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A team of scientists from Brazilian institutions has increased by about 30 times the capacity of a semiconductor material to produce hydrogen by means of water photolysis, a process that consists of dividing the water molecule using light as the only source of energy. The advance contributes to the development of efficient ways to generate green hydrogen, which is the fuel produced using renewable and clean energy.
For photolysis to take place, it is necessary to have photocatalysts suspended in water. A photocatalyst is a semiconductor capable of absorbing light and, from there, generating the charges (electrons and holes) that are necessary to dissociate water molecules (H2O) into hydrogen (H2) and oxygen (O2) though oxidation and reduction reactions. Furthermore, the material must be stable in an aqueous environment.
“Strontium titanate (SrTiO3) is one of the main semiconductor materials applied to photolysis for the production of green hydrogen, as it meets the physicochemical requirements for oxidizing and reducing the water molecule,” says Professor Renato Vitalino Gonçalves (IFSC-USP) , corresponding author of the article that reports this research in ACS Applied Energy Materials. “However, this material has some intrinsic characteristics that limit its photocatalytic potential, such as, for example, its wide bandgap of ~3.2 eV, which restricts its optical absorption to the UV region, which corresponds to only 4% of the solar spectrum”, completes the scientist. Another limitation of this material, common to all semiconductors, is the rapid recombination of electrons and holes, which prevents these charges from flowing freely and promoting oxidation and reduction reactions.
Thus, the Brazilian team, led by Professor Gonçalves, decided to modify strontium titanate to increase its efficiency in photolysis. Initially, the researchers doped the semiconductor with the transition metal molybdenum (Mo) and obtained disaggregated cubic particles with well-defined faces. The unconventional dopant was responsible for making the material capable of absorbing light in the visible region, which represents around 43% of the solar spectrum.
In a second moment, the authors of the work deposited nickel nanoparticles of around 2 nm on the surface of the particles. The result was a junction of two types of semiconductors: Mo:SrTiO3, n-type, and NiO@Ni(OH)2, p-type. “In this new configuration, the photogenerated holes are directed to the NiO@Ni(OH)2 structure, while the electrons migrate to the Mo:SrTiO3 surface, resulting in better charge separation and, consequently, a reduction in the recombination rate of electrons and holes”, explains Gonçalves.
The photocatalysts were placed in suspension in an aqueous solution with 20% methanol as a sacrificial agent – a widely used strategy to increase hydrogen production and also generate high-value by-products for the chemical industry. “When mixed with water, which oxidation is slow, this alcohol is preferentially oxidized”, says Professor Gonçalves. “Even though, the H2 is produced from the reduction of the water molecule and not as a by-product of methanol oxidation”, he adds.
By increasing the absorption of light and decreasing the loss of photogenerated charges, the enhanced material presented an excellent result in the production of hydrogen by photolysis: an increase of its photocatalytic activity of about 30 times compared with the pure semiconductor.
Brazilian scientific cooperation
This scientific work was led by Professor Renato Vitalino Gonçalves, who coordinates the Nanomaterials and Advanced Ceramics Group (NaCA) and the Artificial Photosynthesis and Nanomaterials Laboratory (LAPNano) at IFSC-USP. The synthesis of materials and the study of their structural, optical and electronic properties, as well as their photocatalytic performance for the production of green hydrogen were developed at IFSC-USP, within the doctoral research of Higor Andrade Centurion, supervised by Professor Gonçalves.
The identification and characterization of the nickel nanoparticles in the material was carried out in collaboration with a team from UFABC and LNNano-CNPEM, formed by Professor Flávio Leandro de Souza, postdoctoral student Ingrid Rodriguez-Gutierrez and researcher Jefferson Bettini. In collaboration with Professor Liane M. Rossi (IQ-USP), nickel was quantified using the flame atomic absorption spectroscopy technique.
In addition, with the collaboration of Professor Heberton Wender (UFMS) it was possible to carry out photoluminescence measurements that corroborated the suppression of recombination of charges photogenerated by the formation of the p – n junction.
Finally, computer simulations that made it possible to understand the behavior of the materials were carried out with Professor Matheus M. Ferrer, from UFPel, and Master’s student Lucas Gabriel Rabelo, from IFSC-USP, who also received guidance from Professor Gonçalves.
