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

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.

Control of Membrane Fouling in Organics Filtration Using Ce-Doped Zirconia and Visible Light

“…In fact, after reaching adsorption equilibrium, any further removal of HA under irradiation was not observed. This result corroborates the ones obtained by DRS and previous electron paramagnetic resonance (EPR) characterization [27,33,34,42] since pure zirconia presents a wide band gap value of 5.2 eV, and visible-light irradiation is not able to promote the charge separation. Thus, without photo-induced electrons and hole, neither the direct oxidation of HA nor the generation of radicals capable of degrading HA are possible.…”

“…In agreement with previously reported characterization procedures [27,33,34,41,42], the structure of the resulting phases, obtained via both syntheses, is a mixture of monoclinic and tetragonal ZrO 2 (Figure S1). In both pure and doped oxides, the monoclinic phase presents some degree of anisotropy, as suggested by the variable width of the diffraction peaks [34,42]. None of the samples shows the presence of diffraction peaks related to the formation of CeO 2 phases, indicating that cerium was successfully inserted in the ZrO 2 matrix.…”

“…However, this mechanism does not seem probable for the humic acid photodegradation under visible light, considering that HA solutions do not have higher absorption in the visible region. As discussed in previous works [27,33,34,42], EPR studies indicated the generation of photo-induced holes and electrons, and the formation of • OH radicals when Ce-doped ZrO 2 was irradiated. Hence, to elucidate HA degradation mechanism with doped zirconia, a series of experiments were performed with disodium ethylenediaminetetraacetate (Na 2 -EDTA) (pointed as scavenger for holes [76] and a chelating agent that prevents HA adsorption on the oxide surface [73,77]) and isopropanol (commonly used as scavenger of • OH radical [77,78]).…”

Nanostructured Equimolar Ceria-Praseodymia for Total Oxidations in Low-O2 Conditions

“…Five "u" and five "v" peaks were identified, related to 3d 5/2 and 3d 3/2 states, respectively. The v • , v', u • and u' peaks were ascribed to Ce 3+ ions, while the other six peaks were assigned to the 4+ oxidation state [33,34]. A similar deconvolution was performed on the Pr 3d XP spectrum, in the 968-923 eV region (Figure 5b), ascribing four peaks (herein denoted as a • , a', b • and b') to Pr 3+ ions and other six peaks to the 4+ oxidation state [35].…”

The influence of chemical prehistory on the structure, photoluminescent properties, surface and biological characteristics of Zr0.98Eu0.02O1.99 nanophosphors

“…Nanocrystalline zirconia is the most suitable host matrix for the creation of high-performance solid-state light-emitting devices based on Ln 3+ ions due to its high refractive index, the band gap in the range from 4 to 6 eV, transparency in the visible and infrared spectral regions, as well as low frequency of phonons (470 cm −1 ) [6,7]. To date, nanophosphors based on ZrO 2 have been used in LEDs [8], fuel cells [9], solar panels [10], gas sensors [11], and photocatalytic systems [12,13]. The photostability and high lifetime of Ln 3+ (the range of milliseconds) ion radiation in the ZrO 2 biologically-inert matrix made it possible to use such luminescent markers for medical purposes for detecting, visualizing, diagnosing and treating diseases [14][15][16][17].…”