Professor Elson Longo (CDMF-UFSCar), founding member and former president of B-MRS, is the corresponding author of an article that appears in the Top 100 2018 ranking of the journal Scientific Reports in the area of Materials Science. The ranking highlights the most read articles in 2018, among those published that year in the journal of the Nature group. The paper was published on January 30, 2018 and received 1,042 views throughout the year.
Entitled Towards the scale-up of the formation of nanoparticles on alpha-Ag2WO4 with bactericidal properties by femtosecond laser irradiation, the article is signed by eleven authors, six of them from Brazilian institutions, including the researcher Camila Cristina de Foggi (UNESP), who is also a B-MRS member.
The work proposes a new process to produce bactericidal nanocomposites based on silver nanoparticles and semiconductor materials. The method increases 32 times the bactericidal action of the nanocomposite and, at the same time, generates a new class of spherical nanoparticles.
[Paper: One material, multiple functions: graphene/Ni(OH)2 thin films applied in batteries, electrochromism and sensors. Eduardo G. C. Neiva, Marcela M. Oliveira, Márcio F. Bergamini, Luiz H. Marcolino Jr & Aldo J. G. Zarbin. Scientific Reports 6, 33806 (2016). doi:10.1038/srep33806. Link para o artigo: http://www.nature.com/articles/srep33806]
A lot of science and some serendipity to discover the recipe for a multifunctional nanocomposite.
A recently published paper in the journal Scientific Reports, from the Nature group, reports a study carried out in universities of the state of Paraná (Brazil) on a material based on nickel hydroxide Ni(OH)2 – a composite of great technological interest [See box]. The group of authors developed an innovative method to fabricate a material based on nickel hydroxide graphene and nanoparticles, prepared thin films with this material and demonstrated the efficiency of these films when used as rechargeable battery electrodes, glycerol sensors and electrochromic materials.
The work was carried out within the doctoral research of Eduardo Guilherme Cividini Neiva, under the guidance of Professor Aldo José Gorgatti Zarbin, in the Chemistry Post-Graduation Program of the Federal University of Paraná (UFPR). Neiva began his research on nickel nanoparticles during his undergraduate years, guided by Professor Zarbin. In the master’s program, still with Zarbin, he developed a preparation route of nickel metal nanoparticles for electrochemical applications. After completing the master’s program, Neiva and Zarbin set out to continue the research in Neiva’s doctorate, including graphene in the preparation of nickel metal based nanoparticles to obtain nickel and graphene nanocomposites with different properties. “Most of my scientific interests focus on the preparation of materials with carbon nanostructures, such as nanotubes and graphene,” states Professor Zarbin, who is the corresponding author of the article in Scientific Reports.
They were surprised by the first laboratory results. In the presence of graphene oxide (as a precursor of graphene in the preparation of the material), the process took a different course. At that time, Neiva and Zarbin saw the potential of these particularities: if well understood, they could be controlled and used to prepare nanocomposites, not only of nickel metal, but also of nickel hydroxide, which would open up new application possibilities. “There is a phrase by Louis Pasteur I like very much, which applies perfectly in this case: “Chance favors the prepared mind,” declares Zarbin.
Based on this, student and advisor created a simple and direct process for the fabrication of graphene and nickel hydroxide nanocomposites. In this innovative process, both components are synthesized together in a one-step reaction. Using this technique, Neiva manufactured the nanocomposites. Pure nickel hydroxide samples were also produced in order to compare them with the nanocomposites.
The samples were studied through a series of techniques: X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), thermogravimetry, field emission scanning electron microscope (FEG-MEV), and also by means of transmission electron microscopy (TEM) images carried out by Professor Marcela Mohallem Oliveira, from the Federal Technological University of Paraná (UTFPR). The comparison between the two materials was favorable to the nanocomposite. “Graphene played a key role in the stabilization of particles at the nanometer scale, increasing the chemical and electrochemical stability of the nanoparticles, and increasing the conductivity of the material, which is fundamental for an improvement in the desired applications,” acknowledges Aldo Zarbin.
