The surface chemistry of semiconductor electrode is crucial for determining its resulting J-E behavior, as all photocarriers need to initially travel across the surface and then perform fuel-forming reactions in electrolyte.87–89 Combining surface analysis with PEC measurements under various conditions offer insights into how the changes in surface conditions of photoelectrodes influence the PEC performance. In 1990s, Lewerenz and coworkers pioneered an advanced experimental setup combining the capability of both electrochemistry and ultra-high vacuum (UHV) measurements. In the works presented in this thesis, we extensively applied an air-free transfer technique to transfer sample from glovebox to the X-ray photoelectron spectroscopy (XPS) without air exposure, with the purpose of preserving pristine nature of sample surface. Both of these protocols share the same goal of understanding the correlation between the applied electrochemistry and the surface chemistry of semiconductors.
Overall, the surface of semiconductor electrodes can influence the electrochemical behaviors through the following aspects:
(a) Surface states: Different from atoms in the bulk, atoms sitting at the surface of a crystal may not be considered as fully coordinated, which may lead to additional states within
the band gap of semiconductors. Due to these surface states, photogenerated carriers may first charge these surface states instead of directly transferring from band edges into electrolyte. This will lead to Fermi level pinning as well as increased charge recombination, thus limiting the PEC performance of a semiconductor photoelectrode.
However, the presence of surface states may be challenging to directly visualize experimentally, requiring systematic studies. For example, the surface states of hematite photoanode for the OER have been extensively investigated by different groups using various techniques, such as intensity modulated impedance spectroscopy (IMIS).90–92 These detrimental surface states may be removable by depositing catalysts or tailoring material synthesis conditions. Identifying the chemical origins of surface states may benefit rational approaches for enhancing the device efficiency.
(b) Surface dipole: Covalent functionalization of semiconductor surface by organic functional groups can produce surface dipoles and shift band edges, by a magnitude based on groups’ electronegativity.93 The shifts in band edges may favorably optimize the interfacial energetics in contact with solution potentials to increase the maximum obtainable Voc of photoelectrode.94 In addition, these surface functional groups may also passivate surface states, decrease surface recombination and improve chemical stability.95,96 For example, methyl-terminated Si produced a surface dipole of -0.4 eV relative to Si, while substantially increasing both chemical and electrochemical stability of Si .97
(c) Surface stoichiometry: For compound semiconductors, surface stoichiometry can also influence the resulting J-E behavior of photoelectrodes, which is one of the major findings in this thesis. The surface stoichiometry of electrodes may be altered by in-situ surface reaction in aqueous electrolytes. To correlate such surface changes with the changes in electrochemical behavior requires careful surface analysis. Very recently, Choi and coworkers showed that a Bi-rich surface of BiVO4 photoanode can produce a favorable shift in band edges, resulting in improved PEC performance compared with that with a stoichiometric surface.98 As early as 1980s, Spicer and coworkers also proposed an
unified defect model to for various Ⅲ-Ⅴ semiconductors based on UHV measurements.99 Their results revealed that non-stoichiometric surfaces of InP and GaAs produce mid-gap surface states at specific energy levels. An In- and P-rich surface of InP can produce surface states located below the conduction band minimum by ~0.1 eV and 0.4 eV respectively. A Ga- and As-rich surface of GaAs can produce surface states located above the valence band maximum by ~0.5 eV and 0.75 eV respectively. In this thesis, following this classic unified defect model, we will demonstrate how the surface non-stoichiometry of p-InP and p-GaAs electrodes changes their PEC behaviors.
(d) Interfacial layer: In-situ chemical or electrochemical formation of surface layers over semiconductor photoelectrodes may potentially improve the resulting PEC performance.
Back in 1980s, Lewerenz and coworkers demonstrated a surface transformation of n-type CuInSe2 photoanode into p-type CuISe3-Se0, when in contact with acidic electrolytes containing I2/I- redox couple.88 Such a surface transformation favorably improved the electrode’s PEC performance, due to in-situ construction of heterostructure.100 Later in 2002, Lewerenz and coworkers showed a similar conversion of p-InP by an in-situ incorporation of chloride ions following surface corrosion in hydrochloric acid, into an efficient photocathode in contact with V3+/V2+ redox couple.101,102 More recently, Lewerenz and coworkers again successfully realized a surface conversion of the top AlInP layer into passivating oxide for a high efficient tandem solar fuel device. However, the stability of this in-situ transformed layer under operational conditions remains unknown.
Furthermore, the formation of thin SiOx layer on Si surface by the RCA cleaning is also crucial for realizing high-performance Si photoanodes for the OER.51,52,103 All these examples demonstrated the significant impact of surface conversion in electrolyte towards the resulting PEC performance, which will also be a focus of this thesis. However, leveraging such interfacial layers to improve PEC performance requires an electrochemical or chemical capability to optimize its formation and composition by systematically tuning electrolyte and potential.