Water splitting and stabilizing catalyst surfaces

Dr. Neil Canter, Contributing Editor | TLT Tech Beat March 2014

Researchers use an alternative approach to semiconductors that is known as the Dye Sensitized Photoelectrosynthesis Cell to split water.

KEY CONCEPTS
• A Dye Sensitized Photoelectrosynthesis Cell (DSPEC) that contains a ‘core-shell’ structure with a thin layer of titanium dioxide on the outside of an inner transparent conducting oxide core is active in splitting water into hydrogen and oxygen.
• In a process known as atomic layer deposition, a thin titanium dioxide nanolayer coating is applied to the nanostructured core of the DSPEC through a series of cycles.
• The nanolayer coating dramatically improves the activity of the photoanode and acts as an inert material to prevent the catalyst from losing activity at high pH where the water oxidation rates increase dramatically.

PREPARATION OF IMPROVED CATALYSTS that work on surfaces is continuing at the nanoscale where large surface areas can lead to better contact with substrates. One area that continues to be active is to find an efficient approach for producing hydrogen by splitting the element from water.

Two areas where improvement is needed are catalyst performance and durability. The two main types of catalysts, heterogeneous and homogeneous can be used but, as pointed out in a previous TLT article, each has its advantages and disadvantages (1).

Thomas Meyer, Arey Distinguished Professor of Chemistry at the University of North Carolina in Chapel Hill, N.C., reflected on the problems with using heterogeneous catalysts in two applications: solar water splitting and surface water oxidation. He says, “Options for suitable catalysts are contained within a restricted tool kit. The most desirable approach will be to develop a crossover species that combines the surface stability properties of a heterogeneous catalyst with the ability to make facile synthetic modifications to improve performance and durability.”

Several techniques have been tried to modify surfaces by attaching catalysts but they have limitations. Meyer says, “Surface bonding to metal oxides has been used routinely with carboxylate and phosphonate chemical links. This is a process that can be described as dipping or painting on the oxide surface.”

While surface modifications can improve catalyst performance, the modified species formed tends to be unstable toward loss from the surface, so durability is still in question.

In the past, semiconductors have been used to convert solar energy into the chemical energy needed to split water in a similar fashion to photosynthesis. Meyer says, “Initial development on solar conversion with direct excitation of semiconductors was first investigated in the 1970s by Honda and Fujushima. Ultraviolet light from the sun was absorbed by titanium dioxide to create a hole in the valence band with electron flow to a platinum cathode for hydrogen evolution with both electrons immersed in an electrolyte solution.”

The semiconductors approach is appealing for its simplicity, but finding semiconductors that use visible light from the sun and are stable for long periods of time under illumination has been a major challenge.

Meyer described an alternate approach, the Dye Sensitized Photoelectrosynthesis Cell or DSPEC, in which there is a nanoparticle titanium dioxide electrode attached to a chromophorecatalyst assembly linked to the surface by phosphonate groups. Adaptation of this approach led Meyer and his fellow researchers to develop a DSPEC that is active toward water splitting but only in a “core-shell” structure with a thin layer of titanium dioxide on the outside of an inner transparent conducting oxide (TCO) core.

ATOMIC LAYER DEPOSITION
A working photoanode for the DSPEC was developed through a research collaboration with the group of Greg Parsons at North Carolina State University in Raleigh, N.C. They started by depositing a thin layer of titanium dioxide on a transparent conducting oxide, either tin-doped indium oxide or antimony-doped tin oxide onto a fluoride-doped tin oxide glass substrate to make a nanostructured core. To this core structure is added a thin titanium dioxide nanolayer coating by atomic layer deposition (ALD).

Attached to the titanium dioxide nanolayer is a chromophore- catalyst assembly based on polypyridyl complexes of ruthenium with phosphonic acid groups for surface binding. The combined core shell assembly photoanode was then connected to a platinum cathode to complete the water-splitting cell.

In the ALD process, the reactive precursor, titanium tetrachloride, is first added by vapor phase deposition and then treated with water to form defined layers of titanium dioxide at a specific thickness. Figure 3 shows a transmission electron micrograph image showing the titanium dioxide coating on the core structure after 60 ALD cycles. This number of cycles enables a 3.6-nanometer titanium dioxide layer to be deposited.


Figure 3. The use of atomic layer deposition enables a nanolayer coating of titanium dioxide to be applied to the core structure, as shown in this transmission electron micrograph. (Courtesy of the University of North Carolina)

The effect of this ALD process is to dramatically improve the activity of the photoanode. Meyers says, “Water oxidation at the anode proceeds by a complex mechanism that involves a series of stepwise proton coupled electron transfer reactions. Electron flow moves from the chromophore, which reacts to visible light at an optimal wavelength of 450 nanometers, into the titanium dioxide shell and then into the conducting TCO core.”

“Getting the electrons to the cathode on pure titanium dioxide is plagued by back electron transfer to the assembly on the outside, which shuts down performance. With a thin nanolayer of titanium dioxide on the TCO core, the electrons rapidly reach the core where they are transferred to the cathode for hydrogen generation.”

In a second advance, ALD was also used to benefit water oxidation catalysis on an oxide surface. In this application, ALD was used to deposit a thin titanium dioxide nanolayer after using phosphonate groups to bind a water oxidation catalyst. Addition of a titanium dioxide overlayer by ALD stabilized binding to the surface allows the water oxidation process to be run at a higher pH. Meyer says, “Acid conditions have been used in the past because phosphonate and, especially carboxylate, surface links are unstable and tend to hydrolyze from the surface.”

The rate of hydrolysis increases at a higher pH but if the pH is increased by adding high concentrations of buffers, water oxidation rates increase dramatically with rate enhancements of up to 10⁶ observed.

With the use of the right concentration of buffer, the researchers have achieved impressive water oxidation performance over a wide range of pH values, even up to pH 11. Meyer says, “The titanium dioxide nanolayer functions as an inert material that prevents the surface-modified catalyst from being hydrolyzed and lost from the surface.”

From a durability standpoint, the photoanode catalyst is very stable. Meyer says, “We ran the catalyst for over 10,000 cycles with surprisingly good results.”

The focus of future work will be to optimize the efficiency and stability of the molecular assembly-based DSPEC for water splitting and continue to pursue stabilized catalyst surfaces. Meyer says, “We will evaluate different DSPEC core shell materials by changing the metal oxide to see if efficiency can be improved and are looking for improvement by factors of 10. A second objective is to improve light absorptivity by increasing the fraction of the solar spectrum that is used. Currently, we are only using a small part of what is available.”

Further information can be obtained in two recent articles (2, 3) or by contacting Meyer at tjmeyer@unc.edu.

REFERENCES
1. Canter, N. (2013), “Heterogenized homogeneous nanocatalysts,” TLT, 69 (2), pp. 12-13.
2. Alibabaei, L., Brennaman, M., Norris, M., Kalanyan, B. Song, W., Losego, M., Concepcion, J., Binstead, R., Parsons, G. and Meyer, T. (2013), “Solar water splitting in a molecular photoelectrochemical cell,” Proceedings of the National Academy of Sciences, 110 (50), pp. 20008-20013.
3. Vannucci, A., Alibabaei, L., Losego, M., Concepcion, J., Kalanyan, B., Parson, G. and Meyer, T. (2013), “Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation,” Proceedings of the National Academy of Sciences, 110 (52), pp. 20918-20922.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.