15.3 In-situ and operando surface characterization techniques

In-situ and operando techniques revolutionize surface science by allowing real-time observation of dynamic processes under realistic conditions. These methods bridge the gap between idealized lab experiments and real-world applications, providing crucial insights into surface phenomena as they happen.

From monitoring catalyst restructuring during reactions to studying electrode surfaces in batteries, these techniques offer a window into the complex world of surfaces in action. They combine structural, chemical, and functional information, enabling a deeper understanding of surface processes across various fields.

Importance of in-situ and operando characterization

Real-time observation of surface processes

• In-situ and operando surface characterization techniques allow for the real-time observation and analysis of surface processes as they occur under realistic conditions
• These techniques provide valuable insights into the dynamic nature of surface phenomena, such as adsorption, desorption, reaction kinetics, and surface restructuring
• In-situ measurements capture the transient states and intermediate species involved in surface reactions, which are crucial for understanding reaction mechanisms and kinetics
• Examples of surface processes that can be observed in real-time include:
  • Adsorption and desorption of molecules on surfaces (CO on metal surfaces)
  • Surface restructuring during catalytic reactions (reconstruction of metal nanoparticles)
  • Formation and evolution of surface species during chemical reactions (intermediates in heterogeneous catalysis)

Bridging the gap between laboratory and real-world conditions

• Operando techniques combine surface characterization with simultaneous measurement of catalytic activity or other performance metrics, enabling direct correlation between surface properties and functionality
• By studying surfaces under operating conditions, in-situ and operando techniques bridge the gap between idealized laboratory conditions and real-world applications, providing more relevant and actionable information
• Examples of operando studies include:
  • Monitoring surface structure and composition of catalysts during catalytic reactions (in-situ XPS of metal nanoparticles during CO oxidation)
  • Studying electrode surfaces in batteries or fuel cells during charge-discharge cycles (in-situ Raman spectroscopy of lithium-ion battery electrodes)
  • Investigating corrosion processes under realistic environmental conditions (in-situ AFM of metal surfaces in corrosive media)

In-situ vs Operando Techniques

Low Energy Electron Diffraction (LEED)

• LEED is a surface-sensitive technique that provides information about the long-range order and symmetry of surface structures
• LEED uses low-energy electrons (20-200 eV) to probe the surface, resulting in diffraction patterns that reflect the periodic arrangement of surface atoms
• In-situ LEED allows for the monitoring of surface structure changes during adsorption, desorption, or reaction processes
• Examples of in-situ LEED studies include:
  • Observing surface reconstruction of single-crystal surfaces upon adsorption of gases (Si(111) surface reconstruction upon H2 adsorption)
  • Monitoring the growth of thin films or nanostructures on surfaces (epitaxial growth of metal oxides on metal surfaces)

Scanning Tunneling Microscopy (STM)

• STM is a high-resolution imaging technique that provides atomically resolved images of surface topography and electronic structure
• STM relies on the quantum tunneling of electrons between a sharp tip and the surface, allowing for the mapping of surface features with sub-angstrom resolution
• In-situ STM enables the visualization of dynamic surface processes, such as adatom diffusion, island growth, and catalytic reactions, with atomic-scale precision
• Examples of in-situ STM studies include:
  • Observing the diffusion and aggregation of adatoms on surfaces (diffusion of metal adatoms on metal surfaces)
  • Monitoring the growth and dissolution of surface nanostructures (growth of metal clusters on oxide surfaces)
  • Imaging catalytic reactions at the atomic scale (dissociation of molecules on metal surfaces)

X-ray Photoelectron Spectroscopy (XPS)

• XPS is a surface-sensitive technique that provides information about the chemical composition and oxidation states of surface species
• XPS uses high-energy X-rays to excite core-level electrons from surface atoms, resulting in photoelectrons with characteristic binding energies
• In-situ XPS allows for the monitoring of surface chemical changes during reactions, such as the formation of intermediates or the evolution of oxidation states
• Examples of in-situ XPS studies include:
  • Monitoring the oxidation state changes of metal nanoparticles during catalytic reactions (reduction of metal oxides during CO oxidation)
  • Studying the formation and decomposition of surface species during chemical reactions (formation of surface carbonates during CO2 adsorption)
  • Investigating the surface composition of electrodes in batteries or fuel cells during operation (evolution of solid-electrolyte interphase in lithium-ion batteries)

Complementary nature of techniques

• While LEED and STM provide structural and topographic information, XPS focuses on the chemical aspects of surfaces, making these techniques complementary in understanding surface phenomena
• Combining multiple in-situ and operando techniques can provide a comprehensive picture of surface processes, linking structure, composition, and reactivity
• Examples of complementary studies include:
  • Combining in-situ LEED and XPS to study the structure and composition of surfaces during adsorption and reaction processes (adsorption of organic molecules on metal surfaces)
  • Using in-situ STM and XPS to investigate the relationship between surface morphology and chemical state during catalytic reactions (evolution of metal nanoparticles during CO oxidation)

