Photocatalytic Reduction of Carbon Dioxide on TiO2 Heterojunction Photocatalysts-A Review.

The photocatalytic reduction of carbon dioxide to renewable fuel or other valuable chemicals using solar energy is attracting the interest of researchers because of its great potential to offer a clean fuel alternative and solve global warming problems. Unfortunately, the efficiency of CO2 photocatalytic reduction remains not very high due to the fast recombination of photogenerated electron-hole and small light utilization. Consequently, tremendous efforts have been made to solve these problems, and one possible solution is the use of heterojunction photocatalysts. This review begins with the fundamental aspects of CO2 photocatalytic reduction and the fundamental principles of various heterojunction photocatalysts. In the following part, we discuss using TiO2 heterojunction photocatalysts with other semiconductors, such as C3N4, CeO2, CuO, CdS, MoS2, GaP, CaTiO3 and FeTiO3. Finally, a concise summary and presentation of perspectives in the field of heterojunction photocatalysts are provided. The review covers references in the years 2011-2021.

1. Introduction

Since the 18th century, together with the fast development of human society and the extensive use of fossil energy, environmental pollution has become increasingly serious with great environmental, social and economic impacts. Emissions of CO2 and other greenhouse gases are steadily increasing. The anthropogenic source of greenhouse gas emissions is fossil fuels combustion, mainly coal, natural gas, and oil, along with soil erosion and deforestation. These gases warm the planet by trapping heat in the atmosphere and are the principal factor in global warming. The average increase in temperature since the preindustrial age has already reached almost one degree Celsius, and this rise looks set to continue [1].

The chance of CO2 transformation into clean fuel could provide a progressive solution for both the future deficiency of fossil fuels and problem with global warming. In recent years, a large number of methods, such as chemical, electrochemical, biochemical, photochemical, and thermochemical techniques, have been developed for converting CO2 to light hydrocarbons and alcohols [2,3]. Among the varied methods, CO2 photocatalytic reduction has been receiving great attention and proved to be a promising alternative technology, once it is possible to produce greener gases and/or gases with industrial and fuel applications, using sunlight to activate the semiconductor materials, which result in the photoreduction of gaseous pollutants. This method is one of the promising processes, which not only remove carbon dioxide, but can also transform it into energy valuable products, such as methane, formaldehyde, methanol, CO and other useful compounds [4,5,6,7].

Photocatalysis can be defined as a change in the rate of a photochemical reaction by the activation of a photocatalyst (semiconductor) with sunlight or artificial light (ultraviolet or visible radiation). This process is very efficient and attractive from the economical and eco-friendly point of view. This method is based on the use of a photocatalyst, usually a semiconductor, illuminated with energy equal to or higher than its energy of the bandgap.

It is well known that CO2 is an extremely stable molecule with high thermodynamic stability, being that its reduction is extremely complicated. The photocatalytic reduction of CO2 has complex reaction mechanisms and pathways involving a proton-assisted multi-electron reduction process with high energy barriers, complicated activation and CO2 adsorption, and selectivity of different products as shown in Table 1.

Table 1

Main products of CO2 reduction and the corresponding potential (pH = 7).

Reaction	E° (V vs. NHE)	Product	Reference
H_2O+2e^−→2OH^−+ H_2	−0.41	Hydrogen	[8]
CO_2+e^−→CO2·−	−1.90	•CO2− anion radical	[9]
2CO_2+2H^++2e^−→H_2C_2O_4	−0.87	Oxalate	[8]
CO_2+2H^++2e^−→HCOOH	−0.61	Formic acid	[10]
CO_2+2H^++2e^−→CO+ H_2O	−0.53	Carbon monoxide	[9,10]
CO_2+4H^++4e^−→HCHO+ H_2O	−0.48	Formaldehyde	[8,9,10]
CO_2+6H^++6e^−→CH_3OH+ H_2O	−0.38	Methanol	[9,10]
2CO_2+12H^++12e^−→C_2H_5OH+3H_2O	−0.33	Ethanol	[8]
2CO_2+14H^++14e^−→C_2H_6+4H_2O	−0.27	Ethane	[8]
CO_2+8H^++8e^−→CH_4+2H_2O	−0.24	Methane	[8,9,10]

The photocatalytic system for the reduction of CO2 makes use of a photocatalyst suspension in a solvent with dissolved carbon dioxide, and irradiation with solar energy can drive the photoreduction of CO2. Hole scavengers, such as H2O2, Na2SO3/Na2S, CH3OH, and triethanolamine, are ordinarily added to the reaction mixture to decrease the electron–hole recombination and avoid reoxidation by generated holes or the oxygen which is produced from water [9].

Inoue et al. reported, in 1979, CO2 photocatalytic reduction using several semiconductors (dispersed in water) as photocatalysts. They studied TiO2, WO3, CdS, ZnO, GaP, and SiC for the photocatalytic reduction of CO2, and concluded that TiO2 and SiC materials had higher photocatalytic activity for this reaction [11]. Recently, several photocatalysts, such as TiO2, g C3N4, ZnIn2S4, Bi2WO6, graphene (GR), CdS, SrNb2O6, and ZnO, were investigated for CO2 photocatalytic reduction. However, the TiO2 is the most prevalently used due to its chemical stability, resistance toward corrosion, and mainly low cost [12].

TiO2 has naturally three polymorphic phases: brookite, anatase, and rutile [13]. TiO2 has a large band gap and, therefore, the solar light utilization rate of TiO2 is only 4%. Therefore, the photocatalytic performance of TiO2 using solar energy is limited [14]. TiO2 has a relatively high recombination rate of photoinduced electron/hole (e−/h+) pairs [10]; hence, only a fraction of the generated e−/h+ pairs are available for photoreaction [15,16,17]. In recent years, there has been an effort to increase the photocatalytic activity of TiO2. Several strategies have been suggested to efficiently separate pairs of photogenerated electrons and holes in semiconductor photocatalysts, thereby increasing the efficiency of photocatalysis. Some of the most important ones are, for example, doping metals or non-metals, or creating photocatalysts with heterojunction. The formation of heterojunction photocatalysts, where the generated electron–hole pairs are efficiently separated, has emerged as one of the most promising approaches (Figure 1).

Figure 1

Schematic illustration of the electron–hole separation on an example of heterojunction photocatalyst type-II. Adapted according to refs. [18,19].

A heterojunction is the interface between two diverse materials which has a different band structure, and it can lead to band alignments. Many types of heterojunctions have been studied that are efficient for increasing the photoactivity of materials. These include conventional heterojunctions (type-I, type-II and type-III), surface heterojunctions, p–n heterojunctions, direct Z-scheme heterojunctions, and graphene-semiconductor (graphene-SC) heterojunctions [18]. In the conventional heterojunction photocatalysts, there are three types: the type-I have a straddling gap (Figure 2a), type-II have a staggered gap (Figure 2b), and the type-III a broken gap (Figure 2c).

Figure 2

Schematic illustration of the three different types of separation of electron–hole pairs in the case of conventional light-responsive heterojunction photocatalysts: (a) type-I, (b) type-II, and (c) type-III heterojunctions. Adapted according to Refs. [18,19].

Figure 2a shows the type-I heterojunction photocatalyst. The valence band (VB) and the conduction band (CB) of the semiconductor A are, respectively, lower and higher than the matching bands of semiconductor B. For that reason, after irradiation, e− and h+ cumulate at the CB and the VB levels of the semiconductor B, which has lower Eg. Since both e− and h+ cumulate on one and the same semiconductor, they cannot be effectively separated; therefore, it is not suitable for application in photocatalysis. Figure 2b represents the type-II heterojunction photocatalyst. The VB and the CB levels of semiconductor A are higher than the matching VB and CB levels of semiconductor B. In this case, the migration of photogenerated charges can occur in opposite directions, namely, the e− are accumulated in one semiconductor, while the h+ are accumulated in the other semiconductor, resulting in a spatial separation of e−/h+ pairs. This separation prevents the rapid recombination of photogenerated charges. A semiconductor with appropriate band positions acts as a scavenger of e− and h+, allowing these charges to react separately. The type-III, as can be seen in Figure 2c, has an architecture similar to the type-II heterojunction photocatalyst; however, there is no overlapping of band gaps, thereby being inadequate for photocatalytic applications [18,19].

The p–n heterojunction (Figure 3) combines the p-type semiconductor and n-type semiconductor. The Fermi level is closer to the valence band in a p-type semiconductor. On the other hand, in the case of an n-type semiconductor, it will shift toward the conduction band [18]. This configuration can increase migration of the electron–hole through the heterojunction for increasing the photocatalytic efficiency by giving an additional electric field. In this type of heterostructure, before light irradiation, the e− on the n-type semiconductor diffuse across the p–n interface to the p-type semiconductor, abandoning positive holes (h+). In the meantime, the positive holes of the p-type semiconductor diffuse into the n-type semiconductor, abandoning negative electrons. This diffusion of electrons and holes continues until the Fermi levels of the semiconductors are equal. As a result, an internal electric field is formed on the p–n interface. The electrons and holes, which are photogenerated in p-type and n-type semiconductors, travel due to the impact of the internal electric field from the conduction band of the n-type to the valence band of the p-type, respectively, following the spatial separation of electrons and holes, and prolong their lifetime. Consequently, the efficiency of e−/h+ separation in the case of the p–n heterojunction is quicker than that of type-II heterojunction photocatalysts because of the synergic effect of the band alignment and the internal electric field [18]. For instance, it is very often that the combination of TiO2 (n-type) with a p-type semiconductor for the formation of a p–n heterojunction occurs [10].

Figure 3

Schematic illustration of the electron–hole separation under the influence of the internal electric field of a p–n heterojunction photocatalyst under light irradiation. Adapted according to Refs. [18,19].

However, for these types of heterojunctions mentioned above (conventional type-II and p–n heterojunction types), the redox capability of the photocatalyst is decreased, due to the oxidation and reduction processes take place on the semiconductor with lower oxidation and reduction potentials, respectively [10,18].

Another type of heterojunction is the Z-scheme photocatalytic system. The Z-scheme system for a liquid phase was reported in 1979 by A. J. Bard [20]. Since this discovery, the Z-scheme heterojunctions have become one of the major topics of interest for scientific researchers, to overcome the problems of the abovementioned heterojunction photocatalysts, such as the redox ability of the material [18].

The conventional Z-scheme photocatalytic system is formed with two semiconductors (PS I and PS II), which are not in physical contact, and a dissolved redox mediator consisting of an electron acceptor/donor (A/D) pair (Figure 4a). During the photocatalytic reaction, photogenerated electrons migrate from the CB of the PS II to the VB of the PS I through an A/D pair via following redox reactions.

Figure 4

Schematic representation of (a) electron–hole separation on the conventional Z-scheme photocatalytic system; (b) the electron–hole separation on all-solid-state Z-scheme photocatalysts; and (c) electron–hole separation on a direct Z-scheme heterojunction photocatalyst. Adapted according to Refs. [18,19].

