2D ma­te­ri­als in de­vices sep­a­rate salts in sea­wa­ter

Iran Daily - - Science & Technology -

lim­ited in size by the in­trin­sic rough­ness of a ma­te­rial’s sur­face, which is usu­ally at least ten times big­ger than the hy­drated di­am­e­ter of small ions.

Ear­lier this year, graphene-ox­ide based mem­branes de­vel­oped at the NGI at­tracted con­sid­er­able at­ten­tion as promis­ing can­di­dates for new fil­tra­tion tech­nolo­gies.

This re­search — uti­liz­ing the new tool­kit of 2D ma­te­ri­als — demon­strates the real-world po­ten­tial of pro­vid­ing clean drink­ing water from salt water.

To bet­ter un­der­stand the fun­da­men­tal mech­a­nisms be­hind ion trans­port, a team led by Sir An­dre Geim of The Univer­sity of Manch­ester made atom­i­cally flat slits mea­sur­ing just sev­eral angstroms in size.

Th­ese chan­nels are chem­i­cally in­ert with smooth walls on the angstrom scale.

The re­searchers made their slit de­vices from two 100-nm thick crys­tal slabs of graphite mea­sur­ing sev­eral mi­crons across that they ob­tained by shav­ing off bulk graphite crys­tals.

They then placed rec­tan­gu­lar-shaped pieces of 2D atomic crys­tals of bi­layer graphene and mono­layer MOS2 at each edge of one of the graphite crys­tal slabs be­fore plac­ing another slab on top of the first.

This pro­duces a gap be­tween the slabs that has a height equal to the spac­ers’ thick­ness.

Geim ex­plained, “It’s like tak­ing a book, plac­ing two match­sticks on each of its edges and then putting another book on top.

“This cre­ates a gap be­tween the books’ sur­faces with the height of the gap be­ing equal to the matches’ thick­ness. In our case, the books are the atom­i­cally flat graphite crys­tals and the match­sticks are the graphene, or MOS2 mono­lay­ers.”

The as­sem­bly is held to­gether by van der Waals forces and the slits are roughly the same size as the di­am­e­ter of aqua­por­ins, which are vi­tal for liv­ing or­gan­isms.

The slits are the small­est size pos­si­ble since slits with thin­ner spac­ers are un­sta­ble and col­lapse be­cause of at­trac­tion be­tween op­po­site walls.

Ions flow through the slits if a volt­age is ap­plied across them when they are im­mersed in an ionic so­lu­tion, and this ion flow con­sti­tutes an elec­tric cur­rent.

The team mea­sured the ionic con­duc­tiv­ity as they passed through chlo­ride so­lu­tions via the slits and found that ions could move through them as ex­pected un­der an ap­plied elec­tric field.

Dr. Gopi Kalon, a post­doc­toral re­searcher who led the ex­per­i­men­tal ef­fort, said, “When we looked more care­fully, we found that big­ger ions moved through more slowly than smaller ones like potas­sium chlo­ride.”

Dr. Ali Es­fan­diar, who is the first au­thor of the pa­per, added, “The clas­si­cal view­point is that ions with a di­am­e­ter larger than the slit size can­not per­me­ate, but our re­sults show that this ex­pla­na­tion is too sim­plis­tic.

“Ions in fact be­have like soft ten­nis balls rather than hard bil­liard ones, and large ions can still pass — ei­ther by dis­tort­ing their water shells or maybe shed­ding them al­to­gether.

The new re­search as pub­lished in Science, showed that th­ese newly ob­served mech­a­nisms plays a key role for de­sali­na­tion us­ing the size ex­clu­sion and is a key step to cre­at­ing high-flux water de­sali­na­tion mem­branes.

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