Featured paper: Kinetic model for more efficient organic solar cells.

Posted on Monday 31 de August de 2020

Back cover of the J. Mater. Chem. C highlights the paper of the Brazilian team.

Unlike other solar cells that have dominated the market for a long time, such as silicon cells, the organic ones are thin, light, flexible and semi-transparent. With these characteristics, they become very attractive for specific segments. In Brazil, for example, which has national production, some of the largest installed surfaces in the world can be seen in business buildings, as well as some installations in shopping centers, trucks and bus stops.

Although the organic version of solar cells also offer advantages in large-scale production (simpler industrial processes with lower carbon footprint, such as the roll-to-roll), conquering big markets largely depends on an ongoing efficiency improvement to convert sunlight into electricity. To overcome this challenge, it is essential to develop materials with suitable properties and to combine different materials within the device.

A scientific team from the Brazilian Federal University of Paraná (UFPR) studied in detail, using experimental and theoretical tools, the charge generation mechanism in organic solar cells – a complex process that is not yet fully understood. In practice, the results of this work help choosing which materials should be used and how they should be synthesized, so that their properties enhance the efficiency in converting light into electricity. The research paper was reported in the Journal of Materials Chemistry C (impact factor 7.059), where it was highlighted on the back cover.

Unraveling the exciton dissociation

In the sandwich of layers that forms solar cells, the active layer (responsible for absorbing light and generating electric charges) is composed of semiconductor materials that, for organic devices, are polymers or other carbon-based molecules. When excited by light, these materials do not generate free electric charges, as is the case with inorganic semiconductors. They generate excitons, which are electron–hole pairs connected by forces of attraction between the negative charge of the first and the positive charge of the second.

In order to generate free charges, which form the electric current, it is necessary to break this connection, in a phenomenon called exciton dissociation. One way to achieve this is to create, in the active layer, an interface between an electron donating material and an electron acceptor. “Depending on the combination of these two materials, exciton dissociation processes can occur at a very low time scale, resulting in a more efficient charge generation,” explains Leandro Benatto, corresponding author of the paper. “However, this process is still not well understood,” he adds.

In their work, Leandro and the other authors focused specifically on trying to understand the exciton dissociation and the generation of free charges at the interface between the donor and acceptor material. The team carried out photoluminescence experiments, which are generally used to measure the efficiency in generating free charges in systems of this type, and developed a mathematical model that simulates the process. The experimental and theoretical results were very similar, proving the model’s accuracy. “We developed a model that simulates the kinetics of the process, including the several stages of exciton dissociation and considering the main characteristics of the interface,” he says. “Based on the kinetic model, it was possible to reproduce the experimental results in a comprehensible manner and more clearly observe the main factors that influence the efficiency of the free charge generation process in donor/acceptor interfaces,” he adds.

Fullerenes vs. Non Fullerenes

The study that produced the article was coordinated by two professors from the Physics Department of UFPR, Marlus Koehler and Lucimara Stolz Roman, who have a longstanding partnership in the theoretical and experimental study of organic solar cells. “The theoretical part began to be developed in 2019, at the end of my PhD in Physics at UFPR under the guidance of Professor Marlus, and continued in my postdoctoral work at the Nanostructured Devices Laboratory (DINE) under the coordination of Professor Lucimara,” says Leandro. Also participating in the research were Maiara de Jesus Bassi, PhD student in Physics in the group of Professor Lucimara, and Luana Cristina Wouk, PhD in Physics who was also under the supervision of Professor Lucimara Roman, and currently working at CSEM Brazil, a private applied research center, which helped contextualize the problem in the large-scale development scenario.

