How photoelectrodes change in contact with water

Every green leaf is able to convert solar energy into chemical energy, storing it in chemical compounds. However, an important sub-process of photosynthesis can already be technically imitated -- solar hydrogen production: Sunlight generates a current in a so-called photoelectrode that can be used to split water molecules. This produces hydrogen, a versatile fuel that stores solar energy in chemical form and can release it when needed.

Photoelectrodes with many talents

At the HZB Institute for Solar Fuels, many teams are working on this vision. The focus of their research is on producing efficient photoelectrodes. These are semiconductors that remain stable in aqueous solutions and are highly active: Not only can they convert sunlight into electrical current, but they may also act as catalysts to accelerate the splitting of water. Among the best candidates for inexpensive and efficient photoelectrodes is bismuth vanadate (BiVO4).

What happens when in water?

"Basically, we know that just by immersing bismuth vanadate in the aqueous solution the chemical composition of the surface changes," says Dr. David Starr of the HZB Institute for Solar Fuels. And his colleague Dr. Marco Favaro adds: "Although there are a great many studies on BiVO4, it has not been clear until now exactly what implications this has on the surface electronic properties once they come into contact with the water molecules." In this work, they have now investigated this question.

Doped BiVO4 under water vapor

They studied single crystals of BiVO4 doped with molybdenum under water vapor with resonant ambient pressure photoemission spectroscopy at the Advanced Light Source at Lawrence Berkeley National Laboratory. A team led by Giulia Galli at the University of Chicago then performed density functional theory calculations to help interpret the data and to untangle the contributions of individual elements and electron orbitals to the electronic states.

Polarons on the surface detected

"In situ resonant photoemission has allowed us to understand how the electronic properties of our BiVO4 crystals changed upon water adsorption," Favaro says. The combination of measurements and calculations showed that due to excess charge, generated by either doping or defects on certain surfaces of the crystal, so-called polarons may form: negatively charged localized states, where water molecules can easily attach and then dissociate. The hydroxyl groups formed via water dissociation help to stabilize further polaron formation. "The excess electrons are localized as polarons at VO4 units on the surface," Starr summarizes the results.

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Solar hydrogen: Better photoelectrodes through flash heating

Solar energy can directly drive electrochemical reactions at the surface of photoelectrodes. Photoelectrodes consist of semiconducting thin films on transparent conductive-glass substrates that convert light into electricity. Most photoelectrochemical studies have focused on water splitting, a thermodynamically uphill reaction that could offer an attractive pathway for the long-term capture and storage of solar energy by producing 'green' hydrogen.

Metal-oxide thin film photoelectrodes are particularly promising for these diverse functions. They comprise abundant elements, potentially offering infinite tunability to achieve the desired properties -- at potentially low costs.

Made from plasma

At the HZB Institute for Solar Fuels, several teams focus on developing such photoelectrodes. The usual method to produce them is pulsed laser deposition: an intense laser pulse hits a target containing the material and ablates it into a highly energetic plasma deposited on a substrate.

Quality needs heat

Further steps are needed to improve the quality of the deposited thin film. In particular thermal processing of the metal-oxide thin-film reduces defects and imperfections. However, this creates a dilemma: Reducing atomic defects concentration and improvements in crystalline order of the metal-oxide thin films would require thermal processing temperatures between 850 and 1000 degrees Celsius -- but the problem is that the glass substrate melts at 550 degrees Celsius.

Flash-heating the thin film

Dr. Ronen Gottesman from the HZB Institute for Solar Fuels has now solved this problem: After deposition, using high-powered lamps, he flash-heats the metal-oxide thin film. This heats it up to 850 degrees Celsius without melting the underlying glass substrate.

"The heat efficiently reduces structural defects, trap states, grain boundaries, and phase impurities, which would become more challenging to mitigate with an increasing number of elements in the metal-oxides. Therefore, new innovative synthesis approaches are essential. We have now demonstrated this on photoelectrodes made of Ta2O5, TiO2, and WO3, which we heated to 850 °C without damaging the substrates," says Gottesman.

Record performance for α-SnWO4

The new method was also successful with a photoelectrode material that is considered a very good candidate for solar water splitting: α-SnWO4. Conventional furnace heating leaves behind phase impurities. Rapid thermal processing (RTP) heating improved crystallinity, electronic properties, and performance, leading to a new record performance of 1 mA/cm2 for this material, higher by 25% than the previous record.

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