Strain‐Induced Redistribution of Point Defects in ZnO Nanoparticles

Powder compaction results in local strains in ZnO nanoparticle ensembles, which alter the abundance of intrinsic defects that determine the electronic structure of the semiconductor. Defect formation energies obtained by density functional theory (DFT) calculations predict that oxygen vacancies (VO) and hydroxyl groups substituting oxygen lattice sites (OHO) represent the most abundant point defects in ZnO nanoparticles grown by gas-phase synthesis, where particle nucleation and growth occurs at 1000 K, in oxygen-poor and—due to hydrocarbons in the precursor solution—hydrogen-containing atmosphere. Experimentally, we show with vis–near-infrared diffuse reflectance and electron paramagnetic resonance spectroscopy that uniaxial ZnO nanoparticle powder compaction produces grain-size dependent and strain-induced depletion of lattice hydrogen (OHO). Related trends can be consistently explained by DFT via stress-induced destabilization of protonated oxygen, leading to hydrogen release. Moreover, we found that polyaromatic surface carbon covering ZnO grains as extrinsic defects serves as a strain absorber and compensates for the compaction-induced changes in the concentration of point defects. These findings provide key insights for the defect engineering of particulate ZnO nanostructures in physical contact and are of high relevance for the exploitation of their flexo- and piezoelectric activity.