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 CO 2 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 CO 2 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 CO 2 have huge development potential. Using CO 2 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 CO 2 (Ola and Maroto-Valer, 2015;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 CO 2 . Moreover, the extremely low water solubility of CO 2 (about 30 mM under 25°C at 1 atm) results in the low conversion efficiency of CO 2 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 CO 2 into carbonhydrates as well as the release of O 2 by water oxidation provides a very promising solution to reduce the CO 2 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 CO 2 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).  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 CO 2 molecule is very stable and the photocatalytic CO 2 reduction is a series of uphill reactions (ΔG 0 >0, see in Table 1) , a large energy input is necessary to covert CO 2 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 CO 2 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 CO 2 , whereas the VB edge position (E VB ) should be more positive than the potential for oxidizing H 2 O to O 2 . So far, the photocatalytic activity of numerous photocatalysts, etc. TiO 2 (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), WO 3 (Chen et al., 2012;Shi et al., 2019;, SnO 2 (He et al., 2015;You et al., 2020), Cu 2 O 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), Bi 2 WO 6 Liu et al., 2021), BiVO 4 (Mao et al., 2012;Wei Z. H. et al., 2018), BiOBr (Ye et al., 2016;Wu et al., 2018), g-C 3 N 4 (He et al., 2015;Nie et al., 2018;Thi Thanh Truc et al., 2019;Wang et al., 2020a) and graphene Xiong et al., 2016;Shehzad et al., 2018a;Bie et al., 2019), have been investigated intensively, in which few of them can realize the synergism of photocatalytic CO 2 reduction and water oxidation. In particular, the low-cost TiO 2 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 TiO 2 (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 TiO 2 results in the low charge separation efficiency, therefore reducing its photocatalytic performance further. Moreover, the low selectivity toward photoreduction products of CO 2 based on aqueous TiO 2 suspension photocatalytic system should also be improved.
In the past few decades, various of strategies have been developed to enhance the photocatalytic performance of TiO 2 . Among them, the nanostructured TiO 2 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 TiO 2 , contributing to the adsorption and activation of CO 2 Xiong et al., 2018;Tu et al., 2020). Obviously, the preference adsorption of CO 2 molecules at the surface oxygen vacancy sites of TiO 2 can reduce the reactive barrier of CO 2 photoreduction reactions, in which one oxygen atom of CO 2 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 . To sum up, the significance of surface oxygen vacancies on defected TiO 2 has been ascertained in the enhancement of CO 2 adsorption, activation, dissolution, and stabilization of reaction intermediates . In addition, metal/ nonmetal ion doping is used to introduce additional energy level between the band gap of TiO 2 , 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 TiO 2 displays enhanced photoreduction efficiency of CO 2 due to the injection of photosensitized electrons from the energy level of dye molecule to the CB of TiO 2 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;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 CO 2 , even for the practical application in the future.
As is known, construction of heterojunction between TiO 2 and cocatalyst with matching electronic band structures can significantly promote the separation of photogenerated eand h + , enlarge the spectra response range, while the physicochemical properties of some special cocatalyst can promote the photocatalytic CO 2 reduction or water oxidation to a certain extent, thereby resulting in high photoreduction efficiency of CO 2 over the heterostructured phocatalytic system with enhanced reduction products selectivity (Ma et al., 2014;Wang et al., 2014;Nguyen et al., 2020).
In addition to photocatalysts, photoreactors as well as reaction modes also play vital roles in affecting the photoreduction efficiency of CO 2 . Generally, the two key parameters which determine the types of photoreactors

Reactions
E 0 (V) vs. NHE at pH 7 Frontiers in Chemistry | www.frontiersin.org April 2021 | Volume 9 | Article 637501 utilized in CO 2 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 CO 2 solubility, resulting in the formation of CO 3 2− and HCO 3 − . However, these species are difficult to be reduced in comparison with CO 2 (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 CO 2 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 CO 2 atmosphere can reduce the generation of H 2 , thus enhancing the selectivity for CO 2 reduction. Obviously, the solid-vapor mode is more suitable for CO 2 photoreduction in the presence of H 2 O.
In this review, we will mainly focus on the recent advances of photocatatytic CO 2 reduction processes with water oxidation using TiO 2 -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, CO 2 reduction promoter, water oxidation promoter and surface plasmon resonance (SPR) source) as well as the photoreduction mechanisms of CO 2 are discussed in detail. In the end, we will look forward to the prospects, opportunities and challenges of photocatalytic CO 2 reduction, predict the research directions of this field in the future, and put forward our opinions on the construction of efficient multifunctional integrated photocatalytic CO 2 reduction systems. We believe that this review will provide some useful guidelines for the construction of heterostructured photocatalysts for photoreduction of CO 2 with high performance in the future.
