Microscale garbage “collectors” cleans polluted water


By Michael Berger. Copyright © Nanowerk

3D rendered Molecule (Abstract) with Clipping Path(Nanowerk Spotlight) The construction of artificial micro- and nanomotors is a high priority in the nanotechnology field owing to their great potential for diverse potential applications, ranging from targeted drug delivery, on-chip diagnostics and biosensing, or pumping of fluids at the microscale to environmental remediation.

Particular attention has been given to self-propelled chemically-powered micro/nanoscale motors, such as catalytic nanowires (read more: “”Another nanotechnology step towards ‘Fantastic Voyage'”), microtube engines (read more: “Microbots transport, assemble and deliver micro- and nanoscale objects”) or spherical Janus microparticles (read more: “Novel motor system powered by polymerization”). In new work, researchers in Germany have now reported the first example of micromotors for the active degradation of organic pollutants in solution.

The novelty of this work lies in the synergy between internal and external functionality of the micromotors. “Previously, some groups tried to demonstrate the use of catalytic nanomotors for biomedical applications – including ours – on-chip biosensors and capture of bio species,” Dr. Samuel Sánchez, Group Leader Smart Nano-Bio-Systems, Max Planck Institute for Intelligent Systems, tells Nanowerk. “However, the toxicity of the fuel employed still limits their real applications. We imagined that environmental applications might be another field to explore, where the use of hydrogen peroxide is not controversial.” In that direction, Wang’s group reported the removal of oil droplets (“‘Microsubmarines’ designed to help clean up oil spills”) from solution, not degrading them.

Now, Sánchez and his collaborators went one step beyond that and demonstrated the total removal of contaminants using micromotors. Indeed, the chemical is used for the self-propulsion and for the remediation when interacts with the outer layer of the micromachine. “We have demonstrated the ability of self-propelled micromotors to oxidize organic pollutants in aqueous solutions through a Fenton process,” explains Sánchez.

“The combination of mixing and releasing iron ions in liquids results in a rate of removal of model pollutant (rhodamine 6G) ca. 12 times higher than when the Fenton oxidation process is carried out with nonpropelling metallic iron tubes.” Reporting their results in The November 1, 2013 online edition of ACS Nano (“Self-Propelled Micromotors for Cleaning Polluted Water”), the research team from Max Planck, the Leibniz Institute for Solid State and Materials Research Dresden, and the Chemnitz University of Technology demonstrates that micromotors boost the Fenton oxidation process (read more about Fenton reactions at the bottom of this article) without applying external energy, and complete degradation of organic pollutants is achieved.

 

          Schematic process for the degradation of polluted water into inorganic products by multifunctional micromotors

Schematic process for the degradation of polluted water (rhodamine 6G as model contaminant) into inorganic products by multifunctional micromotors. The self propulsion is achieved by the catalytic inner layer (Pt), which provides the motion of the micromotors in H2O2 solutions. The remediation of polluted water is achieved by the combination of Fe2+ ions with peroxide, generating OH• radicals, which degrade organic pollutants. (Reprinted with permission from American Chemical Society)

Sánchez notes that, if desired, the micromotors can be easily recovered using a magnet once the water purification process has been completed and the excess of hydrogen peroxide can be easily decomposed to pure water and oxygen under visible light. The team fabricated their tubular bubble-propelled micromotors containing small amounts of metallic iron (from 20 to 200 nm layer thickness) as outer layer and platinum as inner layer. The mechanism of degradation is based on Fenton reactions relying on spontaneous acidic corrosion of the iron metal surface of the micromotors in the presence of hydrogen peroxide, which acts both as a reagent for the Fenton reaction and as main fuel to propel the micromotors.

Moreover, the ability of self-propelled, tubular micromotors to improve mixing results in a synergetic effect that enhances water remediation without applying external energy. This work can pave the way for the use of multifunctional micromotors for environmental applications where the use of hydrogen peroxide is not a major drawback but a co-reagent. Sánchez adds that the high efficiency of the oxidation of organic pollutants achieved by the Fe/Pt catalytic micromotors reported in this work is of importance for the design of new and faster water treatments, such as the decontamination of organic compounds in wastewaters and industrial effluents. The aim of this study was to fabricate an autonomous microscopic cleaning system that is working without external energy input in a much faster and convenient way.

