Transmission electron microscopy image of hematite nanorods.
Transmission electron microscopy image of hematite nanorods.

Using sunlight to split water into hydrogen and oxygen efficiently could revolutionize energy generation. Photoelectrochemical (PEC) cells can convert solar photons into hydrogen and oxygen, but finding the right electrode material has proved challenging.

“Hematite has been the best candidate for the photoanode in PEC cells for years,” says Flavio L. Souza of the Federal University of ABC, Brazil, “because of its intrinsic characteristics and abundance, which could enable cheap photoanode technology.”

But despite these promising theoretical predictions and decades of research, the poor electronic transport properties of hematite have hampered the development of a PEC device. Now, however, a team of collaborators led by Souza with Ricardo H. R Castro at University of California-Davis is uncovering the secrets of hematite, which could lead to new breakthroughs in PEC technology.

“We focused on a simple and easily scalable method to manufacture hematite precursor in powder and film/electrode forms simultaneously,” explains Souza.

The process starts by mixing chemical precursors dissolved in water and controlling the pH with Cl- ions, which are vital to the crystallization process and formation of nanorods.

“The Cl species are the responsible for the ‘magic’, favoring the formation of a nano-columnar morphology, which is subsequently calcined to turn the oxyhydroxide into hematite,” says Souza. “This method creates a highly desirable forest of nanorods and wires, which should lead to better electronic transport behavior because there are fewer interfaces to act as traps for electrons.”

However, it is essential that all the Cl be removed from the final hematite for this to work. This has proved tricky in practice because, explains Castro, Cl species can persist up to 900°C, way above typical photoanode processing temperatures.

“We report for the first time that the persistence of Cl- in the hematite structure or on the surface suppresses important properties and reduces PEC device performance,” he says.

Unexpectedly, however, the researchers discovered that the poisoning effect of Cl- species opens up an unprecedented opportunity for manipulating the spin mobility. Their work reveals that the Cl- species act as local charge capacitors, pinning the spin mobility and ‘poisoning’ the hematite surface. The weak ferromagnetic fingerprint of hematite is not discernable until Cl- is entirely eliminated from the material.

“While Cl- removal only happens at high annealing temperatures, the most exciting aspect of our data is that we show that Cl- can manipulate the spin mobility, offering the possibility of tuning hematite properties according to the application,” points out Souza.

The results provide an insight into the manufacture and control of the fundamental properties of hematite via a simple chemical route, which could be helpful in the development of future PEC devices.

Carvalho-Jr et al., J. Alloys Compounds 799 (2019) 83-88