The world is on a quest for clean energy, and it’s one step closer to getting there, thanks to the work of chemist Jimmy Yu. A discovery by the CUHK chemist and his team has paved the way to creating energy simply by exposing water to sunlight.
The secret ingredient is red phosphorous, the most stable and commonly found of three forms of that element. In the sun, it breaks up water to give off bubbles of hydrogen gas, a clean fuel.
That helps make it the world’s simplest photocatalyst. ‘Simple is beautiful,’ Professor Yu says. ‘This element is so abundant that it will never be used up.’
A photocatalyst operates much as chlorophyll does in a plant, absorbing energy from light and causing a chemical reaction. The process of photocatalysis is simply a form of artificial photosynthesis.
There are hundreds, if not thousands, of materials that can be used as photocatalysts. But most of them are heavy-metal oxide compounds that are expensive and complicated to produce. Very rare elements are often used to enhance their efficiency.
So chemists have been searching for a single element that can perform the same function. Phosphorous makes up around 0.1 per cent of the earth’s crust, meaning there are hundreds of billions of tons of it that can be extracted fairly easily.
White phosphorous is the form many chemistry students are familiar with, so unstable and dangerous that it combusts in air and must be kept in oil. Black phosphorous, like red, is very stable, but extremely expensive to manufacture since the necessary crystallization process is hard to execute.
Red phosphorous is a common and widely produced substance that is used as a flame retardant and that gives matches their red colour. It is also used in fire-prevention materials and fireworks.
Professor Yu has been working on materials with environmental applications since 1995. But it was only recently that he and one of his doctoral students, Feng Wang, turned their attention to red phosphorous. They were examining a large group of elements that can act as semiconductors, a prerequisite for acting as a photocatalyst.
The key finding was that red phosphorous can act as a semiconductor, a property that other researchers had not recognized. The conventional wisdom was that red phosphorus was really only useful as an insulator.
The discovery came as a shock to Professor Yu, a chemistry professor who is also the head of United College at CUHK. He and his student double and triple checked the results.
‘It was a very exciting development—we were surprised by its photocatalytic properties,’ Professor Yu says. ‘That had never been reported for a single element. That’s as simple as you can get.’ Then Professor Yu worked with a colleague who specializes in theoretical calculations to explain why and how the element acts as a photocatalyst. Now Professor Yu and his team are experimenting with various crystal forms of red phosphorous, since different crystals vary in their effectiveness in generating hydrogen.
A Japanese scientist, Akira Fujishima, performed the earliest work on splitting hydrogen out of water. He discovered that titanium dioxide could produce the gas, findings reported in Nature in 1972. But the compound is more difficult to produce than red phosphorous, and also requires ultraviolet light to break apart water molecules. Only around four per cent of regular sunlight is UV, so titanium dioxide requires artificial light to be effective.
By contrast, red phosphorous works across the spectrum of natural sunlight, making it ideal for use in the generation of clean power. The light acts as an irradiation source that stimulates the catalyst and induces chemical reactions.
Professor Yu’s water-splitting process functions at room temperature, with the phosphorous separating the individual elements in H2O. Hydrogen has a very high fuel capacity, meaning it creates more energy than any other chemical fuel. Unlike petrol-based fuels, there’s no greenhouse-gas byproduct when it burns, with only water left at the end of the reaction.
Professor Yu’s research has been some of the most ground-breaking in the world, in terms of recognition by his peers. He made the ranks of the ‘World’s Most Influential Scientific Minds’ in 2014, as compiled by Reuters, who rated him as one of the most-cited researchers in the field of materials science over the last 11 years. His research put him in the topone per cent of academics in his field in terms of citations over that time frame.
Still, it will be some time before red phosphorus can be used to produce energy for commercial or industrial applications. The material is less efficient than traditional photocatalysts, and the amount of hydrogen it can produce is currently very small.
The prevalence of fossil fuels has negated the need for investing in the technology necessary to bring photocatalysts to a commercial scale. Even conventional solar power is considerably more efficient. But with a greater emphasis on the environmental degradation that traditional fuels cause, the thinking may change.
Inspired by Professor Yu, another member of his group has uncovered a way to use other elements to generate hydrogen from water. The research associate, Dr. Gang Liu, is now honing the process at the Institute of Metal Research in Shenyang.
‘The final goal is to develop an environmentally friendly way to generate clean fuel,’ Professor Yu says. ‘We hope to offer some possible solutions.’
