Two-dimensional nanomaterials for photocatalytic CO₂ reduction to solar fuels

Abstract

As a “kill two birds with one stone” approach, photocatalytic CO₂ reduction to solar fuels can save supplying energy and simultaneously protect our environment. Specifically, the use of CO₂ as the starting carbon source can help with the required emission cuts. Meanwhile, it directly generates short-chain hydrocarbon products such as CH₄, CH₃OH, C₂H₆ and so on, which can serve as a renewable energy source (solar fuels) to alleviate the increasingly tense energy crisis. Two-dimensional (2D) nanomaterials possess several extraordinary advantages, including large surface-to-volume ratio, abundant active sites, atomic thickness, and a high fraction of coordinated unsaturated surface sites, making them promising candidates with high photocatalytic activity for CO₂ reduction. This review summarizes a series of typical 2D nanomaterials for photocatalytic CO₂ conversion, such as graphene-based photocatalysts, graphitic carbon nitride-based photocatalysts, 2D metal oxide-based photocatalysts, 2D metal chalcogenide-based photocatalysts, 2D metal oxyhalide-based photocatalysts, and layered double hydroxide-based photocatalysts. Furthermore, based on the characteristics of 2D materials and the current status of research on photocatalytic CO₂ reduction, the challenges and opportunities of 2D materials as prospective photocatalysts for CO₂ reduction will also be discussed.

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1. Introduction

Along with the process of global industrialization, global warming from greenhouse gases caused by the ongoing burning of fossil fuels has drawn increasing attention.^1–4 Carbon dioxide (CO₂) is believed to be a major greenhouse gas that is present in the atmosphere and plays a significant role in global warming.^5–11 Therefore, many great strides are made through the fixation and conversion of greenhouse CO₂ to reduce the regional and global CO₂ emissions. To date, several efficient methods have been explored to reduce the CO₂ concentration in the atmosphere, such as thermochemical,^12,13 biological,¹⁴ electrochemical^15–17 and photocatalytic¹⁸ methods. Compared with other methods, photocatalytic reduction of CO₂ motivated by the natural photosynthesis in green plants is one of the most promising and compelling approaches. Photochemical CO₂ reduction has also been regarded as a “kill two birds with one stone” approach in terms of saving supplying energy and simultaneously protecting our environment.¹⁹ On the one hand, the use of CO₂ as the starting carbon source can help with the required emission cuts. On the other hand, it directly generates short-chain hydrocarbon products such as CH₄, CH₃OH, C₂H₆ and so on, which can serve as a green and renewable energy source (solar fuels) to alleviate the increasingly tense energy crisis.^3,20 Since the initial report on photoelectrocatalytic reduction of CO₂ to organic compounds (formic acid, formaldehyde, methanol and methane) was conducted by Inoue and co-workers in 1979,¹⁸ various types of catalysts have been reported in the field of CO₂ photoreduction, including metal oxides,^18,21–35 metal sulfides,^36–42 graphitic carbon nitride, graphene oxide, carbon nanofiber,^43,44 carbon quantum dots,⁴⁵ and metal–organic frameworks.^46–48 In particular, the pace of development of CO₂ photoreduction has recently increased enormously, owing to the promoting effect of advanced technologies (e.g. nanotechnology and in-site advanced characterization) on the development of novel photocatalysts. Unfortunately, the commercial and practical applications of these photocatalysts in the reduction of CO₂ are still limited because of the low CO₂ conversion efficiencies caused by the rapid electron–hole carrier recombination rate and low light utilization.²⁰ Therefore, diverse strategies such as band gap tuning, morphology control, surface oxygen vacancy engineering, exposure of highly active crystal facets, utilization of co-catalysts and heterojunction construction have been explored to improve the activity of photocatalytic CO₂ reduction.^{1,4,6,19,20,49–53}

Ultrathin two-dimensional (2D) nanomaterials represent an emerging class of nanomaterials that possess sheet-like structures that are only single- or few-atoms thick (typically less than 5 nm), but their lateral size is larger than several hundred nanometers.^54,55 Since the discovery of mechanically exfoliated graphene from graphite using Scotch tape by Novoselov and co-workers in 2004,⁵⁶ ultrathin 2D nanomaterials have attracted significant attention owing to their unprecedented physical and chemical properties and versatile applications in electronics,^57,58 optoelectronics,^59–61 energy storage and conversion,^62–66 catalysis,^63,67–71 environment,^6,72,73 sensors,^74–77 biomedicine,^78–80 and so on. At the same time, graphene-like ultrathin 2D nanomaterials have also sprung up, for instance, graphitic carbon nitride (g-C₃N₄),^81–87 hexagonal boron nitride (h-BN),^88–90 transition metal dichalcogenides (TMDs),^91–95 layered metal oxides,^96,97 layered double hydroxides (LDHs),^97–99 MXenes,¹⁰⁰ black phosphorus (BP),^101,102 silicene,^103,104 inorganic perovskites,^105,106 and organic–inorganic hybrid perovskites.¹⁰⁷

Thus far, the utilization of 2D nanomaterials in the field of photocatalysis has fascinated many researchers, and has rapidly become one of the hottest research themes. In comparison with zero-dimensional (0D) and one-dimensional (1D) materials, 2D nanomaterials possess several extraordinary advantages, endowing them with promising potential for photocatalytic applications (see Fig. 1). Firstly, 2D nanomaterials have larger surface-to-volume ratio over their bulk counterparts.^6,73,108,109 Thus, it is easy to understand that there are more effective active sites on the surface of ultrathin 2D nanomaterials, which can significantly improve the photocatalytic activity. Secondly, the atomic thickness is of great benefit to light harvesting and mass transport.^55,110,111 Above all, the ultrathin structure can minimize the charge migration distance from the bulk to the surface, reducing carrier recombination and potentially improving the photocatalytic performance.¹¹² Thirdly, the high fraction of coordinated unsaturated surface sites can function as active centers and come into contact with the substrate intimately.^113,114 Thus, ultrathin 2D nanomaterials are excellent platforms to prepare multicomponent photocatalysts to meet the requirements for various photocatalytic applications.^54,115,116

Fig. 1 Schematic illustration of the superior advantages of 2D nanomaterials for photocatalytic applications in comparison with 0D and 1D materials.

Given their extraordinary properties, 2D nanomaterials provide a wide range of opportunities for constructing diverse forms of composite photocatalysts with high activity and selectivity for CO₂ reduction. Particularly, preparing fine-tuned and robust 2D nanomaterial photocatalysts is necessary to meet the practical requirements of CO₂ reduction. Although there are some comprehensive reviews on 2D materials covering photocatalysts,^6,72,73 a review on 2D nanomaterials as photocatalysts for CO₂ reduction has never emerged. So we aim to prepare an overview of the recent progress in preparing 2D nanomaterial photocatalysts for CO₂ reduction and provide insights into their unique properties. In this feature article, we summarize a series of typical 2D nanostructured photocatalysts for CO₂ reduction, including graphene-based photocatalysts, graphitic carbon nitride-based photocatalysts, 2D metal oxide-based photocatalysts, 2D metal chalcogenide-based photocatalysts, 2D metal oxyhalide-based photocatalysts, and layered double hydroxide-based photocatalysts. Furthermore, based on the characteristics of 2D nanomaterials and the current status of research on photocatalytic CO₂ reduction, the challenges and opportunities of 2D nanomaterials as prospective photocatalysts for CO₂ reduction will also be discussed.

2. Nanostructured 2D materials as efficient photocatalysts for CO₂ reduction

2.1 Graphene-based photocatalysts

Graphene, as a single-layer sp²-bonded carbon nanosheet with honeycomb rings, is a zero bandgap semiconductor, possessing extraordinary properties, including large specific surface area, high electronic conductivity, high transparency and good flexibility.^56,108 All of these make graphene an ideal candidate for constructing new types of graphene-based photocatalysts to efficiently enhance the photocatalytic activity, such as photocatalytic pollutant degradation and hydrogen production.^{11,72,73,117,118} Until 2011, investigations on photocatalytic CO₂ reduction using graphene-based photocatalysts were not reported.¹¹⁹ This is a relatively recent advancement compared to the development of other photocatalysts. Since the first exploratory study by Liang coupling graphene with TiO₂ to improve the visible-light photocatalytic CO₂ reduction activity, there has been an exponential increase in the scientific research on the subject of graphene-based nanocomposites for CO₂ conversion. A summary of developments of graphene-based photocatalysts for CO₂ reduction is shown in Table 1. Because of coupling graphene, there are some fascinating advantages of graphene-based photocatalysts for CO₂ reduction, such as suppressing photogenerated carrier recombination, increasing the specific surface area, improving CO₂ adsorption and activation, enhancing the photostability, improving nanoparticle (NP) dispersion, reducing NP size and enhancing light absorption.⁶ Therefore, a variety of articles on graphene-based photocatalysts for CO₂ reduction have been reported.^6,55,72,120 In view of the dimensionality and size difference, 2D graphene-based photocatalysts composed of two components can be classified into representative categories of graphene-based photocatalysts for photocatalytic CO₂ reduction, namely: 0D/2D graphene-based photocatalysts, 1D/2D graphene-based photocatalysts and 2D/2D graphene-based photocatalysts (Fig. 2).

