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Rekha Bai1 Sujeet Chaudhary1 Dinesh K. Pandya1

1, Indian Institute of Technology Delhi, New Delhi, , India

Photoelectrochemical (PEC) water splitting using nanostructured semiconductor materials has been attracting great attention for production of clean and high efficiency renewable energy. Broad absorption region, effective charge transfer, and high photostability of semiconductor photoanode are the governing features for the improved PEC performance. Vertically aligned one dimensional (1-D) metal oxide semiconductor nanostructure arrays are superior photoelectrodes for high solar energy conversion efficiencies owing to large specific surface area, increased light absorption due to light scattering effect, short diffusion length, ideal geometrical structures for fast electron transport and less charge recombination probability. ZnO-NRs photoanode is emerging as the promising candidate owing to the direct bandgap, higher exciton binding energy, ease of crystallization, facile tailoring of nanostructures, anisotropic growth, and higher electron mobility. CdS is considered to be the most appropriate visible-light sensitizing absorber for ZnO, comprising of direct band gap (~2.4 eV), high absorption coefficient, and similar crystal structure/lattice match, which facilitates better charge transfer between ZnO/CdS nano-architecture (staircase type-II band alignment) required for efficient solar energy conversion applications. In this work, we have fabricated vertically aligned 1-D ZnO/CdS core/shell NRs array by ion-flux controlled two-step chemical bath deposition technique, exhibiting an extended optical absorption from ultraviolet to visible region and enhanced photoresponse. FESEM and HRTEM studies confirm the uniform topotaxial decoration by thin layer (~40 nm) of (002) oriented CdS QDs on the hexagonal prismatic (002) ZnO NRs. The enlarged interface between ZnO and CdS together in close intimacy facilitates effective charge transfer from the excited CdS shell to ZnO NRs core upon visible light illumination. Our optimized ZnO/CdS core/shell NR-array exhibits anodic visible photocurrent density as high as 8.5 mA/cm2 at +1.0 V (vs Ag/AgCl) and excellent photostability in 0.1 M Na2SO4 aqueous solution. The observed high current density in our core/shell NRs is interpreted in terms of the nucleation controlled growth via ion-by-ion deposition mechanism and subsequently type-II band aligned defect-free ZnO/CdS interface.

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