The micromotors offer this ability to move the catalyst around without external actuation or addition of catalyst (iron salts) to achieve water remediation, removal of organic dyes, etc. However, as the researchers point out, this is an application especially for microscale environments. “It is, unfortunately, clear that we would not use the micromotors in a large reactor vessel to clean huge amounts of water,” says Sánchez. Nevertheless, the high efficiency of the oxidation of organic pollutants achieved by the Fe/Pt catalytic micromotors reported here is of importance for the design of new and faster water treatments, such as the decontamination of organic compounds in wastewaters and industrial effluents.

“We have proven that the usefulness of the micromotors lies not solely in their capacity to move, but to exploit their motion using their external surface to enhance useful catalytic reactions,” says Sánchez. “This work could open a new research line towards coupling a variety of catalytic reactions in self-propelled devices where the presence of hydrogen peroxide is not a disadvantage. We expect that a rich variety of contaminants can be in the next years be cleaned.”

Watch the micromotor in action. A synergetic effect is achieved taking advantage of the release of the iron ions from the outer layer of the micromotors and their active motion in the solution.

About Fenton reactions The Fenton method is one of the most popular advanced oxidation processes for the degradation of organic pollutants, utilizing the hydroxyl radical (OH•) as its main oxidizing agent. The generation of OH• in the Fenton method occurs by reaction of H2O2 in the presence of Fe(II). However, one disadvantage of these processes is that Fe ions in solution must be removed after the treatment to meet regulations for drinking water. In order to diminish and, in the best scenario, solve the problems caused by the presence of Fe ions in treated effluents and decrease the costs of recovery, the use of heterogeneous Fenton catalysts is a promising strategy that could allow for the degradation of pollutants by Fenton processes without the requirement of dissolved iron salts. The micromotors fabricated in this work can be included as a new type of heterogeneous Fenton catalyst. With this method the remaining iron in the solution is one to three orders of magnitude lower than in conventional Fenton processes.

Read more: http://www.nanowerk.com/spotlight/spotid=33493.php#ixzz2mN10wW9l

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Gold-palladium nanocatalysts set new mark for breakdown of nitrites


Ordered QDs(Nanowerk News) Chemical engineers at Rice University have found a new catalyst that can rapidly break down nitrites, a common and harmful contaminant in drinking water that often results from overuse of agricultural fertilizers.
Nitrites and their more abundant cousins, nitrates, are inorganic compounds that are often found in both groundwater and surface water. The compounds are a health hazard, and the Environmental Protection Agency places strict limits on the amount of nitrates and nitrites in drinking water. While it’s possible to remove nitrates and nitrites from water with filters and resins, the process can be prohibitively expensive.
gold and palladium nanoparticles can rapidly break down nitrites
Researchers at Rice University’s Catalysis and Nanomaterials Laboratory have found that gold and palladium nanoparticles can rapidly break down nitrites.
“This is a big problem, particularly for agricultural communities, and there aren’t really any good options for dealing with it,” said Michael Wong, professor of chemical and biomolecular engineering at Rice and the lead researcher on the new study. “Our group has studied engineered gold and palladium nanocatalysts for several years. We’ve tested these against chlorinated solvents for almost a decade, and in looking for other potential uses for these we stumbled onto some studies about palladium catalysts being used to treat nitrates and nitrites; so we decided to do a comparison.”
Catalysts are the matchmakers of the molecular world: They cause other compounds to react with one another, often by bringing them into close proximity, but the catalysts are not consumed by the reaction.
In a new paper in the journal Nanoscale (“Supporting palladium metal on gold nanoparticles improves its catalysis for nitrite reduction “), Wong’s team showed that engineered nanoparticles of gold and palladium were several times more efficient at breaking down nitrites than any previously studied catalysts. The particles, which were invented at Wong’s Catalysis and Nanomaterials Laboratory, consist of a solid gold core that’s partially covered with palladium.
Over the past decade, Wong’s team has found these gold-palladium composites have faster reaction times for breaking down chlorinated pollutants than do any other known catalysts. He said the same proved true for nitrites, for reasons that are still unknown.
“There’s no chlorine in these compounds, so the chemistry is completely different,” Wong said. “It’s not yet clear how the gold and palladium work together to boost the reaction time in nitrites and why reaction efficiency spiked when the nanoparticles had about 80 percent palladium coverage. We have several hypotheses we are testing out now. “
He said that gold-palladium nanocatalysts with the optimal formulation were about 15 times more efficient at breaking down nitrites than were pure palladium nanocatalysts, and about 7 1/2 times more efficient than catalysts made of palladium and aluminum oxide.
Wong said he can envision using the gold-palladium catalysts in a small filtration unit that could be attached to a water tap, but only if the team finds a similarly efficient catalyst for breaking down nitrates, which are even more abundant pollutants than nitrites.
“Nitrites form wherever you have nitrates, which are really the root of the problem,” Wong said. “We’re actively studying a number of candidates for degrading nitrates now, and we have some positive leads.”
Source: Rice University

