Developing High-Performance Piezoelectric ZnS:Mn²⁺ Nanoparticles for Enhanced Mechanoluminescence and Piezocatalysis

Piezoelectric materials, featured by their non-centrosymmetric structures and unique charge separation properties under mechanical stimulation, hold significant promise for applications spanning pressure sensing, optogenetics, and piezocatalysis. However, existing materials often exhibit large particle sizes, posing challenges in specific applications like optogenetics, where nanoscale dimensions are required for barrier-free circulation through blood vessels. Furthermore, reducing particle size to the nanoscale offers increased surface area and enhanced reactant adsorption, a pivotal factor in catalytic performance. Thus, developing a straightforward method to produce highly piezoelectric nanomaterials is of great significance.<br/>Herein, we introduce a novel approach for synthesizing ZnS:Mn²⁺ nanoparticles, employing the self-assembly of ZnS:Mn²⁺ quantum dots, followed by high-temperature calcination. During calcination, the ZnS:Mn²⁺ quantum dot assemblies transform into individual nanoparticles, accompanied by a phase transition from sphalerite to wurtzite structure. This sintering process is spatially confined within a silica layer, preventing nanoparticle interconnection. Subsequent etching of the silica layer using NaOH results in ZnS:Mn²⁺ nanoparticles with an average size of approximately 150 nm. These ZnS:Mn²⁺ nanoparticles exhibit exceptional piezoelectricity, featuring a piezoelectric coefficient (d₃₃) of up to 25 pm/V as measured via Piezoresponse Force Microscopy (PFM). Notably, this d₃₃ value is around eight times higher than the literature-reported d₃₃ value of ZnS (3.2 pm/V). It is worth mentioning that the d₃₃ value exhibits variations dependent on calcination temperature and duration. Furthermore, the ZnS:Mn²⁺ nanoparticles display robust mechanoluminescence, both when dispersed in a PDMS film and an aqueous solution under focused ultrasound excitation. This mechanoluminescent emission from the nanoparticles has potential applications in controlling neural stem cell activities..<br/>Moreover, the piezoelectric ZnS:Mn²⁺ nanoparticles demonstrate excellent performances in piezocatalytic dye degradation, effectively decomposing organic dyes like rhodamine B and methylene blue. The degradation process is attributed to the generation of reactive oxygen species from the combination of charge carriers and OH^-/O₂ in the solution, a mechanism supported by Electron Spin Resonance (ESR) measurements. The introduction of h⁺, hydroxyl radical, and superoxide radical scavengers into the reaction system significantly reduces the degradation rate, further substantiating the catalytic mechanism. Compared to undoped ZnS nanoparticles synthesized in a similar manner, ZnS:Mn²⁺ nanoparticles exhibit superior piezocatalytic performance due to defect-induced polarization. Intriguingly, both mechanoluminescence and piezocatalysis are contingent on the non-centrosymmetric wurtzite structure of ZnS and its associated defect-induced polarization, each showcasing different optimal dopant concentrations and operating under distinct ultrasound frequencies.