liquid phase exfoliation

File:Early TEM image of LPE graphene Dublin.tifLiquid phase exfoliation was first described in detail in a paper by a research team in Ireland in 2008,{{cite journal |last1=Hernandez |first1=Yenny |last2=Nicolosi |first2=Valeria |last3=Lotya |first3=Mustafa |last4=Blighe |first4=Fiona M. |last5=Sun |first5=Zhenyu |last6=De |first6=Sukanta |last7=McGovern |first7=I. T. |last8=Holland |first8=Brendan |last9=Byrne |first9=Michele |last10=Gun'Ko |first10=Yurii K. |last11=Boland |first11=John J. |last12=Niraj |first12=Peter |last13=Duesberg |first13=Georg |last14=Krishnamurthy |first14=Satheesh |last15=Goodhue |first15=Robbie |last16=Hutchison |first16=John |last17=Scardaci |first17=Vittorio |last18=Ferrari |first18=Andrea C. |last19=Coleman |first19=Jonathan N. |title=High-yield production of graphene by liquid-phase exfoliation of graphite |journal=Nature Nanotechnology |date=September 2008 |volume=3 |issue=9 |pages=563–568 |doi=10.1038/nnano.2008.215|pmid=18772919 |arxiv=0805.2850 |bibcode=2008NatNa...3..563H |s2cid=205443620 }} although a very short description of a similar process was published by the Manchester group around the same time.{{cite journal |last1=Blake |first1=Peter |last2=Brimicombe |first2=Paul D. |last3=Nair |first3=Rahul R. |last4=Booth |first4=Tim J. |last5=Jiang |first5=Da |last6=Schedin |first6=Fred |last7=Ponomarenko |first7=Leonid A. |last8=Morozov |first8=Sergey V. |last9=Gleeson |first9=Helen F. |last10=Hill |first10=Ernie W. |last11=Geim |first11=Andre K. |last12=Novoselov |first12=Kostya S. |title=Graphene-Based Liquid Crystal Device |journal=Nano Letters |date=1 June 2008 |volume=8 |issue=6 |pages=1704–1708 |doi=10.1021/nl080649i|pmid=18444691 |arxiv=0803.3031 |bibcode=2008NanoL...8.1704B |s2cid=14620203 }} While other papers had previously described methods to exfoliate layered crystals in liquids,{{cite journal |last1=Nicolosi |first1=Valeria |last2=Chhowalla |first2=Manish |last3=Kanatzidis |first3=Mercouri G. |last4=Strano |first4=Michael S. |last5=Coleman |first5=Jonathan N. |title=Liquid Exfoliation of Layered Materials |journal=Science |date=21 June 2013 |volume=340 |issue=6139 |pages=1226419 |doi=10.1126/science.1226419|hdl=2262/69769 |s2cid=177513486 |hdl-access=free }} these papers were the first to describe exfoliation in liquids without any previous ion intercalation or chemical treatment.

LPE involves inserting layered crystals into appropriate stabilizing liquids and then adding energy to remove nanosheets from the layered crystals. A number of different methods have been used to supply energy to the liquid. The earliest and most common is ultrasonication.{{cite journal |last1=Hernandez |first1=Yenny |last2=Nicolosi |first2=Valeria |last3=Lotya |first3=Mustafa |last4=Blighe |first4=Fiona M. |last5=Sun |first5=Zhenyu |last6=De |first6=Sukanta |last7=McGovern |first7=I. T. |last8=Holland |first8=Brendan |last9=Byrne |first9=Michele |last10=Gun'Ko |first10=Yurii K. |last11=Boland |first11=John J. |last12=Niraj |first12=Peter |last13=Duesberg |first13=Georg |last14=Krishnamurthy |first14=Satheesh |last15=Goodhue |first15=Robbie |last16=Hutchison |first16=John |last17=Scardaci |first17=Vittorio |last18=Ferrari |first18=Andrea C. |last19=Coleman |first19=Jonathan N. |title=High-yield production of graphene by liquid-phase exfoliation of graphite |journal=Nature Nanotechnology |date=September 2008 |volume=3 |issue=9 |pages=563–568 |doi=10.1038/nnano.2008.215|pmid=18772919 |arxiv=0805.2850 |bibcode=2008NatNa...3..563H |s2cid=205443620 }} In order to scaleup the process, high shear mixing was introduced in 2014.