Carbon Gels-Modified TiO2: Promising Materials for Photocatalysis Applications

Carbon gels are a kind of porous organic polymer, which play pivotal roles in electrode, supercapacitor, hydrogen storage, and catalysis. Carbon gels are commonly prepared by the condensation of resorcinol and formaldehyde. The as-prepared polymers are further aged and sintered at a high temperature in an inert atmosphere to form cross-linked and intertwined porous structures. Owing to its large specific area and narrow pore size distribution, this kind of material is very appropriate for mass transfer, substrate absorption, and product desorption from the pores. In recent years, carbon gels have been discovered to function as effective hybrid materials with TiO2 for photocatalytic applications. They could act as efficient deep-traps for photo-induced holes, which decreases the recombination probability of photo-induced carriers and lengthens their lifetime. In this mini-review, we will discuss the state-of-the-art paragon examples of carbon gels/TiO2 composite materials applied in photo(electro)catalysis. The major challenges and gaps of its application in this field will also be emphasized.


Introduction
Nowadays, environmental pollution and energy shortage are two main crises facing the whole world. The excessive utilization of fossil fuels produces various pollutants, which was discharged into water and air environments. There is an urgent need for the use of green and renewable energy resources for human society. Sunlight, considered as an inexhaustible and environmentally friendly energy source, has aroused the attention from both academics and industries. As Fujishima and Honda first reported TiO 2 could be used to catalyze the water-splitting process evolving hydrogen and oxygen under sunlight illumination in 1972 [1], and Carey et al. initially used TiO 2 photocatalysis as an advanced oxidation technique for the removal of pollutants in aqueous solution [2,3], photocatalysis has been developed rapidly and becomes the focus of various disciplines including catalysis [4][5][6][7], materials chemistry [8][9][10], environmental chemistry [11], energy chemistry [12,13], surface chemistry [14][15][16], and processing chemistry and chemical engineering [17,18]. Among various photocatalysts, TiO 2 being extremely stable under light irradiation, highly acidic, and basic conditions; non-toxic, earth-abundant, and easily-recyclable; and reusable without much loss of activity, has been profoundly investigated from the 1970s up till now [19][20][21][22][23][24][25][26][27]. Moreover, with the high photo-induced hole oxidation potential (E vb + = 2.7 V vs. NHE (Normal Hydrogen Electrode) at pH = 7); the ability to produce similarly highly oxidative OH• radicals (E = 2.8 V vs. NHE), H 2 O 2 , and HOO•; and the appropriate photo-induced Figure 1. A schematic diagram of the reaction of resorcinol and formaldehyde to synthesize carbon aerogel. Copied with permission from [54], Copyrights 1989 Springer [54].

Carbon Gels-Modified TiO2 Photocatalysis
Owing to their large specific surface area, ordered porous structure, and high electric conductivity, carbon gels have been applied in various fields including hydrogen storage, electrode materials, and heterogeneous catalyst support. Although carbon gels are often used as conductive materials, they also can be used to construct a composite semiconductor photocatalyst with TiO2.
In 2010, Zhao and co-workers firstly used carbon gels to modify the TiO2 photocatalyst to improve its adsorption and conductivity performance [84]. They prepared a TiO2/carbon aerogel (CA) photoelectrode material. The CA was synthesized by the base-catalyzed polycondensation between resorcinol and formaldehyde. The as-synthesized wet gels were initially solvent-exchanged by acetone to replace water. The organic gel was transformed to CA by heating in an argon atmosphere at 950 °C. The CA material was immersed into the sol-gel process of TiO2 formation. The as-formed mixed TiO2/CA was sintered at different high temperatures to yield the final photoelectrode material. Various characterization techniques such as scanning electronic microscopy (SEM), X-ray diffraction (XRD), Raman spectrometry, and N2 isothermal adsorption and desorption experiments were applied to determine the morphology (as shown in Figure 2), crystal structure, and porosity property. The TiO2/CA photoelectrode possesses both excellent photocatalytic properties owing to the anatase TiO2 semiconductor photoresponsive component and outstanding electrochemical properties of CA material, such as its extremely high conductivity, low electrochemical impedance, and large specific surface area. Thus, the TiO2/CA photoelectrode demonstrated much higher photocurrent density and degradation efficiency for highly opaque methylene blue wastewater under a −0.6 V potential bias and 365 nm light irradiation compared with the TiO2/ITO photoelectrode. The authors attributed the high efficiency to the following factors. Firstly, applying −0.6 V bias would form an electrical double layer between the electrode and bulk solution, causing the pollutants methylene blue, which possesses positive charges, to adsorb on TiO2/CA electrode surface. The excellent conductivity of CA materials would enhance the electrosorption process and the large specific surface area would promote the dispersion of TiO2 nano-crystallite on the electrode film. The electrosorption effect would increase the transparency of the opaque wastewater, which would greatly increase the photocatalytic degradation efficiency of TiO2.

