Atomic Layer Deposited TiO₂–IrO_x Alloys Enable Corrosion Resistant Water Oxidation on Silicon at High Photovoltage

Abstract

We synthesized by atomic layer deposition (ALD) TiO2–IrO x alloys that enable high photovoltages and catalyze water oxidation on silicon metal–insulator–semiconductor (MIS) photoanodes. The ratio of TiO2 to IrO x was precisely controlled by varying the number of ALD cycles for each precursor. Silicon with a 2 nm surface SiO2 layer was coated with TiO2–IrO x alloys ranging in composition from 18 to 35% iridium relative to the sum of titanium and iridium concentrations. IrO x catalyzed oxygen evolution and imparted a high work function to the TiO2–IrO x alloys, enabling photovoltages during water oxidation that exceeded 600 mV. TiO2 imparted stability and inhibited corrosion of the underlying silicon light absorber. After annealing in forming gas (5% H2/95% N2), TiO2–IrO x alloys were stable for 12 h of continuous water oxidation in 1 M H2SO4. Key properties of the MIS junction affecting electrochemical operation were also extracted by electrochemical impedance spectroscopy. This work demonstrates that alloying by ALD is a promising approach for designing corrosion resistant Schottky contacts with optimized electronic and materials properties for catalyzed, solar driven water oxidation.

Quantifying Losses and Assessing the Photovoltage Limits in Metal–Insulator–Semiconductor Water Splitting Systems

“…Finally, we note that several MIS water splitting systems have achieved over 600 mV of photovoltage through a combination of using relatively high doped Si and using high‐quality oxides with minimal surface states 30,45,53. As shown by the black region in Figure 6, even an ideal system with optimized insulator thickness may still fall significantly short of the photovoltage limits.…”

“…Many insulators have been utilized in silicon‐based MIS photocatalyst systems. These include SiO 2 ,28–41 TiO 2 ,42–47 HfO 2 ,48,49 Al 2 O 3 ,42,50–53 SrTiO 3 ,54 and ZrO 2. 55 Initially, MIS systems were motivated by their ability to improve the chemical stability of semiconductors, but recently many design strategies have been identified to improve the efficiency of these systems.…”

Semiconductor-based photocatalysts for photocatalytic and photoelectrochemical water splitting: will we stop with photocorrosion?

“…90 Other photoelectrodes with a similar sandwich structure were also reported by the Chidsey group. 91 Furthermore, Gu and coworker designed a GaInP 2 photocathode with a graded catalytic-protective layer by annealing an amorphous-MoS x / TiO x -GaInP 2 stack at 450 C. 92 Annealing treatment resulted in a large degree of mutual penetration between MoS x , TiO 2 and GaInP 2 , while there was a relatively clear boundary between the components before annealing. As a result, the annealed MoS x / TiO x -GaInP 2 photocathode showed an enhanced stability compared to the sample without annealing.…”

Recent Advances in Materials Design Using Atomic Layer Deposition for Energy Applications

“…The TiO 2 –IrO x coated Si photoanode achieved high photovoltage of 600 mV and stability of upto 12 h. This work highlights the importance of ALD technique for designing corrosion resistant Schottky contacts with optimized electronic and materials properties for catalyzed, solar driven water oxidation. [ 124 ]…”