Aldo José Gorgatti Zarbin (on the left side) and Eduardo Guilherme Cividini Neiva, the main authors of the paper, standing at the FEG-MEV equipment of the Materials Chemistry Group of UFPR.
The next stage consisted of
processing the nanocomposites and the nanoparticles of pure nickel hydroxide to obtain thin films, a format that allows using them in the desired applications. “The deposition of materials in the form of films, covering different surfaces, is a great technological challenge, even greater when dealing with multicomponent materials and insoluble, infusible and intractable materials (all characteristics of the material reported in this article)”, explains Zarbin.
To overcome this challenge, Neiva used a processing route, known as liquid/liquid interfacial method, developed in 2010 by the research group led by Zarbin, the Materials Chemistry Group of UFPR. This route, besides simple and cheap, explains Professor Zarbin, allows depositing complex materials in the form of homogeneous and transparent films on various types of materials, including plastics. “The route is based on the high energy at the interface of two immiscible liquids (e.g., water and oil), where the material is initially stabilized to minimize this energy, allowing its subsequent transfer to substrates of interest,” he explained.
With the nanocomposites, Neiva obtained thin transparent films of about 100 to 500 nm in thickness, with nanoparticles of about 5 nm in diameter, distributed homogeneously on the graphene sheets. The pure nickel hydroxide, however, generated films formed by porous spherical nanoparticles of 30 to 80 nm in diameter, distributed heterogeneously, forming agglomerates in some regions.
In the final phase of the work, the films deposited on glass and indium tin oxide were tested in three applications, in which the nanocomposite performed better than pure nickel hydroxide. As a material for rechargeable alkaline battery electrodes, the nanocomposite exhibited high energy and high power – two positive points that are not easily found in the same material. The nanocomposite also showed good performance as an electrochemical sensor. In fact, experiments designed by Professors Márcio Bergamini and Luiz Marcolino Jr, also from UFPR, showed that the nanocomposite is a sensitive sensor of glycerol (a compound known commercially as glycerin and used in several industries). Finally, the nanocomposite acted as an efficient electrochromic material. With these characteristics, the films of the UFPR group have a chance to leave the laboratory and be part of innovative products. “This depends on partners who are interested in scaling the method and testing it on real devices,” says Zarbin.
For now, in addition to scientific articles such as the one published in the journal Scientific Reports, the work generated several patents, both on the deposition method of thin films and on their applications in gas sensors, transparent electrodes, photovoltaic devices and catalysts. “And we have now developed a flexible battery, which was only possible thanks to the film deposition technique we developed,”, adds Professor Zarbin.
The work, which was developed within the macro projects “INCT of carbon nanomaterials” and “Nucleus of Excellence in Nanochemistry and Nanomaterials”, received funding from the federal agencies Capes and CNPq, and the Araucária Foundation, an agency for scientific and technological development of the state of Paraná.
This figure, sent by the authors of the paper, summarizes the main contributions of the paper. In the center, a flask with two liquids and the film at the interface represents the method for thin films processing. A diagram of the film is on the left, with the nickel hydroxide nanoparticles on the graphene sheet. To the right, a photograph of the film deposited on a quartz substrate shows the homogeneity and transparency of the film (it is possible to read text below it). And to the right, from top to bottom, the three applications are shown by a discharge curve (battery), of a transmittance variation curve by the applied potential (electrochromism) and an analytical curve showing the linear variation of the intensity of the current as a function of glycerol concentration in the medium (sensor).
O artigo científico de membros da comunidade brasileira de pesquisa em Materiais em destaque neste mês é:
E. Longo, L. S. Cavalcante,D. P. Volanti, A. F. Gouveia, V. M. Longo, J. A. Varela, M. O. Orlandi and J. Andrés. Direct in situ observation of the electron-driven synthesis of Ag filaments on α-Ag2WO4 crystals. Scientific Reports 3, 2013, article number 1676. DOI: 10.1038/srep01676.
