Recent Development of Photocatalysts Containing Carbon Species : A Review

Undoubtedly, carbon-based (nano)composites can be promising photocatalysts with improved photocatalytic activity due to the coupling effect from the incorporation of carbon species. In this mini-review, we focus on the recent development of photocatalysts based on carbon-based (nano)composites. TiO2 is well-known as a typical photocatalyst. Special attention is paid to the various types of carbon–TiO2 composites such as C-doped TiO2, N–C-doped TiO2, metal–C-doped TiO2, and other co-doped C/TiO2 composites. Various synthetic strategies including the solvothermal/hydrothermal method, sol–gel method, and template-directed method are reviewed for the preparation of carbon-based TiO2 composites. C/graphitic carbon nitride (g-C3N4) composites and ternary C-doped composites are also summarized and ascribed to the unique electronic structure of g-C3N4 and the synergistic effect of the ternary interfaces, respectively. In the end, we put forward the future perspective of the photocatalysts containing carbon species based on our knowledge.

This current mini-review gives an overview of the synthesis and photocatalytic properties of photocatalysts containing carbon species, especially C-doped TiO 2 .In Section 2, various methods including the solvothermal/hydrothermal method, sol-gel method, and template-directed method for the synthesis of C-doped TiO 2 are summarized.In addition, C-doped TiO 2 with pores such as mesoporous and hollow structures is introduced via typical examples.Finally, the existing problems, future development trends, and potential applications of these promising photocatalysts are put forward.We expect that this review would catch a glimpse of the current developments of these photocatalysts containing carbon species.

