Recent Advances of Photocatalytic Hydrogenation of CO 2 to Methanol

: Constantly increasing hydrocarbon fuel combustion along with high levels of carbon dioxide emissions has given rise to a global energy crisis and environmental alterations. Photocatalysis is an effective technique for addressing this energy and environmental crisis. Clean and renewable solar energy is a very favourable path for photocatalytic CO 2 reduction to value-added products to tackle problems of energy and the environment. The synthesis of various products such as CH 4 , CH 3 OH, CO, EtOH, etc., has been expanded through the photocatalytic reduction of CO 2 . Among these products, methanol is one of the most important and highly versatile chemicals widely used in industry and in day-to-day life. This review emphasizes the recent progress of photocatalytic CO 2 hydrogenation to CH 3 OH. In particular, Metal organic frameworks (MOFs), mixed-metal oxide, carbon, TiO 2 and plasmonic-based nanomaterials are discussed for the photocatalytic reduction of CO 2 to methanol. Finally, a summary and perspectives on this emerging ﬁeld are provided.


Introduction
Today's global issues and challenges include the energy crisis and environmental concerns [1].Carbon dioxide comes primarily from the combustion of carbon sources such as fossil fuels and other natural sources.Developing renewable energy technologies to reduce pollutant emissions has become a significant area of research for the development of a sustainable planet.
In light of this, various advanced nanomaterials for CO 2 reduction have been reported, including alkali hydroxide [2,3], CuNi@g-C 3 N 4 /TiO 2 [4,5], Pd50-Ru50/MXene [6,7], CoSA-Ti 3 C 2 T x [8,9], CuSAs/TCNFs carbon nanofibers [10], and copper selenide (Cu 2 xSe(y) nanocatalysts) [11].Gawande and his co-workers recently reviewed advanced Ag-based nanomaterials used for various photocatalytic applications, including CO 2 hydrogenation processes [12].One promising approach to addressing issues related to climate change and the energy crisis is photocatalytic CO 2 reduction into value-added chemicals.Photocatalysis is considered to be a promising method for CO 2 conversion into valuable products, such as methanol, methane, formaldehyde, ethanol, and higher hydrocarbons [13].Photosynthesis is an ideal method for effectively resolving issues relating to energy and the environment by reducing CO 2 into value-added chemicals and fuels [14].The photocatalytic performance of a photocatalyst is highly dependent upon its electronic band structure and bandwidth energy.For an effective photocatalyst, the bandgap energy must be less than 3 eV to expand the light absorption in the visible area and use solar power efficiently.To date, a variety of photocatalysts including P-and F-co-doped carbon nitride (PFCN) [15], RuSA-mC 3 N 4 [16], Cu-ZIF [17,18], single Cu 2 O particle [19,20], g-C 3 N 4 -TiO 2 [21], (Pd/Pt)SA/g-C 3 N 4 [22], O-doped g-C 3 N 4 (OCN-Tube) [23], Cu-TiO 2 [24], Ni-nanocluster loaded on TiO 2 (Ni/TiO 2 [Vo] ) [25], aerogel flow-reactor [26], porous-g-C 3 N 4 /TiO 2 -nanotube [27][28][29], carbon-doped TiO 2 [30][31][32], RGO-NH 2 -MIL-125(Ti) [33], Cu porphyrin-based MOF [34], Due to its role as a greenhouse gas, the conversion of CO2 to alternative chemicals has enabled many options.However, CO2 is highly thermodynamically stable, which means that converting it into valuable chemicals requires a large amount of energy.A promising photo-reduced product of CO2 is methanol.Methanol has multiple applications, including fuel transportation, biodiesel transesterification, and electricity generation [40][41][42].The CO2 conversion to methanol is a highly suitable method for reducing CO2 emissions into the atmosphere.Methanol is greener than gasoline and has a high energy density.In addition to being a feasible clean fuel, methanol is also an important feedstock for chemical industries.The internal combustion engines can directly use it if stored at atmospheric pressure (atm P) due to its high octane number [43,44].In light of all the above advantages of methanol, we examine the various strategies to enhance the photocatalytic CO2 conversion to CH3OH using carbon-based, TiO2 based, MOFs based, mixed metal-oxide, and plasmonic-based photocatalysts.

Scope and Focus of This Review
CO2 emissions are one of the greatest environmental concerns globally today.It is extremely necessary to convert this atmospheric CO2 into valuable fuel or chemicals through catalysis.Photocatalysis is one of the ideal paths for CO2 conversion into valueadded products.
There has not been a review on photocatalytic selective hydrogenation of CO2 to methanol yet.Though there are some reviews on CO2 reduction into value-added chemicals reported, none of these reviews focused solely on methanol [43][44][45][46][47][48][49][50][51][52][53][54].Thus, in this review, we highlighted the photocatalytic CO2 reduction to selectively solar fuel such as Due to its role as a greenhouse gas, the conversion of CO 2 to alternative chemicals has enabled many options.However, CO 2 is highly thermodynamically stable, which means that converting it into valuable chemicals requires a large amount of energy.A promising photo-reduced product of CO 2 is methanol.Methanol has multiple applications, including fuel transportation, biodiesel transesterification, and electricity generation [40][41][42].The CO 2 conversion to methanol is a highly suitable method for reducing CO 2 emissions into the atmosphere.Methanol is greener than gasoline and has a high energy density.In addition to being a feasible clean fuel, methanol is also an important feedstock for chemical industries.The internal combustion engines can directly use it if stored at atmospheric pressure (atm P) due to its high octane number [43,44].In light of all the above advantages of methanol, we examine the various strategies to enhance the photocatalytic CO 2 conversion to CH 3 OH using carbon-based, TiO 2 based, MOFs based, mixed metal-oxide, and plasmonic-based photocatalysts.

Scope and Focus of This Review
CO 2 emissions are one of the greatest environmental concerns globally today.It is extremely necessary to convert this atmospheric CO 2 into valuable fuel or chemicals through catalysis.Photocatalysis is one of the ideal paths for CO 2 conversion into value-added products.
There has not been a review on photocatalytic selective hydrogenation of CO 2 to methanol yet.Though there are some reviews on CO 2 reduction into value-added chemicals reported, none of these reviews focused solely on methanol [43][44][45][46][47][48][49][50][51][52][53][54].Thus, in this review, we highlighted the photocatalytic CO 2 reduction to selectively solar fuel such as methanol by using photoactive supporting materials, including carbon, TiO 2 , MOF, mixed metal-oxide, and plasmonic based photocatalysts (Figure 2).We believe that a detailed overview of the catalytic performance of various photocatalysts for CO 2 reduction to methanol would be useful to a broad community of scientists with interests in nanotechnology, materials chemistry, inorganic chemistry, organic chemistry, and chemical engineering.
Catalysts 2022, 12, x FOR PEER REVIEW 3 of 39 methanol by using photoactive supporting materials, including carbon, TiO2, MOF, mixed metal-oxide, and plasmonic based photocatalysts (Figure 2).We believe that a detailed overview of the catalytic performance of various photocatalysts for CO2 reduction to methanol would be useful to a broad community of scientists with interests in nanotechnology, materials chemistry, inorganic chemistry, organic chemistry, and chemical engineering.
Figure 2. The supporting materials used in photocatalytic CO2 reduction to fuels.

Applications
Photocatalytic CO2 reduction is a promising method for converting CO2 into valuable fuels and chemicals by utilising solar energy.An everyday variety of products utilize utilise methanol as a chemical building block, such as paints, plastics, and construction materials.In this section, a variety of photocatalysts for CO2 reduction to methanol are included, including MOF-based, mixed-metal oxide-based, carbon-based, TiO2-based and plasmonic-based photocatalysts.

Photocatalyst Based on MOFs
The metal-organic framework (MOF) is a hybrid (inorganic-organic) crystalline porous material that consists of metal ions surrounded by organic linkers.Due to an internal hollow structure, it has a remarkably large internal surface area, since the metal ions serve as nodes that bind the linker arms together [55].In contrast to other porous materials, MOFs exhibit unparalleled structural diversity-atomic structural uniformity, uniform pore structures, tunable porosity, as well as flexibility in network topology, and chemical utility.MOFs' cage-like structure is currently being exploited in numerous fields, including purification, gas separation and storage, liquid separation, sensing, gas storage, and catalysis [49].The various types of MOF-based photocatalysts, such as MOF-based, MOF composites, MOF-derived, MOFs as support, and single-site MOFs, are employed for the reduction of CO2.
In recent years, MOF-based materials for photoreduction of CO2 have attracted significant research interest [50].MOF materials are easily designed with convenient metallic sites, specific heteroatoms, and an orderly structure of functional organic ligands [54].They can efficiently increase the efficiency of photocatalytic activity and electron-hole separation.MOF porosity can help to expose channels for reactant adsorption and more

Applications
Photocatalytic CO 2 reduction is a promising method for converting CO 2 into valuable fuels and chemicals by utilising solar energy.An everyday variety of products utilize utilise methanol as a chemical building block, such as paints, plastics, and construction materials.In this section, a variety of photocatalysts for CO 2 reduction to methanol are included, including MOF-based, mixed-metal oxide-based, carbon-based, TiO 2 -based and plasmonic-based photocatalysts.

