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Development of nanostructured carbon-based catalysts for photocatalytic conversion of CO2 into added-value fuels Author Yi Cai Director Francisco Balas Nieto Master in Nanostructured Material for Nanotechnology Applications 2019/2020 Development of nanostructured carbon-based catalysts for photocatalytic conversion of CO2 into added-value fuels Yi Cai Abstract Nowadays, energy shortages, carbon resource shortages, and global warming are the major challengs. The usage of photocatalysis to realize the conversion of CO2 into added-value fuels can alleviate the above problems. As a visible light photocatalyst without metal components, g-C3N4 has unique electronic structure, high catalytic activity, good chemical and thermal stability, which has caused wide attention. This article mainly summarized the basic properties and structure of g-C3N4. Besides, we introduced the commonly used preparation methods and modification methods of g-C3N4, and prospects the application of g-C3N4. Keywords: g-C3N4; photocatalysis; CO2 reduction INDEX Development of nanostructured carbon-based catalysts for photocatalytic conversion of CO2 into added-value fuels ..................................................................................................................... 2 Abstract ............................................................................................................................................ 2 1 Introduction .................................................................................................................................. 4 2 Photocatalytic reduction of CO2 ................................................................................................. 4 2.1 Basic principles and reaction mechanism ....................................................................... 5 2.2 Application of carbon-based photocatalytic catalysts in CO2 reduction ...................... 6 2.2.1 Graphene and its derivatives ................................................................................. 6 2.2.2 GDY ......................................................................................................................... 7 2.2.3 g-C3N4 ...................................................................................................................... 7 3 Advantages and challenges of g-C3N4 catalyst ........................................................................... 8 4 g-C3N4 catalyst and the effect of its preparation method g-C3N4 on performance ................. 8 4.1 Hard-templating method .................................................................................................. 9 4.2 Soft-templating method .................................................................................................. 10 4.3 No-templating method .................................................................................................... 12 5 Modification of g-C3N4 photocatalytic material in CO2 reduction ........................................ 14 5.1 Surface engineering ......................................................................................................... 14 5.1.1 Functional group modification ............................................................................ 15 5.1.2 Surface defects modification ............................................................................... 16 5.2 Semiconductor compound .............................................................................................. 16 5.2.1 Type II heterojunction ......................................................................................... 17 5.2.2 Type I and Type III heterojunction ..................................................................... 18 5.2.3 Schottky junction ................................................................................................. 19 5.2.4 Z type heterojunction ........................................................................................... 20 5.3 Element doping ................................................................................................................ 21 5.3.1 Non-metallic element doping ............................................................................... 21 5.3.2 Metal doping ......................................................................................................... 23 5.4 Specific surface area control modification technology ................................................. 24 5.4.1 Mesoporous g-C3N4 .............................................................................................. 25 5.4.2 Nanoflakes ............................................................................................................. 