Recent Advances in Synthesis and Applications of Carbon-Doped TiO2 Nanomaterials

TiO2 has been widely used as a photocatalyst and an electrode material toward the photodegradation of organic pollutants and electrochemical applications, respectively. However, the properties of TiO2 are not enough up to meet practical needs because of its intrinsic disadvantages such as a wide bandgap and low conductivity. Incorporation of carbon into the TiO2 lattice is a promising tool to overcome these limitations because carbon has metal-like conductivity, high separation efficiency of photogenerated electron/hole pairs, and strong visible-light absorption. This review would describe and discuss a variety of strategies to develop carbon-doped TiO2 with enhanced photoelectrochemical performances in environmental, energy, and catalytic fields. Emphasis is given to highlight current techniques and recent progress in C-doped TiO2-based materials. Meanwhile, how to tackle the challenges we are currently facing is also discussed. This understanding will allow the process to continue to evolve and provide facile and feasible techniques for the design and development of carbon-doped TiO2 materials.


Introduction
Environmental pollution and energy crisis have been the most urgent issues in recent years [1][2][3][4][5]. Photocatalysis is a promising strategy to alleviate, and even work out these problems because it could efficiently decompose organic pollutants or produce chemical energy using semiconductors and renewable solar energy [5][6][7]. Titanium dioxide (TiO 2 ), as one kind of nontoxic, high stable, and low-cost materials, has received special interest in environmental, catalysis, and energy areas [8][9][10][11]. However, the photocatalytic efficiency of pure TiO 2 is not enough up to meet practical needs because of its wide bandgap (~3.2 eV), and the fast recombination of photoinduced charge pairs, leading to a considerable energy consumption, poor visible-light photocatalytic activity, and low quantum efficiency [9,12,13]. Therefore, how to significantly promote charge separation is of significance in meeting practical needs.
Tremendous efforts have been devoted to varying the morphology, structure, and chemical composition of TiO 2 by doping metal and/or nonmetal elements, surface sensitization, and coupling with narrow band-gap semiconductors [8,[12][13][14]. Among these strategies, metal doped TiO 2 usually suffers from photocorrosion, poor stability, low doping amount, and no noticeable change in the band gap of TiO 2 [8,[14][15][16], while TiO 2 is coupled with other semiconductors, the additional cost and undermined stabilization have also disadvantageous effects on the practical employment of TiO 2 composites [15]. Notably, non-metal doping has attracted much more interest of investigators than metal doping in improving the photoelectronic performances of TiO 2 and in shifting its absorption edge to the long wavelength region [13,14]. Especially, carbon doped TiO 2 manifests promising advantages beyond other non-metal doping, which may be ascribed to following aspects [12,14,[17][18][19][20]: (1) carbon holds metallic conductivity; (2) carbon serves as a trapping center and transport channel for photogenerated electrons, promoting separation efficiency of photoinduced electron/hole pairs; and (3) carbon can also act as a sensitizer to sensitize TiO 2 under visible light irradiation, thus aggregating a number of thermal energy, facilitating charge transfer from the bulk of TiO 2 into oxidation reaction sites, and further generating lots of active species. Notably, carbon is always indicated that it can be permeated to the lattice of TiO 2 so as to substitute a lattice O or Ti atom, and then form a Ti-C or C-O-Ti bond, which produces a hybrid orbital just above the valence band of TiO 2 and bestows a significant enhancement in visible-light driven photocatalytic activity. Therefore, coupling TiO 2 with carbon materials including activated carbon [21,22], carbon nanofibers [23,24], carbon nanotubes [25][26][27], carbon sphere [28,29], carbon dot [30,31], graphene [32][33][34], and carbon doping [8,9,14,35] have been widely investigated and proved to hold significant potential over other types of modification methods in environmental, energy, and biomedical fields.
Among carbon materials, carbon doping has been attracting special interest because the introduction of carbon atom causes an electron coupling effect between carbon and TiO 2 or introduces a localized occupied state into TiO 2 to narrow the bandgap of TiO 2 [8,9,14]. For example, Zegeye et al. [36] developed hybrid carbon-doped TiO 2 /S composite as a positive electrode material for lithium-sulfur batteries, manifesting enhanced cycle stability and rate performance. Our groups designed and constructed several carbon-doped TiO 2 -based materials such as C-doped TiO 2 hollow spheres [14], C-doped TiO 2 single-crystal nanorods [8], and hierarchical SiO 2 @C-doped TiO 2 hollow spheres [9]. Further investigation found that the as-synthesized carbon-doped TiO 2 materials manifested superior photocatalytic performance toward the degradation of organic pollutants (RhB-. MB, MV, 4-NP, etc.).
In the past several years, many encouraging achievements have been exhibited in the research area of TiO 2 -based materials. Especially, carbon-doped TiO 2 composites have attracted an increasing attention in environmental and energy science because they can exhibit sizeable potential superiorities as adsorbent, support, and sensitizer, promoting photogenerated electrons migrating to semiconductors [7,37]. In this review, we systematically summarize the currently available synthesis strategies and applications of carbon-doped TiO 2 materials. Notably, we further highlight recent progress in the design and construction of carbon-doped TiO 2 composites with a multifunctional nature. Finally, challenges and outlooks are outlined and discussed, identifying prospective areas for related research in this field.