Photocatalysts of C-doped TiO 2
2.1.Photocatalysts of TiO 2 /C Composites Titanium dioxide (TiO 2 ) is well-known as a typical photocatalytic metal oxide.However, it displays low photocatalytic efficiency under visible-light irradiation due to its intrinsic large band gap.The incorporation of carbon species into TiO 2 for the fabrication of C-doped TiO 2 is a promising strategy for improving the photocatalytic efficiency, especially under visible light irradiation.In early 2006, Shanmugam et al. synthesized TiO 2 @C core-shell composite nanoparticles in the range of 15-35 nm in a specially-made Swagelok cell at different temperatures [30].They discovered that the TiO 2 @C composites showed high photocatalytic activity for 4-chlorophenol degradation, compared with Degussa P25.Then, Sullivan et al. also presented visible light active C-doped TiO 2 photocatalysts [31], displaying the activities in the oxidation of 4-chlorophenol under visible light irradiation.In their work, through the initial He heat treatments, one can tune the extent of graphitic carbon laydown.After that, C and N single and co-doping TiO 2 films of single-layer electrodes were prepared as dye-sensitized solar cell devices [32], enhancing their short circuit current and efficiency, and improving the electron lifetime, charge transport, and recombination resistance.They achieved the highest cell efficiency of 10.2%, as a result of a balance between the band gap, surface area, and diffused reflection of the photoanode.More recently, visible light active carbon-doped TiO 2 with anatase/rutile multi-phases coated on granular activated carbon was applied to the removal of nonylphenol in solution [33].They discovered the decreased band gap from 3.17 eV to 2.72 eV after C doping.
The solvothermal method is one traditional route for the synthesis of metal oxides.In the literature, the calcination-assisted solvothermal method was developed to fabricate the C-TiO 2 composites coupled with three colors (blue, green, and red) of emitting up-conversion phosphors [34], displaying the unique optical property via absorbing UV, visible, and near-infrared (NIR) lights.These composites exhibited photocatalytic activities for the degradation of rhodamine B (RhB) solution under NIR light irradiation and the destruction of continuous NO gas flow under the irradiation of UV, visible light-emitting diodes (LEDs), and infrared laser.It was found that the calculated photon efficiency under infrared light irradiation was much higher than those under UV and visible light irradiations, and the green light emitting up-conversion phosphor coupled C-TiO 2 composite presented the superior photocatalytic performance over the blue and red ones [34].
The solvothermal method was also applied to prepare TiO 2 /graphene composites with a chemical bonding interface and enhanced photocatalytic activity in the photodegradation of formaldehyde in air using tetrabutyl orthotitanate as the Ti source [35].As for the TiO 2 /graphene nanocomposites with 2.5 wt% graphene, numerous TiO 2 nanocrystals that were densely deposited onto the graphene sheets were obtained due to interfacial interactions and preferential heterogeneous nucleation (Figure 1a,b).One can observe the fringe spacing of ca.3.51 Å matching the (101) crystallographic plane of anatase TiO 2 (Figure 1d), exhibiting a high surface energy.Interestingly, in their work, they found the graphene content to exhibit an obvious influence on the photocatalytic activity and synergetic effect between graphene and TiO 2 nanoparticles.The authors indicated that the TiO 2 /graphene composite with chemical bonding caused an intimate interaction between TiO 2 nanoparticles and graphene nanosheets (Figure 2a).The formation of a C-Ti bond significantly contributed to the enhanced photocatalytic activity of the TiO 2 /graphene composite.It is energetically feasible to transfer the photogenerated feasible to transfer the photogenerated electrons on the conduction band (CB) of TiO2 to graphene (Figure 2b).It was reported that the energy level of the valence band (VB) is 2.81 V, the energy level of the conduction band (CB) is −0.39 V, and the calculated Fermi energy level of graphene is −0.08 V versus a normal hydrogen electrode, respectively.The authors suggested that the design of new heterogeneous photocatalysts can be applied in environmental protection, water splitting, and photoelectrochemical conversion.There have been a few reports on the synthesis of C-doped TiO2 composites via the sol-gel method.For example, the sol-gel method was applied to prepare the core-shell structure of TiO2carbon nanoparticles by utilizing the organic group in the Ti precursor [36].One can observe the  feasible to transfer the photogenerated electrons on the conduction band (CB) of TiO2 to graphene (Figure 2b).It was reported that the energy level of the valence band (VB) is 2.81 V, the energy level of the conduction band (CB) is −0.39 V, and the calculated Fermi energy level of graphene is −0.08 V versus a normal hydrogen electrode, respectively.The authors suggested that the design of new heterogeneous photocatalysts can be applied in environmental protection, water splitting, and photoelectrochemical conversion.There have been a few reports on the synthesis of C-doped TiO2 composites via the sol-gel method.For example, the sol-gel method was applied to prepare the core-shell structure of TiO2carbon nanoparticles by utilizing the organic group in the Ti precursor [36].One can observe the From Ref. [35].Reprinted with permission from ACS.
There have been a few reports on the synthesis of C-doped TiO 2 composites via the sol-gel method.For example, the sol-gel method was applied to prepare the core-shell structure of TiO 2 -carbon nanoparticles by utilizing the organic group in the Ti precursor [36].One can observe the core-shell morphology with a ~1.5-nm thick amorphous carbon layer, acting as a barrier on the grain boundary to obstruct the growth of anatase TiO 2 , resulting in a decrease in its crystal size of nine nm (Figure 3a-c).The Electron Energy Loss (EELS) spectra showed an average decrease in the Ti valence state in both C/TiO 2 and reduced TiO 2 (Figure 3d).They reported a large number of oxygen vacancies for these materials, inducing the valence band tails, to enhance visible light absorption.The C/TiO 2 showed excellent solar-driven photocatalytic activities on the photodegradation of phenol and methyl blue, and an excellent hydrogen generation rate, which was 10 times higher than that of the reference TiO 2 by photosplitting water.
Catalysts 2018, 8, x FOR PEER REVIEW 4 of 21 core-shell morphology with a ~1.5-nm thick amorphous carbon layer, acting as a barrier on the grain boundary to obstruct the growth of anatase TiO2, resulting in a decrease in its crystal size of nine nm (Figure 3a-c).The Electron Energy Loss (EELS) spectra showed an average decrease in the Ti valence state in both C/TiO2 and reduced TiO2 (Figure 3d).They reported a large number of oxygen vacancies for these materials, inducing the valence band tails, to enhance visible light absorption.The C/TiO2 showed excellent solar-driven photocatalytic activities on the photodegradation of phenol and methyl blue, and an excellent hydrogen generation rate, which was 10 times higher than that of the reference TiO2 by photosplitting water.The sol-gel method was also used to synthesize TiO2/C nanosheets with a unique, multi-layer, cross-linked hole structure via the assistance of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), which is a polymerization of aniline and carbonization [37].They discovered that the TiO2/C nanosheets showed an amazing photocatalytic capacity under both UV and sunlight irradiations due to C-doping, as well as a unique morphology, structure, and mixed phases of anatase and rutile, with high crystallization.The sol-gel method was developed to prepare a C-doped TiO2 catalyst for the visible light-induced photocatalytic degradation of ethylene [38].They evaluated the effects of key parameters such as the visible light intensity, temperature, and feed composition (ethylene, oxygen, and water vapor) on the photocatalytic reaction rate.They indicated that the oxidation rate of ethylene was improved significantly with an increase of the visible light intensity, temperature, and oxygen and ethylene concentrations.Hassan et al. applied a modified sol-gel route to synthesize C-TiO2 thin films based on the self-assembly technique, exploiting Tween80 as a pore directing agent and carbon source [39].It was reported that the addition of carbon enhanced the photocatalytic activity of TiO2 for methyl orange degradation under visible light irradiation.Recently, an in situ modified sol-gel method was used to synthesize the mesoporous C-doped TiO2 crystals The sol-gel method was also used to synthesize TiO 2 /C nanosheets with a unique, multi-layer, cross-linked hole structure via the assistance of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF 4 ]), which is a polymerization of aniline and carbonization [37].They discovered that the TiO 2 /C nanosheets showed an amazing photocatalytic capacity under both UV and sunlight irradiations due to C-doping, as well as a unique morphology, structure, and mixed phases of anatase and rutile, with high crystallization.The sol-gel method was developed to prepare a C-doped TiO 2 catalyst for the visible light-induced photocatalytic degradation of ethylene [38].They evaluated the effects of key parameters such as the visible light intensity, temperature, and feed composition (ethylene, oxygen, and water vapor) on the photocatalytic reaction rate.They indicated that the oxidation rate of ethylene was improved significantly with an increase of the visible light intensity, temperature, and oxygen and ethylene concentrations.Hassan et al. applied a modified sol-gel route to synthesize C-TiO 2 thin films based on the self-assembly technique, exploiting Tween80 as a pore directing agent and carbon source [39].It was reported that the addition of carbon enhanced the photocatalytic activity of TiO 2 for methyl orange degradation under visible light irradiation.Recently, an in situ modified sol-gel method was used to synthesize the mesoporous C-doped TiO 2 crystals with an average grain size of 22 nm and oxygen vacancy by the limited-oxygen atmosphere originating from the decomposition of urea [40].These materials displayed 97.3% removal efficiency of RhB in nine minutes under UV light, and they also showed the complete RhB degradation under natural sunlight irradiation.
The porous structure is of great importance for increasing and improving the photoactivity of C-doped TiO 2 nanomaterials due to their large surface area and pore volume.In the previous literature, the mesoporous C-doped anatase TiO 2 nanomaterials with excellent visible light photocatalytic activity in the degradation of toluene in the gas phase were synthesized by a one-step "green" synthetic approach with low-cost inorganic Ti(SO 4 ) 2 and glucose as precursors [41].They indicated that the new electronic states above the valence band edge were directly responsible for the electronic origin of the band gap narrowing and visible light photoactivity of the C-doped TiO 2 .After that, mesoporous C-doped anatase TiO 2 nanomaterials were also prepared by a one-pot green synthetic approach and post-thermal treatment using sucrose as a carbon-doping source [42].The enhancement effect of post-thermal treatment between 100-300 • C was proved by the photodegradation of gas-phase toluene.The promoting effect of the post-thermal treatment can be attributed to the changes of the catalysts' surface and optical properties.The recombination of electron-hole pairs is effectively inhibited after thermal treatment due to the reduction of surface defects.
Zhou et al. synthesized uniformly carbonaceous and nitrogenous species sensitized TiO 2 quantum dots employing an oleic acid and oleylamine system and the mesoporous structure of C/N-TiO 2 nanospheres with significantly enhanced photocatalytic activity via an emulsion-based bottom-up self-assembly [43].They obtained the narrow size distribution of monodisperse C/N-TiO 2 quantum dots at an average size of 3.8 nm due to the effect of oleylamine and oleic acid, and mesostructured C/N-TiO 2 nanospheres ranging from 100 nm to 300 nm in diameter, which were composed of these C/N-TiO 2 quantum dots.Moreover, the mesoporous structure of the C/N-TiO 2 nanospheres was stable in the degradation reactions irradiated by visible light.By measuring the photodegradation of RhB under visible light irradiation, they reported the water miscible mesostructured C/N-TiO 2 nanospheres with improved catalytic area and RhB adsorption, and a red shift of the absorption edge to the visible light region, possessing improved photocatalytic activity.
The template-directed method has been a promising strategy for the synthesis of hollow structures.In Shi's work, carbon-doped TiO 2 hollow spheres with a unique three-dimensional network structure and high visible light photocatalytic activity were synthesized using carbon spheres as the template [44].By the decoration of methylene blue solution under visible light irradiation, carbon-doped TiO 2 hollow spheres were reported to possess higher visible light-induced photocatalytic activity than commercial P25, which could be attributed to their visible light absorption characteristic created by C-doping and the unique hollow three-dimensional network structure.The template-directed self-assembly and calcination method was also explored to create C-doped hollow TiO 2 spheres in situ [45].The hollow and porous structure, as well as a highly ordered and well-defined morphology, were obtained.A number of broken spheres were observed in the sample prepared using one gram of tetrabutyl orthotitanate.The hollow sphere had the uniform shell thickness of ∼20 nm in the sample prepared using two grams of tetrabutyl orthotitanate (Figure 4a,b).Increasing the amount of tetrabutyl orthotitanate to three grams, four grams, and five grams, hollow TiO 2 spheres with shell thickness of ∼29 nm (Figure 4c,d), 45 nm (Figure 4e,f), and 59 nm (Figure 4g,h) were obtained, respectively.C-doped hollow TiO 2 spheres, as photocatalysts for the degradation of RhB under visible light irradiation, exhibited a superior photocatalytic activity to commercial P25 and pure TiO 2 -based photocatalysts.Using a multi-walled carbon nanotube mat as a "rigid" template and carbon-doping source, a Cdoped mesoporous anatase TiO2 with a high surface area was obtained [46].TiO2 nanoparticles have an average diameter of 10 nm, surface area of 112-129 m 2 g −1 , and total pore volume of 0.74-0.85m 2 g −1 .Using banana (Musa acuminata) stem fiber as a sacrificial template, visible light active C-doped mesoporous anatase TiO2 multi-tubes with highly improved visible light photocatalytic activity were synthesized [47].The as-obtained materials showed a high surface area of 99 m 2 g −1 , a mesoporous structure made of 15 ± 3 nm nanoparticles, an enhanced light absorption property in the whole visible light region, good thermal stability of the anatase phase up to 750 o C, and an excellent photocatalytic activity for the reduction of Cr(VI) to Cr(III) under the visible light exposure.
Recently, carbon composites derived from different environmentally friendly renewable biomass attracted considerable attention due to their unique properties and wide applications [48][49][50].Liu et al. fabricated visible light-active C/TiO2 composites by the calcination method using C16H36O4Ti as the titanium source and bamboo powder with different sizes as the carbon source [51].The as-obtained composites displayed the photocatalytic activities both under visible light irradiation and UV irradiation.After four cycles, bamboo C/TiO2 composites still exhibited high stability.Moreover, they prepared biomass charcoal-doped titanium dioxide (C/TiO2) composites by the microwave-hydrothermal and calcination methods using tetrabutyl titanate as the titanium source and lignin as the carbon source [52].The composites had a large BET-specific surface area of 203.58 m 2 g −1 , a total pore volume of 0.16 cm 3 g −1 , and an average pore diameter of 22.9 nm.Experimental results indicated that both the microwave-hydrothermal temperature and time played an important role in the microstructure and photocatalytic activity.The core-shell structured CdS-decorated TiO2/carbon microspheres were synthesized with controlled hydrolysis and the sonochemical method [53].The pure commercial TiO2 had only the degradation efficiency of 4.96% for the RhB, and the TiO2/carbon reached 18.92%, while the CdS-decorated TiO2/carbon microspheres achieved 95.24% under visible light irradiation.Using a multi-walled carbon nanotube mat as a "rigid" template and carbon-doping source, a C-doped mesoporous anatase TiO 2 with a high surface area was obtained [46].TiO 2 nanoparticles have an average diameter of 10 nm, surface area of 112-129 m 2 g −1 , and total pore volume of 0.74-0.85m 2 g −1 .Using banana (Musa acuminata) stem fiber as a sacrificial template, visible light active C-doped mesoporous anatase TiO 2 multi-tubes with highly improved visible light photocatalytic activity were synthesized [47].The as-obtained materials showed a high surface area of 99 m 2 g −1 , a mesoporous structure made of 15 ± 3 nm nanoparticles, an enhanced light absorption property in the whole visible light region, good thermal stability of the anatase phase up to 750 • C, and an excellent photocatalytic activity for the reduction of Cr(VI) to Cr(III) under the visible light exposure.
Recently, carbon composites derived from different environmentally friendly renewable biomass attracted considerable attention due to their unique properties and wide applications [48][49][50].The as-obtained composites displayed the photocatalytic activities both under visible light irradiation and UV irradiation.After four cycles, bamboo C/TiO 2 composites still exhibited high stability.Moreover, they prepared biomass charcoal-doped titanium dioxide (C/TiO 2 ) composites by the microwave-hydrothermal and calcination methods using tetrabutyl titanate as the titanium source and lignin as the carbon source [52].The composites had a large BET-specific surface area of 203.58 m 2 g −1 , a total pore volume of 0.16 cm 3 g −1 , and an average pore diameter of 22.9 nm.Experimental results indicated that both the microwave-hydrothermal temperature and time played an important role in the microstructure and photocatalytic activity.The core-shell structured CdS-decorated TiO 2 /carbon microspheres were synthesized with controlled hydrolysis and the sonochemical method [53].The pure commercial TiO 2 had only the degradation efficiency of 4.96% for the RhB, and the TiO 2 /carbon reached 18.92%, while the CdS-decorated TiO 2 /carbon microspheres achieved 95.24% under visible light irradiation.

