{"id":4216,"date":"2015-05-18T06:52:53","date_gmt":"2015-05-18T06:52:53","guid":{"rendered":"http:\/\/revoscience.com\/en\/?p=4216"},"modified":"2015-05-18T06:52:53","modified_gmt":"2015-05-18T06:52:53","slug":"plugging-up-leaky-graphene","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/plugging-up-leaky-graphene\/","title":{"rendered":"Plugging up leaky graphene"},"content":{"rendered":"

New technique may enable faster, more durable water filters.<\/strong><\/em><\/span><\/p>\n

\"download<\/a>CAMBRIDGE, Mass<\/strong>\u00a0—\u00a0For faster, longer-lasting water filters, some scientists are looking to graphene \u2014thin, strong sheets of carbon \u2014 to serve as ultrathin membranes, filtering out contaminants to quickly purify high volumes of water.<\/span><\/p>\n

Graphene\u2019s unique properties make it a potentially ideal membrane for water filtration or desalination. But there\u2019s been one main drawback to its wider use: Making membranes in one-atom-thick layers of graphene is a meticulous process that can tear the thin material \u2014 creating defects through which contaminants can leak.<\/span><\/p>\n

Now engineers at MIT, Oak Ridge National Laboratory, and King Fahd University of Petroleum and Minerals (KFUPM) have devised a process to repair these leaks, filling cracks and plugging holes using a combination of chemical deposition and polymerization techniques. The team then used a\u00a0process it developed previously<\/span><\/a>\u00a0to create tiny, uniform pores in the material, small enough to allow only water to pass through.<\/span><\/p>\n

Combining these two techniques, the researchers were able to engineer a relatively large defect-free graphene membrane \u2014 about the size of a penny. The membrane\u2019s size is significant: To be exploited as a filtration membrane, graphene would have to be manufactured at a scale of centimeters, or larger.<\/span><\/p>\n

In experiments, the researchers pumped water through a graphene membrane treated with both defect-sealing and pore-producing processes, and found that water flowed through at rates comparable to current desalination membranes. The graphene was able to filter out most large-molecule contaminants, such as magnesium sulfate and dextran.<\/span><\/p>\n

Rohit Karnik, an associate professor of mechanical engineering at MIT, says the group\u2019s results, published in the journal\u00a0Nano Letters<\/em>, represent the first success in plugging graphene\u2019s leaks.<\/span><\/p>\n

\u201cWe\u2019ve been able to seal defects, at least on the lab scale, to realize molecular filtration across a macroscopic area of graphene, which has not been possible before,\u201d Karnik says. \u201cIf we have better process control, maybe in the future we don\u2019t even need defect sealing. But I think it\u2019s very unlikely that we\u2019ll ever have perfect graphene \u2014 there will always be some need to control leakages. These two [techniques] are examples which enable filtration.\u201d<\/span><\/p>\n

Sean O\u2019Hern, a former graduate research assistant at MIT, is the paper\u2019s first author. Other contributors include MIT graduate student Doojoon Jang, former graduate student Suman Bose, and Professor Jing Kong.<\/span><\/p>\n

A delicate transfer<\/strong><\/span><\/p>\n

\u201cThe current types of membranes that can produce freshwater from saltwater are fairly thick, on the order of 200 nanometers,\u201d O\u2019Hern says. \u201cThe benefit of a graphene membrane is, instead of being hundreds of nanometers thick, we\u2019re on the order of three angstroms \u2014 600 times thinner than existing membranes. This enables you to have a higher flow rate over the same area.\u201d<\/span><\/p>\n

O\u2019Hern and Karnik have been investigating graphene\u2019s potential as a filtration membrane for the past several years. In 2009, the group began fabricating membranes from graphene grown on copper \u2014 a metal that supports the growth of graphene across relatively large areas. However, copper is impermeable, requiring the group to transfer the graphene to a porous substrate following fabrication.<\/span><\/p>\n

However, O\u2019Hern noticed that this transfer process would create tears in graphene. What\u2019s more, he observed intrinsic defects created during the growth process, resulting perhaps from impurities in the original material.<\/span><\/p>\n