Photocatalyst Based on MOFs
The metal-organic framework (MOF) is a hybrid (inorganic-organic) crystalline porous material that consists of metal ions surrounded by organic linkers.Due to an internal hollow structure, it has a remarkably large internal surface area, since the metal ions serve as nodes that bind the linker arms together [55].In contrast to other porous materials, MOFs exhibit unparalleled structural diversity-atomic structural uniformity, uniform pore structures, tunable porosity, as well as flexibility in network topology, and chemical utility.MOFs' cage-like structure is currently being exploited in numerous fields, including purification, gas separation and storage, liquid separation, sensing, gas storage, and catalysis [49].The various types of MOF-based photocatalysts, such as MOF-based, MOF composites, MOF-derived, MOFs as support, and single-site MOFs, are employed for the reduction of CO 2 .
In recent years, MOF-based materials for photoreduction of CO 2 have attracted significant research interest [50].MOF materials are easily designed with convenient metallic sites, specific heteroatoms, and an orderly structure of functional organic ligands [54].They can efficiently increase the efficiency of photocatalytic activity and electron-hole separation.MOF porosity can help to expose channels for reactant adsorption and more active sites, thus resulting in an excellent catalytic performance.This can increase the efficiency of charge transfer and solar power while inhibiting the recombination of photo-induced electrons.Based on the above merits, the researchers attempt to use various MOFs for the photocatalytic reduction of CO 2 .
MOFs have porous and channel structures that provide anchored centres for photocatalysis, giving them their structural and functional characteristics.Wang et al. reported composites (Cd 0.2 Zn 0.8 S/UiO-66-NH 2 ) with various UiO-66-NH 2 compositions via the solvothermal method, which is used for CO 2 photoreduction and hydrogen evaluation under visible light [55].The electron microscopic (TEM/HR-TEM) images of CZS/UN20 confirmed that Cd 0.2 Zn 0.8 S NPs were evenly distributed on the surface of the UiO-66NH 2 cubes, and the lattice spacing (0.313 nm), which corresponds to the (111) plane of Cd 0.2 Zn 0.8 S (Figure 3a,b).The composite (Cd 0.2 Zn 0.8 S/UiO-66-NH 2 ) showed the largest photocurrent density, implying effective photo-generated charge transfer (between Cd 0.2 Zn 0.8 S and UiO-66-NH 2 ), which is reliable for improved photocatalytic performance (Figure 3c).The UiO-66-NH 2 content significantly influences the photoactivity of Cd 0.2 Zn 0.8 S. The CZS@UN20 sample shows the higher methanol evolution rate among the as-prepared samples (6.8 µmol h −1 g −1 ) under irradiation of visible light; it was larger (3.4 times) compared to pure Cd 0.2 Zn 0.8 S (Figure 3d).Because of the transfer and charge separation (between Cd 0.2 Zn 0.8 S and UiO-66-NH 2 ), the photo-induced electrons absorbed by these materials could be transferred over the surface for the reduction of CO 2 .Apart from photocatalytic performance, the lifetime of the catalyst is very important.Furthermore, the photocatalyst (Cd 0.2 Zn 0.8 S/UiO-66-NH 2 ) showed excellent stability after four recycles in the system of photoreduction CO 2 (Figure 3e).A plausible interface electron transfer behaviour and a related mechanism of photocatalytic CO 2 reduction were demonstrated in Figure 3f.
The Z-scheme heterojunction has a high electron-hole pair separation efficiency and redox ability and a broad light response range.The Z-scheme heterojunction is a wonderful alternative for converting CO 2 into value-adding compounds because of the advantages listed above.The artificial Z-scheme photocatalyst is typically made up of two connected semiconductor photocatalysts: one for oxidation and the other for reduction [56].The Z-scheme gets its name from the fact that it connects the two photosystems in a fashion that looks like the letter "Z".In particular, the engineered Z-scheme direct photocatalysts that mimic the natural photosynthetic system provide a number of advantages, including improved light uptake, spatially separated reductive and oxidative active sites, and well-preserved strong redox capacity.Furthermore, photogenerated separation was made possible by the heterojunction at the interfaces, which boosted the charge participation in catalysed conversion reactions [57].Heterojunctions (Z-scheme) are an effective way to isolate photogenerated electron holes and improve photocatalytic activity in semiconductors.On the other hand, heterojunction-based MOFs are rarely documented.In a similar context, Liu et al. have recently designed a Z-scheme O-ZnO/rGO/UiO-66-NH 2 (OZ/R/U) heterojunction that was obtained by pairing UiO-66-NH 2 and rGO (reduced graphene oxide) with O-ZnO (oxygen-poor) by a facile solvothermal method [58].The SEM and TEM images of the OZ/R/U catalyst displayed that the UiO-66-NH 2 and O-ZnO were well dispersed over the rGO NS with close contact (Figure 4a,b).The OZ/R/U catalyst has a much higher photocurrent intensity than the OZnO and UiO-66-NH 2 catalysts.As a result, it confirms the high charge transfer and separation ability of the OZ/R/U (Figure 4c).The optimal rGO content was 1.5 wt% for the photoactivity of the composite OZnO/rGO/UiO-66-NH 2 photocatalyst (Figure 4d).Under visible light irradiation, the ternary photocatalyst demonstrated excellent photocatalytic performance for CO 2 reduction into methanol and formic acid, as well as good stability.The results showed that this ternary composite could effectively reduce CO 2 to HCOOH and CH 3 OH and that activity was far better than ZnO/rGO/UiO-66-NH 2 and O-ZnO/UiO-66-NH 2 .Under visible light irradiation, the yield of HCOOH and CH 3 OH over the ternary composite (O-ZnO/rGO/UiO-66-NH 2 ) achieved 6.41 and 34.83 µmol g −1 h −1 , respectively (Figure 4e).The photoinduced electrons on the CB of O-ZnO are transferred to the VB of UiO-66-NH 2 through rGO and combined with the photoinduced holes, resulting in the accumulation of electrons on the CB of UiO-66-NH 2 and the accumulation of holes over the VB of O-ZnO.The CB level of UiO-66-NH 2 is more negative than the reduction potential of CO 2 .The electrons accumulated over CB of UiO-66-NH 2 can readily reduce CO 2 to methanol.This Z-scheme charge transfer process of the above two semiconductors can reduce the rate of recombination of photogenerated charge carriers while maintaining the photogenerated electrons' high reduction capacity at more negative CB of UiO-66-NH 2 and the strong oxidation capacity of the photoinduced holes over more positive VB of O-ZnO.This Z-scheme photocatalytic system improved charge separation efficiency by transferring electrons from the CB of O-ZnO to the rGO NS and then recombining those electrons to holes in the VB of UiO-66-NH 2 (Figure 4f).This boosted composite photocatalyst activity was assigned to the development of a photocatalytic system (Z-scheme), which inhibits photogenerated charge carriers' recombination, while maintaining the high reduction capacity of UiO-66-NH 2 and the strong oxidizing ability of O-ZnO (Figure 4f).The Z-scheme heterojunction has a high electron-hole pair separation efficiency and redox ability and a broad light response range.The Z-scheme heterojunction is a wonderful alternative for converting CO2 into value-adding compounds because of the advantages listed above.The artificial Z-scheme photocatalyst is typically made up of two connected semiconductor photocatalysts: one for oxidation and the other for reduction [56].The Z-scheme gets its name from the fact that it connects the two photosystems in a fashion that looks like the letter "Z".In particular, the engineered Z-scheme direct photocatalysts that mimic the natural photosynthetic system provide a number of advantages, including improved light uptake, spatially separated reductive and oxidative active sites, and well-preserved strong redox capacity.Furthermore, photogenerated separation was made possible by the heterojunction at the interfaces, which boosted the charge participa-  The copper single-atoms (CuSA) on the UiO-66-NH2 support (CuSA@UiO-66-N have been developed for photoreduction of CO2 into liquid fuels in water medium (Fi 5a) [59] under visible-light irradiation.The morphology of the CuSA@UiO-66-NH2 ph catalyst was studied by HR-TEM and AC-STEM analysis, which clearly verified th currence of a Cu single-atom catalyst marked with red circles (Figure 5b-d).The im of elemental mapping of (CuSA@UiO-66-NH2), furthermore, demonstrated that the a N, C, O, Cu and Zr are uniformly dispersed on the support (Figure 5e).Remarkab was observed that the photocurrent density of CuSA@UiO-66-NH2 was more than UiO NH2, and the CuNPs@UiO-66-NH2 catalysts showed that a larger number of carriers w produced under visible light illumination (Figure 5f).Remarkably, the develo CuSA@UiO-66-NH2 reached the solar-induced CO2 conversion to ethanol and meth (f) Photocatalytic mechanism over O-ZnO/rGO/UiO-66-NH 2 composites.Reproduced from [58] with permission.
The copper single-atoms (Cu SA ) on the UiO-66-NH 2 support (Cu SA @UiO-66-NH 2 ) have been developed for photoreduction of CO 2 into liquid fuels in water medium (Figure 5a) [59] under visible-light irradiation.The morphology of the CuSA@UiO-66-NH 2 photocatalyst was studied by HR-TEM and AC-STEM analysis, which clearly verified the occurrence of a Cu single-atom catalyst marked with red circles (Figure 5b-d).The images of elemental mapping of (Cu SA @UiO-66-NH 2 ), furthermore, demonstrated that the atoms N, C, O, Cu and Zr are uniformly dispersed on the support (Figure 5e).Remarkably, it was observed that the photocurrent density of Cu SA @UiO-66-NH 2 was more than UiO-66-NH 2 , and the Cu NPs @UiO-66-NH 2 catalysts showed that a larger number of carriers were produced under visible light illumination (Figure 5f).Remarkably, the developed Cu SA @UiO-66-NH 2 reached the solar-induced CO 2 conversion to ethanol and methanol with an evolution rate of about 4.22 µmol h −1 g −1 and 5.33 µmol h −1 g −1 , respectively.The yields were significantly higher as compared to pure counterparts (Figure 5g).Liu et al. developed (ZIF-8/g-C 3 N 4 ) composites by developing various compositions of nanoclusters of ZIF-8 over the surface of g-C 3 N 4 [60].The ZIF-8 nanoclusters were deposited over g-C 3 N 4 nanotubes (NT); the overall tubular morphology remained the same (Figure 6a).Additionally, some nanoparticles protruded from the tube wall of g-C 3 N 4 nanotubes.The corresponding TEM image indicated that the ZIF-8 was effectively inserted on the surface of the g-C 3 N 4 NT (Figure 6b).After ZIF-8 surface grafting, charge separation and transfer efficiency were retarded.Further confirmation of this conclusion came from the photoelectrochemical measurements.Among the three samples, a TCN sample achieved the highest stable photocurrent value, indicating the rapid charge separation (Figure 6c,d).Due to the incorporation of ZIF-8 nanoclusters, ZIF-8@g-C 3 N 4 composites could adsorb more CO 2 than the g-C 3 N 4 NT without compromising light absorption ability.The (ZIF-8/g-C 3 N 4 ) composites had improved photocatalytic functioning for the reduction of CO 2 due to the higher charge separation efficacy and CO 2 capture ability of ZIF-8 from the g-C 3 N 4 NT, where the highest methanol evolution rate arrived at 0.75 mmol h −1 g −1 under the ZIF-8/g-C 3 N 4 composite (Figure 6e).The methanol production rate for bulk g-C 3 N 4 and g-C 3 N 4 NT was 0.24 and 0.49 mmol h −1 g −1 respectively, under similar conditions.However, the methanol was not produced on pristine ZIF-8 nanocrystals.Remarkably, the number of grafted ZIF-8 nanoclusters was critical for the conversion of ZIF-8 and synergetic nanostructural patterns.By combining nanostructure semiconductor and MOF grafts, the ZIF-8/g-C 3 N 4 photocatalyst exhibited an enhanced capacity of CO 2 adsorption, light-harvesting ability, and charge separation efficiency; it therefore, exhibited an excellent increment in effective photocatalytic methanol generation.Typically, a higher CO 2 adsorption capability accompanies a greater efficiency of photocatalytic CO 2 reduction for a semiconductor photocatalyst.
