doping of zno nanorod scaffold for photoelectrochemical water splitting: comparison of seed layer and nanorod doping

Photoelectrochemical (pec) hydrogen fuel production through water splitting is deemed to be a sustainable and green approach to alleviate environmental and energy crisis compared to other technologies. in this method, photoelectrodes with high surface area, large light absorption coefficient, and significant charge mobility are greatly desirable for efficient hydrogen evolution. furthermore, in order to address the charge recombination issue in semiconductors, formation of a suitable heterojunction with high charge separation efficiency is considered crucial in order to reach higher efficiency values. among numerous options investigated as the scaffold of such a heterojunction, 1d zno nanorods have represented great promises as they create a favorable path for charge transfer and hinder the agglomeration of the second material. in addition, high aspect ratio (length/diameter) of this structure contributes to light trapping and catalysis of the water splitting reactions. in this study, a two-step process was implemented to prepare zno nanorods. in the first step, a seed layer was deposited on fluorine-doped tin oxide substrate via dip coating and then the rods were grown through a chemical bath deposition. in order to improve the conductivity of the nanorods and create more space for deposition of the second material, doping of both the seed layer and the nanorods with 1 wt. % aluminum was investigated using al(no3)3 as the al source in both cases. four samples namely undoped seed-undoped rod, doped seed-undoped rod, undoped seed-doped rod, and doped seed-doped rod were prepared. structural, morphological, and optical properties of the obtained samples were characterized utilizing x-ray diffraction (xrd), scanning electron microscopy (sem), and diffused reflective (drs) spectroscopy methods, respectively. in line with the xrd results, all samples exhibited wurtzite structure with (002) diffraction peak at 2θ = 34.5° being the most dominant, which was associated with their preferential growth along the c-axis. consequently, more intense (002) peaks of the undoped seed-doped rod and doped seed-doped rod samples arise from their higher aspect ratio. moreover, sem images revealed that doping of both the seed layer and the nanorods remarkably reduce their final diameter (from ~78 nm for undoped seed-undoped rod to ~55 nm for doped seed-doped rod), while increasing their length for samples with doped rods, which is totally consistent with the xrd results. however, the undoped seed-doped rod sample has larger diameter in comparison with the undoped seed-undoped rod sample, which is due to higher potential of the doped bath at the beginning of the growth process. in terms of doped seed, the higher energy barrier for the diffusion of zn2+ ions in the zno structure may be the reason behind the diameter reduction. however, in the case of doped rod, al(oh)4- species created in the bath act as caping agents and hamper radial growth, while improving longitudinal growth. accordingly, doping in both cases results in zno nanorods with smaller diameters and doping only the chemical bath increases their final lengths. on top of that, drs results illustrate that doping decreases the samples bandgap. in total, as the doped seed-doped rod sample has the highest aspect ratio and also the lowest bandgap, it can be a better scaffold for fabrication of the photoelectrodes for pec water splitting process.