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Artificial Photosynthesis

Deshan Zhang , Chenho Tung , Lizhu Wu

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (7) : 1590 -1599.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (7) : 1590-1599 DOI: 10.7536/PC220323
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Artificial Photosynthesis
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Abstract

Photosynthesis in nature stores solar energy in chemical bonds through a green and efficient way. Mimicking the structure and function of the active center of natural photosynthesis, inert chemical bonds of small molecules (H2O, CO2 and N2 etc.) can be activated and converted for energy conversion. Herein, we summarize the important progress of artificial photosynthesis in water splitting into oxygen and hydrogen, carbon dioxide and nitrogen reduction. Meanwhile, we analyze the design ideas and working principles of related photochemical conversion systems, and discuss the challenges and future development of artificial photosynthesis.

Contents

1 Introduction

2 Artificial photosynthesis

2.1 Water oxidation

2.2 Proton reduction of water

2.3 Carbon dioxide reduction

2.4 Nitrogen reduction

3 Conclusion and outlook

Graphical abstract

Keywords

solar energy / artificial photosynthesis / water splitting / carbon dioxide reduction / nitrogen fixation

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Deshan Zhang, Chenho Tung, Lizhu Wu. Artificial Photosynthesis. Progress in Chemistry, 2022, 34(7): 1590-1599 DOI:10.7536/PC220323

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1 Introduction

In the 21st century, energy and environmental problems have seriously restricted the sustainable development of human society, which is a major challenge facing mankind, so it is imperative to develop new clean energy. Solar energy reserves are huge, and the solar radiation received by the earth per second can replace tens of thousands of tons of coal combustion heat, but the sunlight itself has the problems of low energy density, dispersion, intermittence and low conversion efficiency, so it is of great significance to realize the efficient storage and conversion of solar energy for social development[1,2].
In nature, green plants and microorganisms can use photosynthesis under mild conditions to achieve the activation and efficient conversion of a variety of stable natural small molecules, such as H2O, CO2, and N2. Photosynthetic system II (PSII) absorbs solar photons, and photogenerated holes are transferred to the oxygen-evolving catalytic active center Mn4CaO5 cluster to activate H2O[3]. PS II absorbs photons to give four electrons and four protons from H2O efficiently, accompanied by the formation of O2 in the catalytic center, which realizes the oxidation of H2O. The electrons produced in PSⅡ are transferred to PSⅠ along the electron transport chain of the photosynthetic system, and PSⅠ is excited by photons to produce high-energy electrons that are transferred to Fe redox proteins. In some green algae and anaerobic organisms, hydrogenase in PS I receives electrons and reduces protons to produce H2. In green plants, CO2 and protons are reduced by high-energy electrons in the catalytic center of [NiFeS] metalloenzyme to produce carbohydrates such as glucose (Fig. 1)[4,5]. Microorganisms such as rhizobia and cyanobacteria can fix and reduce N2 in the air to ammonia through their nitrogenase. The Fe protein in nitrogenase transfers electrons to the binding activation site FeMo-co of N2, and the energy input accompanied by ATP hydrolysis realizes the fixation and reduction of N2 under mild conditions.
With the help of energy provided by photons, photosynthetic centers use photochemical reactions to store solar energy in chemical bonds, so that many difficult reactions (such as H2O oxidation, CO2 reduction, N2 fixation) can be carried out under mild reaction conditions. As early as the beginning of the 19th century, scientists tried to achieve chemical conversion through artificial photosynthesis[6]. After a hundred years of development, artificial photosynthesis has made remarkable achievements in activating inert bonds such as C-H bonds and X-H bonds, and constructing chemical bonds such as C-C bonds and C-X bonds[7⇓⇓~10]. For example, the hydrogen evolution cross-coupling reaction combines the organic conversion of photocatalytic activation of inert bonds with the reduction of protons by photogenerated electrons to produce hydrogen, avoids the use of sacrificial agents, and produces the only by-product of the reaction, ——H2, while constructing C — C bonds/C — X bonds[11⇓⇓⇓⇓~16].
In order to simulate efficient natural photosynthesis and expand the application scope of artificial photosynthesis, inert bonds such as O — H bond, C = O bond and N ≡ N bond are effectively activated, and artificial photosynthesis begins to try to realize the conversion of small molecules such as H2O, CO2 and N2. With the development of related research and the deeper understanding of the active center and structure of natural photosynthetic system, scientists have designed many artificial photosynthetic systems based on molecular photosensitizers and semiconductor materials, and the efficiency of light energy utilization and catalytic performance have been continuously improved[17⇓⇓⇓⇓⇓~23]. This paper summarizes the important progress in the field of artificial photosynthesis in recent years, and discusses the future development direction and challenges in this field.

