Solar water Splitting (Tandem Cell Configuration)
Having nearly optimum band gap and sufficiently positive valence band position makes Hematite (α-Fe2O3) and Tantalum Nitride (Ta3N5) promising semiconductors for PEC water oxidation.
Tantalum Nitride (Ta3N5)
Despite promising properties, multiple drawbacks preclude efficient PEC water splitting on Ta3N5: (1) the harsh synthesis conditions – anodization of Ta-foil followed by high-temperature ammonolysis. From synthetic point of view, this method is highly energy intensive, produces a sizable quantity of chemical wastes, inefficient on chemical utilization, and provides highly reducing conditions which prevents its integration in the tandem cell configuration.
The efforts are focused to investigate and develop ALD method to directly deposited crystalline phase of Ta3N5 on commercially available TCOs at lower temperatures which otherwise requires considerably high-temperature annealing in ammonia (highly reducing condition).
Hematite (α-Fe2O3)
Although Hematite is a robust photoanode for PEC water oxidation, its surface has a low catalytic activity toward water oxidation reaction. The surface modification with various catalytic and non-catalytic materials have shown to significantly improve the PEC characteristics of hematite, however the mechanism at which it effect its performance remain elusive.
The aim of this project is to investigate one of the long-standing question in this area that is how the co-catalyst is interfaced with semiconductor and what is the role of co-catalyst. We utilized a series of (photo)electrochemical techniques to assess the effect of the co-catalyst on the PEC performance of catalytically modified hematite electrodes.
Atomic Layer Deposition (ALD)
Atomic layer deposition has emerged as a promising method for the synthesis of thin films both in industrial and laboratory scales. ALD provides an atomic scale control over the film thickness which enables to conformally deposit thin film on high aspect ratio substrates. In addition, the processing temperature and conditions of ALD deposition are milder in comparison to the conventional methods, thus, the ALD provides a viable method for stacking multiple layers of materials- each with specific functionality- on top of each other. Moreover, the ALD process is performed under vacuum which makes it suitable for the synthesis of non-oxide and air sensitive materials which otherwise requires a harsh and reactive synthesis conditions. These advantages are crucially important as it provides a potent synthesis method to study the structure-function relationship of the material.
In a typical ALD process the precursor and co-reactant are sequentially introduced into the deposition chamber in a timely fashion. In general, an ALD cycle is comprised of 4 elementary steps (as shown in the above Figure). Initially, the precursor is introduced in the deposition chamber in excess quantity (step 1). It reacts with all of the surface sites and forms chemical bonds (chemisorption) and the excess precursors are physisorbed. Next, the chamber is purged with a chemically inert carrier gas to remove the loosely bonded precursors (step 2). Ideally, this step provides an atomically clean surface ready for the next step. Subsequently, the co-reactant is introduced into the chamber to react with the fist layer (step3) followed by a purging step to remove the gaseous products and excess co-reactants (step 4). As a result, the ALD process is a surface self-limiting reaction where upon formation of a monolayer, the deposition is self-terminated. Together, these four steps complete one ALD cycle that corresponds to a film with few angstrom thickness (the thickness depends on the material and deposition conditions). To deposit a desirable thickness this process is therefore repeated several times.
Thermo-Catalytic Ammonia Splitting
Coming soon!