In a membrane reactor, Maina et al. demonstrated the controlled incorporation of TiO 2 nanoparticles and Cu II -doped TiO 2 nanoparticles over ZIF-8 films via the rapid thermal deposition (RTD) method (Figure 7a) [61].The presence of TiO 2 NPs in the MOF matrix was emphasized with the red square (Figure 7c).Under UV irradiation, the Cu-TiO 2 @ZIF-8 hybrid film exhibited powerful photocatalytic activity.The results indicate that, compared with only the quantity generated by pure ZIF-8 film single, the yields of CH 3 OH and CO raised by 50% and 188%, respectively (Figure 7d).Furthermore, when Cu@TiO 2 nanoparticles are loaded on MOF, the photocatalytic CO 2 reduction to CO and CH 3 OH is considerably enhanced (Figure 7e).When the loading of Cu@TiO 2 nanoparticles was 7 µg, the Cu-TiO 2 @ZIF-8 displayed excellent catalytic efficacy.Compared to the pure ZIF-8 film, CO and CH 3 OH yields were 23.3% and 70% respectively.This is attributed to a synergistic effect caused by semiconductor NPs' ability to generate photogenerated electrons under light illumination and the CO 2 adsorption potential of MOFs.
In 2018, Cardoso et al. prepared a MOF-based Ti/TiO 2 NT-ZIF-8 photocatalyst by growing films of ZIF-8 over Ti@TiO 2 nanotube electrodes through a layer-by-layer method (Figure 8a) [62].Figure 8b shows the addition of ZIF-8 composites to the TiO 2 NTs. Figure 8c displays the image captured by TEM for Ti/TiO 2 NT-ZIF-8: ZIF-8 nanoparticles can be observed between and within TiO 2 nanotubes.The photo-electrocatalytic CO 2 reduction by (Ti/TiO 2 NT-ZIF-8) electrodes was executed in sodium sulphate saturated with CO 2 at a constant potential (+0.1 V) under UV-visible light illumination.Notably, 0.7 mmol/L of methanol and 10 mmol/L of ethanol were generated in 3 h (Figure 8d).Because of the lack of ZIF-8 films, the Ti 4+ species in Ti@TiO 2 NT may have a lower ability for CO 2 absorption (Figure 8e).In a membrane reactor, Maina et al. demonstrated the controlled incorporation of TiO2 nanoparticles and Cu II -doped TiO2 nanoparticles over ZIF-8 films via the rapid thermal deposition (RTD) method (Figure 7a) [61].The presence of TiO2 NPs in the MOF matrix was emphasized with the red square (Figure 7c).Under UV irradiation, the Cu-TiO2@ZIF-8 hybrid film exhibited powerful photocatalytic activity.The results indicate that, compared with only the quantity generated by pure ZIF-8 film single, the yields of CH3OH and CO raised by 50% and 188%, respectively (Figure 7d).Furthermore, when Cu@TiO2 nanoparticles are loaded on MOF, the photocatalytic CO2 reduction to CO and CH3OH is considerably enhanced (Figure 7e).When the loading of Cu@TiO2 nanoparticles was 7 µg, the Cu-TiO2@ZIF-8 displayed excellent catalytic efficacy.Compared to the pure ZIF-8 film, CO and CH3OH yields were 23.3% and 70% respectively.This is attributed to a synergistic effect caused by semiconductor NPs' ability to generate photogenerated electrons under light illumination and the CO2 adsorption potential of MOFs.In 2018, Cardoso et al. prepared a MOF-based Ti/TiO2NT-ZIF-8 photocatalyst by growing films of ZIF-8 over Ti@TiO2 nanotube electrodes through a layer-by-layer method (Figure 8a) [62].Figure 8b shows the addition of ZIF-8 composites to the TiO2 NTs. Figure 8c displays the image captured by TEM for Ti/TiO2NT-ZIF-8: ZIF-8 nanoparticles can be observed between and within TiO2 nanotubes.The photo-electrocatalytic CO2 reduction by (Ti/TiO2NT-ZIF-8) electrodes was executed in sodium sulphate saturated with CO2 at a constant potential (+0.1 V) under UV-visible light illumination.Notably, 0.7 mmol/L of methanol and 10 mmol/L of ethanol were generated in 3 h (Figure 8d).Because of the lack 4+  As a consequence of the interaction between Cu/ZnOx nanoparticles and their ligands and Zr 6 SBUs, these ultra-small well-mixed NPs do not agglomerate as much and do not undergo phase separation.The Cu/ZnOx@MOF catalysts produce extremely higher activity with yield (2.59 g MeOH kg Cu −1 h −1 ), and higher selectivity for the CO 2 hydrogenation to obtain methanol, and strong stability for over 100 h.The catalyst was prepared by the in situ reduction of post-synthetic metalized UiO-bpy (Figure 9a).The ideal precatalyst structure of Zn@UiO-bpy-Cu via post-synthetic Cu co-ordination to bpy is shown in Figure 9b.As shown in Figure 9c, the CuZn@UiO-bpy has enriched sites that enhance the yield of methanol.The Cu and ZnO x combine well to form Zn and Zr, which leads to improved hydrogenation of CO 2 .The STEM-HAADF images of ultrasmall Cu/ZnO x and CuZn@UiO-bpy catalyst clearly showed the Cu and Zn were uniformly dispersed and mixed across the entire MOF particle and nanoparticles of 0.5−2.0nm in size (Figure 9d,e).The UiO-bpy proved to have an effective selectivity for methanol production (Figure 9f).As a consequence of the interaction between Cu/ZnOx nanoparticles and their ligands and Zr6 SBUs, these ultra-small well-mixed NPs do not agglomerate as much and do not undergo phase separation.The Cu/ZnOx@MOF catalysts produce extremely higher activity with yield (2.59 g MeOH kg Cu −1 h −1 ), and higher selectivity for the CO2 hydrogenation to obtain methanol, and strong stability for over 100 h.The catalyst was prepared by the in situ reduction of post-synthetic metalized UiO-bpy (Figure 9a).The ideal precatalyst structure of Zn@UiO-bpy-Cu via post-synthetic Cu co-ordination to bpy is shown in Figure 9b.As shown in Figure 9c, the CuZn@UiO-bpy has enriched sites that enhance the yield of methanol.The Cu and ZnOx combine well to form Zn and Zr, which leads to improved hydrogenation of CO2.The STEM-HAADF images of ultrasmall Cu/ZnOx and CuZn@UiO-bpy catalyst clearly showed the Cu and Zn were uniformly dispersed and mixed across the entire MOF particle and nanoparticles of 0.5−2.0nm in size (Figure 9d,e).The UiO-bpy proved to have an effective selectivity for methanol production (Figure 9f).It was crucial to enhance the stability of metal-oxide QDs for reaction patterns containing H 2 O so that they could be employed in photocatalytic CO 2 reduction.Recently, Li et al. designed the (g-C 3 N 4 /CuO@MIL-125(Ti)) photocatalyst for the reduction of CO 2 with the proximity of H 2 O by encapsulating CuO quantum dots with pores of MOFs of MIL-125 (Ti) via a complexation-oxidation method (Figure 10a) [64].In aqueous reaction systems, the photocatalyst displays significantly enhanced stability due to the protection offered by the MIL-125(Ti) framework.Because of the close contact between the CuO QDs and the Ti active site in MIL-125 (Ti), electron transfers between the confined CuO QDs and the Ti active site were observed (Figure 10b).The presence of water greatly increases the photocatalytic activity of g-C 3 N 4 /CuO@MIL-125(Ti) composites for the photoreduction of CO 2 .Among the prepared catalysts, the resultant 2.5% g-C 3 N 4 /1%CuO@MIL-125(Ti) photocatalyst performed better for photocatalytic reduction of CO 2 to CH 3 OH, CO, CH 3 CHO, and CH 3 CH 2 OH, implying water (reductant), with yields of 997.2, 180.1, 531.5, and 1505.7 umol/g, respectively (Figure 10c).Figure 10d-g depicts the structural morphologies of the 2.5% g-C 3 N 4 /1% CuO@MIL-125(Ti) photocatalyst.This study developed an efficient method for enhancing charge separation efficiency and stability of metallic-oxide QD-adapted photocatalyst.Furthermore, MIL-125(Ti), due to its high specific surface area and porous structure, provides an encapsulating structure for CuO QDs to prevent aggregation and provides excellent stability and recyclability.Moreover, the unique heterostructure of the composite ensures high light absorption and efficient electron transport from g-C 3 N 4 nanosheets and MIL-125(Ti) to QDs as well.The recent research progress of MOF-based materials/photocatalysts for photocatalytic reduction of CO 2 is summarised in Table 1.
125 (Ti) via a complexation-oxidation method (Figure 10a) [64].In aqueous reaction systems, the photocatalyst displays significantly enhanced stability due to the protection offered by the MIL-125(Ti) framework.Because of the close contact between the CuO QDs and the Ti active site in MIL-125 (Ti), electron transfers between the confined CuO QDs and the Ti active site were observed (Figure 10b).The presence of water greatly increases the photocatalytic activity of g-C3N4/CuO@MIL-125(Ti) composites for the photoreduction of CO2.Among the prepared catalysts, the resultant 2.5% g-C3N4/1%CuO@MIL-125(Ti) photocatalyst performed better for photocatalytic reduction of CO2 to CH3OH, CO, CH3CHO, and CH3CH2OH, implying water (reductant), with yields of 997.2, 180.1, 531.5, and 1505.7 umol/g, respectively (Figure 10c).Figure 10 (d-g) depicts the structural morphologies of the 2.5% g-C3N4/1% CuO@MIL-125(Ti) photocatalyst.This study developed an efficient method for enhancing charge separation efficiency and stability of metallicoxide QD-adapted photocatalyst.Furthermore, MIL-125(Ti), due to its high specific surface area and porous structure, provides an encapsulating structure for CuO QDs to prevent aggregation and provides excellent stability and recyclability.Moreover, the unique heterostructure of the composite ensures high light absorption and efficient electron transport from g-C3N4 nanosheets and MIL-125(Ti) to QDs as well.The recent research progress of MOF-based materials/photocatalysts for photocatalytic reduction of CO2 is summarised in Table 1.

