Electrochemical Characterization of the Titanium/Pbs Interface By Ambient Pressure X-Ray Photoelectron Spectroscopy
Authors: Valero-Vidal Carlos , Favaro Marco , Yu Yi , Crumlin Ethan J 
Published: February 2020

A novel operando Ambient Pressure Tender X-Ray Photoelectron Spectroscopy (T-APXPS) technique, developed by beamline 9.3.1 at the Advanced Light Source (ALS) [1], allows for the first time to study in detail the corrosion phenomena at the metallic biomaterial/body fluid electrified interface under operando electrochemical conditions. “Dip and Pull” method was used in order to create a stable nanometer-thick aqueous layer on the metallic biomaterial surface (Figure 1(a)). The ionically sustained electrical contact between the liquid layer on the sample surface and the bulk electrolyte is ensured by keeping the bottom part of each electrode immersed in the electrolyte. By using synchrotron radiation in the range of tender X-rays (at an energy of 4keV), it is possible to detect photoelectrons ejected from the solid/liquid interface, directly probing complex (electro-)chemical phenomena occurring at such important spatial region.
Titanium is one of the most commonly used metal biomaterials (such as dental implants or restoration) because of its biocompatibility and chemical stability in biological environments [2]. This metal is characterized by the spontaneous formation of an oxide film (mainly composed of amorphous TiO2) on the surface, which protects the bulk material for further oxidation (passive metal). Thus, the formation of the surface oxide is responsible for both biocompatibility and corrosion resistance (related to the long-term durability of the biomaterial implanted into the body and metal ion release) [3].
Changes in the chemical composition of the Ti/TiO2/electrolyte junction during cathodic/anodic polarization in simulated body fluids were characterized using operando T-APXPS technique.

Experimental.
An electrochemical characterization of pure polycrystalline titanium in phosphate buffer solution (PBS) was carried out. Polarization curves were used in order to determine the different potential domains (cathodic, cathodic-anodic transition and passive domain). Potentiostatic tests were conducted between potentials comprised in the range -1VAg/AgCl and 1VAg/AgCl to determine the effect of the applied potential (belonging to the different potential domains) in the electrochemical reactions occurring at the biomaterial/electrolyte interface. During the potentiostatic tests, XPS spectra of Ti2p, O1s, C1s, P2s, and Cl2p were acquired under ultra high vacuum (UHV) and hydrated conditions (i.e., T-APXPS measurements were performed exposing the sample to a water gas pressure of about 16Torr prior any electrochemical treatment).

Results

Figure 1(b) shows the Ti2p spectra obtained at different conditions: (i) UHV, (ii) hydrated conditions, and (iii) during operando electrochemical conditions when potentials of -1.0VAg/AgCl (cathodic domain) and 1.0VAg/AgCl (passive domain) were applied to the working electrode (WE). In those spectra, attenuation in the Ti2p intensities upon water vapor exposure and subjected to polarization conditions were both measured. This observation reveals the formation of a thin electrolyte film (of about ~15 nm) on the titanium (WE).
In this talk, our findings related to the titanium polarization at different applied potentials will be shown. New features on the higher binding energy (BE) side of the Ti2p spectra were observed after increasing the applied potential towards more anodic values. The change in the Ti2p photoelectron peak points to a change of the oxidation state of titanium, with the consequent increase of the Ti+4 concentration. In addition, a demonstration of the functionality and electrochemical activity of the formed thin electrolyte film on the titanium surface will be discussed.
O1s photoelectron spectra are reported in Figure 1(c). It is possible to observe that the core level peaks from the electrolyte elements (EE) undergo a rigid shift in BE (with respect to the Fermi level of the grounded WE) that follows the variation of the applied potential (U) accordingly to the relation BEEE = BE0
EE - eU. This effect demonstrates that the electrolyte layer on the WE is conductive and connected to the bulk electrolyte, allowing potential control at the solid/liquid electrified interface. The O1s spectra shows two main components, centered at 533.6eV and 536.4eV, attributed to the liquid phase water from the thin film and the gas phase water filling the chamber, respectively. A shift of 2.0eV was observed between the liquid phase peaks obtained at 1.0VAg/AgCl and -1.0VAg/AgCl, which match perfectly with the applied potential difference.
In summary, the newly developed T-APXPS technique at ALS allows the operando investigation of corrosion phenomena occurring at the metallic biomaterials/electrolyte interface. The advantage of using such technique relies on the simultaneous acquisition of electrochemical and spectroscopy data which allows a direct probing of the electrochemical process occurring at the biomaterial/liquid interface, including changes in the electrolyte species and evaluation of the metal degradation under simulated biological conditions.

References

[1] S. Axnanda et al. Sci. Rep 5 (2015).
[2] S. Virtanen et al. Acta Biomater. 4 (2008) 468.
[3] C.N.Kraft. Arthroplast. Microcirc. (2005) 31.

Figure 1