Rlvents in nanotechnology

Ali Abo-Hamad , Maan Hayyan , Mohammed AbdulHakim AlSaadi , Mohd Ali HashimUMCiL), University of Malaya, Kuala Lumpur 50603, Malaysiaty of Malaya, Kuala Lumpur 50603, MalaysiaMalaya,NANOCA

The current vision foreshadowed an

1.2. DESs as analogs of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5521.3. Properties of DESs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552

1.3.1. Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5531.3.2. Solvation properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554

2. Main applications of DESs in nanotechnology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554

Corresponding author at: Department of Civil Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia. Tel./fax: +60 3 7967 5311.E-mail address: maan_hayyan@yahoo.com (M. Hayyan).

Chemical Engineering Journal 273 (2015) 551567

Contents lists available at ScienceDirect1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5521.1. History of DESs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552Contentshttp://dx.doi.org/10.1016/j.cej.2015.03.0911385-8947/ 2015 Elsevier B.V. All rights reserved.Keywords:Deep eutectic solventIonic liquidNanomaterialCarbon nanotubeElectrodepositionDispersion

DESs and nanomaterials. This review revealed the recent studies that devoted to the impact of involvingDESs in nanotechnology and potential applications.

2015 Elsevier B.V. All rights reserved.entirely new areas of nanotech builton DESs.

a r t i c l e i n f o

Article history:Received 30 October 2014Received in revised form 28 February 2015Accepted 9 March 2015Available online 25 March 2015a b s t r a c t

Deep eutectic solvents (DESs) have recently received a great interest in diverse elds including nanotech-nology due to their unique properties as new green solvents, efcient dispersants and as large-scalemedia for chemical and electrochemical synthesis of advanced functional nanomaterials. DESs have alsoan active role in improving the size and morphology of nanomaterials during synthesis stage. Moreover,DESs conned in nano-size pores or tubes show distinct behavior from those in the same types but inlarger scales. Therefore, a numerous studies sprung up to expose the importance of the synergy betweenare in chemical and electrochemicalsynthesis.aUniversity of Malaya Centre for Ionic Liquids (bDepartment of Chemical Engineering, UniversicDepartment of Civil Engineering, University ofdNanotechnology & Catalysis Research Centre (

h i g h l i g h t s

DESs can play a primary role innanotechnology.

Up-to-date articles pertaining to DEScontribution in nanotechnology werereviewed.

The most current DESs applicationsKuala Lumpur 50603, MalaysiaT), University of Malaya, Kuala Lumpur 50603, Malaysia

g r a p h i c a l a b s t r a c ta,b a,c, a,d a,b

Potential applications of deep eutectic soeviewjournal homepage: www.elsevier .com/locate /cejChemical Engineering Journal

. . .es a. . .nomrucytic. . .esi. . .s . .icat. . .. . .. . .. . .

a

be eve

convecombnaturwhat

ineentional ones for the use in synthetic processes [11]. Having a (MOs), DESs have opened the door for new interesting avenues inamounts [10].

1.2. DESs as analogs of ILs

ILs have been introduced as new alternative solvents to replace

numerous applications [16]. This can be achieved through theproper combining between salts and HBDs. As a result, it is possi-ble to design a task-specic DES that could meet the electrochemi-cal or physicochemical properties required for certain area ofapplication. [17]. As they are able to dissolve some metal oxidesthe cases of using DESs at a commercial scale are still in nite DESs are close to ILs regarding the fact of being adoptable inditional ILs besides having almost similar solvation properties, put-ting forward many potential applications in different elds ofchemistry and electrochemistry. So far and due to economic rea-sons,

1.3. Properties of DESsn more environmentally benign compared to the earlier tra- and make it real in near future.etals were successfully electrodeposited such as Ag, Zn, Sn, Cr,nd Cu [8,9]The increasing interest of DESs is attributed to their potential to

formance and following up with new ndings and applications.Our hope is to encourage researchers to follow up deeply withthese aspects in order to spread the use of DESs among the worldlized as a medium for electrochemical deposition and differentmmetal cleaning prior to electroplating. Later, DESs were also uti- This review is to focus on nanotechnology side that has employedDESs in the discovery of new routs of synthesis, improving the per-2.1. Media to disperse nanoparticles and control morphology . . .2.1.1. Dispersants for nanomaterials to form nanocomposit2.1.2. Media to split nanomaterials . . . . . . . . . . . . . . . . . . . .2.1.3. Media to obtain special sizes and morphologies of na2.1.4. Electrolytes for electrochemical systems with nanost2.1.5. Media to produce efficient and recyclable nano-catal

2.2. Reaction media for nanomaterial production . . . . . . . . . . . . .2.2.1. Solvents and reactants for the physicochemical synth2.2.2. Electrolytes for nanomaterial electrodeposition . . . . .2.2.3. Media for sputter deposition to produce nanoparticle2.2.4. DES-based nano production: an overview about appl

2.3. Applications of nanosized DESs . . . . . . . . . . . . . . . . . . . . . . . .3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

A DES is generally a combination of two or more componentswhich are able to associate with each other. The concept of DESwas rst introduced by Abbott et al. [1] as a mixture of two or morecomponents that forms a eutectic, the melting point of this eutecticmixture is lower than both of the individual components. The mostpopular component among all DESs is choline chloride (ChCl)which is similar to B vitamins, and it is a biodegradable and non-toxic salt [1,2]. Over the last decade, there has been a rapid devel-opment in DESs as designer solvents for various applications [3].Furthermore, DESs, as a new type of green solvent, have somerecognized properties, such as high viscosity, high thermal stabilityand low vapor pressure [4].

1.1. History of DESs

A new solvent foundation was laid in 2003 reported by Abbottet al. and dubbed deep eutectic solvents (DESs) [1]. This termusually refers to a mixture of a halide salt and a hydrogen bonddonor (HBD) to produce liquid [5]. The rst appearance of DESwas as mixture of salt based on quaternary ammonium cationand a hydrogen donor (amine, imides, and carboxylic compounds).This eutectic phenomenon was rst introduced through a mixtureof urea and ChCl with a 2:1 molar ratio and melting points 133 Cand 302 C, respectively. The result was a eutectic mixture thatmelts at 12 C [6]. Physicochemical properties for DESs are similarto those of ILs, therefore, exploiting them attracted other research-ers [7]. Copper(II) oxide and lithium chloride were successfully dis-solved in this DES. Hence, DESs started to be used as solvents for

552 A. Abo-Hamad et al. / Chemical Engination nature of cation and anion with at least one of them isally organic and melts below some arbitrary temperatures ismade them dened as molten salts [12]. However, based on. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 555nd nanofluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 555. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 556aterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 556

tured electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557s of nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557ions, advantages and challenges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 560. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565

melting point range of ILs, they can be described either as roomtemperature ILs when they melt at room temperature or as nearroom temperature IL if they melt below 100 C. One of the key fea-tures that distinguish ILs form other competitive solvents is thelow vapor pressure comparably [13].

Recently, a special class of ILs was introduced and believed to bea new generation of ILs which known as DESs. These solvents aregenerally based on mixtures of quaternary ammonium or phos-phonium salt and an uncharged hydrogen bond donor such asamide, acid or alcohol with a much lower melting point than thatof any of its individual components have recently gained attention.In literature, DESs are sometimes referred to analogs of ILs [14],even though they could not be dened as true ILs as they are notcompletely comprised of ionic species. The syntheses of these com-pounds proceed simply by mixing together two components(cheap, renewable and biodegradable), which are able to form aeutectic mixture. This does not include any additional solvent orformation of by-products, and results in easily biodegradable prod-ucts [15].

Regarding material synthesis eld, a comparison between theILs and DESs use shows rareness for DES cases because of theirrecent appearance in the few past years. Nevertheless, seminalwork was reported in ionothermal synthesis for both solvents.

There are unlimited opportunities to prepare numerous DESsbecause of the high exibility to choose their individual compo-nents as well as their composition. Thus, a plenty of room is avail-able for the development of fundamental research in eld of DESs.Different properties can be attained from DES production andenvisaged applications can be achieved especially in high-tech pro-duction and processes that demand low costing materials [13].

ring Journal 273 (2015) 551567chemistry in particular [18]. Solubility limitations are selected bythe HBD of the DES. A great interest of DESs as novel solvents arealso placed on the employment of electrochemical procedures to

lart:2n

66.01 1.12 37 (25 C) 48.91 [5,98100]36.15 1.18 259 (25 C) [5,95,98100]32.65 1.20 450 (20 C) [5,98,100]

503 (20 C) [100]Liquid at RTb 1.342 77 (40 C) 35.9 [96]Liquid at RTb 115 (22 C) [101]Liquid at RTb 77 (22 C) [101]Liquid at RTb 85000 (25 C) [102]5.55 1.30 58.94 [98,99,103]49.34 1.23 51.29 [98]

ineeTable 1Some physicochemical properties for different types of DESs.

