Recent Advances in TiO2-Based Heterojunctions for Photocatalytic CO2 Reduction With Water Oxidation: A Review

Photocatalytic conversion of CO₂ into solar fuels has gained increasing attention due to its great potential for alleviating the energy and environmental crisis at the same time. The low-cost TiO₂ with suitable band structure and high resistibility to light corrosion has proven to be very promising for photoreduction of CO₂ using water as the source of electrons and protons. However, the narrow spectral response range (ultraviolet region only) as well as the rapid recombination of photo-induced electron-hole pairs within pristine TiO₂ results in the low utilization of solar energy and limited photocatalytic efficiency. Besides, its low selectivity toward photoreduction products of CO₂ should also be improved. Combination of TiO₂ with other photoelectric active materials, such as metal oxide/sulfide semiconductors, metal nanoparticles and carbon-based nanostructures, for the construction of well-defined heterostructures can enhance the quantum efficiency significantly by promoting visible light adsorption, facilitating charge transfer and suppressing the recombination of charge carriers, resulting in the enhanced photocatalytic performance of the composite photocatalytic system. In addition, the adsorption and activation of CO₂ on these heterojunctions are also promoted, therefore enhancing the turnover frequency (TOF) of CO₂ molecules, so as to the improved selectivity of photoreduction products. This review focus on the recent advances of photocatalytic CO₂ reduction via TiO₂-based heterojunctions with water oxidation. The rational design, fabrication, photocatalytic performance and CO₂ photoreduction mechanisms of typical TiO₂-based heterojunctions, including semiconductor-semiconductor (S-S), semiconductor-metal (S-M), semiconductor-carbon group (S-C) and multicomponent heterojunction are reviewed and discussed. Moreover, the TiO₂-based phase heterojunction and facet heterojunction are also summarized and analyzed. In the end, the current challenges and future prospects of the TiO₂-based heterostructures for photoreduction of CO₂ with high efficiency, even for practical application are discussed.

Introduction

Energy and environmental crizes are two major bottlenecks restricting the sustainable development of human society. For a long time, the excessive consumption of fossil fuels has caused severe energy shortages, and the CO₂ released during the combustion process has become the main factor leading to global warming (Stott et al., 2000; Meinshausen et al., 2009; Solomon et al., 2009; Höök and Tang, 2013). It is urgent to develop and utilize renewable clean energy while reducing the concentration of CO₂ in the atmosphere (Brockway et al., 2019; Shindell and Smith, 2019). Notably, as a simple form of carbon storage, the rich carbon resources contained in CO₂ have huge development potential. Using CO₂ as a carbon feedstock to prepare carbon-based fuels can help alleviate the energy crisis and global warming at the same time, and has become a current research hotspot in the fields of both energy and environment (Olah et al., 2011; Kondratenko et al., 2013; Aresta et al., 2014; Ganesh, 2014; Li et al., 2019). However, the liner molecule with high thermodynamic stability and kinetic inertness makes it a great challenge for the activation and conversion of CO₂ (Ola and Maroto-Valer, 2015; Wei L. et al., 2018; Li et al., 2019; Nguyen et al., 2020). A lot of energy needs to be injected to break the C=O bond (dissociation energy about 750 kJ mol⁻¹) in CO₂ (Kim et al., 2017). Moreover, the extremely low water solubility of CO₂ (about 30 mM under 25°C at 1 atm) results in the low conversion efficiency of CO₂ in the aqueous reaction system (Xie et al., 2014). Therefore, a highly efficient reaction mode is also in great demand.

Fortunately, the natural photosynthesis motivated by solar energy to covert CO₂ into carbonhydrates as well as the release of O₂ by water oxidation provides a very promising solution to reduce the CO₂ level in atmosphere, which inspires the development of artificial photosynthesis systems (Barber, 2009; Dogutan and Nocera, 2019; Zhang and Sun, 2019). As shown in Figure 1A, the water oxidation process takes place in the photosystem II (PSII) of green plants to provide electrons and protons for the CO₂ fixation and conversion in the photosystem I (PSI). Since a series of pioneering works devoted by Fujishima and Honda on semiconductor photocatalysis in the 1970s (Fujishima and Honda, 1972; Inoue et al., 1979), substantial efforts have been made for the combination of the two individual processes within a single artificial architecture to mimic the natural photosynthesis (shown in Figure 1B) during the past decades (Ma et al., 2014; Tu et al., 2014; White et al., 2015; Liu X. et al., 2016; Li et al., 2019; Xu and Carter, 2019).

FIGURE 1

FIGURE 1. Schematic diagrams of (A) natural photosynthesis and (B) semiconductor photocatalytic reduction of CO₂. Reproduced from Liu et al. (2016) with permission from Wiley-VCH and Wei et al. (2018) with permission from the Royal Society of Chemistry.

In a typical semiconductor photocatalytic process, the semiconductor photocatalyst is excited by the incident photons with energy greater than or equal to its bandgap energy (E_g), where electrons (e^-) are excited to the conduction band (CB) to participate in the reduction reactions, remaining holes (h⁺) in the valence band (VB) for the oxidation reactions. Considering that CO₂ molecule is very stable and the photocatalytic CO₂ reduction is a series of uphill reactions (ΔG⁰>0, see in Table 1) (Wei L. et al., 2018), a large energy input is necessary to covert CO₂ into solar fuels, corresponding to the photons in the ultraviolet or shortwave visible regions. In addition, the band structure of ideal semiconductors needs to meet the redox potentials of CO₂ reduction and water oxidation reactions at the same time, as listed in Table 2 (Habisreutinger et al., 2013; White et al., 2015). Generally, the CB edge position (E_CB) should be more negative than the potential for reducing CO₂, whereas the VB edge position (E_VB) should be more positive than the potential for oxidizing H₂O to O₂. So far, the photocatalytic activity of numerous photocatalysts, etc. TiO₂ (Habisreutinger et al., 2013; Ma et al., 2014; Ola and Maroto-Valer, 2015), ZnO (Lee K. M. et al., 2016; Nie et al., 2018; Kegel et al., 2018), WO₃ (Chen et al., 2012; Shi et al., 2019; Wang H. et al., 2019), SnO₂ (He et al., 2015; You et al., 2020), Cu₂O (An et al., 2014; Shi et al., 2019), CdS (Kuai et al., 2015; Kuehnel et al., 2017; Wei Z. H. et al., 2018; Bie et al., 2019; Low et al., 2019), Bi₂WO₆ (Cao et al., 2018; Liu et al., 2021), BiVO₄ (Mao et al., 2012; Wei Z. H. et al., 2018), BiOBr (Ye et al., 2016; Wu et al., 2018), g-C₃N₄ (He et al., 2015; Nie et al., 2018; Thi Thanh Truc et al., 2019; Wang et al., 2020a) and graphene (An et al., 2014; Xiong et al., 2016; Shehzad et al., 2018a; Zhao H. et al., 2018; Bie et al., 2019), have been investigated intensively, in which few of them can realize the synergism of photocatalytic CO₂ reduction and water oxidation. In particular, the low-cost TiO₂ with suitable band structure and high resistibility to light corrosion is a very promising candidate, which has become the benchmark in this field (Habisreutinger et al., 2013; Ma et al., 2014; Ola and Maroto-Valer, 2015). However, the wide band gap of TiO₂ (3.2 eV for anatase) responses to UV light only, which accounts for only 3–5% of the incoming solar spectrum, thus restricting the conversion efficiency of solar energy. Besides, the fast recombination of photo-induced e^-/h⁺ pairs within TiO₂ results in the low charge separation efficiency, therefore reducing its photocatalytic performance further. Moreover, the low selectivity toward photoreduction products of CO₂ based on aqueous TiO₂ suspension photocatalytic system should also be improved.

TABLE 1

TABLE 1. The possible reactions during the photocatalytic CO₂ reduction process.

TABLE 2

TABLE 2. Electrochemical potentials of H₂O oxidation and CO₂ reduction into various products.

In the past few decades, various of strategies have been developed to enhance the photocatalytic performance of TiO₂. Among them, the nanostructured TiO₂ with single crystalline phase exhibited the decreased recombination rate of charge carriers, comparing to the polycrystalline samples that possess large amount of grain boundaries and defects acting as recombination centers (Tu et al., 2014; Xu et al., 2015). Moreover, crystal facet engineering has been adopted to tune the surface energy and active sites of TiO₂, contributing to the adsorption and activation of CO₂ (Liu L. et al., 2016; Xiong et al., 2018; Tu et al., 2020). Obviously, the preference adsorption of CO₂ molecules at the surface oxygen vacancy sites of TiO₂ can reduce the reactive barrier of CO₂ photoreduction reactions, in which one oxygen atom of CO₂ is trapped by a bridging oxygen vacancy defect to induce affinity interactions (Lee et al., 2011; Liu L. et al., 2016; Tan et al., 2017). Moreover, the localized electrons of oxygen vacancies can form adventitious energy levels, extending the photoresponsive range of semiconductor photocatalyst. Besides, surface oxygen vacancies with typical defect states can trap electrons or holes to inhibit their recombination (Wang et al., 2018). To sum up, the significance of surface oxygen vacancies on defected TiO₂ has been ascertained in the enhancement of CO₂ adsorption, activation, dissolution, and stabilization of reaction intermediates (Nguyen et al., 2020). In addition, metal/nonmetal ion doping is used to introduce additional energy level between the band gap of TiO₂, resulting in the reduced band width and enhanced visible light adsorption (Tu et al., 2014; Ola and Maroto-Valer, 2015; Shehzad et al., 2018b; Patil et al., 2019). Dye sensitized TiO₂ displays enhanced photoreduction efficiency of CO₂ due to the injection of photosensitized electrons from the energy level of dye molecule to the CB of TiO₂ with more negative potential, while the superior visible light responsibility of dye molecules can also improve the utilization of incident light (Ma et al., 2014; Ola and Maroto-Valer, 2015; Lee J. S. et al., 2016; Woo et al., 2019). Although these strategies have proven to be effective, the charge separation efficiency, light energy utilization and product selectivity still need to be further improved to fulfill the demand of more efficient photoreduction of CO₂, even for the practical application in the future.