2 Artificial photosynthesis

2.1 Oxidation of water molecule

Photosynthetic enzyme PSⅡ is the only biological system in nature that can oxidize water molecules to produce oxygen by sunlight, and the most important structure is the oxygen-evolving active center (OEC). After decades of exploration by numerous scientists, in 2011, Shen Jianren and others took the lead in proposing a high-resolution structural Mn4CaO5 of natural photosynthetic oxygen evolution active center through the analysis of protein crystal structure[24]. By simulating the coordination environment of natural protein, Zhang Chunxi et al. Synthesized the artificial photosynthetic oxygen evolution catalyst Mn4CaO4, which is similar to the natural catalytic center structure (Fig. 2), but there is still a gap between the artificial photosynthetic oxygen evolution catalyst and the natural Mn4CaO5 structure, and the stability and efficiency of the catalyst need to be improved urgently[25].
O2[26]. Since this pioneering work, Ru-based catalysts have been widely used in the study of artificial photosynthetic oxygen evolution, and the formation principle of O = O bond has been widely discussed. Thummel et al. Synthesized the first mononuclear Ru water oxidation catalyst. When the axial ligand is 4-methylpyridine, the catalytic cycle number (TON) of the mononuclear complex can reach 580[27]. Sun Licheng and Sakai et al. Synthesized RuⅢ-O-RuⅣ-O-RuⅢ trinuclear Ru-based catalyst by mononuclear metal complex Ru(bda)(pic)2, which showed good water oxidation activity under light conditions[28]. Ruthenium complexes of bipyridine carboxylic acid are the most effective water oxidation catalysts among the current molecular catalysts. Privalov, LIobet and Sun Licheng introduced isoquinoline into them through reasonable modification, which significantly improved their catalytic performance[29]. Under acidic conditions, using Ce(NH4)2(NO3)6 as oxidant, the turnover frequency (TOF) and TON can reach 300 s-1 and 8360 ± 91, which is equivalent to the production of 720 mL O2 per milligram of catalyst per minute, and is equivalent to the reaction rate of natural PSⅡ. The reaction mechanism is that two Ru = O molecules form Ru-O-O-Ru through intermolecular dimerization, and then undergo further oxidation to form O = O bond, which is called I2m pathway. We report a novel oxygen-bridged "green dimer" catalyst (fig. 3) formed by air oxidation of a mononuclear Ru (bda) -based catalyst[30]. It is worth noting that the reaction mechanism of the monomer follows the I2m pathway, and the Ru (Ⅲ) -O-Ru (Ⅱ) oxygen bridge of the dimer formation prevents the coupling process of Ru = O, thus switching to the WNA pathway of nucleophilic attack of H2O to form O2.
Fertile metal catalysts, such as iron-based complexes, also exhibit catalytic oxygen evolution activity. Bernhard and Collins et al. Reported that an iron-centered tetraamine-based macrocyclic ligand (Fe-TAML) was able to catalyze the oxidation of H2O to produce O2 as a molecular catalyst in homogeneous solution[31]. The four-coordinate neutral organic ligand Fe complex synthesized by Costas et al. Can have TON greater than 350 and 1000 under the conditions of ceric ammonium nitrate (pH = 1) and sodium periodate (pH = 2), respectively[32]. Gupta and Dhar et al. Were the first to report an example of a molecular catalyst oxidizing H2O via a FeⅤ(O) intermediate (TON=220,TOF=0.76 s-1)[33]. The biuret modified tetraamine macrocyclic ligand can stably bind to Fe and does not produce Fe nanoparticles during oxidation.
Mimicking the natural Mn4CaO5 cluster structure, the complexes with Co4O4 cubane structure also have the activity to oxidize H2O. Patzke et al. Used acetic acid and H2O as ligands to synthesize a cubic structure very close to the active center of natural PS II, which is conducive to accelerating the transfer of charges and promoting the reaction[34]. The mononuclear Co complex synthesized by Ding Yong et al. Showed good photocatalytic water oxidation performance under the condition of using [Ru(bpy)3]2+ as a photosensitizer and Na2S2O8 as an electron acceptor, and the maximum TON could reach 1610[35].
Cu is an ideal active site for H2O oxidation because of its various oxidation States. In 2016, Cao Rui et al. Reported the first work of a water-soluble Cu-based pyridine complex for photocatalytic H2O oxidation, and the TOF and TON of the catalyst were (1.58±0.03)×10-1s-1 and 11.61 ± 0.23, respectively, and the introduction of F in the pyridine ligand well improved the stability of the metal center[36]. Our research group explored the photocatalytic oxygen evolution performance of Cu-based tetrazolium carboxylic acid coordination polymer[37]. In boric acid buffer solution (pH = 9) with Ru(bpy)3Cl2 as the photosensitizer and Na2S2O8 as the sacrificial agent, the TOF can reach 1.68 s-1. The nitrogen-containing electron-rich conjugated carboxylic acid ligand can effectively reduce the reaction overpotential and improve the photocatalytic efficiency.
Polyoxometalates (POM) and nanomaterials have also been used to prepare artificial photosynthetic oxygen evolution catalysts. For example, Lin Wenbin and Wang Enbo designed and synthesized four POM-based Co phosphate molecular catalysts with similar structures, and introduced Ge and As into the artificial OEC structure for the first time, which can maintain the stability of the molecular structure under photocatalytic conditions (Fig. 4)[38]. Kim and Ryu et al. Selected carbon dots as photosensitizer and Co-POM as H2O oxidation catalyst to construct a spherical donor-acceptor complex[39]. This supramolecular hybrid system has photocatalytic oxygen evolution activity with a TON of 552, which is significantly higher than that of the traditional [Ru(bpy)3]2+ catalytic system.
Using solar energy to achieve efficient conversion of H2O to O2 is an important goal of artificial photosynthesis. Although the reaction needs to overcome thermodynamic and kinetic barriers, the performance and stability of OER catalysts are steadily improving with the deepening of related research.

