Microemulsion-based synthesis of nanocrystalline materials Ashok K. Ganguli,* Aparna Ganguly and Sonalika Vaidya Received 23rd February 2009 First published as an Advance Article on the web 22nd September 2009 DOI: 10.1039/b814613f Microemulsion-based synthesis is found to be a versatile route to synthesize a variety of nanomaterials. The manipulation of various components involved in the formation of a microemulsion enables one to synthesize nanomaterials with varied size and shape. In this tutorial review several aspects of microemulsion based synthesis of nanocrystalline materials have been discussed which would be of interest to a cross-section of researchers working on colloids, physical chemistry, nanoscience and materials chemistry. The review focuses on the recent developments in the above area with current understanding on the various factors that control the structure and dynamics of microemulsions which can be effectively used to manipulate the size and shape of nanocrystalline materials. 1. Introduction The large number of unusual properties and applications associated with nanomaterials has triggered enormous interest among scientists from varied fields of research especially due to the interdisciplinary nature of this subject. Nanoscience and nanotechnology today is practised by chemists, biologists, physicists, material scientists and engineers who have put in tremendous efforts to understand new phenomena and develop technologies in this field. A major contribution to the development of this field has been made by chemists working primarily on the theme to design and control of nanostructures and also to functionalize them using both low-temperature solution-based routes and high-temperature (thermodynamic) methods. The microemulsion method is one among the various low-temperature routes to tailor nano- particles. The term ‘microemulsion’ was first coined by J. H. Schulman in 1959, 1 and since then its use has grown considerably and has received justified acclaim from the nanomaterials community. There have been several important reviews published on this subject especially during 1993–2006 by Pileni, 2 Eastoe, 3 Lopez-Quintela, 4 Capek, 5 Holmberg, 6 and Uskokovic. 7 In this review, we have given a brief introduction to the concepts and principles involved in microemulsions and their applications to nanomaterial synthesis. We then build on the information available in the previous reviews and focus on the developments in the past ten years, especially discussing the current understanding on the various factors controlling the structure and dynamics of microemulsions, and their manipulation to control the synthesis of nanocrystalline powders and related systems. In spite of the significant work carried out earlier in this field, many aspects of microemulsion Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India. E-mail: [email protected]; Fax: 91-11-26854715; Tel: 91-11-26591511 Ashok K. Ganguli Prof. Ashok Kumar Ganguli obtained his PhD from the Indian Institute of Science, Bangalore in 1990. He sub- sequently worked at Dupont Company, Wilmington, USA and Ames Laboratory, Iowa State University, USA. before joining IIT Delhi in 1995 where currently he is a full professor. His interests are in the synthesis and properties of nanocrystalline materials, complex metal oxides with dielectric and superconducting properties and polar inter- metallics. He has published over 125 papers in international journals and around 15 in conference proceedings and books, and was awarded the Materials Research Society of India Medal for 2006, and the Chemical Research Society of India medal for 2007. Aparna Ganguly Ms Aparna Ganguly obtained her BSc (Hons) in chemistry from Sri Venkateshwara College, University of Delhi in 2002. Later she obtained her MSc in chemistry from University of Delhi with specialisation in physical chemistry in 2004. Currently she is working as a joint PhD student of Prof. A. K. Ganguli (IIT Delhi) and Dr T. Ahmad (Jamia Millia Islamia) on microemulsion routes to synthesize functionalised nanostructures. 474 | Chem. Soc. Rev., 2010, 39, 474–485 This journal is c The Royal Society of Chemistry 2010 TUTORIAL REVIEW www.rsc.org/csr | Chemical Society Reviews Downloaded by University of Groningen on 19 October 2010 Published on 22 September 2009 on http://pubs.rsc.org | doi:10.1039/B814613F View Online
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Microemulsion-based synthesis of nanocrystalline materials
Ashok K. Ganguli,* Aparna Ganguly and Sonalika Vaidya
Received 23rd February 2009
First published as an Advance Article on the web 22nd September 2009
DOI: 10.1039/b814613f
Microemulsion-based synthesis is found to be a versatile route to synthesize a variety of
nanomaterials. The manipulation of various components involved in the formation of a
microemulsion enables one to synthesize nanomaterials with varied size and shape. In this
tutorial review several aspects of microemulsion based synthesis of nanocrystalline materials have
been discussed which would be of interest to a cross-section of researchers working on colloids,
physical chemistry, nanoscience and materials chemistry. The review focuses on the recent
developments in the above area with current understanding on the various factors that control
the structure and dynamics of microemulsions which can be effectively used to manipulate the size
and shape of nanocrystalline materials.
