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 of the advantages, the current state-of-the art plasma doped materials presents low doping percentages. Conesquently 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 capable of intensively doping material lattices termed High Performance Plasma Doping (HPPD). 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 succesful HPPD. Thus, HAp disks with OHˉ lattice vacancies have been prepared and treated for short times with NTP. All HPPD samples have been oxygen doped successfully, displaying conductivity enhancement up to one order of magnitude. In addition, doping for the entire material bulk has been achieved, reaching a 50 % of doping replacement efficiency. The proposed mechanism, based on oxygen diffusion through the OHˉ HAp columns, has been 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 on the HPPD HAp is 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 ºC and 6 bar of CO2) is achieved, reporting a total yield of 537.85 ± 3.40 μmol/gc. Finally, the implications of HPPD and its extension towards other materials are both discussed and highlighted by performing a state-of-the-art comparison.