Table 1 Summary of various graphene-based photocatalysts toward CO₂ reduction

Dimensionality	Photocatalyst	Light source	Main products	Efficiency	Ref.
2D	Graphene oxide (GO)	300 W halogen lamp	CH3OH 0.172 mmol gcat^−1 h^−1		122
0D/2D	TiO2-less defective graphene	Ultraviolet (365 nm) and visible illumination	CH4 7 times higher than P25 under visible light		119
	TiO2/RGO	15 W energy-saving daylight bulbs	CH4 0.135 μmol gcat^−1 h^−1		151
	TiO2/GO	450 W Xe lamp (250–388 nm)	CO 700 μmol gcat^−1 h^−1		152
	TiO2–graphene 2D sandwich-like hybrid nanosheets	300 W Xe lamp	CH4 8 μmol g^−1 h^−1, C2H6 16.8 μmol g^−1 h^−1		131
	Anatase nitrogen-doped TiO2 nanoparticles with exposed {001} facets deposited on the graphene sheets (N–TiO2-001/GR)	15 W energy-saving daylight lamp	CH4 3.70 μmol gcat^−1 (10 h)	AQY = 0.0603%	121
	Doped-oxygen-rich TiO2–GO (OTiO2–GO)	15 W energy-saving daylight bulbs	CH4 1.718 μmol gcat^−1 (6 h)		153
	Graphene oxide supported oxygen-rich TiO2	500 W Xe lamp (>400 nm)	CH4 3.450 μmol gcat^−1 (8 h), CO, C2H4 and C2H6		154
	Graphitic carbon/hierarchically mesostructured TiO2	300 W Xe lamp	CH4 8.2 μmol gcat^−1, CO 53.6 μmol gcat^−1 (320 min)		155
	TiO2/graphene–tourmaline	500 W Xe lamp (<400 nm)	CH3OH 0.72 mmol g^−1 h^−1	QE = 0.1072	156
	P25/boron doped graphene nanosheets (P25/B–GR)	300 W Xe lamp	CH4 2.50 μmol g^−1 (2 h)		157
	Cu/GO	300 W halogen lamp	CH3OH and CH3CHO total 6.84 μmol gcat^−1 h^−1		130
	Reduced graphene-coated gold nanoparticles (rGO–AuNPs)	Xe lamp	HCOOH (>90%) CH3OH	QY = 1.52%	129
	ZnO/RGO	500 W Xe lamp	CH3OH 45.8 μmol gcat^−1 (10 h)		123
	ZnO/RGO	300 W Xe lamp	CH3OH 263.17 μmol gcat^−1 (3 h)		124
	Cu2O/RGO	150 W Xe lamp	CO 46 ppmg^−1	AQY = 0.344%	125
	Cu2O/RGO	500 W Xe lamp	CH3OH 41.5 μmol gcat^−1 (10 h)		126
	BiVO4/RGO	500W Xe lamp	CH3CH2OH 15.4 μmol gcat^−1		127
	Cu2Se–graphene	Visible light (300–450 nm)	CH3OH 2.63 μmol g^−1 h^−1		158
	CsPbBr3 QDs/GO	100 W Xe lamp with an AM 1.5 G filter	CO 58.7 μmol g^−1 CH4 29.6 μmol g^−1(12 h)		133
	Fe2O3/N-doped graphene	300 W Xe lamp	CO 4 times higher than G–Fe2O3		159
	NiOx–Ta2O5–rGO	400 W metal halide lamp	CH3OH 3.4 times higher than NiOx–Ta2O5		128
	Noble metal modified rGO/TiO2 (Pt, Pd, Ag and Au)	15 W energy-saving daylight bulbs	CH4 1.70 μmol gcat^−1 (6 h)		160
	TiO2/CdS/RGO/Pt	AM 1.5	CH4 0.0867 μmol h^−1 (50 mg photocatalyst)		161
	Porphyrin–graphene	Xe lamp (>400 nm)	CH4 57.38 μmol m^−2 h^−1, C2H2 112.89 μmol m^−2 h^−1		132
	Cobalt phthalocyanine immobilized on GO (CoPc–GO)	20 W white cold LED flood light	CH3OH 78.789 μmol gcat^−1 h^−1		134
	Ruthenium trinuclear polyazine complex immobilized on graphene oxide	20 W white cold LED flood light	CH3OH 3977.57 ± 5.60 μmol gcat^−1 (48 h)		135
	Chemically converted graphene coupled multianthraquinone substituted porphyrin (CCGCMAQSP)	450 W Xe lamp (>420 nm)	HCOOH 110.55 μmol (0.5 mg photocatalyst 2 h)		136
	CCG–BODIPY	450 W Xe lamp (>420 nm)	HCOOH 144.2 ± 1.8 μmol (0.5 mg photocatalyst 2 h)		137
	CCG–isatin-porphyrin	450 W Xe lamp (>420 nm)	CH3OH 11.21 μM (0.5 mg photocatalyst 1 h)		138
1D/2D	CdS nanorod-reduced graphene oxide	300 W Xe lamp (>420 nm)	CH4 2.51 μmol h^−1 g^−1		141
	WO3 nanobelt-graphene	300 W Xe lamp (>400 nm)	CH4 0.89 μmol (0.1 g, 8 h)		155
	Silver nanowire-doped reduced graphene oxide (RGO) integrated with a CdS nanowire network	300 W Xe lamp (>420 nm)	CO 3.648 μmol g^−1 CH4 1.153 μmol g^−1 (4 h)		162
2D/2D	TiO2 nanosheet-graphene	100 W mercury vapor lamp (365 nm)	CH4 9.5 μmol m^−2 h^−1		143
	Graphene–Ti0.91O2 hollow spheres	300 W Xe lamp	CH4 1.14 μmol g^−1 h^−1 CO 8.91 μmol g^−1 h^−1		144
	Sandwich-like graphene/g-C3N4	15 W energy-saving daylight lamp	CH4 5.87 mmol gcat^−1 (10 h)		145
	RGO/protonated g-C3N4	15 W energy-saving daylight lamp	CH4 13.93 mmol gcat^−1 (10 h)	Photochemical quantum yield 0.560%	146
Z-scheme system	Fe2V4O13/RGO/CdS	Visible light (>420 nm)	CH4 2.30 μmol g^−1 h^−1 O2 3.80 μmol g^−1 h^−1		148
	Metal sulfides/RGO/CoOx–BiVO4	300 W Xe lamp (>420 nm)	CO		149
	CdS/RGO/TiO2	300 W Xe lamp	CH4 0.115 μmol g^−1 h^−1		150

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Fig. 2 Schematic illustration of (a) 0D/2D, (b) 1D/2D, and (c) 2D/2D heterointerfaces.

2.1.1 0D/2D graphene-based photocatalysts. Because TiO₂ is a widely used semiconductor and highly effective photocatalyst for CO₂ reaction, most researchers focus on its modification and application.^1,7,19 Since the first report by Liang et al.¹¹⁹ that coupling of graphene with P25 TiO₂ is one of the most feasible ways to improve the photocatalytic activity for CO₂ conversion, numerous similar graphene/TiO₂ composite catalysts have been reported. Liang et al. synthesized solvent-exfoliated graphene/P25 TiO₂ photocatalysts with low graphene defect densities, which exhibited high photocatalytic ability for photocatalytic CO₂ reduction under visible-light. By variation of the graphene processing method and nanocomposite loading, 0.55 wt% graphene–TiO₂ nanocomposites exhibited the highest photoreductive activity under visible light illumination, and approximately 7-fold improvement in the photoreduction of CO₂ to CH₄ compared to pure P25 alone. The low defect density in solvent-exfoliated graphene contributed to improving the electrical mobility and facilitating photoreduction reactions by allowing more photoexcited electrons to effectively transfer to reactive sites (Fig. 3). In addition, P25 coupling with graphene exhibited high photocatalytic activity in the visible-light range, resulting from extending the optical absorption to longer wavelengths.

Fig. 3 Proposed photocatalytic mechanism for graphene–TiO₂ nanocomposites. The color scheme for the atoms is: carbon (gray), hydrogen (white), oxygen (red), and titanium (blue). Upon illumination, the photoexcited electron is injected into the graphene nanoplatelets, leaving behind a TiO₂ confined hole (green). Due to its lower density of defects, electrons in SEG are able to diffuse farther (depicted as the yellow pathway), thus sampling a larger surface area for adsorbed CO₂. Reprinted with permission from ref. 119. Copyright 2011 American Chemical Society.

Recently, the synthesis of crystals with highly reactive facets has been one of the most effective strategies to achieve high photoconversion efficiency of CO₂. Ong et al. reported a facile and reproducible solvothermal method to engineer anatase nitrogen-doped TiO₂ nanoparticles with exposed {001} facets (N–TiO₂-001) deposited on graphene (GR) sheets (N–TiO₂-001/GR).¹²¹ The N–TiO₂-001/GR nanocomposites exhibited high photocatalytic ability under visible light for the reduction of CO₂ to CH₄, which was approximately 11-fold stronger than that of pure TiO₂-001. The enhancement of photocatalytic activity was attributed to the effective charge anti-recombination of graphene, high catalytic activity of {001} facets relative to the {101} facets, and high absorption of the visible light region. Moreover, the as-synthesized photocatalysts were active even under irradiation by a low-power 15 W energy-saving daylight lamp.

Unlike the zero bandgap nature of pristine graphene, graphene oxide (GO) has a wider bandgap and may serve as a potential candidate for a metal-free photocatalyst. Hsu et al. reported that graphene oxides were excellent visible-light-responsive photocatalysts for the photoconversion of CO₂ to methanol by the modified Hummer's method.¹²² The CO₂ conversion rate of the modified graphene oxide (GO-3) was 0.172 mmol g_cat⁻¹ h⁻¹ under visible light, which was six-fold higher than that of pure TiO₂. They also confirmed that methanol was produced directly through photocatalytic CO₂ reduction instead of photo-dissociation of GOs by isotope tracer analyses with ¹³CO₂. Moreover, a variety of oxygen moieties spreading throughout the GO surface endow it with good solution processability, providing an accessible platform to construct various graphene-semiconductor composites for CO₂ conversion.¹²⁰ Apart from graphene/TiO₂ composites, composites of graphene and other photocatalysts, such as graphene/ZnO,^123,124 graphene/Cu₂O,^125,126 graphene/BiVO₄,¹²⁷ and graphene/NiO_x/Ta₂O₅ (ref. 128) have also been reported to be promising for the catalytic conversion of CO₂ into solar fuels, as shown in Table 1.

The dispersion of noble metal nanoparticles such as Pt, Pd, Ag and Au on photocatalysts has been demonstrated to enhance the visible-light-responsive photocatalytic conversion of CO₂ with H₂O into hydrocarbon fuels. The high activity is mainly because of the injection of hot electrons or the facilitated generation of electron–hole pairs by the enhanced near field on plasmonic nanoparticles. For example, Kumar et al. synthesized reduced graphene-coated gold nanoparticles (r-GO–AuNPs), which exhibited excellent photocatalytic activity for CO₂ reduction under visible-light.¹²⁹ Compared to Pt–AuNPs, the as-synthesized catalysts showed excellent selectivity for HCOOH (>90%) and higher conversion efficiency. The photocatalytic activity was excellent and mainly attributed to reduced graphene layers, which could play a role in highly efficient electron acceptors and transporters to enhance utilization of hot electrons for plasmonic photocatalysts, as shown in Fig. 4. In 2014, Chen's group reported graphene oxide decorated with copper nanoparticles by a simple microwave process, which demonstrated significant enhancement in the photocatalytic activity toward solar fuel production.¹³⁰

Fig. 4 Schematic of graphene-coated gold nanoparticles for CO₂ photoconversion. (A) HR-TEM image. (B) Energy dispersive X-ray mapping image of r-GO–AuNPs (Au (blue), carbon (red)). (C) Schematic representation of hot-electron generation and the role of the graphene layer as an efficient electron acceptor and transporter in generating hydrogen and subsequent conversion of CO₂ into formic acid (HCOOH) and methanol (CH₃OH). Reprinted with permission from ref. 129. Copyright 2016 American Chemical Society.

Apart from the photocatalytic conversion of CO₂ into single-carbon molecules, such as CO, CH₄, CH₃OH, and HCOOH, the reduction of CO₂ to multi-carbon compounds by graphene-based nanocomposite photocatalysts has been scarce. Notably, some recent studies have proven that this process can be achieved over graphene-based nanocomposite photocatalysts. Tu et al. fabricated 2D sandwich-like graphene–TiO₂ (G/TiO₂) hybrid nanosheets in a binary ethylenediamine (En)/H₂O solvent by a novel in situ simultaneous reduction-hydrolysis technique (SRH).¹³¹ When the prepared GO was reduced into graphene by En, TiO₂ nanoparticles through the hydrolysis of titanium(IV) (ammonium lactato) dihydroxybis were formed simultaneously. During this process, a large number of TiO₂ nanoparticles could be in situ uniformly loaded onto the graphene nanosheets to hinder their collapse and restacking, then formed a 2D sandwich-like structure (Fig. 5). On one hand, the specific structure exhibited higher specific surface area, more adsorption and reaction sites. On the other hand, the hybrid could favor the migration of photogenerated electrons from TiO₂ to graphene, and thus improved the charge separation efficiency by the formation of d–π electron orbital overlap. Moreover, abundant Ti³⁺ was found on the surface of the G/TiO₂ hybrid, caused by the reducing agent En. These Ti³⁺ ions could serve as photoinduced electron trapping sites and facilitated the electron–hole pair separation rate. As a result of these advantages, such assembled G/TiO₂ exhibited efficient photocatalytic activity for the conversion of CO₂ to valuable hydrocarbons (CH₄ and C₂H₆) in the presence of water vapor. So, the authors concluded that the synergistic effect of the surface-Ti³⁺ sites and graphene was beneficial for the generation of C₂H₆, and the production rate of C₂H₆ increases with increase in the content of the incorporated graphene. Because of the G–TiO₂ hybrid systems, ˙CH₃ radicals may be adsorbed on the surface of graphene via π-conjugation between the unpaired electron of the radical and aromatic regions of graphene. The electron-rich graphene may help to stabilize the ˙CH₃ species, which restrained the combination of ˙CH₃ with H⁺ and e⁻ to form CH₄. Meanwhile, subsequent increasing accumulation of ˙CH₃ on the graphene increased the chances of formation of C₂H₆ by the coupling of ˙CH₃ instead of H⁺. Furthermore, porphyrin–graphene composite photocatalysts have been utilized for the photocatalytic reduction of CO₂ to CH₄ and acetylene (C₂H₂) under visible light irradiation.¹³² Bismuth vanadate/reduced graphene oxide (BiVO₄/RGO) composites exhibited high photocatalytic activity of CO₂ conversion into C₂H₅OH (15.4 μmol g_cat⁻¹) under the illumination of simulated sunlight.¹²⁷

Fig. 5 SEM image (a, b) and schematic illustration (c) of sandwich like G/TiO₂. Reprinted with permission from ref. 131. Copyright 2013 Wiley-VCH.