The initial idea of the work was to understand the difference between two types of electron acceptor molecules: those derived from fullerene (a carbon allotrope), which have excellent performance in the collection and transport of electrons but have a limited spectrum of light absorption, and compounds not derived from fullerenes, which in recent years have optimized the collection and transport properties. “This is a very interesting topic since, recently, the efficiency of organic solar cells based on non-fullerenes surpassed the efficiency of those based on fullerenes, although, a few years ago, it could not be imagined that fullerenes would be surpassed,” reports Leandro. “Currently, laboratory produced organic solar cells based on non-fullerenes have reached 18% efficiency,” he adds.

This research received funding from Brazilian agencies Capes, CNPq and FAPEMIG, INCT–Nanocarbono and COPEL (Companhia Paranaense de Energia).

The authors of the paper, from the left: Leandro Benatto, Maiara de Jesus Bassi, Luana Cristina Wouk, Lucimara Stolz Roman and Marlus Koehler.

[Paper: Kinetic model for photoluminescence quenching by selective excitation of D/A blends: implications for charge separation in fullerene and non-fullerene organic solar cells. L. Benatto, M. de Jesus Bassi, L. C. Wouk de Menezes, L. S. Roman and M. Koehler. J. Mater. Chem. C, 2020,8, 8755-8769].

Featured paper: Nanocontainers for intelligent anticorrosion coatings.

Posted on Tuesday 30 de June de 2020

SEM image of a cluster of zinc hydroxydolibate crystals. Each crystal is a nanocontainer.

Traditional coatings are often effective in protecting steel structures from corrosion by providing a physical barrier that prevents contact between the substrate and corrosive elements. However, when wear generates cracks or pores in these coatings, the substrates are exposed and eventually corrode.

A scientific team from the Brazilian Federal University of Paraná (UFPR) developed a nanotechnology-based coating that offers active protection against corrosion. When the coating shows a flaw, exposing the substrate to corrosive elements, two anti-corrosion mechanisms are activated without human intervention. The coating has very special additives to carry out this intelligent procedure: nanocontainers capable of storing a compound that inhibits corrosion (molybdate) and release it, on demand, in the presence of corrosive elements.

UFPR’s nanocontainers are structures of nanometric dimensions that appear in the form of overlapping two-dimensional sheets (lamellae), separated by a space of a few angstroms. It is in this interlamellar space that molybdate is stored. When a nanoreservoir comes into contact with corrosive environments, such as the sodium chloride (NaCl) solution used in the UFPR experiments, a negative ion exchange reaction occurs: molybdate anions leave the interlamellar space and chloride anions occupy these spaces. In this exchange, the molybdate is free to form a protective film on the exposed substrate surface, while the corrosive anions are “trapped” in the nanoreservoir.

Schematic representation of the active protection mechanism against corrosion provided by the coating additive with intelligent lamellar nanoreservoir.

“The main contribution of the work is related to the active corrosion protection of steel. While traditional coatings provide only barrier protection, that is, passive protection, the intelligent coating developed in this work promotes active protection, releasing the molybdate corrosion inhibitor from the lamellar nanoreservoir on demand, through an anion exchange mechanism,” summarizes Débora Abrantes Nunes Leal, doctoral student of the Graduate Program in Materials Engineering and Science at UFPR and corresponding author of the article reporting the research, recently published in ACS Applied Materials & Interfaces. “In addition to releasing the inhibitor in a controlled manner, the lamellar nanoreservoir also “captures” the chloride anions, acting as a trap for these species and, therefore, also contributes to reducing the corrosive environment,” concludes the doctoral student.

Industrial scale use

Produced by means of a quick and simple process and using low-cost, abundant raw materials with low environmental impacts, such as zinc, UFPR’s nanocontainers have the potential for large-scale manufacturing process. When used as coatings additives, as in the UFPR research, nanocontainers could increase the useful life of steel structures found in atmospheres with high concentrations of corrosive compounds, such as, for example, marine atmospheres or close to chemical plants.

