CUHK Passions and Pursuits

49 T he 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 phosphorus, 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 said. ‘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. Red phosphorus 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 and his team were first examining a large group of elements that could act as semiconductors, a prerequisite for acting as a photocatalyst. They found that red phosphorus could 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. Professor Yu and his team then experimented with various crystal forms of red phosphorus, since different crystals vary in their effectiveness in generating hydrogen. Red phosphorus 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 phosphorus separating the individual elements in H 2 O. 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 Reuters’s ‘World’s Most Influential Scientific Minds’ in 2014. 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. ‘The final goal is to develop an environmentally friendly way to generate clean fuel,’ Professor Yu said. ‘We hope to offer some possible solutions.’ c c Under sunlight, red phosphorus breaks up water to generate bubbles of hydrogen Tapping Water and Sunlight Jimmy Yu’s quest for clean fuel