|Nofrijon Sofyan||Department of Metallurgical and Materials Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Akhmad Herman Yuwono||Department of Metallurgical and Materials Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Aga Ridhova||Department of Metallurgical and Materials Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Joseph Wu||College of Engineering, Mathematics and Science, University of Wisconsin-Platteville, Wisconsin 53818, USA|
The characteristics of nano rosette TiO2 hydrothermally grown on a glass substrate at different reaction times and acid concentrations has been examined. The hydrothermal reaction was performed at 170°C for 3, 4, 5, and 6 hours whereas the crystallization was achieved through calcination at 450°C for 90 minutes. The growth mechanism was observed by employing the hydrothermal reaction under different acid concentrations: 0%, 12.5%, 25%, and 50% v/v HCl. The morphology, formation, crystallization, and growth mechanism of the nano rosette TiO2 were characterized using a field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD). The electron images showed that after 3 hours of hydrothermal reaction time, the nucleation process has just taken place; the formation of the nano rosette was completed after 6 hours. The results also showed that the acid environment plays a dominant role in determining the three-dimensional (3D) architecture of the nano rosette TiO2. Structural studies from XRD showed that different acid concentrations resulted in different crystalline formations. The nano rosette rutile TiO2 crystal structure was formed after 6 hours of hydrothermal reaction under 1:1 distilled water and HCl with a structure indexed to rutile P42/mnm with lattice parameters of a = 4.557(6) Å and c = 2.940(5) Å.
Growth mechanism; Hydrothermal; Nano rosette; Rutile; Titania
In the last several years, the self-assembly architecture of nanoscale building blocks has attracted the attention of many researchers for use in many nanostructured multifunctional materials. The nanoscale building blocks are usually in the forms of one-dimensional (1D), 2D, or 3D architectures of hierarchical nanostructures. The 3D architectures, especially 3D nanostructures assembled from 1D and 2D nanoscales to form building blocks, have become a hotspot and received special attention due to their unique properties and promising applications (Jia et al., 2017). These 3D nanostructures include nanowires (Zhu et al., 2018), nanorods (Govindaraj et al., 2017), nanosheets (Zhong et al., 2015), and nanoflowers (Ma et al., 2017).
One of the compounds that has attracted the attention of many researchers interested in nanostructured multifunctional application is titanium dioxide (TiO2). Titanium dioxide is a polymorph compound that has several crystal structures with unique properties. Depending on how it is formed, TiO2 may have brookite, anatase, and or rutile crystal structures (Banfield & Veblen, 1992). Due to these unique properties, many researchers have had numerous breakthroughs in the wide range of applications for TiO2. For example, TiO2 has been applied in the fields of sensors (Bai & Zhou, 2014), photocatalysts (Nakata & Fujishima, 2012), environmental remediation (Ochiai & Fujishima, 2012), photovoltaic cells (Hagfeldt & Grätzel, 1995), batteries (Longoni et al., 2017), and some other electrochemical devices (Yasin et al., 2016).
Among the several structures of TiO2, rutile is more thermodynamically stable than the other two forms of anatase and brookite. This is especially true because the two other crystal structures will transform to rutile when heated at 600–700°C (Hanaor & Sorrel, 2011). Nevertheless, each of these structures has its own application. For example, anatase is usually considered to be the most photoactive of the three polymorphs and thus it is used for the degradation of organic pollutants (Wang et al., 2015; Fisli et al., 2017) and dye-sensitized solar cells (Hagfeldt & Grätzel, 1995; Sofyan et al., 2017); brookite also has an application in the field of photocatalysis (Xie et al., 2009), whereas rutile is usually used the most in perovskite solar cells (Saif et al., 2012).
Recently, the use of the 3D nanostructures of TiO2, specifically the hierarchical flower-like TiO2 nanostructures, has increased due to their unique and excellent optical, electrical, and electronic properties (Bu et al., 2015). Many investigators have tried to find a facile way to synthesize the 3D flower-like nanostructures of TiO2 that are useful in many applications. For example, the 3D nano-flower hierarchical structures of TiO2 have been proven to enhance its photocatalytic properties (Zhou et al., 2013). Xiao et al. (2017) and Govindasamy et al. (2016) have reported that the use of a combination of compact TiO2 layers with the growth of TiO2 nanorods as an electron transporting layer has improved the performance of perovskite solar cells.
The characteristics of nano rosette TiO2 grown on a glass substrate via a hydrothermal reaction using different reaction times and acid concentrations have been examined. The morphological study from secondary electron images shows that after 3 hours of hydrothermal reaction time, the nucleation process has just taken place; the formation of the 3D hierarchical architecture of the nano rosettes is completed after 6 hours. The morphological and structural studies using different acid concentrations showed that the acid environment is a dominant factor in determining the 3D architecture of nano rosette TiO2. In pure water, there was no tendency to form 3D structures except for the anatase nanoparticle TiO2. In an acid environment, however, depending on the acid concentration, there is the driving force to form 3D structures. The formation of 3D architecture with Mimosa pudica petal-shaped rutile TiO2 occurred in a water and HCl ratio of 1:1, confirmed by the X-ray diffractograms indexed to rutile P42/mnm with lattice parameters of a = 4.557(6) Å and c = 2.940(5) Å. For the reaction pathway, the growth of TiO2 is determined by the reaction environment. In the absence of acid, there is more OH– and the growth of TiO2 occurs via an edge-sharing mechanism to form anatase crystal structures. In the presence of an acid environment, however, there is less OH– and thus the growth occurs via a corner-sharing mechanism to form rutile crystal structure. This reaction pathway also determines the growth mechanism to form the 3D hierarchical nano rosette architecture.
This work was funded by the Directorate of Research and Community Services (DRPM) Universitas Indonesia under the grant Hibah PITTA No. 2504/UN2.R3.1/HKP.05. 00/2018.
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