In this section, the rational design, fabrication, photocatalytic performance and photoreduction mechanism of CO 2 over the TiO 2 -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.

TiO 2 Based Z-scheme S-S Heterojunction for CO 2 Photoreduction
Coupling n-type TiO 2 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;Iqbal et al., 2020;Zhang et al., 2020). As shown in Figure 3, the contacting of the two semiconductors leads to the diffusion of eand h + , then forms a space-charge region at the interface of the p-n heterojunction . As a result, a strong builtin electrical field is created which can drive the photoinduced eand h + to transfer in the opposite directions, therefore enhancing the separation efficiency of charge carriers. In addition to p-n heterojunction, TiO 2 -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, eand 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 efrom 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 eand 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.
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;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 eand h + with weaker reduction and oxidation ability, so that ein 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 TiO 2 -based all-solid-state indirect and direct Z-scheme heterojunctions as well as their application for photocatalytic CO 2 reduction with water oxidation will be reviewed and discussed in detail. Photocatalytic CO 2 reduction performance of the typical TiO 2based all-solid-state Z-scheme heterojunctions are listed in Table 3.
Construction of indirect Z-scheme system between TiO 2 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 CO 2 . As reported by Tahir, ZnFe 2 O 4 /Ag/TiO 2 nanocomposite was fabricated by physical mixing Ag/TiO 2 nanorods and ZnFe 2 O 4 nanospheres in methanol solution under continuous stirring (Tahir, 2020). Compared to the point contact between 0D TiO 2 nanoarticles and 0D ZnFe 2 O 4 nanospheres, the stronger interaction between 0D ZnFe 2 O 4 nanospheres and 1D TiO 2 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 ZnFe 2 O 4 , graphic-C 3 N 4 (g-C 3 N 4 ) is more preferred for the construction of TiO 2based Z-scheme heterojunction due to its fully visible light utilization, improved CO 2 adsorption capacity (derived from its surface π bond) and proper band structure for CO 2 photoreduction with H 2 O 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-C 3 N 4 was coated on the surface of Au/TiO 2 hybrid to form a Z-scheme photocatalyst ( Figure 5A,B) for visible-light-driven (VLD) photocatalytic CO 2 reduction (Wang et al., 2020a). In particular, the efficient separation of photogenerated e − /h + pairs within anatase TiO 2 is attributed to the formation of {001}/{101} facet heterojunction. Then, photogenerated electrons in the CB of TiO 2 are directionally transferred through Au and recombine with photogenerated holes in the VB of g-C 3 N 4 , thereby boosting the photoreduction of CO 2 by photogenerated electrons in the CB of g-C 3 N 4 (shown in Figure 5C). Notably, the high selectivity toward CO 2 photoreduction (>99%) was realized with few H 2 generation, while the selectivity for CH 4 generation (63.3%) was also enhanced compared to pure g-C 3 N 4 (1.4%). The following work of this group (Wang et al., 2020a) devoted to further improving the selectivity for CH 4 generation with high apparent quantum efficiency (AQE) under visible light irradiation, in which 3D ordered macroporous (3DOM) TiO 2 @C was coupled with g-C 3 N 4 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-C 3 N 4 , which can reduce the adsorbed CO 2 to CH 4 directly in the presence of H 2 O, thereby improving the selectivity for CH 4 generation (81.7%). In addition, the strong visible light adsorption by g-C 3 N 4 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 TiO 2 and g-C 3 N 4 could also be strengthened by Al-O links which was introduced into the Z-scheme through impregnation (Wu et al., 2019). Specifically, TiO 2 nanotubes (TNTs) fabricated via anodization of Ti foils are dipped 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.
Frontiers in Chemistry | www.frontiersin.org April 2021 | Volume 9 | Article 637501 6 TABLE 3 | Photocatalytic CO 2 reduction performance on typical TiO 2 -based S-S (Z-scheme), S-M, S-C, multicomponent, phase and facet heterojunctions.  in AlCl 3 solution followed by calcination to obtain Al-O-modified TNTs, which is then combined with porous g-C 3 N 4 (PCN) via solid sublimation and transition of urea/NaHCO 3 hybrid to from Al-O linked PCN/TNT composites. Results show that the low charge transfer efficiency at the interface between TiO 2 and g-C 3 N 4 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 CO 2 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 TiO 2 and CdS (Kuai et al., 2015). The remarkably prolonged photoluminasence (PL) decay time of CdS/rGO/TiO 2 (2.4 ns) reveals the different electron migration mechanism compared to CdS/TiO 2 (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 TiO 2 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 TiO 2 , respectively. Although the photoreduction efficiency of CO 2 is still low on CdS/rGO/ TiO 2 , 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 CH 4 generation is also promising.
