Rubbing Powders: Direct Spectroscopic Observation of Triboinduced Oxygen Radical Formation in MgO Nanocube Ensembles.

Thomas Schwab,John W C Dunlop,Keith Mckenna,Daniel Thomele,Korbinian Aicher,Oliver Diwald

doi:10.1021/acs.jpcc.1c05898

Thomas Schwab, John W C Dunlop + Show 4 more

Open Access

https://doi.org/10.1021/acs.jpcc.1c05898

Copy DOI

Abstract

Powder compaction-induced surface chemistry in metal oxide nanocrystal ensembles is important for very diverse fields such as triboelectrics, tribocatalysts, surface abrasion, and cold sintering of ceramics. Using a range of spectroscopic techniques, we show that MgO nanocube powder compaction with uniaxial pressures that can be achieved by gentle manual rubbing or pressing (p ≥ 5 MPa) excites energetic electron–hole pairs and generates oxygen radicals at interfacial defect structures. While the identification of paramagnetic O– radicals and their adsorption complexes with O2 point to the emergence of hole centers, triboemitted electrons become scavenged by molecular oxygen to convert into adsorbed superoxide anions O2– as measured by electron paramagnetic resonance (EPR). By means of complementary UV-photoexcitation experiments, we found that photon energies in the range between 3 and 6 eV produce essentially the same EPR spectroscopic fingerprints and optical absorption features. To provide insights into this effect, we performed density functional theory calculations to explore the energetics of charge separation involving the ionization of low-coordinated anions and surface-adsorbed O2– radicals at points of contact. For all selected configurations, charge transfer is not spontaneous but requires an additional driving force. We propose that a plausible mechanism for oxygen radical formation is the generation of significant surface potential differences at points of contact under loading as a result of the highly inhomogeneous elastic deformations coupled with the flexoelectric effect.

Highlights

Friction between metal oxide interfaces can have important implications for surface chemistry and occurs naturally in the course of handling and compaction of nanoparticle powders
This study is structured as follows: in the first part, we describe the experimental findings we obtained in the course of MgO nanocube powder compaction-induced charge separation
We find that the ozonide ions remain after pumping at room temperature, which serves as clear evidence for their encapsulation in the internal pores

Summary

Introduction

Friction between metal oxide interfaces can have important implications for surface chemistry and occurs naturally in the course of handling and compaction of nanoparticle powders. While there are a large number of triboinduced phenomena reported for metal oxides, such as tribocharging, triboelectricity, or triboluminescence,[1−6] there exists almost no mechanistic and chemical understanding of these processes at the atomistic scale.[1] A major challenge in characterizing contact phenomena in powders is the difficulty in precisely determining the contact area between the particles.[7] a variety of processes can occur between particles in contact and under powder compaction. A rational connection between triboinduced chemical activation steps including their energetics and the application of processing parameters, which induce the triboelectric phenomena, is very much needed

Methods

Results

Conclusion