High-performance plasma doping as an advantageous vacancy engineering approach for the catalytic activation of materials: the case example of hydroxyapatite

Abstract

Aiming to transition towards sustainable design processes, plasma doping methods have been investigated as ultra-fast and solvent-free alternatives to chemical doping strategies. Despite their advantages, the current state-of-the-art plasma-doped materials present low doping percentages. Consequently, their acceptance as a replacement to conventional methods is still disfavoured. In this work, we propose a change in the paradigm by presenting a new approach termed high-performance plasma doping (HPPD) capable of intensively doping material lattices. To do so, HPPD exploits the higher number of available sites in vacancy-engineered materials for introducing dopants through non-thermal plasma (NTP) treatment. For this purpose, hydroxyapatite (HAp) is presented as a representative case example of successful HPPD. Thus, HAp disks with OH⁻ lattice vacancies are prepared and treated for short times with NTP. All the HPPD samples are oxygen-doped successfully, displaying conductivity enhancement of up to one order of magnitude. In addition, doping for the entire material bulk is achieved, reaching a doping replacement efficiency of 50%. The proposed mechanism, based on oxygen diffusion through the OH⁻ HAp columns, is corroborated through density functional theory (DFT) calculations. Results reveal the key role of lattice vacancies as charge imbalances, exercising an electronic pull on reactive gas species. Further assessment of the HPPD HAp is done through catalytic CO₂ conversion reactions. Thus, the synthesis of C1–C3 products (including ethanol and formic acid, among others) from CO₂ under mild conditions (150 °C and 6 bar of CO₂) is achieved, realizing a total yield of 537.85 ± 3.40 µmol g_c⁻¹. Finally, the implications of HPPD and its extension towards other materials are discussed and highlighted by performing a state-of-the-art comparison.