How does the Zn-precursor nature impact on carrier transfer into ZnO/Zn-TiO2 nanostructures? organic vs. inorganic anions

The formation of heterostructures is one of the most recognized alternatives to improve visible-light absorption and carrier mobility into metal oxide-based composite photoanodes. Despite this fact, it has not been deeply elucidated how precursor nature influences photoanodes preparation on the abovementioned performance behavior. Aiming to clarify the role of the synthesis precursors on the crystal growth during formation of metal oxide-based heterostructures, we studied the morphology and photoelectrochemical (PEC) performance of ZnO nanorods/TiO2 nanoparticles based thin film obtained by the sol-gel method. By using zinc nitrate (N) and zinc acetate (A) as precursors for ZnO nanorods preparation, ZnO-N/TiO2 (N-ZT) and ZnO-A/TiO2 (A-ZT) based n-n type heterojunctions were established, where the growth of ZnO nanorods depended on the used precursor anion. By using the inorganic precursor, large ZnO nanorods were formed containing a high Zn+ electron donor density. This allowed the carrier flow addressing into the N-ZT film, also supported by a high density of Ti3+ species. Conversely, small ZnO nanorods were produced with the use of the organic precursor, as result of Zn2+ dissolution during ZnO formation. This induced the decrease of Zn+ and Ti3+ content into the A-ZT film. Aditionally, Zn2+ was thermally diffused from ZnO to TiO2, promoting the Zn2+-doping of TiO2 and increasing the amount of interfacial and acceptor energy levels in both the N-ZT and A-ZT films. Nonetheless, the donor energy level content was higher in the former material, which facilitated charge carrier separation and transport. Therefore, an improved electron lifetime and a lowered recombination rate were achieved.