title: The influence of structure and processing on the behavior of TiO2 protective layers for stabilization of n-Si/TiO2/Ni photoanodes for water oxidation.
abstract: Light absorbers with moderate band gaps (1-2 eV) are required for high-efficiency solar fuels devices, but most semiconducting photoanodes undergo photocorrosion or passivation in aqueous solution. Amorphous TiO2 deposited by atomic-layer deposition (ALD) onto various n-type semiconductors (Si, GaAs, GaP, and CdTe) and coated with thin films or islands of Ni produces efficient, stable photoanodes for water oxidation, with the TiO2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO2 in these electrodes and the structure and energetic defect states of the material are not yet well-elucidated. We show herein that TiO2 films with a variety of crystal structures and midgap defect state distributions, deposited using both ALD and sputtering, form rectifying junctions with n-Si and are highly conductive toward photogenerated carriers in n-Si/TiO2/Ni photoanodes. Moreover, the photovoltage of these electrodes can be modified by annealing the TiO2 in reducing or oxidizing environments. All of the polycrystalline TiO2 films with compact grain boundaries investigated herein protected the n-Si photoanodes against photocorrosion in pH = 14 KOH(aq). Hence, in these devices, conduction through the TiO2 layer is neither specific to a particular amorphous or crystalline structure nor determined wholly by a particular extrinsic dopant impurity. The coupled structural and energetic properties of TiO2, and potentially other protective oxides, can therefore be controlled to yield optimized photoelectrode performance.
authors.affiliation: †Division of Chemistry and Chemical Engineering, California Institute of Technology, 210 Noyes Laboratory, Pasadena, California 91125, United States.|‡Joint Center for Artificial Photosynthesis, California Institute of Technology, 1200 E. California Blvd., Pasadena, California 91125, United States.
authors.forename: Matthew T
authors.initials: MT
authors.lastname: McDowell
authors.affiliation: †Division of Chemistry and Chemical Engineering, California Institute of Technology, 210 Noyes Laboratory, Pasadena, California 91125, United States.|‡Joint Center for Artificial Photosynthesis, California Institute of Technology, 1200 E. California Blvd., Pasadena, California 91125, United States.
authors.forename: Michael F
authors.initials: MF
authors.lastname: Lichterman
authors.affiliation: †Division of Chemistry and Chemical Engineering, California Institute of Technology, 210 Noyes Laboratory, Pasadena, California 91125, United States.
authors.forename: Azhar I
authors.initials: AI
authors.lastname: Carim
authors.affiliation: †Division of Chemistry and Chemical Engineering, California Institute of Technology, 210 Noyes Laboratory, Pasadena, California 91125, United States.|‡Joint Center for Artificial Photosynthesis, California Institute of Technology, 1200 E. California Blvd., Pasadena, California 91125, United States.
authors.forename: Rui
authors.initials: R
authors.lastname: Liu
authors.affiliation: †Division of Chemistry and Chemical Engineering, California Institute of Technology, 210 Noyes Laboratory, Pasadena, California 91125, United States.|‡Joint Center for Artificial Photosynthesis, California Institute of Technology, 1200 E. California Blvd., Pasadena, California 91125, United States.
authors.forename: Shu
authors.initials: S
authors.lastname: Hu
authors.affiliation: ‡Joint Center for Artificial Photosynthesis, California Institute of Technology, 1200 E. California Blvd., Pasadena, California 91125, United States.|§Beckman Institute, California Institute of Technology, Pasadena, California 91125, United States.
authors.forename: Bruce S
authors.initials: BS
authors.lastname: Brunschwig
authors.affiliation: †Division of Chemistry and Chemical Engineering, California Institute of Technology, 210 Noyes Laboratory, Pasadena, California 91125, United States.|‡Joint Center for Artificial Photosynthesis, California Institute of Technology, 1200 E. California Blvd., Pasadena, California 91125, United States.|§Beckman Institute, California Institute of Technology, Pasadena, California 91125, United States.|∥Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, United States.
authors.forename: Nathan S
authors.initials: NS
authors.lastname: Lewis
keywords: heterojunction interfaces; oxygen evolution reaction; photocorrosion; photoelectrochemistry; solar fuels; water splitting