Photocatalysts of N-C-Doped TiO 2
In comparison with C-doped TiO 2 composites, N-C-doped TiO 2 composites have been found to exhibit improved photocatalytic activity.Wang and Lim synthesized a highly visible light photoactive carbon and nitrogen co-doped TiO 2 with well-developed mesoporosity and a large BET surface area of 102 m 2 g −1 via the solvothermal method [54], and they used these photocatalysts for bisphenol A degradation in a photoreactor irradiated with flexible strips of visible light-emitting diode lights.They reported that under five hours of irradiation with white, blue, green, and yellow light-emitting diode lights, the extents of bisphenol A degradation were >99%, >99%, 84%, and 24%, respectively, while the corresponding percentages of mineralization were 70%, 60%, 45%, and 9%, respectively.The hydrothermal method was developed to synthesize the nitrogen and carbon co-doped titanium dioxide photocatalysts using nitrogen from ammonia and carbon from alcohols with different chain lengths (methanol, ethanol, isopropanol, 1-butanol, 2-butanol, and tertrabutanol) as carbon precursors [55].The authors observed the increasing activity of phenol decomposition with the chain length of the alcohol precursor.They discovered that the highest intrinsic photoactivity for N and C co-doped TiO 2 , which was prepared from 2-butanol and 1-butanol as carbon precursors, exhibited a strong resistance to deactivation during multiple catalyst reuse compared with pristine TiO 2 and commercial Degussa P25 photocatalysts.In Xu et al.'s work, the hydrothermal treatment was employed to prepare N-doped TiO 2 /C nanocomposites with high visible light photocatalytic activity in the presence of L-lysine as a ligand to control the nanocrystal growth and a source of nitrogen and carbon [56].As-prepared nanocomposites had improved absorption in the visible light region and exhibited a higher photocatalytic activity, compared with pure TiO 2 , commercial P25, and previously reported N-doped TiO 2 photocatalysts.The molar ratio of L-lysine/TiCl 4 and the pH value were found to be the important factors affecting the photocatalytic activity of the N-TiO 2 /C composites.
The peptization-reflux method was reported to synthesize visible light-activated C-N-TiO 2 with a diameter of ca.7 nm at 120 • C [57], showing a shift of the absorption edge to a lower energy, a stronger absorption in the visible region, and much higher photocatalytic activity for methyl orange degradation under visible light irradiation, compared to TiO 2 .Authors suggested that it could be assigned to the synergistic effect between the retained alkoxyls and the nitrogen doping.The mild one-step approach was adopted to prepare N-doping carbon-TiO 2 nanohybrids [58].Before calcination, a morphism crystal phase was obtained.After calcination, the crystalline structure of the anatase phase and the characteristic peak for C at 26.2 • were observed in an XRD pattern (Figure 5a).The XPS spectrum exhibits C1s, O1s, N1s, Ti2p, and Cu2p3/2 (substrate) peaks after calcination (Figure 5b).One can observe the formed N-TiO 2 with an average diameter of eight nm and the nicely distributed of the N-carbon dots on N-TiO 2 via TEM (Figure 5c), and the junction between N-carbon and N-TiO 2 (Figure 5d).The N-doping carbon-TiO 2 had a large BET specific surface area of 279.43 m 2 g −1 , a pore volume of 0.5750 cm 3 /g, and a pore area of 96.66 m 2 /g (Figure 6a).N-doping carbon-TiO 2 nanohybrids displayed an absorption in the visible band, while P25 showed the band gap absorption onset at 380 nm (Figure 6b).N-doping carbon-TiO 2 nanohybrids also had high re/oxidative current peaks, compared with that of P25 (Figure 6c).These N-doping carbon-TiO 2 nanohybrids showed excellent photocatalytic ability towards organic (RhB) and inorganic pollutant (K 2 Cr 2 O 7 ) degradations under visible light irradiation (Figure 6d).Using polystyrene spheres as templates and polyaniline as both C and N doping sources, mesoporous C and N co-doped TiO2 hybrid shells were synthesized [59], which exhibited enhanced photocatalytic activity in the degradation of organic dyes and H2 production under visible light irradiation, compared to their undoped counterpart.The co-doped C and N and the presence of graphite carbon were reported to be attributed to the enhanced visible light absorption and charge separation efficiency.They authors observed the synergistic effect in photoelectrochemical reactions exemplified by the concurrent oxidization of perfluorooctanoic acid and hydrogen generation over C-N co-doped TiO2 nanotube arrays as a photoanode [60].Using polystyrene spheres as templates and polyaniline as both C and N doping sources, mesoporous C and N co-doped TiO2 hybrid shells were synthesized [59], which exhibited enhanced photocatalytic activity in the degradation of organic dyes and H2 production under visible light irradiation, compared to their undoped counterpart.The co-doped C and N and the presence of graphite carbon were reported to be attributed to the enhanced visible light absorption and charge separation efficiency.They authors observed the synergistic effect in photoelectrochemical reactions exemplified by the concurrent oxidization of perfluorooctanoic acid and hydrogen generation over C-N co-doped TiO2 nanotube arrays as a photoanode [60].Using polystyrene spheres as templates and polyaniline as both C and N doping sources, mesoporous C and N co-doped TiO 2 hybrid shells were synthesized [59], which exhibited enhanced photocatalytic activity in the degradation of organic dyes and H 2 production under visible light irradiation, compared to their undoped counterpart.The co-doped C and N and the presence of graphite carbon were reported to be attributed to the enhanced visible light absorption and charge separation efficiency.They authors observed the synergistic effect in photoelectrochemical reactions exemplified by the concurrent oxidization of perfluorooctanoic acid and hydrogen generation over C-N co-doped TiO 2 nanotube arrays as a photoanode [60].
By a thermal treatment of titanium substrates embedded in aluminosilicates or alumina particles, C-N co-doped TiO 2 thin film photoanodes were prepared at 500 • C for three hours [61].The as-prepared C-N co-doped TiO 2 photoanodes showed a significant water oxidation with the high photo currents of about 100 mu A cm −2 at zero bias when using alumina as the reactant medium under a standard one SUN illumination of a solar simulator.This photoanode showed a stable photocurrent for about 10 h in the long-term stability test, strong absorption to the visible light region, and improved electron donor density.An in situ carbonization method was also reported to prepare worm-like C-N co-doped TiO 2 nanocrystallites with high surface areas for efficient photocatalysis using tetrakis (dimethylamino) titanium as both titanium oxide and nitrogen-dopant sources, and the mesoporous carbon molecular sieves CMK-3 as a hard template as well as carbon doping source [62].The carbon and nitrogen elements were proven to be responsible for the enhanced visible light-driven photocatalytic activity.

