Hollow Flower-Sphere TiO2 Nanoreactors: Enabling Ultrahigh-Loading and Speciation-Controlled Cu Sites for Solar H2 Evolution
Overcoming the intrinsic loading ceiling of oxide-supported single-atom catalysts remains a long-standing challenge, because oxide frameworks generally provide limited capacity for accommodating high densities of isolated metal species. Here, we report a hollow TiO2 nanoreactor that effectively addresses the long-standing loading limitation of oxide-supported catalysts by coupling high-capacity ion exchange with structural confinement. The multiscale framework is derived from a sodium titanate hollow flower-sphere assembled from ultrathin nanosheets. It enables broad accessibility of exchange sites and facilitates high Cu uptake prior to oxide formation. Subsequently, during Ar-assisted transformation into oxygen-vacancy-rich TiO2, the incorporated Cu species remain highly dispersed within the framework, while vacancy-mediated metal–support interactions further enhance their stability. As a result, controllable Cu speciation is achieved at ultrahigh loadings of 7.4 wt% as spatially isolated single atoms and 12.4 wt% as single-atom/subnanometer-cluster hybrids. The optimized hybrid catalyst delivers a hydrogen evolution rate of 28.8 mmol g−1 h−1 under simulated sunlight, surpassing conventional low-loading Cu/TiO2 systems under comparable conditions. This strategy is readily extendable to other transition metals (Fe, Co, and Ni), establishing a structural design principle for constructing high-density and speciation-controlled metal sites on oxide supports.