Preparation of Highly Crystalline TiO2 Nanostructures by Acid-assisted Hydrothermal Treatment of Hexagonal-structured Nanocrystalline Titania/Cetyltrimethyammonium Bromide Nanoskeleton (1009.1283v2)

Abstract: Highly crystalline TiO2 nanostructures were prepared through a facile inorganic acid-assisted hydrothermal treatment of hexagonal-structured assemblies of nanocrystalline titiania templated by cetyltrimethylammonium bromide (Hex-ncTiO2/CTAB Nanoskeleton) as starting materials. All samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The influence of hydrochloric acid concentration on the morphology, crystalline and the formation of the nanostructures were investigated. We found that the morphology and crystalline phase strongly depended on the hydrochloric acid concentrations. More importantly, crystalline phase was closely related to the morphology of TiO2 nanostructure. Nanoparticles were polycrystalline anatase phase, and aligned nanorods were single crystalline rutile phase. Possible formation mechanisms of TiO2 nanostructures with various crystalline phases and morphologies were proposed.

• The paper demonstrates that HCl concentration critically dictates the crystalline phase, yielding pure anatase at low levels and mixed phases at higher concentrations.
• The paper employs hydrothermal synthesis combined with XRD and TEM, revealing a transition from polycrystalline nanoparticles to single crystalline nanorods.
• The paper highlights that optimizing synthesis parameters modulates crystallite sizes and band-gap shifts, enhancing applications in photocatalysis and solar energy conversion.

Synthesis of Highly Crystalline TiO<sub\>2</sub> Nanostructures via Acid-Assisted Hydrothermal Methodology

The research article by Shuxi Dai et al. focuses on the meticulous preparation of highly crystalline titanium dioxide (TiO<sub\>2</sub>) nanostructures through an acid-assisted hydrothermal treatment of hexagonal-structured nanocrystalline titania/cetyltrimethylammonium bromide (CTAB) nanoskeleton as starting materials. Noteworthy is the critical role of hydrochloric acid (HCl) concentration in determining the morphological and structural crystallinity of these nanoscale materials. This paper is pivotal in advancing the understanding of how precise control over synthesis parameters influences TiO<sub\>2</sub> nanostructure characteristics, permitting customized applications in photocatalysis, pollution control, and solar energy conversion.

Methodology and Results

The researchers employed a hydrothermal synthesis approach, which is typically favored for its advantageous moderate reaction conditions, including lower temperatures and reduced reaction times compared to traditional sol–gel methods. Using X-ray diffraction (XRD) and transmission electron microscopy (TEM), the paper thoroughly examined the crystalline phases and morphologies of the TiO<sub\>2</sub> nanostructures at varying concentrations of HCl.

Key findings include:

1. Phase and Morphology Dependency: It was evident that the crystalline phase—either anatase or rutile—was closely linked with the TiO<sub\>2</sub> nanostructure morphology. Specifically, nanoparticles yielded a polycrystalline anatase phase while aligned nanorods formed a single crystalline rutile phase.
2. HCl Influence on Crystalline Transition: Adjusting the HCl concentration significantly altered the crystalline outcomes. Increasing concentrations from 0.1 M to 1 M resulted in pure anatase phases, while concentrations between 2 M and 7 M led to mixed anatase-rutile structures, peaking at rutile formation at 5 M.
3. Optimization of Crystallite Sizes: With Scherrer's equation, the paper found that crystallite sizes varied from 13 nm to 19 nm depending on HCl molarities. Interestingly, pure anatase structures were particularly smaller around 14.5 nm, signifying a correlation between crystallinity and HCl concentration.
4. UV-Vis Spectroscopy Insights: The UV-visible absorption spectra analysis revealed band-gap variations consonant with phase changes. Nanostructures exhibited red-shifts and blue-shifts correlating with the anatase and rutile phase transitions as HCl concentration was modulated.

Proposed Mechanisms and Implications

The authors proposed two formation mechanisms influencing the crystal phase transitions: dissolution-recrystallization and in situ transformation. Under varying HCl concentrations, these mechanisms modulate the interaction and growth dynamics of TiO<sub\>2</sub> in hexagonal nanocrystalline titania frameworks.

In terms of implications, the controlled synthesis of TiO<sub\>2</sub> structures at finer granularity offers a significant edge in tailoring materials for enhanced performance in environmental and energy applications. The paper suggests that fine-tuning the hydrothermal synthesis parameters can significantly influence not only the materials' properties but their applicability in fields requiring high photoactivity or stability.

Future Prospects

Future research can explore the mechanistic exploration of TiO<sub\>2</sub> phase transitions under different solvent conditions, as well as the exploration of functional doping to further enhance the properties of synthesized TiO<sub\>2</sub> nanostructures. Developing alternative methods that minimize energy-intensive processes while maximizing purity and desired phase formation remains a lucrative challenge for the material science community working with semiconductor oxides. This research offers a vital scaffold for these prospective investigations and innovations.