Researchers at Rice University and the University of Houston created an efficient, simple-to-manufacture core/shell photoanode with a highly active oxygen evolution electrocatalyst shell (FeMnP) and semiconductor core (rutile TiO2) for the photoelectrochemical oxygen evolution reaction (PEC-OER) for solar water splitting.
The lab of Kenton Whitmire, a Rice professor of chemistry, teamed up with researchers at the University of Houston and discovered that growing a layer of an active catalyst directly on the surface of a light-absorbing nanorod array produced an artificial photosynthesis material that could split water at the full theoretical potential of the light-absorbing semiconductor with sunlight. The results appear in two new studies. The first, on the creation of the catalytic films, appears in Chemistry: A European Journal. The second, which details the creation of photoanodes, appears in ACS Nano.
The Rice team combined three of the most abundant metals—iron, manganese and phosphorus—into a precursor that can be deposited directly onto any substrate without damaging it. The resulting thin film contains the same ratio of iron, manganese, and phosphorus as the initial precursor.
To demonstrate the material, the lab placed the precursor into its custom chemical vapor deposition (CVD) furnace and used it to coat an array of light-absorbing, semiconducting titanium dioxide nanorods. The combined material—a photoanode—showed excellent stability while reaching a current density of 10 milliamps per square centimeter, the researchers reported.
Whitmire said the catalyst is grown from a molecular precursor designed to produce it upon decomposition, and the process is scalable. The Rice lab combined iron, manganese and phosphorus (FeMnP) into a molecule that converts to a gas when vacuum is applied. When this gas encounters a hot surface via CVD, it decomposes to coat a surface with the FeMnP catalyst.
The researchers claim their film is “the first heterobimetallic phosphide thin film” created from iron, manganese and phosphorus that starts out as a single precursor. The resulting films contain stable hexagonal arrays of atoms that, until now, had only been seen at temperatures above 1,200 degrees Celsius. The Rice films were created at 350 degrees C in 30 minutes.
Temperatures above 1,200 ˚ C destroy the semiconductor array. But these films can be made at low temperatures, allowing them to evenly coat and interact with the photo absorber and create a hybrid electrode.—Kenton Whitmire
The researchers coated the three-dimensional arrays of titanium dioxide nanorods with the metallic-looking film. The composite material showed potential as a high-surface-area semiconductor for photoelectrochemical cells.
Growing the transition metal coating directly onto the nanorods allows for maximum contact between the two, Whitmire said. The metallic, conductive interface between the semiconductor and the active catalytic surface is key to the way this device works, he said.
The film also has ferromagnetic properties, in which the atoms’ magnetic moments align in the same direction. The film has a low Curie temperature, the temperature at which some materials’ magnetic properties need to be induced. That could be useful for magnetic refrigeration, the researchers said.
Having established the technique, Whitmire said it will now be much easier to investigate hybrid catalysts for many applications, including petrochemical production, energy conversion and refrigeration.
Rice postdoctoral researcher Andrew Leitner is lead author of the Chemistry: A European Journal paper. Co-authors are graduate students Desmond Schipper and Binod Kumar Rai; former postdoctoral researcher Jing-Han Chen; graduate alumnus Adam Colson, now an assistant professor of chemistry at Boise State University; and Emilia Morosan, a professor of physics and astronomy, of chemistry and of materials science and nanoengineering, all at Rice; and Irene Rusakova, a senior research scientist at the University of Houston.
Schipper and Zhenhuan Zhao, a postdoctoral fellow at the University of Houston, are lead authors of the ACS Nano paper. Co-authors are Leitner and University of Houston graduate students Fan Qin, Zhiming Wang, Kamrul Alam and Shuo Chen; undergraduate student Lixin Xie, research professor Dezhi Wang; Zhifeng Ren, the MD Anderson Chair Professor; and Jiming Bao, an associate professor of electrical and computer engineering.
The National Science Foundation, the Robert A. Welch Foundation, the Gordon and Betty Moore Foundation and Rice University supported the research.
Leitner, A. P., Schipper, D. E., Chen, J.-H., Colson, A. C., Rusakova, I., Kumar Rai, B., Morosan, E. and Whitmire, K. H. (2017), “Synthesis of Hexagonal FeMnP Thin Films from a Single-Source Molecular Precursor” Chem. Eur. J. doi: 10.1002/chem.201700203
Desmond E. Schipper, Zhenhuan Zhao, Andrew P. Leitner, Lixin Xie, Fan Qin, Md Kamrul Alam, Shuo Chen, Dezhi Wang, Zhifeng Ren, Zhiming Wang, Jiming Bao, and Kenton H Whitmire (2017) “A TiO2/FeMnP Core/Shell Nanorod Array Photoanode for Efficient Photoelectrochemical Oxygen Evolution” ACS Nano doi: 10.1021/acsnano.7b00704