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 CO 2 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 TiO 2 -based direct Z-scheme heterojunction to improve the utilization of visible light. Typically, ZnInS 2 nanosheets were decorated onto the surface of 1D TiO 2 nanobelts via hydrothermal process (Yang et al., 2017). The authors claimed the Z-scheme electron transfer mechanism between ZnInS 2 and TiO 2 based on the experimental results of CH 4 generation. Since the CB potential of TiO 2 is lower than the redox potential of CO 2 /CH 4 , it is reasonable to believe that photogenerated electrons in the CB of ZnInS 2 are retained due to the Z-scheme electron transfer mechanism and take in charge for photocatalytic CO 2 reduction to produce CH 4 . However, stronger evidence is needed to prove this conjecture. In another work, a similar 3D hierarchical nanostructure was constructed by depositing CuInS 2 nanoplates on TiO 2 nanofibers (Xu et al., 2018b). DFT calculations revealed the higher Fermi level (E F ) of CuInS 2 than that of TiO 2 , which forces electrons transfer from CuInS 2 to TiO 2 after their contact and creates a build in internal electric filed at the interface. The recombination of photogenerated electrons in the CB of TiO 2 and photogenerated holes in the VB of CuInS 2 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 CuInS 2 facilitate the photocatalytic reduction of CO 2 to produce CH 4 and CH 3 OH 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 TiO 2 as well as the increased electron density of CdS, suggesting that photogenerated electrons migrates from TiO 2 to CdS, which agrees well with Z-scheme mechanism. The ternary semiconductor of Zn 3 In 2 S 6 was also used by She et al. for the construction of direct Z-scheme heterojunction with TiO 2 (She et al., 2018). Higher CO and CH 4 yields were realized on Zn 3 In 2 S 6 /TiO 2 in comparison with ZnInS 2 /TiO 2 and CuInS 2 / TiO 2 , 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 TiO 2 to narrow its band gap for the improved visible light adsorption is also an efficient strategy to further enhance the photocatalytic performance of the TiO 2 -based Z-scheme heterojunction. As reported by Truc et al., E g of TiO 2 (3.2 eV) was reduced to 2.91 eV after Nb doping (Thi Thanh Truc et al., 2019). The as-obtained Nb-TiO 2 was grinded with melamine followed by calcination at 550°C to form Nb-TiO 2 /g-C 3 N 4 heterojunction with a clear boundary at the interface. The well matched lattice spacing of the TiO 2 {101} (0.353 nm) and g-C 3 N 4 {002} (0.320 nm) facets benefits to the electron transfer at the interface following Z-scheme mechanism, resulting in high efficiency for photocatalytic CO 2 reduction. The advantages of low cost, full visible light spectrum responsibility (400-700 nm) and superior generation rate of CH 4 (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-TiO 2 /g-C 3 N 4 a promising VLD photocatalyst for practical application in the future to reduce the CO 2 level in the atmosphere. Moreover, the high O 2 yield of Nb-TiO 2 /g-C 3 N 4 (1702 μmol g cat −1 h −1 ) indicates that the artificial Z-scheme system can mimic the nature photosynthesis by green plants well.
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 TiO 2 from photo-oxidation. As   . It is worth nothing that the high specific surface area as well as porous structure of MOFs benefits for CO 2 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 CO 2 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 CO 2 reduction from basic research to practical application.

TiO 2 Based S-M Heterojunction for CO 2 Photoreduction
As reported by previous literatures, TiO 2 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 CO 2 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 CO 2 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 CO 2 (shown in Figure 7C). In this section, strategies for coupling TiO 2 with different metal nanoparticles as well as their different enhancement mechanisms of photocatalytic performance will be reviewed and discussed in detail. Photocatalytic CO 2 reduction performance of the typical TiO 2based S-M heterojunctions are listed in Table 3.