Texto de divulgação: Observação ao vivo da formação de nanofilamentos de prata por uma nova rota de síntese
Quando, no Instituto de Química do campus de Araraquara da Unesp, o microscópio eletrônico de transmissão mostrou o crescimento de protuberâncias nanométricas nos bastões de tungstato de prata que estavam sendo analisados, a equipe de pesquisadores se surpreendeu bastante. Na verdade, os cientistas estavam estudando as propriedades fotoluminescentes dos cristais de tungstato, mas, dando sequência à investigação dessas protuberâncias e após repetir o experimento e caracterizar as amostras, eles acabaram concluindo que se tratava de nanofilamentos de prata, gerados a partir da matriz de tungstato. Os pesquisadores tinham descoberto uma rota de síntese inovadora para esse material, chamada de eletrossíntese por ser produzida por elétrons.
Esta sequência de imagens de microscopia eletrônica de transmissão obtidas de cinco em cinco segundos, reproduz aproximadamente o que os cientistas viram nessa oportunidade. A setinha azul mostra os nanofilamentos crescendo.
Imagens extraídas do artigo da Scientific Reports.
A eletrossíntese se baseia no fenômeno, conhecido para os iniciados e provavelmente surpreendente para os leigos, da interação dos elétrons emitidos pelos microscópios eletrônicos com os objetos que estão sendo observados. Nesses microscópios, sejam eles de transmissão (MET) ou de varredura (MEV), feixes de elétrons são direcionados para as amostras. Da interação entre ambos resultam códigos que acabam gerando imagens que ampliam os objetos observados em até milhões de vezes. Porém, como “efeito colateral”, a alta energia desses elétrons pode produzir modificações nos materiais observados, como, por exemplo, o desgaste das amostras.
No caso da pesquisa com tungstato de prata, o efeito colateral foi, além de surpreendente, positivo e construtivo, dando início a um avanço relevante para a Ciência e Engenharia de Materiais. A pesquisa gerou um artigo assinado por oito pesquisadores: sete brasileiros ligados à Unesp e à UFSCar, e participantes do Centro Multidisciplinar para o Desenvolvimento de Materiais Cerâmicos (CMDC), e um cientista da Universidade Jaume I, da Espanha. O paper foi publicado, em abril deste ano, na Scientific Reports, periódico de acesso aberto do grupo Nature lançado em 2011.
Importância da descoberta
Nanopartículas de prata têm aplicações de impacto, principalmente devido a suas propriedades bactericidas. De fato, revestimentos de prata são realizados, por meio de métodos de deposição, para impedir a proliferação de bactérias em diversos materiais. Nesse sentido, os nanofilamentos de prata gerados por eletrossíntese a partir do tungstato de prata são ainda mais interessantes, desde que a radiação os torna três vezes mais bactericidas do que materiais similares obtidos por outras rotas.
Alguns aspectos da fabricação dos nanofilamentos de prata via eletrossíntese podem ser controlados. Por exemplo, ao se aumentar a energia dos elétrons, aumenta também a velocidade de crescimento dos nanofilamentos. Por isso o uso de microscópios de transmissão é mais eficiente do que o de microscópios de varredura na síntese dos nanofilamentos.
Entretanto, além das aplicações deste novo material, o importante avanço inicial trazido por esta pesquisa feita no Brasil foi a possibilidade de observar o crescimento dos nanofilamentos in situ e em tempo real através dos mesmos microscópios eletrônicos que estavam promovendo seu crescimento. Os pesquisadores puderam analisar o processo de nucleação (formação inicial da nanopartícula de prata a partir do cristal de tungstato) e, em seguida, o crescimento dos nanofilamentos. Complementando essas observações com técnicas de caracterização de materiais, cálculos e teorias, os cientistas puderam apresentar uma explicação científica de por que o material se comporta dessa maneira.
A explicação, ou mecanismo de crescimento, se baseia na compreensão da estrutura dos cristais de tungstato de prata. Ao receberem a irradiação de elétrons, os diversos clusters que constituem os cristais (AgO4, AgO2, WO6 e outros) se desorganizam e reorganizam, ocorrendo transferências de elétrons por meio de reações de redução-oxidação (redox). Dessas reações surgem as nanopartículas de prata, que brotam na superfície dos cristais e crescem axialmente formando os nanofilamentos.