The work was funded mainly by the São Paulo research foundation (FAPESP) and, through the RCGI, by FAPESP/Shell. It also had financial support from the research foundation of Rio Grande do Sul (FAPERGS).
Paper reference: Constructing Particulate p−n Heterojunction Mo:SrTiO3/NiO@Ni(OH)2 for Enhanced H2 Evolution under Simulated Solar Light. Higor A. Centurion, Lucas G. Rabelo, Ingrid Rodriguez-Gutierrez, Mateus M. Ferrer, Jefferson Bettini, Heberton Wender, Liane M. Rossi, Flavio L. Souza, and Renato V. Gonçalves. ACS Appl. Energy Mater. 2022, 5, 12727−12738. https://doi.org/10.1021/acsaem.2c02337.
Corresponding author contact: rgoncalves@ifsc.usp.br.
Brazilian Materials Research Society (B-MRS) declares its total rejection of the serious aggression against democracy and the rule of law in our country, which took place yesterday in Brasília, with the invasion by terrorists of the headquarters of the three powers of the Republic.
B-MRS stands in solidarity with the Brazilian executive,, judiciary and legislative bodies in defense of the Federal Constitution, its values and principles.
It is essential to verify the facts and punish those responsible for terrorist acts, without amnesty, and with ‘Democracy forever’!
B-MRS Executive Board
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In 2022, we returned to the ‘new normal’ with in-person activities interspersed with virtual ones and masks disappearing from our daily lives thanks to the vaccination of most of the population against Covid-19.
That is why we were able to meet again at our annual event in Foz do Iguaçu last September. The strength and resilience of our community were present at all times. Despite the lack of funding and difficult working conditions we had almost 1,200 participants and 22 partner companies present in Foz do Iguaçu!
We hope that the celebratory cry of students at our traditional conference party can always be fulfilled: “Online event never again!”. In 2023, we want science to be heard and respected, with the changes to come in the country. However, as a scientific society, we will continue to fight and act together with our peers and the community in defense of education as a pillar for a fairer society, and of science and technological development as tools to achieve decent living conditions for the entire population.
An excellent end of the year to all! May we recharge our energies to overcome the many challenges ahead, hoping for a better country and a better world.
B-MRS Executive Board
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Researchers from the Brazilian Federal University of Mato Grosso do Sul (UFMS) have developed a photosensitive nanomaterial that is very efficient to produce electricity from organic compounds used as fuel and sunlight as the energy source. The process, which is relatively clean and inexpensive, is carried out in a device called a photo fuel cell. In the work, the team used methanol (CH3OH) as fuel, a liquid alcohol that stores a large amount of energy and, when used, generates much less carbon emissions than fossil fuels. The compound has gained attention as a fuel for clean energy generation, mainly because it can be produced from biomass.
“The photo fuel cell developed in our work contains a simple technology capable of converting methanol into energy using only solar energy as an external driving force, operating with simple, stable, abundant materials free of noble metals, which makes the cost of the process considerably low compared to conventional fuel cells”, says Professor Heberton Wender, a corresponding author of the article that reports this advance in the journal ACS Advanced Materials and Interfaces.
Fuel cells are devices that directly convert the chemical energy of a fuel into electrical energy through electrochemical reactions, with low or zero carbon emissions. Used for decades to supply energy in satellites and spacecraft, fuel cells are already present in homes, businesses, industries and electric cars, and are becoming increasingly relevant in the face of the need to generate energy in the cleanest way possible to mitigate climate change. However, the materials needed to efficiently catalyze electrochemical reactions in fuel cell electrodes are generally based on expensive and scarce elements such as noble metals. Therefore, several alternatives are being investigated; among them, the development of fuel photocells.
In these devices, inexpensive photosensitive materials based on abundant elements help drive reactions through the electrons and holes they generate when excited by sunlight. One such material is titanium dioxide (TiO2). This compound, which is usually in the form of a white powder, is not easily degraded by light and is simple to prepare. However, it has an important limitation: it only absorbs ultraviolet radiation, failing to take advantage of other wavelengths that are also present in sunlight, such as the so-called visible light.