1st component 1st componentTm/f (C)a

2nd component 2nd componentTm/f (C)a

DES moratio 1s

ChCl 302305 U 132135 1:2ChCl 302305 TU 170176 1:2ChCl 302305 1,3-DU 101104 1:2ChCl 302305 EG 13 1:2ChCl 302305 G 20 1:2

1:31:4

ChCl 302305 TFA 7375 1:2ChCl 302305 AA 13 1:1.6

1:2ChCl 302305 ZC 293 1:2MPB 230234 G 20 1:3MPB 230234 EG 13 1:4

A. Abo-Hamad et al. / Chemical Engexplore the mechanisms of electron-transfer reaction [19]. DESsare characterized by high conductivities, viscosities, and surfacetensions, they also have lower vapor pressure in comparison toother solvents [20]. Due to such benecial properties, they havefound plenty of various applications. In Table 1 a list of most com-mon properties of DESs are presented.

1.3.1. ConductivityEach material has a certain level capability to transmit electric

current which determine its electrical conductivity value. Variousfactors play roles in this respect such as charge carriers availablein the material or as-called iconicity, their mobility and the tem-perature. For ILs case, less ionic conductivity range was found com-pared to aqueous electrolytes of high concentrated solutions.Reduced ion mobility accounts for the moderate conductivitiescaused by the presence of large-sized ions or ion aggregationwhich affects the available charge carrier amount [21]. However,ionic conductivity of ILs and DESs are dominated by their viscosity

MPB 230234 TEG 7 1:5

Tm/f: melting point/freezing point range, ChCl: choline chloride, U: urea, TU: thiourea, DUtriethylene glycol, MPB: methyltriphenylphosphonium bromide, AA: acrylic acid, ZC: zin

a According to MSDS.b Not reported but liquid at room temperature (RT).

Table 2Conductivity of some DESs, ILs and organic solvents.

Solvent system Conductivity (mS cm1)

ChCl:Urea (1:2 M ratio) 0.75 (25 C)ChCl:Ethylene glycol (1:2) 7.61 (25 C)ChCl:Glycerol (1:2) 1.05 (25 C)ChCl:Malonic acid (1:1) 0.55 (25 C)ChCl:CrCl36H2O (1:3) 0.55 (30 C)ChCl:ZnCl2 (1:2) 0.06 (42 C)ZnCl2:Urea (1:3.5) 0.18 (42 C)Ethylammonium chloride:Acetamide (1:1.5) 0.688 (40 C)EMC:Ethylene glycol (1:3) 5.429 (25 C)EMC:Glycerol (1:3) 0.602 (25 C)MPB:Ethylene glycol (1:3) 1.092 (25 C)MPB:Glycerol (1:3) 0.062 (25 C)[EMIM][N(Tf)2] 8.4 (25 C)[BMIM][N(Tf)2] 3.9 (25 C)[EMIM][BF4] 13.0 (25 C)[EMIM][PF6] 5.2 (25 C)[BMIM][PF6] 1.5 (25 C)Acetone 0.02 (20 C)*

Ethanol 1.345 106 (25 C)*

EMC: N,N-diethyl ethanol ammonium chloride, MPB: methyl triphenyl phospho-nium bromide, [EMIM]: 1-ethyl-3-methylimidazolium, [BMIM]: 1-butyl-3-methylimidazolium, [N(Tf)2]: bis(triuoromethylsulfonyl)imide, BF4: tetrauo-roborate, PF6: hexauorophosphate.The data are taken from references [7,17,24,26,27,105].

* According to MSDS.dDES freezingpoint (C)

Density(g cm3)

Viscosity(cP)

Surface tension(mNm1)

Ref.

12 1.25 750 (25 C) 52 [1,9597]69 [1]70 [1]

ring Journal 273 (2015) 551567 553in the rst place, therefore, temperature is also as important todetermine their conductivity [2224].

Giving a general description of DES conductivity as low or highmight me sometimes misleading. Researchers have usually men-tioned the poor conduction of DESs when a comparison is madewith normal ILs [3,17,19]. This is because the higher viscositiesof some DESs compared to ILs [25]. By contrast DESs have beenwidely considered as good conductors when compared with con-ventional organic solvents.

21 1.19 49.58 [104]

: dimethyl urea, EG: ethylene glycol, G: glycerol, TFA: 2,2,2-triuoroacetamide, TEG:c chloride.

Table 3Comparison between the solubility (in ppm) of some MOs in three ChCl based DESsand in two aqueous solutions of sodium chloride and hydrochloric acid at 50 C aftertwo days [18].

MO DES 1a DES 2b DES 3c NaCl HCl

TiO2 4 0.5 0.8 0.8 36V2O3 365 148 142 3616 4686V2O5 5809 4593 131 479 10,995Cr2O3 4 3 2 13 17CrO3 6415 10,840 7 12,069 2658MnO 6816 0 12 0 28,124Mn2O3 5380 0 7.5 0 25,962MnO2 114 0.6 0.6 0 4445FeO 5010 0.3 2 2.8 27,053Fe2O3 376 0 0.7 11.7 10,523Fe3O4 2314 6.7 15 4.5 22,403CoO 3626 13.6 16 22 166,260Co3O4 5992 30 18.6 4.0 142,865NiO 151 5 9.0 3.3 6109Cu2O 18,337 219 394 0.1 53,942CuO 14,008 4.8 4.6 0.1 52,047ZnO 16,217 1894 469 5.9 63,896

a DES 1:ChCl:Malonic acid (1:1 M ratio).b DES 2:ChCl:Urea (1:2 M ratio).c DES 3:ChCl:Ethylene glycol (1:2 M ratio).

Table 4Solubility of sodium chloride in different DESs at 60 C [32].

DES Molar ratio Solubility w/100w

ChCl:Zinc(II) chloride 1:1 43.68ChCl:Zinc(II) chloride 1:2 60.00ChCl:Zinc(II) chloride 1:3 63.00ChCl:Tin(II) chloride 1:3 2.08ChCl:Tin(II) chloride:Zinc(II) chloride 1:1:1 4.27

Examples for electrical conductivities of some common DESs,ILs and organic solvents were collected and presented in Table 2[7,2629]. Figures were found comparable sometimes to ILs andgave the impression about the high chances of using DESs in elec-trochemical applications [30,31].

1.3.2. Solvation propertiesFor all chemical and electrochemical synthesis technology it is

quite important to have a solvent or an electrolyte which is capableof dissolving precursors during the reaction time and under syn-thesis conditions. Therefore, it is worthy to study the solvationproperties of solvents before we can determine where they canbe highly exploited.

plication for any modifying process to attain better exploitationin some applications such as sensors and biosensors. To tacklethese difculties, ILs have emerged as green alternatives to vola-tile organic solvents [41]. They have been considered as convenientmedia to disperse, solve, and split nanomaterials. Particularly, dueto the ability of ILs and DESs as their analogs to interact with some

55 92154

223301

392

564

673

1 3 5 11 18 29

45

2007 2008 2009 2010 2011 2012 2013 2014

Year

93%

7%

554 A. Abo-Hamad et al. / Chemical Engineering Journal 273 (2015) 551567Abbott et al. investigated the solubilities of Fe3O4, ZnO, CuO indifferent DESs [6]. Various MOs and salts were then tested for theirsolubility in DESs. Results were compared with the solubilities inaqueous solutions of 0.181 mmol L1 NaCl and 3.14 mmol L1

HCl. All are summarized in the Table 3 below.In different types of DESs, various solubility values were

reported for some types of sodium salts at different temperaturesand for a range of mole ratios (salt:HBD). In general, most of theresults showed that increasing the temperature and decreasingthe molar ratio can increase the solubility. It was also found thatthe chemical structure of the DES affects the sodium salts solubility[32]. High sodium salts solubility in certain types of DESs can beemployed very well in electrochemistry if they are used as elec-trolytes to produce sodium metal at mild temperatures. Someexamples are given in Table 4 to illustrate the different solubilitiesof NaCl salt in some types of DESs. Molar ratio effect was alsoinvestigated for one of the studied DES.

2. Main applications of DESs in nanotechnology

The rst combination of nanotechnology and ILs was publishedin 2001 [33], however, the one related to DES was reported as lateas 2008 introducing the use of the DES a solvent for the chemicalsynthesizing of gold nanoparticles. Later on, several examples innanotechnology elds were reported on the use of DESs, see Fig. 1.