27 5.4.3 g-C3N4 with different morphologies .................................................................... 28 6 Conclusions ................................................................................................................................. 29 Bibliography .................................................................................................................................. 30 1 Introduction Carbon dioxide (CO2) is the main greenhouse gas in the atmosphere, and the increasing concentration of CO2 is one of the most serious problems which contribute to global warming and climate change. Due to the development of industry and society, carbon dioxide emissions mainly come from the burning of fossil fuels in energy use. As the demand for fossil resources continues to grow in the coming decades, it is essential to reduce carbon dioxide emission. The main ways to reduce carbon dioxide can be divided into emission reduction and post-treatment. The post-treatment of carbon dioxide can be further divided into deep sea burial, chemical adsorption storage and chemical conversion. Compared with traditional, high-energy-consuming methods such as capture or geological storage, the resource utilization of CO2 is a more promising approach which can simultaneously alleviate the greenhouse effect and energy crisis. Various approaches have been developed in basic research, such as thermal catalysis, photocatalysis, electrocatalysis, photo-electric synergistic catalysis and organic catalytic conversion, to convert CO2 molecules into high value-added chemicals. Among them, CO2 conversion driven by solar energy is an environmentally friendly treatment method which has outstanding advantages such as mild reaction conditions and so on. The photocatalytic reduction of CO2 is similar to the photosynthesis of plants in nature. It takes the greenhouse gas CO2 emitted by humans and the abundant water resources on the earth as raw materials. Then CO2 is reduced to various energy-containing reducing products through the action of light energy. It is consumed by the existing forms of energy utilization again, and finally completes the carbon cycle process, realizing the balanced development of energy demand for human development and sustainable use of environmental resources. 2 Photocatalytic reduction of CO2 Inspired by photosynthesis, solar energy is used to produce hydrocarbons and O2 from CO2 and H2O. In this process, energy can be simultaneously generated when CO2 in the atmosphere can be reduced. Therefore, photocatalytic reduction of CO2 is the most important way in artificial photosynthesis. 2.1 Basic principles and reaction mechanism When the photocatalyst is excited by radiant light, the generated charge carriers will migrate to the surface of the photocatalyst and interact with the CO2 molecules adsorbed on the surface. The principle of photocatalytic reduction of CO2 is shown in Fig 11. In particular, the reaction of photocatalytic reduction of CO2 includes CO2 reduction and H2O oxidation, involving the cleavage of C=O bonds and the formation of C-H bonds. Therefore, the photoreduction of CO2 is not a single-electron reaction process, but a proton-induced multi-electron reaction to produce various products, including the following processes: CO2 + 2H+ + 2e−→HCOOH Eredox= − 0.20 V（1） CO2 + 2H+ + 2e−→CO + H2O Eredox= − 0.12 V（2） CO2 + 4H+ + 4e−→HCHO + H2O Eredox= − 0.07 V（3） CO2 + 6H+ + 6e−→CH3OH + H2O Eredox= + 0.03 V（4） CO2 + 8H+ + 8e−→CH4 + 2H2O Eredox= + 0.17 V（5） Thermodynamically, CO2 is an extremely stable molecule. The dissociation energy of the C=O double bond is as high as 750 kJ⋅mol-1, which is significantly higher than the C-H bond (430 kJ⋅mol-1) and C-C bond (336 kJ⋅mol-1) in the reduction product, which means that a large amount of energy needs to be injected into the system to trigger the activation and conversion process of CO2. At the same time, the C (Ⅳ) is in the highest oxidation state in the CO2 molecule, and various products can be obtained according to the number of electrons obtained in the reaction. In the photocatalytic reaction, CO2 can usually be reduced to carbon monoxide (CO), methane (CH4), formic acid (HCOOH) or ethanol (CH3OH) and other substances, which with the side reaction of water reduction to generate hydrogen, and it will be significantly reduced the selectivity of the target product. Fig. 1. Schematic illustration of reaction steps in photocatalytic CO2 reduction with H2O From the current research status, the photoreduction of CO2 still faces the difficulties of low conversion rate and poor selectivity. In order to obtain a higher efficiency, the catalyst not only needs to have a suitable energy band structure, but also needs to optimize its surface structure and electronic structure to improve the ability of catalyst to adsorb and activate CO2 which could inhibit the formation of by-products. Therefore, how to overcome these obstacles has become an important issue in the design of efficient photocatalysts for CO2 photocatalytic reduction. 