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,” according to Binghamton University researcher Louis Piper. “Metal oxides are cheap, abundant, and green. 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.”
Here’s how it works: semiconductors are an important class of materials in between metals and insulators. They are 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, a large band gap is required, whereas for artificial photosynthesis, a band gap corresponding to green light is needed. Metal oxides provide a means of tailoring the band gap.
But 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, hole conducting metal oxides are required.
Knowing this, Piper has begun studying layered metal oxides systems, which can be combined 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 whilst using environmentally benign and cheap metal alternatives.
By reinforcing metal oxides’ ‘good bits’ and downplaying the rough spots, Piper is convinced that the development of new and exciting types of metal oxides that can be tailored for specific applications are well within our reach.
“We’re talking battery storage, fuel cells, touch screen technology, and all types of computer switches,” said Piper. “We’re in the middle of a very important gold rush and its 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.”
