A research team led by Prof. ZHONG Chao from the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences, in collaboration with Prof. MA Guijun from ShanghaiTech University proposed a strategy for constructing conformal and conductive biofilms to enable stable, scalable, and sustainable visible light-driven overall water splitting. 

The study was published in Science Advances on June 12. 

Inspired by natural Z-scheme systems, researchers developed various semi-artificial photosynthesis systems to capture and utilize solar energy. Typically, these systems, constructed by anchoring semiconductive nanoparticles on the surface of biological enzymes, facilitate light-driven reduction reactions, including hydrogen production, CO₂ reduction, and N₂ fixation. 

However, enzymes are susceptible to environmental exposure, which highlights the ongoing challenge of developing efficient, stable, and sustainable semi-artificial Z-scheme systems. 

Biofilms are entities of extracellular matrix with enwrapped bacterial cells. They possess numerous advantageous traits including self-regeneration, self-adaption, resilience, and responsiveness to the surrounding environment. Due to these unique characteristics, biofilms could serve as a promising candidate for the development of semi-artificial Z-scheme systems. 

In this study, researchers proposed a method that integrated conductive biofilms with high-efficiency photocatalysts. The resulting system consistently produced a 2:1 ratio of H₂ and O₂, indicating the success of visible light-driven overall water splitting. 

"Natural biofilms typically demonstrate low conductivity," said Dr. WANG Xinyu, one of the first authors of this paper. "Our study has engineered conductive biofilms specifically for Escherichia coli (E. coli) by utilizing an innovative in situ polymerization technique, thereby enhancing the biofilms' conductivity properties." 

Conductivity results demonstrated that the hybrid biofilms retained conductivity both in aqueous solution and under dry state. These results also indicated their potential applications in bioelectronic devices. 

The researchers fabricated the hybrid Z-scheme sheet samples through a layer-by-layer method. A mixture of photocatalysts was drop-casted onto a glass plate, followed by the growth of biofilms and their in situ polymerization process. 

They found that the resulting sheet exhibited the capability of functioning as a free-standing sheet while being resistant to sonication. The conductive biofilms enabled efficient charge separation, which subsequently facilitated visible light-driven overall water splitting. The vertical interparticle electron transfer relied on physical contact between particles, and thus, the inclusion of conductive substances within the photocatalyst layer further enhanced the efficiency of photocatalytic reactions. 

Additionally, the researchers utilized light-assisted Kelvin probe force microscopy (AM-KPFM) to clarify the microscale process of electron transfer. Experimental results showed that the surface photovoltage difference was recorded as 69 mV and the built-in electric field was determined to be 95 kV m^-1, significantly exceeding values obtained from systems with only non-conductive E. coli biofilms. 

These findings indicated that the underlying conductive biofilms ensure a highly effective charge separation process, which is crucial for the exceptional photocatalytic performance observed. 

This work not only advances the frontier of engineered living energy materials but also holds promise for other bio-integrated systems and devices. 

Schematics of natural (A) and artificial (B) Z-scheme systems. (Image by SIAT)