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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.