Among the whole articles that have been published since 2001combining ILs or DESs with nano-science, there are 45 articlesstudying DESs, over about 673 articles on ILs. This includes almost40 review articles covering some areas of IL applications in nan-otechnology, see Fig. 2. In addition, there are more than 500patents studying various types of using ILs/DESs in nano-ledscience.

Carbon nanomaterials (CNMs) such as graphene, single-walledand multi-walled carbon nanotubes (SWCNTs and MWCNTsrespectively) have received a considerable concern in this respect.According to our statistics, approximately 50% of the IL

1 3 6 13 24 34

0

100

200

300

400

500

600

700

800

2001 2002 2003 2004 2005 2006

Num

ber o

f Ar

cles

ILs & Nanotechnology ArclesDESs & Nanotechnology Arcles

Fig. 1. Trends of article number studying the appcontribution in nanotechnology was placed for carbon nano-material projects. Silica nanoparticles come in the second placein terms of their applications with ILs.

IL applications have been spread over a wide range of eldssuch as chemical and biochemical reactions, electrochemistry,polymer science and nano-chemistry [34]. However, tendenciesof DES use are same for those of ILs. So far, all recently reportedDES applications in nanotechnology have already been studiedbefore for IL before but with the use of ILs instead. This meansthere is still a wide range of possibilities to employ DESs as alter-natives to ILs in nano-science as the last (ILs) have paved the roadfor that since 2001. In this review, we aim to summarize the recentroles of DESs played in every eld of nanotechnology. A list of DESsthat have been used so far in nanotechnology eld is provided inTable 5 .

In fact, ILs have shown a special properties when they are innano-quantities, either if they are conned in nano pores of nanos-tructured materials [35,36] or are interdependently existed innano-scale volumes like nano-droplets [37] and nano-lms [38].

Most nanomaterials are likely to aggregate with each othernaturally [39,40]. This may limit their benign properties requiredfor particular applications like the capability to absorb certaincompounds. Another negative aspect for aggregation is the com-

ILs & Nanotechnology

DESs & Nanotechnology

Fig. 2. The proportion of DES and IL articles in nanotechnology elds.lication of DESs and ILs in nanotechnology.

DES Molar ratio Role

ChCl:U 1:2 Electrolyte (nanostructure sensoductpositis of

is ofis of

positis oficro

positemid m

is ofd m

l, AA

ineenanoparticles, many cases have been reported regarding the dis-persion of solid nanomaterials.

In synthesis chemistry, it is generally believed that solvents areof essential signicance in all synthetic processes [42]. It is worthmentioning that the main role of these solvents is to homogenizeall the reagents in the reaction media. Organic and non-organic sol-vents have been widely used, they are chose depending on thereaction peculiarities and how the solvent may deal with them.The same criterion is followed in nanoparticle synthesis.Recently, ILs and DESs have been the solvent of choice for the con-venient synthesis of nanoparticles due to thermal stability, gooddispersibility, large ionic conductivity, wide electrochemical win-dow [43]. Two different routes of nano-synthesis can be carriedout by using ILs/DESs as solvents for chemical syntheses or elec-trolytes for electrochemical ways.

In a recent study [44], DESs were reviewed for their recent

Media for nanoparticles proElectrolyte (nanoparticle deMedia for chemical synthesDispersant

2:1 ExfoliationChCl:1,3DU 1:2 Media for chemical synthesChCl:TU 1:2 Media for chemical synthesChCl:EG 1:2 Dispersant

Electrolyte (nanoparticle deMedia for chemical synthesNanodroplet embedded in m

ChCl:CrCl3.6H2O 1:2 Electrolyte (nanoparticle deChCl:AA 2.3:12.0:11.6:11.3:1 Dispersant and media for chChCl:PTSA 1:1 Structure-directing agent anChCl:HMP 1:1 DispersantChCl:ZnCl2 1:1 Nano-connementChCl:GA:G 1:0.25:0.25 Media for chemical synthesChCl:MA 1:1 Structure-directing agent anCA:DU 1:1.5 Nanocatalytic assembly

ChCl: choline chloride, U: urea, DU: dimethyl urea, TU: thiourea, EG: ethylene glycoGA: gallic acid, G: glycerol, MA: malonic acid, CA: citric acid.Table 5DESs used so far in nanotechnology and their main roles.

A. Abo-Hamad et al. / Chemical Engapplications as designer solvents to produce different sorts ofnanomaterials. Nanomaterial productions were dened in thisstudy in 6 aspects: shape-controlled nanoparticles, electrode-posited lms, metalorganic frameworks, colloidal assemblies,hierarchically porous carbons, and DNA/RNA architectures. Therole played by DESs was investigated deeply on how to directchemistry at the nanoscale. According to literature, DES can actas template, carbon or metal source and as a reactant or auxiliaryagent in nanomaterial production. The reactivity of DESs was foundaffected by many factors such as hydrogen bonding, surface ten-sion, viscosity and polarity.

There has to be known that there are a plenty of elds for theuse of original ILs in nanotechnology, but those for their analogs(DESs) are still few in comparison. Our goal here is to cover therecent occupation of DESs in every single area of nanotechnology.This work is organized to give a complete and concise summarylooking at subjects from a broader perspective. Hopefully, this willadd an engineering-looking character to the work away fromchemistry sophistication.

2.1. Media to disperse nanoparticles and control morphology

Agglomeration tendency of nanoparticles is one of the basicchallenges limiting their applications. It leads to a considerabledecrease in the surface, and as known, the lower surface area, withrespect to the bulk volume, the lower chemical and catalytic activ-ity of the particles.

Generally, metal nanoparticles can only show good stability intheir suspensions by the coordination of some kinds of surfactants.These surfactants somehow help prevent nanoparticles agglomera-tion by forming a protective layer surrounding the surface [45].Uniquely, ILs as liquid solvents with a special ionic nature havethe ability to act as satisfying stabilizers for a wide range of nano-particles even without using any surfactants [41,46]. The impor-tance of such dispersion media is hugely considered, either earlyduring the nanomaterial synthesis or later for preparation of fur-ther application. The last is to obtain special properties and/orget as higher benet as possible.

For rst stages of nanomaterial production, it is highly signi-cant to choose a suitable solvent which can effectively preventproducts from being bundled or cluster-assembled. ILs and DESs

Ref.

r) [63]ion by sputter deposition technique [67,68]ion) [77,8084,106110]nanoparticles [6973,75,94,111117]

[77][57]

nanoparticles [118]nanoparticles [86]

[52]ion) [76,78,79,107,110,119123]nanoparticles [74,124126]structure [93]ion) [127]cal synthesis of nanoparticles [85]edia for chemical synthesis of nanoparticles [58]

[128][91]

nanoparticles [129]edia for chemical synthesis of nanoparticles [49]

[64]

: acrylic acid, PTSA: para-toluene sulfonic acid, HMP: tris(hydroxymethyl) propane,

ring Journal 273 (2015) 551567 555have been widely used as efcient dispersants during the synthesisreaction of nanoproducts. They have played roles in determiningthe shape, size and morphologies [4749]. They are also capableof splitting small-scale structures into nano-scale and this aspectis added to their use as a functional dispersing solvent. Manyexamples of graphene-ILs hybrids fabrication, obtained by usingILs solvents for carbon nanotube (CNT) or graphite exfoliation, sup-port this case [50,51]. After nanomaterials production, ILs may beused to form nano-hybrids (i.e. nanouids or nanocomposites) inorder to enhance special properties of the nanomaterials in themixture or get the mixture itself ready for certain application.However, in spite of the large number of articles discussing the dis-persibility of nanomaterials in ILs, the reported cases of using DESsfor the same purposes are still few in comparison, but they areexisted at least. The aim of the following sections is to cover andsummarize all relevant cases available so far for different DES dis-persing purposes.

2.1.1. Dispersants for nanomaterials to form nanocomposites andnanouids

As mentioned earlier, the contribution of DESs in nanomaterialdispersion is still limited. Up to date, there is no large number ofdeep studies reporting the dispersibility of nanomaterials in DESsas there is for ILs. Table 6 is summarizing some of the related casesof DES as dispersants. Examples were selected from studies wherereaders can nd at least a mentioning about the DES ability to

Di

1.3:1

Symapoco

EleanSiCnaDES as epoxy resincufabepna

Longer storage time was noted for epoxy [128]

EleMWco(co

ineeChCl Ethylene glycol 1:2 SiC and Al2O3nanoparticles

ChCl Tris(hydroxymethyl)propane 1:1 Graphitenanoplatelets(GNP)

ChCl Urea 1:2 Pristine MWCNTs(P-MWCNTs) andoxidized MWCNTs(O-MWCNTs)Table 6Examples of the use of DESs as efcient dispersants.

