New mesoporous crystalline Si exhibits increased rate of H2 production; potential use in Li-ion batteries also
11 April 2014
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| Schematic of mesoporous silicon Image: Donghai Wang/Penn State. Click to enlarge. |
Researchers at Penn State have devised a new process for the bottom-up synthesis of mesoporous crystalline silicon materials with high surface area and tunable primary particle/pore size via a self-templating pore formation process.
The nanosized crystalline primary particles and high surface areas enable an increased rate of photocatalytic hydrogen production from water and extended working life. These advantages also make the mesoporous silicon a potential candidate for other applications, such as optoelectronics, drug delivery systems and even lithium-ion batteries. A paper on their work is published in Nature Communications.
The standard method for manufacturing porous silicon is a subtraction method, similar to making a sculpture. The etching process uses to produce porous silicon results in the loss of material, said said Donghai Wang, assistant professor of mechanical engineering, who led the research.
Instead, Wang’s team uses a chemically based method that builds up the material rather than removing it. The researchers start with silicon tetrachloride, a very inexpensive source of silicon. They then treat the material with a sodium potassium alloy.
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| Micrograph of mesoporous silicon with sodium chloride and potassium chloride salts embedded in the matrix. Image: Donghai Wang/Penn State. Click to enlarge. | Micrograph of mesoporous silicon showing holes where salts were removed. Image: Donghai Wang/Penn State. Click to enlarge. |
Once the bonds break, the chlorine binds with the sodium, potassium and silicon, potassium chloride and sodium chloride—table salt—become solid, forming a material composed of crystals of salt embedded in silicon. The material is then heat-treated and washed in water to dissolve the salt, leaving pores that range from 5 to 15 nanometers.
Because sodium potassium alloy is highly reactive, the entire procedure must be done away from the oxygen in the air, so the researchers carry out their reaction in an argon atmosphere.
I believe that the process can be scaled up to manufacturing size. There are some processes that use sodium potassium alloy at industrial levels. So we can adapt their approaches to make this new type of porositic silicon.
—Donghai Wang
Because these silicon particles have lots of pores, they have a large surface area and act as an effective catalyst when sunlight shines on this porous silicon and water. The energy in sunlight can excite an electron that then reduces water, generating hydrogen gas. This process is aided by the material’s larger-than-normal band gap, which comes from the nanoscale size of the silicon crystallites.
The researchers are also looking into using this porous silicon as the anode in a lithium ion battery.
The US Department of Energy and the Defense Threat Reduction Agency funded this work.
Resources
Fang Dai, Jiantao Zai, Ran Yi, Mikhail L. Gordin, Hiesang Sohn, Shuru Chen & Donghai Wang (2014) “Bottom-up synthesis of high surface area mesoporous crystalline silicon and evaluation of its hydrogen evolution performance,” Nature Communications 5, Article number: 3605 doi: 10.1038/ncomms4605
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