Abstract
In the present work, the activity of Ce and Er-doped ZrO2 nanopowders for sun-driven photocatalytic water oxidation has been investigated. ZrO2 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 (Er3+) and with entirely empty levels (Ce4+) in the ZrO2 matrix for the sun-driven photocatalytic water oxidation reaction. The samples have been characterized by means of UV-Vis spectroscopy, X-Ray diffraction (XRD), N2 adsorption, X-ray photoelectron spectrophotometry (XPS) and transmission electronic microscopy (TEM) with energy dispersive spectroscopy (EDS). With respect to the bare ZrO2 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 ZrO2 at room temperature. XRD data confirmed that dopants were mainly inserted in the t-ZrO2 phase. The photocatalytic O2 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 ZrO2 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-ZrO2 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-ZrO2/m-ZrO2. Instead, the increased ability of the RE-doped ZrO2 to harvest visible light was found to have a secondary role on the photocatalytic activity of the Ce-doped ZrO2 material.
Highlights
It is nowadays a scientific goal to develop clean energy technologies that satisfy the growing energetic demand and reduce the economic and environmental impacts of the use of fossil fuels at a global scale
Ce- and Er- doped ZrO2 were successfully synthesized through hydrothermal method, with molar concentration up to 10% on a rare earth oxide (REO) basis
The structural and morphological analyses point out that the rare earth ions have been inserted in the matrix, while the optical analysis confirms that the doping has increased the ability of zirconia of harvesting visible light
Summary
It is nowadays a scientific goal to develop clean energy technologies that satisfy the growing energetic demand and reduce the economic and environmental impacts of the use of fossil fuels at a global scale. Several efforts have been done in the research community to enhance the performance of photocatalytic systems that are able to exploit solar-light irradiation for industrial applications such as pollutant abatement (Suib, 2013) and solar fuels production (Maeda and Domen, 2010). The latter process can be achieved by photocatalytic water splitting with the production of O2 and H2, or other C-containing fuels if the reducing power is used to directly reduce CO2. Semiconductors with a narrow band gap, compatible with visible light absorption, often may not generate sufficient potential for oxidation and reduction reactions
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