{{cite journal |last1=Paton |first1=Keith R. |last2=Varrla |first2=Eswaraiah |last3=Backes |first3=Claudia |last4=Smith |first4=Ronan J. |last5=Khan |first5=Umar |last6=O’Neill |first6=Arlene |last7=Boland |first7=Conor |last8=Lotya |first8=Mustafa |last9=Istrate |first9=Oana M. |last10=King |first10=Paul |last11=Higgins |first11=Tom |last12=Barwich |first12=Sebastian |last13=May |first13=Peter |last14=Puczkarski |first14=Pawel |last15=Ahmed |first15=Iftikhar |last16=Moebius |first16=Matthias |last17=Pettersson |first17=Henrik |last18=Long |first18=Edmund |last19=Coelho |first19=João |last20=O’Brien |first20=Sean E. |last21=McGuire |first21=Eva K. |last22=Sanchez |first22=Beatriz Mendoza |last23=Duesberg |first23=Georg S. |last24=McEvoy |first24=Niall |last25=Pennycook |first25=Timothy J. |last26=Downing |first26=Clive |last27=Crossley |first27=Alison |last28=Nicolosi |first28=Valeria |last29=Coleman |first29=Jonathan N. |title=Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids |journal=Nature Materials |date=June 2014 |volume=13 |issue=6 |pages=624–630 |doi=10.1038/nmat3944|pmid=24747780 |bibcode=2014NatMa..13..624P |s2cid=43256835 |url=http://sro.sussex.ac.uk/id/eprint/84627/1/__smbhome.uscs.susx.ac.uk_akj23_Documents_Scalable%20production%20of%20large%20quantities.pdf }} This method proved extremely useful and inspired a number of other methods of generating shear in the suspension, including wet ball milling, homogenization, microfluidization and wet jet milling.{{cite journal |last1=Del Rio Castillo |first1=A. E. |last2=Pellegrini |first2=V. |last3=Ansaldo |first3=A. |last4=Ricciardella |first4=F. |last5=Sun |first5=H. |last6=Marasco |first6=L. |last7=Buha |first7=J. |last8=Dang |first8=Z. |last9=Gagliani |first9=L. |last10=Lago |first10=E. |last11=Curreli |first11=N. |last12=Gentiluomo |first12=S. |last13=Palazon |first13=F. |last14=Prato |first14=M. |last15=Oropesa-Nuñez |first15=R. |last16=Toth |first16=P. S. |last17=Mantero |first17=E. |last18=Crugliano |first18=M. |last19=Gamucci |first19=A. |last20=Tomadin |first20=A. |last21=Polini |first21=M. |last22=Bonaccorso |first22=F. |title=High-yield production of 2D crystals by wet-jet milling |journal=Materials Horizons |date=2018 |volume=5 |issue=5 |pages=890–904 |doi=10.1039/c8mh00487k|arxiv=1804.10688 |s2cid=96459022 }}

The simplest stabilizing liquids are solvents with surface energy close to the layered crystal being exfoliated. In practice, liquids with surface tensions close to 70 mJ/m² are used.{{cite journal |last1=Hernandez |first1=Yenny |last2=Lotya |first2=Mustafa |last3=Rickard |first3=David |last4=Bergin |first4=Shane D. |last5=Coleman |first5=Jonathan N. |title=Measurement of Multicomponent Solubility Parameters for Graphene Facilitates Solvent Discovery |journal=Langmuir |date=2 March 2010 |volume=26 |issue=5 |pages=3208–3213 |doi=10.1021/la903188a|pmid=19883090 }} In addition aqueous surfactant solutions are often used.{{cite journal |last1=Lotya |first1=Mustafa |last2=Hernandez |first2=Yenny |last3=King |first3=Paul J. |last4=Smith |first4=Ronan J. |last5=Nicolosi |first5=Valeria |last6=Karlsson |first6=Lisa S. |last7=Blighe |first7=Fiona M. |last8=De |first8=Sukanta |last9=Wang |first9=Zhiming |last10=McGovern |first10=I. T. |last11=Duesberg |first11=Georg S. |last12=Coleman |first12=Jonathan N. |title=Liquid Phase Production of Graphene by Exfoliation of Graphite in Surfactant/Water Solutions |journal=Journal of the American Chemical Society |date=18 March 2009 |volume=131 |issue=10 |pages=3611–3620 |doi=10.1021/ja807449u|pmid=19227978 |arxiv=0809.2690 |s2cid=16624132 }} Less common, but useful for certain applications, is using molecular or polymeric additives to stabilise the exfoliated nanosheets.