2.2 Application of carbon-based photocatalytic catalysts in CO2 reduction There are many kinds of photocatalysts which can be used in the photoreduction of CO2. They can be divided into metal and non-metal photocatalysts. The former includes oxides, sulfides, bismuth oxyhalides (BiOX), metal organic framework compounds (MOFs) and so on. The latter mainly includes carbon-based catalysts and h-BN. This article will focus on the specific introduction of carbon-based catalysts and examples of their realization of the photocatalytic conversion of CO2. 2.2.1 Graphene and its derivatives Graphene and its derivatives (GO, rGO) are a well-known catalyst. Since graphene was first reported in 2004, it has been widely used in many research fields due to its excellent mechanical, thermal, optical and electronic properties. As shown in Fig 22, the use of graphene-based materials for CO2 photoreduction shows the following advantages: (1) Ultra-thin 2D morphology gives graphene a high specific surface area and provides abundant adsorption sites for the reaction substrate. (2) Graphene is a zero band gap material with excellent conductivity and electron mobility. (3) It can form π-π conjugate with CO2 molecules to promote CO2 activation. Zou3 obtained a TiO2/graphene composite material rich in Ti3+ sites by using the simultaneous reduction-hydrolysis technology. The abundant Ti3+ in TiO2 particles can capture photogenerated electrons to inhibit recombination, and synergize with graphene for photocatalytic reaction to achieve CO2 reduction and coupling to generate CH4 and C2H6. Fig.2. Graphene-based photocatalysts for CO2 photoreduction. 2.2.2 GDY Graphene (GDY) is an allotrope of graphene. It is a 2D planar structure composed of spand sp2-hybrid carbon, with a band gap ranging from 0.46 to 1.22 eV4. In terms of electrical properties, GDY has a moderate band gap and high electron mobility (104~105 cm2⋅V-1⋅s-1). The abundant C≡C makes the electron conjugation degree of GDY high, which is very beneficial to the adsorption and activation of CO2. From the structural point of view, GDY contains periodic arrangement of triangular holes, which can effectively reduce its density4-5. Due to the many uncontrollable factors in the reaction of preparing 2D GDY, including monomer stability, side reactions, monomer orientation coupling6, low yield, etc., there are only a few literature reports7 about the use of 2D GDY in CO2 photoreduction reactions. 2.2.3 g-C3N4 g-C3N4 is a new type of non-metallic semiconducting polymer material. It consists of C and N atoms through sp2-hybridization to form a large π conjugated system with a high degree of delocalization, generally composed of anti-triazine ring structural which has a larger binding energy and more stable. Most of the literature usually uses N-rich molecules such as urea, cyanamide and melamine as precursors to form g-C3N4 at high temperatures8. It has more suitable Eg (≈2.77 eV) and CB positions (≈-1 V). At the same time, the spectral absorption covering the ultraviolet and visible regions can meet the needs of CO2 reduction. 3 Advantages and challenges of g-C3N4 catalyst In recent years, g-C3N4 has become a popular material in the field of photocatalysis due to its special energy band structure and easy manufacturing advantages. It can be excited by visible light irradiation, so it has great potential value for the utilization and conversion of solar energy; due to the existence of quantum confinement effect, the position of CB of g-C3N4 moves up slightly, and the reduction ability is further enhanced. It forms π-π conjugation with CO2, which significantly promotes the adsorption and activation of CO2. However, g-C3N4 catalyst still has some bottlenecks that limit its photocatalytic activity, such as high photo-generated electron-hole pair recombination efficiency, limited surface catalytic reaction active sites and low specific surface area. But most importantly, g-C3N4 is a very ideal modification platform, and its performance can be optimized by controlling the preparation method of the material, as well as the doping, compounding, defect construction, surface functional group control and other ways. 4 g-C3N4 catalyst and the effect of its preparation method g-C3N4 on performance In recent years, some new properties of g-C3N4 have been gradually discovered by scientific researchers, which has further promoted the upsurge of research on g-C3N4 semiconductor materials. So far, g-C3N4 with a variety of morphologies has been successfully synthesized, including nanofibers, nanotubes, nanosheets, spheres, etc. These studies have strongly promoted the deep development of g-C3N4 in the field of photocatalysis. Using nitrogen-containing organic molecules as precursors, it is the simplest and direct method to prepare g-C3N4 materials under high temperature (500~600 ℃) pyrolysis polymerization in air or inert atmosphere. However, g-C3N4 prepared by this method had shortcomings such as low specific surface area, which greatly limits its application in the field of catalysis. In comparison, the template method is currently the main method for preparing g-C3N4 with high specific surface area and large pore size. According to the type of template used, it can be divided into hard template method, soft template method and no template method. 