In recent years, a variety of graphene-based photocatalysts coupling perovskite quantum dots,¹³³ transition metal complexes,^134,135 supramolecular metal complexes,¹³² and bio-enzymes^136–138 have emerged for CO₂ reduction, due to their excellent visible-light absorption, suitable redox potential, high quantum yield and high selectivity of products.

2.1.2 1D/2D graphene-based photocatalysts. Generally, a 1D nanostructure has a high aspect ratio with the diameter ranging from 1 to 100 nm, and it can be tube-, rod-, wire-, fiber- or belt-shaped. Compared to 0D nanostructures, 1D nanostructures have some advantages, such as high aspect ratio, large specific surface area, and excellent electronic or ionic charge transport properties.¹³⁹ After coupling with graphene, the photocatalytic activity of 1D/2D graphene-based photocatalysts is apparently higher than that of 0D/2D graphene-based photocatalysts. This view was demonstrated via an experiment wherein the graphene–TiO₂ nanowire nanostructure exhibited higher photocatalytic activity for the degradation of methylene blue (MB) than the graphene–TiO₂ nanoparticle nanostructure under solar simulator illumination.¹⁴⁰

Furthermore, Yu et al. successfully prepared a typical RGO–CdS nanorod composite photocatalyst for the photocatalytic reduction of CO₂ to CH₄ by a microwave-assisted hydrothermal method.¹⁴¹ RGO is regarded as an effective co-catalyst for enhancing photocatalytic CO₂ reduction, and the optimal RGO–CdS photocatalyst exhibits excellent reduction activity of CO₂ to CH₄, which was 10 times higher than that of pure CdS NRs. This was because RGO not only acted as an electron acceptor and transporter to efficiently separate the photogenerated charge carriers, but also enhanced the adsorption and activation of CO₂ molecules over the RGO–CdS composite, thus enhancing the photocatalytic reduction of CO₂ to CH₄. Furthermore, Liu and co-workers explored a ternary hybrid structure of 1D silver nanowire-doped RGO integrated with a CdS nanowire network, which exhibited enhanced visible-light-driven photocatalytic performance toward artificial photosynthesis, including the liquid-phase selective photoreduction of nitroaromatic compounds to amines, splitting of water to hydrogen, and the gas-phase CO₂ reduction. Wang et al. developed graphene–WO₃ nanobelt composites by a facile in situ hydrothermal method.¹⁴² By loading graphene, the conduction band minimum value of WO₃ can be elevated, leading to enhancement of its photocatalytic activity for CO₂ reduction.

2.1.3 2D/2D graphene-based photocatalysts. Compared to 0D/2D and 1D/2D nanocomposites, 2D/2D nanocomposites possess a relatively larger face-to-face contact area (Fig. 2), which accelerates the transfer and separation of photogenerated electron–hole pairs, thus enhancing their photocatalytic performance.^72,73,84 For instance, by loading single-wall carbon nanotubes (SWCNTs) and graphene respectively, on TiO₂ nanosheets, Liang et al. found that the G/TiO₂ composites exhibited higher photocatalytic activity for CO₂ reduction than the SWCNT/TiO₂ composites for CH₄ production.¹⁴³ This was because the 2D/2D structure of the G/TiO₂ composites had a larger contact interface than the 1D/2D structure of the SWCNT/TiO₂ composites, resulting in a superior electron migration rate and CO₂ photoreduction activity. Moreover, Tu et al. fabricated titania-graphene-based 2D/2D composite photocatalysts by synthesizing a robust hollow structure of alternating Ti_0.91O₂ nanosheets and graphene nanosheets (G–Ti_0.91O₂) with polymer spheres as sacrificial templates via a layer-by-layer assembly technique, as shown in Fig. 6.¹⁴⁴ The molecular-scale 2D contact between Ti_0.91O₂ nanosheets and graphene nanosheets enabled the photoinduced electron to migrate fast from Ti_0.91O₂ nanosheets to graphene, resulting in an enhanced lifetime of the photoinduced charge carriers. In addition, the hollow structure could act as a photon trap that allows the multiscattering of incident light. The above two factors led to the prepared samples exhibiting nine times higher photocatalytic activity than commercial P25 TiO₂ for the photocatalytic conversion of CO₂ into CO and CH₄. Photocatalytic CO₂ reduction have also been reported in g-C₃N₄/graphene-based composites for photocatalytic CO₂ reduction,^84,145,146 which will be discussed in detail below.

Fig. 6 Schematic illustration of the procedure for preparing the LBL-assembled multilayer-coated spheres consisting of titania nanosheets and GO nanosheets, followed by microwave reduction of GO to G. Reprinted with permission from ref. 144. Copyright 2015 Wiley-VCH.

2.1.4 Others. In recent years, by mimicking natural photosynthesis in green plants, all-solid-state Z-scheme systems using solid conductive medium as redox mediators have been proved to be an efficient tool to achieve high efficiency photocatalytic water splitting and CO₂ reduction.^52,53 Graphene or reduced graphene oxide (RGO) was found to act as a promising solid electron mediator in the Z-scheme photocatalysis system to enhance the performance of photocatalytic CO₂ reduction.¹⁴⁷ Li et al. fabricated a Fe₂V₄O₁₃/RGO/CdS Z-scheme system perpendicularly growing on a stainless-steel mesh for the photocatalytic reduction of CO₂ to CH₄.¹⁴⁸ The introduction of the RGO interlayer provides a high-speed charge channel in the Fe₂V₄O₁₃/RGO/CdS heterostructure due to its superelectronic mobility, leading to more efficient spatial separation of the photogenerated charge and subsequent high photoconversion efficiency of CO₂ (Fig. 7). In the same year, similar research on a CdS NSs/rGO/TiO₂ Z-scheme system was explored for the photocatalytic reduction of CO₂ to CH₄.¹⁴⁹ In addition, Iwase et al. successfully achieved Z-schematic water splitting under visible light irradiation upon combining photocorrosive CuGaS₂ as an H₂-evolving photocatalyst, an RGO electron mediator, and visible light-driven CoO_x-loaded BiVO₄ as an O₂-evolving photocatalyst.¹⁵⁰ They also found that the Z-schematic photocatalyst composite is active in CO₂ reduction when using water as the sole electron donor under visible light. However, the selectivity of CO₂ reduction was not very high.

Fig. 7 Schematic illustration of the photocatalytic conversion of CO₂ into CH₄ over a Fe₂V₄O₁₃/RGO/CdS Z-scheme system. (Fe₂V₄O₁₃ ECB: −0.55 eV, EVB: 1.28 eV; CdS ECB: −0.52 eV, EVB: 1.88 eV vs. NHE). Reprinted with permission from ref. 148. Copyright 2015 Royal Society of Chemistry.

2.2 Graphitic carbon nitride-based photocatalysts

As a fascinating two-dimensional conjugated polymer, graphitic carbon nitride (g-C₃N₄) has attracted widespread attention as a metal-free and visible-light-active photocatalyst in the arena of solar energy conversion and environmental remediation.^{82,87,163,164} This is due to its unique band structure, excellent thermal and chemical stability, non-toxicity, relative ease of synthesis and earth-abundant nature. Since Wang and co-workers first reported g-C₃N₄ as a novel photocatalyst that exhibited photoactivity for H₂ generation under visible-light irradiation,¹⁶⁵ the number of articles on g-C₃N₄-based photocatalysts shot up dramatically. However, the photocatalytic rate of pristine g-C₃N₄ is still low, due to the high recombination rate of charge carriers, small specific surface area, low electric conductivity, and lack of absorption above 460 nm.^84,85,166 To overcome these shortcomings, some measures on structural modifications, copolymerization, elemental doping, coupling with metals or noble metals, incorporation with other semiconductors or metal phosphides, and so on have been taken to improve its photocatalytic efficiency.^{82,84,85,87,163,164,167–171} In this part, we present a summary of g-C₃N₄-based photocatalysts for CO₂ reduction, as shown in Table 2.

Table 2 Summary of various graphitic carbon nitride-based photocatalysts toward CO₂ reduction