Plugging graphene\u2019s leaks<\/strong><\/span><\/p>\n

To plug graphene\u2019s leaks, the team came up with a technique to first tackle the smaller intrinsic defects, then the larger transfer-induced defects. For the intrinsic defects, the researchers used a process called \u201catomic layer deposition,\u201d placing the graphene membrane in a vacuum chamber, then pulsing in a hafnium-containing chemical that does not normally interact with graphene. However, if the chemical comes in contact with a small opening in graphene, it will tend to stick to that opening, attracted by the area\u2019s higher surface energy.<\/span><\/p>\n

The team applied several rounds of atomic layer deposition, finding that the deposited hafnium oxide successfully filled in graphene\u2019s nanometer-scale intrinsic defects. However, O\u2019Hern realized that using the same process to fill in much larger holes and tears \u2014 on the order of hundreds of nanometers \u2014 would require too much time.<\/span><\/p>\n

Instead, he and his colleagues came up with a second technique to fill in larger defects, using a process called \u201cinterfacial polymerization\u201d that is often employed in membrane synthesis. After they filled in graphene\u2019s intrinsic defects, the researchers submerged the membrane at the interface of two solutions: a water bath and an organic solvent that, like oil, does not mix with water.<\/span><\/p>\n

In the two solutions, the researchers dissolved two different molecules that can react to form nylon. Once O\u2019Hern placed the graphene membrane at the interface of the two solutions, he observed that nylon plugs formed only in tears and holes \u2014 regions where the two molecules could come in contact because of tears in the otherwise impermeable graphene \u2014 effectively sealing the remaining defects.<\/span><\/p>\n

Using a technique they developed last year, the researchers then etched tiny, uniform holes in graphene \u2014 small enough to let water molecules through, but not larger contaminants. In experiments, the group tested the membrane with water containing several different molecules, including salt, and found that the membrane rejected up to 90 percent of larger molecules. However, it let salt through at a faster rate than water.<\/span><\/p>\n

The preliminary tests suggest that graphene may be a viable alternative to existing filtration membranes, although Karnik says techniques to seal its defects and control its permeability will need further improvements.<\/span><\/p>\n

\u201cWater desalination and nanofiltration are big applications where, if things work out and this technology withstands the different demands of real-world tests, it would have a large impact,\u201d Karnik says. \u201cBut one could also imagine applications for fine chemical- or biological-sample processing, where these membranes could be useful. And this is the first report of a centimeter-scale graphene membrane that does any kind of molecular filtration. That\u2019s exciting.\u201d<\/span><\/p>\n

This research was supported in part by the Center for Clean Water and Clean Energy at MIT and KFUPM, the U.S. Department of Energy, and the National Science Foundation.<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

New technique may enable faster, more durable water filters. CAMBRIDGE, Mass\u00a0—\u00a0For faster, longer-lasting water filters, some scientists are looking to graphene \u2014thin, strong sheets of carbon \u2014 to serve as ultrathin membranes, filtering out contaminants to quickly purify high volumes of water. Graphene\u2019s unique properties make it a potentially ideal membrane for water filtration or […]<\/p>\n","protected":false},"author":6,"featured_media":4219,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[14],"tags":[],"class_list":["post-4216","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-innovation"],"featured_image_urls":{"full":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1-150x150.jpg",150,150,true],"medium":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"medium_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"1536x1536":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"2048x2048":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"ultp_layout_landscape_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"ultp_layout_landscape":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"ultp_layout_portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"ultp_layout_square":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"newspaper-x-single-post":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"newspaper-x-recent-post-big":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"newspaper-x-recent-post-list-image":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",95,63,false],"web-stories-poster-portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",275,183,false],"web-stories-publisher-logo":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",96,64,false],"web-stories-thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2015\/05\/download-1.jpg",150,100,false]},"author_info":{"info":["Amrita Tuladhar"]},"category_info":"Innovation<\/a>","tag_info":"Innovation","comment_count":"0","_links":{"self":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/4216","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/comments?post=4216"}],"version-history":[{"count":0,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/4216\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media\/4219"}],"wp:attachment":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media?parent=4216"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/categories?post=4216"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/tags?post=4216"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}