Engineered 2D Metal Oxides for Photocatalysis as Environmental Remediation: A Theoretical Perspective

Abstract

Modern-day society requires advanced technologies based on renewable and sustainable energy resources to meet environmental remediation challenges. Solar-inspired photocatalytic applications such as water splitting, hydrogen evolution reaction (HER), and carbon dioxide reduction reaction (CO₂RR) are unique solutions based on green and efficient technologies. Considering the special electronic features and larger surface area, two-dimensional (2D) materials, especially metal oxides (MOs), have been broadly explored for the abovementioned applications in the past few years. However, their photocatalytic potential has not been optimized yet to the level required for practical and commercial applications. Among many strategies available, defect engineering, including cation and anion vacancy creations, can potentially boost the photocatalytic performance of 2D MOs. This mini-review covers recent advancements in 2D engineered materials for various photocatalysis applications such as H₂O₂ oxidation, HER, and CO₂RR for environmental remediation from theoretical perspectives. By thoroughly addressing the fundamental aspects, recent developments, and associated challenges—the author’s recommendations in compliance with future challenges and prospects will pave the way for readers.

1. Introduction

Numerous MOs have large bandgap energies, which provide attractive energy levels for redox reactions but frequently suffer from poor electron conductivity, reducing photocatalytic frequencies. Given the difficulty of accelerating the transfer of photogenerated electrons in pure MOs, reducing the migration direction will be a more efficient way to boost photocatalysis. Therefore, we can shrink the third dimension while extending the scale of the remaining two dimensions, resulting in a thin assembly with a large surface fraction. This 2D structure minimizes the distance between bulk and surface-active sites for electron migration and maintains a high specific surface area. Certainly, fabricating MOs based on 2D materials is a cost-effective method for optimizing surface area and charge transfer, thus achieving a proficient photocatalytic efficiency [1]. Because most MOs lack layered architectures, only a few 2D MOs were first recognized for photocatalysis. However, with the development of new synthetic approaches and procedures, 2D nanostructures, such as TiO₂, Fe₂O₃, Cu₂O, ZnO, WO₃, SnO, In₂O₃, CeO₂, and HNb₃O₈, were developed and applied to photocatalysis [2,3]. Because of their non-layered structures, several 2D MOs are challenging to shape using the straightforward ultrasonic exfoliation technique. As a result, several other techniques were used to control the shape of such materials. For instance, a lamellar organic/inorganic hybrid approach has been suggested to fabricate ultrathin TiO₂ nanostructures [2]. The solvothermal method was used to produce lamellar TiO₂ octylamine hybrid precursors using Ti isopropoxide (Ti source), octylamine (capping reagent), and 2-phenyl ethanol (solvent) (Figure 1a–c) [1,4]. The powder obtained from the ultrasound-based exfoliation was washed to remove octylamine and obtain smooth, ultrathin (~98 nm) TiO₂ nanostructures.

Additionally, Bi₂W₂O₉ was exfoliated to achieve single-crystalline nanostructures. Due to the layered composition of Bi₂W₂O₉ ([Bi₂O₂]²⁺ and [W₂O₇]²⁻ layers), layers of WO₃ were obtained by carefully etching the [Bi₂O₂]²⁺ layers with acids and stabilizing the layers of WO₃ with a surfactant (tetrabutylammonium hydroxide). These exfoliated WO₃ nanostructures had a higher bandgap than bulk WO₃, which was attributed to the quantum confinement effect. Besides an exfoliation process, wet chemical techniques were used to prepare MOs nanostructures directly. Several ultrathin MOs have been developed using a self-assembly technique involving ethylene glycol (co-surfactant) and polyethylene oxide–polypropylene oxide–polyethylene oxide [1]. These MOs include TiO₂, Fe₃O₄, Co₃O₄, ZnO, MnO₂, and WO₃. Furthermore, perovskite oxides have recently experienced a renaissance because of their improved efficiency in photocatalysis and solar cell applications. In order to produce their layered perovskite nanosheets, numerous stratiform perovskites can be easily exfoliated [1].

Various MOs, such as SnO₂, WO₃, TiO₂, Fe₂O₃, and ZnO, have been broadly examined as photocatalysts over the last four decades [6,7]. TiO₂ was the most studied because of its high stability, adequate electronic structure, biocompatibility, and superior light absorption properties. The 2D nanostructures of TiO₂ obtained by exfoliating layered titanate have attracted interest for their potential use as photocatalysts [8]. These nanostructures exhibit semiconducting properties such as their bulk counterparts and contain anatase and rutile phases but with somewhat higher bandgap due to confinement effects. Ti_0.91O_20.36 nanostructures, for example, demonstrated an energy gap value of 3.8 eV, which was greater when compared to anatase titania (3.2 eV) [9]. The top-down multistep process was well known for the exfoliation and intercalation of layered MOs to form their bulk counterparts [10]. For instance, layered titanates were initially developed in the case of TiO₂ nanostructures through a traditional solid-state reaction between an alkali metal carbonate solution and TiO₂ at high temperatures (Figure 1d). Numerous layered MOs, including titanoniobate (Ti₅NbO₁₄, Ti₂NbO₇, and TiNbO₅), perovskite oxides (KLnNb₂O₇ and RbLnTa₂O₇), HCa_2-xSr_xNb₃, HNb₃O₈, and WO₃ were fabricated via similar solid-state reactions and wet-chemical exfoliation approaches [11,12,13]. Zhou et al. recently developed freestanding, single-layer Bi₂WO₆ nanostructures from cetyltrimethylammonium bromide using a wet chemical method. Several active sites were generated on the surface of the single-layer nanostructures since Bi atoms were not saturated on the single layer, which directly produced holes upon light irradiation. Quick exciton separation at a highly photoactive surface revealed that single layer Bi₂WO₆ has outstanding photocatalytic efficiency for photo degradation of Rhodamine B [14]. Titanoniobate nanostructures have shown improved photocatalytic efficiency in eliminating organic pollutants [15]. By using a simple wet chemical technique, Tae et al. recently investigated the development of multiple titanate nanostructures with diamond-like shapes with a typical lateral size of 30 nm [16]. The current review is arranged as follows; firstly, the defect engineering tools, such as anion and cation vacancy creations, are thoroughly discussed. Moreover, third section deals with emerging environmental remedies such as water splitting, HER, and CO₂ reduction via theoretical aspect. The last section gives brief summary with challenges and future recommendations.