Carbon Gels-Modified TiO 2 Photocatalysis
Owing to their large specific surface area, ordered porous structure, and high electric conductivity, carbon gels have been applied in various fields including hydrogen storage, electrode materials, and heterogeneous catalyst support. Although carbon gels are often used as conductive materials, they also can be used to construct a composite semiconductor photocatalyst with TiO 2 .
In 2010, Zhao and co-workers firstly used carbon gels to modify the TiO 2 photocatalyst to improve its adsorption and conductivity performance [84]. They prepared a TiO 2 /carbon aerogel (CA) photoelectrode material. The CA was synthesized by the base-catalyzed polycondensation between resorcinol and formaldehyde. The as-synthesized wet gels were initially solvent-exchanged by acetone to replace water. The organic gel was transformed to CA by heating in an argon atmosphere at 950 • C. The CA material was immersed into the sol-gel process of TiO 2 formation. The as-formed mixed TiO 2 /CA was sintered at different high temperatures to yield the final photoelectrode material. Various characterization techniques such as scanning electronic microscopy (SEM), X-ray diffraction (XRD), Raman spectrometry, and N 2 isothermal adsorption and desorption experiments were applied to determine the morphology (as shown in Figure 2), crystal structure, and porosity property. The TiO 2 /CA photoelectrode possesses both excellent photocatalytic properties owing to the anatase TiO 2 semiconductor photoresponsive component and outstanding electrochemical properties of CA material, such as its extremely high conductivity, low electrochemical impedance, and large specific surface area. Thus, the TiO 2 /CA photoelectrode demonstrated much higher photocurrent density and degradation efficiency for highly opaque methylene blue wastewater under a −0.6 V potential bias and 365 nm light irradiation compared with the TiO 2 /ITO photoelectrode. The authors attributed the high efficiency to the following factors. Firstly, applying −0.6 V bias would form an electrical double layer between the electrode and bulk solution, causing the pollutants methylene blue, which possesses positive charges, to adsorb on TiO 2 /CA electrode surface. The excellent conductivity of CA materials would enhance the electrosorption process and the large specific surface area would promote the dispersion of TiO 2 nano-crystallite on the electrode film. The electrosorption effect would increase the transparency of the opaque wastewater, which would greatly increase the photocatalytic degradation efficiency of TiO 2 . ×80 k) and TiO2/CA (C ×500, D ×80 k, E ×450 k). Copied with the permission from [84], Copyrights 2010 ACS [84].
In 2011, Wu and co-workers reported that mesoporous and macroporous carbon aerogels could couple with TiO2 to construct a hybrid CA/TiO2 composite photocatalyst [85]. The composite photocatalyst was prepared by a sol-gel process mixing Ti(iPrO)4 with CA in an alcoholic solution. The as-prepared CA was sintered at 400 °C under N2 atmosphere. Two CA materials with different pore diameters were hybridized with TiO2. The one with mesoporous structure displayed much higher activity on methyl orange degradation compared with pristine TiO2 and TiO2/CA with macroporous structure as shown in Figure 3. By analyzing XRD (X-ray diffraction), SEM (scanning electron microscopy), and BET (Brunner-Emmet-Teller measurements) experimental results, the authors determined that the mesoporous structure of TiO2/CA125 photocatalyst promoted the substrates adsorption and products desorption because of the suitable pore size. The micropores could not accommodate the considerably large methyl orange dye molecule entering the pore, while the macropore is too large to confine the dye molecule inside the pore for effective adsorption and surface photocatalytic reaction.  