XPS Analysis
Carbon-doped TiO 2 is that carbon element is incorporated into the lattice of TiO 2 by replacing some of the lattice titanium or oxygen atoms, forming Ti−O-C or Ti−C bond, respectively. Therefore, X-ray photoelectron spectroscopy (XPS) measurement was widely used to obtain the information of Ti−O-C or Ti−C bond because XPS has a perfect match of its probe length (about 10 nm) to the size of particles, and exhibits its ability to probe the chemical identity of the elements present [38]. Therefore, the binding energy of C1s according to XPS analysis is considered as a solid evidence to confirm the formation of carbon doping [8,9,14,[39][40][41][42], as shown in Table 1. This section focuses on addressing and discussing the difference of two chemical bonds (Ti−O-C or Ti−C band) as an evidence to distinguish whether carbon is incorporated into the lattice of TiO 2 , adsorbed on the surface, or the interstitial position of the TiO 2 lattice. XPS measurement was usually employed to obtain the chemical state and binding energy of carbon-doped or carbon-decorated TiO 2 composites, ascertaining fundamental information on the interaction between C dopant and TiO 2 [5,8,35,40,41,48,[50][51][52][53]. When carbon is doped into the TiO 2 lattice by replacing the lattice titanium, the Ti-O-C bond can be observed in the C1s spectra, as shown in Figure 1. Therefore, many investigators believe that the existence of the Ti-O-C bond can be effective evidence for ascertaining carbon doping in their as-synthesized TiO 2 -based materials [16,36,37]. Similarly, S. Ivanov and coworkers [55] insisted that the existence of the Ti−O−C structure could be used as evidence for confirming the formation of C-doped TiO 2 . However, the existence of the Ti−O−C bond is not completely ascribed to the existence of carbon doping. In most cases, surface and/or interstitial amorphous carbonate dopants in the TiO 2 lattice also result in the formation of Ti-O-C [15,40,41] because carboxyl or oxygen-containing carbon in GO sheets exhibits strong covalent binding ability to TiO 2 [12,41,56], narrowing band-gap energy and prompting its visible light absorption. Accordingly, the existence of the Ti-O-C bond is not solid evidence for ascertaining carbon incorporated into TiO 2 lattice, but it might be considered as a result of specifically interstitial carbon doping. Undoubtedly, more convincing evidence is required for confirming the formation of carbon doping in the TiO 2 lattice [39,53]. O-C [15,40,41] because carboxyl or oxygen-containing carbon in GO sheets exhibits strong covalent binding ability to TiO2 [12,41,56], narrowing band-gap energy and prompting its visible light absorption. Accordingly, the existence of the Ti-O-C bond is not solid evidence for ascertaining carbon incorporated into TiO2 lattice, but it might be considered as a result of specifically interstitial carbon doping. Undoubtedly, more convincing evidence is required for confirming the formation of carbon doping in the TiO2 lattice [39,53].