DES type Type of dispersednanomaterial

Type of salt(component1)

Type of HBD (component 2) Molarratio(Salt:HBD)

ChCl Acrylic acid 2.3:12.0:11.6:1

MWCNT

556 A. Abo-Hamad et al. / Chemical Engdisperse nanoparticles during their preparation even it is foranother intended application.

To give a clear example, a study was conducted recently regard-ing the ability of DES for DNA solubilization. In this study, Mondalet al. [52] investigated the suitability of using DES for dual func-tionalization of DNA (Salmon testes). The process was carried outusing Fe3O4 nanoparticles and protonated layered dititanate sheets(H2Ti2O5H2O) as functionalizing agents in DES medium ofChCl:ethylene glycol (1:2 M ratio). Eventually and after 6 h, hybridmaterial was obtained with antibacterial and magnetic propertiessee Fig. 3. High pure DES was successfully recycled and reusedfor solubilization following the same method. The study involvedalso DNA stability in the hybrid material and applications weresuggested in biosensing and biomedicine elds.

2.1.2. Media to split nanomaterialsThe hallmark of nanomaterials is to have in the number size dis-

tribution at least 50% of particles with one or more external dimen-sions in nano size (i.e. between 1 and 100 nm) [53].

Some micro-structures are in fact a group of bundled particlesin nano-size but they are stuck together in somehow to form largerstructures. Graphite for example, is a multi-layer graphene sheetswith a much larger overall external dimension [54,55]. If the gra-phene sheets are rolled to form cylindrical shape, CNTs will beformed. Hence, rolling one graphene sheet layer leads to SWCNTsbut to have seamless cylinders of more layers, multi graphene lay-ers need to be rolled and MWCNT is formed [56]. Based on deep

Fig. 3. Pictorial imagination for the dual functionalized DNA hybrid material [52].understanding of those facts, many researches have been con-ducted to nd out how we could unbundle the bulky aggregationof materials to reach their smallest possible formula. Later, thisprocess was called exfoliation or split of nanomaterials.

DESs are holding some similar properties with IL and thus playthe same role that could be played by ILs in some area of applica-tions. However, it seems that the door of applying DESs to split orexfoliate small-sized materials has just been opened. A recentstudy done by Boulos and co-workers [57] revealed the trans-formation of human hair into functional nano-dimensional mate-rial using ChCl:Urea DES (2:1 M ratio, respectively). SEM resultsshowed that before DES treatment a typical hair was observed witharound 50 lm diameter and some cuticle cells about 5 lm sizewere covering the hair surface. After hair treatment with DES, itwas clearly seen that cuticle cells were completely exfoliated fromhair. More analyses were conducted to recognize the size and mor-

ring agent toricate GNP/DES/oxy resinnocomposite

composition containing DES (more than 60 days)DES could act as an effective GNP dispersingmedium, which positively affected electricalvolume resistivity of epoxy composites

ctrodeposition of Ni/CNT composites on

pper substrateating)

Poor dispersion stability in DES was reported forP-MWCNTs while it was very high for O-MWCNTs (1 year)

[77]spersion purpose Remarks & details Ref.

nthesis ofcroporously(acrylic acid)CNTmposites

DES is polymerizable solvent helped to dispersesubstances and incorporate carbon nanotubesThe composite has great potential inbiomedicine, energy and environmentalapplications

[85]

ctrodeposition of Agd formation of Ag//Al2O3nocomposite lm

DES helped in high dispersed-phase loadings ofelectrodeposited AgImproved lm properties compared to puremetal

[119]

ring Journal 273 (2015) 551567phology of the exfoliated cuticle cells. TEM images detected smallparticles in large numbers with a granular morphology which areprobably melanin pigments of hair cells. These small particles werealso in nano-size (1520 nm) as HRTEM images showed. What ismore, TEM images proved the presence of macrobrils nano-rodswith a diameter of around 200 nm which may be existed due tounraveling of cortical cells to their nano-structural componentsunder the DES effect especially in protein denaturing.

2.1.3. Media to obtain special sizes and morphologies of nanomaterialsThe role of ILs and DESs as morphology directing agents for the

produced materials was frequently investigated in various studieswhen they are included in the reaction media. In fact, apart fromthe role as reaction media which will be discussed later in details,IL and DES natures were found to play a crucial role in determiningthe size and morphology of the produced nanomaterials. Changesmight take place either during the production reaction or afterreaction for dispersing purposes in colloidal applications. In thissection, we aim to report some cases where the effect of DESs onshape, size and morphology is investigated. Sometimes it isreferred to template to describe this role of DES.

Gutirrez et al. [58] reported the functions of DES based on ChCland para-toluene sulfonic acid (1:1 M ratio) in a process to preparea porous carbon. The DES was used as a media and catalyst for fur-furyl alcohol condensation. In addition to that, it was a media forthe following carbonization process to produce different porous

ineecarbon monoliths including MWCNTs. Researchers found that theused DES acted also as structure-directing agent for the producedcarbons. DES segregations were found to be responsible for form-ing macroporous network during polycondensation.

Oh et al. [49] used a DES of ChCl and malonic acid as both areaction medium and structure-directing agent to synthesizehighly monodisperse gold microparticles. The product had a dis-tinctive surface nanoroughness and highly dened diameters con-trolled precisely under different reductive conditions. Thesynthesis was not supported by any surfactants or polymers, sug-gesting that the DES has essential role as a structure directingreagent and particle stabilizer. Different structures were obtainedat different temperatures (At 70 C, isotropic small particles withalmost 100 nm diameter, whilst three-dimensionally networkednanostructures were obtained at 90 C). This was explained bythe highly dependency of the DES properties on temperature.

2.1.4. Electrolytes for electrochemical systems with nanostructuredelectrodes

Many studies have covered the use of ILs as electrolytes in elec-trochemical systems such as sensors, actuators, capacitors, batter-ies, solar and fuel cells [5962]. All these systems need of coursehighly specied requirements from every involved component tobe serving properly. In fact, the use of ILs as elements in electro-chemical devices has been well identied. ILs electrolytes arebecoming very popular in electrochemistry applications in generaland when nanomaterials are accompanied in particular. However,only a single study was reported so far for DES use in electrochemi-cal sensing. In 2014, Zheng et al. [63] used a mixture of PH 4.0 acet-ate buffer solutionwith a ChCl:Urea DES (1:2 M ration respectively)as an electrolyte for the electrochemical sensing of quercetin.MWCNTs modied graphite electrode was used as a working elec-trode in a three-electrode system with platinum and saturatedcalomel as auxiliary and reference electrodes respectively. Theyobtained a good linear relationship between quercetin concentra-tion in the electrolyte and the oxidation peak current within con-centration range from 9.95 107 to 4.76 107 mol L1. Thismethod was found much easier and less expensive than the IL/CNTs composite modied electrodes system used for the samepurpose.

2.1.5. Media to produce efcient and recyclable nano-catalyticassembly

For the synthesis of Imidazo[1,2-a]pyridines, Lu and coworkers[64] have developed a recyclable efcient nano-catalytic system tobe used for the reaction. The optimum catalytic system achievedwas based on CuFeO2 nanoparticles in a eutectic solvent of citricacid-dimethyl urea melt (1:1.5 M ratio). The study involved a ser-ies of reactions using three-based components as reactants,namely 2-aminopyridine, aldehydes and alkynes. Reactions wererst run using various solvents as a part of the catalytic systemsuch as toluene, ethanol, dimethyl formamide as well as sometypes of DESs. Copper salts along with different metallic oxidesnanoparticles were subjected to testify their catalytic efciency.Eventually, the system of citric aciddimethylureaCuFeO2 nano-particles was selected based on the best resulted yield, 95%. Thiscatalytic assembly was then used for the further optimization ofreaction time and temperature. However, according to this study,DES has helped not only to recover the catalyst nanoparticles (keptsame efciency for 6 times) but also it played a catalytic rolebecause of its acidity.

2.2. Reaction media for nanomaterial production

A. Abo-Hamad et al. / Chemical EngWhen using ILs as solvents, nanomaterial production can beachieved in many different ways: (1) chemically, (2)electrochemically, (3) photochemically, (4) sonochemically (usingultrasound technique), (5) by microwave irradiation, (6) by gas-phase synthesis using sputtering, (7) or by plasma electrolysis[45]. This section illustrates the recent applications of DESs as sol-vents or liquid media in nano-synthetic chemistry andelectrochemistry.

2.2.1. Solvents and reactants for the physicochemical synthesis ofnanomaterials

ILs were accompanied with the eld of reaction chemistrythrough the use of chloroaluminate(III) ILs. During 1990s, neutralILs were developed and a wide range of reactions were performedand spread rapidly [65].