Dimensionality	Photocatalyst	Light source	Main products	Efficiency	Ref.
2D	Porous g-C3N4	300 W Xe lamp (>420 nm)	CO		173
	U–g-C3N4 (derived from urea)	300 W Xe lamp (>420 nm)	CH3OH 15.1 μmol C2H5OH 10.8 μmol (0.2 g photocatalyst 12 h)	AQE = 0.18	174
	Bulk g-C3N4	300 W Xe lamp	CH3CHO		175
	g-C3N4 nanosheets	300 W Xe lamp	CH4		175
	C3N4–MCF (using MCF as templates)	UV-visible light	CH3CHO 8.0 μmol g^−1, CH4 0.05 μmol g^−1 (1 h)		176
	C3N4–SBA-15 (using SBA-15 as templates)	UV-visible light	CH3CHO 3.5 μmol g^−1, CH4 0.78 μmol g^−1 (1 h)		176
	Ultra-thin nanosheet assemblies of g-C3N4	300 W Xe lamp	CH4 1.39 μmol h^−1 g^−1, CH3OH 1.87 μmol h^−1 g^−1		177
	Sulfur-doped g-C3N4	300 W Xe lamp	CH3OH 1.12 μmol g^−1 (3 h)		178
	C3N4 derived from urea and barbituric acid	300 W Xe lamp (>420 nm)	CO 31.1 μmol h^−1 (30 mg photocatalyst)		179
	Amine-functionalized g-C3N4	300 W Xe lamp	CH4 0.34 μmol h^−1 g^−1, CH3OH 0.28 μmol h^−1 g^−1		180
	Hydroxyl-rich g-C3N4 nanosheets	300 W Xe lamp (>420 nm)	CH4 7.47 μmol g^−1 (8 h)		181
	K-incorporated amino-rich C3N4	1 sun simulated sunlight from a Xe lamp	CO and CH4 5.4 and 5.6 times higher than urea-polymerized g-C3N4		182
	Thiourea derived g-C3N4	300 W Xe lamp (>200 nm)	CO 10.5 μmol gcat^−1 CH4 1.2 μmol gcat^−1 (5 h)		183
0D/2D	Pt/g-C3N4	300 W Xe lamp	CH4, CH3OH and HCHO		184
	Pt/g-C3N4	15 W energy-saving daylight bulb	CH4 13.02 μmol gcat^−1 (10 h)		185
	Pd/g-C3N4	300 W Xe lamp	CO, C2H5OH and CH4		186
	N–TiO2/g-C3N4	300 W Xe lamp	CO 14.73 μmol (0.1 g photocatalyst 12 h)		187
	TiO2/B-doped g-C3N4	300 W Xe lamp (>420 nm)	CH4 106 μmol (0.2 g photocatalyst 8 h)		188
	CdS/g-C3N4	250 W lamp 365 nm	Methyl formate 1352.07 μmol h^−1 gcat^−1		189
	ZnO/g-C3N4	500 W Xe lamp	CO, CH3OH, CH4 and C2H5OH, total 45.6 μmol h^−1 gcat^−1		190
	WO3/g-C3N4	Light-emitting diode (435 nm)	CH3OH		191
	In2O3/g-C3N4	500 W Xe lamp	CH4 159.2 ppm (20 mg photocatalyst 4 h)		192
	CoOx/g-C3N4	300 W Xe lamp (>420 nm)	CO		193
	AgX/g-C3N4 (X = Cl and Br)	15 W energy-saving daylight Lamp	CH4 10.92 μmol gcat^−1 (10 h)		194
	AgCl/C3N4	15 W energy-saving daylight lamp	CH4		195
	CeO2/g-C3N4	300 W Xe lamp	CO 0.590 μmol h^−1 CH4 0.694 μmol h^−1 (50 mg photocatalyst)		196
	SnO2/B–P codoped g-C3N4	300 W Xe lamp (>420 nm)	CH4 49 μmol (0.2 g photocatalyst 8 h)		197
	Ru complex/C3N4	450 W Xe lamp	CO and HCOOH		201
	RuP/C3N4	Visible light (>400 nm)	HCOOH 67.7 μmol CO 17.7 μmol	AQY = 5.7%	202
		400 W, high-pressure Hg lamp	HCOOH 1.5 μmol g^−1 CO	AQY = 5.7%	204
	Ru(II) complex/C3N4	Visible light (>400 nm)	HCOOH		206
	RuRu′/Ag/C3N4 nanosheets	Visible light (>400 nm)	HCOOH		205
	Zeolitic imidazolate framework (Co-ZIF-9)/g-C3N4	Visible light (>420 nm)	CO 20.8 μmol	AQY = 0.9%	152
	[Co(bpy)n]^2+/g-C3N4	Xe lamp (300–795 nm)	CO	AQY = 1.6%	198
	Co4@g-C3N4	300 W Xe lamp (>420 nm)	CO 107 μmol g^−1 h^−1		200
	Co-porphyrin/C3N4	300 W Xe lamp (>400 nm)	CO 17 μmol g^−1 h^−1	AQY = 0.8%	199
	UiO-66/g-C3N4	300 W Xe lamp (400 nm < λ < 800 nm)	CO 9.9 μmol gCN^−1 h^−1		208
	Carbon nanodot/g-C3N4	Xe lamp (>400 nm)	CH4 29.23 μmol gcat^−1, CO 58.82 μmol gcat^−1 (10 h)	AQY = 0.076%	209
	Red phosphor/g-C3N4	500 W Xe lamp	CH4 295 μmol h^−1 g^−1		210
	B4C/C3N4	300 W Xe lamp (405 nm < λ < 723 nm).	CH4 0.84 μmol g^−1 h^−1		211
1D/2D	NaNbO3 nanowires/g-C3N4	300 W Xe lamp (>420 nm)	CH4 6.4 μmol h^−1 g^−1		218
	Pt–g-C3N4/KNbO3	300 W Xe lamp (>420 nm)	CH4 0.25 μmol h^−1 (100 mg)		219
2D/2D	Sandwich-like graphene/g-C3N4	15 W energy-saving daylight bulb	CH4 5.87 μmol gcat^−1 (10 h)		190
	rGO/g-C3N4	15 W energy-saving daylight bulb	CH4 13.93 μmol gcat^−1 (10 h)	AQY = 0.560%	146
	Mg–Al–LDH/C3N4	500 W Xe lamp	CH4 3.7 μmol (200 mg photocatalyst 24 h)	QE = 0.093%	221
	MnO2/g-C3N4	300 W Xe lamp	CO 9.6 μmol g^−1 h^−1		222
	AgBr/g-C3N4/nitrogen-doped graphene	300 W Xe lamp (>400 nm)	CH3OH 105.89 μmol g^−1, C2H5OH 256.45 μmol g^−1	QE = 3.41%	220
	Bi2WO6/C3N4	300 W Xe lamp (>420 nm)	CO 5.19 mmol g^−1 h^−1		223
Z-scheme	WO3/g-C3N4	Light-emitting diode	CH3OH 2.4 times higher than pure g-C3N4		191
	Ag3PO4/g-C3N4	500 W Xe lamp (>420 nm)	CH4, CH3OH, C2H5OH and CO, total 57.5 μmol h^−1 gcat^−1		224
	SnO2−x/g-C3N4	Simulated sunlight irradiation	CH4, CH3OH and CO, total 22.7 μmol h^−1 gcat^−1		170
	BiOI/g-C3N4	300 W Xe lamp (>400 nm)	CO 4.86 μmol g^−1 h^−1, CH4 0.18 μmol g^−1 h^−1		225
	Bi2WO6/g-C3N4	300 W Xe lamp (>420 nm)	CO 5.19 μmol g^−1 h^−1		223
	ZnO/g-C3N4	300 W Xe lamp	CH3OH 0.6 μmol h^−1 g^−1		226

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Different from graphene, g-C₃N₄ is a metal-free polymeric semiconductor photocatalyst with a moderate band gap of 2.7–2.8 eV.¹⁷² Because the conduction band (CB) edge of g-C₃N₄ is sufficiently negative to reduce CO₂ into hydrocarbons, such as CH₄ (−0.24 V), CH₃OH (−0.38 V), HCHO (−0.48 V) and HCOOH (−0.61 V), the photocatalytic CO₂ conversion could be achieved by the direct use of g-C₃N₄ photocatalysts (Fig. 8).^82,168 Dong et al. first reported that g-C₃N₄ fabricated from melamine hydrochloride could effectively convert CO₂ to CO photocatalytically in the presence of water vapor under visible-light without any cocatalyst.¹⁷³ Mao et al. and Niu et al. revealed the effect of g-C₃N₄ microstructures on the selectivity of photocatalytic CO₂ reduction under visible light.^174,175 Ovcharov and co-workers synthesized porous C₃N₄ using bulk and hard template (SBA-15 and MCF) pyrolysis of melamine.¹⁷⁶ They found that acetaldehyde was the major product of the photocatalytic CO₂ reduction regardless of the morphology of C₃N₄. Recently, Xia et al. prepared hierarchical g-C₃N₄ assembled by amine-functionalized ultra-thin nanosheets by a stepwise NH₃-mediated thermal exfoliation approach.¹⁷⁷ The as-synthesized g-C₃N₄ photocatalyst exhibited remarkably enhanced CO₂ photoreduction performance compared to the unmodified g-C₃N₄ due to reasons including enhanced light harvesting, increased CO₂ adsorption, strong redox ability of charge carriers, more surface active sites, and improved charge carrier transfer and separation.

Fig. 8 The redox potentials of the relevant reactions with respect to the estimated position of the g-C₃N₄ band edges at pH 7.

Doping is one of the most effective strategies for improving photocatalytic activity due to its effectiveness in broadening the optical response range of semiconductor materials.¹⁶⁸ For instance, Wang et al. synthesized sulfur-doped g-C₃N₄ (S–CN) with excellent photocatalytic CO₂ reduction activity by simple thermolysis of thiourea.¹⁷⁸ The band gap of S–CN was narrowed from 2.7 to 2.63 eV (Fig. 9), so more light can be absorbed and utilized to improve the photocatalytic activity. Then, DFT studies confirmed that the photoinduced electrons could easily jump from the VB to the impurity state or from the impurity state to the CB of S–CN owing to the sulfur doping, which induced additional electrons, resulting in spin polarization. As a result, the photocatalytic activity for CO₂ reduction to CH₃OH of the appropriately tuned S–CN was nearly 2.5 times that of undoped g-C₃N₄.

Fig. 9 (a) Schematic of the band structure of pure g-C₃N₄ (left) and S-doped g-C₃N₄ (right); (b) comparison of photocatalytic CH₃OH production over S-doped g-C₃N₄ (TCN) and undoped g-C₃N₄ (MCN) at 3 h under UV-vis light irradiation. Reprinted with permission from ref. 178. Copyright 2015 Elsevier.

Besides this, various methods of g-C₃N₄ surface engineering were shown to be effective for enhancing photocatalytic CO₂ reduction activity, including copolymerization,¹⁷⁹ amine functionalization,¹⁸⁰ hydroxylation,¹⁸¹ and chemical activation with KOH.¹⁸² Qin and co-workers fabricated barbituric acid-modified g-C₃N₄ by the copolymerization of urea with barbituric acid co-monomer.¹⁷⁹ The modified samples showed enhanced optical absorption, suppressed electron–hole recombination rates, and improved charge transfers, which made them efficient heterogeneous photocatalysts for conversion of CO₂ to CO under visible light. Hence, the optimal sample showed 15-fold-enhanced photocatalytic activity for CO₂ conversion into CO compared to the parent sample.

In 2016, Wang et al. reported thiourea derived g-C₃N₄ (TCN) with inferior activity in CO₂ reduction under visible light irradiation, but TCN exhibited higher CO₂ conversion efficiency at 200–400 nm UV light irradiation compared to g-C₃N₄ synthesized from urea (UCN) and commercial TiO₂ (P25).¹⁸³ They found that –NH_x species present in abundance, that was responsible for CO₂ activation, the enhanced reducing ability of photo-excited electrons under UV light and the higher photo-excited electron–hole separation efficiency could all contribute to the excellent performance of TCN in the CO₂ photocatalytic reduction using >200 nm UV light (Fig. 10). However, its visible-light activity was relatively weak because of its limited surface area.

Fig. 10 Schematic mechanism of CO₂ photocatalytic reduction with H₂O over TCN and UCN under >200 nm UV-vis light irradiation. Reprinted with permission from ref. 183. Copyright 2016 Elsevier.

Apart from the above-mentioned nanostructure engineering of g-C₃N₄, such as structural modifications, elemental doping, and surface engineering, a cocatalyst or other semiconductor materials can be loaded onto the surface of g-C₃N₄ to construct the heterojunctions, which enhance their charge separation and photostability, and lead to excellent photocatalytic performance for CO₂ reduction. Next, according to the different dimensions and components, we categorized g-C₃N₄-based heterojunction photocatalysts into three types, such as 0D/2D g-C₃N₄-based photocatalysts, 1D/2D g-C₃N₄-based photocatalysts and 2D/2D g-C₃N₄-based photocatalysts.

