stability of the heterolayered nanohybrids were attributed to the electronic coupling between the two components and the protection of LDHs lattice by the layered titanate.

LDHs constitute a series of excellent photocatalytic materials, especially the photocatalysts driven by visible light, because they have an adjustable band gap ranging from 2.2 eV to 3.2 eV. In addition, the advantages of LDH such as low cost and facile synthesis make them closer to practical applications.

2.4 MXene

A large family of early transition metal carbides and/or nitrides with the layered structures is named "MXene", which has received widespread attention as a rapidly rising star in the field of photocatalysis [61,62]. MXene consists of M_n+1X_n stacked slices. M is the transition metal, including Ti, Ta, Zr, Mo, etc. X represents the C or N elements held together by hydrogen bonds or van der Waals interactions. At the same time, the surfaces of the MXene layers are typically terminated with OH, O and F. Thus, the general formula M_n+1X_nT_x is generated, herein T_x represents the x surface terminating group (Fig. 7a) [63,64]. MXene has excellent structural stability, tunable bandgap (0.92-1.75 eV), ideal electronic structure, and efficient electrons and holes anisotropic mobility, becoming one of the most potential photocatalytic nanomaterials in environmental governance [65]. In general, most MXenes serve as co-catalysts in the photocatalytic reactions, and researchers use MXene to modify semiconductor photocatalysts to improve their rapid photoexcited carriers recombination, thereby increasing the quantum efficiency and photocatalytic activity. Among these modification strategies, MXene Schottky junction and heterostructure can effectively accelerate the photocatalytic performance by adjusting the electronic structure. Cai et al. successfully synthesized Ag₃PO₄/Ti₃C₂ Schottky junction catalyst by electrostatic self-assembly method (Fig. 7b) [66]. Due to the formation of sufficiently and closely contacted interface between the two phases Ti₃C₂ enhanced the photodegradation activity and photocorrosion resistance of Ag₃PO₄ by capturing the unidirectional electron flow across Schottky barrier (Fig. 7c). It can be observed that the photocatalytic performance of Ag₃PO₄/Ti₃C₂ for tetracycline hydrochloride has a relatively small loss after 8 cycles, which still maintained 68.4%. In addition, the high-efficiency photocatalytic reduction of Cr⁶⁺ over AgI/Ti₃C₂ system also strongly indicated that Ti₃C₂ as a versatile "electron sink" can be used in a wide range of environmental treatments. Another advantage of MXene as a co-catalyst is that it has a layered structure and abundant surface functional groups, which can provide favorable conditions for the construction of heterojunctions. Cao et al. fabricated the 2D/2D heterojunction by growing BiWO₆ nanosheets in situ on the Ti₃C₂ nanosheets (Fig. 7d) [67]. The ultra-thin structure and abundant terminal groups of Ti₃C₂ provided a large contact area and tight interface for the Ti₃C₂/Bi₂WO₆ heterojunction photocatalyst, which greatly improves the charge transfer capacity. Thus, Ti₃C₂/Bi₂WO₆ heterojunction demonstrated 4.6 times higher activity than that of primitive Bi₂WO₆ ultrathin materials for the photocatalytic production of CH₄ and CH₃OH, respectively (Figs. 7e and f).

Figure 7

Figure 7. (a) Crystal structure of MXene. (b) Schematic representation of single 2D Ti₃C₂ sheets and Ag₃PO₄/Ti₃C₂ synthesis. (c) The mechanism of photodegradation and anti-photocorrosion of Ag₃PO₄/Ti₃C₂ Schottky catalyst. Reprinted with permission [66]. Copyright 2018, Elsevier. (d) Schematic illustration of the synthetic of 2D/2D heterojunction. (e) TEM image of ultrathin Ti₃C₂/Bi₂WO₆ nanosheets. (f) Photocatalytic activity of TB0 to TB5. Reprinted with permission [67]. Copyright 2018, Wiley-VCH.

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Fabrication of MXene-based junction provides an effective route to construct a high-efficiency photocatalytic system, which is attributed to its stronger light absorption and faster charge transfer. In addition, the large specific surface area allows more reactive sites. The unique optical properties, electronic structure, stability and hydrophilicity make MXene widely concerned in its surface modification and application.

2.5 Transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) such as MoS₂, MoSe₂ and WS₂ that consist of a hexagonal layer of metal atoms M sandwiched between two MX₂ stoichiometric chalcogen layers have attracted enormous research enthusiasm in the field of photocatalysis (Fig. 8a) [68,69]. Benefiting from its intrinsic layered structure and weak van der Waals interactions between layers, the bulk TMDs can be exfoliated into corresponding thin-layered nanosheets [70–72]. Koyyada et al. successfully prepared hexagonal WS₂ nanosheets (e-h-WS₂) with a narrow band gap and higher specific surface area through a liquid phase exfoliation process in dimethylformamide solution (Figs. 8b and c) [73]. Compared with the hexagonal WS₂ (h-WS₂), the monolayered or few-layered WS₂ (e-h-WS₂) nanosheets showed enhanced photocatalytic activity and excellent stability in the degradation of various organic dye pollutants under simulated solar light irradiation. In addition, the high anisotropy and unique crystal structure of TMDs make them widely used as co-catalysts for the modification of semiconductor photocatalysts [74,75]. The unique 2D layered structure of TMDs can be used as an effective support for the anchoring of semiconductor nanoparticles, allowing efficient charge separation, more dispersed active sites and stability [76]. Islam et al. synthesized heterodimensional nanostructures of ultrathin MoS₂ nanosheets (2D) uniformly intersperaed with ZnO nanoparticles (0D) by ultrasound-assisted and wet chemical processes (Fig. 8d) [77]. Benefiting from the rapid charge carriers migration between ZnO and MoS₂ with the intimate interface and the large adsorption surface, the optimized quasi-0D/2D hybrid nanomaterial exhibited excellent photocatalytic ability, which degraded more than approximately 90% of various organic dyes and tetracycline within 30 min. It is worth noting that nano-sized MoS₂ as the eminent O₂-activator for the oxidation reactions can boost the generation of ^•O₂⁻ [78]. Guo et al. reported a structure-controlled nano-flower-like 3D microsphere photocatalyst with excellent photocatalytic activity (Fig. 8e) [79]. The abundant active-edge sites and unique physicochemical properties of MoS₂ nanosheets enabled 3D BiOI/MoS₂ microspheres to show greatly inhibited recombination of photogenerated electrons and holes, achieving an optimum photocatalytic activity for degradation of methyl orange and tetracycline as high as 95.6% and 91.5% within 75 min, respectively.

