Self aggregating metal surfactant complexes: Precursors for nanostructures

Accepted Manuscript Title: Self aggregating metal surfactant complexes: Precursors for nanostructures Author: Ravneet Kaur S.K. Mehta PII: DOI: Reference:

S0010-8545(13)00281-6 http://dx.doi.org/doi:10.1016/j.ccr.2013.12.014 CCR 111810

To appear in:

Coordination Chemistry Reviews

Received date: Revised date: Accepted date:

23-9-2013 17-12-2013 17-12-2013

Please cite this article as: R. Kaur, S.K. Mehta, Self aggregating metal surfactant complexes: Precursors for nanostructures, Coordination Chemistry Reviews (2013), http://dx.doi.org/10.1016/j.ccr.2013.12.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Manuscript

Self aggregating metal surfactant complexes: Precursors for nanostructures Ravneet Kaur and S.K. Mehta* Department of Chemistry and Centre of Advanced Studies in Chemistry,

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Panjab University, Chandigarh – 160 014 (India)

*Corresponding author. Tel.: +91-172 2534423 Fax: +91-172-2545074 E-mail address: [email protected] (S.K. Mehta) 1

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ABSTRACT Metallosurfactants or metal surfactant complexes have been the focus of interest in recent years owing to the advancement in the field of interdisciplinary research. These complexes exemplify a perfect blend of organometallic chemistry and interface science. They display the properties

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characteristic to both, a surfactant (self assembly and surface activity) and a metal ion (redox and catalytic property). Exploiting the properties arising out of this interesting class of compounds

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can lead to fascinating applications. The metallosurfactants apart from being excellent catalytic systems can most certainly be used in metal nanoparticle synthesis. Because of the

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supramolecular assemblies formed, particularly micelles and vesicles, they are promising candidates as nanosized reactors. The route involving metallosurfactant has got several

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advantages over conventional methods for nanoparticle fabrication. Metallosurfactant can itself act as a metal ion source and nanoparticle protector, thereby eliminating the need for a capping agent. The present review focuses on the use of metallosurfactants in the field of nanochemistry.

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Metallosurfactants have been reviewed with an aim to present them as scaffolds for

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nanomaterials, revealing their advantages over conventional nanostructure fabrication methods.

Keywords: Metallosurfactants; nanoparticle synthesis; micelles; self assembly; liquid crystal templating; microemulsion; thermal decomposition. 2

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Contents 1. Introduction 2. Metallosurfactants as building blocks for nanoparticles

2.2.1. Micelles 2.2.2. Microemulsions 2.2.3. Other self assembly structures

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2.3. True Liquid Crystal Templating (TLCT) technique

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2.2. Liquid phase colloidal synthesis using self assembled structures

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2.1. Thermal decomposition of metal surfactant complexes

2.4. Using other reduction techniques

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2.4.1. Sonochemical reduction 2.4.2. Electrostatically induced reduction 2.4.3. Pressure driven reduction

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3. Conclusions and outlook Acknowledgements

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References

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1. Introduction Surfactants have always sparked a researcher‟s interest due to their peculiar and interesting properties which include-contrasting hydrophobic and hydrophilic nature; tendency to

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self associate; ability to solubilize both polar as well as non polar components etc. [1]. The everincreasing advancement in the field of colloid and surface science provides fresh challenges not only for improvement in the already existing materials, but also for the fabrication of novel

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materials. Although, existing amphiphilic systems undoubtedly find various potential applications ranging from DNA transport across membranes [2], to their use as templates for

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polymerization [3], to novel reaction media [4]; but there is always a need for rational design of surfactants possessing the desired functionalities. Obtaining tailor made surfactants with desired

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properties can certainly provide a big boost to the application of such systems. They can be efficiently used as scaffolds for nanostructure materials or as mimics for natural products. The

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incorporation of various moieties e.g. peptides, metals or even carbohydrates etc. into the amphiphilic systems has led to the fabrication of numerous new classes of surfactants. Such systems possess the tendency to self-aggregate and form a variety of supramolecular assemblies

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[5]. The design and preparation of such new type of amphiphilic molecules requires organization

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and control of architecture during the early stages of formation. Amongst all the classes of new amphiphilic complexes known, metallosurfactants or

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metal surfactant complexes have gained much attention in the recent past as they open up a vast arena of applications [6-8]. Complexing metal ions to the conventional surfactant systems or ligands lead to the formation of metallosurfactants possessing unique properties. The complexes formed, possess not only the properties of metals such as acid-base, redox and magnetic properties but they also retain amphiphilic character i.e. surface activity. This basic idea of incorporation of a metal ion into the surfactant molecule provides a facile means of localizing both amphiphilic and metallic properties at the interface presenting a fascinating blend of organometallic chemistry and surface science. Such surfactants [9-13], show applications in the field of MRI contrast agents [6-8,14], as templates for mesoporous materials [15,16], metallomesogens [17,18], optoelectronic devices [19,20], solvatochromic probes [21], homogeneous catalysis [22,23], medicine [24,25], electronic energy-transfer processes [26,27], antimicrobial or antibacterial agents [28-30], logic gates [31] and nanoparticles [32]. 4

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Due to the conventional surfactant like properties, these metallic compounds are capable of forming aggregates of various sizes and morphologies including micelles, vesicles and bilayers incorporating metal ions. The aggregation behavior and self assembly of metallosurfactants have been studied for quite some time [12,13]. Till now, the

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metallosurfactants have been reviewed with a special emphasis on metallomicelles and metalloaggregates and their use in catalysis [33,34]. The morphology of aggregates formed by

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the surfactant ligands also undergoes a change on coordination with a metal ion. Owen and Butler [35] excellently reviewed the metal induced phase transition behavior for

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metallosurfactants in order to develop an understanding towards micelle to vesicle transition. However, to the best of our knowledge, there have been no reports devoted to nanoparticle

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synthesis using metallosurfactants except the studies investigating the thermal decomposition of metal surfactant complexes [36]. Very little information [30] is available on their possible role as

desired functionality on nanoparticles.

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nanosized reactors for the formulation of nanostructures and their further tailoring to confer the

Nanosized metal and semiconductor colloids have drawn remarkable interest due to their

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well-defined, size dependent, optical and electronic properties [37,38], leading to potential applications in optoelectronics [39], catalysis [40], reprography [41,42] etc. Needless to say,

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nanoparticles have invaded our everyday lives, but there will always be a dearth of facile

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approaches to their synthesis. The current synthetic routes to metallic nanostructures are limited and accompanied by many disadvantages which include elimination of contaminants or unwanted products, pH stabilization, requirement of high temperature and pressure, presence of inert atmosphere during the synthesis process, to name a few. Therefore, it is of great interest to develop facile templating approaches towards nanostructure synthesis. For the fulfillment of above mentioned conditions, metallosurfactants can be of great help and offer several advantages over the conventional nanoparticle synthesis procedures. The use of metal complexes or metallosurfactants as single-molecule precursors to synthesize nanoparticles has been identified as a relatively simple and highly efficient route to high-quality, crystalline nanostructures in good yields. This “one-pot” synthetic route allows efficient size and shape control by a simple variation of reaction conditions [43]. The idea that a metal surfactant complex can act both as the metal ion source as well as nanoparticle protector opens up a new method for nano synthesis while eliminating the need for any stabilizing or capping agents. 5

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The present review therefore focuses on this aspect of metallosurfactants which is hitherto unexplored, the efficient and controlled synthesis of nanostructures using metal surfactant complexes. It highlights the use of metallosurfactants as building blocks, outlining different approaches to nanoparticle synthesis. The contents have been subdivided to four major

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sections based on the synthetic procedures of nanoparticle fabrication which include; (i) thermal treatment of metal surfactant complexes to obtain nanoparticles, (ii) liquid phase colloidal

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synthesis exploiting the self assembly nature of metallosurfactants, (iii) True Liquid Crystal

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nanoparticle fabrication have been outlined in Scheme 1.

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Templating approach, (iv) use of other reduction techniques. The various approaches used for

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N A N O P A R T I C L E S

Scheme 1. Schematic representation of various approaches to nanoparticle fabrication employing metallosurfactants.

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2. Metallosurfactants as building blocks for nanoparticles 2.1. Thermal decomposition of metal surfactant complexes Thermal decomposition is one of the most established routes to nanoparticle synthesis [44-47]. Metal complexes showing low decomposition temperatures yielding metal as the end

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product can be fairly well utilized to synthesize nanoparticles by thermal treatment. The same procedure could be applied to metal surfactant complexes with an advantage to directly obtain

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highly crystalline and monodisperse nanoparticles, without any size-selection. It can thus prove to be a facile and inexpensive route to nanoparticle synthesis. Pioneering work has been done in

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this area by Hyeon et al. [48,49]. For the first time, synthesis of monodisperse γ-Fe2O3 nanocrystallites using an iron surfactant complex was reported. The metal–surfactant complex

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makes an excellent reactant species for the synthesis of monodisperse nanostructures [50]. Aging of the iron-oleic acid metal complex or metal oleate complex at high temperature (300 °C) resulted in the formation of monodisperse iron nanoparticles. However, controlled

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oxidation using trimethylamine oxide resulted in the transformation of iron nanoparticles to monodisperse γ-Fe2O3 nanocrystallites (Fig. 1). Particle size was obtained in the range of 4 to 16

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nm. The synthetic procedure developed in this study was employed to obtain monodisperse nanocrystals of bimetallic oxide as well, by Hyeon et al. [51] e.g. high-temperature aging lead to

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the formation of highly crystalline and monodisperse cobalt ferrite nanocrystals. The general

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synthetic procedure used has been represented in Scheme 2.