The conventional Z-scheme photocatalytic system is formed with two semiconductors (PS I and PS II). Unfortunately, this type of heterojunction photocatalyst has the one limitation; they can solely be used in the liquid phase, in which they are not in physical contact, and an electron acceptor/donor (A/D) pair (Figure 4a), named the redox mediator. In this case, both oxidative and reductive photocatalysts are photoexcited, producing electrons and holes. After that, an e− photogenerated in the oxidative photocatalyst reacts with the A, forming an electron donor (D) (Equation (1)). Furthermore, a hole in the reductive photocatalyst reacts with the D, producing an electron acceptor (Equation (2)).

Therefore, the acceptor (A) is reduced to a donor (D) when it reacts with the electrons from the conduction band of the photocatalyst I. Then, the donor (D) is oxidized and produces the acceptor (A) due to the reaction with the holes from the valence band.

In this type of heterojunction, electron–hole separation and a redox ability is achieved, due to the fact that electrons are cumulated in photocatalyst I, with higher reduction potential, while holes are cumulated in photocatalyst II, with higher oxidation potential. The conventional Z-scheme photocatalysts can only be constructed in the liquid phase, thereby limiting their wide application in photocatalysis [18,19].

Later, in 2006, Tada et al. [21] suggested a solid-state Z-scheme photocatalytic system consisting of two photocatalysts (PS I and PS II) connected by a solid-phase electron mediator. This mediator can lead the electrons to go from the oxidative photocatalyst to the reductive photocatalyst, eliminating the inactive charge carriers [19]. Furthermore, this system (Figure 4b) can be applied in practically all experimental conditions, markedly extending it using. However, noble metals (such as Pt, Ag, and Au), which are rare and expensive, are used usually as mediators of electrons in this system, being a limitation to their practical application. In addition, this type of mediator can also absorb incident light, decreasing the photocatalyst’s light utilization [18,19].

In 2013, Yu et al. [22] suggested a heterojunction photocatalyst with the direct Z-scheme. In this case, there is a combination of two different photocatalysts, without an electron mediator. Figure 4c shows that the construction of this direct Z-scheme is the same as the all-solid-state Z-scheme, except that the rare and expensive electron mediators are not required in this system [18,19]. Similarly, e− and h+ are spatially separated on the material with the higher reduction potential and oxidation potential of the direct Z-scheme heterojunction photocatalyst, respectively. The fabrication cost of this direct Z-scheme is low and comparable to that of conventional type-II heterojunction systems. Furthermore, the electron–hole transfer on the direct Z-scheme heterojunction is physically more favorable than that on the type-II heterojunction due to the electrostatic attraction between electrons and holes. In particular, in the case of the direct Z-scheme photocatalysts, the transfer of e− from the CB of the PS II to the h+ rich VB of the PS I is easier, due to the electrostatic attraction between the electrons and the holes. Moreover, without the use of liquid-phase or noble metal electron mediators, the direct Z-scheme photocatalysts have greater potential for wide practical applications [18,19].

The structure of a direct Z-scheme catalyst and p–n heterojunction is similar to that of a type-II heterojunction catalyst. For that reason, it is essential to study the charge-carrier migration mechanism for the different types of heterojunction photocatalysts through various characterization methods, so as to differentiate them. Therefore, various characterization methods could be used for this purpose, such as radical scavenging, photocatalytic reduction testing, metal loading, X-ray photoelectron spectroscopy (XPS), effective mass calculation, internal electric-field simulation and effective mass calculation. Using only a single characterization method cannot provide exact information on the charge-carrier migration mechanism for the heterojunction photocatalyst. Therefore, a comprehensive investigation through a combination of different characterization methods is always essential to describe the type of heterojunction photocatalysts [10,18].

In this review, the most promising semiconductors with heterojunction with TiO2 photocatalysts for CO2 photoreduction, such as C3N4 [23,24,25,26,27,28,29,30,31,32,33], CeO2 [34,35,36,37,38], CuO [39,40,41,42,43], CdS [44,45,46,47], MoS2 [48,49,50,51], and others [52,53,54], are summarized.

2. TiO2 Heterojunction Photocatalysts

2.1. g-C3N4/TiO2

Graphitic carbon nitride (g-C3N4) is a metal-free organic semiconductor, with special physicochemical properties, such as photocatalytic stability [55], electronic band structure, sufficient negative potential of conduction band, chemical and high thermal stability, and low cost. Due to its optical bandgap size (~2.7 eV), it can be activated by visible light, being an appropriate solar light harvesting photocatalyst [56,57,58]. However, this has some disadvantages that reduce its photocatalytic activity, such as high recombination of photogenerated charge carriers, low surface area, and low electrical conductivity [59]. These disadvantages can be overcome by combining them with other heterojunction semiconductors. The combination of wide-band TiO2 and small-band g-C3N4 as a visible light sensitizer to create a heterojunction structure can mask the light response of both photocatalysts, due to the special electronic band structure [60,61]. For this reason, we can harvest more light of the sun through a coupling of g-C3N4 and TiO2, forming a g-C3N4/TiO2 heterojunction. In addition to the CO2 photocatalytic reduction, the resulting heterojunction between TiO2 and C3N4 is used, for example, in the photocatalytic oxidation of NO [62], and for organic pollutants degradation in waste water [63]. CO2 photocatalytic reduction using g-C3N4/TiO2 heterojunction photocatalysts are tabulated in Table 2.

Table 2

CO2 photoreduction using g-C3N4/TiO2 heterojunction photocatalysts.

Photocatalysts		CO2 Photoreduction Condition			Yield of Products	Type of Heterojunction		Ref.
Type	Prepared	Reaction Mixture	Light Source	Conditions
TiO2−x/g-C3N4	Solid state synthesis	CO2 (99.999%), 5 mL of solution containing 4 mL of methyl cyanide (MeCN) solvent, 1 mL of triethanolamine (TEOA), bipyridine (bpy) (10 mmol L−1) and 25 μL of 20 mmol L−1 CoCl2 aqueous solution	300 W xenon lamp	43 mL quartz vessel with a rubber septum; 25 °C; circulation cooling system.Photocatalyst concentration in 1 g L−1	CO = 77.8 μmol g−1 h−1	Type-II	*	[25]
(0.3/1)TiO2/g-C3N4	Simple mechanical mixing of pure g-C3N4 and commercial TiO2 Evonik P25	CO2 with a certified maximum of hydrocarbons less than 1 ppm (SIAD Technical Gases, CZ)	8 W Hg lamp	Cylindrical stirred batch reactor, with internal volume of 355 cm3Photocatalyst concentration in 0.28 g L−1	CH4 = 70 μmol gcat.−1CO = 23 μmol gcat.−1 after 8 h	Type-II	‡	[26]
TiO2@g-C3N4-20%	Stirring method	CO2 and 50 mL 0.08 mol L−1 NaHCO3 solution	300 W Xe lamp with a 420 nm optical filter	quartz glass tube with a volume of 60 mLPhotocatalyst concentration in 1 g L−1	CH3OH ~50 μmol gcat−1 after 4 h	Type-II(see Ref. [27])	-	[27]
HCNS@TiO2	Templating method combined with the subsequent kinetically-controlled coating process	CO2 (high purity) and H2O (400 mL)	Visible-light (300 W Xenon lamp)	cylindrical Pyrex glass photoreactor with 500 mL of volumePhotocatalyst concentration in 1 g L−1	CH3OH = 52.1 μmol gcat−1CH4 = 21.3 μmol gcat−1 after 6 h	Type-II	†	[28]
70:30 g-C3N4-N-TiO2	Hydrothermal method and thermal treatment(in situ method)	Deionized H2O + CO2 (99.999%)	300 W Xe arc lampIntensity 100 mW/cm2	780 mL gas-closed circulation Teflon systemPhotocatalyst concentration in 0.13 g L−1	CO = 14.73 μmol after 12 h	Type-II	¥	[29]
Nb-TiO2/g-C3N4	Solid state synthesis	CO2 (99.99%) flow rate 20 mL/min; water vapor was used as hole scavenger	Two 30 W white bulbs	continuous gas system with a reactor (40 mL) located in the center of a dark cover cask using as a reaction chamber (24 L)Photocatalyst concentration in 2.5 g L−1	CO = 420 μmol g−1 h−1HCOOH = 698 μmol g−1 h−1CH4 = 562 μmol g−1 h−1	Z-scheme	§	[30]
8 mass % g-C3N4/Ag-TiO2	Solvent evaporation followed by calcination	CO2 flow rate 3 mL/min; water vapor was used as hole scavenger	300 W xenon lamp	70 mL cylindrical photoreactorPhotocatalyst concentration in 0.7 g L−1	CH4 = 28.0 μmol g−1CO = 19.0 μmol g−1 after 3 h	Type-II	ƗƗ	[31]
Phosphate–oxygen (P–O) bridged TiO2/g-C3N4	Impregnation-solid state synthesis	CO2 + 3 mL H2O; water vapor was used as a hole scavenger	300 W xenon lamp	cylindrical steel reactor (volume of 100 mL and area of 3.5 cm2)Photocatalyst concentration in 2 g L−1	CH4 = 40 μmol g−1 h−1CO = 15 μmol g−1 h−1	Z-scheme	I	[32]
(Au, C3N4)/TiO2	Immersing (or dipping) method	CO2 + 5 mL H2O	300 W Xenon arc lamp	100 mL sealed steel container with cooling waterPhotocatalyst: Two pieces of samples (0.5 cm2/sample	CO = 0.138 µmol cm−2h−1 CH4 = 0.032 µmol cm−2h−1	Z-scheme	II	[33]

* Reprinted from [25], Copyright (2019), with permission from Elsevier. ‡ Reprinted with permission from [26]. Copyright 2016 American Chemical Society. † Reprinted from [28], Copyright (2020), with permission from Elsevier. ¥ Reprinted from [29], Copyright (2014), with permission from Elsevier. § Reprinted from [30], Copyright (2019), with permission from Elsevier. ƗƗ Reprinted from [31], Copyright (2017), with permission from Elsevier. I Reprinted from [32], Copyright (2017), with permission from Elsevier. II Reprinted from [33], Copyright (2019), with permission from Elsevier.

Shi et al. [25] reported yTiO2−x/g-C3N4 heterojunction photocatalysts with efficient solar-driven CO2 reduction. The 0D/2D heterostructure of oxygen vacancy-abundant TiO2 quantum dots referred in the g-C3N4 (MCN) nanosheets (TiO2−x/g-C3N4), were synthesized by in situ pyrolysis of NH2-MIL-125 (Ti) and melamine with their different mass ratios (g/g) of 5:0.4, 5:0.15, 5:0.1, and 5:0.05. The samples were named yTiO2−x/MCN (y = 8, 3, 2 and 1, which is identical to the % of Ti-MOF out of melamine). All yTiO2−x /MCN photocatalysts showed magnificent photocatalytic reduction performance of CO2 compared with MCN. The authors concluded that the overall rapid decay of electron–hole pairs was ascribed to the interfacial charge transfer, which was attended by relaxation of recombination mediated by shallow trapped sites. Extremely fast interfacial charge transfers significantly increased charge separation. Thus, e− in shallow trapped sites could be readily trapped by carbon dioxide. Moreover, coupling with the synergetic advantage of powerful visible light absorption, high adsorption of CO2 and large specific surface area, TiO2−x/g-C3N4 demonstrated an excellent CO evolution rate. This research shows detailed insights into optimizing the heterojunction structure for robust solar CO2 conversion. The 2TiO2-MCN performed the highest CO formation, roughly five times that of parent g-C3N4. These results show that the significant photoreduction performance of CO2 is also connected with the unique structures, and interface composition of the 0D/2D structure, such as defects in the photocatalyst as well as high specific surface area for enhancing CO2 adsorption and supporting charge carrier separation. In these experiments, only carbon monoxide and a small amount of hydrogen were detected [25].

