Stabilize semiconductors against corrosion in acidic or alkaline water environments to enable the construction of fully integrated and robust solar fuels devices.
We use atomic layer deposition and sputtering to deposit protective coatings onto technologically important semiconductors such as silicon and gallium arsenide, which are otherwise unstable – dissolving or forming insulating oxide coatings on their surfaces – when under operating conditions required for efficient solar fuels devices. Materials are targeted as protective coatings if they are likely to be:
- Chemically stable under the operating conditions of interest;
- Conductive, to allow efficient transfer of charge from the semiconductor to the interface with water;
- Catalytic, to favor direction of reactive charges to desired reactions over direction to corrosion reactions;
- Transparent and anti-reflective, to void interfering with the ability to capture sunlight;
In addition, protective coatings should have favorable energy-band alignments with the underlying semiconductor to maximize the power of the device.
Nickel Oxide Protective Coatings for Self-Passivating Semiconductors
Semiconductors that are capable of absorbing the majority of the visible solar spectrum typically corrode – either dissolving or forming insulating coatings on their surfaces – under the conditions required for the oxidation half-reaction in an efficient, integrated, solar fuels device. The lack of semiconductors that are inherently stable under oxidative conditions and in contact with acidic or alkaline water had hindered the development of integrated solar fuels devices.
In the Lewis group, we developed techniques to deposit a multifunctional nickel oxide (NiOx) coating onto n-type Si and n-type indium phosphide. The reactively sputtered NiOx coating is a p-type conductor – an electronically ideal partner for an n-type photoanode – that is transparent, anti-reflective, and chemically stable under oxidative conditions in alkaline water. Furthermore, the NiOx coating is catalytically active for the oxidation of water to oxygen gas. The NiOx coating enabled illuminated silicon and InP electrodes immersed in strongly alkaline water – electrodes which would fail immediately without the coating – to be used to robustly drive water oxidation for over 1200 hours. Accounting for the ~ 20% capacity factor of sunlight, the 1200 hours of continuous operation is comparable to more than 8 months of outdoor operation.
Electronically “Leaky” Titanium Dioxide Protective Coatings
Silicon, gallium arsenide, and gallium phosphide are technologically important semiconductors that have band gaps that allow absorption of a significant portion of the solar spectrum (1.14 eV, 1.42 eV, and 2.26 eV, respectively). However, under the conditions required for the oxidation half-reaction in an efficient, integrated solar fuels device – contact with acidic or alkaline water while at positive potential – these materials corrode rapidly; an insulating oxide forms on the surface of silicon, while gallium arsenide and gallium phosphide dissolve.
Thin (~ 2 nm) titanium dioxide coatings had been found to protect small photoanodes; however, coatings a few nm thick are difficult to deposit uniformly and thus often contain through-film pinholes that limit the ability to scale up. In addition, it was believed that thicker titanium dioxide coatings would act as insulators and prevent the flow of electronic charge, and that therefore the coatings needed to be thin enough to allow charges to pass via tunneling. Specifically, titanium dioxide is a wide-band-gap (3.05 eV) semiconductor with a valence band positioned at a potential significantly (> 2 V) more positive than the valence bands of Si, GaAs, GaP, or most other semiconductors capable of absorbing significant amounts of sunlight. For a photoanode, positive charges (holes) generated in the valence band of the light absorber need to pass into the electrolyte, and would not be able to pass through titanium dioxide on their way to the electrolyte due to the large (> 2V) potential barrier.
In the Lewis group, we used atomic layer deposition (ALD) to develop a titanium dioxide coating that, together with a Ni catalyst coating, protects semiconductors from corrosion while allowing conduction of holes.3 The titanium dioxide coating can be much thicker (up to 140 nm) than tunneling barriers (few nm), and thus can be prepared pinhole-free over much larger areas than tunneling barriers. The titanium dioxide coating is transparent to visible light, and the nickel layer can be deposited as islands or thin films that allow transmission of light. Without the nickel layer, the TiO2 coating does not conduct holes; thus the nickel layer is key to the function of the “leaky” ALD TiO2. Si photoanodes protected by TiO2/Ni coatings and immersed in strongly alkaline water (1 M KOH) and under simulated solar illumination continuously oxidized water for over 100 hours, while protected GaAs and GaP photoanodes were stable under the same conditions for over 25 hours. We also demonstrated that the TiO2/Ni coatings could protect cadmium telluride photoanodes for over 100 hours of stable sunlight-driven water oxidation,4 and have shown that the coatings protect Si microwire photoanodes.5 Without the TiO2/Ni coating, all of these photoanodes (Si, GaAs, GaP, and CdTe) would have failed within a few seconds.
We have explored the mechanism of conduction – why are TiO2/Ni coatings electronically “leaky”? – using several experimental techniques. We have shown using operando X-ray photoelectron spectroscopy to examine TiO2/Ni/electrolyte interfaces and found that the metallic Ni coating serves as an electrical contact to the electrolyte.6 We have also used solid-state measurements and X-ray photoelectron spectroscopy to characterize the behavior and electronic structure of interfaces between Si and TiO2/Ni coatings.7
- McDowell, M. T.; Lichterman, M.; Carim, A. I.; Liu, R.; Hu, S.; Brunschwig, B. S.; Lewis, N. S., The influence of structure and processing on the behavior of TiO2 protective layers for stabilization of n-Si/TiO2/Ni photoanodes for water oxidation. ACS Appl. Mater. Interfaces 2015, 7 (28), 15189-15199.
- Sun, K.; McDowell, M. T.; Nielander, A. C.; Hu, S.; Shaner, M. R.; Yang, F.; Brunschwig, B. S.; Lewis, N. S., Stable solar-driven water oxidation to O2(g) by Ni-oxide coated silicon photoanodes. J. Phys. Chem. Lett. 2015, 6, 592-598.
- Hu, S.; Shaner, M. R.; Beardslee, J. A.; Lichterman, M.; Brunschwig, B. S.; Lewis, N. S., Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation. Science 2014, 344 (6187), 1005-1009.
- Lichterman, M. F.; Carim, A. I.; McDowell, M. T.; Hu, S.; Gray, H. B.; Brunschwig, B. S.; Lewis, N. S., Stabilization of n-cadmium telluride photoanodes for water oxidation to O2(g) in aqueous alkaline electrolytes using amorphous TiO2 films formed by atomic-layer deposition. Energy Environ. Sci. 2014, 7, 3334-3337.
- Shaner, M. R.; Hu, S.; Sun, K.; Lewis, N. S., Stabilization of Si microwire arrays for solar-driven H2O oxidation to O2(g) in 1.0 M KOH(aq) using conformal coatings of amorphous TiO2. Energy Environ. Sci. 2015, 8, 203-207.
- Lichterman, M. F.; Hu, S.; Richter, M. H.; Crumlin, E. J.; Axnanda, S.; Favaro, M.; Drisdell, W.; Hussain, Z.; Mayer, T.; Brunschwig, B. S.; Lewis, N. S.; Liu, Z.; Lewerenz, H.-J., Direct observation of the energetics at a semiconductor/liquid junction by operando X-ray photoelectron spectroscopy. Energy Environ. Sci. 2015, 8, 2409-2416.
- Hu, S.; Richter, M. H.; Lichterman, M. F.; Beardslee, J.; Mayer, T.; Brunschwig, B. S.; Lewis, N. S., Electrical, photoelectrochemical and photoelectron spectroscopic investigation of the interfacial transport and energetics of amorphous TiO2/Si heterojunctions. J. Phys. Chem. C 2016, 120 (6), 3117-3129.
by Kimberly Papadantonakis, June 2016.