Fig. 1. Synthesis of monodisperse γ-Fe2O3 under controlled oxidation. Reprinted from Ref. [48]. Copyright 2003, with permission from The Royal Society of Chemistry. The procedure developed for γ-Fe2O3 nanoparticle synthesis was also utilized to get the nanocrystals of other transition metals [49]. The procedure employed offered several advantages 7

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over the conventional methods used for nano synthesis. The process gave a high reaction yield, on an ultra-large scale generating multi grams of nanoparticles in a single step reaction, without employing a size-selection process. The use of non-toxic and relatively inexpensive reagents such as metal chlorides made it an environment friendly and economical procedure. Even

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particle size could be efficiently controlled by simply varying the experimental conditions.

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Scheme 2. The overall scheme for the ultra-large scale synthesis of monodisperse nanocrystals. Reprinted from Ref. [49]. Copyright 2004, with permission from Macmillan Publishers Ltd. [Nature Materials].

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To develop a basic understanding of the mechanism involved in nanoparticle formation, along with TEM, thermal decomposition behavior of the solid-state precursor i.e. iron–oleate

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was also monitored by employing techniques such as Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and temperature-programmed infrared spectroscopy [50]. Based on the infrared spectra and TGA/DSC patterns, it was inferred that in the first step one oleate ligand dissociated from the iron oleate precursor in the temperature range of 200–240 °C and the other two oleate moieties dissociated at ~300 °C following a CO2 elimination pathway. The TEM images (Fig. 2) of the samples without aging, recorded at 310 °C revealed that the nanoparticles were not obtained, whereas the TEM image taken at 320 °C showed the formation of relatively uniform nanoparticles in size range of 8-11 nm. The TEM images taken after aging, at a temperature of 320 °C for 10, 20 and 30 min. depicted the formation of monodisperse nanoparticles of 12 nm size. On the contrary, aging at 240 °C for one day, did not produce nanoparticles, however, after keeping for three days ~14 nm sized nanoparticles were obtained but with high polydispersity. 8

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Based on the above results, successful monodisperse nanoparticle fabrication was attributed to the separation of nucleation and growth. According to the proposed mechanism, nucleation took place at 200–240 °C which was triggered by the removal of one oleate moiety from the iron-oleate precursor. The major growth took place at ~300 °C with the dissociation of

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remaining two oleate moieties. Consequently, the separation of nucleation and growth, taking place at two different temperatures, ultimately lead to the formation of monodisperse

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nanocrystals. The growth process also showed time-dependence, i.e. when the precursor was aged at low temperature of 240 °C (close to the nucleation temperature). For one day, there was

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no nanoparticle formation but after three days, polydisperse nanocrystals were obtained. These results fairly well implied the temperature dependent behavior of nucleation and growth kinetics.

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In the year 2004, Hyeon [52] also obtained a patent on the synthesis of monodisperse and highly crystalline nanoparticles of metals, metal oxides, multi metallic oxides and alloys, without employing a size selection process.

320 °C (10 min.)

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320 °C (0 min.)

320 °C (20 min.)

320 °C (30 min.)

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310 °C (0 min.)

Fig. 2. TEM images (upper: low magnification, bottom: higher magnification) of the iron oxide nanoparticles at various reaction time intervals. Reprinted from Ref. [49]. Copyright 2004, with permission from Macmillan Publishers Ltd. [Nature Materials]. Further, extending this approach, Joo et al. [53] synthesized semiconductor nanocrystals of PbS, ZnS, CdS, and MnS with diverse sizes and morphologies through thermal decomposition. Metal-oleylamine complexes obtained by the reaction of oleylamine and metal chloride at high temperature were used as metal precursors. Elemental sulfur was introduced into the reaction mixture of oleylamine complexes at 90 oC. This reaction mixture was heated at 220, 9

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320, 160 and 280 oC to produce PbS, ZnS, CdS and MnS nanocrystals, respectively. The shape of the nanocrystals was controlled by changing the metal complex to sulfur ratio e.g. rods, bipods, and tripods of CdS nanocrystals were obtained with 1:6 molar ratio of cadmium:sulfur (Fig. 3) whereas spherical CdS nanocrystals (5.1 nm) were generated from cadmium:sulfur with

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2:1 molar ratio. A 1:1 molar ratio of MnCl2:sulfur yielded rod-shaped MnS nanocrystals with an average size of 20 nm (thickness) x 37 nm (length). For obtaining novel bullet-shaped MnS

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nanocrystals with an average size of 17 nm (thickness) x 44 nm (length), MnCl 2:sulfur was used in 2:1 molar ratio whereas shorter bullet-shaped MnS nanocrystals were obtained using a 3:1

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molar ratio of MnCl2:sulfur (Fig. 4). All the synthesized nanoparticles were highly crystalline.

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Fig. 3. TEM images of CdS nanocrystals. (a) Mixture of rods, bipods, and tripods (Inset: HRTEM image of a single CdS bipod-shaped nanocrystal), (b) Spherical nanoparticles. Reprinted from Ref. [53]. Copyright 2003, with permission from American Chemical Society.

Fig. 4. TEM images of MnS nanostructures. (a) MnS nanorods (Inset:HRTEM image of a single MnS nanorod), (b) Long bullet-shaped MnS nanocrystals, (c) Short bullet-shaped MnS nanocrystals, (d) Hexagon-shaped MnS nanocrystals. Reprinted from Ref. [53]. Copyright 2003, with permission from American Chemical Society. 10

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Going a step further, Ye et al. [54] successfully reported a solvent-free technique to synthesize highly crystalline and monodisperse Fe3O4 nanocrystallites. The reaction was carried out at room temperature instead of refluxing temperatures (∼265–350 oC). Solid state reaction of inorganic Fe(II) and Fe(III) salts with NaOH produced Fe3O4 nanoparticles. This reaction, when

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carried out in the presence of oleic acid–oleylamine adduct, gave monodisperse and highly crystalline Fe3O4 nanocrystals in a self-assembled, two-dimensional and uniform periodic array

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as shown in Scheme 3. This approach was more advantageous compared to the previous ones because inorganic ferrous, ferric and base solids were used for the synthesis instead of aqueous

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or organic solutions. Synthesized Fe3O4 nanoparticles were extracted directly in hexane, while

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the unreacted components and the byproducts insoluble in hexane were separated easily.

Scheme 3. The overall scheme for synthesis of monodisperse Fe3O4 nanoparticles. Reprinted from Ref. [54]. Copyright 2006, with permission from American Scientific Publishers. Therefore, it can be rightly said that the thermal treatment of the metal-surfactant complexes efficiently leads to the fabrication of metal nanostructures in high yields without a further size selection process. Even the morphology of nanostructures could be controlled by tuning the metal surfactant complex ratio. It is a facile and general approach involving the use of inexpensive reagents employing the complexes in solid state. Subsequent section takes into account the liquid phase synthesis of nanoparticles utilizing metal surfactant complexes.

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2.2. Liquid phase colloidal synthesis using self assembled structures The dual character of surfactants i.e. hydrophobicity and hydophilicity is responsible for their peculiar behavior at interfaces leading to a variety of self aggregating structures namely micelles, microemulsions, vesicles, bilayers etc. [55]. Due to their amphiphilic nature,

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metallosurfactants self assemble to generate different types of aggregates in water and organic solvents [56-60] and show different photophysical properties [61]. These microemulsions and

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micellar systems can be successfully employed as reaction media to prepare novel, functional materials with well-defined nanoscale structures since they tend to retain their morphology.

