Low-Dimensional Photocatalysts for CO2 Conversion: Comparison
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The ongoing energy crisis and global warming caused by the massive usage of fossil fuels and emission of CO2 into atmosphere continue to motivate researchers to investigate possible solutions. The conversion of CO2 into value-added solar fuels by photocatalysts has been suggested as an intriguing solution to simultaneously mitigate global warming and provide a source of energy in an environmentally friendly manner. There has been considerable effort for nearly four decades investigating the performance of CO2 conversion by photocatalysts, much of which has focused on structure or materials modification. In particular, the application of low-dimensional structures for photocatalysts is a promising pathway. Depending on the materials and fabrication methods, low-dimensional nanomaterials can be formed in zero dimensional structures such as quantum dots, one-dimensional structures such as nanowires, nanotubes, nanobelts, and nanorods, and two-dimensional structures such as nanosheets and thin films. These nanostructures increase the effective surface area and possess unique electrical and optical properties, including the quantum confinement effect in semiconductors or the localized surface plasmon resonance effect in noble metals at the nanoscale. 

  • photocatalysis
  • carbon dioxide conversion
  • nanostructures
  • low-dimensional photocatalysts
  • solar fuels

1. Background

The fast-developing modern technology and explosive world population growth have resulted in a huge demand for and consumption of energy. According to an investigation from the U.S. Energy Information Administration, more than 600 quadrillion Btus of energy were spent in 2020 and it is expected that the demand will continue to skyrocket annually. To meet this huge energy demand every year, energy production has been predominately dependent on fossil fuels such as coal, oil, or natural gas. The energy production by fossil fuels is inextricably linked to the gigantic CO

2

 emission of more than 30 billion metric tons every year and, in turn, the accumulated CO

2

 in our atmosphere is deemed to be the main cause of many environmental problems such as global warming and erratic weather patterns. In this context, there is great motivation to find a way of reducing atmospheric CO

2

 and producing energy at the same time. For these problems, CO

2

 conversion by photocatalyst materials under light illumination could be an expedient solution. This is because natural sunlight provides clean, renewable, and abundant energy, and photocatalysts can be activated by light energy from the Sun, while simultaneously consuming CO

2

 for energy production.

In 1978, by Halmann [1], the first demonstration of the photocatalytic conversion of CO

2

 in aqueous solution into liquid fuels such as methanol, formic acid, and formaldehyde was achieved over p-type gallium phosphide semiconductor. The same year, the photoartificial synthesis by SrTiO

3

 photocatalysts for CH

4

 production through the gas-solid phase reaction of CO

2

 and H

2

O was reported by Hemminger [2]. In 1979, another pioneering work by Fujishima and his coworkers introduced the artificial synthesis of solar fuels from a CO

2

-saturated electrolyte under light illumination. In this study, liquid CO

2

 was converted with various semiconductor photocatalysts such as TiO

2

, ZnO, CdS, GaP, SiC, and WO

3

 to produce methane, methanol, formaldehyde and formic acid [3]. Since these historical works mentioned above, several semiconductor materials have been investigated for the conversion of CO

2

 into useful fuels, including graphitic carbon nitride (g-C

3

N

4), graphene, conjugated polymers, covalent organic framework, metal organic frameworks, metal chalcogenides, metal oxides, black phosphorus, bismuth-based materials, and perovskites [4,5,6,7,8,9,10,11,12,13,14]. Moreover, a variety of strategies and approaches have been applied to improve the photocatalyst performance through elemental doping, solid solution, heterostructure, nanostrucutralization, surface engineering and modification, crystal facet engineering, cocatalysts utilization, or dimensionality tailoring [15,16,17,18,19].

), graphene, conjugated polymers, covalent organic framework, metal organic frameworks, metal chalcogenides, metal oxides, black phosphorus, bismuth-based materials, and perovskites [4][5][6][7][8][9][10][11][12][13][14]. Moreover, a variety of strategies and approaches have been applied to improve the photocatalyst performance through elemental doping, solid solution, heterostructure, nanostrucutralization, surface engineering and modification, crystal facet engineering, cocatalysts utilization, or dimensionality tailoring [15][16][17][18][19].

