Novel microsphere photocatalysts show good performance for water splitting and water cleaning

22 December 2014

Researchers at the National University of Singapore and the Agency for Science, Technology and Research (A*STAR) in Singapore have produced novel microsphere catalysts that can improve water quality in daylight and also generate hydrogen as a green energy source. The novel multielement Au/La-SrTiO₃ microspheres were synthesized by a solvothermal method using monodisperse gold and La-SrTiO₃ nanocrystals as building blocks. A paper on their work was published in Chemistry – An Asian Journal.

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Photocatalytic materials use sunlight to create electrical charges, which provide the energy needed to drive chemical reactions in molecules attached to the catalyst’s surface. In addition to decomposing harmful molecules in water, photocatalysts are used to split water into its components of oxygen and hydrogen.

He-Kuan Luo, Andy Hor and colleagues from the A*STAR Institute of Materials Research and Engineering (IMRE) set out to improve an existing catalyst. Oxygen-based compounds such as strontium titanate (SrTiO₃₎ are promising, as they are robust and stable materials and are suitable for use in water. One of the team’s innovations was to enhance its catalytic activity by adding small quantities of the metal lanthanum, which provides additional usable electrical charges.

Catalysts also need to capture a sufficient amount of sunlight to catalyze chemical reactions. To enable the photocatalyst to harvest more light, the scientists attached gold nanoparticles to the lanthanum-doped SrTiO₃ microspheres. These gold nanoparticles are enriched with electrons and hence act as antennas, concentrating light to accelerate the catalytic reaction.

The porous structure of the microspheres results in a large surface area, as it provides more binding space for organic molecules to dock to. A single gram of the material has a surface area of about 100 square meters. The large surface area plays a critical role in achieving a good photocatalytic activity, noted Luo.

To demonstrate the efficiency of these catalysts, the researchers studied how they decomposed the dye rhodamine B in water. Within four hours of exposure to visible light 92% of the dye was gone—faster than conventional catalysts that lack gold nanoparticles.

Both the conduction and valence bands of Au/La-SrTiO₃ microspheres thus show favorable potential for proton reduction under visible light. The superimposed effect of Au nanoparticles and La doping in Au/La-SrTiO₃ microspheres led to high photocurrent density in photoelectrochemical water splitting and good photocatalytic activity in photodegradation of rhodamine B.

The team showed that the microparticles with gold nanoparticles performed better in water-splitting experiments than those without, further highlighting the versatility and effectiveness of these microspheres.

Resources

• Wang, G., Wang, P., Luo, H.-K. Hor, T. S. A. (2014) “Novel Au/La-SrTiO— microspheres: Superimposed effect of gold nanoparticles and lanthanum doping in photocatalysis,” Chemistry – An Asian Journal 9, 1854–1859 doi: 10.1002/asia.201402007