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Dong and Hao [62] synthesized reverse vesicles using an iron surfactant i.e. Ferrum laurate [Fe(OOCC11H23)3] without any change in their shape and produced solid shells on solvent

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evaporation. Similarly, Suades et al. [63] reported the formation of vesicles from metal carbonyl metallosurfactants. Synthesis of nanomaterials from metal surfactant complexes is thus proved to be an extremely efficient method, since the size and shape could be predictively controlled by a

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slight adjustment in surfactant chain length and concentration. 2.2.1. Micelles

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This synthesis approach is highly useful when the metal ions exhibit a tendency to form

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metal-surfactant complexes in aqueous solution around cmc (critical micelle concentration) value. Highly ordered mesoporous silica nanoparticles (MSNs) with tunable pore-size and shapes

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were prepared with the aid of transition metal-chelating surfactant micelle complex using various metal ions such as Co2+, Ni2+, Cu2+, and Zn2+ ions [64]. Metal ions showed a tendency for complex formation with micelles of Pluronic P123 surfactant in the aqueous solution by chelating to the hydrophilic domain, i.e. the poly(ethylene oxide) group of P123 surfactant. This metal–surfactant complex due to the inherent self-assembly behavior of surfactant resulted in hexagonal packing along with silicate species. The different complexation abilities of P123 surfactant with various transition metal ions played a decisive role in the formation of ordered MSNs based on different stabilization constants of the metal-P123 complexes. In comparison to a size of 300 nm obtained in aqueous solution for natural P123 micelle (metal ion free), the size of metal-chelating P123 micelle observed was 25 nm. The Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and solid-state

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Magnetic Resonance (NMR) measurements revealed that metal ions were quantitatively involved 12

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in the metal-P123 complex formation inside the surfactant micelles which further act as precursor for nanosynthesis. The metal-chelating surfactant micelle complexes formed were successfully utilized as templates to fabricate replicated small MSNs in a silica framework aided by the co-hydrolysis of tetraethylorthosilicate (TEOS) and their precursors as represented in

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Scheme 4.

Scheme 4. (a) Schematic diagram of MSNs preparation by a transition metal-chelating P123 micelle template, (b) comparison of micelle size between natural P123 micelle and the Cu2+chelated P123 micelle complex. Reprinted from Ref. [64]. Copyright 2012, with permission from Elsevier. This study enabled the facile design and construction of MSNs overcoming the disadvantages of the already known procedures, particularly low yields and difficult synthetic routes [65-69]. Camargo et al. [70] proposed a new approach to use metallosurfactants as templates for mesoporous silica hosts in micellar solutions of cetyl trimethyl ammonium bromide (CTAB). New optical material (blue-emitting) was prepared by the encapsulation of luminescent Iridium surfactant into the mesopores of mesoporous silica (MCM-41). A single step preparation technique was followed involving templated synthesis of MCM-41 in micellar solutions of 13

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Iridium surfactant and CTAB (Scheme 5). Similar type of approach has been used for TLCT

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synthesis as discussed in the next section.

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Scheme 5. Schematic representation of the strategy used for the encapsulation of the iridium– surfactant complex. The metallosurfactant is mixed with CTAB to form micelles which are then used for the synthesis of MCM-41 via sol–gel process. Reprinted from Ref. [70]. Copyright 2011, with permission from The Royal Society of Chemistry.

Veisz and Kiraly [71] presented an interesting route to nanoparticle formation utilizing the surfactant-complex premicellar and postmicellar aggregates, formed below and above cmc

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values, respectively. Microcrystallization originated in the aqueous solution due to the presence

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of an electrostatic interaction between the complex anions and organic cations, forming a type of salt in 1 : 2 stoichiometry similar to already known [72] laurylammonium and cetylammonium

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salts of [PdCl4]2-. The addition of a reducing agent i.e. hydrazine to the micellar solution of K2[PdCl4]/ CnTABr (alkyl trimethyl ammonium bromide) lead to the formation of Pd particles in nano range. The reduction process therefore, was mediated by the metallomicelles. The [C14TA]2[PdBr4]-surfactant aggregates embedded in the micelles were the real precursors for Pd2+ to Pd0 reduction by hydrazine which was completed within seconds. A suitable microenvironment for the preparation of ultrafine Pd particles was provided by a high local concentration of the stabilizing agent at the reduction center which proceeded via the arrested growth mechanism. The aggregation of metal atoms was prevented at an early stage of the reaction by strong and rapid adsorption of surfactant molecules on the nascent particles which were in close contact with each other. As the alkyl chain length of the stabilizing agent increased, the size distribution and the mean diameter of particles decreased (Fig. 5).

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Fig. 5. The mean diameter of Pd nanoparticles plotted against the alkyl chain length of the stabilizing agent. Reprinted from Ref. [71]. Copyright 2003, with permission from American Chemical Society.

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Kinetics of particle formation was studied using time-resolved UV-vis spectroscopy. Seed formation was a very fast process which proceeded with disappearance of the characteristic

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precursor species peaks at 251 and 342 nm in just about 20 s (Fig. 6). The rapid nucleation

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followed by limited aggregation lead to the formation of zerovalent Pd atoms showing a transition from metal clusters to metal colloids. The evolution of the UV-vis spectrum for the Pd

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clusters was in complete agreement with the one calculated using Mie theory for small spherical Pd particles. Quite interestingly, the mechanism of nanoparticle synthesis proposed for micellar system was opposite to the mechanism proposed for the nonmicellar synthesis of metal nanoclusters, which involved a rather slow and continuous nucleation followed by fast, autocatalytic surface growth.

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Fig. 6. The kinetics of nucleation of Pd nanoparticles, obtained from time-resolved UV-vis spectra. (Inset: diminution of the spectrum of the metallomicelles from 1 to 20 s.). Reprinted from Ref. [71]. Copyright 2003, with permission from American Chemical Society.

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TEM micrograph for Pd particles indicated that the cubooctahedral morphology was preferred as compared to spherical. The complex-micelle aggregates played a vital role in the

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nanoparticle formation mechanism and the final morphology was mediated by the postmicellar aggregates. The structure of these metalloaggregates possessed specificity for the resulting cubooctahedral shape. This dependence of the nanoparticle shape on the metalloaggregate shape

2.2.2. Microemulsions

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has also been exemplified in the subsequent examples.

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Microemulsions are stable, colloidal „nano-dispersions‟ of oil in water or water in oil, stabilized with the aid of a surfactant. They can truly be considered as nanoreactors to carry out chemical reactions and particularly, to synthesize nanomaterials. The main advantage of this technique is that by controlling one of the synthesis parameters tailor-made products at nanoscale level with desirable properties could be produced. Using this approach, palladium nanoparticles were obtained in high yields from a W/O

microemulsion (reverse micelle) of bis(N-octylethylenediamine) palladium (II) chloride i.e. ([Pd(octen)2]Cl2)/water/chloroform) by Iida et al. [73]. NaBH4 was added as reducing agent in 5 times more molar ratio than palladium complex, and shaken vigorously for 30 min., giving a dark yellowish sol. Cryo TEM images revealed the aggregate structure of the palladium complex surfactants (Fig. 7 (a)). The shape of the palladium nanoparticles obtained (Fig. 7 (b)) was similar to the shape of aggregates formed by palladium surfactant. This was reasoned out based 16

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on the fact that palladium was located in the head group of the metal complex surfactant. In this system, the palladium complex surfactant acted both as a metal ion source and nanoparticle stabilizer. As the alkylethylenediamine ligand can form complexes with other transition metals as well, this method could be universally applied to obtain metal nanoparticles with excellent

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yield.

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Fig. 7 (a) Cryo-TEM photograph for the microemulsions of [Pd(oct-en)2]Cl2/water/CHCl3, (b) TEM photograph for the Pd(0) nanoparticles. ([[Pd(oct-en)2]Cl2] = 0.50 mol kg-1 in CHCl3, ωo =50). Reprinted from Ref. [73]. Copyright 2002, with permission from The Chemical Society of Japan.

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On similar lines, Harada et al. [74,75] reported the formation of metal nanoparticles using microemulsion systems of noble metallosurfactants. Alkylethylenediamine complexes of silver and gold metal were reduced using NaBH4 to give the corresponding metal nanoparticles in metal complex/heptane/water system. Decreasing water content and increasing alkyl chain length of metallosurfactant resulted in smaller size as well as a smaller size distribution of silver nanoparticles formed.

Going one step further, Li et al. [76] have proposed that the interfacial activity of microemulsions could be exploited to couple self-assembly and nanoparticle synthesis. Materials with complex organization could be produced as a result of the interdigitation of surfactant molecules attached to specific nanoparticle crystal faces. Three different types of nanostructures for barium chromate viz, linear chains, rectangular superlattices and long filaments were synthesized by varying the reactant molar ratio (Fig. 8). The reactions were carried out using a barium surfactant, i.e. Barium bis(2-ethylhexyl)sulphosuccinate (Ba(AOT)2). The reverse 17

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micelles of Ba(AOT)2 were added to sodium chromate (Na2CrO4)-containing NaAOT microemulsion, giving different molar ratios of [Ba2+]:[CrO42-]≈1 with water content ω = [H2O]: [NaAOT]=10. The nanoparticle precipitates were obtained after 3 hrs. Therefore, if suitable

and higher-order colloids in nano size range.

(c)

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precursors are utilized, it is possible to extend this approach to produce one-dimensional „wires‟

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Fig. 8. TEM image (a) ordered chains of prismatic BaCrO4 nanoparticles prepared in AOT microemulsions at [Ba2+]:[CrO2-4] molar ratio≈1 and ω≈10 (b) Rectangular superlattice of BaCrO4 nanoparticles (Inset: electron diffraction pattern) (c) Bundles of BaCrO4 nanofilaments prepared at [Ba2+]:[CrO2-4] ≈5 : 1 and ω=10; scale bar =500 nm. (d) Higher magnification image of single filaments elongated along the crystallographic a axis; scale bar =200 nm (Inset: electron diffraction pattern recorded from an individual fibre) (e) Spherical BaCrO4 nanoparticles prepared at [Ba2+]:[CrO2-4] ≈1 : 5 and ω= 10; scale bar =50 nm. Reprinted from Ref. [76]. Copyright 1999, with permission from Macmillan Publishers Ltd: [Nature].