As a classic noble metal cocatalyst, Pt has been widely used in the photocatalysis field for both water splitting and CO 2 reduction. In particular, TiO 2 loaded with Pt nanoparticles has been demonstrated to be efficient for photoreduction CO 2 to CH 4 . As reported by Fresno et al., a series of Pt/TiO 2 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 CH 4 generation at the optimum Pt content (>0.58 wt% of TiO 2 ), 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 CH 4 (shown in Figure 8C). However, the adsorption features of CO can not be observed on bare TiO 2 , 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/ TiO 2 was synthesized by the hydrolysis of Titanium (IV) butoxide (TEOT) in the presence of H 2 PtCl 6 ·6H 2 O, resulting in the doping of Pt 2+ into the lattice of TiO 2 and loading of Pt nanoparicles (Pt 0 ) on the surface of TiO 2 . The low recombination efficiency of photogenetated e − /h + pairs due to the deposition of Pt 0 as well as strong visible light adsorption attributed to Pt 2+ doping significantly enhance photocatalytic performance of Pt 2+ -Pt 0 /TiO 2 with higher quantum yield (1.42%) for CO 2 conversion than that of bare TiO 2 (0.36%). Moreover, plenty of electrons enriched by Pt 0 and protons supplied by water oxidation benefit for the high selectivity toward CH 4 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 CO 2 to CH 4 . On this basis, Pt/TiO 2 were deposited on porous supports with large surface area (etc. COK-12 (Tasbihi et al., 2018a) and Al 2 O 3 foam (Tasbihi et al., 2018b)) to promote active sites exposure and achieve higher CH 4 yield. Coupling TiO 2 with noble metals that can induce the LSPR effect has also been adapted by researchers for efficient photocatalytic CO 2 reduction. As reported by Wang et al., 0D/ 2D Au/TiO 2 was synthesized by in situ growth of Au nanoparticles on the surface of TiO 2 nanosheets via chemically reduction . The hot electrons induced by the LSPR effect of Au under visible light irradiation (550 nm) could  inject into the CB of TiO 2 and reduce CO 2 to CO. However, as the only electron source (TiO 2 can not be excited by visible light), the limited hot electrons cannot further reduce CO to CH 4 . Interestingly, the Au/TiO 2 hybrid yielded CH 4 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 TiO 2 to Au. Moreover, h + remained in the VB of TiO 2 could oxidize water to provided plenty of protons for CH 4 generation. On this basis, facet engineering was introduced by Wang et al. for the rational design of interface between Au and exposed facet of TiO 2 to realize higher charge separation efficiency (Wang et al., 2021). Results showed that the lower height of Schottky barrier on the Au/TiO 2 {101} interface resulted in more smooth migration of photogenerated electrons from the CB of TiO 2 to Au compared to the Au/TiO 2 {001} interface, thereby exhibiting better performance for photocatalytic CO 2 reduction. Notably, the architecture of TiO 2 among Au/TiO 2 heterojunction also plays an important role for efficient photoreduction of CO 2 (Jiao et al., 2015;Khatun et al., 2019). Jiao et al. (Jiao et al., 2015) prepared 3D ordered macroporous (3DOM) TiO 2 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 CO 2 reduction, which was benefited for CH 4 generation. In another work, electrochemical anodization was used to fabricate TiO 2 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 CH 4 yield (14.67% of CO 2 was converted to CH 4 ) under visible light irradiation made Au-TNTs a very promising solar-driven photocatalyst to convert CO 2 into hydrocarbon fuels. Similarly, plasmonic Ag were electronchemical deposited into the inner space of TiO 2 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 TiO 2 as well as migration of hot electrons induced by the SPR effect of Ag nanoparticles can be found in the synchronousillumination 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 CO 2 photoreduction. Although promoting photoreduction efficiency of CO 2 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 TiO 2 nanoparticles (Yu et al., 2016). CH 3 OH generated in CO 2 -saturated 1 M NaHCO 3 solution is the main product of photocatalytic CO 2 reduction, which is more valuable than the primary products such as CO and CH 4 . In another work, Ag(I) adsorbed by TiO 2 nanorod arrays were completely reduced by cold plasma within 30 s to form uniformly distributed Ag nanoparticles . This fast and efficient strategy is very promising for the fabrication of metal nanoparticles decorated semiconductor photocatalysts on a large scale.
Bimetallic nanoalloys that combined advantages of the two metal components are efficient cocatalysts for photocatalytic CO 2 reduction and have been introduced in the TiO 2 -based photocatalytic systems. As reported by Neaţu et al., Au and Cu species were deposited on TiO 2 nanopartcles stepwisely followed by calcining in H 2 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 CO 2 and direct the generation of CH 4 . Therefore, high VLD photocatalytic activity with outstanding CH 4 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  nanoparticles are also been used to enhance photocatalytic performance of TiO 2 for selective reduction of CO 2 . Among them, the combination of bimetallic nanoalloys and modified TiO 2 (etc. hydrogenated black TiO 2 (TiO 2-x H x ) (Ziarati et al., 2020) and N-doped TiO 2 (Tan et al., 2018)) exhibited considerably enhanced visible light utilization and charge separation efficiency, which could become the future development trend of TiO 2 -based S-M heterojunction for solar-driven CO 2 photoreduction. Moreover, construction of S-M heterojunction with hierachical architecture is also in great demand (Ziarati et al., 2020).

