Featured paper: Aluminum flakes to produce carbon nanotubes.


[Paper: High-yield synthesis of bundles of double- and triple-walled carbono nanotubes on aluminum flakesThiago H.R. da Cunha, Sergio de Oliveira, Icaro L. Martins, Viviany Geraldo, Douglas Miquita, Sergio L.M. Ramos, Rodrigo G. Lacerda, Luiz O. Ladeira, Andre S. Ferlauto. Carbon 133(2018) 53-61.]

Aluminum flakes to produce carbon nanotubes

Scanning electron microscopy image of carbon nanotube bundles obtained by the method of the CTNano team.
Scanning electron microscopy image of carbon nanotube bundles obtained by the method of the CTNano team.

A team of scientists from institutions in Minas Gerais made a promising contribution to the production of carbon nanotubes. These hollow cylinders, whose carbon walls are only 1 atom thick, are already part of some products (batteries, automotive materials, water filters), but their industrial production is still incipient and needs solutions to lower costs and to increase efficiency, among other challenges.

The Brazilian researchers introduced a novelty in a stage of one of the most consolidated techniques for the mass production of nanotubes, chemical vapor deposition (CVD). As a result, the team was able to produce double- and triple-walled nanotube bundles (somewhat similar to two or three hollow cylinders, one inside the other). Thin, long and of high purity, the nanotubes had diameters of 3 to 8 nanometers, lengths up to 50 thousand times the diameter (from 150 to 300 micrometers) and 90% of carbon in their composition.

“The main contribution of this work is the presentation of a scalable and cost effective process for the synthesis of carbon nanotube bundles with large surface area (625 m2/g) and aspect ratio (50000:1),” says Thiago Henrique Rodrigues da Cunha, researcher of the Nanomaterials Technology Center (CTNano) of the Brazilian Federal University of Minas Gerais (UFGM) and corresponding author of this paper, which was recently published in the journal Carbon (impact factor 2017 = 7,082).

The method, in addition to generating good quality nanotubes, allows producing relatively large quantities of this material using relatively low amounts of raw materials. “Even using small systems, it is possible to obtain carbon nanotubes at a kilogram/day scale,” says the researcher. As the nanotubes obtained showed a very large ratio between surface area and mass (more than 625 square meters weighing only one gram), the production of nanotubes by this method could reach a few million square meters per day.

With the nanotubes obtained and a type of alcohol, the scientific team prepared a paste which was distributed over filter paper, forming a film that was separated from the paper when the paste dried. The black film was 40 micrometers thick and was flexible and foldable. Macroscopic aggregates of carbon nanotubes like this are commonly called buckypapers.

On the left, carbon nanotube film (buckypaper) produced by the team. On the right, an airplane made with this buckypaper.
On the left, carbon nanotube film (buckypaper) produced by the team. On the right, an airplane made with this buckypaper.

“The buckypaper produced from these nanotubes exhibited great surface area and good electrical conductivity, which makes them particularly interesting in the manufacture of electrodes for batteries and supercapacitors,” says Thiago da Cunha, who adds that the CTNano team is already working to use the buckypapers in these energy storage devices. A patent on the process was deposited at the end of 2017. “Our intention is to introduce this technology to potential partners in order to convert it into a high value-added product,” reveals Cunha.

The secret of the process

Scanning electron microscopy image of carbon nanotube bundles that grew from both sides of an aluminum flake.
Scanning electron microscopy image of carbon nanotube bundles that grew from both sides of an aluminum flake.

The CVD nanotube production processes take place inside a tube furnace into which gas containing carbon and catalytic nanoparticles are inserted. Subjected to high temperatures, the gas decomposes, and the carbon atoms deposit on top and around the nanoparticles, forming tubes (the nanotubes). The nanoparticles can be prepared in the same furnace used for nanotube growth.

The secret of the method developed by the Minas Gerais team lies precisely in the preparation of the catalytic nanoparticles. In broad lines, it is a matter of preparing a powder containing iron (Fe) and cobalt (Co) on aluminum flakes (material that had never before been mentioned in the scientific literature as a support for the growth of nanoparticles). The mixture is then subjected to temperatures of 350 to 650 °C for 4 hours, in an atmosphere similar to the air we breathe. This process, known as calcination, produces nanoparticles of iron and/or cobalt oxides. Then, the catalyst nanoparticles, still on the aluminum flakes, are introduced into the CVD furnace, whose internal temperature is brought to 730 °C. The ethylene gas (C2H4) is then introduced, which supplies the carbon so that the nanotubes grow perpendicular to the aluminum flakes.

Scientists observed an interesting advantage of using this new medium. During the calcination, a thin layer of aluminum oxide is formed on the surface of the aluminum that encapsulates the nanoparticles and prevents them from agglomerating or spreading. In addition, in the next step of the process, the aluminum oxide acts as a matrix of the nanotubes, driving their growth in the form of aligned bundles.

To test whether the calcination temperature of the nanoparticles would influence their performance as catalysts, the CTNano team carried out some experiments. The conclusion was that calcination at temperatures of 500-550 °C produces more mixed oxide nanoparticles (containing both iron and cobalt, of the CoFe2O4 formula) and produces better results in the production of nanotubes, both quantitatively (yield) and qualitative (diameter of the nanotubes).

“Unlike other methods described in the literature, which generally display low yield and are dependent on relatively expensive techniques (evaporation, sputtering) for the preparation of the catalyst, we describe in this paper a simple method to produce a catalyst in powder form, which can be used for continuous production of few-walled nanotubes using the chemical vapor deposition technique (CVD),” summarizes Thiago da Cunha.

CTNnano

The work was funded by the Brazilian agencies Fapemig (Minas Gerais State Research Foundation) and CNPq, as well as Petrobras. The work was carried out at CTNano, except for the microscopy images, conducted at the UFMG Microscopy Center.

