Collaborating through a Strategic Research Initiative funded by the U of I College of Engineering, Elif Ertekin (mechanical science and engineering), Ed Seebauer (chemical and biomolecular engineering) and Lane Martin and Angus Rockett (materials science and engineering) are designing atomic scale oxide heterojunctions for converting light to energy. A recent breakthrough by the team could spark a wave of interest in the field.
“We realize there is an untenable energy problem in the United States and around the world,” Martin noted. “We can’t continue to burn through fossil fuels producing the energy requirements of the world. It will both hinder the potential development of up-and-coming economies and impact environmental stability and our long-term health. We’re hoping to produce technologies that will either bridge us to the next thing or even be the next thing to allow us to produce energy with as minimal impact on the environment around us while also continuing growth.”
A photocatalytic energy system uses a material to absorb a photon of energy to create electron hole pairs, which are then separated to produce charge carriers that can catalyze chemical reactions.
“The goal is to take water and provide the appropriate charge to split it apart so that you have hydrogen and oxygen,” Martin explained. “You can burn the hydrogen and get water back. We’re trying to design a substance that absorbs light to do the splitting reaction.”
Through a semiconductor, the energy that is absorbed then needs to be carried to the substance’s surface in order to be dumped into the power grid.
“Molecular hydrogen is a dream fuel because it has no carbon footprint,” Seebauer explained. “The problem is there aren’t many materials that are actually good photocatalysts, especially to make hydrogen cheaply from water.”
To create the dream fuel from an environmental perspective, Illinois has formed its own version of the “Dream Team” with four research operations in one campus that have diverse perspectives, but all speak similar languages.
Martin is an experimentalist, specializing in the study, design, and control of novel oxides at the atomic level for a range of applications. Ertekin brings advanced computer modeling capabilities that allow the production of materials inside a computer to explore the atomic nature of electron transfer in the systems produced in Martin’s lab and to drive the design of new candidate materials. Seebauer, also an experimentalist, uses defect engineering to control the identity and concentration of atomic scale defects that enable deterministic control of the properties of a material. Rockett rounds out the team by providing device-level models and understanding of how all of these parts come together for a real application.
“We now have a critical mass of faculty on this campus who are exploring photocatalysis for energy production,” Seebauer said. “It’s a big area and Illinois needs to play in this arena. There were some special discoveries that each individual has developed independently and now it was time to put those discoveries and insights together to solve an important global problem.”
“For a lot of us, it’s the first foray into these areas,” Martin added. “We are bringing skill sets from very disparate backgrounds and combining them together to solve these complex problems. We have a team that ranges from the atom scale all the way to the device scale.”
Martin’s group has been interested in using earth-abundant materials that are readily available and haven’t been used for other applications (like silicon).
“We’re using stable oxides, derived from the same structures and elements common in rocks in the Earth’s crust, and applying the same high-tech atomistic control and processing traditionally applied to conventional semiconductor materials (such as silicon and gallium arsenide) to produce device-level quality complex materials,” Martin said. “Our goal is to produce oxide materials that absorb light better and do catalytic reactions more efficiently by creating model structures, characterizing their properties, and making rudimentary devices based on them to test their feasibility.
The focus of the project has been on using titanium dioxide as the catalyst. It is a stable compound, produces good chemical activity, and has the ability to form the right charge carriers that migrate to the surface.
“One drawback, however, is that it absorbs solar light only in the ultraviolet portion of the spectrum, not the visible, or the infrared,” Seebauer said. “Because those longer wavelengths are where most of the light is, you’re throwing most of it away. That’s not helpful if you really want to produce a lot of energy.”
There have been various attempts to try to get around this problem. First, some researchers have added foreign elements such as nitrogen directly into the titanium dioxide. That allows the titanium dioxide to absorb a larger fraction of solar radiation in the visible region. However, inserting the nitrogen also produces a highly defective crystalline structure, which kills the lifetime of the charges you’re trying to create and thus not allowing it to reach the surface.
Both Martin and Seebauer have experience using the second option, which is to use a heterojunction, an interface that occurs between two layers of crystalline semiconductors. Fittingly, they are using a similar concept that Nick Holonyak, Jr, a world-renowned Illinois professor of electronic and computer engineering and physics, used some 50 years ago to invent the light emitting diode, which depends on a semiconductor heterojunction.
“In this case, however, instead of emitting light like an LED, you want to absorb the light,” continued Seebauer. “You make the heterojunction such that you have a thin layer of this great photo catalyst that sits on top of a substrate made of a different semiconductor that’s much more capable of absorbing the solar light.”
In recent experiments, Martin has found a new class of oxide materials that fits that bill – including so-called correlated electron metallic oxides such as strontium ruthenate, lanthanum nickelate, and others. This work is currently published in the journal Advanced Materials and Advanced Energy Materials. Martin argues that the reason it works so well is that the curious electronic structure of the correlated metal oxides permits absorption of wavelengths of light across the entire solar spectrum and the production of hot electrons which allow the reactions to take place.
“The hot electrons are excited from their normal ground state that they would occupy at room temperature and they don’t immediately decay back in the way that you would expect,” Seebauer explains. “They stay in that state long enough that they can be injected into an overlayer of titanium dioxide. Once that happens, they continue to live long enough that they make it all the way to the free surface where they do photo chemistry.
The rest of the team now has its sights set on proving and verifying the reason for such interesting results in the lab.
“Our goal now is to predict how the interface structure between those two materials affects the polarization which affects the surface chemistry which can then affect the overall rate of conversion of solar energy to water splitting and hydrogen production,” Ertekin said.
Rockett is then seeking to understand at a device level what the band structure looks like, the implications of the band alignments for the overall performance, and finding ways to back-engineer better alignments to get higher performance.
The intent of the SRI program is to place Illinois in a clear leadership position in promising new and growing areas of engineering research. Early stage funding supports the initiation and exploration of promising interdisciplinary research. The goal here is to use the SRI project as a springboard for future funding in the area.
“You need funding to get results, but you need results to get the funding,” Martin said. “The SRI bridges that gap. “At the end of the day, we’d like to multiply
the contributions from the College many times over. We hope the ultimate outcome of the SRI proposal is that we provide a footprint for Illinois in this community and more importantly that we help to solve a difficult and pressing problem for our planet.”
-Mike Koon, College of Engineering