Superiority of 2D Materials as Photocatalysts

Ultrathin 2D materials with single or few atom thicknesses (>5 nm) have a high surface atom ratio, large surface area, and intrinsic quantum confined electrons that exhibit extraordinary optical, mechanical, and electronic properties and have great potential for the research of transistors, catalysis, optoelectronic, conversion, and the energy storage devices [17,18,19,20,21,22]. These materials present unique physiochemical behavior such as electronic anisotropy, high surface activity, planar conductivity, and tunable energy structure [23]. With the decrease in thickness of bulk substances to the atomic level, atomic structures will go through the apparent distinctions, including length and angle of bonds, coordination number, formation of surface defects, and disordering of surface atoms. As a result, ultrathin 2D materials exhibit not only bulk properties but also new features. Semiconductor photocatalytic materials have gained interest as they give promising solutions for environmental pollution and energy storage [24,25,26,27,28,29]. These materials split the water into hydrogen and oxygen, eliminating pollutants and reduction in CO₂ by solar light as an external driving force [30,31,32,33,34,35]. Light absorption, migration, separation of charges, and surface redox reactions are significant steps for photocatalysis. Upon exposure to irradiation, the photocatalysts absorb the light and produce electron–hole pairs in the conduction and valance band (CB and VB), respectively. These photogenerated electron–hole pairs diffuse to the surface of the material and then migrate to the active sites before the surface reactions. The recombination of charges happens; texture, crystal structure, particle size, and crystallinity affect the separation efficiency. In the end, target molecules adsorb on the material’s surface and undergo charge injection and desorption to make ultimate products [36,37,38,39,40]. Currently, many semiconductors exist for photocatalysis with tunable electronic and crystal structures. Although remarkable achievements are made to optimize the photocatalysis process, many photocatalysts show relatively low photocatalytic performance, which depends on the rational design of such materials. Ultrathin materials have awakened a new aspect of this field because of the appropriate band structure. Ultrathin 2D configuration can harvest more ultraviolet-visible radiations and have a large specific surface area. However, the absorption of photons is very limited in bulk materials due to the reflection and transmission of grain boundaries [1,19,41,42,43]. Additionally, as atomic thickness decreases the migration distance, charge carriers quickly move to the surface area in 2D materials. It reduces the recombination possibility and enhances photocatalysis. Lastly, unique 2D structures with a high surface-atom ratio render many active sites for accelerating the reaction processes. Moreover, atomic escape energies become relatively small due to the decrease in thickness. Surface defects play a role in the target molecule adsorption, building strong interplay, super activation process, and charge transfer. These features help photocatalysts display various features and numbers of scientific reports in this regard [44,45,46]. It is urgent and desirable to present an inclusive review on this field to encourage further developments in this niche.

2. Defect Engineering

2.1. Anion Vacancies

The physiochemical properties and electronic structure of 2D materials are successfully modified using atomically thick oxygen vacancies (Vos), impacting catalytic efficiency [47]. The Vos are among the defects in transition-MOs that have received the most attention due to their low formation energy [48]. In addition to changing the electronic structure and carrier concentration, engineered V_O result supports the activation of molecules such as CO₂, N₂, and O₂ and improves photocatalytic efficiency. By using electrons transferred from the excited BiOBr of the N₂ molecule over the contact, the N₂ molecule could be successfully reduced to NH₃. In the assembly of BiOBr, a cluster of electrons for back transfer has been found by Zhang et al. These clustered electrons can modify the adsorbed N₂ molecule and produce N-atoms stretching (1.078 to 1.133 Å) for free molecular nitrogen [49].

Further, the introduction of V_O in the assembly of ZnAl-LDH (layered double hydroxides) promotes CO₂ activation, such as N₂ activation [50]. The formation of varied unsaturated coordinate Zn ions close to the V_Os caused an increase in the density of V_O, to be seen as the thickness of obtained product ranges from 210 to 2.7 nm, gradually decreasing. The production of surface defects (V_O and Ti³⁺) in TiO₂ nanosheets was offered by Kong et al. via plasma engraving technique [51].

The electrical structure of the 2D TiO₂ nanosheets undergoes considerable variation due to the produced defects, with the bandgap energy reduced from 3.12 to 2.88 eV with CB and VB edge upshifting to create a defective site in the forbidden bandgap. Compared to pure TiO₂, the H₂ production activity increased drastically due to the creation of this defective state. The V_O in WO₃ atomic layers stimulates the formation of intermediate bands in the bandgap and the adsorption and activation of CO₂ into radical COOH• species [52]. More CO and O₂ are produced in the infrared range due to the crucial role played by V_O in WO₃ layers. Lei et al. accomplished the regulated synthesis of V_O-rich and V_O-deficient In₂O₃ nanostructures via quickly heating In(OH)₃ nanostructures in an oxygen-rich atmosphere [53]. The atomic force microscopy (AFM) image in Figure 2 showed that the In₂O₃ nanostructures thickness was about 0.9 nm, demonstrating the controlled atomic thickness of In₂O₃. Electron spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS) spectra showed evidence of V_O. The production of ultrathin V_O-enriched In₂O₃ nanostructures with a peak maxima area was indicated by the 531.4 eV peaks, which also showed that more V_O-rich nanostructures were generated than V_O-poor ones or their equivalent bulk counterparts.

Furthermore, higher V_O signals were seen in ESR at g = 2.004, suggesting the abundant presence of oxygen vacancies in V_O-rich In₂O₃. The electronic configuration of In₂O₃ nanostructures with high V_O material was considerably changed by V_O development. The V_O in the In₂O₃ sample had smaller bandgap energy, and an upshift was seen in the VB tip, according to DRS (Diffuse reflectance spectroscopy) and XPS analysis. Density functional theory (DFT) showed valence band maxima caused a significant DOS. V_O-rich In₂O₃ nanostructures were shown to be more prevalent than V_O-poor In₂O₃ nanostructures, according to a novel concentration of defects. The consequence was a greater carrier level and stronger electric field in the V_O of In₂O₃. The irradiation further energized the electrons in CB. As a result, for H₂O oxidation, V_O-rich In₂O₃ nanostructures performed better than V_O-weak In₂O₃ nanostructures and bulk In₂O₃ by 2.5 and 15 times, respectively. These results confirmed anion vacancy’s effectiveness in electrical configuration engineering.

