Researchers create nanostructures for efficient and sustainable degradation of pollutants

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Au-BiFeO3 nanocrystals are efficient and sustainable photocatalysts for environmental purification and provide insights into advanced material design for solar energy utilization. Credit: Tokyo Tech

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Au-BiFeO3 nanocrystals are efficient and sustainable photocatalysts for environmental purification and provide insights into advanced material design for solar energy utilization. Credit: Tokyo Tech

The need for sustainable and environmentally friendly solutions has accelerated global demand for green and renewable technologies. In this regard, semiconductor photocatalysts have proven to be an attractive solution, due to their potential in reducing pollutants and efficiently utilizing solar energy. Photocatalysts are materials that initiate chemical reactions when exposed to light.

Despite their advances, commonly used photocatalysts suffer from reduced photocatalytic activity and a narrow operating range within the visible light spectrum. Furthermore, they are difficult to extract from water-based solutions, limiting their applications in continuous processes.

Bismuth ferrite (BiFeO3), with its narrow band gap and magnetic properties, is an attractive alternative photocatalyst. The narrow band gap of BiFeO3 allows efficient use of light in the visible region to excite electrons from the valence band to the conduction band, leaving empty holes. The excited electrons and holes can both cause chemical reactions that lead to the breakdown of contaminants in an aqueous solution.

Furthermore, the ferromagnetic property allows easy recovery of BiFeO3 out of the solution. However, similar to common photocatalysts, BiFeO3 also suffers from rapid recombination of electron-hole pairs, which significantly limits its photocatalytic activity.

To address this problem, a team of researchers led by Associate Professor Tso-Fu Mark Chang from the Institute of Innovative Research at Tokyo Institute of Technology, Japan, has developed novel gold (Au) nanoparticle-decorated BiFeO.3 nanocrystals. Their research was published online in the journal ACS applied nanomaterials on April 5.

Dr. Chang explains: “The integration of Au nanostructures into BiFeO3 can introduce more active sites for photodegradation reactions, due to the unique localized surface plasmon resonance of Au nanoparticles and the transfer of the excited electrons into the BiFeO3 to the gold domain suppresses the recombination of electron-hole pairs. The newly developed Au-decorated BiFeO3 nanocrystals take advantage of the synergistic features of both mechanisms.”

The researchers produced the Au-BiFeO3 nanocrystals via a hydrothermal synthesis method and a simple solution process to decorate BiFeO3 with different amounts of Au. The team optimized the photocatalytic activity of the Au-BiFeO3 nanocrystals by evaluating their efficacy in degrading methylene blue (MB), a common denim dye. MB is highly soluble in water and poses a significant risk to aquatic life and human health. This also makes it the ideal contaminant to test the efficacy of photocatalysts.

Experiments showed that the sample containing 1.0% Au by weight showed the best activity, reaching an impressive degradation efficiency of 98% within 120 minutes under a 500-watt xenon lamp. Furthermore, it also retained 80% of its initial activity after four 120-minute cycles, demonstrating excellent stability. Moreover, there was a negligible effect of Au on the magnetic properties of BiFeO3indicating excellent recyclability.

The researchers also studied the mechanisms by which Au enhances photocatalytic activity. When an Au-BiFeO3 nanocrystal is illuminated by light at suitable wavelengths, electrons in BiFeO3 are enthusiastic about the conduction band.

Unlike the recombination that occurs in bare BiFeO3the introduction of Au, which has a less negative fermi level than the conduction band of BiFeO3facilitates the transfer of excited electrons from the conduction band to the Au domain, promoting hole accumulation in BiFeO3. This improves the photocatalytic activity of BiFeO3, which makes it easier to induce the formation of hydroxy radicals in aqueous solutions. These hydroxyl radicals are very active and easily attack MB molecules in the aqueous solution, converting them into harmless products.

“These findings advance our understanding of gold-semiconductor interactions in photocatalysis and pave the way for the design and development of advanced nanocrystal materials,” notes Dr. Chang up. “Overall, our study highlights the promising activity and recyclability of Au-BiFeO3underscoring its potential in efficient and sustainable degradation of environmental pollutants.”

More information:
Jhen-Yang Wu et al., Tunable photocatalytic properties of Au-decorated BiFeO3 nanostructures for dye photodegradation, ACS applied nanomaterials (2024). DOI: 10.1021/acsanm.4c01702