Of the strategies researched to boost the efficiency of CO

2 conversion, decreasing the dimensionality and constructing nanostructures of the photocatalyst have attracted a lot of attention owing to their favorable advantages in photocatalysis: first, nanostructured photocatalysts suppress the carrier recombination due to their higher crystallinity compared to non-nanostructured materials [20,21,22]; second, the implementation of low dimensionality modifies the electronic structure of bulk materials due to the quantum confinement effect in semiconductors or localized surface plasmon resonance effect in noble metals at the nanoscale; third, low-dimensional materials possess larger surface-area-to-volume ratio in comparison with bulk materials, providing more reaction sites. All three features can contribute to improved solar-driven catalytic reactions [19,23].

 conversion, decreasing the dimensionality and constructing nanostructures of the photocatalyst have attracted a lot of attention owing to their favorable advantages in photocatalysis: first, nanostructured photocatalysts suppress the carrier recombination due to their higher crystallinity compared to non-nanostructured materials [20][21][22]; second, the implementation of low dimensionality modifies the electronic structure of bulk materials due to the quantum confinement effect in semiconductors or localized surface plasmon resonance effect in noble metals at the nanoscale; third, low-dimensional materials possess larger surface-area-to-volume ratio in comparison with bulk materials, providing more reaction sites. All three features can contribute to improved solar-driven catalytic reactions [19][23].

2. The Main Fundamentals of CO2 Photoconversion into Solar Fuels and Hydrocarbon Species

2.1. Nature of CO

2

Carbon dioxide (CO

2

) is one of the primary greenhouse gases but, at the same time, it is the main resource for solar fuel production when coupled with proton donors such as H

2

O for photocatalytic CO

2

 conversion. Hence, understanding the nature of the gaseous CO

2

 molecule itself is necessary for efficient utilization of photocatalysts. CO

2

 is a stable linear molecule among carbon compounds because of it is in the highest oxidation state of carbon, C

+4

 [19]. The CO

2

 molecule has two C=O bonds with a dissociation energy of ~750 kJ/mol, which is quite larger than those in other chemical bonds such as C-H (~430 kJ/mol) and C-C (~336 kJ/mol). For this reason, the reduction of CO

2

 to produce solar fuels requires additional energy to break the C=O bond and form, for example, a C-H bond [24]. Owing to its stability and strong bonding, photocatalytic reduction of CO

2

 into solar fuels can be achieved primarily with the support of proton donors such as H

2

O or H

2

 [25].

2.2. CO

2

 Adsorption on the Surface of Photocatalysts

The adsorption and activation of CO

2

 on a solid surface is one of the essential steps in achieving CO

2

 reduction and photocatalytic performance. The adsorption mechanism of CO

2 on several semiconductor photocatalysts has been investigated [26,27]. For instance, the adsorption of CO

 on several semiconductor photocatalysts has been investigated [26][27]. For instance, the adsorption of CO

2

 on TiO

2

 surface has been investigated by Minot and coworkers [26]. Various adsorption modes of CO

2

 over a rutile TiO

2

 surface have been studied using first-principles calculations. The oxygen atom of CO

2

 molecules favors the interaction with the acidic titanium cation over the surface forming a Ti–OCO bond [26]. The adsorption of CO

2

 on a photocatalyst surface includes the interaction between the CO

2

 molecule and the surface atoms of the photocatalyst. This absorption may occur with a charge transfer from the photocatalyst to the linear and stable CO

2

 molecule which can induce the formation of partially charged and bent adsorbate, 

COδ⋅−2

, and this adsorbate can form three different molecular structures, as shown in 

Figure 1: oxygen coordination, carbon coordination, and mixed coordination [24,28,29]. The beneficial feature of 

: oxygen coordination, carbon coordination, and mixed coordination [24][28][29]. The beneficial feature of 

COδ⋅−2

, is that it has decrease in the lowest unoccupied molecular orbital (LUMO) of the CO

2

 energy level as linear CO

2

 molecule transformed into the bent structure. This would facilitate the charge transfer between a photocatalyst and 

COδ⋅−2, [24,28,29].