Figure 8

Figure 8. (a) Crystal structure of TMDs with a typical formula MX₂. (b) scheme of exfoliation procedure for hexagonal WS₂ catalyst. (c) SEM images of h-WS₂ platelets and e-h-WS₂ platelets. Reprinted with permission [73]. Copyright 2019, Elsevier. (d) Schematic illustrations of a "bottom-up" wet chemical process to obtain heterodimensional ZnO/MoS₂ nanocomposites and the proposed photocatalytic mechanism under visible-light irradiation. Reprinted with permission [77]. Copyright 2018, Wiley-VCH. (e) Photocatalytic mechanism of the 3D BiOI/MoS₂ microspheres under visible-light irradiation. Reprinted with permission [79]. Copyright 2021, Elsevier.

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The layered structure of TMDs can be used as an effective support for the anchoring of semiconductor nanoparticles. At the same time, by formation of heterojunction between TMDs nanosheets and semiconductors, more effective charge transfer can be achieved. A lot of pioneer work has found that their photocatalytic activity originated from active sites anchored at exposed and uncoordinated marginal sites. In view of these advantages, TMDs are expected to become one of the star materials to replace noble metals for co-catalysis.

3. Metal-free layered photocatalytic nanomaterials

In the "green" 21^st century, compared with the high cost and easy corrosion of metal-based photocatalysts, metal-free layered photocatalysts nanomaterials have received extensive attention due to their abundant reserves, good durability and environmental sustainability. Metal-free layered photocatalytic materials can be divided into graphite carbon nitride, hexagonal boron nitride and black phosphorus (Table S2 in Supporting information).

3.1 Graphitic carbon nitride

Franklin prepared graphitic carbon nitride (g-C₃N₄) by the thermal decomposition of mercuric thiocyanate in 1922 [80]. After that, the investigation of g-C₃N₄ has begun. In the structure of g-C₃N₄, C and N atoms are connected by sp² hybridization with σ bonds to form a hexagonal structure, which is called a triazine ring. At the same time, the triazine ring is linked by the terminal N atom to form an extended layer structure. Compared with the s-triazine ring (C₃N₃), g-C₃N₄ with ordered tri-s-triazine units (C₆N₇) proves to be the most stable structure (Fig. 9a) [81]. The g-C₃N₄ as a metal-free polymer semiconductor photocatalyst was first reported by Wang et al. in 2009, which can realize photocatalytic water splitting to produce hydrogen [82]. This research opens a new direction for realizing the direct conversion of solar energy to chemical energy via metal-free photocatalyst. Meanwhile, due to the earth-abundant reserves, simple synthesis, moderate band gap (2.7 eV), ideal electronic structure and high stability of g-C₃N₄, it has become one of the most popular layered photocatalytic nanomaterials in the field of environmental pollution [83,84]. However, the photocatalytic activity of g-C₃N₄ is still limited in practical applications due to its low visible light absorption, small specific surface area, and high photogenerated electron and hole recombination efficiency [85].

Figure 9

Figure 9. (a) Crystal structure of g-C₃N₄. (b) Schematic illustration of O-g-C₃N₄ nanofibers formation. (c) Optimized structures of pristine g-C₃N₄ and O-g-C₃N₄. (d) FT-IR spectra of bulk g-C₃N₄ and O-g-C₃N₄. (e) The high-resolution XPS spectra upon Ar⁺ spluttering of O. (f) Density of state (DOS) of pristine g-C₃N₄ (blue) and O-g-C₃N₄ nanofibers (red). Reprinted with permission [93]. Copyright 2018, Elsevier. (g) Top-down process for preparation of foam-like holey ultrathin g-C₃N₄ nanosheets. (h) TEM images of g-C₃N₄ nanosheets. Reprinted with permission [96]. Copyright 2016, Wiley-VCH.

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Among various modification strategies for optimizing photocatalytic performance, as one of the simplest and most effective methods, element doping including alkali metals (Li, Na, K) [86,87], transition metals (Cu, Fe, W) [88,89] and non-metals [90] can ameliorate the environmental management of g-C₃N₄ photocatalyst by adjusting the electronic structure. Based on the layered structure of g-C₃N₄, Xiong et al. inserted K atoms into the interlayer of g-C₃N₄, which served as a transmission channel by bridging the layer, greatly improving the separation and transfer of photoexcited electron-hole pairs between adjacent layers [91]. Furthermore, the non-metal elements doping can continue the metal-free characteristics of g-C₃N₄, which breaks the symmetry of g-C₃N₄ by forming covalent bonds and results in the efficient separation of photogenerated charge carriers [92]. Zeng et al. successfully synthesized oxygen-doped g-C₃N₄ nanorods (O-g-C₃N₄) with a well-defined porosity (Fig. 9b) [93]. Compared with the original g-C₃N₄, the interplanar spacing of O-g-C₃N₄ nanofibers was expanded by 1.204 Å, which resulted in a larger specific surface area (114.2 m²/g) and stronger spin strength (Fig. 9c). FT-IR and high-resolution XPS spectroscopy together proved the introduction of homogeneously distributed O atoms (Figs. 9d and e). In addition, the O doping expanded the band gap of g-C₃N₄ (Fig. 9f). Therefore, O-g-C₃N₄ showed improved light absorption capacity and charge separation efficiency and an excellent photocatalytic degradation ratio of 2,4-dinitrophenol.