2.2.1 0D/2D g-C₃N₄-based photocatalysts. The incorporation of noble metals such as Pt and Pd, loaded on g-C₃N₄ or other semiconductors as cocatalysts is a fascinating strategy for improving CO₂ photoreduction activity and/or selectivity.^184–186 Yu and co-workers prepared Pt-deposited g-C₃N₄ nanocomposites using NaBH₄ as the reducing agent, and they found that the deposited Pt acted as an effective cocatalyst, which enhanced their activity and selectivity of g-C₃N₄ for photocatalytic CO₂ conversion into CH₄, CH₃OH and HCHO.¹⁸⁴ It is believed that the Pt cocatalyst not only facilitated the interfacial electron transfer from g-C₃N₄ to Pt NPs due to the lower Fermi level of Pt, but also lowered the overpotential for the CO₂ reduction. After that, Ong et al. synthesized Pt/g-C₃N₄ hybrid nanostructures via a safe and more environmentally friendly approach using ethylene glycol as both the solvent and the reducing agent.¹⁸⁵ The optimal sample exhibited 5.1-fold enhancement of CH₄ yield in comparison with pure g-C₃N₄. By depositing different facets of Pd cocatalysts on g-C₃N₄, Bai et al. found that photocatalytic CO₂ reduction could occur better on Pd{111} facets than Pd{100} facets.¹⁸⁶

Besides metal/g-C₃N₄-based photocatalysts, composites of g-C₃N₄ and semiconductor photocatalysts, such as g-C₃N₄/TiO₂,^187,188 g-C₃N₄/CdS,¹⁸⁹ g-C₃N₄/ZnO,¹⁹⁰ g-C₃N₄/WO₃,¹⁹¹ g-C₃N₄/In₂O₃,¹⁹² g-C₃N₄/CoO_x (ref. 193) and g-C₃N₄/other semiconductors have also been reported for photocatalytic conversion of CO₂ into solar fuels, as shown in Table 2. For example, Zhou et al. synthesized composites (g-C₃N₄–N–TiO₂) consisting of g-C₃N₄ and N-doped TiO₂ by a facile calcination, which exhibited enhanced photocatalytic performance for CO₂ reduction in the presence of water vapor at room temperature.¹⁸⁷ They found that the product composition could be tuned by varying the preparation parameters. In another study, In₂O₃/g-C₃N₄ composite photocatalysts were prepared through a simple solvothermal method by Cao and co-workers, and the hybrid structures exhibited improved photocatalytic activities for H₂ generation and CO₂ reduction.¹⁹² Such a remarkable improvement in the photocatalytic performance was attributed to the highly effective interfacial charge transfer across the In₂O₃/g-C₃N₄ heterojunction (Fig. 11).

Fig. 11 Schematic illustration of the photocatalytic process for H₂ evolution and CO₂ reduction on the In₂O₃–g-C₃N₄ nanohybrids. Reprinted with permission from ref. 192. Copyright 2014 Elsevier.

In 2016, Ong and co-workers reported AgX/protonated g-C₃N₄ (X = Cl and Br) heterojunction photocatalysts via a sonication-assisted deposition–precipitation approach.¹⁹⁴ The highest photocatalytic performance of the optimal 30AgBr/pCN sample was 34.1 and 4.2 times greater than that of pure AgBr and pCN, respectively. Comparing different halide ions, the photocatalytic activity of 30AgBr/pCN was 1.3 times greater than that of the optimal 30AgCl/pCN. The improvement in the photocatalytic activity of 30AgBr/pCN was ascribed to the formation of the AgBr/pCN heterojunction. The overlapping band potentials of pCN and AgBr matched well, which could form a type II heterojunction to enhance the separation efficiency of photogenerated electron–hole pairs (Fig. 12). Additionally, the formation of metallic Ag upon reduction of some AgBr nanoparticles during the photocatalytic process played a prominent role in the localized SPR effect by absorbing the visible light, thus generating electron–hole pairs, and further promoting the interfacial charge transfer across the Ag/AgBr interface. As a continuation from the previous work, the same research group found a similar phenomenon on an AgCl/C₃N₄ hybrid photocatalyst by using a protonation-assisted deposition–precipitation method.¹⁹⁵

Fig. 12 Schematic illustration of the band structures of (a) Ag/AgCl/pCN and (b) Ag/AgBr/pCN hybrid nanocomposites for the photoreduction of CO₂ with H₂O to CH₄ under light irradiation. Reprinted with permission from ref. 194. Copyright 2016 Elsevier.

Li et al. fabricated mesostructured CeO₂/g-C₃N₄ (m-CeO₂/g-C₃N₄) using SiO₂ nanoparticles as a hard template.¹⁹⁶ The remarkably enhanced performance of the heterogeneous nanocomposites for CO₂ photoreduction was ascribed to the synergistic influence of mutual activations of g-C₃N₄ and CeO₂. The migration of photogenerated electrons from g-C₃N₄ to m-CeO₂ and their trapping by Ce⁴⁺ led to the prevention of combination of photogenerated electrons and holes in g-C₃N₄ (g-C₃N₄ activation) and the generation of Ce³⁺ from Ce⁴⁺ reduction by the trapped electrons in m-CeO₂ (CeO₂ activation), both of which benefited the activity enhancements of CO₂ reduction (Fig. 13).

Fig. 13 Schematic illustration of the proposed charge transfer mechanism in the CeO₂/g-C₃N₄ heterojunction photocatalysts upon light irradiation. Reprinted with permission from ref. 196. Copyright 2016 Elsevier.

Recently, Raziq et al. synthesized SnO₂-coupled boron and phosphorus co-doped g-C₃N₄ (SO/B–P–C₃N₄) nanocomposites for effective photocatalytic conversion of CO₂.¹⁹⁷ Compared to the bare g-C₃N₄ nanosheets, the resulting SO/B–P–C₃N₄ nanocomposite showed enhanced photocatalytic performance for CO₂ conversion to CH₄ by ∼9 times under visible-light, which was due to the extended visible-light absorption from 460 to 550 nm after B–P co-doping, and the improved charge separation via the dopant-induced surface states and the coupled SO.

In addition, coupling cobalt,^{152,193,198–200} ruthenium,^201–207 rhenium and zirconium²⁰⁸ complexes on g-C₃N₄ nanosheets to form heterogeneous photocatalysts has also been regarded as a promising approach to improve the CO₂ reduction of g-C₃N₄ under visible light irradiation. Maeda et al. did a series of research on ruthenium complex/g-C₃N₄ composites for photocatalytic CO₂ reduction. In 2013, they developed a Ru complex/C₃N₄ heterogeneous photocatalyst system, by dispersing g-C₃N₄ powder in a methanol solution containing an appropriate amount of trans(Cl)-[Ru{4,4′-(CH₂PO₃H₂)₂-2,2′-bipyridine}(CO)₂Cl₂] (RuCP) with continuous stirring at room temperature overnight.²⁰¹ In this system, the ruthenium complex and C₃N₄ worked as the catalytic and the light-harvesting unit, respectively. The simple binary hybrid photocatalyst showed efficient photocatalytic conversion of CO₂ into HCOOH under visible-light, with a selectivity of >80% and a turnover number (TON) of >200 using triethanolamine as an electron donor. They clarified that the origin of the HCOOH produced was derived from CO₂ reduction via using isotope tracer experiments with ¹³CO₂. After that, they optimized the mononuclear Ru complexes by using four different Ru complexes, which are represented as RuH, RuMe, RuP, and RuCP, to increase the efficiency of the photocatalytic CO₂ reduction (Fig. 14).²⁰² RuP/C₃N₄ exhibited the best performance, with a selectivity of >80%, high TON (>1000) and apparent quantum yield value (5.7%) at 400 nm. Then, they found that the solvent had a strong influence on product selectivity.²⁰³ In 2016, they designed a new hybrid system RuRu′/Ag/mpg-C₃N₄ involving Ag-modified mesoporous g-C₃N₄ (mpg-C₃N₄) and a Ru^II binuclear complex for CO₂ reduction.²⁰⁴ The photocatalytic CO₂ reduction of this system followed the Z-scheme electron-transfer mechanism including step-by-step photoexcitation of C₃N₄ and RuRu′, as shown in Fig. 15. The hybrid RuRu′/Ag/mpg-C₃N₄ photocatalyst produced a TON of ∼33 000 for 48 h (with respect to the molar amount of RuRu′), which was 30 times greater than that reported previously using C₃N₄ modified with a mononuclear Ru(II) complex. However, the selectivity and TON of this system during CO₂ reduction in aqueous media obviously dropped (at most <80% and 660, respectively). Very recently, by replacing mesoporous g-C₃N₄ with g-C₃N₄ (NS–C₃N₄) nanosheets in this system, RuRu′/Ag/NS–C₃N₄ exhibited a good TON (>2000) and high CO₂ reduction selectivity (approximately 98%) for photocatalytic reduction of CO₂ to HCOOH in aqueous solution.²⁰⁵ These values represent the highest yet reported for a powder-based photocatalytic system for CO₂ reduction under visible light in an aqueous environment.

Fig. 14 CO₂ reduction using a Ru complex/C₃N₄ hybrid photocatalyst, along with structures of the Ru complexes used. Reprinted with permission from ref. 202. Copyright 2015 Wiley-VCH.

Fig. 15 Z-scheme CO₂ reduction using a hybrid of C₃N₄ and a binuclear Ru(II) complex. Reprinted with permission from ref. 204. Copyright 2016 American Chemical Society.

Besides this, coupling some metal free nanoparticles, such as carbon nanodots (CDs),²⁰⁹ red phosphor,²¹⁰ or boron carbide (B₄C)²¹¹ with g-C₃N₄ to form a heterogeneous photocatalyst could also improve its performance for photocatalytic CO₂ reduction. Recently, Ong et al. developed 0D/2D carbon nanodot (CD)-hybridized protonated g-C₃N₄ (pCN) (CD/pCN) heterojunction photocatalysts for the photocatalytic reduction of CO₂ by means of electrostatic attraction.²⁰⁹ The total amounts of CH₄ and CO generated from the optimal sample (3 wt% of CDs) were 3.6 and 2.28 times higher than those generated with pure pCN, respectively, which was mainly attributed to the synergistic interaction between pCN and CNDs (Fig. 16). That is to say, the well contacted heterojunction interfaces accelerated the effective migration of photoexcited electrons from pCN to CNDs, and then hampered the charge recombination. This was confirmed by the first-principles density functional theory (DFT) calculation.

Fig. 16 Charge transfer and separation in the enhanced photoreduction of CO₂ to CO and CH₄ using CND/pCN samples under visible-light or simulated solar irradiation. Reprinted with permission from ref. 209. Copyright 2017 Springer.

2.2.2 1D/2D g-C₃N₄-based photocatalysts. There are a lot of papers on 1D/2D g-C₃N₄-based photocatalysts for H₂ evolution and organic pollutants degradation.^212–217 However, the number of papers on 1D/2D g-C₃N₄-based photocatalysts for CO₂ reduction is very limited. For example, Shi and co-workers reported visible-light-responsive g-C₃N₄/NaNbO₃ nanowire photocatalysts via introducing polymeric g-C₃N₄ on NaNbO₃ nanowires.²¹⁸ The g-C₃N₄/NaNbO₃ heterojunction photocatalysts exhibited a remarkable enhancement of photocatalytic activity for CO₂ conversion into CH₄, which was almost 8 times higher than that of pure C₃N₄ under visible light. This was mainly due to the improved separation and transfer of photogenerated electron–hole pairs at the intimate interface of g-C₃N₄/NaNbO₃ heterojunctions, which originated from the well-aligned overlapping band structures of g-C₃N₄ and NaNbO₃. Similar performance was found in g-C₃N₄/KNbO₃ heterojunction photocatalysts by the same research group.²¹⁹

2.2.3 2D/2D g-C₃N₄-based photocatalysts. Recently, the incorporation of 2D g-C₃N₄ photocatalysts with other 2D nanomaterials forming 2D/2D g-C₃N₄-based heterojunction photocatalysts has drawn increasing attention. A larger 2D/2D face-to-face contact area could promote the charge transfer and separation efficiently to suppress the electron–hole recombination.⁸⁴ Therefore, it is a promising approach to construct efficient photocatalysts.

