Experimental Approach for Determining Semiconductor/liquid Junction Energetics by Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

Semiconductor/liquid junctions have long been investigated due to their simplicity of construction and economical possibility of fuel generation ^1-4, with some such systems obtaining efficiencies over 17%.⁵ These systems operate based on the formation of a rectifying junction at the interface between the semiconductor electrode and the electrolyte. The energetics of semiconductor/liquid junctions are similar to those of a semiconductor/metal, Schottky, junction ³ where an electrolyte assumes the role of the metal. The semiconductor Fermi level, E_F, is the electrochemical potential of the electron in the semiconductor and is analogous to the chemical potential of an electron in solution. In a liquid junction cell the difference in the chemical potential of the electron between the two phases results in the transfer of charge from one phase to another at equilibrium. Since the ions in the electrolyte are free to move while the fixed charges in the semiconductor cannot, a space-charge (or depletion) region forms within the semiconductor with an accompanying electric field. This electric field shifts the Fermi level (electrochemical potential) of the semiconductor to be equal to the chemical potential of the electron in the solution ⁶. The resulting electric field in the semiconductor only exists close (~ 1 µm) to the solution interface and the energy of the electron levels in this region are viewed as being "bent" by the field. The "band bending" in the semiconductor space-charge region results in a barrier to current flow in one direction while allowing conduction in the opposite direction, producing a "rectifying junction". Under illumination, this electrical field in the near-surface region of the semiconductor can separate light-generated electrons and holes, such that the device can be operated in a manner analogous to a solid-state photovoltaic device. Figure 1 demonstrates these basic concepts.

Figure 1: Solid/liquid junction. Illustration showing the band diagram and charge carrier density for (a) flat-band, (b) accumulation, (c) depletion and (d) inversion of an n-type semiconductor/liquid junction with n_e the free electron concentration, n_h the free hole concentration and n_i the intrinsic carrier concentration. The width of space-charge region is show as an accumulation layer d_acc, a depletion layer d_dep or an inversion layer d_inv. For further discussion, see ²⁹. Abbreviations are as follows: CBM: Conduction Band Minimum; VBM: Valence Band Maximum; E_F: Fermi Energy; U: the applied potential with respect to flat band; U_FB the flat band potential; μ_​es​- : the chemical potential in the solution as described in reference ²³. Please click here to view a larger version of this figure.

X-ray photoelectron spectroscopy (XPS) is a widely-used technique for determining both chemical (i.e., oxidation) states and electronic effects such as energy band relations in solid materials. Because of the very small inelastic mean free path (IMFP) of photoelectrons in air, including IMFPs on the mm scale even at millibar pressures⁷, and in order to avoid changes of the probed surfaces during measurements, XPS generally has to be performed under ultra-high vacuum (UHV) conditions. Numerous reviews of the XPS technique have been written ^8-10. In XPS, typically, electrons from core levels of the constituent elements of the sample are ejected into the vacuum by the absorption of X-rays. Upon irradiation with X-rays of an energy hν, electrons are ejected from the sample having a kinetic energy E_Kvac with respect to the vacuum level E_VAC. Figure 2 shows (a) the general geometry of an XPS instrument, (b) a simulated XPS spectra of TiO₂ with core levels (CL), Auger lines and a measurement of the work function, and (c) the relation of photon energy to kinetic and binding energies. The conservation of energy requires

_hν = E_B + E_Kvac + φ (1)

where E_B is the binding energy of the photoelectron from the core level, and φ is the work function of the sample. E_B is referenced to the Fermi level of the sample, E_F. The position of E_F can be determined by measurement of the valence band maximum of a noble metal (i.e. Gold or Silver) and fitting the Fermi function when the photon energy is well known (i.e. Al Kα). Otherwise this procedure is used to calibrate the photon energy, i.e. at electron synchrotrons that produce X-rays of variable energy.

Figure 2: XPS Schematic. Illustration of the XPS method: (a) standard XPS geometry; (b) Simulated XPS spectra of TiO₂ with core levels (CL), Auger lines and work function measurement; (c) Energy band relations for TiO₂ and definitions of kinetic energies E_Kvac, binding energies E_B and work function φ. Please click here to view a larger version of this figure.