Owing to the layered structure, thin-layer g-C₃N₄ nanostructures with p-conjugated system and appropriate energy band gap can be easily synthesized [94], which allows g-C₃N₄ a large specific surface area and short charge transfer distance, greatly promoting the applications of g-C₃N₄ in photocatalysis [95]. Li et al. weakened the van der Waals force between layers in the bulk g-C₃N₄ by a post-calcination method, and synthesized g-C₃N₄ nanosheets with a thickness of about 2 nm (Figs. 9g and h) [96]. The thin-layer g-C₃N₄ nanosheets are excellent in photodegradation of pollutants and hydrogen production (57.20 µmol/h), which is attributed to the larger specific surface area brought by the porous ultra-thin structure, enhanced in-plane photo-generated carrier transfer, more exposed new edges and catalytic active site. In addition to calcination in air, argon or ammonia, liquid exfoliation is another typical preparation method that uses the chemical or physical effects of solvents (strong acid, methanol, formamide, etc.) and external forces (ultrasonic wave, pressure, thermal energy, etc.) to prepare 2D g-C₃N₄ nanosheets to improve the photocatalytic activity [97,98]. Combining g-C₃N₄ with other components to prepare nanocomposites is another effective strategy to improve the lifetime and transport efficiency of photogenerated charge carriers of g-C₃N₄ [99]. Based on the above-mentioned researches on 2D g-C₃N₄ nanosheets, the development of 2D/2D composites of g-C₃N₄ nanosheets and other 2D materials will provide favorable conditions for the high-efficiency charge transfer via large closely-connected interfaces in the photocatalytic process [100,101]. Putri et al. used the combined sonication-assisted electrostatic self-assembly method to hybridize positively-charged oxygen-doped g-C₃N₄ and negatively-charged boron-doped rGO to synthesize 2D-2D p-n heterojunction nanocomposites, which realized improved interfacial charge carrier transport and photocatalytic activity [102]. g-C₃N₄ is widely used in the environmental field due to its non-toxicity, simple synthesis, low cost and high stability. As a type of metal-free layered photocatalyst, more researches on g-C₃N₄ are devoted to interlayer doping and preparation of ultra-thin g-C₃N₄. In addition, the g-C₃N₄ nanosheets consist of tri-s-triazines interconnected by tertiary amines provide the structural feasibility for fabrication of g-C₃N₄-based composite nanomaterials. Benefiting from the above structural advantages, g-C₃N₄ will show broader application prospects in photocatalytic environmental purification.

3.2 Hexagonal boron nitride

Boron nitride (BN) composed of equal stoichiometric amounts of boron (B) and nitrogen (N) atoms has a variety of crystalline forms, such as hexagonal BN (h-BN), cubic BN (c-BN) and wurtzite BN (w-BN) [103,104]. Among them, h-BN as the stablest crystalline phase under ambient conditions has received the most attention. h-BN has a typical layered structure with the alternately linked B and N atoms within the layer by sp² hybridized B-N covalent bonds, and the 2D layers are held together by weak van der Waals forces (Fig. 10a) [105,106]. The metal-free h-BN nanomaterial has many unique physicochemical properties, such as low density, high stability, non-toxicity, etc., which has been widely reported as a robust substrate for semiconductor photocatalysts [107]. In particular, when the bulk h-BN is exfoliated into 2D nanosheets, the negatively-charged h-BN nanosheets will attract photogenerated holes and improve the photoexcited charge carriers separation efficiency [108]. Limited by the wide band gap (5.5 eV to 6.0 eV) of h-BN, the researchers usually doped C and O to narrow the band gap and enhance photocatalytic activity [109,110]. Huang et al. synthesized sustainable and stable carbon-doped h-BN (BCN) nanosheets through a simple calcination method (Fig. 10b) [111]. The ternary BCN nanosheets showed outstanding photocatalytic performance for water splitting and CO₂ reduction, which can be attributed to the delocalized 2D electronic system formed by sp² carbon incorporation (Fig. 10c). Based on the layered structure of h-BN, the thin-layered h-BN as an excellent metal-free photocatalyst support or cocatalyst facilitates the homogeneous dispersion of semiconductor photocatalysts for improved photocatalytic performance. Liu et al. successfully prepared a novel porous BN/TiO₂ hybrid nanosheet by solvothermal method (Fig. 10d) [112]. Owing to the formation of B-O-Ti bond between the boron dangling bonds at the edges of the BN porous nanosheet structure and TiO₂ nanoparticles (Fig. 10e), the optimal porous BN/TiO₂ hybrid nanosheets (38 wt%) exhibited a photocatalytic degradation efficiency of 99% for Rhodamine B under visible light within 6 h (Fig. 10f). In addition, the h-BN nanosheets can be combined with various 2D structures by layer-to-layer stacking, which can inherit the advantages of h-BN and hetero-components. Jiang et al. prepared h-BN/g-C₃N₄ metal-free heterojunction photocatalysts using an in-situ growth method [113]. The enhanced photocatalytic degradation activity of h-BN/g-C₃N₄ nanosheets is mainly attributed to the large specific surface area of the composite structure and the unique properties of h-BN as a photoexcited hole transfer promoter.

Figure 10

Figure 10. (a) Crystal structure of h-BN. (b) Typical TEM dark-field image of BCN sample and the elemental mapping images of B, C and N of the enlargement of selected-area in the picture. Scale bar: 300 nm. (c) The optimized structure of B₁₆N₁₆ and B₁₁C₁₂N₉ with the corresponding calculated energy band. Reprinted with permission [111]. Copyright 2015, Nature. (d) Schematic diagram for the photocatalytic reaction of Rhodamine B by porous BN/TiO₂ hybrid nanosheets with new chemical bonding specie B-O-Ti under simulated visible light and TEM images for starting porous BN nanosheets and HRTEM images for a hole decorated by TiO₂ nanoparticles. (e) XPS spectra of B 1s, O 1s with porous and non-porous BN nanosheets, porous and non-porous BN/TiO₂ hybrid nanosheets. (f) Photodegradation of Rhodamine B under visible light λ > 420 nm by P25 and porous BN/TiO₂ hybrid nanosheets. Reprinted with permission [112]. Copyright 2017, Elsevier.

