Photosynthesis is a fundamental biochemical process by which green plants, algae, and certain bacteria convert solar energy into chemical energy. Through this process, light energy is harnessed to synthesize glucose (C₆H₁₂O₆) and oxygen (O₂) from carbon dioxide (CO₂) and water (H₂O). The overall reaction can be expressed as follows:
$$6CO_2+6H_2O+light energy→C_6H_12O_6+6O_2$$
Central to this process is chlorophyll, a green pigment located in plant chloroplasts, which absorbs light energy and facilitates a cascade of photochemical reactions. The energy captured is stored in the chemical bonds of glucose, serving as a primary source of energy for plant growth and reproduction.
In contrast, artificial photosynthesis seeks to emulate the natural process but with a divergent objective: rather than producing glucose, the system is designed to generate renewable fuels-such as hydrogen, methane, or ethanol-and industrially valuable intermediates like synthesis gas (syngas). These synthetic pathways hold promise for addressing global energy challenges.
The concept has historical roots in the early 20th century, when Italian chemist Giacomo Ciamician envisioned a future in which humanity could replicate the energy-converting capabilities of plants to generate clean power. However, substantive progress was not realized until the latter half of the 20th century, propelled by breakthroughs in material science, nanotechnology, and electrochemistry.
A seminal advancement occurred in 1972 with the work of Akira Fujishima and Kenichi Honda, who demonstrated the photocatalytic splitting of water using titanium dioxide (TiO₂). This discovery marked the inception of practical artificial photosynthesis, providing a foundational proof-of-concept for solar-driven catalytic systems. Subsequent research has focused on the development of photocatalysts-materials that absorb light and mediate redox reactions crucial to fuel generation.
Hydrogen produced via water splitting is a clean energy carrier capable of powering fuel cells, offering a zero-emission alternative to conventional fossil fuels. Additionally, synthetic hydrocarbons produced through carbon dioxide reduction can be seamlessly integrated into existing fuel infrastructure, including internal combustion engines. Beyond energy production, artificial photosynthesis provides environmental co-benefits by sequestering atmospheric CO₂ and transforming it into economically valuable compounds.
Compact artificial photosynthesis systems have the potential to deliver decentralized, off-grid energy solutions, particularly in rural and underdeveloped regions. Furthermore, products such as methanol and syngas serve as key precursors in the manufacture of chemicals, plastics, and pharmaceuticals. The simultaneous generation of oxygen and reduction of greenhouse gases further supports agricultural and ecological sustainability.
Components in artificial photosynthesis
Artificial photosynthesis systems typically consist of three key components:
Light harvesting
The initial and critical phase of artificial photosynthesis involves the efficient capture of solar energy, a process known as light harvesting. This step is typically facilitated by semiconductors, pigments, or engineered molecular systems capable of absorbing and transferring light energy. Materials such as titanium dioxide (TiO₂), perovskites, and organic dyes are widely utilized due to their favorable light absorption spectra and tunable electronic properties. Ongoing research has significantly advanced the design and performance of artificial light-harvesting systems (LHSs), leading to improved solar energy utilization through innovative materials and structural engineering.
One promising direction involves the supramolecular approach, which constructs supramolecular LHSs through the self-assembly of host-guest complexes. Jia et al. (2024) reported the use of β-cyclodextrin combined with fluorescent molecules to achieve high-efficiency energy transfer via Förster Resonance Energy Transfer (FRET). These systems leverage aggregation-induced emission (AIE) effects to avoid self-quenching, enabling the formation of stable nanoparticles in aqueous environments. This strategy facilitates efficient light capture and transfer in water-based systems, which is particularly relevant for bio-inspired and environmentally benign designs.
Another innovative strategy is the nanocellulose-based approach. Nanocellulose, a renewable and biodegradable material, offers a tunable optical platform for integrating photoreactive entities. As demonstrated by Samyn et al. (2023), nanocellulose can encapsulate light-sensitive molecules or photosynthetic organisms, enhancing photon absorption and directing energy toward catalytic centers. Its structural flexibility and optical transparency make it an ideal scaffold for constructing artificial leaves and hybrid systems that emulate natural photosynthesis.