, [24][28][29].

Catalysts 11 00418 g001 550
Figure 1.

 Schematic illustration of the different types of CO

2

 adsorption modes (Adapted from [24]).

In the photocatalytic reaction, the photocatalyst donates electron to the adsorbed species on the surface to initiate the reduction process of CO

2

 in the presence of protons. As shown in 

Table 1, the solar fuel production is determined by the number of electrons and protons included in the reaction [20,24]. For example, two electrons are required for CO evolution, while methane production is an eight-electron reaction. The adsorption of CO

, the solar fuel production is determined by the number of electrons and protons included in the reaction [20][24]. For example, two electrons are required for CO evolution, while methane production is an eight-electron reaction. The adsorption of CO

2

 on the photocatalyst surface can be improved by a variety of strategies. First, decreasing the structural dimensionality of photocatalyst can improve the surface area of the photocatalyst to allow more adsorption. Second, enhanced density of active sites by incorporation of surface defects, such as oxygen and sulfur vacancies, can improve the CO

2

 adsorption. Third, utilization of noble metal nanoparticles can help improve the adsorption due to lowered activation energy of the CO

2 reduction [24,30].

 reduction [24][30].

Table 1.

 Standard electrochemical potentials of CO

2

 and H

2O at 25 °C and atmospheric pressure [20,24].

O at 25 °C and atmospheric pressure [20][24].

Reaction Eo at pH 7

(V vs. NHE)		 Solar Fuel
CO2 reduction CO2 + 2H+ + 2e− → CO + H2O −0.51 CO
CO2 + 8H+ + 8e− → CH4 + 2H2O −0.24 CH4
CO2 + 6H+ + 6e− → CH3OH + H2O −0.39 CH3OH
2CO2 + 12H+ + 12e− → C2H5OH + 3H2O −0.33 C2H5OH
CO2 + 2H+ + 2e− → HCOOH −0.58 HCOOH
CO2 + 4H+ + 4e− → HCHO + H2O
−0.48
HCHO
H2O oxidation
2H2O → O2 +4H+
+0.81
O2

2.3. The Mechanism of Efficient CO

2

 Photoconversion

Upon absorption of light over the photocatalyst, charge carrier pairs are generated to achieve the photosynthesis of solar fuels accompanying the water splitting as shown in 

Table 1

. To execute the conversion of CO

2

 into useful fuels from the thermodynamic point of view, the conduction band minimum (CBM), and the valence band maximum (VBM) of a photocatalyst should bracket the redox potential of CO

2

 and the oxidation potential of water, respectively, as shown in 

Table 1

 and 

Figure 2

. As results of the reactions, various solar fuels can be formed dependent on the number of electrons and protons in the presence of CO

2

 and water under the illumination.

Catalysts 11 00418 g002 550
Figure 2.

 The energy band structures of semiconductor photocatalysts and the corresponding redox potentials of CO

2 reduction into solar fuels (Adapted from [32]).

 reduction into solar fuels (Adapted from [31]).

The artificial photosynthesis of solar fuels using semiconductor photocatalysts consists of three essential steps, as described in 

Figure 3

. Firstly, incident photons of light with energy higher than that of semiconductor band gap (E

g

) induce the generation of the electron-hole pairs (Process (i)). Secondly, the photogenerated charge carriers are transferred to the photocatalyst surface (Process (ii)). Thirdly, the electrons and holes react on the surface of photocatalyst with CO

2

 and H

2

O for evolution of solar fuels (Process (iii)). For efficient photocatalytic conversion of CO

2

 into fuels, the ideal semiconductor photocatalyst should have optimized band gap for efficient light harvesting and photocarrier generation, facile charge separation and transportation, vigorously activated sites and high surface area for maximum adsorption of CO

2 and water [30,31].

 and water [30][32].

Catalysts 11 00418 g003 550

Figure 3.

 A schematic diagram of CO

2

 photocatalytic conversion process over photocatalyst. Process (

i

): light absorption and generation of photocarriers via a semiconductor photocatalyst. Process (

ii

): charge carrier separation and transfer to the surface of photocatalyst. Process (

iii

): reactions of CO

2

 and H

2

O with electrons and holes, respectively to produce solar fuels.

