Hierarchical Surface Patterns upon Evaporation of a ZnO Nanofluid Droplet: Effect of Particle Morphology

Abstract

Surface structures with tailored morphologies can be readily delivered by the evaporation-induced self-assembly process. It has been recently demonstrated that ZnO nanorods could undergo rapid chemical and morphological transformation into 3D complex structures of Zn(OH) nanofibers as a droplet of ZnO nanofluid dries on the substrate via a mechanism very different from that observed in the coffee ring effect. Here, we have investigated how the crystallinity and morphology of ZnO nanoparticles would affect the ultimate pattern formation. Three ZnO particles differing in size and shape were used, and their crystal structures were characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). Their dispersions were prepared by sonication in a mixture of isobutylamine and cyclohexane. Residual surface patterns were created by drop casting a droplet of the nanofluid on a silicon substrate. The residual surface patterns were analyzed by scanning electron microscopy (SEM) and microfocus grazing incidence X-ray diffraction (μGIXRD). Nanofluid droplets of the in-house synthesized ZnO nanoparticles resulted in residual surface patterns consisting of Zn(OH) nanofibers. However, when commercially acquired ZnO powders composed of crystals with various shapes and sizes were used as the starting material, Zn(OH) fibers were found covered by ZnO crystal residues that did not fully undergo the dissolution and recrystallization process during evaporation. The difference in the solubility of ZnO nanoparticles was linked to the difference in their crystallinity, as assessed using the Scherrer equation analysis of their XRD Bragg peaks. Our results show that the morphology of the ultimate residual pattern from evaporation of ZnO nanofluids can be controlled by varying the crystallinity of the starting ZnO nanoparticles which affects the nanoparticle dissolution process during evaporation.

Morphogenesis of polycrystalline dendritic patterns from evaporation of a reactive nanofluid sessile drop

“…The molecular and particulate species generated in situ upon evaporative drying collude with and modify the solvent flows, as they undergo self-assembly and self-organisation under conditions far from equilibrium. As we report in a different report, [26] the complex residual pattern from a reactive ZnO nanofluids also depends on the nanoparticle morphology and crystallinity. Our results demonstrate the important role of the interplay between the in situ generated clusters from reactive ZnO nanoparticles and solvent flows in forming the hierarchical polycrystalline residual patterns.…”

Dendritic surface patterns from Bénard‐Marangoni instabilities upon evaporation of a reactive ZnO nanofluid droplet: A fractal dimension analysis

“…The EISA process based on reactive ZnO nano/microfluids can result in a plethora of hierarchical surface patterns composed of fibres or dendrites [3,[5][6]. For instance, the dendritic patterns triggered by Bénard-Marangoni (BM) instabilities [7][8] were analogous to the foliage of red algae, Spanish dagger, or spider plant [3].…”

“…Here, the analysis was performed on the residual patterns from evaporation of sessile drops containing three types of ZnO particles differing in size and shape: the in-house synthesised ZnO nanoparticles (ca 9 nm in size), and commercially acquired ZnO nanopowder (Sigma-Aldrich, <100 nm particle size, ~80% Zn basis), (ca 36-142 nm) and ZnO powder (Sigma-Aldrich, ACS reagent, ≥99.0% (KT)), (ca 61-292 nm) dispersed in a mixture of cyclohexane (Fisher Chemicals, 99%) and isobutylamine (Sigma-Aldrich, 99%) [6]. (See section SI.01 in the Supporting Information (SI) for transmission electron microscopy (TEM) analysis of the particles and their size distribution, and SI.02 for ZnO nano/microfluid preparation.)…”

Bénard–Marangoni Dendrites upon Evaporation of a Reactive ZnO Nanofluid Droplet: Effect of Substrate Chemistry

“…Their surface chemistry was assessed with the static contact angle (CA) measurement and their roughness by atomic force microscopy (AFM). In Page 5 | 33 addition, three types of ZnO particles with different sizes and crystallinities (26) were dispersed in a mixture of cyclohexane and isobutylamine, which were used to form the evaporating sessile drops. The resulted hierarchical surface patterns were analyzed by scanning electron microscopy (SEM).…”