Photocatalysts of Metal-C Doped TiO 2 Composites
Metal-C doped TiO 2 composites were reported to improve their photocatalytic activity due to the coupling effect of the incorporation of carbon species and metal nanoparticles.The eggshell membrane templating method was developed to synthesize porous organic carbon-doped titania (C-TiO 2 ) nanomaterials and their composites with Ag nanoparticles (Ag/C-TiO 2 ) [63].The doped organic carbon was composed of the active carbon and carbonate species, forming a layer around the surface of TiO 2 nanoparticles.In the degradation of methylene blue under visible light irradiation, both C-TiO 2 and Ag/C-TiO 2 nanomaterials showed higher photocatalytic activity compared with the pure TiO 2 material, commercial Degussa P25.These results can be assigned to the coupling effect of the incorporation of carbon species and Ag nanoparticles.Ag/C-TiO 2 nanomaterials were also prepared by a one-step synthetic approach through the thermal oxidation of an AgNO 3 -impregnated TiC precursor [64].The loaded silver nanoparticles were uniformly distributed on the TiO 2 surface in the Ag 0 metallic state, further enhancing the visible light absorption due to the surface plasmon resonance, yet inhibiting the charge carrier recombination by conduction band electron trapping.The authors proposed the band structure and a tentative visible light photocatalytic disinfection mechanism of Ag/C-TiO 2 with four pathways.They demonstrated the enhanced visible light photocatalytic performance for the disinfection of E. coli and E. fecalis in comparison with C-TiO 2 .
Mo/C co-doped TiO 2 powders were reported by thermal oxidizing a mixture of TiC and MoO 3 powders in the air [65].A 32-nm red shift of the spectrum in the visible absorption onset for these materials was observed.Mo/C co-doped TiO 2 displayed enhanced photocatalytic activity in the decomposition of methylene blue, and the photocurrent was about four times as high as that of pure anatase TiO 2 .A calcination-assisted solvothermal reaction was used to prepare the Nd and C co-doped TiO 2 (Nd-C-TiO 2 ) without the addition of any other C precursors [66].The as-prepared sample had a dramatically improved visible absorption, covering the UV and whole visible light range from 200 to 900 nm.Nd-C-TiO 2 was reported to represent the excellent destruction ability of NOx gas and degradation activity of methyl orange solution under visible light irradiation, which was superior to pure TiO 2 , C-TiO 2 , and Nd-TiO 2 .
A hydrothermal approach was reported to synthesize C-Fe co-doped TiO 2 sheets with dominant {001} facets using TiC and Fe 2 O 3 as the reactants in an HNO 3 -HF mixed solution [67], resulting in the aggregates of highly truncated bipyramidal microsheets with the length of one to two µm and the thickness of 100-200 nm (Figure 7).The incorporation of Fe into anatase TiO 2 did not affect the growth of TiO 2 microsheets.The corresponding SAED pattern demonstrated the flat and square surface belonging to the (001) plane of anatase TiO 2 (Figure 7a).One can find the (020) and (200) atomic planes with a lattice spacing of 0.19 nm and a vertical angle (Figure 7b).These composites exhibited the narrow band gap, strong visible light absorption, and high separation efficiency of photogenerated carriers, compared with the C-doped TiO 2 sheets.The authors suggested that these materials might become promising visible light photocatalysts for organic pollutant degradation and efficient H 2 evolution.The synergistic effect of C-Ag co-doped TiO2 photocatalysts was observed within the GGA plus U framework [68].C and Ag co-doping could induce a synergistic effect, compared with single C and single Ag doping.After co-doping, the visible absorption was stronger for both the near and far distance co-doped systems than those of the single-doped systems.First-principle calculation was applied to estimate all the band structures, density of states, and absorption spectra of pure, Nddoped, C-doped, and Nd-C co-doped TiO2 based on the density functional theory [69].Recently, Ag and Cu co-doped TiO2 deposited on polyurethane was synthesized for the conversion of TiO2 into solar fuels under visible light irradiation [70].The as-synthesized materials exhibited very high photocatalytic activity for the reduction of gaseous CO2 to produce CH4 and CO fuels.The Ag and Cu dopants also enhanced the separation of the electron-hole pairs of the doped TiO2 photocatalysts, exhibiting the highest photocatalytic reduction of CO2.A silver/carbon co-doped titanium dioxide photocatalyst was also fabricated for improving dye degradation under visible light irradiation through the hydrolysis of titanium tetrachloride followed by calcination at 500 o C [71].The co-doping of TiO2 with Ag and carbon resulted in an increase in the surface area of the photocatalyst and altered the ratio of anatase to the rutile phase.In comparison with undoped TiO2, doping TiO2 using carbon showed higher photocatalytic activity toward methyl orange degradation under visible light irradiation.

Photocatalysts of Other Co-Doped C/TiO2 Composites
The synergetic effect is one of the key factors improving the photocatalytic efficiency of co-doped C/TiO2 composites.Xu and Zhang demonstrated the controlled synthesis of C-Cl co-doped anatase TiO2 nanocrystals with mesoporous structures and high surface areas between 158.1-469.6 m 2 g −1 by the hydrolysis of titanium tetraisopropoxide in chloroform containing trace amounts of water at a low temperature of 150 °C without a subsequent thermal procedure [72].C-Cl co-doped anatase TiO2 exhibited higher photoactivity in the photodegradation of RhB under visible light irradiation, compared with C-TiO2 and Cl-TiO2, which was attributed to the larger surface area and the synergetic effect of the co-doping.Hierarchically meso/nanoporous S and C co-doping TiO2 was fabricated by a hydrolysis and calcination method using Ti(OC4H9)4 and K2S2O8 as precursors [73].The S and C co-doped TiO2 was found to exhibit high photocatalytic H2 generation efficiency in a water/methanol sacrificial reagent system under the irradiation of UV light.Experimental results indicated that the high photocatalytic efficiency is dependent on the comprehensively competing effects of the co-doping of S and C, crystallization, specific surface area, and light absorption capability.
The synthesis of S and C co-doped hierarchically meso/nanoporous TiO2 with excellent thermal stability and photocatalytic activity was reported by Xu et al. [74].The sulfur nanoparticles in the The synergistic effect of C-Ag co-doped TiO 2 photocatalysts was observed within the GGA plus U framework [68].C and Ag co-doping could induce a synergistic effect, compared with single C and single Ag doping.After co-doping, the visible absorption was stronger for both the near and far distance co-doped systems than those of the single-doped systems.First-principle calculation was applied to estimate all the band structures, density of states, and absorption spectra of pure, Nd-doped, C-doped, and Nd-C co-doped TiO 2 based on the density functional theory [69].Recently, Ag and Cu co-doped TiO 2 deposited on polyurethane was synthesized for the conversion of TiO 2 into solar fuels under visible light irradiation [70].The as-synthesized materials exhibited very high photocatalytic activity for the reduction of gaseous CO 2 to produce CH 4 and CO fuels.The Ag and Cu dopants also enhanced the separation of the electron-hole pairs of the doped TiO 2 photocatalysts, exhibiting the highest photocatalytic reduction of CO 2 .A silver/carbon co-doped titanium dioxide photocatalyst was also fabricated for improving dye degradation under visible light irradiation through the hydrolysis of titanium tetrachloride followed by calcination at 500 • C [71].The co-doping of TiO 2 with Ag and carbon resulted in an increase in the surface area of the photocatalyst and altered the ratio of anatase to the rutile phase.In comparison with undoped TiO 2 , doping TiO 2 using carbon showed higher photocatalytic activity toward methyl orange degradation under visible light irradiation.