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The layered h-BN nanosheets are called "white graphene" because of their similar crystal structure. In contrast to the huge progress made on carbon analogs, the researches on h-BN are quite limited. The wider band gap of h-BN limits its applicability in environmental and energy fields. The excellent performance of h-BN is mainly manifested by the extremely-high chemical stability, which makes it a promising photocatalyst support material. In addition, structural characteristics of h-BN still deserve attention, which will further promote the research on h-BN as a kind of promising metal-free solar-driven photocatalyst.

3.3 Black phosphorus

As one of the most abundant elements on earth, phosphorus has four allotropes: white, red, violet and black phosphorus. Among them, black phosphorus (BP) is the most thermodynamically stable allotrope due to its orthorhombic crystal structure [114]. Bulk BP is constructed by the single layer of phosphorus atoms (called phosphorene) with an interlayer spacing of 0.53 nm by weak van der Waals force [115]. This non-planar folded hexagonal structure consists of pleated phosphorus atoms connected by sp³ hybrid covalent bonds, which is similar to the puckered honeycomb structure (Fig. 11a) [116].

Figure 11

Figure 11. (a) Crystal structure of BP. (b) Low resolution TEM image of bulk BP and (c) 2D BP. Reprinted with permission [118]. Copyright 2016, American Chemical Society. (d) Mechanism of photocatalytic reactive oxygen species generation over ultrathin BP nanosheet. (e) Schematic illustration of the subband structure of ultrathin BP nanosheet as well as the photoexcitation and photocatalytic processes occurring therein. Reprinted with permission [119]. Copyright 2018, American Chemical Society. (f) Side-view differential charge density map of g-C₃N₄ and phosphorene. Reprinted with permission [122]. Copyright 2018, Wiley-VCH.

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As a strong competitor of graphene in photocatalytic applications, 2D black phosphorous has received extensive attention due to their adjustable band gap, strong light absorption, high charge mobility and large specific surface area. In 2014, Liu et al. used the tape micro-pyrolysis method for the first time to prepare a single-layer or a few-layer phosphorene [117]. Subsequently, the 2D BP was confirmed to be an outstanding catalyst for the conversion of solar energy to chemical energy in 2016 [118]. They prepared dispersed 2D BP flakes in N-methylpyrrolidone (NMP) under an inert atmosphere by the typical liquid exfoliation (Figs. 11b and c). The enhanced photocatalytic oxidation ability of 2D BP on triethylamine (TEA) was attributed to the increased band gap (from 0.3 eV to 2.1 eV), the promoted electron transfer, and the oxidative and reductive ability of photogenerated electrons and holes. Noteworthily, Wang et al. studied the subband structure of ultrathin BP nanosheets caused by low-dimensional many-body effects, which allowed BP nanosheets to possess optically switchable photocatalytic properties and light-dependent reactive oxygen species production ability (Fig. 11d) [119]. Compared with the bulk BP, ultrathin BP nanosheets generated singlet oxygen species (¹O₂) and ^•OH under ultraviolet or visible light irradiation, respectively (Fig. 11e). The above-mentioned examples all showed that monolayers or few-layers of phosphorene exhibited outstanding photocatalytic performance. As a 2D material formed by van der Waals forces, the strategy for photocatalytic activity improvement of BP is focused on designing 2D/2D heterojunctions [120,121]. Ran et al. successfully prepared a new type of 2D/2D metal-free phosphorene/g-C₃N₄ van der Waals heterojunction through a simple self-assembly technology [122]. The 2D/2D heterostructure with a large interface area of phosphorene/g-C₃N₄ exhibited a 13-fold increase in solar energy conversion efficiency compared to primitive g-C₃N₄. The calculations of differential charge density proved that there is a strong electronic interaction between phosphorene and g-C₃N₄ nanosheets, which greatly improved the interfacial charge mobility (Fig. 11f). In addition, 2D phosphorene has massive reactive sites, which further contributed to enhanced photocatalytic activity.

As a direct band gap layered semiconductor, the band gap of BP can be adjusted from 2.0 eV to 0.3 eV by controlling the thickness, which demonstrated that BP has a wide range of optical absorption. The tunable bandgap and high hole mobility at room temperature make 2D BP more suitable for photocatalytic applications. In addition, fabrication of BP-based 2D/2D heterostructure can achieve enhanced separation of spatial photogenerated carriers through the large-area strong electronic coupling between the different phases. BP's unique structure and physicochemical properties make it expected to become the one of the most valuable materials in the post-graphene era.

4. Photocatalytic oxidation for environmental applications

Photocatalysis has been recognized as a potential advanced oxidation process (AOP) for effective water remediation and air purification using sunlight [123,124]. The layered photocatalytic materials have high expectations due to their unique structure, low cost, non-toxicity and high oxidizing abilities in solar light [125]. Based on the characteristics of the layered structure, the 2D materials after exfoliation exhibit outstanding photooxidation activity [126]. In addition, the photooxidation ability of the photocatalysts can be further improved by appropriate strategies, such as morphological control, element doping, heterojunction construction [127].

