Infrared spectroscopy is a powerful tool for analyzing surfaces, revealing the chemical makeup and bonding of molecules on materials. It works by measuring how infrared light interacts with surface species, giving us clues about their structure and behavior.

This technique fits into the broader study of vibrational spectroscopy, which looks at how molecules vibrate when hit with energy. By understanding these vibrations, we can learn a lot about what's happening on surfaces at the molecular level.

Infrared Spectroscopy for Surface Analysis

Principles and Applications

• Infrared spectroscopy is based on the absorption of infrared radiation by molecules, causing vibrations of chemical bonds
• The frequency of the absorbed radiation depends on the mass of the atoms, the strength of the bonds, and the geometry of the molecule
• Surface species can be identified by their characteristic infrared absorption bands, which are sensitive to the chemical environment and orientation of the molecules on the surface (CO adsorption on metal surfaces)
• Infrared spectroscopy provides information about the chemical composition, structure, and bonding of surface species, as well as their interaction with the substrate (hydrogen bonding, dipole-dipole interactions)
• The technique is non-destructive and can be applied to a wide range of materials, including metals (gold, platinum), semiconductors (silicon, gallium arsenide), and insulators (silica, alumina)

Surface Species Analysis with Infrared Spectroscopy

Types of Surface Species

• Adsorbed molecules, such as gases (CO, NO, H2), organic compounds (hydrocarbons, alcohols, acids), and inorganic species (water, ammonia)
• Chemical functional groups, such as hydroxyl (-OH), carbonyl (C=O), carboxyl (-COOH), amine (-NH2), and thiol (-SH) groups
• Surface-bound intermediates and reaction products, such as metal carbonyls (Fe(CO)5), carboxylates (formate, acetate), and nitrosyls (NO adsorbed on rhodium)
• Thin films and coatings, including self-assembled monolayers (alkanethiols on gold), polymer films (polyethylene, polystyrene), and oxide layers (titanium dioxide, zinc oxide)
• Surface defects and impurities, such as oxygen vacancies (reduced metal oxides), substitutional atoms (boron-doped silicon), and adsorbed contaminants (hydrocarbons, silicones)

Applications in Surface Science

• Investigating the adsorption and desorption kinetics of molecules on surfaces (CO adsorption on platinum)
• Studying surface reactions and catalytic processes (methanol synthesis on copper-based catalysts)
• Characterizing the structure and orientation of self-assembled monolayers (alkanethiols on gold)
• Monitoring the growth and properties of thin films (atomic layer deposition of oxide films)
• Identifying surface contaminants and evaluating cleaning procedures (hydrocarbon removal from silicon wafers)

Infrared Spectroscopy Instrumentation

Spectrometer Design

• Fourier-transform infrared (FTIR) spectrometers are commonly used, which employ an interferometer to generate an interferogram and a Fourier transform to convert it into a spectrum
• Transmission mode: The sample is placed between the infrared source and the detector, and the transmitted radiation is measured (suitable for thin, transparent samples)
• Reflection mode: The infrared beam is reflected from the sample surface, and the reflected radiation is collected by the detector (used for opaque samples)
  • External reflection: The beam is reflected from the sample surface at a grazing angle, probing the surface and near-surface region
  • Attenuated total reflection (ATR): The beam undergoes total internal reflection within a high-refractive-index crystal in contact with the sample, probing the surface and near-surface region

Advanced Techniques

• Polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS): A polarization modulator is used to alternately generate s- and p-polarized radiation, which interact differently with surface species, enhancing surface sensitivity
• In situ and operando studies can be performed using specialized sample cells that allow for temperature, pressure, and atmosphere control, enabling the investigation of surface processes under realistic conditions (high-pressure CO adsorption, temperature-programmed desorption)
• Combining infrared spectroscopy with other surface-sensitive techniques, such as sum-frequency generation (SFG) vibrational spectroscopy, provides complementary information on surface structure and orientation (water adsorption on mineral surfaces)

Interpreting Infrared Spectra of Surfaces

Band Assignment and Interpretation

• Identify absorption bands by their position (wavenumber) and assign them to specific chemical bonds and functional groups based on reference spectra and literature data (C-H stretching in hydrocarbons at ~2900 cm^-1)
• Determine the orientation of adsorbed molecules by comparing the relative intensities of absorption bands for different molecular vibrations (stretching vs. bending modes) and considering surface selection rules (upright vs. tilted orientation of alkanethiols on gold)
• Evaluate the strength of surface-adsorbate interactions by analyzing the shift in absorption band position relative to the gas-phase or liquid-phase spectra (red-shift for CO adsorption on metals indicates strong chemisorption)
• Quantify surface coverage and adsorbate density using the intensity of characteristic absorption bands and appropriate calibration methods (using reference compounds with known surface density)

Reaction Monitoring and Kinetics

• Monitor surface reactions and processes by tracking changes in the intensity, position, and shape of absorption bands as a function of time, temperature, or reactant exposure (CO oxidation on platinum, ethylene hydrogenation on palladium)
• Determine reaction rates, activation energies, and reaction orders by analyzing the time-dependent changes in band intensities and applying kinetic models (Langmuir-Hinshelwood, Eley-Rideal mechanisms)
• Identify reaction intermediates and elucidate reaction pathways by detecting transient species and monitoring their evolution during the reaction (formate intermediate in methanol synthesis on copper)

Data Integration and Interpretation

• Combine infrared spectroscopy data with other surface-sensitive techniques (X-ray photoelectron spectroscopy, scanning probe microscopy) to obtain a comprehensive understanding of surface composition and structure (adsorption of organic molecules on metal oxides)
• Correlate spectroscopic data with theoretical calculations (density functional theory) to validate proposed surface structures and bonding configurations (adsorption of small molecules on metal surfaces)
• Use infrared spectroscopy to guide the design and optimization of surface-based technologies, such as heterogeneous catalysts, gas sensors, and functional coatings (development of selective catalysts for environmental remediation)