As is known, construction of heterojunction between TiO₂ and cocatalyst with matching electronic band structures can significantly promote the separation of photogenerated e^- and h⁺, enlarge the spectra response range, while the physicochemical properties of some special cocatalyst can promote the photocatalytic CO₂ reduction or water oxidation to a certain extent, thereby resulting in high photoreduction efficiency of CO₂ over the heterostructured phocatalytic system with enhanced reduction products selectivity (Ma et al., 2014; Wang et al., 2014; Wei L. et al., 2018; Nguyen et al., 2020).

In addition to photocatalysts, photoreactors as well as reaction modes also play vital roles in affecting the photoreduction efficiency of CO₂. Generally, the two key parameters which determine the types of photoreactors utilized in CO₂ photoreduction are the phases involved (i.e., gas-solid, liquid-solid) and the mode of operation (i.e., batch, semi-batch or continuous). In the solid-liquid cases, photocatalysts are usually dispersed in alkaline mediums (aqueous solution) which can realize higher CO₂ solubility, resulting in the formation of CO₃²⁻ and HCO₃⁻. However, these species are difficult to be reduced in comparison with CO₂ (Corma and Garcia, 2013). In order to overcome the above drawbacks, the solid-vapor mode has been widely applied where the generation rate of the products for CO₂ photoreduction can be improved significantly (Xie et al., 2014; Xie et al., 2016; Xiong et al., 2017b). In addition, the exposure of photocatalysts in a CO₂ atmosphere can reduce the generation of H₂, thus enhancing the selectivity for CO₂ reduction. Obviously, the solid-vapor mode is more suitable for CO₂ photoreduction in the presence of H₂O.

In this review, we will mainly focus on the recent advances of photocatatytic CO₂ reduction processes with water oxidation using TiO₂-based heterojunction as photocatalysts. Different categories of heterojunctions, including S-S heterojunction (Figure 2A), S-M heterojunction (Figure 2B), S-C heterojunction (Figure 2C), multicomponent heterojunction, phase heterojunction and facet heterojunction (Figure 2D) are reviewed individually. In addition, the unique functions of cocatalysts among different heterostructured photocatalytic systems (etc. photosensitizer, CO₂ reduction promoter, water oxidation promoter and surface plasmon resonance (SPR) source) as well as the photoreduction mechanisms of CO₂ are discussed in detail. In the end, we will look forward to the prospects, opportunities and challenges of photocatalytic CO₂ reduction, predict the research directions of this field in the future, and put forward our opinions on the construction of efficient multifunctional integrated photocatalytic CO₂ reduction systems. We believe that this review will provide some useful guidelines for the construction of heterostructured photocatalysts for photoreduction of CO₂ with high performance in the future.

FIGURE 2

FIGURE 2. Separation and transfer of photogenerated charge carriers in the TiO₂-based (A) S-S heterojunction, (B) S-M heterojunction, (C) S-C heterojunction and (D) phase and facet heterojunction; (E) Crystalline structures of TiO₂ (anatase) and TiO₂ (rutile).

Photoreduction of CO₂ to Solar Fuels on TiO₂-Based Heterojunctions

From the perspective of semiconductor photocatalysis, the premise of high photocatalytic efficiency is the efficient separation and rapid transfer of photogenerated e^-/h⁺ pairs, thereby prolonging their lifetimes and inhibiting their recombination. The strategy for the construction of heterojunction by coupling semiconductor (TiO₂) with a secondary substance including other semiconductors (Aguirre et al., 2017; Yang et al., 2017; She et al., 2018; Xu et al., 2018a; Jin et al., 2019; Low et al., 2019; Thi Thanh Truc et al., 2019; Wu et al., 2019), metal nanoparticles (Xie et al., 2013; Neaţu et al., 2014; Jiao et al., 2015; Xiong et al., 2015; Lee K. Y. et al., 2016; Yu et al., 2016; Cheng et al., 2017; Tahir et al., 2017; Xiong et al., 2017C; Chong et al., 2018; Low et al., 2018; Tan et al., 2018; Tasbihi et al., 2018a; Wei Y. et al., 2018; Zhao Y. et al., 2018; Khatun et al., 2019; Wang R. et al., 2019; Zeng et al., 2020; Ziarati et al., 2020; Wang et al., 2021) and carbon-based nanostructures (Xia et al., 2007; Tu et al., 2013; Gui et al., 2014; Chowdhury et al., 2015; Xiong et al., 2016; Lin et al., 2017; Tan et al., 2017; Wang et al., 2017; Li et al., 2018; Olowoyo et al., 2018; Shehzad et al., 2018a; Zhang J. et al., 2018; Zubair et al., 2018; Bie et al., 2019; Olowoyo et al., 2019; Wang R. et al., 2019; Rodríguez et al., 2020) has been generally applied. Since different phases (etc. anatase, brookite, or rutile) and exposed facets (etc. (001) or (101)) of TiO₂ exhibit various of band structure and reactivity, TiO₂-based phase heterojunction (Reñones et al., 2016; Hwang et al., 2019; Jin et al., 2019; Chen et al., 2020; Lee J. S. et al., 2016; Xiong et al., 2020) or facet heterojunction (Yu et al., 2014; Cao et al., 2016; Liu L. et al., 2016; Xiong et al., 2016; Xiong et al., 2017a; Di Liberto et al., 2019) are also fabricated for photocatalytic CO₂ reduction, which exhibits enhanced photocatalytic efficiency in comparison with pristine TiO₂. On the one hand, the heterostructure facilitates the separation of photoinduced charge carriers which then transfer across the heterointerface to restrain recombination. On the other hand, the additional active sites introduced by the cocatalysts favor for the adsorption and activation of CO₂, thus enhancing the photoreduction efficiency of CO₂. Besides, the promoted quantum efficiency and product selectivity can also be expected by the constructed heterojunctions, since light-excitation attributes, band structure, and separation efficiency of photogenerated charge carriers of heterojunctions play vital roles in the selectivity of CO₂ photoreduction products. Moreover, various cocatalysts with different reactive active sites can also affect the product selectivity greatly, where the adsorption/activation of CO₂ as well as the adsorption/desorption of the intermediates are tuned (Fu et al., 2019).

In this section, the rational design, fabrication, photocatalytic performance and photoreduction mechanism of CO₂ over the TiO₂-based typical categories of heterojunctions (S-S, S-M, S-C, multicomponent, phase and facet heterojunction) will be reviewed and discussed in detail. In addition, the selectivity toward photoreduction products is another focus. The relative mechanism was concluded and analyzed in the certain case.

TiO₂ Based Z-scheme S-S Heterojunction for CO₂ Photoreduction

Coupling n-type TiO₂ with a p-type semiconductor possessing matching energy band structure to form a p-n heterojunction is one of the most classic S-S heterojunction (Zeng et al., 2014; Aguirre et al., 2017; Lee et al., 2017; Zhang L. et al., 2018; Iqbal et al., 2020; Zhang et al., 2020). As shown in Figure 3, the contacting of the two semiconductors leads to the diffusion of e^- and h⁺, then forms a space-charge region at the interface of the p-n heterojunction (Wang et al., 2014). As a result, a strong built-in electrical field is created which can drive the photoinduced e^- and h⁺ to transfer in the opposite directions, therefore enhancing the separation efficiency of charge carriers. In addition to p-n heterojunction, TiO₂-based non-p-n heterojunctions are also common (Yang et al., 2017; She et al., 2018; Jin et al., 2019; Low et al., 2019; Thi Thanh Truc et al., 2019; Wu et al., 2019). Typically, two closely integrated semiconductors with staggered band configurations can form a type II-1 heterojunction (shown in Figure 4A) (Zhang and Jaroniec, 2018), in which the band bending facilitates the charge transfer at the heterointerface. Specifically, e^- and h⁺ are separated individually in both semiconductor 1 (SC-1) and semiconductor 2 (SC-2) under the irradiation of incident light. The difference in energy level leads to the transfer of e^- from the CB of SC-1 with more negative potential to the CB of SC-2. Meanwhile, h⁺ can transfer from the VB of SC-2 to the VB of SC-1 with more positive potential. Similar to the p-n heterojunction, the reverse migration of e^- and h⁺ in the type II-1 heterojunction improves the separation efficiency of charge carriers, thus endowing the enhanced photocatalytic performance of the heterostructured system. However, the way of carrier transfer in the above heterojunctions will lead to a decrease in their redox ability, making it difficult to ensure the optimal photocatalytic activity.

FIGURE 3

FIGURE 3. Schematic diagram showing the energy band structure and electron-hole pair separation in the p-n heterojunction. Reproduced from Wang et al. (2014) with permission from the Royal Society of Chemistry.

FIGURE 4

FIGURE 4. Photogenerated charge carrier transfer process for two types of non p-n heterojunctions: (A) type II-1, and (B) type II-2 (direct Z-scheme). Reproduced from Zhang and Jaroniec (2018) with permission from Elsevier and Copyright Clearance Center.

Recently, study on the construction of all-solid-state Z-scheme heterojunction has gained great attention of researchers (Kuai et al., 2015; Aguirre et al., 2017; Takayama et al., 2017; Yang et al., 2017; She et al., 2018; Low et al., 2019; Thi Thanh Truc et al., 2019; Wang et al., 2019; Raza et al., 2020; Wang et al., 2020). Comparing to the p-n and type II-1 heterojunctions, the carrier transfer mode in the Z-scheme heterojunction is more favorable for photocatalytic application. In general, the band bending at the interface of direct Z-scheme heterojunction (type II-2) is conducive to the recombination of photoinduced e^- and h⁺ with weaker reduction and oxidation ability, so that e^- in the more negative CB of SC-1 and h⁺ in the more positive VB of SC-2 can be remained (shown in Figure 4B). As a result, both high separation efficiency and optimal redox ability of photoinduced charge carriers can be realized, thus endowing the high photocatalytic performance of the Z-scheme system. In this section, recent advances for the construction of TiO₂-based all-solid-state indirect and direct Z-scheme heterojunctions as well as their application for photocatalytic CO₂ reduction with water oxidation will be reviewed and discussed in detail. Photocatalytic CO₂ reduction performance of the typical TiO₂-based all-solid-state Z-scheme heterojunctions are listed in Table 3.

TABLE 3

TABLE 3. Photocatalytic CO₂ reduction performance on typical TiO₂-based S-S (Z-scheme), S-M, S-C, multicomponent, phase and facet heterojunctions.

