Catalytic reactions at semiconductor surfaces play a critical role in advancing technologies for energy conversion, environmental remediation, and chemical synthesis. Two prominent classes of such reactions are photocatalysis and electrocatalysis, where semiconductor surfaces facilitate redox processes under light illumination or applied voltage, respectively. The efficiency of these reactions depends on the nature of active sites, surface modifications, and the interplay between charge carriers and adsorbates. In-situ characterization techniques, such as X-ray absorption spectroscopy (XAS) and atomic force microscopy (AFM), provide insights into dynamic surface processes.
Active sites on semiconductor surfaces, including steps, kinks, and vacancies, serve as preferential locations for catalytic reactions due to their altered electronic and geometric properties. For instance, oxygen vacancies on titanium dioxide (TiO₂) surfaces enhance photocatalytic activity by introducing defect states within the bandgap, which act as electron traps and reduce charge recombination. Similarly, step edges on transition metal dichalcogenides (TMDCs) like MoS₂ exhibit higher electrocatalytic activity for hydrogen evolution due to their metallic character and favorable hydrogen adsorption energy. The density and distribution of these active sites can be controlled through synthesis conditions, such as annealing in reducing or oxidizing atmospheres.
Surface modifications further optimize catalytic performance by introducing co-catalysts or dopants. Co-catalysts, such as platinum (Pt) or cobalt oxide (Co₃O₄) nanoparticles, are often deposited on semiconductor surfaces to lower activation barriers for specific reactions. In photocatalytic water splitting, Pt-loaded TiO₂ improves hydrogen evolution by acting as an electron sink, while Co₃O₄ facilitates oxygen evolution by providing hole transfer pathways. Doping with foreign elements, such as nitrogen (N) or sulfur (S), modifies the electronic structure of semiconductors, extending light absorption into the visible spectrum. For example, N-doped TiO₂ exhibits enhanced photocatalytic activity under visible light due to mid-gap states introduced by nitrogen incorporation.
Photocatalytic water splitting on TiO₂ is a well-studied model system. Under ultraviolet (UV) illumination, TiO₂ generates electron-hole pairs that drive water oxidation and reduction at the surface. The process involves hole-mediated oxidation of water to oxygen at surface titanium sites and electron-mediated reduction of protons to hydrogen at co-catalyst sites. The overall efficiency is limited by charge recombination, which can be mitigated through nanostructuring or heterojunction formation. Rutile-anatase heterojunctions, for instance, improve charge separation due to interfacial band alignment.
Electrocatalytic CO₂ reduction on semiconductor surfaces offers a pathway to sustainable fuel production. Copper (Cu)-based semiconductors, such as Cu₂O, selectively convert CO₂ to hydrocarbons like methane (CH₄) or ethylene (C₂H₄) at moderate overpotentials. The reaction mechanism involves multi-step proton-coupled electron transfers, with surface Cu⁰ and Cu⁺ sites stabilizing key intermediates like *CO and *CHO. Surface roughness and oxidation state influence product selectivity; for example, oxide-derived Cu surfaces favor C₂ products due to enhanced *CO dimerization.
In-situ characterization techniques reveal dynamic changes during catalysis. XAS probes the electronic structure and coordination environment of active sites under reaction conditions. For TiO₂ photocatalysis, operando XAS identifies Ti³⁺ species as transient intermediates during water oxidation. AFM, particularly in liquid environments, visualizes morphological changes, such as step retreat or island growth, during electrocatalysis. These techniques bridge the gap between idealized models and real-world catalytic systems.
Surface modifications also extend to molecular functionalization, where organic ligands or metal complexes are grafted onto semiconductor surfaces to tailor reactivity. For example, rhenium (Re) bipyridyl complexes on TiO₂ enhance CO₂ reduction by providing well-defined coordination sites for CO₂ activation. Similarly, organic monolayers on silicon (Si) surfaces passivate dangling bonds and introduce new catalytic functionalities.
Despite progress, challenges remain in achieving high selectivity and stability. In photocatalysis, competing side reactions, such as photocorrosion, degrade semiconductor surfaces over time. Protective coatings, such as thin Al₂O₃ layers, mitigate this issue while permitting charge transfer. In electrocatalysis, electrode fouling by reaction intermediates necessitates periodic regeneration or the use of pulsed potentials.
Future directions include the design of hybrid systems combining semiconductors with molecular catalysts for synergistic effects. Advances in operando microscopy and spectroscopy will further elucidate structure-activity relationships at the atomic scale. By understanding and engineering semiconductor surfaces, the next generation of catalytic materials will enable efficient and sustainable chemical transformations.