Photosystem-semiconductor hybrid nanostructures for photoelectrochemical applications

Abstract

In the natural photosynthesis of plant or cyanobacteria, functional proteins and redox cofactors are sophisticatedly self-assembled in the thylakoid membrane, exhibiting nearly $100 %$ of light-harvesting quantum efficiency. Photosystem I (PSI) is an integrated membrane protein complex that catalyzes the transfer of electrons using light energy. The photon energy absorbed by PSI also provides a proton motive force used to produce organic compounds. This finding has inspired engineers to pursue biomimetic and bio-inspired designs for efficient light-harvesting and charge extraction. However, the efficiency of nano-bio hybrid photosystems is often limited by the poorly-oriented assembly of photosystems with nanomaterials. In this work, PSI was assembled with protonated-carbon nitride (p-$C_3N_4$) nanosheets through electrostatic interactions, resulting in linker-free PSI/$p-C_3N_4$ hybrid nanostructures that generate favorable electronic interfacial structures for charge transfer through a Z-scheme system. Surface-modified carbon nitrides can provide photocatalytic activity under visible light irradiation conditions and a new electron transfer path in the hybrid structure with PSI. Electrostatic hybridization between protonated sites on the $p-C_3N_4$ nanosheets and the partially negatively charged lumen side of PSI provides a favorable orientation for efficient electron transfer. The PSI/$p-C_3N_4$ hybrids exhibit a photocurrent density more than 28 times higher than randomly oriented PSI. This study provides a vital strategy to construct an efficient biochemical photoelectrode based on natural photosystems coupled with semiconductor carbon nanostructures through simple self-assembly in a favorable orientation for efficient and stable interfacial electron transfer. In addition, the designed photocathode was coupled with a PSII-based photoanode to design a biophotovoltaic system that can be driven with light energy without external voltage. A structure capable of increasing a surface area and a deposition amount of PSII was applied using a mesoporous $TiO_2$ film with PSII. Compared to the photovoltage when the single photo-anode is used, the photocathode is connected to the photoanode and provides an effect of increasing the open voltage. This work also shows that when coupling the PSI and PSII-based photoelectrodes, a photocurrent of $7.7 \mu A cm^ {-2}$ under bias-free conditions and a cell power up to $0.9 \mu W cm^{−2}$ can be achieved. Also, it is essential for the stability of the electrode used to prevent protein oxidation or damage that may occur due to exposure to liquid electrolytes when performing PS-based photoelectrode. This study intends to simply protect the PSI from the external environment by depositing a metal oxide thin film on the PSI-based electrode with the atomic layer deposition (ALD) method. PSI was deposited on a transparent conductive substrate, and an amorphous $TiO_2$ passive layer was deposited on the electrodes. The $TiO_2$ layer immobilized PSI and prevented exposure to a liquid electrolyte to supplement the stability of electrode use. In addition to a passivation effect, an interaction with PSI was provided according to the photoelectrical properties of the metal oxide further providing an increase in photo-induced electron extraction efficiency. $TiO_2$ and PSI can be replaced with various materials, which are expected to enable changes in the electron transfer structure and can be applied to various fields such as solar cells and biosensors. In conclusion, PS-based photoelectrodes were designed with nanostructured semiconductors to use the advantages of the PSs and to compensate for the limitations of protein-based electrodes. The improved efficiency and stability of the bio-photoelectrode was achieved and it showed the potential of bio photovoltaic cells as a sustainable low-cost solar cell.