TiO 2 Based S-C Heterojunction for CO 2 Photoreduction
Recently, coupling TiO 2 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;Zubair et al., 2018;Bie et al., 2019), graphene oxide (GO) (Chowdhury et al., 2015;Tan et al., 2017) and reduced graphene oxide (rGO) 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)  to construct TiO 2 -carbon heterojunction for photocatalytic reduction of CO 2 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 TiO 2 Frontiers in Chemistry | www.frontiersin.org April 2021 | Volume 9 | Article 637501 nanoparticles with increased exposure of active sites and enhanced CO 2 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 TiO 2 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 TiO 2 -based S-C heterojunction and result in the improved quantum efficiency. In this section, S-C heterojunctions including TiO 2 -GR series, TiO 2 -CNT and TiO 2 -CQDs are reviewed and discussed in detail, respectively. Photocatalytic CO 2 reduction performance of the typical TiO 2based S-C heterojunctions are listed in Table 3.

Coupling TiO 2 With Graphene and Its Derivatives
Construction of TiO 2 -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 TiO 2 -graphene nanocomposites. As reported by Tu et al., in situ simultaneous reduction-hydrolysis technique was developed for the fabrication of TiO 2 -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 TiO 2 nanoparticles and loaded on rGO through Ti-O-C bonds. The abundant surface Ti 3+ sites generated from En reduction could trap photogenerated electrons efficiently, thereby decreasing the recombination efficiency of charge carriers. Moreover, the synergism of Ti 3+ sites and garphene favors for the generation of C 2 H 6 , which is inspiring for C-C coupling during the photoreduction process of CO 2 . In another work, the suspension of GO and TiO 2 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/TiO 2 nanocomposites exhibited greater yields of CH 4 (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/TiO 2 in reducing Frontiers in Chemistry | www.frontiersin.org April 2021 | Volume 9 | Article 637501 CO 2 (Olowoyo et al., 2019). Results reveal that the high electron density of rGO has significant influence on the TiO 2 bands and endows visible light responsibility of the composite. Moreover, the different electron migration paths within rGO/TiO 2 under different light sources were observed. Compared to the electron transfer from TiO 2 to rGO under UVA, irradiation by visible light leads to the direct generation of electorns and holes in rGO or TiO 2 , respectively. Both of the two pathways are efficient for photogenerated charge separation and favor for methanol production. In addition, the large adsorption capacity of CO 2 is another feature of TiO 2 /graphene that contribute to photocatalytic CO 2 reduction (Chowdhury et al., 2015). As reported by Chowdhury et al., TiO 2 /GO nanocomposites was obtained from the aqueous suspension of GO and TiO 2 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 TiO 2 )) led to the high adsorption amount of CO 2 (1.88 mmol g −1 ), which facilitated its activation and photocatalytic reduction. Considering that photoreduction of CO 2 requires the participation of electrons and protons produced by photooxidation of H 2 O, the adsorption capability of H 2 O molecule by photocatalyst is also critical. In view of the compete adsorption of H 2 O and CO 2 on the active sites, proper partial pressures of CO 2 and H 2 O are in great demand that determines the CH 4 yield on GO-doped oxygen-rich TiO 2 (Tan et al., 2017). Notably, the CO 2 adsorption capacity and charge separation efficiency can be further improved by introducing nitrogen dopants into rGO to form NrGO/TiO 2 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 CO 2 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 CO 2 conversion with high efficiency. Furthermore, construction of well-defined nanostructure offers another thought to improve photocatalytic performance of TiO 2 / graphene heterojunction (Zubair et al., 2018). Typically, TiO 2 nanotube arrays (TNT) with attractive 1D vectorial charge transfer can suppress recombination of photogenerated charge carriers efficiently. Graphene quantum dots (GQDs) decorated on TiO 2 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 CO 2 adsorption and activation. Consequently, a 5.6 fold CH 4 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 CO 2 to produce CH 4 (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 CH 4 generation (99.1%, compared to CO yield) was also proved by Zhao et al. on (Pt/TiO 2 )@rGO system , in which vectorial electron transfer from TiO 2 to rGO through Pt was also demonstrated.