2.2. Cation Vacancies

The cation vacancies, similar to anion vacancies, have a controlling influence headed for electronic, physical, and chemical characteristics of metallic compounds, resulting in various electronic and orbital arrangements [54]. Metal cation vacancies are additionally challenging for the design and management of vacancies because of their superior energy growth, making them complicated to determine their function [48]. Numerous photocatalytic materials with cation vacancies have been developed, and researchers are now examining how they may affect the effectiveness of the photocatalysts [55,56,57]. The structure-activity connection may be thoroughly explained using 2D atomic layers with and without limited cationic vacancies. Furthermore, tuning such associated vacancies is made easy by the little escape energy of 2D materials.

Moreover, Vv engineering reached larger DOs close to the VB edge. Tailored vacancies boosted light harvesting with a high Vv concentration and increased the catalyst’s electrical conductivity. Successful charge separation made it possible to prolong the carrier’s lifetime (i.e., 74.5 to 143.6 ns). The photocatalytic efficiency of ~398.3 mol g⁻¹ h⁻¹ for the production of CH₃OH has been attained using Vv. By using a straightforward hydrothermal process and temperature variation, Jiao et al. created thick ZnIn₂S₄ unit cells with few or multi Zn vacancies [58]. The AFM measurements suggested that the ZnIn₂S₄ produced unit-cell of thick layers in the direction of the c-axis, while the transmission electron microscopy (TEM) pictures showed a sheet-like shape. The Zeta-potentials, EPR, and PAS measurements of examined products’ Zn vacancy-rich and poor properties showed that ZnIn₂S₄ layers with different Zn vacancy concentrations were effectively generated, offering two appropriate models for exploring the relationship between Zn vacancies and photocatalyst activities. Because of the high Zn vacancies, the ZnIn₂S₄ layers had much better excitons separation performance, as evidenced via PL analysis, ultrafast transient absorption, and surface photovoltage.

Along with improved CO₂ adsorption and hydrophilicity, the abundant V_Zn also improved light harvesting from the 440 to the near-infrared range. The dispersal of charge was analyzed by DFT, where the determination was focused on charges in space closer to CB’s edge. It appears that electrons are stimulated to CB more rapidly because the Zn vacancy raises the charge density of nearby sulfur atoms. According to Song et al., Ti vacancies in single layer H_1.07Ti_1.73O₄·H₂O nanosheets result in the creation of many radical O species which engage with water molecules via H₂ bonds to generate surface coordination. Consequently, photocatalytic efficiency toward H₂ production increased by 10.5 times [59].

Further, the BiVO₄ unit cell nanostructures have produced varying levels of vanadium vacancies (Vv in Figure 3). Moreover, X-ray fluorescence (XRF) and positron annihilation spectrometry (PAS) were used to quantify the concentration of Vv at an atomic level. For BiVO₄, the Vv was trapped for two samples using the shortest life component (~200 ps) confirmed from PAS [60]. Higher Vv levels were indicated for the reasonable positron power period for BiVO₄ nanostructures rich in Vv. According to DFT calculations, the presence of Vv may cause an innovative defect level in the bandgap energy of prepared nanostructures, allowing electrons to be stimulated farther into CB.

2.3. Other Vacancies Types

The vacancy pairs and voids have been studied to promote the activity of photocatalysts [26,61]. The vacancy pairs usually are formed at the facet with high energy to modulate the local atom structure of photocatalysts, thus improving the photocatalytic activity. For example, the Bi−O vacancy pairs can be formed on the high-energy Bi₂WO₆ (100) facet [62] and serve as a charge separation center (in Bi₂WO₆) to enhance photocatalytic activity [63]. Similar to the role of the Bi−O vacancy pairs in Bi₂WO₆, the Bi−O vacancy pairs in Bi₂MoO₆ also served as a charge separation center, which was reported by Xia et al. [63]. Moreover, these Bi−O vacancy pairs were validated by aberration-corrected scanning transmission electron microscopy in annular dark field mode (STEM-ADF) and positron annihilation spectroscopy, in which dim sites can quickly identify such mixed vacancies. These Bi−O vacancy pairs contributed to modulating the local atom structure of Bi₂MoO₆, thus improving the CO₂ photoreduction activity. Voids called pits are common volume vacancies in 2D materials. Specifically, when more nearby atoms escape from the lattices, it forms a small region without atoms, which is considered a cluster of vacancies in 2D materials. Vacancy voids are usually formed in ultrathin 2D materials, which is attributed to the escaping of lots of nearby atoms in a small region from the lattices, such as WO₃ [64], BiOCl [65], and C₃N₄ [66] 2D nanomaterials. For instance, Wei et al. [67] reported a three-atomic-layer-thin cerium oxide nanosheet with almost 20% voids occupancy, in which massive pits surrounding cerium sites with average coordination numbers of 4.6 played a vital role in lowering the oxygen activation energy and preventing catalysts poisoning.

3. Environmental Remediation

3.1. Theoretical Insights

Besides the enormous work on lab experiments of 2D photocatalysts and heterojunctions, theoretical studies offered considerable interest. Numerous studies on 2D materials such as CdS/InSe, GeS/WS₂, GeS, PdSeO₃, C₂N/WS₂, Pd₃P₂S₈, and GeSe have been published in the literature to be effective photocatalysts for photocatalytic reactions variety based on DFT calculations [68,69,70,71]. Numerous studies have been conducted to develop a photocatalyst capable of concurrently splitting water into its component in visible light. A PdSeO₃ monolayer was proposed as a potential 2D material for complete photocatalytic water splitting based on PdSeO₃ layered structure [72]. From DFT calculations, PdSeO₃ monolayer cleavage energy (0.42 J m⁻²) was comparable with graphene (0.37 J m⁻²), implying a superior probability of attaining PdSeO₃ monolayer using the strategy of mechanical exfoliation. According to HSE06 + SOC theory, indirect (2.84 eV) and direct (3.07 eV) bandgap energies for the PdSeO₃ monolayer have been assessed, meaning that the PdSeO₃ monolayer would effectively harvest light. Additionally, the PdSeO₃ monolayer exhibited the best water splitting at a stoichiometric ratio of 2:1 with the help of co-catalysts or sacrificial reagents. The OI sites are supposed to be the active sites for H₂ evolution, whereas the Se sites are anticipated to be the active sites for H₂O oxidation.

