Use of potential determining ions to control energetics and photochemical charge transfer of a nanoscale water splitting photocatalyst

Abstract

Tetrabutylammonium (TBA) stabilized H[Ca₂Nb₃O₁₀] nanosheets catalyze hydrogen evolution from aqueous methanol under illumination with UV light. Here we show that surface treatment with protons, potassium, and strontium potential-determining cations (PDIs) in aqueous solution modifies the electrostatic, energetic and photocatalytic properties of this nanomaterial. Attachment of cations to the nanocrystals was verified with elemental dispersive spectroscopy. Zeta potentials were measured as −40 mV (TBA+, pH = 4.8), −50 mV (K⁺, pH = 4.3), and −20 mV (Sr²⁺, pH = 4.4). Photoelectrochemical measurements in methanol containing 0.1 M tetraethylammonium chloride revealed anodic current photoonset potentials/Fermi energies ranging between −0.59 V (Sr²⁺) and −0.71 V (at pH = 7, vs. NHE). The photocatalytic proton reduction ability of the modified nanocrystals was assessed in aqueous methanol at pH = 1. Here, K_xH_1−x[Ca₂Nb₃O₁₀] evolved hydrogen at 350 μmol H₂ h⁻¹, Sr_xH_1−2x[Ca₂Nb₃O₁₀] at 70 μmol H₂ h⁻¹, and H[Ca₂Nb₃O₁₀] at 160 μmol H₂ h⁻¹. In addition, the photocatalytic activity was found to increase (20–160 μmol H₂ h⁻¹) with solution pH. These observed activity variations can be quantitatively understood using a linear free energy relationship between the proton reduction rate constant and the free energy of proton reduction. This shows that the photocatalytic activity of the nanocrystals depends on the electrochemical potentials/Fermi energies of the modified catalysts. The effect of the PDI charge on the nanomaterial energetics can be rationalized by considering the surface potential. The latter can be related to the particle surface charge and the concentration of counterions in solution using the Grahame equation. These results provide a quantitative basis for the understanding and manipulation of nanomaterial photocatalysts with PDIs.