2.2 Proton reduction of water molecule

In the photosynthetic system, hydrogenase acts as a hydrogen evolution catalyst (HEC) to convert four electrons and four protons produced in the process of oxygen evolution of PSⅡ into H2,[FeFe], and each active site of hydrogenase can produce nearly ten thousand H2 molecules per second. With the clearer understanding of the structure of hydrogenase in recent years, artificial photosynthetic catalysts have attracted wide attention to improve their catalytic activity by drawing on the structure and function of natural catalysts[40,41].
The [2Fe2S] cluster in natural HEC is considered to be the active site for hydrogen production, so researchers have designed and synthesized many artificial hydrogenases based on this structure. [42]. Åkermark and Sun Licheng et al. First attempted to synthesize the first covalent binary catalyst by phenylacetylene ligation using Ru-based pyridine complexes as photosensitizers and hydrogenase mimics as catalytic centers, but these ingenious binary molecules could not catalyze the production of H2[43]. We used a thermostable and chemically stable Re (Ⅰ) -based complex as a photosensitizer, which was covalently linked to the Fe center in the [FeFe] hydrogenase mimics, and generated a H2 driven by visible light. The photocatalyst had a good charge separation effect and a slow charge recombination process, which was similar to the catalytic process of hydrogenase[44]. Ferrocene as an electron donor, Re complex as a photosensitizer and hydrogenase mimics were covalently linked to form a ternary catalyst, in which multi-step photoinduced electron transfer and Fe (Ⅰ) Fe (0) species were generated to promote photocatalytic hydrogen production[45]. Subsequently, photosensitizers based on Ru, Ir, Re and other metals are connected with the catalytic center through coordination or covalent bonds, and the catalyst shows good photocatalytic hydrogen production performance by adjusting the interaction between the photosensitizer and the catalyst.
In 2011, we reported the first work on the combination of quantum dots and [FeFe] hydrogenase mimics for photocatalytic hydrogen production. Water-soluble CdTe quantum dots are cheaper and have a wide range of light absorption compared with noble metal molecular photosensitizers. Under room temperature conditions with ascorbic acid as a sacrificial agent, the TON and TOF can reach up to 505 and 50 h-1, which is an efficient artificial photosynthetic hydrogen production system[46].
To further improve the stability of the catalyst, we combined polyacrylic acid (PAA) and natural HEC mimics (PAA-g-Fe2S2) as the catalyst (process shown in Fig. 6), and CdSe as a photosensitizer for photocatalytic hydrogen production[47]. The introduction of hydrophilic PAA can introduce the active site Fe2S2 into the aqueous solution, and can well prevent the aggregation of quantum dots and improve the quantum yield of the material. PAA shortens the distance between the photosensitive unit and the active center, which improves the electron transfer efficiency and promotes the TON to 27135.