Akimove et al. used a hybrid rigid-body MD-extended Hückel theory approach (MD/EHT) to investigate electronic and nuclear dynamics in prolonged systems. Static electronic configuration was also inspected for Ru complexes deposited on the Ta₂O₅ using various anchor groups to determine their photocatalytic CO reduction potential [73]. Calculations revealed that different tilting angles of deposited structures result in a different atmosphere for the catalyst’s active site. The horizontal arrangement of the chromophore in the presence of the PO₃H₂ anchor can weaken Ru-CO and Ru-Cl, thus facilitating light-induced reactions by dropping activation energies. The smaller separation between the Ru core and Ta₂O₅ surface is critical if H₂O or CO₂ are partly organized on the substrate subsequent to their reaction involvement.

On the other hand, chromophore vertically coordinated with the COOH group results in tighter Ru-Cl and Ru-CO bonding, significantly reducing the likelihood of reacting species reaching the reaction core. Variations in tilting angle were marginal for Ru complexes attached by an OH anchor, implying extremely small wave function variations and, thus, minute non-adiabatic couplings over time. Conversely, PO₃H₂ exhibited more significant variations, suggesting the possibility of a deeper non-adiabatic coupling between the acceptor and donor states. The performance of various 2D MOs is illustrated in

Table 1. Various 2D MOs for environmental remediation with adopted strategies for enhancing the performance.

Application	2D Material	Synthesis	Engineering Tool	Reaction Condition	Performance	Ref.
H2 evolution	ZnIn2S4	Solvothermal	Sulfur vacancies	20 vol% TEOA with 3 wt% Pt, Xe lamp (>420 nm)	1504.9 μmol g−1 h−1	[74]
	2D ZnIn2S4/2D g-C3N4		Heterojunction	20 vol% TEOA with 3 wt% Pt, Xe lamp (>420 nm)	6095.1 μmol g−1 h−1	[74]
	ZnIn2S4	Hydrothermal	O doping	0.25 M Na2SO3 and 0.35 M Na2S solution, Xe lamp (>420 nm)	2120 μmol −1 h−1	[75]
			Pristine sample	0.25 M Na2SO3 and 0.35 M Na2S solution, Xe lamp (>420 nm)	471.11 μmol g−1 h−1	[75]
			Monolayer	10 mL TEOA, Xe lamp (>400 nm)	1.723 mmol g−1 h−1	[76]
			Bilayer	10 mL TEOA, Xe lamp (>400 nm)	0.799 mmol g−1 h−1	[76]
			Monolayer + Sulfur vacancies	10 mL TEOA, Xe lamp (>400 nm)	13.478 mmol g−1 h−1	[76]
			Pristine sample	0.25 M Na2S and 0.25 M Na2SO3 solution with 2 wt% Pt, Xe lamp (>420 nm)	263.9 μmol g−1 h−1	[77]
	0D AgIn5S8/2D ZnIn2S4	Hydrothermal	Heterostructure	0.25 M Na2S and 0.25 M Na2SO3 with 2 wt% Pt, Xe lamp (>420 nm)	949.9 μmol g−1 h−1	[77]
CO2 reduction	TiO2	Hydrothermal	Pristine sample	A mixture of pure CO2 gas and H2O vapor, Xe lamp (>400 nm)	CH4, 1.643 μmol g−1 h−1	[78]
			Surface acidification by H2SO4	A mixture of pure CO2 gas and H2O vapor, Xe lamp (>400 nm)	CH4, 1.907 μmol g−1 h−1	[78]
		Solvothermal	Pristine sample	100 mL H2O, Xe lamp	CO, 0.15 μmol g−1 h−1	[79]
		In situ ion exchange method	Treated by Lewis base [WO4]2−	100 mL H2O, Xe lamp	CO, 3.05 μmol g−1 h−1	[79]
	WO3	Solvothermal	Pristine sample	0.2 mL H2O, silicon nitride lamp (0.8–17 μm)	No product	[80]
			Poor Vo	0.2 mL H2O, silicon nitride lamp (0.8–17 μm)	CO, 6 μmol g−1 h−1	[80]
			Rich Vo	0.2 mL H2O, silicon nitride lamp (0.8–17 μm)	CO, 2.75 μmol g−1 h−1	[80]
	Sr2Bi2Nb2TiO12	Solvothermal	Pristine sample	4 M H2SO4 with 1.3 g NaHCO3, Xe lamp	CO, 2.62 μmol g−1 h−1	[81]
			Rich Vo	4 M H2SO4 with 1.3 g NaHCO3, Xe lamp	CO, 17.11 μmol g−1 h−1	[81]
	CuIn5S8	Pristine sample	CO2 reduction	2 mL H2O, PLS-SXE 300/300UV Xe lamp (AM 1.5G filter, >420 nm)	CO, 1.3 μmol g−1 h−1, CH4, 1.6 μmol g−1 h−1	[82]
		Hydrothermal	Sulfur vacancies	2 mL H2O, PLS-SXE300/300UV Xe lamp (AM 1.5G filter, >420 nm)	CH4, 8.7 μmol g−1 h−1	[83]
	ZnIn2S4	Hydrothermal	Zn vacancies	2 mL H2O, PLS-SXE300/300UV Xe lamp (AM 1.5G filter)	CO, 33.2 μmol g−1 h−1	[58]
			Pristine sample	2 mL H2O, PLS-SXE300/300UV Xe lamp (AM 1.5G filter)	CO, 9.22 μmol g−1 h−1	[58]

.

3.2. H₂O Oxidation

Photocatalytic water splitting was already observed as a potentially game-changing strategy for producing safe and renewable H₂. Hydrogen, with the highest calorific value and the highest energy content by weight, has been identified as a possible energy carrier for storing energy from the sun in chemical bond energy between two H atoms. Due to the slow dynamics of the four-hole half-reaction mechanism in water oxidation, it was critical for optimum splitting performance. As a result, increasing the efficiencies for light-derived H₂O oxidation application by reasonable photocatalyst structure design is highly desirable. In order to design vis-light active semiconductors for H₂O splitting, the E_g (bandgap energy) and band positions must be optimized, charge separation must be adequate, charge movement must be easy, and the semiconductor must be durable in aqueous solutions. The 2D architecture with a high defect density may represent an ideal structure for increasing O₂ generation activity.