In 2011, Wu and co-workers reported that mesoporous and macroporous carbon aerogels could couple with TiO 2 to construct a hybrid CA/TiO 2 composite photocatalyst [85]. The composite photocatalyst was prepared by a sol-gel process mixing Ti(iPrO) 4 with CA in an alcoholic solution. The as-prepared CA was sintered at 400 • C under N 2 atmosphere. Two CA materials with different pore diameters were hybridized with TiO 2 . The one with mesoporous structure displayed much higher activity on methyl orange degradation compared with pristine TiO 2 and TiO 2 /CA with macroporous structure as shown in Figure 3. By analyzing XRD (X-ray diffraction), SEM (scanning electron microscopy), and BET (Brunner-Emmet-Teller measurements) experimental results, the authors determined that the mesoporous structure of TiO 2 /CA125 photocatalyst promoted the substrates adsorption and products desorption because of the suitable pore size. The micropores could not accommodate the considerably large methyl orange dye molecule entering the pore, while the macropore is too large to confine the dye molecule inside the pore for effective adsorption and surface photocatalytic reaction. In 2011, Wu and co-workers reported that mesoporous and macroporous carbon aerogels could couple with TiO2 to construct a hybrid CA/TiO2 composite photocatalyst [85]. The composite photocatalyst was prepared by a sol-gel process mixing Ti(iPrO)4 with CA in an alcoholic solution. The as-prepared CA was sintered at 400 °C under N2 atmosphere. Two CA materials with different pore diameters were hybridized with TiO2. The one with mesoporous structure displayed much higher activity on methyl orange degradation compared with pristine TiO2 and TiO2/CA with macroporous structure as shown in Figure 3. By analyzing XRD (X-ray diffraction), SEM (scanning electron microscopy), and BET (Brunner-Emmet-Teller measurements) experimental results, the authors determined that the mesoporous structure of TiO2/CA125 photocatalyst promoted the substrates adsorption and products desorption because of the suitable pore size. The micropores could not accommodate the considerably large methyl orange dye molecule entering the pore, while the macropore is too large to confine the dye molecule inside the pore for effective adsorption and surface photocatalytic reaction.  In 2011, Zhao and co-workers reported that CA/TiO 2 hybrid material could act as a very promising candidate for a photocatalysis enhanced electroadsorption (PES)-based dye pollutant elimination process [86]. They prepared the CA/TiO 2 electrode by dispersing the as-prepared CA material into titania tetrabutylate sol-gel. The CA material was synthesized from the poly-condensation of resorcinol and formaldehyde, the displacement of water by immersing in acetone, and final sintering under argon at a high temperature forming the final porous aerosol microstructure. The morphology, surface composition, and properties were characterized by SEM, EDS (Energy Dispersive Spectroscopy), TG/DTA (Thermogravimetric Analysis/Differential Thermal Analysis), BET, EIS (Electrochemical Impedance Spectroscopy), and Raman spectrometry. Furthermore, the material demonstrated excellent alizarin red (AR) dye pollutant elimination ability by combining adsorption, electroadsorption, and photocatalysis in a three-in-all strategy. In a 400 mg/L high concentration AR pollutant solution, the hybrid material exhibited 97.3% TOC (Total Organic Carbon) removal in 240 min for the PES process in comparison with 59% in the photocatalysis process without applied electric field and 22% in the electrosorption process without illumination. Not only did it possess high AR removal efficiency, but this material also exhibited very good stability, providing 92.3% AR removal after five cycles. Moreover, the authors proposed a mechanism for the synergistic effect of photocatalysis and electrosorption as shown in Figure 4. The adsorbed dye molecules labile bonds were initially cleaved via single-electron-transfer by TiO 2 photo-induced hole species or hydrogen abstraction by hydroxyl radicals and other reactive oxygen species (ROS), and the dye molecule radical cations were hydroxylated and mineralized step by step via radical addition, atom abstraction, and bond cleavage mechanism. The photo-induced electrons were extracted to the other electrode by circuit to inhibit hole-electron recombination, and thus improve the mineralization efficiency. Owing to the excellent porosity and conductivity, CA could act as very effective adsorbent and electrode materials. Under applied positive potential, negative-charged dye-molecules would be attracted to the positive electrode, enhancing adsorption. The TiO 2 photocatalyst mineralized the dye, which avoided saturated adsorption of CA material, while CA adsorbed and enriched dye pollutant to keep high pollutant concentration on the TiO 2 surface to make the photocatalytic reaction proceed rapidly. Photocatalysis