Coupling TiO 2 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 TiO 2 /CNT heterojunction. Moreover, CNT can also serve as support to reduce the aggregation of TiO 2 nanoparticles, thus resulting in the formation of highly dispersed heterointerface and large exposure of active sites. As reported by Xia et al., the multiwalled CNT (MWCNT)/TiO 2 hybrid fabricated via sol-gel method yielded C 2 H 5 OH as the main photoreduction product of CO 2 under UV light irradiation (Xia et al., 2007). In a further study, MWCNT/TiO 2 with core-shell nanostructure was demonstrated to be visible light active (due to the excellent visible light adsorption ability of CNT) that can convert CO 2 to CH 4 (Gui et al., 2014). A following work carried out by this group introduced Ag nanoparticles to MWCNT/TiO 2 system (Ag-MWCNT@TiO 2 ) for the further enhanced photocatalytic performance (Gui et al., 2015). In particular, the Schottky barrier between Ag and TiO 2 prevents backflow of photogenerated electrons that transferred from TiO 2 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 CH 4 formation rate (0.91 μmol g cat −1 h −1 ) in comparison with the binary system (MWCNT@TiO 2 , 0.17 μmol g cat −1 h −1 ). Further studies revealed the mechanism of electron transfer between TiO 2 and CNT (Olowoyo et al., 2019). Specifically, strong attachment between MWCNT and the {101} facet of anatase TiO 2 introduces common orbitals within the band gap of TiO 2 , which is the fundamental of charge transfer between the two phases and enables visible light responsibility of the composites. Electron transfer from TiO 2 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 TiO 2 due to the combination of sonothermal and hydrothermal methods results in extremely high photocatalytic activity with CH 3 OH 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 TiO 2 /CNT composite in the medium of supercritical CO 2 (Rodríguez et al., 2020). Although photocatalytic activity of the product is relatively weak (CO, 8.1 μmol g cat −1 h −1 ; CH 4 , 1.1 μmol g cat −1 h −1 ), it can also provide a novel strategy for the synthesis of TiO 2 -based photocatalyst and leave a large space for performance improvement.

Coupling TiO 2 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 TiO 2 . 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-TiO 2 nanocomposites. Particularly, TiO 2 sensitized by NCQDs can be activated by visible light, while electrons transferred from rutile TiO 2 in P25 to NCQDs prevented recombination of photogenerated e − /h + pairs, thus enhancing photoreduction efficiency of CO 2 . Although the photoelectric properties of CQDs are attractive, relative research about coupling CQDs with TiO 2 or other semiconductors to construct heterostructured photocatalyst for photocatalytic CO 2 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 TiO 2 to obtain heterojunction for photocatalytic CO 2 reduction with high efficiency. For instance, Zhang et al. coated TiO 2 on electrospun carbon nanofibers to promote active sites exposure as well as charge separation and transfer of the nanocomposites . 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 CO 2 . The local photothermal effect induced by carbon species is also highlighted by Wang et al. among the hybrid carbon@TiO 2 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 CO 2 reduction. This result indicates that not only component, but also architecture of the heterojunction photocatalysts plays important role in improving photocatalytic performance.

TiO 2 Based Multicomponent Heterojunction for CO 2 Photoreductions
Construction of TiO 2 -based multicomponent heterojunction to introduce two different functional co-catalysts for efficient VLD photocatalytic CO 2 reduction has been widely adopted, in which TiO 2 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 CH 4 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 CO 2 photoreduction (Xie et al., 2014). However, the consequent increase in H 2 production is unfavorable and should be suppressed to realize further enhanced photoreduction efficiency of CO 2 . Xie et al. coated MgO amorphous layers on Pt/TiO 2 hybrid to improve chemisorption of CO 2 , which was then reduced to CH 4 directly by photogenerated electrons enriched on adjacent Pt nanoparticles with high efficiency, thus benefiting for the selective formation of CH 4 . The similar function of Cu 2 O was demonstrated by Xiong et al. from the Pt-Cu 2 O/TiO 2 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 TiO 2 to Pt. Therefore, TiO 2 / 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 TiO 2 ( Figure 11A Figure 11D,F). As a result, the separation efficiency of photogenerated charge carriers is greatly improved, so as to the enhanced photocatalytic performance with CH 4 and CH 3 OH as the main products. In another work, Z-scheme heterojunction that is favoring for the recombination of inefficient charge carriers was constructed by coupling TiO 2 and ZnFe 2 O 4 using Ag as electron mediator (Tahir, 2020). Superior CO yield (1025 μmol g cat −1 h −1 ) accompanied with the generation of CH 4 (132 μmol g cat −1 h −1 ) and CH 3 OH (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 ZnFe 2 O 4 should not be ignored which facilitate the recovery of photocatalyst from solid-liquid suspension, although the solidgas 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-TiO 2 as photocatalyst (Xu and Carter, 2019). In order to further investigate the synergism of the LSPR effect and chemisorption of CO 2 on the improvement of photoreduction efficiency of CO 2 , TiO 2 /MNPs/AS heterojunctions as Au/Al 2 O 3 / TiO 2  and Au@TiO 2 hollow spheres (THS) @CoO  were synthesized. On this basis, MgAl layered double oxides (MgAl-LDO) were developed to provide both Lewis basic sites (MgO) and Lewis acid sites (Al 2 O 3 ) for CO 2 chemisorption and H 2 O dissociation among the Pt/MgAl-LDO/ TiO 2 nanocomposite, respectively (Chong et al., 2018). Benefiting from the generation of monodentate carbonate (m-CO 3 2-) on the surface of MgO accompanied with the supply of H + from H 2 O dissociation by Al 2 O 3 , the activation of CO 2 became easier and led to the increased CH 4 yield by 11 folds compared to Pt/TiO 2 .