Challenges of in-situ and operando characterization

Instrumentation and experimental setup

• One major challenge in in-situ and operando surface characterization is the need for specialized instrumentation that can operate under realistic reaction conditions, such as high temperatures, high pressures, or controlled atmospheres
• The presence of gas-phase molecules or liquid environments can interfere with the surface-sensitive techniques, requiring careful design of experimental setups to minimize signal attenuation or background noise
• Examples of instrumentation challenges include:
  • Designing high-temperature sample holders for in-situ LEED or STM studies (maintaining sample stability and minimizing thermal drift)
  • Developing high-pressure cells for in-situ XPS measurements (maintaining UHV conditions while introducing gases)
  • Integrating electrochemical cells with surface characterization techniques for operando studies (minimizing electrolyte interference and maintaining potential control)

Data interpretation and analysis

• The interpretation of in-situ and operando data can be complex, as the measured signals often represent a convolution of multiple surface processes occurring simultaneously
• The time resolution of in-situ and operando techniques may be limited by the acquisition time of the measurement, making it challenging to capture very fast or transient surface events
• Examples of data interpretation challenges include:
  • Deconvoluting overlapping peaks in in-situ XPS spectra to identify individual surface species (distinguishing between different oxidation states or chemical environments)
  • Resolving fast surface processes in in-situ STM images (capturing intermediate states or short-lived species)
  • Correlating surface structure and composition changes with catalytic activity or selectivity in operando studies (establishing structure-activity relationships)

Spatial resolution and surface sensitivity

• The spatial resolution of some in-situ techniques may be lower compared to their ex-situ counterparts, due to the constraints imposed by the reaction environment or the need for compromise between resolution and data acquisition speed
• The limited penetration depth of surface-sensitive techniques may not provide a complete picture of the bulk properties or the behavior of subsurface species, which can also play a role in surface processes
• Examples of spatial resolution and surface sensitivity limitations include:
  • Achieving atomic resolution in in-situ STM images under high-pressure conditions (maintaining tip stability and minimizing noise)
  • Probing subsurface species or bulk properties with surface-sensitive techniques like XPS (limited information depth of a few nanometers)
  • Resolving nanoscale heterogeneities or local variations in surface composition or reactivity (spatial averaging of surface properties)

Applications of in-situ and operando techniques

Heterogeneous catalysis

• In heterogeneous catalysis, in-situ and operando techniques can provide valuable insights into the structure-activity relationships of catalytic surfaces
• By monitoring surface structure, composition, and oxidation states during catalytic reactions, these techniques can help identify active sites, elucidate reaction mechanisms, and guide the rational design of improved catalysts
• In-situ studies can also reveal the role of surface defects, such as steps or vacancies, in catalytic activity and selectivity
• Examples of in-situ and operando studies in heterogeneous catalysis include:
  • Monitoring the surface restructuring of metal nanoparticles during CO oxidation using in-situ TEM (observing the formation of metal-oxide interfaces)
  • Studying the evolution of surface species during selective hydrogenation reactions using in-situ FTIR spectroscopy (identifying the role of surface hydrides in selectivity)
  • Investigating the influence of support materials on catalytic activity using operando XPS (probing the metal-support interactions during reactions)

Corrosion science

• In corrosion science, in-situ and operando techniques can shed light on the mechanisms of corrosion initiation and progression under realistic environmental conditions
• By monitoring surface changes during exposure to corrosive media, these techniques can help identify the critical factors that influence corrosion rates, such as surface composition, defects, or local pH variations
• In-situ studies can also provide insights into the formation and stability of protective passive layers, which are crucial for corrosion resistance
• Examples of in-situ and operando studies in corrosion science include:
  • Monitoring the early stages of corrosion initiation using in-situ AFM (observing the formation of localized corrosion sites)
  • Studying the composition and thickness of passive layers using in-situ XPS (probing the role of alloying elements in passivation)
  • Investigating the influence of environmental factors on corrosion rates using operando electrochemical techniques (measuring corrosion currents under different conditions)

Energy storage and conversion

• In energy storage applications, such as batteries and fuel cells, in-situ and operando techniques can elucidate the surface processes that govern charge transfer and energy conversion
• By monitoring surface structural and chemical changes during charge-discharge cycles or fuel cell operation, these techniques can help optimize electrode materials, electrolyte compositions, and cell designs for improved performance and durability
• In-situ studies can also provide insights into the degradation mechanisms of energy storage devices, such as surface passivation, electrode dissolution, or electrolyte decomposition, guiding the development of mitigation strategies
• Examples of in-situ and operando studies in energy storage and conversion include:
  • Monitoring the formation and evolution of solid-electrolyte interphase (SEI) layers in lithium-ion batteries using in-situ Raman spectroscopy (understanding the role of SEI in battery performance)
  • Studying the surface composition and oxidation states of fuel cell electrodes using operando XPS (identifying the active sites for oxygen reduction reaction)
  • Investigating the structural changes of electrode materials during cycling using in-situ XRD (probing the reversibility and stability of phase transformations)