and electroadsorption synergistically ensured highly efficient high-concentration dye solution decontamination, which is important not only in academics, but also in industrial engineering. In 2011, Zhao and co-workers reported that CA/TiO2 hybrid material could act as a very promising candidate for a photocatalysis enhanced electroadsorption (PES)-based dye pollutant elimination process [86]. They prepared the CA/TiO2 electrode by dispersing the as-prepared CA material into titania tetrabutylate sol-gel. The CA material was synthesized from the poly-condensation of resorcinol and formaldehyde, the displacement of water by immersing in acetone, and final sintering under argon at a high temperature forming the final porous aerosol microstructure. The morphology, surface composition, and properties were characterized by SEM, EDS (Energy Dispersive Spectroscopy), TG/DTA (Thermogravimetric Analysis/Differential Thermal Analysis), BET, EIS (Electrochemical Impedance Spectroscopy), and Raman spectrometry. Furthermore, the material demonstrated excellent alizarin red (AR) dye pollutant elimination ability by combining adsorption, electroadsorption, and photocatalysis in a three-in-all strategy. In a 400 mg/L high concentration AR pollutant solution, the hybrid material exhibited 97.3% TOC (Total Organic Carbon) removal in 240 min for the PES process in comparison with 59% in the photocatalysis process without applied electric field and 22% in the electrosorption process without illumination. Not only did it possess high AR removal efficiency, but this material also exhibited very good stability, providing 92.3% AR removal after five cycles. Moreover, the authors proposed a mechanism for the synergistic effect of photocatalysis and electrosorption as shown in Figure 4. The adsorbed dye molecules labile bonds were initially cleaved via single-electron-transfer by TiO2 photo-induced hole species or hydrogen abstraction by hydroxyl radicals and other reactive oxygen species (ROS), and the dye molecule radical cations were hydroxylated and mineralized step by step via radical addition, atom abstraction, and bond cleavage mechanism. The photo-induced electrons were extracted to the other electrode by circuit to inhibit hole-electron recombination, and thus improve the mineralization efficiency. Owing to the excellent porosity and conductivity, CA could act as very effective adsorbent and electrode materials. Under applied positive potential, negative-charged dye-molecules would be attracted to the positive electrode, enhancing adsorption. The TiO2 photocatalyst mineralized the dye, which avoided saturated adsorption of CA material, while CA adsorbed and enriched dye pollutant to keep high pollutant concentration on the TiO2 surface to make the photocatalytic reaction proceed rapidly. Photocatalysis and electroadsorption synergistically ensured highly efficient high-concentration dye solution decontamination, which is important not only in academics, but also in industrial engineering. Furthermore, Zhao and co-workers discovered that TiO2/CA material could have the capacity to act as an effective photoelectrode to extend the TiO2 absorption spectrum to the visible-light region and mineralize Rhodamine 6G dye pollutants under visible-light irradiation in an aerobic condition under −0.9 V bias [87]. They figured out that H2O2 was generated on the electrode surface by the reduction of dioxygen. Further, the in-situ generated H2O2 coordinated with TiO2, forming a Furthermore, Zhao and co-workers discovered that TiO 2 /CA material could have the capacity to act as an effective photoelectrode to extend the TiO 2 absorption spectrum to the visible-light region and mineralize Rhodamine 6G dye pollutants under visible-light irradiation in an aerobic condition under −0.9 V bias [87]. They figured out that H 2 O 2 was generated on the electrode surface by the reduction of dioxygen. Further, the in-situ generated H 2 O 2 coordinated with TiO 2 , forming a visible-light responsive surface Ti-peroxide complex. Upon light irradiation, this surface complex was excited and injected electrons to the TiO 2 conduction band. The conduction band electrons' decomposed surface adsorbed H 2 O 2 to hydroxyl radicals. The hydroxyl radicals that possess strong oxidative ability (E 1/2 = 2.4 V vs. NHE) can be directly applied to mineralize pollutants. This mechanism was shown in Figure 5.