Pileni and co-workers [77-79] extensively investigated the influence of the microstructure of the microemulsion phases on the shape and homogeneity of the copper particles. For this purpose, the phase diagram of Cu(AOT)2 at room temperature was mapped. Cu(AOT)2/isooctane/water microemulsion systems, used as templates for the reduction of Cu2+, produced crystalline nanoparticles employing hydrazine as a reducing agent, under N2 atmosphere. Based on the phase diagrams, it was concluded that, (i) spherical particles formed within reverse cylindrical micelles and in the three-phase region of a lamellar liquid crystalline phase (multilamellar spherulites) in equilibrium with a bicontinuous bilayer and oil phase; (ii) mixtures 18

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of spheres and cylinders crystallized within interconnected cylinder phases. Particle formation was investigated with respect to hydration effects of the ionic head group. Similarly, a cobalt complex of AOT i.e. Co(AOT)2 was used to synthesize monodisperse cobalt nanocrystals [80]. Reverse micelles of 5x10-2 M Co(AOT)2 lead to the formation of an

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isotropic phase with water content, ω =[H2O]/[AOT] =32. The amount of NaBH4 used as a reducing agent, was defined by the R value, i.e. R=[NaBH4]/[Co(AOT)2]. Immediately after the

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addition of NaBH4, solution changed color (pink to black), giving the first indication for colloidal cobalt nanoparticle formation. Based on the R value, two types of behavior could be

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distinguished: (i) At R<1, low volume of NaBH4, reverse micelles maintained their stability by acting as nanoreactors, in which the nucleation and growth of cobalt nanocrystals occurred (ii) At R≥1, i.e. the supersaturate regime, micelles got destroyed owing to the limiting water

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concentration, ω. In this case, two kinds of nanocrystal populations were observed. Further, exploring AOT metallosurfactants, silver nanodisks with controlled particle size

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were produced using silver(I) di-(ethylhexyl)sulfosuccinate (Ag(AOT)) [81,82] in the presence of hydrazine as reducing agent. Synthesis was carried out by mixing two solutions. First one, a

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reverse micelle solution of 60% 0.1 M Ag(AOT) and 40% 0.1 M Na(AOT) solubilized in isooctane with water content, ω =[H2O]/[AOT] kept at 2. Second one, 0.1 M Na(AOT) in

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isooctane, with water replaced by hydrazine (N2H4). The hydrazine concentration was varied

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from 0.5-1.66 M, and its amount was defined by y =[N2H4]/[AOT]. On mixing two solutions, a dark color appeared. The color of solution varied with initial synthesis conditions, from red to green and gray as the reducing agent concentration increased (Fig. 9).

Fig. 9. Solutions of nanocrystals dispersed in reverse micelles and obtained at various hydrazine concentrations (y =[N2H4]/[AOT] ) 4.9, 5.8, 6.6, 8.2, and 12.3). Reprinted from Ref. [81]. Copyright 2003, with permission from American Chemical Society. 19

Page 19 of 67

Wang et al. [83] synthesized poly pyrrole (PPy) / Prussian Blue (PB) core/shell nanoparticles via one-step miniemulsion polymerization technique using iron metallosurfactant i.e. EPE-Fe (poly(ethylene glycol)-b-poly-(propyleneglycol)-b-poly(ethylene glycol) terminated with pentacyano(4-(dimethylamino) pyridine)ferrate). PPy/PB core/shell nanoparticles were

ip t

obtained by the addition of Py monomers to the miniemulsion droplets of EPE-Fe. With the addition of Fe3+, coordination of metal with the pentacyanoferrate at the periphery of

cr

miniemulsion droplets resulted in the formation of PB nanoshells. The inner oxidation polymerization of Py occurred simultaneously, leading to PPy core, eventually yielding the

us

PPy/PB core/shell nanoparticles (Scheme 6). The suspension obtained was added dropwise to methanol to get nanoparticle precipitates with a size of ca. 200 nm (Fig. 10). As estimated from

an

the TEM images, the core radius of PPy and the thickness of the PB shell were ca. 100 nm and

Ac ce p

te

d

M

ca. 2 nm, respectively.

Scheme 6. Schematic illustration of PPy/PB core/shell nanoparticles produced via one-step miniemulsion polymerization. Reprinted from Ref. [83]. Copyright 2011, with permission from The Royal Society of Chemistry.

Fig. 10. TEM micrographs of the PPy/PB core/shell nanoparticles at lower magnification (a), at higher magnification (inset: EDX mapping images of PPy/PB core/shell nanoparticles, upside: 20

Page 20 of 67

nitrogen element; downside: iron) (b) Reprinted from Ref. [70]. Copyright 2011, with permission from The Royal Society of Chemistry. Very recently, Yuan et al. following the same miniemulsion polymerization technique [84] prepared DTPA-diamide-like Gd (III) complex to yield nanosized colloids which acted as highly

ip t

efficient MRI contrast agents. The Gd (III) surfactant was functionalized with double hydrophobic chains, endowing it with the ability to self-assemble forming a stable emulsion to

cr

be incorporated into polymer colloids (Scheme 7). Miniemulsion polymerizations of vinyl monomers were performed in the presence of metallosurfactant to give colloids in the size range

us

of 50–110 nm, without any need of cosurfactant (Fig. 11). Narrow size distribution was obtained by varying the ratio of metallosurfactant and vinyl monomers. XPS analysis showed the presence

an

of Gd element on the polymer colloids. The surfactant was firmly attached on the surface of the colloids through the two long alkyl chains. The giant Gd (III) complex formed a kind of stable electric double layer on the colloidal particles, hence, preventing aggregation. ICP-AES further

M

confirmed that over 60% of the Gd (III) complexes were attached to the polymer colloids. With the double hydrophobic chains, the Gd (III) complexes were firmly attached on the polymer

Ac ce p

te

d

latex, limiting local rotational freedom, and resulting in greatly enhanced relaxivity for MRI.

Scheme 7. Schematic representation of the rotational motion of the Gd(III)-based metallosurfactant before and after incorporation into a polymer colloid. Reprinted from Ref. [84]. Copyright 2013, with permission from The Royal Society of Chemistry.

21

Page 21 of 67

ip t

us

cr

Fig. 11. TEM images of polymer colloids obtained with metallosurfactants; 6.0 mM (A), 3.0 mM (B) and 1.5 mM (C). Reprinted from Ref. [84]. Copyright 2013, with permission from The Royal Society of Chemistry. 2.2.3. Other self assembly structures

As is already known, amphiphilic molecules self assemble to supramolecular aggregates

an

at the interface. A variety of micellar structures, including bilayers, long ordered, disordered as well as short cylinders, and spheres, could be formed at the mica-solution interface. The shapes

M

of micellar aggregates depend on the surfactant geometry, such as the shape of the head group, size of the tail and counterions etc. Kawasaki et al. [85] used this self-assembly of surfactants at the solid-solution interface as a template to synthesize nanostructures.

d

Synthesis of crystalline, uniform platinum nanosheets (width=3.5 nm), was carried out by

te

the reduction of a Pt salt (H2PtCl6) with hydrazine at Highly Oriented Pyrolytic Graphite (HOPG)-solution interface in the presence of Tween 60. Tween 60 acting as a template structure

Ac ce p

formed hemicylindrical micellar self-assemblies [86]. Nonionic/cationic mixed hemicylindrical micelles formed using the surfactant dodecyldimethylamine oxide at HOPG-solution interface act as templates to synthesize one-dimensional self-assemblies of Pt nanoparticles on HOPG surface via template directed sintering process [87]. But during the drying process, the selfassembled structures tend to break which indicated that the surface micelles were stable only in aqueous media. The low stability of surface micelles was therefore, unfavorable for the formation of nanostructures. To counter this effect, the self-assembly of surfactant-platinum complex nanofibers on HOPG surface was reported using surfactant solution of hexadecyltrimethylammonium hydroxide (C16TAOH) with hydrogen hexachloroplatinate (IV) (H2PtCl6) [86]. The HOPG plates covered with surfactant self-assemblies were immersed in water initially and then reduced with hydrazine. After carrying out reduction for 30 min., the HOPG substrates were cleaned and dried in a laminar flow bench for approximately 30 min. 22

Page 22 of 67

(Fig. 12). In a similar fashion, gold ions were reduced confined within surfactant nanofibers on HOPG surfaces, yielding gold nanoparticles. Hence, fibrous self-assemblies of a surfactant-metal complex formed on the HOPG surface could be efficiently used as reactive sites for the formation of metal nanoparticles. In contrast to the surfactant hemicylindrical micelles which

ip t

were stable only at HOPG-solution interface, the fibrous surfactant self-assembly remained stable in air, even after being soaked in water. Atomic Force Microscope (AFM) cross-sectional

cr

profiles showed that the nanofibers possessed a width of 5-15 nm and a thickness of 3 nm. The driving force for the formation of the fibrous self-assembly on the HOPG surface was the

us

favorable interaction between the surfactant-metal complex and HOPG surface.