Construction of indirect Z-scheme system between TiO₂ and another semiconductor using noble metals such as Pt (Wang et al., 2020a), Au (Wang et al., 2020b) and Ag (Tahir, 2020) as electron mediators has gained increased attention due its enhanced separation efficiency of photogenerated e⁻/h⁺ pairs with the recombination of inefficient charge carriers, thereby improving the photoreduction efficiency of CO₂. As reported by Tahir, ZnFe₂O₄/Ag/TiO₂ nanocomposite was fabricated by physical mixing Ag/TiO₂ nanorods and ZnFe₂O₄ nanospheres in methanol solution under continuous stirring (Tahir, 2020). Compared to the point contact between 0D TiO₂ nanoarticles and 0D ZnFe₂O₄ nanospheres, the stronger interaction between 0D ZnFe₂O₄ nanospheres and 1D TiO₂ nanorods is beneficial to the transfer of photogenerated electrons and holes at the interface. At the same time, the migration of these charge carriers along the 1D nanostructure is more efficient, which significantly inhibits their recombination. Moreover, the UV irradiation induced Z-scheme carrier transfer pathway ensures the high redox capability of the remaining carriers with the recombination of inefficient species within Ag nanoparticles, resulting in the superior CO generation rate of 1025 μmol g_cat⁻¹ h⁻¹. Compared to ZnFe₂O₄, graphic-C₃N₄ (g-C₃N₄) is more preferred for the construction of TiO₂-based Z-scheme heterojunction due to its fully visible light utilization, improved CO₂ adsorption capacity (derived from its surface π bond) and proper band structure for CO₂ photoreduction with H₂O oxidation (Wu et al., 2019; Wang et al., 2020a). Moreover, it can also trap photogenerated electrons to enhance charge separation efficiency within the heterojunction. On this basis, g-C₃N₄ was coated on the surface of Au/TiO₂ hybrid to form a Z-scheme photocatalyst (Figure 5A,B) for visible-light-driven (VLD) photocatalytic CO₂ reduction (Wang et al., 2020a). In particular, the efficient separation of photogenerated e⁻/h⁺ pairs within anatase TiO₂ is attributed to the formation of {001}/{101} facet heterojunction. Then, photogenerated electrons in the CB of TiO₂ are directionally transferred through Au and recombine with photogenerated holes in the VB of g-C₃N₄, thereby boosting the photoreduction of CO₂ by photogenerated electrons in the CB of g-C₃N₄ (shown in Figure 5C). Notably, the high selectivity toward CO₂ photoreduction (>99%) was realized with few H₂ generation, while the selectivity for CH₄ generation (63.3%) was also enhanced compared to pure g-C₃N₄ (1.4%). The following work of this group (Wang et al., 2020a) devoted to further improving the selectivity for CH₄ generation with high apparent quantum efficiency (AQE) under visible light irradiation, in which 3D ordered macroporous (3DOM) TiO₂@C was coupled with g-C₃N₄ using Pt as electron mediator (3DOM-CNPTC) (shown in Figure 5D,E). The DFT calculation revealed the enrichment of photogenerated electorns by abundant N-sites on the interface between Pt and g-C₃N₄, which can reduce the adsorbed CO₂ to CH₄ directly in the presence of H₂O, thereby improving the selectivity for CH₄ generation (81.7%). In addition, the strong visible light adsorption by g-C₃N₄ and Pt as well as the multiple scattering of incident light within the 3DOM structure (Figure 5F) result in the high AQE of the Z-scheme heterojunction (5.67%), which is 140 folds than that of P25 (0.04%). Interestingly, the interaction between TiO₂ and g-C₃N₄ could also be strengthened by Al-O links which was introduced into the Z-scheme through impregnation (Wu et al., 2019). Specifically, TiO₂ nanotubes (TNTs) fabricated via anodization of Ti foils are dipped in AlCl₃ solution followed by calcination to obtain Al-O-modified TNTs, which is then combined with porous g-C₃N₄ (PCN) via solid sublimation and transition of urea/NaHCO₃ hybrid to from Al-O linked PCN/TNT composites. Results show that the low charge transfer efficiency at the interface between TiO₂ and g-C₃N₄ caused by lattice mismatch of the two components can be significantly improved by introducing Al-O links to replace surface hydroxyl groups, thereby enhancing the separation efficiency of photogenerated charge carriers and benefiting for photoreduction of CO₂ with increased yields of acetic acid, formic acid and methanol. According to Kuai’s research, rGO could also serve as electron mediator for the construction of Z-scheme heterojunction between TiO₂ and CdS (Kuai et al., 2015). The remarkably prolonged photoluminasence (PL) decay time of CdS/rGO/TiO₂ (2.4 ns) reveals the different electron migration mechanism compared to CdS/TiO₂ (0.38 ns), which follows the carrier transfer mode in type II heterojunction. Obviously, the presence of rGO leads to the establishment of Z-scheme system, in which photogenerated electrons in the CB of TiO₂ are extracted by rGO and then transferred to the VB of CdS to recombine with photogenerated holes there, resulting in the enrichment of photogenerated electrons and holes in the CB of CdS and the VB of TiO₂, respectively. Although the photoreduction efficiency of CO₂ is still low on CdS/rGO/TiO₂, the attempt to construct Z-scheme heterojunction with low-cost carbon material instead of noble metal as electron mediator is successful, while the high selectivity for CH₄ generation is also promising.

FIGURE 5

FIGURE 5. (A) SEM and (B) HRTEM images of (Au/A-TiO₂)@g-C₃N₄ catalyst; (C) Schematic for photorecatalytic CO₂ reduction with H₂O to produce CH₄ and CO over (Au/A-TiO₂)@g-C₃N₄. Reproduced from Wang et al., 2020a with permission from Elsevier and Copyright Clearance Center. (D) TEM and (E) HRTEM images of 3DOM-CNPTC photocatalyst; (F) The mechanism of 3DOM-CNPTC catalyst for photocatalytic CO₂ reduction with H₂O to CH₄. Reproduced from Wang et al. (2020) with permission from Elsevier and Copyright Clearance Center.

Recently, construction of direct Z-scheme system by coupling two different semiconductors with matching geometric and band structure has become research hotspot in the field of photocatalytic CO₂ reduction, which is more facile to synthesis and more convenient for charge transfer at the interface. In particular, semiconductors with narrow band gap are more preferred in the TiO₂-based direct Z-scheme heterojunction to improve the utilization of visible light. Typically, ZnInS₂ nanosheets were decorated onto the surface of 1D TiO₂ nanobelts via hydrothermal process (Yang et al., 2017). The authors claimed the Z-scheme electron transfer mechanism between ZnInS₂ and TiO₂ based on the experimental results of CH₄ generation. Since the CB potential of TiO₂ is lower than the redox potential of CO₂/CH₄, it is reasonable to believe that photogenerated electrons in the CB of ZnInS₂ are retained due to the Z-scheme electron transfer mechanism and take in charge for photocatalytic CO₂ reduction to produce CH₄. However, stronger evidence is needed to prove this conjecture. In another work, a similar 3D hierarchical nanostructure was constructed by depositing CuInS₂ nanoplates on TiO₂ nanofibers (Xu et al., 2018b). DFT calculations revealed the higher Fermi level (E_F) of CuInS₂ than that of TiO₂, which forces electrons transfer from CuInS₂ to TiO₂ after their contact and creates a build in internal electric filed at the interface. The recombination of photogenerated electrons in the CB of TiO₂ and photogenerated holes in the VB of CuInS₂ under the guidance of the internal electric filed leads to the accomplishment of high efficient Z-scheme charge transfer pathway. As a result, photogenerated electrons enriched in the CB of CuInS₂ facilitate the photocatalytic reduction of CO₂ to produce CH₄ and CH₃OH in the presence of protons provided by water oxidation. In situ irradiated X-ray photoelectron spectroscopy (ISI-XPS) was also applied to provide direct evidence of Z-scheme electron transfer mechanism (Low et al., 2019). The binding energy shifts of Ti 2p (by 0.3 eV) and Cd 3 days (by -0.2 eV) under light irradiation indicate the decreased electron density of TiO₂ as well as the increased electron density of CdS, suggesting that photogenerated electrons migrates from TiO₂ to CdS, which agrees well with Z-scheme mechanism. The ternary semiconductor of Zn₃In₂S₆ was also used by She et al. for the construction of direct Z-scheme heterojunction with TiO₂ (She et al., 2018). Higher CO and CH₄ yields were realized on Zn₃In₂S₆/TiO₂ in comparison with ZnInS₂/TiO₂ and CuInS₂/TiO₂, which could be attributed to the higher crystallinity of the two constituents that favored for charge separation (shown in Figure 6). In addition, modification on TiO₂ to narrow its band gap for the improved visible light adsorption is also an efficient strategy to further enhance the photocatalytic performance of the TiO₂-based Z-scheme heterojunction. As reported by Truc et al., E_g of TiO₂ (3.2 eV) was reduced to 2.91 eV after Nb doping (Thi Thanh Truc et al., 2019). The as-obtained Nb-TiO₂ was grinded with melamine followed by calcination at 550 °C to form Nb-TiO₂/g-C₃N₄ heterojunction with a clear boundary at the interface. The well matched lattice spacing of the TiO₂ {101} (0.353 nm) and g-C₃N₄ {002} (0.320 nm) facets benefits to the electron transfer at the interface following Z-scheme mechanism, resulting in high efficiency for photocatalytic CO₂ reduction. The advantages of low cost, full visible light spectrum responsibility (400–700 nm) and superior generation rate of CH₄ (562 μmol g_cat⁻¹ h⁻¹), CO (420 μmol g_cat⁻¹ h⁻¹) and HCOOH (698 μmol g_cat⁻¹ h⁻¹), make Nb-TiO₂/g-C₃N₄ a promising VLD photocatalyst for practical application in the future to reduce the CO₂ level in the atmosphere. Moreover, the high O₂ yield of Nb-TiO₂/g-C₃N₄ (1702 μmol g_cat⁻¹ h⁻¹) indicates that the artificial Z-scheme system can mimic the nature photosynthesis by green plants well.

FIGURE 6

FIGURE 6. XRD patterns of (A) ZnInS₂/TiO₂ series, (B) CuInS₂/TiO₂ series and (C) Zn₃In₂S₆/TiO₂ series. Reproduced from Yang et al. (2017) and Xu et al. (2018) with permission from Elsevier and Copyright Clearance Center, and She et al. (2018) with permission from Wiley-VCH, respectively.