To demonstrate the efficiency of this photoelectrocatalysis system, azo dye Rhodamine 6G was applied as a model pollutant in the activity test. The results showed that, upon -0.9 V bias and visible-light irradiation (λ > 420 nm), the TiO 2 /CA photocathode could reach removal efficiency for Rhodamine 6G at 90.3% and TOC at 83.3% in 300 min. GC-MS (Gas Chromatography-Mass Spectroscopy) and HPLC (High-performance Liquid Chromatography) tracing of intermediates indicated that less intermediates were generated under photoelectrocatalytic conditions than under traditional photocatalytic conditions. This catalyst system demonstrated excellent pollutant removal efficiency and low energy consumption. More importantly, this example provided us a new approach to explore the visible-light responsive unconventional TiO 2 -based photocatalytic materials. visible-light responsive surface Ti-peroxide complex. Upon light irradiation, this surface complex was excited and injected electrons to the TiO2 conduction band. The conduction band electrons' decomposed surface adsorbed H2O2 to hydroxyl radicals. The hydroxyl radicals that possess strong oxidative ability (E1/2 = 2.4 V vs NHE) can be directly applied to mineralize pollutants. This mechanism was shown in Figure 5. To demonstrate the efficiency of this photoelectrocatalysis system, azo dye Rhodamine 6G was applied as a model pollutant in the activity test. The results showed that, upon -0.9 V bias and visible-light irradiation (λ > 420 nm), the TiO2/CA photocathode could reach removal efficiency for Rhodamine 6G at 90.3% and TOC at 83.3% in 300 min. GC-MS (Gas Chromatography-Mass Spectroscopy) and HPLC (High-performance Liquid Chromatography) tracing of intermediates indicated that less intermediates were generated under photoelectrocatalytic conditions than under traditional photocatalytic conditions. This catalyst system demonstrated excellent pollutant removal efficiency and low energy consumption. More importantly, this example provided us a new approach to explore the visible-light responsive unconventional TiO2-based photocatalytic materials. Zhao and co-workers reported that TiO2 nanorod cluster (NRC) material could be loaded on carbon aerogel by a hydrothermal/calcination method [88]. The as-prepared TiO2 NRC/CA composite material could be used as photocathode for the decomposition and mineralization of a notorious pollutant p-nitroaniline. They compared the pollutant removal efficiency and activity of TiO2 NRC/CA with TiO2 NRC/graphite material. The results indicated that TiO2 NRC/CA material could provide superior 98.2% PNA (p-Nitroaniline) removal efficiency in 180 min under visible-light irradiation with −0.6 V bias in comparison with 79.8% for TiO2 NRC/graphite. From various characterization experiments, the authors ascribed the higher efficiency of TiO2 NRC/CA to the CA's three-dimensional structure, which provided greater surface area and larger conductivity. More TiO2 photocatalyst could be loaded on CA rather than on graphite. Further, more H2O2 and hydroxyl radicals could be formed on electrode surface. Owing to the greater level of TiO2 loading, more surface Ti-peroxide complex and photocurrent could be yielded under visible-light irradiation. All these factors made TiO2 NRC/CA excellent photocathode materials for p-nitroaniline pollutants mineralization. Moreover, the authors proposed the decomposition pathways for this photoelectrocatalytic system as shown in Figure 6. They discovered the new important intermediates 1,2,4-trihydroxybenzene during the decomposition processes of p-nitroaniline and reasonably explained its mineralization mechanism. Zhao and co-workers reported that TiO 2 nanorod cluster (NRC) material could be loaded on carbon aerogel by a hydrothermal/calcination method [88]. The as-prepared TiO 2 NRC/CA composite material could be used as photocathode for the decomposition and mineralization of a notorious pollutant p-nitroaniline. They compared the pollutant removal efficiency and activity of TiO 2 NRC/CA with TiO 2 NRC/graphite material. The results indicated that TiO 2 NRC/CA material could provide superior 98.2% PNA (p-Nitroaniline) removal efficiency in 180 min under visible-light irradiation with −0.6 V bias in comparison with 79.8% for TiO 2 NRC/graphite. From various characterization experiments, the authors ascribed the higher efficiency of TiO 2 NRC/CA to the CA's three-dimensional structure, which provided greater surface area and larger conductivity. More TiO 2 photocatalyst could be loaded on CA rather than on graphite. Further, more H 2 O 2 and hydroxyl radicals could be formed on electrode surface. Owing to the greater level of TiO 2 loading, more surface Ti-peroxide complex and photocurrent could be yielded under visible-light irradiation. All these factors made TiO 2 NRC/CA excellent photocathode materials for p-nitroaniline pollutants mineralization. Moreover, the authors proposed the decomposition pathways for this photoelectrocatalytic system as shown in Figure 6. They discovered the new important intermediates 1,2,4-trihydroxybenzene during the decomposition processes of p-nitroaniline and reasonably explained its mineralization mechanism. Shi and co-workers discovered that TiO2 photocatalytic activity of DMP (dimethyl phthalate) degradation could be enhanced by