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[PAPER: Bacterial Nanocellulose/MoS2 Hybrid Aerogels as Bifunctional Adsorbent/Photocatalyst Membranes for in-Flow Water Decontamination. Elias P. Ferreira-Neto, Sajjad Ullah, Thais C.A. da Silva, Rafael R. Domeneguetti, Amanda P. Perissinotto, Fábio S. de Vicente, Ubirajara P. Rodrigues-Filho, and Sidney J. L. Ribeiro. ACS Appl. Mater. Interfaces 2020, 12, 37, 41627–41643.]
A team of researchers from Brazilian universities has developed a new material capable of decontaminating water, simultaneously eliminating organic and inorganic pollutants, such as dyes and heavy metals, respectively. The material is in the form of a membrane with the potential to be used as an active filter: as water passes through the membrane, pollutants are adsorbed and degraded. The material can be reused several times, without losing its properties.
The membrane is composed of a three-dimensional network of cellulose nanofibers, coated with molybdenum disulfide (MoS2) nanosheets. Each material performs its function in the filter. Molybdenum disulfide is primarily responsible for adsorbing pollutants and degrading them through photocatalysis [see box]. Nanocellulose operates mainly as support for photocatalysts. Firstly, it allows the construction of an easy to handle macroscopic membrane. In addition, its structure of interwoven nanofibers with a rough surface offers an exceptionally large surface area to contain the photocatalysts. Finally, the flexibility and strength of the nanocellulose allows the membrane to withstand the water flow pressure.
“Although several excellent photocatalysts have been previously developed, one of their disadvantages is the difficult separation and recovery of nanometric materials, thus the idea of producing membranes,” says Elias Ferreira-Neto, postdoctoral fellow at the Laboratory of Photonic Materials, at the Chemistry Institute of UNESP Araraquara. “This study is a first step in the area,” adds Elias, corresponding author of the article reporting on the development of membranes, recently published in Applied Materials & Interfaces (impact factor = 8,758).
Membrane production: from bacterial hydrogel to hybrid aerogel
The “recipe” developed by the Brazilian researchers to produce the membranes involves several steps and requires mastering different processes for the synthesis of materials.
In the first step, bacteria from a non-pathogenic strain, placed in an appropriate medium, perform a metabolic process that generates the bacterial nanocellulose hydrogel as a by-product. This highly porous material is composed of 1% of interwoven cellulose nanofibers and 99% of water. The hydrogel is then washed in order to eliminate impurities.
Subsequently, the surface of the nanofibers is coated with controlled structured molybdenum disulfide nanosheets, regularly distributed on the surface of the nanofibers. Finally, this hybrid hydrogel is transformed into aerogel through a drying process, which replaces the pore water with air. The final result is an aerogel membrane composed of bacterial nanocellulose and molybdenum disulfide.
The researchers built a small photoreactor in order to verify the ability of the new membranes to remove the organic and inorganic pollutants in the water. In this device, the contaminated water passes through the membrane, which is illuminated to generate the photocatalytic effect. In the assays, the researchers used an organic pollutant (methylene blue, a compound used as a dye and as a drug) and an inorganic contaminant (hexavalent chromium, a toxic and carcinogenic compound, still used in various industries).
By measuring the presence of contaminants in the water after filtering, the researchers found that the membrane was able to eliminate approximately 96% of the paint and 88% of the heavy metal after 120 minutes of reactor circulation. “The efficiency achieved is in the range of photocatalytic materials of nanoparticulate-like molybdenum disulfide, which is excellent, given that in the supported material, the active surface area exposed to light is much smaller,” says Elias. However, to make the membranes suitable for real applications outside the laboratory, the researchers intend to further increase this efficiency. “The modification of the materials prepared with other photocatalytic nanostructures, such as titanium dioxide (TiO2) and bismuth vanadate (BiVO4), can greatly increase the efficiency of the materials we have already obtained,” says Elias. In addition, the researchers plan to test the action of the membranes against other organic and inorganic compounds that pollute waters, such as drugs, pesticides and other heavy metals.