Dukovic and King et al. Complexed mercaptopropionic acid (MPA) modified CdS nanorods with hydrogenase (CaI), which promoted the electron transfer from CdS to CaI iron-sulfur clusters through specific binding driven by electrostatic force, and the molar ratio of CdS to CaI could control the rate of charge transfer[48]. Under the illumination of 405 nm, the TOF of the catalyst can reach 380~900 s-1, the quantum yield is about 20%, and the generation rate of H2 is limited by the photon absorption rate (~1 ms-1), which is not affected by the catalytic efficiency of CaI. Before photocatalyst deactivation, the hydrogen generation time of the catalyst can reach 4 H, and the total TON is up to 106.
The research on iron hydrogenase has greatly promoted the development of artificial photosynthetic hydrogen production. Further, we used mercaptopropionic acid modified CdSe quantum dots and NiCl2·6H2O in situ synthesized core-shell structured artificial hydrogen-producing photocatalyst Nih-CdSe/CdS, which can stably produce H2 at a rate of 153μmol·h-1·mg-1[49]. In numerous metal ions (Cu+, Cu2+,Fe2+ 、Fe3+ 、Ni2+ 、Mn2+ 、RuC l 5 2 -, Ag+, and PtC l 4 2 -) combined with quantum dots to form hollow nanospheres,The combination of Ni2+ and CdTe QDs showed the highest hydrogen production activity, and the TON could reach 137 500 under visible light irradiation for 42 H[50]. Krauss, Holland, and Eisenberg et al. Used dihydrolipoic acid (DHLA) -modified CdSe nanocrystals as a photosensitizer and water-soluble Ni2+-DHLA as a catalyst to achieve efficient proton reduction under illumination[51]. Under the condition of pH = 4. 5, the TON of the catalytic system can reach up to 600 000, and the catalytic activity can last for 360 H, and the quantum yield in water is more than 36%.
Mulfort et al. attached a cobaloxime catalyst to the surface of a core-shell CdSe/ZnS quantum dot (Fig. 7). Transient absorption spectroscopy studies showed that the average time of photoinduced electron transfer was 105 ps, followed by a slower charge recombination process (time constant > 3 ns)[52]. Under the condition of using triethanolamine as sacrificial agent, the TON of the catalyst exceeded 10000 within 10 H. Inspired by the active center and residue regulation of natural enzymes, Cao Rui et al. Synthesized three water-soluble polymers with cobalt porphyrin units for photocatalytic hydrogen production[53]. The modulation of the photocatalytic performance was achieved by replacing the side chain groups of the polymer. The addition of the polymer can well improve the solubility and stability of the complex, so that the catalyst has higher hydrogen production performance, and the photocatalytic TON is more than 27400.
In order to further improve the efficiency of the artificial photosynthetic hydrogen production catalyst, our research group used a self-assembly method to combine CdSe/CdS quantum dots and cocatalyst Pt nanoparticles (Fig. 8)[54]. The catalytic system shortens the distance between the light absorption unit and the catalytic unit, and greatly improves the transfer efficiency of interfacial electrons. The internal quantum yield of the photocatalyst reaches 65%,TON>1.64×107, and the catalytic activity of the photocatalyst is close to that of natural HEC, which is the highest catalytic efficiency of artificial photosynthetic hydrogen production at present.
With the in-depth understanding of the structure and catalytic mechanism of natural catalytic active centers, the efficiency of artificial photosynthetic hydrogen production system has been greatly improved. Designing cheaper, stable and efficient artificial HEC is of great significance for promoting energy change and social development.

