X-ray Photoelectron Spectroscopy (XPS)

XPS (X-ray Photoelectron Spectroscopy) is a high-sensitivity spectroscopic technique capable of analyzing the surface chemistry of a material at the nanometer scale. It is particularly used for qualitative and quantitative analysis of surface composition within a depth of 1–10 nm. XPS has become one of the fundamental characterization methods in nanotechnology materials science and surface engineering by providing critical information on elemental composition, vine species oxidation states and surface functionalization like.

XPS operates based on Einstein’s photoelectric law. Monochromatic X-rays directed at the surface excite and eject electrons from inner atomic orbitals. The kinetic energy (Eₖ) of these ejected electrons is measured and the binding energy (E_b) within the atom is calculated using the following equation:

Eb = h⋅ν − E_k − ϕ

Where:

• h⋅ν → energy of the X-ray photon
• ϕ → spectrometer work function
• E_k → measured kinetic energy of the electron

The binding energy of each element is characteristic. Additionally, the chemical environment (chemical environment) causes slight shifts in these energies. Therefore, different chemical forms of the same element can be distinguished (for example, C–C, C=O, COOH carbon types).

Through the peak position, peak area, and peak shape in an XPS spectrum, detailed information is obtained about the element’s:

• Type
• Surface concentration (% atomic)
• Chemical bonding type (oxide hydroxide carboxyl etc.)

XPS provides detailed information on the oxidation state bonding structure and surface functional groups present on nanoscale materials.

Examples:

• Detection of –OH and –COOH groups on the surface of graphene oxide
• Analysis of nitrogen doping on carbon nanotubes
• Identification of oxygen vacancies in ZnO nanoparticles

With XPS the following properties of coatings can be determined in fine detail:

• Chemical integrity
• Layer thickness (via depth profiling)
• Substrate-coating interface interactions

Applications:

• Investigation of biocompatible TiO₂ coatings on medical implants
• Detection of oxide layers on semiconductors
• Assessment of interfacial quality in energy devices (e.g. solar cells)

XPS is used to track the presence of chemical bonds and functional group changes between polymer and matrix nanoparticles.

Examples:

• Verification of Si groups bonded to the surface after silanization
• Analysis of the GO → rGO transformation after reduction
• Control of surface bonding in functionalizations involving metal nanoparticles

• Provides detailed surface-specific chemical information (1–10 nm)
• Distinguishes elemental composition and chemical bonding types
• Enables non-destructive and quantitative analysis
• Allows 3D analysis through depth profiling and mapping

• Requires vacuum conditions (limited for biological samples)
• Surface roughness can affect signal quality
• Cannot analyze light elements such as Li
• Limited depth resolution; only surface analysis is possible