Mixed-Metal-Oxide-Based Photocatalyst
Mixed metal oxides (MMOs), containing two or more types of metals and oxygen, have been broadly employed as photocatalysts for the reduction of CO 2 .The semiconducting properties of their aqueous suspensions irradiated with visible light have been a prominent subject of research.Mixed metal oxide (MMO) is one of the most important photocatalysts, with features that differ from ordinary oxides in circumstances such as acid-base, redox, and surface area.These have been widely explored for several catalytic applications due to their excellent chemical-thermal stability compared to single oxides [70][71][72].

Transition and Non-Transition Metals
Singhal et al. reported Ni/InTaO 4 as supported catalysts for the CO 2 photoreduction to methanol under visible light irradiation by the facile sol-gel method.Loading of 1 wt% Ni on InTaO 4 -produced methanol with a higher yield and slowed down recombination by reducing the bandgap slightly.The bandgap of InTaO 4 and Ni/InTaO 4 catalysts was noted at 2.6 eV and 2.54 eV, respectively.The SEM image shows the agglomerated nature of the catalyst, and EDX confirmed the existence of elements (In, Ta, O, and Ni).TEM displays the size of nanomaterials at 50-80 nm.The HR-TEM demonstrated the crystallinity of InTaO 4 , which was a good match for the XRD pattern.XPS measurement of the catalyst showed the presence of Ni and NiO due to four peaks at different binding energies.The catalyst displayed sharp diffraction peaks, indicating monoclinic InTaO 4 .The material crystalline phase is not altered by Ni loading [73].Recently, the bimetallic oxide was derived from MOFs developed by Cheng et al. [74].
Kumar et al. proposed a highly efficient rGO-covered magnetically separable coreshell-structured microsphere photocatalyst (rGO@CuZnO@Fe 3 O 4 ) for CO 2 reduction below visible light illumination (Figure 11a) [75].In order to allow simultaneous CO 2 reduction and water oxidation, the semiconductor should have a wide bandgap.Neither Fe 3 O 4 nor ZnO by themselves can convert CO 2 to methanol because electrons can only move from the valence band (VB) to conduction band (CB) under the influence of UV light.The morphology of the surface and core-shell structure of microspheres were examined by FE-SEM tomography.The structure of rGO@CuZnO@Fe 3 O 4 remains the same after GO reduction to rGO (Figure 11b).The HR-TEM image clearly showed the 1D lattice plane at about 0.26 nm interplanar distance, corresponding to the (002) plane of ZnO.Additionally, the amorphous zone (0.64 nm thickness) related to rGO was visual (Figure 11c,d).The rGO@CuZnO@Fe 3 O 4 catalyst has shown excellent catalytic activity for a methanol yield (2656 µmol g −1 ) among the various catalysts prepared (Figure 11e).The advantages of this catalyst include a high yield of methanol, no sacrificial donor requirement, ease of recovery, and effective recycling (Figure 11f).The methanol yield increased with Cu loading from 0.25 to 1.0 wt%; however, the photocatalytic performance did not improve with a further increase in Cu content.The enhancement in the reduction in methanol yield due to Cu content can be attributed to the trapping of photo-generated electrons, which reduces the recombination process of electron holes (Figure 11g).This catalyst has a low price, higher electron mobility, and environmental preservation than ZnO.The large surface area and excellent charge-carrier mobility of rGO can lead to the development of a highly efficient, reusable photocatalyst for CO 2 reduction.
Gao et al. synthesized separate Vv-rich and Vv-poor o-BiVO 4 atomic layers with the thickness of a unit of a cell at the gram-scale using an intermediate lamellar hybrid approach (Figure 12a) [76].The single-unit-cell o-BiVO 4 layers with a high content of vanadium vacancies (Vv) are high in the production of methanol(398.3µmol g −1 h −1 ) compared to atomic layers of V v -poor o-BiVO 4 (Figure 12c).At the atomic level, a correlation between defect sites and photoreduction of CO 2 was investigated.A density-functional theory calculation shows that vanadium (V) vacancies introduce a new defect level and greater hole concentration around Fermi levels, resulting in enhanced electronic conductivity and photoabsorption (Figure 12b).The increased surface photovoltage of layers with vanadium voids is confirmed by the enhanced carrier lifetime of the o-BiVO 4 layers, which is shown through time-persistent fluorescence spectra.
Yu et al. prepared a novel efficient visible light 2D photocatalyst (Ti 3 C 2 /Bi 2 WO 6 nanosheets) via in situ progress of ultrathin nanosheets of Bi 2 WO 6 with a surface of Ti 3 C 2 nanosheets (Figure 13a) [77].The generation of the 2D/2D heterojunction was characterized by TEM studies; the results demonstrated the clean ultrathin nanosheets along with lucent features, nanoparticles of Ti 3 C 2 @ Bi 2 WO 6 nanosheets, and the element composition and distribution (Figure 13b-d).The Ti 3 C 2 @Bi 2 WO 6 heterostructured hybrids with distinct atomic layers had much higher photoreduction activity than pure Bi 2 WO 6 , with a 4.6 times higher overall yield of CH 3 OH and CH 4 than pure Bi 2 WO 6 (Figure 13g).Under solar light irradiation, photo-induced electrons are excited and jump from the VB to the CB of Bi 2 WO 6 .The Bi 2 WO 6 has a more negative CB potential compared to the EF of Ti 3 C 2 with terminal -O; photoinduced electrons can then be transferred from Bi 2 WO 6 to Ti 3 C 2 via an ultra-thin layered heterojunction.The photo-induced electrons that have accumulated on the surface of Ti 3 C 2 can then interact with the CO 2 molecules that have been adsorbed.The photo-induced electrons can be quickly transferred from the bulk of Bi 2 WO 6 to the heterojunction interface up to the Ti 3 C 2 surface because of the unique atomic layer 2D/2D heterostructure.Thus, a significant enhancement in the efficiency of photocatalytic CO 2 reduction was observed.This 2D photocatalyst possesses a high contact area and a small distance of charge transport.As a result, electrons are efficiently transferred from the Bi 2 WO 6 (photocatalyst) to the Ti 3 C 2 (cocatalyst) (Figure 13h).The enhancement of photocatalytic functioning is due to excellent CO 2 adsorption ability and effective separation of charge carrier.The present work demonstrated that Ti 3 C 2 nanosheets can be employed as effective cocatalysts for the photoreduction of CO 2 .(g) Methanol yield using Cu wt% in ZnO.Reproduced from [75] with permission.
Gao et al. synthesized separate Vv-rich and Vv-poor o-BiVO4 atomic layers with the thickness of a unit of a cell at the gram-scale using an intermediate lamellar hybrid approach (Figure 12a) [76].The single-unit-cell o-BiVO4 layers with a high content of vanadium vacancies (Vv) are high in the production of methanol(398.3µmol g −1 h −1 ) compared  Yu et al. prepared a novel efficient visible light 2D photocatalyst (Ti3C2/Bi nanosheets) via in situ progress of ultrathin nanosheets of Bi2WO6 with a surface of nanosheets (Figure 13a) [77].The generation of the 2D/2D heterojunction was chara ized by TEM studies; the results demonstrated the clean ultrathin nanosheets along lucent features, nanoparticles of Ti3C2@ Bi2WO6 nanosheets, and the element compos and distribution (Figure 13b-d).The Ti3C2@Bi2WO6 heterostructured hybrids with di atomic layers had much higher photoreduction activity than pure Bi2WO6, with a 4.6 higher overall yield of CH3OH and CH4 than pure Bi2WO6 (Figure 13g).Under solar irradiation, photo-induced electrons are excited and jump from the VB to the C Bi2WO6.The Bi2WO6 has a more negative CB potential compared to the EF of Ti3C2 terminal -O; photoinduced electrons can then be transferred from Bi2WO6 to Ti3C2 v ultra-thin layered heterojunction.The photo-induced electrons that have accumulate the surface of Ti3C2 can then interact with the CO2 molecules that have been adsorbed photo-induced electrons can be quickly transferred from the bulk of Bi2WO6 to the he junction interface up to the Ti3C2 surface because of the unique atomic layer 2D/2D h ostructure.Thus, a significant enhancement in the efficiency of photocatalytic CO2 re tion was observed.This 2D photocatalyst possesses a high contact area and a smal tance of charge transport.As a result, electrons are efficiently transferred from the Bi (photocatalyst) to the Ti3C2 (cocatalyst) (Figure 13h).The enhancement of photocat functioning is due to excellent CO2 adsorption ability and effective separation of ch carrier.The present work demonstrated that Ti3C2 nanosheets can be employed as tive cocatalysts for the photoreduction of CO2.Viswanathan et al. synthesized a photocatalyst (Fe-N/Na (1−x) La x TaO (3+x) ) via a hydrothermal method.Using a variety of approaches, researchers evaluated the effect of doping or co-doping LNTO (lantana) with iron (Fe) and nitrogen (N) on the photo-physical properties of catalysts.The dopant iron (Fe-3d) energy level of the orbital was lower than the Ta-5d energy level.The new energy level is under the conduction band.The Fe 3+ ion (dopant) retains the capacity to capture and de-capture the charge carriers.It can readily capture photogenerated electrons from the CB of NaTaO 3 .A decrease in the PL line intensity suggests that dopants/co-dopants (Fe or N) can reduce recombination, therefore enhancing the charge carrier's lifetime.The enhancement in activity is because of co-doping with N and Fe over lantana (LNTO).The XPS and XRD results show that the Fe 3+ occupies the Ta 5 + ions site, La 3+ ions Na + ion sites, and Nitrogen (N) takes O 2− sites.N and Fe co-doping in a Ta matrix results in the confining of the bandgap because of the formation of secondary energy levels between the bandgap and the absorption of visible light.The presence of dopants (La 3+ and Fe 3+ ) in the tantalate (Ta) lattice assures electro-neutrality.The charge trapping and de-trapping of (Fe 3 /Fe 4+ ) ions efficiently decreases the recombination of the charge carrier.The co-doping of LNTO through N and Fe increases the absorption of visible light because of the synergetic effect.It hinders charge carrier recombination and boosts charge transfer interfacial, enhancing photocatalytic CO 2 reduction to CH 3 OH in aqueous alkaline media higher than that of pure NaTaO 3 [78].
Kumar et al. developed a Z-scheme heterostructured (rGO/InVO 4 /Fe 2 O 3 ) photocatalyst for the reduction of CO 2 to methanol over visible light illumination by the depositionprecipitation method.The existence of a few dark spots over the sheet surface suggested the presence of iron oxide (Fe 2 O 3 ) in the nanocomposites.In Raman spectra, the presence of comparable bands of all constituents at 468, 924, 1329, and 1587 cm −1 proved the existence of Yan et al. [80] developed a polymorph of indium oxide as a photocatalyst for the formation of CO and CH 3 OH.Zhang et al. reported an efficient, highly selective, and stable photocatalyst (In 2 O 3−x (OH) y ) with a rod-like nano-crystallized structure for hydrogenation of gaseous carbon dioxide to methanol and an evolution rate (0.06 mmol gcat −1 h −1 ) under solar irradiation [81].The SEM image showed the rod length of the nanocrystal superstructure to be about 2.6 µm (Figure 14a).The nanoporous nature of the materials produced, made up of nanocrystalline superstructures, has been confirmed by TEM studies (Figure 14b,c).The photocatalytic stability of the In 2 O 3−x (OH) y nano-crystallized structure sample was illustrated.The stabilized rate of methanol within the first 15 h led to a significant enhancement in the 16th h optimized at 250 • C in the light for 20 h (Figure 14d).DFT measurements of the energy profile for the production of methanol via CO 2 hydrogenation over In 2 O 3−x (OH) y (111) were investigated.An acetal intermediate was formed by adding a hydride (H 2 CO 2 *).The lowest energy issue was chosen as the probable intermediate (Figure 14e).