Photocatalysts of Other Co-Doped C/TiO 2 Composites
The synergetic effect is one of the key factors improving the photocatalytic efficiency of co-doped C/TiO 2 composites.Xu and Zhang demonstrated the controlled synthesis of C-Cl co-doped anatase TiO 2 nanocrystals with mesoporous structures and high surface areas between 158.1-469.6 m 2 g −1 by the hydrolysis of titanium tetraisopropoxide in chloroform containing trace amounts of water at a low temperature of 150 • C without a subsequent thermal procedure [72].C-Cl co-doped anatase TiO 2 exhibited higher photoactivity in the photodegradation of RhB under visible light irradiation, compared with C-TiO 2 and Cl-TiO 2 , which was attributed to the larger surface area and the synergetic effect of the co-doping.Hierarchically meso/nanoporous S and C co-doping TiO 2 was fabricated by a hydrolysis and calcination method using Ti(OC 4 H 9 ) 4 and K 2 S 2 O 8 as precursors [73].The S and C co-doped TiO 2 was found to exhibit high photocatalytic H 2 generation efficiency in a water/methanol sacrificial reagent system under the irradiation of UV light.Experimental results indicated that the high photocatalytic efficiency is dependent on the comprehensively competing effects of the co-doping of S and C, crystallization, specific surface area, and light absorption capability.
The synthesis of S and C co-doped hierarchically meso/nanoporous TiO 2 with excellent thermal stability and photocatalytic activity was reported by Xu et al. [74].The sulfur nanoparticles in the sulfur hydrosol were applied to serve as the seeds for the formation of bimodal meso/nanopores TiO 2 upon calcination treatment.The authors observed remarkably enhanced photocatalytic activity and excellent cyclic stability on the decomposition of methylene blue in the visible light region, which was attributed to the numerous oxygen vacancies, acidic sites on the surface of TiO 2 , and large specific surface area.The sol-gel method combining with high-energy ball milling was employed to synthesize the (N, S, and C) co-doped TiO 2 calcined at the different temperatures (400-700 • C), employing butyl titanate as the titanium source and thiourea as the doping agent [75].The (N, S, and C) co-doping and the calcination temperature played an important role in the microstructure and photocatalytic activity of the samples, judging from the photocatalytic reduction of Cr(VI) as a model compound under visible light irradiation.As for the control of TiO 2-500 , it did not cause the concentration decrease of Cr(VI) in aqueous solution upon visible light irradiation for 50 minutes (Figure 8a).In the presence of (N, S, and C) co-doped TiO 2-x samples, a quick concentration decrease of Cr(VI) was observed.Moreover, the concentration of Cr(VI) decreased with the increase of calcination temperature from 500 • C to 700 • C. It was found that the decrease in the visible absorption, the separation rate of the electron-hole pairs, the specific surface area, and the increase in crystal size for (N, S, and C) co-doped TiO 2-x samples resulted in the decrease in photocatalytic performance (Figure 8b).sulfur hydrosol were applied to serve as the seeds for the formation of bimodal meso/nanopores TiO2 upon calcination treatment.The authors observed remarkably enhanced photocatalytic activity and excellent cyclic stability on the decomposition of methylene blue in the visible light region, which was attributed to the numerous oxygen vacancies, acidic sites on the surface of TiO2, and large specific surface area.The sol-gel method combining with high-energy ball milling was employed to synthesize the (N, S, and C) co-doped TiO2 calcined at the different temperatures (400-700 o C), employing butyl titanate as the titanium source and thiourea as the doping agent [75].The (N, S, and C) co-doping and the calcination temperature played an important role in the microstructure and photocatalytic activity of the samples, judging from the photocatalytic reduction of Cr(VI) as a model compound under visible light irradiation.As for the control of TiO2-500, it did not cause the concentration decrease of Cr(VI) in aqueous solution upon visible light irradiation for 50 minutes (Figure 8a).In the presence of (N, S, and C) co-doped TiO2-x samples, a quick concentration decrease of Cr(VI) was observed.Moreover, the concentration of Cr(VI) decreased quickly with the increase of calcination temperature from 500 •C to 700 •C.It was found that the decrease in the visible absorption, the separation rate of the electron-hole pairs, the specific surface area, and the increase in crystal size for (N, S, and C) co-doped TiO2-x samples resulted in the decrease in photocatalytic performance (Figure 8b).

Photocatalysts of C/g-C3N4 Composites
Graphitic carbon nitride (g-C3N4) is a research hotspot material as a promising photocatalyst due to its good photocatalytic performance and unique electronic structure.A sonochemical method was applied to fabricate the mesoporous g-C3N4/graphene and mesoporous g-C3N4/ graphene oxide nanocomposites with high visible light photocatalytic activity (NO removal ratios of 64.9% and 60.7%, respectively) [76].The authors suggested that this photocatalytic performance could be