Ong and co-workers fabricated sandwich-like graphene–g-C₃N₄ nanocomposites via a one-pot impregnation-thermal reduction approach using urea and graphene oxide as precursors for the reduction of CO₂ to CH₄ under visible light.¹⁹⁰ Moreover, they found that the absorption band edge of the as-synthesized sample exhibited a slight red shift. This phenomenon was attributed to the formation of a C–O–C bond as a covalent cross-linker between graphene and g-C₃N₄. The optimal sample exhibited 2.3 times higher activity than pure g-C₃N₄, which was ascribed to the increased electron charge transfer across the interface of the graphene–g-C₃N₄ heterojunction to impede recombination. The article is the first report that the graphene-g-C₃N₄ hybrid nanocomposites showed visible-light photocatalytic activity for CO₂ reduction to CH₄. In another closely related study by the same research group in the same year, they synthesized a novel 2D/2D rGO–hybridized pCN (rGO/pCN) nanostructure constituted of the negatively charged rGO and the positively charged pCN by π–π stacking and electrostatic attraction (Fig. 17).¹⁴⁶ The amounts of CH₄ evolution of the rGO/pCN photocatalysts were remarkably larger than that of rGO/pure g-C₃N₄ (rGO/CN) under a low-power energy-saving daylight bulb, on account of forming an intimate contact between rGO and pCN across the heterojunction interfaces for efficient charge transfer and separation over rGO/pCN.

Fig. 17 Schematic illustration of the charge transfer and separation in the rGO/pCN nanocomposite for CO₂ reduction with H₂O to CH₄ under visible light illumination. Reprinted with permission from ref. 146. Copyright 2015 Elsevier.

In the same year, AgBr nanoparticles (NPs) supported on g-C₃N₄-decorated nitrogen-doped graphene (NG) ternary nanocomposites (ACNNG-x) were constructed by Li and his group, which presented excellent photocatalytic performance for organic contaminant degradation and CO₂ reduction under visible light.²²⁰ Compared with the single component, this ACNNG-x superhybrid photocatalytic system could greatly overcome the drawbacks and realized high charge separation efficiency, strong redox ability, and long-term stability. Such excellent performance was attributed to the distinctive heterojunction system, which could successfully improve the photoinduced carrier separation efficiency and help to suppress the recombination of electrons–holes, leaving more charge carriers to form reactive species.

In addition, 2D–2D Mg–Al–LDH/C₃N₄,²²¹ MnO₂/g-C₃N₄,²²² and Bi₂WO₆/g-C₃N₄ (ref. 223) have also been reported for photocatalytic CO₂ reduction.

2.2.4 Others. In contrast to the above type II isotype heterojunctions, which will weaken the reduction and oxidation ability of the photoinduced holes and electrons, a solid-state Z-scheme heterojunction has been constructed to overcome the weakness for enhanced photocatalysis. For example, Ohno et al. synthesized WO₃/g-C₃N₄ nanocomposites using a planetary mill, which exhibited a significant improvement in photocatalytic CO₂ reduction under visible-light.¹⁹¹ They concluded that the Z-scheme charge transfer occurred over the WO₃/g-C₃N₄ photocatalyst, which led to the efficient charge transfer and separation between the two semiconductors. The above electron behavior was confirmed by double-beam photoacoustic spectroscopy (DB-PA) and WO₃/g-C₃N₄ could utilize both the high oxidation ability of WO₃ and high reduction ability of g-C₃N₄.

He and co-workers fabricated an Ag₃PO₄/g-C₃N₄ composite photocatalyst by an in situ deposition method and investigated it in the photocatalytic reduction of CO₂ to fuels (CH₄, CO, CH₃OH, and C₂H₅OH) for the first time.²²⁴ The optimal Ag₃PO₄/g-C₃N₄ photocatalyst showed a CO₂ conversion rate of 57.5 μmol h⁻¹ g_cat⁻¹, which was 6.1 times higher than that of pristine g-C₃N₄ under simulated sunlight irradiation (Fig. 18a). On account of this phenomenon, a Z-scheme mechanism was proposed. Since the low conduction band of Ag₃PO₄ (E_CB = 0.45 eV) indicated that the electrons cannot reduce CO₂ to fuels, the traditional type II heterojunction (also known as a double-charge transfer mechanism) could not explain the above performance of Ag₃PO₄/g-C₃N₄. However, Ag nanoparticles were formed due to the photoreduction of Ag⁺ from Ag₃PO₄ under light irradiation, which might act as a charge transmission bridge to construct the Ag₃PO₄/g-C₃N₄ Z-scheme system (Fig. 18b). Because the CB edge of Ag₃PO₄ is more negative than the Fermi level of metallic Ag, the photogenerated electrons moved from the CB of Ag₃PO₄ to metallic Ag. Simultaneously, the holes in the VB of g-C₃N₄ moved from the VB of g-C₃N₄ to metallic Ag and combined with the electrons. This type of charge transmission efficiently enhanced the separation of electron–hole pairs and allowed the electrons and holes to remain on the CB of g-C₃N₄ and VB of Ag₃PO₄, respectively. Due to the negative E_CB of g-C₃N₄, these electrons have strong reducibility and can easily reduce CO₂ to CO, CH₄, CH₃OH, and CH₃CH₂OH, which is consistent with the results in Fig. 18a.

Fig. 18 (a) Photocatalytic reduction of CO₂ over Ag₃PO₄/g-C₃N₄ composites; (b) photocatalytic mechanism scheme of the Ag₃PO₄/g-C₃N₄ composite. Reprinted with permission from ref. 224. Copyright 2015 American Chemical Society.

In addition, the same group synthesized another solid-state Z-scheme SnO_2−x/g-C₃N₄ photocatalyst by a simple calcination approach for photocatalytic CO₂ reduction.¹⁷⁰ Based on the Z-scheme mechanism, the electrons from the CB of SnO_2−x combined with the photogenerated holes from the VB of g-C₃N₄, resulting in a strong reducing power of the excited electrons in g-C₃N₄ to reduce CO₂ to energy fuels such as CO, CH₄, and CH₃OH. Due to the direct Z-scheme mechanism, without employing any electron mediators, the rate of photocatalytic CO₂ reduction of the optimal composites was 4.3- and 5-fold enhancement compared to those of g-C₃N₄ and P25.

Though the number of reports on constructing Z-scheme photocatalysts for enhanced catalytic activity has gone up rapidly, direct experimental evidence on how the photogenerated electrons transfer across the junctions, especially all-solid state Z-scheme photocatalysts, has never been reported. Recently, a series of BiOI/g-C₃N₄ photocatalysts were prepared via a simple deposition approach by Wang and co-workers to clarify the charge transfer process.²²⁵ The as-synthesized BiOI/g-C₃N₄ composites exhibited more highly efficient photocatalytic activity than pure g-C₃N₄ and BiOI and the product yields changed remarkably depending on the reaction conditions such as irradiation light wavelength (Fig. 19). What's more important was that an indirect Z-scheme charge transfer mode was verified by detecting the electron mediator I₃⁻/I⁻ pairs experimentally. The reaction mechanism was further proposed based on the detection of the intermediate ˙OH and H₂O₂. This work may be useful for rationally designing new types of Z-scheme photocatalysts and providing some illuminating insights into the Z-scheme transfer mechanism.

Fig. 19 Schematic diagram of photoinduced electron–hole separation processes: (a) double-charge transfer mechanism and (b) Z-scheme mechanism. Reprinted with permission from ref. 225. Copyright 2016 American Chemical Society.

Besides this, some other g–C₃N₄-based Z-scheme photocatalysts, such as Bi₂WO₆/g-C₃N₄ (ref. 223) and ZnO/g-C₃N₄ (ref. 226) have also been constructed, which showed high-efficient performance in the photocatalytic CO₂ conversion to fuels.

2.3 2D metal oxide nanomaterial-based photocatalysts

Over the past four decades, various metal oxides such as TiO₂, ZnO, SnO₂, WO₃ and Fe₂O₃ have been extensively explored as photocatalysts for CO₂ photoreduction to solar fuels. Because of the excellent stability, nontoxicity, low cost and easy availability, TiO₂ has been one of the most widely studied photocatalysts, while TiO₂ based 2D nanosheets through exfoliation of layered titanate have also drawn considerable attention.²²⁷ The reports on TiO₂ based 2D nanosheets focused on photodegradation and water splitting, but the amount of CO₂ reduction is limited. Xu et al. synthesized anatase TiO₂ single crystals composed of TiO₂ ultrathin nanosheets (2 nm in thickness) with 95% of exposed {100} facet, which exhibited about 5 times higher activity in both H₂ evolution and CO₂ reduction than the TiO₂ cuboids with 53% of exposed {100} facets.²²⁸ For the TiO₂ nanosheets, on one hand, the higher percentage of exposed {100} facets and larger surface area could offer more surface active sites. One the other hand, the superior electronic band structure which resulted from the higher percentage of the {100} facet contributed to expanding their activity.

Zou's group reported a series of TiO₂ based photocatalysts for photocatalytic CO₂ conversion to fuels,^{29,144,229–232} among which robust hollow spheres consisting of alternating TiO₂ nanosheets and graphene nanosheets for CO₂ conversion have been mentioned above.¹⁴⁴ After that, the same group designed multilayer Ti_0.91O₂/CdS hollow spheres consisting of alternating ultrathin Ti_0.91O₂ nanosheets and CdS nanoparticles via layer-by-layer self-assembly to achieve an all solid-state Z-scheme system with 7-times enhancement of the CH₄-production rate relative to pure Ti_0.91O₂ hollow spheres.²³² This was due to the efficient separation of photogenerated electron–hole pairs, which greatly prolonged the lifetime of charge carriers. In 2015, Liu and co-workers reported nanosheet-assembled hierarchical titanate yolk@shell microspheres with engineered multilevel microstructures and efficient photocatalytic CO₂ reduction performance by a one-pot solvothermal method.²³³

Moreover, ultrathin, single-crystal WO₃ nanosheets with about 4–5 nm in thickness, which corresponded to six repeating unit cells of monoclinic WO₃ along the c axis, were synthesized with a laterally oriented attachment of tiny WO₃ nanocrystals formed via a solid–liquid phase arc discharge route.²⁵ Because of size-quantization effects in this ultrathin nanostructure that extended the WO₃ band gap, the WO₃ nanosheets exhibited enhanced performance for photocatalytic reduction of CO₂, which did not happen in the bulk form (Fig. 20).

Fig. 20 (a) Calculated band positions of WO₃ nanosheets and commercial WO₃, relative to the redox potential of CO₂/CH₄ in the presence of water; (b) CH₄ generation over the nanosheets and commercial powder as a function of visible light irradiation time (λ > 420 nm). Reprinted with permission from ref. 25. Copyright 2012 American Chemical Society.

