An Electrochemical, Microtopographical and Ambient Pressure X-Ray Photoelectron Spectroscopic Investigation of Si/TiO₂/Ni/Electrolyte Interfaces

Abstract

The electrical and spectroscopic properties of the TiO 2 /Ni protection layer system, which enables stabilization of otherwise corroding photoanodes, have been investigated in contact with electrolyte solutions by scanning-probe microscopy, electrochemistry and in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Specifically, the energy-band relations of the p + -Si/ALD-TiO 2 /Ni interface have been determined for a selected range of Ni thicknesses. AP-XPS measurements using tender X-rays were performed in a three-electrode electrochemical arrangement under potentiostatic control to obtain information from the semiconductor near-surface region, the electrochemical double layer (ECDL) and the electrolyte beyond the ECDL. The degree of conductivity depended on the chemical state of the Ni on the TiO 2 surface. At low loadings of Ni, the Ni was present primarily as an oxide layer and the samples were not conductive, although the TiO 2 XPS core levels nonetheless displayed behavior indicative of a metal-electrolyte junction. In contrast, as the Ni thickness increased, the Ni phase was primarily metallic and the electrochemical behavior became highly conductive, with the AP-XPS data indicative of a metal-electrolyte junction. Photoelectron spectroscopy can be used to directly characterize the energy relations of semiconductor/liquid junctions that underlie the operation of photoelectrochemical cells, 1 provided that the kinetic energy of the emitted photoelectrons can elastically penetrate the water film on the electrode surface. Conventional X-ray photoelectron spectroscopy (XPS) experiments are performed in ultra-high vacuum (UHV) in the absence of electrolyte, and thus do not allow for electrochemical control of an operating device during collection of XPS data. Recent theoretical work has shown that the inclusion of structured solvation layers on electrodes can alter the surface dipole by 0.5-0.7 eV (1.9-2.1 eV) for IrO 2 (WO 3 ).2 Established in-system techniques that allow analyses of (photo)electrodes after electrochemical operation enable assessment of aspects of the surface chemistry and of the associated energetic behavior.3-5 However, such experiments are limited in scope and interpretation due to the rinsing, drying and outgassing procedures required prior to insertion of the sample into the UHV analysis chamber. In contrast, the use of tender X-rays having photon energies in the 2.3-5.2 keV energy range allows generation of photoelectrons that have a substantially increased inelastic mean free path. This approach allows "operando" XPS studies in conjunction with a classical three-electrode potentiostatic arrangement and also facilitates investigation of the influence of the applied potential on the band-edge energies of metal, semiconductor and hybrid electrodes at such interfaces.6,7 Band bending and band-edge shifts can thus be determined directly by this spectroscopic technique. 7 We describe herein surface-sensitive analysis techniques for the characterization of TiO 2 /Ni/electrolyte...

Unravelling the electrochemical double layer by direct probing of the solid/liquid interface

“…1) was successfully created on a polycrystalline gold electrode by the ‘dip and pull' method39404142 (Supplementary Fig. 2) and characterized within the double-layer potential region ranging from −450 to +650 mV (versus Ag/AgCl/Cl − (sat.)…”

Operando Analyses of Solar Fuels Light Absorbers and Catalysts

“…p + -Si/TiO 2 as well as p + -Si/TiO 2 /Ni electrodes, with Ni thicknesses in the range of 0.5 to 10 nm, were contacted with 1.0 M KOH(aq) by use of the "dip and pull" method [18][19][20]. Operando APXPS data were collected while the electrode was under potential control between -1.4 V and +0.4 V vs Ag/AgCl.…”

“…We describe herein the use of operando ambient pressure X-ray photoelectron spectroscopy (APXPS) at the Advanced Light Source using tender X-rays in the 2 to 7 eV region to create photoelectrons that have sufficiently long inelastic mean-free paths (IMFP) to probe the semiconductor surface; the space charge region, the adjacent Helmholtz layer, and the bulk 3 electrolyte/water region [18][19][20]. Improving the activity of electrocatalysts composed of earthabundant elements, while achieving stability, is also of interest in the development of a stable, efficient solar-fuels generator.…”

Characterization of Electronic Transport through Amorphous TiO2 Produced by Atomic Layer Deposition

“…A mass spectrometer collected and analyzed the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 12 ejected secondary ions. The count rates of 30 Si, 48 Ti, 12 C + 133 Cs and 14 N + 133 Cs were collected as a function of sputtering cycle. SIMS data were collected for all TDMAT and TiCl 4 a-TiO 2 films.…”

“…The Si|TiO 2 |Ni|electrolyte interface has been probed via ambient pressure XPS. 17,48 It has been shown that for ultra-thin layers of Ni (< 1 nm) no conduction was observed in Fe(CN) 6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 37 measurements using different excitation energies and varying the electron emission angle confirmed the presence of a metallic Ni phase below the surface. In addition, cross-sectional TEM and SEM images of the TiO 2 |Ni interface region further support the presence of metallic Ni at the interface of the TDMAT a-TiO 2 .…”