hybridizing a CA material and constructing secondary hydrophilic pores [89]. They synthesized TiO2/CA composite photocatalysts with different CA pore diameters. They discovered that materials comprising CA with 9.3 nm mesopores exhibited the highest photocatalytic degradation efficiency, while CA with smaller pore diameters of 3.4 and 4.3 nm generated inferior results even compared with the pristine TiO2 photocatalyst. Contact angle analysis and DMP adsorption tests indicated that CA-9.3 possessed the least ability to adsorb DMP owing to its larger wettability and hydrophilicity. However, as TiO2 nanoparticles dispersed in both inner and outer space of CA-9.3, photo-induced electrons on the outer surface migrated to the inner surface through CA conductive porous structure, inhibiting electron-hole recombination and improving the charge separation process. Thus, the generation of hydroxyl radicals by photo-induced holes on the outer surface was enhanced by this effect, which increased the DMP photodegradation rate. Moreover, the generation rate of hydroxyl radical by other routes was also increased. In the inner space of hydrophilic secondary pore, dioxygen, photo-electrons, and water molecules were enriched by this micro-reactor. This encapsulation effect enhanced the hydroxyl radical generation by the dioxygen reduction route in comparison with the lack of porous structure of pristine TiO2. This mechanism was illustrated in Figure 7. Shi and co-workers discovered that TiO 2 photocatalytic activity of DMP (dimethyl phthalate) degradation could be enhanced by hybridizing a CA material and constructing secondary hydrophilic pores [89]. They synthesized TiO 2 /CA composite photocatalysts with different CA pore diameters. They discovered that materials comprising CA with 9.3 nm mesopores exhibited the highest photocatalytic degradation efficiency, while CA with smaller pore diameters of 3.4 and 4.3 nm generated inferior results even compared with the pristine TiO 2 photocatalyst. Contact angle analysis and DMP adsorption tests indicated that CA-9.3 possessed the least ability to adsorb DMP owing to its larger wettability and hydrophilicity. However, as TiO 2 nanoparticles dispersed in both inner and outer space of CA-9.3, photo-induced electrons on the outer surface migrated to the inner surface through CA conductive porous structure, inhibiting electron-hole recombination and improving the charge separation process. Thus, the generation of hydroxyl radicals by photo-induced holes on the outer surface was enhanced by this effect, which increased the DMP photodegradation rate. Moreover, the generation rate of hydroxyl radical by other routes was also increased. In the inner space of hydrophilic secondary pore, dioxygen, photo-electrons, and water molecules were enriched by this micro-reactor. This encapsulation effect enhanced the hydroxyl radical generation by the dioxygen reduction route in comparison with the lack of porous structure of pristine TiO 2 . This mechanism was illustrated in Figure 7. Wang and co-workers reported that Ce-doped TiO2/CA could be applied as potent visible-light responsive electrosorptive photocatalytic materials for 4-chlorophenol degradation [90]. They discovered that cerium doping could introduce defect sites above the top of the valence band red-shifting TiO2 absorption spectrum from 387 nm to 532 nm. Furthermore, carbon aerogel was hybridized with Ce-TiO2 photocatalyst by an impregnation method. The as-synthesized Ce-TiO2/CA material was used as photoelectrode to degrade 4-chlorophenol under 0.6 V bias and 500 W Xenon lamp irradiation. This material exhibited 75 times larger photocurrent than Ce-TiO2/FTO (fluorine-doped tin oxide), which was ascribed to the lesser electron-hole recombination rate by the CA porous structure, which facilitated the charge separation processes, as mentioned above. Furthermore, for the 4-chlorophenol (4-CP) degradation, Ce-TiO2/CA generated 97.3% 4-CP removal in 4.5 h, in sharp contrast with 65.4% for Ce-TiO2/FTO under other identical photoelectrochemical conditions. This enhancing effect was mainly because of the highly-developed porous structure and excellent conductivity, which were pivotal for 4-CP adsorption and the charge separation and migration processes. This mechanism was illustrated in Figure 8.  Wang and co-workers reported that Ce-doped TiO 2 /CA could be applied as potent visible-light responsive electrosorptive photocatalytic materials for 4-chlorophenol degradation [90]. They discovered that cerium doping could introduce defect sites above the top of the valence band red-shifting TiO 2 absorption spectrum from 387 nm to 532 nm. Furthermore, carbon aerogel was hybridized with Ce-TiO 2 photocatalyst by an impregnation method. The as-synthesized Ce-TiO 2 /CA material was used as photoelectrode to degrade 4-chlorophenol under 0.6 V bias and 500 W Xenon lamp irradiation. This material exhibited 75 times larger photocurrent than Ce-TiO 2 /FTO (fluorine-doped tin oxide), which was ascribed to the lesser electron-hole recombination rate by the CA porous structure, which facilitated the charge separation processes, as mentioned above. Furthermore, for the 4-chlorophenol (4-CP) degradation, Ce-TiO 2 /CA generated 97.3% 4-CP removal in 4.5 h, in sharp contrast with 65.4% for Ce-TiO 2 /FTO under other