Effects of catalyst material and atomic layer deposited TiO2 oxide thickness on the water oxidation performance of metal–insulator–silicon anodes

Abstract

We report on the effects on water oxidation performance of varying (1) the nanoscale TiO 2 thickness and (2) the catalyst material in catalyst/TiO 2 /SiO 2 /Si anodes. Uniform films of atomic layer deposited TiO 2 are prepared in the thickness range $1-12 nm on degenerately-doped p + -Si, yielding water oxidation overpotentials at 1 mA cm À2 of 300 mV to 600 mV in aqueous solution (pH 0 to 14). Electron/hole transport through Schottky tunnel junction structures of varying TiO 2 thickness was studied using the reversible redox couple ferri/ferrocyanide. The dependence of the water oxidation overpotential on ALD-TiO 2 thickness, with all other anode design features unchanged, exhibits a linear trend corresponding to $21 mV of added overpotential at 1 mA cm À2 per nanometer of TiO 2 for TiO 2 thicknesses greater than $2 nm. For thinner TiO 2 layers, an approximately thickness-independent overpotential is observed.The linear behavior for anodes with thicker TiO 2 layers is consistent with the predicted effect of bulk TiO 2 -limited electronic conduction on the voltage required to sustain the current density across the TiO 2 /SiO 2 insulator stack. Eight different oxygen evolution catalysts of thickness 1-3 nm are studied. For the anodes investigated, 3 nm of Ir or Ru gave the best water oxidation performance, but both thinner layers and other catalysts can be quite effective, suggesting the potential for reduced materials cost. Lastly, a flat band voltage analysis of solid state thin film capacitors was done for five different gate metals on n-Si to probe junction energetics directly relevant to a photoanode. The results are consistent with a Schottky junction in which the Fermi level at the semiconductor surface is unpinned. Broader contextEnergy storage is likely to be a requirement for grid-scale replacement of fossil fuel sources by renewable energy. Its value lies not only in the ability to compensate for intermittency, but also in the economics of peak leveling and its national security implications. The possibility of synthesizing fuels in a renewable and clean fashion has long fascinated academic and industrial researchers alike, but the high cost and poor efficiencies of such processes have prevented practical implementation of solar fuel or electro-fuel synthesis. In an effort to address these problems, researchers have been pursuing the development of photoelectrochemical cells: all-in-one units that convert solar energy into energy stored in chemical bonds. However, materials that are stable under harsh reducing and, especially, oxidizing conditions are rarely optimized for solar absorption and transport of electronic carriers. The ability to combine the properties of wide bandgap and stable materials, like TiO 2 , and efficient small bandgap absorbers, like silicon, constitutes a major advance in making viable photoelectrochemical cells. This report looks further at a novel device structure for fuel synthesis in which high quality absorbers are coupled to high quality water oxidation catalysts via an ALD...

Engineering Interfacial Silicon Dioxide for Improved Metal–Insulator–Semiconductor Silicon Photoanode Water Splitting Performance

“…However, based on previously reported values, 8 this thickness reduction will only improve overpotential by ~20 mV, which is significantly less than the observed improvements, so the majority of the improvement is due to the reduction in SiO2 thickness. The effect of forming gas temperature on TiO2-protected photoanode performance is a topic of ongoing study.…”

“…7 Previously reported research has shown that atomic layer deposited (ALD) TiO2 can effectively protect Si photoanodes, yielding efficient, stable devices. [7][8][9] Recently, we have investigated the tunnel barrier to hole transport presented by an interfacial layer (IL) of SiO2 in TiO2-protected Si photoanodes. 10 The SiO2 tunnel oxide can easily dominate the device resistance due to its large bandgap and relative lack of bulk oxide traps, compared to the TiO2 corrosion protection layer.…”

“…13 When initiation of ALD is limited, the film forms in an islanded morphology, leading to high RMS roughness and incomplete film coverage for ultrathin films. [14][15] In our prior research, [7][8] 16 However, methods of reliably producing ultrathin (< 1.3 nm) SiO2 interlayers between an ALD-TiO2 protective film and the Si(100) substrate, and their effects on anode Si anode performance, have yet to be reported.…”

Electrocatalytic Oxygen Evolution on Iridium Oxide: Uncovering Catalyst-Substrate Interactions and Active Iridium Oxide Species

“…al. (15). However, the characteristics of an interlayer formed during thermal preparation of Ru-or Ir oxide are not known.…”

Isolating the Photovoltaic Junction: Atomic Layer Deposited TiO2–RuO2 Alloy Schottky Contacts for Silicon Photoanodes

“…1,2,[7][8][9][10][11][12][13] Incorporation of such protection layers into the MIS junction, however, has compromised the photovoltages commonly reported for these devices. An ideal MIS junction using silicon has a theoretical maximum open circuit voltage of 700-800 mV.…”