Plasma defect-engineering of bulk oxygen-deficient zirconia

Oxygen-deficient zirconia (ZrO2-x) has recently emerged as a promising material for light absorption and photocatalytic applications. However, the economic and environmentally friendly manufacture of bulk ZrO2-x remains challenging and has limited widespread adoption. In this study, we present a novel low-pressure (300 Pa) plasma treatment (H2 gas at 500°C for 5 hours) capable of producing fully-dense bulk ZrO2-x without significant structural modifications. EPR (electron paramagnetic resonance) and XPS (X-ray photoelectron spectroscopy) characterisation of the plasma treated zirconia indicate the formation of Zr3+ ions and F2+ (VO··) centres. The increase of oxygen vacancies is also supported by the greater exothermic heat flow and relative mass gain observed through TGA (thermogravimetric analysis) and DSC (differential scanning calorimetry) analyses. Diffuse reflectance spectroscopy (DRS) reveals a substantial enhancement in light absorption, with an average increase of 66.2% and >65% absolute absorption across the entire spectrum (200-3000 nm). XPS and DRS measurements suggest significant reduction in both direct (from 4.84 to 2.61 eV) and indirect (from 3.19 to 1.45 eV) bandgap transition. By effectively enhancing the light absorption capability, reducing bandgap transitions, and maintaining the structural integrity of zirconia, low-pressure plasma treatments offer a promising and scalable approach for the environmentally friendly production of next-generation ZrO2-x materials.