Entirely carried out in Brazil, more precisely in the state of São Paulo, the work gather the expertise in bacterial cellulose materials from the group led by Professor Sidney Ribeiro (Institute of Chemistry at UNESP Araraquara), and the experience in photocatalysis from the group conducted by Professor Ubirajara Rodrigues Filho (Institute of Chemistry of São Carlos – USP). The research also involved the collaboration of Professor Fábio Simões de Vicente, from the Physics Department of UNESP Rio Claro, to characterize the porosity and textural properties of the materials.
The study is part of Elias Ferreira-Neto’s post-doctoral project, funded by FAPESP. In his doctorate, under the guidance of Professor Ubirajara, and during the research internship abroad conducted with a researcher specialized in aerogels, Elias acquired great experience in the development of inorganic nanoparticles and aerogels for photocatalysis. In these studies, he could identified the great potential of these materials as photocatalysts, as well as their main limitation, the low mechanical resistance and, particularly, the low resistance to capillary pressure in liquid medium.
In the context of Elias’ postdoctoral fellowship, the collaboration between Brazilian research groups could overcome this limitation through the development of hybrid aerogels that combine the excellent mechanical properties of the cellulose support with the photocatalytic and adsorptive properties of the MoS2 nanosheets.
Professor Carlos Alejandro Figueroa (University of Caxias do Sul, UCS), B-MRS member, was appointed editor of Applied Surface Science (Elsevier), a renowned journal in the area of Physics and Chemistry applied to surfaces and interfaces, 6,182 impact factor.
Figueroa is the second researcher of a Latin American institution within the journal’s team of 30 editors.
At the beginning of 2020, Bruno César da Silva was very excited. After six months of participating in an international selection process, he had obtained a position as a temporary researcher at a research institute in the city of Grenoble (France). Paid by the French government, he would commit to studying in detail the properties of semiconductor nanowires and some of their possible applications. In fact, these tiny wires may be used in future electronic and optoelectronic devices, but, for that, they need to be better understood and controlled.
In his master’s and doctorate in Physics, carried out at UNICAMP, Bruno had studied semiconductor nanowires, always under the guidance of excellent professors, and he had acquired solid knowledge in the set of techniques required by the post. “Finding someone who has a background in all the topics the job asked for is difficult, but I was lucky, was willing and had the opportunity to gain experience in all these areas,” says Bruno. In addition, Bruno is the main author of two articles published in renowned scientific journals (Nano Letters and Scientific Reports). Despite not having an expressive number of articles, Bruno’s production, generated during his doctorate, drew attention for its quality.
But Bruno’s training in scientific research started in high school, within the Junior Scientific Initiation Program of CNPq (the main federal research agency in Brazil). He then continued his undergraduate studies in Physics at the University of Lavras, where he undertook undergraduate research as a fellow of FAPEMIG (the research foundation in Minas Gerais State). While still an undergraduate student, Bruno had his first international research experience, at a Spanish university, as a fellow of the Science without Borders Program, from the Brazilian government. “My training is the result of public policies and investments,” says Bruno. In fact, at all stages, Bruno had scholarships financed with public resources. He started with less than 100 reais in high school and reached about 3,600 reais at the end of his doctorate with a scholarship from FAPESP, the São Paulo State research foundation.
Sudden change of plans
Bruno had a flight ticket and accommodations booked to begin his life in Grenoble when in mid-March the WHO declared the Covid-19 pandemic. As a result, hiring at the research institute was suspended, and Bruno started looking for other opportunities.
He searched in Brazil and abroad. He sought postdoctoral fellowships and jobs as a researcher or data scientist – a professional area linked to mathematics and computing.
Without opportunities and no income, Bruno, aged 32, returned to his parents’ home in Jacareí (SP). He requested emergency government assistance and pledged to finalize scientific articles based on the doctoral results, while taking online courses to redirect his career.
Finally, six months after the scheduled date, Bruno took on his temporary position at the French research institute. And he has been working with nanowires for over a month, very happy. “Here, we, the newly graduated doctors, are treated like professionals, and we pay taxes like any worker,” he jokes, making an implicit comparison with the situation that young PhD grant holders experience in Brazil.
After his experience in Grenoble, he plans to participate in a selection process to become a research professor at a Brazilian university. However, Bruno says that if the situation of scarce resources for science continues in Brazil, he will continue to build a career abroad. “I think it is important to return the investment in my education to the Brazilian society through teaching and conducting research that can contribute to the development of my country. We are able to do quality science in Brazil, but we need the necessary conditions,” he concludes.