In a typical TiO 2 /C/AS system, nanocarbon is served as electron channel to guide photogenerated electrons flow from TiO 2 to the AS while photogenerated holes left in the VB of TiO 2 , thereby resulting in the spatial separation of photoinduced redox reactions with enhanced CO 2 photoreduction efficiency. As reported by Jung et al., mesoporous TiO 2 and a few layers of Frontiers in Chemistry | www.frontiersin.org April 2021 | Volume 9 | Article 637501 MoS 2 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 CO 2 reduction. Compared to semiconductors as MoS 2 (Jung et al., 2018), CuGaS 2 (Takayama et al., 2017) or WSe 2 (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  Frontiers in Chemistry | www.frontiersin.org April 2021 | Volume 9 | Article 637501 17 more valuable products as CH 4 and CH 3 OH (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 CO 2 reduction performance of the typical TiO 2 -based multicomponent heterojunctions above are listed in Table 3. In the future, the development of noble metal-free multicomponent heterojunction based on TiO 2 for efficient solar-driven photocatalytic CO 2 reduction will become one of the main directions in this field.

TiO 2 Based Phase and Facet Heterojunctions for CO 2 Photoreduction
TiO 2 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;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 TiO 2 (including anatase, rutile, brookite, and TiO 2 (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 TiO 2 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 CO 2 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 TiO 2 nanofibres composed of anatase and rutile nanoparticles were synthesized by the calcination of electrospun TiO 2 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 CO 2 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 CO 2 . A hydrothermal method was developed by Xiong et al. for the fabrication of anatase-rutile heterophase TiO 2 nanoparticles using K 2 TiO(C 2 O 4 ) 2 ·2H 2 O 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 TiO 2 (TiO 2-x ), which could trap photogenerated eto 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 TiO 2 -EG10 sample synthesized using 30 ml H 2 O and 10 ml EG as solvent exhibits the optimized photocatalytic activity in reducing CO 2 to CH 4 (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 eand h + to suppress charge recombination can further enhance the photoreduction efficiency of CO 2 by TiO 2 . On this basis, a phase-selective A d /R o TiO 2 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 Ti 3+ defects (Hwang et al., 2019). The existence of multi-internal energy bands of Ti 3+ defect sites in A d reduces the band gap of A d /R o TiO 2 to 2.62 eV, while the newly conduction band (-0.27 eV) is well match the redox potential of CO 2 /CH 4 (-0.24 V vs. NHE). As a result, the VLD A d /R o TiO 2 exhibits enhanced reactivity to convert CO 2 into CH 4 (3.98 μmol g cat −1 h −1 ), which is higher than metal (W, Ru, Ag, and Pt)-doped P25. To further improve the CH 4 generation selectivity, 0.1% mass Pt was loaded on the H 2 O 2 modified TiO 2 (M-TiO 2 , containing two phases of anatase and rutile) through photoelectrodeposition (Pt/M-TiO 2 ) . As electron sinks, photogenerated electrons Frontiers in Chemistry | www.frontiersin.org are enriched by Pt nanoparticles and form high charge density areas near their surface, which is favoring for the photoreduction of CO 2 to CH 4 in cooperation with sufficient protons generated by water oxidation. Notably, the yield of CH 4 on Pt/M-TiO 2 is about 60 times that of bare M-TiO 2 , whereas no CO formation can be observed, indicating its high selectivity toward photoreduction products of CO 2 . Compared with the common method that fabricates phase heterojunction by calcining TiO 2 gel, the MOFs (NH 2 -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 CO 2 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 CO 2 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 CO 2 reduction . Generally, Sr 2+ ions were introduced to the TiCl 4 -involed hydrothermal system for the fabrication of SrCO 3 -modified brookite/anatase TiO 2 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 SrCO 3 can improve the adsorption of CO 2 /H 2 O, which can also serve as an efficient cocatalyst for the selective reduction of CO 2 to CH 4 . Especially, 1.0 w/w% SrCO 3 / HPJs composite shows the selectivity of ca. 7.45 for CH 4 /CO (19.66/2.64 μmol g cat −1 h −1 ), which is ca. 32.4 folds compared with pristine brookite TiO 2 (0.79/3.46 μmol g cat −1 h −1 ). This work can also provide guidance for the development of heterojunctions composed of TiO 2 HPJs and alkaline earth metal carbonates via a facile one pot hydrothermal route.