Photocatalysts Based on Carbon Materials
Carbon-based photocatalysts have become increasingly popular for the reduction of CO2 because of their excellent physicochemical and electrochemical properties.Several excellent carbon-based supports such as graphene, CNT, carbon dots, graphitic carbonnitride, and conducting polymers have been employed for several applications over the years.The use of different carbonaceous materials as supports for photocatalysis is highly beneficial, since carbon has fair photocatalytic activity, high surface area, high electrical conductivity, higher dispersion, and visible light absorption capability [82,83].
Gusain et al. synthesized rGO-CuO nanocomposites for CO2 reduction to methanol over visible light illumination.The deposition of CuO nanorods over rGO enhanced photocatalytic efficiency and produced a higher yield of methanol compared to pure CuO nanorods [84].In a similar line, Liu et al. prepared cuprous oxide decorated to various morphologies of rGO(Cu2O/rGO photocatalysts) for CO2 reduction under visible light.As-prepared and well characterized rhombic dodecahedral Cu2O/rGO catalysts exhibited the maximum methanol yield [85].
Yu et al. reported a ternary composite (Ag2CrO4/g-C3N4/GO) for the photoreduction of CO2 to methane and methanol by using silver chromate NPs (photosensitizer) and graphene oxide (cocatalyst) [86].The dispersion of Ag2CrO4 nanoparticles over the surface of Reproduced from [81] with permission.