Using a surfactant or polymer metal complex as reactant showed several advantages such

an

as; (i) reduction in the amount of precursor metal ions needed for the synthesis of nanoparticles, because the precursor metal ions were totally impregnated on the surfactant or polymer surface prior to reduction, (ii) regulating the shape, arrangement and size of nanoparticles with the aid of

Ac ce p

te

d

M

different surfactants.

Fig. 12. Tapping-mode AFM images (in air) of a fibrous self-assembly on an HOPG surface: (a) before reduction, (b) after reduction, (c) phase-contrast image for panel b. Reprinted from Ref. [86]. Copyright 2005, with permission from American Chemical Society. 23

Page 23 of 67

Taking cue from the strategy followed by Brust et al. [88] to synthesize thiol-derivatised gold nanoparticles which consisted of growing metallic particles with simultaneous attachment of the thiol monolayers on the growing nuclei, Manna et al. [89] synthesized silver nanoparticles passivated with a surfactant, N-hexadecylethylenediamine silver nitrate. Amine-protected silver were

prepared

from

N-hexadecylethylenediamine

silver

nitrate

ip t

nanocomposites

(Ag(hexden)2NO3) complex. The nanoparticles were free from surface contamination caused by

cr

an external phase-transfer agent. The major advantage of the procedure was the formation of silver nanoparticles from a single reactant source i.e. Ag(hexden)2NO3.

us

The TEM image (Fig. 13 (a)-(c)) showed a two-dimensional array of the silver particles with an edge to edge spacing of 3.8-3.9 nm between silver cores (approximately double of N-

an

hexadecylethylenediamine chain length). The closest center to center distance between neighboring particles calculated using the first scattering peak of the Small Angle X-Ray Scattering (SAXS) spectrum was in complete agreement with the TEM result. The large edge-

M

edge spacing observed between the cores, compared to the usually shorter ones for alkanethiols and fatty acid protected nanoparticles, was attributed to the dense packing of the diamine

represented in Fig. 13 (d) and (e).

d

monolayer on the metal core leading to weak interaction between neighboring particles as

te

Although a lot of surfactants have been used for complexation with metals, however a

Ac ce p

few studies report the complexation ability of polymer towards metals and further use them as nanoreactors.

Jang et al. [90] have reported the novel fabrication and aqueous self-assembly of amphiphilic nanocrystallo-polymers driven by hydrophobic interaction. Hydrophobic Au nanocrystals with alkyl chain monolayer were used as the hydrophobic component of amphiphiles, which could be uniformly arranged in the core of the nanostructures while the hydrophilic polymer backbone was conjugated to Au by a thiolate bond.

24

Page 24 of 67

(d)

(b)

an

us

(e)

cr

ip t

(a)

d

M

(c)

Ac ce p

te

Fig. 13. TEM photographs (left-hand side) and particle size distributions (right-hand side) of Nhexadecylethylenediamine protected silver nanoparticles at (a) 1.5, (b) 4.5, and (c) 9.0 mM concentration in n-heptane. Schematic illustration of nanoparticle structure: (d) the normal bilayer between N-hexadecylethylenediamine protected silver nanoparticles, (e) conventional interdigitated monolayer between alkanethiol- or fatty acid-protected nanoparticles. Reprinted from Ref. [89]. Copyright 2001, with permission from American Chemical Society. For the synthesis, a water-soluble biocompatible polymer with a poly(amino acid) structure, Poly(2-hydroxyethyl L-aspartamide) (PHEA), was used which could be modified to form graft structures. PHEA conjugated with undecanethiols (P-g-C11SH) by forming an ester bond. These conjugated undecanethiols acted as a link between PHEA and the Au nanocrystals. Ligand place exchange was a key step for the formation of amphiphilic nanocrystallopolymers. Undecanethiols conjugated onto PHA participated in ligand place exchange to give PHEA grafted with alkanethiolate protected Au nanocrystals (P-g-AuNC) as in Fig. 14 (b). This polymer obtained was termed as nanocrystallopolymer which has dynamic self assembling and surface-active properties. Hydrophobic Au nanocrystals were tightly packed inside a self25

Page 25 of 67

aggregate to form a core, and hydrophilic PHEA was located at the surface of the core part to form a corona layer. Therefore, it became possible to incorporate bulky nanocrystals and simultaneously control the shape from spherical aggregates to cylinders using this approach (Fig. 15).

ip t

Using other types of inorganic nanocrystals with hydrophobic and place-exchangeable ligands, self-assembling amphiphilic nanocrystallo-polymers could be fabricated. The key factor

cr

to control the morphology is the hydrophilic to hydrophobic ratio achieved by controlling the length of each block in the block copolymer. The nanostructures synthesized in this manner are

materials. (c)

(d)

Ac ce p

te

(b)

d

M

an

(a)

us

expected to exhibit unique physical properties that bridge the gap between molecular and bulk

Fig. 14. Synthetic route of amphiphilic nanocrystallo-polymer (a) PHEA grafted with undecanethiols, (b) PHEA grafted with dodecanethiolate-protected Au nanocrystals, (c) Water solution of nanocrystallo-polymers and chloroform solution of Au nanocrystals, (d) The surface tension of aqueous solution of alkanethiolate-protected Au nanocrystals (P-g-Au NC) as a function of concentration. Reprinted from Ref. [90]. Copyright 2008, with permission from The Royal Society of Chemistry.

26

Page 26 of 67

(b)

ip t

(a)

us

cr

(c)

an

Fig. 15. TEM images of nanostructures self-assembled from PHEA grafted with Au nanocrystals and dodecyl chains. (a) Spherical aggregates, (b) Core–shell type unimolecular micelles, (c) Cylinders; Scale bars: (a) 200 nm, (b) 200 nm (inset :40 nm), (c) 80 nm (inset :40 nm). Reprinted from Ref. [90]. Copyright 2008, with permission from The Royal Society of Chemistry.

M

2.3. True Liquid Crystal Templating (TLCT) technique

Using lyotropic liquid crystals for producing mesoporous metallic materials with uniform

d

pore size in the nano range is currently a topic of great interest. This has lead to the development

te

of a new technique “True Liquid Crystal Templating” (TLCT). In this approach, the surfactant initially forms a lyotropic liquid crystal phase with well defined geometry that ultimately

Ac ce p

proceeds to nanoparticle formation. The TLCT approach was first introduced by Attard and coworkers [91] where nonionic polyoxyethylene surfactant (C12EO8) was used with tetramethylorthosilicate (TMOS) as silica precursor. Pre-formed liquid crystal mesophases of neutral and cationic surfactants were used to template silicates using a sol–gel methodology where the pore morphology of the final silicate could be pre-determined by the structure of the liquid crystal phase on which it was templated. In seeking new routes to mesoporous silicates containing metal or metal oxide nano particles, King et al. [92] used the hexagonal H1 mesophase [93,94] of some Ru (II)–bipyridine surfactants [95,96]. This approach was also used by Bruce et al. [97] to prepare nanoparticulate Ru containing silicates. Using TLCT, preparation of mesostructured silicates doped with RuO2 was carried out for further use as effective catalysts for alkene hydrogenation and water oxidation. Mesoporous silicates containing Pd and Ir nanoparticles were synthesized using the salts 27

Page 27 of 67

K[AuCl4], K2[PdCl4], K2[PtCl4] and Na3[IrCl6] dissolved in water (10 wt%, pH =2 with HCl) and mixed with C12EO8 surfactant in 1:1 ratio. The resulting mixtures existed in the H1 phase. The mesophases so obtained were then employed in the TLCT synthesis of mesoporous silicates. The morphology of the resulting materials could be predicted due to the application of TLCT. An

ip t

even distribution of uniformly sized metal nanoparticles was achieved as evident by TEM images in Fig. 16.

bipyridine

complex

of

Ru

(II)

with

a

conventional

cr

On comparing the structural properties of silica products obtained by using a surfactant cationic

surfactant,

CTAB

us

(cetyltrimethylammonium bromide), it was established that templating with the Ru metallosurfactant lead to better results and well-ordered mesoporous silica. Catalytically active

Ac ce p

te

d

M

an

RuO2 nanoparticles obtained were uniformly distributed within the silica pores [98].

