Atomic layer deposited transition metal oxide-titania alloys for solar driven water oxidation
Keywords: Chemistry.

Photoelectrochemical (PEC) water splitting is a promising approach for synthesizing chemical fuels from sunlight. First demonstrated by Fujishima and Honda in 1972, PEC cell components and design strategies have proliferated in recent years. Regardless of the specific device architecture, however, any efficient PEC device requires (1) a high yield of energetic photogenerated carriers and (2) a mechanism for extracting these photogenerated carriers, (3) a corrosion-resistant anode at the pH and operating potential of the device, and (4) an effective catalyst for water oxidation. This dissertation addresses these challenges in the context of a photoanode, where water oxidation is tightly coupled to one of the light-absorbing elements of the PEC device. Metal-insulator-semiconductor (MIS) junctions are a promising photoanode design that electronically couples a high-quality semiconductor to an efficient water oxidation catalyst. The photovoltage produced by an MIS junction depends on the strength of the built-in field, or Schottky barrier height. This built-in field, in turn, depends on the difference in work function between the semiconductor and the metal, taking charges and interface fields into account. For optimal performance, a high work function metal induces a field that sweeps photogenerated holes from an n-type semiconductor to the electrolyte interface for water oxidation. In addition to generating large photovoltages, the ideal Schottky contact to an n-type semiconductor photoanode must also catalyze water oxidation and protect the underlying semiconductor from corrosion. In this work, I use atomic layer deposition (ALD) to fabricate alloys of TiO2 and transition metal oxides (specifically RuOx and IrOx) that function as the "M" of an MIS photoanode. Alloying TiO2 with these noble metal oxides combines the corrosion resistance of TiO2 with the high work function and catalytic activity of RuOx and IrOx. These alloys represent an ultra-thin analogue to the dimensionally stable anode used industrially for chlorine evolution. By investigating the chemical and electronic properties of these alloys, I unravel some of the key design principles for corrosion resistant Schottky contacts in MIS photoanodes. First, I demonstrated that ALD TiO2 protects the underlying silicon from corrosion and stabilizes RuOx and IrOx during water oxidation in acid. Second, I found that the electronic properties of TiO2 could be altered by alloying with metal oxides that have the desired work function. TiO2 makes a poor Schottky contact to n-type silicon, and its conductivity is difficult to control. Alloying TiO2 with high work function, conductive metal oxides like RuOx or IrOx not only enables high photovoltages but also guarantees high conductivity. By comparing the electronic properties of TiO2-RuOx alloys with TiO2-IrOx alloys, I also determined that the density of states at the alloy/SiO2 interface was critical for charge transport through the MIS junction. Finally, I gained insight into the relationship between catalytic activity and stability for RuOx and IrOx, two of the most commonly used water oxidation catalysts in acid. While IrOx is more stable than RuOx, its catalytic activity nonetheless degrades slowly over time. Though I used silicon as a model semiconductor, this ALD alloying approach may be particularly valuable for semiconductors that must rely on MIS junctions to generate large photovoltages (because forming a p-n junction is problematic). ALD enables unusually precise control over both the film thickness and composition. The ability to create graded structures by ALD presents a unique opportunity to control the composition these protection layers as a function of depth, placing valuable metal atoms where they are needed most—at the electrode/electrolyte interface for catalysis and at the insulator/metal interface for efficient tunneling. As such, ALD is capable of addressing many of the challenges associated with fabricating carrier selective contacts in photoelectrochemical and photovoltaic devices.