The photocatalytic separation of water into its components is observed as the Holy Grail of chemistry because it requires merely a renewable energy source, photocatalysts as a medium, and H₂O as a reaction source. Although a significant move forward has been made, the efficacy of water splitting is quite restricted in the mainstream photocatalytic approaches. Generally, water oxidation is recognized as a slow and inefficient route toward photocatalytic water-splitting schemes because of the complex four-holes redox method. As a result, it is critical to propose a photocatalyst with a robust solar H₂O oxidation system. Additionally, an appealing design strategy for meeting these criteria is combining defective 2D materials such as MoS₂, graphene, and g-C₃N₄ with suitable semiconductors. Because of this, Di et al. engineered rational pit defects in 2D BiOCl nanostructures by partially digging pits on previously prepared BiOCl nanosheets with ethylene glycol [65]. TEM and STEM images clearly showed the engineered pit defects on the (001) exposed facet (Figure 4).

According to DFT calculations, photogenerated electrons gravitate toward the (001) facet in BiOCl, while hs⁺ (holes) migrate in the (110) direction. Since O₂ production is a complete hole participating reaction, this appears on the (110) facet via hs⁺ extensive migration track to the (110) facet, which eventually includes significant electron–hole recombination. Furthermore, the incorporated pit defects reduced the migration distance of hs⁺, thus increasing hs⁺ utilization. According to the DFT measurement, the engineered pit defects often marginally improve DOS at CB and VB edges, thus raising carrier concentration and facilitating electron excitation. Additionally, the abundance of unsatisfied chemical bonds accompanying defects created a favorable chemical atmosphere for reaction molecules to chemisorb and foster photocatalytic water oxidation reactions. Therefore, the pit-rich BiOCl-nanosheet could generate O₂ at 56.85 mol g⁻¹ h⁻¹, between three and eight times faster than the BiOCl-nanosheet and bulk BiOCl. In another study, ultrasonic exfoliation of lamellar hybrid intermediate (Zn₂Se₂)(propylamine) resulted in the formation of four atomic thin freestanding single-layers ZnSe [84]. Although the size of ZnSe had been atomically reduced, the local atomic structure had undergone remarkable improvements. Simultaneously, the bond lengths for Se–Se was improved from 4.012 to 4.11. These findings established surface distortion in single-layer structures, which resulted in decreased surface energy and exemplary stability of fabricated structures in single-layers.

Additionally, surface deformation can increase DOS at the CB tip, ensuring an even higher charge carrier transfer rate. The ZnSe single layer exhibit high light-harvesting, improved exciton separation, and lower resistance to charge carriers due to their single-layer configuration with surface defects. As a result, single layers ZnSe nanostructures demonstrated a 195-fold increase in photocatalytic efficiency for H₂O oxidation following Xe lamp irradiation relative to bulk ZnSe. Correspondingly, defects engineered in other 2D photocatalysts, including V_Os confined in In₂O₃; pits formed in WO₃ nanosheet; or surface distortions formed in ZnSe, SnS₂, and SnS nanosheet, will show superior photocatalytic water oxidation behaviors [52,64,85]. Liu et al. developed a variety of pore structures in WO₃ nanostructures using a rapid heating technique on previously exfoliated WO₃•2H₂O-nanostructures [86]. Given the photogenerated hs⁺ migration direction was along 001 facets in the x-direction in W-O-W chains, the photogenerated hs⁺ almost certainly experienced many charge carrier recombination, severely impairing photocatalytic efficiency. The pores formed effectively shorten the diffusion path of hs⁺ and promote H₂O oxidation to form O₂ at the WO₃ surface. Additionally, an abundance of dangling bonds along the pore environment provided favorable conditions for facile chemisorption of molecular reactions, which increased O₂ evolution kinetics. Photocatalytic H₂O oxidation efficiency was increased by 18 times when pore-rich WO₃. Moreover, ultrathin nanostructures were compared to bulk WO₃. It demonstrates an important technique for increasing conversion efficiency with a 2D structure for photocatalytic H₂O oxidation.

3.3. H₂ Evolution

Although photocatalytic sunlight conversion to H₂ fuels is an optimal choice for achieving sustainable energy in the future, its comparatively poor energy conversion performance severely limits its practical applications. Numerous remarkable catalytic materials have been used in photocatalytic H₂ production; however, the majority are still said to have very low photocatalytic efficiency, which cannot satisfy the requirements of practical industrial applications. The photocatalytic H₂ processing efficiency can be significantly increased by combining a 2D structure with abundant surface defects [87].

3.3.1. Mechanism

The nanomaterials’ composition, atomic arrangement, and morphology play a key aspect. The electron−hole pairs are yielded when the laser’s energy is greater than their bandgaps [88,89]. The photogenerated charges within the nanomaterials react with adsorbed H₂O on the surface. However, the charge carriers also undergo recombination or are trapped at internal defects before reaching the surface. Ida et al. found that both time and energy played two key factors during the water splitting to give hydrogen (Figure 5) [90], which requires 4e⁻ generations. Thus, the surface of nanomaterials has impinged with four photons for a short period (at least 4 ms). The layered structures can aid light absorption and largely shorten the traveling distance of the photogenerated charges to reduce their recombination [88,90]. In 1958, Parsons et al. found that the catalytic H₂ evolution occurred on the catalysts’ surface [91]. Moreover, Norskov et al. simulated the activity to understand the efficiency [92]. The water splitting involves two basic reactions (Equations (1)–(3)):

2H⁺ + 2e⁻ → H₂ → E⁰ = 0.00 V

(1)

2H₂O → O₂ + 4H⁺ + 4e⁻ → E⁰ = +1.23 V

(2)

All:2H₂O → O₂ + 2H₂

ΔE⁰ = 1.23 V, ΔG⁰ = +237.2 kJ mol⁻¹

(3)

The water splitting to H₂ by solar energy is one of the most eco-friendly methods. A high H₂ production over photocatalysts of the semiconductor materials can be achieved by sunlight or visible light. It is reliable that the semiconductor can adsorb sufficient energy and results in the creation of electron−hole pairs.