2.3 Carbon dioxide reduction

Different from hydrogen-producing microorganisms, green plants in nature use protons produced in PSⅡ to fix CO2 in the air, which plays an important role in maintaining the balance of carbon and oxygen in nature. The reduction process of CO2 involves the transfer of multiple electrons, and the common products of artificial conversion are CO, HCOOH, CH4, etc[55⇓~57]. Efficient, stable and highly selective conversion of CO2 is the key to artificial photosynthesis.
[NiFe] carbon monoxide dehydrogenase is a common metalloenzyme in organisms, which can efficiently convert CO2 into CO, and the TOF value can reach 40000 s-1(pH=8,70℃). In 2010, Armstrong et al. Took the lead in constructing a light-driven CO2 reduction catalyst using TiO2 nanoparticles modified by carbon monoxide dehydrogenase and Ru-based pyridine complex photosensitizer (Fig. 9)[58]. With the efficient catalytic activity of natural enzyme and the photosensitive property of TiO2, the effective reduction of CO2 to CO was realized, which proved the feasibility of constructing an artificial photosynthetic system to realize the reduction of CO2 by natural enzyme.
Based on the composition and structure of the native enzyme, attempts have been made to develop various types of metal complexes to construct efficient and stable photoreduced CO2 systems. Bonin and Robert et al. Introduced a trimethylammonium group to the para-position of the benzene ring of iron porphyrin, and reduced CO2 to CO with 100% selectivity under visible light irradiation[59]. Due to the introduction of the electropositive group, the catalyst has high stability and selectivity, and the activity does not decrease within 60 H. Lau et al. Found that the Fe3+ metal center could reduce CO2 to HCOOH by changing the central metal of the pentadentate nitrogen heterocyclic ligand, and the reduction product was CO when the central ion was Co2+ (Fig. 10). The author attributed the reason to the difference of the electron donating ability of the metal ion in the M-CO2H intermediate[60].
Inspired by the multimetallic centers in natural enzymes, multinuclear metal catalysts have also been designed for photocatalytic CO2 reduction. For example, the recently reported dinuclear cobalt catalyst based on pyridine ligand efficiently converts CO2 to CO or HCOO- through the synergistic effect between two cobalt ions[61]. Under acidic conditions, the selectivity of CO formation is 99%, and the maximum TON is 829; In acetonitrile solution, the selectivity for the formation of HCOO- was 97% with a maximum TON of 821. Ru, Ni, Co, Mn, Fe, Zn and other polynuclear metal complexes have shown excellent catalytic activity in the photoreduction of CO2[62⇓⇓~65].
Similar to the photoinduced electron transfer process of photosynthesis, metal complex catalysts can be combined with photosensitizers to form supramolecular photocatalysts to achieve efficient electron transfer. Using CuInS2/ZnS quantum dots, Weiss et al. Photosensitized the iron tetraphenylporphyrin (FeTPP) catalyst with ultrafast electron transfer (< 200 fs) between the quantum dots and the metal complex to efficiently reduce CO2 to CO with 84% selectivity (16%H2), which is 18 times higher than that of molecular photosensitizers, and has good stability, which is stable for 40 H under 4.5 m W illumination[66]. The recently reported CO2 photoreduction system of CuInS2 QDs and Co-based porphyrin complexes achieved efficient conversion of CO2-CO with selectivity greater than 99%[67]. The TON of the catalyst is more than 80000, and the quantum yield is more than 5% when the catalyst concentration is high, which is the highest catalytic efficiency reported in pure water environment.
The catalyst performance can be effectively improved by reasonably regulating the nanoheterostructure between semiconductor materials. Ho et al. Grew CdS nanoparticles on WO3 hollow microspheres to construct a Z-type photoconversion system, which is beneficial to the separation of photogenerated hole-electron pairs[68]. Electrons migrate to the surface of CdS particles, and holes aggregate on the surface of WO3, which can efficiently reduce CO2 to CH4 under visible light irradiation. Compared with single-phase CdS and WO3, the efficiency can be improved by 10 times and 100 times. We loaded ZnSe QDs on CdS nanorods to construct novel DOR nanoheterostructures (Fig. 11)[69]. The surface-immobilized ZnSe QDs not only contribute to the ultrafast separation of electrons and holes (2 ps), but also can facilitate the transfer of interfacial holes. The accumulation of photogenerated holes on ZnSe QDs and the distribution of photogenerated electrons on CdS can be observed by surface photovoltage microscopy. The reduction rate of CO2 to CO reached 11.3μmol·g-1·h-1, the selectivity was more than 85%, and the catalytic performance could be maintained for 80 H with good stability.
Catalytic systems for photoreduction of CO2 are usually combined with sacrificial agents, which results in a waste of hole oxidation capacity. We have pioneered the use of solar energy to combine CO2 reduction and organic conversion processes[70]. Under visible light irradiation, CdSe/CdS QDs catalyzed the reduction of CO2 to CO with a selectivity of up to 96% and a reaction rate of up to 412.8 mmol·g-1·h-1 in the presence of triethylamine. More importantly, when 1-phenylethanol derivatives were added to the system, the C — C bond oxidative coupling reaction could be efficiently catalyzed to form the corresponding pinacol with a yield of 98% (Fig. 12). The efficient coupling of CO2 reduction and organic conversion under illumination provides a new idea for constructing an effective artificial photoreduction system of CO2 and achieving efficient solar energy conversion.
Artificial photosynthesis has shown good performance in the field of reducing CO2. Achieving efficient photochemical conversion under low CO2 concentration is one of the future development goals of artificial photocatalysts.

