Unveiling Morphology–Structure\nInterplay on\nHydrothermal WO<sub>3</sub> Nanoplatelets for Photoelectrochemical\nSolar Water Splitting

Photoelectrochemical (PEC) water splitting offers a sustainable\nroute for hydrogen production, leveraging noncritical semiconductor\nmaterials. This study introduces a seed layer-free hydrothermal synthesis\napproach for semiconductor photoanodes based on tungsten trioxide\n(WO<sub>3</sub>) nanoplatelets. Aiming to boost the efficiency of\nphotoelectrochemical water splitting through optimization of the\nsynthesis parameters of bare WO<sub>3</sub>, focusing on temperature,\ntime, and layer thickness, we systematically explored their effects\non the morphological, structural, and optical characteristics of WO<sub>3</sub> photoanodes. Combining a low-temperature regime (90 °C\nfor 12 h) with a multilayer strategy (up to six-layers) resulted in\nsignificant improvements in photocurrent. Particularly, the five-layer\nsample exhibited a remarkable increase of over 70% compared to the\nsingle-layer photoanode. Morphological aspects, particularly the fractal\ndimension of nanoplatelets and the emergence of the (220) crystalline\norientation, usually neglected, were found to play pivotal roles in\nmodulating the PEC response. Rietveld refinement of X-ray diffraction\npatterns further underscored the importance of crystallographic facets,\nvolume unit cell expansion, and microstrain in influencing photocurrent\noutcomes. Furthermore, we adapted the Mott–Schottky equation\nto incorporate the fractal dimension reflecting the nanostructures’\nnature, usually set to a planar interface. Our findings highlight\nthe interchange between nanoplatelet morphology and structural parameters\nin determining the PEC efficiency of WO<sub>3</sub> photoanodes.