3.3.2. Recent Progress

In order to increase the H₂ generation performance, 2D materials with atomic thickness, such as Cu₂O (cubic), were created [93]. AFM analysis revealed ~0.62 nm thickness, which corresponds to the four atomic-level thicknesses of Cu₂O in the [011] direction. Consequently, cubic Cu₂O surface energy was reduced to the order of (111) (100) (110). Since (110) and (011) surfaces are identical facets (011), they also demonstrated high surface energy, which results in a high operation. Due to their significantly decreased thickness, atomically thin Cu₂O-nanostructures had a different electronic composition than bulk equivalents. DFT simulations revealed significantly improved DOSs at the VB edge for prepared nanostructures in contrast to bulk Cu₂O. Simultaneously, expanded CB edge in 2D Cu₂O-nanostructures was investigated in comparison to bulk Cu₂O, demonstrating that atomically thick Cu₂O has high carrier mobility and a minimal E_g. Taking advantage of these benefits, a photocatalytic H₂ evolution rate greater than 36 times that of 2D Cu₂O nanostructures was achieved following visible light irradiation. These findings unequivocally demonstrated that 2D materials might have significant benefits for H₂ evolution and a variety of remarkable activities. By combining the ultrathin structure and V_O, the photocatalytic H₂ production rate of defect-rich K₄Nb₆O₁₇ nanosheet can be significantly increased to 1661 mol g⁻¹ h⁻¹, approximately 7- and 21-fold of defect-free K₄Nb₆O₁₇ nanosheet, respectively [94]. Not only does the surface V_Os close the E_g and increase light absorption, but they also act as charge separation centers, effectively separating photogenerated electron–hole pairs. Apart from facilitating charge isolation, the V_Os offer a bridge between two elements, allowing for the formation of close junctions. Zhang et al. [95] obtained 1T MoS₂ monolayers supported on the end-faces of Bi₁₂O₁₇Cl₂ sheet to create a 2D Janus Cl₂-Bi₁₂O₁₇-MoS₂ bilayer junctions, which is different from Van der Waals heterostructures (Figure 6a–k). Electrons were separated by the internal electric field of Bi₁₂O₁₇ of visible light Bi₁₂O₁₇Cl₂, which migrated to MoS₂ through the Bi–S bonds, and holes were formed at the internal electric field of Cl₂ end-faces. Thus, the Janus bilayers gave a superior photocatalytic hydrogen evolution rate of 33 mmol h⁻¹ g⁻¹ and a carrier lifetime of 3.446 ps (Figure 6m–o) [95]. In another case, the direct Z-Scheme photocatalyst 2D/2D Fe₂O₃/g-C₃N₄ generates H₂ almost 13 times compared to g-C₃N₄ [96]. With the variance of work functions among Fe₂O₃ (4.34 eV) and g-C₃N₄ (4.18 eV), the switching of electrons will occur from g-C₃N₄ to Fe₂O₃ at an intimate 2D/2D interface. Thus, at the Fe₂O₃/g-C₃N₄ interface, a built-in electric field is created, which becomes advantageous for photoinduced charge carrier transfer and separation. Additionally, a direct Z-scheme system that relies on the band structures of Fe₂O₃ and g-C₃N₄ has been established. After all, electrons formed in the CB of Fe₂O₃ will pass through the intimate 2D/2D interface to the VB of g-C₃N₄ and afterward recombine with the hs⁺ through d-p conjugation, hence impeding photogenerated charge carrier recombination. As an effect, photoinduced electrons and holes accumulate in g-C₃N₄ at CB and Fe₂O₃ (VB), respectively. Not only will this direct Z-scheme method increase exciton separation performance, but it also generates a major driving force for the light-driven splitting of water, thus increasing the ability of g-C₃N₄.

3.4. CO₂ Reduction

Over the past century, fossil fuel burning has raised CO₂ levels in the atmosphere, leading to widespread climate change. By using an effective photocatalyst, conversions of CO₂ to chemicals such as CO, CH₄, CH₃OH, HCOOH, and HCHO is a feasible approach to mitigate greenhouse influence and discourse the energy crisis [97,98]. However, the activation of CO₂ is challenging since the C-O bond has higher dissociation energy (750 kJ mol⁻¹) and requires several electrons. Further, the development of CO₂-intermediate via single e⁻ transmission was recognized in the reduction strategy as a rate-limiting path.

3.4.1. Mechanism

The CO₂ photoreduction is the energy conversion process involving three main steps: (i) absorbing photons and generating charge carriers; (ii) separating photogenerated carriers and transferring them from the interior to the photocatalyst surface; and (iii) reducing the CO₂ into value-added fuels and chemicals by photogenerated electrons [99,100]. Then the final products will be desorbed from the catalyst surface to facilitate the subsequent reaction. Note that the CO₂ reduction with H₂O is a very complicated reaction involving two mechanisms: (i) the 1e⁻ transfer process and (ii) the multiple proton-coupled e⁻ transfer process. In Figure 7A and mechanism I, the single electron can activate CO₂, forming CO₂*⁻ intermediate [101], which requires a potential of −1.9 V versus NHE (normal hydrogen electrode). Thus, this process is thermodynamically unfavorable and is a large recombination energy between the linear CO₂ and the CO₂*⁻ intermediate. In mechanism II, the route bypasses the CO₂*⁻ formation [102], and different products are yielded, e.g., CO, HCOOH, HCHO, CH₃OH, CH₄, and multi-carbon products (e.g., CH₃CH₂OH, C₂H₆, and C₂H₄),

Table 2. Possible products of CO₂ and water reduction with corresponding reduction potential (versus NHE at pH 7 in aqueous solution). Adapted from [106,107,108] Copyright 2019, Elsevier; 2020 The Royal Society of Chemistry; 2019, American Chemical Society.

Product	Reaction	E0 (V vs. NHE)
Hydrogen	$2{\mathrm{H}}_{2 }\mathrm{O}+2{\mathrm{e}}^{-} \to 2{\mathrm{OH}}^{-}+{ \mathrm{H}}_2$	−0.41 V
Carbon monoxide	${\mathrm{CO}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-} \to \mathrm{CO}+$ ${\mathrm{H}}_{2 }\mathrm{O}$	−0.51 V
Formic acid	${\mathrm{CO}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-} \to \mathrm{HCOOH}$	−0.58 V
Oxalic acid	$2{\mathrm{CO}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-} \to { \mathrm{H}}_2{\mathrm{C}}_2{\mathrm{O}}_4$	−0.87 V
Methanol	${\mathrm{CO}}_2+6{\mathrm{H}}^{+}+6{\mathrm{e}}^{-} \to { \mathrm{CH}}_3\mathrm{OH}+{ \mathrm{H}}_{2 }\mathrm{O}$	−0.39 V
Methane	${\mathrm{CO}}_2+8{\mathrm{H}}^{+}+8{\mathrm{e}}^{-} \to { \mathrm{CH}}_4+2{\mathrm{H}}_{2 }\mathrm{O}$	−0.24 V
Ethanol	$2{\mathrm{CO}}_2+12{\mathrm{H}}^{+}+12{\mathrm{e}}^{-} \to { \mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+3{\mathrm{H}}_{2 }\mathrm{O}$	−0.33 V
Ethane	$2{\mathrm{CO}}_2+14{\mathrm{H}}^{+}+14{\mathrm{e}}^{-} \to { \mathrm{C}}_2{\mathrm{H}}_6+4{\mathrm{H}}_{2 }\mathrm{O}$	−0.27 V

. In addition to the supply of electrons and protons, The C–C coupling needs more supply of electrons and protons, restricting yield and selectivity. Thus, stable intermediates should have participated in C–C couplings [103,104].