2.4 Nitrogen reduction

The N ≡ N bond in nitrogen N2 is very stable (bond energy is 946 kJ·mol-1), and the Haber-Bosch method used in industry needs to be carried out under high temperature and high pressure, while microorganisms such as rhizobia and cyanobacteria in nature use nitrogenase to reduce N2→NH3 under mild environmental conditions. With the further understanding of the structure of MoFe protein and FeMo-co in nitrogenase, researchers have tried to achieve the conversion of N2→NH3 through artificial photosynthesis[71][72⇓~74].
King et al. Used CdS nanocrystals to sensitize MoFe protein and achieved efficient reduction of N2→NH3 under 405 nm illumination[75]. CdS nanorods absorb light energy to replace the chemical energy generated by ATP hydrolysis in the natural enzyme catalysis process, and the photogenerated high-energy electrons are transferred to the FeMo-co site to realize N2 reduction. The TOF of the catalytic system reaches 75 min-1, which is 63% of the efficiency of natural ATP hydrolysis to reduce N2, the quantum yield is about 3.3%, and the TON of the catalytic system reaches 1.1×104.
Inspired by the structure of nitrogenase, Liu Hong et al. Doped Fe into the semiconductor material SrMoO4, which reduced the band gap width of the original material to absorb visible light (Fig. 13), and the Fe-Mo-O active center formed in the catalyst significantly enhanced the adsorption effect on N2[76]. Fe doping can also reduce the charge recombination, increase the migration rate of photogenerated carriers, and promote the transfer of interfacial charges. At the optimal doping concentration, the formation rate of NH3 is up to 93.1μmol·g-1·h-1. Tsang et al. Introduced Fe atoms onto the MoS2 monolayer to construct an inorganic catalyst containing a four-membered ring structure [Fe-S2-Mo] similar to the catalytic active site FeMo-co of nitrogenase[77]. This structure can promote electron clustering at Fe. The theoretical simulation of the adsorption process shows that the introduction of Fe single atom significantly improves the adsorption capacity of the catalyst for N2 and inhibits the hydrogen evolution process. The catalyst can achieve efficient reduction of N2 under visible light and without the addition of any sacrificial agent. The solar-to-NH3 energy conversion efficiency reaches 0.24% when the temperature increases to 270 ° C. Zhang Lizhi et al. Introduced rare earth metal La into the oxygen-deficient Mo O 3 - x, and the monatomic La coordinated with the oxygen in the lattice to form O-La-O sites[78]. The 5d orbital electron of La fills the antibonding orbital of N2, which activates the inert N ≡ N bond and promotes the formation of NH3. Monatomic La enhances the charge transfer efficiency of the Mo O 3 - x support and promotes photocatalytic nitrogen fixation, and the formation rate of NH3 is 209μmol·g-1·h-1 under visible light irradiation without any sacrificial agent.
On the surface of photocatalyst, the construction of defect sites by facet engineering and other methods can capture photogenerated charges and promote the separation of hole-electron[79⇓⇓~82]. Simulating the adsorption and activation of nitrogen by FeMo coenzyme during natural nitrogen fixation, Zhang Lizhi et al. Constructed oxygen vacancies on the (001) crystal face of BiOBr nanosheets, which significantly promoted the transfer of surface charges and enhanced the adsorption effect on N2[83]. The oxygen vacancy photocatalyst has a good activation effect on the inert bond of N2, and activates the adsorbed N2 molecules to be reduced by photogenerated electrons to generate NH3. The external quantum efficiency of the system reaches 0. 23% within 1 H under the irradiation of monochromatic light at 420 nm without the addition of noble metal cocatalyst. The BiOCl oxygen vacancy catalyst achieves N2 activation under illumination via a proton-assisted electron transfer pathway[84]. The introduction of oxygen vacancies on different crystal planes of BiOCl can affect the adsorption structure and activation degree of N2, thus regulating the reduction products, such as NH3 can be selectively generated on the (001) crystal plane, while the reduction product of N2 on the (010) crystal plane is N2H4.
In 2017, Hirai et al. Constructed a large number of surface vacancies on the traditional semiconductor material TiO2 (Fig. 14), and ultraviolet illumination achieved efficient conversion of N2 and H2O to NH3 and O2[85]. The active site catalyzing N2 reduction in the system is the Ti3+ on the oxygen vacancy, which can effectively adsorb N2 and has a good capture effect on the conduction band electrons, thus promoting the light-driven N2 reduction process, and the conversion efficiency of light energy to chemical energy reaches 0.02%.
Recently, Li Lu's team used Mo atoms to replace Ti atoms connected to oxygen vacancies in TiO2 to regulate the electronic structure of vacancy active sites[86]. The results show that the doped Mo atoms provide more adsorption sites for N2 and facilitate the separation of photogenerated carriers. The theoretical calculation shows that the d electron of Mo migrates to the antibonding orbital of N2 during the reduction process, which promotes the activation of N ≡ N bond. The adsorption energy of NH3 can be reduced by the hydrogen spillover effect induced by loading Pt nanoparticles on the surface of the catalyst. The introduction of Mo and Pt also optimized the light-harvesting ability and band structure of TiO2. Under visible light irradiation at room temperature, the NH3 generation rate of the N2 photoreduction system reached 3700μg·h-1·g-1, indicating a good stability.
At present, artificial photosynthetic nitrogen fixation catalysts mainly focus on semiconductor materials, through element doping, vacancy introduction, crystal face engineering and other methods, to achieve the reduction of N2. It still needs more exploration to take more ideas to design artificial photosynthetic conversion system to achieve effective adsorption and activation of N2.