Photocatalysts of C/g-C 3 N 4 Composites
Graphitic carbon nitride (g-C 3 N 4 ) is a research hotspot material as a promising photocatalyst due to its good photocatalytic performance and unique electronic structure.A sonochemical method was applied to fabricate the mesoporous g-C 3 N 4 /graphene and mesoporous g-C 3 N 4 / graphene oxide nanocomposites with high visible light photocatalytic activity (NO removal ratios of 64.9% and 60.7%, respectively) [76].The authors suggested that this photocatalytic performance could be attributed to the high surface area and pore volume, improved visible light utilization, enhanced reduction power of electrons, and promoted separation of charge carriers.In Shi's work, a thermal treatment was also developed during the preparation of the ordered mesoporous carbon/g-C 3 N 4 composites with efficient photocatalytic activity under visible light irradiation [77], which exhibited very high photocatalytic activities in degrading RhB dye, and the high degradation rate was almost 10 times that of pristine g-C 3 N 4 , which was partly due to the ordered mesoporous carbon.After five cycles, these composites retained good stability and photodegradation efficiency.The authors suggested that the combined effects came from the enhanced visible light absorption, enriched the dye adsorption on the catalyst, and subsequent efficient separation of the photogenerated electrons and holes.Recently, an in situ synthesis of C-doped TiO 2 @g-C 3 N 4 core-shell hollow nanospheres with remarkably enhanced visible light photocatalytic activity for water splitting to produce H 2 were reported [78], exhibiting the H2 generation rate of 35.6 mu mol g −1 h −1 .The heterojunction, in situ growth, and unique hollow structure favored the enhanced photocatalytic performance.
Hydrothermal treatment was also applied to synthesize the C/g-C 3 N 4 composites with a mixture of g-C 3 N 4 and glucose solution [79].The authors discovered that carbon doping by glucose is an efficient way to improve the photocatalytic performance of g-C 3 N 4 due to the synergistic effect between carbon and g-C 3 N 4 , leading to increased visible light absorption, enhanced absorptivity, and improved photoinduced electron-hole separation efficiency.A 97% degradation of Methylene blue (MB) by C2/g-C 3 N 4 composite was observed within 2.0 hours under visible light irradiation, while the pure g-C 3 N 4 only caused a 73% degradation of MB (Figure 9).The photocatalytic activity of C/g-C 3 N 4 composites increased with the weight of the glucose precursor from 0.0025 g to 0.005 g.By adding a small amount of citric acid into urea as the precursor, N-doped graphitic carbon-incorporated g-C 3 N 4 could be synthesized during thermal polymerization [80].The as-obtained materials retained the original framework of g-C 3 N 4 , and showed remarkably enhanced visible light harvesting, and promoted photoexcited charge carrier separation and transfer.They demonstrated an H 2 evolution rate of 64 mu mol h -1 on N-doped graphitic carbon-incorporated g-C 3 N 4 under visible light irradiation.This polymerized method was also employed to obtain the visible light-driven N-doped carbon dots/g-C 3 N 4 photocatalyst composite with enhanced visible light photocatalytic activity for the degradation of indomethacin [81].This enhanced photocatalytic activity might be ascribed to the unique up-converted Photoluminescence (PL) properties, efficient charge separation, as well as band gap narrowing of the N-doped carbon dots.The authors indicated that this composite possesses a very desirable performance for the toxicity reduction and mineralization of indomethacin under long exposures of visible light irradiation.attributed to the high surface area and pore volume, improved visible light utilization, enhanced reduction power of electrons, and promoted separation of charge carriers.In Shi's work, a thermal treatment was also developed during the preparation of the ordered mesoporous carbon/g-C3N4 composites with efficient photocatalytic activity under visible light irradiation [77], which exhibited very high photocatalytic activities in degrading RhB dye, and the high degradation rate was almost 10 times that of pristine g-C3N4, which was partly due to the ordered mesoporous carbon.After five cycles, these composites retained good stability and photodegradation efficiency.The authors suggested that the combined effects came from the enhanced visible light absorption, enriched the dye adsorption on the catalyst, and subsequent efficient separation of the photogenerated electrons and holes.Recently, an in situ synthesis of C-doped TiO2@g-C3N4 core-shell hollow nanospheres with remarkably enhanced visible light photocatalytic activity for water splitting to produce H2 were reported [78], exhibiting the H2 generation rate of 35.6 mu mol g −1 h −1 .The heterojunction, in situ growth, and unique hollow structure favored the enhanced photocatalytic performance.
Hydrothermal treatment was also applied to synthesize the C/g-C3N4 composites with a mixture of g-C3N4 and glucose solution [79].The authors discovered that carbon doping by glucose is an efficient way to improve the photocatalytic performance of g-C3N4 due to the synergistic effect between carbon and g-C3N4, leading to increased visible light absorption, enhanced absorptivity, and improved photoinduced electron-hole separation efficiency.A 97% degradation of Methylene blue (MB) by C2/g-C3N4 composite was observed within 2.0 hours under visible light irradiation, while the pure g-C3N4 only caused a 73% degradation of MB (Figure 9).The photocatalytic activity of C/g-C3N4 composites increased with the weight of the glucose precursor from 0.0025 g to 0.005 g.By adding a small amount of citric acid into urea as the precursor, N-doped graphitic carbonincorporated g-C3N4 could be synthesized during thermal polymerization [80].The as-obtained materials retained the original framework of g-C3N4, and showed remarkably enhanced visible light harvesting, and promoted photoexcited charge carrier separation and transfer.They demonstrated an H2 evolution rate of 64 mu mol h -1 on N-doped graphitic carbon-incorporated g-C3N4 under visible light irradiation.This polymerized method was also employed to obtain the visible light-driven Ndoped carbon dots/g-C3N4 photocatalyst composite with enhanced visible light photocatalytic activity for the degradation of indomethacin [81].This enhanced photocatalytic activity might be ascribed to the unique up-converted Photoluminescence (PL) properties, efficient charge separation, as well as band gap narrowing of the N-doped carbon dots.The authors indicated that this composite possesses a very desirable performance for the toxicity reduction and mineralization of indomethacin under long exposures of visible light irradiation.The precipitation method was introduced to fabricate robust g-C3N4 nanosheets/carbons/NiS multiple heterojunctions for photocatalytic H2 generation [82].In this work, the authors used the The precipitation method was introduced to fabricate robust g-C 3 N 4 nanosheets/carbons/NiS multiple heterojunctions for photocatalytic H 2 generation [82].In this work, the authors used the nanocarbons as effective H 2 -evolution co-catalysts and conductive electron bridges to collect photogenerated electrons, resulting in the downshift of the valence band of g-C 3 N 4 and facilitating the fast oxidation of triethanolamine and charge carrier separation.They also discovered that the amorphous carbons with high electrical conductivity and weak electrocatalytic H 2 -evolution activity are suitable interfacial bridges between g-C 3 N 4 and NiS co-catalysts for maximizing H 2 generation.More recently, a thermal treatment method was employed to synthesize a metal organic framework-derived, nitrogen-doped, carbon-modified g-C 3 N 4 heterostructure composite with enhanced photocatalytic activity for bisphenol A degradation with peroxymonosulfate under visible light irradiation [83], which had the hierarchical porosity, conductive network, and abundant exposed active sites for peroxymonosulfate activation.In this research, the as-prepared heterostructure composite exhibited remarkably improved photocatalytic activities toward bisphenol A degradation in the presence of peroxymonosulfate under visible light irradiation, displaying approximately 8.6 times the apparent rate constant of bare g-C 3 N 4 .In general, all these examples mentioned above demonstrated that the carbon doping is an efficient strategy to improve the photocatalytic property of g-C 3 N 4 and that a synergistic effect exists between carbon and g-C 3 N 4 .

Photocatalysts of Ternary C-Doped Composites
Similar to metal C-doped TiO 2 composites, ternary C-doped composites also displayed the synergistic effect from the ternary interfaces.Shen et al. synthesized the magnetic Fe 3 O 4 @C @Cu 2 O composites with a bean-like core/shell nanostructure by a self-assembly approach [84].They obtained as-prepared magnetic composites with a size of about 420 nm and shells that were composed of several nanoparticles in the range of five to 10 nm.The carbonaceous layer with bound hydrophilic groups was reported to inherit from the starting materials, acting as both the linker and the stabilizer between Fe 3 O 4 and Cu 2 O.The Fe 3 O 4 @C@Cu 2 O composites were found to exhibit ferromagnetic behavior, good dispersibility in aqueous solution, and universal and powerful visible light photocatalytic activity for the degradation of RhB, methyl orange (MO), and alizarin red (AR), which was superior to commercial Cu 2 O and Degussa P-25 powders.They suggested that these novel magnetic composites might have applications in dye solution treatment, the degradation of organic pollutants, and environmental cleaning.
The multi-step process of hydrothermal reaction and microwave irradiation was used to synthesize one-dimensional coaxial nanostructures composed of ZnO nanorod cores, intermediate amorphous carbonaceous layers, and CdS nanoparticle sheaths (i.e.ZnO/C/CdS nanocables) [85].A single coaxial ZnO/C/CdS nanocable was obtained, in which ZnO nanoparticles were encapsulated by an amorphous carbonaceous layer with the thickness of about five nm, and CdS nanoparticles with the size of about 5.5 nm were uniformly deposited on the surface of the carbon layer.In addition, the thickness of the amorphous carbonaceous layer was reported to be easily tuned from three to 10 nm by controlling the amount of glucose from 0.1 to 0.3 g.As for the high concentration of Cd 2+ , more CdS nanoparticles were anchored onto the amorphous carbonaceous layer, and the size of the CdS nanoparticles became larger with the increasing of the Cd 2+ concentration.The thickness of the carbonaceous layer and the amount of CdS nanoparticles can be tuned by controlling the glucose intake and the cadmium salt concentration, respectively.The as-prepared ZnO/C/CdS nanocables exhibited much higher activity than that of pure ZnO nanorods, pure CdS, and ZnO/C core-shell nanocables for the degradation of RhB and methylene blue (MB) under visible light irradiation, which was due to stronger adsorption and the synergistic effect from the ternary ZnO/C/CdS interface.
In the previous literature, Li et al. also found the synergistic effect in sandwiched TiO 2 @Au@C hollow spheres.The hydrothermal reaction and calcination were applied to the fabrication of uniform double-shelled and sandwiched TiO 2 @Au@C hollow and porous spheres [86].An HR-TEM image of the nanocomposite showed the (101) crystallographic planes of TiO 2 , the (111) plane of Au nanoparticles, and the coated carbon layer with a thickness of ca. 10 nm in Figure 10b,c displayed the presence of individual nanoparticles on a thick and layered shell.It was found that these microspheres displayed high photocatalytic activity and photocatalytic stability for the degradation of 4-nitroaniline, and the remarkable photocatalytic activity for hydrogen generation from methanol/water solutions.TiO 2 @Au@C achieved a visible light photodegradation of 92.65% for 4-nitroaniline, while P25 only displayed a visible light photodegradation of 17.61% of 4-nitroaniline (Figure 11).It was also observed that the photodegradation reactions followed a pseudo-first-order kinetics with the rate constants (0.0012 min −1 , 0.0004 min −1 , 0.0050 min −1 , 0.0046 min −1 , and 0.015 min −1 ) for 4-nitroaniline and the rate constants (0.0009 min −1 , 0.0014 min −1 , 0.0036 min −1 , 0.0033 min −1 , and 0.081 min −1 ) for 4-nitrophenol by P25, hollow TiO 2 , hollow TiO 2 @C, hollow TiO 2 /Au, and double-shelled and sandwiched TiO 2 @Au@C hollow spheres, respectively, under visible light irradiation.They proposed the synergistic effect of coupling TiO 2 hollow spheres with Au nanoparticles and a C shell in photocatalytic performance.