Read more: http://www.nanowerk.com/nanotechnology_news/newsid=33409.php#ixzz2llzmTSeK

Researcher Realizes Water-Splitting Solar Cell Structure Using Nanoparticles


Published on June 18, 2013 at 6:47 AM

QDOTS imagesCAKXSY1K 8Due to the fluctuating availability of solar energy, storage solutions are urgently needed. One option is to use the electrical energy generated inside solar cells to split water by means of electrolysis, in the process yielding hydrogen that can be used for a storable fuel. Researchers at the HZB Institute for Solar Fuels have modified so called superstrate solar cells with their highly efficient architecture in order to obtain hydrogen from water with the help of suitable catalysts. This type of cell works something like an “artificial leaf.”

This complex solar cell is coated with two different catalysts and works like an “artificial leaf”, using sunlight to split water and yield hydrogen gas.

But the solar cell rapidly corrodes when placed in the aqueous electrolyte solution. Now, Ph.D. student Diana Stellmach has found a way to prevent corrosion by embedding the catalysts in an electrically conducting polymer and then mounting them onto the solar cell’s two contact surfaces, making her the first scientist in all of Europe to have come up with this solution. As a result, the cell’s sensitive contacts are sealed to prevent corrosion with a stable yield of approx. 3.7 percent sunlight.

Hydrogen stores chemical energy and is highly versatile in terms of its applicability potential. The gas can be converted into fuels like methane as well as methanol or it can generate electricity directly inside fuel cells. Hydrogen can be produced through the electrolytic splitting of water molecules into hydrogen and oxygen by using two electrodes that are coated with suitable catalysts and between which a minimum 1.23 volt tension is generated. The production of hydrogen only becomes interesting if solar energy can be used to produce it. Because that would solve two problems at once: On sunny days, excess electricity could yield hydrogen, which would be available for fuel or to generate electricity at a later point like at night or on days that are overcast.

New approach with complex thin film technologies

At the Helmholtz Centre Berlin for Materials and Energy (HZB) Institute for Solar Fuels, researchers are working on new approaches to realizing this goal. They are using photovoltaic structures made of multiple ultrathin layers of silicon that are custom-made by the Photovoltaic Competence Centre Berlin (PVcomB), another of the HZB’s institutes. Since the cell consists of a single – albeit complex – “block,” this is known as a monolithic approach. At the Institute for Solar Fuels, the cell’s electrical contact surfaces are coated with special catalysts for splitting water. If this cell is placed in dilute sulphuric acid and irradiated with sun-like light, a tension is produced at the contacts that can be used to split water. During this process, it is the catalysts, which speed up the reactions at the contacts, that are critically important.