For a long time, stability is one of the main defects facing the photocatalysts that restricts their long-term performance. The photocatalytic activity decreased continuously in the process of illumination due to photocorrosion. Construction of Z-scheme heterojunction can also protect the narrow band gap semiconductor coupled with TiO₂ from photo-oxidation. As reported by Aguirre et al., XPS spectra of Cu₂O exhibited an increased Cu(II) content with the extension of illumination time, indicating that Cu(I) was partially oxidized by photogenerated holes (Aguirre et al., 2017). On the contrary, Cu(I) in the Cu₂O/TiO₂ heterojunction showed no obvious change in valence state, revealing the protection of TiO₂ toward Cu₂O by injecting photogenerated electrons into the VB of Cu₂O to recombine with photogenerated holes there, which also demonstrated the Z-scheme electron transfer mechanism between TiO₂ and Cu₂O. Interestingly, a stable direct Z-scheme heterojunction can also be formed between TiO₂ and metal organic frameworks (MOFs) as PCN-224(Cu) (Wang L. et al., 2019). It is worth nothing that the high specific surface area as well as porous structure of MOFs benefits for CO₂ adsorption, while the alternative ligands endow MOFs with adjustable band structure and spectra response range, thus providing a series of promising candidates for the design and construction of direct Z-scheme systems for efficient photocatalytic CO₂ reduction. With deepening of the research on the Z-scheme photocatalytic system and the recognition of its photocatalytic performance, more and more different types of Z-scheme photocatalysts have been developed (Raza et al., 2020; Zeng et al., 2020), accelerating the process of photocatalytic CO₂ reduction from basic research to practical application.

TiO₂ Based S-M Heterojunction for CO₂ Photoreduction

As reported by previous literatures, TiO₂ modified by metal nanoparticles exhibits enhanced photocatalytic performance due to the promoted charge carrier separation efficiency, expanded light adsorption range as well as high selectivity toward reduction products. In general, Schottky barrier at the interface of semiconductor and metal prevents recombination of photogenerated e⁻/h⁺ pairs (Ma et al., 2014; Wang et al., 2014; Ola and Maroto-Valer, 2015; Li et al., 2019). Specifically, the higher work function of metal (W_m) than that of semiconductor (W_s) results in the higher Fermi level of semiconductor (E_Fs) than that of metal (E_Fm) (shown in Figure 7A). Contacting semiconductor with metal leads to charge transfer at the interface until the alignment of their Fermi levels. During the process, migration of electrons from semiconductor to metal results in band bending of the semiconductor and creates a space charge region at the interface (Schottky barrier), which could prevent backflow of photogenerated electrons to inhibit their recombination with photogenerated holes (shown in Figure 7B). The promoted charge separation efficiency benefits for photocatalytic CO₂ reduction as well as the enhanced water oxidation efficiency. In addition, metals can enrich electrons to create high electron density regions on their surfaces, which are favoring for photoreducing CO₂ to hydrocarbons in the presence of water due to their lower redox potential than CO. Besides, local surface plasmon resonance (LSPR) effect of certain metals can enhance visible light adsorption of the Schottky heterojunction and inject hot electrons into the CB of semiconductor, thereby facilitating the photoreduction of CO₂ (shown in Figure 7C). In this section, strategies for coupling TiO₂ with different metal nanoparticles as well as their different enhancement mechanisms of photocatalytic performance will be reviewed and discussed in detail. Photocatalytic CO₂ reduction performance of the typical TiO₂-based S-M heterojunctions are listed in Table 3.

FIGURE 7

FIGURE 7. Band structure of semiconductor (A) before contact and (B) after contact with metal to form the Schottky barrier; (C) the SPR effect induced hot electron transfer. Reproduced from Bai et al. (2015) with permission from the Royal Society of Chemistry.

As a classic noble metal cocatalyst, Pt has been widely used in the photocatalysis field for both water splitting and CO₂ reduction. In particular, TiO₂ loaded with Pt nanoparticles has been demonstrated to be efficient for photoreduction CO₂ to CH₄. As reported by Fresno et al., a series of Pt/TiO₂ photocatalysts with different Pt loading amount were fabricated by treating P25 in the Pt precursor-contained aqueous solution via the deposition-precipitation procedure (Tasbihi et al., 2018a). The as-obtained sample (Figure 8A) displays ca. 100% selectivity toward CH₄ generation at the optimum Pt content (>0.58 wt% of TiO₂), which can be ascribed to the strong chemisorption of CO on Pt nanoparticles (proved by the FTIR spectra in ATR mode (shown in Figure 8B) along with in-situ NAP-XPS analysis) and the further reduction of CO to CH₄ (shown in Figure 8C). However, the adsorption features of CO can not be observed on bare TiO₂, which yields CO as the main product under the same condition. The correspondingly net selectivities and quantum yield indices (QYI) are shown in Figure 8D. In another work, Pt/TiO₂ was synthesized by the hydrolysis of Titanium (IV) butoxide (TEOT) in the presence of H₂PtCl₆∙6H₂O, resulting in the doping of Pt²⁺ into the lattice of TiO₂ and loading of Pt nanoparicles (Pt⁰) on the surface of TiO₂ (Xiong et al., 2015). The low recombination efficiency of photogenetated e⁻/h⁺ pairs due to the deposition of Pt⁰ as well as strong visible light adsorption attributed to Pt²⁺ doping significantly enhance photocatalytic performance of Pt²⁺-Pt⁰/TiO₂ with higher quantum yield (1.42%) for CO₂ conversion than that of bare TiO₂ (0.36%). Moreover, plenty of electrons enriched by Pt⁰ and protons supplied by water oxidation benefit for the high selectivity toward CH₄ formation (E_red/SCE = −0.48 V). Compared with the formation of CO (E_red/SCE = -0.77 V), this reaction is more feasible in thermodynamics. In summary, the kinetic feasibility (strong chemisorption of CO on Pt) and thermodynamic convenience contribute to photoreduction of CO₂ to CH₄. On this basis, Pt/TiO₂ were deposited on porous supports with large surface area (etc. COK-12 (Tasbihi et al., 2018a) and Al₂O₃ foam (Tasbihi et al., 2018b)) to promote active sites exposure and achieve higher CH₄ yield.

FIGURE 8

FIGURE 8. (A) TEM images of 0.5 Pt/TiO₂ and 0.5 Pt/TiO₂-COK-12, (B) FTIR spectra of the TiO₂, 0.5 Pt/TiO₂ and 0.5 Pt/TiO₂-COK-12 catalysts after reaction, in the CO stretching region, (C) Schematic representation of the proposed reaction pathway over TiO₂ and Pt/TiO₂ catalysts, and (D) Net selectivities and quantum yield indices (QYI) obtained with the different catalysts. The QYI with TiO₂ is 1 by definition. Reproduced from Tasbihi et al. (2018a) with permission from Elsevier and Copyright Clearance Center.

Coupling TiO₂ with noble metals that can induce the LSPR effect has also been adapted by researchers for efficient photocatalytic CO₂ reduction. As reported by Wang et al., 0D/2D Au/TiO₂ was synthesized by in situ growth of Au nanoparticles on the surface of TiO₂ nanosheets via chemically reduction (Wang R. et al., 2019). The hot electrons induced by the LSPR effect of Au under visible light irradiation (550 nm) could inject into the CB of TiO₂ and reduce CO₂ to CO. However, as the only electron source (TiO₂ can not be excited by visible light), the limited hot electrons cannot further reduce CO to CH₄. Interestingly, the Au/TiO₂ hybrid yielded CH₄ as the main product under 300 W Xe lamp irradiation that contained a certain amount of UV light. Specifically, recombination of photogenerated electrons and holes was suppressed owing to the Schottky barrier that facilitated transfer of e⁻ from the CB of TiO₂ to Au. Moreover, h⁺ remained in the VB of TiO₂ could oxidize water to provided plenty of protons for CH₄ generation. On this basis, facet engineering was introduced by Wang et al. for the rational design of interface between Au and exposed facet of TiO₂ to realize higher charge separation efficiency (Wang et al., 2021). Results showed that the lower height of Schottky barrier on the Au/TiO₂{101} interface resulted in more smooth migration of photogenerated electrons from the CB of TiO₂ to Au compared to the Au/TiO₂{001} interface, thereby exhibiting better performance for photocatalytic CO₂ reduction. Notably, the architecture of TiO₂ among Au/TiO₂ heterojunction also plays an important role for efficient photoreduction of CO₂ (Jiao et al., 2015; Khatun et al., 2019). Jiao et al. (Jiao et al., 2015) prepared 3D ordered macroporous (3DOM) TiO₂ to support Au nanopaticles, which were uniformly dispersed in the inner wall of the 3DOM structure. The multiple scattering of incident light within the 3DOM structure enhanced light utilization efficiency of the heterojunction, while the ordered macroporous also improved the mass transfer efficiency of the reactants. In addition, the SPR effect of Au induced by visible light irradiation provided extra electrons for photocatalytic CO₂ reduction, which was benefited for CH₄ generation. In another work, electrochemical anodization was used to fabricate TiO₂ nanotubes (TNTs) with light adsorption edge in the visible region, indicating its weak photocatalytic activity illuminated by visible light (Khatun et al., 2019). Coupling with Au by electronchemical deposition significantly improved visible light adsorption of TNTs and promoted charge separation efficiency due to the LSPR effect of Au nanoparticles. The excellent CH₄ yield (14.67% of CO₂ was converted to CH₄) under visible light irradiation made Au-TNTs a very promising solar-driven photocatalyst to convert CO₂ into hydrocarbon fuels. Similarly, plasmonic Ag were electronchemical deposited into the inner space of TiO₂ nanotube arrays (Figure 9A) to investigate the enhancement of SPR effect on photocatalytic performance, while the morphology and structure of as-obtained Ag-TNTAs-E are shown in Figures 9B,C (Low et al., 2018). The direct evidences of the existence of Schottky barrier between Ag and TiO₂ as well as migration of hot electrons induced by the SPR effect of Ag nanoparticles can be found in the synchronous-illumination X-ray photoelectron spectroscopy (SIXPS) spectra based on the shift of Ti 2p_3/2 peak before and after illumination (Figures 9D,E). Moreover, the SPR effect of Ag nanoparticles was strengthened by the multiple scatted light in TNTAs (Figure 9F), while the derived near field effect accelerated charge transfer at the heterointerface to promote separation efficiency of photogenerated e⁻/h⁺ pairs, thereby endowing enhanced VLD activity of Ag-TNTAs for CO₂ photoreduction. Although promoting photoreduction efficiency of CO₂ and clarifying the involved mechanisms are the focus of current research, the improvement of photocatalyst synthesis methods also deserves attention. The silver mirror reaction was adopted by Yu et al. to deposited Ag on TiO₂ nanoparticles (Yu et al., 2016). CH₃OH generated in CO₂-saturated 1 M NaHCO₃ solution is the main product of photocatalytic CO₂ reduction, which is more valuable than the primary products such as CO and CH₄. In another work, Ag(I) adsorbed by TiO₂ nanorod arrays were completely reduced by cold plasma within 30 s to form uniformly distributed Ag nanoparticles (Cheng et al., 2017). This fast and efficient strategy is very promising for the fabrication of metal nanoparticles decorated semiconductor photocatalysts on a large scale.