CTNano emerged in 2010 based on the motivation to develop products, processes and services using carbon nanotubes and graphene, in order to meet industrial demands in line with the training of qualified human resources. The research realized in CTNano has already originated 26 patents and contributed to the development of more than 200 researchers in the area. According to Thiago da Cunha, CTNano will inaugurate, in 2018, its own headquarters with an area of approximately 3,000 m², located in the Technology Park of Belo Horizonte (BH-TEC).

Authors of the paper, from UFMG, except for Viviany Geraldo, who is a professor at the Federal University of Itajubá (UNIFEI).
Authors of the paper, from UFMG, except for Viviany Geraldo, who is a professor at the Federal University of Itajubá (UNIFEI).

 

Featured paper: Changing the properties and morphology of graphene nanoribbons with nitrogen.


The scientific paper by members of the Brazilian community in Materials research featured this month is:

Josue Ortiz-Medina,  M. Luisa García-Betancourt,  Xiaoting Jia,  Rafael Martínez-Gordillo,  Miguel A. Pelagio-Flores,  David Swanson,  Ana Laura Elías,  Humberto R. Gutiérrez,  Eduardo Gracia-Espino,  Vincent Meunier, Jonathan Owens,  Bobby G. Sumpter, Eduardo Cruz-Silva,  Fernando J. Rodríguez-Macías,  Florentino López-Urías,  Emilio Muñoz-Sandoval, Mildred S. Dresselhaus,  Humberto Terrones,  Mauricio Terrones. Nitrogen-Doped Graphitic Nanoribbons: Synthesis, Characterization and Transport. Advanced Functional Materials 2013, 23, 3755-3762. DOI 10.1002/adfm.201202947.

Changing the properties and morphology of graphene nanoribbons with nitrogen

Multiple layers of graphene with the shape of ribbons (narrow and long) are called graphitic nanoribbons. These materials have been studied to control their properties by various methods, such as doping, in which are introduced atoms of “foreign” elements in the graphene carbon lattice.

In a study led by scientists at Pennsylvania State University with the participation of researchers from institutions in the United States, Mexico, Spain and Brazil, nitrogen-doped graphitic nanoribbons were manufactured by the chemical vapor deposition (CVD) method and showed new features, linked with the introduction of nitrogen, such as highest semiconductor performance, promising for applications in electronic devices, chemical reactivity and a very particular morphology on its edges.  The research was published in the journal Advanced Functional Materials.

“This article showed by the first time that it is possible to make doping with nitrogen on the same synthesis by CVD of graphite nanoribbons, and that you can control the level of doping during synthesis,” highlights Fernando Rodríguez-Macías, foreign visiting professor at the Brazilian Federal University of Pernambuco (UFPE) and one of the authors of the scientific paper. A Mexican national, Rodríguez-Macías came to UFPE in 2012, during his sabbatical year to work as a foreign visiting professor in the Department of Fundamental Chemistry and in the Graduate Program in Materials Science of the University. “I have prolonged my stay for another year to continue until 2014 doing collaborative studies for the production of carbon nanostructures, of bionanotechnology and toxicity of nanomaterials,” says the professor. “I am also teaching preparation and characterization of materials,” he adds.

The doped nanoribbons

The article’s authors showed that different concentrations of nitrogen generate controlled changes in material behavior. In particular, scientists have proven that the more nitrogen introduced into the structure of graphene, the most predominant the semiconductor behavior of nanoribbons. As an explanation to this phenomenon, the researchers suggested, based on theoretical calculations, that nitrogen atoms of doped nanoribbons act as scattering centers of electrons and decrease the conductive behavior of undoped graphene. “The control of doping level allows you to change the electrical properties of the nanoribbons, which can be useful for applications in transistors and other electronic devices,” says Rodríguez-Macías.

In addition, the paper also shows that the reactivity of nanoribbons can change with the doping level. Pure graphene, explains UFPE’s visiting professor, is very inert and has limited interactions with many chemical substances; on the other hand, nanoribbons doped with nitrogen are more reactive, which makes them useful for applications in sensors and catalysis.

As to the morphology, the article’s authors found that the nitrogen-doped nanoribbons have loops on their edges, uniting different graphene sheets. “This morphology is not presented by undoped graphite nanoribbons,” says Rodríguez-Macías.

This figure, sent by Professor Fernando Rodríguez-Macías, shows the nitrogen-doped graphitic nanoribbons in three scales. The scanning electron microscopy (top left corner) shows how these ribbons are made up of several layers and feature a curved surface with roughness. The transmission electronic microscopy (bottom left corner) shows that the nanoribbon layers are graphene sheets. The high resolution transmission electronic microscopy (right) shows that the layers of graphene on the nanoribbons edges form loops uniting different graphene sheets.

Collaborators

Almost all work of materials synthesis of the paper of Advanced Functional Materials was developed at Pennsylvania State University; the characterization was done in collaboration with other researchers and laboratories, reports UFPE’s visiting professor.

The participation of UFPE in the article happened through the doctoral student Miguel Angel Pelagio-Flores in the analysis and theoretical modeling of doped nanoribbons, and through professor Fernández-Macías himself, who, in addition to having participated in the discussion of results and review of the article from his office at UFPE, was doctoral advisor of the first author of the article, Josué Ortiz-Medina, while professor of a Mexican institution, IPICYT. “Ortiz-Medina did most of the experimental work of the article, besides being an important part of the characterization and theoretical studies of these new nanomaterials, when he was in exchange at Penn State in the laboratory of professor Terrones,” contextualizes the professor.

In total, 19 authors sign the article, among them MIT’s Professor Mildred Dresselhaus, reference in carbon science.