Reli et al. [26] studied the TiO2/g-C3N4 materials with the ratio of TiO2 to g-C3N4 ranging from 0.3/1 to 2/1 (TiO2/g-C3N4 ratio of 0.3/1, 0.5/1, 1/1, and 2/1) for the photoreduction of CO2 and photoreduction of N2O. They reported that the production rate of methane is almost linear during the first 8 h of irradiation; on the other hand, the carbon monoxide yields increased rapidly in the first two hours and then are almost constant. The hydrogen was also detected. The hydrogen is generated from the water splitting. The most photoactive photocatalyst was (0.3/1)TiO2/g-C3N4, in the presence of which they observed the highest yields of the products. On the other hand, the lowest product formation was achieved over pristine g-C3N4. The authors concluded that the highest photoactivity of the (0.3/1)TiO2/g-C3N4 photocatalyst can be clarified by the combination of several aspects, such as adsorption edge energy, surface area (SBET), crystallite size and efficient charge carrier separation. The key parameter is the efficient charge separation [26].

Zhang et al. [27] described the synthesis of hollow TiO2@g-C3N4 nanocomposites for CO2 photocatalytic reduction under visible irradiation. In this work was reported the utilization of four TiO2@g-C3N4 composites with different mass ratios of g-C3N4 with respect to composites of 11.1%, 14.3%, 20% and 33.3%, labeled as TiO2@g-C3N4-11.1%, TiO2@g-C3N4-14.3%, TiO2@g-C3N4-20%, and TiO2@g-C3N4-33.3%, respectively. The results indicated that TiO2@g-C3N4 photocatalysts displayed higher photocatalytic activity, compared with pure g-C3N4 and the TiO2 does not showed photocatalytic activity under visible light irradiation. The increased photocatalytic activity of TiO2@g-C3N4 nanocomposites was attributed to the higher photo-induced electron–hole separation efficiency and enhanced photoinduced electron migration. Furthermore, the sample with the best photocatalytic performance for the CO2 reduction was the TiO2@g-C3N4-20%. They concluded that with the decrease in g-C3N4 content, the yield of methanol decreased, due to the fact that TiO2 has no catalytic activity in visible light, so the higher amount of TiO2 weakened the absorbing ability of visible light and reduced the photocatalytic efficiency of the TiO2@g-C3N4-11.1% and TiO2@g-C3N4-14.3% composite materials [27].

Dehkordi et al. [28] reported a hierarchical g-C3N4@TiO2 hollow sphere with brilliant activity for CO2 photocatalytic reduction under visible irradiation. These samples have TiO2 shell onto the surface of hollow carbon nitride sphere (HCNS) and are named HCNS@TiO2. In this work, the photocatalytic efficiency of the HCNS@TiO2 photocatalyst was compared with the commercial photocatalyst TiO2 (P25), g-C3N4 and P25/g-C3N4. The obtained results showed that the P25/g-C3N4 and HCNS@TiO2 samples had a superior efficiency for the conversion of CO2 to valuable products under visible light irradiation, once P25 and pristine g-C3N4 showed the small yield of CH3OH production due to the poor visible light activity and the fast rate of electron–hole recombination, respectively. Furthermore, the heterojunction photocatalyst formed through the combination of TiO2 and g-C3N4 with a special hierarchical hollow structure (HCNS@TiO2) showed to be promising with higher potential than each pristine photocatalysts in the CO2. In this reaction, methanol was the primary product in the beginning of irradiation; consequently, the authors concluded that the solar fuel (CH3OH/CH4) can be obtained by the control of the reaction time of CO2 photoreduction. The authors concluded that the nanocomposite photocatalytic activity could be ascribed to its special structure, providing properties, such as multiple light reflection, light harvesting, and an improved active site. They also observed that the improvement in the photocatalytic performance of the HCNS@TiO2 was obtained due to the increased light absorption. The efficiency of CO2 photoreduction over the HCNS@TiO2 photocatalyst was approximately 5 and 10 times higher than in the presence of pristine g-C3N4 and P25, respectively [28]. The decisive parameter responsible for the increasing the photocatalytic performance of HCNS@TiO2 photocatalyst is the synergistic heterojunction creation between the hollow g-C3N4 sphere with TiO2, which makes a rapid electron transfer at the interface between HCNS and TiO2 and increases charge carrier separation [28].

Furthermore, the interest in studying the efficiency of heterojunction materials for CO2 photoreduction has been increasing, and some studies have also been reported using the combination of g-C3N4 with doped TiO2, for instance, TiO2 doped with amine species (N), and modified with metals, for example Ag and Au. Therefore, the most relevant reported works are mentioned here in this review.

For instance, Zhou et al. described the selective photoreduction of carbon dioxide to CO, using the graphitic carbon nitride (g-C3N4)-N-TiO2 heterostructure as an effective photocatalyst [29]. In this work, the authors prepared photocatalysts of graphitic carbon nitride and in situ N-modified titanium dioxide (g-C3N4-N-TiO2 composites), using precursors that incorporate urea and Ti(OH)4 with various mass ratios (80:20, 70:30, 60:40, 50:50, 40:60, 30:70, 20:80, and 10:90). The greater ratios of urea to Ti(OH)4 (60:40 and more) result in the photocatalysts of g-C3N4 and N-doped TiO2, while smaller ratios (till 50:50) only show in N-doped TiO2. The selectivity of the photocatalytic reaction is interesting in the presence of these photocatalysts. In the presence of N-doped TiO2 samples, CH4 and CO were generated, while in the presence of g-C3N4 and N-TiO2, only CO was performed; the product selectivity may connect with the formed g-C3N4. Among the as-prepared samples, 70:30 g-C3N4 and N-TiO2 composites present the highest CO formation, due to the visible light absorption and lowest electrons and holes recombination. As can be seen in the CO2 photoreduction reactions in Table 1, eight electrons are required for the formation of one CH4 molecule; however, only two e− are necessary for one CO molecule production. For that reason, the CO2 photoreduction into CO is a more dynamic, favored process.

Based on this fact and with the obtained results, the authors designed a mechanism for the increase in photocatalytic performance, where charge carriers are created and transmitted between the interface of g-C3N4 and N-TiO2 during irradiation. Therefore, the holes in g-C3N4 (h+ created in g-C3N4 and transmitted from the valence band of TiO2) might oxidize the H2O absorbed on the surface of g-C3N4, producing O2 and H+. Furthermore, the electrons in N-TiO2 (created in N-TiO2 and the e− transmitted from g-C3N4) can reduce the CO2 into C1 intermediates. No methane was produced when a high ratio of urea and Ti(OH)4 (60:40 or more) was used. This was due to the presence of g-C3N4 and N-TiO2; the H+ may not capture the photogenerated e−, due to the formation of aromatic heterocycles of g-C3N4, which are electron rich, where the protons can be stabilized by the conjugated aromatic heterocycles and, thus, they have difficulty in taking part in the formation of CH4. Furthermore, the e− in the conduction band of g-C3N4 can quickly be transferred to the conduction band of N-TiO2 for CO2 photoreduction into CO. For that reason, the g-C3N4-N-TiO2 photocatalyst formed is selective for the production of CO during the CO2 photoreduction. On the other hand, for low ratios of urea to Ti(OH)4 (till 50:50), the H● radicals or H+ ions formatted during CO2 photocatalytic reduction can be quickly consumed by adsorbed carbon dioxide; thus, CO and CH4 were simultaneously analyzed, due to the absence of conjugated aromatic system on these samples [29].

Another example of the combination of g-C3N4 with doped TiO2 was reported by Truc et al. [30], using TiO2 doped with niobium. Truc et al. [30] studied the photoactivity of niobium doped TiO2/g-C3N4 direct Z-scheme photocatalytic system for effective CO2 conversion into valuable fuels. They prepared three Nb-TiO2/g-C3N4 samples with 25%, 50%, and 75% of the mole percentages of Nb-TiO2.

The authors observed that g-C3N4 did not reduce CO2 under visible light irradiation, due to the high recombination rates of photoexcited e− and h+. However, CO2 photocatalytic reduction under visible irradiation was possible in the presence of Nb-TiO2 and Nb-TiO2/g-C3N4 materials, obtaining different products (CH4, CO, and HCOOH). In the presence of a pure Nb-TiO2 photocatalyst, the products were CO and CH4. Nb dopant in TiO2 lattice led to the creation of the Ti3+, which was as an intermediate band between the valence band and the conduction band of the TiO2, reducing the e− and h+ recombination. Furthermore, when the Nb-TiO2/g-C3N4 was used as photocatalyst, not only CO and CH4 were produced, but also HCOOH was obtained [30].

As expected, the photocatalytic activity for CO2 reduction was higher for the Nb-TiO2/g-C3N4 samples, when compared to the Nb-TiO2, g-C3N4 and TiO2 samples. The authors attributed this to the direct Z-scheme mechanism, where photoexcited e− in the Nb-TiO2 CB combined with the photoexcited h+ in the g-C3N4 VB avoided the existence of e− in the g-C3N4 CB and h+ in the Nb-TiO2 VB. Therefore, this Nb-TiO2/g-C3N4 system had more available electron–hole pairs when compared with the pure Nb-TiO2 photocatalyst. Furthermore, the potential energy of the generated electrons of Nb-TiO2/g-C3N4, (~−1.2 V) was more negative than the generated electron of Nb-TiO2, (~−0.2 V), so the generated electron of the Nb-TiO2/g-C3N4 required lower energy during the reduction of CO2 when compared with that of the Nb-TiO2 [30].

The best photocatalyst for the photoreduction in CO2 under visible light irradiation was the 50Nb-TiO2/50 g-C3N4. The higher efficiency of this sample was due to the higher numbers of produced and consumed e− and h+ when compared with the Nb-TiO2 and the other Nb-TiO2/g-C3N4 photocatalysts. In the 50Nb-TiO2/50 g-C3N4 sample, the Nb-TiO2 mole resembled the mole of g-C3N4; therefore, the photogenerated electrons in the Nb-TiO2 CB would achieve photogenerated holes in the g-C3N4 VB. Therefore, the amounts of e− in the g-C3N4 CB and h+ in the Nb-TiO2 VB of the 50Nb-TiO2/50 g-C3N4 sample were considerably higher than in the presence of other photocatalysts. Based on this work, they concluded that the Nb-TiO2/g-C3N4 photocatalysts have more charge carriers available for different valuable fuels. Additionally, the produced electrons of the Nb-TiO2/g-C3N4 in the conduction band of the g-C3N4, for which the potential energy is around—1.2 V, are enough strong to produce not only CO and CH4, but also HCOOH during the reduction of CO2 [30].

