Bird-nest structured ZnO/TiO2 as a direct Z-scheme photoanode with enhanced light harvesting and carriers kinetics for highly efficient and stable photoelectrochemical water splitting

Titanium dioxide (TiO2) is a promising photoanode material for photoelectrochemical (PEC) water splitting, but the poor light harvesting and sluggish carriers kinetics are the main bottleneck problems. Here, the open bird-nest structured ZnO/TiO2 (BN ZnO/TiO2) and sealed core-shell structured ZnO/TiO2 (CS ZnO/TiO2) were first fabricated by a facile seed-dependent hydrothermal approach, in which the distribution and density of ZnO seeds on the TiO2 substrate were accurately controlled by adjusting the spin-coating speed of ZnO seed solution. The BN ZnO/TiO2 showed the photocurrent densities of 1.05 mA cm−2 at 0.6 V vs. RHE (VRHE) and 2.75 mA cm−2 at 1.23 VRHE, which are 4.32 and 2.27 times higher than TiO2, and 1.98 and 1.38 times higher than CS ZnO/TiO2, respectively. The BN ZnO/TiO2 retained 96.2 % of initial photocurrent after 8 h of continuous irradiation, much better than CS ZnO/TiO2 (73.5 %). The greatly increased PEC performance for BN ZnO/TiO2 is ascribed to the synergistic effect of Z-scheme heterostructure and bird-nest structure. The Z-scheme band alignment for ZnO/TiO2 heterojunction, which is beneficial for suppressing the recombination of photogenerated carriers, was proposed based on different methods. Moreover, the bird-nest structure with high surface area efficiently enhances light harvesting by forming light trap and greatly promotes carriers kinetics processes due to larger interface contacting areas with electrolyte, more direct pathways for fast separation and transfer of carriers.