DESs are classied as a type of green solvent with many promi-nent advantages for chemical synthesis of nanomaterials in manyreferences. DESs were not only used as solvents, in some cases theywere also used as reactants to produce the intended nanoparticles.In this respect they played multiple roles as a solvent and precur-sor at the same time. All aspects of DESs in chemical synthesis ofnanomaterials are summarized in Table 7 as shown below. Eachone is described through: type of DES, type of nanomaterial pro-duced, reaction method, composition and conditions. In this table,it is clear that ChCl:Urea based DES have been used mostly forchemical syntheses of nanomaterials especially metal oxidenanoparticles.

2.2.2. Electrolytes for nanomaterial electrodepositionElectrodeposition is a process leading to the formation of solid

materials by electrochemical reactions in a liquid phase.Passivation phenomenon is one of the basic problems inhibitingthe deposition progress as a result of poor electrolytic solvationability toward metal oxides and hydroxides. As mentioned earlier,high solubility of metal oxides, salts and hydroxides in DESs givesthem bonus prominence over conventional electrolytes based onaqueous solutions or organic solvents [26].

The most popular electrodeposition setup is composed of athree-electrode electrochemical cell (a specially designed cathode,a counter electrode or anode and a reference electrode [66]). Adirect current method with a two-electrode system can also beused for nanoparticle electrodeposition. Simply in this process,positive metallic ions of the electrolyte are reduced at cathodeelectrode and form neutral metal particles deposited on the sur-face. Applications of metal electrodeposition technique includephotoactive semiconductors, corrosion-resistant functionalizedsurfaces, developing the magnetic properties of materials andbuilding up functional components for electronic industries suchas circuit boards.

DESs have been widely used as electrolytes to produce nanopar-ticles electrochemically. With a setup of two or three electrodes,DESs have been successfully used as electrolytes and in similar tech-niques used for ILs beforehand. All available examples for this caseare described in Table 8. ChCl:Urea based DES have taken the rstplace in this respect regarding the number of studies it is involvedin. This DES was the most common electrolyte especially for plat-inum and palladium electrodeposition. While, ChCl:ethylene glycolbasedDES have come in the second placewithmost interests shownfor Ni nanostructures compounds electrodeposition.

2.2.3. Media for sputter deposition to produce nanoparticlesThere are several effective synthesis methods for nanoparticles

such as chemical synthesis, microwave assisted synthesis andphysical vapor deposition (PVD). In fact, physical sputtering

ring Journal 273 (2015) 551567 557deposition has been shown to offer a clean strategy for formingNPs on solvent surface with low vapor pressure such as ILs andDESs.

Table 7Chemical and physicochemical nanomaterials production by the means of DESs as reaction media.

DES type Nanomaterial produced Reaction medium and composition Reaction type Remarks/conditions Ref.

Salt HBD Molar ratio(Salt:HBD)

ChCl Acrylic acid 2.3:12.0:11.6:11.3:1

Macroporous poly(acrylic acid)CNTcomposites

DES with ethylene glycoldimethacrylate (EGDMA) as crosslinkerand benzoyl peroxide as a thermalinitiator

Frontalpolymerizationprocess

At 130 C [85]

ChCl Ethylene glycol 1:2 SnO2 nanocrystalline 100 ml 0.05 M of SnCl2.2H2O inDES + 4 ml of H4N2.H2O 85% (slowly)

Homogeneousprecipitation

At room temperature (25 3 C) [124]

ChCl 1,3-Dimethylurea

Prussian blue nanospheres FeCl3.6H2O and K4Fe(CN)6.3H2O wereadded separately into DES

Coordination ofFe(CN)6-4 ion withFe3+ ion in theDES

At 80 C [118]

ChCl Urea 1:2 Spherical Fe3O4 magneticnanoparticles

1st Step: 2.164 g (8 mmol) of FeCl3.6H2Oand 1.194 g (6 mmol) of groundFeCl2.4H2O were added to 15.585 g ofDES2nd Step: adding 2.613 g (46.7 mmol) ofKOH to the mixture

Co-precipitation 1st Step: 600 rpm and 80 C for 20 min2nd Step: 600 rpm and 80 C for 1.5 h

[111]

ChCl Urea 1:2 PbS nano/micro superstructures 1st Step: solvent Preparation(6 mL) deionized water + 30 ml DES2nd Step: Preparation of solutions:Sol 1: (12 mmol, 0.9134 g)thioacetamide + 12 ml from solventSol 2: (12 mmol, 5.3206 g) lead(IV)acetate + 12 ml solvent3rd Step: mixing Sol 1 + Sol 2

Combining thePb2+ and S2-

precursors in hotDES

1st Step: stirring at 80 C2nd step: stirring both of solutions in an oil bathat 80 C until transparent3rd Step: using a ask with a condenser tomixing at 140 C till the solution turns black

[115]

ChCl Urea 1:2 Nanostructured Ni compounds:nanocrystals Ni(NH3)6Cl2nanosheet-like NiCl2nanoower-like a-Ni(OH)2mesoporous, nanoower-likeNiO

DES + NiCl2.6H2OFor Ni(NH3)Cl2 and nanosheet-like NiCl:0.5 M NiCl2 in DESFor nanoower-like a-Ni(OH)2 andnanoower-like NiO: 0.1 M NiCl2 in DES

Ionothermalstrategy

For Ni(NH3)Cl2 and nanosheet-like NiCl: Teonautoclave 150 C for 4 hFor Ni(NH3)Cl2 and nanosheet-like NiCl: heatedunder ambient atmospheric pressure

[94]

ChCl Urea 1:2 Nickel phosphide nano-particlesNi2P (25%) supported on amorphousand mesoporousNi3(PO4)2-Ni2P2O7

27.92 g (0.2 mol) of ChCl + 24.02 g(0.4 mol) of urea + 5.94 g (0.02 mol) ofNi(H2PO)2.6H2O + 1.66 g (0.02 mol) ofNH4H2PO2

Ionothermalstrategy

At 323 K in N2 for 30 minthen 423 K, during which the solution turnedblack from light green with violently release ofgases (as bubbles) for 5 h

[69]

ChCl para-Toluenesulfonic acid

1:1 Hierarchical porous MWCNTcomposites

functionalized MWCNTs (0.01, 0.03 and0.05 g respectively) + (1.0 g) DES +(0.20 g) furfuryl alcohol (FA)

furfuryl alcohol(FA)condensationcatalyzed by aprotic DES

The suspension (DES + CNT) was stirred for 24 hat room temperature before addition of FApolycondensation and carbonization: FA +(DES + CNT) suspension, the resulting solutionwas vortexed for 2 min and treated thermally(rst at 37 C for 8 h and then at 90 C for 4 days)The resulted condensed FA polymers treated for4 h at 210 C followed by another 4 h at 800 C(1.0 C min1 heating ramp) under a N2atmosphere

[58]

ChCl Urea 1:2 CuCl nanoparticles (14 mL) DES containingpolyvinylpyrrolidone PVP + (2.2232 g)CuCl2.2H2O + (1.3754 g) ascorbic acid +(50 mL) hydrochloric acid (0.1 mol.L1)

Oxidationreductionreaction

Reaction time is 1 h, reaction temperature is25 C in the presence of polyvinylpyrrolidone(PVP) in DES

[112]

ChCl Ethylene glycol 1:2 SnO2/reduced graphene oxidenanocomposite

1st Step: solutions preparationSol 1: 40 mg GO + 100 ml DESSol 2: 2.25 g SnCl2.2H2O + 100 ml DES2nd Step: mixing sol 1 + sol 2

Oxidationreductionreaction betweenSn2+ andgraphene oxide

2nd Step: ultrasonically (200 W) for about 4 h [125]

558A.A

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Table 7 (continued)

DES type Nanomaterial produced Reaction medium and composition Reaction type Remarks/conditions Ref.