Photocatalysts of Other C-doped Composites
The semiconductor ZnO has been attracting considerable attention because of its large exciton binding energy (60 meV) and possible applications to UV light emitters, gas sensors, transparent electronics, photocatalysis, solar cell, environmental remediation, and surface acoustic wave devices.Novel ZnO@C hybrid composites were obtained using a metal organic framework as a precursor [87].The resultant ZnO hierarchical aggregates with a three-dimensional cubic morphology and ZnO@C hybrid composites within a highly porous carbonaceous species were obtained.The hybrid composites showed relatively high photocatalytic decomposition activity and significantly enhanced methanol/water solutions.TiO2@Au@C achieved a visible light photodegradation of 92.65% for 4nitroaniline, while P25 only displayed a visible light photodegradation of 17.61% of 4-nitroaniline (Figure 11).It was also observed that the photodegradation reactions followed a pseudo-first-order kinetics with the rate constants (0.0012 min −1 , 0.0004 min −1 , 0.0050 min −1 , 0.0046 min −1 , and 0.015 min −1 ) for 4-nitroaniline and the rate constants (0.0009 min −1 , 0.0014 min −1 , 0.0036 min −1 , 0.0033 min −1 , and 0.081 min −1 ) for 4-nitrophenol by P25, hollow TiO2, hollow TiO2@C, hollow TiO2/Au, and doubleshelled and sandwiched TiO2@Au@C hollow spheres, respectively, under visible light irradiation.They proposed the synergistic effect of coupling TiO2 hollow spheres with Au nanoparticles and a C shell in photocatalytic performance.

Photocatalysts of Other C-doped Composites
The semiconductor ZnO has been attracting considerable attention because of its large exciton binding energy (60 meV) and possible applications to UV light emitters, gas sensors, transparent electronics, photocatalysis, solar cell, environmental remediation, and surface acoustic wave devices.Novel ZnO@C hybrid composites were obtained using a metal organic framework as a precursor [87].The resultant ZnO hierarchical aggregates with a three-dimensional cubic morphology and ZnO@C hybrid composites within a highly porous carbonaceous species were obtained.The hybrid composites showed relatively high photocatalytic decomposition activity and significantly enhanced

Photocatalysts of Other C-doped Composites
The semiconductor ZnO has been attracting considerable attention because of its large exciton binding energy (60 meV) and possible applications to UV light emitters, gas sensors, transparent electronics, photocatalysis, solar cell, environmental remediation, and surface acoustic wave devices.Novel ZnO@C hybrid composites were obtained using a metal organic framework as a precursor [87].The resultant ZnO hierarchical aggregates with a three-dimensional cubic morphology and ZnO@C hybrid composites within a highly porous carbonaceous species were obtained.The hybrid composites showed relatively high photocatalytic decomposition activity and significantly enhanced adsorption capacities for organic pollutants.The hydrothermal method was reported for the synthesis of ZnO@C coaxial gemel hexagonal microrods with a thin carbon layer on their surface by using furfural as the carbon precursor, and they showed high photocatalytic activity and stability [88].The thickness of the surface carbon layer was found to increase with the hydrothermal time until 16 h.The carbon layer was reported to play a significantly important role in the photocatalytic activities and photostabilities of the ZnO@C microrods for the UV-irradiated photodegradation of methylene blue solution.Zhou et al. described the deposition of carbon on the surface of ZnO nanoparticles via an adsorption and calcination process [89].The as-prepared ZnO/C exhibited much higher photocatalytic properties and adsorption capacity for MB, compared with pure ZnO and carbon, because of the formation of heteroarchitectures, which was tested by photocatalytic decomposition and adsorption of the dye methylene blue in aqueous solution.ZnO/C showed a pseudo-first-order rate constant of 0.326 min −1 , which was 10 times larger than that of pure ZnO.
By combining the electrospinning technique and thermal decomposition process, C-doped ZnO nanofibers with high visible light photocatalytic activity were fabricated using the precursor solution consisting of polyacrylonitrile and zinc acetate [90].The C-doped ZnO nanofibers possessed a much higher photocatalytic degradation rate of RhB compared with the pure ZnO nanofibers under visible light irradiation, which was attributed to the formation of the new energy states in ZnO because of the carbon doping, which might reduce the band gap of the ZnO.By the solvothermal treatment of zinc acetate in an ethylene glycol-ethanol mixture, C-doped ZnO ball-in-ball hollow microspheres were fabricated for photocatalytic and photoelectrochemical applications [91].The C-doped ZnO ball-in-ball hollow microspheres displayed higher photocatalytic active than ZnO particles by the photodegradation of phenol and MO (Figure 12a).The hollow microspheres exhibited ca.8.9-fold and 10.5-fold higher activity in comparison with pristine ZnO nanoparticles as the photocatalyst for organic pollutant degradation and as a photoanode material for photoelectrochemical water splitting, respectively (Figure 12a).The C-doped ZnO hollow microspheres also showed stable performance in recycling reactions (Figure 12b).As for the photocurrent-voltage (I-V) curves, C-doped ZnO displayed an obviously high photocurrent (Figure 12c), and an about 10.5 times higher transient photocurrent than ZnO nanoparticles (Figure 12d).
Catalysts 2018, 8, x FOR PEER REVIEW 15 of 21 adsorption capacities for organic pollutants.The hydrothermal method was reported for the synthesis of ZnO@C coaxial gemel hexagonal microrods with a thin carbon layer on their surface by using furfural as the carbon precursor, and they showed high photocatalytic activity and stability [88].The thickness of the surface carbon layer was found to increase with the hydrothermal time until 16 h.The carbon layer was reported to play a significantly important role in the photocatalytic activities and photostabilities of the ZnO@C microrods for the UV-irradiated photodegradation of methylene blue solution.Zhou et al. described the deposition of carbon on the surface of ZnO nanoparticles via an adsorption and calcination process [89].The as-prepared ZnO/C exhibited much higher photocatalytic properties and adsorption capacity for MB, compared with pure ZnO and carbon, because of the formation of heteroarchitectures, which was tested by photocatalytic decomposition and adsorption of the dye methylene blue in aqueous solution.ZnO/C showed a pseudo-first-order rate constant of 0.326 min −1 , which was 10 times larger than that of pure ZnO.By combining the electrospinning technique and thermal decomposition process, C-doped ZnO nanofibers with high visible light photocatalytic activity were fabricated using the precursor solution consisting of polyacrylonitrile and zinc acetate [90].The C-doped ZnO nanofibers possessed a much higher photocatalytic degradation rate of RhB compared with the pure ZnO nanofibers under visible light irradiation, which was attributed to the formation of the new energy states in ZnO because of the carbon doping, which might reduce the band gap of the ZnO.By the solvothermal treatment of zinc acetate in an ethylene glycol-ethanol mixture, C-doped ZnO ball-in-ball hollow microspheres were fabricated for photocatalytic and photoelectrochemical applications [91].The C-doped ZnO ball-in-ball hollow microspheres displayed higher photocatalytic active than ZnO particles by the photodegradation of phenol and MO (Figure 12a).The hollow microspheres exhibited ca.8.9-fold and 10.5-fold higher activity in comparison with pristine ZnO nanoparticles as the photocatalyst for organic pollutant degradation and as a photoanode material for photoelectrochemical water splitting, respectively (Figure 12a).The C-doped ZnO hollow microspheres also showed stable performance in recycling reactions (Figure 12b).As for the photocurrent-voltage (I-V) curves, C-doped ZnO displayed an obviously high photocurrent (Figure 12c), and an about 10.5 times higher transient photocurrent than ZnO nanoparticles (Figure 12d).The two-step hydrothermal method was reported to prepare Bi 2 WO 6 -coated carbon microspheres using glucose as a carbon source [92].By the photocatalytic degradation of gas-phase benzene under visible light irradiation, it was revealed that the photocatalytic performance of the Bi 2 WO 6 was greatly enhanced by the introduction of carbon microspheres.The Bi 2 WO 6 @C microspheres displayed a photocatalytic benzene conversion of 42.6% and benzene mineralization of 80.0%, respectively.As a direct wide band-gap semiconductor, ZnS nanostructure displayed unique properties, including excellent electrical transport properties, good thermal stability, and high electronic mobility.The density functional theory calculations were used to investigate the origin of the experimentally observed changes in the visible light responsive photoactivity of hexagonal wurtzite ZnS induced by (N, C) co-doping [93].They performed the accurate comparative analyses of geometric and electronic structures for the different doping models.For the mono N-doped or C-doped ZnS systems, the substituted doping was found to just induce a slight band gap narrowing (less than 0.2 eV), while the interstitial doping led to a more significant optical absorption red shift.For the N-C co-doped systems, the electron transition from the impurity states in the band gap to the conduction band will induce a large red shift, suggesting visible light absorption.The calculated results of 2N-C co-doped ZnS indicated that the trimer doping induces a larger red shift of photoelectron transition and passivates the partially occupied states by the charge compensation effect in a donor-acceptor pair.This work provided a solid basis for the rationalization of the experimentally observed red shift of optical absorption in wurtzite ZnS as a consequence of (N, C) co-doping.Recently, a hydrothermal method was developed to synthesize well-dispersed carbon-doped ZnSn(OH) 6 submicrocube photocatalysts at 433 K using glucose as the carbon-doping source [94], which exhibited higher photocatalytic performance by the decomposition of methylene blue in aqueous solution under visible light irradiation, compared with pure ZnSn(OH) 6 .