Professor Ana Flávia Nogueira (UNICAMP), B-MRS member, is the winner of the Award for Brazilian Women in Chemistry and Related Sciences in the “Leadership in Academia” category, for her research trajectory in emerging technologies for solar cells.
The award is promoted by divisions of the American Chemical Society (ACS) and the Brazilian Chemical Society (SBQ), and is supported by B-MRS. The award ceremony took place on October 15, during the symposium on combating inequality in science, which was part of the 43rd Annual Meeting of the SBQ.
From September 5 to 7, 2020, a few dozen students and young researchers, together with some professors, dedicated the afternoons of the holiday of Brazil’s Independence Day to participate, online, in the 1st National Meeting of the B-MRS University Chapters (I ENUC).
The event brought together members of University Chapters (UCs) and people interested in meeting them. The program was built around two axes: the UCs’ presentations about their teams, institutions, actions carried out and future projects, and the invited lectures on topics of interest to the UCs.
ENUC emerged from the desire of the UC from the Federal University of Pernambuco (UFPE) to interact with members of other B-MRS UCs. In this first edition, the event was fully organized and carried out by the members of UC-UFPE.
UCs in action: learn by doing
In his lecture, Professor Newton Barbosa (UFPA), national coordinator of the UCs program reflected that in the scientific career being competent in the chosen area of knowledge is essential, but it is not enough, especially at the present moment in the history of science and technology, in which problem solving requires the interaction between people and areas. Skills such as communication, flexibility, and proactive leadership, he said, are also necessary for the researcher. The scientist explained that “the idea of the UCs program is precisely to help develop these skills, using the learn-by-doing method, with students carrying out team projects.”
“Research alone will not fill our CV,” added Karolyne Santos da Silva, president of UC UFPE. “We will also need to organize events, occupy administrative positions, publicize our work in society, among other things,” added the doctoral student, who was the event coordinator.
A wide range of projects carried out by the chapters were presented throughout the event, which covered from the creation of the unit’s logo and internal statute, to lectures and interviews for laypeople, workshops to motivate girls to work in science and technology, and seminars for researchers. Support for social action campaigns is also part of the activities carried out by the UCs, always with the dual objective of learning-by-doing and generating a positive impact on the environment.
The social impact of research
In one of the invited lectures, Professor Roberto Faria (IFSC-USP), who was president of B-MRS between 2012 and 2015, spoke briefly about the history of the UCs program. “I wanted the students’ greater participation in B-MRS, as they are the country’s greatest wealth for the future,” said the former president of B-MRS. Faria said that it all started in 2013, during his first term as president of B-MRS. At a scientific event in San Francisco (USA), Professor Faria spoke with the coordinator of the Materials Research Society (MRS) university chapters program, who was none other than Mildred Dresselhaus, a scientist internationally renowned for her work with carbon nanomaterials, who passed away in 2017. One year after this conversation, the B-MRS UCs Program was operating with 4 active units. Before ending his speech, Faria invited the members of the UCs to reflect on the relationship of materials research with the development of Brazil and the problems of Brazilians, to find solutions to the contradiction between the country’s natural wealth and the low quality of life of a large part of its population.
The social impact of the research was also addressed in a lecture by professor Eduardo Martinelli (UFRN), scientific director of B-MRS and coordinator of the Materials area at CAPES, the federal government agency, responsible for quality in undergraduate and postgraduate institutions in Brazil. Martinelli shared the work that CAPES has been doing to create metrics that objectively reveal the impacts generated by postgraduate courses on people’s quality of life. “Society invests in us, researchers, and we need to answer how we benefit society.” This concern also appeared in the presentations of the UCs, in actions aimed at showing laypeople the presence and importance of science in everyday life.
Diversity, representativeness and multidisciplinarity
With units in all Brazilian regions, members of different ethnic groups and a good proportion of women on the boards, the UCs program is very close to the ideal of diversity and representativeness. Thus, these two concepts were intensely debated throughout the event, starting with the opening lecture by professor Mônica Cotta (UNICAMP), president of B-MRS (the first woman to occupy this position in the Society). “It is not by chance that today we have two women at the opening of the meeting, the president of the Society and the event coordinator,” stated Professor Petrus Santa Cruz (UFPE), tutor of the UC -UFPE.
