Theoretical study of the electronic structure of Zr(OH)4 and the effects of impurities and defects

Abstract

Zirconium hydroxide (Zr(OH)4) is emerging as an important material in the remediation of toxic substances. The electronic structure of Zr(OH)4, which is a major factor in photochemical applications, has been studied using ab-initio density functional theory with a particular focus on the band gap and on impurity- or defect-induced states in the gap. The band gap (Eg) of Zr(OH)4, which has proven difficult to measure experimentally, is computed to be ∼5.8 eV, which may overestimate the true value by as much as 0.3 eV. The gap is essentially independent of the degree of disorder, the number of layers in the periodic-slab model and structural changes occurring during outgassing at ≤500 K. Fine structure in the O 2s density of states is found to depend on whether the O is bonded to H. Carbonates and adsorbed H2O, the most common contaminants in Zr(OH)4, do not yield states in the gap. However, bridging sulfites formed by adsorption of SO2 are a typical low-level contaminant, and these yield states at 0.44 or 0.86 eV above the valence band maximum, depending on the nature of the sulfite bonding. Chlorine, another low-level contaminant, adsorbs preferentially on Zr(OH)4 surfaces vs. in the bulk but does not produce states in the gap. Two classes of defects have been considered. One is formed by removing an intact Zr(OH)4 unit, which leaves a “hole” in the lattice but does not yield any states in the gap. The other consists of paramagnetic Zr, ZrO or ZrOZr defects (where “” is an unpaired electron) formed by removing an OH radical or an H atom. These produce either occupied (Zr) or empty (ZrO or ZrOZr) states well into the gap and are potentially capable of explaining experimentally-observed photoluminescence and optical absorption at energies well below the predicted Eg.