Salt HBD Molar ratio(Salt:HBD)

ChCl Thiourea 1:2 a-chitin nanobers DES with 10%, w/w Pure chitin powderthen adding 10 mL of distilled water

Solvation Stirring at 500 rpm at 100 C for 2 h [86]

ChCl Ethylene glycol 1:2 Nanoporous Ag lm 0.01 or 0.1 M AgCl in DES Galvanicreplacementreaction

At room temperature 22 (2 C) or at 50 C, byimmersing a cleaned copper alloy foils into AgCl-DES solution without stirring

[126]

ChCl Urea 1:2 Gold Nanoparticles 25 mL DES + L-ascorbic acid (0.05 g) +HAuCl44H2O (0.015 g)

Reduction ofHAuCl4 by L-ascorbic acid

At 30 C, under magnetic agitating [113]

ChCl Urea 1:2 nano-sized SnO crystals 2.25 g SnCl2.2H2O was dispersed in100 ml DES

Ionothermalstrategy

At 100 C with a heating rate of 5 C.min1 undervigorous stirring

[114]

ChCl Urea 1:2 Cu2+-doped ZnO nanocrystals 1st step: 0.36 g of ZnO powder + 150 g ofDES2nd step: bad solvent preparationequalvolumes of deionized water andethylene glycol3rd step: 5 mL of ZnO-containing DESwas injected into 160 mL of the badsolvent

Facile, greenantisolventapproach

At 70 C water bath, vigorous stirring for 30 min [116]

ChCl Gallic acid:glycerol

1:0.25:0.25 gum Arabic coated gold nanoparticles Aqueous solution of DES 0.01% (w/v) +15 mg gum Arabic + (0.05 M) HAuCl4 indifferent volumes

Reduction ofHAuCl4 by DES

Continuous stirring under ambient conditions [129]

ChCl Urea 1:2 Mesoporous NiO 0.1 M NiCl2 in DES Homogeneousprecipitation

50 mL 0.1 M NiCl2/DES in a three-neck askunder ambient atmosphere pressure at 150 C for40 min.Followed by adding 10 mL deionized water undervigorous magnetic stirring.After another 20 min reaction, cooling in ice bath

[117]

ChCl Malonic acid 1:1 Gold Microstructures with SurfaceNanoroughness

200lL (0.15 M) HAuCl4 + 20 mL ofDES + 200lL of (0.1 M) ascorbic acid

Reduction ofHAuCl4 byascorbic acid

Stirring for 3 h at 50 C [49]

ChCl Urea 1:2 Fe2O3 nanospindles 0.1 M FeCl36H2O in DES Ionothermalstrategy

40 mL of 0.1 M FeCl36H2O/DES in a three neckask, at 200 C, under ambient atmosphericpressureFollowed by adding 40 mL deionized water withvigorous stirring for 10 minThen cooling rapidly with ice

[75]

ChCl Urea 1:2 Ni layered double hydroxide (Ni LDH)as a-Ni(OH)2 nanoowerNinanoparticlesNi mesoporous ower-like structure

0.1 M NiCl2 in DES Ionothermalstrategy

1st step: 50 mL of 0.1 M NiCl2/DES in a three neckask, at 150 5 C, under magnetic stirring2nd step: annealing the resulted Ni LDH under Aratmosphere at 300 C for 4 h with and withoutOCN- addition

[71]

ChCl Urea 1:2 Co Fe layered double hydroxide (CoFeLDH) nanosheets

0.1 M Co+2 and 0.035 M Fe+3 in DES fromCoCl26H2O and FeCl36H2O

Ionothermalstrategy

50 mL of Co+2,Fe+3/DES in a round-bottom ask,at 210 5 C, under magnetic stirringFollowed by adding deionized waterThen cooling rapidly with ice

[72]

ChCl Urea 1:2 Mesoporous Co3O4 sheets ornanoparticles

0.1 M CoCl2 in DES Ionothermalstrategy

50 mL of 0.1 M CoCl2/DES in a three neck ask, at150, 180, and 210 C, for 40 min under magneticstirringFollowed by adding 100 mL deionized water withvigorous stirring for 1 minThen cooling rapidly with ice

[73]

(continued on next page)

A.A

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551567

559

Table7(con

tinu

ed)

DES

type

Nan

omaterialprod

uced

Reactionmed

ium

andcompo

sition

Reactiontype

Rem

arks/con

dition

sRef.

Salt

HBD

Molar

ratio

(Salt:HBD)

ChCl

Urea

1:2

MnCO3mesocrystalsnan

owire-like

MnOx

0.5M

Mn+2in

DESfrom

MnCl 2.4H2O

Ionothermal

strategy

and

facile

thermal

conversion

process

1ststep

:50

mlof

0.5M

MnCl 2/DES

inau

toclave

at12

0,15

0,an

d18

0C,for

4h

Coo

lingdo

wnto

room

tempe

rature

then

washing

theMnCO3precipitates

withmethan

olan

dwater

andethan

ol,cen

trifugingan

ddrying

2ndstep

:an

nealingtheMnCO3at

300C

for4h

then

gradua

lcoo

lingprocessto

room

tempe

rature

[70]

ChCl

Ethylen

eglycol

1:2

NiPalloynan

oparticles

Amixture

ofNiCl 26H2Oan

dNaH

2PO2H

2Oin

DES

,molar

ratios

(1:0.2)

Ionothermal

strategy

50mlof

reaction

solution

inDES

inathreeneck

ask,

at15

0C,for

3h

Coo

lingdo

wnto

room

tempe

rature

then

centrifugingtheprecipitateaccompa

niedby

washingwithde

ionized

water

andmethan

ol,and

dryingunde

rvacu

um

[74]

560 A. Abo-Hamad et al. / Chemical EngineeRaghuwanshi et al. [67] reported the formation and growthmechanisms of gold nanoparticles (AuNPs) in eco-friendly deepeutectic solvents (DES; ChCl and urea). Gold nanoparticles wereformed in DES by using a low-energy, soft-sputtering depositiontechnique under very clean conditions in a vacuum chamber.This low-energy deposition technique allowed synthesizingAuNPs either directly or in a sequential way with diameters overa wide range from 1 to 100 nm.

Data analysis revealed that for a prolonged gold-sputtering timethere was no change in the size of the particles. Only the concen-tration of AuNPs increased linearly in time. More surprisingly,the self-assembly of AuNPs into a rst and second shell orderedsystem was observed directly by in situ SAXS for prolonged gold-sputtering times. The self-assembly mechanism was explained bythe templating nature of DES combined with the equilibriumbetween specic physical interaction forces between the AuNPs.

Later on, the samegroup [68] aimed in otherwork to estimate theeffect of sputtering timeon the structureparameters and to evaluatethe growth mechanisms of the AuNPs. They managed to establish adeep understanding of the time dependent growth and the stabilityof AuNPs with respect to the effect of ionic liquids as surroundingenvironments. The results showed formation of spherical nanopar-ticles with a mean diameter of 50.5 nm, see Fig. 4. For extendedsputtering times, the number density of AuNPs increased linearlyand a very pronounced 1st and 2nd shell ordering was observed.For shorter gold-sputtering times of 30 s the gold atoms started toaggregate on the surface, however, prolongation of gold-sputteringtime led to an increase in the concentration of gold atoms on theDES surface. After dispersion into the bulk they self-organized intoclosed-packed ordered domains (clusters of AuNPs).

2.2.4. DES-based nano production: an overview about applications,advantages and challenges

From previous examples, DESs have shown a versatility to beused in different reaction methods. They provide a platform toserve the several physicochemical reaction types perfectly suchas ionothermal reaction, precipitation, polymerization, con-densation and reductionoxidation reactions including replace-ment reactions.

Nanostructured materials obtained ionothermally using DESmedia were found highly valuable in various application. The bene-ts of DESs exceeded being safe, cheap and convenient alternativeto conventional solvents. Their role to determine shape, size andmorphology seemed to be controllable which considered as impor-tant to reach the required efciency in advanced application.Production of nanostructured catalysts for example is still a chal-lenge requiring the efforts to adjust their synthesis conditions.Zhao et al. [69] produced Ni2P nanoparticles supported on meso-porous and amorphous Ni3(PO4)2Ni2P2O7 using ChCl:Urea basedDES as media for ionothermal reaction. Produced nanostructuredsystem showed catalytic activity for hydrodesulfurization of diben-zothiophene, hydrodenitrogenation of quinoline and hydrogena-tion of tetralin. Ionothermal method in ChCl:Urea media can alsofabricate calcite type MnCO3 biominerals with mesocrystal struc-ture [70]. The porous and nanowire-like MnOx nanostructurescan be obtained through a facile thermal conversion process fromthe diverse MnCO3 precursors, which are demonstrated as effectiveand efcient adsorbents to remove organic waste (e.g. Congo red)from water.

Layered double hydroxides (LDHs) such as (a-Ni(OH)2, a-Co(OH)2 and FeCo LDH) and the derived metal oxides can beionothermally synthesized from DES based reaction system.These nanomaterials can be used as potential energy storage elec-

ring Journal 273 (2015) 551567trodes [7173].Anode materials with nanostructures for application in lithium

ion batteries can be synthesized from DESs. Amorphous and

Table 8DES-based electrolytes for nanoparticles electrodeposition.

DES type Electrolyte composition Nanomaterialproduced

Electrochemical cell information Electrodepositionmethod

Electrodeposition conditions & remarks Ref.