FIGURE 9

FIGURE 9. (A) Schematic illustration of electrochemical deposition methods for loading Ag NPs into the TNTAs, (B) SEM image (top view) and (C) TEM image (side view) of Ag/TNTAs-E, Comparison of high-resolution SIXPS spectra of (D) Ti 2p and (E) Ti 2p_3/2 for TNTAs and Ag/TNTAs-E in the dark and under 520 nm LED light irradiation, and (F) Schematic illustration of the enhanced SPR effect of Ag NPs in the TNTAs structure. Reproduced from Low et al. (2018) with permission from Elsevier and Copyright Clearance Center.

Bimetallic nanoalloys that combined advantages of the two metal components are efficient cocatalysts for photocatalytic CO₂ reduction and have been introduced in the TiO₂-based photocatalytic systems. As reported by Neaţu et al., Au and Cu species were deposited on TiO₂ nanopartcles stepwisely followed by calcining in H₂ atmosphere to form Au-Cu alloy (Neaţu et al., 2014). In this case, Au is served as visible light harvester due to its LSPR effect while Cu can covalently bind with CO reduced from CO₂ and direct the generation of CH₄. Therefore, high VLD photocatalytic activity with outstanding CH₄ selectivity (97%) was achieved. Other bimetallic nanoalloys, such as Au-Ag (Tahir et al., 2017), Au-Pd (Ziarati et al., 2020), Ag-Pd (Tan et al., 2018) and Pt-Ru (Wei Y. et al., 2018) nanoparticles are also been used to enhance photocatalytic performance of TiO₂ for selective reduction of CO₂. Among them, the combination of bimetallic nanoalloys and modified TiO₂ (etc. hydrogenated black TiO₂ (TiO_2-xH_x) (Ziarati et al., 2020) and N-doped TiO₂ (Tan et al., 2018)) exhibited considerably enhanced visible light utilization and charge separation efficiency, which could become the future development trend of TiO₂-based S-M heterojunction for solar-driven CO₂ photoreduction. Moreover, construction of S-M heterojunction with hierachical architecture is also in great demand (Ziarati et al., 2020).

TiO₂ Based S-C Heterojunction for CO₂ Photoreduction

Recently, coupling TiO₂ with carbon-based nanomaterials including graphene and its derivatives (etc. graphene (GR) (Tu et al., 2013; Xiong et al., 2016; Biswas et al., 2018; Jung et al., 2018; Shehzad et al., 2018; Zhao et al., 2018; Zubair et al., 2018; Bie et al., 2019), graphene oxide (GO) (Chowdhury et al., 2015; Tan et al., 2017) and reduced graphene oxide (rGO) (An et al., 2014; Kuai et al., 2015; Sim et al., 2015; Tan et al., 2015; Lin et al., 2017; Olowoyo et al., 2019)), carbon nanotubes (CNTs) (Xia et al., 2007; Gui et al., 2014; Gui et al., 2015; Olowoyo et al., 2018; Rodríguez et al., 2020) and carbon quantum dots (CQDs) (Li et al., 2018; Wang K. et al., 2019) to construct TiO₂-carbon heterojunction for photocatalytic reduction of CO₂ has been widely concerned. The unique physicochemical properties of nanocarbon that responsible for the enhanced photocatalytic performance of the S-C heterojunction can be concluded as follows: 1) the large surface area and high mechanical stability of nanocarbon could provide a stable support for the uniformly distributed TiO₂ nanoparticles with increased exposure of active sites and enhanced CO₂ adsorption capacity; 2) the high charge carrier mobility, large capacitance of nanocarbon as well as the formation of Ti-O-C bond at the highly dispersed S-C interface facilitates the migration of electrons from TiO₂ to carbon materials, thereby enhancing the separation efficiency of photogenerated e⁻ and h⁺ and inhibiting their recombination; 3) the optical properties of carbon materials, such as good optical transparency and wide spectrum adsorption range (especially for CQDs, expands to near IR region), contribute to the utilization of visible light of the TiO₂-based S-C heterojunction and result in the improved quantum efficiency. In this section, S-C heterojunctions including TiO₂-GR series, TiO₂-CNT and TiO₂-CQDs are reviewed and discussed in detail, respectively. Photocatalytic CO₂ reduction performance of the typical TiO₂-based S-C heterojunctions are listed in Table 3.

Coupling TiO₂ With Graphene and Its Derivatives

Construction of TiO₂-carbon heterojunction using graphene or its derivatives as the guest/host component derives improved photocatalytic performance due to its excellent electrical properties and chemical stability. It is worth nothing that the path of graphite-GO-rGO has been generally adopted by researchers to obtain graphene, whereas various of strategies have been developed for the fabrication of TiO₂-graphene nanocomposites. As reported by Tu et al., in situ simultaneous reduction-hydrolysis technique was developed for the fabrication of TiO₂-graphene 2D sandwich-like hybrid nanosheets (Tu et al., 2013). During the process, GO was reduced to graphene (rGO) by ethylenediamine (En) while Ti (IV) was hydrolyzed to TiO₂ nanoparticles and loaded on rGO through Ti-O-C bonds. The abundant surface Ti³⁺ sites generated from En reduction could trap photogenerated electrons efficiently, thereby decreasing the recombination efficiency of charge carriers. Moreover, the synergism of Ti³⁺ sites and garphene favors for the generation of C₂H₆, which is inspiring for C-C coupling during the photoreduction process of CO₂. In another work, the suspension of GO and TiO₂ in ethanol was ultrasonicated and refluxed to form Ti-O-C bonds, while GO was partially reduced to rGO during the process (Shehzad et al., 2018a). The tightly connected two phases improve charge separation at the heterointerface, while the enlarged light absorption coefficient is attributed to the reduced bandgap energy by the formation the Ti-O-C bonds. As a result, the rGO/TiO₂ nanocomposites exhibited greater yields of CH₄ (12.75 μmol g_cat⁻¹ h⁻¹) and CO (11.93 μmol g_cat⁻¹ h⁻¹) than anatase for 4 folds. Theoretical calculation was applied by Olowoyo et al. to investigate the enhanced photocatalytic performance of rGO/TiO₂ in reducing CO₂ (Olowoyo et al., 2019). Results reveal that the high electron density of rGO has significant influence on the TiO₂ bands and endows visible light responsibility of the composite. Moreover, the different electron migration paths within rGO/TiO₂ under different light sources were observed. Compared to the electron transfer from TiO₂ to rGO under UVA, irradiation by visible light leads to the direct generation of electorns and holes in rGO or TiO₂, respectively. Both of the two pathways are efficient for photogenerated charge separation and favor for methanol production. In addition, the large adsorption capacity of CO₂ is another feature of TiO₂/graphene that contribute to photocatalytic CO₂ reduction (Chowdhury et al., 2015). As reported by Chowdhury et al., TiO₂/GO nanocomposites was obtained from the aqueous suspension of GO and TiO₂ under ultrasonication flowed by continuous stirring. The synergism of physisorption (intermolecular electrostatic interactions as van der Waals forces or London dispersion forces) and chemisorption (coordination of O atoms with surface Lewis acid center (Ti sites), or coordination of C atom with surface Lewis acid center (oxygen functionalities of GO or TiO₂)) led to the high adsorption amount of CO₂ (1.88 mmol g⁻¹), which facilitated its activation and photocatalytic reduction. Considering that photoreduction of CO₂ requires the participation of electrons and protons produced by photooxidation of H₂O, the adsorption capability of H₂O molecule by photocatalyst is also critical. In view of the compete adsorption of H₂O and CO₂ on the active sites, proper partial pressures of CO₂ and H₂O are in great demand that determines the CH₄ yield on GO-doped oxygen-rich TiO₂ (Tan et al., 2017). Notably, the CO₂ adsorption capacity and charge separation efficiency can be further improved by introducing nitrogen dopants into rGO to form NrGO/TiO₂ system (Lin et al., 2017). On the one hand, the positive electrostatic potential regions created by nitrogen dopants on the surface of NrGO benefit for CO₂ adsorption and activation. On the other hand, the injection of electrons from quaternary-N species existed in NrGO matrix to the delocalized π-system facilitates the transfer of electrons and inhibits the recombination of photogenerated e⁻/h⁺ pairs. These findings are very helpful for the design of photocatalytic system based on heteroatom-doped graphene for CO₂ conversion with high efficiency. Furthermore, construction of well-defined nanostructure offers another thought to improve photocatalytic performance of TiO₂/graphene heterojunction (Zubair et al., 2018). Typically, TiO₂ nanotube arrays (TNT) with attractive 1D vectorial charge transfer can suppress recombination of photogenerated charge carriers efficiently. Graphene quantum dots (GQDs) decorated on TiO₂ nanotube (G-TNT) promotes charge transfer at the heterointerface, which could also enhance the light utilization of the heterojunction due to its superior visible light responsibility. Besides, the high surface area of the heterojunction contributes to the exposure of active sites that favor for CO₂ adsorption and activation. Consequently, a 5.6 fold CH₄ yield was obtained on G-TNT in comparison with pure TNT. On this basis, Pt nanoparticles were employed for the construction of rGO/Pt-TNT ternary composite with promoted visible light responsibility and photoreduction selectivity of CO₂ to produce CH₄ (Sim et al., 2015). In particular, the LSPR effect of Pt expands light adsorption range of the heterojunction to 450 nm that can be activated by visible light. Similarly, the high selectivity for CH₄ generation (99.1%, compared to CO yield) was also proved by Zhao et al. on (Pt/TiO₂)@rGO system (Zhao H. et al., 2018), in which vectorial electron transfer from TiO₂ to rGO through Pt was also demonstrated.