The Existence of Ti-C Bond
When carbon is doped into the TiO 2 lattice by replacing lattice oxygen, Ti-C bond can be seen in the XPS spectra of C1s, as shown in Figure 2. it has been proved that the appearance of the Ti-C bond is considered as a solid evidence for confirming the formation of carbon doping in TiO 2 -based materials [35,36,50]. For example, Dhanasekaran P. and coworkers ascertained the formation of carbon doped titanium oxide according to the peak at 282.7 eV (Ti-C bond) [50]. Recently, Wang et al. [35] successfully constructed carbon-doped TiO 2 nanotubes by referring the binding energy of 282.0 eV in the XPS spectra of C1s. Unlike Ti-O-C bond, the existence of Ti−C is the only result of carbon replacing oxygen in the TiO 2 lattice. Clearly, the existence of the Ti-C bond can be used as the most direct evidence for identifying the formation of carbon doping, as introduced in our previous workers [8,9,14,42]. Similarly, the absence of the Ti−C bond is considered as carbon decorated TiO 2 other than carbon doped TiO 2 , suggesting that carbon species do not substitute oxygen species and dope into the TiO 2 lattice as reported in many publications [7,12,16,18,[57][58][59][60].
Catalysts 2020, 10, x FOR PEER REVIEW 4 of 16 When carbon is doped into the TiO2 lattice by replacing lattice oxygen, Ti-C bond can be seen in the XPS spectra of C1s, as shown in Figure 2. it has been proved that the appearance of the Ti-C bond is considered as a solid evidence for confirming the formation of carbon doping in TiO2-based materials [35,36,50]. For example, Dhanasekaran P. and coworkers ascertained the formation of carbon doped titanium oxide according to the peak at 282.7 eV (Ti-C bond) [50]. Recently, Wang et al. [35] successfully constructed carbon-doped TiO2 nanotubes by referring the binding energy of 282.0 eV in the XPS spectra of C1s. Unlike Ti-O-C bond, the existence of Ti−C is the only result of carbon replacing oxygen in the TiO2 lattice. Clearly, the existence of the Ti-C bond can be used as the most direct evidence for identifying the formation of carbon doping, as introduced in our previous workers [8,9,14,42]. Similarly, the absence of the Ti−C bond is considered as carbon decorated TiO2 other than carbon doped TiO2, suggesting that carbon species do not substitute oxygen species and dope into the TiO2 lattice as reported in many publications [7,12,16,18,[57][58][59][60].

EPR Analysis
Generally, electron paramagnetic resonance (EPR) analysis was widely used to evaluate the formation of defect sites in carbon or other dopants doped TiO2-based materials because EPR can characterize the unpaired electrons or paramagnetic centers [40,61,62]. If carbon doping was formed in the TiO2 lattice, a stronger EPR signatable 1l at g = 2.003 ± 0.001 was more able to be observed than that of pure TiO2 or undoped TiO2, which could be ascribed to the unpaired electron trapped on surface oxygen vacancies, proving that carbon element could be incorporated into the crystal lattice of TiO2-based materials [40,61].

Strategies for the Synthesis of C-Doped TiO2 Materials
Various strategies including hydrothermal technique, template strategy, thermal oxidation of TiC, and sol-gel process have been introduced to construct carbon-doped TiO2 materials with

EPR Analysis
Generally, electron paramagnetic resonance (EPR) analysis was widely used to evaluate the formation of defect sites in carbon or other dopants doped TiO 2 -based materials because EPR can characterize the unpaired electrons or paramagnetic centers [40,61,62]. If carbon doping was formed in the TiO 2 lattice, a stronger EPR signatable 1l at g = 2.003 ± 0.001 was more able to be observed than that of pure TiO 2 or undoped TiO 2 , which could be ascribed to the unpaired electron trapped on surface oxygen vacancies, proving that carbon element could be incorporated into the crystal lattice of TiO 2 -based materials [40,61].

