Oligomer cross-linking strategy enabling quasi-single crystal TiO2 thin films for enhanced photoelectrochemical performance: mechanism and process engineering

Grain boundaries and intrinsic defects in polycrystalline TiO 2 thin films prepared via traditional sol–gel methods severely compromise their carrier transport and optoelectronic efficiency. Addressing this bottleneck, this study proposes a biomimetic “oligomer cross-linking strategy” utilizing triethylamine (TEA) to effectively modulate the sol–gel transition process. Unlike the granular stacking observed in acetic acid-mediated systems, this process engineers a continuous, dense anatase quasi-single crystal (QSC) structure with high (001) orientation on glass substrates. This structural continuity pushes optical properties toward theoretical limits, yielding a refractive index of 2.4987 at 1000 nm and visible transmittance of 95.5%. Crucially, in photoelectrochemical (PEC) applications, the QSC film delivers a photocurrent density of 1.51 mA/cm 2 , doubling that of polycrystalline controls. Furthermore, the optimized film achieves a superior Applied Bias Photon-to-Current Efficiency (ABPE) of 0.87%. Kinetic analysis via Electrochemical Impedance Spectroscopy (EIS) reveals that the QSC structure reduces the interfacial charge transfer resistance ( R CT ) by approximately 50% (768.6 Ω vs. 1434.0 Ω). These results demonstrate that the elimination of grain boundary barriers significantly accelerates hole injection and carrier separation. This work offers a scalable process engineering paradigm for fabricating high-performance oxide films by bridging precursor molecular design with macroscopic film quality.