Fig. 16. TEM images of mesoporous silicates containing (a) Pd, (b) Ir (the black dots are metallic nanoparticles inside the pores), (c) Fe-containing sample viewed down the [100] direction. The arrows indicate Fe nanoparticles inside the channels, (d) Fe-containing sample viewed down the pore axis under an over-focus condition. Reprinted from Ref. [92]. Copyright 2006, with permission from The Royal Society of Chemistry. The same group, using a one-pot approach, employing TLCT on neutral surfactants and simple metal salt precursors, prepared mesostructured, mesoporous silicates in which bimetallic nanoparticles were deposited (Fig. 17) [99]. It was also possible to extend this technique to produce advanced, highly anisotropic nano-particulate media. But further refinement of the 28

Page 28 of 67

TLCT technique was required to attain truly monodispersed particle sizes as desired. Hondow et al. [100] in a recent report prepared new metal-doped mesoporous silicas in a single step from a mixture of metallo and organo surfactants by slight modification in the standard liquid–crystal templating route. Macrobicyclic hexamine ligands i.e. „sarcophagines‟ along with copper and

ip t

cobalt were investigated as templating agents in the synthesis of mesoporous silicas. The materials obtained were well ordered, hexagonally arranged metal incorporated materials

M

an

us

cr

possessing high surface area.

te

d

Fig. 17. TEM images of mesostructured silica containing (a) PtCo, calcined at 400 oC, (b) PdRu. The arrow indicates wall distortion. Reprinted from Ref. [99]. Copyright 2006, with permission from The Royal Society of Chemistry.

Ac ce p

2.4. Using other reduction techniques

Since the advent of nanoscience, the most commonly employed method for nanoparticle fabrication is the simple reduction process. Various reduction methods used extensively for nanosynthesis processes include chemical, photochemical, electrochemical and sonochemical reduction. The present section discusses the methods of nanofabrication which do not proceed simply by the addition of a reducing agent but require an external trigger or driving force or input of energy such as sonication, conditions of high pressure or temperature etc.

2.4.1. Sonochemical reduction Nanoscale particles of palladium were prepared by sonochemical reduction of Pd(O2CCH3)2, and myristyltrimethylammonium bromide, CH3(CH2)12N(CH3)3Br (NR4X) in 1:2 molar ratio using tetrahydrofuran (THF) or methanol by Dhas and Gedankan at room temperature [101]. UV–vis spectroscopy indicated the initial formation of a PdII–NR4X complex, 29

Page 29 of 67

which reduced to Pd0 using sonication. Sonochemical effect is based on the idea of cavitational collapse which induces the generation of a reaction site where temperature is raised to several thousands of K for a period of μs. The exceptionally high temperature and pressure conditions generated in situ during cavitational collapse, combined with high rates of cooling, lead to the

ip t

formation of large amounts of reducing radicals which acted as reducing agents for the PdII→Pd0 reaction. Apart from its stabilizing effect to keep the Pd nanoclusters free from agglomeration,

cr

NR4X also acted as a reducing agent.

Elemental analysis of the resulting precipitates revealed that the THF process gave NR4X

us

stabilized-Pd clusters, whereas the methanol process yielded pure Pd agglomerates. XRD and TEM with SAED techniques revealed the size and morphology of the Pd clusters. TEM of NR 4X

an

stabilized-Pd as in Fig. 18 showed the presence of spherical particles, 10–20 nm in size. Pure Pd consisted of dense agglomerates, whereas NR4X stabilized-Pd existed as thin crystallites. The particle size of 70 nm obtained for stabilized-Pd nanoparticles (X-ray line broadening) indicated

M

a more than three-fold increase in size upon calcination. (b)

Ac ce p

te

d

(a)

(c)

Fig. 18. TEM and associated SAED pattern (inset) of as-formed Pd particles (a) methanol process, (b) THF process, (c) alcoholic THF process. Reprinted from Ref. [101]. Copyright 1998, with permission from The Royal Society of Chemistry. 30

Page 30 of 67

These Pd nanoclusters were catalytically active towards carbon–carbon coupling, or Heck reaction, to a moderate extent of 30% conversion. The hydrogenation of cyclohexene to cyclohexane was also carried out using sonochemically generated Pd materials. The formation of crystalline Pd products suggested that an interfacial and/or bulk process was involved in the

ip t

sonochemical reduction. It was evident that the choice of solvent had a direct impact on the chemical composition, aggregation, dispersity and consequently, the catalytic activity of the

cr

nanoparticles.

us

2.4.2. Electrostatically induced reduction

Xia and co-workers [102] have reported a facile method for preparing ultrathin Au

an

nanowires using [(oleylamine)AuCl] complex chains formed through aurophilic attraction. On parallel lines, a facile and high-yielding method to synthesize unique, porous gold nanobelts was reported by Li et al. [103]. The synthesis procedure was based on the self-organization tendency

M

of nanoparticles. Initially, the formation of nanobelts of the complex N-C12-N(AuCl4)2 which act as precursor was triggered through an electrostatic interaction between negatively charged AuCl4

d

ions and positively charged quaternary ammonium headgroups of N-Cn-NBr2. On reduction with NaBH4, porous gold nanobelts were obtained with shrunken sizes. Schematic illustration has

te

been given in Scheme 8. Compared to solid gold nanobelts, the synthesized porous Au nanobelts

Ac ce p

exhibited improved catalytic activity towards the reduction of 4-nitrophenol, owing to the larger surface area and more number of active sites. This work demonstrated for the first time that metal-surfactant complex precursors could be used as reactive templates for the controlled formation of 1D beltlike, porous metal nanostructure. The bolaform surfactant formed a complex with HAuCl4 due to a strong interaction between two cationic quaternary ammonium headgroups and AuCl4- anions, which played a crucial role in the formation of the porous gold nanobelts. Therefore, it was worthwhile to pay attention to the complex formed between N-C12-NBr2 and HAuCl4. When 0.25 mM N-C12-NBr2 and 0.2 mM HAuCl4, were mixed the precursor complex N-C12-N(AuCl4)2 was immediately formed via electrostatic as well as vander Waals interactions. This metal-surfactant complex then self-assembled and crystallized into curled nanobelts of 500 nm width, 50-100 nm thickness, and several micrometers length (Fig. 19 (e) and (f)). As the reducing agent, NaBH4 was added to the suspension of the complex nanobelts, obtained in the first step, the N-C12-N(AuCl4)2 complex 31

Page 31 of 67

reduced to gold nanocrystals, leading to the formation of porous gold nanobelts with shrunken

M

an

us

cr

ip t

sizes (i.e. about 100-200 nm width and less than 20 nm thickness) (Figs. 19 (a)-(d)).

(e)

(d)

(f)

(b)

Ac ce p

(a)

te

d

Scheme 8. Schematic representation of nanobelt formation. Reprinted from Ref. [103]. Copyright 2010, with permission from American Chemical Society.

(c)

Fig. 19. SEM images of gold products obtained from metal surfactant complex precursors formed at different N-C12-NBr2 concentrations: (a) 0 (b) 0.1 mM, (c) 2.5 mM, (d) 5 mM 32

Page 32 of 67

[HAuCl4] = 0.2mM, (e) and (f) ESEM images of metal-surfactant complex precursor obtained from 0.25 mM N-C12-NBr2 and 0.2 mM HAuCl4. Reprinted from Ref. [103]. Copyright 2010, with permission from American Chemical Society. 2.4.3. Pressure driven reduction

ip t

Fan et al. [104] synthesized silver nanoparticles by reduction of a supercritical solution of CO2 containing 0.06 wt% silver bis(3,5,5-trimethyl-1-hexyl) sulfosuccinate (Ag-AOT-TMH) and 0.5 wt% perfluorooctanethiol (stabilizing ligand) using NaBH4 under conditions of high pressure.

cr

The objective of the study was to devise a novel organometallic complex possessing superior

us

CO2 solubility. The idea was to fabricate silver nanoparticles in CO2 via simple reduction process in the presence of either fluorous or nonfluorous stabilizing thiols. Ag-AOT-TMH was used as the metal precursor because of its facile synthesis procedure, which required only ion

an

exchange with the CO2-soluble ionic surfactant, AOT-TMH to give Ag-AOT-TMH. Reduction of Ag-AOT-TMH lead to silver nanoparticles in the sizes ranging from 1 to 6 nm, which were

M

further, characterized using energy-dispersive spectroscopy (EDS) and TEM (Fig. 20). However, attempts to generate nanoparticles from Ag-AOT-TMH using a completely nonfluorous system were futile as effective stabilization could not be achieved with PEG or hydrocarbon based

d

thiols. Hence, it was inferred that only the fluorinated thiols gave the nanoparticle dispersion

te

successfully. This was further confirmed by EDS spectrum which revealed the presence of silver along with fluorine and sulfur, indicating that the fluorinated-thiol acted as the stabilizing agent.