Salt HBD Molarratio(Salt:HBD)

Anode (counterelectrode)

Cathode (workingelectrode)

Referenceelectrode

ChCl Urea 1:2 ChCl/urea/PdCl2 Nano-sized Pdlm

Pd sheet Rotating Cusubstrate

Applying directcurrent (DC) or pulsecurrent plating (PP),two-electrode system

At 70 C, plating time 90 min, Theapplied current density for DC platingj = 0.05, 0.1 and 0.2 mA cm2 and for PPplating jav = 0.1 mA cm2

[106]

ChCl Urea 1:2 19.3 mM H2PtCl6 DESssolution

Pt nanoowers Pt wire Glassy carbon disk(GC, 3 mm)

Pt quasi-referenceelectrode

Cyclic voltammetry(CV), three-electrodesystem

At 80 C, potential scan range: 1.5 to0.2 V, scan rate of 50 mV s1 and 80cycles, electrodeposition occurs at 1.29V

[82]

ChCl Urea 1:2 19.3 mM H2PtCl6 DESssolution

Triambicicosahedral(TIH) Ptnanocrystals(TIH Pt NCs)

Pt wire Glassy carbon (GC) Pt Programmedelectrodepositionroutine threeelectrode system

At 80 C, the electrochemical potentialwas rstly stepped from open circuitpotential (OCP) to nucleation potential(EN) of 1.80 V (vs. Pt), and then stayedat EN for 45s to generate Pt nuclei on theGC electrode surface. The developmentof the Pt nuclei into TIH Pt NCs wasachieved by applying a square-wavepotential (f = 10 Hz) with the lower (EL)and upper (EU) potentials of 1.30 and0.30 V, respectively

[83]

ChCl Urea 1:2 0.1 M [CuCl2 2H2O] inDES with 10 wt%0.05 mm Al2O3 or 10 wt%13 mm SiC

Nano-structuredCu (brightmetalliccoatings)

Pt Platinum 1 mm or2 mm diameter,made in-house

Ag wirequasi-referenceelectrode

Potential stepchronoamperometry,three-electrodesystem

At 20 2 C, at various scan rates: 10 to50 mV s1 under diffusion control atapplied potentials of either 0.80 V or1.00 V

[107]ChCl Ethylene glycol 1:2

ChCl Ethylene glycol 1:2 1 M NiCl26H2O in DES NanostructuredNi lms

Electrolytic Niplate

Brass (Cu0.64Zn0.36alloy)

(a) constant voltagemode (CVM), (b) pulsevoltage mode (PVM),and (c) reverse pulsevoltage mode (RVM),two-electrode system

At 90 C, (a) Constant voltage mode(CVM), Upos = 1.0 V; (b) pulse voltagemode (PVM), Upos = 1.0 V; (c) reversepulse voltage mode (RVM), Upos = 1.0 V,Uneg = 1.0 Vtfwd = 2 s, trev = 1 s,Upos = 1.0V, and Uneg = 1.0 V are adopted for theRVM

[76]

ChCl Ethylene glycol 1:2 Mixture of either Al2O3(50 nm) or silicon carbide(50 nm or 2 mm) withDES

Agnanocompositeswith nano-aluminaparticles or SiCnanoparticles

Iridium oxidecoated Ti mesh

Ni substrate Constant potentialdifference, two-electrode system

1.2 V was applied between the workingand counter electrodes using a 12 Vpower supply

[119]

ChCl Ethylene glycol 1:2 DES containing 0.3 MNiCl26H2O

NanostructuredNiO lms

Ni plate indium tin oxide(ITO) glass with3 cm 2.5 cm

Direct current (DC),two-electrode system

At 35 3 C, at constant potential (1.0V) electrodeposition duration 10 s, 30 sand 60 s for Ni lm

[120]

ChCl Ethylene glycol 1:2 DES containing 0.3 MNiCl26H2O, 0.1 M KMnO4and 0.1 MH2O2

NanostructuredNiO lms

Ni plate ITO glass of 3 2.5cm2

Direct current (DC),two-electrode system

At 70, 80 and 90 C, potential (1.0 V) [121]

ChCl Ethylene glycol 1:2 DES containing 1 MNiCl26H2O

NanocrystallineNi lm

Electrolytic Niplate

Brass foil(Cu0.64Zn0.36 alloy)

Direct current (DC),two-electrode system

At 25 3 C, working voltage: 1.0 V, for60 min

[122]

ChCl Urea 1:2 DES containing 0.045 MSmCl3 + 0.018 M CoCl2

Samarium andcobalt SmCoNanostructures

Pt spiral Vitreous carbon rods,copper and nano-porous aluminatemplates (50 nmporous diameter)

Ag|AgCl/NaCl 3 Mmounted ina Luggincapillarycontainingthe DESsolvent

Cyclic voltammetry(CV), three-electrodesystem

At 70 C, 50 mV s1, electrodepositionstarts from 1.6 V, electrodepositionstarts from 1.6 V (negative-goingsweep)

[108]

ChCl Urea 1:2 19.3 mM H2PtCl6/DESs Pt Nanocrystals Pt wire Glassy carbon disk(GC, U = 3 mm)

Pt quasi-referenceelectrode

Programmedelectrodepositionmethod, three-electrode system

At 80 C, potential step from 1.20 V (vsPt) to 1.50 V, and this potential wasmaintained for 1 s to generate Pt nuclei.The growth of the Pt nuclei into concavetetrahexahedral (THH) Pt NCs wasachieved by applying a square-wavepotential (f = 10 Hz) with the lower (EL)and upper (EU) potential limits of 1.30and 0.30 V, respectively

[84]

(continued on next page)

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Table 8 (continued)

DES type Electrolyte composition Nanomaterialproduced

Electrochemical cell information Electrodepositionmethod

Electrodeposition conditions & remarks Ref.

Salt HBD Molarratio(Salt:HBD)

Anode (counterelectrode)

Cathode (workingelectrode)

Referenceelectrode

ChCl Urea 1:2 10 mM K2PdCl4 DESsolution

Pd nanoparticles Pt Glassy carbon foil Ag/AgClmini-referenceelectrode(eDAQ)

Cyclic voltammetry(CV), three-electrodesystem

To deposit as many particles as possible,the approximate cathodic limit of theelectrochemical window of the DES (at32.5 C) was used (ca. 1.8 V). A loweroverpotential of 1.4 V was appliedduring the growth pulse to minimize thesize dispersion of the particles twodifferent temperatures were used foreach electrodeposition sequence: 32.5and 44.5 C

[109]

ChCl Urea 1:2 0.3 M NiCl2 inDES + 0.1 g/L MWCNT

Ni/MWCNTcoating

Ni 1 cm2 Cu plate Ag quasi-referenceelectrode

Galvanostatic, three-electrode system

At 60 C, galvanostatically at a currentdensity of -2 mA/cm2 for 30 min andconstant agitation using a magneticstirrer (900 rpm)

[77]

ChCl Chromium chlorideCrCl36H2O

1:2 DES Nano-chromiummagneticdomains

Platinised Tianode

CNT growing onsubstrates thatinclude titaniumnitride coated siliconchips or polishedstainless steel

Two-electrode system At 20 C, in an open beaker, Currents ofno more than 0.15 A were used

[127]

ChCl Urea 1:2 DES containing 0.6 MNiCl26H2O

Ninanocrystalline

Two parallel Niplates

Mg3.0Nd0.2Zn0.4Zr (wt.%, NZ30 K)alloy

Galvanostatic method/ two-electrode system

The deposition current density was3.0 mA/cm2 and deposition time was60 min. The temperature was kept at70 C

[81]

ChCl/ChCl Urea/Ethylene glycol 1:2/1:2 FOR Ag: DES + 4-Mercapto phenyl boronicacid (MPBA, 5 mM)stabilizer and sodiumborohydride (0.15 MNaBH4) as a reducingagentFOR Au: DES + (0.15 MNaBH4) as a reducingagentFOR Pd: DES

Thin lms ofnoble metallicnanoparticlessuch as Au, Agand Pd

FOR Ag: silverwire (0.5 mm)diameterFOR Au: gold wire(0.5 mm)diameterFOR Pd:Palladiumwire (0.5 mm)diameter

FOR Ag: Pt/Rh (or W)WireFOR Au: gold disc1 mm diameter

Chronopotentiometricmode / two-electrodesystem / anodicdissolution technique

The DES medium was kept underconstant magnetic stirring in anelectrochemical cell of 10 ml capacity.During the process, a mild gas evolutionwas seen at the cathode in DES medium.FOR Ag: at 50 C, current density14 mA cm2, 30 min1 hFOR Au: at 50 C, current density0.3 A cm2 for a duration of 20 minFOR Pd: at room temperature, currentdensity 1 A cm2 for 30 min

[110]

