Researchers develop new type of material in nanotube form

Structuring of layered double hydroxides by groups from Brazil and Belgium extends surface area and enhances the capacity to adsorb different elements and chemical compounds.

By Elton Alisson  |  Agência FAPESP – Layered double hydroxides (LDHs) are a new category of material that has attracted interest on the part of researchers in recent years owing to their flexibility and many applications as catalysts, sensors and drug carriers. LDHs are two-dimensional mixed metal solids built up from sheets of divalent and trivalent cations (positively charged ions that donate two and three electrons, respectively, when bonding to other atoms) interspersed with anions (negatively charged ions).

A group of researchers at the University of São Paulo’s Physics and Chemistry Institutes (IF- and IQ-USP) in Brazil, in collaboration with colleagues at the University of Leuven (KU Leuven) in Belgium, structured LDHs into nanotubes, sheets of the material rolled up to form cylinders with a diameter equivalent to a billionth of a meter.

They increased the surface area of the material by creating small cylindrical pores and hollows. Because the pores can house various elements and chemical structures, they can be used to confer the material with additional properties.

The study resulted from a project supported by FAPESP via a Young Investigator Grant, and an article describing it was included in a special issue of Chemical Communications, published by the Royal Society of Chemistry.

Entitled “2017 Emerging Investigators,” the special issue features research carried out by internationally recognized scientists who are in the early stages of their independent careers and are making outstanding contributions to their respective fields. One of them is Danilo Mustafa, a professor in IF-USP’s Materials Physics & Mechanics Department, the principal investigator for the project and the supervisor of PhD student Alysson Ferreira de Morais, the first author of the article.

“The invitation to publish the results of my research in this special issue represented international recognition for the work I’ve been doing during my scientific career, and especially for this study, on which I embarked in 2013 during a postdoctoral research internship at KU Leuven with a scholarship from FAPESP,” Mustafa told Agência FAPESP.

Luminescent materials

As part of his postdoctoral research, Mustafa partnered with colleagues in Belgium to develop nanometric LDHs doped with ions from rare earths. Their aim was to obtain a new luminescent material that could capture solar energy more efficiently for use in photochemical and photovoltaic systems.

Rare earths are a group of 17 minerals with magnetic and luminescent properties, used in such products as catalysts, high-efficiency light bulbs, smartphones and TV screens. The metals are also of enormous value to the defense and renewable-energy industries.

Mustafa and colleagues developed these materials by partially replacing the divalent and trivalent cations in the metallic layers with rare-earth ions and introducing a photosensitizer that was adsorbed (retained) between the layers. The purpose of the photosensitizer was to absorb solar energy and transfer it efficiently to the rare-earth ions.

Based on the results of this successful project, they decided to develop LDHs not in the traditional form, but rather as micrometric flakes in order to increase their surface area.

“Although LDHs are versatile and can accommodate molecules interspersed between the metal layers, they have a spatial limitation that rules out the inclusion of large molecules or complexes such as quantum dots,” Mustafa said. Quantum dots are nanoscale particles or crystals of semiconducting material with a wide array of properties and applications, such as embedding in light-emission technologies.

To surmount this space constraint, they thought of structuring the material in the form of nanotubes with hollow main cavities so that larger structures could fit inside the material.

Pursuing a new strategy that entailed the use of a polymer as a mold to shape the LDHs into nanotubes, followed by its removal at the end of the process, they succeeded in achieving their goal and obtained cylindrical nanotubes composed of a mixture of aluminum, zinc and europium.

“This was the first time LDHs had been structured in the form of self-supporting nanotubes,” Mustafa said.

To explore and enhance the material’s luminescent properties, the researchers placed the nanotubes in contact with quantum dots of cadmium telluride (CdTe), a crystalline compound of cadmium and tellurium with semiconducting and photovoltaic properties.

The results of their analysis showed that the nanotubes interacted with the quantum dots of CdTe to produce a new class of luminescent material.

“This unique LDH nanotube morphology and the possibility of interacting with different compounds extends the range of applications for LDHs, offering opportunities in catalysis, devices and active biological materials such as drug carriers,” Mustafa said.

The researchers now plan to investigate the mechanisms underlying the formation of LDHs with this kind of structure and to explore their potential applications.

The article “Hierarchical self-supported ZnAlEu LHD nanotubes hosting luminescent CdTe quantum dots” (doi: 10.1039/C7CC02097J), by Mustafa et al., can be read by subscribers to Chemical Communications at pubs.rsc.org/en/content/articlelanding/2017/cc/c7cc02097j#!divAbstract.