Notably, the mechanism is different when CO₂ is reduced by CH₄ and H₂ (

Table 3. The reaction process in which CO₂ is reduced by H₂ and CH₄ and the required ΔH_298K (kJ mol⁻¹). Adapted from [112] Copyright 2016 Royal Society of Chemistry.

Reaction	$\Delta {\mathrm{H}}_{298\mathrm{K} }(\mathrm{k}\mathrm{J} \mathrm{m}\mathrm{o}{\mathrm{l}}^{-1})$
${\mathrm{CO}}_2+{ \mathrm{H}}_2 \to \mathrm{CO}+$ ${\mathrm{H}}_{2 }\mathrm{O}$	41.2
${\mathrm{CO}}_2+4{\mathrm{H}}_2 \to { \mathrm{CH}}_4+2{\mathrm{H}}_{2 }\mathrm{O}$	−252.9
${\mathrm{CO}}_2+3{\mathrm{H}}_2\to { \mathrm{CH}}_3\mathrm{OH}+{ \mathrm{H}}_{2 }\mathrm{O}$	−49.5
${\mathrm{CO}}_2+{ \mathrm{CH}}_4 \to 2\mathrm{CO}+$ $2{\mathrm{H}}_{2 }$	247

) [109]. Two pathways involve in the CO₂ conversion to CO: (i) H₂ only acts as the reducing agent and does not participate in the intermediate formation; (ii) formate is the intermediate [110]. The CO₂ conversion with H₂ to CH₄ is called a Sabatier reaction, where CO is the intermediate determining two different reduction pathways. The mechanism of CO₂ reduction to CH₃OH is similar to CO₂ methanation [111]; CO₂ is converted to CO, then the CO is formed into CH₃OH. Note that the whole process can occur without the CO’s participation.

The CO₂ reduction to valuable chemicals with high yield and selectivity of product is still a big challenge. Firstly, the CO₂ molecule is highly stable due to its high dissociation energy of C=O (>750 kJ mol⁻¹). Thus, it requires significant energy input. Secondly, HER may compete when water is present as a reducing agent, which consumes H⁺ and electrons to produce H₂, and CO₂ is low solubility in water, so water reduction prefers to occur. Thirdly, various products can be generated during CO₂ reduction. Thermodynamically, methanol and methane tend to be formed due to less potential energy required. Regarding kinetics, CO and formic acid are easily formed due to the lower number of electrons required [113,114,115]. Thus, some conditions should be satisfied to enhance the CO₂ reduction: (i) photocatalysts should have an appropriate band structure; (ii) the band position matches well with the potential required for CO₂ reduction. The CB edge potential should be lower than the potential for CO₂ reduction, and the VB edge potential should be higher than the oxidation potential of the reducing agent. Additionally, the bandgap (the ideal one is 1.75–3.0 eV) should be close to the potential required for CO₂ reductions [116]. Furthermore, the efficient CO₂ adsorption on the surface is the basis for excellent photocatalytic activity; the adsorbed CO₂ and photogenerated electrons combine quickly to yield products.

3.4.2. Recent Progress

A theoretical potential of −1.9 V vs. NHE is required for initialization, and a higher overpotential is desired for exploited potentials. In 1979, Inoue et al. found the photoconversion of CO₂, and numerous photocatalysts have been well developed [102]. Due to the strong thermodynamic strength of CO₂ (G = −394.4 kJ mol⁻¹), the CO₂ activation to reactive intermediates on the surface of photocatalysts is a significant concern [117]. Thus, CO₂ adsorption and activation are critical for photocatalytic CO₂ conversions. Defective 2D photocatalysts exhibit exceptional CO₂ photoreduction activity. For instance, by reducing ultrathin ZnAl-LDH, V_O defects were created [50]. The ZnVO complexes acted as traps for CO₂ and H₂O molecules, promoting charge separation and enhancing CO₂ photoreduction activity to give CO. The presence of anion and cation vacancies can promote photocatalytic CO₂ reduction. A lamellar hybrid intermediate approach has been applied to prepare Bi₂WO₆ layers [118]. Sodium oleate was used to bind with Bi³⁺, forming lamellar Bi oleate complexes by self-assembling oleate ions in a head-to-head or tail-to-tail bilayer sequence to form a mesostructure. As Na₂WO₄ was injected and hydrothermally refined, whereas self-exfoliation was subjected to Bi₂WO₆ developed into a single-unit cell sheet, which is used for CO₂ photoreduction. Moreover, the constant bubbling of incredibly pure CO₂ gas was acquired in an aqueous solution. An average rate of 75 ${\mathrm{mol}}_{{\mathrm{CH}}_3\mathrm{OH}}$ g⁻¹ h⁻¹ was observed over Bi₂WO₆ layers over a 5 h period, which is ~3 and 0.5 times faster than Bi₂WO₆ nanocrystals and bulk Bi₂WO₆, respectively. Engineered Zn vacancies into ZnIn₂S₄ increased charge separation efficiency [58], resulting in a CO formation rate of 33.2 mol g⁻¹ h⁻¹, which was ~3.6-fold of Zn vacancy-deficient ZnIn₂S₄. In a 2D/2D g-C₃N₄/NiAl-LDH hybrid heterojunction [119], the negatively charged g-C₃N₄ acted as nucleation sites for the in situ growth of NiAl-LDH, giving an intimate interface. The g-C₃N₄/NiAl-LDH exhibited a significantly higher CO conversion rate of 8.2 mol h⁻¹ g⁻¹ than pure g-C₃N₄ and NiAl-LDH. Moreover, the selectivity toward CO is ~82%. In addition, the g-C₃N₄/NiAl-LDH improved CO generation and the evolution of H₂ and O₂. Further, Xie et al. found that o-BiVO₄ layers with V vacancies in the forbidden band with a higher Fermi-level h⁺ density displayed a high rate of CH₃OH formation [60]. The V_V-rich o-BiVO₄ layers can enhance light absorption and carrier separation with a carrier lifetime of 143.6 ns. A CH₃OH formation rate was up to 398.3 mol g⁻¹ h⁻¹, which is ~1.4-fold of the V_V-deficient o-BiVO₄. The V_V-rich o-BiVO₄ layers also displayed outstanding cycling resistance for a 96 h reaction. Tonda et al. [120] reported a 2D/2D/2D heterojunction of Bi₂WO₆, reduced graphene oxide, and g-C₃N₄ (BWO/RGO/CN) for efficient photoreductions. Of note, RGO here acted as a capturer of the electrons from CN and as a redox mediator to improve the charge transfer between BWO and CN. The BWO/RGO/CN with 1 wt% RGO and 15 wt% BWO showed enhanced activity towards the reduction in CO₂ to yield CH₄ and CO using visible-light, compared to the P25 and other synthesized catalysts. BWO/RGO/CN achieved a H₂ selectivity of 92%. It was attributed to the 2D/2D/2D structure with large interfacial contact, which is favor to the rapid charge transfer and the hindering of the recombination of photoinduced electrons and holes.

