Enhanced visible photoluminescent and structural properties of ZnO/KIT-6 nanoporous materials for white light emitting diode (w-LED) application
Graphical abstract
Introduction
It is well established that semiconductors exhibit the unusual electrical and optical properties compared to bulk materials when the particle size decreases to nanometer scale [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Porous materials have exhibited a wide variety of applications in bioengineering, catalysis, environmental engineering and sensor systems owing to their high surface to-volume ratio [2]. A wide variety of techniques such as pulsed laser deposition, sputtering, thermal evaporation and condensation, solid state reaction, and chemical method have been employed to fabricate such nanostructures. Among these techniques, the template-assisted synthesis, which involves confined growth of nanostructures because of volume space effect, provides a simple, low-cost, and high-yield synthetic route for a large variety of materials. Ordered mesoporous silica KIT-6 is one prominent example and was used to construct nanostructures because of its uniform pore size, large surface areas, and high thermal stability [3]. Therefore, convenient to stabilize highly dispersed ultrafine metal or oxide nanocrystals, nanowires, quantum dots, and clusters in the channels of KIT-6. Typically, such methods employ mesoporous silica as the hard templates, into which a solution-based precursor of the desired metal or metal-oxide is introduced, followed by heating or hydrogenation to form the desired metal oxide or metal and silica composites [4]. The silica template is dissolved away to leave a replica mesoporous structure of the target compound. The hard templating route has opened new way to great varieties of mesoporous transition metal-oxides.
ZnO is an n-type direct band-gap semiconductor (3.37 eV) with an exciton-binding energy of 60 meV [5], [19], [20], [21], [22], [23], [24] with very important optical properties which can be used in the fields such as short wavelength lasers, blue light emitting diodes, UV detectors, gas sensors etc [1], [9], [11], [17]. A recent and growing interest has been devoted to assemblies made of semiconductors incorporated in mesoporous materials such systems are often called nanocomposites. A variety of synthesis techniques such as sol–gel [6], impregnation [7], [8] and molecular capping [9] have been used for the dispersion of ZnO particles in inorganic [10], [11], [12], [19], [22] matrices. However, the aggregation of ZnO particles is unavoidable. The discovery of highly ordered mesoporous materials [13] makes the control of nanoparticles size possible since the nanocrystallites are well confined into the pore system whose size is tuneable by different methods. This should lead to potential applications in optoelectronics such as white light emitting diode (w-LED), since the size-dependent optical properties are expected as a result of quantum size effects.
In present work, we made the successful preparation of ordered mesoporous of ZnO supported in mesoporous silica (or ZnO/KIT-6) via the wetness impregnation synthetic route. The obtained materials have been thoroughly characterized by various techniques such as XRD, N2-adsorption, high-resolution (HR-SEM), EDS, elemental mapping, and HRTEM. The XRD, N2-adsorption, and HRTEM analysis showed that the structural order of the materials is retained even after the incorporation of Zn in the silica framework of KIT-6. The PL studies were used nanocomposite properties of the ZnO/KIT-6. The results green emissions show a significant enhancement due to quantum size-effect compared to the emission of the bulk counterpart reported in the earlier studies.
Section snippets
Experimental section
KIT-6 material were prepared by Ref. [14] using a triblock copolymer (P123, Sigma–Aldrich) poly (ethylene oxide)–poly (propylene oxide)–poly (ethylene oxide) as structure directing agent. In a typical synthesis, 6 g P123 was dissolved in 217 g of distilled water and 11.8 g of concentrated HCl (35%), to this 6 g of 1-butanol was added under stirring at 35 8C. After 1-h stirring 12.9 g of TEOS was added drop-wise at 35 °C. The mixture is added under stirring for 24 h at 35 °C. The final solution
Results and discussion
Fig. 1 (A) shows the small-angle XRD patterns of the mesoporous ZnO synthesized by using mesoporous silica KIT-6-X with Ia3d structure and different pore sizes as hard templates. This synthetic approach is based on the absorption of hydrophilic zinc nitrate solution into hydrophilic channels of KIT-6 by capillary forces due to the interaction between the polar solvent and the hydrophilic part of the channels. ZnO forms inside the channels of KIT-6 during calcination. The obtained mesoporous ZnO
Conclusion
The wet impregnation method was employed to prepare the nanoporous of the ZnO supported in KIT-6. The prepared ZnO/KIT-6 with high loadings of ZnO keeps well ordered nanoporous structure of KIT-6. It is found that for Ia3d, the pore size determines the extent of the complete replication can be achieved, retaining the original symmetry of silica template. In this paper reports an interesting finding in the study of nanoparticle insertion in a mesoporous matrix, wherein the nanoparticles change
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