identical photoelectrochemical conditions. This enhancing effect was mainly because of the highly-developed porous structure and excellent conductivity, which were pivotal for 4-CP adsorption and the charge separation and migration processes. This mechanism was illustrated in Figure 8. Wang and co-workers reported that Ce-doped TiO2/CA could be applied as potent visible-light responsive electrosorptive photocatalytic materials for 4-chlorophenol degradation [90]. They discovered that cerium doping could introduce defect sites above the top of the valence band red-shifting TiO2 absorption spectrum from 387 nm to 532 nm. Furthermore, carbon aerogel was hybridized with Ce-TiO2 photocatalyst by an impregnation method. The as-synthesized Ce-TiO2/CA material was used as photoelectrode to degrade 4-chlorophenol under 0.6 V bias and 500 W Xenon lamp irradiation. This material exhibited 75 times larger photocurrent than Ce-TiO2/FTO (fluorine-doped tin oxide), which was ascribed to the lesser electron-hole recombination rate by the CA porous structure, which facilitated the charge separation processes, as mentioned above. Furthermore, for the 4-chlorophenol (4-CP) degradation, Ce-TiO2/CA generated 97.3% 4-CP removal in 4.5 h, in sharp contrast with 65.4% for Ce-TiO2/FTO under other identical photoelectrochemical conditions. This enhancing effect was mainly because of the highly-developed porous structure and excellent conductivity, which were pivotal for 4-CP adsorption and the charge separation and migration processes. This mechanism was illustrated in Figure 8.  Szilágyi and co-workers reported an atomic-layer-deposition (ALD) method to prepare resorcinol-formaldehyde aerogel (RFA) and resorcinol-formaldehyde carbon aerogel (RFCA)/TiO 2 composite photocatalysts [91]. They initially prepared RFA and RFCA by the modified Pekala's three-step method; the first step to synthesize RF hydrogel, the second step dry RF hydrogel in supercritical condition forming RFA, and the last step to yield RFCA by sintering RFA under a N 2 atmosphere at a high temperature. The as-synthesized RFA and RFCA were utilized as substrates for ALD of TiO 2 at 80 • C and 250 • C. TiO 2 prepared at 80 • C was amorphous, while that at 250 • C was crystalline. Furthermore, the authors conducted the photo-degradation experiments of methyl orange dye. Surprisingly, they discovered that RFCA/amorphous TiO 2 exhibited better photocatalytic activity than RFCA/crystalline TiO 2 . Further, the RFCA material without TiO 2 deposition exhibited the best photocatalytic activity towards methyl orange degradation. These results were shown in Figure 9. The authors ascribed these unconventional results to the fact that the oxide deposition reduced the surface area and the functional group content of RFCA, thus blocking and decreasing the activity sites. The ALD treatment at a higher temperature more significantly deteriorated the RFCA porous structure and decreased its functional group contents. This report was interesting, demonstrating that amorphous carbonaceous material such as resorcinol-formaldehyde carbon aerogel could not only exhibit semiconductive photocatalytic activity, but also generate higher ability towards organic pollutant degradation under illumination than traditional metal-oxide materials. Szilágyi and co-workers reported an atomic-layer-deposition (ALD) method to prepare resorcinol-formaldehyde aerogel (RFA) and resorcinol-formaldehyde carbon aerogel (RFCA)/TiO2 composite photocatalysts [91]. They initially prepared RFA and RFCA by the modified Pekala's three-step method; the first step to synthesize RF hydrogel, the second step dry RF hydrogel in supercritical condition forming RFA, and the last step to yield RFCA by sintering RFA under a N2 atmosphere at a high temperature. The as-synthesized RFA and RFCA were utilized as substrates for ALD of TiO2 at 80 °C and 250 °C. TiO2 prepared at 80 °C was amorphous, while that at 250 °C was crystalline. Furthermore, the authors conducted the photo-degradation experiments of methyl orange dye. Surprisingly, they discovered that RFCA/amorphous TiO2 exhibited better photocatalytic activity than RFCA/crystalline TiO2. Further, the RFCA material without TiO2 deposition exhibited the best photocatalytic activity towards methyl orange degradation. These results were shown in Figure 9. The authors ascribed these unconventional results to the fact that the oxide deposition reduced the surface area and the functional group content of RFCA, thus blocking and decreasing the activity sites. The ALD treatment at a higher temperature more significantly deteriorated the RFCA porous structure and decreased its functional group contents. This report was interesting, demonstrating that amorphous carbonaceous material such as resorcinol-formaldehyde carbon aerogel could not only exhibit semiconductive photocatalytic activity, but also generate higher ability towards organic pollutant degradation under illumination than traditional metal-oxide materials. Figure 9. Photocatalytic results of (A) resorcinol-formaldehyde aerogel (RFA) and its composite, (B) resorcinol-formaldehyde carbon aerogel (RFCA) and its composites, and (C) specific surface area and heating corrected data of RFCA and its composites. Copied with the permission from [91], Copyrights 2019 Elsevier [91].