In the lecture, Professor Cotta also spoke about the role of scientific societies and, in particular, about the past, present and future of B-MRS. “For me, the future is related to university chapters, because it is their members who are able to better communicate with new generations,” she said. The president showed that the history of B-MRS is linked to the highly multidisciplinary nature of materials research, which requires merging the knowledge of physicists, chemists, biologists, engineers, medical doctors and other professionals.
Belonging to a generation more accustomed to a multidisciplinary approach, the members of the chapters naturally included the subject in their presentations, which showed the efforts by students from different areas of knowledge.
The most evident result of the event was enabling each UC to get to know the other units. In the evaluation of the organizing team, the meeting allowed to reflect on the effectiveness of all projects, and the possibilities for adapting each action to the different realities of the country. “I highlight the exchange of experiences and ideas between the various UCs, which reflect the cultural plurality of Brazilian society and the various areas that compose the materials community,” says Professor Mônica Cotta, president of B-MRS, who participated in the entire event.
The I ENUC was also a generator of joint projects. “Without a shadow of a doubt, ENUC promoted in an unprecedented way the integration between the various UCs in our program, which can already be seen in practical terms with the organization of a cycle of webinars, which was a result of the event,” said Professor Newton Barbosa, national coordinator of the UCs Program, who also followed the event in its entirety.
In addition, the event provided a broader contact between the members of the program with the B-MRS board and with other people who work or have worked in the Society. “It was especially motivating to be able to observe the maturity, commitment and involvement of these students with the values we share at B-MRS and in the academic area in general, such as respect for science and scientific method, ethical values, diversity and representativeness in all aspects,” expressed the president of the Society.
Another important result of the I ENUC, according to Professor Barbosa, was to have perfected the idea of what it means to be a member of a B-MRS UC. “Being part of a chapter means, at a very young scientific age, participating in the discussions of macro problems of Materials Science and Engineering. Being able to think and propose, in a professional and respectful way, solutions to these problems. Go beyond the laboratory bench and become a professional with multiple skills,” summarized the program coordinator.
Communication and sponsorships
The event also featured a lecture by Verónica Savignano, responsible for Communications at B-MRS. The scientific journalist presented all the communication channels of B-MRS, their audiences and contents, in order to open interaction possibilities with the UCs. Speaking to a generation that is much more accustomed to dissemination (via social networks) than the previous generation, the journalist strived to generate a reflection on the criteria and guidelines (ethical, aesthetic and technical) that should guide the work of communication. During the questions, the discussion revolved around the concepts of disinformation, infodemics, pseudoscience and fake news.
Finally, in another invited lecture, Rosely Maier Queiroz, former financial director of UC-UFPE, shared a step-by-step on how to get sponsorship and manage the budget of UC projects, with specific tips for this time of pandemic and economic crisis.
Independence, Technology and Fight
“I think the date of this event is symbolic, as the independence of a country depends on technological independence,” said Professor Petrus Santa Cruz, when opening the meeting. “This year there is no Independence parade because of the pandemic, but there is a fight alert, the fight for education, for science, for the reduction of social inequalities,” said Karolyne, ending the event.
The event was recorded and is available on the UC-UFPE YouTube channel.
[Paper: Effect of the incorporation of poly(ethylene oxide) copolymer on the stability of perovskite solar cells. Jeann Carlos da Silva, Francineide Lopes de Araújo, Rodrigo Szostak, Paulo Ernesto Marchezi, Raphael Fernando Moral, Jilian Nei de Freitas and Ana Flávia Nogueira. J. Mater. Chem. C, 2020,8, 9697-9706].
Thanks to the contributions of research groups from different countries, perovskite-based solar cells have quickly become competitive in terms of energy conversion efficiency – the percentage of solar energy that is converted into electrical energy – reaching values above 25%. Unfortunately, the good efficiency achieved for these solar cells does not remain throughout their use, mainly because of the instability of their active layer. Composed of materials from the perovskite family, this layer of the sandwich-like solar cell is responsible for absorbing light. Due to moisture, as well as light itself, perovskite degrades and threatens the life cycle of a solar cell.