Strategies for the Synthesis of C-Doped TiO 2 Materials
Various strategies including hydrothermal technique, template strategy, thermal oxidation of TiC, and sol-gel process have been introduced to construct carbon-doped TiO 2 materials with enhanced photoelectrochemical performances [16,17], as shown in Table 1. This section would pay attention to the description and discussion of synthetic methods involved in developing C-doped TiO 2 -based materials.

Hydrothermal Method
As a simple and mature method, the hydrothermal technique has been widely used to construct C-doped TiO 2 materials because the morphology and structure of products are easily controlled by changing hydrothermal conditions [36,39,40,63]. In a typical hydrothermal synthesis, TiO 2 precursor and carbon source are dispersed or dissolved in acidic or alkaline solution, and then the mixture is transferred into a Teflon-lined autoclave, sealed, and heated in an electric oven (100-180 • C), forming crystallized C-TiO 2 structure. Finally, C-doped TiO 2 is successfully synthesized by removing organic residues via calcination under an air atmosphere. For example, mesoporous C-doped TiO 2 was able to be prepared via one-pot hydrothermal synthesis using TiCl 4 and sucrose as TiO 2 precursor and carbon source, respectively [36]. Aragaw et al. [53] introduced the preparation of Sn-C codoped single crystal TiO 2 , in which SnCl 4 and glucose were used as tin and carbon dopant precursors, respectively. Qi D and coworkers reported C-doped TiO 2 /MCF-F photocatalyst using silica mesoporous cellular foam (MCF) as host material and glucose as carbon source. Recently, our group in situ constructed C-doped TiO 2 single-crystal nanorod using CPS@TiO 2 as TiO 2 precursor and carbon dopant source [8], this preparation process and morphology are more clearly illustrated in Figure 3. carbon source, respectively [36]. Aragaw et al. [53] introduced the preparation of Sn-C codoped single crystal TiO2, in which SnCl4 and glucose were used as tin and carbon dopant precursors, respectively. Qi D and coworkers reported C-doped TiO2/MCF-F photocatalyst using silica mesoporous cellular foam (MCF) as host material and glucose as carbon source. Recently, our group in situ constructed C-doped TiO2 single-crystal nanorod using CPS@TiO2 as TiO2 precursor and carbon dopant source [8], this preparation process and morphology are more clearly illustrated in Figure 3. In addition to being a one-pot and mature technique, hydrothermal method is advantageous in tuning the structure, morphology, and physical-chemical performances of final TiO2 materials by changing TiO2 precursor and hydrothermal conditions. Especially, hybrid functional TiO2-based materials can be designed and constructed by adding other required species. Although the whole preparation process seems to be very simple, each step including the selection of the titanium dioxide precursor, hydrothermal condition, washing, and calcination presents a decisive role in tuning the morphology, structure, property, and yield of C-TiO2 materials [63].

Template-Directed Method
Compared with solid/nonporous titanium dioxide nanoparticles, hollow/porous TiO2 materials are more attractive because of a higher surface area and multiple interparticle scattering [14,64]. Precise control of the particle size, hollow/porous structure, and shell thickness of TiO2 spheres has been a pursuing object because it is a key factor determining their properties [14]. In general, two In addition to being a one-pot and mature technique, hydrothermal method is advantageous in tuning the structure, morphology, and physical-chemical performances of final TiO 2 materials by changing TiO 2 precursor and hydrothermal conditions. Especially, hybrid functional TiO 2 -based materials can be designed and constructed by adding other required species. Although the whole preparation process seems to be very simple, each step including the selection of the titanium dioxide precursor, hydrothermal condition, washing, and calcination presents a decisive role in tuning the morphology, structure, property, and yield of C-TiO 2 materials [63].