4.1 Oxidation degradation of pollutants in water

Water pollution by pharmaceuticals and industrial pollutants and the search for effective strategies to deal with organic pollution are the main challenges that need to be resolved in the near future [128]. Till now, the applied wastewater treatment technologies mainly include biodegradation, chemical oxidation and electrochemical conversion, etc. [129,130]. However, in dealing with micropollutants that pose a huge threat to ecosystem, the above-mentioned methods are still not effective enough. With the advantages of low energy consumption and safety, photocatalytic technology has attracted more and more attention in the field of water remediation [131]. Ciprofloxacin (CIP) is a broad-spectral antibacterial agent commonly used for treating bacterial infections. The difficult degradation of CIP will lead to the increase of bacterial resistance, which seriously threatens human health [132]. Zeng et al. synthesized a bismuth oxybromide composite consisting of BiOBr and Bi₂₄O₃₁Br₁₀ phases by a simple solvothermal process with different additives including triethanolamine (TEOA), sodium hydroxide (NaOH) and ammonium hydroxide (NH₃ H₂O), which were named as S-TEOA, S-NaOH and S-NH₃, respectively (Fig. 12a) [133]. S-TEOA nanosheets with a high percentage of (001) facet of Bi₂₄O₃₁Br₁₀ had excellent adsorption properties and more surface oxygen vacancies, which resulted in efficient degradation of CIP under visible light irradiation (Fig. 12b). The adsorption mode of S-TEOA and CIP was uncovered as monodentate coordination and electrostatic interaction, which was conducive to the photogenerated charges at the interface to participate in the chemical reaction, thereby improving the photocatalytic degradation activity of the sample S-TEOA (Fig. 12c). As we well know, various active species such as ^•OH, photo-generated holes (h⁺) and superoxide radical (^•O₂⁻) play an important role in the photocatalytic oxidation process of semiconductors. In this work, the adsorbed CIP was oxidized by photogenerated h⁺, ^•O₂⁻ and ^•OH into final products including CO₂, H₂O, and small molecules (Eqs. 1–8).

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Figure 12

Figure 12. (a) Nitrogen adsorption–desorption isotherms of the sample S-TEOA, S-NaOH and S-NH₃ (inset is SEM image of the S-TEOA). (b) Photocatalytic performance for CIP solution under visible light irradiation of CIP solution containing 150 mg of P25 and the samples S-TEOA, S-NaOH, S-NH₃. (c) Diagrammatic adsorption modes of CIP on the surface of the sample S-TEOA. Reprinted with permission [133]. Copyright 2017, Royal Society of Chemistry. (d) SEM and (e) TEM images of HCNS/rGO. (f) Visible-light photocatalytic performance of bulk-g-C₃N₄, HCNS, HCNS/rGO, and CNCF samples for the removal NO from air. Reprinted with permission [139]. Copyright 2016, Royal Society of Chemistry.

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4.2 Oxidation degradation/removal of air pollutants

With the rapid development of modern industry, air pollutants such as SO_x, NO_x and volatile organic compounds (VOC) become more and more serious and attract increasing attention [134]. For the removal of nitrogen oxides (NO_x) contributing to formation of photochemical smog, acid rain and tropospheric ozone in the air, a variety of technologies have been developed including electrochemical reduction and selective catalytic reduction, which can effectively remove high-concentration NO_x [135,136]. However, for the removal of low-concentration NO_x (NO and NO₂) at the ppb (parts per billion) level, photocatalysis is considered a promising green technology [137,138]. Wu et al. used electrostatic self-assembly technology to couple hollow porous carbon nitride nanospheres (HCNS) and reduced graphene oxide (rGO) to prepare HCNS/rGO metal-free heterojunction, which can remove NO in the air at 600 ppb level under visible light irradiation (Figs. 12d and e) [139]. The NO removal rate of the optimal sample reached 64% (Fig. 12f). In order to obtain more excellent photochemical stability and recyclability, HCNS/rGO was immobilized on C₃N₄ carbonized polymer nanofibers (CNCF) to form a photocatalytic film by electrospinning and carbonization. Furthermore, the photo-oxidation mechanism of photocatalytic removal of NO was given. HCNS/rGO generated reactive free radicals ^•O₂⁻ and ^•OH under visible light irradiation, which reacted with NO and produce HNO₂ and HNO₃. At the same time, holes also directly photo-oxidized NO to NO₃⁻. The above processes can be described by the reaction shown in Eqs. 9–17.

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More examples of photocatalytic oxidation for environmental applications have been provided in supporting documents (Figs. S1 and S2 in Supporting information).

5. Photocatalytic reduction for environmental applications

In artificial photocatalysis, photocatalytic reduction as the half-reaction of photocatalytic reaction also has an equally important position in environmental applications. In diversified photocatalytic reactions based on the strong reduction activity of layered photocatalysts, the CO₂ and Cr(Ⅵ) reduction have been paid intensive attentions [140,141].

5.1 Reduction of heavy metal pollutants

Heavy metal ions have the characteristics of strong toxicity, long half-life, easy accumulation in organisms and refractory degradation, etc. which mainly come from wastewater discharge of metallurgical industry, electroplate factory and nuclear industry. Photocatalytic technology as a potential solution has attracted extensive attention in the field of treating heavy metal pollutants including Cr(Ⅵ), Pb(Ⅱ), Cu(Ⅱ) and U(Ⅵ) in water due to its simplicity, non-toxicity, and high efficiency [142–145]. The photocatalytic mechanism is as follow (Eq. 18):

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here, Mⁿ⁺ and M represent metal oxides and photocatalytic reduction products, respectively.

In industrial production, hexavalent chromium (Cr(Ⅵ)), as the common heavy metal pollutant, has attracted attention due to its high solubility and carcinogenicity [146,147]. The hexavalent chromium (Cr(Ⅵ)) and trivalent chromium (Cr(Ⅲ)) exist in water as two stable oxidation states [148]. The strong toxicity of Cr(Ⅵ) makes it strictly limited in drinking water (0.05 mg/L), which is about 100 times that of Cr(Ⅲ) [149]. Photoreduction of Cr(Ⅵ) to Cr(Ⅲ) is considered to be an effective method for removing Cr(Ⅵ) from wastewater with low energy consumption [150,151]. However, the photoreduction process needs to be realized under acidic conditions [152]. Jia et al. designed a series of solid solutions BiOBr_1-xI_x to dispose Cr(Ⅵ) over a wide range pH values by introducing iodine into BiOBr [153]. With pH ranging from 2 to 10, the removal rate of Cr(Ⅵ) remained at 98%, indicating that satisfactory synergistic effect of photoreduction and adsorption (Fig. 13a). The adsorption and photoreduction of Cr(Ⅵ) and the fixation process of Cr(Ⅲ) under different pH values were proposed. Under acidic conditions, photoreduction dominates the removal of Cr(Ⅵ), which was described in the following Eqs. 19–21 (Figs. 13b and c).