The bacteriochlorophyll aggregate approach draws inspiration from natural photosynthetic organisms, specifically green sulfur bacteria. Malina et al. (2024) demonstrated that aggregates of bacteriochlorophyll can form artificial antennas capable of extending the spectral range of light absorption into the near-infrared region. These biomimetic structures exhibit energy transfer efficiencies exceeding 90%, closely resembling the highly efficient light-harvesting mechanisms of natural chlorosomes. Their performance under low-light conditions underscores their potential for applications in diffuse-light or indoor environments.
Catalysis
Catalysts facilitate the chemical reactions needed to split water into hydrogen and oxygen or reduce carbon dioxide into hydrocarbons. Examples include: The development of highly efficient artificial or synthetic photoactive materials for CO2 reduction faces difficulties due to their often low thermodynamic stability and high rate of charge carrier recombination during the photocatalytic process. Materials like cobalt oxide or manganese mimic the role of chlorophyll. The development of nanostructured materials enhances the stability and efficiency of photocatalysts, addressing challenges like charge carrier recombination(Murali et al., 2020). Another approch is to use a selective and stable photocatalyst for the photocatalytic reduction of CO2 into C2H6. This process occurs via a multi-electron transfer pathway and does not require external sacrificial reagents (Wu et al., 2019). The core component of this composite catalyst is chlorophyll Cu (Chl-Cu), which was extracted from silkworm excrement and then modified. Chl-Cu incorporates a porphyrin structure that acts as an antenna for light absorption, while a Cu cation serves as the active center.Initially, C2 hydrocarbons like C2H2, C2H4, and C2H6 were observed to generate on chlorophyll-a/graphene. However, by replacing the Mg2+ cation with Cu2+ in the porphyrin center and further modifying it with graphene, the catalyst achieved high selectivity, with only C2H6 detected over an 18-hour reaction period . The photocatalyst demonstrated significant activity and selectivity for CO2 reduction, achieving a C2H6 yield rate of 68.23 μmol m-2 h-1 under visible light and an apparent quantum efficiency of 1.26% at 420 nm.In this system, the porphyrin rings are excited by light, generating electron-hole pairs. The photo-induced holes oxidize water to produce oxygen, while graphene functions as an adsorption center and an electron acceptor for the CO2 reduction process Materials like nickel-iron alloys drive the reactions under an applied voltage. Graphene serves as an effective electron collector, improving the overall efficiency of CO2 reduction processes(Zhou et al., n.d.).
Energy Storage
The final step converts the products into storable fuels. Hydrogen gas, for instance, can be stored and later used in fuel cells. Researchers are working on systems that integrate these components seamlessly, optimizing efficiency and scalability.
References
- Zhou, Y., Wu, C., & Zou, Z. (n.d.). Artificial Photosynthesis: Photocatalytic Fixation of CO2. https://doi.org/10.1149/ma2016-01/38/1934
- Murali, A., Sakar, M., Malarvizhi, N., Priya, S., Kader, M. A., Selvaraj, M., & Bensingh, R. J. (2020). Hierarchically nanostructured functional materials for artificial photosynthesis (pp. 229-255). Elsevier. https://doi.org/10.1016/B978-0-12-819552-9.00007-5
- Wu, T., Zhu, C., Han, D., Kang, Z., Niu, L., & Niu, L. (2019). Highly selective conversion of CO2 to C2H6 on graphene modified chlorophyll Cu through multi-electron process for artificial photosynthesis. Nanoscale, 11(47), 22980-22988. https://doi.org/10.1039/C9NR07824J
- Malina, T., Bína, D., Collins, A. M., Alster, J., & Pšenčík, J. (2024). Efficient two-step excitation energy transfer in artificial light-harvesting antenna based on bacteriochlorophyll aggregates. https://doi.org/10.1016/j.jphotobiol.2024.112891
- Samyn, P., Rastogi, V. K., & Van Erps, J. (2023). Role of Nanocellulose in Light Harvesting and Artificial Photosynthesis. Catalysts, 13(6), 986. https://doi.org/10.3390/catal13060986
- Jia, D., Luo, Q., Liu, S., Hou, C., & Liu, J. (2024). An Artificial Light‐Harvesting System based on Supramolecular AIEgen Assembly. Chemistry – European Journal. https://doi.org/10.1002/chem.202402438