4.1 Hard-templating method The hard-templating method is used to design and prepare g-C3N4 materials with various structures and morphologies ranging from millimeters to nanometers. The introduction of different morphologies and highly regular pore structures can effectively adjust the structural properties and surface interaction of g-C3N49-10, promoting the mass transfer and diffusion process of the catalyst surface, helping optimize its semiconductor energy band structure and light absorption characteristics, and improving g-C3N4 photocatalytic performance10. Compared with traditional methods, the penetration of the precursor solution into the pore structure of the template will induce the synthesis of g-C3N4 with different morphologies. The filling of the precursor solution in the template hole channel directly affects the final morphology of the carbon nitride product. There are three main steps to prepare g-C3N4 by hard-templating method: (1) Immerse the precursor into the template. (2) Dry and roast at high temperature to make the precursor in the pores of the template polycondensate to form g-C3N4. (3) The template is removed by using HF or NH4HF2 solvent to obtain g-C3N4 materials with a mesoporous structure11-12. Goettmann13prepared mesoporous g-C3N4 by hard-templating method. Using mesoporous SiO2 spheres with a controllable nanostructure as a hard template, the cyanamide precursor was uniformly dispersed in the mesopores of the SiO2 spheres, and then thermally polymerized. The template was removed with NH4HF2 solvent to thereby greatly reducing the energy gap (~0.7 eV) 35. Fig. 3. (A) Illustration for fabrication of the S doping methyl-modified g-C3N4. (B) The formation energy (Ef) of substitute possible N atoms by S atoms in g-C3N4 networks. 5.1.2 Surface defects modification Defect engineering is an interesting strategy to optimize the photocatalytic efficiency of semiconductor materials. As we all know, defect engineering of g-C3N4 can effectively enhance charge separation, optimize the energy band structure and extend the light response. Therefore, various surface defects of g-C3N4, such as carbon vacancies, nitrogen vacancies, cyanamide defects and structural edge defects, have been extensively studied in recent years to improve the photocatalytic performance of g-C3N4 in CO2 reduction. Zhang36 devised a new strategy to prepare g-C3N4 with rich porous structure and heterostructure defects doped with sulfur atoms by processing g-C3N4 in the presence of CH3CN and H2S. In this special gas environment, the original g-C3N4 nanosheets are etched to produce nanopores. In addition, due to the incomplete conversion of H2S, the melon unit is destroyed to form CS bonds, cyano groups and S. The prepared photocatalyst showed excellent energy band structure, extended light absorption and fast carrier transport, which greatly improved the photocatalytic performance of H2O decomposition. 5.2 Semiconductor compound Semiconductor recombination refers to the usage of semiconductor materials with different energy levels and matching relative energy band positions to recombine on the surface of g-C3N4. g-C3N4 mainly plays two roles. The first is to provide electrons for the system, because g-C3N4 has a two-dimensional layered structure with many small in-plane repeating units, and the repeating units have a conjugate structure, so g-C3N4 has one The large conjugate structure facilitates electron transmission; the second is to cooperate with other semiconductors to form a heterojunction, promoting the separation and transmission of photogenerated carriers, and the photoelectric conversion efficiency. According to the heterojunction formed by the recombination of carbon nitride and semiconductor, it can be roughly divided into type II heterojunction, type I heterojunction, type III heterojunction, Schottky junction and Z type heterojunction. 