Protection against corrosion

The PVcomB photovoltaic cells’ main advantage is their “superstrate architecture”: Light enters through the transparent front contact, which is deposited on the carrier glass; there is no opacity due to catalysts being mounted onto the cells, because they are located on the cell’s back side and are in contact with the water/acid mixture. This mixture is aggressive, that is to say, it is corrosive, so much so that Diana Stellmach had to first replace the usual zinc oxide silver back contact with a titanium coat approximately 400 nanometers thick. In a second step, she developed a solution to simultaneously protect the cell against corrosion with the mounting of the catalyst: She mixed nanoparticles of RuO2 with a conducting polymer (PEDOT:PSS) and applied this mixture to the cell’s back side contact to act as a catalyst for the production of oxygen. Similarly, platinum nanoparticles, the sites of hydrogen production, were applied to the front contact.

Stable H2-Production

In all, the configuration achieved a degree of efficacy of 3.7 percent and was stable over a minimum 18 hours. “This way, Ms. Stellmach is the first ever scientist anywhere in Europe to have realized this kind of water-splitting solar cell structure,” explains Prof. Dr. Sebastian Fiechter. And just maybe anywhere in the World, as photovoltaic membranes with different architectures have proved far less stable.

Yet the fact remains that catalysts like platinum and RuO2 are rather expensive and will ultimately have to give way to less costly types of materials. Diana Stellmach is already working on that as well; she is currently in the process of developing carbon nanorods that are coated with layers of molybdenum sulphide and which serve as catalysts for hydrogen production.

Watch the “artificial leaf” in action: http://www.helmholtz-berlin.de/aktuell/pr/mediathek/video/energieversorgung/superstratzelle_de.html

Source: http://www.helmholtz-berlin.de/

Researchers Determine Critical Factors for Improving Performance of a Solar Fuel Catalyst


October 3, 2012

Contact: Veronika Szalai

False-color SEMs of cross-sectioned hematite films. 
False-color scanning electron micrographs of cross-sectioned hematite films grown by sputter deposition and then annealed at two different temperatures.  The physical structure and the tin dopant atom distributions in the hematite films differ depending on the annealing temperature.  Hematite annealed at higher temperatures has better catalytic performance for splitting water.

Hydrogen gas that is created using solar energy to split water into hydrogen and oxygen has the potential to be a cost-effective fuel source if the efficiency of the catalysts used in the water-splitting process can be improved. By controlling the placement of key additives (dopant atoms) in an iron oxide catalyst, researchers from the NIST Center for Nanoscale Science and Technology have found that the final location of the dopants and the temperature at which they are incorporated into the catalyst crystal lattice determine overall catalytic performance in splitting water.* The iron oxide hematite is a promising catalyst for water splitting because it is stable in water and absorbs a large portion of the solar spectrum. It is also abundant in the earth’s crust, making it inexpensive. Unfortunately, pure hematite has only modest catalytic activity, falling well short of its predicted theoretical maximum efficiency. Incorporating dopants such as tin atoms into hematite’s lattice improves performance, but it is a challenge to accurately measure the dopant concentration, making it difficult to understand and optimize their effects on catalyst performance. Using thin films of hematite doped with tin, the researchers produced highly active samples that enabled them to measure and characterize the spatial distribution of dopants in the material and their role in catalysis. The researchers determined that as a result of the sample preparation protocol they followed, a dopant gradient extends from the interface with the dopant source to the catalyst surface, where the measured concentration is low compared with previous estimates from similarly prepared samples. Contrary to prior results, they found that only a small dopant concentration is needed to improve catalytic activity. The researchers believe this study creates a path for improving the rational design of inexpensive catalysts for splitting water using solar energy.

*Effect of tin doping on α-Fe2O3 photoanodes for water splitting, C. D. Bohn, A. K. Agrawal, E. C. Walter, M. D. Vaudin, A. A. Herzing, P. M. Haney, A. A. Talin, and V. A. Szalai, The Journal of Physical Chemistry C 116, 15290–15296 (2012).
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