Template-Directed Method
Compared with solid/nonporous titanium dioxide nanoparticles, hollow/porous TiO 2 materials are more attractive because of a higher surface area and multiple interparticle scattering [14,64]. Precise control of the particle size, hollow/porous structure, and shell thickness of TiO 2 spheres has been a pursuing object because it is a key factor determining their properties [14]. In general, two main methods are introduced to prepare hollow TiO 2 spheres. One is template-free technique to construct hollow TiO 2 spheres via physical phenomenon [65]. Although the strategy can realize one-pot and large-scale production, a distinguished disadvantage is the concomitant production of TiO 2 spheres with an ill-shaped and fragile structure [13,14]. Another is the template-directed method that effectively tunes the shell and pore size of hollow TiO 2 spheres, overcoming the disadvantage of the template-free technique [14]. For example, Zou et al. [48] prepared C-doped hollow TiO 2 spheres using carbon sphere as a template. Matos and coworkers reported an easy and ecofriendly method to develop pristine anatase phase of C-doped TiO 2 using a biomass-derived molecule as a biotemplate [66]. Ji et al. [13] constructed C-doped TiO 2 nanotubes using surface-sulfonated polystyrene as a template through calcination. Our group introduced an in situ synthetic method for the development of hierarchical SiO 2 @C-doped TiO 2 spheres with a uniform hollow structure using cationic polystyrene spheres as templates, as shown in Figure 4 [9], and then the facile strategy was further broadened to prepare various functionalized C-doped TiO 2 materials including C-doped hollow TiO 2 [14], C-doped porous TiO 2 [42], and C-doped TiO 2 single-crystal nanorods [8]. Although the template-directed method can develop various C-doped TiO 2 materials with a regular and tuned morphology at the nano-and/or microscale via controlling the structure of templates [63], its extensive applications may be limited because of the preparation cost and insufficient characterization of templates.
Catalysts 2020, 10, x FOR PEER REVIEW 6 of 16 [14], C-doped porous TiO2 [42], and C-doped TiO2 single-crystal nanorods [8]. Although the templatedirected method can develop various C-doped TiO2 materials with a regular and tuned morphology at the nano-and/or microscale via controlling the structure of templates [63], its extensive applications may be limited because of the preparation cost and insufficient characterization of templates.

TiC Calcination
Titanium carbide (TiC) holds many fascinating properties including fast electron transfer, easy modification, and superior stabilization [15,44], making it more potential for the construction of Cdoped TiO2 materials via simple calcination because of its high visible-light absorption efficiency and fast charge transfer. Yang et al. [15] prepared a C-doped layer on the TiC nanosphere with efficient visible light-photocatalytic H2 production through in situ calcination of TiC. Figure 5 clearly demonstrated the preparation process, morphology, and structure of the as-synthesized porous Cdoped TiO2. Li et al. [54] reported C-doped TiO2 multiple-phase composites exhibiting excellent ionic and electronic conductivity through the calcination of TiC at 600 °C for 10 h. Unfortunately, it is very

TiC Calcination
Titanium carbide (TiC) holds many fascinating properties including fast electron transfer, easy modification, and superior stabilization [15,44], making it more potential for the construction of C-doped TiO 2 materials via simple calcination because of its high visible-light absorption efficiency and Catalysts 2020, 10, 1431 7 of 16 fast charge transfer. Yang et al. [15] prepared a C-doped layer on the TiC nanosphere with efficient visible light-photocatalytic H 2 production through in situ calcination of TiC. Figure 5 clearly demonstrated the preparation process, morphology, and structure of the as-synthesized porous C-doped TiO 2 . Li et al. [54] reported C-doped TiO 2 multiple-phase composites exhibiting excellent ionic and electronic conductivity through the calcination of TiC at 600 • C for 10 h. Unfortunately, it is very difficult in preparing functionalized C-doped TiO 2 with controlled morphology, size, and hollow/porous structure via the calcination of TiC nanoparticle. Therefore, it is very necessary to combine other promising strategies for the development of C-doped TiO 2 materials with controllable structure and properties.