5.2.1 Type II heterojunction For the type II heterojunction, the band edges of the two semiconductors are arranged staggered, causing the conduction band electrons of the high conduction band semiconductor migrate to the conduction band of the low conduction band semiconductor, and the valence band electrons of the high valence band semiconductor to the low valence band semiconductor. The migration of the valence band causes the photogenerated electrons and holes to accumulate in the low conduction band and low valence band, respectively, inhibiting their recombination. Liu37 synthesized ZnIn2S4-g-C3N4 nano-layered composite by hydrothermal method. The band structure of ZnIn2S4 (2.34~2.48 eV) and g-C3N4 can be matched to form a type II heterojunction, and the interface between the two close contact, effectively transfer and separate photo-generated carriers (as shown in Fig 4). The same type of substrate can also be used as a basis to construct a homogeneous g-C3N4 type II heterojunction. For example, Dong38 used dicyandiamide and urea as precursors, and obtained similar substrates through heat treatment to form g-C3N4/ g-C3N4 metal-free type II heterojunction. Under visible light irradiation, CN-D (with dicyandiamide as the precursor) The conduction band electrons of carbon nitride obtained by the system migrate to the conduction band of CN-U (carbon nitride obtained by using urea as the precursor system), and the valence band holes of the latter migrate to the valence band of the former (as shown in the Fig 5), to achieve effective separation of electrons and holes. Fig.4. Mechanism for the enhanced photocatalytic activity of ZnIn2S4-g-C3N4 composites Fig.5. Illustration of photogenerated electrons and holes transfer process on type Ⅱ g-C3N4/ g-C3N4 heterostructure under visible light irradiation 5.2.2 Type I and Type III heterojunction For type I heterojunction, the conduction band and valence band positions of semiconductor 1 are more negative and positive than those of semiconductor 2, respectively, as shown in Fig 6a. The electrons and holes generated during light irradiation accumulate in the semiconductor with a narrow band gap through migration. In this case, the electron-hole pairs are not effectively separated, but the photocatalytic activity is reduced. For type III heterojunction, the conduction band edge and valence band edge of semiconductor 1 are completely higher than the conduction band edge of semiconductor 2 (Fig 6b). The conduction band and valence band edge of the two semiconductors are completely staggered because they are not effective. Ground separation of electron-hole pairs cannot improve their activity, so there are few studies on type I and type III heterojunctions based on g-C3N4. Fig.6. Illustration of photogenerated electrons and holes transferprocess on type Ⅰ and Ⅲ heterostructures 5.2.3 Schottky junction Similar to the TiO2/Pt system, noble metals such as Pt39-40, Au41 and Pd42 are generally used as promoters to form Schottky junctions with g-C3N4. Noble metals have a high work function and can be used as electron traps to capture and store photogenerated electrons. It is effectively separated from the space of the photo-generated holes, improving its hydrogen production activity. There are also studies on using transition metals as co-catalysts to form Schottky junctions with g-C3N4 to improve its activity43-44. Bi et al. used solvothermal method to use melamine and nickel acetylacetonate as precursors, and loaded Ni on g-C3N4 to form Schottky junction. The band bending phenomenon was observed, which can effectively separate photogenerated carriers and greatly improve the yield of H2. In addition to metals, organic carbon materials, graphene, and carbon nanotubes can act as co-catalysts to accept g-C3N4 photogenerated electrons due to their excellent electrical conductivity. Ong45 prepared 2D/2D rGO/pCN heterojunction catalysts through electrostatic self-assembly, which formed an intimate contact between the two, which effectively separated carriers at the heterojunction interface and prevented the recombination of electron-hole pairs. , Effectively improve the performance of photocatalytic reduction of CO2. 5.2.4 Z type heterojunction The Z-type heterojunction is similar to the type II heterojunction in the energy band structure, but the electron hole flow direction is different, so its redox performance is different. The conduction band electrons of the low conduction band semiconductor combine and annihilate with the valence band holes of the low valence band semiconductor, making the valence band holes of the former and the conduction band electrons of the latter are effectively separated, and the redox effect is exerted. Therefore, this heterojunction not only has a wide spectral absorption range, but also has a high redox capability, which effectively solves the problem of reduced redox properties caused by carrier migration in type II heterojunctions. The most typical is the Z-type heterojunction composed of WO3 and g-C3N4. The band gap of WO3 is 2.7~2.8 