The improvement of CO₂ adsorption also is critical to enhancing photocatalytic efficiency. As Ti₃C₂ MXene has good electrical conductivity and exposed metal sites, ultrathin Ti₃C₂/Bi₂WO₆ was obtained [121], exhibiting a strong interface between Ti₃C₂ and Bi₂WO₆. The –O or OH group on the Ti₃C₂ surface aids in capturing photoinduced e⁻ from Bi₂WO₆. Since Bi₂WO₆’s CB potential exceeds Ti₃C₂’s Fermi stage, the photoinduced e⁻ passes from Bi₂WO₆ to Ti₃C₂. Ti₃C₂/Bi₂WO₆-nanosheet pores with increased specific surface area promoted CO₂ adsorption, which reacts to CH₄ and CH₃OH with photoinduced e⁻. Of note, O₂ can be converted to an H₂O by-product in the photocatalytic CO₂ conversion (Figure 8a). The alkalinization of co-catalyst (Ti₃C₂ MXene) with P25 was applied by Ye et al., which stemmed in a substantial development in photocatalytic CO₂RR [122]. The 5% Ti₃C₂(OH)₂ doped P25 (TC-OH/P25) shows significant outcomes towards CH₄ release than unmodified TC/P25 (Figure 8b). Accordingly, DFT investigations revealed that CO₂ adsorption energy on TCF (F termination) exceeded that of TC-OH (OH termination). As a response, CO₂ molecules are adsorbed onto TC-OH, forming activated CO₃²⁻. Facilitating activation sites, charging isolation, sufficient CO₂ adsorption, and outstanding electrical conductance of alkalinized MXene resulted in significant photocatalytic progress.

Xu et al. [125] reported TiO₂/Ti₃C₂ composites for CO₂RR to generate CH₄ (Figure 6c). The rice crust morphology of TT550 and TT650 was observed to differ from Ti₃C₂ (Figure 8d–f). The e⁻ is efficiently transferred to TiO₂, and the unique morphology provides abundant active sites to enhance photocatalytic performance. However, evaluating TiO₂/Ti₃C₂T_x systems was difficult because of the different morphology and surface modification by MXene, and TiO₂ phases. They recently fabricated an ultrathin heterojunction of Ti₃C₂/Bi₂WO₆ (Figure 8g). The separation and transport of photogenic charges were improved due to intensive electronic coupling and physical effects. Ti₃C₂ (2%)/Bi₂WO₆ showed the highest CH₄ release rate (Figure 8h). Moreover, the large interfacial contact area of 2D/2D heterojunction offered shortened diffusion lengths at the interfaces, generating superior charge mobility in contrast to 1D/2D and 0D/2D ones. These 2D/2D heterojunctions have engendered a new potential in photocatalytic CO₂RR.

4. Conclusions and Perspective

Engineered 2D materials are excellent for elementary photocatalytic processing and have many possible commercial uses. This review focuses on significant progress in applying 2D materials for photocatalytic solar conversion. Different surface defect forms, for instance, anion-cation vacancies, have been used to adjust the microstructure, atom coordination number, electronic structure, carrier concentration, or electrical conductivity of 2D MOs, thereby improving photocatalytic efficiency. The current study summarizes defect engineering, including anion and cation vacancy creation and several photocatalytic applications (water splitting, HER, and CO₂RR).

Despite rapid development in 2D MOs for photocatalysis, this field faces many obstacles. Apart from the recent advances outlined here, research in this field is still in its infancy; issues and challenges in the proposal, synthesis, and application of defective 2D MOs remain. Though various top-down and bottom-up approaches were used to synthesize 2D materials beyond graphene, large-scale preparation of 2D materials remains difficult. The mass development with specified surface defects would be critical for photocatalytic applications. In order to investigate diverse and abundant synthetic strategies for defect-rich 2D MOs with an atomic-scale thickness on a large scale are additionally assorted, and plentiful synthesis approaches should be investigated.

A variety of 2D materials, particularly those with a defect-rich design, would be unstable physicochemically. In the course of the store, along with photoreaction procedures, isolated nanosheets can endure irreversible aggregation and structural disintegration, resulting in the loss of advanced structural characteristics. Moreover, along with surface Vos, certain faults would be filled by ambient H₂O or O₂ during long-term photocatalytic action, negating the microenvironment’s distinct benefits.

Photocatalysts based on 2D MOs can be critical in solving the environmental and energy problems associated with photochemical conversion aided by sunlight. Furthermore, most photocatalytic activity is still in the manual trial-and-error stage, with many of the reaction mechanisms unclear. Certain underdeveloped, highly efficient 2D photocatalysts can be ignored due to the restricted preparation processes. With the introduction of machine learning and DFT computing, increased emphasis must be placed on developing more stable and efficient 2D materials and heterojunctions in two dimensions.

Likewise, environmental considerations about the solar-powered 2D device are essential for commercial applications. However, no research is being conducted on this subject at the moment. Biocompatibility evidence for 2D components used in biomedical applications may be used to approximate their environmental impact. While stable 2D binary compounds such as MoS₂ and MXenes are nontoxic, unstable MXenes such as tellurene are harmful. Further investigation of the 2D material’s long-term environmental impact is recommended.