3. Strategies for Enhancement in the Light-Driven CO2 Conversion over Low-Dimensional Photocatalysts

Applying low-dimensional structure to the photocatalytic system itself is a proven way for obtaining high CO

2

 conversion performance. However, further improvement is available by designing the structures or modifying the photocatalytic materials so that characteristics of photocatalysts or photocatalytic system are engineered. Here, we focus on these two major strategies for further enhancement of CO

2

 conversion by low-dimensional photocatalysts. 

Table 2

 summarizes the examples mentioned in this section.

Table 2.

 Summary of low-dimensional photocatalysts used in photocatalytic reduction of CO

2

 into solar fuels. NC: Nanocrystal, GQD: Graphene Quantum Dot, NF: Nanofiber, NS: Nanosheet, CNT: carbon nanotube, Mt: montmorillonite, m-CN: modified g-C

3

N

4

, NR: nanorod, NW: nanowire, CND: carbon nano dot, p-CN: protonated g-C

3

N

4

, PGCN: porous g-C

3

N

4

, TEOA: triethanolamine, bpy: bipyridine, C

3

N

4

: Melon-based polymeric carbon nitride, UTNS: ultrathin nanosheet, P-g-C

3

N

4

: Phosphorus doped g-C

3

N

4

.

Dimensionality Morphology Photocatalyst Light Source Reducing Agent Main Product Activity

[µmol∙g−1∙h−1]		 Ref.
0D QD

3–12 nm		 CsPbBr3 300 W Xe lamp H2O CO

CH4		 4.26

1.53		 [45][33]
QD

2.3 nm		 Cs3Bi2I9 300 W Xe lamp H2O CO 1.15 [46][34]
QD

2.9 nm		 Cs3Bi2Br9 300 W Xe lamp H2O CO 26.95
QD

2.4 nm		 Cs3Bi2Cl9 300 W Xe lamp H2O CO 21.01
NC

9.5 nm		 Cs2AgBiBr6 AM 1.5G Ethyl acetate CO

CH4		 2.35

1.6		 [47][35]
QD

9.45 nm		 FAPbBr3 300 W Xe arc lamp H2O CO

CH4		 181.25

16.9		 [48][36]
QD

5.86 nm		 GQD 300 W Xe lamp

420 nm cutoff filter		 H2O CH3OH 0.695 [11]
1D NR CeO2 300 W Xe lamp H2O CO 0.020 [49][37]
NT TiO2 300 W Xe arc lamp

320 nm < λ < 780 nm		 H2O CH4 2.128 [21]
NR TiO2 300 W Xe arc lamp

320 nm < λ < 780 nm		 H2O CH4 1.41 [21]
NT P-g-C3N4 300 W Xe lamp H2O/TEOA CO

CH4		 2.37

1.81		 [50][38]
NT PGCN 300 W Xe lamp H2O/MeCN /TEOA CO 103.6 [51][39]
NT Bi12O17Cl2 300 W Xe lamp H2O CO 48.6 [52][40]
NT Bi12O17Br2 300 W Xe lamp H2O CO 34.5 [53][41]
2D NS g-C3N4 300 W Xe arc lamp MeCN/TEOA

(4:1)		 CO

CH4		 5.407

1.549		 [54][42]
UTNS g-C3N4 300 W Xe lamp H2O CH4

CH3OH		 1.39

1.87		 [55][43]
UTNS SiC 300 W Xe lamp H2O CO

CH4		 1.29

3.11		 [56][44]
UTNS Bi2MoO6 300 W Xe lamp H2O CO 3.62 [57][45]
0D/1D QD/NW Black P/WO3 300 W Xenon arc lamp H2O CO

C2H4		 ~ 135

~ 11		 [13]
QD (10 nm)/NT WS2/Bi2S3 300 W Xe arc lamp H2O CH3OH

C2H5OH		 9.55

6.95		 [58][46]
QD (3.5 nm)/NW Ti3C2/Cu2O 300 W Xe lamp H2O CH3OH 78.50 [59][47]
0D/2D QD (1.6 nm)/NS CuO/WO3 300 W Xe lamp