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Figure 13

Figure 13. (a) Effect of pH_i on the removal of Cr(Ⅵ) and (f) Cr(T) by BiOBr_0.25I_0.75. (b) High resolution of Cr 2p spectrum after photoreduction by BiOBr_0.25I_0.75 at pH 5. (c) Process for the removal of chromium on BiOBr_0.25I_0.75. Reprinted with permission [153]. Copyright 2021, Elsevier. (d) Schematic of the band gap structure of BiOBr_0.25I_0.75. (e) Comparison of photocatalytic activity for the Cr(Ⅵ) reduction experiment with Bi₂O₃, BiOBr, and Bi₂₄O₃₁Br₁₀. Reprinted with permission [154]. Copyright 2014, American Chemical Society.

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The strategy of substituting Br with I not only promoted the adsorption of Cr(Ⅵ) on BiOBr_0.25I_0.75, but also upshifted the CB position, enhancing the photoreduction ability for Cr(Ⅵ) to Cr(Ⅲ). Similarly, Shang et al. successfully synthesized Bi₂₄O₃₁Br₁₀ with benign reduction activity by adjusting the proportions of Bi, O and Br (Fig. 13d) [154]. Compared with BiOBr, the Bi₂₄O₃₁Br₁₀ with the up-shifted CB bottom almost totally removed Cr(Ⅵ) within 40 min (Fig. 13e). More examples of photocatalytic reduction of heavy metal pollutants have been provided in supporting documents (Fig. S3).

5.2 Reduction of the organic pollutants

The photocatalytic degradation of organic pollutants can be divided into reduction by photogenerated electrons or oxidative degradation by holes and ^•OH [155]. As a representative of organic pollutants, polybrominated diphenyl ethers (PBDEs) were classified as a persistent organic pollutant under the Stockholm Convention in 2012. Its degradation mechanism involves the widely studied photocatalytic reductive debromination. The photocatalytic reduction of PBDEs is usually carried out in anoxic solvents using selected sacrificial organic agents (such as methanol) [156], whereas photocatalytic oxidation is usually carried out in aqueous solutions [157]. More examples of photocatalytic reduction of the organic pollutants have been provided in supporting documents (Figs. S4a-c in Supporting information).

5.3 Reduction of NO₃⁻ and BrO₃⁻

Managing the nitrogen cycle in water is one of the major challenges that society faces in the 21^st century as identified by the National Society of Engineers. The use of nitrogen fertilizers in agriculture can lead to nitrate contamination of surface and groundwater. As an emerging transformation technology, photocatalytic nitrate reduction has attracted attention in the nitrate removal field to ensure the safety of drinking water resources [158]. The photocatalytic reduction of nitrate to nitrite, ammonium and nitrogen is described by Eqs. 22–24:

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Bromides naturally existing in surface water bodies are oxidized to bromate (BrO₃⁻) in drinking water during disinfection processes such as ozonation, hypochlorination and chloramination. As another typical carcinogenic inorganic pollutant, it has been widely concerned. Photocatalytic technology is an effective strategy to remove BrO₃⁻. During the photocatalytic reduction of BrO₃⁻, the photogenerated electrons act as reducing species for BrO₃⁻ reduction (Eq. 25):

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More examples of photocatalytic reduction of NO₃⁻ and BrO⁻₃ have been provided in supporting documents (Figs. S4d-f in Supporting information).

5.4 CO₂ reduction

Global warming is increasing year by year, which is caused by the large amount of CO₂ gas emitted from the burning of fossil fuels [159,160]. In addition to decreasing the fossil fuel consumption, the development of clean, efficient and renewable green technologies is urgently needed to convert excess CO₂ into valuable products [161]. Semiconductor photocatalytic CO₂ reduction provides a strategy to kill two birds with one stone, which solves the problems of greenhouse effect and energy crisis at the same time. In this system, the photocatalysts convert solar energy into chemical energy, that is, reducing CO₂ to valuable compounds, such as carbon monoxide, methane, methanol, and ethanol [162,163]. The CO₂ reduction capability is related to the energy band position of photocatalysts [164]. The bottom of the conduction band of photocatalysts must be more negative than the reduction potential of various reduction products of CO₂, which are listed in Eqs. 26–30.

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Therefore, band structure engineering is one of the effective strategies to control the selectivity of CO₂ reduction products [165]. More examples of photocatalytic CO₂ reduction have been provided in supporting documents (Fig. S5 in Supporting information).

6. Summary and outlook

A gradual increase in the proportion of layered photocatalysts has been observed from published articles related to the field of photocatalytic environmental applications in the last decade (by WOS) (Fig. 14a), indicating that layered photocatalytic nanomaterials as one of the promising material families, have received more and more attention due to their high efficiency and rich tunability in structure and property. The structure of the layered materials consisting of stacked sheets enables many interesting design and modification possibilities, such as the tailorable nanostructure, controllable accessibility to sites and characteristics, adjustable pore structure and specific surface area.

Figure 14

Figure 14. (a) The results for number of publications on photocatalysis for environmental applications and the share of literature on layered photocatalytic nanomaterials among them from 2011 to 2021. (b) The proportion of photocatalysts for each search query in the number of publications on layered photocatalytic nanomaterials. Date from Web of Science (WOS).