In addition to the above 2D transition metal oxides, Cu(II) nanocluster-grafted Nb₃O₈⁻ nanosheets,²³⁴ SnNb₂O₆ nanosheets,^70,235 ultrathin Bi₂WO₆ nanosheets and hierarchical microspheres scaffolded from ultrathin ZnGa₂O₄ nanosheets²² used as photocatalysts for CO₂ reduction have been reported as shown in Table 3.

Table 3 Summary of various 2D metal chalcogenides, metal chalcogenides, metal oxyhalides, layered double hydroxides and other 2D-based photocatalysts toward CO₂ reduction

Categories	Photocatalyst	Light source	Main products	Efficiency	Ref.
2D metal chalcogenide	TiO2 ultrathin nanosheets (2 nm)	300 W Xe lamp (λ > 300 nm)	CH4 5.8 ppm g^−1 h^−1		228
	Graphene/Ti0.91O2 hollow spheres	300 W Xe lamp	CH4 1.14 μmol g^−1 h^−1, CO 8.91 μmol g^−1 h^−1		144
	Ti0.91O2/CdS	300 W Xe lamp	CH4 0.1 μmol h^−1 (0.01 g photocatalyst)		232
	Amine-functionalized TiO2 nanosheet-assembled yolk@shell microspheres	300 W Xe lamp (>400 nm)	CH3OH 2.1 μmol h^−1 g^−1		162
	WO3 nanosheets	300 W Xe lamp (>420 nm)	CH4 16 μmol g^−1 (14 h)		25
	Cu(II) nanocluster-grafted Nb3O8^− nanosheets	Hg–Xe lamp (240–300 nm)	CO		234
	SrNb2O6 nanoplates	Xe lamp (λ = 300–780 nm)	CO 1.66 μmol, CH4 0.33 μmol (0.01 g photocatalyst 10 h)		235
	Monolayer SnNb2O6	300 W Xe lamp (>420 nm)	CH4 110.9 μL h^−1 g^−1		70
	Hierarchical microspheres scaffolded from ultrathin ZnGa2O4 nanosheets	300 W Xe lamp with an IR cut filter	CH4 6.9 ppm h^−1 (0.1 g photocatalyst)		22
2D metal chalcogenide	MoS2–TiO2	UV-vis light	CH4 10.6 μmol g^−1 h^−1		244
	MoS2/Bi2WO6	300 W Xe lamp (>420 nm)	CH3OH 36.7 μmol gcat^−1, C2H5OH 36.6 μmol gcat^−1 (4 h)		245
	WSe2–graphene	500 W Xe lamp (>300 nm)	CH3OH 5.0278 μmol g^−1 h^−1	QE = 0.8159	246
2D metal oxyhalide	BiOCl nanoplates	500 W Xe lamp	CO 8.1 μmol g^−1 (8 h)		257
	BiOCl nanosheets	300 W Xe lamp (a 250–380 nm UVREF filter)	CH4 41.48 μmol g^−1 (8 h)		256
	Bi4O5Br2 nanosheets	300 W Xe lamp (>400 nm)	CO 2.73 μmol g^−1 h^−1, CH4 2.04 μmol g^−1 h^−1	AQY = 0.21%	255
	BiOI/g-C3N4	300 W Xe lamp (>400 nm)	CO 4.86 μmol g^−1 h^−1, CH4 0.18 μmol g^−1 h^−1		225
	Olive-green few-layered BiOI	300 W Xe lamp (>420 nm)	CO 0.615 μmol h^−1, CH4 0.063 μmol h^−1 (0.15 g photocatalyst)	AQY = 0.14%	258
	BiOI nanosheets	300 W Xe lamp	CO 0.259 μmol h^−1 CH4 0.089 μmol h^−1 (0.05 g photocatalyst)		259
	Bi2O4-decorated BiOBr nanosheets	Xe lamp	CH4 3.7 μmol g^−1, CO 5.2 μmol g^−1 (2 h)		276
	BiOBr nanosheets	Xe lamp	CO 4.45 μmol g^−1 h^−1		277
Layered double hydroxides	Zn–Cu–M(III) (M = Al, Ga) LDHs	500 W Xe lamp	CO 620 μmol h^−1 gcat^−1		262
	Zn–Cu–Ga LDH	500 W Xe lamp	CH3OH 0.49 μmol h^−1 gcat^−1		264
	Zn–Cu–Ga LDH	500 W Xe lamp	CO and CH3OH		265
	Ag or Au/Zn–Ga–LDH	500 W Xe lamp	CO 30 nmol h^−1 gcat^−1, CH3OH 201 nmol h^−1 gcat^−1		266
	M^2+–M^3+ LDHs (M^2+ = Mg^2+, Ni^2+, and Zn^2+; M^3+ = Al^3+, Ga^3+, and In^3+)	200 W Hg–Xe lamp	CO		268
	Ni–Al LDH (Ni/Al = 4)	400 W high pressure Hg lamp	CO and CH4		269
	Fluorinated Mg–Al LDH and Ni–Al LDH	400 W high pressure Hg lamp	CO 93.3 μmol (0.5 g photocatalyst 20 h)		278
	M^2+–M^3+ LDHs (M^2+ = Co^2+, Ni^2+, Cu^2+, Zn^2+; M^3+ = V^3+, Cr^3+, Mn^3+, Fe^3+)	400 W high-pressure Hg lamp	CO 147.1 μmol (0.5 g photocatalyst 5 h)		271
	Mg–Al–LDH/C3N4	500 W Xe lamp	CH4 3.7 μmol (200 mg photocatalyst 24 h)	QE = 0.093%	221
	Cu–M–Al LDH (M = Mg, Zn, Ni)	500 W Xe lamp (380–760 nm)	CH3OH 0.210 mmol g^−1 h^−1		273
	Ce^3+–Ce^4+ LDHs	300 W Xe lamp	CO 1.68 μmol g^−1 h^−1		274
	Ru nanoparticle modified Mg–Al LDHs	300 W Xe lamp	CH4 277 mmol h^−1 g^−1		272
Others	Carbon-doped BN nanosheets	300 W Xe lamp (>420 nm)	CO 9.3 μmol (50 mg photocatalyst 2 h)		275

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2.4 2D metal chalcogenide-based photocatalysts

2D metal chalcogenide nanosheets, such as MoS₂, WS₂, SnS₂, TiS₂, MoSe₂, WSe₂, etc., have been attracting much attention recently owing to their excellent catalytic activity and low cost compared to noble metals.^{15,55,91,92,94,236–238} Among 2D metal chalcogenide nanosheets, MoS₂, composed of hexagonal layers of Mo atoms sandwiched between two layers of sulfur atoms, has been one of the most promising semiconductors due to its thickness-dependent band gap and abundance in nature as molybdenite.^95,239 Bulk MoS₂ material is a semiconductor with an indirect band gap of about 1.3 eV, which is unsuitable for photocatalytic reactions because of its insufficient oxidation/reduction potential. However, the isolated MoS₂ nanosheets possess a direct bandgap of 1.96 eV owing to the quantum confinement effect, which endows MoS₂ nanosheets with extraordinary photoinduced catalytic ability.^91,239 With these extraordinary properties, MoS₂ has frequently acted as a catalyst or non-noble metal co-catalyst combined with other photocatalysts to form a heterojunction to enhance the photocatalytic activity for photodegradation and water splitting.^{91,93,236,239–243} Until now, besides MoS₂, many other TMD nanosheets, such as WS₂ and NiS₂, have also been reported as excellent cost effective catalysts or cocatalysts.^94,237 Although there have been numerous reports on 2D metal chalcogenide nanosheets as electrocatalysts or photocatalysts for the production of hydrogen, only few reports focus on the photocatalytic reduction of CO₂ by using 2D metal chalcogenides as photocatalysts.

Recently, Tu et al. fabricated 2D MoS₂–TiO₂ hybrid nanojunctions, which exhibited photocatalytic activity for CO₂ conversion in aqueous solution.²⁴⁴ MoS₂ nanosheets proved to be an efficient co-catalyst, showed superior activity and selectivity for reducing CO₂ into CH₃OH in aqueous solution to the metal cocatalysts (like Pt, Au, and Ag) under UV-vis light irradiation. This was because the Mo-terminated edges with the metallic character and a high d-electron density benefit the electron transfer and the stabilization of CH_xO_y intermediates (Fig. 21).

Fig. 21 (a) Comparison of the photocatalytic activity of pure TiO₂ nanosheets and MoS₂/TiO₂ hybrid nanosheets with different MoS₂ amounts for CO₂ photoreduction. (b) Schematic illustration of the charge separation and transfer in the MoS₂/TiO₂ system for photoreduction of CO₂ to CH₃OH. Mo sites especially Mo-terminated edges on MoS₂ nanosheets benefit the stabilization of intermediate products (CH_xO_y). Reprinted with permission from ref. 244. Copyright 2017 Royal Society of Chemistry.

In the same year, few-layer MoS₂ loaded hierarchical flower-like Bi₂WO₆ (MoS₂/Bi₂WO₆) nanocomposites were synthesized via an impregnation-calcination method by Dai et al.²⁴⁵ The as-synthesized MoS₂/Bi₂WO₆ nanocomposites exhibited considerable enhancement of activity for photocatalytic reduction of CO₂ under visible light as compared to pure Bi₂WO₆. This was because the few layered MoS₂ as a cocatalyst has an intimate interfacial contact with Bi₂WO₆, which enhanced the visible light absorption, charge transfer and separation, thus boosting their photocatalytic activity. Furthermore, a possible photocatalytic mechanism was also proposed. The CO₃²⁻, HCO₃⁻ and H₂CO₃ generated in CO₂ aqueous solution would be the reactive substrates during the photoreduction reaction, proving the thermodynamic feasibility of CO₂ photoreduction.

Ali and co-workers reported WSe₂–graphene nanocomposites prepared via ultra-sonication and tested them in terms of the photocatalytic reduction of CO₂ to CH₃OH under irradiation with UV/visible light.²⁴⁶ Adding a sacrificial agent (Na₂S/Na₂SO₃) to the WSe₂–graphene nanocomposites could further improve the photocatalytic efficiency. After coupling with graphene, the binary structure promoted the electron transport and decelerated the recombination of the electron–hole pairs compared to pure WSe₂. This was the first article which reported that the WSe₂ chalcogenide family could be used as the photocatalyst for CO₂ reduction.

2.5 2D metal oxyhalide-based photocatalysts

Bismuth oxyhalides, BiOX (X = Cl, Br, and I), have recently attracted much attention for their unique layered structure, and excellent optical and electrical properties.^247,248 Bismuth oxyhalides have a layered structure consisting of [Bi₂O₂]²⁺ layers sandwiched between two slabs of halogen ions, resulting in the formation of self-built internal static electric fields and thus inducing outstanding photocatalytic activity.^249,250 Particularly, BiOX (X = Cl, Br and I) 2D nanosheets and 3D hierarchical structures with 2D nanoflakes displayed remarkable photocatalytic activity for photocatalytic removal of pollutants in air, water and biological contaminants.^251–254 For instance, Wang et al. prepared porous BiOCl micro-flowers constructed from ultrathin nanosheets (4 nm) with nearly 100% {001} facets exposed, which possessed a strong photooxidative ability to degrade the dye pollutants (RhB, MB, MO) directly and easily.²⁵¹ After that, Guan and co-workers reported high solar photocatalytic activity in ultrathin BiOCl nanosheets (2.7 nm) with almost fully exposed active {001} facets for photodegradation of RhB.²⁵³ However, because of the high positive conduction band minimum (CBM) of BiOX photocatalysts, the photocatalytic reduction activity was very low and the number of reports on photoreduction of CO₂ or H₂O using BiOX was very few.^247,248,255 Given this predicament, many strategies, including bismuth-rich,²⁵⁵ thickness-ultrathin,²⁵⁶ oxygen vacancy,²⁵⁷ and construction of Z systems,²²⁵ have been employed to achieve photocatalytic CO₂ reduction activity, as shown in Table 3.

