Interdigitated MgO/Au front selective contacts for enhanced photocatalytic systems based on TiO2 as light-absorber

Abstract

Selective charge extraction is a central limitation in heterogeneous photocatalysis, where short carrier diffusion lengths and interfacial recombination strongly restrict efficiency. Here we translate the photovoltaic concept of selective contacts to photocatalytic systems by designing interdigitated front architectures on a planar TiO₂ absorber. Magnesium-magnesium oxide (Mg-MgO_x) is used as an electron-selective layer, while gold nanoparticles act simultaneously as holeselective contacts, plasmonic enhancers, and charge-transfer mediators toward gas-phase reactants. Interdigitated Mg-MgO_x/Au patterns with controlled coverage and perimeter were fabricated on sol-gel anatase TiO₂ films and tested for hydrogen production from ethanol-water vapor mixtures under UV illumination. While Au nanoparticles alone strongly enhance activity through efficient hole extraction and plasmonic effects, the addition of Mg-MgO_x enables selective electron collection. However, performance is not governed by Mg-MgO_x coverage alone. Samples with similar Mg-MgO_x area fractions but different geometries show markedly different hydrogen production rates. To rationalize this behavior, we introduce a dimensionless geometric parameter η, defined as the normalized ratio between selective-contact perimeter and coverage. Hydrogen production follows a Langmuir-Hinshelwood-type kinetic model when expressed as a function of η, demonstrating that interfacial length, rather than area alone, controls the number of photogenerated carriers that reach adsorbed reactants before recombining. Highly interdigitated samples show up to a twofold increase in hydrogen production compared to non-patterned Au/TiO₂ references, despite having comparable material composition.