Coupling TiO₂ With CNT

Generally, charge transfer along the 1D CNT leads to high separation efficiency of photogenerated e⁻/h⁺ pairs and endows superior photocatalytic performance of TiO₂/CNT heterojunction. Moreover, CNT can also serve as support to reduce the aggregation of TiO₂ nanoparticles, thus resulting in the formation of highly dispersed heterointerface and large exposure of active sites. As reported by Xia et al., the multi-walled CNT (MWCNT)/TiO₂ hybrid fabricated via sol-gel method yielded C₂H₅OH as the main photoreduction product of CO₂ under UV light irradiation (Xia et al., 2007). In a further study, MWCNT/TiO₂ with core-shell nanostructure was demonstrated to be visible light active (due to the excellent visible light adsorption ability of CNT) that can convert CO₂ to CH₄ (Gui et al., 2014). A following work carried out by this group introduced Ag nanoparticles to MWCNT/TiO₂ system (Ag-MWCNT@TiO₂) for the further enhanced photocatalytic performance (Gui et al., 2015). In particular, the Schottky barrier between Ag and TiO₂ prevents backflow of photogenerated electrons that transferred from TiO₂ to Ag. The synergism of MWCNT and Ag nanoparticles greatly enhances separation efficiency of photogenerated charge carriers and restricts their recombination, thereby resulting in higher CH₄ formation rate (0.91 μmol g_cat⁻¹ h⁻¹) in comparison with the binary system (MWCNT@TiO₂, 0.17 μmol g_cat⁻¹ h⁻¹). Further studies revealed the mechanism of electron transfer between TiO₂ and CNT (Olowoyo et al., 2019). Specifically, strong attachment between MWCNT and the {101} facet of anatase TiO₂ introduces common orbitals within the band gap of TiO₂, which is the fundamental of charge transfer between the two phases and enables visible light responsibility of the composites. Electron transfer from TiO₂ with higher density of initial states to MWCNT under irradiation of both UVA and visible light is more probable basing on the computation results. In addition, the tight contact between MWCNT and TiO₂ due to the combination of sonothermal and hydrothermal methods results in extremely high photocatalytic activity with CH₃OH generation rate of 2360 μmol g_cat⁻¹ h⁻¹, much higher than that of the similar photocatalytic systems. A recent study reported the fabrication of TiO₂/CNT composite in the medium of supercritical CO₂ (Rodríguez et al., 2020). Although photocatalytic activity of the product is relatively weak (CO, 8.1 μmol g_cat⁻¹ h⁻¹; CH₄, 1.1 μmol g_cat⁻¹ h⁻¹), it can also provide a novel strategy for the synthesis of TiO₂-based photocatalyst and leave a large space for performance improvement.

Coupling TiO₂ With CQDs

As a new class of 0D carbon-based nanomaterial, the excellent photoelectric properties of CQDs, such as wide spectral response range, photo-induced charge transfer ability, up-conversion function and anti-photocorrosion property, make it a promising cocatalyst to enhance photocatalytic performance of traditional semiconductor photocatalyst as TiO₂ (Li et al., 2018; Wang K. et al., 2019). As reported by Li et al., N, S-containing CQDs (NCQDs) was synthesized via a microwave-assisted method using thiourea and citric acid as precursors, which then assembled with P25 under continuous stirring at 80°C to form NCQDs-TiO₂ nanocomposites. Particularly, TiO₂ sensitized by NCQDs can be activated by visible light, while electrons transferred from rutile TiO₂ in P25 to NCQDs prevented recombination of photogenerated e⁻/h⁺ pairs, thus enhancing photoreduction efficiency of CO₂. Although the photoelectric properties of CQDs are attractive, relative research about coupling CQDs with TiO₂ or other semiconductors to construct heterostructured photocatalyst for photocatalytic CO₂ reduction is still few. In our opinion, as an important characteristic distinguished from other carbon-based materials, up-conversion function of CQDs is worth developing to improve the light energy utilization of heterostructured photocatalysts, especially for the wide band gap semiconductor-contained systems.

In addition to carbon-based nanomaterials analyzed above, other carbon forms can also combine with TiO₂ to obtain heterojunction for photocatalytic CO₂ reduction with high efficiency. For instance, Zhang et al. coated TiO₂ on electrospun carbon nanofibers to promote active sites exposure as well as charge separation and transfer of the nanocomposites (Zhang J. et al., 2018). Besides, the heat produced by carbon nanofibers due to its photothermal conversion function accelerates the diffusion kinetics of reactants and products during photocatalytic process, thus further enhancing the photoreduction efficiency of CO₂. The local photothermal effect induced by carbon species is also highlighted by Wang et al. among the hybrid carbon@TiO₂ hollow spheres (Wang et al., 2017). Moreover, the multiple scattering of incident light within the hollow structure improves light utilization of the hybrid (shown in Figure 10) and contributes to improve the quantum efficiency of photocatalytic CO₂ reduction. This result indicates that not only component, but also architecture of the heterojunction photocatalysts plays important role in improving photocatalytic performance.

FIGURE 10

FIGURE 10. (A) FESEM images of TNS, CSTS, T60, T120 and T180, and TEM images of T60, T120 and T180, and STEM image of T60 and the corresponding elemental mapping images of C, O and Ti; (B) Comparison of the photocatalytic CH₄ or CH₃OH evolution rate of carbon@TiO₂ composite samples and P25 (under simulated solar light); and (C) Photoexcitation process of the carbon@TiO₂ composite photocatalyst with hollow structure. Reproduced from Wang et al. (2017) with permission from the Royal Society of Chemistry.

TiO₂ Based Multicomponent Heterojunction for CO₂ Photoreductions

Construction of TiO₂-based multicomponent heterojunction to introduce two different functional co-catalysts for efficient VLD photocatalytic CO₂ reduction has been widely adopted, in which TiO₂ combined with any two of another semiconductor (AS), metal nanoparticles (MNPs) and nanocarbon (C) to form ternary composites is currently the most studied system. Previous research revealed the highest selectivity of Pt for CH₄ generation compared to other noble metal cocatalysts (Pt > Pd > Au > Rh > Ag) due to its excellent electron extraction ability that derives high electron density around it and facilitates CO₂ photoreduction (Xie et al., 2014). However, the consequent increase in H₂ production is unfavorable and should be suppressed to realize further enhanced photoreduction efficiency of CO₂. Xie et al. coated MgO amorphous layers on Pt/TiO₂ hybrid to improve chemisorption of CO₂, which was then reduced to CH₄ directly by photogenerated electrons enriched on adjacent Pt nanoparticles with high efficiency, thus benefiting for the selective formation of CH₄. The similar function of Cu₂O was demonstrated by Xiong et al. from the Pt-Cu₂O/TiO₂ nanocomposite (Xiong et al., 2017c). Notably, the charge separation efficiency of the ternary system decreased with the increasing amount of MgO, indicating that excess MgO may restrict electrons transfer from TiO₂ to Pt. Therefore, TiO₂/MNPs/AS ternary heterojunctions with rational designed architecture and efficient carrier migration path are necessary. According to Meng’s research, MnO_x and Pt were selectively deposited on the {001} and {101} facet of TiO₂ (Figure 11A,B,C), respectively (Meng et al., 2019). The series connection of S-M (Pt and TiO₂{101}), facet (TiO₂{101} and {001}) and p-n (TiO₂{101} and MnO_x) heterojunction accelerated migration of photogenerated electrons along the path of MnOx→TiO₂{001}→TiO₂{101}→Pt while photogenerated holes in the opposite direction (shown in Figure 11D,F). As a result, the separation efficiency of photogenerated charge carriers is greatly improved, so as to the enhanced photocatalytic performance with CH₄ and CH₃OH as the main products. In another work, Z-scheme heterojunction that is favoring for the recombination of inefficient charge carriers was constructed by coupling TiO₂ and ZnFe₂O₄ using Ag as electron mediator (Tahir, 2020). Superior CO yield (1025 μmol g_cat⁻¹ h⁻¹) accompanied with the generation of CH₄ (132 μmol g_cat⁻¹ h⁻¹) and CH₃OH (31 μmol g_cat⁻¹ h⁻¹) should be attributed to the enhanced charge separation efficiency under UV light irradiation. It is worth nothing that the magnetic properties of ZnFe₂O₄ should not be ignored which facilitate the recovery of photocatalyst from solid-liquid suspension, although the solid-gas mode is undertaken in this study. Moreover, Ag could also promote visible light adsorption of the ternary system due to its strong LSPR effect, which had been demonstrated by Xu et al. using MgO-Ag-TiO₂ as photocatalyst (Xu and Carter, 2019). In order to further investigate the synergism of the LSPR effect and chemisorption of CO₂ on the improvement of photoreduction efficiency of CO₂, TiO₂/MNPs/AS heterojunctions as Au/Al₂O₃/TiO₂ (Zhao Y. et al., 2018) and Au@TiO₂ hollow spheres (THS)@CoO (Zhu et al., 2019) were synthesized. On this basis, MgAl layered double oxides (MgAl-LDO) were developed to provide both Lewis basic sites (MgO) and Lewis acid sites (Al₂O₃) for CO₂ chemisorption and H₂O dissociation among the Pt/MgAl-LDO/TiO₂ nanocomposite, respectively (Chong et al., 2018). Benefiting from the generation of monodentate carbonate (m-CO₃^2-) on the surface of MgO accompanied with the supply of H⁺ from H₂O dissociation by Al₂O₃, the activation of CO₂ became easier and led to the increased CH₄ yield by 11 folds compared to Pt/TiO₂.

FIGURE 11

FIGURE 11. SEM images of as-prepared (A) TiO₂ and (B) TiO₂-MnO_x-Pt (TMP); (C) Schematic diagram of selective photodeposition process of MnO_x nanoflakes and Pt nanoparticles on anatase TiO₂ {001} and {101} facets; Schematic diagram of proposed photocatalytic CO₂ reduction mechanism of sample TMP. The relative band energy positions of TiO₂, Pt, and MnO_x(A) before contact and (B) after contact and under irradiation. Reproduced from Meng et al. (2019) with permission from the American Chemical Society.