Li et al., for example, used the heterostructured g-C3N4/Ag-TiO2 photocatalyst for the CO2 photocatalytic conversion [31]. These authors reported for the first time the preparation of heterostructured g-C3N4/Ag-TiO2 materials via a facile solvent evaporation and by a calcination process with g-C3N4 and Ag-TiO2 as precursors. They prepared four different g-C3N4/Ag-TiO2 samples with various masses of g-C3N4 and Ag-TiO2 and compared them with the commercial TiO2 photocatalyst (Degussa P25), g-C3N4, and AgTi samples. As expected, the results showed that TiO2 obtained the lower CO2 conversion, and no significant amount of CH4 was formed during the 3 h irradiation. Using g-C3N4, the CO2 conversion was higher in comparison with TiO2; however, the CH4 yields were still very low. The AgTi sample showed higher performance than TiO2, due to the Ag nanoparticles (NPs) on the AgTi sample not only making the separation of generated charge carriers on TiO2 by UV irradiation easy, but also increasing the energy of trapped e− through the Ag surface plasmon resonance effect with the visible light irradiation. Due to this fact, there were more e− with higher energy for CH4 formation during CO2 reduction. Using the CN/AgTi composite samples, both the conversion of CO2 and solar fuel (CH4 and CO) yields enhanced with the higher amount of g-C3N4 to AgTi mass ratio from 0 to 8%. They also observed that the rate of electrons consumed was higher when the composite samples were used.

The results showed that the 8CN/AgTi sample (with g-C3N4 to AgTi mass ratio of 8%) obtained the highest photoconversion of CO2 after 3 h of irradiation. Based on this result, the authors concluded that the coupling of g-C3N4 and AgTi had a synergistic effect in the photocatalytic reduction of CO2. However, when the g-C3N4 to AgTi mass ratio increased to 12%, this led to in an evident decrease in the photocatalytic reduction of CO2; this decreasing trend is ordinary, and it is possible to attribute it to the fact that an excessive amount of g-C3N4 resulted in shielding of the active site on the TiO2 surface.

The authors reported that the combination of g-C3N4 and AgTi enhanced the generation of electrons and holes under sunlight. These photogenerated electrons moved through the heterojunction between carbon nitride and titanium dioxide, and further from titanium dioxide to silver nanoparticles with a lower Fermi level, avoiding the electron–hole recombination, and led to electrons cumulating on Ag nanoparticles deposited on the surface of TiO2 in the g-C3N4 /Ag TiO2. After that, the e− cumulated on the Ag nanoparticles were further energized by the surface plasmon resonance effect. Therefore, the CN/AgTi samples showed higher photocatalytic performance.

The Ag nanoparticles on the TiO2 surface in the CN/AgTi composite had a significant role once they decelerated the e−/h+ pairs recombination due to extracting e− from conductive band of TiO2, and also used the surface plasmon resonance effect to increase the energy level of e− cumulating on the surface. Therefore, the bounteous energetic electrons on Ag nanoparticles generated from the activation by solar irradiation of the both TiO2 and g-C3N4 parts were answerable for the important synergy of the combination of g-C3N4 and AgTi in photoreduction of CO2 in the presence of water vapor [31].

Liu et al. studied the P–O functional bridges effects on the electron and hole transfer and separation of TiO2/g-C3N4 photocatalysts for the CO2 reduction [32]. The authors prepared four P–O bridge TiO2/g-C3N4 composite samples with various molar % ratios of phosphate to TiO2 (1, 5, 10 and 15%) and compared them with the g-C3N4 and TiO2/g-C3N4 samples. All photocatalysts exhibited excellent photocatalytic activity for CO2 reduction, being that CH4 was the principal product obtained as well as CO in a small amount. The sample TiO2/g-C3N4 with the P–O bridge with 10% molar ratio showed the best performance for this reaction, with photoactivity approximately 2 and 3 times higher than for pure samples. The P–O functional bridges increased the heterojunction coupling between TiO2 and g-C3N4, thereby significantly enhancing the charge transfer and separation, obtaining higher photocatalytic activity.

Based on this study, the authors concluded that the photoactivity of g-C3N4 was significantly improved due to the connection with P–O-bridged TiO2 (in a proper amount). The characterization of the P–O bridge TiO2/g-C3N4 composite samples, using surface photovoltage and photoluminescence spectroscopy, showed that the improvement on the e−/h+ separation of g-C3N4 after coupling with the P–O bridged TiO2, resulted from the P–O bridge between TiO2 and g-C3N4 that promotes effectively the electrons’ transference from g-C3N4 to TiO2 [32].

Sun et al. [33] reported the preparation of a Z-scheme heterostructure with r-TiO2 (rutile) modified with gold and g-C3N4 quantum dots to achieve a recyclable and high-efficiency photocatalyst for CO2 reduction. The photocatalytic activity of (Au, C3N4)/TiO2 composite was compared with the C3N4/TiO2, r-TiO2, and bulk C3N4. The obtained products were CO, CH4 and O2; however, using the pristine r-TiO2 and bulk g-C3N4 samples, no significant CH4 yield was observed. The higher-energy products require a higher reduction potential. Carbon dioxide reduction to methane is an 8-electrons process, which requires the photogenerated charge to have a long lifetime. The CB band of the pure r-TiO2 is not negative enough to transfer CO2 to CH4. However, for the (Au, C3N4)/TiO2 composite, the yield amount of carbon monoxide and methane was markedly enhanced. The authors concluded that a Z-scheme heterostructure was formed at the r-TiO2 and g-C3N4 interface, instead of type-II heterojunction [33].

The results demonstrated that the (Au, C3N4)/TiO2 photocatalyst has four and five times higher photoactivity in comparison with the bulk g-C3N4 and pristine r-TiO2, respectively. This improvement on the photocatalytic performance was attributed to the excellent Z-scheme heterojunction formed in the interface of r-TiO2 and g-C3N4 [33].

2.2. CeO2/TiO2

Ceria or cerium oxide (CeO2) is a rare earth metal oxide that has attracted the interest of researchers due to the fact that the valences of ceria, such as Ce4+ and Ce3+, enhance light absorption ability and increase electron transfer. This material is an n-type semiconductor with a large bandgap energy (2.7–3.2 eV), non-toxic, readily available, and chemically stable [64]. CeO2 present two oxidation states, Ce (IV) and Ce (III), which give it unique chemical, mechanical, and magnetic properties. Furthermore, the Ce (III) and Ce (IV) oxidation states can be easily converted from one to another [64,65]. The surface oxygen vacancies of CeO2 from the reversible characteristics of Ce3+ and Ce4+ can promote its photocatalytic performance. During the reduction of Ce4+ ions into Ce3+, there occurs a formation of oxygen vacancies on the photocatalyst surface, which consequently act as electron trap centers that can inhibit the recombination [64]. Recently, the utilization of CeO2 as a coupling was reported since the Ce4+/Ce3+ displacement can accelerate the charge separation and impurity levels caused by CeO2 coupled TiO2 to be excited in the visible region [66]. CO2 photocatalytic reduction using CeO2/TiO2 heterojunction photocatalysts are tabulated in Table 3.

Table 3

CO2 photoreduction using CeO2/TiO2 heterojunction photocatalysts.

Photocatalysts		CO2 Photoreduction Condition			Yield ofProducts	Type of Heterojunction		Ref.
Type	Prepared	Reaction Mixture	Light Source	Conditions
Mes-CeTi-1.0	Template method using a nanocasting route	CO2 + H2O	Xe arc lamp 300 W	stainless steel reactor (volume of 1500 mL)Photocatalyst concentration in0.07 g L−1	CH4 = 11.5 mmol gcat−1CO = ~70 mmol gcat−1 after 325 min	-	-	[34]
CeO2-TiO2	Stirring method and calcination method	CO2 and 300 mL of 0.1 mol L−1 NaOH solution (for 30 min before irradiation)During irradiation CO2 was continuously bubbled	Visible light—500 W Xenon lamp, and 2 mol L−1 sodium nitrite solution (to remove UV light)	Pyrex glass reactor (500 mL) Photocatalyst concentration in 1 g L−1	CH3OH = 18.6 μmol gcat−1 after 6 h	Type-II	*	[35]
CeO2/TiO2-4	Gas bubbling-assisted membrane precipitation (GBMP) method	CO2 and H2O	300 W Xe lamp and an optical filter with the absorbed light wavelength of <420 nm	Glass reactor (basal diameter of 4 cm)Photocatalyst amount 0.1 g	CO = 2.06 μmol after 6 h	Type-II	‡	[36]
CeO2/TiO2(R-TiCe0.1)	Hydrothermal method	CO2 and H2O (Gaseous CO2 of 8 kPa was in site produced by the reaction of NaHCO3 with a H2SO4 solution (0.5 M).)	500 W Xenon lamp	reactor connected with mechanical vacuum pumpPhotocatalyst amount 10 mg	CO = 61.9 μmol g−1CH4 = 23.5 μmol g−1 after 6 h	Type-II	†	[37]
0.2CeO2/TiO2	One-pot hydrothermal method	CO2 and H2O (Gaseous CO2 of 8 kPa was produced in situ by the reaction of NaHCO3 with a H2SO4 solution (0.5 M).)	300 W Xenon lamp	reactor connected with mechanical vacuum pumpPhotocatalyst amount 10 mg	CO = 46.6 μmol g−1CH4 = 30.2 μmol g−1 after 6 h	Type-II	¥	[38]

* Reprinted from [35], Copyright (2015), with permission from Elsevier. ‡ Reprinted with permission from [36]. Copyright 2014 American Chemical Society. † Republished with permission of Royal Society of Chemistry, from [37] copyright 2016; permission conveyed through Copyright Clearance Center, Inc. ¥ Reprinted from [38], Copyright (2016), with permission from Elsevier.

Wang et al. [34] prepared three photocatalysts with various Ce/Ti molar ratios, 1:2, 1:1, and 2:1. All prepared CeO2-TiO2 composites had higher photoactivity for the CO2 photoreduction to CH4 and CO, when compared with Mes-CeO2, Mes-TiO2 and commercial TiO2 photocatalyst (P25). This enhancement of the photocatalytic efficiency for these CeO2-TiO2 photocatalysts was achieved due to the ordered large specific surface area, mesoporous architecture, 2D open-pore system that facilitates the diffusion of the reactant into the bulk of photocatalyst and consequently provides fast intraparticle molecular transfer, and also due to the absorption in the visible range due to the CeO2 species photosensitization. The heterojunction between CeO2 and TiO2 also contributes to the enhancement of the CeO2-TiO2 composites, once the photogenerated e− in the TiO2 can be transferred for the CeO2 under the internal electric field, improving the e−/h+ separation in the TiO2, leading to the improvement of photoactivity under irradiation [34].

The authors also confirmed by XPS analysis that the presence of CeO2 can increase significantly the chemisorbed of oxygen species on the surface of the ordered mesoporous CeO2-TiO2 composites. These O species can easily trap e− and produce surface O• with outstanding reduction ability. Additionally, the mixture of Ce3+/Ce4+ oxidation states on the CeO2-TiO2 surface show that the partial metal in photocatalysts is not completely oxidized, and therefore, Ce3+ can react with holes and avoid the recombination of photogenerated e−/h+, leading to a higher quantum effectiveness of CO2 photoreduction [34].