The problem has been the focus of many researchers in the area, among them, those from the Laboratory of Nanotechnology and Solar Energy (LNES) at Unicamp (Brazil), led by Professor Ana Flávia Nogueira. In recently reported research in the Journal of Materials Chemistry C (impact factor 7.059), LNES members were able to produce more stable perovskite films which allowed manufacturing solar cells with lower efficiency losses over time.
The strategy adopted was to add to the perovskite a compound that gives it stability without affecting its crystalline structure, from which important properties emerge for solar cell performance. The chosen additive, a copolymer (polymer formed by two different monomers), was added in different concentrations to a solution of lead iodide and methylammonium iodide, which, when crystallized, formed a modified and more stable perovskite film.
The researchers used the spin coating technique to prepare filmes of pure perovskite and “additivated” perovskite. In a material degradation test, the authors exposed the samples to ambient light and humidity for nine days and observed their degradation, which was visible to the naked eye by the yellowing of the films, whose original color is almost black. In the samples with additive, the degradation was delayed by a few days when compared to the pure perovskite samples.
Another test carried out by the team showed the films’ ability to regenerate after an initial degradation caused by exposure to a humidifier. The samples with the additive not only degraded less, but also spontaneously regenerated, almost entirely, thirty seconds after removing the moisture source – a phenomenon known as healing – as can be seen in this video.
“This work demonstrated that incorporating a copolymer based on poly(ethylene oxide) to the perovskite layer can delay and, in some cases, even reverse the degradation process of the film with relation to moisture and lighting,” summarizes Jeann Carlos da Silva, co-author of the article.
To study in detail the structure and composition of the films, the authors used a series of characterization techniques, including an X-ray diffraction technique (in situ GWAXS), available at the Brazilian National Synchrotron Light Laboratory (LNLS), which allowed to monitor the manufacturing process of the films. Based on the set of characterization results, the authors were able to explain the mechanism that generates the protective effect in perovskite films with additives. According to them, the effect occurs mainly due to the interaction performed by the copolymer, through hydrogen bonds, with the methylammonium cation of the perovskite. In films without the additive, light and moisture cause part of the methylammonium to shift into the gas state and then leave the perovskite structure, generating the degradation, which is partially irreversible. In the films with the additive, the copolymer retains the methylammonium, which generates films that are more stable and have greater regenerative capacity.
“This study also allowed to investigate the crystallization dynamics of the perovskite containing the copolymer, as well as to understand the formation mechanisms of perovskite/copolymer in humidity and lighting conditions,” highlights Francineide Lopes de Araújo, co-author of the article. “In addition, through characterization techniques such as in situ X-ray diffraction, the study explores an important area in order to understand the material, offering an important contribution to the scientific community and opening new investigation perspectives for the application of polymers in the process of forming and manufacturing perovskite solar cells,” she adds.
Finally, the scientific team manufactured solar cells using perovskite films with and without additives as active layer, and compared their energy conversion efficiency. Initially, the presence of the copolymer decreased the efficiency of the devices, since, as it is an insulating material, it impairs the transfer of electrical charges. However, in the stability tests, when the devices were exposed to humidity and light for twenty days, the perovskite cells with additives performed better.
In numbers: while pure perovskite solar cells started at 17% efficiency and maintained 47% of that value at the end of the test, perovskite devices containing 1.5 mg mL-1% copolymer had an initial efficiency of around 15 %, but retained 68% of efficiency after the 20 days of testing.
“Unfortunately, the problem of stability of perovskite solar cells could not be definitively solved through this research, however, an important way to protect the material was explored, mainly against aggressive exposure to moisture and light, which in the future can be combined with other protection mechanisms,” summarizes Jeann Carlos da Silva. “The research also reinforces the feasibility of incorporating extrinsic compounds to perovskite as protective agents,” he adds.
This study began at LNES in 2016, in the master’s research of Jeann Carlos da Silva, shortly after the development, in that same laboratory, of the first perovskite solar cell prototype in Brazil. The research was completed with the collaboration of the postdoctoral fellow Francineide Lopes de Araújo and other members and former members of the group, always under the guidance of Professor Ana Flávia.
The study was funded by Brazilian agencies FAPESP, CNPq and CAPES, and is the subject of the project “Perovskite Solar Cells for Artificial Photosynthesis” of the Center for Innovation on New Energies (CINE) with support from Shell and Fapesp.