In a typical TiO₂/C/AS system, nanocarbon is served as electron channel to guide photogenerated electrons flow from TiO₂ to the AS while photogenerated holes left in the VB of TiO₂, thereby resulting in the spatial separation of photoinduced redox reactions with enhanced CO₂ photoreduction efficiency. As reported by Jung et al., mesoporous TiO₂ and a few layers of MoS₂ were assembled with graphene aerogel via one-pot hydrothermal method to construct a 3D hierarchical structure (Jung et al., 2018). In addition to high separation efficiency of photogenerated e⁻/h⁺ pairs, the improvement of light utilization and mass transfer efficiency based on the 3D structure of graphene with efficient visible light adsorption (Biswas et al., 2018) is also important factor that contribute to photocatalytic CO₂ reduction. Compared to semiconductors as MoS₂ (Jung et al., 2018), CuGaS₂ (Takayama et al., 2017) or WSe₂ (Biswas et al., 2018), noble metal nanoparticles are more efficient for the accumulation of photogenerated electrons transferred through electron channel (graphene) and favoring for the generation of more valuable products as CH₄ and CH₃OH (Tan et al., 2015; Meng et al., 2019; Tahir, 2020). However, the high cost as well as relative low resistance toward photocorrosion restricts their application on a large scale. Photocatalytic CO₂ reduction performance of the typical TiO₂-based multicomponent heterojunctions above are listed in Table 3. In the future, the development of noble metal-free multicomponent heterojunction based on TiO₂ for efficient solar-driven photocatalytic CO₂ reduction will become one of the main directions in this field.

TiO₂ Based Phase and Facet Heterojunctions for CO₂ Photoreduction

TiO₂ Based Phase Heterojunction

Phase heterojunction composed of different crystal phases of the same semiconductor exhibits greater photocatalytic activity than the single-phased photocatalyst (Ma et al., 2014; Wei L. et al., 2018; Nguyen et al., 2020). This is because the contact of crystal phases with different energy band structure leads to an increase in carrier separation efficiency at the interface. Moreover, the unique interfacial trapping sites may become new photocatalytic active sites. As is known, there are four main crystal phases of TiO₂ (including anatase, rutile, brookite, and TiO₂ (B)) existed in nature, in which rutile is the most thermodynamically stable phase and can be obtained by calcining the other three polymorphs (Ma et al., 2014). Among them, as the two most widely studied TiO₂ crystal phases with photocatalytic activity, the difference in the lattice structure of anatase and rutile leads to different electronic band structures, and ultimately results in a difference in band gap width. Compared to rutile (E_g = 3.02 eV for bulk material), anatase tends to show higher photocatalytic activity due to its wider band gap (E_g = 3.20 eV for bulk material) that gives stronger redox ability to the photogenerated carriers. In addition, the higher concentration of oxygen vacancies in anatase leads to more efficient charge separation, whereas the larger specific surface area leads to more active sites exposure, which are also important factors for its better photocatalytic performance than rutile. However, the narrower band gap enables rutile to respond to photons close to the visible region. Besides, the higher crystallinity results in the better charge carrier mobility within rutile. At present, the integration of the advantages of anatase and rutile to construct a phase heterojunction for photocatalytic CO₂ reduction has attracted increasing attention of researchers. As a classic anatase-rutile phase heterojunction, the commercial Degussa P25 has been regarded as a benchmark for both photocatalytic oxidation and reduction reactions. As reported by Reñones et al., hierarchical TiO₂ nanofibres composed of anatase and rutile nanoparticles were synthesized by the calcination of electrospun TiO₂ fibers under Ar atmosphere (Reñones et al., 2016). The phase composition of the fibers depend on the calcination conditions, in which higher rutile amount (81:19) was obtained under static Ar atmosphere (Fibers B) than the dynamic sample (Fibers A, 93:7). Notably, the faster charge transport along the grain boundaries in fibers is attributed to the improved nanocrystals connection, whereas the fibers with higher anatase content exhibits lower recombination efficiency of e^-/h⁺ pairs, thus endowing greater photocatalytic efficiency for reducing CO₂ to CO (10.19 μmol g_cat⁻¹ h⁻¹). In addtion, the overall apparent quantum yields (AQY) of Fibers B (0.036%) is also higher than P25 (0.030%), indicating the enhanced utilization of incident light. However, the large amount of hydrogen evolution (19.94 μmol g_cat⁻¹ h⁻¹) during the photocatalytic process needs to be suppressed to further improve the photoreduction efficiency of CO₂. A hydrothermal method was developed by Xiong et al. for the fabrication of anatase-rutile heterophase TiO₂ nanoparticles using K₂TiO(C₂O₄)₂∙2H₂O as Ti source, which simplifies the synthesis process and makes the reaction conditions more mild (Xiong et al., 2020). Ethylene glycol (EG) was added to the hydrothermal system in order to introduce oxygen vacancy into TiO₂ (TiO_2-x), which could trap photogenerated e^- to restrain the recombination of e⁻/h⁺ pairs, thereby enhancing the photocatalytic performance of the catalysts. Electron paramagnetic resonance (EPR) spectra revealed that the concentration of oxygen vacancy increased with the increasing amount of EG during the formation process of TiO_2-x. However, the excess oxygen vacancies, especially the bulk oxygen vacancies, will act as the recombination center of carriers, resulting in the lower photocatalytic efficiency of as-obtained anatase-rutile heterojunction. Thus, the concentration and distribution of oxygen vacancies should be reasonably designed and constructed to promote the performance of the semiconductor photocatalysts. As reported by Xiong et al. (Xiong et al., 2020), the TiO₂-EG10 sample synthesized using 30 ml H₂O and 10 ml EG as solvent exhibits the optimized photocatalytic activity in reducing CO₂ to CH₄ (43.2 μmol g_cat⁻¹ h⁻¹), which is 54 times higher than that of P25 (0.8 μmol g_cat⁻¹ h⁻¹). Usually, the optimization of oxygen vacancy concentration in photocatalyst is based on the feedback of experimental results rather than theoretical design. Therefore, the critical oxygen vacancy concentration in different photocatalytic systems is various, which is difficult to give a definite value. Considering the poor visible light responsibility of anatase/rutile phase heterojunction, it is necessary to reduce its band gap width to improve the utilization of incoming solar spectrum. In addition, the combination of disordered anatase (A_d) with more active sites and ordered rutile (R_o) for fast transport of e^- and h⁺ to suppress charge recombination can further enhance the photoreduction efficiency of CO₂ by TiO₂. On this basis, a phase-selective A_d/R_o TiO₂ was prepared by treating P25 in the sodium alkyl amine solutions at room temperature and ambient atmosphere, in which anatase among P25 was selective reduced to produce more Ti³⁺ defects (Hwang et al., 2019). The existence of multi-internal energy bands of Ti³⁺ defect sites in A_d reduces the band gap of A_d/R_o TiO₂ to 2.62 eV, while the newly conduction band (-0.27 eV) is well match the redox potential of CO₂/CH₄ (-0.24 V vs. NHE). As a result, the VLD A_d/R_o TiO₂ exhibits enhanced reactivity to convert CO₂ into CH₄ (3.98 μmol g_cat⁻¹ h⁻¹), which is higher than metal (W, Ru, Ag, and Pt)-doped P25. To further improve the CH₄ generation selectivity, 0.1% mass Pt was loaded on the H₂O₂ modified TiO₂ (M-TiO₂, containing two phases of anatase and rutile) through photoelectrodeposition (Pt/M-TiO₂) (Lee J. S. et al., 2016). As electron sinks, photogenerated electrons are enriched by Pt nanoparticles and form high charge density areas near their surface, which is favoring for the photoreduction of CO₂ to CH₄ in cooperation with sufficient protons generated by water oxidation. Notably, the yield of CH₄ on Pt/M-TiO₂ is about 60 times that of bare M-TiO₂, whereas no CO formation can be observed, indicating its high selectivity toward photoreduction products of CO₂. Compared with the common method that fabricates phase heterojunction by calcining TiO₂ gel, the MOFs (NH₂-MIL-125) derived method was adopted by Chen et al. (Chen et al., 2020) to fabricate anatase-rutile junction with large surface area and porous structure that favored for CO₂ adsorption. Besides, the in situ phase transformation from anatase (211) plane to rutile (211) plane results in the highly dispersed anatase/rutile interface for efficient interfacial charge separation, inhibiting the recombination of photogenerated e^-/h⁺ pairs significantly. Moreover, the N-doped carbon layer (generated by the pyrolysis of organic ligands) coating on the anatase/rutile heterostructure promotes the electric conductivity of the photocatalytic system, and expandes its light absorption range to 700 nm. The synergism of N-doped carbon and paragenetic anatase/rutile heterostructure derives the enhanced photoreduction efficiency of CO₂ to CO, which is 7.6 folds compared with P25. Although the added value of the product (CO) is limited, this method has guiding significance for the design and synthesis of other MOFs derived heterostructured photocatalytic systems. In addition to rutile, anatase can also form a phase heterojunction with brookite for photocatalytic CO₂ reduction (Jin et al., 2019). Generally, Sr²⁺ ions were introduced to the TiCl₄-involed hydrothermal system for the fabrication of SrCO₃-modified brookite/anatase TiO₂ heterophase junctions (HPJs). Similar to the anatase-rutile system, the interfacial electron transfer from brookite to anatase promotes the photogenerated charge separation. Moreover, the surface modification of HPJs by SrCO₃ can improve the adsorption of CO₂/H₂O, which can also serve as an efficient cocatalyst for the selective reduction of CO₂ to CH₄. Especially, 1.0 w/w% SrCO₃/HPJs composite shows the selectivity of ca. 7.45 for CH₄/CO (19.66/2.64 μmol g_cat⁻¹ h⁻¹), which is ca. 32.4 folds compared with pristine brookite TiO₂ (0.79/3.46 μmol g_cat⁻¹ h⁻¹). This work can also provide guidance for the development of heterojunctions composed of TiO₂ HPJs and alkaline earth metal carbonates via a facile one pot hydrothermal route.