The Removal of Organic Pollutants
Exploiting effective strategies to remove organic pollutants in wastewater is very necessary according to environmental safety regulations due to increasing concerns about drinking water safety [67,68]. Photocatalysis has been considered as a promising way to remove organic contaminants including refractory organic pollutants by using solar energy because it can mineralize various organic pollutants to produce CO2, H2O, and other harmless small molecules. It is well known that pure TiO2 holds poor visible-light absorption, low quantum yield, and undesired recombination of photogenerated charges. Notably, C-doped TiO2 not only can effectively promote charge separation, but also can shift the optical response of TiO2 from UV to visible spectral region, leading to an enhanced photocatalytic performance for the removal of various organic pollutants (Table 2) [8,9,39,69]. For example, Ji et al. [13] reported that C-doped TiO2 nanotubes demonstrated much better photocatalytic activity toward the degradation of UDMH than bare TiO2 under UV and visible light, as shown in Figure 6. Figure 6c demonstrated its photodegradation mechanism: under light irradiation, photogenerated electrons transferred more efficiently to conduction band of TiO2 because carbon in the carbon-TiO2 nanotubes could act as an electron trap, inhibiting charge recombination [70,71]. Furthermore, carbon also could improve adsorption of pollutant molecules, promoting photocatalytic performance because adsorption was normally the first step in photocatalysis [67,72].

The Removal of Organic Pollutants
Exploiting effective strategies to remove organic pollutants in wastewater is very necessary according to environmental safety regulations due to increasing concerns about drinking water safety [67,68]. Photocatalysis has been considered as a promising way to remove organic contaminants including refractory organic pollutants by using solar energy because it can mineralize various organic pollutants to produce CO 2 , H 2 O, and other harmless small molecules. It is well known that pure TiO 2 holds poor visible-light absorption, low quantum yield, and undesired recombination of photogenerated charges. Notably, C-doped TiO 2 not only can effectively promote charge separation, but also can shift the optical response of TiO 2 from UV to visible spectral region, leading to an enhanced photocatalytic performance for the removal of various organic pollutants (Table 2) [8,9,39,69]. For example, Ji et al. [13] reported that C-doped TiO 2 nanotubes demonstrated much better photocatalytic activity toward the degradation of UDMH than bare TiO 2 under UV and visible light, as shown in Figure 6. Figure 6c demonstrated its photodegradation mechanism: under light irradiation, photogenerated electrons transferred more efficiently to conduction band of TiO 2 because carbon in the carbon-TiO 2 nanotubes could act as an electron trap, inhibiting charge recombination [70,71]. Furthermore, carbon also could improve adsorption of pollutant molecules, promoting photocatalytic performance because adsorption was normally the first step in photocatalysis [67,72]. Yu et al. [39] found that C-doped TiO2 encapsulated with nano-sized graphene manifested superior visible-light performance for phenol degradation than those of anatase TiO2, P25, and bare C-doped TiO2. Zhang et al. [5] confirmed that C-doped TiO2/MCFF exhibited good adsorptive ability and visible-light photocatalytic performance for degrading methyl orange. Our previous works also ascertained that C-doped TiO2-based materials manifested stronger visible-light performances toward the removal of various contaminants including rhodamine B (RhB), methylene blue (MB), and p-nitrophenol [8,9,14]. The enhanced photocatalytic performance may be attributed to the synergistic effect between C-doping and TiO2, in which carbon can generate electrons under visible light illumination, and then photoelectrons would be transferred to the contribution band of TiO2, improving its efficient light harvesting and photocatalytic activity [17,73,74]. It is should be pointed out that the porous/hollow structure of photocatalysts is very beneficial to increase the surface area of photocatalyst and promote the contact probability of catalyst and substrate by decreasing diffusion limitation [9,14,75], which leads to better photocatalytic degradation of organic pollutants.  Yu et al. [39] found that C-doped TiO 2 encapsulated with nano-sized graphene manifested superior visible-light performance for phenol degradation than those of anatase TiO 2 , P25, and bare C-doped TiO 2 . Zhang et al. [5] confirmed that C-doped TiO 2 /MCFF exhibited good adsorptive ability and visible-light photocatalytic performance for degrading methyl orange. Our previous works also ascertained that C-doped TiO 2 -based materials manifested stronger visible-light performances toward the removal of various contaminants including rhodamine B (RhB), methylene blue (MB), and p-nitrophenol [8,9,14]. The enhanced photocatalytic performance may be attributed to the synergistic effect between C-doping and TiO 2 , in which carbon can generate electrons under visible light illumination, and then photoelectrons would be transferred to the contribution band of TiO 2 , improving its efficient light harvesting and photocatalytic activity [17,73,74]. It is should be pointed out that the porous/hollow structure of photocatalysts is very beneficial to increase the surface area of photocatalyst and promote the contact probability of catalyst and substrate by decreasing diffusion limitation [9,14,75], which leads to better photocatalytic degradation of organic pollutants.