Recently, ambient-pressure XPS, AP-XPS, experiments have been made possible due to the construction of differentially pumped electrostatic lens equipped ambient-pressure XPS analyzer systems. One approach to doing XPS at a solid/liquid interface is to separate the vacuum and the solution with a thin membrane through which XPS is carried out{Kolmakov, 2011 #176}^11-13. This technique requires the use of extremely thin membranes of materials such as silicon or graphene, as opposed to allowing measurements on thicker semiconductor materials. While standard XPS is carried out under UHV (10^-9– 10^-11 Torr), in AP-XPS the sample is at tens of Torr pressure while the analyzer remains under HV/UHV conditions. The resulting large pressure difference is realized by multiple stages of differential pumping ^7,14. As a result, measurement conditions much closer to a normal working environment can be realized. Studies on gold oxidation ¹⁵, lithium-oxygen redox reactions ¹⁶, and catalytic reactions ¹⁷ have been carried out in such systems. Further development and refinement of the technique ¹⁸ has allowed use of an electrochemical cell as the sample with the ability to apply a potential difference between the working electrode and the solution in a three-electrode electrochemical cell, which we term operando AP-XPS. The surface of the working electrode under a thin meniscus of electrolyte is analyzed by the operando AP-XPS technique. Figure 3 shows (a) a general schematic of the endstation as well as (b-d) pictures of the various parts of the endstation and (e) the materials under investigation. As a result, the solid working electrode as well as the thin (~13 nm) electrolyte layer can be investigated simultaneously, provided that the photoelectrons have a sufficient kinetic energy to penetrate through the electrolyte overlayer and escape unscattered, i.e. without energy loss, to the analyzer/detector. The use of ~ 4 keV X-rays produces photoelectrons with sufficient kinetic energy (~3.5 keV for Ti 2p and O 1s core levels) to make this possible ¹⁸.

Figure 3: Operando AP-XPS setup. (a) Scheme of the operando XPS setup. The working electrode and the hemispherical electron energy analyzer (HEA) were grounded together. The potential of the working electrode was changed with respect to the reference electrode. The PEC-beaker containing the electrolyte could be lowered whereas the three-electrode mount could be moved in all three directions. (b) View into the high-pressure analysis chamber. The X-ray beam enters through the window on the left, the three-electrode setup is on the top, the electrolyte beaker on the bottom, and the electron analyzer cone is in the center. (c) Three-electrode setup pulled up and in measurement position (compare to (a)). (d) Photo of the actual "tender" X-Ray operando AP-XPS analyzer and the analysis chamber that is directly connected to the analyzer. (e) The energy band relations of the p⁺-Si/TiO₂/H₂O(l.)/H₂O(g.) system under applied potential U. The working electrode (Si) and analyzer are grounded. In the three-electrode configuration the Fermi energy is shifted by U with respect to the reference electrode. The definitions of kinetic energies E_Kvac, binding energies E_B, work function φ and the ionization energy of H₂O (g.) E_IE are given. For p⁺-Si/TiO₂/Ni/H₂O(l.)/H₂O(g.) electrodes, a thin film of Ni/NiO_x would also be present at the solid/liquid interface, and would influence band bending as discussed in the text. For a further analysis of the importance of the Ni/NiO_x film, please see ²⁷. Please click here to view a larger version of this figure.

We have recently demonstrated that the combination of atomic-layer deposition (ALD)-grown TiO₂ with a Ni catalyst can effectively stabilize a variety of semiconductors in alkaline media, including Si, GaP, GaAs ¹⁹, CdTe ²⁰, and BiVO₄²¹ against photocorrosion. This advancement enables the use of technologically advanced semiconductors for energy converting devices such as solar fuel generators. Further investigation of the working principles of TiO₂ in these systems was undertaken to evaluate the nature of the semiconductor/liquid junction in the presence or absence of Ni ^22-24. Direct observation of these junctions using the operando AP-XPS approach produces data which demonstrate the working principles (accumulation, Fermi level pinning, depletion, inversion) behind these systems. Furthermore, this approach provides a tool by which a semiconductor/liquid junction or photocatalyst^25,26 may be interrogated such that the fundamental operating characteristics may be understood and optimized. We describe herein the manner in which such investigations may be undertaken, the conditions that are required for these experiments to work, and the means by which the data collected may be understood. We describe, in sections 1-2, the preparation of the electrodes which were used in our experiments, before presenting more general directions (sections 3-5) regarding the collection of data using this approach.