1. Introduction
The large number of unusual properties and applications
associated with nanomaterials has triggered enormous interest
among scientists from varied fields of research especially due
to the interdisciplinary nature of this subject. Nanoscience and
nanotechnology today is practised by chemists, biologists,
physicists, material scientists and engineers who have put
in tremendous efforts to understand new phenomena and
develop technologies in this field. A major contribution to
the development of this field has been made by chemists
working primarily on the theme to design and control of
nanostructures and also to functionalize them using both
low-temperature solution-based routes and high-temperature
(thermodynamic) methods. The microemulsion method is one
among the various low-temperature routes to tailor nano-
particles. The term ‘microemulsion’ was first coined by
J. H. Schulman in 1959,1 and since then its use has grown
considerably and has received justified acclaim from the
nanomaterials community. There have been several important
reviews published on this subject especially during 1993–2006
by Pileni,2 Eastoe,3 Lopez-Quintela,4 Capek,5 Holmberg,6 and
Uskokovic.7 In this review, we have given a brief introduction
to the concepts and principles involved in microemulsions and
their applications to nanomaterial synthesis. We then build on
the information available in the previous reviews and focus
on the developments in the past ten years, especially discussing
the current understanding on the various factors controlling
the structure and dynamics of microemulsions, and their
manipulation to control the synthesis of nanocrystalline
powders and related systems. In spite of the significant work
carried out earlier in this field, many aspects of microemulsion
Department of Chemistry, Indian Institute of Technology, Hauz Khas,New Delhi 110016, India. E-mail: [email protected];Fax: 91-11-26854715; Tel: 91-11-26591511
Ashok K. Ganguli
Prof. Ashok Kumar Ganguliobtained his PhD from theIndian Institute of Science,Bangalore in 1990. He sub-sequently worked at DupontCompany, Wilmington, USAand Ames Laboratory, IowaState University, USA. beforejoining IIT Delhi in 1995where currently he is a fullprofessor. His interests are inthe synthesis and properties ofnanocrystalline materials,complex metal oxides withdielectric and superconductingproperties and polar inter-
metallics. He has published over 125 papers in internationaljournals and around 15 in conference proceedings and books, andwas awarded the Materials Research Society of India Medal for2006, and the Chemical Research Society of India medalfor 2007.
Aparna Ganguly
Ms Aparna Ganguly obtainedher BSc (Hons) in chemistryfrom Sri VenkateshwaraCollege, University of Delhiin 2002. Later she obtainedher MSc in chemistry fromUniversity of Delhi withspecialisation in physicalchemistry in 2004. Currentlyshe is working as a joint PhDstudent of Prof. A. K. Ganguli(IIT Delhi) and Dr T. Ahmad(Jamia Millia Islamia) onmicroemulsion routes tosynthesize functionalisednanostructures.
474 | Chem. Soc. Rev., 2010, 39, 474–485 This journal is �c The Royal Society of Chemistry 2010
TUTORIAL REVIEW www.rsc.org/csr | Chemical Society Reviews
based synthesis are yet to be understood, to cater to the
increasing demands of precisely tailored nanomaterials. Hence
a lot of excitement prevails among scientists to explore its vast
horizon. This review attempts to put into perspective the
understanding available at present, and to foresee future
developments, which may become popular and meaningful
in the coming years.
1.1 Colloidal solutions
The use of colloidal gold had started from the ancient Roman
period to colour glasses red, mauve or yellow by varying the
concentration of gold. Paracelsus, the alchemist of 16th
century is claimed to be the first to have prepared a gold
colloid (Aurum Potabile). Inspired by his work, Michael
Faraday8 prepared a gold colloid solution in 1857. The finely
divided gold particles exhibited different optical properties
which Faraday recognized as being dependent on the size of
the particles. Colloidal solutions have since become a topic
of intense research due to their interesting properties and
applicability. To prevent the particles from aggregating,
stabilizers such as citrate ions are added which are adsorbed
on the surface of the particles inducing a surface charge and
hence repulsion from other particles to prevent agglomeration.