Ye et al. reported olive-green few-layered (2.8 nm with four layers) BiOI with expanded spacing of the (001) facets and oxygen vacancy, which showed much higher photocatalytic activity for CO₂ reduction than pure bulk BiOI under visible or near-infrared (NIR) light (Fig. 22).²⁵⁸ The enhanced photocatalytic activity was due to the expanded facets spacing and oxygen vacancy of few-layered BiOI, thereby enhancing the separation efficiency of photoinduced carriers and photon absorption efficiency.

Fig. 22 (a) Model showing the direction of the internal electric field in BiOI-100; (b) model showing the direction of the internal electric field in BiOI-001 and (c) the photocatalytic mechanism of facet-dependent photocatalytic activity of BiOI nanosheets for CO₂ photoreduction. Reprinted with permission from ref. 259. Copyright 2016 Elsevier.

In the same year, the same group fabricated Bi₄O₅Br₂ microspheres assembled using ultrathin nanosheets (about 3.7 nm) via a glycerol precursor route.²⁵⁵ Due to the thickness-ultrathin and bismuth-rich strategies, the as-synthesized Bi₄O₅Br₂ displayed excellent photocatalytic reduction activity for CO₂ conversion compared to pure BiOBr and ultrathin BiOBr under visible light. Furthermore, they found that the thickness-ultrathin strategy was responsible for enhancement of the CO generation while the bismuth-rich strategy was responsible for improving the CH₄ generation for photoreduction of CO₂.

2.6 Layered double hydroxides-based photocatalysts

Layered metal hydroxides (LMHs) include layered single metal hydroxides (LSHs) and layered double hydroxides (LDHs).^99,260 This section mainly discusses LDH-based photocatalysts for photocatalytic CO₂ conversion. LDHs, of the general formula [M^II_1−xM^III_x(OH)₂]^x+·(An)^−x/n·mH₂O (in which M = metal and A = interlayer anion), widely treated as basic materials for CO₂ capture and storage, are also applied in photocatalytic CO₂ reduction, because of their strong sorption capacity for CO₂ in the layered space and tunable semiconductor properties (photocatalytically active sties, bang gap etc.) as a result of the choice of metal cations.^98,261,262 In 2011, Ahmed et al. first reported that Zn–Cu–M(III) (M = aluminum, gallium) LDHs could be used as a photocatalyst for CO₂ conversion using hydrogen as a reductant under UV-vis light.²⁶² Among the various LDHs, Zn–Al LDH showed good promise for CO₂ reduction to CO, whereas Cu-containing LDH was effective for CO₂ reduction to methanol. In order to improve the photocatalytic activity for CO₂ reduction, a reverse photofuel cell was designed using WO₃ as the photooxidation catalyst of water and a Cu–Zn–Ga LDH as the photoreduction catalyst of CO₂ (Fig. 23).²⁶³ Furthermore, the same group did a series of research studies to improve the photocatalytic activity and selectivity of LDH (Cu, Zn, Ga, Al) via different methods, like changing the rate of Cu and Zn, replacing interlayer carbonate anions, and using Pt, Pd and Au as cocatalysts.^264–267

Fig. 23 Schematic illustration of the flow of materials and electrons during photocatalytic CO₂ conversion in reverse photofuel cell-1 consisting of WO₃ and LDH1. Reprinted with permission from ref. 263. Copyright 2014 Royal Society of Chemistry.

Besides this, Teramura and Tanaka's group synthesized LDHs to reduce CO₂ into CO in water, and found that various M²⁺–M³⁺ LDHs (M²⁺ = Mg²⁺, Ni²⁺, and Zn²⁺; M³⁺ = Al³⁺, Ga³⁺, and In³⁺) exhibited photocatalytic activity.²⁶⁸ After that, the same group found that Ni–Al LDH from chloride exhibited the highest activity and selectivity for CO formation among the various kinds of LDHs, and the added Cl⁻ could enhance the selectivity to CO.²⁶⁹ Cl⁻ in an aqueous solution acts as a hole scavenger under UV light irradiation and it is oxidized into ClO⁻, ClO₃⁻, and ClO₄.²⁷⁰ In 2016, they prepared 16 different kinds of transition metal containing M²⁺–M³⁺ LDHs (M²⁺ = Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺; M³⁺ = V³⁺, Cr³⁺, Mn³⁺, and Fe³⁺) and investigated their photocatalytic activities for CO₂ reduction in an aqueous NaCl solution under UV light irradiation.²⁷¹ Among these LDHs, Ni–V LDH exhibited the highest activity for the CO evolution. It is estimated that a combination of V³⁺ and Cl⁻ played a significant role in the high photocatalytic activity and selectivity toward CO formation in an aqueous solution.

In general, the above mentioned LDH based materials suffer from the efficiency bottleneck of a μmol h⁻¹ g⁻¹ (catalyst) or lower rate. Recently, Ren et al. designed a novel 2D nanostructured catalyst constructed using an ultrathin Mg–Al LDH matrix and Ru nanoparticles (denoted as Ru@FL-LDHs) (Fig. 24).²⁷² In this composite, the exfoliated LDHs and Ru nanoparticles activated CO₂ and H₂, respectively. The maximum yield rate for CH₄ generation of Ru@FL–LDHs in a flow-type reaction system is 277 mmol h⁻¹ g⁻¹ (catalyst), which remarkably exceeds all previous reports on LDH-based catalysts. The high-efficient and continuous CO₂ hydrogenation is achieved via using the flow-type reaction system, which makes it possible to achieve industrial production in the future.

Fig. 24 (a) Photothermal conversion of CO₂ over Ru loaded catalysts under different flow rates (F.R.) of CO₂ and H₂ mixture. Samples: S1. Ru@FL-LDHs; S2, Ru@LDHs; S3, Ru@SiO₂. (b) Cumulative yield of CH₄ over Ru@FL-LDHs. Reaction conditions: catalyst, 150 mg; light source, 300 W xenon lamp (light intensity, ≈10 suns); H₂/CO₂ flow rates (F.R.), 20.5 mL min⁻¹/5.0 mL min⁻¹, or 100.5 mL min⁻¹/25.0 mL min⁻¹. (c) Schematic exhibition of targeting and simultaneous activation of CO₂ and H₂ as well as the hydrogenation process over Ru@FL-LDHs. Reprinted with permission from ref. 272. Copyright 2017 Wiley-VCH.

In addition, Cu–M–Al LDHs (M = Mg, Zn, Ni),²⁷³ Ce³⁺–Ce⁴⁺ LDHs,²⁷⁴ and the self-assembly of Mg–Al–LDH/C₃N₄ (ref. 221) have also been reported for photocatalytic CO₂ reduction.

2.7 Other 2D nanosheets

In addition to the above mentioned six types of materials, carbon-doped h–BN nanosheets were synthesized, which had functionalities to catalyse hydrogen and oxygen evolution from water as well as CO₂ reduction under visible light illumination.²⁷⁵ The ternary B–C–N alloy featured a delocalized two-dimensional electron system with sp² carbon incorporated in the h–BN lattice where the bandgap could be adjusted by the amount of incorporated carbon to produce unique functions. Such photocatalysts of lightweight elements provide a new insight into fabricating active metal-free photocatalysts and demonstrate the great potential of using economic materials for efficient solar fuel production.

3. Conclusion and outlook

Overall, in this review, we have outlined the recent progress in constructing 2D nanomaterials-based photocatalysts for CO₂ reduction. Because these 2D materials possess many fascinating properties, such as good physical and chemical stability, large surface area, high electron mobility, ultrathin lamellar nature and abundant surface active sites, use of some ultrathin nanomaterials-based photocatalysts that are few-atoms thick or the introduction of 2D materials into various semiconductor-based photocatalysts results in remarkable improvement of photocatalytic activity for CO₂ reduction. Although a number of above-mentioned literature studies have demonstrated that 2D nanomaterials-based photocatalysts have achieved some significant advances in photocatalytic CO₂ reduction, there is still a very long way to go to achieve the process of artificial photosynthesis for practical application using 2D based nanomaterials as photocatalysts due to the unsatisfactory light harvesting, low conversion efficiency, lack of selectivity to specified products, and complex photocatalytic mechanisms.

Firstly, from the material synthesis point of view, it is still a great challenge to realize large-scale, high and uniform quality 2D nanomaterials with controllable thickness for industry or commercialization in the future. Because the photocatalytic activity of photocatalysts depends on their composition and morphology, ultrathin 2D nanomaterials with desired morphologies and compositions should be fabricated to achieve high activity and selectivity for photocatalytic CO₂ conversion.

Furthermore, the underlying mechanisms of enhanced photocatalytic efficiency toward CO₂ reduction by 2D nanomaterial photocatalysts are still not well understood and must be systematically studied, including adsorption/desorption of CO₂ and intermediate products. Owing to the complex multielectron transfer processes and various redox potentials in photocatalytic CO₂ conversions, a variety of products, such as CH₄, HCOOH, CH₂O, and CH₃OH, can be formed after the photocatalytic CO₂ reduction. It was found that the presence of 2D nanomaterials can affect the formation of different products during the photocatalytic CO₂ conversion. The exploration of reaction mechanisms is vital to elucidate the fundamental improvements and further optimization of the photocatalytic activity and selectivity in the future. This depends not only on advanced characterization techniques, but also on theoretical computational simulations of different aspects. Advanced materials characterization techniques such as scanning tunneling microscopy (STM), electron microcopy, atomic force microscopy, X-ray absorption fine-structure (XAFS) and terahertz time-domain spectroscopy could be of great help. Furthermore, the in situ monitoring of the CO₂ concentration during the photocatalytic CO₂ reduction or isotopic ¹³CO₂ labeling should also be performed to demonstrate the carbon-containing products originating from CO₂ photoreduction. In addition, theoretical calculations using density functional theory (DFT) could illustrate the roles of 2D nanomaterials in photocatalysis, which could provide us with efficient guidance for their rational experimental design.

In addition, there is no standard protocol to evaluate the photocatalytic performance in CO₂ reduction. It is because the final activity and selectivity of CO₂ conversion is determined by certain conditions, including the intensity and spectrum of light irradiation, pH of the solution, reaction temperature, humidity, temperature, CO₂ pressure, the amount and kind of sacrificial agent, and mass of photocatalysts. However, apparent quantum efficiency (AQE) under the same photocatalytic reaction conditions could measure the CO₂ photoreduction efficiency, which is calculated by using the number of used photons and the incident photon number, according to eqn (1) (when the products from the CO₂ photoreduction system are complex, the number of reacted electrons is the sum of the reacted electrons involved in the generation of each product^4,48,279,280).

Finally, the exploration of novel 2D nanomaterials made of earth abundant, nontoxic, light-stable, scalable and low cost materials or modification of the existing 2D nanomaterials by fine tuning the structural, compositional, band-gap and surface reaction sites, is highly encouraged. Such modification will further promote light harvesting and retarding the charge recombination to realize artificial photosynthesis for practical application.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21473090, U1663228), the National Basic Research Program of China (973 Program, 2013CB632404) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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