According to Débora, there is already a market, mainly in Europe, still incipient, of intelligent coatings to combat corrosion for the aeronautical and petrochemical industries. However, these coatings use as reservoirs of anticorrosive compounds, mainly polymeric microcapsules or double lamellar hydroxides, which are more complex to manufacture, costly and have a greater environmental impact than the lamellar hydroxyses used in UFPR’s nanocontainers . According to the authors of the article, these materials had never been used to store and release corrosion inhibitors before this work, neither in industry nor in the scientific literature.

All the work reported in the article of ACS Applied Materials and Interfaces was carried out by Débora Abrantes Nunes Leal within her still-in-progress doctoral thesis, which started in 2017. The research is guided by Professor Cláudia Marino (advisor), who is an expert in corrosion and protection of materials, and by Professor Fernando Wypych (co-advisor), whose expertise is the synthesis and characterization of lamellar materials. The work received funding from the Brazilian agencies Capes, CNPq and Finep.

Currently, the research on nanocontainers continues, and the authors indicate that they will soon present new results, including those related to the use of other corrosion inhibitors.

The authors of the paper. From the left: Débora Abrantes Leal, Fernando Wypych, Cláudia Eliana Bruno Marino.

[Paper: Zinc-Layered Hydroxide Salt Intercalated with Molybdate Anions as a New Smart Nanocontainer for Active Corrosion Protection of Carbon Steel. Débora Abrantes Leal, Fernando Wypych, and Cláudia Eliana Bruno Marino. ACS Appl. Mater. Interfaces 2020, 12, 17, 19823–19833. https://doi.org/10.1021/acsami.0c02378.]

Featured paper: Nanorods to develop new anti-inflammatory drugs.

Posted on Friday 30 de August de 2019

[Paper: Characterization of the structural, optical, photocatalytic and in vitro and in vivo anti-inflammatory properties of Mn²⁺ doped Zn₂GeO₄ nanorods. Suzuki, V. Y.; Amorin, L. H. C; Lima, N. M; Machado, E. G; Carvalho, P. E.; Castro, S. B. R.; Souza Alves, C. C.; Carli, A. P.; Li, Maximo Siu; Longo, Elson; Felipe La Porta. J. Mater. Chem. C, 2019, 7, 8216. DOI: 10.1039/c9tc01189g]

nanobastoes A team of researchers from Brazilian universities found, in cylindrical nanostructures known as nanorods, an anti-inflammatory effect equivalent to that achieved by commercial drugs. Researchers have also demonstrated the effectiveness of these nanorods as catalysts (accelerators) in the degradation of a pollutant. These applications are even more relevant considering that the scientific team was able to produce large quantities of the material through a simple and fast process. The work carried out shows the potential of these nanorods for the development of new medicines and for the treatment of effluents.

The work originated about three years ago when Professor Felipe de Almeida La Porta, who had recently joined the faculty of the Federal Technological University of Paraná (UTFPR), Londrina campus, was implementing a research group on nanotechnology and computational chemistry at this university. “Our laboratory was investigating some classes of emerging materials, with the perspective of aligning theory and practice, thus driving new discoveries and applications,” says La Porta. One of the materials studied by the group was zinc germanate (Zn2GeO4), a versatile semiconductor with well-known applications in sensors, catalysts, batteries and other devices.

Together with undergraduate researcher Victor Yuudi Suzuki, the professor started a project in which he synthesized pure Zn2GeO4 nanorods at the UTFPR laboratory with very small percentages of manganese ions. To produce this series of nanorods, they used “microwave assisted hydrothermal synthesis.” The method consists, in broad lines, of mixing aqueous solutions containing certain compounds, heating the final solution in a microwave oven and allowing the compounds to react for a certain period of time at controlled pressure and temperature. In this study, the manganese ion-doped Zn2GeO4 was prepared, and the reactions were performed at 140 °C for 10 minutes. The resulting material from these reactions was collected at room temperature, then washed and dried, which generate the nanorods.