λ > 400 nm		 H2O CO 1.58 [60][48]
ND (4.4 nm)/NS CND/p-CN Xe arc lamp H2O CO

CH4		 5.88

2.92		 [61][49]
QD/NS TiO2/g-C3N4 300 W Xe lamp

λ > 400 nm		 MeCN/TEOA CO 77.8 [62][50]
QD (5nm)/NS Au/TiO2 300 W Xe arc lamp H2O CO

CH4		 19.75

70.34		 [63][51]
QD (7nm) /NS CsPbBr3/Bi2WO6 300 W Xe lamp

λ > 400 nm		 H2O CO/CH4 503 µmol∙ g−1 [64][52]
1D/2D NF/NS TiO2/MoS2 350 W Xe lamp H2O CH4

CH3OH		 2.86

2.55		 [65][53]
NT/NS CNT/g-C3N4 200 W Hg and solar simulator H2O CO

CH4		 410

74		 [66][54]
NR/NS/NF Au/TiO2/BiVO4 300 W Xe lamp H2O CO

CH4		 2.5

7.5		 [67][55]
2D/2D NS/NS Ti3C2/Bi2WO6 300 W Xe lamp H2O CH4

CH3OH		 1.78

0.44		 [68][56]
NS/NS Bi2WO6/BiOI 500 W Xe arc lamp

λ < 400 nm		 H2O CH4 2.92 [69][57]
NS/NS Mt/m-CN 35 W Xenon lamp H2O/H2 CO

CH4		 505

330		 [70][58]
NS/NS SnS2/TiO2 300 W Xe lamp H2O CH4 23 [71][59]
NS/NS g-C3N4/BiVO4 300 W Xe lamp

λ ≥420 nm		 H2O CO

CH4		 5.19

4.57		 [72][60]
NS/NS PGCN/Bi12O17Cl2 300 W xenon lamp H2O CH4 24.4 [73][61]
NP-NS/NS Pd-g-C3N4 /RGOA 300 W Xe lamp H2O CH4 6.4 [74][62]

3.1. Construction of Junction Formed by Low-Dimensional Structures

Of the diverse strategies to promote CO

2 conversion performance of photocatalysts, designing or formation of junctions in photocatalytic systems has seen some success by modifying optical and electrical properties through materials and interfaces [63]. Junctions are constructed by coupling of two or more semiconductor materials or metal materials. The formation of a junction allows valuable properties from various materials to be more available in a single system. That helps more efficient light absorption, charge separation and transfer, or more stable performance. Since the photocatalytic systems with junctions have single or multiple interfaces, the engineering of the interfacial characteristics between the materials is essential for efficient photocatalyst performance. Especially, the interface characteristics can influence on carrier behaviors through bulk or at interfaces (ex. Shockley-Read-Hall recombination).

 conversion performance of photocatalysts, designing or formation of junctions in photocatalytic systems has seen some success by modifying optical and electrical properties through materials and interfaces [51]. Junctions are constructed by coupling of two or more semiconductor materials or metal materials. The formation of a junction allows valuable properties from various materials to be more available in a single system. That helps more efficient light absorption, charge separation and transfer, or more stable performance. Since the photocatalytic systems with junctions have single or multiple interfaces, the engineering of the interfacial characteristics between the materials is essential for efficient photocatalyst performance. Especially, the interface characteristics can influence on carrier behaviors through bulk or at interfaces (ex. Shockley-Read-Hall recombination).

The formation of junctions implies the presence of an internal electric field in the nanomaterial. This internal electric field can contribute to enhanced carrier behaviors such as carrier separation and transfer for the photo-induced charge carriers and to, in the end, the performance of light-driven CO

2 conversion. The internal electric field can be induced by growth of a low-dimensional semiconductor on a low-dimensional semiconductor [23,107]. The combination of two or multiple low-dimensional materials could integrate the advantages of both single units and mitigate the shortcomings of single unit by the synergistic effect [142].

 conversion. The internal electric field can be induced by growth of a low-dimensional semiconductor on a low-dimensional semiconductor [23][63]. The combination of two or multiple low-dimensional materials could integrate the advantages of both single units and mitigate the shortcomings of single unit by the synergistic effect [64].