TiO₂ Based Facet Heterojunction

The difference of geometrical and electronic structures between different crystal facets of the same semiconductor results in the distinctness of their photocatalytic activity. Facet engineering has been applied to control the exposed crystal facet of semiconductors, in order to increase the exposure of active sites and promote the adsorption and activation of substrates, so as to achieve enhanced photocatalytic performance. In terms of anatase TiO₂, the {101} has the lowest surface energy (0.44 J m⁻²) among the low-index facets (including {001}, {010} and {101}) and dominant for CO₂ adsorption basing on first-principles calculations (Yu et al., 2014). The photogenerated electrons transferred from the {101} of TiO₂ to CO₂ facilitates its activation and reduction. Moreover, the enriched photogenerated holes on the {001} of TiO₂ can accelerate the oxidation reactions. Photocatalytic CO₂ reduction over anatase TiO₂ with coexposed {001} and {101} facets was reported by Yu et al. for the first time with the propose of “facet heterojunction” concept (Yu et al., 2014). The facet ratio of {001} and {101} can be tuned by adjusting the amount of HF during the fabrication process, as see in Figures 12A,B, while the correspondingly schematic illustration is displayed in Figure 12C. In particular, the formation of facet heterojunction between {001} and {101} contributes to the transfer and separation of photogenerated carriers, which is beneficial to the enrichment of e⁻ and h⁺ on {101} and {001}, respectively (shown in Figure 12D). As a result, photocatalytic CO₂ reduction occurs selectively on the {101} facet, while the optimized photocatalytic performance is realized when the facet ratio of {101} to {001} is 45–55, in which CO₂ is reduced to CH₄ with a generation rate of 1.35 μmol g_cat⁻¹ h⁻¹ (Figure 12E). On this basis, oxygen vacancies were introduced to the {101} and {001} facets coexposed anatase TiO₂ for the further enhancement of photocatalytic performance, where TiO₂ fabricated via hydrothermal route in the presence of HF was reduced by NaBH₄ to generate surface oxygen defects (Liu L. et al., 2016). In addition to the high charge separation efficiency at the interface of {001} and {101} facets, the visible light responsibility attributed to the newly generated Ti³⁺ energy state as well as the improved CO₂ adsorption and activation derived from the synergism of Ti³⁺ and oxygen vacancies resulted in the enhanced photoreduction efficiency of CO₂. An explicit atomistic model of the interface was applied for further investigation of charge transfer between coexposed {101} and {001} facets of anatase TiO₂ (Di Liberto et al., 2019). The first principles calculations revealed that the localization of h⁺ on oxygen ion of the {001} side and the migration of e⁻ to Ti ion of the {101} side promoted the charge separation and suppressed their recombination, hence responsible for the enhanced photocatalytic activity of the facet junction. In another work, HF was also used by Cao et al. (Cao et al., 2016) for tuning the ratio of coexposed {101} and {001} facets of anatase TiO₂ in a hydrothermal system where nanotube titanic acid (NTA) was used as precursor to facilitate the mass transfer of HF, thereby simplifying the generation of {001} facet. Notably, the anatase TiO₂ nanocrystals with coexposed 51% {001} and 49% {101} facets exhibit the highest photocatalytic activity for reducing CO₂ to CH₄ (1.58 μmol g_cat⁻¹ h⁻¹) among pure TiO₂ series, which can be further improved to 4.0 μmol g_cat⁻¹ h⁻¹ by decorating 1wt% Pt⁰ nanoparticles on their surfaces. The effect of Pt loading for the enhanced photocatalytic performance of anatase TiO₂ facet heterojunction ({101}/{001}) was further investigated by Xiong et al. (Xiong et al., 2017a). Pt precursors (H₂PtCl₆ or Pt (NH₃)₄Cl₂) and deposition methods (photodeposition or chemical reduction) had significant influence on size, distribution, and valence states of Pt nanoparticles, hence led to the difference of photocatatic performance. In particular, the well dispersed Pt nanoparticles fabricated by chemical reducing H₂PtCl₆ (HC) possessed suitable particle size and high Pt⁰/Pt^II ratio (1.15), which led to efficient separation of photogenerated carriers and high efficiency for photoreduction CO₂. Coupling with graphene is also an efficient strategy to enhance the activity of anatase TiO₂ facet heterojunction ({101}/{001}) for the photoreduction of CO₂ (Xiong et al., 2016). Charge transfer between {101} and {001} facets along with the migration of electrons from TiO₂ to graphene greatly improved the separation efficiency of photogenerated carriers, thus increasing the yield of CO by photoreducing CO₂. Photocatalytic CO₂ reduction performance of the typical TiO₂-based phase or facet heterojunctions above are listed in Table 3.

FIGURE 12

FIGURE 12. FESEM images of (A) HF4.5 and (B) HF9; Schematic illustration of (C) tuning the ratio of {101} to {001} facets of anatase TiO₂ by adjusting the amount of HF, and (D) charge transfer at the interface of {101}-{001} facet heterojunction of anatase TiO₂; (E) Comparison of the photocatalytic CH₄-production activity of P25 and the TiO₂ samples prepared by varying HF amount. Reproduced from Yu et al. (2014) with permission from the American Chemical Society.

Conclusions and Prospects

This review summarizes the recent advances in the rational design, fabrication and photocatalytic performance of TiO₂-based heterojunctions for converting CO₂ into solar fuels with water oxidation. Generally, photocatalytic CO₂ reduction that mimic the nature photosynthesis of green plants exhibits great potential for the reduction of CO₂ level in the atmosphere and storage of solar energy in hydrocarbon fuels, so as to alleviate the impact of energy crisis and climate change on the development of human society. However, some obstacles, such as low solar energy conversion efficiency, slow generation rate and poor selectivity toward reduction products of CO₂, and common photocorrosion phenomenon facing the current photocatalytic systems, restricts the practical application of this very promising technology. In recent years, tremendous efforts have been devoted to fabricate TiO₂-based heterojunctions, in order to realize enhanced photocatalytic performance for CO₂ conversion, thus giving new life to this traditional and systematically studied photocatalyst. Although the composition, morphology, architecture and photocatalytic mechanism of TiO₂-based heterojunctions are various, they have much in common that favors for photoreduction of CO₂ as follows: 1) the efficient electron transfer at the heterointerface that promotes spatial separation of photogenerated e⁻/h⁺ pairs and prolongs their lifetime to participate in the photoinduced redox reactions; 2) the expanded light adsorption range and enhanced visible light responsibility, making it possible for solar-driven photocatalytic CO₂ reduction; 3) the enlarged CO₂ adsorption capacity due to the high specific surface area with highly exposed active sites that combine CO₂ by chemical action, which could also activate the adsorbed CO₂ molecules and facilitate hydrocarbon generation; 4) the increase in selectivity toward specific photoreduction products of CO₂ is attributed to the contribution of the cocatalysts.

Although considerable progress has been made on TiO₂-based heterojunction for photocatalytic CO₂ reduction, it is still far from practical application. On the one hand, the formation of multi-carbon products has always been a bottleneck in this field. The study of CO₂ photoreduction intermediates combined with in-situ analysis technology and theoretical calculations needs to be more in-depth in order to clarify the formation mechanisms of different hydrocarbons and guide for the rational design of photocatalysts for the generation of multi-carbon product with high selectivity. On the other hand, pre-defined design of the components and their spatial arrangement in the heterojunction for the optimized phtotcatalytic performance is still a great challenge. It is worth nothing that the difference in synthesis conditions limits the flexibility of component selection, which also complicates the synthesis procedure and reduces the yield of expected heterojunction photocatalyst. At the same time, the randomness arrangement of different components in many cases and the variability of the catalyst morphology and structure affect the photocatalytic performance significantly, which also makes it difficult to clarify the contribution of each component and the synergism mechanism. How to overcome the above limitations to select components of the heterojunction based on photocatalytic performance only, and achieve precise control at the structural unit level, thus realizing efficient synergy of each component for photocatalytic CO₂ reduction is one of the main directions of future development in this field. To our knowledge, recent advances in DNA origami superlattice structure (Tian et al., 2016;Tian et al., 2020) may provide a possible solution and guide the design and construction of well-ordered heterojunction photocatalysts in the future. Specifically, the precisely control of the topological structure, arrangement sequence, and assembly quantity of each building block (polyhedral DNA frame) during the self-assembly process makes it possible for heterojunction photocatalysts with controllable structure and adjustable performance, which is expected to become the future research hotspot in the field of photocatalytic CO₂ reduction. In addition, efficient solar harvesting systems are in great demand to replace artificial light sources with high energy consumption, since efficient photoreduction of CO₂ is based on high light intensity. On the one hand, upconversion quantum dots can be introduced into the heterostructured photocatalyst, in which some of the long-waved visible light in the incident light can be converted into short-waved partial that can excite the photocatalyst to generate e⁻ and h⁺ pairs. On the other hand, focusing lens system can be added to the photoreactor to enhance the concentration of sunlight, then the photocatalyst can operate under higher light intensity and exhibit optimized activity. Although the efficiency of photocatalytic CO₂ reduction is far less than that of electrocatalysis, relying entirely on solar energy will become its irreplaceable advantage. Furthermore, the enrichment of CO₂ in air will become an important consideration in the design of heterojunction photocatalysts, which meet the needs of practical applications. Fortunately, the research on CO₂ storage and controlled release provides the possible solution while the visible light-triggered capture and release of CO₂ from stable MOFs become the most promising candidate (Park et al., 2011; Li et al., 2016; Lyndon et al., 2015). Obviously, combination of photocatalyst and the above MOFs can realize the efficient recycling of CO₂ and improve the efficiency of its conversion into solar fuel, which is expected to become a research hot spot in the future.

In summary, the heterojunction photocatalysts with well-organized structure, optimized solar energy conversion efficiency, ideal turnover frequency of CO₂, and high reduction product selectivity are still the direction of efforts in the future. We hope that this review can inspire new ideas to guide the design and synthesis of high-performance photocatalysts for photoreduction of CO₂ into solar fuels, thus accelerating the industrialization process of this very promising technology and providing practical help to alleviate energy and environmental crises.

Author Contributions

KL designed and wrote the review with input from CT, YT, SW, and QM for conceiving, writing, and editing the manuscript.

Funding

This study was supported by Shenzhen Science and Technology Innovation Program (Grand No. GJHZ20190819151807167), Post-doctoral Foundation Project of Shenzhen Polytechnic (Grand No. 6020330008K0), the Fundamental Research Funds for the Central Universities (Grand No. 020514380141), and China Postdoctoral Science Foundation (Grand No. 2020M671437).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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