Stabilization of ZnO quantum dots by preferred 1:2 interaction with a liquid crystal molecule
Graphical abstract
Introduction
The bulk phase of zinc oxide (ZnO) has a band gap of 3.37 eV, and its exciton binding energy is 60 mV at room temperature. This material absorbs light in the UV region (200–375 nm) and emits close to 520 nm as a result of structural defects such as zinc and oxygen vacancies, or due to surface impurities [[1], [2], [3]]. Bulk ZnO is an n-type semiconductor [4] with excellent mechanical and thermal stability that can have the crystalline structures of hexagonal wurtzite, zinc blende and rock salt, in which bonding is characterized by a tetrahedral geometry and sp3 hybridization [[5], [6], [7]].
Quantum dots (QDs) are semiconducting nanomaterials with diameters ranging from 2 to 10 nm and three-dimensional (3D) quantum confinement effects that lead to distinct optical and electronic properties [8]. ZnO QDs have received special attention due to their low toxicity and have found many applications in light emitting devices [9,10], electrochemical sensors [11,12], biological markers [13,14] and photocatalysis [15,16]. Visible fluorescence emission ranging from blue to yellow under ultraviolet (UV) light excitation can be obtained by controlling the nanoparticles' diameter [1] and their structural defects [3].
As a photocatalyst, ZnO QDs have already been used under excitation by natural and artificial UV and visible light in their pure form or combined with other materials for the degradation of various potentially toxic molecules [17,18]. Associating the photocatalytic activity of ZnO with QD nanostructures is lucrative, as decreasing semiconductor size increases surface area, allows abundant surface states in various morphologies and opens new possibilities for device design [19,20].
Molecular systems with complementary energy levels have been investigated in conjunction to enlarge the visible light absorption for photovoltaic applications, where the stability of the combined system, especially under light irradiation, is essential for long lifetimes of the devices. Liquid crystals (LCs) combine molecular order and fluidity, and they have been used for many technological applications, in displays [21], in organic electronic devices such as OLEDs [22], OFETs [23] and OSCs [24], and in nanocomposite systems with metallic nanoparticles [25], carbon nanotubes [26] and quantum dots [27] in order to improve the electro-optical properties of the combined systems [24,28,29]. Liquid crystalline perylene derivatives absorb and emit visible light, form long-range ordered π-aggregates in columnar superstructures and show high electron mobility with pronounced n-type semiconducting character [30]. We previously demonstrated that OLED performance was improved by mixing two perylene derivatives compared to the individual materials alone [31].
In the present work, ZnO QDs were synthesized in the presence of a LC perylene derivative to combine two n-type semiconductors and to increase absorption range, but with superimposed photoluminescence. The presence of the perylene did not affect the ZnO nanoparticle growth, and it proved to be efficient in the stabilization of the ZnO QD colloidal dispersions, despite the strong tendency of nanoparticles to aggregate in a bulk liquid crystal host [32]. The experimental determination of supramolecular interactions supplemented by theoretical modeling allowed the determination of the number of ZnO units in the ZnO nanoparticles. The prepared ethanolic dispersion demonstrated that ZnO was selective for photodegradation of methylene blue, with no photochemical degradation effect on the perylene derivative.
Section snippets
Materials
Commercial zinc acetate dihydrate (Zn(CH3COO)2·2H2O) ACS ˃ 99% (Sigma-Aldrich), sodium hydroxide (NaOH) ACS reagent ≥ 99.38% (Vetec), and ethanol P.A. ACS 99.5% (Sigma-Aldrich) was used as received. A 100 mL three-necked round bottom glass flask was used as a reactor. A hot plate with magnetic stirrer was used to control temperature and to homogenize the reaction.
The synthesis and characterization of the perylene based liquid crystalline material,
Results and discussion
Fig. 1 shows the absorbance spectra obtained over the temporal evolution of the colloidal dispersion of ZnO QDs in ethanol at room temperature, after the 2 h reaction was stopped. In a previous work, the experimental conditions using this synthetic procedure were determined using 2-propanol, where without stabilizer the ZnO nanoparticles grew up to the limit of the associated energy value of bulk ZnO (3.37 eV) with 120 min of reaction [34]. Here, using ethanol and without stabilizer, a slight
Conclusion
ZnO quantum dots were successfully synthesized in the presence of a perylene liquid crystal derivative, where the nanoparticle growth was not impeded by the organic molecules. This procedure improved the stability of the colloidal dispersion, which can be associated to the strong coupling between nanoparticles and molecules. Absorption titration experiments allowed the determination of a selective 2:1 stoichiometric ratio between liquid crystal molecules and ZnO nanoparticles. Theoretical
CRediT authorship contribution statement
Wallison C. Costa: Formal analysis, Investigation. Crislaine Sandri: Formal analysis. Samara de Quadros: Formal analysis, Investigation. Ana L.E.P. Silva: Investigation. Juliana Eccher: Investigation, Project administration. Lizandra M. Zimmermann: Supervision, Project administration, Writing - original draft. Jose R. Mora: Formal analysis. Harald Bock: Formal analysis, Writing - review & editing. Ivan H. Bechtold: Supervision, Project administration, Writing - original draft.
Declaration of competing interest
No conflict of interest.
Acknowledgments
The authors are grateful to CNPq, CAPES, FAPESC-ACAFE, INCT-INEO, H2020-MSCA-RISE-2017 (OCTA, #778158) and CAPES-COFECUB (# 803-14) for financial support, and to LCME-UFSC for TEM facilities.
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