{{cite journal |last1=Ciesielski |first1=Artur |last2=Samorì |first2=Paolo |title=Supramolecular Approaches to Graphene: From Self-Assembly to Molecule-Assisted Liquid-Phase Exfoliation |journal=Advanced Materials |date=August 2016 |volume=28 |issue=29 |pages=6030–6051 |doi=10.1002/adma.201505371|pmid=26928750 |bibcode=2016AdM....28.6030C |s2cid=205266229 |url=https://hal.archives-ouvertes.fr/hal-03631849/file/islandora_148772.pdf }}{{cite journal |last1=Ciesielski |first1=Artur |last2=Samorì |first2=Paolo |title=Grapheneviasonication assisted liquid-phase exfoliation |journal=Chem. Soc. Rev. |date=2014 |volume=43 |issue=1 |pages=381–398 |doi=10.1039/c3cs60217f|pmid=24002478 }}{{cite journal |last1=May |first1=Peter |last2=Khan |first2=Umar |last3=Hughes |first3=J. Marguerite |last4=Coleman |first4=Jonathan N. |title=Role of Solubility Parameters in Understanding the Steric Stabilization of Exfoliated Two-Dimensional Nanosheets by Adsorbed Polymers |journal=The Journal of Physical Chemistry C |date=24 May 2012 |volume=116 |issue=20 |pages=11393–11400 |doi=10.1021/jp302365w}}

A very wide range of 2D materials have been produced by LPE. The first material to be exfoliated was graphene in 2008. This was followed in 2011 by the exfoliation of BN, MoS2 and WS2.{{cite journal |last1=Coleman |first1=Jonathan N. |last2=Lotya |first2=Mustafa |last3=O’Neill |first3=Arlene |last4=Bergin |first4=Shane D. |last5=King |first5=Paul J. |last6=Khan |first6=Umar |last7=Young |first7=Karen |last8=Gaucher |first8=Alexandre |last9=De |first9=Sukanta |last10=Smith |first10=Ronan J. |last11=Shvets |first11=Igor V. |last12=Arora |first12=Sunil K. |last13=Stanton |first13=George |last14=Kim |first14=Hye-Young |last15=Lee |first15=Kangho |last16=Kim |first16=Gyu Tae |last17=Duesberg |first17=Georg S. |last18=Hallam |first18=Toby |last19=Boland |first19=John J. |last20=Wang |first20=Jing Jing |last21=Donegan |first21=John F. |last22=Grunlan |first22=Jaime C. |last23=Moriarty |first23=Gregory |last24=Shmeliov |first24=Aleksey |last25=Nicholls |first25=Rebecca J. |last26=Perkins |first26=James M. |last27=Grieveson |first27=Eleanor M. |last28=Theuwissen |first28=Koenraad |last29=McComb |first29=David W. |last30=Nellist |first30=Peter D. |last31=Nicolosi |first31=Valeria |title=Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials |journal=Science |date=4 February 2011 |volume=331 |issue=6017 |pages=568–571 |doi=10.1126/science.1194975|pmid=21292974 |bibcode=2011Sci...331..568C |hdl=2262/66458 |s2cid=23576676 |hdl-access=free }} Since, the a wide range of 2D materials have been exfoliated including molybdenum diselenide, tungsten diselenide, gallium sulphide, molybdemum trioxide, nickel(II) hydroxide, germanium monosulfide, SnP3, black phosphorus etc.{{cite journal |last1=Hu |first1=Chen-Xia |last2=Shin |first2=Yuyoung |last3=Read |first3=Oliver |last4=Casiraghi |first4=Cinzia |title=Dispersant-assisted liquid-phase exfoliation of 2D materials beyond graphene |journal=Nanoscale |date=2021 |volume=13 |issue=2 |pages=460–484 |doi=10.1039/d0nr05514j|pmid=33404043 |s2cid=230784148 }}

Recent work has shown that liquid phase exfoliation can be used to produce 2D-nanoplatelets from non-layered 3D-strongly bonded bulk materials.{{cite journal |last1=Kaur |first1=Harneet |last2=Coleman |first2=Jonathan N. |title=Liquid-Phase Exfoliation of Nonlayered Non-Van-Der-Waals Crystals into Nanoplatelets |journal=Advanced Materials |date=September 2022 |volume=34 |issue=35 |pages=2202164 |doi=10.1002/adma.202202164|pmid=35470487 |bibcode=2022AdM....3402164K |hdl=2262/101345 |s2cid=248390135 |hdl-access=free }} This is intuitively unexpected as these 3D-solid bulk crystals consists of strong bonds in all the three-directions. Nevertheless, many non-layered materials such as boron, silicon, germanium, iron disulfide, iron oxide, iron trifluoride, manganese telluride, have been converted to 2D nanoplatelets when sonicated in appropriate solvents.