eV, which is close to the band gap of g-C3N4, and the conduction band and valence band sides are more positive than g-C3N4, which can form staggered band-edge potential energy, satisfying the structure of Z-type heterojunction46. The research of Chen47 showed that when g-C3N4 is the main part, it formed a Z-type heterojunction with WO3, and the valence band holes of g-C3N4 quickly recombined with the conduction band electrons of WO3, resulting in photogenerated electrons of g-C3N4 and WO3. Photo-generated holes accumulate, and its photocatalytic activity for degrading BF was more than doubled compared with g-C3N4 and WO3 (as shown in Fig 7). Fig.7. Separation mechanisms of photogenerated electrons andholes on WO3/g-C3N4 heterojunction 5.3 Element doping Doping usually refers to the purposeful incorporation of a small amount of other elements or compounds into the matrix to change the crystal phase structure, electronic distribution or surface state. It is a commonly used modification method for preparing high-efficiency catalysts. 5.3.1 Non-metallic element doping The doping of non-metals48-51, such as B, S, O, P, F, C, etc., can replace the C, N, and H elements in the 3-s-triazine structural unit, resulting in the replacement of lattice defects and effectively suppressing recombination of electron-hole to improve the photocatalytic performance of g-C3N4. Zhai et al. applied first principles to study the changes caused by S and O doping quantum g-C3N4. It is found that S and O doping can replace N atoms in different positions in g-C3N4 and cause changes in the surrounding C-N bonds, significantly reducing the HOMO-LUMO energy gap. The macroscopic improvement of optical performance was manifested in broadening the light response range and increasing the light absorption intensity, and the higher the doping concentration, the more obvious the improvement in optical performance. It can be seen that non-metal doping is of great significance for improving photocatalytic activity, and has important research value in the field of photocatalytic reduction of CO2. Li34 doped g-C3N4 with S to investigate the reaction mechanism of reducing CO2. It is found that the doping of S is easier to replace the N atoms at the edge of g-C3N4 and form hybridization with C atoms. Since the atomic radius of S atoms (r=100 pm) is larger than that of N atoms (r=65 pm), the crystal structure changes significantly after doping. Besides, the number of free electrons increases after light excitation, and the Fermi level moves up. It exhibits typical n-type doping. Han et al. used P-doped g-C3N4 to prepare BP@ g-C3N4 catalyst for photocatalytic reduction of CO2. After P doping, it was easy to replace the C atoms in g-C3N4, which significantly changed the crystal structure of g-C3N4 and appears as n-type doping. After the doping, the Fermi energy level has shifted significantly, and a new energy band was generated on the basis of the original energy band structure, which had an important influence on the adjustment of the g-C3N4 energy band structure. The reduction rate of CO2 increased from 2.65 μmol·g-1·h-1 to 6.54 μmol·g-1·h-1, which significantly improved the photocatalytic performance. Taking O and S doped g-C3N4 as an example, Table 2 showed the HOMO-LUMO band gap binding energy of different doping sites. It can be seen from Table 2 that the HOMO-LUMO energy gap was significantly reduced when the elements are doped, indicating that the doping of O and S can significantly improve the energy level structure and electronic distribution of g-C3N4. In addition, different elements doping at the same site and the same element doping at different sites play different roles in the regulation of energy levels. The photocatalytic reduction after O and S doping was shown in Fig 8. Different doping sites and different doping elements had different effects on the light absorption properties. Lowering the band gap energy can effectively increase the light absorption range, improve the light absorption and catalytic activity. When CO2 molecules were adsorbed on the catalyst surface, more energy was used to overcome the energy barrier, so that the reduction yield of CO2 was improved. Table 2. The total energy, HOMO-LUMO energy gap, minimum frequency and impurity formation energy of doped structures at different sites of O and S Structure Energy/eV HOMO-LUMO energy Frequency/cm-1 Formation gap energy/eV (g-C3N4)6 -793.481 1.863 33.209 (g-C3N4)6-N2-O -790.149 0.256 17.174 -0.682 (g-C3N4)6-N3-O -790.330 0.574 26.571 -0.863 (g-C3N4)6-N8-O -790.283 0.371 24.827 -0.816 (g-C3N4)6-N2-S -787.122 0.326 23.748 1.714 (g-C3N4)6-N3-S -787.433 0.684 24.892 1.403 (g-C3N4)6-N8-S -787.621 0.617 25.196 1.215 Fig 8. S, O doped g-C3N4 photocatalytic reduction of CO2 reaction mechanism 5.3.2 Metal doping After the metal