Comparing the efficiency of the three different composites, no significant differences were obtained in the obtained yields of CO and CH4 after 325 min of irradiation. The authors analyzed the stability of the composites after irradiation and concluded that these composite were stable after the photocatalytic test [34].

Abdullah et al. [35] reported a CeO2-TiO2 composite for the photoreduction of CO2 into CH3OH under Vis irradiation. They demonstrated that the methanol yield in the presence of CeO2-TiO2 was three times higher than that of pure TiO2. The researchers concluded that this improvement is due to the existence of active anatase phase of titanium dioxide with a small crystalline size, and the uniform structure and smaller bandgap of the CeO2-TiO2 photocatalyst increased the visible light absorption and produced more e−/h+ pairs. Furthermore, the existence of both Ce3+ and Ce4+ oxidation states on the surface of CeO2-TiO2 avoided the recombination of the photogenerated e−/h+. In this case, the e− are captured by the Ce4+, and these trapped electrons are moved to the adsorbed oxygen in order to produce superoxide anion radicals, while Ce3+ reacts with the generated h+, reducing the e−/h+ recombination. As can be seen in the schematic representation in Table 3, the CeO2 has a more negative CB energy, due to the possibility of photoexcited e− transference from CB of CeO2 to CB of TiO2, decreasing the recombination rate of the charge carriers [35].

Jiao et al. [36] prepared four CeO2/TiO2-n photocatalysts, with the weight ratio of CeO2 to TiO2 of n/100, obtaining the samples CeO2/TiO2-16, CeO2/TiO2-8, CeO2/TiO2-4, and CeO2/TiO2-2, and compared the photoactivity of the samples with the 3D ordered macroporous TiO2 (3DOM) and the commercial TiO2 (P25). The outcomes demonstrated that the CeO2/TiO2-2, CeO2/TiO2-4 and CeO2/TiO2-8 composites had higher photoactivity than the TiO2 and P25 samples, showing that the synergetic effect of TiO2 and CeO2 increased the photocatalytic efficiency. The CeO2 sample showed the lower CO production amount. The best photocatalytic performance was obtained with the sample CeO2/TiO2-4. However, the amount of CO decreased with the increase in the CeO2 loading amount (>4), showing that the amounts of CeO2 nanolayers have optimal value. The composite sample with the higher amount of CeO2, CeO2/TiO2-16, showed lower performance than the 3DOM TiO2 and P25 samples, which can be explained due to the fact that the CeO2 sample almost did not have photocatalytic activity for this reaction condition, and this can be the possible reason for the lower photocatalytic activity of this sample. The authors proposed a type-II photocatalytic mechanism for the CO2 photoreduction using the CeO2/TiO2 composite, shown in Table 3. The increase in photoactivities during the photocatalytic reduction of CO2 under Vis irradiation is due to the synergistic effect of the heterojunction between CeO2 and TiO2 and photonic crystals. They concluded that the heterojunction between CeO2 and TiO2 increases the charge carriers separation, and the absorption efficiency of solar irradiation can be enhanced due to the slow light effect of the 3D ordered macroporous structure and the ordered macroporous facilitates the diffusion of the reactants [36].

Zhao et al. [37] investigated the effect of the TiO2 polymorph phases, brookite, anatase, and rutile on the CeO2/TiO2 composites efficiency for the photocatalytic reduction for CO2. They prepared CeO2/TiO2 composite using anatase, brookite and rutile, identified as A-TiCe, B-TiCe and R-TiCe, respectively. The results showed that the higher amount of CO yield produced was achieved using the sample rutile TiO2/CeO2 (R-TiCe). This enhancement in CO2 photoreduction using the rutile TiO2/CeO2 sample was justified due to the formation of Ti defects at the CeO2-rutile interfaces that improves the energy-band structure of rutile, facilitating the e−/h+ pairs’ separation. To go further, the authors prepared samples of rutile TiO2/CeO2 with different mass ratios of CeO2/TiO2, with CeO2 5.9, 12.9 and 24.3 wt.%, obtaining the samples, R-TiCe0.05, R-TiCe0.1, R-TiCe0.2, respectively. They compared the activity of these rutile TiO2/CeO2 composites with the rutile TiO2, CeO2 and P25. The CeO2 had the lower photocatalytic activity. The best photocatalyst for the CO2 photoreduction was the R-TiCe0.1 with the yield of CO. This result can be explained once the activity of CeO2 is markedly lower when compared with the TiO2, suggesting that in the CeO2/TiO2 composites, the TiO2 is the principal active composition, and CeO2 acts as a promoter [37].

Wang et al. [38] synthesized CeO2/TiO2 samples with CeO2 40, 20 and 10 wt.%, identified as 0.4 CeO2/TiO2, 0.2 CeO2/TiO2 and 0.1 CeO2/TiO2, respectively. The activity of the CeO2/TiO2 composites was compared with the CeO2 and TiO2 samples. Comparing all the samples, the best photocatalyst was the 0.2 CeO2/TiO2. Furthermore, they observed that the CeO2/TiO2 composites’ photoactivity enhances with the higher CeO2 amount and reaches a maximum at 20 wt.% CeO2 content (sample 0.2 CeO2/TiO2), since for the sample with higher CeO2 content, the photocatalytic activity decreased. TiO2 is the principal active composition, while CeO2 acts as a promoter in the CeO2/TiO2 composites. CeO2 content is the dominant factor on the enhancement of CO2 photoreduction under simulated sunlight illumination. This work showed that CeO2 extends the light absorption of the CeO2/TiO2 composite to the Vis range and enhances the e−/h+ separation, due to the presence of Ce3+ [38].

2.3. CuO/TiO2

Copper oxide, CuO, is an p-type semiconductor nanomaterial with a bandgap between 1.2 and 1.9 eV, among this narrow direct bandgap. This material has various properties, such as high electrical conductivity, good semiconducting nature, thermal stability, low toxicity and low cost. The bandgap of CuO should favor the Vis light absorption and enhance the photoactivity [41,67,68]. CuO has been used as photocatalyst for the CO2 photocatalytic conversion to solar fuels [69,70]. CuO exhibits spontaneous CO2 adsorption (ΔH= −45 kJ mol−1) in comparison with TiO2. The energy levels of the CO2-adsorbed species, such as –O–Cu–O–, can lead to an improvement in the visible-light absorption and efficient separation of electrons and holes that favors the photocatalytic activity of CuO [69,71]. Moreover, CuO presents selectivity to the formation of value-added solar fuels, such as CH3OH and CH4 in the photocatalytic CO2 reduction [72]. For the above reasons, CuO/TiO2 photocatalysts with heterojunction were also studied for the CO2 photocatalytic reduction. CO2 photocatalytic reduction using CuO/TiO2 heterojunction photocatalysts is tabulated in Table 4.

Table 4

CO2 photoreduction using CuO/TiO2 heterojunction photocatalysts.

Photocatalysts		CO2 Photoreduction Condition			Yield of Products	Type of Heterojunction		Ref.
Type	Prepared	Reaction Mixture	Light Source	Conditions
CuO/TiO2(AB)	Impregnation method	pure CH3OH solution (30 mL), and pure CO2 gas	250 W Hg lampintensity 3900 μW/cm2 at 365 nm	ideal mixing 50 mL quartz tubePhotocatalyst concentration in 1 g L−1	HCOOCH3 = ~1800 μmol gcat−1 after 4 h	-	-	[41]
3 wt.% CuO/TiO2	Impregnation method	CO2 (Ultra high purity grade), 130 mL of 0.2 M KHCO3 and 0.1 M Na2SO3 aqueous solutions	500 W high pressure Hg lamp with a peak light intensity at 365 nm	quartz reactorPhotocatalyst concentration in 2.77 g L−1	methanol = 12.5 μmol g−1ethanol = 27.1 μmol g−1 after 6 h	--	-	[42]
1.0CuO-TiO2	Stirring method followed by calcination	CO2 (99.99% purity) and 30 mL of methanol	250 W high pressure mercury lamp with the radiation peak at about 365 nm	slurry reactor systemPhotocatalyst concentration in 1 g L−1	Methyl formate ~1600 μmol g−1 h−1	Z-scheme	*	[39]
CuO loaded TiO2 nanotube	Hydrothermal method	CO2 (flow rate of 30 mL min−1) and ultrapure water, and NaHCO3 (0.1 M)	400 W high-pressure mercury lamp with a quartz filter	flow system with an inner-irradiation-type reaction vessel at ambient pressurePhotocatalyst amount 0.5 g	100% CO2 conversion into CH4 and CH3OH after 2.5 h	Type-I	‡	[43]

* Reprinted from [39], Copyright (2011), with permission from Elsevier. ‡ Reprinted from [43], Copyright (2018), with permission from Elsevier.

Zhao et al. successfully prepared CuO-incorporated TiO2 photocatalysts by an impregnation method, to be used as photocatalysts for the CO2 reduction into methyl formate in methanol [41]. They observed that the heterojunction photocatalyst CuO/TiO2(AB) had higher methyl formate yield than the pristine photocatalysts. It was caused by its mixed-phase heterojunction structure and higher specific surface area, resulting in an effective separation, an enhanced UV-light response, and a smaller recombination rate of photogenerated electrons and holes. CuO/TiO2(AB) also demonstrated sufficient stability. The methyl formate yield reproducibility was higher than 90% in cyclic runs [41]. The experimental conditions are resumed in Table 4.

Li et al. [42] dealt with CO2 photocatalytic reduction to produce CH3OH and C2H5OH over CuO-loaded titania powders suspended in H2O with Na2SO3, which was the hole scavenger and promoted the formation of ethanol. The authors prepared four composites with a copper amount between 1 and 7 wt.% (7 wt.% CuO/TiO2, 5 wt.% CuO/TiO2, 3 wt.% CuO/TiO2, and 1 wt.% CuO/TiO2). They observed that yields of methanol and ethanol are enhanced with a CuO amount until 3 wt.%, and for the samples with 5 and 7 wt.%, the yields are decreased, being in this case 3 wt.%, the ideal amount of CuO loading. Loading of CuO enhances CH3OH and C2H5OH yields due to the higher amount of active sites. Copper is an electron catcher and inhibits e−/h+ recombination. However, the samples with a higher amount of copper (>3 wt.% CuO) cannot further enhance the CH3OH and C2H5OH yields due to the excess of CuO, which covers the surface of TiO2, decreasing the TiO2 photoexciting capacity, thereby reducing the photoactivity [42].

Another example of CuO and TiO2 heterojunction was reported by Qin et al. [39]. They studied the photocatalytic reduction of carbon dioxide in CH3OH to methyl formate in the presence of CuO–TiO2 photocatalysts. The methanol was used as the hole scavenger, which can react with the photogenerated holes in the VB, and CO2 was reduced by the e− in the VB. The authors prepared samples with 0.5, 1, 3 and 5 weight percentage of CuO and compared their photocatalytic activity with TiO2. The coupling of TiO2 with CuO led to the rapid increase in the photoactivity because TiO2 and CuO created composite photocatalysts, and electron and hole recombination was reduced.

