(Nanowerk News) When organic pollutants such as dyes, agricultural chemicals and pharmaceuticals enter waterways around the world, they can harm the environment and human health – and eliminating them can be extremely difficult.
Photocatalysts – substances that absorb energy from light and use it to speed up the rate of chemical reactions – can break down organic pollutants in a process called mineralization, turning them into water, carbon dioxide and other harmless molecules. But there’s a catch: most photocatalysts require UV light to work, making them impractical and expensive to use on a large scale.
To solve that problem, the researchers set their sights on finding photocatalysts that could harness more of the sun’s spectrum. “If you can use sunlight, it’s cheaper and more widely available than UV light,” says Magnus Rønning, professor of catalysis in the Department of Chemical Engineering, NTNU.
Gold nanoparticle key
Nano-sized discs made of the mineral bismuthite are promising photocatalysts, and researchers have found that adding gold nanoparticles to their surface increases their sensitivity to the visible part of the solar spectrum. However, while there are several ways to deposit those gold nanoparticles on the surface, most methods provide limited control over the size and shape of the particles you produce.
“Often you’ll get a distribution of sizes and shapes (that) you can’t control, so you have a mix of sticks, balls and cubes,” says Rønning.
Now, Rønning and colleagues at NTNU have found a way to create gold nanoparticles of uniform size and shape on the surface of bismuth nanodiscs. Their research was recently published in the journal Photochemistry and Photobiology (“Optimizing the shape anisotropy of gold nanoparticles for better light harvesting and photocatalytic applications”), opens up the possibility to study the effect of nanoparticle size and shape on catalyst performance, which in turn makes it possible to maximize their light harvesting capacity.
Testing different shaped nanoparticles
The researchers used tiny gold ores as nucleation sites where they grew gold nanoparticles in various shapes – bars, scratched rods with rough surfaces, and spheres – by adjusting the concentration and pH of the solution in which the particles grew.
These nanoparticles have a surfactant coating on their surface, which reduces the chance that they will agglomerate, before being deposited on the bismuthite nanodisk.
“With this, we can maintain fairly fine control over the size and shape of these particles,” says Rønning. Samples were prepared and characterized by PhD candidate Jibin Antony at NTNU’s NanoLab, with his own catalytic reaction run in the Catalysis Group laboratory in the Department of Chemical Engineering.
The researchers tested how well the resulting photocatalyst could break down an organic pollutant known as methylene blue. As well as being a widespread organic contaminant, methylene blue is a useful test case to see how well a photocatalyst will work on other pollutants.
“It’s quite representative as an organic contaminant, but it’s also a relatively complex molecule,” says Rønning. “If it works on methylene blue, it should also work well on other organic materials.”
Another advantage of using methylene blue is that its decomposition is well understood, allowing researchers to investigate not only how much methylene blue is left at the end of the process, but what it has been converted into.
While that’s not something Rønning and colleagues looked at in their work on gold nanoparticles, in a related paper the researchers saw that adding silica to bismuthite nanodisks did change the degradation product of methylene blue. “In the end, you want full mineralization and not just conversion into something as dangerous or undesirable as methylene blue,” says Rønning.
Stems are better than balls
Rønning and his colleagues found that photocatalysts with rod-shaped gold nanoparticles performed 14% better than spherical ones. But there is still room for improvement. “Even after three hours of reaction, you still have some contaminants left,” says Rønning. “So, yeah, it worked. But we still need something that works better.”
Several photocatalysts are already used in commercial wastewater treatment and air purification systems. The technology also holds promise for hydrogen splitting – producing inexpensive hydrogen fuel using only water and sunlight. But to make that possible, researchers need to find a way to make the catalysts much more effective than they currently are.
The main improvement needed for better photocatalysts lies in how many photons are actually used to drive the reaction. “In good cases, maybe 1%,” says Rønning. “If we can get this up to, say, 10%, it will be closer to practical application.”
While changing the size and shape of the gold nanoparticles is unlikely to result in such a large increase in efficiency, it is a start. “We need to improve the catalyst to make this commercially viable,” says Rønning. “This is a step in that direction.”