Conclusions and Future Perspectives
In this review, we summarized the recent advances on the synthesis, photocatalytic properties, and potential applications of photocatalysts based on carbon-based (nano)composites.According to the results from represented examples, these carbon-based (nano)composite photocatalysts exhibited improved photocatalytic properties compared to their individual components.It is worth noting that the synergistic effect is one of the key factors responsible for their photocatalytic performance.Based on our knowledge, some future research trends and directions can be expected as follows.
(1) The exploitation of the precursor sources is important for the properties and applications of carbon-based (nano)composites.Various raw materials should be developed as carbon sources.Obviously, different types of biomass such as wood, grass, and bamboo are promising precursor sources of carbon, to which researchers should pay more attention.(2) There have been many examples indicating that the as-prepared carbon-based (nano)composites exhibited improved photocatalytic activity compared with P25 and their individual components.Then, we need to focus on the practical applications of these carbon-based (nano)composites prepared in labs.From an application point of view, it is not enough to obtain carbon-based (nano)composite photocatalysts with only high photocatalytic activity.We should also consider other factors such as the raw materials, production cost, production cycle, yield, and so on.Photocatalytic activity is a key and important factor, but it is not the only factor.It is also important to develop environmentally friendly methods for the large-scale and industrial-scale production of carbon-based nanocomposite photocatalysts at low cost and with high efficiency.(3) There are many examples mentioning the synergistic effect of the incorporation of carbon species in the carbon-based nanocomposite.This effect still needs to be scrutinized in depth, e.g., by using theoretical modeling and computer simulation.Moreover, the relationships between the preparation, structures, properties, and applications of carbon-based nanocomposites need to be dissected in detail.We believe that more and more rapid progresses will be made in the research of carbon-based nanocomposite photocatalysts, since they have broad potential applications in the environmental treatment and renewable energy fields.

Figure 2 .
Figure 2. Schematic illustrations for the interfacial charge transfer effect in TiO2/graphene composites: (a) the chemical bonding between TiO2 nanoparticles and graphene nanosheets; (b) the reported energy level of the valence band (VB) and conduction band (CB) vs. NHE (normal hydrogen electrode).From Ref. [35].Reprinted with permission from ACS.

Figure 2 .
Figure 2. Schematic illustrations for the interfacial charge transfer effect in TiO2/graphene composites: (a) the chemical bonding between TiO2 nanoparticles and graphene nanosheets; (b) the reported energy level of the valence band (VB) and conduction band (CB) vs. NHE (normal hydrogen electrode).From Ref. [35].Reprinted with permission from ACS.

Figure 2 .
Figure 2. Schematic illustrations for the interfacial charge transfer effect in TiO 2 /graphene composites: (a) the chemical bonding between TiO 2 nanoparticles and graphene nanosheets; (b) the reported energy level of the valence band (VB) and conduction band (CB) vs. NHE (normal hydrogen electrode).From Ref.[35].Reprinted with permission from ACS.
Liu et al. fabricated visible light-active C/TiO 2 composites by the calcination method using C 16 H 36 O 4 Ti as the titanium source and bamboo powder with different sizes as the carbon source [51].

Figure 8 .
Figure 8.(a) Variation of Cr(VI) concentration (pH 1.5 and C0 = 10 mg/L) with irradiation time in the presence of TiO2-500 and (N, S, and C) co-doped TiO2-x samples under visible light (λ >420 nm) irradiation, together with the results from the blank experiments (no photocatalyst, but with visible light irradiation); (b) plots of [(C0−Ct)/C0, t = 50 min] in comparison with Eg values, specific surface area, and crystal size for all (N, S, and C) co-doped-TiO2-x samples.From Ref. [75].Reprinted with permission from Elsevier.

Figure 8 .
Figure 8.(a) Variation of Cr(VI) concentration (pH 1.5 and C0 = 10 mg/L) with irradiation time in the presence of TiO 2-500 and (N, S, and C) co-doped TiO 2-x samples under visible light (λ > 420 nm) irradiation, together with the results from the blank experiments (no photocatalyst, but with visible light irradiation); (b) plots of [(C 0 − C t )/C 0 , t = 50 min] in comparison with Eg values, specific surface area, and crystal size for all (N, S, and C) co-doped-TiO 2-x samples.From Ref. [75].Reprinted with permission from Elsevier.

Figure 12 .
Figure 12.(a) The reaction rate for methyl orange (MO) and phenol photodegradation under UV-VIS irradiation; (b) the recycling of MO photodegradation using ZEG-12; (c) photocurrent-voltage (I-V) curves of ZnO in dark and under irradiation; (d) transient photocurrent (I-t plots) at 0.85 V vs. RHE.From Ref. [91].Reprinted with permission from Elsevier.

Figure 12 .
Figure 12.(a) The reaction rate for methyl orange (MO) and phenol photodegradation under UV-VIS irradiation; (b) the recycling of MO photodegradation using ZEG-12; (c) photocurrent-voltage (I-V) curves of ZnO in dark and under irradiation; (d) transient photocurrent (I-t plots) at 0.85 V vs. RHE.From Ref. [91].Reprinted with permission from Elsevier.