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In the present work, the activity of Ce and Er-doped ZrO 2  nanopowders for sun-driven photocatalytic water oxidation has been investigated. ZrO 2  powders with tunable amounts of tetragonal, monoclinic and cubic polymorphs have been synthesized by introducing Ce and Er (from 0.5 to 10 mol % on an oxide basis) through hydrothermal method. The aim of this work is to investigate the role of rare earth (RE) ions rich of electrons (Er 3+ ) and with entirely empty levels (Ce 4+ ) in the ZrO 2  matrix for the sun-driven photocatalytic water oxidation reaction. The samples have been characterized by means of UV-Vis spectroscopy, X-Ray diffraction (XRD), N 2  adsorption, X-ray photoelectron spectrophotometry (XPS) and transmission electronic microscopy (TEM) with energy dispersive spectroscopy (EDS). With respect to the bare ZrO 2  mainly containing monoclinic (m-) phase, an increasing amount of rare-earth (RE) dopant was found to improve the specific BET surface area and to stabilize the tetragonal (t-) or cubic (c-) polymorphs of ZrO 2  at room temperature. XRD data confirmed that dopants were mainly inserted in the t-ZrO 2  phase. The photocatalytic O 2  evolution from water under AM 1.5 G simulated sunlight illumination of the prepared samples have been correlated with their optical, structural and chemical properties. The effect of the dopant concentration on the chemical-physical and photocatalytic properties of the Er- and Ce-doped ZrO 2  materials was elucidated. The samples with 5% of RE oxide were the most active, i.e., three times more than pure zirconia. Their superior photocatalytic activity was found to be mainly correlated to two factors: (i) an optimal surface concentration of RE ions of about 3.7%, which increased charge carriers separation in the photocatalysts surface due more superficial defects of the t-ZrO 2  and a higher surface area, thus enhancing the reaction kinetics, (ii) a controlled amount of monoclinic vs. tetragonal (or cubic) polymorphs of zirconia with an optimum ratio of about 70/30 of t-ZrO 2 /m-ZrO 2 . Instead, the increased ability of the RE-doped ZrO 2  to harvest visible light was found to have a secondary role on the photocatalytic activity of the Ce-doped ZrO 2  material.

Insights Into the Sunlight-Driven Water Oxidation by Ce and Er-Doped ZrO2