However, as mentioned above, these authors also concluded that higher CuO loading (>1.0%, in this case) decreases the photoactivity because of the CuO particles’ agglomeration. The most active photocatalyst was 1.0CuO–TiO2 (1 wt.% of CuO). The authors concluded that the heterojunction between TiO2 and CuO was the decisive parameter for enhancing the photoactivity of the samples [39].

Razali et al. [43] prepared p–n type CuO-TiO2 nanotube samples with improved ability for carbon dioxide photoconversion into fuels. They concluded that the higher photocatalytic efficiency of CuO—TiO2 photocatalyst is attributed to the restraint of e− photogeneration and h+ recombination, as the p–n heterojunction between the CuO particles and TiO2 nanotube facilitates the charge separation between electrons and holes, due to the presence of an electrostatic field at the junction. Electrons in the CB of CuO transfer into the CB of TiO2, whereas holes in the VB of TiO2 transfer to the VB of CuO. The charge transfers and separation between both semiconductors may prohibit the recombination of electrons and holes, thus increasing the photocatalytic performance of the CuO-loaded TiO2 nanotube [43].

2.4. CdS/TiO2

Cadmium sulfide (CdS) is a semiconductor material from the II–VI group with a direct bandgap of 2.4 eV [73,74]. CdS is used for carbon dioxide photocatalytic reduction under UV light irradiation [11,75]. This photocatalyst has ideal properties, such as the capability of converting light energy into chemical, optical, photophysical and photochemical energy [73]. However, this material has some disadvantages, such as fast e−/h+ recombination, and photocorrosion vulnerability in aqueous solution due to oxidation by photo-generated holes during photocatalytic reaction [73]. Nevertheless, the photocatalytic activity of this semiconductor can be enhanced, for instance, by doping with metal elements or by the combination with other semiconductors [73]. To date, CdS is widely used in the TiO2/CdS coupled heterojunction to improve the photoelectron conversion efficiency of photocatalysis and solar cell [74]. TiO2/CdS combination is reported as the one of the most representative hybrid semiconductors, once the valence and conduction bands of the CdS are appropriately located in relation to those of TiO2 for higher charge separation, and also CdS can absorb a main part of visible light, as it is possible to use sunlight [44,76]. CO2 photocatalytic reduction using CdS/TiO2 heterojunction photocatalysts is tabulated in Table 5.

Table 5

CO2 photoreduction using CdS/TiO2 heterojunction photocatalysts.

Photocatalysts		CO2 Photoreduction Condition			Yield of Products	Type of Heterojunction		Ref.
Type	Prepared	Reaction Mixture	Light Source	Conditions
TiO2/CdS-3	Conventional hydrothermal technique	Ar or CO2 (both 99.99%) for 1 h, and aqueous isopropanol solution (1.0 M, 100 mL)	450 W Xe arc lamp in combination with 320 nm or 420-nm-cutoff filters	airtight glass reactor (120 mL) with a quartz disc for light penetrationPhotocatalyst concentration in 1 g L−1	methane = ~18 µmol (after 10 h)CO = ~2.5 µmol (after 10 h)Under UV-vis irradiation	Type-II	*	[44]
TiO2/CdS	Ionic layer adsorption and reaction (SILAR) method	CO2 and H2O vapor (from 84 mg of NaHCO3 and 0.3 mL of HCl solution (4 M))	300 W Xenon arc lamp	200 mL Pyrex reactor(purged with N2 gas)Photocatalyst: Film with 4 cm2	11.9 mmol h−1 m−2 for CH4 production	Z-scheme	‡	[45]
CdS-TiO2-8	Hydrothermal method	CO2 and 10 mL cyclohexanol	250 W high pressure mercury lamp	batch slurry bed reactor with inner capacity of 50 mLPhotocatalyst concentration in 2 g L−1	cyclohexyl formate = 20.2 µmol gcat−1h−1cyclohexanone = 20 µmol gcat−1 h−1	Z-Scheme	†	[46]
CdS-TiO2 S3 (45%)	Hydrothermal method	N2 and CO2	125 W Hg lamp (350–400 nm)For the visible light, the UV wavelengths <400 nm were removed using a sodium nitrite solution (2.0 M)	Pyrex reactor with an effective volume of 125 mLPhotocatalyst concentration in 1.44 g L−1	Under UV-vis irradiation:CO = ~15.5 µmol gcat−1CH4 = ~3.0 µmol gcat−1after 8 hUnder visible light irradiation:CO = ~10.3 µmol gcat−1 CH4 = ~1.5 µmol gcat−1after 8 h	Type-II	¥	[47]

* Reprinted from [44], Copyright (2016), with permission from Elsevier. ‡ Reprinted from [45], © 2022 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. † Reprinted from [46], Copyright (2014), with permission from Elsevier. ¥ Reprinted from [47], Copyright (2014), with permission from Elsevier.

Park et al. [44] reported the photocatalytic conversion of CO2 to CH4 in the presence of TiO2/CdS in an isopropanol (IPA) solution under UV-Vis and Vis light irradiation. IPA is frequently used as a sacrificial e− donor, such as an h+ scavenger. The authors prepared three TiO2/CdS composites, TiO2/CdS-5, TiO2/CdS-3 and TiO2/CdS-1, with varied amounts of loaded CdS to TiO2, approximately 33.7%, 23.6%, and 11.4%, respectively. However, they did not observe a significant difference on the obtained yields with the CdS amount on the TiO2, so they only reported the results obtained using the sample TiO2/CdS-3 as a photocatalyst for the CO2 photoreduction. The authors analyzed the H2 evolution and the production of CO and CH4, using Ar or CO2 gas to purge (before irradiation) the aqueous TiO2 and TiO2/CdS suspensions with isopropanol.

During the H2 evolution, the results showed that using Ar-purged gas, the TiO2/CdS composite sample had better photocatalytic activity in comparison with the pristine TiO2. As expected, the production of H2 in the Ar atmosphere was higher than in the CO2 atmosphere, due to the competition for electrons. In contrast, for the CO production, it was observed that using CO2-purged gas, the TiO2/CdS composite sample had better photocatalytic activity in comparison with the pristine TiO2. This result was observed due to the better adsorption of CO2 on CdS, as well as the increased charge carrier separation and transfer on the TiO2/CdS composite sample.

Regarding the CH4 production, the results showed that regardless of the purge gas used (Ar or CO2), the TiO2/CdS composite sample had higher catalytic activity when compared with the TiO2 sample. It is well known that CdS-modified TiO2 is more active than pure TiO2 for the formation of CH4. However, in this case, some part of the CH4 obtained was formed due to the presence of IPA, both making a contribution to the observed CH4 yields. Furthermore, the authors did not discard the idea that the presence of hydrocarbon contaminants during the preparation of the catalyst can be considered for a fraction of the obtained yields.

The authors also investigated the photoactivity of the TiO2/CdS sample (in CO2-purged gas) under Vis light irradiation. In this case, only the CdS photocatalyst was capable of excitation, and no significant differences on the CH4 production were obtained using UV-vis and visible light irradiation, suggesting that the CdS plays a significant role in CO2 fixation and in photocatalyzing the transference of multi-electrons to CO2.

With this work, the authors concluded that the presence of CdS on TiO2 enhanced the production rate of CH4 and enhanced the total CH4 yields. They reported that this enhancement can be ascribed to the easy transference of e− from the CdS to surface-bound CO2, resulting in to the formation of •CO2− that binds to the positively charged surface of CdS, and also due to the surface-bound bicarbonate geometry that increases the production of CH4 due to smaller energy barriers in comparison with the linear O=C=O molecule [44].

Low et al. [45] described a direct Z-scheme TiO2/CdS composite with high efficiency for the photocatalytic reduction of CO2. They compared the photocatalytic activity of the TiO2/CdS composite with the TiO2, CdS and commercial P25 samples. The TiO2/CdS composite formed 3.5-, 5.4-, and 6.3-times higher amounts of CH4 than the TiO2, CdS and commercial P25, respectively. They compared the type II and direct Z-scheme possibility for the mechanism of their TiO2/CdS composites during activation. With a simple test of the ●OH production (using coumarin to trap ●OH and produce fluorescent products, it is possible to analyze by fluorescence spectroscopy) they concluded that it was possible to produce this radical. On the other hand, using the CdS photocatalyst was not obtained, due to the position of the VB (around 1.8 V) being lower than the potential of this reaction (E0(OH−/●OH) = 2.4 V). So, the ●OH was formed on the TiO2 side of the composite, following the direct Z-scheme mechanism of the charge-transfer process, as shown in the schematic illustration in Table 5. The enhanced performance obtained in the presence of the TiO2/CdS composite can be explained due to the e−/h+ availability (according to the enhanced photocurrent for this sample), due to the direct Z-scheme heterojunction [45].

Song et al. [46] prepared four CdS–TiO2 samples with various molar ratios, named CdS–TiO2-X (X is molar ratios of TiO2/CdS: CdS–TiO2-10, CdS–TiO2-9, CdS–TiO2-8, and CdS–TiO2-6. All composites had higher activity for the CO2 photoreduction than CdS and TiO2. This can be explained due to the interaction between TiO2 and CdS that improved the photocatalytic reduction capacity of CO2.

They observed that the increase in the CdS amount in the composites until 8:1 increased the efficiency, obtaining the CdS–TiO2-8, optimal photoactivity for the production of cyclohexyl formate (CF) and cyclohexanone (CH). However, for CdS content higher than 8:1 TiO2/CdS molar ratios, a decrease in the photocatalytic activity was observed, indicating that a high amount of CdS in the TiO2 photocatalyst decreases the photogenerated e− on the TiO2 and then leads to a smaller photocatalytic activity. The exactly 1 mole of excited TiO2 has to correspond to 1 mole of excited CdS; otherwise, the exceeded e− or h+ recombine to decrease the reaction rates. In addition, for the higher than 8:1 TiO2/CdS molar ratios, there were difficulties in the light absorbance once the higher amount of CdS aggregated on the surface of the TiO2 nanosheets, which hampered the absorption of light by the TiO2. Furthermore, they concluded that the CO2 absorbed in cyclohexanol can be decreased to CF, and the cyclohexanol oxidized into CH on the conduction band and valence band of the TiO2/CdS photocatalyst, respectively (as can be seen in the scheme of Table 5) [46].

Ahmad Beigi et al. [47] reported the preparation of CdS/TiO2 nanocomposites for the photocatalytic reduction of CO2 to CO and CH4 under UV-vis and visible light irradiation. For this study, four CdS/TiO2 samples were synthesized with different weight ratios of CdS in TiO2: S1 (9%), S2 (23%), S3 (45%) and S4 (74%). All CdS/TiO2 nanocomposites had higher photocatalytic activity than the TiO2 and CdS samples. The CO was the majoritarian product of this reduction reaction. The presence of CdS greatly improved the photocatalytic efficiency of the TiO2, and the best performance was achieved using the composite CdS/TiO2 S3 (45%), which was the optimal ratio of CdS/TiO2 for CO2 photoreduction. The enhancement used the CdS/TiO2 S3 (45%) composite, due to the large specific surface area and low crystal size of this sample.