One of the advantages of semiconductor QDs is the quantum size effect which is responsible for the optical properties of the photocatalyst. Apart from the acceleration of charge separation and transfer process, the contact between 0D semiconductor and 1D semiconductor provides the nanocomposite with an additional properties such as excessive electroactive sites, high surface area, and homogenous dispersion [143]. 1D Bi

One of the advantages of semiconductor QDs is the quantum size effect which is responsible for the optical properties of the photocatalyst. Apart from the acceleration of charge separation and transfer process, the contact between 0D semiconductor and 1D semiconductor provides the nanocomposite with an additional properties such as excessive electroactive sites, high surface area, and homogenous dispersion [65]. 1D Bi

2

S

3

 nanotubes have outstanding ability to absorb visible and near infrared light. The tubular structure of Bi

2

S

3 provides the photocatalytic reaction with more active sites than other morphologies [58]. The remarkable optical and electronic properties of tungsten disulfide (WS

 provides the photocatalytic reaction with more active sites than other morphologies [46]. The remarkable optical and electronic properties of tungsten disulfide (WS

2

) QDs can be realized due to the quantum confinement effect. WS

2

 QDs can be also dispersed uniformly on the surface of Bi

2

S

3

 forming Bi–S channels to facilitate the charge carrier separation transfer process. The designed 0D/1D nanocomposite exhibited outstanding photocatalytic reduction of CO

2

 into CH

3

OH and C

2

H

5

OH of 38.2 µmol∙g

−1

 and 27.8 µmol∙g

−1

 after 4 h radiation, respectively. The improved photoreduction performance is related to the following features. Firstly, the 0D/1D nanocomposite provided combined optical and electrical properties of both WS

2

 QDs and Bi

2

S

3

 nanotubes causing high visible and near infrared light absorption. Secondly, the enlarged surface area of the nanocomposite provided more active adsorptions site for CO

2. Thirdly, the low resistive QDs–NTs interface due to the Bi–S bonds plays a critical role for accelerated charge carrier separation [58]. CsPbBr

. Thirdly, the low resistive QDs–NTs interface due to the Bi–S bonds plays a critical role for accelerated charge carrier separation [46]. CsPbBr

3 is widely used in the photocatalytic reactions but it suffers from the high rate of recombination during the interface transfer due to the strong reductive ability of electrons [64]. Hence, the suppression of undesired electron loss throughout the transfer process at the interface is critical factor for efficient utilization of CsPbBr

 is widely used in the photocatalytic reactions but it suffers from the high rate of recombination during the interface transfer due to the strong reductive ability of electrons [52]. Hence, the suppression of undesired electron loss throughout the transfer process at the interface is critical factor for efficient utilization of CsPbBr

3

. Li et al. fabricated 0D/2D nanocomposite of CsPbBr

3

/Bi

2

WO

6

 via ultrasonic method with intimate contact at the interface to improve the charge separation and transfer. The Bi–Br bonds which is formed at the QDs–NSs interface is responsible for the strong interfacial interaction. The decoration of Bi

2

WO

6

 with CsPbBr

3

 QDs could enhance the CO and CH

4

 yield by factor of 9.5 over that of pristine CsPbBr

3 [64].

 [52].

The building of 1D semiconductor materials on 2D semiconductors is an efficient strategy for efficient CO

2

 conversion. Coupling of TiO

2

 nanofibers with light harvesting semiconductors such as MoS

2 nanosheets is an efficient way to overcome the fast recombination of charge carriers and enhance the light absorption efficiency [65]. The electronic properties of MoS

 nanosheets is an efficient way to overcome the fast recombination of charge carriers and enhance the light absorption efficiency [53]. The electronic properties of MoS

2

 nanosheets can be tuned by control of the thickness. The superior conversion activity of CO

2

 into hydrocarbon species, that is, CH

4

 and CH

3

OH, resulted from the improved light harvesting, sufficient reactive sites for CO