{{cite journal |last1=Kaur |first1=Harneet |last2=Coleman |first2=Jonathan N. |title=Liquid-Phase Exfoliation of Nonlayered Non-Van-Der-Waals Crystals into Nanoplatelets |journal=Advanced Materials |date=September 2022 |volume=34 |issue=35 |pages=2202164 |doi=10.1002/adma.202202164|pmid=35470487 |bibcode=2022AdM....3402164K |hdl=2262/101345 |s2cid=248390135 |hdl-access=free }} This raises many open questions on the mechanism of liquid-phase exfoliation process.{{cite journal |last1=Li |first1=Zheling |last2=Young |first2=Robert J. |last3=Backes |first3=Claudia |last4=Zhao |first4=Wen |last5=Zhang |first5=Xun |last6=Zhukov |first6=Alexander A. |last7=Tillotson |first7=Evan |last8=Conlan |first8=Aidan P. |last9=Ding |first9=Feng |last10=Haigh |first10=Sarah J. |last11=Novoselov |first11=Kostya S. |last12=Coleman |first12=Jonathan N. |title=Mechanisms of Liquid-Phase Exfoliation for the Production of Graphene |journal=ACS Nano |date=22 September 2020 |volume=14 |issue=9 |pages=10976–10985 |doi=10.1021/acsnano.0c03916|pmid=32598132 |hdl=2262/93628 |s2cid=220269811 |hdl-access=free }} For layered materials, the energy required to break inter-plane (perdominately van der Waals) bonds forces is small compared to that required to break in-plane ionic or covalent bonds. Then, the exfoliation procedure results in the formation of 2D-nanosheets.{{cite journal |last1=Backes |first1=Claudia |last2=Campi |first2=Davide |last3=Szydlowska |first3=Beata M. |last4=Synnatschke |first4=Kevin |last5=Ojala |first5=Ezgi |last6=Rashvand |first6=Farnia |last7=Harvey |first7=Andrew |last8=Griffin |first8=Aideen |last9=Sofer |first9=Zdenek |last10=Marzari |first10=Nicola |last11=Coleman |first11=Jonathan N. |last12=O’Regan |first12=David D. |title=Equipartition of Energy Defines the Size–Thickness Relationship in Liquid-Exfoliated Nanosheets |journal=ACS Nano |date=25 June 2019 |volume=13 |issue=6 |pages=7050–7061 |doi=10.1021/acsnano.9b02234|pmid=31199123 |arxiv=2006.14909 |s2cid=189813507 }} However, for non-layered 3D-strongly bonded materials, with minimal difference in bonding between different atomic planes, there is no "easily exfoliated" direction and sonication should yield quasi spherical particles.{{cite journal |last1=Kaur |first1=Harneet |last2=Coleman |first2=Jonathan N. |title=Liquid-Phase Exfoliation of Nonlayered Non-Van-Der-Waals Crystals into Nanoplatelets |journal=Advanced Materials |date=September 2022 |volume=34 |issue=35 |pages=2202164 |doi=10.1002/adma.202202164|pmid=35470487 |bibcode=2022AdM....3402164K |hdl=2262/101345 |s2cid=248390135 |hdl-access=free }} Nevertheless, near isotropic materials such as silicon have been exfoliated to give high-aspect ratio platelets.{{cite journal |last1=Wang |first1=Mengxia |last2=Zhang |first2=Fang |last3=Wang |first3=Zhengping |last4=Xu |first4=Xinguang |title=Liquid-Phase Exfoliated Silicon Nanosheets: Saturable Absorber for Solid-State Lasers |journal=Materials |date=9 January 2019 |volume=12 |issue=2 |pages=201 |doi=10.3390/ma12020201|pmid=30634424 |pmc=6356386 |doi-access=free |bibcode=2019Mate...12..201W }} Therefore, developing an understanding of the mechanisms by which non-layered materials are exfoliated will be important, in particular because the application scope of such nonlayered 2D-nanoplatelets is broad, ranging from biomedical applications to energy storage to opto-electronics.{{cite journal |last1=Kaur |first1=Harneet |last2=Coleman |first2=Jonathan N. |title=Liquid-Phase Exfoliation of Nonlayered Non-Van-Der-Waals Crystals into Nanoplatelets |journal=Advanced Materials |date=September 2022 |volume=34 |issue=35 |pages=2202164 |doi=10.1002/adma.202202164|pmid=35470487 |bibcode=2022AdM....3402164K |hdl=2262/101345 |s2cid=248390135 |hdl-access=free }}

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