Ac ce p

Ion exchange of silver with other ionic surfactants composed of oxygenated hydrocarbon tails was unsuccessful because of the unstable Ag+ counterion in the synthesis procedure. (a)

(b)

Fig. 20. (a) TEM image of silver nanoparticles formed by reducing a CO2 solution containing Ag-AOT-TMH and fluorinated-thiol stabilizing ligands using NaBH4 as reducing agent, (b) EDS 33

Page 33 of 67

measurements of the silver nanoparticles. Reprinted from Ref. [104]. Copyright 2006, with permission from American Chemical Society. An attempt to produce stable metal and metal oxide nanoparticles using a metallosurfactant [M(CH3COO)4] [C12H25NH3]2 has also been made by our group [105] by

ip t

simply using a reducing agent, such as NaBH4 or hydrazine. The detailed synthesis and characterization of the metallosurfactants has already been published elsewhere [106,107]. The

cr

main idea behind using a metallosurfactant is that, not only it provides the metal ion but also a hydrophobic chain to cap the metal nanoparticle formed in situ. The nanoparticle synthesis was reducing

agent

was

added

to

the

violet

us

carried out in water-heptane biphasic redox system. Freshly prepared aqueous solution of colored

copper

surfactant

complex

([Cu(CH3COO)4][C12H25NH3]2) in n-heptane phase and stirred vigorously for approximately 2

an

hrs. This resulted in the formation of wine red colored Cu nanoparticles. To study the effect of polymer on the nanoparticles, Polyacrylic acid (PAA) was used to cap the nanoparticles. It was

M

observed that Cu nanoparticles remained stable for weeks in the presence of PAA, but in the absence of PAA, Cu nanoparticles converted to CuO nanoparticles (Scheme 9). The size of Cu and CuO nanoparticles was 30 and 10 nm, respectively. Both types of nanoparticles synthesized

Ac ce p

te

d

acted as excellent catalysts for reduction reactions.

[Cu (CH 3COO) 4 ]2 [C12H 25NH3+ ]2 (hep tan e) + 4e (aq)

(1)

Cu (C12H 25NH 2 )(hep tan e) + 4CH 3COO (aq) + H 2 ↑ air Cu (C12 H 25 NH 2 )  CuO(C´12 H 25 NH 2 )

(2)

Scheme 9. Pictorial representation and reaction scheme for preparation Cu nanoparticles. 34

Page 34 of 67

To summarize, metallosurfactants can be efficiently used as nano reactors. The metalloamphiphiles are of considerable interest to develop metal-based catalytically active nanostructures (Table 1). The ability of metallosurfactants to self assemble into specific architectures and resulting into the formation of nanostructures looks promising, but has not yet

ip t

reached its full potential. The engineering of metalloamphiphiles to design nanoreactors as desired or drug-delivery systems or even components in quantum electronics can be further

cr

explored resulting in numerous commercial applications. Overall, it seems that the studies towards metalloamphiphiles and the exploitation of such materials in nanoparticle fabrication

an

bound to produce exciting results for years to come.

us

and catalysis, holds tremendous promise for future advances. The line of metallosurfactants is

Table 1 Metallosurfactants as nanoreactors.

4.

5. 6. 7. 8. 9. 10. 11.

d

te

3.

Ac ce p

2.

Method of Nanoparticles Morphology of preparation synthesized nanoparticles Thermal γ-Fe2O3 Spherical decomposition nanocrystals Iron oleic acid complex Thermal Fe, γ-Fe2O3 Nanocrystallites decomposition (ƞ 5C5H5)CoFe2(CO)9 Thermal Co-Fe alloy Nanocrystals oleic acid complex decomposition Metal oleylamine complex Thermal PbS, ZnS, Cube-shaped, rods, decomposition CdS, and bipods, tripods and MnS bullet-shaped nanocrystals Fe(II) and Fe(III) oleic acid– Thermal Fe3O4 Nanocrystallites oleylamine adduct decomposition Transition metal-P123 micelle Micellar mesoporous Nanoparticles, complex synthesis silica nanorods Iridium surfactant and CTAB Micellar mesoporous Nanoparticles synthesis silica K2[PdCl4]/CnTABr Micellar Pd Cubooctahedral synthesis particles [Pd(octen)2]Cl2 Microemulsion Pd Nanoparticles media Bis (doden)AgNO3 Microemulsion Ag Nanoparticles media Bis (doden)AuNO3 Microemulsion Au Nanoparticles media

M

S.No Metal surfactant . complex 1. Iron oleate complex

Ref. 50 49 51 53

54 64 70 71 73 74 74

35

Page 35 of 67

13.

Cu(AOT)2

Cu

14.

Co(AOT)2

15.

Ag(AOT)

15.

EPE-Fe

17.

Gd (III) complex

18.

H2PtCl6- C16TAOH complex

19.

Au-PHEA polymer complex

20.

PdII–NR4X

21.

N-C12-N(AuCl4)2

22.

Ag(hexden)2NO3

23. 24. 25.

27.

Ag AOT-TMH tris(bipyridine)ruthenium(II) K2[PdCl4], Na3[IrCl6] and C12EO8 complex K2[PtCl4], Na2[Co(EDTA)] and C12EO8 complex [Cu(CH3COO)4][C12H25NH3]2

Microemulsion media Microemulsion media Microemulsion media Miniemulsion polymerization Miniemulsion polymerization Fibrous selfassembly on an HOPG surface Self-assembly structure Sonochemical reaction Electrostaticall y driven reduction Biphasic reduction Reduction TLCT TLCT

3.

Conclusions and Outlook

Ag

Nanodisks

82

PPy/PB core/shell Gd

nanoparticles

83

Nanoparticles

84

80

86

90

Pd

Nanocrystals, cylinders Nanoclusters

Au

Nanobelts

103

Ag

Nanoparticles

89

Ag Ru Pd, Ir

Nanoparticles Nanoparticles Nanoparticles

104 97 92

TLCT

PtCo

Nanoparticles

99

Biphasic reduction

Cu

Nanoparticles

105

d

M

an

Pt

78

Nanofibres

te

Ac ce p

26.

76

Co

Spherical, linear chains, rectangular superlattices and long filaments Mixture of spheres and cylinders Nanocrystals

ip t

Microemulsion BaCrO4 media

cr

Ba(AOT)2

us

12.

Au

101

It can be effectively concluded that rational designing and modification offers a possibility to control various properties of metallosurfactants. The structural framework of the metallosurfactant can be fine tuned according to the need and desired applications. The solid state metallosurfactants and even the self assembled structures obtained in solution form can be efficiently utilized in nanoparticle synthesis. Metalloaggregate structures are excellent scaffolds 36

Page 36 of 67

for nanometer sized structures as they have dynamic surface-active properties and offer several advantages over existing conventional methods. Therefore, owing to the self-assembly process, diverse morphologies can be obtained, resulting in a regular and controlled arrangement of functionalized or derivatised nanostructures. This approach presents several advantages over the

ip t

conventional methods of nanoparticle fabrication which include (a) control over nanostructure morphology as well as size by changing the metallosurfactant ratio (b) facile approach without

cr

using expensive reagents and laborious conditions of temperature, pressure or inert atmosphere (c) absence of stabilizing or capping or phase transfer agent to name a few. This subject area has

us

rightly attracted new found research interest from both organometallic and nanosciences. This can prove to be a revolution in the field of surfactant and nanochemistry by acting as a bridge

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between two fields.

Acknowledgements

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SKM and RK are thankful to Department of Science and Technology (DST) and Council of

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Graphical Abstract (for review)

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Graphical Abstract

Various approaches to nanoparticle fabrication using metallosurfactants

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*Highlights (for review)

Highlights

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 Metallosurfactants as nanoreactors are advantageous over conventional nanofabrication routes.  Synthetic procedure is general and could be applied to get nanocrystals for several metals.  Precise control over nanostructure morphology and size by changing only metallosurfactant ratio

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Figure(s)

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Fig. 1. Synthesis of monodisperse γ-Fe2O3 under controlled oxidation. Reprinted from Ref. [42]. Copyright 2003, with permission from The Royal Society of Chemistry.

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Fig. 2. TEM images (upper: low magnification, bottom: higher magnification) of the iron oxide nanoparticles at various reaction time intervals. Reprinted from Ref. [43]. Copyright 2004, with permission from Macmillan Publishers Ltd. [Nature Materials].

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Fig. 3. TEM images of CdS nanocrystals. (a) Mixture of rods, bipods, and tripods (Inset: HRTEM image of a single CdS bipod-shaped nanocrystal), (b) Spherical nanoparticles. Reprinted from Ref. [47]. Copyright 2003, with permission from American Chemical Society.

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Fig. 4. TEM images of MnS nanostructures. (a) MnS nanorods (Inset:HRTEM image of a single MnS nanorod), (b) Long bullet-shaped MnS nanocrystals, (c) Short bullet-shaped MnS nanocrystals, (d) Hexagon-shaped MnS nanocrystals. Reprinted from Ref. [47]. Copyright 2003, with permission from American Chemical Society.

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Fig. 5. The mean diameter of Pd nanoparticles plotted against the alkyl chain length of the stabilizing agent. Reprinted from Ref. [56]. Copyright 2003, with permission from American Chemical Society.

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Fig. 6. The kinetics of nucleation of Pd nanoparticles, obtained from time-resolved UV-vis spectra. (Inset: diminution of the spectrum of the metallomicelles from 1 to 20 s.). Reprinted from Ref. [56]. Copyright 2003, with permission from American Chemical Society.

