Metal oxides vital to green energy
Harnessing the energy of sunlight can be as simple as tuning the optical and electronic properties of metal oxides at the atomic level by making an artificial crystal or super-lattice “sandwich.”
“Metal oxides are cheap, abundant and ‘green,’” said Louis Piper, assistant professor of physics at Binghamton University in Binghamton, NY. “And as the study proved, quite versatile. With the right touch, metal oxides can be tailored to meet all sorts of needs, which is good news for technological applications, specifically in energy generation and flat screen displays.”
This is how it happens: Semiconductors are an important class of materials in between metals and insulators. They end up defined by the size of their band gap, which represents the energy required to excite an electron from the occupied shell to an unoccupied shell where it can conduct electricity. Visible light covers a range of 1 (infrared) to 3 (ultraviolet) electron volts. For transparent conductors, you need a large band gap, whereas for artificial photosynthesis, you need a band gap corresponding to green light. Metal oxides provide a means of tailoring the band gap.
While metal oxides are very good at electron conduction, they are very poor “hole” conductors. Holes refer to absence of electrons, and can conduct positive charge. To maximize their technologically potential, especially for artificial photosynthesis and invisible electronics, you need hole conducting metal oxides.
Knowing this, Piper has begun studying layered metal oxide systems, which can combine to selectively ‘dope’ (replace a small number of one type of atom in the material), or ‘tune’ (control the size of the band gap). Recent work revealed that a super-lattice of two hole-conducting copper oxides could cover the entire solar spectrum. The goal is to improve the performance while using environmentally benign and cheap metal alternatives.
For instance, indium oxide is one of the most widely used oxides used in the production of coatings for flat screen displays and solar cells. It can conduct electrons really well and is transparent. But it is also rare and very expensive. Piper’s current research aims to use much cheaper tin oxide layers to get electron and hole conduction with optical transparency.
“It’s going to be a case of some serious detective work,” Piper said. “We’re working in a world where physics and chemistry overlap. And we’ve reached the theoretical limit of our calculations and fundamental processes. Now we need to audit those calculations and see where we’re missing things. I believe we will find those missing pieces by playing around with metal oxides.”
By reinforcing metal oxides’ ‘good bits’ and downplaying the rough spots, Piper feels the development of new and exciting types of metal oxides can tailor specific applications are well within reach.
“We’re talking battery storage, fuel cells, touch screen technology and all types of computer switches,” Piper said. “We’re in the middle of a very important gold rush and it’s very exciting to be part of that race to strike it rich. But first we have to figure out what we don’t know before we can figure out what we do. One thing’s for sure: metal oxides hold the key.”