As in the works reported above, the ratio of CdS in the composites was crucial to the photocatalytic performance of the composites. In this case, the authors also reported that the crystal size and specific surface area were the parameters that influenced the performance of these composites for the CO2 reduction. Therefore, a certain amount of CdS can enhance the TiO2 photocatalytic activity, and the porous structure of this CdS/TiO2 composite can have reacting sites for electrons transference to the reactant and avoid the recombination of the e−/h+ [47].

2.5. MoS2/TiO2

Molybdenum disulfide, MoS2, is a typical representative of two-dimensional (2D) transition metal chalcogenides (transition metal dichalcogenides—TMDs) [77]. Any one layer of MoS2 contains three atomic layers (S–Mo–S) stacked together [78]. This material is used as a substitute for noble metal co-catalysts, due to its properties, such as high activity, low cost, excellent chemical stability and abundance, and the band gap being around 1.3 to 1.9 eV [79]. Coupling TiO2 with MoS2 [49,80] leads to the creation of a heterojunction structure, which can speed up the electron transfer and reduce the photogenerated electrons and holes recombination. This heterojunction composite was recently studied for application on photocatalytic systems. MoS2/TiO2 composites were widely investigated as photocatalysts for photocatalytic degradation, hydrogen evolution, and CO2 reduction; with this combination, the reduction of the electron/hole recombination should be possible, and also the presence of MoS2 provides large catalytically active sites for photocatalytic progress [79]. CO2 photocatalytic reduction using MoS2/TiO2 heterojunction photocatalysts is tabulated in Table 6.

Table 6

CO2 photoreduction using MoS2/TiO2 heterojunction photocatalysts.

Photocatalysts		CO2 Photoreduction Condition			Yield of Products	Type of Heterojunction		Ref.
Type	Prepared	Reaction Mixture	Light Source	Conditions
10% MoS2/TiO2	Calcined at 300 °C for 4 h with argon shielding gas	100 mL deionized H2O which was preprocessed for 30 min with CO2 (99.99%) of 100 kPa	Xe-arc lamp 300 W acting	500 cm3 cylindrical reactorPhotocatalyst concentration in 0.5 g L−1	CO = 268.97 μmol gcat−1CH4 = 49.93 μmol gcat−1 after 6 h	Type-II	*	[49]
10% MoS2/TiO2	In situ growing MoS2 nanosheets onto TiO2 nanofibers by hydrothermal method	CO2 and H2O vapor were in situ generated by the reaction of NaHCO3 (0.12 g) and H2SO4 aqueous solution (0.25 mL, 2 M)	350 W Xe lamp	200 mL homemade Pyrex reactorPhotocatalyst concentration in 0.25 g L−1	CH4 = 2.86 μmol g−1 h−1CH3OH = 2.55 μmol g−1 h−1	Type-II	‡	[50]
0.5 wt% MoS2/TiO2	Hydrothermal method	200 mL of 1 M NaHCO3 solution and pure CO2	300 W Xenon arc lamp.	airtight quartz glass reactorPhotocatalyst concentration in 0.5 g L−1	CH3OH = 10.6 μmol g−1 h−1	-		[51]

* Reprinted from [49], Copyright (2019), with permission from Elsevier. ‡ Reprinted from [50], © 2022 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Peng-yao Jia et al. in 2019 [49] synthesized the MoS2/TiO2 heterojunction composites with different mass ratios of MoS2, 0, 5, 10 and 15 wt.%, obtaining the samples of TiO2, 5 wt.% MoS2/TiO2, 10 wt.% MoS2/TiO2, and 15 wt.% MoS2/TiO2, respectively. The obtained results showed that the 10% MoS2/TiO2 sample had higher photocatalytic activity for the photoreduction of CO2, obtaining higher amounts of CO and CH4 than the other composite materials. The achieved yields of CH4 and CO on the 10% MoS2/TiO2 heterojunction photocatalyst were approximately 5 times and 16 times higher than for pure TiO2 (P25). This result can be explained due to the lower band gap energy of this material, and also this material showed the lowest e−/h+ recombination by PL characterization; the photocurrent characterization indicates that this sample had more enhancement in e− and h+ separation. The authors proposed a type-II heterojunction mechanism for this sample as can be seen in Table 6. Once the CB edge potential of MoS2 (−0.93 V) is more negative than that of TiO2 (−0.55 V), the migration of e− from the surface of MoS2 to accumulate in the TiO2 is possible, due to the contact in the interface [49].

Xu et al. [50] described the preparation of 1D/2D TiO2/MoS2 nanostructured photocatalysts for increased photocatalytic CO2 reduction. The authors prepared different TiO2/MoS2 samples, with 1%, 5%, 7.5%, 10%, 15% and 25% of MoS2, labelled as TMx, where T and M are TiO2 and MoS2, respectively, and x denotes the mol.% of MoS2 to TiO2.

They observed that the formation rate of CH4 and CH3OH was markedly increased with higher MoS2 loading. The maximum value for the CH4 and CH3OH yields was reached for the TM10 sample. On the other side, only CH3OH was observed as the product in the presence of pure TiO2.

The authors concluded that the enhanced photocatalytic efficiency of TM10 is attributed to the increased light absorption, implying that more optical energy is absorbed after hybridization; the increased specific surface area nominates a higher amount of accessible reactive sites between TiO2/MoS2 and CO2 molecules; there is a higher CO2 adsorption capacity since CO2 adsorption is the beginning step for the next reduction processes; and there is enhanced charge separation, owing to the presence of MoS2 nanosheets as a cophotocatalyst [50].

In the TiO2/MoS2 samples, the photoinduced e− in TiO2 transfers to MoS2, reaching more efficient electron–hole separation. Thus, the number of catalytically active e− is significantly improved over pure TiO2, favoring the 8 e− reaction for producing CH4. For that reason, the TMx showed higher photocatalytic CO2 reduction activity and better selectivity of CH4 than the pure TiO2 photocatalyst. However, the next increasing amount of MoS2 resulted in a decrease in the photocatalytic activity (e.g., TM25 and TM15), probably because of the severe charge carrier recombination and the shielding effects toward light absorption or the transfer of electrons, owing to the presence of a high amount of MoS2.

In this study, the author also analyzed the stability of the TM10 catalyst in consecutive reutilizations and concluded that TM10 is stable without loss of photoactivity for four cycles.

Test with an isotope tracer confirmed that the products of CO2 photocatalytic reduction solely originated from the CO2 source. The DFT calculation demonstrated that TiO2 has a higher work function than MoS2, resulting in electrons transfer from MoS2 to TiO2 upon their contact, which supports the charge carrier separation of upon photoexcitation as MoS2 acts as a cophotocatalyst. Moreover, the hybridization with MoS2 increases light harvesting and enhances the CO2 adsorption of TiO2, further contributing to the superior photocatalytic efficiency of the TiO2/MoS2 hybrid [50].

Tu et al. [51] described the preparation of two-dimensional MoS2–TiO2 hybrid nanojunctions, for the CO2 photocatalytic reduction to CH3OH. They prepared MoS2/TiO2 photocatalysts with 3, 2, 1, and 0.5 wt.% contents of MoS2.

All samples proved photocatalytic activity for CO2 photoreduction into CH3OH, the 0.5 wt.% MoS2/TiO2 sample being the one with the best photocatalytic performance for this reaction. Using this sample, CH3OH production was almost three times higher than using pure TiO2. However, for the samples with higher MoS2 content (1, 2, and 3 wt.%) a gradual decrease in the photocatalytic activity was obtained. This occurs due to the fact that the photons in the photocatalytic system are absorbed by the excess of black MoS2 nanosheets, and probably decrease the light intensity through shielding the light reached on the TiO2 surface (i.e., “shielding effect”).

It was found that the two-dimensional MoS2/TiO2 hybrid composites presented high photocatalytic activity for CO2 photoreduction. They concluded that loaded MoS2 nanosheets minimize the charge carrier recombination and enhance the conversion performance of the CO2 photoreduction into CH3OH due to the e− transfer from TiO2 to MoS2 [51].

2.6. Other Semiconductors

In this section, studies on the CO2 photoreduction using composite materials with heterojunction with TiO2 not so often used until now are shown. CO2 photocatalytic reduction using GaP/TiO2, CaTiO3/TiO2 and FeTiO3/TiO2 heterojunction photocatalysts are tabulated in Table 7.

Table 7

CO2 photoreduction using GaP/TiO2, CaTiO3/TiO2 and FeTiO3/TiO2 heterojunction photocatalysts.

Photocatalysts		CO2 Photoreduction Condition			Yield of Products	Type of Heterojunction		Ref.
Type	Prepared	Reaction Mixture	Light Source	Conditions
1:10 GaP/TiO2	Mechanically milling of Commercial TiO2 Evonik P25 and GaP Aldrich powders	CO2 and water	1500 W high pressure Xe lamp	gas–solid Pyrex batch photoreactor of cylindrical shape (V = 100 mL, Φ = 94 mm, height = 15 mm) Photocatalyst concentration in 3 g L−1	CH4 = 118.18 μM g−1 after 10 h	Z-scheme	*	[52]
20% FeTiO3/TiO2	Hydrothermal method	30 mL distilled water containing sodium bicarbonate (NaHCO3, 0.08 M)	500 W high-pressure Xe lamp. A Pyrex glass tube cut off light with λ < 300 nm and a 2 M NaNO2 solution was applied to cut off λ < 400 nm	quartz reaction vessel, connected to a gas chromatograph.Photocatalyst concentration in 1.7 g L−1	CH3OH = 0.462 μmol g−1 h−1 under UV-vis irradiation and CH3OH = 0.432 μmol g−1 h−1 under visible light irradiation.	-	-	[53]
13.4% CaTiO3/TiO2	In situ hydrothermal method	CO2 and water	300 W Xe lamp	Quartz tube reactor, with 43 mL volumePhotocatalyst concentration in 0.23 g L−1	CO = 11.72 μmol g−1 h−1	Z-scheme	‡	[54]

* Reprinted from [52], Copyright (2014), with permission from Elsevier. ‡ Republished with permission of Royal Society of Chemistry, from [54] copyright 2019; permission conveyed through Copyright Clearance Center, Inc.

2.6.1. GaP/TiO2

Gallium phosphide, GaP, is a semiconductor material with an indirect band gap of 2.3 eV, insoluble in water. This semiconductor is not often used as a photocatalyst due to the low oxidizing power of its VB; however, the conduction band (CB) position allows the CO2 reduction once it is 1.26 V more negative than that of CO2/CH4 (E0 = −0.24 V) and 0.97 V more negative than CO2/CO (E0 = −0.53 V), as can be seen in the reactions from Table 1.