ChCl Ethylene glycol 1:2 DES containing 0.1 MSnCl22H2O

Nanoporous thinlm of Sn

Sn plate Cu foil 0.03 mm Direct current (DC),two-electrode system

At variable applied voltages and ambienttemperature (25 3 C) without thedeaeration processe.g. at 0.5 V, for 2 min and at 0.7 V for5 min (200300 nm average size and3040 nm pore size)

[123]

ChCl Urea 1:2 DES containing:(a) 0.4 M ZnCl20.1 MNiCl20.05 M NH4H2PO2or (b) 0.4 M ZnCl20.1 MNiCl20.1 M NH4H2PO2

NanostructuredZnNiP alloylm

Ni Cu plate 20 50 mm Ag quasi-referenceelectrode

Three-electrodesystem

At 50 2 C current densities of4 mA cm2, 6 mA cm2 and 8 mA cm2

Varied metal contents were dependenton current density and electrolytecomposition

[80]

ChCl Ethylene glycol 1:2 DES containing:(a) 0.30 M NiCl26H2O0.05 M NaH2PO2H2O(b) NiCl26H2O0.1 MNaH2PO2H2O(c) NiCl26H2O0.15 MNaH2PO2H2O

NanostructuredNiP alloy lm

Ni Brass (CuZn alloy) Ag quasi-referenceelectrode

Cyclic voltammetry(CV), three-electrodesystem

At 30 2 C starts at0.520 V for electrolyte (a)0.550 V for electrolyte (b)0.600 V for electrolyte (c)

[79]

ChCl Ethylene glycol 1:2 DES containing 0.1 MCoCl26H2O0.05 MSnCl22H2O

NanostructuredCoSn alloy lm

Sn plate Cu foil (0.05 mmthickness)

Ag wire Three-electrodesystem

At variable potentials (0.8 V, 1.0 Vand 1.2 V) and temperatures (75 C and25 C)Electrodeposition times varied to obtainsame electric quantity

[78]

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ineeA. Abo-Hamad et al. / Chemical Engcrystalline NiP nanoparticles were produced in (ChCl:Ethyleneglycol) DES with coreshell structure. Fig. 6 illustrates the coreshell procedure to obtain NiP nanoparticles [74]. Fe2O3 nanospin-dles were also synthesized ionothermally in ChCl:U and showed ahigh capacity, good cycle stability and rate performance for theproduced electrode in lithium ion battery [75].

Crystal growth mechanism was studied most often in everyionothermal strategy. As each reported study represented a uniquereaction mechanism, no generalization can be conrmed. Fig. 7summarizes some recent examples for the different mechanismssuggested for ionothermal production of nanostructures.

Applications of electrochemically produced nanomaterials werealmost to fabricate protecting lms with high enhanced corrosionresistance. This was reported for single metal coating (such as Ni[76]) and multi-ingredient coating (such as Ni/MWCNT composite

Fig. 4. Cryo-TEM micrographs of gold nanoparticles in DES synthesized by sputter depo[68].

Fig. 5. SEM images of the pure PVDF lm (a) and the DES-NiCl2@PVDF

Fig. 6. Schematic illustration of the synthetic procedure f0

ring Journal 273 (2015) 551567 563[77], CoSn alloy [78], NiP alloy [79], ZnNiP [80] and MgNdZnZr alloy [81]). DES electrolytes have offered a exibility toobtain several lm compositions for different coating purposes.The key factor was to adjust the dosage between chemical compo-nents dissolved in DESs.

Catalytic activities of electrochemically produced nanoparticleswere also a matter of study for different reactions [8284]. Specialmorphologies obtained were controllable by changing time, cur-rent density, temperature as well as the applied potential. Thishelped to study the various activities obtained and helped to iden-tify the growth mechanism of the produced particles.

However, the environmental aspects of DESs after beingemployed were not assessed. This clearly shows the need ofenvironmental studies and studying the environmental impact ofDESs, for instance, handling DESs after nanoparticles preparation,

sition at 20 mA and 0.05 mbar (argon pressure). (a and a ) 300 s without stabilizer

lm (b). Inset of (b) is the corresponding magnied SEM picture [93].

or the coreshell structured NiP nanoparticles [74].

inee564 A. Abo-Hamad et al. / Chemical Engdischarging them or using them in recycling processes. A few stud-ies reported the successful recycling of DESs after using them as

Fig. 7. Schematic illustrations for some suggested mechanisring Journal 273 (2015) 551567media to disperse and produce nanoparticles and nanocomposites[52,58,64,85,86].

ms to produce nanostructured materials ionothermally.

As common with most studies, the basic concern on nano-material properties is placed on the solid phase only. Recently,

ineesome reports have emerged and revealed novel physicochemicalproperties of ILs and DESs which designed somehow to be innano-scale dimensions. Many techniques have been used to studythe liquid in such scales. For IL case in general, ILs nanostructurescan be formed (1) as nano-thick lms on a substrate surface [87],(2) nanodroplet by immobilization in nanowells of mesoporousmaterials [37] or by molecular beam deposition route [88], (3)and as connement by conning the IL in nano-sized spaces ofnanomaterials such as the tubular space in CNT [89] or the nar-row pores in nanoporous silica [90]. Connement is the mostreported case to study the physicochemical behavior of IL innano-volumes. It was also the rst route where DES has begunto be studied in nano scales.

Chen et al. [91] used a SWCNT to conne a DES based on ChCland zinc chloride (1:1 M ratio) in an intriguing way. The procedurefor encapsulation involved thermal treatment of synthesizedSWCNT to remove end-caps, loading both SWCNTs and DES sepa-rately in H-type quartz tube, sealing the tube under vacuum(107 torr) and then mixing at 200 C for 72 h. The morphologyand melting behavior of the encapsulated DES were studied usingdifferent techniques such as high-resolution transmission elec-tron microscopy (HRTEM) and in situ TEM electron beam irradia-tion. The results showed that the tube size play the main role inmorphology determination. With an increasing size of SWCNTsdiameter, a tendency was observed to form single-chain, dou-ble-helix, zigzag tube, and nally, random tube morphologies.Thermal stability of the conned DES was found to be much higherthan in the bulk DES. These results and others aim to get furtherunderstanding of the structure and phase behavior of nano-con-ned DESs and improve their use in solar cells, lubricants andcatalysts.

Thermochromic materials have a signicant role in builtenvironment. They are usually formed as crystalline or polymericsolid or as solutions of transition metal complexes including thesolvent molecule as donor [92]. Gu and coworkers [93] discoveredthat DESs with their solvolysis properties can be used to dissolvemetal complexes and enable thermochromism. Two types ofChCl-based DESs (i.e. ChCl:U and ChCl:EG, 1:2 M ratios in both)were used to dissolve different transition metal chlorides.However, only NiCl26H2O exhibited a stable and prominent ther-mochromic behavior within a wide range of temperature fromroom temperature to about 150 C. This was utilized to fabricatea thermochromic poly(vinylidene uoride) (PVDF) composite lmby embedding DESNiCl2 nanosized droplets in PVDF micropores(Fig. 5). Color-temperature relation of the composite lm was fullyreversible owing to the uniformly dispersed nanodroplets on theligaments and channels of the porous structure.

3. Conclusion

This summarized the up-to-date studies of employing DESs indiverse nanotechnology areas. The unique properties of DESs offerthem some advantages to in comparison to ILs. DESs have beenused as dispersants, exfoliating agents and templates for nano-2.3. Applications of nanosized DESs

DESs have taken a wide interest since they have been discov-ered. Many studies have emerged in an accelerating rate coveringthe physical and chemical properties of these promising solventsand proposing various applications in different eld of science.

A. Abo-Hamad et al. / Chemical Engmaterials. Their application as media to synthesize nanoparticleschemically, physically, physicochemically or electrochemicallyresembles those for ILs. However, some positive features can beadded to DES use over ILs such as their easy preparation and beingmuch eco-friendlier. DESs like ILs can be obtained in nano-sizewith many intriguing features, but these investigations are stillin their infancy compared to conventional ILs that have made greatstrides in the eld of nano-environment. Various applications canbe predicted for DESs in the near future especially the onesreported previously in IL area. Although there have been hugeefforts placed on studying ILs applications in nanotechnology, thedegree of DES contribution is still limited. Hence, it is highlyrecommended to encourage research to follow up with the restof applications available from pre-established ones for ILs.Investigations can be conducted not only by employing analready-known or famous DES in particular, but also by designingspecic-task DESs based on available materials to be suited forthe desired application in nanotechnology.

Acknowledgments

The authors would like to express their thanks to University ofMalaya HIR-MOHE (D000003-16001) and University of MalayaCentre for Ionic Liquids (UMCiL) for their support to this research.

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