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For all layered photocatalytic nanomaterials that have been reviewed, g-C₃N₄ gave the maximum number of search results compared to other photocatalysts, followed by Sillén-phase bismuth-based photocatalysts represented by BiOX (X = Cl, Br and I, Fig. 14b). The burgeoning expansion study of nanoscale layered g-C₃N₄ and BiOX photocatalysts over the past decade has indisputably witnessed unique properties with extraordinary ameliorated photocatalytic applications in environmental remediation. Especially as a metal-free semiconductor photocatalyst, g-C₃N₄ has become the research hotspot for pollutant photodegradation, reduction of high-valent metal ions and greenhouse gases due to its unique layered structure and simple modification methods. For the selection of the modification strategies, in addition to considering the structural characteristics of the material itself, the mechanism exploration of the whole photocatalytic process will also provide the accurate guides for the development of efficient layered photocatalysts with high stability and activity. Compared with the photocatalytic reduction process utilizing photogenerated electrons, the multi-step mechanisms of photocatalytic oxidative degradation are that holes oxidize water molecules to generate ^•OH or directly act on pollutants. In addition, the anionic ^•O₂⁻ generated from the reduction of adsorbed oxygen molecules by photogenerated electrons also have strong oxidizing properties. Meanwhile, the protonation of superoxide will generate hydroperoxide radicals, hydrogen peroxide and eventually form ^•OH radicals. It is worth noting that the non-selective pollutant treatment degree depends not only on the photocatalytic performance, but also depends on the structural stability of the degradation products. Therefore, the molecular-level assessment of the photocatalytic process is of immeasurable significance for the development of efficient photocatalytic materials for environmental purification.

This review summarizes the basic knowledge of the layered photocatalytic nanomaterials based on their classification, structure and composition, the modification strategy for optimizing the photocatalytic efficiency in environmental field, and the overall development status of layered photocatalytic materials. Although significant progress has been made in layered photocatalytic materials in the purification of water and air pollutants, there are still challenges in current small-scale photocatalytic research, which is far from practical applications. In general, it is very necessary to explore the correlation among photocatalyst activity, involved reaction mechanism and the structural properties of photocatalysts at the micro-atomic scale, which will depend on the development of more in-situ characterization techniques. Based on the mastery of the above-mentioned laws, in addition to focusing on high photocatalytic activity, the durability of the photocatalytic environmental treatment system should also be paid more attention to ensure industrial-grade long-term stability. In addition, the ideal photocatalyst needs to consider the economic benefits of industrial scale. Therefore, it is very important to develop the photocatalyst composed of abundant elements on the earth with high stability, scalability and excellent photocatalytic activity. Meanwhile, the strong collaboration between materials science and engineering is required to achieve best consequences.

A clean environment is the basic requirement to maintain the sustainable development of life on earth. The photocatalytic technology is committed to solving environmental pollution problems in a greener way. A large number of reports on layered photocatalytic materials have proven their potential in environmental applications. There is no doubt that layered photocatalytic nanomaterials will be explored in more detail to meet scalability requirements and solve global environmental problems.

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.

Acknowledgments

This work was jointly supported by the National Natural Science Foundation of China (Nos. 51972288 and 51672258), the Fundamental Research Funds for the Central Universities (No. 2652018287).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.05.037.
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Figure 1 Schematic illustration of layered photocatalytic nanomaterials for environmental applications.

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Figure 2 (a) Crystal structure of Aurivillius layered perovskite oxides. (b) SEM images and formation of C-Bi bond of Bi₂WO₆@rGO. Reprinted with permission [19]. Copyright 2017, Elsevier. (c) The schematic illustration for the charge separation mechanism and CO₂ reduction reaction. (d) CO production rate over polarized ultrathin Bi₂MoO₆. Reprinted with permission [20]. Copyright 2020, Royal Society of Chemistry. (e) Crystal structure of SrBi₂Nb₂O₉. (f) Photocatalytic CH₄ production curves over SrBi₂Nb₂O₉ series samples annealed at 400 ℃. Reprinted with permission [21]. Copyright 2021, Elsevier.

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Figure 3 (a) Crystal structures of Ruddlesden Popper, (b) Dion-Jacobaen, (d) (111) layered and (e) (110) layered perovskite oxides. (c) Degradation of various organic pollutants (4-CP, phenol, MO, RhB) by using hollow spheres La₂NiO₄ and aggregated nanoplate La₂NiO₄ as the catalyst under visible-light irradiation and in the dark. Reprinted with permission [25]. Copyright 2019, American Chemical Society. (f) Mechanism of photocatalytic CO₂ reduction over BaLa₄Ti₄O₁₅ with Ag cocatalysts loaded by several methods. Reprinted with permission [28]. Copyright 2011, American Chemical Society.

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Figure 4 (a) Crystal structure of BiOX (X = Cl, Br, I). (b) SEM images and the corresponding schematic representation for BiOCl-100. (c) The schematic representations for photodeposited BiOCl samples with Pt and MnO_x. Reprinted with permission [39]. Copyright 2019, Wiley-VCH. (d) Schematic illustration of the separation and (e) migration of electrons and holes in the bulk of C-doped Bi₃O₄Cl. Reprinted with permission [41]. Copyright 2016, Wiley-VCH. (f) The conduction charge density neat the band edges of the BiOBr model with oxygen vacancy and (N_c N_v)^1/2 value of atomic sites at different distances around the oxygen vacancy. Reprinted with permission [42]. Copyright 2018, American Chemical Society.

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Figure 5 (a) Crystal structure of BiOIO₃. (b) The morphology evolution and the corresponding schematic illustration for BIO-S and BIO-L. (c) The dipole moment (z-axis) of IO₃ polyhedra and the BiOIO₃ unit cells with different c-lattice lengths. Reprinted with permission [44]. Copyright 2020, Wiley-VCH. (d) Crystal structure of Bi₄MO₈X (M = Ta, Nb; X = Cl, Br). (e) Diagram of band bending of Bi₄NbO₈Br under different conditions. (f) The concentration of ^•O₂⁻ over Bi₄NbO₈X (X = Cl, Br) under light, ultrasonic and simultaneous light and ultrasonic irradiation within 2 h. Reprinted with permission [51]. Copyright 2019, Wiley-VCH.

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Figure 6 (a) Structure of LDHs. (b) Leu-LDHs catalyzed chemoselective O-methylation of substituted phenols and thiophenols with DMC and the schematic structural models of Mg-Al LDHs and Leu-LDHs. Reprinted with permission [57]. Copyright 2013, Elsevier. (c) Schematic illustration for synthesis of nanoPt intercalated Zn-Ti LDHs composite (Zn-Ti LDHs; subsequent exchange of PtCl₆²⁻ with anions of CO₃²⁻; followed by photoreduction to form the assembly of Pt/Zn–Ti LDHs. Reprinted with permission [58]. Copyright 2014, Elsevier.