2

 adsorption, and the intimate 1D–2D chemical contact between MoS

2

 and TiO

2 which could be favorable for facile and efficient charge separation upon photoexcitation [65]. The increase of the contact area between the two semiconductor nanomaterials is much more favorable to enhance the photocatalytic performance over the photocatalyst. In other words, constructing the 2D/2D interface is favorable for highly separated charge carriers at the interface. Wang et al. prepared 2D/2D heterojunctions by growth of ultrathin tin disulfide (SnS

 which could be favorable for facile and efficient charge separation upon photoexcitation [53]. The increase of the contact area between the two semiconductor nanomaterials is much more favorable to enhance the photocatalytic performance over the photocatalyst. In other words, constructing the 2D/2D interface is favorable for highly separated charge carriers at the interface. Wang et al. prepared 2D/2D heterojunctions by growth of ultrathin tin disulfide (SnS

2

) onto TiO

2 nanosheets via a hydrothermal method [71]. The production yield of CH

 nanosheets via a hydrothermal method [59]. The production yield of CH

4

 over SnS

2

/TiO

2

 was much higher than that of pristine SnS

2

 and TiO

2

 nanosheets. The reason for such outstanding performance originates from the increment of the contract area between SnS

2

 and TiO

2 nanosheets [71].

 nanosheets [59].

The photocatalytic systems with multiple junctions, that is, with multiple interfaces, displays excellent photocatalytic activity toward solar fuels generation compared to one with/without single junction [67,144,145]. Recently, Macyk and coworkers designed two heterointerface-based photocatalyst, TiO

The photocatalytic systems with multiple junctions, that is, with multiple interfaces, displays excellent photocatalytic activity toward solar fuels generation compared to one with/without single junction [55][66][67]. Recently, Macyk and coworkers designed two heterointerface-based photocatalyst, TiO

2

/C

3

N

4

/Ti

3

C

2

, via the interfacial assembly of Ti

3

C

2

 QDs on the TiO

2

/C

3

N

4

 binary nanocomposite to boost the charge separation and transfer and providing strong redox ability in CO

2 photoreduction reaction [145]. The as-synthesized composite exhibited enhanced light absorption, suppressed electron-hole recombination, and demonstrated stable photocurrent sensitivity. The fabricated composite could overcome the disadvantage of TiO

 photoreduction reaction [67]. The as-synthesized composite exhibited enhanced light absorption, suppressed electron-hole recombination, and demonstrated stable photocurrent sensitivity. The fabricated composite could overcome the disadvantage of TiO

2

/C

3

N

4

 nanocomposites with a single junction by providing more efficient transport channels of electrons-hole pairs due to the strong interaction between Ti

3

C

2

 QDs and TiO

2

/C

3

N

4

 NS. Theoretical studies demonstrated that construction of two interfacial electric fields between TiO

2

/C

3

N

4

 and Ti

3

C

2

/C

3

N

4

 is due to electron transfer processes at the two interfaces. The interfacial built-in electric fields can promote the charge carrier separation and the photocatalytic reduction of CO

2

 into CO and CH

4

. Tonda et al. fabricated another multijunction system with a Bi

2

WO

3

/RGO/g-C

3

N

4

 2D/2D/2D architecture using two-step hydrothermal method for utilization in CO

2 and water reduction into useful fuels [144]. This ternary heterojunction exhibited highly improved characteristics in light harvesting ability, CO

 and water reduction into useful fuels [66]. This ternary heterojunction exhibited highly improved characteristics in light harvesting ability, CO

2

 adsorption capacity, photocurrent responses, and interfacial contact area. The photoconversion performance of CO

2

 over Bi

2

WO

3

/RGO/g-C

3

N

4

 was dramatically enhanced toward CH

4

 and CO evolution. The performance of Bi

2

WO

3

/RGO/g-C

3

N

4

 was 2.5 times higher and 3.8 times higher than those of Bi

2

WO

3

/RGO and RGO/g-C

3

N

4, respectively [144].

, respectively [66].