Professor La Porta and his research group were able to optimize one of the process steps, the crystallization of materials, thus reducing the synthesis time from hours to a few minutes, but maintaining the quality of the material and the possibility to control its shape.

After preparing the samples, they traveled from Londrina (state of Paraná) to São Carlos (São Paulo state) to characterize the materials at the Center for Functional Materials Development (CDMF) at the Federal University of São Carlos (UFSCar) and at the Institute of Physics at the University of São Paulo (USP). Together with the local researchers, they were able to analyze the shape, structure and luminescence of the four types of nanorod compositions produced: manganese-free and with 1, 2 and 4% of this element incorporated into the structure of Zn2GeO4.

Finally, knowing that compounds containing zinc, germanium or manganese exhibit considerable effects on living things, the team contacted some collaborators to investigate these properties in the nanorods. Thus, several experiments were performed at the Departments of Chemistry and Pharmacy of the Federal University of Juiz de Fora and at the Federal University of Vales do Jequitinhonha and Mucuri, both in the state of Minas Gerais.

The authors of the paper. From the left: Victor Suzuki, Luís Amorin, Felipe La Porta, Maximo Si Li, Elson Longo, Sandra de Castro, Paloma de Carvalho, Alessandra Carli, Emanuelle Machado, Caio Alvez, Nerilson Lima.

To study the anti-inflammatory action, the team performed in vitro tests (in contact with cells in laboratory containers) and also in vivo tests (using rats with paw edema, within the norms of the Brazilian code for laboratory animal use). Both types of experiments revealed that nanorods with about 4% manganese were the most effective in controlling inflammation. The in vitro tests showed these nanostructures were able to modulate molecules that regulate inflammation without causing cell death (without cytotoxicity). In the in vivo experiments, the nanorods reduced the induced rat paw edema with results similar to that of the application of dexamethasone, a well-known drug of the corticoid group.

“At first, we thought that combining these elements to form a ternary oxide could somehow potentiate these effects. But we had no idea the results would be so significant. Given that the drugs currently available in therapy are proving to be less effective every day, these results may encourage the use of these nanorods, for example in the production of a new pharmaceutical formulation, especially for cases of inflammation,” says Felipe La Porta, who is the corresponding author of the paper that was recently published by the research team in the Journal of Materials Chemistry C (impact factor 6,641).

In addition to proving the potential of the material for this application in the health area, the authors of the paper have experimentally verified the ability of nanorods to degrade a chemical dye widely found in industrial effluents, known as methylene blue. For this application, 2% manganese nanostructures were the most efficient, completely decomposing the dye in 10 minutes. “Due to the manufacture simplicity of this system, coupled with its excellent properties, this material is also promising for cleaning various environmental pollutants, and can be easily recovered at the end of this process,” adds Prof La Porta.

In the center, a cluster of 4% manganese zinc germanate nanorods. Clockwise: photoluminescence measurements of the samples; representation of the structure of manganese-doped zinc germanate; pollutant degradation mechanism and methylene blue degradation measures; anti-inflammatory action of nanorods and other treatments in induced-edema rat paw.

The superior properties that the Brazilian scientific team found in the nanorods with manganese can be related to the structural defects observed in these samples. In fact, the three-dimensional network of atoms that forms zinc germanate is crystalline, that is, organized in regular patterns. The introduction of manganese generates irregularities, and new properties emerge.

The scientific paper that reports this work was selected to be part of the Materials and Nano Research in Brazil collection, prepared by the Royal Society of Chemistry in celebration of the 18th B-MRS Meeting, and can therefore be accessed free of charge until October 15 of this year, here.

The work was carried out with funding from Brazilian research support agencies: the federal CNPq and Capes, and the state Araucaria Foundation, Fapesp and Fapemig.

Featured paper. A lot of science and some serendipity to discover the recipe for a multifunctional nanocomposite.

Posted on Monday 28 de November de 2016

[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 processing method of thin films. 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). — 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).