doped with g-C3N4, electrons are transferred from the metal to the adjacent N or C atoms, changing the electron density of the N or C atoms, and then affecting the electronic structure and energy band position of g-C3N4. Metals such as Cu52, Ag, Au53, Fe54, Pt55-56 can be doped into g-C3N4, causing lattice defects. Li57 found that the doping of Cu and Mo change the electron distribution and energy band structure of g-C3N4, and at the same time changed the reaction path on the surface of g-C3N4, which promoted the adsorption and activation of CO2 on the catalyst surface, effectively reducing the activation energy during reduction process. Beenish52 used Cu doped with rod-shaped g-C3N4 to investigate the performance and product distribution of CO2 reduction in different reaction systems. It was found that Cu in the form of Cu2+ was doped in g-C3N4, and significantly affected the energy level structure of g-C3N4. In different reaction systems, different main products are obtained by reduction. In the CO2-H2O reaction system, the main product is CH4, and the reaction rate was 1.84 times that of pure g-C3N4, In the CO2-CH4 reaction system, the main product For CO and H2, the reaction rate was 1.33 times that of pure g-C3N4. Yu37 used Pt-doped g-C3N4 to reduce CO2 to prepare hydrocarbons. It is found that Pt exists in g-C3N4 in the form of atoms, but after the reduction reaction occurs, part of Pt exists in the form of ions. And the different doping amounts of Pt made the reduction products of CO2 different selectivity, and the main product of CO2 reduction by pure g-C3N4 was CH3OH. When the doping amount of Pt is 5%, the main products of reducing CO2 were CH4 and HCHO; when the doping amount is 10%, the main products was CH4. The reaction mechanism of metal-doped g-C3N4 applied to photocatalytic reduction of CO2 was shown in Fig 9. The Fermi level of g-C3N4 is close to the bottom of CB. The metal doping makes the Fermi level move, and the dopant orbital hybridizes with the C or N electron orbital to form a new electron orbital, which effectively improve the oxidation-reduction performance of the catalyst and overcome the energy barrier of photocatalytic reduction of CO2. When the light source is illuminated, the valence band electrons on the catalyst were excited to the conduction band. Then a series of reduction reactions occur on the conduction band, and the holes on the valence band undergo oxidation reactions. Fig. 9. Metal-doped g-C3N4 photocatalytic reduction of CO2 reaction mechanism 5.4 Specific surface area control modification technology Usually g-C3N4 prepared from N-containing precursors (such as urea, melamine, dicyandiamine, etc.) through high-temperature polycondensation is shown as bulk particles or lamellar polymers with a small specific surface area (<10 m2/g ), greatly restricting its application. Regardless of the doping modification technology or the semiconductor compound modification, the interaction only occurs on the surface of g-C3N4. Therefore, effectively regulating the g-C3N4 nanostructure and expanding its specific surface area not only makes the performance of g-C3N4 more stable, but also increases reactive sites, which is an effective way to improve the photocatalytic activity of g-C3N4. 5.4.1 Mesoporous g-C3N4 The introduction of nano-scale porous structure into the bulk g-C3N4 can significantly increase the specific surface area of g-C3N4, which is beneficial to increase the reaction contact area and reactive sites, thereby improving its catalytic performance. Using ordered silicon-based materials as hard templates, porous carbon nitride materials with adjustable pore structure and pore size can be synthesized. It was reported that Zhao et al. use SBA-15 and a new type of cross-linked bimodal mesoporous SBA-15 (CLBM-SBA-15) as a hard template to prepare mesoporous g-C3N4. The resulting g-C3N4 (CLBMSBA-15) had a morphology similar to mesoporous SiO2, and numerous pores are formed on the surface of g-C3N4. In the photocatalytic degradation test of methyl orange, the activity of mesoporous g-C3N4 was nearly 15.3 times that of bulk g-C3N4 (the results were shown in Fig 7). Such high photocatalytic performance was due to the fact that the mesoporous g-C3N4 has high specific surface area, pore volume and active reaction sites. Shi et al. synthesized mpg-CN in situ using TEOS as a mesoporous template and cyanamide as a precursor, and the specific surface area of mpg-CN reached 152 m2/g. Its photocatalytic effect on the degradation of RhB was remarkable. This can be attributed to the high specific surface area and high electron-hole pair separation efficiency of mpg-CN, which enhanced the dye adsorption capacity of mpg-CN. And mpg-CN still performed well after being recycled for 3 times. 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