3.2. Modification of Low-Dimensional Nanomaterials

Modification of nanostructured low-dimensional photocatalysts themselves are another beneficial strategy for the enhancement of photocatalytic CO

2

 conversion because it helps the properties of photocatalysts to be engineered. The modification can be achieved using several approaches such as introduction of surface oxygen vacancies or the formation of a porous structure.

The efficient utilization of the solar spectrum can be controlled by tuning the band structure of the semiconductor using the elemental doping [19]. Yu et al. synthesized oxygen-doped g-C

3

N

4

 nanotubes via exfoliation and a 3D g-C

3

N

4 curling condensation method [146]. It was found that the synthesized photocatalyst consisted of curled nanosheets that had a uniform tubular structure with 20–30 nm of diameter. The oxygen atoms can substitute for the C or N atoms in g-C

 curling condensation method [68]. It was found that the synthesized photocatalyst consisted of curled nanosheets that had a uniform tubular structure with 20–30 nm of diameter. The oxygen atoms can substitute for the C or N atoms in g-C

3

N

4

 under high temperature oxidation conditions. The oxygen doping of 1D g-C

3

N

4

 helped the conduction band to be at a more positive potential causing a narrower band gap and efficient light harvesting. This structure exhibited a significant methanol evolution rate of 0.88 µmol∙g

−1

∙h

−1

 under visible light radiation. Wu et al. synthesized self-doped black TiO

2

 nanotubes arrays using a one-step aluminothermic reduction for solar-driven conversion of CO

2 into CO [147]. It is found that the average diameter of the nanotubes was 75–85 nm with 5–7 nm of wall thickness. The oxygen vacancies can act as active sites for CO

 into CO [69]. It is found that the average diameter of the nanotubes was 75–85 nm with 5–7 nm of wall thickness. The oxygen vacancies can act as active sites for CO

2

 molecules for efficient photogenerated charge carrier separation. The visible light absorption of black TiO

2

 was largely enhanced by virtue of the oxygen vacancies. The resulted photocatalytic conversion was 185.39 µmol∙g

−1

∙h

−1

 of CO evolution rate under visible light.

The introduction of defects into semiconductors can improve the photocatalytic activity of CO

2 into solar fuels ascribed to the promotion of photogenerated charge carrier separation and the extended light absorption [148]. Liu and coworkers prepared Bi

 into solar fuels ascribed to the promotion of photogenerated charge carrier separation and the extended light absorption [70]. Liu and coworkers prepared Bi

12

O

17

Cl

2 nanotubes with surface oxygen defects via solvothermal method [52]. The tubular structure plays crucial role for accelerating the photogenerated charge carrier separation, while the oxygen defects on the surface act as active centers for CO

 nanotubes with surface oxygen defects via solvothermal method [40]. The tubular structure plays crucial role for accelerating the photogenerated charge carrier separation, while the oxygen defects on the surface act as active centers for CO

2

 activation. It is found that the absorption of Bi

12

O

17

Cl

2

 nanotubes is improved in the visible region compared to its bulk counterpart. The defective ultrathin tubular structure of Bi

12

O

17

Cl

2

 provides effective CO

2

 conversion into CO with production yield of 16.8 times higher than bulk Bi

12

O

17

Cl

2

. The higher photocatalytic conversion rate can be attributed to faster charge separation on the surface of Bi

12

O

17

Cl

2

 nanotubes.

The porosity of nanostructured semiconductors provides an additional feature to increase the surface area of photocatalysts, and subsequently is favorable for the solar-driven reduction of CO

2 into valuable fuels [149]. Huang et al. used a template-free method to prepare porous g-C

 into valuable fuels [71]. Huang et al. used a template-free method to prepare porous g-C

3

N

4 with increased surface area [150]. It is reported that the porous g-C

 with increased surface area [72]. It is reported that the porous g-C

3

N

4

 nanotubes had excellent photocatalytic conversion of CO

2

 into CO of 40 µmol∙g

−1

 within 4 h illumination. The CO yield was higher than that of bulk g-C

3

N

4

 by a factor of 5.6 originated from the higher surface area of the porous tubular structure, and the improved charge carrier separation and transfer process.

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