Photocatalysts Based on Carbon Materials
Carbon-based photocatalysts have become increasingly popular for the reduction of CO 2 because of their excellent physicochemical and electrochemical properties.Several excellent carbon-based supports such as graphene, CNT, carbon dots, graphitic carbonnitride, and conducting polymers have been employed for several applications over the years.The use of different carbonaceous materials as supports for photocatalysis is highly beneficial, since carbon has fair photocatalytic activity, high surface area, high electrical conductivity, higher dispersion, and visible light absorption capability [82,83].
Gusain et al. synthesized rGO-CuO nanocomposites for CO 2 reduction to methanol over visible light illumination.The deposition of CuO nanorods over rGO enhanced photocatalytic efficiency and produced a higher yield of methanol compared to pure CuO nanorods [84].In a similar line, Liu et al. prepared cuprous oxide decorated to various morphologies of rGO(Cu 2 O/rGO photocatalysts) for CO 2 reduction under visible light.
As-prepared and well characterized rhombic dodecahedral Cu 2 O/rGO catalysts exhibited the maximum methanol yield [85].
Yu et al. reported a ternary composite (Ag 2 CrO 4 /g-C 3 N 4 /GO) for the photoreduction of CO 2 to methane and methanol by using silver chromate NPs (photosensitizer) and graphene oxide (cocatalyst) [86].The dispersion of Ag 2 CrO 4 nanoparticles over the surface of the g-C 3 N 4 sheet was investigated through SEM and TEM imaging (Figure 15a,b).The triplet composites displayed an increased conversion of CO 2 with a TOF of 0.30 h −1 , which was higher than bare g-C 3 N 4 under sunlight irradiation.The photocatalytic activity was enhanced because of expanded light absorption, greater adsorption of CO 2, and efficient charge separation.Ag 2 CrO 4 nanoparticles can improve the g-C 3 N 4 light absorption, and GO cocatalyst (electron acceptor), besides promoting charge transfer, also provides higher adsorption of CO 2 and catalytic sites.Notably, charge transfer takes place between Ag 2 CrO 4 and g-C 3 N 4 via a direct Z-scheme mechanism, which enhances the photocatalytic system's redox ability while promoting charge separation (Figure 15e).Jain et al. developed a cobalt phthalocyanine embedded over g-C3N4 as a hybrid photocatalyst (denoted as g-C3N4/CoPc-COOH) for selective reduction of CO2 into CH3OH by employing a sacrificial electron donor (triethylamine (TEA) in the presence of visible light (Figure 16a) [87].The immobilization of the CoPc-COOH complex over the g-C3N4 support was confirmed by HR-TEM (Figure 16b).The resultant g-C3N4/CoPc-COOH hybrid provided greatly increased affinity of CO2 through active Co +2 sites, charge separation  16a) [87].The immobilization of the CoPc-COOH complex over the g-C 3 N 4 support was confirmed by HR-TEM (Figure 16b).The resultant g-C 3 N 4 /CoPc-COOH hybrid provided greatly increased affinity of CO 2 through active Co 2+ sites, charge separation through the g-C 3 N 4 surface, as well as large surface areas for efficient CO 2 conversion.They have demonstrated and investigated that the hybrid photocatalyst offered a much improved yield of methanol than the reported catalysts.The higher methanol yield over the hybrid photocatalyst was observed after 24 h, employing TEA as an electron contributor (Figure 16c).Notably, the durable interaction between -COOH with heteroatoms of g-C 3 N 4 support makes a robust hybrid photocatalyst that ultimately inhibits metal leaching during photoreduction.The Gong group investigated the production of a family of polymeric C3N4/CdSe QDs (p-CNCS) with various particle sizes of CdSe for CO2 photoreduction into methanol, targeting maximum selectivity and activity by the quantum confinement effect [88].Based on the quantum confinement effect, the energy of the CB electrons was modified to a suitable value, which was deficient for H2 formation but sufficient for methanol formation, and which enhances activity and selectivity for methanol production from the photocatalytic CO2 reduction reaction (CO2RR).CdSe QDs were packed on p-C3N4 via an impregnation method, producing p-CNCS as a photocatalyst (Figure 17b-d).The correlation between the band energy and photocatalytic activity have been discussed.Moreover, the high surface area and appropriate dangling bonds of p-C3N4 (Polymeric carbon nitride nanosheet) provide sufficient loading sites for CdSe quantum dots, allowing a strong interaction between p-C3N4 and CdSe to the heterojunction (Figure 17e,f).The heterojunction within 0D/2D materials could transport photoinduced holes and electrons to CdSe and p-C3N4, enhancing charge separation and increasing CdSe stability.The Gong group investigated the production of a family of polymeric C 3 N 4 /CdSe QDs (p-CNCS) with various particle sizes of CdSe for CO 2 photoreduction into methanol, targeting maximum selectivity and activity by the quantum confinement effect [88].Based on the quantum confinement effect, the energy of the CB electrons was modified to a suitable value, which was deficient for H 2 formation but sufficient for methanol formation, and which enhances activity and selectivity for methanol production from the photocatalytic CO 2 reduction reaction (CO2RR).CdSe QDs were packed on p-C 3 N 4 via an impregnation method, producing p-CNCS as a photocatalyst (Figure 17b-d).The correlation between the band energy and photocatalytic activity have been discussed.Moreover, the high surface area and appropriate dangling bonds of p-C 3 N 4 (Polymeric carbon nitride nanosheet) provide sufficient loading sites for CdSe quantum dots, allowing a strong interaction between p-C 3 N 4 and CdSe to the heterojunction (Figure 17e,f).The heterojunction within 0D/2D materials could transport photoinduced holes and electrons to CdSe and p-C 3 N 4 , enhancing charge separation and increasing CdSe stability.Bafaqeer et al. synthesized 2D/2D heterojunction composite catalyst (2D ZnV2O6/pCN) via combing ZnV2O6 and g-C3N4 (protonated -CN) for selective pho duction of CO2 with H2O via the solvothermal method [89].The HR-TEM was emplo to examine the crystalline structure of the pCN merged with ZnV2O6 nanosheets (Fi 18b,c).The surface charge conversion with protonation of g-C3N4 acts as a moderator trapped photo-excited electrons.The performances of as-prepared catalysts were stu in gas-phase and liquid photocatalytic methods under light irradiation (UV/visible).methanol formation rate of the 2D/2D ZnV2O6/pCN(100%) composite catalyst was µmol g-cat − 1 higher than that of the pure ZnV2O6 and pCN catalysts (Figure 18d,e).W bombarded with light, the pCN in the heterostructured photocatalysts functions as a sitizer, absorbing photons and exciting electron and hole pairs.Photoexcited electron pCN might be transported to ZnV2O6 CB because the CB edge potential of pCN (1.12 is more negative than that of ZnV2O6 (0.87 eV).The protonation of g-C3N4 can opera an excellent acceptor and trap for photoexcited electrons, allowing the photo-indu Bafaqeer et al. synthesized 2D/2D heterojunction composite catalyst (2D/2D ZnV 2 O 6 /pCN) via combing ZnV 2 O 6 and g-C 3 N 4 (protonated -CN) for selective photoreduction of CO 2 with H 2 O via the solvothermal method [89].The HR-TEM was employed to examine the crystalline structure of the pCN merged with ZnV 2 O 6 nanosheets (Figure 18b,c).The surface charge conversion with protonation of g-C 3 N 4 acts as a moderator and trapped photoexcited electrons.The performances of as-prepared catalysts were studied in gas-phase and liquid photocatalytic methods under light irradiation (UV/visible).The methanol formation rate of the 2D/2D ZnV 2 O 6 /pCN(100%) composite catalyst was 3742 µmol g-cat −1 higher than that of the pure ZnV 2 O 6 and pCN catalysts (Figure 18d,e).When bombarded with light, the pCN in the heterostructured photocatalysts functions as a sensitizer, absorbing photons and exciting electron and hole pairs.Photoexcited electrons on pCN might be transported to ZnV 2 O 6 CB because the CB edge potential of pCN (1.12 eV) is more negative than that of ZnV 2 O 6 (0.87 eV).The protonation of g-C 3 N 4 can operate as an excellent acceptor and trap for photoexcited electrons, allowing the photo-induced electrons to be quickly transported to the ZnV 2 O 6 and converted to methanol.The photoexcited electron-hole pairs could be effectively separated in this way.As a result, the created junction between pCN and ZnV 2 O 6 in heterostructured photocatalysts reduces electron and hole recombination during the charge carrier transfer process, resulting in improved photocatalytic activity of pCN/ZnV 2 O 6 heterojunctions.In addition to the synergistic effect of ZnV 2 O 6 and pCN nanosheet heterojunctions, combining pCN with ZnV 2 O 6 resulted in increased activity.Furthermore, the hierarchical structure, rich 2D coupling surfaces, increased interfacial contact, and charge separation could all help to improve photoactivity and product selectivity.The photocatalytic stability of the 2D/2D ZnV 2 O 6 /pCN composite catalyst with a moderator was excellent.
Catalysts 2022, 12, x FOR PEER REVIEW 25 of 39 electrons to be quickly transported to the ZnV2O6 and converted to methanol.The photoexcited electron-hole pairs could be effectively separated in this way.As a result, the created junction between pCN and ZnV2O6 in heterostructured photocatalysts reduces electron and hole recombination during the charge carrier transfer process, resulting in improved photocatalytic activity of pCN/ZnV2O6 heterojunctions.In addition to the synergistic effect of ZnV2O6 and pCN nanosheet heterojunctions, combining pCN with ZnV2O6 resulted in increased activity.Furthermore, the hierarchical structure, rich 2D coupling surfaces, increased interfacial contact, and charge separation could all help to improve photoactivity and product selectivity.The photocatalytic stability of the 2D/2D ZnV2O6/pCN composite catalyst with a moderator was excellent.Kumar et al. synthesized CNT−TiO2 photocatalysts for photoreduction of CO2 and water splitting by joined sonothermal and hydrothermal method [90].Figure 19a-d shows the effect of CNT on TiO2 morphology.The (101) plane of anatase TiO2 (lattice spacing 0.342 nm) is confirmed by HRTEM.The CNT content increases the attachment of spherical TiO2.The prepared CNT−TiO2 photocatalysts showed increased photocatalytic activity for the reduction of CO2 compared to bare TiO2 in the presence of visible light.As prepared catalysts, the 2.0 CNT−TiO2 showed better performance for the yield of methanol under UV light (Figure 19g,h).According to a computational study, the binding of CNT to TiO2 NPs was desirable at (101) surfaces rather than (001) facets (Figure 19e).The photoexcitation of this composite over visible light leads to charge transfer within CNT and TiO2 and the formation of isolated charge carriers, while UV light excitation leads to charge transfer in all directions from TiO2 to CNT and CNT to TiO2 (Figure 19i).Kumar et al. synthesized CNT-TiO 2 photocatalysts for photoreduction of CO 2 and water splitting by joined sonothermal and hydrothermal method [90].Figure 19a-d shows the effect of CNT on TiO 2 morphology.The (101) plane of anatase TiO 2 (lattice spacing 0.342 nm) is confirmed by HRTEM.The CNT content increases the attachment of spherical TiO 2 .The prepared CNT-TiO 2 photocatalysts showed increased photocatalytic activity for the reduction of CO 2 compared to bare TiO 2 in the presence of visible light.As prepared catalysts, the 2.0 CNT-TiO 2 showed better performance for the yield of methanol under UV light (Figure 19g,h).According to a computational study, the binding of CNT to TiO 2 NPs was desirable at (101) surfaces rather than (001) facets (Figure 19e).The photoexcitation of this composite over visible light leads to charge transfer within CNT and TiO 2 and the formation of isolated charge carriers, while UV light excitation leads to charge transfer in all directions from TiO 2 to CNT and CNT to TiO 2 (Figure 19i).Tang et al. investigated a special variety of CD (carbon-dots) as a hole receiver for mCD/CN composite when produced via the flexible microwave method [38].The co-residence and morphological information of both m CD and CN structures were investigated through HR-TEM.The as-prepared CN was graphene-like nanosheets, and m CD has a graphitic structure with a featured-spacing of about 0.23 nm (Figure 20a,b).The m CD/CN composite was c.a. 12 times more active than s CD/CN (sonication method) for the conversion of CO 2 .Remarkably, the m CD/CN nanocomposite generated methanol and oxygen from H 2 O and CO 2 with a selectivity of around 100% methanol (Figure 20d) and an internal quantum efficiency of 2.1% in the visible area, which was validated through isotopic labelling.Furthermore, the unique m CD captured holes from CN (carbon nitride) and hindered methanol adsorption, which resulted in the oxidization of water instead of methanol and the enhancement of the selectivity for reduction of CO 2 to alcohols (Figure 20c).Tang et al. investigated a special variety of CD (carbon-dots) as a hole receiver for mCD/CN composite when produced via the flexible microwave method [38].The co-residence and morphological information of both m CD and CN structures were investigated through HR-TEM.The as-prepared CN was graphene-like nanosheets, and m CD has a graphitic structure with a featured-spacing of about 0.23 nm (Figure 20a,b).The m CD/CN composite was c.a. 12 times more active than s CD/CN (sonication method) for the conversion of CO2.Remarkably, the m CD/CN nanocomposite generated methanol and oxygen from H2O and CO2 with a selectivity of around 100% methanol (Figure 20d) and an internal quantum efficiency of 2.1% in the visible area, which was validated through isotopic labelling.Furthermore, the unique m CD captured holes from CN (carbon nitride) and hindered methanol adsorption, which resulted in the oxidization of water instead of methanol and the enhancement of the selectivity for reduction of CO2 to alcohols (Figure 20c).