TiO 2 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 TiO 2 , 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 CO 2 adsorption basing on first-principles calculations (Yu et al., 2014). The photogenerated electrons transferred from the {101} of TiO 2 to CO 2 facilitates its activation and reduction. Moreover, the enriched photogenerated holes on the {001} of TiO 2 can accelerate the oxidation reactions. Photocatalytic CO 2 reduction over anatase TiO 2 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 CO 2 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 CO 2 is reduced to CH 4 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 TiO 2 for the further enhancement of photocatalytic performance, where TiO 2 fabricated via hydrothermal route in the presence of HF was reduced by NaBH 4 to generate surface oxygen defects . 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 Ti 3+ energy state as well as the improved CO 2 adsorption and activation derived from the synergism of Ti 3+ and oxygen vacancies resulted in the enhanced photoreduction efficiency of CO 2 . An explicit atomistic model of the interface was applied for further investigation of charge transfer between coexposed {101} and {001} facets of anatase TiO 2 (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 TiO 2 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 TiO 2 nanocrystals with coexposed 51% {001} and 49% {101} facets exhibit the highest photocatalytic activity for reducing CO 2 to CH 4 (1.58 μmol g cat −1 h −1 ) among pure TiO 2 series, which can be further improved to 4.0 μmol g cat −1 h −1 by decorating 1wt% Pt 0 nanoparticles on their surfaces. The effect of Pt loading for the enhanced photocatalytic performance of anatase TiO 2 facet heterojunction ({101}/{001}) was further investigated by Xiong et al. (Xiong et al., 2017a). Pt precursors (H 2 PtCl 6 or Pt (NH 3 ) 4 Cl 2 ) 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 H 2 PtCl 6 (HC) possessed suitable particle size and high Pt 0 /Pt II ratio (1.15), which led to efficient separation of photogenerated carriers and high efficiency for photoreduction CO 2 . Coupling Frontiers in Chemistry | www.frontiersin.org April 2021 | Volume 9 | Article 637501 with graphene is also an efficient strategy to enhance the activity of anatase TiO 2 facet heterojunction ({101}/{001}) for the photoreduction of CO 2 (Xiong et al., 2016). Charge transfer between {101} and {001} facets along with the migration of electrons from TiO 2 to graphene greatly improved the separation efficiency of photogenerated carriers, thus increasing the yield of CO by photoreducing CO 2 . Photocatalytic CO 2 reduction performance of the typical TiO 2based phase or facet heterojunctions above are listed in Table 3.

CONCLUSIONS AND PROSPECTS
This review summarizes the recent advances in the rational design, fabrication and photocatalytic performance of TiO 2based heterojunctions for converting CO 2 into solar fuels with water oxidation. Generally, photocatalytic CO 2 reduction that mimic the nature photosynthesis of green plants exhibits great potential for the reduction of CO 2 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 CO 2 , 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 TiO 2 -based heterojunctions, in order to realize enhanced photocatalytic performance for CO 2 conversion, thus giving new life to this traditional and systematically studied photocatalyst. Although the composition, morphology, architecture and photocatalytic mechanism of TiO 2 -based heterojunctions are various, they have much in common that favors for photoreduction of CO 2 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 CO 2 reduction; 3) the enlarged CO 2 adsorption capacity due to the high specific surface area with highly exposed active sites that combine CO 2 by chemical action, which could also activate the adsorbed CO 2 molecules and facilitate hydrocarbon generation; 4) the increase in selectivity toward specific photoreduction products of CO 2 is attributed to the contribution of the cocatalysts.
Although considerable progress has been made on TiO 2 -based heterojunction for photocatalytic CO 2 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 CO 2 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 Frontiers in Chemistry | www.frontiersin.org April 2021 | Volume 9 | Article 637501 20 photocatalytic CO 2 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 CO 2 reduction. In addition, efficient solar harvesting systems are in great demand to replace artificial light sources with high energy consumption, since efficient photoreduction of CO 2 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 CO 2 reduction is far less than that of electrocatalysis, relying entirely on solar energy will become its irreplaceable advantage. Furthermore, the enrichment of CO 2 in air will become an important consideration in the design of heterojunction photocatalysts, which meet the needs of practical applications. Fortunately, the research on CO 2 storage and controlled release provides the possible solution while the visible light-triggered capture and release of CO 2 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 CO 2 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 wellorganized structure, optimized solar energy conversion efficiency, ideal turnover frequency of CO 2 , 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 CO 2 into solar fuels, thus accelerating the industrialization process of this very promising technology and providing practical help to alleviate energy and environmental crises.