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Figure 7 (a) Crystal structure of MXene. (b) Schematic representation of single 2D Ti₃C₂ sheets and Ag₃PO₄/Ti₃C₂ synthesis. (c) The mechanism of photodegradation and anti-photocorrosion of Ag₃PO₄/Ti₃C₂ Schottky catalyst. Reprinted with permission [66]. Copyright 2018, Elsevier. (d) Schematic illustration of the synthetic of 2D/2D heterojunction. (e) TEM image of ultrathin Ti₃C₂/Bi₂WO₆ nanosheets. (f) Photocatalytic activity of TB0 to TB5. Reprinted with permission [67]. Copyright 2018, Wiley-VCH.

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Figure 8 (a) Crystal structure of TMDs with a typical formula MX₂. (b) scheme of exfoliation procedure for hexagonal WS₂ catalyst. (c) SEM images of h-WS₂ platelets and e-h-WS₂ platelets. Reprinted with permission [73]. Copyright 2019, Elsevier. (d) Schematic illustrations of a "bottom-up" wet chemical process to obtain heterodimensional ZnO/MoS₂ nanocomposites and the proposed photocatalytic mechanism under visible-light irradiation. Reprinted with permission [77]. Copyright 2018, Wiley-VCH. (e) Photocatalytic mechanism of the 3D BiOI/MoS₂ microspheres under visible-light irradiation. Reprinted with permission [79]. Copyright 2021, Elsevier.

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Figure 9 (a) Crystal structure of g-C₃N₄. (b) Schematic illustration of O-g-C₃N₄ nanofibers formation. (c) Optimized structures of pristine g-C₃N₄ and O-g-C₃N₄. (d) FT-IR spectra of bulk g-C₃N₄ and O-g-C₃N₄. (e) The high-resolution XPS spectra upon Ar⁺ spluttering of O. (f) Density of state (DOS) of pristine g-C₃N₄ (blue) and O-g-C₃N₄ nanofibers (red). Reprinted with permission [93]. Copyright 2018, Elsevier. (g) Top-down process for preparation of foam-like holey ultrathin g-C₃N₄ nanosheets. (h) TEM images of g-C₃N₄ nanosheets. Reprinted with permission [96]. Copyright 2016, Wiley-VCH.

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Figure 10 (a) Crystal structure of h-BN. (b) Typical TEM dark-field image of BCN sample and the elemental mapping images of B, C and N of the enlargement of selected-area in the picture. Scale bar: 300 nm. (c) The optimized structure of B₁₆N₁₆ and B₁₁C₁₂N₉ with the corresponding calculated energy band. Reprinted with permission [111]. Copyright 2015, Nature. (d) Schematic diagram for the photocatalytic reaction of Rhodamine B by porous BN/TiO₂ hybrid nanosheets with new chemical bonding specie B-O-Ti under simulated visible light and TEM images for starting porous BN nanosheets and HRTEM images for a hole decorated by TiO₂ nanoparticles. (e) XPS spectra of B 1s, O 1s with porous and non-porous BN nanosheets, porous and non-porous BN/TiO₂ hybrid nanosheets. (f) Photodegradation of Rhodamine B under visible light λ > 420 nm by P25 and porous BN/TiO₂ hybrid nanosheets. Reprinted with permission [112]. Copyright 2017, Elsevier.

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Figure 11 (a) Crystal structure of BP. (b) Low resolution TEM image of bulk BP and (c) 2D BP. Reprinted with permission [118]. Copyright 2016, American Chemical Society. (d) Mechanism of photocatalytic reactive oxygen species generation over ultrathin BP nanosheet. (e) Schematic illustration of the subband structure of ultrathin BP nanosheet as well as the photoexcitation and photocatalytic processes occurring therein. Reprinted with permission [119]. Copyright 2018, American Chemical Society. (f) Side-view differential charge density map of g-C₃N₄ and phosphorene. Reprinted with permission [122]. Copyright 2018, Wiley-VCH.

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Figure 12 (a) Nitrogen adsorption–desorption isotherms of the sample S-TEOA, S-NaOH and S-NH₃ (inset is SEM image of the S-TEOA). (b) Photocatalytic performance for CIP solution under visible light irradiation of CIP solution containing 150 mg of P25 and the samples S-TEOA, S-NaOH, S-NH₃. (c) Diagrammatic adsorption modes of CIP on the surface of the sample S-TEOA. Reprinted with permission [133]. Copyright 2017, Royal Society of Chemistry. (d) SEM and (e) TEM images of HCNS/rGO. (f) Visible-light photocatalytic performance of bulk-g-C₃N₄, HCNS, HCNS/rGO, and CNCF samples for the removal NO from air. Reprinted with permission [139]. Copyright 2016, Royal Society of Chemistry.

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Figure 13 (a) Effect of pH_i on the removal of Cr(Ⅵ) and (f) Cr(T) by BiOBr_0.25I_0.75. (b) High resolution of Cr 2p spectrum after photoreduction by BiOBr_0.25I_0.75 at pH 5. (c) Process for the removal of chromium on BiOBr_0.25I_0.75. Reprinted with permission [153]. Copyright 2021, Elsevier. (d) Schematic of the band gap structure of BiOBr_0.25I_0.75. (e) Comparison of photocatalytic activity for the Cr(Ⅵ) reduction experiment with Bi₂O₃, BiOBr, and Bi₂₄O₃₁Br₁₀. Reprinted with permission [154]. Copyright 2014, American Chemical Society.

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Figure 14 (a) The results for number of publications on photocatalysis for environmental applications and the share of literature on layered photocatalytic nanomaterials among them from 2011 to 2021. (b) The proportion of photocatalysts for each search query in the number of publications on layered photocatalytic nanomaterials. Date from Web of Science (WOS).

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