TiO 2 Based Photocatalysts
Titanium dioxide (TiO 2 ) is a wide-bandgap semiconductor.In a variety of energy and environmental applications, TiO 2 is employed as a photocatalyst because of its high stability, efficient photoactivity, low price, and safety for people and the environment.TiO 2 has properties such as nontoxicity, easy availability, chemical stability, and the capability to oxidize in the presence of radiation.TiO 2 -based photocatalysts will be able to solve the main global problems related to pollution of the environment and renewable energy requirements [91,92] [93].The structural and compositional characteristics of as-prepared catalysts were investigated through XRD, XPS, and TEM images (Figure 21a-d).The TEM and HR-TEM revealed that the size of the Cu 2 O and TiO 2 nanoparticles were approximately 5 and 10 nm, respectively (Figure 21d,e).The enhancement in the photoactivity of the as-prepared photocatalyst was because of the synergistic effect produced by pillaring K 2 Ti 4 O 9 (layered semiconductor) with TiO 2 and the eventual loading of Cu 2 O nanoparticles as cocatalyst.The increment in the surface area of the photocatalyst TiO 2 /Ti 4 O 9 /Cu 2 O was from (25 to 145 m 2 g −1 ) analysed through nitrogen adsorption-desorption isotherms (NADI).The methanol production was double that of the pure K 2 Ti 4 O 9 due to the enhanced activity of the photocatalyst (Figure 21f).Incorporating Cu 2 O nanoparticles onto mesoporous solid surfaces extended solar radiation absorption, mobility of electron and charge separation at the surface.
Jiang and co-workers synthesized hybrid carbon@TiO 2 composite hollow spheres for CO 2 photo-reduction by the simple and benign method of employing colloidal carbon spheres (Figure 22a) [94].The nanostructure, hollow spherical structure, and elemental mapping of the as-prepared composite were confirmed by FESEM, TEM and STEM analysis (Figure 22b-f).The obtained carbon@TiO 2 composite nanostructure displayed good photoreduction of CO 2 operation and selectivity over artificial solar light compared to TiO 2 (P25).The methanol generation rate was (9.1 µmol g −1 h −1 ) greater than pure TiO 2 (Figure 22g).The carbon composition of the carbon@TiO 2 composites considerably affected photocatalytic activity.The improvement in the photocatalytic performance of the carbon@TiO 2 photocatalyst was because of enhanced specific surface area (110 m 2 g −1 ), the CO 2 absorption capacities, and a native photothermal impact near the photocatalyst due to the carbon.According to the electrochemical impedance spectra (EIS), carbon composition could affect the efficiency of charge transfer of carbon@TiO 2 composites.
Yadav et al. successfully loaded Au nanoparticles on S 8 -TiO 2 (S8-TiO 2 (40)-Au photocatalyst) via the sol immobilization method.The as-prepared catalysts employed or CO 2 photoreduction to methanol and HER under visible light irradiation [95].Sharma et al. developed a nanocomposite photocatalyst NiO-TiO 2 /ACF via the sol-gel method for photoreduction of CO 2 with H 2 O into methanol by immobilization of Ni loaded TiO 2 on initiated carbon fibers (ACFs) under UV and visible light illumination (Figure 23a) [96].The SEM images of ACF, ACF-TiO 2 and NiO-TiO 2 /ACF photocatalyst indicated that the even distribution of NiO/TiO 2 nanoparticles was found over the surface of ACF with high crystallinity.The ACF surface was almost clean after pre-treatment with a hierarchical porous structure capable of acting as active sites for adsorption (Figure 23b-d).The yield of methanol over NiO-TiO 2 /ACF photocatalyst was 986.3 and 755.1 µmol g −1 under visible light and UV, respectively (Figure 23e).Increasing the photocatalytic activity of Ni-loaded TiO 2 was achieved with the use of the ACF support, which inhibited electron-hole recombination.The ACFs (surface area: 163.9 m 2 /g) and NiO were used to increase CO 2 adsorption capacity and also to alter the electronic absorption properties of TiO 2 .Moreover, NiO co-doped TiO 2 beat the recombination rate issue of electron-hole and induced the formation of Ti 3+ and oxygen vacancies for CO 2 conversion into CH 3 OH over UV/visible light irradiation (Figure 23f).produced from [93] with permission.
Jiang and co-workers synthesized hybrid carbon@TiO2 composite hollow spheres CO2 photo-reduction by the simple and benign method of employing colloidal car spheres (Figure 22a) [94].The nanostructure, hollow spherical structure, and eleme mapping of the as-prepared composite were confirmed by FESEM, TEM and STEM a ysis (Figure 22b-f).The obtained carbon@TiO2 composite nanostructure displayed g photoreduction of CO2 operation and selectivity over artificial solar light compared TiO2 (P25).The methanol generation rate was (9.1 µmol g -1 h -1 ) greater than pure TiO2 (Figure 22g).The carbon composition of the carbon@TiO2 composites considerably affected photocatalytic activity.The improvement in the photocatalytic performance of the carbon@TiO2 photocatalyst was because of enhanced specific surface area (110 m 2 g -1 ), the CO2 absorption capacities, and a native photothermal impact near the photocatalyst due to the carbon.According to the electrochemical impedance spectra (EIS), carbon composition could affect the efficiency of charge transfer of carbon@TiO2 composites.23a) [96].The SEM images of ACF, ACF-TiO2 and NiO-TiO2/ACF photocatalyst indicated that the even distribution of NiO/TiO2 nanoparticles was found over the surface of ACF with high crystallinity.The ACF surface was almost clean after pre-treatment with a hierarchical porous structure capable of acting as active sites for adsorption (Figure 23b-d).The yield of methanol over NiO-TiO2/ACF photocatalyst was 986.3 and 755.1 µmol.g −1 under visible light and UV, respectively (Figure 23e).Increasing the photocatalytic activity of Ni-loaded TiO2 was achieved with the use of the ACF support, which inhibited electron-hole recombination.The ACFs (surface area: 163.9 m 2 /g) and NiO were used to increase CO2 adsorption capacity and also to alter the electronic absorption properties of TiO2.Moreover, NiO codoped TiO2 beat the recombination rate issue of electron-hole and induced the formation of Ti 3+ and oxygen vacancies for CO2 conversion into CH3OH over UV/visible light irradiation (Figure 23f).24a) [97].The FESEM, TEM, and HR-TEM images of TiO 2 /Ni(OH) 2 photocatalyst revealed that the hierarchical nanostructure.The thickness of nanosheets of Ni(OH) 2 was nearly 20 nm and was well deposited on TiO 2 surface (Figure 24b-d).The TiO 2 /Ni(OH) 2 composite nanofibre exhibited unusually enhanced activity of CO 2 photoreduction than the bare TiO 2 fibers.The yield of CH 3 OH over this hybrid photocatalyst was increased after loading of 15 wt% Ni(OH) 2 and irradiation time (Figure 24e,f).The TiO 2 /Ni(OH) 2 photocatalyst displayed an increased activity of CO 2 reduction, selectivity, the efficiency of charge separation, and increased the density of the CO 2 on photocatalyst surface due to the existence of Ni(OH) 2 nanosheets as cocatalyst.The clusters of Ni(OH) 2 /Ni can act as electron sinks and stimulate the photo-generated electron separation from TiO 2 to a cluster of Ni(OH) 2 /Ni and toward CO 2 molecules, which can play a vital role as active sites and during the photocatalytic process, reducing photo-excited charge carrier recombination (Figure 24g).Later on, modified TiO 2 /rGO/CeO 2 composite photocatalyst was investigated for CO 2 photoconversion by the ultrasonication method [98].
Catalysts 2022, 12, x FOR PEER REVIEW 32 of 39 ward CO2 molecules, which can play a vital role as active sites and during the photocatalytic process, reducing photo-excited charge carrier recombination (Figure 24g).Later on, modified TiO2/rGO/CeO2 composite photocatalyst was investigated for CO2 photoconversion by the ultrasonication method [98].Reproduced from [97] with permission.

Photocatalysts with Plasmonic Properties
Recently, plasmonic photocatalysis has enabled rapid advances in improving the photocatalytic efficiency for CO 2 reduction under irradiation with visible light.It employs precious metal NPs dispersed on semiconductor photocatalysts and has outstanding properties such as localized plasmonic surface resonance (LSPR), which contributes to the strong absorption of visible light and the excitation of active charge carriers.The conduction of electrons on the nanoparticle surface offers plasmonic materials extraordinary optical characteristics.The interaction between free electrons in metal nanoparticles and incident light is known as the plasmonic effect.Plasmonic metal nanoparticles with high light absorptivity have been shown to represent a new class of photocatalysts with features that differ dramatically from those of typical semiconductor photocatalysts.Plasmonic nanoparticles have unique optical, electrical, and thermal properties [99][100][101][102][103][104][105][106][107].In this section, we focused on plasmonic photocatalysts for the photoreduction of CO 2 to methanol.
Becerra et al. developed an efficient method for the preparation of plasmonic photocatalyst (Aux@ZIF-67) [69].The decoration of Au NPs over ZIF-67 was investigated for the reduction of CO 2 under irradiation with sunlight.The plasmonic system showed greatly enhanced photocatalytic activity for CO 2 reduction to CH 3 OH with excellent selectivity.The plasmonic Au NPs (size 30-40 nm) significantly enhanced the absorption of visible light, increased separation of charge, and contributed to selectivity.This nanocomposite demonstrated the advantages of high specific surface area (SSA) as well as electrochemical properties.An optimal production rate of methanol (~2.5 mmol g −1 h −1 ) was achieved for the conversion of CO 2 .This deposited Au NPs favours both the activity and the selectivity of synthesized plasmonic photocatalyst because of the injection of energetic electrons on the surface of the ZIF-67 derived from the plasmonic response.The localized surface plasmon resonance (LSPR) at the edges of rod-and triangular nanostructures decreases the recombination of electron-hole pairs during the photocatalytic reduction of CO 2 .The production of CH 3 OH (~2.5 mmol g −1 h −1 )and C 2 H 5 OH (0.5 mmol g −1 h −1 ) were achieved from Au nanorods and Au nanotriangle, respectively.This Au@ ZIF-67 plasmonic system has been significantly improved in terms of photocatalytic activity to reduce CO 2 and improve cost-efficiency.Yadav et al. reported plasmonic photocatalyst (Au/Ti x Si 1−x O 2 ) for photoreduction of CO 2 employing an LED light source by deposition of plasmonic Au NPS on mesoporous titania with isolated silica (Ti x Si 1−x O 2 ).Following a DFT study, an excellent adsorption of CO 2 on the surface was observed, which could have been due to the incorporation of silica sites.The methanol production was 1835 µmol gcat −1 using Au/Ti x Si 1−x O 2 material with 28 mol% Si in titania lattice and 1.0 wt% Au nanoparticle deposition [108].
Ye et al. reported plasmonic catalysts (Cu/ZnO) for photoreduction of CO 2 into methanol that promoted visible light illumination under atmospheric pressure.The rich Cu-ZnO interfaces are recognized as active sites for the production of CH 3 OH (Figure 25a).The production rate of methanol rose from 1.38 to 2.13 µmol g −1 min −1 , and the noticeable activation energy was reduced from 82.4 to 49.4 kJ mol −1 (Figure 25b).N 2 sorption isotherms specified that the physicochemical characteristics of the decreased Cu/ZnO catalyst were mesoporous in nature (Figure 25c).The mechanism fuses the photo-generated hot electrons onto Cu NPs, and these electrons can transfer to the ZnO via interfaces of metal-support (Figure 25d).The activation of reaction intermediates co-operatively promoted by hot-electrons over Cu and ZnO leads to the photo-promoted synthesis of methanol [109].