In this context, the initial idea was born for the work of the UFMS team, which was developed within Luiz Felipe Plaça’s doctorate under the guidance of Prof. Heberton. “We thought of using self-doped titanium dioxide, that is, with self-induced structural defects, using a simple, inexpensive process that can be easily scaled up in the future”, says the researcher. “That was when we decided to use heat treatment in a reducing atmosphere with small amounts of sodium borohydride (NaBH4)”, he details. The idea generated great results. The treatment made it possible to control the density of defects in the titanium dioxide nanoparticles and, in this way, increase their ability to absorb radiation, including part of the visible spectrum of sunlight. Furthermore, the material lost its characteristic white color and turned black.
The black titanium dioxide was placed on a transparent conductive glass substrate and used as the photoanode of the fuel cell. The photoanode is the component responsible for absorbing sunlight and transforming it into electrons and holes that will reduce oxygen and oxidize fuel, respectively, generating the desired electric current at the end of the process. With the black titanium dioxide photoanodes, the efficiency of the fuel cell showed a very considerable increase in its ability to produce electric current from methanol and solar energy. “The improved device, without the use of noble metals, showed a 2,000% increase in maximum output power”, says Professor Heberton. “This represents an impressive efficiency and puts self-doped titanium dioxide on the list of the most promising materials to be used as photoanodes in photocells fueled by methanol or alternative fuels such as ethanol, glycerol, other alcohols and even organic pollutants with higher molar mass”.
As it can be supplied with organic pollutants – a possibility that was explored in other work by the team, the photocell could be used to decontaminate water without additional energy costs and even generate a little extra electricity for external use in low-power devices. “In a hypothetical scenario, it would be possible to purify water from effluents in rural properties while producing energy”, points out Professor Cauê Alves Martins, who is also a corresponding author of the ACS Applied Materials and Interfaces article.
In addition to the laboratory photocell, the authors prepared, with the new photoanode, a prototype of a small portable device: a microfluidic photo fuel cell. The device, which fits in the palm of a hand, can be produced in less than an hour at a cost of less than $2.00. To develop the prototype, the team had the participation of an undergraduate student in Physics Engineering at UFMS, Pedro Lucas S. Vital, who, guided by Prof. Cauê, accepted the challenge of preparing the cell using a 3D printer. The device was also tested, with good results. Despite being a good prototype, the engineering of the device can yet be improved to increase the power density via scale out, with more devices operating together, comment Heberton and Cauê.
The work is the result of a well-established collaboration between two research groups at the UFMS Institute of Physics: Nano&Photon, coordinated by Professor Heberton Wender, and the Electrochemistry Research Group, led by Professor Cauê Alves Martins. Researchers from the Institute of Chemistry of São Carlos (IQSC-USP) also participated in the work.
Paper reference: Black TiO2 Photoanodes for Direct Methanol Photo Fuel Cells. Luiz Felipe Plaça, Pedro-Lucas S. Vital, Luiz Eduardo Gomes, Antonio Carlos Roveda Jr., Daniel Rodrigues Cardoso, Cauê Alves Martins, and Heberton Wender. ACS Applied Materials & Interfaces. DOI: 10.1021/acsami.2c04802.
Corresponding authors contact: heberton.wender@ufms.br, caue.martins@ufms.br.
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The call for symposium proposals to compose the XXI B-MRS Meeting is open until December 3rd this year. The event will be held from October 1st to 5th, 2023 in Maceió (Alagoas, Brazil), chaired by Professors Carlos Jacinto da Silva and Mario Roberto Meneghetti, from the Federal University of Alagoas (UFAL).
Proposals can be submitted by teams of researchers (Ph.D.), preferably of international composition, wishing to organize a symposium within the event on a research topic in the field of Materials Science and Technology – from the design, synthesis and characterization of materials to their applications in the most diverse segments. The list of approved final symposia will be published on December 16 this year.
To submit a symposium proposal, simply fill out, in English, the online form available at http://sbpmat.org.br/proposed_symposium/ .
In addition, the event’s organizing committee invites the community to send suggestions of plenary speakers (internationally renowned scientists who can give a motivational lecture on advances made over time in a particular research topic, as well as the challenges and perspectives for the future). The plenary lectures should interest a wide audience, with different levels of training and thematic specialties.
Suggestions for the plenary lectures must be submitted by November 20th of this year through this Google form.
XXI B-MRS Meeting website: https://www.sbpmat.org.br/21encontro/ .