Why Binding Energy increases with the increasing Oxidation State - XPS analysis
Understanding Binding Energy and Oxidation States in XPS Analysis
The Concept of Binding Energy
- Binding energy is likened to a fingerprint, unique for each element, crucial for identifying elements in X-ray Photoelectron Spectroscopy (XPS).
- Each element, such as lead, copper, silver, or gold, has a distinct binding energy that helps in their identification through XPS spectra.
- The binding energy values are compared against a reference table containing data for all detectable elements except hydrogen and helium.
Oxidation State and Its Impact on Binding Energy
- Oxidation state refers to the loss or gain of electrons by an atom; positive oxidation states indicate electron loss while negative indicates gain.
- As oxidation states increase (e.g., titanium from 0 to +4), the binding energy also increases due to greater positive charge attracting remaining electrons more strongly.
Mechanism Behind Increased Binding Energy
- When titanium loses electrons and becomes positively charged (e.g., Ti²⁺), it exerts a stronger attractive force on its remaining electrons.
- Removing additional electrons (e.g., forming Ti⁴⁺) further increases the positive charge, making it increasingly difficult to remove remaining electrons due to higher binding energy requirements.
Observing Variations in Binding Energy
- High-resolution XPS spectra reveal variations in binding energies across different oxidation states of titanium through distinct peaks.
- For iron with two oxidation states (Fe²⁺ and Fe³⁺), changes in electronic configuration affect its binding energy significantly.
Case Study: Iron's Oxidation States
- Iron's electronic configuration shows that upon losing two electrons (to form Fe²⁺), it experiences increased binding energy due to enhanced nuclear attraction on remaining electrons.
- Further oxidation leads to even higher positive charges which result in progressively higher binding energies as more electrons are removed.