In 1978, Halmann [81] used GaP for the photoelectrochemical reduction of CO2. In this case, GaP was used in the liquid junction of solar cells, and the obtained products were formic acid, formaldehyde and methanol [52,81]. Furthermore, recently, Barton et al. also used GaP and found that a highly selective CO2 photoreduction to CH3OH occurred when a GaP electrode with pyridine was used. In this case, pyridine served as a cocatalyst [52,82]. Regardless of the fact that electrons from the GaP conduction band can reduce CO2 to methane, we need to look at the oxidation reaction as well. For example, often water or water vapor is chosen as the hole trap (oxidation step). In this case, water cannot be used as a hole trap because the GaP valence band (E0 = 0.80 V) does not have sufficient potential for water oxidation (E0 = 0.82 V). Therefore, the pristine GaP cannot be used for the CO2 photocatalytic reduction.

In line with this, Giuseppe Marcì et al. [52], for the first time, evaluated the GaP/TiO2 composites for the photocatalytic reduction of carbon dioxide. The suitable position of VB and CB of the semiconductors not only allows for heterojunction photocatalysts (GaP/TiO2) to have efficient electron–hole separation, but also enables both H2O oxidation and CO2 reduction.

Giuseppe Marcì et al. [52] reported GaP/TiO2 photocatalysts with significant efficiency during the photocatalytic reduction of carbon dioxide to the formation of methane. The researchers concluded that decreasing the mass ratio of the GaP:TiO2 enhances the photoactivity of the photocatalyst, and the highest efficiency was observed in the presence of photocatalysts with a 1:10 mass ratio. The photocatalytic effectiveness of the photocatalysts was connected with the band structures of the semiconductors and also with the efficient electron–hole transfer between GaP and TiO2 in the heterojunction photocatalysts.

2.6.2. FeTiO3/TiO2

Another interesting alternative to improve the TiO2 photocatalytic reduction of CO2 is the combination with ternary oxides, such as ilmenite (FeTiO3) and perovskite (CaTiO3); these heterojunction composite materials have not been studied very much for the photoreduction of CO2. However, the works reported by Truong et al. [53] and Lin et al. [54] showed that FeTiO3/TiO2 and CaTiO3/TiO2, respectively, are promising materials for this photocatalytic reaction.

Ilmenite (FeTiO3) is a semiconductor material with a band gap energy between 2.59 and 2.90 eV. This is one of the most abundant minerals used as raw material for the production of TiO2 and Ti. FeTiO3 has been studied by several researchers, due to its optic, semiconductive and magnetic properties, low-cost and high abundancy (as natural ilmenite), being an alternative semiconductor for photoactivated processes [83,84,85,86,87,88]. This semiconductor has been used for the formation of hetero-interfaces, with other different semiconductors, such as p–n junctions and Schottky contacts for effective carrier separation [89]. Recently, it was reported the high efficiency of FeTiO3 as a photocatalyst for hydrogen production [83]. Furthermore, several works have been reported with the preparation and utilization of FeTiO3-TiO2 composites as photocatalysts for the degradation of organic pollutants, showing that this combination improves the photocatalytic activity [84,85,90,91].

Truong et al. [53] showed the photocatalytic reduction of CO2 using the FeTiO3/TiO2 photocatalyst. The authors reported the preparation of a heterojunction sample of FeTiO3/TiO2 with various Fe/Ti mole ratios of 70%, 50%, 20%, and 10% [53]. All FeTiO3/TiO2 composites had a significantly higher photoactivity for the carbon dioxide reduction under both radiation sources (UV–Vis and visible light) in comparison with the TiO2 and P25 samples. This can be explained due to the heterojunction effect between the two semiconductors, and also the higher activity in the visible light due to the combination of TiO2 with FeTiO3. In the FeTiO3/TiO2 composites, the e− in the valence band of TiO2 transfer to FeTiO3 VB, while the h+ are subsequently created in TiO2 CB. Furthermore, the e−/h+ are photogenerated, owing to the narrow bandgap of FeTiO3 [53,91]. The obtained results showed that 20% FeTiO3/TiO2 sample had the best photocatalytic activity. In cases with a higher amount of FeTiO3 (50% and 70%), the CH3OH production decreased. This was explained by the smaller surface area, and also the high metal amount in FeTiO3/TiO2, which can represent recombination centers, resulting in reduced photocatalytic efficiency. The enhanced photoactivity along with an increasing amount of FeTiO3 is reasonable, due to the higher number of active sites for the carbonate species reduction. The optimal FeTiO3 amount for the highest photocatalytic efficiency is 20 wt.%. The authors concluded with this study that the unique band structure, the heterojunction effect of two materials, and the FeTiO3 narrow bandgap were responsible for the significant photocatalytic effectiveness on selective CH3OH production during CO2 photoreduction [53].

2.6.3. CaTiO3/TiO2

CaTiO3 is a titanium-based perovskite-type oxide, and an n-type semiconductor with a large band gap between 3.0 and 3.5 eV. This is an alkaline earth metal titanate that is non-toxic, with chemical stability, optical properties, a low cost and an eco-friendly nature. Currently, it is being used for several applications, such as electronic devices, photocatalytic degradation of dyes, water splitting for H2 production and CO2 reduction [92]. CaTiO3 has been studied for the preparation of heterostructured photocatalysts systems to improve their photocatalytic activity, in order to promote separation and photogenerated charge carrier transportation, also leading to the improvement in their visible light response. For example, coupling CaTiO3 with TiO2 was studied for organic pollutants’ photodegradation [93]; however, for the CO2 reduction, only one study was reported to date.

Lin et al. [54] synthesized four CaTiO3/TiO2 composite samples with different amounts of TiO2, 0.4, 0.3, 0.2 and 0.1 g, obtaining samples named 8.6%CaTiO3/TiO2, 13.4%CaTiO3/TiO2, 24.2%CaTiO3/TiO2 and 66.7%CaTiO3/TiO2, respectively, with the weight contents of CaTiO3 obtained by XRD quantification.

The activity of these four CaTiO3/TiO2 had higher photoactivity for the CO2 photoreduction than the TiO2 and P25 samples. The best sample for this reduction reaction was 13.4%CaTiO3/TiO2, being that the photocatalytic activity of this sample was six times higher than the TiO2. The results also showed that the CaTiO3 and TiO2 ratio influenced the photocatalytic activity efficiency for the CO2 photoreduction, i.e., the increase in CaTiO3 content above 13.4%CaTiO3/TiO2 decreased the CO evolution. The authors reported that when an excess of CaTiO3 content was introduced, a nanocubic morphology was obtained instead of a nanosheets morphology, and also the specific surface area decreased, being the reason for the decrease in the CO production. The authors concluded that the enhancement in the CaTiO3/TiO2 composites catalytic activity for CO2 photoreduction was attributed to the similar crystal structures and the matched band structures of the CaTiO3/TiO2 heterojunction photocatalysts that simplified the photogenerated electron–hole separation, as well as the increased surface basicity of the CaTiO3/TiO2 samples that provided more abundant active sites for adsorption of CO2 and, therefore, increased the photoreduction CO2 [54].

2.7. Semiconductor-Covalent Organic Framework Z-Scheme Heterojunctions

The integration of covalent organic frameworks (COFs) with inorganic materials gives possibilities to develop new type of composite materials [94]. These materials have high surface areas and novel functionalities relevant to photocatalysis, chemical adsorption, and magnetic resonance imaging. The disadvantages of these materials associated with challenging, multi-step synthesis were overcome by Zhu et al. [94], who reported a one-pot synthesis approach, using a wide range of metal oxides to catalyze the synthesis of highly crystalline and porous COFs.

A series of COF semiconductor Z-scheme photocatalysts integrating semiconductors (TiO2, Bi2WO6 and α-Fe2O3) with COFs (COF-316/318) were synthesized and characterized by Zhang et al. [95]. Prepared photocatalysts showed high photocatalytic CO2 conversion to CO efficiency, with H2O as an electron donor in the gas–solid CO2 reduction without additional photosensitizers and sacrificial agents. This is the first report of a covalent-bonded COF-inorganic semiconductor Z-scheme applied for artificial photosynthesis. The COF-318-TiO2 Z-scheme heterojunction photocatalyst showed the highest CO production rate, which was about six times higher than the pure COF-318, and TiO2 was also much higher than the physical mixture composites. Experiment studies and density functional theory (DFT) confirmed the efficient electron transfer from semiconductors to COFs by covalent coupling, resulting in the electrons being accumulated in cyano/pyridine of COF for the reduction of CO2 and positive holes remaining in the semiconductor for the oxidation of H2O. This work found a new method to create a covalent bond linked organic–inorganic Z-scheme heterojunction and showed a new perspective in the field of photocatalysis.

3. Final Conclusions

Nowadays, energy depletion and environmental pollution is one of the most discussed topics. The photocatalytic reduction of carbon dioxide into valuable and clean fuels can be one of the sustainable solutions to reduce carbon dioxide emissions. Although photocatalytic CO2 reduction has received unprecedented attention from scientists worldwide, its widespread use is limited due to the low selectivity, stability and especially the low efficiency of the photocatalytic system. The most studied photocatalyst in recent years is TiO2 because it is cheap, non-toxic and environmentally friendly. Unfortunately, TiO2 has some limitations, such as its activation, especially in the UV region, or the rapid recombination of generated electrons and holes. These imperfections can be tuned by doping TiO2 with metals or non-metals or by creating TiO2 heterojunction photocatalysts with other semiconductors.

In this review, TiO2 heterojunction photocatalysts were discussed to further increase the photocatalytic efficiency of TiO2 photocatalysts. In the last few years, several studies have been published on the preparation of TiO2 heterojunction photocatalysts suitable for photocatalytic CO2 reduction. Using these materials with the heterojunction, it was possible to improve the catalytic activity for the photoreduction of CO2, due to the efficient electron transference in the interface, supporting the separation of the e−/h+ pairs and consequently reducing the e−/h+ recombination. In addition, the activity in the visible light range was improved because it was possible to utilize sunlight more effectively; there was higher adsorption of CO2 due to the highly specific surface area; and there was an increase in selectivity specific CO2 photoreduction products due to the contribution of the cocatalysts. In the case of heterojunction photocatalysts, there is always an optimal amount or ratio of semiconductors used. The use of the metal as a dopant TiO2, which then forms a heterojunction with C3N4, also proved to be very advantageous.

Further research in this area should focus on the following aspects:

To create heterojunction photocatalysts, it is essential to find materials that have the appropriate band structure for redox reactions, are active in the visible light region, and are stable.

Efforts are underway to develop not too complex, efficient and effective methods for preparing heterojunction photocatalysts that could be produced in larger quantities. The most appropriate physicochemical properties of each semiconductor, such as the appropriate morphology, crystallite size, phase composition, etc., should be considered when developing preparation methods.

The migration pathways of photogenerated electron–hole pairs need to be thoroughly studied. Heterojunction photocatalysts can have different arrangements (e.g., heterojunction type II or Z-scheme heterojunction) and, thus, different migration pathways for electron–hole separation, which need to be thoroughly studied and confirmed using advanced characterization techniques.

To better understand the mechanism of migration pathways, knowledge from modeling methods or theoretical calculations should be used.

We hope that this review will encourage new approaches to the preparation of heterojunction photocatalysts, help optimize existing photocatalysts and create new efficient heterojunctions to achieve the higher efficiencies that are necessary for practical applications.