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Fig. 7 (a) Cryo-TEM photograph for the microemulsions of [Pd(oct-en)2]Cl2/water/CHCl3, (b) TEM photograph for the Pd(0) nanoparticles. ([[Pd(oct-en)2]Cl2] = 0.50 mol kg-1 in CHCl3, ωo =50). Reprinted from Ref. [58]. Copyright 2002, with permission from The Chemical Society of Japan.

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(b) Fig. 8. TEM image (a) ordered chains of prismatic BaCrO4 nanoparticles prepared in AOT microemulsions at [Ba2+]:[CrO2-4] molar ratio≈1 and ω≈10 (b) Rectangular superlattice of BaCrO4 nanoparticles (Inset: electron diffraction pattern) (c) Bundles of BaCrO4 nanofilaments prepared at [Ba2+]:[CrO2-4] ≈5 : 1 and ω=10; scale bar =500 nm. (d) Higher magnification image of single filaments elongated along the crystallographic a axis; scale bar =200 nm (Inset: electron diffraction pattern recorded from an individual fibre) (e) Spherical BaCrO4 nanoparticles prepared at [Ba2+]:[CrO2-4] ≈1 : 5 and ω= 10; scale bar =50 nm. Reprinted from Ref. [61]. Copyright 1999, with permission from Macmillan Publishers Ltd: [Nature].

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Fig. 9. Solutions of nanocrystals dispersed in reverse micelles and obtained at various hydrazine concentrations (y =[N2H4]/[AOT] ) 4.9, 5.8, 6.6, 8.2, and 12.3). Reprinted from Ref. [66]. Copyright 2003, with permission from American Chemical Society.

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Fig. 10. TEM micrographs of the PPy/PB core/shell nanoparticles at lower magnification (a), at higher magnification (inset: EDX mapping images of PPy/PB core/shell nanoparticles, upside: nitrogen element; downside: iron) (b) Reprinted from Ref. [70]. Copyright 2011, with permission from The Royal Society of Chemistry.

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Fig. 11. TEM images of polymer colloids obtained with metallosurfactants; 6.0 mM (A), 3.0 mM (B) and 1.5 mM (C). Reprinted from Ref. [68]. Copyright 2013, with permission from The Royal Society of Chemistry.

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Fig. 12. Tapping-mode AFM images (in air) of a fibrous self-assembly on an HOPG surface: (a) before reduction, (b) after reduction, (c) phase-contrast image for panel b. Reprinted from Ref. [70]. Copyright 2005, with permission from American Chemical Society

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Fig. 13. TEM photographs (left-hand side) and particle size distributions (right-hand side) of N-hexadecylethylenediamine protected silver nanoparticles at (a) 1.5, (b) 4.5, and (c) 9.0 mM concentration in n-heptane. Schematic illustration of nanoparticle structure: (d) the normal bilayer between N-hexadecylethylenediamine protected silver nanoparticles, (e) conventional interdigitated monolayer between alkanethiol- or fatty acid-protected nanoparticles. Reprinted from Ref. [73]. Copyright 2001, with permission from American Chemical Society.

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Fig. 14. Synthetic route of amphiphilic nanocrystallo-polymer (a) PHEA grafted with undecanethiols, (b) PHEA grafted with dodecanethiolate-protected Au nanocrystals, (c) Water solution of nanocrystallo-polymers and chloroform solution of Au nanocrystals, (d) The surface tension of aqueous solution of alkanethiolate-protected Au nanocrystals (P-g-Au NC) as a function of concentration. Reprinted from Ref. [74]. Copyright 2008, with permission from The Royal Society of Chemistry.

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Fig. 15. TEM images of nanostructures self-assembled from PHEA grafted with Au nanocrystals and dodecyl chains. (a) Spherical aggregates, (b) Core–shell type unimolecular micelles, (c) Cylinders; Scale bars: (a) 200 nm, (b) 200 nm (inset :40 nm), (c) 80 nm (inset :40 nm). Reprinted from Ref. [74]. Copyright 2008, with permission from The Royal Society of Chemistry.

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Fig. 16. TEM images of mesoporous silicates containing (a) Pd, (b) Ir (the black dots are metallic nanoparticles inside the pores), (c) Fe-containing sample viewed down the [100] direction. The arrows indicate Fe nanoparticles inside the channels, (d) Fe-containing sample viewed down the pore axis under an over-focus condition. Reprinted from Ref. [76]. Copyright 2006, with permission from The Royal Society of Chemistry.

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Fig. 17. TEM images of mesostructured silica containing (a) PtCo, calcined at 400 oC, (b) PdRu. The arrow indicates wall distortion. Reprinted from Ref. [83]. Copyright 2006, with permission from The Royal Society of Chemistry.

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Fig. 18. TEM and associated SAED pattern (inset) of as-formed Pd particles (a) methanol process, (b) THF process, (c) alcoholic THF process. Reprinted from Ref. [85]. Copyright 1998, with permission from The Royal Society of Chemistry.

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Fig. 19. SEM images of gold products obtained from metal surfactant complex precursors formed at different N-C12-NBr2 concentrations: (a) 0 (b) 0.1 mM, (c) 2.5 mM, (d) 5 mM [HAuCl4] = 0.2mM, (e) and (f) ESEM images of metal-surfactant complex precursor obtained from 0.25 mM N-C12-NBr2 and 0.2 mM HAuCl4. Reprinted from Ref. [87]. Copyright 2010, with permission from American Chemical Society.

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Fig. 20. (a) TEM image of silver nanoparticles formed by reducing a CO2 solution containing Ag-AOT-TMH and fluorinated-thiol stabilizing ligands using NaBH4 as reducing agent, (b) EDS measurements of the silver nanoparticles. Reprinted from Ref. [88]. Copyright 2006, with permission from American Chemical Society.

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Table(s)

Table 1 Metallosurfactants as nanoreactors.

6. 7. 8. 9. 10.

Ref. 50 49 51 53

54 64 70 71 73 74 74

te

d

11.

cr

5.

us

4.

an

3.

M

2.

Method of Nanoparticles Morphology of preparation synthesized nanoparticles Thermal γ-Fe2O3 Spherical decomposition nanocrystals Iron oleic acid complex Thermal Fe, γ-Fe2O3 Nanocrystallites decomposition (ƞ 5C5H5)CoFe2(CO)9 Thermal Co-Fe alloy Nanocrystals oleic acid complex decomposition Metal oleylamine complex Thermal PbS, ZnS, Cube-shaped, rods, decomposition CdS, and bipods, tripods and MnS bullet-shaped nanocrystals Fe(II) and Fe(III) oleic acid– Thermal Fe3O4 Nanocrystallites oleylamine adduct decomposition Transition metal-P123 micelle Micellar mesoporous Nanoparticles, complex synthesis silica nanorods Iridium surfactant and CTAB Micellar mesoporous Nanoparticles synthesis silica K2[PdCl4]/CnTABr Micellar Pd Cubooctahedral synthesis particles [Pd(octen)2]Cl2 Microemulsion Pd Nanoparticles media Bis (doden)AgNO3 Microemulsion Ag Nanoparticles media Bis (doden)AuNO3 Microemulsion Au Nanoparticles media

ip t

S.No Metal surfactant . complex 1. Iron oleate complex

Ba(AOT)2

Microemulsion BaCrO4 media

13.

Cu(AOT)2

14.

Co(AOT)2

15.

Ag(AOT)

16.

EPE-Fe

17.

Gd (III) complex

18.

H2PtCl6- C16TAOH complex

19.

Au-PHEA polymer complex

20.

PdII–NR4X

Ac

ce p

12.

Microemulsion media Microemulsion media Microemulsion media Miniemulsion polymerization Miniemulsion polymerization Fibrous selfassembly on an HOPG surface Self-assembly structure Sonochemical

76

Co

Spherical, linear chains, rectangular superlattices and long filaments Mixture of spheres and cylinders Nanocrystals

Ag

Nanodisks

82

PPy/PB core/shell Gd

nanoparticles

83

Nanoparticles

84

Pt

Nanofibres

86

Au

Nanocrystals, cylinders Nanoclusters

90

Cu

Pd

78 80

101

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Ag(hexden)2NO3

23. 24. 25.

Ag AOT-TMH tris(bipyridine)ruthenium(II) K2[PdCl4], Na3[IrCl6] and C12EO8 complex K2[PtCl4], Na2[Co(EDTA)] and C12EO8 complex [Cu(CH3COO)4][C12H25NH3]2

Ag

Nanoparticles

89

Ag Ru Pd, Ir

Nanoparticles Nanoparticles Nanoparticles

104 97 92

TLCT

PtCo

Nanoparticles

99

Biphasic reduction

Cu

Nanoparticles

105

an M d te ce p

27.

103

Ac

26.

Nanobelts

ip t

22.

Au

cr

N-C12-N(AuCl4)2

us

21.

reaction Electrostaticall y driven reduction Biphasic reduction Reduction TLCT TLCT

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