This is a kind of electrostatic stabilisation. Steric stabilisation
can be achieved with bulky organic molecules being present on
the metal surface providing a protective shield, as is the case
with surfactants. The stabilizer should coordinate to the
particle strongly enough to prevent agglomeration but should
also be easily removable from the metal surface.
Colloidal solutions have found application in synthesis of
novel materials, plastics and ceramics, and more recently in
nanotechnology. Due to their optical and electronic properties
they find use as biosensors, especially gold colloids, which are
being extensively studied for this purpose.9 Among other
important applications, silver colloids have been found to
have anti-bacterial activity, which is being exploited in textiles.
1.2 Surfactant aggregates
The word surfactant is derived from ‘‘surface active agent’’
and is known to reduce the interfacial tension between two
immiscible phases. They are mostly organic molecules with
a polar head group (hydrophilic) and a long alkyl chain
(hydrophobic part). Depending on the size of these two chains,
an empirical number, Hydrophilic–Lipophilic Balance (HLB),
has been assigned to the surfactants. It is a measure of the
degree to which it is hydrophilic or lipophilic. Griffin proposed
an HLB scale for non-ionic surfactants and the HLB number 1
was assigned to the most lipophilic molecule while 20 was
assigned to the most hydrophilic molecule. Various methods
have been described in literature to calculate the HLB number
of the surfactants. For instance, HLB values of polyhydric
alcohol fatty acid esters can be calculated using eqn (1) in
which Mh is the weight of the hydrophobic group and Mw is
the molecular weight.
HLB ¼ 20 1�Mh
Mw
� �ð1Þ
For fatty acid esters (Tween type), the HLB value can be
calculated using eqn (2) where E is the weight percentage of
oxyethylene and P is the weight percentage of polyhydric
alcohol.
HLB = (E + P)/5 (2)
Pasquali et al. in their studies has developed different
equations to calculate the HLB number for various kinds of
surfactants.10 The HLB value of the surfactant depends on its
structure and thus decides its action in the solution. The
application of the surfactant can be predicted from its HLB
number, for example w/o type of emulsions can be formed
using a surfactant with low HLB number while o/w emulsions
can be formed with surfactants having a high HLB number.
The other factor which is important when surfactants are
discussed is the critical micellar concentration (CMC). At
low concentrations, the surfactant dissolves in the aqueous
phase but when the concentration exceeds the critical micellar
concentration (CMC), the surfactant molecules organize
spontaneously to form aggregates such as micelles, vesicles
etc. Formation of such micelles is an entropy driven process.
Water molecules in the liquid state can be considered to have a
3-D structure of hydrogen bonds similar to ice with cavities. In
liquid water, there is always an equilibrium existing between
the destruction and formation of hydrogen bonds, which
results in movement of free water molecules through cavities.
In the presence of a hydrocarbon, the cavities are occupied by
the hydrocarbon molecules that results in restricted movement
of water; consequently water molecules surrounding the
hydrophobic solute become more ordered. During micellization,
there is a transfer of non-polar surfactant chains from an
ordered aqueous environment to the hydrocarbon-like
environment of the micelles, resulting in the disordering of
water molecules surrounding the non-polar molecules, thereby
increasing the entropy of the system and stabilizing the
microemulsion. In a micelle the hydrophobic tail of the
surfactant points towards the core while the polar head group
forms an outer shell. Such an assembly maintains a favourable
contact with water. Micelles can thus solubilize significant
amounts of non-polar molecules attributed to the hydro-
phobic core inside. Similarly, surfactants or amphiphiles may
Sonalika Vaidya
Ms Sonalika Vaidya obtainedher BSc (Hons) in chemistryfrom Hindu College, Univer-sity of Delhi in 2002. Sheobtained second position inthe university at her under-graduate level. She has beenawarded the Rastogi Awardin all the three years of herundergraduate studies forholding first position at collegelevel. Later she obtained herMSc in Chemistry from IITDelhi in 2004 after which shejoined for her PhD and iscurrently working on core–shell nanostructures inProfessor Ganguli’s group.
This journal is �c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 474–485 | 475
A. K. G. thanks CSIR and DST (Govt. of India) for financial
assistance and IIT Delhi for facilities; S. V. thanks CSIR and
A. G. thanks UGC for fellowships.
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