3 Conclusion and prospect

Inspired by natural photosynthesis, artificial photosynthetic systems have successfully stored light energy in chemical reactions, realized the conversion of various types of chemical bonds, and made progress in the decomposition of H2O, the reduction of CO2 and the fixation and reduction of N2. Researchers have been working hard to build more efficient, stable and inexpensive artificial photosynthetic systems.
At present, there are still some challenges in the development of artificial photosynthesis: (1) artificial photosynthetic nitrogen fixation is still in its infancy, the efficiency of existing photocatalytic systems is still in the micromolar level, and the ability of artificial photocatalytic systems to adsorb and activate N2 molecules still needs to be improved; (2) The activation and conversion of inert C(sp3)-H bonds of alkanes such as CH4 and C2H6 by artificial photoconversion systems need to be further studied.
In the future, artificial photosynthesis should be carried out from the following aspects: (1) reasonably regulating the structure of the catalyst, taking into account the light utilization rate, further improving the redox capacity, and realizing the activation of more types of inert bonds; (2) improving the catalytic efficiency of the artificial photosynthetic system by introducing substrate molecular adsorption sites, surface engineering, defect engineering and the like; (3) The theoretical calculation and experimental exploration are further combined to have a clearer understanding of the catalytic principle and optimization method.
It is believed that with the deepening of related research, artificial photosynthesis can use solar energy to achieve more efficient conversion of chemical bonds, so that photochemical conversion can play an important role in more fields.

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Funding

National Natural Science Foundation of China(22088102)

Ministry of Science and Technology of China(2017YFA0206903)

Strategic Priority Research Program of the Chinese Academy of Science(XDB17000000)

Key Research Program of Frontier Sciences of the Chinese Academy of Science(QYZDY-SSW-JSC029)

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