Electrochemical Application
A variety of experiments have been proved that TiO 2 is a very promising electrode material for electrochemical applications due to its low cost, ideal capacitive response, and good cyclic stability [42,[86][87][88], however, TiO 2 has many disadvantages including low conductivity, fast charge recombination, and high photochemical stability, leading to a poor electrochemical performance [48,55,89,90]. The introduction of carbon doping is a potential tool to efficiently improve the electrical conductivity of TiO 2 -based materials because carbon holds good corrosion resistance, cyclic stability, and a long service lifetime during charge/discharge processes [58,86,87], as shown in Table 3. Shen et al. [86] found that hierarchical carbon-doped TiO 2 beads featured higher electronic conductivity than P25 and anatase TiO 2 beads. Our previous work also demonstrated that C-doped porous TiO 2 electrodes [42] increased ion diffusion channels and accelerated ion transfer, leading to an enhanced electrochemical performance, as shown in Figure 7. Notably, mesoporous/hollow electrode materials can exhibit high accessible area and ionic transport [11,36,42,58], resulting in enhanced electrochemical performances. Besides, the synergistic effect of multicomponent materials is of significance in promoting the electrochemical properties of carbon-doped TiO 2 [50,56,91,92].

Summary and Outlook
Although carbon doping can efficiently enhance photoelectrochemical properties of TiO 2 , it is still a very challenging task in obtaining a high doping amount of carbon in the TiO 2 lattice because C-doped TiO 2 is probably more difficult to be prepared than other non-mental doping [16], especially for single crystal or single-crystal-like TiO 2 due to its high crystallinity [8,91], which hinders its further applications in environmental, energy, and catalytic fields. Co-doping with two or more dopants is a promising way to further enhance the properties of TiO 2 compared to their single doped or undoped TiO 2 counterparts due to a strong synergistic effect between these codopants within the TiO 2 matrix [53,95,96]. Zegeye et al. [36] reported that MC-Meso C-dopedTiO 2 /S showed the best cycling stability and enhanced electrochemical property for lithium-sulfur batteries. Zhou et al. [7] found that In 2 O 3 and carbon codoped TiO 2 could oxidize Hg 0 and manifested higher visible-light photoactivity compared with P25. Our previous works showed that C/N-TiO 2 hollow sphere [95] and C/Bi-TiO 2 single crystal nanorod [97] both exhibited an enhanced photocatalytic performance toward the removal of refractory organic pollutants. Unfortunately, it is extremely difficult in figuring out the contribution of carbon doping in codoped TiO 2 -based materials. On the other hand, how to obtain uniform dispersion of carbon doping in the TiO 2 lattice also remains a big challenge [86]. Therefore, it is envisioned that future research will provide new insights in optimizing existing strategies and developing new techniques, which better construct C-doped TiO 2 materials with a high doping amount and controlled distribution of carbon. Funding: The authors would like to thank the "Blue Project" in Jiangsu Colleges and Universities (2019-69) for financial supports to carry out this work.

Conflicts of Interest:
The authors declare no conflict of interest.