The performance of the state-of-the-art examples of TiO2/CA photo(electro)catalysis is summarised in Table 1.  Figure 9. Photocatalytic results of (A) resorcinol-formaldehyde aerogel (RFA) and its composite, (B) resorcinol-formaldehyde carbon aerogel (RFCA) and its composites, and (C) specific surface area and heating corrected data of RFCA and its composites. Copied with the permission from [91], Copyrights 2019 Elsevier [91].
The performance of the state-of-the-art examples of TiO 2 /CA photo(electro)catalysis is summarised in Table 1.  [91] Besides the activity improvements of the TiO 2 /CA photo(electro)catalyst, its life cycle assessment (LCA) of the environmental impacts should also be considered. Although we lack the data of the environmental effects of the CA synthesis and the hybrid process of TiO 2 and CA nano-materials, according to the previously reported LCA of environmental impacts for seven different fabrication routes of TiO 2 nanomaterials by mass unit, surface-area unit, and photocatalytic activity unit [92], or the reported LCA cradle to gate environmental impacts of five different non-doped and metal-doped TiO 2 nanomaterials prepared by the sol-gel method by the function unit of photocatalytic carbamazepine and methyl orange degradation [93], we can infer that the TiO 2 /CA photo(electro)catalyst will generate more environmental impacts because the fabrication of CA materials requires an intensive energy-input process and the use of a considerable amount of organic precursor resorcinol and formaldehyde, while the use of formaldehyde will contribute to environmental concern. Further, during the photo(electro)catalytic process of pollutant removal, the added bias would generate much greater consumption of electricity energy. However, the greatly enhanced pollutants removal efficiency would come as a trade-off to the added extra environment impacts for TiO 2 /CA materials compared with non-doped TiO 2 photocatalysts. Further, a thorough investigation using quantitative software to assess the environmental impacts of the TiO 2 /CA photo(electro)catalyst "from cradle to grave" based on the function unit of the photo(electro)catalytic removal of certain organic pollutants by LCA should be conducted in the future.

Conclusions
We have outlined the state-of-the-art examples of carbon aerogels materials hybridized with TiO 2 applied as both catalysts for photoelectrochemical cells and photocatalysts for environmental pollutants degradation. Although still in its blossoming stage in comparison with other traditional inorganic metal oxides, metal chalcogenides, and surface plasmonic metallic photocatalysts and photo-electrocatalysts, CA-modified TiO 2 material still exhibited miscellaneous advantages. Firstly, carbon aerogels modification could greatly increase the surface area of TiO 2 owing to the great porosity and highly-developed hierarchical porous structures. The increase of surface area would enhance the adsorption of pollutants on TiO 2 . Furthermore, the introduction of porous structure would enhance electron-hole pair separation by the facilitated TiO 2 photoelectron transfer between inner surface and outer surface in the interconnected pore structure. Secondly, owing to CA's excellent conductivity, upon hybridization with CA, TiO 2 semiconductive material became much more conductive, and the electron-transfer impedance was greatly reduced. This effect was crucial for the increase of photocurrent during the photoelectrochemical degradation of organic pollutants. Last, but not least, the doping of carbon aerogel would introduce defect sites in TiO 2 bulk solid, thus generating a dopant energy level in the top of the valence band and in the bottom of the conduction band, thus providing the narrower band gap for more efficient photon absorption. This would generate an apparent red-shift phenomenon for TiO 2 , extending its absorption limit from 387 nm to approximately 532 nm. Compared with other crystalline carbonaceous materials, metal oxides, metal chalcogenides, and metal surface plasmonic photocatalysts, CA/TiO 2 photocatalysts still have a long way to proceed for more mature applications in environment remediation. The main limitations and challenges for CA/TiO 2 photocatalysts include the following aspects. Initially, the degradation efficiency should be further improved to fulfill the requirements of practical use in industry wastewater treatment. To meet this end, an elaborate, intricate, and novel synthesizing method should be developed. Much higher performance of hydroxyl radical generation and pollutants adsorption should be achieved. Secondly, in order to improve the activity of the composite photocatalyst, the mechanism of how the photoelectrons, photo-induced holes, dioxygen, water, organic pollutants, and the degradation intermediates interacted in the composite photocatalyst, and in particular, how the porous structure influenced the whole photocatalytic process should be more clearly demonstrated. We do believe that if these issues can be resolved successfully, the future of CA/TiO 2 photocatalysis would be promising.