Conclusions and Future Perspectives
In this review, we discussed the recent research advances of photocatalytic CO duction into methanol.Photocatalysts such as MOFs, mixed-metal oxide, carbon, TiO plasmonic-based photocatalytic reduction of CO2 to methanol were systematically marized.The photocatalytic activity, photo-excited charge transfers and separation ciency, CO2 capture capacities, and stability can be enhanced by supporting synthetic cedures, semiconductor, metal, and ligand replacement, and incorporation of photoa

Conclusions and Future Perspectives
In this review, we discussed the recent research advances of photocatalytic CO 2 reduction into methanol.Photocatalysts such as MOFs, mixed-metal oxide, carbon, TiO 2 and plasmonic-based photocatalytic reduction of CO 2 to methanol were systematically summarized.The photocatalytic activity, photo-excited charge transfers and separation efficiency, CO 2 capture capacities, and stability can be enhanced by supporting synthetic procedures, semiconductor, metal, and ligand replacement, and incorporation of photoactive responsive units.A detailed overview of the catalytic performance of various photocatalysts for CO 2 reduction to methanol would be helpful to researchers.The selectivity and catalytic activity that are mostly directed by the electronic environment of the metals could be attuned by a combination of support, doping of heteroatoms, and employing a system with multiple metals.The photoreduction of CO 2 to valuable chemicals and to fuels has gradually become important because of its efficiency in simultaneously solving global warming and energy crisis problems.Indeed, photoactive materials retain their specific benefits with light-driven CO 2 reduction, and all of these efficient variation strategies will offer directions to the rational design of photocatalysts with improved catalytic performance.It is hoped that this review will play a crucial and interesting role in serving future developments in this important field.

Figure 1 .
Figure 1.The number of publications for the keyword search "Photocatalytic reduction of CO2 to methanol" as found in Web of Science (dated: 12 November 2021).

Figure 1 .
Figure 1.The number of publications for the keyword search "Photocatalytic reduction of CO 2 to methanol" as found in Web of Science (dated: 12 November 2021).

Figure 2 .
Figure 2. The supporting materials used in photocatalytic CO 2 reduction to fuels.

Figure 7 .
Figure 7. (a) A schematic representation for the synthesis of Cu-TiO 2 /ZIF-8 membranes; (b) SEM image of porous substrate; (c) The high-resolution (HR-TEM) image of the TiO 2 @ZIF-8 hybrid; (d) Effect of Cu−TiO 2 nanoparticle loading on product yields; (e) The impact of membrane content on yield of product.Reproduced from [61] with permission.

1 Figure 13 .
Fe 2 O 3 , InVO 4, and rGO in the composites.In comparison to Fe 2 O 3 and InVO 4 , notably rGO/InVO 4 /Fe 2 O 3 had a lower PL intensity, indicating less charge recombination and thus greater photocatalytic activity.The XRD pattern indicated the appearance of Fe 2 O 3 and InVO 4 into the composite The photo-induced electron transfer occurred from the CB of Fe 2 O 3 into the VB of InVO 4 and further to the CB of InVO 4 over the Z-scheme system for the CO 2 photoreduction.Triethylamine (TEA) was employed as a sacrificial electron donor to achieve methanol yields as high as 16.9 mmol gcat −1 employing the Z-scheme-based photocatalyst.The recycling ability of the photocatalyst demonstrated greater stability and efficient recyclability.The synergistic effect of the ternary rGO/InVO 4 /Fe 2 O 3 photocatalyst leads to effective separation of charge carriers and charge mobility over the surface of the catalysts, which results in an effective reduction of CO 2 and an enhancement in the yield of methanol [79].(a) Schematic representation for preparation of ultrathin Ti 3 C 2 /Bi 2 WO 6 nanosheets; (b-d) TEM, enlarged image, elemental mapping; (e) FESEM; (f) Transient photocurrent spectra of the prepared catalyst; (g) Photocatalytic activity; (h) Energy profile diagram of Ti 3 C 2 and Bi 2 WO 6 and process electron transfer.Reproduced from [77] with permission.

Figure 14 .
Figure 14.Characterization of In 2 O 3−x (OH) y photocatalyst: (a-c) SEM/TEM and HR-TEM images; (d) Stability of catalytic performance; (e) The mechanism for CO 2 hydrogenation into CH 3 OH.Reproduced from[81] with permission.

Figure 15 .
Figure 15.(a,b) Characterization (SEM/TEM images) of the Ag 2 CrO 4 /g-C 3 N 4 /GO composite photocatalyst; (c) CH 3 OH formation of methanol over different samples of CN, under induced sunlight irradiation (1 h); (d) CH 3 OH and CH 4 formation over various catalysts; (e) The Z-scheme mechanism for reduction of CO 2 .Reproduced from [86] with permission.

Catalysts 2022 ,
12,  x FOR PEER REVIEW 23 of 39 g-C3N4 support makes a robust hybrid photocatalyst that ultimately inhibits metal leaching during photoreduction.

Figure 17 .
Figure 17.(a) Pattern of p-C3N4; (b) Sketch for p-CNCS; (c,d) The HR-TEM images of p-CNC Methanol generation selectivity employing with various size of particles; (f) Catalytic perform of as-prepared catalysts.Reproduced from [88] with permission.

Figure 17 .
Figure 17.(a) Pattern of p-C 3 N 4 ; (b) Sketch for p-CNCS; (c,d) The HR-TEM images of p-CNCS; (e) Methanol generation selectivity employing with various size of particles; (f) Catalytic performance of as-prepared catalysts.Reproduced from [88] with permission.

Figure 20 .
Figure 20.(a) HR-TEM images of CN; (b) HR-TEM images of m CD/CN nanocomposite; (c) The fundamental insights for high methanol selectivity; (d) Yield of methanol; (e) Schematic diagram for photoreduction of CO2 over the m CD/CN nanocomposites.Reproduced from [38] with permission.

Figure 20 .
Figure 20.(a) HR-TEM images of CN; (b) HR-TEM images of m CD/CN nanocomposite; (c) The fundamental insights for high methanol selectivity; (d) Yield of methanol; (e) Schematic diagram for photoreduction of CO 2 over the m CD/CN nanocomposites.Reproduced from [38] with permission.

Figure 22 .Figure 22 .
Figure 22.(a) The mechanism for the preparation of carbon@TiO2 composite hollow structure; (be) FESEM and TEM images of samples T60 and T120; (f) STEM image of T60 and comparable elemental mapping images of C, O and Ti; (g) The photocatalytic activity of carbon@TiO2 composite samples; (h) Photo-excitation process of the carbon@TiO2 composite.Reproduced from [94] with permission.Yadav et al. successfully loaded Au nanoparticles on S8-TiO2 (S8−TiO2 (40)−Au photocatalyst) via the sol immobilization method.The as-prepared catalysts employed or CO2 photoreduction to methanol and HER under visible light irradiation [95].Sharma et al. developed a nanocomposite photocatalyst NiO-TiO2/ACF via the sol-gel method for pho-Figure 22.(a) The mechanism for the preparation of carbon@TiO 2 composite hollow structure; (b-e) FESEM and TEM images of samples T60 and T120; (f) STEM image of T60 and comparable elemental mapping images of C, O and Ti; (g) The photocatalytic activity of carbon@TiO 2 composite samples; (h) Photo-excitation process of the carbon@TiO 2 composite.Reproduced from [94] with permission.

Figure 25 .
Figure 25.(a) HR-TEM image of Cu/ZnO catalyst; (b) Methanol production; (c) N2 sorption therms; (d) The mechanism of methanol preparation.Reproduced from [109] with permission Fan et al. established a plasmonic photocatalyst (Cu/TiO2) by employing both hy thermal and microwave-assisted processes [110].Both Cu NPs and unique TiO2 film play the properties of light harvesting as per LSPR.The charge carrier recombination decreased by the deposition of Cu NPs, that were found during fluorescence quenc The photocatalytic activity of Cu/TiO2 films was excellent because of charge transfer ciency and LSPR absorption of Cu NPs.Recently, Wang et al. prepared plasmonic ph catalysts (Ag NPs/ACFs) by deposition of plasmonic Ag NPs over acid-ached carbo bers (ACFs) coupled with ultrasonication for photoreduction of CO2 to methanol u visible light irradiation [111].

Figure 25 .
Figure 25.(a) HR-TEM image of Cu/ZnO catalyst; (b) Methanol production; (c) N 2 sorption isotherms; (d) The mechanism of methanol preparation.Reproduced from [109] with permission.Fan et al. established a plasmonic photocatalyst (Cu/TiO 2 ) by employing both hydrothermal and microwave-assisted processes [110].Both Cu NPs and unique TiO 2 film display the properties of light harvesting as per LSPR.The charge carrier recombination was decreased by the deposition of Cu NPs, that were found during fluorescence quenching.The photocatalytic activity of Cu/TiO 2 films was excellent because of charge transfer efficiency and LSPR absorption of Cu NPs.Recently, Wang et al. prepared plasmonic photocatalysts (Ag NPs/ACFs) by deposition of plasmonic Ag NPs over acid-ached carbon fibers (ACFs) coupled with ultrasonication for photoreduction of CO 2 to methanol under visible light irradiation [111].

Table 1 .
Reported MOF-based photocatalysts for reduction of CO 2 to fuels.
. Nogueira et al. reported nanocomposite TiO 2 /Ti 4 O 9 /Cu 2 O for photocatalytic CO 2 reduction through loading of Cu 2 O on TiO 2 -pillared tetratitanate (K 2 Ti 4 O 9 ) Xu et al. synthesized hybrid TiO 2 /Ni(OH) 2 photocatalysts for photoreduction of CO 2 by deposition of perpendicularly arranged Ni(OH) 2 nanosheets on the TiO 2 fibers by electrospinning and the wet-chemical precipitation method (Figure